Monolithic solid-state laser

Abstract

The invention relates to a monolithic solid-state laser (10, 110), comprising a laser resonator (12) including a gain medium (14), a holder (22, 122) supporting the laser resonator, and a pusher (30, 130). The laser resonator comprises a self-supported portion (28b, 128), e.g., a cantilever portion or a bridge portion, and the pusher is arranged for applying a transverse load on the self-supported portion (28b, 128).

Description

MONOLITHIC SOLID-STATE LASERBackground of the Invention

[0001] The invention generally relates to a monolithic solid-state laser. The invention further relates to a device for treating or ablating biological tissue comprising a monolithic solid-state laser.

[0002] Lasers have become versatile and indispensable tools in many fields of technology. Nevertheless, for industrial applications, the reliability of lasers can still be improved. Solid-state lasers (SSL) represent the dominant laser design due to their compactness and low operational costs compared to, e.g., gas lasers or dye lasers. Solid-state lasers use solid-state gain media, such as, e.g., crystals or glasses doped with transition-metal or rare-earth ions. Also, semiconductor lasers belong to the solid- state lasers, but the invention relates to solid-state lasers that are neither waveguide lasers (e.g., fiber lasers) nor semiconductor lasers (e.g., diode lasers).

[0003] A solution for building rugged, high-power, solid-state lasers is to fabricate the whole laser resonator as a monolithic structure without air gaps. These monolithic lasers are intrinsically stable and reliable. In particular, misalignment concerns are minimized, and the closed cavity is immune to contamination.

[0004] Monolithic solid-state lasers provide a benefit in almost every aspect of laser engineering, because they combine positive characteristics that are challenging to realize with other laser designs. An accurately designed monolithic structure usually results in a compact and stable setup that allows very reliable laser operation.

[0005] Additionally, a monolithic laser can be very price-competitive due to the not required time-consuming alignment step allowing for semi-automated production, which is the case for nearly all semiconductor lasers. During the production of a laser system, the most time-consuming process step is to get the setup to lase at all, even far-off the optimum operation point. Afterwards, the consecutive optimization procedure is straight forward and quickly performed. An advantage of a monolithic laser is that such a system lases directly from the start, and, therefore, the tedious initial setup procedure is avoided. Unfortunately, an optimization of the performance of such a laser is usually not possible due to the missing adjustment possibilities. Minor adjustments may be performed by applying external fields and heat, but the options are very limited.

[0006] WO 2010 / 145802 discloses a monolithic, side pumped solid-state laser, comprising a laser resonator structure comprised of a laser gain medium, the laser resonator structure having end faces forming a linear optical path resonant cavity there between. At least one of the end faces comprises an at least partially reflecting laser mirror. The laser gain medium receives pump light from at least one diode laser via a side face. The laser further comprises a conductive cooler having contact faces that contact the laser gain medium. A pump light reflector is arranged opposite to the side face with respect to the longitudinal axis of the gain medium in order to improve the utilization of the pump light and to achieve a more homogeneous pump power density distribution within the gain medium.

[0007] However, achieving an optimal pump power density distribution remains a difficult task. Furthermore, even if the pump power density distribution is designed with great care, it may nevertheless happen that a monolithic solid-state laser does not operate in optimal condition. One may illustrate this by considering the example of a simple monolithic laser consisting of a linear laser cavity formed by a solid material (e.g., YAG) in a rod-like shape with a high aspect ratio (length-to-diameter ratio). On average, the laser light travels a certain distance (or a certain number of roundtrips) within the cavity before it leaves the cavity through the outcoupling mirror. If 10% of intracavity power is coupled out, the average distance corresponds to 10 roundtrips and in case of 20% outcoupling rate, the average distance is 5 roundtrips. Any deviation from the ideal alignment of the laser cavity results in a loss of photons due to multiple reflections and to the limiting aperture of the gain medium. The lower the outcoupling rate, the larger is the average number of roundtrips and therefore, the higher is also the sensitivity of the laser cavity to misalignment. For this reason, lasers with high gain and the resulting higher optimum outcoupling rates, as employed in configurations such as semiconductor lasers or fiber lasers, are less sensitive to misalignment of the mirrors. Additionally, if the linear cavity comprised a waveguide structure with a certain numerical aperture, the waveguide structure would reduce the loss of photons due to misalignment. Nevertheless, in a laser cavity without a waveguide, the mechanically most critical parameter affecting the laser mode is the correct alignment of the highly reflective mirror and of the outcoupling mirror, regardless of the pumping scheme.Summary of the Invention

[0008] In a first aspect, the invention relates to a monolithic solid-state laser, comprising a laser resonator including a gain medium, a holder supporting the laser resonator, and a pusher. The laser resonator comprises a self-supported portion (e.g., a cantilever portion or a bridge portion) and the pusher is arranged for applying a transverse load (force) on the self-supported portion.

[0009] It will be appreciated that the pusher allows bending the self-supported portion and thus (slightly) deforming the cavity. The pusher may, therefore, be used, in particular, to optimize the cavity geometry and to reduce the loss of photons.

[0010] As used herein, the expression monolithic solid-state laser, designates a solid- state laser, the resonator of which consists of a single piece of gain medium (e.g., glass or crystal) and, possibly, other components rigidly attached to the gain medium (e.g., by diffusion bonding or by optical-contact bonding) and is closed by dielectric mirror coatings or thanks to total internal reflection. The monolithic laser design provides immunity to contamination inside the cavity, where typically the highest fluence is present. In order to achieve stability of the cavity, one can make use of curved surfaces, thermal lenses and / or other optical elements or effects.

[0011] The laser resonator preferably defines a linear optical cavity and includes a first and a second end mirror. One of the end mirrors preferably has a high reflectivity at the laser wavelength (e.g., a reflectivity of, preferably, at least 99%), whereas the other one is preferably configured as an output coupler. The laser resonator preferably has a cross-sectional area of less than 7.5 mm2. The laser resonator may be rodshaped (preferably with a circular or elliptical cross-section). Such rod may, e.g., have a diameter of less than 3 mm, preferably of less than 2 mm, and most preferably less than 1 mm. The advantage of a rod having a small diameter and / or cross-sectional area is that the threshold pump power (in absolute figures) is relatively low and that a diode laser (array) can thus be used to optically pump the gain medium. The length of the laser resonator preferably lies in the range from 1 cm to 4 cm, more preferably in the range from 1.5 cm to 3.5 cm. Preferably, the laser resonator has a length-to- diameter ratio in the range from 10:1 to 60:1. According to a preferred embodiment, the laser resonator comprises a laser rod having a length of 2.1 cm and a diameter of 0.8 mm.

[0012] Preferably, the self-supported portion comprises a free end (a cantilever end portion) of the laser resonator. The pusher may in that case be used to correct misalignment of the mirror carried by the free end (i.e. , the output coupler or the highly reflective mirror.) It is worthwhile noting that one or both ends of the laser resonator may be configured as self-supporting portions (free ends). The monolithic solid-state laser may comprise a pusher at each free end of the laser resonator for applying a transverse load. If the monolithic solid-state laser comprises plural pushers, they may be arranged so as to apply loads in different transversal directions (e.g., mutually orthogonal transversal directions) and thus allowing correction of misalignment in more than one direction.

[0013] The length of the self-supported portion preferably amounts to between 0.5 and 3.0 times the (transversal) diameter of the laser resonator. More preferably, the length of the self-supported portion amounts to between 1.0 and 2.0 times the (transversal) diameter of the laser resonator. It is worthwhile noting that in case of a resonator rod having a diameter of about 0.8 mm, a transversal load of the order of a few Newtons may be sufficient to optimize the performance (in particular in terms of output power and / or beam quality).

[0014] The holder may comprise or be configured as a thermally conductive cooler. Such a cooler may comprise an active or passive heat exchanger that transfers the heat generated within the laser resonator to the environment or to a fluid medium (coolant) circulating in the cooler.

[0015] The monolithic solid-state laser may be optically side-pumped, e.g., using one or more flash lamps, or, more preferably, one or more laser diodes as the pump source(s).

[0016] The pump source(s) may, preferably, be arranged alongside the gain medium. The monolithic solid-state laser may include one or more pump light reflectors to (re-)inject pump light not absorbed by the gain medium (again) into the latter and thereby improve the overall efficiency of the optical pumping. The pump source(s) and the pump light reflector(s) may be arranged to create a (nearly) homogenous pump power density distribution within the gain medium.

[0017] The pusher preferably comprises an actuator for exerting and / or adjusting the transverse load on the self-supported portion.

[0018] The actuator may, e.g., comprise a linear actuator. Examples of linear actuators include an adjustment screw, a micrometer screw, a piezoelectric linear actuator, and / or an electromechanical linear actuator.

[0019] According to a preferred embodiment, the pusher comprises a flexible linkage for converting a displacement of the actuator into the transverse load on the selfsupported portion by elastic deformation (of the flexible linkage). The flexible linkage could, e.g., comprise a spring member pushing on the self-supported portion. An advantage of the use of a flexible linkage is that it may be designed so as to limit the maximum load that is exerted on the self-supported portion and thus to avoid mechanical failure.

[0020] Due to the high sensitivity of such a monolithic solid state (non-waveguide) laser resonator to the correct alignment of the mirrors, a pusher as used in accordance with the invention may be highly effective for optimizing laser performance. The pusher may mechanically deform the laser resonator (e.g., by tilting an end face), which results in a change of the laser mode(s). Since the sensitivity of the laser cavity to misalignment depends on the outcoupling rate, the invention is most effective for 20% and lower outcoupling rates.

[0021] When tested, this optimization step turned out to be very quick and not comparable to the time-consuming mirror alignments of a laser cavity built from discrete elements. The reason for this is that the monolithic solid state laser can be produced ready-to-use, and that the pusher merely serves to compensate for imponderabilities of the production process (within the tolerances of the same) which lead to maybe acceptable but less than optimal performance.

[0022] It may be advantageous to design the system slightly off the optimal operation point in order to have a defined adjustment direction. For example, the monolithic solid state laser could be designed such that the pump light density leads to a thermal lens that is slightly offset from the nominal resonator axis in a defined direction. In this way, one can make sure that the resonator has to be deformed in that specific direction in order to reach the maximum output power, whereas the amount of deformation (or the amount of transverse load) remains subject to the optimization process. In other words, by designing the laser in a certain way, one can eliminate an optimization variable (the direction of the applied load). The monolithic solid-state laser may thus preferably comprise an optical pump arrangement configured to inject pump light into the gainmedium generating therein a pump power density distribution that is imbalanced in a certain transversal direction. The pusher is in that case preferably arranged for applying the transverse load in that transversal direction. It should be noted that for a given laser design, the transversal direction for application of the transverse load might always be the same. This could facilitate retrofitting existing monolithic solid state lasers with a pusher.

[0023] In a further aspect, the invention relates to a treatment device, e.g., for treating or ablating biological tissue, which comprises a monolithic solid-state laser. According to an embodiment of such a treatment device, the monolithic solid-state laser may be configured to operate at a wavelength in the mid-infrared (MIR) range, e.g., in the range between 1700 nm and 3200 nm. Such a treatment device could, e.g., be used for treating, cutting or ablating biological tissue.

[0024] In the present document, the verb “to comprise” and the expression “to be comprised of’ are used as open transitional phrases meaning “to include” or “to consist at least of’. Unless otherwise implied by context, the use of singular word form is intended to encompass the plural, except when the cardinal number “one” is used: “one” herein means “exactly one”. Ordinal numbers (“first”, “second”, etc.) are used herein to differentiate between different instances of a generic object; no particular order, importance or hierarchy is intended to be implied by the use of these expressions. Furthermore, when plural instances of an object are referred to by ordinal numbers, this does not necessarily mean that no other instances of that object are present (unless this follows clearly from context). When this description refers to “an embodiment”, “one embodiment”, “embodiments”, etc., this means that the features of those embodiments can be used in the combination explicitly presented but also that the features can be combined across embodiments without departing from the invention, unless it follows from context that features cannot be combined.Brief Description of the Drawings

[0025] 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 schematic longitudinal cross-sectional view of a monolithic, side pumped solid-state laser in accordance with an embodiment of the invention;Fig. 2: is a schematic elevation view of the laser of Fig. 1 ;Fig. 3: is a schematic elevation view of a monolithic solid-state laser in accordance with a further embodiment of the invention.Detailed Description of Preferred Embodiments

[0026] Fig. 1 illustrates a monolithic, side pumped solid-state laser 10, comprising a laser resonator 12 made of a laser gain medium 14 having the shape of a rod. The laser resonator 12 extends along the longitudinal axis of the laser 10 and is, preferably, of elliptical or circular cross-section. The resonator 12 comprises a high reflector (HR) laser mirror 16a deposited on a first end face of the gain medium 14 and an output coupler (OC) laser mirror 16b deposited on the opposite, second end face of the gain medium 14. The OC mirror 16b has preferably a reflectivity in the range from 80 % to 99 %, more preferably from 92% to 98%. The laser mirrors 16a, 16b may be directly applied on the end faces of the gain medium. For certain applications, however, intracavity elements, such as, e.g., a passive element for Q-switching, or an etalon for wavelength-narrowing could be introduced between the gain medium and a laser mirror, e.g. by diffusion bonding. The laser mirrors 16a, 16b may have a convex, a concave, an aspherical convex or an aspherical concave shape. One or both of the laser mirrors 16a, 16b may have an inclination with respect to the optical axis, e.g., to compensate an asymmetry.

[0027] Examples of laser gain media include rare-earth-doped YAG (yttrium aluminium garnet), YSGG (yttrium scandium gallium garnet) or YLF (yttrium lithium fluoride). Suitable dopants include: neodymium, ytterbium, erbium, thulium, chromium and / or holmium. As particularly interesting laser gain media, one may cite, e.g., Er: YAG, Er: YSGG, Ho:YAG, Tm:YAG or Ho:Tm:YAG. The solid-state laser 10 could be configured to operate at a wavelength between 1700 nm and 3200 nm, e.g. at 2940 nm, in which case the laser would be particularly suitable for treating or ablating biological tissue.

[0028] Turning again to Fig. 1 , the gain medium 14 is side-pumped by a pump source 18. In the illustrated embodiment, the pump source 18 comprises a least one semiconductor laser, in particular a diode laser array arranged on support plate 20, such as, e.g., a heat spreader.

[0029] The solid-state laser 10 comprises a cooler 22 that holds the laser resonator 12. The laser resonator 12 is fixed between a first part 22a and a second part 22b ofthe cooler 22. The laser 10 also comprises a base plate 24, which carries the cooler 22 and the pump source arrangement. The cooler could comprise a body of thermally conductive material (e.g., copper or aluminium). Preferably, the cooler 22 is actively cooled by means of a cooling circuit or one or more fans. Depending on the application, passive cooling of the cooler 22 may, however, be sufficient.

[0030] A reflector 25 is arranged on a side of the laser gain medium 14 opposite the pump source 18. A fraction of the pump light generated by the pump source 18 that is not absorbed by the gain medium 14 and leaves the latter on the opposite side face is thus reflected on reflector 25 and reinjected into the gain medium 14. The reflector 25 may comprise one or more mirrors reflective in the wavelength range of the pump light. Additionally, or alternatively, the reflector 25 could be directly applied on the laser resonator 12 as a reflective coating. Also, the internal faces of the cooler 22 could be coated with a reflective coating to improve the power efficiency of the laser and / or the beam quality.

[0031] It should be noted that the gain medium 14 is preferably in contact with and supported by the cooler 22 on almost the entire length of the resonator. In particular, the cooler 22 may have contact faces contacting the gain medium 14, that are arranged with respect to the longitudinal axis of the laser resonator 12 such that, at least approximately, cylindrically symmetrical cooling of the laser resonator is achieved. It should be noted that the cooler 22 preferably has contact faces 26 extending over the length of the optical pumping zone. Heat transfer from the gain medium 14 to the cooler 22 may be improved by applying a thermal compound between the gain medium 14 and the cooler 6, such as, e.g., a thermal heat sink paste, or a liquid metal such as gallium, or a mixture of a liquid metal and particles of a solid metal.

[0032] The laser resonator 12 comprises one or more self-supported portions. In the illustrated embodiment, the resonator 12 has free ends 28a, 28b which are configured as cantilever portions. Free end 28a extends into a hollow space 30 within the cooler 22, whereas free end 28b protrudes to the outside of the cooler 22. The length of the free ends 28a, 28b (in longitudinal direction) preferably amounts to between 1.0 and 2.0 times the (transversal) diameter of the laser resonator 12. The laser resonator 12 could, e.g., have a diameter from 0.7 mm to 1 .2 mm and the length of each free end could be from 0.7 to 2.4 mm. The free ends 28a, 28b act as stress-reducing elements that improve lasing stability and decrease thermal lensing effects.

[0033] At the free end 28b of the laser resonator 12, the monolithic solid-state laser 10 comprises a pusher 30 for applying a transverse load. This transverse load causes a slight deformation of the laser resonator, resulting in a tilting of the end face, which can be used to correct a misalignment of the optical cavity. In case of a resonator rod having a diameter of about 0.8 mm, a transversal load of the order of a few Newtons may be sufficient to optimize the performance (in particular in terms of output power and / or beam quality).

[0034] The mechanical pusher 30 is fixed on the holder of the laser resonator 12 (i.e. , corresponding to the cooler 22 in the illustrated embodiment). As best viewed in Fig. 2, the pusher 30 comprises a flexible linkage and a linear actuator 34. The flexible linkage comprises a first leg configured as a spring member 32 that contacts the free end 28b and a second leg 36 arranged opposite the actuator 34. Both legs extend from a hub portion 38 that is pivotably mounted on the holder of the laser resonator 12. The flexible linkage is configured and arranged so as to convert a displacement of the actuator 34 into the transverse load on the self-supported portion by elastic deformation of the flexible linkage, in particular by elastic deformation of the spring member 32. The linear actuator includes an adjustment screw, e.g., a micrometer screw, for precisely adjusting the load on the free end 28b.

[0035] The invention allows bringing a monolithic, side pumped solid-state laser closer to its optimal operation point. While the greatest care can be taken to design the laser such that it works optimally, any real-life implementation of the resulting design will deviate from the ideal, theoretical situation. Non-optimal performance may be due to various parameters, e.g., misalignment of the cavity under operating conditions, pump light inhomogeneity (or more generally: any deviation from the assumed pump light density distribution), non-radial heat transfer, manufacturing tolerances, etc. Whereas previous monolithic solid-state laser offered no possibility to easily remediate the laser configuration, the invention creates an improved situation in that by mechanical adjustment of the laser cavity, laser performance can be improved.

[0036] It may be advantageous to deliberately design the system slightly off the optimal operation point in order to have a defined adjustment direction, i.e., a defined direction for application of the transverse load. For example, a laser can be designed such that the pump light distribution leads to a slightly off-axis thermal lens within the gain medium. By doing so, it may be possible to define at the laser design stage thedirection of the deformation of the laser resonator that allows reaching optimal performance, e.g., in terms of output power and / or beam quality. When the direction of the mechanical load is known in advance, the pusher including its fixation on the laser holder can be designed integrally with the laser.

[0037] Fig. 3 shows the front of a monolithic solid-state laser 110 according to a further embodiment of the invention. The laser 110 comprises a laser resonator with a cantilever end portion 128. A pusher 130 is arranged for applying a transverse load on the cantilever end portion 128. The construction of the pusher 130 is different from that of the pusher of Fig. 1 and 2 but its principle of operation is similar. The pusher 130 comprises a flexible linkage featuring a first leg configured as a spring member 132 that contacts the cantilever end portion 128 and a second leg configured as a lever arm 136. Both legs extend from a hub portion 138 that is pivotably mounted on the holder 122 of the laser resonator. The load on the cantilever end portion 128 may be adjusted by moving the lever arm 136. A retaining screw 134 allows clamping the lever arm 136. The thread of the retaining screw 134 passes through an arc-shaped groove in the lever arm 136 and is inserted into a screw hole in the holder 122. When the screw 134 is loose, the lever arm 136 may be rotated about the pivot axis and the spring member 132 follows until it gets into contact with the cantilever end portion 128. When the lever arm 136 is rotated further, the spring member 132 pushes on the cantilever end portion 128 and causes it to bend. By tightening the retaining screw 134, the pusher 130 may be locked in position when the laser 110 has reached its optimal operation point.

[0038] While specific embodiments have been described herein in detail, those skilled in the art will appreciate that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. 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

Claims1 . A monolithic solid-state laser, comprising a laser resonator including a gain medium; a holder supporting the laser resonator, wherein the laser resonator comprises a self-supported portion; and wherein the monolithic solid-state laser comprises a pusher arranged for applying a transverse load on the self-supported portion.

2. The monolithic solid-state laser as claimed in claim 1 , wherein the self-supported portion comprises a free end of the laser resonator.

3. The monolithic solid-state laser as claimed in claim 1 or 2, wherein the holder comprises a thermally conductive cooler.

4. The monolithic solid-state laser as claimed in any one of claims 1 to 3, wherein monolithic solid-state laser is optically side-pumped.

5. The monolithic solid-state laser as claimed in claim 4, comprising a pump source arranged alongside the gain medium.

6. The monolithic solid-state laser as claimed in any one of claims 1 to 6, wherein the pusher comprises an actuator for exerting and / or adjusting the transverse load on the self-supported portion.

7. The monolithic solid-state laser as claimed in claim 6, wherein the actuator comprises a linear actuator, e.g., an adjustment screw, a micrometer screw, a piezoelectric linear actuator, or an electromechanical linear actuator.

8. The monolithic solid-state laser as claimed in claims 6 or 7, wherein the pusher comprises a flexible linkage for converting a displacement of the actuator into the transverse load on the self-supported portion by elastic deformation.

9. The monolithic solid-state laser as claimed in claim 8, wherein the flexible linkage comprises a spring member pushing on the self-supported portion.

10. The monolithic solid-state laser as claimed in any one of claims 1 to 9, comprising an optical pump arrangement configured to inject pump light into the gain medium generating therein a power density distribution that is imbalanced in a certain transversal direction and wherein the pusher is arranged for applying the transverse load in said transversal direction.

11. A treatment device, e.g., for treating or ablating biological tissue, comprising a monolithic solid-state laser as claimed in any one of claims 1 to 10.

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