Question

Page 4 3. (a) Explain howa p-njunction can be used to make a diode laser. 4 marks (b) Sketch the ideal diode laser characteristic, indicating the spontaneous emission and stimulated emission regions and the threshold current. 2 marks State the threshold condition for laser action to occur in a laser diode of length L, absorption coefficient a, and gain coefficient y. The reflectivities of the cleaved planes of the laser diode are ry and F2 respectively. 1 marks Calculate the threshold forward current density required for lasing to occur in a diode laser with 1 In 2L 1 = 10 cm-1 15 cm1. All symbols have their usual meanings. 3 marks B=2.1 x 10-2 cm/A and a =

Answer 1

gallium arsenide (Ga As) p-n junction 3 a) The basic stsucture of used as 1 a diocle laser is shoon in type n-tybe Laser beam Adive Poliahel Walls The pair ot parallel planes perpendicular to the plane of the junction jts po lished When a feusard The injected election move hols thom he p-side to the n- helombine, photons f.adiatid auway all diictions at low currert a threshold cussunt is Aeached at which Hhe emited photons timulate the emiuion ot internal heflected everal times while the tuwo hemaining sides of the oliode ase Aaughaned bias is applied to the laser oliode o cursent tlows from the M-side to the p-side and side As the electhons and holis emited These plotons ase heabsonbed are This is the pontaneous emission ohich ocs in ussent is increased,eventually As photons hee phatons polished walls, stimulatiy more are at photons, all coherent with them. mose and more when photons beam bewmes Ailfiuently intense thom it emerges aut jundion the

b) laser diode characteristics spontancou stimulated emission Cmission rain saturation Laser dioda bias currend Thethold cusrent These is a minimum threshoRd cusrent for lagng Jintensity disectly propartional to The current action with the ught intersly