Mode converter and mode converter production method

Abstract

A mode converter 100 is to be provided between a fiber 10 capable of single-mode propagation and a fiber 11 capable of high-order-mode propagation and comprises an optical waveguide 17 that is formed using a photocurable resin 50 and connects a core 10a of the fiber 10 and a core 11a of the fiber 11. The optical waveguide 17 converts a single-mode optical signal emitted from the fiber 10 to a high-order-mode optical signal and inputs the high-order-mode optical signal into the fiber 11.

Description

Mode converter and method for manufacturing the mode converter

[0001] The present disclosure relates to a mode converter and a method for manufacturing a mode converter.

[0002] With the spread of the Internet, the transmission capacity of information networks is expanding year by year. Methods for expanding the transmission capacity of optical fibers include time division multiplexing transmission technology, wavelength division multiplexing transmission technology, and space division multiplexing transmission technology. Space division multiplexing transmission technology includes multicore fiber transmission, in which multiple cores are arranged in one optical fiber core, and multimode transmission, in which multiple modes are multiplexed in one optical fiber core.

[0003] In multimode transmission, optical signals can be converted from single mode to higher-order modes and multiplexed with single-mode signals for mode multiplexed transmission. It is possible to generate higher-order mode optical signals by inserting a single-mode optical signal into a multimode fiber.

[0004] However, when a single-mode optical signal is transmitted through a multimode fiber, a certain distance is required for conversion from the single mode to a higher-order mode due to the linearity of light. Furthermore, while bending the optical fiber midway facilitates conversion from the single mode to a higher-order mode, it is extremely difficult to control the number of modes and their power ratios. Non-Patent Documents 1 and 2 have proposed technologies for generating optical signals in desired higher-order modes at desired ratios.

[0005] Takayoshi Mori, Yasushi Sakamoto, Takashi Yamamoto, and Yuji Azuma, "Wideband WDM Coherent Optical MIMO Transmission Using GI-MMF with Selective Mode Excitation," IEICE Technical Report, OFT2012-35, 2012, pp. 37-42. Nobutomo Hanzawa, Kunimasa Saito, Yasushi Sakamoto, Takashi Matsui, Kyozo Tsujikawa, Masanori Koshiba, and Fumihiko Yamamoto, "Mode-Multiplexed Transmission Using Waveguide-Type Mode Multi / Demultiplexers," Laser Research, Vol. 41, No. 6, pp. 432-436, 2013.

[0006] Non-Patent Document 1 proposes a method for connecting a single-mode fiber and a multimode fiber in a spatial optical system, in which a higher-order mode is generated by axial misalignment and then multiplexed with the fundamental mode for transmission. The technology in Non-Patent Document 1 requires precise control of the amount of axial misalignment in order to reduce loss during coupling for multiplexing the higher-order mode with the fundamental mode. Therefore, a jig that allows for axial misalignment of less than 1 μm is required, which poses a problem of complicated device configuration and mode adjustment.

[0007] Non-Patent Document 2 proposes a method of multiplexing and transmitting modes by mode conversion using a waveguide-type mode multiplexer / demultiplexer. In the technology of Non-Patent Document 2, in order to generate multiple modes, it is necessary to prepare a light source equal to the number of modes, or to split one light source into optical fibers equal to the number of modes using a demultiplexer and connect them to a mode converter. Furthermore, multiple waveguide-type mode multiplexers / demultiplexers that are appropriately adjusted so that a single-mode optical signal can be converted into a desired higher-order mode are required, which poses the problem of complicated device configuration and mode adjustment.

[0008] Furthermore, in Non-Patent Document 1, there is a problem that reflected light occurs at the fiber end face and lens surface, which couples with the optical signal to generate noise components, thereby degrading the transmission performance of the optical signal.In Non-Patent Document 2, there is a problem that excessive loss occurs due to field mismatch when connecting a rectangular waveguide and a circular optical fiber at the end face, thereby degrading the transmission performance of the optical signal.

[0009] The present disclosure has been made in consideration of the above circumstances, and an object of the present disclosure is to provide a mode conversion technology that can easily convert a single-mode optical signal into a higher-order mode optical signal with low loss.

[0010] One aspect of the present disclosure is a mode converter disposed between a first optical fiber capable of single-mode propagation and a second optical fiber capable of propagating a higher-order mode, the mode converter comprising an optical waveguide formed using a photocurable resin, connecting a core of the first optical fiber and a core of the second optical fiber, the optical waveguide converting a single-mode optical signal output from the first optical fiber into an optical signal in a higher-order mode and inputting the optical signal into the second optical fiber.

[0011] One aspect of the present disclosure is a method for manufacturing a mode converter, comprising filling a hollow housing with photocurable resin, connecting a first optical fiber capable of single-mode propagation to one connection portion of the housing, and connecting a second optical fiber capable of propagating a higher-order mode to the other connection portion of the housing, irradiating the photocurable resin with light of a wavelength that increases the refractive index of the photocurable resin and hardens it from the core of the first optical fiber, and irradiating the photocurable resin with light of the same wavelength from the core of the second optical fiber, thereby forming an optical waveguide connecting the core of the first optical fiber and the core of the second optical fiber.

[0012] The present disclosure provides a mode conversion technique that can easily convert a single-mode optical signal into a higher-order mode optical signal with low loss.

[0013] FIG. 1 is a diagram showing an example of a connection configuration of a mode converter according to an embodiment. FIG. 2 is a diagram showing an example of a mode multiplexing transmission system according to an embodiment. FIG. 3 is a diagram showing another example of a mode multiplexing transmission system according to an embodiment. FIG. 4 is a diagram illustrating a mode conversion mechanism of a mode converter according to an embodiment. FIG. 5 is a diagram illustrating a method for manufacturing a mode converter according to an embodiment. FIG. 6 is a diagram illustrating another example of a mode conversion mechanism of a mode converter according to an embodiment. FIG. 7 is a diagram illustrating another example of a mode conversion mechanism of a mode converter according to an embodiment. FIG. 8 is a diagram illustrating another example of a mode conversion mechanism of a mode converter according to an embodiment. FIG. 9 is a diagram illustrating an example of an end face of a fiber capable of single-mode propagation. FIG. 10 is a diagram illustrating an example of an end face of a fiber capable of propagating a higher-order mode.

[0014] Next, several embodiments will be described in detail with reference to the drawings. In the description, the same components are designated by the same reference numerals and duplicated explanations will be omitted.

[0015] 1 is a diagram illustrating an example of a connection configuration of a mode converter 100 according to an embodiment of the present disclosure. As shown in FIG. 1 , a fiber 10 (first optical fiber) capable of single-mode propagation at an arbitrary wavelength and a fiber 11 (second optical fiber) capable of propagating a higher-order mode are connected to the mode converter 100. Note that the fiber 10 capable of single-mode propagation may be an SMF (single mode fiber), and the fiber 11 capable of propagating a higher-order mode may be an FMF (few mode fiber) or an MMF (multi-mode fiber).

[0016] The mode converter 100 of this embodiment is disposed between a fiber 10 capable of single-mode propagation and a fiber 11 capable of higher-order mode propagation. The mode converter 100 includes an optical waveguide formed using a photocurable resin, connecting the core of the fiber 10 with the core of the fiber 11. The optical waveguide converts a single-mode optical signal output from the fiber 10 into a higher-order mode optical signal and inputs the higher-order mode optical signal into the fiber 11. The optical waveguide may also convert a higher-order mode optical signal output from the fiber 11 into a single-mode optical signal and input the higher-order mode optical signal into the fiber 10. The single mode is also referred to as the fundamental mode.

[0017] The modulator 13 modulates the fundamental mode optical signal emitted from the light source 12 and propagates the modulated signal through the fiber 10 capable of single-mode propagation. The mode converter 100 converts the fundamental mode optical signal propagated through the fiber 10 into an optical signal in a higher-order mode, and propagates the converted optical signal through the fiber 11 capable of propagating in a higher-order mode.

[0018] The mode converter 100 not only converts the fundamental mode into a higher-order mode, but also converts a portion of the optical signal in the fundamental mode into a higher-order mode, and multiplexes the remaining optical signal in the fundamental mode with the converted optical signal in the higher-order mode, and then inputs the multiplexed signals into the fiber 11 that can propagate the higher-order mode, causing them to propagate.

[0019] 2 is a diagram showing an example of a mode multiplexing transmission system using the mode converter 100 of this embodiment. The mode converter 100 converts an optical signal propagated through a fiber 10 into a higher-order mode, and propagates the mode-multiplexed optical signal into a fiber 11 that can propagate the higher-order mode. The optical signal propagated to the mode converter 100 through the fiber 10 is an optical signal modulated by a modulator 13.

[0020] The mode-multiplexed optical signal propagates through fiber 11 and is then output to mode demultiplexer 14. Mode demultiplexer 14 demultiplexes the mode-multiplexed optical signal into each mode. The demultiplexed optical signals for each mode are received by receiver 15 via fibers 20 and 21, respectively.

[0021] 2, the mode-multiplexed modulated optical signal is demultiplexed into three modes by the mode demultiplexer 14. The optical signal in the fundamental mode is received by the receiver 15 via a fiber 20 capable of single-mode propagation. The optical signals in two higher-order modes are split into two fibers 21 capable of higher-order mode propagation and received by each receiver 15. The number of multiplexed modes is not limited to three and may be two or more.

[0022] Fig. 3 is a diagram showing another example of a mode multiplexing transmission system using the mode converter 100 of this embodiment. Fig. 3 shows an example in which an optical signal from a light source 12 is mode-converted by the mode converter 100, demultiplexed into each mode by a mode demultiplexer 14, modulated by a modulator 13, and each mode is combined for mode multiplexing transmission.

[0023] Specifically, an optical signal in a fundamental mode emitted from a light source 12 is propagated to a mode converter 100 via a fiber 10 capable of single-mode propagation. The mode converter 100 converts the optical signal in the fundamental mode into an optical signal in a higher mode, and mode-multiplexes the optical signal in the higher mode with the optical signal in the fundamental mode, and the resulting signal propagates through a fiber 11 capable of propagating in the higher mode. A mode demultiplexer 14 demultiplexes the mode-multiplexed optical signal into three modes. The demultiplexed optical signals in each mode are propagated to a modulator 13 via fibers 20 and 21, respectively.

[0024] Each modulator 13 modulates an optical signal of a corresponding mode. Each modulated optical signal is propagated to the mode multiplexer 16 via fibers 20 and 21. The mode multiplexer 16 multiplexes the propagated optical signals and performs mode-multiplexed transmission using a fiber 41 capable of propagating higher-order modes. A mode demultiplexer 24 connected to the fiber 41 demultiplexes the combined optical signal into three modes. The demultiplexed optical signals of each mode are received by the receiver 15 via fibers 30 and 31, respectively. The optical signal of the fundamental mode propagates through the fiber 30 capable of single-mode propagation and is received by the receiver 15. The optical signals of the two higher-order modes propagate through two fibers 31 capable of propagating higher-order modes, respectively, and are received by each receiver 15. Here, large-capacity transmission by mode multiplexing is performed through the fiber 41.

[0025] FIG. 4 is a diagram illustrating the mode conversion mechanism of the mode converter 100 according to this embodiment. FIG. 4 is a cross-sectional view (side cross-sectional view) of the mode converter 100 taken along a plane parallel to the longitudinal direction of the fibers 10 and 11. The mode converter 100 includes an optical waveguide 17 (core portion) having a high refractive index and propagating an optical signal, and a cladding 18 surrounding the optical waveguide 17 and having a refractive index lower than that of the optical waveguide 17. One end of the optical waveguide 17 is connected to a core 10a of a fiber 10 capable of single-mode propagation, and the other end is connected to a core 11a of a fiber 11 capable of propagating a higher-order mode. The end face of the optical waveguide 17 on the fiber 10 side has the same shape as the core 10a of the fiber 10, and the end face of the optical waveguide 17 on the fiber 11 side has the same shape as the core 11a of the fiber 11.

[0026] Each fiber 10, 11 has a core 10a, 11a and a cladding 10b, 11b. Generally, the fiber 11 capable of propagating higher-order modes has a larger core diameter than the fiber 10 capable of propagating single-mode signals. That is, the optical waveguide 17 of the mode converter 100 has a tapered shape in which the diameter (the cross-sectional diameter of the optical waveguide 17) decreases from one end on the fiber 11 side toward one end on the fiber 10 side. A single-mode optical signal incident on the mode converter 100 from the core 10a of the fiber 10 capable of propagating single-mode signals experiences an expanded electric field distribution in the optical waveguide 17, changing the optical confinement conditions and converting a portion of the signal into a higher-order mode optical signal.

[0027] The mode converter 100 can form the optical waveguide 17 using, for example, a self-written optical waveguide. A self-written optical waveguide is an optical waveguide with a high refractive index that is formed by irradiating a photocurable resin with light of a specific wavelength.

[0028] 5A and 5B are diagrams illustrating a manufacturing method of the mode converter 100 according to this embodiment. First, a hollow housing that will become the mode converter 100 is filled with photocurable resin 50. The housing is assumed to be provided with an injection port (not shown) for injecting the photocurable resin 50.

[0029] The housing of the mode converter 100 also includes connection portions 51 and 52 for connecting the fibers 10 and 11. The fiber 10 capable of single-mode propagation is connected to one connection portion 51 of the housing of the mode converter 100, and the fiber 11 capable of propagating a higher-order mode is connected to the other connection portion 52. In this way, the fibers 10 and 11 are connected to the photocurable resin 50.

[0030] A light source 19A is connected to the fiber 10 capable of single-mode propagation. A light source 19B is connected to the fiber 11 capable of higher-order mode propagation. The light sources 19A and 19B output light of a wavelength that increases the refractive index of the photocurable resin 50 and hardens it. The light output from the light source 19A is irradiated onto the photocurable resin 50 from the core 10a of the fiber 10, and the light output from the light source 19B is irradiated onto the photocurable resin 50 from the core 11a of the fiber 11. The light irradiation from each fiber 10 and 11 is preferably performed simultaneously. This increases the refractive index of the irradiated portion of the photocurable resin 50 and hardens it, forming an optical waveguide 17 (self-written optical waveguide) connecting the core 10a of the fiber 10 and the core 11a of the fiber 11.

[0031] Alternatively, the cladding 18 may be formed by the following procedure: After forming the optical waveguide 17, the fibers 10 and 11 are removed, a light source that outputs light of a wavelength that hardens the photocurable resin 50 to form the cladding 18 is installed near the housing of the mode converter 100, and the cladding 18 is formed by irradiating the photocurable resin 50 filled in the housing of the mode converter 100 with light of the wavelength.

[0032] Furthermore, after forming the optical waveguide 17, the light sources 19A and 19B may be removed from the fibers 10 and 11, respectively, and instead light sources that output light of a wavelength that hardens the photocurable resin 50 to form the cladding 18 may be connected to the fibers 10 and 11, respectively, to form the cladding by irradiating the fibers 10 and 11 with light of the wavelength. In this case, the light of the wavelength that hardens the photocurable resin 50 to form the cladding is irradiated onto the photocurable resin 50 filled in the housing of the mode converter 100 by propagating through the fibers 10 and 11 in the cladding mode.

[0033] After the optical waveguide 17 is formed, the light sources 19A and 19B are removed from the fibers 10 and 11. By this procedure, the mode converter 100 can be fabricated.

[0034] In forming the optical waveguide 17, the wavelength of the light irradiated onto the photocurable resin 50 may be within the wavelength band capable of single-mode transmission of the fiber 10, i.e., the signal wavelength band, in order to increase the refractive index and harden the photocurable resin 50. The wavelength of the light may be any wavelength, such as ultraviolet, visible, or infrared, as long as it is capable of increasing the refractive index and hardening the photocurable resin 50.

[0035] A light source with a single-mode propagation mode can be used as the light source 19A. A light source with a desired higher-order mode propagation mode can be used as the light source 19B. The propagation modes of the light sources 19A and 19B may be any propagation mode that allows the optical waveguide 17 formed in the photocurable resin 50 to be formed into a shape that enables mode conversion between single mode and higher-order mode. Alternatively, the light sources 19A and 19B may be wavelength-tunable light sources that can emit light with a wavelength that increases the refractive index of the photocurable resin 50 and hardens it, and light with a communication wavelength that performs mode multiplexing transmission.

[0036] 6, the end face 10c of the fiber 10 capable of single-mode propagation connected to the mode converter 100 may be formed into a convex spherical shape. This spreads the electric field distribution of light propagating from the core 10a of the fiber 10 to the photocurable resin 50, forming the optical waveguide 17 in a tapered shape, and converting the propagating optical signal from the fundamental mode to a higher-order mode. By connecting the fiber 11 capable of propagating higher-order modes to the connection portion 52 of the mode converter 100, the fundamental-mode optical signal incident from the fiber 10 capable of single-mode propagation is converted to an optical signal in the higher-order mode and propagates to the core 11a of the fiber 11.

[0037] As shown in Fig. 7, the end face 11c of the fiber 11 capable of propagating a higher-order mode and connected to the mode converter 100 may be formed into a concave spherical shape. This makes it possible to condense the light incident from the fiber 11 and improve the coupling efficiency with the light emitted from the fiber 10 capable of propagating a single mode when forming the optical waveguide 17. Note that, although the end face 10c of the fiber 10 in Fig. 7 is formed into a convex spherical shape, the end face 10c of the fiber 10 does not have to be a convex spherical shape.

[0038] Fig. 8 is a diagram illustrating another mode conversion mechanism of the mode converter 100 according to this embodiment. The example shown in Fig. 8 shows a case where each of the fibers 10 and 11 has two cores 10a and 11a. As in the case where each of the fibers 10 and 11 has one core 10a and 11a, two optical waveguides 17 can be formed by irradiating each of the cores 10a and 11a with light of a wavelength that increases the refractive index of the photocurable resin 50 and causes it to harden.

[0039] 8 shows an example of two optical waveguides 17, the number of optical waveguides 17 is not limited to two. It is sufficient that the number of cores and their arrangement in the fiber 10 capable of single-mode transmission correspond equally to the number of cores and their arrangement in the fiber 10 capable of higher-order mode transmission.

[0040] 9 is a diagram showing an example of the core arrangement of a fiber 10 capable of single-mode transmission. Fig. 9 is a diagram showing the end face (cross section) of the fiber 10, and shows an example in which seven cores 10a capable of single-mode transmission are arranged in a hexagonal close-packed structure. The cores 10a of the fiber 10 may be located anywhere on the fiber end face, and there is no limit to the number of cores.

[0041] 10 is a diagram showing an example of a core arrangement of a fiber 11 capable of transmitting a higher-order mode. Fig. 10 is a diagram showing the end face (cross section) of the fiber 11, and shows an example in which seven cores 11a capable of transmitting a higher-order mode are arranged in a hexagonal close-packed structure. The number of cores 11a of the fiber 11 and their arrangement need only correspond equally to the number of cores and their arrangement in the fiber 10 capable of single-mode propagation connected by the mode converter 100, and is not limited to a structure in which seven cores are arranged in a hexagonal close-packed state.

[0042] The mode converter 100 of the embodiment not only converts a fundamental mode optical signal output from the fiber 10 capable of single-mode propagation into a higher-order mode optical signal, but also converts a higher-order mode optical signal output from the fiber 11 capable of propagating a higher-order mode optical signal into a fundamental mode optical signal.

[0043] The mode converter 100 of the present embodiment described above is a mode converter 100 disposed between a fiber 10 capable of single-mode propagation and a fiber 11 capable of higher-order mode propagation, and includes an optical waveguide 17 formed using a photocurable resin 50, which connects the core 10 a of the fiber 10 and the core 11 a of the fiber 11, and the optical waveguide 17 converts a single-mode optical signal emitted from the fiber 10 into an optical signal in a higher-order mode and inputs the optical signal into the fiber 11.

[0044] The mode converter 100 is manufactured by filling a hollow housing with photocurable resin 50, connecting a fiber 10 capable of single-mode propagation to one connection of the housing, and connecting a fiber 11 capable of propagating a higher-order mode to the other connection of the housing, irradiating the photocurable resin 50 with light of a wavelength that increases the refractive index of the photocurable resin 50 and hardens it from the core 10 a of the fiber 10, and irradiating the photocurable resin 50 with light of the same wavelength from the core 11 a of the fiber 11, thereby forming an optical waveguide 17 connecting the core 10 a of the fiber 10 and the core 11 a of the fiber 11.

[0045] According to this embodiment, it is possible to provide a mode conversion technique that can easily convert a single-mode optical signal into a higher-order mode optical signal with low loss.

[0046] Specifically, it is possible to provide a mode converter that can easily realize mode multiplexing by generating a desired number of higher-order modes with one mode converter 100 and that does not require precise mode adjustment. Furthermore, since one mode converter 100 is used in this embodiment, it has a simple and compact configuration and does not require a large number of parts, making it economical.

[0047] The present disclosure is not limited to the above-described embodiments, and various modifications and combinations are possible.

[0048] 100: Mode converter 10, 20, 30: Fiber capable of single-mode propagation (first optical fiber) 10a: Core of fiber capable of single-mode propagation 10b: Clad of fiber capable of single-mode propagation 11, 21, 31, 41: Fiber capable of higher-order mode propagation (second optical fiber) 11a: Core of fiber capable of higher-order mode propagation 11b: Clad of fiber capable of higher-order mode propagation 12: Light source 13: Modulator 14, 24: Mode splitter 15: Receiver 16: Mode multiplexer 17: Optical waveguide 18: Clad 19A, 19B: Light source that emits light to photocurable resin

Claims

1. A mode converter disposed between a first optical fiber capable of propagating a single mode and a second optical fiber capable of propagating a higher-order mode, comprising an optical waveguide formed using a photocurable resin and connecting the core of the first optical fiber and the core of the second optical fiber, wherein the optical waveguide converts a single-mode optical signal emitted from the first optical fiber into an optical signal in a higher-order mode and inputs the converted signal into the second optical fiber.

2. The mode converter according to claim 1, wherein the optical waveguide has a tapered shape.

3. The mode converter according to claim 1, wherein the end face of the first optical fiber connected to the mode converter has a convex spherical shape.

4. The mode converter according to claim 1, wherein the end face of the second optical fiber connected to the mode converter has a concave spherical shape.

5. A mode converter according to claim 1, wherein the optical waveguide converts a higher-order mode optical signal output from the second optical fiber into a single-mode optical signal and inputs the single-mode optical signal into the first optical fiber.

6. A method for manufacturing a mode converter, comprising filling a hollow housing with photocurable resin, connecting a first optical fiber capable of single-mode propagation to one connection part of said housing, and connecting a second optical fiber capable of propagating a higher-order mode to the other connection part of said housing, and irradiating said photocurable resin with light of a wavelength that increases the refractive index of said photocurable resin and hardens it from the core of said first optical fiber, and irradiating said photocurable resin with light of the same wavelength from the core of said second optical fiber, thereby forming an optical waveguide connecting the core of said first optical fiber and the core of said second optical fiber.

7. The method for manufacturing a mode converter according to claim 6, wherein the optical waveguide has a tapered shape.

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