Filter device

Abstract

The filter device (10) includes a first multi-core optical fiber (20) having a plurality of first cores, a first lens (30), a filter (40) for transmitting outgoing light from the first lens (30) by rotating around a rotation axis by a predetermined rotation angle with respect to a plane orthogonal to an optical axis, a second lens (50), and an optical fiber (60) having a core for incoming outgoing light from the second lens (50). When a reference axis passing through the optical axis and parallel to the rotation axis is defined, and an orthogonal axis orthogonal to the optical axis and the reference axis is defined, and a first orthogonal direction and a second orthogonal direction are defined in a direction toward one side with respect to the reference axis and a direction toward the other side with respect to the reference axis, respectively, a direction on a circumference of the first multi-core optical fiber is set to minimize a separation distance, which is a sum of distances from the reference axis to first cores farthest from the reference axis in the first orthogonal direction and the second orthogonal direction when an end surface of the first multi-core optical fiber is observed along a central axis of the first multi-core optical fiber.

Description

Technical Field

[0001] This invention relates to a filter device. Background Technology

[0002] The demand for Internet communication services is increasing year by year, with a growing expectation for faster and higher-capacity optical communication. In the past, to meet this demand, technologies such as Wavelength Division Multiplexing (WDM) and digital coherence have been used to increase transmission capacity.

[0003] In recent years, space division multiplexing (SDM) technology using multi-core optical fibers has attracted attention as a new multiplexing technology. It is believed that SDM technology can further achieve high speed and high capacity. With the progress of SDM technology research and development, the demand for optical isolators and filters using multi-core optical fibers is increasing. Here, as a filter, for example, a device is known in which a multi-core optical fiber, a filter made of a dielectric multilayer film, and an optical fiber (typically a multi-core optical fiber) having the same number of cores as the multi-core optical fiber are arranged in this order along a certain axis. This filter, for example, has the function of allowing light of a specific wavelength band to pass through. Light rays emitted from each core of the multi-core optical fiber are incident on the incident surface of the filter, pass through the filter, and then incident on the corresponding core of the optical fiber (for example, see Patent Document 1).

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Patent No. 5598882 Summary of the Invention

[0007] The problem the invention aims to solve

[0008] In such a filtering device, when the filter is configured such that its incident surface is parallel to a plane orthogonal to the axis, light rays emitted from the multi-core fiber may be reflected at its incident surface. This reflected light is generally referred to as "reflected back light." The reflected back light may be incident on the transmitting side of the communication device via the multi-core fiber or undergo multiple reflections, which may degrade the optical properties of the signal light.

[0009] Therefore, in the past, the reflected light has been reduced by arranging the filter so that the incident surface of the filter is tilted relative to a surface orthogonal to the axis.

[0010] However, while tilting the filter in this way reduces reflected light, it also introduces a deviation in the transmission loss of light exiting from each core of the multi-core fiber and passing through the filter, potentially hindering proper optical signal transmission. See below for reference.Figure 25A and Figure 25B Let me explain in detail. Figure 25A This is a graph showing the transmission loss characteristics of a certain optical filter. The filter comprises two multi-core optical fibers, each with seven cores, and a filter disposed between these two fibers. The two multi-core optical fibers have identical structures. Specifically, one of the seven cores extends along the central axis of the multi-core fiber as the center core. The remaining six cores are located at the vertices of a regular hexagon centered on the center core, extending along the axial direction as peripheral cores. The filter is a short-pass filter (a filter that allows light with wavelengths shorter than a specific wavelength to pass through). The filter is tilted.

[0011] like Figure 25A As shown, the cut-off wavelength of this filter is approximately 1520 nm. Figure 25B It will be Figure 25A The chart is magnified through the part where the loss begins to increase. Figure 25B In the graph, the solid line 101 on the shortest wavelength side (left) represents the transmission spectrum of light emitted from one peripheral core, and the solid line 102 on the longest wavelength side (right) represents the transmission spectrum of light emitted from another peripheral core. Furthermore, in this example, the transmission spectra of multiple peripheral cores are substantially identical and superimposed, therefore the number of transmission spectra (solid lines) in the graph does not correspond to the number of cores.

[0012] Figure 25B The transmission spectra shown are all shifted towards shorter wavelengths compared to the transmission spectra of emitted light from the same core before the filter was tilted (illustration omitted), but the amount of shift varies for each core. In this example, the shift of transmission spectrum 101 is the largest, and the shift of transmission spectrum 102 is the smallest. As the transmission spectra shift in this way, the maximum value of the transmission spectrum deviation (i.e., the deviation between the transmission spectrum on the shortest wavelength side and the transmission spectrum on the longest wavelength side) increases compared to before the filter was tilted. In this example, the maximum value of the deviation of the transmission spectrum (cutoff wavelength) at a transmission loss of 3 dB is approximately 0.6 nm.

[0013] When the transmission spectrum deviates in this way in each core, the transmission loss of the emitted light from each core at any given wavelength also deviates. If the deviation in transmission loss increases, the intensity of the light propagating in each core of the optical fiber (in this case, a multi-core fiber) will deviate, potentially resulting in inadequate transmission of optical signals. Figure 25B As shown, the greater the maximum value of the transmitted spectrum deviation, the greater the deviation of the transmission loss. Therefore, in order to properly transmit light after passing through the optical filter using optical fiber, it is crucial to reduce the maximum value of the transmitted spectrum deviation.

[0014] This invention was made to solve the above-mentioned problems. That is, one of the objectives of this invention is to provide a filter device that can appropriately transmit optical signals while reducing reflected light.

[0015] Solution for solving the problem

[0016] The filter device (10) of the present invention comprises:

[0017] A first multi-core optical fiber (20) is columnar and has a plurality of first cores (C1 to C7) extending along the axial direction and a common cladding (21) surrounding the plurality of first cores (C1 to C7).

[0018] The first lens (30) has an optical axis (z-axis) located on the central axis of the first multi-core optical fiber (20), which collimates the light rays emitted and diverging from each of the first cores (C1 to C7) and focuses the collimated light rays from each of the first cores (C1 to C7).

[0019] A filter (40) having a first surface (40a) into which light rays emitted from the first lens (30) enter and a second surface (40b) facing the first surface (40a) and into which light rays that have passed through it exit, is used to allow light of a specific wavelength to pass through with arbitrary transmission intensity. The filter (40) is rotated by a predetermined rotation angle (η) about a rotation axis (r1) extending in a specific direction perpendicular to the optical axis (z-axis) of the first lens (30) from a position on the first surface (40a) that is parallel to a surface (xy plane) orthogonal to the optical axis (z-axis) of the first lens (30).

[0020] The second lens (50) converges the light rays emitted from the filter (40) from each of the first cores (C1 to C7); and

[0021] The optical fiber (60) is columnar and has a core that allows all light rays from each of the first cores (C1 to C7) emitted from the second lens (50) to be incident on and extend along the axial direction.

[0022] In this filter device (10),

[0023] When a first orthogonal direction (-x-axis direction) is defined as a direction originating from a reference axis (y-axis) that passes through the optical axis (z-axis) and is parallel to the rotation axis (r1) and extending along an orthogonal axis (x-axis) that is orthogonal to both the optical axis (z-axis) and the reference axis (y-axis), and pointing towards one side relative to the reference axis (y-axis), and a second orthogonal direction (+x-axis direction) is defined as a direction originating from the reference axis and extending along the orthogonal axis towards the other side relative to the reference axis (y-axis),

[0024] The circumferential orientation of the first multi-core fiber (20) is set to minimize the separation distance, where the separation distance is the sum of the distance between the first core (first separation core) farthest from the reference axis (y-axis) in the first orthogonal direction (-x-axis direction) to the reference axis (y-axis) and the distance between the first core (second separation core) farthest from the reference axis (y-axis) in the second orthogonal direction (+x-axis direction) to the reference axis (y-axis) when observing the end face (20a) of the first multi-core fiber (20) along the central axis (z-axis) of the first multi-core fiber (20).

[0025] In addition, an optical fiber provided with a core "that allows all light beams from each first core to enter" refers to an optical fiber provided with a core that allows the exit light from each core of the first multi-core fiber to enter. That is, reflected return light is not included in the above-mentioned "all light beams".

[0026] In addition, in this specification, "converging (Japanese: 集光)" refers to that a lens concentrates light beams (strictly speaking, the principal rays of the light beams) from a plurality of light sources (e.g., a plurality of first cores of the first multi-core fiber) to one point, and "focusing (Japanese: 収束)(focusing (Japanese: 集束))" refers to that a lens reduces the diameter of light beams from one light source (e.g., each first core of the first multi-core fiber) and concentrates the light beams to one point.

[0027] Effects of the Invention

[0028] According to the present invention, an optical signal can be appropriately transmitted while reducing reflected return light. Brief Description of Drawings

[0029] Figure 1 It is a top view of the optical filter according to the first embodiment of the present invention.

[0030] Figure 2 It is a diagram showing an end face of a multi-core fiber functioning as an exit member.

[0031] Figure 3 It is a diagram showing an incident angle α of a principal ray incident on an optical filter and a light ray angle θ.

[0032] Figure 4 It is a top view of an optical filter prepared for investigating the relationship between the incident angle α and the wavelength offset Δλ, and is a diagram showing a situation where a light ray R passes through the optical filter.

[0033] Figure 5 It is a diagram showing the transmission loss characteristic based on measurement using Figure 4 's optical filter.

[0034] Figure 6 It is a chart defining the relationship between the incident angle α and the wavelength offset Δλ.

[0035] Figure 7A It is a diagram showing the relationship between the incident angle α, the rotation angle η of the filter, the ray angle θ, and the angle Φ of each core in an orthogonal coordinate system.

[0036] Figure 7B This is a diagram used to illustrate how the angle Φ is calculated.

[0037] Figure 8A This is a diagram showing the number of cores and core configuration of a multi-core optical fiber.

[0038] Figure 8B This is a diagram showing the number of cores and core configuration of another multi-core optical fiber.

[0039] Figure 8C This is a diagram showing the number of cores and core configuration of another multi-core optical fiber.

[0040] Figure 9A It is shown Figure 8A A diagram of the end face when a multi-core optical fiber is configured as orthogonal.

[0041] Figure 9B It is stipulated that from Figure 9A A graph showing the relationship between the incident angle α and the wavelength offset Δλ of the light emitted from each core of a multi-core optical fiber.

[0042] Figure 10A It is shown Figure 8A A diagram of the end face of a multi-core optical fiber configured as parallel.

[0043] Figure 10B It is stipulated that from Figure 10A A graph showing the relationship between the incident angle α and the wavelength offset Δλ of the light emitted from each core of a multi-core optical fiber.

[0044] Figure 11A It is shown Figure 8B A diagram of the end face when a multi-core optical fiber is configured as a diagonal fiber.

[0045] Figure 11B It is stipulated that from Figure 11A A graph showing the relationship between the incident angle α and the wavelength offset Δλ of the light emitted from each core of a multi-core optical fiber.

[0046] Figure 12A It is shown Figure 8B A diagram of the end face of a multi-core optical fiber configured as parallel.

[0047] Figure 12B It is stipulated that from Figure 12A A graph showing the relationship between the incident angle α and the wavelength offset Δλ of the light emitted from each core of a multi-core optical fiber.

[0048] Figure 13A It is shown Figure 8C A diagram of the end face when a multi-core optical fiber is configured as orthogonal.

[0049] Figure 13B It is stipulated that from Figure 13A A graph showing the relationship between the incident angle α and the wavelength offset Δλ of the light emitted from each core of a multi-core optical fiber.

[0050] Figure 14A It is shown Figure 8C A diagram of the end face of a multi-core optical fiber configured as parallel.

[0051] Figure 14B It is stipulated that from Figure 14A A graph showing the relationship between the incident angle α and the wavelength offset Δλ of the light emitted from each core of a multi-core optical fiber.

[0052] Figure 15 This is a top view of the filter device according to the second embodiment of the present invention.

[0053] Figure 16 This is a diagram illustrating the oblique grinding direction of a multi-core optical fiber that functions as an ejection component.

[0054] Figure 17A This is a diagram showing the number of cores and core configuration of a multi-core optical fiber whose end face has been obliquely polished.

[0055] Figure 17B This is a diagram showing the number of cores and core configuration of another multi-core optical fiber whose end face has been obliquely polished.

[0056] Figure 18 This shows the filter with a rotation angle η = 2.9°. Figure 17A A diagram illustrating an example of the relative positions of multi-core optical fibers.

[0057] Figure 19 This is under the condition that the rotation angle η = 2.9° is specified. Figure 17A A graph showing the relationship between the rotation angle ψ and the maximum deviation Dmax of the oblique grinding of multi-core optical fibers.

[0058] Figure 20 This shows the filter with a rotation angle η = -2.9°. Figure 17A A diagram illustrating an example of the relative positions of multi-core optical fibers.

[0059] Figure 21 This is under the condition that the rotation angle η = -2.9° is specified. Figure 17A A graph showing the relationship between the rotation angle ψ and the maximum deviation Dmax of the oblique grinding of multi-core optical fibers.

[0060] Figure 22 This is under the condition that the rotation angle η = 1.8° is specified. Figure 17A A graph showing the relationship between the rotation angle ψ and the maximum deviation Dmax of the oblique grinding of multi-core optical fibers.

[0061] Figure 23 This is under the condition that the rotation angle η = -1.8° is specified. Figure 17A A graph showing the relationship between the rotation angle ψ and the maximum deviation Dmax of the oblique grinding of multi-core optical fibers.

[0062] Figure 24 This is under the condition that the rotation angle η = 2.9° is specified. Figure 17B A graph showing the relationship between the rotation angle ψ and the maximum deviation Dmax of the oblique grinding of multi-core optical fibers.

[0063] Figure 25A This is a graph showing the transmission loss characteristics of the filter device.

[0064] Figure 25B yes Figure 25A A magnified view of a portion of the chart. Detailed Implementation

[0065] (First Implementation)

[0066] Figure 1 This is a top view showing an example of the filter device 10 according to the first embodiment of the present invention. Figure 1 As shown, the filter device 10 includes a multi-core optical fiber 20 as a first multi-core optical fiber, a first lens 30, a filter 40, a second lens 50, and a multi-core optical fiber 60 as an optical fiber. These components are arranged in the order described above along axis A1. An orthogonal coordinate system is established for the filter device 10. The z-axis extends along axis A1 with the positive direction being from the first lens 30 toward the filter 40. The y-axis is orthogonal to the z-axis (i.e., axis A1) and extends with the positive direction being from the paper towards the viewer. The x-axis is orthogonal to both the z-axis and the y-axis. Hereinafter, the multi-core optical fiber will also be referred to as "MCF". Furthermore, in this specification, for ease of observation, the figures are shown with variations in the dimensions and angles of specific components (e.g., MCF 20 and MCF 60).

[0067] MCF 20 is cylindrical, with at least the central axis of its end in the +z direction aligned with axis A1. The end face 20a of MCF 20 is parallel to a plane (xy plane) orthogonal to axis A1. Figure 2 This is a diagram showing the end face 20a of the MCF 20. (See diagram below.) Figure 2As shown, MCF 20 comprises seven cores C1 to C7 as the first core and a common cladding 21 surrounding these cores C1 to C7. Core C4 extends along the central axis of MCF 20 (hereinafter also referred to as "central core C4"). Cores C1 to C3 and C5 to C7 are located at the vertices of a regular hexagon centered on central core C4 and extend along the axial direction (hereinafter also referred to as "peripheral cores C1 to C3 and C5 to C7" respectively). In other words, peripheral cores C1 to C3 and C5 to C7 extend along the axes of MCF 20 other than the central axis. The distance between adjacent cores (core spacing) is 38 μm. Cores C1 to C7 and cladding 21 are formed of glass with quartz as the main component. The refractive index of cores C1 to C7 is greater than the refractive index of cladding 21. MCF 20 is a single-mode optical fiber. MCF 20 is an example of a multi-core optical fiber used in filter device 10 (described later). Furthermore, the materials of the cores C1 to C7 and the cladding 21 are not limited to glass with quartz as the main component, but can also be formed from other materials. Additionally, in this specification, the term "cylinder" also includes cases where the axis is bent.

[0068] like Figure 1 As shown, the +z-axis end of the MCF 20 is held through a cylindrical collar 22. The end face 22a of the collar 22 and the end face 20a of the MCF 20 are on the same plane. This is because the end face 20a of the MCF 20 is ground together with the end face 22a of the collar 22 while it is inserted into the collar 22. Figure 1 In the diagram, the MCF20 inside the collar 22 is shown in dashed lines, but the illustrations of cores C1 to C7 are omitted.

[0069] Light rays propagating in the cores C1 to C7 of the MCF 20 are emitted from the end face 20a toward the first lens 30. That is, the MCF 20 functions as an emission component. Figure 1 The diagram only shows the light rays emitted from cores C1 to C7, specifically from cores C1, C4, and C7 (see reference). Figure 2 The principal rays of the emitted light. Rays B1, B4, and B7 correspond to the principal rays of the emitted light from cores C1, C4, and C7, respectively. The principal rays of the emitted light from each core C1 to C7 are parallel to each other (see reference). Figure 1 The light rays (B1, B4, and B7) are emitted, but each emitted light is a diverging light that spreads as it travels.

[0070] The first lens 30 is an aspherical lens with a focal length of 2.5 mm. The central axis (optical axis) of the first lens 30 is located on axis A1. The first lens 30 collimates (parallelizes) the diverging light rays emitted from each core C1 to C7. In other words, the first lens 30, for example, collimates (parallelizes) the diverging light rays emitted from core C1. Figure 1Only its principal ray B1 is shown in the diagram. That is, the first lens 30 is a so-called collimating lens. The first lens 30 converges the collimated rays from each core C1 to C7 to a focal point (in... Figure 1 In the diagram, only rays B1, B4, and B7 are shown. Hereinafter, rays collimated in this way will also be referred to as "collimated rays (parallel rays)".

[0071] Filter 40 is a short-pass filter. Short-pass filters are known filters made of dielectric multilayer films, so a detailed description is omitted. Filter 40 has an incident surface 40a as a first surface and an exit surface 40b as a second surface, parallel to and facing the incident surface 40a. Light rays emitted from the first lens 30 are incident on the incident surface 40a. Filter 40 is configured such that the focal point of the first lens 30 is located on the incident surface 40a. Therefore, light rays from each core C1 to C7 are focused onto the incident surface 40a by the first lens 30, pass through the filter 40, and exit from the exit surface 40b. Furthermore, the lens disposed between MCF 20 and filter 40 is not limited to the first lens 30; any lens capable of substantially collimating the light emitted from each core C1 to C7 of MCF 20 can be used. For example, a spherical lens or a GRIN lens can also be used.

[0072] The filter 40 has a rotation axis r1 extending along the y-axis at the position where the axis A1 intersects the incident surface 40a (i.e., the focal point of the first lens 30). The filter 40 rotates about the rotation axis r1 by a rotation angle η from a position where the incident surface 40a is parallel to a plane orthogonal to the axis A1. The magnitude of the rotation angle η is 0° < η < 90°. This reduces reflected light. Hereinafter, it is defined that when viewing the filter 40 from the +y-axis direction (i.e., from...),... Figure 1 When viewed in the direction shown, the rotation angle η of the filter 40 when rotating counterclockwise around the rotation axis r1 has a positive value, and the rotation angle η of the filter 40 when rotating clockwise around the rotation axis r1 has a negative value.

[0073] exist Figure 1 In the example shown, when viewed from the +y-axis direction, filter 40 is rotated counterclockwise by a rotation angle η. Therefore, the position of the light rays emitted from the exit surface 40b is located in the +x-axis direction relative to the position of the light rays incident on the incident surface 40a. The light rays emitted from filter 40 from each core C1 to C7 (in...) Figure 1 The principal rays shown in the diagram are all collimated (parallel), but they travel in mutually divergent directions (see reference). Figure 1 (Light rays B1, B4, and B7). Furthermore, the principal ray B4 emitted from filter 40 is parallel to axis A1.

[0074] The second lens 50 is an aspherical lens with a focal length of 2.5 mm. The second lens 50 is offset by a predetermined distance in the +x axis direction so that the principal ray B4 emitted from the filter 40 travels along the central axis (optical axis) of the second lens 50. The second lens 50 is positioned at the intersection point of the principal rays emitted from the filter 40 (at... Figure 1 (Illustration omitted) The position is moved away from the focal length in the +z axis direction. The second lens 50 refracts the light rays from each core C1 to C7 emitted from the filter 40 in a manner that makes the principal rays of the aforementioned light rays parallel to each other (see reference). Figure 1 The light rays B1, B4, and B7). Additionally, the second lens 50 converges the light rays from each core C1 to C7 (in... Figure 1 (Only the principal ray is shown in the diagram).

[0075] MCF 60 has the same structure as MCF 20. That is, MCF 60 is cylindrical, with seven cores (not shown) extending along the axial direction and a common cladding (not shown) surrounding these cores. MCF 60 is a single-mode fiber. The central axis of at least the -z-axis end of MCF 60 is aligned with the central axis of the second lens 50. The -z-axis end of MCF 60 is held in a cylindrical ferrule 62. The end face 60a of MCF 60, while penetrating into the ferrule 62, is ground together with the end face 62a of the ferrule 62. Thus, the end face 60a of MCF 60 and the end face 62a of the ferrule 62 are located in the same plane (xy plane). Figure 1 In the image, the MCF 60 inside the collar 62 is shown by a dashed line.

[0076] The end face 60a of MCF 60 is located at the convergence point of the light rays from each core C1 to C7. Thus, the light rays from each core C1 to C7 emitted from the second lens 50 are incident on the corresponding cores of MCF 60 with low loss. That is, MCF 20 and MCF 60 are optically coupled via filter 40, and through the first lens 30 and the second lens 50. Furthermore, the lens disposed between filter 40 and MCF 60 is not limited to the second lens 50; for example, it could be a spherical lens or a GRIN lens. Additionally, when MCF 20 is replaced with another MCF with a different core number and / or core configuration, it is preferable that MCF 60 is also replaced with another MCF having the same number of cores and the same core configuration as the replaced MCF. Furthermore, MCF 60 functions as a light-receiving component that receives the light emitted from the second lens 50, but the light-receiving component is not limited to a multi-core fiber. For example, the light-receiving component could also be a fiber with the same number of cores as MCF 20 (in... Figure 1 In the example, there are 7 single-core fiber groups of the same number of single-mode single-core fibers, wherein each core of the single-mode single-core fiber is surrounded by a separate cladding.

[0077] The above is a description related to the structure of the filter device 10.

[0078] In such a filter device 10, when the filter 40 is rotated about the rotation axis r1, although the reflected light can be reduced, the deviation of the transmission spectrum (especially the maximum deviation) of the light from each core C1 to C7 of the MCF 20 increases, and the deviation of the transmission loss at any given wavelength increases. As a result, it may be impossible to properly transmit optical signals using the MCF 60. This is believed to be due to the increased deviation of the incident angle α of the light from each core C1 to C7 incident on the filter 40 caused by the rotation of the filter 40.

[0079] Reference Figure 3 Let me explain in detail. Figure 3 This is a diagram showing the incident angles α (α1, α2, α3) of the principal rays B4, B1, and B7 incident on filter 40 (angle θ will be described later). Furthermore, in Figure 3 The illustration of the principal ray emitted from the exit surface 40b of the filter 40 is omitted. Figure 3 As shown, when filter 40 rotates counterclockwise around rotation axis r1 by a rotation angle η (η>0), the normal N of filter 40 rotates counterclockwise from axis A1 by an angle η. Consequently, the incident angle α1 of principal ray B4 (ray from core C4) is equal to angle η (α1=η), the incident angle α2 of principal ray B1 (ray from core C1) is larger than incident angle α1 (α2>α1), and the incident angle α3 of principal ray B7 (ray from core C7) is smaller than incident angle α1 (α3<α1). Conversely, when filter 40 does not rotate (i.e., the incident surface 40a is parallel to the xy plane), due to the symmetry of cores C1 and C7 relative to core C4, incident angles α2 and α3 are equal (α2=α3). That is, when filter 40 rotates, the deviation between incident angles α2 and α3 increases. It is assumed that the incident angle α of light rays from cores C2, C3, C5, and C6 will also produce the same deviation.

[0080] Therefore, the inventors of this application investigated the relationship between the incident angle α of light incident on the filter 40 and the wavelength shift Δλ (described later) of the transmitted spectrum of that light, and studied the circumferential orientation of the MCF20 (and the MCF 120 and 220 described later) that can reduce the maximum value of the transmission spectrum deviation. This will be explained in detail below.

[0081] Figure 4This is a top view of a filter 40 prepared for investigating the relationship between the incident angle α and the wavelength shift Δλ. A ray R is incident along axis A2 onto the incident surface 40a of the filter 40. The ray R passes through the filter 40 and exits from the exit surface 40b. The filter 40 is capable of rotating about a rotation axis r1 by a rotation angle η relative to a surface orthogonal to axis A2. Therefore, the incident angle α of the ray R is equal to the rotation angle η. The inventors of this application measured the transmission loss of the ray R emitted from the filter 40 when the rotation angle η changes by 0.5° each time within the range of -2.5° ≤ η ≤ 2.5° (in other words, when the incident angle α of the ray R changes by 0.5° each time within the range of -2.5° ≤ α ≤ 2.5°).

[0082] Figure 5 This is a graph showing the transmission loss characteristics based on the above measurements. In the graph, solid line L1 represents the transmission spectrum of light ray R with an incident angle α = 0°; solid lines L2 and L3 represent the transmission spectra of light ray R with incident angles α = 0.5° and -0.5°, respectively; solid lines L4 and L5 represent the transmission spectra of light ray R with incident angles α = 1° and -1°, respectively; solid lines L6 and L7 represent the transmission spectra of light ray R with incident angles α = 1.5° and -1.5°, respectively; solid lines L8 and L9 represent the transmission spectra of light ray R with incident angles α = 2° and -2°, respectively; and solid lines L10 and L11 represent the transmission spectra of light ray R with incident angles α = 2.5° and -2.5°, respectively. Furthermore, transmission spectra L2 and L3 are substantially identical and are superimposed on the graph.

[0083] like Figure 5 As shown, when the magnitude (absolute value) of the incident angle α is equal, the behavior of the transmitted spectrum is similar (e.g., solid lines L10 and L11). Furthermore, all transmitted spectra L2 to L11 are shifted towards shorter wavelengths relative to transmitted spectrum L1, and the larger the magnitude of the incident angle α, the greater the shift. Hereinafter, the wavelength shift of transmitted spectra L2 to L11 relative to transmitted spectrum L1 when the transmission loss is 3dB (i.e., transmittance 50%) will be defined as "wavelength shift Δλ". That is, wavelength shift Δλ is "the wavelength shift of the transmitted spectrum of light with an incident angle α at a transmission loss of 3dB relative to the transmitted spectrum of light with an incident angle of 0°".

[0084] Figure 6 This is a graph that defines the relationship between the incident angle α and the wavelength offset Δλ. The 11 measured values ​​in the graph are based on... Figure 5 The values ​​are marked on the graph. Additionally, the results obtained by calculating the relationship between the incident angle α and the wavelength offset Δλ based on the following analytical formula (1) are shown. Here, analytical formula (1) is derived with reference to the following literature.

[0085] Mitsunobu Kohiyama, "Optical Film Fiber Film", 1st edition, Edotron Co., Ltd., 2006, p. 301-346

[0086] [Number 1]

[0087]

[0088] Here, in analytical equation (1), λ0 represents the cut-off wavelength (=1522.4nm) at an incident angle α=0°, n1 represents the refractive index in air (=1.0), and n2 represents the refractive index of filter 40. Regarding n2, it is set to n2=1.65 to make the analytical value fit the measured value. According to... Figure 6 The graphs show that the behavior of analytical equation (1) is very consistent with the measured values. Therefore, in the following study, we will assume that the wavelength shift Δλ is calculated based on analytical equation (1). In addition, according to analytical equation (1), the larger the incident angle α, the greater the wavelength shift Δλ.

[0089] Furthermore, in this example, the wavelength offset Δλ is specified as the wavelength offset when the transmission loss is 3dB, but it is not limited to this. For example, the wavelength offset Δλ could also be specified as the wavelength offset when the transmission loss is 2dB, 4dB, or 5dB. This is because, as... Figure 5 As shown, at least in the range of 2dB to 5dB transmission loss, transmission spectra 1 to 11 are approximately parallel to each other, so the wavelength shift of each transmission spectrum 2 to 11 is approximately the same as the wavelength shift Δλ when the transmission loss is 3dB (i.e., very consistent with analytical equation (1)).

[0090] Furthermore, the incident angle α can be determined based on "the rotation angle η of the filter 40" and "the angle θ of the light ray emitted from the first lens 30 that forms an acute angle with the axis A1 (refer to...)". Figure 3 The values ​​are derived from "( )" and "angles Φ of each peripheral core C1 to C3 and C5 to C7 (described later)". Figure 7A and Figure 7B Please provide an explanation. Figure 7A It is a diagram showing the relationship between the incident angle α, rotation angle η, ray angle θ, and angle Φ in an orthogonal coordinate system. Figure 7B This is a diagram showing the end face 20a of the MCF 20. (See diagram below.) Figure 7B As shown, the angle Φ of any peripheral core is defined as the deflection angle. The deflection angle is the angle from the positive portion of the x-axis to the line segment connecting the center of any peripheral core to the origin. Figure 7B In the example, the angles Φ of the peripheral cores C1, C2, C5, C7, C6, and C3 are 0°, 60°, 120°, 180°, 240°, and 300°, respectively. For the central core C4, the angle Φ is not defined. Furthermore, in... Figure 7A In the diagram, angle Φ is shown as an angle that increases counterclockwise from the "negative" part of the x-axis when viewed from the +z-axis direction. This is because the light rays from the peripheral cores C1 to C3 and C5 to C7 of the MCF 20 are refracted through the first lens 30.

[0091] Figure 7A The vector B represents the ray vector of a ray traveling from any core C1 to C7 through the first lens 30. When the half-line extending from the origin in the direction of angle Φ is defined as half-line b, the ray vector B lies in a plane passing through half-line b and the z-axis, forming a ray angle θ with the unit vector ez (not shown) extending along the +z-axis. Therefore, the ray vector B can be expressed as the following equation (2).

[0092] [Number 2]

[0093]

[0094] Figure 7A The vector N is the normal vector derived from the exit surface 40b of the filter 40. The normal vector N lies in the zx plane and forms a rotation angle η with the unit vector ez. Furthermore, in Figure 7A In this context, the normal vector N is defined as the normal vector drawn from the exit surface 40b, although it is different from... Figure 3 and Figure 4 The direction of the normal N shown is opposite, but this is because it is easier to illustrate by defining the angle between it and the unit vector ez extending along the +z axis as the rotation angle η. It is worth noting that... Figure 7A With Figure 3 and Figure 4 The rotation angle η shown is the same value. The normal vector N can be expressed as the following equation (3).

[0095] [Number 3]

[0096]

[0097] The incident angle α is the angle between the ray vector B (Equation (2)) and the normal vector N (Equation (3)). Therefore, according to the definition of the inner product, the incident angle α can be derived by the following equation (4).

[0098] [Number 4]

[0099]

[0100] Furthermore, since the ray angle θ from the central core C4 is 0°, the second term within parentheses in equation (4) has a value of 0 when calculating the incident angle α of the ray from the central core C4 (because the value of sinθ is 0). That is, in this case, the angle Φ is not included in equation (4). Therefore, even if the angle Φ is not defined for the central core C4, there is no particular problem, and the incident angle α of the ray from the central core C4 can be appropriately calculated using equation (4).

[0101] Based on the above, the incident angle α of the light rays from each core C1 to C7 incident on the filter 40 can be calculated based on the rotation angle η of the filter 40, the light ray angle θ, and the angle Φ of the core corresponding to the light ray. By calculating the incident angle α, the wavelength offset Δλ can be calculated using equation (1). The inventors of this application applied MCFs with various core numbers and core configurations to the filter device 10, and used equations (4) and (1) to study the circumferential orientation that can reduce the maximum value of the deviation of the transmitted spectrum.

[0102] Figures 8A to 8C This is a diagram showing the end face of the MCF used in the study. Figure 8A The MCF is MCF 20. The MCF 20 structure uses... Figure 2 The description has been provided, so detailed explanations are omitted. The ray angle θ from the central core C4 is 0°. The ray angle θ from the peripheral cores C1 to C3 and C5 to C7 are equal to each other due to the symmetry with respect to the ray from the central core C4, and are each 0.87° (refer to...). Figure 1 Additionally, the rotation angle η is set to 2.9°. This is in Figure 8B and Figure 8C The same applies to the example. Furthermore, the peripheral cores C1 to C3 and C5 to C7 are equivalent to an example of the "outermost peripheral core".

[0103] Figure 8B The MCF 120 differs from the MCF 20 only in the number and configuration of its cores. The MCF 120 has four cores C1 to C4 as the first core and a common cladding 121 surrounding these cores C1 to C4. Cores C1 to C4 are located at the vertices of a square centered on the center of end face 120a. That is, cores C1 to C4 are “peripheral cores”. The core spacing is 50 μm. The ray angle θ from cores C1 to C4 is equal to each other due to symmetry with respect to the central axis of the MCF 120, and is 0.81°. Furthermore, cores C1 to C4 are an example of “outermost peripheral cores”.

[0104] Figure 8CThe MCF 220 differs from the MCF 120 only in its core configuration. The MCF 220 comprises four cores C1 to C4 as the first core and a common cladding 221 surrounding these cores C1 to C4. Cores C1 to C4 are arranged in a linear configuration with a quadratic symmetry centered on the end face 220a. That is, cores C1 to C4 are "peripheral cores". The core spacing is 50 μm. The ray angle θ from cores C1 and C4 is equal to each other, at 1.7°, due to symmetry with respect to the central axis of the MCF 220. The ray angle θ from cores C2 and C3 is also equal to each other, at 0.57°, due to symmetry with respect to the central axis of the MCF 220.

[0105] Figure 9A This is shown in the MCF 20 (refer to) Figure 8A The diagram shows the end face 20a with the circumferential orientation set such that the straight line passing through cores C1, C4, and C7 is orthogonal to the y-axis, which serves as the reference axis (i.e., the axis that passes through axis A1 and is parallel to the rotation axis r1). Hereinafter, this orientation will also be referred to as the "orthogonal type". In this example, when the core furthest from the y-axis in the -x-axis direction is defined as the "first separating core" and the core furthest from the y-axis in the +x-axis direction is defined as the "second separating core", the first separating core is core C7, and the second separating core is core C1. Furthermore, when the sum of the "distance from the y-axis to the first separating core" and the "distance from the y-axis to the second separating core" is defined as the "separation distance", the separation distance is 75 μm in this example. Additionally, the -x-axis direction corresponds to an example of a "first orthogonal direction" along the x-axis, which serves as the orthogonal axis, and towards one side relative to the y-axis; the +x-axis direction corresponds to an example of a "second orthogonal direction" along the x-axis and towards the other side relative to the y-axis. In this embodiment, the rotation axis r1 is also an axis that passes through the axis A1, so the rotation axis r1 can also be used as a reference axis.

[0106] Figure 9B This is a graph that defines the relationship between the incident angle α and the wavelength offset Δλ of the light rays from each core C1 to C7 when the MCF 20 is set to orthogonal. The incident angle α can be calculated according to equation (4), and the wavelength offset Δλ can be calculated according to equation (1). For example, first, the incident angle α is calculated by substituting η = 2.9°, θ = 0.87°, and Φ = 180° into equation (4), and then the wavelength offset Δλ of the light rays from core C7 can be calculated by substituting the incident angle α along with the other physical quantities mentioned above into equation (1). Furthermore, the dashed line 70 in the graph represents the analytical equation (1).

[0107] according to Figure 9BThe graph shows that the incident angle α of the light from the first separating core C7 is the smallest, and the incident angle α of the light from the second separating core C1 is the largest. Therefore, the wavelength offset Δλ of the light from the first separating core C7 is the smallest, and the wavelength offset Δλ of the light from the second separating core C1 is the largest. The maximum value of the transmission spectrum deviation is equal to the difference between the minimum value Δλmin and the maximum value Δλmax of the wavelength offset Δλ. Hereinafter, this difference (Δλmax-Δλmin) will be referred to as the "maximum value of the transmission spectrum deviation Dmax", or simply "maximum value of the deviation Dmax" or "Dmax". The maximum value of the transmission spectrum deviation Dmax when MCF 20 is set to orthogonal type is 0.87nm. In addition, the incident angle α of the light from the central core C4 is equal to the rotation angle η (refer to...). Figure 3 and Figure 4 Therefore, according to Figure 9B The graph shows that the incident angle α of the light rays from the central core C4 is 2.9°. For Figure 10B The same applies.

[0108] Figure 10A This diagram shows the end face 20a when the circumferential orientation of the MCF 20 is set such that the straight line passing through cores C1, C4, and C7 is parallel to the y-axis. Hereinafter, this orientation will also be referred to as "parallel type". In this example, the first separating cores are cores C2 and C5, and the second separating cores are cores C3 and C6. Furthermore, the separation distance is 65 μm.

[0109] Figure 10B This is a graph that specifies the relationship between the incident angle α and the wavelength offset Δλ of light rays from each core C1 to C7 when MCF 20 is set to parallel configuration. According to... Figure 10B The graph shows that the incident angle α of light from the first separating cores C2 and C5 is the smallest, while the incident angle α of light from the second separating cores C3 and C6 is the largest. Therefore, the wavelength shift Δλ of light from the first separating cores C2 and C5 is the smallest, while the wavelength shift Δλ of light from the second separating cores C3 and C6 is the largest. The maximum value of the transmission spectrum deviation, Dmax, when MCF 20 is set to parallel mode is 0.75 nm.

[0110] according to Figures 9A to 10B The separation distance in the parallel configuration (65 μm) is shorter than that in the orthogonal configuration (75 μm). Furthermore, the deviation of the incident angle α in the parallel configuration is smaller than that in the orthogonal configuration; therefore, the deviation of the wavelength offset Δλ (i.e., Dmax) in the parallel configuration is smaller.

[0111] Based on the above, it can be seen that there is a correlation between the core position along the x-axis and the incident angle α. More specifically, it can be seen that the shorter the separation distance, the smaller the deviation of the incident angle α. As a result, the deviation of the wavelength shift Δλ (Δλmax - Δλmin) can be reduced, that is, the maximum value of the transmission spectrum deviation Dmax can be reduced. Orthogonal type is the circumferential orientation of MC F 20 when the separation distance is maximum (i.e., Dmax is maximum), and parallel type is the circumferential orientation of MC F 20 when the separation distance is minimum (i.e., Dmax is minimum). The Dmax of the parallel type (0.75nm) is about 13% smaller than that of the orthogonal type (0.87nm). This means that by setting the circumferential orientation of MCF 20 to parallel type, Dmax can be reduced by up to about 13%.

[0112] In addition, according to Figure 9A and Figure 10A Regardless of whether MCF 20 is set to orthogonal or parallel type, the peripheral cores C1 to C3 and C5 to C7 are linearly symmetrical with respect to the y-axis. However, when MCF 20 is set to orthogonal type, two of the six peripheral cores (cores C1 to C3 and C5 to C7) (cores C1 and C7) are located on the x-axis. Conversely, when MCF 20 is set to parallel type, none of the peripheral cores are located on the x-axis. Therefore, the circumferential orientation of MCF 20 when the separation distance is minimized (i.e., Dmax is minimized) can also be defined as follows. That is, for an MCF (MCF 20) with a core configuration where "the peripheral cores are located at the vertices of a regular hexagon centered on the center of the end face of the MCF", the separation distance can be minimized by setting the MCF to one of the two types (orthogonal and parallel) where the peripheral cores are linearly symmetrical with respect to the y-axis, but the peripheral cores are not located on the x-axis (parallel type).

[0113] Figure 11A This is shown in the MCF 120 (refer to) Figure 8B The circumferential orientation of the core is set such that the straight line passing through cores C2 and C4 is orthogonal to the y-axis, as shown in the diagram of end face 120a. Hereinafter, this orientation will also be referred to as "diagonal type". In this example, the first separating core is core C2, and the second separating core is core C4. Furthermore, the separation distance is 71 μm.

[0114] Figure 11B This is a graph that specifies the relationship between the incident angle α of light rays from cores C1 to C4 and the wavelength offset Δλ when the MCF 120 is set to diagonal configuration. According to... Figure 11BThe graph shows that the incident angle α and wavelength shift Δλ of the light from the first separating core C2 are the smallest, while the incident angle α and wavelength shift Δλ of the light from the second separating core C4 are the largest. The maximum deviation of the transmission spectrum Dmax when the MCF 120 is set to diagonal type is 0.81 nm.

[0115] Figure 12A This diagram shows the end face 120a when the circumferential orientation of the MCF 120 is set such that the straight line passing through cores C2 and C3 (or cores C1 and C4) is parallel to the y-axis. Hereinafter, this orientation will also be referred to as "parallel type". In this example, the first separating cores are cores C2 and C3, and the second separating cores are cores C1 and C4. Furthermore, the separation distance is 50 μm.

[0116] Figure 12B This is a graph that specifies the relationship between the incident angle α and the wavelength offset Δλ of light rays from each core C1 to C4 when the MCF 120 is set to parallel configuration. According to... Figure 12B The graph shows that the incident angle α and wavelength shift Δλ of the light from the first splitting cores C2 and C3 are the smallest, while the incident angle α and wavelength shift Δλ of the light from the second splitting cores C1 and C4 are the largest. The maximum value of the transmission spectrum deviation Dmax when the MCF 120 is set to parallel mode is 0.57 nm.

[0117] according to Figures 11A to 12B The separation distance in the parallel configuration (50 μm) is shorter than that in the diagonal configuration (71 μm). Furthermore, the deviation of the incident angle α in the parallel configuration is smaller than that in the diagonal configuration, therefore the deviation of the wavelength offset Δλ (i.e., Dmax) in the parallel configuration is smaller.

[0118] Based on the above, similar to the case of MCF 20, the shorter the separation distance, the smaller the deviation of the incident angle α. As a result, the maximum deviation of the transmitted spectrum, Dmax, can be reduced. The diagonal orientation is the circumferential orientation of the MCF 120 when the separation distance is at its maximum, while the parallel orientation is the circumferential orientation of the MCF 120 when the separation distance is at its minimum. The Dmax of the parallel orientation (0.53 nm) is approximately 29% smaller than that of the diagonal orientation (0.81 nm). This means that by setting the circumferential orientation of the MCF 120 to parallel, Dmax can be reduced by a maximum of approximately 29%.

[0119] In addition, according to Figure 11A and 12ARegardless of whether the MCF 120 is set as a diagonal or parallel type, the peripheral cores C1 to C4 are linearly symmetrical with respect to the y-axis. However, when the MCF 120 is set as a diagonal type, two of the four peripheral cores (cores C1 to C4) (cores C2 and C4) are located on the x-axis. Conversely, when the MCF 120 is set as a parallel type, none of the peripheral cores are located on the x-axis. Therefore, similar to the case of the MCF 20, the circumferential orientation of the MCF 120 with the minimum separation distance can also be specified as follows. That is, for the MCF (MCF 120) having a core configuration where "the peripheral cores are located at the vertices of a square centered on the center of the end face of the MCF", the separation distance can be minimized by setting the MCF to one of the two types (diagonal and parallel) where the peripheral cores are linearly symmetrical with respect to the y-axis, and the type where the peripheral cores are not located on the x-axis (parallel type).

[0120] Figure 13A This is shown in the MCF 220 (refer to) Figure 8C The circumferential orientation of the end face 220a is set such that the straight line passing through cores C1 to C4 is orthogonal to the y-axis. Hereinafter, this orientation will also be referred to as "orthogonal type". In this example, the first separating core is core C1, and the second separating core is core C4. Furthermore, the separation distance is 150 μm.

[0121] Figure 13B This is a graph that specifies the relationship between the incident angle α and the wavelength offset Δλ of light rays from each core C1 to C4 when the MCF 220 is set to orthogonal configuration. According to... Figure 13B The graph shows that the incident angle α and wavelength shift Δλ of the light from the first separating core C1 are the smallest, while the incident angle α and wavelength shift Δλ of the light from the second separating core C4 are the largest. The maximum value of the transmission spectrum deviation Dmax when the MCF 220 is set to orthogonal mode is 1.7 nm.

[0122] Figure 14A This diagram shows the end face 220a when the circumferential orientation of the MCF 220 is set such that the straight line passing through cores C1 to C4 is parallel to the y-axis. Hereinafter, this orientation will also be referred to as the "parallel type". In the parallel type case, cores C1 to C4 are arranged along the y-axis, therefore cores C1 to C4 are both the first and second separation cores. Consequently, the separation distance is 0 μm.

[0123] Figure 14B This is a graph that specifies the relationship between the incident angle α and the wavelength offset Δλ of light rays from each core C1 to C4 when the MCF 220 is set to parallel configuration. According to... Figure 14BThe graph shows that the incident angle α and wavelength shift Δλ of light rays from cores C2 and C3 (i.e., cores closer to axis A1 (z-axis)) are the smallest, while the incident angle α and wavelength shift Δλ of light rays from cores C1 and C4 (i.e., cores farther from axis A1) are the largest. The maximum deviation of the transmitted spectrum, Dmax, when the MCF 220 is set to parallel mode is 0.23 nm.

[0124] according to Figures 13A to 14B The separation distance (0 μm) in the parallel configuration is shorter than that in the orthogonal configuration (150 μm). Furthermore, the deviation of the incident angle α in the parallel configuration is smaller than that in the orthogonal configuration, therefore the deviation of the wavelength offset Δλ (i.e., Dmax) in the parallel configuration is smaller.

[0125] Based on the above, similar to the cases of MCF 20 and MCF 120, the shorter the separation distance, the smaller the deviation of the incident angle α. As a result, the maximum deviation of the transmitted spectrum, Dmax, can be reduced. Orthogonal configuration refers to the circumferential orientation of the MCF 220 at its maximum separation distance, while parallel configuration refers to the circumferential orientation of the MCF 220 at its minimum separation distance. The Dmax in the parallel configuration (0.23 nm) is approximately 87% smaller than the Dmax in the orthogonal configuration (1.7 nm). This means that by setting the circumferential orientation of the MCF 220 to parallel, Dmax can be reduced by a maximum of approximately 87%.

[0126] In addition, according to Figure 13A and Figure 14A Regardless of whether the MCF 220 is set to orthogonal or parallel type, the peripheral cores C1 to C4 are linearly symmetrical with respect to the y-axis. However, when the MCF 220 is set to orthogonal type, all four peripheral cores (cores C1 to C4) are located on the x-axis; conversely, when the MCF 220 is set to parallel type, none of the peripheral cores are located on the x-axis. Therefore, similar to the cases of MCF 20 and 120, the circumferential orientation of the MCF 220 to minimize the separation distance can be specified as follows: That is, for an MCF (MCF 220) having a core configuration where the peripheral cores are arranged along a straight line passing through the center of the end face of the MCF, the separation distance can be minimized by setting the MCF to one of the two types (orthogonal and parallel) where the peripheral cores are linearly symmetrical with respect to the y-axis, but the peripheral cores are not located on the x-axis (parallel type).

[0127] Based on the above research results, when applying MCFs 20, 120, or 220 to the filter device 10, it is preferable to set the circumferential orientation of these MCFs to be parallel. This minimizes the separation distance between MCFs 20, 120, and 220, thus minimizing the deviation of the incident angle α of light from each core incident onto the filter 40. Consequently, the maximum value Dmax of the transmission spectrum deviation of light from each core can be minimized, maximally suppressing the deviation in transmission loss of these rays at any given wavelength. As a result, optical signals can be transmitted appropriately while reducing reflected light.

[0128] Furthermore, when the MCF has a core configuration where "the peripheral cores are located at the vertices of a regular polygon centered on the center of the MCF's end face," the MCF can also have one or more other peripheral cores as inner cores located radially inward from the peripheral cores. The inner cores do not need to be symmetrical; their configuration is acceptable. This is because the separation distance is determined solely by minimizing the maximum deviation, Dma x, and this separation distance is a value determined independently of (and unaffected by) the core configuration of the inner cores.

[0129] Furthermore, the number of chips and chip configuration of the MCF are not limited to... Figures 8A to 8C Examples are listed below. Furthermore, the core configuration of the MCF can also be asymmetrical with respect to the center of the MCF's end face. In other words, the first and second separating cores can be offset from each other in the y-axis direction, and the distance from the y-axis to the first separating core can be different from the distance from the y-axis to the second separating core. Even in such cases, the deviation of the incident angle α can be minimized by setting the circumferential orientation of the MCF in a way that minimizes the separation distance, thus achieving the aforementioned effect.

[0130] Furthermore, the filter 40 can also rotate in a manner where the rotation angle η has a negative value. Even in this case, the aforementioned effect can be achieved by setting the circumferential orientation of the MCF in a way that minimizes the separation distance.

[0131] (Second Implementation)

[0132] Figure 15 This is a top view illustrating an example of the filter device 310 according to the second embodiment of the present invention. In this embodiment, the same reference numerals are used for components identical to those in the first embodiment, and detailed descriptions of structures identical to those in the first embodiment are omitted. Figure 15As shown, the filter device 310 differs from the filter device 10 in that it has an MCF 320 and a collar 322 instead of an MCF 20 and a collar 22, and an MCF 360 and a collar 362 instead of an MCF 60 and a collar 62. Furthermore, the position of the second lens 50 differs from its position in the filter device 10.

[0133] like Figure 15 As shown, the end face 320a of the MCF 320 is obliquely ground (oblique grinding) at a predetermined grinding angle (e.g., 8°) relative to the xy plane in a predetermined oblique direction. This reduces reflected light at the end face 320a. When viewed from a direction perpendicular to the end face 320a, the end face 320a is elliptical in shape. Hereinafter, the direction along the major axis of the end face 320a (an example of the oblique grinding reference axis) from the far end relatively far from the filter 40 toward the proximal end relatively close to the filter 40, when viewed along the central axis of the end face 320a, is defined as the "oblique grinding direction".

[0134] Figure 16 This is a diagram showing end face 320a. Figure 16 The solid arrow 81 indicates the oblique grinding direction of end face 320a, and the dashed arrow 80 indicates the reference direction, which serves as the calculation reference for the oblique grinding angle described later. Figure 16 As shown, the reference direction 80 extends along the +y axis. Hereinafter, the angle formed by rotating any oblique grinding direction counterclockwise from the reference direction 80 is defined as the "oblique grinding rotation angle ψ with a positive value". In this example, the oblique grinding direction 81 rotates counterclockwise from the reference direction 80 to form an angle of 270°, therefore the oblique grinding rotation angle ψ of the oblique grinding direction 81 is 270° (in other words, -90°).

[0135] like Figure 15 As shown, the end of the MCF 320 in the +z axis direction is held through a cylindrical collar 322. The end face 320a of the MCF 320 and the end face 322a of the collar 322 are both obliquely ground together. However, the end 322a1 of the end face 322a of the collar 322 in the oblique grinding direction 81 is not obliquely ground, but is parallel to the xy plane. The end 322a1 is the so-called grinding allowance (Japanese: けんましろ).

[0136] MCF 320 has seven cores C1 to C7 and a common cladding 321 surrounding these cores C1 to C7. MCF 320 has the same core configuration as MCF 20. In addition, the circumferential orientation of MCF 320 is set to parallel (i.e., the orientation with the minimum separation distance).

[0137] Light rays propagating in the cores C1 to C7 of the MCF 320 are emitted from the end face 320a toward the first lens 30. Figure 15 The diagrams only show the cores C3, C4, and C2 (see reference). Figure 16 The emitted light rays are principal rays B3, B4, and B2. Principal rays B3, B4, and B2 travel obliquely toward the grinding direction 81 (in this example, the +x axis direction), and are therefore oblique relative to axis A1.

[0138] The first lens 30 is positioned away from the focal length along the +z axis from the center of the central core C4 at the end face 320a. Therefore, the principal ray B4 emitted from the first lens 30 is parallel to the axis A1. The first lens 30 collimates and converges the light rays from each core C1 to C7.

[0139] The filter 40 is configured such that the focal point of the emitted light from the first lens 30 is located on the incident surface 40a. Light incident on the incident surface 40a passes through the filter 40 and exits from the exit surface 40b. The filter 40 has a rotation axis r2 extending along the y-axis at the position where the light converges on the incident surface 40a. The filter 40 is rotated by an angle η about the rotation axis r2 from a position parallel to the xy-plane. This reduces reflected light. Furthermore, the principal ray B4 emitted from the filter 40 is parallel to the axis A1.

[0140] The second lens 50 is configured such that its central axis coincides with axis A1. The second lens 50 refracts light rays emitted from the filter 40 from each core C1 to C7 in such a manner that the principal rays of these light rays are parallel to each other (see reference). Figure 15 The light rays B3, B4, and B2). Additionally, the second lens 50 converges the light rays from each core C1 to C7 (in... Figure 15 (Only the principal ray is shown in the diagram).

[0141] MCF 360 has the same structure as MCF 320. The end of MCF 360 in the -z-axis direction is held in place by a cylindrical collar 362. The end face 360a of MCF 360 and the end face 362a of collar 362 are both obliquely ground. This reduces reflected light at end face 360a. The end face 362a of collar 362 has a grinding allowance at its end 362a1 in the oblique grinding direction. MCF 360 is linearly symmetrical with respect to the x-axis to MCF 320.

[0142] The relative positions of the second lens 50 and the MCF 360 are determined such that the end face 360a is located at a position where the light rays emitted from the second lens from each core C1 to C7 converge.

[0143] The above is a description related to the structure of the filter device 310.

[0144] The inventors of this application applied two types of MCFs, whose circumferential orientation is set to parallel, to the filter device 310. By varying the oblique grinding rotation angle ψ within the range of 0°≤ψ≤360°, the relationship between the oblique grinding rotation angle ψ and the maximum value Dmax of the deviation of the transmitted spectrum was investigated.

[0145] Figure 17A and Figure 17B This is a diagram showing the end face of the MCF used for the investigation. Figure 17A The MCF is MC F 320. (Using...) Figure 16 The structure of MCF 320 has been described, therefore detailed explanations are omitted. The oblique grinding direction 82 can rotate 360° counterclockwise from the reference direction 80 (illustration omitted). The ray angle θ from the center core C4 is 0° (refer to...). Figure 15 The average angle θ of the rays from the peripheral cores C1 to C3 and C5 to C7 is 0.87°. In this example, the rotation angle η is set to one of four values: η ± 2.9° and ± 1.8°.

[0146] Figure 17B The MCF is MCF 420, differing only from MCF 320 in the number and configuration of cores. MCF 420 has four cores C1 to C4 and a common cladding 421 surrounding these cores. MCF 420 has the same core configuration as MCF 220. The oblique grinding direction 83 can rotate 360° counterclockwise from the reference direction 80 (not shown). The light angle θ from cores C1 and C4 is 1.7°. The light angle θ from cores C2 and C3 is 0.57°. In this example, the rotation angle η is set to 2.9°.

[0147] Figure 18 This shows filter 40 with a rotation angle η = 2.9° and MCF 320 after oblique grinding (see reference). Figure 17A A diagram illustrating an example of the relative positional relationship of the components. In this example, the end face 320a of the MCF 320 was obliquely ground with a rotation angle ψ of 90°. Figure 18 The diagram of light rays emitted from the exit surface 40b through the filter 40 is omitted. For Figure 20 The same applies.

[0148] Figure 19 It is stipulated that... Figure 18The graph shows the relationship between the "oblique grinding rotation angle ψ" and the "maximum deviation Dmax" when the oblique grinding rotation angle ψ of the MCF 320 varies within the range of -180°≤ψ≤180°. The solid line 90 in the graph represents an MCF whose end face is not obliquely ground (i.e., the parallel type MCF 20 of the first embodiment, see reference 1). Figure 10A and Figure 10B The maximum value of the deviation, Dmax, is reached when )). Furthermore, in Figure 10B In the figure, Dmax = 0.75nm, but this is because it is recorded with two significant digits. The strictly defined value, as shown by the solid line 90, is slightly smaller than 0.75nm. Figure 19 When 0° < ψ < 180°, the Dmax of the obliquely ground MCF 320 is equal to or smaller than the Dmax of the parallel-type MCF 20 that is not obliquely ground.

[0149] Figure 20 This diagram illustrates an example of the relative positional relationship between the filter 40 and the MCF 320 after oblique grinding, with a rotation angle η = -2.9°. In this example, the end face 320a is obliquely ground with an oblique grinding rotation angle ψ of -90°.

[0150] Figure 21 It is stipulated that... Figure 20 The graph shows the relationship between the "oblique grinding rotation angle ψ" and the "maximum deviation Dmax" when the oblique grinding rotation angle ψ of the MCF 320 varies within the range of -180°≤ψ≤180°. According to... Figure 21 When -180° < ψ < 0°, the Dmax of the MCF 320 after oblique grinding is equal to or smaller than the Dmax of the parallel type MCF 20 that has not been obliquely ground.

[0151] Figure 22 It is stipulated that Figure 18 The graph shows the relationship between the "oblique grinding rotation angle ψ" and the "maximum deviation Dmax" when the rotation angle η of the filter 40 is changed to 1.8° and the oblique grinding rotation angle ψ of the MCF 320 is varied within the range of -180°≤ψ≤180°. The solid line 91 in the graph represents the maximum deviation Dmax when an MCF with an unobliquely ground end face (i.e., the parallel type MCF 20 of the first embodiment) is used. Figure 22 When 0° < ψ < 180°, the Dmax of the obliquely ground MCF320 is equal to or smaller than the Dmax of the parallel-type MCF20 that is not obliquely ground.

[0152] Figure 23 It is stipulated that Figure 20The graph shows the relationship between the "oblique grinding rotation angle ψ" and the "maximum deviation Dmax" when the rotation angle η of the filter 40 is changed to -1.8° and the oblique grinding rotation angle ψ of the MC F 320 varies within the range of -180°≤ψ≤180°. Figure 23 When -180° < ψ < 0°, the Dmax of the obliquely ground MCF 320 is equal to or smaller than the Dmax of the unobliquely ground parallel-type MCF 20.

[0153] Based on the above, it can be seen that there is a fixed tendency between the oblique grinding rotation angle ψ and the maximum deviation Dmax. More specifically, when the rotation angle η has the same magnitude but different signs, it can be seen that the behavior of Dmax is linearly symmetrical about ψ = 0°. Furthermore, it can be seen that when the rotation angle η has a positive value (refer to...), Figure 19 and Figure 22 By performing oblique grinding on MCF 320 in a manner that ensures the oblique grinding rotation angle ψ satisfies 0° < ψ < 180°, the maximum deviation Dmax can be maintained or further reduced. On the other hand, it is known that when the rotation angle η has a negative value (refer to...), Figure 21 and Figure 23 By performing oblique grinding on MCF 320 in a manner that ensures the oblique grinding rotation angle ψ satisfies -180° < ψ < 0°, the maximum deviation Dmax can be maintained or further reduced.

[0154] In contrast, Figure 24 It is specified that MCF 420 (refer to) is achieved when the rotation angle η = 2.9°. Figure 17B The graph shows the relationship between the "oblique grinding rotation angle ψ" and the "maximum deviation Dmax" when the oblique grinding rotation angle ψ varies within the range of -180°≤ψ≤180°. The solid line 92 in the graph represents the MCF used when the end face is not obliquely ground (i.e., the parallel type MCF 220 of the first embodiment (see reference)). Figure 14A and Figure 14B The maximum value of the deviation, Dmax, is reached when )). Furthermore, in Figure 14B In the figure, Dmax = 0.23 nm, but this is because it is recorded with two significant digits. The strictly defined value, as shown by solid line 92, is slightly smaller than 0.23 nm. Figure 24 When the rotation angle η has a positive value, the Dmax of the MCF 420 after oblique grinding is larger than the Dmax of the parallel type MCF 220 that has not been obliquely ground.

[0155] Based on the line symmetry of the behavior of Dmax when using MCF 320, it is inferred that the behavior of Dmax when η = -2.9° is the same as the behavior of Dmax when η = 2.9° (refer to...). Figure 24 The behavior of ")" is the same after reversing relative to ψ = 0°. Here, according to Figure 24 The behavior of Dmax at η = 2.9° is linearly symmetric with respect to ψ = 0°. Therefore, when using MCF 420, it is speculated that the behavior of Dmax at η = -2.9° is consistent with the behavior of Dmax at η = 2.9°. Based on this speculation, it is also assumed that the Dmax of the obliquely ground MCF 420 is larger than that of the unobliquely ground parallel-type MCF 220 when the rotation angle η is negative. In other words, it is assumed that if the MCF 420 is obliquely ground, the maximum value of the deviation Dmax, independent of the sign of the rotation angle η, will increase.

[0156] Therefore, as with the core configuration of MCF 320, when the separation distance is greater than zero, "when η>0, MCF 320 is obliquely ground in a manner that satisfies 0°<ψ<180°, and when η<0, MCF 320 is obliquely ground in a manner that satisfies -180°<ψ<0°". Thus, compared with the structure without oblique grinding, the maximum deviation Dmax can be maintained or further reduced. However, as with the core configuration of MCF 420, such an effect cannot be obtained when the separation distance is zero.

[0157] Based on the above findings, when the circumferential orientation of the MCF is set to minimize the separation distance (refer to the first embodiment), if this distance is greater than zero, it is preferable to perform oblique grinding on the MCF within the range of the oblique grinding rotation angle ψ corresponding to the sign of the rotation angle η, as described above. Therefore, compared to a structure without oblique grinding, the maximum deviation Dmax can be maintained or further reduced, thus enabling appropriate transmission of optical signals while suppressing reflected light. On the other hand, when the separation distance is zero, it is preferable not to perform oblique grinding on the MCF to prevent the maximum deviation Dmax from increasing.

[0158] Furthermore, MCFs can also have inner peripheral cores. Additionally, the number and configuration of cores in an MCF are not limited to... Figure 17A and Figure 17B The examples listed above. Furthermore, the core configuration of the MCF may not be symmetrical with respect to the center of the MCF's end face. Even in this case, when the separation distance is greater than zero, the deviation of the incident angle α can be further reduced by obliquely grinding the MCF along the aforementioned oblique grinding direction, thus achieving the aforementioned effect.

[0159] The filter devices involved in the embodiments have been described above, but the present invention is not limited to the above embodiments. Various modifications can be made as long as they do not depart from the purpose of the present invention.

[0160] For example, filter 40 is not limited to a short-pass filter, but can also be other filters that allow light of a specific wavelength band to pass through with arbitrary transmission intensity (e.g., a long-pass filter, a band-pass filter, or a gain-flattening filter). When using a long-pass filter as the filter, λ0 in the above analytical expression (1) represents the cut-on wavelength when the incident angle α = 0°. When using a band-pass filter as the filter, λ0 represents the center wavelength when the incident angle α = 0°. When using a gain-flattening filter as the filter, λ0 can be appropriately set according to the wavelength distribution.

[0161] Furthermore, in the second embodiment, the end face of the MCF is obliquely ground, thus causing a deviation in the light angle θ of the light rays emitted from the first lens 30 from each core. Therefore, the deviation in the light angle θ can be reduced by moving the MCF a predetermined distance in the direction opposite to the oblique grinding direction.

[0162] Alternatively, the incident surface 40a of the filter 40 may not be located at the focal point of the emitted light from the first lens 30. In this case, the rotation axes r1 and r2 of the filter 40 can be set to any axis passing through the filter 40 along the y-axis direction.

[0163] Furthermore, the MCF is not limited to a cylindrical shape; for example, it can also be a cylindrical shape with an elliptical or polygonal cross-section orthogonal to the axis. In this case, the oblique grinding direction of the MCF is defined as the direction observed along the central axis of the end face of the MCF, from the far end relatively far from the filter 40 toward the proximal end relatively close to the filter 40, where the oblique grinding reference axis is a line segment that intersects the end face with a plane that passes through the center of the end face of the MCF, is orthogonal to the end face, and is parallel to the tilt direction (the direction in which the end face of the MCF is tilted relative to the xy plane).

[0164] Furthermore, in the above-described embodiments, MCF 60 (or MCF 360) has the same number of cores and the same core configuration as MCF 20 (or MCF 320), but is not limited thereto. MCF 60 (or MCF 360) can have any cores capable of allowing emitted light from each core of MCF 20 (or MCF 320) to be incident, or it can have a different number of cores and core configuration than MCF 20 (or MCF 320). For example, MCF 60 (or MCF 360) may have one or more cores in addition to the seven cores described above.

[0165] Similarly, a single-core fiber optic assembly only needs to have a single-mode single-core fiber that can provide incident light from each core of the MCF 20 (or MCF 320), and its number can be more than the number of cores of the MCF 20 (or MCF 320).

[0166] In these cases, there are cores or single-core optical fibers from each core C1 to C7 of MCF 20 (or MCF 320) that do not receive incident light, but even with such a structure, it can still achieve the same effect as the first embodiment.

[0167] Furthermore, MCF 20 (or MCF 320) can also be multimode fiber. In this case, MCF 60 (or MCF 360) can also be multimode fiber with more than 7 cores. Alternatively, a single-core fiber ensemble can also have more than 7 multimode single-core fibers.

[0168] Furthermore, when using a single-core fiber group as the light-receiving component, the second lens 50 can also be a lens array having the same number of lenses as the MCF 20 (or MCF 320). Each lens in the lens array converges the light rays from the corresponding core of the MCF 20 (or MCF 320) in such a way that the light rays are incident on the corresponding single-core fiber.

[0169] Furthermore, it is also possible that not all cores C1 to C7 of MCF 20 (or MCF 320) are used for light propagation. For example, only cores C1 to C4 may be used for light propagation, while cores C5 to C7 may not be used. In this case, MCF60 (or MCF 360) only needs to have more than the number of cores used for light propagation as MCF 20 (or MCF 320). That is, MCF60 (or MCF 360) does not necessarily need to have the same number of cores as MCF 20 (or MCF 320). Alternatively, a single-core fiber ensemble only needs to have more than the number of single-core fibers used for light propagation as MCF 20 (or MCF 320). That is, a single-core fiber ensemble does not necessarily need to have the same number of single-core fibers as MCF 20 (or MCF 320).

[0170] Explanation of reference numerals in the attached figures

[0171] 10: Filter device; 20, 120, 220: Multi-core optical fiber; 20a, 120a, 220a: End face of multi-core optical fiber; 21, 121, 221: Cladding; 22: Loop; 22a: End face of lobing; 30: First lens; 40: Filter; 40a: Incident surface; 40b: Emission surface; 50: Second lens; 60: Multi-core optical fiber; 60a: End face of multi-core optical fiber.

Claims

1. A filter device, comprising: The first multi-core optical fiber is cylindrical and has a plurality of first cores extending along the axial direction and a common cladding surrounding the plurality of first cores. The first lens has an optical axis located on the central axis of the first multi-core optical fiber, which collimates the diverging light rays emitted from each of the first cores and focuses the collimated light rays from each of the first cores. A filter having a first surface into which light emitted from a first lens is incident, and the focal point of the light emitted from the first lens is located on the first surface, and a second surface facing the first surface into which light emitted through itself is emitted, for allowing light of a specific wavelength to pass through with arbitrary transmission intensity, the filter being rotated by a predetermined rotation angle from a position of the first surface parallel to a surface orthogonal to the optical axis of the first lens about a rotation axis located at the focal point and extending in a specific direction perpendicular to the optical axis; The second lens converges the light rays emitted from each of the first cores from the filter. as well as The optical fiber is cylindrical and has a core that allows all light rays from each of the first cores, emitted from the second lens, to enter and extend along the axial direction. When a first orthogonal direction is defined as a direction starting from a reference axis that passes through the optical axis and is parallel to the rotation axis, along an orthogonal axis orthogonal to both the optical axis and the reference axis, and toward one side relative to the reference axis; and a second orthogonal direction is defined as a direction starting from the reference axis, along the orthogonal axis, and toward the other side relative to the reference axis,... The circumferential orientation of the first multi-core optical fiber is set to minimize the separation distance, which is the sum of the distance from the first core farthest from the reference axis in the first orthogonal direction and the distance from the first core farthest from the reference axis in the second orthogonal direction when the end face of the first multi-core optical fiber is viewed along the central axis of the first multi-core optical fiber.

2. The filter device according to claim 1, wherein, The plurality of first cores includes a plurality of peripheral cores extending along an axis other than the central axis of the first multi-core optical fiber. When observing the end face of the first multi-core optical fiber along its central axis, The peripheral cores are arranged along a straight line passing through the center of the end face, or the outermost peripheral cores farthest from the center are located at the vertices of a regular polygon centered on the center. The outermost core or the peripheral cores arranged in a straight line are linearly symmetrical with respect to the reference axis and are not located on the orthogonal axis.

3. The filter device according to claim 1, wherein, The optical fiber is a second multi-core optical fiber whose core is surrounded by a common cladding. The second lens refracts light rays emitted from the filter from each of the first cores in such a manner that the principal rays of the light rays are parallel to each other.

4. The filter device according to claim 1, wherein, The optical fiber is a single-core optical fiber group comprising multiple single-core optical fibers, each of which includes a core and a cladding surrounding the core.

5. The filter device according to claim 1, wherein, The rotation angle of the filter is greater than 0° and less than 90°.

6. The filter device according to any one of claims 1 to 5, wherein, The end face of the first multi-core optical fiber is obliquely ground at a predetermined grinding angle relative to a predetermined tilt direction orthogonal to the optical axis. The z-axis is defined as extending along the optical axis in a positive direction from the first lens toward the filter, the y-axis is defined as extending along the reference axis in a positive direction from one end of the reference axis toward the other end, and the x-axis extends along the first orthogonal direction and the second orthogonal direction; The definition is that, when the filter is viewed from the positive direction of the y-axis, the rotation angle of the filter when it rotates counterclockwise about the rotation axis has a positive value, and the rotation angle of the filter when it rotates clockwise about the rotation axis has a negative value; and The oblique grinding direction is defined as the direction observed along the oblique grinding reference axis from the far end relatively far from the filter towards the proximal end relatively close to the filter, viewed along the central axis of the end face of the first multi-core optical fiber. The angle formed by rotating the oblique grinding direction counterclockwise from the positive direction of the y-axis is defined as the oblique grinding rotation angle with a positive value. The oblique grinding reference axis is a line segment that intersects the end face with a plane passing through the center of the end face, orthogonal to the end face, and parallel to the oblique direction. When the aforementioned provisions are made, if the separation distance is greater than 0... When the rotation angle of the filter is positive, the end face of the first multi-core optical fiber is obliquely ground with the oblique grinding rotation angle being greater than 0° and less than 180°. When the rotation angle of the filter has a negative value, the end face of the first multi-core optical fiber is obliquely ground in a manner where the oblique grinding rotation angle is a value greater than -180° and less than 0°.

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