This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on “Modulation Formats”.
1. _____________ is essentially a crude form of Amplitude shift keying.
a) Analog modulation
b) Digital intensity modulation
d) Receiver structure
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.
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.
b) 2 to 8%
c) 10 t0 50%
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).
5. ________ does not require an external modulator.
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
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.
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. State whether the given statement is true or false.
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.
9. External modulation for ________ modulation format allows the most sensitive coherent detection mechanism.
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
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
Explanation: Polarization shift keying is abbreviated as PolSK. PolSK requires additional receiver complexity than other modulation formats.
Sanfoundry Global Education & Learning Series – Optical Communications.
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