Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Unit : Basic Electronics
Optical Fiber
Optical fiber:
- An optical fiber is essentially a "light pipe" or waveguide for optical frequencies.
- The fiber is typically drawn from a boule of glass to a diameter of -25 μm.
- The fine glass fiber is relatively flexible and can be used to guide optical signals over distances of kilometers without the necessity of perfect alignment between source and detector.
- This significantly increases the applications of optical communication in areas such as telephone and data transmission.
- One type of optical fiber has an outer layer of very pure fused silica (Si02), with a core of germanium doped glass having a higher index of refraction (Fig. a).'
- Such a step-index fiber maintains the light beam primarily in the central core with little loss at the surface.
- The light is transmitted along the length of the fiber by internal reflection at the step in the refractive index.
- Losses in the fiber at a given wavelength can be described by an attenuation coefficient α.
- The intensity of the signal at a distance x along the fiber is then related to the starting intensity by the usual expression.
- The attenuation is not the same for all wavelengths, however, and it is therefore important to choose a signal wavelength carefully.
- plot of α. vs. λ for a typical silica glass fiber is shown in Fig. given below.
- It is clear that dips in a near 1.3 and 1.55 μm provide "windows" in the attenuation, which can be exploited to reduce the degradation of signals.
- The overall decrease in attenuation with increasing wavelength is due to the reduced scattering from small random inhomogeneities which result in fluctuations of the refractive index on a scale comparable to the wavelength.
- This type of attenuation, called Rayleigh scattering, decreases with the fourth power of wavelength.