Intro to OFC

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Introduction to OFC

Introduction to Fibre Optics, theory and principle of Fibre Optics,

propagation of light through fibre, Fibre Geometry, Fibre Types.

FIBRE OPTICS :

Optical Fibre is new medium, in which information (voice, Data or Video) istransmitted through a glass or plastic fibre, in the form of light, following the

transmission sequence give below :

(1) Information is encoded into electrical signals.

(2) Electrical signals are converted into light signals.

(3) Light travels down the fibre.

(4) A detector changes the light signals into electrical signals.

(5) Electrical signals are decoded into information.

ADVANTAGES OF FIBRE OPTICS :

Fibre Optics has the following advantages :

(I) Optical Fibres are non conductive (Dielectrics)

- Grounding and surge suppression not required.

- Cables can be all dielectric.

(II) Electromagnetic Immunity :

- Immune to electromagnetic interference (EMI)

- No radiated energy.

- Unauthorised tapping difficult.

(III) Large Bandwidth (> 5.0 GHz for 1 km length)

- Future upgradability.

- Maximum utilization of cable right of way.

- One time cable installation costs.

(IV) Low Loss (5 dB/km to < 0.25 dB/km typical)

- Loss is low and same at all operating speeds within the fibre's

specified bandwidth long, unrepeated links (>70km is operation).

(v) Small, Light weight cables.

- Easy installation and Handling.

- Efficient use of space.

(vi) Available in Long lengths (> 12 kms)

- Less splice points.

(vii) Security

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- Extremely difficult to tap a fibre as it does not radiate energy that can

be received by a nearby antenna.

- Highly secure transmission medium.

(viii) Security - Being a dielectric

- It cannot cause fire.

- Does not carry electricity.

- Can be run through hazardous areas.

(ix) Universal medium

- Serve all communication needs.

- Non-obsolescence.

APPLICATION OF FIBRE OPTICS IN COMMUNICATIONS :

- Common carrier nationwide networks.

- Telephone Inter-office Trunk lines.

- Customer premise communication networks.

- Undersea cables.

- High EMI areas (Power lines, Rails, Roads).

- Factory communication/ Automation.

- Control systems.

- Expensive environments.

- High lightening areas.

- Military applications.

- Classified (secure) communications.

Transmission Sequence :

(1) Information is Encoded into Electrical Signals.

(2) Electrical Signals are Coverted into light Signals.

(3) Light Travels Down the Fiber.

(4) A Detector Changes the Light Signals into Electrical Signals.

(5) Electrical Signals are Decoded into Information.

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- Inexpensive light sources available.

- Repeater spacing increases along with operating speeds because

low loss fibres are used at high data rates.

Principle of Operation - Theory

Total Internal Reflection - The Reflection that Occurs when a Ligh Ray

Travelling in One Material Hits a Different Material and Reflects Back

into the Original Material without any Loss of Light.

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THEORY AND PRINCIPLE OF FIBRE OPTICS

Speed of light is actually the velocity of electromagnetic energy in vacuum

such as space. Light travels at slower velocities in other materials such as glass.

Light travelling from one material to another changes speed, which results in light

changing its direction of travel. This deflection of light is called Refraction.

The amount that a ray of light passing from a lower refractive index to a

higher one is bent towards the normal. But light going from a higher index to a

lower one refracting away from the normal, as shown in the figures.

As the angle of incidence increases, the angle of refraction approaches 90o

to the normal. The angle of incidence that yields an angle of refraction of 90o is the

critical angle. If the angle of incidence increases amore than the critical angle, the

light is totally reflected back into the first material so that it does not enter the

second material. The angle of incidence and reflection are equal and it is called

Total Internal Reflection.

By Snell's law, n1 sin 1 = n2 sing 2

The critical angle of incidence c where 2 = 90o

Is c = arc sing (n2 / n1)

At angle greater than c the light is reflected, Because reflected light

means that n1 and n2 are equal (since they are in the same material), 1 and 2

are also equal. The angle of incidence and reflection are equal. These simple

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principles of refraction and reflection form the basis of light propagation through an

optical fibre.

PROPAGATION OF LIGHT THROUGH FIBRE.

The optical fibre has two concentric layers called the core and the cladding.

The inner core is the light carrying part. The surrounding cladding provides the

difference refractive index that allows total internal reflection of light through the

core. The index of the cladding is less than 1%, lower than that of the core.

Typical values for example are a core refractive index of 1.47 and a cladding index

of 1.46. Fibre manufacturers control this difference to obtain desired optical fibre

characteristics.

Most fibres have an additional coating around the cladding. This buffer

coating is a shock absorber and has no optical properties affecting the

propagation of light within the fibre.

Figure shows the idea of light travelling through a fibre. Light injected into

the fibre and striking core to cladding interface at grater than the critical angle,

reflects back into core, since the angle of incidence and reflection are equal, the

reflected light will again be reflected. The light will continue zigzagging down the

length of the fibre.

BRBRAITT, Jabalpur6

1

Angle of incidence

n1

n2

2

n1

n2

1

2

n1

n2

1 2

Angle ofreflectio

Light is bent awayfrom normal

Light does not entersecond material


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Light striking the interface at less than the critical angle passes into the

cladding, where it is lost over distance. The cladding is usually inefficient as a light

carrier, and light in the cladding becomes attenuated fairly. Propagation of light

through fibre is governed by the indices of the core and cladding by Snell's law.

Such total internal reflection forms the basis of light propagation through a

optical fibre. This analysis consider only meridional rays- those that pass through

the fibre axis each time, they are reflected. Other rays called Skew rays travel

down the fibre without passing through the axis. The path of a skew ray is typically

helical wrapping around and around the central axis. Fortunately skew rays are

ignored in most fibre optics analysis.

The specific characteristics of light propagation through a fibre depends on

many factors, including

- The size of the fibre.

- The composition of the fibre.

- The light injected into the fibre.

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FIBRE GEOMETRY

An Optical fibre consists of a core of optically transparent material usually

silica or borosilicate glass surrounded by a cladding of the same material but a

slightly lower refractive index.

Fibre themselves have exceedingly small diameters. Figure shows cross

section of the core and cladding diameters of commonly used fibres. The

diameters of the core and cladding are as follows.

Core ( m) Cladding ( m)

8 125

50 125

62.5 125

100 140

BRBRAITT, Jabalpur8

Jacket

Cladding

Core

Cladding

Angle ofreflection

Angle ofincidence

Light at less thancritical angle isabsorbed in jacket

Jacket

Light is propagated by

total internal reflection

Jacket

Cladding

Core

(n2)

(n2)

Fig. Total Internal Reflection in an optical Fibre


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Fibre sizes are usually expressed by first giving the core size followed by

the cladding size. Thus 50/125 means a core diameter of 50m and a cladding

diameter of 125m.

FIBRE TYPES

The refractive Index profile describes the relation between the indices of

the core and cladding. Two main relationship exists :

(I) Step Index

(II) Graded Index

The step index fibre has a core with uniform index throughout. The profile

shows a sharp step at the junction of the core and cladding. In contrast, the

graded index has a non-uniform core. The Index is highest at the center and

gradually decreases until it matches with that of the cladding. There is no sharp

break in indices between the core and the cladding.

By this classification there are three types of fibres :

(I) Multimode Step Index fibre (Step Index fibre)

(II) Multimode graded Index fibre (Graded Index fibre)

(III) Single- Mode Step Index fibre (Single Mode Fibre)

(1) STEP INDEX MULTIMODE FIBRE

This fibre is called "Step Index" because the refractive index changes

abruptly from cladding to core. The cladding has a refractive index somewhat

lower than the refractive index of the core glass. As a result, all rays within a

certain angle will be totally reflected at the core-cladding boundary. Rays striking

the boundary at angles grater than the critical angle will be partially reflected and

BRBRAITT, Jabalpur9

125 8 125 50 125 62.5 125 100

Core Cladding

Typical Core and Cladding Diameters


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partially transmitted out through the boundary. After many such bounces the

energy in these rays will be lost from the fibre.

The paths along which the rays (modes) of this step index fibre travel differ,

depending on their angles relative to the axis. As a result, the different modes in a

pulse will arrive at the far end of the fibre at different times, resulting in pulse

spreading which limits the bit-rate of a digital signal which can be transmitted.

The maximum number of modes (N) depends on the core diameter (d),

wavelength and numerical aperture (NA)

( x d x N A

N= 0.5 x (----------------------) 2

( )

This types of fibre results in considerable model dispersion, which results

the fibre's band width.

(2) GRADED INDEX MULTI-MODE FIBRE

This fibre is called graded index because there are many changes in the

refractive index with larger values towards the center. As light travels faster in a

lower index of refraction. So, the farther the light is from the center axis, the grater

is its speed. Each layer of the core refracts the light. Instead of being sharply

reflected as it is in a step index fibre, the light is now bent or continuously refracted

in an almost sinusoidal pattern. Those rays that follow the longest path by

travelling near the outside of the core, have a faster average velocity. The light

travelling near the center of the core, has the slowest average velocity.

As a result all rays tend to reach the end of the fibre at the same time. That

causes the end travel time of different rays to be nearly equal, even though they

travel different paths.

The graded index reduces model dispersing to 1ns/km or less.

Graded index fibres have core diameter of 50, 62.5 or 85 m and a

cladding diameter of 125 m. The fibre is used in applications requiring a wide

bandwidth a low model dispersion. The number of modes in the fibre is about half

that of step index fibre having the same diameter & NA.

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dxNAN= 0.25 x ( ---------------- )2

()

(3) SINGLE MODE FIBRE.

Another way to reduce model dispersion is to reduce the core's diameter,

until the fibre only propagates one mode efficiently. The single mode fibre has an

exceedingly small core diameter of only 5 to 10 m. Standard cladding diameter


High order

Mode

DispersionRefractive

Index Profile

Low Order ModeMulti mode Step Index

Input

Pulse

Output

Pulse

n1

n2

Single Mode Step Index

n1n2

Dispersion

Multi mode Graded Index

n1

n2


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is 125 m. Since this fibre carries only one mode, model dispersion does not

exists. Single mode fibres easily have a potential bandwidth of 50to 100GHz-km.

The core diameter is so small that the splicing technique and measuring

technique are more difficult. High sources must have very narrow spectral width

and they must be very small and bright in order to permit efficient coupling into the

very small core dia of these fibres.

One advantage of single mode fibre is that once they are installed, the

system's capacity can be increased as newer, higher capacity transmission

system becomes available. This capability saves the high cost of installing a new

transmission medium to obtain increased performance and allows cost effective

increases from low capacity system to higher capacity system.

As the wavelength is increased the fibre carries fewer and fewer modes

until only one remains. Single mode operation begins when the wavelength

approaches the core diameter. At 1300 nm, the fibre permits only one mode, it

becomes a single mode fibre.

As optical energy in a single mode fibre travels in the cladding as well as in

the core, therefore the cladding must be a more efficient carrier of energy. In a

multimode fibre cladding modes are not desirable, a cladding with in efficient

transmission characteristic can be tolerated. The diameter of the light appearing at

the end of the single mode fibre is larger than the core diameter, because some of

the optical energy of the mode travels in the cladding. Mode field diameter is the

term used to define this diameter of optical energy.

OPTICAL FIBRE PARAMETERSOptical fibre systems have the following parameters.

(I) Wavelength.

(II) Frequency.

(III) Window.

(IV) Attenuation.

(V) Dispersion.

(VI) Bandwidth.



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WAVELENGTH

It is a characterstic of light that is emitted from the light source and is

measures in nanometers (nm). In the visible spectrum, wavelength can be

described as the colour of the light.For example, Red Light has longer wavelength than Blue Light, Typical

wavelength for fibre use are 850nm, 1300nm and 1550nm all of which are

invisible.

FREQUENCY

It is number of pulse per second emitted from a light source. Frequency is

measured in units of hertz (Hz). In terms of optical pulse 1Hz = 1 pulse/ sec.

WINDOWA narrow window is defined as the range of wavelengths at which a fibre

best operates. Typical windows are given below :

Window Operational Wavelength

800nm - 900nm 850nm

1250nm - 1350nm 1300nm

1500nm - 1600nm 1550nm



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Gamma rays

Rontgen rays

U.V. rays

Visible Light

Infra Red

Thermal Rays

U.H.F.

M.F.

L.F.

RadioFrequencies

1

0-12

10-8

10-6

10-4

100

1

04

106

102

10-2

10-10

1Mm

1Km

1m

1mm

1m

1nm

1pm

WAVELENGTHINNM


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ATTENUATION

Attenuation is defined as the loss of optical power over a set distance, a

fibre with lower attenuation will allow more power to reach a receiver than fibre

with higher attenuation.

Attenuation may be categorized as intrinsic or extrinsic.

INTRINSIC ATTENUATION

It is loss due to inherent or within the fibre. Intrinsic attenuation may occur

as

(I) Absorption - Natural Impurities in the glass absorb light energy.

(II) Scattering - Light rays travelling in the core reflect from smallimperfections into a new pathway that may be lost through the

cladding.

(1) Absorption - Natural Impurities in the Glass Absorb Light Energy.

Or

(2) Scattering - Light Rays Travelling in the Core Reflect from small

Imperfections into a New Pathway that may be Lost through the cladding.


Light

Ray

Light

Ray

Light is lost


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EXTRINSIC ATTENUATION

It is loss due to external sources. Extrinsic attenuation may occur as

(I) Macrobending - The fibre is sharply bent so that the light

travelling down the fibre cannot make the turn & is lost in the

cladding.

(II) Microbending - Microbending or small bends in the fibre caused by

crushing contraction etc. These bends may not be visible with the

naked eye.

Attenuation is measured in decibels (dB). A dB represents the comparison

between the transmitted and received power in a system.

DISPERSION

It is defined as the spreading of light pulse as it travels down the fibre.

ecause of the spreading effect, pulses tend to overlap, making them unreadable

by the receiver.


Micro bend

Micro bend

Fig. Loss and Bends

Micro bend


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BANDWIDTH

It is defined as the amount of information that a system can carry such that

each pulse of light is distinguishable by the receiver.

System bandwidth is measured in MHz or GHz. In general, when we say

that a system has bandwidth of 20 MHz, means that 20 million pulses of light per

second will travel down the fibre and each will be distinguishable by the receiver.

NUMBERICAL APERTURE

Numerical aperture (NA) is the "light - gathering ability" of a fibre. Light

injected into the fibre at angles greater than the critical angle will be propagated.

The material NA relates to the refractive indices of the core and cladding.

NA = n12 - n2

2

where n1 and n2 are refractive indices of core and cladding respectively.

NA is unitless dimension. We can also define as the angles at which rays

will be propagated by the fibre. These angles form a cone called the acceptance

cone, which gives the maximum angle of light acceptance. The acceptance cone

is related to the NA

= arc sing (NA) or

NA = sin

where is the half angle of acceptance

The NA of a fibre is important because it gives an indication of how the

fibre accepts and propagates light. A fibre with a large NA accepts light well, a

fibre with a low NA requires highly directional light.

In general, fibres with a high bandwidth have a lower NA. They thus allow

fewer modes means less dispersion and hence greater bandwidth. A large NA

promotes more modal dispersion, since more paths for the rays are provided NA,

although it can be defined for a single mode fibre, is essentially meaningless as a

practical



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characteristic. NA in a multimode fibre is important to system performance

and to calculate anticipated performance.

Total Internal Reflection

(Summary)

* Light Ray A : Did not Enter Acceptance Cone - Lost

* Light Ray B : Entered Acceptance Cone - Transmitted through the Core

by Total Internal Reflection.

NA = 0.275 (For 62.5 m Core Fiber)



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DISPERSION

Dispersion is the spreading of light pulse as its travels down the length of

an optical fibre. Dispersion limits the bandwidth or information carrying capacity of

a fibre. The bit-rates must be low enough to ensure that pulses are farther apart

and therefore the greater dispersion can be tolerated.

There are three main types of dispersion in a fibre -

(I) Modal Dispersion

(II) Material dispersion

(III) Waveguide dispersion

MODAL DISPERSIONModal dispersion occurs only in Multimode fibres. It arises because rays

follow different paths through the fibre and consequently arrive at the other end of

the fibre at different times. Mode is a mathematical and physical concept

describing the propagation of electromagnetic waves through media. In case of

fibre, a mode is simply a path that a light ray can follow in travelling down a fibre.

The number of modes supported by a fibre ranges from 1 to over 100,000. Thus a

fibre provides a path of travels for one or thousands of light rays depending on itssize and properties. Since light reflects at different angles for different paths (or

modes), the path lengths of different modes are different. Thus different rays take

a shorter or longer time to travel the length of the fibre. The ray that goes straight

down the center of the core without reflecting, arrives at the other end first, other

rays arrive later. Thus light entering the fibre at the same time exist the other end

at different times. The light has spread out in time.

The spreading of light is called modal dispersion. Modal dispersion is that

type of dispersion that results from the varying modal path lengths in the fibre.

Typical modal dispersion figures for the step index fibre are 15 to 30 ns/ km. This

means that for light entering a fibre at the same time, the ray following the longest

path will arrive at the other end of a 1 km long fibre 15 to 30 ns after the ray,

following the shortest path. Fifteen to 30 billionths of a second may not seem like

much, but dispersion is the main limiting factor on a fibre's bandwidth. Pulse

spreading results in a pulse overlapping adjacent pulses as shown in figure.

Eventually, the pulses will merge so that one pulse cannot be distinguished from



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another. The information contained in the pulse is lost Reducing dispersion

increases fibre bandwidth.

Model dispersion can be reduced in three ways :



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(I) Use a smaller core diameter, which allows fewer modes.

(II) Use a graded -index fibre so that light rays that allow longer paths

also travel at a faster velocity and thereby arrive at the other end of

the fibre at nearly the same time as rays that follow shorter paths.

(III) Use a single-mode fibre, which permits no modal dispersion.

MATERIAL DISPERSION

Different wavelengths (colours) also travel at different velocities through a

fibre, even in the same mode, as

n = c/v

where n is index of refraction, c is the speed of light in vacuum and v is the speed

of the same wavelength in the material. The value of V in the equation changes for

each wavelength, Thus Index of refraction changes according to the wavelength.

Dispersion from this phenomenon is called material dispersion, since it arises from

material properties of the fibre.

Each wave changes speed differently, each is refracted differently. White

light entering the prism contains all colours. The prism refracts the light and its

changes speed as it enters the prism. Red light deviates the least and travels the

fastest. The violet light deviates the most and travels the slowest.

The amount of material dispersion depends on two factors :



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(I) The range of light wavelengths injected into the fibre. A source does

not normally emit a single wavelength, it emits several. This range of

wavelengths, expressed in nanometer is the spectral width of the

source. An LED has a much higher spectral width than a LASER -

about 35 nm for a LED and 2 to 3 nm for a LASER.

(II) The centre operating wavelength of the sources

Around 850nm, longer (reddish) wavelengths travel faster than the shorter

(Bluish) ones. At 1550nm however the situation is reversed. The shorter

wavelengths travel faster than the longer ones. At some point, the cross over must

occur where the bluish and reddish wavelengths travel at the same speed. This

crossover occurs around 1300nm, the zero-dispersion wavelength. At

wavelengths below 1300nm, dispersion is negative. So wavelengths travel or

arrive later. Above 1300 nm, the wavelengths lead or arrive faster.

This dispersion is expressed in Pico seconds per kilometer per nanometer

of source spectral width (ps/km/nm).

WAVEGUIDE DISPERSION :

Waveguide dispersion, most significant in a single- mode fibre, occurs

because optical energy travels in both the core and cladding, which have slightly

different refractive indices. The energy travels at slightly different velocities in the

core and cladding because of the slightly different refractive indices of the

materials. Altering the internal structures of the fibre, allows waveguide dispersion

to be substantially changed, thus changing the specified overall dispersion of the

fibre.

BANDWIDTHAND DISPERSION :

A bandwidth of 400 MHz -km means that a 400 MHz-signal can be

transmitted for 1 km. It means that the product of frequency and the length must

be 400 or less. We can send a lower frequency for a longer distance, i.e. 200 MHz

for 2 km or 100 MHz for 4 km.

Multimode fibres are specified by the bandwidth-length product or simply

bandwidth.

Single mode fibres on the other hand are specified by dispersion,

expressed in ps/km/nm. In other words for any given single mode fibre dispersion



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is most affected by the source's spectral width. The wider the source spectral

width, the greater the dispersion.

Conversion of dispersion to bandwidth can be approximated roughly by the

following equation.

0.187BW = --------------------------

(Disp) (SW) (L)

Disp = Dispersion at the operating wavelength in seconds/ nm/ km.

SW = Spectral width of the source in nm.

L = Fibre length in km.

So the spectral width of the source has a significant effect on the

performance of a single mode fibre.

OPTICAL WINDOWS :

Attenuation of fibre for optical power varies with the wavelengths of light.

Windows are low-loss regions, where fibre carry light with little attenuation. The

first generation of optical fibre operated in the first window around 820 to 850 nm.

The second window is the zero-dispersion region of 1300 nm and the third window

is the 1550 nm region.

High loss regions, where attenuation is very high occur at 730, 950, 1250

and 1380 nm. One wishes to avoid operating in these regions. Evaluation of

losses in a fibre must be done with respect to the transmitted wavelength.

Figure shows a typical attenuation curve for a low loss multimode fibre.

Making the best use of the low loss properties of the fibre requires that the

sources emit light in the low loss region of the fibre. Plastic fibres are best

operated in the visible light area around 650 nm. One important feature of

attenuation in an optical fibre is that the constant at all modulation frequencies

within the bandwidth. Attenuation in a fibre has two main causes.

(I) Scattering

(II) Absorption



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We can obtain losses less than 2.5 dB/km in the first window at 850 nm.

Graded index fibres in the second window with loss below 1 dB/km and in the thrid

window below 0.5 dB/km are obtained. Even lower losses are regarded as



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feasible for monomode fibres in all the three windows. Typically minimum loss in

the three windows for the multimode fibre is 2.5 dB/km, 0.44 dB, km and 0.22

dB/km respectively. The corresponding figures for a monomode fibre are 1.9

dB/km, 0.32 dB/km and 0.048 dB/km.

1. OPTICAL TRANSMITTERS

In optical line systems we need light sources in the infrared spectrum part. The

wavelengths used are in one of the following windows of optical fibres, i.e. 850 nm,

1300 nm or 1550 nm.

The features of an ideal source for fibre optic communication systems are as

follows

High brightness

Small emission area (< Fibre Core).

small emission cone angle (< Fibre NA).

Fast response to electrical modulation.

Long life

Emission wavelength compatible with fibre.

Only 2 semiconductor devices approach these ideals :

(1) Light emitting diode (L.E.D.).

(2) Semiconductor LASER (Light amplification by stimulated emission of

radiation).

1.1LED'S (Light Emitting Diode)

Led's are generally manufactured from crystalline materials such as

galliumarsenide.

The LED has a relatively wide lobe of radiation.

The transfer characteristic of the LED is linear.

The LED has a bandwidth of approximately 10.000 GHz (= 20

nm).

The LED represents in most respects a good compromise between

performance and cost.



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Light emitting diodes are composed of a PN junction with "doped"

semiconductor layers. Injected electrons will recombine with "holes" in the Player

where this phenomenon results in the emission of photons.

There are two types of LEDs :

(1) Surface emitting LED's

(2) Edge emitting LED's.

The edge emitting LED has more or less a similar composition as a LASER. The

beam is relatively directed and thus the efficiency of the coupled light into the fibre is

higher than the surface LED. The light generated by LED's is incoherent. The photons

are neither in phase with each other nor do they possess the same frequency. This fact

limits the application possibilities of LEDs.

In optical line systems not using monomode fibres, the transmitted pulses from

LED sources suffer from pulse broadening, caused by chromatic dispersion. The

bandwidth of LEDlight pulses depends on the DC current supply.The optical power of a

LED can be controlled by an external current producing the injection electrons. The

relation between optical power P and controlling current is given in figure.

Fig. : Power Characteristic of a LED

The linearity of LEDs is fairly good. There is no threshold. The temperature

range is rather wide and the curves show little influence of temperature variations (0 ......

80o).



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Introduction to OFC

The modulation frequencies are limited to about 100 MHz due to the delay in

recombination time of the carriers in the active area.

The lifetime of LEDs is rather long = 106

working hours (110 years), but their

output power is low as compared to LASER.

1.2 LASERS

In LASERS, spontaneous photon emission is generated between 2 parallel

reflecting surfaces. Since the dimensions of this device are very small, the beam is not

so well collimated as in a HENE gas LASER.

However, the external beam is quite narrow and is generated within such a small

area that very high power levels can be launched into even the smallest optical fibre.

The LASER has a bandwidth of approximately 1000 GHz ( = 2 nM). The

transfer characteristic of a LASER is non linear (see figure), so a LASER needs a more

complicated stabilizing of the working point, which changes with temperature and aging,

etc.

LASERS working in the windows 850 nM and 1.3 M are usually made from Al

Ga AS material. LASER for 1.55 M are made of quaternary compound in Ga AS P.

A LASER has a threshold phenomenon in its light/current response. For that

reason a bias current must be supplied to make the LASER work, in the linear slope

region. Also, feedback must be adopted to keep both output power and variations in

temperature between certain limits.

In a LASER's life time the optical power and the light/current characteristic

shows a gradual degradation of the performance (see figure).

In practical systems the bias current is controlled automatically to obtain an

optical output power. By observing the value of the bias current and comparing this

value with a chosen alarm threshold value, a laser can be replaced in. Whenever there

is an increase in the biasing current by more than 50% of the initial value, the laser is

said to have lived its life. An alarm is initiated to indicate end of laser's lifetime in

system.

1.2.1 LASER SAFETY

The high degree of collimation and brightness of some LASER beams makes

them a serious hazard to the human eye and, therefore, suitable safety precautions

have to be taken in their operation.



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Semiconductor LASERS for optical transmission generally have a lower

brightness and poor collimation, can also be a hazard if viewed under particularly

unfavourable conditions.

With certain exceptions the radiation from LEDs is quite safe to the naked eye.

Optical line equipments have a laser safety switch off function, to switch off the LASER

during break in fibre somewhere on route.

However, no energised optical source or illuminated fibre should be viewed

through a naked eye or microscope.

1.3 TYPICAL CHARACTERISTICS OF LIGHT SOURCE

PROPERTY LED LASERSINGLELASER

Spectral width (nm) 200100 15 02

Rise time (ns) 2250 0.11 0.11

Modulation B.W (MHz) < 300 < 2000 ~ 2000

Coupling efficiency Very low Moderate Moderate

Compatible fibre mode M.M. (Si and GI) M.M. GI & SM Single

Temperature sensitivity Low High High

Circuit Complexity Simple Complex Complex

Lifetime (hours)10

5

10

4

10

5

10

4

10

5

Costs Low High Highest

Path length Moderate Long Very long

Data rate Moderate High Very high



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Introduction to OFC

Fig. : Laser Light/Current Characteristic

Fig. : Laser Transmitter

1.4 LASING SPECTRUM

When the drive current is near threshold, lasers produce multimode spectra. As

the current increases, total line width decreases and number of longitudinal modes

decreases. At sufficiently high currents, the spectrum contains just one mode. The light

from LASER beam is confined to a narrow angular region.



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Introduction to OFC

Spectral width : laser produce range of wavelengths called spectral width, usually

quoted as FWHM (Full width at half maximum power ) or 3 db point. Wider the spectral

width more is the dispersion.

Line width : semiconductor laser produces a series of lines at a number of discrete

wavelengths. Width of these lines is called line width.

1.5 APPLICATIONS OF LED AND LASER

For low data rates and for shorter routes, LEDs are a good choice for the

transmitter because the driving circuitry needed is very simple. On the other hand, for

high speed data and long haul systems, LASERS are selected for transmitters. The

driving circuitry is quite complex, but LASER sources have the following advantages.

Larger output power.

Narrow spectral width

Suitable emission geometry.



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Introduction to OFC

The disadvantage with laser is that they require large driving and modulating

currents. Their lifetime is shorter as compared to LEDs. The output power varies with

time and temperature. This necessitates the use of feed back and regulation circuitries

for maintaining output power constant.

Laser Types:

1. Fabry-Perot (FP) :- Generates many wavelengths (MLM = multi

ongitudinal mode ). Linewidth 2 nm. Spectral width 5-8 nm used in PDH.

2. Distributed Feed Back (DFB):- All Other wavelengths are

reduced (more than 30 db except one (SLM = single longitudinal mode).

Linewidth is 5x10-6 nm. Spectral width 0.4 nm. They are used in SDH.

3. Distributed Bragg Reflector (DBR) :- Mostly used as Tunable

Laser for WDM working.

CHIRP: Chirp is a gradual shift in frequency as shown in figure.

When input power is applied , there is an abrupt change in carriers (electron-

holes) flux density in the cavity resulting change in Refractive Index of cavity, causing

change in central wavelength . Central wavelength shifts towards longer wavelength.

DFB lasers have less chirp problem than FP lasers. Modulating signal also spreads the

spectral width of laser by twice the frequency of modulating signal. Laser chirp can be

reduced by using external modulator.



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Introduction to OFC

2.0 OPTICAL DETECTORS

A photodetector converts light into an electric current. In a fibre optic

communication system, detectors received the transmitted pulses and convert them,

with as little loss as possible into electronic pulses that can be used by a telephone,

computer, other terminal at the receiving end or at an intermediate repeater. In this rolethe detector must have -

High efficiency.

Fast response.

Low noise.

Small size and light weight.

Long life and reliability.

Low cost.

Important detector properties:

1. RESPONSIVITY It is the ratio of the output current of the detector to the optic

input power. Responsivity p = i/P i.e. A/W

2. SPECTRAL RESPONSE This refers to the curve of the detector responsivity as

a function of wavelength. Different detectors must be used in different optical

windows.

3. RISE TIME the rise time (Tr) is the time for the detector output current to

change from 10 % to 90 % of its final value when the optic input power variation is

a step. The 3 db modulation bandwidth of the detector is :

F3 db = o.35/Tr

4. QUANTUM EFFICIENCY Efficiency n is defined as the ratio of number of

emitted electron to the number of incident photons.

Two types of Photodiodes most nearly meet the above requirements:

(1) Pin photodiode.

(2) Avalanche photodiode

2.1 PIN PHOTODIODES

Pin photodiode is relatively easy to fabricate, highly reliable, has low noise and is

compatible with low voltage amplifier circuits. In addition, it is sensitive over an

extremely large bandwidth because there is no gain mechanism. PIN photodiodes have

resistance intrinsic layer, sandwiched between P and N layers, the depletion layer

spread over intrinsic layer under influence of high field due to reverse bias. The device

operates under reverse bias, meaning that in the absence of light, only a small leakage



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Introduction to OFC

current will flow. Under reverse bias, a moderate electric field of about 105

volts per cm

exists in the vicinity of the junction, that depletes the region of all free carriers.

When a photon enters the depletion region, it is absorbed and generates an

electron and a hole, both of which are rapidly drawn to the opposite electrodes. Then

they are collected and appear as a current in the external circuit. Because there is no

gain mechanism in a PIN photodiode, the maximum efficiency of the device is unity and

the gain bandwidth product is equal to the bandwidth itself. The ultimate bandwidth of a

PIN photodiode is limited by the time, it takes to collect the charge. This time is inversely

proportional to the width of the depletion region and directly proportional to the velocity

of the charge carriers in the region of high electric field. PIN photodiodes have lower

capacitance, high quantum efficiency and high speed of response.

2.2AVALANCHE PHOTODIODES (APD)

APDs are operated at reverse voltage high enough so that when the carriers are

separated in the electric field, they collide with the atoms in the semiconductor crystal

lattice. The collisions ionize the lattice atoms, generating a second electronhole pair.

Each of the secondary carriers, alongwith the initial carrier, also collides with the lattice

and large numbers of carriers are eventually collected at the electrodes. This gives the

device internal gain. The principal source of noise in APDs is the avalanche process

that gives the APD gain. An existing transmission system can often be upgraded simply

by a change in optical detectors (receivers), i.e. from PIN to APD. The detectors, in

general, must have a good sensitivity, which is limited by noise and is influenced by :

(i) Thermal noise from the input resistance to the pre amplifier.

(ii) Short noise from the photo diode.

(iii) Noise from the first stage of the amplifier.

(iv) Dark current noise.

The actual output of a detector depends on the quantum efficiency. For the

same quantum efficiency, an APD has better sensitivity than PIN diode. Silicon

detectors are useful only up to 1.1 M. For longer wavelength, germanium baseddevices are used. The GeAPD is useful in the 1.01.5 M range. In GaAS PIN diodes

in conjunction with a low noise FET amplifier offer sensitivity of the order of 65 dBM. In

contrast to APDs, PIN diodes offer inferior sensitivities.They are mainly useful for low

bitrate and of short route length.

2.3 COMPARISON OF DETECTORS (Optical)

Characteristic PIN(Si) APD (Si) APD (Ge)



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Introduction to OFC

Detection range (m) 0.41.1 0.41.1 0.41.6

Wavelength of peak sensitivity 0.80.9 0.50.9 1.11.4

Quantum efficiency (%)6.08.0

0.82 (m)

4.08.0

0.82 (m)

4.05.0

1.15 (m)

Dark current (nA) 0.15.0 0.0215 10200

Rise time (ns) 0.53.0 0.10.5 0.10.2

Detecting area 46 115 5070

2.4 MATERIAL FOR SOURCES AND DETECTORS

High capacity communication system operate at 1.3 and 1.55 micrometers. Suchsystems employ Indium gallium arsenide (In Ga As) or germanium as the detecting

semiconductor.

2.5BIT ERROR RATE (BER)

In a digital optical communication system, the sensitivity can be determined by

calculating how often a transmitted digital 1 will be mistakenly identified by the receiver

as a zero or vice versa. This is known as Bit Error Rate. For most signal applications a

bit error rate of 109

, i.e., one mistake per 109

transmitted bit is sufficiently low. The

sensitivity of all optical detectors decreases, however, with increasing bit rate, becausethe total noise also depends increasingly on bandwidth a characteristic of white noise

source that is intrinsic to these detectors.

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