Sensitive density-fluctuation measurements using wavelength-modulation spectroscopy with high-order-harmonic detection

We discuss experimental and theoretical results which show that when wavelength-modulation spectroscopy is used to monitor concentration fluctuations of gaseous species, greater sensitivity may be obtained if one uses high-order detection. We also show that, depending on the ambient concentration being monitored, there are regions in which the commonly used second derivative would show a negligible variation of signal magnitude with concentration fluctuations, whereas measurement with a higher harmonic would result in a much improved signal. Theoretical results for the measurements of any transition that can be described by the Voigt profile are given. The technique discussed is illustrated by presenting the results of measurements of wavelength-modulation spectroscopy of lines in the oxygenA band. Different detection harmonic orders are suitable for different ambient concentrations, and a related criterion that helps in the determination of a suitable detection harmonic order is given.     

Harmonic detection with tunable diode lasers —two-tone modulation

The minimum absorption detectable by a tunable diode laser spectrometer utilising harmonic techniques is often limited by interference fringes generated by scattered light. The sensitivity of the spectrometer to absorption can be increased by applying a jitter modulation. In this paper, the theory of harmonic response for single- and two-tone modulation over optical fringes and Lorentzian absorption lines is developed and compared to experimental measurements. A simple analytic expression for the two-tone harmonic line shape is derived. This expression provides a physical understanding of the effects of the second modulation, and a means to unravel the effects of the second modulation on the linewidth and line shape. For a specific choice of the jitter frequency and phase, it is possible to simultaneously minimise the fringe signal and increase the harmonic absorption signal. The results of this investigation are applicable to trace gas detection using tunable diode lasers, and to other areas of spectroscopy and magnetic resonance where harmonic techniques are used.This work was supported by the Natural Sciences and Engineering Research Council of Canada, and the Ontario Ministry of the Environment     

Picosecond spectroscopy of intermolecular interactions in dye solutions

Some new experimental data on the time development of ultrashort superfluorescence (SF) in the dye solution are presented. The correlation between temporal behaviour change and the change of the spectral and spatial intensity distribution gives direct evidence of the important role of the selffocusing phenomenon in the formation of the ultrashort SF pulse synchronized with the pumping one. The oriental relaxation of solvent molecules in the dye solution not only influences the duration of the dye SF via the selffocusing mechanism but also causes a temporal change of spontaneous emission spectrum. At least two different relaxation times were found in the fluorescence spectrum of the dye in several polar solutions.     

Sub-Doppler excited molecule energy transfer spectroscopy

A sub-Doppler spectroscopic technique involving the detection of vibrational energy transfer in excited molecules with selected velocity components at a microphone diaphragm is demonstrated. The technique is applied to the investigation of thev ₃ R(16) transition in HCN and the results compared with a previously derived theoretical model.     

Detection of methane in air using diode-laser pumped difference-frequency generation near 3.2 μm

Spectroscopic detection of the methane in natural air using an 800 nm diode laser and a diode-pumped 1064 nm Nd:YAG laser to produce tunable light near 3.2 µm is reported. The lasers were pump sources for ring-cavity-enhanced tunable difference-frequency mixing in AgGaS₂. IR frequency tuning between 3076 and 3183 cm^–1 was performed by crystal rotation and tuning of the extended-cavity diode laser. Feedback stabilization of the IR power reduced intensity noise below the detector noise level. Direct absorption and wavelength-modulation (2f) spectroscopy of the methane in natural air at 10.7 kPa (80 torr) were performed in a 1 m single-pass cell with 1 µW probe power. Methane has also been detected using a 3.2 µm confocal build-up cavity in conjunction with an intracavity absorption cell. The best methane detection limit observed was 12 ppb m (Hz.)^–1/2.     

Application of pressure-tuned diode lasers to high-resolution spectroscopy

The application of the high-pressure chamber with a pressure transmitting medium (liquid at room temperature) to diode-laser tuning was found to be a useful method for high-resolution spectroscopy. Ammonia absorbtion spectra in the 10 μm region were recorded with a resolution approaching the Doppler limit.     

Polarization selective optical heterodyne detection for dramatically improved sensitivity in laser spectroscopy

The noise characteristic of available laser sources limits the sensitivity of many types of nonlinear spectroscopy. We show how to maximize the sensitivity by optimizing the strength of a local oscillator wave in a heterodyne detection scheme without altering the amplitude of the wave being detected. The intensity profile of the optimum local oscillator closely matches that of the incident probe wave, but the optimum intensity is much less than that of the probe under realistic conditions. A general signal-to-noise analysis applicable to all nonlinear spectroscopy techniques is presented along with specific applications to coherent Raman spectroscopy, two-photon absorption, saturation spectroscopy, and optical coherent transient techniques. A simple optimization procedure employing polarization selection rules is described. Detailed calculations are performed for the case of TEM₀₀ waves interacting via a third-order nonlinear susceptibility and for the case where the sample is simultaneously probed at many different frequency combinations. Supported by the National Science Foundation and Office of Naval Research.     

Dependence of harmonic signals on sample-gas parameters in wavelength-modulation spectroscopy for precise absorption measurements

4 and N₂O, are given. Received: 4 March 1998/Revised version: 27 July 1998     

Amplitude modulation of the cw laser light in the TEM mn mode by means of a mechanical chopper

Amplitude modulation of the laser beam in a TEM _mn mode by means of a mechanical chopper is investigated on the basis of the chopping model represented by a moving system of infinitely long, parallel slots and mark spaces. The cases of rectangular and axial symmetry of the laser beam are both treated. The explicit expressions for the waveform of the modulated normalized transmitted laser power are deduced and their consequences investigated. It is found that in the case of rectangular symmetry, unlike the case of axial symmetry, the TEM _mn modes give, for a constant value of the mode numbern and for any value ofm, the same time dependence of the amplitude modulated laser power. The notion of the equivalent modulation widths is introduced and conditions for the efficient amplitude modulation are found.     

Conditions for the harmonic-like and efficient amplitude modulation of the cw gaussian laser beam by means of a mechanical chopper

The values of the chopping disc slot and mark space widths relative to the radius of the Gaussian beam are found, which achieve optimum harmonic-like and efficient amplitude modulation of the cw laser beam. The simple approximation for the waveform of the modulated laser power valid for these optimum or near optimum values of the chopping disc parameters is presented.     

Diode laser spectrometer for high-resolution spectroscopy in the visible range

A high-resolution diode laser spectrometer operating at 657 nm is described. To achieve a narrow linewidth and a high power, a master-slave laser system is employed. The master laser is an extended cavity diode laser whose linewidth is reduced to less than 100 Hz by the FM sideband technique. The slave laser is an AR-coated diode laser and characteristics of injection locking are experimentally studied. The injection current of the slave laser is utilized to stabilize the output power or to produce pulsed output. Using this spectrometer, we probed the intercombination line of Ca and observed high-contrast optical Ramsey fringes with a linewidth of 10 kHz. A velocity-selective Ramsey fringe is also observed in the pulse-mode operation.     

Precise measurement of methane in air using diode-pumped 3.4-μm difference-frequency generation in PPLN

3 volume multi-pass absorption cell with an 18-m path length. The methane mixing ratio was determined by comparing the direct optical absorption measured in the sample with that measured in a reference gas at 100 torr and room temperature. Relative accuracy of better than 1 ppb (parts in 10⁹, by mole fraction) was achieved in measurements of natural air that contained 1700–1900 ppb methane. The typical measurement time for each sample was 60 seconds. The accuracy was limited by residual interference fringes in the multi-pass cell that resulted from scattering. Without the use of reference samples, the relative accuracy was 20 ppb; it was limited by the long-term reproducibility of the spectroscopic baseline, which was affected by drift in the optical alignment coupled to changes in the ambient temperature. This work demonstrates the use of diode-pumped difference-frequency generation (DFG) in PPLN in a high-precision infrared spectrometer. Compact, room-temperature solid-state gas sensors can be built based on this technology, for accurate real-time measurements of trace gases in the 3–5 μm spectroscopic region. Received: 2 October 1996     

Study of relaxation of molecular iodine by the method of saturated absorption spectroscopy

The paper describes detailed experimental studies of the width, shape and shifts of saturated absorption resonances at the X(v=0, j=13)B(v=43, j=12) transition of molecular iodine. Use of the geometry of oppositely travelling and unidirectional waves in the experiment and study of the luminescence decay under pulsed excitation allowed estimation of the cross-sections of elastic and inelastic scattering of I₂ molecules on the upper and lower levels of transition. The observed nonlinear dependence of width and shift of the saturation resonance on iodine pressure is explained by the influence of elastic collisions with no phase mismatch.     

Direct observation of excimer-laser photoablation products from polymers by picosecond-uv-laser mass spectroscopy

A bistable device driven by the incident light polarization is created on the basis of a phase electro-optical modulator from LiNbO₃. The light transmitted through the device does not vary its intensity. The switching takes place between states with different polarization. The device action at different parameters is experimentally investigated.     

Second-harmonic detection with tunable diode lasers — Comparison of experiment and theory

A series of experiments are carried out by current modulating a tunable diode laser, and slowly ramping the wavelength to scan weak absorption lines in gases at pressures ranging from 2 to 60 Torr. A lock-in amplifier detects the second harmonic (2f) of the modulation frequency, and the experimental 2f signals are compared with theory. Detailed measurements are made on Lorentzian, Voigt, and Gaussian line profiles, over a wide range of modulation amplitudes. Excellent agreement between experiment and calculation is obtained in all cases. This quantitative understanding enables one to derive true lineshapes and linewidths of very weak absorption lines from measurements of 2f lineshapes only. Results are applicable to trace gas detection using tunable diode lasers, and to other areas of spectroscopy and magnetic resonance where harmonic detection techniques are routinely employed to monitor weak signals.This work was supported in part by the National Science and Engineering Research Council, Canada     

Effects of modulation on signals obtained in saturated absorption spectroscopy experiments

An analysis of the useful signals detected in saturated absorption spectroscopy experiments is given. Both Gaussian and plane-wave fields are considered. The various cases of unmodulated, square-wave modulated and sinusoidally modulated saturating beams are examined. It is found that square-wave and sinusoidal modulations give different results. The first is identical to no modulation, and detection at the fundamental frequency records a fraction of the total signal which is also a square wave. The second gives fundamental or higher-harmonic signals each with a very different dependence on the saturation parameter. The signal at the fundamental saturates faster and more completely than that for square-wave modulation. The results suggest experimental methods for the determination of the absolute value of the saturation parameter.     

Difference-frequency laser spectroscopy detection of acetylene trace constituent

5 sbandpectrum. Spectroscopic detection of 28 ppmm/Hz^1/2 was performed. This corresponds to a minimum detectable mixing ratio of ∼1 ppb in conjunction with a 100-m White cell and balanced detection. Received: 3 April 1998/Revised version: 5 June 1998     

Test of an optical parametric oscillator (OPO) as a compact and fast tunable stokes source in coherent anti-stokes raman spectroscopy (CARS)

An optical parametric oscillator (OPO), as a novel kind of broadband Stokes source, is employed for coherent anti-Stokes Raman spectroscopy (CARS). Compared to the conventional dye laser configuration OPO-CARS offers practical advantages. The tunable OPO allows a fast and comfortable frequency tuning. The excitation bandwidth of about 35 cm^–1 (FWHM) limits the spectral range of effective and stable single pulse CARS generation but can be used to enhance selected spectral structures.     

Improved resolution in VUV isochromat spectroscopy

The energy resolution of a VUV isochromat spectrometer employing the traditional energy selective Geiger counter can be significantly improved. The variance of the optical resolution function with the usual CaF₂ entrance window is (240 meV)². With SrF₂ we obtain (113 meV)² at room temperature and (73 meV)² at 70°C. A direct confirmation of these data, which were derived from a moment analysis of the threshold behaviour of an Au isochromat is provided by a measurement of image potential states at Cu(001).     

Superhigh resolution spectroscopy in methane with cold molecules

Use of optical selection of cold particles in saturated absorption spectroscopy is discussed. Owing to this we observed directly the splitting of a methane absorption line (=3.39 m) due to recoil effect in a light beam of 0.5 cm in diameter. The obtained width of nonlinear resonance of about 1 kHz corresponds to an effective temperature of particles of about 10^–1 K. The new direct absolute frequency measurement of the central hyperfine component of theF ₂ ⁽²⁾ line in methane (7–6 transition) made under these conditions gave the following value 88376181600.7±0.5 kHz. The behaviour of the resonance intensity of saturated absorption in the transit-time conditions is analysed. It has been shown that with an absorption cell inside a cavity the saturation resonance intensity in the transit-time region increases sharply.