Measurement of the isotopic composition of atmospheric methane is a valuable tool for understanding the sources and sinks of the global carbon budget. One promising carbon isotope ratio measurement technology is optical spectroscopy using inter-band cascade (IC) lasers. Ongoing development of these light sources has the goal of providing, from a package operating near room temperature, a single mode laser source in the wavelength range of 3 &mgr;m. The spectral features of methane are sufficiently strong at this wavelength that a path length of about 100 m should suffice for measuring 12- and 13-C isotopes in air without pre-concentrating the sample. Experimental IC lasers are described and their use for isotope sensing by wavelength modulation spectroscopy is evaluated.
The development of high power, single mode and continuously tunable diode lasers in the visible to near-infrared region, and antimonide diode lasers operating in the 2 - 3 micrometer region near room temperature, is opening measurement opportunities in wavelength regions hitherto inaccessible by diode lasers. The spectroscopic properties of antimonide-based diode lasers operating in the 2.2 - 2.3 micrometer region have been examined for application to high sensitivity monitoring of carbon monoxide and formaldehyde. In a second application, nonlinear- up-conversion of diode laser output in order to access strong electronic transitions of atoms and molecules in the UV wavelength region is described. A tunable 308 nm beam was generated by sum frequency mixing high power diode laser output with single frequency ArPLU laser output, and high sensitivity absorption and laser induced fluorescence detection of the hydroxyl radical was demonstrated. Finally, direct doubling of the near-infrared output from an external cavity diode laser/power amplifier module was used to generate a tunable, near-UV laser beam to demonstrate the feasibility of optical flux monitoring of Group III atomic beams in an MBE chamber by spatially resolved optical absorption/LIF detection.
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.
Diode laser spectroscopy provides exceptional sensitivity and selectivity for real-time characterization of reacting systems and gas streams. High frequency wavelength modulation techniques achieve species detection limits that are routinely in the ppm range and can reach sub-ppb levels under favorable conditions. Narrow laser linewidths guarantee selective detection of key species even in the presence of myriad other components. Diode laser spectroscopy is also relatively immune from interference by black body radiation or chemiluminescence. Prototype diode-laser based systems have been demonstrated successfully for trace gas detection in turbulent, high temperature particle-laden streams, for oxygen quantitation in flames, for free radical characterization in a plasma etching reactor and for greenhouse gas flux measurements in air. We also discuss the availability of laser wavelengths, compatibility with fiber optics, cost safety and expectations for new laser development.