Diode laser based absorption spectroscopy (DLAS) is widely used for gas detection in variety of applications across the energy, petrochemical and mining industries. Recent developments in near and mid infrared diode lasers have improved the sensitivity of gas measurement based on high resolution spectra of target species. The availability of near-infrared diodes that can operate at room temperature has expanded the application of spectroscopy technique in hand-held gas detection devices. DLAS in conjunction with optical fibres has also been applied for the distributed sensing and monitoring of various gases in real-time basis. This paper, after introducing methane characteristics and semiconductor diode lasers, comprehensively reviews development in spectroscopic methane sensing techniques in accordance with methane absorption lines in the near infrared spectrum.

A review of developments in near infrared methane detection based on tunable diode laser

We present the design and initial investigation of a fibre optical system which may be used both for intra-cavity and for ring-down measurements of absorption losses. The system consists of a fibre loop containing a length of erbium-doped fibre pumped at 980 nm, with gain adjustment below or above threshold for the two types of operation. The fibre loop is constructed from standard fibre optical components and includes a micro-optical gas cell. The intended application is for measurement of levels of trace gases which possess near-IR absorption lines within the gain bandwidth of the erbium fibre amplifier. We discuss the key issues involved in operation of the system and the level of sensitivity required. Our initial experimental investigations have demonstrated that ring-down times of several microseconds can be obtained, which can be altered through adjustment of the attenuation or gain factor of the loop. Gain control is one of the most important issues and we explain how this may be achieved.

https://iopscience.iop.org/article/10.1088/0957-0233/12/7/316/pdf

An investigation of an optical fibre amplifier loop for intra-cavity and ring-down cavity loss measurements

In this paper we investigate the prospects for realization of evanescent-wave gas sensors using gas absorption lines within the transmission window of silica fibres. Our work focuses on methane and uses D-fibre as the evanescent-field sensor. We discuss the three major obstacles, namely, low sensitivity, high background signal levels from interference effects and system degradation through surface contamination. We explain how sensitivity may be improved through sol-gel coatings, how background signal levels may be reduced by use of polarization-maintaining fibre and suggest a way of compensating for surface contamination by continuous monitoring of the birefringence of the D-fibre. Improved performance can be obtained through use of a distributed-feedback laser instead of a light-emitting diode, but at much greater cost. In the long term, the likely application for the evanescent-field sensor is in distributed measurement systems.

Prospects for fibre-optic evanescent-field gas sensors using absorption in the near-infrared

A diode-laser sensor system based on absorption spectroscopy techniques has been developed to monitor CH4 nonintrusively in high-temperature environments. Fundamental spectroscopic parameters, including the line strengths of the transitions in the R(6) manifold of the 2ν3 band near 1.646 μm, have been determined from high-resolution absorption measurements in a heated static cell. In addition, a corrected expression for the CH4 partition function has been validated experimentally over the temperature range from 400 to 915 K. Potential applications of the diode-laser sensor system include process control, combustion measurements, and atmospheric monitoring.

Tunable diode-laser absorption measurements of methane at elevated temperatures

/pdf/environmental-monitoring-a-comprehensive-review-on-optical-e9xrtshk.pdf

Environmental Monitoring: A Comprehensive Review on Optical Waveguide and Fiber-Based Sensors

Remote simultaneous detection of methane and acetylene by using two single-mode diode lasers emitting at 1.66 and 1.53 mu m, respectively, coupled with 4-km optical fibers has been demonstrated by laboratory simulations. The lasers were modulated at different frequencies to measure in real time the absorption of the two chemical species without interference. The gases could be detected quantitatively by the ratio of the phase-sensitive-detected fundamental and second-harmonic signals. When the absorption pathlength was 10 cm, the minimum detectable concentrations of methane and acetylene in air were 5 and 3 p.p.m., respectively, for a signal averaging time of 1 s. >

Long-distance simultaneous detection of methane and acetylene by using diode lasers coupled with optical fibers

Long-distance Simultaneous Detection Of Methane And Acetylene By Using Diode Lasers in Combination With Optical Fibers

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Long-distance simultaneous detection of methane and acetylene by using diode lasers coupled with optical fibers

Long-distance Simultaneous Detection Of Methane And Acetylene By Using Diode Lasers in Combination With Optical Fibers