Optical communication system, optical communication method, and construction method

Abstract

This optical communication system comprises: a laser light source; an optical modulator that generates an optical signal by modulating light from the laser light source on the basis of a first electric signal; a wavelength dispersion addition unit that adds a prescribed amount of wavelength dispersion to the optical signal generated by the optical modulator; and a photoelectric conversion unit that generates a second electric signal at a level corresponding to the intensity of the optical signal having added thereto, by the wavelength dispersion addition unit, the wavelength dispersion.

Description

Optical communication system, optical communication method, and construction method

[0001] This application claims priority from Japanese Patent Application No. 2024-020740, filed February 15, 2024, the disclosure of which is incorporated herein in its entirety by reference.

[0002] Patent Document 1 (JP Patent Publication No. 10-51391A) discloses the following fixed station device for mobile communications: That is, the fixed station device for mobile communications is a mobile communication device in which a radio base station that receives a radio signal from a mobile station and a demodulator that demodulates the radio signal are connected by an optical fiber transmission line, and the fixed station device for mobile communications comprises: a compressor that compresses and amplifies a level difference of the radio signal received at the radio base station, an electrical-to-optical converter that converts the radio signal from the compressor from an electrical signal to an optical signal and inputs the converted signal to the optical fiber transmission line, and an optical-to-electrical converter that converts the optical signal transmitted by the optical fiber transmission line into an electrical signal and outputs the electrical signal to the demodulator without passing through an expander.

[0003] Furthermore, Patent Document 2 (Japanese Patent Laid-Open Publication No. 2005-175826) discloses the following radio-on-fiber transmission system: That is, the radio-on-fiber transmission system is a radio-on-fiber transmission system in which a transmitter that converts a radio signal received via an antenna into an optical signal and transmits the optical signal, and a receiver that receives the optical signal transmitted from the transmitter and demodulates the radio signal, are connected by an optical fiber, and the transmitter includes a reception level detector that detects the reception level of the radio signal received via the antenna, a transmission signal controller that controls amplification or attenuation processing to be performed on the radio signal received via the antenna in accordance with the reception level detected by the reception level detector, and a control unit that transmits control information relating to the reception level detected by the reception level detector in association with the radio signal controlled by the transmission signal controller. and an electrical / optical conversion unit that converts the radio signal associated with the control information into an optical signal and transmits it to the receiving device via an optical fiber. The receiving device comprises an optical / electrical conversion unit that converts the optical signal transmitted from the transmitting device via the optical fiber into an electrical signal, a control information extraction unit that extracts the control information transmitted by the transmitting device in association with the radio signal from the electrical signal converted by the optical / electrical conversion unit, and a receiving signal control unit that controls amplification or attenuation processing to be performed on the electrical signal converted by the optical / electrical conversion unit based on a reception level obtained from the control information extracted by the control information extraction unit so as to offset the processing performed by the transmitting signal control unit.

[0004] Japanese Patent Laid-Open No. 10-51391 Japanese Patent Laid-Open No. 2005-175826

[0005] The optical communication system disclosed herein comprises a laser light source, an optical modulator that generates an optical signal by modulating light from the laser light source based on a first electrical signal, a chromatic dispersion adding unit that adds a predetermined amount of chromatic dispersion to the optical signal generated by the optical modulator, and an opto-electrical conversion unit that generates a second electrical signal at a level corresponding to the intensity of the optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit.

[0006] One aspect of the present disclosure may be realized as a semiconductor integrated circuit that realizes part or all of an optical communication device, as an optical communication method having steps that represent characteristic processing of the optical communication device, or as a program for causing a computer to execute such steps.

[0007] FIG. 1 is a diagram illustrating a configuration of an optical communication system according to a first embodiment of the present disclosure. FIG. 2 is a table illustrating various parameters used in a simulation of the CNR of an RF signal in the optical communication system according to the first embodiment of the present disclosure. FIG. 3 is a graph illustrating an example of a simulation result of the CNR of an RF signal in the optical communication system according to the first embodiment of the present disclosure. FIG. 4 is a graph illustrating an example of a simulation result of the CNR of an RF signal in the optical communication system according to the first embodiment of the present disclosure. FIG. 5 is a diagram illustrating an example of a communication sequence in the optical communication system according to the first embodiment of the present disclosure. FIG. 6 is a flowchart illustrating an installation method for an optical communication system according to the first embodiment of the present disclosure. FIG. 7 is a table illustrating various parameters used in a simulation of the CNR of an RF signal in an optical communication system according to a modification of the first embodiment of the present disclosure. FIG. 8 is a graph illustrating an example of a simulation result of the CNR of an RF signal in the optical communication system according to a modification of the first embodiment of the present disclosure. FIG. 9 is a graph illustrating an example of a simulation result of the CNR of an RF signal in the optical communication system according to a modification of the first embodiment of the present disclosure. Fig. 10 is a diagram illustrating a configuration of an optical communication system according to a second embodiment of the present disclosure. Fig. 11 is a diagram illustrating a configuration of an optical communication system according to a modified example of the second embodiment of the present disclosure. Fig. 12 is a diagram illustrating a configuration of an optical communication system according to a modified example of the second embodiment of the present disclosure. Fig. 13 is a diagram illustrating a configuration of an optical communication system according to a third embodiment of the present disclosure.

[0008] Conventionally, techniques have been developed to improve communication quality in optical communication systems.

[0009] [Problem to be Solved by the Present Disclosure] The techniques described in Patent Documents 1 and 2 require a complex circuit configuration in order to improve communication quality.

[0010] The present disclosure has been made to solve the above-mentioned problems, and its purpose is to provide an optical communication system, an optical transmission method, and an installation method that can improve communication quality in an optical communication system with a simple configuration.

[0011] Effect of the Present Disclosure According to the present disclosure, it is possible to improve communication quality in an optical communication system with a simple configuration.

[0012] [Description of Embodiments of the Present Disclosure] First, the contents of the embodiments of the present disclosure will be listed and described. (1) An optical communication system according to an embodiment of the present disclosure includes a laser light source, an optical modulator that generates an optical signal by modulating light from the laser light source based on a first electrical signal, a chromatic dispersion adding unit that adds a predetermined amount of chromatic dispersion to the optical signal generated by the optical modulator, and an opto-electrical conversion unit that generates a second electrical signal at a level corresponding to the intensity of the optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit.

[0013] In this way, by adding chromatic dispersion to an optical signal generated by external modulation and outputting an electrical signal at a level corresponding to the intensity of the optical signal to which chromatic dispersion has been added, it is possible to attenuate noise components caused by, for example, the relative noise intensity of a laser light source. Therefore, compared to configurations that increase the modulation depth or use a laser light source with a low relative noise intensity, it is possible to improve the receiving sensitivity of electrical signals in an optical communication system with a simple configuration without being affected by the nonlinearity of the photoelectric conversion unit or increasing the power consumption of the optical modulator. Therefore, it is possible to improve the communication quality in an optical communication system with a simple configuration.

[0014] (2) In the above (1), the chromatic dispersion adding section may add the chromatic dispersion to the optical signal in an amount such that a carrier-to-noise ratio (CNR) of the second electrical signal is equal to or greater than a predetermined value.

[0015] With this configuration, by adding chromatic dispersion to the optical signal, it is possible to reduce the noise component to less than a predetermined value and improve the communication quality in the optical communication system.

[0016] (3) In the above (2), the predetermined value may be a value that is set based on the CNR of the third electrical signal when the photoelectric conversion unit generates a third electrical signal having a level corresponding to the intensity of the optical signal generated by the optical modulator to which no chromatic dispersion has been added.

[0017] With this configuration, for example, compared to a configuration in which the transmission distance of the optical signal is zero, it is possible to improve the CNR of the electrical signal and enhance the communication quality in the optical communication system.

[0018] (4) In any one of the above (1) to (3), the chromatic dispersion adding section may be an optical transmission medium provided between the optical modulator and the photoelectric conversion section.

[0019] With this configuration, chromatic dispersion can be added to an optical signal with a simple configuration in the transmission path of the optical signal.

[0020] (5) In any one of (1) to (4) above, the optical communication system may include a first laser light source that is the laser light source, a second laser light source that is the laser light source, a first optical modulator that is the optical modulator, a second optical modulator that is the optical modulator, a first opto-electrical conversion unit that is the opto-electrical conversion unit, and a second opto-electrical conversion unit that is the opto-electrical conversion unit, wherein the first optical modulator may generate a downstream optical signal that is the optical signal by modulating light from the first laser light source, and the second optical modulator may generate an upstream optical signal that is the optical signal by modulating light from the second laser light source, and the chromatic dispersion adding unit may generate a downstream optical signal that is the optical signal by modulating light from the second laser light source. The first optical-to-electrical conversion unit may add chromatic dispersion to the downstream optical signal generated by the first optical modulator and to the upstream optical signal generated by the second optical modulator, the first optical-to-electrical conversion unit may generate the second electrical signal at a level corresponding to the intensity of the downstream optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit, the second optical-to-electrical conversion unit may generate the second electrical signal at a level corresponding to the intensity of the upstream optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit, the wavelengths of the downstream optical signal and the upstream optical signal may be equal to or greater than 1530 nm and equal to or less than 1565 nm, and the difference between the wavelengths of the downstream optical signal and the upstream optical signal may be 20 nm or less.

[0021] With this configuration, it is possible to further reduce noise components in optical signals in an optical communication system that performs bidirectional communication using the C-Band (Conventional-band).

[0022] (6) In any one of (1) to (4) above, the optical communication system may include a first laser light source that is the laser light source, a second laser light source that is the laser light source, a first optical modulator that is the optical modulator, a second optical modulator that is the optical modulator, a first opto-electrical conversion unit that is the opto-electrical conversion unit, and a second opto-electrical conversion unit that is the opto-electrical conversion unit, wherein the first optical modulator may generate a downstream optical signal that is the optical signal by modulating light from the first laser light source, and the second optical modulator may generate an upstream optical signal that is the optical signal by modulating light from the second laser light source, and the chromatic dispersion adding unit may generate the downstream optical signal generated by the first optical modulator and the upstream optical signal generated by the second optical modulator. The chromatic dispersion may be added to the generated upstream optical signal, the first opto-electrical conversion unit may generate the second electrical signal at a level corresponding to the intensity of the downstream optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit, the second opto-electrical conversion unit may generate the second electrical signal at a level corresponding to the intensity of the upstream optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit, the wavelengths of the downstream optical signal and the upstream optical signal may be 1260 nm or more and 1360 nm or less, the difference between the wavelengths of the downstream optical signal and the upstream optical signal may be 5 nm or less, and the length of the optical fiber used to transmit the downstream optical signal and the upstream optical signal may be 5 km or less.

[0023] This configuration makes it possible to further reduce noise components in optical signals in an optical communication system that performs bidirectional communication using the O-Band (Original-Band).Furthermore, it is possible to ensure a CNR of at least a predetermined value while suppressing an increase in the amount of attenuation of the optical signal.

[0024] (7) In any of (1) to (6) above, the optical communication system may further include a branching unit that branches the optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit, and the optical communication system may include a plurality of the photoelectric conversion units, and each of the photoelectric conversion units may generate the second electrical signal at a level corresponding to the intensity of the optical signal branched by the branching unit.

[0025] With this configuration, it is possible to improve the communication quality in an optical communication system that performs one-to-many communication.

[0026] (8) An optical communication method according to an embodiment of the present disclosure is an optical communication method in an optical communication system equipped with a laser light source, and includes the steps of generating an optical signal by modulating light from the laser light source based on a first electrical signal, adding a predetermined amount of chromatic dispersion to the generated optical signal, and generating a second electrical signal at a level corresponding to the intensity of the optical signal to which the chromatic dispersion has been added.

[0027] In this way, by adding chromatic dispersion to an optical signal generated by external modulation and outputting an electrical signal at a level corresponding to the intensity of the optical signal to which chromatic dispersion has been added, it is possible to attenuate noise components caused by, for example, the relative noise intensity of a laser light source. Therefore, compared to configurations that increase the modulation depth or use a laser light source with a low relative noise intensity, it is possible to improve the receiving sensitivity of an electrical signal in an optical communication system with a simple configuration without being affected by the nonlinearity of the photoelectric conversion unit or increasing the power consumption of the optical modulator. Therefore, it is possible to improve the communication quality in an optical communication system with a simple configuration.

[0028] (9) An installation method according to an embodiment of the present disclosure is a method for installing an optical communication system including a laser light source, an optical modulator that generates an optical signal by modulating light from the laser light source based on a first electrical signal, and an opto-electrical conversion unit that generates a second electrical signal at a level corresponding to the intensity of the optical signal, and includes the steps of: acquiring an effective length of an optical fiber between the optical modulator and the opto-electrical conversion unit; determining, based on the acquired effective length, an amount of chromatic dispersion to be added to the optical signal so as to make the CNR of the second electrical signal equal to or greater than a predetermined value; and connecting a chromatic dispersion adding unit that adds the determined amount of chromatic dispersion to the optical fiber.

[0029] In this way, by determining the amount of chromatic dispersion based on the effective length of the optical fiber and connecting a chromatic dispersion adding section that adds the determined amount of chromatic dispersion to the optical fiber, it is possible to attenuate noise components caused by, for example, the relative noise intensity of a laser light source in an optical communication system that has a chromatic dispersion adding section added. Therefore, compared to configurations that increase the modulation depth or use a laser light source with a low relative noise intensity, it is possible to improve the receiving sensitivity of electrical signals in an optical communication system with a simple configuration without being affected by the nonlinearity of the photoelectric conversion section or increasing the power consumption of the optical modulator. Therefore, it is possible to improve the communication quality in an optical communication system with a simple configuration.

[0030] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the drawings, identical or corresponding parts are designated by the same reference numerals, and their description will not be repeated. Furthermore, at least some of the embodiments described below may be combined in any manner.

[0031] First Embodiment [Configuration and Basic Operation] Fig. 1 is a diagram illustrating the configuration of an optical communication system according to a first embodiment of the present disclosure. Referring to Fig. 1, an optical communication system 301 includes a master station 101 and a slave station 201. The master station 101 and the slave station 201 are connected to each other via an optical fiber 191 having a length La kilometers. For example, the optical fiber 191 is a standard single-mode fiber. The length La is, for example, 1 kilometer or less. In this specification, a "standard single-mode fiber" refers to a single-mode fiber conforming to G.652.B or G.652.D defined by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T). The standard single-mode fiber has a zero-dispersion wavelength of approximately 1300 nm.

[0032] The master station 101 includes an RF signal generator 10, an LD (Laser Diode) 11A, an optical modulator 12A, an optical coupler 13A, a PD (Photo Diode) 14A, an amplifier 15A, an optical fiber 16, and an adapter 17. The optical fiber 16 is an example of a chromatic dispersion adding unit.

[0033] The slave station device 201 includes an LD 11B which is the LD 11, an optical modulator 12B which is the optical modulator 12, an optical coupler 13B which is the optical coupler 13, a PD 14B which is the PD 14, and an amplifier 15B which is the amplifier 15.

[0034] For example, the optical fiber 16 is an optical transmission medium provided in the transmission path between the optical modulator 12A in the master station device 101 and the PD 14B in the slave station device 201, and also provided in the transmission path between the PD 14A in the master station device 101 and the optical modulator 12B in the slave station device 201. More specifically, the optical fiber 16 is a standard single-mode fiber with a length of Lb kilometers that is provided as an excess in the master station device 101. The adapter 17 is a connector that connects the optical fiber 16 and the optical fiber 191.

[0035] LD11 is an example of a laser light source. LD11A is an example of a first laser light source. LD11B is an example of a second laser light source. Optical modulator 12A is an example of a first optical modulator. Optical modulator 12B is an example of a second optical modulator. PD14 is an example of an opto-electrical conversion unit. PD14A is an example of a first opto-electrical conversion unit. PD14B is an example of a second opto-electrical conversion unit.

[0036] The master station device 101 and the slave station device 201 transmit and receive optical signals, including analog signals, via an optical fiber 191 .

[0037] (Downstream Optical Signal) The RF signal generator 10 in the master station 101 receives a digital signal containing communication data from a base station (not shown). The master station 101 converts the received digital signal into an analog signal to generate, for example, an RF signal in the millimeter wave band. The RF signal generated by the RF signal generator 10 is an example of a first electrical signal. The RF signal generator 10 outputs the generated RF signal to the optical modulator 12A.

[0038] The LD 11A outputs light of wavelength λ1 to the optical modulator 12A. For example, the LD 11A outputs light of the C-band to the optical modulator 12A. That is, the wavelength λ1 is equal to or greater than 1530 nm and equal to or less than 1565 nm. For example, the wavelength λ1 is 1565 nm.

[0039] The optical modulator 12A generates a downstream optical signal OPd, which is an optical signal, by modulating the light from the LD 11A based on the RF signal. More specifically, the optical modulator 12A is an external modulator conforming to the MZM (Mach-Zehnder Modulator) or EML (Electro-Absorption Modulated Laser) method. The optical modulator 12A generates a downstream optical signal OPd with a wavelength λ1 by modulating the light received from the LD 11A based on the RF signal received from the RF signal generating unit 10. The optical modulator 12A transmits the generated downstream optical signal OPd to the slave station equipment 201 via the optical coupler 13A and the optical fibers 16 and 191.

[0040] The optical fiber 16 adds a predetermined amount of chromatic dispersion to the downstream optical signal OPd generated by the optical modulator 12 A. More specifically, when the downstream optical signal OPd generated by the optical modulator 12 A is transmitted through the optical fiber 16, an amount of chromatic dispersion corresponding to the length Lb of the optical fiber 16 is added to the downstream optical signal OPd.

[0041] Furthermore, as the downstream optical signal OPd is transmitted from the master station 101 to the slave station 201 via the optical fiber 191 , chromatic dispersion is added to the downstream optical signal OPd by an amount corresponding to the length La of the optical fiber 191 .

[0042] The PD 14B in the slave station device 201 generates an RF signal at a level corresponding to the intensity of the downstream optical signal OPd to which chromatic dispersion has been added by the optical fibers 16 and 191. More specifically, the PD 14B receives the downstream optical signal OPd from the master station device 101 via the optical fiber 191 and the optical coupler 13B, and generates an RF signal by photoelectrically converting the received downstream optical signal OPd. The RF signal generated by the PD 14B is an example of a second electrical signal. The PD 14B outputs the generated RF signal to the amplifier 15B.

[0043] The amplifier 15B amplifies the RF signal received from the PD 14B. The RF signal amplified by the amplifier 15B is transmitted to a mobile communication terminal (not shown) via an antenna.

[0044] (Upstream Optical Signal) The LD 11B in the slave station device 201 outputs light of wavelength λ2 to the optical modulator 12B. For example, the LD 11B outputs light in the C-band to the optical modulator 12B. That is, the wavelength λ2 is equal to or greater than 1530 nm and equal to or less than 1565 nm. For example, when the fiber length Sum, which is the sum of the length Lb of the optical fiber 16 and the length La of the optical fiber 191, is within a numerical range Rg1 described below, the difference between the wavelength λ1 and the wavelength λ2 is equal to or less than 20 nm. As an example, the wavelength λ2 is 1545 nm.

[0045] The optical modulator 12B generates an upstream optical signal OPu, which is an optical signal, by modulating the light from the LD 11B based on the RF signal. More specifically, the optical modulator 12B is an external modulator conforming to MZM or EML. The optical modulator 12B receives a millimeter-wave RF signal containing communication data from a mobile communication terminal (not shown) via an antenna. The optical modulator 12B generates an upstream optical signal OPu with a wavelength λ2 by modulating the light received from the LD 11B based on the RF signal received from the mobile communication terminal. The optical modulator 12B transmits the generated upstream optical signal OPu to the parent station 101 via the optical coupler 13B and the optical fiber 191.

[0046] As the upstream optical signal OPu is transmitted from the slave station 201 to the master station 101 via the optical fiber 191 , chromatic dispersion is added to the upstream optical signal OPu by an amount corresponding to the length La of the optical fiber 191 .

[0047] The optical fiber 16 in the parent station 101 adds a predetermined amount of chromatic dispersion to the upstream optical signal OPu generated by the optical modulator 12B. More specifically, as the upstream optical signal OPu generated by the optical modulator 12B is transmitted through the optical fiber 16, an amount of chromatic dispersion corresponding to the length Lb of the optical fiber 16 is added to the upstream optical signal OPu.

[0048] The PD 14A generates an RF signal at a level corresponding to the intensity of the upstream optical signal OPu to which chromatic dispersion has been added by the optical fiber 16. More specifically, the PD 14A receives the upstream optical signal OPu from the slave station device 201 via the optical fibers 191 and 16 and the optical coupler 13A, and generates an RF signal by photoelectrically converting the received upstream optical signal OPu. The RF signal generated by the PD 14A is an example of a second electrical signal. The PD 14A outputs the generated RF signal to the amplifier 15A.

[0049] The amplifier 15A amplifies the RF signal received from the PD 14A and transmits the amplified RF signal to a base station device (not shown).

[0050] (CNR of RF Signal) The CNR of the RF signal after amplification by the amplifier 15 is expressed by the following equation (1).

[0051] Here, BN is the signal bandwidth of the RF signal. RIN is the relative noise intensity of the LD 11. m is the optical modulation depth. R is the conversion efficiency of photoelectric conversion in the PD 14. Pr is the light receiving power of the PD 14. e is the elementary charge. Id is the dark current contained in the RF signal output by the PD 14. Ie is the input-equivalent noise of the amplifier 15. H1 is a transfer function indicating the input / output characteristics of the LD 11. H2 is a transfer function indicating the input / output characteristics of the optical modulator 12.

[0052] The transfer functions H1 and H2 are expressed by the following equations (2) and (3), respectively.

[0053] Here, αLD is the transient chirp of the LD 11. αEX is the transient chirp of the optical modulator 12. ω is the center frequency of the RF signal received by the optical modulator 12. For example, ω is the average value of the center frequency of the bandwidth of the RF signal received by the optical modulator 12A and the center frequency of the bandwidth of the RF signal received by the optical modulator 12B. ωc is the adiabatic chirp of the LD 11. j is the imaginary unit. The phase θ in the transfer functions H1 and H2 is expressed by the following equation (4).

[0054] where L is the transmission distance of an optical signal through a standard single-mode fiber, D is the dispersion parameter, λ is the wavelength of the optical signal transmitted through the standard single-mode fiber, and c is the high velocity in a vacuum.

[0055] FIG. 2 is a table showing various parameters used in a simulation of the CNR of an RF signal in the optical communication system according to the first embodiment of the present disclosure.

[0056] 3 and 4 are graphs showing an example of simulation results of the CNR of an RF signal in the optical communication system according to the first embodiment of the present disclosure. In FIGS. 3 and 4, the horizontal axis represents the transmission distance L [km], and the vertical axis represents the CNR [dB]. FIG. 3 shows simulation results illustrating the relationship between the transmission distance L of an optical signal through a standard single-mode fiber and the CNR of the RF signal after amplification by the amplifier 15, when the various parameters shown in FIG. 2 are used. FIG. 4 shows simulation results illustrating the relationship between the transmission distance L of an optical signal through a standard single-mode fiber and the CNR of the RF signal after amplification by the amplifier 15, when the RIN is set to −160 dB / Hz, compared to the simulation results shown in FIG. 3.

[0057] 3 and 4, the CNR of the RF signal after amplification by the amplifier 15 reaches a maximum value when the transmission distance L is L1, L3, or L5, and reaches a minimum value when the transmission distance L is L2, L4, or L6. This is because the amplitude noise component contained in the optical signal due to the relative noise intensity RIN of the LD 11 is converted into a phase component and attenuated by chromatic dispersion of the optical fiber 16 or 191 when the transmission distance L is L1, L3, or L5. L1 is the smallest value of the transmission distance L when the CNR reaches a maximum value. L2 is the second smallest value of the transmission distance L when the CNR reaches a maximum value. L3 is the third smallest value of the transmission distance L when the CNR reaches a maximum value.

[0058] For example, the optical fiber 16 adds chromatic dispersion to the optical signal in an amount such that the CNR of the RF signal generated by the PD 14 is equal to or greater than a predetermined threshold Th1.

[0059] The threshold value Th1 is set based on a reference value Vs, which is the CNR of the RF signal when the PD 14 generates an RF signal at a level corresponding to the intensity of the optical signal generated by the optical modulator 12 and to which no chromatic dispersion has been added. The reference value Vs is the CNR of the RF signal after amplification by the amplifier 15 when the length Lb of the optical fiber 16 and the length La of the optical fiber 191 are zero and the optical couplers 13A and 13B are directly connected. For example, the threshold value Th1 is a value 6 dB smaller than the reference value Vs. Note that the threshold value Th1 may be a value 3 dB smaller than the reference value Vs or may be the same value as the reference value Vs.

[0060] For example, the fiber length Sum, which is the sum of the length Lb of the optical fiber 16 and the length La of the optical fiber 191, is set to a numerical range Rg1, Rg2, or Rg3 in which the CNR is equal to or greater than the threshold value Th1. The numerical ranges Rg1, Rg2, and Rg3 are ranges that include L1, L2, and L3, respectively. Hereinafter, each of the numerical ranges Rg1, Rg2, and Rg3 will also be referred to as the numerical range Rg.

[0061] More specifically, the length Lb of the optical fiber 16 is set so that if the length La of the optical fiber 191 is not within the numerical range Rg, the fiber length Sum is a value within the smallest numerical range Rg that exceeds the length La.

[0062] For example, when the length La of the optical fiber 191 is less than L1 kilometers, the length Lb of the optical fiber 16 is set so that the fiber length Sum is closer to L1 kilometers.

[0063] For example, when the length La of the optical fiber 16 is 5 kilometers, the fiber length Sum is set to a value closer to L2 kilometers.

[0064] [Operation Flow] FIG. 5 is a diagram illustrating an example of a communication sequence in the optical communication system according to the first embodiment of the present disclosure.

[0065] Referring to FIG. 5, first, the master station device 101 converts a digital signal received from the base station device into an analog signal to generate an RF signal in the millimeter wave band (step S11).

[0066] Next, the master station 101 generates a downstream optical signal OPd by modulating the light from the LD 11A based on the RF signal (step S12).

[0067] Next, the parent station device 101 transmits the generated downstream optical signal OPd to the child station device 201 via the optical fibers 16 and 191. As the downstream optical signal OPd is transmitted via the optical fibers 16 and 191, chromatic dispersion is added to the downstream optical signal OPd in ​​an amount corresponding to the length Lb of the optical fiber 16 and the length La of the optical fiber 191 (step S13).

[0068] Next, the slave station device 201 generates an RF signal by photoelectrically converting the downstream optical signal OPd received from the master station device 101 (step S14).

[0069] Next, the slave station device 201 amplifies the generated RF signal and transmits the amplified RF signal to a mobile communication terminal (not shown) via an antenna (step S15).

[0070] Next, the slave station device 201 receives a millimeter wave band RF signal containing communication data from a mobile communication terminal (not shown) via an antenna (step S16).

[0071] Next, the slave station device 201 generates an upstream optical signal OPu by modulating the light from the LD 11B based on the RF signal (step S17).

[0072] Next, the slave station device 201 transmits the generated upstream optical signal OPu to the master station device 101 via the optical fiber 191. As the upstream optical signal OPu is transmitted via the optical fibers 191 and 16, chromatic dispersion is added to the upstream optical signal OPu in an amount corresponding to the length La of the optical fiber 191 and the length Lb of the optical fiber 16 (step S18).

[0073] Next, the master station 101 generates an RF signal by photoelectrically converting the upstream optical signal OPu received from the slave station 201 (step S19).

[0074] Next, the master station device 101 amplifies the generated RF signal and transmits the amplified RF signal to a base station device (not shown) (step S20).

[0075] FIG. 6 is a flowchart illustrating a construction method for an optical communication system according to the first embodiment of the present disclosure.

[0076] 6 , first, the installer of the optical communication system 301 obtains the effective length of the already laid optical fiber 191 in the optical communication system 301 that does not include the optical fiber 16. More specifically, the installer transmits a measurement optical signal from the master station 101 to the slave station 201 via the optical fiber 191, and calculates the length La of the optical fiber 191 based on the measurement result of the CNR of the RF signal generated in the slave station 201. The installer may also actually measure the length La of the optical fiber 191 (step S31).

[0077] Next, the installer determines the amount of chromatic dispersion to be added to the optical signal so as to make the CNR equal to or greater than the threshold value Th1, based on the simulation results of the effective length and CNR of the optical fiber 191. More specifically, the installer determines the length Lb of the optical fiber 16 when the fiber length Sum is within the minimum numerical range Rg that exceeds the length La (step S32).

[0078] Next, the installer connects the optical fiber 16 to which the determined amount of chromatic dispersion is to be added to the optical fiber 191. More specifically, the installer connects the optical fiber 16 having the determined length Lb between the optical fiber 191 and the optical coupler 13A via the adapter 17 (step S33).

[0079] In addition, in step S31, instead of obtaining the effective length of the optical fiber 191 that has already been laid, the installer may obtain the effective length of the optical fiber 191 when the optical fiber 191 is laid between the parent station device 101 and the child station device 201 based on the distance between the installation position of the parent station device 101 and the installation position of the child station device 201 before laying the optical fiber 191.

[0080] In addition, the installer may determine the length Lb of the optical fiber 16 in step S32 without using the CNR simulation results. More specifically, if the transient chirp αEX of the optical modulator 12 is sufficiently smaller than the transient chirp αLD of the LD 11, the CNR will reach its maximum value when the transfer function H1 is zero. Furthermore, if the adiabatic chirp ωc of the LD 11 is sufficiently smaller than the center frequency ω of the RF signal, the transfer function H1 can be approximated by the following equation (5).

[0081] The fiber length Sum when the transfer function H1 becomes zero is expressed by the following equation (6).

[0082] Here, n is a natural number. For example, the installer determines the fiber length Sum and the length Lb of the optical fiber 16 based on equation (6).

[0083] In addition, in the optical communication system 301 according to the embodiment of the present disclosure, the master station device 101 is configured to include the optical fiber 16 and the adapter 17, but this is not limiting. Instead of the master station device 101, the slave station device 201 may be configured to include the optical fiber 16 and the adapter 17.

[0084] Furthermore, in the optical communication system 301 according to the embodiment of the present disclosure, the optical modulator 12 is configured to generate an optical signal by modulating the light from the LD 11 based on an RF signal, but this is not limited to this. The optical modulator 12 may be configured to generate an optical signal by modulating the light from the LD 11 based on an electrical signal including an RF signal and a digital control signal, for example, at 1 Gbps or less. In this case, in the optical communication system 301, the frequency of the RF signal is sufficiently higher than the frequency of the control signal, so the effect of chromatic dispersion added to the optical signal by the optical fiber 16 on the control signal can be ignored. Therefore, in the optical communication system 301, the control signal can be transmitted without significantly degrading the communication quality of the control signal.

[0085] In the optical communication system 301 according to the embodiment of the present disclosure, the optical fiber 16 is configured to add chromatic dispersion to the optical signal in an amount that makes the CNR of the RF signal equal to or greater than the threshold value Th1, but this is not limiting. The optical fiber 16 may also be configured to add chromatic dispersion to the optical signal in an amount that improves the CNR compared to when the optical fiber 16 is not provided.

[0086] In addition, in the optical communication system 301 according to the embodiment of the present disclosure, the threshold value Th1 is set based on the reference value Vs, but this is not limiting. The threshold value Th1 may be set based on the average CNR of the RF signal in the simulation results, instead of the reference value Vs.

[0087] Furthermore, although the optical communication system 301 according to the embodiment of the present disclosure has been described as including the optical fiber 16, this is not limiting. The optical communication system 301 may be configured to include an FBG (Fiber Bragg Grating) instead of the optical fiber 16. The FBG includes an optical fiber and a diffraction grating formed in the core of the optical fiber. One end of the FBG is connected to the optical coupler 13, and the other end is connected to the optical fiber 191 via the adapter 17.

[0088] Furthermore, in the optical communication system 301 according to the embodiment of the present disclosure, the fiber length Sum is set based on the results of a simulation using the wavelength λ1 of the downstream optical signal as the wavelength λ of the optical signal transmitted through the standard single-mode fiber, but this is not limited to this. The optical communication system 301 may also be configured to set the fiber length Sum based on the results of a simulation using the wavelength λ2 of the upstream optical signal instead of the wavelength λ1 of the downstream optical signal as the wavelength λ of the optical signal transmitted through the standard single-mode fiber. The optical communication system 301 may also be configured to set the fiber length Sum based on the results of a simulation using the average value λave of the wavelengths λ1 and λ2 as the wavelength λ of the optical signal transmitted through the standard single-mode fiber. Furthermore, in the optical communication system 301, if at least one of the wavelengths λ1 and λ2 is variable, the wavelength λ1 and the wavelength λ2 may be the average value of the maximum and minimum values ​​of three or more wavelengths that can be used as the wavelengths λ1 and λ2.

[0089] Furthermore, in the optical communication system 301 according to the embodiment of the present disclosure, the LDs 11A and 11B are configured to output light in the C-band, but this is not limited thereto. The LDs 11A and 11B may also be configured to output light in the O-band. That is, the wavelengths λ1 and λ2 may be 1260 nm or more and 1360 nm or less. In this case, for example, when the fiber length Sum is a value within the numerical range Rg1, the difference between the wavelengths λ1 and λ2 is 5 nm or less. Furthermore, in this case, taking into account the attenuation of the optical signal in the optical fibers 16 and 191, it is preferable that the length La of the optical fiber 191 be less than the smallest value of the transmission distances L at which the CNR reaches a minimum value. As an example, the length La of the optical fiber 191 is, for example, 5 kilometers or less.

[0090] FIG. 7 is a table illustrating various parameters used in a simulation of the CNR of an RF signal in an optical communication system according to a modified example of the first embodiment of the present disclosure.

[0091] 8 and 9 are graphs showing an example of simulation results of the CNR of an RF signal in an optical communication system according to a modification of the first embodiment of the present disclosure. In FIGS. 8 and 9, the horizontal axis represents the transmission distance L [km], and the vertical axis represents the CNR [dB]. FIG. 8 shows simulation results illustrating the relationship between the transmission distance L of the optical signal through the optical fiber 16, 191 and the CNR of the RF signal after amplification by the amplifier 15, when the various parameters shown in FIG. 7 are used. FIG. 9 shows simulation results illustrating the relationship between the transmission distance L of the optical signal through the optical fiber 16, 191 and the CNR of the RF signal after amplification by the amplifier 15, when the RIN is set to −160 dB / Hz, as compared with the simulation results shown in FIG. 8.

[0092] 8 and 9 , the CNR after amplification by the amplifier 15 reaches a minimum value when the transmission distance L is L7. For example, the optical fiber 16 adds chromatic dispersion to the optical signal in an amount such that the CNR of the RF signal generated by the PD 14 is equal to or greater than a predetermined threshold Th2. In the simulation results shown in FIGS. 8 and 9 , the transmission distance L at which the CNR reaches a maximum value is longer than in the simulation results shown in FIGS. 3 and 4 . Therefore, in the optical communication system 301 using the O-band, for example, when the length La of the optical fiber 191 is equal to or greater than L7, the length Lb of the optical fiber 16 required to ensure a CNR equal to or greater than a predetermined value is longer than when the C-band is used. Therefore, the length La of the optical fiber 191 is preferably less than L7, and more preferably equal to or less than 5 kilometers.

[0093] Next, other embodiments of the present disclosure will be described with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals and their description will not be repeated.

[0094] Second Embodiment This embodiment relates to an optical communication system 302 that includes a plurality of slave station devices 201, as compared with the optical communication system 301 according to the first embodiment. The optical communication system 302 is the same as the optical communication system 301 according to the first embodiment except for the contents described below.

[0095] 10 is a diagram illustrating a configuration of an optical communication system according to a second embodiment of the present disclosure. Compared to the optical communication system 301, the optical communication system 302 includes a master station device 102 instead of the master station device 101, and further includes a slave station device 202 and a splitter 24.

[0096] Compared to the master station device 101, the master station device 102 further includes an LD 11C instead of the LD 11, an optical modulator 12C instead of the optical modulator 12, a PD 14C instead of the PD 14, and an amplifier 15C instead of the amplifier 15, and also includes an optical coupler 23A instead of the optical coupler 13A.

[0097] Compared to the slave station device 201, the slave station device 202 has an LD11D that is LD11 instead of the LD11B, an optical modulator 12D that is optical modulator 12 instead of the optical modulator 12B, an optical coupler 13D that is optical coupler 13 instead of the optical coupler 13B, a PB14D that is PB14 instead of the PD14B, and an amplifier 15D that is amplifier 15 instead of the amplifier 15B.

[0098] The RF signal generator 20 in the master station 102 receives a digital signal containing communication data from a base station device (not shown). The master station 101 converts the received digital signal into an analog signal to generate, for example, an RF signal in the millimeter wave band. The RF signal generated by the RF signal generator 10 is an example of a first electrical signal. The RF signal generator 10 outputs the generated RF signal to the optical modulator 12C.

[0099] The LD 11C outputs light of wavelength λ3 to the optical modulator 12C. For example, the LD 11C outputs light of the C-band to the optical modulator 12C. That is, the wavelength λ1 is equal to or greater than 1530 nm and equal to or less than 1565 nm.

[0100] The optical modulator 12C generates a downstream optical signal OPd, which is an optical signal, by modulating the light from the LD 11C based on the RF signal. The optical modulator 12C transmits the generated downstream optical signal OPd to the splitter 24 via the optical coupler 23A and the optical fibers 16 and 191. The splitter 24 is an example of a branching unit.

[0101] The optical modulator 12A also transmits the downstream optical signal OPd to the splitter 24 via the optical coupler 23A and the optical fibers 16 and 191 .

[0102] The splitter 24 splits the downstream optical signal OPd to which chromatic dispersion has been added by the optical fibers 16 and 191. More specifically, the splitter 24 outputs the downstream optical signal OPd transmitted by the optical modulator 12 in the parent station device 102 to the child station devices 201 and 202.

[0103] The PD 14B in the slave station device 201 generates an RF signal at a level corresponding to the intensity of the downstream optical signal OPd split by the splitter 24. More specifically, the PD 14B receives the downstream optical signal OPd with wavelength λ1 split by the splitter 24 via the optical coupler 13B, and generates an RF signal by photoelectrically converting the received downstream optical signal OPd. The PD 14B outputs the generated RF signal to the amplifier 15B.

[0104] The amplifier 15B amplifies the RF signal received from the PD 14B. The RF signal amplified by the amplifier 15B is transmitted to a mobile communication terminal (not shown) via an antenna.

[0105] The PD 14D in the slave station device 202 generates an RF signal at a level corresponding to the intensity of the downstream optical signal OPd branched by the splitter 24. More specifically, the PD 14D receives the downstream optical signal OPd with wavelength λ3 branched by the splitter 24 via the optical coupler 13D, and generates an RF signal by photoelectrically converting the received downstream optical signal OPd. The PD 14D outputs the generated RF signal to the amplifier 15D.

[0106] The amplifier 15D amplifies the RF signal received from the PD 14D. The RF signal amplified by the amplifier 15D is transmitted to a mobile communication terminal (not shown) via an antenna.

[0107] The LD 11D in the slave station device 202 outputs light of wavelength λ4 to the optical modulator 12D. For example, the LD 11D outputs light of the C-band to the optical modulator 12D. That is, the wavelength λ4 is not less than 1530 nm and not more than 1565 nm.

[0108] The optical modulator 12D generates an upstream optical signal OPu with a wavelength of λ4 by modulating the light from the LD 11D based on the RF signal. The optical modulator 12D transmits the generated upstream optical signal OPu to the parent station 102 via the optical coupler 13D, the splitter 24, and the optical fiber 191.

[0109] The optical modulator 12B generates an upstream optical signal OPu with wavelength λ2 by modulating the light from the LD 11B based on the RF signal, and transmits the generated upstream optical signal OPu to the parent station 102 via the optical coupler 13B, the splitter 24, and the optical fiber 191.

[0110] The PD 14A in the master station 102 receives an upstream optical signal OPu having a wavelength λ2 from the slave station 201 via the splitter 24, the optical fibers 191 and 16, and the optical coupler 23A, and generates an RF signal at a level corresponding to the intensity of the received upstream optical signal OPu. The PD 14A outputs the generated RF signal to the amplifier 15A. The amplifier 15A amplifies the RF signal received from the PD 14A and transmits the amplified RF signal to a base station (not shown).

[0111] The PD 14C receives an upstream optical signal OPu having a wavelength of λ4 from the slave station 201 via the splitter 24, the optical fibers 191 and 16, and the optical coupler 23A, and generates an RF signal at a level corresponding to the intensity of the received upstream optical signal OPu. The PD 14C outputs the generated RF signal to the amplifier 15C. The amplifier 15C amplifies the RF signal received from the PD 14C and transmits the amplified RF signal to a base station (not shown).

[0112] For example, the difference between the transmission distance L between the master station equipment 102 and the slave station equipment 201 and the transmission distance L between the master station equipment 102 and the slave station equipment 202 is 300 meters or less. If the LD 11 is configured to output light in the O-band, the difference between the transmission distance L between the master station equipment 102 and the slave station equipment 201 and the transmission distance L between the master station equipment 102 and the slave station equipment 202 is 1 kilometer or less.

[0113] The installer of the optical communication system 302 determines the length Lb of the optical fiber 16 for each of the slave station devices 201 and 202 when the fiber length Sum falls within the numerical range Rg, and calculates the average value Lbave of the determined lengths Lb. The installer then connects the optical fiber 16 having the determined average length Lbave between the optical fiber 191 and the optical coupler 23A via the adapter 17.

[0114] 11 is a diagram illustrating a configuration of an optical communication system according to a modification of the second embodiment of the present disclosure. Compared to the optical communication system 302, the optical communication system 303 includes slave station devices 203 and 204 instead of the slave station devices 201 and 202, and an optical coupler 25 instead of the splitter 24.

[0115] The slave station equipment 203 does not include the optical coupler 13B, as compared with the slave station equipment 201. The slave station equipment 204 does not include the optical coupler 13D, as compared with the slave station equipment 202.

[0116] The PD 14B in the slave station equipment 203 receives the downstream optical signal OPd with wavelength λ1 from the master station equipment 102 via the optical fiber 191 and the optical coupler 25, and generates an RF signal by photoelectrically converting the received downstream optical signal OPd.

[0117] The PD 14D in the slave station equipment 204 receives the downstream optical signal OPd with wavelength λ3 from the master station equipment 102 via the optical fiber 191 and the optical coupler 25, and generates an RF signal by photoelectrically converting the received downstream optical signal OPd.

[0118] The optical modulator 12B in the slave station device 203 generates an upstream optical signal OPu with wavelength λ2 by modulating the light from the LD 11D based on the RF signal, and transmits the generated upstream optical signal OPu to the master station device 102 via the optical coupler 25 and the optical fiber 191.

[0119] The optical modulator 12D in the slave station device 204 generates an upstream optical signal OPu with wavelength λ4 by modulating the light from the LD 11D based on the RF signal, and transmits the generated upstream optical signal OPu to the master station device 102 via the optical coupler 25 and the optical fiber 191.

[0120] 12 is a diagram illustrating a configuration of an optical communication system according to a modification of the second embodiment of the present disclosure. Referring to FIG. 12 , compared to the optical communication system 302, the optical communication system 304 includes a master station 103 instead of the master station 102.

[0121] Compared to the master station device 101, the master station device 103 includes an optical coupler 33A instead of the optical coupler 23A.

[0122] The optical modulator 12 A in the parent station 103 transmits the downstream optical signal OPd to the splitter 24 via the optical coupler 33 A and the optical fibers 16 and 191 .

[0123] The PD 14B in the slave station device 201 receives the downstream optical signal OPd with wavelength λ1 split by the splitter 24 via the optical coupler 13B, and generates an RF signal by photoelectrically converting the received downstream optical signal OPd.

[0124] The PD 14D in the slave station device 202 receives the downstream optical signal OPd with wavelength λ1 split by the splitter 24 via the optical coupler 13D, and generates an RF signal by photoelectrically converting the received downstream optical signal OPd.

[0125] The PD 14A in the master station 103 receives the upstream optical signals OPu with wavelengths λ2 and λ4 via the splitter 24, optical fibers 191 and 16, and optical coupler 33A, and generates an RF signal by photoelectrically converting the received upstream optical signals OPu.

[0126] Next, other embodiments of the present disclosure will be described with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals and their description will not be repeated.

[0127] Third Embodiment This embodiment relates to an optical communication system 305 that performs FSO (Free Space Optics) communication, as compared with the optical communication system 301 according to the first embodiment. The optical communication system 305 is the same as the optical communication system 301 according to the first embodiment except for the contents described below.

[0128] 13 is a diagram illustrating a configuration of an optical communication system according to a third embodiment of the present disclosure. Compared to the optical communication system 301, the optical communication system 305 includes a master station device 105 instead of the master station device 101 and a slave station device 205 instead of the slave station device 201.

[0129] The master station equipment 105 does not include the adapter 17 and further includes a lens 51A, as compared with the master station equipment 101. The slave station equipment 205 further includes a lens 51B, as compared with the slave station equipment 201.

[0130] The optical modulator 12A in the master station 105 modulates the light from the LD 11A to generate a downstream optical signal OPd with a wavelength λ1. The optical modulator 12A transmits the generated downstream optical signal OPd to the slave station 205 via the optical coupler 13A, the optical fiber 16, and the lens 51A.

[0131] The PD 14B in the slave station device 205 receives the downstream optical signal OPd from the master station device 105 via the lens 51B and the optical coupler 13B, and generates an RF signal by photoelectrically converting the received downstream optical signal OPd.

[0132] The optical modulator 12B in the slave station 205 modulates the light from the LD 11B to generate a downstream optical signal OPd with a wavelength λ2. The optical modulator 12B transmits the generated downstream optical signal OPd to the master station 105 via an optical coupler 13B and a lens 51B.

[0133] The PD 14A in the master station 105 receives the downstream optical signal OPd from the slave station 205 via the lens 51A, the optical fiber 16, and the optical coupler 13A, and generates an RF signal by photoelectrically converting the received downstream optical signal OPd.

[0134] In the optical communication system 305, compared to the optical communication system 301, it is not necessary to take into consideration the length La of the optical fiber 191, and therefore the length Lb of the optical fiber 191 can be determined more easily and accurately.

[0135] The above-described embodiments should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

[0136] Each process (each function) in the above-described embodiments is realized by a processing circuit (circuitry) including one or more processors. The processing circuit may be configured as an integrated circuit or the like that combines one or more memories, various analog circuits, and various digital circuits in addition to the one or more processors. The one or more memories store programs (instructions) that cause the one or more processors to execute each of the processes. The one or more processors may execute each of the processes according to the program read from the one or more memories, or may execute each of the processes according to a logic circuit designed in advance to execute each of the processes. The processor may be any of various processors suitable for computer control, such as a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), a DSP (Digital Signal Processor), an FPGA (Field Programmable Gate Array), and an ASIC (Application Specific Integrated Circuit). Note that the physically separated processors may cooperate with each other to execute the processes. For example, the processors installed in the physically separated computers may cooperate with each other via a network such as a LAN (Local Area Network), a WAN (Wide Area Network), or the Internet to execute the processes. The program may be installed into the memory from an external server device or the like via the network, or may be distributed in a state stored on a recording medium such as a CD-ROM (Compact Disc Read Only Memory), a DVD-ROM (Digital Versatile Disc Read Only Memory), or a semiconductor memory, and then installed into the memory from the recording medium.

[0137] The above description includes the following additional features: [Supplementary Note 1] An optical communication system comprising: a laser light source; an optical modulator that generates an optical signal by modulating light from the laser light source based on a first electrical signal; a chromatic dispersion adding unit that adds a predetermined amount of chromatic dispersion to the optical signal generated by the optical modulator; and an opto-electrical conversion unit that generates a second electrical signal at a level corresponding to the intensity of the optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit, wherein the chromatic dispersion adding unit is an optical fiber that adds the chromatic dispersion to the optical signal in an amount such that the CNR of the second electrical signal is equal to or greater than a predetermined value.

[0138] 10, 20 RF signal generation unit 11, 11A, 11B, 11C, 11D LD 12, 12A, 12B, 12C, 12D Optical modulator 13, 13A, 13B, 13C, 13D, 23A, 25, 33A Optical coupler 14, 14A, 14B, 14C, 14D PD 15, 15A, 15B, 15C, 15D Amplifier 16 Optical fiber 17 Adapter 24 Splitter 51A, 51B Lens 101, 102, 103, 105 Master station 191 Optical fiber 201, 202, 203, 204, 205 Slave station 301, 302, 303, 304, 305 Optical communication system Vs Reference value Th1 Threshold Rg, Rg1, Rg2, Rg3 Numerical range

Claims

1. An optical communication system comprising: a laser light source; an optical modulator that generates an optical signal by modulating light from the laser light source based on a first electrical signal; a chromatic dispersion adding unit that adds a predetermined amount of chromatic dispersion to the optical signal generated by the optical modulator; and an opto-electrical conversion unit that generates a second electrical signal at a level corresponding to the intensity of the optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit.

2. The optical communication system according to claim 1, wherein the chromatic dispersion adding section adds the chromatic dispersion to the optical signal in an amount such that the CNR (Carrier to Noise Ratio) of the second electrical signal is equal to or greater than a predetermined value.

3. The optical communication system described in claim 2, wherein the predetermined value is a value set based on the CNR of the third electrical signal when the photoelectric conversion unit generates a third electrical signal at a level corresponding to the intensity of the optical signal generated by the optical modulator to which no chromatic dispersion has been added.

4. An optical communication system according to any one of claims 1 to 3, wherein the chromatic dispersion adding section is an optical transmission medium provided between the optical modulator and the photoelectric conversion section.

5. The optical communication system comprises a first laser light source that is the laser light source, a second laser light source that is the laser light source, a first optical modulator that is the optical modulator, a second optical modulator that is the optical modulator, a first opto-electrical conversion unit that is the opto-electrical conversion unit, and a second opto-electrical conversion unit that is the opto-electrical conversion unit, wherein the first optical modulator generates a downstream optical signal that is the optical signal by modulating light from the first laser light source, the second optical modulator generates an upstream optical signal that is the optical signal by modulating light from the second laser light source, the chromatic dispersion adding unit adds chromatic dispersion to the downstream optical signal generated by the first optical modulator and the upstream optical signal generated by the second optical modulator, the first opto-electrical conversion unit generates the second electrical signal at a level corresponding to the intensity of the downstream optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit, and the second opto-electrical conversion unit generates the second electrical signal at a level corresponding to the intensity of the upstream optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit, 5. The optical communication system according to claim 1, wherein the wavelength of the downstream optical signal and the wavelength of the upstream optical signal are equal to or greater than 1530 nm and equal to or less than 1565 nm, and the difference between the wavelength of the downstream optical signal and the wavelength of the upstream optical signal is equal to or less than 20 nm.

6. The optical communication system comprises a first laser light source that is the laser light source, a second laser light source that is the laser light source, a first optical modulator that is the optical modulator, a second optical modulator that is the optical modulator, a first opto-electrical conversion unit that is the opto-electrical conversion unit, and a second opto-electrical conversion unit that is the opto-electrical conversion unit, wherein the first optical modulator generates a downstream optical signal that is the optical signal by modulating light from the first laser light source, the second optical modulator generates an upstream optical signal that is the optical signal by modulating light from the second laser light source, the chromatic dispersion adding unit adds chromatic dispersion to the downstream optical signal generated by the first optical modulator and the upstream optical signal generated by the second optical modulator, the first opto-electrical conversion unit generates the second electrical signal at a level corresponding to the intensity of the downstream optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit, and the second opto-electrical conversion unit generates the second electrical signal at a level corresponding to the intensity of the upstream optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit, 5. The optical communication system according to claim 1, wherein the wavelength of the downstream optical signal and the wavelength of the upstream optical signal are 1260 nm or more and 1360 nm or less, the difference between the wavelength of the downstream optical signal and the wavelength of the upstream optical signal is 5 nm or less, and the length of the optical fiber used to transmit the downstream optical signal and the upstream optical signal is 5 km or less.

7. The optical communication system according to any one of claims 1 to 6, further comprising a branching unit that branches the optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit, and the optical communication system comprises a plurality of photoelectric conversion units, each of which generates the second electrical signal at a level corresponding to the intensity of the optical signal branched by the branching unit.

8. An optical communication method in an optical communication system equipped with a laser light source, comprising the steps of: generating an optical signal by modulating light from the laser light source based on a first electrical signal; adding a predetermined amount of chromatic dispersion to the generated optical signal; and generating a second electrical signal at a level corresponding to the intensity of the optical signal to which the chromatic dispersion has been added.

9. A method for constructing an optical communication system comprising a laser light source, an optical modulator that generates an optical signal by modulating light from the laser light source based on a first electrical signal, and an opto-electrical conversion unit that generates a second electrical signal at a level corresponding to the intensity of the optical signal, the method comprising the steps of: acquiring the effective length of the optical fiber between the optical modulator and the opto-electrical conversion unit; determining, based on the acquired effective length, an amount of chromatic dispersion to be added to the optical signal so as to make the CNR of the second electrical signal equal to or greater than a predetermined value; and connecting a chromatic dispersion adding unit that adds the determined amount of chromatic dispersion to the optical fiber.

Images