The development of compact mid-IR sources using frequency- converted diode lasers has been demonstrated to be applicable for the ultra sensitive, selective, and real time detection of many trace gas species in the infrared spectroscopic fingerprint region, which contains virtually all the fundamental vibrational modes of molecules. This development of infrared laser sources has taken advantage of recent significant technological advances of semiconductor diode lasers and solid state lasers, new nonlinear optical materials, optical fiber and novel data acquisition techniques. Such sensors are able to detect molecules at the parts-per-billion level in ambient air using infrared absorption spectroscopy either by monitoring trace gases in an open path or multi-pass cell configuration. Real world applications ranging from urban, industrial, rural emission studies to spacecraft habitat monitoring are described.

We have characterized the spectral noise density and frequency modulation performance of an 8.5 micron quantum- cascade diode (QC) laser operating continuously at liquid nitrogen temperatures. The phase noise is measured by fixing the laser frequency in the half-height region of a molecular resonance and measuring the fluctuations in absorbance; these fluctuations are then accurately converted into measurements in the fluctuations of the absolute frequency. A Fourier analysis of the intrinsic spectral/phase noise show a 1/f² dependence up to the measurement bandwidth limit of approximately 1 MHz. The laser linewidth is < 1 MHz when measured over several milliseconds. Servo locking schemes will be discussed with the implication that QC laser sources can be stabilized to a high degree. The frequency modulation performance of a QC laser has been measured and synchronous detection of f, 2f and 3f absorption signals (nitrous oxide at 8.5 micrometers ) has been achieved with direct modulation of the injection current.

Laser characteristics have been evaluated for mid-infrared quantum-cascade (QC) lasers operating in a continuous mode at cryogenic temperatures. These tests were performed to determine the suitability of QC lasers for use in various spectroscopic applications, including Doppler-limited molecular absorption spectroscopy and pressure-limited LIDAR instrumentation. Using rapid-scanning techniques, direct absorbance measurements of nitric oxide, ammonia and nitrous oxide have been performed with QC lasers, operating at either 5.2 or 8.5 micrometers . Measured Doppler-limited absorption profiles show no distortion with increased averaging (up to 10³ - 10⁴ samples averaged), thereby minimizing the need for sophisticated data acquisition systems which re- register successive data streams to accommodate for laser frequency jitter and drift. Additionally, the high tuning rates (2.5 cm^-1 in 0.6 milliseconds; 5 - 10 kHz sweep repetition rate) achieved with the QC lasers allow for the measure of relatively rapid transient phenomena or a high degree of signal averaging in a short time. Noise- equivalent absorbances of 3 X 10^-6 have also been obtained without optimizing the optical arrangement.

We report here on ultra-compact diode-pumped single frequency, TEM_oo mode Nd:YVO₄ lasers at 1064 nm and 1340 nm. The lasers incorporate coupled cavity design operating in single longitudinal mode with a line-width of less than 20 kHz and frequency drift of 50 MHz over 10 minutes. These lasers produce more than 500 mW output at 1064 nm and more than 200 mW output at 1340 nm wavelength and are pumped by a 1.2 W laser diode. In addition, the lasers also feature excellent output stability with a output power variation of less than 1% over 8 hours and less than 0.2% amplitude noise in the 10 Hz to 50 MHz frequency range. The dimensions of the laser head are 3 X 3 X 12 cm and the universal 100 to 240 VAC input power control unit has been dimensions of 5 X 14 X cm. The total power consumption of the laser system is only 12 W. Therefore these ultra-compact, single frequency lasers will have a great potential for analytical instrumentation, Raman spectroscopy, biomedical diagnostics, optical communication as well as space exploration.

A fast diode laser sensor has been applied for micrometeorological flux measurement of methane emissions from rice paddy fields in order to assess the quality of data on methane fluxes and allow a comparison with simultaneously recorded data provided by the closed chamber method. Systematic differences between chamber and the eddy correlation technique have been found as closed chamber measurements report about 70% higher emissions than eddy correlation measurements. This demonstrates that diode laser spectroscopy is a valuable tool for quality assurance in atmospheric research.

We report on our recent advances with cavity ring-down spectroscopy using mid-infrared cw lasers. An external high- finesse cavity is excited on a single fundamental mode with a tunable laser operating in the 3 micrometers region. After excitation the laser power is turned off for a short time and the subsequent decay of the field stored inside the cavity is observed. The effective pathlength covered by the laser light inside the cavity during the decay amounts to several km, depending on the mirror reflectivity. Measurement of the decay time gives the photon losses and thus enables the detection of weakly absorbing species inside the cavity. This approach is closely related to cavity ring-down spectroscopy with pulsed lasers. However the cw approach exhibits several advantages concerning spectral resolution and detection sensitivity. Application of this method to online monitoring of trace gases seems to be very promising. We demonstrate detection of hydrocarbons, like membrane and ethylene on the ppb level.

The development of a mid-infrared cavity ringdown spectrometer for trace gas measurements is described. The device employs a novel light source based on periodically poled lithium niobate (PPLN). Narrow linewidth (<EQ 0.08 cm^-1 FWHM) mid-infrared radiation (at energies up to 15 (mu) J) is generated by three serial elements: a broadband optical parametric generator, a tunable spectral filter, and an optical parametric amplifier. Currently, spectral filtering is accomplished by an air-spaced Fabry-Perot etalon that allows 15 cm^-1 of narrowband continuous tuning anywhere between 6200 - 6780 cm^-1 and 3200 - 2620 cm^-1. This can, in principle, be extended to the entire PPLN transparency window (2220 - 7690 cm^-1) using multiple PPLN crystals and a suitable tuning element. The high gain of PPLN allows pumping by compact, high-repetition-rate solid-state laser sources, thereby minimizing the sensor size and allowing rapid spectral scans. Operation is demonstrated using both a 1 kHz Nd:YAG and a novel 120 Hz passively Q-switched Nd:YAG microlaser. Performance of the cavity ringdown sensor is characterized in terms of sensitivity, spectral coverage (segmented scans up to 350 cm^-1 long), measurement speed, and measurements in the presence of atmospheric background gases. Issues relevant to the ultimate portable implementation of the sensor are addressed, including the use of two alternative frequency filtering/tuning mechanisms (a fiber-optic etalon and an acousto-optically tunable filter plus an air-speed etalon) and implementation of frequency calibration.

Accurate measurements of formaldehyde (CH₂O), a trace gas found throughout the atmosphere, are important for furthering our understanding of hydrocarbon oxidation processes in the atmosphere. During the 1997 North Atlantic Regional Experiment numerous trace gases, including CH₂O, were measured onboard a WP3 aircraft operated by the National Oceanic and Atmospheric Administration to study continental transport and photochemistry over remote regions of the North Atlantic Ocean. A highly sensitive tunable diode laser absorption spectrometer was employed in acquiring ambient CH₂O measurements on 10 different flights during this campaign. A second instrument, based on chemical derivatization of ambient CH₂O with DNPH, was also operated on the WP3 aircraft. This paper will briefly summarize the aircraft TDLAS system employed and discuss the level of agreement obtained between both instruments. This will be followed by a brief discussion of the results, and concludes with a preliminary comparison of the measurements with a 0-dimensional box model constrained by the measurements of other species during the campaign.

Rapid and accurate ambient measurements of the tropospheric trace gas formaldehyde (CH₂O) have been made by the NCAR low altitude tunable diode laser absorption spectrometer on both aircraft and ground based platforms. Field sensitivities of 80 - 120 pptv in 1 minute (40 - 60 pptv in 5 min) were typical of the first aircraft version of the instrument, providing good resolution for studying formaldehyde's role in the oxidative mechanisms of the troposphere. Recently the instrument has been modified to provide simultaneous detection of a second tropospherically interesting molecule, hydrogen peroxide (H₂O₂), as well as enhanced measurement precision and instrument stability. The optic assembly of the new Dual Channel Airborne Laser System (DCALS) has been designed to be more mechanically stable and better thermally conditioned. Other improvements include measures to mitigate optical noise, stabilize cell pressure, and minimize sample perturbation. Measurements of formaldehyde by DCALS at a ground site during the 1999 Southern Oxidants Study show improved sensitivities of 30 - 100 pptv in 1 minute, and much better long term instrument stability.

During the STREAM98 campaign conducted from Timmins (Canada) in July 1998, simultaneous airborne measurements of chemical tracers were performed using a Cessna Citation II twinjet as carrier platform. We present an intercomparison of N₂O measurements using an in-situ GC (University of Frankfurt/Main, Germany) and a tunable diode laser instrument (TRISTAR) operated by the Max Planck Institute for Chemistry, Mainz, Germany. The duty cycle of the GC is one second measurement per 120 s interval, whereas the sampling rate of the TDLAS is 1 Hz. Both instruments are in situ calibrated using working gas standards traced against primary standards provided by NOAA. The maximum deviation between the two independent measurements of N₂O is less than two years, which is of the order of the uncertainties of both instruments.

We have developed an instrument to fly on the NASA WB-57F aircraft which measures simultaneously two atmospheric tracers, CO and N₂O. These gases have lifetimes which make them appropriate dynamical tracers in the upper troposphere and lower stratosphere. To measure these species for studies of current interest we need a continuous record with a sensitivity of about 1 ppbv, and an absolute accuracy of about 1 percent. To trace the effect of small scale features requires a response time around 1 second. We describe the details of a tunable diode laser-based instrument designed for automatic operation aboard the WB- 57F aircraft at altitudes between the surface and 65,000 feet. A single diode laser, cooled by liquid nitrogen, is used to provide radiation at absorption lines of CO (2190.018 cm^-1) and N₂O (2190.350 cm^-1). Fully reflective optics directs the infrared beam through a long pass cell (36 m path length) of small volume (0.3 l) operated at relatively low pressure (40 mb). The instrument operates autonomously for up to 8 hours. Calibration is achieved by the introduction of standard, calibrated mixtures into the inlet sample flow.

Argus is a two channel, tunable diode laser instrument which measures atmospheric methane and nitrous oxide in the upper troposphere and stratosphere up to an altitude of 32 km using second harmonic detection. Investigations of stratospheric transport from mid-latitudes into the tropics is the current focus of our work which requires high precision and high accuracy data. Argus was designed for use on remotely piloted aircraft or light-payload balloons and weighs less than 20 kg. The two channels each have their own laser, optics, detector and signal processing chains. We sample methane at 3.3 micrometers and nitrous oxide at 4.5 micrometers at a rate of 0.1 Hz. The gas is sampled in a Herriott cell which has a total path length of 18.8 m. Each laser is current- and temperature-controlled by a dedicated microprocessor. We sweep the laser at 10 Hz and record direct absorption spectra during the ramping of the laser current. The lasers are modulated with a 40 kHz sine wave; a phase sensitive amplifier and integrator detects the second harmonic data. The analysis is performed offline by applying direct fits to the measured spectra using the non-linear Marquardt- Levenberg algorithm. We have recently flown Argus and ATLAS together on the ER-2 platform for comparison. Precision of Argus is 0.6% and accuracy is estimated to 4 - 17% increasing with altitude.

A new, near-infrared, diode laser based hygrometer, designed for operation in unmanned aerial vehicles, is presented. This instrument is designed to accurately and rapidly measure moisture from sea level to the lower stratosphere. It is compact, lightweight and requires very little power. All system control and data processing are accomplished using a stand-alone digital signal processor super- controller. The usefulness of this approach for atmospheric measurements as well as other industrial applications will be presented.

Small, robust, powerful and easy to operate instruments based on atomic absorption spectrometry with diode lasers has been developed. The instruments allow measurements of trace element concentrations as low as about 1 pg/g. This was demonstrated by the measurement of trace elements on silicon wafers and ultra pure chemical reagents in semiconductor industry, the detection of chlorine in polymers, and element selective detection in GC and HPLC.

The work on the development of compact diode laser-based lidar systems at SRI International is reviewed. Two systems, a pseudorandom modulation lidar, and a mobile remote sensor for natural gas pipeline leak detection are described in detail, and experimental results are presented. Methods to enhance signal detection by digital filtering are also reviewed.

A dual tunable diode laser absorption spectrometer (TDLAS) for continuous field measurement of nitric acid and nitrogen dioxide eddy covariance fluxes is described and preliminary field results are presented. The dual TDLAS simultaneously measures nitric acid (HNO₃) and nitrogen dioxide (NO₂) by direct absorption spectroscopy over a long path enclosed in an astigmatic Herriott multipass cell. The technique provides sufficient precision and time response (200 ppt RMS in 1 second) needed to record ambient variations and deposition rates by the eddy-covariance method. Real-time fitting of the integrated spectra over multiple absorption features makes the system appropriate for continuous field measurements while retaining the highly selective quality of direct absorption measurements and minimizing potential interferences. This method also produces an absolute, spectroscopic determination of concentration within the multipass cell, eliminating the need for calibrated gas mixtures in the field.

A novel Tunable Diode Laser Absorption Spectrometer has been developed for trace gas flux measurements based on micrometeorological techniques. Up to 2 different species can be measured simultaneously with high temporal resolution (< 1 sec) using individual lead-salt diode lasers. The instruments response time is ultimately determined by the gas exchange time through the compact multi-reflection cell (Aerodyne Model AMAC-36 Astigmatic Herriott Cell, 0.3 l volume, total path 36 m). The lasers are operated in a time multiplexed mode using a novel modulation scheme, which combines laser operation in a pulsed-current mode with a combination of rapid scanning and two-tone frequency modulation. The latter has the potential to improve the signal-to-noise ratio of phase-sensitive detection when compared to standard lock-in techniques because of the reduction of instrument noise at higher detection frequencies. The stability and the detection limit of the instrument will be characterized. It has been used to measure CH₄ and N₂O fluxes via the eddy covariance technique from rice paddies and tropical ecosystems during two recent field campaigns.

We report on a laser active imager suitable for the visualization of natural gas leaks and volatile organic compounds emitted by oil refineries. The described backscatter-absorption gas-imaging (BAGI) system employs a raster scanner in conjunction with a tunable continuous wave (cw) laser source. The imager creates real-time video imagery of a scene, while illuminating it with infrared laser light at a wavelength that is absorbed by the gas to be detected. Thus, gas plumes that otherwise cannot be seen by the human eye appear in BAGI images as dark clouds. In order to produce the high intensity infrared light that is needed to image natural gas and refinery by-products, we used a nonlinear frequency-conversion technique that employs the quasi-phase-matched crystal periodically poled LiNbO₃. The crystal serves as the active medium in a cw optical parametric oscillator (OPO) that is pumped by a diode-pumped Nd:YAG laser. The output frequencies were selected to coincide with absorption features of general aliphatic species (2935 and 2968 cm^-1), aromatics, such as benzene and toluene (3033 cm^-1), and methane (3018 cm^-1). The crystal was engineered to cover the desired spectral range using a fan-out design. This allows tuning of the OPO between 2832 and 3145 cm^-1 in idler wavelength by simply translating the crystal at a fixed temperature. Presented data demonstrate the performance of this system for imaging species of interest at relevant concentrations and ranges up to about 30 m.

On-road remote sensors can measure the emissions of motor vehicles under real-world conditions. The most sensitive and versatile remote sensor reported to date is based on Tunable Infrared Laser Differential Absorption Spectroscopy (TILDAS). This study applied this TILDAS remote sensor to the measurement of the emissions from heavy-duty diesel trucks (HDDTs). The remote sensor could operate with an optical pathlength of 88 m, or more than 5 times that of competing instruments. Remote sensing of NO₂ emissions was demonstrated for the first time. Good agreement was obtained when comparing the TILDAS measurements with the on- board measurements of an instrumented HDDT. The distribution of NO emissions from HDDT was found not to be skewed. HDDTs are estimated to contribute about 3/5 of the on-road NO_x emission inventory. These emissions are underestimated by a factor of 2.2 in the latest EPA inventory.

Remote sensing measurements of CO to CO₂ ratios, and hence the CO emissions from on-road vehicles are being made in Houston and Dallas metropolitan areas in the state of Texas. A near infrared tunable diode laser absorption spectrometer system integrated with License Plate Recognition, speed and acceleration measurement systems, as been used to monitor over 345,000 vehicles in the first six months at the end of 1998 and beginning of 1999. About 1.5% of the vehicles were found to be gross emitters (emitting more than 6% CO) and about 12% of the vehicles were determined to be high emitters (emitting between 2% and 6% CO). Some 73% of the vehicles emitted less than 1% CO. Five percent of the vehicles generated approximately 26% of the CO emissions. The next fifteen percent of the vehicles generated another 34% of the CO emissions. The fraction of high emitters had a small linear increase with the age of the vehicle up to 1990 model year and a relatively rapid increase for older vehicles.

In this paper we discuss the development of a tunable diode laser absorption spectroscopy probe to simultaneously measure, in situ concentrations of 4 major species (CO, C₂H₂, CH₄, and H₂O) and the gas phase temperature in a pool fire. The difficulty in making these types of measurements is intrinsic to the environment itself. A large fire is composed of very hot (> 1000 K), turbulent gases and highly radiating and absorbing soot particles. Fiber optic cables are used to transport laser radiation into the fire via a water-cooled probe. This paper focuses on probe design issues, such as the optimization of open path, multipass optics for a turbulent, particulate- laden flow, and the application of high-frequency wavelength modulation spectroscopy to frequency-domain multiplexing of diode lasers.

A dual infrared tunable diode laser system (IR-TDL) has been developed for the simultaneous detection of multiple gaseous components in cigarette smoke. The high spectral resolution (0.001 cm^-1) and rapid time response (20 Hz) of the TDL system are ideal for separating the absorptions from the multitude of gas phase components found in this matrix. The combustion products are sampled into a 0.3 liter, 18 meter multiple pass absorption cell with a flow response time of 0.15 seconds, which provides ample time resolution to observe variations within each 2-second puff. Two independent beam paths allow simultaneous detection in two wavelength regions; the first for ethylene and ammonia and the second for formaldehyde. Rapid scan-sweep integration with direct absorption permits absolute gas concentrations to be determined on-line. A nonlinear least squares procedure is used for `fingerprint' fitting of up to four gases with each diode. Results demonstrating the instrument sensitivity and time response, along with potential caveats, for several gaseous components will be presented.

We will show the results from a Tunable Diode Laser (TDL) spectrometer installation monitoring the O₂ concentration and the temperature in an olivine pellet production plant. The spectrometer has been operating continuously for more than two years. In the pelletizing process a reduction of magnetite and sintering takes place at a temperature around 1250 degree(s)C. To achieve a high and predictable quality of the produced pellets the oxygen concentration and the temperature has to be measured in-situ inside the process furnace. A specially designed high temperature sensor was mounted on the furnace wall and an optical fiber was used to carry the probing light from the TDL spectrometer to the measurement point. The TDL spectrometer operates at two absorption lines in the near infrared wavelength region to measure the oxygen concentration and the temperature simultaneously. The temperature is measured using the relative intensity of the two absorption lines and the concentration is calculated from the temperature compensated absorbance. The accuracy of the concentration and temperature measurements at 1 s response time was 0.1 vol.% and 50 degree(s)C, respectively. In order to validate the TDL measurements the pelletizing process furnace temperature was varied between 100 degree(s)C up to 1300 degree(s)C while the oxygen and temperature readings from the TDL spectrometer was recorded. The temperature measurements were also correlated with temperature measurements using thermocouples inside the furnace. The O₂ absorption line parameters were determined in a controlled laboratory experiment using a heated measurement path. This work shows that it is possible to build and field a TDL spectrometer to measure O₂ and temperature in-situ in a steel making process furnace.

Two new double resonance signals were observed in H₂CS using the 10R4 CO₂ laser line. Both radio-frequency transitions have been assigned. Their behavior in a magnetic field up to 10.5 kG has been studied. One signal was pumped by the 9P20 laser line and the other two by the 10R4 laser line. A 10R4 signal observed at 15.8 MHz was assigned to the 11_3.8 yields 11_3.9 transition in the v₆ ground state of H₂CS. The other 10R4 signal, which was observed at 202.8 MHz, was assigned to the 11_2.9 yields 11_2.10 transition in the excited state. This transition is the first to be reported in an excited vibrational state of the electronic ground state. The frequency of this transition was found to be strongly influenced by Coriolis coupling between the v₄ and v₆ states.

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A mid-IR tunable diode laser absorption spectrometer (TDLAS) equipped with a multiple-pass gas cell was used to measure breath samples from a number of student volunteers at the University of Oklahoma. Test subjects included one to two pack-a-day cigarette smokers and non-smokers. The concentrations of four different molecules, N₂O, ¹²CO₂, ¹³CO₂ and CO, were measured by each laser scan in the 2206.1 cm^-1 to 2207 cm^-1 spectral range. The average concentration of nitrous oxide (N₂O) increased slightly for smokers versus non-smokers and was generally higher (12%) than the approximately 255 ppm concentration measured in ambient air. Carbon monoxide concentrations, however, were much higher in breath samples from cigarette smokers. Ambient concentrations of carbon monoxide, approximately 0.4 ppm, increased from approximately 1.0 ppm in non-smokers to levels over 13.4 ppm in smokers. These measurements provide clear evidence of the well-known effect that cigarette smoking has on replacing oxygen with carbon monoxide in human hemoglobin. Carbon dioxide concentrations of smokers were generally decreased by approximately 12%. Mid-IR laser measurements also provided ¹³CO₂/¹²CO₂ isotope ratio values, and smokers had a approximately 30% greater concentration of isotopic ¹³C in their breath. The possible mechanisms for ¹³CO₂ isotopic increases are at present unknown. Overall, long-path TDL spectroscopy of exhalation products is a uniquely powerful tool. The TDL systems can be used for noninvasive diagnosis of a wide range of metabolisms and pathologies.

This paper will give a brief overview of the technology available at Laser Components and describe recent improvements in reliability and quality control. Results of recently demonstrated pulsed operation of lead salt lasers at and above room temperature will be presented. More traditional cryogenic laser operation promises to be straightforward in the future thanks to the recent development of a compact tunable IR laser source based on a lead salt laser combined with an innovative miniature Stirling cooler. This device will operate unattended and without any routine maintenance for prolonged periods and is suitable for a wide variety of laboratory and industrial applications.