Improved waveguide for an optical polarization rotator

Abstract

According to an example aspect of the present disclosure, there is provided a twisted waveguide arranged to rotate the polarization of light passed through the twisted waveguide, wherein the twisted waveguide is made of polymer and arranged to be connected between waveguides on a Photonic Integrated Circuit, PIC, the twisted waveguide being arranged to rotate the polarization of light by about 45 degrees.

Description

IMPROVED WAVEGUIDE FOR AN OPTICAL POLARIZATION ROTATORFIELD

[0001] Embodiments of the present disclosure relate in general to optics and more specifically, to a waveguide for an optical polarization rotator.BACKGROUND

[0002] Polarization rotation has been traditionally difficult to achieve in planar waveguide platforms due to the inherent orientation of the optical axis perpendicular and parallel to the surface plane. Wideband, adiabatic polarization rotation would need a gradual rotation of the axis. For this, quasi-3D structures may be used to create an asymmetric waveguide geometry which rotates the polarization. Rotation may also be done using bireffingent wave-plates, but inserting these into optical waveguides is a challenging assembly task. There is therefore a need to provide an improved optical waveguide, e.g., for Photonic Integrated Circuits, PICs.SUMMARY

[0003] According to some aspects, there is provided the subject-matter of the independent claims. Some embodiments are defined in the dependent claims.

[0004] According to an aspect of the present disclosure, there is provided a twisted waveguide arranged to rotate the polarization of light passed through the twisted waveguide, wherein the twisted waveguide is made of polymer and arranged to be connected between waveguides on a Photonic Integrated Circuit, PIC, the twisted waveguide being arranged to rotate the polarization of light by about 45 degrees.

[0005] According to another aspect, there is provided a polarization rotator comprising the twisted waveguide. According to yet another aspect, there is provided an optical isolator comprising the polarization rotator.BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIGURE 1 illustrates a 45° polarization rotator according to a first embodiment of the present disclosure;

[0007] FIGURE 2 illustrates a 45° polarization rotator according to a second embodiment of the present disclosure;

[0008] FIGURE 3 illustrates an arrangement capable of supporting at least some embodiments of the present disclosure.EMBODIMENTS

[0009] Polarization rotation is a key component, e.g., in integrated optical isolators, polarization diversity circuits, and polarization controllers used in Polarization-Division Multiplexing, PDM, transmission links. Specifically, for the integrated optical isolator a robust integrated solution would be required that achieves a polarization rotation of 45 degrees.

[0010] At least one challenge is how to realize polarization rotation in a planar Photonic Integrated Circuit, PIC, where waveguides are dielectric, processing may be done using surface micromachining and circuits often work in the terahertz range. There is a need to provide an improved optical waveguide for PICs, like an optical waveguide with fully 3D geometry that rotates the polarization of light by an arbitrary but preset amount, like 45 degrees. The optical waveguide should be suitable to be fabricated on a PIC as well.

[0011] FIGURE 1 illustrates a 45° waveguide polarization rotator according to a first embodiment of the present disclosure. Polarization rotator 1 shown in FIGURE 1 may be a part of an optical isolator. For instance, the optical isolator may comprise a polarization divider and a Faraday rotator in addition to polarization rotator 1. Polarization rotator 1 may comprise in-coupling waveguide 10, twisted waveguide 11 and out-coupling waveguide 12. Twisted waveguide 11 may be in between in-coupling waveguide 10 and out-coupling waveguide 12, i.e., twisted waveguide 11 may be coupled to in-coupling waveguide 10 from one end and to out-coupling waveguide 12 from another end. Twisted waveguide 11 may be arranged to be connected between waveguides 10 and 12 on a Photonic Integrated Circuit, PIC. The polarization may be rotated with an arbitrary amount and with mode-matching atthe input and the output of twisted waveguide 11 for coupling to optical waveguides 10 and 12.

[0012] In some embodiments, twisted waveguide 11 may be made of polymer. The use of polymer makes it possible to rotate twisted waveguide 11 by an arbitrary amount, e.g., to cause polarization rotation of about 45 degrees. For instance, twisted waveguide 11 may be manufactured as an optical waveguide using, e.g., Two Photon Polymerization, 2PP, techniques. In FIGURE 1, the cross-sections of parts A - E of polarization rotator 1 are shown, wherein the parts B-D are parts of twisted waveguide 11. In some embodiments, parts A and E may be parts of twisted waveguide 11 as well.

[0013] Twisted waveguide 11 may comprise twisted portion 13, wherein twisted portion 13 is twisted from the shape of part B to the shape of part D via the shape of part C. Twisted portion 13 may be rotated along the light propagation direction with an angle by which the polarization should be rotated, e.g., about 45 degrees. Thus, twisted waveguide 11 may be arranged to cause said polarization rotation along a longitudinal axis X of twisted waveguide 11 with twisted portion 13 with an angle of rotation of about 45 degrees. That is, said polarization rotation may be effected by twisted waveguide 11 , wherein twisted portion 13 is twisted from the shape of part B to the shape of part of D, along its longitudinal axis X with twisted portion 13 with an angle of rotation that corresponds to an amount of polarization rotation of about 45 degrees. The longitudinal axis X may refer to a propagation direction of light. That is, there may be an incoming light beam from in-coupling waveguide 10 to twisted waveguide 11. Twisted waveguide 11 may rotate polarization of the incoming light beam by about 45 degrees to generate an outgoing light beam. The outgoing light beam with rotated polarization, compared to the polarization of the incoming light beam, may then be provided to waveguide 12 from twisted waveguide 11.

[0014] In some embodiments, twisted waveguide 11 may be arranged to rotate the polarization of light exactly by 45 degrees. However, as the skilled person understands, it may not be always possible to achieve polarization rotation of 45 degrees exactly. So in general, twisted waveguide 11 may be arranged to rotate the polarization of light about, or approximately, by 45 degrees.

[0015] As shown in FIGURE 1 , twisted waveguide 11 may comprise a portion AB preceding twisted portion 13, such as rectangular or square portion, when looking at the arrangement from the point of view of light incoming to twisted portion 13 from in-couplingwaveguide 10. Portion AB refers to a portion of the twisted waveguide 11 between parts A and B. Similarly, for example portion DE refers to a portion of the twisted waveguide 11 between parts D and E.

[0016] Preceding portion AB may act as an interface to in-coupling waveguide 10. In some embodiments, an optimal cross-section of preceding portion AB may be something else, like a circle, an ellipse with different width and height, or T-shaped. Moreover, in some embodiments, the cross-section of preceding portion AB may be asymmetric with respect to the axes, like rectangular, an ellipse with different width and height, or T-shaped. If preceding portion AB is asymmetric, i.e., a portion wherein the cross-section of the waveguide does not have a 90 degree rotational symmetry, cross-coupling between orthogonal polarization states is suppressed, and such shapes are therefore particularly beneficial. The cross-section A may be converted to cross-section B and cross-section D may be converted to cross-section E using linear tapers.

[0017] Alternatively, or in addition, twisted waveguide 11 may comprise twisted portion 13 and portion DE following twisted portion 13, such as a rectangular or square portion, when looking at the arrangement from the point of view of light incoming to twisted portion 13 from in-coupling waveguide 10. Following portion DE may act as an interface to out-coupling waveguide 12. In some embodiments, an optimal cross-section E of following portion DE may be something else, like a circle, an ellipse with different width and height, or T-shaped. If following portion DE is asymmetric, i.e., a portion wherein the cross-section of the waveguide does not have a 90-degree rotational symmetry, cross-coupling between orthogonal polarization states is suppressed, and such shapes are therefore particularly beneficial. If both, preceding portion AB and following portion DE have such shapes, crosscoupling between orthogonal polarization states is suppressed throughout the structure.

[0018] In some embodiments, if following portion DE is acting as an interface to out- coupling waveguide 12, said interface may have in the middle a circular cross-section E without a defined optical axis. Said interface may further provide coupling of light to out- coupling waveguide without turning an optical axis of the light. Hence, following portion DE may convert the waveguide of light as asymmetric, e.g., by linear tapering. After that, cross-section E of following portion DE may be tapered to correspond to the field of out- coupling waveguide 12. Circular cross-section in the middle of following portion DE makesit possible that during tapering the polarization won’t turn as it would if tapering would be done at once from twisted portion 13 to out-coupling waveguide 12.

[0019] FIGURE 2 illustrates a 45° waveguide polarization rotator according to a second embodiment of the present disclosure. Similarly, as in the first embodiments shown in FIGURE 1, polarization rotator 2 may comprise in-coupling waveguide 20, twisted waveguide 21 and out-coupling waveguide 22, corresponding to in-coupling waveguide 10, twisted waveguide 11 and out-coupling waveguide 12 of FIGURE 1, respectively. Twisted waveguide 21 may comprise twisted portion 23, like twisted portion 13 of FIGURE 1.

[0020] Arbitrary polarization rotation in the twisted portion 23, in this example 45°, may be followed by a waveguide portion 25 that has at its input circular cross section D which due to the rotational symmetry of the cross section does not have a defined optical axis. This is beneficial because for an arbitrary angle polarization rotation, the waveguide shape C should be converted to match out-coupling waveguide 22 without simultaneously turning the optical axis, as this would cause further polarization rotation. According to the second embodiment shown in FIGURE 2, this problem is solved by using an intermediate waveguide section 25.

[0021] The waveguide geometry may then be converted back to match the geometry of out-coupling waveguide 22. Thus, circular waveguide portion 25, which can also be a separate waveguide coupled to twisted waveguide 21, may be used to ensure transition between the rotated, birefringent waveguide 21 and out-coupling waveguide 22 may be done without rotating the optical axis. The cross-section shapes A - E of the twisted waveguide 21 are also shown in FIGURE 2. There may be a part after circular waveguide portion 25, wherein the cross-section changes from circular to match the shape of out-coupling waveguide 22. Thus, the cross-section of waveguide portion 25 may not be constant.

[0022] An input end of twisted waveguide 21 may be adjusted to match to waveguide 20 (Note: only interfaces of waveguides 20 and 22 are shown in FIGURE 2). For instance, one end of twisted waveguide 21 may comprise first spot-size converter 24, first spot-size converter 24 being arranged to couple twisted waveguide 21 to in-coupling waveguide 20. The cross-section of the spot-size converter 24 may vary suitably slowly for the transition to be adiabatic (that is, optical power is not coupled between local waveguide modes). For instance, a ratio between a change in lateral dimension and change in length may be about 1 / 20. First spot-size converter 24 may be used to reduce coupling losses.

[0023] In polarization rotator 1 an input end and an output end of twisted waveguide 11 may be also adjusted to match to waveguides 10 and 12 (Note: only interfaces of waveguides 10 and 12 are shown in FIGURE 1). For instance, one end of twisted waveguide 11 may comprise a first spot-size converter at twisted waveguide input 14. Alternatively, or in addition, another end of twisted waveguide 11 may comprise a second spot-size converter at twisted waveguide output 15, second spot-size converter 15 being arranged to couple twisted waveguide 11 to out-coupling waveguide 12.

[0024] Polarization rotators 1 and 2, and especially twisted waveguides 11 and 21, may be manufactured using 2PP processes. Three-dimensional micro-structuring of photosensitive materials by 2PP is effective for the fabrication of 3-D structures having a resolution of 100 nm or better.

[0025] Polarization rotators as shown in FIGURES 1 and 2 may be used in PICs. That is, in-coupling waveguides 10, 20, twisted waveguides 11, 21 and out-coupling waveguides 12, 22 may be PIC waveguides.

[0026] In some embodiments, an optical isolator may comprise polarization rotator 1 or 2. An optical isolator is a component that allows light to pass only in one direction. Their primary use is to suppress back-reflections to a laser and thus improve the laser frequency and power stability. An optical isolator that is integrated on a PIC can reduce the assembly time and cost of optical modules, especially if the laser is also integrated on the PIC. In its most common configuration of an optical isolator which uses a Faraday rotator section, polarization rotation of specifically 45 degrees may be required. Therefore, the angle of 45 degrees enables an improved optical isolator.

[0027] FIGURE 3 illustrates an arrangement capable of supporting at least some embodiments of the present disclosure. More specifically, in FIGURE 3 is schematically shown arrangement 30 for manufacturing twisted waveguides, like twisted waveguides 11 and 21, by 3D- printing and by two-photon polymerization. Arrangement 30 may comprise computer 31, laser 32 with a positioning mechanism (not shown), a sensor 33 and lens 34 for providing feedback on the progress of the waveguide manufacturing process taking place in photoresist volume 35. Computer 31 may control laser 32 over wireless link 36, for example.

[0028] For two-photon polymerization and 3-D materials processing, computer- controlled positioning systems may be combined with laser 32 that is for example a nearinfrared Ti:sapphire femtosecond laser. By using a scanner on the Z-axis of an X-Y positioning system and a 2-D translational stage for the sample, one can move a tiny beam waist three-dimensionally inside a photosensitive or photoresist resin and write complex 3- D structures. In general, various materials may be used for two-photon polymerization, such as organic / inorganic polymer or epoxide based photoresist.

[0029] In some embodiments, twisted waveguide 11 and / or 21 may be manufactured by 3D writing process where the spatially scanned focal point of a high-intensity laser beam inside a volume of monomers of an optical grade polymer may perform selective two-photon polymerization.

[0030] It is to be understood that the embodiments of the disclosure disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0031] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

[0032] As used herein, a plurality of items, structural elements, compositional elements, and / or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present disclosure may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments,examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present disclosure.

[0033] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the preceding description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the disclosure. One skilled in the relevant art will recognize, however, that the disclosure can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the disclosure.

[0034] While the forgoing examples are illustrative of the principles of the present disclosure in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the disclosure. Accordingly, it is not intended that the disclosure be limited, except as by the claims set forth below.

[0035] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, a singular form, throughout this document does not exclude a plurality.INDUSTRIAL APPLICABILITY

[0036] At least some embodiments of the present disclosure find industrial application in PICs.ACRONYMS LIST2PP Two Photon PolymerizationPDM Polarization-Division Multiplexing PIC Photonic Integrated CircuitREFERENCE SIGNS LIST

Claims

CLAIMS:

1. A twisted waveguide arranged to rotate the polarization of light passed through the twisted waveguide, wherein the twisted waveguide is made of polymer and arranged to be connected between waveguides on a Photonic Integrated Circuit, PIC, the twisted waveguide being arranged to rotate the polarization of light by about 45 degrees.

2. A twisted waveguide according to claim 1, wherein the twisted waveguide is arranged to cause said polarization rotation along a longitudinal axis of the twisted waveguide with a twisted portion with an angle of rotation of about 45 degrees.

3. A twisted waveguide according to claim 1 or claim 2, wherein the twisted waveguide is arranged to rotate the polarization of light by 45 degrees.

4. A twisted waveguide according to any of the preceding claims, wherein one end of the twisted waveguide comprises a first spot-size converter, the first spot-size converter being arranged to couple the twisted waveguide to an in-coupling waveguide.

5. A twisted waveguide according to any of the preceding claims, wherein another end of the twisted waveguide comprises a second spot-size converter, the second spot-size converter being arranged to couple the twisted waveguide to an out-coupling waveguide.

6. A twisted waveguide according to any of the preceding claims, comprising a twisted portion and a portion preceding the twisted portion, the preceding portion being a circleshaped or square portion and acting as an interface to an in-coupling waveguide.

7. A twisted waveguide according to any of claims 1 to 5, comprising a twisted portion and a portion preceding the twisted portion, the preceding portion being a portion wherein the cross-section of the waveguide does not have a 90 degree rotational symmetry and acting as an interface to an in-coupling waveguide.

8. A twisted waveguide according to claim 7, wherein the preceding portion is rectangular, ellipse-shaped with different width and height, or T-shaped.

9. A twisted waveguide according to any of the preceding claims, comprising a twisted portion and a square portion following the twisted portion, the following square portion acting as an interface to an out-coupling waveguide.

10. A twisted waveguide according to any of claims 1 to 8, comprising a twisted portion and a portion following the twisted portion, the following portion acting as an interface to an out-coupling waveguide, said interface having in the middle a circular cross-section without a defined optical axis and providing coupling of light to the out-coupling waveguide without turning an optical axis of the light.

11. A twisted waveguide according to any of claims 1 to 8, comprising a twisted portion and a portion following the twisted portion, the following portion being a portion wherein the cross-section of the waveguide does not have a 90 degree rotational symmetry and acting as an interface to an in-coupling waveguide.

12. A twisted waveguide according to claim 11, wherein the following portion is rectangular, ellipse-shaped portion with different width and height, or T-shaped.

13. A twisted waveguide according to any of the preceding claims, wherein the twisted waveguide is manufactured by 3D writing process where the spatially scanned focal point of a high-intensity laser beam inside a volume of monomers of an optical grade polymer performs selective two-photon polymerization.

14. A polarization rotator comprising the twisted waveguide according to any of the preceding claims.

15. A polarization rotator according to claim 14, further comprising an in-coupling waveguide and an out-coupling waveguide, wherein the twisted waveguide is coupled to the in-coupling waveguide from one end and to the out-coupling waveguide from another end.

16. A polarization rotator according to claim 15, wherein the in-coupling and the out- coupling waveguides are PIC waveguides.

17. An optical isolator comprising the polarization rotator according to any of claims 14 to

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