JP2012160782A - Multimode light transmission system and method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize simplification of a configuration and a high speed in a multimode light receiving device.SOLUTION: A multimode light receiving device has: mode convergence devices 22to 22to which coherent modulation signal light transmitted in a multimode fiber 100 is input and which converges a plurality of modes in the transmitted coherent modulation signal light to a basic mode; and coherent optical receivers 23to 23receiving the coherent modulation signal light converged to the basic mode by the mode convergence devices 22to 22.

Description

  The present invention relates to a multimode optical transmission system and method using a higher-order mode using a communication multimode fiber.

  In recent years, an optical transmission system using a digital coherent technology has attracted attention due to an increase in demand for expansion of communication capacity. With this technology, it is possible to suppress chromatic dispersion and polarization mode dispersion, which are considered as signal degradation of an optical transmission system, using digital signal processing technology. On the other hand, problems that cannot be compensated for by digital signal technology include nonlinear effects and fiber fuses. One means for solving this problem is to increase the core diameter of the optical fiber.

  However, if the core diameter is increased beyond a certain value, the single mode operation condition cannot be satisfied and a higher mode is generated. The occurrence of this higher-order mode has caused signal degradation in the past, but this higher-order mode can be actively utilized by applying MIMO (Multiple Input Multiple Output) technology to optical fiber communications. As a result, transmission capacity can be increased.

  There has already been a report that the MIMO technology has been applied to optical fiber communication to increase the frequency utilization efficiency, and a transmission distance of 800 Mbps and 2.8 km has been realized (for example, see Non-Patent Document 1).

  In realizing optical MIMO communication, a method of using a 90-degree optical mixer in a receiving system as shown in FIG. 11 to receive multimode light has been proposed (for example, Non-Patent Document 1). A spatial 90-degree optical mixer that can be used in such a receiving system has also been proposed (see, for example, Non-Patent Document 2). FIG. 12 shows a specific example of the spatial 90-degree optical mixer of Non-Patent Document 2.

  Further, in the reception system shown in FIG. 11, a mode-dependent multimode coupler 21 is necessary. Since directional couplers using multimode fibers have different propagation constants for each mode, a mode-dependent branching ratio is generated, and as a result, a mode-dependent multimode coupler 21 can be realized. Mode multiplex transmission is realized using the mode-dependent multimode coupler 21 (see, for example, Non-Patent Document 3).

R. C. J. et al. Hsu, A .; Shah, and B.A. Jalali, “Coherent Optical Multiple-Input Multiple-Output Communication,” IEICE Electron. Express, Vol. 1, No. 1 13, pp. 392-397, (2004) Akhil R.D. Shah, Rick C.I. J. et al. Hsu, Alireza Tarihat, Ali H. et al. Sayed, and Bahr Jalali, “Coherent Optical MIMO (COMIMO),” JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. 8, pp. 2410-2419 (2005) Hideaki Kimura, “Mode Multiplex Transmission Experiment Using 1.3μ SMF”, 1998 IEICE General Conference, B-10-210 (1998) Simon Haykin, “Adaptive Filter Theory,” Prentice Hall 4 edition, pp. 442-444 (2001)

  The multi-mode spatial 90-degree optical mixer of Non-Patent Document 2 has a problem that the enlargement of the size and the optical adjustment become complicated because a large number of optical elements are arranged. On the other hand, since a silica-based PLC type 90 degree optical mixer can be used for a single mode light receiver, the configuration is easier than a spatial 90 degree optical mixer. However, the optical receiver for single mode cannot receive the higher-order mode, so that optical MIMO communication cannot be realized.

  Also, when transmitting multimode signal light, the multimode light receiver has a large light receiving area, and therefore the reception speed is low. However, the optical receiver for single mode cannot receive the higher-order mode, so that optical MIMO communication cannot be realized.

  In this way, when MIMO technology is applied to optical fiber communication and transmission technology that actively uses higher-order modes is used, a single-mode light receiver cannot be used. It was difficult.

  The present invention has been made in view of the above-described circumstances, and an object of the present invention is to realize a simplified configuration and higher speed in a multimode optical receiver.

  In order to achieve the above object, the multimode optical receiver and multimode optical transmission system of the present invention are based on a plurality of modes in the transmitted multimode signal light with respect to the signal light transmitted through the multimode fiber. It is characterized by convergence to a mode.

  Specifically, in the multimode optical receiver of the present invention, a coherent modulated signal light transmitted through a multimode fiber is input, and a plurality of modes included in the coherent modulated signal light are converged to a basic mode. Mode converging means for outputting to the fiber, and coherent light receiving means for receiving the coherent modulated signal light converged to the fundamental mode by the mode converging means.

  Since the mode converging means is provided, it is possible to convert the fundamental mode light without removing the higher order mode light out of the transmission path. As a result, the multimode optical receiver of the present invention can use a single-mode light receiver for the coherent light-receiving means without dropping high-order mode information. Moreover, it can amplify using the optical device for single modes. Therefore, it is possible to achieve a simplified configuration and higher speed in the multimode optical receiver.

  In the multimode optical receiver according to the present invention, the mode converging means includes a collimator lens that converts the coherent modulation signal light transmitted through the multimode fiber into parallel light, and parallel light from the collimator lens as the single mode fiber. And a condensing lens for condensing light at the incident end.

  In the multimode optical receiver of the present invention, the mode converging means is a tapered optical fiber having a mode field diameter decreasing from one end to the other end, the one end being connected to the multimode fiber, The other end may be connected to the single mode fiber.

  Specifically, the multimode optical transmission system of the present invention includes a multimode optical transmission apparatus that transmits multimode coherent modulated signal light having different dispersion states for each mode to a multimode fiber, and the multimode. A multimode optical receiver according to the present invention for receiving the coherent modulated signal light transmitted through the fiber.

  Since the multimode optical transmission system of the present invention includes the multimode optical receiver according to the present invention, the configuration of the multimode optical receiver can be simplified and the speed thereof can be increased.

  Specifically, in the multimode optical reception method of the present invention, a coherent modulated signal light transmitted through a multimode fiber is input, and a plurality of modes included in the coherent modulated signal light are converged to a basic mode to obtain a single mode. A mode convergence procedure for outputting to the fiber and a coherent light reception procedure for receiving the coherent modulated signal light converged to the fundamental mode in the mode convergence procedure are sequentially provided.

  Since it has a mode convergence procedure, it is possible to convert to a fundamental mode without removing light in a higher-order mode outside the transmission path. As a result, the multimode optical receiving method of the present invention can use a single-mode optical receiver in a coherent light receiving procedure without dropping high-order mode information. Moreover, it can amplify using the optical device for single modes. Therefore, it is possible to achieve a simplified configuration and higher speed in the multimode optical receiver.

  Specifically, the multimode optical transmission method of the present invention includes a multimode optical transmission procedure for transmitting a multimode coherent modulated signal light having different dispersion states for each mode to a multimode fiber, and the multimode. And a multimode optical receiving method according to the present invention for receiving the coherent modulated signal light transmitted in the optical transmission procedure.

  Since the multimode optical transmission method of the present invention executes the multimode optical reception method according to the present invention, the configuration of the multimode optical receiver can be simplified and the speed thereof can be increased.

  According to the present invention, it is possible to use a single-mode light receiver without dropping high-order mode information by converting high-order mode light to the basic mode without removing it out of the transmission path. . Furthermore, optical amplification, which was difficult in multimode, is enabled, and high-efficiency and high-speed multimode optical transmission can be realized.

1 illustrates an example of a multimode optical transmission system according to a first embodiment. A specific example of the multimode optical receiver 20 will be shown. 2 is an example of a mode converging device according to the first embodiment. Is a block diagram showing the configuration of the experimental system for verifying the characteristics of the mode concentrator 22 _{2-22 5.} An example of the verification result of the characteristics of the mode converging device 54 is shown. The constellation map of the transmission signal obtained by the simulation system of FIG. 1 is shown. The constellation map of the received signal after the coherent light receiver obtained by the simulation system of FIG. 1 is shown. The relationship between the training signal by a MIMO adaptive equalization process and an error (x-xc) is shown. The constellation map of the received signal after the MIMO adaptive equalization process obtained by the simulation system of FIG. 1 is shown. It is an example of the mode converging device which concerns on Embodiment 2. FIG. An example of a conventional multimode optical receiving system is shown. A specific example of a 90-degree optical mixer that can be used in a multimode optical reception system will be described.

  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 following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

(Embodiment 1)
FIG. 1 shows an example of a multimode optical transmission system according to the first embodiment. In the multimode optical transmission system shown in FIG. 1, a multimode optical transmitter 10 and a multimode optical receiver 20 are connected by a multimode fiber 100.

  The multimode optical transmission method according to the first embodiment includes a multimode optical transmission procedure in which the multimode optical transmitter 10 transmits multimode coherent modulated signal light to the multimode fiber 100, and the multimode optical receiver 20 is in multimode. And a multi-mode optical receiving method for receiving coherent modulated signal light transmitted in the optical transmission procedure. The multimode optical reception method has a mode convergence procedure and a coherent light reception procedure in this order. The coherent modulation signal light is, for example, BPSK (Binary Phase Shift Keying) signal light. In the present embodiment, as an example, a 4-input, 5-output optical MIMO transmission having the configuration shown in FIG. 1 will be described.

Multimode optical transmission device 10 includes a laser light source 11, a single-mode coupler 12, an optical modulator _131-134, a mode exciter ₁₄ 1-14 _4, a multi-mode coupler 15, a. The light from the laser light source 11 and 4 branches by single mode coupler 12, carried out individually modulate the respective light 4 branched by the optical modulator _131-134, to generate a signal of 4 channels. Signal format of an optical modulator _131-134 is BPSK. After the signal of each channel mode exciter 14 ₁ enters the -14 ₄ is excited higher order modes are multiplexed by the multi-mode coupler 15 having four inputs and one output.

Here, the mode exciters 14 _{1 to} 14 ₄ have different mode ratios for excitation. For example, the mode exciter 14 ₁ is excited as the mode ratio of the first higher-order mode is high, the mode exciter 14 ₂ is excited as mode ratio of the second order mode is high, mode excitation vessel 14 ₃ is excited as the mode ratio of the third order mode is high, the mode exciter 14 ₄ excites as mode ratio of the fourth order mode increases. As described above, the coherent modulated signal light of each channel includes all modes, but the mode ratio is different for each channel. It transmits to the multimode fiber 100 in such a mode distribution state.

The multimode coupler 15 is preferably a mode-dependent type having a different branching ratio for each input port and for each mode. For example, the multi-mode coupler 15, to match the mode ratio of excitation of each mode exciter ₁₄ 1-14 _4, to couple the multimode light from each mode exciter ₁₄ 1-14 _4. That is, the first higher order mode has a high coupling ratio of the multi-mode light from the mode exciter 14 _1, the second higher order mode has a high coupling ratio of the multi-mode light from the mode exciter 14 _2, third order mode has a high multimode optical coupling ratio from the mode exciter 14 _3, 4-th order mode has a high coupling ratio of the multi-mode light from the mode exciter 14 _4.

  The combined 4-channel signal light is incident on the transmission multimode fiber 100. A multimode fiber generally has several hundred higher-order modes during transmission, but this time, up to fourth higher-order modes are considered. In the transmission multimode fiber, the propagation speed in the fiber is different, so the higher-order mode is delayed with respect to the fundamental mode.

  For each higher-order mode in the transmission multimode fiber, the first higher-order mode is 1 bit, the second higher-order mode is 3 bits, and the third higher-order mode is The 7-bit, fourth higher-order mode gave a 13-bit delay. In addition, energy exchange (mode conversion) between modes during transmission and loss depending on the mode are not considered.

The multimode optical receiver 20 includes a multimode coupler 21, mode converging devices 22 _{1 to} 22 ₅ , coherent light receivers 23 ₁ to 23 _5, and a MIMO adaptive equalizer 25. A signal after being transmitted through the multimode fiber 100 is branched by a multimode coupler 21 having one input and five outputs.

Here, the multimode coupler 21 is preferably a mode-dependent type having a different branching ratio for each output port and for each mode. For example, the multi-mode coupler 21, to match the mode ratio for receiving the respective coherent light receiver ₂₃ 1-23 ₅ branches the multi-mode light to each mode concentrator ₂₂ 1-22 _5. That is, the first higher order mode has a high branching ratio to the mode concentrator 22 _1, the second higher order mode has a high branching ratio to the mode concentrator 22 _2, third-order modes of the mode concentrator high branching ratio to 22 _3, 4-th order mode has a high branching ratio of the mode concentrator 22 _4.

Mode concentrator 22 _{1-22 5} is coherent modulated signal light is transmitted through the multimode fiber 100 is input, a plurality of modes included in the coherent modulated signal light is converged to the fundamental mode and outputs a single mode fiber. For example, the mode concentrator 22 _{1-22 5,} the multi-mode light input from the multi-mode fiber 100 to reduce the mode field diameter by passing through a space lens system, and outputs the incident single-mode fiber. The mode converging units 22 _{1 to} 22 ₅ can convert the fundamental mode component with low loss while maintaining the amplitude and phase information of the higher-order mode light.

Here, the loss of each mode in the mode concentrator ₂₂ 1-22 _5, the fundamental mode 5 dB, the first higher-order mode is 7 dB, the second-order mode is 11 dB, the third higher order modes 13 dB and the fourth higher-order mode were 17 dB.

On the other hand, in FIG. 1, if not through the mode concentrator 22 _{1-22 5,} it becomes difficult to bond to the single mode fiber, in particular splice loss at higher modes becomes large. In this case, the loss of each mode is 10 dB for the fundamental mode, 70 dB for the first higher-order mode, 110 dB for the second higher-order mode, 130 dB for the third higher-order mode, and the fourth higher-order mode. The mode was 170 dB.

The coherent light receivers 23 ₁ to 23 ₅ receive the coherent modulated signal light converged to the fundamental mode by the mode converging units 22 _{1 to} 22 ₅ , respectively. The MIMO adaptive equalizer 25 receives light reception signals from the coherent light receivers 23 ₁ to 23 ₅ and performs signal restoration by digital signal processing.

FIG. 2 shows a specific example of the multimode optical receiver 20. Here, for easy understanding, a mode converger 22 ₁ of the modes concentrator ₂₂ 1-22 _5, the case of the coherent light receiver 23 _one of the coherent light receiver ₂₃ 1-23 ₅ will be described. The coherent light receiver 23 ₁ includes a local light source 24 ₁ , a 90-degree optical mixer 31 ₁ , and balanced receivers 32 ₁ I and 32 ₁ Q.

The optical signal coupled to the single mode fiber by the mode converging unit 22 ₁ is multiplied in the 90-degree optical mixer 31 ₁ together with the local light from the local light source 24 ₁ and is output to the balanced receivers 32 ₁ I and 32 ₁ Q. The The balanced receivers 32 ₁ I and 32 ₁ Q convert the respective optical signals into in-phase components (I components) and quadrature components (Q components) of the baseband electrical signal. Subsequently, the MIMO adaptive equalizer 25 performs sampling and quantization by digital signal processing to restore the signal.

Here, if the incident multimode 90-degree optical mixer 31 ₁ designed for single mode, the high-order mode component will be missing. Therefore, so as not to lack the high-order mode component, it is used in this embodiment mode concentrator 22 _1. Due to the use of mode concentrator 22 _1, it is possible to realize an optical amplifier using a single mode optical amplifier 26 ₁ while retaining the information in higher order modes. Note that the mode-dependent loss in the coherent light receivers 23 ₁ to 23 ₅ is not taken into consideration.

In the MIMO adaptive equalizer 25, a tap coefficient of the FIR filter can use an RLS (Recursive Least Square) algorithm (see, for example, Non-Patent Document 4). The number of taps was 15, the number of training signals was 100, the forgetting factor λ was 1.0 × 10 ⁻³ , and the step size σ was 10. The transmission signal is x1, x2, x3, x4, the reception signal after the coherent optical receivers 23 ₁ to 23 ₅ is y1, y2, y3, y4, y5, and the restored signal after the MIMO adaptive equalizer 25 is xc1, xc2. , Xc3, and xc4.

Here, the operation of the mode converging device according to the present invention will be specifically described with reference to FIGS. 3, 4, and 5.
Figure 3 is a diagram showing the mode structure of the concentrator 22 _{1-22 5} using spatial lens system. The multimode signal light from the multimode fiber 41 is converted into a parallel beam by the collimator lens 42, is incident on the single mode fiber 44 by the condenser lens 43, and is coupled to the single mode fiber 44.

Figure 4 is a block diagram showing the configuration of the experimental system for verifying the characteristics of the mode concentrator 22 _{1-22 5.} In this experimental system, laser light emitted from a laser light source 51 having a wavelength of 1550 nm is incident on an optical modulator 53 and modulated by a pulse signal generated by a pulse generator 52, and the mode output is changed by a mode exciter 54. Excited and incident on a multi-mode fiber 55 having a fiber length of 3 km, a higher-order mode component is converged to a fundamental mode by a mode converging unit 56, and incident on a single-mode fiber 57 having a fiber length of 2 m. An optical oscilloscope 58 was used.

FIG. 5 shows an example of the verification result of the characteristics of the mode converging device 56. 5A shows a pulse time waveform before the mode exciter 54, FIG. 5B shows a time waveform after mode excitation and transmission through the multimode fiber 55, and FIG. The time waveform after transmission of the single mode fiber 57 when not used is shown, and FIG. 5D shows the time waveform after transmission when the mode converging device 56 is used.
The pulse width before transmission is 100 ps. As shown in FIG. 5 (b), after mode excitation and transmission through a multimode fiber, a pulse is present after 600 ps and 1.3 ns after the first pulse (pulse on the horizontal scale '1'). By doing so, it can be seen that a higher-order mode exists.

  Comparing the time waveform shown in FIG. 5 (b) with the time waveform shown in FIG. 5 (c), when the mode converging device is not used, the pulse is detected at 600 ps and 1.3 ns after the first pulse. It can be seen that the higher order mode components are lost after the single mode fiber transmission. On the other hand, when the time waveform shown in FIG. 5B is compared with the time waveform shown in FIG. 5D, when the mode converging device is used, the time is 600 ps after the first pulse and 1.3 ns later. It can be seen that the higher order mode components are not lost after the single mode transmission of the pulses. From the above, it can be seen that this mode converging device has a function as mode convergence for converging higher-order mode components to the fundamental mode.

FIG. 6 shows a constellation map of transmission signals obtained by the simulation system of FIG. 6 (a) shows the transmission signal x1, FIG. 6 (b) shows the transmission signal x2, FIG. 6 (c) shows the transmission signal x3, and FIG. 6 (d) shows the transmission signal x4.
FIG. 7 shows a constellation map of the received signal after the coherent light receiver obtained by the simulation system of FIG. FIGS. 7 (a) to 7 (e) show received signals y1, y2, y3, y4, and y5 when there is no mode converging unit. FIGS. 7 (f) to 7 (j) respectively show mode converging units. The received signals y1, y2, y3, y4, and y5 are shown.
FIG. 8 shows the relationship between the training signal by the MIMO adaptive equalization process and the error (x-xc). FIGS. 8A to 8D show cases where there is no mode converging device, and FIGS. 8E to 8H show cases where there is a mode converging device, respectively.
FIG. 9 shows a constellation map of the received signal after the MIMO adaptive equalization processing obtained by the simulation system of FIG. FIGS. 9A to 9D each show a case where there is no mode converging device, and FIGS. 9E to 9H each show a case where there is a mode converging device.

  FIG. 9 shows that when the mode converging unit is not used, the transmission signal shown in FIG. 6 cannot be decoded into the constellation map. That is, when the mode converging unit is not used, higher-order mode components are lost, so that signal restoration by the MIMO adaptive equalizer becomes impossible. On the other hand, when the mode converging device is used, it can be seen that the transmission signal shown in FIG. 6 is decoded into the constellation map. That is, it was shown that the signal can be restored by the MIMO adaptive equalizer without missing necessary intensity and phase information by converting the higher order mode component into the fundamental mode component.

From the above, it mode concentrator 22 _{1-22 5} for converging higher mode components in the fundamental mode has a function as a coherent multi-mode optical receiver 20 seen by the embodiment. Furthermore, according to the present embodiment, it is possible to realize optical amplification using the single-mode optical amplifier (reference numerals 26 ₁ and 26 ₂ shown in FIG. ₂ ) while having high-order mode information.

  Although the signal format in this embodiment is BPSK, any coherent modulation can be used instead of BPSK. Coherent modulation is not limited to a modulation format such as QPSK (Quadrature Phase Shift Keying) or QAM (Quadrature Amplitude Modulation).

(Embodiment 2)
A second embodiment of mode convergence means according to the present invention will be described.
In the present embodiment, a tapered optical fiber as shown in FIG. 10 is used as a mode converging device instead of the spatial lens system mode converging device shown in FIG. 3 in the first embodiment.

  In the case of the tapered optical fiber 45 whose mode field diameter decreases from one end to the other end, one end is connected to the multimode fiber 41 and the other end is connected to the single mode fiber 44. The mode field diameter is reduced by passing the multimode light input from the multimode fiber 41 through the tapered optical fiber 45, and enters and outputs the single mode fiber 44. With this mode converging device, it is possible to convert the fundamental mode component with low loss while maintaining the amplitude and phase information of the higher-order mode light. The mode converging device using the tapered optical fiber 45 can obtain the same characteristics as the spatial lens system mode converging device.

  The preferred embodiments of the multimode optical receiver and the mode converging device according to the present invention have been described above. However, the embodiments of the present invention are not limited to the above-described embodiments, and the scope of the present invention is not limited thereto. It is all within the scope of the present invention to replace, change, add, increase / decrease the number of components, change the design of the shape, change the design parameter, etc.

  INDUSTRIAL APPLICABILITY The present invention is applicable to a multimode optical receiver that realizes high-speed and high-efficiency multimode transmission for signal light having different electric field distributions for each mode in a multimode optical transmission system.

10: Multi-mode optical transmitter 11: Laser light source 12: Single mode coupler 13 ₁ , 13 ₂ , 13 ₃ , 13 ₄ , 53: Optical modulator 14 ₁ , 14 ₂ , 14 ₃ , 14 ₄ , 54: Mode exciter 15, 21: Multi-mode coupler 20: Multi-mode optical receivers 22 ₁ , 22 ₂ , 22 ₃ , 22 ₄ , 22 ₅ , 56: Mode converging devices 23 ₁ , 23 ₂ , 23 ₃ , 23 ₄ , 23 ₅ : Coherent Light receivers 24 ₁ and 24 ₂ : Local light source 25: MIMO adaptive equalizers 26 ₁ and 26 ₂ : Optical amplifiers 31 ₁ and 31 ₂ : 90 degree optical mixers 32 ₁ I, 32 ₁ Q, 32 ₂ I, and 32 ₂ Q ,: balanced receiver 41: multimode fiber 42: collimating lens 43: condenser lens 44, 57: single mode fiber 51: laser light source 52: pulse Enereta 55,100: multi-mode fiber 58: light oscilloscope

Claims (6)

Mode converging means for receiving coherent modulated signal light transmitted through a multimode fiber, converging a plurality of modes included in the coherent modulated signal light into a fundamental mode, and outputting to a single mode fiber;
Coherent light receiving means for receiving the coherent modulated signal light converged to the fundamental mode by the mode convergence means;
A multi-mode optical receiver characterized by comprising: The mode convergence means includes:
A collimating lens that collimates the coherent modulated signal light transmitted through the multimode fiber; and
A condensing lens that condenses the parallel light from the collimating lens at the incident end of the single mode fiber;
The multimode optical receiver according to claim 1, further comprising: The mode converging means is a tapered optical fiber whose mode field diameter decreases from one end to the other end, the one end is connected to the multimode fiber, and the other end is connected to the single mode fiber. The multimode optical receiver according to claim 1, wherein: A multi-mode optical transmitter that transmits multi-mode coherent modulated signal light having different dispersion states for each mode to a multi-mode fiber;
The multimode optical receiver according to any one of claims 1 to 3, which receives the coherent modulated signal light transmitted through the multimode fiber;
A multimode optical transmission system. A mode convergence procedure in which coherent modulated signal light transmitted through a multimode fiber is input, and a plurality of modes included in the coherent modulated signal light are converged to a fundamental mode and output to a single mode fiber;
A coherent light receiving procedure for receiving the coherent modulated signal light converged to the fundamental mode in the mode convergence procedure;
In order. A multi-mode optical transmission procedure for transmitting multi-mode coherent modulated signal light having different dispersion states for each mode to the multi-mode fiber for each channel;
The multimode optical reception method according to claim 5, wherein the coherent modulated signal light transmitted in the multimode optical transmission procedure is received;
A multimode optical transmission method comprising:

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