Polarization-maintaining circulator

Abstract

The present application relates to the technical field of optics. Disclosed is a polarization-maintaining circulator (100), comprising a dual-core waveguide (21) for realizing a port I and a port III, a first birefringent crystal (22), a first Faraday rotator (23), a second birefringent crystal (24), a lens (25) and a port II, which are sequentially arranged in a first direction, wherein the first birefringent crystal (22), the first Faraday rotator (23) and the second birefringent crystal (24) are all parallel flat-plate structures, such that light inputted via the port I is transmitted to the port II in the first direction, and light inputted via the port II is transmitted to the port III in a second direction, the first direction being opposite to the second direction. By means of the above means, the present application simplifies the device structure, and achieves miniaturization of the circulator.

Description

Polarization-Maintaining Circulator Technical Field

[0001] The application embodiment relates to the field of optical technology, and specifically to a polarization-maintaining circulator. Background Art

[0002] An optical circulator is a multi-port, non-reciprocal optical passive device that restricts optical signal transmission along a specified port sequence and prevents reverse transmission. Optical circulators are widely used in optical communications and optical sensing for single-fiber bidirectional communication, optical signal addition and drop, and dispersion compensation.

[0003] Currently, optical circulators are mainly divided into two major designs. One is that ports 1, 2, and 3 face three different directions, and the device has a rectangular appearance. This structure takes up a lot of space and is extremely inconvenient to arrange. The other is that ports 1 and 3 face the same direction, while port 2 faces the opposite direction. The device has a cylindrical appearance. Although this design solves the problem of large space occupation of the previous solution, the overall length is still long. In addition, both of the above solutions are parallel optical path designs and require the use of two or three collimators (such as waveguides + lenses) and polarization beam splitters, at least one or more Faraday rotators, half-wave plates, Wollaston wedge pairs, or polarization beam combiner components. This results in many parts, complex processes, and instability.

[0004] Summary of the Invention

[0005] In view of the above problems, an embodiment of the present application provides a polarization-maintaining circulator to simplify the device structure and achieve miniaturization of the device.

[0006] An embodiment of the present application provides a polarization-maintaining circulator, configured to transmit light input from port one to port two in a first direction, and to transmit light input from port two to port three in a second direction, wherein the first direction and the second direction are opposite. The polarization-maintaining circulator includes: a dual-core waveguide, a first birefringent crystal, a first Faraday rotator, a second birefringent crystal, a lens, and port two, which are sequentially arranged along the first direction. The first birefringent crystal, the first Faraday rotator, and the second birefringent crystal are all parallel plate structures. Port one is a polarization-maintaining port for inputting divergent linearly polarized light. , the light input from the first port enters the first birefringent crystal as o light or e light; the first birefringent crystal is used to make the light input from the first port form a positive displacement toward the first birefringent crystal and then emit it to the first Faraday rotator; the first Faraday rotator is used to rotate the polarization state of the light input from the first port by an angle α along a first rotation direction and then emit it to the second birefringent crystal, wherein α=45°; the second birefringent crystal is used to make the light input from the first port form a positive displacement toward the second birefringent crystal and then emit it to the lens; the lens is used to perform optical shaping on the light input from the first port and then emit it to the second port; the second port is used to output the The light input from the port 1 is transmitted from the port 1 to the port 2; the port 2 is also used to input light and direct the input light toward the lens; the lens is also used to optically converge the light input from the port 2 and direct it toward the second birefringent crystal; the second birefringent crystal is also used to cause the light input from the port 2 to form a reverse second birefringent crystal displacement and then direct it toward the first Faraday rotator, the first Faraday rotator is also used to rotate the polarization state of the light input from the port 2 by the angle α along the first rotation direction and then direct it toward the first birefringent crystal; the first birefringent crystal is also used to cause the light input from the port 2 to form a reverse second birefringent crystal displacement and then direct it toward the first Faraday rotator. The first birefringent crystal is displaced and directed toward the third port, wherein the polarization state of the light input from the second port in the first birefringent crystal is rotated 90° relative to the polarization state of the light input from the first port, and the o-light and the e-light are transformed, so that the light input from the second port deviates from the light input from the first port in the first birefringent crystal, and the lateral walk-off generated between the light input from the first port and the light input from the second port is equal to the dual-core spacing of the dual-core waveguide, so that the light input from the second port is coupled into the third port; the third port is used to output the light input from the second port, thereby realizing the transmission of light from the second port to the third port.

[0007] The polarization-maintaining circulator provided in the present application is designed for non-parallel light paths, realizes the transmission of optical signals along a specified port sequence, and prevents the optical signals from transmitting in the opposite direction. It does not require complex optical components, and the number of required optical components is small, which is conducive to product miniaturization and reduced production costs. In addition, the first birefringent crystal, the first Faraday rotator, and the second birefringent crystal are all parallel plate structures, which are simple in structure and easy to process and manufacture.

[0008] The above description is only an overview of the technical solutions of the embodiments of the present application. In order to more clearly understand the technical means of the embodiments of the present application, they can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the embodiments of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The accompanying drawings are only used to illustrate the embodiments and are not to be considered as limiting the present application. In addition, the same reference symbols are used to represent the same components throughout the drawings. In the drawings:

[0010] FIG1a is a schematic diagram of birefringence phenomenon and light displacement of a positive birefringence crystal provided by an embodiment of the present application;

[0011] FIG1 b is a schematic diagram of birefringence phenomenon and light displacement of a positive birefringence crystal provided by an embodiment of the present application;

[0012] FIG2 a is a schematic diagram of birefringence phenomenon and light displacement of a negative birefringence crystal provided by an embodiment of the present application;

[0013] FIG2 b is a schematic diagram of the birefringence phenomenon and light displacement of a negative birefringence crystal provided by an embodiment of the present application;

[0014] FIG3 is a schematic diagram of the optical path of a three-port full-polarization-maintaining waveguide polarization-maintaining circulator with all-optical waveguide input and output provided in Example 1 of the present application;

[0015] FIG4 is a schematic diagram of the structure and optical axis of a birefringent crystal A1 provided in an embodiment of the present application;

[0016] FIG5 is a schematic diagram of the structure and optical axis of a birefringent crystal B1 provided in an embodiment of the present application;

[0017] FIG6 a is a schematic diagram showing the displacement of forward light and reverse light in a birefringent crystal A1 according to an embodiment of the present application;

[0018] FIG6 b is a schematic diagram of the displacement of forward light and reverse light in the birefringent crystal B1 according to an embodiment of the present application;

[0019] FIG7 is a schematic diagram of the optical paths of the o-light incident from port 1 and the e-light incident from port 2 in the polarization-maintaining circulator provided in Example 1 of the present application;

[0020] FIG8 is a schematic diagram showing the displacement of linearly polarized light incident from port 1 to port 2 (in the first direction) and from port 2 to port 3 (in the second direction) in the crystal after the linearly polarized light as o-light incident from port 1 enters the birefringent crystal 22 in the polarization-maintaining circulator provided in Example 1 of the present application;

[0021] FIG9 is a schematic diagram of the optical path of e-light incident from port three in the polarization-maintaining circulator provided in Example 1 of the present application;

[0022] FIG10 is a schematic diagram of the reverse isolation displacement of linearly polarized light incident from port 3 to port 2 in the polarization-maintaining circulator provided in Example 1 of the present application after it enters the birefringent crystal 22 as e-light;

[0023] FIG11 is a schematic diagram of the optical path of a 4-port full-polarization-maintaining waveguide polarization-maintaining circulator with all-optical waveguide input and output provided in Example 2 of the present application;

[0024] FIG12 is a schematic diagram of the optical paths of the e-light incident from port 3 and the o-light incident from port 4 in the polarization-maintaining circulator provided in Example 2 of the present application;

[0025] FIG13 is a schematic diagram showing the displacement of linearly polarized light incident from port 3 to port 4 and from port 4 to port 1 in the polarization-maintaining circulator provided in Example 2 of the present application after entering the birefringent crystal 22 as e-light;

[0026] FIG14 is a schematic diagram of the optical path of a single-stage isolated polarization-maintaining circulator with three ports and non-full polarization-maintaining waveguides and all-optical waveguide input and output, provided in Example 3 of the present application;

[0027] FIG15 a is a schematic diagram of the structure and optical axis of a birefringent crystal C1 provided in an embodiment of the present application;

[0028] FIG15 b is a schematic diagram showing the displacement of forward light and reverse light in the birefringent crystal C1 according to an embodiment of the present application;

[0029] FIG16a is a schematic diagram showing the displacement of linearly polarized light incident from port 1 to port 2 in the polarization-maintaining circulator provided in Example 3 of the present application after it enters the birefringent crystal 22 as o-light;

[0030] FIG16 b is a schematic diagram showing the displacement of light incident from port 2 to port 3 in the polarization-maintaining circulator provided in Example 3 of the present application after entering the birefringent crystal 27 as o-light and e-light;

[0031] FIG17a is a schematic diagram of the optical path of o-light incident from port three in the polarization-maintaining circulator provided in Example 3 of the present application;

[0032] FIG17 b is a schematic diagram of the optical path of e-light incident from port three in the polarization-maintaining circulator provided in Example 3 of the present application;

[0033] FIG17c is a schematic diagram of the reverse isolation displacement of linearly polarized light incident from port 3 to port 2 in the polarization-maintaining circulator provided in Example 3 of the present application after it enters the birefringent crystal 22 as o-light;

[0034] FIG17 d is a schematic diagram of the reverse isolation displacement of linearly polarized light incident from port 3 to port 2 in the polarization-maintaining circulator provided in Example 3 of the present application after entering the birefringent crystal 22 as e-light;

[0035] FIG18 is a schematic diagram of the optical path of another 3-port non-full-polarization-maintaining waveguide single-stage isolated polarization-maintaining circulator with all-optical waveguide input and output provided in Example 4 of the present application;

[0036] FIG19a is a schematic diagram showing the displacement of linearly polarized light incident from port 1 to port 2 in the polarization-maintaining circulator provided in Example 4 of the present application after it enters the birefringent crystal 27 as o-light;

[0037] FIG19 b is a schematic diagram showing the displacement of light incident from port 2 to port 3 in the polarization-maintaining circulator provided in Example 4 of the present application after entering the birefringent crystal 24 as o-light and e-light.

[0038] FIG20 a is a schematic diagram of the optical path of o-light incident from port three in the polarization-maintaining circulator provided in Example 4 of the present application;

[0039] FIG20 b is a schematic diagram of the optical path of e-light incident from port three in the polarization-maintaining circulator provided in Example 4 of the present application;

[0040] FIG20 c is a schematic diagram of the reverse isolation displacement of linearly polarized light incident from port 3 to port 2 in the polarization-maintaining circulator provided in Example 4 of the present application after entering the birefringent crystal 27 as o-light;

[0041] FIG20 d is a schematic diagram of the reverse isolation displacement of linearly polarized light incident from port 3 to port 2 in the polarization-maintaining circulator provided in Example 4 of the present application after entering the birefringent crystal 27 as e-light;

[0042] FIG21 a is a schematic diagram of the optical path of a three-port non-full-polarization-maintaining waveguide double-stage isolated polarization-maintaining circulator with all-optical waveguide input and output provided in Example 5 of the present application;

[0043] FIG21 b is a schematic diagram of the structure and optical axis of a birefringent crystal D1 provided in an embodiment of the present application;

[0044] FIG21c is a schematic diagram showing the displacement of forward light and reverse light in the birefringent crystal D1 according to an embodiment of the present application;

[0045] FIG22 a is a schematic diagram showing the displacement of linearly polarized light incident from port 1 to port 2 in the polarization-maintaining circulator provided in Example 5 of the present application after it enters the birefringent crystal 22 as o-light;

[0046] FIG22 b is a schematic diagram showing the displacement of light incident from port 2 to port 3 in the polarization-maintaining circulator provided in Example 5 of the present application after entering the birefringent crystal 27 as o-light and e-light;

[0047] FIG23a is a schematic diagram of the optical path of o-light incident from port three in the polarization-maintaining circulator provided in Example 5 of the present application;

[0048] FIG23 b is a schematic diagram of the optical path of e-light incident from port three in the polarization-maintaining circulator provided in Example 5 of the present application;

[0049] FIG23 c is a schematic diagram of the reverse isolation displacement of linearly polarized light incident from port 3 to port 2 in the polarization-maintaining circulator provided in Example 5 of the present application after it enters the birefringent crystal 22 as o-light;

[0050] FIG23 d is a schematic diagram of the reverse isolation displacement of linearly polarized light incident from port 3 to port 2 in the polarization-maintaining circulator provided in Example 5 of the present application after entering the birefringent crystal 22 as e-light;

[0051] FIG24 is a schematic diagram of the optical path of a three-port fully polarization-maintaining circulator with parallel light output from port two provided in Example 6 of the present application;

[0052] FIG25 a is a schematic diagram of the optical path of a three-port non-full-polarization-maintaining single-stage isolated polarization-maintaining circulator with parallel light output from port two provided in Example 7 of the present application;

[0053] FIG25 b is a schematic diagram of the optical path of another three-port non-full-polarization-maintaining single-stage isolated polarization-maintaining circulator with parallel light output from two ports provided in Example 8 of the present application;

[0054] FIG26 is a schematic diagram of the optical path of a three-port non-full-polarization-maintaining double-stage isolated polarization-maintaining circulator with parallel light output from port two provided in Example 9 of the present application;

[0055] FIG27 is a schematic diagram of the optical path of a polarization-maintaining circulator provided in Example 10 of the present application, in which a filter is added to the polarization-maintaining circulator shown in FIG24 ;

[0056] FIG28 a is a schematic diagram of the optical path of a polarization-maintaining circulator provided in Example 11 of the present application, in which a filter is added to the polarization-maintaining circulator shown in FIG25 a ;

[0057] FIG28 b is a schematic diagram of the optical path of a polarization-maintaining circulator provided in Example 12 of the present application, in which a filter is added to the polarization-maintaining circulator shown in FIG25 b ;

[0058] FIG29 is a schematic diagram of the optical path of a polarization-maintaining circulator provided in Example 13 of the present application, in which a filter is added to the polarization-maintaining circulator shown in FIG26 ;

[0059] FIG30 a is a schematic diagram showing the structure and optical axis of a first birefringent crystal as a combined crystal provided in an embodiment of the present application;

[0060] FIG30 b is a schematic diagram of the structure and optical axis of a first birefringent crystal as a combined crystal provided in another embodiment of the present application. DETAILED DESCRIPTION

[0061] The following embodiments of the technical solution of the present application will be described in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application and are therefore only examples and are not intended to limit the scope of protection of the present application.

[0062] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned figure descriptions are intended to cover non-exclusive inclusions.

[0063] In the description of the embodiments of this application, the technical terms "first" and "second" are used only to distinguish different objects and should not be understood to indicate or imply relative importance or implicitly specify the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "plurality" is more than two, unless otherwise clearly and specifically defined.

[0064] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0065] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists, A and B exist at the same time, and B exists. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0066] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0067] In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the embodiments of the present application.

[0068] During the development of the present invention, the inventors discovered that existing circulators suffer from complex structures, high costs, and poor process stability. In response to these issues, the present invention provides a minimalist converging optical circulator that utilizes a birefringent crystal and a Faraday rotator to process the converging light. Ports one and three of the circulator face the same direction, resulting in a simple structure, significantly reduced dimensions, and a simplified manufacturing process, significantly improving product manufacturability.

[0069] For the phenomenon of crystal birefringence and light displacement, please refer to Figures 1a and 1b. Taking a light beam incident on positively birefringent crystal A as an example, when the polarization state of the light beam incident on positively birefringent crystal A is misaligned with both the o-ray and e-ray of positively birefringent crystal A, the light beam is decomposed into o-ray and e-ray in positively birefringent crystal A. The o-ray follows the law of refraction, and the angle of the light beam remains unchanged, entering and exiting positively birefringent crystal A in the same direction as the incident light. However, the e-ray does not follow the law of refraction and deviates from the optical path in positively birefringent crystal A, resulting in a certain displacement d before exiting positively birefringent crystal A. At this time, the relative displacement of the o-ray and e-ray is d. The birefringence and light displacement phenomenon caused by a light beam incident on negatively birefringent crystal B is shown in Figures 2a and 2b, and will not be further described here.

[0070] Since the e-ray is deflected in opposite directions by positive and negative birefringent crystals, for simplicity, the following explanation will use a positive birefringent crystal as an example. The angle θ between the optical axis of positive birefringent crystal A and the normal to the incident surface is called the walk-off angle. This walk-off angle θ is used to control the relative displacement of the o-ray and e-ray after the light passes through positive birefringent crystal A. When 0°<θ<90°, as shown in Figure 1a, the e-ray is deflected upward, resulting in a displacement of d; when -90°<θ<0°, as shown in Figure 1b, the e-ray is deflected downward, resulting in a displacement of -d. Therefore, by changing the angle θ, the relative displacement of the o-ray and e-ray at the output end can be changed.

[0071] According to one aspect of an embodiment of the present application, a polarization-maintaining circulator is provided, configured to transmit light input from port 1 to port 2 in a first direction, and transmit light input from port 2 to port 3 in a second direction, the first and second directions being opposite. Unless otherwise specified, in the figures of the embodiments of the present application, ① represents port 1, ② represents port 2, ③ represents port 3, and ④ represents port 4. Unless otherwise specified, port 1 in all figures refers to port 1 as described in the text of the present application, port 2 refers to port 2 as described in the text of the present application, and port 3 refers to port 3 as described in the text of the present application. For details, please refer to FIG3 , which illustrates the structure of the polarization-maintaining circulator provided in an embodiment of the present application. As shown in the figure, the polarization-maintaining circulator 100 includes a dual-core waveguide 21, a first birefringent crystal 22, a first Faraday rotator 23, a second birefringent crystal 24, a lens 25, and port 2, arranged in sequence along a first direction to realize port 1 and port 3. Port 1 is a polarization-maintaining port, and the first birefringent crystal 22, the first Faraday rotator 23, and the second birefringent crystal 24 are all parallel plate structures.

[0072] The following describes the embodiments of the present application based on the light transmission process between ports.

[0073] 1. Port 1 to Port 2:

[0074] Port 1 is used to input divergent linearly polarized light with a fixed polarization direction. The light input from port 1 enters the first birefringent crystal 22 as either o-light or e-light (Figure 3 illustrates o-light as an example). The first birefringent crystal 22 is used to cause the light input from port 1 to undergo a first crystal forward shift before being directed toward the first Faraday rotator 23. The first Faraday rotator 23 is used to rotate the polarization state of the light input from port 1 by an angle α (45°) along a first rotation direction before being directed toward the second birefringent crystal 24. The second birefringent crystal 24 is used to cause the light input from port 1 to undergo a second crystal forward shift before being directed toward the lens 25. The lens 25 is used to optically shape the light input from port 1 (either converging or collimating) before being directed toward port 2. Port 2 is used to output the light input from port 1, enabling light transmission from port 1 to port 2.

[0075] In the first direction, the polarization state of the o-light from the first birefringent crystal 22 is aligned with the polarization state of the o-light or e-light from the second birefringent crystal 24 after passing through the first Faraday rotator 23. In the specific embodiment shown in FIG3 , along the first direction, the polarization state of the o-light from the first birefringent crystal 22 is aligned with the polarization state of the e-light from the second birefringent crystal 24 after passing through the first Faraday rotator 23. That is, as shown in FIG3 , when light input from port 1 enters the first birefringent crystal 22 as o-light, it is emitted in the same direction as the incident light and does not deviate from the optical path. After passing through the first Faraday rotator 23, the polarization state of the light is rotated 45° (either clockwise or counterclockwise). After the rotation, the polarization state of the light is aligned with the polarization state of the e-light from the second birefringent crystal 24, and the optical path deviates in the second birefringent crystal 24.

[0076] In some other embodiments, along the first direction, the polarization state of the O light of the first birefringent crystal 22 can also be aligned with the polarization state of the O light of the second birefringent crystal 24 after passing through the first Faraday rotator 23. The O light of the first birefringent crystal 22 is also incident and emitted from the second birefringent crystal 24 in the same direction, and no optical path deviation occurs.

[0077] 2. Port 2 to Port 3:

[0078] Port 2 is used to input divergent linearly polarized light with a fixed polarization direction and directs the input light toward lens 25. Lens 25 is used to optically converge the light input from port 2 and direct it toward second birefringent crystal 24. Second birefringent crystal 24 is used to cause the light input from port 2 to undergo a second crystal reverse shift before directing it toward first Faraday rotator 23. In other embodiments, the light input from port 2 may also be randomly polarized light, as will be explained in detail below using specific embodiments.

[0079] The first Faraday rotator 23 is used to rotate the polarization state of the light input from port 2 by an angle of α (45°) along a first rotation direction before it is directed toward the first birefringent crystal 22. The Faraday rotator's ability to rotate the polarization state of a light beam has been mentioned above. Accordingly, in Figure 3, viewed from left to right, the polarization states of the light from port 1 to port 2 and from port 2 to port 3 are both rotated clockwise or counterclockwise when passing through the first Faraday rotator 23. Therefore, after passing through the first Faraday rotator 23, the polarization state of the light input from port 2 undergoes a 90° rotation in the first birefringent crystal 22 relative to the light input from port 1, resulting in a conversion between the o- and e-light beams.

[0080] Based on this, in the optical path, between port 2 and the output end of the first Faraday rotator 23 along the second direction, the optical paths of the light input from port 1 and the light input from port 2 completely overlap, except that the transformation and relative displacement of the o-light and the e-light occur in the first birefringent crystal 22.

[0081] Light continues to enter the first birefringent crystal 22, which is used to cause the light input from port 2 to undergo a reverse shift in the first crystal before being directed toward port 3. As shown above, the polarization state of the light input from port 2 in the first birefringent crystal 22 is rotated 90° relative to the polarization state of the light input from port 1. The o-light and e-light are transformed, causing the light input from port 2 to deviate from the light input from port 1 in the first birefringent crystal 22. The lateral walk-off between the light input from port 1 and the light input from port 2 is equal to the dual-core spacing of the dual-core waveguide 21, allowing the light input from port 2 to be coupled into port 3. Port 3 is used to output the light input from port 2, enabling light transmission from port 2 to port 3. The lateral walk-off is equal to the relative displacement formed by the sum of the forward displacement of the first crystal, the forward displacement of the second crystal, the reverse displacement of the second crystal, and the reverse displacement of the first crystal.

[0082] To facilitate understanding of the specific embodiments, the following description uses the example of light input from port 1 being incident normally on the first birefringent crystal 22. The functions provided by the embodiments of the present invention are equally applicable to the case of light input from port 1 being incident obliquely. To illustrate this, two birefringent crystals are first used as examples. In the specific embodiments, different solutions can be achieved by using different alignment methods for the o- and e-lights of these two birefringent crystals. For details, please refer to Figures 4 and 5. Figure 4 schematically illustrates the structure and optical axis of birefringent crystal A1, while Figure 5 schematically illustrates the structure and optical axis of birefringent crystal B1.

[0083] As shown in FIG4 , the birefringent crystal A1 is a parallel plate structure. The incident surface of the light input from port 1 is the plane where the x and y axes are located, and the first direction is the positive direction of the z axis. A three-dimensional rectangular coordinate system is established, wherein the normal line of the incident surface of the light input from port 1 into the birefringent crystal A1 is parallel to the z axis. The angle θ between the optical axis of the birefringent crystal A1 and the normal line of the incident surface of the light input from port 1 (i.e., the z axis) is A1 The angle between the line where the optical axis of the birefringent crystal A1, the e-light oscillation plane, intersects the xy plane, and the y-axis is Figure 4 FIG6a shows a schematic diagram of light displacement in birefringent crystal A1 (in FIG6 and subsequent light displacement diagrams, the first direction is defined as the forward direction and the second direction is defined as the reverse direction). An x- and y-axis rectangular coordinate system is formed on the incident surface of birefringent crystal A1, with the incident point of light input from port 1 as the origin. Both the relative displacement of light input from port 1 and the relative displacement of light input from port 2 are referenced to the xyz coordinate system of light input from port 1. As shown in FIG6a , after light input from port 1 is incident normally on birefringent crystal A1, the forward o-light component follows the law of refraction and does not undergo displacement in the xy plane. It exits birefringent crystal A1 at the origin, meaning that the forward o-light component has a displacement of DoxI=0 along the x-axis and DoyI=0 along the y-axis in birefringent crystal A1. The forward e-light component does not follow the law of refraction, with a displacement along the x-axis set to DexI=2a and a displacement along the y-axis set to DeyI=0. Correspondingly, the normally incident reverse o light also does not shift, and the reverse e light shifts along the x-axis in the birefringent crystal A1 by rDexI = -2a, rDeyI = 0. Of course, other shift values ​​can be used according to specific circumstances and are not limited here.

[0084] As shown in FIG5 , birefringent crystal B1 is also a parallel plate structure. A three-dimensional rectangular coordinate system is established in birefringent crystal B1 in the same manner as birefringent crystal A1. The angle θ between the optical axis of birefringent crystal B1 and the normal to the light incident surface of port 1 (i.e., the z-axis) is B1 The angle between the line where the e-light oscillation plane where the optical axis of the birefringent crystal B1 intersects the xy plane and the y-axis is Figure 5 Figure 6b shows a schematic diagram of light displacement in birefringent crystal B1. The coordinate system and reference method are the same as those for birefringent crystal A1. As shown in Figure 6b, the displacement of normally incident forward e-ray in birefringent crystal B1 is DexII = -a, DeyII = a. Correspondingly, the displacement of normally incident reverse e-ray in birefringent crystal B1 is rDexII = a, rDeyII = -a. Of course, other displacement values ​​can be used depending on the specific situation and are not limited here.

[0085] In the embodiment of the present application, the first birefringent crystal 22 is the birefringent crystal A1, and the second birefringent crystal 24 is the birefringent crystal B1. The angle θ1 between the normal of the incident surface of the first birefringent crystal 22 and the optical axis of the first birefringent crystal 22 at port 1 is a first walk-off angle, where -90° < θ1 < 0° or 0° < θ1 < 90°. The first walk-off angle is used to control the forward and reverse displacements of the first crystal.

[0086] In the embodiment of the present application, the angle θ2 between the normal of the incident surface of the second birefringent crystal 24 and the optical axis of the second birefringent crystal 24 of the light input from port 1 is the second walk-off angle, -90°<θ2<0° or 0°<θ2<90°, and the second walk-off angle is used to control the forward displacement of the second crystal and the reverse displacement of the second crystal.

[0087] In the embodiment of the present application, an xyz coordinate system is established with the first direction as the positive direction of the z-axis; the e-light oscillation plane where the optical axis of the first birefringent crystal 22 is located intersects with the xy plane as a first straight line, and the angle between the first straight line and the y-axis is The e-light oscillation plane where the optical axis of the second birefringent crystal 24 is located intersects with the xy plane to form a second straight line. The angle between the second straight line and the y axis is in, or

[0088] for For details, please refer to Figure 4 for the birefringent crystal A1. Description of For details, please refer to Figure 5 for the birefringent crystal B1. By and The relationship is set to or It is ensured that the polarization state of the light emitted from the first birefringent crystal 22 can be aligned with the o-light or e-light of the second birefringent crystal 24 after passing through the first Faraday rotator 23. satisfy or

[0089] Regarding whether the port maintains polarization:

[0090] In the embodiment shown in FIG3 , all ports are polarization-maintaining ports, providing a full polarization-maintaining solution. Ports 1, 2, and 3 are all polarization-maintaining ports. The polarization state of port 2 is aligned with the polarization state of light input from port 1 along a first direction and emitted through lens 25. The polarization state of port 3 is aligned with the polarization state of light input from port 2 along a second direction and emitted through first birefringent crystal 22. The following description will be made using Examples 1, 2, 6, and 10 as examples.

[0091] In some other embodiments, port 1 is a polarization-maintaining port, and whether port 2 and port 3 are polarization-maintaining is not limited. For example, port 2 can be a polarization-maintaining port or a non-polarization-maintaining port, and port 3 can be a polarization-maintaining port or a non-polarization-maintaining port. The resulting circulator can be a full polarization-maintaining circulator or a non-full polarization-maintaining circulator. The following description will use Examples 3, 4, 5, 7, 8, 9, 11, 12, and 13 as examples.

[0092] Regarding the characteristics of the input / output light of port 2:

[0093] In the embodiment shown in Figure 3, port two is a single-core waveguide 26, which is used to output converging light or input diverging light. Lens 25 is used to converge light input from port one to form converging light, which is then coupled into single-core waveguide 26. Single-core waveguide 26 is used to output converging light. Single-core waveguide 26 is used to input diverging linearly polarized light and direct it toward lens 25. Lens 25 is used to converge light input from single-core waveguide 26 to form converging light. The following description will use Examples 1 to 5 as examples.

[0094] In some embodiments, port 2 is used to output or input parallel light; lens 25 is used to collimate the light input from port 1 into parallel light and output it to port 2, which is used to output the parallel light; port 2 is used to input parallel light and direct it toward lens 25, which is used to converge the light input from port 2 into concentrated light. The following description will use Examples 6 to 13 as examples.

[0095] The specific embodiments provided in this application are described in detail below.

[0096] Example 1

[0097] Please continue to refer to FIG3 , which is a schematic diagram of the optical path of a three-port full-polarization-maintaining waveguide polarization-maintaining circulator with all-optical waveguide input and output provided in Example 1 of the present application.

[0098] In this embodiment, the first birefringent crystal 22 is the birefringent crystal A1, and the second birefringent crystal 24 is the birefringent crystal B1. Along the first direction, the polarization state of the light input from port 1 in the first birefringent crystal 22 and the second birefringent crystal 24 is oe.

[0099] Please refer to Figure 7, which illustrates the optical paths of the o-light input to port 1 and the e-light input to port 2 in the polarization-maintaining circulator provided in Example 1. An xyz coordinate system is established, with the first direction being the positive z-axis. As shown in Figure 7, the o-light input to port 1 undergoes no optical path deviation in the first birefringent crystal 22 and exits the exit surface of the first birefringent crystal 22 along the same xy coordinates as its incident position. After passing through the first Faraday rotator 23, the polarization state of the light is rotated 45°, aligning with the polarization state of the e-light input to the second birefringent crystal 24. After entering the second birefringent crystal 24, the light undergoes optical path deviation and exits the second birefringent crystal 24. For the light e input from port 2, the optical paths of the light o input from port 1 and the light e input from port 2 overlap between the input end of the second birefringent crystal 24 along the second direction and the output end of the first Faraday rotator 23 along the second direction. However, after passing through the first Faraday rotator 23, the polarization state of the light e input from port 2 is rotated by 45° in the same direction as the light o input from port 1. Since the polarization state of the light o input from port 1 is rotated by 45° in this direction when passing through the first Faraday rotator 23, the polarization state of the light e input from port 2 is rotated by 45° in the first birefringent crystal 22. The total rotation is 90° relative to the o-light input from port 1, as shown in the polarization direction changes of the o-light input from port 1 and the e-light input from port 2 in the lower portion of FIG7 . In the first birefringent crystal 22, the polarization state of the e-light input from port 2 is perpendicular to the polarization state of the o-light input from port 1, that is, the o-light and the e-light are transformed, so that the polarization state of the e-light input from port 2 is aligned with the polarization state of the e-light input from the first birefringent crystal 22. Then, the e-light input from port 2 deviates from the optical path after entering the first birefringent crystal 22, thereby deviating from the o-light input from port 1.

[0100] This embodiment also provides a schematic diagram of the projected positions of the beam's entrance and exit points at each birefringent crystal in the xy plane, as shown in Figure 8. After the o-light input from port one enters the first birefringent crystal 22 at point A (i.e., the origin of the coordinate system), it undergoes no displacement within the first birefringent crystal 22 and continues to exit at point A (0, 0). After passing through the first Faraday rotator 23, the o-light input from port one enters the second birefringent crystal 24 and propagates in the e-light polarization state, experiencing displacements of DexII = -a and DeyII = a along the x- and y-axes, respectively, and exits at point B (-a, a). After the e-light input from port two enters the second birefringent crystal 24 at point B, it undergoes displacements of DexII = a and DeyII = -a along the x- and y-axes, respectively, and exits at point A. After passing through the first Faraday rotator 23, the e-light input from port 2 enters the first birefringent crystal 22 and continues to propagate in the e-light polarization state, experiencing displacements DexII = -2a and DeyII = 0 along the x- and y-axes, respectively. It exits at point C (-2a, 0) and couples into port 3. Based on this, the o-light input from port 1 enters the first birefringent crystal 22 at point A, and the e-light input from port 2 exits the first birefringent crystal 22 at point C. The two cores of the dual-core waveguide 21 are aligned with points A and C, and the polarization states of the two cores are aligned with the o-light polarization state and the e-light polarization state of the first birefringent crystal 22, respectively.

[0101] Please refer to Figure 9, which illustrates the optical path of the e-light incident from port 3 in the polarization-maintaining circulator provided in Example 1. The light input from port 3 enters the first birefringent crystal 22 as the e-light. The e-light input from port 3 undergoes optical path deflection within the first birefringent crystal 22 and exits the first birefringent crystal 22. After passing through the first Faraday rotator 23, its polarization state is rotated 45°, aligning with the polarization state of the o-light from the second birefringent crystal 24. After entering the second birefringent crystal 24, the light remains on the same optical path and exits the exit surface of the second birefringent crystal 24 along the same xy coordinates as its incident position. In the second birefringent crystal 24, the polarization state of the e-light input from port 2 differs by 90° from the polarization state of the e-light input from port 3. This means that the o-light and the e-light are transformed, causing the e-light input from port 3 to deviate from the optical path of the e-light input from port 2 and be unable to enter port 2.

[0102] This embodiment also provides a schematic diagram of the projected positions of the incident and exit points of the light beams in the optical path shown in FIG9 at each birefringent crystal in the xy plane, as shown in FIG10 . As previously described, the exit point of the e-light beam from port three at the first birefringent crystal 22 is point C. It exits at point C and couples into port three. The e-light beam from port three then enters the first birefringent crystal 22 at point C with the polarization state e. Within the first birefringent crystal 22, it undergoes a displacement of DexII = 2a and DeyII = 0 along the x- and y-axes, respectively, and exits at point A. After passing through the first Faraday rotator 23, the e-light beam from port three enters the second birefringent crystal 24 with the polarization state o, without any displacement, and still exits at point A. Consequently, the exit point A of the e-light beam from port three deviates from the entry point B of the e-light beam from port two at the second birefringent crystal 24, preventing it from coupling into port two, thus achieving reverse isolation from port three to port two. According to the aforementioned coupling of the light input from port 2 into port 3, it can be seen that this embodiment also achieves reverse isolation from port 2 to port 1.

[0103] In another embodiment, the optical path arrangement is the same as that of the above embodiment 1, except that the crystal The polarization state of the light inputted from port 1 is oo in the first birefringent crystal 22 and the second birefringent crystal 24, or the polarization state of the light inputted from port 1 is eo in the first birefringent crystal 22 and the second birefringent crystal 24, or the polarization state of the light inputted from port 1 is ee in the first birefringent crystal 22 and the second birefringent crystal 24, by reasonably setting the optical axes of the first birefringent crystal 22 and the second birefringent crystal 24. By rationally designing the polarization relationship between the o-light and e-light and properly matching the dual-core spacing and polarization direction of the dual-core waveguide, the circulator function of circular coupling and reverse isolation can be realized. The specific polarization state changes and optical displacement are similar and will not be elaborated here.

[0104] Example 2

[0105] FIG11 is a schematic diagram of the optical path of a 4-port full-polarization-maintaining waveguide polarization-maintaining circulator with all-optical waveguide input and output provided in Example 2 of the present application.

[0106] In this embodiment, the first birefringent crystal 22 is the birefringent crystal A1, and the second birefringent crystal 24 is the birefringent crystal B1. Along the first direction, the polarization state of the light input from port 1 in the first birefringent crystal 22 and the second birefringent crystal 24 is oe.

[0107] The polarization-maintaining circulator 100 of this embodiment is used to transmit light input from port one to port two in a first direction, transmit light input from port two to port three in a second direction, transmit light input from port three to port four in the first direction, and transmit light input from port four to port one in the second direction, wherein the first direction and the second direction are opposite.

[0108] The polarization-maintaining circulator includes: a first polarization-maintaining dual-core waveguide 21 for realizing port one and port three, a first birefringent crystal 22, a first Faraday rotator 23, a lens 25, a second birefringent crystal 24, and a second polarization-maintaining dual-core waveguide 26 for realizing port two and port four, which are arranged in sequence along a first direction. The first birefringent crystal 22, the first Faraday rotator 23, and the second birefringent crystal 24 are all parallel plate structures.

[0109] 1. Port 1 to Port 2:

[0110] Port 1 is used to input divergent linearly polarized light with a fixed polarization direction. The light input from port 1 is o-light (this embodiment uses o-light as an example, but e-light can also be used) and enters the first birefringent crystal 22. The first birefringent crystal 22 is used to cause the light input from port 1 to undergo a first crystal forward displacement before being directed toward the first Faraday rotator 23. The first Faraday rotator 23 is used to rotate the polarization state of the light input from port 1 by an angle α (45°) along a first rotation direction before being directed toward the lens 25. The lens 25 is used to converge the light input from port 1 to form converged light before being directed toward the second birefringent crystal 24. The second birefringent crystal 24 is used to cause the light input from port 1 to undergo a second crystal forward displacement before being directed toward port 2. Port 2 is used to output the light input from port 1, thereby achieving light transmission from port 1 to port 2.

[0111] 2. Port 2 to Port 3:

[0112] Port 2 is used to input divergent linearly polarized light with a fixed polarization direction. The light input from port 2 enters the second birefringent crystal 24 as o light or e light with the same polarization state as the light input from port 1. The second birefringent crystal 24 is used to make the light input from port 2 form a reverse displacement of the second crystal and then emit it to the lens 25. The lens 25 is used to converge the light input from port 2 to form converged light and then emit it to the first Faraday rotator 23. The first Faraday rotator 23 is used to rotate the polarization state of the light input from port 2 along the first rotation direction by an angle of α (45°) and then emit it to the first birefringent crystal 22. The first birefringent crystal 22 is used to make the light input from port 2 form a reverse displacement of the second crystal and then emit it to the lens 25. The input light forms a reverse displacement in the first crystal and is emitted to port three. The polarization state of the light input from port two in the first birefringent crystal 22 is rotated 90 degrees relative to the polarization state of the light input from port one, and the o-light and e-light are transformed, so that the light input from port two deviates from the light input from port one in the first birefringent crystal 22. The lateral walk-off generated between the light input from port one and the light input from port two is equal to the dual-core spacing of the first polarization-maintaining dual-core waveguide 21, so that the light input from port two is coupled into port three; port three is used to output the light input from port two, realizing the transmission of light from port two to port three.

[0113] 3. Port 3 to Port 4:

[0114] Port three is used to input divergent linearly polarized light with a fixed polarization direction. The light input from port three enters the first birefringent crystal 22 as o light or e light with the same polarization state as the light input from port two. The first birefringent crystal 22 is used to cause the light input from port three to form a first crystal forward displacement and then be directed to the first Faraday rotator 23. The first Faraday rotator 23 is used to rotate the polarization state of the light input from port three by an angle of α (45°) along the first rotation direction and then be directed to the lens 25. The lens 25 is used to converge the light input from port three to form converged light and then be directed to the second birefringent crystal 24. The second birefringent crystal 24 is used to cause the light input from port three to form a converged light and then be directed to the second birefringent crystal 24. The input light forms a forward displacement in the second crystal and is emitted toward port four. The polarization state of the light input from port three in the second birefringent crystal 24 is rotated 90° relative to the polarization state of the light input from port two, and the o-light and e-light are transformed, so that the light input from port three deviates from the light input from port two in the second birefringent crystal 24. The lateral walk-off generated between the light input from port two and the light input from port three is equal to the dual-core spacing of the second polarization-maintaining dual-core waveguide 26, so that the light input from port three is coupled into port four. Port four is used to output the light input from port three, thereby realizing light transmission from port three to port four.

[0115] 4. Port 4 to Port 1:

[0116] Port 4 is used to input divergent linearly polarized light with a fixed polarization direction. The light input from port 4 enters the second birefringent crystal 24 as o-light or e-light with the same polarization state as the light input from port 3. The second birefringent crystal 24 is used to cause the light input from port 4 to undergo a second crystal reverse shift and then be directed toward the lens 25. The lens 25 is used to converge the light input from port 4 to form converged light, which is then directed toward the first Faraday rotator 23. The first Faraday rotator 23 is used to rotate the polarization state of the light input from port 4 by an angle α (45°) along a first rotation direction and then be directed toward the first birefringent crystal 22. The first birefringent crystal 22 is used to cause the light input from port 4 to undergo a first crystal reverse shift and then be directed toward port 1. In the first birefringent crystal 22, the polarization state of the light input from port 4 is rotated 90° relative to the polarization state of the light input from port 3, resulting in a shift in the o-light and e-light polarization states. At this point, the light has the same polarization state as the light input from port 1, and thus enters port 1 in the reverse direction of the light input from port 1, achieving light transmission from port 4 to port 1.

[0117] In this embodiment, the optical paths, polarization state changes, and displacement changes from port 1 to port 2 and from port 2 to port 3 are similar to those in Example 1, and reference may be made to the descriptions of Figures 7 and 8 above. The optical transmission from port 3 to port 4 and from port 4 to port 1 is described below.

[0118] Please refer to Figure 12, which shows the optical paths of the e-light incident from port 3 and the o-light incident from port 4 in the polarization-maintaining circulator provided in Example 2. The e-light input from port 3 undergoes optical path deviation in the first birefringent crystal 22 and exits from the first birefringent crystal 22. After passing through the first Faraday rotator 23, the polarization state of the light is rotated 45°, aligning with the polarization state of the o-light from the second birefringent crystal 24. After entering the second birefringent crystal 24, the light path remains unchanged and exits from the exit surface of the second birefringent crystal 24 along the same xy coordinates as its incident position. For the O light input from port 4, the optical paths of the E light input from port 3 and the O light input from port 4 overlap between the input end of the second birefringent crystal 24 along the second direction and the output end of the first Faraday rotator 23 along the second direction. However, after passing through the first Faraday rotator 23, the polarization state of the O light input from port 4 is rotated 45° in the same direction as the E light input from port 3. In the first birefringent crystal 22, the polarization state of the O light input from port 4 is rotated by a cumulative 90° relative to the E light input from port 3. In the first birefringent crystal 22, the polarization state of the O light input from port 4 is perpendicular to the polarization state of the E light input from port 3. In other words, the E light and the O light are transformed, so that the polarization state of the O light input from port 4 is aligned with the polarization state of the O light input from the first birefringent crystal 22. At this point, the polarization state of the O light input from port 4 is the same as the polarization state of the O light input from port 1. Therefore, the O light enters port 1 in the opposite direction of the O light input from port 1, achieving light transmission from port 4 to port 1.

[0119] This embodiment also provides a schematic diagram of the xy-plane projection positions of the incident and exit points of the light beams in FIG12 at each birefringent crystal; see FIG13 for details. After the e-light beam input from port three enters the first birefringent crystal 22 at point C, it undergoes a displacement of DexII = 2a and DeyII = 0 along the x- and y-axes, respectively, within the first birefringent crystal 22, and exits at point A. After passing through the first Faraday rotator 23, the e-light beam input from port three enters the second birefringent crystal 24 and propagates in the o-polarization state without displacement. It continues to exit at point A and is coupled into port four. After entering the second birefringent crystal 24 at point A, the o-light beam input from port four remains unchanged and exits at point A. After passing through the first Faraday rotator 23, the o-light beam input from port four enters the first birefringent crystal 22 and continues to propagate in the o-polarization state without displacement. It continues to exit at point A and is coupled into port one.

[0120] In summary, the incident point of the e-light input from port two on the second birefringent crystal 24 is point B (corresponding to the position of port two), the exit point of the e-light input from port three on the second birefringent crystal 24 is point A (corresponding to the position of port four), the two cores of the second dual-core waveguide 26 are aligned with points B and A, and the polarization states of the two cores are respectively aligned with the e-light polarization state and the o-light polarization state of the second birefringent crystal 24; the incident point of the e-light input from port three on the first birefringent crystal 22 is point C, the exit point of the o-light input from port four on the first birefringent crystal 22 is point A (corresponding to the position of port one), the two cores of the first dual-core waveguide 21 are aligned with points A and C, and the polarization states of the two cores are respectively aligned with the o-light polarization state and the e-light polarization state of the first birefringent crystal 22.

[0121] According to the aforementioned sequential coupling of input light from each port into the next port, it can be seen that input light from each port cannot be coupled into the previous port, thereby achieving reverse isolation from each port to the previous port.

[0122] In another embodiment, the optical path arrangement is the same as that of the above embodiment 2, except that the crystal The polarization state of the light inputted from port 1 is oo in the first birefringent crystal 22 and the second birefringent crystal 24, or the polarization state of the light inputted from port 1 is eo in the first birefringent crystal 22 and the second birefringent crystal 24, or the polarization state of the light inputted from port 1 is ee in the first birefringent crystal 22 and the second birefringent crystal 24, by reasonably setting the optical axes of the first birefringent crystal 22 and the second birefringent crystal 24. By rationally designing the polarization relationship between the o-light and e-light and properly matching the dual-core spacing and polarization direction of the dual-core waveguide, the circulator function of circular coupling and reverse isolation can be realized. The specific polarization state changes and optical displacement are similar and will not be elaborated here.

[0123] Example 3

[0124] Figure 14 is a schematic diagram of the optical path of a single-stage isolated polarization-maintaining circulator with three ports and all-optical waveguide input and output, provided in Example 3 of the present application. In this embodiment, port 1 is a polarization-maintaining port, port 2 is a polarization-maintaining port or a non-polarization-maintaining port, and port 3 is a polarization-maintaining port or a non-polarization-maintaining port.

[0125] First, based on the aforementioned birefringent crystals A1 and B1, another birefringent crystal C1 will be described. Specifically, please refer to Figures 15a and 15b, which schematically illustrate the structure and optical axis of birefringent crystal C1. As shown in Figure 15a, birefringent crystal C1 is a parallel plate structure. A three-dimensional rectangular coordinate system is established with the light incident surface of port one as the plane containing the x and y axes and the first direction as the positive direction of the z axis. The normal to the incident surface of the light input from port one on birefringent crystal C1 is parallel to the z axis. The angle θ between the optical axis of birefringent crystal C1 and the normal to the incident surface of the light input from port one (i.e., the z axis) is C1 The angle between the line where the e-light oscillation plane where the optical axis of the birefringent crystal C1 intersects the xy plane and the y-axis is Figure 15a FIG15 b shows a schematic diagram of light displacement in birefringent crystal C1. The rectangular coordinate system establishment and reference method are the same as those described above for birefringent crystals A1 and B1. The same applies below and will not be repeated here. As shown in FIG15 b, the displacement of the forward o light along the x-axis is DoxIII = 0, and the displacement along the y-axis is DoyIII = 0; the displacement of the forward e light along the x-axis is DexIII = a, and the displacement along the y-axis is DeyI = a / tan(45°) = a; the displacement of the reverse o light along the x-axis is rDoxIII = 0, and the displacement along the y-axis is rDoyIII = 0; the displacement of the reverse e light along the x-axis is rDexIII = -a, and the displacement along the y-axis is rDeyIII = -a / tan(45°) = -a. Of course, other displacement values ​​can be used according to specific circumstances and are not limited here.

[0126] In addition to the structure shown in FIG3 , the polarization-maintaining circulator of this embodiment further includes a third birefringent crystal 27 disposed along the first direction between the second birefringent crystal 24 and the lens 25. The third birefringent crystal 27 is a parallel plate structure. Light input from port one forms a third crystal forward displacement in the third birefringent crystal 27, and light input from port two forms a third crystal reverse displacement in the third birefringent crystal 27, thereby realizing a single-stage isolated polarization-maintaining circulator.

[0127] In the embodiment of the present application, the first birefringent crystal 22 utilizes the aforementioned birefringent crystal A1, the second birefringent crystal 24 utilizes the aforementioned birefringent crystal B1, and the third birefringent crystal 27 utilizes the aforementioned birefringent crystal C1. For details, refer to the previous description and are not repeated here. Along the first direction, the polarization state of light input from port 1 in the first birefringent crystal 22, the second birefringent crystal 24, and the third birefringent crystal 27 is oeo.

[0128] In the polarization-maintaining circulator provided in Example 3 of the present application, (the displacement of the linearly polarized light incident from port 1 to port 2 in the crystal after it enters the birefringent crystal 22 as o light is shown in FIG16a, and the displacement of the light incident from port 2 to port 3 in the crystal after it enters the birefringent crystal 27 as o light and e light is shown in FIG16b), the first and third ports of the first dual-core waveguide 21 are aligned according to points A and C in the displacement diagram, and the polarization states of the two cores are aligned with the polarization states of the o light and the e light of the first birefringent crystal 22, respectively, so as to achieve sequential coupling from port 1 to port 2 and from port 2 to port 3. In the polarization-maintaining circulator, the o light incident from port 3 is Please refer to Figure 17a for the optical path of the e-light incident from port three, please refer to Figure 17b for the optical path of the e-light incident from port three, please refer to Figure 17c for the reverse isolation displacement of the linearly polarized light incident from port three from port three to port two in the crystal after entering the birefringent crystal 22 as o-light, please refer to Figure 17d for the reverse isolation displacement of the linear polarized light incident from port three from port three to port two in the crystal after entering the birefringent crystal 22 as e-light, the exit points D and E of the o-light and e-light incident from port three are both deviated from point B where port two is located, so reverse isolation from port three to port two can be achieved. The contents and related principles shown in the figure are consistent with the relevant description of the above-mentioned embodiment 1, and will not be elaborated here.

[0129] The angle θ3 between the normal of the incident surface of the third birefringent crystal 27 and the optical axis of the third birefringent crystal 27 is the third walk-off angle, where -90° < θ3 < 0° or 0° < θ3 < 90°. The third walk-off angle is used to control the forward and reverse displacements of the third crystal. An xyz coordinate system is established with the first direction as the positive z-axis. The e-light oscillation plane where the optical axis of the second birefringent crystal 24 is located intersects with the xy plane as a second straight line. The angle between the second straight line and the y-axis is The e-light oscillation plane where the optical axis of the third birefringent crystal 27 is located intersects with the xy plane to form a third straight line. The angle between the third straight line and the y axis is in, or 270°, used to couple light of any polarization state input from port two into port three.

[0130] for For details, please refer to FIG15a regarding the birefringent crystal C. By and The relationship is set to or and will and The relationship is set to Or 270°, to ensure that the polarization state of the light exiting the first birefringent crystal 22 can be aligned with the o light or e light of the second birefringent crystal 24 after passing through the first Faraday rotator 23, and after entering the third birefringent crystal 27, the polarization state of the light in the second birefringent crystal 24 is compared to achieve oe conversion or eo conversion. satisfy or 270°.

[0131] In another embodiment, the optical path arrangement is the same as that of the above embodiment 3, except that the crystal The polarization state of the o light input from port 1 in the first birefringent crystal 22-the second birefringent crystal 24-the third birefringent crystal 27 is ooe, or the polarization state of the e light input from port 1 in the first birefringent crystal 22-the second birefringent crystal 24-the third birefringent crystal 27 is eoe, or the polarization state of the e light input from port 1 in the first birefringent crystal 22-the second birefringent crystal 24-the third birefringent crystal 27 is eeo, by reasonably setting the optical axes of the first birefringent crystal 22, the second birefringent crystal 24 and the third birefringent crystal 27 The circulator function of circular coupling and reverse single-stage isolation can also be achieved by matching the appropriate dual-core spacing and polarization direction of the dual-core waveguide, based on the relative displacement of the o-light and e-light, as well as the reasonable design of the polarization state relationship between the o-light and e-light between each birefringent crystal. The specific polarization state changes and light displacement are similar and will not be elaborated here.

[0132] Example 4

[0133] FIG18 is a schematic diagram of the optical path of another single-stage isolated polarization-maintaining circulator with three ports and non-full polarization-maintaining waveguides, provided in Example 4 of the present application, with all-optical waveguide input and output. In this embodiment, port 1 is a polarization-maintaining port, port 2 is a polarization-maintaining port or a non-polarization-maintaining port, and port 3 is a polarization-maintaining port or a non-polarization-maintaining port. This embodiment differs from Example 3 in that the third birefringent crystal 27 is inserted in a different position. This is described in detail below:

[0134] In addition to the structure shown in FIG3 , the polarization-maintaining circulator of this embodiment further includes a third birefringent crystal 27 disposed between the dual-core waveguide 21 and the first birefringent crystal 22. The third birefringent crystal 27 is a parallel plate structure. Light input from port one forms a third crystal forward displacement in the third birefringent crystal 27, and light input from port two forms a third crystal reverse displacement in the third birefringent crystal 27.

[0135] In the embodiment of the present application, the first birefringent crystal 22 utilizes the aforementioned birefringent crystal B1, and the second birefringent crystal 24 utilizes the aforementioned birefringent crystal A1. For details, see the previous description and are not repeated here. The third birefringent crystal 27 utilizes the aforementioned birefringent crystal C1. Along the first direction, the polarization state of light input from port 1 is oeo in the third birefringent crystal 27, the first birefringent crystal 22, and the second birefringent crystal 24.

[0136] In the polarization-maintaining circulator provided in Example 4 of the present application, the displacement of the linearly polarized light incident from port one into the birefringent crystal 27 as o light and then from port one to port two in the crystal is shown in FIG19a . The displacement of the light incident from port two into the birefringent crystal 24 as o light and e light and then from port two to port three in the crystal is shown in FIG19b . Port one and port three of the first dual-core waveguide 21 are aligned according to points A and D in the displacement diagram, and the polarization states of the two cores are respectively aligned with the polarization state of o light and the polarization state of e light of the third birefringent crystal 27, thereby realizing sequential coupling from port one to port two and from port two to port three. Please refer to Figure 20a for the optical path of the o light incident from port three in the polarization-maintaining circulator, and please refer to Figure 20b for the optical path of the e light incident from port three. Please refer to Figure 20c for the reverse isolation displacement of the linearly polarized light incident from port three from port three to port two in the crystal after entering the birefringent crystal 27 as o light. Please refer to Figure 20d for the reverse isolation displacement of the linear polarized light incident from port three from port three to port two in the crystal after entering the birefringent crystal 27 as e light. The exit points C and E of the o light and e light incident from port three are both deviated from point B where port two is located, so reverse isolation from port three to port two can be achieved. The contents and related principles shown in the figure are consistent with the relevant description of the above-mentioned embodiment 1, and will not be repeated here.

[0137] The angle θ3 between the normal of the incident surface of the third birefringent crystal 27 and the optical axis of the third birefringent crystal 27 of the light input from port 1 is the third walk-off angle, where -90°<θ3<0° or 0°<θ3<90°. The third walk-off angle is used to control the forward displacement and the reverse displacement of the third crystal.

[0138] The xyz coordinate system is established with the first direction as the positive direction of the z axis; the e-light oscillation plane where the optical axis of the first birefringent crystal 22 is located intersects with the xy plane as the first straight line, and the angle between the first straight line and the y axis is The e-light oscillation plane where the optical axis of the third birefringent crystal 27 is located intersects with the xy plane to form a third straight line. The angle between the third straight line and the y axis is in, or 270°, used to couple light of any polarization state input from port two into port three.

[0139] for For details, please refer to FIG15a regarding the birefringent crystal C. By and The relationship is set to or and will and The relationship is set to or 270°, ensuring that the polarization state of the light exiting the third birefringent crystal 27 enters the first birefringent crystal 22 and realizes oe conversion or eo conversion compared to the polarization state of the light in the third birefringent crystal 27, and the polarization state of the light exiting the first birefringent crystal 22 can be aligned with the o light or e light of the second birefringent crystal 24 after passing through the first Faraday rotator 23. satisfy or 270°.

[0140] In another embodiment, the optical path arrangement is the same as that of the above embodiment 4, except that the crystal The polarization state of the o light input from the first port in the third birefringent crystal 27-first birefringent crystal 22-second birefringent crystal 24 is oee, or the polarization state of the e light input from the first port in the third birefringent crystal 27-first birefringent crystal 22-second birefringent crystal 24 is eoe, or the polarization state of the e light input from the first port in the third birefringent crystal 27-first birefringent crystal 22-second birefringent crystal 24 is eoo, by reasonably setting the optical axes of the first birefringent crystal 22, the second birefringent crystal 24 and the third birefringent crystal 27. The circulator function of circular coupling and reverse single-stage isolation can also be achieved by matching the appropriate dual-core spacing and polarization direction of the dual-core waveguide, based on the relative displacement of the o-light and e-light, as well as the reasonable design of the polarization state relationship between the o-light and e-light between each birefringent crystal. The specific polarization state changes and light displacement are similar and will not be elaborated here.

[0141] Example 5

[0142] The reverse isolation function achieved by three birefringent crystals (first birefringent crystal 22, second birefringent crystal 24, and third birefringent crystal 27) and one Faraday rotator (i.e., first Faraday rotator 23) is referred to as single-stage isolation. To achieve even better isolation, the number of Faraday rotators can be increased to form a dual-stage isolation.

[0143] FIG21 is a schematic diagram of the optical path of a three-port non-full-polarization-maintaining waveguide double-stage isolated polarization-maintaining circulator with all-optical waveguide input and output provided in Example 5 of the present application.

[0144] In this embodiment, port one is a polarization-maintaining port, port two is a polarization-maintaining port or a non-polarization-maintaining port, and port three is a polarization-maintaining port or a non-polarization-maintaining port.

[0145] First, another birefringent crystal D1 used in this embodiment is described. Specifically, please refer to Figures 21b and 21c, which schematically illustrate the structure and optical axis of the birefringent crystal D1. As shown in Figure 21b, the birefringent crystal D1 is a parallel plate structure. A three-dimensional rectangular coordinate system is established with the light incident surface of port one as the plane where the x and y axes are located and the first direction as the positive direction of the z axis. The normal to the incident surface of the light input from port one on the birefringent crystal D1 is parallel to the z axis. The angle θ between the optical axis of the birefringent crystal D1 and the normal to the incident surface of the light input from port one (i.e., the z axis) is D1 The angle between the line where the optical axis of the birefringent crystal D1, the e-light oscillation plane, intersects the xy plane, and the y-axis is Figure 21b Figure 21c shows a schematic diagram of light displacement in birefringent crystal D1. The rectangular coordinate system and reference method are the same as those described above for birefringent crystals A1 and B1, and the same applies below, so no further explanation is given. As shown in Figure 21c, the displacement of the forward o light along the x-axis is DoxIII = 0, and the displacement along the y-axis is DoyIII = 0; the displacement of the forward e light along the x-axis is DexIII = 0, and the displacement along the y-axis is DeyI = -2a; the displacement of the reverse o light along the x-axis is rDoxIII = 0, and the displacement along the y-axis is rDoyIII = 0; the displacement of the reverse e light along the x-axis is rDexIII = 0, and the displacement along the y-axis is rDeyIII = 2a. Of course, other displacement values ​​can be used depending on the specific situation and are not limited here.

[0146] In addition to the structure shown in FIG3 , the polarization-maintaining circulator of this embodiment further includes a second Faraday rotator 28 and a third birefringent crystal 27, which are sequentially disposed along the first direction between the second birefringent crystal 24 and the lens 25 (i.e., based on the third embodiment shown in FIG14 , the second Faraday rotator 28 is added between the second birefringent crystal 24 and the third birefringent crystal 27). Both the second Faraday rotator 28 and the third birefringent crystal 27 are parallel plate structures. The second Faraday rotator 28 is used to rotate the polarization state of light input from port 1 by an angle α along a second rotation direction before directing it toward the third birefringent crystal 27. The second Faraday rotator is also used to rotate the polarization state of light input from port 2 by an angle α along the second rotation direction before directing it toward the second birefringent crystal 24. Light input from port 1 experiences a third crystal positive shift in the third birefringent crystal 27, while light input from port 2 experiences a third crystal negative shift in the third birefringent crystal 27.

[0147] In the embodiment of the present application, the first birefringent crystal 22 is the aforementioned birefringent crystal B1, the second birefringent crystal 24 is the aforementioned birefringent crystal D1, and the third birefringent crystal 27 is the aforementioned birefringent crystal C1. For details, refer to the previous description and are not repeated here. Along the first direction, the polarization state of light input from port 1 in the first birefringent crystal 22, the second birefringent crystal 24, and the third birefringent crystal 27 is oeo.

[0148] Figure 22a is a schematic diagram of the displacement of linearly polarized light incident from port one in the polarization-maintaining circulator provided in Example 5 of the present application after entering the birefringent crystal 22 as o light and then moving from port one to port two in the crystal. Figure 22b is a schematic diagram of the displacement of light incident from port two in the polarization-maintaining circulator provided in Example 5 of the present application after entering the birefringent crystal 27 as o light and e light and then moving from port two to port three in the crystal. Port one and port three of the first dual-core waveguide 21 are aligned according to points A and B in the displacement diagram, and the polarization states of the two cores are aligned with the polarization state of o light and the polarization state of e light of the first birefringent crystal 22, respectively, thereby achieving sequential coupling from port one to port two and from port two to port three. Please refer to Figure 23a for the optical path of the o light incident from port three in the polarization-maintaining circulator, and please refer to Figure 23b for the optical path of the e light incident from port three. Figure 23c is a schematic diagram of the reverse isolation displacement of the linearly polarized light incident from port three to port two in the polarization-maintaining circulator provided in Example 5 of the present application after entering the birefringent crystal 22 as o light, and Figure 23d is a schematic diagram of the reverse isolation displacement of the linearly polarized light incident from port three to port two in the polarization-maintaining circulator provided in Example 5 of the present application after entering the birefringent crystal 22 as e light. The exit points E and A of the o light and e light incident from port three are both deviated from point B where port two is located, so reverse isolation from port three to port two can be achieved. The contents and related principles shown in the figure are consistent with the relevant description of the above-mentioned Example 1, and will not be repeated here.

[0149] The angle θ3 between the normal line of the incident surface of the third birefringent crystal 27 and the optical axis of the third birefringent crystal 27 of the light input from port 1 is the third walk-off angle, wherein -90°<θ3<0° or 0°<θ3<90°. The third walk-off angle is used to control the forward displacement and the reverse displacement of the third crystal.

[0150] The xyz coordinate system is established with the first direction as the positive direction of the z axis; the e-light oscillation plane where the optical axis of the second birefringent crystal 24 is located intersects with the xy plane as the second straight line, and the angle between the second straight line and the y axis is The e-light oscillation plane where the optical axis of the third birefringent crystal 27 is located intersects with the xy plane to form a third straight line. The angle between the third straight line and the y axis is in, 90°+α, 180°+α or 270°+α, in this embodiment Meet the above The third birefringent crystal 27 is used to couple the light of any polarization state input from the second port into the third port.

[0151] Regarding the specific implementation of bipolar isolation, please refer to Figures 21a, 23a, and 23b. Light of any polarization state incident from port three is incident on a first birefringent crystal 22. The first birefringent crystal 22 is configured to decompose the light of any polarization state incident from port three into three-incident o-light and three-incident e-light with mutually perpendicular polarization states. The first birefringent crystal 22 then generates a first crystal forward shift for each of the three-incident o-light and the three-incident e-light, respectively, and then outputs the shift to a first Faraday rotator 23. The first Faraday rotator 23 then rotates the polarization states of the three-incident o-light and the three-incident e-light by an angle α (45°) along a first rotation direction and then outputs the shift to a second birefringent crystal 24. The second birefringent crystal 24 then generates a second crystal forward shift for each of the three-incident o-light and the three-incident e-light, and then outputs the shift to a second Faraday rotator 28. In the optical path between the first birefringent crystal 22, the first Faraday rotator 23, and the second birefringent crystal 24, the polarization states of the o-light and e-light input from port 3 in the second birefringent crystal 24 are rotated 90° relative to the o-light and e-light input from port 2, respectively. This transformation causes the optical paths of the o-light and e-light input from port 3 in the second birefringent crystal 24 to deviate from the optical paths of the o-light and e-light input from port 2 in the second birefringent crystal 24, achieving a first-level isolation effect from port 3 to port 2. The second Faraday rotator 28 is used to rotate the polarization states of the o-light and e-light input from port 3 by an angle of α (45°) along a second rotation direction before outputting the polarization states to the third birefringent crystal 27. The third birefringent crystal 27 is configured to cause the o-light input from port 3 and the e-light input from port 3 to generate a third crystal forward displacement. In the optical path between the second birefringent crystal 24, the second Faraday rotator 28, and the third birefringent crystal 27, the o-light polarization state and the e-light polarization state of the o-light input from port 3 and the e-light input from port 2, both of which have the same polarization state in the second birefringent crystal 24, are rotated by an angle α (45°) along the second rotation direction by the second Faraday rotator 28. This causes the o-light and e-light polarization states of the o-light input from port 3, which travels in the first direction, to be rotated by a cumulative 90° relative to the e-light polarization state of the e-light input from port 2, which travels in the second direction. This causes the o-light and e-light polarization states of the o-light input from port 3 and port 2, both of which have the same polarization state in the second birefringent crystal 24, to be shifted in the third birefringent crystal 27. This causes the o-light input from port 3 and port 2 to deviate from the optical path of the e-light input from port 2 in the third birefringent crystal 27, thereby achieving a second-level isolation effect from port 3 to port 2. The sum of the forward displacement of the first crystal, the forward displacement of the second crystal, and the forward displacement of the third crystal ultimately causes the port three input o light and the port three input e light output by the third birefringent crystal 27 to deviate from the optical path of the light incident on port two, and after being converged by the lens 25, they cannot be coupled into the single-core waveguide 26, thereby achieving a double-stage isolation effect from port three to port two.

[0152] According to the aforementioned sequential coupling of input light from each port into the next port, it can be seen that input light from port two cannot be coupled into port one, thus achieving reverse isolation from port two to port one.

[0153] In another embodiment, the optical path arrangement is the same as that of the above embodiment 5, except that the crystal Angle and / or Faraday rotation direction, so that the polarization state of the port 1 input o light in the first birefringent crystal 22-second birefringent crystal 24-third birefringent crystal 27 is ooe, or the polarization state of the port 1 input o light in the first birefringent crystal 22-second birefringent crystal 24-third birefringent crystal 27 is ooo, or the polarization state of the port 1 input o light in the first birefringent crystal 22-second birefringent crystal 24-third birefringent crystal 27 is oee, or the polarization state of the port 1 input e light in the first birefringent crystal 22-second birefringent crystal 24-third birefringent crystal 2 7 is eoe, or the polarization state of the e-light input from port 1 in the first birefringent crystal 22-the second birefringent crystal 24-the third birefringent crystal 27 is eeo, or the polarization state of the e-light input from port 1 in the first birefringent crystal 22-the second birefringent crystal 24-the third birefringent crystal 27 is eee, or the polarization state of the e-light input from port 1 in the first birefringent crystal 22-the second birefringent crystal 24-the third birefringent crystal 27 is eoo, by reasonably setting the optical axes of the first birefringent crystal 22, the second birefringent crystal 24 and the third birefringent crystal 27 Based on the angle and relative displacement of o-light and e-light, as well as the reasonable design of the polarization state relationship between o-light and e-light between each birefringent crystal, while matching the appropriate dual-core spacing and polarization state direction of the dual-core waveguide, the circulator function of circular coupling and reverse double-stage isolation can also be realized. The specific polarization state changes and light displacement are similar and will not be elaborated here.

[0154] Example 6

[0155] FIG24 is a schematic diagram of the optical path of a three-port fully polarization-maintaining polarization-maintaining circulator with parallel light output from port 2 provided in Example 6 of the present application. The difference between this embodiment and Example 1 is that port 2 of this embodiment does not use a single-core waveguide, but outputs parallel light into free space. Lens 25 is used to collimate the light input from port 1 into parallel light and output it to port 2, which is used to output the parallel light; and port 2 is used to input parallel light and direct the parallel light toward lens 25, which is also used to converge the light input from port 2 to form converged light and output it to port 3. The optical path transmission and principles of the rest of this embodiment are similar to those of Example 1, and reference may be made to the description of Example 1.

[0156] Example 7

[0157] Figure 25a is a schematic diagram of the optical path of a three-port non-full-polarization-maintaining single-stage isolated polarization-maintaining circulator with parallel light output from port 2 provided in Example 7 of the present application. The difference between this embodiment and Example 3 is that port 2 of this embodiment does not use a single-core waveguide, and it outputs parallel light into free space. Lens 25 is used to collimate the light input from port 1 into parallel light and output it to port 2, which is used to output the parallel light; and port 2 is used to input parallel light and direct the parallel light toward lens 25, which is also used to converge the light input from port 2 to form converged light and output it to port 3. The optical path transmission and principles of the rest of this embodiment are similar to those of Example 3, and reference may be made to the description of Example 3.

[0158] Example 8

[0159] Figure 25b is a schematic diagram of the optical path of another type of three-port non-full-polarization-maintaining single-stage isolated polarization-maintaining circulator with parallel light output from port 2, provided in Example 8 of the present application. The difference between this embodiment and Example 4 is that port 2 of this embodiment does not use a single-core waveguide, and outputs parallel light into free space. Lens 25 is used to collimate the light input from port 1 into parallel light and output it to port 2, which is used to output the parallel light; and port 2 is used to input parallel light and direct the parallel light toward lens 25, which is also used to converge the light input from port 2 to form converged light and output it to port 3. The optical path transmission and principles of the rest of this embodiment are similar to those of Example 4, and reference may be made to the description of Example 4.

[0160] Example 9

[0161] FIG26 is a schematic diagram of the optical path of a three-port non-full-polarization-maintaining, double-stage isolated polarization-maintaining circulator with parallel light output from port 2 provided in Example 9 of the present application. This embodiment differs from Example 5 in that port 2 of this embodiment does not use a single-core waveguide, but outputs parallel light into free space. Lens 25 is used to collimate the light input from port 1 into parallel light and output it to port 2, which is used to output the parallel light; and port 2 is used to input parallel light and direct the parallel light toward lens 25, which is also used to converge the light input from port 2 to form converged light and output it to port 3. The optical path transmission and principles of the rest of this embodiment are similar to those of Example 5, and reference may be made to the description of Example 5.

[0162] Examples 10-13

[0163] In some embodiments, a filter can be added between the lens and port 2, located at the focal point of lens 25. Please refer to Figures 27-29, where Figure 27 is a schematic diagram of the optical path of a polarization-maintaining circulator according to Example 10 of the present application, which adds a filter to the polarization-maintaining circulator shown in Figure 24; Figure 28a is a schematic diagram of the optical path of a polarization-maintaining circulator according to Example 11 of the present application, which adds a filter to the polarization-maintaining circulator shown in Figure 25a; Figure 28b is a schematic diagram of the optical path of a polarization-maintaining circulator according to Example 12 of the present application, which adds a filter to the polarization-maintaining circulator shown in Figure 25b; and Figure 29 is a schematic diagram of the optical path of a polarization-maintaining circulator according to Example 13 of the present application, which adds a filter to the polarization-maintaining circulator shown in Figure 26. In the illustrated embodiments, port 2 is used to output or input parallel light. The polarization-maintaining circulator 100 further includes a filter disposed between lens 25 and port 2, located at the focal point of lens 25. The filter is used to reflect all or part of the parallel light output from port 1 to port 2.

[0164] Combined crystal:

[0165] Continuing with FIG3 , the refractive indices of the light o inputted from port 1 and the light e inputted from port 2 are different in the first birefringent crystal 22. Therefore, the optical path length of the light o inputted from port 1 is not equal to the optical path length of the light e inputted from port 2 in the first birefringent crystal 22, resulting in an optical path length difference between the two. Similar situations may also exist in other embodiments.

[0166] Therefore, in some embodiments, the first birefringent crystal 22, and / or the second birefringent crystal 24, and / or the third birefringent crystal 27 are combined crystals. In addition to achieving the same displacement, the combined crystals can also be used to compensate for optical path difference.

[0167] Specifically, please refer to FIG30a and FIG30b, which provide two exemplary explanations by taking the first birefringent crystal 22 as a combined crystal as an example. As shown in FIG30a, the incident surface is also taken as the x-axis and y-axis plane, and the first direction is taken as the positive direction of the z-axis to establish a coordinate system. The first birefringent crystal 22 can be composed of a sub-birefringent crystal 221 and a sub-birefringent crystal 222. The sub-birefringent crystal 221 is With the birefringent crystal 222 The same, and the sub-birefringent crystal 221 satisfies 0°<θ H-1 <90°, the sub-birefringent crystal 222 satisfies 0°<θ H-2 ≤90°, the positions of the sub-birefringent crystal 221 and the sub-birefringent crystal 222 can be interchanged to achieve the same function. As shown in FIG30b, the first birefringent crystal 22 can also be composed of a sub-birefringent crystal 223 and a sub-birefringent crystal 224. The angular difference is 90° or 270°, and the sub-birefringent crystal 223 satisfies 0°<θ H-3 ≤90°, the sub-birefringent crystal 224 satisfies 0°<θ H-4 <90°, the same function can be achieved by exchanging the positions of the sub-birefringent crystal 223 and the sub-birefringent crystal 224. The second birefringent crystal 24 and the third birefringent crystal 27 are similar and will not be described in detail here.

[0168] Taking the example of a combined crystal as the first birefringent crystal 22, the sum of the relative displacements of the light input at port 1 in the two sub-birefringent crystals forms a displacement equal to the relative walk-off of the o-ray and e-ray in the first birefringent crystal 22. Combined crystals can be used to compensate for the optical path difference in a single birefringent crystal, or they can be used to compensate for the optical path difference of multiple birefringent crystals in a full optical path. When compensating a single crystal, the polarization states of the o-ray and e-ray in the two sub-birefringent crystals, both of which are positively birefringent or negatively birefringent, can be interchanged, thereby reducing the total optical path difference. Alternatively, the two sub-birefringent crystals can be positively birefringent and negatively birefringent, respectively, and the polarization states (both o-ray and e-ray) are maintained identically in the positive and negative birefringent crystals, thereby reducing the optical path difference. By properly setting the angle θ and the thickness of the sub-birefringent crystals, the optical path difference of the o-ray and e-ray in each sub-birefringent crystal can be varied. When compensating for the optical path difference of the entire optical path, the sub-birefringent crystal in the combined crystal that compensates for the optical path difference not only achieves the same displacement change as the other sub-birefringent crystal in the combined crystal, but also has its optical path difference configuration of o-light and e-light opposite to the direction of the sum of the optical path differences of the other birefringent crystals in the entire optical path, thereby reducing the total optical path difference or even making the optical path difference zero, thereby achieving the function of compensating for the optical path difference of the entire optical path.

[0169] Finally, it should be noted that the above embodiments are merely illustrative, and the deviation, direction, and distance of the optical path do not constitute limitations on the present application. Based on the structure proposed in the present application, and with reasonable design of the structures and optical axes of the first birefringent crystal 22, the second birefringent crystal 24, and the third birefringent crystal 27, the rotation direction of the Faraday rotator, and the relationship between the polarization states of the o-light and the e-light, all solutions for implementing an optical circulator should be included within the scope of protection of the present application.

Claims

1. A polarization-maintaining circulator, characterized in that: Used to transmit light input from port 1 to port 2 in a first direction, and to transmit light input from port 2 to port 3 in a second direction, wherein the first direction is opposite to the second direction; The polarization-maintaining circulator comprises: a dual-core waveguide, a first birefringent crystal, a first Faraday rotator, a second birefringent crystal, a lens and the port 2, which are sequentially arranged along the first direction and are used to realize the port 1 and the port 3; the first birefringent crystal, the first Faraday rotator and the second birefringent crystal are all parallel plate structures; wherein, The port 1 is a polarization-maintaining port, used for inputting divergent linearly polarized light, and the light inputted from the port 1 enters the first birefringent crystal as o light or e light; The first birefringent crystal is used to make the light input from the first port shift forward toward the first birefringent crystal and then be emitted toward the first Faraday rotator; The first Faraday rotator is used to rotate the polarization state of the light input from the port 1 by an angle α along a first rotation direction and then emit it toward the second birefringent crystal, wherein α=45°; The second birefringent crystal is used to make the light input from the first port shift toward the second birefringent crystal and then be emitted toward the lens; The lens is used to optically shape the light input from the port 1 and then emit it to the port 2; The port 2 is used to output the light input by the port 1, so as to realize the transmission of the light from the port 1 to the port 2; The port 2 is also used to input light and direct the input light toward the lens; The lens is also used to optically converge the light input from the second port and then emit it to the second birefringent crystal; The second birefringent crystal is also used to make the light input from the second port to be emitted to the first Faraday rotator after being shifted by the second birefringent crystal in the reverse direction. The first Faraday rotator is further used to rotate the polarization state of the light input from the second port by the angle α along the first rotation direction and then emit it toward the first birefringent crystal; The first birefringent crystal is also used to make the light input from the port 2 form a reverse first birefringent crystal displacement and then be emitted to the port 3, wherein the polarization state of the light input from the port 2 in the first birefringent crystal is rotated by 90° relative to the polarization state of the light input from the port 1, and the o light and the e light are transformed, so that the light input from the port 2 deviates from the light input from the port 1 in the first birefringent crystal, and the lateral walk-off generated between the light input from the port 1 and the light input from the port 2 is equal to the dual-core spacing of the dual-core waveguide, so that the light input from the port 2 is coupled into the port 3; The port three is used to output the light input by the port two, so as to realize the transmission of the light from the port two to the port three.

2. The polarization-maintaining circulator according to claim 1, characterized in that: The angle θ1 between the normal of the incident surface of the first birefringent crystal and the optical axis of the first birefringent crystal at the first port input is a first walk-off angle, wherein -90°<θ1<0° or 0°<θ1<90°, and the first walk-off angle is used to control the forward displacement of the first birefringent crystal and the reverse displacement of the first birefringent crystal; and / or, The angle θ2 between the normal of the incident surface of the second birefringent crystal and the optical axis of the second birefringent crystal of the light input from the port 1 is the second walk-off angle, -90°<θ2<0° or 0°<θ2<90°, and the second walk-off angle is used to control the forward second birefringent crystal displacement and the reverse second birefringent crystal displacement.

3. The polarization-maintaining circulator according to claim 1, characterized in that: Establishing an xyz coordinate system with the first direction as the positive direction of the z-axis; The e-light oscillation plane where the optical axis of the first birefringent crystal is located intersects with the xy plane to form a first straight line, and the angle between the first straight line and the y axis is The e-light oscillation plane where the optical axis of the second birefringent crystal is located intersects with the xy plane to form a second straight line, and the angle between the second straight line and the y axis is in, or 4. The polarization-maintaining circulator according to claim 1, characterized in that: Along the first direction, the polarization state of the o-light of the first birefringent crystal is aligned with the polarization state of the o-light or the e-light of the second birefringent crystal after passing through the first Faraday rotator.

5. The polarization-maintaining circulator according to claim 1, characterized in that: Both port 2 and port 3 are polarization-maintaining ports. The polarization state of port 2 is aligned with the polarization state of light input from port 1 along the first direction and emitted through the lens, and the polarization state of port 3 is aligned with the polarization state of light input from port 2 along the second direction and emitted through the first birefringent crystal.

6. The polarization-maintaining circulator according to claim 1, characterized in that: The port 2 is a polarization-maintaining port or a non-polarization-maintaining port, the port 3 is a polarization-maintaining port or a non-polarization-maintaining port, the polarization-maintaining circulator further comprises a third birefringent crystal disposed between the second birefringent crystal and the lens along the first direction, the third birefringent crystal is a parallel plate structure, the light input from the port 1 forms a positive third birefringent crystal displacement in the third birefringent crystal, and the light input from the port 2 forms a reverse third birefringent crystal displacement in the third birefringent crystal; The angle θ3 between the normal line of the incident surface of the third birefringent crystal and the optical axis of the third birefringent crystal of the light input from the port 1 is a third walk-off angle, -90°<θ3<0° or 0°<θ3<90°, and the third walk-off angle is used to control the forward displacement of the third birefringent crystal and the reverse displacement of the third birefringent crystal; The xyz coordinate system is established with the first direction as the positive direction of the z-axis; the e-light oscillation plane where the optical axis of the second birefringent crystal is located intersects with the xy plane as the second straight line, and the angle between the second straight line and the y-axis is The e-light oscillation plane where the optical axis of the third birefringent crystal is located intersects with the xy plane to form a third straight line, and the angle between the third straight line and the y axis is in, Or 270°, for coupling light of any polarization state input from the port two into the port three.

7. The polarization-maintaining circulator according to claim 1, characterized in that: The port 2 is a polarization-maintaining port or a non-polarization-maintaining port, the port 3 is a polarization-maintaining port or a non-polarization-maintaining port, the polarization-maintaining circulator further comprises a third birefringent crystal disposed between the dual-core waveguide and the first birefringent crystal, the third birefringent crystal is a parallel plate structure, the light input from the port 1 forms a positive third birefringent crystal displacement in the third birefringent crystal, and the light input from the port 2 forms a reverse third birefringent crystal displacement in the third birefringent crystal; The angle θ3 between the normal line of the incident surface of the third birefringent crystal and the optical axis of the third birefringent crystal of the light input from the port 1 is a third walk-off angle, -90°<θ3<0° or 0°<θ3<90°, and the third walk-off angle is used to control the forward displacement of the third birefringent crystal and the reverse displacement of the third birefringent crystal; An xyz coordinate system is established with the first direction as the positive direction of the z-axis; the intersection of the e-light oscillation plane where the optical axis of the first birefringent crystal is located and the xy plane is a first straight line, and the angle between the first straight line and the y-axis is The e-light oscillation plane where the optical axis of the third birefringent crystal is located intersects with the xy plane to form a third straight line, and the angle between the third straight line and the y axis is in, Or 270°, for coupling light of any polarization state input from the port two into the port three.

8. The polarization-maintaining circulator according to claim 1, characterized in that: The second port is a polarization-maintaining port or a non-polarization-maintaining port, the third port is a polarization-maintaining port or a non-polarization-maintaining port, the polarization-maintaining circulator further comprises a second Faraday rotator and a third birefringent crystal which are sequentially arranged between the second birefringent crystal and the lens along the first direction, the second Faraday rotator and the third birefringent crystal are both parallel plate structures, the second Faraday rotator is used to rotate the polarization state of the light input from the first port by the angle α along the second rotation direction and then emit it to the third birefringent crystal, and is used to rotate the polarization state of the light input from the second port by the angle α along the second rotation direction and then emit it to the second birefringent crystal, the light input from the first port forms a positive third birefringent crystal displacement in the third birefringent crystal, and the light input from the second port forms a reverse third birefringent crystal displacement in the third birefringent crystal; The angle θ3 between the normal line of the incident surface of the third birefringent crystal and the optical axis of the third birefringent crystal of the light input from the port 1 is a third walk-off angle, -90°<θ3<0° or 0°<θ3<90°, and the third walk-off angle is used to control the forward displacement of the third birefringent crystal and the reverse displacement of the third birefringent crystal; The xyz coordinate system is established with the first direction as the positive direction of the z-axis; the e-light oscillation plane where the optical axis of the second birefringent crystal is located intersects with the xy plane as the second straight line, and the angle between the second straight line and the y-axis is The e-light oscillation plane where the optical axis of the third birefringent crystal is located intersects with the xy plane to form a third straight line, and the angle between the third straight line and the y axis is in, 90°+α, 180°+α or 270°+α, for coupling light of any polarization state input from the port 2 into the port 3, and for realizing the transmission from the port 2 to the port 1, Double-stage isolation effect from port three to port two.

9. The polarization-maintaining circulator according to any one of claims 1 to 8, characterized in that: The first birefringent crystal, and / or the second birefringent crystal, and / or the third birefringent crystal are combined crystals. In addition to achieving the same displacement, the combined crystals can also be used to compensate for optical path differences.

10. The polarization-maintaining circulator according to claim 1, characterized in that: The port 2 is a single-core waveguide, which is used to output convergent light or input divergent light; the lens is used to converge the light input from the port 1 to form convergent light, and couple it into the single-core waveguide, and the single-core waveguide is used to output the convergent light; the single-core waveguide is also used to input divergent linear polarized light and emit the divergent linear polarized light to the lens, and the lens is also used to converge the light input from the single-core waveguide to form convergent light; or, The port two is used to output or input parallel light; the lens is used to collimate the light input from the port one into parallel light and output it to the port two, and the port two is used to output the parallel light; the port two is also used to input parallel light and direct the parallel light toward the lens, and the lens is also used to converge the light input from the port two to form converged light.

11. The polarization-maintaining circulator according to claim 10, characterized in that: When the port two is used to output or input parallel light, the polarization-maintaining circulator also includes a filter arranged between the lens and the port two and located at the focal position of the lens, and the filter is used to reflect all or part of the parallel light output from the port one to the port two.

12. The polarization-maintaining circulator according to claim 1, characterized in that: Used to transmit light input from port one to port two in a first direction, transmit light input from port two to port three in a second direction, transmit light input from port three to port four in the first direction, and transmit light input from port four to port one in the second direction, wherein the first direction is opposite to the second direction; The polarization-maintaining circulator comprises: a first polarization-maintaining dual-core waveguide for realizing the port one and the port three, a first birefringent crystal, a first Faraday rotator, a lens, a second birefringent crystal, and a second polarization-maintaining dual-core waveguide for realizing the port two and the port four, which are arranged in sequence along the first direction, wherein the first birefringent crystal, the first Faraday rotator, and the second birefringent crystal are all parallel plate structures; wherein The port one is used to input divergent linear polarized light, and the light input from the port one enters the first birefringent crystal as o light or e light; the first birefringent crystal is used to make the light input from the port one shift positively toward the first birefringent crystal and then emit to the first Faraday rotator; the first Faraday rotator is used to rotate the polarization state of the light input from the port one by an angle α along a first rotation direction and then emit to the lens, wherein α=45°; the lens is used to converge the light input from the port one to form converged light and then emit to the second birefringent crystal; the second birefringent crystal is used to make the light input from the port one shift positively toward the second birefringent crystal and then emit to the port two; the port two is used to output the light input from the port one, so as to realize the transmission of light from the port one to the port two; The port 2 is also used to input divergent linear polarized light, and the light input from the port 2 enters the second birefringent crystal as o light or e light with the same polarization state as the light input from the port 1; the second birefringent crystal is also used to make the light input from the port 2 form a reverse second birefringent crystal displacement and then emit it to the lens; the lens is also used to converge the light input from the port 2 to form converged light and then emit it to the first Faraday rotator; the first Faraday rotator is also used to rotate the polarization state of the light input from the port 2 by the angle α along the first rotation direction and then emit it to the first birefringent crystal; the first birefringent crystal is also used to make the light input from the port 2 form a reverse second birefringent crystal displacement and then emit it to the lens; The first birefringent crystal is displaced and directed toward the port three, wherein the polarization state of the light input from the port two in the first birefringent crystal is rotated 90° relative to the polarization state of the light input from the port one, and the o light and the e light are transformed, so that the light input from the port two deviates from the light input from the port one in the first birefringent crystal, and the lateral walk-off generated between the light input from the port one and the light input from the port two is equal to the double-core spacing of the first polarization-maintaining double-core waveguide, so that the light input from the port two is coupled into the port three; the port three is used to output the light input from the port two, so as to realize the transmission of light from the port two to the port three; The port three is also used to input divergent linear polarized light, and the light input from the port three enters the first birefringent crystal as o light or e light with the same polarization state as the light input from the port two; the first birefringent crystal is used to make the light input from the port three shift positively toward the first birefringent crystal and then emit it to the first Faraday rotator; the first Faraday rotator is used to rotate the polarization state of the light input from the port three by the angle α along the first rotation direction and then emit it to the lens; the lens is used to converge the light input from the port three to form converged light and then emit it to the second birefringent crystal; the second birefringent crystal is used to make the light input from the port three shift positively toward the second birefringent crystal and then emit it to the port Four; wherein the polarization state of the light input from the port three in the second birefringent crystal is rotated 90° relative to the polarization state of the light input from the port two, and the o light and the e light are transformed, so that the light input from the port three deviates from the light input from the port two in the second birefringent crystal, and the lateral walk-off generated between the light input from the port two and the light input from the port three is equal to the double-core spacing of the second polarization-maintaining double-core waveguide, so that the light input from the port three is coupled into the port four; the port four is used to output the light input from the port three, so as to realize the transmission of light from the port three to the port four; The port four is also used to input divergent linear polarized light, and the light input from the port four enters the second birefringent crystal as o light or e light with the same polarization state as the light input from the port three; the second birefringent crystal is also used to make the light input from the port four form a reverse second birefringent crystal displacement and then emit it to the lens; the lens is also used to converge the light input from the port four to form converged light and then emit it to the first Faraday rotator; the first Faraday rotator is also used to rotate the polarization state of the light input from the port four by the angle α along the first rotation direction and then emit it to the first birefringent crystal; the first birefringent crystal is also used to make the light input from the port four form a reverse first birefringent crystal displacement and then emit it to the port one, wherein the polarization state of the light input from the port four in the first birefringent crystal forms a 90° rotation relative to the polarization state of the light input from the port three, the o light and the e light are transformed, and at this time they are the same as the polarization state of the light input from the port one, so they enter the port one in the reverse direction of the light input from the port one, thereby realizing the transmission of light from the port four to the port one.

13. The polarization-maintaining circulator according to claim 12, characterized in that: The first birefringent crystal and / or the second birefringent crystal are combined crystals. In addition to achieving the same displacement, the combined crystal can also be used to compensate for optical path difference.

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