Optical module, optical fiber amplifier, and multicore erbium-doped optical fiber

Abstract

The present invention provides an optical module provided with a first optical waveguide assembly in which light exits from an optical waveguide at a first end face, a second optical waveguide assembly in which light enters an optical waveguide at a second end face, a first relay lens system which is provided with a first lens and a second lens and in which the first lens has a first focal plane on the first end face, while the first lens and the second lens share a second focal plane, a second relay lens system which is provided with a third lens and a fourth lens and in which the third lens has a third focal plane on the second end face, while the third lens and the fourth lens share a fourth focal plane, and an optical functional element which is disposed midway between the second lens and the fourth lens and includes a first polarization separation element, a Faraday rotator, a half-wave plate including two types of regions in which directions of fast axes are different from each other by 45 degrees, and a second polarization separation element.

Description

Optical module, optical fiber amplifier and multi-core erbium-doped optical fiber

[0001] This disclosure relates to an optical module, an optical fiber amplifier, and a multi-core erbium-doped optical fiber. This application claims priority to Japanese Patent Application No. 2024-139584, filed on August 21, 2024, and incorporates by reference all of the contents of said Japanese application.

[0002] Patent Document 1 and Non-Patent Document 1 describe a specific configuration in which the propagation directions of signal light are reversed between nearest neighboring cores in a multi-core erbium-doped optical fiber amplifier.

[0003] Patent Document 2 describes an example of a multi-core optical fiber component that utilizes a spatial optical system.

[0004] JP 2014-116466 A International Publication No. 2022 / 019019

[0005] H. Takeshita, et al., "Novel Bidirectional Multicore EDFA Based on Twin Turbo Cladding Pumping Using Bidirectional Pumping and Recycling," 2022 27th OptoElectronics and Communications Conference (OECC) and 2022 International Conference on Photonics in Switching and Computing (PSC), Toyama, Japan, 2022, TuC1-2.

[0006] The present disclosure provides a first optical waveguide assembly including a plurality of optical waveguides, with light entering and exiting each of the plurality of optical waveguides at a first end face; a second optical waveguide assembly including a plurality of optical waveguides, with light entering and exiting each of the plurality of optical waveguides at a second end face; a first relay lens system including a first lens and a second lens, with the first lens having a first focal plane at the first end face and the first lens and the second lens sharing a second focal plane; a second relay lens system including a third lens and a fourth lens, with the third lens having a third focal plane at the second end face and the third lens and the fourth lens sharing a fourth focal plane; and a second relay lens system disposed between the second lens and the fourth lens, with a first polarization an optical functional element including a splitter element, a Faraday rotator, a half-wave plate including two types of regions whose fast axes are oriented 45 degrees apart from each other, and a second polarization splitter element sandwiching the Faraday rotator and the half-wave plate between it and the first polarization splitter element, wherein a double relay lens system is formed by sharing a fifth focal plane of the second lens in the first relay lens system and a sixth focal plane of the fourth lens in the second relay lens system, the first optical waveguide assembly and the second optical waveguide assembly have waveguide arrangements that are similar to each other, and the imaging magnification of the double relay lens system is equal to the similarity ratio of the waveguide arrangements in the first optical waveguide assembly and the second optical waveguide assembly.

[0007] FIG. 1 is a diagram showing an outline of an optical module according to a first embodiment and a light propagation path. FIG. 2 is a diagram showing an outline of a half-wave plate used in the optical module according to the first embodiment. FIG. 3 is a diagram showing an outline of an optical module according to the first embodiment and a light propagation path. FIG. 4 is a diagram showing an outline of an optical module according to a second embodiment and a light propagation path. FIG. 5 is a diagram showing an outline of an optical module according to the second embodiment and a light propagation path. FIG. 6 is a diagram showing an outline of an optical module according to a third embodiment. FIG. 7 is a diagram showing an outline of an optical fiber amplifier including an optical module according to this embodiment. FIG. 8 is a diagram showing an outline of a gain-flattening filter in an optical fiber amplifier including an optical module according to this embodiment. FIG. 9 is a diagram explaining the characteristics of a gain-flattening filter in an optical fiber amplifier including an optical module according to this embodiment. FIG. 10 is a diagram showing an outline of a gain-flattening filter in an optical fiber amplifier including an optical module according to this embodiment. FIG. 11 is a diagram explaining the characteristics of an optical fiber amplifier including an optical module according to this embodiment.

[0008] An optical amplifier includes at least one optical isolator to prevent laser oscillation. A multi-core optical fiber amplifier, for example, needs to include an optical isolator for each core according to the propagation direction. A multi-core optical fiber amplifier, for example, needs to connect signals traveling in the same direction together to the multi-core optical isolator.

[0009] The present disclosure provides an optical isolator that utilizes a spatial optical system in an optical waveguide assembly that includes a plurality of optical waveguides.

[0010] First, embodiments of the present disclosure will be listed and described.

[0011] The optical module of a first aspect includes a first optical waveguide assembly having a plurality of optical waveguides, with light entering and exiting each of the plurality of optical waveguides at a first end face; a second optical waveguide assembly having a plurality of optical waveguides, with light entering and exiting each of the plurality of optical waveguides at a second end face; a first relay lens system having a first lens and a second lens, with the first lens having a first focal plane at the first end face and the first lens and the second lens sharing a second focal plane; a third lens and a fourth lens, with the third lens having a third focal plane at the second end face and the third lens and the fourth lens sharing a fourth focal plane; and a second relay lens system disposed between the second lens and the fourth lens. an optical functional element including a first polarization separation element, a Faraday rotator, a half-wave plate including two types of regions whose fast axes are oriented 45 degrees apart from each other, and a second polarization separation element sandwiching the Faraday rotator and the half-wave plate between it and the first polarization separation element, wherein a double relay lens system is formed by sharing a fifth focal plane of the second lens in the first relay lens system and a sixth focal plane of the fourth lens in the second relay lens system, the first optical waveguide assembly and the second optical waveguide assembly have waveguide arrangements that are similar to each other, and the imaging magnification of the double relay lens system is equal to the similarity ratio of the waveguide arrangements in the first optical waveguide assembly and the second optical waveguide assembly.

[0012] According to the optical module of the first aspect, an optical isolator utilizing a spatial optical system can be provided in an optical waveguide assembly including a plurality of optical waveguides.

[0013] An optical module according to a second aspect includes a first optical waveguide assembly having a plurality of optical waveguides, with light entering and exiting each of the plurality of optical waveguides at a first end face; a second optical waveguide assembly having a plurality of optical waveguides, with light entering and exiting each of the plurality of optical waveguides at a second end face; a first relay lens system having a first lens and a second lens, with the first lens having a first focal plane at the first end face and the first lens and the second lens sharing a second focal plane; a third lens and a fourth lens, with the third lens having a third focal plane at the second end face and the third lens and the fourth lens sharing a fourth focal plane; and a relay lens system disposed between the second lens and the fourth lens, with a frame. an optical module including an optical functional element including a Faraday rotator and a half-wave plate including two types of regions whose fast axes are oriented 45 degrees apart from each other; a first polarization separation element between the first lens and the second lens; and a second polarization separation element between the third lens and the fourth lens, wherein a double relay lens system is formed by sharing a fifth focal plane of the second lens in the first relay lens system and a sixth focal plane of the fourth lens in the second relay lens system; the first optical waveguide assembly and the second optical waveguide assembly have waveguide arrangements that are similar to each other; and an imaging magnification of the double relay lens system is equal to the similarity ratio of the waveguide arrangements in the first optical waveguide assembly and the second optical waveguide assembly.

[0014] According to the optical module of the second aspect, an optical isolator utilizing a spatial optical system can be provided in an optical waveguide assembly including a plurality of optical waveguides.

[0015] An optical module according to a third aspect is the optical module according to the first or second aspect, wherein at least one of the first optical waveguide assembly and the second optical waveguide assembly is a multi-core optical fiber.

[0016] According to the optical module of the third aspect, an optical isolator between cores of a multi-core optical fiber can be provided.

[0017] An optical module according to a fourth aspect is the optical module according to any one of the first aspect to the third aspect, further comprising: a third optical waveguide assembly including a plurality of optical waveguides; a fifth lens; and a dichroic mirror provided between the third lens and the fourth lens, wherein the dichroic mirror transmits light propagating between the third lens and the fourth lens and reflects light emitted from the third optical waveguide assembly and transmitted through the fifth lens, and the light reflected by the dichroic mirror is incident on the optical waveguide included in the second optical waveguide assembly.

[0018] According to the optical module of the fourth aspect, the structure for introducing the excitation light can be integrated with the optical isolator.

[0019] The optical fiber amplifier of a first aspect is an optical fiber amplifier including: an erbium-doped optical fiber having a first end and a second end; a first pumping light combiner connected to the first end; a first optical isolator which is an optical module according to any one of the first to fourth aspects and connected to the first pumping light combiner; a second pumping light combiner connected to the second end; and a second optical isolator which is an optical module according to any one of the first to fourth aspects and connected to the second pumping light combiner.

[0020] According to the optical fiber amplifier of the first aspect, in an optical fiber amplifier having a plurality of waveguides, the configuration of the optical isolator can be simplified.

[0021] An optical fiber amplifier according to a second aspect is an optical fiber amplifier comprising: a first erbium-doped optical fiber having a first end and a second end; a second erbium-doped optical fiber having a third end and a fourth end; a gain-flattening filter connected between the second end and the third end; a first pumping light combiner connected to the first end; a first optical isolator which is an optical module according to any one of the first to fourth aspects and which is connected to the first pumping light combiner; a second pumping light combiner connected to the fourth end; and a second optical isolator which is an optical module according to any one of the first to fourth aspects and which is connected to the second pumping light combiner.

[0022] According to the optical fiber amplifier of the second aspect, in an optical fiber amplifier having a plurality of waveguides, the configuration of the optical isolator can be simplified.

[0023] An optical fiber amplifier according to a third aspect is an optical fiber amplifier comprising: a first erbium-doped optical fiber having a first end and a second end; a second erbium-doped optical fiber having a third end and a fourth end; a gain-flattening filter connected between the second end and the third end; a first optical isolator that is an optical module according to any one of the first to fourth aspects and that is connected to the first end; and a second optical isolator that is an optical module according to any one of the first to fourth aspects and that is connected to the fourth end.

[0024] According to the optical fiber amplifier of the third aspect, in an optical fiber amplifier having a plurality of waveguides, the configuration of the optical isolator can be simplified.

[0025] In an optical fiber amplifier of a fourth aspect, the gain flattening filter includes a fourth optical waveguide assembly including a plurality of optical waveguides, with light entering and exiting each of the plurality of optical waveguides at a third end face; a fifth optical waveguide assembly including a plurality of optical waveguides, with light entering and exiting each of the plurality of optical waveguides at a fourth end face; a sixth lens, a seventh lens, with the sixth lens having a seventh focal plane at the third end face, with the sixth lens and the seventh lens sharing an eighth focal plane; a third relay lens system including an eighth lens and a ninth lens, with the eighth lens having a ninth focal plane at the fourth end face, with the eighth lens and the ninth lens sharing a tenth focal plane; an eleventh focal plane of the seventh lens in the third relay lens system and a twelfth focal plane of the ninth lens in the fourth relay lens system are shared to form a second dual relay lens system, the fourth optical waveguide assembly and the fifth optical waveguide assembly have waveguide arrangements that are similar to each other, and an imaging magnification of the second dual relay lens system is equal to a similarity ratio of the waveguide arrangements in the fourth optical waveguide assembly and the fifth optical waveguide assembly.

[0026] According to the optical fiber amplifier of the fourth aspect, the difference between the cores in the flattened filter module can be reduced.

[0027] An optical fiber amplifier according to a fifth aspect is the optical fiber amplifier according to the fourth aspect, wherein the gain flattening filter has an insertion loss of 3 dB or less for pumping light.

[0028] According to the optical fiber amplifier of the fifth aspect, the conversion efficiency in the optical fiber amplifier can be increased.

[0029] A multi-core erbium-doped optical fiber according to a first aspect is a multi-core erbium-doped optical fiber having one or more erbium-doped cores and one or more non-erbium-doped cores.

[0030] According to the multi-core erbium-doped optical fiber of the first aspect, manufacturing costs can be reduced by doping erbium only in the cores where it is necessary.

[0031] A multi-core erbium-doped optical fiber according to a second aspect is the doped optical fiber according to the first aspect, which has m (m is an integer of 2 or more) erbium-doped cores and m non-erbium-doped cores, and the mth combinations in ascending order of the spacing between two cores are all combinations of an erbium-doped core and a non-erbium-doped core.

[0032] According to the multi-core erbium-doped optical fiber of the second aspect, noise between cores can be suppressed.

[0033] [Details of the embodiments of the present disclosure] Specific examples of the optical module of the present disclosure will be described below with reference to the drawings. The present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0034] In the description of the specification and drawings relating to each embodiment, components having substantially the same or corresponding functions may be designated by the same reference numerals, and redundant explanations may be omitted. For ease of understanding, the scale of each part in the drawings may differ from the actual scale.

[0035] The vertical direction may be offset to the extent that the effect of the embodiment is not impaired. The vertical direction may include approximately vertical.

[0036] For example, "substantially perpendicular" means that even if two lines or two surfaces are not perfectly perpendicular to each other, they can be treated as being perpendicular to each other within a range that is permissible in manufacturing.

[0037] The following embodiments will be described, and at least some of the embodiments described below may be combined in any manner.

[0038] First Embodiment An optical module according to a first embodiment will be described using a specific example. Fig. 1 is a diagram illustrating an outline of an optical module 1, which is an example of the optical module according to the first embodiment, and a propagation path of light.

[0039] The optical module 1 is an optical isolator used in, for example, an erbium-doped optical fiber amplifier (EDFA). The optical module 1 can set the propagation direction of an optical signal in each of multiple cores of a multi-core optical fiber. The optical module 1 has a multi-core optical fiber input / output.

[0040] The optical module 1 includes an optical fiber 10, an optical fiber 20, a relay lens system 30, a relay lens system 40, and an optical functional element 50. The optical module 1 includes, in order from the optical fiber 10, the relay lens system 30, the optical functional element 50, the relay lens system 40, and the optical fiber 20. In Fig. 1, the solid and dotted lines connecting the optical fiber 10 and the optical fiber 20 schematically show the path along which an optical signal propagates. This also applies to the following figures.

[0041] The optical fiber 10 is a multi-core optical fiber. An optical signal is input to the optical module 1 from some of the cores of the optical fiber 10. The optical module 1 outputs the optical signal from the remaining cores of the optical fiber 10.

[0042] The optical fiber 20 is a multi-core optical fiber. An optical signal is input to the optical module 1 from some of the cores of the optical fiber 20. The optical module 1 outputs the optical signal from the remaining cores of the optical fiber 20.

[0043] Each of the optical fibers 10 and 20 is, for example, a four-core optical fiber in which cores are arranged at the vertices of a square with sides of 40 μm in a cross section perpendicular to the direction in which the optical fiber extends. Each of the optical fibers 10 and 20 is, for example, a four-core optical fiber with a mode field diameter of 10.0 μm and a cladding diameter of 125 μm at a wavelength of 1550 nm.

[0044] The core arrangement of the optical fiber 20 in a cross section perpendicular to the direction in which the optical fiber 20 extends is the same as the core arrangement of the optical fiber 10 in a cross section perpendicular to the direction in which the optical fiber 20 extends. The core arrangement of the optical fiber 20 in a cross section perpendicular to the direction in which the optical fiber 20 extends may be an arrangement similar to the core arrangement of the optical fiber 10 in a cross section perpendicular to the direction in which the optical fiber 10 extends. In the optical module 1, light emitted from any of the multiple cores in the optical fiber 10 is incident on any of the cores in the optical fiber 20. In the optical module 1, light emitted from any of the multiple cores in the optical fiber 20 is incident on any of the cores in the optical fiber 10.

[0045] The relay lens system 30 includes, in order from the optical fiber 10, a lens 31 and a lens 32. A focal plane FP1 of the lens 31 is disposed at the end face 10S of the optical fiber 10. The lenses 31 and 32 share a focal plane FP3. Because the lenses 31 and 32 share the focal plane FP3, the lenses 31 and 32 function as a relay lens system.

[0046] In the present disclosure, for example, when a statement is made that the focal plane of a lens is located on an end surface, it does not necessarily mean that the focal plane of the lens exactly coincides with the end surface. For example, if the focal plane of the lens is within a manufacturing tolerance range relative to the end surface, it also includes a case where the focal plane of the lens is located on the end surface. For example, when a statement is made that a first lens and a second lens share a focal plane, it does not necessarily mean that the focal plane of the first lens exactly coincides with the focal plane of the second lens. For example, if the focal plane of the second lens is within a manufacturing tolerance range relative to the focal plane of the first lens, it also includes a case where the first lens and the second lens share a focal plane. The same or similar applies to the following descriptions.

[0047] The relay lens system 40 includes, in order from the optical fiber 20, a lens 41 and a lens 42. A focal plane FP2 of the lens 41 is disposed at the end face 20S of the optical fiber 20. The lenses 41 and 42 share a focal plane FP4. Because the lenses 41 and 42 share the focal plane FP4, the lenses 41 and 42 function as a relay lens system.

[0048] The focal plane FP5 of the lens 32 and the focal plane FP6 of the lens 42 coincide with each other. That is, the relay lens system 30 and the relay lens system 40 are arranged so that the focal plane FP5 of the lens 32 and the focal plane FP6 of the lens 42 coincide with each other. By arranging the relay lens system 30 and the relay lens system 40 so that the focal plane FP5 of the lens 32 and the focal plane FP6 of the lens 42 coincide with each other, the relay lens system 30 and the relay lens system 40 form a dual relay lens system.

[0049] In the optical module 1, the optical fiber 10 and the optical fiber 20 are multi-core optical fibers having the same cross-sectional shape. The similarity ratio between the core arrangement in the optical fiber 10 and the core arrangement in the optical fiber 20 is 1. The imaging magnification of the dual relay lens system formed by the relay lens system 30 and the relay lens system 40 is equal to the similarity ratio of the core arrangement, which is the waveguide arrangement in the optical fiber 10 and the optical fiber 20. The imaging magnification of the dual relay lens system formed by the relay lens system 30 and the relay lens system 40 is 1. In the present disclosure, for example, when it is said that the imaging magnification of a lens system is equal to the similarity ratio, it is not limited to the case where the imaging magnification of the lens system is strictly equal to the similarity ratio. For example, if the imaging magnification of a lens system is within a manufacturing allowable range with respect to the similarity ratio, this is also included in the case where the imaging magnification of a lens system is equal to the similarity ratio. This also applies to the following description.

[0050] Each of the lenses 31 and 41 is an aspherical lens having, for example, a focal length of 0.6 mm and a diameter of 1.5 mm. Each of the lenses 32 and 42 is a plano-convex lens having, for example, a focal length of 6.0 mm and a diameter of 3.0 mm.

[0051] The optical functional element 50 functions to change the propagation path of light depending on the direction of light propagation. The optical functional element 50 includes a polarization separation element 51, a Faraday rotator 52, a half-wave plate 53, and a polarization separation element .

[0052] The polarization separation element 51 and the polarization separation element 54 are elements in which light propagates in different directions depending on the polarization. The polarization separation element 51 and the polarization separation element 54 are, for example, birefringent crystals. The polarization separation element 51 and the polarization separation element 54 are each made of yttrium orthovanadate (YVO4) having a thickness of 2 mm. ４ It is a Yttria (Yttria cerium orthovanadate) single crystal.

[0053] The Faraday rotator 52 is an element that rotates the polarization direction of light. The Faraday rotator 52 rotates the polarization direction by 45 degrees. The Faraday rotator 52 is, for example, a rare-earth-doped iron garnet with a thickness of 0.45 mm and a cylindrical permanent magnet arranged around its periphery.

[0054] The half-wave plate 53 is a wave plate having regions divided into the same number as the number of cores included in each of the optical fiber 10 and the optical fiber 20. The half-wave plate 53 is a half-wave plate that rotates the polarization direction of linearly polarized light.

[0055] 2 is a diagram illustrating an outline of a half-wave plate 53 used in the optical module 1, which is an example of an optical module according to the first embodiment. As shown in FIG. 2 , the half-wave plate 53 is divided into regions corresponding to the core arrangements of the optical fiber 10 and the optical fiber 20. The half-wave plate 53 is divided into four regions corresponding to the number of cores in the optical fiber 10 and the optical fiber 20. The half-wave plate 53 has a region 53a, a region 53b, a region 53c, and a region 53d. The regions 53a, 53b, 53c, and 53d are provided corresponding to the core arrangements of the optical fiber 10 and the optical fiber 20, respectively.

[0056] In FIG. 2 , arrows indicate the direction of the fast axis. Regions 53a and 53d have fast axes that are in the same direction. The fast axes of regions 53a and 53d are tilted downward at an angle α with respect to the horizontal direction in FIG. 2 . Regions 53b and 53c have fast axes that are in the same direction. The fast axes of regions 53b and 53c are tilted upward at an angle α with respect to the horizontal direction in FIG. 2 . The angle α is 22.5 degrees, or 45 / 2 degrees. The fast axes of regions 53a and 53b differ by 45 degrees. In half-wave plate 53, the fast axes of adjacent regions differ by 45 degrees. Half-wave plate 53 is a wave plate formed by arranging half-wave plates whose fast axes are tilted by 45 degrees with respect to adjacent regions.

[0057] The following describes the propagation of light in the Faraday rotator 52 and the half-wave plate 53 included in the optical functional element 50. Here, the description is given assuming that the polarization separation element 51 in the optical functional element 50 is a birefringent crystal.

[0058] When an ordinary ray is incident on the Faraday rotator 52 from the polarization separation element 51, the Faraday rotator 52 rotates the polarization direction of the ordinary ray by 45 degrees. When the ordinary ray, whose polarization direction has been rotated by 45 degrees, enters the first region of the half-wave plate 53, it is folded back by the half-wave plate 53 and converted into, for example, an ordinary ray in the polarization separation element 54. When light enters the first region of the half-wave plate 53 from the polarization separation element 54, the ordinary ray in the polarization separation element 54 is converted into an extraordinary ray in the polarization separation element 51 by the half-wave plate 53 and the Faraday rotator 52. The first region of the half-wave plate 53 is, for example, region 53a or region 53d.

[0059] When an ordinary ray whose polarization direction has been rotated by 45 degrees by the Faraday rotator 52 enters the second region of the half-wave plate 53, it is reflected by the half-wave plate 53. After being reflected by the half-wave plate 53, the orientation of the fast axis differs by 45 degrees between the first region and the second region, and therefore the ordinary ray is converted into an extraordinary ray in the polarization separation element 54. When light enters the second region of the half-wave plate 53 from the polarization separation element 54, the ordinary ray in the polarization separation element 54 is converted into an ordinary ray in the polarization separation element 51 by the half-wave plate 53 and the Faraday rotator 52. The second region of the half-wave plate 53 is, for example, region 53b or region 53c.

[0060] As described above, the optical functional element 50 can convert an ordinary ray in the polarization separation element 51 into an ordinary ray in the polarization separation element 54, and can convert the ordinary ray in the polarization separation element 54 into an extraordinary ray in the polarization separation element 51, by using the Faraday rotator 52 and the half-wave plate 53. The optical functional element 50 can convert an ordinary ray in the polarization separation element 51 into an extraordinary ray in the polarization separation element 54, and can convert the ordinary ray in the polarization separation element 54 into an ordinary ray in the polarization separation element 51, by using the Faraday rotator 52 and the half-wave plate 53.

[0061] In other words, the optical functional element 50 can convert, for each core, the signal light incident from the optical fiber 10 from an ordinary ray to an ordinary ray, and can convert the signal light incident from the optical fiber 20 from an ordinary ray to an extraordinary ray, by the Faraday rotator 52 and the half-wave plate 53. The optical functional element 50 can convert, for each core, the signal light incident from the optical fiber 10 from an ordinary ray to an extraordinary ray, and can convert the signal light incident from the optical fiber 20 from an ordinary ray to an ordinary ray, by the Faraday rotator 52 and the half-wave plate 53.

[0062] The following describes how signal light propagates in the optical module 1 using the optical functional element 50. First, we will describe the case where light is incident from the optical fiber 10 in the optical module 1. Fig. 1 shows the propagation path of signal light when it propagates from the optical fiber 10 to the optical fiber 20.

[0063] A description will be given of a case where signal light A is incident from one core of the optical fiber 10. The signal light A passes through the relay lens system 30 and enters the polarization separation element 51 in the optical functional element 50.

[0064] Signal light A1, which is an ordinary ray in the polarization separation element 51 and has entered the polarization separation element 51, propagates straight through the polarization separation element 51 and enters the Faraday rotator 52. When the signal light A1 propagates through the Faraday rotator 52 and the half-wave plate 53, the signal light A1 is converted into an extraordinary ray in the polarization separation element 54. The signal light A1 converted into an extraordinary ray in the polarization separation element 54 propagates obliquely through the polarization separation element 54.

[0065] Signal light A2, which is an extraordinary ray in the polarization separation element 51 and has entered the polarization separation element 51, propagates obliquely through the polarization separation element 51 and enters the Faraday rotator 52. When the signal light A2 propagates through the Faraday rotator 52 and the half-wave plate 53, the signal light A2 is converted into an ordinary ray in the polarization separation element 54. The signal light A2 converted into an ordinary ray in the polarization separation element 54 propagates straight through the polarization separation element 54.

[0066] The signal light A1 and the signal light A2 are each converted between an ordinary ray and an extraordinary ray by the Faraday rotator 52 and the half-wave plate 53. By converting between the ordinary ray and the extraordinary ray, the signal light A1 and the signal light A2 travel the same distance in the same direction in the polarization separation element 51 or the polarization separation element 54, and therefore exit from the same location in the polarization separation element 54. The signal light A1 and the signal light A2 exiting from the polarization separation element 54 propagate through the relay lens system 40 and enter the optical fiber 20. It is assumed that the signal light A1 and the signal light A2 exiting from the polarization separation element 54 are focused at the center of the end face of the core of the optical fiber 20. The signal light A1 and the signal light A2 entering the optical fiber 20 propagate through the optical fiber 20 as signal light Ap.

[0067] A case will be described in which signal light B is incident from one core of the optical fiber 10. The signal light B passes through the relay lens system 30 and enters the polarization separation element 51 in the optical functional element 50.

[0068] The signal light B1, which is an ordinary ray in the polarization separation element 51 and has entered the polarization separation element 51, propagates straight through the polarization separation element 51 and enters the Faraday rotator 52. When the signal light B1 propagates through the Faraday rotator 52 and the half-wave plate 53, the signal light B1 is converted into an ordinary ray in the polarization separation element 54. The signal light B1 converted into an ordinary ray in the polarization separation element 54 propagates straight through the polarization separation element 54.

[0069] Signal light B2, which is an extraordinary ray in the polarization separation element 51 and has entered the polarization separation element 51, propagates obliquely through the polarization separation element 51 and enters the Faraday rotator 52. When the signal light B2 propagates through the Faraday rotator 52 and the half-wave plate 53, the signal light B2 is converted into an extraordinary ray in the polarization separation element 54. The signal light A2 converted into an extraordinary ray in the polarization separation element 54 propagates obliquely through the polarization separation element 54.

[0070] The signal light B1 and the signal light B2 are not converted between ordinary rays and extraordinary rays by the Faraday rotator 52 and the half-wave plate 53. Because no conversion between ordinary rays and extraordinary rays occurs, the signal light B1 and the signal light B2 exit from different positions on the polarization separation element 54. The signal light B1 and the signal light B2 exiting from the polarization separation element 54 propagate through the relay lens system 40 and enter the optical fiber 20. The signal light B1 and the signal light B2 exiting from the polarization separation element 54 are focused at a position offset from the center of the end face of the core of the optical fiber 20. The signal light B1 and the signal light B2 entering the optical fiber 20 are focused at a position offset from the center of the end face of the core of the optical fiber 20, and are therefore not entered into the core of the optical fiber 20.

[0071] As described above, the optical module 1 can allow the signal light A emitted from the optical fiber 10 to propagate to the optical fiber 20 and prevent the signal light B emitted from the optical fiber 10 from propagating to the optical fiber 20 .

[0072] A case where light is incident from the optical fiber 20 in the optical module 1 will be described. Fig. 3 is a diagram showing an outline of the optical module 1, which is an example of the optical module according to the first embodiment, and a propagation path of light. Fig. 3 shows the propagation path when signal light propagates from the optical fiber 20 to the optical fiber 10.

[0073] A case will be described in which signal light C is incident from one core of the optical fiber 20. The signal light C passes through the relay lens system 40 and enters the polarization separation element 54 in the optical functional element 50.

[0074] The signal light C1, which is an ordinary ray in the polarization separation element 54 and has entered the polarization separation element 54, propagates straight through the polarization separation element 54 and is incident on the half-wave plate 53. When the signal light C1 propagates through the half-wave plate 53 and the Faraday rotator 52, the signal light C1 is converted into an ordinary ray in the polarization separation element 51. The signal light C1 converted into an ordinary ray in the polarization separation element 51 propagates straight through the polarization separation element 54.

[0075] The signal light C2, which is an extraordinary ray in the polarization separation element 54 and has entered the polarization separation element 54, propagates obliquely through the polarization separation element 54 and enters the half-wave plate 53. When the signal light C2 propagates through the half-wave plate 53 and the Faraday rotator 52, the signal light C2 is converted into an extraordinary ray in the polarization separation element 51. The signal light C2, which has been converted into an ordinary ray in the polarization separation element 51, propagates obliquely through the polarization separation element 54.

[0076] The signal light C1 and the signal light C2 are not converted between ordinary rays and extraordinary rays by the Faraday rotator 52 and the half-wave plate 53. Because no conversion between ordinary rays and extraordinary rays occurs, the signal light C1 and the signal light C2 exit from different positions on the polarization separation element 51. The signal light C1 and the signal light C2 exiting from the polarization separation element 51 propagate through the relay lens system 30 and enter the optical fiber 10. The signal light C1 and the signal light C2 exiting from the polarization separation element 51 are focused at a position offset from the center of the end face of the core of the optical fiber 10. The signal light C1 and the signal light C2 entering the optical fiber 10 are not entered into the core of the optical fiber 10 by being focused at a position offset from the center of the end face of the core of the optical fiber 10.

[0077] A case will be described in which signal light D is incident from one core of the optical fiber 10. The signal light D passes through the relay lens system 40 and enters the polarization separation element 54 in the optical functional element 50.

[0078] The signal light D1, which is an ordinary ray in the polarization separation element 54 and has entered the polarization separation element 54, propagates straight through the polarization separation element 54 and is then incident on the half-wave plate 53. When the signal light D1 propagates through the half-wave plate 53 and the Faraday rotator 52, the signal light D1 is converted into an extraordinary ray in the polarization separation element 51. The signal light D1 converted into an extraordinary ray in the polarization separation element 51 propagates obliquely through the polarization separation element 51.

[0079] The signal light D2, which is an extraordinary ray in the polarization separation element 54 and has entered the polarization separation element 54, propagates obliquely through the polarization separation element 54 and enters the half-wave plate 53. When the signal light D2 propagates through the half-wave plate 53 and the Faraday rotator 52, the signal light D2 is converted into an ordinary ray in the polarization separation element 51. The signal light D2 converted into an ordinary ray in the polarization separation element 51 propagates straight through the polarization separation element 54.

[0080] The signal light D1 and the signal light D2 are each converted between an ordinary ray and an extraordinary ray by the Faraday rotator 52 and the half-wave plate 53. By converting between the ordinary ray and the extraordinary ray, the signal light D1 and the signal light D2 travel the same distance in the same direction in the polarization separation element 51 or the polarization separation element 54, and therefore exit from the same location in the polarization separation element 51. The signal light D1 and the signal light D2 exiting from the polarization separation element 51 propagate through the relay lens system 30 and enter the optical fiber 10. The signal light D1 and the signal light D2 exiting from the polarization separation element 51 are focused at the center of the end face of the core of the optical fiber 10. The signal light D1 and the signal light D2 entering the optical fiber 10 propagate through the optical fiber 10 as signal light Dp.

[0081] As described above, the optical module 1 can allow the signal light D emitted from the optical fiber 20 to propagate through the optical fiber 10 , and can prevent the signal light C emitted from the optical fiber 20 from propagating through the optical fiber 20 .

[0082] As described above, the optical module 1 functions as an optical isolator.

[0083] According to the optical module of the first embodiment, an optical isolator can be provided by utilizing a spatial optical system.

[0084] Second Embodiment An optical module according to a second embodiment will be described using a specific example. The optical module according to the second embodiment differs from the optical module according to the first embodiment in the position of the polarization separation element. Fig. 4 is a diagram showing an outline of an optical module 2, which is an example of an optical module according to the second embodiment, and the propagation path of light.

[0085] The optical module 2 includes an optical fiber 110, an optical fiber 120, a relay lens system 130, a relay lens system 140, and an optical functional element 150. The optical module 2 includes, in order from the optical fiber 110, the relay lens system 130, the optical functional element 150, the relay lens system 140, and the optical fiber 120.

[0086] The optical fiber 110 is a multi-core optical fiber. An optical signal is input to the optical module 2 from some of the cores of the optical fiber 110. The optical module 2 outputs the optical signal from the remaining cores of the optical fiber 110.

[0087] The optical fiber 120 is a multi-core optical fiber. An optical signal is input to the optical module 2 from some of the cores of the optical fiber 120. The optical module 2 outputs the optical signal from the remaining cores of the optical fiber 120.

[0088] Each of the optical fibers 110 and 120 is, for example, a six-core optical fiber in which cores are arranged at the vertices of a regular hexagon with sides of 25 μm in a cross section perpendicular to the direction in which the optical fibers extend. Each of the optical fibers 110 and 120 is, for example, a six-core optical fiber with a mode field diameter of 10.0 μm and a cladding diameter of 125 μm at a wavelength of 1550 nm.

[0089] The core arrangement of the optical fiber 120 in a cross section perpendicular to the direction in which the optical fiber 120 extends is the same as the core arrangement of the optical fiber 110 in a cross section perpendicular to the direction in which the optical fiber 110 extends. The core arrangement of the optical fiber 120 in a cross section perpendicular to the direction in which the optical fiber 120 extends may be an arrangement similar to the core arrangement of the optical fiber 110 in a cross section perpendicular to the direction in which the optical fiber 110 extends. In the optical module 2, light emitted from any of the multiple cores in the optical fiber 110 is incident on any of the cores in the optical fiber 120. In the optical module 2, light emitted from any of the multiple cores in the optical fiber 120 is incident on any of the cores in the optical fiber 110.

[0090] The relay lens system 130 includes, in order from the optical fiber 110, a lens 131 and a lens 132. A focal plane FP11 of the lens 131 is disposed on the end face 110S of the optical fiber 110. The lenses 131 and 132 share a focal plane FP13. Because the lenses 131 and 132 share the focal plane FP13, the lenses 131 and 132 function as a relay lens system.

[0091] The relay lens system 140 includes, in order from the optical fiber 120, a lens 141 and a lens 142. A focal plane FP12 of the lens 141 is disposed at the end face 120S of the optical fiber 120. The lenses 141 and 142 share a focal plane FP14. Because the lenses 141 and 142 share the focal plane FP14, the lenses 141 and 142 function as a relay lens system.

[0092] The focal plane FP15 of the lens 132 and the focal plane FP16 of the lens 142 coincide with each other. That is, the relay lens system 130 and the relay lens system 140 are arranged so that the focal plane FP15 of the lens 132 and the focal plane FP16 of the lens 142 coincide with each other. By arranging the relay lens system 130 and the relay lens system 140 so that the focal plane FP15 of the lens 132 and the focal plane FP16 of the lens 142 coincide with each other, the relay lens system 130 and the relay lens system 140 form a dual relay lens system.

[0093] In the optical module 2, the optical fiber 110 and the optical fiber 120 are multi-core optical fibers having the same cross-sectional shape. The similarity ratio between the core arrangement in the optical fiber 110 and the core arrangement in the optical fiber 120 is 1. The imaging magnification of the dual relay lens system formed by the relay lens system 130 and the relay lens system 140 is set equal to the similarity ratio of the core arrangement, which is the waveguide arrangement in the optical fiber 110 and the optical fiber 120. The imaging magnification of the dual relay lens system formed by the relay lens system 130 and the relay lens system 140 is 1.

[0094] The lenses 131 and 141 are each an aspherical lens with a focal length of 0.6 mm and a diameter of 1.5 mm, for example. The lenses 32 and 42 are each a plano-convex lens with a focal length of 3.6 mm and a diameter of 2.5 mm, for example.

[0095] The optical functional element 150 affects the propagation path of light differently depending on the direction of light propagation. The optical functional element 150 includes a polarization separation element 151, a Faraday rotator 152, a half-wave plate 153, and a polarization separation element 154. The Faraday rotator 152 and the half-wave plate 153 may be referred to as the optical functional element.

[0096] The polarization separation element 151, the Faraday rotator 152, the half-wave plate 153, and the polarization separation element 154 have the same configuration as the polarization separation element 51, the Faraday rotator 52, the half-wave plate 53, and the polarization separation element 54, respectively, and therefore the above description will be referred to for the common configuration. Here, the polarization separation element 151, the Faraday rotator 152, the half-wave plate 153, and the polarization separation element 154 will be described, focusing on the differences between the polarization separation element 51, the Faraday rotator 52, the half-wave plate 53, and the polarization separation element 54, respectively.

[0097] The polarization separation element 151 is provided between the lens 131 and the lens 132. The half-wave plate 153 is divided into six regions in accordance with the number of cores.

[0098] The following describes how signal light propagates in the optical module 2 using the optical functional element 150. First, a case where light is incident from the optical fiber 110 in the optical module 2 will be described. Fig. 4 shows the propagation path of signal light when it propagates from the optical fiber 110 to the optical fiber 20.

[0099] The conversion between ordinary and extraordinary rays in the Faraday rotator 152 and the half-wave plate 153 is the same as in the optical module 1 .

[0100] Signal light A from one core in optical fiber 110 is converted such that the ordinary ray in polarization separation element 151 becomes an extraordinary ray in polarization separation element 154, and the extraordinary ray in polarization separation element 151 becomes an ordinary ray in polarization separation element 154. Therefore, the position from which signal light A emerges is expanded in polarization separation element 154. Because the position from which signal light A emerges is expanded, the angle of incidence becomes larger than the angle at which signal light A can enter the core of optical fiber 120, and signal light A cannot enter optical fiber 120, as shown in FIG.

[0101] Signal light B from one core in optical fiber 110 is converted such that the ordinary ray in polarization separation element 151 becomes an ordinary ray in polarization separation element 154, and the extraordinary ray in polarization separation element 151 becomes an extraordinary ray in polarization separation element 154. Therefore, as shown in Fig. 4, the position from which signal light B emerges does not expand in polarization separation element 154. Because the position from which signal light B emerges does not expand, the angle of incidence becomes smaller than the angle at which signal light B can enter the core in optical fiber 120, and the signal light Bp enters optical fiber 120 and propagates.

[0102] A case where light is incident from the optical fiber 120 in the optical module 2 will be described. Fig. 5 is a diagram showing an outline of the optical module 2, which is an example of an optical module according to the second embodiment, and a propagation path of light. Fig. 5 shows the propagation path when signal light propagates from the optical fiber 120 to the optical fiber 110.

[0103] Signal light C from one core in optical fiber 120 is converted so that the ordinary ray in polarization separation element 154 becomes an ordinary ray in polarization separation element 151, and the extraordinary ray in polarization separation element 154 becomes an extraordinary ray in polarization separation element 151. Therefore, as shown in Fig. 5 , the position from which signal light C emerges does not expand in polarization separation element 151. Because the position from which signal light C emerges does not expand, the angle of incidence becomes smaller than the angle at which signal light C can enter the core in optical fiber 110, and the signal light Cp enters optical fiber 110 and propagates.

[0104] Signal light D from one core in optical fiber 120 is converted such that the ordinary ray in polarization separation element 154 becomes an extraordinary ray in polarization separation element 151, and the extraordinary ray in polarization separation element 154 becomes an ordinary ray in polarization separation element 151. Therefore, the position from which signal light D emerges is expanded in polarization separation element 151. Because the position from which signal light D emerges is expanded, the angle of incidence becomes larger than the angle at which the signal light D can enter the core in optical fiber 110, and the signal light cannot enter optical fiber 110, as shown in FIG.

[0105] As described above, the optical module 2 functions as an optical isolator.

[0106] According to the optical module of the second embodiment, an optical isolator can be provided by utilizing a spatial optical system.

[0107] Third Embodiment An optical module according to a third embodiment will be described using a specific example. The optical module according to the third embodiment includes a configuration in which excitation light is incident on the optical module according to the first embodiment. Fig. 6 is a diagram showing an outline of an optical module 3, which is an example of the optical module according to the third embodiment, and the propagation path of light.

[0108] The optical module 3 includes an optical fiber 210 , an optical fiber 220 , a relay lens system 230 , a relay lens system 240 , an optical functional element 250 , an optical fiber bundle 260 , a lens 271 , and a dichroic mirror 272 .

[0109] The optical fiber 210, the optical fiber 220, the relay lens system 230, the relay lens system 240, and the optical functional element 250 have the same configuration as the optical fiber 10, the optical fiber 20, the relay lens system 30, the relay lens system 40, and the optical functional element 50, respectively. The optical fiber 210, the optical fiber 220, the relay lens system 230, the relay lens system 240, and the optical functional element 250 each have the same configuration, so the above description can be referred to and the description will be omitted here.

[0110] The optical fiber bundle 260 is an optical fiber bundle including a plurality of single-core pumping light delivery optical fibers. The single-core pumping light delivery optical fiber is, for example, an optical fiber bundle in which single-core optical fibers having a mode field diameter of 6.0 μm at a wavelength of 980 nm and a cladding diameter of 125 μm are arranged in a square. In order to reduce the connection loss to the optical fiber 220 via a lens system, the core near the optical input / output end face may be enlarged by heat treatment so that the mode field diameter at the optical input end face of the optical fiber 220 is 10.0 μm at a wavelength of 1550 nm. The mode field diameter at the optical input end face of the optical fiber 220 is 5.9 μm at a wavelength of 980 nm. Therefore, the core near the optical output end face is enlarged by heat treatment so that the mode field diameter at the optical output end face of the single-core pumping light delivery optical fiber including the optical fiber bundle 260 is 17.7 μm at a wavelength of 980 nm.

[0111] The dichroic mirror 272 is an optical glass wedge prism having a first incident and exit surface on which a dielectric multilayer film is deposited so as to reflect 98% or more of excitation light with a wavelength of 975±10 nm at an incident angle of 45 degrees. The dielectric multilayer film is laminated so as to transmit 98% or more of signal light with a wavelength of 1550±50 nm. The dichroic mirror 272 has a second incident and exit surface on which an anti-reflection film with a reflectance of 0.1% is deposited. The dichroic mirror 272 has a square incident and exit surface with sides of 4.0 mm, and is plate-shaped with a thickness of 2.0 mm and a wedge angle of 1 degree.

[0112] The optical module according to the third embodiment functions as an optical isolator and can also input excitation light.

[0113] <Optical Fiber Amplifier> An optical fiber amplifier including an optical module according to this embodiment will be described. Fig. 7 is a diagram showing an outline of an optical fiber amplifier 4, which is an example of an optical fiber amplifier including an optical module according to this embodiment.

[0114] The optical fiber amplifier 4 is a multi-core erbium-doped optical fiber amplifier (EDFA) that propagates pump light in the same direction as the signal propagates.

[0115] The optical fiber amplifier 4 includes an optical isolator 301, an optical isolator 302, a gain-flattening filter module 303, a pumping light combiner 304, and a pumping light combiner 305. The optical fiber amplifier 4 includes a pumping laser 306a, a pumping laser 306b, a pumping laser 307a, and a pumping laser 307b. The optical fiber amplifier 4 further includes a multi-core erbium-doped optical fiber 308a (hereinafter referred to as EDF 308a) and a multi-core erbium-doped optical fiber 308b (hereinafter referred to as EDF 308b).

[0116] The optical isolator 301 and the optical isolator 302 are, for example, an optical module 1 which is an example of the optical module according to the first embodiment, or an optical module 2 which is an example of the optical module according to the second embodiment, respectively.

[0117] The pumping light combiner 304 combines the pumping light output from each of the pumping lasers 306a and 306b with the signal light of the corresponding core, and outputs the combined light to the EDF 308a. The pumping light combiner 305 combines the pumping light output from each of the pumping lasers 307a and 307b with the signal light of the corresponding core, and outputs the combined light to the EDF 308b.

[0118] Each of the EDFs 308a and 308b has a mode field diameter of 3.3 μm at a wavelength of 980 nm, a mode field diameter of 5.6 μm at a wavelength of 1550 nm, and a cladding diameter of 125 μm. Each of the EDFs 308a and 308b is a four-core optical fiber in which cores are arranged at the vertices of a square with sides of 32 μm. Each of the EDFs 308a and 308b pairs two of the four cores located at diagonal corners of the square, and the first pair of cores has a diameter of 5.6 × 10 ２４ m -３ The second pair of cores of each of the EDFs 308a and 308b are undoped.

[0119] The EDF 308a amplifies the signal light emitted from the pump light multiplexer 304 by the pump light while passing through the erbium-doped core. In the EDF 308a, the light propagating from the gain-flattening filter module 303 passes through the core of the EDF 308a that is not doped with erbium.

[0120] The EDF 308b amplifies the signal light emitted from the pump light multiplexer 305 by the pump light while passing through the erbium-doped core. In the EDF 308b, the light propagating from the gain-flattening filter module 303 passes through the core of the EDF 308b that is not doped with erbium.

[0121] As described above, each of the EDFs 308a and 308b has one or more erbium-doped cores and one or more non-erbium-doped cores. For example, when there are m (m is an integer of 2 or more) erbium-doped cores and m non-erbium-doped cores, it is preferable that the first m combinations in ascending order of the spacing between two cores are all combinations of an erbium-doped core and a non-erbium-doped core. By having the first m combinations in ascending order of the spacing between two cores be all combinations of an erbium-doped core and a non-erbium-doped core, noise between the cores can be suppressed.

[0122] The following describes the configuration of the gain flattening filter module 303. Fig. 8 is a diagram showing an outline of the gain flattening filter module 303 in the optical fiber amplifier 4, which is an example of an optical fiber amplifier equipped with the optical module according to this embodiment.

[0123] The gain-flattening filter module 303 includes an optical fiber 310, an optical fiber 320, a relay lens system 330, a relay lens system 340, and a gain-flattening filter 350. The gain-flattening filter module 303 includes, in order from the optical fiber 310, the relay lens system 330, the gain-flattening filter 350, the relay lens system 340, and the optical fiber 320.

[0124] The optical fiber 310 is a multi-core optical fiber. An optical signal is input to the gain-flattening filter module 303 from some of the cores of the optical fiber 310. The gain-flattening filter module 303 outputs the optical signal from the remaining cores of the optical fiber 310.

[0125] The optical fiber 320 is a multi-core optical fiber. An optical signal is input to the gain-flattening filter module 303 from some of the cores of the optical fiber 320. The gain-flattening filter module 303 outputs the optical signal from the remaining cores of the optical fiber 320.

[0126] The core arrangement of the optical fiber 320 in a cross section perpendicular to the direction in which the optical fiber 320 extends is the same as the core arrangement of the optical fiber 310 in a cross section perpendicular to the direction in which the optical fiber 320 extends. The core arrangement of the optical fiber 320 in a cross section perpendicular to the direction in which the optical fiber 320 extends may be an arrangement similar to the core arrangement of the optical fiber 310 in a cross section perpendicular to the direction in which the optical fiber 310 extends. In the gain-flattening filter module 303, light emitted from any of the multiple cores in the optical fiber 310 is incident on any of the cores in the optical fiber 320. In the gain-flattening filter module 303, light emitted from any of the multiple cores in the optical fiber 320 is incident on any of the cores in the optical fiber 310.

[0127] The relay lens system 330 includes, in order from the optical fiber 310, a lens 331 and a lens 332. A focal plane FP31 of the lens 331 is disposed on the end face 310S of the optical fiber 310. The lenses 331 and 332 share a focal plane FP33. Since the lenses 331 and 332 share the focal plane FP33, the lenses 331 and 332 function as a relay lens system.

[0128] The relay lens system 340 includes, in order from the optical fiber 320, a lens 341 and a lens 342. A focal plane FP32 of the lens 341 is disposed on the end face 320S of the optical fiber 320. The lenses 341 and 342 share a focal plane FP34. Since the lenses 341 and 342 share the focal plane FP34, the lenses 341 and 342 function as a relay lens system.

[0129] The focal plane FP35 of the lens 332 and the focal plane FP36 of the lens 342 coincide with each other. That is, the relay lens system 330 and the relay lens system 340 are arranged so that the focal plane FP35 of the lens 332 and the focal plane FP36 of the lens 342 coincide with each other. The relay lens system 330 and the relay lens system 340 are arranged so that the focal plane FP35 of the lens 332 and the focal plane FP36 of the lens 342 coincide with each other, whereby the relay lens system 330 and the relay lens system 340 form a dual relay lens system.

[0130] The characteristics of the gain-flattening filter 350 will now be described. FIG. 9 is a diagram showing an example of the characteristics of the gain-flattening filter module 303 in the optical fiber amplifier 4, which is an example of an optical fiber amplifier equipped with the optical module according to this embodiment. FIG. 9 shows the transmission loss spectrum for a dielectric multilayer film in the gain-flattening filter 350 at an incident angle of 4 degrees. The horizontal axis of FIG. 9 represents wavelength (unit: nanometers), and the vertical axis of FIG. 9 represents transmission loss (unit: decibels). The gain of an erbium-doped optical fiber amplifier is wavelength-dependent. The gain-flattening filter 350, which is a gain-flattening filter, has transmission characteristics that flatten the gain spectrum of the erbium-doped optical fiber amplifier (optical amplifier) ​​in the wavelength band of the signal light, for example, from 1528 nm to 1564 nm. Specifically, the deviation of the gain from the average core-to-core gain in the optical functional device 50, which is a gain-flattening filter, is preferably ±0.5 dB or less in the signal wavelength band.

[0131] Another example of an optical fiber amplifier including the optical module according to this embodiment will now be described. Fig. 10 is a diagram showing an outline of an optical fiber amplifier 5, which is an example of an optical fiber amplifier including the optical module according to this embodiment.

[0132] The optical fiber amplifier 5 is a multi-core erbium-doped optical fiber amplifier that supports counter-propagation.

[0133] The optical fiber amplifier 5 includes an optical isolator 401 in place of the optical isolator 301 and the pumping light combiner 304 in the optical fiber amplifier 4, and an optical isolator 402 in place of the optical isolator 302 and the pumping light combiner 305. The optical fiber amplifier 5 includes EDFs 408a and 408b in place of the EDFs 308a and 308b in the optical fiber amplifier 4. The optical fiber amplifier 5 includes pumping lasers 406a, 406b, 406c, and 406d. Furthermore, the optical fiber amplifier 5 includes pumping lasers 407a, 407b, 407c, and 407d.

[0134] All cores of each of the EDFs 408a and 408b are doped with erbium. The optical fiber amplifier 5 performs both forward pumping and backward pumping in each of the EDFs 408a and 408b.

[0135] Each of the optical isolator 401 and the optical isolator 402 is, for example, an optical module 3 which is an example of an optical module according to the third embodiment.

[0136] Fig. 11 is a diagram illustrating the characteristics of an optical fiber amplifier 5, which is an example of an optical fiber amplifier equipped with an optical module according to this embodiment. The horizontal axis of Fig. 11 represents conversion efficiency (unit: %), and the vertical axis represents noise figure (unit: decibels). Loss in the graph in Fig. 11 represents the insertion loss of the pumping light in the gain-flattening filter module, and Power Ratio in the graph represents the ratio of the pumping light power in forward pumping to the pumping light power in backward pumping. Ex in Fig. 11 represents the results for the optical fiber amplifier 4.

[0137] 11, it is possible to increase the noise figure while increasing the conversion efficiency by performing both forward and backward pumping using the optical fiber amplifier 5. From the results for the optical fiber amplifier 4 shown in Ex of Fig. 11, it is sufficient for the insertion loss for the pump light in the gain-flattened filter module to be 3 dB or less, and even better if it is 1 dB or less.

[0138] Conventionally, when designing a gain-flattening filter, the filter is designed to flatten the gain of the signal light. However, the attenuation of the pump light has not been considered. As shown in Figure 11, the pump light is not attenuated, so the conversion efficiency can be improved by effectively utilizing the pump light.

[0139] 1, 2, 3 Optical module 4, 5 Optical fiber amplifier 10, 20, 110, 120, 210, 220, 310, 320 Optical fiber 10S, 20S, 110S, 120S, 310S, 320S End face 30, 40, 130, 140, 230, 240, 330, 340 Relay lens system 31, 32, 41, 42, 131, 132, 141, 142, 231, 232, 241, 242, 331, 332, 341, 342 Lens 50, 150, 250 Optical functional element 51, 54, 151, 154, 251, 254 Polarization separation element 52, 152, 252 Faraday rotator 53, 153, 253 Half-wave plates 53a, 53b, 53c, 53d Region 260 Optical fiber bundle 271 Lens 272 Dichroic mirror 301, 302, 401, 402 Optical isolator 303 Gain-flattening filter module 304, 305 Pumping light multiplexer 306a, 306b, 307a, 307b, 406a, 406b, 406c, 406d, 407a, 407b, 407c, 407d Pumping laser 308a, 308b, 408a, 408b Multi-core erbium-doped optical fiber (EDF) 350 Gain-flattening filter A, A1, A2, Ap, B, B1, B2, Bp, C, C1, C2, Cp, D, D1, D2, Dp Signal light FP1, FP2, FP3, FP4, FP5, FP6, FP11, FP12, FP13, FP14, FP15, FP16, FP31, FP32, FP33, FP34, FP35, FP36 Focal plane α Angle Ex Results for optical fiber amplifier 4

Claims

1. A first optical waveguide assembly comprising a plurality of optical waveguides, with light entering and exiting each of the plurality of optical waveguides at a first end face; a second optical waveguide assembly comprising a plurality of optical waveguides, with light entering and exiting each of the plurality of optical waveguides at a second end face; a first relay lens system comprising a first lens and a second lens, with the first lens having a first focal plane at the first end face and the first lens and the second lens sharing a second focal plane; a second relay lens system comprising a third lens and a fourth lens, with the third lens having a third focal plane at the second end face and the third lens and the fourth lens sharing a fourth focal plane; an optical functional element disposed intermediate the second lens and the fourth lens, the optical functional element comprising: a first polarization separation element, a Faraday rotator, a half-wave plate including two types of regions whose fast axes are oriented 45 degrees apart from each other; and a second polarization separation element sandwiching the Faraday rotator and the half-wave plate between the first polarization separation element and the Faraday rotator; wherein a double relay lens system is formed by sharing a fifth focal plane of the second lens in the first relay lens system and a sixth focal plane of the fourth lens in the second relay lens system; the first optical waveguide assembly and the second optical waveguide assembly have waveguide arrangements that are similar to each other; and an imaging magnification of the double relay lens system is equal to a similarity ratio of the waveguide arrangements in the first optical waveguide assembly and the second optical waveguide assembly.

2. A first optical waveguide assembly comprising a plurality of optical waveguides, with light entering and exiting each of the plurality of optical waveguides at a first end face; a second optical waveguide assembly comprising a plurality of optical waveguides, with light entering and exiting each of the plurality of optical waveguides at a second end face; a first relay lens system comprising a first lens and a second lens, with the first lens having a first focal plane at the first end face and the first lens and the second lens sharing a second focal plane; a second relay lens system comprising a third lens and a fourth lens, with the third lens having a third focal plane at the second end face and the third lens and the fourth lens sharing a fourth focal plane; an optical functional element disposed between the second lens and the fourth lens, comprising a Faraday rotator and a half-wave plate including two types of regions whose fast axes are oriented 45 degrees apart from each other; and a first polarization separation element disposed between the first lens and the second lens. an optical module comprising: a second polarization separation element between the third lens and the fourth lens; a double relay lens system formed by sharing a fifth focal plane of the second lens in the first relay lens system and a sixth focal plane of the fourth lens in the second relay lens system; the first optical waveguide assembly and the second optical waveguide assembly having waveguide arrangements that are similar to each other; and an imaging magnification of the double relay lens system equal to a similarity ratio of the waveguide arrangements in the first optical waveguide assembly and the second optical waveguide assembly.

3. The optical module according to claim 1 or 2, wherein at least one of the first optical waveguide assembly and the second optical waveguide assembly is a multi-core optical fiber.

4. An optical module according to any one of claims 1 to 3, comprising: a third optical waveguide assembly comprising a plurality of optical waveguides; a fifth lens; and a dichroic mirror provided between the third lens and the fourth lens, wherein the dichroic mirror transmits light propagating between the third lens and the fourth lens and reflects light emitted from the third optical waveguide assembly and transmitted through the fifth lens, and the light reflected by the dichroic mirror is incident on the optical waveguides provided in the second optical waveguide assembly.

5. An optical fiber amplifier comprising: an erbium-doped optical fiber having a first end and a second end; a first pumping light combiner connected to the first end; a first optical isolator that is an optical module described in any one of claims 1 to 4 and that is connected to the first pumping light combiner; a second pumping light combiner connected to the second end; and a second optical isolator that is an optical module described in any one of claims 1 to 4 and that is connected to the second pumping light combiner.

6. An optical fiber amplifier comprising: a first erbium-doped optical fiber having a first end and a second end; a second erbium-doped optical fiber having a third end and a fourth end; a gain-flattening filter connected between the second end and the third end; a first pumping light combiner connected to the first end; a first optical isolator that is an optical module described in any one of claims 1 to 4 and that is connected to the first pumping light combiner; a second pumping light combiner connected to the fourth end; and a second optical isolator that is an optical module described in any one of claims 1 to 4 and that is connected to the second pumping light combiner.

7. An optical fiber amplifier comprising: a first erbium-doped optical fiber having a first end and a second end; a second erbium-doped optical fiber having a third end and a fourth end; a gain-flattening filter connected between the second end and the third end; a first optical isolator that is an optical module according to any one of claims 1 to 4 and that is connected to the first end; and a second optical isolator that is an optical module according to any one of claims 1 to 4 and that is connected to the fourth end.

8. The gain flattening filter comprises: a fourth optical waveguide assembly having a plurality of optical waveguides, light entering and exiting each of the plurality of optical waveguides at a third end face; a fifth optical waveguide assembly having a plurality of optical waveguides, light entering and exiting each of the plurality of optical waveguides at a fourth end face; a third relay lens system having a sixth lens and a seventh lens, the sixth lens having a seventh focal plane at the third end face and the sixth lens and the seventh lens sharing an eighth focal plane; a fourth relay lens system having an eighth lens and a ninth lens, the eighth lens having a ninth focal plane at the fourth end face and the eighth lens and the ninth lens sharing a tenth focal plane; and a dielectric multilayer film filter disposed between the seventh lens and the ninth lens and flattening the gain spectrum, wherein the eleventh focal plane of the seventh lens in the third relay lens system and the twelfth focal plane of the ninth lens in the fourth relay lens system are shared to form a second dual relay lens system, 8. The optical fiber amplifier according to claim 6, wherein the fourth optical waveguide assembly and the fifth optical waveguide assembly have waveguide arrangements that are similar to each other, and an imaging magnification of the second dual relay lens system is equal to a similarity ratio of the waveguide arrangements in the fourth optical waveguide assembly and the fifth optical waveguide assembly.

9. The optical fiber amplifier according to claim 8, wherein the gain-flattening filter has an insertion loss of 3 dB or less for pumping light.

10. A multi-core erbium-doped optical fiber having one or more erbium-doped cores and one or more non-erbium-doped cores.

11. The multi-core erbium-doped optical fiber according to claim 10, having m (m is an integer of 2 or greater) erbium-doped cores and m non-erbium-doped cores, wherein the mth combinations in ascending order of the spacing between two cores are all combinations of an erbium-doped core and a non-erbium-doped core.

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