Hot zone temperature measurement by 1f modulation diode laser absorption spectroscopy with logarithmic signal conversion

A new version of diode laser absorption spectroscopy (DLAS) is developed for determining the temperature of hot zones. The technique is based on a combination of slow scanning of the radiation frequency of a probe diode laser (DL) in the vicinity of the absorption line of a test molecule and fast modulation of the DL frequency with an amplitude on the order of the absorption line width. A specific feature of the developed DLAS version is the use of a two-beam differential scheme, logarithmic processing of signals and detection of absorption at the first harmonic of the modulation frequency. The two-beam differential registration scheme and the logarithmic processing of the absorption signals make it possible to significantly reduce nonselective absorption of probe radiation and excess noises of laser radiation. The use of the first harmonic provides an increase in the DLAS sensitivity. The developed technique is tested by measuring the temperatures of water vapour in atmospheric air in the range of 700–1100 K. These results are compared with those of commercial thermocouples. The difference between the temperature measured by a standard thermocouple and its average value determined by the developed technique is 40 K for a temperature of 1000 K and 30 K for a temperature of 800 K.