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 B_{T}, 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

B_{T} = 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 B_{T}, 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

B_{T} = 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, Δ_{t}, is

Δ_{t }= 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

B_{T} = bit rate

2Γ = duration of pulse.

$B_{T} = \frac{1}{2\times0.15\times 10^{-6} }$

B_{T} = 3.3 MHz

14. The optical source used in fiber is an injection laser with a relative spectral width σ_{λ}/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:

λ_{RMS} 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 σ_{λ}/λ= 0.01 is given. The RMS spectral width can be calculated as follows:

Given

Wavelength = 0.70μm = 0.70 × 10^{-6}

σ_{λ}/λ = 0.0011

σ_{λ} = 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).