The field of the invention is that of Pockels cells used in particular in the fields of amplification of the laser beams and wavelength switches.
The electro-optical materials change the polarization of light by applying an electric voltage within the material. These materials are often associated with the polarizers, so as to produce electro-optical switches, also called Pockels cells. A polarizer selectively passes a light beam whose polarization is in a predetermined state. An electro-optical material allows to change this state by applying a voltage. It is thus possible to control the transmission of the light beam by means of an electric voltage.

Many Pockels cells use configurations have been implemented in the field of laser beam amplification. One of these configurations, also called "Q-switch" is to activate a Pockels cell in the laser cavity, so as to increase the losses of an oscillator (or equivalently to reduce the cell transmission ): the laser pulse is then trapped in the cavity. By suddenly applying a voltage to the Pockels cell, it removes the cavity losses, which allows to release the laser pulse. This phenomenon is causing laser oscillator Q-switched (short pulses typically between 5 ns and 40 ns).

Pockels cells to two transverse fields crystals mounted in thermal compensation possible to produce optical switches with unimportant voltages. These cells use two electro-optical crystals in series thereby reducing the level of the voltage to be applied to activate them. The crystals used are generally so-called biaxial crystals: their natural birefringence depends on their cutting axis (X or Y axis) and also varies greatly with temperature. The mounting of crystals in thermal compensation allows precisely orienting the crystals relative to the other, to compensate for the natural birefringence of one with that of the other and this compensation is effective over a range of temperatures extremely off - 40 ° to + 60 °, for example. The quality of compensation, causing the temperature range over which the Pockels cell is operational, depends on the relative setting accuracy of the two crystals but also their similarity. There is talk of crystals "twins" who must, as their name suggests, be as similar as possible. For this reason, the two crystals of these cells are generally polished together to have identical dimensions.

The choice of crystals (most similar possible) and assembly ensure the correct operation of the Pockels cell over very wide temperature ranges, subject to not disturb the thermal compensation implementation of the crystals.

Figure 1 shows a Pockels cell 100 to two crystals 10a, 10b mounted in thermal compensation according to the prior art. The twin crystals are preset and pasted on a common base 12 which also serves as an electrode. A metal plate 1 1 A and 1 1b bonded to each crystal 10a, 10b is used to apply tension and change the transmission of the cell. The base 12 is fixed to a support structure 13.

But such Pockels cell is often disturbed by external factors inherent in the intended applications. The laser oscillators or amplifiers require the pumping of an amplifying medium: it is necessary to bring to the amplifying medium of the light energy so as to store it to restore it in a second time in the form of a coherent monochromatic beam and . This is the laser effect.

This light energy to the gain medium is not perfect, because part of the energy needed to produce the 200 is converted into heat.

We saw previously that cells Pockels two crystals set in compensation work properly on wide temperature range provided that the crystals retain their similarity to maintain the temperature compensation. However, if the thermal power generated by the pumping induces a temperature difference between the two crystals as illustrated in FIG 1 by the arrow 150 (in decreasing intensity), then a disparity is created and the cell 100 no longer functions correctly.

The evacuation of the generated heat and power does not generally pose a problem in a "terrestrial" very constraining environment: it prevents the heat generated power is transmitted to the cell, either away or in the thermally insulating of the heat source as disclosed for example in EP 1 532 482. the first solution is not desirable in a space environment where equipment must occupy a minimum volume. The second solution is also difficult to implement efficiently in a small space without weakening the rigidity of the equipment.

In a more constrained spatial type environment for example, where space is very limited and the use of proscribed evacuation fluids, the thermal power generated by the pump is discharged mostly by conduction through the carriers equipment. In this constrained environment, the Pockels cell is most often located in the immediate environment of the heat source. The thermal power generated by the pumping system is then transmitted inevitably to the Pockels cell.

Accordingly, to this day remains a need for a Pockels cell simultaneously giving satisfaction to all of the above requirements in terms of heat dissipation, and environmental constrained spatial type for example where space is very limited and the use of proscribed evacuation fluids.

Rather than suppress transmission of the thermal power to the Pockels cell to avoid the creation of a temperature gradient between the two crystals, the Pockels cell according to the invention includes means for this power is transmitted to the two crystals of the cell symmetrically in order not to break the clearing assembly.

More specifically the invention relates to a Pockels cell which comprises:

- two similar electro-optical crystals oriented thermal compensation on

- a common horizontal metal base to the two crystals, and

- a support structure.

It is mainly characterized in that it comprises between the base and the bearing structure of a thermally conductive element with symmetric configuration according to a vertical plane passing between the two crystals, to symmetrically distribute to the base, a heat flux generated in the carrier structure asymmetrically according to this vertical plane.

Thus, when a heat source located nearby, generates a thermal power which is dissipated in the bearing structure of the asymmetrically equipment, the heat flux can not be transmitted to the crystals by means of the thermally conductive member . With this element, which is located in a plane of symmetry for the right cell in the middle of the two crystals, the unbalanced heat flux in the bearing structure is distributed symmetrically in each crystal the Pockels cell: this prevents the establishing a temperature gradient between the crystals. The compensation is preserved regardless of the heat flow dissipated in the cell. This solution is compact (requires no volume increase) and is independent of the heat flow to be dissipated.

This thermally conductive member is for example a vertical blade.

It can also be made:

- a horizontal frame to be in contact with the supporting structure, with its hollow center,

- a horizontal plate on which is mounted the base and connected to the frame by

- an arm located in the vertical plane passing midway between the two crystals.

According to another embodiment, the thermally conductive member is a horizontal plate provided with thermal braids disposed on the edges of the plate and perpendicular to the vertical plane passing through the middle of the two crystals.

The invention also relates to a Q-switched laser cavity laser comprising a Pockels cell as described, or a wavelength switch which comprises a laser and the laser output, a Pockels cell as described, and a device for changing the wavelength.

Other features and advantages of the invention will become apparent from reading the following detailed description, given by way of example and with reference to the accompanying drawings in which:

Figure 1, already described, schematically shows a Pockels cell according to the prior art disturbed by an external source of heat,

2 schematically shows a Pockels athermal cell according to the invention subjected to an external heat source,

Figures 3 schematically show an example of a thermally conductive element athermal Pockels cell according to the invention in perspective (fig 3a), with a view of the thermally conductive member alone in perspective (fig 3b)

4 schematically shows another example of a thermally conductive element athermal Pockels cell according to the invention, with an overall perspective view,

Figure 5 illustrates the influence of the heat flow density (abscissa) on the difference of T ° between the cells (on the ordinate) with and without conductive element.

From one figure to another, the same elements are identified by the same references.

In the following description, the terms "top", "bottom", "front", "rear", "side", "horizontal", "vertical" are used with reference to the orientation of the figures described. To the extent that the cell or the thermally conductive members may be positioned in

other orientations, the directional terminology is given by way of illustration and not limitation.

Rather than avoid the transmission of the thermal power to the Pockels cell to avoid the creation of a temperature gradient, the invention ensures that this power is transmitted to the two crystals in the cell symmetrically, to to break the thermal compensation assembly.

Pockels cell according to the invention described in connection with Figures 2, 3 and 4 comprises two crystals 10a, 10b electrooptical similar parallelepiped oriented thermal compensation in the direction Y of the radiation on a base 12 common to the two horizontal metal crystals . They are arranged one after the other on the base at a distance from each other. Each is provided with electrodes on two surfaces opposite to each other. The common base 12 also serves as a common electrode to the two crystals. A metal plate 1 1 a, 1 1 b attached to each crystal 10a, 10b constitutes the second electrode and used to apply tension and change the transmission of the cell. The surface of the other electrode 1 1 a of a crystal 10a is rotated 90 ° relative to the surface of the other electrode 1 1b of the other crystal 10b in the Y direction radiation. The base 12 is fixed to a support structure 13.

An external heat source 200 in the vicinity of the supporting structure 13, generates a thermal power which is dissipated in the carrier structure of the cell asymmetrically relative to the plane 160 vertical. This heat source may also be in contact with the supporting structure.

In the following it is considered that the heat source 200 is a heat source generating a stream 150 of asymmetrical heat in the carrier structure as shown in the example of FIG 2, but the description applies equally to a source cold generating cold flow.

According to the invention, the heat flux 150 present in the bearing structure 13 is not transmitted to the crystals 10a, 10b than via

a thermally conductive member located between the base 12 and the supporting structure 13, and in contact with them. This element has a symmetrical configuration with respect to plane 1 60 vertical (XZ according therefore) of symmetry passing between the two crystals 10a, 10b and in the middle of them. The flow of asymmetrical heat is therefore distributed symmetrically towards the base 12 and thus in each of the two crystals 10a, 10b, thereby preventing the establishment of a temperature gradient between the crystals. It thus enables symmetrical between the two crystals the temperature gradient present in the carrier structure, in order to preserve the thermal compensation. The compensation is preserved regardless of the heat flow dissipated in the cell.

This solution is compact (requires no volume increase) and is independent of the flow of heat to dissipate.

2 shows a preferred embodiment implementing a member thermally conductive form of blade 15 in the plane 1 60 of vertical symmetry between the two crystals, and a length equal to the width (along X) of the base. She is thin along Y to better channel conduction while maintaining its strength. According to this embodiment the base 12 and the supporting structure 13 are separated by a cushion of air (or other) except at the blade.

The vertical blade 15 and the base 12 may form a single piece. The vertical blade 15 and the supporting structure 13 can form a single piece. Finally, the vertical blade 15, the base 12 and the support structure 13 can form a single piece as shown in Figure 2. Portions of the base and of the support structure were dug symmetrically relative to the plane 160 at the blade level to promote thermal conduction by the blade and reduce the conduction through the air space between the base and the bearing structure.

This solution is interesting taking account of planned environments (space) because it can be easily miniaturized.

Thermal simulations were used to compare the induced thermal gradient in a standard Pockels cell as shown in Figure 1, the one calculated in the same conditions with a heatproof Pockels cell as shown in Figure 2. These calculations presented Figure 5 simulate the behavior thermal a Pockels cell in a typical case of exchange (heat flux of 80 mW / mm 2 on a surface of 75 mm 2 ). Natural convection, radiation and conduction are considered in these simulations. It is noted that the thermal gradient between the crystals of the Pockels cell is 3.5 ° C in a representative case of the prior art and 0.6 ° C in an athermal configuration according to the invention. The difference is even more important that the flow of external heat is considered important. In typical conditions considered, the temperature gradient in the athermal Pockels cell is divided by nearly 6 with respect to a Pockels cell of the prior art.

A second example of athermal Pockels cell according to the invention can be achieved, which allows horizontally channel the thermal power of the external heat source to the vertical plane of symmetry of the Pockels cell. The thermally conductive element shown in Figures 3 consists of a horizontal frame 151 (as XY) of preferably hollow, vertical thickness (along Z) predetermined to be in contact with the supporting structure not shown in Figures 3. in its recessed center is placed a horizontal plate 152 preferably solid for thermal reasons, vertical thickness less than the vertical thickness of the frame 151 and on which is mounted the base 12. it is connected to the frame 151 by a ( or two) arm 153 full or hollow, located in the vertical plane 1 60 through the middle of the two crystals. The vertical thickness (in the Z direction) of the arm 153 is for example equal to that of the frame 151 but may be lower; as in the example of the blade 15 of Figure 2, its width (along Y) is much lower than that of the wafer 152 to concentrate its conduction channel. Other configurations are possible. For example, the dimension of the arm 153 in the Z direction is such that the plate is raised relative to the frame 151.

A third example of athermal cell according to the invention may be performed wherein the thermally conductive element shown Figure 4 is a horizontal plate 155 provided with thermal braids or heat pipes 156 disposed on the edges of the tray, perpendicular to the vertical plane 160 passing in the middle of the two crystals, for symmetrical dissipation

heat in the cell. This plate 155 may be rectangular as shown in the example of FIG but not necessarily; more typically it has a symmetrical shape with respect to the vertical plane 1 60 passing between the crystals. It is intended to be in contact with the supporting structure not shown in Figure 4.

So far it has been assumed in the description that the carrier structure was subjected to a heat source from only one direction "lateral" as illustrated in Figures 2 and 4, but of course it can come from many directions and lateral / or originating from below of the support structure 13.

The applications are the production of Q-switched laser oscillators (by a Pockels cell). Similar application is to use the Pockels cell to introduce or take out an existing light pulse in a laser amplifier. As for the Q-switch is used the ability to modify the polarization of the laser beam by applying a voltage. For both applications, the Pockels cell is associated with a polarizer for controlling the transmission of the laser beam.

We can also associate the Pockels cell to any other sensing element to the polarization of the laser light. For example, if instead of associating the cell with a polarizer, it is associated with a harmonic generator (to generate new wavelengths or new color beam from a change in polarization) it is possible to create a wavelength switch, instead of a device changing the transmission.

CLAIMS
1. Pockels cell (100) having two crystals (10a, 10b) similar electro-optic oriented in temperature compensation on a base (12) common to the two horizontal metal crystals, and a support structure (13), characterized in that it comprises between the base (12) and the supporting structure (13) a thermally conductive member with symmetric configuration according to a vertical plane (1 60) passing between the two crystals, to symmetrically distribute to the base (12), a heat flux ( 150) generated in the carrier structure (13) asymmetrically with respect to the vertical plane (1 60).

Pockels cell according to the preceding claim, characterized in that the thermally conductive element is a vertical blade (15).

Pockels cell according to claim 2, characterized in that the vertical blade (15) and the base (12) form one piece.

Pockels cell according to claim 2, characterized in that the vertical blade (15) and the supporting structure (13) form one piece.

Pockels cell according to claim 2, characterized in that the vertical blade (15), the base (12) and the supporting structure (13) form one piece.

Pockels cell according to claim 1, characterized in that the thermally conductive element is constituted:

- a frame (151) horizontal to be in contact with the supporting structure (13) with its hollow center,

- a plate (152) horizontal on which is mounted the base (12) and which is connected to the frame (151) by

- an arm (153) located in the vertical plane (160) passing between the two crystals.

7. Pockels cell according to claim 1, characterized in that the thermally conductive element is a plate (155) provided with horizontal thermal braids (156) disposed on the edges of the plate perpendicular to the vertical plane (160) passing between the two crystals.

8. Laser Q-switched laser cavity comprising a Pockels cell according to one of the preceding claims.

9. wavelength switch which comprises a laser and the laser output, a Pockels cell according to one of claims 1 to 7.