RESONATOR MIRROR FOR AN OPTICAL RESONATOR OF A LASER APPARATUS, AND LASER APPARATUS

Abstract

The invention relates to a resonator mirror (4) for an optical resonator (1) of a laser device (2), especially of a gas laser or a slab waveguide laser, comprising a reflective surface (6) with a structured area (5) which spans across a region of the reflective surface (6) centered about the optical axis (5). According to one variant of the principle underlying the invention, the structured area (5) has at least one reflective surface cross-section (8, 18, 28, 38, 48, 58, 68) which is offset with respect to the reflective surface (6) outside the structured area (5) and parallel to the optical axis (A) by half of a predefined wavelength or by a whole multiple of half the predefined wavelength. According to another variant, the structured area (5) has at least two surface cross-sections (8, 18, 28, 38, 48, 58, 68) which are offset against each other and parallel to the optical axis (A) by half of a predefined wavelength or by a whole multiple of half the predefined wavelength. In addition, the invention relates to a laser device (2) whose optical resonator (1) comprises a resonator mirror (4) designed in such a manner.

Claims

1-16. (canceled)

17. A laser device, comprising: an optically-active medium having a plurality of frequency bands; first and second resonator mirrors, the resonator mirrors arranged around the optically-active medium to form an unstable optical resonator, the unstable optical resonator having an optical axis; wherein the second resonator mirror includes a structured area, the structured area occupying less than 30% of the second resonator mirror, the structured area located on the optical axis, the structured area having at least one reflective surface that is offset against a reflective surface of the second resonator mirror outside the structured area, the offset being along the optical axis and equal to half or multiples-of-half of a selected wavelength in a desired frequency band; wherein the structured area introduces reflection losses for frequency bands to be suppressed, the desired frequency band thereby having higher net amplification than the suppressed frequency bands during operation of the laser device; and wherein the first resonator mirror and the structured area form a stable optical resonator near the optical axis, the stable optical resonator seeding the unstable optical resonator during operation of the laser device.

18. The laser device of claim 17, wherein the optically-active medium is a gas mixture that contains carbon dioxide.

19. The laser device of claim 17, wherein the optically-active medium is a gas mixture that contains carbon monoxide.

20. The laser device of claim 17, wherein the at-least-one reflective surface of the structured area is raised with respect to the reflective surface of the second resonator mirror outside the structured area.

21. The laser device of claim 17, wherein the at-least-one reflective surface of the structured area is recessed with respect to the reflective surface of the second resonator mirror outside the structured area.

22. The laser device of claim 17, wherein the structured area occupies less than 15% of the second resonator mirror.

23. The laser device of claim 22, wherein the structured area occupies less than 5% of the second resonator mirror.

24. The laser device of claim 17, wherein the at-least-one reflective surface of the structured area is flat.

25. The laser device of claim 17, wherein the at-least-one reflective surface of the structured area has curvature.

26. The laser device of claim 25, wherein the curvature of the at-least-one reflective surface of the structured area corresponds to a curvature of the reflective surface of the second resonator mirror outside the structured area.

27. The laser device of claim 17, wherein the structured area has a plurality of stepped reflective surfaces, respectively offset against each other by half or multiples-of-half of the selected wavelength.

28. The laser device of claim 27, wherein the stepped reflective surfaces are circular in shape.

29. The laser device of claim 28, wherein the stepped reflective surfaces are concentrically arranged with respect to each other.

30. The laser device of claim 27, wherein the stepped reflective surfaces are rectangular in shape.

31. The laser device of claim 17, wherein the structured area has one stepped reflective surface laterally spanning the second resonator mirror.

32. The laser device of claim 17, wherein the structured area is limited to an area about the optical axis having a diameter of a few millimeters.

33. The laser device of claim 17, wherein the structured area is limited to an area about the optical axis having a diameter of less than one millimeter.

34. The laser device of claim 17, wherein the unstable laser-resonator is a negative-branch unstable resonator.

35. The laser device of claim 17, wherein the first resonator mirror includes another structured area.

36. The laser device of claim 35, wherein the structured areas of the first and second resonator mirrors are complementary, one structured area being raised with respect to the reflective surface outside thereof, the other structured area being recessed with respect to the respective reflective surface outside thereof.

Description

[0032] Possible embodiments of the invention are explained in detail below with reference to the drawings. The following Figures show:

[0033] FIG. 1: an optical resonator of a laser device designed as a slab waveguide laser;

[0034] FIG. 2: a structured area of a resonator mirror according to a first embodiment of the invention viewed from the top;

[0035] FIG. 3: the structured area of the first embodiment in a schematic cross-section;

[0036] FIG. 4: the progression of curved surface cross-sections of a variant of the first embodiment in a detailed schematic diagram;

[0037] FIG. 5: a structured area of a resonator mirror according to a second embodiment of the invention viewed from the top;

[0038] FIG. 7: a structured area of a resonator mirror according to a third embodiment of the invention viewed from the top;

[0039] FIG. 8: the structured area of the third embodiment in a schematic cross-section;

[0040] FIG. 9: a structured area of a resonator mirror according to a fourth embodiment of the invention viewed from the top;

[0041] FIG. 10: the structured area of the fourth embodiment in a schematic cross-section;

[0042] FIG. 11: a structured area of a resonator mirror according to a fifth embodiment of the invention viewed from the top;

[0043] FIG. 12: the structured area of the fifth embodiment in a schematic cross-section;

[0044] FIG. 13: a structured area of a resonator mirror according to a sixth embodiment of the invention viewed from the top;

[0045] FIG. 14: the structured area of the sixth embodiment in a schematic cross-section;

[0046] FIG. 15: a resonator mirror with a structured area according to a seventh embodiment in a perspective view.

[0047] Corresponding parts in all Figures have the same reference symbols.

[0048] FIG. 1 schematically illustrates the layout of an optical resonator 1 of a laser device 2 designed as a slab waveguide laser. The optical resonator 1 is constrained on its flat sides by two electrodes 3 opposite each other. During operation of the laser device 2, for example, between the electrodes 3 there is a high-frequency alternating field which excites a gas or gas mixture introduced into a discharge chamber located between the electrodes 3.

[0049] 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.

[0050] 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 FIG. 1 for better illustration.

[0051] 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.

[0052] 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).

[0053] 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.

[0054] FIGS. 2 and 3 show a section of a resonator mirror 4 with a structured area 5 according to a first embodiment as viewed from the top and in cross-section. The progression of the represented section is designated with III in FIG. 2.

[0055] 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 FIGS. 2 and 3) and are offset against each other respectively by a distance D that is equal to half the wavelength to be selected.

[0056] 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 FIG. 4. The curvature of the resonator mirror 4 or of the design surfaces K is shown significantly exaggerated. FIG. 4 in this respect shows a marginally modified variant of the first embodiment in that the reflective surface cross-sections 8 are recessed against each other or with respect to the reflective surface 6. The distance D of the reflective surface cross-sections 8 with respect to each other 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.

[0057] In the embodiment only shown as an example in FIGS. 2 and 3, a selection of amplification should occur in the region of 9.3 μm, that is, the distance D in this case is around 4.65 μm. The shape accuracy of the resonator mirror should be at least one twentieth of the wavelength to be selected. Production tolerance in this concrete application is preferably less than ±500 nm, and especially preferably about ±250 nm. The distances between reflective surface 6 and the reflective surface cross-sections 8 offset against each other are equal to whole multiples of D and hence essentially whole multiples of half the wavelength to be selected.

[0058] 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.

[0059] FIGS. 5 and 6 show a second embodiment of the structured area 5 with reflective surface cross-sections 18 viewed from the top or in a cross-section. The progression of the section shown in FIG. 6 is designated with VI in FIG. 5. The surface cross-sections 18 are disposed concentrically about the optical axis A. The structured area 5 of the second embodiment essentially corresponds to that of the first embodiment.

[0060] 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.

[0061] In the second embodiment of FIGS. 5 and 6, only two grooved and recessed surface cross-sections 18 are provided as examples. Another circular ring surface cross-section 18 runs within that of the curved plane defined by the reflective surface 6. The number and configuration of the surface cross-sections 18 can deviate from this. Preference is given to embodiments with 2 to 20 grooved and recessed or ribbed and raised surface cross-sections 18.

[0062] FIGS. 7 and 8 show a third embodiment of the structured area 5 with reflective surface cross-sections 28 viewed from the top or in a cross-section. The progression of the represented section is designated with VIII in FIG. 7.

[0063] 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-CO.sub.2 lasers.

[0064] FIGS. 9 and 10 show a fourth embodiment of the structured area 5 with reflective surface cross-sections 38 viewed from the top or in a cross-section. The progression of the represented section is designated with X in FIG. 9.

[0065] 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.

[0066] 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.

[0067] FIGS. 11 and 12 show a fifth embodiment of the structured area 5 with reflective surface cross-sections 48 viewed from the top or in a cross-section. The progression of the represented section is designated with XII in FIG. 11.

[0068] 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.

[0069] FIGS. 13 and 14 show a sixth embodiment of the structured area 5 with reflective surface cross-sections 58 in a perspective view or in a cross-section.

[0070] 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 FIGS. 2 and 3, a step structure is hereby specified, and this structure has numerous surface cross-sections 58 offset against each other and parallel to the optical axis A.

[0071] FIG. 15 shows a seventh embodiment of the invention in a perspective view. The structured area 5 of the seventh embodiment has only one single surface cross-section 68 stepped and elevated with respect to the reflective surface 6. The surface cross-section 68 spans across the entire width of the resonator mirror 4. In contrast to the other embodiments, the structured area 5 here is not limited to a region radially constrained about the optical axis A, rather the structured area 5 of the seventh embodiment exhibits an overall lateral expansion L that corresponds to the width of the resonator mirror 4. The stepped and elevated surface cross-section 68 spans in the middle across the width of the resonator mirror 4 and runs parallel to the flat sides of the resonator chamber.

[0072] 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.

[0073] 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.

LIST OF REFERENCE SYMBOLS

[0074] 1 optical resonator [0075] 2 laser device [0076] 3 electrode [0077] 4 resonator mirror [0078] 5 structured area [0079] 6 reflective surface [0080] 7 outcoupling window [0081] 8 surface cross-section [0082] 18 surface cross-section [0083] 28 surface cross-section [0084] 38 surface cross-section [0085] 48 surface cross-section [0086] 58 surface cross-section [0087] 68 surface cross-section [0088] A optical axis [0089] D distance [0090] I radial expansion [0091] L overall expansion [0092] E1 plane [0093] E2 plane [0094] K design surface