Spatial multiplexing optical transmission system and multi-core fiber

Abstract

To provide a spatial multiplexing optical transmission system using a multicore fiber, which does not use a tone signal and does not require any compensation circuit for phase fluctuations in a receiving circuit.SOLUTION: One of multiple cores is used as an optical length control channel for compensating for optical length fluctuations that occur in a multicore fiber. Coherent optical communication or quantum communication / quantum key delivery is performed using another core in the multicore fiber whose optical length has been controlled by the optical length control channel.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a spatial multiplexing optical transmission system and a multi-core fiber. This application claims priority based on Japanese Patent Application No. 2024-077233, filed on May 10, 2024, the contents of which are incorporated herein by reference. [Background technology]

[0002] To increase the capacity of backbone optical transmission networks, there has been much interest in improving the spectral efficiency of wavelength division multiplexing (WDM) transmission systems by introducing multilevel coherent optical transmission technology. In wavelength division multiplexing transmission systems, the optical carrier signal generated in the transmitter and the local oscillator signal generated in the receiver generally have different optical frequencies, and these fluctuate independently due to factors such as temperature drift. This requires digital signal processing in the receiver circuit to compensate for the carrier frequency discrepancy, which creates the problem of increasing the load on the digital processing circuit as the signal multilevel level increases. This makes it difficult to achieve real-time transmission of high-level signals.

[0003] As a method for reducing the load of this digital signal processing, optical phase-locked loop circuits (see, for example, Non-Patent Document 1) and optical injection locking circuits (see, for example, Non-Patent Documents 2 and 3) have been reported, which synchronize the phase of local light with a tone signal in an analog manner. These analog circuit methods can achieve highly accurate optical phase synchronization regardless of the signal multilevel. Furthermore, a self-homodyne detection method, which uses a tone signal distributed along with a signal as a local light source, does not require a special phase-locked circuit and has a simple system configuration (see, for example, Non-Patent Document 4 and Patent Document 1). This self-homodyne detection method is not limited to coherent optical communications, but is also applied to quantum communications and quantum key distribution (see, for example, Non-Patent Document 5).

[0004] Another method that does not use tone signals is a transmission method in which light sources that oscillate at the same optical frequency are placed independently in the transmitter and receiver, and a homodyne receiving circuit is constructed using these light sources (see, for example, Patent Document 2). [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] K. Kasai, J. Hongo, M. Yoshida, and M. Nakazawa, “Optical phase-locked loop for coherent transmission over 500 km using heterodyne detection with fiber lasers”, IEICE Electron. Express, February 2007, vol. 4, no. 3, pp. 77-81 [Non-patent document 2] S. Adhikari, S. Sygletos, AD Ellis, B. Inan, SL Jansen, and W. Rosenkranz, “Enhanced self-coherent OFDM by the use of injection locked laser”, in Proceedings of the Optical Fiber Communication Conference (OFC), Los Angeles, 2012, JW2A [Non-patent document 3] Wang, K. Kasai, M. Yoshida, and M. Nakazawa, “120 Gbit / s injection-locked homodyne coherent transmission of polarization-multiplexed 64 QAM signals over 150 km”, Opt. Express, December 2014, vol. 22, no. 25, pp. 31310-31316 [Non-patent document 4] Miyazaki, T. Kubota, F. “PSK self-homodyne detection using a pilot carrier for multibit / symbol transmission with inverse-RZ signal”, IEEE Photonics Technol. Lett., 2005, 17, p.1334-1336 [Non-patent document 5] T. Hirano, H. Yamanaka, M. Ashikaga, T. Konishi, and R. Namiki, “Quantum cryptography using pulsed homodyne detection”, Phys. Rev. A68, 2003, 042331 [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-064049 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-216437 Summary of the Invention [Problem to be solved by the invention]

[0007] However, analog circuit methods using tone signals as described in Non-Patent Documents 1 to 3 have the problem of system complexity because one tone signal is individually distributed to each channel of a WDM signal and the receiver requires as many phase-locked loop circuits as there are channels. Also, self-homodyne methods as described in Non-Patent Documents 4 and 5 and Patent Document 1 do not require a phase-locked loop, but use a tone signal as local light whose S / N ratio has deteriorated after fiber transmission, which has the problem of limiting the transmission distance.

[0008] Furthermore, the method described in Patent Document 2, in which light sources oscillating at the same optical frequency are arranged in the transmitter and receiver, has the problem that phase fluctuations caused by variations in the optical length in the fiber transmission line are directly imparted to the homodyne reception signal, and therefore a compensation circuit for phase fluctuations due to the optical transmission line is required in the receiver circuit.

[0009] The present invention has been made to solve the above problems, and has an object to provide a spatial division multiplexing optical transmission system using a multicore fiber, which does not use a tone signal and does not require any compensation circuit for phase fluctuations in the receiving circuit, and a multicore fiber. [Means for solving the problem]

[0010] In order to achieve the above object, the spatial multiplexing optical transmission system according to the present invention is characterized in that, in a spatial multiplexing optical transmission system using a multicore fiber having a plurality of cores, any one of the plurality of cores is used as an optical length control channel for compensating for optical length fluctuations occurring in the multicore fiber, and coherent optical communication or quantum communication / quantum key distribution is performed using other cores in the multicore fiber that have been subjected to optical length control by the optical length control channel.

[0011] The spatial multiplexing optical transmission system according to the present invention preferably uses light sources having the same optical frequency as the transmitting light source and the local light source for the coherent optical communication or the quantum communication / quantum key distribution, and further uses the multi-core fiber subjected to the optical length control as the optical transmission path, thereby eliminating the need for any compensation circuit for phase fluctuations in the receiving circuit.

[0012] The spatial multiplexing optical transmission system according to the present invention preferably uses a multicore fiber with extremely small crosstalk (leakage) between cores, so that the coherent optical communication and the quantum communication / quantum key distribution can be simultaneously performed in different cores without interfering with each other.

[0013] In the spatial multiplexing optical transmission system according to the present invention, it is effective to use a frequency-stabilized single-frequency light source in an optical lattice clock as an optical reference signal source for detecting the optical length fluctuation occurring in the multicore fiber. In this case, it is also effective to synchronize the phases of the transmitting light source and the local light source of the coherent optical communication or the quantum communication / quantum key distribution to the phase of the optical reference signal source by using a negative feedback control circuit, for example, a phase-locked loop circuit.

[0014] In the spatial multiplexing optical transmission system according to the present invention, a crosstalk suppression structure may be provided that separates a core of the multicore fiber used as the optical length control channel from a core that performs the quantum communication and quantum key distribution. In this case, the crosstalk suppression structure can suppress the occurrence of crosstalk at the connection portion between the multicore fiber and the fan-in element and the fan-out element, even when signals with a large difference in intensity are simultaneously transmitted.

[0015] In the spatial multiplexing optical transmission system according to the present invention, at least one core of the multicore fiber other than the core that performs the quantum communication and quantum key distribution may be doped with a rare earth element in at least a portion thereof. In this case, when the multicore fiber is used to transmit an optical reference signal for compensating for optical length fluctuations and simultaneously perform quantum communication and quantum key distribution, by doping a portion of at least one core other than the core that transmits the quantum communication and quantum key distribution signal with a rare earth element, it is possible to selectively optically amplify the cores without imparting an optical path length difference between the cores.

[0016] The spatial multiplexing optical transmission system according to the present invention may further include a side pumping fiber adjacent to at least one core other than the core performing the quantum communication and quantum key distribution. With this configuration, an optical amplification mechanism for optically pumping a rare-earth element-doped core can be realized without using an optical amplifier fan-in element and an optical amplifier fan-out element.

[0017] When at least a portion of at least one of the cores is doped with a rare earth element, a notch that blocks only the core doped with the rare earth element can be made from the outer periphery of the multicore fiber, and a bandpass optical filter can be inserted therein, thereby eliminating amplified spontaneous emission (ASE) noise that occurs during optical amplification.

[0018] In the spatial multiplexing optical transmission system according to the present invention, it is effective to compensate for the optical length fluctuations occurring in the multicore fiber, such as slow fluctuations caused by temperature or pressure changes, or fast fluctuations caused by thermal vibrations of glass molecules constituting the optical fiber, by negative feedback control using an optical fiber stretcher or an optical modulator.

[0019] The multi-core fiber according to the present invention is a multi-core fiber having a plurality of cores, and includes one core used for transmitting an optical reference signal and 10 cores used for transmitting the optical reference signal as an optical length control channel for compensating for optical length fluctuations occurring in the multi-core fiber. -6 It is characterized by the provision of a crosstalk suppression structure that separates it from other cores used to transmit signals with weaker optical intensities.

[0020] The multicore fiber according to the present invention can suppress the occurrence of crosstalk at the connection points between the multicore fiber and the fan-in element and the fan-out element, due to the crosstalk suppression structure, even when signals with large intensity differences are transmitted simultaneously.

[0021] In the multicore fiber according to the present invention, the weak signal is a signal used for quantum communication / quantum key distribution, and at least one core other than the other cores used to transmit the weak signal may be at least partially doped with a rare earth element. With this configuration, even when transmitting a quantum communication / quantum key distribution signal, which cannot be transmitted using an amplifier, the signal can be selectively optically amplified within the cores without imparting an optical path length difference between the cores. [Effects of the Invention]

[0022] According to the present invention, it is possible to provide a spatially multiplexed optical transmission system using a multicore fiber, which does not use a tone signal and does not require any compensation circuit for phase fluctuations in the receiving circuit. As described above, according to the present invention, since phase fluctuation compensation by digital signal processing in the receiving circuit is completely unnecessary, the load on digital signal processing is reduced, and it becomes easy to realize a receiving circuit that demodulates and analyzes data in real time. Furthermore, since there is no need to distribute tone signals for the number of channels during WDM transmission, the system configurations for coherent optical communication and quantum communication / quantum key distribution can be simplified. Furthermore, when an optical reference signal and a quantum communication / quantum key distribution signal are simultaneously transmitted, they can be transmitted over a multicore fiber without crosstalk. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a block diagram showing a spatial multiplexing optical transmission system according to a first embodiment of the present invention. [Figure 2] FIG. 10 is a block diagram showing a modified example of the spatial multiplexing optical transmission system according to the first embodiment of the present invention. [Figure 3] FIG. 10 is a block diagram showing a spatial multiplexing optical transmission system according to a second embodiment of the present invention. [Figure 4] FIG. 10 is a block diagram showing a modified example of the spatial multiplexing optical transmission system according to the second embodiment of the present invention. [Figure 5] 1A and 1B are perspective views showing an example in which a crosstalk suppression structure provided at the input end and / or output end of an N-core fiber transmission line in the spatial multiplexing optical transmission systems of the first and second embodiments of the present invention is made up of a protrusion, and FIG. 1B is an example in which a crosstalk suppression structure is made up of a groove and a partition plate. [Figure 6] 5A and 5B are perspective views showing an example in which the propagation direction crosstalk suppression structure provided at the input end and / or output end of the N-core fiber transmission line in the spatial multiplexing optical transmission system according to the first and second embodiments of the present invention is made up of air holes, and FIG. 5B is a perspective view showing an example in which the propagation direction crosstalk suppression structure is further combined with the structure shown in FIG. 5B. [Figure 7] FIG. 1 is a perspective view showing a configuration in which cores are selectively optically amplified in an N-core fiber transmission line in the spatial multiplexing optical transmission systems according to the first and second embodiments of the present invention. [Figure 8] FIG. 1 is a perspective view showing a configuration in which cores are selectively optically amplified in an N-core fiber transmission line and crosstalk between cores in the propagation direction is suppressed in the spatial multiplexing optical transmission systems of the first and second embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0024] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. A first embodiment of a spatial multiplexing optical transmission system according to the present invention is shown in Fig. 1. This system comprises an N-core fiber transmission line 1, a transmitting section, and a receiving section.

[0025] The N-core fiber transmission line 1 is a multi-core fiber and has multiple core fibers (cores) #1 to #N. The multiple core fibers are made of glass such as silica or quartz doped with germanium dioxide (GeO2) for refractive index adjustment. The N-core fiber is configured such that the multiple core fibers are covered with a clad made of glass such as silica, and the clad is covered with a coating made of plastic, vinyl, or the like.

[0026] First, the transmitter will be described. Here, core #1 is used for optical length control. Specifically, the transmitter includes an optical reference signal source 2 for detecting optical length fluctuations in the N-core fiber transmission line 1, and its output light is split into two by a two-branch coupler 3. One of the optical reference signals is coupled to core #1, one of the N cores in the N-core fiber transmission line 1, via an optical circulator 4 and a fan-in element 5. This light is amplified by an optical amplifier 13 connected to core #1 of the fan-out element 12 in the receiver (described below), and is then input back into the N-core fiber transmission line 1 via the fan-out element 12 in the reverse direction, traveling back and forth through the N-core fiber transmission line 1. It is then output again in the opposite direction from the fan-in element 5 and input into a balanced photodetector 6 via the optical circulator 4. The other optical reference signal split by the two-branch coupler 3 is also input into the balanced photodetector 6, where it undergoes delayed self-homodyne detection. The transmitter uses this detection signal to detect information about optical length fluctuations that occur during a round trip through the N-core fiber transmission line 1. The detected fluctuation information is then fed back to the optical length control mechanism 8 via the negative feedback control circuit 7, and the optical length fluctuations in the N-core fiber transmission line 1 are compensated for.

[0027] In the transmitter, coherent transmission or quantum communication / quantum key distribution is performed using a total of N-1 cores, i.e., cores #2 to #N other than core #1, of the N-core fiber transmission line 1, whose optical length fluctuations have been compensated for using the optical reference signal source 2 as described above. The transmitter has a frequency-stabilized light source 9 as its transmission light source. This light source may be separate from the optical reference signal source 2. In the transmitter, the output is branched into N-1 beams using an N-1 branching coupler 10, and each branched beam corresponds to communication data or key information and is modulated by N-1 optical modulators 11. Each optically modulated signal is then coupled to cores #2 to #N of the N-core fiber via a fan-in element 5. The light from each of these cores propagates through the N-core fiber transmission line 1, whose optical length is controlled by an optical length control mechanism 8, and is input to the receiver.

[0028] Next, the receiving section will be described. In the receiving section, the light propagating through each core in the N-core fiber transmission line 1 is spatially demultiplexed by the fan-out element 12. As described above, the optical reference signal propagating through core #1 is input to the optical amplifier 13 via the optical circulator 4-2. The amplified light is then input in the reverse direction to the N-core fiber transmission line 1 again via the optical circulator 4-2 and the fan-out element 12, and is used to detect optical length fluctuations. As shown in the figure, injection locking of a semiconductor laser (LD) 14 may be used instead of the optical circulator 4-2 and the optical amplifier 13.

[0029] On the other hand, the optical signals propagated through cores #2 to #N are homodyne detected as follows. A frequency-stabilized light source 9-2 oscillating at the same optical frequency as the transmitting light source is arranged as a local light source in the receiving section, and its output light is branched into N-1 beams using an N-1 branching coupler 10-2. Then, the optical signals propagated through cores #2 to #N are individually homodyne-detected using the branched N-1 local light beams and N-1 homodyne receiving circuits 15. At this time, the optical length fluctuation in the N-core fiber transmission line 1 is compensated for by the light from the optical reference signal source 2 using core #1, and therefore phase fluctuation compensation in the homodyne receiving circuit 15, which was conventionally required, is completely unnecessary, which is a major characteristic effect of the present invention.

[0030] Furthermore, by using a fiber in which the crosstalk between cores is significantly reduced by widening the core spacing or by providing holes between the cores as the N-core fiber transmission line 1, coherent optical communication and quantum communication / quantum key distribution can be carried out simultaneously in different cores without interfering with each other.

[0031] The spatial multiplexing optical transmission system of the first embodiment of the present invention uses, as the optical reference signal source 2 in the transmitting section, for example, 10 -15A single-frequency laser phase-locked to an optical lattice clock that can provide the following stability is effective. Furthermore, as the frequency-stabilized light source 9 and frequency-stabilized light source 9-2 to be placed in the transmitter and receiver, respectively, a single-frequency laser whose oscillation frequency is absolutely stabilized by the absorption of gas molecules such as acetylene, or a single-frequency laser phase-locked to a pulsed light source with a controlled carrier-envelope offset frequency, is effective.

[0032] Furthermore, it is desirable to arrange multiple light sources with different oscillation frequencies as the frequency-stabilized light source 9 and frequency-stabilized light source 9-2 arranged in the transmitter and receiver, respectively, to increase the capacity using the WDM system. Alternatively, multiple sidebands may be generated around the output light of the frequency-stabilized light source 9 and frequency-stabilized light source 9-2 using an optical modulator, and each sideband component may be used as a carrier signal for each channel or as a local light source in the WDM system.

[0033] Furthermore, a fiber stretcher having an optical fiber attached to a piezoelectric element or an optical phase modulator utilizing the Pockels effect may be used as the optical length control mechanism 8. Furthermore, the N-core fiber transmission line 1 may be one in which the elongation of the optical fiber is suppressed by coating the surface with a material having a thermal expansion coefficient different from that of the optical fiber, thereby suppressing the temperature change of the optical length.

[0034] Furthermore, a multi-core rare-earth doped optical amplifier or Raman amplifier capable of bidirectional amplification may be used in the N-core fiber transmission line 1, and the transmission distance may be increased by multiple repeaters.

[0035] A modified example of the first embodiment of the spatial multiplexing optical transmission system according to the present invention is shown in Fig. 2. The difference from the embodiment shown in Fig. 1 is that the optical reference signal source 2 is also used as a frequency reference for the frequency-stabilized light sources 9 and 9-2, and this difference will be described below.

[0036] In the transmitting section, a portion of the output of the optical reference signal source 2 is branched using a two-branch coupler 3-2, and the branched optical reference signal is used to synchronize via a negative feedback control circuit 7-2 so that the phase of the frequency-stabilized light source 9 matches that of the optical reference signal. Meanwhile, in the receiving section, a portion of the optical reference signal transmitted through core #1 of the N-core fiber transmission line 1 is branched via an optical circulator 4-2 and a two-branch coupler 3-3, and the branched optical reference signal is used to synchronize via a negative feedback control circuit 7-3 so that the phase of the frequency-stabilized light source 9-2 matches that of the optical reference signal. As a result, a space-division multiplexing optical transmission system in which the optical reference signal source 2 is used as the optical phase reference for the entire system can be realized, enabling more stable communications.

[0037] A second embodiment of the spatial multiplexing optical transmission system according to the present invention is shown in Fig. 3. The difference from the first embodiment shown in Fig. 1 is that the receiving section also has an optical reference signal source 2-2 with the same specifications as the transmitting section, but different from the transmitting section, and the receiving section detects information about optical length fluctuations in the N-core fiber transmission line 1. The differences in this optical length fluctuation detection system will be described below.

[0038] In the transmitting section, the output light of the optical reference signal source 2 is coupled to core #1 in the N-core fiber via the fan-in element 5. The optical reference signal propagated through core #1 of the N-core fiber transmission line 1 passes through the optical length control mechanism 8 and is input to the receiving section. Next, in the receiving section, the optical reference signal propagated through the N-core fiber transmission line 1 is spatially demultiplexed by the fan-out element 12. In the receiving section, the separated light of the optical reference signal propagated through core #1 is input to the balanced photodetector 6 together with the output light from the optical reference signal source 2-2 arranged in the receiving section, and information on fluctuations in the optical length occurring during propagation through the N-core fiber transmission line 1 is detected from these homodyne detection signals. Then, the detected fluctuation information is fed back from the balanced photodetector 6 via the negative feedback control circuit 7 to the optical length control mechanism 8 connected to the N-core fiber transmission line 1, thereby compensating for the optical length fluctuations in the N-core fiber transmission line 1.

[0039] Due to the loop length constraints in the negative feedback control system, it is difficult to compensate for high-speed fluctuation components, such as GAWBS (Guided Acoustic-Wave Brillouin Scattering) noise, detected by the balanced photodetector 6 using the optical path length control mechanism 8. Therefore, phase noise associated with high-speed optical path fluctuations remains in the signal light (communication data or key information) transmitted from the transmitter to the receiver. To eliminate the influence of this high-speed phase noise component on the demodulation characteristics, the high-speed phase noise component added to the signal light can be added to the phase of the local oscillator light in an in-phase relationship and then canceled out during homodyne detection. To achieve this phase noise compensation, an optical phase modulator 16 is inserted in the optical path of the local oscillator light output from the frequency-stabilized light source 9-2 in the receiver. The high-speed component of the optical path fluctuation information detected by the balanced photodetector 6 is used as a modulation signal, and the timing is adjusted using a delay line 17 to drive the optical phase modulator 16.

[0040] In this high-speed phase noise compensation mechanism using the optical phase modulator 16, the compensation target is not limited to GAWBS noise, but can collectively compensate for all phase noise components in the high frequency band that are difficult for the optical length control mechanism 8 to respond to.

[0041] A modified example of the second embodiment of the spatial multiplexing optical transmission system according to the present invention is shown in Fig. 4. In this case, too, an optical reference signal source 2-2 is used in the receiving section. The difference from the embodiment shown in Fig. 3 is that the optical reference signal sources 2 and 2-2 are also used as frequency references for the frequency-stabilized light sources 9 and 9-2, and this difference will be described below.

[0042] In the transmitter, a portion of the output of optical reference signal source 2 is branched using two-branch coupler 3-2, and the phase of frequency-stabilized light source 9 is synchronized with the branched optical reference signal via negative feedback control circuit 7-2. Meanwhile, in the receiver, a portion of optical reference signal source 2-2 is branched using two-branch coupler 3-3, and the phase of frequency-stabilized light source 9-2 is synchronized with the branched optical reference signal via negative feedback control circuit 7-3. This makes it possible to realize a space-division multiplexing optical transmission system in which optical reference signal sources 2 and 2-2 serve as the optical phase reference for the entire system.

[0043] The first and second embodiments of the spatial multiplexing optical transmission system of the present invention use an optical reference signal, a coherent optical communication signal, and a quantum communication / quantum key distribution signal, etc., to compensate for optical length fluctuations. In this case, the quantum communication / quantum key distribution signal is very weak compared to the optical reference signal and the coherent optical communication signal, and its strength is 10 -6 times or less. When transmitting such a weak signal with a large intensity difference and an optical reference signal through an N-core fiber, there is a problem that crosstalk occurs at the input ends of the fan-in element 5 and the fan-out element 12. By employing the multi-core fiber configuration described below, it is possible to suppress the occurrence of crosstalk in the N-core fiber transmission line 1.

[0044] 5(a) and 5(b) are diagrams showing a crosstalk suppression structure of the N-core fiber transmission line 1 at a connection point of the N-core fiber transmission line 1 with the fan-in element 5 and / or the fan-out element 12. Here, for simplicity, an example where N=2 is shown.

[0045] As shown in Fig. 5(a), in the N-core fiber transmission line 1, core #1 is used for the quantum communication / quantum key distribution signal, which is a weak signal, and core #2 is used for the weak signal. 6 To suppress crosstalk between core #1 and core #2, which transmits an optical reference signal used for optical length control or coherent transmission and has an intensity more than twice as strong as core #1, a protrusion is provided to separate core #1 and core #2. The side of this protrusion may be coated with a metal such as aluminum to prevent light from passing through it.

[0046] The fan-in element 5 and the fan-out element 12 each include a lens that introduces an optical signal into the core at the connection point with the N-core fiber transmission line 1. The height of the protrusion is preferably a size equivalent to the focal length of the lens, specifically 5 mm to 15 mm, and particularly preferably around 10 mm. The width of the protrusion that separates core #1 and core #2 is preferably equal to or larger than the diameter of the core, specifically 5 μm to 15 μm or larger, so as to suppress leakage of the optical signal into the core.

[0047] Although it is desirable to provide a crosstalk suppression structure on both the fan-in element 5 and the fan-out element 12, it may be provided on either one. In FIG. 5(a), the protrusion is provided in the center to divide the core into left and right halves, but the shape is not limited to that shown in FIG. 5(a). Any shape and arrangement may be used as long as the crosstalk suppression structure can be provided to separate the core used for optical length control from the core performing coherent transmission or quantum communication / quantum key distribution. For example, a core for a weak signal (e.g., quantum communication / quantum key distribution signal) may be provided in the center, a circular protrusion serving as a crosstalk suppression structure may be provided around it, and a core for a high-intensity optical reference signal or coherent transmission may be provided around the outer periphery.

[0048] As another aspect of the crosstalk suppression structure, as shown in FIG. 5(b), a groove for fitting a partition plate may be provided between core #1 and core #2 in the N-core fiber transmission line 1, and the partition plate may be inserted into the groove to form a crosstalk suppression structure. In this case, any material for the partition plate may be used as long as it can absorb infrared light. A structure in which a commercially available infrared light absorbing sheet is attached to an aluminum plate, which is easy to process, is also effective. Furthermore, the thickness of the partition plate (the width of the groove separating core #1 and core #2) is preferably equal to or greater than the diameter of the core, specifically, 5 μm to 15 μm or more, so as to suppress leakage of optical signals into the cores. Furthermore, it is desirable that the partition plate be configured to protrude 5 mm to 15 mm, particularly about 10 mm, when inserted into the groove.

[0049] The grooves and partition plates in Fig. 5(b) are not limited to the shapes shown in Fig. 5(b), and may be of any shape or arrangement as long as a crosstalk suppression structure is provided to separate the cores used for transmitting weak signals for quantum communication and quantum key distribution from cores for transmitting optical reference signals with high optical intensity used for optical length control or for coherent transmission. For example, a core for transmitting weak signals may be provided in the center, a circular groove may be provided around it, and a cylindrical partition plate may be inserted thereto to form a crosstalk suppression structure, and a core for transmitting optical reference signals with high optical intensity or for coherent transmission may be provided around it.

[0050] As described above, as shown in Figures 5(a) and 5(b), by processing the tip and terminal ends of the N-core fiber transmission line 1 to provide a crosstalk suppression structure between cores, it is possible to suppress the occurrence of crosstalk at the connection part of the N-core fiber transmission line 1 with the fan-in element 5 and / or the fan-out element 12.

[0051] 6A and 6B are diagrams showing a fiber structure that suppresses crosstalk at a connection portion of the N-core fiber transmission line 1 with the fan-in element 5 and / or the fan-out element 12, and also suppresses crosstalk that occurs during light propagation in the longitudinal direction (propagation direction) of the N-core fiber transmission line 1. As shown in Fig. 6A, a plurality of air holes with a diameter of 5 μm to 10 μm extending in the propagation direction are provided between core #1 and core #2 as a propagation direction crosstalk suppression structure that suppresses crosstalk that occurs in the propagation direction of the N-core fiber transmission line 1. As shown in Fig. 6B, by combining this propagation direction crosstalk suppression structure with the inter-core crosstalk suppression structure of Fig. 5B, crosstalk that occurs at a connection portion with the fan-in element 5 and / or the fan-out element 12 and in the propagation direction within the N-core fiber transmission line 1 can be suppressed.

[0052] In the first and second embodiments of the spatial multiplexing optical transmission system of the present invention, the transmission distance can be extended by arranging rare-earth doped optical amplifiers or Raman amplifiers capable of bidirectional amplification at intervals of 50 km to 100 km in the N-core fiber transmission line 1 and performing multiple repeaters. At each repeater point, the N-core fiber transmission line 1 is connected to the rare-earth doped optical amplifiers or the Raman amplification pumping light source using an optical amplification fan-in element and an optical amplification fan-out element (not shown).

[0053] However, rare-earth doped optical amplifiers and Raman amplifiers cannot be used in quantum communication and quantum key distribution. Furthermore, the use of optical amplifier fan-in and fan-out elements increases the cost of equipment and the possibility of crosstalk. Below, we present an example configuration that selectively amplifies cores and eliminates ASE noise generated in the cores without creating an optical path length difference between each core, without using rare-earth doped optical amplifiers or Raman amplifiers. This configuration also eliminates the need for optical amplifier fan-in and fan-out elements.

[0054] Fig. 7 shows an example of an N-core fiber transmission line 1 that can selectively amplify optical signals in cores and remove ASE noise generated in the cores. In Fig. 7, the N-core fiber transmission line 1 is composed of a multi-core fiber in which some of the cores are doped with rare earth elements and the other cores are not doped with rare earth elements. The multi-core fiber also includes an ASE removal optical filter that cuts ASE noise imparted to the cores doped with rare earth elements, and a side pumping fiber provided near the cores doped with rare earth elements.

[0055] In FIG. 7, among cores #1 to #N of the N-core fiber transmission line 1, only cores other than those used for quantum communication and quantum key distribution, i.e., cores used for transmitting optical reference signals and coherent optical communication signals for compensating for optical length fluctuations, are doped with rare-earth elements at least partially within the fiber cores (for simplicity, an example (N=2) is shown in FIG. 7). The portions within the fiber cores correspond to relay points within the N-core fiber transmission line 1. Examples of rare-earth elements include erbium, ytterbium, thulium, bismuth, and praseodymium. The concentration of the rare-earth element is preferably 10,000 ppm·m to 20,000 ppm·m, particularly preferably around 15,000 ppm·m, where the product of the concentration and the length of the fiber core doped with the rare-earth element is 10,000 ppm·m to 20,000 ppm·m, particularly preferably around 15,000 ppm·m. Furthermore, to reduce the optical path length difference between the rare-earth-element-doped core and the undoped core, it is desirable to keep the length of the fiber core doped with the rare-earth element short. For example, it is preferable to set the length of the doped fiber core to 1.5 m at a high concentration of 10,000 ppm. In this way, by doping only specific cores of the N-core fiber transmission line 1 with a rare earth element, it is possible to selectively amplify the light of specific cores without imparting an optical path length difference between cores #1 to #N.

[0056] 7, a notch may be provided in the rare-earth-element-doped core to block only the rare-earth-element-doped core from the outer periphery of the N-core fiber transmission line 1, and a band-pass optical filter (optical filter for removing ASE) may be inserted there. The band-pass optical filter is preferably a filter that cuts the wavelength band of the fluorescence spectrum specific to the doped rare-earth element. With this configuration, it is possible to remove ASE noise generated in the rare-earth-element-doped core, enabling communication with a higher S / N ratio.

[0057] In Figure 7, the coating surrounding the rare-earth element-doped core is removed, and part of the cladding is removed, and a side pumping fiber is placed almost adjacent to the rare-earth element-doped core (adjacent at an interval of several micrometers). With this configuration, the pumping light input from the side pumping fiber can be coupled to the rare-earth element-doped core, optically exciting the rare-earth element-doped core. By adopting this configuration, an optical amplification mechanism can be realized without using optical amplification fan-in elements and optical amplification fan-out elements.

[0058] Figure 8 is a diagram showing a fiber structure in which the propagation direction crosstalk suppression structure shown in Figure 6(a) is provided for the N-core fiber transmission line 1 having the optical amplification mechanism shown in Figure 7, and crosstalk that occurs in the process of light propagating in the fiber longitudinal direction is suppressed. Using an N-core fiber transmission line 1 in which a hole structure extending in the propagation direction is provided between core #1 and core #2, processing similar to that shown in Figure 7 may be performed. [Industrial Applicability]

[0059] As explained in detail above, optical length control can be performed on a multicore fiber by placing light sources with the same optical frequency at the transmitter and receiver, using a multicore fiber as the optical transmission path, and using any one core of the multicore fiber as an optical length control channel to compensate for optical length fluctuations that occur within the multicore fiber. Furthermore, by performing communications using a multicore fiber with such optical length control, phase fluctuation compensation by digital signal processing in the receiver circuit is completely unnecessary, thereby reducing the load on digital signal processing and facilitating the realization of a receiver circuit that demodulates and analyzes data in real time. Furthermore, there is no need to distribute tone signals for the number of channels during WDM transmission, which simplifies the system configuration for coherent optical communications as well as quantum communications and quantum key distribution. [Explanation of symbols]

[0060] 1 N-core fiber transmission line 2,2-2 Optical reference signal source 3, 3-2, 3-3 2-branch coupler 4,4-2 Optical Circulator 5 Fan-in elements 6 Balanced Photodetector 7,7-2,7-3 Negative feedback control circuit 8 Optical length control mechanism 9,9-2 Frequency-stabilized light source 10,10-2 N-1 branch coupler 11 Optical Modulator 12 Fan-out elements 13 Optical Amplifier 14 Semiconductor laser (LD) 15 Homodyne receiving circuit 16 Optical Phase Modulator 17 Delay Line

Claims

1. In a spatial multiplexing optical transmission system using a multicore fiber having multiple cores, One of the plurality of cores is used as an optical length control channel for compensating for optical length fluctuations occurring in the multi-core fiber, and coherent optical communication or quantum communication / quantum key distribution is performed using other cores in the multi-core fiber whose optical lengths have been controlled by the optical length control channel. A spatial multiplexing optical transmission system.

2. 2. The spatial multiplexing optical transmission system according to claim 1, wherein a light source having the same optical frequency is used as a transmission light source and a local light source for the coherent optical communication or the quantum communication / quantum key distribution, and further, the multi-core fiber subjected to the optical length control is used as an optical transmission path.

3. 2. The spatial multiplexing optical transmission system according to claim 1, wherein a multi-core fiber with extremely small crosstalk is used as the multi-core fiber, so that the coherent optical communication and the quantum communication / quantum key distribution can be simultaneously performed in different cores without interfering with each other.

4. 2. The spatial multiplexing optical transmission system according to claim 1, wherein a frequency-stabilized single-frequency light source is used in an optical lattice clock as an optical reference signal source for detecting the optical length fluctuation occurring in the multi-core fiber.

5. 5. The spatial multiplexing optical transmission system according to claim 4, wherein the phases of a transmitting light source and a local light source for the coherent optical communication or the quantum communication / quantum key distribution are synchronized with the phase of the optical reference signal source by a negative feedback control circuit.

6. 2. The spatial multiplexing optical transmission system according to claim 1, further comprising a crosstalk suppression structure that separates a core used as the optical length control channel of the multicore fiber from a core that performs the quantum communication and quantum key distribution.

7. 2. The spatial multiplexing optical transmission system according to claim 1, wherein a rare earth element is doped in at least a portion of at least one core of the multicore fiber other than the core that performs the quantum communication and quantum key distribution.

8. 8. The spatial multiplexing optical transmission system according to claim 7, further comprising a side pumping fiber adjacent to at least one core other than the core for performing the quantum communication and quantum key distribution.

9. 9. The spatial multiplexing optical transmission system according to claim 1, wherein the optical length fluctuation occurring in the multicore fiber is a slow fluctuation caused by a temperature change or a pressure change, a fast fluctuation caused by thermal vibration of glass molecules in the optical fiber, or both of these fluctuations, and is compensated for by negative feedback control using an optical fiber stretcher or an optical modulator.

10. A multicore fiber having a plurality of cores, As an optical length control channel for compensating for optical length fluctuations occurring in the multi-core fiber, one core used for transmitting an optical reference signal and 10 cores used for transmitting the optical reference signal are provided. -6 A crosstalk suppression structure is provided to separate the cores from other cores used to transmit signals with a weak optical intensity of Multicore fiber characterized by:

11. the weak signal is a signal used for quantum communication / quantum key distribution, At least one core other than the other cores used for transmitting the weak signal is doped with a rare earth element at least in part. The multi-core fiber according to claim 10.

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