Quantum cascade lasers are often operated in pulsed regime for low-power applications due to the large thermal dissipation required for continuous wave operation. The typical pulse length is of the order of 100 ns with a duty cycle below 1%. Fourier transform infrared spectrometers, commonly used in the mid-infrared, typically have a spectral resolution of the order of 3 GHz and rely on the acquisition of a path-difference interferogram. As a consequence, when measuring devices operated in pulsed regime such spectrometers can only measure the spectrum averaged over several pulses.
We propose a method to determine the absolute instantaneous frequency of a pulsed laser with a precision of 10 MHz. First, the light from the laser is sent through a 30 cm long Fabry-Perot resonator under vacuum. The temporal waveform of the transmitted signal, which is measured using an HgCdTe detector, contains fringes corresponding to constructive and destructive interference occurring as function of time. This experiment allows to determine the chirp rate. The Fabry-Perot cavity is then filled with a known gas exhibiting an absorption line lying within the laser emission range, which can be used as an absolute frequency reference. By combining this measurement with the chirp rate, we obtain the instantaneous frequency of the laser as a function of time. Complex spectral behavior of pulsed DFB lasers, such as mode-hopping and dual-wavelength lasing, can also be properly identified using this technique.