Quantum cascade lasers (QCLs) provide access to the mid-infrared region of the electromagnetic spectrum, at wavelengths between approximately 3 and 10 microns. This region corresponds to the stretching of many molecular bonds and is therefore useful for rovibrational spectroscopy. In the TAMU laboratory, QCLs provide access to species such as CO, CO2, H2O, PO2, and N2O.
The QCL diagnostics are arranged in the generic direct absorption setup. A laser beam is split into two components: one for monitoring the time-resolved incident intensity and one for monitoring the time-resolved transmitted intensity It, which can be used in conjunction with the Beer-Lambert law to find the species time histories. The two beam intensities are monitored by semiconductor photodetectors with typical bandwidths of ~200 kHz. The wavelength and power output of the QCL are controlled by current and temperature controllers. The wavelength of the laser is typically monitored using an interferometric wavemeter.
The color of the QCL may either be fixed at a desired wavelength, or it may be rapidly scanned over a wavelength range of interest. The fixed-wavelength configuration can provide species time histories with the temporal resolution of the detector bandwidth, while the scanned-wavelength configuration can provide spectrally resolved information with the temporal resolution of the laser modulation speed.
Schematic of a generic direct absorption diagnostic. TEC: thermoelectric cooler, BP: bandpass.