Q2 Discuss the requirement for population inversion in order that stimulated emission may dominate over spontaneous emission in a laser. Illustrate your answer with the aid of energy level diagram provided that the laser has four energy levels. (4 marks) (b) Lasers have mirrors in their structure. Justify why such mirrors are compulsory for the operation of laser. With the aid of diagrams, explain your justification in the sense of spontaneous and stimulated emission mechanism. (5 marks) (a) (c) The laser cavity of semiconductor lasers comes in a few types; among them are distributed feedback (DFB), vertical cavity surface-emitting (VCSEL) and Fabry-Perot (FP) laser. Differentiate DFB, VCSEL and FP lasers in terms of their structure and mechanism. (6 marks) (d) A planar light emitting diode (LED) is fabricated from gallium arsenide which has a refractive index of 3.6. (i) Calculate the optical power emitted into air as a percentage of the internal optical power for the device when the transmission factor at the crystal-air interface is 0.68 (2 marks) (ii) When the optical power generated internally is 50% of the electric power supplied, determine the external power efficiency. (2 marks)