Method for the correction of the wave surface of an optical beam by means of a deformable and mirror used in this method

The present invention relates to a method to correct the wavefront of an optical beam using a deformable mirror, and a mirror for implementation of the method.

In general, the wavefront of a optical beam is affected by defects that are detrimental to the quality of the transmitted optical beam, which impairs the performance of the associated optical system this beam.

In astronomical observation, for example, a Large Telescope includes a primary mirror, secondary mirror and an optical system for analyzing and observe the beam incident was successively reflected from the primary and secondary mirrors.

The wavefront of the optical beam which is received by the telescope is affected by defects caused by atmospheric turbulence, inflections of the telescope structure, misalignment optical, large-scale defects mirror primary...

In the case of a high laser beam energy, the beam wave surface may be particularly affected by defects caused by manufacturing tolerances that are binding and thermal deformation of the order of optical components laser, and the effects of laser pumping.

To correct the wavefront defects an optical beam in the case of a telescope or laser, it is known to use deformable mirrors which may be a flat shape, parabolic or any other geometry which is adapted to the optical system wherein the mirrors are integrated. Most of the time, the mirror has two parallel planar faces, and actuators are used to apply forces and / or impose movements to generate deformations of the mirror.

In the case of a telescope for example, it is known to provide a deformable mirror to correct aberrations of the wavefront. To perform correction, occasionally deforms the mirror from a plurality of actuators that are selectively ordered according to the defects corrected. However, such a correction requires the presence of a number important to actuators, which leads to a structure complex and difficult to implement because the settings are very difficult to control and thus to achieve.

The object of the invention is to overcome these Disadvantages based on a new approach is to refer to the vibration modes of mirror.

Specifically, using the example of a plate that is excited by hitting a shot hammer, the plate will begin to vibrate. vibration will decompose resulting in several vibrations various elementary that is commonly called the eigenmodes or natural vibration modes of the plate. Suppose that the wave correction area desired is a sine wave, for example. In Generally, the plate will not vibrate naturally like a wave, so it takes change the shape of the plate so that it can vibrate like a wave. Actuators are used to mechanically deforming the plate by his mobile behavior will then have a natural tendency to deform following a wave. To be effective, actuators are divided into the areas of extreme deformation rather than in other areas, when is chosen to position the actuators perpendicular to the mirror surface.

One understands the need to use a large number of actuators to deform the mirror in the form of corrections to the surface an optical wave beam incident on this mirror.

According to the invention, we will consider mirror vibration normal modes to optimize its deformation based on corrections.

To this end, the invention provides a method to correct the wavefront of an optical beam, in particularly a laser beam, by means of a mirror deformable, the method of deforming the surface mirror with actuators according to the form the correction to the wavefront of the optical beam incident on the mirror, which method is characterized in that it consists in modifying the structure of the mirror so as to match the deformations of the mirror substantially following the first eigenmodes or lowest frequency modes natural vibration of said mirror with corrections the wavefront to be performed.

Thus, depending on the shape of the correction to the beam wavefront perspective, it gives the mirror structure or geometry such that after deformation the natural modes of vibration the mirror substantially correspond to the shape of corrections.

The invention also relates to a device deforming mirror to correct the surface wavelength of an optical beam, particularly a laser beam, the mirror comprising a substrate having a face is covered with a reflective surface, and actuators to deform the surface at least locally the mirror according to the shape of the correction wave surface to perform, which device is characterized in that the mirror comprises means such its structure is changed so that the first modes own or modes of lower natural vibration frequency of said mirror substantially correspond to the form of corrections to the wave surface beam.

For example, the above means that used to modify the structure to mirror his deformation using actuators is made following essentially the first eigen modes of the mirror correspond to the shape of the corrections are up: <ul><li>by ribs parallel to each other and located on the face of the substrate which is opposite to the reflective surface, the number, spacing and shape of the cross section of each rib being determined according to the shape of the correction to bring,</li><li>by a mirror whose substrate has a variable thickness, or</li><li>by a mirror whose substrate is constituted several layers of materials having rigidities different.</li></ul>

Specifically, the mirror will be deformed provided by actuators, but the mirror structure is such that it will take a limited number of these actuators to deform the mirror following forms which substantially correspond to the first eigenmodes vibration of the mirror that match themselves substantially forms Correction to Bring the wavefront of the beam.

A mirror according to the invention has many advantages among which may include to quote<ul><li>a quality improvement correction, especially near the edges mirror and between the actuators,</li><li>a significant decrease in the number of actuators needed to deform the mirror,</li><li>a significant reduction in the cost of manufacturing and </li><li>a reduction in the space occupied by actuators.</li></ul>

In general, such a mirror can for example, be used to correct the wavefront an optical beam in a laser amplifier great energy.

Other advantages, features and details of the invention emerge from the description Explanatory which follows, with reference to the drawings attached, given by way of example and in which :<ul><li>Figure 1 is a schematic sectional view a mirror and a deforming device of this mirror for carrying out the method according the invention,</li><li>Figure 2 is a sectional view of a mirror according to one embodiment of the invention, and</li><li>Figure 3 is an example of a form correction wave to bring to a wavefront optical beam.</li></ul>

A deformable mirror 1 according to the invention may be deformed by means of a deformation device 3 which is schematically illustrated in Figure 1. The mirror 1 comprises two opposite main plane faces and 1b, the face being for example one that is for receiving a beam wavefront optical.

The forming device 3 includes a plurality of actuators 5 which are supported by a barrel 7 for example. Each actuator 5 is connected to mirror 1 by a device interface 9.

For example, each device interface 9 may include a mobile 9a rod in translation, which is connected to the actuator 5 on the one hand, and a yoke 9b itself attached to the surface 1b of the mirror 1 on the other.

All actuators are controlled from 5 U1 a control unit which is controlled by a unit U2 control. The U2 control unit's function analyzing the beam wavefront and determine the corrections, and transmits orders to U1 controller for deforming the mirror 1 according to the desired shape by a selective action on the 5 actuators.

According to one embodiment shown in 2, the mirror 1 is a plane mirror that includes a reflecting surface 10 which is deposited on one face a substrate 12.

In general, the substrate 12 is with or cooperates with means 14 which have function to give it a better ability to 'the deformation in the desired behavior.

For example, the opposite surface of substrate 12 is equipped with these means 14 which are example ribs 16 parallel to each other. These ribs 16 can be obtained by milling substrate or be related thereto by gluing. These ribs 16 have a rectangular cross section, but this section may be trapezoidal, triangular or some form. These means 14 are not necessarily consist of the same material as the substrate 12.

Alternatively, these means 14 may be constituted by a mirror whose thickness is variable or whose substrate is composed of several layers having different thicknesses and / or consisting of layers having different rigidities.

These means 14 are determined from the shape corrections to bring to the surface the wave optical beam. Thus, suppose the correction to be a sinusoidal S, as is schematically illustrated in Figure 3. In this case, means 14 will give some form Such a structure, after deformation of the mirror 1 by actuators 5, the first natural modes of vibration of the mirror 1 substantially correspond to the shape S sine of the correction to the surface beam of the wave incident on the mirror.

Specifically, in the example application mirror 1 has a square section of about 40 x 40 cm², and consists of a metallic reflecting surface 10 deposited on a substrate 12 made of glass and having a thickness of the order of 15 mm.

In general, from the family of correction surfaces that we wish to bring mirror 1, the predominant areas in terms of local curvatures are sinusoids and cosinusoïdes oriented in a preferred direction and whose wavelengths are 1, 2/3 and 1/2 times the length the mirror 1.

Depending on the correction accuracy desired, the structure of 1 above may be mirror modified by the addition of thirteen ribs 16 in section rectangular right, regularly spaced the surface 1b of the mirror 1 and which extend over the entire length mirror 1. These ribs are cut in 16 the substrate 12 or attached by gluing, for example. The actuators 5 that will ensure distortion 1 mirror are mounted to the right of the ribs 16 to form thirteen rows of <u>not</u> actuators. The number of actuators 5 is selected as a function of the precision Sought and other desired correction modes but which are not common in terms of curvature local, the number of actuators 5 which may vary from row to the other. In this example, should by use against a larger number of actuators, of about one and a half to correct the same Default prior art.