Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1992-06-18
1996-07-09
Phan, James
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359210, 359300, 359305, 372 21, G02B 2608
Patent
active
055350410
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a device for stabilizing the reflectivity of a phase conjugation mirror. This device is applicable to improving the reflectivity of mirrors having phase conjugation through stimulated Brillouin scattering at high pulse repetition rate.
2. Discussion of Background
Stimulated Brillouin scattering is a physical mechanism well suited to the conjugation of the phase of short pulses within the 1-10 ns range of duration. Applications relate to the correction of optical phase distortion from various causes, for example within solid-state laser media such as YAG:Nd. A geometry very often used to produce such phase conjugation mirrors is represented in FIG. 1: the incident wave which has suffered aberration is focused in the non-linear medium, which is most often a gas (CH.sub.4, Xe, . . . ) or a liquid (acetone . . . ). Beyond a peak power threshold, the creation is observed of a reflected pulse, with a high efficiency (up to 90%) which is phase conjugated with the incident wave.
If this performance is achieved in the case of single incident pulses, nevertheless the fact of raising the repetition rate leads to a drop or to fluctuations in reflectivity as a function of time, which is correlated with a degradation in the quality of the phase conjugation (fidelity). These harmful effects are certainly due to the heating of the non-linear medium as a result of its non-zero absorption, which leads to the formation of turbulences which degrade the process.
Several solutions have already been envisaged for overcoming this reduction in performance at high rate. These solutions provide for circulating the fluid in the gas cell so as to renew it in the interaction zone with each pulse. This solution requires the production of laminar fluxes in the interaction region and complicates the use of phase conjugation mirrors in systems where the size constraints are severe.
SUMMARY OF THE INVENTION
The invention provides for displacing the interaction zone in the active medium with the aid of appropriate optical means so that this interaction zone is different from one pulse to the next.
Moreover, according to the invention, this displacement is done without any movement of the active medium.
The invention therefore relates to a device of the reflectivity (sic) of a phase conjugation mirror, characterised in that it includes a means of displacing relative to the mirror an incident beam to be reflected.
A device for focusing the incident beam enables the incident beam to be focused at a point situated inside the phase conjugation mirror.
The means of displacement then make it possible to displace the focusing point inside the phase. conjugation mirror.
BRIEF DESCRIPTION OF THE DRAWINGS
The various subjects and characteristics of the invention will emerge in the description which will follow and in the attached figures which represent:
FIG. 1, the functioning of a phase conjugation mirror already described earlier;
FIGS. 2a and 2b, an illustrative embodiment of the device of the invention in which the focusing point moves over a circle in a plane perpendicular to the optical axis of the system;
FIG. 3, a variant embodiment of the device of FIGS. 2a and 2b;
FIG. 4, an illustrative embodiment of the device of the invention in which the focusing point moves over a line in a plane perpendicular to the optical axis of the system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Before describing the illustrative embodiments of the invention, it is expedient to point out that these examples may contain characteristics specific to themselves but which are not necessarily essential characteristics of the invention.
The essential characteristic of the invention is that the beam to be reflected is displaced relative to the phase conjugation mirror.
The device of FIG. 2a is the simplest solution a problem posed (sic).
This solution, which is the simplest since it makes it possible to not append any additional optical element in the path of the beam, is repr
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Applied Physics B. Photophysics and Chemistry. vol. 36, 1985, Heidelberg De pp. 83-92; I. D. Carr et al.: "Performance of a Nd:YAG Oscillator/Amplifier with Phase-Conjugation via".
Soviet Journal Of Quantum Electronics. vol. 20, No. 7 Jul. 1990, New York US, pp. 770-772; B. I. Denker, et al.: "Compact lase with stimulated Brillouin scattering mirror operated at pulse repetition frequency up to 150 Hx".
Ayral Jean-Luc
Huignard Jean-Pierre
Tournois Pascal
"Thomson-CSF"
Phan James
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