Question

A low loss fibre has average loss of 3dB/km at 900 nm. Compute the length over which a) Power decreases by 50%b) Power decreases by 75 %.​

Answer 1

Explanation:

One of the important properties of optical fiber is signal attenuation. It is also known as

fiber loss or signal loss. The signal attenuation of fiber determines the maximum distance

between transmitter and receiver. The attenuation also determines the number of

repeaters required, maintaining repeater is a costly affair.

Another important property of optical fiber is distortion mechanism. As the signal pulse

travels along the fiber length it becomes broader. After sufficient length the broad pulses

starts overlapping with adjacent pulses. This creates error in the receiver. Hence the

distortion limits the information carrying capacity of fiber.

2. Attenuation

Attenuation is a measure of decay of signal strength or loss of light power that occurs as

light pulses propagate through the length of the fiber.

In optical fibers the attenuation is mainly caused by two physical factors absorption and

scattering losses. Absorption is because of fiber material and scattering due to structural

imperfection within the fiber. Nearly 90 % of total attenuation is caused by Rayleigh

scattering only.

Micro bending of optical fiber also contributes to the attenuation of signal. The rate at

which light is absorbed is dependent on the wavelength of the light and the characteristics

of particular glass. Glass is a silicon compound; by adding different additional chemicals

to the basic silicon dioxide the optical properties of the glass can be changed.

The Rayleigh scattering is wavelength dependent and reduces rapidly as the wavelength

of the incident radiation increases.

The attenuation of fiber is governed by the materials from which it is fabricated, the

manufacturing process and the refractive index profile chosen. Attenuation loss is

measured in dB/km.