Michel Lefebvre

Precision of multiplex CARS temperature measurements

The potential measurement accuracy of coherent anti-Stokes Raman scattering (CARS) in N2 was studied. A test was conducted using BOXCARS and nonresonant background suppression in a stabilized tube furnace from room temperature up to 1700 K. A computer program was developed for quick data processing. The influence of referencing on temperature measurement accuracy was analyzed. Absence of referencing produced errors up to 100 K on the mean temperature and on the standard deviation. Referencing with a time-averaged reference gave accurate values for the mean temperature. Referencing on a shot-to-shot basis improved the mean slightly and produced marginal gains in the standard deviation of individual measurements. The latter stays in the 10–50 K range in the temperature domain. Simulations show that part of the standard deviation (20%) is attributable to shot noise alone. The rest probably results from dye laser spectral noise and beam wandering.

Spectroscopic analysis and chemical kinetics modeling of a diamond deposition plasma reactor

Abstract The quality and the growth rate of diamond films produced in a microwave bell jar plasma reactor are strong functions of the plasma characteristics at the plasma-surface interface (temperatures, species concentrations). These local parameters are shown to be functions of the plasma operating conditions (dissociation in volume) and of the boundary conditions at the surface. Spectroscopic measurements of some plasma parameters based on coherent anti-Stokes Raman and optical emission spectroscopy are presented; the deposition results are correlated with these measurements. A zero-dimensional chemical kinetics model is developed for studying the influences of the gas temperature and the electron temperature on the production of atomic hydrogen.

Dual-cavity doubly resonant optical parametric oscillators: demonstration of pulsed single-mode operation

The operating points of pulsed dual-cavity doubly resonant optical parametric oscillators have been investigated, taking into account the influence of the optical dispersion. A diagram is proposed to determine the spectral separation of doubly resonant positions for any optical lengths of both cavities. From the analysis of the distribution of doubly resonant coincidences, original conditions for stable single-mode operation are specified. This approach is validated by use of a type II phase-matched β-barium borate crystal. Frequency stability and tuning characteristics are also reported. To our best knowledge, this is the first demonstration of single-mode operation that uses a dual-cavity doubly resonant optical parametric oscillator in the nanosecond pulsed regime.

Compact, single-frequency, doubly resonant optical parametric oscillator pumped in an achromatic phase-adapted double-pass geometry.

We report on a nested-cavity, doubly resonant optical parametric oscillator (NesCOPO) architecture for widely tunable, mid-IR, single-frequency generation. By use of an achromatic phase-adapted double-pass pumping scheme, this new, low-threshold, semimonolithic architecture only requires two free-standing cavity mirrors and a nonlinear crystal with a mirror coating deposited on its input facet while the other facet is antireflection coated. It is thus as simple and compact as any basic linear optical parametric oscillator cavity, is easily tunable, and displays low sensitivity to mechanical vibrations. Using a high-repetition-rate (4.8 kHz) microlaser as the pump source of the NesCOPO, we demonstrate a compact source that provides pulsed, stable single-frequency output over a wide spectral range (3.8-4.3 μm) with a high peak power (up to 50 W), which are properties well suited for practical gas sensing applications.

High-energy single-longitudinal mode nearly diffraction-limited optical parametric source with 3 MHz frequency stability for CO2 DIAL

We report on a 2.05 microm nanosecond master oscillator power amplifier optical parametric source for CO2 differential-absorption lidar. The master oscillator consists of an entangled-cavity nanosecond optical parametric oscillator based on a type II periodically poled lithium niobate crystal that provides highly stable single-longitudinal-mode radiation. The signal emission is amplified by a multistage parametric amplifier to generate up to 11 mJ in a nearly diffraction-limited beam with an M2 quality factor of approximately 1.5 while maintaining single-longitudinal-mode emission with a frequency stability better than 3 MHz rms. This approach can be readily applied to the detection of various greenhouse gases.

Temperature measurement by single-shot dual-line CARS in low-pressure flows

A CARS method, adapted for diagnostics in low-pressure unsteady flows is described. The technique employs two narrow-band dye lasers and a single-frequency pump laser. Single-shot rotational temperature is obtained from the ratio of the intensities of two isolated Q lines. Temperature-measurement accuracy is discussed. The advantage of referencing the CARS signals from the flow in a cell filled with the same gas is shown. Demonstrative experiments are performed using N2 at pressures of 102–103 Pa.

Entangled-cavity optical parametric oscillator for mid-infrared pulsed single-longitudinal-mode operation

We report a pulsed doubly resonant optical parametric oscillator that uses an original entangled-cavity geometry. This compact source (total volume of 1 L, including the pump laser) displays single-frequency operation (linewidth, <100 MHz), a high repetition rate (>10 kHz), low threshold (<10 muJ), and wide tuning in the mid-infrared. These properties qualify pulsed doubly resonant optical parametric oscillators as powerful tools for applications in such fields as nonlinear spectroscopy, lidar, and pollutant detection.

Multispecies high-energy emitter for CO 2 , CH 4 , and H 2 O monitoring in the 2 μm range

We demonstrate the first emitter, based on a single optical source device, capable of addressing three species of interest (CO₂, CH₄, and H₂O) for differential absorption Lidar remote sensing of atmospheric greenhouse gases from space in the 2 μm region. It is based on an amplified nested cavity optical parametric oscillator. The single frequency source shows a total conversion efficiency of 37% and covers the 2.05-2.3 μm range.

Actively mode-locked optical parametric oscillator.

We report on what we believe to be the first demonstration of active mode locking of an optical parametric oscillator. An acousto-optic modulator is inserted into a nearly degenerate (approximately 1064 nm) and doubly resonant optical parametric oscillator based on periodically poled LiNbO3 and pumped with the second harmonic of a quasi-continuous-wave single-frequency Nd:YAG laser. When the modulation frequency is matched to the free spectral range of the cavity (120 MHz), a pulsed regime is observed, with pulse durations as short as 700 ps.

Single-shot time-domain coherent anti-Stokes Raman scattering: application to temperature and velocity measurements in supersonic flows

It is shown that a new approach in time-domain coherent anti-Stokes Raman spectroscopy can be used to perform single-shot measurements of the static temperature and velocity in low-pressure supersonic flows. Instead of repetitively probing the macroscopic polarizibility coherently driven by a short-duration pump and Stokes pulses with variable delay times, we create the coherence with a long-duration pump and a short-duration Stokes pulse and probe it continuously with the same pump pulse.The decay of the anti-Stokes signal reveals the static temperature. The beating between two signals that are created in opposite crossing directions with respect to the flow direction gives the velocity of the flow.