Photoconductivity and optical fiber

Photoconductivity

1. The conductivity of semiconducting materials depends upon the number of free electrons in the conduction of band and electrons in the conduction band.
2. Bombardments of semiconductor by photon with energy equal or greater than band gap creates electrons- holes pairs. Both electrons and holes participate in conduction. This process is called photography.
3. It is different from photoelectric effect in the sense that an electron-holes pair is generated whose energy is related to band gap energy instead of free electron alone whose energy is related to Fermi level.
4. The current produced in photography is directly related to the intensity of incident light.
5. The phenomena of photoconductivity is utilized in photography light meters. Cadmium sulphide is commonly used for detection of visible light as in light meters.
6. Photoconductivity is also underlying principle of photovoltaic cell, kwon to common as solar cell.
7. Used for the conversion of solar energy into electricity.

Optical fiber:

Construction and principle:

1. Optical fiber is system of glass fiber made up of different layers having different refractive index.
2. These system consist of transmitter (a semiconducting layer) to convert electrical signals to light signals in which optical fiber is used to transmit light signals.
3. In communication industry photodiode is used to converts light signals back to electrical signal.

Optical fiber consist of

Core

Cladding

Coating

The core transmits signals while cladding constraint to the core. The outer coating protects the cores and cladding from environment.

5. Typically both the core and cladding are made of special type of glass with carefully controlled refractive index.
6. The index of refraction are selected that refractive index of cladding is less than refractive index of core. Core is optically denser than cladding.
7. Once the light enters the core from source it is reflected internally and propagate along the length if the fiber.
8. Internal reflection is accomplished by varying the index of refraction and cladding glass materials. Usually there are two type of design.

Types of optical fiber:

1. Step-index optical fiber
2. Graded-index optical fiber

• Single mode fiber

1. In step-index optical fiber there is sharp change in refractive index between the core and cladding. In this design output pulse will be broader than input one. It is because light rays traveling in different trajectories have a verity of path length.
2. In graded-index optical fiber there is graded change in refractive index. It is possible to avoid the broadening of pulse in this fiber. This results in a helical path for light rays instead of zig-zack path in step index fiber.

• Here impurities such as Borron-oxide or Germenium dioxide are added to Silica glass such that index of refraction is gradually varied in parabolic manner over the cross-section. This enables light to travels faster where to the periphery than the center.

1. Both step-index and graded-index optical fiber are termed as multimode optical fiber.
2. The third type of optical signal made of in which light travels largely parallel to the fiber axes with little distortion of light pass.
3. These are use for long transmission line due to constant value of intensity of light.

Typical diameter and refractive index:

$$\frac{core diameter}{cladding diameter}=\frac{62.5 \mu m}{125 \mu m};\frac{100 \mu}{140 \mu m};…….;\frac{8000 \mu m}{1200 \mu m}$$

Refractive index of core=1.45

% difference in index of refraction=1% to 2%

$$\frac{core diameter}{cladding diameter}=\frac{50}{125};\frac{62.5}{125};\frac{85}{125}$$

Core index of refraction=1.45

Index difference=1% to 2% in glass index

Simple cladding: difficult fabrication low to zero model dispersion

$$\frac{core diameter}{cladding diameter}=\frac{9}{125}$$

Core index=1.45

Index difference=1% to 2%

No model dispersion

Difficult for fabrication

Optical fiber properties:

1. Core and cladding material are selected not only on the basis of that refractive index but also on the basis of mechanical strength. Light loss, manufacture ability and dispersion properties.
2. The density and refractive index are critical for the optical fiber and are related as,

$$n =\frac{rho+10.4}{8.6}$$where

n=refractive index

\(\rho\)=densities

3. High purity silica based glass are used as optical fiber diameter ranging from 5 to 100\(\mu\)m.
4. The fiber are carefully fabricated to be free from flaws.

References:

Callister, W.D and D.G Rethwisch. Material Science and Engineering. 2nd. New Delhi: Wiley India, 2014.

Lindsay, S.M. Introduction of Nanoscience . New York : Oxford University Press, 2010.

Patton, W.J. Materials in industry . New Delhi : Prentice hall of India, 1975.

Poole, C.P. and F.J. Owens. Introduction To Nanotechnology. New Delhi: Wiley India , 2006.

Raghavan, V. Material Science and Engineering. 4th . New Delhi: Pretence-Hall of India, 2003.

Tiley, R.J.D. Understanding solids: The science of Materials. Engalnd : John wiley & Sons , 2004