Multi-core fiber wavelength selective optical coupler

Abstract

A multi-core fiber wavelength selective optical coupler according to the present application includes a multi-core fiber and a coupling optical fiber, wherein the optical propagation characteristics of the coupling optical fiber are different from those of the multi-core fiber. The coupling optical fiber is configured such that the propagation constants of the two fiber cores are identical at a wavelength at which coupling is required in an optical coupling region, and the propagation constants of the two fiber cores are significantly different at a wavelength at which coupling is to be avoided. Thus, a multi-core fiber wavelength selective optical coupler having low loss and high yield compared to the prior art can be realized.

Description

Technical Field

[0001] This invention relates to a wavelength-selective optical coupler, which is an optical device that can be applied to optical communication and optical sensors utilizing multi-core optical fibers. Background Technology

[0002] A wavelength-selective optical coupler is an optical device that uses optical fibers at both the input and output ends. For multi-wavelength input light, a specific wavelength (wavelength 1) exhibits high optical coupling at a specific output end, while another wavelength (wavelength 2) exhibits low optical coupling at the aforementioned output end and high optical coupling at an output end different from the aforementioned output end. High optical coupling typically refers to optical coupling of 90% or more, while low optical coupling refers to less than 10%.

[0003] Fiber wavelength-selective optical couplers (hereinafter referred to as wavelength-selective optical couplers) come in various types, with free-space and all-fiber being representative examples. The free-space method uses optical fiber as the input / output end, but guides the light into free space in the middle and uses filters to achieve wavelength selectivity. The all-fiber method does not require guiding the optical signal into free space; instead, it utilizes the principle of optical coupling between multiple closely spaced fiber cores. The device according to the present invention is an all-fiber design, which has the advantages of simple structure and low insertion loss compared to the free-space method.

[0004] There are various methods for manufacturing all-fiber optical couplers. Non-patent document 1 and patent document 1 describe an all-fiber optical coupler manufactured by placing two optical fibers side by side, then melting and stretching them at high temperature until a specified wavelength selectivity is achieved (tapering method).

[0005] Furthermore, as described in Patent Document 2, there is also a device that uses the side surfaces of two optical fibers to be polished so that the distance between the fiber cores is approximately a few μm, thereby inducing optical coupling and then utilizing the periodicity of the optical coupling and the intensity dependence of the optical coupling on the wavelength to achieve wavelength selection.

[0006] Furthermore, as part of efforts to improve the data transmission efficiency of optical communication, multi-core optical fibers containing multiple cores in a single fiber have been developed, and extensive research and development activities and application trials are underway [Non-Patent Literature 2]. To use multi-core optical fibers, various types of optical devices are required, especially wavelength-selective coupling devices for multi-core optical fibers are needed to realize multi-core optical amplifiers.

[0007] In methods for implementing wavelength selective coupling devices for multi-core optical fibers, as described in Non-Patent Document 3, there is a method of installing a free-space optical system between two multi-core optical fibers and coupling light of another wavelength emitted from a single fiber to the emitting fiber. However, this technology requires a large number of components to achieve precise optical alignment and has a complex structure, thus posing manufacturing difficulties.

[0008] In addition, when using the tapering method described above on multi-core optical fibers to achieve an all-fiber coupler without using a free-space optical system, the optical interference between the fiber cores may increase due to the smaller fiber core spacing in the processing area, thus preventing the desired function from being achieved.

[0009] Furthermore, as described in Patent Document 3, there is a method that chemically or mechanically processes the sides of a multi-core optical fiber and processes the sides of the fiber to be optically coupled with a selected core, thereby achieving an optical coupler by bonding the two together. In this method, since the structure of the multi-core optical fiber, such as the core spacing, does not change, the problems that occur in the tapered method can be avoided. Although the concept of this method has been around for a long time, its actual implementation and public disclosure are relatively recent. Specifically implemented "all-fiber multi-core optical fiber wavelength selective optical couplers," as described in Patent Documents 4 and 5 and Non-Patent Document 4, achieve optical coupling using optical fibers with processed sides.

[0010] Figure 1 Conceptual illustrations are provided of the main components of the prior art wavelength-selective optical coupler mentioned in Patent Documents 4 and 5 and Non-Patent Document 4. In this wavelength-selective optical coupler, a single-core optical fiber (hereinafter referred to as the coupling fiber) 102 is used for coupling with the multi-core optical fiber 101. Specifically, a specially designed fiber is used during device fabrication to ensure that the light propagation characteristics in the core of the multi-core optical fiber are as similar as possible to the light propagation characteristics in the core of the coupling fiber.

[0011] Figure 2 Viewed from the side Figure 1 A schematic diagram of the cross-section of the main part of the optical coupler is provided. Light 132 of wavelength 1 incident on the core 113 of the single-core fiber 102 is coupled to a selected core 111 of the multi-core fiber 101 in the optical coupling region 141 and then transmitted. Additionally, light 131 of wavelength 2 incident on the selected core 111 of the multi-core fiber is transmitted within the same core after passing through the optical coupling region.

[0012] The wavelength selectivity of this device is achieved by utilizing the wavelength dependence of the optical coupling strength at wavelengths 1 and 2. Figure 3 The optical coupling characteristic curves of this type of wavelength-selective optical coupler relative to the length of the optical coupling region are illustrated. For example... Figure 3As shown, depending on the length of the optical coupling region, the coupling rates of wavelength 1 and wavelength 2 exhibit periodic changes as shown by curves 201 and 202, respectively. Therefore, in order to achieve wavelength selectivity and meet the requirements... Figure 3 The conditions shown in 203 are used to create the system so that the coupling of wavelength 1 is maximized and the coupling of wavelength 2 is minimized.

[0013] However, this approach presents several technical challenges. First, to achieve the condition that the coupling rate at wavelength 1 is maximized while the coupling rate at wavelength 2 is minimized, the device needs to be fabricated by increasing the fiber polishing fineness and extending the length of the optical coupling region. This leads to increased insertion loss and reduced fabrication efficiency. In particular, because the insertion loss increases significantly when the optical coupling polishing fineness alters the fiber core structure, increasing the polishing fineness may result in a substantial decrease in yield.

[0014] Second, even when the coupling conditions between the multi-core fiber and the coupling fiber are adjusted to optimal conditions to fabricate a wavelength-selective optical coupler, the wavelength selectivity remains fundamentally limited by the fiber propagation characteristics determined during the fabrication of the corresponding fiber. This limitation makes it difficult to fabricate high-performance, high-yield wavelength-selective optical couplers.

[0015] The actual optical loss of the fabricated device is about 1.5dB to 2.5dB, which is too high for practical applications.

[0016] References [Patent Document 1]: US Patent 5,150,439, "Simplified WDM Fused Fiber Coupler Design"; [Patent Document 2]: US Patent 4,556,279, "Passive Fiber Optic Multiplexer"; [Patent Document 3]: US Patent 5,625,728, "Method of coupling a multi-core optical fiber to a plurality of single-core optical fibers"; [Patent Document 4]: Japanese Patent JP7161985; [Patent Document 5]: US Patent US2022 / 019542 A1, "Optical coupler and optical amplifier"; [Non-Patent Literature 1]: MN Mclandrich et al., Polarization Independent Narrow Channel Wavelength Division Multiplexing Fiber Couplers for 1.55 μm, Journal of Lightwave Technology, Vol.9, No.4, p.442, 1991; [Non-Patent Literature 2]: Richardson, D., Fini, J. & Nelson, L, “Space-division multiplexing in optical fibers”, Nature Photon, 7, 354-362 (2013). https: / / doi.org / 10.1038 / nphoton.2013.94; [Non-Patent Literature 3]: J. Sakaguchi et al., “19-core MCF transmission system using EDFA with shared corepumping coupled via free-space optics”, Opt. Express 22, 90-95 (2014), https: / / doi.org / 10.1364 / OE.22.000090; [Non-Patent Document 4]: Y. Wakayama et al., “FIFO-less Core-pump Combiner for Multicore Fiber Amplifier”, Optical Fiber Communication Conference (OFC) 2021 (2021), paper M3D.3, Optica Publishing Group, Jun. 2021, p. M3D.3. doi: 10.1364 / OFC.2021.M3D.3. Summary of the Invention

[0017] Technical issues This invention proposes an optical device with lower loss than the existing side-polished wavelength selective optical coupler described above and which is easier to manufacture.

[0018] Problem Solving Methods Existing inventions use coupling optical fibers with the same core optical propagation characteristics as multi-core optical fibers to fabricate optical couplers. In contrast, the device of the present invention is characterized by fabricating and using coupling optical fibers with core optical propagation characteristics different from those of multi-core optical fibers. Here, "different optical propagation characteristics" refers to different characteristics of the change in the optical propagation constant relative to wavelength.

[0019] Figure 4 The optical coupling characteristic curves of the wavelength-selective optical coupler according to the present invention relative to the length of the optical coupling region are illustrated. That is, when the propagation constants of the two optical fibers are the same at the wavelength requiring optical coupling (wavelength 1), and the propagation constants are significantly different at the wavelength where optical coupling should be avoided (wavelength 2), the optical coupling ratio relative to the optical coupling length has the following characteristics: Figure 4 The characteristics shown are as follows. In particular, as shown by the coupling curve (302) for wavelength 2, the coupling rate 302 for wavelength 2 exhibits a periodic variation between 0 and a small coupling rate value. Therefore, it has the advantage of making it easier to determine the conditions (303) that can maximize wavelength selectivity compared to the prior art.

[0020] The propagation constant β of light with wavelength λ in an optical fiber can be written as β = 2πn eff / λ, where n eff This is the effective refractive index of the optical fiber for light of that wavelength. Therefore, the consistency of the propagation constant can be described in terms of the consistency of the effective refractive index at the corresponding wavelength. The following will use the effective refractive index to explain the detailed technical aspects.

[0021] Optical coupling occurs over a region of a certain length along the propagation direction. For the device according to the invention, the length of this optical coupling region is in the range of several hundred micrometers to several millimeters. Therefore, the upper limit of the effective refractive index difference at the wavelength (wavelength 1) where high optical coupling is required needs to be determined by considering whether optical coupling of more than 90% can be achieved in a shorter optical coupling region. Preferably, the effective refractive index difference between the two optical fibers at wavelength 1 is 0.0004 or less. In this case, the decrease in coupling rate due to phase difference during transmission in the optical coupling region can be controlled within a practical range. Furthermore, the lower limit of the effective refractive index difference at another wavelength (wavelength 2) needs to be determined by considering whether the optical coupling magnitude can be suppressed to less than 10% when the length of the optical coupling region is longer. Preferably, the refractive index difference between the two optical fibers at wavelength 2 is 0.0008 or more.

[0022] Furthermore, fiber optic manufacturers and fiber optic device manufacturers are often different, which frequently results in the absence of fiber optic combinations that meet the aforementioned level of consistency in effective refractive index. Therefore, as another technique, this invention proposes a method to modify the optical properties of the portion of the fiber (coupling fiber) that is optically coupled to the multi-core fiber, so that the effective refractive index of the wavelength to be optically coupled is consistent with or close to the effective refractive index of the multi-core fiber.

[0023] Figure 5 A schematic diagram of the device constructed according to the present invention has been provided. However, the number of fiber cores, etc., shown in the figures are for illustrative purposes only and do not limit the scope of the invention.

[0024] The wavelength-selective optical coupler according to the present invention differs from devices based on the prior art in that it uses a combination of different optical fibers to achieve optical coupling, wherein the difference between the effective refractive index in the core of the multi-core optical fiber and the effective refractive index in the coupling optical fiber is less than 0.0004 at the optical coupling wavelength, and the difference between the effective refractive indices in another wavelength region is greater than 0.0008.

[0025] for Figure 5 For the multi-core optical fiber 501, the sides of the core 511 near which optical coupling is required are ground, and the sides of the portion of the coupling optical fiber 502 where optical coupling is required are also ground. However, the degree of side grinding should be controlled to a level that will not cause deformation of the corresponding core structure of each optical fiber used for coupling. In the coupling region 541, the ground sides of each optical fiber are pressed together and fixed by mechanical means or adhesive.

[0026] Furthermore, in order to improve the wavelength selectivity of the device, the coupling fiber 502 may have an optical coupling region that modifies the optical properties in a way that improves the uniformity of the effective refractive index at the optical coupling wavelength.

[0027] As mentioned above, methods for altering optical properties include: melting and stretching the optical fiber at high temperatures using gas heaters, lasers, or arc discharges to reduce its diameter and thereby change the propagation constant; inducing diffusion of the optical fiber constituent material at high temperatures to thereby change the refractive index; using ultraviolet light to change the refractive index; and forming fiber gratings to thereby change the effective refractive index.

[0028] Invention Effects The device according to the present invention has the advantage of easily achieving high wavelength selectivity and low insertion loss compared to the device according to the prior art. Attached Figure Description

[0029] Figure 1The main component of existing wavelength-selective optical couplers.

[0030] Figure 2 The main cross-section of wavelength-selective optical couplers in existing technologies.

[0031] Figure 3 The optical coupling characteristic curves of wavelength-selective optical couplers relative to the length of the optical coupling region in existing technologies.

[0032] Figure 4 The optical coupling characteristic curve of the wavelength-selective optical coupler according to the present invention is relative to the length of the optical coupling region.

[0033] Figure 5 : A conceptual configuration diagram of the device according to the present invention.

[0034] Figure 6 Examples of the invention.

[0035] Symbol explanation: 101: Multi-core optical fiber; 102: Optical fiber for coupling; 111: The core of a multi-core optical fiber used for optical coupling; 112: The core of a multi-core optical fiber not used for optical coupling; 113: The core of an optical fiber used for optical coupling; CSV: Cross-sectional view of region A-A'; 131: Light at wavelength 2 that requires minimal optical coupling; 132: The wavelength of light with the highest required optical coupling is 1. 141: Optical coupling region; 201: Optical coupling characteristic curves of devices relative to wavelength 1 according to the prior art; 202: Optical coupling characteristic curves of devices relative to wavelength 2 according to the prior art; 203: Optical coupling length selected to optimize wavelength selectivity; 301: Optical coupling characteristic curve of the device according to the present invention relative to wavelength 1; 302: Optical coupling characteristic curve of the device according to the present invention relative to wavelength 2; 303: Optical coupling length selected to optimize wavelength selectivity; 501: Multi-core optical fiber; 502: Optical fiber for coupling; 503, 504: Fiber optic fixing blocks; 511: The core of a multi-core optical fiber used for optical coupling; 512: The core of a multi-core optical fiber not used for optical coupling; 513: The core of an optical fiber used for optical coupling; 514: Fiber core used for optical coupling measurements; 541: Optical coupling region.

[0036] Preferred Implementation The embodiment described herein is a wavelength selective optical coupler using a multi-core optical fiber with four cores (hereinafter referred to as a four-core fiber).

[0037] The design / fabrication method here is as follows: a 1550nm wavelength signal light is transmitted through a four-core optical fiber, while another 980nm wavelength light beam enters through a separate single-mode optical fiber and is coupled to a nearby specific fiber core for transmission. The main structure of the coupling section is similar to... Figure 5 Similarly, its production process is in Figure 6 A summary diagram is provided.

[0038] First, a multi-core fiber 501 and a coupling fiber 502 were prepared. Specifically, a fiber with a numerical aperture (NA) higher than that of conventional communication fibers was selected as the coupling fiber. A high-temperature tapering process (a process of melting and stretching the fiber at high temperature) was used to reduce the diameter of a region approximately 10 mm long to 80 μm, thereby achieving a similarity in optical propagation constant at 980 nm wavelength to that of the multi-core fiber that is less than 0.0002. Under these conditions, the effective refractive index difference at 1550 nm wavelength was measured to be greater than 0.001. Figure 6 A conceptual illustration of the 502 pairs of optical fibers used for coupling via a tapered process is provided in (a) of the diagram.

[0039] Next, the two optical fibers are fixed to blocks 503 and 504, which have curved grooves, and polished together with the blocks so that the distance between the outermost edge of the fiber core of each optical fiber used for optical coupling and the polished surface is about 5 μm or less. Figure 6 (b) in the middle.

[0040] Next, as Figure 6 As shown in (c), the polished surfaces of the two fiber blocks are pressed together, 980 nm light is input into the core 513 of the coupling fiber 502, and the relative positions of the two blocks are adjusted to optimize the 980 nm light output from the opposite side 514 of the core 511 of the multi-core fiber 501. Finally, epoxy resin is used for fixation.

[0041] Measurements showed that the insertion loss of 1550 nm light input to core 511 of the multi-core fiber in the fabricated wavelength-selective optical coupler was 0.3 to 0.5 dB, while the insertion loss of the coupled wavelength, 980 nm, was approximately 0.04 to 0.7 dB. This value is significantly lower than the insertion loss (1.5 to 2.5 dB) of existing devices, reaching a level suitable for practical application in optical systems.

[0042] Therefore, if the device according to the present invention is applied to an optical communication system, it has the advantage of improving communication quality by reducing optical loss. Detailed Implementation

[0043] In the above embodiments, the following situation was described: when the effective refractive index of the fiber core used for coupling with the multi-core fiber and the effective refractive index of the fiber core used for coupling are inconsistent at the target wavelength, the effective refractive index of the fiber used for coupling is changed by a tapering process, and the device is fabricated by optical coupling. However, the present invention does not exclude the case where the difference between the effective refractive index of the multi-core fiber core and the effective refractive index of the fiber core used for coupling is less than a specific value (0.0002) at the target wavelength. In this case, a fiber used for coupling that has not undergone a tapering process can be used, and wavelength-selective optical coupling can be achieved through the same process as in the above embodiments. Furthermore, the present invention can be applied to the fabrication of wavelength-selective optical couplers operating in multiple wavelength regions, including the 1550 nm / 980 nm wavelength region.

[0044] Furthermore, although the multi-core optical fiber was described using a four-core optical fiber as an example in the foregoing embodiments, the present invention can be applied to wavelength selective optical couplers using multi-core optical fibers with two, six, or seven cores.

[0045] Industry availability The wavelength-selective optical coupling device according to the present invention can be used to input pump light into a multi-core fiber with low loss in a multi-core fiber amplifier. Furthermore, it can be used in sensors using multi-core fibers to couple or separate signal light of different wavelengths.

[0046] sequence list No related matters.

Claims

1. A wavelength-selective optical coupling device, characterized in that: It includes a multi-core optical fiber and a coupling optical fiber for being configured adjacent to the multi-core optical fiber and for coupling or separating light of a predetermined wavelength. The multi-core optical fiber is side-polished without altering its core structure, and the coupling optical fiber is side-polished to the extent that the structure of the core used for optical coupling is not changed. In the optical coupling region where the polished portions of the two optical fibers are tightly bonded, optical coupling of over 90% occurs in a specific wavelength region 1. And to enable it to achieve less than 10% optical coupling in wavelength region 2, which is different from wavelength region 1. The difference between the effective refractive index of the core of the multi-core optical fiber in wavelength region 1 and the effective refractive index of the corresponding core of the coupling optical fiber is less than 0.0004, and the difference between the effective refractive indices in wavelength region 2 is greater than 0.0008.

2. The wavelength-selective optical coupling device according to claim 1, characterized in that: The effective refractive index of the fiber portion used for coupling in the wavelength region 1 is different from the effective refractive index of the input / output portion of the coupling fiber.

3. The wavelength-selective optical coupling device according to claim 2, characterized in that: The optical coupling region of the coupling optical fiber is obtained by changing the effective refractive index of the wavelength region 1 by melting and stretching the same coupling optical fiber at high temperature.

4. The wavelength-selective optical coupling device according to claim 2, characterized in that: The optical coupling region of the coupling optical fiber is obtained by inducing the diffusion of material within the optical fiber through high-temperature treatment of the same coupling optical fiber, thereby changing the effective refractive index of the wavelength region 1.

5. The wavelength-selective optical coupling device according to claim 2, characterized in that: The optical coupling region of the coupling optical fiber is obtained by changing the effective refractive index of the wavelength region 1 by forming a fiber grating in the same coupling optical fiber.

6. The wavelength-selective optical coupling device according to claim 1, characterized in that: The coupling fiber is a single-core fiber.

7. The wavelength-selective optical coupling device according to claim 1, characterized in that: The coupling fiber is a multi-core fiber.

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