Optical branching device and optical amplifier

Abstract

This optical branching device comprises: one or more first cores; a second core different from the first cores; and a wavelength branching waveguide that selectively extracts, from among light propagating through the second core, specific light having a predetermined wavelength, and distributes the specific light between the first cores at a predetermined branching ratio. This optical amplifier comprises: the optical branching device; a signal input unit that inputs signal light to the first cores; a light generation unit that generates the specific light and introduces the specific light into the optical branching device; and a rare earth-doped fiber that amplifies the signal light included in the first cores on the basis of the specific light.

Description

Optical branching device and optical amplifier

[0001] The present disclosure relates to an optical branching device and an optical amplifier.

[0002] Non-Patent Document 1 discloses a technology for a core-pumped optical amplifier including a pumping light multiplexer, a pumping laser, a rare-earth-doped multi-core fiber, and an isolator. According to this technology, pumping light is branched into a multi-core fiber for transmitting pumping light and multiplexed into the rare-earth-doped multi-core fiber, thereby performing optical amplification. Here, the rare-earth-doped multi-core fiber is a fiber doped with a rare-earth metal such as neodymium (Nd), erbium (Er), thulium (Tm), holmium (Ho), or ytterbium (Yb).

[0003] Takafumi Ohtsuka and Hirotaka Sakuma and Takahiro Suganuma and Tetsuya Hayashi and Takemi Hasegawa and Hidehisa Tazawa, "Optical Amplifiers Using Multicore Erbium Doped Optical Fibers", SEI Technical Review, No. 94, 82-87, April 2022.

[0004] According to the technology disclosed in Non-Patent Document 1, an element for branching pumping light is required for each core of a multi-core fiber for transmitting pumping light, and further, an alignment mechanism for optically coupling between cores of the multi-core fiber for transmitting pumping light and cores of the rare-earth-doped multi-core fiber is required in a pumping light multiplexer that multiplexes the pumping light into a rare-earth-doped multi-core fiber. Therefore, there is a problem that the overall configuration becomes complicated.

[0005] The present disclosure has been made in view of the above-mentioned problems, and has as its object to provide an optical branching device and an optical amplifier that can simplify the overall configuration by eliminating the need for an element that branches light and an alignment mechanism for optical coupling.

[0006] In order to solve the above-mentioned problems, an optical branching device according to one aspect of the present disclosure comprises one or more first cores, a second core different from the first cores, and a wavelength branching waveguide that selectively extracts specific light having a predetermined wavelength from the light propagating through the second cores and distributes the specific light among the first cores at a predetermined branching ratio.

[0007] An optical branching device according to one aspect of the present disclosure may include one or more first cores, and a wavelength branching waveguide into which specific light having a predetermined wavelength is input and which distributes the specific light among the first cores at a predetermined branching ratio.

[0008] An optical amplifier according to one aspect of the present disclosure includes an optical branching device, a signal input unit that inputs signal light into a first core, a light generating unit that generates specific light and introduces it into the optical branching device, and a rare-earth doped fiber that amplifies the signal light contained in the first core based on the specific light.

[0009] According to the present disclosure, it is possible to provide an optical branching device and an optical amplifier that can simplify the overall configuration by eliminating the need for an element that branches light and an alignment mechanism for optical coupling.

[0010] FIG. 1 is a schematic diagram showing a structure of an optical branching device according to an embodiment of the present disclosure. FIG. 1 is a schematic diagram showing an example of an optical branching device configured with a multicore fiber. FIG. 2 is a configuration diagram of a system for multiplexing pump light into an optical branching device. FIG. 3 is a diagram showing the arrangement of cores and wavelength branching waveguides used in model calculations. FIG. 4 is a diagram showing a power distribution in an xz cross section for a fundamental mode of a wavelength of 1550 nm. FIG. 5 is a diagram showing the coupling efficiency to a core and a wavelength branching waveguide for a fundamental mode of a wavelength of 1550 nm. FIG. 6 is a diagram showing a power distribution in an xz cross section for a fundamental mode of a wavelength of 980 nm. FIG. 7 is a diagram showing the coupling efficiency to a core and a wavelength branching waveguide for a fundamental mode of a wavelength of 980 nm. FIG. 8 is a schematic diagram showing a modified example of the structure of an optical branching device according to an embodiment of the present disclosure. FIG. 9 is a configuration diagram showing an example of an optical amplifier according to an embodiment of the present disclosure.

[0011] Next, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description, the same components are designated by the same reference numerals and redundant description will be omitted.

[0012] [Structure of Optical Branching Device] Fig. 1 is a schematic diagram showing the structure of an optical branching device according to an embodiment of the present disclosure. Fig. 2 is a schematic diagram showing an example of an optical branching device configured with a multi-core fiber.

[0013] The optical branching device 1 includes one or more first cores CR1, a second core CR2 different from the first core CR1, and a wavelength branching waveguide BR. The first core CR1 and the second core CR2 may be multiple cores included in a multicore fiber, as shown in FIG.

[0014] The wavelength branching waveguide BR selectively extracts specific light having a predetermined wavelength from the light propagating through the second core CR2 and distributes the specific light among the first cores CR1 at a predetermined branching ratio. The wavelength branching waveguide BR may distribute the specific light equally among the first cores CR1. Alternatively, the wavelength branching waveguide BR may be disposed in a cladding region between the first core CR1 and the second core CR2.

[0015] The wavelength branching waveguide BR may be formed inside a fiber using a femtosecond laser or the like. The end face shape of the wavelength branching waveguide BR may be various shapes such as a circle, a square, an ellipse, etc. The cross-sectional shape of the wavelength branching waveguide BR may be any shape that allows the specific light to easily propagate, and may be various shapes such as a circle, a square, etc.

[0016] 1, the distance between the center of the first core CR1 or the second core CR2 and the wavelength branching waveguide BR is defined as length L1, and the length in the longitudinal direction where the first core CR1 or the second core CR2 and the wavelength branching waveguide BR are adjacent to each other over the length L1 is defined as length L2. By adjusting the values ​​of lengths L1 and L2, only specific light having a predetermined wavelength can be coupled to the wavelength branching waveguide BR.

[0017] The predetermined wavelength is defined as wavelength λ1, and the wavelength branching waveguide BR is capable of propagating light having wavelength λ1. For example, light having wavelength λ1 is pump light used in the rare-earth doped fiber RFB described below. Furthermore, the first core CR1 and the second core CR2 are capable of propagating signal light having wavelength λ2, which is different from wavelength λ1, in addition to light having wavelength λ1.

[0018] For example, a femtosecond laser processing system used to fabricate the wavelength branching waveguide BR includes a pulsed light source, an attenuator, an objective lens, a stage, etc. Processing parameters for processing using a femtosecond laser include the pulse energy, repetition frequency, and pulse width of the light output from the pulsed light source, the magnification and numerical aperture (NA) of the objective lens, the scanning speed of the stage, and the number of scans.

[0019] By adjusting and setting the processing parameters and the direction of stage movement, it is possible to create any desired refractive index modulation portion at any desired position. Here, the "refractive index modulation portion" refers to a region with an increased refractive index formed by laser processing. The stage moves in three directions: the x-, y-, and z-directions, and by setting the amount of movement for each direction and processing, it is possible to form a wavelength branching waveguide BR.

[0020] As shown in Fig. 2, the wavelength branching waveguide BR may be adjacent to a plurality of first cores CR1 and distribute specific light selectively extracted from the second core CR2 among the plurality of first cores CR1 at a predetermined branching ratio. Fig. 2 shows a state in which the wavelength branching waveguide BR is adjacent to three first cores CR1. The number of first cores CR1 adjacent to the wavelength branching waveguide BR may be one or more.

[0021] By adjusting the lengths L1 and L2, a specific wavelength from a core that propagates signal light of multiple wavelengths can be coupled to the wavelength branching waveguide BR and then coupled to another core. By controlling the core to be coupled depending on the wavelength, core switching by wavelength can also be realized.

[0022] 3 is a configuration diagram of a system for multiplexing pump light into an optical branching device. One end of the optical branching device 1 is connected to a transmission multicore fiber and a pump light transmission fiber FB via a pump light multiplexer MG. Here, the transmission multicore fiber MCF transmits signal light of wavelength λ2 in each core. Furthermore, the pump light transmission fiber FB transmits pump light of wavelength λ1.

[0023] By the pumping light multiplexer MG, signal light of wavelength λ2 is coupled from each core of the transmission multicore fiber MCF to each core of the optical branching device 1. Pumping light of wavelength λ1 is coupled to the second core CR2 of the optical branching device 1. For example, a beam splitter may be used as the pumping light multiplexer MG. The pumping light multiplexer MG for realizing the above-mentioned coupling is not limited to the example given here.

[0024] 3, the optical branching device 1 has the same number of cores as the transmission multicore fiber MCF and includes a wavelength branching waveguide BR therein. Pumping light coupled to the second core CR2 is branched to the wavelength branching waveguide BR1 via the wavelength branching waveguide BR.

[0025] Next, the state of coupling between the first core CR1 or the second core CR2 and the wavelength branching waveguide will be described. Fig. 4 is a diagram showing the arrangement of the cores and the wavelength branching waveguide used in the model calculation.

[0026] In Fig. 4, the second core CR2 and the wavelength branching waveguide BR are arranged in parallel with an interval of length L1. In the calculation, the wavelength λ2 of the signal light is set to 1550 nm, and the wavelength λ1 of the pumping light is set to 980 nm. The core radius of the second core CR2 and the core radius of the wavelength branching waveguide BR are set to the same, 4.5 µm. The relative refractive index difference of the second core CR2 and the wavelength branching waveguide BR with respect to the cladding region is set to the same, 0.35%. The length L1 is set to 12 µm.

[0027] 5A is a diagram showing the power distribution in the xz cross section for the fundamental mode of 1550 nm wavelength. Fig. 5B is a diagram showing the coupling efficiency to the core and the wavelength branching waveguide for the fundamental mode of 1550 nm wavelength. In Fig. 5B, the solid line shows the coupling efficiency in the wavelength branching waveguide BR, and the dashed line shows the coupling efficiency in the first core CR1.

[0028] 5A and 5B, it can be seen that the second core CR2 and the wavelength branching waveguide BR are coupled at a period of 2700 μm.

[0029] 6A is a diagram showing the power distribution in the xz cross section for the fundamental mode of 980 nm wavelength. 6B is a diagram showing the coupling efficiency to the core and the wavelength branching waveguide for the fundamental mode of 980 nm wavelength. The solid line shows the coupling efficiency in the wavelength branching waveguide BR, and the dashed line shows the coupling efficiency in the second core CR2.

[0030] 6A and 6B, it can be seen that the second core CR2 and the wavelength branching waveguide BR are coupled at a period of 11,500 μm.

[0031] For example, when applying this technology to a two-core fiber amplifier, it is desirable to leave the signal light with a wavelength of 1550 nm in the second core CR2, leave half of the pump light with a wavelength of 980 nm in the second core CR2, and couple the remaining half to the first core CR1 via the wavelength branching waveguide BR.

[0032] 5A, 5B, 6A, and 6B, by setting the length L2 to 5300 μm, 95% or more of the signal light with a wavelength of 1550 nm can be retained in the second core CR2. Also, 55% of the pump light with a wavelength of 980 nm can be retained in the second core CR2, and 45% can be coupled to the wavelength branching waveguide BR and then coupled to one first core CR1.

[0033] Furthermore, when this technology is applied to a four-core fiber amplifier, it is desirable to leave the signal light with a wavelength of 1550 nm in the second core CR2, leave 25% of the pump light with a wavelength of 980 nm in the second core CR2, and couple the remaining 75% to the three first cores CR1 via the wavelength branching waveguide BR. In this case, the pump light with a wavelength of 980 nm may be coupled by 25% between the first cores CR1.

[0034] 5A, 5B, 6A, and 6B, by setting the length L2 to 15,200 μm, 95% or more of the signal light with a wavelength of 1,550 nm can be retained in the second core CR2. Also, 25% of the pump light with a wavelength of 980 nm can be retained in the second core CR2, and 75% can be coupled to the wavelength branching waveguide BR and then coupled to the three first cores CR1.

[0035] The wavelength branching waveguide BR does not have to equally distribute the pump light among the multiple first cores CR1. The wavelength branching waveguide BR may distribute the pump light among the first cores CR1 at a predetermined branching ratio. When there is a core deviation in the intensity of the signal light propagating through the first cores CR1, a gain difference may be provided among the first cores CR1 by adjusting the coupling ratio of the pump light. The gain difference may then be set so that the power of the signal light after amplification is equal.

[0036] 7 is a schematic diagram illustrating a modified example of the structure of an optical branching device according to an embodiment of the present disclosure. The optical branching device 1 may include one or more first cores CR1 and a wavelength branching waveguide BR. Unlike the optical branching device 1 illustrated in FIG. 1, the optical branching device 1 illustrated in FIG. 7 does not include a second core CR2.

[0037] The wavelength branching waveguide BR may receive specific light having a predetermined wavelength and distribute the specific light among the first cores CR1 at a predetermined branching ratio. The wavelength branching waveguide BR may distribute the specific light equally among the first cores CR1. Furthermore, the wavelength branching waveguide BR may be disposed in a cladding region surrounding the first cores CR1.

[0038] In FIG. 7, the incident surface through which the specific light is input to the wavelength branching waveguide BR may be disposed on the end face or side face of the optical fiber including the first core CR1.

[0039] For example, if an incident surface through which specific light is input is located at the end face of the optical fiber, the pumping light can be directly incident on the wavelength branching waveguide BR via the pumping light multiplexer MG. The wavelength branching waveguide BR can be located at any position in the cladding region, and in particular, it may be located at a position distant from the first core CR1. This makes it possible to minimize coupling of pumping light leaking due to axial misalignment during coupling with the first core CR1.

[0040] Furthermore, if an incident surface through which specific light is input is disposed on the side of the optical fiber, the pumping light can be directly incident from the side without passing through the end face of the optical fiber. Methods for inputting pumping light include using a micromirror, polishing the side face and connecting it, or filling the connection with matching oil and directly connecting the side face of the optical fiber and the end face of the pumping light transmitting fiber FB. The method for inputting pumping light is not limited to the examples given here.

[0041] 8 is a configuration diagram illustrating an example of an optical amplifier according to an embodiment of the present disclosure. The optical amplifier 10 includes the optical branching device 1 described above, a transmission multicore fiber MCF (signal input unit) that inputs signal light to the first core CR1, a light generation unit LD that generates specific light (pump light) and introduces it into the optical branching device 1, and a rare-earth doped fiber RFB that amplifies the signal light included in the first core CR1 based on the specific light.

[0042] Additionally, the optical amplifier 10 may include a pumping light multiplexer MG, a pumping light transmission fiber FB, and an isolator IS.

[0043] In the above description, the "predetermined wavelength" which is the wavelength of the specific light may be set to a wavelength near 980 nm or near 1480 nm, which are wavelengths often used as excitation light.

[0044] [Effects of the embodiment] As described in detail above, an optical branching device according to one aspect of the present disclosure comprises one or more first cores, a second core different from the first cores, and a wavelength branching waveguide that selectively extracts specific light having a predetermined wavelength from the light propagating through the second cores and distributes the specific light among the first cores at a predetermined branching ratio.

[0045] This eliminates the need for an element that branches light and an alignment mechanism for optical coupling, simplifying the overall configuration. In particular, the wavelength branching waveguide allows specific light to be distributed between the first cores, eliminating the need for fan-in components that were necessary in the configuration of a core-pumped optical amplifier. This simplifies the configuration of the optical amplifier. In addition, the optical fiber for inputting light can be changed to a single-core fiber, reducing the number of connection points and the time and loss required for connection alignment. The good connectivity with the optical fiber, which is the transmission path, also reduces connection loss.

[0046] The wavelength branching waveguide may be disposed in the cladding region between the first core and the second core, thereby making it possible to create the wavelength branching waveguide using a femtosecond laser or the like.

[0047] An optical branching device according to one aspect of the present disclosure may include one or more first cores and a wavelength branching waveguide that receives specific light having a predetermined wavelength and distributes the specific light among the first cores at a predetermined branching ratio. This eliminates the need for an element that branches the light and an alignment mechanism for optical coupling, thereby simplifying the overall configuration. In particular, because the wavelength branching waveguide can distribute the specific light among the first cores, the fan-in components required in the configuration of a core-pumped optical amplifier are unnecessary.

[0048] Furthermore, the specific light can be input directly into the wavelength branching waveguide. The incident surface through which the specific light is input into the wavelength branching waveguide can be located at a position away from the first core, which minimizes the coupling of leaked light into the core due to axial misalignment during coupling.

[0049] An incident surface through which the specific light is input may be arranged on the end face or side face of the optical fiber including the first core. This allows the specific light to be input directly to the wavelength branching waveguide. Furthermore, when the incident surface through which the specific light is input is arranged on the side face of the optical fiber, the pump light can be directly incident from the side face without passing through the end face of the optical fiber. This makes it possible to minimize coupling of leaked light due to axial misalignment during coupling to the core.

[0050] The wavelength branching waveguide may equally distribute the specific light among the first cores, thereby making it possible to equalize the amplification factor among the cores when amplifying the signal light using the specific light.

[0051] An optical amplifier according to one aspect of the present disclosure includes the above-described optical branching device, a signal input unit that inputs signal light into a first core, a light generation unit that generates specific light and introduces it into the optical branching device, and a rare-earth doped fiber that amplifies the signal light contained in the first core based on the specific light.

[0052] This eliminates the need for an element that branches light and an alignment mechanism for optical coupling, simplifying the configuration of the optical amplifier. In particular, the wavelength branching waveguide allows specific light to be distributed between the first cores, eliminating the need for fan-in components that were necessary in the configuration of core-pumped optical amplifiers. This simplifies the configuration of the optical amplifier. In addition, the optical fiber for inputting light can be changed to a single-core fiber, reducing the number of connection points and the time and loss required for connection alignment. The good connectivity with the optical fiber, which is the transmission path, also reduces connection loss.

[0053] The predetermined wavelength may be set to be in the vicinity of 980 nm or 1480 nm, thereby allowing wavelengths that are often used as pumping light to be branched by the wavelength branching waveguide.

[0054] Although the contents of the present disclosure have been described above based on the embodiments, the present disclosure is not limited to these descriptions, and various modifications and improvements are possible, which will be apparent to those skilled in the art. The descriptions and drawings that form part of this disclosure should not be understood as limiting the present disclosure. Various alternative embodiments, examples, and operating techniques will be apparent to those skilled in the art from this disclosure.

[0055] Of course, the present disclosure includes various embodiments not described herein. Therefore, the technical scope of the present disclosure is defined only by the invention-specifying matters according to the scope of the claims that are appropriate from the above description.

[0056] REFERENCE SIGNS LIST 1 Optical branching device 10 Optical amplifier BR Wavelength branching waveguide CR1 First core CR2 Second core FB Pumping light transmission fiber IS Isolator L1, L2 Length LD Light generation unit MCF Transmission multi-core fiber MG Pumping light multiplexer RFB Rare earth doped fiber

Claims

1. An optical branching device comprising: one or more first cores; a second core different from the first cores; and a wavelength branching waveguide that selectively extracts specific light having a predetermined wavelength from the light propagating through the second cores and distributes the specific light among the first cores at a predetermined branching ratio.

2. The optical branching device according to claim 1, wherein the wavelength branching waveguide is disposed in a cladding region between the first core and the second core.

3. An optical branching device comprising: one or more first cores; and a wavelength branching waveguide that receives specific light having a predetermined wavelength and distributes the specific light among the first cores at a predetermined branching ratio.

4. An optical branching device according to claim 3, wherein an incident surface for inputting the specific light is disposed on an end face or a side face of the optical fiber including the first core.

5. An optical branching device according to any one of claims 1 to 4, wherein the wavelength branching waveguide equally distributes the specific light among the first cores.

6. An optical amplifier comprising: an optical branching device according to any one of claims 1 to 4; a signal input unit that inputs signal light into the first core; a light generation unit that generates the specific light and introduces it into the optical branching device; and a rare-earth doped fiber that amplifies the signal light contained in the first core based on the specific light.

7. An optical amplifier according to claim 6, wherein the predetermined wavelength is set to a value near 980 nm or near 1480 nm.

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