Atmospheric methane trends over the last few years have been increasing at a rate of 7-12 parts per billion (ppb) per year after brief a pause in the first decade of this century. The reasons for the pause and subsequent increase remains unclear. Thus, there is a critical need for additional, precise and accurate methane observations to understand the natural and anthropogenic processes that drive the trends in atmospheric methane and to constrain its sources, and sinks. At NASA Goddard Space Flight Center (GSFC), in collaboration with Freedom Photonics Inc., we have been developing a lidar to measure atmospheric methane using Integrated Path Differential Absorption (IPDA) from an airborne platform as a precursor to a future space mission. In this paper we present the design of a laser transmitter operating at ~1651 nm based on a newly developed Distributed Bragg Grating (DBR) seed laser and an Optical parametric oscillator (OPO). The DBR is rapidly step-tuned over the methane absorption at several discrete wavelengths. This multi-wavelength approach enables us to sample the entire methane lineshape and reduce systematic errors.
The global and regional quantification of methane fluxes and identification of its sources and sinks has been highlighted as one of the goals of the NASA 2017 Earth Science Decadal Survey. Detecting methane from space and airborne platforms with an active (laser) remote sensing instrument presents several unique technology and measurement challenges. The instrument must have a single frequency, narrow-linewidth light source, and photon-sensitive detector at the right spectral region to make continuous measurements from orbit, day and night, all seasons and at all latitudes. It must have a high signal to noise ratio and must be relatively immune to biases from aerosol/cloud scattering, spectroscopic and meteorological data uncertainties, and instrument systematic errors.
At Goddard Space Flight Center (GSFC), in collaboration with industry, we have developed an airborne instrument to measure methane. Our instrument is a nadir-viewing lidar that uses Integrated Path Differential Absorption (IPDA), to measure methane near 1.65 μm. We sample the absorption line using multiple wavelengths from a narrow linewidth laser source and a sensitive photodetector. This measurement approach provides maximum information content about the CH4 column, and minimizes biases in the XCH4 retrieval.
In this paper, we will review our progress to date and discuss the technology challenges, options and tradeoffs to measure methane from space and airborne platforms.
We report on the fabrication and characterization of Er:YGG films suitable for waveguide amplifiers that could in principle be used in integrated path differential absorption lidar systems. Presented is our fabrication technique, comprising pulsedlaser- deposition growth of ~10 μm-thick crystalline films, their channeling via ultraprecision ductile dicing with a diamond-blade, producing optical quality facets and sidewalls, and amplifier performance. Net gain at 1572 nm and 1651 nm is obtained for the first time in Er-doped YGG waveguide amplifiers. Additionally, in a channel waveguide a maximum internal gain of 3.5 dB/cm at the 1533nm peak was realized. Recent crystal film quality improvements promise further performance enhancements needed for the intended application for high-peak power sources in the 1.6-μm spectral region targeting Earth observation systems for monitoring greenhouse gases.
NASA Goddard’s CO2 Sounder is a pulsed, multiple-wavelength, IPDA-lidar. It was flown onboard the NASA DC-8 to measure atmospheric CO2 column concentrations (XCO2) in the lower stratosphere and troposphere of the Arctic region of North America as part of the 2017 ASCENDS airborne campaign. Eight flights covering 40,000 km were flown in late July and August over Alaska and Canada’s Northwest Territories, including a northern transit east of the Rockies and a return transit partly over the ocean between Alaska and California. The Arctic flights were coordinated with the 2017 Arctic-Boreal Vulnerability Experiment (ABoVE) campaign. The metrological conditions were challenging: a non-uniform CO2 distribution, a dynamic atmosphere and varied surface-reflectivity. To assess the accuracy of our lidar the aircraft’s scientific payload included the AVOCET and Picarro instruments. These two instruments measured in-situ XCO2 during the flights and column XCO2 from 47 separate descent spirals from ~12 km altitude to near ground at local airfields distributed throughout the measurement region. Each spiral maneuver allows a direct comparison between the retrievals of XCO2 from the lidar against those computed from insitu instruments. The CO2 Sounder worked very well during all phases of the campaign. Analysis to date shows the lidar measured column concentrations are in close agreement with in-situ column measurements with a precision of better than 0.8 ppm with 1 second averaging. In addition, preliminary analyses of measurements to the ubiquitous cloud tops also produced column concentrations and information on the vertical XCO2 structure.
Our understanding of the Mars atmosphere and the coupled atmospheric processes that drive its seasonal cycles is limited by a lack of observation data, particularly measurements that capture diurnal and seasonal variations on a global scale. As outlined in the 2011 Planetary Science Decadal Survey and the recent Mars Exploration Program Analysis Group (MEPAG) Goals Document, near-polar-orbital measurements of height-resolved aerosol backscatter and wind profiles are a high-priority for the scientific community and would be valuable science products as part of a next-generation orbital science package. To address these needs, we have designed and tested a breadboard version of a direct detection atmospheric wind lidar for Mars orbit. It uses a single-frequency, seeded Nd:YAG laser ring oscillator operating at 1064 nm (4 kHz repetition rate), with a 30-ns pulse duration amplified to 4 mJ pulse energy. The receiver uses a Fabry-Perot etalon as part of a dual-edge optical discrimination technique to isolate the Doppler-induced frequency shift of the backscattered photons. To detect weak aerosol backscatter profiles, the instrument uses a 4x4 photon-counting HgCdTe APD detector with a 7 MHz bandwidth and < 0.4 fW/Hz1/2 noise equivalent power. With the MARLI lidar breadboard instrument, we were able to measure Doppler shifts continuously between 1 and 30 m/s by using a rotating chopper wheel to impart a Doppler shift to incident laser pulses. We then coupled the transmitter and receiver systems to a laser ranging telescope at the Goddard Geophysical and Astronomical Observatory (GGAO) to measure backscatter and Doppler wind profiles in the atmosphere from the ground. We measured a 5.3 ± 0.8 m/s wind speed from clouds in the planetary boundary layer at a range of 4 to 6 km. This measurement was confirmed with a range-over-time measurement to the same clouds as well as compared to EMC meteorological models. Here we describe the lidar approach and the breadboard instrument, and report some early results from ongoing field experiments.
The Lunar Orbiter Laser Altimeter (LOLA) instrument [1-3] on NASA’s Lunar Reconnaissance Orbiter (LRO) mission, launched on June 18th, 2009, from Kennedy Space Center, Florida, will provide a precise global lunar topographic map using laser altimetry. LOLA will assist in the selection of landing sites on the Moon for future robotic and human exploration missions and will attempt to detect the presence of water ice on or near the surface, which is one of the objectives of NASA’s Exploration Program.
Our present knowledge of the topography of the Moon is inadequate for determining safe landing areas for NASA’s future lunar exploration missions. Only those locations, surveyed by the Apollo missions, are known with enough detail. Knowledge of the position and characteristics of the topographic features on the scale of a lunar lander are crucial for selecting safe landing sites. Our present knowledge of the rest of the lunar surface is at approximately 1 km kilometer level and in many areas, such as the lunar far side, is on the order of many kilometers. LOLA aims to rectify that and provide a precise map of the lunar surface on both the far and near side of the moon.
LOLA uses short (6 ns) pulses from a single laser through a Diffractive Optical Element (DOE) to produce a five-beam pattern that illuminates the lunar surface. For each beam, LOLA measures the time of flight (range), pulse spreading (surface roughness), and transmit/return energy (surface reflectance). LOLA will produce a high-resolution global topographic model and global geodetic framework that enables precise targeting, safe landing, and surface mobility to carry out exploratory activities. In addition, it will characterize the polar illumination environment, and image permanently shadowed regions of the lunar surface to identify possible locations of surface ice crystals in shadowed polar craters.
Trace gases and their isotopic ratios in planetary atmospheres offer important but subtle clues as to the origins of a planet's atmosphere, hydrology, geology, and potential for biology. An orbiting laser remote sensing instrument is capable of measuring trace gases on a global scale with unprecedented accuracy, and higher spatial resolution that can be obtained by passive instruments.
For Earth we have developed laser technique for the remote measurement of the tropospheric CO2, O2, and CH4 concentrations from space. Our goal is to develop a space instrument and mission approach for active CO2 measurements. Our technique uses several on and off-line wavelengths tuned to the CO2 and O2 absorption lines. This exploits the atmospheric pressure broadening of the gas lines to weigh the measurement sensitivity to the atmospheric column below 5 km and maximizes sensitivity to CO2 changes in the boundary layer where variations caused by surface sources and sinks are largest. Simultaneous measurements of O2 column use a selected region in the Oxygen A-band. Laser altimetry and atmospheric backscatter can also be measured simultaneously, which permits determining the surface height and measurements made to thick cloud tops and through aerosol layers.
We use the same technique but with a different transmitter at 1.65 um to measure methane concentrations. Methane is also a very important trace gas on earth, and a stronger greenhouse gas than CO2 on a per molecule basis. Accurate, global observations are needed in order to better understand climate change and reduce the uncertainty in the carbon budget. Although carbon dioxide is currently the primary greenhouse gas of interest, methane can have a much larger impact on climate change. Methane levels have remained relatively constant over the last decade but recent observations in the Arctic have indicated that levels may be on the rise due to permafrost thawing. NASA’s Decadal Survey underscored the importance of Methane as a greenhouse gas and called for a mission to measure CO2, CO and CH4. Methane has absorptions in the mid-infrared (3.3 um) and the near infrared (1.65 um). The 3.3 um spectral region is ideal for planetary (Mars) Methane monitoring, but unfortunately is not suitable for earth monitoring since the Methane absorption lines are severely interfered with by water. The near infra-red overtones of Methane at 1.65 um are relatively free of interference from other atmospheric species and are suitable for Earth observations. The methane instrument uses Optical Parametric Generation (OPG) along with sensitive detectors to achieve the necessary sensitivity. Our instrument generates and detects tunable laser signals in the 3.3 or 1.65 um spectral regions with different detectors in order to measure methane on Earth or Mars. For Mars, the main interest in methane is its importance as a biogenic marker.
We report on an airborne demonstration of atmospheric methane (CH4) measurements with an integrated path differential absorption lidar using an optical parametric amplifier and optical parametric oscillator laser transmitter and sensitive avalanche photodiode detector. The lidar measures the atmospheric CH4 absorption at multiple, discrete wavelengths near 1650.96 nm. The instrument was deployed in the fall of 2015, aboard NASA’s DC-8 airborne laboratory along with an in situ spectrometer and measured CH4 over a wide range of surfaces and atmospheric conditions from altitudes of 2 to 13 km. We will show the results from our flights, compare the performance of the two laser transmitters, and identify areas of improvement for the lidar.
Atmospheric methane (CH4) is the second most important anthropogenic greenhouse gas with approximately 25 times the radiative forcing of carbon dioxide (CO2) per molecule. CH4 also contributes to pollution in the lower atmosphere through chemical reactions leading to ozone production. Recent developments of LIDAR measurement technology for CH4 have been previously reported by Goddard Space Flight Center (GSFC). In this paper, we report on a novel, high-performance tunable semiconductor laser technology developed by Freedom Photonics for the 1650nm wavelength range operation, and for LIDAR detection of CH4. Devices described are monolithic, with simple control, and compatible with low-cost fabrication techniques. We present 3 different types of tunable lasers implemented for this application.
We report on an airborne demonstration of atmospheric methane (CH4) measurements with an Integrated Path Differential Absorption (IPDA) lidar using an optical parametric oscillator (OPO) and optical parametric amplifier (OPA) laser transmitter and a sensitive avalanche photo detector. The lidar measures the CH4 absorption at multiple, discrete wavelengths around 1650.9 nm. In September 2015, the instrument was deployed on NASA’s DC-8 airborne laboratory and measured atmospheric methane over a wide range of topography and weather conditions from altitudes of 3 km to 13 km. In this paper, we will review the results from our flights, and identify areas of improvement.
Er:YGG planar waveguide amplifiers (PWAs) are promising candidates to meet the needs of greenhouse-gas differentialabsorption LIDAR applications. We report pulsed–laser-deposition growth of this doped crystal and net-gain performance (internal gain ~2 dB/cm for 0.7-at.% Er-doping) in a 0.9-cm-long uncoated single-pass PWA. Rapid fabrication is also demonstrated with optimized parameters, where crystal growth rates approaching 20 microns/hour have been realized. We compare Er-doping concentrations ranging from 0.5 at.% - 4 at.%, and report on their spectroscopic properties. Furthermore, we show the ability to tailor the deposited crystal properties, controlling the waveguide and gain characteristics. Finally, we discuss the spectroscopy and potential performance of this relatively unstudied material for PWAs in the eye-safe regime.
Atmospheric methane (CH4) is the second most important anthropogenic greenhouse gas, with approximately 25 times the radiative forcing of carbon dioxide (CO2) per molecule. Yet, lack of understanding of the processes that control CH4 sources and sinks and its potential release from stored carbon reservoirs contributes significant uncertainty to our knowledge of the interaction between carbon cycle and climate change. At Goddard Space Flight Center (GSFC) we have been developing the technology needed to remotely measure CH4 from orbit. Our concept for a CH4 lidar is a nadir viewing instrument that uses the strong laser echoes from the Earth’s surface to measure CH4. The instrument uses a tunable, narrow-frequency light source and photon-sensitive detector to make continuous measurements from orbit, in sunlight and darkness, at all latitudes and can be relatively immune to errors introduced by scattering from clouds and aerosols. Our measurement technique uses Integrated Path Differential Absorption (IPDA), which measures the absorption of laser pulses by a trace gas when tuned to a wavelength coincident with an absorption line. We have already demonstrated ground-based and airborne CH4 detection using Optical Parametric Amplifiers (OPA) at 1651 nm using a laser with approximately 10 μJ/pulse at 5kHz with a narrow linewidth. Next, we will upgrade our OPO system to add several more wavelengths in preparation for our September 2015 airborne campaign, and expect that these upgrades will enable CH4 measurements with 1% precision (10-20 ppb).
We demonstrate the airborne measurement of atmospheric methane using a pulsed lidar at 1650 nm using an integrated path differential absorption scheme. Our seeded nanosecond-pulsed optical parametric amplifier (OPA)-based instrument works up to the highest altitudes flown (<10 km). The obtained absorption profile is in good agreement with theoretical predictions based on the HITRAN database.
We report on the development effort of a nanosecond-pulsed optical parametric amplifier (OPA) for remote trace gas measurements for Mars and Earth. The OPA output has ∼500 MHz linewidth and is widely tunable at both near-infrared and mid-infrared wavelengths, with an optical-optical conversion efficiency of up to ∼39% . Using this laser source, we demonstrated open-path measurements of CH 4 (3291 and 1652 nm), CO 2 (1573 nm), H 2 O (1652 nm), and CO (4764 nm) on the ground. The simplicity, tunability, and power scalability of the OPA make it a strong candidate for general planetary lidar instruments, which will offer important information on the origins of the planet's geology, atmosphere, and potential for biology.
We report on ground and airborne atmospheric methane measurements with a differential absorption lidar using an
optical parametric amplifier (OPA). Methane is a strong greenhouse gas on Earth and its accurate global mapping is
urgently needed to understand climate change. We are developing a nanosecond-pulsed OPA for remote measurements
of methane from an Earth-orbiting satellite. We have successfully demonstrated the detection of methane on the ground
and from an airplane at ~11-km altitude.
We present current and near-term uses of high-power fiber lasers and amplifiers for NASA science and spacecraft
applications. Fiber lasers and amplifiers offer numerous advantages for the deployment of instruments on exploration
and science remote sensing satellites. Ground-based and airborne systems provide an evolutionary path to space and a
means for calibration and verification of space-borne systems. NASA fiber-laser-based instruments include laser
sounders and lidars for measuring atmospheric carbon dioxide, oxygen, water vapor and methane and a pulsed or
pseudo-noise (PN) code laser ranging system in the near infrared (NIR) wavelength band. The associated fiber
transmitters include high-power erbium, ytterbium, and neodymium systems and a fiber laser pumped optical parametric
oscillator. We discuss recent experimental progress on these systems and instrument prototypes for ongoing
At NASA's Goddard Space Flight Center we are developing next generation laser transmitters for future spaceflight,
remote instruments including a micropulse altimeter for ice-sheet and sea ice monitoring, laser spectroscopic
measurements of atmospheric CO2 and an imaging lidar for high resolution mapping of the Earth's surface. These laser
transmitters also have applicability to potential missions to other solar-system bodies for trace gas measurements and
surface mapping. In this paper we review NASA spaceflight laser transmitters used to acquire measurements in orbit
around Mars, Mercury, Earth and the Moon. We then present an overview of our current spaceflight laser programs and
describe their intended uses for remote sensing science and exploration applications.
At NASA's Goddard Space Flight Center, we are developing the next generation laser transmitters for future remote
sensing applications including a micropulse altimeter for ice-sheet monitoring, laser spectroscopic measurements and
high resolution mapping of the Earth's surface as well as potential missions to other planets for trace gas measurement
and mapping. In this paper we will present an overview of the spaceborne laser programs and offer insights into future
spaceborne lasers for remote sensing applications.
We report on the atmospheric pressure measurements using a fiber-based laser system using the
oxygen A-band at 765 nm. Remote measurements of atmospheric temperature and pressure are
required for a number of scientific applications including greenhouse gas monitoring, weather
prediction, and climate modeling.
Trace gases in planetary atmospheres offer important clues as to the origins of the planet's hydrology, geology,
atmosphere, and potential for biology. We report on the development effort of a nanosecond-pulsed optical parametric
amplifier (OPA) for remote trace gas measurements for Mars and Earth. The OPA output light is single frequency with
high spectral purity and is widely tunable both at 1600 nm and 3300 nm with an optical-optical conversion efficiency of
~40%. We demonstrated open-path atmospheric measurements of CH4 (3291 nm and 1651 nm), CO2 (1573 nm), H2O
(1652 nm) with this laser source.
Many fundamental questions about planetary evolution require monitoring of the
planet's atmosphere with unprecedented accuracy at both high and low latitudes, over both
day and night and all seasons. Each planetary atmosphere presents its own unique challenges.
For the planets/moons that have relatively low surface pressure and low trace gas
concentrations, such as Mars or Europa, the challenge is to have enough sensitivity to
measure the trace gas of interest. For Earth, the challenge is to measure trace gases with very
high precision and accuracy in the presence of other interfering species.
An orbiting laser remote sensing instrument is capable of measuring trace gases on a global
scale with unprecedented accuracy, and higher spatial resolution that can be obtained by
passive instruments. For Mars, our proposed measurement uses Optical Parametric
Amplifiers (OPA) and Integrated Path Differential Absorption (IDPA) in the 3-4 um spectral
range to map various trace gas concentrations from orbit on a global scale. For earth, we
propose to use Erbium Doped Fiber Amplifier technology (EDFA) and IDPA at 1.57 and
OPA at 1.65 μm to measure carbon dioxide and methane concentrations respectively.
We report on a lidar approach to measure atmospheric CO2 column concentration being developed as a candidate for
NASA's ASCENDS mission. It uses a pulsed dual-wavelength lidar measurement based on the integrated path
differential absorption (IPDA) technique. We demonstrated the approach using the CO2 measurement from aircraft
in July and August 2009 over various locations. The results show clear CO2 line shape and absorption signals, which
follow the expected changes with aircraft altitude from 3 to 13 km. The column absorption measurements show
altitude dependence in good agreement with column number density estimates calculated from airborne in-situ
measurements. The approaches for O2 measurements and for scaling the technique to space are discussed.
NASA Goddard Space Flight Center (GSFC) has been engaging in Earth and planetary science remote sensing
instruments development for many years. The latest instrument was launched in 2008 to the moon providing the most
detailed topographic map of the lunar surface to-date. NASA GSFC is preparing for several future missions, which for
the first time will perform active spectroscopic measurements from space. In this paper we will review the past, present
and future of space-qualified lasers for remote sensing applications at GSFC.
We report on the development effort of a nanosecond-pulsed seeded optical parametric generator (OPG) for remote trace
gas measurements. The seeded OPG output light is single frequency with high spectral purity and is widely tunable both
at 1600nm and 3300nm with an optical-optical conversion efficiency of ~40%. We demonstrated simultaneous tuning
over the methane (CH4) absorption line at idler wavelength, 3270.4nm, and carbon dioxide (CO2) absorption line at
signal wavelength, 1578.2nm. In this paper, we will also discuss open-path atmospheric measurements with this newly
developed laser source.
Mounting concern regarding global warming and the increasing carbon dioxide (CO2) concentration has stimulated
interest in the feasibility of measuring CO2 mixing ratios from space. Precise satellite observations with adequate spatial
and temporal resolution would substantially increase our knowledge of the atmospheric CO2distribution and allow
improved modeling of the CO2 cycle. Current estimates indicate that a measurement precision of better than 1 part per
million (1 ppm) will be needed in order to improve estimates of carbon uptake by land and ocean reservoirs. A 1-ppm
CO2 measurement corresponds to approximately 1 in 380 or 0.26% long-term measurement precision. This requirement
imposes stringent long-term precision (stability) requirements on the instrument In this paper we discuss methods and
techniques to achieve the 1-ppm precision for a space-borne lidar.
The Lunar Orbiter Laser Altimeter (LOLA) instrument on NASA's Lunar Reconnaissance Orbiter (LRO) mission,
scheduled to launch in October 2008, will provide a precise global lunar topographic map using laser altimetry. LOLA
uses short pulses from a single laser through a Diffractive Optical Element (DOE) to produce a five-beam pattern that
illuminates the lunar surface. For each beam, LOLA measures the time of flight (range), pulse spreading (surface
roughness), and transmit/return energy (surface reflectance). LOLA will produce a high-resolution global topographic
model and global geodetic framework that enables precise targeting, safe landing, and surface mobility to carry out
exploratory activities. In addition, it will characterize the polar illumination environment, and image permanently
shadowed polar regions of the lunar surface to identify possible locations of surface ice crystals in shadowed polar
We describe an integrated detection system based on upconverting phosphor particles bound to capture sites on the inside surfaces of rectangular wick capillaries. This deice can be used with either antibody or nucleic acid to detect specific micro-organisms. The system uses a high- power, 980 nm, semiconductor diode laser to illuminate 200 X 300 X 20 micrometers capture surfaces. The rectangular capillary wicks are held in a tray that is inserted into the detection system, positioning the capture surface at the object plane of the optical system. Phosphorescent light emitted from the capture surface is collected by a high numerical aperture microscope objective and directed through a series of filters onto either a CCD camera or a photomultiplier. A combination of band-reject filters attenuates the 980 nm laser excitation light and its harmonic at 490 nm, and a tunable liquid crystal filter provides for rapid scanning from 400 to 750 nm. The data acquisition and control is controlled by a laptop PC with a custom GUI interface developed using LabWindows/CVI. The system can detect a single phosphor particle bound to a capture surface.
In recent years there has been renewed interest in using modulation spectroscopy for a variety of scientific and engineering applications. Modulation spectroscopy is often implemented with diode lasers which have the advantages of small size, non-intrusiveness, speed, and ease of use. Several theoretical frameworks have been developed to describe modulation spectroscopy and its limiting cases: frequency modulation spectroscopy (FMS), and wavelength modulation spectroscopy (WMS). In this paper we examine these frameworks and describe a general approach to modulation spectroscopy which takes into account the properties of semiconductor lasers.
We have fabricated single-frequency diode lasers from a number of III-V semiconducting compounds. These diode lasers were specifically designed for laser absorption spectroscopy. Their emission wavelengths span the internal of 0.76 to 2.7 micrometers . Water vapor, CO, CO2, NH3, CH4 HF, and O2 have been detected using them. After a brief review of their physical structure and principles of operation, we present representative output characteristics of these lasers, along with a discussion of several important applications.
Laser absorption spectroscopy using III-V semiconductor laser diodes has several advantages for gas sensing applications, as compared with traditional methods employing tunable dye laser and II-VI (e.g., lead salt) laser sources. These advantages include room-temperature operation, reduced cost, and compact size. Limited coverage of spectroscopy wavelengths by high-performance III-V lasers has prevented their widespread application to gas sensing. At those fixed wavelengths, performance of commercially available devices has been limited by multimode emission and/or inadequate wavelength tuning and mode hops. These spectra can, however, be greatly improved by incorporating frequency-selective structures. We have developed single-mode distributed-feedback (DFB) GaAs/AlGaAs quantum well lasers applicable to laser spectroscopy of molecules absorbing in the wavelength interval from 760 to 840 nm. These devices exhibit low threshold current (< 20 mA), high efficiency (> 40%), high output power (> 25 mW), and narrow linewidth (< 3.0 MHz). The lasers display smooth, continuous, single-mode wavelength tuning over 5 nm. Typical temperature and current wavelength-tuning coefficients are 0.065 nm/ degree(s)C and 0.0075 nm/mA (approximately -3.5 GHz/mA), respectively. In preliminary tests, they have been applied to the detection of H2O vapor and O2 gas.
Near-infrared semiconductor diode lasers are useful for spectroscopic monitoring of a number of chemical species at parts-per-million levels and below. Although these devices are typically used in point sensing geometries, they can also be used in several remote sensing configurations. In this paper we discuss some of the properties of these lasers that are relevant for point detection, and we discuss a simple scheme to measure temperature and pressure. In the context of remote sensing, we discuss several modulation methods that allow for range- resolved detection of chemicals and aerosols using continuous wave diode lasers.
Proc. SPIE. 2366, Optical Instrumentation for Gas Emissions Monitoring and Atmospheric Measurements
KEYWORDS: Signal to noise ratio, Optical filters, Digital signal processing, Spectroscopy, Interference (communication), Semiconductor lasers, Frequency modulation, Electronic filtering, Signal detection, Filtering (signal processing)
In recent years there has been renewed interest in using diode laser based sensors for environmental monitoring, industrial process control, and medical diagnostics applications. Diode lasers have the advantages of small size, non-intrusiveness, speed, ease of use, and high detection sensitivity. Several spectroscopic detection techniques can be employed with diode lasers, and digital signal processing algorithms can be used to enhance the detection sensitivity of a system. In our laboratory we used the following digital signal processing techniques to enhance the sensitivity and accuracy of near- and mid-infrared diode laser sensors: digital bandpass, Wiener, Kalman, and matched filtering, and a general least-squares fit. These digital signal processing algorithms have enhanced the signal-to-noise ratio of our sensors by an order of magnitude.
SRI International has designed and built several instruments that use tunable diode lasers and frequency modulation spectroscopy. These instruments have been used for flux measurements of trace gases, explosives detection, and environmental monitoring. A detection sensitivity of 2X10-6 with a stability of 0.1% over 10 hours has been demonstrated using a GaAlAs laser and an oxygen absorption line at 760.56 nm.
Near-infrared diode-laser-based systems using laser-absorption molecular spectroscopy can sensitively monitor atmospheric gases, pollutants, and toxic gases. They can also monitor trace gases on the human breath for medical diagnostics. The detection levels are equal to or less than parts per million. Sarnoff/SRI has made and tested room-temperature InGaAsP/InP DFB lasers operating at 1.39, 1.6, and 1.65 micrometers . All of these devices had output powers of 10 mW or more. The current-tuning rates varied from -580 to -1240 MHz/mA. The temperature tuning rate was about 0.1 nm/K for all devices. Continuous tuning ranges were 7 nm for the 1.39 micrometers lasers and 5 nm for the 1.6 and 1.65 micrometers lasers. We observed H2O at 1.39 micrometers , CO and CO2 at 1.6 micrometers , and CH4 at 1.65 micrometers . We monitored the ratio of 13CO2 to 12CO2 on human breath samples as the initial step towards clinical trials for medical diagnostics.
Frequency modulations spectroscopy (FMS) with infrared lasers is an attractive technique for a number of environmental chemical sensing problems. The technique combines high detection sensitivity with high detection speed and, when implemented with tunable infrared laser sources, is capable of detecting numerous chemical species in the atmosphere. To date, the technique has been demonstrated with semiconductor diode lasers and carbon dioxide lasers, and absorptions at the 10-7 level have been detected. We will review the principles and status of FMS for chemical sensing and discuss applications in atmospheric and environmental monitoring.