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Laser Fiber Optics Trainer

Scientech 2506

Product Tutorials

Ver. 1.1

Designed & Manufactured by-

An ISO 9001:2008 company

Scientech Technologies Pvt. Ltd.

94, Electronic Complex, Pardesipura, Indore - 452 010 India,

+ 91-731 4211100, : [email protected] , : www.ScientechWorld.com

mailto:[email protected]://www.scientechworld.com/http://www.scientechworld.com/mailto:[email protected]


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Scientech 2506

Scientech Technologies Pvt. Ltd. 2

Laser Fiber Optics Trainer

Scientech 2506

Table of Contents

1. Introduction 3

2. Description of Scientech 2506 Transmitter Unit 4

3.

Description of Scientech 2506 Receiver Unit 5

4.

Experiments

• Experiment1 6Study of Characteristics of LASER Diode

5. Specification for LASER Diodes 7

• Experiment 2 13Study of operation of ACC and APC modes of operation

• Experiment 3 15Study of the AC characteristics of an Intensity Modulation LASERand Fiber Optics System

• Experiment 4 18Study of the rise time and fall time pulse width distortion of a

LASER and determine the Propagation Delay

•

Experiment 5 20Study of various types of losses in optical Fiber

• Experiment 6 22Study of the performance of an analogue and digital, free space

communication system

• Experiment 7 23Determination of Numerical Aperture of Optical fiber

6. Warranty 25

7. List of Accessories 25


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Introduction

The LASER Diode/Fiber Optics Design Kit model Scientech 2506 has been

developed to conduct studies on LASER diodes, optical fibers and opticalcommunication methods, by transmission either through an optical waveguide or free

space. The experiments included for study, introduce the student to concepts

underlying this state-of-art technology in a simple style. The Trainer includes all the

accessories to conduct most of the experiments.

Seven experiments based on the Model Scientech 2506 Kit have been included in this

Manual with full details. The user can design a number of other interesting

experiments and small projects based on the Kit.

Contents of the Kit:

1.

LASER Diode Design Module Scientech 2506 TX unit 1No

2. LASER Diode Design Module Scientech 2506 RX Unit 1No

3. Two meter PMMA Fiber Patch cord (cable 1) 1No

4. Two-meter Glass Fiber Patch cord (cable 2) 1No

5. Numerical Aperture Measurement Jig


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Description of Transmitter Unit

The schematic diagram is depicted below.

Figure 1

The LASER controller and driver circuit facilitates study of LASER diode

characteristics in the automatic current control and automatic power control modes,selectable through the toggle switch provided on the module. Intensity modulation

with analogue or digital signals is through Vin. The optical intensity/carrier level is

settable by the knob marked set IF. The LD current and the monitor photo detector

current are monitored on the leads marked VL and VM. Optical power is coupled to

an optical fiber using the terminal SMA connectors. In the case of free space

communication the output from the module links to the RX Unit through a line-of-

sight path.


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Description of Receiver Unit

Scientech 2506 RX Unit comprises two parts

a)

The optical power meter, and

b) The linear detector

Figure 2

These are described below:

The Optical Power Meter converts the optical power incident at the port marked FO

PIN through an SMA terminated optical fiber and facilitates display of the optical

power Po in decibels referred to a milli watt. The two wires marked Po, are connected

to the 0-2000 m V range of the DMM. The optical power, in decibels, is given byDMM Reading/ 10 in decibels referred to 1mw. As an example, if the meter reading is

-193 then the optical power Po is -193/10 dBm, or simplified,-19.30 dBm. For higher

sensitivity, in some experiments, one may use the 0-200.0 mV range of the DMM. In

such cases too, the DMM reading has to be divided by 10. As an example, if the

DMM reading in the 0-200.0mV range is -133.5 then the optical power, Po, is read as

-13.35 dBm.

The Optical Power meter section can be used in experiments involving other

equipment too. The linear detector is activated through the port marked FO PT.

The detector receives a LASER (any optical) signal and demodulates the intensity

modulated signal on the carrier. For continuous wave light, a DC voltage proportionalto the intensity results The SMA connector facilitates coupling to an optical fiber.


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Experiment 1

Objective:

Study of Characteristics of LASER Diodes(i)

Optical Power (Po) of LASER Diode vs LASER Diode Forward Current (IF)

(ii)

Monitor photodiode current (IM) vs LASER Optical Power Output (Po)

Equipments Needed:

1.

Digital Multi-meters

Basic Definitions and Concepts:

LEDs and LASER Diodes are the commonly used sources in optical communicationsystems, whether the system transmits digital or analogue signals. In the case of

analogue transmission, direct intensity modulation of the optical sources is possible provided the optical output from the source can be varied linearly as a function of the

modulating electrical signal amplitude. LEDs have a linear optical output with

relation to the forward current over a certain region of operation. It may be mentioned

that in many low-cost, short-haul and small bandwidth applications. LEDs at 660nm,

850nm, and 1300nm are popular. While direct intensity modulation is simple to

realize, higher performance is achieved by fm modulating the base-band signal prior

to intensity modulation.

LASER Diodes are used in telecom, data com and video communication applications

involving high speeds and long hauls. All single mode optical fiber communicationsystems use LASERs in the 1300nm and 1550 nm windows. LASERs with very small

line widths also facilitate realization of wavelength division multiplexing (WDM) forhigh density communication over a singe fiber. The inherent properties of LASER

diodes that make them suitable for such applications are high coupled optical power

into the fiber (Typically greater than 1 mW), high stability of optical intensity, small

line-widths (less than 0.05 nm in special devices), high speed (several GHz) and high

linearity (over a specified region suitable for analogue transmission). Special LASERs

also provide for generation/ amplification of, optical signals within an optical fiber.These fibers are known as erbium doped fiber amplifiers; LASER diodes for

communication applications are available in the wavelength regions 650nm, 780nm,850nm, 980nm, 1300nm and 1550nm.

Even though a variety of LASER diode constructions are available there are a number

of common features in all of them. We have selected a very simple device (650nm/2.5

mw) to demonstrate the functioning of a LASER diode.


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Specifications of typical LASER diode at 650 nm are summarized below.

Symbol Parameter Typical Unit

PO CW output power 2.5 mW

IOP Operating current 30 mA

WP Wavelength at peak emission 650 Nm

MTTF Mean Time to Failure 10,000 hrs

Specifications for LASER Diodes

Monitor Photo Detector (MPD) Automatic Power Control and Automatic

Current Control Modes of Operation

Figure 3


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A LASER diode has a built in photo detector, which one can employ to monitor the

optical intensity of the LASER at a specified forward current. This device is also

effectively utilized in designing an optical negative feedback control loop, to stabilize

the optical power of a LASER in the steep lasing region. The electronic circuitscheme that employs the monitor photodiode to provide a negative feedback for

stabilization of optical power is known as the Automatic Power Control Mode (APC).

If a closed loop employs current control alone to set optical power then this mode is

called the Automatic Current Control Mode (ACC).

The disadvantage of ACC scheme is that the optical power output may not stable at agiven current due to the fact that small shifts in the lasing characteristics occur with

temperature changes and ageing. The disadvantage of the APC is that the optical

feedback loop may cause oscillations, if not designed properly.

Schematic of a LASER diode Operation (ACC Mode)

Figure 4

Precautions to be observed while handling LASER Diodes:

• LASERs are highly ESD sensitive, use proper ESD protected facility

• No LASER beam should LASER diode be viewed directly of from a highlyreflecting surface

• LASERs are highly sensitive to transients


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Procedure with Block Schematic for Po vs IF experiments:

The schematic diagram for study of the LASER DIODE Po as a function of LASERDIODE forward current IF is shown below and is self explanatory.

Figure 5

1. Connect the 2 meter PMMA FO cable (cable 1) to TX Unit and couple the

LASER light to the power meter FO PIN on the RX Unit as shown. Select ACCmode of operation.

2. Set DMM 1 to the 2000 mV range. On the RX side connect the wires marked Po

to it. Turn it on. The power meter is now ready for use. Po = (reading)/10 dBm.

It required change the DMM range to 200mV.

3. Set DMM2 to the 200.0 mV range and connect it between the wire VL and

ground on the TX unit. IF = VL /100Ω.

4. Adjust the SET IF on the TX knob to the extreme anticlockwise position to

reduce IF to zero. The power meter reading will normally be below -40dBm or

out of range.

5. Slowly turn the SET IF knob clockwise to increase IF and Po, Note IF and Po

readings. Take closer reading prior to and above the LASER threshold ofLASER Diode.

6. Plot the graph Po vs IF on a semi log graph sheet. Determine the slopes prior to

lasing and after lasing. Record the LASER threshold LASER Diode current. It

should look like as shown below:


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Figure 6Table of Readings: (ACC Mode/PMMA Cable)

Sr. No VL (mV) IF = VL /100mA Po (dBm)

1

2

3

4

Inferences: From the above Table it is seen that the LASER optical output does not

increase appreciably for IF below the threshold LASER Diode current I th. Above Ith,Po increases steeply Po is very steep. The LASER threshold LASER Diode may be

determined from the graph or by recording closer readings.

Procedure with Block Schematic for IM vs Po Experiment:

The schematic diagram for study of the monitor photodiode current as a function of

LASER DIODE optical output Po is shown in figure 6 and is self explanatory.

1. Connect the 2-meter PMMA FO cable to Po port of and couple the LASER lightto FO PIN the power meter as shown.

2. Set DMM 1 to the 2000 mV range. On the RX Unit, connect the Wires marked

Po to it. Turn it on. The power meter is now ready for use. Po = (Reading)/10dBm


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Figure 7

3. Set DMM2 to the 200.0mV range and connect it between the VM and ground on

the TX Unit.

4. Adjust the SET If knob to the extreme anticlockwise position to reduce IM to the

minimum value. There will be a negligible offset voltage.

5. Change Po in suitable Steps and note the VM readings. Record up to the extremeclockwise position.

6. Plot the graph IM vs Po on a semi-log graph sheet IM = (VM)/ (100K).


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Table of Readings, ACC/PMMA Cable

Sr. No Po (dBm) VM (mV) IM (µ)=(VM)/ 100k

1

2

3

4

Inferences:

From the above Table it is seen that the MPD photo current sharply increases, above

the threshold LASER Diode Po, following the pattern of the first part of experiment.

The threshold LASER Diode Po and IF (threshold LASER Diode) may be computed

from the plot.


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Experiment 2

Objective:

Study of the operations of the Automatic Current Control (ACC») and the Automatic

Power Control (APC) modes

Figure 8

In the ACC mode the feedback to the LASER driver is derived from the load current

IF. V0 tracks the Vref (or Vin DC). As already noted, this may not ensure constant

optical power for a given Vref, if shifts in LASER threshold of LASER Diode occur

due to change in temperature and ageing. On the other hand the APC circuit derives

its feedback from the monitor photo current; IM which is proportional to Po, Here, VM

tracks Vref. For a given Vref, we get a constant optical power output, irrespective of

changes in temperature and ageing procedure with Block Schematic. The schematic

diagram for the ACC and APC modes is shown below.

Figure 9

1. Connect the 2-meter PMMA FO cable 1 the LASER port of the TX Unit.

Couple the LASER to the power meter as shown.

2. Set DMM 1 to the 2000 mV range. On the Rx Unit, connect the wires marked

Po to it. Turn it on. The power meter is now ready for use. Po = (Reading)/10

dBm


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3. On the Tx Unit set DMM2 to the 200.0 mV range and connect it between the

VM and ground. On the Tx Unit set DMM 3 to 200.0 m V range and connect it

between the VL and ground.

4. Adjust the SET IF knob to the extreme anticlockwise position.

5. Set the ACC/APC select switch to ACC Mode. Slowly turn the SET. IF knobclockwise to increase Po to a suitable value. Note the readings Po1 and Vo (Vo=

Vref in the ACC Mode when system gain is unity) readings. Next switch to the

APC Mode and note P02 and the VM readings (VM = Vref in the APC mode for

system gain = unity).

Record up to the extreme clockwise position.

Table of Readings ACC and APC Operations /PMMA

S. No. ACC Po(dBm) Mode Vo=Vref(mV) APC VM=Vref (mV) Mode Po (dBm)

1

2

3

Inferences:

From the above table it is seen that the V0 tracks Vref in the ACC mode and VM

tracks Vref in the APC mode. The corresponding optical outputs Po are different.


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Design and Evaluation of a LASER Diode Linear Intensity Modulation System

Experiment 3

Objective:Study of the AC characteristics of an Intensity Modulation LASER and Fiber

Optics System:

I) Vin (AC) vs Vout (AC) for fixed carrier power Po and signal frequency, F0

II) Vin max vs Po for known distortion free out at fixed F0

Basic Principles and Definitions:

The intensity modulation/demodulation system is realized using the L T 2506 TX

Unit and the RX Unit linked through an optical fiber. We use the 2-meter PMMA

fiber cable.

Figure 10

The LASER carrier power, Po is set by adjusting the SET.IF knob in the middle

LASER region.

Selection of optimum carrier power is essential to minimize distortion. Limiting depthof modulation also ensures distortion free transmission. The bandwidth of the system

in the present case is limited by the photo detector. We may choose to operate in the

ACC or APC mode to obtain output proportional to the modulating signal Vin.

An ideal IM transmission system will have the relationship Vout = G.Vin, where G is

a factor dependent on the LASER DIODE conversion efficiency, loss in the optical

transmission path and the LASER photo detector conversion efficiency. Distortion

results from the LASER DIODE being biased in the non-linear region. Bandwidth is

limited by the slowest device in the system; in this case it is the phototransistor. Speed

can be increased by using a PIN diode, which is inherently a faster device.


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Procedure with Block Schematic

1. Connect one end of the PMMA FO cable (cable 1) to the LASER port on the Tx

Unit. The other end is first connected to FO PIN (on Rx Unit) to set the carrier

power level of the LASER. Then it is removed and given to FO PT (Rx Unit) tostudy the response of the IM system.

2. Set DMM to the 2000mV range. Connect the wires marked Po to it. The powermeter is now ready for use. Po = Reading)/10 dBm.

3. On the Tx Unit, connect Vin to a function generator (10Hz to 500 KHz; sine

wave output, 10mV to 2000 mVp-p output). Give the function generator output

to CHI, as shown.

Figure 11

4. On the Rx Unit connect Vout to CH2 of the Dual Trace Oscilloscope.

5. Plug the AC mains for both systems

6. With the PMMA Fo cable connected to the power meter, adjust the SET IF knob

to set the optical carrier power Po to a suitable level say, -15 dBm. Next

disconnect the cable from the power meter and connect to FO PT.

7. Set signal frequency and amplitude to 2 KHz and 100mV respectively. Observethe transmitted and received signals on the oscilloscope. Set Rin suitably to getVout = Vin or a known gain. The system gain is now set. Next, vary Vin in

suitable values from 10mV to 1000 mVp-p and note the value of out. Tabulate

and plot a graph Vout vs Vin.


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Table of Readings: Vout vs Vin

Frequency = 2 KHz; Carrier Level Po = -15dBm; Initial Gain = minimum position

S. No. Vin (mVp-p) Vout (mVp-p) G =V0/Vin

1

2

3

8.

Set signal frequency to 2 KHz and Po to -25.0 dBm. Disconnect Vin before Pomeasurement. Set Vin to its maximum value for distortion free Vout. Note the

values of Vin and Vout. Repeat this for other values of Po and record changein gain if any. You may additionally observe the waveforms in the

oscilloscope DC coupled position too. Table of Readings: Vin maximum vs PoFrequency =2 KHz; Initial Gain=Unity

S. No. Po(dBm) Vin max (mVp-p) Vout (mVp-p) Gain= V0/Vin

1

2

3

You may also compare the ACC and the APC modes in the case of IM. The

experiment may be repeated for other settings of gain.


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Design and Evaluation of a LASER Diode Digital Intensity Modulation System

Experiment 4

Objective: Study of the rise time and fall time pulse width distortion of a LASER and Fiber

Optics digital transmission system and determine the Propagation Delay


The intensity modulation/demodulation for digital system is realized using the

LT2506 Tx and the Rx units linked through an optical fiber. We use the 2-meter

PMMA fiber cable. The LASER carrier power, Po is set by adjusting the SET IF knob.

It is set below the lasing threshold LASER Diode for state '0'. An 'I'1 at the input will

drive the LASER to LASER mode and in effect high intensity. The Rin for the linear

detector can be set to values to study the responses.

Figure 12

1. On the Tx Unit, connect one end of the PMMA FO cable to the LASER Po. On

the Rx Unit, the other end is first connected to FO PIN to set the carrier power

level, Po. Then it is removed and given to FO PT. Setting of P O may not have

any effect with some LASER diodes.

2. Set DMM to the 2000 mV range. Connect the wires marked Po to it. The power

meter is now ready for use. Po = (Reading)/10 dBm.

3. Connect Vin to a function generator (10Hz to 500 KHz; square wave, TTL

output). Give the function generator output to CH I.

4. With the power off, measure the Rin, i.e. the resistance across Vout for Rx Unit.

Now adjust SET Rin to 1000ohms. Next connect Vout of the Rx Unit to CH 2 of

the Dual Trace Oscilloscope.


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5. With the PMMA cable connected to the power meter, adjust the SET IF knob to

set the optical carrier power to a suitable value, say -17 dBm. Disconnect the

cable from the power meter and connect to FO PT.

6. Set signal frequency to 10 KHz. Observe the transmitted and received signals on

the oscilloscope. Compare the rise and fall times of the transmitted and received

pulses. Also determine the propagation delay.

7. Repeat for other values of Rin from Step 4.

Results:

Case 1

Po=15 dbm, Vin=TTL Fo=10 KHz R load=100ohms

Rise Time Distortion: µsFall Time Distortion: µs

Propagation Delay: µs

Case II

Po= -15 dbm, Vin = TTL Fo=10 KHz R load=200ohms

Rise Time Distortion: µs

Fall Time Distortion: µs

Propagation delay: µs


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Transmission of LASER through an Optical Fiber:

Experiment 5

Objective:Study of various types of losses that occur in optical fibers and measure the loss in dB

of optical fiber patch cords individually and also connected in tandem using an in-lineadaptor.

Basic Definitions:

Attenuation in an optical fiber is a result of a number of effects. This aspect is well

covered in the books referred. We will confine our study to attenuation in a fiber due

to macro bending and estimate the losses in the patch cords. The loss as a function of

the length of the fiber is not measurable here as the lengths of fibers under

consideration are too short. The optical power at a distance, L, in an optical fiber is

given by PL = Po 10(L/100) where Po is the launched power and is the attenuationcoefficient in decibels per unit length. The typical attenuation coefficient value for the

PMMA fiber under consideration here is 0.3 dB per meter for light at a wavelength of660 nm. For the GI fibers it is of the order of 3 to 4 dB-per kilometer at 850 nm.

Loss in fibers expressed in decibels is given by

10 log (Po /PF)

Where, Po is the launched power and PF is power at the far end of the fiber. Typical

losses at connector junctions may vary from 0.3 dB to 0.8 dB.

The loss equation for a simple fiber optic link is given as:

Pin (dBm)-P out (dBm) = LJ1 + LFIB1 + LJ2 + LFIB2 + LJ3 (dB); where, LJ1 (dB) is the

loss at the LED-connector junction, LFIB1 (dB) is the loss in cable LJ2 (dB) is the

insertion loss at a splice or in-line adaptor, LFIB2 (dB) is the loss in cable 2 and LJ3 (dB) is the loss at the connector -detector junction. Losses in fibers also occur at fiber-

fiber joints or splices due to axial displacement, angular displacement, separation,

mismatch of cores diameters, mismatch of numerical apertures, improper cleaving and

cleaning at the ends.

In the experiment we will be studying the losses due to core differences and losses

due to core mismatches. Loss as a function of length is not possible because thelengths under consideration are too small. Loss as a function of wavelength is not

possible as we have only one source.

The patch cords designated for the experiments are as follows:

Cable 1: 2-meter PMMA SI/MM (BLACK Jacket)

Cable 2: 2-meter Glass fiber (ORANGE Jacket)

The experiment may be repeated for other patch cords if available.



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LASER Free Space Communication

Experiment 6

Objective:Study of the performance of an analogue and digital, free space communication

system


In this experiment a free space intensity modulation/demodulation system is realized

using the TX and RX Units. The system can work effectively up to a few tens of

meters. All experiments are identical to Experiments 3 and 4 except that instead of an

optical fiber to couple the transmitter output to the detector, we employ transmission

of the LASER beam through free space. The scheme is depicted in the diagram below.

Since the procedures are identical to Experiments 3 & 4, detailed descriptions are

skipped here. To obtain effective results, the line-of sight system should LASERDiode be properly aligned and then retained in a stable position.

Figure 15


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Determination of Numerical Aperture of Optical Fibers

Experiment 7

Objective:Determination the numerical Aperture of the Optical Fibers available

Basic Definitions:

Numerical aperture of any optical system is a measure of how much light can be

collected by the optical system. It is the product of the refractive index of the incident

medium and the sine of the maximum ray angle.

NA == ni. sinθmax

ni for air is 1, hence NA = sinθmax

For a step-index fiber, as in the present case, the numerical aperture is given by N= (n core

2 – n cladding

2)

For very small differences in refractive indices the equation reduces to

NA = n core (2!) ½

Where ! is the fractional difference in refractive indices.


The schematic diagram of the numerical aperture measurement system is shown

below and is self explanatory.

Figure 16

1. Connect one end of the PMMA FO cable to Po of TX Unit and the other end to

the NA jig, as shown.

2. Plug the AC mains. Light through the LASER Diode should appear at the end of

the fiber on the NA Jig. Turn the Set IF knob clockwise to set to maximum Po.

The light intensity of LASER Diode should increase.


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3. Hold the white screen, provided in the Kit vertically at a distance of 15 mm (L)

from the emitting fiber end and view the red spot on the screen. A dark room

will facilitate good contrast. Position the screen-cum-scale to measure the

diameter (W) of the spot. Choose the largest diameter.

4. Compute NA from the formula

NA= sinθmax = √ W / (4L2+W2)

Tabulate the reading and repeat the experiment for 10mm, 20mm, and 25mm

distance.

5. In case the fiber is under filled, the intensity within the spot may not be evenly

distributed. To ensure even distribution of light in the fiber, first remove twists

on the fiber and then wind 5 turns of the fiber on to the mandrel as shown. Use

an adhesive tape to hold the windings in position. Now view the spot. The

intensity will be more evenly distributed within the core.

Table of Readings,

Inferences:

The numerical aperture as recorded in the manufacturer's data sheet is 0.5 typically.The value measured here is 0.437. The lower reading recorded is mainly due to the

fiber being under filled. The acceptance angle is given by 2 θmax. The value of 52°recorded in the experiment is close to the range of 55-60°. The lower reading is again

due to the fiber being under filled.

S. No. L(mm) W(mm) NA θ (degrees)

1

2

3

4

5


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Warranty

1. We guarantee this product against all manufacturing defects for 24 months from

the date of sale by us or through our dealers.2.

The guarantee will become void, if

a. The product is not operated as per the instruction given in the Learning

Material.

b.

The agreed payment terms and other conditions of sale are not followed.

c.

The customer resells the instrument to another party.

d.

Any attempt is made to service and modify the instrument.

3.

The non-working of the product is to be communicated to us immediately giving

full details of the complaints and defects noticed specifically mentioning thetype, serial number of the product and date of purchase etc.

4. The repair work will be carried out, provided the product is dispatched securely

packed and insured. The transportation charges shall be borne by the customer.

Hope you enjoyed the Scientech Experience.

List of Accessories

Quantity

1. Transmitter........................................................................................................1

2. Receiver.............................................................................................................1

3. Fiber Optic Cable (Plastic).................................................................................1

4. Fiber Optic Cable (Glass)...................................................................................1

5.

Mains Cords.......................................................................................................2

6. NA measuring Jig...............................................................................................1

7. NA Measurement Scale......................................................................................1

8. Mandrel...............................................................................................................1

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