JP6529925B2 - Mode multiplexer / demultiplexer, optical transmission system and relay transmission system - Google Patents

Description

  The present disclosure relates to a mode multiplexer / demultiplexer in a mode multiplexing optical transmission system using a several mode optical fiber in which a plurality of propagation modes propagate as a transmission line and the optical transmission system.

  Several mode optical fibers using a plurality of propagation modes have been proposed as a technique for increasing transmission capacity. In particular, mode multiplex transmission using a plurality of propagation modes has attracted attention as a new large-capacity transmission method because the transmission capacity can be improved by a factor of the number of modes.

  In the transmission using this few mode optical fiber, since inter-mode crosstalk occurs in the transmission path, a Multiple-Input Multiple-Output (MIMO) equalizer is used at the receiving end as a compensation means. However, if there is a mode dependent loss (hereinafter referred to as "MDL"), performance degradation of the transmission system becomes a problem even if the MIMO equalizer is used. (See, for example, Non-Patent Document 1.) Also, if the differential mode delay (hereinafter referred to as DMD) at the receiving end is large, the load on digital processing (DSP) related to MIMO increases and the length becomes long. In order to realize distance transmission, it is an issue to reduce DSP load. (For example, refer to Non-Patent Document 2.) In order to alleviate the influence of MDL and DMD, therefore, it has been proposed to use a mode scrambler that causes coupling between modes (for example, see Non-Patent Document 3) ). Also, ring core optical fibers have been proposed to positively cause coupling between modes in an optical fiber transmission line (see, for example, Non-Patent Documents 4 and 5).

  The “single mode optical fiber” and the “multimode (several mode) optical fiber” described in the present specification respectively propagate in an optical fiber propagating in a single mode in the wavelength of an optical signal to be transmitted and in a multimode Means an optical fiber.

P. J. Winzer, et al. , “Mode-dependent loss, gain, and noise in MIMO-SDM systems”, in Proc. ECOC 2014, paper Mo. 3.3.2, 2014. S. O. Arik, D. Askarov, J., et al. M. Kahn, Effect of mode coupling on signal processing complexity in mode-division multiplexing, J.P. Lightwave Technol. 31 (3) (2013) 423-431. Lobato, A. Ferreira, F .; Rabe, J .; Kuschnerov, M .; Spinnler, B .; Lankl, B .; “Mode scrambles and reduced-search maximum-likelihood detection for mode-dependent-loss-impaired transmission”, in Optical Communication (ECOC 2013), 39th European Conference and Exhibition, pp. 1-3, 22-26 Sept. 2013 N. Fontaine, R. Ryf, M. Hirano, and T. Sasaki, "Experimental investigation of crosstalk accumulation in a ring-core fiber", in Proc. IEEE Photon. Soc. Summer Top. Meeting Series, 2013, pp. 111-112. F. Feng, G. S. Gordon, X. Q. Jin, D .; C. O'Brien, F .; P. Payne, Y. Jung, Q. Kang, J. K. Sahu, S. U. Alam, D. J. Richardson, and T. D. Wilkinson, "Experimental Characterization of a Graded-Index Ring-Core Fiber Supporting 7 LP Mode Groups", at Optical Fiber Communication Conference, OSA Technical Digest (online) (Optical Society of America, 2015), paper Tu2D. 3. T. Birks, I. Gris-Sanchez, S. Yerolatsitis, S. Leon-Saval, and R.S. Thomson, “The photonic lantern”, Adv. Opt. Photon. 7, 107-167 (2015). Tsutomu Saito, Kengo Watanabe, Katsunori Imamura, Masato Shiino, Fiber Bundle Type Fan-out for Multi-Core Fiber, The IEICE General Conference, B-10-26, 2012 Watanabe Kengo, Saito Kosuke, MCF fiber bundle type fanout loss improvement, The Institute of Electronics, Information and Communication Engineers General Conference, B-10-6, 2015

  However, in Non-Patent Document 4 and Non-Patent Document 5, as a mode multiplexer / demultiplexer for a ring core optical fiber, a phase plate or spatial phase modulator which is a spatial type is used, and loss or It was a challenge to reduce the inter-channel difference in delay. Therefore, in order to solve the above-mentioned problems, the present invention is provided with a mode multiplexer / demultiplexer capable of suppressing the difference between loss and delay between channels even if the number of propagation modes is increased in a transmission line using a ring core optical fiber An object of the present invention is to provide an optical transmission system and a relay transmission system.

  In order to achieve the above object, in the mode multiplexer / demultiplexer according to the present invention, the core of the plurality of optical fibers or the plurality of cores of the planar lightwave circuit is the ring core with respect to the cross section of the ring core optical fiber through which the plurality of modes propagate. We decided to use a low-loss mode multiplexer / demultiplexer placed in contact with the ring core of the optical fiber.

Specifically, the mode multiplexer / demultiplexer according to the present invention is a mode multiplexer / demultiplexer including a plurality of first ports connected to a single mode optical fiber and one second port connected to a ring core optical fiber. There,
It has the same number of cores as the first port connecting the first port and the second port, and
The core is
It is characterized in that it is disposed in a tapered shape in which the core interval is shortened from the first port to the second port, and is disposed in contact with the ring core of the ring core optical fiber at the surface of the second port.

  The core of the mode multiplexer / demultiplexer converts the fundamental mode coupled to the first port into an arbitrary mode (fundamental mode and higher-order mode) and couples it to a ring core optical fiber connected to the second port, Any mode (basic mode and higher order mode) coupled from the optical fiber to the second port is converted to the basic mode and output to the first port of each. Therefore, the mode multiplexer / demultiplexer also has a mode scrambler function. That is, when the transmission line of the ring core optical fiber and the mode multiplexer / demultiplexer have the mode scrambler function, the influence of the propagation mode loss difference and the propagation mode delay difference in mode multiplexing transmission can be mitigated, and the transmission capacity and transmission It is possible to increase the distance.

  Therefore, the present invention can provide a mode multiplexer / demultiplexer that can suppress the difference between loss and delay between channels even if the number of propagation modes increases in a transmission line using a ring core optical fiber.

  The core of the mode multiplexer / demultiplexer according to the present invention is characterized in that the core is disposed in an annular shape on the surface of the second port. A ring core optical fiber has a toroidal electric field distribution and a small electric field spread at the center. Therefore, the core of the mode multiplexer / demultiplexer does not need to be disposed at the center of the ring core optical fiber, and may be disposed in an annular shape on the ring core.

  The cores of the mode multiplexer / demultiplexer according to the present invention are arranged in the plane of the second port so that the center of gravity of the regular N-shape formed at each core center coincides with the center of the ring core optical fiber It is characterized by In order to make the electric field distribution of each core uniform, the cores are arranged so that the center of gravity of a regular N-shape with the center of each core at the top coincides with the center of the ring core optical fiber.

The core of the mode multiplexer / demultiplexer according to the present invention is:
The diameter is equal to or greater than the core width of the ring core optical fiber,
Preferably, the core center is disposed at the center of the core width of the ring core optical fiber in the surface of the second port. In addition, the loss can be reduced.

  The core may be a core of a small diameter optical fiber that connects the first port and the second port.

  The core may be an optical waveguide of a three-dimensional planar lightwave circuit (PLC).

The optical transmission system according to the present invention is
N (N is an integer of 2 or more) optical transmitters that transmit signal light in N single-mode optical fibers,
1 ring core optical fiber that propagates N modes in the ring core,
The mode multiplexer / demultiplexer connected to the single mode optical fiber at the first port, connected to the ring core optical fiber at the second port, and N signal light coupled to the first port A mode multiplexer coupled to the ring core optical fiber as N modes;
The mode multiplexer / demultiplexer is connected to the ring core optical fiber at the second port, and M (M is an integer greater than or equal to N) signal lights propagated in N modes in the ring core optical fiber A mode splitter that splits into the first port,
M optical receivers that receive the respective signal lights demultiplexed by the mode demultiplexer;
Equipped with

The optical transmission system according to the present invention is
One relay unit is
1 ring core optical fiber that propagates N modes in the ring core,
The mode multiplexer / demultiplexer is connected to a single mode optical fiber at the first port, connected to the ring core optical fiber at the second port, and N signal lights coupled to the N first ports are N A mode multiplexer coupled to the ring core optical fiber as a plurality of modes;
The mode multiplexer / demultiplexer, wherein the second port is connected to the ring core optical fiber, and the signal light propagated in N modes in the ring core optical fiber is split into N first ports With a duplexer,
N optical amplifiers for amplifying the N signal lights demultiplexed by the mode demultiplexer and outputting the amplified signal light to a single mode optical fiber;
And two or more of the relay units are connected.

The optical transmission system according to the present invention is
One relay unit is
1 ring core optical fiber that propagates N modes in the ring core,
An optical amplifier disposed in the ring core optical fiber for amplifying signal light propagating through the ring core optical fiber;
The mode multiplexer / demultiplexer is connected to a single mode optical fiber at the first port, connected to the ring core optical fiber at the second port, and N signal lights coupled to the N first ports are N A mode multiplexer coupled to the ring core optical fiber as a plurality of modes;
The mode multiplexer / demultiplexer, wherein the second port is connected to the ring core optical fiber, and the signal light propagated in N modes in the ring core optical fiber is split into N first ports With a duplexer,
And two or more of the relay units are connected.

  In order to achieve the above object, in the optical transmission system and the relay transmission system according to the present invention, the transmission path is a ring core optical fiber in which a plurality of modes propagate, and the mode multiplexer / demultiplexer comprises a ring core optical fiber, an amplifier and a transmitter. In order to connect with the receiver, or when there is a difference in propagation mode loss in the transmission path, each channel receives the same loss, and in the case where there is a difference in propagation mode delay, each channel receives the same delay . Therefore, the loss and delay of each channel become equal, and the load on the DSP can be reduced.

  Therefore, the present invention can provide an optical transmission system and a relay transmission system capable of suppressing the difference between loss and delay among channels even if the number of propagation modes increases in a transmission line using a ring core optical fiber.

  The present invention uses a mode multiplexer / demultiplexer having a plurality of cores each of which can transmit and receive various modes simultaneously when the ring core optical fiber is used as a transmission channel, whereby the modes in the transmission channel and the mode multiplexer / demultiplexer It is possible to provide a transmission system capable of exhibiting the scramble function, alleviating the effects of loss difference and delay difference of propagation modes in multimode optical fiber transmission, and expanding transmission capacity and transmission distance.

  That is, the present invention is a mode multiplexer / demultiplexer capable of suppressing the difference between loss and delay between channels even if the number of propagation modes increases in a transmission line using a ring core optical fiber, an optical transmission system including the same, and a relay transmission system. Can be provided.

It is a figure explaining an example of the light transmission system concerning the present invention. It is a figure explaining an example of the relay transmission system which concerns on this invention. It is a figure explaining an example of the relay transmission system which concerns on this invention. It is a figure explaining the refractive index distribution of a ring core optical fiber. Delta ₁ is 0.25 [%], describes a number of propagation modes and the effective refractive index difference Δneff in the case where the central layer radius _{a 1} of _the ring core optical fiber is varied in the ring core width a2-a1 is 1.8 [[mu] m] It is a figure to do. It is a figure explaining the core arrangement | positioning with respect to the ring core optical fiber in the 2nd port of the mode multiplexer / demultiplexer based on this invention, and the number of propagation modes. It is a figure explaining the calculation result of electric field distribution and coupling efficiency of 3 mode single core multiplexer / demultiplexer and 3 mode ring core optical fiber. It is a figure explaining the calculation result of electric field distribution and coupling efficiency of a mode multiplexer / demultiplexer (3 cores) and 3 mode ring core optical fiber which concern on this invention. It is a figure explaining the calculation result of electric field distribution and coupling efficiency of a mode multiplexer / demultiplexer (5 cores) and 5 mode ring core optical fiber which concern on this invention. It is a figure explaining the arrangement relationship of the ring core optical fiber and the core in the 2nd port of the mode multiplexer / demultiplexer (3 cores) which concerns on this invention. In the mode multiplexer / demultiplexer (three cores) concerning the present invention, it is a figure showing the calculation result of the relation between the distance from the gravity center of core arrangement, and loss. In the mode multiplexer / demultiplexer (three cores) concerning the present invention, it is a figure showing the calculation result of the relation between core radius and loss. It is a figure explaining the arrangement relationship of the ring core optical fiber and the core in the 2nd port of the mode multiplexer / demultiplexer (3 cores) which concerns on this invention. It is a figure explaining the case where the mode multiplexer / demultiplexer which concerns on this invention is the bundle structure which bundled the small diameter fiber. It is a figure explaining the case where the mode multiplexer / demultiplexer which concerns on this invention is the bundle structure which bundled the small diameter fiber. It is a figure explaining the case where the mode multiplexer / demultiplexer which concerns on this invention is the bundle structure which bundled the small diameter fiber. It is a figure explaining the case where the mode multiplexer / demultiplexer which concerns on this invention is the bundle structure which bundled the small diameter fiber. When the mode multiplexer / demultiplexer according to the present invention has a bundle structure in which small diameter fibers are bundled, it is a diagram for explaining a connection state with a ring core optical fiber. It is a figure explaining the case where the mode multiplexer / demultiplexer based on this invention is a three-dimensional waveguide structure. FIG. 7 is a diagram for explaining a connection state with a ring core optical fiber in the case where the mode multiplexer / demultiplexer according to the present invention has a three-dimensional waveguide structure. It is a figure explaining the mode multiplexer-demultiplexer based on this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present invention is not limited to the embodiments described below. These implementation examples are merely illustrative, and the present invention can be implemented in various modifications and improvements based on the knowledge of those skilled in the art.

(Embodiment 1)
FIG. 21 is a diagram for explaining the mode multiplexer / demultiplexer 101 of the present embodiment. The mode multiplexer / demultiplexer 101 is a mode multiplexer / demultiplexer including a plurality of first ports 11 connected to a single mode optical fiber 50 and one second port 12 connected to a ring core optical fiber 55,
The core 13 has the same number of cores as the first port connecting the first port 11 and the second port 12;
The core 13 is
It is characterized in that it is disposed in a tapered shape in which the core interval is shortened from the first port 11 to the second port 12 and is in contact with the ring core of the ring core optical fiber 55 on the surface of the second port 12.

  The core 13 is characterized in that it is annularly arranged on the surface of the second port 12. The cores 13 are characterized in that, in the plane of the second port 12, the center of gravity of the regular N-shape formed at each core center coincides with the center of the ring core optical fiber 55.

  FIG. 6 is a view for explaining an arrangement example of cores in the second port 12 of the mode multiplexer / demultiplexer 101. As shown in FIG. The ring core optical fiber 55 has a toroidal electric field distribution and a small electric field spread at the center. Therefore, the core 13 need not be disposed at the center of the ring core optical fiber 55, and may be disposed on the ring core 13 in an annular shape. Further, in order to make the electric field distribution of each core 13 uniform, the core 13 is arranged such that the center of gravity of a regular N-gon having the center of each core 13 at the top coincides with the center of the ring core optical fiber 55 (the center of the ring core 53). Place.

  FIG. 6A is a view for explaining the arrangement of the cores 13 when the propagation mode of the ring core optical fiber 55 is 3 (LP01, LP11a, LP11b mode). In this case, the mode multiplexer / demultiplexer 101 needs at least three cores 13. FIG. 6B is a view for explaining the arrangement of the core 13 when the propagation mode of the ring core optical fiber is 5 (LP01, LP11a, LP11b, LP21a, LP21b mode). In this case, the mode multiplexer / demultiplexer 101 needs at least five cores 13. FIG. 6C is a view for explaining the arrangement of the cores 13 when the propagation mode of the ring core optical fiber is 7 (LP01, LP11a, LP11b, LP21a, LP21b, LP31a, LP31b mode). In this case, the mode multiplexer / demultiplexer 101 needs at least seven cores 13. Each core 13 is disposed in contact with the ring core 53 of the ring core optical fiber 55.

  In the mode multiplexer / demultiplexer 101, a plurality of cores 13 are arranged in a tapered shape, and adiabatic conversion occurs because the structure changes gradually between the first port 11 and the second port 12 (for example, non-patent document) 6). For this reason, the mode multiplexer / demultiplexer 101 forms N modes in each core 13 at the second port 12 from the basic mode coupled to the first port 11.

  FIG. 7 shows the electric field distribution of the core in the three mode single core multiplexer / demultiplexer (A), the electric field distribution of the three mode ring core optical fiber (B), the three mode single core multiplexer / demultiplexer and the three mode ring core optical fiber It is a figure explaining the calculation result (C) of the coupling efficiency with. FIG. 7 is a comparative example. As shown in FIG. 7B, the electric field distribution of the ring core optical fiber is annular in any mode.

The refractive index distribution of the core portion of the single core multiplexer / demultiplexer is a step index shape. In this example, the core radius is 7 μm, and the relative refractive index difference Δ is 0.4%. Refractive index profile of the ring core optical fiber is as in Figure 4, so that the effective refractive index difference [Delta] n _eff between propagation modes is 0.0005 or less, the center layer radius _{a 1} is 7 [[mu] m], the outer peripheral radius _{a 2} Is 11 μm, and the core portion refractive index difference Δ is 1.15%. In the above structure, the number of propagation modes expected 5 mode, without affecting the effective refractive index difference [Delta] n _eff by providing a suitable trench structure on the _outside, it is possible to control the number of propagation modes.

The mode coupling efficiency η was calculated by the overlap integration of the electric field of each mode. At this time, the wavelength was 1550 nm.
Here, E _i is the electric field of the input mode (single core), and E _o is the electric field of the output mode (ring core).

When a single core and a ring core are connected, the coupling efficiency is about 0.1 in the LP01 mode, and about 0.3 in the LP11 mode. The loss can be calculated from the sum of the coupling efficiency for all modes (LP01, LP11a and LP11b modes) of the ring core in each mode of a single core.
For example, since the coupling efficiency from a single core LP01 mode to a ring core is 0.12 + 1.2e-12 + 5.1e-13 ≒ 0.12, it becomes 10 × log 10 (0.12) = − 9.2 dB, which is a loss. Is approximately 9 dB.
Similarly, for single-core LP11a and LP11b modes, the sum of the coupling efficiencies for all modes to the ring core is approximately 0.30, so 10 × log 10 (0.30) = − 5.2 dB is obtained.
Therefore, it is understood that the connection loss between the three-mode single-core multiplexer / demultiplexer and the three-mode ring core optical fiber is approximately 5 to 10 dB.

  FIG. 8 shows the electric field distribution (A) of the core of the mode multiplexer / demultiplexer 101 (three cores), the electric field distribution (B) of the three mode ring core optical fiber, and the coupling between the mode multiplexer / demultiplexer 101 and the three mode ring core optical fiber It is a figure explaining the calculation result (C) of efficiency. As shown in FIG. 8B, the electric field distribution of the ring core optical fiber is annular in any mode.

  In this example, the core of the mode multiplexer / demultiplexer 101 has a core radius of 3.0 [μm], a relative refractive index difference Δ of 0.35 [%], and a distance of 8. It was 5 [μm]. The refractive index distribution of the ring core optical fiber is as shown in FIG.

  Calculation as described in FIG. 7 shows that when the three cores of the mode multiplexer / demultiplexer 101 are connected to the ring core, the coupling efficiency is about 0.66 between LP01 modes and about 0.57 between LP11 modes. Similar to the description of FIG. 7, the loss is calculated as the sum of the coupling efficiency to all the ring core modes (LP01, LP11a and LP11b modes) for each mode of three cores, and the loss is about 2 to 3 dB, which is a single core It improves about 3 to 6 dB as compared with the connection with.

  FIG. 9 shows the electric field distribution (A) of the core of the mode multiplexer / demultiplexer 101 (five cores), the electric field distribution (B) of the five mode ring core optical fiber, and the coupling between the mode multiplexer / demultiplexer 101 and the five mode ring core optical fiber It is a figure explaining the calculation result (C) of efficiency. As shown in FIG. 9B, the electric field distribution of the ring core optical fiber is annular in any mode.

  In this example, the core of the mode multiplexer / demultiplexer 101 has a core radius of 4.0 [μm], a relative refractive index difference Δ of 0.35 [%], and a distance of 8. It was 5 [μm]. The refractive index distribution of the ring core optical fiber is as shown in FIG.

  When calculation is made as described in FIG. 7, when connecting the five cores of the mode multiplexer / demultiplexer 101 and the ring core, approximately 0.80 between LP01 modes, approximately 0.78 between LP11 modes, 0. 0 between LP21 modes. The coupling efficiency is about 64. Similar to the explanation of FIG. 7, the loss can be calculated as a sum of coupling efficiency of about 1 to 2 dB for each of the five core modes from the sum of coupling efficiency to all modes (LP01, LP11a, LP11b, LP21a, LP21b modes) of the ring core Become. If FIG. 8 and FIG. 9 are compared, loss improvement can be anticipated, so that the number of modes increases. This is because the core of the mode multiplexer / demultiplexer 101 is disposed on the ring core in the plane of the second port, and the number of cores increases, so that the overlapping area of the electric field increases.

  Next, the arrangement of cores in the plane of the second port of the mode multiplexer / demultiplexer 101 will be described. FIG. 10 is a view for explaining the arrangement relationship between the ring core of the ring core optical fiber and the cores of the mode multiplexer / demultiplexer 101 at the second port of the mode multiplexer / demultiplexer 101 (three cores). The distance d from the center of gravity of the triangle connecting the three core centers and the core radius a are important parameters.

  The calculation result of the loss (connection loss between the mode multiplexer / demultiplexer 101 and the ring core optical fiber 55) generated in the mode multiplexer / demultiplexer 101 when the distance d is changed as shown in FIG. 13 is shown in FIG. The core radius a is 3.0 μm. For example, when the loss is to be 3 dB or less, the distance d from the center of gravity needs to be in the range of 6.5 to 10 [μm].

  FIG. 12 shows the loss when the core radius a of the mode multiplexer / demultiplexer 101 whose distance d from the center of gravity is 8.5 [μm] (the connection between the mode multiplexer / demultiplexer 101 and the ring core optical fiber 55 Calculation results of loss). For example, when it is desired to reduce the loss to 3 dB or less, the core radius a needs to be in the range of 2.0 to 7.5 μm.

From the calculation results in FIG. 11 and FIG. 12, the core diameter of mode multiplexer / demultiplexer 101 (3 cores) is equal to or larger than the ring core width, and a loss of 3 dB or less can be realized if the core center is from the inner diameter to the outer peripheral radius . Not only in the 3-core mode multiplexer / demultiplexer, but also in the N-core mode multiplexer / demultiplexer, the core diameter is equal to or greater than the ring core width, and the core center is arranged from the inner diameter to the outer peripheral radius of the ring core. A coupler can be realized.
Preferably, the core diameter of the mode multiplexer / demultiplexer is equal to or greater than the core width of the ring core optical fiber, and the core center is disposed at the center of the core width of the ring core optical fiber in the surface of the second port.

As described above, the mode multiplexer / demultiplexer of this embodiment
(1) By arranging the cores so that the core interval is narrowed from the first port to the second port, the first port to the second port form a plurality of modes from a single mode, and the second port to the first port Form a single mode from multiple modes,
(2) The connection loss between the core and the ring core can be reduced by arranging a plurality of cores in an annular shape at the second port and contacting the ring core of the ring core optical fiber of the transmission path.

Second Embodiment
FIG. 14 is a diagram for explaining the core 13 of the mode multiplexer / demultiplexer 101 described with reference to FIG. The core 13 of the mode multiplexer / demultiplexer 101 of the present embodiment is a core of the small diameter optical fiber 21 that connects the first port 11 and the second port 12. That is, the mode multiplexer / demultiplexer 101 of this embodiment is a bundle type structure in which the small diameter optical fibers 21 are bundled by the coating 23. The position of the core 13 may be fixed by placing the glass rod 22 at an appropriate position between the small diameter optical fibers 21 (for example, Non-Patent Document 7). FIGS. 15 to 17 are diagrams for explaining an arrangement example of the small diameter optical fiber 21 of the mode multiplexer / demultiplexer 101. FIG. FIGS. 15 and 16 show an example of three cores, and FIG. 17 shows an example of five cores.

  FIG. 18 shows a specific example of a mode multiplexer / demultiplexer composed of small diameter optical fibers. In the mode multiplexer / demultiplexer 101 of FIG. 18, a single mode optical fiber 50 is connected to the first port 11, and a ring core optical fiber 55 is connected to the second port 12. The small diameter optical fiber 21 bundled in the mode multiplexer / demultiplexer 101 is tapered, and a plurality of modes are formed in the mode multiplexer / demultiplexer.

  When the mode multiplexer / demultiplexer 101 is used as a multiplexer, the signals of a plurality of single mode optical fibers 50 are sent to the ring core optical fiber 55, and when it is used as a demultiplexer, the signals of the ring core optical fiber 55 are made plural. It is possible to send to a single mode optical fiber 50.

  For example, when the mode multiplexer / demultiplexer 101 has six cores, the cladding diameter of the small diameter optical fiber 21 is about 45 μm, and it is possible to reduce excess loss by using a trench structure (for example, non-patent literature 8). Although the non-patent document 8 has seven cores, the mode multiplexer / demultiplexer 101 does not require a small diameter optical fiber at the center.

(Embodiment 3)
FIG. 19 is a view for explaining the core 13 of the mode multiplexer / demultiplexer 101 described with reference to FIG. The core 13 of the mode multiplexer / demultiplexer 101 of the present embodiment is an optical waveguide of a three-dimensional planar lightwave circuit (PLC). The core 13 of the present embodiment may be rectangular in cross section, and may be square or rectangular. The laminated structure of the PLC is characterized in that the core position can be relatively easily determined.

  FIG. 20 shows a specific example of a mode multiplexer / demultiplexer in which the core 13 is formed of PLC. The core 13 of the PLC has a single mode and is tapered such that the core spacing narrows from the first port 11 to the second port 12, and a plurality of modes for light propagating from the first port 11 to the second port 12 Is formed. In the mode multiplexer / demultiplexer 101 of FIG. 20, a single mode optical fiber 50 is connected to the first port 11, and a ring core optical fiber 55 is connected to the second port 12.

  When the mode multiplexer / demultiplexer 101 is used as a multiplexer, the signals of a plurality of single mode optical fibers 50 are sent to the ring core optical fiber 55, and when it is used as a demultiplexer, the signals of the ring core optical fiber 55 are made plural. It is possible to send to a single mode optical fiber 50.

(Embodiment 4)
FIG. 1 is a diagram for explaining an optical transmission system 301 according to the present embodiment. The optical transmission system 301 is
N (N is an integer of 2 or more) optical transmitters 31 for transmitting signal light through N single mode optical fibers 50;
One ring core optical fiber 55 that propagates N modes in the ring core,
The mode multiplexer / demultiplexer 101 described in FIG. 21 is connected to the single mode optical fiber 50 at the first port 11, connected to the ring core optical fiber 50 at the second port 12, and coupled to the N first ports 11. A mode multiplexer 101A that couples the received signal light to the ring core optical fiber 50 as N modes;
The mode multiplexer / demultiplexer 101 described with reference to FIG. 21 is connected to the ring core optical fiber 55 at the second port 12 and M signal lights propagated in N modes by the ring core optical fiber 55 (M is N or more) Mode splitter 101 B for demultiplexing to the first port 11 of
M optical receivers 32 for receiving the respective signal lights demultiplexed by the mode demultiplexer 101 B,
A digital signal processor (DSP) 34 and a MIMO signal processor (MIMO-DSP: multiple-input and multiple-output-digital signal processor) 35;
Equipped with
In FIG. 1, N = 3 is described.

  The optical transmitters 31 each include a coherent transmitter circuit (Tx: Transmitter), and transmit signal light to the mode multiplexer 101A via the single mode optical fiber 50. The mode multiplexer 101A sends out the signal light to the optical receiver 32 via the ring core optical fiber 55 and the mode splitter 101B.

  The optical receivers 32 each include a coherent receiver circuit (Rx: Receiver), and receive signal light using local light from a local oscillator circuit (LO: Local Oscillator) 33. The optical receiver 32 converts the frequency of the received signal light to a constant frequency, and sends the signal light to the DSP 34.

  The DSP 34 performs arithmetic processing of the signal light, and sends the signal light subjected to the arithmetic processing to the MIMO signal processor 35. The MIMO signal processor 35 includes a multiplication circuit and an addition circuit, and performs multiplication processing and addition processing on the signal light processed by the DSP 34 by the multiplication circuit and the addition circuit.

The following description will be made using a specific example of the optical transmission system 301 of the present embodiment.
Ring core optical fiber
First, the ring core optical fiber 55 will be described. FIG. 4 is a view for explaining the refractive index distribution of the ring core optical fiber 55. As shown in FIG. The horizontal axis is the radius of the ring core optical fiber, and the vertical axis is the refractive index.

For example, a ring core type fibers in Non-Patent Document 4 is a central layer radius _{a 1} is 3.6 [mu] m, the outer peripheral radius _{a 2} is 5.4μm ring core portion relative refractive index difference Δ of the ring core is 0.25%, propagation mode The number is 3 (LP01, LP11a, b mode) at a wavelength of 1550 nm. In order to limit the number of propagation modes, it is necessary to adjust the central layer radius a ₁ , the outer circumferential radius a _{2 of} the ring core portion, and the relative refractive index difference Δ. In the present specification, for the sake of simplicity, the number of propagation modes is converted to the LP mode by the weak waveguide approximation.

FIG. 5 is a diagram for explaining the relationship between the central layer radius a ₁ of the ring core type fiber, the effective refractive index difference Δneff, and the number of propagation modes. At this time, the ring core width a ₂ -a ₁ is 1.8 μm, the relative refractive index difference Δ ₁ is 0.25%, and the wavelength is 1550 nm. When the central layer radius a ₁ was changed, the effective refractive index difference Δneff and the group delay difference DMD were calculated based on the finite element method. At this time, the effective refractive index difference Δneff is a value obtained by subtracting the effective refractive index of the high-order mode from the effective refractive index of the basic mode, and is calculated at an actual wavelength of 1550 nm.

From FIG. 5, for example,
2.0 [μm] <center layer radius a ₁ <4.0 [μm]: propagation mode number 3,
4.0 [μm] <central layer radius a1 <7.4 [μm]: the number of propagation modes is 5,
7.4 [μm] <central layer radius a1: It is possible to realize an optical fiber in which the number of propagation modes is 7 or more.

  Further, from FIG. 5, in order to reduce Δneff, it is necessary to increase the central layer radius a1, but when focusing on the LP11 mode, when the central layer radius a1 is 5.0 [μm], 0.0005 It can be seen that the Δneff of can be realized.

  In Non-Patent Document 4, a coupled number mode optical fiber is studied, and as a design guideline, it is assumed that an effective refractive index difference Δneff between propagation modes is 0.0005 or less. Therefore, as the ring core optical fiber 55 of the present embodiment, a ring core optical fiber having an effective refractive index difference Δneff of 0.0005 or less is used.

[Mode multiplexer, mode splitter]
The mode multiplexer 101A and the mode demultiplexer 101B are the mode multiplexer / demultiplexer 101 described in the first to third embodiments. Therefore, the mode multiplexer 101A can form a plurality of modes from a single mode from the first port 11 to the second port 12, and can reduce connection loss with the ring core optical fiber 55. The mode demultiplexer 101 B can form a single mode from a plurality of modes from the second port 12 to the first port 11, and can reduce connection loss with the ring core optical fiber 55.

[Operation of optical transmission system]
In this embodiment, as shown in FIG. 1, an example in which signals of three channels are transmitted using signal light of three modes is shown. Signals of the three channels in the mode multiplexer _{101A (x 1, x 2,} x 3) is distributed to the three modes (fundamental mode _{LP 01} and the high-order mode _{LP 11a, LP 11b),} functions as a three-mode optical fiber To the ring core optical fiber 55.

Due to mode coupling and mode loss differences in the ring core optical fiber 55, the ratio of modes included in each channel signal may change before and after transmission. The signal light after transmission is separated by the mode demultiplexer 101 B, and is received by the coherent receiving circuit functioning as a digital coherent receiver included in the optical receiver 32. The received signal is subjected to DPS 34 to obtain amplitude / phase information of _three received signals (y ₁ , y ₂ , y ₃ ). Furthermore, the MIMO signal processor 35 obtains the restored signal (x _c1 , x _c2 , x _c3 ) by _performing MIMO processing on this (y ₁ , y ₂ , y ₃ ).

Here, the MIMO signal processor 35 performs multiplication processing and addition processing of the received signal (y ₁ , y ₂ , y ₃ ) processed by the DSP 34 by the multiplication circuit and the addition circuit to obtain a restored signal ( Obtain x _c1 , x _c2 , x _c3 ).
(Operation A) xc1 = y1 · w11 + y2 · w12 + y3 · w13
(Operation B) xc2 = y1 · w21 + y2 · w22 + y3 · w23
(Operation C) xc3 = y1 · w31 + y2 · w32 + y3 · w33

The MIMO signal processing unit 35 is configured by a finite impulse response (FIR) filter. The FIR filter is composed of a plurality of taps consisting of a delay unit, a multiplier and an adder, and compensates for the mode delay difference of the ring core optical fiber, the spread of impulse response due to differential mode delay (DMD) and crosstalk due to mode coupling. Restore the signal. Here, the multiplier coefficients (tap coefficients; w ₁₁ , w ₁₂ , w ₁₃ , w ₂₁ , w ₂₂ , w ₂₃ , w ₃₁ , w ₃₂ , w ₃₃ ) use a training signal sequence whose signal pattern is known. It is set so that x _c1 = x ₁ , x _c2 = x ₂ , x _c3 = x ₃ .

  In the conventional mode multiplexer, Ch1 is input to the transmission fiber as LP01, Ch2 to LP11a, and Ch3 to LP11b mode. However, the optical transmission system 301 is characterized in that the mode multiplexer / demultiplexer 101 is used as the mode multiplexer 101A, and each of Ch1, Ch2 and Ch3 is transmitted including all the LP01, LP11a and LP11b modes. As a result, in the mode multiplexer 101A, each channel includes all modes, and coupling between modes occurs longitudinally in the transmission line, thereby mode scrambling occurs and mode-dependent loss and delay effects are mitigated. can do.

  Further, in FIG. 1, the mode multiplexer / demultiplexer 101 is used for both of the mode multiplexer 101A and the mode demultiplexer 101B, but the mode multiplexer / demultiplexer is used for the mode multiplexer 101A or the mode demultiplexer 101B. The same effect can be obtained by using 101.

Embodiment 5
FIG. 2 is a diagram for explaining the mode multiplex transmission relay system 302. As shown in FIG. The mode multiplex transmission relay system 302
One relay unit α is
One ring core optical fiber 55 that propagates N modes in the ring core,
The mode multiplexer / demultiplexer 101 described in FIG. 21 is connected to the single mode optical fiber 50 at the first port 11, connected to the ring core optical fiber 55 at the second port 12, and coupled to the N first ports 11. A mode multiplexer 101A that couples the received signal light to the ring core optical fiber 55 as N modes;
The mode multiplexer / demultiplexer 101 described with reference to FIG. 21 is connected to the ring core optical fiber 55 at the second port 12, and the signal light propagated in N modes at the ring core optical fiber 55 is N first ports 11. Mode splitter 101B for demultiplexing into
N optical amplifiers 41 for amplifying the N signal lights demultiplexed by the mode demultiplexer 101 B and outputting the amplified signal light to a single mode optical fiber 50;
And two or more relay units α are connected.

The mode multiplexing transmission relay system 302 has N port mode multiplexers 101 A, N for transmitting _N (N is 2 or more) signals (x ₁ , x ₂ , x ₃ ,,, x _N ). A ring core optical fiber 55 that propagates more than modes, an N port mode splitter 101B, and N optical amplifiers 41 are one relay unit α, and a mode splitter 101B of one relay unit and mode combination of the next relay unit It is characterized in that an optical amplifier 41 is installed between the wavers 101A.

  Since a single mode is provided downstream of the mode demultiplexer 101A, it is possible to use an optical amplifier compatible with a single mode as the optical amplifier 41. Also, by randomly connecting the ports of the mode multiplexer 101B between the relay units to the mode multiplexer 101A, a mode scramble effect can be obtained.

N optical receivers 32 are connected to the first port 11 of the mode branching filter 101B of the last relay unit. The optical receiver 32 receives the optical signals (y ₁ , y ₂ , y ₃ ,..., Y _N ), and the DSP 34 performs signal processing to obtain N restored signals.

Embodiment 6
FIG. 3 is a diagram for explaining the mode multiplex transmission relay system 303. As shown in FIG. The mode multiplex transmission relay system 303
One relay unit β is
One ring core optical fiber 55 that propagates N modes in the ring core,
An optical amplifier 42 disposed in the ring core optical fiber 55 for amplifying the signal light propagating through the ring core optical fiber 55;
The mode multiplexer / demultiplexer 101 described in FIG. 21 is connected to the single mode optical fiber 50 at the first port 11, connected to the ring core optical fiber 55 at the second port 12, and coupled to the N first ports 11. A mode multiplexer 101A that couples the received signal light to the ring core optical fiber 55 as N modes;
The mode multiplexer / demultiplexer 101 described with reference to FIG. 21 is connected to the ring core optical fiber 55 at the second port 12, and the signal light propagated in N modes at the ring core optical fiber 55 is N first ports 11. Mode splitter 101B for demultiplexing into
And two or more relay units β are connected.

  The mode multiplexing transmission relay system 303 propagates N port mode multiplexer 101A, N modes or more, in order to transmit N (N is 2 or more) signals (x1, x2, x3,..., XN). A ring core optical fiber 55, one optical amplifier 42, an N-port mode splitter 101B as one relay unit β, and an optical amplifier 42 installed between the ring core optical fiber 55 and the mode splitter 101B Do.

  Since the subsequent stage of the ring core optical fiber 55 is multimode, an optical amplifier compatible with multimode is used as the optical amplifier 41. As compared with the mode multiplexing transmission relay system 302 of FIG. 2, the mode multiplexing transmission relay system 303 does not have to install optical amplifiers for the number of modes, and only one optical amplifier is required. Also, by randomly connecting the ports of the mode multiplexer 101B between the relay units to the mode multiplexer 101A, a mode scramble effect can be obtained.

(Other embodiments)
The mode multiplex transmission relay system may be configured by combining the relay unit α described in FIG. 2 and the relay unit β described in FIG.

(Effect of the invention)
As described above, the transmission system according to the present embodiment uses the mode multiplexer / demultiplexer in which the transmission path is a ring core optical fiber and a plurality of cores are annularly arranged, thereby reducing the loss of the signal placed in the mode. It has mode scrambling function in which input / output and each channel are assigned to all modes, mode-dependent loss and delay difference can be mitigated, and long distance and large capacity transmission can be realized.

  The present invention can realize an increase in capacity and an increase in distance of optical fiber transmission by using high-order modes in the fiber.

11: first port 12: second port 13: core 21: small diameter optical fiber 22: glass rod 23: coating 31: light transmitter 32: light receiver 33: local oscillator circuit (LO: Local Oscillator)
34: Digital signal processor (DSP: Digital Signal Processor)
35: MIMO signal processor (MIMO-DSP: Multiple-Input and Multiple-Output-Digital Signal Processor)
41, 42: optical amplifier 50: single mode optical fiber 53: ring core 55: ring core optical fiber 101: mode multiplexer / demultiplexer 101A: mode multiplexer 101B: mode demultiplexer 301: optical transmission system

Claims (7)

Ring core optical fiber,
A plurality of first port that is connected to the single-mode optical fiber,
And one second port connected to the ring core optical fiber,
The same number of cores as the first port connecting the first port and the second port;
A mode demultiplexer comprising,
The core is
The diameter is equal to or greater than the core width of the ring core optical fiber,
Core spacings are arranged in a tapered manner such that the spacing between the first port and the second port is short ,
Mode aggregation , wherein in the surface of the second port, each core center is arranged at the center of the core width of the ring core optical fiber in an annular shape so as to contact the ring core of the ring core optical fiber Wave. Ring core optical fiber,
A plurality of first port that is connected to the single-mode optical fiber,
And one second port connected to the ring core optical fiber,
The same number of cores as the first port connecting the first port and the second port;
A mode demultiplexer comprising,
The core is
The diameter is equal to or greater than the core width of the ring core optical fiber,
Core spacings are arranged in a tapered manner such that the spacing between the first port and the second port is short ,
In the plane of the second port so as to be in contact with the ring core of the ring core optical fiber, as the positive N prismatic centroids are formed in the respective core center and the center of the ring core optical fiber match, and each core A mode multiplexer / demultiplexer characterized in that the center is located at the center of the core width of the ring core optical fiber . 3. The mode multiplexer / demultiplexer according to claim 1, wherein the core is a core of a small diameter optical fiber connecting the first port and the second port. The mode multiplexer / demultiplexer according to claim 1 or 2 , wherein the core is an optical waveguide of a planar lightwave circuit (PLC) formed in three dimensions. N (N is an integer of 2 or more) optical transmitters that transmit signal light in N single-mode optical fibers ,
The mode multiplexer / demultiplexer according to any one of claims 1 to 4 , wherein N modes can be propagated by the ring core of the ring core optical fiber, and the first port is connected to the single mode optical fiber , A mode multiplexer for coupling signal light coupled to the N first ports to the ring core optical fiber as N modes;
A mode demultiplexer according to any one of claims 1 to 4, and ring core optical fiber of the mode multiplexer to the ring core optical fiber is common in front Symbol ring core optical fiber at the N mode A mode splitter that splits the propagated signal light into M (M is an integer greater than or equal to N) first ports;
M optical receivers that receive the respective signal lights demultiplexed by the mode demultiplexer;
Optical transmission system comprising: One of the relay unit,
A mode demultiplexer according to any one of claims 1 to 4, it is possible propagation of N modes in the ring core of the ring core optical fiber, connected to the single mode optical fiber at the first port, A mode multiplexer for coupling signal light coupled to the N first ports to the ring core optical fiber as N modes;
The mode multiplexer / demultiplexer according to any one of claims 1 to 4 , wherein the ring core optical fiber and the ring core optical fiber of the mode multiplexer are common, and the ring core optical fiber propagates in N modes. A mode splitter that splits the received signal light into N first ports;
N optical amplifiers for amplifying the N signal lights demultiplexed by the mode demultiplexer and outputting the amplified signal light to a single mode optical fiber;
A relay transmission system, comprising: two or more relay units connected to each other. One of the relay unit,
A mode demultiplexer according to any one of claims 1 to 4, it is possible propagation of N modes in the ring core of the ring core optical fiber, connected to the single mode optical fiber at the first port, A mode multiplexer for coupling signal light coupled to the N first ports to the ring core optical fiber as N modes;
The mode multiplexer / demultiplexer according to any one of claims 1 to 4 , wherein the ring core optical fiber and the ring core optical fiber of the mode multiplexer are common, and the ring core optical fiber propagates in N modes. A mode splitter that splits the received signal light into N first ports;
An optical amplifier disposed in the ring core optical fiber for amplifying signal light propagating through the ring core optical fiber;
A relay transmission system, comprising: two or more relay units connected to each other.