Communications MCQ Set 1
1. What is the full form of ETDM?
a) Electronic tube di-cyclic mechanism
b) Electrical time division multiplexing
c) Emphasis tier division mechanism
d) Electrical tube dielectric medium
Answer: b [Reason:] ETDM is the major baseband digital strategy. It allows for greater exploitation of available fiber bandwidth.
2. The practical limitations of the speed of electronic circuits have been pushed towards operational frequencies around ___________
a) 100 MHz
b) 120 MHz
d) 80 Hz
Answer: c [Reason:] The speed of the circuitry in the fiber optic communication plays an important role in its performance. It is pushed around 100 GHz frequency allowing for 100 Gbit/s feasibility.
3. A strategy used for increasing the bitrate of digital optical fiber systems beyond the bandwidth capabilities of the drive electronics is known as ___________
a) Optical time division multiplexing
b) Electrical time division multiplexing
c) Frequency division multiplexing
d) Code division multiplexing
Answer: a [Reason:] OTDM is favourable for long distance transmission of signal. It is designed to push the bitrate of the fiber systems beyond the bandwidth limits to gain performance.
4. ____________ semiconductor laser sources provide low duty cycle pulse streams for subsequent time multiplexing.
a) Diameter preferred
b) Mode locked
Answer: b [Reason:] Mode locked semiconductor laser sources were used at the transmitter side. They provide effective distribution of time multiplexing providing low duty cycle pulse streams.
5. ______________ are the devices which are employed to eliminate the laser chirp.
a) Optical intensity modulators
d) Optical Isolators
Answer: a [Reason:] Optical intensity modulators eliminate the laser chirp. This laser chirp may result in dispersion of the transmitted pulses as they propagate within the single mode fiber, thus limiting the achievable transmission distance.
6. _____________ provides operation at high transmission rate.
a) Optical intensity modulators
d) Electro-absorption modulators
Answer: d [Reason:] Electro-absorption modulators are employed at the transmitter and receiver sections. They provide operation at high transmission rate and for field trial.
7. In __________ the microwave frequency are modulated with an optical carrier and transmitted using a single wavelength channel.
a) Subcarrier multiplexing
d) Code division multiplexing
Answer: a [Reason:] Optical Subcarrier multiplexing (SCM) is transmitted using a single wavelength channel. It enables multiple broadband signals to be transmitted over single-mode fiber.
8. Which of the following techniques is easy to implement?
a) Amplitude shift keying
b) Phase shift keying
c) Frequency shift keying
Answer: c [Reason:] Frequency shift keying has an advantage of being simple to implement at the modulator as well as demodulator side. It is formed by up converting to a narrowband channel at high frequency employing frequency.
9. Which of the following is the disadvantage of SCM?
a) Source nonlinearity
d) Narrow bandwidth
Answer: The problem associated with SCM is source nonlinearity. The distortion caused by this becomes noticeable when several subcarriers are transmitted from a single optical source.
10. In CATV, the signal must be received with a carrier to noise ratio of between __________
a) 90 and 100 dB
b) 10 and 30 dB
c) 60 and 70 dB
d) 45 and 55 dB
Answer: d [Reason:] The CATV multichannel spectrum tends to minimize the required bandwidth. The carrier to noise ratio must be between to avoid degradation of picture quality.
11. The IF signal can be input to a demodulator to recover the baseband signal. State whether the given statement is true or false.
Answer: a [Reason:] The IF signal is obtained through SCM at the receive terminals. The baseband video signal in a CATV is obtained through IF signal by using it with a demodulator input.
Communications MCQ Set 2
1. _____________ refers to any spurious or undesired disturbances that mask the received signal in a communication system.
Answer: b [Reason:] Noise is an unwanted and undesirable quantity. It affects the received signal in a communication system. In optical fiber communication systems, noise is due to the spontaneous fluctuations rather than erratic disturbances.
2. How many types of noise are observed because of the spontaneous fluctuations in optical fiber communication systems?
Answer: d [Reason:] There are three types of noise because of the spontaneous fluctuations in optical fiber communication systems. These are thermal noise, the dark current noise and quantum noise. These noise types are not caused by the electronic interference.
3. ______________ is caused due to thermal interaction between the free electrons and the vibrating ions in the conduction medium.
a) Thermal noise
b) Dark noise
c) Quantum noise
d) Gaussian noise
Answer: a [Reason:] Thermal noise is basically a spontaneous fluctuation caused due to thermal interaction of electrons and ions. It is especially prevalent in resistors at room temperature. Thermal noise is measured in the form of current and is called as thermal noise current.
4. A small leakage current still flows from the device terminals even if there is no optical power incident on the photo detector. State whether the given statement is true or false.
Answer: a [Reason:] A reverse leakage current that flows from the device terminals is called as dark current. This dark current contributes to the total system noise. This gives random fluctuations about the average particle flow of the photocurrent.
5. ___________ distribution provides the description the random statistics of light emitted in black body radiation.
Answer: d [Reason:] Incoherent light is emitted by independent atoms and therefore there is no phase relationship between the emitted photons. The property dictates an exponential intensity distribution which is identical to Bose-Einstein distribution.
6. The probability of zero pairs being generated when a light pulse is present is given by equation –
a) P (0/1) = exp(-Zm)
b) P(x) = exp (Zm)
c) P(y) = x (0) + x(1)
d) P (z) = P(-Zm)
Answer: a [Reason:] The probability of zero pairs being generated when a light pulse is present is given by equation –
P (0/1) = exp(-Zm)
Where, P (0/1) represents the system error probability p (e) and Zm is variance of the probability distribution.
7. The minimum pulse energy needed to maintain a given bit-error-rate (BER) which any practical receiver must satisfy is known as ___________
a) Minimal energy
b) Quantum limit
c) Point of reversed
d) Binary signaling
Answer: b [Reason:] A perfect photo detector emits no electron-hole pairs in the absence of illumination. The error probability determines a standardized fundamental limit in digital optical communications. This limit is termed as quantum limit.
8. A digital optical fiber communication system requires a maximum bit-error-rate of 10-9. Find the average number of photons detected in a time period for a given BER.
Answer: c [Reason:] The probability of error is given by-
P (e) = exp(-Zm)
Where, Zm= No. of photons
Here P (e) =10-9, therefore Zm is calculated from above relation.
9. For a given optical fiber communication system, P (e) =10-9,Zm= 20.7, f = 2.9×1014, η= 1. Find the minimum pulse energy or quantum limit.
Answer: a [Reason:] The minimum pulse energy or quantum limit is given by –
Where, Zm= Number of photons
h= Planck’s constant
η= Quantum efficiency .
10. An analog optical fiber system operating at wavelength 1μmhas a post-detection bandwidth of 5MHz. Assuming an ideal detector and incident power of 198 nW, calculate the SNR ( f = 2.99×1014Hz)
Answer: c [Reason:] The SNR is given by –
S/N = ηP0/2hfB
Where, η= 1 (for ideal detector)
P0= incident power
h= Planck’s constant
B= Bandwidth .
11. The incident optical power required to achieve a desirable SNR is 168.2nW. What is the value of incident power in dBm?
a) -37.7 dBm
b) -37 dBm
c) – 34 dBm
d) -38.2 dBm
Answer: a [Reason:] Incident power in denoted byP0. It is given by –
Where P0(watts) = incident power in Watts/milliWatt.
12. In the equation given below, what does τstands for?
d) Refractive index
Answer: b [Reason:] In the given equation, Zmm is the variance of the probability distribution. The number of electrons generated in time τis equal to the average of the number of photons detected over this time periodZm. Hence, τ is the time and P0 is the incident power, ηis the quantum efficiency and fis the frequency.
Communications MCQ Set 3
1. The nonlinear effects in optical fibers are large. State whether the given statement is true or false.
Answer: b [Reason:] The nonlinear effect arises from the interactions between light waves and the material transmitting them and thus affects the optical signals. The nonlinear effects are usually small in optical fibers. They have power levels of up to few milliWatts.
2. How many categories of nonlinear effects are seen in optical fibers?
Answer: b [Reason:] The nonlinear effects are separated on the basis of their characteristics. There are two such categories; one is scattering effect and the other is Kerr effect.
3. Which of the following is not related to Kerr effects?
a) Self-phase modulation
b) Cross-phase modulation
c) Four-wave mixing
d) Stimulated Raman Scattering
Answer: d [Reason:] Stimulated Raman Scattering is related to scattering. The other effects include modulation and mixing which are parts of Kerr effect.
4. Linear scattering effects are _______ in nature.
Answer: a [Reason:] Linear scattering effects are elastic because the scattered wave frequency is equal to incident wave frequency. Nonlinear scattering effects are purely inelastic.
5. Which thing is more dominant in making a fiber function as bidirectional optical amplifier?
a) Core material
b) Pump source
c) Cladding material
d) Diameter of fiber
Answer: b [Reason:] Brillouin gain is always greater than Raman gain. It exists for light propagation in opposite direction to the pump source. Also Brillouin frequency shifts and gain bandwidth are much smaller than Raman. Raman amplification occurs for light propagating in either direction. Thus, pump source is more important in making a fiber function as bidirectional optical amplifier.
6. _________ semiconductor laser sources generally have broader bandwidths.
d) Silicon hybrid
Answer: b [Reason:] Pulsed semiconductor lasers have broader bandwidths. Therefore, these sources prove to be inefficient pump sources. They prove inefficient especially for narrow gain spectrum.
7. Nonlinear effects which are defined by the intensity – dependent refractive index of the fiber are called as
a) Scattering effects
b) Kerr effects
c) Raman effects
d) Tomlinson effects
Answer: b [Reason:] Kerr effects are nonlinear effects. Nonlinear effects are divided into scattering and Kerr effects. Scattering effects include scattering of phonon whereas Kerr effects include intensity refractive index parameters.
8. Self-phase modulation causes modifications to the pulse spectrum. State whether the given statement is true or false.
Answer: a [Reason:] Kerr effect results in different transmission phase for the peak of the pulse compared with leading and trailing edges. Self-phase modulation can broaden the frequency spectrum of the pulse as the time varying phase creates a time varying frequency.
9. Self-phase modulation can be used for
a) Enhancing the core diameter
b) Wavelength shifting
c) Decreasing the attenuation
d) Reducing the losses in the fiber
Answer: b [Reason:] Self phase modulation is related to phase change. It imposes a positive frequency sweep on the pulse which in turn enables wavelength or frequency shifting.
10. The beating between light at different frequencies or wavelengths in multichannel fiber transmission causes
b) Amplitude modulation of channels
c) Phase modulation of channels
d) Loss in transmission
Answer: c [Reason:] Phase modulation is related to frequency and wavelength shifting. In multichannel fiber transmission, phase modulation causes generation of modulation sidebands at new frequencies. This phenomenon is called as four-wave mixing.
11. What is different in case of cross-phase modulation from self-phase modulation?
a) Overlapping but same pulses
b) Overlapping but distinguishable pulses
c) Non-overlapping and same pulses
d) Non-overlapping but distinguishable pulses
Answer: b [Reason:] In cross phase modulation, variation in intensity of one pulse width modulates the refractive index of the fiber which causes phase modulation of the overlapping phases. In self-phase modulation, this phase modulation broadens the pulse spectrum.
12. When three wave components co-propagate at angular frequency w1, w2, w3, then a new wave is generated at frequency w4, which is given by
a) w4 = w1-w2-w3
b) w4 =w1+w2+w3
c) w4 =w1+w2-w3
d) w4 =w1-w2+w3
Answer: c [Reason:] This type of frequency mixing is called as four-wave mixing. This frequency combination is problematic for multichannel optical communication as they become phase matched if the channel wavelengths are near to zero dispersion wavelengths.
13. _____________ results from case of nonlinear dispersion compensation in which the nonlinear dispersion compensation in which the nonlinear dispersion compensation in which the nonlinear chirp caused by self-phase modulation balances, postpones, the temporal broadening induced by group velocity delay.
a) Four wave mixing
b) Phase modulation
c) Soliton propagation
d) Raman scattering
Answer: c [Reason:] Soliton propagation is a nonlinear dispersion phenomenon. It limits the propagation distance that can be achieved when acting independently. It balances broadening of light pulse.
Communications MCQ Set 4
1. ____________________ lasers are presently the major laser source for optical fiber communications
Answer: c [Reason:] Injection laser coupling using discrete lasers have proved to fruitful. Such lenses provide for a relaxation for an alignment tolerances normally required for fiber coupling. Certain non-semiconductor sources are making its lace in the optical fiber communication. At slowly present, injection lasers are mostly used as laser sources.
2. In Nd: YAG lasers, the maximum doping levels of neodymium is ____________
a) 0.5 %
b) 1.5 %
c) 1.8 %
d) 2 %
Answer: b [Reason:] The Nd: YAG laser structure is formed by doping of yttrium- aluminum -garnet (YAG) with neodymium. The energy levels for lasing transition and pumping are provided by neodymium ions. The maximum doping level of neodymium in YAG is around 1.5 %.
3. Which of the following is not a property of Nd: YAG laser that enables its use as an optical fiber communication source?
a) Single mode operation
b) Narrow line-width
c) Long lifetime
d) Semiconductors and integrated circuits
Answer: d [Reason:] Nd: YAG laser is a non-semiconductor laser. It does not include the use of semiconductors and thus cannot take advantage of well-developed technology associated with integrated circuits. Single mode operation, narrow line-width, lifetime are the properties that are useful for optical communication.
4. The Nd: YAG laser has a narrow line-width which is ________________
a) < 0.01 nm
b) > 0.01 nm
c) > 1 mm
d) > 1.6 mm
Answer: a [Reason:] The Nd: YAG laser has several properties which make it an active optical source. One of such properties is its narrow line-width. It is less than 0.01 nm which is useful for reducing dispersion of optical links.
5. The strongest pumping bands is a four level system of Nd: YAG laser at wavelength of range_________________
a) 0.25 and 0.56 nm
b) 0.75 and 0.81 nm
c) 0.12 and 0.23 nm
d) 1 and 2 nm
Answer: b [Reason:] The Nd: YAG laser is a four level system. It consists of number of pumping bands and fluorescent transitions. The strongest pumping bands are the wavelengths of 0.75μm and 0.81μm. and gives lasing transition at 1.064μmand 1.32μm. Single mode emission is usually obtained at these wavelengths.
6. The Nd: YAG laser is costlier than earth-doped glass fiber laser. State whether the following statement is true or false
Answer: a [Reason:] The most important requirement of the Nd: YAG laser is pumping and modulation. These two requirements tend to give a cost disadvantage in comparison with earth-doped glass fiber laser. Also it is easier and less expensive to fabricate glass fiber in earth-doped laser.
7. It is a resonant cavity formed by two parallel reflecting mirrors separated by a mirror separated by a medium such as air or gas
a) Optical cavity
b) Wheatstone’s bridge
d) Fabry-perot resonator
Answer: d [Reason:] Resonant cavity is formed between two mirrors where fiber core doped with earth ions is placed. This cavity is 250-500 μmlong and 5 to 15 μm wide. A Fabry-perot resonator oscillates at resonant frequency for which there is high gain.
8. In a three level system, the threshold power decreases inversely with the length of the fiber gain medium. State whether the given statement is true or false.
Answer: b [Reason:] If the imperfection losses are low then in a four level system the threshold power decreases inversely with the length of the fiber gain medium. A three level consists of an optimum length. This optimum length gives the minimum threshold power which is independent of the value of imperfection losses.
9. Which of the following co-dopant is not employed by neodymium and erbium doped silica fiber lasers?
a) Phosphorus pent oxide
Answer: c [Reason:] Silica based glass fibers are proved to be the best host material till date. These silica fibers are doped with neodymium and erbium. These dopants include co-dopants such as phosphorus pent-oxide, germanium and alumina.
10. Dopants levels in glass fiber lasers are generally ___________
c) Same as that of GRIN rod lens laser
d) Same as that of semiconductor laser
Answer: a [Reason:] Dopant levels are low in glass fibers (nearly 400 parts per million). This is because of increasing in concentration quenching which increases with the doping level. It may cause the reduction in the population of the upper lasing level as well as crystallization within the glass matrix.
11. _______________ fibers include addition of lead fluoride to the core glass in order to raise the relative refractive index.
Answer: d [Reason:] Up-conversion pumping of laser material is used to convert an infrared laser output to a visible laser output. ZBLANP is host material on which laser action at all wavelengths can be obtained by pumping. The relative refractive index is increased by addition of lead fluoride which makes it a very interesting host material.
12. The lasing output of the basic Fabry-perot cavity fiber is restricted to between ____________
a) 1 and 2 nm
b) 5 and 10 nm
c) 3 and 6 nm
d) 15 and 30 nm
Answer: b [Reason:] the gain spectrum of rare earth ions may be seen over a wavelength range of 50 nm. The lasing output will thus be narrow unless the dielectric on the mirror is arranged. Such a narrow line-width is not used for a broadband optical source.
13. In Fabry-perot laser, the lower threshold is obtained by
a) Increasing the refractive index
b) Decreasing the refractive index
c) Reducing the slope efficiency
d) Increasing the slope efficiency
Answer: c [Reason:] The finesse of Fabry-perot cavity provides a measure of its filtering properties. When the finesse is high the splitting ratio is low thus lowering the laser threshold in an optical cavity without mirror. In Fabry-perot laser, mirrors are present and thus lower threshold is obtained by reducing the slope efficiency.
14. When did the non-semiconductor laser developed?
Answer: c [Reason:] Non-semiconductor sources are crystalline and glass wave-guiding structures. They are doped with rare earth ions and are good optical sources. The development of these sources started in the year 1985. Example: Nd: YAG laser.
15. Y3Al5 O12 Is a molecular formula for _____________
a) Ytterbium aluminate
b) Yttrium oxide
c) Ytterbium oxy-aluminate
d) Yttrium-aluminum garnet
Answer: d [Reason:] The atomic number of Yttrium is 39. It is the base element of Yttrium-aluminum garnet. Y3Al5 O12, doped with rare earth ion neodymium to form Nd: YAG laser structure.
Communications MCQ Set 5
1. ___________________ is defined as a process by which the wavelength of the transmitted signal is changed without altering the data carried by the signal.
a) Wavelength conversion
c) Sigma management
d) Wavelength dispersion
Answer: a [Reason:] Wavelength conversion observes the changes in the length of the wave. It does not proportionate with the data carried by the signal or wave.
2. The device which is used to perform wavelength conversion is called as ___________
b) Wavelength Gyrator
c) Wavelength Circulator
d) Wavelength translator
Answer: d [Reason:] Wavelength translator changes the frequency of the wave and hence it is also called as frequency changer. It does not affect the data carried by the wave.
3. A wavelength converter is termed as _______ if the converted wavelength is longer than the original signal wavelength.
a) Down converter
b) Up converter
Answer: b [Reason:] A wavelength converter is capable of receiving an incoming signal at any wavelength at the input port and produces output at the output port. A converter is termed as up converter when the output signal wavelength is longer than the original signal wavelength.
4. The ___________ converters cannot process different modulation formats.
b) Optoelectronic wavelength
d) Magnetic simulating
Answer: b [Reason:] In optoelectronic wavelength converters, the information contained in the intensity, frequency, phase of the signal is required to be reprocessed for the purpose of wavelength conversion. It does not process all the modulation formats.
5. The optical medium, in case of optical wavelength conversion is ___________
Answer: c [Reason:] The implementation of optical wavelength conversion involves non-linearity of the optical medium. It can be either active or passive, each providing different nonlinear effects.
6. The process of imposing the nonlinear response of the medium onto the control signal is known as ______________ scheme.
d) Repeater mixing
Answer: c [Reason:] The cross-modulation scheme involves changes produced due to the intensity variation of the intensity-modulated input signal. It takes place in the active cavity.
7. How many approaches are adopted by the cross-modulation scheme?
Answer: a [Reason:] Based on the properties of the nonlinear medium, the cross-modulation scheme can be divided into four main approaches. These are cross-gain modulation, cross-phase modulation, cross-absorption modulation, differential polarization modulation.
8. __________ wavelength converters make use of a passive optical medium to exploit non-linear effects.
Answer: d [Reason:] The nonlinear effects include four-wave mixing and difference frequency generation. Coherent wavelength converters use a passive medium to extend the changes of nonlinear effects.
9. A _____________ wavelength converter utilizes the nonlinear properties of a semiconductor optical amplifier to perform the conversion process.
a) Cross-gain modulation
b) Cross-phase modulation
c) Cross-absorption modulation
d) Differential polarization modulation
Answer: a [Reason:] Cross-gain modulation wavelength converter is also called as XGM wavelength converter. It uses semiconductor optical amplifier(SOA) along with its nonlinear properties for the conversion process.
10. The intensity modulated data on one signal wavelength is called as _______
a) Dispersed data
b) Pump signal
c) Probe signal
d) Frequency signal
Answer: b [Reason:] Pump signal is intensity modulated data. It produces variations in the carrier density within the SOA which provides inverted gain modulation in the SOA medium.
11. The probe signal is inverse to that of the pump signal. State whether the given statement is true or false.
Answer: a [Reason:] The gain modulations of the pump signal are imprinted onto the probe signal. Thus, the probe signal acquires the inverse copy of the pump signal, thereby contributing to the wavelength conversion with the pump signal.
12. In the XGM converter, the transfer function maintains the rectangular shape. State whether the given statement is true or false.
Answer: b [Reason:] By default, the ideal transfer function should be rectangular in shape. But it does not apply the same for XGM converter as the amplitude gradually decreases.
13. The speed of operation of XGM wavelength conversion is determined by the _______________ of the SOA.
a) Depletion level
b) Hole concentration
c) Carrier dynamics
d) Electron concentration
Answer: c [Reason:] The carrier dynamics deals with the interaction time between the input and the probe signal. On increasing the interaction time, the speed of operation of XGM wavelength conversion is increased.
14. ____________ is defined as the deviation in the emission frequency with respect to time when a laser is driven by a time-varying current source.
a) Intensity probe
d) Frequency chirp
Answer: d [Reason:] Frequency chirp occurs during the process of XGM and XPM. It is often termed as instantaneous frequency variation.
15. When frequency chirp shifts the optical frequency towards the shorter wavelength, it is known as ________
a) Red shift
b) Green shift
c) Yellow shift
d) Blue shift
Answer: d [Reason:] When frequency chirp shifts the optical frequency towards the shorter wavelength, it is known as blue shift. Similarly, when frequency chirp shifts the optical frequency towards the longer wavelength, it is known as red shift.