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 mum at various subthreshold current levels. The change in refractive index with increased bias current is obtained from the peak wavelength shifts of the Fabry-Perot 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-Perot modal spacings. (C) 2003 American Institute of Physics.