A diode laser based natural gas leak detector has been developed that can measure methane concentrations over six
orders of magnitude, from ambient (1.7 ppm) to pure gas levels. The detection method utilizes a small multipass cell
and wavelength modulation absorption spectroscopy. At high methane concentration, various forms of unmodulated
absorption spectroscopy are used. The instrument is a handheld unit that operates on less than 2 W of power and
weighs 1.4 kg (including battery). A small pump on the unit pulls outside gas into the enclosed optical cell through an
extendable probe. The response time of the instrument is approximately 1 - 2 sec.
Backscatter gas imaging uses laser absorption spectroscopy to detect the presence of a gas by illuminating a region with light from an infrared laser and imaging the returned light. Contrast can be enhanced by comparing the back-scattered intensity on and off the absorption feature. Wavelength modulation spectroscopy can provide just such a capability, but the detector signal must be processed with a lock-in amplifier, which is incompatible or prohibitively expensive with most array detectors. Images can be recorded using a single photodiode by spatially modulating the laser or the detected image. This paper describes initial experiments to demonstrate the feasibility of a combined wavelength- and spatially- modulated gas imager. It is based on a single near-infrared laser, a single detector, lock-in detection, and a commercial micromirror array. The gases imaged include water vapor, mono-deuterated water vapor, acetylene and hydrogen cyanide. Doppler imaging is demonstrated using heterodyne detection and spatial image modulation.
Visible/near-infrared diode lasers are well-suited for use as spectroscopic light sources in detection of a wide variety of gases by optical absorption. The high spectral resolution of these devices permits the selective detection of targeted species, while their characteristics of low cost, room temperature operation, and compatibility with fiber optics make them attractive for instrument development. A partial list of industrially or environmentally significant gases that may be measured by near-IR diode laser spectroscopy includes oxygen, water vapor, methane, acetylene, carbon monoxide, carbon dioxide, hydrogen halides, ammonia, hydrogen sulfide, and nitrogen oxides. This paper describes recent work at Southwest Sciences in development of diode laser-based instrumentation for industrial or environmental monitoring applications. Instrumentation utilizing a 1.393 micrometers DFB diode laser for measurement of trace moisture contamination in high purity process gases is described. In addition, recent laboratory studies to characterize the performance of new types of diode lasers in gas sensing applications are discussed, including vertical cavity surface emitting lasers in the 650 to 960 nm region and antimonide-based lasers in the 2.6 micrometers region.
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