Optical Communications Questions and Answers – Dispersion – Chromatic Dispersion

«
»

This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on “Dispersion – Chromatic Dispersion “.

1. What is dispersion in optical fiber communication?
a) Compression of light pulses
b) Broadening of transmitted light pulses along the channel
c) Overlapping of light pulses on compression
d) Absorption of light pulses
View Answer

Answer: b
Explanation: Dispersion of transmitted optical signal causes distortion of analog as well as digital transmission. When the optical signal travels along the channel, the dispersion mechanism causes broadening of light pulses and thus in turn overlaps with their neighboring pulses.
advertisement

2. What does ISI stands for in optical fiber communication?
a) Invisible size interference
b) Infrared size interference
c) Inter-symbol interference
d) Inter-shape interference
View Answer

Answer: c
Explanation: Dispersion causes the light pulses to broaden and overlap with other light pulses. This overlapping creates an interference which is termed as inter-symbol interference.

3. For no overlapping of light pulses down on an optical fiber link, the digital bit rate BT must be:
a) Less than the reciprocal of broadened pulse duration
b) More than the reciprocal of broadened pulse duration
c) Same as that of than the reciprocal of broadened pulse duration
d) Negligible
View Answer

Answer: a
Explanation: The digital bit rate and pulse duration are always inversely proportional to each other.
B T < = ½ Γ
Where B T = bit rate
2Γ= duration of pulse.

4. The maximum bit rate that may be obtained on an optical fiber link is 1/3Γ. State whether the given statement is true or false.
a) True
b) False
View Answer

Answer: b
Explanation: The digital bit rate is function of signal attenuation on a link and signal to noise ratio. For the restriction of interference, the bit rate should be always equal to or less than 1/2Γ.

5. 3dB optical bandwidth is always ___________ the 3dB electrical bandwidth.
a) Smaller than
b) Larger than
c) Negligible than
d) Equal to
View Answer

Answer: b
Explanation: Optical bandwidth is half of the maximum data rate. For non-return:0 (NRZ), bandwidth is same as bit rate. The bandwidth B for metallic conductors is defined by electrical 3dB points. Optical communication uses electrical circuitry where signal power has dropped to half its value due to modulated portion of modulated signal.

advertisement

6. A multimode graded index fiber exhibits a total pulse broadening of 0.15μsover a distance of 16 km. Estimate the maximum possible bandwidth, assuming no intersymbol interference.
a) 4.6 MHz
b) 3.9 MHz
c) 3.3 MHz
d) 4.2 MHz
View Answer

Answer: c
Explanation: The maximum possible bandwidth is equivalent to the maximum possible bitrate. The maximum bit rate assuming no inter-symbol interference is given by
B T = ½ Γ
Where B T = bandwidth.

7. What is pulse dispersion per unit length if for a graded index fiber, 0.1μs pulse broadening is seen over a distance of 13 km?
a) 6.12ns/km
b) 7.69ns/km
c) 10.29ns/km
d) 8.23ns/km
View Answer

Answer: b
Explanation: The dispersion mechanism causes broadening of light pulses. The pulse dispersion per unit length is obtained by dividing total dispersion of total length of fiber.
Dispersion = 0.1*10-6/13= 7.69 ns/km.

8. Chromatic dispersion is also called as intermodal dispersion. State whether the given statement true or false.
a) True
b) False
View Answer

Answer: b
Explanation: Intermodal delay is a result of each mode having a different group velocity at a single frequency. The intermodal delay helps us to know about the information carrying capacity of the fiber.

9. Chromatic dispersion is also called as intermodal dispersion. State true or false
a) True
b) False
View Answer

Answer: b
Explanation: Intermodal delay, the name only suggests, includes many modes. On the other hand chromatic dispersion is pulse spreading that takes place within a single mode. Chromatic dispersion is also called as intermodal dispersion.
advertisement

10. The optical source used in a fiber is an injection laser with a relative spectral width σλ/λ of 0.0011 at a wavelength of 0.70μm. Estimate the RMS spectral width.
a) 1.2 nm
b) 1.3 nm
c) 0.77 nm
d) 0.98 nm
View Answer

Answer: c
Explanation: The relative spectral width σλ/λ= 0.01 is given. The rms spectral width can be calculated as follows:
σλ /λ = 0.0011
σλ = 0.0011λ
= 0.0011*0.70*10-6
= 0.77 nm.

11. In waveguide dispersion, refractive index is independent of
a) Bit rate
b) Index difference
c) Velocity of medium
d) Wavelength
View Answer

Answer: d
Explanation: In material dispersion, refractive index is a function of optical wavelength. It varies as a function of wavelength. In wavelength dispersion, group delay is expressed in terms of normalized propagation constant instead of wavelength.

Sanfoundry Global Education & Learning Series – Optical Communications.
To practice all areas of Optical Communications, here is complete set of 1000+ Multiple Choice Questions and Answers.

advertisement
advertisement
advertisement
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn