The methods of tunable diode-laser absorption spectroscopy (TDLAS) in combination with newly developed sensor configurations have been proven capable of providing non-intrusive, in situ, species-specific measurements of gas properties in a wide range of practical applications, even in harsh environments as e.g. in engines and power-plants. While in the past most frequently line-of-sight approaches have been used, characterized by a laser on one end and a detector on the other, methods detecting laser light that is backscattered off surfaces of natural material characterized by low reflectivity have just started to be applied in field measurements.

In this Applied Optics article, Christopher S. Goldenstein and his colleagues present a compact and robust fiber-coupled near-infrared TDLAS sensor for stand-off measurements of gas temperature, pressure, and composition. The sensor utilizes a fiber bundle with six multi-mode fibers surrounding one single-mode fiber to transmit and receive backscattered laser light in a hand-held unit, which contains the transmitter and the receiver. Scanned-wavelength-modulation spectroscopy with 1f -normalized 2f -detection and fast (80-200 kHz) wavelength modulation is used to provide calibration-free measurements and reduce the influence of cavity-noise formed by the overlapping of transmitted and reflected laser light.

The capabilities of this new sensor type are demonstrated by measuring (i) temperature, pressure, and the mole fraction of H2O in a propane flame and (ii) the concentration of CH4 at stand-off distances of 15 cm and 10 m, respectively. The fraction of photons collected by the system ranged from 10⁴ to 1 ppm at stand-off distances from 10 cm to 10 m, respectively. This new sensor design enables a more practical utilization as a hand-held or wearable detector and has the potential to become integrated as an analysis tool even under conditions of limited optical access.

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