Lead-salt Diode Laser Research Articles

Analytical applications of infrared semiconductor lasers in atmospheric trace gas monitoring

Sensitivity, specifity, time time resolution and cost effective simultaneous measurements of several components using operational systems are the main requirements for atmospheric research and air pollution monitoring. Tunable diode lasers adsorption spectroscopy (TDLAS) is increasingly beeing used to measure atmospheric trace gas concentrations down to low ppb-levels (10 −9 volume mixing ratio). This optical 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 can further improve sensitivity and detection speed of modern instrumentation

Improved multipass optics for diode laser spectroscopy

Feedback between optical elements can be a major source of noise when trying to attain high sensitivity in infrared absorption experiments. We find that a conventional White-cell optical arrangement introduces étaloning fringes that modulate the peak-to-peak amplitude of our signals by 1 part in 16 666, a fractional change of 6×10−5. Although relatively small, this ‘‘noise’’ is systematic and adds coherently with averaging, obscuring interesting absorption features. An easily constructed multipass optical system suited for performing high-resolution infrared spectroscopy in molecular beams is described. The design is based on a variation of the White cell and has been optimized for use with lead salt diode lasers. One of the key components in the improved design is the addition of an oscillating mirror for spoiling optical feedback generated by laser scatter and/or poor mode coupling of the laser to the multipass optics.

In‐Situ Gas Analysis with Infrared Lasers

AbstractIn the first part of the paper we present the experimental validation of models describing the NOx removal from flue gases through the SNCR process. The Selective Non‐Catalytic NOx 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, NH3, N2O, NO2, HCN behind the reactor. After the injection of ethane at concentrations of up to 1000 ppm the shift (up to 220 K) of the temperature window of the SNCR process was measured.In the second part a Laser In‐Situ Ammonia‐monitor (LISA) based on a 13CO2‐wave guide laser is used to establish an on‐line control of the SNCR in an industrial environment by measuring in‐situ the excess ammonia behind the reduction zone. With differential absorption spectroscopy and a special referencing scheme sensitivities in the order of several ppm·m of NH3 at atmospheric pressure have been achieved even in heavily dust laden flue gases. With a multi‐channel LISA system the 2‐dimensional NH3 distribution in the flue gas channel of an experimental coal burner system could be measured and the persistence of spatial inhomogeneities, caused by incomplete mixing of NH3 and the flue gas could be shown.In the third part we present first results aimed at the measurement of the local O2‐concentration in combustion systems. With a thermally stabilized 5 mW NIR diode laser at 760 nm we detected oxygen at atmospheric pressure with a sensitivity of less than 500 ppm·m using two beam absorption spectroscopy.

Combined wavelength and frequency modulation spectroscopy: a novel diagnostic tool for materials processing

By applying both low-frequency wavelength modulation and high-frequency phase modulation to a laser diode, we develop a sensitive, high-bandwidth chemical diagnostic tool that is applicable to a variety of gas-phase processing environments. Specific chemical species are identified and monitored through their infrared absorption spectra, and the modulation methods allow for sensitive detection that is free of window and other reflection-driven interference fringes. Absorbance limits of 5.3 x 10(-8) and 1.9 x 10(-7) are obtained for an AlGaAs diode laser and a lead-salt diode laser, respectively. We discuss applications to plasma etching and chemical vapor deposition.

Dynamic resonant peak locking scheme for diode laser modulation spectroscopy

We describe a diode laser frequency control scheme that dynamically locks the laser to an absorption resonance line while simultaneously allowing the quantitative measure of absorption strengths as weak as a few parts in 10 6 . The method exploits a novel modulation scheme, and is capable of locking the laser to both stable and transient species in an electrically noisy environment such as that found in plasma etching reactors. We present an experimental demonstration using a lead salt diode laser operating near an rt plasma etching diode reactor and in an industrial-type triode reactor.

Stabilization of 3.3 and 5.1 μm lead-salt diode lasers by optical feedback

Optical stabilization of tunable lead-salt diode lasers (TDL) in the mid-infrared is presented for the first time. By introducing an external feedback mirror both the linewidth is substantially narrowed and the frequency of the TDL is stabilized and controlled via this mirror in a frequency-offset locked scheme. We achieved narrowing of the linewidth by 1–2 orders of magnitude or better. The TDL is offset-locked to a CO gas laser by a heterodyne technique: the beatnote between the two lasers is used to control the length of the external resonator. By this scheme we gain the capability of absolute frequency measurements with sub-Doppler accuracy. The improved spectral properties of the diode laser provide a new tool for high-resolution molecular spectroscopy in the mid-infrared.

Infrared-infrared double resonance study of methyl alcohol

In this work we carried out an infrared-infrared (IR-IR) double resonance (DR) study of methyl alcohol using a CO 2 laser and a lead salt diode laser, in order to confirm previously proposed far-infrared (FIR) laser assignments for the 9P(34) and 9P(16) CO 2 laser lines. For the 9P(34) CO 2 laser line it is confirmed that the pump radiation excites the torsionally excited Q(125, 9) transition of methyl alcohol in agreement with the assignments proposed earlier. From the study it was noticed that due to the pump radiation, transitions which are not connected by dipole selection rules either to the lower or upper pumped level were also affected. Thus it appears that four-level IR-IR DR experiments are unlikely to provide useful information for transition assignments. The diode laser study was coupled with a recent spectroscopic study using a high resolution Fourier transform spectrometer to detect and identify hot band and perturbation allowed transitions in CH 3OH.

Novel étalon fringe rejection technique for laser absorption spectroscopy

When used for trace-gas detection, laser absorption spectroscopy is usually limited by false absorption signals that are traceable to interferometric effects induced by windows and other pairs of optical surfaces. Here we introduce a new technique that can selectively reject these étalon fringes while preserving the true absorption signal over a wide range of étalon free spectral range to absorption linewidth ratios. We present a theoretical analysis and experimental verification by using a tunable lead salt diode laser.

Line broadening in the ν3 band of SO 2: Studied with diode laser spectroscopy

A spectrometer with pulsed lead salt diode lasers was used to study line broadening in the strongest band of SO 2, ν 3 near 7.3 μm. A Fourier analysis technique was used to determine the linewidths; this technique increases the usable pressure range for closely spaced lines. The results give a decrease of both air- and self-broadening coefficients with increasing J″ and show an overall good agreement with theoretical data.

Frequency modulation and wavelength modulation spectroscopies: comparison of experimental methods using a lead-salt diode laser

Wavelength modulation spectroscopy (WMS) and one-tone and two-tone frequency modulation spectroscopy (FMS) are compared by measuring the minimum detectable absorbances achieved using a mid-IR lead-salt diode laser. The range of modulation and detection frequencies spans over 5 orders of magnitude. The best results, absorbances in the low-to-mid 10(-7) range in a 1-Hz bandwidth, are obtained by using high-frequency WMS (10-MHz detection frequency) and are limited by detector thermal noise. This sensitivity can provide species detection limits well below 1 part per billion for molecules with moderate line strengths if multiple-pass cells are used. High-frequency WMS is also tested by measuring the absorbance due to tropospheric N(2)O at 1243.795 cm(-1). WMS at frequencies < 100 kHz is limited by laser excess (1/f) noise. Both of the FMS methods, which require modulating the laser at frequencies >/= 150 MHz, give relatively poor results due to inefficient coupling of the modulation waveform to the laser current. The re ults obtained agree well with theory. We also discuss the sensitivity limitations due to interference fringes from unintentional étalons and the effectiveness of étalon reduction schemes.

Persistent infrared spectral hole burning of NO−2 ions in potassium halide crystals. I. Principle and satellite hole generation

New features are resolved within the internal vibrational mode spectra of NO−2 defects in KCl, KBr, and KI crystals at low temperatures using high-resolution Fourier transform spectroscopy and persistent infrared spectral hole (PIRSH) burning separately and together. With interferometry it has been discovered that the vibrational linewidths of the different modes range over a factor of 300—from 0.01 cm−1 to ∼3 cm−1 and, with PIRSH burning, it has been demonstrated that the narrowest lines are inhomogeneously broadened while the broadest ones are homogeneously broadened. PIRSH’s have been found in some internal modes and combination bands of the NO−2 molecule when pumped with low-intensity single-mode lead salt diode lasers; however, detectable persistent holes are not produced in all of the modes because of a competition between hole production and relaxation by tunneling at low temperatures. This competition results in a hole burning intensity, below which hole relaxation overwhelms hole production and only small holes may be produced. The most unusually shaped absorption features are the V-shaped notches in the reorientational tunneling fine structure at the NO−2 bending mode frequency in KCl and KBr. Of all the internal modes that do show pronounced PIRSH burning, these V-notched absorption bands exhibit the most striking behavior. Multiple satellite PIRSH’s are detected at frequencies away from the single-mode laser burn frequency with a broadband probe beam produced by a high-resolution Fourier transform interferometer. An explanation for these satellite holes is derived from temperature, plastic deformation, and uniaxial stress dependence measurements on the KCl:NO−2 absorption spectrum. We find that the inhomogeneous broadening of the KCl:NO−2 ν2 reorientational tunneling fine structure is dominated by degenerate rotor level splitting produced by random crystal strains. Degenerate perturbation theory of the rotor level splitting in the strain field is found to match very closely the V-shaped inhomogeneous distribution of levels associated with the KCl:NO−2 reorientational tunneling fine structure. The general conclusion is that whenever strain splitting of a doubly degenerate level dominates the inhomogeneous broadening, then the absorption spectrum displays zero strength in the distribution at zero splitting and a linear increase in absorption coefficient away from this frequency generating the observed V-shaped notch in the absorption profile.

Diode laser spectroscopy of the ν3 vibration of the HO 2 radical

The infrared spectrum of the ν 3 vibration of the hydroperoxyl radical has been examined using a tunable lead salt diode laser. Fifty-nine transitions were observed in the wavenumber interval between 1040 and 1120 cm −1. This work extends the earlier measurements of Johns, McKellar, and Riggin to significantly higher rotational states and results in an improved set of spectroscopic constants which should predict all atmospherically relevant HO 2 transition frequencies with a precision approaching a thousandth of a cm −1. The incorporation of these line positions in atmospheric transmission models using HITRAN or ATMOS data bases will be useful in remote sensing measurements of HO 2.

High-frequency intensity noise of lead-salt diode lasers

The high-frequency intensity noise spectra of mid-infrared diode lasers have been investigated. Theoretical estimates are compared with experimental results of spectral noise measurements. Lasers with different lateral structures showed comparable characteristics, although at different noise levels. Relative intensity noise as structures 10−17 Hz−1 was observed; this is close to the quantum noise limit for detection and hence suitable for small as trace-gas analysis systems.

Low-frequency amplitude-noise characteristics of lead-salt diode lasers fabricated by molecular-beam-epitaxy

Low-frequency amplitude-noise measurements for several lead-salt diode lasers fabricated by molecular-beam-epitaxy are presented. The influence of current fluctuations of the current source and of the magnitude of the injection current was investigated. The total noise is compared to the noise in single modes.

Tunable diode laser IR spectrometer for in situ measurements of the gas phase composition and particle size distribution of Titan’s atmosphere

A new instrument, the Probe Infrared Laser Spectrometer (PIRLS), is described for in situ sensing of the gas composition and particle size distribution of Titan's atmosphere on the NASA/ESA Saturn Orbiter/Titan Probe Cassini Mission. For gas composition measurements, several narrow bandwidth (0.0001 cm(-l)) tunable lead-salt diode lasers operating near 80 K at selected, mid-IR wavelengths (3-16 microm) are directed over a pathlength defined by a small reflector extending over the edge of the probe spacecraft platform; volume mixing ratios of 10(-9) should be measurable for several species of interest. A cloud particle size spectrometer using a diode laser source at 0.78 microm shares the optical path and deployed reflector; a combination of imaging and light scattering techniques will be used to determine sizes of haze and cloud particles and their number density as a function of altitude.

An optical isolator for mid-infrared diode lasers

A simple, compact Faraday isolator has been constructed and demonstrated for use in conjunction with a lead-salt diode laser operating in the 5.4 μm spectral region. We obtained an isolation ratio of 25 dB with an n-doped InAs crystal 6 mm in diameter by 1.3 mm thick, housed in a Sm 2Co 17 magnet assembly. The insertion loss was 1.5 dB.

Quantum-limited FM-spectroscopy with a lead-salt diode laser

Ultrasensitive absorption spectroscopy of NO2 was performed with a tunable lead-salt diode laser (TDL) using a single-tone high-frequency modulation (FM) technique. With a detection bandwidth of 200 kHz, an optical density of 2.7 × 10−5 was detectable at SNR of 1. The detectable optical density could be further improved by reducing the detection bandwidth in agreement with the √Δf relationship, reaching 2.5 × 10−6 at a detection bandwidth of 1.56 kHz. Normalized to 1 Hz bandwidth, the demonstrated performance would then correspond to a detectable optical density of 5.9 × 10−8. This detection limit agrees well with the calculated “quantum limited” performance based on the measured laser power, modulation index, noise figure of the electronic components, and other parameters of the apparatus. These measurements and calculations show that by implementation of the FM technique, the sensitivity of the present TDL absorption spectrometers (TDLAS) can be improved by at least a factor of 10 and possibly even of 100. Such a sensitivity improvement would greatly extend the applicability of TDLAS for trace gas analysis, especially in atmospheric monitoring.

Quantum noise-limited FM spectroscopy with a lead-salt diode laser

This paper reports the demonstration of quantum-noise-limited sensitivity in frequency modulation spectroscopy (FMS) with a lead-salt diode laser operating in the 4.8-microm region. Experimentally determined sensitivities of 10(-7) in a 1-Hz bandwidth have been demonstrated for detection of CO and CO(2) in a single-pass absorption cell. The high sensitivity obtained with our FMS apparatus can be attributed to the use of improved lead-salt diode lasers, a new optical fringe reduction method, and careful attention to optical processing of the diode laser beam.

Wideband noise characteristics of a lead-salt diode laser: possibility of quantum noise limited TDLAS performance

The wideband noise characteristics of a PbEuSe molecular beam epitaxy diode laser have been measured up to 500 MHz. The cutoff of the frequency dependent (1/f type) laser noise contribution was found to be 170 MHz for this particular laser. Above this cutoff frequency the photon shot noise dominates, as was demonstrated. A noise reduction of more than 2 orders of magnitude was observed in the shot noise limited domain when compared with the 1/f noise dominated region below 1 MHz. This finding indicates that a similar 2 orders of magnitude sensitivity improvement can be achieved in tunable diode laser absorption spectroscopy when frequency modulation techniques are applied instead of the more conventional derivative modulation below 1 MHz.

Vibrational relaxation of the 2ν2 level of NH3

Lead salt diode lasers are used in conjuction with a Q-switched CO2 laser to measure collisional energy transfer from the 2ν2 level of NH3. The (V–T,R) relaxation rate from the 2ν2 level is found to be approximately an order of magnitude faster than the comparable relaxation rate from the ν2 level. (V–V) transfer from the ν2 to the 2ν2 levels plays an insignificant role in the dynamics of energy transfer in the ν2 mode of NH3.