Device and method for amplifying laser radiation with a laser crystal

Abstract

The present invention relates to a device (10) for amplifying laser radiation with a laser crystal (1), in particular a plate-shaped one, and with at least two reflection devices (2, 3) for the laser radiation to be amplified, which are configured to reflect the laser radiation such that the laser radiation is passed back and forth multiple times through the laser crystal (1), wherein the reflection devices (2, 3) are designed as retroreflectors. The invention further relates to a method for amplifying laser radiation in which the laser radiation to be amplified is passed back and forth multiple times through a laser crystal (1), in particular a plate-shaped one, by means of two reflection devices (2, 3), wherein the reflection devices (2, 3) are designed as retroreflectors.

Description

The present invention relates to a device for amplifying laser radiation with a laser crystal, in particular a plate-shaped one, and with at least two reflection devices for the laser radiation to be amplified, which are configured to reflect the laser radiation such that the laser radiation is passed back and forth through the laser crystal multiple times. The invention further relates to a method for amplifying laser radiation in which the laser radiation to be amplified is passed back and forth through a laser crystal, in particular a plate-shaped one, multiple times by means of two reflection devices. Such devices are known, for example, as so-called InnoSlab amplifiers, as described in the following article: InnoSlab Amplifiers - IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 21, NO. 1, JANUARY / FEBRUARY 2015 - DOI: 10.1109 / JSTQE.2014.2333234. In such devices, the laser radiation to be amplified is guided as a beam through a plate-shaped laser crystal (slab). The reflection devices are designed as curved mirrors and guide the laser radiation to be amplified back and forth through the laser crystal multiple times in a zigzag pattern. Pumping is achieved by optical excitation, for example, by pump radiation supplied to the laser crystal. InnoSlab amplifiers of this type are sensitive to various influences, which can lead to strong angular and positional fluctuations as well as drifts. They can even potentially cause the entire system to become misaligned. These problems can occur particularly in combination with the spatial hole burning effect. Spatial hole burning occurs when laser radiation overlaps within the laser crystal due to its zigzag path. Against this background, the task arises to enable a more stable operation that is less sensitive to negative influences when amplifying laser radiation using a laser crystal, especially a plate-shaped one. To solve the problem, a device for amplifying laser radiation according to claim 1 is proposed. This device for amplifying laser radiation comprises a laser crystal, in particular a plate-shaped one, and at least two reflection devices for the laser radiation to be amplified, which are configured to reflect the laser radiation such that the laser radiation is passed back and forth through the laser crystal multiple times, wherein the reflection devices are designed as retroreflectors. The retroreflectors reflect incident laser radiation primarily in a direction parallel to the direction of incidence. According to the invention, this makes it possible to implement a propagation scheme for the laser radiation through the laser crystal in which the overlap of the laser radiation within the crystal is reduced or avoided. Laser radiation incident on the retroreflectors can propagate through the crystal parallel to the laser radiation reflected back by the retroreflectors. Undesired overlap of reciprocating laser radiation within the crystal can be avoided. Consequently, the effect of spatial hole burning can be reduced or completely prevented. This enables more stable operation, less sensitive to negative influences, when amplifying laser radiation using a disk-shaped laser crystal. According to an advantageous embodiment of the invention, the retroreflectors each have a plane of symmetry, and the planes of symmetry of the two retroreflectors are spaced apart from one another. Such an offset of the symmetry planes of the retroreflectors allows the laser radiation to be guided back and forth through the laser crystal along non-overlapping, parallel paths. The volume of the laser crystal can therefore be utilized effectively without the risk of unwanted overlaps of the reciprocating laser radiation. According to an advantageous embodiment of the invention, the retroreflectors are arranged such that the laser radiation to be amplified is guided in parallel paths in the laser crystal. According to an advantageous embodiment of the invention, at least one, preferably both, of the retroreflectors are designed as angle reflectors. Angle reflectors can comprise two or more mirrors oriented perpendicular to each other. According to an advantageous embodiment of the invention, at least one, preferably both, of the retroreflectors each comprise two mirrors whose mirror surfaces enclose an angle of 90°. Preferably, the retroreflectors comprise exactly two mirrors whose mirror surfaces enclose an angle of 90°. Such an embodiment of a retroreflector with exactly two mirrors has proven to be particularly suitable for use in a device for amplifying laser radiation, since the propagation path of the laser radiation through the laser crystal can be adjusted by selecting the properties of the two mirrors and their arrangement relative to each other. According to an advantageous embodiment of the invention, at least one, preferably both, of the mirrors are designed as plane mirrors. The use of a plane mirror as a mirror in the retroreflector offers the advantage that a parallel beam of light remains essentially parallel even after reflection from the plane mirror. Therefore, the collinearity of the laser radiation can be maintained by using two plane mirrors. Particularly preferably, both retroreflectors each have two plane mirrors, so that the laser radiation remains essentially parallel during amplification in the device. According to an advantageous embodiment of the invention, at least one, preferably both, of the mirrors are curved, in particular convexly curved. A curved mirror allows the reflected beam of light to be widened. For example, one of the retroreflectors may comprise a curved, in particular convexly curved, mirror, so that the laser beam is widened upon reflection from this mirror. According to an advantageous embodiment of the invention, the mirrors are arranged to form an angle of 45° with a plane, in particular a plane of symmetry, and the planes, in particular planes of symmetry, of the two retroreflectors are spaced apart from one another. By aligning the planes of the two retroreflectors in this way, the propagation path of the laser radiation through the laser crystal can be adjusted so that the reciprocating beams of the laser radiation do not overlap. According to an advantageous embodiment of the invention, the mirrors of a retroreflector are arranged such that the laser radiation can be coupled out of the device through a slit between these mirrors. This embodiment enables a beam path through the laser crystal in which the laser radiation is guided closer to an optical central axis of the device, which runs through the slit between the mirrors, with each reflection at a retroreflector. In this respect, after entering the device, the laser radiation can first be guided through outer regions of the laser crystal and then through a region in the center of the laser crystal before being guided through the slit and exiting the device.Alternatively, according to an advantageous embodiment of the invention, the mirrors of a retroreflector can be arranged such that the laser radiation can be coupled into the device through a slit between these mirrors. Such an embodiment enables a beam path through the laser crystal in which, with each reflection at a retroreflector, the laser radiation is guided further away from an optical central axis of the device that runs through the slit between the mirrors. In this respect, after entering the device, the laser radiation can first be guided through inner regions of the laser crystal and then through a region far from the center of the laser crystal. According to an advantageous embodiment of the invention, a deflecting mirror is arranged between at least one of the two retroreflectors and the laser crystal. The deflecting mirror allows the laser radiation to be deflected from a plane of the laser crystal, for example by 90°. This enables a compact design of the device, in which the respective retroreflector does not have to be arranged in the same plane as the laser crystal. Preferably, a deflecting mirror is arranged between each of the two retroreflectors and the laser crystal. This allows for a particularly compact design. According to an advantageous embodiment of the invention, the deflecting mirror(s) is / are designed as dichroic deflecting mirrors. With a dichroic deflecting mirror, pump radiation can be coupled into the laser crystal from the side of the respective deflecting mirror. According to an advantageous embodiment of the invention, a deflecting mirror is arranged between each of the two retroreflectors and the laser crystal, the deflecting mirrors being configured to couple pump radiation into the laser crystal. The two deflecting mirrors allow pump radiation to be coupled into the laser crystal from two opposite sides. According to an advantageous embodiment of the invention, at least one of the retroreflectors is configured to couple pump radiation into the laser crystal. For example, at least one, preferably both, of the retroreflectors may each have two mirrors whose mirror surfaces enclose an angle of 90°, wherein at least one of the two mirrors of a retroreflector is configured as a dichroic mirror. Preferably, both retroreflectors are configured to couple pump radiation into the laser crystal, so that the pump radiation can be coupled into the laser crystal from two opposite sides. Another object of the invention is a laser device comprising a laser radiation source and a device for amplifying laser radiation as described above, which is configured to amplify laser radiation generated by the laser radiation source. The laser device can achieve the same technical advantages and effects as described in connection with the device according to the invention for amplifying laser radiation. The laser radiation source can be, for example, a laser diode, a solid-state laser, or a gas laser. It can be a continuous-wave or a pulsed laser radiation source. Another object of the invention is a method for amplifying laser radiation, in which the laser radiation to be amplified is repeatedly passed back and forth through a laser crystal, in particular a plate-shaped one, by means of two reflection devices, wherein the reflection devices are designed as retroreflectors. The same technical advantages and effects can be achieved with this method as described in connection with the device according to the invention for amplifying laser radiation. According to an advantageous embodiment of the invention, the laser radiation to be amplified is guided in parallel paths within the laser crystal. This prevents unwanted overlapping of the reciprocating laser radiation. Alternatively or in addition to the advantageous embodiments explained above, the advantageous embodiments and features explained in connection with the device according to the invention can be used in the method. Further details and advantages of the invention will be explained below with reference to the exemplary embodiments shown in the figures. Figure 1 shows a prior art device for amplifying laser radiation in a perspective view; Figure 2 shows a prior art device for amplifying laser radiation in a schematic representation; Figure 3 shows a schematic representation of a first embodiment of the invention; Figure 4 shows a schematic representation of a device for amplifying laser radiation according to a second embodiment of the invention; Figure 5 shows a schematic representation of a device for amplifying laser radiation according to a third embodiment of the invention; and Figure 6 shows a schematic representation of a device for amplifying laser radiation according to a fourth embodiment of the invention. Figure 1 shows a perspective view of a prior art device 10 for amplifying laser radiation. The device 10 comprises a plate-shaped laser crystal 1, which is optically pumped to amplify the incoming laser radiation 6 and thereby obtain amplified outgoing laser radiation 7. The device 10 further comprises two reflection devices 2, 3, which reflect the laser radiation 6 to be amplified such that the laser radiation is repeatedly passed back and forth through the laser crystal 1. In the prior art device, the reflection devices 2, 3 are designed as curved mirrors. The laser radiation is therefore passed back and forth through the laser crystal 1 along a zigzag path. The laser crystal 1 comprises two optical surfaces for coupling in and out the laser radiation, which, in the representation according to Fig. 1, are located in an xy-plane. Furthermore, the laser crystal 1 comprises two large surfaces via which the laser crystal 1 can be cooled. These are located in an xz-plane in the representation according to Fig. 1. The device 10 also includes two dichroic deflecting mirrors 4, 5, which deflect the laser radiation to be amplified by 90° and simultaneously enable the coupling of a pump radiation 8, 9 into the laser crystal 1 via the aforementioned optical surfaces. Figure 2 shows a schematic representation of another prior art device 10 for amplifying laser radiation. This device 10 also comprises a plate-shaped laser crystal 1 and two reflection devices 2, 3 for the laser radiation 6 to be amplified. These reflection devices are configured to reflect the laser radiation such that it passes back and forth multiple times through the laser crystal 1. The reflection devices 2, 3 are designed as curved mirrors. It is evident that the paths of the laser radiation within the laser crystal 1 overlap. This can lead to the effect of spatial hole burning. Figure 3 schematically shows a first embodiment of a device 10 for amplifying laser radiation according to the invention. The device can be used in a laser device with a laser radiation source to amplify the laser radiation generated by the laser radiation source. The device 10 comprises a plate-shaped laser crystal 1 and two reflection devices 2, 3 for the laser radiation 6 to be amplified, which are configured to reflect the laser radiation such that the laser radiation is guided back and forth through the laser crystal 1 multiple times. Optionally, one or more deflecting mirrors can be provided, as explained in relation to the prior art according to Fig. 1. This allows optical pumping of the laser crystal 1, as in the prior art. In contrast to the prior art, the reflection devices 2, 3 in the embodiment shown in Fig. 3 are designed as retroreflectors. The retroreflectors 2, 3 reflect incident laser radiation primarily in a direction parallel to the direction of incidence. The laser radiation travels through the laser crystal 1 along parallel paths, thus reducing or even eliminating unwanted overlaps of the laser radiation within the laser crystal 1. In this case, the laser radiation passes through the laser crystal 1 a total of six times. There is one path of the incident laser radiation through the laser crystal 1 to the second reflection device 3 and five paths of the laser radiation, each resulting from reflection at one of the reflection devices 2, 3 and running parallel to the path of the incident laser radiation. The two retroreflectors 2, 3 each have a plane of symmetry S2, S3. These planes of symmetry S2, S3 are arranged parallel to each other and separated by a distance D. This arrangement allows for an offset of the reciprocating laser radiation and further reduces the risk of unwanted overlaps of the laser radiation within the laser crystal 1. In this embodiment, the retroreflectors 2, 3 are configured as angle reflectors, each with two mirrors 21, 22, 31, 32 whose mirror surfaces form an angle of 90°. According to this embodiment, the mirrors 21, 22, 31, 32 are all designed as plane mirrors. Thus, a first retroreflector 2 is formed by two plane mirrors 21, 22 whose mirror surfaces form an angle of 90°. A second retroreflector 3 is formed by two plane mirrors 31, 32 whose mirror surfaces form an angle of 90°. The mirrors 21, 22, 31, 32 each form an angle of 45° with the plane of symmetry S2, S3. The mirrors 21, 22 of the first retroreflector 2 are arranged such that the laser radiation can be coupled out of the device 10 through a slit 23 between these mirrors 21, 22. Figure 4 shows a second embodiment of a device 10 for amplifying laser radiation according to the invention. Device 10 is similar to the device of the first embodiment, except that the laser radiation passes through the laser crystal 1 a total of eight times. Figure 5 shows a third embodiment of a device 10 for amplifying laser radiation according to the invention. The device 10 is similar to the device according to the first embodiment, except that the laser radiation passes through the laser crystal 1 a total of five times and is coupled out through a slit 33 between the mirrors 31, 32 of the second reflection device 3. Figure 6 shows a fourth embodiment of a device 10 for amplifying laser radiation according to the invention. The device 10 is similar to the device according to the third embodiment, except that one of the two mirrors 22 of the first reflection device 2 is curved, in particular convexly curved. The laser radiation is therefore expanded upon reflection at this mirror 22. In the embodiments described above according to the invention, the laser radiation is first guided by means of the reflection devices 2, 3 through regions of the laser crystal 1 that are located further away from the plane of symmetry S2, S3 of the reflection devices 2, 3, and is then guided through regions of the laser crystal 1 that are closer to the plane of symmetry S2, S3 of the reflection devices 2, 3. In this respect, the laser radiation is guided from the outside to the inside of the laser crystal 1. According to a modification of the described embodiments, it is possible for this beam direction to be reversed, i.e.,The laser radiation is first guided by the reflection devices 2, 3 through regions of the laser crystal 1 that lie close to the plane of symmetry S2, S3 of the reflection devices 2, 3, and then through regions of the laser crystal 1 that lie further away from the plane of symmetry S2, S3 of the reflection devices 2, 3. In such a modification, the laser radiation is guided from the inside out through the laser crystal 1. In the embodiments described above according to the invention, a deflecting mirror can optionally be arranged between at least one of the two retroreflectors and the laser crystal. This deflecting mirror(s) can be designed as dichroic deflecting mirrors so that pump radiation can be coupled into the laser crystal. Reference symbol: 1 Laser crystal 2 Reflection device 3 Reflection device 4 Deflection mirror 5 Deflection mirror 6 (To be amplified) incoming laser radiation 7 (Amplified) outgoing laser radiation 8 Pump radiation 9 Pump radiation 10 Device for amplifying laser radiation 21 Mirror 22 Mirror 23 Slit 31 Mirror 32 Mirror D Distance S2, S2 Plane of symmetry x, y, z Spatial directions QUOTES INCLUDED IN THE DESCRIPTION This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited non-patent literature IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 21, NO. 1, JANUARY / FEBRUARY 2015 - DOI: 10.1109 / JSTQE.2014.2333234

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Claims

Device (10) for amplifying laser radiation with a, in particular plate-shaped, laser crystal (1) and with at least two reflection devices (2, 3) for the laser radiation to be amplified, which are configured to reflect the laser radiation in such a way that the laser radiation is passed back and forth several times through the laser crystal (1), characterized in that the reflection devices (2, 3) are designed as retroreflectors. Device (10) according to claim 1, characterized in that the retroreflectors each have a plane of symmetry (S2, S3) and the planes of symmetry (S2, S3) of the two retroreflectors are arranged apart from each other. Device (10) according to one of the preceding claims, characterized in that the retroreflectors are arranged such that the laser radiation to be amplified is guided in parallel paths in the laser crystal (1). Device (10) according to one of the preceding claims, characterized in that at least one, preferably both, of the retroreflectors are designed as angle reflectors. Device (10) according to one of the preceding claims, characterized in that at least one, preferably both, of the retroreflectors each have two mirrors (21, 22, 31, 32), the mirror surfaces of which enclose an angle of 90°. Device (10) according to claim 5, characterized in that at least one, preferably both, of the mirrors (21, 22, 31, 32) are designed as plane mirrors. Device (10) according to claim 5, characterized in that at least one, preferably both, of the mirrors (22) are curved, in particular convexly curved. Device (10) according to one of claims 5 to 7, characterized in that the mirrors (21, 22, 31, 32) each enclose an angle of 45° with a plane, in particular a plane of symmetry (S2, S3), and the planes, in particular planes of symmetry (S2, S3), of the two retroreflectors are arranged spaced apart from each other. Device (10) according to one of claims 5 to 8, characterized in that the mirrors (21, 22, 31, 32) of a retroreflector are arranged spaced apart from each other such that the laser radiation can be coupled out of or into the device (10) through a slit (23, 33) between these mirrors (21, 22, 31, 32). Device (10) according to one of the preceding claims, characterized in that a deflecting mirror (4, 5) is arranged between at least one of the two retroreflectors and the laser crystal (1). Device (10) according to claim 10, characterized in that the deflecting mirror (4, 5) is designed as a dichroic deflecting mirror. Device (10) according to one of the preceding claims, characterized in that a deflecting mirror (2, 3) is arranged between each of the two retroreflectors (2, 3) and the laser crystal (1), wherein the deflecting mirrors (2, 3) are configured to couple a pump radiation into the laser crystal (1). Device (10) according to one of the preceding claims, characterized in that at least one of the retroreflectors is configured to couple a pump radiation into the laser crystal (1). Laser device comprising a laser radiation source and a device (10) according to any of the preceding claims, which is configured to amplify laser radiation generated by the laser radiation source. Method for amplifying laser radiation in which the laser radiation to be amplified is repeatedly passed through a laser crystal (1), in particular a plate-shaped one, by means of two reflection devices (2, 3), wherein the reflection devices (2, 3) are designed as retroreflectors. Method according to claim 15, characterized in that the laser radiation to be amplified is guided in parallel paths in the laser crystal (1).

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