DE102022128256A1 - Electro-optical modulator with reflection protection - Google Patents

Abstract

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).

Description

Background of the invention

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 before and/or after the electro-optical modulator from laser radiation reflected by the electro-optical modulator. The invention further relates to a laser system with such a modulator and a method using such a modulator.

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

The 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. The respective Pockels cell has an electro-optical crystal and two electrodes that are arranged opposite one another on the crystal. The Pockels cell and other optical elements of the laser system are inclined relative to one another in order to avoid 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 available from US 2014/0203194 A1 known.

In the known laser systems, the reflection protection can only be achieved with comparatively high expenditure, especially for the adjustment of the laser system.

Object of the invention

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

Description of the invention

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.

1. a) Eine Stirnfläche des bahnführenden Elements zum Einstrahlen des Laserstrahls in den elektrooptischen Modulator;
2. b) Eine Rückfläche des bahnführenden Elements zum Austreten des Laserstrahls aus dem elektrooptischen Modulator, wobei die Rückfläche der Stirnfläche gegenüberliegt;
3. c) Eine erste Seitenfläche und eine zweite Seitenfläche des bahnführenden Elements, wobei die Seitenflächen zwischen der Stirnfläche und der Rückfläche liegen und die Stirnfläche und die Rückfläche verbinden,

wobei die Stirnfläche und die erste Seitenfläche einen spitzen ersten Neigungswinkel einschließen.The electro-optical modulator according to the invention has a path-guiding element for guiding the laser beam and the following elements:

1. a) An end face of the path-guiding element for irradiating the laser beam into the electro-optical modulator;
2. b) a rear surface of the path-guiding element for exiting the laser beam from the electro-optical modulator, the rear surface being opposite the front surface;
3. c) a first side surface and a second side surface of the web-guiding element, the side surfaces being located between the front surface and the rear surface and connecting the front surface and the rear surface,

wherein the front surface and the first side surface enclose an acute first angle of inclination.

A laser beam that hits the front face is reflected to a certain extent by the front face. The front 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 front face at an angle of incidence that differs from 0° is reflected from the front 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 front face. The angle of incidence between the perpendicular to the front face and the beam direction of the laser beam is measured when it hits the front 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 front face.

Due to the inclination of the front surface to the first side surface, the oblique orientation of the reflected laser radiation with respect to the direction of incidence of the laser beam already occurs when the first side surface of the electro-optical modulator is aligned parallel to the direction of incidence of the laser beam. The directional deviation of the reflected radiation can be further increased in a simple manner depending on the alignment of the first side surface with respect to the direction of incidence of the laser beam. This advantageously provides protection against reflection over a large angular range of the alignment of the electro-optical modulator with respect to the laser path. This advantageously facilitates the assembly of the electro-optical modulator under Avoiding reflections in the original laser path is made easier and can be done in a time-saving manner.

The front surface, the rear surface and the side surfaces are formed in particular on a surface of the path-guiding element (i.e. not on side surfaces 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 the second side surface. When it hits the front surface of the electro-optical modulator, the laser beam is refracted in accordance with Snell's law. The angle of incidence of the laser beam hitting the front surface is now preferably selected, taking into account the refraction of the laser beam at the front surface due to Snell's law, such that the beam direction of the laser beam in the electro-optical modulator runs parallel to the first and/or the second side surface. Such a selection of the angle of incidence also means that the direction in which the laser beam is reflected at the front surface deviates even further from the beam direction.

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 on the rear surface is radiated away from the path on which the laser is propagated to the rear surface. As a result, 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 surface. The protection of the laser source against reflected laser radiation is thus advantageously increased.

In an advantageous variant, the side surfaces are parallel to one another. This makes it easier to guide the respective refracted laser beam parallel to the side surfaces of the electro-optical modulator in order to achieve good beam quality of the laser beam. In particular, the laser beam propagates from the front surface to the rear surface of the electro-optical modulator.

In an advantageous design, the rear surface and the front surface run parallel to each other. This makes it easier to guide the laser beam.

In particular, this can ensure 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.

In an advantageous embodiment, the path-guiding element is designed as a birefringent crystal. The birefringent crystal is particularly suitable for adjusting the polarization of the laser beam. In addition, the crystal has stable optical properties due to the solid crystal structure. In particular, the front surface and the rear surface of the crystal are advantageously designed with great dimensional stability. The angle of incidence is particularly preferably selected so that the laser beam passes through the path-guiding element designed as a birefringent crystal parallel to its optical axis.

The electro-optical modulator can be designed as a Pockels cell. A Pockels cell enables the polarization of the laser beam to be adjusted quickly and precisely as it passes through the electro-optical modulator. In particular, the Pockels cell has the aforementioned birefringent crystal, with two electrodes preferably being arranged opposite one another on the birefringent crystal.

1. a) Eine Laserquelle zur Erzeugung des Laserstrahls;
2. b) Einen vorgenannten elektrooptischen Modulator;
3. c) eine Laserbahn, die sich von der Laserquelle ausgehend durch den elektrooptischen Modulator hindurch erstreckt und auf der der Laserstrahl nach seiner Erzeugung von der Laserquelle zu dem elektrooptischen Modulator und durch den elektrooptischen Modulator propagieren kann, wobei das Lot auf die Stirnfläche in Bezug auf die Richtung eines Einstrahlabschnitts der Laserbahn mit einem spitzen Einfallswinkel geneigt ist, wobei der Einstrahlabschnitt der Laserbahn an die Stirnfläche des elektrooptischen Modulators angrenzt und außerhalb des elektrooptischen Modulators ausgebildet ist.

A laser system according to the invention for modulating a laser beam under reflection protection has the following elements:

1. a) A laser source for generating the laser beam;
2. b) An electro-optical modulator as mentioned above;
3. 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 perpendicular 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.

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. This means that the path on which the laser radiation from the front face in the direction of the laser source, from the beam section of the laser path through which the laser beam passes when it first hits the front face, even with such an alignment of the electro-optical modulator. Depending on the angle of inclination of the front face to the first side face, reflection of the laser beam on the front face in the direction of the beam section can be avoided with different alignments of the electro-optical modulator. Advantageously, the laser source is protected from reflection over a large angle range of the alignment of the electro-optical modulator in relation to the beam section. This makes it easier to install the modulator while protecting the laser source and can be carried out in a time-saving manner.

The laser path runs in particular along the path that a laser beam travels on its way from the laser source when it is 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 located in front of the end face on the laser path and is closest to it.

In a preferred embodiment, the side surfaces of the path-guiding element are inclined at an acute angle of deflection relative to the beam section of the laser path. The deflection angle advantageously ensures good beam quality of the laser beam when it passes through the electro-optical modulator and after it exits 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 hits the front surface 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 so that the laser beam refracted at the inclined front surface (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 pivoting angle relative to the laser path, in which the refracted laser beam (or at least partial beams, in particular a proper partial beam) runs parallel to the side surfaces of the birefringent crystal.

In a further development of the aforementioned embodiment, the swivel angle is between 0.1° and 5°. In this value range, the swivel angle can be adjusted to a typically small first and second inclination angle in order to achieve a good beam quality of the laser beam when passing through the modulator.

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 which polarization the laser beam has that passes through the laser system. In particular, a polarizer in front of the electro-optical modulator can be used to provide the laser beam with a specific polarization 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 to generate and/or temporally shape laser pulses, among other things.

In a preferred variant, the laser system has several electro-optical modulators that are 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 the electro-optical modulator at the end. Such an arrangement makes it possible to tailor the laser pulses precisely.

In a preferred variant, the laser system has an EUV source, wherein the EUV source is arranged on the laser path starting from the laser source behind the electro-optical modulator. After passing through the modulator, the laser beam can be directed at the EUV source, for example at tin drops, in order to generate EUV radiation. The electro-optical modulator can be used to generate laser pulses, in particular together with the aforementioned polarizers, through which the laser radiation can only pass with a specific polarization.

1. a) Aussenden des Laserstrahls aus einer Laserquelle;
2. b) Durchstrahlen eines vorgenannten elektrooptischen Modulators mit dem Laserstrahl aus der Laserquelle, wobei der Laserstrahl durch die Stirnfläche des elektrooptischen Modulators in den elektrooptischen Modulator eintritt, wobei die Stirnfläche in Bezug auf die Richtung der Laserbahn des Laserstrahls an der Stirnfläche vor dem Eintritt in den elektrooptischen Modulator geneigt ist, sodass der Laserstrahl mit einem spitzen Einfallswinkel auf die Stirnfläche trifft.

A method for modulating a laser beam under reflection protection comprises the following steps:

1. a) emitting the laser beam from a laser source;
2. b) irradiating an electro-optical modulator mentioned above with the laser beam from the laser source, whereby the laser beam enters the electro-optical modulator through the front face of the electro-optical modulator, whereby the front face is arranged at the front face with respect to the direction of the laser path of the laser beam is tilted before entering the electro-optical modulator so that the laser beam hits the front surface at an acute angle of incidence.

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.

In an advantageous embodiment of the method, the laser beam irradiates an EUV source after passing through the electro-optical modulator in order to generate EUV radiation. Within the scope of the method, laser pulses can be generated and/or formed at high frequency by changing the polarization of the laser beam in the electro-optical modulator, in particular in conjunction with polarizers.

Further advantages of the invention emerge from the description and the drawing. Likewise, the features mentioned above and those described below can each be used individually or in combination. The embodiments shown and described are not to be understood as an exhaustive list, but rather are exemplary in nature for the description of the invention.

Detailed description of the invention and drawing

• 1 shows a schematic of a laser system with an electro-optical modulator with reflection protection;
• 2 shows schematically the laser system with an EUV source.

1 shows a schematic 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 by 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 on the laser path 20 behind the laser source 18 as seen in the beam direction SR. The laser path 20 is in particular the path along which the laser beam 16 runs if it is aligned as intended. The first polarizer 22a polarizes the laser beam 16 with a linear first polarization P1, 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.

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.

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 on 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 radiate the laser beam 16 into the birefringent crystal 34, while the rear surface 42 serves to allow the laser beam 16 to exit the birefringent crystal 34. The front surface 40 and the rear surface 42 run parallel to each other, which simplifies the beam guidance of the laser beam 16 and the optical properties of the electro-optical modulator 14.

A first reflection portion 44a of the laser beam 16 is reflected on the front face 40 when it is irradiated into the birefringent crystal 34. The front face 40 runs at an incline to the first side face 38a, so that an acute first inclination angle NW1 is formed between the front face 40 and the first side face 38a. In addition, the first side face 38a and the second side face 38b are pivoted out at a pivot angle AW relative to the beam direction SR of the laser beam 16 before hitting the birefringent crystal 34. Due to the first inclination angle NW1 and the pivot angle AW, the front face 40 has an acute angle of incidence EW relative to the beam direction SR of the laser beam 16 on the beam section 26 along the y-axis. Due to the angle of incidence EW, the laser beam 16 is reflected from the front surface 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 reflecting back into the laser source 18 on the path on which the laser beam 16 is propagated to the front surface 40 and damaging or disrupting it.

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

When 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 front surface 40 with an inclination to the first side surface 38a, so that an obtuse second inclination angle NW2 is formed between the front surface 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 irradiation section 26. In this way, similar to the front surface 40, the risk of damage to the laser source 18 by laser radiation reflected at the rear surface 42 is greatly reduced or prevented.

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 the 1 the laser beam 16 in the beam direction SR behind the Pockels cell 30 has a polarization P2 in the xy plane parallel to the x-axis, ie the polarization P2 is rotated by 90° with respect to the polarization P1.

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. When the polarizer 22b is aligned perpendicular to the x-axis (in 1 not shown), 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.

2 shows schematically the laser system 12 with the laser source 18, the modulator system 10 and with an EUV source 46. 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 drop generator 48, which emits tin drops 50 into the path of the laser beam 16. The laser beam 16 heats the tin drops 50 to plasma, generating EUV radiation 52, among other things for use in EUV lithography.

Taking a look at all the figures of the drawing together, 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.

QUOTES INCLUDED IN THE DESCRIPTION

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

Cited patent literature

• US 20160156150 A1 [0003]
• US 20140203194 A1 [0004]

Claims (15)

Electro-optical modulator (14) for modulating a laser beam (16) with 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 the laser beam (16) to exit from the electro-optical modulator (14), the rear face (42) being 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 front surface (40) and the rear surface (42) and connect the front surface (40) and the rear surface (42), wherein the front surface (40) and the first side surface (38a) enclose an acute first angle of inclination (NW1). Electro-optical modulator according to Claim 1 , wherein the first inclination angle (NW1) is between 87° and 89.9°, in particular between 88° and 89°. 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 inclination angle (NW2). Electro-optical modulator according to one of the preceding claims, wherein the side surfaces (38a, 38b) are parallel to each other. Electro-optical modulator according to one of the preceding claims, wherein the rear surface (42) and the front surface (40) run parallel to each other. Electro-optical modulator according to one of the preceding claims, wherein the path-guiding element (32) is designed as a birefringent crystal (34). Electro-optical modulator according to Claim 6 , wherein the electro-optical modulator (14) comprises a Pockels cell (30). Laser system (12) for modulating a laser beam (16) with 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) can propagate after its generation from the laser source (18) to the electro-optical modulator (14) and through the electro-optical modulator (14), wherein the perpendicular 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). 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 beam section (26) of the laser path (20). Laser system according to Claim 9 , where the swing angle (AW) is between 0.1° and 5°. Laser system according to one of the Claims 8 until 10 , wherein a polarizer (22a, 22b) is arranged on the laser path (20) in front of and/or behind the electro-optical modulator (14). Laser system according to one of the Claims 8 until 11 , wherein several electro-optical modulators (14) are arranged one behind the other along the laser path (20). Laser system according to one of the Claims 8 until 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). Method for modulating a laser beam (16) under reflection protection, comprising the steps: a) emitting the laser beam (16) from a laser source (18); b) passing it through an electro-optical modulator (14) according to one of the Claims 1 until 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). Procedure according to Claim 14 , wherein the laser beam (16) after passing through the electr an EUV source (46) is irradiated by an optical modulator (14) to generate EUV radiation (52).

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