Question

(a) The optical gain coefficient of a semiconductor laser with 150 μm cavity length is shown in Fig 1. The loss coefficient and the refractive index of the semiconductor laser are 21.6 cm" and 3.5 respectively. Find (i)The minimum gain coefficient required for lasing (ii) The frequency separation between the longitudinal modes of the laser (iii) The number of longitudinal modes emitted (b) This laser is required to produce a single mode output at a wavelength of 1.32 unm If gratings are used for this purpose, determine the design pitch of the grating that must be used (ii)Indicate the reasons for the great interest in such single mode semiconductor lasers Note. The power reflectivity of a surface between two materials of refractive indices ni and n is given by: Optical Gain 100 50 Coefficient cm 1.298 1.300 1.336 1.338 Wavelength (um) Fig. 1.

Answer 1

Solution Givenha t The optical qcin coefficteni ot a Semi- /i, tu length Conducor laser ofth 150 .unm gven Groph s 298 300 336 1338 wave length (m ) rom he ?raph , 4he minimum qain co-elttctent for lasfngis Cqain co-ctficient)min 50 cm requenc seperaton bel uwee o 4he longt tudinal modes dI the laser

Tro m tl)a ve length Sepe ra-Hon Sothe* ー> he qraphs 338.298 u V I V2 3냐 v2-v2) ogf tudina modes emted (ti)The nmbe înal from he raph are ontadinal modes mitked