Towards simultaneous calibration-free and ultra-fast sensing of temperature and species in the intrapulse mode

We report on exploiting the down-chirp phenomenon seen in quantum cascade lasers (QCLs), when modulated with long pulses, for the purpose of performing calibration-free and temporally resolved measurements. Intrapulse spectra of a native species (e.g., H₂O), common to combustion environments, were generated near λ = 7.62 μm at repetition rates as high as 3.125 MHz. Two-line absorption spectroscopy was employed to infer calibration-free temperature from the chirp-induced intrapulse spectra. In this study, such temperature measurements were limited to rates of 250 kHz due to spectral distortion at higher repetition rates. We demonstrate the ease at which accurate temperatures and H₂O compositions can be achieved using simple and compact QCLs operated in the intrapulse mode. The sensor is also applicable to other species, and has the potential to be integrated into commercial technologies.