FIBRE OPTICS ASSIGNMENT

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					FIBRE OPTICS ASSIGNMENT

4) i) EXPLAIN IN DETAIL ABOUT THE DIFFERENT TYPES OF ABSORPTION LOSSES IN FIBER OPTIC CABLE. ANSWER:Material absorption is loss mechanism related to material composition and fabrication process of the fiber which results in some dissipation of optical power. There are basically three types of absorption losses 1) atomic effects 2) intrinsic losses 3) extrinsic losses ATOMIC EFFECTS:It is caused due to Imperfections in the atomic structures of fiber materials. Imperfection corresponds to missing molecules and high density structure. Losses are much accountable if the fiber is exposed to ionising radiation. Radiation damages the internal structure of the material. Higher the radiation level larger the attenuation will occur. INTRINSIC EFFECTS:An absolutely pure silica glass will have little absorption due to material structure in the near infrared region. It occurs when material is in perfect stage (no density variations).it results from electronic absorption bands in the near infrared region. Electronic absorption bands are associated with band gap of amorphous glass material. It may be observed that there is a fundamental absorption edge, the peaks of which are centered in the ultraviolent region. This is due to the simulation of electrons transition within the glass by higher energy excitations. Ultra violet edge of the electron absorption bands of both amorphous and crystalline materials follow the empirical relation αuv= CeE/Eo (urbach’s rule) E is photon energy Effect of the losses may be minimized by suitable choice of core and cladding composition.

EXTRINSIC LOSSES:In practical optical fiber are prepared by conventional melting techniques a major source of signal attenuation is extrinsic absorption from transition metal element impurities (copper, cobalt etc). Transition element contamination reduced to acceptable levels by glass refining techniques such as vapour phase oxidation which largely eliminates the effects of these metallic impurities. Another major extrinsic loss mechanism is caused by absorption due to water dissolved in glass. These hydroxyl groups are bonded into the glass structure and have fundamental stretching vibrations which occur at wavelength between 2.7 and 4.2µm depending on group position in the glass network. It may be noted that only significant absorption band in the region below a wavelength of 1µm is the second overtone at 0.95µm which causes attenuation of about 1dBkm-1 for one part per million of hydroxyl. At longer wavelength the first overtone at 1.38µm and its sideband at 1.24 µm are strong absorbers giving attenuation of about 2dBkm-1 ppm and 4dBkm-1 ppm respectively.

ii) EXPLAIN IN DETAIL ABOUT DIFFERENT TYPES OF SCATTERING LOSSES IN A FIBER OPTIC CABLE? ANSWER:Scattering losses are more in the case of multimode fibers than in the single mode fiber. It is due to the large diameter and large compositional variations. There are two types of scattering losses 1) Linear losses 2) Nonlinear losses

LINEAR SCATTERING LOSSES:Optical power linearly transfers one propagating mode to a different one. Linear scattering process may cause the attenuation of the operating mode power by means of transferring it to leaky or radiation mode. High scattering losses is experienced in multimode fiber due to dopants. There are two types of linear losses 1) Rayleigh scattering losses 2) Mie scattering losses

RAYLEIGH SCATTERING LOSSES:Rayleigh scattering is the dominant intrinsic loss mechanism in the low absorption window between the ultra violet and infrared absorption tails. It results from the in homogeneities of a random nature occurring on a small scale compared with the wavelength of light. In homogeneities may occur due to density fluctuations, refractive index fluctuation, and compositional variations. The compositional variation s may be reduced by improved fabrication but the index fluctuations caused by freezing in of density in homogeneities are fundamental and cannot be avoided. The subsequent scattering due to the density fluctuations, which is in almost all direction, produces attenuation proportional to 1/λ4 following the scattering formula. RAYLEIGH =1/λ4 For a single component glass is given by:

γR = 8π3/3����4 n8 p2 βc K Tf

Where n = refractive index of silica p = photo elastic coefficient βc isothermal compressibility K Boltzmann’s constant Tf annealing temperature λ wavelength The Rayleigh scattering coefficient is related to the transmission loss factor of the fiber following the relation

α=exp (-γR L)

Where L is the length of the fiber

MIE SCATTERING LOSSES:Linear scattering losses may also occur at in homogeneities which are comparable in size to the guided wavelength. These result from the non perfect cylindrical structure of the waveguide and may be caused by fiber imperfection such as irregularities in the core –cladding interface, core cladding refractive index differences along the fiber length, diameter fluctuation, strains and bubbles. The scattering created by such in homogeneties is mainly in the forward direction and is called Mie scattering. The in homogeneities may be reduced by: 1) Removing imperfections due to the glass manufacturing process 2) Carefully controlled extrusion and coating of the fiber 3) Increasing the fiber guidance by increasing the relative index difference NON LINEAR SCATTERING LOSSES:When using high optical power levels, non linear scattering occurs. Scattering loses the optical power to shift from one mode to another with different frequency levels, either in forward or backward direction. Scattering are inelastic, due to the shift in frequency. There are two types of non linear scattering 1) Stimulated Brilloven scattering 2) Stimulated Raman scattering STIMULATED BRILLOVEN SCATTERING:Stimulated Brilloven scattering may be regarded as the modulation of light through thermal molecular vibrations within the fiber. The scattered light appears as the upper and lower sidebands which are separated from the incident light by the modulation frequency. The incident photon in the scattering process produces a photon of acoustic frequency as well as a scattered photon. This produces an optical frequency shift which varies with the scattering angle because the frequency of the sound wave varies with the acoustic wavelength. Frequency shift is maximum in the backward direction and reduced to zero in the forward direction. Brilloven scattering is only significant above a threshold a power density. The threshold optical power is given by PB =4.4*10-3 d2λ2αdBv watts

Where d = fiber core diameter λ = operating wavelength αdB Brilloven scattering loss coefficient v= source bandwidth in gigahertz STIMULATED RAMAN SCATTERING:Stimulated Raman scattering is similar to stimulated Brilloven scattering except that a high a frequency optical photon rather than an acoustic photon is generated in the scattering process. It can occur in both forward and backward direction in an optical fiber and may have an optical power threshold of up to three orders of magnitude higher than the Brilloven threshold in a particular fiber. The threshold optical power for stimulated Raman scattering ii a single mode fiber is given by PB =5.9*10-2 d2λαdB watts

Where d =fiber core diameter λ = operating wavelength αdB Raman scattering loss coefficient Losses introduced by a non linear scattering can be avoided by the use o a suitable optical signal level. Stimulated Brilloven and stimulated Raman scattering are not usually seen in multimode fiber because their relatively large core diameter make the threshold optical power levels extremely high.


				
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