Linewidth enhancement factor of a type-I quantum-cascade laser

Experimental results using the amplified spontaneous emission spectroscopy of a type-I quantum-cascade laser are presented. Using the Hakki–Paoli method, the optical gain spectra of the laser are extracted for the wavelength of 8.2 μm at various subthreshold current levels. The change in refractive index with increased bias current is obtained from the peak wavelength shifts of the Fabry–Pérot spectrum. A low value of −0.5 for the linewidth enhancement factor is found. A group index of around 3.47 has also been determined from Fabry–Pérot modal spacings.