Optical module and optical fiber amplifier

Abstract

This optical module comprises: a first optical waveguide assembly in which light is emitted from an optical waveguide at a first end surface; a second optical waveguide assembly in which light is incident on an optical waveguide at a second end surface; a first relay lens system which is provided with a first lens and a second lens, the first lens having a first focal plane at the first end surface, and the first lens and the second lens sharing a second focal plane; a second relay lens system which is provided with a third lens and a fourth lens, the third lens having a third focal plane at the second end surface, and the third lens and the fourth lens sharing a fourth focal plane; and an optical function element. The first relay lens system and the second relay lens system form a double relay lens system in which a fifth focal plane of the second lens and a sixth focal plane of the fourth lens are shared. The optical functional element is disposed between the second lens and the fourth lens.

Description

Optical modules and optical fiber amplifiers

[0001] This application claims priority to Japanese Patent Application No. 2024-072354, filed April 26, 2024, and incorporates by reference all of the contents of said Japanese application.

[0002] Patent Document 1 describes an example of a multi-core optical fiber component that utilizes a spatial optical system.

[0003] Non-Patent Document 1 describes that the flatness of the gain spectrum of a multi-core erbium-doped optical fiber amplifier is impaired due to the different angles of incidence on the dielectric multilayer film for each core.

[0004] International Publication No. 2022 / 019019

[0005] Setsufumi Otsuka and five others, "Optical Amplifier Using Multi-Core Erbium-Doped Fiber," Sumitomo Electric Technical Review, No. 200, pp. 81-86

[0006] The present disclosure relates to a first optical waveguide assembly including a plurality of optical waveguides, with light exiting from the optical waveguides at a first end face; a second optical waveguide assembly including a plurality of optical waveguides, with light entering the 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; and 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. the first relay lens system and the second relay lens system form a double relay lens system that shares a fifth focal plane of the second lens and a sixth focal plane of the fourth lens, the arrangement of the plurality of optical waveguides of the first optical waveguide assembly at the first end face and the arrangement of the plurality of optical waveguides of the second optical waveguide assembly at the second end face are similar to each other, the imaging magnification of the double relay lens system is equal to the similarity ratio of the similar shapes, and the optical functional element is an optical module disposed between the second lens and the fourth lens.

[0007] FIG. 1 is a diagram showing an outline of an optical module according to a first embodiment. FIG. 2 is a diagram showing characteristics of an optical functional element used in the optical module according to the first embodiment. FIG. 3 is a diagram showing an outline of an optical module according to a second embodiment. FIG. 4 is a diagram showing an outline of an optical module according to a third embodiment. FIG. 5 is a diagram showing an outline of an optical module according to a fourth embodiment. FIG. 6 is a diagram showing an outline of an optical module according to a fifth 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 an optical fiber amplifier including an optical module according to this embodiment. FIG. 9 is a diagram explaining the arrangement of optical functional elements in the optical module according to this embodiment. FIG. 10 is a diagram showing an outline of an optical module of a reference example.

[0008] [Problem to be Solved by the Present Disclosure] In a multi-core erbium-doped optical fiber amplifier, there have been cases where the gain of all cores in the multi-core erbium-doped optical fiber amplifier cannot be flattened even though the amplifier is equipped with a gain-flattening filter.

[0009] The present disclosure provides an optical module that reduces variations in characteristics between waveguides in an optical waveguide assembly that includes a plurality of optical waveguides.

[0010] Effect of the Present Disclosure According to the optical module of the present disclosure, in an optical waveguide assembly including a plurality of optical waveguides, variations in characteristics between the waveguides can be reduced.

[0011] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described.

[0012] An optical module according to a first aspect includes a first optical waveguide assembly having a plurality of optical waveguides, with light exiting from the optical waveguides at a first end face; a second optical waveguide assembly having a plurality of optical waveguides, with light entering the 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; the first relay lens system and the second relay lens system form a double relay lens system that shares a fifth focal plane of the second lens and a sixth focal plane of the fourth lens, the arrangement of the plurality of optical waveguides of the first optical waveguide assembly at the first end face and the arrangement of the plurality of optical waveguides of the second optical waveguide assembly at the second end face are similar in shape to each other, the imaging magnification of the double relay lens system is equal to the similarity ratio of the similar shapes, and the optical functional element is an optical module disposed between the second lens and the fourth lens.

[0013] According to the optical module of the first aspect, when an optical functional element having angle dependency is used, the angle of incidence at the optical functional element can be made uniform, thereby reducing the variation in characteristics between waveguides in an optical waveguide assembly having a plurality of optical waveguides.

[0014] An optical module according to a second aspect is the optical module according to the first aspect, wherein at least one of the first optical waveguide assembly and the second optical waveguide assembly is a multi-core optical fiber.

[0015] According to the optical module of the second aspect, it is possible to reduce variations in characteristics between cores of a multi-core optical fiber.

[0016] An optical module according to a third aspect is the optical module according to either the first or second aspect, further comprising a dielectric multilayer filter as the optical functional element.

[0017] According to the optical module of the third aspect, in an optical module using a dielectric multilayer filter having angle dependency as an optical functional element, it is possible to reduce variations in characteristics between the waveguides in the optical waveguide assembly.

[0018] An optical module of a fourth aspect is the optical module of the third aspect, wherein the dielectric multilayer filter comprises a dielectric multilayer, and the dielectric multilayer is arranged in a space between a first plane parallel to the fifth focal plane and 5 mm away on the optical axis from the fifth focal plane toward the second lens, and a second plane parallel to the sixth focal plane and 5 mm away on the optical axis from the sixth focal plane toward the fourth lens.

[0019] According to the optical module of the fourth aspect, in an optical module using a dielectric multilayer film having angle dependency as an optical functional element, it is possible to reduce variations in characteristics between the waveguides in the optical waveguide assembly.

[0020] An optical module of a fifth aspect is the optical module of the third aspect, wherein the dielectric multilayer filter comprises a dielectric multilayer, and the dielectric multilayer is disposed in a space between a first plane parallel to the fifth focal plane and 1 mm away on the optical axis from the fifth focal plane toward the second lens, and a second plane parallel to the sixth focal plane and 1 mm away on the optical axis from the sixth focal plane toward the fourth lens.

[0021] According to the optical module of the fifth aspect, in an optical module using a dielectric multilayer film having angle dependency as an optical functional element, it is possible to reduce variations in characteristics between waveguides in an optical waveguide assembly.

[0022] An optical module according to a sixth aspect is the optical module according to any one of the third to fifth aspects, wherein the dielectric multilayer filter has a transmission characteristic that flattens the gain spectrum of the optical amplifier under specific operating conditions.

[0023] According to the optical module of the sixth aspect, in an optical module using a gain flattening filter having angle dependency as an optical functional element, it is possible to reduce variations in characteristics between waveguides in an optical waveguide assembly.

[0024] An optical module according to a seventh aspect is the optical module according to any one of the first to sixth aspects, in which the optical functional element includes an optical isolator.

[0025] According to the optical module of the seventh aspect, in an optical module including an optical isolator as an optical functional element, it is possible to reduce variations in characteristics between the waveguides in the optical waveguide assembly.

[0026] An optical module of an eighth aspect is an optical module of any one of the first to sixth aspects, in which the first lens and the third lens are gradient index lenses, the first lens and the first optical waveguide assembly are connected in physical contact at the first end face, and the third lens and the second optical waveguide assembly are connected in physical contact at the second end face.

[0027] According to the optical module of the eighth aspect, the simplified optical system can reduce variations in characteristics between the waveguides in the optical waveguide assembly.

[0028] An erbium-doped optical fiber amplifier according to a first aspect is an erbium-doped optical fiber amplifier equipped with the optical module according to any one of the third to sixth aspects, in which the dielectric multilayer filter has transmission characteristics that flatten the gain spectrum of the erbium-doped optical fiber under specific operating conditions.

[0029] According to the erbium-doped optical fiber amplifier of the first aspect, the variation in characteristics between the cores of the multi-core optical fiber can be further reduced.

[0030] [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. Note that 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.

[0031] In the description of the specification and drawings of each embodiment, components having substantially the same or corresponding functions may be designated by the same reference numerals to avoid redundant explanation. In addition, the scale of each part in the drawings may differ from the actual scale to facilitate understanding.

[0032] The parallel and perpendicular directions may be misaligned to the extent that the effect of the embodiment is not impaired. The corners may be rounded. The parallel and perpendicular directions may include approximately parallel and approximately perpendicular, respectively.

[0033] For example, "substantially parallel" means that even if two lines or two surfaces are not completely parallel to each other, they can be treated as parallel to each other within the range allowed for manufacturing. "Substantially perpendicular" also means that two lines or two surfaces are perpendicular as long as their relative positions are within the range allowed for manufacturing.

[0034] The following embodiments will be described, and at least some of the embodiments described below may be combined in any manner.

[0035] First Embodiment An optical module according to a first embodiment will be described using a specific example. Fig. 1 is a diagram showing an outline of an optical module 1, which is an example of the optical module according to the first embodiment.

[0036] The optical module 1 flattens the gain spectrum in an erbium-doped optical fiber amplifier (EDFA). The optical module 1 includes a gain-flattening filter. FIG. 1 shows an optical system including a gain-flattening filter used in an erbium-doped optical fiber amplifier. The optical module 1 has a multi-core optical fiber input and output.

[0037] 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 this 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 light emitted from the core of the optical fiber 10 propagates to the core of the optical fiber 20. This is the same in the following figures.

[0038] The optical fiber 10 is a multi-core optical fiber. An optical signal is input to the optical module 1 from the optical fiber 10. The optical fiber 20 is a multi-core optical fiber. The optical module 1 outputs an optical signal from the optical fiber 20.

[0039] 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.

[0040] 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. Note that the core arrangement of the optical fiber 20 may be similar to the core arrangement of the optical fiber 10. 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.

[0041] 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.

[0042] 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. Furthermore, 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 applies to the following description.

[0043] 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.

[0044] 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 double relay lens system.

[0045] 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 arrangement of the multiple cores at the end face 10S of the optical fiber 10 and the arrangement of the multiple cores at the end face 20S of the optical fiber 20 need only be similar to each other. 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 set to be equal to the similarity ratio of the core arrangement, which is the waveguide arrangement in the optical fiber 10 and the optical fiber 20. In other words, if the optical waveguide arrangements in the optical fiber 10 and the optical fiber 20 are similar in shape, the imaging magnification of the dual relay lens system formed by the relay lens system 30 and the relay lens system 40 should be equal to the similarity ratio of the similar shapes. 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 homothetic ratio, it is not limited to the case where the imaging magnification of the lens system is strictly equal to the homothetic ratio. For example, if the imaging magnification of the lens system is within a manufacturing allowable range with respect to the homothetic ratio, this is included in the case where the imaging magnification of the lens system is equal to the homothetic ratio. The same applies to the following description.

[0046] Each of the lenses 31 and 41 is an aspherical lens having, for example, a focal length of 0.7 mm and a diameter of 2.5 mm. Each of the lenses 32 and 42 is a doublet lens having, for example, a focal length of 19.0 mm and a diameter of 12.7 mm.

[0047] The optical functional element 50 performs a specific function on light passing through it. One example of the optical functional element 50 is a gain-flattening filter. The optical functional element 50 is a plate-shaped element. The optical functional element 50 has a square entrance and exit surface with each side measuring 2.0 mm. The optical functional element 50 also has a thickness of 2.0 mm and a wedge angle of 1 degree.

[0048] The optical functional element 50, which is a gain-flattening filter, has a dielectric multilayer film laminated on a first incident and exit surface of a wedge prism made of optical glass, and an anti-reflection film with a reflectance of 0.1% laminated on a second incident and exit surface of the optical functional element 50, which is different from the first incident and exit surface.

[0049] The characteristics of a gain-flattening filter will be described as an example of an optical functional element 50. FIG. 2 is a diagram showing an example of the characteristics of the optical functional element 50 used in the optical module 1, which is an example of the optical module according to the first embodiment. FIG. 2 shows a transmission loss spectrum for a dielectric multilayer film in the optical functional element 50 at an incident angle of 4 degrees. The horizontal axis of FIG. 2 represents wavelength (unit: nanometers), and the vertical axis of FIG. 2 represents transmission loss (unit: decibels). The gain of an erbium-doped optical fiber amplifier is wavelength-dependent. The optical functional element 50, 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 gain in the optical functional element 50, which is a gain-flattening filter, is preferably ±0.5 dB or less in the signal wavelength band.

[0050] In the optical functional element 50, when the angle of incidence changes, for example, the transmission loss spectrum shifts in parallel along the wavelength axis. In particular, when the angle of incidence increases, the spectrum shifts in parallel toward shorter wavelengths.

[0051] The optical functional element 50 is disposed between the lens 32 in the relay lens system 30 and the lens 42 in the relay lens system 40 .

[0052] According to the optical module of the first embodiment, it is possible to reduce the core dependency on the angle of incidence of light incident on the optical functional element. By reducing the core dependency on the angle of incidence of light incident on the optical functional element, the optical module of the first embodiment can reduce the core dependency of the optical function even when using an optical functional element whose characteristics in the operating band are incident angle dependent. In other words, according to the optical module of the first embodiment, it is possible to reduce the variation in characteristics between waveguides. Therefore, the optical module of the first embodiment can reduce the core dependency to the degree associated with core eccentricity between input and output multi-core optical fibers (typically, about 0.1 dB).

[0053] In the above example, a multi-core optical fiber is used as the optical waveguide assembly, but the optical waveguide assembly is not limited to a multi-core optical fiber. The optical waveguide assembly may include a plurality of optical waveguides. The optical waveguide assembly may be, for example, a fiber bundle formed by bundling a plurality of optical fibers. Furthermore, a multi-core optical fiber and an optical waveguide assembly may be combined. In other words, at least one of the first optical waveguide assembly and the second optical waveguide assembly may be a multi-core optical fiber. In the present disclosure, when the waveguide arrangement of the first optical waveguide assembly in a cross section perpendicular to the extension direction of the first optical waveguide assembly is similar in shape to the waveguide arrangement of the second optical waveguide assembly in a cross section perpendicular to the extension direction of the second optical waveguide assembly, the first optical waveguide assembly and the second optical waveguide assembly are said to have waveguide arrangements that are similar in shape to each other. The same applies to the following description.

[0054] Second Embodiment An optical module according to a second embodiment will be described using a specific example. Fig. 3 is a diagram showing an outline of an optical module 2, which is an example of the optical module according to the second embodiment. The optical module according to the second embodiment uses a graded index (GRIN) lens that is in physical contact with the optical fiber.

[0055] The optical module 2 flattens the gain spectrum of the erbium-doped optical fiber amplifier, similar to the optical module 1. The optical module 2 includes a gain-flattening filter. Fig. 3 shows an optical system including a gain-flattening filter used in an erbium-doped optical fiber amplifier. The optical module 2 has a multi-core optical fiber input and output.

[0056] 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 50. The optical module 2 includes, in order from the optical fiber 110, the relay lens system 130, the optical functional element 50, the relay lens system 140, and the optical fiber 120.

[0057] Regarding the configuration of the optical module 2 that is common to the optical module 1, the description of the optical module 1 should be referred to, and detailed description thereof will be omitted here.

[0058] The optical fiber 110 is a multi-core optical fiber. An optical signal is input to the optical module 2 from the optical fiber 110. The optical fiber 120 is a multi-core optical fiber. The optical module 2 outputs the optical signal from the optical fiber 120.

[0059] Each of the optical fibers 110 and 120 is, for example, a seven-core optical fiber having a mode field diameter of 10.0 μm at a wavelength of 1550 nm, a cladding diameter of 125 μm, and cores arranged at the vertices and center of a regular hexagon with sides of 35 μm in a cross section perpendicular to the direction in which the optical fiber extends.

[0060] 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. Note that 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 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 other words, the arrangement of the multiple cores at the end face 110S of the optical fiber 110 and the arrangement of the multiple cores at the end face 120S of the optical fiber 120 may be similar to each other. 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.

[0061] The relay lens system 130 includes, in order from the optical fiber 110, a lens 131 and a lens 132. The lens 131 is a gradient index lens. The lens 131 is connected to the end face 110S of the optical fiber 110 via physical contact (PC). Here, a physical contact connection refers to a connection that does not create a gap in the optical path or a connection that does not include an interface with air in the optical path. Examples of physical contact connections include a connection in which the end faces are fixed in contact with each other, a connection in which the end faces are fixed and maintained in a state in which a material with refractive index matching function is held between them, a fusion splice, or bonding using an adhesive, particularly bonding using an adhesive with refractive index matching function. The focal plane FP11 of the lens 131 is located at the end face 110S of the optical fiber 110. The lens 131 and the lens 132 share a focal plane FP13. By sharing the focal plane FP13, the lens 131 and the lens 132 function as a relay lens system.

[0062] The relay lens system 140 includes, in order from the optical fiber 120, a lens 141 and a lens 142. The lens 141 is a gradient index lens. The lens 141 is connected to the end face 120S of the optical fiber 20 in physical contact. A focal plane FP12 of the lens 141 is disposed on the end face of the optical fiber 20. 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.

[0063] The optical fiber 10, which is a multi-core optical fiber, and the lens 131 may be connected by physical contact. The optical fiber 20, which is a multi-core optical fiber, and the lens 141 may be connected by physical contact. By using physical contact, the number of glass-to-air interfaces requiring anti-reflection coatings is reduced by four compared to the optical module according to the first embodiment. The reduction in the number of glass-to-air interfaces requiring anti-reflection coatings makes it easier to reduce the insertion loss of the entire optical system in the optical module 2.

[0064] 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 double relay lens system.

[0065] 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. Therefore, the imaging magnification of the dual relay lens system formed by the relay lens system 130 and the relay lens system 140 is 1. When the optical waveguide arrangements in the optical fiber 110 and the optical fiber 120 are similar in shape, the imaging magnification of the dual relay lens system formed by the relay lens system 130 and the relay lens system 140 should be equal to the similarity ratio of the similar shapes.

[0066] Each of the lenses 131 and 141 has, for example, a pitch of 0.25, a central refractive index of 1.5901, and a gradient coefficient of 0.9995 mm. －１ The lens 131 is a gradient index lens having a focal length of 6.0 mm and a diameter of 3.0 mm. The exit end 131S of the lens 131 and the exit end 141S of the lens 141 are polished to have an angle of 4 degrees. The lens 132 and the lens 142 are each a plano-convex lens having a focal length of 6.0 mm and a diameter of 3.0 mm, for example.

[0067] The optical functional element 50 is disposed between the lens 132 in the relay lens system 130 and the lens 142 in the relay lens system 140 .

[0068] The optical module according to the second embodiment has the same effects as the optical module according to the first embodiment. Furthermore, according to the optical module 2 according to the second embodiment, by using a gradient index lens for the lens adjacent to the optical fiber, the number of surfaces on which anti-reflection coatings are formed can be reduced by four, and the insertion loss of the optical module can be reduced.

[0069] Third Embodiment An optical module according to a third embodiment will be described using a specific example. Fig. 4 is a diagram showing an outline of an optical module 3, which is an example of the optical module according to the third embodiment. The optical module according to the third embodiment further comprises the function of an optical isolator in addition to the optical module according to the first embodiment.

[0070] Like the optical module 1, the optical module 3 flattens the gain spectrum of the erbium-doped optical fiber amplifier. The optical module 3 also functions as an optical isolator. The optical module 3 includes an optical isolator and a gain-flattening filter. Fig. 4 shows an optical system including a gain-flattening filter used in an erbium-doped optical fiber amplifier. The optical module 3 has a multi-core optical fiber input and output.

[0071] The optical module 3 includes an optical fiber 10, an optical fiber 20, a relay lens system 230, a relay lens system 240, an optical functional element 50, and an optical isolator 60. The optical module 3 includes, in order from the optical fiber 10, the relay lens system 230, the optical isolator 60, the optical functional element 50, the relay lens system 240, and the optical fiber 20.

[0072] Regarding the configuration of the optical module 3 that is common to the optical module 1, the description of the optical module 1 should be referred to, and detailed description thereof will be omitted here.

[0073] The relay lens system 230 includes, in order from the optical fiber 10, a lens 231 and a lens 232. The focal plane of the lens 231 is disposed at the end face 10S of the optical fiber 10. The lenses 231 and 232 share a focal plane. Because the lenses 231 and 232 share a focal plane, the lenses 231 and 232 function as a relay lens system.

[0074] The relay lens system 240 includes, in this order from the optical fiber 20, a lens 241 and a lens 242. The focal plane of the lens 241 is disposed at the end face 20S of the optical fiber 20. The lenses 241 and 242 share a focal plane. Because the lenses 241 and 242 share a focal plane, the lenses 241 and 242 function as a relay lens system.

[0075] Each of the lenses 231 and 241 is, for example, a plano-convex lens with a focal length of 2.0 mm and a diameter of 1.5 mm. Note that, as in the optical module according to the second embodiment, each of the lenses 231 and 241 may be a gradient index lens. Each of the lenses 232 and 242 is, for example, a plano-convex lens with a focal length of 12.0 mm and a diameter of 6.0 mm.

[0076] Specifically, the optical isolator 60 transmits light propagating from the optical fiber 10 to the optical fiber 20 and blocks light propagating from the optical fiber 20 to the optical fiber 10. The optical isolator 60 includes a polarizer 61, a Faraday rotator 62, a half-wave plate 63, and a polarizer 64. The optical isolator 60 is also an example of an optical functional element. The optical module according to this embodiment may include only an optical isolator as the optical functional element, or may include an optical isolator in combination with another optical functional element, for example, a gain-flattening filter.

[0077] The optical functional element 50 is disposed between the lens 232 in the relay lens system 230 and the lens 242 in the relay lens system 240 .

[0078] The optical module according to the third embodiment has the same effects as the optical module according to the first embodiment. Furthermore, the gain-flattening filter is often disposed downstream of the optical isolator. The optical module according to the third embodiment combines two types of optical modules, a gain-flattening filter and an optical isolator, thereby reducing the number of fusion splices and free-space optical splices between multi-core optical fibers by one each. Reducing the number of splices reduces loss and crosstalk after output from the erbium-doped optical fiber.

[0079] Fourth Embodiment An optical module according to a fourth embodiment will be described using a specific example. Fig. 5 is a diagram showing an outline of an optical module 4, which is an example of an optical module according to the fourth embodiment. In the optical module according to the fourth embodiment, the core diameters of the input and output optical fibers are different.

[0080] The optical module 4 flattens the gain spectrum of the erbium-doped optical fiber amplifier, similar to the optical module 1. The optical module 4 also functions as an optical isolator. The optical module 4 includes an optical isolator and a gain-flattening filter. Fig. 5 shows an optical system including a gain-flattening filter used in an erbium-doped optical fiber amplifier. The optical module 4 has a multi-core optical fiber input and output.

[0081] The optical module 4 includes an optical fiber 310, an optical fiber 320, a relay lens system 230, a relay lens system 240, an optical functional element 50, and an optical isolator 60. The optical module 4 includes, in order from the optical fiber 310, the relay lens system 230, the optical isolator 60, the optical functional element 50, the relay lens system 240, and the optical fiber 320.

[0082] Regarding the configuration of the optical module 4 that is common to the optical module 3, the description of the optical module 3 should be referred to, and detailed description thereof will be omitted here.

[0083] The optical fiber 310 is a multi-core optical fiber. An optical signal is input to the optical module 4 from the optical fiber 310. The optical fiber 310 is a six-core optical fiber having a mode field diameter of 6.0 μm at a wavelength of 1550 nm, a cladding diameter of 125 μm, and cores arranged at the vertices of a regular hexagon with sides of 25 μm. The core near the end face 310S of the optical fiber 310 is expanded by heat treatment to form an expanded portion 310a so that the mode field diameter at the end face 310S of the optical fiber 310 becomes 10.0 μm.

[0084] The optical fiber 320 is a multi-core optical fiber. The optical module 4 outputs an optical signal from the optical fiber 320. The optical fiber 320 is a six-core optical fiber having a mode field diameter of 10.0 μm at a wavelength of 1550 nm, a cladding diameter of 125 μm, and cores arranged at the vertices of a regular hexagon with sides of 25 μm.

[0085] The optical module according to the fourth embodiment has the same effects as the optical module according to the third embodiment. Furthermore, the optical module according to the fourth embodiment can achieve low-loss connection between multi-core optical fibers having different core diameters.

[0086] Fifth Embodiment An optical module according to a fifth embodiment will be described using a specific example. Fig. 6 is a diagram showing an outline of an optical module 5, which is an example of an optical module according to the fifth embodiment. In the optical module according to the fourth embodiment, the core arrangement in the input optical fiber is different from the core arrangement in the output optical fiber.

[0087] The optical module 5 flattens the gain spectrum of the erbium-doped optical fiber amplifier, similar to the optical module 1. The optical module 5 also functions as an optical isolator. The optical module 5 includes an optical isolator and a gain-flattening filter. Fig. 6 shows an optical system including a gain-flattening filter used in an erbium-doped optical fiber amplifier. The optical module 5 has a multi-core optical fiber input and output.

[0088] The optical module 5 includes an optical fiber 410, an optical fiber 420, a relay lens system 430, a relay lens system 440, an optical functional element 50, and an optical isolator 60. The optical module 5 includes, in order from the optical fiber 410, the relay lens system 430, the optical isolator 60, the optical functional element 50, the relay lens system 440, and the optical fiber 420.

[0089] Regarding the configuration of the optical module 5 that is common to the optical module 3, the description of the optical module 3 should be referred to, and detailed description thereof will be omitted here.

[0090] The optical fiber 410 is a multi-core optical fiber. An optical signal is input to the optical module 5 from the optical fiber 410. The optical fiber 410 is, for example, a four-core optical fiber having a mode field diameter of 6.0 μm at a wavelength of 1550 nm, a cladding diameter of 125 μm, and cores arranged at the vertices of a square with sides of 25 μm in a cross section perpendicular to the direction in which the optical fiber extends.

[0091] The optical fiber 420 is a multi-core optical fiber. The optical module 5 outputs an optical signal from the optical fiber 420. Each of the optical fibers 420 is, for example, a four-core optical fiber having a mode field diameter of 10.8 μm at a wavelength of 1550 nm, a cladding diameter of 125 μm, and cores arranged at the vertices of a square with sides of 45 μm in a cross section perpendicular to the direction in which the optical fiber extends.

[0092] The relay lens system 430 includes, in this order from the optical fiber 410, a lens 431 and a lens 432. The focal plane of the lens 431 is disposed at the end face 410S of the optical fiber 410. The lenses 431 and 432 share a focal plane. Because the lenses 431 and 432 share a focal plane, the lenses 431 and 432 function as a relay lens system.

[0093] The relay lens system 440 includes, in this order from the optical fiber 420, a lens 441 and a lens 442. The focal plane of the lens 441 is disposed at the end face 420S of the optical fiber 420. The lenses 441 and 442 share a focal plane. Because the lenses 441 and 442 share a focal plane, the lenses 441 and 442 function as a relay lens system.

[0094] The focal plane of lens 432 coincides with the focal plane of lens 442. That is, relay lens system 430 and relay lens system 440 are arranged so that the focal plane of lens 432 coincides with the focal plane of lens 442. By arranging relay lens system 430 and relay lens system 440 so that the focal plane of lens 432 coincides with the focal plane of lens 442, relay lens system 430 and relay lens system 440 form a double relay lens system.

[0095] In the optical module 5, the optical fiber 420 is a multi-core optical fiber having a cross-sectional shape similar to that of the optical fiber 410. The arrangement of multiple cores at the end face 410S of the optical fiber 410 and the arrangement of multiple cores at the end face 420S of the optical fiber 420 need only be similar to each other. The similarity ratio between the core arrangement in the optical fiber 410 and the core arrangement in the optical fiber 420 is 1.8. The imaging magnification of the dual relay lens system formed by the relay lens system 430 and the relay lens system 440 is 1.8 times. In other words, when the optical waveguide arrangements in the optical fiber 410 and the optical fiber 420 are similar in shape, the imaging magnification of the dual relay lens system formed by the relay lens system 430 and the relay lens system 440 should be equal to the similarity ratio of the similar shapes.

[0096] Lens 431 is, for example, a plano-convex lens with a focal length of 1.0 mm and a diameter of 1.5 mm. Lens 441 is, for example, a plano-convex lens with a focal length of 1.8 mm and a diameter of 1.5 mm. Note that, as in the optical module according to the second embodiment, lenses 431 and 441 may each be a gradient index lens. Lens 432 and 442 are, for example, plano-convex lenses with a focal length of 10.8 mm and a diameter of 6.0 mm.

[0097] The optical module according to the fifth embodiment has the same effects as the optical module according to the third embodiment.

[0098] <Optical fiber amplifier> An optical fiber amplifier including the optical module according to this embodiment will be described below. Fig. 7 is a diagram showing an outline of an optical fiber amplifier 6, which is an example of an optical fiber amplifier including the optical module according to this embodiment.

[0099] The optical fiber amplifier 6 is an erbium-doped optical fiber amplifier. The optical fiber amplifier 6 is a core-pumped multi-core erbium-doped optical fiber amplifier.

[0100] The optical fiber amplifier 6 includes a pumping light combiner 510 , a plurality of pumping lasers 520 , a pumping fan-in 530 , an erbium-doped optical fiber 564 , an isolator 540 , and a gain-flattening filter 550 .

[0101] The pumping light multiplexer 510 multiplexes the signal light La and the pumping light Lb, and outputs multiplexed light Lc obtained by multiplexing the signal light La and the pumping light Lb.

[0102] The pumping light multiplexer 510 includes optical fibers 561, 562, and 563. In FIG. 7 , crosses indicate fusion splice points where the optical fibers are fused together. Signal light La is input from the optical fiber 561 to the pumping light multiplexer 510. Pumping light output from each of the multiple pumping lasers 520 is input to the pumping fan-in 530. Pumping light Lb multiplexed by the pumping fan-in 530 is output to the optical fiber 562a. The optical fiber 562a and the optical fiber 562 are fusion-spliced. Pumping light Lb is input from the optical fiber 562 to the pumping light multiplexer 510. The pumping light multiplexer 510 multiplexes the signal light La and the pumping light Lb, and outputs the multiplexed multiplexed light Lc to the optical fiber 563.

[0103] The erbium-doped optical fiber 564 amplifies the signal light La by the pump light Lb contained in the input multiplexed light Lc. The erbium-doped optical fiber 564 is fusion-spliced ​​to the optical fiber 563. The erbium-doped optical fiber 564 is also fusion-spliced ​​to the optical fiber 565. The erbium-doped optical fiber 564 amplifies the signal light La by the pump light Lb contained in the multiplexed light Lc, and outputs amplified light Ld to the optical fiber 565 fusion-spliced ​​to the erbium-doped optical fiber 564.

[0104] The isolator 540 has an optical fiber 565 and an optical fiber 566, and transmits light propagating from the optical fiber 565 to the optical fiber 566, while blocking light propagating from the optical fiber 566 to the optical fiber 565. The isolator 540 outputs the amplified light Ld input from the optical fiber 565 to the optical fiber 566 as amplified light Le. The isolator 540 also blocks the light input from the optical fiber 566.

[0105] The gain flattening filter 550 flattens the gain of the input amplified light Le. The gain flattening filter 550 includes an optical fiber 567 and an optical fiber 568. The optical fiber 567 is fusion-spliced ​​to the optical fiber 566. The gain flattening filter 550 flattens the gain of the amplified light Le input from the optical fiber 567 and outputs the amplified light Le to the optical fiber 568 as output light Lf.

[0106] In the optical fiber amplifier 6, the gain flattening filter 550 may be, for example, the optical module according to the first embodiment or the optical module according to the second embodiment.

[0107] Another example of an optical fiber amplifier including the optical module according to this embodiment will now be described. Fig. 8 is a diagram showing an outline of an optical fiber amplifier 7, which is an example of an optical fiber amplifier including the optical module according to this embodiment.

[0108] The optical fiber amplifier 7 is an erbium-doped optical fiber amplifier. The optical fiber amplifier 7 is a core-pumped multi-core erbium-doped optical fiber amplifier.

[0109] The optical fiber amplifier 7 includes a pumping light multiplexer 610 , a pumping laser 620 , an erbium-doped optical fiber 664 , and an isolator / gain flattening filter 650 .

[0110] The pumping light multiplexer 610 multiplexes the signal light La1 and the pumping light Lb1, and outputs multiplexed light Lc1 obtained by multiplexing the signal light La1 and the pumping light Lb1.

[0111] The pumping light multiplexer 610 includes optical fibers 661, 662, and 663. In FIG. 8 , the marks "x" indicate fusion splice points where the optical fibers are fused and spliced. Signal light La1 is input from the optical fiber 661 to the pumping light multiplexer 610. Pumping light Lb1 is output from the pumping laser 620 to the optical fiber 622a. The optical fibers 662a and 662 are fusion spliced. Pumping light Lb1 is input from the optical fiber 662 to the pumping light multiplexer 610. The pumping light multiplexer 610 multiplexes the signal light La1 and the pumping light Lb1, and outputs the combined multiplexed light Lc1 to the optical fiber 663.

[0112] The erbium-doped optical fiber 664 amplifies the signal light La1 using the pump light Lb1 contained in the input multiplexed light Lc1. The erbium-doped optical fiber 664 is fusion-spliced ​​to the optical fiber 663. The erbium-doped optical fiber 664 is also fusion-spliced ​​to the optical fiber 665. The erbium-doped optical fiber 664 amplifies the signal light La1 using the pump light Lb1 contained in the multiplexed light Lc1, and outputs amplified light Ld1 to the optical fiber 665 fusion-spliced ​​to the erbium-doped optical fiber 664.

[0113] The isolator / gain-flattening filter 650 flattens the gain of the input amplified light Ld1. The isolator / gain-flattening filter 650 includes an optical fiber 665 and an optical fiber 668. The optical fiber 665 is fusion-spliced ​​to the erbium-doped optical fiber 664. The isolator / gain-flattening filter 650 flattens the gain of the amplified light Ld1 input from the optical fiber 665 and outputs it to the optical fiber 668 as output light Lf1. The isolator / gain-flattening filter 650 also blocks the light input from the optical fiber 668.

[0114] In the optical fiber amplifier 7, for example, the optical modules according to the third to fifth embodiments can be applied as the isolator / gain flattening filter 650.

[0115] Next, the arrangement of the optical functional elements in the optical module according to this embodiment will be described with reference to Fig. 9. The arrangement of the optical functional elements in the optical module according to this embodiment will be described with reference to Fig. 9.

[0116] The optical functional element 50 may be disposed in a space between a plane PP5 parallel to the focal plane FP5 of the lens 32 and a first distance away from the focal plane FP5 of the lens 32 in a direction closer to the lens 32, and a plane PP6 parallel to the focal plane FP6 of the lens 42 and a first distance away from the focal plane FP6 of the lens 42 in a direction closer to the lens 42. The first distance may be, for example, 5 mm, and preferably 1 mm.

[0117] In order to explain the effects of the optical module according to this embodiment, an optical module of a reference example will be described below. Fig. 10 is a diagram showing an overview of an optical module 1z, which is an example of the optical module of the reference example.

[0118] The optical module 1z includes an optical fiber 10z, an optical fiber 20z, a lens 31z, a lens 41z, and an optical functional element 50z. The optical module 1z includes, in order from the optical fiber 10z, the lens 31z, the optical functional element 50z, the lens 41z, and the optical fiber 20z.

[0119] The optical functional element 50z is provided at a position where the end face of the optical fiber 10z is imaged by the lens 31z and at a position where the end face of the optical fiber 20z is imaged by the lens 41z.

[0120] As shown in Figure 10, the angle of incidence on the optical functional element 50z varies depending on the position of the core of the optical fiber. If the angle of incidence on the optical functional element 50z varies, the optical function of the optical functional element 50z varies. In other words, the optical module 1z has core dependency. Therefore, the optical function of the optical module 1z varies depending on the core, and uniform processing cannot be performed between the cores.

[0121] According to the optical module of this embodiment, by using a double relay lens system, the angle of incidence on the optical functional element can be made uniform regardless of the position of the core of the optical fiber. In other words, the optical module of this embodiment can reduce core dependency. Therefore, the optical module of this embodiment has the same optical effect between cores, allowing uniform processing between cores.

[0122] 1, 2, 3, 4, 5 Optical module 6, 7 Optical fiber amplifier 10, 20, 110, 120, 310, 320, 410, 420 Optical fiber 10S, 20S, 110S, 120S, 310S, 320S, 410S, 420S End face 30, 130, 230, 430 Relay lens system 31, 32, 131, 132, 231, 232, 431, 432 Lens 40, 140, 240, 440 Relay lens system 41, 42, 141, 142, 241, 242, 441, 442 Lens 131S, 141S Exit end 50 Optical functional element 60 Optical isolator 61, 64 Polarizer 62 Faraday rotator 63 Half-wave plate 510, 610 Pumping light combiner 520, 620 Pumping laser 530 Pumping fan-in 540 Isolator 550 Gain flattening filter 650 Isolator / gain flattening filter 561, 562, 562a, 563, 565, 566, 567, 568, 661, 662, 662a, 663, 665, 668 Optical fiber 564, 664 Erbium-doped optical fiber 310a Enlarged section 1z Optical module 10z, 20z Optical fiber 31z, 41z Lens 50z Optical functional element FP1, FP2, FP3, FP4, FP5, FP6, FP11, FP12, FP13, FP14, FP15, FP16 Focal plane PP5, PP6 plane La, La1 signal light Lb, Lb1 pump light Lc, Lc1 combined light Ld, Ld1, Le amplified light Lf, Lf1 output light

Claims

1. A first optical waveguide assembly comprising a plurality of optical waveguides, light being emitted from the optical waveguides at a first end face; a second optical waveguide assembly comprising a plurality of optical waveguides, light being incident on the optical waveguides at a second end face; a first relay lens system comprising a first lens and a second lens, wherein the first lens has a first focal plane at the first end face and the first lens and the second lens share a second focal plane; a second relay lens system comprising a third lens and a fourth lens, wherein the third lens has a third focal plane at the second end face and the third lens and the fourth lens share a fourth focal plane; and an optical functional element, wherein the first relay lens system and the second relay lens system form a double relay lens system wherein the fifth focal plane of the second lens and the sixth focal plane of the fourth lens are shared, an arrangement of the plurality of optical waveguides of the first optical waveguide assembly at the first end face and an arrangement of the plurality of optical waveguides of the second optical waveguide assembly at the second end face are similar in shape to each other; an imaging magnification of the double relay lens system is equal to a similarity ratio of the similar shapes; and the optical functional element is disposed between the second lens and the fourth lens.

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

3. The optical module according to claim 1 or 2, wherein the optical functional element includes a dielectric multilayer filter.

4. The optical module described in claim 3, wherein the dielectric multilayer filter comprises a dielectric multilayer, and the dielectric multilayer is disposed in a space between a first plane parallel to the fifth focal plane and 5 mm away on the optical axis from the fifth focal plane toward the second lens, and a second plane parallel to the sixth focal plane and 5 mm away on the optical axis from the sixth focal plane toward the fourth lens.

5. The optical module described in claim 3, wherein the dielectric multilayer filter comprises a dielectric multilayer, and the dielectric multilayer is disposed in a space between a first plane parallel to the fifth focal plane and 1 mm away on the optical axis from the fifth focal plane toward the second lens, and a second plane parallel to the sixth focal plane and 1 mm away on the optical axis from the sixth focal plane toward the fourth lens.

6. An optical module according to any one of claims 3 to 5, wherein the dielectric multilayer filter has a transmission characteristic that flattens the gain spectrum of the optical amplifier under specific operating conditions.

7. The optical module according to any one of claims 1 to 6, wherein the optical functional element includes an optical isolator.

8. An optical module according to any one of claims 1 to 6, wherein the first lens and the third lens are gradient index lenses, the first lens and the first optical waveguide assembly are connected in physical contact at the first end face, and the third lens and the second optical waveguide assembly are connected in physical contact at the second end face.

9. An erbium-doped optical fiber amplifier equipped with the optical module according to any one of claims 3 to 6, wherein the dielectric multilayer filter has transmission characteristics that flatten the gain spectrum of the erbium-doped optical fiber under specific operating conditions.

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