Theoretical analysis of pure effects of strain and quantum confinement on differential gain in InGaAsP/lnP strained-layer quantum-well lasers

The pure effects of both strain and quantum confinement on differential gain of InGaAsP/InP strained-layer quantum-well lasers (SL-QWL´s) are studied on the basis of valence band structures calculated by k·p theory. Using an InGaAsP quaternary compound as an active layer makes it possible to distinguish the effect of strain (both tensile and compressive) from the quantum-confinement effect when keeping the emission wavelength constant. The essential features of strain-induced changes in the valence band structures are extracted from the k·p results by four characterization parameters: the averaged density of states (DOS), the subband energy spacings, the joint density of electron and hole states, and the squared optical matrix elements. Each of them is then directly correlated to differential gain in SL-QWL´s. In tensile-strained quantum wells, all of these factors are significantly improved compared with unstrained wells, while only the averaged DOS is improved in compressive-strained wells. Due to these characteristic features, it is concluded that the intrinsic potential of tensile-strained QWL´s for improving differential gain is twice as high as that of compressive-strained ones. On the basis of the essential features of the strain-induced changes in valence band structures, we also discuss basic design principles for SL QWL´s with larger differential gain