Lead-salt Diode Laser Research Articles

Three channel diode laser spectrometer for the determination of line-shift coefficients in the ν3 band of NO 2

The implementation of a three channel lead salt diode laser spectrometer and its application for the measurement of line-shift coefficients in the ν 3 band of NO 2 are presented. The spectrometer allows the determination of line shifts larger than 5.10 −4 cm −1 and is suitable also for the investigation of relatively crowded spectra of molecules like NO 2 or SO 2. For the first time it was possible to determine shift coefficients for seven lines of the ν 3 band of NO 2 in the case of collisions with He, Ne, Ar, Kr, and Xe. Moreover, the spin splitting and the relative intensity of the doublet components were measured for eight unresolved doublets. The shift and broadening coefficients were determined for lines covering the quantum number ranges 24 ⩽ N″ ⩽ 38, 1⩽ K a ″ ⩽ 5, the spin splittings and the intensity ratios for lines within the intervals 20 ⩽ N″ ⩽ 38, 1 ⩽ K a ″ ⩽ 5. The broadening data show an excellent agreement with experimental data recently published. The results for the spin splitting correspond to the theoretical expectation, the intensity ratios are in good agreement with experimental and theoretical data. These coincidences are prerequisite for the validation of the line-shift data. All shift coefficients measured are negative; in the best cases the uncertainties were less than 2%. The absolute values of the line-shift coefficients for the different noble gases follow the trend ¦δ NO 2He ¦<¦δ NO 2 ¦<¦δ NO 2Ar ¦ <¦δ NO 2Kr ¦<¦δ NO 2Xe ¦ . Within the studied quantum number range an increase of the absolute size ¦δ NO 2noble gas ¦ with increasing N″ is observed.

Finite-element modeling predicts possibility of thermoelectrically cooled lead-salt diode lasers

Finite-element thermal modeling shows that thermoelectric cooled operation of IV-VI semiconductor diode lasers is possible by replacing the thermally resistive lead-salt substrate with copper. Contrasting thermal models reveal a 63 K decrease in active region temperature under normal operating conditions when PbTe is replaced with copper. This improved device structure can be obtained by using epitaxial-lift-off techniques similar to those developed for III-V semiconductor devices.

Argon broadening of the R(48), R(50) and R(52) lines of CO2 in the (0001) ← (0000) band

A c.w. narrow-line lead-salt diode laser was used to measure the fully resolved absorption line shape of the R(50) line of CO2 in the v3 band (0001) ← (0000) at 2380.72 cm−1 in shock-heated mixtures of CO2Ar. The line strength S (cm−2 atm−1) and collisional-broadening coefficient 2γCO2Ar (cm−1 atm−1) were determined from rapid-scanning experiments conducted over the temperature range 300–1860 K. Collisional-broadening coefficients for the R(48), R(50) and R(52) lines were determined from fixed-frequency experiments conducted over the temperature range 300–2290 K. For the assumed form γ(T)γ(Tref) = (TTref)−n, the exponent n was determined to be 0.61 with γ(Tref) = 0.0498 (cm−1 atm−1) (HWHM) and Tref = 298 K. No J″ dependence to γCO2Ar was observed for this limited range of high-J″ transitions.

Pulse propagation effects in bulk InSb crystal

Based upon the time dependent perturbation technique for coherent ultrafast pulsed radiation-semiconductor interaction, the transient induced polarization has been considered for the analysis of change in the pulse shape while propagating through the material. Using the electromagnetic treatment, the output field amplitude is obtained. Numerical estimations are made for a sample of InSb crystal duly irradiated by lead-salt (PbEuSeTe) diode laser operating in the 2.6 – 6.6 μm wavelength range. The analytical results are in very good agreement with the available experimental and theoretical results.

High-resolution PIRSS using a tunable infrared diode laser: the Davydov doublet of the antiferroelectric (2 × 1) monolayer CO adsorbed on NaCl(001)

The temperature dependence of the Davydov doublet of the antiferroelectric (2 × 1) monolayer CONaCl(001) has been studied between 11 and 25 K by means of polarization infrared surface spectroscopy (PIRSS) using a tunable lead salt diode laser with a resolution better than 5 × 10 −3cm −1. At 11 K the linewidth of the symmetric vibrational exciton, polarized perpendicular to the surface, was determined to be 0.035 cm −1, the linewidth of the antisymmetric vibrational exciton, polarized parallel to the surface, to be 0.025 cm −1, the sharpest spectral line of an adsorbate observed. To obtain phase relaxation times T 2 in the low-temperature region, we fitted the parameters of the exchange dephasing model to the experimental data. A satisfactory fit for both the frequency and the linewidth is obtained at temperatures below 25 K. Above 25 K the antisymmetric vibrational exciton becomes extremely broad and finally disappears, and a successful fit was obtained only for the linewidth of the symmetric vibrational exciton. It is shown how the vibrational energy relaxation time T 1 may be obtained by varying the spectral power of the laser light.

Stabilizing Lead-Salt Diode Lasers: Understanding and Controlling Chaotic Frequency Emission

Lead-salt tunable diode lasers (TDLs) are the only devices currently available that can generate tunable monochromatic radiation at arbitrary wavelengths between 3 and 30 micrometers and are particularly useful for high-resolution spectroscopy over a wide range of spectral regimes. Detailed observations of TDLs show that the observed instrumental linewidth is actually a temporal average of many narrow (less than 0.5 megahertz) emission "modes." The time scale characteristic of these "modes," which appear to be of relatively constant intensity, is of the order of a microsecond. The laser's behavior is highly suggestive of a chaotic process, that is, seemingly random excursions of a dynamic variable (frequency) within a bounded range. This report shows experimentally that TDL emissions are indeed chaotic. Furthermore, in a simple and robust fashion, this chaotic behavior has been successfully controlled with the use of recent techniques that take advantage of chaos to produce a narrow band laser output.

A continuously tunable long-wavelength cw IR source for high-resolution spectroscopy and trace-gas detection

A new widely tunable source in the infrared for use in high-resolution spectroscopy and trace-gas detection is described. This spectroscopic source is based on Difference Frequency Generation (DFG) in gallium selenide (GaSe) and is continuously tunable in the 8.8–15.0 μm wavelength region. Such a DFG source operates at room temperature which makes it a useful alternative to a lead-salt diode-laser- based detection system that requires cryogenic temperatures and numerous individual diode lasers.

Explosives detection with a frequency modulation spectrometer

An explosives detection instrument was designed and tested at SRI International. The instrument uses frequency modulation spectroscopy with midinfrared lead-salt diode lasers to perform high-sensitivity detection of characteristic nitrogen-containing decomposition products of explosives. Ultimately, the instrument should be capable of detecting and identifying subpicogram levels of plastic explosives, which would be suitable for screening passengers at airports. Using the laboratory breadboard instrument and two different explosive vapor generators, we demonstrated a lower limit of detection of 5-10 pg for cyclotrimethylene trinitramine and linearity of the signal over an order of magnitude.

MBE grown buried heterostructure separate confinement multiple quantum well Pb 0.9854Eu 0.0146Se xTe 1− x/Pb 0.981Sn 0.019Te tunable diode lasers for high resolution sepctroscopy

Lead-salt diode lasers are useful as tunable infrared sources for high resolution molecular spectroscopy in the 3–;30 μm spectral range. PbEuSeTe/PbSnTe separate confinement multiple quantum well (SCMQW) diode lasers in the buried heterostructure (BH) configuration have been prepared for the first time. Pb 0.981Sn 0.019Te quantum wells 170–220 Å thick and Pb 0.9854Eu 0.0146Se y Te 1− y barriers 500–1000 Å thick have been fabricated in different thickness combinations using molecular beam epitaxy technology. A record continuous wave operating temperature of 200 K for PbSnTe based devices has been achieved. Threshold current density as low as 6.67 kA cm −2 has been measured at 160 K.

Spectroscopic trace gas analysis using semiconductor diode lasers

Sensitivity, specificity, high time resolution, cost effective simultaneous measurements of several species and operational systems are the main requirements for in situ trace gas analysis in atmospheric research and process monitoring applications. Tunable diode laser absorption spectroscopy is increasingly being used to measure atmospheric trace gas concentrations down to low ppb-levels and the technique fulfills the requirements for trace gas analysis in the atmosphere for most of the smaller molecules with resolved absorption spectra. Semiconductor lead-salt diode-lasers give access to the mid infrared spectral region, where the most important atmospheric constituents have strong rotational vibrational absorption bands. The application of high frequency modulation (FM) schemes further improves sensitivity and detection speed of modern instrumentation. In this paper the FM technique will be reviewed with emphasis to the work, which has been done in our laboratory and findings from recent investigations, as well as novel approaches will be discussed. Some applications related to quality control and ISO calibration in addition to measurement challenges for fast and high sensitivity applications will be addressed.

Prognosis for a mid-infrared magnetic rotation spectrometer for the in situ detection of atmospheric free radicals

Mid-infrared magnetic rotation spectroscopy (MRS) experiments on nitric oxide (NO) are quantitatively modeled by theoretical calculations. The verified theory is used to specify an instrument that can make in situ measurements on NO and NO(2) in the Earth's atmosphere at a sensitivity level of a few parts in 10(12) by volume per second. The prototype instrument used in the experiments has an extrapolated detection limit for NO of 30 parts in 10(9) for a 1-s integration time over a 12-cm path length. The detection limit is an extrapolation of experimental results to a signal-to-noise ratio of one, where the noise is considered to be one-half the peak-to-peak baseline noise. Also discussed are the various factors that can limit the sensitivity of a MRS spectrometer that uses liquid-nitrogen-cooled lead-salt diode lasers and photovoltaic detectors.

Experiments on linewidth reduction by fast electrical feedback

We report our first results on broadband electrical feedback applied to tunable lead-salt diode lasers. We use a heterodyne technique in combination with electronic frequency discrimination to reduce the diode-laser linewidth. The loop band width is about 10 MHz. We present results on high-frequency modulation to study the frequency response of the diode laser. From this experiment we are able to estimate the carrier tuning rate of the used lead-salt diode laser. Preliminary results in linewidth reduction by fast electrical feedback promise the possibility to apply the fast electrical feedback technique to lead-salt diode lasers.

Development of a line-narrowed and frequency-stabilized TDL system for sub-Doppler molecular spectroscopy

The spectral resolution of TDL spectrometers is generally limited by the finite spectral width of the TDL radiation. Usually, the spectral purity of lead-salt diode lasers is sufficient to perform Doppler-limited studies but is inadequate to resolve sub-Doppler spectroscopic features like saturated-absorption signals. The application of external optical feedback allowed us to reduce the linewidth of tunable diode lasers by two orders of magnitude down to 200 kHz. In a frequency-offset locking scheme we stabilized and controlled the frequency of the narrowed line. This technique enabled us to use the TDL as saturating and probing laser in a saturation experiment: we observed a nonlinear absorption signal (Lamb dip) in an OCS absorption line with very high resolution. The current performance of our TDL system is reported and the resolution and accuracy limits are discussed.

Optoacoustic Lead-Salt Diode Laser Detection of Trace Species in a Flow System

An experimental setup for kinetic and product studies in a flow system has been developed. For detection, a nonresonant optoacoustic technique combined with a tunable lead-salt diode laser has been employed. The performance of the system was tested on ammonia in argon, and a detection limit (S/N = 2) of 8 × 1012 molecules cm−3 (corresponding to 320 ppb at atmospheric pressure) was obtained at a pressure of about 7 kPa when frequency modulation was employed. At 1.5 kPa the detection limit was about 2 × 1014 molecules cm−1. With a multipass cell the sensitivity was increased by a factor of four.

Laser excess noise and interferometric effects in frequency-modulated diode-laser spectrometers

Tunable Diode-Laser Absorption Spectroscopy (TDLAS) is increasingly being used to measure trace-gas concentrations down to low part-per-billion levels (1 ppbv = 10−9 volume mixing ratio). Semiconductor lead-salt diode lasers give access to the mid-infrared spectral region and the application of high-Frequency Modulation (FM) schemes can further improve the sensitivity and detection speed of modern instrumentation. Several factors influence or even limit spectrometer performance. The central elements in such spectrometers are lead-salt diode lasers. Experimental data will be presented, which demonstrate that high-frequency excess-noise contributions above several MHz can be attributed to mode hopping and mode partition noise during multimode laser operation. Additionally it will be discussed how a FM-TDLAS spectrometer can be interpreted as an optimized Michelson interferometer for absolute distance measurements and, therefore, is extremely sensitive towards drift effects. The higher the modulation frequency, the higher is the drift sensitivity of the spectrometer due to interferometric effects. These drift effects are a second factor affecting ultrasensitive measurements. While wideband-laser noise characteristics call for high modulation frequencies, the aforementioned interferometric effects in the spectrometer require low modulation frequencies.

Optical heterodyne detection and microwave rectification up to 26 GHz using quantum well infrared photodetectors

We have demonstrated heterodyne detection up to an intermediate frequency of 26.5 GHz using quantum well infrared photodetectors. A CO/sub 2/ laser and a lead-salt tunable diode laser were used as the infrared sources. Heterodyne detection experiments measure the high frequency behavior of photoexcited electrons and their transport properties. We have also carried out microwave rectification experiments which measure the high frequency behavior associated with the dark-current electron-transport processes.

Absorption spectroscopy of N_2O with phase-diffusion-noise sidebands on a lead-salt tunable diode laser

The broad phase-diffusion-noise spectrum of a lead-salt tunable diode laser (TDL) has been exploited for the detection of absorbances produced from low-pressure N(2)O vapor in the 4.47-µm spectral region. This effect has previously been observed in the detection of electronic transitions in rubidium, cesium, and molecular oxygen with anAlGaAs TDL operating in the 0.700-0.800-µm region. For the first time to our knowledge, this paper presents evidence of this phenomenon when a midinfrared rovibrational absorption line is monitored. The technique is also compared with traditional TDL spectroscopic methods for its applicability to trace-vapor monitoring.

An experimental and modeling study of the selective noncatalytic reduction of NO by ammonia in the presence of hydrocarbons

This paper presents an experimental and modeling investigation of the NO x removal from flue gases through the SNCR process in the presence of hydrocarbon additives. The selective non-catalytic NO x reduction process was realized in a high-temperature flow reactor. A lead salt diode laser system, combined with second derivative spectroscopy, monitored the concentration of NO, NH 3, N 2O, NO 2, and HCN downstream of the reactor. After injection of the hydrocarbon additive (methane or ethane) at concentrations of up to 1000 vppm, the shift of the temperature window of the SNCR process and the by-product formation was measured. The experimental results have then been compared with the results of the modeling study. Both experimental and theoretical results show a shift of the temperature window towards lower temperatures in the presence of additives, as well as a decreased temperature range of ammonia slip and the formation of NO 2 as a by-product at the cold side of the temperature window. Nitrous oxide was neither predicted by the model nor experimentally detected. In oxygen-deficient gas mixtures, HCN-formation was observed up to 300 vppm. HCN-formation from NO-ethane interaction could be attributed to the reaction of HCCO with NO, but there is an additional route of HCN-production from the interaction of ethane with ammonia, possibly from the reaction of NH 2 with unsaturated hydrocarbons. Thus, there is a strong demand for reliable kinetic data on these reactions and on the reaction sequence HCCO + NO → HCNO + CO,HCNO + H → HCN + OH.

Pressure Broadening of NO 2 by NO 2, N 2, He, Ar, and Kr Studied with TDLAS

The line broadening in the strongest band of NO 2, the ν 3 band near 1616.85 cm −1, was studied using a spectrometer with pulsed lead salt diode lasers. The examination uses self-broadening experiments as well as broadening with N 2, He, Ar, and Kr to obtain information on the intermolecular interactions. For a set of quantum numbers with 16 ≤ N″ ≤ 48, 0 ≤ K″ a ≤ 12, the broadening coefficients and spin splittings were determined with an accuracy between 1.5 and 5%. The measured values for the spin splittings are in good agreement with the theoretical values from A. Perrin.

Double Modulation Technique in Pulsed Jet Experiments with Tunable Diode Lasers

Abstract A double modulation technique was used to achieve higher sensitivity in absorption measure­ ments with a pulsed supersonic jet. The wavelength of a lead-salt diode laser was modulated at a frequency of 10 kHz and detected at twice the frequency by a lock-in amplifier, reducing significantly the influence of the low frequency excess laser noise. The output of this lock-in amplifier was measured by a boxcar integrator, driven at 80 Hz, the frequency of the pulsed jet. In case the duration of the jet pulse is long enough, a second lock-in amplifier can also be used instead of the boxcar integrator. The optical path of the diode laser radiation through the jet was increased 16 times by using White type multireflection optics. The achieved value for the minimum detectable absorption of Ar - CO complexes in the jet was 2 x 10-5 in relative absorption, limited by the excess noise of the diode laser.