Optical isolator and method for manufacturing same

Abstract

This optical isolator 10 comprises: a first optical fiber 20 including a plurality of first cores; a first lens 30; isolator unit groups 40, 50; a second lens 60; and a second optical fiber 70 including a plurality of second cores, arranged in this order along a predetermined axis in a forward direction. When the shortest distance among the distances between each of a plurality of return light beams emitted from the second core and each of the plurality of first cores in a front view of an end face of the first optical fiber is defined as a minimum distance, the value of the minimum distance varies between a predetermined upper limit value and a predetermined lower limit value as the an isolator unit group rotates around the axis, and the isolator unit group is fixed at a position where the value of the minimum distance falls within a range from a value obtained by adding the lower limit value to 30% of the variation width of the minimum distance to the upper limit value.

Description

Optical isolator and its manufacturing method

[0001] The present invention relates to a polarization-independent optical isolator for a multi-core optical fiber and a method for manufacturing the same.

[0002] Research and development of space division multiplexing (SDM) technology using multi-core optical fibers has been progressing with the aim of increasing the transmission capacity per optical fiber and reducing the cost per bit. For example, conventionally, single-core optical fibers have been used in optical amplifiers, and conversion from a single-core optical fiber to a multi-core optical fiber, or conversion from a multi-core optical fiber to a single-core optical fiber, has been performed using a fan-in / fan-out (FIFO) device. However, there has been a problem of reduced transmission capacity due to increased loss caused by the insertion of a FIFO device. Therefore, in recent years, optical amplifiers using multi-core optical fibers (hereinafter also referred to as "MCF optical amplifiers") have been proposed (see Non-Patent Document 1). The MCF optical amplifier allows the FIFO device to be omitted, thereby suppressing the reduction in transmission capacity caused by the insertion loss of the FIFO device.

[0003] Yuta Wakayama and two others, "FIFO-less core-pumped multi-core fiber amplifier with fiber Bragg grating-based gain-flattening filter," The European Conference on Optical Communication, 2022

[0004] (Problem to be Solved by the Invention) An MCF optical amplifier requires an optical isolator using a multi-core optical fiber, but there is a problem that the dimensions of the optical isolator must be increased to suppress an increase in crosstalk due to returned light. That is, a polarization-independent optical isolator for optical fibers is generally configured to transmit output light from a core of an input-side optical fiber to make it incident on a corresponding core of an output-side optical fiber, while separating the output light from the corresponding core of the output-side optical fiber (in other words, returned light) to suppress coupling of the returned light with the corresponding core of the input-side optical fiber (i.e., to isolate the returned light). When such an optical isolator is used in a multi-core optical fiber, although it can effectively suppress coupling of returned light from each core of the output-side multi-core optical fiber with the corresponding core of the input-side multi-core optical fiber, there is a possibility that crosstalk will increase if the returned light couples with "cores other than the corresponding core." For this reason, an increase in crosstalk due to returned light has been suppressed by increasing the dimensions of the optical isolator. The dimensions of an optical isolator tend to increase as the number of cores in a multi-core optical fiber increases. The above-mentioned problem is not limited to MCF optical amplifiers, but is common to all polarization-independent optical isolators for multi-core optical fibers.

[0005] The present invention has been made to address the above-mentioned problems, and an object of the present invention is to propose a technique that enables miniaturization of an optical isolator for a multi-core optical fiber while suppressing an increase in crosstalk caused by optical feedback.

[0006] (Means for Solving the Problems) An optical isolator (10) for a multi-core optical fiber according to a first invention comprises: a first optical fiber (20) having a plurality of first cores (C1 to C4), a first lens (30), an isolator unit group (G), a second lens (60), and a second optical fiber (70) having a plurality of second cores (C1 to C4), which are arranged in this order in the forward direction along a predetermined axis (A). At least one of the first optical fiber and the second optical fiber is a multi-core optical fiber. the first lens collimates each of the light rays (b1 to b4) in any polarization state emitted from the first core of the first optical fiber; the isolator unit group is composed of one or more isolator units (40, 50) arranged in series, each including a pair of polarization separation elements (42, 46, 52, 56) and a Faraday rotator (44, 54) provided between the pair of polarization separation elements, and transmits each of the light rays traveling in the forward direction, while transmitting each of the return light rays (br1 to br4), which are light rays traveling in the reverse direction, in a state where the return light rays are separated into first light rays (br11 to br41) and second light rays (br12 to br42) whose polarization planes are orthogonal to each other; and the second lens converges each of the light rays transmitted through the isolator unit group so that each of the light rays is incident on the corresponding second core of the second optical fiber. When viewed from the front of the end face of the first optical fiber, if the shortest distance between each of the multiple return beams and each of the multiple first cores is defined as the minimum distance (dmin), the value of the minimum distance varies between a predetermined upper limit value (dminu) and a predetermined lower limit value (dminl) as the isolator unit group rotates around the axis, and the isolator unit group is fixed at a position such that the value of the minimum distance is within the range from the value obtained by adding the lower limit value to the value obtained by multiplying the fluctuation range of the minimum distance by 30% to the upper limit value.

[0007] A method for manufacturing an optical isolator (10) for a multi-core optical fiber according to a second invention includes the steps of: preparing an input-side collimator (Cin) including a first optical fiber (20) having a plurality of first cores (C1 to C4) and having an end face that is bevel-polished, and the first lens (30) configured to collimate each of light rays (b1 to b4) in any polarization state that are output from the first core; and preparing an output-side collimator (Cout) including a second optical fiber (70) having a plurality of second cores (C1 to C4), and the second lens (60) configured to converge each of the light rays so that each of the light rays enters a corresponding second core of the second optical fiber; a step of preparing an isolator unit group (G) configured to transmit each of the light rays traveling in a forward direction, which is a direction from the input-side collimator toward the output-side collimator, while transmitting each of the return light rays (br1 to br4), which are light rays traveling in a reverse direction, in a state where the return light rays are separated into first light rays (br11 to br41) and second light rays (br12 to br42) whose polarization planes are orthogonal to each other; and the first lens, the isolator unit group, the second lens, and the second optical fiber are arranged in this order along a predetermined axis (A) in the forward direction, and fixing the isolator unit group relatively to the input collimator and the output collimator so that a specific angle formed by an oblique polishing direction of the first optical fiber and an inclination direction of the birefringent plate located furthest forward in the forward direction is included in a specific angle range. In this manufacturing method, at least one of the first optical fiber and the second optical fiber is a multi-core optical fiber.When viewed from the front of the end face of the first optical fiber, the shortest distance among the distances between each of the multiple return beams and each of the multiple first cores is defined as the minimum distance (dmin), the value of the minimum distance fluctuates between a predetermined upper limit value (dminu) and a predetermined lower limit value (dminl) as the isolator unit group rotates around the axis, and the specific angle range is an angle range corresponding to a specific distance range ranging from the value obtained by adding the lower limit value to the value obtained by multiplying the fluctuation range of the minimum distance by 30% to the upper limit value.

[0008] Effect of the Invention According to the present invention, in an optical isolator for a multi-core optical fiber, the optical isolator can be miniaturized while suppressing an increase in crosstalk caused by returned light.

[0009] 2 is a side view of an optical isolator according to an embodiment of the present invention, showing an example of an optical path in the forward direction. FIG. 3 is a plan view of the optical isolator, showing an example of an optical path in the forward direction. FIG. 4 is a side view of the optical isolator, showing an example of an optical path in the forward direction. FIG. 5 is a plan view of the optical isolator, showing an example of an optical path in the reverse direction. FIG. 6 is a front view of an end face of a multi-core optical fiber on an input side. FIG. 7 is a front view of an end face of a multi-core optical fiber on an output side. FIG. 8 is a diagram showing the position of returned light in a reference state superimposed on the front view of FIG. 2. FIG. 9 is a diagram showing the position of returned light when the isolator unit group is rotated by a rotation angle φ around an axis from the reference state superimposed on the front view of FIG. 2. FIG. 10 is a graph defining the relationship between the separation amount s and the distance dmin and the rotation angle φ. FIG. 11 is a graph defining the relationship between the rotation angle φ and the absolute values ​​of the distance dmin and the maximum reverse crosstalk XTrmax according to condition A. FIG. 12 is a graph defining the relationship between the rotation angle φ and the absolute values ​​of the distance dmin and the maximum reverse crosstalk XTrmax according to condition B. 10 is a graph defining the relationship between the rotation angle φ and the absolute value of the distance dmin and the maximum backward crosstalk XTrmax according to condition C. FIG. 11 is a front view of a multi-core optical fiber used in condition C. FIG. 12 is a graph showing the wavelength dependence characteristics of forward and backward crosstalk.

[0010] An optical isolator 10 according to an embodiment of the present invention will now be described with reference to the drawings. The optical isolator 10 is a polarization-independent optical isolator, i.e., a type in which the insertion loss does not change regardless of the polarization state of incident light. FIGS. 1A and 1C are side views of the optical isolator 10, and FIGS. 1B and 1D are plan views of the optical isolator 10. As shown in FIGS. 1A to 1D, the optical isolator 10 includes a multi-core optical fiber 20, a lens 30, an isolator unit group G, a lens 60, and a multi-core optical fiber 70 (hereinafter, the multi-core optical fiber will also be referred to as an "MCF"). These components are arranged in the above order along an axis A. A rectangular coordinate system is set for the optical isolator 10. The z-axis extends parallel to the axis A, with the direction from the MCF 20 toward the MCF 70 being the positive direction. The y-axis is orthogonal to the z-axis, and extends such that the upward direction in the plane of the paper is the positive direction in FIGS. 1A and 1C. The x-axis is perpendicular to the y-axis and z-axis, and extends in a positive direction in the upward direction of the paper in FIGS. 1B and 1D.

[0011] In this specification, the +z-axis direction is defined as the forward direction of the optical isolator 10, and the −z-axis direction is defined as the backward direction of the optical isolator 10. Therefore, the MCF 20 functions as an input-side multi-core optical fiber, and the MCF 70 functions as an output-side multi-core optical fiber. FIGS. 1A and 1B show the optical path of the light beam emitted from the input-side MCF 20 (strictly speaking, the light beam emitted from a core C1, which will be described later) in the forward direction, while FIGS. 1C and 1D show the optical path of the light beam emitted from the output-side MCF 70 (strictly speaking, the light beam emitted from a core C1, which will be described later) in the backward direction. The light beam emitted from the output-side MCF 70 is the light beam reflected by each optical component arranged downstream in the forward direction of the optical isolator 10 (so-called return light).

[0012] In this specification, for ease of viewing, the dimensions of specific components (e.g., MCFs 20 and 70), the angles of light rays, etc. are changed in the drawings. The MCFs 20 and 70 correspond to examples of a "first multi-core optical fiber" and a "second multi-core optical fiber", respectively. The lenses 30 and 60 correspond to examples of a "first lens" and a "second lens", respectively.

[0013] The MCF 20 is cylindrical, and the central axis of at least its end in the +z-axis direction coincides with the axis A. This end of the MCF 20 is inserted into and held by a ferrule (not shown). The end face 20a of the MCF 20 and the end face of the ferrule are bevel-polished together, with the polishing angle being, for example, 8°. Here, when the end face of any MCF is viewed from the front, if the direction from the "distal end, which is the position farthest from the corresponding lens," to the "proximal end, which is the position closest to the corresponding lens," is defined as the "bevel-polished direction," then the bevel-polished direction of the MCF 20 is the +y-axis direction (see FIG. 1A ).

[0014] 2 is a front view of the end face 20a (i.e., a view of the end face 20a viewed along the central axis). As shown in FIG. 2, the MCF 20 includes four cores C1 to C4 and a common cladding CL surrounding these cores C1 to C4. The cores C1 to C4 are located at the vertices of a square centered on the center of the end face 20a, and the core pitch is 44 μm. The cores C1 to C4 extend in a direction parallel to the central axis of the MCF 20. The mode field diameter of the MCF 20 is 10.6 μm.

[0015] 1A and 1B, the explanation continues. The lens 30 is a collimating lens with a focal length of 2.5 mm, and more specifically, is an aspherical lens having a rotationally symmetric curved surface. The optical axis of the lens 30 is located on the axis A. The curved surface of the lens 30 may be non-rotationally symmetric as long as it can collimate the light rays from each of the cores C1 to C4. The lens 30 may also be a spherical lens or a GRIN lens, or a lens with one flat surface.

[0016] A collimator is configured by the ferrule in which the MCF 20 is housed and the lens 30. This collimator functions as an input-side collimator Cin.

[0017] The isolator unit group G is composed of two isolator units 40 and 50 connected in series. The isolator unit 40 includes a pair of birefringent plates 42 and 46 arranged along the axis A, and a Faraday rotator 44 provided between the pair of birefringent plates 42 and 46. The isolator unit 50 is located after the isolator unit 40 in the forward direction, and includes a pair of birefringent plates 52 and 56 arranged along the axis A, and a Faraday rotator 54 provided between the pair of birefringent plates 52 and 56.

[0018] The birefringent plates 42, 46, 52, and 56 are all wedge-shaped birefringent single crystals made of rutile (TiO2) and have the same optical properties. The birefringent plates 42, 46, 52, and 56 are rectangular prisms with right-angled trapezoidal cross sections, and are identical in size and shape. The birefringent plates 42, 46, 52, and 56 are examples of "polarization separation elements." Note that the cross-sectional shape of the birefringent plates 42, 46, 52, and 56 is not limited to a right-angled trapezoid and may be any rectangular shape. Furthermore, the birefringent single crystals may be made of materials other than rutile. Furthermore, the birefringent plates 42, 46, 52, and 56 are not limited to birefringent single crystals, and other birefringent optical elements may be used.

[0019] The birefringent plate 42 is located in front of the birefringent plate 46 in the forward direction. The birefringent plate 42 has four surfaces 42a, 42b, 42c, and 42d that constitute its side surface. The surfaces 42a and 42b face each other, and the surfaces 42c and 42d face each other. The surface 42b is parallel to the xy plane, and the surface 42a is inclined at a predetermined taper angle (e.g., 5°) relative to the surface 42b. The birefringent plate 42 is arranged so that the surfaces 42a and 42b serve as the entrance and exit surfaces, respectively, for light rays traveling in the forward direction. The width (length in the z-axis direction) of the surface 42c is narrower than the width of the surface 42d. In other words, the surface 42c is a narrow surface, and the surface 42d is a wide surface.

[0020] The birefringent plate 46 has four surfaces 46a, 46b, 46c, and 46d that constitute its side peripheral surface. Surfaces 46a and 46b face each other, and surfaces 46c and 46d face each other. Surface 46b is parallel to the xy plane, and surface 46a is inclined relative to surface 46b by the above-mentioned taper angle. The birefringent plate 46 is arranged so that surfaces 46b and 46a serve as the entrance and exit surfaces, respectively, for light rays traveling in the forward direction. The width of surface 46c is narrower than the width of surface 46d. In other words, surface 46c is a narrow surface, and surface 46d is a wide surface.

[0021] As is clear from the above description, within the isolator unit 40, the inclined surfaces 42a and 46a face outward, while the vertical surfaces 42b and 46b face inward. Here, the direction from the narrow surface (surface 42c or 46c) toward the wide surface (surface 42d or 46d) is defined as the "inclination direction" (in FIG. 1A, the inclination direction of the birefringent plate 42 is the -y-axis direction, and the inclination direction of the birefringent plate 46 is the +y-axis direction). In this case, the birefringent plates 42 and 46 are arranged so that their inclination directions form an angle of 180°. In particular, in this embodiment, the taper angle of the surface 42a is equal to the taper angle of the surface 46a. Therefore, it can also be said that the birefringent plates 42 and 46 are arranged so that the surfaces 42a and 46a are parallel to each other. The optical axis of the birefringent plate 42 and the optical axis of the birefringent plate 46 extend in different directions, and form an angle of 45° therebetween.

[0022] The Faraday rotator 44 rotates the plane of polarization of the incident light by 45° in a particular direction (for example, clockwise in the forward direction and counterclockwise in the reverse direction).

[0023] The birefringent plate 52 is located in front of the birefringent plate 56 in the forward direction. The configuration of the birefringent plate 52 in the isolator unit 50 is the same as the configuration of the birefringent plate 42 in the isolator unit 40. The configuration of the birefringent plate 56 in the isolator unit 50 is the same as the configuration of the birefringent plate 46 in the isolator unit 40. Therefore, in the isolator unit 50, the inclined surfaces 52a and 56a face outward, and the vertical surfaces 52b and 56b face inward. The birefringent plates 52 and 56 are arranged so that their inclination directions form an angle of 180°. In particular, in this embodiment, the taper angle of the surface 52a is equal to the taper angle of the surface 56a. Therefore, it can also be said that the birefringent plates 52 and 56 are arranged so that the surfaces 52a and 56a are parallel to each other. Furthermore, the optical axis of the birefringent plate 52 and the optical axis of the birefringent plate 56 extend in different directions, and form an angle of 45° therebetween.

[0024] The Faraday rotator 54 rotates the polarization plane of the incident light beam by 45° in a specific direction opposite to the specific direction (for example, counterclockwise in the forward direction and clockwise in the reverse direction).

[0025] The isolator units 40 and 50 are arranged so that the optical axis of the birefringent plate 46 and the optical axis of the birefringent plate 52 (the optical axes of adjacent birefringent plates) are orthogonal to each other. That is, within the isolator unit group G, adjacent isolator units are arranged so that the "optical axis of the birefringent plate of the upstream isolator unit in the direction of light propagation" and the "optical axis of the birefringent plate of the upstream isolator unit in the direction of light propagation" are orthogonal to each other. This provides the effect of preventing the separated light beams separated and output by the upstream isolator unit from being further separated by the downstream isolator unit (described below). Note that, as long as this effect is obtained, the direction in which the Faraday rotator 54 rotates the polarization plane of the incident light beam may be the same as the direction of the Faraday rotator 44.

[0026] The lens 60 is a collimating lens with a focal length of 2.5 mm, and more specifically, is an aspherical lens with a rotationally symmetric curved surface. The optical axis of the lens 60 is located on the axis A. The curved surface of the lens 60 may be non-rotationally symmetric as long as it can converge the collimated light. The lens 60 may also be a spherical lens or a GRIN lens, or a lens with one flat surface.

[0027] The MCF 70 is cylindrical, and at least the central axis of its end in the -z axis direction coincides with the axis A. This end of the MCF 70 is inserted into and held by a ferrule (not shown). The end face 70a of the MCF 70 and the end face of the ferrule are obliquely polished together, with the polishing angle being, for example, 8°. The oblique polishing direction of the MCF 70 is the -x axis direction (see FIG. 1B). That is, in this embodiment, the oblique polishing direction of the MCF 70 is shifted by 90° from the oblique polishing direction of the MCF 20. However, the angle formed by the oblique polishing directions of the MCFs 20 and 70 is not limited to this.

[0028] 3 is a front view of the end face 70a (i.e., a view of the end face 70a viewed along the central axis). As shown in FIG. 3, the MCF 70 includes four cores C1 to C4 and a common cladding CL surrounding these cores C1 to C4. The cores C1 to C4 are located at the vertices of a square centered on the center of the end face 70a, and the core pitch is 44 μm. That is, the core pitch of the MCF 20 and the core pitch of the MCF 70 are equal to each other. The cores C1 to C4 extend in a direction parallel to the central axis of the MCF 70.

[0029] 1A and 1B, a collimator is configured by the lens 60 and the ferrule housing the MCF 70. This collimator functions as an output collimator Cout.

[0030] The input collimator Cin, the isolator unit group G, and the output collimator Cout are housed in a sleeve (not shown), both ends of which are sealed with rubber. The MCFs 20 and 70 extend to the exterior from both rubber-sealed ends. In this specification, "reducing the size of the optical isolator 10" means reducing the size of this sleeve (particularly, reducing it radially).

[0031] Next, the optical paths in the forward and reverse directions will be described. For ease of understanding, in Figures 1A and 1B, of the light beams b1 to b4 emitted from the cores C1 to C4 (see Figure 2), respectively, of the MCF 20, only the light beam b1 from the core C1 is shown, and in Figures 1C and 1D, of the light beams (returned light) br1 to br4 emitted from the cores C1 to C4 (see Figure 3), respectively, of the MCF 70, only the light beam br1 from the core C1 is shown.

[0032] First, in the forward direction, as shown in Figures 1A and 1B, light rays b1 to b4 in any polarization state that have propagated through each of the cores C1 to C4 of the MCF 20 are emitted from the end face 20a toward the lens 30. These light rays b1 to b4 are divergent light rays that diverge as they travel, and because the end face 20a is bevel-polished, they enter the lens 30 at an angle that is inclined by a predetermined angle with respect to the axis A. The lens 30 collimates (parallelizes) these incident light rays b1 to b4. The collimated light rays b1 to b4 are each emitted at an exit angle that corresponds to the incident position and angle on the lens 30, and enter the isolator unit 40. The birefringent plate 42 separates each of these incident light rays b1 to b4 into first light rays b11 to b41 and second light rays b12 to b42, whose planes of polarization are orthogonal to each other (only the first light ray b11 and the second light ray b12 are shown in FIGS. 1A and 1B ). The first light rays b11 to b41 and the second light rays b12 to b42 emitted from the birefringent plate 42 have their planes of polarization rotated by 45° in specific directions by the Faraday rotator 44, and then pass through the birefringent plate 46 to enter the isolator unit 50. Before entering the isolator unit 50, the first light rays b11 to b41 are ordinary rays and the second light rays b12 to b42 are extraordinary rays.

[0033] As described above, the optical axis of the birefringent plate 46 and the optical axis of the birefringent plate 52 are perpendicular to each other. Therefore, the first light rays b11 to b41 and the second light rays b12 to b42 travel through the isolator unit 50 without being further separated. This prevents the number of light rays emitted from the isolator unit 50 from increasing. In addition, upon entering the isolator unit 50, the first light rays b11 to b41 change from ordinary rays to extraordinary rays, and the second light rays b12 to b42 change from extraordinary rays to ordinary rays. This eliminates the propagation delay that occurred between the first light rays b11 to b41 and the second light rays b12 to b42 immediately after passing through the isolator unit 40. Hereinafter, the first light rays and the second light rays will also be simply referred to as "light rays." The light beams b11 to b41 and the light beams b12 to b42 have their polarization planes rotated in the opposite direction by the Faraday rotator 54.

[0034] The light rays b11 to b41 and the light rays b12 to b42 transmitted through the birefringent plate 56 are incident on the lens 60. The lens 60 converges these light rays b11 to b41 and the light rays b12 to b42. The lens 60 and the MCF 70 are respectively positioned so that the light rays b11 to b41 and the light rays b12 to b42 emitted from the lens 60 are incident on the bevel-polished end face 70a of the MCF 70 at appropriate angles of incidence and converge on the end face 70a. Therefore, the light rays b11 and b12 converge on the corresponding core C1, the light rays b21 and b22 converge on the corresponding core C2, the light rays b31 and b32 converge on the corresponding core C3, and the light rays b41 and b42 converge on the corresponding core C4. This configuration allows the optical isolator 10 to achieve extremely low coupling loss in the forward direction.

[0035] 1C and 1D , in the reverse direction, the light rays br1 to br4, which are return light having propagated through the cores C1 to C4 of the MCF 70, are emitted from the end face 70a toward the lens 60 and are incident on the lens 60 at a predetermined angle with respect to the axis A. The light rays br1 to br4 collimated by the lens 60 are emitted at respective exit angles according to the incident positions and angles on the lens 60, and are incident on the isolator unit 50. The birefringent plate 56 separates each of these incident light rays br1 to br4 into first return light rays br11 to br41 and second return light rays br12 to br42, the polarization planes of which are orthogonal to each other (only the first return light br11 and the second return light br12 are shown in FIGS. 1C and 1D ). The first returning beams br11 to br41 and the second returning beams br12 to br42 emitted from the birefringent plate 56 have their polarization planes rotated by 45° in the opposite direction by the Faraday rotator 54, and then pass through the birefringent plate 52 and enter the isolator unit 40. The first returning beams br11 to br41 change from ordinary rays to extraordinary rays when they enter the birefringent plate 52. The second returning beams br12 to br42 change from extraordinary rays to ordinary rays when they enter the birefringent plate 52.

[0036] In the isolator unit 40, the first returning beams br11 to br41 and the second returning beams br12 to br42 travel without being separated any further, and their polarization planes are each rotated by 45° in a specific direction by the Faraday rotator 44. The first returning beams br11 to br41 change from extraordinary rays to ordinary rays when they are incident on the birefringent plate 46, and change from ordinary rays to extraordinary rays when they are incident on the birefringent plate 42. The second returning beams br12 to br42 change from ordinary rays to extraordinary rays when they are incident on the birefringent plate 46, and change from extraordinary rays to ordinary rays when they are incident on the birefringent plate 42. The beams br11 to br41 and the beams br12 to br42 that have passed through the birefringent plate 42 are each refracted by the lens 30 and reach the MCF 20 or its vicinity.

[0037] FIG. 4A is a front view of the end face 20a of the MCF 20 when the components of the optical isolator 10 are positioned as shown in FIGS. 1C and 1D . In other words, FIG. 4A is a front view of the end face 20a when the beveled polishing direction of the MCF 20 is oriented in the +y-axis direction and the tilt direction of the birefringent plate 42 is oriented in the −y-axis direction. FIG. 4A shows the positions of the light beams br11 to br41 and the light beams br12 to br42 traveling in the opposite directions superimposed on each other. As shown in FIG. 4A , the split light beams (br11, br12), (br21, br22), (br31, br32), and (br41, br42) of the light beams br1 to br4 emitted from the cores C1 to C4 of the MCF 70 are all separated by equal distances from the cores C1 to C4 of the MCF 20, respectively. The split amounts s of these split light beams are equal to each other. By separating the returning light in this manner, coupling of the returning light with the corresponding core is effectively suppressed, thereby achieving high isolation. This method works well when the optical isolator is used in a single-core optical fiber. However, this method does not take into consideration the possibility of the returning light coupling with other cores (cores other than the corresponding core). Therefore, when used in an optical isolator for an MCF (i.e., the optical isolator 10), there is a possibility that crosstalk (backward crosstalk) caused by the returning light may increase, which necessitates an increase in the size of the optical isolator. For example, in the example of Figure 4A, the light ray br22 (the returning light from core C2 of the MCF 70) is far from the corresponding core C2 of the MCF 20, thereby achieving high isolation. However, because it is very close to core C4, there is a possibility that crosstalk of core C4 caused by the light ray br22 may increase.

[0038] Therefore, the present inventors have investigated a technology that can reduce the size of an optical isolator for an MCF while suppressing the increase in crosstalk caused by optical feedback. In this specification, crosstalk XTr caused by optical feedback is defined as the ratio of the power Pinj of a light beam emitted from any core Cj (j: an integer from 1 to 4) of the MCF 70 and input in the reverse direction to the power Poutk of the light beam when the light beam is coupled with another core Ck (k: an integer from 1 to 4 excluding j) of the MCF 20 and output in the reverse direction. The power Poutk is the larger of the two powers Poutk of the ordinary and extraordinary rays. For example, when j = 1, three crosstalk XTrs can be measured: "Pout2 / Pin1," "Pout3 / Pin1," and "Pout4 / Pin1." In other words, a total of 12 crosstalk XTrs can be measured. The inventors of the present application thought that if the value of the maximum reverse crosstalk XTrmax, which is the maximum value of these 12 crosstalk XTr, could be reduced, the increase in crosstalk XTr could be effectively suppressed (hereinafter, the maximum reverse crosstalk will also be simply referred to as "maximum crosstalk").

[0039] Hereinafter, the direction in which the light rays br11 to br41 and the light rays br12 to br42 are separated from one another is defined as the "separation direction." Furthermore, the state in which the light rays br11 to br41 and the light rays br12 to br42 are distributed at the positions shown in FIG. 4A is defined as the "reference state," and the angle between the y-axis and the separation direction in the reference state is defined as the "reference angle θ." In the example of FIG. 4A , the reference angle θ is −45°. Furthermore, in a front view of the end face 20a of the MCF 20, the shortest distance between each of the light rays br11 to br41 and the light rays br12 to br42 and each of the cores C1 to C4 is defined as the "distance dmin." In the example of FIG. 4A , the distance dmin is the distance between the light ray br22 and the core C4 (or the distance between the light ray br41 and the core C2).

[0040] Here, when the isolator unit group G is rotated around the axis A by an angle φ from the reference state, as shown in FIG. 4B , the light rays br11 to br41 and the light rays br12 to br42 also rotate by the angle φ around the corresponding cores C1 to C4. That is, the rotation angle of the isolator unit group G is equal to the angle between the separation direction of any pair of separated light rays in the reference state and the separation direction of the separated light rays after rotation. The inventors of the present application focused on this point and discovered that the distance dmin periodically changes when the isolator unit group G is rotated around the axis A. They then considered that the maximum crosstalk XTrmax could be reduced by rotating the isolator unit group G from the reference state so that the distance dmin becomes a relatively large value. In this embodiment, the isolator unit group G is rotated counterclockwise when viewed from the −z-axis direction. However, the rotation direction is not limited to this.

[0041] FIG. 5 is a graph showing the relationship between the separation amount s, the distance dmin, and the rotation angle φ using the optical isolator 10. This graph plots the rotation angle φ and the distance dmin when the distance dmin is maximized as the rotation angle φ is increased or decreased. According to this graph, within a relatively small range of the separation amount s, the distance dmin increases as the separation amount s increases, regardless of the rotation angle φ. Furthermore, once the separation amount s reaches a certain level, the distance dmin decreases with increasing separation amount s. This is thought to be because the arrival position of the separated light beam approaches other cores (cores other than the corresponding core) as the separation amount s increases. As the separation amount s increases further, the distance dmin also increases. This is thought to be because the arrival position of the separated light beam also moves away from other cores as the separation amount s increases. Meanwhile, the rotation angle φ that maximizes the distance dmin increases with increasing distance dmin. From the above, it can be seen that increasing the separation amount s is sufficient to increase the distance dmin. However, since the separation amount s depends on the focal lengths of the lenses 30 and 60 and the separation angles of the birefringent plates 42, 46, 52 and 56, increasing the separation amount s results in an increase in the size of the optical isolator 10.

[0042] Therefore, in order to achieve both "miniaturization of the optical isolator 10" and "reduction of the maximum crosstalk XTrmax," the inventors adjusted the positional relationship of the components of the optical isolator 10 so that the separation amount s was 156 μm, and investigated the relationship between the rotation angle φ, the distance dmin, and the maximum crosstalk XTrmax. A calculated value (theoretical value) was used for the distance dmin, and a measured value was used for the maximum crosstalk XTrmax. Hereinafter, this investigation condition will be referred to as "Condition A." The investigation results under Condition A are shown in FIG. 6. For ease of explanation, in FIG. 6, the maximum crosstalk XTrmax is shown as its absolute value. This also applies to FIGS. 7 and 8, which will be described later.

[0043] In addition, the inventors conducted two measurements under different investigation conditions to investigate the relationship between the separation amount s, the distance dmin, and the maximum crosstalk XTrmax in more detail. Under the first investigation condition, Condition B, lenses with a focal length of 1.8 mm were used instead of lenses 30 and 60. The separation amount s was 114 μm, and the reference angle θ was −45°. The investigation results under Condition B are shown in FIG. 7 . Meanwhile, under Condition C, the second investigation condition, lenses with a focal length of 1.8 mm were used instead of lenses 30 and 60. Additionally, MCF120 (see FIG. 9 ) was used instead of MCF20 and MCF70. As shown in FIG. 9 , MCF120 includes seven cores C0 to C6 and a common cladding CL surrounding these cores C0 to C6. Core C0 is located on axis A, and cores C1 to C6 are located at the vertices of a regular hexagon centered at the center of end face 220a, with a core pitch of 37 μm. That is, the core pitch of the input-side multi-core optical fiber (MCF120) and the core pitch of the output-side multi-core optical fiber (MCF120) are equal to each other. The mode field diameter of the MCF120 is 10 μm. The separation amount s is 114 μm, and the reference angle θ is -45°. The investigation results under condition C are shown in Figure 8.

[0044] According to the graphs in Figures 6 to 8, under all conditions A to C, the distance dmin periodically varies within a 360° range as the rotation angle φ changes. Specifically, under conditions A and B, the distance dmin varies in 90° cycles, while under condition C, the distance dmin varies in 60° cycles. This is thought to be because the MCF core arrangement is four-fold symmetric under conditions A and B (360° / 4 = 90°) and six-fold symmetric under condition C (360° / 6 = 60°). Under condition B, the upper limit value dminu and the lower limit value dminl of the distance dmin are both smaller than under condition A. This is thought to be because the separation amount s (= 114 μm) under condition B is smaller than the separation amount s (= 156 μm) under condition A. Furthermore, under condition C, the upper limit value dminu and the lower limit value dminl are both smaller than under condition A. This is thought to be because the separation amount s (=114 μm) under condition C is smaller than the separation amount s under condition A, and in addition, the number of cores and core arrangement (including core pitch) under both conditions are different from each other.

[0045] Furthermore, under conditions B and C, the behavior of |XTrmax| with changes in the rotation angle φ closely matches the behavior of the distance dmin. However, under condition A, the behavior of |XTrmax| and the behavior of the distance dmin match only within a certain angular range of the rotation angle φ (0°≦φ≦15° and 75°≦φ≦90°), and the behaviors of the two do not match within other angular ranges (15°<φ<75°). This is thought to be because the larger |XTrmax| (i.e., the smaller XTrmax) becomes, the greater the influence of various external factors (e.g., stray light). This measurement confirmed that external factors begin to affect the value of XTrmax only after |XTrmax| exceeds 50 dB. Furthermore, when |XTrmax| is 50 dB or less, it was confirmed that |XTrmax| increases as the distance dmin increases (i.e., XTrmax can be reduced). If the results of conditions B and C are applied to condition A, it is estimated that |XTrmax| will be greater than 50 dB (XTrmax can be reduced to less than −50 dB) when the rotation angle φ satisfies 15°<φ<75° (in other words, when dmin≧25 μm). In particular, it is estimated that |XTrmax| will be well over 60 dB (XTrmax can be reduced to −60 dB or less) when the rotation angle φ satisfies 22.5°≦φ≦67.5° (in other words, when dmin≧33 μm).

[0046] Based on the above investigation results, the inventors of the present application have concluded that by setting the rotation angle φ so that the value of the distance dmin is "a value within the range from the intermediate value dminm to the upper limit value dminu," the distance dmin can be made relatively large, and as a result, XTrmax can be suitably reduced. Note that the intermediate value dminm is the intermediate value between the upper limit value dminu and the lower limit value dminl. Hereinafter, the range from the intermediate value dminm to the upper limit value dminu is also referred to as the "specific distance range" of the distance dmin.

[0047] Furthermore, the inventors set the separation amount s to a value such that the lower limit dminl is equal to or less than the core pitch p of the input MCF. Specifically, under condition A, the separation amount s is 156 μm, which is a value selected from the range of separation amount s (see FIG. 5 ) such that the lower limit dminl (= 15.8 μm) is equal to or less than the core pitch p of the MCF 20 (= 44 μm). Similarly, under condition B, the separation amount s is 114 μm, which is a value selected from the range of separation amount s such that the lower limit dminl (= 5.2 μm) is equal to or less than the core pitch p of the MCF 20 (= 44 μm). Under condition C, the separation amount s is 114 μm, which is a value selected from the range of separation amount s such that the lower limit dminl (= 7.1 μm) is equal to or less than the core pitch p of the MCF 120 (= 37 μm).

[0048] The reason for setting the separation amount s in this manner is as follows. That is, the present invention was originally conceived in consideration of the problem that, in optical isolators for MCFs, backward crosstalk XTr tends to increase as the dimensions are reduced. One criterion for evaluating backward crosstalk XTr is forward crosstalk XT. Here, forward crosstalk XT is a well-known parameter defined as the ratio of "power Poutj when a light beam traveling in the forward direction is coupled with a corresponding core Cj of the MCF 70 and output in the forward direction" to "power Poutj when the light beam is coupled with another core Ci (i: an integer from 1 to 4 excluding j) of the MCF 70 and output in the forward direction." The fact that "maximum backward crosstalk XTrmax (in other words, the backward crosstalk XTr with the lowest evaluation) is equal to or less than forward crosstalk XT" can be one evaluation criterion indicating that an increase in XTrmax has been appropriately suppressed. Here, if the distance dmin is greater than the core pitch pin (dmin > pin) at any rotation angle φ, XTrmax < XT holds (i.e., the above evaluation criterion is satisfied), and there is less need to control the rotation angle φ. Therefore, when the objective is to satisfy the above evaluation criterion, the present invention is actually useful when "the separation amount s is set to a value such that the upper limit value dminu (rather than the lower limit value dminl) is equal to or less than the core pitch pin." However, even when "the separation amount s is set to a value such that the lower limit value dminl is equal to or less than the core pitch pin," it is still possible to sufficiently achieve both "miniaturization of the optical isolator 10" and "reduction of the maximum crosstalk XTrmax" compared to conventional techniques. Furthermore, the evaluation criterion for the maximum reverse crosstalk XTrmax is not limited to the above criterion, and other evaluation criteria may be adopted. Based on the above, in order to ensure a high degree of design freedom, the separation amount s is set (limited) to a value such that "dminl ≦ pin" holds.

[0049] The setting range of the separation amount s is not limited to the above. For example, the separation amount s may be set to a value such that the "lower limit value dminl" is equal to or less than the "value obtained by multiplying the core pitch pin by a predetermined percentage of 100% or more." This percentage may be, for example, 120% or 150%. The larger this percentage, the greater the degree of freedom in design. On the other hand, the smaller this percentage (within a range of 100% or more), the more compact the optical isolator 10 can be.

[0050] To verify the above study results, the inventors set the rotation angle φ of optical isolator 10 that satisfies condition A to 22.5° and measured various optical characteristics including maximum crosstalk XTrmax. Note that this rotation angle φ = 22.5° is the rotation angle corresponding to dmin selected from the range of distance dmin from the median value dminm to the upper limit value dminu.

[0051] 10 is a graph showing the wavelength dependency characteristics of all 12 crosstalk XTr (reverse crosstalk XTr) and 12 crosstalk XT (forward crosstalk XT) caused by light rays traveling in the forward direction. According to the graph of FIG. 10, it was confirmed that in the range where the wavelength λ satisfies 1525 nm≦λ≦1570 nm, all of the 24 crosstalk XT and XTr in the forward and reverse directions were less than −50 dB.

[0052] The wavelength dependence of isolation and wavelength dependence of insertion loss IL were also investigated for each of the cores C1 to C4 of the MCF 20 in the temperature range from -5°C to 70°C. The cutback method was used to measure the insertion loss IL. As a result, it was confirmed that the isolation exceeded 60 dB when the wavelength was 1550 nm and the temperature was 25°C, but that when the wavelength was 1550 nm, the isolation decreased as the temperature increased, and when the temperature was 25°C, the isolation decreased as the wavelength increased. This is due to the characteristics of the Faraday rotators 44 and 54, and a similar tendency is observed in optical isolators for single-core optical fibers. Furthermore, the insertion loss IL was in the range of 0.24 dB≦IL≦0.44 dB, which was equivalent to the insertion loss IL of optical isolators for single-core optical fibers.

[0053] In addition, the polarization dependent loss, return loss, and polarization mode dispersion were also investigated, and it was confirmed that all of them were equivalent to the polarization dependent loss, return loss, and polarization mode dispersion of an optical isolator for a single-core optical fiber.

[0054] The dimensions of the optical isolator 10 fixed at a rotation angle φ of 22.5° were 5.5 mm in outer diameter × 29 mm, which was equivalent to the dimensions of an optical isolator for a single-core optical fiber, and significantly smaller than the dimensions (e.g., 41 mm × 14 mm × 14 mm) of a conventional optical isolator for a multi-core optical fiber.

[0055] Next, a description will be given of a manufacturing method of the optical isolator 10. When manufacturing the optical isolator 10, first, the input-side collimator Cin, the output-side collimator Cout, and the isolator unit group G are prepared. Next, the input-side collimator Cin, the isolator unit group G, and the output-side collimator Cout are arranged so that the MCF 20, the lens 30, the isolator unit group G, the lens 60, and the MCF 70 are arranged in this order along the axis A in the forward direction.

[0056] Next, the isolator unit group G is fixed relative to the input-side and output-side collimators Cin and Cout so that the specific angle θs formed by the oblique polishing direction of the MCF 20 and the tilt direction of the birefringent plate 42 (in other words, the birefringent plate located furthest forward in the forward direction) is within a predetermined specific angle range, which will be described later. Here, the specific angle range is an angle range corresponding to the specific distance range. In this embodiment, the collimators Cin and Cout and the isolator unit group G are fixed by welding them into the above-mentioned sleeve (not shown). The optical isolator 10 is manufactured in this manner.

[0057] The position at which the isolator unit group G is fixed may be determined based on the measured value of the maximum crosstalk XTrmax. Specifically, first, the maximum crosstalk XTrmax due to the returned light is measured while rotating the isolator unit group G around the axis A within a range in which the specific angle θs does not deviate from a specific angle range. For example, in the optical isolator 10 under condition A, the maximum crosstalk XTrmax fluctuates in a 90° cycle as shown in FIG. 6 . Then, the isolator unit group G is fixed at a rotational position (i.e., rotation angle φ) at which the maximum crosstalk XTrmax is equal to or less than a predetermined crosstalk threshold XTth. For example, the theoretical value of the crosstalk XTr corresponding to the median value dminm of the distance dmin can be used as the crosstalk threshold XTrth.

[0058] As described above, with the optical isolator 10 for MCF according to this embodiment, even if the separation amount s is reduced to reduce its dimensions, an increase in the maximum crosstalk XTrmax can be suppressed by adjusting the rotation angle φ to set the distance dmin to as large a value as possible. Therefore, the optical isolator 10 can be miniaturized while suppressing an increase in the reverse crosstalk (crosstalk caused by returned light) XTr. Furthermore, optical characteristics other than the crosstalk XTr can also be achieved that are equivalent to those of an optical isolator for a single-core optical fiber.

[0059] The optical isolator and the manufacturing method thereof according to the embodiment have been described above, but the present invention is not limited to the above embodiment, and various modifications are possible without departing from the object of the present invention.

[0060] For example, the isolator unit group G is not limited to a configuration in which the value of dmin is fixed at a position selected from a range between the intermediate value dminm and the upper limit value dminu. For example, the selected value may be a value selected from a range R in which dmin ranges from the upper limit value dminu to the value obtained by multiplying the fluctuation range of dmin (= dminu - dminl) by a predetermined percentage satisfying 30% or more and adding the lower limit value dminl to the value obtained. In this case, range R corresponds to a "specific distance range." This configuration also allows for appropriate suppression of an increase in crosstalk XTr compared to the reference state (φ = 0°). Furthermore, this percentage may be set to, for example, 40% or a larger value (e.g., 60% or 80%). The higher the percentage, the more appropriately the increase in crosstalk XTr can be suppressed.

[0061] Furthermore, the core arrangement of the MCFs 20 and 70 is not limited to being rotationally symmetric, but may be non-rotationally symmetric. In this case, the distance dmin varies in a 360° cycle.

[0062] In addition, a bundle optical fiber in which a plurality of single-core optical fibers (SCFs) each having one core are bundled may be used as one of the optical fibers on the input side or the output side. When a bundle optical fiber is used, the length of a side of a polygon formed by connecting the centers of the cores of each SCF corresponds to the "core pitch."

[0063] Furthermore, the number of isolator units included in the isolator unit group G is not limited to two. The isolator unit group G may be configured to include one isolator unit, or may be configured to include three or more isolator units arranged in series.

[0064] 10: Optical isolator, 20: Multi-core optical fiber, 30: Lens, 40: Isolator unit, 50: Isolator unit, 60: Lens, 70: Multi-core optical fiber

Claims

1. An optical isolator for a multi-core optical fiber, comprising a first optical fiber having a plurality of first cores, a first lens, an isolator unit group, a second lens, and a second optical fiber having a plurality of second cores, which are arranged in this order along a predetermined axis in the forward direction, and at least one of the first optical fiber and the second optical fiber is a multi-core optical fiber, wherein the first lens collimates each of the light beams in any polarization state emitted from the first core of the first optical fiber, the isolator unit group is composed of one or more isolator units arranged in series, each of which includes a pair of polarization separation elements and a Faraday rotator provided between the pair of polarization separation elements, and transmits each of the light beams traveling in the forward direction, while transmitting each of the return light beams traveling in the reverse direction, separated into a first light beam and a second light beam whose polarization planes are orthogonal to each other, and the second lens converges each of the light beams transmitted through the isolator unit group so that each of the light beams is incident on the corresponding second core of the second optical fiber, An optical isolator, wherein, when viewed from the front on the end face of the first optical fiber, the shortest distance between each of the multiple return beams and each of the multiple first cores is defined as a minimum distance, the value of the minimum distance varies between a predetermined upper limit value and a predetermined lower limit value as the isolator unit group rotates about the axis, and the isolator unit group is fixed at a position such that the value of the minimum distance is within a range from a value obtained by adding the lower limit value to a value obtained by multiplying the fluctuation range of the minimum distance by 30% to the upper limit value.

2. An optical isolator as claimed in claim 1, wherein the group of isolator units are fixed at positions such that the value of the minimum distance falls within a range from the upper limit value to a value obtained by multiplying the fluctuation range of the minimum distance by a predetermined rate that satisfies 30% or more and adding the lower limit value to the upper limit value.

3. An optical isolator according to claim 2, wherein said percentage is 60%.

4. An optical isolator according to any one of claims 1 to 3, wherein the lower limit of the minimum distance is equal to or less than the core pitch of the first optical fiber.

5. An optical isolator according to claim 4, wherein the core pitch of said first optical fiber and the core pitch of said second optical fiber are equal to each other.

6. An optical isolator according to any one of claims 1 to 3, wherein the central axes of the first optical fiber and the second optical fiber are positioned on the axis.

7. An optical isolator according to any one of claims 1 to 3, wherein the isolator unit group includes two isolator units, and the polarization separation element is a single crystal having a wedge-shaped birefringence.

8. A method for manufacturing an optical isolator for a multi-core optical fiber, comprising the steps of: preparing an input collimator including a first optical fiber having a plurality of first cores, the end faces of which are bevel-polished, and the first lens configured to collimate each of the light rays in any polarization state emitted from the first core; preparing an output collimator including a second optical fiber having a plurality of second cores, and the second lens configured to converge each of the light rays so that each of the light rays is incident on a corresponding second core of the second optical fiber; and preparing an isolator unit group including one or more isolator units arranged in series, the isolator unit group including a pair of birefringent plates having a rectangular prism shape and a Faraday rotator provided between the pair of birefringent plates, and configured to transmit each of the light rays traveling in a forward direction, which is a direction from the input collimator to the output collimator, while transmitting each of the return light rays traveling in a reverse direction, with the first light ray and the second light ray having mutually orthogonal polarization planes. the step of arranging the input side collimator, the isolator unit group, and the output side collimator so that the first optical fiber, the first lens, the isolator unit group, the second lens, and the second optical fiber are arranged in this order along a predetermined axis in the forward direction; and the step of fixing the isolator unit group relatively to the input side collimator and the output side collimator so that a specific angle formed by an oblique polishing direction of the first optical fiber and an inclination direction of the birefringent plate located at the frontmost stage in the forward direction is included within a specific angle range, wherein at least one of the first optical fiber and the second optical fiber is a multi-core optical fiber, and when the shortest distance among the distances between each of the plurality of return beams and each of the plurality of first cores in a front view of an end face of the first optical fiber is defined as a minimum distance, the value of the minimum distance varies between a predetermined upper limit value and a predetermined lower limit value with the rotation of the isolator unit group about the axis,the specific angle range is an angle range corresponding to a specific distance range ranging from a value obtained by multiplying a fluctuation width of the minimum distance by 30% and adding the lower limit value to the upper limit value.

9. A method for manufacturing an optical isolator as described in claim 8, wherein the fixing step further includes the steps of: measuring a maximum reverse crosstalk, which is the maximum value of crosstalk caused by each of the return lights, while rotating the isolator unit group around the axis so that the specific angle falls within the specific angle range; and fixing the isolator unit group at a rotational position where the maximum reverse crosstalk is equal to or less than a predetermined crosstalk threshold.

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