Experimental study of optical gain and loss in 3.4-3.6 μm interband cascade lasers

The optical gain and loss of lasers operating over the wavelength range of 3.4 to 3.6 μm were measured using the Hakki-Paoli technique. The optical loss was found to increase with increasing temperature, from 38 cm^-1 at 80 K to 62 cm^-1 at 200 K for a typical sample. Above the maximum continuous-wave (CW) lasing temperature, the modal gain was observed to saturate before the device reached threshold. The reason for the gain saturation is the active area overheating in the CW mode. Some of the samples exhibited strong mode-grouping effects. These effects indicate the presence of 'leaky modes' in these structures and lead to a significant reduction of net gain at certain frequencies.