91. When the input and output power in an optical fiber is 120μW & 3μW respectively and the length of the fiber is 8 km. What is the signal attenuation per km for the fiber?
a) 3dB/km
b) 2dB/km
c) 1dB/km
d) 4dB/km
Answer:2. 2dB/km
Explanation:
Signal attenuation per unit length is given by
αdBL = 10 logPi / Po
αdBL = 10 log(120/3)
αdBL = 10 log40
αdBL = 16 dB
αdB = 16 dB/L
αdB = 16 dB/8
αdB = 2dB/km.
92. If the input power 100μW is launched into 6 km of fiber, the mean optical power at the fiber output is 2μW. What is the overall signal attenuation through the fiber assuming there are no connectors or splices?
a) 15.23dB
b) 16.98dB
c) 17.12dB
d) 16.62dB
Answer:2. 16.98dB
Explanation:
Signal attenuation per unit length is given by
αdBL = 10 logPi / Po
αdBL = 10 log(100/2)
αdBL = 10 log50
αdBL = 16 .987dB
93. A device that reduces the intensity of light in optical fiber communications is ________
Compressor
Barometer
Optical attenuator
Reducer
Answer:3. Optical attenuator
Explanation:
Optical attenuators are devices that can be used to attenuate a light beam, i.e., to reduce its optical power. The amount of attenuation is often specified in terms of an optical density or in decibels, sometimes in percent of transmitted or blocked optical power. An optical attenuator is a device that affects the intensity of light and incurs a loss in transmission.
A compressor compresses the signal before transmission. It does not affect the intensity of light.
94. _______ may be defined as the ratio of input and output optical power for a particular optical wavelength.
Decibel
Watt
Noise
Any of the above
Answer:1. Decibel
Explanation:
Decibel may be defined as the ratio of input and output optical power for a particular optical wavelength.
Signal attenuation refers to the loss in transmission and it needs a logarithmic unit to express. Decibel is mainly used for comparing two power levels. It has the advantage that the operations of multiplication and division reduce to addition and subtraction.
95. Earlier the attenuation level of optical fiber is upto
1 dB/km
2 dB/km
5 dB/km
20 dB/km
Answer:2. 20 dB/km
Explanation:
Earlier, attenuation was 20 dB/km which is now reduced to 0.16 dB/km at 1.55 µm wavelength in the single-mode fiber and further research is being done to reduce this.
If the attenuation is reduced to the low level, the cost of optical fiber link will be reduced as optical amplifiers and dispersion compensators will be placed at long distances and their total requirement will go down in the long haul optical link.
