Time-resolved spectral characterization of a pulsed external-cavity quantum cascade laser

Spectrally tunable narrow-linewidth mid-infrared sources are used in a variety of spectrometric optical systems for detection, identification, and/or quantification of chemical species. However, in the pulsed regime they often display a varying spectrum in time, either from shot-to-shot or during the pulse itself, with consequences on the measurement accuracy, resolution, and repeatability. This is, for instance, the case of pulsed quantum cascade lasers (QCL), mainly because of strong transient thermal effects in the optical waveguide. Unfortunately, little information has been published on this subject because mid-infrared time-resolved spectrometers are extremely scarce. In this paper, we explain how this can be circumvented by using time-gated frequency upconversion in a nonlinear crystal. We apply this principle to characterize a pulsed external cavity QCL (EC-QCL) at 7.8 mu m, using AgGaS2 as the nonlinear crystal and a Q-switched Nd:YAG laser as the pump source. The upconverted near infrared spectrum is conveniently analyzed with a high resolution lambdameter and an optical spectrum analyzer. We evidence frequency chirp at an average rate of -50 MHz/ns and mode hops spanning 15 GHz for the EC-QCL. These results are compared to published data.