Patent Application
20210242656
The invention relates to a resonator mirror for an optical resonator 5 of a laser device, especially a gas laser or a slab waveguide laser with a gaseous, optically active medium and a laser device with a resonator mirror designed in such a manner.
Laser devices designed as slab waveguide lasers or slab lasers typically comprise resonators that are formed from a combination of a waveguide resonator and a positive or negative branch unstable optical resonator. In gas-filled slab waveguide lasers a carbon dioxide (CO2) containing gas mixture mostly works as an optically active medium injected into a discharge chamber formed between two flat electrodes. The gas or the gas mixture is excited by a high-frequency electromagnetic field applied between the electrodes. The discharge chamber or the optical resonator is constrained in front by reflective elements that are designed as a resonator mirror and are typically made of metal, especially copper, in case of high-performance CO2 lasers. The resonator mirrors are executed and arranged in such a way that an unstable resonator, mostly an unstable confocal resonator, is built parallel to the flat sides of the electrodes.
It is known that carbon dioxide (CO2), an optically active medium, has several frequency bands or wavelength ranges potentially suitable for laser amplification at 9.3 μm, 9.6 μm, 10.3 μm and 10.6 μm. The laser transition at 10.6 μm is usually dominant during laser amplification. However, for certain applications it has proven advantageous to use laser beams, especially those in the 9.3 μm or 9.6 μm band. To generate laser beams of these wavelengths, it is known from WO 2011/154272 A1, for example, that at least one of the electrodes constraining the discharge chamber is to be equipped with a passivation layer containing silicon dioxide (SiO2). In addition, the clearance between electrodes is adjusted in such a way that the laser beam of the 10.6 μm and 10.3 μm band experiences greater attenuation than those of the 9.3 μm or 9.6 μm band. Oscillation of longer-wave modes in the resonator can be suppressed in this manner.
Another possibility—initiating a wavelength-selective amplification of the laser beam—involves providing coated optics, in particular coated resonator mirrors, where there is greater absorption in the wavelength range to be suppressed. However, losses thus created inevitably lead to a highly localized heat input on the resonator mirrors which must also be dissipated. For this reason, the area of application of this type of coated resonator mirrors especially at high power densities is limited by their damage threshold, beyond which delaminations of the dielectric layers or mode burns for example can occur.
In principle, the wavelength selectivity of the optical resonator can also be induced by inserting other optical elements such as transmissive optical gratings or Fabry-Perot etalons. This method, however, generally requires radical design modifications to the resonator structure, especially if slab waveguide lasers are used. Moreover, it must be ensured that the laser beam propagating away from the optical axis does not damage peripheral components or endanger users of the laser device.
The task of the present invention is to specify the means for wavelength-selective modification of resonator losses, which are especially suitable for use in laser devices with high power densities.
The task is accomplished by a resonator mirror for an optical resonator with the other features of patent claim 1. Advantageous improvements of the invention are dealt with in the dependent claims.
A resonator mirror for an optical resonator of a laser device, especially a gas laser or a gas-filled slab waveguide laser, comprises a reflective surface with a structured area which spans an area of the reflective surface centered about the optical axis. The structured area has either at least one reflective surface cross-section which is offset against the reflective surface outside the structured area and parallel to the optical axis, or at least two surface cross-sections which are offset to each other and parallel to the optical axis. In both cases the offset of at least one surface cross-section with respect to the reflective surface outside the structured area or the offset of at least two surface cross-sections against each other is equal to half of a predefined wavelength or a whole multiple of half of the predefined wavelength. In other words, the at least one surface cross-section is raised or recessed with respect to the other reflective surface outside the structured area. The same also applies to the case when the structured area itself already has at least two reflective surface cross-sections. The surface cross-sections here can also be recessed or raised in steps with respect to each other.
The principle underlying the invention is based on multiple-beam interference of laser beams circulating in the optical resonator during laser amplification. At least one step, whose height is equal to half the wavelength to be selected, is introduced into the reflective surface of the resonator mirror constraining the optical resonator. Thus the sub-beams reflected on the surface cross-sections offset against each other, then overlap precisely and constructively when they possess the desired wavelength. For other wavelengths there is no completely constructive interference or the propagation direction of the reflected spherical waves changes marginally, and so they propagate away from the optical axis and ultimately exit the resonator or are absorbed by an element constraining the optical resonator. With this in mind, the resonator mirror is equipped with a structure which introduces wavelength-dependent losses of a “geometric nature”. The beam of the undesirable wavelength range, therefore, must not be absorbed by the resonator mirror. In fact this beam component is reflected at an angle to the optical axis so that the optical resonator for these wavelengths experiences additional circulation losses.
It goes without saying that in terms of generation of structural interference in the selected wavelength range it is irrelevant whether the offset between the reflective surfaces or surface cross-sections is equal to half the wavelength to be selected or a whole multiple thereof. What is important is that the offset is along or parallel to the optical axis.
Since numerous circulations typically take place in the optical resonator during laser amplification, ideally only one single step is needed to introduce sufficiently large losses in the wavelength range or wavelength ranges to be suppressed so that the net amplification, adjusted for losses, for the selected wavelength is greater than that in the other wavelength ranges. This takes place, for example, when the reflective surface cross-section is offset with respect to the reflective surface outside the structured area. Another possibility is to offset at least two reflective surface cross-sections of the structured area against each other. The reflective surface or surface cross-sections run at least approximately parallel to each other, that is, they can have curvatures marginally deviating from each other if necessary. During stationary operation with saturated amplification, the laser device runs on the desired or the selected wavelength under any circumstances, provided the net amplification in this area is the greatest.
The reflective surface is preferably composed of a broadband reflective metal, especially gold, silver, chromium, nickel, aluminum, copper or molybdenum, or an alloy containing a broadband reflective metal. Apart from high reflectivity, good thermal conductivity and mechanical stability are decisive factors here. Wavelength selection takes place only through the use of reflective components. Transmissive components are not needed to modify the amplification behavior of the optical resonator. This favors the use of the resonator mirror especially for applications in the high-performance range where only highly reflective resonator mirrors limit the optical resonator. In this case, the optical resonator is typically configured as an unstable resonator. Resonator mirrors composed of a broadband reflective metal or metal alloy are especially suited for use in gas lasers or slab waveguide lasers. The gas laser or slab waveguide laser in a concrete embodiment contains carbon dioxide as the optically active medium and at least one of the resonator mirrors constraining the optical resonator in front is completely made of copper. Copper exhibits good reflection properties in the relevant middle infrared range and also has good thermal conductivity so that the thermal losses arising on the resonator mirror can dissipate easily. In other embodiments the reflective surface of the resonator mirror is made of a reflective metal coating such as gold, silver, chromium or nickel, and this is applied to a substrate which is made of silicon or a carbide, especially silicon carbide or tungsten carbide, for example.
In preferred embodiments the expansion of the structured area is limited to a radially constrained region about the optical axis. In other words, the structured area inducing the constructive interference is only introduced near the optical axis. This type of design is especially suited for an unstable resonator. Since the wavelength of the beam to be amplified in the central area near the optical axis is considered a decisive factor here, it is sufficient to limit the surface structure to this area. The further advantage here is that this type of configuration minimizes additional losses in other areas of the resonator or resonator mirror. In preferred embodiments the expansion of the structured area is limited to an area about the optical axis whose diameter is only a few millimeters or is even in the submillimeter range. In comparison to the size of the reflective surface of the resonator mirror, only a very small area is occupied by the structured area, which in particular takes up less than 30%, preferably less than 15%, especially preferably 5% or less of the entire mirror surface of the resonator mirror.
The reflective surface cross-sections and/or the reflective surface outside the structured area can be level (i.e. flat). In preferred embodiments the reflective surface exhibits a concave or convex camber at least outside the structured area. In this connection, it is preferred that at least one surface cross-section or preferably all surface cross-sections disposed in the structured area have a curvature which corresponds to the curvature of the reflected surface outside the structured area, that is, the progression of the reflective surface and the reflective surface cross-sections of the structured area follows the same mathematical design conditions. In other words, the progression of the reflective surface or the surface cross-sections corresponds to the progression of a mirror surface of an unstructured mirror with the same focal length, which are offset against each other in the direction of the optical axis. In a spherically curved mirror, the progression of the reflective surface and the reflective surface cross-sections thus follows the spherically curved sections that are offset against each other along the optical axis. In other embodiments the reflective surface or the reflective surface cross-sections exhibit a parabolic curvature. The advantage in these designs is that the phase surface of the reflected beam remains unchanged precisely in the wavelength to be selected compared to a conventional mirror that has no surface cross-sections offset against each other. For all other wavelengths the resonator mirror induces a phase surface interference, causing the associated waves inside the resonator to propagate in a manner unsuitable for laser amplification.
The reflective surface outside the structured area is spherically shaped or like an elliptical paraboloid or a rotational paraboloid, for example. The reflective surfaces in the structured area are preferably sections that are offset in parallel to the surrounding reflective surface and exhibit the same curvature progression. In other words, the reflective surface and the reflective surface cross-sections of the structured area span sections of the spherically or parabolically curved design surfaces, which are offset against each other parallel to the optical axis by half the wavelength to be selected or a whole multiple of half the wavelength to be selected.
The at least one surface cross-section or the at least two surface cross-sections, for example, have a stepped, ribbed, rectangular, quadratic, circular ring and/or circular disk design. Stepped designs, for example, can have several ribs running parallel to each other parallel. The step height of this type of step structure with respect to the optical axis is equal to half the wavelength to be selected or a whole multiple thereof.
In preferred embodiments, there are several, especially three or more circular ring and/or circular disk surface cross-sections which are offset against each other concentrically. The surface cross-sections, for example, are alternately offset against each other so that all reflective surface cross-sections run parallel to only two reflection surfaces offset with respect to the optical axis. The path difference between the sub-beams reflected on the offset surface cross-sections is double the offset accordingly. Special preference is given to circular ring and/or circular disk surface cross-sections concentrically disposed against each other in one direction parallel to the optical axis and offset against each other by half the predefined wavelength or a whole multiple of half the predefined wavelength. Designs of this type have several, especially more than two, reflection planes offset against each other. In this case, the structured area has a shape similar to a step pyramid with a round base area.
The concentrically disposed, circular ring and/or circular disk surface cross-sections are preferably centered about the optical axis.
In an especially preferred embodiment, the structured area exhibits a concave or convex curvature which corresponds to the curvature of all surface cross-sections disposed in the structured area. The radial expansion of the surface cross-sections adjoining each other is sized depending on the curvature in such a way that each surface cross-section describes a contour that lies completely in an intermediate area between two planes coplanar with each other, whereby the coplanar planes run perpendicular to the optical axis and exhibit a mutual clearance equal to half the predefined wavelength or a whole multiple of half the predefined wavelength. The radial expansion of the surface cross-sections decreases at the edge in curved resonator mirrors.
In another preferred embodiment, the structured area exhibits several surface cross-sections that run parallel to each other and form a step structure monotonously increasing in one direction.
In one embodiment the reflective surface outside the structured area exhibits a concave or convex curvature. The structured area span only across the area of the resonator mirror centered about the optical axis. The at least one surface cross-section, which is offset against the reflective surface outside the structured area, exhibits a curvature progression deviating from the curvature progression of the other reflective surface. In particular, the at least one surface cross-section disposed in the structured area can runs flat in contrast to the other reflective surface.
In another variant of this embodiment, the structured area exhibits at least two surface cross-sections offset against each other and running in a curved or flat manner. These designs are intended especially for optical resonators that exhibit a stable central subsection in the area of the optical axis, which assumes a function similar to that of a seed laser during operation. To build this type of resonator, for example, a flat section of another resonator mirror is disposed opposite and coplanar with the flat area of the resonator mirror centered about the optical axis.
In another embodiment the structured area is formed by a single stepped and offset surface cross-section spanning in the middle across the entire lateral expansion of the resonator mirror. In other words, the resonator mirror has one single ribbed raised area opposite the other reflective surface or one grooved recessed area opposite the other reflective surface, which span across the entire width of the resonator mirror.
In addition, the invention relates to a laser device with a gas or gas mixture serving as the optically active medium, which is introduced into a resonator that is constrained in front by reflective elements. At least one of the reflective elements is constructed as a resonator mirror with the features described above. The advantageous application of this type of resonator mirror in such laser devices results directly from the previous description and from the fact that gases or gas mixtures, especially carbon monoxide (CO), carbon dioxide (CO2) or CO or CO2 containing gas mixtures, generally exhibit several frequency bands suitable for amplification. According to the invention, the wavelength-dependent selection takes place in such a way that one fully constructive interference only occurs for the given wavelength. For all other wavelengths there is one, at least partially, destructive interference or one interference changing the direction of propagation, so that a wavelength-dependent resonator quality is introduced by additional circulation losses in the wavelength ranges to be suppressed.
A slab waveguide laser is an especially preferable laser device. The gas or gas mixture serving as the optically active medium is introduced into an optical resonator that is constrained laterally by two flat electrodes with their flat sides disposed opposite each other. In addition, the optical resonator is preferably constrained on its front sides by reflective elements in such a way that an unstable resonator is formed.
The electrodes are preferably thermally attached to a cooling system circulating cooling fluid to dissipate heat. The advantage of this type of diffusion-cooled laser devices is that the optical medium does not need to be circulated. This results in a reduced maintenance requirement, among others. In addition, the cooling system is designed so that the gas or gas mixture serving as the optically active medium can be cooled reliably during operation of the laser device. The additional heat input brought about by the geometrically induced losses in the wavelength range to be suppressed is negligible in comparison, and so it can be dissipated reliably. There is no need for additional adjustment of the cooling capacity of the cooling system in this matter.
The resonator mirrors can be cooled or uncooled. Cooled resonator mirrors having one or more cooling channels to conduct a cooling fluid are preferable, especially in high-performance applications.
The laser device in preferred embodiments is a high-performance laser with output powers of at least 500 W. Especially preferable are those with more than 1000 W.
The optically active medium in a preferred embodiment comprises carbon dioxide (CO2) and/or carbon monoxide (CO). In carbon dioxide containing laser devices, the at least one reflective surface cross-section is preferably offset against the reflective surface outside the structured area to support the laser amplification in the 9.3 μm band parallel to the optical axis about approximately 4.65 μm or about a whole multiple of approximately 4.65 μm. In another preferred alternative, the at least two surface cross-sections are offset to support laser amplification in the 9.3 μm band parallel to the optical axis about approximately 4.65 μm or about a whole multiple of approximately 4.65 μm. Certain organic materials, especially plastic materials, exhibit elevated absorption in the region of 9.3 μm and so it has proven advantageous to use a laser beam of this wavelength to process this type of materials.
In other application examples, selective amplification takes place in the 9.6 μm or 10.3 μm band. The at least one reflective surface cross-section is offset accordingly here against the reflective surface outside the structured area about approximately 4.8 μm or 5.15 μm or about a whole multiple of approximately 4.8 μm or 5.15 μm. Alternatively, at least two surface cross-sections of the structured area can also be offset accordingly about approximately 4.8 μm or 5.15 μm or about a whole multiple thereof.
Both reflective elements constraining the optical resonator in front are preferably designed as resonator mirrors with the features described above. The structured areas of both resonator mirrors constraining the optical resonator are disposed opposite each other and exhibit identical or complementary structures.
Possible embodiments of the invention are explained in detail below with reference to the drawings. The following Figures show:
Corresponding parts in all Figures have the same reference symbols.
Direct-current excitation of the gas or gas mixture takes place in another embodiment, and this serves as the optically active medium during laser amplification.
The optical resonator 1 is a confocal unstable resonator that is constrained accordingly in front by two concave resonator mirrors 4. The resonator mirrors 4 are curved and disposed in such a way that the amplified laser beam exits the optical resonator 1 after several circulations laterally through an outcoupling window 7 (represented by dashed lines). The curvature of the resonator mirror 4 corresponds to a rotational paraboloid. The focal lengths of this type of resonator mirror 4 typically lie in the range of half the resonator length, which in negative-branch resonators is equal to the sum of the focal lengths of the resonator mirror 4. The focal lengths here generally lie in the range of 10 cm to about 1 m. The diagram is not true to scale. In particular, the curvature is significantly exaggerated in
In the embodiment shown as an example, carbon dioxide serves as the optically active medium during laser amplification. Carbon dioxide exhibits several frequency bands suitable for laser amplification in the region of 9.3 μm, 9.6 μm, 10.3 μm and 10.6 μm. In the application shown, laser beams with a 9.3 μm wavelength should be generated. The optical resonator 1 exhibits a wavelength-dependent resonator quality for the suppression of modes in the other wavelength ranges. One of the two resonator mirrors 4 constraining the optical resonator 1 in front is equipped here with a structured area 5 that only spans across a small part of the reflective surface 6 near the optical axis A.
The resonator mirrors 4 are made of copper which is highly reflective in the middle infrared range and also has good thermal conductivity. The laser device 2 is diffusion-cooled, that is, the electrodes 3 are thermally coupled to a cooling system circulating cooling fluid (not shown in detail).
In preferred embodiments, both resonator mirrors 4 are equipped with structured areas 5 that are disposed opposite each other. Furthermore, the structured areas 5 are preferably identically designed or have complementary structures. In the latter case, disposed accordingly opposite the elevated surface cross-sections of the one resonator mirror 4 are recessed surface cross-sections of the other resonator mirror 4.
The resonator mirror 4 of the first embodiment has a structured area 5 with several reflective surface cross-sections 8 which are offset against each other and opposite the reflective surface 6 that is outside the structured area 5. The offset of the surface cross-sections 8 against each other takes place in one direction parallel to the optical axis A. The surface cross-sections 8 have a circular ring or circular disk design and are centered about the optical axis A. The surface cross-sections 8 and the surface 6 exhibit a parabolic curvature (not shown in detail in
The curvature progression of the reflective surface 6 and of the surface cross-sections 8 mutually correspond, that is, the reflective surface 6 and the surface cross-sections 8 follow identical but offset in parallel design surfaces K whose progression is shown schematically in
In the embodiment only shown as an example in
In the first embodiment, the stepped design of the structured area 5 has only three surface cross-sections 8 raised with respect to the reflective surface 6. In other embodiments, the number of surface cross-sections 8 can deviate from this. Preference is given to 2 to 20 reflective surface cross-sections 8 offset against each other and disposed in a stepped manner. The structured area 5 spans only across a relatively small area of the resonator mirror 4 centered about the optical axis A. The radial expansion I of the surface cross-sections 8 is comparably small, and so the structured area 5 in the illustrated embodiment constitutes no more than 30% of the total reflective surface of the resonator mirror 5. At least 70% of the total reflective surface of the resonator mirror 5 is formed accordingly by the reflective surface 6 outside the structured area 5.
In contrast to the first embodiment, the reflective surface cross-sections 18 are alternately offset so that all reflection surfaces lie in only two parallel design surfaces offset against each other by the distance D. The curvature progression of the reflective surface 6 and of the surface cross-sections 18 mutually correspond, that is, the reflective surface 6 and the surface cross-sections 18 follow two identical but offset in parallel design surfaces. The distance D is equal to half the predefined wavelength for which the resonance condition should be met or a whole multiple thereof, so that only the laser beam with the predefined wavelength experiences a fully constructive interference.
In the second embodiment of
The structured area 5 of the third embodiment comprises numerous ribbed surface cross-sections 28 that run parallel to each other and protrude from the reflective surface 6. Like in the other embodiments, the step height corresponds to the distance D which is equal to half the wavelength to be selected or a whole multiple thereof. The distance of the ribbed surface cross-sections 28 from each other in the plane perpendicular to the optical axis A is of minor importance to the interference effect to be produced and can lie in the range of 50 μm to 100 μm in resonator mirrors 4 that are intended for high-performance-CO2 lasers.
The fourth embodiment can be seen as a variation of the first embodiment in a sense and have numerous surface cross-sections 38 concentrically disposed about the optical axis A and having the circular ring or circular disk design. The concave curvature of the resonator mirror 4 is significantly exaggerated for reasons of illustration.
The parabolic curvature of the reflective surface 6 and the curvature of the reflective surface cross-sections 38 of the structured area 5 mutually correspond. In fourth embodiment, the reflective surface 6 and the reflective surface cross-sections 38 thus span section-by-section along rotational paraboloids that are offset against each other with respect to the optical axis A by distance D. In addition, the lateral expansion of the surface cross-sections 38 varies in such a way that each surface cross-section 38 lies fully between two planes E1, E2 running coplanar with each other and respectively extending perpendicular to optical axis A. In other words, the width of the concentrically disposed surface cross-sections 38 is selected in such a way that the entire surface structure of the structured area 5 lies between the planes E1, E2 whose distance D from each other is equal to half the predefined wavelength or a whole multiple thereof. In curved areas this means that the radial expansion or width of the surface cross-sections 38 decreases as the radial distance from the optical axis A increases.
In the structured area 5 of the fifth embodiment, only one single circular disk surface cross-section 48 is provided. This surface cross-section spans across a region radially constrained about the optical axis A. In contrast to the surrounding reflective surface 6, which exhibits a concave, especially spherical or parabolic curvature, the surface cross-section 48 has a deviating, especially flat, curvature. The resonator mirror 4 of the fifth embodiment is used preferably for the development of resonator configurations where a stable subsection is formed in the region of the optical axis A.
The sixth embodiment comprises a structured area 5 that is executed as a step mirror, that is, there are several reflective surface cross-sections 58 which run parallel to each other and form a step structure that rises monotonously in the lateral direction, that is, in a direction essentially perpendicular to the optical axis A. The step height with respect to the optical axis is equal to the distance D. Similar to the first embodiment in
The illustration of the embodiments in the Figures is not true to scale. In particular, for reasons of presentability, any existing curvatures in the reflective surface 6 or the reflective surface cross-sections 8, 18, 28, 38, 48, 58, 68 are not shown or are significantly exaggerated.
The invention was described above with reference to preferred embodiments. It goes without saying, however, that the invention is not restricted to the concrete design of the embodiments shown, rather the competent person skilled in the art can derive variations using the description without deviating from the essential basic principles of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2016 116 779.7 | Sep 2016 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/067721 | 7/13/2017 | WO | 00 |
