Inter-mode loss difference compensator

Abstract

To suppress wavelength dependency of an inter-mode loss compensation quantity for mode multiplex transmission.SOLUTION: An inter-mode loss difference compensator has: a waveguide 17 which can propagate n excitation modes, where n is an integer equal to or larger than 3; a first mode converter 11 which can convert an arbitrary excitation mode, made incident on the waveguide 17 from a first optical fiber 101 capable of propagating (n-1) excitation modes, into a higher mode; a Mach-Zehnder interferometer 15 which can adjust phase change between a pair of arms for branching and multiplexing as to an excitation mode made incident from the first mode converter 11; and a second mode converter 12 which can convert an excitation mode made incident from the Mach-Zehnder interferometer 15 into the arbitrary excitation mode as to an excitation mode emitted to a second optical fiber capable of propagating the (n-1) modes through the waveguide.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a modal loss difference compensator. [Background technology]

[0002] Mode multiplexing transmission technology is known, which increases communication capacity by utilizing multiple modes propagating through optical fibers. In mode multiplexing transmission systems, there is a problem that transmission quality deteriorates due to loss differences between modes that occur in the transmission fiber and in mode multiplexers / demultiplexers. To solve this problem, mode dependent loss (MDL) compensation devices have been studied (see Non-Patent Documents 1 and 2). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] T. Mizuno et al., “Mode dependent loss equalizer and impact of MDL on PDM-16QAM few-mode fiber transmission,” ECOC 2015, P5.9 (2015) [Non-Patent Document 2] T. Fujisawa et al., “Silica-PLC based mode-dependent-loss equalizer for two LP mode transmission,” OFC 2022, paper M4J.4 (2022) Summary of the Invention [Problem to be solved by the invention]

[0004] However, for example, the technology described in Non-Patent Document 1 uses a spatial optical system, and the amount of compensation for the loss difference between modes is limited to a fixed value by the interpolating spatial filter, and the size of the device becomes large due to the spatial optical system. In addition, the spatial filter uses the difference in the shape of the electric field distribution of the modes to give a higher loss to the fundamental mode, but since the filter itself is a loss medium, there is an issue that it also gives loss to the LP (linear polarization) 11 mode, which is a higher order mode.

[0005] Non-Patent Document 2 reports an inter-modal loss difference compensation device that uses planar lightwave circuit (PLC) technology to suppress loss to the LP11 mode while providing loss to the fundamental mode, LP01 mode, by using a parallel waveguide with two coupling parts.

[0006] Thus, when mode multiplexing transmission and wavelength multiplexing transmission are used in combination, the band for compensating characteristics is limited, which is an issue. The present disclosure has been proposed in view of such circumstances, and has an object to provide a modal loss difference compensator for mode multiplexing transmission in which the wavelength dependency of the amount of modal loss difference compensation is suppressed. [Means for solving the problem]

[0007] The present disclosure provides a modal loss difference compensator that includes a waveguide capable of propagating n excitation modes, where n is an integer equal to or greater than 3; a first mode converter capable of converting any excitation mode incident on the waveguide from a first optical fiber capable of propagating n-1 excitation modes into a higher mode; a Mach-Zehnder interferometer capable of adjusting a phase change between a pair of arms that branch and merge for the excitation mode incident from the first mode converter; and a second mode converter capable of converting the excitation mode incident from the Mach-Zehnder interferometer into any excitation mode for the excitation mode outputted to a second optical fiber capable of propagating n-1 modes via the waveguide. Effect of the Invention

[0008] According to the present disclosure, by aggregating functions for each band, it is possible to contribute to reducing the manufacturing costs of transmission devices. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing a schematic configuration of a modal loss difference compensator. [Diagram 2] 1 is a diagram illustrating a configuration of a modal loss difference compensator according to a first embodiment. [Diagram 3] FIG. 13 is a diagram illustrating the operation of a Y-branch waveguide. [Figure 4] FIG. 1 is a diagram showing the configuration of a Mach-Zehnder interferometer. [Diagram 5] FIG. 2 is a plan view showing a first combination of mode rotators. [Figure 6] FIG. 11 is a plan view showing a second combination of mode rotators. [Figure 7] FIG. 11 is a plan view showing a third combination of mode rotators. [Figure 8] FIG. 13 is a diagram showing mode transitions of E11. [Figure 9] FIG. 13 is a diagram showing another mode transition of E11. [Figure 10] FIG. 13 is a diagram showing mode transitions of E21. [Figure 11] FIG. 13 shows another mode transition of E21. [Figure 12] FIG. 13 is a diagram showing mode transitions of E12. [Figure 13] FIG. 13 shows another mode transition of E12. [Figure 14] 4 is a graph showing a transmission spectrum of the modal loss difference compensator of the first embodiment. [Figure 15] FIG. 2 is a diagram illustrating a configuration of an example in which the modal loss difference compensator according to the first embodiment is used as an equalizer. [Figure 16] FIG. 13 is a diagram showing a configuration of a first modified example. [Figure 17] 1 is a graph showing the relationship between the waveguide width and the effective refractive index and the group refractive index. [Figure 18] 13 is a graph showing a transmission spectrum of a first modified example. [Figure 19] FIG. 13 is a diagram showing a configuration of a second modified example. [Figure 20] 13 is a graph showing a transmission spectrum of a second modified example. [Figure 21] FIG. 11 is a diagram illustrating a configuration of a modal loss difference compensator according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, the modal loss difference compensator of the present embodiment will be described in detail with reference to the drawings. Fig. 1 is a diagram showing a schematic configuration of a modal loss difference compensator 10 of the present embodiment.

[0011] The mode loss difference compensator 10 is formed on a quartz substrate as a silica-based planar lightwave circuit (PLC), and a first mode converter 11, a Mach-Zehnder interferometer (MZI) 15, and a second mode converter 12 are provided in this order along a waveguide 17 extending from an input end 10a to an output end 10b. The waveguide 17 is formed of a core with a rectangular cross section having a predetermined width and height embedded in a cladding of the PLC extending in the horizontal direction, and can propagate four excitation modes (hereinafter simply referred to as modes) of electric field distribution, E11, E12, E21, and E22. Here, the subscripts of the Emn modes indicate the orders of the electric field distribution in the horizontal (m) and vertical (n) directions.

[0012] The Mach-Zehnder interferometer 15 is configured by combining a pair of arms, a first arm 15a and a second arm 15b, which branch and merge, with a first Y branch waveguide 15c and a second Y branch waveguide 15d. Of the pair of arms, the first arm 15a is provided with a phase adjustment unit 15p that applies a phase change Δφ. The phase adjustment unit 15p can be realized, for example, by providing a heater that controls the temperature in the first arm 15a.

[0013] At the input end 10a and output end 10b of the modal loss difference compensator 10, a first optical fiber 101 and a second optical fiber 102 of 2LP (linear polarization) modes are connected to the waveguide 17, respectively. The first optical fiber 101 and the second optical fiber can propagate three modes, LP01, LP11a, and LP11b. Here, the subscripts of LPmn indicate the electric field distribution in the angular direction (m) and the radial direction (n). Also, the subscripts a and b of LP11 indicate that the polarization plane is horizontal or vertical. These LP01, LP11a, and LP11b modes correspond to the E11, E21, and E12 modes in the waveguide 17, respectively.

[0014] 1, modes LP01, LP11a, and LP11b of the first optical fiber 101 are incident on the waveguide 17 from the incident end 10a of the modal loss difference compensator 10, and excite the corresponding modes E11, E21, and E12, respectively. Since there is no mode of the first optical fiber 101 corresponding to the propagation mode E22 of the waveguide 17, the E22 mode of the waveguide 17 is not excited. Note that, in the following, for the purpose of explaining the operation of the modal loss difference compensator 10, the non-excited E22 mode will be treated in the same way as other modes.

[0015] The first mode converter 11 receives the modes E11, E21, E12, and E22 from the input end 10a of the modal loss difference compensator 10 through the waveguide 17. Of the input modes, the first mode converter 11 maintains the fundamental mode E11, but converts the higher modes E21, E12, and E22 into E12, E22, and E21, respectively. The first mode converter 11 can reduce wavelength dependency by being an adiabatic conversion type element.

[0016] The E11, E21, E12, and E22 modes converted by the first mode converter 11 are input to the Mach-Zehnder interferometer 15 through the waveguide 17. The Mach-Zehnder interferometer 15 controls conversion between the input E11, E21, E12, and E22 modes by the phase change Δφ applied by the phase adjustment unit 15p. In detail, when the phase change Δφ=0, the E11, E21, E12, and E22 modes are maintained as shown by the solid line in Fig. 1. When the phase change Δφ=π, the E11 and E21, E12, and E22 modes are swapped with each other as shown by the dashed line.

[0017] When the phase change Δφ is 0<Δφ<π, the Mach-Zehnder interferometer 15 maintains some of the modes of E11, E21, E12, and E22 as when the phase change Δφ=0, and the remaining modes exchange the modes of E11 and E21, E12, and E22 with each other as when the phase change Δφ=π. The ratio at which the modes of E11, E21, E12, and E22 are maintained increases as the phase change Δφ approaches 0, and the ratio at which the modes of E11, E21, E12, and E22 are exchanged with each other increases as the phase change Δφ approaches π.

[0018] The E11, E21, E12 and E21 modes converted by the Mach-Zehnder interferometer 15 are incident on the second mode converter 12 through the waveguide 17. The second mode converter 12 maintains the fundamental mode E11 among the incident modes, but converts the higher modes E21, E12 and E22 into E22, E21 and E12 modes, respectively. As with the first mode converter 11, the second mode converter 12 is an adiabatic conversion type element, thereby making it possible to reduce wavelength dependency.

[0019] The E11, E21, E12, and E21 modes converted by the second mode converter 12 are output from the output end 10b of the modal loss difference compensator 10 through the waveguide 17. The E11, E21, and E12 modes output from the output end 10b are input to the second optical fiber 102 and excite the corresponding LP01, LP11a, and LP11b, respectively. The E22 propagation mode of the waveguide 17 is cut off and not guided because there is no corresponding mode of the second optical fiber 102.

[0020] In this way, the LP01, LP11a, and LP11b modes input from the first optical fiber 101 to the input end 10a of the modal loss difference compensator 10 excite the corresponding E11, E21, and E12 modes in the waveguide 17 of the modal loss difference compensator 10, and are controlled in conversion between E11, E21, E12, and E22 of the waveguide 17 in accordance with the phase change Δφ of the Mach-Zehnder interferometer 15. The modes are output from the output end 10b of the modal loss difference compensator 10 to excite the LP01, LP11a, and LP11b modes of the second optical fiber 102. In detail, when the phase change Δφ=0, the modal loss difference compensator 10 maintains E11, E21, E12, and E22, and the modes LP01, LP11a, and LP11b of the first optical fiber 101 are guided to the modes LP01, LP11a, and LP11b of the second optical fiber 102, respectively, without being converted by the modal loss difference compensator 10.

[0021] When the phase change Δφ=π, the modal loss difference compensator 10 swaps the propagation modes of E11 and E21, and E12 and E22 with each other. The LP11a and LP11b modes of the first optical fiber 101 are swapped with the corresponding E21 and E12 modes by the modal loss difference compensator 10 into the E12 and E21 modes, respectively, and excite the LP11b and LP11a modes of the second optical fiber 102. The LP01 mode of the first optical fiber 101 is converted from the corresponding E11 mode to the E22 mode by the modal loss difference compensator 10, but is cut off and not guided because there is no corresponding mode of the second optical fiber 102.

[0022] When the phase change Δφ is in the range of 0<Δφ<π, the modal loss difference compensator 10 maintains some of the modes of E11, E21, E12, and E22, and the modes of LP01, LP11a, and LP11b of the first optical fiber 101 excite the modes of LP01, LP11a, and LP11b of the second optical fiber 102, respectively, without being converted by the modal loss difference compensator 10. As for the remaining modes of E11, E21, E12, and E22, the modes of E11, E21, E12, and E22 are respectively exchanged with each other, and the modes of LP11a and LP11b of the first optical fiber 101 excite the modes of LP11b and LP11 of the second optical fiber 102, respectively. The LP01 mode of the first optical fiber 101 is converted from the corresponding E11 mode to the E22 mode by the modal loss difference compensator 10, but since there is no corresponding mode of the second optical fiber 102, it is cut off and not guided.

[0023] In the modal loss difference compensator 10, the ratio at which the modes of LP01, LP11a, and LP11b of the first optical fiber 101 are maintained and the modes of LP01, LP11a, and LP11b of the second optical fiber 102 are excited increases as the phase change Δφ approaches 0, and the ratio at which the modes of LP11a and LP11b of the first optical fiber are swapped and the modes of LP11a and LP11b of the second optical fiber 102 are excited, and the ratio at which the mode of the second optical fiber 102 is not excited by the propagation mode LP01 of the first optical fiber 101 increases as the phase change Δφ approaches π.

[0024] In this way, the modal loss differential compensator 10 can control the transmittance of the mode of LP01 traveling from the first optical fiber 101 to the second optical fiber 102. In detail, when the phase change Δφ=0 applied by the Mach-Zehnder interferometer 15, LP01 in the fundamental mode is transmitted without attenuation, but as the phase change Δφ increases, the amount of attenuation of LP01 is increased, and when the phase change Δφ reaches π, LP01 is completely attenuated and does not transmit. On the other hand, although the higher-order modes LP11a and LP11b may be partially or completely replaced with each other, the combined modes of LP11a and LP11b maintain a constant transmittance regardless of the phase change Δφ applied by the Mach-Zehnder interferometer 15.

[0025] The input / output relationship of such a modal loss difference compensator 10 is summarized in Table 1 below. The input in parentheses indicates the corresponding mode of the first optical fiber 101. The excitation ratio of the first and second term modes at the output can be controlled by the phase change Δφ. Note that the input E22 mode is not excited because there is no corresponding propagation mode in the first optical fiber 101 of the 2LP mode. Also, the output E22 mode is not guided and is cut off because there is no corresponding propagation mode in the second optical fiber 102.

[0026] [Table 1]

[0027] Generally, in a 2LP mode waveguide such as the first optical fiber 101 and the second optical fiber 102, the loss of higher-order modes LP11a and LP11b is greater than that of the fundamental mode LP01. The modal loss difference compensator 10 controls the transmittance of the fundamental mode LP01 while maintaining the transmittance of the higher-order modes LP11a and LP11b, thereby making it possible to compensate for the modal loss difference between the fundamental mode LP01 and the higher-order modes LP11a and LP11b.

[0028] Furthermore, in the modal loss difference compensator 10, by using adiabatic conversion type elements for the first mode converter 11 and the second mode converter 12, the modal loss difference compensator 10 can be configured with adiabatic conversion type elements except for the Mach-Zehnder interferometer 15. By using adiabatic conversion type elements, it is possible to reduce the wavelength dependency of the modal loss difference compensator 10. The adiabatic conversion type elements will be described later.

[0029] In the modal loss difference compensator 10, the fundamental mode E11 of the waveguide 17 corresponding to the fundamental mode LP01 incident from the first optical fiber 101 is converted to E11 or a higher mode E22, and the transmittance of LP01 is controlled by cutting off E22, but the transmittance can also be controlled in a similar manner by guiding a desired mode to a mode to be cut off.

[0030] In addition, although the waveguide 17 of the mode loss difference compensator 10 has four modes, E11, E21, E12, and E22, the waveguide 17 may be capable of propagating n modes, where n is an integer equal to or greater than 3, the first mode converter 11 may be capable of converting any incident mode into a higher mode, the second mode converter 12 may be capable of converting a mode incident via the Mach-Zehnder interferometer 15 into any mode, and a first optical fiber 101 and a second optical fiber 102 capable of propagating n-1 modes may be connected to the incident end 10a and the exit end 10b, respectively.

[0031] In such a configuration, the transmittance of a desired mode included in the n-1 modes that can propagate through the waveguide 17 through the modal loss difference compensator 10 can be controlled by the phase change Δφ applied by the phase adjustment unit 15p of the Mach-Zehnder interferometer 15. In detail, the first mode converter 11 converts the desired first mode, the transmittance of which is controlled, between the second mode in which the waveguide 17 is not excited because there is no mode corresponding to the first optical fiber 101 and the second optical fiber 102, into a pair of modes that can be controlled to be swapped in accordance with the phase change Δφ applied by the Mach-Zehnder interferometer 15.

[0032] The Mach-Zehnder interferometer 15 controls the ratio of switching between the pair of modes in response to the phase change Δφ. The ratio of switching between the pair of modes changes in response to the phase change Δφ from a phase change Δφ=0, where the modes are maintained without being switched, to a phase change Δφ=π, where the modes are completely switched.

[0033] The second mode converter 12 is configured to convert the pair of modes transmitted through the Mach-Zehnder interferometer 15 without being swapped back into the original first and second modes incident on the first mode converter 11. Therefore, when the phase change Δφ=0 applied by the Mach-Zehnder interferometer 15 is applied, the first and second modes incident on the first mode converter 11 are converted back to the original first and second modes by the second mode converter 12, and the first mode is incident on the second optical fiber 102 from the output end 10b and excites the corresponding mode, but the second mode is cut off and not guided because there is no corresponding mode propagating through the second optical fiber 102.

[0034] When the ratio at which the pair of modes is switched in the Mach-Zehnder interferometer 15 increases with an increase in the phase change Δφ, the second mode converter 12 converts the original first and second modes into the first and second modes by switching them according to this ratio. When the phase change Δφ reaches π, the second mode converter 12 converts the original first and second modes into the first and second modes by switching them completely, respectively. The original second mode converted into the first mode is input from the output end 10b into the second optical fiber 102 and excites the corresponding mode, but the original first mode converted into the second mode is cut off and not guided because there is no corresponding mode propagating through the second optical fiber 102.

[0035] With this configuration, the modal loss difference compensator 10 can control the transmittance of the desired first mode by the phase change Δφ applied by the Mach-Zehnder interferometer 15. Note that, of the n-1 modes, n-2 modes excluding the first mode may be interchanged with each other, but the total amount passes through the modal loss difference compensator 10 without attenuation.

[0036] (First embodiment) Fig. 2 is a diagram showing the configuration of a mode loss difference compensator 10 according to the first embodiment. In the first embodiment, the first mode converter 11 and the second mode converter 12 in the mode loss difference compensator 10 shown in Fig. 1 are each configured by providing a pair of arms and a mode rotator that branch and join together by combining two Y-branch waveguides.

[0037] 3 is a diagram showing the operation of a Y-branch waveguide. The Y-branch waveguide has a Y shape in which one waveguide branches into two waveguides in the light propagation direction. In the first embodiment, the Y-branch waveguide has a length of about 1 mm or more in the light propagation direction so that the light branched in the Y-branch waveguide is adiabatically converted.

[0038] FIG. 3(a) shows the case where the E11 mode is incident on the Y-branch waveguide. Similarly, FIG. 3(b), FIG. 3(c), and FIG. 3(d) show the cases where the E21, E12, and E22 modes are incident, respectively. In all of FIG. 3(a) to FIG. 3(d), the electric field distribution of each mode is separated in the direction where the Y-branch waveguide branches due to adiabatic conversion by the Y-branch waveguide. Although FIG. 3(a) to FIG. 3(d) show an example where the incident mode is branched by the Y-branch waveguide, in the case of adiabatic conversion, FIG. 3(a) to FIG. 3(d) also apply to the case where two modes merge into one mode by the Y-branch waveguide in the direction opposite to the arrow.

[0039] 4 is a diagram showing the configuration of Mach-Zehnder interferometer 15. In Mach-Zehnder interferometer 15, a branch path from the upstream side to a pair of arms, first arm 15a and second arm 15b, is formed by first Y-branch waveguide 15c, and a junction path where the pair of arms, first arm 15a and second arm 15b, are joined on the downstream side is formed by second Y-branch waveguide 15d. In Mach-Zehnder interferometer 15, a phase change Δφ can be applied between first arm 15a and second arm 15b by phase adjustment unit 15p provided in first arm 15a.

[0040] When no phase change is applied, that is, when the phase change Δφ=0, the mode propagating through the Mach-Zehnder interferometer 15 is maintained as shown in the following equation (1).

[0041]

number

[0042] Where:

[0043]

number

[0044] represents the modes E11, E21, E12, and E22 incident on the Mach-Zehnder interferometer 15,

[0045]

number

[0046] represent the modes E11, E21, E12, and E22 emitted from the Mach-Zehnder interferometer 15. For example, when the E11 mode is input,

[0047]

number

[0048] In addition, θ1, θ2, θ3, and θ4 are the phases of the E11, E21, E12, and E22 modes incident on the Mach-Zehnder interferometer 15, respectively, and θ arm represents the phase change caused by the arm portion.

[0049] On the other hand, when a phase change of π is applied, i.e., when the phase change Δφ=π, conversion occurs between the horizontal peaks, as shown in the following equation 5, and the E11 mode and the E21 mode are converted into each other, and the E12 mode and the E22 mode are converted into each other.

[0050]

number

[0051] Furthermore, when an arbitrary phase change is applied, i.e., when the phase change Δφ is an arbitrary value, the E11 mode and the E21 mode are converted into each other at a ratio according to the phase change Δφ, and the E12 mode and the E22 mode are converted into each other at a ratio according to the phase change Δφ.

[0052] In equation (5), it is assumed that the phase adjustment unit 15p provided in the first arm 15a applies a uniform phase change Δφ to all modes. However, in an actual Mach-Zehnder interferometer 15, the phase change Δφ has wavelength dependence and mode dependence. In this disclosure, an adiabatic conversion type mode rotator, which will be described later, is used to suppress the wavelength dependence of the modal loss difference compensation amount according to the set phase change Δφ.

[0053] Fig. 5 is a plan view showing a first combination of mode rotators. In the mode rotator, a tapered trench is formed at a predetermined depth on the top surface of a core having a predetermined length in the light propagation direction and a rectangular cross section, with an edge formed linearly from one side surface of the core to the other side surface in the light propagation direction, and one or the other of the core sides intersects with one side, and the cross section has a substantially L-shaped cross section in which the trench reaches one side surface. In other words, in the mode rotator, a triangular trench is formed at a predetermined depth on the top surface of a core block having a predetermined length in the light propagation direction, with one side surface as the base and an edge formed linearly from one side surface to the other side surface as the hypotenuse. In the figure, the trench is represented by a gray area.

[0054] As shown in Fig. 5(a), in the first combination, the two mode rotators are arranged such that the tapered trenches face each other and are in contact with each other, and the arrangement of the two trenches is mirror symmetrical with respect to the boundary surface where the two trenches face each other. The mode propagating through the mode rotator of the first combination is transformed as shown in the following equation (6). Note that in the case of the combination shown in Fig. 5(b), the transformation is the same as in the case of the combination in Fig. 5(a), and the transfer matrix is ​​the same as that in equation (6).

[0055]

number

[0056] 2, such a first combination mode rotor is provided on the first arm 11a of the first mode converter 11. Similarly, it is also provided on the first arm 12a of the second mode converter 12.

[0057] FIG. 6 is a plan view showing a second combination of mode rotators. As shown in FIG. 6(a), according to the second combination, the two mode rotators are arranged so that the tapered trenches face each other and are staggered with respect to the boundary surface where the two mode rotators face each other. In other words, the two mode rotators are arranged so as to be rotationally symmetric with respect to one axis extending vertically within the boundary surface. The mode propagating through the mode rotators of the second combination is transformed as shown in the following formula 7. Note that the combination shown in FIG. 6(b) is also transformed in the same way as the combination shown in FIG. 6(a), and the transfer matrix is ​​the same as that of formula 7.

[0058]

number

[0059] When the transfer matrix of the mode rotor of the second combination shown in Equation 7 is compared with the transfer matrix of the mode rotor of the first combination shown in Equation 6, the complex amplitudes of E21 and E12 differ by a phase of π. For the other E11 and E22, there is no phase difference between the mode rotor of the second combination and the mode rotor of the first combination.

[0060] 2, the second combination mode rotor is provided on the second arm 11b of the first mode converter 11, as well as on the second arm 12b of the second mode converter 12.

[0061] 7 is a plan view showing a third combination of mode rotators. In the third combination, a mode rotator in which a trench is formed so as to expand from one side surface to the other side surface in the light propagation direction is followed by a block in which a trench of a predetermined depth is formed so that the edges of the trench extend linearly from the center of the two side surfaces to both side surfaces and the cross section is convex or inverted T-shaped.

[0062] The mode rotator of the third combination operates to interchange E21 and E12. The convex block following the mode rotator ensures that the effective refractive index is matched between the output side of the mode rotator, which has a trench between both sides, and the waveguide 17 of the modal loss difference compensator 10, which does not have a trench.

[0063] 2, the mode rotator of the third combination is provided in the first mode converter 11 after the second Y branch waveguide 11d where the first arm 11a and the second arm 11b join, and in the second mode converter 12, before the first Y branch waveguide 12c where the first arm 12a and the second arm 12b branch.

[0064] In the first embodiment, the mode rotators of the first to third combinations are of the adiabatic conversion type by ensuring a length in the light propagation direction. In addition, as described above, the Y-branch waveguide is also of the adiabatic conversion type. Therefore, the first embodiment has small wavelength dependency, which is desirable from the viewpoint of insertion loss. In addition, the length of the mode rotator can be shortened by applying the FAQUAD (fast quasi-adiabatic dynamics) design method to the mode rotator.

[0065] In the mode loss difference compensator 10 of the first embodiment having such a configuration, in the front stage having a configuration in which the first mode converter 11 is branched into the first arm 11a and the second arm 11b, the incident E12 mode is converted to an E22 mode, but the other E11 and E21 modes are maintained. In the rear stage in the first mode converter 11 where the first arm 11a and the second arm 11b join at the second Y branch waveguide 11d, E21 is converted to E12 by a mode rotator, but the other E11 and E22 modes are maintained.

[0066] As described above, the Mach-Zehnder interferometer 15 converts between the E11 mode and the E21 mode at a ratio according to the phase change Δφ applied by the phase adjustment unit 15p, and converts between the E12 mode and the E22 mode at a ratio according to the phase change Δφ.

[0067] In the second mode converter 12, in the front stage where the first arm 12a and the second arm 12b branch at the first Y branch waveguide 12c, E12 and E21 are swapped with each other by a mode rotator, but the other modes of E11 and E22 are maintained. In the rear stage having a configuration branched to the first arm 12a and the second arm 12b, the modes of the incident E12 and E22 are swapped with each other, but the other modes of E11 and E21 are maintained.

[0068] Specific operation of the modal loss difference compensator 10 of the first embodiment will be described with reference to Fig. 8 to Fig. 13. Fig. 8 shows transition when E11 in the fundamental mode is incident on the modal loss difference compensator 10 and the phase adjustment unit 11p adds a phase change Δφ=0. The E11 mode is excited by the LP01 mode incident on the first optical fiber 101 at the incident end 10a. The E11 mode propagates from the incident end 10a along the waveguide 17 through the first mode converter 11, the Mach-Zehnder interferometer 15, and the second mode converter 12 without being converted from the E11 mode, and reaches the output end 10b. The E11 mode output from the output end 10b is input to the second optical fiber 102 to excite the LP01 mode.

[0069] 9 shows a transition when fundamental mode E11 is incident on the modal loss difference compensator 10 and the phase adjustment unit 11p adds a phase change Δφ=π. The E11 mode propagates through the first mode converter 11 from the input end 10a along the waveguide 17, but is converted to E21 by the Mach-Zehnder interferometer 15 that adds a phase change Δφ=π in the phase adjustment unit 11p. It is further converted to E12 mode by the third combination of mode rotators in the front stage of the second mode converter 12, and is converted to E22 mode after branching and merging to the first arm 12a and the second arm 12b in the rear stage, and reaches the output end 10b. The E22 mode is cut off and not guided because there is no corresponding mode of the second optical fiber 102.

[0070] 10 shows a transition when a high-order mode E12 is incident on the modal loss difference compensator 10 and the phase adjustment unit 11p applies a phase change Δφ=0. The E12 mode is excited by the LP11b mode incident on the input end 10a from the first optical fiber 101. The E12 mode propagates from the input end 10a along the waveguide 17, branches and joins to the first arm 11a and the second arm 11b in the front stage of the first mode converter 11, is converted to the E22 mode, propagates through the third combination of mode rotators in the rear stage, the Mach-Zehnder interferometer 15, and the third combination of mode rotators in the front stage of the second mode converter 12, branches and joins to the first arm 12a and the second arm 12b in the rear stage, is converted to the E12 mode, and reaches the output end 10b. The E12 mode output from the output end 10b enters the second optical fiber 102 and excites the LP11b mode.

[0071] 11 shows a transition when a high-order mode E12 is incident on the modal loss difference compensator 10 and the phase adjustment unit 11p applies a phase change Δφ=π. The E12 mode is excited by the LP11b mode incident on the input end 10a from the first optical fiber 101. The E12 mode propagates from the input end 10a along the waveguide 17, is converted to the E22 mode by branching and merging into the first arm 11a and the second arm 11b in the front stage of the first mode converter 11, propagates through the third combination of mode rotators in the rear stage, is converted to the E12 mode by the Mach-Zehnder interferometer 15, is converted to the E21 mode by the third combination of mode rotators in the front stage of the second mode converter 12, is branched and merging into the first arm 12a and the second arm 12b in the rear stage, and reaches the output end 10b. The mode of E21 output from the output end 10b enters the second optical fiber 102 and excites the mode of LP11a.

[0072] FIG. 12 shows a transition when E21 in a higher order mode is incident on the modal loss difference compensator 10 and the phase adjustment unit 11p adds a phase change Δφ=0. The mode of E21 is excited by the mode of LP11a incident from the first optical fiber 101 to the incident end 10a. The mode of E21 propagates from the incident end 10a along the waveguide 17, branches and joins to the first arm 11a and the second arm 11b in the front stage of the first mode converter 11, is converted to the mode of E12 by the mode rotator of the third combination in the rear stage, is converted to the mode of E21 by the Mach-Zehnder interferometer 15 and the mode rotator of the third combination in the front stage of the second mode converter 12, branches and joins to the first arm 12a and the second arm 12b in the rear stage, and reaches the output end 10b. The mode of E21 output from the output end 10b is input to the second optical fiber 102 to excite the mode of LP11a.

[0073] 13 shows a transition when a high-order mode E21 is incident on the modal loss difference compensator 10 and the phase adjustment unit 11p applies a phase change Δφ=π. The mode of E21 is excited by the mode of LP11a incident on the input end 10a from the first optical fiber 101. The mode of E21 propagates from the input end 10a along the waveguide 17, branches and joins to the first arm 11a and the second arm 11b in the front stage of the first mode converter 11, is converted to the mode of E12 by the mode rotator of the third combination in the rear stage, is converted to the mode of E22 by the Mach-Zehnder interferometer 15, propagates through the mode rotator of the third combination in the front stage of the second mode converter 12, branches and joins to the first arm 12a and the second arm 12b in the rear stage, and is converted to the mode of E12 to reach the output end 10b. The E12 mode output from the output end 10b enters the second optical fiber 102 and excites the LP11b mode.

[0074] 10 to 13 show the transition when the higher-order modes E21 or E12 are incident on the modal loss difference compensator. In these cases, even if a phase change Δφ=π is applied by the phase adjustment unit 11p of the Mach-Zehnder interferometer 15, only a rotation occurs in which the E21 and E12 modes are interchanged, and there is no change in the output intensity.

[0075] FIG. 14 is a graph showing a transmission spectrum of the modal loss difference compensator 10 according to the first embodiment. target is a target transmittance value. FIG. 14(a) shows a case where the phase adjustment unit 15p of the Mach-Zehnder interferometer 15 is set so that E11 of the fundamental mode incident from the input end 10a of the modal loss difference compensator 10 is output from the output end with a transmittance of 0.8. The wavelength range of the transmission spectrum is the S band (1.46 to 1.53 μm), C band (1.53 to 1.565 μm), and L band (1.565 to 1.625 μm) bands used in mode multiplexing transmission technology. Similarly, FIG. 14(b), FIG. 14(c), and FIG. 14(d) show cases where the phase adjustment unit 15p is set so that the transmittance is 50%, 20%, and 0%.

[0076] 14, it was observed that the insertion loss of the higher modes E12 and E21 was 3% or less, that is, the total power of the E12 and E21 modes was 97% or more of the input power. In the first embodiment, since the components are mostly composed of adiabatic conversion elements except for the phase adjustment unit 15p using a heater, it was confirmed that operation over a wide band including S band, C band, and L band was realized.

[0077] (Example) 15 is a diagram showing the configuration of an example to which the modal loss difference compensator 10 of the first embodiment is applied. The example is a 2LP mode transmission system, in which the modal loss difference compensator 10 of the first embodiment is provided as an equalizer 35 between a first optical fiber 101 and a second optical fiber 102 of the 2LP mode. In addition, an S band amplifier 31, a C band amplifier 32, and an L band amplifier 33 are provided in parallel in the first optical fiber 101.

[0078] As described above, the mode loss difference compensator 10 of the first embodiment can realize operation over a wide band of the S band, C band, and L band, so by providing the mode loss difference compensator 10 of the first embodiment as an equalizer 35 in the subsequent stages of the S band amplifier 31, the C band amplifier 32, and the L band amplifier 33, it is possible to equalize the mode gains collectively over all bands by the single equalizer 35. Therefore, by using the mode loss difference compensator 10 of the first embodiment, it is no longer necessary to install an equalizer 35 in each of the S band amplifier 31, the C band amplifier 32, and the L band amplifier 33, and the number of equalizers 35 can be reduced.

[0079] (First Modification) FIG. 16 is a diagram showing the configuration of a first modified example of the mode loss difference compensator 10 of the first embodiment. The first modified example is different from the configuration of the first embodiment in which a phase difference is generated by a phase adjustment unit 15p using a heater in that the first modified example is a passive operation configuration in which the waveguides of the first arm 15a and the second arm 15b of the Mach-Zehnder interferometer 15 are set to a predetermined width and length to generate a phase difference in order to adjust the phase between the first arm 15a and the second arm 15b. The other configurations of the first modified example are the same as those of the first embodiment. Since the modified example is a passive operation, the mode dependent loss (MDL) is fixed.

[0080] 16, in the modified Mach-Zehnder interferometer 15, the width of the waveguide in the first arm 15a is set to 7 μm, and the width of the waveguide in the second arm 15b is set to 5.4 μm. The widths of the waveguides in the first arm 15a and the second arm 15b are selected as follows.

[0081] FIG. 17 is a graph showing the relationship between the waveguide width and the effective refractive index and the group refractive index. In order to suppress the wavelength dependency, it is necessary to reduce the wavelength dependency of the relative phase difference width ΔβL of the first arm 15a and the second arm 15b of the Mach-Zehnder interferometer 15. As shown in FIG. 15, when the waveguide widths of 5.4 μm and 7.0 μm are selected, the group refractive index becomes equal, resulting in a structure with small wavelength dependency. Note that, in the connection of the parts with different waveguide widths, a linear taper may be formed so that the waveguide width transitions continuously to suppress the connection loss.

[0082] Fig. 18 is a graph showing the transmission spectrum of the first modified example. Fig. 18(a) shows the transmission spectrum of the first modified example, and Fig. 18(b) shows the transmission spectrum of the first embodiment for comparison. The wavelength range of the transmission spectrum is the S band, C band, and L band, similar to Fig. 14 showing the transmission spectrum of the first embodiment.

[0083] In the transmission spectrum of the first modified example shown in FIG. 18(a), the lengths of the waveguides with the widths set as described above for the transmittances of 100, 70, 50, 25, and 13% were 0, 288, 390, 520, and 596 μm, respectively. In the transmission spectrum of the first embodiment shown in FIG. 18(b), the heater phase adjustment unit 15p was controlled so that the transmittances were 100, 70, 50, 25, and 13%. As shown in FIG. 18(b), it was confirmed that the transmission spectrum of the first embodiment has a relatively large wavelength dependency due to the heater phase adjustment unit 15p. On the other hand, as shown in FIG. 18(a), it was confirmed that the transmission spectrum of the first modified example, in which the MDL is fixed in a passive type configuration, has a reduced wavelength dependency.

[0084] (Second Modification) FIG. 19 is a diagram showing the configuration of a second modified example of the mode loss difference compensator 10 of the first embodiment. The second modified example differs from the first modified example in which the Mach-Zehnder interferometer 15 is entirely configured in a passive operation in that a heater-based phase adjustment unit is added to the first arm 15a of the Mach-Zehnder interferometer 15 in the configuration of the first modified example shown in FIG. 15, similarly to the configuration of the first embodiment shown in FIG. 2. The other configurations of the second modified example are the same as those of the first modified example. In the second modified example, the transmittance is set to 50% or an arbitrary value in advance in a passive type configuration such as a waveguide having a predetermined width and length of the first arm 15a and the second arm 15b of the Mach-Zehnder interferometer 15, and from that state, the phase control is performed by the heater-based phase adjustment unit 15p, thereby making it possible to suppress the wavelength dependency of the phase control by the phase adjustment unit 15p.

[0085] Fig. 20 is a graph showing the transmission spectrum of the second modified example. The wavelength range of the transmission spectrum is the S-band, C-band, and L-band bands, similar to Fig. 18 showing the transmission spectrum of the first modified example. It was confirmed that the wavelength dependency of the transmission spectrum is reduced in the second modified example as in the first modified example in which the MDL is fixed in a passive type configuration.

[0086] (Second embodiment) Fig. 21 is a diagram showing the configuration of a mode loss difference compensator 10 according to the second embodiment. In the second embodiment, the first mode converter 11 and the second mode converter 12 in the mode loss difference compensator 10 shown in Fig. 1 are configured with long period gratings. More specifically, the first mode converter 11 is configured with a first long period grating 18, and the second mode converter 12 is configured with a second long period grating 19.

[0087] First long period grating 18 and second long period grating 19 operate by adding a periodic structure to the waveguide according to the propagation constant difference of the modes propagating through the waveguide. Also in the second embodiment, first long period grating 18 and second long period grating 19 provide the operation of first mode converter 11 and second mode converter 12, thereby realizing the operation of modal loss difference compensator 10.

[0088] As described above, according to this embodiment, a wideband mode loss difference compensator can be realized by incorporating a Y-branch waveguide and a mode rotator having a phase conversion function, which is an adiabatic conversion element, into an optical waveguide. It is possible to set a variable adjustment type using a heater for the phase conversion function and a fixed type that changes only the relevant part of the waveguide structure, and it is possible to make the structure have small wavelength dependency in the S-band, C-band, and L-band bands. [Explanation of symbols]

[0089] 10. Mode loss difference compensator 10a entrance end 10b Output end 11 First mode converter 11a First Arm 11b Second Arm 11c First Y-branch waveguide 11d Second Y-branch waveguide 12 Second mode converter 12a First Arm 12b 2nd Arm 12c First Y-branch waveguide 12d Second Y-branch waveguide 15 Mach-Zehnder Interferometer 15a 1st Arm 15b 2nd Arm 15c First Y-branch waveguide 15d Second Y-branch waveguide 15p Phase adjustment section 17 Waveguide 18 First long period grating 19 Second long period grating

Claims

1. a waveguide capable of propagating n excitation modes, n being an integer of 3 or more; a first mode converter capable of converting any excitation mode incident on the waveguide from a first optical fiber capable of propagating n-1 excitation modes into a higher-order mode; a Mach-Zehnder interferometer capable of adjusting a phase change between a pair of arms that branch and merge with respect to an excitation mode incident from the first mode converter; a second mode converter capable of converting an excitation mode incident from the Mach-Zehnder interferometer into an arbitrary excitation mode with respect to an excitation mode outputted to a second optical fiber capable of propagating n-1 modes via the waveguide; A modal loss difference compensator including:

2. 2. The modal loss difference compensator according to claim 1, wherein the waveguide is capable of propagating four excitation modes, namely E11, E12, E21, and E22, the first mode converter is capable of converting the E12 mode and the E21 mode into the E22 mode and the E12 mode, respectively, the second mode converter is capable of converting the E21 mode, the E22 mode, and the E12 mode into the E22 mode, the E12 mode, and the E21 mode, respectively, and a first optical fiber and a second optical fiber capable of propagating the LP01 and LP11 modes, respectively, are connected to an input end and an output end of the waveguide.

3. 2. The modal loss difference compensator according to claim 1, wherein the Mach-Zehnder interferometer comprises a pair of arms branched and joined by two Y-branch waveguides.

4. 3. The modal loss difference compensator according to claim 2, wherein the first mode converter includes a pair of arms that branch and merge, each formed of two Y-branch waveguides, for converting between the E12 mode and the E22 mode, and a mode rotator that converts between the E21 mode and the E12 mode, and the second mode converter includes a mode rotator that converts between the E21 mode and the E12 mode, and a pair of arms that branch and merge, each formed of two Y-branch waveguides, for converting between the E12 mode and the E22 mode.

5. 2. The modal loss difference compensator according to claim 1, wherein the Mach-Zehnder interferometer adjusts a phase change between the pair of arms by at least one of a width and a length of the waveguides of the pair of arms.

6. 2. The modal loss difference compensator according to claim 1, wherein the Mach-Zehnder interferometer adjusts a phase change between the pair of arms by providing a heater in one of the pair of arms.

7. 7. The modal loss difference compensator according to claim 1, which is formed by a planar lightwave circuit.

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