Scanning Laser Infrared Molecular Spectrometer:

Instrument Development for Chemical Sensing

Joel M. Steinkraus, Kelly Rickey, Alexander Ksendzov, Warren P. George,

Abdullah S. Aljabri, and David C. Scott

Jet Propulsion Laboratory, California Institute of Technology4800 Oak Grove Drive

M/S 125-177Pasadena, CA 91109

Abstract: The ability to observe and identify the presence of trace gases within an environment is a paramount capability needed to advance earth and planetary atmospheric research. Detection of trace levels of gases is also of interest in defense, industrial, security, medical, and environmental health applications. Current scientific objectives largely focus on identifying the presence of specific gases and isotopologues found in planetary atmospheres within our solar system. The presence and relative amounts of these gases allows scientists to deduce history of the planetary atmosphere and the likelihood that life has or could exist there. One challenge is accurately acquiring the data needed to make reliable conclusions when some of the target gas molecules are present in trace quantities of 10 parts per billion (ppb) or less. Laser gas spectrometers are effective ways of collecting in situ gas measurements, but their precision is directly proportional to the path length of the optical system. The Scanning Laser Infrared Molecular Spectrometer (SLIMS) is a novel solution that achieves very long effective path lengths, which yield ppb and sub-ppb measurements of trace gases. It can also accommodate multiple laser channels covering a wide range of wavelengths resulting in detection of more chemicals of interest. The mechanical design of the mirror cell allows for the large effective path length within a small footprint. The same design provides a robust structure which lends itself to being immune to some of the alignment challenges that similar cells face. The continued forward progress of the SLIMS project will rely on optimizing the optical paths and optical alignment geometries. Missions referred to in this document are for planning and discussion purposes only.

Historical Background: A key focus in the modern scientific community is to discover and understand the history and evolution of planetary atmospheres within our solar system. One of the most effective ways to do this is to look at the makeup of the gases in an atmosphere. By identifying the presence and relative amounts of key gases, logical speculation can be made about how the atmosphere has changed over time. This includes historical chemical records about the presence of water and chemical isotope ratios providing evidence of carbon based signatures of life.

Two targets of particular interest are the planet Venus and Saturn’s moon, Titan. Both of these bodies contain dense atmospheres thicker than that of Earth; and each has unique characteristics that make them prime scientific targets.

Titan: Titan is the second largest moon in the solar system and the only moon to contain a significant atmosphere. This moon’s atmosphere (composed of ~98.4% nitrogen and ~1.6% methane as well as other trace gases) is the only body besides Earth in our solar system containing a nitrogen rich atmosphere. Large amounts of methane, and trace amounts of the chemical building blocks of amino acids exist as well. Their presence on Titan leads scientists to liken the moon to Earth before the presence of oxygen-producing bacteria. By measuring the types and specific amounts of these trace gases a better understanding can be gained as to how they were formed. Methane specifically can be created through many different processes; therefore, identifying the origins of Titan’s methane is a key to understanding the moon as a whole. In the same way water exists as a liquid and a gas on Earth, lakes of liquid methane exist on Titan’s surface.1 Some scientists believe that if methane producing microbes do exist on Titan, then these lakes would be their most likely place of residence.2 Future missions to Titan would explore this possibility by closely examining these lakes and their composition. Much like Earth, Titan also has global weather patterns. Methane clouds occur over the surface daily and winds circulate in the same direction as the moon rotates. Studying weather patterns are another way of understanding change on Titan.

Figure 1: Artist’s Concept of a Titan Montgolfier Balloon probing the atmosphere of Titan. Credit: C. Waste.3

Changes in weather show how an environment is changing and contribute to its evolution. The landscape of the moon is sculpted by winds and liquid methane erosion; both predominantly weather dictated processes. It is believed that heavy methane storms on Titan are seasonal and

only commonly occur in specific latitudes, but light methane drizzles are common over the entire planet at any time.4 When exploring the makeup of other bodies in our solar system, water is a resource that is commonly sought. Liquid water especially is sought since it occurs so rarely in large amounts other than on Earth. Titan’s surface temperature hovers around -180 °C and any trace of water could only exist as ice. However, there is evidence that beneath the surface where the temperature is warmer, there is liquid water. Water on Titan functions much in the same way that lava does on Earth: it stays liquid until it is forced to the surface where it forms cryogenic volcanoes.5 The water comes in contact with the cold atmosphere and quickly hardens and freezes into icy mountains and flood plains.6

Venus: Venus is the closest planet to Earth and the two planets have several important characteristics in common. Venus is only slightly smaller than Earth and resembles it in overall chemical makeup and in gravitational pull. Unlike Earth, Venus’ atmosphere is composed of ~96.5% CO2, ~3.5% N as well as trace amounts of other compounds including water vapor, sulfur dioxide and sulfuric acid. The surface temperature of Venus stays relatively constant at 461 °C with a pressure of 93 atm.7 In the mid and upper atmosphere it rains sulfuric acid; however, because of the high surface temperature it evaporates before reaching the surface. Scientists believe that Venus used to be a planet very similar to Earth with large bodies of water and a very similar atmosphere. However, it is speculated that the oceans evaporated and the water vapor turned into CO2 and H2, the latter escaping into space.8 The geological processes and makeup of Venus and Earth are similar with the exception there being a lack of plate tectonic activity on Venus. This difference is expected to have played a role, at least in part, in Venus’ lack of large bodies of water, its high temperature, and its insufficient magnetic field to provide shielding from solar radiation. Future studies of Venus would likely focus on developing a better understanding of the geothermal processes that occur on and beneath the planet’s surface. In addition to this, chemical interactions that occur in the atmosphere of Venus would be studied in greater detail. Through observation of the concentrations of trace gases that exist at different atmospheric elevations, patterns can be seen which might explain how the atmosphere is changing and at what rate. With this information further speculation can be made as to how a possibly Earth-like planet evolved into the hot and barren one that exists today.

SLIMS: The Scanning Laser Infrared Molecular Spectrometer (SLIMS) is a proposed spectrometer that will detect and analyze trace gas samples. SLIMS is currently a multi project concept that is simultaneously being proposed for possible use in the Titan and Venusian atmospheres as well as for Mars missions. SLIMS will be a long path length, infrared, multi-pass, laser spectrometer capable of detecting gases at a sub part per billion level. The long path length will be created using a new, spherical ring mirror technology that utilizes a single, solid mirror to reflect the beams as seen in Figure 2.9 By creating a ring with a spherically shaped cavity and a highly polished finish, multiple bounces can be made within the mirror ‘cell’ to create a long path length. This technology will be combined with compact, tunable quantum cascade lasers (QCLs) as well as thermoelectrically cooled detectors to produce a powerful yet compact sensing device for interplanetary science missions.

Figure 2: Ray trace for a configuration using the spherical ring mirror absorption cell with a 2° injection angle for the laser probe beam.

Spherical Ring Challenges and Solutions: In order to study the atmosphere of these planets a reliable and highly accurate method of detecting and measuring trace amounts of gas is needed. The inherent challenge in identifying these gases is that their concentration in the atmosphere is often 10 parts per billion (ppb) or less. Absorption spectroscopy is a viable method of detecting trace gases, but it is necessary to produce long path lengths in order to achieve the precision necessary for detection. Multi-pass laser spectrometers, like Herriott cells,10 create long path lengths within a relatively small space using mirrored surfaces. This method is particularly attractive for use in outer-Earth missions because of the restrictions on space. Herriott cells and similarly designed spectrometers serve as work horses for atmospheric research.11 While they are very useful the mirrors and supporting optics require precise alignment. These alignments are critical for the spectrometer to function and degradation of alignment results in decreased signal to noise over the course of the flight. On the other hand, our proposed technology uses a solid, spherical cavity with a highly polished inner surface to create the long path length. This ring mirror removes the possibility of mirror misalignment caused by thermal expansion or vibrations because there is only a single, solid reflecting surface.

a) b)

Figure 3: a) External view of spherical cell geometry validated and tested in the JPL Optical Metrology Laboratory inside one of the Mars Exploration Rover wheels. b) Internal view from axel hub of spherical cell. This system is extremely stable and robust resisting alignment errors caused by mechanical and thermal perturbations.

The robust design also affords a solid surface to which other components of the spectrometer can be anchored. Optically, the ring mirror design provides an exceptionally long path length for a minimum footprint, volume and mass requirement. A stack of four rings capable of operating four different channels, each with a diameter of 0.5 m and a height of .015 m could weigh as little as 0.4 kg and conservatively produce an effective path length of more than 50 m.

In creating a longer path length, more of the laser’s initial signal is absorbed by the sample’s gas particles. If too little is absorbed, the change will be unobservable. Conversely, if there is too much absorption at a specific frequency, all of the energy will be absorbed and no meaningful information will be obtained. CO2 and many of the trace species of interest on Venus have an overlap of many of their absorption frequencies. Venus’ atmosphere is primarily CO2. If an unaltered sample from the atmosphere were analyzed it is likely that the presence of the gases of interest would be overlooked. One solution is to reduce the amount of CO2 present in the sample. CO2 filters use a chemical reaction between solid lithium hydroxide and gaseous carbon dioxide to produce solid lithium carbonate and liquid water. If a sample were to be filtered before it was introduced into the spectrometer then detection of trace gases would be much simpler. Unfortunately, possible complications could arise from the filter contaminating or altering the sample in unexpected ways. Further testing will be needed to understand precisely how CO2 filters alter atmospheric samples.

In many cases open air cells are used to take measurements of atmospheric gases. The gas that moves through the cell is analyzed, and thus the sample is constantly changing. Open air cells are simpler because they don’t require pumps to insert the sample or to evacuate the cell before a new sample.12 More measurements can also be taken with an open cell because there is always a new sample entering the cell. Less attractive, however, is that an open cell gives you no control over the sample. Pressure, concentration and other properties of the gas cannot be controlled with an open cell.

Another challenge of an open cell is maintaining the surface quality of the mirror. The mirror has a highly polished surface and several coatings to minimize the loss per bounce and maximize reflection. As the quality of the surface finish increases, so does the number of passes that can be made with the same amount of loss. As mentioned previously, the number of passes is directly proportional to the precision of the measurement being made. If the mirror quality is degraded, precision is also lost. In conditions where there are heavy winds, the mirror surfaces of an open cell are vulnerable to any solid particles that hit them and can easily be damaged. In the case of Venus, the upper atmosphere is known to contain acids which would corrode and damage the surface of the mirrors should they be exposed. If an open cell is to be used the mirror surface will require a coating to protect it from exposure to the environment while maintaining high reflectivity.

Tunable Quantum Cascade Lasers and SLIMS: QCLs are an improvement on previous generations of lasers in many respects. QCLs are created by molecular beam epitaxial deposition of atomized layers of materials onto a wafer to create a lasing material.13 By varying the thicknesses of the layers and the layer composition the wavelength of the laser can be customized over a wide range in the mid to far infrared region. The benefits of QCLs are extensive. They can be packaged in volumes smaller than a dime as shown in Figure 4, yet still have a high output power. QCLs are also very durable in their design. They are not static

sensitive or structurally fragile. Further improvements have made them even more attractive for scientific use.

Figure 4: Quantum Cascade Lasers offer extremely high power output in a small robust package.

Advancements in technology now enable operation of QCLs with thermoelectric coolers. This eliminates the need for cryogenic cooling and operational limitations caused by the use of liquid nitrogen or helium. Recent research has also led to the development of broadly tunable QCLs. By varying the input voltage and coupling to an external cavity the output wavelength can be altered up to 10% off of the center wavelength.14 For SLIMS this means less mass and longer operational life due to cryogen free cooling. Broad tunability will allow the detection of multiple gas species using a single channel, thus reducing the need and cost of adding multiple channels. Operation over broad spectral ranges now also enables detection of larger chemical species such as sarin, which have very broad absorption features in the infrared spectral region as shown in Figure 5.15

1130 1140 1150 1160 1170 11800.0

1.0x10-4

2.0x10-4

10 (c

m-1)

Wavenumber (cm-1)

Herriott Cell + ECQCL NWIR Spectral Library

Figure 5: Broad scan using an External Cavity Quantum Cascade Laser (EC-QCL) using Freon-125 as a surrogate for sarin. The solid line is the experimental EC-QCL data and the dashed line is from the Northwest Infrared Spectral Library.

One advantage that QCLs have in relation to detection of chemical species is the infrared range in which they operate. In different regions of the infrared spectrum the molecules are bent and stretched differently. The magnitude of the deformations that occur as the wavelength increases absorb more energy due to stronger absorption cross sections thus making detection of trace gases easier (Figure 6).

Figure 6: EC-QCLs allow broad tunability over the molecular fingerprint region of the electromagnetic spectrum.

Detector Developments: As lasers became advanced detectors became the limiting factor. Even though QCLs could operate without liquid nitrogen, detectors still required cryogenic cooling to operate at the desired precision. SLIMS uses a new type of detector that was developed in response to the need for non-cryogenic dependent systems. Like the QCLs, these detectors can make precision measurements using thermoelectric coolers. SLIMS incorporates thermoelectrically cooled detectors allowing the spectrometer to operate independently of cryogenic cooling. The result is a spectrometer capable of long term fully autonomous missions.

Future Study and Development of Spherical Ring Technology: Continuing development of spherical mirrors must focus on improving the mounting technology and the beam shaping optics required to achieve precise optical alignment for the long path geometries. Purely spherical cavity mirrors can only sustain ray paths in a single plane (such as those shown in Figure 2). Use of non-spherical geometries, such as ellipsoids and curved cylinders, theoretically should allow out-of-plane ray trace solutions. Such technology could increase the effective path length even more. Figure 7 is an untested model of one possible alternative to a spherical mirror.

Figure 7: Cross section of a theoretical alternative to the original spherical mirror concept.

To develop non-spherical mirrors, optical ray tracing equations will need to be developed in order to test and optimize beam paths within the cell. Mirror surface quality and laser power are important factors in developing longer effective path lengths. For all configurations, more beam passes mean more instances of lost power when the beam is reflected by the mirror. To minimize these losses and maximize the number of allowable passes, techniques to improve surface quality of the mirrors must advance. Using higher-powered lasers in the cell will allow more passes; however, ultimately reducing loss and minimizing beam overlap which causes optical fringing is the key to increased detection capability.

The research described in this publication was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

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