Optical Soft Copy 2014 QP

8/17/2019 Optical Soft Copy 2014 QP

1/46

11.a) i) Derive an normalized frequency of an optical fiber and explain its use

Normalized frequency.

Electromagnetic waves bound to an optical fiber are described by the fiber's

normalized frequency. The normalized frequency determines how many modes a fiber

can support. Normalized frequency is a dimensionless quantity.

Normalized frequency is also related to the fiber's cutoff wavelength. Normalizedfrequency (V) is defined as:

where n1 is the core index of refraction, n2 is the cladding index of refraction, a isthe core diameter, and λ is the wavelength of light in air.

The number of modes that can exist in a fiber is a function of V. As the value of V

increases, the number of modes supported by the fiber increases. Optical fibers, single

mode and multimode, can support a different number of modes

Single Mode Fiber (Single Mode Fiber Optic Cable):

When the fiber core is so small that only light ray at 0° incident angle can stably

pass through the length of fiber without much loss, this kind of fiber is called single mode

fiber. The basic requirement for single mode fiber is that the core be small enough torestrict transmission to a singe mode. This lowest-order mode can propagate in all fibers

with smaller cores (as long as light can physically enter the fiber).

The most common type of single mode fiber has a core diameter of 8 to 10 μmand is designed for use in the near infrared (the most common are 1310nm and 1550nm).Please note that the mode structure depends on the wavelength of the light used, so thatthis fiber actually supports a small number of additional modes at visible wavelengths.Multi mode fiber, by comparison, is manufactured with core diameters as small as 50um

and as large as hundreds of microns.The following picture shows the fiber structure of a single mode fibe


2/46

Conditions for Single Mode Transmission

To calculate the number of modes Nm in a step-index fiber, Nm can be simplified

as:

Reducing the core diameter sufficiently can limit transmission to a single mode.

The following formula defines the maximum core diameter, D, which limits transmission

to a single mode at a particular wavelength, λ :

If the core is any larger, the fiber can carry two modes.

Mode Field Diameter (MFD)

The typical core diameter of communication single mode fibers is from 8~10um

for operating wavelength 1.31um to 1.5um. Fiber with a core diameter less than about tentimes the wavelength of the propagating light cannot be modeled using geometric optics

as we did in the explanation of step-index multimode fiber. Instead, it must be analyzed

as an electromagnetic structure, by solution of Maxwell's equations as reduced to the

electromagnetic wave equation.So even though the fiber cladding confines the lightwithin the fiber core, some light does penetrate into the cladding, despite the fact that it

nominally undergoes total internal reflection. This occurs both in single mode and

multimode fibers, but this phenomenon is more significant in single mode fibers.For a

Gaussian power distribution (lasers used in communications are Gaussian power

distribution) in a single mode optical fiber, the mode field diameter (MFD) is defined as

the point at which the electric and magnetic field strengths are reduced to 1/e of their

maximum values, i.e., the diameter at which power is reduced to 1/e2 (0.135) of the peak power (because the power is proportional to the square of the field strength ). For single

mode fibers, the peak power is at the center of the core.Mode field diameter is slightly

larger than the core diameter, as shown in the following illustration.


3/46

SINGLE MODE FIBER:


4/46


5/46

11.a) ii) Discuss on transmission of light through graded index fiber

Multimode graded index fiber :

A multimode graded-index fiber has a core of radius (a). Unlike step-index fibers,the value of the refractive index of the core (n1) varies according to the radial distance (r).The value of n1 decreases as the distance (r) from the center of the fiber increases. Thevalue of n1 decreases until it approaches the value of the refractive index of the cladding(n2). The value of n1 must be higher than the value of n2 to allow for proper modepropagation. Like the step-index fiber, the value of n2 is constant and has a slightly lowervalue than the maximum value of n1. The relative refractive index difference (Δ) isdetermined using the maximum value of n1 and the value of n2.

Figure 3-3 shows a possible refractive index profile n(r) for a multimode graded-

index fiber. Notice the parabolic refractive index profile of the core. The profile

parameter

(α) determines the shape of the core's profile. As the value of &agr; increases, the

shape of the core's profile changes from a triangular shape to step as shown in figure 3-4.Most multimode graded-index fibers have a parabolic refractive index profile. Multimode


6/46

fibers with near parabolic graded-index profiles provide the best performance. Unless

otherwise specified, when discussing multimode graded-index fibers, assume that the

core's refractive index profile is parabolic (α=2).

Light propagates in multimode graded-index fibers according to refraction andtotal internal reflection. The gradual decrease in the core's refractive index from the center

of the fiber causes the light rays to be refracted many times. The light rays become

refracted or curved, which increases the angle of incidence at the next point of refraction.Total internal reflection occurs when the angle of incidence becomes larger than the

critical angle of incidence. Figure 3-5 shows the process of refraction and total internal

reflection of light in multimode graded-index fibers. Figure 3-5 also illustrates the

boundaries of different values of core refractive index by dotted lines. Light rays may be

reflected to the axis of the fiber before reaching the core-cladding interface

Figure 3-3. - The refractive index profile for multimode graded-index fibe


7/46

Figure 3-4. - The refractive index profiles for different values of &agr;.

Figure 3-5. - Refractive index grading and light propagation in multimode graded-index

fibers.

The NA of a multimode graded-index fiber is at its maximum value at the fiber

axis. This NA is the axial numerical aperture [NA(0)]. NA(0) is approximately equal

to

∆21n

However, the NA for graded-index fibers varies as a function of the radial distance

(r). NA varies because of the refractive index grading in the fiber's core. The NA

decreases from the maximum, NA(0), to zero at distances greater than the core-cladding

boundary distance (r>a). The NA, relative refractive index difference (Δ), profile

parameter (α), and normalized frequency (V) determine the number of propagating

modes in multimode graded-index fibers. A multimode graded-index fiber with the same

normalized frequency as a multimode step-index fiber will have approximately one-half

as many propagating modes. However, multimode graded-index fibers typically have overone-hundred propagating modes.

Multimode graded-index fibers accept less light than multimode step-index fiberswith the same core Δ. However, graded-index fibers usually outperform the step-

index fibers. The core's parabolic refractive index profile causes multimode graded-indexfibers to have less modal dispersion.

Figure 3-5 shows possible paths that light may take when propagating in

multimode graded-index fibers. Light rays that travel farther from the fiber's axis travel a


8/46

longer distance. Light rays that travel farther from the center travel in core material with

an average lower refractive index.

In chapter 2, you learned that light travels faster in a material with a lower

refractive index. Therefore, those light rays that travel the longer distance in the lower

refractive index parts of the core travel at a greater average velocity. This means that the

rays that travel farther from the fiber's axis will arrive at each point along the fiber atnearly the same time as the rays that travel close to the fiber's axis. The decrease in time

difference between light rays reduces modal dispersion and increases multimode graded-index fiber bandwidth. The increased bandwidth allows the use of multimode graded-

index fibers in most applications.

Most present day applications that use multimode fiber use graded-index fibers.

The basic design parameters are the fiber's core and cladding size and Δ. Standard

multimode graded-index fiber core and cladding sizes are 50/125 μm, 62.5/125

μm, 85/125 μm, and 100/140 μm. Each fiber design has a specific Δ

that improves fiber performance. Typical values of Δ are around 0.01 to 0.02.

Although no single multimode graded-index fiber design is appropriate for all

applications, the 62.5/125 μm fiber with a Δ of 0.02 offers the best overall

performance.

A multimode graded-index fiber's source-to-fiber coupling efficiency and

insensitivity to microbending and macrobending losses are its most distinguishing

characteristics. The fiber core size and Δ affect the amount of power coupled into

the core and loss caused by microbending and macrobending. Coupled power increases

with both core diameter and Δ, while bending losses increase directly with corediameter and inversely with Δ. However, while these values favor high Δs, a

smaller Δ improves fiber bandwidth.

In most applications, a multimode graded-index fiber with a core and cladding size of

62.5/125 μm offers the best combination of the following properties:

l Relatively high source-to-fiber coupling efficiency

l Low loss

l Low sensitivity to microbending and macrobending

l High bandwidth

l Expansion capacity

For example, local area network (LAN) and shipboard applications use multimode

graded-index fibers with a core and cladding size of 62.5/125 μm. In LAN-type

environments, macrobend and microbend losses are hard to predict. Cable tension, bends,and local tie-downs increase macrobend and microbend losses. In shipboard applications,

a ship's cable-way may place physical restrictions, such as tight bends, on the fiber during

cable plant installation. The good microbend and macrobend performance of 62.5/125

μm fiber permits installation of a rugged and robust cable plant. 62.5/125 μm

multimode graded-index fibers allow for uncomplicated growth because of high fiber

bandwidth capabilities for the expected short cable runs on board ships.


9/46

11) b) Explain the features of single mode and multimode step index fiber

Optical fibers are characterized by their structure and by their properties of

transmission. Basically, optical fibers are classified into two types. The first type is single

mode fibers. The second type is multimode fibers. As each name implies, optical fibers

are classified by the number of modes that propagate along the fiber. As previously

explained, the structure of the fiber can permit or restrict modes from propagating in a

fiber. The basic structural difference is the core size. Single mode fibers are manufactured

with the same materials as multimode fibers. Single mode fibers are also manufactured by

following the same fabrication process as multimode fibers.

Single Mode Fibers

o The core size of single mode fibers is small. The core size (diameter) istypically around 8 to 10 micrometers (μm).

o A fiber core of this size allows only the fundamental or lowest order mode

to propagate around a 1300 nanometer (nm) wavelength.

o Single mode fibers propagate only one mode, because the core size

approaches the operational wavelength (λ). T

o The value of the normalized frequency parameter (V) relates core size with

mode propagation.

o In single mode fibers, V is less than or equal to 2.405. When V ≤ 2.405,

single mode fibers propagate the fundamental mode down the fiber core,

while high-order modes are lost in the cladding.

o For low V values (≤1.0), most of the power is propagated in the

cladding material. Power transmitted by the cladding is easily lost at fiber

bends. The value of V should remain near the 2.405 level.

o Single mode fibers have a lower signal loss and a higher information

capacity (bandwidth) than multimode fibers. Single mode fibers are

capable of transferring higher amounts of data due to low fiber dispersion.

o Basically, dispersion is the spreading of light as light propagates along a

fiber. Dispersion mechanisms in single mode fibers are discussed in more

detail later in this chapter. Signal loss depends on the operational

wavelength (λ).

o In single mode fibers, the wavelength can increase or decrease the losses

caused by fiber bending. Single mode fibers operating at wavelengths

larger than the cutoff wavelength lose more power at fiber bends.

o They lose power because light radiates into the cladding, which is lost at

fiber bends. In general, single mode fibers are considered to be low-lossfibers, which increase system bandwidth and length.


10/46

Multimode Fibers

o As their name implies, multimode fibers propagate more than one mode.

Multimode fibers can propagate over 100 modes. The number of modespropagated depends on the core size and numerical aperture (NA).

o As the core size andNA increase, the number of modes increases. Typical

values of fiber core size and NA are 50 to 100 μm and 0.20 to 0.29,

respectively.

o A large core size and a higher NA have several advantages. Light is

launched into a multimode fiber with more ease.

o The higher NA and the larger core size make it easier to make fiber

connections. During fiber splicing, core-to-core alignment becomes less

critical. Another advantage is that multimode fibers permit the use of light

-emitting diodes (LEDs).

o Single mode fibers typically must use laser diodes. LEDs are cheaper, less

complex, and last longer. LEDs are preferred for most applications.

o Multimode fibers also have some disadvantages. As the number of modesincreases, the effect of modal dispersion increases. Modal dispersion(intermodal dispersion) means that modes arrive at the fiber end at slightlydifferent times. This time difference causes the light pulse to spread.

o Modal dispersion affects system bandwidth. Fiber manufacturers adjust thecore diameter, NA, and index profile properties of multimode fibers tomaximize system bandwidth

12)a)i) What is meant by critical bending radiation losses of optical fiber and

explain.

Optical fibers suffer 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, hich causes light energy to be

radiated from the fiber. !n illustration of this situation is shon in. The part of the mode

hich is on the outside of the bend is re"uired to travel faster than that on the inside so

the ave front perpendicular to the direction of propagation is maintained. #ence, part of

the mode in the cladding needs to travel faster than the velocity of light in that medium.!s this is not possible, the energy associated ith this part of the mode is lost through

radiation. The loss can generally be represented by a radiation attenuation coefficient

hich has the form


11/46

here R is the radius of curvature of the fiber bend and c1, c2 are constants hich are

independent of R. $urthermore, large bending losses tend to occur in multimode fibers at

a critical radius of curvature Rc

%t may be observed from the expression given in &". '(.8) that potential macro bending

losses may be reduced by*

'a) designing fibers ith large relative refractive index differences+

'b) operating at the shortest avelength possible.

The above criteria for the reduction of bend losses also apply to singlemode fibers. One

theory based on the concept of a single "uasiguided mode, provides an expression from

hich the critical radius of curvature for a singlemode fiber Rcs can be estimated as*

here -c is the cutoff avelength for the singlemode fiber.

#ence again, for a specific singlemode fiber 'i.e. a fixed relative index difference and

cutoff avelength), the critical avelength of the radiated light becomes progressively

shorter as the bend radius is decreased.


12/46

12.a) ii) Explain the followin in the sinle mode fiber ! "odel #irefrinence

"odel #irefrinence

inglemode fibers ith nominal circular symmetry about the core axis allo the

propagationof to nearly degenerate modes ith orthogonal polari/ations. They are

thereforebimodal supporting #& x11 and #e y 11 modes here the principal axes x and y

are determinedby the symmetry elements of the fiber cross section. #ence in an opticalfiber ith an idealoptically circularly symmetric core both polari/ation modes propagate

ith identicalvelocities. anufactured optical fibers, hoever, exhibit some birefringence

resultingfrom differences in the core geometry 'i.e. ellipticity) resulting from variations intheinternal and external stresses, and fiber bending. The fiber therefore behaves as a

birefringentmedium due to the difference in the effective refractive indices, and hence

phasevelocities, for these to orthogonally polari/ed modes. The modes therefore have

differentpropagation constants x and y hich are dictated by the anisotropy of the fiber crosssection. %n this case x and y are the propagation constants for the slo mode and

the fastmode respectively. hen the fiber crosssection is independent of the fiber length

L in the z direction, then the modal birefringence B$ for the fiber is given by

here - is the optical avelength. 3ight polari/ed along one of the principal axes ill

retain its polari/ation for all L. The difference in phase velocities causes the fiber toexhibit a linear retardation 4' z ) hich depends on the fiber length L in the z direction and

is given by

assuming that the phase coherence of the to mode components is maintained. The phase

coherence of the to mode components is achieved hen the delay beteen the totransit times is less than the coherence time of the source. !s indicated in ection (.11,

the coherence time for the source is e"ual to the reciprocal of the uncorrelated source

fre"uency idth '156 f ).%t may be shon that birefringent coherence is maintained over alength of fiber Lbc 'i.e. coherence length) hen*


13/46

here c is the velocity of light in a vacuum and is the source line idth


14/46

!n illustration of the beat length in a singlemode optical fiber 'a) the polari/ation states against 4' z )+ 'b)

the light intensity distribution over the beat length ithin the fiber

#oever, hen phase coherence is maintained 'i.e. over the coherence length) &"u leadsto a polari/ation state hich is generally elliptical but hich varies periodically along the

fiber. This situation is illustrated in $igure (.28'ahere the incident linear polari/ation

hich is at 79 ith respect to the x axis becomes circular polari/ation at 4 :;52 and

linear again at 4 :;. The process continues through another circular polari/ation at 4 :(;52 before returning to the initial linear polari/ation at 4 : 2;. The characteristic length

L< for this process corresponding to the propagation distance for hich a 2; phase

difference accumulates beteen the to modes is =non as the beat length. %t is given by*

ubstituting for B$ from &". '(.7>) gives


15/46

Typical singlemode fibers are found to have beat lengths of a fe centimeters andthe effect may be observed directly ithin a fiber via ?ayleigh scattering ith use of a

suitable visible source 'e.g. #e@Ae laser)%t appears as a series of bright and dar= bands

ith a period corresponding to the beat length, as shon in $igure The modal birefringence B$ may be determined from these observations of beat length.

12.b) i) Describe the three types of fiber misalinment that contribute to insertion

loss at an optical fiber $oint

! maBor consideration ith all types of fiber@fiber connection is the optical lossencountered at the interface. &ven hen the to Bointed fiber ends are smooth and

perpendicular to the fiber axes, and the to fiber axes are perfectly aligned, a small

proportion of the light may be reflected bac= into the transmitting fiber causingattenuation at the Boint. This phenomenon, =non as $resnel reflection, is associated ith

the step changes in refractive index at the Bointed interface 'i.e. glass@air@glass). The

magnitude of this partial reflection of the light transmitted through the interface may beestimated using the classical $resnel formula for light of normal incidence and is given by

here r is the fraction of the light reflected at a single interface, n1 is the refractive index

of the fiber core and n is the refractive index of the medium beteen the to Bointedfibers 'i.e. for air

n : 1). #oever, in order to determine the amount of light reflected at a fiber Boint,

$resnel reflection at both fiber interfaces must be ta=en into account. The loss in decibelsdue to $resnel reflection at a single interface is given by*

#ence, using the relationships given in &"s '.1) and '.2) it is possible to determinethe optical attenuation due to $resnel reflection at a fiber@fiber Boint %t is apparent that

$resnel reflection may give a significant loss at a fiber Boint even hen all other aspects of

the connection are ideal. #oever, the effect of $resnel reflection at a fiber@fiber

connection can be reduced to a very lo level through the use of an indexmatching fluid


16/46

in the gap beteen the Bointed fibers. hen the indexmatching fluid has the same

refractive index as the fiber core, losses due to $resnel reflection are in theory eradicated.

Cnfortunately, $resnel reflection is only one possible source of optical loss at a fiber Boint.! potentially greater source of loss at a fiber@fiber connection is caused by misalignment

of the to Bointed fibers. %n order to appreciate the development and relative success of

various connection techni"ues it is useful to discuss fiber alignment in greater detail. !nydeviations in the geometrical and optical parameters of the to optical fibers hich are

Bointed ill affect the optical attenuation 'insertion loss) through the connection. %t is not

possible ithin any particular connection techni"ue to allo for all these variations.#ence, there are inherent connection problems hen Bointing fibers ith, for instance*

The three possible types of misalignment hich may occur hen Bointing compatible

optical fibers 'a) longitudinal misalignment+ 'b) lateral misalignment+ 'c) angularmisalignment

'a) different core and5or cladding diameters+'b) different numerical apertures and5or relative refractive index differences+'c) different refractive index profiles+

'd) fiber faults 'core ellipticity, core concentricity, etc.).

The losses caused by the above factors together ith those of $resnel reflection are

usually referred to as intrinsic Boint losses. The best results are therefore achieved ith

compatible 'same) fibers hich are manufactured to the loest tolerance. %n this casethere is still the problem of the "uality of the fiber alignment provided by the Bointing

mechanism. &xamples of possible misalignment beteen coupled compatible optical

fibers are illustrated in $igure 1 %t is apparent that misalignment may occur in three

dimensions* the separation beteen the fibers 'longitudinal misalignment), the offset perpendicular to the fiber core axes 'lateral5radial5 axial misalignment) and the angle

beteen the core axes 'angular misalignment). Optical losses resulting from these three

types of misalignment depend upon the fiber type, core diameter and the distribution of the optical poer beteen the propagating modes. &xamples of the measured optical

losses due to the various types of misalignment are shon in $igure 2. $igure 2'a) shos

the attenuation characteristic for both longitudinal and lateral misalignment of a graded


17/46

index fiber of 0 μm core diameter. %t may be observed that the lateral misalignment gives

significantly greater losses per unit displacement than the longitudinal misalignment.

$or instance, in this case a lateral displacement of 10 μm gives about 1 d< insertion loss

hereas a similar longitudinal displacement gives an insertion loss of around 0.1 d


18/46

Figure 2.Insertion loss characteristics for jointed optical bers with various types ofmisalignment: (a) insertion loss due to lateral and longitudinal misalignment for a

graded inde ber of !" m core diameter.

12. b) ii) %utline the ma$or cateories of multiport fiber optic coupler

!n optical fiber coupler is a device that distributes light from a main fiber into oneor more branch fibers. The latter case is more normal and such devices are =non as

multiport fiber couplers. ?e"uirements are increasing for the use of these devices to

divide or combine optical signals for application ithin optical fiber informationdistribution systems including data buses, 3!As, computer netor=s and

telecommunication access netor=s

$igure 1.Flassification of optical fiber couplers* 'a) core interaction type+ 'b) surface

interaction type

Optical fiber couplers are often passive devices in hich the poer transfer ta=es

place either*

'a) Through the fiber core crosssection by butt Bointing the fibers or by using some form

of imaging optics beteen the fibers 'core interaction type)+ or

'b) Through the fiber surface and normal to its axis by converting the guided core modesto both cladding and refracted modes hich then enable the poersharing mechanism

'surface interaction type).

The mechanisms associated ith these to broad categories are illustrated in

$igure 1 !ctive aveguide directional couplers are also available hich are reali/ed using

integrated optical fabrication techni"ues. uch device types, hoever, are dealt and thus

in this section the discussion is restricted to the above passive coupling strategies.ultiport optical fiber couplers can also be subdivided into the folloing three main

groups as illustrated in $igure 2.


19/46

1. Three and fourportG couplers, hich are used for signal splitting, distribution and

combining.2. tar couplers, hich are generally used for distributing a single input signal to multiple

outputs.

(. avelength division multiplexing 'H) devices, hich are a speciali/ed form of coupler designed to permit a number of different pea= avelength optical signals to be

transmitted in parallel on a single fiber %n this context H couplers either combine the

different avelength optical signal onto the fiber 'i.e. multiplex) or separate the differentavelength optical signals output from the fiber 'i.e. demultiplex).

%deal fiber couplers should distribute light among the branch fibers ith no

scattering lossI or the generation of noise, and they should function ith completeinsensitivity to factors including the distribution of light beteen the fiber modes, as ell

as the state of polari/ation of the light. Cnfortunately, in practice passive fiber couplers do

not display all of the above properties and hence the characteristics of the devices affect

the performance of optical fiber netor=s. %n particular, the finite scattering loss at thecoupler limits the number of terminals that can be connected, or alternatively the span of

the netor=, hereasthe generation of noise and modal effects can cause problems in the specification of the

netor= performance. #ence, couplers in a netor= cannot usually be treated as

individual components ith =non parameters, a factor hich necessitates certaincompromises in their application. %n this section, therefore, a selection of the more

common fiber coupler types is described in relation to the coupling mechanisms, their

performance and limitations

&hree' and four'port couplers

everal methods are employed to fabricate three and fourport optical fiber couplers The lateral offset method, illustrated in $igure ('a), relies on the overlapping of

the fiber end faces. 3ight from the input fiber is coupled to the output fibers according to

the degree of overlap. #ence the input poer can be distributed in a ell defined proportion by appropriate control of the amount of lateral offset beteen the fibers.

This techni"ue, hich can provide a bidirectional coupling capability, is ell

suited for use ith multimode step index fibers but may incur higher excess losses thanother methods as all the input light cannot be coupled into the output fibers.

!nother coupling techni"ue is to incorporate a beam splitter element beteen the

fibers. The semitransparent mirror method provides an ingenious ay to accomplish sucha fiber coupler, as shon in $igure ('b).

! partially reflecting surface can be applied directly to the fiber end face cut at anangle of 79 to form a thinfilm beam splitter. The input poer may be split in any desired

ratio beteen the reflected and transmitted beams depending upon the properties of the

intervening mirror, and typical excess losses for the device lie in the range 1 to 2 d


20/46

mode fibers have been fabricated .%n addition, ith suitable avelengthselective

interference coatings this coupler type can form a H device .! fastgroing category

of optical fiber coupler is based on the use of microoptic components. %n particular, acomplete range of couplers has been developed hich utili/e the beam expansion and

collimation properties of the J?%Arod lens combined ith spherical retroreflecting

mirrors These devices, to of hich are displayed in $igure 7, are miniature opticalassemblies of compact construction hich generally exhibit lo insertion loss 'typically

less than 1 d


21/46

$igure 2* Optical fiber coupler types and functions* 'a) threeport couplers+'b) fourport coupler+ 'c) star coupler+ 'd) avelength division multiplexing and

demultiplexing couplers


22/46

3ight rays from the input fiber F 1 collimate in the first lens before they are

incident

on the mirror. ! portion of the incident beam is reflected bac= and is coupled to fiber F 2,hile the transmitted light is focused in the second lens and then coupled to fiber F (. The

slant surface version of the similar coupler is shon in $igure 7'b).

The parallel surface type, hoever, is the most attractive due to its ease of

fabrication, compactness, simplicity and relatively lo insertion loss. $inally, the

substitution of the mirror by an interference filterG offers application of these devices toH Kerhaps the most common method for manufacturing couplers is the fused

biconical taper '$


23/46

$igure 7. J?%Arod lens microoptic fiber couplers* 'a) parallel surface type+n 'b) slant

surface type

$igure .tructure and principle of operation for the fiber fused biconical taper coupler

The higher order modes, hoever, leave the fiber core because of its reduced si/ein the tapereddon region and are therefore guided as cladding modes. These modes

transfer bac= to guided core modes in the taperedup region of the output fiber ith an

approximately even distribution beteen the to fibers. Often only a portion of the total


24/46

poer is coupled beteen the to fibers because only the higher order modes ta=e part in

the process, the loer order modes generally remaining ithin the main fiber.

%n this case a modedependent 'and therefore avelengthdependent) coupling

ratio is obtained. #oever, hen the aist of the taper is made sufficiently narro, then

the entire mode volume can be encouraged to participate in the coupling process and alarger proportion of input poer can be shared beteen the output fibers. This strategy

gives an improvement in both the poer and modal uniformity of the coupler.

The various loss parameters associated ith fourport couplers may be rittendon ith reference to $igure . #ence, the excess loss hich is defined as the ratio of

poer

input to poer output is given by*

The insertion loss, hoever, is generally defined as the loss obtained for a

particular portto port optical path

The crosstal= hich provides a measure of the directional isolationI achieved by

the

device is the ratio of the bac=scattered poer received at the second input port to the input

poer hich may be ritten as*

$inally, the splitting or coupling ratio indicates the percentage division of optical

poer

beteen the output ports


25/46

1(.a.i) Hescribe the operation of inBection laser

(aser modes

The typical output spectrum for a broadarea inBection laser is shon in $igure

L.22'a). %t does not consist of a single avelength output but a series of avelength pea=s corresponding to different longitudinal 'in the plane of the Bunction, along the

optical cavity) modes ithin the structure., the spacing of these modes is dependent on

the optical cavity length as each one corresponds to an integral number of lengths. They

are generally separated by a fe tenths of a nanometer, and the laser is said to be a

multimode device. #oever, also indicates some broadening of the longitudinal mode

pea=s due to sub pea=s caused by higher order hori/ontal transverse modes.G These

higher order lateral modes may exist in the broadarea device due to the unrestricted

idth of the active region. The correct stripe geometry inhibits the occurrence of the

higher order lateral modes by limiting the idth of the optical cavity, leaving only a

single lateral mode hich gives the output spectrum shon in $igure 1'b) here only

the longitudinal modes may be observed. This represents the typical output spectrum for

a good multimode inBection laser.

$or singlemode operation, the optical output from a laser must contain only a

single longitudinal and single transverse mode. #ence the spectral idth of the emission

from the singlemode device is far smaller than the broadened transition lineidth

discussed in ection L.2.7. %t as indicated that an inhomogeneously broadened laser

can support a number of longitudinal and transverse modes simultaneously, giving a

multimode output. ingle transverse mode operation, hoever, may be obtained by

reducing the aperture of the resonant cavity such that only the T&00 mode is

supported. To obtain singlemode operation it is then necessary to eliminate all but oneof the longitudinal modes.

One method of achieving single longitudinal mode operation is to reduce thelength L of the cavity until the fre"uency separation of the adBacent modes given by &".

as 6 f : c52nL is larger than the laser transition lineidth or gain curve. Then only the

single mode hich falls ithin the transition lineidth can oscillate ithin the laser

cavity. #oever, it is clear that rigid control of the cavity parameters is essential to provide the mode stabili/ation necessary to achieve and maintain this singlemode


26/46

operation.

The structures re"uired to give mode stability are discussed ith regard to the

multi mode inBection laser and similar techni"ues can be employed to produce a laser

emitting a single longitudinal and transverse mode. $or example, the correct H

structure ill restrict the vertical idth of the aveguiding region to less than 0.7

μm alloing only the fundamental transverse mode to be supported and removing anyinterference of the higher order transverse modes on the emitted longitudinal modes.

The lateral modes 'in the plane of the Bunction) may be confined by the

restrictions on the current flo provided by the stripe geometry. %n general, only the

loer order modes are excited, hich appear as satellites to each of the longitudinal

modes. #oever, as ill be discussed, stripe contact devices often have instabilities and

strong nonlinearities 'e.g. =in=s) in their light output against current characteristics.

Tight current confinement as ell as good aveguiding are therefore essential in order

to achieve only the re"uired longitudinal modes hich form beteen the mirror facets

in the plane of the Bunction. $inally, as indicated above, singlemode operation may be

obtained through con trol of the optical cavity length such that only a single

longitudinal mode falls ithin the gain bandidth of the device. $igure 1 shos a

typical output spectrum for a single mode device.#oever, inBection lasers ith short cavity lengths 'around 0 μm) are

difficult to handle and have not been particularly successful. Aevertheless, suchdevices, together

ith the maBor alternative structures hich provide singlemode operation, are dealt

ith in under the title of singlefre"uency inBection lasers.

$igure 1. Typical single longitudinal mode output spectrum from a singlemode inBection laser


27/46

13.a)ii) Compare the optical sources LED and ILD

Light-emitting diodes

A light-emitting diode (LED) is a semiconductor device that emits incoherent

light, through spontaneous emission, when a current is passed through it. Typically LEDsfor the 850-nm region are fabricated using GaAs and AlGaAs. LEDs for the 1300-nm and

1550-nm regions are fabricated using InGaAsP and InP.

The basic LED types used for fiber optic communication systems are the surface-

emitting LED (SLED), the edge-emitting LED (ELED), and the superluminescent diode(SLD). LED performance differences help link designers decide which device is

appropriate for the intended application. For short-distance (0 to 3 km), low-data-rate

fiber optic systems, SLEDs and ELEDs are the preferred optical source. Typically,

SLEDs operate efficiently for bit rates up to 250 megabits per second (Mb/s). Because

SLEDs emit light over a wide area (wide far-field angle), they are almost exclusively used

in multimode systems. For medium-distance, medium-data-rate systems, ELEDs arepreferred.

E"*+%,D-+&% *(D!

Many types of materials including gas, liquid, and semiconductors can form thelasing medium. However, in this chapter we only discuss semiconductor laser diodes.

Semiconductor laser diodes are the primary lasers used in fiber optics. A laser diode emitslight that is highly monochromatic and very directional. This means that the LD's output

has a narrow spectral width and small output beam angle.

A semiconductor LD's geometry is similar to an ELED with light-guiding regions

surrounding the active region. Optical feedback is established by making the front facet

partially reflective. This chapter provides no diagram detailing LD structures because theyare similar to ELEDs in design. The rear facet is typically coated with a reflective layer so

that all of the light striking the facet is reflected back into the active region. The frontfacet is typically left uncoated so that most of the light is emitted. By increasing the drive

current, the diode becomes a laser currents below the threshold current, LDs function asELEDs.

To optimize Frequency response, laser diodes are often biased above this laser

threshold. As a result, in an LD fiber optic system, light is modulated between a high

power level and a lower power level, but never shut off. LDs typically can be modulated.

at frequencies up to over 2 gigahertz (GHz). Some lasers are capable of being modulatedat frequencies over 20 GHz.

There are several important differences between LDs and LEDs. One is that LEDs

usually lack reflective facets and in some cases are designed to suppress reflections back

into the active region. Another is that lasers tend to operate at higher drive currents to


28/46

produce light. A higher driver current results in more complicated drive circuits and more

heat dissipation in the device.

LDs are also much more temperature sensitive than either SLEDs or ELEDs.

Increases in the laser temperature significantly reduce laser output power. Increases in

laser temperature beyond certain limits result in the loss of lasing. When lasers are used inmany applications, the temperature of the laser must be controlled. Typically, electronic

coolers, called thermo-electric (TE) coolers, are used to cool LDs in system application

1/.b)i) 0hat are the possible noise sources that contribute the photo detector noise.

The overall sensitivity of a photodiode results from the random current and voltage

fluctu ations hich occur at the device output terminals in both the presence and

absence of an incident optical signal. !lthough the factors that determine the sensitivity

of the optical receiver are dealt ith in Fhapter M, it is appropriate at this stage to

consider the sources of noise that arise ithin photodiodes, hich do not have an

internal gain mechanism. The photodiode dar= current mentioned in ection 8.8.2corresponds to the level of the output photocurrent hen there is no intended optical

signal present. #oever, there may be some photo generated current present due to

bac=ground radiation entering the device.

The inherent dar= current can be minimi/ed through the use of high"uality, defect

free material hich reduces the number of carriers generated in the depletion region as

ell as those hich diffuse into this layer from the pN and nregions. oreover, the

surface cur rents can be minimi/ed by careful fabrication and surface passivation such

that the surface state and impurity ion concentrations are reduced. Aevertheless, it is the

case that the detector average current $ alays exhibits a random fluctuation about its

mean value as a result of the statistical nature of the "uantum detection process 'seeThis fluctuation is exhibited as shot noise here the mean s"uare current variation i2 is

proportional to $ and the photodiode received bandidth B. Thus the rms value of this

shot noise current is*

Parious figures of merit have traditionally been employed to assess the noise

performance of optical detectors. !lthough these parameters are not alays appropriate

for the evalu ation of the highspeed photodiodes used in optical fiber

communications, it is instructive to define those most commonly utili/ed. These are* thenoise e"uivalent poer ' NEP )+ the detectivity ' D)+ and the specific detectivity ' DG).

The NEP is defined as the incident optical poer, at a particular avelength or ith

a specified spectral content, re"uired to produce a photodetector current e"ual to the

rms noise current ithin a unit bandidth 'i.e. B : 1 #/). To obtain an expression for

the NEP at a specific avelength, &". '8.8) must be rearranged as follos to give*

s


29/46

Then putting the photocurrent I p e"ual to the rms shot noise current in &"

oreover, the photodiode average current F may be represented by ' I p+ I d) here I d is thedar= current ithin the device. #ence*


30/46

The specific detectivity D* is a parameter hich incorporates the area of the photodetector A in order to ta=e account of the effect of this factor on the amplitude of the

device dar= current. This proves necessary hen bac=ground radiation and thermal

generation rather than surface conduction are the maBor causes of dar= current. Therefore

the specific detectivity is given by*

1/.b.ii) 0hat is meant by detector response time.


31/46

PHOTODETECTOR RESPONSE TIME:

• The response time of a photodetector with its output circuit depends mainly on the

following three factors:1- The transit time of the photocarriers in the depletion region. The transit time

depends on thet carrier drift velocity and the depletion layver width w, and is

given by:d d

t d = w

vd

2- Diffusion time of photocarriers outside depletion region.

3- RC time constant of the circuit. The circuit after the photodetector acts like RC low pass filter with a passband given by:

B =1

2π RT C T

RT = Rs || R L and C T = C a + C d

14.a) Draw the block diagram of optical receiver and explain

A fiber optic transmitter is an electro-optic device capable of accepting electrical

signals, converting them into optical signals, and launching the optical signals into an

optical fiber. The optical signals propagating in the fiber become weakened and distortedbecause of scattering, absorption, and dispersion. The fiber optic device responsible for

converting the weakened and distorted optical signal back to an electrical signal is a fiberoptic receiver.

A fiber optic receiver is an electro-optic device that accepts optical signals from

an optical fiber and converts them into electrical signals. A typical fiber optic receiver

consists of an optical detector, a low-noise amplifier, and other circuitry used to produce

the output electrical signal (see figure 7-1). The optical detector converts the incomingoptical signal into an electrical signal. The amplifier then amplifies the electrical signal to

a level suitable for further signal processing. The type of other circuitry contained within

the receiver depends on what type of modulation is used and the receiver electrical output

requirements.


32/46

Block diagram of a typical fiber optic receiver.

Receiver spectral response, sensitivity, Frequency response, and dynamic range

are key receiver performance parameters that can affect overall system operation. The

choice of optical detector materials and structures determines the spectral response.

Silicon (Si), gallium arsenide (GaAs), and gallium aluminum arsenide (GaAlAs) are


33/46

typical detector materials used for receiver operation in the 850-nm wavelength region.

germanium (Ge), indium phosphide (InP), and indium gallium arsenide (InGaAs) are

examples of detector materials used for receiver operation in the 1300-nm and 1550-nm

wavelength regions.

The receiver sensitivity is the minimum amount of optical power required to

achieve a specific receiver performance.

$or digital transmission at a given data rate and coding, this performance isdescribed by a maximum biterror rate '


34/46

The ray path for a meridional ray launched into an optical fiber in air at an input angleless than the acceptance angle for the fiber

Fonsidering the rightangled triangle ABC indicated in $igure

here S is greater than the critical angle at the core@cladding interface. #ence &".

becomes*

Csing the trigonometrical relationship sin2 S cos2 S : 1,

hen the limiting case for total internal reflection is considered, S becomes e"ual to the

critical angle for the core@cladding interface and is given by &". !lso in this limiting

case Q1 becomes the acceptance angle for the fiber Qa

!part from relating the acceptance angle to the refractive indices, serves as

the basis for the definition of the important optical fiber parameter, the numerical

aperture ' NA). #ence the NA is defined as*


35/46

ince the NA is often used ith the fiber in air here n0 is unity, it is simplye"ual to sin Qa. %t may also be noted that incident meridional rays over the range 0 Q1

Qa ill be propagated ithin the fiber. The NA may also be given in terms of the relative

refractive index difference U beteen the core and the cladding hich is defined as*G

The relationships given in &"s for the numerical aperture are a very useful measure of the

lightcollecting ability of a fiber. They are independent of the fiber core diameter and illhold for diameters as small as 8 μm. #oever, for smaller diameters they brea= don as

the geometric optics approach is invalid. This is because the ray theory model is only a

partial description of the character of light. %t describes the direction a plane ave

component ta=es in the fiber but does not ta=e into account interference beteen suchcomponents. hen interference phenomena are considered it is found that only rays ith

certain discrete characteristics propagate in the fiber core. Thus the fiber ill only

support a discrete number of guided modes. This becomes critical in smallcore diameter fibers hich only support one or a fe modes. #ence electromagnetic mode theory must

be applied in these cases

"easurement of efractive index profile

To consider the propagation of light ithin an optical fiber utili/ing the ray theory

model it is necessary to ta=e account of the refractive index of the dielectric medium. Therefractive index of a medium is defined as the ratio of the velocity of light in a vacuum to

the velocity of light in the medium. ! ray of light travels more sloly in an optically

dense medium than in one that is less dense, and the refractive index gives a measure of this effect. hen a ray is incident on the interface beteen to dielectrics of differing

refractive indices 'e.g. glass@air), refraction occurs, as illustrated in $igure.


36/46

%t may be observed that the ray approaching the interface is propagating in a

dielectric of refractive index n1 and is at an angle S1 to the normal at the surface of the

interface. %f the dielectric on the other side of the interface has a refractive index n2hich is less than n1, then the refraction is such that the ray path in this loer index

medium is at an angle S2 to the normal, here S2 is greater than S1. The angles of

incidence S1 and refraction S2 are related to each other and to the refractive indices of the dielectrics by nellRs la of refraction hich states that


37/46

FIGURE 2

%t may also be observed in $igure 2.'a) that a small amount of light is reflected

bac= into the originating dielectric medium 'partial internal reflection). !s n1 is greater than n2, the angle of refraction is alays greater than the angle of incidence. Thus hen

the angle of refraction is M09 and the refracted ray emerges parallel to the interface

beteen the dielectrics, the angle of incidence must be less than M09. This is the limiting

case of refraction and the angle of incidence is no =non as the critical angle Sc, asshon in $igure 2.'b). $rom &". the value of the critical angle is given by* !t angles of

incidence greater than the critical angle the light is reflected bac= into the originating

dielectric medium 'total internal reflection) ith high efficiency 'around MM.MV).


38/46

#ence, it may be observed in $igure 2.'c) that total internal reflection occurs at

the interface beteen to dielectrics of differing refractive indices hen light is incident

on the dielectric of loer index from the dielectric of higher index, and the angle of incidence of the ray exceeds the critical value. This is the mechanism by hich light at a

sufficiently shallo angle 'less than M09 N Sc) may be considered to propagate don an

optical fiber ith lo loss. $igure( illustrates the transmission of a light ray in an opticalfiber via a series of total internal reflections at the interface of the silica core and the

slightly loer refractive index silica cladding. The ray has an angle of incidence S at the

interface hich is greater than the critical angle and is reflected at the same angle to thenormal.

The light ray shon in $igure ( is =non as a meridional ray as it passes through

the axis of the fiber core. This type of ray is the simplest to describe and is generally usedhen illustrating the fundamental transmission properties of optical fibers. %t must also

be noted that the light transmission illustrated in $igure ( assumes a perfect fiber, andthat any discontinuities or imperfections at the core@cladding interface ould probablyresult in refraction rather than total internal reflection, ith the subse"uent loss of the

light ray into the cladding.

#cceptance angle

#aving considered the propagation of light in an optical fiber through total

internal reflection at the core@cladding interface, it is useful to enlarge upon thegeometric optics approach ith reference to light rays entering the fiber. ince only rays

ith a sufficiently shallo gra/ing angle 'i.e. ith an angle to the normal greater than Sc)

at the core@cladding interface are transmitted by total internal reflection, it is clear thatnot all rays entering the fiber core ill continue to be propagated don its length.

The geometry concerned ith launching a light ray into an optical fiber is shon

in $igure 7, hich illustrates a meridional ray A at the critical angle Sc ithin the fiber atthe core@cladding interface. %t may be observed that this ray enters the fiber core at an

angle Qa to the fiber axis and is refracted at the air@core interface before transmission to

the core@cladding interface at the critical angle. #ence, any rays hich are incident into

the fiber core at an angle greater than Qa ill be transmitted to the core@claddinginterface at an angle less than Sc, and ill not be totally internally reflected.


39/46

This situation is also illustrated in $igure 7, here the incident ray B at an angle

greater than Qa is refracted into the cladding and eventually lost by radiation. Thus for

rays to be transmitted by total internal reflection ithin the fiber core they must beincident on the fiber core ithin an acceptance cone defined by the conical half angle Qa.

#ence Qa is the maximum angle to the axis at hich light may enter the fiber in order to

be propagated, and is often referred to as the acceptance angleG for the fiber.

%f the fiber has a regular crosssection 'i.e. the core@cladding interfaces are parallel and there are no discontinuities) an incident meridional ray at greater than the

critical angle ill continue to be reflected and ill be transmitted through the fiber. $rom

symmetry considerations it may be noted that the output angle to the axis ill be e"ual tothe input angle for the ray, assuming the ray emerges into a medium of the same

refractive index from hich it as input.

13.a)1.Draw the bloc4 diaram of %&D .explain the measurement of any two fiber

optic measurements

$ptical time domain re%ectometry 5 %&D6! measurement techni"ue hich is far more sophisticated and hich finds ide

application in both the laboratory and the field is the use of optical time domainreflectometry 'OTH?). This techni"ue is often called the bac=scatter measurement

method. %t provides measurement of the attenuation on an optical lin= don its entire

length giving information on the length dependence of the lin= loss. %n this sense it is

superior to the optical attenuation measurement methods discussed previously hichonly tend to provide an averaged loss over the hole length measured in d< =mN1. hen

the attenuation on the lin= varies ith length, the averaged loss information is

inade"uate.

OTH? also allos splice and connector losses to be evaluated as ell as the

rotation of any faults on the lin=. %t relies upon the measurement and analysis of thefraction of light hich is reflected bac= ithin the fiberRs numerical aperture due to


40/46

?ayleigh scattering #ence the bac=scattering method, hich as first described by


41/46

bac=scatter measurement method is shon in $igure ! light pulse is launched into the

fiber in the forard direction from an inBection laser using either a directional coupler or

a system of external lenses ith a beam splitter 'usually only in the laboratory). The bac=scattered light is detected using an avalanche photodiode receiver hich drives an

integrator in order to improve the received signalto noise ratio by giving an arithmetic

average over a number of measurements ta=en at one point ithin the fiber. This isnecessary as the received optical signal poer from a particular point along the fiber

length is at a very lo level compared ith the forard poer at that point by some 7 to

L0 d< and is also samped ith noise. The signal from the integrator is fed through alogarithmic amplifier and averaged measurements for successive points ithin the fiber

are plotted on a chart recorder.

This provides locationdependent attenuation values hich give an overall picture

of the optical loss don the lin=. ! possible bac=scatter plot is illustrated in $igurehich shos the initial pulse caused by reflection and bac=scatter from the input coupler

folloed by a long tail caused by the distributed ?ayleigh scattering from the input pulse

as it travels don the lin=. !lso shon in the plot is a pulse corresponding to the discrete

reflection from a fiber Boint, as ell as a discontinuity due to excessive loss at a fiber imperfection or fault. The end of the fiber lin= is indicated by a pulse corresponding to

the $resnel reflection incurred at the output end face of the fiber. uch a plot yields theattenuation per unit length for the fiber by simply computing the slope of the curve over

the length re"uired. !lso the location and insertion losses of Boints and5or faults can be

obtained from the poer drop at their respective positions on the lin=. $inally the overalllin= length can be determined from the time difference beteen reflections from the fiber

input and output end faces. tandard methods for these measurements are covered in

T%!5&%!7M to L1 D?efs M2@M7E and they provide very poerful techni"ues for field

measurements on optical fiber lin=s. %n addition, the measurement of splice or connector loss and the measurement of splice or connector return loss utili/ing OTH?

13.b.Discuss the followin .

0D" ,etwor4s wth -ltra 7ih +apacity ,etwor4s


42/46

0avelenth division multiplexed networ4s

Optical fiber netor=s using avelength division multiplexing 'H)

techni"ues can be classified as either broadcastandselect netor=s or avelength

routing netor=s. ! broadcastandselect netor= strategy based on a star coupler isshon in $igure 1.The optical transmission is broadcast to all other nodes using fixed

transmitters and a tunable receiver at the destination node extracts the desired signal from

the entire group of avelength multiplexed transmitted signals %t should be noted that alltransmissions are broadcast to all netor= nodes and hence most of the transmitted poer

is depleted on the receivers hich do not use it. Fonse"uently, as the number of nodes

increases, each station receives a small fraction of the overall transmitted poer.

!lternatively, a avelength routing netor= can be used to avoid this astage of transmitted poer here each node ithin the netor= is provided ith restricted

connection's) to the receiver's). %n avelength routing, instead of distributing the

message over the entire netor=, the signal is routed to the specific destination through

either a single node or using multiple nodes.

Figure &.'roadcastandselect networ

The concept of avelength routing is illustrated in $igure here the physical

bidirectional interconnections beteen five nodes 'i.e. !,


43/46

-(.


44/46


45/46


46/46

Documents

Optical Soft Copy 2014 QP