FREQUENCY-CONVERTING LASER DEVICE

Abstract

A frequency-converting laser device that is efficient but at the same time has a simple structure contains an optical resonator that has two resonator mirrors, specifically a coupling-out mirror and an end mirror. The laser device furthermore contains an optically active medium for generating light of a first frequency and an optically nonlinear medium for converting light of the first frequency into light of another frequency. The optically active medium and the optically nonlinear medium are in this case arranged in a beam path between the resonator mirrors. The laser device furthermore contains a first polarization-influencing laser optic that polarizes the light of the first frequency, reflected by the coupling-out mirror in the direction of the end mirror, such that a frequency conversion of the light thus polarized of the first frequency is suppressed, in particular minimized, when passing through the nonlinear medium.

Claims

1. A laser device, comprising: an optical resonator having two resonator mirrors including a decoupling mirror and an end mirror; an optical active medium for generating light of a first frequency; an optical nonlinear medium for converting light of the first frequency into light of another frequency, wherein said optical active medium and said optical nonlinear medium are disposed in a beam path between said two resonator mirrors; and a first polarization-influencing laser optical unit, for polarizing the light of the first frequency reflected by said decoupling mirror in a direction toward said end mirror such that a frequency conversion of the light of the first frequency thus polarized is suppressed during a passage through said optical nonlinear medium.

2. The laser device according to claim 1, further comprising a second polarization-influencing laser optical unit, which polarizes the light of the first frequency propagating in a direction toward said decoupling mirror such that a frequency conversion of the light of the first frequency thus polarized is promoted during the passage through the optical nonlinear medium.

3. The laser device according to claim 1, wherein said first and second polarization-influencing laser optical units are selected from the group consisting of a wave plate, a quarter-wave plate, a polarization rotator, a Faraday rotator, a quartz crystal rotator, and a liquid crystal rotator.

4. The laser device according to claim 2, wherein: said first polarization-influencing laser optical unit is a quarter-wave plate; and said second polarization-influencing laser optical unit is selected from the group consisting of a polarization rotator, a Faraday rotator, a quartz crystal rotator, and a liquid crystal rotator.

5. The laser device according to claim 2, wherein said first and second polarization-influencing laser optical units are selected from the group consisting of a polarization rotator, a Faraday rotator, a quartz crystal rotator, and a liquid crystal rotator.

6. The laser device according to claim 1, wherein said optical resonator has a linear beam path.

7. The laser device according to claim 2, further comprising a third polarization-influencing laser optical unit, connected downstream of said decoupling mirror and is configured to compensate for an influence of said first polarization-influencing laser optical unit on a frequency-converted light.

8. The laser device according to claim 7, wherein said first polarization-influencing laser optical unit and said third polarization-influencing laser optical unit are formed by quarter-wave plates which are structurally identical but are rotated relative to one another by 90.

9. The laser device according to claim 1, further comprising a quality switch.

10. The laser device according to claim 1, wherein said active optical medium is a solid crystal.

11. The laser device according to claim 1, wherein said optical nonlinear medium has an optical nonlinear crystal in a type I phase match configuration.

12. The laser device according to claim 1, wherein said optical nonlinear medium contains at least two optical nonlinear crystals connected in succession to one another.

13. The laser device according to claim 12, wherein said at least two optical nonlinear crystals connected in succession to one another comprise a first crystal in a type I phase match configuration and a second crystal in a type II phase match configuration.

14. The laser device according to claim 1, further comprising a second polarization-influencing laser optical unit, which polarizes the light of the first frequency propagating in a direction toward said decoupling mirror such that a frequency conversion of the light of the first frequency thus polarized is maximized during the passage through the optical nonlinear medium.

15. The laser device according to claim 9, wherein said quality switch is an electro-optical quality switch, an acousto-optical quality switch or a passive quality switch.

16. The laser device according to claim 10, wherein said solid crystal is a neodymium-doped yttrium-orthovanadate crystal.

17. The laser device according to claim 11, wherein said optical nonlinear crystal is a lithium triborate crystal.

18. The laser device according to claim 13, wherein said at least two optical nonlinear crystals connected in succession to one another are lithium triborate crystals.

19. The laser device according to claim 1, wherein the frequency conversion of the light of the first frequency thus polarized is minimized during the passage through said optical nonlinear medium.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0032] FIG. 1 is a schematic simplified representation of a basic principle of a laser device according to the invention;

[0033] FIG. 2 is an illustration showing a first specific embodiment of the laser device in a representation according to FIG. 1; and

[0034] FIG. 3 is an illustration showing a second specific embodiment of the laser device in a representation according to FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0035] Parts and structures corresponding to one another are always provided with identical reference signs in all figures.

[0036] Referring now to the figures of the drawings in detail and first, particularly to FIG. 1 thereof, there is shown, in roughly schematic form, a laser device 2 having an optical resonator 4. The resonator 4 is formed by two resonator mirrors 6, 8, namely a decoupling mirror 6 and an end mirror 8. It furthermore includes a (laser) medium 10, which is energetically excited (pumped) in operation of the laser device 2 by means of a pump device 12 (only indicated in FIG. 1) by supplying light or electrical energy.

[0037] In operation, the laser medium 10 excited by the pump device 12 emits light of a fundamental frequency f.sub.1, which circulates between the resonator mirrors 6, 8 in a forward direction 14 (oriented from the end mirror 8 onto the decoupling mirror 6) and a reverse direction 16 (oriented from the decoupling mirror 6 onto the end mirror 8). For this light, designated hereinafter as the fundamental wave F, the decoupling mirror 6 and the end mirror 8 are opaque (in the scope of the quality of the resonator mirrors 6, 8 implemented in production).

[0038] Furthermore, an optical nonlinear medium 18 is arranged in the resonator 4, which in operation of the laser device 2 converts a part of the fundamental wave F into light of a second frequency f.sub.2. In the illustrated example, the second frequency f.sub.2 corresponds to an integer multiple of the fundamental frequency f.sub.1 (f.sub.2=n.Math.f.sub.1; with n=2, 3, 4, . . . ). The frequency-converted light of the second frequency f.sub.2 is therefore designated hereinafter as the harmonic wave H.

[0039] The decoupling mirror 6 is configured such that it is transmissive for the harmonic wave H (completely or at least in the scope of the achievable quality of the decoupling mirror 6 as extensively as possible).

[0040] The nonlinear medium 18 is arranged inside the resonator 4, thus between the resonator mirrors 6, 8.

[0041] A first polarizer 20 is on the one hand interconnected between the nonlinear medium 18 and the decoupling mirror 6. In operation of the laser device 2, this first polarizer 20 influences the polarization of the fundamental wave F reflected by the decoupling mirror 6 and thus propagating in the reverse direction 16 such that the fundamental wave F polarized in this manner passes in the reverse direction 16 through the nonlinear medium 18 without triggering a frequency conversion. An emission of frequency-converted light in the reverse direction 16 is thus suppressed by the polarization of the fundamental wave F by means of the first polarizer 20.

[0042] On the other hand, a second polarizer 22 is interconnected between the laser medium 10 and the nonlinear medium 18. In operation of the laser device 2, this second polarizer 22 influences the polarization of the fundamental wave F emitted by the laser medium 10 in the forward direction 14 such that the fundamental wave F polarized in this manner triggers a maximum frequency conversion upon passage through the nonlinear medium 18. An emission of frequency-converted light in the forward direction 14 is thus maximized by the polarization of the fundamental wave F by means of the second polarizer 22.

[0043] Due to the cooperation of the two polarizers 20 and 22, the harmonic wave H is emitted from the nonlinear medium 18 with maximum intensity exclusively in the forward direction 14.

[0044] Upon incidence on the decoupling mirror 6, the harmonic wave H is decoupled from the resonator 4, by which a laser beam L having the second frequency f.sub.2 is generated.

[0045] The end mirror 8, the laser medium 10, the second polarizer 22, the nonlinear medium 18, the first polarizer 20, and the decoupling mirror 6 are arranged in succession along a linear optical axis 23 and thus along a linear beam path.

[0046] A first specific embodiment of the laser device 2, which is only shown generally in FIG. 1, is shown in FIG. 2. The laser device 2 shown in FIG. 2 is a solid-state laser, which includes as the laser medium 10 a neodymium-doped yttrium-orthovanadate crystal (Nd:YVO.sub.4 crystal 24). The Nd:YVO.sub.4 crystal emits light in the infrared range having a light wavelength of .sub.1 1064 nm (.sub.1=1064 nm) to form the fundamental wave F. Accordingly, the fundamental frequency f.sub.1 is 282.0 THz here (f.sub.1=282.0 THz). The fundamental wave F emitted by the laser medium 10 in the forward direction 14 is linearly polarized with a polarization direction which is assigned the angle 0 here and hereinafter.

[0047] The pump device 12 is formed in the example according to FIG. 2 by a diode laser 26, which optically excites the Nd:YVO.sub.4 crystal 24 using a pump laser beam P.

[0048] The second polarizer 22 connected downstream of the Nd:YVO.sub.4 crystal 24 in the forward direction 14 is designed as a Faraday rotator 28, which rotates the polarization direction of the fundamental wave F by an angle of 45.

[0049] The optical nonlinear medium 18 is formed here by a crystal, namely a lithium triborate crystal (LBO crystal 30) in a type I phase match configuration, which induces a frequency doubling of the fundamental frequency f.sub.1. The second frequency f.sub.2 thus has the value of 564.0 THz (f.sub.2=564.0 THz) here. Accordingly, the harmonic wave H generated by the LBO crystal 30 is the second harmonic H2 of the fundamental wave F, which has a wavelength .sub.2 of 532 nm and is thus in the spectral range of green visible light. The LBO crystal 30 is moreover aligned in the beam path of the resonator 4a such that it converts the light of the fundamental frequency f.sub.1 with maximum efficiency into the light of the second frequency f.sub.2 when the light of the fundamental frequency f.sub.1 is linearly polarized with a polarization direction of 45. The Faraday rotator 28 and LBO crystal 30 are matched to one another such that the efficiency of the frequency doubling for the passage of the fundamental wave F is maximized by the LBO crystal 30 in the forward direction 14.

[0050] The second polarizer 22 connected downstream of the LBO crystal 30 in the forward direction 14 is formed in the example from FIG. 2 by a quarter-wave plate 32 matched to the fundamental wave F (and thus to light of the fundamental frequency f.sub.1). The quarter-wave plate 32 is arranged in the beam path of the resonator 4 such that it (re)polarizes the fundamental wave F incident in the forward direction 14 as a linearly polarized wave having a polarization direction of 45 into a circularly polarized light wave.

[0051] The fundamental wave F is reflected at the downstream decoupling mirror 6 and is thus reflected back in the reverse direction 16 onto the quarter-wave plate 32. The fundamental wave F incident in the reverse direction 16 as the circularly polarized light wave is now (re-)polarized by the quarter-wave plate 32 into a linearly polarized light wave having a polarization direction of 135.

[0052] The fundamental wave F polarized in this manner now passes in the reverse direction 16 through the LBO crystal 30. Due to the anisotropy of the LBO crystal 30 and the polarization of the fundamental wave F, the efficiency of the frequency doubling is minimized for the fundamental wave F propagating in the reverse direction 16.

[0053] Upon the passage of the fundamental wave F propagating in the reverse direction 16 through the Faraday rotator 28, the polarization direction of the fundamental wave F is again rotated by 45. The fundamental wave F therefore leaves the Faraday rotator 28 in the reverse direction 16 as a linearly polarized wave having a polarization direction of 180, which corresponds to the original polarization direction of 0. After reflection at the end mirror 8, the fundamental wave F is reflected back on the laser medium 10 (thus the Nd:YVO.sub.4 crystal 24) and the above-described circuit begins again.

[0054] Due to the suppression of the frequency doubling in the reverse direction 16, the second harmonic H2 is emitted from the LBO crystal 30 (at least approximately) exclusively in the forward direction 14. The second harmonic H2 is initially provided here as a linearly polarized light wave having a polarization direction of 135. Since the quarter-wave plate 32 is matched to the fundamental wave F (and the associated wavelength .sub.1), it has no defined polarization-influencing effect on the second harmonic H2. The second harmonic H2 is therefore provided with undefined polarization properties after the passage through the quarter-wave plate 32.

[0055] In this form, the second harmonic H2 is decoupled from the resonator 4 via the decoupling mirror 6 to form the laser beam L. To give the laser beam L a defined polarization property, a third polarizer 34 in the form of a further quarter-wave plate 36 is connected downstream from the decoupling mirror 6 outside the resonator 4. This further quarter-wave plate 36 is designed structurally identical to the quarter-wave plate 32 and is therefore also matched to the wavelength .sub.1 of the fundamental wave F. However, it is rotated by 90 around the optical axis 23 in relation to the quarter-wave plate 32. The further quarter-wave plate 36 in this way compensates for the effect of the quarter-wave plate 32 on the second harmonic H2. After the passage of the laser beam L through the quarter-wave plate 36, the laser beam L is thus provided in linearly polarized form having a polarization angle of 135.

[0056] In an optional refinement of the concept schematically shown in FIG. 1, the laser device 2 from FIG. 2 is configured as a quality-switched laser operated in a pulsed manner. The laser device 2 includes for this purpose as a further component a quality switch 38, which is interconnected in the illustration according to FIG. 2 between the laser medium 10 (thus the Nd:YVO.sub.4 crystal 24 here) and the second polarizer 22 (thus the Faraday rotator 28 here). The quality switch 38 is embodied in the embodiment according to FIG. 2, for example, as an acousto-optical modulator 40 (Bragg cell).

[0057] In a manner known per se, the quality of the resonator 4 is reduced in intervals between each two laser pulses by the quality switch 38, so that the laser activity of the resonator 4 is prevented and therefore a particularly strong excitation of the laser medium 10 (thus of the Nd:YVO.sub.4 crystal 24) is forced. To trigger a laser pulse, the quality of the resonator 4 is temporarily increased by the quality switch, so that the laser activity begins.

[0058] Alternatively thereto, the laser device 2 is operated as a mode-coupled laser. In this embodiment (essentially corresponding in hardware to the embodiment according to FIG. 2), a quality modulator, in particular again an acousto-optical modulator 40, is arranged for this purpose in the resonator 4. This quality modulator modulates the quality of the resonator 4 at a frequency which corresponds to the circulation time of a pulse in the resonator 4.

[0059] The profile of the fundamental wave F and the harmonic wave H (thus of the second harmonic H2 here) is schematically indicated below the resonator 4 in FIG. 2 for the purpose of illustration.

[0060] The embodiment of the laser device 2 shown in FIG. 3 differs from the embodiments described on the basis of FIG. 2 in that the optical nonlinear medium 18 includes here, in addition to the frequency-doubling LBO crystal 30, a second crystal made of lithium triborate (LBO crystal 42), which is connected between the LBO crystal 30 and the first polarizer 20 (again in the form of the quarter-wave plate 32 here) in the beam path of the resonator 4. This second LBO crystal 42 generates, in operation of the laser device under the effect of the fundamental wave F and the second harmonic H2, light of a third frequency f.sub.3, which corresponds to three times the fundamental frequency f.sub.1 (f.sub.3=845.9 THz). This frequency-tripled light is emitted by the LBO crystal 42 as the third harmonic H3. It has a wavelength .sub.3 of 354 nm and is in the ultraviolet range of the electromagnetic spectrum. In the laser device 2 from FIG. 3, both the second harmonic H2 and the third harmonic H3 are decoupled from the resonator 4 via the decoupling mirror 6. The second LBO crystal 42 is preferably a crystal in type II phase match configuration.

[0061] The second LBO crystal 40 is also oriented in the beam path of the resonator 4 such that the frequency conversion (thus the frequency tripling here) is maximized for the fundamental wave F propagating in the forward direction 14. Frequency tripling is not triggered by the fundamental wave F propagating in the reverse direction 16 due to the lack of the second harmonic H2 in the LBO crystal 40. The third harmonic H3 is therefore also (at least approximately) emitted exclusively in the forward direction 14. A frequency doubling in the LBO crystal 30 due to the fundamental wave F propagating in the reverse direction 16 is again suppressed by the polarization of the fundamental wave F by means of the quarter-wave plate 32.

[0062] The original linear polarization destroyed by the quarter-wave plate is reestablished by the third polarizer 34 (also formed here by the quarter-wave plate 36) connected downstream from the decoupling mirror 6 for both the second harmonic H2 and the third harmonic H3.

[0063] In contrast to the embodiment according to FIG. 2, the laser device 2 according to FIG. 3 optionally has a frequency-selective mirror 44 connected downstream of the quarter-wave plate 36. The mirror 44 is transmissive for the light of the third frequency fa, so that the third harmonic H3 decoupled from the resonator 4 to form the laser beam L passes through the mirror 42.

[0064] The second harmonic H2 decoupled from the resonator 4, in contrast, is deflected by the mirror 42. It is reflected in this case, for example, onto a light sensor 46 used for detecting the laser activity.

[0065] The profile of the fundamental wave F and the harmonic wave H (thus the second harmonic H2 and the third harmonic H3 here) is again schematically indicated in FIG. 2 for the purpose of illustration below the resonator 4.

[0066] The subject matter of the invention is particularly clear in the above-described exemplary embodiments, but is in no way restricted thereto. Rather, further embodiments of the invention can be derived from the claims and the above description. In particular, the third polarizer 34 described on the basis of FIGS. 2 and 3 and the quality switch 38 can also be used in other embodiments of the laser device 2 according to the invention. Furthermore, the first polarizer 20 and/or the second polarizer 22 can also be implemented in a way other than that shown in FIGS. 2 and 3. For example, a Faraday rotator, which rotates the polarization direction of the fundamental wave by 45, can be used for the first polarizer 20 instead of the quarter-wave plate 32. Furthermore, other suitable materials than those described by way of example can be used for the laser medium 10 and for the optical nonlinear medium 18.

[0067] The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention.

LIST OF REFERENCE SIGNS

[0068] 2 laser device [0069] 4 resonator [0070] 6 decoupling mirror [0071] 8 end mirror [0072] 10 (laser) medium [0073] 12 pump device [0074] 14 forward direction [0075] 16 reverse direction [0076] 18 (optical nonlinear) medium [0077] 20 (first) polarizer [0078] 22 (second) polarizer [0079] 23 optical axis [0080] 24 Nd:YVO.sub.4 crystal [0081] 26 diode laser [0082] 28 Faraday rotator [0083] 30 LBO crystal [0084] 32 quarter-wave plate [0085] 34 (third) polarizer [0086] 36 quarter-wave plate [0087] 38 quality switch [0088] 40 acousto-optical modulator [0089] 42 LBO crystal [0090] 44 (frequency-selective) mirror [0091] 46 light sensor [0092] f.sub.1 fundamental frequency [0093] f.sub.2 (second) frequency [0094] F fundamental wave [0095] H harmonic wave [0096] H2 (second) harmonic [0097] H3 (third) harmonic [0098] L laser beam [0099] P pump laser beam