Lateral analysis of quasi-index-guided injection

Quasi-index-guided (QIG) semiconductor lasers, in which an effective lateral index step dn(L) over the stripe region is induced through evanescent-field coupling, are theoretically analyzed. The model used to obtain the threshold and above-threshold behavior of a QIG laser is presented and used to study the transition from gain to index guiding. It is found that as dn(L) increases, the threshold current rapidly decreases, the lateral mode confinement improves, and the twin peaks in the far-field move closer. At the critical value of dn(L), index guiding starts to dominate and the device exhibits all features pertinent to an index-guided device. Using parameters appropriate to a 1.3 micron InGaAsP laser, it is shown that an optimum value of dn(L) of about 0.005 exists for the efficient operation of a QIG laser. For this dn(L), index-guiding is weak but sufficient to overcome the disadvantages of a gain-guided device.