1. What is dispersion in optical fiber communication?
Compression of light pulses
Broadening of transmitted light pulses along the channel
Overlapping of light pulses on compression
Absorption of light pulses
Answer: 2. Broadening of transmitted light pulses along the channel
Explanation:
Dispersion is the spreading of the light pulse as its travels down the length of an optical fiber. Dispersion limits the bandwidth or information-carrying capacity of a fiber. The bit- rates must be low enough to ensure that pulses are farther apart and therefore the greater dispersion can be tolerated.
Dispersion of transmitted optical signals 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. ________ is the spreading of light pulse as its travels down the length of an optical fiber.
Transmission
Dispersion
Diffusion
Radiation
Answer: 2. Dispersion
Explanation:
Dispersion is the spreading of the light pulse as its travels down the length of an optical fiber. Optical fiber dispersion describes the process of how an input signal broadens/spreads out as it propagates/travels down the fiber. Normally, dispersion in fiber optic cable includes modal dispersion, chromatic dispersion, and polarization mode dispersion.
3. What does ISI stand for in optical fiber communication?
Invisible size interference
Infrared size interference
Inter-symbol interference
Inter-shape interference
Answer:3. Inter-symbol interference
Explanation:
In telecommunication, intersymbol interference (ISI) is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have a similar effect as noise, thus making the communication less reliable. Dispersion causes the light pulses to broaden and overlap with other light pulses.
4. In waveguide dispersion, refractive index is independent of ______________
Bit rate
Index difference
Velocity of medium
Wavelength
Answer:4. Wavelength
Explanation:
The waveguide dispersion occurs because the propagation constant is a function of the fiber parameters (core radius and difference between refractive indices in fiber core and cladding) and at the same time is a function of wavelength.
The longer the wavelength, the more power in the cladding.
The effect of waveguide dispersion on pulse spreading can be approximated by assuming that the refractive index of a material is independent of wavelength.
In material dispersion, the 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.
5. ______ dispersion occurs because the propagation constant is a function of the fiber parameters.
Intramodal Dispersion
Chromatic Dispersion
Waveguide Dispersion
Intermodal Dispersion
Answer:3. Waveguide Dispersion
Explanation:
The waveguide dispersion occurs because the propagation constant is a function of the fiber parameters.
Waveguide dispersion occurs because a single-mode fiber confines only about 80 percent of the optical power to the core.
The dispersion thus arises, since the 20 percent of the light propagating in the cladding travels faster than the light confined to the core.
The amount of waveguide dispersion depends on the fiber design since the modal propagation constant β is a function of a/ (the optical fiber dimension relative to the wavelength λ; here a is the core radius.)
6. For no overlapping of light pulses down on an optical fiber link, the digital bit rate BT must be ___________
Less than the reciprocal of broadened pulse duration
More than the reciprocal of broadened pulse duration
Same as that of than the reciprocal of broadened pulse duration
Negligible
Answer: 1. Less than the reciprocal of broadened pulse duration
Explanation:
For no overlapping of light pulses down on an optical fiber link the digital bit rate BT, must be less than the reciprocal of the broadened (through dispersion) pulse duration (2τ). Hence:
The digital bit rate and pulse duration are always inversely proportional to each other.
$B_{T}<\frac{1}{2\tau }$
Where
BT = bit rate
2Γ = duration of pulse.
7. Which of the following are types of optical fiber dispersion?
Intramodal Dispersion
Waveguide Dispersion
Material Dispersion
All of the above
Answer:4. All of the above
Explanation:
The term dispersion describes the pulse broadening effect in fibres.
The types of dispersion are
Intramodal Dispersion
Material Dispersion
Waveguide Dispersion
Intermodal Dispersion
Polarization mode Dispersion
8. The maximum bit rate that may be obtained on an optical fiber link is ______
1/2Γ
1/3Γ
1/Γ
1/4Γ
Answer:2. 1/2Γ
Explanation:
For no overlapping of light pulses down on an optical fiber link the digital bit rate BT, must be less than the reciprocal of the broadened (through dispersion) pulse duration (2τ). Hence:
The digital bit rate and pulse duration are always inversely proportional to each other.
$B_{T}<\frac{1}{2\tau }$
Where
BT = bit rate
2Γ = duration of pulse.
The digital bit rate is the 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.
9. The total dispersion produced by the fiber depends directly on its ______.
Bandwidth
Length
Power
Refractive Index
Answer:2. Length
Explanation:
Dispersion is the spreading of the light pulse as its travels down the length of an optical fiber. Dispersion limits the bandwidth or information-carrying capacity of a fiber.
The total dispersion produced by the fiber depends directly on its length. So, dispersion is typically measured in nanoseconds per kilometer (ns/km).
10. 3dB optical bandwidth is always ___________ the 3dB electrical bandwidth.
Smaller than
Larger than
Negligible than
Equal to
Answer:1. Smaller than
Explanation:
The 3db point occurs when the ratio of currents is equal to 1/2. Note that electrical bandwidth occurs at the current ratio is 0.707, as electrical power is proportional to the square of the current. Therefore electrical bandwidth is smaller than the optical bandwidth.
Optical communication uses electrical circuitry where signal power has dropped to half its value due to modulated portion of the modulated signal.
