Laser amplifier with dichroic mirror for coupling a pump laser beam

Abstract

A laser amplifier (1) for amplifying an input laser beam (2) to an amplified output laser beam (3) comprises a laser-active solid rod (4) through which the input laser beam (2) passes at least twice, having two opposing end faces (4a, 4b), and a pumping device (5) for generating at least one pump laser beam (6), which is coupled into the solid rod (4) via one, first end face (4a) of the solid rod (4) in order to optically pump the solid rod (4), wherein the input laser beam (2) is coupled into the solid rod (4) via the other, second end face (4b) of the solid rod (4), and the input laser beam (2a, 2b) amplified in the solid rod (4) is coupled out of the solid rod (4) in order to form the output laser beam (3).According to the invention, a dichroic mirror (9) is arranged perpendicular to the pump laser beam (6) in the beam path of the pump laser beam (6) between the solid rod (4) and the pumping device (5), which is transmissive for the pump laser beam (6) and reflective for the input laser beam (2).

Description

[0001] The invention relates to a laser amplifier for amplifying an input laser beam to an amplified output laser beam, having a laser-active solid-state rod with two opposite end faces through which the input laser beam passes at least twice, and having a pumping device for generating at least one pump laser beam which is coupled into the solid-state rod via the one, first end face of the solid-state rod in order to optically pump the solid-state rod, wherein both the input laser beam is coupled into the solid-state rod via the other, second end face of the solid-state rod and the input laser beam amplified in the solid-state rod is coupled out of the solid-state rod in order to form the output laser beam.

[0002] Such a laser amplifier is known, for example, from EP 1 507 321 A1. In this known laser amplifier, a pump laser beam is coupled into a gain medium via one of its first end faces, and an input laser beam is coupled into the gain medium via its other, second end face. After passing through the gain medium, the input laser beam is deflected from the beam path of the counter-propagating input laser beam by a dichroic plate arranged obliquely to the pump laser beam, which is transmissive to the pump laser beam and reflective to the input laser beam. It is reflected back by a mirror, and coupled back into the gain medium via the first end face by the dichroic plate, co-propagating to the pump laser beam.The input laser beam, which is amplified twice in the gain medium, is coupled out via the second end face and deflected as the output laser beam by means of an optical isolator from the counter-rotating beam path of the incident input laser beam.

[0003] Furthermore, US Pat. No. 6,141,143 discloses a laser amplifier in which a first and a second pump laser beam are coupled into a laser crystal, each via opposite crystal end faces. An input laser beam is coupled into the laser crystal via one crystal end face in the same direction as the first pump laser beam by means of a mirror arranged obliquely to the first pump laser beam, which is transmissive to the first pump laser beam and reflective to the input laser beam. The input laser beam amplified in the laser crystal is output via the other crystal end face and separated from the counter-propagating second pump laser beam by means of a mirror arranged obliquely to the second pump laser beam, which is transmissive to the second pump laser beam and reflective to the amplified output laser beam, in order to form the output laser beam.

[0004] Single-emitter pump diodes with fiber coupling are typically used as the pumping device. The fiber is carefully wound, and the output end is appropriately mounted. The pump laser beam emerging from the output end is imaged onto the end face of the solid-state rod using appropriate optics. The pump radiation is homogenized by propagation along the fiber.

[0005] In contrast, the object of the present invention is to provide an alternative beam guidance for a laser amplifier of the type mentioned above.

[0006] This object is achieved according to the invention in the laser amplifier mentioned at the outset in that a dichroic mirror is arranged in the beam path of the pump laser beam between the solid-state rod and the pumping device at right angles to the pump laser beam, which mirror is transmissive for the pump laser beam and reflective for the input laser beam to be coupled in.

[0007] In contrast to EP 1 507 321 A1, where the input laser beam is deflected from the beam path of the pump laser beam by means of a dichroic plate and reflected back outside the beam path of the pump laser beam by means of a separate mirror, according to the invention the input laser beam is reflected back for a second pass through the solid-state rod by means of a dichroic mirror arranged in the beam path of the pump laser beam at right angles to the pump laser beam.

[0008] In a particularly preferred embodiment, a beam splitter device is arranged in the counter-rotating beam path of the input laser beam to be coupled in and the amplified input laser beam output from the solid-state rod. This beam splitter device separates the amplified input laser beam output from the solid-state rod from the input laser beam to be coupled in, in order to form the output laser beam. In a preferred development with a polarized input laser beam to be coupled in, the beam splitter device has a polarization beam splitter and a λ / 4 plate arranged between the solid-state rod and the polarization beam splitter in order to separate the output amplified input laser beam from the input laser beam to be coupled in due to its polarization at the polarization beam splitter, which is rotated by the λ / 4 plate.For a third and fourth pass of the input laser beam through the solid-state rod, another polarization beam splitter is arranged between the polarization beam splitter and the λ / 4 plate. This further polarization beam splitter is transmissive for the input laser beam to be coupled in and the input laser beam that has passed through the λ / 4 plate four times, and reflective for the input laser beam that has passed through the λ / 4 plate twice. This splitter directs the input laser beam that has passed through the λ / 4 plate twice onto a retro-reflecting mirror. Furthermore, a Faraday rotator can be arranged between the two polarization beam splitters, which changes the polarization of the passing input laser beam and the four-fold amplified input laser beam, in particular in the same rotation direction. The Faraday rotator preferably changes the passing input laser beam rotationally by 45°.The Faraday rotator together with the polarization beam splitters adjacent to both sides can also be designed as a Faraday isolator.

[0009] Preferably, the pumping device comprises a single-emitter pump diode for generating the pump laser beam or a plurality of single-emitter pump diodes for generating a plurality of parallel, spaced-apart pump laser beams. In the latter case, the plurality of single-emitter pump diodes are preferably arranged offset from one another in a direction transverse to the pump laser beams.

[0010] Starting from at least one single-emitter pump diode, the pump laser beam can be guided to the solid-state rod by means of an optical fiber (e.g. glass fiber), and the pump laser beam emerging from the output-side fiber end can be imaged onto the end face of the solid-state rod using appropriate optics. The optical fiber also serves to homogenize the pump laser beam, i.e. to achieve a homogeneous intensity distribution across the fiber cross-section at the output-side fiber end. However, the 1 to 1.5 m long optical fiber must be guided and wound up, which usually requires complex manual work and takes up a lot of space in the laser amplifier. In addition, the waveguiding in multi-mode fibers leads to a coupling of mechanical vibrations to the pump laser radiation, which in turn imposes additional (intensity) noise on the laser beam to be amplified.

[0011] Alternatively, the at least one pump laser beam can also be coupled into the first end face from the at least one single-emitter pump diode using free-beam propagation, i.e., without pump laser beam homogenization. The pump laser radiation from the individual single-emitter pump diodes is imaged directly onto the end face of the solid-state rod, so that there are excited and unexcited regions on the end face. As it propagates along the solid-state rod, the pump laser radiation is distributed. In addition, the input laser beam passes through the pumped solid-state rod at least twice, resulting in further homogenization of the input laser beam, even without a special homogenizer. Free-beam propagation increases the optical efficiency because losses during coupling into the fiber are avoided. In some cases, the optics required for coupling into the optical fiber can also be eliminated.

[0012] Overall, typical fiber coupling losses of 5 to 10 percentage points can be avoided, as no optical fiber is required. Without an optical fiber, the laser amplifier can be constructed more compactly and even automated, as the required optics can be easily positioned and adjusted using robots. The amplified laser beam also exhibits significantly lower noise. An optical fiber, on the other hand, cannot be installed automatically; instead, it requires manual intervention.

[0013] Furthermore, the laser amplifier may comprise a laser beam source, such as a seed laser, for generating the input laser beam.

[0014] Further advantages of the invention will become apparent from the description and the drawings. Likewise, the features mentioned above and those further described can be used individually or in combination. The embodiments shown and described are not intended to be exhaustive, but rather are exemplary in nature for describing the invention.

[0015] It shows: Fig. 1: a first embodiment of the laser amplifier according to the invention with a single single-emitter laser diode and with a double amplifier pass; Fig. 2: a second embodiment of the laser amplifier according to the invention with a single single-emitter laser diode and with a quadruple amplifier pass; Fig. 3a: a third embodiment of the laser amplifier according to the invention with several single-emitter laser diodes and with a double amplifier passage; and Fig. 3b: an intensity distribution of the plurality of single-emitter laser diodes of the laser amplifier according to the invention according to the third embodiment.

[0016] The Fig. The laser amplifier 1 shown in Figure 1 serves to amplify an input laser beam 2 to an amplified output laser beam 3.

[0017] The laser amplifier 1 has a laser-active solid-state rod 4, through which the input laser beam 2 passes twice, with two opposite end faces 4a, 4b, a pump device 5 for generating a pump laser beam 6, and a laser beam source (e.g. seed laser) 7 for generating the input laser beam (e.g. seed laser beam) 2. The solid-state rod 4 is, for example, a cylindrical Yb:YAG crystal having a diameter of, for example, 3 mm, a length of, for example, 10 mm, and a Yb doping of, for example, 3%. The pump device 5 can be formed, for example, by a single-emitter pump diode 8. In the case of a Yb:YAG crystal, the pump device 5 emits a pump wavelength of 941 nm, and the laser beam source 7 emits laser radiation with a wavelength of 1030 nm.

[0018] The pump laser beam 6 is coupled into the solid-state rod 4 via the first end face 4a of the solid-state rod 4 in order to optically pump the solid-state rod 4. Starting from the single-emitter pump diode 8, the pump laser beam 6 can be coupled into the first end face 4a using free-beam propagation. Alternatively, the pump laser beam 6 can also be guided to the solid-state rod 4 via an optical fiber, and the pump laser beam 6 emerging from the output-side fiber end can be imaged onto the first end face 4a using appropriate optics.

[0019] In the beam path of the pump laser beam 6, a dichroic mirror 9 is arranged between the solid rod 4 and the pump device 5, perpendicular to the pump laser beam 6. This dichroic mirror is transmissive for the pump laser beam 6 and reflective for the input laser beam 2. The input laser beam 2 is coupled into the solid rod 4 via the other, second end face 4b of the solid rod 4 and passes through the solid rod 4, thereby being amplified. The input laser beam 2 exits the solid rod 4 via the first end face 4a and is reflected back by the dichroic mirror 9. The reflected input laser beam 2 re-enters the solid rod 4 via the first end face 4a of the solid rod 4, passes through the solid rod 4 again, whereby it is amplified a second time, and exits the solid rod 4 as a twice-amplified input laser beam 2a via the second end face 4b.

[0020] In the counter-rotating beam path of the input laser beam 2 to be coupled in and the doubly amplified input laser beam 2a output from the solid-state rod, a beam splitter device 10 is arranged, which separates the output, doubly amplified input laser beam 2a from the input laser beam 2 to be coupled in. In the case of a polarized input laser beam 2, as in Fig. 1, the beam splitter device 10 comprises a polarization beam splitter 11 and a λ / 4 plate 12 arranged between the solid-state rod 4 and the polarization beam splitter 11. Due to its polarization rotated by the λ / 4 plate 12, the output laser beam 2a, which is coupled out and amplified twice, is separated at the polarization beam splitter 11 from the input laser beam 2 to be coupled in, in order to form the output laser beam 3.

[0021] From Fig. 1 differs from the Fig. 2 by an additional third and fourth pass of the input laser beam 2 through the solid-state rod 4, thereby generating a four-fold amplified input laser beam 2b, which forms the output laser beam 3. For this purpose, a further polarization beam splitter 13 is arranged between the polarization beam splitter 11 and the λ / 4 plate 12. This further polarization beam splitter 13 is arranged obliquely in the beam path of the pump laser beam 6 and obliquely in the beam path of the input laser beam 2, 2b to be coupled in and the four-fold amplified input laser beam 2, 2b. Furthermore, a Faraday rotator 15 is arranged between the two polarization beam splitters 11, 13, which changes the polarization of the passing input laser beam 2 or of the four-fold amplified input laser beam 2b, in particular in the same direction of rotation.

[0022] The polarization beam splitter 13 is transmissive for the input laser beam 2 to be coupled in and for the quadruple-amplified input laser beam 2b, which passes through the λ / 4 plate 12 four times, and reflective for the doubly amplified input laser beam 2a, which passes through the λ / 4 plate 12 twice. The outcoupled, doubly amplified input laser beam 2a is separated from the counter-propagating input laser beam 2 at the polarization beam splitter 13 and directed onto a mirror 14, which reflects the doubly amplified input laser beam 2a. The twice amplified input laser beam 2a enters the solid rod 4 via the second end face 4b, passes through the solid rod 4, is reflected back at the dichroic mirror 9, passes through the solid rod 4 again, whereby it is again amplified twice, and exits the solid rod 4 as a four-fold amplified input laser beam 2b via the second end face 4b.At the polarization beam splitter 11, the four-fold amplified input laser beam 2b is separated from the counter-propagating input laser beam 2 to form the output laser beam 3.

[0023] From Fig. 1 differs from the Fig. 3a is characterized in that the pumping device 5 comprises a plurality of single-emitter pump diodes 8, here only three by way of example, for generating a plurality of parallel-spaced pump laser beams 6, here three. The parallel-spaced pump laser beams 6 of the single-emitter pump diodes 8 lie in a common plane and are offset from one another in a direction transverse to the pump laser beams 6. Fig. Figure 3b shows the corresponding intensity distribution of the pump laser beams 6 incident on the first end face 4a of the solid rod 4. List of reference symbols 1 laser amplifier 2 input laser beam 2a amplified input laser beam 2b amplified input laser beam 3 Output laser beam 4 solid rod 4a first front side 4b second front side 5 Pumping device 6 Pump laser beam 7 Laser beam source 8 single-emitter pump diode 9 dichroic mirrors 10 Beam splitter device 11 polarization beam splitters 12 λ / 4 plate 13 additional polarization beam splitters 14 retro-reflective mirror 15 Faraday rotator QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] EP 1 507 321 A1 [0002, 0007] US 6,141,143

[0003]

Claims

[1] Laser amplifier (1) for amplifying an input laser beam (2) to an amplified output laser beam (3), comprising a laser-active solid-state rod (4) with two opposite end faces (4a, 4b) through which the input laser beam (2) passes at least twice, and comprising a pumping device (5) for generating at least one pump laser beam (6) which is coupled into the solid-state rod (4) via the first end face (4a) of the solid-state rod (4) in order to optically pump the solid-state rod (4), wherein both the input laser beam (2) is coupled into the solid-state rod (4) via the other, second end face (4b) of the solid-state rod (4) and the input laser beam (2a, 2b) amplified in the solid-state rod (4) is coupled out of the solid-state rod (4) in order to form the output laser beam (3), characterized bythat in the beam path of the pump laser beam (6) between the solid rod (4) and the pump device (5) a dichroic mirror (9) is arranged at right angles to the pump laser beam (6), which is transmissive for the pump laser beam (6) and reflective for the input laser beam (2). [2] Laser amplifier according to claim 1, characterized by in that a beam splitter device (10) is arranged in the counter-rotating beam path of the input laser beam (2) to be coupled in and the amplified input laser beam (2a, 2b) coupled out of the solid-state rod (4), which separates the amplified input laser beam (2a, 2b) coupled out of the solid-state rod (4) from the input laser beam (2) to be coupled in in order to form the output laser beam (3). [3] Laser amplifier according to claim 2, characterized bythat the input laser beam (2) to be coupled in is polarized and that the beam splitter device (10) has a polarization beam splitter (11) and a λ / 4 plate (12) arranged between the solid-state rod (4) and the polarization beam splitter (11). [4] Laser amplifier according to claim 3, characterized by that for a third and fourth pass of the input laser beam (2) through the solid-state rod (4), a further polarization beam splitter (13) is arranged between the polarization beam splitter (11) and the λ / 4 plate (12), which is transmissive for the input laser beam (2) to be coupled in and an amplified input laser beam (2b) which has passed through the λ / 4 plate (12) four times, and which is reflective for the input laser beam (2a) which has passed through the λ / 4 plate (12) twice and which directs the input laser beam (2a) which has passed through the λ / 4 plate (12) twice onto a retro-reflecting mirror (14). [5] Laser amplifier according to claim 4, characterized bythat a Faraday rotator (15) is arranged between the polarization beam splitter (11) and the further polarization beam splitter (13). [6] Laser amplifier according to one of the preceding claims, characterized by that the pumping device (5) has at least one single-emitter pump diode (8) for generating the pump laser beam (6). [7] Laser amplifier according to claim 6, characterized by that the pumping device (5) has a plurality of single-emitter pump diodes (8) for generating a plurality of parallel-spaced pump laser beams (6). [8] Laser amplifier according to claim 7, characterized by that the plurality of parallel-spaced pump laser beams (6) of the single-emitter pump diodes (8) are arranged offset from one another in a direction transverse to the pump laser beams (6). [9] Laser amplifier according to one of claims 6 to 8, characterized bythat the at least one pump laser beam (6) is coupled from the at least one single-emitter pump diode (8) in free beam propagation into the first end face (4a) of the solid-state rod (4). [10] Laser amplifier according to one of the preceding claims, characterized by a laser beam source (7) for generating the input laser beam (2).

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