Optical Fiber Systems 2 : Coherent and Phase Modulated ( Optical Communication ) MCQs – Optical Communication MCQs
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Optical Communication MCQs – Optical Fiber Systems 2 : Coherent and Phase Modulated ( Optical Communication ) MCQs
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Practical Constraints of Coherent Transmission
1. Which technology development has helped the field of optical fiber communication?
a) Glass technology
b) Component technology
c) Multiplexing
d) Power
Answer: b
Explanation: Substantial developments in component technology have allowed the initial difficulties in the optical fiber communication to go away. The coherent factor experienced most of the difficulties.
2. __________ dictates the performance characteristics required from components and devices which are to be utilized in coherent optical fiber systems.
a) System considerations
b) Bluetooth technology
c) Multiplexing
d) Practical constraints
Answer: d
Explanation: Practical constraints inhibit the development of coherent optical fiber communications. These constraints are derived from factors associated with the elements of the coherent optical fiber communication.
3. Coherent optical transmission is degraded by the ________ associated with the transmitter and local oscillator lasers.
a) Phase noise
b) White noise
c) Dissipation
d) Power
Answer: a
Explanation: Phase noise is determined by the laser line width. The phase noise associated with both the transmitter and the mid-tier section severely degrades the coherent optical transmission as well as reception.
4. ___________ improves the spectral purity of the device output and noise current.
a) Power dissipation
b) Laser line width reduction
c) Laser line width injection
d) Phase noise
Answer: b
Explanation: Laser line width determines the level of phase noise and long term phase stability. The reduced phase noise is obtained using narrow-line width devices. This improves the spectral purity as well as reduces the noise current.
5. ____________ is the principal cause of line width broadening in the coherent devices.
a) Electromagnetic field
b) Power dissipation
c) Injection laser phase noise
d) Gaussian noise
Answer: c
Explanation: Injection laser phase noise affects the system performance. The system performance considerations include receiver noise, power loss and line width broadening.
6. Which technique was started for narrowing of injection laser line widths?
a) External resonator cavity
b) Long-hauled oscillator
c) Circulator
d) Gyrator
Answer: a
Explanation: Many approaches evolved in time for laser line width problem. The one which sustained and showed effects was the use of external resonator cavity in the lasers.
7. The line width tolerance is wider for heterodyne receivers.
a) False
b) True
Answer: b
Explanation: The laser line width requirements depend on the modulation format, coherent detection mechanism which includes the use of heterodyne and homodyne receivers. The line width tolerance is wider for heterodyne receivers when employing FSK modulation.
8. ___________ is an alternative to reduce phase noise and line width requirements.
a) Homodyne detection
b) Heterodyne detection
c) FSK modulation
d) Phase diversity reception
Answer: d
Explanation: The more sensitive coherent transmission techniques are most affected by phase noise problem. A specially configured reception technique called as phase diversity reception technique is used to overcome phase noise problem.
9. ______________ is the progressive spatial separation between the two polarization modes as they propagate along the fiber.
a) Fiber birefringence
b) Fiber dispersion
c) Fiber separation
d) Fiber coupling
Answer: a
Explanation: In a perfectly formed fiber, both modes would travel together. But, in practice, the fiber contains random manufacturing irregularities. This result in a progressive spatial separation called as fiber birefringence.
10. How many compensator devices are required to provide full polarization-state control?
a) Three
b) One
c) Four
d) Two
Answer: d
Explanation: At least two compensator devices are required to provide full polarization-state control. They can be placed in either the incoming signal path or the local oscillator output path.
11. Which technique was found to be providing an infinite range of polarization control?
a) Homodyne detection
b) Fiber squeezers
c) Heterodyne detection
d) Power dissipation
Answer: b
Explanation: Four fiber squeezers provide an infinite range of adjustment or endless polarization control. Stress was applied to the fiber in the local oscillator path using the squeezers which are angled at 45 degrees to each other.
12. What is the main drawback of the squeezer?
a) Damages the fiber
b) Attenuation
c) Dispersion
d) Signal degradation
Answer: a
Explanation: The squeezers are simple to configure. The main drawback of squeezer is that they tend to damage the fiber and could not be engineered into reliable transducers for practical systems.
13. The use of balanced receiver compensates the losses due to coupling optics.
a) True
b) False
Answer: a
Explanation: The losses due to coupling optics and the suppression of the excess noise in the local oscillator signal are eliminated by the use of balanced receiver. It is also called as balanced-mixer receiver.
14. ___________ is the phenomenon which occurs in the single carrier systems due to small refractive index changes induced by the optical power fluctuations.
a) SBS gain
b) Self-phase modulation
c) FSK modulation
d) Birefringence
Answer: b
Explanation: It occurs only in the single-carrier systems. It affects the phase of the transmitted signal.
Modulation Formats
1. _____________ is essentially a crude form of Amplitude shift keying.
a) Analog modulation
b) Digital intensity modulation
c) Photodetector
d) Receiver structure
Answer: b
Explanation: Many techniques have been developed to amplitude modulate an optical signal. Digital intensity modulation used in direct detection systems is essentially a crude form of ASK in which the received signal is detected using square law detector.
2. Almost _________ of the transmitter power is wasted in the use of external modulators.
a) Half
b) Quarter
c) One-third
d) Twice
Answer: a
Explanation: All external modulators suffer the drawback that around half of the transmitted power is wasted. To avoid this, non-synchronous detection can be employed.
3. The line width in the range ________ of bit rate is specified for ASK heterodyne detection.
a) 8%
b) 2 to 8%
c) 10 t0 50%
d) 70%
Answer: c
Explanation: The ASK modulation scheme can be used with laser sources exhibiting the line widths comparable with the bit transmission rate. For ASK heterodyne detection, line width range of 10 to 50% is usually specified.
4. ______________ is also referred to as on-off keying (OOK).
a) FSK
b) DSK
c) PSK
d) ASK
Answer: d
Explanation: Amplitude shift keying (ASK) involves the locking and assembling of the amplitude of the wave. It involves the carrier wave along with the amplitude wave or transmitted wave and hence referred to as on-off keying.
5. ________ does not require an external modulator.
a) FSK
b) DSK
c) PSK
d) ASK
Answer: a
Explanation: FSK involves the frequency deviation property of the directly modulated semiconductor laser used in wideband systems. Unlike ASK, it does not require an external modulator, which in turn, avoids the wastage of transmitted power.
6. The frequency deviation at frequencies above 1 MHz is typically ____________
a) 10 to 20 mA-1
b) 100 to 500 mA-1
c) 1000 to 2000 mA-1
d) 30 to 40 mA-1
Answer: b
Explanation: The carrier modulation effect occurs at the frequencies above 1 MHz. At the phase of carrier modulation, the frequency deviation is about 100 to 500 mA-1.
7. ___________ offers the potential for improving the coherent optical receiver sensitivity by increasing the choice of signalling frequencies.
a) MFSK
b) MDSK
c) MPSK
d) MASK
Answer: a
Explanation: Multilevel FSK includes 4-level or 8-level FSK. It improves the receiver sensitivity by reducing the deviation and increasing the usage of signalling frequencies.
8. Eight level FSK and binary PSK yields an equivalent sensitivity.
a) False
b) True
Answer: b
Explanation: Binary PSK and 8-level FSK provides an equivalent sensitivity. The main drawback of 8-level FSK is that it yields an equivalent sensitivity to binary PSK at the expense of a greater receiver bandwidth requirement.
Optical Fiber Systems 1 : Intensity Modulation MCQs
9. External modulation for ________ modulation format allows the most sensitive coherent detection mechanism.
a) FSK
b) DSK
c) PSK
d) ASK
Answer: c
Explanation: External modulation for PSK is usually straightforward. It is therefore utilized to provide the modulation format which allows the most sensitive coherent detection mechanism.
10. _________ can potentially provide spectral conservation through the use of multilevel signalling.
a) M-ary PSK
b) MFSK
c) ASK
d) DFSK
Answer: a
Explanation: In M-ary schemes, the spectral efficiency is increased by the factor log2 M.this is purely for M-level schemes which can provide multilevel signalling patterns.
11. The digital transmission on implementation of polarization modulation which involves polarization characteristics of the transmitted optical signal is known as _____________
a) Frequency shift keying
b) Amplitude shift keying
c) Phase shift keying
d) Polarization shift keying
Answer: d
Explanation: Polarization shift keying is abbreviated as PolSK. PolSK requires additional receiver complexity than other modulation formats.
Demodulation Schemes
1. _________ heterodyne detection does not require phase matching between the incoming signal and the local oscillator.
a) Synchronous
b) Asynchronous
c) Noisy
d) Spatial
Answer: b
Explanation: For heterodyne detection, a beat note signal produces the IF signal which is obtained using the square law optical detector. Hence, it does not require phase matching.
2. In ___________ detection, the phase of the local oscillator signal is locked to the incoming signal.
a) Homodyne
b) Heterodyne
c) Spatial
d) Noisy
Answer: a
Explanation: Phase diversity and multiport detection is considered to be a form of heterodyne detection. In case of homodyne detection, the incoming signal is bundled with the phase of the local oscillator signal.
3. The introduction of nonlinear element within the ______ is necessary to enable efficient carrier recovery.
a) PSK
b) ASK
c) FSK
d) PLL
Answer: d
Explanation: Carrier recovery is done by slightly reducing the depth of the phase modulation. This is done by the introduction of the nonlinear within the phase locked loop(PLL).
4. What is the main attraction of the optical homodyne detection?
a) Receiver sensitivity
b) Transmission power
c) Modulation scheme
d) Line width
Answer: a
Explanation: The main attraction of optical homodyne detection is the potential 3dB improvement in the receiver sensitivity. It also eases the receiver bandwidth requirement considerably.
5. The phase difference between the local oscillator and the source must be ________ for high sensitivity reception.
a) Deep
b) High
c) Direct
d) Near zero
Answer: d
Explanation: The phase difference must be held near zero for high sensitivity reception. As it moves towards 90 degrees, the output signal current becomes zero and the detection process will cease.
6. How many strategies of homodyne demodulation have proved to be successful for coherent optical fiber reception?
a) One
b) Three
c) Four
d) Two
Answer: d
Explanation: Two homodyne demodulation strategies have been successful for optical fiber reception. They are the use of either a pilot carrier or a decision-driven optical phase locked loop.
7. In the __________ receiver, the incoming signal is not precisely shifted to the baseband.
a) Intra-dyne
b) Heterodyne
c) Homodyne
d) Optical
Answer: a
Explanation: In homodyne detection, the incoming signal is precisely shifted to the baseband. In intra-dyne receiver, the incoming signal is shifted to a frequency lower than the transmission rate.
8. The use of PLL can be avoided in the intra-dyne detection.
a) False
b) True
Answer: b
Explanation: In intra-dyne receiver, the incoming signal is lower than the transmission rate. In place of PLL, a baseband filter in the form of electrical filtering can be used.
9. Phase diversity reception is also referred to as ______________
a) Homodyne detection
b) Noisy detection
c) Homodyne detection
d) Multiport detection
Answer: d
Explanation: In phase diversity reception, phase is not locked. It also has bandwidth advantage over homodyne detection as it converts the incoming signal directly to baseband.
10. The received signal through polarization diversity reception is linearly polarized.
a) False
b) True
Answer: a
Explanation: The received optical signal through polarization diversity reception is elliptically polarized. It is also uncontrollable unlike coherent receivers.
Receiver Sensitivities
1. ASK with heterodyne detection can achieve same SNR limit as ASK with asynchronous detection.
a) True
b) False
Answer: a
Explanation: The coherent optical systems’ receiver sensitivities depend on the SNR limit and the 3dB improvement in the receiver bandwidth. The ASK with heterodyne and asynchronous detection allows for the same receiver SNR limit.
2. In a ______________ receiver, the analyses of signal and noise phenomena are more complicated than in the IM-DD case.
a) Homodyne
b) Heterodyne
c) Circular
d) Heterogeneous
Answer: b
Explanation: The optical output appearance shows the analysis of signal and noise phenomenon. In a heterodyne receiver, the analyses of signal and noise phenomena are more complicated than in the IM-DD case because the optical detector output appears as an IF signal and not as a baseband signal.
3. FSK modulation is attributed to the use of __________ frequencies unlike ASK modulation.
a) One
b) Three
c) Two
d) Four
Answer: c
Explanation: FSK heterodyne detection has receiver sensitivity of 3dB more than tha ASK. Hence, it is attributed to the use of two frequencies unlike ASK where only one frequency is used.
4. BER of FSK modulation scheme is ________ as/to the ASK modulation scheme.
a) Twice
b) Thrice
c) Unequal
d) Same
Answer: d
Explanation: FSK modulation uses two frequencies. The similar BER can be obtained with the two modulation schemes when the average power is transmitted.
5. ___________ can also be used in place of multilayered filters in the dual filter direct detection FSK receiver.
a) Bragg gratings
b) Ceramic gratings
c) Aluminum arsenide
d) Bragg diodes
Answer: a
Explanation: A dual optical filter uses two multilayered dielectric filters. Bragg gratings are an excellent alternative to the dual dielectric filters as they perform the same function as dual dielectric multilayer filters.
6. FSK synchronous detection is _________ more sensitive than asynchronous heterodyne detection.
a) 0.24 dB
b) 0.45 dB
c) 0.9 dB
d) 0.12 dB
Answer: b
Explanation: During FSK detection, the probability of error is increased with respect to direct detection as a result of amplified noise from the orthogonal polarization. Thus, the synchronous detection is more sensitive than the asynchronous heterodyne detection.
7. The asynchronous heterodyne detection is _________ more sensitive than the dual filter direct detection FSK receiver.
a) 0.9 dB
b) 0.23 dB
c) 0.43 dB
d) 0.40 dB
Answer: d
Explanation: The orthogonal polarization leads to the amplified noise structures and behavior in the FSK receiver. This leads to the loss in the sensitivity and accuracy of the FSK receiver.
8. The use of __________ was undertaken to separate the polarization in an analog to digital conversion.
a) FSK
b) DSP
c) ASK
d) DP-FSK
Answer: b
Explanation: DSP is abbreviated as digital signal processing. Digital signal processing involves the cutting, trimming and manipulating of signals in the process of analog to digital conversion. The signals are converted in the form of numerous pulses.
Multicarrier Systems
1. A major attribute of coherent optical transmission was its ability to provide _______________ for future multicarrier systems and networks.
a) Attenuation
b) Dispersion
c) Frequency selectivity
d) Noisy carriers
Answer: c
Explanation: A coherent optical transmission involves the wavelength, frequency and the distance as its main factors. It provides frequency and wavelength selectivity with narrow channel spacing’s for future multicarrier systems and networks.
2. The channel width is narrow for the coherent systems than the WDM systems.
a) True
b) False
Answer: a
Explanation: The coherent optical systems involve wavelength selectivity with narrow channel spacing. The conventional WDM systems use a far more relaxed channel spacing than the coherent systems.
3. The technique within the coherent multicarrier systems used to broadcast the optical signals over the network is the use of passive _____________
a) Star coupler
b) Optical resonator
c) Optical regenerator
d) Local oscillator
Answer: a
Explanation: In multicarrier systems, the channels are separated via various techniques. The use of passive star coupler ensures that the optical signals are broadcasted over the network within the coherent system.
4. Estimate the minimum transmitter power if number of photons per bit are 150, wavelength 1.3 micrometer with an optical bandwidth of 20 THz.
a) 0.2 mW
b) 0.5mW
c) 1 mW
d) 2.3mW
Answer: b
Explanation: The minimum transmitter power is given by –
Ptx = Np hf B
Here, Ptx = transmitter power, f = frequency, h = Planck’s constant and B = bandwidth.
5. The performance degradation due to nonlinear phase noise is referred to as _____________
a) Munich effect
b) Linear bipolar effect
c) Gordon Mollenauer effect
d) Delta effect
Answer: c
Explanation: The phase noise is a variant due to the RZ signal. The nonlinear phase noise causes severe performance degradation in terms of bandwidth and frequency.
6. ______________ facilitates the doubling of the feasible spectral efficiency through the transmission of independent information in each of the two orthogonal polarizations.
a) WDM
b) Gordon Mollenauer effect
c) EDFA control
d) POLMUX
Answer: d
Explanation: POLMUX is abbreviated as Polarization multiplexing. It provides a different approach to the multilevel modulation. It requires polarization control at the receiver side.
7. ____________ is a transparent multiplexing technique.
a) POLMUX
b) PDM
c) WDM
d) Munich
Answer: a
Explanation: It is a transparent technique as it is not dominated by polarization mode dispersion or polarization-dependent loss. It provides many advantages to the multilevel modulation format in comparison to the non-POLMUX signals at the same data rate.
8. Which of the following is not a drawback of POLMUX?
a) Polarization-sensitive detection at the receiver
b) Receiver complexity
c) Cross-polarization nonlinearities
d) Multilevel modulation
Answer: d
Explanation: The drawbacks of the POLMUX include receiver complexity, polarization sensitivity at the receiver side and the cross polarization nonlinearities. It is advantageous to the multilevel modulation scheme.
9. A multicarrier modulation format in which there has been growing interest to compensate for impairments in optical fiber transmission systems is _______________
a) OFDM
b) EDM
c) WDM
d) ADM
Answer: a
Explanation: OFDM is abbreviated as orthogonal frequency division multiplexing. It combats both fiber chromatic dispersion and polarization mode dispersion.
10. It is suggested that the technique with high white noise is an attractive option for use in long haul systems.
a) False
b) True
Answer: a
Explanation: Long haul systems require a technique which boosts the distance covered by the signals with less use of carrier signal. The technique which exhibits high spectral density is an attractive option for the long haul systems.