The study of the dynamic characteristics of bulk and quantum- well semiconductor lasers

Abstract

A reduced effective differential gain is shown for the first time to arise in diode lasers by including the modulation of the confinement factor with carrier density. This effective differential gain, and not the material gain, is the parameter which actually determines the resonance frequency and therefore the modulation bandwidth of diode lasers. For bulk lasers with short cavity lengths and thin active layers, the effective differential gain can be significantly reduced. The modulation of the confinement factor with carrier density can also adversely effect the differential gain of quantum - well lasers if the confining layer thickness is too thin and the threshold carrier density is too high. The know ledge of the carrier-induced index change is required in the study of the confinement factor modulation with carrier density. The carrier-induced index change was measured using a novel injection-reflection technique in combination with differential carrier lifetime data. The observed relation between index change and injected carrier density at bandgap wavelength is shown for the first time to be nonlinear and is approximately given by δn[act]=-6.1x10^-14 (N)^0.66 for 1.5 μm laser and δn[act]=-1.3x10^-14 (N)0 68 for 1.3 μm laser. The carrier-induced index change for a 1.3 μm laser at 1.53 μm wavelength is smaller and is given by δn[act]=-9.2x10^-16 (N)^0.72. In an attempt to relate the measured dynamics to quantum-well laser parameters, the frequency response of some quantum-well lasers has been measured using optical modulation technique. Quantum-well laser with a larger number of wells has the tendency to exhibit smaller threshold carrier density, and thus smaller K-factor than quantum-well laser with a smaller number of wells. Measurements show that quantum-well lasers with larger external quantum efficiency have smaller K-factors. This is expected since device with larger external quantum efficiency has smaller optical loss and smaller threshold carrier density.