Direct measurement of the linewidth enhancement factor by optical heterodyning of an amplitude-modulated quantum cascade laser

A measurement of the linewidth enhancement factor alpha of a distributed feedback quantum cascade laser is presented. The measurement is based on a heterodyning experiment, in which one of the lasers is modulated at radio frequency. A value of alpha=0.02 +/- 0.20 is obtained for a modulation frequency of 500 MHz. As the frequency is decreased, alpha increases and is consistent with a thermal chirp effect. (c) 2006 American Institute of Physics.