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