Continuous wave operation of quantum cascade lasers is reported up to a temperature of 311 K. Fabry-Perot and DFB devices were fabricated as buried heterostructure lasers with high-reflection facet coatings. Junction-down mounted FP-lasers emitted up to 17 mW and 3 mW of optical power per facet under continuous wave operation at 292 K and 311 K, respectively. The DFB-devices could be operated up to 253 K on a thermoelectric cooler at an emission frequency of 1114 cm^-1 with a side mode suppression rate better than 30 dB. Grating-coupled external cavity quantum cascade lasers based on a bound-to-continuum active region featuring a broad gain spectrum demonstrated frequency tuning of ~10% from 1036 cm^-1 to 1142 cm^-1 with average output power between 0.15 and 0.85 mW under pulsed operation at room temperature.

There is an increasing need in many chemical sensing applications ranging from industrial process control to environmental science and medical diagnostics for fast, sensitive, and selective gas detection based on laser spectroscopy. The recent availability of novel pulsed and cw quantum cascade distributed feedback (QC-DFB) lasers as mid-infrared spectroscopic sources address this need. A number of spectroscopic techniques have been demonstrated. For example, the authors have employed QC-DFB lasers for the monitoring and quantification of several trace gases and isotopic species in ambient air at ppmv and ppbv levels by means of direct absorption, wavelength modulation, cavity enhanced and cavity ringdown spectroscopy. In this work, pulsed thermoelectrically cooled QC-DFB lasers operating at ~15.6 μm were characterized for spectroscopic gas sensing applications. A new method for wavelength scanning based on the repetition rate modulation was developed. A non-wavelength-selective pyroelectric detector was incorporated in the gas sensor giving an advantage of room-temperature operation and low cost. Absorption lines of CO₂ and H₂O were observed in ambient air providing information about the concentration of these species.

We show that by using a high resolution Fourier transform infrared spectrometer we can map the temporal characteristics of a pulsed quantum cascade laser into the wavenumber domain, and hence show that when a square current pulse is applied to a distributed feedback laser a linear sub-microsecond frequency chirp is developed. We describe a mid infrared spectrometer, that is based upon the use of this linear chirp, which can provide a real-time display of the spectral fingerprint of molecular gases. The sensitivity of the spectrometer is based upon the use of long pathlength White or Herriot cells, and the multiplex advantage associated with recording the entire spectral window during each electrical pulse. For a cell with a path length of 9.6 m, dilution measurements made on the ν₉ band transistions of 1,1 difluoroethylene indicate a sensitivity of 30 parts per billion.

We report the application of quantum cascade (QC) lasers to measurement of atmospheric trace gases in both closed path and open path configurations. The QC laser, a recently available commercial device, is Peltier cooled and pulsed, with emission near 965 cm^-1. We use direct absorption with a rapid sweep integration and spectral fits to derive absolute concentrations from tabulated line parameters without calibration. In the closed path configuration, with absorption in a long pathlength multipass cell (210 m, 50 Torr), we examined laser line widths and sensitivity limits. We measured ammonia with a precision of 0.05 nmole/mole (0.05 ppbv) RMS at 1 Hz, limited by detector noise. The laser linewith was 0.007 cm^-1 HWHM, based on measurements of ethylene absorption line shapes with a current pulse width of ~14 ns. In the open path configuration, we measured ammonia in the exhaust of automobiles driving through a probe beam. Atmospheric pressure line broadening and turbulence limit the sensitivity, giving a column density noise level at 20 Hz of 1.4 ppm-m. We observed ammonia column densities up to 40 ppm-m in the exhaust plumes. In future systems we will include a CO₂ channel, allowing normalization to fuel use rate.

Laser-optical sensors are applied whenever sensitive, selective and fast in-situ gas analysis is required. This contribution illustrates the performance of selected near- and mid-infrared spectrometers based on tunable diode-lasers. Applications of lead-salt diode-lasers for NO₂ and CH₄ sensing, of antimonide lasers for CH₄ and HCHO sensing in the 3-4 μm range and a near-infrared gas sensor for CO₂ based on a room temperature 2 μm Indium-Phosphide laser will be presented and spectrometer performance will be discussed.

Inter-subband (Type I) quantum-cascade (QC) lasers have shown the potential to generate tunable mid-IR radiation with narrow intrinsic linewidths (< 160 KHz in 15 mSec sweeps) and excellent amplitude stability (< 3 ppm averaged over minutes). Our bench-scale efforts to develop the Type I distributed feedback (DFB)-QC lasers for fieldable atmospheric chemistry campaigns, where multipass (Herriot or White) cells are used to enhance path-length, have not yet realized performance to the low intrinsic noise levels seen in these devices. By comparison, many operational systems' levels of noise-equivalent-absorbance (NEA) using Pb-salt lasers can routinely achieve at least one-order of magnitude better cw-performance, and with much lower powers. We have found that instability effets from weak back-scattered laser light -primarily from the Herriot cell- results in feedback-implicated technical noise well above the thermal and shot-noise of standard IR detectors. Of more fundamental concern is the fact that planar-stripe DFB-QC lasers undergo beam steering and transverse spatial-mode competitions during current tuning. It is the development of fully automated sub-ppbV sensitive IR chem-sensors. It is possible to reach low-ppm levels of absorptance change-detection (ΔI/I₀) over small wavelength regions with careful alignment to 100 M Herriott cells, but extreme care in spatial filtering is critical. However in the case of optical configurations which preclude significant optical feedback and need for stringent mode coupling alignments, the cw-DFB-QC lasers show great promise to do high resolution sub-Doppler spectroscopy. By serendipitous events, a varient of 'mode- or level-crossing' spectroscopy was probably rediscovered, which may allow very high resolution, sub-Doppler features and/or hyperfine alignments to be probed with 'uni-directional' topologies. We will primarily discuss the basic features of the 'uni-directional' sub-Doppler spectroscopy concept in this report. It shows potential to be exploitable in multi-pass cells or ring configurations. The phenomena of satuation 'dips' in molecular transitions appear to be very accessible with sinusoidally current-modulated DFB-QC lasers. Observations of sub-Doppler structures, either induced by residual AM 'pulsation dips' and/or hyperfine level-crossing effects (due to weak Zeeman splittings by the earth's B-field) can be recovered with good contrast. If this phenomena is indeed implicated with long-lived coherent hyperfine alignments, due perhaps to coherent population trapping in 'dark-states,' then sub-Doppler signals from saturated 'level-crossings' can potentially be seen without recourse to expensive polarization optics, nor elaborate beam shaping and isolation techniques.

We describe research leading to a trace gas detection system based on optical absorption using near-IR diode lasers that is intended to provide early warning of incipient fires. Applications include "high loss" structures such as office buildings, hospitals, hotels and shopping malls as well as airplanes and manned spacecraft where convention smoke detectors generate unacceptably high false alarm rates. Simultaneous or near-simultaneous detection of several gases (typically carbon dioxide, carbon monoxide, acetylene and hydrogen cyanide) provides high sensitivity while reducing the chance of false alarms. Continuous measurement of carbon dioxide concentrations also provides an internal check of instrument performance because ambient levels will not drop below ~350 ppm.

A versatile gas sensor for use in gravitational studies and/or long-term monitoring of biological systems in space is described. The sensor combines two diode lasers in a compact, low power package for quantitative, simultaneous measurements of oxygen, carbon dioxide and potentially, water vapor. Wavelength modulation spectroscopy(WMS) combined with digital signal processor (DSP) control allows this system to meet the stringent weight and power restrictions of a space-borne sensor. The sensor, configured for measurements in plant growth chambers, exhibits a very high level of precision for long-term measurements, with an uncertainty in CO₂ concentrations of better than 3 parts per thousand (ppth), and about 5 ppth for ambient oxygen, all at 1 Hz. Allan variance measurements indicate that increasing the averaging time to 100 sec will improve the precision to 0.3 ppth. The dynamic range for CO₂ detection exceeds four orders of magnitude.

Diode laser absorption offers the possibility of high-speed, robust, and rugged sensors for a wide variety of practical applications. Pressure broadening complicates absorption measurements of gas temperature and species concentrations in high-pressure, high-temperature practical environments. More agile wavelength scanning can enable measurements of temperature and species concentrations in flames and engines as demonstrated by example measurements using wavelength scanning of a single DFB in laboratory flames or a vertical cavity surface emitting laser (VCSEL) in a pulse detonation engine environment. Although the blending of multiple transitions by pressure broadening complicates the atmospheric pressure spectrum of C₂H₄ fuel, a scanned wavelength strategy enables quantitative measurement of fuel/oxidizer stoichiometry. Wavelength-agile scanning techniques enable high-speed measurements in these harsh environments.

In this paper the recent developments in the field of diode-laser photoacoustics will be discussed. Photoacoustic detector designs with high sensitivity and signal-to-noise ratio will be presented. The advantages and disadvantages of different modulation and measurement techniques will be discussed and compared. Simple expressions will be given for an estimation of the sensitivity achievable with an optimized photoacoustic detector. Recent results will be presented for a state-of-the-art dual-resonator differential cell suitable for sensitive trace gas analysis with low electronic and acoustic noise. With this resonant photoacoustic cell and a near-infrared DFB diode laser with 42 mW power at 1.53 mm polar ammonia molecules could be detected with a sensitivity of 200 parts-per-billion volume (ppbv) under flow conditions used to reduce the adsorption problem. By excitation of fundamental vibrations in methane using a pulsed optical parametric oscillator (OPO) with 60 mW output power a sensitivity of 1.2 ppbv has been achieved using this resonant cell. This setup allows sub-ppbv detection of the greenhouse gas methane with a concentration of about 1.7 ppmv in ambient air.

A novel trace-gas sensor system has been developed based on resonant photoacoustics, wavelength modulation spectroscopy, near-infrared diode lasers and optical fiber amplifiers that can achieve parts-per-billion sensitivity with a ten centimeter long sample cell and standard commercially-available optical components. An optical fiber amplifier with 500 mW output power is used to increase the photoacoustic signal by a factor of 25, and wavelength modulation spectroscopy is used to minimize the interfering background signal from window absorption in the sample cell, thereby improving the overall detection limit. This sensor is demonstrated with a diode laser operating near 1532 nm for detection of ammonia that achieves an ultimate sensitivity of less than 6 parts-per-billion. The minimum detectable fractional optical density, α_minl, is 1.8x10^-8, the minimum detectable absorption coefficient, α_min, is 9.5x10^-10 cm^-1, and the minimum detectable absorption coefficient normalized by power and bandwidth is 1.5x10^-9 Wcm^-1/&sqrt; Hz. These measurements represent the first use of fiber amplifiers to enhance photoacoustic spectroscopy, and this technique is applicable to all other species that fall within the gain curves of optical fiber amplifiers.

Significant progress has been made in the performance, qualification and validation of Active Remote Sensing systems to address complex questions in climate science from satellites in low earth orbit. During the past year, ITT has completed the design, qualified the components, and validated the performance of sophisticated Tunable Diode Laser Absorption Spectroscopy systems for airborne and space missions. ITT has shown that measurement of total column CO₂ to an accuracy of 0.5% can be readily achieved using a 5 watt laser, 1 meter telescope and digital signal processing techniques to reject sunlight and noise. Furthermore, the design exploits the proven high reliability of photonic components developed by the telecom industry. ITT testing validated that these components survive launch and multi-year operation in space without significant degradation. Using a scaled sensor, the ground based validation campaign demonstrated the ability to accurately retrieve the CO₂ diurnal cycle as well the automotive induced variations in CO₂ observed in urban settings. These data validate the end-end sensor performance model and retrieval algorithms, which have previously been used to design a space based CO₂ sensor proposed to NASA. ITT will discuss the application of these technologies to other atmospheric constituents. Combined, these results serve to demonstrate that laser based remote sensing of key components of the atmosphere which address global climate change can be achieved from low earth orbit without further development.

Absorption measurements of HCl during plasma etching of poly-silicon are made using the P(4) transition in the first vibrational overtone band near 1.79 μm. Single path absorption provides a real-time HCl monitor during etching of six-inch wafers in a commercial Lam Research 9400SE reactor at the University of Michigan. Wavelength modulation at 10.7 MHz is used to distinguish the absorption signal from the strong plasma emission. The laser center frequency is ramp-tuned at 500 Hz providing an HCl measurement every 2ms. Direct absorption measurements without the plasma are used to calibrate the wavelength modulation signal. The minimum detectable absorbance was 5x(10)^-6 with 50 ms averaging, leading to an HCl detection limit of ~(10)¹²cm^-3. For a given ratio of the feedstock HBr/Cl₂, the measured HCl concentration tracks the average etch rate. These measurements demonstrate the feasibility of a real-time diode laser-based etch rate sensor.

A small size tunable diode laser (TDL) spectrometer with dimensions of only 280 x 190 x 130 mm (11 x 7.5 x 5 inch) was built for automated field use. The water-protected package also contains batteries and a data storage device for an independent operation of 15 hours. Mass of the complete instrument is 5 kg. Smallest absorptions recorded in fully auto-mated operation are in the 10^-5 range. Data is stored as direct absorptions and as derivative as well. The instrument can be equipped with near infrared (NIR) laser diodes depending on target species (e.g. water vapor or methane). A high dyna-mic range is achieved by periodically tuning the laser over various molecular absorption lines of different line strength using temperature scan. The absorption path is set up using a retroreflector, the pathlength can be between 0.5 meters and 100 meters depending on requirements. The instrument is targeted at reliable and sensitive field measurements in fully automated and unattended mode, including e.g. aircraft and environmental measurements.

Open path tunable diode laser gas analyzers are constrained by the interference absorption line features in the modulation wavenumber window or line overlap from peripheral absorption lines, which reduce accuracy in the estimates of object gas concentration and in the locking of the laser and absorption line center wavenumbers. To achieve greater potential resolution and smaller physical instrument size in gas analyzers for atmospheric sampling, laser wavelengths must increase to the 1.5-5 um mid-IR band where unfortunately density and line strength of interference absorption lines also increase. A systematic approach including digital methods is described to select the most suitable Hitran object gas lines in the presence of interference gases H₂O, CO₂, CH₄, and N₂O and record the set of interference lines about the object gas line. A method is described to decompose the sample absorbance spectrum as absorption feature amplitude-position data to compare with the recorded interference line data to identiy the object line and laser center wavenumber shift. The processes of automatic object gas line identity, interference absorbance spectrum synthesis and subtraction from the sample absorbance and of line-lock and concentration analysis from the resultant spectrum are detailed. Examples are presented to illustrate the expected accuracy and error bounds for a dual gas CO₂/H₂O analyzer.

A time-of-flight anemometer is decribed which is composed of multiple IR absorption paths between fiber connected lenses arranged in symmetric configurations which provide spatial and temporal separation of the object gas, H₂O, processes intersecting the tunable diode laser beams. The time series H₂O concentration data from two gas analyzer paths are continuously correlated in real time to provide estimates of time shift between the processes. From beam geometry and time shifts, wind velocity vectors are calculated for a two-dimensional equilateral triangle prism structure and a three-dimensional co-axial structure. The real time convolution process is described and expected resolution and accuracy in the near surface layer are included. The resolution of the wind vectors and scalar variables makes this gas analyzer-anemometer suitable for eddy correlation flux measurements.

A novel instrument that employs a high-finesse optical cavity as an absorption cell has been developed for sensitive measurements of gas mixing ratios using near-infrared diode lasers and absorption spectroscopy techniques. The instrument employs an off-axis trajectory of the laser beam through the cell to yield an effective optical path length of several kilometers without significant unwanted effects due to cavity resonances. As a result, a minimum detectable absorption of ~1.4x10^-5 over an effective optical path of 4,200 meters was obtained in a 1.1-Hz detection bandwidth to yield a detection sensitivity of ~3.1x10^-11 cm^-1 Hz^-1/2. The instrument has been applied for measurements of CO, CH₄, C₂H₂, and NH₃ in the 1530-1650 nm range, and stable isotopes of CO₂ (¹³CO₂, ¹²CO₂, ¹²C¹⁶O¹⁸O) near 2052 nm.

Formaldehyde (CH₂O) is a ubiquitous component of both the remote atmosphere as well as the polluted urban atmosphere. This important gas-phase intermediate is a primary emission product from hydrocarbon combustion sources as well as from oxidation of natural hydrocarbons emitted by plants and trees. Through its subsequent decomposition, formaldehyde is a source of reactive hydrogen radicals, which control the oxidation capacity of the atmosphere. Because ambient CH₂O concentrations attain levels as high as several tens of parts-per-billion (ppbv) in urban areas to levels as low as tens of parts-per-trillion (pptv) in the remote background atmosphere, ambient measurements become quite challenging, particularly on airborne platforms. The present paper discusses an airborne tunable diode laser absorption spectrometer, which has been developed and refined over the past 6 years, for such demanding measurements. The results from a recent study will be presented.

Laser based gas detection and monitoring techniques have now evolved to a mature level. Critical laser performance parameters include spatial beam quality, usable IR power, linear frequency tunability and stability. For continuous-wave, long-path absorption spectroscopy, the development of robust mid-infrared spectroscopic sources has led to numerous selective, sensitive and real-time gas monitoring applications. These new compact and tunable spectroscopic sources (<0.5 cubic feet) can be designed for efficient room-temperature operation in the 2.4 - 4.6 microns wavelength region using standard near-IR telecom lasers that are optically mixed in nonlinear optical materials such as periodically poled LiNbO3 (PPLN). Wavelength multiplexing and flexible dispersion control of PPLN crystals offer convenient narrow-linewidth (100 kHz - 2 MHz), single or multiple-frequency mid-IR operation at the milli-watt level. This permits the sensitive detection of many molecules such as HF, HCl, CH₂O, CH₄, CO₂, CO and N₂O at their strong fundamental rotational-vibrational transitions using direct, dual-beam, 2-f and other advanced spectroscopic detection schemes. At this wavelength region, these new laser sources provide an ideal alternative to cryogenically cooled lead-salt diode lasers. This paper will focus on the comparison of the two technologies with an emphasis on achieving ultra-high sensitivity in ground and airborne applications.

We demonstrate a new tunable mid-IR laser source based on the guided-wave frequency conversion of two diode lasers operating in the near-infrared. Important features of this laser source include portability, room-temperature operation, freedom from thermal cycling, smooth tunability, and modular construction using readily available components. The source, when fully optimized, will maintain the properties which have made lead-salt lasers so useful for atmospheric trace gas detection, including sub-Doppler linewidths, microwatt-level output powers, and amenability to detection techniques based on frequency modulation. We describe the design and construction of the laser source, including its key component: a waveguide fabricated in periodically poled lithium niobate. In addition, we present a laboratory absorption spectrum which illustrates the potential usefulness of this laser source for the detection of atmospheric methane. Difficulties encountered when making the transition from a laboratory tabletop device to a portable device are discussed.

An instrument has been developed for the measurement of water vapor in the troposphere and lower stratosphere. This instrument, dubbed the NASA Langley/Ames Diode Laser Hygrometer (DLH), has been flown on 10 missions aboard NASA's DC-8 aircraft. The DLH utilizes an open-path, double-pass configuration, where the path is defined on one end by a laser transceiver mounted on the interior of a modified window panel, and on the other by a panel of retroreflecting material mounted on the DC-8's outboard engine nacelle. The DLH operates on one of two spectral absorption lines in the 1.4 μm spectral region, in a wavelength-modulated (WM) mode, with the laser locked to the center of the absorption line encountered in a reference cell. The spectral line used is determined by the local conditions - a weak line is used at low altitudes and a stronger one at high altitudes - and is changed at various times during a flight by the operator. Signal detection is accomplished by demodulating the return signal at twice the driving frequency (2F detection). The returned laser power (DC) is also measured. The DLH is calibrated in the laboratory at various combinations of pressure and water vapor density. From the calibration data and a multiparameter spectral model, a set of coefficients is developed, and these coefficients are used to convert the measured 2F/DC ratio, along with local temperature and pressure (which are measured by separate instruments aboard the aircraft), to water vapor mixing ratio.

We report the application of diode laser spectroscopy to the non-intrusive measurement of water vapor and helium content within the enclosures of the Charters of Freedom that were sealed by NBS approximately 50 years ago. The instrument was adapted from the Diode Laser Hygrometer, developed through the NASA Global Tropospheric Experiment program to make open-path water vapor measurements outside the NASA DC-8 research aircraft. Operating in the 1.4-micron wavelength region, laser wavelength scans of water vapor absorption lines enabled the determination of the moisture level and the amount of helium relative to air within the Charters enclousres (an indicator of enclosure leaks). These measurements revealed that at least five of the seven Charters of Freedom enclosures had not leaked since they were sealed approximately 50 years ago. The measurements also revealed that water vapor within these enclosures ranged from 29 to 55% greater than anticipated.

Hollow-glass waveguides may be a viable technology that, in some cases, may supplant heavier multi-pass cells such as White or Herriott cells for performing trace detection using tunable diode laser absorption spectroscopy. We report here a series of experiments for testing the suitability of waveguides for infrared spectroscopy. The loss characteristics of 1 mm bore diameter waveguides have been measured for straight and coiled lengths. The minimum linear loss coefficient is 0.46 (+/- 0.10) dB/m while the bending loss coefficient is 0.42 dB. Using a flow of 1 ppm nitric oxide in nitrogen mixture through a coiled 3 meter length of waveguide (coil diameter 50 cm) we could detect the fundamental R(8.5), Ω = 3/2 transition of NO with a signal-to-noise (RMS) ratio of 44:1 in direct absorption using a single mode, lead-salt diode laser with six minutes of signal averaging. Using direct absorption spectroscopy we have found that the absorption pathlength is approximately equal to the physical length of the waveguide. Broadband FM diode laser spectroscopy produces for the same transition and waveguide sample conditions a comparable signal-to-noise ratio with less than a second of signal averaging. Finally, we have also performed near-infrared spectroscopy of nitrous oxide flowing through a waveguide using a telecommunications diode laser. The RMS baseline noise for these measurements, in absorbance units, was 2 x 10^-5.

The southern area of Italy is characterized by the presence of many active volcanic areas. In Pozzuoli (Napoli, Italy), an urban area characterized by high volcanic risk, a gaseous emitting site is present. CO₂ is one of the emitted gases and the measurement of its atmospheric concentration is an important task in many environmental and scientific applications. A mobile IR laser system, able to measure the CO₂ concentration along rectilinear atmospheric paths up to 1 km length, has been used in that area. The system computes the average concentration by processing the received IR laser radiation propagated along an open-air rectilinear link connecting the transmitter/receiver laser unit and a passive retroreflector device. The laser system scans a spectral range centered on 1580 nm wavelength. An isolated CO₂ absorption line characterizes such range. A one-day measurement campaign has been made and more than 15 different atmospheric propagation links were considered moving the transmitter/receiver unit and some retroreflectors. The links were located over a gaseous emitting area whose extension is less than 100x100 square meters. A meteorological station was present and also temperature and wind measurements have been collected. CO₂ measurements and meteorological data are presented and discussed. Moreover, the spatial configuration of the measurement links is such that ad-hoc tomographic data processing procedures are profitably usable. The two-dimensional time-averaged spatial distribution of the CO₂ at about 1.5 meters over the monitored area is reported as retrieved by the tomographic procedures. Potential and perspectives of tomographic processing are also evidenced.

Mode-locked and noise analysis of hybrid soliton pulse source (HSPS) utilizing uniform fiber Bragg grating is described. The HSPS model is based on a time domain solution of coupled-mode equations including spontaneous emission noise. Relative intensity noise (RIN) is calculated using numerical solutions of these equations. It is found that near transform limited pulses are generated only a few frequencies even if system is properly mode-locked over a wide frequency range. Output pulse is not more affected at the fundamental mode-locking frequency because RIN has not a noise peak at this frequency. It is also found that RIN reduction is possible for mode-locked HSPS by selecting a suitable grating such as Gaussian apodized or linearly chirped.

A novel molecular beam spectrometer for the purpose of trace gas sensing is described. Sensitivity is greatly enhanced and absorption interference by atmospheric H₂O and CO₂ is greatly reduced by using a molecular expansion. The expansion results in rotational cooling and population enhancement of low-lying energy levels. The instrument employs a tunable mid-infrared lead salt diode, which operates in single mode from 2348.5 to 2351.1 cm^-1, and a 36 m Herriott multipass cell. The sample gas is injected axially through a coupling hole in one of the spherical mirrors. The result is an increase of the residence time of the molecular beam in the sampling region. Pulsed operation of the nozzle allows background subtracted spectra to be acquired. The spectrometer can either be operated in fast scan mode, in which the laser frequency is rapidly scanned over the absorption feature of interest, or in frequency modulation mode with 2f-detection. A special adaptation of frequency modulation to the axial sampling system is described. Sample data of CO₂ are presented.