Pierre Lavigne

Large-size Gaussian mode in unstable resonators using Gaussian mirrors

Gaussian modes with large sections have been experimentally produced in Cassegrain resonators using Gaussian reflectivity convex couplers. The far field of the beam, which was coupled through a Gaussian coupler, was found to be free from secondary rings.

Design and characterization of complementary Gaussian reflectivity mirrors.

A mirror with a Gaussian reflectivity profile and a complementary transmission has been designed. This nonabsorptive smooth mirror is based on radially varying the thickness of a high-refractive-index dielectric deposited on a transparent substrate. Prototypes have been fabricated for use around 10 μm. The reflection and transmission profiles of one of them have been measured together with the modifications of the reflected and transmitted wave fronts.

Optical resonators with Gaussian reflectivity mirrors: output beam characteristics.

The characteristics of Gaussian beams which are transmitted through complementary Gaussian reflectivity couplers are analyzed. It is shown that the coupled beam properties are strongly dependent on the coupler focal length. The on-axis far-field brightness can be improved by 1.5 and the misalignment sensitivity minimized when the coupler collimates the transmitted beam.

Optical resonators with Gaussian reflectivity mirrors: misalignment sensitivity

An analytical solution to the problem of the misaligned optical resonator with a Gaussian reflectivity mirror is presented. It is shown that the fundamental mode of such a resonator remains a Gaussian beam which propagates along a tilted axis in the misalignment plane. The exact analytical solution yields simple expressions that characterize the misalignment sensitivity of the resonator by giving the beam-steering angle and the loss variations.

Effects of hard apertures on mode properties of resonators with gaussian reflectivity mirrors

The effects of hard apertures on the energy distribution, the far-field beam quality, the mode volume, and the discrimination against high-order modes in Cassegrain resonators with Gaussian reflectivity mirrors have been investigated both theoretically and experimentally. It has been shown that, in the far field, the fraction of energy in the secondary lobes remains small as long as the ratio of the design beam waist to the radius of the gain medium w_{b}/a is w_{b}/a > 0.7 , deterioration of the beam quality and diffraction losses counterbalanced a more efficient filling of the gain medium to limit the resonator efficiency. At low magnifications, resonators with Gaussian mirrors out perform resonators with Standard mirrors.

Variable reflectivity unstable resonators for coherent laser radar emitters

Beam properties obtained from a TE CO(2) laser in a Cassegrain resonator using various graded reflectivity mirror couplers are compared. It is shown that, experimentally, Gaussian profiles maximize the far-field intensity while parabolic profiles yield more uniform near fields with a lower energy extraction. Both configurations appear suited for coherent ladar systems.

Modes of resonators with internal apertures

Modes of resonators formed by paraxial elements of infinite extent and single internal aperture are shown to be the solutions of two coupled symmetric integral equations involving four parameters: two Fresnel numbers N1 and N2 and two geometrical factors A1 and A2. The modes become the eigenfunctions of an Hermitian operator when N1 = ±N2, A1 = ∓A2; analytical solutions can then be written as generalized prolate spheroidal functions. The same solutions are derived for a resonator in which one mirror is replaced by an infinite phase conjugate mirror. Real nonsymmetric eigenvalue problems are associated with the condition N1A1 = −N2A2; such configurations can generate pairs of modes with same power losses but different oscillation frequencies. Extension to cavities with two internal apertures yields a system of four coupled integral equations with eight independent parameters; again the modes can be the solutions of Hermitian or real nonsymmetric eigenvalue problems under special conditions.

Superthermal electron production from hot underdense plasmas

Very‐high‐energy electrons of up to an energy of ∼2.3 MeV have been observed to be emitted from the hot underdense exploding thin foil plasmas created by 10.6 μm CO2 laser radiation at intensity levels up to ∼4×1014 W/cm2. As a supplement to the electron measurements the forward and backward scattered light components were also measured. Correlation of these measurements shows that either Raman scattering or the high‐temperature version of two‐plasmon decay or both, manifesting themselves near the quarter‐critical density region, are responsible for the production of a hot (Th∼135 keV) tail of electrons at least up to energies of 1 MeV. There are no indications that the Raman forward scattering (as distinct from Raman backward scattering) at lower densities plays any significant role. These experimental results are consistent with the results from a l 1/2 ‐dimensional particle‐in‐cell code simulation with a parabolic density profile resembling the experimental conditions. An apparent anomaly is discussed,...

A compact wide-aperture single-mode TE-CO 2 laser with a low chirp rate

Single mode operation of a TE-CO 2 laser was obtained using a 2 \times 2 \times 30 cm gain module in a Cassegrain resonator with a Gaussian reflectivity convex mirror to produce a fundamental mode of large cross sectional area, and reducing the pressure to 375 torr in a 50 cm long cavity to limit the oscillation to a single longitudinal mode. The 70 ns gain switched pulse (FWHM) had an energy of 175 mJ, a peak power of 2.0 MW, a diffraction limited far-field, and a frequency chirp rate lower than 60 kHz/μs2.

Multiwavelength laser solid–target interaction at 10 microns

Some aspects of multiple‐wavelength interaction on solid targets have been experimentally studied at CO2‐laser wavelengths with 1‐nsec pulses. Contrary to predictions, the use of a two‐line (10.6‐ and 9.6‐μm) source does not reduce the back reflection. This result could be explained by a very short Brillouin interaction length or/and a very low saturation level. The hot‐electron production is also not significantly modified by multiline illumination. A signal at the frequency sum indicates that, even at Iλ2 as low as 5×1013 W cm−2 μm2, the critical surface is not well defined and that both the 9.6‐ and 10.6‐μm light interact in the same resonant zone. Finally, significant sidebands were measured at 8.7 and 12 μm. Some 1 (1)/(2) ‐dimensional simulations with mobile ions indicate that those sidebands probably originate from the beat frequency resonant density at 0.0123nc.