Investigation of the spectral width of quantum cascade laser emission near 5.2 μm by a heterodyne experiment

We report line width measurements of a quantum cascade distributed feedback laser by a heterodyne experiment. At currents slightly above threshold and a laser output power higher than 1 mW, the full width at half maximum of the beat signal was narrower than 0.5 MHz, which gives us an upper limit of the laser line width. As reference laser we used a carbon monoxide laser. Both lasers were operating unstabilized in continuous wave mode emitting light at about 5.2 μm.