Comparison of approaches to modulation spectroscopy with GaAIAs semiconductor lasers: application to water vapor

Background signals in wavelength-modulation spectrometry with frequency-doubled diode-laser light. I. Theory

Various types of background signals appear when wavelength-modulated (WM) diode-laser light is frequency doubled. We present a theoretical analysis of such background signals in terms of a previously derived formalism for WM spectrometry that is based on a Fourier series. Explicit expressions for various nf harmonics of the background signals are derived. The analysis shows that 2f detection will be plagued by significant background signals when frequency-doubled WM diode-laser light is used. It also demonstrates that 4f and 6f detection will experience background signals but not, however, to the same extent as 2f detection. The analysis illustrates clearly how the various nf harmonics of the background signals depend on entities such as modulation amplitude, associated intensity modulation, dispersion of the frequency-doubling material, laser power, and detuning. The background signals can take both positive and negative values, depending on the relation between these entities. Guidelines for how to minimize these background signals are given.

Characterization of background signals in wavelength-modulation spectrometry in terms of a fourier based theoretical formalism

The detectability of wavelength-modulation (WM) diode-laser spectrometric techniques is frequently limited by various background signals. A new theoretical formalism for WM spectrometry, based on Fourier analysis and therefore capable of handling a variety of phenomena including the characterization and the analysis of analytical as well as background WM signals, was recently presented [Appl. Opt. 38, 5803 (1999)]. We report a detailed characterization of WM background signals from multiple reflections between pairs of surfaces in the optical system that act as etalons and from the associated intensity modulation in terms of this new formalism. The agreement between the background signals from a thin glass plate and those predicted by the formalism is good, which verifies the new Fourier analysis-based formalism.

Two-tone frequency-modulation spectroscopy for quantitative measurements of gaseous species: theoretical, numerical, and experimental investigation of line shapes

The capability of retrieving spectral information from line shapes recorded by two-tone frequency-modulation spectroscopy (TTFMS) is investigated. A TTFMS theory accounting for dispersion and nonlinear distortion of diode laser frequency modulation response is presented. The adequacy of the theory for a detailed modeling of line shapes recorded with high resolution is examined. An extensive error analysis of line parameters (i.e., width, intensity, and line center) retrieved by a nonlinear least-squares fitting procedure is made. Plots of residual errors with characteristic signatures that are due to incorrectly assigned modulation parameters and choice of line profile are presented. In least-squares fits to experimental oxygen data with a Voigt profile influence from collisional(Dicke) narrowing is clearly exhibited, and when we used a collisionally narrowed line profile deviations of the model were reduced to less than 0.2%. We demonstrate that accurate quantitative measurements by TTFMS over a wide range of concentrations, temperatures, and pressures are possible.

Theoretical description of frequency modulation and wavelength modulation spectroscopy

We present a comprehensive theory for heterodyne absorption spectroscopy with phase-modulated light. The general equations presented allow for an arbitrary modulation index and an arbitrary modulation frequency. We use this description for three purposes: First, we review the special cases of so-called frequency modulation and wavelength modulation spectroscopy. Second, we present the additional case of large-index, high-frequency modulation. Third, we present an overview of how the absorption signal depends on the experimental parameters of modulation frequency and modulation index. This overview may be helpful to experimentalists in choosing these parameters, for it provides a systematic understanding of how moving around in parameter space changes certain features of the signal, while leaving other features invariant.

Tomographic imaging of fluid flows by the use of two-tone frequency-modulation spectroscopy

Diode laser absorption measurements obtained with two-tone frequency-modulation spectroscopy (TTFMS) are combined with tomography to produce spatially resolved quantitative images of fluid flows. The high sensitivity, good accuracy, and high stability of TTFMS absorption measurements permit optical-absorption tomography to be performed with low noise on extremely weakly absorbing objects. The method is demonstrated on a small oxygen flow with a GaAlAs diode laser operating near 760 nm and with an absorption sensitivity of 1:10(6).

Laser diode wavelength-modulation spectroscopy for simultaneous measurement of temperature, pressure, and velocity in shock-heated oxygen flows

Wavelength modulation at 10 MHz of an AlGaAs laser diode, superposed on repetitive linear scans of wavelength, is applied to measure second-harmonic absorption line shapes of oxygen in the A band. Theoretical expressions of the harmonic line shapes, including the effect of laser amplitude modulation and varying modulation depth, are presented. A least-squares fit of the experimental line shapes to theoretical second-harmonic line shapes permits simultaneous determination of the temperature and the pressure. The use of high-repetition-rate (10-kHz) linear scans of the studied wavelength region permits application of the technique to high-speed unidimensional transient flows generated in a shock tube; velocity is derived from the Doppler shift of the absorption profiles.

Long-atmospheric-path measurements of near-visible absorption lines of O(2) isotopes and H(2)O with a prototype AlGaAs laser transceiver system

Near-visible absorption lines of ambient H(2)O vapor and normal and heavy isotopes of O(2) have been measured over atmospheric paths of up to 0.46 km by using two wavelength-modulated, line-locked AlGaAs laser sources with a retroreflector-telescope system. The absolute signal levels agree with theoretical calculations for the O(2) isotopes to within 2%, which is similar to the accuracy with which the column densities were known. Measurements of (16)O(2) linewidths and line strengths were made, and they agree with literature values to within experimental error. The detection sensitivity for (16)O(18)O was found to be 0.1 part in 10(6) atm. km, correspondingto an absorbance sensitivity of 1 × 10(-5). It is concluded that atmospheric trace-gas sensing will be feasible with this apparatus over distances of several kilometers and at levels under 1 part in 10(6).

Sensitive detection of acetylene absorption in the visible by using a stabilized AlGaAs diode laser

Overtone transitions of C(2)H(2) at ~ 789 nm are investigated by means of an AlGaAs laser operating in an external optical cavity configuration. Relative amplitude noise is of a few parts in 10(6) (ppm) and permits an absorption detection limit of 0.2 ppm/km. Self-broadening and air broadening is measured for two components of the observed band [P(11) at 12646.966 and R(5) at 12688.699 cm(-1)].

Measurement of molecular concentrations and line parameters using line-locked second harmonic spectroscopy with an AlGaAs diode laser

The technique of line-locked wavelength modulation with 2f detection is applied to the measurement of water vapor concentration and absorption line parameters by using an 820-nm AlGaAs communications diode laser. Measurements of the 2f signal as a function of the modulation amplitude yield accurate concentrations and line parameters over a pressure range of an order of magnitude and half-widths from 0.02 to 0.15 cm(-1). Usingtwo different spectral lines, we determined concentrations and line parameters with 1% precision, and the absolute accuracy of the line parameters is 3% or better. The results have been used to calculate calibration curves for a diode laser humidity monitor.

Frequency-modulation spectroscopy for trace species detection: theory and comparison among experimental methods

A variety of frequency-modulation methods for high-sensitivity absorption detection of gas-phase species has evolved in recent years. The distinctions among these methods are mostly semantic. The mathematical derivations for wavelength-modulation spectroscopy and one- and two-tone frequency-modulation spectroscopies are presented; a common terminology is used to permit a comprehensive comparison of predicted detection sensitivities. Applying this formalism, I compare the optimum detection sensitivities of these different methods for a typical laser system, using the same parameters. As long as residual amplitude modulation is minimized by proper adjustment of the detection phase angle, high-frequency wavelength modulation and one- and two-tone frequency-modulation methods all achieve approximately the same sensitivities. The choice among techniques is most strongly driven by the individual laser tuning characteristics, the absorption linewidth, and the detection bandwidth. It is shown that excess laser noise cannot always be excluded from consideration, even at megahertz detection frequencies. Also, detection at harmonics of the modulation or beat frequency may present certain advantages in minimizing residual amplitude-modulation noise.

Laser-absorption mass flux sensor for high-speed airflows

We present a laser-based concept for an air mass flux sensor based on the simultaneous measurement of density and velocity of O(2). The sensor uses wavelength modulation absorption spectroscopy at 10 MHz of an AlGaAs laser diode in the b(1)Sigma(g)(+)(v' = 0)-X(3)Sigma(g)(-)(v'' = 0) system of O(2), combined with second-harmonic detection. The apparatus is demonstrated on a high-speed oxygen flow generated behind the incident shock wave in a shock tube. The measurement time is 0.5 ms for typical absorption levels of 0.5%.

Detection of oxygen using short external cavity GaAs semiconductor diode lasers

High sensitivity detection of O(2) in the near infrared with short external cavity (SXC) AlGaAs semiconductor diode lasers is reported. The mode control provided by the SXC enhances the performance of these diode lasers by allowing a number of single laser modes (up to ten) to be individually selected and continuously scanned over extended frequency intervals. At a constant laser heat sink temperature, this provides nearly complete spectral coverage of up to approximately 40 cm(-1) for overlapping modes. When using second harmonic detection techniques, these SXC controlled lasers were found to provide high sensitivity detection of O(2) at atmospheric pressure. A minimum detectable absorbance of approximately 4 x 10(-6) has been achieved with a SNR of 1. The stability of these laser systems has also allowed the continuous monitoring of an absorption signal over extended time intervals. Absorbances of 1 x 10(-2) have been continuously monitored for durations of up to 15 h with an rms uncertainty of +/-6 x 10(-5). These results are reproducible for measurements made on all the laser modes that could be selected with the SXC.

Tunable-diode-laser frequency-modulation spectroscopy using balanced homodyne detection

Quantum-limited frequency-modulation spectroscopy (FMS) has been performed with tunable diode lasers at 5.4 and 1.3 microm by employing a dual-beam balanced homodyne detection scheme to suppress optical fringes, residual amplitude modulation, and excess laser noise by 15-20 dB. Performing FMS at the two wavelengths involved different lasers (lead salt and InGaAsP, respectively), optics, and detectors, which emphasizes the versatility and applicability of the technique. System sensitivities of 1 x 10(-7) and 2 x 10(-7) were obtained for the respective systems.

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.