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 the 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.
11. ______ is also called as intermodal dispersion.
Waveguide Dispersion
Material Dispersion
Polarization mode Dispersion
Chromatic Dispersion
Answer:4. Chromatic Dispersion
Explanation:
Intramodal dispersion is also called chromatic dispersion because it depends on the range of wavelengths transmitted by a fiber.
Chromatic dispersion is the sum of two components: waveguide dispersion and material dispersion.
Unlike intermodal dispersion, chromatic dispersion can be positive or negative.
When material dispersion, which is always negative, is added to waveguide dispersion, the total can be positive or negative.
The two components of chromatic dispersion can also cancel each other, yielding zero dispersion.
12. 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?
6.12ns/km
7.69ns/km
10.29ns/km
8.23ns/km
Answer: 2. 7.69ns/km
Explanation:
Dispersion is the spreading out of light pulses as they travel along with fiber. It occurs because the speed of light through a fiber depends on its wavelength and the propagation mode.
In its simplest sense, dispersion measures pulse spreading per unit distance in nanoseconds or picoseconds per kilometer. Total pulse spreading, Δt, is
Δt = Dispersion × Distance
Given
Pulse Broading = 0.1 μs = 0.1 × 10-6
Distance = 13 km
Dispersion = (0.1 × 10-6)/13 = 7.69 ns/km.
13. A multimode graded-index fiber exhibits a total pulse broadening of 0.15μs over a distance of 16 km. Estimate the maximum possible bandwidth, assuming no intersymbol interference.
4.6 MHz
3.9 MHz
3.3 MHz
4.2 MHz
Answer:3. 3.3 MHz
Explanation:
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.
$B_{T} = \frac{1}{2\times0.15\times 10^{-6} }$
BT = 3.3 MHz
14. The optical source used in 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.
1.2 nm
1.3 nm
0.77 nm
0.98 nm
Answer:3. 0.77 nm
Explanation:
λRMS is the RMS spectral width of the source, in nm (the RMS spectral width is the standard deviation of the curve of power density versus wavelength)
The relative spectral width σλ/λ= 0.01 is given. The RMS spectral width can be calculated as follows:
Given
Wavelength = 0.70μm = 0.70 × 10-6
σλ/λ = 0.0011
σλ = 0.0011λ
= 0.0011 × 0.70 × 10-6
= 0.77 nm.
15. Dispersion is measured in the unit of time in ______
Second
Nanosecond
Picosecond
Both 2 and 3
Answer:4. Both 2 and 3
Explanation:
Dispersion is the spreading out of light pulses as they travel along with fiber. It occurs because the speed of light through a fiber depends on its wavelength and the propagation mode.
Dispersion is measured in the unit of time either in nanoseconds or picoseconds. The total dispersion produced by the fiber depends directly on its length. So, dispersion is typically measured in nanoseconds per kilometer (ns/km).
