Photonics — DU B. Sc. (Hons) Electronics Previous Year Question Papers

B.Sc.al) Electronics (Core) Instruction for Candidates 2. All questions cany equal marks. 1. (a) A non-reflecting single layer coating is to be deposited on a lens of refractive index 1.78.. Detemine the refractive index of a coating material and the thickness required to produce zero reflection for light of wavelength 550 nm. a) Explain the effect of decreasing the slit width on diffraction pattern, keeping the distance between the two slits unchanged, for double slit diffraction. (c) What is Brewster's law. Plot the reflectance as a fuhction angle of incidence for light polarized parallel and perpendicular to the plane of incidence on the interface from the denser medium. (d) What should be the thickness of a half-wave quartz plate for a wavelength of 590 nm, given the ordinary and extra-ordinary refractive indices as no = 1.5442 and ne = 2. (e) Explain the principle and working of a phototransistor. (I) Explain why the waveguide modes are discrete. (a) Derive an expression for the intensity distribution in a single slit Fraunhofer diffraction pattern. Also give the position of maxima and minima. (6 x 3) (b) Explain with necessary theory how circular rings are obtained in Newton's ring method? Also explain how it is used for measuring wavelength of light. Why the central fringe is dark? (b)Explain the Phenomenon of double refraction in Uniaxial Crytal. What is the significance of optic axis. (5) (c) What is a quarter wave plate? Deduce its thickness for a given wavelength in terms of its refractive indic es. (4) (a) Explain the missing orders in double slit diffraction pattern. The /two slits of a double slit each having a width of 0.140 nm and distance between centres of 0.840 nm. (i) what orders are missing (ii) find number of interference maxima occurring under the central diffraction. maxima. (8) (b) Two spectral lines at a wavelength of 6200 A have separation of 0.652 A. Find the minimum number of lines a diffraction grating must have to just resolve this doublet in the second order. (4) © What is the major advantage of an LCD over other common display devices? Discuss the wiorking of an LCD device. (6) (a) Explain why the semiconductor diode laser is forward biased for its :operation. Draw and explain various regions of its ,light output-current characteristic. Also plot its radiant output as a function of frequency for (i) below threshold (ii) above threshold. (6) (b) Derive the relation between the Einstein coefficients for spontaneous emission and stimulated emission. Explain why Ruby laser is classified as a solid-state laser and C02 laser as a gas laser. (c) With the help of diagram, explain why the intrinsic layer is important imp-I.-ap (7) (a) Why is total intemal reflection cmcial for optical fiber communication? Show mathematically how the critical angle depends on refractive indices of the two media.