This paper presents recent results obtained from tunable diode lasers fabricated from the Pb-salt materials Pb1-x Sn x Se, PbS1-xSex and Pb1x Cd x S. Four general areas are discussed: (1) Reliability improvement and statistical reliability data, (2) Performance improvements in stripe-geometry lasers, (3) Laser performance at wavelengths beyond 25 μm and (4) Laser performance at wavelengths below 4 μm. Improved processing techniques introduced several years ago have resulted in diode lasers with greatly improved shelf-storage stability values. A periodic re-test schedule involving hundreds of lasers has provided stability data which indicate retest survival rates of over 95% for lasers up to 5 years old. Criteria for retest survival and detailed results are presented. Performance improvements in stripe-geometry lasers of Pbi_xSnxSe and PbS1-xSex 4 cm -1 have resulted in lasers with single mode current-tuning ranges in excess of and maximum CW operating temperatures in excess of 100K. While significant improvements in single mode tuning range have been achieved, the ultimate limitations on tuning range are a natural consequence of the relatively narrow spectral gain curve of semiconductor lasers which cause the laser emission in a given mode to jump to another mode which is closer to the center of the gain curve as the device is current (or temperature) tuned. Along with improved single mode tunability devices with threshold current density values as low as 50 amp/cm2 have been obtained. Long wavelength (25-30 μm) lasers have been fabricated from Ploi_xSnxSe with single-mode output powers in excess of 200 μW. During the past year, under a NASA Goddard Space Flight Center-funded program, a reproducible process of obtaining 28 μm lasers has been achieved. Performance characteristics of these lasers are presented. Lasers emitting in the 2.8-4.3 pμm region have, in the past, been difficult to manufacture. Recent developments have increased the yields and performance of these lasers.

A Cd diffusion technique applicable to PbSnSe, PbSSe and PbCdS is presented. The pn-junc-tions of these diffused diodes are characterized by EBIC. The two important laser properties - maximum operation temperature and maximum output power - are discussed in detail. The large continuous tuning range of the diodes make them useful for integrative or derivative spectroscopy methods. An apparatus containing a laser source assembly for eight different diode lasers in a single closed cycle refrigerator unit, a polychromator for mode selection and beam combination and a two path measuring cell is described. Test results on simultan-eous CO and CO2 real time concentration measurements using integrative spectroscopy are presented.

Lead-rare earth-chalcogenide diode lasers have been grown by molecular beam epitaxy. Emission wavelengths shorter than 5-6 μm have been obtained from lead-europium-selenide-telluride (pb1-xEuxSe yTe1-y) double heterojunction diode lasers grown lattice-matched to PbTe substrates. Mesa diodes with :65 μm wide tripes have been fabricated which have a wide range of single longitudinal mode emission at up to ,r1 mW/facet output power. These diodes have operated at up to 147 K CW, which is the highest CW operating temperature ever achieved with lead-chalcogenide diode lasers to our knowledge. The wavelength coverage of the PbTe system has so far been extended to 4.06 μm CW. Longer wavelength coverage is obtained from double heterojunction diode lasers with Pb1-ySnyTe active regions lattice-matched to (Pbi1-ySny)1-xYbxTe confinement layers. In preliminary studies of diode wYth x = 0.034, y = 0.14, the CW emission wavelength varied from 10.7 μm (at 10 K) to 7.1 μm (at 128 K).

Internal quantum efficiencies of Pb Sn Te lasers with double-hetero geometries have been estimated. The efficiencies of conventional PbTe/PbSnTe/PbTe lasers are very low and far from the efficiencies which are limited by Auger-recombination mechanism. The main cause of low efficiency is thought to be due to the misfit dislocations at the PbTe-PbSnTe hetero-interface. To reduce the misfit dislocations, lattice-matched PbTeSe/PbSnTe/PbTeSe lasers are prepared and they show considerable improvements. However, due to the difficulty of cleaving of PbTeSe layers, the efficiencies are not sufficient. By designing a one-side-lattice-matched PbTeSe/PbSnTe/PbTe structure, the lasers whose efficiencies are very close to the theoretical limit can be prepared.

Low threshold 6 - 15 m range lead salt diode lasers have been fabricated from closely lattice-matched PbSnSeTe-PbSeTe double-heterostructure layers grown on PbSnTe or PbTe substrates by liquid phase epitaxy. Temperature dependence of threshold current density and injected carrier lifetimes have been measured for variously doped PbSnSeTe active region lasers. Very low threshold current densities at low temperatures were obtained for compensated active region lasers. For 1 Ltm thick Bi-Tl doped active region lasers, the threshold current densities of 40 - 50 A/cm2 and 200 -250 A/cm2 at 4.2 K and 77 K, respectively, were very reproducibly obtained. The lowest observed threshold was 21 A/cm2 at 4.2 K. These results were used to discuss the factors determining the threshold current density and the highest operating temperatures.

Preliminary results of a program to develop ion implantation confined, shallow mesa stripe (Pb,Sn)Te laser diodes are presented. The practicality of using a shallow mesa stripe to produce single mode laser output and to increase the single mode tuning range are demonstrated. The first results of p-type ion implantation in the lead-tin salts are also reported. It is shown that sodium and lithium both can be used to convert n-type Pb(Te,Se) to p-type. The implant and anneal procedures are described, and electrical characteristics of Li-implanted layers are presented.

Distributed Bragg-reflector (DBR) diode lasers were fabricated from lattice-matched Pb0.817Sn0.183 Te/PbSe0.08Te0.92 wafers grown by liquid phase epitaxy. The DBR lasers operated within a limited range of heat-sink temperatures, 8.5°-38°K, with threshold current density of 3kA/cm2 at 20°K. Single longitudinal-mode operation was obtained up to more than three times the threshold current. The DBR lasers exhibited continuous tuning range of 6 cm-1 near 775 cm-1 (12.9 μm). The average tuning rate was 0.21 cm-1/°K and was much smal-ler than that of the corresponding Fabry-Perot lasers, which was 2.3 cm-1/°K.

High resolution infrared tunable diode laser spectroscopy (TDL) has been applied to the study of cigarette smoke for qualitative and quantitative determinations involved in tobacco blend and cigarette filter developments. As examples of the different types of application of this work, several TDL studies are presented. The measurements of smoke components on a puff-by-puff basis in confined sample chambers and flowing streams were used to study the smoke component deliveries and the effects of filter dilution. The study of isotopes generated during combustion of chemically treated tobaccos was another application of the TDL system to complex gas mixtures without prior separation of compo-nents. The application of the TDL to the study of cigarette filters and smoke delivery simultaneously was demonstrated by using two well resolved absorption lines of two different gases which occur on a single TDL wavelength scan.

Derivative infrared diode laser spectrometry has been used for the rapid and precise (± 50 ppmν) determination of trace (> 50 ppmy) moisture in small (10-100 μL) samples of gas as they expand into an evacuated absorption cell. Adsorption/desorption of the analyte is the most serious potential source of error in the measurement; however, the application of time-resolved (resolution < 10 msec) techniques permits accurate analyses in the presence of these dynamic processes. The experimental method and a simple model which accurately reproduces the concentration-time behavior observed during analysis are presented which permits the analyst to ensure the accuracy of any given measurement or detect and correct erroneous ones. The method has been applied to the determination of moisture in hermetically sealed integrated circuit (IC) packages and is shown to be a superior alternative to existing methods using mass spectrometry (MIL-STD Method 1018.2).

An application of an infrared tunable diode laser for the continuous and automatic analysis of a process stream is described. In a commercially operating continuous compounding process for the manufacture of room temperature vulcanized (RTV) silicone products, the content of crosslinker, methyltriacetoxysilane (MTAS), is measured by means of the infrared absorption of this carbonyl group. This monitoring and subsequent process control plays a crucial role in manufacturing RTVs with the desired physical properties. The high intensity, narrow line width of the laser overcomes two obstacles which preclude the use of more conventional, commercially available process control instruments; in par-ticular, 1) relatively long optical path lengths are used allowing for timely and representative sampling and 2) an off-center absorption frequency is used to discriminate against non-MTAS carbonyl groups.

In the past few years we have made several studies of carbon monoxide (CO) in automotive exhaust with lead-salt tunable diode lasers. The laser system has several advantages over conventional exhaust analysis instrumentation including a fast response time (25 milliseconds) and the ability to measure CO in the presence of water vapor without interference. These studies were done using an engine dynamometer with a 5.7 ℓ V-8 engine. Initially, the CO was measured before and after the exhaust catalytic converter during step changes in the air-fuel ratio to study the dynamic response of the converter. The converter responded differently when the air-fuel ratio was switched from lean to rich than it did when switched from rich to lean. This behavior was attributed to chemical storage effects on the converter bed, among other factors. Next, the engine was closed-loop controlled at a given CO concentration level using the diode laser as a CO sensor rather than the conven-tional oxygen sensor. Limit cycle oscillations of the closed-loop system were substantially reduced by this method. Finally, a linearized model of the engine's CO emissions was developed based on the TDL measurements of CO during oscillation of the air-fuel ratio at different engine operating conditions.

Tunable diode lasers provide important opportunities for combustion research owing to the importance of infrared-active species in combustion. This paper summarizes recent work in the High Temperature Gasdynamics Laboratory at Stanford to develop and apply tunable diode laser techniques for combustion gas measurements. Example results are presented for species concentration and spectral parameters (line strengths and collision linewidths) of CO and NO, obtained over a broad range of temperature (295-3400K) using a room temperature absorption cell, various laboratory flames and a shock tube.

Fully automated tunable diode laser systems have been developed for the long-term, precision monitoring of gases, liquids, and solids. Active laser feedback stabilization allows continuous, unattended monitoring of the chemical or physical characteristics of materials. Specific systems will be described for the measurement of nuclear reactor performance, nonregulated automotive emissions, and extruded plastic quality. Common elements in all these systems are computerized laser stabilization and control, ultra-stable sampling cells, and optimized detectors selected for long-term, maintenance free operation.

The need for accurate measurements of trace gases in tropospheric air is well recognized. An instrumental system ideally suited for such measurements should have the following characteristics: 1. Universality; it should be applicable to a wide variety of gases. 2. Specificity; it should provide positive identification of the target gas. 3. Freedom of interferences from other constituents. 4. Rapid response of one minute or less, permitting real-time measurements. 5. High sensitivity of less than 1 ppbv.

An airborne instrument using tunable diode lasers (TDL's) has been developed to make in situ measurements of trace atmospheric gases. The instrument performance for the measurement of ambient CO is characterized. A technique to suppress TDL excess noise is demonstrated that is expected to lead to an increase in the state-of-the-art sensitivity of the TDL differential absorption technique.

A balloon-borne instrument for stratospheric research has been developed with the capability to simultaneously measure several chemically related species in situ, for a full diurnal cycle. The instrument utilizes tunable infrared diode lasers (TDLs) to provide the radiation in selected wavelength regions for sensitive absorption spectroscopy over a one-km round-trip path. The TDL radiation is directed to a remote retroreflector which is lowered 500 m below the instrument gondola. A HeNe laser and coaligned TV camera with CID imaging are used for retroreflector tracking. Currently the instrument operates with two TDLs, and the capability exists to measure four stratospheric species: NO, NO2, 03, and H2O. The number of operating TDLs can be expanded to four, resulting in the possibility of measuring several additional trace species.

New frequency calibration tables are required to keep abreast of the resolution attainable by currently available tunable lasers. One key to the generation of tables with requisite accuracy involves accurate heterodyne frequency measurements; another key consists of reliable fitting and analysis. Coordinated activity in NBS involves selection of suitable molecular calibration candidates, their frequency measurement and analysis, and dissemination of the results in the form of frequency calibration tables. Current status of these efforts is described.

Quantum phase noise limited Lorentzian power spectral densities were achieved with tunable lead-salt diode lasers. Linewidths as narrow as 22 kHz were observed. A truly programmable infrared synthesizer was produced by frequency-offset-locking the tunable diode lasers to the combination of a stable CO2 (or CO) reference laser and a programmable microwave frequency synthesizer. Absolute frequency accuracy and reproducibility of about ±30 kHz (10-6 cm -) relative to the primary Cs frequency standard may now be obtained with this technique.

Heterodyne spectroscopy at infrared wavelengths is a unique approach to the study of atmospheric species and astrophysical objects. It's coherent detection properties ma4 it 7 the only optical technique to combine ultra-high frequency resolving power (v/A = 106- 107) with diffraction-limited spatial resolution. The use of lead-salt tunable diode lasers (TDL's) as local oscillators in a heterodyne instrument offers the additional advantage of continuous tunability, permitting operation over the entire nominal tuning range of the device. Previous investigators have obtained high signal-to-noise TDL heterodyne spectra of terrestrial atmospheric features in solar absorption and molecular features in sunspots. Until recently, however, the noisy character and low output power of TDL's have precluded their use for heterodyne detection of objects much fainter than the sun. Attempts to observe planets and astrophysical infrared sources have produced only a handful of weak continuum detections. The major categories of TDL excess noise are now fairly well understood, and new device fabrication techniques have produced dramatic improvements in noise reduction, power output, single mode tunability and operation at long wavelengths (X > 10 microns). These next generation devices should result in ground-based instrument performance which rivals the CO2 laser heterodyne technique throughout the 8 to 13 micron atmospheric window.

We report the results of a laboratory study for detecting the important atmospheric molecule, HC1, using a tunable diode laser coupled to a multipass White cell. In contrast to many such prototype studies, the calibration in this work was carried out near the concentration range of interest and verified using three independent techniques. Employing pathlengths of 40-m, we have demonstrated a detection sensitivity (S/N = 1) in the 250 - 300 parts-per-trillion range at pressures around 9 torr.

One of the requirements for a better understanding of stratospheric chemistry is a better data base of stratospheric trace gas measurements. In particular simultaneous measurements of reactive species in the same air mass, which would provide good tests of current models, are almost non-existant. We have built a tunable diode laser absorption spectrometer (TDLAS) capable of making truly simultaneous concentration measurements of more than one species from a balloon platform. At present it is configured to measure NO and NO2 although it can be upgraded to measure the concentrations of 4 species simultaneously.

The laser optogalvanic (LOG) technique for studying molecular spectra has been extended for the first time to the infrared wavelength region. Portions of the NH3 v2 band at 9.5 pm and the NO2 v3 band at 6.2 um have been recorded at Doppler-limited resolution using CW tunable diode lasers to probe DC electrical discharges in pure NH3 and an NO2/He gas mixture. Using adjustable electrode positions and an orthogonal geometry between the probe laser and the discharge axis, two contributions to the optogalvanic signal are identified: one which corresponds to an increase in discharge impedance and is seen only for irradiation of the negative glow region; and a second which corresponds to a decrease in discharge impedance and is seen for irradiation of all other discharge regions.

Tunable diode lasers have made possible new techniques to study the vibrations of molecules in solids and on surfaces. We will focus on two of these: persistent nonphotochemical hole burning (NPHB) for the study of molecules in solids and electroreflectance vibrational spectroscopy (EVS) for the study of molecules on surfaces. We give an overview of both techniques. The system that we use for EVS measurements is described in detail. We also discuss the sources of diode laser intensity noise and the approach taken to measure EVS signals accurately over a wide frequency range.

A method is described to measure the mobility of ions in DC glow discharge plasmas using tunable infrared diode laser spectroscopy. Excellent agreement with earlier time-of-flight drift-tube studies for the mobility of ArH in He was obtained.

Tunable diode lasers are used to probe the dynamics of conventional cw CO2 lasers, and a cw Raman laser operating at 12 μm. The tunability, high resolution and high sensitivity of the diode laser allow us to obtain detailed information concerning vibrational popula-tions, relaxation rates, and gain coefficients which would be unattainable by more conven-tional techniques.

Sub-Doppler double resonance experiments are described in which a fixed frequency CO laser pumps a 6 μm v4 transition in NH3, while a tunable diode laser probes a 10 μm v2 transition having a common lower level. The experiments are done with a precision Stark cell inside the CO laser cavity. A pump transition is Stark-tuned into exact coincidence with the CO laser, which is then Lamb-dip stabilized on the transition. The probe beam overlaps the pump beam colinearly in the Stark cell, and the double resonance signals appear as nar-row transmission peaks on the diode laser frequency scans. The narrowest observed widths are c=d3 MHz (FWHM), a large portion of which is due to unresolved hyperfine structure. An analysis of the various broadening mechanisms indicates that under optimum conditions the diode laser contributes less than 1 MHz to the widths.

A diode-laser spectrometer was used to measure individual R-branch line strengths in the (v4 + v5)° combination band of C2H2. A total band strength of Sv = 63 ±2 cm-1/cm atm was found for the normal isotopic composition of C2H2. Broadening parameters for several R-branch lines were determined with N2 and He as the broadening gases.

Absorption line strengths have been measured at 295 K for the first time for four vibration-rotation transitions of the OH radical, using a tunable, infrared diode-laser in conjunction with a molecular modulation spectrometer. A periodically varying OH concentration was created by photolysis, using a sinusoidally modulated 25374_ Hg lamp, of a flowing mixture of 03 and H2O contained within a quartz-walled White cell. The modulated absorption owing to the OH radical was subsequently observed by phase-sensitive detection. The absolute modulation amplitude of the OH number density was obtained by numerical simulation of the complete time-dependent photochemical system. The strongest transition measured was at 3407.607 cm-1, with a strength of 3.3 ±1.5 x 10-20 cm-1 molecule-1 cm2.

It is suggested that a multimode waveguide Y-coupler may be used as a coherent multiplexer for heterodyne detection. The dependence of the mixing efficiency upon various parameters such as the asymmetry of the coupler, the intermediate frequency, the modal occupation and the dimensions is analysed and discussed. Numerical calculations indicate that the most important parameter is the symmetry of the Y-junction. For a symmetric Y-coupler, 1 cm long, a mixing efficiency of more than 60 per cent and bandwidth of the order of 50 GHz can be achieved. Experiments with AgC2:AgBr infrared fibers and preliminary experiments with As, Se, thin films seem to support the theory. This may be a first step towards the reali-zation of a coherent detector for integrated optics.