MAHALAKSHMImahalakshmiengineeringcollege.com/pdf/ece/VIIsem/EC2402...EC 2402 OPTICAL COMMUNICATION & NETWORKS – IV/VII - V.SENTHAMIZH SELVAN ASST. PROF/ECE in a given time slot,

MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI - 621213

DEPARTMENT : ECE SUBJECT NAME : OPTICAL COMMUNICATION & NETWORKS SUBJECT CODE : EC 2402 UNIT – IV: FIBER OPTIC RECEIVER AND MEASUREMENT

PART -A (2 Marks)

1. What are requirements of an optical receiver ?[AUC NOV 2006]

• Light detector

• Pre amplifier

• Equalizer

• Signal discriminator circuits

2. List out various error sources? [AUC MAY 2013/NOV 2012]

• Quantum noise

• Bulk dark current noise

• Surface leakage current noise

• Thermal noise

• Amplifier noise

3. Why do we prefer trans-impedance pre amplifier rather than high impedance preamplifier? [AUC MAY 2007]

• Since the high impedance produces large input RC time constant, the front end

bandwidth is less than the signal bandwidth. This drawback is overcome in the trans-impedance amplifier.

4. Define threshold level. [AUC NOV 2009]

• A decision circuit compares the signal in each time slot with a certain reference voltage known as threshold level.

5. Define quantum limit? [AUC MAY 2013]

• It is possible to find the minimum received optical power required for a specific bit error rate performance in a digital system. This minimum received power level is known as quantum limit.

EC 2402 OPTICAL COMMUNICATION & NETWORKS – IV/VII - V.SENTHAMIZH SELVAN ASST. PROF/ECE

6. What are the methods used to measure the fiber refractive index profile? [AUC MAY 2012]

• Interferometric method • Near field method • Refracted near field method

7. Define dark current. [AUC NOV 2012]

• It is the current to flow through thr bias current of the device when no light is incident on photo diode.

8. What are the advantages of preamplifier [AUC NOV 2011]

• Low noise level

• High bandwidth

• High dynamic range

• High sensitivity

• High gain

9. List out the advantages of outer diameter measurement. [AUC NOV 2009]

• Speed is large

• More accuracy

• Faster diameter measurements

10. Define effective cutoff wavelength? [AUC April 2004, MAY2010]

• It is defined as wavelength greater than the ratio between the total power to the launched higher order modes and fundamental mode power.

11. Define BER? [AUC MAY2012]

• An approach is to divide the number of errors occurring over a certain time interval t by the number of pulses transmitted during this interval. This is called bit error rate or error rate.

12. What are the requirements of preamplifier. [AUC MAY 2008]

• Preamplifier bandwidth must be greater than or equal to signal bandwidth.

• It must reduce all source of noise

• It must have high receiver sensitivity


13. Compare the performance of APD and PIN diode. [AUC NOV 2008]

S.No Parameters PIN APD 1 Sensitivity Less sensitive (0- 12 dB)

More sensitive (5-15 dB)

2 Biasing Low reverse biased voltage (5 to 10 V)

High reverse biased voltage (20- 400 volts)

3 Wavelength region 300- 1100 nm

400 -1000 nm

4 Gain No Internal gain

Internal gain

PART (B)

1. Explain the fiber optic receiver operation? [AUC NOV 2010] The receiver must first detect weak, distorted signal and then make decisions on what type of

data was sent based on amplified version of the distorted signal. To understand the function of

the receiver, we first examine what happens to the signal as it is sent through the optical data

link which is shown in the following figure.

Fig: Signal path through an optical data link


A digital fiber transmission link is shown in the above figure. The transmitted signal is a

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

slot is referred to as bit period. Electrically there are many ways of sending a given digital

message. One of the simplest techniques for sending binary data is Amplitude Shift Keying (ASK), wherein a voltage level is switched between two values, which are generally on and off.

The resultant signal wave thus consists of a voltage pulse of amplitude V relative to zero voltage

level when a binary 1 occurs and a zero voltage level space when a binary 0 occurs. When a 1

is sent, a voltage pulse of duration Tb occurs, whereas for a 0 the voltage remains at its zero

level.

The function of the optical transmitter is to convert the electric signal to an optic signal.

Here 1 is represented by a pulse of optical power (light) of duration Tb, whereas a 0 is the

absence of any light. The optical signal that gets coupled from the light source to the fiber

becomes attenuated and distorted as it propagates along the fiber waveguide. Upon reaching

the receiver either a pin or an avalanche photodiode converts the optical signal back to an

electric format. The electric signal then gets amplified and filtered. A decision circuit compares

the signal in each time slot with a certain reference voltage known as the threshold level. If the

received signal level is greater than the threshold level, a 0 is assumed to be received. In some

cases an optical amplifier is placed ahead of the photodiode to boost the optical signal level

before photodetection. This is done so that the signal to noise ratio degradation caused by

thermal noise in the receiver electronics can be suppressed. Compared to APD’s or optical

heterodyne detectors, an optical preamplifier provides a large gain factor and a broader

bandwidth.

2. Explain error sources of optical receiver. [AUC NOV 2010] Error Sources: Errors arise from various noise and disturbances associated with the signal detection

system which is shown in the following figure.

Fig: Noise sources and disturbances in the optical pulse detection mechanism.

The term noise is used to describe unwanted components of an electric signal that tend

to disturb the transmission and processing of the signal in a physical system. The noise sources

can be either external or the system (for example atmospheric noise, equipment generated

noise) or internal to the system. Let us consider the internal noise. This noise is caused by the EC 2402 OPTICAL COMMUNICATION & NETWORKS – IV/VII - V.SENTHAMIZH SELVAN ASST. PROF/ECE

spontaneous fluctuations of current or voltage in electronic circuits. The two most

common examples of these spontaneous fluctuations are shot noise and thermal noise. Shot noises arise in electronic devices because of the discrete nature of current flow in the device.

Thermal noises arise from the random motion of electrons in a conductor.

The random arrival rate of signal photons produces a quantum (shot) noise at the

photodetector. When using an avalanche photodiode, an additional shot noise arises from the

statistical nature of the multiplication process. These noise level increases with increasing

avalanche gain M. Additional photodetector noises come from the dark current and leakage

current. These are independent of the photodiode illumination and can generally be made very

small in relation to other noise currents. When an avalanche photodiode is used in low optic

signal level applications, the optimum avalanche gain is determined.

The thermal noises are of Gaussian nature. The primary photocurrent generated by the

photodiode is a time varying Poisson process resulting from the random arrival of photons at the

detector. If the detector is illuminated by an optical signal P(t), then the average number of

electron hole pairs

generated in a time τ is

Where η is the detector quantum efficiency, hv is the photon energy, and E is the energy

received in a time interval τ. The actual number of electron hole pairs n that are generated

fluctuates from the average according to the Poisson distribution

Where Pr(n) is the propobality that n electrons are emitted in an interval τ. The fact that it is not

possible to predict exactly how many electron hole pairs are generated by a known optical

power incident on the detector is the origin of the type of shot noise called quantum noise. The

random nature of the avalanche multiplication process gives rise to another type of shot noise.

For a detector with a mean avalanche gain M and an ionization rate ratio k, the excess noise

factor F(M) for electron injection is

This equation is often approximated by the empirical expression

Where the factor x ranges from 0 to 1, depending on the photodiode material. Intersymbol interference (ISI) results from pulse spreading in the optical fiber. When a pulse is transmitted


in a given time slot, most of the pulse energy will arrive in the corresponding slot at the receiver

as shown in the following figure.

Fig: Pulse spreading in optical signal leads to Intersymbol interference

Due to pulse spreading some of the transmitted energy will progressively spread into

neighbouring time slots as the pulse propagates along the fiber. The presence of this energy in

adjacent time slots results in an interfering signal hence the term Intersymbol interference.

3. Explain fiber optic receiver configuration. [AUC MAY 2011] RECEIVER CONFIGURATION: A schematic diagram of a typical optical receiver is shown in the following figure

Fig: Schematic diagram of a typical optical receiver

The three basic stages of a receiver are a photodetector, an amplifier and an equalizer. The photodetector can be either an avalanche photodiode with a mean gain M or a

pin photodiode for which M=1. The photodiode has a quantum efficiency η and a capacitance

Cd. The detector bias resistor has a resistance Rb which generates a thermal noise current ib(t).

The amplifier has an input impedance represented by the parallel combination of a

resistance Ra and a shunt capacitance Ca. Voltages appearing across this impedance cause

current to flow in the amplifier output. This amplifying function is represented by the voltage

controlled current source which is characterized by a transconductance gm. There are two

amplifier noise sources. The input noise current ia(t) arises from the thermal noise of the

amplifier input resistance Ra, whereas the noise voltage source ea(t) represents the thermal

noise of the amplifier channel. The equalizer that follows the amplifier is used to mitigate the EC 2402 OPTICAL COMMUNICATION & NETWORKS – IV/VII - V.SENTHAMIZH SELVAN ASST. PROF/ECE

effects of signal distortion and Intersymbol interference. The rectangular pulses sent by the

transmitter arrive distorted at the receiver. The binary digital pulse incident on the photodetector

is given by

Here P(t) is the received optical power, Tb is the bit period, bn is an amplitude parameter

representing the nth message digit, and hp(t) is the received pulse shape. The parameter bn can

take on the two values bon and boff corresponding to binary 1 and 0 respectively.if we let the non

negative photodiode input pulse hp(t) be normalized to have unit area

Then bn represents the energy in the nth pulse. The mean output current from the photodiode at

time t resulting from the pulse train is

Wher R0=ηq/hv is the photodiode responsivity.

The mean output voltage is given by

Where A is the amplifier gain.

hb(t) is given by inverse fourier transform of the bias circuit transfer function HB(f).

HB(f) is given by

Where

And

The mean output voltage from the equalizer can be written in the form


where

The Fourier transform of the above eqn is given by

Here Hp(f) is the Fourier transform of the received pulse shape hp(t) and Heq(f) is the transfer

function of the equalizer.

PROBABILITY OF ERROR: There are many ways to measure the rate of error in a digital stream. One common

approach is to divide the number Ne of errors occurring over a certain time interval t by the

number Nt of pulses transmitted during this interval. This is called either the error rate or the bit error rate, which is commonly, abbreviated BER. Thus we have

Where b= 1/ Tb is the bit rate. Error rate is expressed by a number such as 10-6.error rates for

fiber telecommunication system ranges from 10-6 to 10-10. This error rate depends on the

signal to noise ratio at the receiver. To compute the bit error rate at the receiver, we have to

know the propability distribution of the signal at the equalizer output. The shapes of two signal

probability distributions are shown in the following figure.

Fig: probability distribution for two signal levels (0 and 1).

These are

Which is the probability that the output voltage exceeds v when a 1 pulse was sent and


Which is the probability that the output voltage exceeds v when a 0 was transmitted? The functions p(y|1) and p(y|0) are the conditional probability distribution functions. If the threshold voltage is Vth then the error probability Pe is defined as

a and b are the probabilities that either a 1 or 0 occurs respectively. To calculate the error probability we require square noise voltage v2

N, which is superimposed on the signal voltage at the decision time. Many methods have been proposed to calculate the performance of a binary optical fiber receiver. The simplest method is based on Gaussian approximation. It is assumed that when the sequence of optical input pulses is known the equalizer output voltage Hout(t) is a Gaussian random variable. Thus to calculate error probability we need to know the standard deviation of vout(t). Let us assume the noise has a gaussian probability density function with zero mean. If we sample the noise voltage n(t) at any arbitrary time t1, the probability that the measured sample n(t1) falls in the range n to n+dn is given by

Where σ2 is the noise variance and f(n) is the probability density function. And

When a 1 is transmitted the decoder sees a pulse of amplitude V volts plus superimposed noise. In this case the equalizer output voltage v(t) will fluctuate around V, so that the probability density function becomes

where the subscript 1 denotes the presence of a 1 bit. The probability of error that a 1 is decoded as 0 is that the sampled signal plus noise pulse falls below V/2. This is simply given by

The probability of error Pe in decoding of any digit is given by

where

is the error function. A plot of BER versus V/ σ is given below.


THE QUANTUM LIMIT: Consider an ideal photodetector which has unity quantum efficiency and which produces no

dark current,that is no electron hole pairs are generated in the absence of an optical pulse. With this

condition it is possible to find the minimum received optical power required for a specific bit error rate

performance in a digital system. This minimum received power level is known as the quantum limit, since

all system parameters are assumed ideal and the performance is only limited by the photodetection

statistics.

Assume that an optical pulse of energy E falls on the photodetector in a time interval τ. This

can only be interpreted by the receiver as a 0 pulse if no electron hole pairs are generated with the pulse

present. The probability that n=0 electrons are emitted in a time interval τ is

Thus for a given error probability Pr(0), we can find the minimum energy E required at a specific

wavelength λ.

4. Explain the following. [AUC MAY 2012] 1. High impedance FET amplifiers 2. High impedance BJT amplifiers HIGH IMPEDANCE FET AMPLIFIERS: A number of different FETs can be used fir front end receiver designs. For giga bit

per second data rates, the lowest noise receivers are made using GaAs MESFET preamplifiers.

At lower frequencies silicon MOSFETs or JFETs are generally used. The circuit of a simple FET

amplifier is shown below. Typical FETs have very large input resistances Ra.


Fig: simple high impedance preamplifier design using FET

The principal noise sources are thermal noise associated with the channel conductance, thermal

noise from the load or feedback resistor, and noise arising from gate leakage current. A fourth

noise source is FET i/f noise. This was not included. Since the amplifier input resistance is very

large, the input current noise spectral density SI is

Where Igate is he gate leakage current of the FET. In an FET the thermal noise of the conducting

channel resistance is characterized by the transconductance gm. The voltage noise spectral

density is

Where the FET channel noise factor Γ is a numerical constant that accounts the thermal noise

and gate induced noise plus the correlation between these noises. The thermal noise

characteristic W at the equalizer output is

To minimize the noise in a high impedance design, the bias resistor should be made very large.

The effect of this is that the detector output signal is integrated by the amplifier input resistance.


HIGH IMPEDANCE BIPOLAR TRANSISTOR AMPLIFIERS:

The circuit of a simple bipolar grounded emitter transistor amplifier is shown below.

Fig: Simple high impedance preamplifier design using a bipolar transistor.

The input resistance of a bipolar transistor is given by

Where IBB is the base bias current. For a bipolar transistor amplifier the input resistance Ra is

given by the parallel combination of the bias resistors R1 and R2 and the transistor input

resistance Rin. For a low noise design R1 and R2 are chosen to be much greater than Rin, so

that Ra= Rin.

The spectral density of the input noise current source results from shot noise of the

base current.

The spectral height of the noise voltage source is

Here the transconductance gm is related to the shot noise by virtue of the collector current Ic


We know that

Substituting the value of Rin, SI, SE, and gm in the above eqn we get

The contribution Ca to C from the bipolar transistor is a few picofarads. If the photodetector bias

resistor Rb is much larger than the amplifier resistance Ra then R= Ra= Rin, so that

With a high impedance FET preamplifier, the impedance loading the photo detector integrates

the detector output signal. Again to compensate, the amplified signal is differentiated in the

equalizing filter.

5. Explain detail in fiber attenuation measurement. [AUC MAY 2011] FIBER ATTENUATION MEASUREMENT:

Measurement techniques to obtain the total fiber attenuation give either the spectral loss

characteristic or the single wavelength.

Total fiber Attenuation:

A commonly used technique for determining the total fiber attenuation per unit length is

the cutback or differential method. The following figure shows a schematic diagram of the typical

setup for the measurement of spectral loss to obtain the overall attenuation spectrum for the

fiber.


Fig: Arrangement for measurement of spectral loss in optical fibers using the cut back technique.

It consists of a white light source, usually tungsten halogen or xenon arc lamp. The focused light

is then mechanically chopped at a low frequency of a few hundred hertz. This enables the lock

in amplifier at the receiver to perform phase sensitive detection. The chopped light is then fed to

monochromator which utilizes a prism or diffraction grating arrangement to select the required

wavelength at which the attenuation is to be measured. Hence the light is filtered before being

focused onto the fiber by means of microscope objective lens. A beam splitter is used for

viewing optics and a reference signal is used for compensating output power fluctuations. A

mode stripper can also be used at the fiber output end to remove any optical power which is

scattered from the core into the cladding.

The optical power at the receiving end is detected using a pin or APD. In order to obtain

reproducible results the photodetector surface is usually index matched using epoxy resin or an

index matched cell. Finally the electric output from the photodetector is fed to a lock in amplifier,

the output of which is recorded.

The cutback method involves taking a set of optical output power measurements over

the required spectrum using a long length of fiber (usually at least one kilometer). This fiber is

generally uncabled having only a primary protective coating. The fiber is then cut back to a point

a few meters (e.g. 3m) from the input end and maintaining the same launch conditions another

set of power output measurements are taken. The following relationship for the optical

attenuation per unit length αdB for the fiber may be obtained by

L1 and L2 are the original and cut back fiber lengths respectively, and P01 and P02 are the

corresponding output optical powers at a specific wavelength from the original and cut back fiber

lengths. Hence when L1 and L2 are measured in kilometers αdB has units of dB km-1. The above

eqn becomes


Where V1 and V2 correspond to output voltage readings from the original fiber length and the

cut back fiber length respectively. The accuracy of the result obtained for αdB using the method

is largely dependent on constant optical launch conditions.

Spot measurements may be performed using the above set up. However interference

filters are widely used instead of monochromators in order to obtain a measurement for a

particular wavelength. A typical optical configuration for spot attenuation measurements is

shown below.

Fig: Experimental setup for making spot attenuation measurements using interference

filters and employing cut back technique.

The interference filters are located onto a wheel to allow measurement at selection of different

wavelengths. The source spot size is defined by a pin hole and the beam angular width is varied

by using different diaphragms. The determination of optical loss is performed in the same

manner, using the cut back technique.

6. Explain detail in fiber dispersion measurement. [AUC NOV 2009] FIBER DISPERSION MEASUREMENTS: Fiber dispersion depends upon the type of the fiber. In multimode fibers, intermodal

dispersion occurs and tends to be dominant mechanism, whereas in single mode fibers

intermodal dispersion does not exist. Dispersion effects may be measured by taking the impulse

response of the fiber in the time domain, or by measuring the baseband frequency response in

the frequency domain.


If the fiber response is linear with regard to power, a mathematical expression can

be obtained for optical power P0(t) by convoluting the power impulse response h(t) with the

optical input power P1(t) as

Where the asterisk * denotes the convolution. The convolution of h(t) with Pi(t) shown in above

eqn can be evaluated using the convolutional integral where

In the frequency domain the power transfer function H(ω) is the fourier transform of h(t) and

therefore by taking the fourier transform of all the functions we obtain

Where ω is the baseband angular frequency.

4.6.1. Time domain measurement: The most common method for time domain measurement of pulse dispersion in

optical fibers is illustrated below.

Fig: Experimental arrangement for making fiber dispersion measurements in the time

domain.

Short optical pulses (100- 400 ps) are launched into the fiber from a suitable source (e.g.

AlGaAs injection laser) using fast driving electronics. The pulse travel down the length of fiber

under test and are broadened due to various dispersion mechanisms. In multimode fibers

intramodal dispersion is negligible and intermodal dispersion occurs. The pulses are received by

a high speed photodetector and are displayed on a fast sampling oscilloscope and for input

pulse measurement.

After the initial measurement of output pulse width, the long fiber length may be cut

back to a short length and the measurement repeated in order to obtain the effective input pulse

width. If Pi(t) and P0(t) are assumed to have Gaussian shape then


Where τ i(3dB) and τo(3 dB) are the 3 dB pulse widths at the fiber input and output respectively

and τ(3 dB) is the width of the fiber impulse response again measured at half the maximum

amplitude. Hence the pulse dispersion in the fiber in nskm-1 is given by

Where τ(3 dB),τ i(3dB) and τo(3 dB) are measured in ns and L is the fiber length in Km.when the

launched optical pulses and the fiber impulse response are Gaussian the the 3 dB optical

bandwidth for the fiber Bopt may be calculated using

A more convenient method of measuring the temporal dispersion of an optical pulse

within a fiber which does not require a long fiber length is the shuttle pulse technique. This

experimental setup reported by cohen is shown below

Fig: Apparatus used in shuttle pulse technique for time domain measurement in optical

fibers. Both ends of a short fiber length are terminated with partially transparent mirrors

and a pulse launched from a GaAs injection laser travels through one mirror into the fiber then

shuttles back and forth between the fiber ends. This technique has an added advantage in that it EC 2402 OPTICAL COMMUNICATION & NETWORKS – IV/VII - V.SENTHAMIZH SELVAN ASST. PROF/ECE

allows the length dependence of the impulse response to be studied by sampling the pulse after

each 2N-1 transits. The pulse at the output end is displayed on a sampling oscilloscope through

the partially transparent mirror. Hence the pulse broadening may be measured by comparing the

widths of the output pulses. An index matching fluid is also utilized between the fiber end faces

and the mirrors in order to achieve optimum optical transmission.

7. Explain fiber refractive index profile measurement. [AUC MAY 2008] FIBER REFRACTIVE INDEX PROFILE MEASUREMENT: A detailed knowledge of the refractive index profile enables the impulse response of the

fiber to be predicted. There are different methods for measuring the refractive index profile.

1. Interferometric Methods: Interference microscopes (e.g. Mach- Zehnder, Michelson) have been widely used to

determine the refractive index profiled of optical fibers. The technique usually involves the

preparation of a thin slice of fiber which has both ends accurately polished to obtain square and

optically flat surfaces. The slab is often immersed in an index matching fluid, and the assembly

is examined with an interference microscope. Two methods are used; using either a transmitted

light interferometer or a reflected light interferometer. In both cases light from the microscope

travels normal to the prepared fiber slice faces, and differences in refractive indx result in

different optical path lengths. This situation is illustrated in the case of Mach- Zehnder

interferometer in the following figure.

Fig a)the principle of the Mach-Zehnder interferometer b) the interference fringe pattern

obtained with an interference microscope from a graded index fiber.

The fringe displacements for the points within the fiber core are then measured using as

reference the parallel fringes outside the fiber core. The refractive index difference between a


point in the fiber core and the cladding can be obtained from the fringe shift q, which

corresponds to a number of fringe displacements. This difference in refractive index δn is given

by

Where x is the thickness of the fiber slab and λ is the incident optical wavelength. The slab

method gives an accurate measurement of the refractive index profile. A limitation of this

method is time required to prepare the fiber slab.

Another interferometric technique has been developed. In this method the light beam is

incident to the fiber perpendicular to its axis; this is known as transverse shearing interferometry.

Again fringes are observed from which the fiber refractive index profile may be calculated.

Fig: Fiber refractive index profile computed from the interference pattern shown in fig b). 2. Near field scanning Method: The near field scanning method utilizes the close resemblance that exists between the

near field intensity distribution and the refractive index profile, for a fiber with all the guided

modes equally illuminated. When a diffuse Lambertian source (e.g. tungsten filament lamp or

LED) is used to excite all the guided modes then the near field optical power density at a radius

r from the core axis PD(r)may be expressed as a fraction of the core axis near field optical power

density PD(0) following

Where ni(0) and n1(r) are the refractive indices at the core axis and at a distance r from the core

axis respectively, n2 is cladding refractive index and C(r,z) is a correction factor. The correction

factor is used for compensating the leaky modes.


An experimental configuration is shown in following figure.

Fig: Experimental setup for near field scanning measurement of the refractive index

profile. The output from a lambertian source is focused onto the end of the fiber using a

microscope objective lens. A magnified image of the fiber output end is displayed in the plane of

a small active area photodetector. The photodetector which scans the field transversely receives

amplification from the phase sensitive combination of the optical chopper and lock in amplifier.

Hence the profile may be directly plotted on X- Y recorder. The test fiber is generally less than

1m in length to eliminate any differential mode attenuation and mode coupling. A typical

refractive index profile for a step index fiber measured by the near field scanning method is

shown below.

Fig Refractive index profile of a step index fiber measured using the near field scanning

method. It may be observed that the profile dips in the center at the fiber core axis.

Measurements of the refractive index profile may also be obtained from the far field pattern

produced by the laser light scattered by the fiber under test. This technique, generally known as

the scattered pattern method, requires complex analysis of the forward or backward patterns in

order to determine the refractive index profile.


3. End Reflection Method: The refractive index at any point in the cross section of an optical fiber is directly related

to the reflected power from the fiber surface in air at that point following the Fresnel reflection

formula. Hence the fraction of light reflected at the air fiber interface is given by

Where n1is the refractive index at the point on the fiber surface. For small changes in the value

of refractive index:

Therefore combining both the eqn’s we have

The above eqn gives the relative change in the Fresnel reflection coefficient r which

corresponds to the change of refractive index at the point of measurement. However when the

measurement is performed in air the small changes in refractive index δn1 that must be

measured give only very small changes in r. Two experimental arrangements for performing end

reflection measurements are shown below

Fig: Experimental arrangement for end reflection measurement of fiber refractive index profile a) without index matching of fiber input end face b) with index matching of fiber

input end face


Figure a) shows end reflection measurements without index matching of the fiber input end face.

The laser beam is initially directed through a polarizer and a λ/4 plate in order to prevent

feedback of the reflected power from both the fiber end face and the intermediate optics,

causing modulation of the laser output through interference. The circularly polarized light beam

from the λ/4 plate is then spatially filtered and expanded to provide a suitable spot size. A beam

splitter is used to provide both a reference from the input light beam which is monitored with a

solar cell, and two beams from the fiber end face reflection. The reflected beams are used for

measurement via a pin photodiode, lock in amplifier combination and for visual check of the

alignment on the fiber end face using a screen. Focusing on the fiber end face is achieved with

a microscope objective lens, and the fiber end is scanned slowly across the focal spot using

precision translation stages. The reflected optical power is monitored as a function of the fiber

linear position on an X-Y recorder and the refractive index profile may be obtained directly using

Possible reflections from the other fiber end face are avoided by immersing it in an index

matching liquid.

The experimental arrangement shown in fig b) provides increased sensitivity by

immersing the fiber in index matching oil. In this case the laser beam which is again incident on

a polarizer and λ/4 plate is deflected vertically using a mirror. An oil immersion objective is

utilized to focus the beam onto the immersed fiber end. This apparatus has shown sensitivity

comparable with the near field method. However there is a need for careful alignment of the

apparatus in order to avoid stray reflections. Also in both techniques it is essential that the fiber

end face should be perfectly flat because the reflected power is severely affected by surface

irregularities.

8. Explain fiber numerical aperture measurement. [AUC NOV 2011] FIBER NUMERICAL APERTURE MEASUREMENTS: The numerical aperture is an important optical fiber parameter as it affects

characteristics such as the light gathering efficiency and the normalized frequency of the fiber

(V). the numerical aperture of a step index fiber is given by

Where θa is the maximum acceptance angle, n1 is the core refractive index and n2 is the

cladding refractive index.

A simple commonly used technique for measuring the fiber numerical aperture involves

measurement of the far field radiation pattern from the fiber. This measurement may be


performed by directly measuring the far field angle from the fiber using a rotating stage, or by

calculating the far field angle using trigonometry. An experimental arrangement with a rotating

stage is shown below

Fig: fiber numerical aperture measurement using a scanning photodetector and a

rotating stage. The fiber end faces are prepared in order to ensure square smooth terminations. The

fiber output end is then positioned on the rotating stage with its end face parallel to the plane of

the photodetector input, and so that its output is perpendicular to the axis of rotation. Light is

launched into the fiber at all possible angles using an optical system similar to that used in spot

attenuation measurements.

The photodetector may be either a small area device or an aperture large area device, is

placed 10-20 cm from the fiber and positioned in order to obtain a maximum signal with no

rotation (0°). Hence when the rotating stage is turned the limits of the far field pattern may be

recorded. The output power is monitored and plotted as a function of angle, the maximum

acceptance angle being obtained when the power drops a predetermined amount. Thus the

numerical aperture can be found out by using the above eqn.

Another method for finding the numerical aperture is shown below,

Fig: apparatus for trigonometric fiber numerical aperture measurement


Where the end prepared fiber is located on an optical base plate or slab. Again light is launched

into the fiber under test over the full range of its numerical aperture, and the far field pattern from

the fiber is displayed on a screen which is positioned a known distance D from the fiber output

end face. The test fiber is then aligned so that the optical intensity on the screen is maximized.

Finally the pattern size on the screen A is measured using a calibrated vernier caliper. The

numerical aperture can be obtained from simple trigonometric relationships where

It must be noted that the accuracy of the measurement technique is dependent upon the visual

assessment of the far field pattern from the fiber. The above measurements is employed with

only multimode fibers, as the far field patterns from single mode fibers are affected by diffraction

phenomena.


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MAHALAKSHMImahalakshmiengineeringcollege.com/pdf/ece/VIIsem/EC2402...EC 2402 OPTICAL COMMUNICATION & NETWORKS – IV/VII - V.SENTHAMIZH SELVAN ASST. PROF/ECE in a given time slot,