Polarization rotator

Abstract

The present invention relates to a polarization rotator. The polarization rotator of the present invention comprises an optical coupler, the optical coupler comprising a waveguide, the waveguide having at least a first port and a second port at a first end, the first port being configured as an input port for polarized light, the second port being configured as an output port for reflected polarized light, the waveguide having a second end opposite to the first end. It also comprises a birefringent waveplate having a reflective surface on one side, the waveplate being arranged to receive light from the second end of the waveguide and reflect light emitted from the coupler back into the coupler. According to the present invention, the waveplate is also configured to cause the birefringent material to rotate the polarization of the reflected light, the amount of which rotation depends on the rotation angle of the birefringent waveplate relative to the optical coupler.

Description

Background Art

[0001] The present invention relates to polarization rotators, and more particularly to polarization rotators for integrated optics. Existing polarization rotation solutions in integrated optics are based on mode evolution, on "rotator waveguides" that are asymmetric waveguides acting as waveplates, and on thin waveplates made of birefringent material inserted into grooves etched through the waveguide.

[0002] Polarization rotators are optical devices that rotate the polarization axis of a linearly polarized light beam by a selected angle. In free-space optics, these devices can be based on the Faraday effect, on birefringence, or on total internal reflection. For guided waves, the rotation can also be done by mode evolution. Polarization rotators are key components in, for example, optical isolators and polarization diversity receivers.

[0003] A Faraday rotator requires a material that exhibits a magneto-optical effect. When placed in a magnetic field, the material will rotate the plane of linearly polarized light. This effect is irreversible and can therefore be used to make optical isolators. In a polarization rotator based on birefringence, linearly polarized light is decomposed into two components. A phase delay in the "slow" axis of the material causes the plane of the linearly polarized light to rotate. Birefringence can be both a material property and, in the case of waveguides, can be geometrically induced.

[0004] Integrated polarization rotators are usually based on the use of some type of rotator waveguide. Methods based on cross-polarization coupling or mode evolution have also been proposed. Rotators based on mode evolution work by adiabatically modifying the cross section of the waveguide so that the optical axis is gradually rotated by the desired amount. This transformation requires adiabatic treatment so that no power coupling occurs between the different modes, making the device very long and difficult to manufacture with conventional waveguide processes. In addition, it is difficult to achieve rotation angles other than 90°.

[0005] Light that passes through multiple total internal reflections in a particular direction will have its polarization rotated. These devices are broadband because operation does not rely on interference of the light.

[0006] Known integrated polarization rotators are very sensitive to manufacturing errors and process variations because both the orientation of the optical axis and the amount of birefringence need to be precisely controlled. This limits their use in commercial devices. Some techniques, such as inserting thin wave plates into etched grooves, are demanding from both a manufacturing and assembly perspective.

[0007] Purpose of the Invention

[0008] The object of the present invention is to provide a polarization rotator which can be manufactured using waveguide components that are not sensitive to standard manufacturing and whose assembly requirements are less stringent than in known solutions. To achieve this, according to one aspect of the present invention, an optical coupler is provided having at least a first port and a second port and a birefringent waveplate having a reflective surface.

[0009] The polarization rotator of the present invention for integrated optics offers considerable advantages because it uses standard, manufacturing-insensitive waveguide components combined with an assembly of external wave plates. Compared to slit-integrated wave plates, the assembly does not require high translational alignment accuracy, only angular alignment is required to control the rotation of polarization.

[0010] Therefore, the polarization rotator device of the present invention is easy to implement using standard waveguide components and does not require changes to the photonic integrated circuit manufacturing process. The assembly of the wave plate to the optical chip is simple because only precise angular alignment is required. Summary of the invention

[0011] According to a main aspect of the present invention, there is provided a polarization rotator comprising:

[0012] - an optical coupler comprising a waveguide having at least a first port and a second port at a first end, the first port being configured as an input port for polarized light and the second port being configured as an output port for reflected polarized light, the waveguide having a second end opposite to the first end;

[0013] - a birefringent waveplate having a reflective surface on one side, the waveplate being arranged to receive light from the second end of the waveguide and to reflect light emitted from the coupler back into the coupler.

[0014] According to the present invention, the wave plate is further configured so that the birefringent material rotates the polarization of the reflected light by an amount that depends on the rotation angle of the birefringent wave plate relative to the optical coupler.

[0015] A waveguide has an optical axis determined by the principal direction of light propagation in the waveguide.

[0016] In some embodiments, the birefringent wave plate is oriented vertically relative to the waveguide so that the normal to the plane of the wave plate is perpendicular to the second end of the waveguide, and wherein the polarization rotation is determined by the angle through which the wave plate is rotated about an axis perpendicular to the plane of the birefringent wave plate. In some variations of these embodiments, the polarization rotation can be tuned by rotating the wave plate about an axis perpendicular to the plane of the birefringent wave plate.

[0017] In some other embodiments, a birefringent waveplate is positioned laterally relative to the waveguide so that the normal to the plane of the waveplate is perpendicular to the optical axis of the waveguide, and light is directed from the opposite end of the waveguide to the birefringent waveplate by an optical element, such as a mirror or a second waveguide, and wherein the polarization rotation is determined by the angle of rotation of the waveplate about an axis perpendicular to the plane of the birefringent waveplate. In some variations of these embodiments, the polarization rotation is tunable when the waveplate is rotated about the axis perpendicular to the plane of the birefringent waveplate.

[0018] In some other embodiments, light emitted from the second end of the waveguide is coupled back into the coupler through at least one other waveguide disposed between the coupler and the waveplate.In some embodiments, the axis perpendicular to the birefringent waveplate is perpendicular to the optical axis of the waveplate. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A prior art MMI coupler is shown;

[0020] FIG. 2A to FIG. 2C Various basic aspects of one embodiment of a basic polarization rotator of the present invention are shown;

[0021] Figure 3 Another embodiment of the polarization rotator of the present invention is shown;

[0022] Figure 4 shows further details of the polarization rotator of the present invention;

[0023] Figure 5 Another embodiment of the polarization rotator of the present invention is shown;

[0024] Figure 6 Another embodiment of the polarization rotator of the present invention is shown. DETAILED DESCRIPTION

[0025] Figure 1 A prior art 2x2 multimode interference (MMI) coupler 10 is shown with one-to-one imaging between input ports 11a, 11b and output ports 12a, 12b, here with 100% light transmission to the crossover port (11a->12b), and with invariant polarization.

[0026] Figure 2A FIG. 2 shows the basic structure of an MMI coupler 20 that can be used in the polarization rotator of the present invention. The coupler 20 is similar to Figure 1The coupler in FIG. 1 is similar to the coupler in FIG. 1 , but here it is only half the length and terminated with a reflector 23. Thus, the input port 21 and the output port 22 are located on the same side, receiving and transmitting light in opposite directions, while maintaining a contact between the input port and the output port. Figure 1 The coupler 20 can preferably be polarization independent, and instead of an MMI it can also be, for example, a directional coupler. The purpose of the coupler 20 is to route the light reflected from the wave plate 24 into a port different from the input port, since reflecting the light back to the light source is generally not desired and can be detrimental to the optical operation of, for example, a laser. The return path LP (light propagation) of the light in the coupler is provided by a reflector 23 that routes the light into the output port 22. The waveguide of the coupler is indicated by reference numeral 27. The waveguide 27 can be the entire coupler, or the coupler 20 can contain additional layers or materials not shown. For the purposes of this description, the coupler 20 and the waveguide 27 have the same function.

[0027] Figure 2B The principle of the polarization rotator of the present invention is shown. Figure 2A The reflector 23 in the MMI coupler 20 is replaced by a reflective wave plate 24 placed parallel to the end face of the MMI coupler 20. The wave plate 24 will cause polarization rotation in the reflected light field. The wave plate can be made of a first refractive index η o The "fast" axis and has a second refractive index η e The layer 25 of birefringent material with a "slow" axis is composed. The birefringent material is covered with a reflective surface 26 on one side, for example by metallization. By using a reflective wave plate 24 to reflect the light field, the polarization of the light can be rotated as required by aligning the angle of the optical axis of the wave plate 24 relative to the coupler 20.

[0028] Figure 2C A tunable polarization rotator of the invention is shown having a rotatable wave plate 24 in operation. As can be seen, the amount of polarization rotation R' at output port 22 relative to the polarization at input port 21 depends on the amount of rotation R of the optical axis of wave plate 24 relative to waveguide 20.

[0029] In its simplest form, the coupler can be located at the edge of the optical chip, and the reflective waveplate can then be located on the outside of the chip. The reflective waveplate can also be placed vertically in a trench on the optical chip. In contrast to the previous trench integration concept, the trench size is not critical here, as it only needs to be large enough to accommodate the waveplate. Furthermore, the coupler and waveplate can be physically separated and connected to the waveguide (see Figure 5 ), so that the coupler can be freely placed on the optical chip, and the wave plate is located at the edge of the chip, where assembly is unimportant.

[0030] Figure 3Another embodiment of the invention is shown in which a coupler 30 terminates in a mirror 33 that reflects light upward. This allows a birefringent wave plate 34 to be placed on top of the coupler 30. Light fed into the input port 31 of the coupler is reflected upward by the birefringent layer 35 and reflected back from the reflective surface 36 and then reflected forward through the coupler to the output port 32, where the exiting light has a polarization rotation R' corresponding to the angle R at which the wave plate 34 is rotated on top of the coupler.

[0031] Figure 4 In more detail, the FIG. 2A to FIG. 2C . The coupler 40 may be composed of a silicon layer 41 on a silicon dioxide (SiO2) layer 42 (with a silicon substrate 47 located underneath) in a typical SOI configuration. For example, the reflective wave plate 44 may be composed of a birefringent layer 45 of lithium niobate (LiNbO3). For example, the thickness of the layer 45 may be 5-10 μm. For example, the reflective layer 46 may be composed of a high reflective (HR) reflector. The ends of the SOI devices 41, 42, 47 facing the reflector 44 may be provided with an anti-reflective (AR) coating 43.

[0032] Figure 5 Shown with Figure 2C 50, which has an input port 51 and an output port 52 and a waveguide 53. Here, the wave plate 54 is not located directly after the coupler, but is optically connected to the coupler via additional waveguides 55 and 56. Since two intermediate waveguides 55, 56 are arranged between the waveguide 53 and the wave plate 54, the light emitted from the waveguide 53 is coupled to the two intermediate waveguides 55, 56 and returns to the coupler 50 through the same intermediate waveguides 55, 56. This arrangement allows, for example, to position the coupler in the middle of the optical chip 57 and the wave plate at the edge 58 of the chip.

[0033] Light exiting the waveguide will begin to expand at a rate inversely proportional to the beam size. To reduce beam divergence and thereby reduce coupling losses, a wide mode field can be provided at the edge of the second end of the waveguide in the coupler. In the lateral direction, expansion can be accomplished by widening the waveguide. However, the waveguide layers are typically thin, and the beam will begin to expand rapidly in the vertical direction. The more the beam expands as it propagates in free space, the less power is coupled back into the waveguide if the reflection is from a flat surface.

[0034] Since the vertical divergence depends on the waveguide thickness, which is usually fixed, it may be useful to focus the light that expands vertically after reflection back into the waveguide in some way. This can be done, for example, with a cylindrical reflector. The curved surface of the cylindrical reflector is the part of the cylinder on which the light always strikes vertically. The reflected light will then be focused back into the SOI waveguide.

[0035] An alternative way to reduce coupling losses is to collimate the light leaving the waveguide in the vertical direction with a cylindrical lens and reflect this collimated beam onto a flat waveplate. The beam expansion is then mitigated by a cylindrical lens. A cylindrical lens is a lens that focuses light in only one direction, forming a focal line instead of a point.

[0036] Figure 6 Shown with Figure 2C and Figure 4 The embodiment of a coupler similar to the coupler shown in FIG. The coupler 60 may be composed of a silicon layer 61 on a silicon dioxide (SiO2) layer 62 (silicon substrate 67 is located below). The reflective wave plate 64 has a birefringent layer 65 and a reflective layer 66, and the ends of the SOI devices 61, 62, 67 facing the reflector 64 may be provided with an anti-reflection (AR) coating 63.

[0037] exist Figure 6 In the embodiment of FIG. 6 , a cylindrical lens 69 located between the coupler and the wave plate 64 is used to collimate the light beam 68 leaving the SOI device 61. The light returns to the coupler 61 through the same cylindrical lens 69. This arrangement reduces coupling losses caused by the divergence of the light beam 68.

[0038] It should be understood that the disclosed embodiments of the present invention are not limited to the specific structures, process steps or materials disclosed herein, but are extended to equivalents thereof that will be recognized by those of ordinary skill in the relevant art. It should also be understood that the technical terms used herein are used only for the purpose of describing specific embodiments and are not intended to be limiting.

[0039] Reference to an embodiment or embodiments in this specification means that a particular feature, structure, or characteristic described in conjunction with the embodiment is included in at least one embodiment of the invention. Thus, the phrases "in one embodiment" or "in an embodiment" appearing in various places throughout the specification do not necessarily all refer to the same embodiment. Where a value is referred to using the terms "such as," "for example," "about," or "substantially," the exact value is also disclosed.

[0040] As used herein, for convenience, multiple items, structural elements, constituent elements and / or materials may be presented in a common list. However, these lists should be understood as: each member in the list is individually identified as a separate, unique member. Therefore, any individual member on such a list should not be understood as a de facto equivalent of any other member on the same list simply based on its performance in a common group, without any indication to the contrary. In addition, various embodiments and examples of the present invention and alternatives to its various components may be mentioned herein. It should be understood that such embodiments, examples and alternatives should not be interpreted as de facto equivalents of each other, but should be understood as independent and autonomous representations of the present invention.

[0041] In addition, in one or more embodiments, the described features, structures or characteristics may be combined in any suitable manner. In the foregoing description, many specific details, such as examples of lengths, widths, shapes, etc., are provided to provide a thorough understanding of embodiments of the present invention. However, those skilled in the relevant art will recognize that the present invention may be implemented without one or more of the specific details, or with other methods, components, materials, etc. In other cases, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the present invention.

[0042] Although the above examples illustrate the principles of the present invention in one or more specific applications, it is obvious to those skilled in the art that a large number of modifications can be made in form, usage and implementation details without exercising creativity and without departing from the principles and concepts of the present invention. Therefore, the present invention is not intended to be limited except for the limitations of the following claims.

[0043] The verbs "comprise" and "include" in this document are used as open limitations, neither excluding nor requiring the presence of unrecited features. Unless explicitly stated otherwise, the features recited in the dependent claims are freely combinable with each other. Furthermore, it should be understood that the use of "a" or "an" (i.e., the singular) in this document does not exclude the plural.

Claims

1. A polarization rotator, comprising: an optical coupler comprising a waveguide having at least a first port and a second port at a first end, the first port being configured as an input port for polarized light and the second port being configured as an output port for reflected polarized light, the waveguide having a second end opposite the first end; a birefringent wave plate having a reflective surface on one side, the birefringent wave plate being arranged to receive light from the second end of the waveguide and reflect light emitted from the optical coupler back into the optical coupler, wherein The birefringent wave plate is also configured to rotate the polarization of reflected light by a birefringent material of the birefringent wave plate, the amount of rotation depending on the rotation angle of the birefringent wave plate relative to the optical coupler, and the polarization rotation is tunable when the birefringent wave plate is rotated about an axis perpendicular to the plane of the birefringent wave plate.

2. The polarization rotator according to claim 1, wherein: The birefringent waveplate is vertically oriented relative to the waveguide such that a normal to a plane of the birefringent waveplate is perpendicular to the second end of the waveguide, and wherein the polarization rotation is determined by an angle of rotation of the birefringent waveplate about an axis perpendicular to the plane of the birefringent waveplate.

3. The polarization rotator according to claim 1, wherein: The birefringent wave plate is positioned laterally relative to the waveguide such that a normal to a plane of the birefringent wave plate is perpendicular to an optical axis of the waveguide, and light is directed from the second end of the waveguide to the birefringent wave plate by an optical element, and wherein the polarization rotation is determined by an angle through which the birefringent wave plate is rotated about an axis perpendicular to the plane of the birefringent wave plate.

4. The polarization rotator according to claim 3, wherein: The optical element is a mirror or a second waveguide.

5. The polarization rotator according to any one of claims 1 to 4, wherein: Light emitted from the second end of the waveguide is coupled back into the optical coupler through at least one other waveguide arranged between the optical coupler and the birefringent wave plate.

6. The polarization rotator according to any one of claims 1 to 4, wherein: An axis perpendicular to the plane of the birefringent wave plate is perpendicular to the optical axis of the birefringent wave plate.

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