The field of the invention is that of Pockels cells, which are in particular used in the fields of the amplification of laser beams and of wavelength switches.
Electro-optical materials change the polarization of light via the application of an electrical voltage to the interior of the material. These materials are often associated with polarizers, so as to produce electro-optical switches, which are also referred to as Pockels cells. A polarizer lets pass selectively a light beam the polarization of which is in a preset state. An electro-optical material allows this state to be changed via the application of a voltage. It is thus possible to control the transmission of the light beam by way of an electrical voltage.
Pockels cells have been used in many configurations in the field of amplification of laser beams. One of these configurations, which is also referred to as “Q-switching”, consists in activating a Pockets cell located in the laser cavity, so as to increase the losses of an oscillator (or equivalently to decrease the transmittance of the cell): the laser pulse is then trapped in the cavity. By suddenly applying a voltage to the Pockels cell, the losses of the cavity are removed, thereby allowing the laser pulse to be freed. Q-switched laser oscillators (which generate short pulses, typically of duration comprised between 5 ns and 40 ns) are based on this effect.
Pockels cells containing two transverse-field crystals mounted to achieve temperature compensation allow optical switches that switch with low electrical voltages to be produced. These cells employ two electro-optical crystals in series, thereby allowing the level of the voltage to be applied to activate them to be decreased. The crystals used are in general what are called biaxial crystals: their natural birefringence depends on their cutting axis (X or Y axes) and also varies greatly with temperature. Mounting the crystals to achieve temperature compensation, by precisely orientating them with respect to each other, allows the natural birefringence of one crystal to be compensated for by that of the other. This compensation is effective over an extremely wide temperature range, for example from −40° to +60°. The quality of the compensation, which is the origin of the temperature range over which the Pockels cell will be operational, depends on the relative precision with which the two crystals are positioned, but also on their similarity. Matched pairs of crystals are spoken of. As their name indicates, such matched pairs of crystals must be as similar as possible. For this reason, the two crystals of these cells are generally polished together so as to ensure their dimensions are identical.
Suitably choosing and mounting crystals (which are as similar as possible) ensures the Pockels cell will operate correctly over very wide temperature ranges, provided that the temperature compensation achieved via the crystals is not lost.
However, such a Pockels cell is often adversely affected by exterior factors inherent to the targeted applications. Laser amplifiers or oscillators require a gain medium to be pumped: it is necessary to supply to the gain medium light energy so as to store it in order to subsequently deliver it in the form of a coherent monochromatic beam. This is the laser effect.
This supply of light energy to the gain medium is not perfect, because some of the energy required to produce it is transformed into heat 200.
It was seen above that Pockels cells containing two crystals mounted to achieve compensation operate correctly over wide temperature ranges provided that the crystals preserve their similarity so as to maintain the temperature compensation. However, if the thermal power generated by the pumping induces a temperature difference between the two crystals, such as illustrated in
Removal of the thermal power thus generated is generally not a problem in a “terrestrial” environment subject to few constraints: transfer of the generated thermal power to the cell is prevented by either moving it further away, or by thermally insulating it from the heat source as described for example in patent EP 1 532 482. The first solution is not desirable in a space environment as the equipment must occupy a minimum volume. The second solution is also difficult to implement effectively in a small bulk without weakening the rigidity of the equipment.
In an environment subject to more constraints, such as a space environment for example, and in which bulk is very limited and the use of coolants is proscribed, the thermal power generated by the pumping is most often removed by conduction through the structures bearing the equipment. In this constraining environment, the Pockels cell is most often located in the immediate vicinity of the heat source. The thermal power generated by the pumping system is then inevitably transferred to the Pockels cell.
Therefore, there remains to this day a need for a Pockels cell that simultaneously satisfies all the aforementioned requirements, in terms of heat removal, and the requirements of constraining environments, such as the space environment, in which bulk is very limited and the use of coolants is proscribed.
Rather than prevent the transmission of the thermal power to the Pockels cell in order to avoid the creation of a temperature gradient between the two crystals, the Pockels cell according to the invention includes means that ensure this power is transferred to the two crystals of the cell symmetrically, in order to prevent the compensation achieved via the crystals from being lost.
More precisely, one subject of the invention is a Pockels cell that includes:
two similar electro-optical crystals oriented to achieve temperature compensation on
a horizontal metal base common to the two crystals, and
a carrier structure.
It is mainly characterized in that it includes, between the base and the carrier structure, a thermally conductive element, which has a configuration that is symmetric about a vertical plane passing between the two crystals, in order to symmetrically distribute, to the base, a heat flux generated in the carrier structure asymmetrically about this vertical plane.
Thus, when a thermal source located nearby generates thermal power that is dissipated in the carrier structure of the equipment asymmetrically, the heat flux can be transferred to the crystals only by way of the thermally conductive element. By virtue of this element, which is located in a plane of symmetry of the cell right in the middle of the two crystals, the heat flux, which is asymmetric in the carrier structure, is distributed symmetrically in each of the two crystals of the Pockels cell: this prevents a temperature gradient from forming between the crystals. The compensation is therefore preserved whatever the heat flux dissipated in the cell. This solution is compact (requires no increase in volume) and is independent of the heat flux to be dissipated.
This thermally conductive element is for example a vertical strip.
It may also consist:
According to another embodiment, the thermally conductive element is a horizontal platen equipped with heat pipes that are placed on the edges of the platen and perpendicular to the vertical plane passing midway between the two crystals.
Another subject of the invention is a Q-switched laser comprising a laser cavity including a Pockels cell such as described, or a wavelength switch that includes a laser and, immediately after the exit of the laser, a Pockels cell such as described, then a device allowing the wavelength to be changed.
Other features and advantages of the invention will become apparent on reading the following detailed description, which is given by way of nonlimiting example and with reference to the appended drawings, in which:
In all the figures, elements that are the same have been referenced with the same references.
In the rest of the description, the terms “top”, “bottom”, “front”, “back”, “side”, “horizontal” and “vertical” are used with reference to the orientation of the described figures. In so far as the cell or the thermally conductive elements may be positioned with other orientations, the directional terminology is indicated by way of illustration and is nonlimiting.
Rather than prevent the transmission of the thermal power to the Pockets cell in order to avoid the creation of a temperature gradient, the invention acts so that this power is transferred to the two crystals of the cell symmetrically, in order to prevent the temperature compensation achieved via the crystals from being lost.
The Pockets cell according to the invention described with reference to
An external thermal source 200 located nearby the carrier structure 13 generates thermal power that dissipates in the carrier structure of the cell asymmetrically with respect to the vertical plane 160. This thermal source may also make contact with the carrier structure.
Below, the thermal source 200 is considered to be a heat source generating a heat flux 150 that is asymmetric in the carrier structure, as shown in the example of
According to the invention, the heat flux 150 present in the carrier structure 13 is transferred to the crystals 10a, 10b only by way of a thermally conductive element located between the base 12 and the carrier structure 13, and in contact therewith. This element has a configuration that is symmetric with respect to the vertical plane (in XZ therefore) 160 of symmetry passing between the two crystals 10a, 10b and midway therebetween. The asymmetric heat flux is therefore distributed symmetrically to the base 12 and therefore in each of the two crystals 10a, 10b, thereby preventing a temperature gradient from forming between the crystals. It thus allows the temperature gradient present in the carrier structure to be made symmetrical between the two crystals, in order to preserve the temperature compensation. The compensation is therefore preserved whatever the heat flux dissipated in the cell.
This solution is compact (no increase in volume is required) and is independent of the heat flux to be dissipated.
The vertical strip 15 and the base 12 may form a single part. The vertical strip 15 and the carrier structure 13 may form a single part. Lastly, the vertical strip 15, the base 12 and the carrier structure 13 may form a single part as shown in
This solution is advantageous given the envisaged (space) environments, because it may be easily miniaturized.
Thermal simulations have allowed the thermal gradient induced in a standard Pockels cell such as shown in
A second example of an athermal Pockels cell according to the invention may be produced, which allows the thermal power of the external heat source to be channelled to the vertical plane of symmetry of the Pockels cell horizontally. The thermally conductive element shown in
A third example of an athermal cell according to the invention may be produced, in which example the thermally conductive element shown in
Up to now in the description the carrier structure has been considered to have been subjected to a thermal source acting only from a “lateral” direction as illustrated in
The targeted applications are the production of Q-switched laser oscillators (switched with a Pockels cell). A similar application is the use of the Pockets cell to make an already existing light pulse enter into or exit from a laser amplifier. As for Q-switching, the ability to modify the polarization of the laser beam by applying a voltage is used. For these two applications, the Pockels cell is associated with a polarizer allowing the transmission of the laser beam to be controlled.
The Pockels cell may also be associated with any other element sensitive 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 (allowing new wavelengths or new beam colours to be generated on the basis of a change in polarization), it is possible to create a wavelength switch, rather than a transmission-modifying device.
Number | Date | Country | Kind |
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16/01378 | Sep 2016 | FR | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2017/073105 | 9/14/2017 | WO | 00 |