Simultaneous water vapor concentration and temperature measurements using 1.31-micron diode lasers

Abstract

This paper reports the development of a compact, inexpensive sensor for simultaneous water vapor concentration and temperature measurements suitable for aeropropulsion exhaust applications. High sensitivity is achieved with an electronically balanced dual detector strategy that circumvents requirements for custom-fabricated lasers operating at specific wavelengths or high-frequency modulation techniques. Using widely available, broadly tunable InGaAsP diode lasers near 1.31 jxm, simultaneous measurements are demonstrated in a fiber-coupled, wavelength multiplexed configuration with a limiting density sensitivity of 1015 cm~3 in a 50-cm path and an rms standard deviation of 42 K over a range from 300 to 1300 K. Initial results suggest the possibility of extending this temperature range to 1900 K and above using other line pairs. (1) where Iv is the monochromatic laser intensity at frequency v, mea- sured after propagating a pathlength t through a medium with an absorbing species number density N. The strength of the absorp- tion is determined by the temperature-dependent line strength S(T), and the line shape function g(v — v()). The line shape function de- scribes the temperature- and pressure-dependent broadening mech- anism of the fundamental line strength. The temperature dependence of the line strength arises from the Boltzmann population statistics governing the internal energy level population distribution of the absorbing species. The single-mode distributed feedback (DFB) diode lasers used in this work are sufficiently narrow in frequency that they may be con- sidered essentially monochromatic with respect to the absorption line shape. The laser frequency may be tuned over a range that en- compasses the entire line shape function so that the resultant trans- mission can be integrated to remove the pressure and temperature dependence of the line-broadening mechanisms. The recorded ab- sorbance, then, is proportional only to the temperature-dependent line strength and the absorbing species number density. It is usu- ally possible to select an absorbing ground state whose line strength is relatively constant over some target temperature range so that the absorbance is a direct measurement of species number density. Separate temperature measurements may be used to correct for tem- perature variations, if necessary. Alternatively, two absorption transitions may be probed (using one or two lasers, depending on the target transition separation and the laser tuning range). The ratio of the integrated absorbance of each transition is a pure function of temperature,2