OPTICAL COMMUNICATION

By

Shubhanshi Gupta

UNIT 1

Communication may be defined as the transfer of information from one point to another. Within the communication system the

information transfer is frequently achieved by superimposing or modulating the information transfer on to an electromagnetic wave

which acts as a carrier for the information signal. This modulated signal is then transmitted to the required destination where it is

received and the original information signal is obtained by demodulation.

The use of visible light to carry the information is called optical communication and the light travels through a optical fiber cable.

Optical Fiber Communication:

Freq14

to 1015

Hz

1.1 Block Diagram of Optical Communication System: The optical fiber communication

system is similar in basic concepts, the block

diagram is shown:

The block diagram represents the information

source provides an electrical signal to a

transmitter comprising an electrical signal to a

transmitter comprising an electrical signal which

derives an optical source to give modulation of

light wave carrier. The optical sources (LED or

LASER) which provide the optical conversion

used to convey the light travels through fiber

cable in a particular manner. At the receiver end a

optical detector exists, it can be a PIN or APD

photodiode, it converts light energy to electrical

signal. Electrical receiver receives the signal and

converts it into a message format. This is the

working of a optical fiber communication system.

1.1.1 Advantages: Optical fiber communication offers a number of advantages over other communication systems because it has

very low attenuation:

a) Enormous potential bandwidth: The optical fiber communication offers frequency in the range of 1013 to 1016 Hz which yields a far greater potential transmission bandwidth. At present, the bandwidth available to fiber system is not fully utilized but

modulation over three hundred kilometer without repeaters is possible.

b) Small size and weight: Fiber cables have very small diameter, just like a hair, rather than when it cover with jackets for protection still they are very light and small diameter.

c) Electrical Isolation: Optical fibers are fabricated from glass or sometimes a plastic polymer, they are electrical insulators and they do not exhibit earth loop.

d) Immunity to interface and cross talk: Optical fiber form a dielectric waveguide and are therefore free from electromagnetic interference (EMI), radio frequency interference (RFI) or switching transients electromagnetic pulses.

e) Signal Security: The light from optical fiber does not radiate significantly and therefore they provide a high degree of signal security.

f) Low transmission loss: The attenuation in optical fiber cable is very low (around 0.2 dB/km) as compare to other communication channels.

g) Ruggedness and flexibility: Optical fibers are manufactured with very high tensile strength. The fiber may bent to quite small radii or twisted without damage.

h) System reliability and ease of maintenance: It reduces the requirement of intermediate repeaters or live amplifiers to boost the transmitted signal strength. The reliability is high due to predicted life time of 20 years to 30 years.

1.1.2 Disadvantages:

Introduction: Block diagram of optical fiber communication system, Advantages of optical fiber communication.

Optical fiber waveguides: structure of optical wave guide, light propagation in optical fiber using ray theory, acceptance angle,

numerical aperture, skew rays, wave theory for optical propagation, modes in a planar and cylindrical guide, mode volume, single

mode fibers, cutoff wavelength, mode field diameter, effective refractive index and group and mode delay factor for single mode

fiber.

a) Optical cables can be handle with a skilled hand, the perfect joining of cable is most important otherwise signal lost in between the cable.

b) The detection of faulty area is very hard because these cable situated under Mud. c) The whole establishment of optical cables is very expensive. d) Bending loss occurs, so cable must be properly aligned.

1.2 Optical Fiber Waveguide: 1 surrounded by

2. The cladding supports the waveguide structure and reducing the

radiation loss into the surrounding air.

Typically the value of light in a vacuum of refractive indexes of

1.2.1 Ray theory transmission: 1.2.1.1 Reflection and refraction: When a light ray encounters a boundary

called Reflection, or it will bent towards second medium called Refraction.

Refraction affects the refractive index of the medium.

The relationship at the interfac

1 2 sin r

1 2 cos r (1)

1.2.1.2 Total Internal Reflection: 1 2, the angle of refraction is

always greater than the angle of incidence. Thus when angle of refraction is 900 and refracted

emerges parallel to axis, the angle is called critical angle. The critical angle is given by,

At angles of incidence greater than the critical angle the light is reflected back into the originating dielectric medium that is called total

internal Reflection. This is the mechanism by which light at a sufficient shallow angle (less than 900 - c) may be considered to

propagate down an optical fiber with low loss.

1.2.1.3 Numerical Aperture and Acceptance Angle: As per the diagram a meridional ray A enters at the critical angle c within

the fiber at the core cladding interface. The a to the fiber axis and refracted at the air core

sin c =

a will be transmitted to the core

cladding interface at an angle less than c a is called

(NA) is relationship between acceptance angle and refractive indexes. Fig shows a light ray incident on the

fiber core at an angle is which is less than the acceptance angle for the fiber. The ray enters to the fiber from medium

(air) of refractive index number.

0 sin 1 sin (1)

Consider the right angle triangle, then

= (2)

where is greater than the critical angle at core cladding interface. Hence,

0 sin 1 cos (3)

0 sin 1 (1-sin2 )

1/2

When the limiting case for TIR is considered, becomes equal to critical angle,

so

sin c = 2/ 1 so the limiting case will be,

0 sin 12

22)

1/2 (4)

This equation serves as a basic for the definition of Numerical Apertrure, so

The NA may also be given in the terms of relative refractive index difference between core and cladding.

=

Hence,

1.2.1.4 Skew Rays: Skew rays are the rays which does not follow the fiber axis. These rays are not easy to visualize, only the

direction can be predicted in helical path of direction change of 2

the two dimension and the radii of the fiber core at the point of reflection.

When the light input to the fiber is non uniform, rays will therefore tend

to have a smoothing effect on the distribution of light as it is transmitted,

giving more information output.

NA in case of skew rays,

1.2.2 Optical Fiber Modes:

Modes: In a planar guide, TE(E2 = 0) and TM(H2 = 0) modes are

obtained within the dielectric cylinder. Thus two integers, l and m are

necessary in order to specify the modes, the single integer (m) required for the planar guide, for

cylindrical waveguide we refer TE lm and TM ln modes.

Modes in Fiber: There are two fiber modes exists. First is

a) Single mode Fiber b) Multi mode Fiber

The optical fiber is a dielectric waveguide that operates at optical frequencies. The fiber waveguide is normally cylindrical in form.

Single mode fiber sustains only one mode of propagation, whereas multimode fibers contain many hundreds of modes. The diameter

of core of SMF is comparatively very small from MMF.

A disadvantage of MMf is that they suffer from intermodal dispersion but it can be reduced.

0 12

22)

1/2

1 1/2

0 12

22)

1/2

SINGLE MODE FIBER MULTI MODE FIBER

1.2.3 Mode Theory for Circular Waveguide: In optical fibers, the core cladding boundary conditions lead to a coupling between

the electric and magnetic field components. This gives rise to hybrid modes, which means optical waveguide analysis is more complex

than metallic waveguide analysis. Fibers are constructed so that the difference in the core and cladding indexes of refraction is very

small, i.e

The field components are called linearly polarized (LP) modes and labeled as LP jm, where j & m are integers designating mode

solutions.

Figure shows a electric field distribution for several of the lower order guided modes in a symmetrical slab waveguide.

The core of this waveguide is a dielectric slab of index 1 that is sandwiched between two dielectric layers which have refractive

2 1. Fig shows the field patterns of several of the lower order transverse electric (TE) modes.

The order of a mode is equal to the ray congruence or same corresponding to this mode makes with the plane of the waveguide.

The plot shows that the electric fields of the guided modes are not completely confined to the central dielectric slab.

The field varies harm 1 and decay exponentially outside of the region. For low

order modes the fields are tightly concentrated near the center of the slab, will little penetration into the cladding region. On the other

hand, for higher order modes the fields are distributed more towards the edges of the guide and penetrate faster into the cladding

region.

Mode Coupling: As the core and cladding modes propagates along the fiber; mode coupling occurs between the cladding and higher

order core modes. This coupling occurs because the electric fields of the guided core modes are not completely confined to the core

but expend partially into the cladding

.

No of modes in a fiber with cut off conditions:

SMF, V= 12

22)

1/2 =

MMF, M = )2 1

2 2

2) =

Power: P = total optical power

Pclad = avg optical power residing in cladding

1.2.4 Step Index Fiber and Graded Index Fiber: 1.2.4.1 Step Index Fiber: The optical fiber with a core of constant refractive index 1 and a cladding of a slightly lower refractive

2 is known as step index fiber. This is because the refractive index profile for this type of fiber makes a step change at the core

cladding interface.

The refractive index profile,

=

(a) (b) The figure shows a multimode step index fiber (a) and a single mode index fiber (b). The core diameter of SMF is around 2 to 10 m.

The modes in step index fiber is,

1.2.4.2 Graded Index Fiber: Graded index fiber do not have a constant refractive index in the core but a decreasing core index

1 2 beyond the core radius a in the cladding.

The refractive index profile,

= profile parameter; when

= 1, triangular profile

= 2, parabolic profile

less inter modal dispersion than multimode step index fiber.

Number of modes for graded index fibers is:

Ms =

Ng

1.2.4.3 Single Mode Fibers: Single mode fibers have only relatively recently emerged as a viable optical communication medium

they have quickly become the dominant and most widely used fiber type within telecommunications. The advantages of single mode

fibers are:

1. They currently exhibit the greatest transmission bandwidths and lowest losses of the fiber transmission media. 2. They have superior transmission quality over other fiber types because absence of modal noise. 3. They offer a upgrade capability for future wide bandwidth services using faster Transmitter or receiver. 4. They are compatible with the developing integrated optics technology.

1.2.5 Cut off Wavelength:

(1)

where Vc

c

so dividing this equation with the reference equation, we get

(2)

1.2.6 Mode Field diameter and Spot Size: Many losses occur including jointing, micro bend, dispersion and width of radiation

pattern. Therefore, Mode field diameter is a parameter for characterizing single mode fiber properties which takes into account the

wavelength dependent field penetration into the field cladding.

1.2.7 Effective Refractive Index: The rate of change of phase of the fundamental LP01 mode propagating along a straight fiber is

01 01 by the factor

01 = 2

(1)

Sometimes it is defined by a phase index or normalized phase change coefficient, eff ,

eff = (2)

01 eff , where

The effective refractive index can be considered as an average over the refractive index of this medium.

1.2.8 Group Delay and Mode Delay factor: g for a light pulse propagating along a unit length of

fiber is the inverse of the group velocity, Vg, hence

(1)

The group index of a uniform plane wave propagation in a homogenous medium has been determined,

c = 1/2

c = =

01 =

01 =

g =

Nge = for single mode fiber (2)

so, (3)

The effective group index may be written in terms of effective refractive index,

(4)

1.2.9 Fiber Material and Fabrication Techniques: A optical fiber material, must satisfy three conditions:

a) It must be possible to make long, flexible fiber from the material. b) The material must be transparent at a particular optical wavelength in order for the fiber to guide light efficiently. c) Physically compatible materials that have slightly different refractive indexes for the core and cladding.. These requirements can be satisfied by

a) Glass b) Plastics c) Photonic Crystal Fibers

1.2.9.1 Glass Fiber: Glass is made by fusing mixtures of metal oxides, sulfides or solenoids. The resulting material is a randomly

connected molecular network rather than a well defined ordered structure as found in crystalline material.

When glass is heated up from room temperature, it remains a hard solid up to several hundred degrees. As the temperature increases

further, the glass gradually begins to soften until at very high temperature it becomes viscous liquid. An extended temperature range in

which the glass becomes fluid enough to free itself fairly quickly of gas bubbles.

The most common fiber material built by glass is silica (SiO2), which has refractive index of 1.458 at 850nm and slightly similar

refractive index materials are B2O3, GeO2 or P2O5 are added to silica.

E.g. 1) GeO2 - SiO2 core, SiO2 cladding

2) P2O5 - SiO2 core, SiO2 cladding

3) SiO2 core, B2O3 - SiO2 cladding

Active Glass Fiber: Some glass material (atomic no 57.71) resulting new optical and magnetic properties. These new properties allow

the material to perform amplification, attenuation and phase retardation on light passing through it. Doping can be carried out for

silica, tellurite and halide glasses.

1.2.9.2 Plastic Optical Fibers: For high speed services and high bandwidth, graded index polymer (plastics) optical fiber [OF]

designed. The core of these fibers is either polymethacrylate or a perfluronated polymer. These polymers are referred to as PMMA

POF and PFPOF. They offer greater optical signal attenuations than a glass fiber. They are tough and durable.

COMPARISON BETWEEN PMMA & PF POLYMER OPTICAL FIBER:

CHARACTERISTICS PMMA POF PF POF

Core Diameter 0.4 mm 0.125 0.30 mm

Cladding Diameter 1.0 mm 0.25 0.60 mm

Numerical Aperture 0.25 0.20

Attenuation 150 dB/km at 650 nm

b) Photonic Band gap fiber: The structure of index guiding PCF and photonic band gap fiber are same. The fiber has a hollow core that is surrounded by a cladding region which contains air holes running along the fiber length.

c) But the functional principle is analogous to the role of a periodic crystalline lattice in a semiconductor, which blocks electrons from occupying a band gap region. The hollow core acts as a defect in the photonic band gap structure, which

creates a region in which the light can propagate.

1.2.10 Fiber Fabrication: The basic techniques for fabrication of all glass optical waveguide are:

1.2.10.1 Outside Vapor Phase Oxidation: In this method, a layer of SiO2

rotating graphite or ceramic mandrel. The glass soot adheres to this bait rod and layer by layer porous glass preform is built up. By

property controlling the constituents of the metal halide vapor stream during the deposition process, the glass compositions and

dimensions desired for the core and cladding can be incorporated into the perform. When the deposition process is completed, the

mandrel is removed and the porous tube is then vitrified in a dry atmosphere at a high temperature (above 14000) to a clear perform

and it is mounted in a fiber drawing tower and made into a fiber.

1.2.10.2 Vapor Phase Axial Deposition (VAD): It is nearly like OVPO method. In this method, SiO2 particles are formed in the

same way. As these particles emerge from the torches, they are deposited onto the end of surface of a silica glass rod which acts as a

seed. A porous perform is grown in the axial direction by moving the rod upward. When it moves upward, it is transformed into a

solid, transparent rod perform by zone melting with the carbon ring heater.

Any fiber, step index or graded index, can be made by this VAD method.

Advantages:

1) The preform has no central hole. 2) The preform can be fabricated in continuous lengths which can effect process costs and product yields. 3) The deposition chamber and zone melting ring heater are tightly connected to each other in the same enclosure allows the

clean environment.

1.2.10.3 Modified Chemical Vapor Deposition (MCVD): The MCVD was widely adopted to produce very low loss graded

index fibers.

The glass vapor particles arising from the reaction of the constituent metal halide gases and oxygen flow through the inside of a

revolving silica tube. As SiO2 particles are deposited, they are sintered to a clear glass layer by a oxy hydrogen torch which travels

back and forth along the tube. When the desired thickness of glass has been deposited, the vapor flow is shut off and the tube is heated

strongly to cause it to collapse into a solid rod perform.

1.2.10.4 Plasma Activated Chemical Vapor Deposition: In PCVD, a non isothermal microwave plasma operating at low

pressure initiates the chemical reaction. With the silica tube held at temperatures in the range of 1000 12000C to reduce mechanical

stresses in the growing glass films, a moving microwave resonator operating at 2.45 GHz generates plasma inside the tube to activate

the chemical reaction. This process deposits clear glass material directly on the tube wall, there is no soot formation.

UNIT-2

Attenuation: Attenuation of a light signal as it propagates along a fiber is an important consideration in determining the maximum transmission distance between a transmitter and receiver or an in online amplifier. The

basic attenuation in fiber are [1] Absorption [2] Scattering [3] Radiative losses of the optical energy/ fiber bending

losses Signal attenuation is expressed as = 10log10 where, Pi = Input power, Po = Output Power In optical fiber dB L = 10log10 dB=

Signal attenuation per unit length, L = Length of the fiber. [1] Absorption: Absorption is caused by three different

mechanisms

[1.1] Intrinsic Absorption: Due to infrared and ultraviolet region intrinsic absorption is associated with the basic

fiber material (eg.. pure SiO2) and is the principal physical factor that defines the transparency window of a material

over a specified spectral region. It occurs when the material is in a perfect state with no density variations,

impurities, material inhomogeneties and so on. Intrinsic absorption results from electronic absorption bands in the

ultraviolet region and from atomic vibration bands in the near infrared region. Absorption occurs when a photon

interacts with an electron in the valence band and excites it to higher energy level.

[1.2] Extrinsic Absorption: (Due to impurities and OH molecules) A major source of signal attenuation is extrinsic

absorption from transition metal elements impurities. Some most common metallic impurities, found in glasses, are

Cr3+, C2+, Fe2+, Fe3+ etc. These contain absorption losses around 109 of one part. Certain of these impurities like

chromium and copper causes attenuation in excess of 1dB/Km in near infrared region.

Another major intrinsic loss occurs due to water dissolve in the glass. These hydroxyl groups are bound in to the glass structure and create vibrations at wavelength 2.7 and 4.2 m and generate overtones.When the hydroxyl group is in silica then overtones and vibrations occur due to SiO2 near 1.24, 1.13 and 0.88 m. [2] Scattering: Scattering occurs due to the micro irregularities inside the fiber. Scattering results in attenuation as the scattered light may not continue to satisfy the TIR (total internal reflection) in the fiber core. Scattering loss in glass arises from microscopic variations in material density, compositional fluctuations and from structural or defects in homogeneities during fiber manufacture. [2.1] Linear Scattering Loss: Linear Scattering mechanisms cause the transfer of some or all the optical power contained wit in one propagating mode to be transferred linearly in to a different mode. This may generate radio mode or leaky ray. But the frequency of light would not be affected during scattering. [2.1.1] Rayleigh Scattering: This results from random inhomogeneties that are small in size compared with the wavelength. These inhomogeneties exist in the form of refractive index fluctuations which are frozen into amorphous glass fiber upon fiber pulling. Such fluctuations always exist and cannot be avoided.

R = 83/3

4 n

8 p

2 c k Tf

Where: R = Rayleigh Scattering Coefficient = Optical Wavelength. n= refractive index of medium p= average photo

elastic coefficient c = isothermal compressibility - R L), where L= length of the fiber. Attenuation = 10 log dB /Km [2.2.2] Mic

Scattering: Mic scattering is due to non perfect cylindrical structure of the fiber and imperfection like irregularities

in the core cladding interface diameter fluctuations, strains and bubbles may create linear scattering which termed as

Mic Scattering. These Inhomogeneities can be removed by

2.2 Non linear scattering losses

Optical waveguides do not always behaves as completely linear especially at high optical power levels

scattering causes disproportionate attenuation, due to non linear behaviour. Because of this non linear scattering

the optical power from one mode is transferred in either the forward or backward direction to the same, or other

modes, at different frequencies. The two dominant types of non linear scattering are:

a) Stimulated Brillouin Scattering and

b) Stimulated Raman Scattering.

2.2.1 Stimulated Brillouin Scattering:

Stimulated Brillouin scattering (SBS) may be regarded as the modulation of light through thermal molecular

vibrations within the fiber. The scattered light appears as upper and lower sidebands which are separated from

the incident light by the modulation frequency. The incident photon in this scattering process produces a

phonon of acoustic frequency as well as a scattered photon. This produces an optical frequency shift which

varies with the scattering angle because the frequency of the sound wave varies with acoustic wavelength. The

frequency shift is a maximum in the backward direction, reducing to zero in the forward direction, making SBS

a mainly backward process.

The threshold power PB is given by:

PB = 4.4 103d

2 2dB watts

Where, d and are the fiber core diameter and the operating wavelength

dB is the fiber attenuation in decibels per kilometre and

source bandwidth in gigahertz.

2.2.3 Stimulated Raman Scattering:

Stimulated Raman scattering (SRS) is similar to SBS except that a high-frequency optical phonon rather than an

acoustic phonon is generated in the scattering process. Also, SRS can occur in both the forward and backward

directions in an optical fiber, and may have an optical power threshold of up to three orders of magnitude higher

than the Brillouin threshold in a particular fiber.

The threshold optical power for SRS PR in a long single-mode fiber is given by:

PR = 5.9 102d

2dB watts

3. Fiber Bending Losses Optical fiber suffers radiation losses at bends or curves on their paths. This is due to the energy in the

evanescent field at the bend exceeding the velocity of light in the cladding and hence the guidance mechanism

is inhibited, which causes light energy to be radiated from the fiber. The part of the mode which is on the

outside of the bend is required to travel faster than that on the inside so that a wave front perpendicular to the

direction of propagation is maintained. Hence, part of the mode in the cladding needs to travel faster than the

velocity of light in that medium. As this is not possible, the energy associated with this part of the mode is lost

through radiation.

Radiation attenuation coefficient

r = c1 exp c2R)

Where R is the radius of curvature of the fiber bend and c1, c2 are constants large bending losses tend to occur

in multimode fiber at a critical radius of curvature Rc which may be

The macro bending losses may be reduced by:

(a) Designing fiber with large relative refractive index differences;

(b) Operating at the shortest wavelength possible.

4. DISPERSION

Dispersion of the transmitted optical signal causes distortion for both digital and analog transmission along

optical fiber. Signal dispersion limits the maximum possible bandwidth attainable with a particular optical fiber

to the point where individual symbols can no longer be distinguished. When considering digital modulation,

then dispersion mechanisms within the fiber cause broadening of the transmitted light pulses as they travel

along the channel. The effect is known as intersymbol interference (ISI).

For no overlapping of light pulses down on an optical fiber link the digital bit rate BT must be less than the

reciprocal of the b

Hence:

BT

dictates the input pulse

Types of dispersion

a. Intramodal Dispersion i. Material dispersion ii. Waveguide Dispersion

b. Intermodal Dispersion

4.1 Intramodal Dispersion or Chromatic Dispersion

Chromatic or intramodal dispersion may occur in all types of optical fiber and results from the finite spectral

line width of the optical source. Since optical sources do not emit just a single frequency but a band of

frequencies then there may be propagation delay differences between the different spectral components of the

transmitted signal. This causes broadening of each transmitted mode and hence intramodal dispersion. The

delay differences may be caused by the dispersive properties of the waveguide material (material dispersion)

and also guidance effects within the fiber structure (waveguide dispersion).

4.1.1 Material dispersion

Pulse broadening due to material dispersion results from the different group velocities of the various spectral

components launched into the fiber from the optical source. It occurs when the phase velocity of a plane wave

propagating in the dielectric medium varies nonlinearly with wavelength, and a material is said to exhibit

material dispersion when the second differential of the refractive index with respect to wavelength is not zero.

The pulse spread due to material dispersion may be obtained by considering t

defined by Eqs:

Where n1 in a fiber of

length L is therefore:

As the first term in Eq. usually dominates, especially for sources operating over the 0.8 to 0.9

range, then:

Therefore, substituting the expression the rms pulse broadening due to material dispersion is given by:

in terms of a material dispersion parameter M which is defined as:

4.1.2 Waveguide Dispersion

The wave guiding of the fiber results from the variation in group velocity with wavelength for a particular

mode. Considering the ray theory approach, it is equivalent to the angle between the ray and the fiber axis

varying with wavelength which subsequently leads to a variation in the transmission times for the rays, and

hence dispersion

1. For a singl 0.

2. For multimode fibers, where the majority of modes propagate far from cut-off, are almost free of waveguide dispersion and it is generally negligible compared .

4.2 INTERMODAL OR MODAL DISPERSION

Propagation delay differences between modes within a multimode fiber results Pulse broadening. As the

different modes which constitute a pulse in a multimode fiber travel along the channel at different group

velocities, the pulse width at the output is dependent upon the transmission times of the slowest and fastest

modes. This dispersion mechanism creates the fundamental difference in the overall dispersion.

1) Multimode step index fibers exhibit a large amount of intermodal dispersion which gives the greatest pulse broadening.

2) Intermodal dispersion in multimode fibers may be reduced by adoption of an optimum refractive index profile which is provided by the near-parabolic profile of most graded index fibers. Thus graded index

fibers used with a multimode source give a tremendous bandwidth advantage over multimode step index

fibers.

3) In Single-mode Fibers there is no intermodal dispersion.

4.2.1 Multimode step index fibers

Using the ray theory model, the fastest and slowest modes propagating in the step index fiber may be

represented by the axial ray and the extreme meridional respectively. The delay difference between these two

rays when travelling in the fiber core allows estimation of the pulse broadening resulting from intermodal

dispersion within the fiber. As both rays are travelling at the same velocity within the constant refractive index

fiber core, then the delay difference is directly related to their respective path lengths within the fiber.

Hence the time taken for the axial ray to travel along a fiber of length L gives the minimum delay time TMin and:

The extreme meridional ray exhibits the maximum delay time TMax where:

cladding interface:

Where, n2 is the refractive index of the cladding. Furthermore, substituting into Eq

Ts between the extreme meridional ray and the axial ray may be obtained

Hence rearranging Eq

4.2.2 Multimode Graded Index Fibers

Intermodal dispersion in multimode fibers is minimized with the use of graded index fibers. Hence, multimode

graded index fibers show substantial bandwidth improvement over multimode step index fibers. The fiber

shown has a parabolic index profile with a maximum at the core axis the index profile is given by n(r)

2 as:

Figure shows several meridional ray paths within the fiber core. The local group velocity is inversely

proportional to the local refractive index and therefore the longer sinusoidal paths are compensated for by

higher speeds in the lower index medium away from the axis. As these various ray paths may be considered to

represent the different modes propagating in the fiber, then the graded profile reduces the disparity in the mode

transit times.

The dramatic improvement in multimode fiber bandwidth achieved with a parabolic or near-parabolic refractive

index profile is highlighted by consideration of the reduced delay difference between the fastest and slowest

Tg

the rms pulse broadening of a near-parabolic index profile graded index fiber

After substituting values from above equations;

4.3 Modal Noise

The intermodal dispersion properties of multimode optical fibers create another phenomenon which affects the

transmitted signals on the optical channel. It is exhibited within the speckle patterns observed in multimode

fiber as fluctuations which have characteristic times longer than the resolution time of the detector, and is

known as modal or speckle noise.

The speckle patterns are formed by the interference of the modes from a coherent source when the

coherence time of the source is greater than T within the fiber. The coherence

time for a source with uncorrelated so f f. Hence, modal noise occurs when:

The conditions which give rise to modal noise are therefore specified as:

(a) A coherent source with a narrow spectral width and long coherence length.

(b) Disturbances along the fiber which give differential mode delay or modal and spatial filtering;

(c) Phase correlation between the modes.

Modal noise may be prevented on an optical fiber link through suitable choice of the system components.

However, this may not always be possible and then certain levels of modal noise must be tolerated.

4.4 Overall fiber dispersion

4.4.1 Multimode Fiber

The overall dispersion in multimode fibers comprises both chromatic and intermodal terms. The total rms pulse

T is given by:

W caused by delay

due to both material and

waveguide dispersion. Waveguide dispersion is generally negligible compared with material dispersion in

4.4.2 Mono Mode Fibers

The pulse broadening in single-mode fibers results almost entirely from chromatic or intra modal dispersion as

only a single-mode is allowed to propagate. Hence the bandwidth is limited by the finite spectral width of the

g for a light pulse propagating along a unit length of single-

mode fiber may be given,

Where c is the velocity of light in within the fiber core of

refractive index n1 and k is the propagation constant for the mode in a vacuum.

5. Fiber birefringence

Single-mode fibers with nominal circular symmetry about the core axis allow the propagation of two nearly

degenerate modes with orthogonal polarizations. In an optical fiber with an ideal optically circularly symmetric

core both polarization modes propagate with identical velocities. Manufactured optical fibers exhibit some

birefringence resulting from differences in the core geometry resulting from variations in the internal and

external stresses, and fiber bending. The fiber therefore behaves as a birefringent medium due to the difference

in the effective refractive indices, and hence phases velocities, for these two orthogonally polarized modes. The

modes therefore have different x y which are dictated by the anisotropy of the fiber

cross section.

x y are the propagation constants for the slow mode and the fast mode respectively. When the

fiber cross-section is independent of the fiber length L in the

z direction, then the modal birefringence BF for the fiber is given by

The difference in phase velocities causes the fib z) which depends on the fiber

length L in the z direction and is given by

Assuming that, Phase coherence of two mode components is maintained. The phase coherence of the two mode

components is achieved when the delay between the two transit times is less than the coherence time of the

source. The coherence time for the source is equal to the reciprocal of the uncorrelated source frequency width

f ). It may be shown that birefringent coherence is maintained over a length of fiber Lbc (i.e. coherence

length) when:

However, when phase coherence is maintained leads to a polarization state which is generally elliptical but

which varies periodically along the fiber. This situation is illustrated in Figure where the incident linear

polarization which is at 45 with respect to the x axis becomes circular

linear polarization at LB for this process corresponding to the propagation

distance beat length. It is

given by:

So,

Typical single-mode fibers are found to have beat lengths of a few centimetres and the effect may be observed

directly within a fiber via Rayleigh scattering with use of a suitable visible source (e.g. He Ne laser). It appears

as a series of bright and dark bands with a period.

5.1 Polarization mode dispersion (PMD)

Polarization mode dispersion (PMD) is a form of modal dispersion where two different polarizations of light in

a waveguide, which normally travel at the same speed, travel at different speeds due to random imperfections

and asymmetries, causing random spreading of optical pulses. Unless it is compensated, which is difficult, this

ultimately limits the rate at which data can be transmitted over a fiber.

Polarization mode dispersion (PMD) is a source of pulse broadening which results from fiber birefringence and

it can become a limiting factor for optical fiber communications at high transmission rates.

So the differential group delay is given by;

6. OPTICAL FIBER CONNECT IONS

6.1 Fiber Splicing:

A permanent joint formed between two individual optical fibers in the field or factory is known as a fiber splice.

Fiber splicing is frequently used to establish long-haul optical fiber links where smaller fiber lengths need to be

joined, and there is no requirement for repeated connection and disconnection. Splices may be divided into two

broad categories depending upon the splicing technique

1) Fusion splicing or welding: Fusion splicing is accomplished by applying localized heating (e.g. by a flame or an electric arc) at the interface between two butted, pre-aligned fiber ends causing them to

soften and fuse.

2) Mechanical splicing: Mechanical splicing, in which the fibers are held in alignment by some mechanical means, may be achieved by various methods including the use of tubes around the fiber ends (tube splices) or V-grooves

into which the butted fibers are placed.

6.1.1 Fusion splicing

Fusion splicing is the act of joining two optical fibers end-to-end using heat. The goal is to fuse the two fibers

together in such a way that light passing through the fibers is not scattered or reflected back by the splice, and

so that the splice and the region surrounding it are almost as strong as the virgin fiber itself. The source of heat

is usually an electric arc, but can also be a laser, or a gas flame, or a tungsten filament through which current is

passed.

Fusion splicing of single- presents problems of

required for low-loss joints). However,

splice insertion losses below 0.3 dB may be achieved due to a self-alignment phenomenon which partially

compensates for any lateral offset.

6.1.2 Mechanical splicing

The most common mechanical splicing technique is V-groove method. V-groove splices formed by

sandwiching the butted fiber ends between a V-groove glass substrate and a flat glass retainer plate have also

proved very successful in the laboratory. Splice insertion losses of less than 0.01 dB when coupling single-mode

fibers have been reported using this technique.

6.2 Fiber connectors

Optical fiber connectors are used to join optical fibers where a connect/disconnect capability is required. This is

because they must maintain similar tolerance requirements to splices in order to couple light between fibers

efficiently, but they must accomplish it in a removable fashion.

Hence optical fiber connectors may be considered in three major areas, which are:

(a) the fiber termination, which protects and locates the fiber ends;

(b) the fiber end alignment to provide optimum optical coupling;

(c) The outer shell, which maintains the connection and the fiber alignment, protects the fiber ends from the environment and provides adequate strength at the joint.

6.2.1 Cylindrical ferrule connectors

The basic ferrule connector which is perhaps the simplest optical fiber connector design, is illustrated in Figure:

The two fibers to be connected are permanently bonded (with epoxy resin) in metal plugs known as ferrules

which have an accurately drilled central hole in their end faces where the stripped (of buffer coating) fiber is

located. Within the connector the two ferrules are placed in an alignment sleeve which, using accurately

machined components, allows the fiber ends to be butt jointed.

6.2.2 Expanded beam connectors

An alternative to connection via direct butt joints between optical fibers is offered by the principle of the

expanded beam. It shows a connector consisting of two lenses for collimating and refocusing the light from one

fiber into the other. The use of this interposed optics makes the achievement of lateral alignment much less

critical than with a butt-jointed fiber connector. Expanded beam connectors are useful for multi-fiber

connection and edge connection for printed circuit boards where lateral and longitudinal alignment are

frequently difficult to achieve.

6.3 Fiber couplers

An optical fiber coupler is a device that distributes light from a main fiber into one or more branch fibers. The

latter case is more normal and such devices are known as multiport fiber couplers. Requirements are increasing

for the use of these devices to divide or combine optical signals for application within optical fiber information

distribution systems including data buses, LANs, computer networks and telecommunication access networks.

Optical fiber couplers are often passive devices in which the power transfer takes place either:

(a) Through the fiber core cross-section by butt jointing the fibers or by using some form of imaging optics

between the fibers (core interaction type); or

(b) Through the fiber surface and normal to its axis by converting the guided core modes to both cladding and

refracted modes which then enable the power-sharing mechanism.

Multiport optical fiber couplers can also be subdivided into the following three main groups

1. Three- and four-port couplers, which are used for signal splitting, distribution and combining.

2. Star couplers, which are generally used for distributing a single input signal to multiple outputs.

3. Wavelength division multiplexing (WDM) devices, which are a specialized form of coupler designed to

permit a number of different peak wavelength optical signals to be transmitted in parallel on a single fiber.

Ideal fiber couplers should distribute light among the branch fibers with no scattering loss or the generation of

noise, and they should function with complete insensitivity to factors including the distribution of light between

the fiber modes, as well as the state of polarization of the light.

6.3.1 Three- and four-port couplers

Figure shows the structure of a parallel surface type of GRIN-rod lens three port coupler which comprises two

quarter pitch lenses with a semitransparent mirror in between. Light rays from the input fiber F1 collimate in

the first lens before they are incident on the mirror. A portion of the incident beam is reflected back and is

coupled to fiber F2, while the transmitted light is focused in the second lens and then coupled to fiber F3.

The various loss parameters associated with four-port couplers may be written down with reference to Figure.

Hence, the excess loss which is defined as the ratio of power input to power output is given by:

The insertion loss, however, is generally defined as the loss obtained for a particular port-to-port optical path

6.3.2 Star couplers

In an ideal star coupler the optical power from any input fiber is evenly distributed among the output fibers. The

total loss associated with the star coupler comprises its theoretical splitting loss together with the excess loss.

The splitting loss is related to the number of output ports N following:

Splitting loss (star coupler) = 10 log10 N (dB)

For a single input port and multiple output ports where j = 1, N, then the excess loss is given by:

6.3.3 Wavelength division multiplexing couplers

WDM devices are a specialized coupler type which enables light from two or more optical sources of differing

nominal peak optical wavelength to be launched in parallel into a single optical fiber. Hence such couplers

perform as either wavelength multiplexers or wavelength demultiplexers. The important optical parameters

associated with the WDM coupler are the attenuation of the light over a particular wavelength band, the inter

band isolation and the wavelength band or channel separation.

UNIT-3

Optical sources

The function of an optical transmitter is to convert incoming electrical signals into outgoing optical signals. The

major component of the transmitter is the optical source, which either a semiconductor light-emitting diode

(LED) or laser diode. Semiconductor diode devices have the advantages over other light sources of small size,

high efficiency, high reliability, suitable wavelength ranges, small emission areas matching fiber cores, and the

ability to be directly current modulated.

1. LED sources

Light-emitting diodes are simply forward-biased p-n junctions, which emit light by spontaneous emission.

Spontaneous emission (or electroluminescence) is caused by radiative recombination of electron-hole pairs in

that emit over a broad spectral

bandwidth (20-150 nm) and large angular bandwidth. Advantages of LED:

1. Simpler fabrication. There are no mirror facets and in some structures no striped geometry.

2. Cost. The simpler construction of the LED leads to much reduced cost which is always likely to be

maintained.

3. Reliability. The LED does not exhibit catastrophic degradation and has proved far less sensitive to gradual

degradation than the injection laser. It is also immune to self-pulsation and modal noise problems.

4. Generally less temperature dependence. The light output against current characteristic is less affected by

temperature than the corresponding characteristic for the injection laser. Furthermore, the LED is not a

threshold device and therefore raising the temperature does not increase the threshold current above the

operating point and hence halt operation.

5. Simpler drive circuitry. This is due to the generally lower drive currents and reduced temperature

dependence which makes temperature compensation circuits unnecessary.

6. Linearity. Ideally, the LED has a linear light output against current characteristic

This can prove advantageous where analog modulation is concerned.

Drawbacks:

(a) Generally lower optical power coupled into a fiber (microwatts);

(b) Usually lower modulation bandwidth;

(c) Harmonic distortion.

1.1 LED power and efficiency The power generated internally by an LED may be determined by consideration of the excess electrons and

holes in the p- and n-type material respectively. When it is forward biased and carrier injection takes place at

n p is equal since the injected carriers are

created and recombined in pairs such that charge neutrality is maintained within the structure. In extrinsic

materials one carrier type will have a much higher concentration than the other and hence in the p-type region,

for example, the hole concentration will be much greater than the electron concentration. Generally, the excess

minority carrier density decays exponentially with time t according to the relation:

n n t ..... (1)

n

lifetime.

When there is a constant current flow into the junction diode, an equilibrium condition is established. In this

case, the total rate at which carriers are generated will be the sum of

the externally supplied and the thermal generation rates. Hence a rate equation for carrier recombination in the

LED can be expressed in the form

.......(2)

The condition for equilibrium is obtained by setting the derivative in Eq. (2) to zero. Hence:

....... (3)

Equation (3) therefore gives the steady-state electron density when a constant current is flowing into the

junction region.

It is also apparent from Eq. (2) that in the steady state the total number of carrier recombi-nations per second or

the recombination rate rt will be:

(5)

where rr is the radiative recombination rate per unit volume and rnr is the non-radiative recombination rate per

unit volume. Moreover, when the forward-biased current into the device is i, then from Eq. (7.4) the total

number of recombinations per second Rt becomes:

(6)

radiative recombination

rate to the total recombination rate,

(8)

where Rr is the total number of radiative recombination per second. Rearranging Eq. (8) and substituting from

Eq. (6) gives:

(9)

Since Rr is also equivalent to the total number of photons generated per second each photon has an energy

equal to hf joules, then the optical power generated internally by the LED, Pint, is:

he internally generated power in terms of wavelength rather than frequency gives:

For the exponential decay of excess carriers depicted by Eq. (1) the radiative minority r =

n/rr and the non- n/rnr. Therefore, from Eq. (7.7) the internal

quantum efficiency is:

n/rt gives:

Hence,

1.2 The double-heterojunction LED The principle of operation of the DH LED is illustrated in Figure. The device shown consists of a p-type GaAs

layer sandwiched between a p-type AlGaAs and an n-type bAlGaAs layer. When a forward bias is applied

electrons from the n-type layer are injected through the p n junction into the p-type GaAs layer where they

become minority carriers. These minority carriers diffuse away from the junction recombining with majority

carriers (holes) as they do so. Photons are therefore produced with energy corresponding to the bandgap energy

of the p-type GaAs layer. The injected electrons are inhibited from diffusing into the p-type AlGaAs layer

because of the potential barrier presented by the p p heterojunction, Hence, electro luminescence only occurs in

the GaAs junction layer, providing both good internal quantum efficiency and high-radiance emission.

The DH structure is therefore used to provide the most efficient incoherent sources for application within

optical fiber communications. Nevertheless, these devices generally exhibit the previously discussed constraints

in relation to coupling efficiency to optical fibers.

1.3 LED structures Types of LED

a. Planar LED b. Dome LED c. Surface Emitter LED d. Edge-Emitter LED e. Superluminescent LEDs

1.3.1 Planar LED The planar LED is the simplest of the structures that are available and is fabricated by either liquid- or vapor-

phase epitaxial processes over the whole surface of a GaAs substrate. This involves a p-type diffusion into the

n-type substrate in order to create the junction. Forward current flow through the junction gives Lambertian

spontaneous emission and the device emits light from all surfaces. However, only a limited amount of light

escapes the structure due to total internal reflection and therefore the radiance is low.

1.3.2 DOME LED The structure of a typical dome LED is shown in Figure. A hemisphere of n-type GaAs is formed around a

diffused p-type region. The diameter of the dome is chosen to maximize the amount of internal emission

reaching the surface within the critical angle of the GaAs air interface. Hence this device has a higher external

power efficiency than the planar LED. However, the geometry of the structure is such that the dome must be far

larger than the active recombination area, which gives a greater effective emission area and thus reduces the

radiance.

1.3.3 Surface Emitter LED

Surface emitter LED (SLED) has been widely employed within optical fiber communications in which A

method for obtaining high radiance is to restrict the emission to a small active region within the device. These

structures have low thermal impedance in the active region allowing high current densities and giving high-

radiance emission into the optical fiber. The structure of a high-radiance etched well DH surface emitter* for

wavelength band is shown in Figure. The internal absorption in this device is very low due to

the larger band-gap-confining layers, and the reflection coefficient at the back crystal face is high giving good

forward radiance. The emission from the active layer is essentially isotropic, although the external emission

distribution may be considered Lambertian with a beam width of 120 due to refraction from a high to a low

refractive index at the GaAs fiber interface. The power coupled Pc into a multimode step index fiber may be

estimated from the relationship:

P r)ARD(NA)2

Where r is the Fresnel reflection coefficient at the fiber surface, A is the smaller of the fiber core cross-section

or the emission area of the source and RD is the radiance of the source.

1.3.4 Edge emitter LED Edge emitter LED (ELED) has a similar geometry to a conventional contact stripe injection laser

It takes advantage of

light produced in the active layer spreads into the transparent guiding layers, reducing self-absorption in the

active layer. The consequent waveguiding narrows the beam divergence to a half-power width of around 30 in

the plane perpendicular to the junction. However, the lack of waveguiding in the plane of the junction gives a

Lambertian output with a half-power width of around 120. The ELED active layer was heavily doped with Zn

to reduce the minority carrier lifetime and thus improve the device modulation bandwidth. In this way a 3 dB

modulation bandwidth of 600 MHz was obtained. Very high coupled optical power levels into single-mode

have been obtained with InGaAsP ELEDs at drive currents as low as 50 mA.

1.3.5 Superluminescent LED Another device geometry which is providing significant benefits over both SLEDs and ELEDs for

communication applications is the Superluminescent diode or SLD. This device offers advantages of:

(a) A high output power;

(b) A directional output beam; and

(c) A narrow spectral line width.

All of which prove useful for coupling significant optical power levels into optical fiber. The super radiant

emission process within the SLD tends to increase the device modulation bandwidth over that of more

conventional LEDs.

A Superluminescent light emitting diode is, similar to a laser diode, based on an electrically driven pn-

junction that, when biased in forward direction becomes optically active and generates amplified spontaneous

emission over a wide range of wavelengths. The peak wavelength and the intensity of the SLED depend on the

active material composition and on the injection current level. SLEDs are designed to have high single pass

amplification for the spontaneous emission generated along the waveguide but, unlike laser diodes, insufficient

feedback to achieve lasing action. This is obtained very successfully through the joint action of a tilted

waveguide and anti-reflection coated (ARC) facets.

1.4 LED Characteristics Optical output power LED is a very linear device in comparison with the majority of injection lasers and hence it tends to be more

suitable for analog transmission where severe constraints are put on the linearity of the optical source. However,

in practice LEDs do exhibit significant nonlinearities which depend upon the configuration utilized. It is

therefore often necessary to use some form of linearizing circuit technique in order to ensure the linear

performance of the device to allow its use in high-quality analog transmission systems.

(a) Ideal LED characteristics

(b) S ameter dot (c) E m wide stripe and Contact

Light output temperature dependence for three important LED structures emitting at a wavelength of

1.4.2 Output spectrum The spectral linewidth of an LED operating at wavelength band is usually

between 25 and 40 nm at the half maximum intensity points. For materials with smaller bandgap energies

width tend to increase to around 50 to 160 nm.

Examples of these two output spectra are shown in Figure. The increases in linewidth due to increased doping

levels and the formation of band tail states. This becomes apparent in the differences in the output spectra

between surface- and edge-emitting LEDs where the devices have generally heavily doped and lightly doped.

1.5 Modulation bandwidth The modulation bandwidth in optical communications may be defined in either electrical or optical terms. When

the associated electrical circuitry in an optical fiber communication system to use the electrical definition where

the electrical signal power has dropped to half its constant value due to the modulated portion of the optical

signal. This corresponds to the electrical 3 dB point or the frequency at which the output electric power is

reduced by 3 dB with respect to the input electric power. Alternatively, if the 3 dB bandwidth of the modulated

optical carrier (optical bandwidth) is considered, we obtain an increased value for the modulation bandwidth.

OPTICAL SOURCE: LASER

In optical Communication three main types of optical light source are available. These are:

(b) Monochromatic incoherent sources (light-emitting diodes, LEDs);

(c) Monochromatic coherent sources (lasers).

The major requirements for an optical fiber emitter which are outlined below:

1. A size and configuration compatible with launching light into an optical fiber. Ideally, the light output should

be highly directional.

2. Must accurately track the electrical input signal to minimize distortion and noise. Ideally, the source should

be linear.

3. Should emit light at wavelengths where the fiber has low losses and low dispersion and where the detectors

are efficient.

4. Preferably capable of simple signal modulation over a wide bandwidth extending from audio frequencies to

beyond the gigahertz range.

5. Must couple sufficient optical power to overcome attenuation in the fiber plus additional connector losses and

leave adequate power to drive the detector.

6. Should have a very narrow spectral bandwidth in order to minimize dispersion in the fiber.

7. Must be capable of maintaining a stable optical output which is largely unaffected by changes in ambient

conditions (e.g. temperature).

8. It is essential that the source is comparatively cheap and highly reliable in order to compete with

conventional transmission techniques.

A laser is a device that emits light through a process of optical amplification based on the stimulated

emission of electromagnetic radiation. The term "laser" originated as an acronym for "light amplification by

stimulated emission of radiation coherently.

Lasers have many important applications. They are used in common consumer devices such as optical disk

drives, laser printers, and barcode scanners. Lasers are used for both fiber-optic and free-space optical

communication.

2.1 Basic Concepts

2.1.1 Absorption and emission of radiation

The interaction of light with matter takes place in discrete packets of energy or quanta, called photons.

Furthermore, the quantum theory suggests that atoms exist only in certain discrete energy states such that

absorption and emission of light causes them to make a transition from one discrete energy state to another. The

frequency of the absorbed or emitted radiation f is related to the difference in energy E between the higher

energy state E2 and the lower energy state E1 by the expression:

E = E E1 = hf

Where h may be considered

to correspond to electrons occurring in particular energy levels relative to the nucleus. Hence, different energy

states for the atom correspond to different electron configurations, and a single electron transition between two

energy levels within the atom will provide a change in energy suitable for the absorption or emission of a

photon.

This emission process can occur in two ways:

(a) By spontaneous emission in which the atom returns to the lower energy state in an entirely random manner;

(b) By stimulated emission when a photon having an energy equal to the energy difference between the two

states (E E1) interacts with the atom in the upper energy state causing it to return to the lower state with the

creation of a second photon.

It is the stimulated emission process which gives the laser its special properties as an optical source. The photon

produced by stimulated emission is generally of an identical energy to the one which caused it and hence the

light associated with them is of the same frequency. The light associated with the stimulating and stimulated

photon is in phase and has the same polarization. Therefore, in contrast to spontaneous emission, coherent

radiation is obtained.

2.1.2 The Einstein relations

In 1917 Einstein demonstrated that the rates of the three transition processes of absorption, spontaneous

emission and stimulated emission were related mathematically. He achieved this by considering the atomic

system to be in thermal equilibrium such that the rate of the upward transitions must equal the rate of the

downward transitions. The population of the two energy levels of such a system is described by Boltzmann

statistics which give:

where N1 and N2 represent the density of atoms in energy levels E1 and E2, respectively, with g1 and g2 being

the corresponding degeneracies of the levels, K and T is the absolute temperature.

As the density of atoms in the lower or ground energy state E1 is N1, the rate of upward transition or absorption

is proportional to both N f of the radiation energy at the transition frequency f.

Hence, the upward transition rate R12 may be written as:

R12 = N f B12

where the constant of proportionality B12 is known as the Einstein coefficient of absorption.

For spontaneous emission the average time that an electron exists in the excited state before a transition occurs

within the system with energy E2 is N2, then

the spontaneous emission rate is given by the product of N N2A21 where A21,

the Einstein coefficient of spontaneous emission, is equal to the reciprocal of the spontaneous lifetime.

The rate of stimulated downward transition of an electron from level 2 to level 1 may be obtained in a similar

manner to the rate of stimulated upward transition. Hence the rate of stimulated emission is given by

R21 = N2A21 + N f B21

For a system in thermal equilibrium, the upward and downward transition rates must be equal and therefore R12

= R21, or:

N f B12 = N2A21 + N fB21

It follows that:

Substituting values from equations

Planck showed that the radiation spectral density for a black body radiating within a frequency range f to f + df

is given by

after comparing equations,

&

The ratio of the stimulated emission rate to the spontaneous emission rate is given by:

2.1.3 Population inversion

Under the conditions of thermal equilibrium given by the Boltzmann distribution, the lower energy level E1 of

the two-level atomic system contains more atoms than the upper energy level E2, which is normal for structures

at room temperature. However, to achieve optical amplification it is necessary to create a non-equilibrium

distribution of atoms such that the population of the upper energy level is greater than that of the lower energy

level (i.e. N2 > N1). This condition is known as population inversion.

In order to achieve population inversion it is necessary to excite atoms into the upper energy level E2

and hence obtain a non-equilibrium distribution. This process is achieved using an external energy source and is

When the two levels are equally degenerate (or not degenerate), then B12 = B21. Thus

the probabilities of absorption and stimulated emission are equal, providing at best equal populations in the two

levels.

Population inversion may be obtained in systems with three or four energy levels. To achieve population

inversion both systems display a central metastable state in which the atoms spend an unusually long time. It is

from this metastable level that the stimulated emission or lasing takes place.

2.1.4 Optical feedback and laser oscillation

Light amplification in the laser occurs when a photon colliding with an atom in the excited energy state causes

the stimulated emission of a second photon and then both these photons release two more. Continuation of this

process effectively creates avalanche multiplication, and when the electromagnetic waves associated with these

photons are in phase, amplified coherent emission is obtained. To achieve this laser action it is necessary to

contain photons within the laser medium and maintain the conditions for coherence. This is accomplished by

placing or forming mirrors (plane or curved) at either end of the amplifying medium. The optical cavity formed

is more analogous to an oscillator than an amplifier as it provides positive feedback of the photons by reflection

at the mirrors at either end of the cavity. Hence the optical signal is fed back many times while receiving

amplification as it passes through the medium.

Since the structure forms a resonant cavity, when sufficient population inversion exists in the amplifying

medium the radiation builds up and becomes established as standing waves between the mirrors. Thus when the

optical spacing between the mirrors is L, the resonance condition along the axis of the cavity is given by:

s the emission wavelength, n is the refractive index of the amplifying medium

and q is an integer. Alternatively, discrete emission frequencies f is defined by:

The different frequencies of oscillation within the laser cavity are determined by the various integer values of q

and each constitutes a resonance or mode. These f where:

f _ f and as f = c

Hence,

2.1.4 Threshold condition for laser oscillation

The steady-state conditions for laser oscillation are achieved when the gain in the amplifying medium exactly

balances the total losses. Hence, although population inversion between the energy levels providing the laser

transition is necessary for oscillation to be established, it is not alone sufficient for lasing to occur. we assume

the amplifying medium occupies a length L completely filling the region between the two mirrors which have

reflectivities r1 and r2. On each round trip the beam passes through the medium twice. Hence the fractional loss

incurred by the light beam is:

Fractional loss = r1r2 L)

It is found that the increase in beam intensity resulting from stimulated emission is exponential

Therefore if the gain coefficient per unit length produced

round trip gain is given by:

Fractional gain = exp(2CL)

Hence:

exp(2CL) r1r2 L) = 1

and

r1r2 L] = 1

The threshold gain per unit length may be obtained by rearranging the above expression to give:

The second term on the right hand side represents the transmission loss through the mirrors.

2.2 The Semiconductor Injection Laser

Stimulated emission by the recombination of the injected carriers is encouraged in the semiconductor injection

laser (also called the injection laser diode (ILD) or simply the injection laser) by the provision of an optical

cavity in the crystal structure in order to provide the feedback of photons. This gives the injection laser several

major advantages over other semiconductor sources (e.g. LEDs) that may be used for optical communications.

These are as follows:

1. High radiance due to the amplifying effect of stimulated emission. Injection lasers will generally supply

milliwatts of optical output power.

2. Narrow linewidth on the order of 1 nm (10 ) or less which is useful in minimizing the effects of material

dispersion.

3. Modulation capabilities which at present extend up into the gigahertz range and will undoubtedly be

improved upon.

4. Relative temporal coherence which is considered essential to allow heterodyne (coherent) detection in high-

capacity systems, but at present is primarily of use in single-mode systems.

5. Good spatial coherence which allows the output to be focused by a lens into a spot which has a greater

intensity than the dispersed unfocused emission.

Schematic diagram of a GaAs homojunction injection laser with a Fabry Prot cavity

The DH injection laser fabricated from lattice-matched III V alloys provided both carrier and optical

confinement on both sides of the p n junction, giving the injection laser a greatly enhanced performance. This

enabled these devices with the appropriate heat sinking to be operated in a CW mode at 300 K with obvious

advantages for optical communications

2.2.1 Efficiency

It is the differential external quantum efficiency D which is the ratio of the increase in photon output rate for a

given increase in the number of injected electrons. If Pe is the optical power emitted from the device, I is the

current, e is the charge on an electron and hf is the photon energy, then:

Where Eg is the bandgap energy expressed in eV. It may be noted that D gives a measure of the rate of change

of the optical output power with current and hence defines the slope of the output characteristic. The internal

quantum efficiency of the semiconductor laser i,

It is related to the differential external quantum efficiency by the expression

Where A is the loss coefficient of the laser cavity, L is the length of the laser cavity and r1, r2 is the cleaved

mirror reflectivities.

Another paramet T which is efficiency defined as:

As the power emitted Pe changes linearly when the injection current I is greater than the threshold current Ith,

then:

For high injection current (e.g. I = 5Ith T D, whereas for lower currents (I 2Ith) the total efficiency is

lower and around 15 to 25%.

ep in converting electrical input to optical

output is given by:

For the total efficiency we find:

2.2.2 Stripe geometry

The DH laser structure provides optical confinement in the vertical direction through the refractive index step at

the heterojunction interfaces, but lasing takes place across the whole width of the device.

Figure shows the broad-area DH laser where the sides of the cavity are simply formed by roughening the edges

of the device in order to reduce unwanted emission in these directions and limit the number of horizontal

transverse modes. However, the broad emission area creates several problems including difficult heat sinking,

lasing from multiple filaments in the relatively wide active area and unsuitable light output geometry for

efficient coupling to the cylindrical fibers.

To overcome these problems while also reducing the required threshold current, laser structures in which the

active region does not extend to the edges of the device were developed. A common technique involved the

introduction of stripe geometry to the structure to provide optical containment in the horizontal plane.

2.2.3 Laser modes

LASER contains a large number of modes which are generated within the laser cavity. Hence the laser emission

will only include the longitudinal modes contained within the spectral width of the gain curve as shown in

figure.

This gives rise to resonant modes which are transverse to the direction of propagation. These transverse

electromagnetic modes are designated in a similar manner to transverse modes in waveguides by TEMlm where

the integers l and m indicate the number of transverse modes. In the case of the TEM00 mode all parts of the

propagating wave front are in phase. This is not so, however, with higher order modes (TEM10, TEM11, etc.)

where phase reversals produce the various mode patterns. Thus the greatest degree of coherence, together with

the highest level of spectral purity, may be obtained from a laser which operates in only the TEM00 mode.

Higher order transverse modes only occur when the width of the cavity is sufficient for them to oscillate.

The correct stripe geometry inhibits the occurrence of the higher order lateral modes by limiting the width of the

optical cavity, leaving only a single lateral mode which gives the output spectrum.

2.2.4 Single-mode operation

For single-mode operation, the optical output from a laser must contain only a single longitudinal and single

transverse mode. Hence the spectral width of the emission from the single-mode device is far smaller than the

broadened transition linewidth. Single transverse mode operation, however, may be obtained by reducing the

aperture of the resonant cavity such that only the TEM00 mode is supported. To obtain single-mode operation it

is then necessary to eliminate all but one of the longitudinal modes. One method of achieving single

longitudinal mode operation is to reduce the length L of the cavity until the frequency separation of the adjacent

modes given by f = c/2nL is larger than the laser transition linewidth or gain curve. Then only the single mode

which falls within the transition linewidth can oscillate within the laser cavity.

2.2.5 External quantum efficiency gth

The external quantum efficienc ext is defined as the number of photons emitted per radiative electron hole

pair recombination above threshold.

ext=

-0.7 at room tempreture)

Experimentally, ext is calculated from the straight-line portion of the curve for the emitted optical power P versus

drive current I, which gives

ext= =

2.3 Laser Diode Rate Equation

The relationship between optical output power and the diode drive current can be determined by examined by

the rate equations that govern the interaction of photons and electrons in the active region. for a p-n junction

with a carrier confinement region of depth d, the rate of equation are given by

- ...(1)

= stimulated emission+ spontaneous emission+ photon loss

= - - .....(2)

= injection+ spontaneous recombination+ stimulated emission; which governs the number of electrons n

Where; C= coefficient describing the strength of he optical absorption

Rsp=rate of spontaneous emission into lasing mode

ph= photon life time

s= spontaneous recombination lifetime

on solving equation 1 and 2 for a steady state condition will yield an expression for the output power.

In first equation assuming Rsp

Cn- ph

1, this threshold

value can be expressed in terms of the threshold current Jth needed to maintain an inversion level n=nth in steady

=

This expression defines the current requirement to sustain an excess electron density in the laser when

spontaneous emission is the only decay mechanism,

Now consider the photon and electron rate equations in the steady state condition at the lasing threshold

0=Cnth s+Rsp- s ph ......(4)

0= - -Cnth s ........(5)

After adding these two equations, the no of photons per unit volume

s= (J-Jth ph Rsp

UNIT 4

SOURCE TO FIBER POWER LAUNCHING: In implementing an optical fiber link, two of the major system questions are now to launch operation of optical power into a particular

fiber from same type of luminescent source and how to couple optical power from the fiber to other. A measure of the amount of

optical power emitted from a source that can be coupled into a fiber is usually given by the coupling efficiency defined as:

: Power couple into the fiber

: Power emitted from the light source

The launching and coupling efficiency depends on the type of fiber that is attached to the source and on the coupling process.

The optical power that can couple into a fiber depends on the radiance or brightness which is given through a diode drive current.

Radiance is the optical power radiated into a unit solid angle per unit emitting surface areas and is generally specified in terms of

Watts/cm2.

SOURCE OUTPUT PATTERN: The optical power accepting capability of a fiber is represented by a spatial radiation pattern of the

source which is shown in figure:

with the polar axis. The radiance may be a function of

and can also vary from point to point on the emitting surface.

1). Surface emitter LED can be characterized by this output pattern, which means the source is equally bright when viewed from any emission

area pattern follows:

: radiance along the normal to the radiating surface.

2). 0) 0) in the planes parallel and normal. In general,

where, L and T represents lateral power and transverse power distribution.

For edge emitter, L=1

for laser diode, L=100

POWER COUPLING: The optical power coupling of any fiber can be calculated by the symmetric source of brightness (B), area

and solid acceptance angle

Here, the fiber end face is centered over emitting surface of the source and is positioned as close as possible.

So, the power coupled is:

: upper integration limit of radiation

If the source radius is less than fiber core radius a, then ; and for sources areas larger than the fiber core area, = a.

For SLED, , so,

POWER LAUNCHING VERSUS WAVELENGTH: The power launched into a fiber depends upon the brightness of the source,

which is radiance. So a number of modes can propa

So, the radiated power per mode, , from a source at a particular wavelength is given by,

: radiance

EQUILIBRIUM NUMERICAL APERTURE: In a optical fiber setup, the losses occurs in the first few tens of meters of a

multimode system. To achieve a low coupling loss, this should be connected to a system fiber that has a identical NA and core

diameter. A certain amount of optical power is lost at the connecting mechanism of the fiber setup.

If the light emitting area of the LED is less than the cross sectional area of the fiber core, the power coupled into the fiber is NAin and

when the optical power measured in long multimode fibers after the launched mode have come to equilibrium, the effect of

equilibrium NA become apparent. At this point optical power in the fiber:

LASER DIODE TO FIBER COUPLING: In edge emitting LED, the angular output distribution of the laser is greater than the fiber

acceptance angle, and since the laser emitting area is much smaller than the fiber core, spherical or cylindrical lenses can also be used

to improve the coupling efficiency between edge cutting laser diodes and optical fibers. This is also known as vertical cavity surface

emitting laser (VCSELs).

PHOTODETECTORS:

1. PIN PHOTODETECTOR:

The most common semiconductor photo detector is the PIN photodiode.

The device structure consists of p and n regions separated by a very lightly doped n type intrinsic region. In normal operation, a large

reverse bias voltage is applied across the device so that the intrinsic region is fully depleted of carriers.

Operation: When an incident photon has energy greater than or equal to the band gap energy of the semiconductor material, the

photon can give up its energy and excite an electron from the VB to CB. The electrons and holes are called photo carriers. The photo

detector is normally designed so that these carriers are intentionally added in the depletion region, where most of the incident light is

absorbed. This gives rise to a current flow in an external circuit, with one electron flowing for every carrier pair generated. This

current is known as the photocurrent.

The charge carriers move a distance LN or LP for electrons and holes. This distance is known as diffusion length and the time taken for

an electron and hole to recombine is known as carrier lifetime ( and ). The lifetime and the diffusion length are related as:

of energy h and is given by:

: Photo current

: Incident optical power

2. AVALANCHE PHOTODIODE:

Avalanche photodiode (APD) internally multiply the primary signal photocurrent before it enters the input circuitry of the following

amplifier. This increases receiver sensitivity, since the photocurrent is multiplied before encountering the thermal noise associated

with the receiver circuit. In order for carrier multiplication to take place, the photo generated carriers must traverse a region where a

very high electric field is present. In this high field region, a photo generated electron or hole can gain enough energy so that it ionizes

bound electrons in the valance bond upon colliding time. This is known as impact ionization.

The newly created carriers are also accelerated by high electric field, thus known as avalanche effect.

The average number of electron hole pair created by a carrier/unit distance travelled is called ionization rate. The multiplication M for

all carriers generated in the photodiode is defined by:

: Multiplied carrier current

: Primary current

DETECTOR RESPONSE TIME: The response time of a photodiode together with its output circuit depends upon,

a). The transit time of the photo carriers in the depletion region. b). The diffusion time of the photo carriers generated outside the depletion region. c). The RC time constant of the photodiode.

The photodiode parameters responsible for these three factors are absorption coefficient , the depletion region width , the

photodiode junction and package capacitances, the amplifier capacitance. The transit time depends on the carrier drift velocity and

The photodiode response time to an optical input pulse is

: Rise time life time

: fall time life time

Junction capacitances:

Now, photodiode pulse responses under various detector parameters:

TEMPERATURE EFFECT ON AVALANCHE GAIN: The gain mechanism of an avalanche photodiode is very temperature

sensitive because of the dependence of the electron and hole ionization rates. This temperature dependence is particular critical at high

bias voltage, where small changes in temperature can cause large variation in gain.

To maintain a constant gain as the temperature changes, the electric filed in the multiplying region of the p-n junction must also be

changed, which adjusts the applied bias voltage on the photo detector when the temperature changes.

The temperature dependent expression for gain is:

: Breakdown voltage

: varies between 2.5 to 7, as per material

: reverse bias voltage

: multiplies photocurrent

: resistance

So, the breakdown voltage:

OPTICAL RECEIVER OPERATION:

The design of an optical receiver is much more complicated than that of an optical transmitter because the receiver must be able to

detect weak signals, distorted signals and make decisions on what type of data was send based on an amplified and reshaped version

of this distorted signal.

DIGITAL SIGNAL TRANSMISSION:

The transmitted signal is a two level binary data stream consisting of either a 0 or a 1 in a time slot of duration Tb. This time slot is

referred to as a bit period. One technique for sending binary data is amplitude shift keying (ASK) or on-off key (OOK). The resultant

signal wave thus consists of a voltage pulse of amplitude V relative to the zero voltage level when a binary 1 occurs and a zero voltage

level space when a binary 0 occurs. Depending on the coding scheme to be used a binary 1 may or may not fill the time slot Tb.

The function of the optical transmitter is to convert the electric signal to an optical signal, thus in the optical signal emerging from the

LED or laser transmitter 1 is represented by a pulse of optical power (light) of duration Tb, whereas 0 is the absence of any light.

The optical signal that is coupled from the light source to the fiber becomes attenuated and distorted as it propagates along the fiber

waveguide. Upon arriving at the end of the fiber, a receiver converts the optical signal back to an electrical format.

BASIC COMPONENTS OF AN OPTICAL RECEIVER:

As per the diagram, the first element is either a pin or an avalanche photodiode, which produces an electric current that is proportional

to the received powe