49th International Conference on Environmental Systems ICES-2019-358 7-11 July 2019, Boston, Massachusetts

Tunable Laser Absorption Spectroscopy for Human Spaceflight

Christopher M Matty1 and Lance E Christensen2 National Aeronautics and Space Administration, Houston Texas 77058 and Pasadena California 91109

Human Spaceflight consistently has need for measurement of the major constituent gasses in a habitable cabin environment. Traditionally, this measurement has been performed using a mixture of mass spectrometry, electrochemical, and other traditional sensing methods. Tunable laser spectroscopy has been used in industry and research applications since the 1970s but has not been used in primary operational human spaceflight systems aside from a handful of experimental applications. Meanwhile, recent advances in the quality, size, and cost of semiconductor lasers has had a significant impact on the range of feasible applications for tunable laser spectroscopy. In 2018, work started on the Orion Laser Air Monitor (LAM), which is designed to be the primary major constituency monitoring system for the Orion Spacecraft. This paper covers the history leading up to the selection, design, and build of the Orion Laser Air Monitor as a primary major constituent analyzer for human spaceflight.

Nomenclature

TLDAS = Tunable Laser Diode Absorption Spectroscopy QC = Quantum Cascade IC = Interband Cascade TEC = Thermo Electric Cooler TRL = Technology Readiness Level SWAP = System Size Weight and Power IR = Infrared MCA = International Space Station Major Constituent Analyzer CAMS = Submarine Central Atmosphere Monitoring System AGA = Anomaly Gas Analyzer MGM = Multi Gas Monitor CPM = Combustion Product Monitor LAM = Laser Air Monitor SAFFIRE = Spacecraft Fire Experiement GaSb = Gallium Antimodide

I. Introduction UNABLE Laser Diode Absorption Spectroscopy (TLDAS) is currently enjoying a period at the developmental Tforefront. From the first uses of laser spectrometers; the technology has largely been driven by the availability and attendant size of serviceable lasers emitting at the necessary wavelength. Original laser instruments weighed hundreds of pounds and required cryostats or liquid nitrogen for cooling, making them at best awkward and largely impractical for operational use in spacecraft. Modern electronics miniturazation and micro-manufacturing capability, combined with advancement in semiconductor laser technology has allowed tunable semiconductor lasers to shrink to postage-stamp size, along with matching thermoelectric coolers (TEC)s that allow reliable function on an easily portable scale which lends more readily to integration into system and vehicle-level mechanisms. Additionally; the

1 Advanced Environmental Control and Life Support Systems Integrator, International Space Station Program, Johnson Space Center, Houston, Texas 77058. 2 Atmospheric Observations, Jet Propulsion Laboratory, Pasadena, California 91109

Technology Readiness Level (TRL) of semiconductor lasers has significantly increased as a function of the aforementioned technological advancements and attendant field deployment of tunable lasers in unmanned probes and rovers, to the point that use in human spaceflight, with the associated strict requirements for performance and reliability, has become a viable proposition.

II. History and Background of Tunable Laser Diode Adsorption Spectroscopy Tunable semiconductor laser spectrometers have been an essential part of NASA Earth Science since the 1980s1. Early high-altitude aircraft spectrometers used cryogenically-cooled lead-salt lasers to measure chemical species at the parts-per-trillion level, enabling understanding of critical Earth systems such as the dynamics and chemistry of the ozone hole2. As tunable lasers matured towards room-temperature operation, synchronous miniaturization of tunable laser spectrometers permitted their integration into NASA Planetary Science platforms such as the Tunable Laser Spectrometer on the Mars Curiosity Rover to understand geochemical processes and possible life signatures on Mars3. NASA also invests in tunable laser spectrometer demonstrations for monitoring of gases important to human spaceflight on ISS4. Much of the attraction of laser spectroscopy for science and human exploration needs arises from its sensitivity (parts-per-billion or better), System Size, Weight and Power; SWAP, (weight less than few kg, power less than a few watts), fast response (compared with solid state sensors), relative freedom from calibration, and specificity (capability to distinguish isotopes). Sensitivity is primarily derived from the emitted wavelength and optical pathlength. Mid-IR lasers access stronger rovibrational (electronic for O2) transitions than near-IR lasers (Figure 1) and the implications for spectrometer design can be dramatic. The advent of thermoelectrically-cooled lasers at 3.3 µm in the 2000s5 meant it was possible to measure methane at parts-per-trillion with a 36×13-cm dia. instrument that could fit inside a rover6 employing a traditional beam multipass technique like Herriott cells7 proven in rugged environments like open-path configurations in Earth’s stratosphere8. Without such mid-IR laser evolution, it would have been difficult to engineer the cryogenic-cooling or cavity-enhanced solutions (cavity ringdown or integrated cavity output spectroscopy) required to obtain this sensitivity while remaining robust through spaceflight shock and vibration and environmental changes on Mars surface. Many of the trace gas monitoring needs (CO2, H2O, O2, CH4, HCl, HCN, HF, NH3, N2H4) for human exploration can be done with shoebox-sized open-path Figure 1. Rovibrational Tranistion Linestrengths Rovibrational instruments without need of pumps9. transition linestrengths for carbon dioxide (black) and methane (red). Note For chemicals that are sticky (e.g. logarithmic y-axis. HCl) or decompose on surfaces (e.g. N2H4), open-path monitoring can give a clearer assessment of local chemical concentration then if gas were pumped to the analysis region. Tens of Hz or faster measurement rates combined with the high sensitivity afforded by laser spectrometers may prove useful for human breath monitoring needs such as assessing respiratory function of astronauts. Tunable laser spectrometers can achieve millisecond response times because measurement speed is dictated by laser drive electronics as opposed to chemical-physical adsorption or reaction processes. Increasingly, the Earth science community turns to laser spectrometers for fast eddy-covariance measurements of greenhouse gases10. The spectral resolution of tunable semiconductor lasers (few MHz) enables distinguishing individual molecules and isotopologues with one laser. Sub-percent level carbon dioxide and water vapor can be measured using an 8-cm optical pathlength open-pass configuration at 2.68 microns at 1 atm (Figure 2). To understand Martian geochemistry, 13 18 17 the Tunable Laser Spectrometer measures isotopologues of carbon dioxide (CO2, CO2, OCO, OCO) at 2.79

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microns at 0.010 atm. It can be envisioned that tunable laser spectrometers can provide non-invasive, robust, and miniature health monitoring for astronaut metabolic state (breakdown of sugar vs fat), passive indications of health problems like sepsis and liver fibrosis, and active testing (inhalation of isotopically-labeled tracers) for specific diseases like tuberculous11.

III. Legacy to Current Human Spaceflight Sensing Systems Sensing and monitoring of habitable cabin atmosphere and spacesuit interface volumes is largely considered to be a necessity for human spaceflight. The performance, integration, safety, and health considerations of human crew members onboard a spacecraft necessitate monitoring of the pressure, temperature, and chemical composition of the cabin and suit atmospheres. Initially, during the Mercury through Apollo era, US spacecraft used dedicated single- channel sensors for gross measurement of CO2 and O2 levels. With the advent of the Skylab program, a flight mass spectrometer sensor system (affectionately called “the pig” for its round shape) which collectively measured N2, O2, CO2, H2O, H2, and approximate Total Hydrocarbons (THC) in one unit. Longer missions, coupled with increased vehicle size and volume, coupled with high potential measurement accuracy attendant to a mass spectrometer, made mass spectrometry attractive as a means to precisely measure and monitor cabin atmospheric constitucency. The Skylab “pig” paved the way for the current Major Constituent Analyzer (MCA) currently in use onboard the International Space Station (ISS). The MCA itself is a heritage design carried from the Central Atmosphere Monitoring System (CAMS) technology originally used in military submarines12. MCA is a magnetic-sector type mass spectrometer which measures CO2, O2, N2, H2O, CH4, and H2. In contemporary human spaceflight philosophy, the primary cabin atmosphere monitoring system is typically considered as Criticality 1, or Criticality 1, meaning that function and performance of the system are immediately critical to the safe performance of mission by the human crew. Having a long track record of field use for human systems, mass spectrometers generally are held in high regard in providing robust, dependable Criticality 1 level constituent monitoring. However, while mass spectrometers are highly regarded for precision and accuracy; there are also inherent drawbacks to the technology, namely relatively high levels of complexity, difficulty hardening against harsh loads or dirty environments, and typical need for regular calibration in order to maintain specified performance.

IV. Description of Tunable Laser Systems in Development (Laser Air Monitor) NASA is investing in several tunable laser spectrometers that are under development for human exploration missions, which to date have happened on experimental or traial baseis. The Anomaly Gas Analyzer (AGA) takes its heritage from the Multi-Gas Monitor (MGM) which has flown on ISS as a technology demonstration4. AGA measures CO, HCN, HF, HCl, CO2, O2, NH3, and N2H4 using an integrating sphere to increase optical pathlength and a laser photoacoustic sensor for H2H4. The Combustion Products Monitor (CPM)9 is being developed for NASA’s SAFFIRE experiment. CPM uses both single-pass and Herriott-cell multipass channels to measure CO, HCl, HCN, HF, CO2, and O2. In addition to these experimental units; a new class of instrument, the Laser Air Monitor (LAM), began development in 2018. LAM was designed and built from the outset as a criticality 1 type system, to be directly integrated into the Figure 2. Modeled Spectra of Water and Carbon Dioxide Model spectra vehicle (Orion, in this case), and of 1.5% water vapor (black) and 500 ppm CO2 (red) for an 8-cm optical to serve as an immediate part of pathlength at 980 mbar, 298 K. Arrow points to central target wavelength. the operational mission architecture. As previously addressed, the need to function as a Criticality 1 system brings a higher level of engineering, design, build, qualification, and ultimately mission performance requirements, which offer a new avenue for tunable laser systems. 3 International Conference on Environmental Systems

LAM is a two-channel laser spectrometer designed to measure O2 on one channel and CO2 and water vapor on the other. The O2 channel utilizes a diode laser emitting at 760.9 nm into a Herriott multipass cell with optical pathlength araound 230 cm (Figure 3). The CO2/H2O channel utilizes a diode laser emitting at 2683.1 nm into a signal-pass configuration of around 7 cm. Figure 4 shows a schematic of the optical analysis cell. LAM employs 2f wavelength modulation spectroscopy to measure chemical concentrations. To minimize SWAP and allow redundant units to be deployed, LAM does not employ active pressure or temperature control. LAM calibrates the detected signal to concentration as a function of pressure (2.1 - 23.9 PSIA) and temperature (2.2 - 49.4 ⁰C). Both strong and weak CO2 rovibrational transitions are available to aid measurements over a large dynamic range. Table 1 describes the required measurement accuracies and precisions for the 7.5-15.5 PSIA pressure range. Reduced requirements exist for 2.1-7.4 PSIA and 15.6 – 23.9 PSIA. Figure 3. Modeled Spectra of Oxygen Model spectra of 20% O2 for 230 cm Key to the success of flight- optical pathlength, 980mbar, 298K. Arrow points to target spectral feature. qualified LAM and any flight- qualified human exploration tunable laser spectrometer are obtaining lasers that can pass Criticality 1 level requirements. While the technology exists to produce mW or higher levels of single-wavelength emitted light from semiconductor laser chips at room temperature at almost any wavelength through the mid- Figure 4. LAM Schematic Curved surfaces are mirrors. On the left side is the IR, flight qualifying 2683 nm laser which emits a single pass beam (blue) to a room-temperature semiconductor lasers remains in MCT detector on the opposide side. On the right side is the 760 nm laser which its infancy. There is data on the emits a multi-pass beam (red) with several meters optical pathlength which radiation performance of GaSb impinges on a silicon detector on the opposite side. For scale, the distance diode lasers13 but extrapolating between mirrors is around 8 cm. that to other laser materials and internal structures (interband- or quantum- cascade) is not straight-forward. Almost all of what is referred to as ‘lasers’ is actually a hybrid package of components such as thermoelectric coolers, package material, solder joints, thermistors, windows, internal coatings, internal bath gas. Generally, these materials, as well as the laser chips themselves, are sourced from foreign vendors with little flight experience. Working in tandem with laser material growers, fabricators, packagers, and testers to generate a consistent, reliable source of this essential component is a paramount importance. Table 1. LAM Requirements for Orion Parameter measured Range Accuracy Precision Total Pressure 7.5 – 15.5 psia ± 0.3% full scale ± 0.001 psia ppH2O 3.62 – 40.00 mmHg ± 0.2 mmHg N/A ppO2 1.9 – 5.5 psia ± 0.05 psia N/A 5.51 – 15.5 psia ± 0.5 psia ppCO2 0-7.76 mmHg ± 0.31 mmHg N/A

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V. Future Planning and Development The Orion Laser Air Monitor LAM is being built for delivery and integration into the vehicle for initial flight on the Orion Exploration Mission 2 flight currently scheduled for 2023. Nominally, this system will prove to be robust, reliable, and functionaly capable of meeting all objectives for a Criticality 1 human spaceflight system. Once successfully field proven, TLDAS sensors may reach additional applications in human spaceflight, as well as in industry, military applications, and other highly demanding environments where human safety is critical and therefore accuracy, reliability, and operational ruggedness are paramount. While there will remain many applications for traditional mass spectrometry type systems, TLDAS sensor offer multiple advantages for SWAP vs systems currently in use, namely small form factor, relatively low power requirements, stability vs need for calibration, and hardware robustness.

VI. Conclusion TLDAS remains a relatively young technology, and one that has seen a significant boost in recent years due to the availability of small, robust lasers. The implementation of TLDAS into Criticality 1 systems for human spaceflight offers a progressive step toward the verification and qualification of the technology for a wide range of applications, while simultaneously adding a useful new option to the engineering toolkit available to support human spaceflight.

References 1. Menzies RT, et al. “Balloon-Borne Diode-Laser Absorption Spectrometer for Measurements of Stratospheric Trace Species” App. Opt. 22, 2655, 1983.

2. Webster CR, et al. “Chlorine Chemistry on Polar Stratospheric Cloud Particles in the Arctic Winter”, Science 261, 1130, 1993.

3. Webster CR, et al. “Isotope Ratios of H, C, and O in CO2 and H2O of the Martian Atmosphere”, Science 341, 260, 2013.

4. Mudgett PD, et al. “Laser Spectroscopy Multi-Gas Monitor: Results of a Year Long Technology Demonstration on ISS”, ICES-2015-243, 2015.

5. Christensen LE, et al. “Thermoelectrically Cooled Interband Cascade Laser for Field Measurements”, Opt. Eng. 49, 111119, 2010.

6. Mahaffy PR, et al. “The Sample Analysis at Mars Investigation and Instrument Suite”, Space Sci. Rev. 170, 401, 2012.

7. Herriott D, Schulte H, “Folded Optical Delay Lines”, App. Opt. 4, 883, 1965.

8. May R., “Open-path, near-infrared tunable diode laser spectrometer for atmospheric measurements of H2O”, J. Geophys. Res. 103, 19151, 1998.

9. Briggs RM, et al. “Compact Multi-Channel Infrared Laser Absorption Spectrometer for Spacecraft Fire Safety Monitoring”, ICES-2016-283, 2016.

10. LICOR 7700 Methane Analyzer.

11. Murtz M, “Breath Diagnostics using laser spectroscopy”, Optics and Photonics News, 30, Jan. 2005.

12. Mudgett PD, et al. “US Navy Submarine Sea Trial of NASA developed Multi-Gas Monitor”, ICES-2017-167

13. Esquivias I, et al. “Evaluation of the Radiation Hardness of GaSb-based Laser Diodes for Space Applications”, IEEE RADECS Proceedings, 2011.

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