Highly coherent long cavity GaAs/AlGaAs single quantum-well lasers

Measurements of the spectral properties of ridge waveguide graded index separate-confinement heterostructure single-quantum-well GaAs/AlGaAs lasers are discussed. Long cavity lasers (800 μm) exhibit remarkably pure single-longitudinal mode spectra under continuous operation in spite of the short cavity mode spacing. At an output power of 5 mW, the sidemode suppression exceeds 24 dB and the linewidth is 1.5 MHz. The linewidth-power product is 6.4 MHz-mW. Measurements of the linewidth power product as a function of cavity length L gives an L^-2 dependence in agreement with theory for lasers with small internal loss. The results are used to deduce the linewidth enhancement factor α at the gain peak wavelength ant its dependence on the excitation level. A decrease in α was observed for lasers operating at the second quantized state due to a recovery of the differential gain