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.
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 T b. 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 T b 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 T b, 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
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 P r (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 C d. The detector bias resistor has a resistance Rb which generates a thermal noise current i b (t). The amplifier has an input impedance represented by the parallel combination of a resistance R a and a shunt capacitance C a. 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 g m. There are two amplifier noise sources. The input noise current i a (t) arises from the thermal noise of the amplifier input resistance R a, whereas the noise voltage source e a (t) represents the thermal noise of the amplifier channel. The equalizer that follows the amplifier is used to mitigate the
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, b n is an amplitude parameter representing the nth message digit, and h p (t) is the received pulse shape. The parameter b n can take on the two values b on and b off corresponding to binary 1 and 0 respectively.if we let the non negative photodiode input pulse h p (t) be normalized to have unit area Then b n represents the energy in the n th pulse. The mean output current from the photodiode at time t resulting from the pulse train is Wher R 0 =ηq/hv is the photodiode responsivity. The mean output voltage is given by Where A is the amplifier gain. h b (t) is given by inverse fourier transform of the bias circuit transfer function H B (f). H B (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 H p (f) is the Fourier transform of the received pulse shape h p (t) and H eq (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 N e of errors occurring over a certain time interval t by the number N t 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/ T b 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. These are Fig: probability distribution for two signal levels (0 and 1). 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 V th then the error probability P e 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 v 2 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 H out (t) is a Gaussian random variable. Thus to calculate error probability we need to know the standard deviation of v out (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 σ 2 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 P e 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 R a.
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 S I is Where I gate is he gate leakage current of the FET. In an FET the thermal noise of the conducting channel resistance is characterized by the transconductance g m. 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 I BB is the base bias current. For a bipolar transistor amplifier the input resistance R a is given by the parallel combination of the bias resistors R 1 and R 2 and the transistor input resistance R in. For a low noise design R 1 and R 2 are chosen to be much greater than R in, so that R a = R in. 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 g m is related to the shot noise by virtue of the collector current I c
We know that Substituting the value of R in, S I, S E, and g m 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 α db for the fiber may be obtained by L 1 and L 2 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 L 1 and L 2 are measured in kilometers α db has units of db km -1. The above eqn becomes
Where V 1 and V 2 correspond to output voltage readings from the original fiber length and the cut back fiber length respectively. The accuracy of the result obtained for α db 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 P 0 (t) by convoluting the power impulse response h(t) with the optical input power P 1 (t) as Where the asterisk * denotes the convolution. The convolution of h(t) with P i (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 P i (t) and P 0 (t) are assumed to have Gaussian shape then
Where τ i (3dB) and τ o (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 τ o (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 B opt 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
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 P D (r)may be expressed as a fraction of the core axis near field optical power density P D (0) following Where n i (0) and n 1 (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 n 1 is 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 δn 1 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 θ a is the maximum acceptance angle, n 1 is the core refractive index and n 2 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.