JP2019191224A - Mode exchanger - Google Patents

Abstract

To provide a mode exchanger capable of compensating for the difference in a mode loss by a method different from the method for giving a loss only to a specific mode and also relaxing the limit of a transmitting distance associated with the compensation for the difference in the mode loss.SOLUTION: A mode exchanger 301 consists of a waveguide in which n (n is 3 or more) mode propagates and includes a mode conversion circuit 10 in which the number of times of converting the mode is smaller than n-1. By arranging two or more mode exchanging circuits on a transmission line, a lower order mode, out of n-modes, is converted to a higher order mode at least once, and at least one of higher order modes, out of n-modes, is converted to the lower order mode at least once.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to a mode exchanger having a function of exchanging power between propagating modes in a mode multiplexing transmission system using a multimode optical fiber.

  In optical fiber communication systems, non-linear effects and fiber fuses that occur in optical fibers are problematic, and transmission capacity is limited. In order to relax these restrictions, it is necessary to reduce the density of the light guided to the optical fiber, and as shown in Non-Patent Document 1, a large core fiber has been studied.

  However, bending loss reduction, single mode operation area expansion, and effective area expansion are in a trade-off relationship with each other, and there is a problem that the amount of effective area expansion under certain conditions is limited. Therefore, a mode multiplex transmission system that uses a multimode fiber as a transmission fiber and performs parallel transmission using a plurality of modes to propagate has been studied as a technology for realizing a dramatic increase in capacity (for example, non-patent literature). 2).

  In a mode multiplex transmission system, a mode multiplexer / demultiplexer has been proposed in order to propagate a plurality of signals emitted from a transmitter as separate modes in an optical fiber (see, for example, Non-Patent Documents 3 and 4). ).

  In addition, optical MIMO transmission has been proposed in which coupling between modes occurring in a transmission path is compensated by MIMO signal processing at the receiving end. However, in the transmission path, optical characteristics such as transmission loss, chromatic dispersion, and polarization dispersion are also different between modes, and the difference in mode-dependent characteristics causes quality degradation of the restored signal even when MIMO signal processing is performed. For example, in a two-mode multiplex transmission line, a mode loss difference compensator for a three-mode (2LP) mode transmission line that compensates for a loss difference between modes by giving a loss only to the LP01 mode has been proposed. Non-patent document 5).

T.A. Matsui, et al. , “Applicability of Photonic Crystal Fiber With Uniform Air-Hole Structure to High-Speed and Wide-Band Transmission OverTownConventionmm. Lightwave Technol. 27, 5410-5416, 2009. N. Hanzawa et al. , “Demonstration of mode-division multiplexing transmission 10 km two-mode fiber with mode coupler”, OFC2011, paper OWA4 (2011) N. Hanzawa et al. , "Asymmetric parallel waveguide with mode conversion for mode and wavelength division multiplexing transmission", OFC2012, OTU1l. 4). N. Hanzawa et al, "Mode multi / demultiplexing with parallel waveguide for mode division multiplexed transmission", Opt. Express vol. 22, pp. 29321-29330 (2014) T.A. Mizuno et al. , "Mode dependent loss equalizer and impact of MDL on PDM-16QAM new-mode fiber transmission", Proc. ECOC, P.M. 5.9, Valencia (2015). S. Savin, et al., “Tunable mechanically induced long-period fiber gratings” OPTICS LETTERS / Vol. 25, no. 10 / May 15, 2000 Y. Yamashita et al. "Excitation of LP21b and LP02 Modes with PLC-based Tapered Waveform for Mode-division Multiplexing", OECC2016, paper TuE3-4 (2016) K. Saitoh, et al. “PLC-based LP11 mode rotator for mode-division multiplexing transmission”, Opt. Express, vol. 22, pp. 19117-19130 (2014)

  However, as in Non-Patent Document 5, it is difficult to realize the loss difference between modes by giving a loss only to a specific mode when the number of modes is large, and transmission is performed because the loss difference between modes is compensated. There is a problem that the distance is limited.

  Therefore, in order to solve the above-described problem, the present invention can compensate for the mode loss difference by a method different from the method of giving a loss only to a specific mode, and can relax the restriction on the transmission distance accompanying the compensation of the mode loss difference. An object is to provide a mode exchanger.

  In order to achieve the above object, the mode exchanger according to the present invention switches the high-order mode and the low-order mode.

Specifically, the mode exchanger according to the present invention is a PLC type mode exchanger arranged in a transmission system using an n (n is a natural number of 3 or more) mode fiber as a transmission path,
a mode conversion circuit including a waveguide through which n modes propagate, the number of times of mode conversion being less than n−1;
By arranging two or more of the mode conversion circuits in the transmission path,
Of the n modes, the mode of the mode lower order (primary to n / 2 order mode when n is an even number, primary to n / 2 + 0.5 when n is an odd number) at least once, The mode order is converted to the higher order mode (when n is an even number, n / 2 order to n order mode, when n is an odd number, n / 2 + 0.5 to n order mode),
Among the n modes, at least one mode having a higher mode order is converted to a mode having a lower mode order at least once.

  The optical characteristics (transmission loss, chromatic dispersion, polarization dispersion, etc.) between modes tend to deteriorate as the mode becomes higher. For this reason, it is not necessary to cyclically exchange all the modes, and a great effect can be obtained only by replacing the low-order mode with the high-order mode and the high-order mode with the low-order mode. This mode exchanger has a configuration in which a high-order mode and a low-order mode are switched, and the optical characteristics between the modes can be made uniform (mode loss difference compensation).

  Furthermore, since all modes are not cyclically exchanged (the number of mode exchanges is n-1 times), the number of mode exchanges can be reduced (the number of mode exchanges is less than n-1 times). Reducing the number of mode exchanges can prevent an increase in loss and relax the limitation of transmission distance.

  Therefore, the present invention provides a mode switch that can compensate for a mode loss difference by a method different from a method that gives a loss only to a specific mode, and can relax the limitation of transmission distance associated with the compensation of the mode loss difference. it can.

For example, when n is 6, the mode conversion circuit is
The heights of the waveguides (thicknesses of the waveguides in the direction perpendicular to the PLC substrate) are all equal,
A first taper portion in which a waveguide width (the thickness of the waveguide in a direction parallel to the PLC substrate) changes in a non-adiabatic manner so that coupling occurs between the E31 mode and the E13 mode propagating in the waveguide;
A first mode exchanging unit for exchanging LP01 mode and LP21b mode propagating in the waveguide;
A second mode exchanging unit for exchanging the LP01 mode and the LP11a mode propagating in the waveguide and simultaneously exchanging the LP11b mode and the LP21a mode;
A second taper portion in which the waveguide width adiabatically changes so that no coupling occurs between the E31 mode and the E13 mode propagating through the waveguide;
It is characterized by providing.

When n is 6, the mode conversion circuit is
The heights of the waveguides (thicknesses of the waveguides in the direction perpendicular to the PLC substrate) are all equal,
A mode rotator for coupling the LP11a mode and the LP11b mode propagating in the waveguide;
A first taper portion in which a waveguide width (the thickness of the waveguide in a direction parallel to the PLC substrate) changes in a non-adiabatic manner so that coupling occurs between the E31 mode and the E13 mode propagating in the waveguide;
A first mode exchange unit for exchanging the LP11a mode and the LP21b mode propagating in the waveguide;
A second mode exchanging unit for exchanging the LP01 mode and the LP11a mode propagating in the waveguide and simultaneously exchanging the LP11b mode and the LP21a mode;
A second taper portion in which the waveguide width adiabatically changes so that no coupling occurs between the E31 mode and the E13 mode propagating through the waveguide;
It is characterized by providing.

  With such a configuration, the high-order mode and the low-order mode can be interchanged with the maximum number of mode exchanges of three times, and the compensation of the mode loss difference and the relaxation of the transmission distance restriction can be realized.

The waveguide widths of the first mode exchange unit and the second mode exchange unit are periodically changed with a period Λ, and the period Λ corresponds to the effective refractive indexes n1 and n2 and the wavelength λ used for the two modes to be exchanged. The
It is preferable to satisfy.

  The present invention can provide a mode exchanger that can compensate for a mode loss difference by a method different from a method that gives a loss only to a specific mode, and can relax the limitation of transmission distance associated with the compensation of the mode loss difference.

It is a figure explaining the structure of the mode exchanger which concerns on this invention. It is a figure explaining the mode exchange part with which the mode exchanger which concerns on this invention is provided. It is a table | surface explaining the ratio of the output mode with respect to the incident mode in the 1st mode exchange part with which the mode exchanger which concerns on this invention is provided. It is a table | surface explaining the ratio of the output mode with respect to the incident mode in the 2nd mode exchange part with which the mode exchanger which concerns on this invention is provided. It is a figure explaining the 1st taper part with which the mode exchanger concerning the present invention is provided. It is a figure explaining the change of the effective refractive index of LP21b and LP02 mode with respect to waveguide width. It is a figure explaining adiabatic change. It is a figure explaining the 2nd taper part with which the mode exchanger concerning the present invention is provided. It is a figure explaining the mode rotor with which the mode exchanger concerning the present invention is provided. It is a figure explaining operation | movement of the mode exchanger which concerns on this invention. It is a figure explaining the example which arrange | positions the mode exchanger which concerns on this invention in a transmission line. It is a figure explaining the effect when the mode exchanger concerning the present invention is arranged in a transmission line. It is a table | surface explaining the response | compatibility of the emission mode with respect to the incident mode of the mode exchanger which concerns on this invention. It is a figure explaining the structure of the mode exchanger which concerns on this invention. It is a figure explaining operation | movement of the mode exchanger which concerns on this invention. It is a figure explaining the effect when the mode exchanger concerning the present invention is arranged in a transmission line. It is a table | surface explaining the response | compatibility of the emission mode with respect to the incident mode of the mode exchanger which concerns on this invention.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

  In order to reduce the optical characteristic difference between the modes, mode exchange in which a specific mode is converted into another mode in the transmission path is effective. However, for example, in one mode switch, when a mode to be propagated is cyclically converted to another mode (an n-order mode is converted to an n + 1-order mode, and a highest-order mode is converted to a lowest-order mode). , N-1 mode converters are required for the n mode, which is not desirable because the mode converter is downsized and excessive loss is increased.

  Since the optical characteristics between modes tend to deteriorate as the higher-order mode, it is not necessary to cyclically exchange all the modes, and the lower-order mode is changed to the higher-order mode. Therefore, it is considered that a large effect can be obtained only by exchanging the higher order mode with the lower order mode.

That is, the mode exchanger of the present embodiment is a PLC-type mode exchanger arranged in a transmission system using n (n is a natural number of 3 or more) mode fiber as a transmission path,
a mode conversion circuit including a waveguide through which n modes propagate, the number of times of mode conversion being less than n−1;
By arranging two or more of the mode conversion circuits in the transmission path,
Of the n modes, the mode of the mode lower order (primary to n / 2 order mode when n is an even number, primary to n / 2 + 0.5 when n is an odd number) at least once, The mode order is converted to the higher order mode (when n is an even number, n / 2 order to n order mode, when n is an odd number, n / 2 + 0.5 to n order mode),
Among the n modes, at least one mode having a higher mode order is converted to a mode having a lower mode order at least once.
The mode exchanger of the present embodiment is characterized in that when the mode propagating through the waveguide is n mode, the number m of times the mode is converted in the waveguide is less than n-1.

(Example 1)
A first embodiment will be described. FIG. 1 is a diagram for explaining the configuration of the mode converter 301 of this embodiment. The mode converter 301 is a 6-mode exchanger and includes a mode conversion circuit 10. The mode conversion circuit 10
The heights of the waveguides (thicknesses of the waveguides in the direction perpendicular to the PLC substrate) are all equal,
A mode rotator 21 for coupling the LP11a mode and the LP11b mode propagating in the waveguide;
A first tapered portion 22 in which the waveguide width (the thickness of the waveguide in a direction parallel to the PLC substrate) changes non-adiabatically so that coupling occurs between the E31 mode and the E13 mode propagating in the waveguide;
A first mode exchanging unit 23 for exchanging the LP11a mode and the LP21b mode propagating in the waveguide;
A second mode exchanging unit 24 for exchanging the LP01 mode and the LP11a mode propagating in the waveguide and simultaneously exchanging the LP11b mode and the LP21a mode;
A second taper portion 25 in which the waveguide width adiabatically changes so that no coupling occurs between the E31 mode and the E13 mode propagating through the waveguide;
Is provided.

  A straight line is provided between the first taper part 22 and the first mode conversion part 23, between the first mode conversion part 23 and the second mode conversion part 24, and between the second mode conversion part 24 and the second taper part 25. They are connected by the waveguide 30.

  FIG. 2 is a diagram illustrating the structure of the first mode exchange unit 23 and the second mode exchange unit 24. The mode converter 301 is provided with a basic waveguide structure having a height H and a width W designed to propagate six modes. The first mode exchanging unit 23 and the second mode exchanging unit 24 have their basic waveguide widths periodically changed with a period Λ and a waveguide width change amount d. A portion where the waveguide width periodically changes is defined as a grating portion, and a grating length is defined as L.

In the first mode exchanging unit 23 and the second mode exchanging unit 24, the effective refractive indexes n1 and n2 of the two modes of interest among the propagating modes are set. Here, the period Λ is
If these conditions are satisfied, the first mode exchange unit 23 and the second mode exchange unit 24 can combine the two modes. If the grating length L is appropriately designed, the mode power can be exchanged between the first mode exchange unit 23 and the second mode exchange unit 24.

  For example, FIG. 3 shows each mode (leftmost) of incident light when W = 11.0 μm, H = 10.0 μm, Λ = 450 μm, d = 0.5 μm, and L = 6750 μm in the structure of FIG. Column) summarizes the ratio of which mode is converted when exiting. The highest conversion ratio for each incident mode is underlined. Incident LP01 mode power is most converted to LP11a mode power (84.4%), and incident LP11b mode power is most converted to LP21a mode power (82.4%). I understand. That is, the second mode exchanging unit 24 can be configured by setting each parameter as described above.

  W and H can be designed because the waveguide has a structure capable of propagating a desired number of modes (6 modes in this embodiment). As shown in Equation 1, Λ can be calculated from the effective refractive index of the mode determined by the waveguide structure and the wavelength used. For d and L, for example, d is determined first and L is set so that the modes are exchanged, or L is determined first and d is set so that the modes are exchanged. It is possible to realize the mode exchange waveguide shown.

  Further, FIG. 4 shows each mode (leftmost) of incident light when W = 11.0 μm, H = 10.0 μm, Λ = 286 μm, d = 0.5 μm, and L = 2300 μm in the structure of FIG. Column) summarizes the ratio of which mode is converted when exiting. The highest conversion ratio for each incident mode is underlined. It can be seen that the incident LP11a mode power is most converted to the LP21b (E31) mode power (70%), and no strong mode conversion occurs in the other modes. That is, the first mode exchanging unit 23 can be configured by setting each parameter as described above.

  FIG. 5 is a diagram illustrating the configuration of the first taper portion 22. FIG. 5 is viewed from the upper surface of the PLC (the surface in the stacking direction with respect to the substrate). In the 6-mode waveguide, as shown in FIG. 6, when the cross-sectional structure is square (W = H), the effective refractive indexes of the LP21b (E31) mode and the LP02 (E13) mode are close to each other, and the waveguide width and As the height difference | W−H | increases, the difference in effective refractive index between the two modes increases. This result is H = 12 μm and the relative refractive index difference Δ = 0.7%.

  At this time, if the waveguide width changes sufficiently slowly with respect to the propagation distance, each mode does not cause mode conversion, and the effective refractive index changes along each solid line (this is called adiabatic change). . On the other hand, when the change rate of the waveguide width is steep above a certain level, coupling occurs between modes according to propagation. In particular, in the structure of W = H, the difference in effective refractive index between the two modes is small, so that mode conversion is more likely to occur.

  FIG. 7 shows a calculation result of the taper length Lt1 and the output power ratio of the LP21b and LP02 modes when the E31 mode is incident on the tapered waveguide. From the figure, in the region where the taper length is 5000 μm or less, dominant mode coupling of 30% or more occurs. In the mode exchanger, it is not always necessary to realize equal distribution in the tapered portion, and even if power distribution is 30%, which is a difference in distribution ratio of 2 dB or less with respect to equal distribution, the characteristic difference between modes is reduced. A sufficient effect is expected. Therefore, in order to cause mode coupling between the modes of LP21b and LP02 as described above, the taper length should be at least 5000 μm or less.

  For example, in the structure of FIG. 5, when W = 10.2 μm, W1 = 11.0 μm, and Lt = 100 μm, the incident LP21b mode is equally distributed to the LP21b and LP02 modes on the output end side. In the LP02 mode, the power is equally distributed to the LP21b and LP02 modes on the emission end side. That is, the first tapered portion 22 can be obtained by setting each parameter as described above.

  FIG. 8 is a diagram illustrating the second tapered portion 25 in which the waveguide width decreases with respect to the propagation direction. For example, in the structure of FIG. 8, when W = 10.2 μm, W1 = 11.0 μm, and Lt2 = 6000 μm, the incident LP21b and LP02 modes propagate to the output side without causing mode coupling with each other. That is, the second tapered portion 25 can be obtained by setting each parameter as described above.

  The tapered portion 2 may have a two-stage tapered structure including two tapered portions having different taper ratios as described in Non-Patent Document 7. In that case, mode conversion of LP21b and LP02 modes in the tapered portion can be reduced by setting the length of each tapered portion to 10000 μm and 4000 μm as described in Non-Patent Document 7.

  Here, the taper length for preventing mode coupling between the modes of LP21b and LP02 as described above is sufficiently tapered so that mode propagation calculation is performed and the mode is not converted (adiabatic change). Should be lengthened.

  As shown in FIG. 6, in the structure of W = H, the electric field distribution of the waveguide mode having a higher effective refractive index is substantially equal to the LP21b mode of the fiber, and the refractive index of the LP02 mode is lower. Since the electric field distribution in the waveguide mode becomes substantially equal, the waveguide structure at the end face of the connection portion with the fiber is preferably W = H.

  FIG. 9 is a diagram for explaining the structure of the mode rotor 21. FIG. 9A is a perspective view, and FIG. 9B is a cross-sectional view. The mode rotor 21 has a structure in which a part of the waveguide is missing. By appropriately designing the waveguide structure for each parameter, the incident LP11a mode can be converted into the LP11b mode, and the LP11b mode can be converted into the LP11a mode (see, for example, Non-Patent Document 8).

  For example, in the structure of FIG. 9, when H = 10.0 μm, W = 10.2 μm, wt = 1.5 μm, ht = 2.6 μm, L = 1.0475 mm, the incident LP11a modes are LP11a and LP11b. The power is equally branched to the mode, and the incident LP11b mode is equally branched to the LP11a and LP11b modes. That is, the mode rotor 21 can be obtained by setting each parameter as described above.

  FIG. 10 is a diagram illustrating a mode conversion flow for the incident mode of the mode exchanger 301. The mode exchanger 301 is connected in the order of the mode rotor 21, the first taper portion 22, the first mode conversion portion 23, the second mode conversion portion 24, and the second taper portion 25 from the incident side. The mode rotor 21 causes mode coupling between the modes of LP11a and LP11b, and the first taper portion 22 whose waveguide width is increased causes mode coupling between the modes of LP21b and LP021, and the first mode switching unit. 23 exchanges LP11a and LP21b, the second mode exchange unit 23 exchanges LP01 and LP11a, and at the same time exchanges LP11b and LP21a. To do.

  It will be described that the mode exchanger 301 reduces the difference in optical characteristics between modes. Consider inserting 0 to 4 mode exchangers 301 in the 150 km transmission section as shown in FIG. (B) arranges a mode exchanger at the midpoint of the transmission line, (c) arranges a mode exchanger every 50 km, (d) arranges a mode exchanger every 37.5 km, (e) every 30 km A mode exchanger is arranged in For each case, post-transmission mode loss difference (maximum loss difference) and mode dependent loss (MDL) were calculated. The loss in the fiber in the transmission section was assumed to be 0.15 dB / km, 0.18 dB / km, 0.20 dB / km, and 0.23 dB / km in the LP01, LP11a / b, LP21a / b, and LP02 modes, respectively.

  The result is shown in FIG. It can be seen that by increasing the number of inserted mode exchangers 301, the inter-mode loss difference and MDL can be sufficiently reduced. In particular, the maximum loss difference can be reduced to less than half by simply inserting two exchangers.

  FIG. 13 is a table for explaining the correspondence between the incident mode and the emission mode of the mode exchanger 301. The rightmost column indicates the emission mode when two mode exchangers 301 are passed. It can be seen that all lower-order modes (LP01, LP11a, LP11b) are converted to higher-order modes (LP21a, LP21b, LP02) at least once. As for the higher-order mode, LP21a is not converted to the lower-order mode, but LP21b and LP02 are converted to the lower-order mode.

  That is, by inserting two mode converters 301 and switching the high-order mode and the low-order mode, the maximum loss difference could be reduced to half or less. Also, considering the limitation of transmission distance, it is better that the number of mode exchangers to be inserted is smaller. Therefore, in the transmission section as shown in FIG. Both relaxation of distance restrictions can be achieved.

(Example 2)
A first embodiment will be described. FIG. 14 is a diagram for explaining the configuration of the mode exchanger 302 of this embodiment. The mode converter 302 is a 6-mode exchanger and includes a mode conversion circuit 11. The mode conversion circuit is 11,
The heights of the waveguides (thicknesses of the waveguides in the direction perpendicular to the PLC substrate) are all equal,
A first tapered portion 22 in which the waveguide width (the thickness of the waveguide in a direction parallel to the PLC substrate) changes non-adiabatically so that coupling occurs between the E31 mode and the E13 mode propagating in the waveguide;
A first mode exchanging unit 23 for exchanging the LP01 mode and the LP21b mode propagating in the waveguide;
A second mode exchanging unit 24 for exchanging the LP01 mode and the LP11a mode propagating in the waveguide and simultaneously exchanging the LP11b mode and the LP21a mode;
A second taper portion 25 in which the waveguide width adiabatically changes so that no coupling occurs between the E31 mode and the E13 mode propagating through the waveguide;
It is characterized by providing.

  A straight line is provided between the first taper part 22 and the first mode conversion part 23, between the first mode conversion part 23 and the second mode conversion part 24, and between the second mode conversion part 24 and the second taper part 25. They are connected by the waveguide 30.

  The mode converter 302 is a mode converter that does not include the mode rotor 21 of the mode converter 301 of FIG. The structures of the first taper part 22, the first mode exchange part 23, the second mode exchange part 24, and the second taper part 25 are the same as those described in the first embodiment. Compared with the configuration of the first embodiment, this structure does not require a mode exchanger and has an advantage that a mode exchanger can be realized with a simpler configuration.

  FIG. 15 is a diagram illustrating a mode conversion flow for the incident mode of the mode exchanger 302. The mode exchanger 302 is connected from the incident side in the order of the first taper part 22, the first mode exchange part 23, the second mode exchange part 24, and the second taper part 25. The first taper part 22 having a large waveguide width causes mode coupling between the modes of LP21b and LP021, the first mode exchange part 23 exchanges LP01 and LP21b, and the second mode exchange part 23 becomes LP01. The LP 11a is exchanged, and the LP 11b and the LP 21a are exchanged at the same time. The second taper portion 25 having a small waveguide width exchanges the LP 21b and LP02.

  In order to explain that the mode exchanger 302 reduces the difference in optical characteristics between modes, consider inserting 0 to 4 mode exchangers in the 150 km transmission section as shown in FIG. For each case, post-transmission mode loss difference and mode dependent loss (MDL) were calculated. The fiber loss in the transmission line was assumed to be the same as in the first embodiment.

  The result is shown in FIG. It can be seen that by increasing the number of inserted mode exchangers 302, the inter-mode loss difference and MDL can be sufficiently reduced. In particular, the maximum loss difference can be reduced to less than half by simply inserting two exchangers.

  FIG. 17 is a table for explaining the correspondence between the incident mode and the emission mode of the mode exchanger 302. The rightmost column shows the emission mode when two mode exchangers 302 are passed. It can be seen that all the low-order modes (LP01, LP11a, LP11b) are converted to the high-order modes (LP21a, LP21b, LP02) at least once by passing through two switches. As for the higher-order mode, some of LP21b and LP02 are not converted to the lower-order mode, but others are converted to the lower-order mode.

  That is, by inserting two mode converters 302 and switching the higher order mode and the lower order mode, the maximum loss difference could be reduced to half or less. Further, considering the limitation of the transmission distance, it is better that the number of mode exchangers to be inserted is smaller. Therefore, in the transmission section as shown in FIG. Both relaxation of distance restrictions can be achieved.

(Other examples)
In the present embodiment, an example in which the number of modes is 6 is shown. However, in general, when the mode propagating through the waveguide is the n mode, the high-order mode and the low-order mode are interchanged in the waveguide. The effect of reducing the MDL by reducing the number m of times the mode is converted to n-1 is also expected.

In the present specification, an example relating to a planar lightwave circuit using a SiO ₂ -based material (quartz-based material) has been described, but the material may naturally be other. For example, even in the case of a planar lightwave circuit using a semiconductor such as Si-based or InGaAsP, or an organic material such as a polymer, the same effects as those of the embodiments described in this specification can be obtained.

  Also, the wavelength band to be used is about 1.5 to 1.6 μm in the examples described in the present specification, but it may be a mid-infrared region (2 μm or more) having a longer wavelength or a visible light band. Absent.

  The mode exchanger according to the present invention can be applied to large capacity and long distance communication of a mode multiplexing transmission system using a multimode optical fiber.

DESCRIPTION OF SYMBOLS 10, 11: Mode conversion circuit 21: Mode rotor 22: 1st taper part 23: 1st mode conversion part 24: 2nd mode conversion part 25: 2nd taper part 30: Linear waveguide 301, 302: Mode exchanger

Claims (4)

A PLC type mode exchanger arranged in a transmission system using an n (n is a natural number of 3 or more) mode fiber as a transmission path,
a mode conversion circuit including a waveguide through which n modes propagate, the number of times of mode conversion being less than n−1;
By arranging two or more of the mode conversion circuits in the transmission path,
Of the n modes, the mode of the mode lower order (primary to n / 2 order mode when n is an even number, primary to n / 2 + 0.5 when n is an odd number) at least once, The mode order is converted to the higher order mode (when n is an even number, n / 2 order to n order mode, when n is an odd number, n / 2 + 0.5 to n order mode),
A mode exchanger characterized in that at least one mode having a higher mode order among n modes is converted to a mode having a lower mode order at least once. When n is 6, the mode conversion circuit is
The height of the waveguide (the thickness of the waveguide in the direction perpendicular to the substrate of the PLC) is all equal,
A first taper portion in which a waveguide width (the thickness of the waveguide in a direction parallel to the PLC substrate) changes in a non-adiabatic manner so that coupling occurs between the E31 mode and the E13 mode propagating in the waveguide;
A first mode exchanging unit for exchanging LP01 mode and LP21b mode propagating in the waveguide;
A second mode exchanging unit for exchanging the LP01 mode and the LP11a mode propagating in the waveguide and simultaneously exchanging the LP11b mode and the LP21a mode;
A second taper portion in which the waveguide width adiabatically changes so that no coupling occurs between the E31 mode and the E13 mode propagating through the waveguide;
The mode exchanger according to claim 1, further comprising: When n is 6, the mode conversion circuit is
The height of the waveguide (the thickness of the waveguide in the direction perpendicular to the substrate of the PLC) is all equal,
A mode rotator for coupling the LP11a mode and the LP11b mode propagating in the waveguide;
A first taper portion in which a waveguide width (the thickness of the waveguide in a direction parallel to the PLC substrate) changes in a non-adiabatic manner so that coupling occurs between the E31 mode and the E13 mode propagating in the waveguide;
A first mode exchange unit for exchanging the LP11a mode and the LP21b mode propagating in the waveguide;
A second mode exchanging unit for exchanging the LP01 mode and the LP11a mode propagating in the waveguide and simultaneously exchanging the LP11b mode and the LP21a mode;
A second taper portion in which the waveguide width adiabatically changes so that no coupling occurs between the E31 mode and the E13 mode propagating through the waveguide;
The mode exchanger according to claim 1, further comprising: The waveguide widths of the first mode exchange unit and the second mode exchange unit are periodically changed with a period Λ, and the period Λ corresponds to the effective refractive indexes n1 and n2 and the wavelength λ used for the two modes to be exchanged. The
The mode exchanger according to any one of claims 2 and 3, wherein: