Patent Grant
11956070
This application is a 371 U.S. National Phase of International Application No. PCT/JP2019/036318, filed on Sep. 17, 2019. The entire disclosure of the above application is incorporated herein by reference.
The present disclosure relates to an optical transmission system using spatial multiplexing optical fibers.
In an optical fiber communication system, transmission capacity is limited by non-linear effects or a fiber fuse generated in optical fibers. In order to mitigate such limitations, spatial multiplexing technologies are under study, such as parallel transmission using a multi-core fiber containing a plurality of cores in one optical fiber (NPL 1), mode-multiplexing transmission using a multi-mode fiber in which a plurality of propagation modes are present in a core (NPL 2), and a few-mode multi-core fiber combining multi-core and mode-multiplexing (NPL 3).
In the multi-core fiber, in order to reduce crosstalk between signals propagating between the cores, each of the cores can be used as an independent transmission line with a distance of constant value or greater between cores (for example, 30 μm or greater). On the other hand, in transmission using the multi-mode fiber, such crosstalk occurs between the propagation modes due to a connection point in the transmission line and a fiber bending, and thus, a multiple-input multiple-output (MIMO) equalizer that compensates for an inter-mode crosstalk at a reception end is used. In a multi-core structure, a method has also been proposed in which crosstalk between cores is allowed, and a coupled multi-core fiber (with a core spacing of generally 25 μm or less) is used to compensate for crosstalk between the cores by using the MIMO equalizer similarly to the multi-mode fiber transmission (for example, NPL 4).
Existing MIMO transmission systems using an L mode fiber use a transmission system including L transceivers and an L×L MIMO equalizer.
In this case, if one transceiver is a device that transmits and receives polarization multiplexed signals, the optical fiber is designed to propagate L spatial modes (that is, designed to propagate 2L modes combined with polarization and spatial modes), and a 2L×2L MIMO equalizer is employed on a reception side. As used herein, unless otherwise specified, signal beam of light is a polarization multiplexed signal, and the number of modes refers to a total number of polarization and spatial modes.
In transmission using multi-mode fibers or coupled multi-core fibers, the MIMO equalizer is required, and a transmission capacity, a signal quality, a MIMO signal processing load, and the like of a transmission system to be constructed are determined depending on the number of propagation modes of the optical fiber to be installed. That is, in the optical fiber propagating an L mode, a system is constructed to transmit L signals in parallel by using an L×L MIMO equalizer.
Here, a problem of a transmission system using the MIMO equalizer will be described. In a case of a wavelength-division multiplexing transmission system using single mode fibers having low loss characteristics in a wide wavelength, the capacity can be gradually increased by controlling the number of multiplexed wavelengths depending on a capacity demand, and a load on the device side can also be gradually increased by preparing a required transceiver accordingly.
On the other hand, in a transmission system employing the MIMO equalizer, if a system is constructed using optical fibers including more propagation modes allowing for future increases in capacity demand, it is necessary to employ a MIMO equalizer in accordance with the number of modes. The MIMO equalizer is known to increase the signal processing load in proportion to the square of the number of modes. That is, the transmission system employing the MIMO equalizer has a problem in that the transmission capacity and the signal processing load of the MIMO equalizer are determined by the number of propagation modes of the optical fiber, making it difficult to adapt to changes in the transmission capacity and the required quality.
Thus, in order to solve the problems described above, an object of the present invention is to provide an optical transmission system capable of controlling a transmission capacity and a signal processing load of a MIMO equalizer, without depending on the number of propagation modes of the optical fiber.
In order to achieve the above object, in the optical transmission system according to the present invention, the number of inputs and outputs of signal beams of light to and from an optical fiber that allows spatial multiplexing transmission is adjusted as a variable parameter, and a signal is appropriately generated on the reception side to control a transmission capacity and a signal quality of the optical transmission system.
Specifically, an optical transmission system according to the present invention includes an optical fiber with the number of spatial modes being L (L is an integer of 2 or greater),
The present optical transmission system is provided with the P×M MIMO equalizer, includes the receivers (N units) the number of which is equal to the number L of spatial multiplexing of the optical fiber, and changes the number M of the signal beams of light transmitted through the optical fiber in the range of the number L of spatial multiplexing or less. The adjustment of the number M of signal beams of light makes it possible to adjust the transmission capacity and the signal quality of the optical transmission system even after construction of the transmission line, thus allowing performance to be changed without changing the system configuration. Furthermore, the present optical transmission system generates the P combined signals from the L received signals received from the receivers. Changing the method of generating the combined signals or the number P makes it possible to change the performance, without changing the system configuration.
Consequently, according to the present invention, it is possible to provide an optical transmission system capable of controlling the transmission capacity and the signal processing load of a MIMO equalizer, without depending on the number of propagation modes of the optical fiber.
The optical transmission system according to the present invention further includes a controller configured to instruct the optical multiplexer to change the number M of the signal beams of light incident on the optical fiber or the number P of the P combined signals, and
The signal generation apparatus of the optical transmission system according to the present invention is a switch configured to selects a received signal of the N received signals, a phase combiner configured to apply a phase rotation to the N received signals and add the resultant N received signals, or an amplitude combiner configured to change an intensity of a received signal of the N received signals to a certain intensity and add the resultant N received signals.
In this case, further provided are
The optical fiber of the optical transmission system according to the present invention may be a coupled multi-core fiber having inter-mode crosstalk of −30 dB/m or more. In the optical fiber, it is possible to randomly couple all propagation modes and achieve an MDL reduction effect.
According to the present invention, it is possible to provide an optical transmission system capable of controlling a transmission capacity and a signal processing load of a MIMO equalizer, without depending on the number of propagation modes of an optical fiber.
Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention and the present invention is not limited to the embodiments described below. Note that components with the same reference signs in the specification and the drawings are assumed to be the same components.
The optical fiber 11 is a multi-mode fiber or a multi-core fiber. M types of signal beams of light output by the M transmitters 12 are multiplexed by the optical multiplexer 13. The multiplexed signal beams of light are incident on each mode or each core of the optical fiber 11. Here, in a case where the optical fiber 11 is a multi-mode fiber, the optical multiplexer 13 is a mode multiplexer designed so that the input signal is coupled to a propagating mode (for example, NPL 5). In a case where the optical fiber 11 is a multi-core fiber, the optical multiplexer 13 is a fan-in device designed so that each signal is incident on each core (for example, NPL 6).
The optical demultiplexer 14 demultiplexes the signal beams of light propagated in each mode or by each core into N ports (N=L). The N receivers 15 respectively receive N types of signal beams of light resulting from the demultiplexing. The signal generation apparatus 17 generates P combined signals from L received signals received by the receivers 15. The MIMO equalizer 16 compensates for signal degradation caused in the optical fiber 11 and outputs M reconstructed signals. The MIMO equalizer 16 is also capable of compensating for a mode variance, a wavelength variance, and a polarization variance. In the present optical transmission system, a MIMO transmission with M inputs and M outputs is performed, making it possible to perform parallel transmission of M types of signals.
Note that when a polarization multiplexed signal is employed for a transmit signal, the polarization multiplexed signal is propagated on a per-port basis, and a 2M×2M MIMO equalizer is required to obtain M signals on the reception side. In the present embodiment, for the sake of simplicity, a case where the transmit signal is not the polarization multiplexed signal will be described.
In order to acquire an electric field amplitude and phase information of the received signal beam of light, it is only required to employ a receiver including a local oscillation light source, a 90° hybrid, a balance receiver, an analog-digital converter, and a computing machine, as the receivers 15 (for example, NPL 7).
Each of the FIR filters 21-m-n is a signal processing circuit for restoring one of the transmit signals. The FIR filter 21 includes taps equal in number L of spatial modes of the optical fiber 11. Each of the taps includes a delay element with an amount of delay τ and a multiplier of a coefficient w (tap coefficient). In
Appropriate configuration of the delay amount τ and the coefficient w of the tap of each of the FIR filters 21-m-n allows the MIMO equalizer 16 to compensate for signal degradation due to a mode variance, a wavelength variance, and a polarization mode variance, and crosstalk occurring in the optical fiber 11.
At this time, when the tap coefficient increases, an amount of calculation required to evaluate the correct tap coefficient increases. That is, when the number of signals input to the MIMO equalizer increases, a signal processing load in the MIMO equalizer increases accordingly.
An M-mode multiplexing optical transmission system using an existing MIMO equalizer includes M optical transmitters, an L-mode or L-core optical fiber, and N optical receivers, where L=M=N, and uses an M×M MIMO equalizer.
On the other hand, the optical transmission system according to the present embodiment includes M optical transmitters, an L-mode or L-core optical fiber, and N optical receivers, where N=L and M≤N, and further, L received signals received by the optical receivers are combined to reduce the number of signals to P (M≤P≤L), so that a P×M MIMO equalizer is employed. Thus, the optical transmission system of the present embodiment can realize a MIMO signal processing load in accordance with the transmission capacity.
With reference to figures onward from
Furthermore, the optical multiplexer 13 may disperse and input the M transmit signals to the L ports of the optical fiber 11 (increase the number of signals from M to L).
The signal generation apparatus 17 generates P combined signals from the L received signals received by the receivers 15. In the present example, the signal generation apparatus 17 selects any P received signals from the L received signals to form combined signals. The P×M MIMO equalizer 16 reconstructs M transmit signals from the P combined signals.
In a case of an optical transmission system not including the signal generation unit but including an optical fiber through which the L mode propagates and an M×M MIMO equalizer, if M<L, it may not always be possible to reconstruct the transmit signals by the M×M MIMO equalizer, and the quality of the reconstructed signals varies. This is because, in a configuration where M<L, among the propagated optical signals, some of the light is not received by the receiver to lead to a loss or an increase in mode-dependent loss, resulting in a decrease in system performance. In the present optical transmission system, it is possible to extract an appropriate received signal by the signal generation unit and improve the quality of the reconstructed signals.
Next, an effect of the present optical transmission system will be described.
The optical multiplexer 13 is a fan-in device that inputs these signal beams of light to each of the 12 cores of the optical fiber 11. There are 12 incident ports of the optical multiplexer 13 and polarization multiplexed signals will propagate through each of the ports. Thus, the present optical transmission system is configured so that M is an even number, but M may also be an odd number. The optical fiber 11 of
The optical demultiplexer 14 is a fan-out device that inputs 24 types of propagated beams of light output from the 12 cores of the optical fiber 11 to the receivers 15. Such a receiver 15 is a coherent receiver including a 90° hybrid and a balance receiver. The receiver 15 receives the propagated beam of light, the signal generation apparatus 17 generates P signals from the received signals, and the P×M MIMO equalizer 16 reconstructs M signals.
Here, as for calculation of the MDL, as described in NPL 8, in M singular values λ1 to λM obtained by singular value decomposition of an M×M channel matrix of the transmission line, λ max indicating a maximum value and λ min indicating a minimum value may be used to determine the MDL according to the following equation:
MDL (dB)=20 log 10(λ max/λ min).
The vertical axis in
On the other hand, even if P is small (for example, P=M=4), when specific received signals are combined and reconstructed, MDL characteristics equivalent to those of the existing configuration can be achieved. Note that even if P=M=4, there are a few combinations of received signals in which the relative MDL value does not deteriorate significantly (for example, an amount of relative MDL deterioration is 1 dB or less).
According to the results in
Here, it is desirable that P is designed so as to have values corresponding to a region where a reconstruction probability is sufficiently ensured, based on a maximum MDL design value (desired MDL value) that allows the signal to be reconstructed. For example, in
Furthermore, a region having a high cumulative frequency (for example, 90% or 99%) allows the desired MDL value to be achieved for almost all combinations of received signals, and thus, it is not necessary to select the received signal, and the configuration on a receiver side can be simplified without the need of the signal generation apparatus 17. Specifically, in
That is, the MDL is stable for a certain period of time, and thus, the application equalization algorithm 42, included in the MIMO equalizer 16, for controlling received signals to be selected allows a combination of received signals for the desired MDL value to be realized without using the signal generation apparatus 17.
In
In the examples described above, the number of ports of the input/output devices (the optical multiplexer 13 and the optical demultiplexer 14) to and from the optical fiber 11 is the number that allows M=N=L to be configured. However, in a configuration where M<L, it is only required that the input/output devices have a required number of ports. For example, in an optical transmission system in which polarization multiplexed signals are transmitted and received, the L-mode multi-core optical fiber 11 includes L/2 cores, and if the number of transmit signals is M, it is only required that the input/output devices include M/2 ports. An example of the optical multiplexer 13 includes a fan-in device that inputs a transmit signal only to M/2 cores.
In the optical communication system according to the first embodiment, the optical fiber 11 is a multi-mode fiber or a multi-core fiber. Here, when the optical fiber 11 is a coupled multi-core fiber as described in NPL 4, the following advantages are achieved.
In a case of the multi-mode fiber, propagating modes are coupled due to connection points and bent portions, but if a sufficient random coupling is not sufficient, the following problems arise. If signal beams of light for M-mode (M<L) are incident on an L-mode multi-mode fiber, a mode distribution may be biased within the optical fiber. Higher order modes within the optical fiber generally have lower optical characteristics than a fundamental mode. Thus, if the incident signal beam of light is distributed to a higher order mode, the improvement effect of the MDL described in the first embodiment may decrease.
On the other hand, in a case of the coupled multi-core fiber, all the propagation modes are randomly coupled along with the propagation. Thus, even when the signal beams of light for the M mode (M<L) are incident on the L-mode multi-core fiber, a sufficient coupling within the optical fiber causes the mode distribution to not be biased. Thus, when the coupled multi-core fiber is used for the optical fiber 11 of the optical transmission system explained in
As described in NPL 4 or 9, the coupled multi-core fiber has a characteristic in that the core spacing ranges from 16 μm to 25 μm, and a group delay spread increases with respect to a transmission distance in proportion to the square root of the distance.
Here, what amount of coupling gives the random coupling to reduce the impulse response width is calculated.
In the case of the amount of coupling of −50 dB/m, a pulse indicating a large intensity is present on both ends, and a width of the pulse is 40 ns the same as of an accumulated DMD (1 ns/km×40 km). In the case of the amount of coupling of −40 dB/m, although the pulse intensity on both ends decreases, the impulse response width is the same as of the accumulated DMD.
On the other hand, in the case of the amount of coupling of −30 dB/m, the impulse response shape is a Gaussian shape. It is well known that in a case that the inter-mode coupling is strong, the impulse response shape is a Gaussian shape. In the case of the amount of coupling of −20 dB/km, it can be seen that the Gaussian shape is similarly obtained, but a width thereof is further smaller.
As described above, it can be thought that the random coupling occurs in the region of the amount of coupling of −30 dB/m or more where it is possible to reduce the impulse response width. If the amount of coupling is −20 dB/m, a more random coupling occurs, which is more desirable.
For example, the controller 19 instructs the optical multiplexer 13 to change the number M of the signal beams of light incident on the optical fiber 11. The measuring instrument 18 measures a signal quality of the demodulated signal and notifies the controller 19 of the number M of the signal beams of light so that the signal quality exceeds a threshold value.
Specifically, the optical multiplexer 13 includes a multiplexing unit 13b that serves as a mode multiplexer if the optical fiber 11 is a multi-mode fiber and as a fan-in device if the optical fiber 11 is a multi-core fiber, and an optical switch 13a used to input signal beams of light into any M ports from among N ports of the multiplexing unit 13b.
It is possible to control a transmission capacity and a signal quality by changing the number M of signal beams of light propagated through the optical fiber 11. In this case, it is difficult to replace a hardware configuration such as a mode multiplexer and a fan-in device. Thus, when the optical multiplexer 13 has a function (the optical switch 13a) of changing the number M of signal beams of light incident on the optical fiber 11, it is possible to eliminate the need for replacing the devices. For example, the number M of signal beams of light is dynamically changed according to a temporal change of a transmission capacity demand by using the optical switch 13a, thus allowing a system performance to be changed without changing the hardware of the optical transmission line system.
For example, the controller 19 instructs the signal generation apparatus 17 to change the number P of the combined signals. The measuring instrument 18 measures the signal quality of the demodulated signal and notifies the controller 19 of the number P of the combined signals so that the signal quality exceeds a threshold value.
As described in Example 1 of the first embodiment, the MDL value can be controlled by changing the number P of the combined signals. In this case, it is difficult to replace a hardware configuration such as a mode demultiplexer and a fan-out device. Thus, if the switch 31 of the signal generation apparatus 17 dynamically selects, on the basis of a measurement result of the measuring instrument 18, the received signals so as to obtain a desired MDL value, it is possible to eliminate the need for device replacement. That is, it is possible to change a system performance without changing the hardware of the optical transmission line system.
For example, the controller 19 indicates, to the signal generation apparatus 17, an amount of the phase rotation to be applied by the phase rotation unit 32 or an intensity of the received signal to be changed by the multiplier 34. The measuring instrument 18 measures the signal quality of the demodulated signal and notifies the controller 19 of the amount of the phase rotation to be applied or the intensity of the received signal so that the signal quality exceeds a threshold value.
As described in Examples 2 and 3 of the first embodiment, the MDL value can be controlled by changing the amount of the phase rotation or the intensity of the received signal. In this case, it is difficult to replace a hardware configuration such as a circuit and a device. Thus, if the phase rotation unit 32 and the multiplier 34 of the signal generation apparatus 17 dynamically adjust, on the basis of a measurement result of the measuring instrument 18, a phase and an intensity of the received signals so as to obtain a desired MDL value, it is possible to eliminate the need for device replacement. That is, it is possible to change a system performance without changing the hardware of the optical transmission line system.
An optical transmission system according to the present embodiment is described below.
(1) The present optical transmission system includes
(2) In the optical transmission system according to (1) above, the signal generation apparatus uses an adder/multiplier to multiply/add L output signals by a coefficient to create P signals and inputs the P signals to the P×M MIMO equalizer to output M demodulated signals.
(3) In the optical transmission system according to (2) above, the coefficient of the adder/multiplier dynamically changes so that a quality of the demodulated signals is equal to or greater than a desired value.
(4) In the optical transmission system according to (2) to (3) above, M and P are controlled so that the quality of the demodulated signals is equal to or greater than a desired value.
(5) In the optical transmission system according to (1) to (4) above, the optical fiber is a multi-core fiber having two or greater cores.
(6) The optical transmission system according to (5) above is a coupled multi-core fiber with an impulse response width of the optical fiber increasing in proportion to the square root of a distance, that is, the coupled multi-core fiber has an inter-mode crosstalk of −30 dB/m or greater.
In the optical transmission system according to the present invention, it is possible to control the transmission capacity and the signal processing load of a MIMO equalizer, without depending on the number of propagation modes of the optical fiber. Furthermore, in the optical transmission system according to the present invention, it is possible to dynamically optimize performance in accordance with a transmission capacity demand and quality, without changing an optical transmission line, and thus, the system performance can be maximized. That is, the optical transmission system according to the present invention is capable of reducing the number of signals input to the MIMO equalizer from the number of spatial modes of the optical fiber to a certain number that allows a desired MDL to be maintained, to reduce the MIMO processing load.
The present invention can be used as a transmission medium in an optical transmission system.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2019/036318 | 9/17/2019 | WO |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2021/053709 | 3/25/2021 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 20170207850 | Takahashi | Jul 2017 | A1 |
| 20200067624 | Tsuzuki | Feb 2020 | A1 |
| Number | Date | Country |
|---|---|---|
| 2017152811 | Aug 2017 | JP |
| Entry |
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| Number | Date | Country | |
|---|---|---|---|
| 20220360358 A1 | Nov 2022 | US |
