Emission linewidth and $\alpha$-factor of 1.55 $\mu$m-range vertical-cavity surface-emitting lasers based on InGaAs/InGaAlAs quantum wells

The emission linewidth of single-mode vertical-cavity surface-emitting lasers with an active region based on strained InGaAs/InGaAlAs quantum wells in the spectral range of 1.55 $\mu$m was studied. The removal of degeneracy in polarization of the fundamental mode (splitting of the resonance wavelength) and polarization switching (type I) associated with the transition from lasing via the short-wavelength mode to lasing via the long-wavelength one were observed. As the output optical power increased, the emission linewidth dropped to $\sim$30 MHz for both orthogonally polarized modes and was limited by the residual linewidth. The value of the $\alpha$-factor was estimated: for the short-wavelength mode it reached 5, while for the long-wavelength mode it increased to $\sim$9. At an output optical power of more than 1 mW, the emission line broadening is observed, which can be associated with a gain saturation and the increased $\alpha$-factor caused by a strong self-heating of the investigated laser.