THEORETICAL-STUDY ON ENHANCED DIFFERENTIAL GAIN AND EXTREMELY REDUCED LINEWIDTH ENHANCEMENT FACTOR IN QUANTUM-WELL LASERS

Low-chirped lasing operation in semiconductor lasers is desirable for high-speed high-bit-rate optical transmission. This paper addresses this issue with a theoretical investigation of possibility of extreme reductions in the linewidth enhancement factor (alpha factor) in quantum-well (QW) lasers to a value of zero. We show that in reducing the alpha factor it is essential that lasing oscillation be around the peak of the differential gain spectrum, not in the vicinity of the gain peak. The condition for such lasing oscillation is analytically derived. The wavelength dependence of the material gain, the differential gain, and the alpha factor are calculated in detail taking into account the effects of compressive-strain and band mixing on the valence subband structure. Along with the derived condition, we also discuss the effect of p-type modulation doping in compressive-strained QW's. It is shown the alpha factor, the anomalous dispersion part in the spectrum, crosses zero in the region of positive material gain, which makes it possible to attain virtual chirpless operation by detuning.