JP2016082318A - Optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve a problem of an optical transmission path constructed by multi-stage connection of MCF that inter-core deviation of a transmission loss increases in SDM transmission, and also a problem that transmission loss and inter-mode deviation in delay time increase in FMF because the electrical field distribution between propagation modes is different.SOLUTION: Switching of a spatial channel is performed at a connection point of each SDM transmission optical fiber, whereby the deviation between spatial channels of a total transmission characteristic in the whole transmission path length can be reduced. Accordingly, there can be provided an optical transmission system enabling SDM transmission in which the total transmission characteristic is uniform between the spatial channels when passing through the optical transmission path.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical transmission system using a space division multiplexing (SDM) technique.

  In recent years, research and development related to a large-capacity optical communication system using the SDM technology has been actively performed (see, for example, Non-Patent Document 1). In addition, active studies on optical fibers for SDM transmission are also underway. For example, a multi-core fiber (MCF) having a plurality of cores in the same cladding, or a plurality of optical fibers in the same core. A number mode optical fiber (FMF) in which a light wave propagates, and a number mode multi-core optical fiber (FM-MCF) in which a number mode core is multi-core have been developed (for example, non-mode fiber). (See Patent Document 2).

  A conventional single-core single-mode fiber (SC-SMF) has a transmission capacity of about 100 Tbit / d due to restrictions such as a low-loss and a wavelength band capable of optical amplification. It is considered to be limited to about s. However, due to the research and development of the above-mentioned optical fiber for SDM transmission, for example, using MCF having 10 or more cores, it is possible to realize 1 Pbit / s large-capacity optical communication that is 10 times or more than conventional SC-SMF. Has been found.

P. J. et al. Winzer, “Optical networking beyond WDM”, IEEE Photon. J. et al. , Vol. 4, no. 2, pp. 647-651, 2012. K. Nakajima, H .; Takara, and I.K. Morita, “Collaborative research on multi-fiber technology inventing industry, academy and governance”, New Breeze, vol. 25, no. 4, pp. 12-15, 2013. N. Hanzawa et al. "Two-mode PLC-based mode multi / demultiplexer for mode and wavelength division multiplexed transmission", Optics Express vol. 21 pp. 25752-25760 (2013). N. Hanzawa, K .; Saitoh, T .; Sakamoto, K. et al. Tsujikawa, T .; Uematsu, and M.M. Kosiba, “Three-mode PLC-type multi / demultiplexer for mode-division, multiplexing multiplexing transmission,” The 39th European Confection and Coexistence and Excitation. 22-26, 2013.

  However, SDM transmission has a problem in that transmission quality varies due to a deviation in transmission characteristics between spatial channels to be used. In particular, in SDM transmission using MCF, dispersion of transmission loss between cores becomes a problem. In general, when the number of cores in the MCF is increased, there are a plurality of cores having different distances from the center of the optical fiber in the same cladding cross section. In the MCF connection, it is indispensable to control the direction of the rotation axis between the optical fibers facing each other. In general, in the MCF connection, the longer the core is from the center of the optical fiber, the more the connection loss tends to increase. In general, the transmission loss of an optical fiber is also closely related to the thickness of the cladding, and the transmission loss tends to increase as the cladding thickness decreases. For this reason, in MCF, the transmission loss tends to increase as the core is closer to the outer periphery of the clad. For this reason, in an optical transmission line configured by connecting MCFs in multiple stages, there has been a problem that an inter-core deviation of the transmission loss increases. In addition, the FMF has a problem that the transmission loss and the inter-mode deviation of the delay time increase due to the difference in the electric field distribution between the propagation modes.

  Accordingly, an object of the present invention is to provide an optical transmission system capable of performing SDM transmission in which the total transmission characteristics are uniform between spatial channels when passing through an optical transmission line in order to solve the above-described problems. To do.

  To achieve the above object, according to the present invention, in an SDM transmission system using an optical transmission line configured using a plurality of optical fibers for SDM transmission, a spatial channel is switched at a connection point of each optical fiber for SDM transmission. An element to be performed was provided.

  Specifically, an optical transmission system according to the present invention includes N (N is an integer of 2 or more) spatial multiplexing transmission lines that transmit optical signals through a plurality of spatial channels, and the spatial multiplexing transmission lines in series. N−, which connect and combine optical signals so that the spatial channel of the (n−1) th (n is an integer of 2 to N) spatial channel of the spatial multiplexing transmission line and the spatial channel of the nth spatial multiplexing transmission line are different. One spatial channel switching element.

  Here, the spatial channel switching element connects the spatial multiplexing transmission lines so that the total transmission characteristics when passing through all the spatial multiplexing transmission lines are uniform among the multiplexed optical signals.

  According to the present invention, by switching the spatial channel at the connection point of each optical fiber for SDM transmission, it is possible to reduce the deviation between the spatial channels of the total transmission characteristics in the entire transmission path. Therefore, the present invention can provide an optical transmission system that enables SDM transmission in which the total transmission characteristics are uniform between spatial channels when passing through an optical transmission line.

  The spatial multiplexing transmission line of the optical transmission system according to the present invention is a multi-core optical fiber having two or more cores, each core being the spatial channel, and the spatial channel switching element is a cross section of the multi-core optical fiber. The optical signal is exchanged between at least two cores having different distances from the center.

  Here, the spatial channel switching element connects the core of the (n−1) th spatial multiplexing transmission path and the core of the nth spatial multiplexing transmission path with an optical waveguide.

  For the SDM transmission line using the MCF, a means for switching the optical signal propagating between adjacent cores in the spatial signal switching element is adopted.

  The spatial multiplexing transmission line of the optical transmission system according to the present invention is a multimode optical fiber capable of propagating an optical signal in a core in two or more propagation modes and each propagation mode as the spatial channel, and the spatial channel switching The element is characterized in that the propagation modes of at least two or more optical signals are switched.

  For the SDM transmission line using FMF, means for replacing the mode propagating in the same core in the spatial signal switching element is adopted.

  In the spatial multiplexing transmission line of the optical transmission system according to the present invention, the core can propagate two or more propagation modes, and the spatial channel switching element exchanges optical signals between the cores and propagates each optical signal. It is characterized by switching modes.

  For the SDM transmission line using FM-MCF, means for replacing the mode propagating in the same core with the means for switching the optical signal propagating between adjacent cores in the spatial signal switching element is adopted.

Here, there are the following two configurations of the spatial channel switching element.
One said spatial channel switching element is
A first parallel waveguide section for separating an optical signal for each propagation mode;
A tapered region part for converting the propagation mode of each optical signal separated by the first parallel waveguide part into another propagation mode by a tapered waveguide;
A second parallel waveguide portion for multiplexing the respective optical signals whose propagation modes are converted in the tapered region portion;
Having a mode conversion waveguide.

Other spatial channel switching elements are:
Having two parallel waveguides having a coupling portion for coupling the optical signal of the first waveguide to the second waveguide by performing propagation mode conversion;
Mode conversion in which the parallel waveguides are connected such that the first waveguide and the second waveguide of one of the parallel waveguides are connected to the second waveguide and the first waveguide of the other parallel waveguide, respectively. It has a waveguide.

  The present invention can provide an optical transmission system that enables SDM transmission in which the total transmission characteristics are uniform between spatial channels when passing through an optical transmission line.

It is a conceptual diagram which shows the structure of the optical transmission system which concerns on this invention. It is an example of a section structure of MCF of an optical transmission system concerning the present invention. It is a conceptual diagram of the connector of the optical transmission system which concerns on this invention. This is a case where the MCF has two cores. It is a conceptual diagram of the connector of the optical transmission system which concerns on this invention. This is a case where the MCF has 4 cores. It is a figure which shows the cross-sectional structural example of MCF applicable to the optical transmission system which concerns on this invention. It is a conceptual diagram of the connector of the optical transmission system which concerns on this invention. This is a case where any three cores in the MCF are arranged on a straight line. It is a conceptual diagram of the connector of the optical transmission system which concerns on this invention. This is a case where any three cores in the MCF are arranged in a triangular shape. It is a conceptual diagram of the connector of the optical transmission system which concerns on this invention. This is a case where any three cores in the MCF are arranged in a triangular shape. It is a conceptual diagram of the connector of the optical transmission system which concerns on this invention. This is a case where the optical fiber is FMF. It is a conceptual diagram of the connector of the optical transmission system which concerns on this invention. This is a case where the optical fiber is FMF.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are embodiments 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.

[Overall structure]
FIG. 1 is a conceptual diagram showing the configuration of the optical transmission system of this embodiment.
In this optical transmission system, N (N is an integer of 2 or more) spatial multiplexing transmission lines 30 for transmitting optical signals through a plurality of spatial channels and the spatial multiplexing transmission lines 30 are connected in series, and the (n-1) th. N in which optical signals are combined so that the spatial channel of spatial multiplexing transmission line 30- (n-1) of n (n is an integer of 2 or more and N or less) and the spatial channel of nth spatial multiplexing transmission line 30-n are different. -1 spatial channel switching element 40.

  The optical transmission system further includes a transmitter 10 that transmits an optical signal and a receiver 20 that receives the optical signal. The transmitter 10 and the receiver 20 are connected by an optical transmission line in which N spatial multiplexing transmission lines 30 are connected in series by N−1 spatial channel switching elements 40.

  The spatial multiplexing transmission line 30 is an optical fiber for SDM transmission, and is, for example, MCF, FMF, or FM-MCF. The number N of SDM transmission optical fibers to be connected may be two or more. However, the effect of reducing the transmission characteristic deviation between the spatial channels according to the present invention can be expanded by increasing N.

  The spatial channel switching element 40 connects the spatial multiplexing transmission lines 30 so that the total transmission characteristics when passing through all the spatial multiplexing transmission paths 30 are uniform among the multiplexed optical signals. Details will be described below.

Example 1
In the optical transmission system of this embodiment, the MCF is used for the spatial multiplexing transmission path 30.
The spatial multiplexing transmission line 30 is a multi-core optical fiber having two or more cores, and each core is the spatial channel.
The spatial channel switching element 40 exchanges optical signals between at least two cores having different distances from the center of the cross section of the multi-core optical fiber.

  FIG. 2 is an example of a cross-sectional structure of the MCF in this embodiment. The MCF used in this embodiment has four cores arranged on a straight line passing through the center of the optical fiber. Here, the distances from the optical fiber center of the cores 2 and 3 and the cores 1 and 4 are r1 and r2, respectively. In the optical transmission system of the present embodiment, two or more cores having different distances from the center of the optical fiber are used in combination. That is, in FIG. 2, the core 1 and the core 2 are set as the group 1, and the core 3 and the core 4 are set as the group 2. Here, the distance between the cores in each group, that is, the distance between the centers of the core 1 and the core 2 and the distance between the centers of the core 3 and the core 4 are denoted by Λ.

  In general, the connection loss of the MCF shown in FIG. 2 depends on the alignment accuracy of the rotating shaft, and increases as the distance from the core center increases. In general, excess loss caused by inhomogeneous microbending strain tends to increase as the cladding thickness of the optical fiber decreases. Therefore, when the transmission path is constructed by connecting the MCFs shown in FIG. 2 in multiple stages, the loss of the core 1 and the core 4 tends to be larger than the loss of the core 2 and the core 3. Therefore, if the optical signal of each core at the connection point can be replaced between the cores in the above-described group, it is possible to reduce the inter-core deviation of the total loss in the entire transmission path.

The spatial channel switching element 40 connects the core of the spatial multiplexing transmission path 30- (n-1) and the core of the spatial multiplexing transmission path 30-n with an optical waveguide.
The replacement of the optical signal between the cores can be easily realized by using, for example, a planar waveguide as shown in FIG. The planar waveguide shown in FIG. 3 has two cores arranged at an interval equal to the inter-core distance Λ. The optical signal incident on each core propagates in a waveguide having a cross structure and is coupled to different cores at the output end. Thus, by using a planar waveguide having the same structural dimensions as the MCF to be used, the optical signal between the cores can be easily replaced. Further, the planar waveguide can be easily miniaturized, and can be incorporated in an adapter that fits into a general-purpose optical fiber connector.

  FIG. 3 shows a conceptual diagram for replacing the optical signals of two adjacent cores. However, as shown in FIG. 4, two waveguides are combined, or four cores on the same spatial channel switching element. By forming the waveguide, it is possible to replace the optical signals of the four cores at once with one spatial channel switching element.

  3 and 4 show the case where the MCF has four cores on a straight line. As shown in FIG. 5, the optical transmission system of this embodiment includes a plurality of cores having different distances from the center. A multi-core optical fiber can be employed. The optical transmission system according to the present embodiment can also employ an MCF having two or three total cores, that is, an MCF having a plurality of groups having different distances from the optical fiber center. For example, in the optical transmission system of the present embodiment, an optical signal replacement group is formed by two cores having different distances from the center of the optical fiber as in FIG. 2, and each group is within the cross section as shown in FIG. Thus, it is possible to use MCFs laminated in several layers, or MCFs in which each group is arranged radially with respect to the center of the optical fiber as shown in FIG. Further, the optical transmission system of the present embodiment can also be configured with three or more cores as one group. For example, as shown in FIG. 5C, the optical transmission system according to the present embodiment includes an MCF in which three cores arranged on the same straight line are grouped and each group is stacked in a cross section. As shown in (d), it is also possible to use MCFs in which three cores arranged in a triangular shape form one group and each group is arranged radially with respect to the center of the optical fiber.

  FIG. 6 is a conceptual diagram showing the configuration of the optical waveguide of the spatial channel switching element 40 for the MCF having the group group shown in FIG. In the optical waveguide of FIG. 6, a crossed waveguide is formed so that the optical signal of the core 1 is coupled to the core 2, the optical signal of the core 2 is coupled to the core 3, and the optical signal of the core 3 is coupled to the core 1. The optical transmission system employing the MCF having the group group shown in FIG. 5C has the effect of reducing the inter-core deviation of the total loss in the total length of the transmission line by making the number N of MCF a multiple of 3. Can be increased.

  7 and 8 are conceptual diagrams showing the configuration of the optical waveguide of the spatial channel switching element 40 for the MCF having the group group shown in FIG. By combining two waveguides having an L-shaped cross structure, the optical signals of the cores arranged in a triangular shape can be replaced. 7 and 8 also form a cross-type waveguide so that the optical signal of the core 1 is coupled to the core 2, the optical signal of the core 2 is coupled to the core 3, and the optical signal of the core 3 is coupled to the core 1. ing. The optical transmission system that employs the MCF having the group group shown in FIG. 5D also has the effect of reducing the inter-core deviation of the total loss in the entire transmission line length by making the number N of MCF a multiple of 3. Can be increased.

(Example 2)
In the optical transmission system of this embodiment, FMF is used for the spatial multiplexing transmission line 30.
The spatial multiplexing transmission line 30 is a multimode optical fiber that can propagate an optical signal in the core in two or more propagation modes, and each propagation mode is the spatial channel.
The spatial channel switching element 40 switches the propagation modes of at least two or more optical signals.

  When FMF is used for the space division multiplexing transmission path, a transmission loss difference occurs between modes propagating in the same core. In the optical transmission system of this embodiment, the optical signal between different modes propagating in the same core is replaced in the spatial signal switching element 40 of FIG. As a result, the optical transmission system according to the present embodiment can reduce the inter-propagation mode deviation of the total loss over the entire transmission path.

  Further, in an optical transmission system using FMF, the transmission speed between propagation modes is different, so that a time difference between modes occurs in the signal arrival time in the receiver 20. In general, the time difference between modes increases the signal processing load, so it is preferable to reduce it to zero as much as possible. In the optical transmission system of the present embodiment, the mode of propagation propagated in the spatial channel switching element 40 in FIG. 1 is converted, so that the difference in signal arrival time between modes in the receiver 20 can be reduced at the same time.

FIG. 9 is a conceptual diagram of a spatial channel switching element 40 provided in an optical transmission system that transmits an optical signal in two propagation modes. The spatial channel switching element 40 is
A first parallel waveguide section 51 for separating an optical signal for each propagation mode;
A tapered region portion 52 that converts the propagation mode of each optical signal separated by the first parallel waveguide portion 51 into another propagation mode by a tapered waveguide; and
A second parallel waveguide portion 53 for multiplexing the respective optical signals whose propagation modes are converted by the tapered region portion 52;
A mode conversion waveguide 55 having

  Two propagation modes incident from the left side of the mode conversion waveguide 55 are separated into a first propagation mode and a second propagation mode in the first parallel waveguide section 51. Each mode after separation is converted into the second propagation mode and the second propagation mode into the first propagation mode in the tapered region 52 in the middle stage of the mode conversion waveguide 55, respectively. Next, the propagation modes are combined in the second parallel waveguide portion 53 downstream of the mode conversion waveguide 55 and emitted to the output end on the right side.

  The multiplexing / demultiplexing and conversion of each mode in the mode conversion waveguide 55 is realized by appropriately adjusting the refractive index and the structure size of the waveguide according to the refractive index of the next term of each mode actually propagating in the waveguide. it can. In addition, the hatching type of the waveguide in FIG. 9 illustrates the relationship of the modes existing in each waveguide section, and does not indicate that the actual waveguide region is divided by the refractive index or the like. Absent. In addition, FIG. 9 shows a configuration example in which two types of propagation modes are replaced. However, for three or more types of propagation modes, the parallel corresponding to the propagation modes added to the preceding and subsequent stages with respect to the middle tapered region. This can be dealt with by adding waveguide sections in cascade.

FIG. 10 is a conceptual diagram of a spatial channel switching element 40 having another structure included in an optical transmission system that transmits an optical signal in two propagation modes. The spatial channel switching element 40 has two parallel waveguides 61 each having a coupling portion 64 that couples the optical signal of the first waveguide 62 to the second waveguide 63 by performing propagation mode conversion.
Mode conversion in which the parallel waveguide 61 is connected so that the first waveguide 62 and the second waveguide 63 of the parallel waveguide 61a are connected to the second waveguide 63 and the first waveguide 62 of the parallel waveguide 61b, respectively. A waveguide 65 is provided.

The mode conversion waveguide 65 has a first waveguide 61a and a second waveguide 61b having a structure as described in Non-Patent Document 3.
In the mode conversion waveguide 65, the first parallel waveguide 61a and the second parallel waveguide 61b are connected, and the second parallel waveguide 61b is obtained by inverting the first parallel waveguide 61a and has the same function. have. The two propagation modes incident from the port 1 are output to the port 3 after the fundamental mode is converted to the port 2 and the higher order mode is converted to the fundamental mode in the coupling part 64 of the first parallel waveguide 61a. Next, in the second waveguide 61 b, the fundamental mode incident from the port 2 is converted into a higher order mode in the coupling unit 64 and output to the port 4. On the other hand, the basic mode incident from the port 3 is output to the port 4 as it is. That is, since the signal that was in the fundamental mode at the time of incidence is converted into a higher-order mode, and the signal that was in the higher-order mode is converted into the fundamental mode, it has a replacement function between propagation modes.

  FIG. 10 shows an example of a configuration that replaces two types of propagation modes. However, for three or more types of propagation modes, as shown in Non-Patent Document 4, a parallel waveguide section corresponding to the added propagation mode is added. It can respond.

(Example 3)
The optical transmission system of the present embodiment uses FM-MCF for the spatial multiplexing transmission path 30.
The spatial multiplexing transmission line 30 is capable of propagating a propagation mode in which the core is 2 or more,
Spatial channel switching element 40 switches the optical signal between cores and switches the propagation mode of each optical signal.

  In this embodiment, an FM-MCF having a structure as shown in FIG. 2 in which each core can propagate several modes is adopted as the spatial multiplexing transmission line 30. The spatial channel switching element 40 is a combination of the waveguide shown in FIGS. 3 and 4 of the first embodiment and FIGS. 6, 7 and 8 and the mode conversion waveguide 55 shown in FIG. 9 of the second embodiment. Configure. The optical transmission system of this embodiment is a spatial multiplexing transmission line 30. By using the spatial channel switching element 40, it is possible to convert a propagation mode in a desired core and replace an optical signal between desired cores.

(Effect of the invention)
According to the optical transmission system of the present invention, by inserting a spatial signal switching element at a connection point of an optical fiber for SDM transmission, there is an effect of reducing a deviation in transmission characteristics between cores or between propagation modes. Since the effect of reducing the transmission characteristic deviation of the present invention is expanded as the number of connection points increases, it is particularly effective in long-distance optical transmission configured by connecting a large number of optical fibers.

10: Transmitter 20: Receiver 30, 30-1, 30-2,..., 30- (N-1), 30-N: Spatial multiplexing transmission lines 40, 40-1, 40-2,. 40- (N-1): Spatial channel switching element 51: first parallel waveguide section 52: taper region section 53: second parallel waveguide section 55: mode conversion waveguides 61, 61a, 61b: parallel waveguide 62: first waveguide 63: second waveguide 64: coupling portion 65: mode conversion waveguide

Claims (8)

N (N is an integer of 2 or more) spatial multiplexing transmission lines for transmitting optical signals through a plurality of spatial channels;
The spatial multiplexing transmission lines are connected in series, and the spatial channel of the (n-1) th (n is an integer of 2 or more and N or less) spatial channel of the spatial multiplexing transmission path is different from the spatial channel of the nth spatial multiplexing transmission path. N-1 spatial channel switching elements for combining optical signals as follows:
An optical transmission system comprising: The spatial channel switching element is
2. The light according to claim 1, wherein the spatial multiplexing transmission lines are connected so that a total transmission characteristic when passing through all the spatial multiplexing transmission lines is uniform among the multiplexed optical signals. Transmission system. The spatial multiplexing transmission line is a multi-core optical fiber having two or more cores, each core being the spatial channel,
3. The optical transmission system according to claim 1, wherein the spatial channel switching element exchanges optical signals between at least two cores having different distances from the cross-sectional center of the multi-core optical fiber.   4. The optical transmission according to claim 3, wherein the spatial channel switching element connects the core of the (n−1) -th spatial multiplexing transmission line and the core of the n-th spatial multiplexing transmission line with an optical waveguide. system. The spatial multiplexing transmission line is a multimode optical fiber that can propagate an optical signal in the core in two or more propagation modes, and each propagation mode is the spatial channel,
The optical transmission system according to claim 1, wherein the spatial channel switching element switches a propagation mode of at least two or more optical signals. In the spatial multiplexing transmission line, the core can propagate two or more propagation modes,
5. The optical transmission system according to claim 3, wherein the spatial channel switching element exchanges an optical signal between cores and exchanges a propagation mode of each optical signal. 6. The spatial channel switching element is
A first parallel waveguide section for separating an optical signal for each propagation mode;
A tapered region part for converting the propagation mode of each optical signal separated by the first parallel waveguide part into another propagation mode by a tapered waveguide;
A second parallel waveguide portion for multiplexing the respective optical signals whose propagation modes are converted in the tapered region portion;
The optical transmission system according to claim 5, wherein the optical transmission system has a mode conversion waveguide. The spatial channel switching element is
Having two parallel waveguides having a coupling portion for coupling the optical signal of the first waveguide to the second waveguide by performing propagation mode conversion;
Mode conversion in which the parallel waveguides are connected such that the first waveguide and the second waveguide of one of the parallel waveguides are connected to the second waveguide and the first waveguide of the other parallel waveguide, respectively. The optical transmission system according to claim 5, further comprising a waveguide.