|Mesa Photonics is working in four areas of optical absorption spectroscopy. All are aimed at detection of trace compounds, particularly trace gases. Optical absorption is described by the Beer-Lambert law:
where and , respectively the intensity of light before and after passing through a sample, is the concentration of a target compound in the sample, is the target compound’s absorption cross section, and is the length of the optical path through the sample. The cross section, , is a known property of the target compound and distance is a design parameter of the spectrometer. The concentration, , of a compound can then be determined by measuring . Measuring trace concentrations – i.e., determining small values of – requires either determining small changes in optical intensity, – , or using long optical paths, or both.
Our spectroscopy technology emphasizes working with and developing spatially coherent light sources that can be propagated easily over long distances or can be injected efficiently into optical resonators. These approaches provide the long optical paths that are needed for high sensitivity.
Two of Mesa Photonics spectroscopic technologies use near-infrared diode lasers – the types of lasers developed for long haul fiber optic telecommunications – for trace gas analysis. This work will lead highly miniaturized trace gas analyzers that are useful for emissions monitoring, environmental studies, and industrial safety application. The diode lasers have the advantages of being small, operating at low power, and, for most cases, producing eye safe output. The lasers operate at room temperature and are housed in hermetically sealed packages that are only 2.5 × 1.5 × 0.7 cm. The light is emitted through an optical fiber. Mesa Photonics researchers have over 30 years combined experience in diode-laser-based spectroscopy for trace gas detection.
One of the technologies is based on optical absorption spectroscopy with diode lasers and will, we anticipate, provide part per billion detection limits for a variety of gases with pocket-sized analyzers that cost less than half the price of existing optical analyzers. This proprietary method can also be applied to detection carbon monoxide, carbon dioxide, ammonia, hydrogen sulfide, methane, hydrogen chloride, nitric oxide, and ethylene. In the case of hydrogen sulfide detection for the petrochemical industry, the new technology should offer significant advantages over existing methods for hydrogen sulfide monitoring at wellheads and in refineries. It is also likely that our approach will be useful for isotope ratio measurements in carbon dioxide, water vapor, and methane.
Mesa Photonics is also working on detecting trace gases in exhaled breath using a technique called NICE-OHMS, standing for Noise-Immune, Cavity-Enhanced, Optical Heterodyne Spectroscopy. NICE-OHMS was invented by John Hall, Jun Ye, and co-workers at NIST in Boulder, CO. Their work and subsequent research by other groups demonstrated phenomenal detection sensitivity. But, the research used specialized lasers and was limited to low pressure gas samples. We are developing real-world applications including exhaled breath analysis by implementing NICE-OHMS using low-cost, telecommunications diode lasers and atmospheric pressure samples. Sensitivity is not as good as achieved at NIST, but it doesn’t need to be. We can trade off some performance for simplicity, and end up with better than part per billion sensitivity for some of the gases listed above.
We are also investigating new implementations of Fourier transform infrared (FTIR) spectroscopy. This technology area combines recent advances in optical sources, interferometry technology, and a proprietary signal analysis algorithm to implement compact, inexpensive spectrometers. Proof-of-principle applications were successful using near-infrared light sources. The next step is shifting to mid-infrared wavelengths.
Mesa Photonics interests in ultrafast lasers and optical spectroscopy are combined in the development and application of broad-band optical sources generated by femtosecond lasers. We own two ultrashort lasers – a Ti:sapphire oscillator from KM Labs and a 1550 nm fiber laser purchased from Menlo Systems – that are used for continuum generation. Wavelengths generated extend from the near-ultraviolet to 3 µm. New fiber materials will make it possible to extend the long wavelength range to 5 µm, and beyond. Temporal synchronization of the laser pulses simplifies cavity-enhanced spectroscopy techniques, and the broad-band sources can be used for optical spectroscopy with our new FTIR sub-system.