WO2018108251A1

WO2018108251A1 - Laser

Info

Publication number: WO2018108251A1
Application number: PCT/EP2016/080818
Authority: WO
Inventors: Tomi Pekka LEINONEN; Jussi-Pekka PENTTINEN; Mircea Dorel GUINA
Original assignee: Vexlum Oy
Priority date: 2016-12-13
Filing date: 2016-12-13
Publication date: 2018-06-21

Abstract

According to this disclosure a laser is suggested comprising: a resonator (4)having a first cavity (5) and a second cavity (6), an optically pumped gain element (2) arranged to generate fundamental electromagnetic radiation (17) at a fundamental frequency and located in the first cavity (5), and a periodically poled nonlinear optical crystal (3, 103, 123, 151, 152, 161) arranged to convert the fundamental electromagnetic radiation (17) into converted electromagnetic radiation (18, 126, 164) having a converted frequency and located in the second cavity (6), wherein the resonator (4)is resonant for the fundamental electromagnetic radiation (17), and wherein the first cavity (5) and the second cavity (6) are electromagnetically coupled.

Description

Laser

The present invention relates to a laser comprising a resonator, an optically pumped gain element designed to generate fundamental electromagnetic radiation at a fundamental frequency and a periodically poled nonlinear optical crystal designed to convert the fundamental electromagnetic radiation into converted electromagnetic radiation having a converted frequency.

While laser sources providing coherent electromagnetic radiation are available for a broad range of frequencies, there are frequency ranges, which cannot be directly addressed by emission from an electrically or optically pumped gain medium. In these cases, laser sources based on a combi- nation of an electrically or optically pumped gain medium and an optical nonlinear frequency conversion process have been established. The fundamental electromagnetic radiation generated by the gain medium at a fundamental frequency is converted by a nonlinear optical element into converted radiation having a converted frequency being different from the fundamental frequency. WO 2000/025399 A1 discloses an intra-cavity frequency-doubled external-cavity optically-pumped semiconductor laser including a monolithic surface-emitting semiconductor layer structure with a Bragg mirror portion and a gain portion. An external mirror and the Bragg mirror portion define a laser resonant-cavity including the gain-portion of the semiconductor layer structure. A birefringent filter is located in the resonant-cavity for selecting a frequency of the laser-radiation within a gain bandwidth characteristic of the semiconductor structure. Optical nonlinear crystal is located in the resonant cavity between the birefringent filter and the external mirror and arranged to double the selected frequency of the laser radiation.

In the prior art, an optically pumped gain structure with relatively low gain is located in the same cavity with a birefringent phase-matched nonlinear crystals having relatively low conversion efficiency. It turns out that this design limits the degrees of freedom when it comes to optimizing the operational parameters of the laser, such as the output coupling of the laser, which is determined by the conversion efficiency of the nonlinear crystal. Optimization of the output coupling, on the other hand, is critical for increasing the optical-to-optical efficiency of the laser, which compares the optical power used to pump the gain portion and the output of the frequency converted radiation generated. It is thus an objective of the present invention to provide a laser with an optically pumped gain element and a nonlinear optical element in the same resonator, having improved flexibility in the laser design. It is another objective of the present invention to provide a laser with increased optical- to-optical efficiency.

At least one of the above objects is solved by a laser comprising a resonator having a first cavity and a second cavity, an optically pumped gain element arranged to generate fundamental electromagnetic radiation at a fundamental frequency and located in the first cavity, and a periodically poled nonlinear optical crystal arranged to convert the fundamental electromagnetic radiation into converted electromagnetic radiation having a converted frequency and located in the second cavity, wherein the resonator is resonant for the fundamental electromagnetic radiation, and wherein the first cavity and the second cavity are electromagnetically coupled.

The laser design according to the present invention allows to reduce the optical field strength in the second cavity accommodating the nonlinear optical crystal independently of the optical field strength in the first cavity accommodating the gain element. The laser design thus allows optimization of the power densities in the first cavity and in the second cavity to optimal levels for the gain element and for the periodically poled nonlinear optical crystal. In particular, the laser design according to the present invention includes the use of a periodically poled nonlinear crystal in the same resonator with an optically pumped surface emitting gain element. The use of a periodically poled nonlinear crystal in turn allows greater flexibility in the optimization of the conversion efficiency of the nonlinear crystal, i.e. the useful output coupling losses of the laser.

By decreasing the power density in the second cavity, the laser design according to the present invention helps to reduce the probability of damage to the periodically poled nonlinear crystals and hence enables the use of such crystals offering high conversion efficiency and wide range of nonlinear interactions. Similarly, the design also helps to reduce the probability of damages to the coatings on the crystal and to other optical components in the second cavity.

In addition, by separating the gain element and the nonlinear optical crystal in the first and second cavities, the design of the laser reduces the impact of the parasitic losses in the second cavity on the performance of the gain element in the first cavity. This is advantageous in particular for a gain element having a rather low gain and thus requiring a cavity with low losses, or in other words, a cavity with high quality factor - Q. This also allows the use of considerably lossy components in the second cavity while reducing their detrimental effects on the Q of the first cavity. Disk shaped optically pumped gain elements are particular examples of gain elements with relatively low gain. Disk shaped in the sense of the present invention is a gain element whose thickness in the direction of propagation of the fundamental radiation is much smaller than its lateral dimensions, i.e. its dimensions in two directions perpendicular to its thickness. Disk shaped gain elements are used in order to reduce thermal effects such as temperature gradients over thermally low conducting material and thermal lensing by increasing the surface for the heat flow and thus decreasing the heat flux. Particular examples of disk-shaped optically pumped gain elements in the sense of the present invention are bulk solid state materials in form of a disk and disk-shaped semiconductor structures. In a semiconductor structure the thickness of the active material is the total thickness of the quantum wells, i.e. multiplication of the thickness of a single quantum well by the total number of quantum wells which can be stacked onto each other in the gain element. The optical thickness of the entire active region, where the quantum wells are located, is typically several quarter-wavelengths. In an embodiment, the gain element is a disk of a semiconductor resonant periodic gain structure, wherein the quantum wells are positioned in the antinodes of the optical field.

In an embodiment of the present invention, the optically pumped gain element is an optically pumped vertical external-cavity surface-emitting laser structure (OP-VECSEL structure).

In order to enable optical pumping of the gain element, in an embodiment of the present invention the laser comprises a pumping source, e.g. a laser diode, operating at a pumping frequency. In this case, the pumping source is arranged such that the electromagnetic pumping radiation during operation of the laser is guided into the gain element. In an embodiment, a focusing element is located between an output aperture of the pumping source and the gain element.

In an embodiment of the present invention, the optical pumping may provide for a multi-pass pumping, wherein the gain element and the optics for pumping radiation are arranged to provide multiple passes of the pumping radiation through the gain element. This is useful in an embodiment, wherein the gain element is of a bulk solid state material or a semiconductor resonant periodic gain structure.

The present invention suggests the use of a periodically poled nonlinear optical crystal enabling quasi-phase-matched nonlinear frequency conversion. Quasi-phase-matched periodically poled nonlinear optical crystals have many attractive advantages. In the present application, the terms quasi-phase-matched nonlinear optical crystal and periodically poled nonlinear optical crystal are used synonymously. Quasi-phase-matched crystals are advantageous to support a wide range of nonlinear interactions, which might be used in order to address different frequency ranges. Examples for nonlinear interactions which are enabled by quasi-phase-matched crystals are second harmonic generation (SHG), third harmonic generation (THG), sum frequency generation (SFG), difference frequency generation (DFG), optical parametric amplification and Raman conversion.

It is evident for a person skilled in the art that sum and difference frequency generation requires electromagnetic radiation at two frequencies. Thus for sum frequency generation a second crystal or a crystal having two differently poled sections will be required in order to generate a second frequency before the two frequencies can be used for sum frequency generation in the second crystal or section. For difference frequency mixing it would be advantageous if the optically pumped gain element provides radiation at two different frequencies. In order to support parametric amplification, it might be required to design the resonator such that it is resonant for the signal wave and for the idler wave. Furthermore, the concept of quasi phase-matching in an embodiment enables utilization of nonlinear optical crystals having a nonlinear coefficient larger than nonlinear coefficients of nonlinear optical crystals typically used for birefringent phase-matching. The optical efficiency of the laser is dictated by the ratio of output coupling (useful losses) and parasitic losses (e.g. scattering and absorption losses along with light leaking through the reflectors). By use of a periodically poled nonlinear optical crystal according to the present invention, the useful losses can be increased owing to intrinsic higher nonlinear conversion efficiency offered by such crystal. This increase in the useful losses allows for higher tolerable level of parasitic losses which will still lead to better or similar overall efficiency. The high conversion efficiency of the quasi-phase-matched nonlinear optical crystals thus in an embodiment allows the use of intra-cavity components like frequency se- lective elements having higher parasitic losses.

In an embodiment, the propagation direction of the fundamental electromagnetic radiation in the quasi-phase-matched crystal is along the crystal axis avoiding spatial walk-off. In addition, in an embodiment the acceptance angle of the quasi-phase-matched crystal is large.

The quasi-phase-matched nonlinear optical crystals in an embodiment allow operation at or near room temperature by choosing a suitable design of the poling. In another embodiment, the quasi-phase-matched nonlinear crystal allows use of a single crystal with different sections optimized for different nonlinear optical interactions or the same interaction for different frequencies. In an embodiment of the present invention, the periodically poled nonlinear optical crystal is cut under the Brewster angle for the fundamental electromagnetic radiation. Cutting under the Brewster angle reduces reflections from the facets or surfaces of the nonlinear optical crystal and provides polarization selection. The possibility to cut under the Brewster angle is enabled by the fact that periodically poled nonlinear optical crystals do not usually change the polarization of the fundamental electromagnetic radiation in the crystal.

In an embodiment of the present invention, the periodically poled nonlinear crystal has a phase- modulation regime incorporated in the crystal.

On the other hand, the high efficiency of nonlinear optical conversion in periodically poled nonlinear optical crystals while in general being an advantage turn out to be a disadvantage when used for intra-cavity frequency conversion in the same cavity with the gain element. This in particular holds when an optically pumped surface-emitting gain element requiring cavity with high Q is used. Once the single-pass conversion efficiency in the nonlinear optical crystal is high enough, it will overly reduce the cavity Q. Thus, a combination of a periodically poled nonlinear optical crystal and a low gain optically pumped gain element in the same cavity might not work stably or not work efficiently. This is an aspect which is addressed by a laser according to the present invention.

According to the present invention, the resonator in which the optically pumped gain element as well as the periodically poled nonlinear optical crystal is a so-called coupled cavity design. The resonator comprises a first cavity and a second cavity, wherein the two cavities are electromag- netically coupled. While the optically pumped gain element is located in the first cavity, the periodically poled nonlinear optical crystal is located in the second cavity. In this cavity design according to the present invention, an optical field at the fundamental frequency resonates in both cavities while the ratio of the amplitudes of the field strength in the two cavities is set by the coupling mirror or coupling mirrors.

The length of the first cavity defines how many resonant longitudinal modes the first cavity can support and the length of the second cavity similarly how many resonant longitudinal modes the second cavity can support. The Vernier effect, meaning the overlapping resonances of the first and second cavities, together with the gain bandwidth of the gain element, i.e. the bandwidth wherein the gain element provides gain, determine the number of longitudinal modes supported by the resonator for laser operation. In an embodiment, wherein the cavity length of the second cavity is larger, preferably substantially larger than the cavity length of the first cavity, the laser can have multiple operating longitudinal modes. For single mode operation, frequency filtering or selection might then be required. The present invention separates the intra-cavity field intensity of the fundamental optical field in the gain element and the intra-cavity field intensity of the fundamental optical field in the nonlinear crystal by splitting the resonator into the first cavity and the second cavity being electromagnetically coupled with each other. By locating the gain element on the one hand and the nonlinear crystal on the other hand in two separate but coupled cavities, the power density of the fundamental radiation in each of the two elements can be optimized separately. For example, in an embodiment the power density of the fundamental radiation in the gain structure can remain high while the power density for the periodically poled nonlinear optical crystal can be lowered below the damage threshold of the crystal and any optical coatings. In an embodiment of the invention, the power density of the fundamental radiation in the first cavity is higher than in the second cavity. Simultaneously, a high conversion efficiency of the nonlinear optical crystal can be retained without detrimental effects on the gain element.

In an embodiment, the resonator having the first cavity and the second cavity comprises at least three mirrors. In a further embodiment, the first cavity comprises at least a first mirror and a second mirror, wherein at least the second mirror is a coupling mirror, and wherein the second cavity comprises at least a first mirror and a second mirror, wherein the coupling mirror is partly transmissive for the fundamental electromagnetic radiation providing an electromagnetic coupling between the first cavity and the second cavity.

The coupling mirror or coupling mirrors must allow transmission of the fundamental radiation between the first cavity and the second cavity. The coupling mirror is thus only partially reflecting for the fundamental radiation, which is an alternative expression for the coupling mirror being partly transmissive for the fundamental electromagnetic radiation. In an embodiment the coupling mirror has a reflectivity for the fundamental radiation of 99 % or less. In an embodiment the coupling mirror has a reflectivity for the fundamental radiation in a range from 30 % to 99 %.

In a first embodiment the first mirror of the first cavity as well as the second mirror of the first cavity are coupling mirrors, wherein the coupling mirrors are partly transmissive for the fundamental elec- tromagnetic radiation. These two coupling mirrors while forming the end mirrors of the first cavity are located in a beam path between the first mirror and the second mirror of the second cavity, wherein the first mirror and the second mirror of the second cavity have a higher reflectivity for the fundamental electromagnetic radiation than the two coupling mirrors forming the first mirror and the second mirror of the first cavity. The two coupling mirrors thus provide an electromagnetic coupling between the first cavity and the second cavity. It should be understood that the first mirror and the second mirror of the second cavity, in particular all mirrors of the second cavity, of this embodiment are high reflective for the fundamental electromagnetic radiation. Alternatively the first mirror or the second mirror of the second cavity, in particular any of the mirrors of the second cavity, can be partly transmissive for the fundamental electro- magnetic radiation, thus allowing output coupling of the fundamental radiation from resonator. In either case the first mirror and the second mirror, in particular all mirrors, of the second cavity have a higher reflectivity for the fundamental electromagnetic radiation than the coupling mirrors of the first cavity. It should be noted that a design, wherein the first cavity comprises two coupling mirrors can be implemented as a linear resonator, wherein the first cavity and the second cavity are standing wave resonators. Alternatively, this design could also be implemented as a ring resonator, wherein the first cavity is a linear cavity and the second cavity forms a ring. In an alternative, second embodiment the second mirror of the first cavity is the first mirror of the second cavity, and wherein the first mirror of the first cavity and the second mirror of the second cavity have a higher reflectivity for the fundamental electromagnetic radiation than the coupling mirror. It should be noted that the coupling mirror forms part of the first cavity and of the second cavity. Thus, it must be designed to be reflecting for the fundamental radiation in the first cavity and for the fundamental radiation in the second cavity, while allowing transmission of the fundamental radiation between the first cavity and the second cavity. It should be noted that in the second embodiment both, the first cavity and the second cavity may be implemented as linear or standing wave resonators. In an embodiment, the linear resonator geometry for example can be chosen from a group consisting of an l-shaped resonator, a V-shaped resonator, a Z-shaped resonator, an M-shaped resonator or any combination thereof. Alternatively, this design could also be implemented as a ring resonator, wherein the first cavity is a linear cavity and the second cavity forms a ring.

In an embodiment at least one of the above requirements is fulfilled by a single distributed Bragg reflector forming the coupling mirror. However, in an embodiment the coupling mirror may in principle be formed by two distributed Bragg reflectors, wherein the first distributed Bragg reflector forms part of the first cavity and the second one forms part of the second cavity, and wherein between the first and second Bragg reflectors a spacer being transparent for the fundamental radiation is placed. This way the coupling mirror is formed by a Fabry-Perot etalon, which in addition acts as a frequency selective element as described below.

In a further embodiment, the first mirror of the first cavity and the second mirror of the second cavity are highly reflective for the fundamental electromagnetic radiation. In an embodiment the first mirror and the second mirror have a reflectivity of approximately 100 % or of 100 % for the fundamental electromagnetic radiation.

In an alternative embodiment, the second mirror of the second cavity has reflectivity below 100% for the fundamental electromagnetic radiation, and thus forms an output coupler for the fundamental electromagnetic radiation.

In either case the first mirror of the first cavity and/or the second mirror of the second cavity has a higher reflectivity for the fundamental radiation than the coupling mirror(s).

In another embodiment, the second mirror of the second cavity is an output coupler for the converted electromagnetic radiation and thus has a low or no reflectivity for the converted electromagnetic radiation. In an embodiment of the present invention, the gain element is in direct physical contact with the first mirror of the first cavity. This is in particular advantageous in embodiments, wherein the gain element is a semiconductor structure having a resonant periodic gain structure. In this case, the first mirror of the first cavity can be monolithically integrated into the gain element as a structured Bragg mirror.

In an alternative embodiment the first mirror of the first cavity is a distributed Bragg-reflecting mirror on a solid state disk arranged between a heatsink and a disk-shaped gain element.

In an embodiment, wherein the gain element is a disk of a bulk solid state material, direct contact between the gain element and the first mirror of the first cavity can be achieved by bonding the first mirror onto the gain element.

In an embodiment of the present invention, the gain element is in contact, preferably in direct physical contact, with the coupling mirror forming the second mirror of the first cavity.

In an embodiment, the coupling mirror forming the second mirror of the first cavity is monolithically integrated with the gain element. This is in particular advantageous in embodiments, wherein the gain element is a semiconductor resonant periodic gain structure provided in a semiconductor material. In an embodiment, the coupling mirror is a Bragg mirror structured into the same semiconductor material as the gain element. In an alternative embodiment the coupling mirror forming the second mirror of the first cavity is a dielectric Bragg mirror coated onto the gain element.

In a particular embodiment the first mirror of the first cavity is a semiconductor distributed Bragg reflector monolithically integrated with a semiconductor gain element and the coupling mirror form- ing the second mirror of the first cavity is a dielectric mirror coated on the semiconductor gain element.

In an embodiment of the invention, the gain element is mounted in thermal contact with a heat conducting or a heat absorbing element. A heat conducting or heat absorbing element in the sense of the present application may also be denoted as a heat spreader. A heat spreader is used in order to take away the heating of the gain element due to the electromagnetic pump radiation absorbed by the gain element and not being converted into fundamental electromagnetic radiation. The heat spreader in an embodiment may consist of diamond. In a highly integrated embodiment, the gain element, the first mirror of the first cavity, the coupling mirror forming the second mirror of the first cavity as well as the heat spreader are stacked onto each other, wherein the surfaces of each two elements are in direct contact with each other. While with respect to the thermal properties it may be an advantage once the gain element is in direct physical contact with the heat spreader, embodiments are feasible, wherein the gain element is in direct physical contact with the first mirror and the coupling mirror of the first cavity, wherein either the first mirror or the coupling mirror or both the first mirror and the coupling mirror of the first cavity are in direct physical contact with a heat spreader.

Examples and implementations of the gain element, the first mirror of the first cavity, the coupling mirror forming the second mirror of the first cavity and the heat spreader are described in detail with respect to the figures below.

In an embodiment of the invention, the nonlinear optical crystal comprises a material chosen from the group consisting of LN (LiNbOs), LT (LiTaOs), KTP (KT1OPO4) and LBGO (LaBGeOs), for example.

In an embodiment of the present invention, the nonlinear optical crystal for frequency conversion is non-waveguiding for the fundamental electromagnetic radiation. In the sense of the present application, the term non-waveguiding for the fundamental electromagnetic radiation means that the nonlinear optical crystal has a lateral extension, i.e. an extension in a direction perpendicular to the axis of propagation of the fundamental electromagnetic radiation, being larger than the wavelength of the fundamental frequency, in particular at least twice as large as the wavelength of the fundamental frequency and there is no refractive index variation in the transversal direction, i.e. perpen- dicular to the axis of propagation.

In an embodiment of the present invention, the periodically poled nonlinear optical crystal has a poling period, wherein the poling period is chosen such that the crystal can be used for any nonlinear optical process chosen from a group consisting of second harmonic generation (SHG), third harmonic generation (THG), fourth harmonic generation (FHG), difference frequency generation (DIFF F.), sum frequency generation (SUM F.), and Raman conversion or any combination thereof.

In another embodiment, the nonlinear optical crystal comprises a first section having a first poling period and a second section having a second poling period, wherein the first poling period and the second poling period are chosen such that the first section supports a first conversion of the fundamental electromagnetic radiation into an intermediate electromagnetic radiation and such that the second section supports a second conversion of the intermediate electromagnetic radiation into the converted electromagnetic radiation. It is apparent that the first conversion and the second conversion are chosen from a group consisting of second harmonic generation (SHG), third harmonic generation (THG), fourth harmonic generation (FHG), difference frequency generation (DIFF F.), sum frequency generation (SUM F.), and Raman conversion. In a particular embodiment, the first conversion and the second conversion are second harmonic generation processes. Thus, the converted electromagnetic radiation has a converted frequency being the fourth harmonic of the fundamental frequency. In another embodiment, the first conversion is a second harmonic generation process and the second conversion is a sum frequency generation process of the fundamental electromagnetic radiation and the intermediate electromagnetic radiation to generate the converted radiation. In this case, the converted electromagnetic radiation has a converted frequency being the third harmonic of the fundamental frequency.

In order to make the acceptance bandwidth of a periodically poled nonlinear optical crystal wider, in an embodiment the poling periods are chirped. In an embodiment of the invention, at least one of the end surfaces of the periodically poled nonlinear optical crystal is coated with a dielectric anti-reflection coating or with a dielectric high reflective coating in order to control the paths of the fundamental electromagnetic radiation, the intermediate electromagnetic radiation and the converted electromagnetic radiation.

In an embodiment of the invention, the nonlinear crystal has a section for phase matching of the reflected wave. This phase matching section can include electrodes for controlling the refractive index and thus compensating the phase among mixing waves. In an embodiment of the invention, a frequency selective element is located in the resonator to define the fundamental frequency. In a particular embodiment, the frequency selective element is located in the second cavity of the resonator. It shall be understood from the above that any elements introducing losses into the resonator can be introduced into the second cavity to alleviate detrimental effects on the required intra-cavity power density at the fundamental frequency in the first cavity accommodating the gain element.

In an embodiment of the invention, the frequency selective element is chosen for example from a group consisting of a birefringent filter, a Lyot filter, a reflection grating, a high reflecting grating, a grating waveguide mirror, a grating waveguide structure, an etalon, e.g. a Fabry-Perot etalon, and a Bragg grating or any combination thereof.

In an embodiment of the invention the frequency selective element is a Fabry-Perot etalon forming part of the coupling mirror. Expressed in other words the Fabry-Perot etalon in this embodiment is integrated into the coupling mirror. In an embodiment this Fabry-Perot etalon is formed by a first distributed Bragg reflector in direct contact with the gain element, wherein in this case the gain element preferably is a semiconductor structure. In the beam direction away from the first mirror of the resonator the first distributed Bragg reflector is followed by a spacer being transparent for the fundamental radiation, and wherein the spacer is followed by a second distributed Bragg reflector. In this embodiment the coupling mirror is effectively formed by the first and second distributed Bragg reflectors of the Fabry-Perot etalon.

In an embodiment, the frequency selective element is tunable and thus allows for a variation of the fundamental frequency and in turn of the converted frequency generated by the laser. In an embodiment, the frequency selective element is a volume Bragg grating. Volume Bragg gratings, also denoted as volume holographic gratings, consist of a volume of a material comprising a periodic change of the refractive index. In an embodiment, the volume Bragg grating will be used as the second mirror of the second cavity. In another embodiment of the present invention, the laser comprises a polarization selective element located in the resonator. As before, the polarization selective element as a lossy element in an embodiment is located in the second cavity of the resonator.

In an embodiment of the invention, the polarization selective element is chosen for example from a group consisting of a birefringent filter, a Lyot filter, a Brewster plate and a reflection grating or any combination thereof. In a particular embodiment, the polarization selective element and the frequency selective element are one and the same element. In particular, a birefringent filter, a Lyot filter or a reflection grating simultaneously act as a frequency selective element as well as a polarization selective element.

Further advantages, features and applications of the present invention will become apparent from the following description of embodiments and the corresponding figures attached.

Figure 1 is a schematic drawing of the elements forming a laser according to an embodiment of the present invention. Figure 2 is a schematic drawing of an embodiment of a laser according to the present invention.

Figure 3 is a schematic drawing of a further embodiment of a laser according to the present invention. Figure 4 is a schematic drawing of yet another embodiment of a laser according to the present invention.

Figures 5 to 9 are schematic drawings of different embodiments of the first cavity according to the present invention, wherein the first mirror, the gain element and the coupling mirror are stacked onto each other.

Figures 10 and 1 1 are schematic drawings of embodiments of the first cavity according to the present invention, wherein the heat spreader and the gain element are separated from the first mirror and the coupling mirror.

Figure 12 is a schematic drawing of a laser having an l-shaped resonator with intra-cavity second harmonic generation and a birefringent filter as a frequency and as a polarization selective element according to an embodiment of the present invention. Figure 13 is a schematic drawing of a laser having an l-shaped resonator with intra-cavity second harmonic generation and a volume Bragg grating as a frequency selective element and a Brewster plate as a polarization selective element according to an embodiment of the present invention.

Figure 14 is a schematic drawing of a laser having an l-shaped resonator with intra-cavity third harmonic generation and a volume Bragg grating as a frequency selective element and a Brewster plate as a polarization selective element according to an embodiment of the present invention. Figure 15 is a schematic drawing of a laser having a V-shaped resonator with intra-cavity second harmonic generation and a volume Bragg grating as a frequency selective element and a Brewster plate as a polarization selective element according to an embodiment of the present invention.

Figure 16 is a schematic drawing of a laser having a V-shaped resonator with intra-cavity third harmonic generation and a volume Bragg grating as a frequency selective element and a Brewster plate as a polarization selective element according to an embodiment of the present invention.

Figure 17 is a schematic drawing of a laser having a V-shaped resonator with intra-cavity third harmonic generation and a volume Bragg grating as a frequency selective element and a Brewster plate as a polarization selective element according to an embodiment of the present invention.

Figure 18 is a schematic drawing of a laser having a V-shaped resonator with intra-cavity fourth harmonic generation and a volume Bragg grating as a frequency selective element and a Brewster plate as a polarization selective element according to the present invention.

Figure 19 is a schematic drawing of a laser having a V-shaped resonator with intra-cavity second harmonic generation and a reflection grating as a frequency and polarization selective element according to an embodiment of the present invention. Figure 20 is a schematic drawing of a laser having a V-shaped resonator with intra-cavity third harmonic generation and a reflection grating as a frequency and polarization selective element according to an embodiment of the present invention.

Figure 21 is a schematic drawing of a laser having a V-shaped resonator with a dielectric second mirror of the second cavity and a birefringent filter as a polarization and wavelength selective element and an etalon as a frequency selective element, wherein both the second mirror of the second cavity and the folding mirror form output couplers for the converted radiation according to an embodiment of the present invention. Figure 22 is a schematic drawing of a laser having a V-shaped resonator of figure 21 , wherein the birefringent filter forms the output coupler for the converted radiation according to an embodiment of the present invention.

Figure 23 is a schematic drawing of a laser having a V-shaped resonator of figure 21 , wherein the Fabry-Perot etalon as a frequency selective element being integrated into the coupling mirror according to an embodiment of the present invention. Figure 24 is a schematic drawing of a laser having a l-shaped resonator, wherein the first cavity has two coupling mirrors according to an embodiment of the present invention.

Figure 25 is a schematic drawing of a laser having a ring resonator, wherein the first cavity has two coupling mirrors according to an embodiment of the present invention.

In the figures, identical elements have been denoted by identical reference numbers.

Figures 1 to 23 show embodiments of a laser according to the present invention comprising a single coupling mirror, whereas figures 24 and 25 show embodiments of a laser according to the present invention comprising exactly two coupling mirrors.

Figure 1 shows a schematic drawing of the elements forming a laser 1 according to the present invention. The laser 1 comprises a gain element 2, a periodically poled nonlinear optical crystal 3 and a resonator 4. In turn the resonator 4 consists of two cavities 5, 6. In the embodiment, sche- matically depicted in figure 1 , the resonator 4 consists of three mirrors, a first mirror 7, a second mirror 8 and a coupling mirror 9. While the first mirror 7 and the second mirror 8 are high reflecting mirrors for the fundamental electromagnetic radiation at the fundamental frequency, the coupling mirror 9 has a reflectivity at the fundamental frequency which is lower than the reflectivity of the first mirror 7 and the second mirror 8. While the first cavity 5 is formed between the first mirror 7 and the coupling mirror 9, the second cavity 6 is formed between the coupling mirror 9 and the second mirror 8.

In the sense of the present application the first mirror 7 is the first mirror of the first cavity 5, the second mirror 8 is the second mirror of the second cavity 6 and the coupling mirror 9 simultaneously forms the second mirror of the first cavity 5 and the first mirror of the second cavity 6. Thus, the field of the fundamental electromagnetic radiation resonates in the first cavity 5 and in the second cavity 6, while the field strength in the first cavity 5 and in the second cavity 6 is set by the reflectivity of the coupling mirror for the fundamental electromagnetic radiation. The gain element 2 is located in the first cavity 5, which consequently is also denoted as the active cavity. In contrast, the periodically poled nonlinear optical crystal 3 is located in the second cavity 6, which consequently is also denoted as the passive cavity.

The gain element 2 is optically-pumped. Thus, the laser 1 comprises a pump source 10, which during operation of the laser emits electromagnetic radiation incident on the gain element 2 in order to pump the gain element 2. The gain element 2 in turn emits electromagnetic radiation at a fundamental frequency, which is thus noted as the fundamental electromagnetic radiation. As the first mirror 7 of the first cavity 5 and the second mirror 8 of the second cavity 6 are high reflective mirrors hardly any fundamental electromagnetic radiation is emitted from the resonator 4. Instead the fun- damental electromagnetic radiation is converted in the periodically poled nonlinear optical crystal 3 into electromagnetic radiation at a converted frequency, which thus is also denoted as the converted electromagnetic radiation. The converted electromagnetic radiation in this basic configuration is emitted from the resonator 4 through the second mirror 8 of the second cavity 6. In alternative embodiments the converted electromagnetic radiation can be emitted by reflection on a surface of a dichroitic filter, a Brewster plate or a birefringent filter (see figure 23) or by transmission though a folding mirror of the second cavity (e.g. see figures 19 and 21 ).

While figure 1 schematically shows a minimum number of elements forming a laser according to an embodiment of the present invention, figures 2 to 1 1 show a number of embodiments demonstrating how the active cavity 5 may be implemented. In figures 2 to 4 both the active cavity 5 as well as the passive cavity 6 are shown. Figures 5 to 1 1 only show the active cavity 5 for simplicity of the drawings. Any of the designs of the active cavity 5 of figures 2 to 1 1 can be combined with the shapes of resonators and the design of the passive cavity 6 as described in detail below with reference to the implementations of figures 12 to 23.

In all cases described herein, it is assumed that the intracavity power of the fundamental electro- magnetic radiation 17 in the second cavity 6 is lower than in the first cavity 5.

Figures 2 to 9 show embodiments of the active cavity 5, wherein the active cavity is highly integrated in that the elements located in the active cavity 5 and forming the active cavity 5 are stacked onto each other. In contrast figures 10 and 1 1 show two embodiments, wherein the active cavity 5 has a first mirror 7 and a coupling mirror 9, which are bulk optical elements being separated from the gain medium 2. The coupling mirror 9 is the second mirror of the first cavity 5, but simultaneously forms the first mirror of the second cavity 6.

The gain medium 2 in all embodiments of figures 2 to 1 1 might alternatively be formed either by a bulk solid state disk material or a disk-shaped semiconductor structure having a resonant periodic gain, e.g. quantum well structure in the semiconductor material. The design of the resonator 4 having a first cavity 5 and a second cavity 6 allows to adjust the intra- cavity power densities of the fundamental radiation in the first cavity 5 and in the second cavity 6 independently from each other. In this way the requirement of high cavity Q to be fulfilled by the disk-shaped gain elements 2 can be satisfied while simultaneously in the same resonator 4 comparatively lossy elements, in particular a periodically poled nonlinear optical crystal 3, can be lo- cated without negatively influencing the optical-to-optical efficiency of the laser.

Only for simplification of the drawings in figures 2 to 1 1 the pumping source 10 has been omitted. Still all gain elements 2 shown in these figures are optically pumped in the sense of the present invention.

In the laser 1 ' of figure 2 the first mirror 7 of the first cavity is a Bragg reflector being in direct mechanical contact with the gain element 2. Thus the surface of the gain element 2 and a surface of the Bragg mirror 7' are in contact with each other. Furthermore, the other surface of the Bragg mirror 7' is in contact with a heat spreader 1 1 acting as a heat conducting element in the sense of the present application used to avoid negative thermal effect on the gain element 2 and the Bragg mirror 7'. In the embodiment of figure 2 the coupling mirror 9 is realized in bulk optics, e.g. is a dielectric mirror. The coupling mirror 9 is designed as a concave mirror to focus the fundamental radiation into the gain element 2. In the embodiment of the laser 1 " of figure 3, the bulk optics coupling mirror 9 has been replaced by a further Bragg reflector 9', which is in direct contact with the other surface of the gain element 2. Thus in the embodiment of the laser 1 " of figure 3 the first Bragg reflector 9' forming the coupling mirror, the gain element 2, the second Bragg reflector 7' forming the first mirror of the resonator 4 and thus of the first cavity 5 and the heat spreader 1 1 are stacked onto each other.

This highly integrated concept of the first or active cavity 5 is maintained in the embodiment of the laser 1 "' of figure 4. Instead of using the second mirror 8 of the second cavity 6 for focusing the fundamental radiation into the gain element 2 the second mirror 8 in the embodiment of figure 4 is a planar mirror and a focusing or stabilization lens 12 has been introduced in the passive cavity 6 in order to provide the required focusing into the gain element 2 and into the periodically poled crystal 3. The implementation of the active cavity 5 of figure 5 depicts the same design as shown for the embodiment of figures 3 and 4. Thus figure 5 represents an enlarged view of the active cavity 5 of figures 3 and 4.

In figures 6 to 9 the same concept for the active cavity 5 stacking the different elements onto each other is followed. However, the number of elements is varied and the order of stacking these elements onto each other is varied, too.

In the embodiment of the active cavity 5 shown in figure 6 the heat spreader 1 1 is stacked onto the surface of the Bragg reflector 9' forming the coupling mirror.

In the embodiment of figure 7 the heatspreader 1 1 is located between the Bragg reflector 9' forming the coupling mirror again and the gain element 2.

In order to achieve a better temperature stabilization of the gain element 2 the embodiment of figure 8 comprises two heat spreaders 1 1 , 13, wherein the gain element 2 is sandwiched between the two heat spreaders 1 1 , 13. The Bragg reflector forming the first mirror T of the first cavity 5 is bonded onto the first heat spreader 1 1 , while the Bragg reflector 9' forming the coupling mirror is stacked onto the second heat spreader 13. In the embodiment of figure 9 the arrangement of the active cavity 5 still comprises two heat spreaders 1 1 , 13, however, only the heat spreader 13 is in direct contact with the surface of the gain element 2. The Bragg reflector T forming the first mirror of the first cavity 5 is located between the gain element 2 and the heat spreader 1 1. Thus one may argue that the heat spreader 1 1 is located outside the actual active cavity 5 in the sense of the present application.

In the embodiments of figures 10 and 1 1 the first mirror 7 and the coupling mirror 9 of the first cavity 5 are formed by dielectric mirrors in bulk optics, wherein the gain element 2 and the heat spreader (1 1 , 13) are located in the free space between the two mirrors 7, 9. In the embodiment of figure 10 a single heat spreader 1 1 is stacked onto the gain element 2. In the embodiment of the active cavity 5 of figure 1 1 the gain element 2 is sandwiched between two heat spreaders 1 1 , 13.

Figures 12 to 22 show implementations of the laser, which all make use of the same design of the active cavity 5 but vary with respect to the implementation of the passive cavity 6. All lasers of figures 12 to 22 comprise a sandwiched structure of a gain element 2 between a first Bragg reflector 9' forming the coupling mirror of the two cavities 5, 6 and a second Bragg reflector 7' forming the first mirror of the first cavity 5 of the resonator 4. In order to give an example, the gain element 2 of all implementations according to figures 12 to 23 is assumed to be a disk-shaped semiconductor structure with a resonant periodic gain. The first mirror 7' of the first cavity 5 is bonded onto a heat spreader 1 1 , which in turn is mounted on a heat sink 14. The arrangement of the active cavity 5 of figures 12 to 22 could easily be replaced by any of the arrangements as depicted with reference to figures 2 to 1 1 or in figure 23.

In the implementations of figures 12 to 14, a lens 15 is schematically depicted in order to indicate that the gain element 2 is optically pumped by electromagnetic pump radiation 16. This pumping lens has been omitted in figures 15 to 25 for simplification. In all figures the fundamental electromagnetic radiation is depicted by a dotted line 17. The converted radiation is depicted as a dashed line (18, 126, 164).

Below, an emphasis is put on a description of the design of the second or passive cavity 6 of the lasers according to figures 12 to 23. The second cavities 6 of figures 12 to 23 differ from each other with respect to principal cavity geometry, a frequency selective element used in order to tune the fundamental frequency and thus the converted frequency, the design of the periodically poled nonlinear optical crystal(s), implementation of the second mirror of the second cavity of the resonator 4, and the polarization selective element used to control the polarization of the fundamental electromagnetic radiation 17. Figure 12 shows an implementation of the laser 100 with a linear l-shaped resonator. The lasers 1 10, 120 of figures 13 and 14 have the same principal resonator geometry. In contrast the lasers 130, 140, 150, 160, 170, 180, 130', 130", 130"' of figures 15 to 23 have a linear V-shaped resonator geometry. In the laser 100 of figure 12 the second mirror of the second cavity 6 is implemented as a high reflecting dielectric mirror 8' coated onto the end facet 104 of the periodically poled nonlinear optical crystal 103. The poling period of the periodically poled nonlinear optical crystal 103 is chosen such that the crystal 103 converts the fundamental electromagnetic radiation 17 into converted electromagnetic radiation 18, wherein the converted radiation has twice the frequency of the fundamental radiation. Thus, the crystal 103 is a crystal for second harmonic generation.

In addition, the coating forming the second mirror 8' of the second cavity 6 is an anti-reflection coating for the converted SHG radiation 18. The opposite facet 105 of the crystal 103 also carries dielectric coating 107 being high reflective for the converted radiation 18 and providing an anti- reflection coating for the fundamental radiation 17.

Furthermore, the laser 100 in the second cavity 6 comprises a birefringent filter 106 acting as a frequency selective element in the sense of the present application in order to define the fundamental frequency of the fundamental electromagnetic radiation 17 generated in the laser. At the same time the birefringent filter 106 acts as a polarization selecting element in the sense of the present application as it controls the polarization of the mode of the fundamental electromagnetic radiation generated in the laser. Rotating the birefringent filter 106 allows to tune the generated fundamental frequency and thus in turn the frequency of the converted radiation 18.

The birefringent filter 106 as well as the frequency selective elements and polarization selective elements of the upper embodiments according to figures 13 to 23 are located in the second cavity 6 in order to alleviate the influence of their losses on the radiation oscillating in the gain element 2.

The major difference of the laser 1 10 of figure 13 when compared to the laser 100 of figure 12 is that the second mirror of the second cavity 6 of the laser 1 10 is formed by a volume Bragg grating 8" instead of the high reflecting coating forming the second mirror 8' of the second cavity 6 in the laser 100 of figure 12. Consequently, the coating 1 18 on the end facet 104 of the crystal 103 of the laser 1 10 is an anti-reflection coating for the fundamental frequency as well as for the converted second harmonic frequency. The coating 107 on the opposite end facet 105 and the crystal 103 is identical to the coating 107 of figure 12, i.e. it is an anti-reflecting coating for the fundamental frequency and high reflective coating for the second harmonic converted frequency. In order to pin the polarization of the intra-cavity mode of the fundamental radiation 17 in the cavity 6 a Brewster plate 19 as a polarization selective element is located in the second cavity 6. This polarization selective element is required as the frequency selective element of laser 1 10 is formed by the second mirror 8" of the second cavity, wherein this frequency selective element is not polarization selective at the same time.

The volume Bragg grating 8" forming the second mirror of the second cavity is a Bragg grating structured into a block of material, wherein the periodicity of the grating experienced by the fundamental electromagnetic radiation 17 can be tuned to a certain extent by changing the temperature of the volume Bragg grating.

Like the crystal 103 of the laser 100 of figure 12 the periodically poled nonlinear optical crystal 103 of figure 13 is a crystal for generating the second harmonic of the fundamental frequency. The laser 120 of figure 14 differs from the embodiment of the laser 1 10 of figure 13 only with respect to the design of the periodically poled nonlinear optical crystal 123. The crystal 123 comprises a first section 124 and a second section 125 having different poling periods. The poling period of the first section 124 has been chosen such that in the first section 124 the crystal 123 acts as a second harmonic generating crystal converting the fundamental radiation 17 into intermediate radiation having twice the fundamental frequency. The second section 125 instead has a poling period optimized for sum frequency generation, wherein the second section 125 generates converted electromagnetic radiation 120 by sum frequency mixing between the fundamental radiation 17 and the intermediate radiation generated in the first section 124. Thus, the output of the crystal 123 as the converted electromagnetic radiation 126 is a third harmonic when compared to the fundamental radiation 17.

The laser 130 of figure 15 differs from the laser 1 10 of figure 13 in that instead of the l-shaped linear cavity it has a V-shaped linear cavity comprising a cavity folding mirror 131 replacing the stabilization lens 12 of the laser 1 10. The same difference is to be noted between the laser 120 of figure 14 and laser 140 of figure 16.

The volume Bragg gratings 8" of the lasers of figures 13 to 18 have anti-reflection coatings for the fundamental electromagnetic radiation 17 as well as for the converted electromagnetic radiation 18 and 126.

In the laser 130' of figure 21 the volume Bragg grating 8" has been replaced by a dielectric mirror 8 and the Brewster plate 19 has been replaced with a birefringent filter 106. Further the laser 130' comprises an additional etalon 132. The etalon 132 is an etalon in bulk optics located in the second cavity in order to provide a frequency selective element in the sense of the present invention. Output coupling of the frequency converted radiation 18 in this resonator is provided through the second mirror 8 of the second cavity and through the folding mirror 131. The second mirror 8 of the second cavity and the folding mirror 131 are transparent for the converted radiation 18. The laser 130" of figure 22 shows yet another way of coupling the frequency converted radiation 18 out of the resonator. The second mirror 8 of the second cavity, the folding mirror 131 as well as the birefringent filter 106 have high reflective coatings for the frequency converted radiation. Thus, the frequency converted radiation 18 is coupled out of the second cavity by reflection from the birefringent filter 106, which is oriented under an angle with respect to the direction of propagation of the frequency converted radiation 18. The laser 130"' of figure 23 has a V-shaped resonator similar to the one of figure 21. However, the bulk etalon 132 has been replaced by a Fabry-Perot etalon 132' formed on top of the gain element 2 and integrated with the coupling mirror. The Fabry-Perot etalon 132' is formed by a first distributed Bragg reflector 133 in direct contact with the gain element 2, a spacer 134 being transparent for the fundamental radiation, and a second distributed Bragg reflector 135. In this embodiment the first cavity 5 resonates between the first mirror 7' and the first Bragg reflector 133 of the etalon 132' and the second cavity resonates between the second Bragg reflector 135 and the second mirror 8. The two cavities are coupled via the etalon 132'. Thus, the etalon 132' forms part of the coupling mirror in the sense of the present invention. The coupling mirror is formed by the first and second distributed Bragg reflectors 133, 135 of the Fabry-Perot etalon 132'.

In the laser 150 of figure 17 the periodically poled nonlinear optical crystal 123 of figure 16 has been replaced by two separate and distinct periodically poled nonlinear crystals 151 , 152. The first crystal 151 replaces the first section 124 of the crystal 123 of figure 16 and the second crystal 152 replaces the second section 125 of the crystal 123 of figure 16. Thus, the first crystal 151 has a poling period chosen in order to convert the fundamental electromagnetic radiation 17 into second harmonic generation forming an intermediate electromagnetic radiation 153 in the sense of the present application having twice the fundamental frequency. The first crystal 151 thus is a second harmonic generating crystal. The second crystal 152 has a poling period chosen in order to be optimized for sum frequency generation between the intermediate radiation 153 generated by frequency conversion in the first crystal 151 and the fundamental radiation 17. The output 126 thus is again the third harmonic of the fundamental radiation 17. The dielectric coating 154 at the first end of the first crystal 151 is anti-reflecting for the fundamental radiation and high reflecting for the second harmonic radiation 153. The dielectric coating 155 on the second end of the first crystal 151 is anti-reflecting for the fundamental radiation 17 as well as for the intermediate second harmonic radiation 153. The dielectric coating 156 on the first end of the second crystal 152 is anti- reflecting for the fundamental radiation 17 as well as for the intermediate radiation 153. The dielectric coating 157 on the second end of the second crystal 152 is anti-reflecting for the fundamental radiation 17 as well as for the third harmonic radiation 126 as the converted radiation in the sense of the present application.

Figure 18 shows a laser 160, wherein a single periodically poled nonlinear optical crystal 161 in the second cavity 6 is used to generate converted electromagnetic radiation 164 being the fourth harmonic of the fundamental radiation 17. The crystal 161 comprises a first section 162 having a poling period being chosen for optimized second harmonic generation. This first section 162 generates intermediate electromagnetic radiation in the sense of the present application by frequency doubling the fundamental radiation 17. The second section 163 has a poling period chosen for optimized second harmonic generation from the intermediate radiation. Thus, the outputted converted radiation 164 is a fourth harmonic of the fundamental radiation 17.

The dielectric coating 166 on the second end facet 104 of the crystal 161 is anti-reflecting for the fundamental radiation 17 as well as for the fourth harmonic radiation 164.

The laser 170 of figure 19 has a design, which is fairly similar to the design of the laser 130 of figure 15. However, the volume Bragg grating 8" forming the second mirror of the second cavity as well as the frequency selective element in the sense of the present application has been replaced by a high reflection grating 172.

This high reflection grating 172 serves as a frequency selective element as well as a polarization selective element. Consequently, the Brewster plate 19 of the design of the laser 130 of figure 15 can be omitted. The frequency of the fundamental radiation 17 can be tuned by tilting the normal axis of the high reflection grating 172 with respect to the direction of incidence of the fundamental radiation 17 on the grating 172. The periodically poled nonlinear optical crystal 103 has a poling period optimized for second harmonic generation. Consequently, the converted radiation 18 out- putted from the resonator is a second harmonic of the fundamental radiation 17. As the high reflective grating 172 cannot be used as an output coupler for the converted radiation 18, the folding mirror 131 of the laser 170 is transparent for the converted radiation 18 and the coatings 1 18 and 107 have been chosen for an optimized emission of the converted radiation 18 through the folding mirror 131. Thus, the dielectric coating 1 18 on the first end facet 105 of the nonlinear optical crystal 103 is anti-reflecting for the fundamental radiation 17 as well as for the second harmonic converted radiation 18. In contrast, the dielectric coating 107 on the second end facet 104 of the crystal 103 is anti-reflecting for the fundamental radiation 17 and highly reflecting for the second harmonic converted radiation 18. The laser 180 of figure 20 applies the frequency selecting scheme of the laser 170 of figure 19 using a reflection grating 172 to the nonlinear conversion process as described with reference to figure 16. In order to do so, the periodically poled nonlinear optical crystal 123 of the laser 180 has a first section 124 for generating second harmonic intermediate radiation from the fundamental radiation 17. This intermediate radiation then is sum frequency mixed with the fundamental radia- tion 17 in the second section 125 of the crystal 123. In order to provide the output of the converted radiation 126 being the third harmonic of the fundamental radiation 127 through the folding mirror 131 , the crystal 123 comprises correspondingly matched dielectric coatings. The dielectric coating 1 18 on the first end facet 105 of the crystal 123 is anti-reflecting for the fundamental radiation as well as for the third harmonic radiation 126. The dielectric coating 107 on the second end facet 104 of the crystal 123 is anti-reflecting for the fundamental radiation 17 while being highly reflecting for the second harmonic intermediate radiation generated in the first section 124 of the crystal 123. Figures 24 and 25 show embodiments of a laser 190, 200 according to a present invention, wherein the first cavity 5 containing the gain element 2 is formed by two coupling mirrors 9, 9'. The coupling mirror 9 forming the first mirror of the first cavity 5 is a Bragg reflector and the coupling mirror 9' forming the second mirror of the first cavity 5 is another Bragg mirror. The heat spreader 1 1 has been chosen such that it is transparent for the fundamental electromagnetic radiation 17. The non- linear optical crystal 103 located in the second cavity is configured as the crystal 103 in the laser 1 10 of figure 13.

In both designs according to figures 24 and 25 the first cavity 5 is located in a beam path of the fundamental radiation 17 between the first and second mirrors of the second cavity. Thus electro- magnetic coupling between the two cavities is effected by the two coupling mirrors 9, 9'.

In both designs according to figures 24 and 25, the arrangement of the active cavity 5 could easily be replaced by any of the arrangements as depicted with reference to figures 2 to 1 1 , with the exception that the highly reflective mirror 7 or T has to be replaced by a partially reflective mirror 9 or 9', respectively.

The second l-shaped linear cavity 6 of the laser 190 of figure 24 is formed between a first mirror 7" and a second mirror 8. The two mirrors 7", 8 are high reflectors having higher reflectivity for the fundamental electromagnetic radiation than the coupling mirrors.

In contrast, the second cavity of the laser 200 of figure 25 is a unidirectional ring cavity formed between four mirrors 131 being high reflective for the fundamental electromagnetic radiation. Two of these mirrors 131 form the first and second mirrors of the second cavity in the sense of the present application. In order to support a single direction of propagation, only, the second cavity comprises an optical isolator 201 .

For purposes of original disclosure, it is pointed out that all features which are apparent for a person skilled in the art from the present description, the figures and the claims, even if they have only been described with further features, could be combined on their own or together with all the com- binations of the features disclosed herein, if not excluded explicitly or technically impossible. A comprehensive explicit description of all possible combinations of features is only omitted in order to provide readability of the description. While the disclosure has been described with respect to a limited number of embodiments, it will be understood that the disclosure is not limited to those embodiments. Other embodiments comprising various changes do not depart from the scope of the disclosure. In particular, the description of preferred embodiments shall not be understood to be limited to what is explicitly shown and described in the specification and drawings but shall encompass the disclosure of the specification and drawings as a whole.

Reference numbers

I , 1 ', 1 ", Γ', 100, 1 10, 120, 130, 130", 130", 130", 130"' laser

140, 150, 160, 170, 180, 190, 200 laser

2 gain element

3, 103, 123, 151 , 152, 161 periodically poled nonlinear optical crystal

4 resonator

5 first cavity

6 second cavity

7 first mirror of the first cavity

T Bragg reflector as first mirror of the first cavity

7" Bragg reflector as first mirror of the second cavity

8 second mirror of the second cavity 8' dielectric coating as second mirror of the second cavity

8" volume Bragg grating as second mirror of the second cavity

9 coupling mirror

9' Bragg reflector as coupling mirror 10 pump source

I I , 13 heat spreader

12 stabilization lens

14 heat sink

15 focusing lens

16 electromagnetic pump radiation 17 fundamental electromagnetic radiation

18, 126, 164 converted electromagnetic radiation

19 Brewster plate

104 end facet of the crystal 103, 123, 161

105 opposite end facet of the crystal 103,

123, 161

106 birefringent filter

107, 1 18, 154, 155, 156, 157, 166, dielectric coating 124 first section of the crystal 123 125 second section of the crystal 123 131 cavity folding mirror

132, 132' Fabry-Perot etalon

133 first distributed Bragg reflector

134 spacer

135 second distributed Bragg reflector

151 first periodically poled nonlinear optical crystal

152 second periodically poled nonlinear optical crystal

153 intermediate radiation

162 first section of the crystal 161 163 second section of the crystal 161 172 high reflection grating

201 optical isolator

Claims

C l a i m s

A laser (1 , 1 ', 1 ", 1 "', 100, 1 10, 120, 130, 130', 130", 130"' 140, 150, 160, 170, 180, 190,

200) comprising

a resonator (4) having a first cavity (5) and a second cavity (6),

an optically pumped gain element (2) arranged to generate fundamental electromagnetic radiation (17) at a fundamental frequency and located in the first cavity (5), and

a periodically poled nonlinear optical crystal (3, 103, 123, 151 , 152, 161 ) arranged to convert the fundamental electromagnetic radiation (17) into converted electromagnetic radiation (18, 126, 164) having a converted frequency and located in the second cavity (6),

wherein the resonator (4) is resonant for the fundamental electromagnetic radiation (17), and

wherein the first cavity (5) and the second cavity (6) are electromagnetically coupled.

The laser (1 , 1 ', 1 ", 1 "', 100, 1 10, 120, 130, 130', 130", 130"', 140, 150, 160, 170, 180, 190, 200) according to claim 1 , wherein the first cavity (5) comprises at least a first mirror (7, 7', 9) and a second mirror (9, 9'), wherein at least the second mirror is a coupling mirror, and wherein the second cavity comprises at least a first mirror and a second mirror, wherein the coupling mirror (9, 9') is partly transmissive for the fundamental electromagnetic radiation (17) providing an electromagnetic coupling between the first cavity (5) and the second cavity (6).

The laser (190, 200) according to claim 2, wherein the first mirror (9) of the first cavity (5) is a coupling mirror, wherein the coupling mirror (9) is partly transmissive for the fundamental electromagnetic radiation (17) providing an electromagnetic coupling between the first cavity (5) and the second cavity (6), and wherein the first mirror (9) and the second mirror (9') of the first cavity (5) are located in a beam path between the first mirror (7") and the second mirror (8) of the second cavity (6), wherein the first mirror (7") and the second mirror (8) of the second cavity (6) have a higher reflectivity for the fundamental electromagnetic radiation than the first mirror (9) and the second mirror (9') of the first cavity (5).

The laser (1 , 1 ', 1 ", 1 "', 100, 1 10, 120, 130, 130', 130", 130"', 140, 150, 160, 170, 180) according to claim 2, wherein the second mirror (9, 9') of the first cavity (5) is the first mirror of the second cavity (6), and wherein the first mirror (7, 7') of the first cavity and the second mirror of the second cavity (6) have a higher reflectivity for the fundamental electromagnetic radiation (17) than the coupling mirror (9, 9').

The laser (1, 1', 1", 1"', 100, 110, 120, 130, 130', 130", 130"', 140, 150, 160, 170, 180) according to the previous claim, wherein the gain element (2) is in contact with the first mirror (7, 7') of the first cavity.

The laser(1, 1', 1", 1"', 100, 110, 120, 130, 130', 130", 130"', 140, 150, 160, 170, 180, 190, 200) according to any one of the previous claims, wherein the gain element (2) is a disk of a solid state material or a disk of a semiconductor structure.

The laser(1, 1', 1", 1"', 100, 110, 120, 130, 130', 130", 130"', 140, 150, 160, 170, 180, 190, 200) according to any one of the previous claims, wherein the gain element (2) is a semiconductor resonant periodic gain structure.

The laser(1, 1', 1", 1"', 100, 110, 120, 130, 130', 130", 130"', 140, 150, 160, 170, 180, 190, 200) according to any one of the previous claims, wherein the gain element (2) is mounted in thermal contact with a heat conducting element (11, 13) or a heat absorbing element.

The laser(1, 1', 1", 1"', 100, 110, 120, 130, 130', 130", 130"', 140, 150, 160, 170, 180, 190, 200) according to the previous claim, wherein the first mirror (7') of the first cavity is located between the gain element (2) and the heat conducting element (11, 13) or heat absorbing element. 10. The laser (1, 1', 1", 1"', 100, 110, 120, 130, 130', 130", 130"', 140, 150, 160, 170, 180, 190, 200) according to any one of the previous claims, wherein the nonlinear optical crystal (3, 103, 123, 151, 152, 161) is non-waveguiding for the fundamental electromagnetic radiation (17). 11. The laser (1, 1', 1", 1"', 100, 110, 120, 130, 130', 130", 130"', 140, 150, 160, 170, 180, 190, 200) according to any one of the previous claims, wherein the nonlinear optical crystal (3, 103, 123, 151, 152, 161) has a poling period, and wherein the poling period is chosen such that the nonlinear optical crystal (3, 103, 123, 151, 152, 161) supports a conversion of the fundamental electromagnetic radiation (17) into the converted electromagnetic radiation (18, 126, 164) chosen from a group consisting of second harmonic generation, third harmonic generation, sum frequency generation, difference frequency generation, optical parametric amplification, and Raman conversion or any combination thereof. The laser (1 , 1 ', 1 ", 1 "', 100, 1 10, 120, 130, 130', 130", 130"', 140, 150, 160, 170, 180, 190, 200) according to any one of the previous claims, wherein the nonlinear optical crystal (3, 123, 161 ) comprises a first section (124, 162) having a first poling period and a second section (125, 161 ) having a second poling period, wherein the first poling period and the second poling period are chosen such that the first section supports a first conversion of the fundamental electromagnetic radiation (17) into an intermediated electromagnetic radiation and such that the second section supports a second conversion of the intermediate electromagnetic radiation (17) into the converted electromagnetic radiation (18, 126, 164).

The laser (1 , 1 ', 1 ", 1 "', 100, 1 10, 120, 130, 130', 130", 130"', 140, 150, 160, 170, 180, 190, 200) according to any one of the previous claims, wherein a frequency selective element (8", 106, 132, 132', 172) is located in the resonator (4) to define the fundamental frequency.

The laser (1 , 1 ', 1 ", 1 "', 100, 1 10, 120, 130, 130', 130", 130"', 140, 150, 160, 170, 180, 190, 200) according to the previous claim, wherein the frequency selective element is chosen from a group consisting of a birefringent filter (106), a Lyot filter, a reflection grating (172), a grating waveguide mirror, a grating waveguide structure, an etalon (132, 132'), a volume Bragg grating (8") or any combination thereof. 15. The laser (1 , 1 ', 1 ", 1 "', 100, 1 10, 120, 130, 130', 130", 130"', 140, 150, 160, 170, 180, 190, 200) according to any one of the previous claims, wherein a polarization selective element (19, 106, 172) is located in the resonator (4).