DE102017119434A1 - VIBRATION STEAMED POCKEL CELL - Google Patents

Abstract

A Pockels cell (5) has an electro-optic crystal (7) extending along a longitudinal axis (Z) with at least one side surface extending along the longitudinal axis (Z) and at least one support element (11) disposed along the side surface made of a damping material. An adhesive layer (23) extends between the side surface of the crystal (7) and the support member (11), the adhesive layer (23) having a thickness (Dk) ranging from 10 μm to 200 μm.

Description

The present invention relates generally to the mounting of electro-optic crystals, in particular Pockels cells with attenuated electro-optic crystals. Furthermore, the invention relates to Pockels cells for providing an optically stable polarization window.

The activation of a Pockels cell for the polarization adjustment of electromagnetic radiation, in particular laser radiation, is effected by rapid switching of a high voltage applied to the electro-optical crystal of the Pockels cell. The applied high voltage causes via the electro-optical effect an electrical polarization in the crystal, which leads for example to a desired birefringence of the crystal. The birefringence can be used to adjust the state of polarization of laser radiation passed through the crystal of the Pockels cell.

It is also known that such rapid voltage circuits can be accompanied by mechanical vibrations of the crystal, which are caused by a piezoelectric effect occurring simultaneously with the electro-optical effect. Mechanical damping of excited resonances can be effected, for example, by the shaping of the crystal or by special holders. DE 102 37 308 A1 discloses a vibration damping caused by the crystal form. DE 10 2013 012 966 A1 further discloses damping mechanical vibrations by cohesively bonding a crystal bonded to electrodes. Furthermore, crystals are trapped or glued, see, for. B. EP 1 532 482 B1 and WO 2005/059634 A1 ,

However, Pockels cells, in which such piezoelectric vibrations are effectively suppressed by mechanical damping, can be very sensitive to temperature variations. Thus, for example, a built-in such Pockels cells electro-optical crystal, such as a BBO crystal tear by performing the mechanical damping already at a temperature difference of a few degrees Kelvin, so that the Pockels cell is unusable. In general, it is desirable to allow the storage and transport of Pockels cells in a temperature range of -25 ° C to +70 ° C. The thermal expansion is z. B. in the DE 1 972 737 addressed with the help of a rubber layer.

In the prior art, the thermal expansion is considered independently of the vibration problem, so that the known approaches do not allow a very good mechanical damping while robustness to temperature variation.

One aspect of this disclosure is based on the object of providing a Pockels cell which, on the one hand, can be exposed to temperature fluctuations and, on the other hand, damps piezoelectric oscillations during operation. Further tasks relate to the optimization of the vibration damping and the contacting of an electrode coating of an electro-optical crystal in a Pockels cell.

At least one of these objects is achieved by a Pockels cell according to claim 1. Further developments are specified in the subclaims.

In one aspect, a Pockels cell comprises an electro-optic crystal extending along a longitudinal axis with at least one side surface extending along the longitudinal axis, at least one support member disposed along the side surface of a damping material, and an adhesive layer extending between the side surface of the crystal and the support member , The adhesive layer has a thickness which is in the range of 10 .mu.m to 200 .mu.m.

In some embodiments of the Pockels cell, the support member (eg, typically an elastic polymer) and the adhesive layer form a vibration energy absorbing damping system. In particular, the damping material of the carrier element, the crystal and the adhesive layer have mutually adapted thermal expansion coefficients.

In some embodiments, the ratio of the modulus of elasticity (at 20 ° C.) of the damping material of the carrier element to the elastic modulus (at 20 ° C.) of the adhesive layer is in the range of 10 to 200, in particular in the range of 20 to 100 or 30 to 70 and / or Ratio of the modulus of elasticity (at 20 ° C) of the damping material of the carrier element to the modulus of elasticity (at 20 ° C) of the adhesive layer is in the range of 50 to 70 in a thickness of the adhesive layer in the range of 30 .mu.m to 100 .mu.m, z. At 60 with a thickness of the adhesive layer in the range of about 50 microns to about 60 microns. Thus, the damping material of the support member may have a modulus of elasticity (at 20 ° C) in the range of 1 GPa to 80 GPa.

In some embodiments, the Pockels cell further comprises two electrode units arranged on opposite sides of the crystal, in particular flat electrode structures.

In some embodiments, the crystal has two contacting sides extending along the longitudinal axis (Z) and opposing each other with respect to the longitudinal axis, and the Pockels cell further comprises an electrode material coating provided, for example, as a gold coating, on the corresponding contacting side and each with an electrode a high voltage switching system for applying a high voltage waveform is connectable, and the adhesive layer extends between the electrode material coating and the damping material.

In some embodiments, the contacting side corresponds to the side surface along which the carrier element is arranged, and a contact finger extends through the carrier element and is electrically conductively connected at one end to the electrode material coating, in particular adhesively bonded.

In some embodiments, an adhesive surface of the contact finger bonded to the electrode material coating is smaller than 4 mm ² , and in particular rectangular, for example square, with a side length of 1.5 mm.

The damping material is z. Example, a homogeneous polymer material, such as polyoxymethylene, polyetheretherketone (PEEK) or polyamide or a composite material of a polymeric material, for example, for example, polyoxymethylene, polyetheretherketone or polyamideimide, and a support structure, such as a metal deactivator structure and / or a fiber structure of glass and / or carbon fibers. The damping material may also be a composite of polymer layers, such as PEEK, and metal layers, such as copper or aluminum. In this case, at least one subgroup of the metal layers can extend to an inner side of the damping material, in particular in the entire region of the side surface of the crystal, and can be electrically connected to an electrode of a high-voltage switching system for applying a high-voltage profile. The metal layers may comprise metal strips (eg, aluminum and / or copper strips) which are in particular oriented orthogonally and / or parallel to the side surface. Several of the metal layers may be in electrical contact with an outer surface of the damping material with a contact foil.

In some embodiments, the carrier element is divided into two and, in a region arranged in the interior of the carrier element in the assembled state of the carrier element, has a recess for receiving the, in particular cuboidal, crystal. The crystal may be parallelepiped-shaped and the at least one carrier element may be bonded to two opposite sides of the crystal, and on the remaining opposite sides of the crystal, one electrode each of a high-voltage switching system may be provided for applying a high-voltage waveform.

In some embodiments, the Pockels cell has a housing that fixes the support member and the crystal and has at least one opening for light coupling into the crystal and light extraction from the crystal.

Generally, the ratio of the thermal expansion coefficient of the crystal to the coefficient of thermal expansion of the support member may be in the range of 0.5 to 1.5, and / or the ratio of the coefficient of thermal expansion of the adhesive layer to the coefficient of thermal expansion of the support member and / or the crystal may be in the range of 0 , 8 to 1.3 lie.

In some embodiments, prior art problems are solved by matching an assembly comprising the electro-optic crystal and at least one support member bonded to the crystal in a vibration damping structure with respect to the thermal expansion coefficients of the components. For this purpose, a suitable choice of material for the adhesive and the carrier element in dependence on the crystal and a suitable geometric dimensioning of the carrier element, in particular an adhesive layer between the carrier element and crystal.

For this purpose, in some embodiments, the carrier element is designed as a meta material adapted in the thermal expansion coefficient, which can additionally be used as an electrode. Generally, herein the carrier element, if used for applying an electric field, is also referred to as an electrode.

Aspects described herein are based, in part, on the recognition that Pockels cells can be improved by considering the aspects of mechanical damping and thermal expansion adaptation together in the Pockels cell design.

The first aspect of the Pockels cell design refers to the mechanical damping of the crystal by the support having at least one support member. The mechanical damping is achieved in that the crystal is attenuated to the material of the support member is connected with a suitably designed adhesive layer. As a result, caused by the piezoelectric effect temporary deformation of the Crystal, however, secondary effects (such as resonating resonance effects) can be effectively suppressed. This property is advantageous, for example, when a Pockels cell is to be used as a fast electro-optical switch in the case of several frequency bands, for example in a regenerative amplifier with a variably adjustable repetition rate. A correspondingly large attenuation of the mechanical deformation of the crystal due to the piezoelectric effect - possibly with a partial suppression - can be achieved in that the carrier element consists of a damping material, which is positively attached to the crystal, and that lying between the crystal and the carrier element adhesive layer the damping material of the support member forms a correspondingly designed mechanically damping system with respect to crystal deformations, for example by adjusting its modulus of elasticity on the modulus of elasticity of the damping material. For this purpose, the adhesive layer has a thickness in the range of a few 10 .mu.m to a few 100 .mu.m and acts by the appropriate choice of the modulus of elasticity together with the damping material of the support element damping the movement of the crystal.

The second aspect of the Pockels cell design deals with the thermal expansion of the (e.g., BBO) crystal and the support member. If a temperature change occurs, for example as a result of the heating of the Pockels cell during laser operation or due to changing temperatures during transport, different thermal expansion coefficients of the crystal and the surrounding material (essentially the support element) can lead to mechanical stresses in the crystal.

During switching operation, such mechanical stresses may adversely affect the Pockels cell electro-optic switching characteristic. So z. B. reduce the polarization contrast. Furthermore, a Z-direction crystal can often withstand only a few megapascals, so that usually the maximum strain voltage in electro-optic crystals is very limited. Here, the Z-direction corresponds to the direction of propagation of the light in the crystal, which usually runs along a longitudinal axis of the Pockels cell. In most cases, the crystal is largest in the Z direction, so that the greatest mechanical stresses along this direction are induced. Very high mechanical stresses can lead to a rupture of the crystal, which can occur even with a temperature change of a few degrees Kelvin.

It has now been recognized that the procedure known from the state of the art for providing an expansion joint filled with a material which is as elastic as possible to compensate for different geometric thermal expansion properties does not adequately comply with the first aspect of the Pockels cell construction. For such an expansion joint would indeed reduce the formation of mechanical stresses with temperature changes, if not prevent, but causes such. B. a very wide and very elastic expansion joint no satisfactory damping effect.

For this reason, the following strategy for adjusting the coefficients of thermal expansion is proposed. The material of the carrier element is selected in such a way that the thermal expansion coefficient of the material of the carrier element is matched as well as possible to that of the crystal. Here, the smaller the difference in expansion coefficients between, for example, BBO crystal and the surrounding material of the support member, the smaller an expansion joint and the stiffer the filling material of the expansion joint can be carried out, so that the resulting mechanical damping of the crystal in particular by the choice of adhesive material can be adjusted.

In some embodiments, when constructing the Pockels cell, it must be considered that the voltage-applied and opposite conductor regions of the at least one carrier element are arranged as positively as possible on the crystal for the desired structure of the electric field (eg, as homogeneous as possible over the crystal). It follows, inter alia, that the adaptation of the thermal expansion coefficients is to be implemented especially for the carrier element.

As is known, it is not possible to match the thermal expansion coefficients for a metal or alloy electrode designed as a carrier element. However, it was recognized that relevant metals such as copper or aluminum have a lower coefficient of expansion than z. B. have a BBO crystal, while polymers typically have a higher coefficient of expansion than the BBO crystal. Thus, with regard to some exemplary embodiments of the Pockels cell disclosed herein, it has been recognized that a composite material, referred to herein as a metamaterial, as a support member / electrode allows geometric properties to combine the properties of an entrapped metal and a matrix acting polymer. The metamaterial can thus be adjusted in terms of the coefficient of thermal expansion, wherein it due to the incorporated metal additionally in the Compared to the pure polymer has higher rigidity.

In further exemplary embodiments of the Pockels cell disclosed herein, the electrode is implemented as a thin layer on the crystal surface. The carrier element can then be glued to the layer. If the layer is thin enough, the mechanical stress that could be thermally induced by this layer can be neglected. The contacting of the layer can, for example, locally through the support member, z. B. in the Z direction in the middle of the crystal. The support element may, for. B. from a pure or stabilized with deposits polymer. Thus, the support element z. B. from a polymer which has been modified by metal deactivators and / or by glass fibers are used. Such composites may have a thermal expansion coefficient equivalent to that of electro-optic crystals such as BBO. They are thus suitable, among other things, as body material of the electrode which is compensated for thermal expansion.

The embodiments disclosed herein may include i.a. have the following advantages:

You can use a combination of good to very good mechanical vibration damping together with a thermal robustness to temperature fluctuations of z. B. allow ± 50 K. In particular, permissible transport conditions in the range of -25 ° C to 70 ° C can be guaranteed.

The concepts disclosed herein for supporting a crystal in a Pockels cell can be generally used in polarization adjusting applications of the Pockels effect. Thus, the concepts described herein relate in particular to the coupling of electromagnetic radiation to be amplified, in particular of laser pulses, as well as the decoupling of amplified laser pulses, in particular in Q-switched lasers, cavity dumping or regenerative amplification. Other applications include i.a. an intensity and polarization modulation outside a cavity, e.g. B. in the control of a pulse picker. Other applications include CW laser, expander with upstream pulse picker and Q-switching.

In general, the implementation of the concepts proposed herein is independent of crystal geometry. By way of example, the implementation of the concepts proposed here in cuboid crystal forms will be described below. However, the concepts can also be generally applied to polyhedron-shaped crystals (for example, in the form of prisms, in particular as parallelepiped crystals) or cylinder-shaped crystals.

• 1 ein schematisch dargestelltes Lasersystem mit einer schaltbaren Pockels-Zelle,
• 2 eine schematische Darstellung einer beispielhaften Ausführungsform einer Pockels-Zelle in einem Längsschnitt,
• 3 eine schematische Frontansicht z. B. der Pockels-Zelle aus 2 zur Verdeutlichung einer beispielhaften Ausführungsform eines Trägerelements,
• 4 eine schematische Frontansicht z. B. der Pockels-Zelle aus 2 zur Verdeutlichung einer weiteren beispielhaften Ausführungsform eines Trägerelements,
• 5 eine schematische Darstellung einer Ausführungsform einer Pockels-Zelle mit einem Trägerelement aus einem Metamaterial in einem Längsschnitt,
• 6 eine schematische Frontansicht einer weiteren Ausführungsform einer Pockels-Zelle mit einem Metamaterial und
• 7 eine schematische Frontansicht einer weiteren Ausführungsform einer Pockels-Zelle mit einem Metamaterial.

Herein, concepts are disclosed that allow to at least partially improve aspects of the prior art. In particular, further features and their expediencies emerge from the following description of embodiments with reference to the figures. From the figures show:

• 1 a schematically illustrated laser system with a switchable Pockels cell,
• 2 1 is a schematic representation of an exemplary embodiment of a Pockels cell in a longitudinal section;
• 3 a schematic front view z. B. the Pockels cell 2 to illustrate an exemplary embodiment of a carrier element,
• 4 a schematic front view z. B. the Pockels cell 2 to illustrate a further exemplary embodiment of a carrier element,
• 5 a schematic representation of an embodiment of a Pockels cell with a support element of a metamaterial in a longitudinal section,
• 6 a schematic front view of another embodiment of a Pockels cell with a metamaterial and
• 7 a schematic front view of another embodiment of a Pockels cell with a metamaterial.

Aspects described herein are based, in part, on the recognition that optical crystals used in Pockels cells (eg, BBO, KDP, or KTP crystals) have more or less pronounced piezoelectric properties. These can cause that made electrical switching pulses generate mechanical deformation of the crystal. The deformations of the crystal can occur in all three spatial directions. Dependent on u. a. The dimensions, the geometry and the speed of sound of the respective crystal can cause these deformations disturbances. Thus, operation near a resonant frequency (or associated subharmonic) can result in unstable switching behavior, e.g. B. lead to an unstable coupling or decoupling behavior in a regenerative amplifier. The mechanical sensitivity of the crystal is attenuated by a suitably adapted storage and attachment of the optical crystal.

In particular, it was recognized that the vibration behavior of the optical crystal can be influenced by means of a correspondingly adapted storage and attachment of the optical crystal. For this purpose, a damping system is proposed which comprises a carrier element and a bond formed in a small gap between the carrier element and the crystal. It was recognized that by tuning the mechanical parameters such as the moduli of elasticity and the gap thickness a desired damping behavior can be implemented. That damping system can be designed so that when the crystal deforms, possible no ringing occur. In other words, the damping system is intended to damp a vibration behavior / ringing of the crystal due to the rigidity of the carrier element.

The damping system shall be designed for an energy absorption adapted to the deformation occurring during operation. In other words, in the damping system, the adhesive layer and possibly the carrier element can absorb (vibrational) energy by their own deformation. Although this can lead to a slight warming of the Pockels cell, however, the resulting heat can be dissipated with the cooling system of the Pockels cell or via the air.

If, for example, a BBO crystal deforms and the adhesive is too soft, there is essentially no damping for the deformation since the adhesive does not build up any resistance. On the other hand, if the adhesive is very hard and the support element also has a corresponding rigidity, no deformation of the BBO crystal can take place and mechanical stress curves in the BBO crystal can thereby be generated, whereby the polarization change can likewise take place to an uncontrolled extent.

If the bond gap is generally too large, the mechanism of the crystal is not coupled to the carrier element. Accordingly, the damping system is determined (only) by the elastic properties of the adhesive and not by the properties of the support element.

Furthermore, it has been recognized that in a particularly stiff embodiment of the carrier element, the damping behavior of the damping system for suppressing disturbances due to the mechanical deformation of the crystal, in particular over a larger excitation frequency range, can be adjusted.

Thus, the various approaches disclosed herein for supporting the crystal with a damping system may allow, for example, high voltage circuit operation close to a resonant frequency of the crystal. Resonant frequencies of a crystal depend in particular on the dimensions of the crystal such as the extent of the crystal in the direction of the applied electric field, as well as the crystal type, the crystal form, generally the crystal section, an adjacent E-field vector and / or a scattering of mechanical vibrations in original not stimulated spatial axes.

Hereinafter, the concepts proposed herein with reference to FIGS 1 to 7 explained in more detail.

As mentioned above, Pockels cells can be used for fast switching between radiation paths of electromagnetic radiation, in particular laser radiation. In such used Pockels cells, birefringence can be induced by applying a high voltage to a suitable optical crystal (eg, using utility voltages up to and greater than 10 KV). The switchable birefringence allows a temporally adjustable change of the polarization state of the light passing through the crystal. In combination with a polarizer can be in this way z. B. the quality of a laser resonator can be switched. This is z. B. in Q-switched lasers, cavity dumping and in regenerative amplifiers (to adjust the repetition rate (s)) used. Switching the Pockels cell between two voltage states, i. H. the single voltage switching operation is typically very fast (eg, within a few nanoseconds), maintaining a voltage state for an adjustable duration of a polarization window (eg, for a few microseconds). This makes it possible, for example, to select individual (laser) pulses of a pulse train. However, voltage switching processes also cause the mentioned mechanical vibration excitation.

1 schematically shows an exemplary laser system 1 with a gain medium 3 , In the beam path, an optical switch is provided on a Pockels cell 5 based. The Pockels cell 5 points as in 1 schematically illustrated, an electro-optical crystal 7 , Electrode units 9 (exemplified in 1 opposite conductive coatings on contacting sides of the crystal 7 ) and two the crystal 7 holding carrier elements 11 on.

The crystal 7 extends along a longitudinal axis Z of the Pockels cell 5 , along with the laser radiation 13 through the crystal 7 occurs. The laser radiation 13 For example, includes ultrashort laser pulses that come in one with an end mirror 15 circulate formed cavity, or amplified laser radiation of an amplifier.

The crystal 7 is between the electrodes 9 arranged one near the other along the longitudinal axis Z extending side surface of the crystal 7 are arranged. The crystal 7 influenced in dependence on the switchable voltage applied to the polarization of each passing laser radiation 13 , For clarification, in 1 an excited Pockels cell with one in the crystal 7 homogeneously designed electric field 21 indicated.

The carrier elements 11 are also along the side surfaces of the crystal 7 arranged and consist of a damping material. Each of the carrier elements 11 is through an adhesive layer 23 with the crystal 7 , or in the case of a coated crystal with the coating connected.

Next to the Pockels cell 5 the optical switch comprises one in the beam path of the laser radiation 13 arranged wave plate 17 , In the exemplary embodiment of the laser system 1 of the 1 become the Pockels cell 5 and the wave plate 17 in the double passage due to a reflection at the end mirror 15 used.

Furthermore, the optical switch comprises a polarization-dependent beam splitter 19 ,

The excitation of the crystal 7 For example, it is designed so that after the double pass through the wave plate 17 (eg a λ / 8 wave plate) and through the crystal 7 (eg switchable as a + λ / 8 wave plate or -λ / 8 wave plate) during the polarization window, no polarization change is made, while outside the polarization window (eg when the Pockels cell is not excited) the optical switch is used as λ / 2 wave plate acts. In the latter case, ie outside the polarization window, the retrograde laser radiation in the beam splitter 19 reflected. Thus, using the electro-optical effect, the polarization can be adjusted and laser radiation after a desired amplification from the laser system 1 be decoupled.

The optical crystals used in Pockels cells (eg BBO, KDP, KTP crystals) have more or less pronounced piezoelectric properties. These cause the application of an electrical voltage to the crystal 7 depending on the polarity to an expansion or contraction of the crystal 7 generally leads to deformation of the crystal 7 , If the change in voltage occurs quickly enough, mechanical vibrations (acoustic modes) in the crystal can occur 7 be stimulated. As a result, the crystal deforms characteristically and begins to vibrate.

To be on the deformation of the crystal 7 interacting with the carrier elements 11 along with the adhesive layers 23 each one (vibration) damping system.

In the following, two types of exemplary embodiments of attenuated Pockels cells will be described in more detail. The first type is based on a crystal thin film electrode polymer assembly ( 2 to 4 ), the second type is based on a crystal metamaterial assembly ( 5 to 7 ).

2 shows an exemplary embodiment of a Pockels cell 5 ' with a cuboid-shaped electro-optical crystal 7 ' in a sectional view. For example, the crystal has 7 ' front surfaces 31 through, through the polarized light, usually laser radiation, into the crystal 7 ' entry or exit. The crystal 7 ' is traversed by the laser radiation in the Z direction and is usually elongated in the Z direction. The front surfaces 31 For example, have a square shape with a side of a few millimeters z. B. 4 mm. For clarification is in 2 the thickness of the crystal 7 ' shown oversized. The length of the crystal 7 ' in the Z direction is a few millimeters, for example, 5 mm to 30 mm, z. B. 25 mm.

Two opposite sides of the crystal 7 ' are with a thin, z. B. a few microns thick, gold layer 9 ' steamed. With the help of an adhesive layer 23 ' gets on each of the gold layers 9 ' a polymer carrier element 11 ' attached, which can also be glued to the non-coated side surfaces. With view on 3 and 4 can become edge lengths of the Pockels cell 5 ' , ie, give a width of the Pockels cell transverse to the Z direction, of a few tens of millimeters, e.g. B. 20 mm to 30 mm.

Outside on the carrier elements 11 ' can z. B. massive electrodes 25 for connection to a high voltage switching system for applying a high voltage characteristic (voltage source in 2 not shown) are applied. To the electrodes 25 each with one of the gold layers 9 ' electrically conductive can connect to any of the gold layers 9 ' a few 10 microns thick aluminum flag, which is a few millimeters (eg 1 mm to 3 mm) wide and long, applied with elastic (electrically conductive) contact adhesive. Alternatively you can use the electrodes 25 (eg copper / aluminum stamp) directly on the gold layer stick (adhesive layer 27 ).

Through recesses in the support elements 11 ' protrude contact fingers 29 to the electrodes 25 over the adhesive layers 27 with the gold layer 9 ' electrically conductive to connect. The contact fingers 29 may for example consist of aluminum or copper and cylindrical, conical, cuboid or obelisk-shaped. In a alternative embodiment, the connection of the contact fingers 29 with the gold layer 9 ' be designed as a resilient system. For this purpose, the contact finger 29 a perpendicular to the Z-direction spring device containing the contact fingers 29 on the gold layer 9 ' suppressed.

The diameter, generally the volume, of the punch-shaped contact fingers 29 is preferably kept as low as possible, so that the thermal expansion behavior of the Pockels cell 5 ' as little as possible through the contact fingers 29 being affected. For example, a contact finger thickness Df (for example, in the Z direction in FIG 2 indicated) at some 100 microns to a few millimeters, for example, 500 microns or 1.5 mm. The connection of the contact fingers 29 to the gold layer 9 ' has by the bond on a certain elasticity in order to ensure a sufficiently insured with respect to thermal stresses electrical connection.

The sizing of the Pockels cell 5 ' , in particular the associated damping system, depends on the following parameters: thickness dk the bonding gap (ie, the thickness of the adhesive layer 23 ' ), Modulus of elasticity and coefficient of thermal expansion of the adhesive, modulus of elasticity and thermal expansion coefficient of the support elements 11 ' forming polymer, geometry of the electro-optic crystal used, thermal expansion coefficient of the electro-optical crystal and elastic modulus of the electro-optical crystal and the mechanical fracture stress of the electro-optical crystal.

For the carrier elements 11 ' For example, usable polymer materials or aforementioned metamaterials have elastic moduli (at 20 ° C.) of 1 GPa to 50 GPa and thermal expansion of 10 ppm / K to 60 ppm / K. Possible adhesives have in the cured state, for example, elastic moduli (at 20 ° C) of less than or equal to 1 GPa and thermal expansion of greater than 30 ppm / K. Possible thicknesses Dk of the adhesive layer 23 ' (of the adhesive gap) are in the range of 5 microns to 300 microns. Typical thicknesses for sufficiently damping systems are in the range of 30 μm to 200 μm, for example in the range of 50 μm to 100 μm. Due to manufacturing inaccuracies, variations in the range of up to ± 30 μm are possible.

For a carrier element 11 ' with a material that is in thermal behavior to the crystal 5 ' is adapted, the damping system can be analyzed, for example, by varying the parameters of the adhesive and the gap thickness by means of a finite element simulation. It stands for the materials of the support element 11 ' and the materials of the adhesive due to the requirement of an adapted thermal expansion behavior is not always available all material combinations.

It has been found that, in particular for sufficient damping in normal operating situations of a Pockels cell with a BBO crystal, the ratio of the modulus of elasticity (at 20 ° C.) of the damping material of the carrier element 11 ' to the elastic modulus (at 20 ° C) of the adhesive layer 23 ' in the range of 10 to 200, in particular in the range of 20 to 100 or in the range of 30 to 70, lies. In some advantageous embodiments, a ratio of the modulus of elasticity (at 20 ° C) of the damping material of the carrier element resulted 11 ' to the elastic modulus (at 20 ° C) of the adhesive layer 23 ' in the range of 50 to 70 with a thickness of the adhesive layer in the range of 30 microns to 100 microns. A preferred ratio is z. At about 60 for a thickness of the adhesive layer 23 ' in the range of about 50 μm to about 60 μm. With regard to the modulus of elasticity, reference is made to the standard measurement ISO standard of polymers / plastics / elastic materials - eg. B. ISO 527 - referred.

Comparable parameter values also apply to other crystal types used in Pockels cells.

In general, materials for a ratio of the thermal expansion coefficient of the crystal to the coefficient of thermal expansion of the carrier element in the range of 0.5 to 1.5, for example in the range of 0.8 to 1.3, are assumed. Further, a ratio of the coefficient of thermal expansion of the adhesive layer to the coefficient of thermal expansion of the support member and / or the crystal in the range of 0.5 to 5 has been assumed. With regard to the coefficient of thermal expansion, reference is made to the standard measurement ISO standard of polymers / plastics / elastic materials - eg. Eg ISO 11359 - referred.

Thus, an exemplary damping system resulted for the combination of an adhesive with an E-modulus of 58 MPa and a thermal expansion coefficient of 40 ppm / K for the adhesive layer 23 ' , a thickness D of the adhesive layer 23 ' of 50 microns and the polymer polyetheretherketone as an example of a pure polymer for the support elements 11 ' , The polymer is characterized by an E-modulus of about 3.5 GPa and a thermal expansion coefficient of 46 ppm / K. The Pockels cell realized with such a damping system exhibits very good mechanical damping and high thermal robustness.

3 shows a schematic front view of a Pockels cell 33 with a cuboid electro-optical crystal 7 , The cuboidal crystal 7 has a square front surface 31 on, which can be seen centrally in the front view. Accordingly, the crystal points 7 four side surfaces projecting into the plane of the drawing and one (analogous to the front surface 31 ) square back (front) surface.

The crystal 5 is made up of two carrier elements 35 each having a substantially triangular cross-section and extending over the entire length of the crystal 7 extend. Each of the carrier elements 35 indicates the inclusion of one half of the crystal 7 a recess with two perpendicular surfaces 37A . 37B on, in their dimensions to two adjacent side surfaces of the cuboid crystal 5 are adjusted. Next to the surfaces 37A . 37B extend connecting surfaces 37C , In the assembled state of the Pockels cell 33 stands a connection surface 37C a support element 35 in conjunction with a connection surface 37C of the other carrier element 35 , The two recesses are designed so that in the assembled state of the Pockels cell 33 a cuboid recess results, which is substantially positive fit to the crystal 7 adapted to accommodate the same.

On two opposite side surfaces of the crystal 7 (in 3 top and bottom) is an electrically conductive coating 39 provided that allows a homogeneous electric field in the crystal 7 (in 3 z , B. from top to bottom). These are about contact fingers 29 (dashed lines) with external electrodes 25 in electrically conductive connection. The electrodes 25 can be connected to a switchable voltage source. The contact fingers 29 in 3 are conical by way of example and, for example, in the Z direction in the middle in the support elements 35 arranged.

The two support elements 35 are at the connection surfaces 37C glued together. Furthermore, the two support elements 35 each with a coated and an uncoated side surfaces of the crystal 7 bonded. In 3 is an adhesive layer 41 dashed lines, which can be divided into different sections from top left to bottom right. Two sections 41A extend between the connecting surfaces 37C the support elements 35 , two sections 41B extend between each one recess side of the support element 35 and an uncoated side of the crystal 5 and two sections 41C extend between each of the other recess side of the support element 35 and the coating 39 of the crystal 7 ,

The adhesive layer 41 has a thickness Dk which, together with the moduli of elasticity of the adhesive and the carrier elements 35 defines the damping system described above.

The carrier elements 35 in addition to a housing 43 secured in their position. The housing 43 allows positioning of the Pockels cell 33 in the beam path and can be connected in addition to the cooling with a cooling system. Accordingly, water-cooled z. B. operating temperatures in the range of 40 ° C to 50 ° C can be stabilized.

4 illustrates another embodiment of a Pockels cell 45 , at the support elements 47 consist of a polymer in which about 25% glass fiber content has been embedded. By the arbitrary alignment of the glass fibers 49 increases the modulus of elasticity. Furthermore, the coefficient of thermal expansion of the support elements 47 reduced.

The carrier elements 47 are in this embodiment only with the uncoated sides of the crystal 7 glued (adhesive layer 41 ), wherein the materials and the gap thickness were coordinated to form a damping system. Exemplary shows 4 in cross section trapezoidal shaped support elements 47 ,

Alternatively to the in 3 conductive coatings shown can surface electrodes 50 be arranged on the free sides of the crystal, which in turn can be connected to a switchable power supply.

It is generally noted that various aspects of the individual exemplary embodiments described may also be implemented in other embodiments. This concerns z. B. the shape of the support member and the contact.

5 shows a metamaterial-based embodiment of a Pockels cell 5 ' in a sectional view. As in 3 has the Pockels cell 5 ' a cuboid-shaped electro-optical crystal 7 " on. At least one of the four side faces of the crystal is along the longitudinal axis of the crystal 7 " flat with a metamaterial electrode 51 bonded.

Between the metamaterial electrode 51 and thus the crystal is again an adhesive layer 53 , so that forms a damping system.

The metamaterial electrode 51 exists as a combination material along the Crystal longitudinal direction alternating metal layers 51A and polymer layers 51B , Accordingly, the metal layers extend 51A and the polymer layers 51B transverse to the crystal longitudinal axis. The ratio of the respective layer thicknesses Dm, Dp defines the resulting total thermal expansion of the material. Thus, the ratio depending on the polymer and metal can be chosen such that the thermal expansion coefficients of the metamaterial electrode 51 and the crystal 7 " are about the same.

The functionality of the entire assembly depends on the dimensioning of the following properties and sizes: Young's modulus and coefficient of thermal expansion of the adhesive, adhesive gap width, thickness of the individual layers of the metamaterial electrode 51 , For metal layers of aluminum thus elastic moduli z. In the range of 10 GPa to 50 GPa for the metamaterial. When using copper in the metal layers and higher moduli of elasticity in the range of z. B. 50 GPa to 80 GPa can be achieved.

In this case, all parameters are connected and influence each other. By evaluating these parameters, for example in a finite element simulation and considering available materials, a possible parameter set may be as follows: adhesive with an E-modulus of 1.2 GPa and a thermal expansion coefficient of 40 ppm / K with 10 μm adhesive gap width; 20 μm thick copper strips alternate with 10 μm thick PEEK strips in the metamaterial electrode 51 from.

The metamaterial electrode 51 This can result in a resulting thermal expansion of 36 ppm / K and a modulus of elasticity of approximately 70 GPa, thus ensuring excellent mechanical damping and at the same time ensuring very high thermal robustness of the entire assembly.

In the in 5 embodiment shown are opposite outer edge surfaces 52 the metamaterial electrodes 51 with the electrodes 25 over thin bridges 54 connected. The electrodes 25 are in turn with a switchable power supply to generate an electric field in the crystal 7 " connectable. The metamaterial electrode 51 is formed so that on opposite sides of the crystal along the entire length of the crystal electrically contacted metal layers are arranged, preferably up to the adhesive layer 53 protrude. For example, the other two sides can be left blank or bonded with a polymer.

6 shows a front view of another on a metamaterial electrode 51 ' based Pockels cell 57 , In contrast to the embodiment of 5 the metal layers extend 51A and the polymer layers 51B along the crystal axis. Further, the metamaterial electrodes 51 ' only with two opposite sides of the crystal 7 " bonded. As in 5 the metal layers extend 51A in the entire area of the side surfaces up to the adhesive layer 53 , The overall assembly is in a housing 43 fixed, in turn, edge surfaces 52 the metamaterial electrodes 51 with electrodes 25 be in electrical connection.

7 shows a front view of another on a metamaterial electrode 51 " based Pockels cell 59 , In the embodiment of the 7 the metal layers extend 51A and the polymer layers 51B parallel to the side surface of the crystal 7 " , The metal layers 51A are not intended for shaping the electric field. This is done z. B. as in 4 using separately provided electrodes 50 , Alternatively, the contact layer between the crystal and the metamaterial can be an electrically conductive metal layer, which, for example, as in 4 described can be contacted by separate electrodes.

As explained above, the mechanical properties of the Pockels cell, in particular its vibration behavior, are designed by the specific selection of the moduli of elasticity to a damping of the embedded crystal, in addition to the breaking stress by adjusting the thermal expansion coefficients.

It is explicitly pointed out that all features disclosed in the description and / or the claims are considered separate and independent of each other for the purpose of original disclosure as well as for the purpose of limiting the claimed invention independently of the feature combinations in the embodiments and / or the claims should. It is explicitly stated that all range indications or indications of groups of units disclose every possible intermediate value or subgroup of units for the purpose of the original disclosure as well as for the purpose of restricting the claimed invention, in particular also as the limit of a range indication.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 10237308 A1 [0003]
• DE 102013012966 A1 [0003]
• EP 1532482 B1 [0003]
• WO 2005/059634 A1 [0003]
• DE1972737 [0004]

Cited non-patent literature

• ISO standard [0062, 0064]
• ISO 527 [0062]
• Eg ISO 11359 [0064]

Claims (19)

Pockels cell (5) with an electrooptical crystal (7) extending along a longitudinal axis (Z) with at least one side surface extending along the longitudinal axis (Z), at least one along the side surface arranged support element (11, 11 ', 35, 47) made of a damping material and an adhesive layer (23) extending between the side surface of the crystal (7) and the support element (11, 11 ', 35, 47), in which the adhesive layer (23) has a thickness (Dk) which is in the range of 10 μm to 200 μm. Pockels cell (5) after Claim 1 wherein the carrier element (11, 11 ', 35, 47) and the adhesive layer (23) form a vibration energy absorbing damping system. Pockels cell (5) after Claim 1 or 2 , wherein the damping material of the carrier element (11, 11 ', 35, 47), the crystal (7) and the adhesive layer (23) have matched to each other thermal expansion coefficients. Pockels cell (5) according to one of the preceding claims, wherein the ratio of the modulus of elasticity (at 20 ° C) of the damping material of the carrier element (11, 11 ', 35, 47) to the elastic modulus (at 20 ° C) of the adhesive layer (23) in the range of 10 to 200, in particular in the range of 20 to 100 or 30 to 70, is located. Pockels cell (5) according to one of the preceding claims, wherein the ratio of the modulus of elasticity (at 20 ° C) of the damping material of the carrier element (11, 11 ', 35, 47) to the elastic modulus (at 20 ° C) of the adhesive layer (23) in the range of 50 to 70 at a thickness of the adhesive layer (23) in the range of 30 microns to 100 microns, z. B. at 60 at a thickness of the adhesive layer (23) in the range of about 50 microns to about 60 microns, is located. Pockels cell (5) according to one of the preceding claims, wherein the damping material of the carrier element (11, 11 ', 35, 47) has a modulus of elasticity (at 20 ° C) in the range of 1 GPa to 80 GPa. Pockels cell (5) according to one of the preceding claims, further comprising two on opposite sides of the crystal (7) arranged electrode units (9), in particular flat electrode structures (50). A pedestal cell (5) according to any one of the preceding claims, wherein the crystal (7) has two contacting sides extending along the longitudinal axis (Z) and opposing the longitudinal axis (Z) and the Pockels cell (5) further comprises an electrode material coating which is provided, for example, as a gold coating (9 ') on the corresponding contacting side and in each case connectable to an electrode (25) of a high-voltage switching system for applying a high voltage characteristic, and the adhesive layer (23) extends between the electrode material coating and the damping material. Pockels cell (5) after Claim 8 , wherein the contacting side corresponds to the side surface along which the carrier element (11, 11 ', 35, 47) is arranged, and a contact finger (29) extends through the carrier element (11, 11', 35, 47) and on a End is electrically connected to the electrode material coating, in particular glued. Pockels cell (5) after Claim 9 , wherein an adhesive surface of the contact finger (29) glued to the electrode material coating is smaller than 4 mm ² , and in particular is rectangular, for example square, with a side length of 1.5 mm. A pedestal cell (5) according to any one of the preceding claims, wherein the crystal (7) is a BBO crystal. Pockels cell (5) according to one of the preceding claims, wherein the damping material a homogeneous polymer material, for example polyoxymethylene, polyetheretherketone or polyamideimide or a composite material of a polymer material, for example, for example, polyoxymethylene, polyetheretherketone or polyamideimide, and a support structure, for example a metal deactivator structure and / or a fiber structure of glass and / or carbon fibers. A pedestal cell (5) according to the preceding claims, wherein the damping material is a composite of polymer layers (51B), for example PEEK, and metal layers (51A), for example, copper or aluminum. Pockels cell (5) after Claim 13 , wherein at least one subset of the metal layers (51A) extend to an inner side of the damping material, in particular in the entire region of the side surface of the crystal (7 '), and are electrically connectable to an electrode of a high voltage switching system for applying a high voltage waveform. Pockels cell (5) after Claim 13 or 14 wherein the metal layers (51A) comprise aluminum and / or copper strips which are in particular aligned orthogonally and / or parallel to the side surface. Pockels cell (5) after one of Claims 13 to 15 wherein a plurality of the metal layers (51A) are in electrical contact with an outer surface of the damping material with a contact foil. Pockels cell (5) according to one of the preceding claims, wherein the carrier element (11, 11 ', 35, 47) is divided into two and in one, in the assembled state of the carrier element (11, 11', 35, 47) in the interior of the carrier element (11, 11 ', 35, 47) arranged region has a recess for receiving the, in particular cuboid-shaped, crystal (7). Pockels cell (5) according to any one of the preceding claims, wherein the crystal (7) is cuboidal and the at least one carrier element (11, 11 ', 35, 47) is glued to two opposite sides of the crystal (7) and on the remaining sides of the crystal (7) are each provided with an electrode (50) of a high-voltage switching system for applying a high-voltage characteristic. Pockels cell (5) according to one of the preceding claims, further comprising a housing (43) which fixes the carrier element (11, 11 ', 35, 47) and the crystal (7) and at least one opening for a light coupling into the crystal (7) and a light extraction from the crystal (7).

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