WO2018141516A1

WO2018141516A1 - Pockels cell and method for producing a pockels cell

Info

Publication number: WO2018141516A1
Application number: PCT/EP2018/050538
Authority: WO
Inventors: Peter Heist; Uwe Kutschki; Shanna Heinitz; Poggel SVEN
Original assignee: Jenoptik Laser Gmbh
Priority date: 2017-02-02
Filing date: 2018-01-10
Publication date: 2018-08-09
Also published as: DE102017001188B3

Abstract

The invention relates to a Pockels cell, which comprises a first crystal, which exhibits a Pockels effect. The use of a thermal-expansion-adapted second crystal to change the acoustic resonance frequencies of the first crystal is described. Piezoelectric ringing can thereby be reduced.

Description

Pockels cell and method of making a Pockels cell

Technical area

The invention relates to a Pockels cell. Pockels cells are well known, see, for example, W. Koechner: Solid-State Laser Engineering; 6th rev. Ed. 2006, Springer, p. 502+ or https://www.rp-photonics.com/pockels_cells.html. A Pockels cell is understood to mean an arrangement which comprises an electro-optical crystal on which electrodes are arranged and which is mounted so that an electrical voltage (high voltage) can be applied to the electrodes and that a light beam can pass through the crystal. The applied voltage causes an electric field, which penetrates the crystal. The electric field applied to the crystal via the electrodes causes a change in the refractive index ellipsoid due to the electro-optical effect, whereby the light passing through the crystal experiences a phase difference between mutually perpendicular polarized portions.

State of the art

The Pockels cell acts like a retarder plate controlled by the electrical voltage and can therefore be used in conjunction with a polarizer as electro-optical modulator or switch. In particular, Pockels cells are used as fast switches or modulators for laser beams. Fast here means that a high modulation or switching frequency can be achieved. As an electro-optical crystal, BBO (beta-barium borate,

BaB20 ₄ ) is used, because this material has particularly outstanding properties, in particular a very high damage threshold and comparatively little pronounced piezoelectric effects. The former allows the switching of very high laser powers, the latter the use in a wide range of the modulation frequency (or the repetition rate of the laser) f to about 1000 kHz.

However, the electric field may cause a piezoelectric effect in the crystal, which is undesirable in a Pockels cell. The piezoelectric effect undesirable in an electro-optical Pockels cell is small in the BBO, but not completely absent. The alternating electric field excites acoustic oscillations, which in turn influence the birefringence properties via the photo-elastic effect and the reciprocal piezoelectric effect. The electro-optical Pockels cell switches in one In this case, it is no longer ideal between the desired polarization states or, when used as a modulator, deviations of the actual modulation from the desired value can occur. This effect is called ringing, which could be called ringing or ringing in the German language. This piezoelectric ringing has negative effects because the quality of the switching or modulation behavior is impaired. For example, the ringing is disadvantageous if the Pockels cell is arranged as a switch in the resonator of a regenerative amplifier. Then, due to the changed influence on the state of polarization, unwanted pre- and post-pulses of the laser radiation in the amplifier can arise. The effect is particularly pronounced when the excited acoustic vibrations are in resonance with the mechanical natural frequencies of the crystal. In addition, there is the risk of destruction of the crystal due to the mechanical stresses that occur.

Various approaches are known to reduce unwanted ringing. Out

US5221988A a damping system for a Pockels cell is known. Aluminum blocks with specially designed surface structures attached to the crystal are designed to dampen the acoustic vibrations.

From US5075795A an electro-optical switch for high repetition rates is known. The acoustic vibrations of the crystal are absorbed by a block of aluminum or lead mechanically bonded to the crystal. The mechanical connection is made by means of an intermediate part made of quartz glass. The disadvantages of this arrangement will be explained below.

From US3653743A an electro-optical switch with suppression of the acousto-optic effect is known. The lateral surfaces of the crystal are connected with plates made of lead, lead glass or titanium to absorb the sound waves. The disadvantages of this arrangement will be explained below.

From US5079642A an electro-optic modulator with acoustic damping is known. By specially attached couplers attached to the crystal, the acoustic waves are shaped and directed onto an absorber. Silver is recommended as the material for the couplers. For the absorber lead, titanium or tungsten or a mixture of 97% tungsten and 3% epoxy resin is recommended. The disadvantages of this arrangement will be explained below. DE10 2013 012 966 A1 discloses a Pockels cell with a damped electro-optical crystal. The avoidance or reduction of ringing is to be effected here by the fact that the crystal is connected to at least one surface cohesively with one of the electrodes or both electrodes. The disadvantages of this arrangement will be explained below. The above five publications include attenuation of sound waves. This can have the disadvantage that the sound waves are converted into heat energy. Because of energy conservation, this energy must be provided by the high voltage source. As a result, the maximum possible repetition rate can be limited or the high-voltage source must be dimensioned for a high output power. In addition, waste heat can form in the sound absorbers. In addition, damping and / or coupling elements must be attached to the crystal and with this in a fixed mechanical connection in the five aforementioned known arrangements. However, the known Dämpfungsund coupling elements have a deviating from the crystal thermal expansion coefficient. A change in temperature can therefore cause mechanical stresses which impair the birefringence properties of the crystal and can even lead to the destruction of the comparatively brittle electro-optical crystal. This is especially critical, bearing in mind that equipment for industrial use during storage and transportation is exposed to conditions where temperature differences of 60 K or more may occur. It should also be noted that the crystal can have different thermal expansions in different directions. The thermal expansion coefficient of the crystal may be generally anisotropic, ie, directional.

From DE 102014201472 A1 it is known to suppress unwanted mechanical vibrations by using modified rectangular signals for driving the Pockels cell. This has the disadvantage that the Pockels cell switches slower. Object of the invention

The invention is therefore an object of the invention to show a cost-producible Pockels cell, in the work area no disturbance of the operation by the piezoelectric effect (ringing, ringing) occurs and is insensitive to temperature changes.

Advantages of the invention

The Pockels cell according to the invention is inexpensive to manufacture and can switch high laser powers with high modulation frequencies in the range of a few hundred kHz and more. It has no or only a negligible ringing in a wide range of variable modulation frequency. At the same time, it is designed so stably that it meets the typical requirements for an industrial product with regard to storage and transport, in particular with regard to large temperature ranges for operation and storage. In comparison with known arrangements which aim at damping mechanical vibrations, the present invention can already avoid the development of resonant mechanical vibrations. As a result, the conversion of electrical energy from the high voltage source is reduced to heat and the high voltage source is correspondingly less burdened or it can be made smaller. Solution of the task:

The object is achieved by a Pockels cell, comprising

a) a composite crystal

b) a first electrode

c) a second electrode

wherein the composite crystal comprises at least a first crystal and at least one second crystal

and the first crystal has a Pockels effect

and the first electrode is disposed on a first side of the first crystal and the second electrode is disposed on a second side of the first crystal

and at least one beam path is provided for a light beam passing through the first crystal

and the second crystal is connected to the first crystal at a first connection surface, wherein the first connection surface is disposed outside the intended beam path and the first connection surface has a normal y

wherein the first crystal has a coefficient of thermal expansion a _x in a direction perpendicular to y and the first crystal has a thermal expansion coefficient a _z in a direction z perpendicular to x and y and

the second crystal in the direction x has a thermal expansion coefficient a _> a and the second crystal has a thermal expansion coefficient a _Z 2 in the direction z

and the coefficient of thermal expansion a _x differs by less than 30% from the coefficient of thermal expansion 0x2 and the coefficient of thermal expansion a _z deviates by less than 30% from the thermal expansion coefficient a _Z 2. To achieve the object, the use of a second crystal, which has a c-axis, for changing the acoustic resonance frequencies of a first electro-optical crystal, which has a c-axis, wherein

the first crystal and the second crystal have an identical chemical composition or the chemical composition of the second crystal differs from that of the first one only by isotopes,

and

the first and the second crystal have an identical crystal structure and / or consist of different phases of the same material,

and

the second crystal is connected to the first crystal at a first connection surface such that the c-axis of the second crystal is parallel to the c axis of the first crystal, the first crystal being exposed to an electric field and at least one beam path for a light beam is provided, which passes through the first crystal. In addition, there is provided a method of fabricating a Pockels cell comprising a) providing a first crystal, the first crystal exhibiting a Pockels effect, and at least one beam path for a light beam passing through the first crystal

b) attaching a first electrode and a second electrode to the first crystal, c) providing a second crystal,

d) connecting the second crystal to the first crystal at a first connection surface, wherein the first connection surface is arranged outside the intended beam path, wherein the first connection surface has a normal y

and the first crystal has a coefficient of thermal expansion a _x in a direction perpendicular to y, and the first crystal has a thermal expansion coefficient a _z in a direction z perpendicular to x and y, and

and coefficient of thermal expansion a _x differs by less than 30% of the thermal expansion coefficients of 0x2 and the thermal expansion coefficient of a _z by less than 30% deviated from the thermal expansion coefficient a _Z 2nd description

As Pockels effect, also known as a linear electro-optical effect, a change over a certain range linear refractive index by an electric field is called. In crystals, an electric field can be created by applying an electrical voltage. Due to the electric field, a birefringence in the crystal can occur or an existing birefringence can be changed. In this case, the change in birefringence can be linear to the change in the electric field strength of the applied electric field. Materials that exhibit a pockel effect are referred to as electro-optic materials. Crystals that show a Pockel effect are called electro-optic crystals. It is known that inversion-symmetric crystals can not show a Pockels effect, but only a Kerr effect, i. a quadratic dependence of the refractive index on the electric field strength. Inversion-symmetric materials are referred to in the English-language literature as "centrosymmetric." According to the current state of knowledge, only non-centrosymmetric materials that are not inversion-symmetric can show a Pockels effect.

The modulation Δ n of the refractive index n of a material by an electric field can be determined by the Pockels effect according to the formula

to be discribed. Here n ₀ denotes the unmodified refractive index of the material in the selected geometry r _e ff the effective electro-optical tensor, which depends on the crystal orientation and the polarization of the light and E the electric field. A more general description of the Pockels effect is possible by considering the modification of Fletcher's indicatrix. In a birefringent electro-optic crystal, the difference of the refractive indices of the ordinary and extraordinary beams can be increased or decreased by the electric field.

The Pockels effect can be used, inter alia, in the Pockels cell to induce phase differences in birefringent crystals by selective modification of the refractive index, which in turn can lead to polarization rotations in a continuous light beam. By means of a downstream polarizer, the intensity of a light beam, in particular of a laser beam, can be modulated. A Pockels cell can be designated as a device which, on the basis of the Pockels effect, effects a modulation of a light beam. The modulation can be continuous. The modulation can also take place between several, preferably two, discrete states. In this case you can call the modulation as switching. The Pockels cell according to the invention can be designed as a longitudinal or as a transverse Pockels cell. A longitudinal Pockels cell is one in which the electric field applied across the electrodes is longitudinal with respect to the light beam. A transverse Pockels cell is one in which the electric field applied across the electrodes is transverse to the light beam. The Pockels cell according to the invention comprises a composite crystal. As the composite crystal, it may be called a crystal comprising at least a first crystal and a second crystal. The first and second crystals may be separate crystals that have been bonded together to form a composite crystal. The second crystal is connected to the first crystal at a first connection surface. The connection can advantageously be designed so that mechanical vibrations can be transmitted from the first crystal to the second and vice versa. For this purpose, an inelastic compound may be particularly suitable. In the case of an inelastic connection, sound waves can be transmitted between the two crystals mentioned without significant attenuation of the sound waves. The compound can be carried out advantageously flat. However, the connection can also be implemented at a plurality of connection points or connection lines distributed over the connection area. The connection of the first crystal with the second crystal may be cohesive. The compound of the first crystal with the second crystal may be made with a joining agent. Advantageously, a hardenable joining agent can be used. The joining agent may be thermosetting and / or chemically curing and / or light-curing. Suitable joining agents may be organic or inorganic adhesives, for example filled or unfilled acrylate or epoxy resin adhesives, cement- or waterglass-based adhesives, or solders, for example metallic solders or glass solders.

As the first crystal is according to the invention provide such a crystal having a Pockels effect. The first crystal can be made, for example, from BBO (barium metaborate, also referred to as barium borate tetraoxide, Ba (BC> 2) 2), KD ^* P (potassium dideuterium phosphate, KD2PO4), lithium niobate (LiNbOs), KTP (potassium titanyl phosphate, ΚΤΊΟΡΟ4) or RTP (rubidium Titanyl phosphate, RbTiOP0 ₄ ). In the case of BBO, the β-form (β-BBO, also referred to as low-temperature form) can advantageously be used as the first crystal. The o form of the BBO (a-BBO, also referred to as a high-temperature shape) may be unsuitable as the first crystal due to the lack of a Pockel effect. The first crystal may, for example, have a thickness, ie an extension in the y-direction, between 0.5 mm and 10 mm. The length, ie the extension in the z-direction, may for example be between 1 mm and 30 mm.

The first crystal may have an optical axis. It should be noted that the term optical axis is used here in the crystal-optical sense and not in the radial-optical context. The first crystal can have exactly one optical axis, in this case one can speak of a uniaxial crystal. The first crystal may also have two optical axes. In the latter case one can speak of a biaxial crystal.

In particular, it may be advantageous to use a fourth crystal in addition to the first crystal, which also has a Pockels effect. The optical axes of the first and fourth crystals may be transversely perpendicular to each other. In this way, a transverse Pockels cell can be constructed in which the natural birefringence can be compensated. By natural birefringence one can understand a birefringence of the crystal without electric field.

In addition, it may be advantageous to use only one, namely only the first crystal for the modulation of the light. The optical axis of the first crystal can advantageously be arranged longitudinally in a transverse Pockels cell. The optical axis of the first crystal may also be advantageously arranged longitudinally in a longitudinal Pockels cell, for example in a KD ^* P Pockels cell.

The first crystal may have a crystallographic c-axis. In addition, the crystal may have a crystallographic axis. In addition, the crystal may have a crystallographic axis. The position of the c-axis, as well as the a and b axes is commercially available

Crystals mostly specified by the manufacturer. It is customary to label the a-b and c-axes so that the Hermann-Mauguin symbol conforms to the standard of the International Tables for Crystallography. In the trigonal crystal system, for example for the point group 3m, one would call the threefold axis of rotation c-axis, in the hexagonal crystal system the sixfold axis of rotation. In the tetragonal crystal system, for example for the point group 422 or 42m, we would call the vierzählige axis of rotation as c-axis. The crystallographic c-axis may be the optical axis of the crystal in the case of an optically uniaxial crystal. As a result, in the wirteligen crystal systems (trigonal, tetragonal and hexagonal), the latter simplified consideration is possible. In contrast, in the orthorhombic crystal system, for example, for the point group mm2, one of the two-axis rotation axes having the largest lattice parameter may be referred to as the c-axis.

The a-axis and the b-axis may have the same cell parameters. In the case of a BBO crystal, the lattice parameter of the a and b axes may be 0.12532 nm, the lattice parameter of the c axis 0.12717 nm. In the direction of the a-axis, the crystal may have the same thermal expansion coefficient as in the b-direction. The thermal expansion coefficient in the direction of the c-axis may be greater than the coefficient of expansion in the direction of the a-axis and in the direction of the b-axis. The c-axis may be distinguished by having the crystal in this direction the largest coefficient of expansion, i. the maximum value of the directional coefficient of expansion. For example, the coefficient of thermal expansion in the direction of the c-axis may be at least three times as great as in the direction of the a-axis and in the direction of the b-axis.

In the case of a lithium niobate crystal, the thermal expansion coefficient in the direction of the a and b axes can be 10 ^* 10 ^-6 K ^-1 and 14 ^* 10 ^-6 K ^-1, respectively, while the coefficient of thermal expansion in the direction of the c-axis 42 ^* 10 ^"6 K ^{" 1} can be. In this case, the extent in the direction c may therefore be at least three times as great as in the other directions.

In the case of a BBO crystal, the thermal expansion coefficient in the direction of the a and b axes may be 4 ^* 10 ^-6 K ^-1 , while the coefficient of thermal expansion in the direction of the c-axis may be 36 ^* 10 ^-6 K ^-1 . In this case, the extent in the direction c may be nine times greater than in the other directions.

In the case of a KD ^* P crystal, the thermal expansion coefficient in the direction of the a and b axes may be 19 ^* 10 ^-6 K ^-1 , while the coefficient of thermal expansion in the c-axis direction may be 44 ^* 10 ^-6 K ^-1 , In this case, the extent in the direction c can therefore be more than twice as large as in the other directions. In case of a KDP crystal (potassium dihydrogen phosphate KH2PO4), the thermal expansion coefficient in the direction of the a- and b-axes 25 ^* 10 ^"6 ^K" be ^1, while the thermal expansion coefficient in the direction of the c axis 44 ^* 10 ^"6 ^K" May be ¹ . In this case, the extent in direction c can be almost twice as large as in the other directions.

In the case of an RTP crystal, the thermal expansion coefficient in the direction of the a and b axes may be 2 ^* 10 ^-6 K ^-1 , while the thermal expansion coefficient in the direction of the c-axis may be 17 ^* 10 ^-6 K ^-1 . In this case, the extent in the direction c may be more than eight times as large as in the other directions. The crystallographic c-axis of the first crystal can advantageously be arranged in the direction of the light beam, ie in the z-direction. For example, such an arrangement may be advantageous for first crystals of lithium niobate, BBO or KTP.

If the first crystal has a b-axis which is not equivalent to the a-axis, there may be a biaxial crystal having two optical axes. Then, it may be advantageous to use, in addition to the first crystal, a fourth crystal whose crystallographic a-axis can be arranged perpendicular to the direction of the a-axis of the first crystal. The c-axes of the first and the fourth crystal can both lie in the direction z of the light beam. The fourth crystal may be located in the beam path and used to compensate for the natural birefringence of the first crystal. Such compensation can be advantageous, for example, for lithium niobate crystals.

The crystallographic c-axis of the first crystal may also advantageously be perpendicular to the direction of the light beam, i. lie in the x or y direction. For example, such an arrangement may be advantageous for a first crystal of RTP. In such an arrangement, it may be advantageous to use, in addition to the first crystal, a fourth crystal whose crystallographic c-axis can be arranged perpendicular to the direction of the light beam and also perpendicular to the direction of the c-axis of the first crystal. The fourth crystal may be located in the beam path and used to compensate for the natural birefringence of the first crystal.

The crystallographic c axis does not necessarily have to agree with one of the directions x, y or z. The normals of the cut surfaces of the first crystal, in particular those of the The light entry surface and the light exit surface and the normal of the first and second electrodes described below may, but need not, extend in the direction of one of the crystallographic a-b or c-axis. For example, RTP crystals can also be used to advantage in a 45 ° z cut. The Pockels cell also includes a first electrode and a second electrode. The second electrode may be electrically isolated from the first one. The first electrode is disposed on a first side of the first crystal and the second electrode is disposed on a second side of the first crystal. The first and second sides of the first crystal may be opposite sides of the first crystal. The second electrode may be arranged parallel to the first electrode. The first electrode and the second electrode may be formed as flat surfaces. The first and / or the second electrode may be formed as a layer, for example as a metallic layer. The metallic layer may be applied as a coating by means of a known coating method on a surface of the first crystal. The first electrode and / or the second electrode may, for example, have a thickness between 50 nm and 2000 nm. The first and / or the second electrode may alternatively be formed as a conductive foil, for example as a metal foil or as a carbon foil or as a metallically coated or as a conductive plastic foil. Such a film may, for example, have a thickness between 5 μm and 500 μm. The film can then be advantageously arranged in each case on a surface of the first crystal. Alternatively, the first and / or second electrodes may be formed as metallized areas of the second and third crystal described below, respectively, wherein the metallized area of the second and third crystals may be disposed on each surface of the first crystal. Between the first and the second electrode, an electrical voltage U can be applied. The electrical voltage U (t) can be variable, so for example, time-dependent. As a result of the electrical voltage, an electric field can arise in the first crystal. The electrical voltage can be a high voltage. The amount of voltage may be, for example, between 0 and 50 kV

According to the invention, at least one beam path is provided for a light beam that passes through the first crystal. The light beam may pass through the first crystal at least once. The light path may also be provided such that the light beam passes through the first crystal several times. For this purpose, a reflection surface may be provided on one side of the crystals or spaced apart from the crystal. The first crystal may have a light entrance surface and a light exit surface. Advantageously, the light entry surface and the light exit surface be anti-reflective. The light beam incident on the first crystal may have a certain first polarization state. The polarization state can be characterized by the amplitude of two mutually perpendicular field strength vectors of the light, for example E-field vectors of the light, and the transition or phase difference between these vectors. Advantageously, the incident on the first crystal light beam can be linearly polarized. In the same way, the light beam emitted by the first crystal can have a second polarization state. When passing through the beam path in the first crystal, the light beam can undergo a modulation that depends on the magnitude of the instantaneous field strength of the electric field applied to the crystal by means of the electrodes. As a result, the second polarization state can be influenced (modulated). The strength of the modulation can be proportional to the electric field strength of the applied electric field. The modulated parameter of the light beam may be the state of polarization, that is to say, for example, the amplitude ratio of the mutually perpendicular light field strength and / or their phase difference or phase difference. For example, the incident light in a first polarization direction can be linearly polarized and the emergent light can be linearly polarized in a second polarization direction. Then, the second polarization direction can be changed by the voltage applied to the electrodes. That is, the polarization direction of the light can be rotated as it passes through the first crystal and the angle of rotation can depend on the voltage applied to the electrodes. It may also be advantageous to provide a predetermined angle of incidence and a predetermined angle of reflection of the light beam into and out of the crystal. For this purpose, the entry or exit surface can be arranged inclined relative to the light path in the first crystal. The angle of incidence and / or the angle of reflection can be, for example, 0 °. It can also be between 3 ° and 10 °, for example. In this case, back reflexes can be avoided. It can also be designed as a Brewster angle. In this way, reflection losses at the interface can be reduced.

The first crystal may have a greater extent in a direction z than in a direction x and in a direction y. The directions x, y and z can form a rectangular coordinate system. The extension of the crystal in the direction z can be referred to as the length, that in the direction y as the height and in the direction x as the width. For the light path through the first crystal, the direction z can be provided. Then, an entrance aperture and an exit aperture may be determined by the height and width of the first crystal. The composite crystal also includes at least one second crystal. The second crystal is connected to the first crystal at a first connection surface. The first connection surface is arranged outside the intended beam path. The second crystal may be arranged so that it is not traversed by the light beam. This can have the advantage that the second crystal does not need to have optical quality. In addition, a Pockels effect may be dispensable for the second crystal.

The first connection surface has a normal y. The first crystal has in a direction perpendicular to y direction x a coefficient of thermal expansion a _x in a direction perpendicular to x and y direction z a thermal expansion coefficient a _z . The second crystal has in the direction x a thermal expansion coefficient a _> a and in the direction z a thermal expansion coefficient a _Z 2. The second crystal may be stretch-fitted to the first crystal. According to the invention the thermal expansion coefficient a _x deviates by less than 30% from the coefficient of thermal expansion 0x2. The thermal expansion coefficient a _x preferably deviates by less than 10%, particularly preferably less than 5%, from the coefficient of thermal expansion 0x 2. Ideally, a _x and 0x2 can be the same size. According to the invention, the thermal expansion coefficient a _z differs by less than 30% from the thermal expansion coefficient a _Z 2. Preferably, the thermal expansion coefficient a _z deviates by less than 10%, particularly preferably less than 5%, from the thermal expansion coefficient a _Z 2. Ideally, a _z and a _Z 2 can be the same size. The possible deviation is to be considered in terms of amount, since the first crystal in the respective direction x or z can have a smaller or a larger thermal expansion coefficient than the second crystal. For example, the first crystal may be arranged so that the crystallographic c-axis points in the direction z. Then, for example, a _{z can be} considerably larger than a _x , for example 5 to 10 times as large. Therefore, expansion adjustment in the z-direction may be particularly important as compared to that in the direction x.

The first and second crystals may each have a c-axis. The c-axis of the second crystal may advantageously be arranged parallel to the c-axis of the first crystal. An expansion adjustment of both crystals at the first connection surface can be achieved by orienting the second crystal with the crystallographic c-direction parallel to the c-direction of the first crystal. The first crystal and the second crystal may advantageously have the same chemical composition, ie they may be composed of the same lattice atoms which are in the same stoichiometric ratio. The first and second crystals can then have the same chemical formula. For example, the first crystal and the second crystal may be made of the same material and have an identical crystal structure. However, the first crystal and the second crystal may also consist of different phases of the same material. The first crystal and the second crystal may have different isotopes even with otherwise the same chemical composition. In the cases mentioned in this paragraph, the thermal expansion adjustment described above can be achieved by aligning the c axis of the second crystal parallel to the c axis of the first crystal.

For example, the molecular formula of the second crystal may have one or more hydrogen atoms which are wholly or partially replaced by deuterium atoms in the molecular formula of the first crystal. For example, the first crystal may be a KD ^* P crystal while the second crystal is a KDP crystal. An advantage may be that the second crystal can be made from a cheaper material.

However, the second crystal can also consist of a material different from the first crystal, if a thermal expansion adaptation can be produced with the corresponding materials. The first crystal may advantageously be a non-inversion symmetric crystal. The second crystal may or may not be an inversion-symmetric crystal. This can be particularly advantageous if an inversion-symmetric crystal is cheaper than a comparable non-inversion symmetric. For example, the first crystal may be a β-BBO crystal. The second crystal may be an a-BBO crystal. This has the advantage that only the first crystal consists of the relatively expensive β-BBO, while the second crystal consists of the cheaper α-BBO. In this case, different phases of one and the same material can be present.

The second crystal can be used to effect a change in the acoustic resonance frequencies of the first crystal. This may mean that the frequency ranges in which ringing occurs can be shifted. In addition, the frequency ranges in which no ringing occurs can be moved and / or widened. The comparison can between an arrangement with the use according to the invention of the second crystal compared to an arrangement without the use of a second crystal. Advantageously, the second crystal may have a larger volume than the first crystal. By acoustic resonance frequencies one can understand the mechanical natural frequencies of different modes of vibration of the first crystal.

The first connection surface may be on the first electrode or on the second electrode. But it is also possible that the connection surface is located on neither of the two electrodes.

The second crystal may have a metallization. The metallization of the second crystal may extend over the first connection surface and at least one further surface of the second crystal. In particular, when the bonding surface is on an electrode, metallization of the second crystal may be used to apply the voltage to the electrode. Particularly advantageous may be a circumferential metallization of the second crystal. The second crystal may have one or more recesses. One or more electrical connection elements can be arranged in the recesses or laterally next to the second crystal. These can be provided to bring the electrical voltage to the first and / or second electrode.

The first crystal can be frictionally connected to a first contact body. The first contact body may be electrically conductive and / or have an electrically conductive coating. The first contact body may consist of an isotropic material. The first crystal may, for example, be connected in area to the first contact body on an electrode, ie on the first or on the second electrode, wherein this connection may be electrically conductive. The connection can be made by pressing with a certain force. The first crystal may be connected to a first contact body via a first flexible layer. Advantageously, you can produce this connection by adhesion. The force may preferably be directed normal to the flexible layer. The first flexible layer may be formed electrically conductive. The first flexible layer may be provided to compensate for the differential thermal expansion of the first crystal with respect to the first contact body, the differences in thermal expansion also being directional could be. Due to the frictional connection and / or the first flexible layer, thermally induced mechanical stresses in the first crystal can be avoided.

The second crystal can be frictionally connected to a first contact body. The first contact body may be electrically conductive and / or have an electrically conductive coating. The first contact body may consist of an isotropic material. For example, the second crystal may be attached to an electrode, i. at the first or at the second electrode, be connected flat with the first contact body, said compound may be electrically conductive. The connection can be made by pressing with a certain force. The second crystal can be connected to a first contact body via a first flexible layer. Advantageously, you can produce this connection by adhesion. The force may preferably be directed normal to the flexible layer. The first flexible layer may be formed electrically conductive. The first flexible layer may be provided to compensate for the differential thermal expansion of the second crystal with respect to the first contact body, wherein the differences in thermal expansion may also be directional. Due to the frictional connection and / or the first flexible layer, thermally induced mechanical stresses in the second crystal can be avoided.

The first crystal may be connected to a third crystal at a second connection surface. The second connection surface may be parallel to the first connection surface. The second connection surface may be disposed on a side of the first crystal opposite the first connection surface. The first connection surface may be arranged on the first electrode, the second connection surface on the second electrode.

The third crystal can be frictionally connected to a second contact body. The second contact body may be electrically conductive and / or have an electrically conductive coating. The second contact body may consist of an isotropic material. The third crystal may, for example, be connected in area to the second contact body on one electrode, ie on the first or on the second electrode, wherein this compound may be electrically conductive. The connection can be made by pressing with a certain force. The third crystal may be connected to a second contact body via a second flexible layer. Advantageously, you can produce this connection by adhesion. The force may preferably be directed normal to the flexible layer. The second flexible layer may be formed electrically conductive. The second flexible layer may be provided to compensate for the differential thermal expansion of the third crystal with respect to the second Compensate contact body, the differences in thermal expansion may also be directional. Due to the frictional connection and / or the second flexible layer, thermally induced mechanical stresses in the third crystal can be avoided.

The first contact body or the second contact body may be formed as a base plate or housing. The first contact body or the second contact body may have an electrical ground potential. This may mean that this contact body is grounded. Then, the other contact body can be acted upon by the high voltage. In this case, the high voltage source may be formed as an asymmetric voltage source with respect to the ground potential. An apparatus for generating a pulsed laser radiation may comprise a above-described Pockels cell according to any one of the following claims, further comprising a laser resonator, an optical amplifier medium disposed in the laser resonator and a pump source. The light beam for which the beam path is provided in the first crystal may be a laser beam in this case. The pump source serves to pump the laser medium. The laser medium can be for example a laser crystal, a laser disk, an active optical fiber or a semiconductor laser disk. The pump source may be a diode laser. The Pockels cell can be arranged in the laser resonator. The laser resonator can be designed as a laser oscillator. The device can be designed, for example, as a Q-switched laser, which can oscillate itself without input pulses. The laser resonator can also be designed as a laser amplifier, which has an input for at least one laser pulse to be amplified. The laser amplifier can be designed as a regenerative amplifier, for example as ultrashort pulse laser. An ultrashort pulse laser may be provided to generate laser pulses of less than 50 ps or less than 10 ps. The laser resonator may further include a frequency multiplier element. The Pockels cell may be provided for coupling out a laser pulse or a laser pulse train from the laser resonator at a second time. The decoupling can be done by the interaction of the Pockels cell with a polarization-dependent reflection element. The Pockels cell can also be provided for coupling in each case a laser pulse or a laser pulse sequence into the laser resonator at a first time. The preparation of a Pockels cell according to the invention may comprise the following steps: a) providing a first crystal, wherein the first crystal shows a Pockels effect, and at least one beam path is provided for a light beam passing through the first crystal

d) connecting the second crystal to the first crystal at a first connection surface, wherein the first connection surface is located outside the intended beam path. The first connection surface has a normal y, the first crystal having a coefficient of thermal expansion a _x in a direction perpendicular to y, and the first crystal having a thermal expansion coefficient a _z in a direction z perpendicular to x and y

the second crystal in the direction x has a thermal expansion coefficient a> a and the second crystal has a thermal expansion coefficient a _Z 2 in the direction z

and the coefficient of thermal expansion a _x differs by less than 30% from the coefficient of thermal expansion a> a and the coefficient of thermal expansion a _z deviates by less than 30% from the thermal expansion coefficient a _Z 2.

The figures show the following:

1 shows a Pockels cell according to the invention of a first embodiment in an xy-section.

Fig. 2 shows the Pockels cell according to the invention of the first embodiment in a yz-section.

Fig. 3 shows a Pockels cell according to the invention of a second embodiment in an xy-section.

4 shows a Pockels cell according to the invention of a third embodiment in a yz-section.

5 shows a Pockels cell according to the invention of a fourth embodiment in a yz section.

FIG. 6 shows a Pockels cell according to the invention of a fifth exemplary embodiment in a yz section.

FIG. 7 shows a Pockels cell according to the invention of a sixth exemplary embodiment in a yz section.

FIG. 8 shows a Pockels cell according to the invention of a seventh embodiment in a yz section. 9 shows a Pockels cell according to the invention of an eighth embodiment in a yz cut.

10 shows a laser arrangement according to the invention.

Fig. 11 shows by way of example a beneficial effect of the use of the second crystal.

EXAMPLES

1 shows a Pockels cell according to the invention of a first embodiment in an xy-section. The Pockels cell 1 comprises a compound crystal 2. The composite crystal 2 comprises a first crystal 3 and a second crystal 4. Further, the Pockels cell comprises a first electrode 1 1 arranged on a first side of the first crystal and a second electrode 12 on the opposite second side of the first crystal is arranged. The first and second electrodes are formed as metallic layers having thicknesses between 50 nm and 2000 nm. The second crystal 4 is connected to the first crystal 3 at a first connection surface 9. The first connection surface is located on the first side of the first crystal, on which the first electrode is arranged. The first connection surface 9 has a normal pointing in the direction y. The first crystal 3 has a Pockels effect. By applying an electrical voltage U (t) to the terminals 20, an electric field 18 can be generated in the first crystal.

Fig. 2 shows the Pockels cell according to the invention of the first embodiment in a yz-section. There is provided a beam path for a light beam 22 passing through the first crystal 3. The light beam can be modulated by a variable voltage U (t). In this case, a modulation of the polarization state, for example a rotation of the polarization direction is possible. Also possible is a modulation of the phase of the light beam. The first connection surface 9 is arranged outside the intended beam path 22. The c-axes of the first and second crystals are arranged parallel and in the direction z. As a result, an expansion adaptation is achieved on the first connection surface, namely in the direction z and in the direction x. The directions x, y and z form a rectangular coordinate system.

In a specific embodiment of the first embodiment, the first crystal is a KD ^* P crystal and the second crystal is a KDP crystal, both of which are oriented such that the c-axis lies in the z-direction. Both crystals have the same thermal expansion coefficients ciz and a _Z 2 of 44 ^* 10 ^"6 K ^{" 1} in the direction z. The first crystal has a thermal expansion coefficient a _x of 19 ^* 10 ^-6 K ^-1 in the direction x and the second crystal in the direction x a 0x2 coefficient of thermal expansion of 25 ^* 10 ^"6 K ^'1. The thermal expansion coefficient a _x thus deviates by 24% from the thermal expansion coefficient a> a. Thus, in the direction z, there is an ideal, albeit not entirely ideal, thermal adaptation in the direction x. In another specific embodiment of the first embodiment, the first crystal is a beta-BBO crystal and the second crystal is an alpha-BBO crystal, both oriented so that the c-axis lies in the z-direction. Both crystals have the same thermal expansion coefficients a _z or a _Z 2 of 36 ^* 10 ^-6 K ^-1 in the direction _z, and the same thermal expansion coefficients a _x and a> a of 4 ^* 10 ^-6 K ^-1 in the direction x there is an ideal thermal adaptation in both directions x and z.

In another specific embodiment of the first embodiment, the first crystal is a lithium niobate crystal and the second crystal is also a lithium niobate crystal, both oriented so that the c-axis of both crystals are oriented in parallel and the a-axes of both crystals are also oriented parallel to one another. Thus, there is an ideal thermal adaptation in both directions x and z.

Fig. 3 shows a Pockels cell according to the invention of a second embodiment in an xy-section. In this embodiment, the first connection surface 9 is located on a side of the first crystal 3 to which no electrode is attached. The electric field 18 runs here in the direction x. The beam path 22 is provided in the direction z. 4 shows a Pockels cell according to the invention of a third embodiment in a yz-section. In addition to the first crystal 3, a fourth crystal 8 is present. While the electric field 18 in the first crystal is in the direction x, the electric field 19 in the fourth crystal is in the direction y.

5 shows a Pockels cell according to the invention of a fourth embodiment in a yz section. Here, an electric field 18 is generated in the direction z, which is parallel to

Beam path 22 is located. The electrodes 1 1, 12 are arranged on the end faces of the first crystal and formed as ring electrodes. FIG. 6 shows a Pockels cell according to the invention of a fifth exemplary embodiment in a yz section. In this case, in addition to the first 3 and second crystal 4, a third crystal 6 is present, which is connected at a second connection surface 10 with the first crystal. The second crystal has a circumferential metallization 5, the third crystal a circumferential metallization 7. A force 17, for example a clamping force, holds the crystals between a first contact body 15 and a second contact body 16. The second contact body is designed as a base plate or housing bottom and connected to an electrical ground potential 21. In addition, the second contact body is electrically connected to the second electrode 12 via an electrically conductive, second flexible layer 14 and the circumferential metallization 7 of the third crystal. The first contact body is electrically connected to the first electrode 1 1 via an electrically conductive first flexible layer 13 and the circumferential metallization 5. Therefore, the first contact body for connecting the voltage U (t) can be used and the voltage is applied to the ground potential. The contact bodies are made of metal and the two flexible layers ensure that no thermally induced mechanical stresses can occur. They act as strain compensation in the layer plane.

FIG. 7 shows a Pockels cell according to the invention of a sixth exemplary embodiment in a yz section. Here, the first crystal 3 is connected to a first contact body 15.

FIG. 8 shows a Pockels cell according to the invention of a seventh exemplary embodiment in a yz section. Here also a clamping with a clamping force 17 is provided. The first crystal is placed on the second contact body 16 with a second flexible layer 14. The second crystal 4 has a recess 24 in which an electrical connection element 23, for example a spring or a through-connection, is arranged, wherein the connection element electrically connects the first electrode 1 1 to the first contact body 15.

9 shows a Pockels cell according to the invention of an eighth embodiment in a yz cut. Here, the second crystal 4 is connected to the first contact body 1516.

10 shows a laser arrangement according to the invention. The laser arrangement 25 for generating a pulsed laser radiation comprises a laser resonator 26 and a pump light source 30. The laser resonator is delimited by a first mirror 27 and a second mirror 28. In the resonator, an amplifier medium 29 is arranged, which is pumped by the pump source, to achieve a cast inversion. Furthermore, a Pockels cell 1, a phase delay plate 32 and a polarizer 31 are arranged in the laser resonator. The resonator has an open state in which no laser radiation is formed or no amplification of a laser pulse is possible. In addition, the resonator has a closed state in which laser radiation is generated and / or a laser pulse in the resonator can run back and forth between the two mirrors and is respectively amplified during one revolution. By switching the high voltage value at the electrodes of the Pockels cell, the resonator can be switched between the open and the closed state. In a regenerative amplifier, the switching between the open and the closed resonator state causes the coupling in and out of the pulses. An input pulse 33 can be introduced into the resonator in the open state of the resonator. Then this is amplified in the closed state of the resonator. When the desired pulse energy is reached, the resonator is switched back to the open state and the amplified output pulse 34 can leave the resonator. Fig. 11 shows by way of example a beneficial effect of the use of the second crystal. The curve 35 shows the frequency ranges in which ringing occurs in the case where the Pockels cell is conventionally built without using a second crystal. In this case, a 4 mm thick first crystal was used. The curve 36 shows the frequency ranges in which a ringing occurs in the event that the Pockels cell according to the invention in addition to the first crystal comprises a second crystal. In this case, a second crystal with a thickness of 4 mm was adhered to the first crystal. It can thus be shown that, in this case, the measure according to the invention alone makes it possible to use the frequency ranges between 410 kHz and 550 kHz as well as between 200 kHz and 330 kHz without restriction for modulation, without any risk of ringing being feared. By using an even thicker second crystal, the areas above 200kHz where ringing occurs can be completely eliminated.

For completeness, it should be noted that the figures are not drawn to scale. Reference numerals:

1 Pockels cell

2 composite crystal

3 First Crystal

4 second crystal

5 metallization of the second crystal

6 Third Crystal

7 metallization of the third crystal

8 Fourth Crystal

9 First interface

10 second interface

1 1 First electrode

12 Second electrode

13 First flexible layer

14 Second flexible layer

15 First contact body

16 second contact body

17 power

18 electric field

19 Electric field in the fourth crystal

20 connection for electrical voltage

21 Electrical ground potential

22 light beam, beam path

23 Electrical connection element

24 recess

25 Apparatus for generating a pulsed laser radiation

26 laser resonator

27 First mirror

28 Second mirror

29 amplifier medium

30 pump source

31 polarizer

32 phase delay plate

33 input pulse

34 output pulse Frequency ranges with ringing for Pockels cell without second crystal Frequency ranges with ringing for Pockels cell with second crystal

Claims

claims:

1 . Pockels cell (1) comprising

a) a composite crystal (2)

b) a first electrode (1 1)

c) a second electrode (12)

wherein the compound crystal (2) comprises at least a first crystal (3) and at least a second crystal (4)

and the first crystal (3) has a Pockels effect

and the first electrode (1 1) is disposed on a first side of the first crystal (3) and the second electrode (12) is disposed on a second side of the first crystal (3)

and at least one beam path is provided for a light beam (22) passing through the first crystal (3)

and the second crystal (4) is connected to the first crystal (3) at a first connection surface (9),

wherein the first connection surface (9) is arranged outside the intended beam path (22)

and the first connection surface (9) has a normal y

and the coefficient of thermal expansion a _x differs by less than 30% from the coefficient of thermal expansion a> a and the coefficient of thermal expansion a _z differs by less than 30% from the thermal expansion coefficient a _Z 2.

2. Pockels cell (1) according to claim 1, characterized in that the first crystal is provided for the modulation of light with a light propagation direction in the direction z and that the thermal expansion coefficient a _z deviates by less than 10% from the thermal expansion coefficient a _Z 2.

3. Pockels cell (1) according to any one of the preceding claims, characterized in that the first crystal and the second crystal of the same lattice atoms are constructed and the lattice atoms in the same stoichiometric ratio.

4. Pockels cell (1) according to any one of the preceding claims, characterized in that the first crystal is a non-inversion symmetric crystal and the second crystal is an inversionssymmetrischer crystal.

5. Pockels cell (1) according to any one of the preceding claims, characterized in that the first crystal consists of KD ^* P and the second crystal consists of KDP or that the first crystal consists of beta-BBO and the second crystal consists of alpha-BBO ,

6. Pockels cell (1) according to any one of the preceding claims, characterized in that the second crystal is connected to the first crystal cohesively and / or by means of a joining means.

7. Pockels cell (1) according to one of the preceding claims, characterized in that the first connection surface is located on the first electrode or on the second electrode.

8. Pockels cell (1) according to one of the preceding claims, characterized in that the second crystal has a metallization (5) which extends over the first connection surface and at least one further surface of the second crystal

9. Pockels cell (1) according to one of the preceding claims, characterized in that the first crystal is non-positively connected (17) and / or via a first flexible layer (13) with a first contact body (15).

10. Pockels cell (1) according to one of the preceding claims, characterized in that the second crystal is non-positively connected (17) and / or via a first flexible layer (13) connected to a first contact body.

1 1. Pockels cell (1) according to one of the preceding claims, characterized in that the first crystal with a third crystal (6) at a second connection surface (10) is connected,

12. Pockels cell (1) according to one of the preceding claims, characterized in that the third crystal is non-positively and / or via a second flexible layer (14) connected to a second contact body (16).

13. Pockels cell (1) according to any one of the preceding claims, characterized in that the first contact body (15) or the second contact body (16) has an electrical ground potential (21).

14. Pockels cell (1) according to any one of the preceding claims, characterized in that the first flexible layer (13) and / or the second flexible layer (14) are formed electrically conductive.

15. An apparatus for generating a pulsed laser radiation, comprising a Pockels cell (1) according to one of the preceding claims, a laser resonator (26), an arranged in the laser resonator optical amplifier medium (29), a pump source (30), wherein the light beam, for which the Beam path (22) is provided, a laser beam is.

16. Use of a second crystal (4), which has a c-axis, for changing the acoustic resonance frequencies of a first electro-optical crystal (3), which has a c-axis, wherein

the first crystal (3) and the second crystal (4) have an identical chemical composition or the chemical composition of the second crystal differs from that of the first only by isotopes,

and

the second crystal is connected to the first crystal at a first connection surface (9) such that the c-axis of the second crystal is parallel to the c axis of the first crystal, and the first crystal can be supplied with an electric field (18); at least one beam path (22) is provided for a light beam passing through the first crystal (3).

17. A method for producing a Pockels cell (1) comprising

a) providing a first crystal (3), wherein the first crystal shows a Pockels effect, and at least one beam path (22) is provided for a light beam passing through the first crystal (3)

b) attaching a first electrode (1 1) and a second electrode (12) to the first crystal,

c) providing a second crystal (4),

d) connecting the second crystal to the first crystal at a first connection surface (9),

wherein the first connection surface (9) has a normal y

and the first crystal (3) has a coefficient of thermal expansion a _x in a direction perpendicular to y, and the first crystal has a thermal expansion coefficient a _z in a direction z perpendicular to x and y, and