Trace gas detection by laser intracavity photothermal spectroscopy

A novel laser intracavity photothermal detector is described. In this scheme, sample absorption of the pump laser power takes place within the cavity of a probe He-Ne laser causing modulation in the gain and in turn the output power. Comparison of this intracavity detector with two other photothermal techniques, namely, phase fluctuation optical heterodyne spectroscopy and thermal beam deflection, is made in terms of practicality and sensitivity. For in situ measurements, sensitivity of 0.5 x 10(-7) cm(-1) for a probe length of 3 cm has been achieved.     

Measurement of absolute minority species concentration and temperature in a flame by the photothermal deflection spectroscopy technique

It is experimentally demonstrated that absolute concentrations of minority species in flames can be measured by the photothermal deflection spectroscopy (PTDS) technique. In addition, the PTDS signal simultaneously yields the flame temperature the measurement point. Absolute concentration profiles of OH have been measured in a flat-flame burner with methane as fuel. The PTDS measurements agree well with those obtained independently by the absorption technique. The flame temperature measurements by PTDS are also in good agreement with those obtained by the Boltzmann distribution among the rotational levels of OH.     

Measurement of nonlinear absorption coefficients of organic materials by mode-mismatched Z-scan thermal lensing technique

We evaluate a new pump-probe mode-mismatched thermal lens (TL) scheme for the measurement of nonlinear absorption in nitrobenzene, benzene, and chloroform. In this new scheme the pump beam is focused in the presence of a collimated probe beam. Values of the nonlinear absorption coefficients of the materials studied for the wavelength of 532 nm are reported, and we compare the proposed technique with the well-known open Z-scan method.     

High-resolution absorption measurements by use of two-tone frequency-modulation spectroscopy with diode lasers

The capability of two-tone frequency-modulation spectroscopy (TTFMS) in deriving spectral line-shape information was investigated. Two oxygen A-band transitions at 760 nm were selected, and the Voigt profile and two different collisionally narrowed line profiles were employed in their analysis. By means of a least-squares fitting procedure, we obtained accurate information regarding transition strengths and pressure-induced broadening, shift, and narrowing coefficients. Both TTFMS and direct absorption line shapes were modeled with deviations as small as 0.3% over a wide pressure range by use of the collisionally narrowed line profiles. Line parameters measured with TTFMS showed excellent agreement with the parameters measured with direct absorption. The experimental technique used constant-current fast-wavelength scanning, which improved measurement accuracy.     

Doppler-free nonlinear absorption in ethylene by use of continuous-wave cavity ringdown spectroscopy

We report what we believe to be the first systematic study of Doppler-free, nonlinear absorption by use of cavity ringdown spectroscopy. We have developed a variant of cavity ringdown spectroscopy for the mid-infrared region between 9 and 11 microm, exploiting the intracavity power buildup that is possible with continuous-wave lasers. The infrared source consists of a continuous-wave CO2 laser with 1-mW tunable infrared sidebands that couple into a high-finesse stable resonator. We tune the sideband frequencies to observe a saturated, Doppler-free Lamb dip in the nu7, 11(1,10) <-- 11(2,10) rovibrational transition of ethylene (C2H4). Power studies of the Lamb dip are presented to examine the intracavity effects of saturation on the Lamb-dip linewidth, the peak depth, and the broadband absorption.     

Investigation of interferometric noise in fiber-optic gas sensors with use of wavelength modulation spectroscopy

We report on interferometric noise limitation of fiber-optic gas sensors with highly coherent lasers and wavelength modulation spectroscopy. Interference between signal wave and reflected waves causes signal fluctuation in the output, which limits the performance of the sensing system. Sensor resolution limited by interferometric noise is calculated for a fiber-optic gas sensor with the Q(6) absorption line of methane gas at approximately 1650 nm. The results are useful for system designers of this particular type of gas sensor.     

Optical properties of CdTe thin films studied by photothermal deflection spectroscopy

Photothermal deflection spectroscopy (PDS) was used to analyze CdTe thin films which are to be used to construct solar cells. It was found that CdTe thin films grown on a conducting SnO₂ layer deposited on glass show deviations from a stoichiometric composition due to an excess of tellurium. This non-stoichiometry modifies the absorption spectrum obtained from the PDS measurements.     

Measurement of interferometric autocorrelations: comment

Conventional Michelson interferometers that are widely used for autocorrelation of ultrashort optical pulses are incapable of producing two identical replicas of an optical pulse, hence yielding a correct autocorrelation signal. The problem is acute when one records the interferometric autocorrelation of pulses consisting of just a few optical cycles. With the use of sub-10-fs pulses we demonstrate asymmetric autocorrelation traces recorded with a conventional Michelson interferometer, refer to previously reported results that exhibit the same deficiency, and elucidate the origin of the problem. A simple modification of the conventional interferometer configuration yields the correct fringe-resolved autocorrelation trace.     

Determination of thermal diffusivity of a solid surface by photothermal deflection spectroscopy

The Photothermal Deflection Spectroscopy (PDS) method is applied to the measurement of thermal diffusivity in solid surfaces. Thermal diffusivity of Al₂O₃ and γ- LiAlO₂ ceramic samples at different porosities were measured at room temperature. The values so obtained are compared to those found in the literature.     

Estimation of the absorption and the scattering coefficients of natural waters by use of underwater irradiance measurements

The graphic presentation of relationships between a, b, R, and μˉ in a previously published nomogram on calculation of inherent optical properties [Aust. J. Mar. Freshwater Res. 32, 533 (1981)] are replaced by explicit mathematical expressions.     

Measurement and calibration of interferometric imaging aberrations

Phase-shifting interferometry is the standard method for testing figure error on optical surfaces. Instruments measuring spheres and flats are readily available, but the accurate measurement of aspheres requires null correction. One problem with the general (nonull) testing of aspheres is the loss of common path. Systematic errors are introduced into the measurement by the fringe imaging optics. The sources and types of error are reviewed, as well as their effect on a wave-front measurement. These nonnull errors are predicted generally, with third-order analytic expressions derived for a tilted or a defocused test surface. An interferometer is built to test the expressions. The imaging system is a single lens, nominally image telecentric. Measurements are performed on a test surface defocused from -5 to 5 mm. The resulting measurement bias is shown to be in good agreement with third-order aberration theory predictions.     

Sensitive absorption spectroscopy by use of an asymmetric multiple-quantum-well diode laser in an external cavity

We have developed a broadly tunable diode laser system by employing custom-designed asymmetric multiple-quantum-well (AMQW) InGaAsP lasers in an external cavity configuration. Feedback is provided by a diffractive optical element with high coupling efficiency and wavelength selectivity, allowing for single-mode tuning of the output wavelength by varying the external cavity length. This tunable laser system was used experimentally to perform absorption spectroscopy on weak CO(2) lines over a broad wavelength region in the near infrared. An experimental tuning range of 80 nm has been observed for a laser cavity length of 600 mum, which is double the tuning range found with conventional, uncoated quantum-well lasers. We achieved a detection sensitivity of 5 x 10(-6) at 95% confidence over the wavelength range of 1.54-1.62 mum by employing a second-harmonic detection technique. The theoretical predictions of a broad gain profile from an ambipolar rate equation model are found to correspond to the experimentally observed increased tunability of the uncoated AMQW lasers.     

Measurements of collisional broadening coefficients by infrared polarization spectroscopy

We present measurements of collisional broadening coefficients, obtained at atmospheric pressure, by polarization spectroscopy. Using tunable single mode laser radiation at approximately 2 microm, high-resolution infrared polarization spectra were recorded for CO2-Ar and CO2-He binary mixtures. The recorded polarization spectra were fitted with a Lorentzian cubed function form to obtain the broadening coefficients. The full-width at half-maxima (FWHM) collisional broadening rates of CO2 by Ar and He, for the R14 (12 degrees1<--00 degrees0) line, have been determined to be 0.161+/-0.018 cm-1 atm-1 and 0.1823+/-0.0032 cm-1 atm-1, respectively.     

Measurement of acoustic reflection coefficients by an ultrasonicmicrospectrometer

An ultrasonic microspectrometer (UMSM) was developed in order to evaluate the elastic properties of a solid specimen at a small spot on its surface. In this system, spherical-planar-pair (SPP) lenses were used, by which the acoustic reflection coefficient of a liquid/solid interface was measured as a function of the incident angle in the frequency range from 20 to 140 MHz. Using a specimen of fused quartz whose material constants were well known, the measurement accuracy was examined. The phase velocity of a leaky Rayleigh wave was obtained from the phase change of the reflection coefficient with 0.4% accuracy in this frequency range. For a specimen of steel with a large acoustic attenuation, bulk attenuation factors and their frequency dependence were successfully estimated by computer-fitting of the reflection coefficient. As an example of anisotropic materials, the reflection coefficient of X-cut quartz was also measured. Measured phase of the reflection coefficient was in good agreement with numerical calculation     

An approach to the infrared study of materials by photothermal beam deflection spectroscopy

Infrared Fourier transform photothermal beam deflection spectroscopy (PBDS) is briefly outlined. It involves heating a surface by the photothermal effect, using infrared (IR) radiation from a scanning interferometer; energy is transferred to the gas above the surface and the resulting refractive index gradient in the gas is detected by observing the deflection of a laser beam grazing the solid's surface; the deflection is related to the IR radiation which is absorbed, so that the Fourier transform of the deflection signal results in the IR spectrum. PBDS is especially useful for materials which scatter and/or absorb IR radiation so extensively that conventional transmission/absorption techniques are not possible. A variety of examples of applications are given, including spectra of polymers, floor tile, polyester thread and fabric, leather, cellulose, paper, lichen and bone.     

Measurement of tissue optical properties by the use of oblique-incidence optical fiber reflectometry

Fiber-optic-based oblique-incidence reflectometry is a simple and accurate method for measuring the absorption and reduced scattering coefficients mu(a) and mu?(s) of semi-infinite turbid media. Obliquely incident light produces a spatial distribution of diffuse reflectance that is not centered about the point of light entry. The amount of shift in the center of diffuse reflectance is directly related to the medium's diffusion length D. We developed a fiber-optic probe to deliver light obliquely and sample the relative profile of diffuse reflectance. Measurement in absolute units is not necessary. From the profile, it was possible to measure D, perform a curve fit for the effective attenuation coefficient mu(eff), and then calculate mu(a) and mu?(s). This method was verified with Monte Carlo simulations and tested on tissue phantoms. Our measurements of D and mu(eff) had an accuracy of approximately 5%, thus giving us 10% and 5% accuracy for mu(a) and mu?(s), respectively.     

Theory of photothermal spectroscopy in an optically dense fluid

The theory of photothermal spectroscopy in an optically dense fluid is presented. The general case is considered in which the fluid may be flowing or stationary, and the excitation could be cw (modulated or unmodulated) or pulsed (arbitrary pulse length). All three detection schemes, deflection, phase shift, and lensing, are considered. This is the most complete theory of photothermal spectroscopy in fluids to date.