Electrooptical modulator having reflection protection

Abstract

The invention relates to an electrooptical modulator (14) for adjusting the polarisation (P2) of a laser beam (16). An end face (40) of the electrooptical modulator (14), through which end face the laser beam (16) can be radiated into the electrooptical modulator (14), is oriented so as to be inclined at an acute first inclination angle (NW1) with respect to a first lateral face (38a) of the electrooptical modulator (14), wherein the first lateral face (38a) adjoins the end face (40).

Description

[0001] Electro-optical modulator with reflection protection

[0002] Background of the invention

[0003] The invention relates to an electro-optical modulator for modulating a laser beam while protecting components arranged along a laser path of the laser beam upstream and / or downstream of the electro-optical modulator from laser radiation reflected by the electro-optical modulator. The invention further relates to a laser system comprising such a modulator and a method using such a modulator.

[0004] Electro-optical modulators are known from the state of the art.

[0005] US 2016 / 0156150 A1 discloses a laser system for generating EUV radiation with optical switches for controlling the transmission of laser radiation. The optical switches comprise, in particular, polarizers and electro-optical modulators with Pockels cells for modulating the laser radiation. Each Pockels cell has an electro-optical crystal and two electrodes arranged opposite one another on the crystal. The Pockels cell and other optical elements of the laser system are tilted relative to one another to prevent optical feedback caused by reflection of laser radiation in the laser system. A laser system for generating EUV radiation with electro-optical modulators is also known from US 2014 / 0203194 A1.

[0006] In the known laser systems, the reflection protection can only be created with comparatively high effort, especially for the adjustment of the laser system.

[0007] Object of the invention

[0008] It is therefore an object of the present invention to provide an electro-optical modulator with which, with comparatively simple assembly, protection against laser radiation reflected by the electro-optical modulator can be achieved. It is also an object of the invention to provide a laser system with such a modulator and a method using such a modulator.

[0009] Description of the invention

[0010] This object is achieved according to the invention by an electro-optical modulator according to claim 1. The features of a laser system according to the invention are specified in claim 8, and the features of the method according to the invention are specified in claim 13. Advantageous embodiments emerge from the dependent claims.

[0011] The electro-optical modulator according to the invention comprises a path-guiding element for guiding the laser beam and the following elements: a) an end face of the path-guiding element for irradiating the laser beam into the electro-optical modulator; b) a rear face of the path-guiding element for exiting the laser beam from the electro-optical modulator, the rear face being opposite the end face; c) a first side face and a second side face of the path-guiding element, the side faces being located between the end face and the rear face and connecting the end face and the rear face, the end face and the first side face enclosing an acute first angle of inclination.

[0012] A laser beam that hits the end face is reflected to a certain extent by the end face. The end face is inclined with respect to the first side face at the acute first angle of inclination. As a result, a laser beam that radiates onto the end face at an angle of incidence other than 0° is reflected from the end face in a reflection direction that is inclined to the direction of incidence. This prevents the reflected portion from being reflected back onto the path along which the laser beam propagated to the end face. The angle of incidence between the normal to the end face and the beam direction of the laser beam is measured when it hits the end face. In particular, this prevents the reflected laser radiation from damaging the laser source or other optical elements that lie on the laser path of the laser beam before it hits the end face.

[0013] Due to the inclination of the end face relative to the first side face, the oblique orientation of the reflected laser radiation relative to the laser beam incidence direction already occurs when the first side face of the electro-optical modulator is aligned parallel to the laser beam incidence direction. The directional deviation of the reflected radiation can be easily further amplified depending on the alignment of the first side face relative to the laser beam incidence direction. This advantageously provides reflection protection over a large angular range of the electro-optical modulator's alignment relative to the laser path. This advantageously facilitates the assembly of the electro-optical modulator, avoiding reflections into the original laser path, and can be carried out in a time-saving manner.

[0014] The end face, the rear face, and the side faces are formed in particular on a surface of the path-guiding element (i.e., not on side faces of electrodes or other elements of the electro-optical modulator). The path-guiding element is designed to guide the laser beam. In particular, the path-guiding element is a crystal. In an advantageous embodiment, the first angle of inclination is between 87° and 89.9°, in particular between 88° and 89°. The angle of incidence 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 side face. When it strikes the end face of the electro-optical modulator, the laser beam is refracted according to Snell's law.The angle of incidence of the laser beam impinging on the front surface is preferably selected, taking into account the refraction of the laser beam at the front surface due to Snell's law, so that the beam direction of the laser beam in the electro-optical modulator runs parallel to the first and / or second side surface. Such a selection of the angle of incidence also causes the direction in which the laser beam is reflected at the front surface to deviate even further from the incident direction.

[0015] In a preferred embodiment, the rear surface and the first side surface enclose an obtuse second angle of inclination. Due to the inclination of the rear surface, a portion of the laser beam reflected by the rear surface is radiated away from the path along which the laser is propagated to the rear surface. This protects the laser source and / or other optical elements on the laser path from damage or interference caused by the portion of the laser radiation reflected by the rear surface. The protection of the laser source from reflected laser radiation is thus advantageously increased.

[0016] In an advantageous variant, the side surfaces are parallel to each other. This facilitates beam guidance of the refracted laser beam parallel to the side surfaces of the electro-optical modulator, ensuring good beam quality. In particular, the laser beam propagates from the front surface to the rear surface of the electro-optical modulator.

[0017] In an advantageous embodiment, the rear surface and the front surface run parallel to each other. This facilitates the guidance of the laser beam. In particular, this ensures that the laser beam has the same orientation after exiting the modulator as it did before entering the modulator. This simplifies the alignment of optical elements in a laser system.

[0018] In an advantageous embodiment, the path-guiding element is designed as a birefringent crystal. This birefringent crystal is particularly well-suited for adjusting the polarization of the laser beam. Furthermore, the crystal exhibits stable optical properties due to its solid crystal structure. In particular, the front and rear surfaces of the crystal are advantageously designed with high dimensional stability. The angle of incidence is particularly preferably selected so that the laser beam passes through the path-guiding element, which is designed as a birefringent crystal, parallel to its optical axis.

[0019] The electro-optical modulator can be designed as a Pockels cell. A Pockels cell enables the rapid and targeted adjustment of the polarization of the laser beam as it passes through the electro-optical modulator. In particular, the Pockels cell comprises the aforementioned birefringent crystal, with two electrodes preferably arranged opposite one another on the birefringent crystal.

[0020] A laser system according to the invention for modulating a laser beam with reflection protection has the following elements: a) a laser source for generating the laser beam; b) an aforementioned electro-optical modulator; c) a laser path which extends from the laser source through the electro-optical modulator and on which the laser beam can propagate after its generation from the laser source to the electro-optical modulator and through the electro-optical modulator, wherein the normal to the end face is inclined with an acute angle of incidence with respect to the direction of an incident section of the laser path, wherein the incident section of the laser path adjoins the end face of the electro-optical modulator and is formed outside the electro-optical modulator.

[0021] The acute angle of incidence already occurs when the first side surface of the electro-optical modulator is aligned parallel to the incident beam section. Thus, the path along which laser radiation is reflected back from the end surface toward the laser source deviates from the incident beam section of the laser path through which the laser beam passes when it first strikes the end surface, even with such an alignment of the electro-optical modulator. Depending on the angle of inclination of the end surface to the first side surface, reflection of the laser beam at the end surface toward the incident beam section can be avoided with different alignments of the electro-optical modulator. Advantageously, reflection protection of the laser source occurs over a large angular range of the electro-optical modulator's alignment relative to the incident beam section.This makes it easier to install the modulator while protecting the laser source and saves time.

[0022] The laser path runs, in particular, along the path a laser beam travels on its way from the laser source when aligned as intended and the laser system is in operation. The incident beam section is, in particular, a section of the laser path between the end face and a beam-generating or beam-guiding element of the laser system that is positioned upstream of the end face on the laser path and closest to it.

[0023] In a preferred embodiment, the side surfaces of the path-guiding element are inclined at an acute angle relative to the incident beam section of the laser path. This angle advantageously ensures good beam quality of the laser beam as it passes through the electro-optical modulator and after exiting the modulator. Good beam quality is achieved, among other things, by the beam direction of the laser beam in the path-guiding element running parallel to its side surfaces. When the laser beam strikes the end face of the path-guiding element, it is refracted according to Snell's law. The angle of refraction depends on the angle of incidence - the greater the angle of incidence, the greater the angle of refraction.The deflection angle can now be selected such that the laser beam refracted at the inclined end face (or at least partial beams of the refracted laser beam) runs parallel to the side surfaces of the modulator. Preferably, a birefringent crystal of the modulator is provided with a deflection angle relative to the laser path at which the refracted laser beam (or at least partial beams, in particular a regular partial beam) runs parallel to the side surfaces of the birefringent crystal.

[0024] In a further development of the aforementioned embodiment, the deflection angle is between 0.1° and 5°. Within this value range, the deflection angle can be adjusted to a typically small first and second inclination angle to ensure good beam quality of the laser beam when passing through the modulator.

[0025] In an advantageous embodiment, a polarizer is arranged on the laser path in front of and / or behind the electro-optical modulator. The polarizers can be used to select the polarization of the laser beam passing through the laser system. In particular, a polarizer in front of the electro-optical modulator can impart a specific polarization to the laser beam before the laser beam enters the modulator. The polarization of the laser beam can then be specifically changed in the modulator. A polarizer behind the module can be used to control the polarization at which the laser beam can reach a target in the beam direction behind this polarizer. This can be used, among other things, to generate and / or temporally shape laser pulses.

[0026] In a preferred variant, the laser system comprises several electro-optical modulators arranged in series along the laser path. In particular, in this variant, a polarizer is arranged between the electro-optical modulators and along the laser path before and after each end-side electro-optical modulator. Such an arrangement enables the laser pulses to be precisely tailored.

[0027] In a preferred variant, the laser system comprises an EUV source, wherein the EUV source is arranged behind the electro-optical modulator on the laser path extending from the laser source. After passing through the modulator, the laser beam can be directed onto the EUV source, for example, onto tin drops, to generate EUV radiation. The electro-optical modulator can be used to generate laser pulses, particularly in conjunction with the aforementioned polarizers, through which the laser radiation can only pass at a specific polarization.

[0028] A method for modulating a laser beam with reflection protection comprises the following steps: a) emitting the laser beam from a laser source; b) irradiating an aforementioned electro-optical modulator with the laser beam from the laser source, wherein the laser beam enters the electro-optical modulator through the end face of the electro-optical modulator, wherein the end face is inclined with respect to the direction of the laser trajectory of the laser beam at the end face before entering the electro-optical modulator, so that the laser beam strikes the end face at an acute angle of incidence.

[0029] In such a method, reflection of laser radiation at the front surface towards the laser source is prevented over a large angular range of the orientation of the electro-optical modulator.

[0030] In an advantageous embodiment of the method, the laser beam irradiates an EUV source after passing through the electro-optical modulator to generate EUV radiation. Within the scope of the method, laser pulses can be generated and / or shaped at high frequency by changing the polarization of the laser beam in the electro-optical modulator, particularly in conjunction with polarizers. Further advantages of the invention emerge from the description and the drawing. Likewise, the features mentioned above and those explained below can each be used individually or in groups in any desired combination. The embodiments shown and described are not to be understood as an exhaustive list, but rather are exemplary in nature for describing the invention.

[0031] Detailed description of the invention and drawing

[0032] Fig. 1 shows a schematic diagram of a laser system with an electro-optical modulator with reflection protection;

[0033] Fig. 2 shows a schematic of the laser system with an EUV source.

[0034] Fig. 1 schematically shows a cross-section through a modulator system 10 of a laser system 12 with an electro-optical modulator 14 for modulating a laser beam 16 under reflection protection. The laser beam 16 is emitted from a laser source 18 on a laser path 20 with a beam direction SR, wherein the beam direction SR is aligned parallel to a y-axis of a reference system R in the form of a Cartesian coordinate system with an x-axis, the y-axis, and a z-axis. The laser beam 16 then passes through a first polarizer 22a, which is arranged behind the laser source 18 on the laser path 20, viewed in the beam direction SR. The laser path 20 is, in particular, the path along which the laser beam 16 travels when it is aligned as intended. The first polarizer 22a polarizes the laser beam 16 with a linear first polarization PI, which is aligned along the z-direction.The term polarization refers in particular to the direction of the electric field of the laser beam 16.

[0035] After passing through the first polarizer 22a, the laser beam 16 propagates on an incident beam section 26 of the laser path 20 to the electro-optical modulator 14, which is arranged behind the first polarizer 22a in the beam direction SR of the laser beam 16.

[0036] The electro-optical modulator 14 has a Pockels cell 30 with two electrodes 36a, 36b and a path-guiding element 32 in the form of a birefringent crystal 34, wherein the electrodes 36a, 36b are arranged opposite one another on a first side surface 38a and a second side surface 38b of the birefringent crystal 34. The side surfaces 38a, 38b of the birefringent crystal 34 run parallel to one another and connect an end surface 40 of the birefringent crystal 34 and a rear surface 42 of the birefringent crystal 34. The end surface 40 of the birefringent crystal 34 serves to irradiate the laser beam 16 into the birefringent crystal 34, while the rear surface 42 serves to exit the laser beam 16 from the birefringent crystal 34.The front surface 40 and the rear surface 42 run parallel to each other, thereby simplifying the beam guidance of the laser beam 16 and the optical properties of the electro-optical modulator 14.

[0037] A first reflection component 44a of the laser beam 16 is reflected at the end face 40 upon irradiation into the birefringent crystal 34. The end face 40 extends at an inclination relative to the first side face 38a, such that an acute first inclination angle NW1 is formed between the end face 40 and the first side face 38a. Furthermore, the first side face 38a and the second side face 38b are pivoted outward at a pivot angle AW relative to the beam direction SR of the laser beam 16 before impinging on the birefringent crystal 34. Due to the first inclination angle NW1 and the pivot angle AW, the end face 40 has an acute angle of incidence EW relative to the beam direction SR of the laser beam 16 on the incident beam section 26 along the y-axis. Due to the angle of incidence EW, the laser beam 16 is reflected from the end face 40 in a reflection direction that is inclined to the beam direction SR of the laser beam 16 on the irradiation section 26.This prevents the laser beam 16 from radiating back into the laser source 18 on the path along which the laser beam 16 is propagated to the end face 40 and damaging or disrupting the laser source.

[0038] A portion of the laser beam 16 refracted at the front surface 40 propagates through the birefringent crystal 34 and strikes the back surface 42 of the birefringent crystal 34.

[0039] Upon exiting the birefringent crystal 34 at the rear surface 42, a second reflection portion 44b of the laser beam 16 is reflected. The rear surface 42 runs parallel to the end face 40 with an inclination toward the first side surface 38a, such that an obtuse second inclination angle NW2 is formed between the end face 40 and the first side surface 38a. As a result, the second reflection portion 44b of the laser beam 16 is reflected from the rear surface 42 in a second reflection direction, which is also inclined to the beam direction SR of the laser beam 16 on the incident beam section 26. Similar to the front surface 40, the risk of damage to the laser source 18 from laser radiation reflected from the rear surface 42 is thus greatly reduced or eliminated.

[0040] By applying a voltage to the electrodes 36a, 36b of the Pockels cell 30, a polarization P2 of the laser beam 16 is set, which the laser beam 16 has in the beam direction SR behind the Pockels cell 30 (the electro-optical modulator 14). In Fig. 1, the laser beam 16 has a polarization P2 in the xy plane parallel to the x-axis behind the Pockels cell 30 in the beam direction SR, i.e., the polarization P2 is rotated by 90° relative to the polarization P1.

[0041] After passing through the electro-optical modulator 14, the laser beam 16 propagates to a second polarizer 22b, which is arranged behind the electro-optical modulator 14 in the beam direction SR of the laser beam 16. The second polarizer 22b is also aligned parallel to the x-axis, so that the laser beam 16 can pass through the second polarizer 22b. If the polarizer 22b is aligned perpendicular to the x-axis (not shown in Fig. 1), the laser beam 16 cannot pass through the second polarizer 22b. The alignment of the polarization P2 of the laser beam 16 by the electro-optical module 14 relative to the alignment of the second polarizer P2 can therefore be used to generate and / or temporally shape laser pulses.

[0042] Fig. 2 schematically shows the laser system 12 with the laser source 18, the modulator system 10, and an EUV source 46. In Fig. 2, the modulator system 10 of the laser system 12 is symbolized by a box. After passing through the modulator system 10 of the laser system 12, the laser beam 16 propagates to the EUV source 46, which is arranged behind the modulator system 10 in the beam direction SR of the laser beam 16, parallel to the y-axis of the reference system R. The EUV source 46 has a droplet generator 48 that emits tin droplets 50 into the path of the laser beam 16. The laser beam 16 heats the tin droplets 50 to plasma, generating EUV radiation 52, among other things for use in EUV lithography.

[0043] Taking a summary of all the figures of the drawing, the invention relates to an electro-optical modulator 14 for adjusting the polarization P2 of a laser beam 16. An end face 40 of the electro-optical modulator 14, through which the laser beam 16 can be radiated into the electro-optical modulator 14, is aligned obliquely to a first side face 38a of the electro-optical modulator 14 at an acute first angle of inclination NW1, wherein the first side face 38a adjoins the end face 40.

Claims

Patent claims Electro-optical modulator (14) for modulating a laser beam (16) under reflection protection with a path-guiding element (32) for guiding the laser beam (16), comprising: a) an end face (40) of the path-guiding element (32) for radiating the laser beam (16) into the electro-optical modulator (14); b) a rear face (42) of the path-guiding element (32) for exiting the laser beam (16) from the electro-optical modulator (14), wherein the rear face (42) is opposite the end face (40); c) A first side surface (38a) and a second side surface (38b) of the web-guiding element (32), wherein the side surfaces (38a, 38b) lie between the end surface (40) and the rear surface (42) and connect the end surface (40) and the rear surface (42), wherein the end surface (40) and the first side surface (38a) enclose an acute first angle of inclination (NW1).The electro-optical modulator according to claim 1, wherein the first angle of inclination (NW1) is between 87° and 89.9°, in particular between 88° and 89°. The electro-optical modulator according to one of the preceding claims, wherein the rear surface (42) and the first side surface (38a) enclose an obtuse second angle of inclination (NW2). The electro-optical modulator according to one of the preceding claims, wherein the side surfaces (38a, 38b) are parallel to one another. The electro-optical modulator according to one of the preceding claims, wherein the rear surface (42) and the end surface (40) are parallel to one another.

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

7. Electro-optical modulator according to claim 6, wherein the electro-optical modulator (14) comprises a Pockels cell (30).

8. A laser system (12) for modulating a laser beam (16) under reflection protection, comprising: a) a laser source (18) for generating the laser beam (16); b) an electro-optical modulator (14) according to one of the preceding claims; c) a laser path (20) which extends from the laser source (18) through the electro-optical modulator (14) and on which the laser beam (16), after its generation, can propagate from the laser source (18) to the electro-optical modulator (14) and through the electro-optical modulator (14), wherein the normal to the end face (40) is inclined with an acute angle of incidence (EW) with respect to the direction of an incident beam section (26) of the laser path (20), wherein the incident beam section (26) of the laser path (20) adjoins the end face (40) of the electro-optical modulator (14) and is formed outside the electro-optical modulator (14).

9. Laser system according to claim 8, wherein the side surfaces (38a, 38b) of the path-guiding element (32) are inclined at an acute pivoting angle (AW) relative to the irradiation section (26) of the laser path (20).

10. Laser system according to claim 9, wherein the pivot angle (AW) is between 0.1° and 5°.

11. Laser system according to one of claims 8 to 10, wherein a polarizer (22a, 22b) is arranged on the laser path (20) in front of and / or behind the electro-optical modulator (14).

12. Laser system according to one of claims 8 to 11, wherein several electro-optical modulators (14) are arranged one behind the other along the laser path (20).

13. Laser system according to one of claims 8 to 12 with an EUV source (46), wherein the EUV source (46) is arranged on the laser path (20) starting from the laser source (18) behind the electro-optical modulator (14).

14. A method for modulating a laser beam (16) with reflection protection, comprising the steps of: a) emitting the laser beam (16) from a laser source (18); b) irradiating an electro-optical modulator (14) according to one of claims 1 to 7 with the laser beam (16) from the laser source (18), wherein the laser beam (16) enters the electro-optical modulator (14) through the end face (40) of the electro-optical modulator (14), wherein the end face (40) is inclined with respect to the direction of the laser path (24) of the laser beam (16) at the end face (40) before entering the electro-optical modulator (14), so that the laser beam (16) strikes the end face (40) at an acute angle of incidence (EW).

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

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