LU93076B1 - Laser system for harmonics generation - Google Patents

Abstract

A laser system comprises a laser or amplifier diode for emitting laser light at a fundamental frequency on a beam path and an optical resonator. The optical resonator is resonant for light at a harmonic frequency of the fundamental frequency and comprises a periodically poled nonlinear optical medium in the form of a planar waveguide arranged on the beam path for converting light at the fundamental frequency into light at the harmonic frequency. The nonlinear optical medium has an entry face coated with a first optical coating and an exit face coated with a second optical coating. One of the first and second optical coatings is antireflective for light at the fundamental frequency and high-reflective for light at the harmonic frequency; the other of the first and second optical coating is antireflective for light at the fundamental frequency and partially reflective for light at the harmonic frequency. 93076

Description

DESCRIPTION

LASER SYSTEM FOR HARMONICS GENERATION

Field of the Invention [0001] The present invention generally relates to a device for generating radiation at a harmonic frequency of electromagnetic radiation at a fundamental frequency, in particular to a device for second harmonic generation of laser radiation.

Background of the Invention [0002] Laser emission in the short wavelength range, e.g. in the visible range (herein: 380 nm to 800 nm), is in general hard to achieve. Sometimes, the most efficient or the only way to cover this wavelength range relies on nonlinear (NL) interaction processes involving longer wavelength radiation and taking place in a NL medium. These processes require high intensity radiation, which is cheap and easily available in the infrared range (wavelengths in the range from about 800 to 1200 nm). Especially second harmonic generation (SHG, also known as “frequency doubling”) in a medium with second-order NL optical susceptibility is an efficient and therefore economically interesting process. SHG conversion efficiency depends on the NL medium and polarization and it increases with intensity and spectral brightness.

[0003] It may be worthwhile recalling some general requirements regarding the fundamental radiation (i.e. the radiation at the fundamental frequency) and the SH for efficient interaction in the NL medium. The phase matching concept is known to persons skilled in the art and therefore just some comments required for the further explanation are given. The phase matching condition depends on the specific NL medium, on the fundamental radiation as well as its polarization and on some environmental conditions, in particular the temperature of the NL medium. Depending on the way it is achieved, phase matching can be divided into two major classes, “critical” and “non-critical” phase matching. In critical phase matching, the fundamental and the SH do not propagate collinearly, but under a small angle. The angle is indeed the key parameter to control the phase matching and to achieve efficient conversion. In non-critical phase matching, the fundamental and the SH propagate collinearly and the phase matching controlling parameter is the temperature of the NL medium. Furthermore, for lasers in cw or qcw regime the concept of quasi-phase-matching (QPM) is very attractive. QPM is a technique, which allows a positive net flow of energy from the fundamental into the SH by creating a periodic structure in the NL medium. Momentum is conserved, as is necessary for phase-matching, through an additional momentum contribution corresponding to the wave vector of the periodic structure and, consequently, any three-wave mixing process can be phase-matched. For example, all the electric waves involved can be collinear, can have the same polarization, and travel through the NL medium in arbitrary directions. Up to now, lithium niobate (LN), lithium tantalate (LT) and potassium titanyl phosphate (KTP) are commercially available as periodically poled (PP) materials in bulk or waveguide form and allow very efficient SHG.

[0004] In general, SHG can be implemented adopting resonant and non-resonant schemes. Non-resonant or single pass schemes are attractive because of their simplicity. They only require a suitable NL medium, which the focused fundamental propagates through either once or twice without extra length stabilization. Unfortunately, these schemes have relatively low conversion efficiencies especially at low intensities as it is the case for tapered laser diodes (TLDs) operating in cw or qcw regimes. The efficiency can be moderately increased by employing longer NL media but this directly translates into an increase of the price of the overall system. Furthermore, the sensitivity of the NL interaction to temperature variations is very high and it increases linearly with crystal length. For example, for SHG with a 1060 nm pump (fundamental) in a 20 mm long periodically poled lithium niobate (PPLN) crystal, the temperature acceptance bandwidth (FWHM) is about 1°C and follows a sinc2-function for plane waves. If the crystal temperature can be maintained at the optimum phase matching temperature to within +/-0.1°C, then the SHG power is stable, with less than 5% variation. Commercially available temperature controllers work with precisions of +/-0.01 °C, which limits the overall length in the case of PPLN to about 200 mm for stable operation.

[0005] The resonant schemes are based on the increase of intensity, either by placing the NL medium inside a laser cavity (intra-cavity, IC) for the fundamental radiation or with a resonant enhancement cavity (EC). A downside of these schemes is that to achieve efficient SHG the fundamental laser should operate on a single frequency. In the IC scheme, the components are subjected to very high intensities leading to easy damage of the elements. Moreover, the IC scheme is very difficult to implement for TLDs due to the highly astigmatic and elliptic beam profile. On the other hand, employing an EC resonant for the fundamental requires an optical isolator and active length stabilization with a precision that is hardly sustainable in an industrial environment. An alternative way to boost the efficiency is to resonate the SH in an EC containing the NL medium. This SHG scheme is relatively uncommon since it is not as efficient as resonating the fundamental and is, furthermore, considered to require active length stabilization as well as an optical isolator.

[0006] For SHG with a relatively low-power light source like a TLD, the concept of quasi phase matching in PPLN is usually considered most efficient. LN has the highest nonlinear coefficient of the PP materials in the visible wavelength range and this is utterly important because it factors exponentially into the amplification. For laser coagulation, the main treatment for age-related macular degeneration, more than 1.5 W in the green is needed and in order to reach this power, several SHG schemes have been proposed. Single-pass SHG within one long crystal is not economical or even technically possible. The paper A. K. Hansen et al., "Concept for power scaling second harmonic generation using a cascade of nonlinear crystals," Opt. Express 23, 15921-15934 (2015), describes a cascade of two crystals with a dispersion compensating plate between them. While sufficient power levels could be demonstrated, this setup is even more expensive than one using only a single long crystal.

[0007] LU 92476 discloses so-called multi-pass SHG using an EC that is resonant for the SH but not for the fundamental. This SHG scheme has been demonstrated to be capable of providing about 1 W at 532 nm using PPLN as the NL medium. While conversion efficiency can be increased by this scheme in comparison to a single-pass setup, saturation was shown to hit in at about 1 W, preventing the generation of higher powers.

[0008] The paper D. Jedrzejczyk et al., "Efficient high-power frequency doubling of distributed Bragg reflector tapered laser radiation in a periodically poled MgO-doped lithium niobate planar waveguide," Opt. Lett. 36, 367-369 (2011), discloses single-pass SHG in a MgO-doped PPLN planar waveguide (PW) using a distributed Bragg reflector TLD as a pump source. The maximum power of the SH was again about 1 W.

Summary of the invention [0009] It is an object of an aspect of the present invention to provide a new scheme for generating harmonic radiation of a fundamental electromagnetic wave, which overcomes or reduces at least some of the shortcomings of the known schemes.

[0010] Within the context of the invention, a multi-pass (MP) resonant harmonic generation scheme is applied in combination with a non-linear medium featuring a PW geometry.

[0011] A first main aspect of the invention relates to a laser system that comprises a laser or amplifier diode (e.g. a TLD) for emitting laser light at a fundamental frequency, on a beam path, and an optical resonator. The optical resonator (also “optical cavity”) is resonant for light at a harmonic frequency of the fundamental frequency (but not for the fundamental itself) and comprises a PP NL optical medium in the form of a PW arranged on the beam path for converting light at the fundamental frequency into light at the harmonic frequency. The NL optical medium has an entry face coated with a first optical coating and an exit face coated with a second optical coating. One of the first and second optical coatings is antireflective (AR) for light at the fundamental frequency and high-reflective (HR) for light at the harmonic frequency; the other of the first and second optical coating is antireflective for light at the fundamental frequency and partially reflective (PR) for light at the harmonic frequency.

[0012] The coating that is PR for the light at the harmonic frequency provides the output mirror for the generated harmonic light. According to an embodiment, the second optical coating provides this output mirror; in this case both the residual fundamental and the harmonic light leave the NL optical medium via the exit face. Alternatively, it is the first optical coating that provides the output mirror for the harmonic light; in this case, the harmonic light leaves the NL optical medium via the entry face, whereas the residual fundamental light leaves the NL optical medium via the exit face. The second alternative has the advantage that the generated harmonic light is neatly separated from the residual fundamental light. (In the first alternative, some propagation distance behind the NL optical medium and/or an optical filter may be necessary to separate the generated harmonic light from the residual fundamental light.

[0013] As used herein, the term “light” refers to electromagnetic radiation within the wavelength range from 10 nm to 15 pm. Visible light means light within the wavelength range from 380 nm to 800 nm.

[0014] As used herein, a “planar waveguide” or “PW” is an optical waveguide having an essentially planar geometry that restricts transversal propagation of the light beam in one dimension whereas the light beam is not guided in the other transversal dimension.

[0015] The coatings on the entry and exit faces are preferably dielectric thin-film coatings. The coatings are preferably multilayer coatings. In the context of the present document, an HR coating is a coating having a reflectivity of 99% at least for the specified wavelength or frequency. An AR coating is a coating with a reflectivity of 1% at most for the specified wavelength or frequency. A PR coating is a coating with a reflectivity comprised in the range from 1% to 99% for the specified wavelength or frequency. According to an embodiment of the invention, the reflectivity of the coating that is PR for light at the harmonic frequency is comprised in the range from 30% to 90%, more preferably in the range from 40% to 80%, still more preferably in the range from 50% to 65%.

[0016] According to an embodiment of the invention, the PP NL optical medium is PPLT (preferably doped with magnesium oxide (MgO) to increases its resistance to photorefractive damage). It has been mentioned above that neither the multi-pass SHG scheme of LU 92476 nor the single-pass SHG scheme using a PPLN PW reach a power level of 1.5 W or more for a SH at 532 nm. Even though an absorption effect in LN proportional to the intensity of the SH has not been reported in the literature, it is possible that it is LN that prevents the achievement of higher powers. This embodiment combines the use of PPLT with a multi-pass scheme and a PW scheme. Surprisingly, higher powers can be achieved with such a laser system. This result is unexpected, since none of the known schemes previously allowed such high powers and LT features a considerably lower NL coefficient than LN.

[0017] The harmonic frequency may be the SH of the fundamental (i.e. the double of the fundamental frequency) or any other harmonic. In most practical applications, however, the harmonic frequency will be the SH. The wavelength of the fundamental is preferably comprised in the range from 750 nm to 1200 nm.

[0018] As will be appreciated, the present laser system provides, inter alia, a SHG scheme, which may be used, especially, for TLD radiation in cw or qcw regime. The SHG scheme is efficient and may be used with short NL media. Depending on the application, an optical isolator and/or active stabilization of the optical cavity might be unnecessary.

[0019] According to an embodiment, the laser system comprises beam shaping optics configured and arranged so as to focus laser light emitted by the laser or amplifier diode into the optical resonator. It is worthwhile noting that focussing of the laser light may be necessary only in the guided direction (i.e. in the thickness direction of the PW). As regards the unguided direction (which lies in the plane of greatest extension of the PW), the beam shaping optics are preferably configured and arranged so as to collimate the light beam before it enters the PW and thus to avoid excessive expansion of the light beam in the unguided direction.

[0020] The entry and exit faces of the PW may be parallel to each other, e.g., to form a linear resonator. Alternatively, the entry and exit faces of the PW can be arranged at a (non-zero) angle to each other to support a zig-zag pass or also a ring resonator. The entry and exit faces of the PW are perpendicular to the plane of the PW.

[0021] According to an embodiment, the optical resonator is a monolithic ring resonator, i.e. a resonator wherein the resonated harmonic light travels within the PP NL medium on a closed path (loop), crossings not being excluded. In such monolithic ring geometry, the resonated harmonic light is reflected at least once on each roundtrip on a side surface of the PW. Preferably, the optical cavity has a fixed geometry, i.e. no active stabilization of the cavity is provided.

[0022] According to an embodiment, the NL medium has a PP domain structure, which is inclined with respect to the beam path of the fundamental within the NL medium. Preferably, the PP domain structure is inclined with respect to the fundamental’s propagation axis by an angle comprised in the range from 0.5° to 10°, more preferably in the range from 1.7° to 7.5° and still more preferably in the range from 3° to 5°. The entry and exit faces of the PW may also be inclined with respect to the beam path of the fundamental within the NL medium. Preferably, the entry and exit faces are inclined by the same angle as the PP domain structure. Preferably, the entry face is parallel to the PP domain boundaries, as such configuration facilitates handling of back-reflections (which may occur at the entry face and the PP domain boundaries).

The PW could be of trapezoidal shape to support a ring resonator forming a closed beam path.

[0023] The laser system may comprise a temperature controller in thermal contact with the NL medium for controlling the temperature thereof (and, thereby, the phase matching condition). A further temperature controller may be provided in thermal contact with the laser or amplifier diode in order to control the temperature of the laser or amplifier diode and, thereby, the wavelength of the fundamental light.

[0024] According to an embodiment, the laser or amplifier diode comprises a TLD. Such TLD preferably comprises a ridge waveguide section and a tapered gain section, as well as a diode driver configured for driving a current across the ridge waveguide section and/or the gain section, respectively. In the waveguide section of the TLD, which is preferably configured to support a single transverse mode, a grating structure can be integrated to narrow the spectrum, which is beneficial for efficient SHG.

[0025] It is worthwhile mentioning that TLDs are sensitive to back reflection, which can either damage the tapered region or affect the spectral behavior resulting in a broader emitted spectrum. This broadening has been experimentally observed in SHG using a PPLN crystal leading to a saturation of the SHG with increasing power of the fundamental. In the present laser system, back reflections from the periodic domain structure into the TLD can be avoided with the above-mentioned inclined domain structure. Accordingly, a high number of domains (leading to high SHG efficiency) are not problematic in the present invention.

[0026] Preferably, the laser system comprises a feedback loop for adjusting the fundamental frequency of the TLD, the feedback loop including a controller, the diode driver and a sensor for providing a signal indicative of the optical power at the harmonic frequency output by the optical resonator. The controller is preferably configured to control, via the diode driver, the current across the ridge waveguide section and/or through the gain section, a variation of the current translating into a variation of the fundamental frequency. With such a feedback loop, the TLD can be easily tuned to exactly match the SH cavity mode for efficient SHG. Preferably, the controller is configured to control, via the diode driver, the current across the ridge waveguide section. Variations of the current of the tapered section cause stronger wavelength shifts than the same variations on the current of the waveguide section. Additionally or alternatively, the temperature of the laser diode can be changed by the temperature controller in order to tune the fundamental frequency. It should be noted, however, that the current of the tapered section and the temperature of the TLD significantly affect each other (assuming that the temperature of the TLD is not actively kept constant). Both TLD temperature and the current of the tapered section greatly affect the optical power of the fundamental. It has furthermore been shown that the current of the tapered section also has an impact on certain beam parameters, such as astigmatism. Therefore, a feedback based on the current of the ridge section, which minimally affects the characteristics of the generated fundamental beam is preferred.

[0027] A second main aspect of the invention relates to an optical resonator for use in a laser system as described above. The optical resonator comprises a PW comprising a PP NL optical medium for converting light at a fundamental frequency into light at a harmonic frequency (e.g. the SH) of the fundamental frequency. The PW has an entry face coated with a first optical coating and an exit face coated with a second optical coating, one of the first and second optical coatings being AR for light at the fundamental frequency and HR for light at the harmonic frequency, the other of the first and second optical coatings being AR for light at the fundamental frequency and PR partially reflective for light at the harmonic frequency.

[0028] The PP NL optical medium is, preferably, PPLT.

[0029] The entry and exit faces may be parallel to each other, e.g., to form a linear resonator. Additionally and/or alternatively the optical resonator may be configured as a monolithic ring resonator.

[0030] The NL medium preferably has a PP domain structure, which is inclined with respect to the nominal beam path of the light at the fundamental frequency between the entry and exit faces.

Brief Description of the Drawings [0031] By way of example, preferred, non-limiting embodiments of the invention will now be described in detail with reference to the accompanying drawings, in which:

Fig. 1 : is a side view of a laser system according to an embodiment of the invention; Fig. 2: is a top view of the laser system of Fig. 1 ;

Fig. 3: is an illustration of a further embodiment of a PP NL crystal that may be used in the context of the present invention.

Detailed Description of Preferred Embodiments [0032] A laser system 10 according to an embodiment of the invention is shown in Figs. 1 and 2 in the fast and slow axis directions, respectively. In the context of laser or amplifier diodes, the term "fast axis direction" designates the direction perpendicular to the beam axis in which the divergence of the beam after the exit facet of the diode, but before divergence correction, is greatest. The fast axis direction corresponds to the direction of the small extension of the output facet. Conversely, the term "slow axis direction" designates the direction perpendicular to the beam axis in which the divergence of the beam after the exit facet of the diode, but before divergence correction, is least. The slow axis direction corresponds to the direction of the large extension of the facet.

[0033] Laser system 10 comprises a TLD 12, collimating optics 14 (schematically represented as a cylindrical fast-axis collimating lens 14-1 and a cylindrical slow-axis collimating lens 14-2), focusing optics 16 (represented by a cylindrical slow-axis focusing lens), a monolithic optical resonator 18 including a NL medium for SHG and a SH collimating optics 20 at the output of the resonator 18.

[0034] TLD 12 produces a laser beam at the fundamental wavelength. A TLD can be realized as a master-oscillator-power-amplifier or as a single laser cavity consisting of two sections, a waveguide section and a tapered gain section. The oscillator or waveguide section is responsible for the spatial and spectral properties and the tapered region is used for amplification. Both sections can be integrated in a monolithic configuration that significantly increases the robustness of the system. The fundamental beam produced by the TLD 12 is coupled into the optical resonator 18 for SHG.

[0035] Optical resonator 18 comprises a PPLT crystal in the form of a PW 22. The entry face of the PW is coated with a first optical coating 24, which is AR for the fundamental and HR for the SH light. The exit face of the PW is coated with a second optical coating 26, which is also AR for the fundamental but PR for the SH.

[0036] The domains 28, 28’ of the PPLT crystal are inclined with respect to the propagation direction of the fundamental by an angle preferably in the range from 3° to 5°. Any reflections occurring at the domain boundaries or the crystal boundaries are thus directed away from the optical axis, whereby back reflections into the TLD 12 can be largely avoided.

[0037] The fundamental beam is prepared in such a way that the axis perpendicular to the polarization (i.e. the fast axis in case of a TDL) is collimated to about the width of the PW (e.g. about 300 pm). The beam axis parallel to the polarization (i.e. the slow axis in case of a TLD) is focused to the size of the lowest order mode of the PW (about e.g. about 5 pm). The preparation of such beam shape can be achieved with one lens of aspherical shape, which is placed such that the fast axis is collimated; the slow axis is focused due to the strong astigmatism and the low divergence. In the example of Figs. 1 and 2, crossed cylindrical lenses are used to collimate and focus the fundamental beam in the fast and slow axis directions, respectively. Other collimating/focussing optics could be used as well. In the linear setup of Figs. 1 and 2, the PW is arranged perpendicular to the active area of the TLD 12, i.e. in the area spanned by the fast axis and the beam propagation axis, to ensure that the polarization is correct for the SHG.

[0038] Within the resonator 18, the beam is guided in the slow axis direction (Fig. 2) whereas it propagates freely in the fast axis direction (Fig. 1). The optics generate a beam, which allows the resonator 18 to be of very simple monolithic construction, with flat mirrors that are directly coated onto the PW crystal end faces. During a pass through the PW, the beam propagation parameters remain substantially constant, since the beam is guided in the thickness direction of the PW and is collimated in the plane of the PW. The SH beam generated in the NL medium inherits these properties from the fundamental beam. Therefore, at the end of the PW crystal the SH beam can be reflected by a plane mirror and the back-propagating SH beam has nearly the same properties. The same is true after further reflections on the end faces of the PW. Therefore the PW cavity mode is nearly identical to the mode of the in-coupling beam resulting in a stable output and good SH conversion efficiency.

[0039] Figs. 1 and 2 show an embodiment of the PW with parallel planar entry and exit faces. Fig. 3 shows an embodiment of a PW 22’ having a trapezoidal configuration in the plane perpendicular to the polarization. The entry face of the PW 22’ and the domain boundaries of the PP NL medium are parallel. The fundamental beam 30 is coupled into the PW 22’ at an angle such that, in the crystal, it travels parallel to the side faces 34, 36 of the PW. The angle between the normal of the PP domain boundaries and the fundamental’s propagation axis is comprised in the range from 0.5° to 10°, preferably in the range from 1.7° to 7.5° and most preferably in the range from 3° to 5°. The exit face is inclined with respect to the fundamental’s propagation axis by the same angle but in the opposite direction. Both entry and exit faces of the PW are AR-coated for the fundamental light. The coating 24 on the entry face is HR for the SH and the coating 26 on the exit face is PR for the SH (reflectivity e.g. in the range from 50% to 65%). SH light 32 is generated along the path of the fundamental beam 30. At the exit face, one part of the SH is coupled out of the PW whereas the other part is reflected into the crystal. Back-reflected SH light 32 hits the larger side face 34 of the PW 22’, from where it bounces off by total internal reflection into the direction of the entry face 24. From there, SH light is reflected onto the path of the fundamental beam inside the crystal.

[0040] It is, theoretically, possible that the generated SH wavelength does not match any resonator mode, which results in low conversion efficiency. Therefore, to ascertain that the length of one roundtrip of the SH beam in the crystal amounts to an integer number of SH wavelengths in the medium, the TLD may be tuned to the corresponding fundamental frequency.

[0041] One may take advantage from the following property of a TLD: by changing the current through the ridge section of the TLD, a wavelength shift, e.g. of about 1 pm/mA, is generated. Thanks to that dependency, a TLD can be easily tuned to exactly match the SH cavity mode for efficient SHG.

[0042] As an alternative to tuning the TLD wavelength, one could think of changing the temperature of the NL medium to increase the cavity length. However, that would mean that the phase matching between the fundamental and the harmonic can no longer be controlled independently. Accordingly, one preferably controls (a) the temperature of the PP NL medium to control the phase matching and (b) the current through the ridge section of the TLD to control the wavelengths of the fundamental at its harmonic. It will be appreciated that no active mechanical stabilization is needed in this case.

Example [0043] The optics of Figs. 1 and 2 was used to simulate the efficiency of SHG, when coupled into a ring resonator of Fig. 3 with a TLD delivering up to 6 W at 1064 nm. The emitting surface of the TLD is about 1 pm (0.28 NA (numerical aperture)) in the fast axis direction and 425 pm (0.16 NA) in the slow axis direction with an astigmatism of 1.4 mm. The beam is collimated to a width of about 300 pm (1/e2) in fast axis direction by an aspherical cylinder lens and focused in the slow axis direction to a diameter of about 5 pm by a crossed aspherical cylinder lens. The PW made of 8 mole-% MgO doped PP congruent LT placed at the focus position. The length of the PPLT crystal is 10 mm, the thickness is 1.0 mm including the support substrate (PW thickness 10 pm) and the PW width was 1 mm. The PPLT crystal was PP for SHG of 1064 nm to 532 nm and polished to form a monolithic ring resonator in trapezoidal shape with an angle of the end faces and the domain structure of 4.3°, in order to minimize back reflections from the domain structure and support a closed beam pass. The entry face of the PW was AR coated to R < 0.25% for 1064 nm light and HR coated to R>99.5% 532 nm light. The exit face coating of the PW was AR at1064 nm and PR (R = 55°) at 532 nm. The simulation shows that with this arrangement more than 1.5 W at 532 nm can be achieved.

[0044] While specific embodiments have been described herein in brevity and/or in detail, those skilled in the art will appreciate that various modifications and alternatives to those embodiments could be developed in light of the overall teachings of the disclosure. Specifically, features of different embodiments may be freely combined unless it follows from the context of the description of the embodiments that such a combination is not possible or unless such a combination does not make any technical sense. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.

Claims (14)

A laser system comprising: a laser or amplifier diode for emitting laser light at a fundamental frequency onto a beam path; an optical resonator resonant to light at a harmonic frequency of the fundamental frequency, the optical resonator having a periodically poled nonlinear optical medium in the form of a planar waveguide disposed on the beam path for converting light at the fundamental frequency into light harmonic frequency, the nonlinear optical medium having an entrance surface with a first optical coating deposited thereon and an exit surface having a second optical coating deposited thereon, one of the first and second optical coatings being antireflective of light at the fundamental frequency and highly reflective to light with the harmonic frequency, the other of the first and second optical coatings being anti-reflective of light at the fundamental frequency and partially reflective to light having the harmonic frequency. The laser system of claim 1, wherein the periodically poled non-linear optical medium is periodically poled lithium tantalate. A laser system according to claim 1 or 2, comprising beam shaping optics configured and arranged to focus laser light emitted from the laser or amplifier diode into the optical resonator. 4. Laser system according to one of claims 1 to 3, wherein the entrance and exit surfaces are parallel to each other. 5. A laser system according to any one of claims 1 to 3, wherein the optical resonator is a monolithic ring resonator. The laser system of any one of claims 1 to 5, wherein the non-linear medium has a periodically poled domain structure that is inclined with respect to the beam path. 7. A laser system according to any one of claims 1 to 6, comprising a temperature control unit in thermal contact with the non-linear medium for controlling its temperature. The laser system of any one of claims 1 to 7, wherein the laser or amplifier diode comprises a trapezoid laser diode. 9. The laser device of claim 8, wherein the trapezoidal laser diode comprises a ridge waveguide section and a trapezoidal gain section and a diode driver configured to drive a current through the ridge waveguide section and / or the gain section. 10. An optical resonator comprising a planar waveguide comprising a periodically poled nonlinear optical medium for converting light at a fundamental frequency into light at a harmonic frequency of the fundamental frequency, the planar waveguide having an entrance surface with a first optical coating applied thereto and an exit surface having a second optical coating coated thereon, one of the first and second optical coatings being antireflective of fundamental frequency light and high harmonic frequency of light, the other of the first and second optical coatings being antireflective of fundamental frequency light and partially is reflective for light with the harmonic frequency. The optical resonator of claim 10, wherein the periodically poled non-linear optical medium is periodically poled lithium tantalate. 12. An optical resonator according to claim 10 or 11, wherein the entrance and exit surfaces are parallel to each other. 13. An optical resonator according to any one of claims 10 to 12, wherein the optical resonator is a monolithic ring resonator. 14. An optical resonator according to claim 11, wherein the non-linear medium has a periodically poled domain structure inclined with respect to a beam path of the light at the fundamental frequency between the entrance and exit surfaces.