Attenuation in optical Fiber MCQ || Attenuation in optical Fiber Questions and Answers

Answer:3. Photonic Crystal Fiber

Explanation: 

• Conventional optical fibers have several hundreds of losses in transmission.
• Conventional optical fibers have more transmission losses than Photonic Crystal Fiber.
• Photonic crystal fibers have resulted in a reduction in overall transmission losses which were initially several hundred decibels per kilometer even with the most straightforward designs.
• Increased control over the homogeneity of the fiber structures together with the use of highly purified silicon as the base material has now lowered these losses to a level of a very few decibels per kilometer for most PCF types, with a loss of just 0.3 dB km⁻¹ at 1.55 µm for a 100 km span being recently reported.
• In conventional optical fibers, electromagnetic modes are guided by total internal reflection in the core region, which has a slightly raised refractive index.
• The existence of two different guidance mechanisms makes PCFs versatile in their range of potential applications.

Answer: 3. 0.3 dB/km

Explanation: 

Conventional optical fibers have several hundreds of losses in transmission. Photonic crystal fibers have resulted in a reduction in overall transmission losses which were initially several hundred decibels per kilometer even with the most straightforward designs.

Increased control over the homogeneity of the fiber structures together with the use of highly purified silicon as the base material has now lowered these losses to a level of a very few decibels per kilometer for most PCF types, with a loss of just 0.3 dB km⁻¹ at 1.55 µm for a 100 km span being recently reported.

Answer: 1. Less than 1μm

Explanation: 

The high index contrast enables the PCF core to be reduced from around 8 µm in conventional fiber to less than 1 µm, which increases the intensity of the light in the core and enhances the nonlinear effects.

The principles of guidance and the characteristics of index-guided PCFs are similar to those of conventional fiber, there is greater index contrast since the cladding contains air holes with a refractive index of 1 in comparison with the normal silica cladding index of 1.457 which is close to the germanium-doped core index of 1.462.

Answer: 3. dB/km

Explanation: 

Attenuation is the reduction in the power of the light signal as it is transmitted. Attenuation is caused by passive media components such as cables, cable splices, and connectors.

Attenuation is also referred to as transmission loss. Channel attenuation helped to determine the maximum transmission distance prior to signal restoration. Attenuation is usually expressed in the logarithmic unit of decibel. It is given by

α_dBL = 10 log₁₀P_i / P_o

Where

α_dB = signal attenuation per unit length

P_i & P_o = Input and output power.

Answer:3. Both 1 and 2

Explanation: 

• The attenuation of the optical fiber is a result of two factors, absorption, and scattering. The absorption is caused by the absorption of the light and conversion to heat by molecules in the glass.
• Primary absorbers are residual OH+ and dopants used to modify the refractive index of the glass. This absorption occurs at discrete wavelengths, determined by the elements absorbing the light. The OH+ absorption is predominant and occurs most strongly around 1000 nm, 1400 nm, and above 1600 nm.
• The largest cause of attenuation is scattering. Scattering occurs when light collides with individual atoms in the glass and is anisotropic.
• The light that is scattered at angles outside the numerical aperture of the fiber will be absorbed into the cladding or transmitted back toward the source Scattering is also a function of wavelength, proportional to the inverse fourth power of the wavelength of the light.
• Thus if you double the wavelength of the light, you reduce the scattering losses by 2 to the 4th power or 16 times. Therefore, for long-distance transmission, it is advantageous to use the longest practical wavelength for minimal attenuation and maximum distance between repeaters.

Answer: 2. Attenuation

Explanation: 

When a signal is transmitted from transmitter to receiver via optical fiber medium in an optical fiber link, various types of loss mechanisms are responsible for signal degradation. This is termed as attenuation.

Answer: 1. Cutback Method

Explanation: 

The most accurate way of measuring the fiber attenuation coefficient requires transmitting light of a known wavelength through the fiber and measuring the changes over distance. The conventional method, known as the cutback method, involves coupling fiber to the source and measuring the power out of the far end. The fiber is then cut near the source and power measured again.

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.

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

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.

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.

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.

Answer:4. Holey fibers

Explanation: 

A photonic crystal fiber (also called holey fiber, hole-assisted fiber, microstructure fiber, or microstructured fiber). A photonic crystal is a low-loss periodic dielectric medium constructed using a periodic array of microscopic air holes that run along the entire fiber length.