# Optical Communications Questions and Answers – Dispersion – Chromatic Dispersion

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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

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.

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

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

Explanation: The digital bit rate and pulse duration are always inversely proportional to each other.
BT < = $$\frac{1}{2}$$ Γ
Where BT = bit rate
2Γ = duration of pulse.

4. The maximum bit rate that may be obtained on an optical fiber link is 1/3Γ.
a) True
b) False

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

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.

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

Explanation: The maximum possible bandwidth is equivalent to the maximum possible bitrate. The maximum bit rate assuming no inter-symbol interference is given by
BT = $$\frac{1}{2}$$ Γ
Where BT = 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

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.
a) True
b) False

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.
a) True
b) False

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.

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

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

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.

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