Home Science Spectroscopy Optical Fiber

# Optical Fiber

## What is optical fiber cable?

Optical fiber cable or optical fibre cable is a new development in the field of communication technology that can transmit light along the cable over a considerable distance. It is developed mainly for telecommunication industries. In the latter, it is used for the transmission of light over considerable distances in the spectrophotometer.

Optical fiber behaves exactly like an electrochemical sensor. It can transmit light along the cable over a considerable distance.

The core in fiber transmitted light signals and cladding guided the light within the core. Therefore, light transmits along the core is called a wave guide. Optical fiber is made by drawing glass (silicon) or plastic which is slightly thicker than that of human hair.

## Properties of Optical Fiber

Optical fibers have a coaxial core which is surrounded by shielding material of higher refractive index followed by an external coating of a protective buffer. The transmitted light travels by internal reflection within this coaxial core tube. A typical fiber is systematically represented by the following picture,

### Numerical aperture

The numerical aperture (NA) of a fiber is the sine of the largest angle of entering light. It is mathematically represented,
NA = fext b sinÎ± = âˆš(f12 – f22)
where, f2 = refractive index of cladding,
f1 = refractive index of the coaxial core tube, and
fext = refractive index of external medium.

Numerical aperture (NA) values for different fibers are different. Higher core values with respect to cladding mean larger values of NA. However, increasing NA causes higher scattering loss from a greater concentration of dopant.

### Attenuation

The loss of light per unit length at different wavelengths is an equally important property of optical fiber. It is called attenuation and is expressed by unit decibel/km.

Mathematically, attenuation is expressed as,
db = 10 logP0/P
where, P0 = input intensity
P = output intensity of fiber

## Types of optical fiber

The types and uses of the optical fiber depend on the basis of refractive index, core size, numerical aperture, cladding materials, and mode of propagation of light.

• A compound with the glass itself as the protective coating with a core size of 15 to 75 micrometer is used commonly in ultraviolet visible spectroscopy. The numerical aperture (NA) of this type of fiber is 0.5 to 0.8 and the attenuation value is 800 db/km.
• Silica with silicon cladding and a nylon buffer (core size = 100 – 2000 Î¼m, NA = 0.5 – 0.6, attenuation = 200 db/km) is useful for UV and IR regions of the electromagnetic spectrum.
• Silica with suitable cladding material like fluoropolymer, silicon, or doped silica is used for long-distance transmission.

According to the mode of propagation of light, optical fibers are two types, single-mode fibers, and multimode fibers.

1. Single-mode fiber: A single-mode optical cable is glass fiber with a diameter of 8.3 to 10 micrometers with one mode of transmission. It is used mainly for the long-distance transmission of signals.
2. Multimode fiber: Common diameter range for multimode fibers is 50 to 100 micrometers. These types of fiber are used for the short-range transmission of signals.

## Uses of Optical Fiber

• It is mainly used in telecommunication technology and computer technology due to its fixable behavior low cost and weight. It is very useful for telecommunication technology because of the much lower attenuation values than the electricity in electric cables.
• Optical fiber can be changed into selective absorbance sensors by immobilizing suitable reagents on the extremities of fiber. It operates better than an electrochemical sensor because it cannot need of reference electrode.
• For fluorimetric analysis, an optical fiber is fabricated by coating one end with fluorescein isothiocyanate which is pH scale sensitive. Penicillin with a suitable indicator used for enzyme analysis in biosensors.
• It is used to transmit power in MRI machines.
• Recently it is developed for the analysis of oxygen and s-block metals.

## Advantages of optical fiber communication

It is very useful because of the less signal degradation. It is more economically effective than electric cables. Optical fiber is flexible, lightweight, and easy to transport than electric cable.