Linewidth enhancement factor and carrier-induced differential index in InGaAs separate confinement multi-quantum-well lasers

Quantum-well lasers can achieve a low spectral linewidth because of their high differential gain, leading to a lower linewidth enhancement factor α than for bulk lasers. The differential refractive index and the gain of InGaAs separate confinement multi-quantum-well lasers with two different quaternary barrier layers, have been determined from the spontaneous emission spectra below threshold. The measured value of the α factor is about 2.8 up to 3.5 at the gain maximum for both laser structures. The refractive index and the gain spectra are connected via the Kramers–Kronig relation. Therefore, the differential refractive index and the α factor have been deduced from calculated gain spectra with an additional contribution of the intraband transitions of the free carriers.