Optical transmission system

Abstract

Intensity control units 40A and 40B acquire the intensity of backscattered light when inputting signal light of two or more modes for each core in a multicore optical fiber 10, which has two or more cores and in which two or more modes propagate to each of the cores, so that the transmission direction of light differs between adjacent cores, and output a control signal for reducing the input optical power of a high-order mode other than a basic mode of the signal light by a predetermined amount so as to reduce the influence of the high-order mode on the basic mode.

Description

Optical Transmission System

[0001] The present disclosure relates to an optical transmission technology using a few-mode multi-core optical fiber.

[0002] It has been pointed out that the transmission capacity of general-purpose single-mode fiber (SMF) has reached a physical limit, and efforts are being made to develop new spatially multiplexed channels. For example, Non-Patent Document 1 considers the use of uncoupled few-mode multicore fiber (FM-MCF: Few-Mode Multicore Fiber) in order to maximize the spatial multiplicity of optical fiber.

[0003] In optical transmission using uncoupled multi-core optical fibers, if optical coupling (crosstalk (XT)) occurs between cores, the transmission quality deteriorates as the transmission distance increases. In contrast, it has been reported that in optical transmission using single-mode multi-core optical fibers (SM-MCFs), XT noise can be suppressed by approximately 20 dB by performing bidirectional transmission in which the propagation direction of signal light is switched between adjacent cores (see Non-Patent Document 2). Furthermore, bidirectional transmission using few-mode multi-core optical fibers has also been studied (see Non-Patent Document 3).

[0004] In bidirectional transmission using single-mode multi-core optical fibers, it is known that crosstalk light from adjacent cores undergoes Rayleigh backscattering and is added to the signal light as noise light. On the other hand, in few-mode multi-core optical fibers, it has been reported that in addition to this effect, inter-mode coupling occurs during Rayleigh backscattering, which affects the noise light (see Non-Patent Document 3). Such inter-mode coupling reduces the spatial utilization efficiency of the entire system.

[0005] Y. Sagae, T. Matsui, T. Mori, T. Sakamoto, T. Iwaya, T. Sato, K. Saitoh, and K. Nakajima, “Weakly coupled homogeneous 3-Mode 4-Core fiber with stand-ard cladding diameter,” Journal of Lightwave Tech-nology, vol. 41, no. 12, pp. 3950-3956, 2023. Sano, Akihide, et al. "Crosstalk-managed high-capacity long-haul multicore fiber transmission with propagation-direction interleaving." Journal of Lightwave Technology 32.16 (2014): 2771-2779. Iwaya et al., "Study on XT noise in bidirectional transmission of FM-MCF," IEICE General Conference, B-13-16, 2024.

[0006] In order to solve the above problems, an object of the present disclosure is to reduce the influence of inter-mode coupling during backscattering in a bidirectional optical transmission system using a few-mode multi-core optical fiber.

[0007] In order to achieve the above object, the control device, optical transmission system, and method disclosed herein employ a technique of reducing the input optical power of a higher-order mode by a predetermined amount so as to reduce the influence of the higher-order mode on the fundamental mode in bidirectional transmission using a few-mode multi-core optical fiber.

[0008] Specifically, the control device of the present disclosure acquires the intensity of backscattered light when signal light of two or more modes is input to each core such that the light transmission direction differs between adjacent cores of a multi-core optical fiber having two or more cores and two or more modes propagating through each of the cores, and outputs a control signal to reduce the input light power of the higher-order mode by a predetermined amount so as to reduce the influence of higher-order modes other than the fundamental mode on the fundamental mode in the signal light.

[0009] More specifically, the optical transmission system of the present disclosure inputs signal light of two or more modes to each of the cores of a multi-core optical fiber having two or more cores in which two or more modes propagate, such that the light transmission direction differs between adjacent cores, and reduces the input light power of the higher-order modes by a predetermined amount based on the observation result of the intensity of backscattered light in the multi-core optical fiber, so as to reduce the influence of the higher-order modes other than the fundamental mode on the fundamental mode in the signal light.

[0010] In the above optical transmission system, the multi-core optical fiber is made up of a plurality of spans, and comprises: a plurality of transmitters provided for each of the cores and inputting the signal light; a plurality of receivers receiving the signal light; one or more repeaters arranged between the plurality of spans and amplifying the signal light or compensating for differential mode loss; an intensity observation unit observing the intensity of the backscattered light in the multi-core optical fiber; and a control unit transmitting, to the plurality of transmitters and the one or more repeaters, a control signal for reducing the input optical power of the higher-order mode by a predetermined amount based on the observation result by the intensity observation unit, so as to reduce the influence of the higher-order mode on the fundamental mode in the signal light, and the control unit controls the plurality of transmitters and the one or more repeaters so as to maximize a figure of merit (FoM) related to a ratio of spatial utilization efficiencies defined by equations (1) and (2) described below.

[0011] More specifically, the optical transmission method of the present disclosure acquires the intensity of backscattered light when signal light of two or more modes is input to each core of a multi-core optical fiber having two or more cores and two or more modes propagating through each of the cores such that the light transmission direction differs between adjacent cores, and outputs a control signal to reduce the input optical power of the higher-order mode by a predetermined amount so as to reduce the influence of higher-order modes other than the fundamental mode on the fundamental mode in the signal light.

[0012] Each device and each functional unit constituting the multi-core optical fiber transmission system of the present disclosure can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided via a network. The program of the present disclosure is a program for causing a computer to realize each function of the device according to the present disclosure, and a program for causing a computer to execute each procedure of the method executed by the device according to the present disclosure.

[0013] The above disclosures can be combined as much as possible.

[0014] According to the present disclosure, it is possible to reduce the influence of inter-mode coupling during backscattering in an optical transmission system using a few-mode multi-core optical fiber.

[0015] Fig. 1 is a block diagram showing a configuration of a multi-core optical fiber transmission system according to an embodiment of the present disclosure; Fig. 2 is a diagram explaining inter-mode coupling during backscattering; Fig. 3 is a graph showing the relationship between input optical power and signal to XT noise; Fig. 4 is a graph showing the spatial utilization efficiency of a few-mode multi-core optical fiber; Fig. 5 is a graph showing the optimal inter-mode power ratio at which the spatial utilization efficiency is maximized; Fig. 6 is a block diagram showing a configuration of a multi-core optical fiber transmission system according to an embodiment of the present disclosure;

[0016] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings indicate the same components.

[0017] First Embodiment A configuration of a multi-core optical fiber transmission system 100 according to a first embodiment of the present disclosure will be described with reference to FIG. 1 . The multi-core optical fiber transmission system 100 uses an uncoupled multi-core optical fiber 10 having multiple cores, each of which transmits light in several modes. The multi-core optical fiber transmission system 100 maximizes the spatial utilization efficiency of the entire system by performing bidirectional transmission in which the propagation direction of signal light is swapped between adjacent cores and adjusting the input optical power between modes. Specifically, the multi-core optical fiber transmission system 100 evaluates the intensity of backscattered light in each mode using test light that has a different wavelength or intensity from the signal light, and adjusts the input optical power of each mode to maximize the transmission performance of the entire system. Note that this embodiment illustrates a case in which the number of spans of the multi-core optical fiber 10 is one. The multi-core optical fiber transmission system 100 is an example of an "optical transmission system."

[0018] The multi-core optical fiber transmission system 100 mainly includes two pairs of transceivers, two fan-in / fan-out (FIFO) units 11A and 11B, two intensity observation units 30A and 30B, and two intensity control units 40A and 40B. The transceivers are configured to generate and decode several-mode signal light.

[0019] In this embodiment, the transmitting and receiving unit is a transmitting unit T １ and T ２ (Transmitting units 1 and 2), receiving unit R １ and R ２ (Receiving units 1 and 2) Transmitting unit T １ and the receiving unit R ２ is provided at one end side of the multi-core optical fiber 10 (the left end side of the multi-core optical fiber 10 in FIG. 1 ), and a receiving section R １ and the transmitting unit T ２ are provided at the other end side of the multi-core optical fiber 10 (the right end side of the multi-core optical fiber 10 in FIG. 1 ). A transmitter is provided for each core and transmits signal light. A receiver is provided for each core corresponding to the transmitter and receives the signal light.

[0020] The two fan-in fan-outs 11A and 11B are configured to multiplex and demultiplex few-mode signal light from the transmitter and receiver to the multi-core optical fiber 10. The fan-in fan-out 11A is connected to one end of the multi-core optical fiber 10 and is also connected to the transmitter T through an optical fiber 12. １ and the receiving unit R ２ The fan-in fan-out 11B is connected to the other end of the multi-core optical fiber and is also connected to the receiving unit R via an optical fiber 12. １ and the transmitting unit T ２ As described above, the optical transmission path of the multi-core optical fiber transmission system 100 in this embodiment is composed of two pairs of transmitter / receiver units, an optical fiber 12, two fan-in / fan-outs 11A and 11B, and the multi-core optical fiber 10.

[0021] The two intensity observation units 30A and 30B are configured to observe the intensity of backscattered light in the multi-core optical fiber 10. Specifically, the intensity observation unit 30A is configured to observe the intensity of backscattered light in the propagation direction A, that is, the transmission unit T １ The intensity observation unit 30B observes the intensity of the backscattered light of the optical signal transmitted from the transmission unit T. ２ The intensity of backscattered light of the optical signal transmitted from the

[0022] The two intensity control units 40A and 40B control the transmission unit T based on the observation results of the two intensity observation units 30A and 30B. １ and T ２ Specifically, the intensity control unit 40A is configured to evaluate the intensity of the backscattered light in each mode based on the observation results of the intensity observation unit 30A, and then transmit a control signal to the transmission unit T １ The intensity control unit 40B evaluates the intensity of the backscattered light in each mode based on the observation results of the intensity observation unit 30A, and then transmits a control signal to the transmission unit T ２These control signals are set to maximize the transmission performance of the entire system. The backscattered light intensity ratio that maximizes the space utilization efficiency of the system will be described in detail later. The intensity control units 40A and 40B function as "control units."

[0023] As described above, in bidirectional transmission using a few-mode multi-core optical fiber, the two intensity control units 40A and 40B acquire the intensities of backscattered light from the intensity observation units 30A and 30B, and output control signals to the transmitter or repeater to reduce the input optical power of the higher-order mode by a predetermined amount so as to reduce the influence of the higher-order mode on the fundamental mode in the signal light.

[0024] Transmitter T １ and T ２ transmits an optical signal in which the input optical power of each mode is adjusted so that the backscattered light intensity ratio of each mode falls within a predetermined range, based on control signals from the two intensity control units 40A and 40B. This maximizes the space utilization efficiency of the entire system. Each transmitting unit may also have a function to compensate for loss differences between modes. Also, a relay amplifier or a repeater, which is a device for compensating for inter-mode loss differences, may be provided on the transmission path.

[0025] In the present embodiment, two intensity observation units and two intensity control units are provided corresponding to the propagation directions A and B, but the scope of the present disclosure is not limited to this. For example, one intensity observation unit and one intensity control unit may be provided for one span of the multi-core optical fiber. In this case, one intensity observation unit may measure the backscattered light intensity in either the propagation direction A or B, and may also observe the test light intensity (transmitted light intensity) after transmission through the multi-core fiber 10 in the other of the propagation directions A and B. For example, when only the intensity observation unit 30A and the intensity control unit 40A are provided in FIG. 1 , the backscattered light intensity in the propagation direction A may be observed, and the test light intensity (transmitted light intensity) after transmission through the multi-core fiber 10 in the propagation direction B may be observed. On the other hand, when only the intensity observation unit 30B and the intensity control unit 40B are provided in FIG. 1 , the backscattered light intensity in the propagation direction B may be measured, and the test light intensity (transmitted light intensity) after transmission through the multi-core fiber 10 in the propagation direction A may be observed. Then, based on the observation result by the intensity observation unit 1, the intensity control unit 1 controls the transmission unit T １ and T ２ A control signal may be sent to both of the above.

[0026] (Inter-mode coupling) Inter-mode coupling during backscattering in the multi-core optical fiber 10 will be described with reference to Fig. 2. In bidirectional transmission using the multi-core optical fiber 10, higher-order mode components are added to the bidirectional XT noise of the fundamental mode due to the influence of inter-mode coupling during backscattering.

[0027] 2 shows a case where the multi-core optical fiber 10 is a 2LP2-core fiber. In particular, FIG. 2 shows a LP propagating in a core 1. ０１ Mode and LP １１ The signal light of the LP mode propagates in the core 2. ０１ The process of the influence of the fundamental mode on the noise light is shown. ０１ mode, and the higher order mode is LP １１In addition, the input optical power of each mode may be adjusted taking into account the influence of multiple higher-order modes on one fundamental mode.

[0028] Here, the LP input to each core of the multi-core optical fiber 10 is ０１ Mode and LP １１ The input optical power of each mode is expressed as P ０１ and P １１ Also, LP ０１ Mode and LP １１ The output optical power of each mode is expressed as P ０１＿ｏｕｔ and P １１＿ｏｕｔ In addition, the LP propagating in the core 2 is ０１ Mode and LP １１ The noise optical power of each mode is expressed as P ｎｏｉｓｅ０１ and P ｎｏｉｓｅ１１ In addition, in FIG. 2, the fundamental mode, LP ０１ In order to explain that a component of a higher order mode is added to the noise light of the LP mode, ０１ Only the signal light (noise light) of the mode is shown.

[0029] LP ０１ Mode and LP １１ The inter-core optical coupling amount per 1 km of the mode is expressed as h ０１ and h １１ When the signal light is Rayleigh backscattered in the fiber, the backscattering coefficient when it is coupled from the original mode to the same mode is defined as B ｍ→ｍ , and the backscattering coefficient when coupling from the original mode to another mode is B ｍ→ｎ Then, the input optical power P １１ LP for ０１ Mode signal to XT noise ratio κ ０１ P ｎｏｉｓｅ０１ / P ０１＿ｏｕｔ , and the input optical power P １１ LP for １１ Mode signal to XT noise ratio κ １１ P ｎｏｉｓｅ１１ / P １１＿ｏｕｔIn FIG. 2, the fundamental mode, LP, is defined as ０１ Signal to XT noise ratio κ when higher-order mode components are added to the noise light of the mode ０１ To account for this, the signal to XT noise ratio κ １１ is omitted.

[0030] The influence of the signal light in core 2 on the noise light of the signal light in core 1 is also considered by the signal to XT noise ratio κ ０１、 κ １１ Furthermore, even when the number of cores or modes increases, the spatial utilization efficiency of the entire system can be maximized by transmitting an optical signal with the input optical power of each mode adjusted, taking into account the influence between adjacent cores.

[0031] As shown in FIG. 2, the LP propagating in the core 1 ０１ The signal light of the mode is ０１ and backscattering coefficient B ｍ→ｍ The fundamental mode propagating in core 2 is LP ０１ The LP propagating in the core 1 is coupled to the noise light of the LP mode. １１ The signal light of the mode is １１ and backscattering coefficient B ｍ→ｎ The fundamental mode propagating in core 2 is LP ０１ In this case, the LP in core 2 couples into the noise light of the mode. ０１ Mode signal to XT noise ratio κ ０１ As mentioned above, P ｎｏｉｓｅ０１ / P ０１＿ｏｕｔ However, as will be described later, κ ０１ is the LP propagating in core 1 １１ Input optical power P of the signal light in the mode １１ The larger the value, the larger the

[0032] (Input Optical Power and Signal to XT Noise) The relationship between input optical power and signal to XT noise will be described with reference to Fig. 3. Fig. 3 shows the signal to XT noise ratio κ in the same situation as Fig. 2, where bidirectional transmission is performed using a 2LP mode 2-core fiber. ０１ , κ １１Specifically, the change in the LP propagating in the core 1 is shown. １１ Input optical power P of the signal light in the mode １１ The signal to XT noise ratio κ when ０１ , κ １１ In FIG. 3, the change in the XT noise ratio κ ０１ is the solid line L31, and the XT noise ratio κ １１ is indicated by the dashed line L32.

[0033] Here, in calculating the graph, the wavelength λ was set to 1.55 μm and the span length was set to 80 km. ０１ The input optical power P ０１ is fixed at -4.0 dBm. Backscattering coefficient B ｍ→ｍ is -32 dB / km, backscattering coefficient B ｍ→ｎ is 34 dB / km, and the amount of optical coupling between cores h １１ was set to -43 dB. ｍ→ｍ , B ｍ→ｎ , and h １１ depends on the fiber structure and is not limited to the above values.

[0034] As shown in FIG. １１ With increasing κ ０１ This is because the fundamental mode, LP, is increased due to the influence of inter-mode coupling during backscattering. ０１ The bidirectional XT noise of the mode is due to the LP mode, which is a higher order mode. １１ This is because the components added from

[0035] In contrast to this, in the present disclosure, the multi-core optical fiber transmission system is provided with a function for suppressing the backscattered light intensity of higher-order modes, thereby reducing the influence of inter-mode coupling during backscattering and maximizing the space utilization efficiency of the entire system. For example, in the multi-core optical fiber transmission system 100 of FIG. １ and T ２ Based on control signals from the two intensity control units 40A and 40B, the optical signal is transmitted with the input optical power of each mode adjusted so that the backscattered light intensity ratio of each mode falls within a predetermined range, thereby maximizing the space utilization efficiency of the entire system.

[0036] (Spatial Utilization Efficiency) The spatial utilization efficiency of a multi-core optical fiber transmission system will be described with reference to Figure 4. As an index of spatial utilization efficiency, the Figure of Merit (FoM) is defined as the ratio of spectral efficiency (SE) to that of a conventional single-mode fiber (SMF: Single-Mode Fiber (G.652)) by the following formula (1): ｒｅｆ is the standard spatial efficiency of a conventional single-mode fiber.

[0037] Also, assuming that span loss is ideally compensated for in the transmitter or repeater, the SE per all modes or channel when the inter-core XT used in the above equation (1) is assumed is given by the following equation (2): where P is the input optical power, N ｓ is the number of spans, P ＡＳＥ is the ASE noise optical power, χP ３ denotes the nonlinear noise, and κ denotes the signal-to-noise ratio at the receiver.

[0038] In FIG. １１ LP for １１ FoM per spatial channel of the mode (LP １１ ) is shown as a dotted line L41, and the total FoM of all spatial channels in a 2LP mode 2-core fiber is shown as a solid line. １１ The optimum transmission power (input optical power) of one spatial channel of a mode is determined by ASE noise, nonlinear noise, and XT noise, and P １１ = -1.9 dB FoM (LP １１ ) is the maximum.

[0039] On the other hand, LP ０１ The FoM of the mode is LP due to the influence of inter-mode coupling. １１ Input optical power of the mode P １１ It deteriorates as P １１ to −2.8 dB (see arrow A in FIG. 4), the influence of inter-mode coupling is reduced, and the FoM of all spatial channels is maximized. １１The transmitter T １ , T ２ By controlling the input optical power P １１ is an example of "reducing the input optical power of a higher-order mode by a predetermined amount" in this disclosure.

[0040] At this time, LP ０１ Mode transmission power (input optical power) P ０１ and LP １１ Transmission power P of the mode １１ The ratio is P １１ / P ０１ = 1.2 dB, and LP ０１ Mode and LP １１ The ratio of the backscattered light intensity (signal to XT noise) of the mode is κ ０１ / κ １１ In the above, the LP was set to each core of the multi-core optical fiber 10. ０１ Mode and LP ０２ Although the maximization of the spatial utilization efficiency when transmitting signal light of one mode has been described, the spatial utilization efficiency can also be maximized by using the above formulas (1) and (2) when transmitting signal light of another mode to each core.

[0041] (Optimum inter-mode power ratio) １１ The amount of inter-core optical coupling after 1 km propagation of the mode h １１ The optimal inter-mode power ratio P that maximizes the spatial utilization efficiency (total FoM) of the system １１ / P ０１ That is, in the LP mode, h １１ The condition for maximizing the FoM of all spatial channels when σ is changed is shown in Fig. 5. The condition for maximizing the total FoM is on the solid line L5 in Fig. 5 and is given by the following equation (3).

[0042] Here, h １１ The larger (the further to the right of the graph), the greater the LP １１ The signal light of the mode is the LP of the adjacent core. ０１ The proportion of noise light in the P０１ For P １１ By adjusting h to a low value, the influence of inter-mode coupling can be relatively weakened, and the total FoM can be maximized. １１ In the range of ０１ / κ １１ The range of κ is 1.5<κ ０１ / κ １１ <2.5 [dB]. In other words, it was confirmed that the backscattered light intensity (signal to XT noise) was 1.5<κ ０１ / κ １１ <2.5 [dB] １１ By adjusting h, the condition of equation (3) is guaranteed. １１ When P = 43 dB (in the case of FIG. 4), １１ When κ is reduced to -2.8 dB ０１ / κ １１ is 2.0 dB, and 1.5<κ ０１ / κ １１ <2.5 [dB]. In other words, the input optical power P １１ Under such conditions (1.5<κ ０１ / κ １１ The spatial efficiency of the entire system is maximized by controlling the transmitters and repeaters so that the 1.5 dB or 2.5 dB value is satisfied. However, the values ​​1.5 dB and 2.5 dB themselves have no special meaning, and 1.5 < κ ０１ / κ １１ The condition of <2.5 [dB] is ０１ / κ １１ This means that the noise level is in the vicinity of 2.0 dB.

[0043] A configuration of a multi-core optical fiber transmission system 200 according to a second embodiment of the present disclosure will be described with reference to Fig. 6 . In this embodiment, the case where the number of spans of the multi-core optical fiber 10 is M is shown. The multi-core optical fiber transmission system 200 evaluates the intensity of backscattered light in each mode using test light of a different wavelength or intensity from the signal light, and adjusts the input optical power of each mode for each transmission span to maximize the transmission performance of the system. The multi-core optical fiber transmission system 200 is an example of an "optical transmission system."

[0044] The multi-core optical fiber transmission system 200 mainly includes N pairs of transceiver units, two fan-in / fan-out units 11A and 11A, M-1 repeaters 20, a pair of intensity observation units 30A and 30B provided for each span, and a pair of intensity control units 40A and 40B provided for each span. The number of spans and the number of repeaters 20 are arbitrary.

[0045] The transmitting and receiving units are configured to generate and decode several-mode signal light, and N transmitting units T １ -T Ｎ (transmitting units 1, 2, ..., N) and N receiving units R １ -R Ｎ The multi-core optical fiber 10 is made up of receiving units (receiving units 1, 2, ..., N). The transmitting units with odd subscripts and the receiving units with even subscripts are arranged at one end of the multi-core optical fiber 10, and the transmitting units with even subscripts and the receiving units with odd subscripts are arranged at the other end of the multi-core optical fiber 10. This allows for bidirectional transmission.

[0046] The two fan-in fan-outs 11A and 11B have the same configuration as the two fan-in fan-outs 11A and 11B of the multi-core optical fiber transmission system 100 in the first embodiment. The fan-in fan-out 11A is connected to transmitters with odd subscripts and receivers with even subscripts via optical fibers 12. The fan-in fan-out 11B is connected to transmitters with even subscripts and receivers with odd subscripts via optical fibers 12. In this way, the optical transmission path of the multi-core optical fiber transmission system 200 in this embodiment is composed of N pairs of transmitter-receiver units, the optical fiber 12, the two fan-in fan-outs 11A and 11B, and the multi-core optical fiber 10.

[0047] Each of the M-1 repeaters 20 is disposed between each span. The repeaters 20 are configured to adjust the input optical power of each mode. The repeaters 20 include, for example, repeater amplifiers and mode loss difference compensation devices. A transmission section with M spans is constructed by the above transmission path and the M-1 repeaters 20.

[0048] The intensity observation unit 30A is configured to observe the intensity of backscattered light of the optical signal in the propagation direction A in each span. The intensity observation unit 30B is configured to observe the intensity of backscattered light of the optical signal in the propagation direction B in each span.

[0049] The intensity control unit 40A is configured to send a control signal that controls the intensity of the input signal to a transmitter or repeater located upstream in the propagation direction A, based on the observation results of the intensity observation unit 30A. The intensity control unit 40B is configured to send a control signal that controls the intensity of the input signal to a transmitter or repeater located upstream in the propagation direction B, based on the observation results of the intensity observation unit 30B.

[0050] For example, in span 1, the control signal obtained from the backscattered light intensity of the optical signal in propagation direction A is transmitted to transmitters 1, 2, ..., N. On the other hand, the control signal obtained from the optical signal in propagation direction B is transmitted to the leftmost repeater (repeater 1) among the M-1 repeaters 20.

[0051] At this time, the intensity control sections 40A and 40B adjust the input optical power P １１ The transmitter T １ -T Ｎ By controlling the M-1 repeaters 20, the space utilization efficiency of the entire system is maximized. １ -T Ｎ Furthermore, as described above, the backscattered light intensity ratio of each mode is set to 1.5<κ ０１ / κ １１ By setting it to <2.5 [dB], the space utilization efficiency of the system is maximized.

[0052] Each device and each functional unit constituting the multi-core optical fiber transmission system of the present disclosure can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided via a network. The program of the present disclosure is a program for causing a computer to realize each function of the device according to the present disclosure, and a program for causing a computer to execute each procedure of the method executed by the device according to the present disclosure.

[0053] The present disclosure can be applied to the information and communications industry.

[0054] 10: Multi-core optical fiber 11: Fan-in fan-out 12: Optical fiber 20: Repeater 30A, 30B: Intensity observation unit 40A, 40B: Intensity control unit 100, 200: Multi-core optical fiber transmission system T １ , ...T Ｎ : Transmitter R １ , ...R Ｎ : Receiver

Claims

1. A control device that acquires the intensity of backscattered light when signal light of two or more modes is input to each core of a multi-core optical fiber having two or more cores and in which two or more modes propagate through each of the cores so that the light transmission direction differs between adjacent cores, and outputs a control signal to reduce the input light power of the higher-order mode by a predetermined amount so as to reduce the influence of higher-order modes other than the fundamental mode on the fundamental mode in the signal light.

2. An optical transmission system comprising: a multi-core optical fiber having two or more cores in which two or more modes propagate, respectively, wherein signal light of two or more modes is input to each of the cores so that the light transmission direction differs between adjacent cores; and, based on the observation results of the intensity of backscattered light in the multi-core optical fiber, reducing the input optical power of the higher-order modes by a predetermined amount so as to reduce the influence of the higher-order modes other than the fundamental mode on the fundamental mode in the signal light.

3. The optical transmission system according to claim 2, wherein the multi-core optical fiber is made up of a plurality of spans, and comprises: a plurality of transmitters provided for each of the cores and inputting the signal light; a plurality of receivers receiving the signal light; one or more repeaters arranged between the plurality of spans and amplifying the signal light or compensating for differential mode loss; an intensity observation unit observing the intensity of the backscattered light in the multi-core optical fiber; and a control unit transmitting, based on the observation result by the intensity observation unit, to the plurality of transmitters and the one or more repeaters, a control signal for reducing the input optical power of the higher-order mode by a predetermined amount so as to reduce the influence of the higher-order mode on the fundamental mode in the signal light, and wherein the control unit controls the plurality of transmitters and the one or more repeaters so as to maximize a figure of merit (FoM) related to a ratio of spatial utilization efficiencies defined by the following equations (C1) and (C2): SE ｒｅｆ : Spatial utilization efficiency of a reference conventional single-mode fiber SE: Spatial utilization efficiency for all modes in the multi-core optical fiber P: Input optical power N ｓ : Number of spans P ＡＳＥ : ASE noise optical power χP ３ : nonlinear noise κ: signal to XT noise ratio 4. An optical transmission method comprising: acquiring the intensity of backscattered light when signal light of two or more modes is input to each core of a multi-core optical fiber having two or more cores and in which two or more modes propagate through each of the cores so that the light transmission direction differs between adjacent cores; and outputting a control signal to reduce the input optical power of the higher-order mode by a predetermined amount so as to reduce the influence of higher-order modes other than the fundamental mode on the fundamental mode in the signal light.

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