Frequency conversion laser device

Abstract

An efficient, but at the same time simple to construct, frequency conversion laser device (2) comprises an optical resonator (4) having two resonator mirrors (6, 8), namely a coupling-out mirror (6) and an end mirror (8). Furthermore, the laser device (2) comprises an optically active medium (10) for generating light of a first frequency (f1) and an optical nonlinear medium (18) for converting light of the first frequency (f1) into light of a further frequency (f2, f3). The optically active medium (10) and the optical nonlinear medium (18) are arranged in the beam path between the resonator mirrors (6, 8) here. Furthermore, the laser device (2) comprises a first polarization-influencing laser optics (20) which polarizes light of the first frequency (f1) reflected by the coupling-out mirror (6) in the direction of the end mirror (8) in such a way that the frequency conversion of the thus polarized light of the first frequency (f1) when passing through the nonlinear medium (18) is suppressed, in particular minimized.

Description

Technical Field

[0001] The present invention relates to a frequency-converting laser device, specifically to an optical apparatus for generating and optionally guiding, shaping, converting and / or enhancing a laser beam. Background Technology

[0002] For industrial applications such as engraving or marking using laser beams, solid-state lasers, i.e., laser devices whose active optically active medium is formed of crystalline or glassy (i.e., amorphous) solids, are frequently used. Light generated by such solids is typically in the infrared range, especially at wavelengths above 800 nm. However, to date, no suitable (especially commercially available) solid material has been available to generate the short-wavelength light required for many applications.

[0003] A common technique used to generate laser light in the green, blue, violet, or ultraviolet spectral range using solid-state lasers is so-called frequency conversion. Here, a portion of the light initially generated at its fundamental frequency (also called the basic frequency) is converted into light of another frequency through a (optical) nonlinear medium. The frequency of the converted light is often several times that of the fundamental frequency, especially two or three times. The fundamental frequency light and the frequency-converted light generated from it and emitted in the same direction are coherent.

[0004] Nonlinear media are often arranged in the resonant cavity of laser devices, thereby generating frequency-converted light within the resonator. Therefore, in technical terms, this frequency conversion measure is also called "Intra-Cavity Nonlinear Frequency Conversion." In such a resonator, the fundamental frequency light travels through the nonlinear medium in two directions along the path between the resonator mirrors, undergoing a frequency conversion in each direction. Accordingly, the frequency-converted light passing through the nonlinear medium is emitted not only in the forward direction (i.e., towards the coupling mirror of the resonator) but also in the backward direction (i.e., towards the opposite end mirror of the resonator). The portion of the frequency-converted light emitted in the forward direction can be easily utilized because it is coupled out of the resonator, while the portion emitted in the backward direction is generally an undesirable interference signal because it negatively interferes with the frequency-converted light emitted in the forward direction, thus impairing the efficiency and stability of laser operation. Furthermore, the portion of the frequency-converted light emitted in the backward direction also increases the load on the resonator components, especially the active medium (and thus leads to increased wear).

[0005] To overcome this drawback, frequency-converting laser devices are sometimes equipped with bent resonators. For this purpose, a deflector is placed between the two resonator mirrors, which deflects the fundamental frequency light and thus divides the resonator into two arms. The active medium is placed in one arm of the bent resonator, while the nonlinear medium is placed in the second arm. The deflector is transparent to the frequency of the converted light. This ensures that the converted light circulates only in the second arm of the resonator.

[0006] A deflector can also be used to couple the converted light out of the resonator. To further improve the resonator's efficiency, alternatively, a third resonator mirror can be arranged behind the deflector (in the extension of the second resonator arm). This third resonator mirror reflects the frequency-converted light transmitted through the deflector back into the second resonator arm. Thus, the second resonator arm and the third resonator mirror form an independent resonant cavity for the frequency-converted light, forcing a resonant frequency conversion.

[0007] However, the high manufacturing cost of such bent resonators due to their extremely complex structure limits the commercial applicability of corresponding laser devices. In particular, active stabilization measures are often required to coordinate the lengths of the two resonator arms or resonator cavities.

[0008] Furthermore, the bent resonator occupies a relatively large structural space, which also limits the availability of the corresponding laser equipment. Summary of the Invention

[0009] The objective of this invention is to describe an effective yet simple frequency-converting laser device.

[0010] According to the present invention, this task is accomplished by a laser device having the features of claim 1. The advantages and aspects of the invention, which are considered inventive design methods and improvements, are set forth in the dependent claims and the following description.

[0011] The laser device according to the invention includes, in a manner commonly found in laser devices, an optical resonator having two resonator mirrors: a coupling mirror and an end mirror. The coupling mirror is the front side of the resonator. Therefore, the direction of propagation of light projected onto the coupling mirror is referred to as the "forward direction." Conversely, light striking the end mirror propagates in the "backward direction." Furthermore, the resonator includes an optically active medium (laser medium) that generates light of a first frequency during operation of the laser device. This first frequency is also referred to hereinafter as the "fundamental frequency." The light of the first frequency is also correspondingly referred to as the "fundamental wave."

[0012] In addition to resonators, laser devices also include (optical) nonlinear media that, during operation, convert light of a first frequency—in other words, a portion of the fundamental frequency—into light of another frequency. This other frequency is preferably, but not necessarily, an integer multiple of the fundamental frequency, particularly two, three, or four times. To distinguish it from the fundamental frequency, the light of the frequency-converted portion is generally referred to hereinafter as the "converted wave." If the other frequency is an integer multiple of the fundamental frequency, the frequency-converted light is also called a "tuned wave" or simply a "harmonic." When the frequency is doubled, the frequency-converted light is also called the "second harmonic"; when the frequency is tripled, it is called the "third harmonic," and so on.

[0013] The coupling mirror is designed to be (completely or at least partially) transparent to the converted wave. Conversely, both resonator mirrors are preferably opaque to the fundamental wave.

[0014] An optical nonlinear medium is arranged inside the resonator. Therefore, both the optically active medium and the optical nonlinear medium are arranged in the beam path between the resonator mirrors.

[0015] According to the present invention, in addition to the above-described components, the laser device further includes a (first) polarization-influencing laser optics device that polarizes light of a first frequency (i.e., the fundamental wave) reflected from the coupling mirror toward the terminal mirror, thereby suppressing the frequency conversion of the polarized light as it passes through the nonlinear medium. This first polarization-influencing laser optics device (hereinafter simply referred to as the "(first) polarizer") is arranged, in particular, between the nonlinear medium and the coupling mirror in the beam path of the resonator. In other words, the first polarizer causes the fundamental wave to not undergo a frequency conversion in the backward direction through the nonlinear medium, or the frequency conversion that does occur is at least weaker than the frequency conversion without the first polarizer. The frequency conversion occurring due to the fundamental wave passing through the nonlinear medium in the backward direction is minimized, in particular, by the appropriate polarization of the fundamental wave.

[0016] In this context and below, "polarization" or "polarization" is generally understood as a change in polarization characteristics. Therefore, light polarized by the first polarizer has different polarization characteristics than before. For example, the polarization direction of the fundamental wave is rotated by the first polarizer, causing linear polarization to be converted to circular polarization, or vice versa.

[0017] It is known, and described, for example, in DE 690 08 415T2, that depending on the type and / or configuration of the optical nonlinear medium (e.g., depending on the orientation of the crystal axis relative to the propagation direction of the incident wave), there are two types of frequency conversion, referred to as "Type I" and "Type II," respectively. Type I frequency conversion is caused by the interaction of an incident wave with the nonlinear medium and the same polarization. Type II frequency conversion requires the interaction of an orthogonally polarized incident wave with the nonlinear optical medium. Optical nonlinear media that exhibit "Type I" or "Type II" frequency conversion due to their type and / or configuration are hereinafter simply referred to as "Type I media" or "Type II media" (or "Type I crystal" or "Type II crystal" in the case of nonlinear optical crystals). The present invention is based on the understanding that, in optical nonlinear media, especially Type I frequency conversion, often exhibits a significant dependence on the polarization of the incident light. Here, light with a specific polarization direction is converted with maximum efficiency, while light with a polarization direction perpendicular to that specific polarization direction is converted with minimum efficiency or even not converted at all. According to the present invention, this effect is fully utilized to improve the efficiency of resonators. By suppressing the frequency conversion in the backward direction, the converted wave emitted in the backward direction is reduced, either completely or at least partially, thus avoiding the drawbacks mentioned at the beginning. Therefore, high resonator efficiency is achieved in a simple manner.

[0018] In a preferred embodiment, in addition to the first polarizer described above, the laser device also includes a second laser optics device that influences polarization, which is hereinafter (and without limitation) simply referred to as the "second polarizer". This second polarizer has the opposite effect to the first polarizer; specifically, the second resonator polarizes light of a first frequency (i.e., the fundamental wave propagating in the forward direction) propagating in the direction of the decoupled mirror, thereby facilitating, and in particular maximizing, the frequency conversion of the polarized light as it passes through the nonlinear medium.

[0019] Therefore, the fundamental wave passing through the nonlinear medium in the forward direction undergoes a stronger frequency conversion than it would without the first polarizer, caused by the second polarizer, which is arranged, particularly in the beam path of the resonator, between the laser medium and the nonlinear medium.

[0020] The first and second polarizers (if present) are preferably formed by waveplates (also called delay plates), especially λ / 4 waveplates, or by polarization rotators, such as Faraday rotators, quartz crystal rotators, or liquid crystal rotators. In embodiments of the laser device where both the first and second polarizers are present, within the scope of the invention, the two polarizers can be configured as the same or different types. Thus, in a preferred embodiment of the invention, a λ / 4 waveplate is used as the first polarizer, and a polarization rotator is used as the second polarizer. In particular, the polarization rotator is designed here such that it rotates the polarization direction of the incident fundamental wave by 45°. In alternative embodiments of the laser device, the first and second polarizers are each formed by polarization rotators. These polarization rotators are also designed here, in particular, such that they each rotate the polarization direction of the incident fundamental wave by 45°.

[0021] By focusing the frequency conversion onto the fundamental wave traveling in the forward direction through at least one polarizer, a simple resonator design is achieved without sacrificing resonator efficiency. In particular, this makes a bent resonator design neither necessary nor preferred. More precisely, in the preferred embodiment, the resonator has a linear beam path; that is, the resonator mirror, laser medium, optical nonlinear medium, and one or each polarizer are arranged along a straight optical axis. This simple structure allows for high stability of the laser beam generated during the operation of the laser device with minimal overhead. Active stabilizers are not necessary and are therefore not included in the preferred embodiment of the invention.

[0022] Because the first polarizer is coordinated to influence the fundamental wave, it has an a priori uncertain effect on the frequency-converted light (i.e., on the converted wave). Therefore, the laser beam coupled from the resonator is also generated with a priori undefined polarization characteristics. To still ensure defined polarization characteristics of the laser beam, in a preferred embodiment, the laser device includes, in addition to the first and second polarizers (if present), a third polarization-influencing laser optics device (also referred to as the "third polarizer") located downstream of the coupling mirror and thus arranged outside the resonator. This third polarizer is configured to compensate for the effect of the first polarizer on the converted wave (and consequently, on the laser beam coupled from the resonator). In other words, the effect of the first polarizer on the converted wave is compensated for by the third polarizer. In a particularly suitable embodiment of this variant of the invention, the first and third polarizers are formed by λ / 4 waveplates with identical structures but rotated 90° relative to each other about their optical axes. The term "λ / 4" in both polarizers refers to the wavelength of the fundamental wave.

[0023] Generally, laser devices within the scope of this invention can operate as continuously emitted lasers (CW lasers) or as pulsed lasers.

[0024] Preferably, the laser device is a Q-switched laser. In this embodiment, the laser device additionally includes a Q-switch arranged in the beam path of the resonator, particularly between the laser medium and the nonlinear medium or (if present) a second polarizer, by which the Q value of the resonator can be changed. This Q-switch is preferably an active Q-switch, which is based, for example, on electro-optic principles (such as Polkell cells, Kerr cells, or electro-optic modulators) or acousto-optic principles (such as Bragg cells). However, in principle, the laser device within the scope of this invention may also include a passive Q-switch, particularly in the form of a semiconductor saturable absorber mirror (SESAM) or a nonlinear crystal (such as a Cr:YAG crystal). Alternatively, the laser device is a mode-locked laser.

[0025] Laser devices are preferably solid-state lasers. Therefore, the active optically active medium is preferably a solid, especially a neodymium-doped yttrium vanadate crystal (Nd:YVO4 crystal).

[0026] The nonlinear medium preferably comprises a medium configured, in terms of its type and / or configuration (e.g., orientation relative to the propagation direction of the incident wave), for type I frequency conversion (i.e., type I phase-matched frequency conversion). This medium is preferably a solid, i.e., an optically nonlinear (type I) crystal, particularly lithium triborate (LBO) crystal.

[0027] In a particular variant of the invention, especially for generating higher harmonics of the initial fundamental wave, the nonlinear medium has at least two optical nonlinear crystals, particularly LBO crystals, arranged sequentially. In this case, the first crystal of the two crystals is preferably a type I phase-matched crystal. This first crystal is used to generate a first converted wave of a moderate frequency (e.g., a frequency twice the fundamental frequency) from the fundamental wave. In particular, the second crystal, used to generate a second converted wave of a higher frequency (e.g., a wave with a frequency three times the fundamental frequency) through the interaction of the fundamental wave and the first converted wave, can also, in principle, be formed of a type I phase-matched crystal within the scope of the invention. However, a type II phase-matched crystal is preferred for the second crystal. Attached Figure Description

[0028] The embodiments of the present invention will now be explained in more detail with reference to the accompanying drawings. Wherein:

[0029] Figure 1 A simplified schematic diagram illustrating the basic principle of the laser device according to the present invention is shown;

[0030] Figure 2 according to Figure 1 The illustration shows a first specific embodiment of the laser device; and

[0031] Figure 3 according to Figure 1 The illustration shows a second specific embodiment of the laser device.

[0032] In all the drawings, corresponding parts and structures always have the same reference numerals. Detailed Implementation

[0033] Figure 1 A laser device 2 with an optical resonator 4 is roughly schematically shown. The resonator 4 is formed by two resonator mirrors 6 and 8 (i.e., the coupling mirror 6 and the end mirror 8). It also includes a (laser) medium 10, which is energetically excited (“pumped”) by the delivery of optical or electrical energy by means of a pumping device 12, which is only indicated in the figure, during the operation of the laser device 2.

[0034] During operation, the laser medium 10, excited by the pumping device 12, emits light with a fundamental frequency f1. This light circulates between the resonator mirrors 6 and 8 in a forward direction 14 (oriented from the end mirror 8 to the coupling mirror 6) and a backward direction 16 (oriented from the coupling mirror 6 to the end mirror 8). For this light, hereinafter referred to as the fundamental frequency F, the coupling mirror 6 and the end mirror 8 (within the range of Q values ​​realized by the manufacturing technology of the resonator mirrors 6 and 8) are opaque.

[0035] Furthermore, an optical nonlinear medium 18 is arranged in the resonator 4, which converts a portion of the fundamental frequency F into light with a second frequency f2 during the operation of the laser device 2. In the example shown, the second frequency f2 is equivalent to an integer multiple of the fundamental frequency f1 (f2 = n·f1; where n = 2, 3, 4, ...). Therefore, the light with the frequency-converted second frequency f2 is referred to below as the tuned wave H.

[0036] The coupling mirror 6 is designed to be transparent to the tuned wave H (completely transparent or as transparent as possible within the range of Q values ​​achievable by the coupling mirror 6).

[0037] The nonlinear medium 18 is arranged inside the resonator 4, that is, between the resonator mirrors 6 and 8.

[0038] On one hand, a first polarizer 20 is connected between the nonlinear medium 18 and the coupling mirror 6. During the operation of the laser device 2, this first polarizer 20 affects the polarization of the fundamental wave F reflected by the coupling mirror 6 and thus propagating in the backward direction 16, so that the fundamental wave F, thus polarized, passes through the nonlinear medium 18 in the backward direction 16 without triggering a frequency conversion. Therefore, by polarizing the fundamental wave F with the aid of the first polarizer 20, the emission of frequency-converted light in the backward direction 16 can be suppressed.

[0039] On the other hand, a second polarizer 22 is connected between the laser medium 10 and the nonlinear medium 18. During the operation of the laser device 2, this second polarizer 22 affects the polarization of the fundamental wave F emitted from the laser medium 10 in the forward direction 14, causing the fundamental wave F, thus polarized, to trigger a maximum frequency conversion when passing through the nonlinear medium 18. Therefore, by polarizing the fundamental wave F with the aid of the second polarizer 22, the emission of the frequency-converted light in the forward direction 14 can be maximized.

[0040] The interaction of two polarizers 20 and 22 enables the tuned wave H to be emitted from the nonlinear medium 18 with maximum intensity only in the forward direction 14.

[0041] The tuned wave H is coupled out from the resonator 4 when it strikes the coupling mirror 6, thereby generating a laser beam L with a second frequency f2.

[0042] The end mirror 8, laser medium 10, second polarizer 22, nonlinear medium 18, first polarizer 20 and coupling mirror 6 are arranged sequentially to each other along the straight optical axis 23 and thus along a linear beam path.

[0043] Figure 2 It shows Figure 1 The first specific embodiment of the laser device 2 shown in the figure is only a general representation. Figure 2 The laser device 2 shown is a solid-state laser with a neodymium-doped yttrium vanadate crystal (Nd:YVO4 crystal 24) as the laser medium 10. To generate the fundamental frequency F, the Nd:YVO4 crystal emits light in the infrared range with a wavelength λ1 of 1064 nm (λ1 = 1064 nm). Correspondingly, the fundamental frequency f1 is 282.0 THz (f1 = 282.0 THz). The fundamental frequency F emitted from the laser medium 10 in the forward direction 14 is linearly polarized, and its polarization direction is assigned an angle of 0° here and below.

[0044] According to Figure 2 In the example, the pumping device 12 is formed by a diode laser 26, which optically excites the Nd:YVO4 crystal 24 with a pump laser beam P.

[0045] The second polarizer 22, located downstream of the Nd:YVO4 crystal 24 in the forward direction 14, is configured as a Faraday rotator 28, which rotates the polarization direction of the fundamental wave F by an angle of 45°.

[0046] The optical nonlinear medium 18 is formed here by a crystal, specifically a lithium triborate crystal (LBO crystal 30) of type I phase-matched configuration, which causes the fundamental frequency f1 to double. Therefore, the second frequency f2 here has a value of 564.0 THz (f2 = 564.0 THz). Accordingly, the tuned wave H generated by the LBO crystal 30 is the second harmonic H2 of the fundamental frequency F, with a wavelength λ2 of 532 nm, and thus falls within the spectral range of green visible light. Furthermore, the LBO crystal 30 is oriented in the beam path of the resonator 4a such that when the light of the fundamental frequency f1 is linearly polarized at a polarization direction of 45°, it converts the light of the fundamental frequency f1 into light of the second frequency f2 with maximum efficiency. Therefore, the Faraday rotator 28 and the LBO crystal 30 are coordinated to maximize the efficiency of frequency doubling when the fundamental frequency F passes through the LBO crystal 30 in the forward direction 14.

[0047] exist Figure 2 In the example, the second polarizer 22 located downstream of the LBO crystal 30 in the forward direction 14 is formed by a λ / 4 waveplate 32 coordinated with the fundamental wave F (and thus the light with fundamental frequency f1). The λ / 4 waveplate 32 is arranged in the beam path of the resonator 4 such that it (re)polarizes the fundamental wave F, which is a linearly polarized wave with a polarization direction of 45°, into a circularly polarized light wave in the forward direction 14.

[0048] The fundamental wave F is reflected by the downstream coupling mirror 6 and thus returned in the backward direction 16 to be projected onto the λ / 4 waveplate 32. The fundamental wave F, which collided as a circularly polarized light wave in the backward direction 16, is now (re)polarized by the λ / 4 waveplate 32 into a linearly polarized light wave with a polarization direction of 135°.

[0049] The fundamental wave F, thus polarized, now passes through the LBO crystal 30 in the backward direction 16. Due to the anisotropy of the LBO crystal 30 and the polarization of the fundamental wave F, the efficiency of doubling the frequency of the fundamental wave F propagating in the backward direction 16 is minimized.

[0050] When the fundamental wave F propagating in the backward direction 16 passes through the Faraday rotator 28, its polarization direction is rotated again by 45°. Therefore, the fundamental wave F exits the Faraday rotator 28 as a linearly polarized wave with a polarization direction of 180° (which corresponds to the original polarization direction of 0°) in the backward direction 16. After being reflected by the end mirror 8, the fundamental wave F is projected back onto the laser medium 10 (i.e., the Nd:YVO4 crystal 24) and the above cycle begins again.

[0051] Since the frequency doubling in the backward direction 16 is suppressed, the second harmonic H2 is emitted from the LBO crystal 30 (at least approximately) only in the forward direction 14. The second harmonic H2 exists here primarily as a linearly polarized light wave with a polarization direction of 135°. Because the λ / 4 waveplate 32 is coordinated with the fundamental wave F (and its corresponding wavelength λ1), it has no limiting effect on the polarization of the second harmonic H2. Therefore, the second harmonic H2 exists with indeterminate polarization characteristics after passing through the λ / 4 waveplate 32.

[0052] In this configuration, the second harmonic H2 is coupled out from the resonator 4 via the coupling mirror 6 to form the laser beam L. To impart defined polarization characteristics to the laser beam L, a third polarizer 34 in the form of a λ / 4 waveplate 36 is connected downstream of the coupling mirror 6, outside the resonator 4. This additional λ / 4 waveplate 36 is structurally identical to the λ / 4 waveplate 32 and is therefore also aligned with the wavelength λ1 of the fundamental wave F. However, compared to the λ / 4 waveplate 32, it is twisted by 90° about the optical axis 23. Thus, the additional λ / 4 waveplate 36 compensates for the effect of the λ / 4 waveplate 32 on the second harmonic H2. Therefore, after the laser beam L passes through the λ / 4 waveplate 36, the laser beam L exists in a meridionally polarized form with a polarization angle of 135°.

[0053] exist Figure 1 Among the possible improvements to the concept illustrated in the diagram, Figure 2 The laser device 2 is constructed as a Q-switched pulsed laser. For this purpose, the laser device 2 has a Q-switch 38 as an additional component, which, according to... Figure 2 In the diagram, the Q-switch is connected between the laser medium 10 (here, Nd:YVO4 crystal 24) and the second polarizer 22 (here, Faraday rotator 28). According to... Figure 2 In one implementation, the Q switch 38 is exemplary implemented as an acousto-optic modulator 40 (Brag unit).

[0054] In a known manner, the Q value of resonator 4 is intermittently reduced between two laser pulses by Q-switch 38, thereby preventing laser activity of resonator 4 and thus forcing the laser medium 10 (i.e., Nd:YVO4 crystal 24) to be subjected to particularly strong excitation. When a laser pulse is triggered, the Q value of resonator 4 is briefly increased by Q-switch, thereby initiating laser activity.

[0055] For this alternative location, laser device 2 also operates as a mode-locked laser. In this (basically in terms of hardware technology, it is based on...) Figure 2In the corresponding implementation scheme, a Q-value modulator is arranged in the resonator 4, and in particular, an acousto-optic modulator 40 is arranged again. The Q-value modulator modulates the Q value of the resonator 4 at a frequency corresponding to the round-trip time of the pulse in the resonator 4.

[0056] The directions of the fundamental frequency F and the tuned frequency H (here, the second harmonic H2) are used to illustrate... Figure 2 The middle part is schematically indicated below resonator 4.

[0057] Figure 3 The embodiment of the laser device 2 shown is based on Figure 2 The difference in the described implementation is that, in addition to the LBO crystal 30 which doubles the frequency, the optical nonlinear medium 18 here also has a second crystal (LBO crystal 42) made of lithium triborate, which is connected in the beam path of the resonator 4 between the LBO crystal 30 and the first polarizer 20 (again, in the form of a λ / 4 waveplate 32). During operation of the laser device, this second LBO crystal 42, under the influence of the fundamental frequency F and the second harmonic H2, generates light with a third frequency f3, equivalent to three times the fundamental frequency f1 (f3 = 845.9 THz). This light of the third frequency is emitted from the LBO crystal 42 as the third harmonic H3. Its wavelength λ3 is 354 nm and lies in the ultraviolet range of the electromagnetic spectrum. Figure 3 In the laser device 2, both the second harmonic H2 and the third harmonic H3 are coupled out from the resonator 4 via the coupling mirror 6. The second LBO crystal 42 is preferably a type II phase-matched crystal.

[0058] The second LBO crystal 40 is also oriented in the beam path of the resonator 4 such that the frequency conversion (here, a third harmonic) is maximized for the fundamental wave F propagating in the forward direction 14. Due to the absence of the second harmonic H2, the third harmonic of the fundamental wave F propagating in the backward direction 16 is not triggered in the LBO crystal 40. Therefore, the third harmonic H3 is also (at least approximately) emitted only in the forward direction 14. The frequency doubling in the LBO crystal 30 due to the fundamental wave F propagating in the backward direction 16 is suppressed by the polarization of the fundamental wave F using the λ / 4 waveplate 32.

[0059] The third polarizer 34 (also formed by a λ / 4 waveplate 36) located downstream of the coupling mirror 6 restores the original linear polarization that was disrupted by the λ / 4 waveplate for the second harmonic H2 and for the third harmonic H3.

[0060] According to Figure 2 The implementation plans differ, according to Figure 3The laser device 2 optionally has a frequency-selective reflector 44 located downstream of the λ / 4 waveplate 36. The reflector 44 is transparent to light of the third frequency f3, so that the third harmonic H3 coupled from the resonator 4 passes through the reflector 42 to form a laser beam L.

[0061] The second harmonic H2 coupled from the resonator 4 is deflected by the reflector 42. Here, it is projected, for example, onto the optical sensor 46 used to detect laser activity.

[0062] For ease of explanation, Figure 2 Below the resonator 4, the direction of the fundamental wave F and the tuned wave H (here, the second harmonic H2 and the third harmonic H3) is schematically indicated again.

[0063] The subject matter of the invention is particularly evident in the above embodiments, but is by no means limited thereto. Further embodiments of the invention can be derived from the claims and the foregoing description. In particular, according to... Figure 2 and Figure 3 The third polarizer 34 and Q switch 38 described herein are also used in other embodiments of the laser device 2 according to the present invention. Furthermore, the first polarizer 20 and / or the second polarizer 22 can also be used with… Figure 2 and Figure 3 The methods shown are implemented differently. For example, a Faraday rotator can be used instead of the λ / 4 waveplate 32 for the first polarizer 20, which can rotate the polarization direction of the fundamental wave by 45°. Furthermore, suitable materials different from those described in the exemplary description can be used for the laser medium 10 and the optical nonlinear medium 18.

[0064] List of reference numerals

[0065] 2. Laser equipment

[0066] 4 resonators

[0067] 6 Coupled-out mirrors

[0068] 8. End reflector

[0069] 10 (laser) medium

[0070] 12 Pumping Unit

[0071] 14 Forward direction

[0072] 16. Backward direction

[0073] 18 (Optical Nonlinear) Media

[0074] 20 (First) Polarizer

[0075] 22 (Second) Polarizer

[0076] 23 optical axes

[0077] 24 Nd:YVO4 crystal

[0078] 26 Diode Lasers

[0079] 28 Faraday Rotator

[0080] 30 LBO crystal

[0081] 32 λ / 4 waveplate

[0082] 34 (Third) Polarizer

[0083] 36 λ / 4 waveplate

[0084] 38 Q switch

[0085] 40 Acousto-optic modulator

[0086] 42 LBO crystal

[0087] 44 (Frequency Selectable) Mirror

[0088] 46. ​​Optical Sensor

[0089] f1 baseband

[0090] f2 (second) frequency

[0091] F fundamental wave

[0092] H-tuned wave

[0093] H2 (second) harmonic

[0094] H3 (third) harmonic

[0095] L laser beam

[0096] P-pumped laser beam

Claims

1. A laser device (2), wherein the laser device has: - Optical resonator (4), which has two resonator mirrors (6, 8), namely, the output mirror (6) and the end mirror (8). - Optically active medium (10) for generating light of the first frequency (f1). - An optical nonlinear medium (18) for converting light of the first frequency (f1) into light of other frequencies (f2, f3). - in, The optically active medium (10) and the optically nonlinear medium (18) are arranged in the beam path between the resonator mirrors (6, 8), and - A first polarization-influencing laser optics (20) polarizes light of a first frequency (f1) reflected from the coupling mirror (6) toward the end mirror (8), such that the frequency conversion of the light of the first frequency (f1) polarized in this way is suppressed when it passes through the nonlinear medium (18).

2. The laser device (2) according to claim 1. in, The first polarization-affecting laser optics (20) polarizes light of a first frequency (f1) reflected from the coupling mirror (6) toward the end mirror (8), such that the frequency conversion of the light of the first frequency (f1) polarized in this way is minimized when it passes through the nonlinear medium (18).

3. The laser device (2) according to claim 1 or 2, wherein the laser device comprises: The second polarization-influencing laser optics (22) polarizes light of a first frequency (f1) propagating toward the coupling mirror (6), thereby promoting frequency conversion of the light of the first frequency (f1) polarized in this way as it passes through the nonlinear medium (18).

4. The laser device (2) according to claim 3. in, The second polarization-affecting laser optics (22) polarizes light of a first frequency (f1) propagating toward the coupling mirror (6), such that the frequency conversion of the light of the first frequency (f1) polarized in this way is maximized when it passes through the nonlinear medium (18).

5. The laser device (2) according to claim 3. in, A waveplate or polarization rotator is used as the first polarization-affecting laser optical device (20) and the second polarization-affecting laser optical device (22).

6. The laser device (2) according to claim 3. in, A λ / 4 waveplate (32) is used as the first laser optical device (20) that affects polarization, and a polarization rotator is used as the second laser optical device (22) that affects polarization.

7. The laser device (2) according to claim 3. in, The polarization rotator is used as the first laser optical device (20) that affects polarization and the second laser optical device (22) that affects polarization.

8. The laser device (2) according to claim 1 or 2. in, The resonator (4) has a linear beam path.

9. The laser device (2) according to claim 1 or 2, wherein the laser device comprises: A third polarization-affecting laser optics device (34) is located downstream of the coupling mirror (6), and the third polarization-affecting laser optics device is configured to compensate for the influence of the first polarization-affecting laser optics device (20) on the frequency-converted light.

10. The laser device (2) according to claim 9. in, The first polarization-affecting laser optical device (20) and the third polarization-affecting laser optical device (34) are formed by λ / 4 waveplates (32, 36) with the same structure but rotated 90° relative to each other.

11. The laser device (2) according to claim 1 or 2, wherein the laser device comprises: Q switch (38).

12. The laser device (2) according to claim 11. in, The Q switch (38) is an electro-optic or acousto-optic Q switch or a passive Q switch.

13. The laser device (2) according to claim 1 or 2. in, The optically active medium (10) is a solid.

14. The laser device (2) according to claim 13. in, The optically active medium (10) is a neodymium-doped yttrium vanadate crystal (24).

15. The laser device (2) according to claim 1 or 2. in, The optical nonlinear medium (18) includes an optical nonlinear crystal with a type I phase-matching configuration.

16. The laser device (2) according to claim 15. in, The optical nonlinear medium (18) includes lithium triborate crystals (30, 42).

17. The laser device (2) according to claim 1 or 2. in, The optical nonlinear medium (18) comprises at least two consecutive optical nonlinear crystals.

18. The laser device (2) according to claim 17. in, At least two consecutive optical nonlinear crystals are lithium triborate crystals (30, 42).

19. The laser device (2) according to claim 17. in, The at least two consecutive optical nonlinear crystals (30, 42) include a first crystal (30) of type I phase matching configuration and a second crystal (42) of type II phase matching configuration.

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