Methodology for temperature measurements in water vapor using wavelength-modulation tunable diode laser absorption spectrometry in the telecom C-band

A methodology based upon wavelength modulation and two-line thermometry for assessment of gas temperature by tunable diode laser absorption spectroscopy utilizing a standard tunable distributed feedback (DFB) diode laser working in the telecom C-band has been developed. Due to the high density of water lines in this wavelength region at elevated temperatures the methodology does not make use of two individual lines, but rather two groups of lines. The two groups identified have been found to have favorable properties for accurate temperature assessment in the 200–1000°C range, primarily a separation that is within a single DFB laser scan (∼20 GHz), an adequate linestrength, and sufficient difference in temperature dependence; the ratio of the peak values of the 2f-wm-signals, R, was found to increase monotonically with temperature, T, with a relative sensitivity, (ΔR/R)/(ΔT/T), above or around unity for most of the pertinent temperature range. The standard deviation of a temperature measurement with a 1 s integration time was found to be below 0.3%. It is shown that the temperature assessment has virtually no dependence on water concentration but a weak dependence on modulation amplitude.