Coherent light generators – Particular beam control device – Control of pulse characteristics
Patent
1998-03-02
2000-06-06
Lee, John D.
Coherent light generators
Particular beam control device
Control of pulse characteristics
359285, 359305, 372 13, 372 26, 385 7, H01S 310, G02F 111
Patent
active
060728132
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a device for controlling light pulses by a programmable acoustooptic device and its application to pulse-processing devices and in particular to a laser source delivering ultrashort pulses.
Devices employing the interaction of acoustic devices on the behaviour of optical signals are known in the art.
Considering an acoustic medium, and in particular an anisotropic elastooptic medium into which an acoustic wave is introduced, this having the effect of inducing optical index variations in this medium, and if an optical wave colinear with the direction of propagation of the acoustic wave is introduced into this medium, there is an interaction between the acoustic wave and the optical wave. This interaction is due to the index variations on the optical wave. If the optical wave and the acoustic wave are colinear, there is a coupling of a diffracted optical wave .kappa.d to an incident optical wave .kappa.i via an acoustic wave .kappa.ac, as shown in FIG. 1, in which kd=ki+kac.
This acoustooptic interaction couples the incident optical wave of wavenumber .beta..sub.1 to the diffracted optical wave of wavenumber .beta..sub.2 via an acoustic wave of wavenumber K in an anisotropic elastooptic material. optical wave is coupled to the amplitude A.sub.1 (z) of the electric field E.sub.1 of the incident optical wave by the equations: ##EQU1## .kappa. being the coupling constant.
The solutions of equations (2) may be written, for A.sub.2 (0)=0, as: ##EQU2## with: ##EQU3## .omega. being the optical angular frequency, c being the velocity of light in vacuo, the incident wave and the diffracted wave,
At a distance Z, the fraction of energy transferred from A.sub.1 to A.sub.2 is: ##EQU4##
All of the energy of the incident wave is transferred to a diffracted wave at a distance L.sub.c, called the coupling length when: ##EQU5##
If the incident optical wave carries a signal s.sub.i (t) and the acoustic coupling wave carries a signal a(t), the diffracted optical wave in a colinear interaction will carry the signal in the form;
The signal s.sub.d (t) is the result of the filtering of s.sub.i (t) by an impulse response filterirg a(t).
In particular, if the signal s.sub.i (t) is a very short optical pulse and the signal a(t) is a long amplitude-modulated or frequency-modulated acoustic pulse of BT (bandwidth.times.time) product, the signal s.sub.d (t) will be a long amplitude-modulated or frequency-modulated optical pulse having the same BT product faithfully copying the code of the acoustic wave such that .DELTA..beta.=0: ##EQU6## f.sub.opt being the optical frequency, f.sub.ac being the acoustic frequency
In frequency-modulated light-pulse amplification arrangements known as CPA (Chirp Pulse Amplification), the problem consists in generating, from a very short optical pulse, a long frequency-modulated optical signal which will be recompressed after laser amplification.
The compression device subjected to high-peak powers cannot be programmed. It is therefore desirable for the expansion device, which operates with a small optical signal, to be made programmable depending on the defects and instabilities in the laser amplification chain.
This is because, when the expansion device is perfectly matched to the compression device in the absence of a laser amplification chain, the recompressed signal is virtually identical to the very short initial optical signal before expansion. However, in the presence of the amplification chain, which distorts the signals, the recompressed signal is not identical to the initial signal even when the expander and the compressor are perfectly matched.
The only way of compensating for defects in the amplification and in the compression is to use a programmable expansion filter.
The invention therefore relates to a programmable acoustooptic device comprising an elastooptic medium provided with an acoustic transducer capable of generating a modulated acoustic wave in the elastooptic medium along a defined direction, together with means for coupling a polarized incident optical w
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"Thomson-CSF"
Lee John D.
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