Optical circulator

Abstract

Provided is an optical circulator that is reduced in size and weight. This optical circulator comprises a first optical part, a second optical part, and a nonreciprocal part disposed between the first optical part and the second optical part. The first optical part and the second optical part each have a light guide base material, and two first diffraction elements that reflect and diffract first polarized light, and transmit second polarized light, the first diffraction elements being respectively disposed on a first surface and a second surface of the light guide base material and at different positions in the surface direction. The nonreciprocal part includes a λ / 2 plate and a Faraday rotator.

Description

Optical Circulator

[0001] The present invention relates to an optical circulator.

[0002] An optical circulator is an optical device with an optical separation function that outputs input light from one port to only one specific other port, such as outputting input light from a first port to a second port and outputting input light from the second port to a third port. It is used in optical communications for bidirectional communication and branching of optical signals.

[0003] An example of a conventional optical circulator is described in Patent Document 1. This optical circulator is composed of a pair of cube-shaped polarizing beam splitters, and a Faraday rotator and a half-wave plate disposed between the cube-shaped polarizing beam splitters.

[0004] Japanese Patent Application Laid-Open No. 2002-031780

[0005] However, the cube-type polarizing beam splitter used in conventional optical circulators is formed by joining two right-angle prisms made of optical glass, which is relatively large and heavy, preventing the optical circulator from being made smaller and lighter. Therefore, the object of the present invention is to provide a compact and lightweight optical circulator without using a cube-type polarizing beam splitter.

[0006] As a result of extensive research, the inventors have discovered that an optical circulator can be made smaller and lighter than conventional optical circulators that use cube-shaped polarizing beam splitters by using a polarization separation / combination section that separates two different polarized lights in the same incident light beam and combines two polarized lights that are incident on different optical paths into the same optical path, and a non-reciprocal section that consists of a Faraday rotator and a half-wave plate, and the polarization separation / combination section is made of a diffraction element and a light-guiding material.

[0007] That is, it has been found that the above object can be achieved by the following configuration.

[0008] [1] An optical circulator comprising a first optical unit, a second optical unit, and a nonreciprocal unit disposed between the first and second optical units, wherein the first and second optical units each comprise a light-guiding substrate and two first diffraction elements disposed on the first and second surfaces of the light-guiding substrate at different positions in the plane direction, the first diffraction elements reflecting and diffracting the first polarized light and transmitting the second polarized light, and wherein the nonreciprocal units include a λ / 2 plate and a Faraday rotator. [2] The optical circulator according to [1], wherein the first diffraction element is a liquid crystal diffraction element, and wherein λ / 4 plates are disposed on the side opposite the nonreciprocal unit side of the first optical unit, between the first optical unit and the nonreciprocal unit, between the nonreciprocal unit and the second optical unit, and on the side opposite the nonreciprocal unit side of the second optical unit. [3] The optical circulator according to [2], wherein at least one of the liquid crystal diffraction elements includes a cholesteric liquid crystal layer. [4] The optical circulator according to [1], wherein the second optical unit further includes second diffraction elements arranged on each of the first and second surfaces of the light-guiding substrate at different positions in the in-plane direction, the second diffraction elements transmitting the first polarized light and reflecting and diffracting the second polarized light, and the third optical unit is further included between the nonreciprocal portion and the second optical unit, the third optical unit including the light-guiding substrate and the third diffraction elements arranged on each of the first and second surfaces of the light-guiding substrate at different positions in the in-plane direction, the third diffraction elements reflecting and diffracting the first polarized light and transmitting the second polarized light. [5] The optical circulator according to [4], wherein the first diffraction element, the second diffraction element, and the third diffraction element are liquid crystal diffraction elements, and the optical circulator includes λ / 4 plates on the side opposite to the nonreciprocal portion side of the first optical unit, between the first optical unit and the nonreciprocal portion, between the nonreciprocal portion and the third optical unit, and on the side opposite to the nonreciprocal portion side of the second optical unit.

[0009] The present invention can provide a compact and lightweight optical circulator.

[0010] Fig. 1 is a diagram conceptually showing an example of the configuration of the present invention. Fig. 2 is a diagram schematically showing different optical signal paths in the configuration of Fig. 1. Fig. 3 is a diagram conceptually showing an example of another configuration of the present invention. Fig. 4 is a diagram schematically showing different optical signal paths in the configuration of Fig. 3. Fig. 5 is a diagram conceptually showing an example of another configuration of the present invention. Fig. 6 is a diagram schematically showing different optical signal paths in the configuration of Fig. 5.

[0011] Hereinafter, preferred embodiments of the optical circulator of the present invention will be described in detail.

[0012] In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0013] [Optical Circulator] The optical circulator of the present invention has a first optical section, a second optical section, and a non-reciprocal section arranged between the first optical section and the second optical section, wherein the first optical section and the second optical section each have a light-guiding substrate and two first diffraction elements that reflect and diffract a first polarized light and transmit a second polarized light, the first diffraction elements being arranged on the first surface and the second surface of the light-guiding substrate, respectively, at different positions in the surface direction, and wherein the non-reciprocal section includes a λ / 2 plate and a Faraday rotator.

[0014] Fig. 1 is a conceptual diagram showing an example of an optical circulator of the present invention, and Fig. 2 is a diagram for explaining different optical signal paths in the optical circulator shown in Fig. 1.

[0015] 1 and 2 has polarization separation / combination units 20 and 21 each consisting of a diffraction element and a light-guiding substrate, a nonreciprocal unit 30 consisting of a λ / 2 plate and a Faraday rotator, and is provided with first to third ports 10 to 12 for inputting and outputting optical signals. In this configuration, P-polarized light input from the first port 10 is output from the second port 11, and P-polarized light and S-polarized light are input from the second port 11 and output to the third port 12.

[0016] In the optical circulator shown in Figures 1 and 2, the polarization separation / combination section 20 corresponds to the first optical section in the present invention, the non-reciprocal section 30 corresponds to the non-reciprocal section in the present invention, and the polarization separation / combination section 21 corresponds to the second optical section in the present invention.

[0017] The optical circulator shown in FIGS. 1 and 2 has a first optical unit (polarization separation / combination unit) 20, a nonreciprocal unit 30, and a second optical unit (polarization separation / combination unit) 21 arranged in this order. The second port 11 is arranged on the first optical unit 20 side of the optical circulator, and the first port 10 and the third port 12 are arranged on the second optical unit 21 side. The first port 10 and the second port 11 are arranged at approximately the same position (overlapping position) in the planar direction, while the first port 10 and the third port 12 are arranged at different positions in the planar direction. The planar direction is an in-plane direction perpendicular to the arrangement direction of the first optical unit 20, the nonreciprocal unit 30, and the second optical member 21, and is the up-down direction and the direction perpendicular to the paper surface in FIG. 1 . This is also true for other examples.

[0018] The first optical section (polarization separation / combining section) 20 has a light guiding substrate 40 and two diffraction elements 51 .

[0019] The light-guiding substrate 40 is a known light-guiding member capable of guiding light therein. In the illustrated example, the light-guiding substrate 40 is plate-shaped, and its two largest surfaces are referred to as a first surface and a second surface. A diffraction element 51 is disposed on each of the first surface and the second surface. In the following description, the surface on the second port 11 side is referred to as the first surface, and the surface on the first port 10 side is referred to as the second surface. The light-guiding substrate 40 is disposed so that the first surface and the second surface are parallel to the surface direction.

[0020] The diffraction element 51 is a diffraction element that reflects and diffracts the first polarized light and transmits the second polarized light without diffracting it, and corresponds to the first diffraction element in the present invention. That is, the diffraction element (first diffraction element) 51 is a reflective diffraction element. In the example shown in FIGS. 1 and 2 , the first polarized light is S-polarized light, and the second polarized light is S-polarized light. That is, the diffraction element 51 is a diffraction element that reflects and diffracts S-polarized light and transmits S-polarized light without diffracting it. Note that the first polarized light may be P-polarized light and the second polarized light may be S-polarized light. Alternatively, as described below, the first polarized light may be right-handed circularly polarized light and the second polarized light may be left-handed circularly polarized light, or the first polarized light may be left-handed circularly polarized light and the second polarized light may be right-handed circularly polarized light. That is, the first diffraction element may reflect and diffract one circularly polarized light and transmit the other circularly polarized light without diffracting it. This also applies to the diffraction element 52 described below.

[0021] Of the two diffraction elements 51, one diffraction element 51 is arranged on a first surface of the light-guiding substrate 40, and the other diffraction element 51 is arranged on a second surface of the light-guiding substrate 40. The diffraction element 51 arranged on the first surface and the diffraction element 51 arranged on the second surface are arranged at different positions in the surface direction. Note that the diffraction element 51 arranged on the first surface and the diffraction element 51 arranged on the second surface may partially overlap in the surface direction.

[0022] More specifically, the diffraction element 51 arranged on the first surface of the light-guiding substrate 40 is arranged at a position overlapping with the third port 12 in the surface direction. Furthermore, the diffraction element 51 arranged on the first surface of the light-guiding substrate 40 does not overlap with the second port 11 in the surface direction. On the other hand, the diffraction element 51 arranged on the second surface of the light-guiding substrate 40 is arranged at a position overlapping with the second port 11 in the surface direction, that is, on a straight line connecting the first port 10 and the second port 11. Furthermore, the diffraction element 51 arranged on the second surface of the light-guiding substrate 40 does not overlap with the third port 12 in the surface direction.

[0023] The non-reciprocal unit 30 is disposed between the first optical unit 20 and the second optical unit 21. In the illustrated example, the non-reciprocal unit 30 includes a λ / 2 plate 60 and a Faraday rotator 70 from the first optical unit 20 side.

[0024] As is well known, the λ / 2 plate 60 is a retardation plate that has a phase difference of λ / 2 and can rotate the polarization direction of incident polarized light by imparting a phase difference to the incident polarized light. In the optical circulator of the present invention, the λ / 2 plate 60 is positioned with its optical axis adjusted so as to rotate the polarization plane of the incident light by 45°.

[0025] As is well known, the Faraday rotator 70 rotates the plane of polarization of incident light by using a magnetic field, and rotates the plane of polarization of light in the same direction relative to the crystal regardless of the direction of incidence. In the optical circulator of the present invention, the Faraday rotator 70 is configured to rotate the plane of polarization of incident light by 45°.

[0026] The nonreciprocal unit 30, which includes the λ / 2 plate 60 and the Faraday rotator 70, is configured so that the λ / 2 plate 60 rotates the plane of polarization of polarized light incident from the first optical unit 20 side by 45°, and the Faraday rotator 70 further rotates the plane of polarization by 45° in the same direction. That is, the nonreciprocal unit 30 rotates the plane of polarization of polarized light incident from the first optical unit 20 side by 90°. On the other hand, in the nonreciprocal unit 30, the Faraday rotator 70 rotates the plane of polarization of polarized light incident from the second optical unit 21 side by 45°, and then the λ / 2 plate 60 rotates the plane of polarization by 45° in the opposite direction. Therefore, the nonreciprocal unit 30 transmits polarized light incident from the second optical unit 21 side without rotating the plane of polarization.

[0027] The second optical section (polarized light separation / combination section) 21 has a light guiding substrate 41 and two diffraction elements 52 .

[0028] Like the light-guiding substrate 40, the light-guiding substrate 41 is a known light-guiding member capable of guiding light therein. In the illustrated example, the light-guiding substrate 41 is plate-shaped, and if its two largest surfaces are referred to as a first surface and a second surface, a diffraction element 52 is disposed on each of the first surface and the second surface. In the following description, the surface on the second port 11 side is referred to as the first surface, and the surface on the first port 10 side is referred to as the second surface. The light-guiding substrate 41 is disposed so that the first surface and the second surface are parallel to the surface direction.

[0029] The diffraction element 52 is a diffraction element that reflects and diffracts the first polarized light and transmits the second polarized light without diffracting it, and corresponds to the first diffraction element in the present invention. That is, the diffraction element (first diffraction element) 52 is a reflective diffraction element. In the illustrated example, the diffraction element 52 is a diffraction element that reflects and diffracts the S-polarized light and transmits the S-polarized light without diffracting it.

[0030] Of the two diffraction elements 52, one diffraction element 52 is arranged on a first surface of the light-guiding substrate 41, and the other diffraction element 52 is arranged on a second surface of the light-guiding substrate 41. The diffraction element 52 arranged on the first surface and the diffraction element 52 arranged on the second surface are arranged at different positions in the surface direction. Note that the diffraction element 52 arranged on the first surface and the diffraction element 52 arranged on the second surface may partially overlap in the surface direction.

[0031] More specifically, the diffraction element 52 arranged on the first surface of the light-guiding substrate 41 is arranged at a position overlapping with the third port 12 in the planar direction. Furthermore, the diffraction element 52 arranged on the first surface of the light-guiding substrate 41 does not overlap with the second port 11 in the planar direction. Therefore, the diffraction element 52 arranged on the first surface of the light-guiding substrate 41 is arranged at a position overlapping with the diffraction element 51 of the first optical section 20 arranged on the first surface of the light-guiding substrate 40 in the planar direction.

[0032] On the other hand, the diffraction element 52 arranged on the second surface of the light-guiding substrate 41 is arranged at a position overlapping with the second port 11 in the planar direction, i.e., on the straight line connecting the first port 10 and the second port 11. Furthermore, the diffraction element 52 arranged on the second surface of the light-guiding substrate 41 does not overlap with the third port 12 in the planar direction. Therefore, the diffraction element 52 arranged on the second surface of the light-guiding substrate 41 is arranged at a position overlapping with the diffraction element 51 of the first optical unit 20 arranged on the second surface of the light-guiding substrate 40 in the planar direction.

[0033] Next, the operation of the optical circulator having such a configuration will be described with reference to FIGS.

[0034] 1 is a schematic diagram illustrating how an optical signal input to the optical circulator from the first port 10 is output to the second port 11. First, P-polarized light input to the optical circulator from the first port 10 is incident on the diffraction element 52 disposed on the second surface of the light-guiding substrate 41 of the second optical unit 21. As described above, the diffraction element 52 is polarization-dependent and is designed to transmit P-polarized light without diffracting it and to reflect and diffract S-polarized light. Therefore, the P-polarized light incident on the diffraction element 52 is transmitted without being diffracted by the diffraction element 52, and then transmits through the light-guiding substrate 41 to enter the non-reciprocal unit 30.

[0035] The nonreciprocal unit 30 is composed of a Faraday rotator 70 and a λ / 2 plate 60. The Faraday rotator 70 rotates the polarization plane of incident light by 45°, and the optical axis of the subsequent λ / 2 plate 60 is set so that it also rotates the polarization plane of incident light by 45°. Furthermore, while the direction of rotation of the polarization plane of light by the Faraday rotator 70 is independent of the direction of propagation, that of the λ / 2 plate 60 is dependent on the direction of propagation. Therefore, the nonreciprocal unit 30, which combines these elements, can rotate the polarization plane of light by a total of 90° or leave the polarization plane unchanged, depending on the direction of incidence. As described above, the polarization plane of light incident from the λ / 2 plate 60 side (first optical unit 20 side) is rotated by 90°, while the polarization plane of light incident from the Faraday rotator 70 side (second optical unit 21 side) is set to be unchanged. Therefore, P-polarized light incident from the Faraday rotator 70 side of the nonreciprocal unit 30 is transmitted as P-polarized light.

[0036] The P-polarized light that has passed through the non-reciprocal portion 30 is incident on the diffraction element 51 that is disposed on the second surface of the light-guiding substrate 40 of the first optical portion 20. As described above, the diffraction element 51 has polarization dependency and transmits P-polarized light without diffracting it. Therefore, the P-polarized light that has entered the diffraction element 51 is transmitted without being diffracted by the diffraction element 51, and further passes through the light-guiding substrate 40, and is finally emitted to the second port 11.

[0037] FIG. 2 is a schematic diagram illustrating how an optical signal input to the optical circulator from the second port 11 is output from the third port 12. First, the P-polarized light and S-polarized light input to the optical circulator from the second port 11 pass through the light-guiding substrate 40 of the first optical unit 20 and enter the diffraction element 51 arranged on the second surface of the light-guiding substrate 40. The diffraction element 51 is polarization-dependent and is designed to transmit P-polarized light without diffracting it and to reflect and diffract S-polarized light. Therefore, the P-polarized light passes through the diffraction element 51 without being diffracted, and the S-polarized light is reflected and diffracted by the diffraction element 51, resulting in polarization separation. The transmitted P-polarized light travels toward the first port 10. Meanwhile, the reflected S-polarized light is guided within the light-guiding substrate 40 and is again reflected and diffracted by the diffraction element 51 arranged in the reflection direction (the diffraction element 51 arranged on the first surface of the light-guiding substrate 40), and travels toward the third port 12 parallel to the transmitted P-polarized light.

[0038] The separated P-polarized light and S-polarized light enter the non-reciprocal unit 30 from the λ / 2 plate 60 side. As described above, the non-reciprocal unit 30 rotates the plane of polarization of the light that enters from the λ / 2 plate 60 side (the first optical unit 20 side) by 90 degrees. Because the plane of polarization of the light is rotated by 90 degrees, the P-polarized light is converted into S-polarized light, and the S-polarized light is converted into P-polarized light.

[0039] Of the S-polarized light and P-polarized light that have passed through the non-reciprocal portion 30, the S-polarized light passes through the light-guiding substrate 41 of the second optical unit 21 and is incident on the diffraction element 52 arranged on the second surface of the light-guiding substrate 41. As described above, the diffraction element 52 reflects and diffracts the S-polarized light, so the S-polarized light is reflected and diffracted by the diffraction element 52, guided through the light-guiding substrate 40, and reflected and diffracted again by the diffraction element 52 arranged in the reflection direction (the diffraction element 52 arranged on the first surface of the light-guiding substrate 41), changing its traveling direction toward the third port 12. On the other hand, the P-polarized light is incident on the diffraction element 52 arranged on the first surface of the light-guiding substrate 41 of the second optical unit 21. However, as described above, the diffraction element 52 transmits P-polarized light without diffracting it, so the P-polarized light passes through the diffraction element 52, passes through the light-guiding substrate 41, and is incident directly on the third port 12. At this time, the second optical section (polarization separation / combination section) 21, which includes two diffraction elements 52 and a light-guiding substrate 41, is positioned so that the previously reflected S-polarized light and the transmitted P-polarized light follow the same optical path, and therefore the P-polarized light and the S-polarized light can be input into the third port 12 in a combined state.

[0040] As described above, in the optical circulator of the present invention, light incident on the first port 10 exits from the second port 11, and light incident on the second port 11 exits from the third port 12. With this configuration, it can be seen that the optical circulator of the present invention can be used as a small and lightweight optical circulator without using a cube-type polarizing beam splitter.

[0041] Fig. 3 is a conceptual diagram showing another example of an optical circulator according to the present invention, and Fig. 4 is a diagram for explaining different optical signal paths in the optical circulator shown in Fig. 3.

[0042] 3 and 4 has polarization separation / combination units 22 and 23 each consisting of a liquid crystal diffraction element and a light-guiding substrate, a nonreciprocal unit 30 consisting of a λ / 2 plate and a Faraday rotator, and λ / 4 plates 90 to 93, and is provided with first to third ports 10 to 12 for inputting and outputting optical signals. In this configuration, P-polarized light input from first port 10 is output from second port 11, and P-polarized light and S-polarized light are input from second port 11 and output to third port 12.

[0043] In the optical circulator shown in Figures 3 and 4, the polarization separation / combination section 22 corresponds to the first optical section in the present invention, the non-reciprocal section 30 corresponds to the non-reciprocal section in the present invention, and the polarization separation / combination section 23 corresponds to the second optical section in the present invention.

[0044] 3 and 4 has a λ / 4 plate 90, a first optical unit (polarization separation / combining unit) 22, a λ / 4 plate 91, a nonreciprocal unit 30, a λ / 4 plate 92, a second optical unit (polarization separation / combining unit) 23, and a λ / 4 plate 93 arranged in this order, with the second port 11 arranged on the first optical unit 22 side of the optical circulator and the first port 10 and the third port 12 arranged on the second optical unit 23 side. The first port 10 and the second port 11 are arranged at approximately the same position (overlapping position) in the planar direction, and the first port 10 and the third port 12 are arranged at different positions in the planar direction.

[0045] The λ / 4 plates 90, 91, 92, and 93 are retardation plates that have a phase difference of λ / 4 and can impart a phase difference to incident polarized light to change its polarization state. In the optical circulator of the present invention, the λ / 4 plates are arranged with their optical axes oriented such that they convert incident linearly polarized light into circularly polarized light and convert incident circularly polarized light into linearly polarized light.

[0046] The λ / 4 plate 90 is disposed on the side of the first optical unit 22 opposite to the non-reciprocal unit 30. The λ / 4 plate 91 is disposed between the first optical unit 22 and the non-reciprocal unit 30. The λ / 4 plate 92 is disposed between the non-reciprocal unit 30 and the second optical unit 23. The λ / 4 plate 93 is disposed on the side of the second optical unit 23 opposite to the non-reciprocal unit 30.

[0047] The first optical section (polarized light separation / combination section) 22 has a light-guiding substrate 42 and two liquid crystal diffraction elements 80 .

[0048] Like the light-guiding substrate 40, the light-guiding substrate 42 is a known light-guiding member capable of guiding light therein. In the illustrated example, the light-guiding substrate 42 is plate-shaped, and if its two largest surfaces are referred to as a first surface and a second surface, a liquid crystal diffraction element 80 is disposed on each of the first surface and the second surface. In the following description, the surface on the second port 11 side is referred to as the first surface, and the surface on the first port 10 side is referred to as the second surface. The light-guiding substrate 42 is disposed so that the first surface and the second surface are parallel to the surface direction.

[0049] The liquid crystal diffraction element 80 is a diffraction element that reflects and diffracts the first polarized light and transmits the second polarized light without diffracting it, and corresponds to the first diffraction element in the present invention. That is, the liquid crystal diffraction element (first diffraction element) 80 is a reflective diffraction element. In the example shown in FIGS. 3 and 4 , the first polarized light is left-handed circularly polarized light, and the second polarized light is right-handed circularly polarized light. That is, the liquid crystal diffraction element 80 is a diffraction element that reflects and diffracts left-handed circularly polarized light and transmits right-handed circularly polarized light without diffracting it. The liquid crystal diffraction element 80 has at least one cholesteric liquid crystal layer. In the example shown in FIGS. 3 and 4 , the liquid crystal diffraction element 80 reflects and diffracts left-handed circularly polarized light, and therefore has a cholesteric liquid crystal layer with a left-twisted helical structure and does not have a cholesteric liquid crystal layer with a right-twisted helical structure. Note that the first polarized light may be right-handed circularly polarized light, and the second polarized light may be left-handed circularly polarized light. That is, the liquid crystal diffraction element 80 may reflect and diffract right-handed circularly polarized light and transmit left-handed circularly polarized light without diffracting it. This also applies to the liquid crystal diffraction element 81 described later.

[0050] Of the two liquid crystal diffraction elements 80, one liquid crystal diffraction element 80 is disposed on a first surface of the light-guiding substrate 42, and the other liquid crystal diffraction element 80 is disposed on a second surface of the light-guiding substrate 42. The liquid crystal diffraction element 80 disposed on the first surface and the liquid crystal diffraction element 80 disposed on the second surface are disposed at different positions in the surface direction. The liquid crystal diffraction element 80 disposed on the first surface and the liquid crystal diffraction element 80 disposed on the second surface may partially overlap in the surface direction.

[0051] More specifically, the liquid crystal diffraction element 80 arranged on the first surface of the light-guiding substrate 42 is arranged at a position overlapping with the third port 12 in the surface direction. Furthermore, the liquid crystal diffraction element 80 arranged on the first surface of the light-guiding substrate 42 does not overlap with the second port 11 in the surface direction. On the other hand, the liquid crystal diffraction element 80 arranged on the second surface of the light-guiding substrate 42 is arranged at a position overlapping with the second port 11 in the surface direction, that is, on the straight line connecting the first port 10 and the second port 11. Furthermore, the liquid crystal diffraction element 80 arranged on the second surface of the light-guiding substrate 42 does not overlap with the third port 12 in the surface direction.

[0052] The nonreciprocal portion 30 has the same configuration as the nonreciprocal portion 30 of the optical circulator shown in FIGS. 1 and 2, and therefore a description thereof will be omitted.

[0053] That is, the non-reciprocal section 30 rotates the plane of polarization of polarized light incident from the first optical section 22 side by 90°, and transmits polarized light incident from the second optical section 23 side without rotating the plane of polarization.

[0054] The second optical section (polarized light separation / combination section) 23 has a light guiding substrate 43 and two liquid crystal diffraction elements 81 .

[0055] Like the light-guiding substrate 40, the light-guiding substrate 43 is a known light-guiding member capable of guiding light therein. In the illustrated example, the light-guiding substrate 43 is plate-shaped, and if its two largest surfaces are referred to as a first surface and a second surface, a liquid crystal diffraction element 81 is disposed on each of the first surface and the second surface. In the following description, the surface on the second port 11 side is referred to as the first surface, and the surface on the first port 10 side is referred to as the second surface. The light-guiding substrate 43 is disposed so that the first surface and the second surface are parallel to the surface direction.

[0056] The liquid crystal diffraction element 81 is a diffraction element that reflects and diffracts the first polarized light and transmits the second polarized light without diffracting it, and corresponds to the first diffraction element in the present invention. That is, the liquid crystal diffraction element (first diffraction element) 81 is a reflective diffraction element. In the illustrated example, the liquid crystal diffraction element 81 is a diffraction element that reflects and diffracts left-handed circularly polarized light and transmits right-handed circularly polarized light without diffracting it. That is, the liquid crystal diffraction element 81 has a left-twisted cholesteric liquid crystal layer and does not have a right-twisted cholesteric liquid crystal layer.

[0057] Of the two liquid crystal diffraction elements 81, one liquid crystal diffraction element 81 is disposed on a first surface of the light-guiding substrate 43, and the other liquid crystal diffraction element 81 is disposed on a second surface of the light-guiding substrate 43. The liquid crystal diffraction element 81 disposed on the first surface and the liquid crystal diffraction element 81 disposed on the second surface are disposed at different positions in the surface direction. Note that the liquid crystal diffraction element 81 disposed on the first surface and the liquid crystal diffraction element 81 disposed on the second surface may partially overlap in the surface direction.

[0058] More specifically, the liquid crystal diffraction element 81 arranged on the first surface of the light-guiding substrate 43 is arranged at a position overlapping with the third port 12 in the planar direction. Furthermore, the liquid crystal diffraction element 81 arranged on the first surface of the light-guiding substrate 43 does not overlap with the second port 11 in the planar direction. Therefore, the liquid crystal diffraction element 81 arranged on the first surface of the light-guiding substrate 43 is arranged at a position overlapping with the liquid crystal diffraction element 80 of the first optical unit 22 arranged on the first surface of the light-guiding substrate 42 in the planar direction.

[0059] On the other hand, the liquid crystal diffraction element 81 arranged on the second surface of the light-guiding substrate 43 is arranged at a position overlapping with the second port 11 in the planar direction, i.e., on the straight line connecting the first port 10 and the second port 11. Furthermore, the liquid crystal diffraction element 81 arranged on the second surface of the light-guiding substrate 43 does not overlap with the third port 12 in the planar direction. Therefore, the liquid crystal diffraction element 81 arranged on the second surface of the light-guiding substrate 43 is arranged at a position overlapping with the liquid crystal diffraction element 80 of the first optical unit 22 arranged on the second surface of the light-guiding substrate 42 in the planar direction.

[0060] Next, the operation of the optical circulator having such a configuration will be described with reference to FIGS.

[0061] 3 is a schematic diagram illustrating how an optical signal input to the optical circulator from the first port 10 is output from the second port 11. First, P-polarized light input to the optical circulator from the first port 10 is converted to right-handed circularly polarized light by the λ / 4 plate 93. The converted right-handed circularly polarized light enters the liquid crystal diffraction element 81 disposed on the second surface of the light-guiding substrate 43 of the second optical unit 23. As described above, the liquid crystal diffraction element 81 has a left-twisted cholesteric liquid crystal layer, so the right-handed circularly polarized light is transmitted through the liquid crystal diffraction element 81 without being diffracted. The right-handed circularly polarized light then passes through the light-guiding substrate 43 and then enters the λ / 4 plate 92, where it is converted to P-polarized light.

[0062] The optical signal converted into P-polarized light is incident on the non-reciprocal portion 30 from the Faraday rotator 70 side, and as described above, the incident P-polarized light is transmitted as it is.

[0063] The P-polarized light that has passed through the non-reciprocal portion 30 enters the λ / 4 plate 91 and is converted into right-handed circularly polarized light, and then enters the liquid crystal diffraction element 80 that is disposed on the second surface of the light-guiding substrate 42 of the first optical portion 22. As described above, the liquid crystal diffraction element 80 has a left-twisted cholesteric liquid crystal layer, so the right-handed circularly polarized light is transmitted through the liquid crystal diffraction element 80 without being diffracted, and then further transmitted through the light-guiding substrate 42. The right-handed circularly polarized light then enters the λ / 4 plate 90 and is converted back into P-polarized light, and is finally output to the second port 11.

[0064] FIG. 4 is a schematic diagram illustrating how an optical signal input to the optical circulator from the second port 11 is output from the third port 12. First, the P-polarized light and S-polarized light input to the optical circulator from the second port 11 enter the λ / 4 plate 90 and are converted into right-handed circularly polarized light and left-handed circularly polarized light, respectively. Next, the right-handed and left-handed circularly polarized light pass through the light-guiding substrate 42 of the first optical unit 22 and enter the liquid crystal diffraction element 80 disposed on the second surface of the light-guiding substrate 42. As described above, the liquid crystal diffraction element 80 has a left-twisted cholesteric liquid crystal layer. Therefore, the right-handed circularly polarized light passes through the liquid crystal diffraction element 80 without being diffracted, and the left-handed circularly polarized light is reflected and diffracted by the liquid crystal diffraction element 80, resulting in polarization separation. The transmitted right-handed circularly polarized light travels toward the first port 10. On the other hand, the reflected left-handed circularly polarized light is guided within the light-guiding substrate 42, and is reflected and diffracted again by the liquid crystal diffraction element 80 arranged in the reflection direction (the liquid crystal diffraction element 80 arranged on the first surface of the light-guiding substrate 42), and passes through the light-guiding substrate 42, traveling in the direction of the third port 12 parallel to the transmitted right-handed circularly polarized light.

[0065] The separated right-handed circularly polarized light and left-handed circularly polarized light enter the λ / 4 plate 91 and are converted into P-polarized light and S-polarized light, respectively. The converted P-polarized light and S-polarized light enter the non-reciprocal unit 30 from the λ / 2 plate 60 side. As described above, the non-reciprocal unit 30 rotates the plane of polarization of the light incident from the λ / 2 plate 60 side (first optical unit 22 side) by 90 degrees. Because the plane of polarization of the light is rotated by 90 degrees, P-polarized light is converted into S-polarized light, and S-polarized light is converted into P-polarized light.

[0066] The S-polarized light and P-polarized light that have passed through the non-reciprocal portion 30 enter the λ / 4 plate 92 and are converted into left-handed circularly polarized light and right-handed circularly polarized light, respectively. Of these, the left-handed circularly polarized light passes through the light-guiding substrate 43 of the second optical portion 23 and enters the liquid crystal diffraction element 81 arranged on the second surface of the light-guiding substrate 43. Because the liquid crystal diffraction element 81 has a left-twisted cholesteric liquid crystal layer, the left-handed circularly polarized light is reflected and diffracted by the liquid crystal diffraction element 81, guided through the light-guiding substrate 43, and reflected and diffracted again by the liquid crystal diffraction element 81 arranged in the reflection direction (the liquid crystal diffraction element 81 arranged on the first surface of the light-guiding substrate 43), changing its traveling direction toward the third port 12. On the other hand, the right-handed circularly polarized light is incident on the liquid crystal diffraction element 81 arranged on the first surface of the light-guiding substrate 43 of the second optical unit 23. However, as described above, the liquid crystal diffraction element 81 has a left-twisted cholesteric liquid crystal layer, so the right-handed circularly polarized light passes through the liquid crystal diffraction element 81, passes through the light-guiding substrate 43, and enters the third port 13 as is. At this time, the second optical unit (polarization separation / combination unit) 23 including the two liquid crystal diffraction elements 81 and the light-guiding substrate 43 is positioned so that the reflected left-handed circularly polarized light and the transmitted right-handed circularly polarized light follow the same optical path, so that the left-handed circularly polarized light and the right-handed circularly polarized light can be input to the third port 13 in a combined state. Furthermore, by passing through the λ / 4 plate 93 in front of the third port 13, the right-handed circularly polarized light and the left-handed circularly polarized light are converted into P-polarized light and S-polarized light, respectively, and therefore exit from the third port 12 in the same polarization state as when they entered from the second port 11.

[0067] As described above, in the optical circulator of the present invention, light incident on the first port 10 exits from the second port 11, and light incident on the second port 11 exits from the third port 12. With this configuration, it can be seen that the optical circulator of the present invention can be used as a small and lightweight optical circulator without using a cube-type polarizing beam splitter.

[0068] 3 and 4, the diffraction element of the first optical unit 22 and the diffraction element of the second optical unit 23 are both liquid crystal diffraction elements that reflect and diffract one circularly polarized light and transmit the other circularly polarized light without diffracting it, but this is not limited thereto. For example, the diffraction element of the first optical unit 22 may be a liquid crystal diffraction element that diffracts one circularly polarized light and does not diffract the other circularly polarized light, and the diffraction element of the second optical unit 23 may be a diffraction element that diffracts one linearly polarized light and does not diffract the other linearly polarized light. In this case, λ / 4 plates may be provided immediately upstream and downstream of the first optical unit 22 that has the liquid crystal diffraction element, i.e., on the side of the first optical unit 22 opposite the non-reciprocal unit 30, and between the first optical unit 22 and the non-reciprocal unit 30. Alternatively, for example, the diffraction element of the first optical unit 22 may be a diffraction element that diffracts one linearly polarized light and does not diffract the other linearly polarized light, and the diffraction element of the second optical unit 23 may be a liquid crystal diffraction element that diffracts one circularly polarized light and does not diffract the other circularly polarized light. In this case, it is sufficient that λ / 4 plates are provided immediately upstream and immediately downstream of the second optical unit 23 that has the liquid crystal diffraction element, i.e., between the nonreciprocal unit 30 and the second optical unit 23, and on the side of the second optical unit 22 opposite to the nonreciprocal unit 30 side.

[0069] Fig. 5 is a conceptual diagram showing another example of an optical circulator according to the present invention, and Fig. 6 is a diagram for explaining different optical signal paths in the optical circulator shown in Fig. 5.

[0070] The optical circulator shown in Figures 5 and 6 has polarization separation / combination units 24, 25 made up of a liquid crystal diffraction element and a light-guiding substrate, a nonreciprocal unit 30 made up of a λ / 2 plate and a Faraday rotator, and λ / 4 plates 90-93, and is provided with first to third ports 10-12 for inputting and outputting optical signals. In this configuration, P-polarized light and S-polarized light input from the first port 10 are output from the second port 11, and P-polarized light and S-polarized light are input from the second port 11 and output to the third port 12. This configuration can accommodate both P-polarized light and S-polarized light entering the first port 10, and is therefore expected to be usable in a wider range of applications than the configurations shown in Figures 1 to 4.

[0071] In the optical circulator shown in Figures 5 and 6, the polarization separation / combination unit 24 corresponds to the first optical unit in the present invention, the non-reciprocal unit 30 corresponds to the non-reciprocal unit in the present invention, and the polarization separation / combination unit 25 includes the second optical unit and the third optical unit in the present invention.

[0072] 5 and 6, a λ / 4 plate 90, a first optical unit (polarization separation / combination unit) 24, a λ / 4 plate 91, a nonreciprocal unit 30, a λ / 4 plate 92, a third optical unit 25b, a second optical unit 25a, and a λ / 4 plate 93 are arranged in this order. The second port 11 is arranged on the first optical unit 24 side of the optical circulator, and the first port 10 and the third port 12 are arranged on the second optical unit 25a side. The first port 10 and the second port 11 are arranged at different positions in the planar direction, the first port 10 and the third port 12 are arranged at different positions in the planar direction, and the second port 11 and the third port 12 are arranged at different positions in the planar direction.

[0073] 3 and 4, the λ / 4 plates 90, 91, 92, and 93 are retardation plates that have a phase difference of λ / 4 and can change the polarization state by imparting a phase difference to incident polarized light. In the optical circulator of the present invention, the orientation of the optical axis of each λ / 4 plate is adjusted and arranged so that it converts incident linearly polarized light into circularly polarized light and converts incident circularly polarized light into linearly polarized light.

[0074] The λ / 4 plate 90 is disposed on the side of the first optical unit 24 opposite to the non-reciprocal unit 30. The λ / 4 plate 91 is disposed between the first optical unit 22 and the non-reciprocal unit 30. The λ / 4 plate 92 is disposed between the non-reciprocal unit 30 and the third optical unit 25b. The λ / 4 plate 93 is disposed on the side of the second optical unit 25a opposite to the non-reciprocal unit 30.

[0075] The first optical section (polarized light separation / combination section) 24 has a light guiding substrate 44 and two liquid crystal diffraction elements 82 .

[0076] Like the light-guiding substrate 40, the light-guiding substrate 44 is a known light-guiding member capable of guiding light therein. In the illustrated example, the light-guiding substrate 44 is plate-shaped, and if its two largest surfaces are referred to as a first surface and a second surface, a liquid crystal diffraction element 82 is disposed on each of the first surface and the second surface. In the following description, the surface on the second port 11 side is referred to as the first surface, and the surface on the first port 10 side is referred to as the second surface. The light-guiding substrate 44 is disposed so that the first surface and the second surface are parallel to the surface direction.

[0077] Like the liquid crystal diffraction element 80, the liquid crystal diffraction element 82 is a diffraction element that reflects and diffracts the first polarized light and transmits the second polarized light without diffracting it, and corresponds to the first diffraction element in the present invention. That is, the liquid crystal diffraction element (first diffraction element) 82 is a reflective diffraction element. In the example shown in FIGS. 5 and 6 , the first polarized light is left-handed circularly polarized light, and the second polarized light is right-handed circularly polarized light. That is, the liquid crystal diffraction element 82 is a diffraction element that reflects and diffracts left-handed circularly polarized light and transmits right-handed circularly polarized light without diffracting it. The liquid crystal diffraction element 82 has at least one cholesteric liquid crystal layer. In the example shown in FIGS. 5 and 6 , the liquid crystal diffraction element 82 reflects and diffracts left-handed circularly polarized light, and therefore has a cholesteric liquid crystal layer with a left-twisted helical structure and does not have a cholesteric liquid crystal layer with a right-twisted helical structure. Note that the first polarized light may be right-handed circularly polarized light, and the second polarized light may be left-handed circularly polarized light. That is, the liquid crystal diffraction element 82 may reflect and diffract right-handed circularly polarized light and transmit left-handed circularly polarized light without diffracting it. This also applies to the liquid crystal diffraction element 83 and the liquid crystal diffraction element 85 described later.

[0078] Of the two liquid crystal diffraction elements 82, one liquid crystal diffraction element 82 is disposed on a first surface of the light-guiding substrate 44, and the other liquid crystal diffraction element 82 is disposed on a second surface of the light-guiding substrate 44. The liquid crystal diffraction element 82 disposed on the first surface and the liquid crystal diffraction element 82 disposed on the second surface are disposed at different positions in the surface direction. Note that the liquid crystal diffraction element 82 disposed on the first surface and the liquid crystal diffraction element 82 disposed on the second surface may partially overlap in the surface direction.

[0079] More specifically, the liquid crystal diffraction element 82 arranged on the first surface of the light-guiding substrate 44 is arranged at a position overlapping with the third port 12 in the surface direction. Furthermore, the liquid crystal diffraction element 82 arranged on the first surface of the light-guiding substrate 44 does not overlap with the first port 10 or the second port 11 in the surface direction. On the other hand, the liquid crystal diffraction element 82 arranged on the second surface of the light-guiding substrate 44 is arranged at a position overlapping with the second port 11 in the surface direction. Furthermore, the liquid crystal diffraction element 82 arranged on the second surface of the light-guiding substrate 44 does not overlap with the first port 10 or the third port 12 in the surface direction.

[0080] The nonreciprocal portion 30 has the same configuration as the nonreciprocal portion 30 of the optical circulator shown in FIGS. 1 and 2, and therefore a description thereof will be omitted.

[0081] That is, the non-reciprocal section 30 rotates the plane of polarization of polarized light incident from the first optical section 24 side by 90°, and transmits polarized light incident from the second optical section 25a side without rotating the plane of polarization.

[0082] The polarization splitting / combining unit 25 has a second optical unit 25a and a third optical unit 25b. The third optical unit 25b is disposed between the non-reciprocal unit 30 and the second optical unit 25a.

[0083] The second optical unit 25 a has a light-guiding substrate 46 , two liquid crystal diffraction elements 84 , and two liquid crystal diffraction elements 85 .

[0084] Like the light-guiding substrate 40, the light-guiding substrate 46 is a known light-guiding member capable of guiding light therein. In the illustrated example, the light-guiding substrate 46 is plate-shaped, and if its two largest surfaces are referred to as a first surface and a second surface, a liquid crystal diffraction element 84 is disposed on each of the first surface and the second surface, and a liquid crystal diffraction element 85 is disposed on each of the first surface and the second surface. In the following description, the surface on the second port 11 side is referred to as the first surface, and the surface on the first port 10 side is referred to as the second surface. The light-guiding substrate 46 is disposed so that the first surface and the second surface are parallel to the surface direction.

[0085] The liquid crystal diffraction element 84 is a diffraction element that reflects and diffracts the first polarized light and transmits the second polarized light without diffracting it, and corresponds to the first diffraction element in the present invention. That is, the liquid crystal diffraction element (first diffraction element) 84 is a reflective diffraction element. In the illustrated example, the liquid crystal diffraction element 84 is a diffraction element that reflects and diffracts left-handed circularly polarized light and transmits right-handed circularly polarized light without diffracting it. That is, the liquid crystal diffraction element 84 has a left-twisted cholesteric liquid crystal layer but does not have a right-twisted cholesteric liquid crystal layer.

[0086] Of the two liquid crystal diffraction elements 84, one liquid crystal diffraction element 84 is disposed on a first surface of the light-guiding substrate 46, and the other liquid crystal diffraction element 84 is disposed on a second surface of the light-guiding substrate 46. The liquid crystal diffraction element 84 disposed on the first surface and the liquid crystal diffraction element 84 disposed on the second surface are disposed at different positions in the surface direction. The liquid crystal diffraction element 84 disposed on the first surface and the liquid crystal diffraction element 84 disposed on the second surface may partially overlap in the surface direction.

[0087] More specifically, the liquid crystal diffraction element 84 arranged on the first surface of the light-guiding substrate 46 is arranged at a position overlapping with the third port 12 in the planar direction. Furthermore, the liquid crystal diffraction element 84 arranged on the first surface of the light-guiding substrate 46 does not overlap with the second port 11 in the planar direction. Therefore, the liquid crystal diffraction element 84 arranged on the first surface of the light-guiding substrate 46 is arranged at a position overlapping with the liquid crystal diffraction element 82 of the first optical unit 24 arranged on the first surface of the light-guiding substrate 44 in the planar direction.

[0088] On the other hand, the liquid crystal diffraction element 84 arranged on the second surface of the light-guiding substrate 46 is arranged at a position overlapping with the second port 11 in the planar direction. Furthermore, the liquid crystal diffraction element 84 arranged on the second surface of the light-guiding substrate 46 does not overlap with the first port 10 or the third port 12 in the planar direction. Therefore, the liquid crystal diffraction element 84 arranged on the second surface of the light-guiding substrate 46 is arranged at a position overlapping with the liquid crystal diffraction element 82 of the first optical unit 24 arranged on the second surface of the light-guiding substrate 44 in the planar direction.

[0089] The liquid crystal diffraction element 85 is a diffraction element that transmits the first polarized light without diffracting it and reflects and diffracts the second polarized light, and corresponds to the second diffraction element in the present invention. That is, the liquid crystal diffraction element (second diffraction element) 85 is a reflective diffraction element. In the illustrated example, the liquid crystal diffraction element 85 is a diffraction element that reflects and diffracts right-handed circularly polarized light and transmits left-handed circularly polarized light without diffracting it. That is, the liquid crystal diffraction element 85 has a right-twisted cholesteric liquid crystal layer and does not have a left-twisted cholesteric liquid crystal layer. In this way, the liquid crystal diffraction element (second diffraction element) 85 reflects and diffracts the opposite circularly polarized light to the liquid crystal diffraction element (first diffraction element) 84.

[0090] Of the two liquid crystal diffraction elements 85, one liquid crystal diffraction element 85 is arranged on the first surface of the light guiding substrate 46, and the other liquid crystal diffraction element 85 is arranged on the second surface of the light guiding substrate 46. Furthermore, the liquid crystal diffraction element 85 arranged on the first surface and the liquid crystal diffraction element 85 arranged on the second surface are arranged at different positions in the surface direction. Note that the liquid crystal diffraction element 85 arranged on the first surface and the liquid crystal diffraction element 85 arranged on the second surface may partially overlap in the surface direction. Furthermore, the liquid crystal diffraction element 85 arranged on the first surface is arranged at a different position in the surface direction from the liquid crystal diffraction element 84 arranged on the first surface of the light guiding substrate 46 and the liquid crystal diffraction element 84 arranged on the second surface of the light guiding substrate 46. Furthermore, the liquid crystal diffraction element 85 arranged on the second surface is arranged at a position overlapping the liquid crystal diffraction element 84 arranged on the second surface of the light guiding substrate 46.

[0091] More specifically, the liquid crystal diffraction element 85 arranged on the first surface of the light-guiding substrate 46 is arranged at a position that overlaps with the first port 10 in the planar direction. Furthermore, the liquid crystal diffraction element 85 arranged on the first surface of the light-guiding substrate 46 does not overlap with the second port 11 and the third port 12 in the planar direction.

[0092] On the other hand, the liquid crystal diffraction element 85 arranged on the second surface of the light guiding substrate 46 is arranged at a position overlapping with the second port 11 in the surface direction. Furthermore, the liquid crystal diffraction element 85 arranged on the second surface of the light guiding substrate 46 does not overlap with the first port 10 and the third port 12 in the surface direction. Therefore, the liquid crystal diffraction element 85 arranged on the second surface of the light guiding substrate 46 is arranged stacked on the liquid crystal diffraction element 84 arranged on the second surface of the light guiding substrate 46. Furthermore, the liquid crystal diffraction element 85 arranged on the second surface of the light guiding substrate 46 is arranged at a position overlapping with the liquid crystal diffraction element 82 of the first optical unit 24 arranged on the second surface of the light guiding substrate 44 in the surface direction.

[0093] The third optical portion 25 b is disposed between the non-reciprocal portion 30 and the second optical portion 25 a, and includes a light-guiding substrate 45 and two liquid crystal diffraction elements 83 .

[0094] Like the light-guiding substrate 40, the light-guiding substrate 45 is a known light-guiding member capable of guiding light therein. In the illustrated example, the light-guiding substrate 45 is plate-shaped, and if its two largest surfaces are referred to as a first surface and a second surface, a liquid crystal diffraction element 83 is disposed on each of the first surface and the second surface. In the following description, the surface on the second port 11 side is referred to as the first surface, and the surface on the first port 10 side is referred to as the second surface. The light-guiding substrate 45 is disposed so that the first surface and the second surface are parallel to the surface direction.

[0095] The liquid crystal diffraction element 83 is a diffraction element that reflects and diffracts the first polarized light and transmits the second polarized light without diffracting it, and corresponds to the third diffraction element in the present invention. That is, the liquid crystal diffraction element (third diffraction element) 83 is a reflective diffraction element. In the illustrated example, the liquid crystal diffraction element 83 is a diffraction element that reflects and diffracts left-handed circularly polarized light and transmits right-handed circularly polarized light without diffracting it. That is, the liquid crystal diffraction element 83 has a left-twisted cholesteric liquid crystal layer and does not have a right-twisted cholesteric liquid crystal layer. In this way, the liquid crystal diffraction element (third diffraction element) 83 reflects and diffracts the same circularly polarized light as the liquid crystal diffraction element (first diffraction element) 84.

[0096] Of the two liquid crystal diffraction elements 83, one liquid crystal diffraction element 83 is disposed on a first surface of the light-guiding substrate 45, and the other liquid crystal diffraction element 83 is disposed on a second surface of the light-guiding substrate 45. The liquid crystal diffraction element 83 disposed on the first surface and the liquid crystal diffraction element 83 disposed on the second surface are disposed at different positions in the surface direction. Note that the liquid crystal diffraction element 83 disposed on the first surface and the liquid crystal diffraction element 83 disposed on the second surface may partially overlap in the surface direction.

[0097] More specifically, the liquid crystal diffraction element 83 arranged on the first surface of the light-guiding substrate 45 is arranged at a position overlapping with the first port 10 in the planar direction. Furthermore, the liquid crystal diffraction element 83 arranged on the first surface of the light-guiding substrate 45 does not overlap with the second port 11 and the third port 12 in the planar direction. Therefore, the liquid crystal diffraction element (third diffraction element) 83 arranged on the first surface of the light-guiding substrate 45 is arranged at a position overlapping with the liquid crystal diffraction element (second diffraction element) 85 of the second optical unit 25 a arranged on the first surface of the light-guiding substrate 46 in the planar direction.

[0098] On the other hand, the liquid crystal diffraction element 83 arranged on the second surface of the light guiding substrate 45 is arranged at a position overlapping with the third port 12 in the surface direction. Furthermore, the liquid crystal diffraction element 83 arranged on the second surface of the light guiding substrate 45 does not overlap with the first port 10 and the third port 12 in the surface direction. Therefore, the liquid crystal diffraction element 83 arranged on the second surface of the light guiding substrate 45 is arranged at a position overlapping with the liquid crystal diffraction element 82 of the first optical unit 24 arranged on the first surface of the light guiding substrate 44, and the liquid crystal diffraction element (first diffraction element) 84 of the second optical unit 25a arranged on the first surface of the light guiding substrate 46 in the surface direction.

[0099] Next, the operation of the optical circulator having such a configuration will be described with reference to FIGS.

[0100] 5 is a schematic diagram illustrating how an optical signal input to the optical circulator from the first port 10 is output from the second port 11. First, the P-polarized light and S-polarized light input to the optical circulator from the first port 10 are converted into right-handed circularly polarized light and left-handed circularly polarized light, respectively, by the λ / 4 plate 93. The converted right-handed circularly polarized light and left-handed circularly polarized light are incident on the liquid crystal diffraction element (second diffraction element) 85 arranged on the first surface of the light-guiding substrate 46 of the second optical unit 25a. Because the liquid crystal diffraction element 85 has a right-handed twisted cholesteric liquid crystal layer, the right-handed circularly polarized light is reflected and diffracted and guided through the light-guiding substrate 46. The right-handed circularly polarized light is again reflected and diffracted by the liquid crystal diffraction element 85 arranged in the reflection direction (the liquid crystal diffraction element 85 arranged on the second surface of the light-guiding substrate 46), and its optical path changes toward the front of the second port 11. On the other hand, the left-handed circularly polarized light passes through the liquid crystal diffraction element 85 arranged on the first surface of the light-guiding substrate 46, and is incident on the liquid crystal diffraction element (third diffraction element) 83 of the third optical unit 25b, which is arranged on the first surface of the light-guiding substrate 45. At this time, because the liquid crystal diffraction element 83 has a left-twisted cholesteric liquid crystal layer, the left-handed circularly polarized light is reflected and diffracted, and guided through the light-guiding substrate 45, and is reflected and diffracted again by the liquid crystal diffraction element 83 arranged in the reflection direction (the liquid crystal diffraction element 83 arranged on the second surface of the light-guiding substrate 45), and changes to a different optical path that is parallel to the right-handed circularly polarized light.

[0101] The separated right-handed circularly polarized light and left-handed circularly polarized light are converted into P-polarized light and S-polarized light, respectively, by the λ / 4 plate 92 and enter the non-reciprocal section 30 from the Faraday rotator 70 side. As described above, the incident P-polarized light and S-polarized light are transmitted as P-polarized light and S-polarized light, respectively.

[0102] The P-polarized light and S-polarized light that have passed through the non-reciprocal portion 30 enter the λ / 4 plate 91 and are converted into right-handed circularly polarized light and left-handed circularly polarized light, respectively. Of these, the left-handed circularly polarized light passes through the light-guiding substrate 44 of the first optical portion 24 and enters the liquid crystal diffraction element 82 arranged on the first surface of the light-guiding substrate 44. Because the liquid crystal diffraction element 82 has a left-twisted cholesteric liquid crystal layer, the left-handed circularly polarized light is reflected and diffracted, and is guided through the light-guiding substrate 44. The left-handed circularly polarized light is reflected and diffracted again by the liquid crystal diffraction element 82 arranged in the reflection direction (the liquid crystal diffraction element 82 arranged on the second surface of the light-guiding substrate 44), and changes its traveling direction toward the second port 11. On the other hand, the right-handed circularly polarized light is incident on the liquid crystal diffraction element 82 disposed on the second surface of the light-guiding substrate 44 of the first optical unit 24. However, because the liquid crystal diffraction element 82 has a left-twisted cholesteric liquid crystal layer, the right-handed circularly polarized light is transmitted without being diffracted by the liquid crystal diffraction element 82, passes through the light-guiding substrate 44, and enters the second port 11 as is. At this time, the first optical unit (polarization separation / combination unit) 24 including the two liquid crystal diffraction elements 82 and the light-guiding substrate 44 is positioned so that the reflected left-handed circularly polarized light and the transmitted right-handed circularly polarized light follow the same optical path, so that the left-handed circularly polarized light and the right-handed circularly polarized light can be input to the second port 11 in a combined state. Furthermore, by passing through the λ / 4 plate 90 in front of the second port 11, the right-handed circularly polarized light and the left-handed circularly polarized light are converted into P-polarized light and S-polarized light, respectively, and therefore exit from the second port 11 in the same polarization state as when they entered from the first port 10.

[0103] 6 is a schematic diagram illustrating how an optical signal input to the optical circulator from the second port 11 is output from the third port 12. First, the P-polarized light and S-polarized light input to the optical circulator from the second port 11 enter the λ / 4 plate 90 and are converted into right-handed circularly polarized light and left-handed circularly polarized light, respectively. Next, the right-handed circularly polarized light and left-handed circularly polarized light pass through the light-guiding substrate 44 of the first optical unit 24 and enter the liquid crystal diffraction element 82 disposed on the second surface of the light-guiding substrate 44. Because the liquid crystal diffraction element 82 has a left-twisted cholesteric liquid crystal layer, the right-handed circularly polarized light passes through the liquid crystal diffraction element 82 without being diffracted, while the left-handed circularly polarized light is reflected and diffracted by the liquid crystal diffraction element 82, resulting in polarization separation. The transmitted right-handed circularly polarized light continues in the same direction from the second port 11. On the other hand, the left-handed circularly polarized light reflected and diffracted by the liquid crystal diffraction element 82 arranged on the second surface of the light-guiding substrate 44 of the first optical unit 24 is guided within the light-guiding substrate 44, and is reflected and diffracted again by the liquid crystal diffraction element 82 arranged in the reflection direction (the liquid crystal diffraction element 82 arranged on the first surface of the light-guiding substrate 44), passes through the light-guiding substrate 44, and travels in the direction of the third port 12 parallel to the transmitted right-handed circularly polarized light.

[0104] The separated right-handed circularly polarized light and left-handed circularly polarized light enter the λ / 4 plate 91 and are converted into P-polarized light and S-polarized light, respectively. The converted P-polarized light and S-polarized light enter the nonreciprocal portion 30 from the λ / 2 plate 60 side, and the polarization plane of the light is rotated by 90°, so that the P-polarized light is converted into S-polarized light and the S-polarized light is converted into P-polarized light.

[0105] The S-polarized light and P-polarized light that have passed through the non-reciprocal portion 30 enter the λ / 4 plate 92 and are converted into left-handed circularly polarized light and right-handed circularly polarized light, respectively. Of these, the left-handed circularly polarized light passes through the light-guiding substrate 45 of the third optical portion 25 b, passes through the light-guiding substrate 46 of the second optical portion 25 a, and enters the liquid crystal diffraction element 85 and the liquid crystal diffraction element 84 that are disposed on the second surface of the light-guiding substrate 46. Because the liquid crystal diffraction element 85 has a right-twisted cholesteric liquid crystal layer, this left-handed circularly polarized light passes through the liquid crystal diffraction element 85 without being diffracted, and enters the liquid crystal diffraction element 84. Because the liquid crystal diffraction element 84 has a left-twisted cholesteric liquid crystal layer, the left-handed circularly polarized light is reflected and diffracted by the liquid crystal diffraction element 84, guided through the light-guiding substrate 46, and reflected and diffracted again by the liquid crystal diffraction element 84 arranged in the reflection direction (the liquid crystal diffraction element 84 arranged on the first surface of the light-guiding substrate 46), transmitted through the light-guiding substrate 46, and changes its traveling direction toward the third port 12. On the other hand, the right-handed circularly polarized light transmits through the light-guiding substrate 45 of the third optical unit 25b and enters the liquid crystal diffraction element 83 arranged on the second surface of the light-guiding substrate 45. However, because the liquid crystal diffraction element 83 has a left-twisted cholesteric liquid crystal layer, the right-handed circularly polarized light is transmitted through the liquid crystal diffraction element 83 without being diffracted by the liquid crystal diffraction element 83. Next, this right-handed circularly polarized light is incident on the liquid crystal diffraction element 84 arranged on the first surface of the light-guiding substrate 46 of the second optical unit 25 a, but because the liquid crystal diffraction element 84 has a left-twisted cholesteric liquid crystal layer, the right-handed circularly polarized light is transmitted through the liquid crystal diffraction element 84 without being diffracted, and then transmitted through the light-guiding substrate 46 and enters the third port 12 as is. At this time, the polarization separation / combining unit 25 (second optical unit 25 a and third optical unit 25 b) including the liquid crystal diffraction elements 83, 84, 85 and the light-guiding substrates 45, 46 is positioned so that the reflected left-handed circularly polarized light and the transmitted right-handed circularly polarized light follow the same optical path, and therefore the left-handed circularly polarized light and the right-handed circularly polarized light can be input to the third port 12 in a combined state. Furthermore, by passing through the λ / 4 plate 93 in front of the third port 12, the right-handed circularly polarized light and the left-handed circularly polarized light are converted into P-polarized light and S-polarized light, respectively, and are therefore emitted from the third port 12 in the same polarization state as when they entered from the second port 11.

[0106] As described above, in the optical circulator of the present invention, light incident on the first port 10 exits from the second port 11, and light incident on the second port 11 exits from the third port 12. With this configuration, it can be seen that the optical circulator of the present invention can be used as a small and lightweight optical circulator without using a cube-type polarizing beam splitter.

[0107] In the examples shown in Figures 5 and 6, the first diffraction element (liquid crystal diffraction element 82, liquid crystal diffraction element 84), the second diffraction element (liquid crystal diffraction element 85), and the third diffraction element (liquid crystal diffraction element 83) are all liquid crystal diffraction elements, and a λ / 4 plate is provided on the side opposite the non-reciprocal portion 30 of the first optical unit 24, between the first optical unit 24 and the non-reciprocal portion 30, between the non-reciprocal portion 30 and the third optical unit 25b, and on the side opposite the non-reciprocal portion 30 of the second optical unit 25a, but the present invention is not limited to this.

[0108] The first diffraction element, the second diffraction element, and the third diffraction element may all be diffraction elements that diffract one linearly polarized light and do not diffract the other linearly polarized light. In this case, each λ / 4 plate is not required. Alternatively, at least one of the first diffraction element of the first optical unit, the first diffraction element of the second optical unit, the second diffraction element of the second optical unit, and the third diffraction element of the third optical unit may be a diffraction element that diffracts one linearly polarized light and does not diffract the other linearly polarized light, and the remaining diffraction elements may be liquid crystal diffraction elements that diffract one circularly polarized light and do not diffract the other circularly polarized light. In this case, it is sufficient to have λ / 4 plates immediately upstream and immediately downstream of the optical unit that has the liquid crystal diffraction element.

[0109] Examples of the diffraction element include a surface relief type diffraction element, a volume hologram type diffraction element, and a liquid crystal diffraction element, but a liquid crystal diffraction element is preferred because it can maintain high diffraction efficiency regardless of the size of the diffraction angle.

[0110] A known surface relief diffraction element can be used as the surface relief diffraction element. As is well known, a surface relief diffraction element is configured with minute groove-like concave and convex portions alternately arranged in parallel at a predetermined period on the surface. The period of the diffraction structure, the material, the height of the convex portions, etc. may be appropriately set depending on the wavelength range to be diffracted. Furthermore, by adjusting the period of the diffraction structure, the material, the height and shape of the convex portions, etc., the surface relief diffraction element can be made into a diffraction element that reflects and diffracts one linearly polarized light and transmits the other linearly polarized light without diffracting it.

[0111] A known volume hologram diffraction element can be used as the volume hologram diffraction element. A volume hologram diffraction element is configured with linear regions with high refractive index and linear regions with low refractive index alternately arranged in parallel at a predetermined period. The period of the diffraction structure, the material, and the refractive index of each region can be appropriately set depending on the wavelength range to be diffracted. Furthermore, by adjusting the period and material of the diffraction structure, the volume hologram diffraction element can be made into a diffraction element that reflects and diffracts one linearly polarized light and transmits the other linearly polarized light without diffracting it.

[0112] The liquid crystal diffraction element may be of either a transmissive type or a reflective type, and may be appropriately selected in accordance with constraints such as the configuration and arrangement.

[0113] The liquid crystal diffraction element can be a conventionally known liquid crystal diffraction element formed using a composition containing a liquid crystal compound and having a liquid crystal layer with a liquid crystal orientation pattern in which the direction of the optical axis derived from the liquid crystal compound changes while continuously rotating along at least one direction in the plane.

[0114] In a liquid crystal diffraction element (a liquid crystal layer having a liquid crystal orientation pattern), by configuring the liquid crystal compound in the thickness direction to have a cholesteric liquid crystal layer with a helical structure in which the liquid crystal compound is spirally wound and stacked in the thickness direction, it is possible to create a reflective liquid crystal diffraction element that reflects and diffracts one circularly polarized light and transmits the other circularly polarized light without diffracting it.

[0115] Such reflective liquid crystal diffraction elements (cholesteric liquid crystal layers having liquid crystal orientation patterns) are described in WO 2019 / 131950, WO 2019 / 131966, WO 2019 / 189852, WO 2020 / 071169, etc.

[0116] The diffraction angle, diffraction efficiency, and diffraction wavelength of the liquid crystal diffraction element can be adjusted appropriately to suit the application. The diffraction angle depends on the in-plane pitch (one period of the liquid crystal orientation pattern), and the diffraction efficiency depends on the film thickness. The diffraction wavelength depends on the film thickness in the case of a transmission type and on the chiral pitch length (helical pitch) in the case of a reflection type, so it can be adjusted appropriately to suit the application.

[0117] The liquid crystal diffraction element may also be used by laminating or bonding together a plurality of liquid crystal layers (cholesteric liquid crystal layers) that diffract light of different wavelengths and / or polarized light.

[0118] Although the embodiments shown in FIGS. 1 to 6 show the diffraction element bonded to the light guiding substrate, it is not necessarily required that they be bonded.

[0119] The light guiding substrate travels through the interior while undergoing total reflection due to the difference in refractive index between the substrate and air on both the front and back principal surfaces. It is also desirable to suppress unintended interfacial reflections due to the difference in refractive index between the substrate and the diffraction element. Therefore, it is desirable for the light guiding substrate to have a refractive index in the range of 1.45 to 2.0.

[0120] Suitable materials for the light-guiding substrate include, but are not limited to, glass, acrylic, and polycarbonate. From the viewpoints of weight reduction and durability (crack prevention), it is preferable that the light-guiding substrate has a thickness of 0.5 to 5.0 mm.

[0121] Although typical embodiments of the present invention have been described above, the present invention is not limited to these and various embodiments that can be understood by those skilled in the art can be applied as long as they fall within the scope of the claims of the present invention.

[0122] 10 First port 11 Second port 12 Third port 20, 22, 24 First optical section (polarized light separation / combining section) 21, 23, 25a Second optical section (polarized light separation / combining section) 25b Third optical section 30 Non-reciprocal section 40 to 46 Light guiding substrate 51, 52 Diffraction element (first diffraction element) 60 λ / 2 plate 70 Faraday rotator 80 to 82, 84 Liquid crystal diffraction element (first diffraction element) 83 Liquid crystal diffraction element (third diffraction element) 85 Liquid crystal diffraction element (second diffraction element) 90 to 93 λ / 4 plate

Claims

1. An optical circulator comprising a first optical unit, a second optical unit, and a non-reciprocal unit disposed between the first optical unit and the second optical unit, wherein the first optical unit and the second optical unit each comprise a light-guiding substrate and two first diffraction elements disposed on a first surface and a second surface of the light-guiding substrate, respectively, at different positions in the surface direction, the first diffraction elements reflecting and diffracting a first polarized light and transmitting a second polarized light, and the non-reciprocal unit including a λ / 2 plate and a Faraday rotator.

2. An optical circulator as described in claim 1, wherein the first diffraction element is a liquid crystal diffraction element, and λ / 4 plates are provided on the side opposite the non-reciprocal portion of the first optical unit, between the first optical unit and the non-reciprocal portion, between the non-reciprocal portion and the second optical unit, and on the side opposite the non-reciprocal portion of the second optical unit.

3. An optical circulator according to claim 2, wherein at least one of said liquid crystal diffractive elements comprises a cholesteric liquid crystal layer.

4. The optical circulator according to claim 1, wherein the second optical section further comprises second diffraction elements arranged on each of the first and second surfaces of the light-guiding substrate at different positions in the in-plane direction, which transmit the first polarized light and reflect and diffract the second polarized light, and further comprises a third optical section between the non-reciprocal section and the second optical section, and wherein the third optical section comprises a light-guiding substrate and third diffraction elements arranged on each of the first and second surfaces of the light-guiding substrate at different positions in the in-plane direction, which reflect and diffract the first polarized light and transmit the second polarized light.

5. An optical circulator as described in claim 4, wherein the first diffraction element, the second diffraction element, and the third diffraction element are liquid crystal diffraction elements, and a λ / 4 plate is provided on the side opposite the non-reciprocal portion of the first optical unit, between the first optical unit and the non-reciprocal portion, between the non-reciprocal portion and the third optical unit, and on the side opposite the non-reciprocal portion of the second optical unit.

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