Spectral tuning and mode competition in quantum cascade lasers studied by time-resolved Fourier transform spectroscopy

Over the past few years quantum cascade lasers (QCL) have attracted much interest for sensing applications and infrared countermeasures. In most of these applications high average power is needed, which can most easily be achieved by driving the devices in pulsed operation applying high duty cycles. The sensing applications can be divided into two classes: (i) spectroscopy of trace gases, typically covering a small spectral range at high spectral resolution, (ii) spectroscopy of solids, liquids and complex molecules needing a broad spectral tuning range. While for the former application single-mode distributed feedback (DFB) QCL are ideally suited, lower cost Fabry-Perot (FP) QCL might be sufficient for the latter application due to a larger width of characteristic absorption features. However, little is known so far regarding the temperature dependence and temporal tuning of the lasing spectrum of FP QCL in pulsed mode operation. In the present work first a spectroscopic technique is outlined enabling high resolution spectroscopy of QCL at a time resolution of 3 ns. In the second part, we investigate the spectral characteristics and far-field distribution of broad band emitting Fabry-Perot (FP) quantum cascade leasers. The far-field distribution of the lasers provide evidence for excitation of higher lateral modes.