THEORETICAL-ANALYSIS OF DIFFERENTIAL GAIN OF 1.55 MU-M INGAASP/INP COMPRESSIVE-STRAINED MULTIPLE-QUANTUM-WELL LASERS

Basic design principles are formulated for increasing the differential gain of 1.55 mu m InGaAsP/InP compressive-strained multiple-quantum-well (MQW) lasers. An InGaAsP quaternary active layer can provide more freedom of design compared to an InGaAs ternary active layer since the amount of strain and the quantum-well thickness can be independently determined in an InGaAsP material system for a given emission wavelength. Compressive strain ranging from 1% to 1.5% is large enough to reduce the density of states below one-third of that of unstrained wells. No further reduction is expected even if more compressive strain is applied. When the well thickness is, in turn, determined, it is essential to incorporate a trade-off between the conduction and valence subband energy spacings and the squared optical matrix elements. The extra enhancement of differential gain in MQW structures with a modulation p doping is also studied. By designing MQW structures according to these principles, differential gain can be increased to over 2X10(-15) cm(2), which corresponds to a relaxation oscillation frequency of more than 30 GHz at an output power of 20 mW.