Electro-optical modulator with reflection protection

Abstract

The invention relates to an electro-optical modulator (14) for setting a polarization (P2) of a laser beam (16). A front face (40) of the electro-optical modulator (14), through which the laser beam (16) can enter the electro-optical modulator (14), is arranged at an acute first angle of inclination (NW1) with respect to a first side face (38a) of the electro-optical modulator (14), the first side face (38a) adjoining the front face (40).

Description

Field of the Invention

[0001] The present invention relates to an electro - optical modulator for modulating a laser beam, which is used to modulate the laser beam in a case where elements arranged before and after the electro - optical modulator along the laser path are protected from laser radiation reflected by the electro - optical modulator. The present invention also relates to a laser device having such a modulator and a method of using such a modulator. Background Art

[0002] Electro - optical modulators are known from the prior art.

[0003] US2016 / 0156150A1 discloses a laser device for generating EUV radiation, which has an optical switch for controlling the transmission of laser radiation. The optical switch particularly includes a polarizer and an electro - optical modulator having a Pockels cell for modulating the laser radiation. The corresponding Pockels cell has an electro - optical crystal and two electrodes arranged opposite to each other on the crystal. Among them, the Pockels cell and other optical elements of the laser device are inclined relative to each other to avoid optical feedback caused by the reflection of laser radiation in the laser device.

[0004] A laser device for generating EUV radiation having an electro - optical modulator is also known from US2014 / 0203194A1.

[0005] In known laser devices, reflection protection can only be achieved at a relatively high cost, especially for the calibration of the laser device. Summary of the Invention

[0006] Therefore, the object of the present invention is to provide an electro - optical modulator by which protection against laser radiation reflected on the electro - optical modulator can be achieved in a relatively simple installation situation. Another object of the present invention is to provide a laser device having such a modulator and a method of using such a modulator.

[0007] This object is solved by an electro - optical modulator according to claim 1. The features of a laser device having the features of the present invention are given in claim 8, and the features of the method of the present invention are given in claim 13. Advantageous configurations result from the dependent claims referring to them.

[0008] The electro - optical modulator according to the present invention has a path - guiding element (bahnführendes Element) for guiding the laser beam and the following elements:

[0009] a) the front side of the path - guiding element for injecting the laser beam into the electro - optical modulator;

[0010] b) the back side of the path - guiding element For emitting a laser beam from an electro-optical modulator, where the back side is opposite to the front side;

[0011] c) The first side and the second side of the path guiding element, where the sides are located between the front side and the back side and connect the front side and the back side,

[0012] where the front side and the first side enclose a first inclination angle that is an acute angle.

[0013] The laser beam incident on the front side will be reflected by the front side to a certain extent. The front side is inclined with respect to the first side at a first inclination angle that is an acute angle. Therefore, the laser beam incident on the front side at an incident angle different from 0° will be reflected from the front side in a reflection direction inclined with respect to the incident direction. Thus, it is prevented that the reflected part is reflected back onto the path of the laser beam propagating to the front side. Among them, when the laser beam is incident on the front side, the incident angle between the normal (Lot) on the front side and the beam direction of the laser beam is measured. In particular, it is prevented that the reflected laser radiation damages the laser source or other optical elements on the laser path (Laserbahn) before the laser beam is incident on the front side.

[0014] Due to the inclination of the front side with respect to the first side, an inclined orientation of the reflected laser beam with respect to the incident direction of the laser beam has already occurred when the first side of the electro-optical modulator is oriented parallel to the incident direction of the laser beam. Depending on the orientation of the first side with respect to the incident direction of the laser beam, the direction deviation of the reflected beam can be further increased in a simple manner. Therefore, advantageously, reflection protection occurs within a relatively large angular range of the orientation of the electro-optical modulator with respect to the laser path. Therefore, advantageously, the installation of the electro-optical modulator is simplified, reflection back to the original laser path is avoided, and it can be carried out in a time-saving manner.

[0015] The front side, the back side and the sides are particularly formed on the surface of the path guiding element (i.e., not on the sides of the electrodes or other elements of the electro-optical modulator).

[0016] The path guiding element is used to guide the laser beam. In particular, the path guiding element is a crystal.

[0017] In an advantageous configuration, the first inclination angle is between 87° and 89.9°, particularly between 88° and 89°. The incident angle of the laser beam is preferably selected such that the laser beam passes through the path guiding element parallel to the first and / or second sides. The laser beam is refracted according to Snell's law when it is incident on the front side of the electro-optical modulator. Considering the refraction of the laser beam on the front side, the incident angle is now preferably selected according to Snell's law such that the beam direction of the laser beam in the electro-optical modulator is parallel to the first and / or second sides. This selection of the incident angle also results in a further deviation of the reflection direction of the laser beam on the front side from the incident direction.

[0018] In a preferred configuration, the rear face and the first side face enclose a second inclination angle of an obtuse angle. By the inclination of the rear face, the portion of the laser beam reflected on the rear face is deflected upon reflection from the path along which the laser beam propagates to the rear face. Thus, the laser source and / or other optical elements on the laser path are protected from damage or interference by the portion of the laser radiation reflected on the rear face. Accordingly, the protection of the laser source from the reflected laser radiation is advantageously enhanced.

[0019] In an advantageous variant, the side faces are parallel to each other. This simplifies the beam guidance of the corresponding refracted laser beam parallel to the side faces of the electro-optical modulator to achieve good beam quality of the laser beam. In particular, the laser beam propagates from the front face of the electro-optical modulator to the rear face.

[0020] In an advantageous configuration, the rear face and the front face extend parallel to each other. This simplifies the beam guidance of the laser beam. In particular, it is possible to make the laser beam have the same orientation after exiting the modulator as before entering the modulator. This simplifies the orientation of the optical elements in the laser device.

[0021] In an advantageous embodiment, the path guiding element is configured as a birefringent crystal. The birefringent crystal is particularly suitable for setting the polarization of the laser beam. In addition, the crystal has stable optical properties due to its fixed crystal structure. In particular, the front face and the rear face with high shape stability are advantageously constructed on the crystal. Particularly preferably, the angle of incidence is selected such that the laser beam passes through parallel to the optical axis of the path guiding element implemented as a birefringent crystal.

[0022] The electro-optical modulator can be configured as a Pockels cell. The Pockels cell enables the polarization of the laser beam to be set quickly and purposefully when passing through the electro-optical modulator. In particular, the Pockels cell has the aforementioned birefringent crystal, wherein preferably two electrodes are arranged opposite each other on the birefringent crystal.

[0023] The laser device for modulating a laser beam under reflection protection according to the invention has the following elements:

[0024] a) a laser source for generating a laser beam;

[0025] b) one of the aforementioned electro-optical modulators;

[0026] c) a laser path that extends from the laser source through the electro-optical modulator, and the laser beam can propagate from the laser source to the electro-optical modulator and pass through the electro-optical modulator after being generated, wherein the normal on the front face is inclined at an acute angle of incidence with respect to the direction of the incident section of the laser path, wherein the incident section of the laser path is adjacent to the front face of the electro-optical modulator and is constructed outside the electro-optical modulator.

[0027] An acute angle of incidence occurs when the first side of the electro-optic modulator is oriented parallel to the incident section of the laser path. Thus, in this orientation of the electro-optic modulator, the path of the laser beam reflected back from the front face to the laser source has deviated from the incident section when the laser beam first impinged on the front face. Depending on the tilt angle of the front face relative to the first side, it is possible to avoid the reflection of the laser beam on the front face in the direction of the incident section in different orientations of the electro-optic modulator. Advantageously, reflection protection for the laser source occurs within a relatively large angular range of the orientation of the electro-optic modulator relative to the incident section. Thus, the installation of the modulator is simplified in the case of protecting the laser source and can be carried out in a time-saving manner.

[0028] When the laser beam is properly oriented and the laser device is operating, the laser path extends particularly along the path of the laser beam starting from the laser source. The incident section is in particular the part of the laser path between the front face and the laser beam generating element or the laser beam guiding element, which is located in front of the front face and closest to the front face on the laser path.

[0029] In a preferred embodiment, the side face of the path guiding element is inclined at an acute angle of deflection (Ausschwenkwinkel) relative to the incident section of the laser path. Through the angle of deflection, a good beam quality of the laser beam after passing through and emerging from the electro-optic modulator is advantageously achieved. The good beam quality particularly results from the fact that the beam direction of the laser beam in the path guiding element is parallel to its side face. The laser beam is refracted according to Snell's law when it impinges on the front face of the path guiding element. The angle of refraction depends on the angle of incidence - the larger the angle of incidence, the larger the angle of refraction. The angle of deflection can now be selected such that the refracted laser beam (or at least a partial beam of the refracted laser beam) propagates parallel to the side face of the modulator. Preferably, the birefringent crystal of the modulator is arranged with an angle of deflection relative to the laser path such that the refracted laser beam (or at least a partial beam, in particular the ordinary beam) propagates parallel to the side face of the birefringent crystal.

[0030] In a further development of the above embodiment, the angle of deflection is between 0.1° and 5°. Within this value range, the angle of deflection can be adapted to the usually relatively small first and second tilt angles to achieve a good beam quality of the laser beam when it passes through the modulator.

[0031] In an advantageous configuration, a polarizer is arranged before and / or after the electro-optic modulator on the laser path. The polarization of the laser beam passing through the laser device can be selected by the polarizer. In particular, a specific polarization can be set for the laser beam by the polarizer before the electro-optic modulator before the laser beam enters the modulator. Then the polarization of the laser beam can be changed purposefully in the modulator. The polarization of the laser beam reaching the target on the subsequent laser path can be controlled by the polarizer after the modulator. This can be used in particular for generating laser pulses and / or for temporal shaping of laser pulses.

[0032] In a preferred variant, the laser device has a plurality of electro-optical modulators arranged successively along the laser path. In particular, in this variant, polarizers are arranged between the electro-optical modulators and respectively in front of and behind the electro-optical modulators at both ends along the laser path. This arrangement enables precise shaping of the laser pulses.

[0033] In a preferred variant, the laser device has an EUV source, where the EUV source is located behind the electro-optical modulator on the laser path starting from the laser source. The laser beam can be directed at the EUV source, such as a tin droplet, after passing through the modulator to generate EUV radiation. The electro-optical modulator can be used to generate laser pulses, especially in combination with the aforementioned polarizers, through which the laser radiation can only pass in a specific polarization.

[0034] A method for modulating a laser beam under reflection protection, comprising the following steps:

[0035] a) emitting a laser beam from a laser source;

[0036] b) irradiating one of the aforementioned electro-optical modulators with the laser beam from the laser source, where the laser beam enters the electro-optical modulator through the front face,

[0037] where the front face is inclined with respect to the direction of the laser path at the front face before the laser beam enters the electro-optical modulator, such that the laser beam impinges on the front face at an acute angle of incidence.

[0038] In this method, within a large angular range of the orientation of the electro-optical modulator, the laser beam is prevented from being reflected back to the laser source at the front face.

[0039] In an advantageous configuration of the method, the laser beam irradiates an EUV source after passing through the electro-optical modulator to generate EUV radiation. In this method, by changing the polarization of the laser beam in the electro-optical modulator, laser pulses can be generated and / or shaped at a high frequency, especially in combination with polarizers.

[0040] Other advantages of the present invention result from the description and the drawings. Similarly, the features described above and below can be used individually or in any combination. The embodiments shown and described should not be understood as exhaustive, but rather have exemplary features for describing the present invention. Description of the Drawings

[0041] Figure 1 A laser device with an electro-optical modulator with reflection protection is schematically shown;

[0042] Figure 2 A laser device with an EUV source is schematically shown. Detailed Description of the Invention

[0043] Figure 1A cross-section of a modulation system 10 of a laser device 12 is schematically shown, which has an electro-optic modulator 14 for modulating a laser beam 16 under reflection protection. The laser beam 16 is emitted by a laser source 18 in a beam direction SR on a laser path 20, where the beam direction SR is oriented parallel to the y-axis of a reference system R, which is a Cartesian coordinate system having an x-axis, a y-axis, and a z-axis. The laser beam 16 then passes through a first polarizer 22a, which is arranged on the laser path 20 in the beam direction SR behind the laser source 18. The laser path 20 is in particular the path along which the laser beam 16 extends when properly oriented. The first polarizer 22a polarizes the laser beam 16 into a linearly polarized first polarization P1 oriented in the z-direction. The term polarization particularly relates to the direction of the electric field of the laser beam 16.

[0044] After passing through the first polarizer 22a, the laser beam 16 propagates on an incident section 26 of the laser path 20 to the electro-optic modulator 14, which is arranged on the laser path 20 in the beam direction SR behind the first polarizer 22a.

[0045] The electro-optic modulator 14 has a Pockels cell 30 with two electrodes 36a, 36b and a path guiding element 32, which is in the form of a birefringent crystal 34, where the electrodes 36a, 36b are arranged opposite each other on a first side 38a and a second side 38b of the birefringent crystal 34. The sides 38a, 38b of the birefringent crystal 34 extend parallel to each other and connect a front face 40 and a rear face 42 of the birefringent crystal 34. The front face 40 of the birefringent crystal 34 is used to inject the laser beam 16 into the birefringent crystal 34, while the rear face 42 is used to let the laser beam 16 exit the birefringent crystal 34. The front face 40 and the rear face 42 extend parallel to each other, thus simplifying the beam guiding of the laser beam 16 and the optical properties of the electro-optic modulator 14.

[0046] A first reflected portion 44a of the laser beam 16 is reflected on the front face 40 when injected into the birefringent crystal 34. The front face 40 extends obliquely with respect to the first side 38a such that a first tilt angle NW1, which is an acute angle, is enclosed between the front face 40 and the first side 38a. In addition, the first side 38a and the second side 38b are inclined at a deflection angle AW with respect to the beam direction SR of the laser beam 16 before it is incident on the birefringent crystal 34. Due to the first tilt angle NW1 and the deflection angle AW, the front face 40 encloses an incident angle EW, which is an acute angle, with respect to the beam direction SR of the laser beam 16 along the y-axis on the incident section 26. Due to the incident angle EW, the laser beam 16 is reflected from the front face 40 in a reflection direction inclined with respect to the beam direction SR of the laser beam 16 on the incident section 26. Thereby, it is prevented that the laser beam 16 is reflected back to the laser source 18 along the path along which it propagates to the front face 40 and damages or interferes with it.

[0047] The portion of the laser beam 16 refracted at the front face 40 passes through the birefringent crystal 34 and impinges on the back face 42 of the birefringent crystal 34.

[0048] When exiting from the back face 42 of the birefringent crystal 34, the second reflected portion 44b of the laser beam 16 is reflected. The back face 42 extends parallel to the front face 40 and is inclined with respect to the first side face 38a such that a second tilt angle NW2 that encloses an obtuse angle between the front face 40 and the first side face 38a. Accordingly, the second reflected portion 44b of the laser beam 16 is also reflected from the back face 42 in a second reflection direction that is inclined with respect to the beam direction SR of the laser beam 16 on the incident section 26. Thus, similar to the front face 40, the risk of damage to the laser source 18 by the laser radiation reflected from the back face 42 is greatly reduced or eliminated.

[0049] By applying a voltage to the electrodes 36a, 36b of the Pockels cell 30, the polarization P2 of the laser beam 16 is set, which the laser beam 16 has in the beam direction SR after the Pockels cell 30 (electro-optic modulator 14). In Figure 1 it, the laser beam 16 has a polarization P2 in the xy plane parallel to the x-axis in the beam direction SR after the Pockels cell 30, i.e., the polarization P2 is rotated by 90° with respect to the polarization P1.

[0050] After passing through the electro-optic modulator 14, the laser beam 16 propagates to the second polarizer 22b, which is arranged after the electro-optic modulator 14 in the beam direction SR of the laser beam 16. The second polarizer 22b is also oriented parallel to the x-axis, so that the laser beam 16 can pass through the second polarizer 22b. In the case where the polarizer 22b is oriented perpendicular to the x-axis ( Figure 1 not shown in it), the laser beam 16 cannot pass through the second polarizer 22b. Thus, orienting the polarization P2 of the laser beam 16 by the orientation of the electro-optic modulation module 14 with respect to the second polarizer P2 can be used to generate laser pulses and / or perform temporal shaping of the laser pulses.

[0051] Figure 2 A laser device 12 having a laser source 18, a modulation system 10, and an EUV source 46 is schematically shown. Among them, Figure 2 the modulation system 10 of the laser device 12 in it is represented by a box. After passing through the modulation system 10 of the laser device 12, the laser beam 16 propagates to the EUV source 46, which is arranged after the modulation system 10 parallel to the y-axis of the reference system R in the beam direction SR of the laser beam 16. The EUV source 46 has a droplet generator 48, which emits tin droplets 50 into the path of the laser beam 16. The laser beam 16 heats the tin droplets 50 into a plasma, thereby generating EUV radiation 52, especially for EUV lithography.

[0052] Viewed in combination with all the figures, the present invention relates to an electro-optic modulator 14 for setting the polarization P2 of a laser beam 16. A front face 40 of the electro-optic modulator 14 (through which the laser beam 16 can enter the electro-optic modulator 14) is inclined and oriented at a first acute inclination angle NW1 with respect to a first side face 38a of the electro-optic modulator 14, where the first side face 38a adjoins the front face 40.

Claims

1. An electro - optical modulator (14) for modulating a laser beam (16) under reflection protection, the electro - optical modulator having a path - guiding element (32) for guiding the laser beam (16), the electro - optical modulator having: a) a front face (40) of the path - guiding element (32) for injecting the laser beam (16) into the electro - optical modulator (14); b) a back face (42) of the path - guiding element (32) for ejecting the laser beam (16) from the electro - optical modulator (14), where, The back surface (42) is opposite to the front surface (40); c) a first side surface (38a) and a second side surface (38b) of the path guiding element (32), wherein the side surfaces (38a, 38b) are located between the front surface (40) and the back surface (42) and connect the front surface (40) and the back surface (42), wherein the front surface (40) and the first side surface (38a) enclose a first tilt angle (NW1) of an acute angle.

2. The electro - optical modulator according to claim 1, wherein, The first tilt angle (NW1) is between 87° and 89.9°, particularly between 88° and 89°.

3. The electro - optical modulator according to claim 1 or 2, wherein, The back surface (42) and the first side surface (38a) enclose a second tilt angle (NW2) of an obtuse angle.

4. The electro - optical modulator according to any one of the preceding claims, wherein, The side surfaces (38a, 38b) are parallel to each other.

5. The electro - optical modulator according to any one of the preceding claims, wherein, The back surface (42) and the front surface (40) extend parallel to each other.

6. The electro - optical modulator according to any one of the preceding claims, wherein, The path guiding element (32) is configured as a birefringent crystal (34).

7. The electro - optical modulator according to claim 6, wherein, The electro-optical modulator (14) has a Pockels cell (30).

8. A laser device (12) for modulating a laser beam (16) under reflection protection, the laser device having: a) a laser source (18) for generating the laser beam (16); b) an electro - optical modulator (14) according to any one of the preceding claims; c) a laser path (20) that extends from the laser source (18) through the electro - optical modulator (14), and on which the laser beam (16) can propagate from the laser source (18) to the electro - optical modulator (14) and through the electro - optical modulator (14) after its generation, wherein, The normal on the front surface (40) is inclined at an incident angle (EW) of an acute angle with respect to the direction of the incident section (26) of the laser path (20), wherein the incident section (26) of the laser path (20) is adjacent to the front surface (40) of the electro-optical modulator (14) and is configured outside the electro-optical modulator (14).

9. The laser device according to claim 8, wherein, The side surfaces (38a, 38b) of the path guiding element (32) are inclined at a deflection angle (AW) of an acute angle with respect to the incident section (26) of the laser path (20).

10. The laser device according to claim 9, wherein, The deflection angle (AW) is between 0.1° and 5°.

11. The laser device according to any one of claims 8 to 10, wherein, On the laser path (20), polarizers (22a, 22b) are arranged before and / or after the electro-optical modulator (14).

12. The laser device according to any one of claims 8 to 11, wherein, A plurality of electro-optical modulators (14) are arranged in sequence along the laser path (20).

13. The laser device according to any one of claims 8 to 12, the laser device having an EUV source (46), wherein, The EUV source (46) is arranged on the laser path (20) starting from the laser source (18) after the electro-optical modulator (14).

14. A method for modulating a laser beam (16) under reflection protection, the method having the following steps: a) emitting the laser beam (16) from a laser source (18); b) irradiating the laser beam (16) from the laser source (18) through an electro-optical modulator (14) according to any one of claims 1 to 7, wherein, The laser beam (16) enters the electro-optical modulator (14) through the front surface (40) of the electro-optical modulator (14), wherein the front surface (40) is inclined with respect to the direction of the laser path at the front surface (40) before the laser beam (16) enters the electro-optical modulator (14), such that the laser beam (16) irradiates the front surface (40) at an incident angle (EW) of an acute angle.

15. The method according to claim 14, wherein, The laser beam (16) irradiates the EUV source (46) after passing through the electro-optical modulator (14) to generate EUV radiation (52).

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