R.S.M. Chrystie, E.F. Nasir, A. Farooq
Proc. Combust. Inst., Issue 35, 3757-3764, (2015)
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.