EC1402_Optical_Communication

# EC1402_Optical_Communication - EC 1402 OPTICAL...

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EC 1402 OPTICAL COMMUNICATION 2 marks 1. Write the expression for the refractive index in graded index fibers. n(r)= n1[1-2 Δ (r/a) α ] 1/2 for 0<=r<=a n1(1-2 Δ ) 1/2 ~ n1(1- Δ ) =n2 for r>=a r b radial distance from fiber axis a b core radius n1 b refractive index at the core n2 b refractive index at the cladding α b shape of the index profile Δ b index difference 2. Define Numerical aperture of a step index fiber. Numerical aperture (N.A) of the fiber is the light collecting efficiency of the fiber and is the measure of the amount of light rays that can be accepted by the fiber. It is equal to the sine of acceptance. N.A=sin Φ max =(n 1 2 -n 2 2 ) 1/2 where n 1 and n 2 are the refractive indices of core and cladding respectively. 3. Define Mode-field diameter. The fundamental parameter of a single mode fiber is the mode-field diameter. This can be determined from the mode field distribution of the fundamental LPo1 mode. 4. Give the expression for linearly polarized waves. The electric or magnetic field of a train of plane polarized waves travelling in a direction k can be represented in the general form A(x,t) = e i Aoexp[j(wt-k.x)] with x=xe x +ye y +ze z representing a general position vector and k=k x e x +k y e y +k z e z representing the wave propagation vector. 6.What is Snell’s law? The relationship at the interface is known as Snell’s law and is given by n 1 sin Φ 1 =n 2 sin Φ 2 7.What is the necessity of cladding for an optical fiber? a) To provide proper light guidance inside the core b) To avoid leakage of light from the fiber c) To avoid mechanical strength for the fiber d) To protect the core from scratches and other mechanical damages

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8. What are the uses of optical fibers? a) To transmit the information which are in the form of coded signals of the telephone communication, computer data, etc. b) To transmit the optical images (Example : Endoscopy) c) To act as a light source at the inaccessible places. d) To act as sensors to do mechanical, electrical and magnetic measurements. 9. What is the principle used in the working of fibers as light guides? The phenomenon of total internal reflection is used to guide the light in the optical fiber. To get total internal reflection, the ray should travel from denser to rarer i.e. from core to clad region of the fiber and the angle of incidence in the denser medium should be greater than the critical angle of that medium. 10. what are step index and graded index fibers? In the case of graded index fiber, the refractive index of a core is a constant and is larger than the refractive index of the cladding. The light propagation is mainly by meridional rays. In the case of graded index fiber (GRIN fiber) the refractive index of the core varies parabolically from the centre of the core having maximum refractive index to the core-cladding interface having constant minimum refractive index. Here the light propagation is by skew rays. 11. Define acceptance angle.
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EC1402_Optical_Communication - EC 1402 OPTICAL...

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