Method for manufacturing optical fiber connector and method for measuring mpi

Abstract

The present invention achieves a method for measuring MPI with high measurement reproducibility. A first end surface (101a), which is one end surface of the first polarization-maintaining fiber (101), and a second end surface (102a), which is one end surface of the second polarization-maintaining fiber (102), are connected such that the slow axis (101s) at the first end surface (101a) and the slow axis (102s) at the second end surface (102a) are parallel or substantially parallel, and a direction (D1') from the center of the core on the first end surface (101a) toward the center of the core on the second end surface (102a) matches or substantially matches a specific direction (D1).

Description

Method for manufacturing an optical fiber connector and method for measuring MPI

[0001] The present invention relates to a method for manufacturing an optical fiber connection assembly, and also to a method for measuring MPI using an optical fiber connection assembly.

[0002] Optical fibers having a polarization-maintaining function, that is, polarization-maintaining fibers, are widely used. For example, Patent Document 1 discloses polarization-maintaining fibers.

[0003] Japanese Patent Application Publication No. 2018-159926

[0004] Optical modules such as optical transceivers often use polarization-maintaining fibers with a low-refractive-index layer (trench layer or depressed layer) surrounding the core, because the low-refractive-index layer surrounding the core has the effect of minimizing bending loss when the polarization-maintaining fiber is bent at a small bending radius.

[0005] However, in polarization-maintaining fibers, which tend to have shorter fiber lengths than optical fibers used for communication applications, a phenomenon called multi-path interference (MPI) occurs, which raises concerns that the power of light propagating through the polarization-maintaining fiber may fluctuate, resulting in a deterioration in transmission characteristics.

[0006] Here, the measurement method described in ITU-T G. 650.1 is known as a method for measuring MPI. However, when measuring polarization-maintaining fiber, the measurement method described in ITU-T G. 650.1 cannot achieve MPI measurement with high measurement reproducibility. The reason for this is as follows.

[0007] That is, there are four LP11 modes in a polarization-maintaining fiber (LP11ax mode, LP11bx mode, LP11ay mode, and LP11by mode). The MPI value changes depending on which LP11 mode is excited in the polarization-maintaining fiber. However, in the measurement method described in ITU-T G.650.1, the light input to the optical fiber to be measured is light with an unspecified polarization direction (e.g., random polarization). Therefore, when the measurement method described in ITU-T G.650.1 is applied to a polarization-maintaining fiber, the ratio of the four LP11 modes excited in the polarization-maintaining fiber changes each time a measurement is performed. In other words, when the measurement method described in ITU-T G.650.1 is applied to a polarization-maintaining fiber, variation occurs in the measurement results. For this reason, it is not possible to achieve MPI measurements with high measurement reproducibility.

[0008] One aspect of the present invention has been made in view of the above problems, and its object is to realize an MPI measurement method with high measurement reproducibility, and to provide a method for manufacturing an optical fiber connection assembly that contributes to realizing an MPI measurement method with high measurement reproducibility.

[0009] A manufacturing method according to one aspect of the present invention is a method for manufacturing an optical fiber connection body, which includes a connecting step of connecting a first end face, which is the end face of a first polarization-maintaining fiber, and a second end face, which is the end face of a second polarization-maintaining fiber, and in the connecting step, the first end face and the second end face are connected so that the slow axis at the first end face and the slow axis at the second end face are parallel or approximately parallel, and so that the direction from the center of the core at the first end face toward the center of the core at the second end face coincides or approximately coincides with a specific direction.

[0010] A measurement method according to one aspect of the present invention includes an optical fiber connection assembly including a first polarization-maintaining fiber, a second polarization-maintaining fiber having one end face connected to one end face of the first polarization-maintaining fiber, and an optical fiber having one end face connected to the other end face of the second polarization-maintaining fiber, wherein a first end face that is one end face of the first polarization-maintaining fiber and a second end face that is one end face of the second polarization-maintaining fiber are connected so that a slow axis at the first end face and a slow axis at the second end face are parallel or approximately parallel and so that a direction from a center of a core at the first end face to a center of a core at the second end face coincides or approximately coincides with a specific direction, and the measurement method includes a measurement step of measuring an MPI value of the second polarization-maintaining fiber by measuring the power of light output from the other end face of the optical fiber when linearly polarized light whose polarization direction coincides or approximately coincides with the specific direction is input from the one end face of the first polarization-maintaining fiber.

[0011] According to the measurement method of one aspect of the present invention, it is possible to realize a method for measuring MPI with high measurement reproducibility. According to the manufacturing method of one aspect of the present invention, it is possible to provide a method for manufacturing an optical fiber connection assembly that contributes to realizing a method for measuring MPI with high measurement reproducibility.

[0012] 5A and 5B are cross-sectional views showing the transverse cross section of a polarization-maintaining fiber. (a) relates to a PANDA-type polarization-maintaining fiber, (b) relates to a bowtie-type polarization-maintaining fiber, and (c) relates to an elliptical jacket-type polarization-maintaining fiber.

[0023] Fig. 5A is a side view showing the configuration of an optical fiber connection body according to an embodiment of the present invention. (a), (c), and (d) are plan views showing one end face of a first polarization-maintaining fiber included in the optical fiber connection body shown in Fig. 2. (b) is a cross-sectional view showing one end face of a second polarization-maintaining fiber included in the optical fiber connection body shown in Fig. 2. (a), (c), and (d) are plan views showing the other end face of the second polarization-maintaining fiber included in the optical fiber connection body shown in Fig. 2. (b) is a cross-sectional view showing one end face of an optical fiber included in the optical fiber connection body shown in Fig. 2. (a) is a block diagram showing the configuration of a measurement system including the optical fiber connection body shown in Fig. 2. (a) is a graph showing the wavelength dependence of the power of light generated by a light source in the measurement system shown in Fig. 5. (b) is a graph showing the wavelength dependence of the power of light measured by a power meter in the measurement system shown in Fig. 5. 6 is a plan view showing one end face (solid line) of a first polarization-maintaining fiber and one end face (dotted line) of a second polarization-maintaining fiber included in the measurement system shown in FIG. 5. FIG. 7 is a graph showing the electric field distribution of the LP01x mode in the first polarization-maintaining fiber included in the measurement system shown in FIG. 5, and the electric field distributions of the LP11ax mode and the LP11bx mode in the second polarization-maintaining fiber included in the measurement system shown in FIG. 5. FIG. 8 is a plan view showing one end face (solid line) of a first polarization-maintaining fiber and one end face (dotted line) of a second polarization-maintaining fiber included in the measurement system shown in FIG. 5. FIG. 9 is a graph showing the electric field distribution of the LP01x mode in the first polarization-maintaining fiber included in the measurement system shown in FIG. 5, and the electric field distributions of the LP11ax mode and the LP11bx mode in the second polarization-maintaining fiber included in the measurement system shown in FIG. 5. FIG. 10 is a plan view showing one end face (solid line) of a first polarization-maintaining fiber and one end face (dotted line) of a second polarization-maintaining fiber included in the measurement system shown in FIG. 5. in , L out 10 log (L in L out ) + (L in +L out) and the measured MPI. LP01 , I LP11 and the intensity I calculated from the coupling efficiency η MPI± 1 is a graph showing the wavelength dependence of the MPI value (MPI waveform) measured for 2 samples of optical fiber connections prepared as examples. 2 is a graph showing the wavelength dependence of the MPI value (MPI waveform) measured for 6 samples of optical fiber connections prepared as examples. in , L out 10 log (L in L out ) + (L in +L out ) and the measured MPI. LP01 , I LP11 and the intensity I calculated from the coupling efficiency η MPI± 10A is a graph illustrating the wavelength dependence of the angle between two axes, and FIG. 10B is a graph illustrating the wavelength dependence of the angle between two directions.

[0013] In polarization-maintaining fibers, which tend to have shorter fiber lengths than optical fibers used for communications, a phenomenon known as multi-path interference (MPI) can occur, which can cause fluctuations in the power of light propagating through the polarization-maintaining fiber and degrade transmission characteristics. This MPI is particularly likely to occur when the fiber has a low refractive index layer. Consider an optical fiber connection in which a previous optical fiber is connected to one end of the polarization-maintaining fiber while being misaligned, and a subsequent optical fiber is connected to the other end of the polarization-maintaining fiber while being misaligned. At the connection point between the one end of the polarization-maintaining fiber and the end of the previous optical fiber, a portion of the LP01 mode light guided through the previous optical fiber couples to the LP11 mode of the polarization-maintaining fiber, and at the connection point between the other end of the polarization-maintaining fiber and the end of the subsequent optical fiber, a portion of the LP11 mode light guided through the polarization-maintaining fiber couples to the LP01 mode of the subsequent optical fiber. In this case, interference occurs between (1) light that is guided in the upstream optical fiber as the LP01 mode, guided in the polarization-maintaining fiber as the LP11 mode, and guided in the downstream optical fiber as the LP01 mode, and (2) light that is guided in the upstream optical fiber as the LP01 mode, guided in the polarization-maintaining fiber as the LP01 mode, and guided in the downstream optical fiber as the LP01 mode. The phenomenon that causes this interference, or the physical quantity derived from the fluctuation range of the light intensity with respect to the wavelength of the light guided in the polarization-maintaining fiber, is called MPI. If the fiber length of the polarization-maintaining fiber is short, the total amount of confinement loss of the LP11 mode is small, and the light in the LP11 mode is guided from one end to the other without being lost. At the connection point between the other end of the polarization-maintaining fiber and the end of the downstream optical fiber, part of the light in the LP11 mode guided in the polarization-maintaining fiber is likely to be coupled to the LP01 mode of the downstream optical fiber. Therefore, when the polarization-maintaining fiber is short, the LP11 mode tends to remain in the subsequent optical fiber, increasing the effect of MPI.

[0014] A known method for measuring MPI is described in ITU-T G. 650.1. However, when measuring polarization-maintaining fiber, the method described in ITU-T G. 650.1 cannot achieve MPI measurement with high measurement reproducibility. The reasons for this are as follows.

[0015] That is, there are four LP11 modes in a polarization-maintaining fiber (LP11ax mode, LP11bx mode, LP11ay mode, and LP11by mode). The MPI value changes depending on which LP11 mode is excited in the polarization-maintaining fiber. However, in the measurement method described in ITU-T G.650.1, the light input to the optical fiber to be measured is light with an unspecified polarization direction (e.g., random polarization). Therefore, when the measurement method described in ITU-T G.650.1 is applied to a polarization-maintaining fiber, the ratio of the four LP11 modes excited in the polarization-maintaining fiber changes each time a measurement is performed. In other words, when the measurement method described in ITU-T G.650.1 is applied to a polarization-maintaining fiber, variation occurs in the measurement results. For this reason, it is not possible to achieve MPI measurements with high measurement reproducibility. Hereinafter, a method for measuring MPI with high measurement reproducibility and a method for manufacturing an optical fiber connection assembly that contributes to realizing the method for measuring MPI with high measurement reproducibility will be described.

[0016] (Configuration of Polarization-Maintaining Fiber) The configuration of the polarization-maintaining fiber used in each embodiment described below will be described with reference to Fig. 1. Fig. 1 is a cross-sectional view showing the transverse section of the polarization-maintaining fiber. Here, the transverse section refers to a cross section perpendicular to the central axis of the polarization-maintaining fiber.

[0017] The polarization-maintaining fiber 1A shown in FIG. 1A is a PANDA (Polarization-maintaining AND Absorption-reducing) type polarization-maintaining fiber.

[0018] 1A, the PANDA polarization-maintaining fiber 1A includes a core 11, a pair of stress-applying portions 13a1 and 13a2, and a cladding 14. The polarization-maintaining fiber 1A may further include a low-refractive index layer 12 and a coating (not shown) that covers the outer surface of the cladding 14.

[0019] In the cross section of the polarization-maintaining fiber 1A, the axis passing through the center of the core 11 and extending in the arrangement direction of the stress-applying portions 13a1 and 13a2 is the slow axis SA, and the axis passing through the center of the core 11 and perpendicular to the arrangement direction of the stress-applying portions 13a1 and 13a2 is the fast axis FA.

[0020] The core 11 is a columnar region extending in the longitudinal direction of the polarization-maintaining fiber 1A. The refractive index n1 of the core 11 is set to be higher than the refractive index n2 of the cladding 14, which will be described later. That is, the relative refractive index difference of the core 11 with respect to the cladding 14, Δ1 [%] = 100 × (n1 ２ -n2 ２ ) / (2 × n1 ２ ) takes a positive value. The core 11 is made of, for example, silica glass doped with an up-dopant. Examples of up-dopants include germanium, aluminum, and phosphorus. In the PANDA polarization-maintaining fiber 1, the cross-sectional shape of the core 11 is circular with a radius r1. However, the cross-sectional shape of the core 11 is not limited to a circle in the strict sense, as long as it can be approximated by a circle. When the cross-sectional shape of the core 11 is non-circular, the radius r1 of the core 11 is half the average value of the radii (equivalent to diameters) of the core 11 in a direction perpendicular to the central axis of the core 11.

[0021] The low-refractive-index layer 12 is a cylindrical region extending in the longitudinal direction of the polarization-maintaining fiber 1A, and is disposed so as to surround the core 11. The refractive index n3 of the low-refractive-index layer 12 is set lower than the refractive index n2 of the cladding 14, which will be described later. That is, the relative refractive index difference of the low-refractive-index layer 12 with respect to the cladding 14, Δ3 [%] = 100 × (n3 ２ -n2 ２ ) / (2 × n3 ２) takes a negative value. The low-refractive-index layer 12 is made of, for example, silica glass doped with a down dopant. Examples of down dopants include fluorine and boron. In the PANDA-type polarization-maintaining fiber 1, the cross-sectional shape of the low-refractive-index layer 12 is annular (ring-shaped) with an inner radius r2 and an outer radius r3. However, the cross-sectional shape of the low-refractive-index layer 12 is not limited to a strict circular shape, as long as it can be approximated by a circular shape. When the cross-sectional shape of the low-refractive-index layer 12 is non-annular, the inner radius r2 of the low-refractive-index layer 12 is half the average value of the inner radii (corresponding to inner diameters) in a direction perpendicular to the central axis of the core 11, and the outer radius r3 of the low-refractive-index layer 12 is half the average value of the outer radii (outer diameters) in a direction perpendicular to the central axis of the core 11. The low-refractive-index layer 12 of this embodiment is in contact with each of the stress-applying portions 13a1 and 13a2. Therefore, the outer periphery of the low refractive index layer 12 is composed of a part of a circle having an outer periphery with a diameter larger than the shortest distance from the central axis of the core 11 to the stress-applying parts, and parts that contact the stress-applying parts 13a1 and 13a2. The low refractive index layer 12 may be separated from the stress-applying parts 13a1 and 13a2.

[0022] The stress-applying portions 13a1 and 13a2 are each a columnar region extending in the longitudinal direction of the polarization-maintaining fiber 1A. The stress-applying portions 13a1 and 13a2 are arranged to sandwich the core 11. The refractive index n4 of the stress-applying portions 13a1 and 13a2 is set to be lower than the refractive index n2 of the cladding 14, which will be described later. That is, the relative refractive index difference Δ4 [%] of the stress-applying portions 13a1 and 13a2 with respect to the cladding 14 (the outer region of the cladding 14) is Δ4 [%] = 100 × (n4 ２ -n2 ２ ) / (2 × n4 ２) takes a negative value. The stress-applying portions 13a1 and 13a2 are made of, for example, silica glass doped with a down dopant. Examples of down dopants include boron oxide (BO) and fluorine. In the PANDA polarization-maintaining fiber 1A, the cross-sectional shapes of the stress-applying portions 13a1 and 13a2 are circular. However, the cross-sectional shapes of the stress-applying portions 13a1 and 13a2 are not limited to circular. For example, the cross-sectional shape of the stress-applying portion 13a1 may be an ellipse whose minor axis direction is the arrangement direction of the stress-applying portions 13a1 and 13a2, or an ellipse whose major axis direction is the arrangement direction of the stress-applying portions 13a1 and 13a2. The cross-sectional shapes of the stress-applying portions 13a1 and 13a2 may also be crescent or non-circular.

[0023] The cladding 14 is a columnar region extending in the longitudinal direction of the polarization-maintaining fiber 1A and contains the core 11, the low-refractive-index layer 12, and the stress-applying portions 13a1 and 13a2. The central axis of the cladding 14 coincides or substantially coincides with the central axis of the core 11 and the central axis of the low-refractive-index layer 12. The refractive index of the cladding 14 is lower than that of the core 11 and higher than that of the low-refractive-index layer 12 and the stress-applying portions 13a1 and 13a2. The cladding 14 is made of, for example, silica glass. In the PANDA-type polarization-maintaining fiber 1, the cross-sectional shape of the cladding 14 is circular. However, the cross-sectional shape of the cladding 14 is not limited to a circle. For example, the cross-sectional shape of the cladding 14 may be an ellipse whose minor axis corresponds to the arrangement direction of the stress-applying portions 13a1 and 13a2, or an ellipse whose major axis corresponds to the arrangement direction of the stress-applying portions 13a1 and 13a2.

[0024] The inner radius r2 of the low refractive index layer 12 may be (1) larger than the radius r1 of the core 11, or (2) equal to the radius r1 of the core 11.

[0025] When the inner radius r2 of the low-refractive-index layer 12 is larger than the radius r1 of the core 11, i.e., when the low-refractive-index layer 12 is spaced apart from the core 11, the cladding 14 has a region inside the low-refractive-index layer 12 (hereinafter also referred to as the "inner region") and a region outside the low-refractive-index layer 12 (hereinafter also referred to as the "outer region"). In this case, the inner region of the cladding 14 covers the outer surface of the core 11, the low-refractive-index layer 12 covers the outer surface of the inner region of the cladding 14, and the outer region of the cladding 14 covers the outer surface of the low-refractive-index layer 12. Such a low-refractive-index layer 12 is also called a "trench layer."

[0026] On the other hand, when the inner radius r2 of the low refractive index layer 12 is the same as the radius r1 of the core 11, that is, when the low refractive index layer 12 is not spaced apart from the core 11, the low refractive index layer 12 covers the outer surface of the core 11, and the cladding 14 covers the outer surface of the low refractive index layer 12. Such a low refractive index layer 12 is also called a "depressed layer."

[0027] In the PANDA polarization-maintaining fiber 1, the center of the circle forming the outer periphery of the core 11, the center of the circle forming the inner periphery of the low-refractive-index layer 12, and the center of the circle forming the outer periphery of the low-refractive-index layer 12 each coincide with the center of the circle forming the outer periphery of the cladding 14. However, the present invention is not limited to this. That is, the center of the circle forming the outer periphery of the core 11, the center of the circle forming the inner periphery of the low-refractive-index layer 12, and the center of the circle forming the outer periphery of the low-refractive-index layer 12 may each be included in the center of the cladding 14. Therefore, when the diameter of the cladding 14 is 79 μm or more and 126 μm or less, the center of the circle forming the outer periphery of the core 11, the center of the circle forming the inner periphery of the low-refractive-index layer 12, and the center of the circle forming the outer periphery of the low-refractive-index layer 12 may each be included in the center of the cladding 14. Here, the center of the cladding 14 refers to the inner region of a circle with a radius of 0.6 μm, the center of which coincides with the center of the circle forming the outer periphery of the cladding 14.

[0028] The polarization-maintaining fiber 1B shown in FIG. 1(b) is a bowtie type polarization-maintaining fiber.

[0029] 1B, the bowtie type polarization-maintaining fiber 1B includes a core 11, a low refractive index layer 12, a pair of stress-applying portions 13b1 and 13b2, and a cladding 14. The bowtie type polarization-maintaining fiber 1B is obtained by replacing the stress-applying portions 13a1 and 13a2 of the PANDA type polarization-maintaining fiber 13A with the stress-applying portions 13b1 and 13b2.

[0030] The stress-applying portions 13b1 and 13b2 are each a columnar region extending in the longitudinal direction of the polarization-maintaining fiber 1B. The stress-applying portions 13b1 and 13b2 are arranged to sandwich the core 11. The refractive index n4 of the stress-applying portions 13b1 and 13b2 is set to be lower than the refractive index n2 of the cladding 14. That is, the relative refractive index difference Δ4 [%] of the stress-applying portions 13b1 and 13b2 with respect to the cladding 14 is Δ4 [%] = 100 × (n4 ２ -n2 ２ ) / (2 × n4 ２ ) takes a negative value. The stress-applying portions 13b1 and 13b2 are made of, for example, silica glass doped with a down dopant. An example of the down dopant is boron oxide (BO). In the bowtie polarization-maintaining fiber 1B, the cross-sectional shape of the stress-applying portions 13b1 and 13b2 is an isosceles trapezoid with the upper base (shorter base) facing the core 11. However, one or both of the upper and lower bases (longer bases) may be arc-shaped bulging away from the core 11. FIG. 1B illustrates the stress-applying portions 13b1 and 13b2 as isosceles trapezoids with the upper base arc-shaped bulging away from the core 11 and the lower base linear.

[0031] In the cross section of the bowtie polarization-maintaining fiber 1B, the slow axis SA passes through the center of the core 11 and is an axis parallel to the arrangement direction of the stress-applying portions 13b1 and 13b2, and the fast axis FA passes through the center of the core 11 and is an axis perpendicular to the arrangement direction of the stress-applying portions 13b1 and 13b2.

[0032] The polarization-maintaining fiber 1C shown in FIG. 1C is an elliptical jacket type polarization-maintaining fiber.

[0033] 1C, the elliptical jacket type polarization-maintaining fiber 1C includes a core 11, a low refractive index layer 12, a stress-applying portion 13c, and a cladding 14. The bowtie type polarization-maintaining fiber 1B is the PANDA type polarization-maintaining fiber 13A in which the stress-applying portions 13a1 and 13a2 are replaced with the stress-applying portion 13c.

[0034] The stress-applying portions 13c are columnar regions extending in the longitudinal direction of the polarization-maintaining fiber 1B. The stress-applying portions 13c are arranged so as to enclose the core 11. The refractive index n4 of the stress-applying portions 13c is set to be lower than the refractive index n2 of the cladding 14. That is, the relative refractive index difference Δ4 [%] of the stress-applying portions 13b1 and 13b2 with respect to the cladding 14 is Δ4 [%] = 100 × (n4 ２ -n2 ２ ) / (2 × n4 ２ ) takes a negative value. The stress-applying portion 13c is made of, for example, silica glass doped with a down dopant. An example of the down dopant is boron oxide (BO). In the bowtie polarization-maintaining fiber 1C, the cross section of the stress-applying portion 13c is an ellipse whose center coincides or nearly coincides with the center of the core 11.

[0035] In the cross section of the elliptical jacket type polarization-maintaining fiber 1B, the slow axis SA passes through the center of the core 11 and is parallel to the major axis of the stress-applying portion 13c, and the fast axis FA passes through the center of the core 11 and is parallel to the minor axes of the stress-applying portions 13b1 and 13b2.

[0036] In the following description, an optical fiber that has a polarization-maintaining function and has a defined slow axis SA and fast axis FA will be referred to as a “polarization-maintaining fiber.” The PANDA-type polarization-maintaining fiber 1A, the bowtie-type polarization-maintaining fiber 1B, and the elliptical-jacket-type polarization-maintaining fiber 1C described in this section are examples of polarization-maintaining fibers.

[0037] (Configuration of Optical Fiber Connection Body) The configuration of an optical fiber connection body 100 according to one embodiment of the present invention will be described with reference to Fig. 2. Fig. 2 is a side view of the optical fiber connection body 100.

[0038] As shown in Fig. 2, the optical fiber connection assembly 100 includes a first polarization-maintaining fiber 101 and a second polarization-maintaining fiber 102 having one end face 102a connected to one end face 101a of the first polarization-maintaining fiber 101. The first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102 may be connected by fusion splicing or by a connector. As shown in Fig. 2, the polarization-maintaining fiber 100 may further include an optical fiber 103 having one end face 103a connected to the other end face 102b of the second polarization-maintaining fiber 102. The second polarization-maintaining fiber 102 and the optical fiber 103 may be connected by fusion splicing or by a connector.

[0039] The first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102 are optical fibers having a polarization-maintaining function. Examples of optical fibers having a polarization-maintaining function include a PANDA-type polarization-maintaining fiber 1A, a bowtie-type polarization-maintaining fiber 1B, and an elliptical jacket-type polarization-maintaining fiber 1C. The optical fiber 103 may be an optical fiber having a polarization-maintaining function or an optical fiber not having a polarization-maintaining function. Examples of optical fibers having a polarization-maintaining function include a PANDA-type polarization-maintaining fiber 1A, a bowtie-type polarization-maintaining fiber 1B, and an elliptical jacket-type polarization-maintaining fiber 1C. Examples of optical fibers not having a polarization-maintaining function include a single-mode fiber conforming to ITU-T G.652.

[0040] The connection between the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102 will be described with reference to Fig. 3. Fig. 3(a), (c), and (d) are plan views showing one end face 101a of the first polarization-maintaining fiber 101, and Fig. 3(b) is a cross-sectional view showing one end face 102a of the second polarization-maintaining fiber 102. In Fig. 3(c) and (d), one end face 102a of the second polarization-maintaining fiber 102 connected to one end face 101a of the first polarization-maintaining fiber 101 is indicated by a dotted line.

[0041] One end face 102a (hereinafter also referred to as "second end face 102a") of the second polarization-maintaining fiber 102 is connected to one end face 101a (hereinafter also referred to as "first end face 101a") of the polarization-maintaining fiber 1 in a state where the axis is shifted in a specific direction D1. Here, the state where the axis is shifted in the specific direction D1 refers to a state where both of the following conditions 1 and 2 are satisfied.

[0042] Condition 1: The slow axis 101s at the first end face 101a (the slow axis of the first polarization-maintaining fiber 101) and the slow axis 102s at the second end face 102a (the slow axis of the second polarization-maintaining fiber 102) are parallel or approximately parallel.

[0043] Condition 2: The direction D1' from the center of the core (core of the first polarization-maintaining fiber 101) at the first end face 101a toward the center of the core (core of the second polarization-maintaining fiber 102) at the second end face 102a coincides or approximately coincides with the specific direction D1.

[0044] The specific direction D1 may be a predetermined direction or may not be a predetermined direction. In the former case, the method for predetermining the specific direction D1 is not particularly limited. For example, one direction (e.g., the slow axis direction) on the first end face 101a may be determined, and that direction may be the specific direction D1. Alternatively, a first point and a second point on the first end face 101a may be determined, and the direction from the first point to the second point may be determined as the specific direction D1. Alternatively, a first point, a first movement vector, and a second movement vector may be determined on the first end face 101a, and the second point may be a point moved from the first point by the first movement vector, and the third point may be a point moved from the second point by the second movement vector, and the direction from the first point to the third point may be determined as the specific direction D1.

[0045] 3C shows a state in which the slow axis 101s at the first end face 101a and the slow axis 102s at the second end face 102a are parallel, and the direction D1' from the center of the core at the first end face 101a toward the center of the core at the second end face 102a coincides with the specific direction D1. On the other hand, FIG. 3D shows a state in which the slow axis 101s at the first end face 101a and the slow axis 102s at the second end face 102a are substantially parallel, and the direction D1' from the center of the core at the first end face 101a toward the center of the core at the second end face 102a coincides with the specific direction D1.

[0046] Here, the expression "the slow axis 101s at the first end face 101a and the slow axis 102s at the second end face 102a being substantially parallel" means that the angle Δθ1 between these two axes 101s and 102s is 4° or less. Furthermore, the expression "the direction D1' from the center of the core at the first end face 101a toward the center of the core at the second end face 102a" and the specific direction D1 "substantially coincide" means that the angle Δθ2 between these two directions D1 and D1' is 15° or less.

[0047] The specific direction D1 may be (1) the slow axis direction at the first end face 101a (a direction parallel to the slow axis 101s of the first polarization-maintaining fiber 101), (2) the fast axis direction at the first end face 101a (a direction parallel to the fast axis 101f of the first polarization-maintaining fiber 101), or (3) a direction that forms an angle of 45° with the slow axis direction at the first end face 101a.

[0048] The difference between the mode field diameter of the first polarization-maintaining fiber 101 and the mode field diameter of the second polarization-maintaining fiber 102 is preferably 1 μm or less. This makes it possible to suppress the splice loss to 0.05 dB or less when, for example, a first polarization-maintaining fiber 101 having a mode field diameter of 8.5 μm is spliced ​​with a second polarization-maintaining fiber 102 having a mode field diameter of 9.5 μm without axial misalignment. The mode field diameters of the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102 can be measured using light belonging to the wavelength band to be used. For example, if the wavelength band to be used is the C-band (1530 nm or more and 1565 nm or less), light with a wavelength of 1550 nm can be used.

[0049] Furthermore, it is preferable that the ratio of the major axis to the minor axis of the mode field of the first polarization-maintaining fiber 101 and the ratio of the major axis to the minor axis of the mode field of the second polarization-maintaining fiber 102 are each between 1 and 1.2. As will be described later, the optical fiber connector 100 can be suitably used to excite the LP11 mode in the second polarization-maintaining fiber 102 (to couple the LP01 mode of the first polarization-maintaining fiber 101 to the LP11 mode of the second polarization-maintaining fiber 102), but if the above ratio value is greater than 1.2, the excited LP11 modes will become non-uniform with respect to the amount of axial misalignment.

[0050] The connection between the second polarization-maintaining fiber 102 and the optical fiber 103 will be described with reference to Fig. 4. Fig. 4 illustrates a case where the optical fiber 103 does not have a polarization-maintaining function. (a), (c), and (d) of Fig. 4 are plan views showing the other end face 102b of the second polarization-maintaining fiber 102, and (b) of Fig. 4 is a cross-sectional view showing one end face 103a of the optical fiber 103. (c) and (d) of Fig. 4 show the one end face 103a of the optical fiber 103, which is connected to the other end face 102b of the second polarization-maintaining fiber 102, indicated by a dotted line.

[0051] One end face of the optical fiber 103 (hereinafter also referred to as the "fourth end face 103a") is connected to the other end face 102b (hereinafter also referred to as the "third end face 102b") of the second polarization-maintaining fiber 102 in a state where the optical fiber is misaligned in a specific direction D2. Here, if the optical fiber 103 is an optical fiber with a polarization-maintaining function, the state where the optical fiber is misaligned in the specific direction D2 refers to a state where both the following conditions 3 and 4 are satisfied. On the other hand, if the optical fiber 103 is an optical fiber without a polarization-maintaining function, the state where the optical fiber is misaligned in the specific direction D2 refers to a state where only the following condition 4 is satisfied.

[0052] Condition 3: The slow axis 102s at the third end face 102b (the slow axis of the second polarization-maintaining fiber 102) and the slow axis 103s at the fourth end face 103a (the slow axis of the optical fiber 103) are parallel or approximately parallel.

[0053] Condition 4: The direction D2' from the center of the core (core of the second polarization-maintaining fiber 102) at the third end face 102b toward the center of the core (core of the second polarization-maintaining fiber 102) at the fourth end face 103a coincides or approximately coincides with the specific direction D2.

[0054] The specific direction D2 may be a predetermined direction or may be a non-predetermined direction. In the former case, the method for predetermining the specific direction D2 is not particularly limited. For example, one direction (e.g., the slow axis direction) on the third end face 102b may be determined, and the determined direction may be the specific direction D2. Alternatively, a first point and a second point on the third end face 102b may be determined, and the direction from the first point to the second point may be determined as the specific direction D2. Alternatively, a first point, a first movement vector, and a second movement vector on the third end face 102b may be determined, and the second point may be a point moved from the first point by the first movement vector, and the third point may be a point moved from the second point by the second movement vector, and the direction from the first point to the third point may be determined as the specific direction D2.

[0055] 4C shows a state in which direction D2' from the center of the core at the third end face 102b toward the center of the core at the fourth end face 103a coincides with the specific direction D2, while FIG. 4D shows a state in which direction D2' from the center of the core at the third end face 102b toward the center of the core at the fourth end face 103a substantially coincides with the specific direction D2.

[0056] Here, the expression "direction D2' from the center of the core at the third end face 102b toward the center of the core at the fourth end face 103a" being "substantially coincident" with the specific direction D2 means that the angle Δθ4 between these two directions D2, D2' is 15° or less. Note that, if the optical fiber 103 has a polarization-maintaining function, the expression "substantially parallel" between the slow axis 102s at the third end face 102b and the slow axis 103s at the fourth end face 103a means that the angle Δθ3 between these two axes 102s, 103s is 4° or less (not shown).

[0057] The specific direction D2 may be (1) the slow axis direction at the third end face 102b (a direction parallel to the slow axis 102s of the second polarization-maintaining fiber 102), (2) the fast axis direction at the third end face 102b (a direction parallel to the fast axis 102f of the second polarization-maintaining fiber 102), or (3) a direction that forms an angle of 45° with the slow axis direction at the third end face 102b. The specific direction D2 related to the axial misalignment when splicing the optical fiber 103 to the second polarization-maintaining fiber 102 may or may not coincide with the specific direction D1 related to the axial misalignment when splicing the second polarization-maintaining fiber 102 to the first polarization-maintaining fiber 101.

[0058] The difference between the mode field diameter of the second polarization-maintaining fiber 102 and the mode field diameter of the optical fiber 103 is preferably 1 μm or less. This makes it possible to suppress the connection loss to 0.05 dB or less when, for example, a second polarization-maintaining fiber 102 having a mode field diameter of 8.5 μm is spliced ​​with an optical fiber 103 having a mode field diameter of 9.5 μm at an angle of 4° or less. The mode field diameters of the second polarization-maintaining fiber 102 and the optical fiber 103 can be measured using light belonging to the wavelength band to be used. For example, if the wavelength band to be used is the C-band (1530 nm or more and 1565 nm or less), light with a wavelength of 1550 nm can be used.

[0059] (Configuration of Measurement System) The configuration of the measurement system MS including the optical fiber connection assembly 100 will be described with reference to Fig. 5. Fig. 5 is a block diagram showing the configuration of the measurement system MS including the optical fiber connection assembly 100.

[0060] The measurement system MS is composed of an optical fiber connection assembly 100, a light source 104, a polarizer 105, and a power meter 106. The measurement system MS is a system configured based on the narrowband ECL / PM technique of ITUT G650.1, for measuring the MPI value of the second polarization-maintaining fiber 102. In the measurement system MS, the first polarization-maintaining fiber 101 and the optical fiber 103 are single-mode transmission fibers, cut to a fiber length of 5 m or more, and spirally wound around a cylindrical mandrel with a diameter of 150 mm.

[0061] The light source 104 generates light having a wavelength ranging from 1530 nm to 1565 nm. The light source 104 is configured to be able to change the wavelength of the generated light in 0.1 nm steps while maintaining the power of the generated light constant. The light generated by the light source 104 is input to the polarizer 105. The polarizer 105 selectively transmits linearly polarized light contained in the light output from the light source 104. The linearly polarized light that has passed through the polarizer 105 is input to the first polarization-maintaining fiber 101. Note that the linearly polarized light that passes through the polarizer 105 and is input to the first polarization-maintaining fiber 101 is linearly polarized light whose polarization direction (electric field oscillation direction) is parallel to the slow axis of the first polarization-maintaining fiber 101, or linearly polarized light whose polarization direction (electric field oscillation direction) is parallel to the fast axis of the first polarization-maintaining fiber 101.

[0062] The linearly polarized light that has passed through the polarizer 105 is input to the first polarization-maintaining fiber 101. The first polarization-maintaining fiber 101 guides the linearly polarized light output from the polarizer 105 while maintaining its polarization direction. The linearly polarized light that has been guided through the first polarization-maintaining fiber 101 is input to the second polarization-maintaining fiber 102. In the measurement system MS, the polarization extinction ratio of the linearly polarized light that is input to the second polarization-maintaining fiber 102 is −25 dB or less. The second polarization-maintaining fiber 102 guides the linearly polarized light output from the first polarization-maintaining fiber 101 while maintaining its polarization direction. The linearly polarized light that has been guided through the second polarization-maintaining fiber 102 is input to the optical fiber 103. If the optical fiber 103 does not have a polarization-maintaining function, the optical fiber 103 guides the linearly polarized light output from the second polarization-maintaining fiber 102 without maintaining its polarization direction. When the optical fiber 103 has a polarization-maintaining function, the optical fiber 103 guides the linearly polarized light output from the second polarization-maintaining fiber 102 while maintaining the polarization direction. The light guided through the optical fiber 103 is input to the power meter 106. The power meter 106 measures the power of the light output from the optical fiber 103.

[0063] As described above, in the optical fiber connector 100, the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102 are connected in a state where there is an axial misalignment, and the second polarization-maintaining fiber 102 and the optical fiber 103 are connected in a state where there is an axial misalignment.

[0064] As a result, a portion of the LP01 mode of the first polarization-maintaining fiber 101 couples with the LP11 mode of the second polarization-maintaining fiber 102 due to an axial misalignment at the connection point between the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102. Furthermore, a portion of the LP11 mode of the second polarization-maintaining fiber 102 couples with the LP01 mode of the optical fiber 103 due to an axial misalignment at the connection point between the second polarization-maintaining fiber 102 and the optical fiber 103. Therefore, interference occurs between the light that is guided through the first polarization-maintaining fiber 101 in the LP01 mode, the second polarization-maintaining fiber 102 in the LP01 mode, and the optical fiber 103 in the LP01 mode, and the light that is guided through the first polarization-maintaining fiber 101 in the LP01 mode, the second polarization-maintaining fiber 102 in the LP11 mode, and the optical fiber 103 in the LP01 mode. Due to this interference, when the wavelength of the light generated by the light source 104 is swept, the power of the light measured by the power meter 106 changes periodically.

[0065] FIG. 6A is a graph showing the wavelength dependence of the power of light generated by the light source 104. FIG. 6B is a graph showing the wavelength dependence of the power of light measured by the power meter 106. The power of light generated by the light source 104 is constant as shown in FIG. 6A. On the other hand, the power of light measured by the power meter 106 changes periodically as shown in FIG. 6B. The wavelength sweep range is determined to be larger than the change period of the power of light measured by the power meter 106 (e.g., more than five times the change period). The wavelength sweep step is determined to be smaller than the change period of the power of light measured by the power meter 106 (e.g., less than 1 / 10 of the change period). The wavelength sweep range is set to the C-band (1530 nm or more and 1565 nm or less) because the polarization-maintaining fiber 1 is intended for use in telecom devices such as CPO switch modules.

[0066] The MPI value can be calculated from the optical power measured by the power meter 106 according to the method described as the narrowband ECL / PM technique in ITU-T-G.650.1, using the following formula (1): PTP = 1 / (Pin / Time) / (Max / Min ...

[0067] The LP01 mode guided through the first polarization-maintaining fiber 101 includes two modes with different polarization directions (electric field directions). The first mode is the LP01x mode, in which the polarization direction is parallel to the slow axis (hereinafter also referred to as the "slow axis 101s") of the first polarization-maintaining fiber 101. The second mode is the LP01y mode, in which the polarization direction is parallel to the fast axis (hereinafter also referred to as the "fast axis 101f") of the first polarization-maintaining fiber 101.

[0068] When the polarization direction of the linearly polarized light that is transmitted through the polarizer 105 and input to the first polarization-maintaining fiber 101 is parallel to the slow axis 101s, the LP01 mode guided through the first polarization-maintaining fiber 101 becomes the LP01x mode. On the other hand, when the polarization direction of the linearly polarized light that is transmitted through the polarizer 105 and input to the first polarization-maintaining fiber 101 is parallel to the fast axis 101f, the LP01 mode guided through the first polarization-maintaining fiber 101 becomes the LP01y mode.

[0069] The LP11 mode excited in the second polarization-maintaining fiber 102 includes two modes with different polarization directions (electric field directions). The first mode is an LP11x mode whose polarization direction is parallel to the slow axis (hereinafter also referred to as the "slow axis 102s") of the second polarization-maintaining fiber 102. The second mode is an LP11y mode whose polarization direction is parallel to the fast axis (hereinafter also referred to as the "fast axis 102f") of the second polarization-maintaining fiber 102.

[0070] The LP11x mode excited in the second polarization-maintaining fiber 102 includes two modes with different even / odd characteristics. The first mode is LP11ax, which is an even function with respect to the slow axis 102s and an odd function with respect to the fast axis 102f. The second mode is LP11bx, which is an odd function with respect to the slow axis 102s and an even function with respect to the fast axis 102f.

[0071] The LP11y mode excited in the second polarization-maintaining fiber 102 includes two modes with different even / odd characteristics. The first mode is LP11ay, which is an even function with respect to the slow axis 102s and an odd function with respect to the fast axis 102f. The second mode is LP11by, which is an odd function with respect to the slow axis 102s and an even function with respect to the fast axis 102f.

[0072] When the LP01 mode guided through the first polarization-maintaining fiber 101 is the LP01x mode, the LP11 mode excited in the second polarization-maintaining fiber 102 becomes the LP11ax mode and / or the LP11bx mode. On the other hand, when the LP01 mode guided through the first polarization-maintaining fiber 101 is the LP01y mode, the LP11 mode excited in the second polarization-maintaining fiber 102 becomes the LP11ay mode and / or the LP11by mode.

[0073] If the polarization direction of the LP01 mode guided through the optical fiber connected upstream of the second polarization-maintaining fiber 102 (hereinafter also referred to as the "pre-stage optical fiber") is not specified, the ratio between the LP11x mode and the LP11y mode excited in the second polarization-maintaining fiber 102 becomes indefinite. Furthermore, if the direction of the axial misalignment of the second polarization-maintaining fiber 102 with respect to the upstream optical fiber is not specified, the ratio between the LP11ax mode and the LP11bx mode excited in the second polarization-maintaining fiber 102 and the ratio between the LP11ay mode and the LP11by mode excited in the second polarization-maintaining fiber 102 become indefinite. Since the MPI value is affected by these ratios, if the polarization direction of the LP01 mode guided through the upstream optical fiber is not specified, or if the direction of the axial misalignment of the second polarization-maintaining fiber 102 with respect to the upstream optical fiber is not specified, the reproducibility of the MPI measurement becomes low.

[0074] In contrast, in the measurement system MS, the polarization direction of the LP01 mode guided through the first polarization-maintaining fiber 101, which is the upstream optical fiber, is specified, and therefore the ratio of the LP11x mode to the LP11y mode excited in the second polarization-maintaining fiber 102 is specified. Furthermore, in the measurement system MS, the axial misalignment direction of the second polarization-maintaining fiber 102 relative to the first polarization-maintaining fiber 101, which is the upstream optical fiber, is specified, and therefore the ratio of the LP11ax mode to the LP11bx mode excited in the second polarization-maintaining fiber 102 and the ratio of the LP11ay mode to the LP11by mode excited in the second polarization-maintaining fiber 102 are specified. Therefore, the measurement system MS improves the reproducibility of MPI measurements.

[0075] (Axial Misalignment in the Slow Axis Direction) In the measurement system MS, (1) with regard to the connection between the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102, it is preferable that one end face 102a (hereinafter also referred to as the "second end face 102a") of the second polarization-maintaining fiber 102 is connected to one end face 101a (hereinafter also referred to as the "first end face 101a") of the first polarization-maintaining fiber 101 in a state where the end face is misaligned in the slow axis direction Ds of the first polarization-maintaining fiber 101. Note that there are two directions in the slow axis direction Ds of the first polarization-maintaining fiber 101 (the rightward direction and the leftward direction as viewed in FIG. 7), but either direction is acceptable.

[0076] Here, the state in which the first polarization-maintaining fiber 101 is misaligned in the slow-axis direction Ds refers to a state in which both the following conditions 1 and 2 are satisfied.

[0077] Condition 1: The slow axis 101s at the first end face 101a (the slow axis of the first polarization-maintaining fiber 101) and the slow axis 102s at the second end face 102a (the slow axis of the second polarization-maintaining fiber 102) are parallel or approximately parallel.

[0078] Condition 2: The direction D1' from the center of the core (core of the first polarization-maintaining fiber 101) at the first end face 101a toward the center of the core (core of the second polarization-maintaining fiber 102) at the second end face 102a coincides or approximately coincides with the slow axis direction Ds.

[0079] 7A shows a state in which the slow axis 101s at the first end face 101a and the slow axis 102s at the second end face 102a are parallel, and the direction D1' from the center of the core at the first end face 101a toward the center of the core at the second end face 102a coincides with the slow axis direction Ds. On the other hand, FIG. 7B shows a state in which the slow axis 101s at the first end face 101a and the slow axis 102s at the second end face 102a are substantially parallel, and the direction D1' from the center of the core at the first end face 101a toward the center of the core at the second end face 102a coincides with the slow axis direction Ds.

[0080] Here, the expression "the slow axis 101s at the first end face 101a and the slow axis 102s at the second end face 102a being substantially parallel" means that the angle Δθ1 between these two axes 101s and 102s is 4° or less. Furthermore, the expression "the direction D1' from the center of the core at the first end face 101a toward the center of the core at the second end face 102a" and the slow axis direction Ds are "substantially coincident" means that the angle Δθ2 between these two directions Ds and D1' is 15° or less.

[0081] In this way, by using the optical fiber connector 100 in which the second polarization-maintaining fiber 102 is connected to the first polarization-maintaining fiber 101 with its axis shifted in the slow-axis direction Ds, it is possible to evaluate the MPI value when the LP01x mode of the first polarization-maintaining fiber 101 couples with the LP11ax mode of the second polarization-maintaining fiber 102, or the MPI value when the LP01y mode of the first polarization-maintaining fiber 101 couples with the LP11ay mode of the second polarization-maintaining fiber 102.

[0082] For example, when the LP01 mode guided through the first polarization-maintaining fiber 101 is the LP01x mode, the excitation ratio of the LP11ax mode can be set to 90% or more among the LP11ax mode and the LPbx mode excited in the second polarization-maintaining fiber 102. This is because, as shown in Figure 8, the overlap integral between the electric field distribution of the LP01x mode of the first polarization-maintaining fiber 101 and the electric field distribution of the LP11bx mode of the second polarization-maintaining fiber 102 is small, whereas the overlap integral between the electric field distribution of the LP11ax mode of the second polarization-maintaining fiber 102 is large. Therefore, in this case, it is possible to evaluate the MPI value when the LP01x mode of the first polarization-maintaining fiber 101 couples with the LP11ax mode of the second polarization-maintaining fiber 102. If the LP01 mode guided through the first polarization-maintaining fiber 101 is the LP01y mode, for the same reason, the MPI value can be evaluated when the LP01y mode of the first polarization-maintaining fiber 101 couples with the LP11ay mode of the second polarization-maintaining fiber 102.

[0083] The MPI value varies depending on the axial misalignment direction of the second polarization-maintaining fiber 102 with respect to the first polarization-maintaining fiber 101. The MPI value when the LP01x mode of the first polarization-maintaining fiber 101 couples with the LP11ax mode of the second polarization-maintaining fiber 102 and the MPI value when the LP01y mode of the first polarization-maintaining fiber 101 couples with the LP11ay mode of the second polarization-maintaining fiber 102 are the best (minimum) or worst (maximum) MPI values. Therefore, the best or worst MPI value can be measured by aligning or approximately aligning the axial misalignment direction of the second polarization-maintaining fiber 102 with the slow-axis direction Ds.

[0084] The best MPI value can be used when manufacturing an optical fiber connection in which an optical fiber is connected upstream of the second polarization-maintaining fiber 102. For example, when connecting an optical fiber upstream of the second polarization-maintaining fiber 102, adjusting the axial misalignment direction so that the MPI value approaches the best value makes it possible to manufacture an optical fiber connection in which the MPI value is minimized. Furthermore, the worst MPI value can be used when inspecting an optical fiber connection in which an optical fiber is connected upstream of the second polarization-maintaining fiber 102. For example, if the MPI value of an optical fiber connection in which an optical fiber is connected upstream of the second polarization-maintaining fiber 102 exceeds the worst value, the optical fiber connection can be rejected as a defective product.

[0085] (Axial Misalignment in the Fast Axis Direction) Furthermore, in the measurement system MS, (1) with regard to the connection between the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102, it is preferable that one end face 102a (hereinafter also referred to as the "second end face 102a") of the second polarization-maintaining fiber 102 is connected to one end face 101a (hereinafter also referred to as the "first end face 101a") of the first polarization-maintaining fiber 101 in a state where the end face 102a is misaligned in the fast axis direction Df of the first polarization-maintaining fiber 101. Note that, although there are two directions for the fast axis direction Df of the first polarization-maintaining fiber 101 (upward and downward as viewed in FIG. 9 ), either direction is acceptable.

[0086] Here, the state in which the first polarization-maintaining fiber 101 is misaligned in the fast axis direction Df refers to a state in which both of the following conditions 1 and 2 are satisfied.

[0087] Condition 1: The slow axis 101s at the first end face 101a (the slow axis of the first polarization-maintaining fiber 101) and the slow axis 102s at the second end face 102a (the slow axis of the second polarization-maintaining fiber 102) are parallel or approximately parallel.

[0088] Condition 2: The direction D1' from the center of the core (core of the first polarization-maintaining fiber 101) at the first end face 101a toward the center of the core (core of the second polarization-maintaining fiber 102) at the second end face 102a coincides or approximately coincides with the fast axis direction Df.

[0089] 9A shows a state in which the slow axis 101s at the first end face 101a and the slow axis 102s at the second end face 102a are parallel, and the direction D1' from the center of the core at the first end face 101a toward the center of the core at the second end face 102a coincides with the fast axis direction Df. On the other hand, FIG. 9B shows a state in which the slow axis 101s at the first end face 101a and the slow axis 102s at the second end face 102a are substantially parallel, and the direction D1' from the center of the core at the first end face 101a toward the center of the core at the second end face 102a coincides with the fast axis direction Df.

[0090] Here, the expression "the slow axis 101s at the first end face 101a and the slow axis 102s at the second end face 102a being substantially parallel" means that the angle Δθ1 between these two axes 101s and 102s is 4° or less. Furthermore, the expression "the direction D1' from the center of the core at the first end face 101a toward the center of the core at the second end face 102a" and the fast axis direction Df being "substantially coincident" means that the angle Δθ2 between these two directions Df and D1' is 15° or less.

[0091] In this way, by using the optical fiber connector 100 in which the second polarization-maintaining fiber 102 is connected to the first polarization-maintaining fiber 101 with an axial misalignment in the fast axis direction Df, it is possible to evaluate the MPI value when the LP01x mode of the first polarization-maintaining fiber 101 couples with the LP11bx mode of the second polarization-maintaining fiber 102, or the MPI value when the LP01y mode of the first polarization-maintaining fiber 101 couples with the LP11by mode of the second polarization-maintaining fiber 102.

[0092] For example, when the LP01 guided through the first polarization-maintaining fiber 101 is the LP01x mode, the excitation ratio of the LP11ay mode among the LP11ax mode and the LPbx mode excited in the second polarization-maintaining fiber 102 can be made 90% or more. This is because, in this case, as shown in Fig. 10 , the overlap integral between the electric field distribution of the LP01x mode of the first polarization-maintaining fiber 101 and the electric field distribution of the LP11ax mode of the second polarization-maintaining fiber 102 is small, whereas the overlap integral between the electric field distribution of the LP11bx mode of the second polarization-maintaining fiber 102 is large. Therefore, in this case, it is possible to evaluate the MPI when the LP01x mode of the first polarization-maintaining fiber 101 couples with the LP11bx mode of the second polarization-maintaining fiber 102. If the LP01 mode guided through the first polarization-maintaining fiber 101 is the LP01y mode, for the same reason, the MPI value can be evaluated when the LP01y mode of the first polarization-maintaining fiber 101 couples with the LP11by mode of the second polarization-maintaining fiber 102.

[0093] The MPI value varies depending on the axial misalignment direction of the second polarization-maintaining fiber 102. Within this range of MPI variation, the MPI value when the LP01x mode of the first polarization-maintaining fiber 101 couples with the LP11bx mode of the second polarization-maintaining fiber 102 and the MPI value when the LP01y mode of the first polarization-maintaining fiber 101 couples with the LP11by mode of the second polarization-maintaining fiber 102 are the best value (minimum value) or the worst value (maximum value). Therefore, by aligning the axial misalignment direction of the second polarization-maintaining fiber 102 with or approximately aligning it with the fast axis direction Df, the best or worst MPI value can be measured.

[0094] The best MPI value can be used when manufacturing an optical fiber connection in which an optical fiber is connected upstream of the second polarization-maintaining fiber 102. For example, when connecting an optical fiber upstream of the second polarization-maintaining fiber 102, adjusting the axial misalignment direction so that the MPI value approaches the best value makes it possible to manufacture an optical fiber connection in which the MPI value is minimized. Furthermore, the worst MPI value can be used when inspecting an optical fiber connection in which an optical fiber is connected upstream of the second polarization-maintaining fiber 102. For example, if the MPI value of an optical fiber connection in which an optical fiber is connected upstream of the second polarization-maintaining fiber 102 exceeds the worst value, the optical fiber connection can be rejected as a defective product.

[0095] (Axial Misalignment in a Direction That Forms an Angle of 45° with the Slow-Axis Direction) Furthermore, in the measurement system MS, (1) with regard to the connection between the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102, it is preferable that one end face 102a of the second polarization-maintaining fiber 102 (hereinafter also referred to as the "second end face 102a") is connected to one end face 101a of the first polarization-maintaining fiber 101 (hereinafter also referred to as the "first end face 101a") in a state where the end face 102a is axially misaligned in a direction Dm that forms an angle of 45° with the slow-axis direction Ds of the first polarization-maintaining fiber 101. Note that, as described above, there are two directions for the slow-axis direction Ds of the first polarization-maintaining fiber 101. Therefore, there are four directions (upper right, lower right, upper left, and lower left as viewed from the paper in FIG. 9 ) for the direction Dm at which the angle formed with the slow axis direction Ds of the first polarization-maintaining fiber 101 is 45°, but any direction is acceptable.

[0096] Here, the state of axial deviation in the direction Dm at an angle of 45° with respect to the slow axis direction Ds refers to a state in which both the following conditions 1 and 2 are satisfied.

[0097] Condition 1: The slow axis 101s at the first end face 101a (the slow axis of the first polarization-maintaining fiber 101) and the slow axis 102s at the second end face 102a (the slow axis of the second polarization-maintaining fiber 102) are parallel or approximately parallel.

[0098] Condition 2: The direction D1' from the center of the core (core of the first polarization-maintaining fiber 101) at the first end face 101a toward the center of the core (core of the second polarization-maintaining fiber 102) at the second end face 102a coincides or approximately coincides with the direction Dm such that the angle between the direction Ds and the slow axis direction Ds is 45°.

[0099] 11A shows a state in which the slow axis 101s at the first end face 101a and the slow axis 102s at the second end face 102a are parallel, and the angle between the direction D1' from the center of the core at the first end face 101a toward the center of the core at the second end face 102a and the slow axis direction Ds is 45°, and the direction Dm coincides. On the other hand, FIG. 11B shows a state in which the slow axis 101s at the first end face 101a and the slow axis 102s at the second end face 102a are substantially parallel, and the angle between the direction D1' from the center of the core at the first end face 101a toward the center of the core at the second end face 102a and the slow axis direction Ds is 45°, and the direction Dm coincides.

[0100] Here, the expression "substantially parallel" between the slow axis 101s at the first end face 101a and the slow axis 102s at the second end face 102a means that the angle Δθ1 between these two axes 101s and 102s is 4° or less. Furthermore, the expression "substantially coincident" with the direction Dm, where the angle between the direction D1' from the center of the core at the first end face 101a toward the center of the core at the second end face 102a and the slow axis direction Ds is 45°, means that the angle Δθ2 between these two directions Dm and D1' is 15° or less.

[0101] In this way, by using an optical fiber connection assembly 100 in which the second polarization-maintaining fiber 102 is connected to the first polarization-maintaining fiber 101 with its axis offset in a direction Dm such that the angle it forms with the slow-axis direction Ds is 45°, it is possible to evaluate the MPI value when the LP01x mode of the first polarization-maintaining fiber 101 couples with both the LP11ax mode and the LP11bx mode of the second polarization-maintaining fiber 102, or the MPI value when the LP01y mode of the first polarization-maintaining fiber 101 couples with both the LP11ay mode and the LP11by mode of the second polarization-maintaining fiber 102. Knowing such MPI values ​​is useful in understanding the characteristics of the second polarization-maintaining fiber 102.

[0102] (Additional Information Regarding Optical Fiber) In the measurement system MS, the optical fiber 103 is preferably a polarization-maintaining fiber. When the optical fiber 103 is a polarization-maintaining fiber, by setting the axial misalignment of the optical fiber 103 relative to the second polarization-maintaining fiber 102 to be approximately the same as the axial misalignment of the second polarization-maintaining fiber 102 relative to the first polarization-maintaining fiber 101, the connection loss at the connection point between the second polarization-maintaining fiber 102 and the optical fiber 103 can be made approximately the same as the connection loss at the connection point between the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102. This makes it easier to control the amount of coupling between the LP11 mode of the second polarization-maintaining fiber 102 and the LP01 mode of the optical fiber 103, thereby improving the reproducibility of MPI measurements.

[0103] (Relationship between connection loss and MPI) Connection loss L in , L out Here, the relationship between the connection loss L in " refers to the connection loss (in dB) of the LP01 mode at the connection point between the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102, and "connection loss L out " refers to the splice loss (in dB) of the LP01 mode at the splice point between the second polarization-maintaining fiber 102 and the optical fiber 103.

[0104] Connection loss L in , L outThe relationship between MPI and ρ is given by the following equation (2): In the following equation (2), B is a constant. The constant B is given by the following equation (3): In this specification, log represents common logarithm.

[0105] The derivation of the above formulas (2) and (3) will be explained below.

[0106] The electric field of the LP01 mode in the first polarization-maintaining fiber 101 is defined as E1. The electric field E1 is given by E1=Aexp(-iβz)exp(iωt), where t is time, z is position, A is amplitude, ω is angular frequency, and β is the propagation constant of the LP01 mode. Furthermore, the optical intensity of the LP01 mode in the first polarization-maintaining fiber 101 is defined as I1. The optical intensity I1 is given by I=p|E| ２ = pA ２ is given by

[0107] The coupling rate of the optical intensity from the LP01 mode of the first polarization-maintaining fiber 101 to the LP01 mode of the second polarization-maintaining fiber 102 is defined as η １ and the coupling rate of the optical intensity from the LP01 mode of the first polarization-maintaining fiber 101 to the LP11 mode of the second polarization-maintaining fiber 102 is H １ Furthermore, the coupling rate of the optical intensity from the LP01 mode of the second polarization-maintaining fiber 102 to the LP01 mode of the optical fiber 103 is set to η ２ and the coupling rate of the optical intensity from the LP11 mode of the second polarization-maintaining fiber 102 to the LP01 mode of the optical fiber 103 is H ２ Furthermore, the attenuation of the LP11 mode of the second polarization-maintaining fiber 102 at wavelength λ is denoted by α(λ).

[0108] At the end face 102b of the second polarization-maintaining fiber 102 on the optical fiber 103 side, the light intensity I ２LP01 I ２LP01 =η １ I １ Furthermore, at the end face 102b of the second polarization-maintaining fiber 102 on the optical fiber 103 side, the optical intensity I of the LP11 mode is given by ２LP１1 I ２LP11 = H １α(λ)I １ is given by

[0109] The light intensity I of the LP01 mode of the optical fiber 103 that passes through the LP01 mode of the second polarization-maintaining fiber 102 ３LP01a I ３LP01a =η １ η ２ I １ The optical intensity I of the LP01 mode of the optical fiber 103 passing through the LP11 mode of the second polarization-maintaining fiber 102 is given by ３LP01b I ３LP01b = H １ H ２ α(λ)I １ is given by

[0110] The optical fiber 103 receives the electric field E of the LP01 mode of the optical fiber 103 via the LP01 mode of the second polarization-maintaining fiber 102. １ and the electric field E of the LP01 mode of the optical fiber 103 via the LP11 mode of the second polarization-maintaining fiber 102. ２ The electric field E ３ = E １ +E ２ The electric field E ３ is given by the following equation (4).

[0111] Therefore, the optical intensity I of the LP01 mode of the optical fiber 103 ３LP01 is given by the following equation (5): In the following equation (5), 2(η １ η ２ H １ H ２ α(λ) １／２ The cos ψ term contributes to the interference.

[0112] Light Intensity I ３LP01 is converted to dB units, and the difference between the maximum and minimum values ​​is defined as PTP, then PTP is given by the following equation (6): When the PTP given by the following equation (6) is substituted into the definition of MPI described above, MPI is given by the following equation (7):

[0113] Connection loss L in [dB] is L in =-10 log η １ and the connection loss L out [dB] is L out =-10 log η ２ The connection loss L in , L out When is sufficiently small, the coupling efficiency H １ and connection loss L in and the coupling efficiency H ２ and connection loss L out The relationship between the connection loss and the in , L out When is sufficiently small, the coupling efficiency H １ Is, H １ = c １ L in and the coupling efficiency H ２ Is, H ２ = c ２ L out where c １ , c ２ is a constant. Substituting this into the above equation (7), the following equation (8) is obtained. The following equation (8) is nothing but the above equations (1) and (2).

[0114] Here, α(λ) is a parameter specific to the second polarization-maintaining fiber 102. Furthermore, the constants c1 and c2 are parameters that depend on the mode field diameters of the first polarization-maintaining fiber 101 and the optical fiber 103, respectively, but can be considered to be parameters specific to the second polarization-maintaining fiber 102 if it can be assumed that the mode field diameters of the first polarization-maintaining fiber 101 and the optical fiber 103 are constant.

[0115] (MPI estimation using the relationship between connection loss and MPI) Connection loss L in , L out By using the above equation (2) which shows the relationship between the connection loss L in , L out The MPI can be estimated from

[0116] First, the connection loss L in , L out A plurality of samples of the optical fiber connection body 100 are prepared, in which only the connection loss L in , L out The actual connection loss L in , L out The value of the constant B included in the equation (2) is determined so that the MPI estimated using the equation (2) best approximates the actually measured MPI. The connection loss L in , L out This gives the equation for estimating MPI. in , L out This allows the connection loss L to be estimated without actually measuring the MPI. in , L out It is possible to estimate the MPI from

[0117] FIG. 12 shows the actually measured connection loss L in , L out 10 log (L in L out ) + (L in +L out 12 is a scatter diagram showing the relationship between the splice loss L in , L out 12 corresponds to a graph of the estimation formula for estimating MPI from the in , L out This corresponds to the constant B included in the estimation formula for estimating MPI from

[0118] The connection loss of the LP01 mode when there is no axial misalignment between the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102, and the connection loss of the LP01 mode when there is no axial misalignment between the second polarization-maintaining fiber 102 and the optical fiber 103 are expressed as L ΔＥ This connection loss L ΔＥTaking into account the above, assuming linearity between the coupling efficiency [%] of the LP11 mode and the connection loss [dB] of the LP01 mode, the connection loss L in , L out The relationship between and MPI is given by equation (9) below.

[0119] According to the above formula (9), the connection loss L when there is no axial misalignment ΔＥ In view of this, the MPI value changes depending on the connection loss L ΔＥ However, when there is no variation in the characteristics of the first polarization-maintaining fiber 101 and the optical fiber 103 and there is no axial misalignment, the splice loss L ΔＥ If it can be assumed that ρ is constant, it is possible to estimate the relative MPI values ​​using equation (2) above.

[0120] (Relationship between Coupling Efficiency and MPI Waveform) The relationship between the coupling efficiency H and the MPI waveform will be examined. Here, the "coupling efficiency H" refers to the coupling efficiency of the light intensity from the LP11 mode of the second polarization-maintaining fiber 102 to the LP01 mode of the optical fiber 103.

[0121] The optical intensity of the LP01 mode in the second polarization-maintaining fiber 102 is expressed as I LP01 The coupling efficiency of the optical intensity from the LP01 mode of the second polarization-maintaining fiber 102 to the LP01 mode of the optical fiber 103 is η. The optical intensity of the LP11 mode in the second polarization-maintaining fiber 102 is I LP11 and the coupling efficiency of the optical intensity from the LP11 mode of the second polarization-maintaining fiber 102 to the LP01 mode of the optical fiber 103 is H. Then, in the optical fiber 103, the optical intensity I １ Ga I １ =ηI LP01 The light and light intensity I given by ２ Ga I ２ =HI LP11 The resulting electric field E of the light is MPI and intensity I MPIare given by the following equations (10) and (11), respectively.

[0122] Intensity of the constructively interfered light I MPI＋ and the intensity I of the light weakened by interference MPI－ is given by the following equation (12).

[0123] Here, the light intensity I LP01 , light intensity I LP11 , and the coupling efficiency η can be measured at each wavelength. Therefore, the only unknown parameter on the right side of the above equation (11) is the coupling efficiency H. Therefore, the intensity I calculated at each wavelength using the above equation (11) can be calculated as follows: MPI＋ approximates (preferably best approximates) the maximum value of the measured MPI waveform, and / or the intensity I calculated at each wavelength using equation (11) above. MPI－ By determining the coupling efficiency H that approximates (preferably best approximates) the minimum value of the actually measured MPI waveform, the coupling efficiency H can be estimated from the MPI waveform.

[0124] FIG. 13 shows the measured MPI waveform, the intensity I calculated using the above equation (11) at each wavelength. MPI＋ , and the intensity I calculated at each wavelength using the above equation (11) MPI－ The value of the coupling efficiency H in the above formula (11) is calculated using the above formula (11) at each wavelength by the intensity I MPI＋ is the intensity I that best approximates the maximum value of the measured MPI waveform and is calculated at each wavelength using the above equation (11). MPI－ is determined to best approximate the minimum value of the measured MPI waveform.

[0125] Example 1 As a first example, 12 samples were produced of the optical fiber connector 100. In each of the produced samples, PANDA-type polarization-maintaining fibers were used as the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102, and a single-mode fiber was used as the optical fiber 103.

[0126] For six of the twelve samples, optical fiber connectors 100 were produced with the goal of aligning the axial misalignment direction of the second polarization-maintaining fiber 102 with respect to the first polarization-maintaining fiber 101 with the slow axis direction Ds. For the remaining six of the twelve samples, optical fiber connectors 100 were produced with the goal of aligning the axial misalignment direction of the second polarization-maintaining fiber 102 with respect to the first polarization-maintaining fiber 101 with the fast axis direction Df.

[0127] FIG. 14 is a scatter plot showing the relationship between the amount of axial misalignment in the slow axis direction Ds and the amount of axial misalignment in the fast axis direction Df for 12 samples of optical fiber connectors 100 created as examples.

[0128] As can be seen from the scatter diagram shown in Fig. 14, of the six samples created with the goal of matching the axis misalignment direction with the slow axis direction Ds, it was confirmed that the axis misalignment direction of two samples matched the slow axis direction Ds, and the axis misalignment direction of three samples approximately matched the slow axis direction Ds (the angle between the two directions was 15° or less). Also, as can be seen from the scatter diagram shown in Fig. 14, of the six samples created with the goal of matching the axis misalignment direction with the fast axis direction Df, it was confirmed that the axis misalignment direction of two samples matched the fast axis direction Df, and the axis misalignment direction of four samples approximately matched the fast axis direction Df (the angle between the two directions was 15° or less).

[0129] In the optical fiber connector 100, the connection loss L at the connection point between the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102 is in 0.125 dB≦L in It is preferable to satisfy the following condition: in This is because, assuming that all of these factors contribute to the excitation of the LP11 mode in the second polarization-maintaining fiber 102, this condition must be satisfied in order to obtain an accuracy of −50 dB or better, which is the general measurement limit in measuring MPI near the cutoff wavelength.

[0130] Example 2 As a second example, one sample of the optical fiber connector 100 was produced. In the produced sample, PANDA type polarization-maintaining fibers were used as the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102, and a PANDA type polarization-maintaining fiber was used as the optical fiber 103.

[0131] For this sample, the above-described measurement system MS was used to measure the polarization extinction ratio at the connection point between the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102, and at the connection point between the second polarization-maintaining fiber 102 and the optical fiber 103. As a result, the polarization extinction ratio at the connection point between the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102 was −38.8 dB. Furthermore, the polarization extinction ratio at the connection point between the second polarization-maintaining fiber 102 and the optical fiber 103 was −27.8 dB.

[0132] That is, it was confirmed that in the second polarization-maintaining fiber 102, the LP11ax mode and / or LP11bx mode can be excited without exciting the LP11ay mode and / or the LP11by mode, or the LP11ay mode and / or LP11by mode can be excited without exciting the LP11ax mode and / or the LP11bx mode.

[0133] Example 3 As a second example, two samples were prepared of the optical fiber connector 100. In each of the prepared samples, PANDA-type polarization-maintaining fibers were used as the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102, and a single-mode fiber was used as the optical fiber 103.

[0134] In the first sample, the axial misalignment direction of the second polarization-maintaining fiber 102 with respect to the first polarization-maintaining fiber 101 is set to the slow axis direction Ds, and the splice loss L in The axis misalignment amount was set to 1.6 μm so that the splice loss L outIn the second sample, the axial misalignment direction of the second polarization-maintaining fiber 102 relative to the first polarization-maintaining fiber 101 was set to the fast axis direction Df, and the amount of axial misalignment was set to 1.6 μm so that the splice loss was about 0.5 dB. The axial misalignment direction of the optical fiber 103 relative to the second polarization-maintaining fiber 102 was set to the fast axis direction Df, and the amount of axial misalignment was also set to 1.6 μm so that the splice loss was about 0.5 dB. out was set to 1.6 μm so that the difference was about 0.5 dB.

[0135] The MPI was measured for each of the first and second samples using the above-described measurement system MS. The MPI waveforms obtained by this measurement are shown in Fig. 15. Note that the waveforms shown in Fig. 15 have been corrected using as a baseline the transmission waveform obtained when the first polarization-maintaining fiber 101 and the optical fiber 103 are directly connected without the second polarization-maintaining fiber 102.

[0136] 15 , it can be seen that the MPI waveform is an oscillatory waveform with a constant period, whether the axis misalignment direction is set to the slow-axis direction Ds or the fast-axis direction Df. It can also be seen that the amplitude and period of the MPI waveform differ when the axis misalignment direction is set to the slow-axis direction Ds and when the axis misalignment direction is set to the fast-axis direction Df. This means that when the axis misalignment direction is different, the LP11 mode excited in the second polarization-maintaining fiber 102 differs, and therefore the manner of interference occurring in the optical fiber 103 differs. The reason why the manner of interference occurring in the optical fiber 103 differs is because the attenuation and propagation constant of the LP11 mode in the second polarization-maintaining fiber 102 differ depending on whether the LP11 mode excited in the second polarization-maintaining fiber 102 is the LP11ax mode or the LP11bx mode.

[0137] Furthermore, for the first sample whose axis misalignment direction is the slow axis direction Ds, the MPI value calculated from the MPI waveform according to the above formula (1) was −37.3 dB. Note that for the first sample, the splice loss L at the splice point between the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102 was in, and the splice loss L at the splice point between the second polarization-maintaining fiber 102 and the optical fiber 103. out When measured, L in = 0.33 dB, and L out = 0.57 dB.

[0138] On the other hand, for the second sample, whose axis misalignment direction is the fast axis direction Df, the MPI value calculated from the MPI waveform according to the above formula (1) was −23.8 dB. Note that for the second sample, the splice loss L in , and the splice loss L at the splice point between the second polarization-maintaining fiber 102 and the optical fiber 103. out When measured, L in = 0.62 dB, and L out =0.45 dB.

[0139] Although the amount of axial misalignment was set to be constant between the first sample and the second sample, the splice loss L in , L out The values ​​of are significantly different, and it is thought that the MPI values ​​are also affected by differences in connection loss.

[0140] Example 4 As a fourth example, six samples of the optical fiber connector 100 were produced. In each of the produced samples, PANDA-type polarization-maintaining fibers were used as the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102, and a PANDA-type polarization-maintaining fiber was used as the optical fiber 103. In these six samples, the length of the second polarization-maintaining fiber 102 was the same.

[0141] Of the six samples created, for three samples, the axial misalignment direction of the second polarization-maintaining fiber 102 relative to the first polarization-maintaining fiber 101 was set to coincide with the slow-axis direction Ds, and the axial misalignment direction of the optical fiber 103 relative to the second polarization-maintaining fiber 102 was set to the slow-axis direction Ds. Note that these three samples had different amounts of axial misalignment with respect to the slow-axis direction Ds. Furthermore, for the remaining three samples of the six samples created, the axial misalignment direction of the second polarization-maintaining fiber 102 relative to the first polarization-maintaining fiber 101 was set to coincide with the fast-axis direction Df, and the axial misalignment direction of the optical fiber 103 relative to the second polarization-maintaining fiber 102 was set to the fast-axis direction Df. Note that these three samples had different amounts of axial misalignment with respect to the fast-axis direction Df.

[0142] For each of the six samples, the splice loss L at the splice point between the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102 was measured. in , and the splice loss L at the splice point between the second polarization-maintaining fiber 102 and the optical fiber 103. out Furthermore, the MPI of each of the six samples was measured using the measurement system MS.

[0143] FIG. 16 shows the measured splice loss L in , L out 10 log (L in L out ) + (L in +L out 16 is a scatter diagram showing the relationship between the measured splice loss L and the measured MPI. According to the scatter diagram shown in FIG. 16, the measured splice loss L for each of the three samples whose axis misalignment direction is Ds along the slow axis and the three samples whose axis misalignment direction is Df along the fast axis is in , L out 10 log (L in L out ) + (L in +L out It is confirmed that a proportional relationship exists between the measured MPI and the measured MPI.

[0144] The measured splice loss L for the three samples in which the axis misalignment direction is the slow axis direction Dsin , L out The value of the constant B included in the above equation (2) was determined so that the MPI estimated using equation (2) would best approximate the measured MPI. The determined value of B was -28.7 dB. Using the estimation formula obtained by substituting this value into the above equation (2), the connection loss L in , L out The MPI was estimated when the noise level was 0.5 dB. The estimated MPI value was −33.7 dB.

[0145] Similarly, the measured splice loss L for the three samples in which the direction of the axial misalignment is the fast axis direction Df in , L out The value of the constant B included in equation (2) was determined so that the MPI estimated using equation (2) would best approximate the measured MPI. The determined value of B was -19.9 dB. Using the estimation formula obtained by substituting this value into equation (2) above, the connection loss L in , L out The MPI was estimated when the noise level was 0.5 dB. The estimated MPI value was −24.9 dB.

[0146] For six samples of optical fiber connections 100, the angle between the slow axis 101s of the first polarization-maintaining fiber 101 and the slow axis 102s of the second polarization-maintaining fiber 102 was checked and found to be 3.5° or less. This confirms that if the angle between the slow axis 101s of the first polarization-maintaining fiber 101 and the slow axis 102s of the second polarization-maintaining fiber 102 is 4° or less, it is possible to selectively excite only one of the LP11ax mode and the LP11bx mode in the second polarization-maintaining fiber 102.

[0147] Example 5 As a fifth example, one sample of the optical fiber connector 100 was produced. In the produced sample, PANDA type polarization-maintaining fibers were used as the first polarization-maintaining fiber 101 and the second polarization-maintaining fiber 102, and a PANDA type polarization-maintaining fiber was used as the optical fiber 103.

[0148] For the prepared sample, (1) MPI, (2) the light intensity I of the LP01 mode in the second polarization-maintaining fiber 102, and LP01 , (3) the light intensity I of the LP11 mode in the second polarization-maintaining fiber 102 LP11 (4) The coupling efficiency η of the optical intensity from the LP11 mode of the second polarization-maintaining fiber 102 to the LP01 mode of the optical fiber 103 was measured at each wavelength.

[0149] At each wavelength, the measured light intensity I LP01 , I LP11 and the intensity I calculated from the coupling efficiency η using the above formula (11) MPI＋ / I MPI－ The coupling efficiency H included in the above equation (11) was estimated to best approximate the maximum / minimum values ​​of the measured MPI. The estimated value of coupling efficiency H was 5.9%.

[0150] FIG. 17 shows the measured MPI and the measured light intensity I LP01 , I LP11 and the intensity I calculated from the coupling efficiency η using the above formula (11) MPI＋ , and the measured light intensity I LP01 , I LP11 and the intensity I calculated from the coupling efficiency η using the above formula (11) MPI－ 17 is a graph showing the wavelength dependence of the measured light intensity I LP01 , I LP11 and the intensity I calculated from the coupling efficiency η using the above formula (11) MPI＋ The graph of Fig. 1 well reproduces the maximum value of the measured MPI, and the measured light intensity I LP01 , I LP11 and the intensity I calculated from the coupling efficiency η using the above formula (11) MPI－ It can be seen that the graph of Fig. 1 well reproduces the minimum value of the measured MPI.

[0151] (Supplementary Notes on Terminology) The definition of the angle formed by two axes will be explained with reference to FIG. 18(a).

[0152] When two axes A1 and A2 intersect, four angles are generated around the intersection point, and the magnitude of these four angles is one of two angles θ1 and θ2. In this specification, the magnitude of the angle formed by the two axes A1 and A2 refers to the smaller angle θ1 of these two angles θ1 and θ2.

[0153] The definition of the angle formed by two directions will be explained with reference to FIG.

[0154] When the two directions D1 and D2 are expressed as vectors, two angles are generated around the starting point, and the magnitude of these two angles is one of two angles θ1 and θ2. In this specification, the magnitude of the angle formed by the two directions D1 and D2 refers to the smaller angle θ1 of these two angles θ1 and θ2.

[0155] (Summary) The manufacturing method of aspect 1 is a method for manufacturing an optical fiber connection body, comprising: a connecting step of connecting a first end face, which is an end face of a first polarization-maintaining fiber, and a second end face, which is an end face of a second polarization-maintaining fiber; and in the connecting step, the first end face and the second end face are connected so that the slow axis at the first end face and the slow axis at the second end face are parallel or approximately parallel, and so that the direction from the center of the core at the first end face toward the center of the core at the second end face coincides or approximately coincides with a specific direction.

[0156] A manufacturing method according to Aspect 2 is the method for manufacturing the optical fiber connection body according to Aspect 1, characterized in that the specific direction is a slow axis direction at the first end face or a fast axis direction at the first end face.

[0157] A manufacturing method according to Aspect 3 is the method for manufacturing the optical fiber connection body according to Aspect 1, characterized in that the specific direction is a direction in which the angle formed with the slow axis direction at the first end face is 45°.

[0158] A manufacturing method according to aspect 4 is the method for manufacturing an optical fiber connection assembly according to any one of aspects 1 to 3, characterized in that the difference between the mode field diameter of the first polarization-maintaining fiber and the mode field diameter of the second polarization-maintaining fiber is 1.0 μm or less, and the ratio of the major axis to the minor axis of the mode field of the first polarization-maintaining fiber and the ratio of the major axis to the minor axis of the mode field of the second polarization-maintaining fiber are each 1.2 or less.

[0159] A measurement method according to a fifth aspect of the present invention is characterized in that it uses an optical fiber connection assembly including a first polarization-maintaining fiber, a second polarization-maintaining fiber having one end face connected to one end face of the first polarization-maintaining fiber, and an optical fiber having one end face connected to the other end face of the second polarization-maintaining fiber, wherein a first end face that is one end face of the first polarization-maintaining fiber and a second end face that is one end face of the second polarization-maintaining fiber are connected such that a slow axis at the first end face and a slow axis at the second end face are parallel or approximately parallel to each other and such that a direction from a center of a core at the first end face to a center of a core at the second end face coincides or approximately coincides with a specific direction; and it includes a measurement step of measuring an MPI value of the second polarization-maintaining fiber by measuring the power of light output from the other end face of the optical fiber when linearly polarized light whose polarization direction coincides or approximately coincides with the specific direction is input from the one end face of the first polarization-maintaining fiber.

[0160] A measurement method according to Aspect 6 is the measurement method according to Aspect 5, wherein the specific direction is a slow axis direction at the first end facet or a fast axis direction at the first end facet.

[0161] A measurement method according to Aspect 7 is the measurement method according to Aspect 5, wherein the specific direction is a direction that forms an angle of 45° with the slow axis direction at the first end face.

[0162] A measurement method according to Aspect 8 is the measurement method according to any one of Aspects 5 to 7, characterized in that the difference between the mode field diameter of the first polarization-maintaining fiber and the mode field diameter of the second polarization-maintaining fiber is 1.0 μm or less, and the ratio of the major axis to the minor axis of the mode field of the first polarization-maintaining fiber and the ratio of the major axis to the minor axis of the mode field of the second polarization-maintaining fiber are each 1.2 or less.

[0163] A measurement method according to Aspect 9 is the measurement method according to any one of Aspects 5 to 8, characterized in that: the optical fiber is a polarization-maintaining fiber; and a third end face, which is the one end face of the optical fiber, and a fourth end face, which is the other end face of the second polarization-maintaining fiber, are connected so that the slow axis at the third end face and the slow axis at the fourth end face are parallel or approximately parallel, and so that the direction from the center of the core at the third end face toward the center of the core at the fourth end face coincides or approximately coincides with a specific direction.

[0164] A measurement method according to aspect 10 includes calculating a splice loss L at a splice point between the first polarization-maintaining fiber and the second polarization-maintaining fiber using the following formula (1) including a constant B: in , and the splice loss L at the splice point between the second polarization-maintaining fiber and the optical fiber. out The measurement method according to any one of Aspects 5 to 9, further comprising an estimation step of estimating the MPI value from the measured value.

[0165] A measuring method according to aspect 11 includes using the following formula (2) to calculate an intensity I of the LP01 mode light intensity in the second polarization-maintaining fiber, an efficiency η of the light intensity from the LP11 mode of the second polarization-maintaining fiber to the LP01 mode of the optical fiber, and an intensity I calculated from the light intensity I of the LP11 mode in the second polarization-maintaining fiber: MPI＋ and intensity I MPI－and further comprising a determining step of determining a coupling efficiency H of light intensity from the LP11 mode of the second polarization-maintaining fiber to the LP01 mode of the optical fiber so as to approximate the maximum and minimum values ​​of MPI measured in the measuring step.

[0166] (Additional Notes) The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in the above-described embodiments are also included in the technical scope of the present invention.

[0167] 1A, 1B, 1C Polarization-maintaining fiber 11 Core 12 Low refractive index layer 13a1, 13a2 Stress-applying portion 13b1, 13b2 Stress-applying portion 13c Stress-applying portion 14 Cladding FA Fast axis SA Slow axis 100 Optical fiber connector 101 First polarization-maintaining fiber 102 Second polarization-maintaining fiber 103 Optical fiber MS Measurement system 104 Light source 105 Polarizer 106 Power meter

Claims

1. A method for manufacturing an optical fiber connection assembly, comprising a connecting step of connecting a first end face, which is the end face of a first polarization-maintaining fiber, and a second end face, which is the end face of a second polarization-maintaining fiber, wherein in the connecting step, the first end face and the second end face are connected so that the slow axis of the first end face and the slow axis of the second end face are parallel or approximately parallel, and so that the direction from the center of the core of the first end face toward the center of the core of the second end face coincides or approximately coincides with a specific direction.

2. The method for manufacturing an optical fiber connection assembly according to claim 1, wherein the specific direction is the slow axis direction at the first end face or the fast axis direction at the first end face.

3. The method for manufacturing an optical fiber connection assembly according to claim 1, characterized in that the specific direction is a direction that forms an angle of 45° with the slow axis direction at the first end face.

4. A method for manufacturing an optical fiber connection assembly according to any one of claims 1 to 3, characterized in that the difference between the mode field diameter of the first polarization-maintaining fiber and the mode field diameter of the second polarization-maintaining fiber is 1.0 μm or less, and the ratio of the major axis to the minor axis of the mode field of the first polarization-maintaining fiber and the ratio of the major axis to the minor axis of the mode field of the second polarization-maintaining fiber are each 1.2 or less.

5. A measurement method using an optical fiber connection assembly including a first polarization-maintaining fiber, a second polarization-maintaining fiber having one end face connected to one end face of the first polarization-maintaining fiber, and an optical fiber having one end face connected to the other end face of the second polarization-maintaining fiber, wherein the first end face, which is one end face of the first polarization-maintaining fiber, and the second end face, which is one end face of the second polarization-maintaining fiber, are connected so that the slow axis at the first end face and the slow axis at the second end face are parallel or nearly parallel, and so that the direction from the center of the core at the first end face to the center of the core at the second end face coincides or nearly coincides with a specific direction; comprising a measurement step of measuring the MPI value of the second polarization-maintaining fiber by measuring the power of light output from the other end face of the optical fiber when linearly polarized light whose polarization direction coincides or nearly coincides with the specific direction is input from one end face of the first polarization-maintaining fiber.

6. The measurement method according to claim 5, wherein the specific direction is a slow axis direction at the first end face or a fast axis direction at the first end face.

7. The measurement method according to claim 5, wherein the specific direction is a direction that forms an angle of 45° with the slow axis direction at the first end face.

8. A measurement method according to any one of claims 5 to 7, characterized in that the difference between the mode field diameter of the first polarization-maintaining fiber and the mode field diameter of the second polarization-maintaining fiber is 1.0 μm or less, and the ratio of the major axis to the minor axis of the mode field of the first polarization-maintaining fiber and the ratio of the major axis to the minor axis of the mode field of the second polarization-maintaining fiber are each 1.2 or less.

9. The measurement method according to any one of claims 5 to 8, characterized in that the optical fiber is a polarization-maintaining fiber, and the third end face, which is one end face of the optical fiber, and the fourth end face, which is the other end face of the second polarization-maintaining fiber, are connected so that the slow axis at the third end face and the slow axis at the fourth end face are parallel or approximately parallel, and so that the direction from the center of the core at the third end face toward the center of the core at the fourth end face coincides or approximately coincides with a specific direction.

10. Using the following equation (1) including a constant B, the splice loss L at the splice point between the first polarization-maintaining fiber and the second polarization-maintaining fiber is calculated. in , and the splice loss L at the splice point between the second polarization-maintaining fiber and the optical fiber. out The measurement method according to any one of claims 5 to 9, further comprising an estimation step of estimating the MPI value from 11. Using the following equation (2), calculate the light intensity I of the LP01 mode in the second polarization-maintaining fiber. LP01 , the coupling efficiency η of the optical power from the LP11 mode of the second polarization-maintaining fiber to the LP01 mode of the optical fiber, and the optical power I of the LP11 mode in the second polarization-maintaining fiber. LP11 Intensity I calculated from MPI＋ and intensity I MPI－ the measurement method according to any one of claims 5 to 10, further comprising a determining step of determining a coupling efficiency H of light intensity from the LP11 mode of the second polarization-maintaining fiber to the LP01 mode of the optical fiber so as to approximate the maximum and minimum values ​​of MPI measured in the measuring step.

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