Optical Filter Device

Abstract

To provide an optical filter device which properly transmits an optical signal while reducing reflected return light.SOLUTION: An optical filter device 10 includes: a first multicore optical fiber 20 with a plurality of first cores; a first lens 30; an optical filter 40 which is rotated by a prescribed rotational angle about a rotary axis relative to a surface orthogonal to an optical axis, and transmits emission light from the first lens; a second lens 50; and an optical fiber 60. When a direction toward one side and a direction toward the other side relative to a reference axis along an orthogonal axis crossing the optical axis and the reference axis from a reference axis passing the optical axis and parallel to the rotary axis are respectively defined as a first orthogonal direction and a second orthogonal direction, a direction in a circumferential direction of the first multicore optical fiber, in a view of an end surface thereof along its center axis line, is set so as to minimize a separation distance being a sum of distances from a reference axis of a first core separated the most in the first orthogonal direction and in the second orthogonal direction respectively from the reference axis.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an optical filter device. [Background technology]

[0002] The demand for communication traffic via the Internet is increasing year by year, and there is a demand for even faster and larger capacity optical communications. To meet this demand, efforts have been made to increase transmission capacity using wavelength division multiplexing (WDM) technology and digital coherent technology.

[0003] In recent years, space division multiplexing (SDM) technology using multi-core optical fibers has been attracting attention as a new multiplexing technology. It is said that SDM technology can achieve even higher speeds and larger capacities. With the progress of research and development of SDM technology, the demand for optical isolators and optical filter devices using multi-core optical fibers is increasing. Here, as an optical filter device, for example, a device in which a multi-core optical fiber, an optical filter made of a dielectric multilayer film, and an optical fiber having the same number of cores as the multi-core optical fiber (typically a multi-core optical fiber) are arranged in this order along a certain axis is known. The optical filter has a function of transmitting, for example, light rays in a specific wavelength band. A light ray emitted from each core of the multi-core optical fiber is incident on an incident surface of the optical filter, passes through the optical filter, and is incident on a corresponding core of the optical fiber (for example, see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5598882 Summary of the Invention

[0005] In such an optical filter device, when the optical filter is arranged such that its incidence surface is parallel to a plane perpendicular to the axis, a light beam emitted from the multi-core optical fiber may be reflected by the incidence surface. Such reflected light is generally called "reflected back light." The reflected back light may enter a transmitting communication device via the multi-core optical fiber or may degrade the optical characteristics of the signal light by undergoing multiple reflections.

[0006] Therefore, conventionally, an optical filter has been disposed so that the incidence surface of the optical filter is inclined with respect to a plane perpendicular to the axis, thereby reducing the reflected back light.

[0007] However, when the optical filter is tilted in this way, although the reflected back light can be reduced, the transmission loss of the light emitted from each core of the multi-core optical fiber and transmitted through the optical filter varies, and the optical signal may not be transmitted properly. Hereinafter, a specific description will be given with reference to Figs. 25A and 25B. Fig. 25A is a graph showing the transmission loss characteristics of a certain optical filter device. This optical filter device includes two multi-core optical fibers having seven cores and an optical filter disposed between the two multi-core optical fibers. The two multi-core optical fibers have the same configuration. Specifically, one of the seven cores extends as a central core along the central axis of the multi-core optical fiber. The remaining six cores are located at the vertices of a regular hexagon centered on the central core, and extend as peripheral cores along the axis direction. A short-wavelength transmission optical filter (an optical filter that transmits light in a wavelength band shorter than a specific wavelength) is used as the optical filter. The optical filter is tilted.

[0008] As shown in Fig. 25A, the cutoff wavelength of this optical filter is about 1520 nm. Fig. 25B is a graph showing an enlarged view of a portion in Fig. 25A where the transmission loss starts to increase. Of the four solid lines in the graph of Fig. 25B, the solid line 101 located on the shortest wavelength side (left side) indicates the transmission spectrum of light emitted from a certain peripheral core, and the solid line 102 located on the longest wavelength side (right side) indicates the transmission spectrum of light emitted from another certain peripheral core. Note that in this example, the transmission spectra of the multiple peripheral cores are substantially the same and are illustrated overlapping each other, so the number of transmission spectra (solid lines) in the graph does not match the number of cores.

[0009] The transmission spectra shown in FIG. 25B are all shifted to the short wavelength side compared to the transmission spectrum (not shown) of the light emitted from the same core before tilting the optical filter, but the shift amount of each transmission spectrum is different for each core. In this example, the shift amount of transmission spectrum 101 is the largest, and the shift amount of transmission spectrum 102 is the smallest. When each transmission spectrum shifts in this way, the maximum value of the variation of the transmission spectrum (i.e., the variation between the transmission spectrum located on the shortest wavelength side and the transmission spectrum located on the longest wavelength side) increases compared to before tilting the optical filter. In this example, the maximum value of the variation of the transmission spectrum (cutoff wavelength) when the transmission loss is 3 dB is about 0.6 nm.

[0010] In this way, when the transmission spectrum varies among the cores, the transmission loss of light emitted from each core at any given wavelength varies. When the variation in the transmission loss increases, the intensity of the light propagating through each core of the optical fiber (in this example, a multi-core optical fiber) varies, and as a result, the optical signal may not be transmitted properly. As shown in FIG. 25B, the variation in the transmission loss increases as the maximum value of the variation in the transmission spectrum increases. Therefore, in order to transmit the light after passing through the optical filter properly through the optical fiber, it is essential to reduce the maximum value of the variation in the transmission spectrum.

[0011] The present invention has been made to address the above-mentioned problems. That is, an object of the present invention is to provide an optical filter device that can appropriately transmit an optical signal while reducing reflected back light.

[0012] The optical filter device (10) according to the present invention comprises: A first multi-core optical fiber (20) including a plurality of columnar first cores (C1 to C7) extending along an axial direction and a common cladding (21) surrounding the plurality of first cores (C1 to C7); The optical fiber (20) has an optical axis (z-axis) located on the central axis of the first multi-core optical fiber (20), and collimates the light beams emitted and diverging from each of the first cores (C1 to C7), and collimates the light beams from each of the collimated first cores (C1 to C7). To one point A first lens (30) for collecting light; an optical filter (40) comprising: a first surface (40a) on which light rays emitted from the first lens (30) are incident, the first surface being a first surface at which a focal point of a principal ray of the emitted light from the first lens (30) is located; and a second surface (40b) facing the first surface (40a) and from which light rays transmitted through the optical filter (40) are emitted; the optical filter (40) transmits light rays of a specific wavelength band with an arbitrary transmission intensity, the optical filter (40) being rotated by a predetermined rotation angle (η) from a position where the first surface (40a) is parallel to a plane (xy plane) perpendicular to the optical axis (z axis) of the first lens (30) around a rotation axis (r1) located at the focal point and extending in a specific direction perpendicular to the optical axis (z axis); a second lens (50) that converges the light beams from the first cores (C1 to C7) that are output from the optical filter (40); an optical fiber (60) having a columnar core extending along an axial direction, into which all the light beams from the first cores (C1 to C7) emitted from the second lens (50) are incident; Equipped with. In this optical filter device (10), If a direction from a reference axis (y axis) that passes through the optical axis (z axis) and is parallel to the rotation axis (r1) along an orthogonal axis (x axis) that is orthogonal to the optical axis (z axis) and the reference axis (y axis) toward one side of the reference axis (y axis) is defined as a first orthogonal direction (-x axis direction), and a direction toward the other side of the reference axis (y axis) is defined as a second orthogonal direction (+x axis direction), then, The circumferential orientation of the first multi-core optical fiber (20) is set so that, when viewing its end face (20a) along its central axis (z-axis), the separation distance, which is the sum of the distance from the reference axis (y-axis) of the first core (first separated core) that is farthest from the reference axis (y-axis) in the first orthogonal direction (-x-axis direction) and the distance from the reference axis (y-axis) of the first core (second separated core) that is farthest from the reference axis (y-axis) in the second orthogonal direction (+x-axis direction), is minimum. In addition, the optical fiber having a core into which "all light rays from each first core are incident" means an optical fiber having a core into which the outgoing light from each core of the first multi-core optical fiber can be incident. In other words, the reflected return light is not included in the above "all light rays". In addition, in this specification, "collecting" means that a lens collects light rays (strictly speaking, the main rays of light) from multiple light sources (e.g., multiple first cores of a first multi-core optical fiber) to one point, and "converging" means that a lens reduces the diameter of a light ray from one light source (e.g., each first core of a first multi-core optical fiber) and collects it to one point.

[0013] According to the present invention, it is possible to appropriately transmit an optical signal while reducing reflected back light. [Brief description of the drawings]

[0014] [Figure 1] 1 is a plan view of an optical filter device according to a first embodiment of the present invention. [Diagram 2] 1 is a diagram showing an end face of a multi-core optical fiber functioning as an emitting member; [Diagram 3] 1 is a diagram showing an incidence angle α and a ray angle θ of a chief ray incident on an optical filter. [Figure 4] FIG. 2 is a plan view of an optical filter prepared for investigating the relationship between the incident angle α and the wavelength shift amount Δλ, and shows how a light ray R passes through the optical filter. [Diagram 5] 5 is a graph showing transmission loss characteristics based on measurements using the optical filter of FIG. 4. [Figure 6] 1 is a graph defining the relationship between the incident angle α and the wavelength shift amount Δλ. [Figure 7A] 1 is a diagram showing the relationship between the incident angle α, the rotation angle η of the optical filter, the light ray angle θ, and the angle φ of each core in a rectangular coordinate system. [Figure 7B] FIG. 11 is a diagram useful in explaining a method for calculating an angle φ. [Figure 8A] 1 is a diagram showing the number of cores and the core arrangement of a multi-core optical fiber. [Figure 8B] 11 is a diagram showing the number of cores and the core arrangement of another multi-core optical fiber. FIG. [Figure 8C] FIG. 13 is a diagram showing the number of cores and the core arrangement of yet another multi-core optical fiber. [Figure 9A] 8B is a diagram showing an end face of the multi-core optical fiber in FIG. 8A when it is set in an orthogonal type. [Figure 9B] 9B is a graph defining the relationship between the incident angle α of a light beam emitted from each core of the multi-core optical fiber of FIG. 9A and the wavelength shift amount Δλ. [Figure 10A] 8B is a diagram showing an end face of the multi-core optical fiber in FIG. 8A when it is set in a parallel type. [Figure 10B] 10B is a graph defining the relationship between the incident angle α of a light beam emitted from each core of the multi-core optical fiber of FIG. 10A and the wavelength shift amount Δλ. [Figure 11A] 8C is a diagram showing an end face of the multi-core optical fiber in FIG. 8B when it is set in a diagonal shape. [Figure 11B] 11B is a graph defining the relationship between the incident angle α of a light beam emitted from each core of the multi-core optical fiber of FIG. 11A and the wavelength shift amount Δλ. [Figure 12A]8C is a diagram showing an end face of the multi-core optical fiber in FIG. 8B when the multi-core optical fiber is set in a parallel type. [Figure 12B] 12B is a graph defining the relationship between the incident angle α of a light beam emitted from each core of the multi-core optical fiber of FIG. 12A and the wavelength shift amount Δλ. [Figure 13A] 8D is a diagram showing an end face of the multi-core optical fiber in FIG. 8C when it is set in an orthogonal configuration. FIG. [Figure 13B] 13B is a graph defining the relationship between the incident angle α of a light beam emitted from each core of the multi-core optical fiber of FIG. 13A and the wavelength shift amount Δλ. [Figure 14A] 8D is a diagram showing an end face of the multi-core optical fiber in FIG. 8C when it is set in a parallel type. FIG. [Figure 14B] 14B is a graph defining the relationship between the incident angle α of a light beam emitted from each core of the multi-core optical fiber of FIG. 14A and the wavelength shift amount Δλ. [Figure 15] FIG. 4 is a plan view of an optical filter device according to a second embodiment of the present invention. [Figure 16] FIG. 2 is a diagram used to explain the oblique polishing direction of a multi-core optical fiber functioning as an emitting member. [Figure 17A] 1A and 1B are diagrams showing the number and arrangement of cores in a multi-core optical fiber having an obliquely polished end face; [Figure 17B] 13 is a diagram showing the number of cores and the core arrangement of another multi-core optical fiber having an end face bevel-polished. FIG. [Figure 18] 17B is a diagram showing an example of the relative positional relationship between the optical filter and the multi-core optical fiber of FIG. 17A when the rotation angle η is 2.9°; FIG. [Figure 19] 17B is a graph defining the relationship between the oblique polishing rotation angle Ψ of the multi-core optical fiber of FIG. 17A and the maximum value Dmax of the variation when the rotation angle η is 2.9°. [Figure 20] 17B is a diagram showing an example of the relative positional relationship between the optical filter and the multi-core optical fiber of FIG. 17A when the rotation angle η=−2.9°. FIG. [Figure 21]17B is a graph defining the relationship between the oblique polishing rotation angle Ψ of the multi-core optical fiber of FIG. 17A and the maximum value Dmax of the variation when the rotation angle η=−2.9°. [Figure 22] 17B is a graph defining the relationship between the oblique polishing rotation angle Ψ of the multi-core optical fiber of FIG. 17A and the maximum value Dmax of the variation when the rotation angle η is 1.8°. [Diagram 23] 17B is a graph defining the relationship between the oblique polishing rotation angle Ψ of the multi-core optical fiber of FIG. 17A and the maximum value Dmax of the variation when the rotation angle η=−1.8°. [Figure 24] 17C is a graph defining the relationship between the oblique polishing rotation angle Ψ of the multi-core optical fiber of FIG. 17B and the maximum value Dmax of the variation when the rotation angle η is 2.9°. [Figure 25A] 1 is a graph showing the transmission loss characteristics of an optical filter device. [Figure 25B] FIG. 25B is a partially enlarged view of the graph in FIG. 25A. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] (First embodiment) FIG. 1 is a plan view showing an example of an optical filter device 10 according to a first embodiment of the present invention. As shown in FIG. 1, the optical filter device 10 includes a multi-core optical fiber 20 as a first multi-core optical fiber, a first lens 30, an optical filter 40, a second lens 50, and a multi-core optical fiber 60 as an optical fiber. These members are arranged in the above order along an axis A1. An orthogonal coordinate system is set in the optical filter device 10. The z-axis extends on the axis A1 such that the direction from the first lens 30 toward the optical filter 40 is the positive direction. The y-axis is orthogonal to the z-axis (i.e., axis A1) and extends such that the direction toward the front of the paper is the positive direction. The x-axis is orthogonal to the z-axis and the y-axis. Hereinafter, the multi-core optical fiber is also referred to as "MCF". In this specification, the dimensions, angles, and the like of certain members (e.g., MCF 20 and MCF 60) are changed in order to make the drawings easier to understand.

[0016] The MCF 20 is cylindrical, and at least the central axis at the end in the +z-axis direction coincides with the axis A1. The end face 20a of the MCF 20 is parallel to a plane (xy plane) perpendicular to the axis A1. FIG. 2 is a diagram showing the end face 20a of the MCF 20. As shown in FIG. 2, the MCF 20 includes seven cores C1 to C7 as first cores and a common cladding 21 surrounding these cores C1 to C7. The core C4 extends along the central axis of the MCF 20 (hereinafter also referred to as the "central core C4"). The cores C1 to C3 and C5 to C7 are located at the vertices of a regular hexagon centered on the central core C4, and extend along the axis direction (hereinafter also referred to as the "peripheral cores C1 to C3 and C5 to C7", respectively). In other words, the peripheral cores C1 to C3 and C5 to C7 extend along axes other than the central axis of the MCF 20. The distance between adjacent cores (core pitch) is 38 μm. The cores C1 to C7 and the cladding 21 are made of glass containing quartz as a main component. The refractive index of the cores C1 to C7 is greater than that of the cladding 21. The MCF 20 is a single-mode optical fiber. The MCF 20 is an example of a multi-core optical fiber used in the optical filter device 10 (described later). Note that the material of the cores C1 to C7 and the cladding 21 is not limited to glass containing quartz as a main component, and may be other materials. In this specification, the term "cylinder" also includes one with a curved axis.

[0017] 1, the end of the MCF 20 in the +z-axis direction is inserted into and held by a cylindrical ferrule 22. An end face 22a of the ferrule 22 is located on the same plane as an end face 20a of the MCF 20. This is because the end face 20a of the MCF 20 is polished together with the end face 22a in a state where it is inserted into the ferrule 22. In FIG. 1, the MCF 20 in the ferrule 22 is shown by a dashed line, but the cores C1 to C7 are not shown.

[0018] The light beams propagating through each of 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 member. In FIG. 1, among the light beams emitted from each of the cores C1 to C7, only the chief rays of the light beams emitted from the cores C1, C4, and C7 (see FIG. 2) are shown. The light beams B1, B4, and B7 correspond to the chief rays of the light emitted from the cores C1, C4, and C7, respectively. The chief rays of the light emitted from each of the cores C1 to C7 are parallel to each other (see the light beams B1, B4, and B7 in FIG. 1), but each emitted light is a divergent light that diverges as it travels.

[0019] The first lens 30 is an aspheric lens with a focal length of 2.5 mm. The central axis (optical axis) of the first lens 30 is located on the axis A1. The first lens 30 collimates (parallelizes) the light beams emitted from each of the cores C1 to C7 and diverging. In other words, the first lens 30 collimates, for example, the light beam emitted from the core C1 and diverging (only the main light beam B1 is shown in FIG. 1). That is, the first lens 30 is a so-called collimating lens. The first lens 30 collects the light beams from each of the cores C1 to C7 collimated in this way at a focal point (only the light beams B1, B4, and B7 are shown in FIG. 1). Hereinafter, the light beams collimated in this way are also referred to as "collimated light (parallel light)".

[0020] The optical filter 40 is a short-wavelength transmission optical filter. The short-wavelength transmission optical filter is a well-known optical filter made of a dielectric multilayer film, and therefore a detailed description thereof will be omitted. The optical filter 40 has an incident surface 40a as a first surface, and an exit surface 40b as a second surface that faces parallel to the incident surface 40a. The light beam emitted from the first lens 30 is incident on the incident surface 40a. The optical filter 40 is arranged so that the focal point of the first lens 30 is located on the incident surface 40a. Therefore, the light beam from each of the cores C1 to C7 is collected on the incident surface 40a via the first lens 30, passes through the optical filter 40, and is emitted from the exit surface 40b. The lens arranged between the MCF 20 and the optical filter 40 is not limited to the first lens 30, and may be any lens that can substantially collimate the light beam emitted from each of the cores C1 to C7 of the MCF 20. For example, it may be a spherical lens or a GRIN lens.

[0021] The optical filter 40 has a rotation axis r1 extending in the y-axis direction at a position where the axis A1 intersects with the incident surface 40a (i.e., the focal point of the first lens 30). The optical filter 40 is rotated by a rotation angle η around the rotation axis r1 from a position where the incident surface 40a is parallel to a plane perpendicular to the axis A1. The magnitude of the rotation angle η is 0°<η<90°. This reduces reflected back light. In the following, it is defined that when the optical filter 40 is viewed from the +y-axis direction (i.e., when viewed from the direction shown in FIG. 1), the rotation angle η when the optical filter 40 is rotated counterclockwise around the rotation axis r1 has a positive value, and the rotation angle η when the optical filter 40 is rotated clockwise around the rotation axis r1 has a negative value.

[0022] In the example shown in Fig. 1, the optical filter 40 is rotated counterclockwise by a rotation angle η when viewed from the +y-axis direction. Therefore, the position of the light beam emitted from the exit surface 40b is located in the +x-axis direction with respect to the position of the light beam incident on the entrance surface 40a. The light beams from each of the cores C1 to C7 emitted from the optical filter 40 (only the chief ray is shown in Fig. 1) are all collimated light (parallel light), but the chief rays travel in directions away from each other (see the light beams B1, B4, and B7 in Fig. 1). The chief ray B4 emitted from the optical filter 40 is parallel to the axis A1.

[0023] The second lens 50 is an aspheric lens with a focal length of 2.5 mm. The second lens 50 is shifted in the +x-axis direction by a predetermined distance so that the chief ray B4 emitted from the optical filter 40 travels along the central axis (optical axis) of the second lens 50. The second lens 50 is disposed at a position spaced apart from the intersection (not shown in FIG. 1) of the chief rays of the light emitted from the optical filter 40 by the focal length in the +z-axis direction. The second lens 50 refracts the light rays from each of the cores C1 to C7 emitted from the optical filter 40 so that these chief rays are parallel to each other (see the light rays B1, B4, and B7 in FIG. 1). The second lens 50 also converges the light rays from each of the cores C1 to C7 (only the chief rays are shown in FIG. 1).

[0024] The MCF 60 has the same configuration as the MCF 20. That is, the MCF 60 is cylindrical and includes seven cores (not shown) extending along the axial direction and a common cladding (not shown) surrounding these cores. The MCF 60 is a single-mode optical fiber. At least the central axis of the MCF 60 at the end in the -z axis direction coincides with the central axis of the second lens 50. The end in the -z axis direction of the MCF 60 is inserted into and held by a cylindrical ferrule 62. An end face 60a of the MCF 60 is polished together with an end face 62a of the ferrule 62 while being inserted into the ferrule 62. As a result, the end face 60a of the MCF 60 and the end face 62a of the ferrule 62 are located on the same plane (xy plane). In FIG. 1, the MCF 60 in the ferrule 62 is indicated by a dashed line.

[0025] The end face 60a of the MCF 60 is located at a position where the light beams from each of the cores C1 to C7 converge. As a result, the light beams from each of the cores C1 to C7 emitted from the second lens 50 are incident on the corresponding cores of the MCF 60 with low loss. That is, the MCF 20 and the MCF 60 are optically coupled by the first lens 30 and the second lens 50 via the optical filter 40. The lens disposed between the optical filter 40 and the MCF 60 is not limited to the second lens 50, and may be, for example, a spherical lens or a GRIN lens. In addition, when the MCF 20 is replaced with another MCF having a different number of cores and / or core arrangement, it is preferable that the MCF 60 is also replaced with another MCF having the same number of cores and core arrangement as the replaced MCF. Furthermore, the MCF 60 functions as a light receiving member that receives the light emitted from the second lens 50, but the light receiving member is not limited to a multi-core optical fiber. For example, the light receiving member may be a group of single-core optical fibers having the same number of single-mode single-core optical fibers, each core surrounded by an individual cladding, as the number of cores in the MCF 20 (seven in the example of Figure 1). The above is a description of the configuration of the optical filter device 10.

[0026] In such an optical filter device 10, when the optical filter 40 is rotated around the rotation axis r1, the reflected light can be reduced, but the variation in the transmission spectrum of the light beams from each of the cores C1 to C7 of the MCF 20 (particularly the maximum value of the variation) increases, and the variation in the transmission loss at any given wavelength increases, which may result in an inability to properly transmit an optical signal through the MCF 60. This is believed to be because the variation in the incidence angle α of the light beams from each of the cores C1 to C7 to the optical filter 40 increases when the optical filter 40 is rotated.

[0027] A specific description will be given with reference to FIG. 3. FIG. 3 is a diagram showing the angles of incidence α (α1, α2, α3) of the respective principal rays B4, B1, and B7 incident on the optical filter 40 (angle θ will be described later). Note that in FIG. 3, the illustration of the principal rays transmitted through the optical filter 40 and emitted from the emission surface 40b is omitted. As shown in FIG. 3, when the optical filter 40 is rotated counterclockwise around the rotation axis r1 by a rotation angle η (η>0), the normal line N of the optical filter 40 rotates and moves counterclockwise from the axis line A1 by an angle η. As a result, the angle of incidence α1 of the principal ray B4 (light beam from core C4) becomes equal to the angle η (α1=η), the angle of incidence α2 of the principal ray B1 (light beam from core C1) becomes larger than the angle of incidence α1 (α2>α1), and the angle of incidence α3 of the principal ray B7 (light beam from core C7) becomes smaller than the angle of incidence α1 (α3<α1). In contrast, when the optical filter 40 is not rotated (i.e., the incident surface 40a is parallel to the xy plane), the incident angles α2 and α3 are equal to each other (α2=α3) due to the symmetry of the cores C1 and C7 with respect to the core C4. That is, when the optical filter 40 is rotated, the variation between the incident angles α2 and α3 increases. It is considered that the same variation occurs in the incident angles α of the light beams from the cores C2, C3, C5, and C6.

[0028] Therefore, the inventors of the present application have investigated the relationship between the incident angle α of a light beam incident on the optical filter 40 and the wavelength shift amount Δλ (described later) of the transmission spectrum of the light beam, and have studied the circumferential direction of the MCF 20 (and MCFs 120 and 220 described later) that can reduce the maximum value of the variation in the transmission spectrum. This will be described in detail below.

[0029] FIG. 4 is a plan view of an optical filter 40 prepared to examine the relationship between the incident angle α and the wavelength shift amount Δλ. A light ray R is incident on the incident surface 40a of the optical filter 40 along the axis A2. The light ray R passes through the optical filter 40 and is emitted from the emission surface 40b. The optical filter 40 is rotatable by a rotation angle η around a rotation axis r1 with respect to a plane perpendicular to the axis A2. As a result, the incident angle α of the light ray R becomes equal to the rotation angle η. The inventors of the present application measured the transmission loss of the light ray R emitted from the optical filter 40 when the rotation angle η was changed in 0.5° increments within the range of -2.5°≦η≦2.5° (in other words, when the incident angle α of the light ray R was changed in 0.5° increments within the range of -2.5°≦α≦2.5°).

[0030] FIG. 5 is a graph showing the transmission loss characteristics based on the above measurement. In the graph, the solid line L1 shows the transmission spectrum of the light ray R with an incident angle α=0°, the solid lines L2 and L3 show the transmission spectrum of the light ray R with incident angles α=0.5° and -0.5°, respectively, the solid lines L4 and L5 show the transmission spectrum of the light ray R with incident angles α=1° and -1°, respectively, the solid lines L6 and L7 show the transmission spectrum of the light ray R with incident angles α=1.5° and -1.5°, respectively, the solid lines L8 and L9 show the transmission spectrum of the light ray R with incident angles α=2° and -2°, respectively, and the solid lines L10 and L11 show the transmission spectrum of the light ray R with incident angles α=2.5° and -2.5°, respectively. The transmission spectra L2 and L3 are substantially the same and are shown superimposed.

[0031] As shown in FIG. 5, when the magnitude (absolute value) of the incident angle α is the same, the behavior of the transmission spectrum is similar (for example, solid line L10 and solid line L11). Moreover, all of the transmission spectra L2 to L11 are shifted toward the short wavelength side from the transmission spectrum L1, and the shift amount increases as the incident angle α increases. Hereinafter, the wavelength shift amount by which the transmission spectra L2 to L11 are shifted from the transmission spectrum L1 when the transmission loss is 3 dB (i.e., transmittance 50%) is defined as the "wavelength shift amount Δλ." In other words, the wavelength shift amount Δλ is "the wavelength shift amount of the transmission spectrum of the light beam with the incident angle α from the transmission spectrum of the light beam with the incident angle of 0° when the transmission loss is 3 dB."

[0032] Figure 6 is a graph that specifies the relationship between the incident angle α and the wavelength shift amount Δλ. The 11 measured values ​​in the graph are values ​​plotted based on the graph in Figure 5. Also shown are the results of calculating the relationship between the incident angle α and the wavelength shift amount Δλ based on the following analytical formula (1). Here, analytical formula (1) was derived with reference to the following literature. Mitsunobu Kohiyama, "Optical Thin Film Filter Design", 1st Edition, Optronics Co., Ltd., 2006, pp.301-346

number

[0033] Here, in analytical formula (1), λ0 represents the cutoff wavelength (=1522.4 nm) when the incident angle α is 0°, n1 represents the refractive index in air (=1.0), and n2 represents the refractive index of the optical filter 40. n2 is set to 1.65 in order to fit the analytical value to the actual measured value. According to the graph in FIG. 6, analytical formula (1) closely matches the behavior of the actual measured value. For this reason, in the following discussion, the wavelength shift amount Δλ will be calculated based on analytical formula (1). Also, according to analytical formula (1), the wavelength shift amount Δλ increases as the incident angle α increases.

[0034] In this example, the wavelength shift amount Δλ is defined as the wavelength shift amount when the transmission loss is 3 dB, but is not limited thereto. For example, the wavelength shift amount Δλ may be defined as the wavelength shift amount when the transmission loss is 2 dB, 4 dB, or 5 dB. This is because, as shown in FIG. 5, in the transmission loss range of at least 2 dB to 5 dB, the transmission spectra 1 to 11 are substantially parallel to each other, and therefore the wavelength shift amount of each of the transmission spectra 2 to 11 is substantially the same as the wavelength shift amount Δλ when the transmission loss is 3 dB (i.e., it is in good agreement with analytical formula (1)).

[0035] Incidentally, the incident angle α can be derived from the "rotation angle η of the optical filter 40", the "beam angle θ (see FIG. 3) that the light beam emitted from the first lens 30 forms with the axis A1 at an acute angle", and the "angle φ (described later) of each of the peripheral cores C1 to C3 and C5 to C7". A description will be given with reference to FIG. 7A and FIG. 7B. FIG. 7A is a diagram showing the relationship between the incident angle α, the rotation angle η, the beam angle θ, and the angle φ in an orthogonal coordinate system, and FIG. 7B is a diagram showing the end face 20a of the MCF 20. As shown in FIG. 7B, the angle φ of any peripheral core is defined as a deviation angle. The deviation angle is the angle from the positive part of the x-axis to the line segment connecting the center of any peripheral core and the origin. In the example of FIG. 7B, the angles φ of the peripheral cores C1, C2, C5, C7, C6, and C3 are 0°, 60°, 120°, 180°, 240°, and 300°, respectively. The angle φ is not defined for the central core C4. In FIG. 7A, the angle φ is illustrated as an angle that increases counterclockwise from the "negative" portion of the x-axis as viewed from the +z-axis direction. This is because the light rays from each of the peripheral cores C1 to C3 and C5 to C7 of the MCF 20 are refracted by the first lens 30.

[0036] Vector B in Fig. 7A is a ray vector representing a ray traveling from any of the cores C1 to C7 through the first lens 30. If a half line extending from the origin in the direction of angle φ is defined as half line b, the ray vector B is located on a plane passing through the half line b and the z-axis, and forms a ray angle θ with a unit vector ez (not shown) extending in the +z-axis direction. For this reason, the ray vector B can be expressed as the following formula (2).

number

[0037] The vector N in FIG. 7A is a normal vector from the exit surface 40b of the optical filter 40. The normal vector N is located on the zx plane and forms a rotation angle η with the unit vector ez. Note that in FIG. 7A, the normal vector N is defined as a normal vector from the exit surface 40b, and the direction is opposite to that of the normal N shown in FIG. 3 and FIG. 4. This is because it is easier to explain by defining the angle with the unit vector ez extending in the +z axis direction as the rotation angle η, and it should be noted that it has the same value as the rotation angle η shown in FIG. 3 and FIG. 4. The normal vector N can be expressed as the following formula (3).

number

[0038] The incidence angle α is the angle between the light vector B (equation (2)) and the normal vector N (equation (3)). Therefore, by the definition of the dot product, the incidence angle α can be derived as the following equation (4).

number

[0039] In addition, since the ray angle θ of the light from the central core C4 is 0°, when calculating the incident angle α of the light from the central core C4, the value of the second term in the parentheses in formula (4) is 0 (because the value of sinθ is 0). That is, in this case, the angle φ is not included in formula (4). Therefore, there is no particular problem even if the angle φ is not defined for the central core C4, and the incident angle α of the light from the central core C4 can be appropriately calculated using formula (4).

[0040] From the above, it can be seen that the incidence angle α of the light beam from each of the cores C1 to C7 to the optical filter 40 can be calculated from the rotation angle η of the optical filter 40, the light beam angle θ of the light beam, and the angle φ of the core corresponding to the light beam. By calculating the incidence angle α, the wavelength shift amount Δλ can be calculated using formula (1). The inventors of the present application applied MCFs having various numbers of cores and core arrangements to the optical filter device 10, and investigated the circumferential direction that can reduce the maximum value of the variation in the transmission spectrum using formulas (4) and (1).

[0041] 8A to 8C are diagrams showing the end faces of the MCF used in the study. The MCF in FIG. 8A is MCF20. The configuration of MCF20 has been described with reference to FIG. 2, and detailed description will be omitted. The light beam angle θ of the light beam from the central core C4 is 0°. The light beam angles θ of the light beams from the peripheral cores C1 to C3 and C5 to C7 are equal to each other due to symmetry with respect to the light beam from the central core C4, and are 0.87° each (see FIG. 1). The rotation angle η is set to 2.9°. This is the same in the examples of FIG. 8B and FIG. 8C. The peripheral cores C1 to C3 and C5 to C7 correspond to an example of the "outermost core".

[0042] The MCF in FIG. 8B is MCF120, which differs from MCF20 only in the number of cores and the core arrangement. MCF120 includes four cores C1 to C4 as first cores and a common cladding 121 surrounding these cores C1 to C4. The cores C1 to C4 are located at the vertices of a square centered on the center of the end face 120a. That is, the cores C1 to C4 are "peripheral cores". The core pitch is 50 μm. The beam angles θ of the beams from the cores C1 to C4 are equal to each other due to symmetry with respect to the central axis of MCF120, and are each 0.81°. The cores C1 to C4 correspond to an example of an "outermost core".

[0043] The MCF in FIG. 8C is MCF220, which differs from MCF120 only in the core arrangement. MCF220 includes four cores C1 to C4 as first cores and a common cladding 221 surrounding these cores C1 to C4. Cores C1 to C4 are linearly arranged in two-fold symmetry with respect to the center of end face 220a. That is, cores C1 to C4 are "peripheral cores". The core pitch is 50 μm. The beam angles θ of the beams from cores C1 and C4 are equal to each other due to symmetry with respect to the central axis of MCF220, and are 1.7° each. The beam angles θ of the beams from cores C2 and C3 are equal to each other due to symmetry with respect to the central axis of MCF220, and are 0.57° each.

[0044] FIG. 9A is a diagram showing the end face 20a when the circumferential direction of the MCF 20 (see FIG. 8A) is set so that the straight line passing through the cores C1, C4, and C7 is perpendicular to the y-axis (i.e., the axis passing through the axis A1 and parallel to the rotation axis r1) as the reference axis. Hereinafter, such an orientation is also referred to as the "orthogonal type." Hereinafter, if the core farthest from the y-axis in the -x-axis direction is defined as the "first separated core," and the core farthest from the y-axis in the +x-axis direction is defined as the "second separated core," in this example, the first separated core is the core C7, and the second separated core is the core C1. In addition, if the sum of the "distance from the y-axis to the first separated core" and the "distance from the y-axis to the second separated core" is defined as the "separation distance," in this example, the separation distance is 75 μm. The -x-axis direction corresponds to an example of a "first orthogonal direction" that faces one side of the y-axis along the x-axis as an orthogonal axis, and the +x-axis direction corresponds to an example of a "second orthogonal direction" that faces the other side of the y-axis along the x-axis. In this embodiment, the rotation axis r1 also passes through the axis A1, so the rotation axis r1 may be used as the reference axis.

[0045] 9B is a graph defining the relationship between the incident angle α of the light beam from each of the cores C1 to C7 and the wavelength shift amount Δλ when the MCF 20 is set to the orthogonal type. The incident angle α can be calculated from the formula (4), and the wavelength shift amount Δλ can be calculated from the formula (1). For example, the wavelength shift amount Δλ of the light beam from the core C7 can be calculated by first substituting η=2.9°, θ=0.87°, and φ=180° into the formula (4) to calculate the incident angle α, and then substituting the incident angle α into the formula (1) together with the other physical quantities described above. The dashed line 70 in the graph represents the analytical formula (1).

[0046] According to the graph of FIG. 9B, the incident angle α of the light beam from the first separated core C7 is minimum, and the incident angle α of the light beam from the second separated core C1 is maximum. Therefore, the wavelength shift amount Δλ of the light beam from the first separated core C7 is minimum, and the wavelength shift amount Δλ of the light beam from the second separated core C1 is maximum. The maximum value of the variation of the transmission spectrum is equal to the difference between the minimum value Δλmin and the maximum value Δλmax of the wavelength shift amount Δλ. Hereinafter, this difference (Δλmax-Δλmin) is referred to as the "maximum value Dmax of the variation of the transmission spectrum" or simply as the "maximum value Dmax of the variation" or "Dmax". The maximum value Dmax of the variation of the transmission spectrum when the MCF20 is set to the orthogonal type was 0.87 nm. The incident angle α of the light beam from the central core C4 is equal to the rotation angle η (see FIG. 3 and FIG. 4). Therefore, according to the graph of FIG. 9B, the incident angle α of the light beam from the central core C4 is 2.9°. The same is true for FIG. 10B.

[0047] 10A is a diagram showing the end face 20a when the circumferential direction of the MCF 20 is set so that the straight line passing through the cores C1, C4, and C7 is parallel to the y-axis. Hereinafter, such an orientation is also referred to as a "parallel type." In this example, the first separated cores are the cores C2 and C5, and the second separated cores are the cores C3 and C6. The separation distance is 65 μm.

[0048] Fig. 10B is a graph defining the relationship between the incident angle α of the light beams from each of the cores C1 to C7 and the wavelength shift amount Δλ when the MCF 20 is set to the parallel type. According to the graph of Fig. 10B, the incident angle α of the light beams from the first separated cores C2 and C5 is the smallest, and the incident angle α of the light beams from the second separated cores C3 and C6 is the largest. Therefore, the wavelength shift amount Δλ of the light beams from the first separated cores C2 and C5 is the smallest, and the wavelength shift amount Δλ of the light beams from the second separated cores C3 and C6 is the largest. The maximum value Dmax of the variation in the transmission spectrum when the MCF 20 is set to the parallel type was 0.75 nm.

[0049] 9A to 10B, the separation distance (65 μm) in the parallel type is shorter than the separation distance (75 μm) in the orthogonal type. Also, since the variation in the incident angle α in the parallel type is smaller than the variation in the incident angle α in the orthogonal type, the variation in the wavelength shift amount Δλ (i.e., Dmax) is smaller in the parallel type.

[0050] From the above, it is found that there is a correlation between the position of the core in the x-axis direction and the incident angle α. More specifically, the shorter the separation distance, the smaller the variation in the incident angle α, and as a result, it is found that the variation in the wavelength shift amount Δλ (Δλmax-Δλmin) can be reduced, that is, the maximum value Dmax of the variation in the transmission spectrum can be reduced. The orthogonal type is the circumferential orientation of the MCF 20 when the separation distance is maximum (i.e., Dmax is maximum), and the parallel type is the circumferential orientation of the MCF 20 when the separation distance is minimum (i.e., Dmax is minimum). Dmax (0.75 nm) in the parallel type is about 13% smaller than Dmax (0.87 nm) in the orthogonal type. This means that Dmax can be reduced by a maximum of about 13% by setting the circumferential orientation of the MCF 20 to the parallel type.

[0051] 9A and 10A, the peripheral cores C1 to C3 and C5 to C7 are line-symmetrical with respect to the y-axis whether the MCF 20 is set to the orthogonal type or the parallel type. However, when the MCF 20 is set to the 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, whereas when the MCF 20 is set to the parallel type, none of the peripheral cores are located on the x-axis. From this, the circumferential direction of the MCF 20 when the separation distance is minimized (i.e., Dmax is minimized) can also be specified as follows. That is, in an MCF (MCF20) having a "core arrangement in which 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 the type (parallel type) in which the peripheral cores are not located on the x-axis out of the two types (orthogonal type and parallel type) in which the peripheral cores are line-symmetrical with respect to the y-axis.

[0052] 11A is a diagram showing end face 120a when the circumferential direction of MCF 120 (see FIG. 8B) is set so that a straight line passing through cores C2 and C4 is perpendicular to the y-axis. Hereinafter, such an orientation is also referred to as a "diagonal type." In this example, the first separated core is core C2, and the second separated core is core C4. The separation distance is 71 μm.

[0053] Fig. 11B is a graph defining the relationship between the incident angle α of the light beam from each of the cores C1 to C4 and the wavelength shift amount Δλ when the MCF 120 is set to a diagonal line type. According to the graph of Fig. 11B, the incident angle α and the wavelength shift amount Δλ of the light beam from the first separated core C2 are minimum, and the incident angle α and the wavelength shift amount Δλ of the light beam from the second separated core C4 are maximum. The maximum value Dmax of the variation in the transmission spectrum when the MCF 120 is set to a diagonal line type is 0.81 nm.

[0054] 12A is a diagram showing end face 120a when the circumferential direction of MCF 120 is set so that a straight line passing through cores C2 and C3 (or cores C1 and C4) is parallel to the y-axis. Hereinafter, such an orientation is also referred to as a "parallel type." In this example, the first separated cores are cores C2 and C3, and the second separated cores are cores C1 and C4. The separation distance is 50 μm.

[0055] Fig. 12B is a graph defining the relationship between the incident angle α of the light beam from each of the cores C1 to C4 and the wavelength shift amount Δλ when the MCF 120 is set to the parallel type. According to the graph of Fig. 12B, the incident angle α and the wavelength shift amount Δλ of the light beam from the first separated cores C2 and C3 are minimum, and the incident angle α and the wavelength shift amount Δλ of the light beam from the second separated cores C1 and C4 are maximum. The maximum value Dmax of the variation of the transmission spectrum when the MCF 120 is set to the parallel type is 0.57 nm.

[0056] 11A to 12B, the separation distance (50 μm) for the parallel type is shorter than the separation distance (71 μm) for the diagonal type. Also, since the variation in the incident angle α for the parallel type is smaller than the variation in the incident angle α for the diagonal type, the variation in the wavelength shift amount Δλ (i.e., Dmax) is smaller for the parallel type.

[0057] From the above, it can be seen that, as in the case of MCF20, the shorter the separation distance, the smaller the variation in the incident angle α, and as a result, the maximum value Dmax of the variation in the transmission spectrum can be reduced. The diagonal type is the circumferential orientation of MCF120 when the separation distance is maximum, and the parallel type is the circumferential orientation of MCF120 when the separation distance is minimum. Dmax (0.53 nm) in the parallel type is approximately 29% smaller than Dmax (0.81 nm) in the diagonal type. This means that Dmax can be reduced by approximately 29% at most by setting the circumferential orientation of MCF120 to the parallel type.

[0058] 11A and 12A, the peripheral cores C1 to C4 are line-symmetrical with respect to the y-axis whether the MCF 120 is set to a diagonal type or a parallel type. However, when the MCF 120 is set to a diagonal type, two of the four peripheral cores (cores C1 to C4) (cores C2 and C4) are located on the x-axis, whereas when the MCF 120 is set to a parallel type, none of the peripheral cores are located on the x-axis. From this, the circumferential direction of the MCF 120 when the separation distance is minimized can be specified as follows, similar to the case of the MCF 20. That is, in an MCF (MCF 120) having a "core arrangement in which 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 the type (parallel type) in which the peripheral cores are not located on the x-axis out of the two types (diagonal type and parallel type) in which the peripheral cores are line-symmetrical with respect to the y-axis.

[0059] Fig. 13A is a diagram showing end face 220a when the circumferential direction of MCF 220 (see Fig. 8C) is set so that a straight line passing through cores C1 to C4 is perpendicular to the y-axis. Hereinafter, such an orientation is also referred to as "orthogonal type". In this example, the first separated core is core C1, and the second separated core is core C4. The separation distance is 150 μm.

[0060] Fig. 13B is a graph defining the relationship between the incident angle α of the light beam from each of the cores C1 to C4 and the wavelength shift amount Δλ when the MCF 220 is set to the orthogonal type. According to the graph in Fig. 13B, the incident angle α and the wavelength shift amount Δλ of the light beam from the first separated core C1 are minimum, and the incident angle α and the wavelength shift amount Δλ of the light beam from the second separated core C4 are maximum. The maximum value Dmax of the variation of the transmission spectrum when the MCF 220 is set to the orthogonal type is 1.7 nm.

[0061] 14A is a diagram showing the end face 220a when the circumferential direction of the MCF 220 is set so that a straight line passing through the cores C1 to C4 is parallel to the y-axis. Hereinafter, such an orientation is also referred to as a "parallel type." In the parallel type, the cores C1 to C4 are arranged along the y-axis, and therefore the cores C1 to C4 are both the first separated cores and the second separated cores. Therefore, the separation distance is 0 μm.

[0062] Fig. 14B is a graph defining the relationship between the incident angle α of the light beam from each of the cores C1 to C4 and the wavelength shift amount Δλ when the MCF 220 is set to the parallel type. According to the graph in Fig. 14B, the incident angle α and the wavelength shift amount Δλ of the light beam from the cores C2 and C3 (i.e., the cores closer to the axis A1 (z-axis)) are minimum, and the incident angle α and the wavelength shift amount Δλ of the light beam from the cores C1 and C4 (i.e., the cores farther from the axis A1) are maximum. The maximum value Dmax of the variation in the transmission spectrum when the MCF 220 is set to the parallel type was 0.23 nm.

[0063] 13A to 14B, the separation distance (0 μm) in the parallel type is shorter than the separation distance (150 μm) in the perpendicular type. Also, since the variation in the incident angle α in the parallel type is smaller than the variation in the incident angle α in the perpendicular type, the variation in the wavelength shift amount Δλ (i.e., Dmax) is smaller in the parallel type.

[0064] From the above, it can be seen that, as in the cases of MCF20 and MCF120, the shorter the separation distance, the smaller the variation in the incident angle α, and as a result, the maximum value Dmax of the variation in the transmission spectrum can be reduced. The orthogonal type is the circumferential orientation of MCF220 when the separation distance is maximum, and the parallel type is the circumferential orientation of MCF220 when the separation distance is minimum. Dmax (0.23 nm) in the parallel type is approximately 87% smaller than Dmax (1.7 nm) in the orthogonal type. This means that Dmax can be reduced by approximately 87% at most by setting the circumferential orientation of MCF220 to the parallel type.

[0065] 13A and 14A, the peripheral cores C1 to C4 are line-symmetric with respect to the y-axis whether the MCF220 is set to the orthogonal type or the parallel type. However, when the MCF220 is set to the orthogonal type, all of the four peripheral cores (cores C1 to C4) are located on the x-axis, whereas when the MCF220 is set to the parallel type, none of the peripheral cores are located on the x-axis. From this, the circumferential direction of the MCF220 when the separation distance is minimized can be specified as follows, similarly to the cases of the MCF20 and 120. That is, in an MCF (MCF220) having "a core arrangement in which 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 the type (parallel type) in which the peripheral cores are not located on the x-axis out of the two types (orthogonal type and parallel type) in which the peripheral cores are line-symmetric with respect to the y-axis.

[0066] According to the above study, when the MCF 20, 120, or 220 is applied to the optical filter device 10, it is desirable to set the circumferential orientation of these to be parallel. This minimizes the distance between the MCF 20, 120, and 220, and therefore minimizes the variation in the incidence angle α of the light beam from each core to the optical filter 40. Therefore, the maximum value Dmax of the variation in the transmission spectrum of the light beam from each core can be minimized, and the occurrence of variation in the transmission loss of these light beams at any given wavelength can be suppressed to the maximum. As a result, the optical signal can be appropriately transmitted while reducing the reflected back light.

[0067] In addition, when the MCF has a core arrangement in which the peripheral cores are located at the vertices of a regular polygon centered on the center of the end face of the MCF, the MCF may have one or more other peripheral cores as inner cores radially inward of the peripheral cores. The inner cores do not need to be symmetrical and may be arranged in any way. This is because the minimum separation distance is sufficient to minimize the maximum value Dmax of the variation, and the separation distance is a value that is determined regardless of (without being affected by) the core arrangement of the inner cores.

[0068] The number of cores and the core arrangement of the MCF are not limited to the examples shown in Figs. 8A to 8C. The core arrangement of the MCF does not have to be symmetrical with respect to the center of the end face of the MCF. In other words, the first and second separated cores may be shifted from each other in the y-axis direction, and the distance from the y-axis to the first separated core may be different from the distance from the y-axis to the second separated core. Even in such a case, the circumferential direction of the MCF is set so that the separation distance is minimized, thereby minimizing the variation in the incident angle α, and the above-mentioned effect can be achieved.

[0069] Furthermore, the optical filter 40 may be rotated so that the rotation angle η has a negative value. Even in this case, the above-mentioned effects can be achieved by setting the circumferential direction of the MCF so as to minimize the separation distance.

[0070] Second embodiment Fig. 15 is a plan view showing an example of an optical filter device 310 according to a second embodiment of the present invention. In this embodiment, the same members as those in the first embodiment are denoted by the same reference numerals, and detailed description of the same configuration as in the first embodiment is omitted. As shown in Fig. 15, the optical filter device 310 is different from the optical filter device 10 in that it includes an MCF 320 and a ferrule 322 instead of the MCF 20 and the ferrule 22, and includes an MCF 360 and a ferrule 362 instead of the MCF 60 and the ferrule 62. In addition, the position of the second lens 50 is different from that in the optical filter device 10.

[0071] As shown in Fig. 15, the end face 320a of the MCF 320 is polished obliquely (oblique polished) so as to be inclined at a predetermined polishing angle (e.g., 8°) in a predetermined inclination direction with respect to the xy plane. This reduces reflected back light at the end face 320a. The end face 320a has an elliptical shape when viewed from a direction perpendicular thereto. Hereinafter, the direction from the distal end, which is farther away from the optical filter 40, to the proximal end, which is closer to the optical filter 40, along the long axis of the end face 320a (an example of an oblique polishing reference axis), when viewed along the central axis of the end face 320a, is defined as the "oblique polishing direction".

[0072] FIG. 16 is a diagram showing the end surface 320a. The solid arrow 81 in FIG. 16 indicates the oblique grinding direction of the end surface 320a, and the dashed arrow 80 indicates the reference direction that is the calculation reference for the oblique grinding angle described later. As shown in FIG. 16, the reference direction 80 extends along the +y axis direction. Hereinafter, an angle that an arbitrary oblique grinding direction forms counterclockwise with the reference direction 80 is defined as a "positive value oblique grinding rotation angle Ψ". In this example, the oblique grinding direction 81 forms an angle of 270° counterclockwise with the reference direction 80, so the oblique grinding rotation angle Ψ of the oblique grinding direction 81 is 270° (in other words, -90°).

[0073] 15, an end portion of the MCF 320 in the +z-axis direction is inserted into and held by a cylindrical ferrule 322. An end face 320a of the MCF 320 is obliquely polished together with an end face 322a of the ferrule 322. However, an end portion 322a1 of the end face 322a of the ferrule 322 in the oblique polishing direction 81 is not obliquely polished and is parallel to the xy plane. The end portion 322a1 is a so-called polishing allowance.

[0074] The MCF 320 includes seven cores C1 to C7 and a common cladding 321 surrounding these cores C1 to C7. The MCF 320 has a core arrangement similar to that of the MCF 20. The circumferential direction of the MCF 320 is set to be parallel (i.e., the direction in which the separation distance is minimized).

[0075] The light beams propagating through each of the cores C1 to C7 of the MCF 320 are emitted from the end face 320a towards the first lens 30. Fig. 15 shows only the chief rays B3, B4 and B2 of the light beams emitted from the cores C3, C4 and C2 (see Fig. 16), respectively. The chief rays B3, B4 and B2 travel at an incline towards the oblique polishing direction 81 (in this example, towards the +x-axis direction), and are therefore inclined with respect to the axis A1.

[0076] The first lens 30 is disposed at a position spaced apart from the center of the central core C4 on the end face 320a by the focal length in the +z-axis direction. Therefore, the chief ray B4 emitted from the first lens 30 is parallel to the axis A1. The first lens 30 collimates and collects the light rays from each of the cores C1 to C7.

[0077] The optical filter 40 is disposed so that the focal point of the light emitted from the first lens 30 is located on the incident surface 40a. The light beam incident on the incident surface 40a passes through the optical filter 40 and is emitted from the exit surface 40b. The optical filter 40 has a rotation axis r2 extending in the y-axis direction at the position where the light beam is focused on the incident surface 40a. The optical filter 40 is rotated by a rotation angle η around the rotation axis r2 from a position parallel to the xy plane. This reduces reflected back light. The chief ray B4 emitted from the optical filter 40 is parallel to the axis A1.

[0078] The second lens 50 is disposed so that its central axis coincides with the axis A1. The second lens 50 refracts the light beams from each of the cores C1 to C7 emitted from the optical filter 40 so that their chief rays are parallel to each other (see light beams B3, B4, and B2 in FIG. 15). The second lens 50 also converges the light beams from each of the cores C1 to C7 (only the chief rays are shown in FIG. 15).

[0079] The MCF 360 has the same configuration as the MCF 320. An end portion of the MCF 360 in the -z-axis direction is inserted into and held by a cylindrical ferrule 362. An end face 360a of the MCF 360 is obliquely polished together with an end face 362a of the ferrule 362. This reduces reflected return light at the end face 360a. The end face 362a of the ferrule 362 has a polishing allowance at an end 362a1 in the oblique polishing direction. The MCF 360 is in line symmetry with the MCF 320 with respect to the x-axis.

[0080] The positional relationship between the second lens 50 and the MCF 360 is determined so that the end face 360a is located at a position where the light beams emitted from the second lens and the cores C1 to C7 converge. The above is a description of the configuration of the optical filter device 310.

[0081] The inventors of the present application applied two types of MCFs, described below, whose circumferential orientations are set to be parallel, to an optical filter device 310, and by changing the oblique polishing rotation angle Ψ in the range of 0°≦Ψ≦360°, they considered the relationship between the oblique polishing rotation angle Ψ and the maximum value Dmax of the variation in the transmission spectrum.

[0082] 17A and 17B are diagrams showing the end face of the MCF used in the study. The MCF in FIG. 17A is MCF320. The configuration of MCF320 has been described with reference to FIG. 16, so detailed description will be omitted. The oblique polishing direction 82 can be rotated 360° counterclockwise from a reference direction 80 (not shown). The light ray angle θ of the light ray from the central core C4 is 0° (see FIG. 15). The average value of the light ray angle θ of the light ray from the peripheral cores C1 to C3 and C5 to C7 is 0.87°. In this example, the rotation angle η is set to four values ​​of η = ±2.9° and ±1.8°.

[0083] The MCF in FIG. 17B is MCF420, which is different from MCF320 only in the number of cores and the core arrangement. MCF420 has four cores C1 to C4 and a common cladding 421 surrounding these cores C1 to C4. MCF420 has the same core arrangement as MCF220. The oblique polishing direction 83 can be rotated 360° counterclockwise from a reference direction 80 (not shown). The beam angles θ of the beams from the cores C1 and C4 are each 1.7°. The beam angles θ of the beams from the cores C2 and C3 are each 0.57°. In this example, the rotation angle η is set to 2.9°.

[0084] Fig. 18 is a diagram showing an example of the relative positional relationship between the optical filter 40 and the obliquely polished MCF 320 (see Fig. 17A) when the rotation angle η = 2.9°. In this example, the end face 320a of the MCF 320 is obliquely polished so that the oblique polishing rotation angle Ψ is 90°. In Fig. 18, the illustration of the light beam that passes through the optical filter 40 and is emitted from the emission surface 40b is omitted. The same applies to Fig. 20.

[0085] FIG. 19 is a graph defining the relationship between the "angle of oblique polishing rotation Ψ" and the "maximum value of variation Dmax" when the angle of oblique polishing rotation Ψ of the MCF 320 shown in FIG. 18 is changed in the range of -180°≦Ψ≦180°. The solid line 90 in the graph indicates the maximum value of variation Dmax when an MCF whose end face is not obliquely polished (i.e., the parallel type MCF 20 of the first embodiment (see FIGS. 10A and 10B)) is used. Note that in FIG. 10B, Dmax=0.75 nm, but this is because the value is written with two significant digits, and the strict numerical value is slightly smaller than 0.75 nm, as shown by the solid line 90. According to FIG. 19, when 0°<Ψ<180°, Dmax of the obliquely polished MCF 320 is equal to or smaller than Dmax of the parallel type MCF 20 that is not obliquely polished.

[0086] 20 is a diagram showing an example of the relative positional relationship between the optical filter 40 and the obliquely polished MCF 320 when the rotation angle η is -2.9°. In this example, the end face 320a is obliquely polished so that the oblique polishing rotation angle Ψ is -90°.

[0087] Fig. 21 is a graph defining the relationship between the "angle of oblique polishing rotation Ψ" and the "maximum value of variation Dmax" when the angle of oblique polishing rotation Ψ of the MCF 320 shown in Fig. 20 is changed in the range of -180°≦Ψ≦180°. According to Fig. 21, when -180°<Ψ<0°, Dmax of the obliquely polished MCF 320 is equal to or smaller than Dmax of the parallel-type MCF 20 that is not obliquely polished.

[0088] Fig. 22 is a graph defining the relationship between the "angle of oblique polishing rotation Ψ" and the "maximum value of variation Dmax" when the rotation angle η of the optical filter 40 shown in Fig. 18 is changed to 1.8° and the angle of oblique polishing rotation Ψ of the MCF 320 is changed in the range of -180°≦Ψ≦180°. A solid line 91 in the graph indicates the maximum value of variation Dmax when an MCF whose end face is not obliquely polished (i.e., the parallel-type MCF 20 of the first embodiment) is used. According to Fig. 22, when 0°<Ψ<180°, the Dmax of the obliquely polished MCF 320 is equal to or smaller than the Dmax of the parallel-type MCF 20 that is not obliquely polished.

[0089] Fig. 23 is a graph defining the relationship between "angle of oblique polishing rotation Ψ" and "maximum value of variation Dmax" when the rotation angle η of the optical filter 40 shown in Fig. 20 is changed to -1.8° and the angle of oblique polishing rotation Ψ of the MCF 320 is changed in the range of -180°≦Ψ≦180°. According to Fig. 23, when -180°<Ψ<0°, Dmax of the obliquely polished MCF 320 is equal to or smaller than Dmax of the parallel-type MCF 20 that is not obliquely polished.

[0090] From the above, it is understood that there is a certain tendency between the oblique polishing rotation angle Ψ and the maximum variation Dmax. More specifically, it is understood that when the rotation angle η has the same magnitude but different sign, the behavior of Dmax is linearly symmetrical with respect to Ψ=0°. It is also understood that when the rotation angle η has a positive value (see FIGS. 19 and 22), the maximum variation Dmax can be maintained at the same level or reduced by obliquely polishing the MCF 320 so that the oblique polishing rotation angle Ψ satisfies 0°<Ψ<180°. On the other hand, when the rotation angle η has a negative value (see FIGS. 21 and 23), it is understood that the maximum variation Dmax can be maintained at the same level or reduced by obliquely polishing the MCF 320 so that the oblique polishing rotation angle Ψ satisfies -180°<Ψ<0°.

[0091] In contrast, FIG. 24 is a graph defining the relationship between the "angle of oblique polishing rotation Ψ" and the "maximum value of variation Dmax" when the angle of oblique polishing rotation Ψ of the MCF 420 (see FIG. 17B) is changed in the range of -180°≦Ψ≦180° when the rotation angle η is 2.9°. A solid line 92 in the graph indicates the maximum value of variation Dmax when an MCF whose end face is not obliquely polished (i.e., the parallel type MCF 220 of the first embodiment (see FIGS. 14A and 14B)) is used. Note that in FIG. 14B, Dmax=0.23 nm, but this is because the value is written with two significant digits, and the strict numerical value is slightly smaller than 0.23 nm, as shown by the solid line 92. According to FIG. 24, when the rotation angle η has a positive value, Dmax of the obliquely polished MCF 420 is larger than Dmax of the parallel type MCF 220 that is not obliquely polished.

[0092] According to the line symmetry of the behavior of Dmax when MCF320 is used, it is estimated that the "behavior of Dmax when η = -2.9°" when MCF420 is used is equivalent to the behavior of "the behavior of Dmax when η = 2.9° (see FIG. 24)" inverted with respect to Ψ = 0°. Here, according to FIG. 24, the behavior of Dmax when η = 2.9° is line symmetric with respect to Ψ = 0°. Therefore, when MCF420 is used, it is estimated that the "behavior of Dmax when η = -2.9°" coincides with the "behavior of Dmax when η = 2.9°". According to this estimation, even when the rotation angle η has a negative value, it is estimated that the Dmax of the obliquely polished MCF420 becomes larger than the Dmax of the parallel type MCF220 that is not obliquely polished. In other words, it is estimated that when MCF420 is obliquely polished, the maximum value of the variation Dmax increases regardless of the sign of the rotation angle η.

[0093] From this, it can be seen that when the "separation distance is greater than zero" as in the core arrangement of MCF320, the maximum value of variation Dmax can be maintained at the same level or reduced by "angle-polishing MCF320 so that 0°<Ψ<180° is satisfied when η>0, and angle-polishing MCF320 so that -180°<Ψ<0° is satisfied when η<0" as compared to a configuration without angle-polishing, but when the "separation distance is zero" as in the core arrangement of MCF420, such an effect cannot be obtained.

[0094] According to the above considerations, when the circumferential orientation of the MCF is set so that the separation distance is minimized (see the first embodiment), if the distance is greater than zero, it is desirable to obliquely polish the MCF within the range of the oblique polishing rotation angle Ψ according to the sign of the rotation angle η, as described above. This makes it possible to maintain or further reduce the maximum value of variation Dmax as compared to a configuration without oblique polishing, and therefore to appropriately transmit optical signals while suppressing reflected back light. On the other hand, when the separation distance is zero, it is desirable not to obliquely polish the MCF in order to prevent the maximum value of variation Dmax from increasing.

[0095] The MCF may have an inner core. The number and arrangement of the cores of the MCF are not limited to the examples shown in Fig. 17A and Fig. 17B. The arrangement of the cores of the MCF does not have to be symmetrical with respect to the center of the end face of the MCF. Even in this case, when the separation distance is greater than zero, the variation in the incident angle α can be further reduced by obliquely polishing the MCF in the above-mentioned obliquely polishing direction, and the above-mentioned effect can be achieved.

[0096] Although the optical filter device according to the embodiment has been described above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the object of the present invention.

[0097] For example, the optical filter 40 is not limited to a short wavelength transmission optical filter, but may be another optical filter (e.g., a long wavelength transmission optical filter, a band transmission optical filter, or a gain-equalizing optical filter) that transmits light in a specific wavelength band with any transmission intensity. When a long wavelength transmission optical filter is used as the optical filter, λ0 in the above analytical formula (1) represents the cut-on wavelength when the incident angle α=0°. When a band transmission optical filter is used as the optical filter, λ0 represents the center wavelength when the incident angle α=0°. When a gain-equalizing optical filter is used as the optical filter, λ0 can be set appropriately according to the wavelength profile.

[0098] In the second embodiment, since the end face of the MCF is bevel-polished, there is variation in the light angle θ of the light beam from each core emitted from the first lens 30. Therefore, the MCF may be moved a predetermined distance in the direction opposite to the bevel-polishing direction to reduce the variation in the light angle θ.

[0099] Furthermore, the incident surface 40a of the optical filter 40 does not have to be located on the focal point of the light emitted from the first lens 30. In this case, the rotation axes r1 and r2 of the optical filter 40 can be set as any axes passing through the optical filter 40 in the y-axis direction.

[0100] Moreover, the MCF is not limited to a cylindrical shape, and may be, for example, a cylindrical shape with an elliptical or polygonal cross section perpendicular to the axis. In this case, the oblique polishing direction of the MCF is defined as the direction from the distal end farther away from the optical filter 40 to the proximal end closer to the optical filter 40 along the "oblique polishing reference axis," which is a line segment where "a plane passing through the center of the end face of the MCF, perpendicular to the end face, and parallel to the tilt direction (the direction in which the end face of the MCF is tilted with respect to the xy plane)" intersects with the end face, as viewed along the central axis of the end face.

[0101] Furthermore, in the above embodiment, the MCF60 (or MCF360) has the same number of cores and the same core arrangement as the MCF20 (or MCF320), but is not limited to this. The MCF60 (or MCF360) may have a different number of cores and a different core arrangement from the MCF20 (or MCF320) as long as the MCF60 (or MCF360) has a core into which the emitted light from each core of the MCF20 (or MCF320) can be incident. For example, the MCF60 (or MCF360) may have one or more cores in addition to the seven cores described above. Similarly, the number of single-core optical fibers in the single-core optical fiber group may be greater than the number of cores in MCF20 (or MCF320), so long as the single-mode single-core optical fibers are capable of receiving the output light from each core of MCF20 (or MCF320). In these cases, there will be cores or single-core optical fibers into which the emitted light from each core C1 to C7 of MCF20 (or MCF320) does not enter, but even with such a configuration, the same effect as in the first embodiment can be achieved.

[0102] Furthermore, MCF 20 (or MCF 320) may be a multimode optical fiber. In this case, MCF 60 (or MCF 360) may be a multimode optical fiber having seven or more cores. Alternatively, the group of single-core optical fibers may include seven or more multimode single-core optical fibers.

[0103] Furthermore, when a group of single-core optical fibers is used as the light receiving member, the second lens 50 may be a lens array having lenses in the same number as the number of cores of the MCF 20 (or the MCF 320). Each lens of the lens array converges a light beam from each corresponding core of the MCF 20 (or the MCF 320) so that the light beam is incident on each corresponding single-core optical fiber.

[0104] Furthermore, all the cores C1 to C7 of the MCF 20 (or MCF 320) may not be used for the propagation of the light beam. For example, only the cores C1 to C4 may be used for the propagation of the light beam, and the cores C5 to C7 may not be used for the propagation of the light beam. In this case, the MCF 60 (or MCF 360) may have cores equal to or greater than the number of cores of the MCF 20 (or MCF 320) used for the propagation of the light beam. That is, the MCF 60 (or MCF 360) does not necessarily have to have cores equal to or greater than the number of cores of the MCF 20 (or MCF 320). Alternatively, the single-core optical fiber group may have single-core optical fibers equal to or greater than the number of cores of the MCF 20 (or MCF 320) used for the propagation of the light beam. That is, the single-core optical fiber group does not necessarily have to have single-core optical fibers equal to or greater than the number of cores of the MCF 20 (or MCF 320). [Explanation of symbols]

[0105] 10: optical filter device, 20, 120, 220: multi-core optical fiber, 20a, 120a, 220a: end face of multi-core optical fiber, 21, 121, 221: cladding, 22: ferrule, 22a: end face of ferrule, 30: first lens, 40: optical filter, 40a: incident surface, 40b: exit surface, 50: second lens, 60: multi-core optical fiber, 60a: end face of multi-core optical fiber

Claims

1. A first multi-core optical fiber including a plurality of columnar first cores extending along an axial direction and a common cladding surrounding the plurality of first cores; a first lens having an optical axis located on a central axis of the first multi-core optical fiber, collimating light beams emitted from each of the first cores and diverging, and converging the collimated light beams from each of the first cores to one point; an optical filter comprising: a first surface onto which light rays emitted from the first lens are incident, the first surface being a focal point of a principal ray of light emitted from the first lens; and a second surface opposed to the first surface and from which light rays transmitted through the optical filter are emitted; the optical filter transmits light rays of a specific wavelength band with an arbitrary transmission intensity; the optical filter being rotated by a predetermined rotation angle from a position where the first surface is parallel to a plane perpendicular 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; a second lens that converges the light beams from the first cores outputted from the optical filter; an optical fiber having a columnar core extending along an axial direction, into which all the light beams from the first cores emitted from the second lens are incident; Equipped with A direction from a reference axis that passes through the optical axis and is parallel to the rotation axis along an orthogonal axis that is orthogonal to the optical axis and the reference axis toward one side of the reference axis is defined as a first orthogonal direction, and a direction from the reference axis toward the other side of the reference axis is defined as a second orthogonal direction. a circumferential direction of the first multi-core optical fiber is set such that, when an end face of the first multi-core optical fiber is viewed along its central axis, a sum of a distance from the reference axis of a first core that is farthest from the reference axis in the first orthogonal direction and a distance from the reference axis of a first core that is farthest from the reference axis in the second orthogonal direction is minimum; Optical filter device.

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

3. 3. The optical filter device according to claim 1, the optical fiber is a second multi-core optical fiber in which the cores are surrounded by a common cladding; The second lens refracts the light beams from the first cores outputted from the optical filter so that their chief rays are parallel to each other. Optical filter device.

4. 3. The optical filter device according to claim 1, The optical fiber is a single-core optical fiber group including a plurality of single-core optical fibers, each of the single-core optical fibers including one of the cores and a cladding surrounding the core. Optical filter device.

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

6. 6. The optical filter device according to claim 1, an end face of the first multi-core optical fiber is polished obliquely so as to be inclined at a predetermined polishing angle in a predetermined inclination direction with respect to a plane perpendicular to the optical axis; The z-axis extends on the optical axis such that a direction from the first lens to the optical filter is a positive direction, the y-axis extends on the reference axis such that a direction from one end to the other end of the reference axis is a positive direction, and the x-axis extends in the first orthogonal direction and the second orthogonal direction, When the optical filter is viewed from the positive direction of the y axis, the rotation angle has a positive value when the optical filter is rotated counterclockwise around the rotation axis, and the rotation angle has a negative value when the optical filter is rotated clockwise around the rotation axis, The oblique polishing direction is defined as a direction from a distal end farther away from the optical filter to a proximal end closer to the optical filter along an oblique polishing reference axis, which is a line segment that passes through the center of the end face of the first multi-core optical fiber, is perpendicular to the end face, and is parallel to the tilt direction and intersects with the end face. The oblique polishing direction is defined as a direction from a distal end farther away from the optical filter to a proximal end closer to the optical filter along the central axis of the end face. The angle that the oblique polishing direction forms counterclockwise with respect to the positive direction of the y-axis is defined as an oblique polishing rotation angle having a positive value. When the separation distance is greater than 0, When the rotation angle of the optical filter has a positive value, the end face of the first multi-core optical fiber is obliquely polished such that the oblique polishing rotation angle is a value greater than 0° and less than 180°; When the rotation angle of the optical filter has a negative value, the end face of the first multi-core optical fiber is obliquely polished so that the oblique polishing rotation angle is a value greater than −180° and less than 0°. Optical filter device.

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