Reduction of linewidth enhancement factor in strained 1.55-μm InGaAsP lasers

The linewidth enhancement factor, α, is an important parameter of semiconductor laser operation as it affects the spectral linewidth, increasing it by a factor of (1 + α2) compared to the value predicted by the Schalow-Towns formula. Bulk lasers typically show α-values of 6–7; however, this value has been reduced by the use of quantum well structures to 3-4. Recently, the introduction of strained layers into quantum well lasers has led to further reductions in the α-value. In this work we analyse from first principles, for the first time, the physical mechanisms responsible for the reduction of the α-parameter in strained layer lasers operating in both cw and dynamic regimes. We measure α-values of ~2 in 1% compressively strained lasers operating d.c. and under modulation conditions. Agreement between experiment and theory is found.