A compact single-ended optical sensor for temperature measurements via laser absorption spectroscopy

The tunable diode laser absorption spectroscopy (TDLAS) has been employed in various fields successfully. However, most current TDLAS sensors have low spatial resolution, which is the inherent disadvantage of TDLAS techniques. To address this issue, a compact single-ended optical sensor based on the laser absorption spectroscopy is developed in this manuscript. The laser is delivered to the region of interest by a quartz rod, providing the sensor with millimeter-level spatial resolution. And the design of probe makes the singled-end sensor convenient and capable for measurements in diverse situations. In order to validate the feasibility of this structure, temperature of the flame produced with a McKenna burner is measured via absorption of H₂O 7185.59 cm⁻¹ and 7444.35 cm⁻¹ transitions. Applying the scanned-wavelength modulation spectroscopy with second-harmonic detection and first-harmonic normalization (WMS-2f/1f) strategy, an enhancement of the signal-to-noise ratio over the short absorption path can be obtained. The temperature obtained by the single-ended sensor agrees well with that measured by a B-type thermocouple across different radial positions and heights, regardless of variations in equivalence ratios, confirming the accuracy of the sensor. In addition, a CFD simulation is performed to investigate the nonuniformity in the measurement region and the results show that the nonuniformity can be negligible.