Optical Manual Final

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Fiber Optic Manual

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TABLE OF CONTENTS

Introduction ........................................................................................ iii

ACTIVITIES

IDENTIFYING THECOMPO NENTS.............................................................. 1

MAK INGA LIGHT GUIDE ........................................................................... 5

FIBER OPTIC CABLE TR A NSMISSIO N .......................................................... 9

CO NNECTORS A ND SPLICES ...................................................................... 15

I NDEX MATCHING....................................................................................

FIBER TER MINATIO NS...............................................................................

SPEEDOF OPTO-ELECTR O NIC DEVICES.....................................................

FIBER OPTIC TR A NSMITTERS.....................................................................

R ECEIVER AMPLIFIER DESIGNS...................................................................

APPENDIX

DEVICE PIN DIAGR AMS.............................................................................

R ECOMMENDED TEST EQUIPMENT...........................................................

SHIPMENT DAMAGEOR MISSINGPARTSCLAIMS......................................

LISTOF R EFERENCES.................................................................................

GLOSSARY.....................................................................................................

20

24

29

35

42

49

50

51

52

54

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- ii -



Introduction

This manual is an action-filled guide for completing nine stimulating activities related to fiber

optic communications.The manual is compatible wit h most classroom texts and is ideal for

creating a lab to go wit h almost any vocational or secondary-education fiber optics course.

For best results we suggest using t he "Hardware Kit" from Industrial Fiber Optics t hat

contains all t he necessary fiber optic, opto-electronic and electronic components required to

complete t hese nine activities.T o ac hieve t he best results and understand t he electronics

terminology here, we suggest t hat you have a minimum of one year of electronics experience.

Please read t he manual carefully w hen completing t hese activities.

Upon completing t he activities, you will have gained a better understanding of fiber optics

from having worked wit h real fiber optics hardware and learning tec hniques, and from gaining

hands-on experience. In "real-life" practice, t he components may c hange, but t he principles

remain t he same.

Sincerely,

Department of Electronics and

Communication,

Arya Institute of Engineering

and Technology,Kukas,Jaipur

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ACTIVITY I: COMPONENT IDENTIFICATION

In this activity you will identify the components in the IF-LMH kit. Table 1.1 is a list of components. Part numbers

have been included where necessary.

Table 1.1 IF-LMH Parts list.

General Description

Red LED (Flat-top T 1 3/4 pkg.)

Red LED (Blue w/pink dot)

Green LED (Blue w/white dot)

Infrared LED (Blue w/copper dot)

Sidelooker Device Housing (black)

Cinch Nut (black)

Penlight, 2-volt 60 mA

Photodiode (Sidelooker package)

Phototransistor (Sidelooker package)

Phototransistor (T 1 3/4 pkg.)

Photodarlington (Sidelooker package)

TTL open-collector hex inverterCMOS hex inverter

Operational amplifier

General-purpose NPN transistor

General-purpose NPN transistor

High-speed PNP transistor

Switching diode

0.01wf Mylar® capacitor

0.001wf ceramic capacitor

Fiber optic splice

Fiber optic retention clip

Fiber optic simplex receptacleFiber optic simplex assembly

1000wm core plastic optical fiber

Vinyl tubing 3/16 inch I.D.

1/4-watt carbon film resistors

2000-grit polishing paper

3 mm polishing film

Eye dropper

Industry Standard

P/N

OP950

LPT80A or PT1928

SHF300

OP560

74LS054069

LM741

2 N3904

P N2222

MPS3638A

1 N914

228051-1

228046-1

228042-1228087-1

SeeTable 1.2

IFO Reorder

P/N

750020

IF-E96

IF-E93A

IF-E91C

410509

410511

790005

770065

770025

770100

770050

730055730035

720075

710040

710025

710055

710010

640035

640030

2280511

2280461

22804212280871

IFCE1000

310005

290010

290024

860010

Quantity in kit

1 1 1

1

1

1

1

1

1

1 1

11

1

2

1

1

3

1

1

1

2

12

3 meters

15 cm

50

1 1

1

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Procedure

1.

2.

3.

Place all the electrical and fiber optic components contained in the kit on the left side of a work space with aflat surface such as a table approximately 60 x 90 cm (2 x 3 feet) in size. (The left side will contain unidentified components; on the right side will be the identified components.)

Locate the 3-meter length of 1000wm core plastic optical fiber and move it to the right side of your work space.

Identify the 15 cm (6 inches) length of vinyl tubing. It has an

outside diameter of 7.5 mm (5/16 inch) and an inside

diameter of 4.8mm (3/16 inch). Move it to the right side of your workspace. 1

1460.eps

4.

5.

6.

7.

Identify the 2000-grit polishing paper. It is approximately 50

x 50 mm (2 x 2 inches) in size, shiny black on the front side

and medium-gray on the back. Identify the 3wm polishing paper. It will be light pink in color and approximately 50 x 50 mm

(2 x 2 inches) in size also. Set both items to the right.

Select the red LED using Figure 1.1 to aid in its identification. The

red LEDwill emit red light when electrical current flows through

it. Place the LED to the right side of your workspace.

Identify the 3 fiber optic LEDs in the kit, which are the devices in a blue device housing.They are red, green and infrared light- producing. An electrical pin diagram of these devices can be seen

on page 49. The red LED has a pink dot on the device housing, thegreen has a white dot, and the infrared has a copper dot.

Identify the 2-volt penlight. It looks like a small

2

Figure 1.1 Red LED.

Figure 1.2 The 2-volt penlight.

8.

incandescent bulb and is approximately the same size asthe LEDs. A diagram of the penlight is shown inFigure

1.2.

Identify the 3 sidelooker style photodetectors in the kit.

They are a photodiode, phototransistor and photodarlington. A diagram of these devices can be seen

146

2

ps.e

on page 49. The photodiode has a blue stripe on the back,the phototransistor has a black stripe on the back and the

photodarlington has no marking.

1

Figure 1.3 A 14-pin integrated circuit (IC)

9. Identify the SFH300 phototranistor which is in a T 1 3/4 in a dual-in-line-package (DIP).

package. See page 49 for diagram of this part.

10. The three integrated circuits (ICs) contained in this kit are shipped in a plastic tube for protection. A 14-pin

DIP IC is shown in Figure 1.3. Part numbers are found on the topside of the ICs. Identify each of the generic

parts numbers LM741, 4069, and 74LS05.R etur n the ICs to their protective tube and set them to the right.

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s

1 . e

p

1 4

6



11.

12.

13.

Identify the four different transistors in the kit. All four transistorsare packaged inwhat is commonly called a TO-92 case. ATO-92

package is shown in Figure 1.4. On the flat side will be markingsor lettering.R ead the markings on each device and identify their

part numbers in Table 1.1. When all four transistors have been identified, set them to the right side of your work space.

Find the three 1 N914 silicon switching diodes. They are axial two-

leaded devices, slightly smaller than a resistor and have a copper base color with black band. The lead closest to the black band is

the device's cathode.Locate the fiber optics splice (P/ N 228051-1), simplex receptacle

(P/ N 228042-1), two retention clips (P/ N 228046-1), black Figure 1.4 A TO-92 case.

sidelooker device housing (410510), black cinch nut (410509) and two simplex assemblies (P/ N 228087-1).Use the component diagrams shown in Figure 1.5 to aid in identification.

Simplex Plug

1097.eps 1100.eps

1096.eps

Figure 1.5 Simplex plug assemblies, splices and fiber retention clips found in the IF-LMH kit.

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13.

14.

Identify the 100 pf ceramic disk capacitor (light brown in color)and the .01 Mylar ® capacitor, which has a green body. The

Mylar ® capacitor is shown in Figure 1.6.

The resistors listed in Table 1.2 are contained in a 2 x 4 inchreclosable plastic bag. Identify each by color and quantity.

Table 1.2 Color code bands for resistors in kit.

Description Quantity Color Code Figure 1.6 .01w f Mylar ® capacitor.

47; 100;

150;

220;

390;

470;

560;

820;

1k ; 2.2k ;

3.9k ;4.7k ;

5.6k ;

8.2k ;

10k ;

22k ;

39k ;

47k ;

56k ;

82 k ; 100 k ;

2 4

2

2

2

2

2

2

4

2

2

2

2

2

4

2

2

2

2

2

4

Yellow PurpleBlack

Brown Black Brown

Brown Green Brown

R edR edBrown

Orange WhiteBrown

Yellow PurpleBrown

Green BlueBrown

GrayR edBrown

Brown Black R ed

R edR edR ed

Orange WhiteR ed

Yellow PurpleR ed

Green BlueR ed

GrayR edR ed

Brown Black Orange

R edR ed Orange

Orange White Orange

Yellow Purple Orange

Green Blue Orange

GrayR ed Orange

Brown Black Yellow

1464.eps

Figure 1.7 Illustration of a typical resistor.

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1 4 6 3 . e p s



ACTIVITY II: MAKING A LIGHT GUIDE

Overview

In this activity you will construct a simple light guide using water and a length of vinyl tubing.The water and vinyl

tubing will act as the core, while air will act as the claddi ng or boundary layer. The experiment will demonstrate how

effective even a simple light guide is for coupling energy from a light source to a detector. You will also observe how thelight guide can carry light "around a cor ner"with relatively little loss compared to when light 1348.eps

travels in a straight line. Cladding

Core

Materials Required

Red LED

Phototransistor (T 1 3/4 package)

Vinyl tubing, 15 cm

150;resistor

Eye dropper

Single-edge razor blade or sharp knife*

Figure 2.1 Cross section of an optical fiber with a light raytraveling down the core.

Paper towels*

Small, shallow, water-tight pan*

Miscellaneous electrical test leads*

Multimeter*

Distilled Water*

Solderless breadboard*

Variable voltage power supply*

* Not included in the IF-LMH kit. For suggestions on recommended test equipment see APPENDIX.

Procedure

1.

2.

3.

4.

5.

6.

Using a single-edge razor or sharp k nife trim a small amount from the ends of the vinyl tubing so that they are

clean and square (90 degrees).

Insert the red flat-topped LED into one end of the vinyl tube. Be sure to insert the LED all the way into thetubing to ensure a tight fit.

Insert the phototransistor (T 1 3/4 package) into the other end of the vinyl tubing. Push the phototransistor in

completely for a tight fit.

Tur n on the variable voltage power supply and adjust the output to + 5 volts DC. Set

the function of the multimeter to read "Current" on the 2 mA scale.

On your solderless breadboard connect the electrical circuits as shown in Figure 2.2.Use the device diagrams

found in the APPENDIX to identify anode and cathode on the LED, and collector and emitter on the

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phototransistor. (The phototransistor and 1465.eps

+ 5 volts

multimeter circuit willfunction as an inexpensive

radiometer to evaluate thelight guide. This type of circuit

photodetector/multimeter)will be used as a radiometer

150;

RedLED Light Pipe

+ -

Multimeter

7.

8.

throughout this manual.)

Light should be visible fromthe red LED at this point. If

not, check the electricalconnections to the LED.

The multimeter should

indicate current flow through

Photo-transistor

Figure 2.2 Test circuits for evaluating light guide.

the phototransistor. If not, check the electrical connections and correct polarity for the phototransistor.

To obtain best r esults in thi s act ivity, you may need to d im the r oom l ig hts or cover the l ig ht g uide with a

dark cloth or box. Thi s will minimize the chance of ambient l ig ht being captur ed by the photot r an si stor , and

im pr ove the accur acy of your measur ements.

9. In Table 2.1 record the current measured by the multimeter (LEDO N).

10. Disconnect the 150;resistor from the + 5volt power supply, which will tur n the LED

off. Table 2.1 Empirical data for 15 cm (6-inch) light guide

with air core.11. In Table 2.1 record the current measured

12.

by the multimeter through the phototransistor with the LED off.

R emove the vinyl tubing, red LED and

phototransistor as an assembly from the

solderless breadboard. Pull the red LED

LEDs

Red

LED OFF LED ON

from the vinyl tubing (leaving the phototransistor in), and slowly fill the vinyl tubing with distilled water usingthe eyedropper. Do not hurry when filling the tubing; try to put in a drop at a time to avoid leaving any air

bubbles in the tubing.Bubbles will scatter some of the light being transmitted through the water.

13.

14.

15.

R e-insert the red LED in the vinyl tubing and push in completely for a tight fit to prevent water from leaking

out. Make certain there are no air bubbles inside the tubing between the red LED and phototransistor. R efill as

necessary during the experiment if any water leaks out.

R e-connect the red LED and phototransistor to the circuit on the solderless breadboard. R e-connect the 150; resistor to the +5 volt power supply. InTable 2.2 record the current measured by the multimeter (LEDO N).

Disconnect the 150;resistor from the + 5 volt power supply, which will tur nthe LEDoff. InTable 2.2

record the current measured by the multimeter through the phototransistor with the LED off.

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16. Gently make a 90-degree bend in the light guide and repeat steps 14 and 15. Be careful to not let any water leak out from the light guide ² refill if necessary. R ecord the data inTable 2.3.

17. Dip the light guide into a pan

of water.Describe below

Table 2.2 Empirical data for 15 cm light guide with

water core.

what happens to currentmeasured by the multimeter,

and what happens to the redLED light. (It may help to dim

Source

Red

LED OFF LED ON

the room lights to view the Table 2.3 Empirical data for light guide with 90-degree

LED light better.) bend.

18. Tur n off the power supply and

retur n all items to their proper storage containers andlocations.

Source

Red

LED OFF LED ON

Analysis & Questions

W hat i s the amount of l ig ht in mill iwatts (mW) that falls on the photot r an si stor when using the r ed LED with the

l ig ht g uide and water cor e [ assuming the r espon sivity of the photot r an si stor to be 50 mill iam per es/ mill iwatt

(mA / mW)]? W hat i s it with no water in the cor e?

Does the l ig ht g uide send mor e or less l ig ht onto the photot r an si stor with water in the cor e? W hy?

Did the 90-de gr ee bend signi f icantly chang e the amount of l ig ht hitt ing the photot r an si stor ? W hy or W hy not ?

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C alculate the crit ical ang le of the l ig ht g uide with the cor e. A ssume canola water has a r ef r act ive index of 1.33

and the cladd ing has a r ef r act ive index of 1.0.

U!sin1n1 n2

n1r ef r act ive index of the cladd ing , 1.0

n2r ef r act ive index of the cor e, 1.45

Do you know of an y other l iquids which may t r ap mor e l ig ht in side the vin yl tubing than the water used in thi s

experiment ?

HOMEWORK PROJECT

Find at least two common diameters of core and cladding used in communication-grade optical fibers. Be sure

to include units of size (meters, inches, centimeters, etc.) The periodicals in the List of References are a good place

to look for such information.

-8-



ACTIVITY III: FIBER OPTIC CABLE TRANSMISSION

Overview

In the preceding activity you constructed basic light guides using simple materials. In practice, few people make

their own optical fiber. Commercially available optical fiber has much superior "performance" and is much more

convenient to obtain. Optical fibers are composed of one of the following materials: Glass Plastic Other

More than 99 percent of all fiber optics cable used in the world is made from glass or plastic. The category "other"includes exotic optical materials such as silicon or gallium arsenide, which is used for ultraviolet or infrared lightapplications.The remaining activities in this manual will utilize a commercially available plastic core optical fiber. It is

inexpensive, easy to use, safe and inmost cases handled very much like the copper wire or cable that it replaces.In each field "performance" has its ownmeaning. In fiber optics one of the terms that defines optical fiber

performance is attenuation, or light loss per unit of travel. In this activity you will measure the light transmitted throughseveral lengths of optical fiber and use the measured results to determine what is k nown as the attenuation coefficient,

E , of the fiber.

Materials Required

Green LED (IF-E93A-blue w/white dot)

Red LED (IF-E96-blue w/pink dot)

Phototransistor LPT80A

Sidelooker device housing (black)

Infrared LED (IF-E91C-blue w/copper dot)

390;resistor

Cinch Nut (black)

* Not included in the IF-LMH kit.

Procedure


Small flat bladed screwdriver*

Multimeter*

1-meter measuring device*




1.

2.

3.

Cut 2 mm (.1 inch) off the ends of the 3 meter optical fiber with a single-edge razor blade or sharp k nife. Try toobtain a precise 90-degree angle (square).

While observing one end of the optical fiber, aim the other end toward a sunlit window or other strong light

source. As the "aimed" end faces the light source, the "viewing " end should brightenwith the transmittedlight.

Figure 3.1 shows a cross-sectional view of a sidelooker device housing with a component inserted. The

component can either be an LED or photodetector, like the ones in the kit provided with this manual. Withthe phototransistor in hand, look at the pictorial of the device on page 49 of the manual, and notice the raised

section of the body. This dome shaped protrusion acts as a micro-lens. Note in Figure 3.1 that the micro-lens

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lines up with the fiber entry point in the device housing. When a fiber is installed flush to the micro-lens, itcaptures the light and helps focus it on the photodetector. The same principle applies in reverse for the case of an LED component.

4. Identify and set aside the red, green, and infrared LEDs 1527.eps

5.

6.

in the blue fiber optic device housings.

Using a small flat bladed screwdriver, gently insert the

phototransistor (LPT80A) into the black sidelooker

device housing.

Thread the black cinchnut onto the sidelooker housingcontaining the phototransistor. Insert the ends of the 3-meter optical fiber into the red LED and phototransistor

sidelooker device housings. Push in the fiber until theend seats against the red LED and phototransistor micro

Figure 3.1 Cross-sectional view of fiber optic

housing with photodetector installed.

7.

8.

lens. Tighten the cinchnut for a snug fit to lock the fiber in place.

Using a solderless breadboard, electrically connect the red LED and phototransistor as shown in the circuit

diagram in Figure 3.2. Use the device diagrams found in the Appendix to identify anode and cathode on theLED, and collector and emitter on the phototransistor. (A radiometer or optical power meter can be used

instead of the phototransistor, device mount and multimeter if your lab has this equipment available.)

Set the function of the multimeter to read "Current" on the 2 mA scale.

9. Tur n on the variable

voltage power supply andadjust the output voltageto + 5 volts DC. From the

bottom of the device

1517.eps

390;

Multimeter + -

+ 5 volts

10.

housing red light should

now be visible from theLED. If not, check your

power supply, electricalconnections andcomponents.

R ecord the current

measured by the

LED

Fiber Cable

Photo-transistor

Figure 3.2 Test circuits for measuring cable transmission.multimeter in Table 3.1.

11.

12.

13.

14.

15.

R eplace the red LEDwith the green LED. Be sure to re-insert the fiber and tighten the cinchnut snugly.

R ecord the phototransistor current measured by the multimeter in Table 3.1. R eplace the green LEDwith

the infrared LED, then repeat Step 13.

R emove the 3-meter optical fiber assembly from both sidelooker device housings.

Measure a distance of one meter from one end of the 3-meter fiber assembly.

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16. Cut the cable with a single-edge razor blade or sharp k nife at the 1-meter point. Try to obtain a precise 90-degree angle (square).

17. Insert the 1-meter optical fiber cable into the sidelooker device housings containing the infrared LED and

phototransistor, then push in place until the ends seat against the micro lens. Tighten the cinchnuts snugly.

18. R ecord the current measured by the

multimeter in Table 3.1.Table 3.1 Measured phototransistor current with 3- and

1-meter lengths of 1000wm core plastic fiber.

19.

20.

Complete the remainder of Column

3 in Table 3.1 using the red and

greenLEDs.Tur n off the variable voltage power

supply, then unhook and retur n allthe components to their proper storage container and locations.

LED

RedGreen

Infrared

i3meter i1meter


In general, the output power from a fiber cable at a given length is defined by the equation:

P l ! P o eEl P o launch power L leng th of

f iber

E attenuat ioncoeff icient

From the equation above, another equation can be derived to determine the attenuation coefficient,E, from havingmeasured the optical power output at two different optical fiber lengths. That equation is:

E! ln( P 1/ P 2) L2L1

P 2 power output of f iber leng th2

P 1 power ouput of f iber leng th1

L2 leng th of f iber 2 L1 leng th of

f iber 1

lnnatur al lo grithumof x

In this activity we did not measure optical power, but rather phototransistor current, which is linearly proportional to

the optical power. Substituting the phototransistor current for optical power and rewriting the equation above for the 1-and 3-meter optical fiber lengths used in this activity the equation becomes:

E! ln( I 1 / I 2) L2L1

I 2measur ed curr ent for 1meter f iber

I 1measur ed curr ent for 3meter f iber

L2 leng th of 1meter f iber L1 leng th of

3meter f iber

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C om plete Table 3.2 to

calculate the f iber attenuat ion

coeff icients for each of the Table 3.2 Table for calculating attenuation coefficients.

LED s f r om the data r ecor ded in

Table 3.1.

Ar e the attenuat ion coeff icient

values the same for the

d i ffer ent LED s? W hy or why

not ?

LED

Red

Green

Infrared

i(3meter )

i(1meter )

ln

i(3meter )

i(1meter )E!

ln ii((31meter ))

meter

2

The launch power ( curr ent ) into a f iber can be calculated by r earr anging the f ir st equat ion described in thi s

act ivity. The r earr ang ed equat ion for calculat ing launch power (when the attenuat ion i s known ) i s shown below:

P o! P l eEl

P o launch power l leng th of

f iber

E attenuat ioncoeff icient

P l power at given leng th

C alculate the launch power for each LEDusing Table 3.3. (S ubst itute the photot r an si stor curr ent for power in

thi s equat ion because the photot r an si stor curr ent i s l inear ly pr opor t ional to the opt ical power . )

Table 3.3 Calculation of launch power (equivalent current) for each LED.

LED

RedGreen

Infrared

P 1meter E

eE(1) P o! P 1meter eE(1)

C alculate the photot r an si stor curr ent pr oduced by the l ig ht f r om the thr eeLED s having t r aveled down a 5-meter

plast ic opt ical f iber . U se Table 3.4 as a g uide. (ObtainE f r omTable 3.2 and P 0 f r omTable 3.3. )

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C alculate the photot r an si stor curr ent pr oduced by l ig ht f r om the thr ee LED s t r avel ing down a 10-meter plast ic

opt ical f iber , using Table 3.5 as a g uide. (Obtain a f r om Table 3.2 and P 0 f r om Table 3.3. )

Table 3.4 Calculation of phototransistor current for a 5-meter fiber length.

LED

Red

Green

Infrared

P

o

E l

5

55

eE( 5) P 5meter ! P o eE(5)

Table 3.5 Calculation of phototransistor current for a 10-meter fiber length.

LED

Red

Green

Infrared

P

o

E

l

10

10

10

eE(10) P 10meter ! P 0 eE(10)

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P lot measur ed photot r an si stor curr ents for the 1-meter and 3-meter leng ths, and the calculated photot r an si stor

curr ents for 5- and 10-meter f iber leng ths for all thr eeLED s in Figure 3.3.

Figure 3.3 Phototransistor current created at the end of various fiber

lengths for three different LED light sources.

HOMEWORK PROJECTFind the optical wavelength, in nanometers, of the colors red and green. An excellent source for such information would be a good encyclopedia or physics book.

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ACTIVITY IV: CONNECTORS AND SPLICES

Overview

In the previous experiment you lear ned that while having

many advantages, fiber optics technology is not "perfect" because

some light is lost as it travels down the optical core. Light loss insidethe fiber, or attenuation, is called "intrinsic" loss. This intrinsic losscan be categorized as either scattering or absor pt ion. With high-

quality raw materials, super-quality clean rooms and moder n manufacturing methods, intrinsic fiber loss can be reduced almost tozero. The clarity of today's premium quality optical glass fiber is

comparable to a slab of window glass one mile thick losing only 50 percent of the light intensity passing through.

Fiber optics systems cannot always be installed with a singleuninterrupted length of optical fiber. Often, two or more fiber

lengths must be joined in order to obtain a necessary length, or route through buildings and enclosures. Losses from theseconnections are called "extrinsic" losses because they occur outside

100

10

1

0.1

0.01

GH 4001; Datacomgrade

Launch NA = 0.1

400 500 600 700 800 900 1000

the optical fiber core and cladding boundary.The two most common extrinsic losses due to joining or

connecting optical fibers occur at:

Splices - Permanent connections of two optical fiber lengths

that may be thermally fused or mechanically applied.

Wavelength (nm)

Figure 4.1 Attenuation of 1000wm core plastic fiber versus wavelength.

Connectors - Junctions that allow an optical fiber to readily be attached or detached from a light source, detector or

another fiber.

In this activity you will work with and measure the attenuation in a fiber optic connector and splice.

Materials required

Phototransistor LPT80A from activity III

2-meter fiber from activity III

Red LED (IF-E96-blue housing pink dot)

Simplex assembly 228087-1

Retention clips 228046-1 (2)

Splice 228051-1

Infrared LED (IF-E91C-blue housing-copper dot)

Simplex receptacle 228042-1

390;resistor


1-meter fiber from activity III

Multimeter*


Needle-nose pliers*



Solderless breadboard* 18-

gauge wire stripper*

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T r a n s m i s s i o n L o s s ( d B / m )



Procedure #1: Fiber Connectors

1. Take the 1-meter optical fiber cable from

the previous activityand use an 18 gauge

wire stripper to remove5mm (3/16 inch) of

1518.eps

390;

Multimeter + -

+ 5 volts

the fiber jacket fromone end.Be careful not

to k nick the fiber whilestripping or you'll lose

LED

Fiber Cable

Simplex

Photo-transistor

2.

light later in theexperiment.

Use the needle nose

pliers to push the

stripped cable end intothe large opening in the

Receptacleor Splice

Figure 4.2 Test circuits for measuring cable transmission with bulkhead connectors and splices.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

simplex assembly. Continue pushing the cable until the fiber tip is flush with the end of the simplex assembly body. R epeat Steps 1 and 2 above for one end of the 2-meter optical fiber cable.

Assemble the electrical circuit shown in Figure 4.2 on the solderless breadboard.Use the red LED device

housing as the light source to start this activity.Set the function of the multimeter to measure "Current", on the 20 mA scale.

Insert the bare end of the 2-meter optical fiber cable into the red LED/device housing and push until it seats

against the microlens. Tighten the cinchnut snugly to lock the fiber in place.

Insert the simplex assembly end of the 2-meter optical fiber into one port of the simplex receptacle (partnumber 228042-1). Push until the fingers on the simplex receptacle fully capture the flanges on the simplex

assembly.

Insert the simplex assembly end of the 1-meter optical fiber cable into the remaining port of the simplex

receptacle. Push until the fingers on the simplex receptacle fully capture the flanges on the simplex assembly.

Insert the bare end of the 1-meter optical fiber into the phototransistor device housing and push until it seats

against the micro lens. Tighten the cinchnut snugly to lock the fiber in place.

Tur n on the variable voltage power supply and adjust the output voltage to + 5 volts DC. From the bottom of

the device housing red light should now be visible from the LED. If not, check your power supply, electrical

connections and components.

R ecord the current measured by the multimeter in R ow 2, Column 3 of Table 4.1.

Tur n off the power supply.

R emove the fiber end from the red LED device housing.

- 16 -



13. R eplace the red LEDwith the infrared LED on the solderless breadboard. Insert the end of the 2-meter fiber into the infrared LED device housing, until it seats against the micro lens, and then securely tighten the cinchnut.

Table 4.1 Measured phototransistor current with 2- and 1-meter14.

15.

Tur n on the power supply.

R ecord the current measured by

fibers joined with a simplex receptacle.

16.

the multimeter in R ow 3, column 3 of Table 4.1. (If the current

measured is zero, check to

determine if the LED is installedcorrectly.)

R emove the 1-meter fiber from the

simplex receptacle and rotate thesimplex assembly, including fiber,

LED

Red

Infrared

Position

#1#2

#3

#4

i sim plex

17.

18.

19.

clockwise 90 degrees. R e-insert the simplex assembly in the simplex receptacle and press it into place until thefingers on the simplex receptacle fully capture the flanges on the simplex assembly.

R e-measure the phototransistor current with the multimeter and record the current in R ow 4, Column 3 of

Table 4.1.

R epeat Steps 16 and 17 two more times and record the measured results in rows 5 and 6 of Table 4.1

respectively.


Procedure #2: Fiber Splices

1.

2.

3.

4.

5.

R emove the 2-meter and 1-meter fibers from the simplex receptacle. Leave the other end of the fiber cables

attached to the LED and phototransistor device housings.Be careful in the next few steps to not pull on thesecomponents.

Using a sharp k nife or single edge razor, cut off the simplex assemblies from the free ends of the 1- and 2-

meter fiber scables. To preserve length, cut close to the shoulder of the simplex assembly where the fiber

cables enter.

Trim the 1-meter cable further by laying it on a flat surface and cutting an additional 1 mm (0.04 inch) off the

end with a sharp k nife or single-edge razor blade. Try to obtain a precise square cut, at a 90-degree angle tothe length of the cable.

Pick up the retention clip (part number 228046-1) and look at it closely. Identify the end of the retention clip

opposite the triangular cutout or "V" groove. Insert the free end of the 1-meter fiber cable into this side of theretention clip.

Place the tip of the retention clip end on a hard, flat surface and press down on the cable until the end is flush

with the end of the retention clip. R epeat Steps 3 through 5 for the 2-meter fiber cable.

- 17 -



6.

7.

8.

Using needle-nose pliers gently grip the 1-meter fiber cable just behind the retention clip. Push the retention clip and fiber into the fiber splice (part number 228051-1) until the back end of the retention clip is flush withthe rear of the splice.

R epeat steps 6 above for the 2-meter cable and retention clip. Continue pushing the 2-meter cable until both

fibers make physical contact inside the splice. The fiber cores should touch when the backs of both retention clips are even with the rear of the splice. (Both clips will be fully inside the splice).


Table 4.2 Measured phototransistor9. R ecord the current measured by the multimeter in R ow

3, column 2 of Table 4.2.

current with 2-meter and 1-meter optical

fibers spliced together.

10.

11.

12.

13.


R eplace the infrared LEDwith the red LED, tighten the

cinchnut and tur n on the power supply.

R ecord the current measured by the multimeter in R ow

2, Column 2 of Table 4.2.

Tur n off the power supply and multimeter.

LED

Red

Infrared

i spl ice

14.

15.

Disconnect all the electrical connections from the multimeter and power supply that are attached to the

breadboard. R emove the fiber from the LED and phototransistor device mounts.You can leave the electricalcircuit intact on the solderless breadboard and the 1- and 2-meter fibers spliced together, because you will be

using them in the next activity.R etur n all remaining items to their proper storage containers and locations.


F r om Activity III, Table 3.1, C olumn 2 , copy the measur ed photot r an si stor curr ents for the r ed and in f r ar ed

LED s and r ecor d them in Row s 2 and 3, C olumn 3 of Table 4.3. F or r ow s 4 thr ou g h 6 (in f r ar ed LED ) write in

the same number s as those in Row 3.

C opy the measur ed data f r om C olumn 3 of Table 4.1 and r ecor d it in C olumn 4 of Table 4.3.

Divide C olumn 4 by C olumn 3 and mult i ply by 100 for r ow s 2 thr ou g h 6 in Table 4.3. Recor d your r esults in

C olumn 5.

I s the f iber' s t r an smi ssion mor e or less after the f iber connector i s in stalled in the 3-meter f iber ? W hy?

W hat happen s to the measur ed photot r an si stor curr ent when the sim plex assembly and 1-meter f iber ar e r otated

to d i ffer ent posit ion s within the sim plex r eceptacle? Describe below the physical cond it ion s that ar e occurring

and why. (Dr awing a pictur e mig ht be helpful. )

- 18 -



F r om Row s 2 and 3, C olumn 3 in Table 4.3 , copy the measur ed photot r an si stor curr ent for the 3-meter

cont inuous f iber and r ecor d it in C olumn 2 of Table 4.4. C opy the data f r om C olumn 2 of Table 4.2 to C olumn

3 in Table 4.4.

Divide C olumn3

by C olumn 2 ,

mult i ply by 100 for

Row s 2 and 3 in

Table 4.3 Comparison of transmission characteristics of a continuous 3-meterfiber optic cable to those of a 3-meter fiber length with fiber optic connectorinstalled.

Table 4.4 and

write the r esults in

C olumn 4of Table

4.4.

I s the t r an smi ssion

gr eater for the 3-

meter f iber with

the spl ice in stalled

or with the sim plex

r eceptacle? I s thi s

LED

Red

Infrared

R otation

#1

#2

#3

#4

i Act ivity III i sim plex

% Transmission

what you expected ? W hy or why not ?

Table 4.4 Comparison of the transmission characteristics of acontinuous 3-meter fiber optic cable and those of a length

with a splice in it.

I n your own wor ds, state at

least t wo ad vanta g es and

d i sad vanta g es of f iber

connector s ver sus f iber

spl ices. Li st at least t wo for

each.

LED

Red

Infrared

i Act ivity III i spl ice % Transmission

HOMEWORK PROJECT

In a large metropolitan phone book find the names, addresses and phone numbers of two fiber optics companies. Ask them to send you information about their products or services.

- 19 -



ACTIVITY V: INDEX MATCHING

Overview

Extrinsic losses, as we noted in the previous activity, occur

outside the optical fiber. As demonstrated by the rotation of the

simplex assembly, physical alignment of two optical fibers has a

great deal of effect on the integrity of optical coupling between

them. The most commonly used method to minimize loss

caused by alignment mismatch is to maintain very close

tolerances on the diameters and concentricity of the optical fiber

and fiber terminations.

Another common extrinsic loss is caused by Fresnel

reflections (sometimes called Fresnel losses). These reflections

are analogous to voltage standing waves (measured as ratios or

VSWR ) that reduce power transfer in radio frequency (RF)

applications.The greater the electrical mismatch (higher VSWR ),

n1

Reflected

Incident

n2

Transmitted

the more power is reflected back. Fresnel reflections

demonstrate this same effect in a different part of the

electromagnetic spectrum. See Figure 5.1. The greater the

mismatch of refractive indexes as light travels from one medium

to the other, the greater the amount of optical energy that is

reflected back. For a uniformly polarized light beam at near

perpendicular angles the generalized equation for Fresnel

reflections is:

1468.eps

Figure 5.1 Diagram of incident light ray being split into two components at the boundarybetween optical materials with different refractive

indices.

J%!n2n

1

2

n2 n1

n1r ef r act ive index of material 1

n2r ef r act ive index of material 2

Fresnel losses in an optical fiber connection/splice can bereduced by a method k nown as index matching.This involves

filling the microscopic air gap between the two optical cores

with a material that has a refractive index very close to that of

the core materials. In this activity you will demonstrate how

index matching can reduce Fresnel losses in a fiber optic splice.

Figure 5.2 Magnified view of air gap betweentwo optical fiber cores in a splice that causes

Fresnel reflections.

- 20 -



Materials Required

Phototransistor, LPT80A in black sidelooker housing

Red LED (IF-E96-blue housing-pink dot)

1 & 2-meter optical fibers joined together in a splice


Eyedropper

390;resistor


Procedure #1: Index matching

Needle-nose pliers*


Multimeter*


Miscellaneous electrical leads*

1.

2.

Begin this activity by using the red LED housing as the light source.

Assemble the

electrical + 5 volts

1520.eps

circuits shown in Figure 5.3

on your

solderless breadboard.

390;

Multimeter +

-

3.

4.

Set the fu

nctio

n of the

multimeter tomeasure

"Current" on the 2 mA scale.

Insert the

LEDFiber Cable

Splice

Photo-transistor

unattached end

of the 2-meter optical fiber into

Figure 5.3 Test diagram for measuring photodiode current with a fiber

splice installed.

the red LED device housing and push until it seats against the micro lens. (The other end should still beinstalled in the splice from Activity IV along with the 1-meter fiber. If there is no splice joining the 1-meter and2-meter fibers, go back to Activity IV for instructions on doing so.)

5.

6.

7.

8.

Insert the free end of the 1-meter optical fiber in the phototransistor housing. Tighten the cinch nuts to lock

the fiber ends in place.

Tur n on the power supply and set the output voltage to + 5 volts DC.R ed light from the red LED should be

visible from the bottom of the device housing. If not, check the power supply, electrical connections andcomponents.

R ecord the current measured by the multimeter in R ow 2, Column 2 of Table 5.1.

Tur n off the power supply and remove the fiber end from the red LED device housing.

- 21 -



9. R eplace the red LEDwith the infrared LED on the solderless breadboard. Insert the end of the2-meter fiber into the infrared LED housing until

Table 5.1 Measurements of phototransistorcurrent when 2-meter and 1-meter fibers arespliced together with and without index-matching.

10.

11.

it seats against the micro lens, and then securely

tighten the cinch nut.


R ecord the current measured by the multimeter

in Table 5.1, R ow 3, Column 2.

LED

Red

Infrared

i spl ice iindexmatched

12.13.

14.

15.

16.

17.

R emove the 1-meter fiber from the splice ² there should be a retention clip attached.Using the eyedropper place one drop of glycerin on the core (the clear center portion) of the 1-meter fiber.

Firmly hold the 2-meter fiber where it enters the splice (behind the retention clip). Insert the 1-meter fiber and

retention clip into the fiber splice.

Using needle-nose pliers, gently grip the 1-meter optical fiber behind the retention clip and push it into the

splice until the fiber cores touch. The fiber cores should touch when the back of both retention clips are

approximately even with the rear of the splice.

R ecord the current measured by the multimeter in R ow 3,

Column 3 of Table 5.1.

Tur n off the power supply and remove the fiber end from the

infrared LEDhousing..eps1067

18.

19.

20.

21.

22.

23.

R eplace the infrared LED the red LED. R e-insert and seat the

fiber, then securely tighten the cinch nut.


R ecord the current measured by the multimeter in R ow 2,

Column 3 of Table 5.1.

Tur n off the power supply and multimeter.

R emove the 2-meter and 1-meter fibers from the splice.

R emove the fibers from the LED and phototransistor device housings.

Figure 5.4 Correct position for placing a drop of glycerin on the 1-meter fiber end.

24. Tur n off the variable voltage power supply, then unhook and retur n all the components to their proper storage

container and locations.

- 22 -




C opy the data f r om Table 5.1 into Table 5.2.

F or Row s 2 and 3 in Table 5.2 , d ivide C olumn 3 by C olumn 2 , subt r act one, and mult i ply the r esult by 100.

W rite the r esults in C olumn 4.

E xplain the chang e in t r an smi ssion ind icated by C olumn 4 in Table 5.2.

E xplain in your own wor ds

what "index-matching" mean s. Table 5.2 Comparison of the transmission characteristics of a fiber

splice with and without index-matching.

C alculate thema gnitude of a

F r esnel r eflect ion using the

equat ion at the int r oduct ion to

LED

Red

Infrared

i spl ice iindexmatched % Improvement

thi s act ivity. A ssume the r ef r act ive index of M ed ium #1 , the f iber cor e, to be 1.49 and M ed ium #2 , air , to be 1.0.

B y the boundar y cond it ion s shown in Figure 5.2 , how man y F r esnel r eflect ion s ar e ther e for an opt ical r ay

passing f r om one f iber into the other ? C alculate the incr ease in f iber t r an smi ssion i f a per fectly index-matched

g el f illed the g ap bet ween the f iber s.

HOMEWORK PROJECTIn an optics or physics books look up the detailed equations for Fresnel reflections that include effects of

polarization and incident angles.

- 23 -



ACTIVITY VI: FIBER TERMINATIONS

Overview

In the preceding activity we lear ned about

Fresnel losses and how to reduce them. As you may

already k now, the preparation of the fiber optic"end", or core/cladding, is very important in determining the coupling between two fibers. At

the top of Figure 6.1 is an illustration of an idealfiber termination. It has a completely flat surfacewith no irregularities or imperfections to scatter

light outside the expected light cone. The bottomhalf of Figure 6.1 depicts a fiber end with a rough

end surface which causes light rays to scatter over a

much greater angle. When the light rays arescattered over a greater angle, the adjoiningdetector or fiber does not capture as much of theexiting light.

In this activity we will go through the varioussteps of terminating an optical fiber and measure

improvements in coupling with each step. The baseline measurement will be an optical fiber cut

with a wire cutter. (Using a fiber with such a poor

1470.eps

Nor mal light

cone

Expanded light

cone due to

imperfect surface

terminationwould never be acceptable in realapplications. We do so here only for educational

purposes.) The fiber will then be cut and polished in

successive steps to create a proper fiber endtermination.

Materials Required

Figure 6.1 (Top) Fiber with a perfectly flat termination,arrows depicting the cone of light exiting the fiber core.(Bottom) A poor fiber polish/termination showing theresultant increased divergence of exiting light rays.

Phototransistor, LPT80A in black sidelooker housing

2-meter optical fiber


2000 grit polishing paper (dark gray color)

3wm polishing film (pink color)

390;resistor

1-meter optical fiber


* Not included in the IF-LMH kit


Paper Towels*


Diagonal-cutting pliers*

Water, light oil or glycerin*

18-gauge wire stripper*


Multimeter*

- 24 -



Procedure #1: Fiber Polishing

1.

2.

3.

4.

Using the diagonal-cutting pliers, cut 19 mm (3/4 inch) off the 1-meter fiber cable end removed from thesplice in the previous activity. This removes the retention clip and a small amount of fiber from the cable.

Assemble the circuit diagram shown in Figure 6.2 on the solderless breadboard.

Insert the fiber end cut with the diagonal-cutting pliers into the infrared LED device housing, then push in

place until it seats against the micro lens. Tighten the cinch nut to lock the fiber in place.

Insert the other end of the 1-meter fiber assembly into the phototransistor device housing and push it into

place, then tighten the cinch nut.

5.

6.

Set the function of the

multimeter tomeasure "Current", on the 2 mA scale.

Tur n on the power

1521.eps

390;

Multimeter + -

+ 5 volts

7.

supply and set theoutput voltage to + 5voltsDC.

Measure the current

through the

phototransistor withthe multimeter andrecord the results in

IRLED

1-meter Fiber Cable

Photo-transistor

8.

R ow 2, Column 2 of

Table 6.1.

R emove the opticalfiber end from the

infrared LEDdevice housing.

Figure 6.2 Test diagram for measuring the effects of various fiber terminations.

9.

10.

11.

12.

13.

14.

Cut 1 mm (.040 inch) off the end of the 1-meter fiber with a single-edge razor blade or sharp k nife, trying to

achieve a square (90 degree) edge.

Insert the newly cut fiber end into the infrared LED housing and push into place. Tighten the cinch nut to

secure the fiber.

Measure the phototransistor current and record it in R ow 3, Column 2 of Table 6.1.

R emove the 1-meter fiber end from the infrared LED housing.

Place the dark gray, gritty side of the 2000-grit polishing paper gritty-side-up on a hard, flat surface.

Wet the center of the 2000-grit polishing paper with water, light oil or glycerin.

- 25 -



15. Hold the 1-meter fiber upright, at right angles to the polishing paper, and polish the fiber tip with a gentle"figure-8" motion as shown inFigure 6.3. You may

Table 6.1 Transmission data measured for a1-meter optical fiber with three different endpreparations.

get the best results by supporting the upright fiber

against some flat object such as a small piece of wood.Complete about 20 "figure-8" strokes. It isn'tnecessary to press down on the fiber ² just makegentle contact between the fiber and paper.

Fiber Termination

Wire cutter

Sharp knife

Phototransistor

current

16. R epeat Steps 14 and 15 using the 2-meter fiber end. Polished

17. Place the smooth side of the 3wm polishing filmdown on a hard, flat surface and wet the center of thefilm with water, light oil or glycerin.

18.

19.

20.

21.

22.

Take the 2-meter fiber end just polished on the 2000-grit paper and polish it using the 3wm polishing film.Use 20 of the same "figure-8" strokesdepicted in Figure 6.3.

Visually compare the end of the 1-meter fiber

polished with 2000-grit paper to the 2-meter

fiber end polished with 3wm film. (Goodlighting and/or a magnifying lens or microscope may be helpful.)

Polish the free end of the 1-meter fiber, using20 of the same "figure-8" strokes, on the 3

wm polishing film.

Insert the 1-meter optical fiber end just

polished into the infrared LED device

housing. Push into place and tighten the cinchnut.

Measure the phototransistor current and

record it in R ow 4, Column 2 of Table 6.1. 1095.eps

23.

24.

Tur n off the multimeter and power supply.

R inse off the 3wm polishing film and the

Figure 6.3 Position and pattern of the optical fiber

during polishing.

2000-grit polishing paper inwater. Use dishwashing soap as needed. Dry each with a paper towel.

25. R etur n all items to their proper storage containers and locations.

- 26 -




I n the space below , d r aw pictur es to show the d i ffer ences bet ween the f iber end pol i shed with 2000 grit paper

and the end pol i shed with the 3wm pol i shing f il m.

C opy the data f r om Table 6.1 into Table 6.2.

I n Row 2 , C olumn 3 of Table 6.2 write the number r esult ing f r om d ivid ing the number in Row 2 , C olumn 2 by

the number in Row 4 , C olumn 2.

I n Row 3, C olumn 3 of Table 6.2 write the number r esult ing f r om d ivid ing the number in Row 3, C olumn 2 by

the number in Row 4 , C olumn 2.

Table 6.2 Calculations for determining losses due to fiber end preparation.

Fiber

Termination

Wire cutter

Sharp knife

Polished

Phototransistor

current

Column #3 Losses %

I nC olumn 4 , r ecor d the number s r esult ing f r om subt r act ing the number s in

C olumn 3 f r om 1 and mult i plying

by 100 per cent.

Ar e the incr eased losses with poor er f iber terminat ion what you expected ? W hy or why not ?

- 27 -



Describe how the sur face textur e caused low photot r an si stor curr ent with poor er terminat ion s in thi s act ivity.

Relate your an swer to crit ical ang le and f iber end terminat ion. Dr awing an illust r at ion may be helpful.

Describe how poor f iber terminat ion s on both ends of a f iber , along with l ig ht exit ing and entering , causes

r educed opt ical power at the photodetector .

HOMEWORK PROJECT

Find at least one company that manufactures fiber optic splices or connectors. (AMP, the supplier of fibers used in this kit, does not count.)Try looking in the buyers guides or the periodicals found in the List of References.

- 28 -



ACTIVITY VII: SPEED OF OPTO-ELECTRONIC DEVICES

Overview

Where optical fiber is used for data communications, semiconductor technology produces the most suitable light

sources and photodetectors. Components manufactured using semiconductor technology are: Fast Small The most cost-effective devices on the market

Easily interfaced with electronic circuitsIn this activity you will broaden your experience to include the most common types of photodetectors ² photodiodes,

phototransistors, and photodarlingtons. Avalanche photodiodes have been omitted here because of their high cost ² $50

each or more, even in quantities of 1,000 pieces. You will also focus your attention on the key characteristic of LEDs and photodetectors that is important for data communications ² speed or bandwidth. Bandwidth is a measure of data transfer

rate that can be defined by numerous terms including rise and fall times, 3 dB bandwidth frequency, and baud rate. R iseand fall times will be used as a relative measure of speed in this manual. The faster the rise and fall times, the greater the

data transfer capability of an LED or photodetector.Following, you will set up some basic equipment and electrical circuits to make rise and fall time measurements of

several LEDs and photodetectors. From those measurements you will observe that each light source and photodetector has its own characteristics.Some devices have high light output and slow rise/fall times, while others have high rise/fall

times and low light output. In fiber optic applications you will find that there are no "best" light sources or detectors. (In the real world the performance of a device is linked to its cost ² somewhat like sports cars.) The best choice for an LED

or photodetector is entirely dependent upon the specific size, load, environment, available voltage, and other factors. In fact, you may find applications where there is no suitable LED or photodetector available. In those situations, the

skills/k nowledge you acquire from this manual will help you determine a compromise among various requirements toarrive at an acceptable solution.

Materials Required

2-volt penlight Photodarlington - OP560 sidelooker - no stripe on back

Phototransistor LPT80A in black sidelooker housing 1-meter 1000wm fiber

74LS05, TTL open collector inverter

47;resistors (2)

1 k ;resistor

Infrared LED (IF-E91C-blue housing-copper dot)Device Housing w/Green

Red LED (IFE96-blue housing-pink dot)

Photodiode - OP950 sidelooker style - blue stripe

* Not contained in the IF-LMH kit.

150;resistors (2)

10 k ;resistor

Dual trace 40 MHz oscilloscope with probes*

Solderless breadboard*40 MHz square wave signal generator*


Needle-nose pliers*

- 29 -





7.

8.

9.

10.

Tur n the variable voltage power supply on and adjust the output voltage to + 5 volts DC.

Connect the signal generator output to pins 1, 3, 5, 9and 11 of the 74LS05 IC.

Verify that the 74LS05 gate is driving the LED

properly by observing the electrical signal present at

pin 2 of the 74LS05 hex inverter with theoscilloscope. The observed signal should be similar to that

shown in Figure 7.2.

Connect the other oscilloscope probe to the junction of the phototransistor collector and the 150; resistor. The observed voltage on the oscilloscopewill be in phase with that on pin 2 of the inverter, 1094.esp

11.

and inverted from the signal generator output.

Expand the sweeptime/timebase and the vertical

amplitude settings on the oscilloscope to measure

Figure 7.3 Proper oscilloscope display for

measuring the rise time of the phototransistor.

the rise time at the phototransistor collector. (If you are uncertain about the definitions of rise and fall timessee the Glossary at the rear of this manual.) See Figure 7.3 for a typical oscilloscope display for measuringrise time. R ecord the rise time in Table 7.1.

12.

13.

14.

15.

Adjusting the oscilloscope as required, measure the fall time of the phototransistor, then record the time in

Table 7.1.

Tur n the power supply and signal generator off.

Using a pair of needle nose pliers carefully remove the phototransistor from the device housing and replace it

with the photodarlington, part number OP560. R eplace the 150;resistor connected to the collector of the

phototransistor with a 47;resistor. (Check the photodarlingtonpinconnection intheAppendixof thismanual.)

Tur n the power supply and signal

16.

generator on.

R educe the signal generator

Table 7.1 Measured values of photodetector rise and fall times,with an infrared LED as the optical source.

17.

frequency to 1 k Hz.

Adjust the sweeptime/timebase and

vertical amplitude settings of theoscilloscope and measure the rise

and fall times present at thecollector of the photodarlington.R ecord your measured results in Table 7.1.

Detector

Phototransistor

PhotodarlingtonPhotodiode

Rise time Fall time

18. Tur n the power supply and signal generator off.

- 31 -



19.

20.

21.

22.

23.

R emove the 47;resistor and connect a 10 k ;resistor to circuit ground.R eplace the photodarlingtonwiththe photodiode (part number OP950-blue stripe on back) in the device housing.Electrically connect the

photodiode's cathode to + 5 volts and its anode to the 10 k ;resistor onthe solderless breadboard.

Tur n on the power supply and signal generator.

Increase the operating frequency of the signal generator to 100 k Hz.

With the oscilloscope, measure the rise and fall times at the junction of the 10 k resistor and the photodiode

anode. R ecord the results in Table 7.1.

Tur n off the power supply and signal generator.

Procedure #2: LEDs

1.

2.

3.

4.

5.

6.7.

8.

Copy your measurements from R ow 4, Table 7.1, in R ow 2 of Table 7.2.

R eplace the infrared LEDwith the red LED on the solderless breadboard. Be sure to re-insert the fiber and

tighten the cinch nut snugly.


Adjust the oscilloscope sweeptime/timebase and vertical amplitude as required to measure the rise and fall

time of the red LED using the photodiode as the detector. R ecord your results in R ow 3 of Table 7.2.


R emove the red LED from the breadboard.R emove the 1-meter fiber from the photodiode and red LED housings and place the fiber to one side.

R emove the photodiode from its

housing and reinstall it on the breadboard.

Table 7.2 Measured values of rise and fall times of four optical

sources using the photodiode as the optical detector.

9. Using needle-nose pliers carefully

remove the greenLED from its

device mount and install it onto the breadboard facing directly into the photodiode. Place the LED so thedistance between it and the

phototransistor face is less than .5mm (.020 inch).

LED

Infrared

Red

Green

Penlight

Rise time Fall time

10.

11.

12.

13.

R eplace the 150;resistor (inseries with the + 5 volt power supply and the greenLED) with a 47;resistor.


Set the frequency of the signal generator to 1 k Hz.

Press the green LED directly against the photodiode and measure the rise and fall times at the anode of the

photodiode, using the oscilloscope. R ecord your results in R ow 4 of Table 7.2.

- 32 -



14.

15.

16.

17.

18.

19.


R emove the green LED from the breadboard and

replace it with the penlight. (The penlight has no polarity.)


Slow the signal generator frequency down to 20Hz.

See Figure 7.4.

Measure the rise and fall times of the penlight and

record the results in Table 7.2.


20. R etur n all items to their proper storage containers

and locations.

1092.eps

Figure 7.4 Oscilloscope trace showing rise and

fall times when a photodiode is used as thedetector and penlight as an optical source.


W hich of the photodetector s tested in Table 7 .1 had the fastest ri se and fall t imes?

Ar e ther e an y detector s tested in Table 7.1 for which the ri se and fall t imes ar e signi f icantly d i ffer ent ? I f so, which ones?

The upper 3 d B f r equency band width of a device can be determined f r om the ri se t ime, or the fall t ime, by using

the equat ion below:

f 3d B! .35Xr

Xr ri se t ime, 10 to90%

f 3d B3 d Bband width in Hz

C opy the photodetector ri se and fall t ime data r ecor ded in Table 7.1 to Table 7.3.

Taking the long er of the ri se t imes and fall t imes for each detector , calculate the 3 d B f r equency band width and

r ecor d it in C olumn 4 of Table 7.3.

Table 7.3 Calculation of detector upper frequency 3 dB bandwidth using an infrared LEDas the optical source.

Detector Rise time Fall time f 3d B

Phototransistor

Photodarlington

Photodiode

- 33 -



W hich photodetector has the larg est band width?

C opy the l ig ht sour ce ri se and fall t ime data f r om Table 7.2 to Table 7.4.

Taking the long er of the ri se t imes and fall t imes for each l ig ht sour ce, calculate the 3 d B f r equency band width

and r ecor d it in C olumn 4 of Table 7.4.

W hich LED i s the fastest in Table 7.4? I s thi s what you expected ? W hy?

W hy ar e incandescent bulbs not used as f iber opt ics l ig ht sour ces? (U se the data in thi s act ivity to formulate your

an swer . )

Table 7.4 Calculations of light source upper frequency 3dBbandwidth using a photodiode as the optical detector.

U sing in format ion as r equir ed f r om

Activity III , determine i f a par t icular LED

emits the gr eatest amount of opt ical

power , has the best opt ical f iber

t r an smi ssion and the fastest ri se/fall

t imes. W hich one? I f one does not meet

all the criteria, pick the best LED in each

cate g or y and l i st it below.

LED

Infrared

Red

Green

Penlight

Rise time Fall time f 3d B

HOMEWORK PROJECT

Determine if the rise and fall times measured in this activity are typical of these devices by researching the

speed of comparable LEDs and photodetectors. Try looking inmanufacturers' data books. Device data books arenormally available inmost vocational, trade and engineering libraries or can be obtained from manufacturersthemselves such as: AmericaBright, Agilent,Honeywell, Fairchild, Optek, Seimens,Toshiba, Stanley and Sharp.

- 34 -



ACTIVITY VIII: FIBER OPTIC TRANSMITTERS

Overview

As you lear ned in your main course, all fiber optic systems have three major elements:

Transmitter R eceiver Optical fiber

Figure 8.1 depicts thesemajor elements.Thetransmitter and receiver

contain smaller elements or building blocks, some of

which you should recognizefrom previous activities. Sofar with the instructions in this manual we have made alight guide, characterized and

terminated optical fibers, and

SignalIn

SignalOut

Dr iver

Amplif ier

TR ANSMITTER

LightSource

RECEIVER

Detector

Source-to-f iber connection

Fiber-to-detector connection

1473.eps

OpticalFiber

evaluated LEDs and Figure 8.1 Basic elements in fiber optic links.

detectors. In this activity we

shall investigate one of the last two elements in a fiber optic system the driver for the light source. As you have studied,there are two commonly used light sources in fiber optics, LEDs and laser diodes. The drivers covered in this activity arefor visible and infrared LEDs. We will not discuss drivers for laser diodes because they are outside the scope of thismanual. They can be very sophisticated, complex, and cost thousands of dollars. There are also optical safetyconsiderations when using laser diodes. Inmore advanced fiber optics classes, or in your job, you will find the information

you lear ned here about driving LEDs is a good primer for laser diode driver design.

Materials Required


PN2222

MPS3638A

47;resistor

470;resistors (2)

Assorted resistors

0.01wf capacitor

1 k ;resistors (2)

100 pf capacitor

1N914 (3)

* Not contained in the IF-LMH kit

2N3904

74LS05 TTL Hex inverter

220;resistor

40 MHz signal generator*

Multimeter*

Oscilloscope*



Miscellaneous electrical leads*

- 35 -



Procedure #1: Digital Circuits

The word "digital" in fiber optics means much the same as it does in electronics.Digital gates, circuits or systems are

those inwhich there are two defined states ² a "high" or digital 1, and "low", a digital 0. See Figure 8.2 for an

illustration. In all digital systems a "low" or digital 0 does not necessarily mean zero voltage or zero optical signal. A digital0 means a lower state than the digital 1 state. In the electronics industry, the definitions of a digital 1 and 0 are defined

within a logic family. For example, the established limits for the electronicTTL logic family are that Vhigh must be between 2.0 and 5.0 volts andVlow between 0 and 0.8 volts.

A very simple electronic circuit that can be connected to the output of anyTTL or CMOS logic gate for driving aLED is shown in Figure 8.3. It will drive an LEDwith up to 50 mA of current, and a frequency of several megahertz.

1.

2.

Calculate the resistor value,R c, needed to permit a current of 20 mA through the LED in Figure 8.3 when

the transistor is saturated. Assume theVce(sat)=.2 volts and theVf for the LED to be 1.8 volts.

Rc! 5V f V ce( sat ) I c

Calculate the maximum value of the base resistor, R b,

needed to drive the transistor into saturation if Vi was high

connected to + 5 volts. Assume V be=.7 volts and

hfe(min)= 50.

Rb! 5V be( sat ) I c h fe

low

Figure 8.2 The two states of digital and

3. Choose resistors from the kit that are closest to the fiber optic systems.

calculatedR c and one-half the calculated R b. See Table

1.2 for choices.

4.

5.

6.

Assemble the circuit shown in Figure 8.3 on your solderless breadboard. Use the pin diagrams found in the

APPENDIX to identify device connections.

Tur n on the variable voltage power supply and adjust the output+ 5 volts

voltage to + 5 volts DC.

RcConnect the end of R b marked Vi to + 5 volts. The red LED shouldnow be on. If not, check the power supply and electrical

7.

8.

connections.

With the multimeter measure the transistor collector-to-emitter voltage and the voltage across the LED, then record the results in

Table 8.1.

ChangeVi from the + 5 volts to ground.The red LED should now

Vi

Rb

RedLED

2N3904

1475.eps

9.

be off.

With the multimeter, measure the collector-to-emitter voltage

across the transistor and record the result in Table 8.1.

- 36 -

Figure 8.3 Single NPN transistor

switching circuit for driving a fiber

optic LED.



10. Tur n on the signal generator and set it for a symmetrical

square wave with a frequency of 100 k Hz and aVhigh

between 3.4 volts and 5 volts, and Vlow greater than 0

Table 8.1 Measured data taken from the

circuit shown in Figure 8.3.

11.

12.

13.

volts and less than 0.7 volts. Check the frequency andamplitude with an oscilloscope.

DisconnectVi from ground and connect it to the signalgenerator output.

Using the oscilloscope measure the rise and fall times of

the LED drive circuit at the collector of the 2 N3904transistor. R ecord your results in Table 8.1.

Increase the frequency of the signal generator until the

Measurement

Vce (LED on)

Vf (LED)

Vce (LED off)

Rise time

Fall time

Period3 dB

Data

digital signal on the collector of the transistor reduces to70 percent of its peak value. Measure the period of the frequency and record it in Table 8.1.

14. Tur n off the power supply and signal generator.

Fiber optic communication systems are often duplex in configuration, and have transmitters and receivers adjacent

on a printed circuit board. The on/off current of the LED driver may cause power supply ripples or generate noise thatcould affect the adjacent receiver. Driving the LED in a push/pull arrangement that shunts the current when the LED is

not on can reduce power supply ripple. (This provides a more constant current drain from the power supply.) Figure 8.4

is a modification of the circuit in Figure 8.3 that does exactly that.

15.

16.

17.

18.

19.

R eco

nfigure the electrical compo

nents o

nthe breadboardto match those with the circuit shown in Figure 8.4.

Tur n on the power supply. Connect the unattached end of

R b markedVi to + 5 volts.

With the multimeter measure the voltage across the

transistor and record your results in Table 8.2.

ChangeVi from the + 5 volts to ground.The red LED should now be lit.

Measure the voltage across the LEDwith the multimeter

Vi

Rb

+ 5 volts

Rc

2N3904RedLED

1476.eps

20.

and record your results in Table 8.2.

Tur n on the signal generator and set it for a symmetrical

square-wave with a frequency of 100 k Hz, with the same

voltage amplitudes as in Step 18.

Figure 8.4 Transistor shunt circuit for driving a fiber optic LED that would reduce

power supply current ripple.

21. DisconnectVi from ground and connect it to the signal generator output.

22. Using the oscilloscope measure the rise and fall time of the LED drive circuit at the collector of the 2 N3904

transistor.R ecord the results in Table 8.2.

- 37 -



23. Increase the frequency of the signal generator until the digital signal on the collector of the transistor reducesto 70 percent of its peak value. Measure the period of the frequency and record it in Table 8.2.

24. Tur n off the power supply and signal generator. Disconnect the signal generator fromR b.

Table 8.2 Measured data on the LED drive

Procedure #2: Digital Circuits II (Optional) circuit shown in Figure 8.4.

Measurement Data1.

2.

R emove the LED drive circuit from the breadboardand replace it with the circuit shown in Figure 8.5.

Use the same value for R c as used in Procedure #1.

Perform all the necessary steps needed to measure

the data required to complete Table 8.3.

Vce

Vf

Rise time

Fall time

Period3 dB

Table 8.3 Measured data for the high-speedLED drive circuit shown in Figure 8.5.

Measurement

Vce (LED on)

Vce (LED off)

Rise time

Fall time

Period3dB

Data

Signal 1

0.01wf

2

470;

3 4

1K

1N914

1K

Rc

MPS

+ 5 volts

LED

Generator

74LS05

3638A 1N914

1N914

1477.eps

Figure 8.5 Ten-megabit push-pull LED drive circuit .

- 38 -






I s the measur ed volta g e acr oss the collector of 2N 3904 t r an si stor in Figure 8.3 for the LED " on" and " off "

com par e to what you expected ? W hy or why not ?

W ith the LED " on" in Figure 8.3 calculate the " on" curr ent using themeasur ed data in thi s act ivity for V ce ( sat )

and V f .

U sing the measur ed ri se t ime f r om Table 8.1 , calculate the 3 d B band width for the cir cuit shown in Figure 8.3.

f 3 d B! .35Xr


f 3 d B 3 d Bband widthin Hz

H ow does the calculated band width com par e to the measur ed band width?

C alculate the aver a g e curr ent used by the LED d river in Figure 8.3 , assuming it i s being d riven at a 50 per cent duty cycle.

U sing the measur ed value for V f , calculate the curr ent thr ou g h the LED in Figure 8.4.

C alculate the curr ent thr ou g h the 2N 3904 in Figure 8.4 when it i s on and the LED i s off.

W hat i s the aver a g e curr ent thou g h the cir cuit shown in Figure 8.4 , assuming that it i s being d riven at a 50

per cent duty cycle?

- 40 -



C om paring the peak curr ent and aver a g e curr ent for the cir cuits in Figures 8.3 and 8.4 , which would cause the

gr eatest power supply ri pple? B y how much?

W hat i s the d i ffer ence bet ween the cir cuits in Figure 8.3 and 8.4? (H I N T : C on sider inver ted and non-inver ted

funct ion s. )

A ssuming h fe i s 100 for the P N2222 , calculate the DC LEDcurr ent d r awn for the cir cuit shown in Figure 8.6

with no in put signal.

W hat i s the maximum l inear volta g e swing of the cir cuit shown in Figure 8.6 ? (H I N T : Determine the an swer

f r om em pirical data. )

W hat i s the 3 d B band width of the cir cuit shown in Figure 8.6 ?

HOMEWORK PROJECT

Design a digital LED drive circuit using an N-channel enhancement-mode MOSFET that will drive the LED

with a current of 100 mA and a frequency of 5 MHz. The circuit must be powered from + 8 volts and the in putmust be electrically compatible with CMOS logic devices operating from the same + 8 volt power supply.

- 41 -





Procedure #1: Basic Transimpedance Function

The simplest transimpedance device is just a resistor. Its output is measured in volts per ampere. A photodiode and

resistor termination is shown in Figure 9.2. The transfer function of this receiver circuit is:

V o! P i R x

P iO pt ical power incident upon photod iode

r espon sivity of photodetector

R xvalue of r esi stor , ohm s

1.

2.

3.

4.

Assemble the circuit shown in Figure 9.2 on the solderless breadboard. Start with the resistor R x at a value of 100 k ;.

Tur n on the power supply and adjust the output voltage to + 5 volts DC.

Tur n on the signal generator and oscilloscope.

Adjust the signal generator to produce an output with a symmetrical 10 k Hz square wave frequency, and a

TTL amplitude as follows (3.4 volts < Vhigh < 5.0 volts and 0 volts < Vlow < .7 volts). Check the frequencyand amplitude

+ 5 voltswith your 1523.eps

5.

oscilloscope.

Connect the

signal generator

output to pin 1 of the 74LS05 IC on the breadboard.

IR

LED

47;

1-meter opticalf iber

Photo-diode

Vo

6. On the

oscilloscopeToSignal

114

74LS05

7

2Rx

observe the signalat the node

markedVo

(between the

Generator

Figure 9.2 Photodiode terminated with a resistor.

7.

8.

9.

10.

11.

12.

photodiode and the resistor, R x).

Adjust the oscilloscope settings to measure the peak-to-peak voltage, and the rise time and fall time at Vo.R ecord the measurements in Table 9.1.

Tur n off the signal generator and power supply.

R eplace the 100 k ;resistor with a 47 k ;resistor.

Tur n on the power supply and signal generator, measureV p-p, tr and tf atVo with the oscilloscope and recordthe results in Table 9.1.

R epeat Steps 8 through 10, replacing the 47 k ;resistor with a 10 k ;resistor.

Place a .001wf capacitor in parallel with the 10 k ;resistor.

- 43 -



13. Adjust the oscilloscope and Table 9.1 Measured data for various termination resistors, R x, in thesignal generator to circuit shown in Figure 9.2.

measureV p-p, tr and tf at

14.

Vo. R ecord thesemeasurements in Table

9.1.

Tur n off the signal

generator and power

supply.

Rx

100 k ;

47k ;

10 k ;

10 k ; ||.001wf

Vp-p tr tf

The transimpedance resistor used as a fiber optic receiver has two major drawbacks. The first is that the impedance of

the load must be high, as compared to R x , or the gain of the receiver goes down. The second is that the capacitance of the load can greatly affect the bandwidth of the circuit, as can be seen by the equation:

1 f 3d B! 2T R x(C d C r C l )

R x r esi stancevalue

C d capacitance of photodetector

C r capacitance of r esi stor ,R x C l capacitance of load

Procedure #2: Operational Amplifiers

Fortunately there are much better fiber optic receiversthan the photodiode and single resistor. One example, an operational amplifier design, is shown in Figure 9.4. Its main

advantage over the previous circuit is that the operationalamplifier buffers the load impedance from the transimpedance

gain resistor, R x. The receiver gain is still determined by thefeedback resistor, and the 3 dB frequency by the circuitcapacitances at the operational amplifier inverting terminal.

+ 5 volts

Photodiode

RxRL

CL

Figure 9.3 Receiver showing a parasitic load resistor and capacitor.

- 44 -

1 5 2 4 . e p



1.

2.

3.

4.

5.

R eplace the simple resistor located on the breadboard with the circuit shown in Figure

9.4. (Ignore Cl for now.)

Tur n on the power supply and adjust its output

to provide a voltage of +/- 5 voltsDC.

Tur n on the signal generator.

For the three values of R f listed in Table 9.2,measureV

p-p, t

r and t

f with an oscilloscope.

R ecord the measured results in Table 9.2.

Place one end of the .001wf capacitor (C1) atthe output of the operational amplifier and the

Photodiode

1 k;

2

3

-

+

LM741

Rf

+5 volts

7

4

-5 volts

6Vo

C1

1525.eps

6.

other end to ground as shown in the dashed

outline in Figure 9.4.

R ecord in the last row of

Figure 9.4 Non-inverting fiber optic receiver using anoperational amplifier.

Table 9.2 V p-p, tr and tf of this receiver circuit with the

Table 9.2 Measured data for various termination resistors, R f , in thecircuit shown in Figure 9.4.

7.

added load capacitance.

Tur n off the signal generator

and power supply.

Rf

100 k ;

47k ;

10 k ;

10 k ; ||.001wf

Vp-p tr tf

Procedure #3: Discrete Designs

Although operational amplifiers are readily available and easy to use, they sometimes lack the required frequency

bandwidth for some applications. For these applications, discrete transistor amplifiers are often the answer. Examples of two discrete bipolar-transistor amplifier circuits are shown in Figure 9.5.

1.

2.

3.

4.

5.

R eplace the operational amplifier and associated circuitry on the breadboard with the circuit shown in Figure

9.5 (a). R eposition the photodiode and device mount as needed.

Tur n on the power supply and adjust the output connected to the discrete transistor receiver to + 5 volts DC.

Tur n on the signal generator.

Measure the V p-p, tr and tf with an oscilloscope for the values of R f listed in Table 9.3. R ecord the results. (Itwill be helpful if you set your oscilloscope in put for AC coupling.)

Tur n off the signal generator and power supply.

- 45 -



To achieve high frequency bandwidth and Table 9.3 Measured data for various termination resistors, R f ,

low electrical noise, discrete designs use state- in the circuit shown in Figure 9.5.

of-the-art transistors and circuit miniaturization techniques to reduce the package size as much

as possible.Very often these circuits are builtusing the bare transistor chip (no package, just

the semiconductor die), chip resistors and chipcapacitors.

Most discrete designs are obsolete thesedays (except for the very highest frequency

Rf

100 k ; 47k ;

10 k ;

10 k ; ||.001wf

Vp-p tr tf

bandwidth applications) because special integrated circuitsoptimized for the transimpedance function are available. One

such integrated circuit available from Signetics is part number NE5212. It has an impressive 3 dB frequency bandwidth of

125 MHz and a transimpedance gain of 7000V/A.Thisdevice is also inexpensive and very easy to use.

Procedure #4: Phototransistor andPhotodarlington Receivers

All the discussions in this activity have involved

amplifiers with high-frequency bandwidth capabilities.

Phototransistor and photodarlington devices, although muchslower, do not require a high-speed amplifier attached to

Photo-diode

470;

2N3904

47;

Rf

+ 12 volts

2N3904

Vo

150;

1481.eps (a)them. All of the circuits shown so far in this activity will work

by substituting the phototransistor or photodarlington for the

photodiode.Phototransistors and photodarlingtons are great detectors

for low-speed applications, and their receivers are often designed for simplicity. Examples of amplifiers for a

phototransistor or photodarlington type photodetector are

Photo-diode

2N3904

R3

+ 12 volts

PN2222

Vo

shown in Figures 9.6 (a) and (b). Typical applications of such receivers include appliances, games and motor controls.

R1 2N3904

R4

1.

2.

R eplace the photodiode in the device housing

with the phototransistor. R eplace the 47; resistor in Figure 9.2 with a 100;resistor.

R emove the receiver circuitry on the breadboard

and replace it with the circuitry shown in Figure

Rf

(b)

R2

Figure 9.5 Fiber optic receivers using a discrete9.6 (a). (Ignore the dashed resistor in parallel

bipolar transistor.with the 390;resistor.)

3. Tur n on the power supply and adjust the output

voltage to + 5 volts DC.

- 46 -



1483.eps

4. Tur n on the signal generator.

5. Observe the signal at the emitter of the phototransistor Photo-transistor

with an oscilloscope. R ecordV p-p, tr and tf inTable 9.4.

6.

7.

Measure V p-p, tr and tf at pin 2 of the 4069 logic gateand record in Table 9.4.

Place successively smaller value resistors in parallel with

the 390;resistor shown inFigure 9.6 (a)until theoutput of 4069 no longer is a periodic signal.R ecord

below the value of the equivalent resistor that just

(a)

4069

14

7

2

8.

9.

removes the periodic signal at pin 2 on the 4069 IC.

R =


R eplace the phototransistor in the device housing with

12 volts

Photo-transistor

2.2 k;

-

LM741

22 k;

+ 12 V

10.

11.

the photodarlington.


Again place successively smaller value resistors in parallel

with the 390;resistor until a periodic signal isno

100 k;

.01wf

(b)

+ - 12 V

longer present at the output of 4069. R ecord below thevalue of the equivalent resistor that just removes the

periodic signal at pin 2 on the 4069 IC.

R =

Figure 9.6 (a)Inverting phototransistor CMOS

receiver; (b)non-inverting receiver with AC-coupled buffer amplifier.

12. Tur n off the power supply and signal generator.

13. R etur n all items to their proper storage containers and locations.

Table 9.4 Measured data for various termination resistors, R f , in the circuitshown in Figure 9.6 (a).

Location Vp-p tr tf

Vemitter

V4069

- 47 -

3 9 0 ;

1 4 8 4 . e p s




W hat happen s to the ri se and fall t imes at the r eceiver as the r esi stor value i s r educed in Figure 9.2? W hat

happen s to the peak-to- peak volta g e?

W hat happen s to the ri se and fall t imes acr oss the terminat ion r esi stor (R X ) when capacitance i s added in the

cir cuit shown in Figure 9.2?

W hat ar e the ad vanta g es and d i sad vanta g es of the r eceiver cir cuit shown in Figure 9.4 com par ed to the cir cuit

shown in Figure 9.2?

H ow does the add it ion of the load capacitance affect the ri se and fall t imes in the cir cuit shown in Figure 9.4 as

com par ed to Figure 9.2? W hy?

C alculate the band width of the r eceiver shown in Figure 9.4 with the 10 k g ain r esi stor in stalled using the

equat ion:

f 3 d B! .35Xr


f 3 d B 3 d Bband widthin Hz

C alculate the band width of the bi polar t r an si stor r eceiver in Figure 9.5 using the 10 k feedback r esi stor

in stalled.

H ow much mor e g ain does the photodar l ing ton have than the photot r an si stor in the cir cuit in Figure 9.6 (a)?

(Hint : U se the r at io of the equivalent r esi stances which just r emoved the period ic signal at the output of 4069. )

HOMEWORK PROJECTFrom the information received from the fiber optic vendors inActivity IV, write a paragraph describing a

product or service that each company provides.

- 48 -



DEVICE PIN DIAGRAMS

Photodiode (Blue stripe on back)1. Anode2. Cathode

1

2 Red LED

1. Anode

2. Cathode

1460.eps

12

Phototransistor (Black stripe on back)

1.Emitter 2. Collector

Photodarlington ( No stripe on back)


1

21058.eps 1485.eps

SFH300: Phototransistor in

T 1 3/4 package


2N3904, 2N2222, MPS3638ATransistors

1.Emitter

2.Base3. Collector

1 2 REF 5X 1 2

BOTTOMSIDE

Fiber Optic LEDs: IF-E91C, IF-E93A and IF-E96

1. Cathode2. Anode

V cc 6A 6Y 5A 5Y 4A 4Y

14 13 12 11 10 9 8

OffsetTr im 1 8 NC

2 -In 7 +Vs

3 6 +In Output

1 2 3 4 5 6 7

1487.eps

-Vs 4 5 OffsetTr im 1A 1Y 2A 2Y 3A 3Y GND

1488.eps

LM741 Operational Amplifier 74LS05 and 4069 Hex inverters

- 49 -

1 4 8 6 . e p s

1 5 2 6 . e p s



RECOMMENDED TEST EQUIPMENT

The activities in this manual include lists of recommended parts and equipment. Following is a general description of each item to help you in proper selection.

18-gauge wire stripper

The 18-gauge wire stripper can be a tool sized for stripping only 18 gauge wire; a stripper sized for multiple gauges; or an adjustable wire stripper. When using an adjustable wire stripper, be sure it is not sizedtoo small or the optical fiber cladding can be nicked,which could cause you to make inaccuratedeterminations from the results.

Miscellaneous electrical test leads

A generic description for all the electrical (test)leads required to electrically connect the power supply,signal generators, multimeter and oscilloscope to thecircuits on the solderless breadboard used in thismanual. It is recommended that oscilloscope probes beattached to the oscilloscope in puts whenmonitoringcircuit activity for best measurement accuracy.

Multimeter

We suggest using aFluke 73 digital multimeter or equivalent. An analog multimeter is also suitable if itcan accurately measureDC current to at least .01mA. (DC current is the most critical measurementmade bythe multimeter)

Oscilloscope

This instrument and its in put measurement probesare critical to making measurements of circuit

performance inACTIVITIES VII through IX. It issuggested that you use an oscilloscope with dual trace

capabilities and an electrical bandwidth of at least 40MHz. If you are using a digital sampling oscilloscope,we suggest one with a bandwidth rating of 80 MHz or more. All oscilloscope probes should have a bandwidthin excess of 40 MHz; have been recently calibrated;and be in good working condition. Very often a goodoscilloscope can produce inaccurate results due to poor or damaged probes.

Signal generator

The signal generator is used to produce twowaveforms. One is a square-wave, TTL- compatiblesignal at a frequency from 10Hz to 40 MHz.The other is an analog sine wave with variable amplitude from 10millivolts to 2 volts peak-to-peak and a frequency from2 k Hz to 20 MHz.You may choose to use separatesignal generators for the analog and digital functions.

Solderless breadboard

"Solderless breadboard" is a generic term used todescribe a piece of equipment that allows electriccircuits to be assembled and tested without soldering.Suggested board capacity for completing the activities in this manual is 1360 total contacts and a size of 16.5 x 8 cm (6.5 x 3.125 inches). A solderless breadboard withelectrical binding posts for electrical in put power isoptional.

Variable voltage power supply

In this manual the variable voltage power supplyshould have two independent and isolated sourcescapable of delivering from 0 to 12 volts. Current ratingfor both supplies should exceed 100 mA. A chassisground isolated from the negative terminal is necessaryfor the negative 12 volt output.

- 50 -



SHIPMENT DAMAGE OR MISSING PARTS CLAIMS

Shipment Damage Claims

If damage to an I ndust rial F iber O pt ics product should occur during shipping, it is imperative that it be reported

immediately, both to the carrier and the distributor or salesperson from whom the item was purchased. DO NOT CO NTACT I NDUS T R I AL FI BER O P T IC S .

Time is of the essence because damage claims submitted more than five days after delivery may not be honored. If

damage has occurred during shipment, please do the following:

Make a note of the carrier company; the name of the carrier employee; the date; and the time of the delivery.

Keep all packing material.

In writing, describe the nature of damage to the product.

In the event of severe damage, do not attempt to use the product (including attaching it to a power source).

Notify the carrier immediately of any damaged product.

Notify the distributor from whom the purchase was made.

Missing Parts Claims

I ndust rial F iber O pt ics products are warranted against missing parts and defects inmaterials and workmanship for

90 days. Since soldering and incorrect assembly can damage electrical components, no warranty can be made after assembly has begun. If any parts become damaged, replacements may be obtained from most radio/electronics supplyshops. R efer to the parts list in Table 1.1 of this manual for identification.

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LIST OF REFERENCES

Following is a list of books, magazines, and other items that may be useful in the study of fiber optics. If the title doesnot mention fiber optics, you still may consider it a worthwhile source of information, since fiber optic technology spansmultiple disciplines.

BOOKS

Introductory

F iber O pt ics: A Brig ht N ew W ay to C ommunicate, Billings,Dodd, Mead & Company, NewYork, NY 1986TheRewiring of America: The F iber Revolut ion, David Chaffee, Academic Press, Inc., Orlando, FL 32887, 1988

Under stand ing F iber O pt ics, Second Edition, Hecht,Howard W. Sams, 201 West 103rd Street, Indianapolis, IN 46290, 1993

F iber O pt ics C ommunicat ion s, Experiments & Projects, Boyd, Howard W. Sams, 4300 West 62nd Street,Indianapolis, IN1982

Technician s Guide to F iber O pt ics, Sterling, AMP Incorporated,Harrisburg, PA 17105, 1987 (Paperback), Delmar Publishers, 2 Computer Drive West, Box 15-015, Albany, NewYork 12212-9985 (Hardbound version)

F iber O pt ics H andbook , Second Edition, Hentschel, Hewlett Packard, 1988

Advanced

F iber O pt ic C ommunicat ion s, Third Edition, Palais, Prentice-Hall Publishing, 1987

O pt ical F iber T r an smi ssion, Basch,Howard W. Sams, 201 West 103rd Street, Indianapolis, IN 46290, 1986

College Level

An I nt r oduct ion to O pt ical F iber s, Cherin, McGraw-HillBook Company, 1983

F iber O pt ic H andbook for Engineer s and S cient i sts, Allard, McGraw-Hill Publishing, NewYork, NY 1990

F iber O pt ics, Daly, CR C Press, 1986

F iber O pt ics in C ommunicat ion S ystem s, Elion and Elion, Marcel Dekker, Inc. 1978

P ulse C ode F ormats for F iber O pt ic C ommunicat ion s, Morris, MarcelDekker, Inc. 1983

O pt ical F ibr e S en sing and Signal P r ocessing , Culshaw, Peter Peregrinus LTD., 1984

P rinci ples of O pt ical F iber M easur ements, Marcuse, Academic Press, 1974

S emiconductor Devices for O pt ical C ommunicat ion s, Kressel, S pringer-Verlag, Inc., 1980S emiconductor Laser and H eter o junct ion LED s, Butler and Kressel, Academic Press, Inc., 1977

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Other

Laser Receiver s, R oss, JohnWiley & Sons, Inc., 1966

N oi se in E lect r onic C ir cuits, Ott, JohnWiley & Sons, 1976

Safety

S afety with Laser s and Other O pt ical S our ces, Stiney and Wolbarsht, Plenum Press, 1980

S afeU se of Laser s, A NSI Standard Z136.1, LIA, 12424 R esearch Parkway, Suite 130, Orlando,FL 32826

S afeU se of O pt ical F iber C ommunicat ion s S ystem s U t il izing Laser Diodes & LED S our ces, A NSI Standard Z136.2,LIA, 12424 R esearch Parkway, Suite 130, Orlando, FL 32826

A U ser' s M anual for O pt ical W ave g uide C ommunicat ion s, Gallawa, U.S. Department of Commerce

F iber O pt ics, Lacy, Prentice-Hall, Inc., 1982

MONTHLY PUBLICATIONS

The first two listings are jour nals available to members of the respective professional societies. The last four are trade

magazines often available free of charge.

A ppl ied O pt ics, Optical Society of America, 1816 Jefferson Place, NW, Washington, DC 20036

O pt ical Engineering , SPIE, P. O. Box 10,Bellingham, WA 98227

F iber opt ic P r oduct N ew s, Gordon Publications, Inc.,Box 1952,Dover, NJ 07801

Laser F ocus W or ld , PenWell Publishing Co., 1421 S. Sheridan, Tulsa, OK 74112

Lig ht wave M a g a zine, PenWell Publishing Co., 1421 S. Sheridan, Tulsa, OK 74112 P hotonics S pect r a, Laurin Publishing Co., Berkshire Common, P.O. Box 4949, Pittsfield, MA 01202-4949

BUYER 'SGUIDES

F iber opt ic P r oduct N ew s Buying Guide, Gordon Publications, Inc.,Box 1952, Dover, NJ 07801

Lig ht wave 2003 Buyer' s Guide, PenWell Publishing Co., 1421 S. Sheridan, Tulsa, OK 74112

ORGANIZATIONS

Optical Society of America, 1816 Jefferson Place, NW., Washington, DC 20036

Society of Photo-Optical Instrumentation Engineers (SPIE), P. O. Box 10, Bellingham, WA 98227

Laser Institute of America, 12424 R esearch Parkway, Suite 130, Orlando, FL 32826

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GLOSSARY

This glossary contains definitions to help readers understand the meanings of technical terms as they are used in thismanual, and as they relate to electronics, fiber optics, and lasers. Some words have different meanings when used in other contexts.

A Absorption - In an optical fiber, the loss of optical power resulting from conversion of that power into heat. See also:Scattering.

Acceptance angle - The angle within which an optical fiber willaccept light for transmission along its core. This angle is

measured from the centerline of the core.

U

Acousto-optic modulator - A device that varies the amplitudeand phase of a light beam (for example, from a laser) by soundwaves.

Active port diameter - In a light source or detector, the

diameter of the area in which light can be guided into or from an optical fiber.

Analog - A signal that varies continuously (sound or water waves, for example). Analog signals have a frequency and

bandwidth measured in Hertz (Hz).

Angle of incidence - The angle formed between a ray of lightstriking a surface and a line drawn perpendicular to that surfaceat the point of incidence (the point at which the light ray strikesthe surface).

Angstrom ( Å ) - A unit of length often used to characterizelight. An Angstrom is equal to 0.1 nm or 10-10 meters. The wordis often spelled out as Angstrom(s) because the special symbol isnot available on typewriters and older printers.

Angular misalignment loss - The optical power loss caused byangular deviation from the optimum alignment of a source to an

optical fiber, fiber-to-fiber, or fiber-to-detector. See also:Extrinsic joint loss; Intrinsic joint loss; Lateral offset loss.

Anode -The positive electric terminal in a laser, semiconductor or other electronic component. The electron stream normallyflows toward this terminal. See also: Cathode.

Anti-Reflection (AR) coating - A thin layer of material, or acomposite of materials, applied to an optical surface to reducereflectance and increase transmittance of light.

Attenuation - Loss of optical power.

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Attenuation coefficient - The rate of diminution of averageoptical power ² the sum of the scattering and absorption coefficients.

Attenuation-limited operation -The condition prevailingwhen the received signal amplitude (rather than distortion) limitsa fiber optic system's performance. See also: Bandwidth-limitedoperation; Distortion-limited operation.

Avalanche photodiode (APD) - A semiconductor photodetector that includes detection and amplification stages.Electrons and hole pairs generated between the p and n

junctions are accelerated in a high electronic field region wherethey collide with ions to create other electron hole pairs, or current amplification. APDs can detect faint signals, but requirehigher operating voltages than other semiconductor

photodetectors.

Axis - A straight line, real or imaginary, passing through a bodyand indicating its center.

Axial ray - A light ray that travels along an optical fiber's axis.See also: Meridional ray; Skew ray.

B Backscattering - The portion of scattered light which retur ns in a direction generally opposite the direction of propagation. Seealso: Rayleigh scattering.

Bandwidth -The range of frequencies which can be handled bya device or system within specific limits. See also: Fiberbandwidth.

Bandwidth-limited operation - The condition prevailing when the system's frequency bandwidth, rather than the amplitude (or

power) of the signal limits, performance.This condition can bereached when the fiber optic cable's material and modaldispersion distort the shape of the waveform beyond specifiedlimits. See also: Attenuation-limited operation; Distortion-limited operation.

Beam divergence - The increase in beam diameter as distancefrom the source increases.

Beamsplitter - A device for dividing an optical beam into two or more separate beams; often a partially reflecting mirror. See also:Coupler; Splitter.

Birefringence -The separation of a light beam (as it penetrates adoubly refracting object) into two diverging beams, commonlyk nown as ordinary and extraordinary beams.

Bit Error Rate (BER) - In digital applications, the ratio of bitsreceived in error to total number of bits sent.BER s of 10-9 (oneerror bit in one billion sent) are typical.

Boltzman Constant - A constant which is equal to 1.38 x 10-23 joules per Kelvin. Commonly expressed by the symbol K.



Buffer -See: Fiber buffer.

C

Coupling loss - The amount of power in a fiber optic link lost atdiscrete junctions such as source-to-fiber, fiber-to-fiber, or fiber-to-detector.

Cable - A single fiber or a bundle, sometimes includingstrengthening strands of opaque material covered by a protective

jacket.

Cathode - Also k nown as the negative electric terminal, in alaser, semiconductor or other electronic component. Theelectron stream normally flows from the cathode to the anode.See also: Anode.

Characteristic angle - The angle at which a given mode travels

down an optical fiber. See also: Mode.Cladding - A layer of glass or other transparent materialsurrounding the light-carrying core of an optical fiber. It has alower refractive index than the core. Coatings may be appliedover the cladding.

Cladding mode - A mode that is confined in the cladding of an optical fiber by virtue of the material surrounding the claddinghaving a lower refractive index. See also: Mode.

Coaxial -Having the same centerline. A cross-section of coaxialcable reveals concentric circles.

Coherent - Light, as in a laser beam, whose waves haveidentical frequencies and are in phase with each other. Onlylasers produce coherent light.

Collimated - Light rays, within a light beam, that are parallel toeach other, as within a laser beam.

Combiner - A passive device in which optical power fromseveral in put fibers is collected at a common point in a singlefiber. See also: Coupler.

Connector - A device mounted at the end of a fiber optic cable,light source, receiver or housing that mates to a similar device tocouple light optically into and out of optical fibers. A connector

joins two fiber ends or one fiber end and a light source or detector.

Core -The central portion of an optical fiber that carries light.

Coupler - A device which connects three or more fiber ends,dividing one in put among two or more outputs or combiningtwo or more in puts into one output. See also: Beam splitter

Coupling efficiency -The fraction of available output from aradiant source which is captured and transmitted by an opticalfiber. The coupling efficiency of a Lambertian radiator is usually

equal to the sin25maximumfor the optical fiber being used.

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Critical angle -The smallest angle of incidence at which lightwill undergo total inter nal reflection.

Cutback technique - A technique for measuring fiber attenuation or distortion by performing two transmission measurements. One measurement is taken at the full length of the fiber; the other when a portion has been cut from the fulllength.

D Dark current - A parasitic output current that a photodetector

produces in the absence of light and operational voltages.

Decibel (dB) - A logarithmic unit of measure used to expressgain or loss and relative power levels. Ten times the base-ten logarithm of that ratio:

d B! 10 log10 P 2 / P 1 Demodulator - A circuit that separates an information signalfrom its carrier.

Detector - A device that generates an electrical signal when illuminated by light or infrared radiation. The most common detectors in fiber optics are APDs, photodiodes, photodarlingtonsand phototransistors.

Diffuse reflection -R eflection from a surface that makes its

texture appear matte or dull. Opposite of the spectral reflection occurring from a mirror.

Digital - A signal format in which the information beingconveyed is contained in the presence or absence of signal in sequential time periods, resulting in "zero bits" and "one bits"represented in varying sequences.

Diode - An electronic device that allows current to flow in onlyone direction.

Diode laser - See: Injection Laser Diode

Discrete -R eferring to an individual component that is completein itself. Examples of such are resistors, printed circuit boards,transistors and LEDs.

Dispersion -Distortion of an electromagnetic signal caused bythe varying propagation characteristics (speed) of differentwavelengths. In an optical fiber the optical pulses spread out.

Distortion - Change in a signal's wave shape. Examples of distortion include dispersion and clipping in an amplifier circuit.

Distortion-limited operation - The condition prevailing when distortion of a received signal, rather than its amplitude (or

power), limits performance. The condition reached when asystem distorts the shape of the wave form beyond specifiedlimits. In a fiber-optic system, it usually results from dispersion.See also: Attenuation-limited operation; Bandwidth-limited

operation.



Duplex -Dual. A fiber-optic cable that contains two opticalfibers.

E Electron - A particle which orbits the atomic nucleus. It

possesses a unit negative electrical charge of 1.6 x10

-19

coulombs.

Electro-optic effect -The change of a material's refractiveindex or the change of its birefringence under the influence of an electric field. One material that exhibits this effect is lithium-niobate.

End finish -Quality of the surface at an optical-fiber's end,commonly described as mirror, mist, hackle, chipped, cracked,or specified by final grit size of the polishing medium (1wm, 0.3wm, etc.).

Endoscope - A medical-optical fiber bundle used to examine theinside of the human body.

End separation loss -The optical power loss caused byincreasing the longitudinal distance between the end of an optical fiber and a source, detector, or fiber. See also: Extrinsic

joint loss.Equilibrium length - For a specific excitation condition, thelength of a multimode optical wave guide necessary to attain stable distribution of optical power among the propagatingmodes.

Equilibrium mode distribution (EMD ) - The condition in amultimode optical fiber inwhich the relative power distribution among the propagating modes is independent of length.Synonym:Steady-state condition. See also: Equilibrium length;Mode; Mode coupling.

Extraordinary ray - A ray that has a non-isotropic speed in adoubly refracting crystal. It does not necessarily obey Snell's lawupon refraction at the crystal interface. See also: Birefringence;Ordinary ray.

Extrinsic joint loss - Light loss caused by imperfect alignmentof fibers in a connector or splice. Contributors include angular misalignment, lateral offset, end separation and end finish.Generally synonymous with Insertion loss. See also: Angularmisalignment loss; End separation loss; Intrinsic joint loss;Lateral offset loss.

F Fall time -Typically specified as the time required for a signal tofall from 90 percent to 10 percent of its original negative or

positive amplitude. See also: Rise time

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Ferrule - A component of a fiber-optic connection that holds afiber in place and aids in its alignment.

Fiber -The optical wave guide, or light-carrying core or conductor.Generally refers to the combination of optical coreand cladding.

Fiber amplifier - A special length of fiber that will amplifyoptical signals entering its active area. Very similar in operation to a laser amplifier.

Fiber bandwidth - The frequency at which the magnitude of the fiber transfer function decreases to a specified fraction of thezero frequency (DC) value. Often, the specified value is one-half

the optical power at zero frequency.

Fiber buffer - A material protecting optical fibers from physicaldamage by providing mechanical isolation between them andexter nal influences.

Fiber bundle - An assembly of un buffered fibers. These areusually used as a single transmission channel to transmit light or images which may be either coherent or incoherent.

Fiber-optic link - Any optical transmission channel designed toconnect two end terminals.

Fiber optics - A branch of optical technology that deals with thetransmission of radiant energy through optical wave guidesgenerally made of glass or plastic.

Fresnel reflection - R eflection losses that occur at the in put andoutput faces of any optical material due to the differences in

refractive indexes between them. Similar to "standing waveratio" in electronics.

G Graded-index fiber - An optical fiber which has a gradualrefractive index change from the center to the edge. This type of fiber has much less dispersion than step-index fiber.

H Hertz (Hz) - A unit of frequency equivalent to one cycle per second.

I Inclusion -The presence of an impurity within a body of glass.

Incoherent light - Light that is made up of rays which lack a

fixed phase relationship. Most light is incoherent. LEDs producethis type of radiation. See Also: Coherent.

Index-matching material - A liquid or cement whose refractiveindex is nearly equal to that of a fiber's core index; used toreduce Fresnel reflections from an optical fiber's end face. SeeAlso: Fresnel reflection; Refractive index.

Index of refraction - See: Refractive index.

Infrared - Wavelengths longer than 700 nm and shorter than lmm. Infrared radiation cannot be seen, but it can be felt as heat.

Infrared light-emitting diode (ILED) - A semiconductor diodevery similar to LEDs, but which emits infrared radiation rather



than visible light. The manufacturing methods for both LEDs aresimilar, but they are composed of different semiconductor materials.

Injection laser diode - A solid state semiconductor deviceconsisting of at least one p-n junction capable of emittingcoherent, stimulated radiation under specified conditions.

Insertion loss -The optical power loss caused by insertion of an optical component such as a connector, splice, or coupler into a

previously continuous light path.

Interference - 1.The additive process whereby the amplitudesof two or more waves are systematically attenuated and

reinforced. 2. The process whereby a given wave is split into twoor more waves by, for example, reflection and refraction of beamsplitters, and then reunited to form a single wave.

Intrinsic joint loss - Loss caused by fiber-parameter (e.g., coredimensions, profile parameter) mismatches when two non-identical fibers are joined.See also: Extrinsic joint loss; Lateraloffset loss; Angular misalignment loss.

J Jacket - A layer of material surrounding a fiber but not bondedto it.

Joule - One unit of measure for energy (usually for smallamounts).

K Kilo - A prefix in the SI system meaning one thousand (1 x 103).Abbreviation k.

L Laser - 1. An acronym for "Light Amplification byStimulatedEmission of R adiation." Laser light is highly directional, occupiesa very narrow band of wavelengths and is more coherent than ordinary light. 2. "Laser" also refers to the apparatus whichcreates laser light.

Lateral offset loss - Optical power loss caused by transverse or lateral deviation from optimum alignment of source-to-optical-fiber, fiber-to-fiber, or fiber-to-detector. See also: Angularmisalignment loss.

Launch angle - The angle between a light ray and the opticalaxis of an optical fiber or bundle.

LED -S

ee: Light Emitting Diode.Light - 1.Electromagnetic radiation visible to the human eye. 2.Commonly, the term is applied to electromagnetic radiationwith

properties similar to those of visible light, including the invisiblenear-infrared radiation used in fiber optic systems.

Light emitting diode (LED) - A p-n junction semiconductor device that emits incoherent optical radiation when biased in theforward direction.

Loss - See: Absorption; Angular misalignment loss;Insertion loss; Intrinsic joint loss; Lateral offset loss;Rayleigh scattering.

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M Macrobending loss - Light losses caused by light rays exiting awave guide because the incident angle is less than the criticalangle, due to fiber bends greater than the fiber's diameter. Doesnot cause radiation losses.

Mega - A prefix in the SI system meaning one million (1 x 106).Abbreviation, M.

Meridional ray - A ray that passes through the optical axis of an optical fiber (in contrast to a skew ray, which does not). See also:Axial ray; Numerical aperture; Skew ray.

Microbending loss - In an optical fiber, light loss caused bysharp curvatures involving local axial displacements of a fewmicrometers and spatial wavelengths of a few millimeters. Such

bends may result from fiber coating, cabling, packaging andinstallation. Note: Microbending can cause significant radiation losses and mode coupling.

Micro - A prefix in the SI system meaning one millionth (1 x 10-

6). Abbreviation,w.

Micrometer - A unit of length in the SI system equal to 1 x 10-6

meters. Abbreviation,wm.

Milli - A prefix in the SI system meaning one thousandth (1 x

10-3). Abbreviation, m.

Modal noise - The noise generated at the exit aperture of awave guide when a coherent light source is used. The effect iscaused by interference between modes in the wave guide. Seealso: Mode; Interference.

Mode - In any cavity or transmission line, one of the elec-tromagnetic field distributions that satisfies Maxwell's equationsand boundary conditions.The field patter n of a mode dependson the wavelength, refractive index, and cavity or wave guidegeometry. See also: Equilibrium mode distribution.

Mode coupling - In an optical fiber, the exchange of power among modes. The exchange will reach statistical equilibriumafter propagation over a finite distance that is designated theequilibrium length. See also: Equilibrium modedistribution.

Modulate -To modify a single-frequency carrier frequency witha superimposed signal containing information, e.g., amplitudemodulation, frequency modulation, phase modulation, pulsemodulation. Used whether the carrier is a light-frequency signalor a radio-frequency (RF) signal.

Multifiber cable - An optical cable that contains two or morefibers, each of which provides a separate information channel.See also: Fiber bundle; Cable.

Multimode fiber - A fiber that permits propagation of morethan one mode. The number of modes in a fiber is defined by

boundary conditions and Maxwell's equations.The corediameter of multimode fibers can range from 25 - 2,000 microns.See also: Mode.



N n - A symbol used to represent refractive index.

Nanometer - One trillionth of a meter, one millionth of amillimeter. A common unit of measure for wavelengths of high-frequency energy such as light. Abbreviation, nm.

Near-infrared -The shortest wavelengths in the infrared region (700 to 2,000 nm), just slightly longer than those of visible light.Often called near-IR .

Noise currents - Any noise voltage or current that prevents

precise measurements.Dark current and thermal noise (fromamplifiers and resistors) contribute to noise in fiber opticsystems.

Noise equivalent power (NEP) - The rms value of optical power which is required to produce an rms signal-to-noise ratioof 1/1. NEP is an indication of noise level which defines theminimum detectable signal level

Normal - Also referred to as l ine normal . An imaginary line thatforms a right angle with a surface or with other lines. The word"normal" is often used rather than "perpendicular" when measuring/describing incident, reflected and refractive angles.

90r

Numerical aperture (NA) - The numerical aperture of an optical fiber defines the characteristic of a fiber in terms of itsacceptance of impinging light.The larger the numerical aperturethe greater the ability of a fiber to accept light. See also:Acceptance angle; Critical angle.

O Optical spectrum -Generally, the portion of electromagneticspectrum within the wavelength region extending from theultraviolet at 20 nm to the far-infrared at 100wm.

Optical time domain reflectometry - A method for

characterizing a fiber whereby an optical pulse is transmittedthrough the fiber and the resulting backscatter and reflectionsare measured as a function of time. Useful in estimatingattenuation coefficient as a function of distance and identifyingdefects and other localized losses. See also: Back-scattering;Rayleigh scattering; Scattering.

Optical wave guide - Any structure having the ability to guidea flow of radiant energy along a path parallel to its axis and, atthe same time, to contain that energy within or adjacent to itssurface.

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Optoelectronics -The field of electronics that deals with LEDs,lasers, and photodetectors, or any other electronic devices that

produce, respond to or utilize optical radiation.

Ordinary ray - A ray that has isotopic speed in a doublyrefracting crystal. It obeys Snell's law upon refraction at thecrystal surface. See also: Birefringence; Extraordinaryray.

P Photoconductivity - A conductivity increase exhibited by somenonmetallic materials, resulting from free carriers generatedwhen photon energy is absorbed in electronic transitions. An example of a photoconductive material is cadmium sulfide (CdS).

Photocurrent -The current that flows through a photosensitivedevice (such as a photodiode) as the result of exposure to radiant

power. See also: Dark current; Photodetector.

Photodarlington - A light detector in which a phototransistor iscombined in a device with a second transistor to amplify itsoutput current.

Photodetector - A detector that responds to incident light upon its surface. See Also: Photodarlington; Phototransistor;Photodiode.

Photon - In the quantum theory of physics, the fixed, elementalunit of light energy. Light can be viewed either as a wave of electromagnetic energy or as a series of photons.

Phototransistor - A transistor that detects light and amplifies

the resulting electrical signal. Light falling on the base-emitter junction generates a current, which is amplified inter nally.

Photovoltaic effect - Production of a voltage difference across a p-n junction resulting from the absorption of photon energy.Thevoltage difference is caused by inter nal drift of holes andelectrons. See also: Photon.

Photodiode - A two-electrode, radiation-sensitive junction formed in a semiconductor material in which the reverse currentvaries with illumination. Photodiodes are used for the detection of optical power and for the conversion of optical power toelectrical power. See also: Avalanche photodiode.

Planck's Constant - A universal constant (h) which describesthe ratio of a quantum of radiant energy (E) to the frequency (v)of its source. It is expressed:

Plastic clad silica (PCS) fiber - A fiber with a glass core and plastic cladding.This fiber has very high numerical aperture andhigh light transmission.

Polarization -The electric field vector motion in an electromagnetic wave.Different types of polarization describedifferent types of sources.

Propagating -Transmitting or moving energy along a path.

Pulse dispersion - The widening of an optical pulse as it travelsthe length of a fiber. This property ² which limits the useful

bandwidth of the fiber ² is usually expressed in nanoseconds of





an in jection laser diode above lasing threshold. See also:Spontaneous emission; Injection Laser Diode.

T Total internal reflection - The total reflection of light back intoa material when it strikes a boundary at an angle exceeding thecritical angle. This is the property that keeps light confinedwithin an optical fiber. See also: Critical Angle.

Transducer - A device designed to convert one form of energyinto another. Audio speakers for radio, TV and tape decks,convert audio-frequency electrical energy into audible sound. A

phonograph cartridge converts mechanical movement of theneedle into an electrical signal that activates the speakers.

Transmittance - The ratio of the radiant power transmitted byan object to the incident radiant power. Thus, sunglasses maytransmit to your eyes only 10 percent of the light being radiated

by the sun.

Transmitter - A device such as a transducer which converts an electrical signal into an optical wave for transmission over a fiber cable.See also: Receiver.

U Ultraviolet - An invisible portion of the optical spectrum whosewavelengths begin immediately beyond the violet end of thevisible spectrum.Ultraviolet wavelengths range fromapproximately 20 to 380 nm. These are the most damaging of the sun's rays to human skin and eyes.

V Velocity of light -The speed of light in a vacuum, in roundnumbers, is 300,000 kilometers per second, or 186,000 miles

per second. It is less than .01% slower in air.

W Wavelength -The distance an electromagnetic wave travels in the time it oscillates through a complete cycle. Wavelengths of light are most often measured in nanometers (nm) or micrometers (mm).

X

Y

Z

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Table 3. Metric Pr efixesand Their Meanings.

Pr efix

ter a

giga

mea

kilo

hecto

deca

deci

centi

milli

micro

nano

pico

f emto

Symbol

T

G

M

k

h

da

dc

m

w

n

p

f

Multiple

1012 (tr illion)

109 (billion)

106 (million)

103(thousad

102(hundred

101 (ten)

10-1 (tenth)

10-2 (hundredth)

10-3 (thousandth)

10-6 (millionth)

10-9 (billionth)

10-12(tr illionth)

10-15 (quadr illionth



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