Abstract
A new scheme for a calibration-free diode laser absorption spectroscopy (DLAS) sensor for measuring the parameters of harsh zones is proposed. The key element of the scheme is a micro-prism retroreflector (MPRR). The MPRR facilitates an increase in the mechanical stability of the sensor and a decrease in the background thermal radiation in the hot areas of a tested zone. Reduction in the broadband thermal emission allowed the application of a differential logarithmic conversion (LC) technique for elimination of the residual amplitude modulation and other sources of non-selective attenuation of the probing laser beam. LC allows the use of a 1f-wavelength modulation spectroscopy (WMS) detection scheme. Combination of LC and a 1f-WMS algorithm provided a new modification of calibration-free DLAS, which could be particularly useful for probing harsh zones with pronounced strong turbulence and high levels of acoustic and electrical noise. The influence of the experimental parameters and characteristics of the main electronic components of the recording and processing system on the accuracy of the integral line intensity determination is investigated theoretically and experimentally. The proposed optical scheme of a DLAS sensor and algorithm for the data processing allowed the integral intensity of an absorption line to be obtained. The potential for the scheme was exemplified with a single water vapor absorption line at 7185.6 cm−1. Simultaneous detection of several absorption lines and data processing using the developed algorithm provides the final goal of a DLAS sensor—determination of temperature and partial pressure of a test molecule in a probed gas volume. The developed scheme allows the spatial multiplexing of the radiation of different diode lasers (DLs), which can be used if various test molecules are to be detected, or absorption lines of a test molecule are detected over different wavelength intervals.
Highlights
Tunable diode laser (DL) absorption spectroscopy (DLAS) is a widely used technique for contactless diagnostics of various gaseous media
diode laser absorption spectroscopy (DLAS) sensors are routinely used for the detection of important air components H2 O, CO, CO2, CH4, etc. [1,2,3,4,5,6,7], and are commonly used for measuring temperature and total and partial pressures in hot zones [8,9,10,11]
Several applications of the logarithmic conversion (LC) combined with wavelength modulation spectroscopy (WMS) DLAS have been published: the LC in a double beam scheme results in reduction in the excess noise of the laser radiation [30,31]; a large linear dynamic range of the output signal of a sensor using the LC was demonstrated [32]; detection of CH4 using
Summary
Tunable diode laser (DL) absorption spectroscopy (DLAS) is a widely used technique for contactless diagnostics of various gaseous media. Several applications of the LC combined with WMS DLAS have been published: the LC in a double beam scheme results in reduction in the excess noise of the laser radiation [30,31]; a large linear dynamic range of the output signal of a sensor using the LC was demonstrated [32]; detection of CH4 using. The idea of the technique is based on: (i) the use of a micro-prism retro reflector (MPRR), which provides efficient autofocusing of the transmitted laser beam into the optical fiber; (ii) logarithmic processing of the photodetector signal; and (iii) inference of the absorbance on the selected test molecule absorption lines from the first harmonic of the WMS signal.
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