Optical Resonator, Method of Manufacturing the Optical Resonator and Applications Thereof

Abstract

An optical resonator (100) comprises an optical waveguide device (10) having an optical axis (OA) and extending with a longitudinal length between two waveguide end facets (11), resonator mirrors (13) being arranged for enclosing a resonator section (14) of the optical waveguide device (10), and a ferrule (20) having two ferrule facets (21), wherein the optical waveguide device (10) is mounted to the ferrule (20) and the ferrule (20) extends along the full longitudinal length of optical waveguide device (10). Furthermore, an optical apparatus (200) including the optical resonator (100) and a method of manufacturing the optical resonator (100) are described.

Claims

1-31. (canceled)

32. Optical resonator, comprising an optical waveguide device having an optical axis (OA) and extending with a longitudinal length between two waveguide end facets; resonator mirrors including dielectric mirrors each having a stack of dielectric layers and being arranged on the waveguide end facets for enclosing a resonator section of the optical waveguide device; and a ferrule having two ferrule facets, wherein the optical waveguide device is mounted to the ferrule, wherein the ferrule extends along the full longitudinal length of optical waveguide device, the resonator mirrors provide a passive optical cavity, one of the resonator mirrors has a reflectivity of at least 99.9% and the other resonator mirror has a reflectivity of at least 99%.

33. Optical resonator according to claim 32, wherein the optical resonator is adapted for light coupling via a direct contact of the ends of the optical waveguide device including the resonator mirrors with adjacent waveguides.

34. Optical resonator according claim 32, wherein the waveguide end facets are aligned with the ferrule facets.

35. Optical resonator according to claim 32, wherein the waveguide end facets and the resonator mirrors project beyond the ferrule facets.

36. Optical resonator according to claim 34, wherein the resonator mirrors at least partially cover the ferrule facets.

37. Optical resonator according to claim 32, wherein exposed surfaces of the resonator mirrors are aligned with the ferrule facets.

38. Optical resonator according to claim 32, wherein the dielectric layers in at least one of the stacks of dielectric layers have varying thicknesses, wherein the thicknesses of the dielectric layers are selected for adjusting a group velocity dispersion of the optical resonator.

39. Optical resonator according to claim 32, wherein the dielectric mirrors are arranged such that less sensitive dielectric layers are exposed at the outer ends of the optical resonator.

40. Optical resonator according to claim 32, wherein the resonator mirrors are arranged such that reflecting surfaces thereof are orthogonal to the optical axis (OA) of the optical waveguide device at least in the centre of the waveguide end facets.

41. Optical resonator according to claim 32, wherein the resonator mirrors are curved mirrors with a curvature being selected for optimizing back-reflection of light fields into the optical waveguide device.

42. Optical resonator according to claim 41, wherein at least one of the resonator mirrors has a radius of curvature selected such that it compensates for a diffraction related beam expansion of the light field exiting the optical waveguide device.

43. Optical resonator according to claim 32, wherein at least one of the resonator mirrors is a semi-transparent mirror.

44. Optical resonator according to claim 32, wherein the optical waveguide device is configured as a single mode optical waveguide with a waveguide core and a waveguide cladding.

45. Optical resonator according to claim 44, wherein the optical waveguide device has a core diameter of the waveguide core adapted to a core diameter of an optical single mode fibre.

46. Optical resonator according to claim 44, wherein the optical waveguide device has an outer diameter equal to or below 125 m and a mode field diameter equal to or below 100 m.

47. Optical resonator according to claim 32, wherein the optical waveguide device has an inner tapered section.

48. Optical resonator according to claim 32, wherein the optical waveguide device includes multiple sections forming a combined waveguide, wherein each section has specific waveguide properties.

49. Optical resonator according to claim 32, wherein the optical waveguide device comprises at least one optical fibre and the ferrule comprises a fibre ferrule.

50. Optical resonator according to claim 49, wherein the at least one optical fibre includes a single mode fibre, a polarization maintaining fibre, a dispersion compensated fibre, a highly nonlinear fibre, a hollow core fibre, a single-crystal fibre, a photonic crystal fibre, an ultra-violet compatible fibre or a mid-infrared compatible fibre, a multi-mode fibre, or a dielectric material which along its length is reflectively coated.

51. Optical resonator according to claim 49, wherein the optical waveguide device includes multiple optical fibres being at least one of spliced and stacked together.

52. Optical resonator according to claim 49, wherein the optical waveguide device includes multiple optical fibres being coupled in series along the length of the optical waveguide device forming a combined waveguide.

53. Optical resonator according to claim 32, wherein the optical waveguide device has a length equal to or below 5 cm.

54. Optical resonator according to claim 32, wherein the optical waveguide device has a length equal to or below 2 cm.

55. Optical resonator according to claim 32, wherein the optical waveguide device has a length such that the resonator has a free-spectral range of at least 1 GHz.

56. Optical resonator according to claim 1, wherein the optical waveguide device has a length such that the resonator has a free-spectral range of at least 10 GHz.

57. Optical resonator according to claim 32, wherein the ferrule has an outer shape being adapted to a standardized optical connector mating sleeve.

58. Optical apparatus, including a light source device, and at least one optical resonator according to claim 32.

59. Optical apparatus according to claim 58, wherein the optical resonator is connected via fibre optical connectors with other components of the optical apparatus.

60. Method of manufacturing an optical resonator according to claim 32, comprising the steps of (a) fixing the optical waveguide device in the ferrule, (b) shortening and polishing ends of the ferrule including ends of the optical waveguide device, so that two ferrule facets and two waveguide end facets are created, the waveguide end facets having a mutual distance equal to a desired length of the resonator section, and (c) providing the resonator mirrors at the waveguide end facets.

61. Method according to claim 60, wherein step (a) includes inserting the optical waveguide device into the ferrule, and gluing the optical waveguide device in the ferrule.

62. Method according to claim 60, wherein step (c) includes ion-beam sputtering dielectric layers on the waveguide end facets and on the ferrule facets.

63. Method according to claim 60, wherein step (b) includes removing a part of the ferrule having non-constant, conic inner shape after the waveguide has been inserted into the ferrule.

64. Method of using an optical resonator according to claim 32, for at least one of optical pulse generation, based on non-linear optical effects, frequency comb generation, microwave generation, channel generation for optical telecommunication, resonant super-continuum generation, Brillouin frequency shift generation, reference frequency generation, and optical filtering, comprising light coupling into or out of the optical resonator.

65. Method according to claim 64, wherein the optical resonator is used for the optical pulse generation, which includes solution-pulse generation.

Description

[0049] Further advantages and details of the invention are described in the following with reference to the attached drawings, which show in:

[0050] FIGS. 1 to 4: schematic cross-sectional views of the optical resonator according to the first embodiment of the invention;

[0051] FIG. 5: a schematic cross-sectional view of the optical resonator according to the second embodiment of the invention;

[0052] FIG. 6: a schematic illustration of a laser apparatus according to a preferred embodiment of the invention; and

[0053] FIG. 7: a schematic illustration of a conventional fibre-based optical resonator (prior art).

[0054] Embodiments of the invention are described in the following with exemplary reference to an optical resonator including an optical fibre as the optical waveguide. It is emphasized that the invention is not restricted to the use of optical fibres, but rather possible with other types of optical waveguides, e.g. (potentially doped) solid rods of glass, crystals or semiconductor material. Furthermore, reference is made in particular to the use of a single-mode optical fibre as the optical waveguide. The implementation of the invention is not restricted to this example, but rather possible with the other types of optical fibres as mentioned above.

[0055] The drawings show enlarged cross-sectional views of the optical resonator, wherein the details of the resonator are schematically shown for illustrative purposes. In practice, in particular the thickness of the resonator mirrors and the diameter of the optical waveguide are essentially smaller compared with the dimensions of the ferrule.

[0056] FIGS. 1 to 5 illustrate embodiments of the inventive optical resonator 100, comprising a ferrule 20 and an optical waveguide 10 with resonator mirrors 13. FIGS. 1 to 4 show the first embodiment of the optical resonator 100, wherein the resonator mirrors 13 comprise dielectric mirrors (stacks of dielectric layers) arranged on waveguide end facets 11 of the optical waveguide 10. FIG. 5 shows an example of the second embodiment of the inventive optical resonator 100, wherein the resonator mirrors 13 comprise Bragg waveguide reflectors included in the optical waveguide 10.

[0057] According to FIG. 1A, the optical waveguide 10 comprises an optical fibre that is mounted inside the ferrule 20. The optical fibre 10 is a standard single-mode fibre, e.g. made of plastics or glass, with an outer diameter of e.g. 125 m. Instead of the single-mode fibre any other fibre type can be used as mentioned above, in particular a polarization maintaining fibre, a dispersion compensated fibre, a highly nonlinear fibre (for enhancing non-linearity), a doped gain fibre (for supporting lasing) or mid-infrared compatible fibre (e.g. for molecular spectroscopy).

[0058] The optical waveguide 10 has waveguide end facets 11 carrying the resonator mirrors 13. A lateral waveguide face 12 is provided by the lateral surface of the optical waveguide 10 facing in radial directions relative to the optical axis OA. The lateral waveguide face 12 is fully covered by the ferrule 20. A resonator section 14 is spanned between the resonator mirrors 13.

[0059] The ferrule 20 is a straight cylindrical component having a longitudinal bore with an inner diameter adapted to the outer diameter of the optical waveguide at least at the ends thereof. The central axis of the ferrule simultaneously defines the optical axis OA of the optical waveguide 10. The ferrule 20 is made of a ceramic, e.g. zirconia or aluminium nitride, or a metal, e.g. steel with an outer diameter of a standard off-the-shelf ferrule, e.g. 3.175 mm and a longitudinal length below 1 cm.

[0060] The optical fibre 10 and the ferrule 20 are polished at both axial ends and coated with reflective coatings providing the resonator mirrors 13. At least on one side, the reflective resonator mirror 13 is semi-transparent (e.g. 99,999% reflection or 0.0001% transmission), to allow light coupling into and out of the optical resonator 100.

[0061] The resonator mirrors 13 are made of stacks of dielectric layers 15 (see enlarged sectional image). The dielectric layers 15 are designed with alternatingly higher or lower refractive indices and thicknesses such that the desired reflectivity is obtained according to the requirements of the practical application of the optical resonator 100. As an example, stack with a number of up to hundred layers made of high refractive index transparent materials such as oxides or fluorides with thickness below one optical wavelength.

[0062] The design of the dielectric resonator mirrors 13 can be done by using standard software tools for designing dielectric mirrors. The resonator mirrors 13 are created e. g. by ion beam sputtering. Advantageously, the dielectric layers 15 can be designed for adjusting the GVD of the resonator mirrors, in particular for dispersion engineering, e.g. to achieve normal, anomalous, zero or more complex group velocity dispersion profiles as required by the different practical applications of the optical resonator 100.

[0063] According to FIG. 1A, the waveguide end facets 11 and the ferrule facets 21 are aligned by the shortening and polishing step of the method for manufacturing the optical resonator 100 (see below). Accordingly, the resonator mirrors 13 are deposited on the common plane (or curved, see FIG. 10) surface provided by the waveguide end facets 11 and the ferrules facets 21. Although it would be sufficient to restrict the resonator mirrors 13 to the cross-section of the optical waveguide 10, larger resonator mirrors 13 can be preferred, which at least partially also cover the ferrule facets 21 (as shown).

[0064] With further practical examples, the ferrule length is approximately 10 mm and the used fiber is a standard single mode silica fiber (SMF28) or a polarization maintaining highly-nonlinear silica fiber (HNLF). The resulting free-spectral range of the resonators is approximately 10 GHz. The resonance width is approximately 100 kHz to 15 MHz, resulting in a finesse in the order of 10.sup.3 to 10.sup.5. The coating is applied to the resonator end facet via ion-beam sputtering resulting in a mechanically robust reflector coating and the device can be connected manually via standard fiber-optic mating sleeves to a standard PM or non-PM fiber without measurably affecting the reflector coating's quality.

[0065] FIG. 1B illustrates one end of another implementation of the first embodiment of the optical resonator with the optical waveguide 10 inside of the ferrule 20, wherein a curved waveguide end facet 11 extends in longitudinal direction beyond the ferrule facet 21. According to the curved shape of the waveguide end facet 11, the resonator mirror 13 has a curved shape as well. Advantageously, the convex surface structure improves light back reflection into the optical waveguide 10 (optical fibre). The optical resonator can be dimensioned e. g. as mentioned with reference to FIG. 1A. The radius of curvature of the waveguide end facet 11 is selected in a range of e.g. half a waveguide diameter to 30 mm.

[0066] FIG. 10 illustrates another modification of the invention, wherein the waveguide end facet 11 of the optical waveguide 10 and the ferrule facet 21 of the ferrule 20 provide a curved end surface carrying the resonator mirror 13. Again, light back reflection into the waveguide is improved by the curved shape of the resonator mirror. The optical resonator and the radius of curvature can be dimensioned e. g. as mentioned with reference to FIGS. 1A and 1B.

[0067] FIGS. 2A and 2B illustrate another implementation of the first embodiment of the optical resonator 100, wherein the resonator mirrors 13 on the waveguide end facets 11 of the optical waveguide 10 are covered by the ferrule 20. The exposed surfaces of the resonator mirrors 13 are aligned with the ferrule facets 21. This alignment can be provided with a plane facet shape (FIG. 2A) or a curved facet shape (FIG. 2B).

[0068] FIG. 3 illustrates the first embodiment of the optical resonator 100 similar to the implementation of FIG. 1, wherein the optical waveguide 10 has a tapered section 16 within the ferrule 20. At the tapered section 16, the optical waveguide 10 has an outer diameter of e.g. 10 m. The spacing between the tapered section 16 and the inner surface of the ferrule 20 can be filled by a glue.

[0069] Another modification of the first embodiment is shown in FIG. 4, wherein the optical waveguide 10 includes multiple waveguide sections 17 (schematically illustrated). The materials and lengths of the waveguide section 17 are selected for providing predetermined waveguide properties, e.g. for designing nonlinear optical properties of the optical waveguide 10. One example could be having a central photonic crystal (holey fiber) spliced to two ends of standard single mode fiber on which the mirrors are deposited.

[0070] The second embodiment of the optical resonator 100 is schematically shown in FIG. 5. With this embodiment, the resonator mirrors 13 are formed by Bragg waveguide reflectors in the material of the optical waveguide 10, e. g. an optical fibre. The optical waveguide 10, including the resonator mirrors 13, is completely covered by the ferrule 20. The Bragg waveguide reflectors can be designed similar to the design of the dielectric mirrors such that the reflectivity/transmission of the resonator mirrors 13 is adjusted. As an example, the reflectivity of the Bragg waveguide reflectors can be adjusted by the longitudinal length thereof. Preferably, the resonator mirrors 13 have a longitudinal length in a range from 10 m to 5 mm.

[0071] The optical resonator 100 according to the invention, e.g. the implementation of FIG. 1A or 10, is manufactured according to the following method.

[0072] Firstly, an optical fibre for providing the optical waveguide 10 and the ferrule 20 are prepared with an axial length slightly above the final length of the optical resonator 100 to be obtained. The optical fibre 10 is glued into the ferrule 20 as it is known from standard procedures of mounting optical fibres to fibre ferrules.

[0073] Both sides of the ferrule 20 including the optical waveguide 10 are shortened and polished to align the waveguide end facets 11 and the ferrule facets 21, to adjust the optical path length of the optical fibre 10 and to achieve low surface roughness. Shortening and polishing, in particular wet polishing is provided on both sides of the ferrule 20. Optionally, grinding of the surfaces can be provided before polishing. Polishing on both sides is continued until the desired length of the optical waveguide 10 is reached.

[0074] Preferably, the ferrule 20 is shortened at least to the point where the inner bore diameter of the ferrule 20 reaches its narrowest width. Advantageously, this results in a precise centre alignment and a removal of excess glue at the ends of the optical fibre 10. The glue comprises e.g. an epoxy glue.

[0075] Subsequently, the resonator mirrors 13 are deposited by ion beam sputtering. The dielectric layers 15 are deposited according to the GVD to be obtained. Finally, the optical resonator is ready for use, e.g. in a laser apparatus 200 of FIG. 6.

[0076] The exemplary embodiment of FIG. 6 shows a laser apparatus 200 with a laser source device 210 and at least one optical resonator 100 according to the invention. The optical resonator 100 is connected with the laser source device 210 via a circulator 220 and optical fibres 230 connecting the laser source device 210 with the circular resonator 220 and the circular resonator 220 with the optical resonator 100.

[0077] FIG. 6 illustrates the connection of the resonator 100 with the optical fibre 230 via fibre optical connectors 231, and in particular ferrule connecting sleeves 240, which accommodate one end of the optical resonator 100 and corresponding fibre optical connectors (fibre ferrules) 231 attached to the optical fibres 230.

[0078] With the setup of FIG. 6, a reflection signal R of the optical resonator 100 can be obtained via the circular resonator 220, while a transmission signal T of the optical resonator 100 is obtained at the second end thereof.

[0079] It is noted that the application of the invention is not restricted to the setup of FIG. 6, but rather possible e.g. with one of the following applications.

[0080] As an example, the optical resonator 100 can be included in a solution-pulse generator, wherein the optical resonator is driven by a cw laser. With this embodiment, the longitudinal length of the optical resonator 100 (cavity length) is selected according to the desired pulse repetition rate. The free-spectral range (FSR) of the optical fibre resonator is larger than the Brillouin gain bandwidth to avoid unwanted stimulated Brillouin scattering. Furthermore, the resonator mirrors 13 are adjusted for a weakly anomalous GVD.

[0081] According to a further application, the optical resonator 100 is included in a frequency comb generator using four-wave mixing, a microwave generator and a general generator for optical telecommunication (see EP 1 988 425 B1). With this embodiment, the cavity length is selected according to the desired frequency comb line spacing. The FSR is larger than the Brillouin gain bandwidth to avoid unwanted stimulated Brillouin scattering. The resonator mirrors 13 are provided with weakly anomalous GVD. For intrinsically low noise systems (in particular for telecom applications) strongly anomalous GVD is provided.

[0082] In the case of a channel generator, the length of the optical resonator 100 is selected such that the generated lines are spaced according to telecom standard, e.g. by 12.5, 25 or 50 GHz.

[0083] According to another application of the optical resonator, it is provided as the active element in a resonant supercontinuum generator. With this embodiment, the optical resonator 100 is driven by a modulated laser source device. The GVD is engineered according to the desired spectral energy distribution of the supercontinuum radiation to be generated. Phase matching allows an enhancement of the spectral power in certain wavelength regions, similar to dispersive waves. The cavity length and the FSR are matched to the pulse repetition rate of the driving laser source device.

[0084] A Brillouin laser is a further example of an application of an optical resonator 100, which is driven by a cw laser. With this application, the cavity length and the FSR are matched to stimulated Brillouin shift frequency. The GVD is engineered such that competing non-linear effects, such as four-wave mixing are suppressed, e.g. via normal GVD.

[0085] According to another application, the optical resonator 100 can provide a cw laser, which is driven by a multi- or single-mode pump light. The cavity length is selected in accordance with the finesse and doping concentration of the optical fibre 10. With this embodiment, a GVD design is not necessary.

[0086] Alternatively, a mode-locked laser can be provided by the optical resonator 100, which is driven by multi- or single-mode pump light. The cavity length is selected according to the desired pulse repetition rate and the resonator mirrors 13 are adjusted for providing a weak anomalous GVD to allow for solution effects.

[0087] Finally, the optical resonator can provide a passive reference cavity or optical filter. It is used in combination with narrow or broadband laser sources. For obtaining the filter function, the cavity length is selected according to the desired FSR. For broadband applications, the GVD and higher order dispersion terms are close to zero. For narrowband applications, priority is given to optimizing the coating of the resonator mirrors 13 for high cavity finesse.

[0088] The features of the invention disclosed in the above description, the drawings and the claims can be of significance both individually as well as in combination or sub-combination for the realisation of the invention in its various embodiments.