Above: A low-power dissipation interband cascade (IC) laser fabricated at MDL inside a package designed specially for TLS. This device emits at 3.27 µm and was designed specifically to target absorption lines of methane.

Tunable Laser Spectrometers

Since the 1980s JPL has pioneered the development and deployment of Tunable Laser Spectrometers for NASA for in situ atmospheric and planetary applications.

The vast majority of gaseous chemical substances exhibit fundamental vibrational absorption bands in the mid-infrared spectral region (≈ 2–10 µm), and the absorption of light by these fundamental bands provides a nearly universal means for their detection. A main feature of optical techniques is the non-intrusive in situ detection capability for trace gases.

Tunable laser spectrometers offer a wide diversity of capability for highly sensitive measurement of atmospheric gases or those evolved (pyrolysis heating, laser ablation, etc.) from solid samples. Because it is based on IR laser absorption of individual rotational lines within a vibrational band, the method is very sensitive (parts-per-billion to parts-per-trillion), direct, non-invasive, easy to calibrate and unambiguous in its species identification and isotope ratio determinations without interference.

Although mass spectrometers have strength in wide survey capability and uniqueness in noble gas detection and noble gas isotope ratios, small planetary instruments are not well suited to low abundance H2O and confuse H₂O, NH₃, and CH₄ isotopic species that overlap in mass number, especially when present in similar abundances. TLS is the preferred detection method for a variety of gases including H₂O, CH₄, N₂O, CO, CO₂, NO, NO₂, HNO₃, O₂, O₃, HCl, HF, and for stable isotopes in C, H, and O.

Laser sources are now available at room temperature with several to tens of milliwatts continuous-wave (CW) output power spanning a wide wavelength range. Detectors are single-element, and miniature all-solid-state spectrometers are already being developed. Three detection methods (direct absorption, second-harmonic, and cavity ring-down) will be employed as appropriate.

Tunable laser spectroscopy can be applied in a variety of simple configurations: closed mutipass cell (Herriott cell or cavity ringdown cell), open cradle cell, or direct to target ranging using topographically-scattered and return laser light. Hand-held portable systems are also feasible for many specific applications. Using tunable laser spectrometers, trace molecules in the Earth’s atmosphere or the atmosphere surrounding another planet may be precisely identified and studied by measuring their infrared absorption spectrum. Such measurements can reveal a wealth of information about the atmosphere, its composition, chemistry, evolution, and winds. Furthermore, as demonstrated by TLS, part of the Sample Analysis at Mars (SAM) instrument suite on the Mars Curiosity rover, isotope ratios of gases released from heating rocks can reveal details about the planet’s evolution and volatile history. NASA is developing ideas to conduct ice-core sampling at the Martian poles to measure water isotopes using a TLS instrument to reveal the climate record over millions of years past- just as is done on Earth.

Dr. Webster is a Senior Research Scientist and former Director of MDL. He pioneered the application of tunable laser spectroscopy to a wide variety of research in molecular structure, photochemistry, electrical discharge physics, and especially Earth atmospheric science. He is an expert in numerous laser techniques from UV to IR and has built several miniaturized state-of-the-art instruments for use on Earth aircraft and planetary missions.

Tunable Laser Spectrometers

Products & Processes

TLS Mars

Harvard Collaboration

Spacecraft Fire

TLS Development

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