Fiber Optics Technology and Step Index Fiber

Optical Fiber

By- Ramesh Pagidipalli

Fiber-Optic Communication Fiber-optic communication is a method of transmitting

information from one place to another place by sending pulses of light through an optical fiber.

Fiber-optic communication systems have revolutionized the telecommunications industry and have played a major role in the advent of the Information Age.

Because of its advantages over electrical transmission, optical fibers have largely replaced copper wire communications in core networks in the developed world.

Researchers at Bell Labs have reached internet speeds of over 100 petabits per second

Optical Fiber

Optical Fiber Communication

Optical Fiber

Optical Fiber An optical fiber is a cylindrical dielectric waveguide made of

low-loss materials such as silica glass. It has a central core in which the light is guided, embedded in

an outer Cladding of slightly lower refractive index Light rays incident

on the core-cladding boundary at angles greater than the critical angle undergo total internal

Reflection and are guided through the core without refraction..

Optical Fiber

Optical Fiber

Optical Fiber

History 1880 Alexander G. Bell

Photo phone, transmit sound waves over beam of light

1930: TV image through uncoated fiber cables Few years later image through a single glass fiber

1951: Flexible fiberscope: Medical applications 1956: The term “fiber optics” used for the first time 1958: Paper on Laser & Maser

Optical Fiber

History Cont’d 1960: Laser invented 1967: New Communications medium: cladded fiber 1960s: Extremely lossy fiber:

More than 1000 dB /km 1970: Corning Glass Work NY, Fiber with loss of less

than 2 dB/km 70s & 80s : High quality sources and detectors Late 80s : Loss as low as 0.16 dB/km 1990: Deployment of SONET systems

Optical Fiber

Optical Fiber: Advantages Capacity: much wider

bandwidth (10 GHz) Crosstalk immunity Immunity to static interference

Lightening Electric motor Florescent light

Higher environment immunity Weather, temperature, etc.

http://www.tpub.com/neets/book24/index.htmOptical Fiber

Optical Fiber: Advantages Safety: Fiber is non-metalic

No explosion, no chock Longer lasting Security: tapping is difficult Economics: Fewer repeaters

Low transmission loss (dB/km) Fewer repeaters Less cable Remember: Fiber is non-conductive

Hence, change of magnetic field hasNo impact!

Optical Fiber

Optical Fiber: Disadvantages Higher initial cost in installation Interfacing cost Strength Lower tensile strength Remote electric power More expensive to repair/maintain Tools: Specialized and sophisticated

Optical Fiber

Light Spectrum Light frequency is

divided into three general bands

Remember: When dealing with

light we use wavelength: =c/f c=300E6 m/sec

Optical Fiber

Optical Fiber Architecture

TransmitterInputSignal

Coder orConverter

LightSource

Source-to-FiberInterface

Fiber-to-lightInterface

LightDetector

Amplifier/ShaperDecoder

Output

Fiber-optic Cable

Receiver

TX, RX, and Fiber Link

Optical Fiber

Optical Fiber Architecture – Components

Light source: Amount of light emitted is

proportional to the drive current

Two common types: LED (Light Emitting

Diode) ILD (Injection Laser

Diode) Source–to-fiber-coupler

(similar to a lens): A mechanical interface to

couple the light emitted by the source into the optical fiber

InputSignal

Coder orConverter

LightSource

Source-to-FiberInterface

Fiber-to-lightInterface

LightDetector

Amplifier/ShaperDecoder

Output

Fiber-optic Cable

Receiver

Light detector: PIN (p-type-intrinsic-n-type) APD (avalanche photo diode) Both convert light energy into

current

Optical Fiber

A little about Light When electrons are excited and

moved to a higher energy state they absorb energy

When electrons are moved to a lower energy state loose energy emit light photon of light is generated

Energy (joule) = h.f Planck’s constant: h=6.625E-23

Joule.sec f is the frequency

http://www.student.nada.kth.se/~f93-jhu/phys_sim/compton/Compton.htm

E=h.f

Optical Fiber

Refraction Refraction is the change in direction

of a wave due to a change in its speed Refraction of light is the most

commonly seen example Any type of wave can refract when

it interacts with a medium Refraction is described by Snell's law,

which states that the angle of incidence is related to the angle of refraction by :

The index of refraction is defined as the speed of light in vacuum divided by the speed of light in the medium: n=c/v

Optical Fiber

Optical Fiber Construction Core – thin glass center

of the fiber where light travels.

Cladding – outer optical material surrounding the core

Buffer Coating – plastic coating that protects the fiber.

Optical Fiber

Fiber Types Based on Material Optical fibers are made up of materials like silica and plastic. The basic optical fiber material must have the following properties:

(i) Efficient guide for the light waves (ii) Low scattering losses (iii) The absorption, attenuation and dispersion of optical energy

must be low.

Optical Fiber

Fiber Types Based on Material Based on the material used for fabrication, they are classified into two types:

Glass fibers

Plastic fibers

Optical Fiber

Fiber Types Based on Material Glass fibers : The glass fibers are generally fabricated by

fusing mixtures of metal oxides and silica glasses.

Examples:

SiO2 core; P2 O3 – SiO2 cladding GeO2 – SiO2 core; SiO2 claddingP2O5 – SiO2 core; SiO2 cladding

Optical Fiber

Fiber Types Based on Material Plastic fibers :

The plastic fibers are of low cost. Although they exhibit considerably greater signal attenuation than glass fibers, but these are tough and durable. Due to its high refractive index differences between the core and cladding materials, plastic fibers yield high numerical aperture and large angle of acceptance.

Optical Fiber

Optical Fibers Based on Modes

Mode is the one which describes the nature of propagation waves in a Fiber.

i.e. it is the allowed direction whose associated angles satisfy the conditions for total internal reflection and constructive interference.

Optical Fiber

Optical Fibers Based on ModesBased on the number of modes that propagates through the optical fiber, they are classified as:

Single mode fibers (V<2.405) Multi mode fibers (V>2.405)

Optical Fiber

2π2 1anV

Here, n1 = refractive index of the core; a = radius of the core; λ = wavelength of the light propagating through the fiber; Δ = relative refractive indices difference.

Single Mode Fibers:

In a fiber, if only one mode is transmitted through it, then it is said to be a single mode fiber.

A typical single mode fiber may have a core radius of 3 μm and a numerical aperture of 0.1 at a wavelength of 0.8 μm.

The condition for the single mode operation is given by the V number of the fiber which is defined as such that V ≤ 2.405.

Optical Fiber

Single Mode Fibers:characteristics:

Only one path is available.V-number is less than 2.405Core diameter is small No dispersion Higher band width (1000 MHz)Used for long haul communicationFabrication is difficult and costly

Optical Fiber

Multi Mode Fibers:

If more than one mode is transmitted through optical fiber, then it is said to be a multimode fiber.

The larger core radii of multimode fibers make it easier to launch optical power into the fiber and facilitate the end to end connection of similar powers.

Optical Fiber

Multi Mode Fibers:Properties More than one path is available V-number is greater than 2.405 Core diameter is higher Higher dispersion Lower bandwidth (50MHz) Used for short distance communication Fabrication is less difficult and not costly

Optical Fiber

Modes in Fiber

Optical Fiber

Optical Fiber Based on Refractive Index Profile Based on the refractive index profile of the core and cladding, the optical fibers are classified into two types:

Step index fiber

Graded index fiber.

Optical Fiber

Step Index Fiber In a step index fiber, the refractive index

changes in a step fashion, from the center of the fiber, the core, to the outer shell, the cladding.

It is high in the core and lower in the cladding. The light in the fiber propagates by bouncing back and forth from core-cladding interface.

The step index fibers propagate both single and multimode signals within the fiber core.

Optical Fiber

Types Of Optical Fiber

Single-mode step-index Fiber

Multimode step-index Fiber

n1 coren2 cladding

n2 claddingn1 core

Lightray

Index profile

Optical Fiber

Step index single mode fibers :

The light energy in a single-mode fiber is concentrated in one mode only.

This is accomplished by reducing and or the core diameter to a point where the V is less than 2.4.

In other words, the fiber is designed to have a V number between 0 and 2.4.

Optical Fiber

Step index single mode fibers :

This relatively small value means that the fiber radius and , the relative refractive index difference, must be small.

No intermodal dispersion exists in single mode fibers because only one mode exists.

With careful choice of material, dimensions and , the total dispersion can be made extremely small, less than 0.1 ps /(km nm), making this fiber suitable for use with high data rates.

Optical Fiber

Single-mode step-index FiberAdvantages: Minimum dispersion: all rays take same path, same time to

travel down the cable. A pulse can be reproduced at the receiver very accurately.

Less attenuation, can run over longer distance without repeaters.

Larger bandwidth and higher information rateDisadvantages: Difficult to couple light in and out of the tiny core Highly directive light source (laser) is required Interfacing modules are more expensive

Optical Fiber

Step index multimode fibers : In this fibers light propagates in many modes. The total number of modes MN increases with

increase in the numerical aperture. For a larger number of modes, MN can be

approximated by

Optical Fiber

2

12 2

9.42

dnVM N

where d = diameter of the core of the fiber and V = V – number or normalized frequency.

Step index multimode fibers : The normalized frequency V is a relation

among the fiber size, the refractive indices and the wavelength. V is the normalized frequency or simply the V number and is given by

Optical Fiber

21

1 )2(2 N.A 2

naaV

where a is the fiber core radius, is the operating wavelength, n1 the core refractive index and the relative refractive index difference.

Step index multimode fibers :To reduce the dispersion, the N.A should not be decreased beyond a limit for the following reasons:

First, injecting light into fiber with low N.A becomes difficult. Lower N.A means lower acceptance angle, which requires the entering light to have a very shallow angle.

Second, leakage of energy is more likely, and hence losses increase. The core diameter of the typical multimode fiber varies between 50 m and about 200 m, with cladding thickness typically equal to the core radius.

Optical Fiber

What do the fiber terms 9/125, 50/125 and 62.5/125 (micron)

Remember: A micron (short for micrometer) is one-millionth of a meter

Typically n(cladding) < n(core)

Optical Fiber

Any Questions

Optical Fiber