Laser intracavity absorption spectroscopy

Emission spectra of multimode lasers are very sensitive to spectrally selective extinction in their cavity. This phenomenon allows the quantitative measurement of absorption. The sensitivity of measurements of intracavity absorption grows with the laser pulse duration. The ultimate sensitivity obtained with a cw laser is set by various perturbations of the light coherence, such as quantum noise, Rayleigh scattering, four-wave mixing by population pulsations, and stimulated Brillouin scattering. It depends on the particular laser type used, and on its operative parameters, for example pump power, cavity loss, cavity length, and length of the gain medium. Nonlinear mode-coupling dominates the dynamics of lasers that feature a thin gain medium, such as dye lasers, whereas Rayleigh scattering is more important in lasers with a long gain medium, such as doped fibre lasers, or the Ti:sapphire laser. The highest sensitivity so far has been obtained with a cw dye laser. It corresponds to 70000 km effective length of the absorption path. The ultimate spectral resolution is determined by the spectral width of mode emission, which is 0.7 Hz in this dye laser. High sensitivity and high temporal and spectral resolution allow various practical applications of laser intracavity spectroscopy, such as measurements and simulations of atmospheric absorption, molecular and atomic spectroscopy, process control, isotope separation, study of free radicals and chemical reactions, combustion diagnostics, spectroscopy of excited states and nonlinear processes, measurements of gain and of spectrally narrow light emission. Intracavity absorption in single-mode lasers shows enhanced sensitivity as well, although not as high as in multimode lasers. Received: 10 May 1999 / Published online: 29 July 1999     

High-sensitivity intracavity laser absorption spectroscopy with vertical-external-cavity surface-emitting semiconductor lasers

We report the demonstration of high-sensitivity intracavity laser absorption spectroscopy with multiple-quantum-well vertical-external-cavity surface-emitting semiconductor lasers (VECSEL's). A detection limit of 3x10(-10) cm (-1) has been achieved. The spectrotemporal dynamics of a VECSEL in the 1030-nm wavelength region has been studied. The laser was operating cw at room temperature, with a baseline signal-to-noise ratio as high as 400. The laser was optically pumped with a threshold as low as 80 mW and was broadly tunable over a spectral range of ~75 nm .     

Sensitivity calibration in intracavity laser spectroscopy

The possibility of sensitivity calibration in intracavity laser spectroscopy is investigated as applied to the measurement of the absorption k _abs and gain k _gain coefficients from lines with known k _abs and k _gain. The lines of a YAG:Nd³⁺ crystal placed inside the cavity of a basic Nd:glass laser were used as absorption and gain lines.     

Quantum noise in intracavity laser spectroscopy

The limitations on the intracavity laser spectroscopy (ILS) method are considered which are caused by the statistical character of the multimode laser radiation. Stochastic processes (shot photon noise) are shown to result in a heavily “jagged” function, instead of a smooth one, which describes a specific realization of the generation spectrum. The increase of fluctuations with time between two successive measurements of the spectrum has also been investigated. The threshold sensitivity for the ILS method has been found which is determined by the stochastic character of the multimode generation.     

Parasitic selection in intracavity laser detection spectroscopy

The origin of the parasitic structure in the spectra of multimode dye lasers in the presence of additional optical surfaces in the cavity has been investigated. Methods are proposed to diminish the influence of parasitic structures in real experiments on the results of measurements by intracavity laser spectroscopy when studying low absorption with coefficients down to ～10^-9cm^-1.     

Fiber laser intracavity absorption spectroscopy for in situ multicomponent gas analysis in the atmosphere and combustion environments

Intracavity absorption spectroscopy with a broadband Er³⁺-doped fiber laser is applied for the measurements of several molecular species revealing quantitative information about the gas concentration, temperature and chemical reactions in flames. The spectral range of measurements extends from 6200 cm⁻¹ to 6550 cm⁻¹ with the proper choice of the fiber length and by moving an intracavity lens. With a pulsed laser applied in this experiment, the sensitivity to absorption corresponds to an effective absorption path length of 3 km assuming the cavity is completely filled with the sample. For a cw laser, the effective absorption path length is estimated to be 50 km. Absorption spectra of various molecules such as CO₂, CO, H₂O, H₂S, C₂H₂ and OH were recorded separately in the cell and/or in low-pressure methane and propane flames. The presented measurements demonstrate simultaneous in situ detection of three molecular products of chemical reactions at different flame locations. Variation of the relative strengths of OH absorption lines with the temperature enables the estimation of the local flame temperature. The sensitivity of this laser does not depend on the broadband cavity losses and it can be used for in situ measurements of absorption spectra in hostile environments such as contaminated samples, flames or combustion engines. The presented technique can be applied for various diagnostic purposes, such as in environmental, combustion and plasma research, in medicine and in the determination of stable isotope ratios.     

The optical parametric oscillator as a means for intracavity absorption spectroscopy

In view of the recent progress in the detection of narrow and weak absorption lines as provided by the intracavity technique which utilizes lasers with broad homogeneous emission lines, it appears to interesting to investigate also other light sources with regard to their suitability for intracavity spectroscopic measurements. A promising candidate is an optical parametric oscillator (OPO) pumped by a one-mode field and oscillating in several signal and idler modes, since, as will be shown, this light source exhibits a strong competition effect which, as known from the laser, gives rise to a drastic enhancement in detection sensitivity. In this paper, we calculate the enhancement factor utilizing a simple OPO model which takes account of the spontaneous production of signal and idler photons (parametric fluorescence). The result is similar to that for a laser operated such that spontaneous emission is the only mechanism which prevents a mode with higher losses to be extinguished. Moreover, the OPO offers a possibility to perform the measurement at a frequency which is complementary (with respect to the pump frequency) to the frequency of the absorption line to be detected.     

Temperature measurements in flames using water molecule overtone . spectra detected by intracavity laser absorption spectroscopy

Received: 14 June 1996 / Revised version: 6 September 1996     

Line shape studies in CW dye laser intracavity absorption

The line shape of the signals observed by intracavity absorption in an atomic beam of barium is studied as a function of absorber density. Complex structure is observed consisting of both absorption and enhancement features. Comparison is made with models of intracavity absorption, and it is concluded that the rate equation model in its present form does not explain the structure. On the other hand the super-regen model does seem able to partially account for the observed structure. The complexity of the line shape will directly affect those workers who are using intracavity absorption as a spectroscopic technique.     

An alternate model of CW dye laser intracavity absorption

A new model is presented for intracavity absorption in a CW multimode dye laser based on the similarity between the laser and a self-quenched super- regenerative oscillator. The model suggest that the primary interaction mechanism between the atomic absorber and the laser is through a modification of the background radiation level from which the oscillation builds up. Numerical calculations are carried out leading to a prediction of the spectral line shape to be observed in intracavity absorption as well as the dependence of the signal on absorber density and pump power.     

IR diode laser absorption spectroscopy in an no seeded molecular beam

Diode laser spectroscopy of a molecular beam containing 5–10% NO in He at pressures < 1 bar showed that the rotational temperature of the NO in the beam could be measured in an ordinary molecular beam apparatus using absorption spectroscopy. Molecular beams used for various purposes, such as surface scattering, can thus be characterized without excessive complication of the experimental set-up. The sensitivity of the method is good enough to allow characterization of low intensity beams, e.g. radical beams.     

The temporal development of intracavity absorption in a dye laser

Intercavity absorptions is known to lead to an enormous enhancement in the detectability of absorbing species. Here the time development of this enhancement of this enhancement is investigated in order to fix a typical time scale such measurements. This is done with a Pockles cell in front and inside a flashlamp pumped dye laser resonator. The enhancement of absorption steeply increases at laser threshold and then levels off during the letter part of the laser pulse. By Q-switching a dye laser it is demonstrated that on a time resolution of 20 nsec extinctions as low as 10^-4 can be measured with an enhancement of two orders of magnitude.     

Specific features of complete-absorption measurement in selective intracavity laser spectroscopy

For multimode pulsed dye lasers (of nanosecond and microsecond duration with frequency-dependent losses in the cavity), experimental studies and theoretical calculations have been made of the dependence of the value of the equivalent line width, used as a measure of selective absorption, on the main parameters characterizing the laser, the cavity, and the absorbent. Analytical relations have been obtained which describe the main aspects of complete absorption in lines with dispersion, Gaussian, or mixed spectra profiles, and their experimental testing has been carried out. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 3, pp. 327–332, May–June, 2000.