Linewidth Enhancement Factor of a Type-I Quantum-Cascade Laser

Experimental results using the amplified sppontaneous emission (ASE) spectroscopy of a type-I quantum-cascade (QC) laser are presented. Using the Hakki-Paoli method, the optical gain spectra of the laser are extracted for the wavelength of 8.2mum at various sub-threshold 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 dtermined from Farby-Perot modal spacings.