F. Stoeckel

Laser optical feedback tomography

We describe a new method for imaging in three-dimensional turbid media, laser optical feedback tomography. This technique is based on the resonant sensitivity of a short-cavity laser to frequency-shifted optical feedback from ballistic photons retrodiffused from the medium. The advantage of the method is that the detector is the laser source itself, which provides optical amplification with self-aligned spatial and temporal coherent detection.

Quantitative measurements of very weak H2O absorption lines by time resolved intracavity laser spectroscopy

Time resolved quasicontinuous intracavity dye laser absorption spectroscopy (ICLAS) experiments have been made without and with absorbing species inside the cavity. In the absence of an absorber, a near Gaussian shape with a half‐width decreasing as the square root of the time has been found for the spectrum. With atmospheric water vapor as the absorber, absorption lines in the 600 nm range appear on the spectrum, and the absorption intensity increases with time. This time evolution leads to a quantitative determination of the absorption coefficient of the lines. We confirm that ICLAS follows the Lambert–Beer law with an equivalent length of the cell given by Leq = ct (c is light velocity and t is the time between the laser turn on and the time of observation). This experiment result is in good agreement with time‐dependent theories.

Jet cooled NO2 intra cavity laser absorption spectroscopy (ICLAS) between 11200 and 16150 cm−1

We have combined the high sensitivity of the ICLAS technique with the rotational cooling effect of a slit jet expansion in order to observe and to understand the visible and near infrared NO2 spectrum. By this way, an equivalent absorption pathlength of several kilometers through rotationally cooled molecules has been achieved. Due to the vibronic interaction between the two lowest electronic states, X2A1 and A 2B2, this spectrum is vibronically dense and complex. Moreover, the dense room temperature rotational structure is perturbed by additional rovibronic interactions. In contrast, the rotational analysis of our jet cooled spectrum is straightforward. The NO2 absorption spectrum is vanishing to the IR but, owing to the high sensitivity of the ICLAS technique, we have been able to record the NO2 spectrum down to 11200 cm−1 with a new Ti:sapphire ICLAS spectrometer. As a result 249 2B2 vibronic bands have been observed (175 cold bands and 74 hot bands) in the 11200–16150 cm−1 energy range. Due to the cooling effect of the slit jet we have reduced the rotational temperature down to about 12 K and at this temperature the K = 0 subbands are dominant. Consequently, we have analysed only the K = 0 manifold for N ⩽ 7 of each vibronic band. The dynamical range of the band intensities is about one thousand. Due to the strong vibronic interaction between the X 2A1 and A 2B2 electronic states, we observed not only the a1 vibrational levels of the A 2B2 state but also the b2 vibrational levels of the X 2A1 state interacting with the previous ones. By comparison with the calculated density of states, we conclude that we have observed about 65% of the total number of 2B2 vibronic levels located in the studied range. However, there are more missing levels in the IR because of the weakness of the spectrum in this range. The correlation properties of this set of vibronic levels have been analysed calculating the power spectrum of the absorption stick spectrum which displays periodic motions: the dominant period, at 714 ± 20 cm−1, corresponds to the bending motion of the A 2B2 state. The other observed periods remain unassigned. In contrast the next neighbor spacing distribution (NNSD) shows a strong level repulsion, i.e. a manifestation of quantum chaos. These two observations, apparently contradictory, can be rationalized as follows: the short time dynamics, for t < 10−12 s, is “regular” while for longer times the dynamics becomes “chaotic”. We suggest that this behavior may be observed directly with a pump and probe fs laser experiment.

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 (VECSELs). A detection limit of 3 x 10(-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 approximately 75 nm .

Intracavity-laser-absorption spectroscopy of the visible overtone transition of methane in a supersonically cooled jet

Abstract We report the CH-stretching overtone spectrum of jet-cooled methane in the visible range. Due to the very weak intensity of the transition, the intracavity-laser-absorption-spectroscopy technique (ICLAS) was used with a free jet of methane from a slit orifice placed inside a dye-laser cavity. In this way, an equivalent pathlength through the jet of 800 m was achieved. The rotational cooling drastically reduces the congestion of the spectrum: whereas the room-temperature spectrum is extremely congested and complicated, the jet spectrum is sparse and reduced to well-isolated lines. These results show that rotational congestion is mainly responsible for the overtone band shape of methane and that, in consequence, the contribution of the background vibrational states is not predominant. The experimental method developed in this work can be extended to study the overtone transitions of a wide class of jet-cooled compounds.

High resolution spectra of GeH4 v=6 and 7 stretch overtones. The perturbed local mode vibrational states

Here we report the preliminary analysis of the high resolution spectra of 70GeH4 (6000) and (7000) stretch overtone bands obtained by intracavity laser absorption spectroscopy (ICLAS). The observed P(J+1), Q(J), and R(J−1) transitions agree well with the combination difference relations, indicating that the Coriolis coupling in the degenerate states is negligible. This is an important feature for a spherical top molecule near the local mode limit. The vibration–rotational energy levels of upper states as well as those of some perturber states are obtained. The improved local mode parameters of GeH4 are also derived.

Rotationally resolved overtone transitions of CHD3 in the visible range

Absorption spectra in the 14 900–18 700 cm−1 range have been recorded for gaseous CHD3 at high resolution using the intracavity laser absorption spectroscopy (ICLAS) technique. The observed transitions correspond to the N=6 and 7 overtones of the C–H chromophore. Five bands were identified and rotational constants determined for four of them. The comparison of our experimental data with the calculated band origins and rotational constants obtained by Lewerenz and Quack is discussed. Absolute band intensities are given for seven bands corresponding to the N=5, 6, and 7 polyads.

Rotational structure of overtone transitions of CHD3 near 13 500 cm−1

The absorption spectrum of CHD3 has been recorded for the N=5 polyad near 13 500 cm−1 by the intracavity laser absorption spectroscopy (ICLAS) technique with a spectral resolution of 0.02 cm−1. The rotational structure of the four observed bands has been resolved and the rotational constants given. The behavior of these constants versus the quantum number of the C–H stretching and bending modes are also discussed.

INTRACAVITY LASER ABSORPTION SPECTROSCOPY DETECTION OF SINGLET CH2 RADICALS IN HYDROCARBON FLAMES

Abstract The spectrum of methylene, the CH 2 radical, in the singlet electronic state was detected with a high signal/noise ratio in methane-oxygen-nitrogen flames using intracavity laser absorption spectroscopy. The spectra were recorded at different pressures including first measurements at atmospheric pressure. The maximum concentration of the singlet CH 2 radical in a 30 Torr flat flame was estimated based on literature data from laser photolysis experiments done at room temperature. The value of 2 × 10 13 cm −3 seems to be higher than that predicted by the model calculations. The dependence of the calculated maximum concentration of singlet CH 2 on the equivalence ratio is in qualitative agreement with measured values.

Laser relaxation-oscillation frequency imaging.

We describe a new imaging technique based on modification of laser relaxation frequency induced by coherent optical feedback from an external target. A direct comparison (both theoretical and experimental) is made with laser feedback interferometry techniques, in which there is a modification of the lasers steady state. We show that, for a laser with a cavity damping rate gamma(c) higher than the population damping rate, gamma(1) , the modification of the laser relaxation frequency can be several orders of magnitude more sensitive than the perturbation of the lasers output power. Application of this technique to imaging is reported.