Lasing gain and threshold current density in GaAs/AlGaAs multi-quantum-well lasers are theoretically analyzed. Split energy levels in multi-quantum-well structure are calculated. The lasing gain and electron lifetime by the spontaneous emission effect are calculated taken into account both the split energy levels and broadening effect by the intraband relaxation of electron wave. It is found that the gain can not be much higher than that in conventional double heterostructure, because the gain is broadened by the intraband relaxation effect even in the quantum-well structure. Conditions of laser structure to get much lower threshold current density are discussed. The most important condition to reduce the threshold current density is to increase confinement factor of the optical field into the well layers.

A theoretical analysis of the lasing gain and the threshold current in GaAs-AlGaAs Graded-Index Separate Confinement Hetero (GRIN-SCH) structure lasers is given. Energy levels and the state densities for electron distribution in both the well layer and the optical guiding regions are formulated. Confinement ratios of the injected electrons and holes and the optical field in the well layer are examined. The injection current, the electron lifetime due to the radiative recombination, the lasing gain and the threshold current are examined by taking into account the intraband relaxation effect of the electron wave. Optimum conditions to obtain low threshold current density are numerically discussed.

Laser module fabricated with silicon platform technology is very attractive for low-cost modules. The technology enables passive optical alignment of an LD to an optical fiber. Our marker design for passive alignment allows positioning accuracy within 1 µm of LD. However, coupling efficiency is a key issue because that by conventional butt coupling scheme is low with about 10 dB coupling loss. We investigated optical coupling characteristics in various types of coupling scheme: conventional flat end fibers, cone fibers, integrated GRIN rod lenses on the platform and the coupling with new-type LDs integrated with spot size transformer. Improvement of coupling efficiency with 3 dB and 7.5 dB compared to flat-end fiber is achieved by using the cone fiber and the GRIN rod lens, respectively, although 1-dB coupling tolerances for alignment deteriorated with these schemes. We obtained high efficient coupling with 3.5 dB coupling loss and wide alignment tolerance of 2.3 µm simultaneously with a new-type LD integrated with spot size transformer owing to its expanded spot size characteristics.