Inter-mode loss difference compensation device

Abstract

To provide an inter-mode loss difference compensation device that improves manufacturing tolerance and contributes to improvement in manufacturing yield.SOLUTION: An inter-mode loss difference compensation device includes a first waveguide and a second waveguide each capable of propagating m propagation modes (m being an integer of 3 or more), a third waveguide connected between the first waveguide and the second waveguide and capable of propagating n propagation modes (n being an integer greater than m), and a mode converter provided in the third waveguide and configured to convert one or more propagation modes having an order of m or less into propagation modes having an order greater than m and equal to or less than n.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a modal loss differential compensation device. [Background technology]

[0002] In mode-division multiplexing transmission, which increases communication capacity by utilizing multiple modes propagating through optical fibers, non-patent documents 1 and 2 disclose a mode loss differential compensation device that improves transmission quality by suppressing the loss differential between modes that occurs in transmission fibers and mode multiplexers / demultiplexers.

[0003] Non-Patent Document 1 discloses an intermodal loss differential compensation device that uses a spatial optical system to set the amount of intermodal loss compensation using an interpolating spatial filter.

[0004] In Non-Patent Document 2, a PLC (Planar Lightwave Circuit) technology is used to generate an LP by using a parallel waveguide with two coupling parts. 11 While suppressing the loss to the mode, the fundamental mode LP 01 A modal loss differential compensation device is disclosed that provides loss to the modes. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] T. Mizuno et al., "Mode dependent loss equalizer and impact of MDL on PDM-16QAM few-mode fiber transmission", ECOC2015, P5.9 (2015) [Non-patent document 2] T. Fujisawa et al., "Silica-PLC based mode-dependent-loss equalizer for two LP mode transmission", OFC2022, paper M4J.4 (2022) Summary of the Invention [Problem to be solved by the invention]

[0006] For example, in a long-period fiber Bragg grating (LPFG), which couples a mode propagating within the core to a cladding mode, there are an infinite number of cladding modes to couple to, so a design that couples only specific modes poses the problem of reducing manufacturing tolerances.

[0007] The present disclosure has been made in view of the above-mentioned problems, and has an object to provide an inter-modal loss difference compensation device that improves manufacturing tolerances and contributes to improving manufacturing yields. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems, a modal loss difference compensation device according to one embodiment of the present disclosure includes a first waveguide and a second waveguide capable of propagating m propagation modes (m is an integer greater than or equal to 3), a third waveguide connected between the first waveguide and the second waveguide and capable of propagating n propagation modes (n is an integer greater than m), and a mode converter disposed within the third waveguide and capable of converting one or more propagation modes having an order less than or equal to m into a propagation mode greater than m and less than or equal to n. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to provide an inter-modal loss difference compensation device that improves manufacturing tolerances and contributes to improving manufacturing yields. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram illustrating a configuration of an inter-modal loss difference compensation device according to an embodiment of the present disclosure. [Figure 2] FIG. 1 is a schematic diagram showing the structure of a mode converter using an LPFG. [Figure 3] FIG. 10 is a diagram showing the relationship between the LPFG length and the amount of mode conversion and loss. [Figure 4A] FIG. 1 is a first diagram showing the wavelength dependence of conversion efficiency. [Figure 4B] FIG. 2 is a second diagram showing the wavelength dependence of conversion efficiency. [Figure 4C] FIG. 3 is a third diagram showing the wavelength dependence of conversion efficiency. [Figure 4D] FIG. 4 is a fourth diagram showing the wavelength dependence of conversion efficiency. [Figure 5A] FIG. 1 is a diagram showing an example of a refractive index profile of a 4LP mode fiber. [Figure 5B] FIG. 1 is a diagram showing an example of a refractive index profile of a 6LP mode fiber. [Figure 6A] FIG. 1 is a first diagram showing the propagation length dependence of conversion efficiency in a 4LP mode fiber. [Figure 6B] FIG. 2 is a second diagram showing the propagation length dependence of the conversion efficiency in a 4LP mode fiber. [Figure 7A] FIG. 1 is a first diagram showing the propagation length dependence of conversion efficiency in a 5LP mode fiber. [Figure 7B] FIG. 2 is a second diagram showing the propagation length dependence of the conversion efficiency in a 5LP mode fiber. [Figure 8A] FIG. 1 is a first diagram showing the wavelength dependence of conversion efficiency in a 5LP mode fiber. [Figure 8B] FIG. 2 is a second diagram showing the wavelength dependence of conversion efficiency in a 5LP mode fiber. DETAILED DESCRIPTION OF THE INVENTION

[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] [1. Configuration example of inter-mode loss difference compensation device 1] 1 is a diagram illustrating a configuration of an intermodal loss difference compensation device 1 according to an embodiment of the present disclosure. The intermodal loss difference compensation device 1 includes a first waveguide FB1, a second waveguide FB2, and a third waveguide FB3.

[0013] In particular, the first waveguide FB1 and the second waveguide FB2 are capable of propagating m propagation modes (m is an integer equal to or greater than 3). The third waveguide FB3 is connected between the first waveguide FB1 and the second waveguide FB2 and is capable of propagating n propagation modes (n is an integer greater than m).

[0014] For example, the first waveguide FB1, the second waveguide FB2, and the third waveguide FB3 may be optical fibers, and at least one of the first waveguide FB1, the second waveguide FB2, and the third waveguide FB3 may have a multi-core structure instead of a single core.

[0015] In particular, the first waveguide FB1 and the second waveguide FB2 may be two LP mode fibers. 01 Mode and LP 11 The LP mode may be propagated. 11 The third waveguide FB3 may be a 4LP mode fiber. 01 Mode, LP 11 Mode, LP 21 Mode and LP 02 The LP mode may be propagated. 11 Mode and LP 21 Each mode has two degenerate modes, so there are a total of six propagation modes.

[0016] Here, the LP propagating from the first waveguide FB1 01 Mode and LP 11 The modes are introduced into the third waveguide FB3 and are respectively 01 Mode and LP 11 The LP of the third waveguide FB3 11 The mode is maintained as it is. 01 A part of the mode is converted into LP by the mode converter MC. 02is converted into a mode.

[0017] The mode converter MC is disposed in the third waveguide FB3 and is configured to be able to convert one or more propagation modes having an order equal to or less than m into a propagation mode greater than m and equal to or less than n. For example, the mode converter MC can propagate five or more LP modes, and converts the LP 01 LP mode 02 It may be a device that converts the signal into a mode.

[0018] The mode converter MC may be configured by a long-period fiber Bragg grating (LPFG), or may be configured by a phase-modulatable filter such as a spatial optical element.

[0019] LP of the third waveguide FB3 01 Mode, LP 11 Mode, LP 21 Mode and LP 02 The mode is introduced into the second waveguide FB2.

[0020] The second waveguide FB2 is 01 Mode and LP 11 It is configured to be able to propagate the LP mode, but the cutoff 02 The LP mode incident from the first waveguide FB1 to the third waveguide FB3 is not propagated. 01 LP where the mode is output from the third waveguide FB3 to the second waveguide FB2 02 By controlling the amount of mode conversion, 01 Any loss can be applied to the mode only.

[0021] In addition, the LP 01 The mode in the third waveguide FB3 to which the mode is coupled is 02 The mode converter MC is not limited to LP mode. 01 The mode in the third waveguide FB3 to which the mode is coupled is introduced from the third waveguide FB3 to the second waveguide FB2, and then: It may be a mode whose propagation is limited by cutoff in the second waveguide FB2, e.g., LP 21 It may be a mode.

[0022] [2. Configuration example of mode converter MC] Fig. 2 is a schematic diagram showing the structure of a mode converter using an LPFG. Fig. 3 is a diagram showing the relationship between the LPFG length, the amount of mode conversion, and the loss. The vertical axis of Fig. 3 shows the conversion efficiency (Conversion Ratio), and the horizontal axis shows the LPFG length LG. The conversion efficiency was calculated using a simple model that takes into account the coupling of only the strongly confined mode. Here, the mode converter MC is configured using an LPFG, and the LP 01 LP mode 02 1 shows a design example of a device 1 for compensating for inter-modal loss differences by converting into modes.

[0023] For example, the first waveguide FB1, the second waveguide FB2, and the third waveguide FB3 each include a core 11, a cladding 12, and a coating 13.

[0024] The optical fiber constituting the first waveguide FB1 and the second waveguide FB2 is a 2LP mode fiber having a step-shaped refractive index profile. Regarding the step-shaped refractive index profile in the 2LP mode fiber, the radius of the core 11 is 7 μm, and the relative refractive index difference between the core 11 and the cladding 12 is 0.4%.

[0025] The optical fiber constituting the third waveguide FB3 is a 4LP mode fiber having a step-shaped refractive index profile. Regarding the step-shaped refractive index profile of the 4LP mode fiber, the radius of the core 11 is 7 μm, and the relative refractive index difference between the core 11 and the cladding 12 is 0.7%.

[0026] The amount of refractive index modulation of the LPFG in the mode converter MC is such that, when the grating period is Λ as shown in FIG. 2, the refractive index increases uniformly by 0.001 in a section of Λ / 2.

[0027] The above-described structures of the first waveguide FB1, the second waveguide FB2, the third waveguide FB3, and the LPFG in the mode converter MC are merely examples, and the present invention is not limited to these examples.

[0028] In the above structure, LP 01 The propagation constant of the mode is β 01 , LP 02 The propagation constant of the mode is β 02 Then, when the wavelength is 1550 nm, the LP 01 Mode to LP 02 The grating period Λ for coupling into the mode is calculated to be 231.6 μm according to equation (1).

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[0029] In addition, in FIG. 3, the curve C11 is 01 Mode to LP 01 The conversion efficiency to LP mode, curve C12, 01 Mode to LP 02 As shown in Figure 3, as the LPFG length LG increases, the LP 01 Mode to LP 02 For example, at about 15.5 mm, the conversion efficiency to the LP mode is almost 02 You can see that it is converted into mode.

[0030] LP you want to stimulate 02 The excitation rate of the mode is T 01→02 Then, the LPFG length LG to be set is expressed as in equation (2). C =15.5mm.

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[0031] [3. Wavelength dependence of conversion efficiency] Next, the wavelength dependence of the conversion efficiency will be explained. Fig. 4A is a first diagram showing the wavelength dependence of the conversion efficiency. Fig. 4B is a second diagram showing the wavelength dependence of the conversion efficiency. Fig. 4C is a third diagram showing the wavelength dependence of the conversion efficiency. Fig. 4D is a fourth diagram showing the wavelength dependence of the conversion efficiency. In Figs. 4A, 4B, 4C, and 4D, the vertical axis represents the conversion efficiency (Conversion Ratio), and the horizontal axis represents the wavelength of the propagated light.

[0032] The wavelength dependence of the conversion efficiency can be controlled by controlling the LPFG length LG. Figures 4A, 4B, 4C, and 4D show the relationship between the conversion efficiency and wavelength when the LPFG length LG is 5.1 mm, 7.6 mm, 9.7 mm, and 15.5 mm, respectively. In each figure, curve C11 indicates the relationship between the conversion efficiency and wavelength when the LPFG length LG is 5.1 mm, 7.6 mm, 9.7 mm, and 15.5 mm. 01 Mode to LP 01 The conversion efficiency to LP mode, curve C12, 01 Mode to LP 02 The figure shows the conversion efficiency to the mode.

[0033] 4A, 4B, 4C, and 4D show that the longer the LPFG length LG, the greater the conversion efficiency and wavelength dependency. Therefore, the longer the LPFG length LG, the more it needs to be designed to match the required wavelength characteristics.

[0034] For example, as shown in the results for the LPFG length LG = 15.5 mm, the wavelength dependence of mode conversion can be set to a large value. By installing such a structure according to the desired band, it is possible to set any spectrum for any wavelength. Note that wavelength dependence also depends heavily on the refractive index profile and refractive index modulation period of the optical fiber. The refractive index profile and refractive index modulation period of the optical fiber may also be set.

[0035] [4. Example of LPFG configuration capable of propagating higher-order modes] Next, an example of the configuration of an LPFG capable of propagating higher-order modes will be described. Fig. 5A is a diagram showing an example of the refractive index profile of a 4LP mode fiber. Fig. 5B is a diagram showing an example of the refractive index profile of a 6LP mode fiber.

[0036] In Figure 5A, the bold line indicates the refractive index profile of the 4LP mode fiber, and the dashed line indicates the effective refractive index of the propagation mode propagating through the 4LP mode fiber. In Figure 5B, the bold line indicates the refractive index profile of the 6LP mode fiber, and the dashed line indicates the effective refractive index of the propagation mode propagating through the 6LP mode fiber. These indicate that four and six LP modes exist in the core 11, respectively.

[0037] When the optical fiber in which the mode converter MC made of LPFG is provided is a 4LP mode fiber, as shown in FIG. 5A, 01 Compared to the effective refractive index of the mode (1.4528), LP 11 The effective refractive index of the mode (1.4503) is close to the effective refractive index of the cladding mode (1.4443). 11 Mode-to-cladding mode coupling is a concern.

[0038] Therefore, the third waveguide FB3 provided with the LPFG may be an optical fiber capable of propagating higher modes instead of a four LP mode fiber. For example, the third waveguide FB3 may be an optical fiber capable of propagating five or more LP modes.

[0039] For example, in the 6LP mode fiber shown in Figure 5B, 11 The effective refractive index of the mode is 1.4545, and the LP propagating through the 4LP mode fiber 11 Therefore, when an LPFG is installed, the effective refractive index of the LP 11 It is expected that coupling from the mode to the cladding mode will be reduced.

[0040] Figure 6A is a first diagram showing the dependence of the conversion efficiency on the propagation length in a 4LP mode fiber. Figure 6B is a second diagram showing the dependence of the conversion efficiency on the propagation length in a 4LP mode fiber. In Figures 6A and 6B, the vertical axis represents the conversion efficiency (Transmission) and the horizontal axis represents the propagation distance of the propagated light. These conversion efficiencies were obtained by analysis that also took into account coupling with the cladding mode.

[0041] In FIG. 6A, curve C11 is 01 Mode to LP 01 The conversion efficiency to LP mode, curve C12, 01 Mode to LP 02 In FIG. 6B, the curve CH11 shows the conversion efficiency to the LP mode. 11 Mode to LP 11 The figure shows the conversion efficiency to the mode.

[0042] At a wavelength of 1550 nm, LP 01 Mode to LP 02 In this structure, when the grating period Λ is about 16.9 mm, the LP 01 Mode to LP 02 It was confirmed that the LP 01 The propagation loss of the mode was 0.1 dB per mm.

[0043] In Figure 6B, the oscillations that occur as the propagation distance increases are due to coupling to the cladding mode. To suppress such unintended propagation losses, it is necessary to use a structure that strongly confines the propagation mode.

[0044] Fig. 7A is a first diagram showing the dependence of the conversion efficiency on the propagation length in a 5LP mode fiber. Fig. 7B is a second diagram showing the dependence of the conversion efficiency on the propagation length in a 5LP mode fiber. In Fig. 7A and Fig. 7B, the vertical axis represents the transmittance, and the horizontal axis represents the propagation distance of the propagated light.

[0045] In FIG. 7A, curve C11 is 01 Mode to LP 01 The conversion efficiency to LP mode, curve C12, 01 Mode to LP 02 In FIG. 7B, the curve CH11 shows the conversion efficiency to the LP mode. 11 Mode to LP 11 The figure shows the conversion efficiency to the mode.

[0046] At a wavelength of 1550 nm, LP 01 Mode to LP 02 In this structure, when the grating period Λ is about 24.0 mm, the LP 01 Mode to LP 02 It was confirmed that the LP 01 The propagation loss of the mode was 0.0098 dB per mm. Therefore, by increasing the propagation mode, the LP 11 It can be seen that excess loss due to mode conversion is suppressed. Increasing the number of propagation modes is equivalent to using a structure with strong confinement.

[0047] Fig. 8A is a first diagram showing the wavelength dependence of the conversion efficiency in a 5LP mode fiber. Fig. 8B is a second diagram showing the wavelength dependence of the conversion efficiency in a 5LP mode fiber. In Fig. 8A and Fig. 8B, the vertical axis represents the transmittance, and the horizontal axis represents the wavelength of the propagated light.

[0048] In FIG. 8A, curves CV1, CV2, CV3, and CV4 represent the LPFG lengths LG of 9.7 mm, 11.7 mm, 13.3 mm, and 24.0 mm, respectively. 01 The figure shows the relationship between the conversion efficiency and wavelength when a mode is input.

[0049] In FIG. 8B, curves CV1, CV2, CV3, and CV4 represent the LPFG lengths LG of 9.7 mm, 11.7 mm, 13.3 mm, and 24.0 mm, respectively. 11 The figure shows the relationship between the conversion efficiency and wavelength when a mode is input.

[0050] By controlling the LPFG length LG, it is possible to control the wavelength dependence of the conversion efficiency. 01 When a mode is input, it can be seen that the longer the LPFG length LG, the greater the conversion efficiency and wavelength dependency. Therefore, it is necessary to design it optimally according to the required wavelength characteristics.

[0051] Also, LP 11 When a mode is input, the longer the LPFG length LG, the higher the excess loss. However, even when the LPFG length LG is the longest at 24 mm, we confirmed that the conversion efficiency is 0.25 dB or less across the entire C-band.

[0052] LP 01 Not just mode but LP 11 If you want to give a loss of any spectral shape to a mode, you can use LP 11 LP mode 12 It is also possible to couple the optical fiber to a mode. Note that the wavelength dependency is also greatly dependent on the refractive index profile and the refractive index modulation period of the optical fiber. The refractive index profile and the refractive index modulation period of the optical fiber may be set.

[0053] [Effects of the embodiment] As described above in detail, the modal loss difference compensation device according to this embodiment includes a first waveguide and a second waveguide capable of propagating m propagation modes (m is an integer greater than or equal to 3), a third waveguide connected between the first waveguide and the second waveguide and capable of propagating n propagation modes (n is an integer greater than m), and a mode converter disposed within the third waveguide and capable of converting one or more propagation modes having an order less than or equal to m into a propagation mode greater than m and less than or equal to n.

[0054] This makes it possible to provide a modal loss difference compensation device that improves manufacturing tolerances and contributes to improving manufacturing yields. In particular, the first and second waveguides do not propagate propagation modes with orders greater than m due to cutoff, so that loss can be applied to propagation modes with orders equal to or less than m. As a result, the loss difference between modes can be compensated for.

[0055] In the differential modal loss compensation device according to this embodiment, the mode converter may be configured with a long-period fiber Bragg grating. This allows the wavelength dependence of the conversion efficiency to be controlled via the length of the long-period fiber Bragg grating. By configuring the long-period fiber Bragg grating to match the structure of the desired band, it is possible to set an arbitrary spectrum for the wavelength.

[0056] Furthermore, in the modal loss compensation device according to this embodiment, at least one of the first waveguide, the second waveguide, and the third waveguide may have a multi-core structure, which can improve manufacturing tolerances.

[0057] In the modal loss difference compensation device according to the present embodiment, the first waveguide and the second waveguide are two LP mode fibers, the mode converter is capable of propagating five or more LP modes, and 01 LP mode 02 This allows the LP mode to be converted by cutoff in the first waveguide FB1 or the second waveguide FB2.02 Limits the propagation of modes and reduces the LP 01 Loss can be applied to the modes, so that the loss difference between the modes can be compensated for.

[0058] 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.

[0059] 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. [Explanation of symbols]

[0060] 1. Mode loss compensation device 11 cores 12 Clad 13 Coating FB1 1st waveguide FB2 2nd waveguide FB3 3rd waveguide LG LPFG length MC mode converter

Claims

1. a first waveguide and a second waveguide capable of propagating m propagation modes (m is an integer of 3 or more); a third waveguide connected between the first waveguide and the second waveguide and capable of propagating n propagation modes (n is an integer greater than m); a mode converter disposed within the third waveguide and capable of converting one or more propagation modes having an order equal to or less than m into a propagation mode greater than m and equal to or less than n; A mode loss difference compensation device.

2. 2. The modal loss compensation device according to claim 1, wherein the mode converter is constituted by a long-period fiber Bragg grating.

3. The modal loss difference compensation device according to claim 1 , wherein at least one of the first waveguide, the second waveguide, and the third waveguide has a multi-core structure.

4. the first waveguide and the second waveguide are 2LP mode fibers; The mode converter is capable of propagating five or more LP modes, ０１ Mode LP ０２ The inter-modal loss difference compensation device according to any one of claims 1 to 3, which converts the inter-modal loss difference into a mode.

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