Thierry Debuisschert

High beam quality unstable cavity infrared optical parametric oscillator

We have implemented an Optical Parametric Oscillator (OPO) with an unstable cavity. The cavity is a telescope made of a concave mirror and a convex mirror. This latter is coated only in its center an is the pupil of the cavity. The nonlinear crystal is a 5 cm long LiNbO3. The OPO is pumped by a 20 ns Q-switch Nd:YAG laser and is operated at 1.9 micrometers for the signal and 2.4 micrometers for the idler. The OPO threshold is 7 mJ. For a pump energy of 30 mJ, the signal and idler energy produced is 9 mJ. Those results are comparable to those obtained with plane mirrors cavities. The near field of the output beam has an annular distribution imposed by the pupil. The distribution is not uniform but has 2 lobes located on opposite sides one from another. This uniformity can be improved increasing in the magnification factor from 1.2 to 1.33. The far field shows a central peak with side lobes in the vertical direction. This intensity distribution can be calculated from the diffraction of a plane wave by a uniform annular pupil similar to the near field intensity distribution. This indicates that the field is close to the diffraction limit in this direction. In the horizontal direction, there is peak which is twice as large as the previous one, and there is no side lobes. The field can be estimated to be 2 times the diffraction limit in this direction. This experiment shows that infrared OPOs with unstable cavities can produce beams close to the diffraction limit with a good energy conversion.

Nanosecond optical parametric oscillators

Abstract The main characteristics of nanosecond OPOs are presented: nonlinear crystals, tunability ranges, output energies, phase-matching conditions. A semianalytical model and a completely numerical model are described. They point out the importance of the complex intensity distribution that appears in the transverse planes of the output pulses. Techniques to improve the spectral and spatial quality of the output beams such as intracavity Fabry–Perot filters or unstable cavity are detailed. Finally a modelisation and experimental results about intracavity OPO are presented.

High-sensitivity detection of methane-combining photoacoustic spectroscopy and optical parametric oscillator

We have combined a LiNbO3 optical parametric oscillator (OPO) and a photoacoustic (PA) open cell to perform air pollution measurements. THe OPO is pumped by a Nd:YAG laser working at 10 Hz. It is tunable between 1.55 micrometers and 4.5 micrometers . The PA cell is designed to be isolated from outside acoustic noise. The OPO output wavelength is tuned between 3.39 micrometers and 3.44 micrometers to monitor absorption lines of methane. The characteristics of the pulses are E equals 2 mJ, (tau) equals 15 ns, (Delta) (nu) equals 2 cm-1. The minimum detected concentration is 1 ppm. Decreasing the background noise, detection of concentrations as low as 20 ppb is expected.