Optical communication system, optical communication method, and construction method

Abstract

The optical communication system of the present application has: a laser light source; an optical modulator that generates an optical signal by modulating light from the laser light source based on a first electric signal; a dispersion adding section that adds a prescribed amount of dispersion to the optical signal generated by the optical modulator; and an optical-electric conversion section that generates a second electric signal of a level corresponding to the intensity of the optical signal to which the dispersion has been added by the dispersion adding section.

Description

Technical Field

[0001] This invention relates to optical communication systems, optical communication methods, and construction methods.

[0002] This application claims priority to Japanese Patent Application No. 2024-020740, filed on February 15, 2024, and incorporates in its entirety the disclosure of that application. Background Technology

[0003] Patent Document 1 (Japanese Patent Application Publication No. 10-51391) discloses a fixed base station device for mobile communication. Specifically, the fixed base station device for mobile communication is an apparatus that connects a wireless base station receiving wireless signals from a mobile station to a demodulator demodulating the wireless signals via an optical fiber transmission line. This fixed base station device for mobile communication includes: a compressor that compresses and amplifies the level difference of the wireless signals received by the wireless base station; an electro-optical converter that converts the wireless signals from the compressor into electrical signals and inputs them to the optical fiber transmission line; and an optical-to-electrical converter that converts the optical signals transmitted through the optical fiber transmission line into electrical signals and outputs them to the demodulator without passing through an extender.

[0004] Furthermore, Patent Document 2 (Japanese Patent Application Publication No. 2005-175826) discloses a fiber optic wireless transmission system as follows. That is, the fiber optic wireless transmission system is a system that connects a transmitting device and a receiving device via optical fiber. The transmitting device converts a wireless signal received via an antenna into an optical signal and transmits it. The receiving device receives the optical signal transmitted from the transmitting device and demodulates the wireless signal. The transmitting device includes: a receiving level detection unit that detects the receiving level of the wireless signal received via the antenna; a transmission signal control unit that controls the amplification or attenuation processing of the wireless signal received via the antenna based on the receiving level detected by the receiving level detection unit; and a control information transmission unit that transmits control information related to the receiving level detected by the receiving level detection unit and the wireless signal controlled by the transmission signal control unit. The transmitting device is associated with and transmits the control information; and an electro-optical conversion unit converts the wireless signal associated with the control information into an optical signal and transmits it to the receiving device via an optical fiber. The receiving device includes: an optical-to-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 wireless signal from the electrical signal converted by the optical-to-electrical conversion unit; and a reception signal control unit that controls the amplification or attenuation processing performed on the electrical signal converted by the optical-to-electrical conversion unit based on the reception level obtained from the control information extracted by the control information extraction unit, so as to counteract the processing performed by the transmission signal control unit.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 10-51391;

[0008] Patent Document 2: Japanese Patent Application Publication No. 2005-175826. Summary of the Invention

[0009] The optical communication system of the present invention comprises: a laser source; an optical modulator that generates an optical signal by modulating light from the laser source based on a first electrical signal; a dispersion addition unit that adds a predetermined amount of dispersion to the optical signal generated by the optical modulator; and a photoelectric conversion unit that generates a second electrical signal with a level corresponding to the intensity of the optical signal to which the dispersion has been added by the dispersion addition unit.

[0010] One aspect of the present invention can be implemented as a semiconductor integrated circuit that implements part or all of an optical communication device, or as an optical communication method that takes the characteristic processing of an optical communication device as steps, or as a program for causing a computer to execute the above steps. Attached Figure Description

[0011] Figure 1 This diagram illustrates the structure of an optical communication system according to the first embodiment of the present invention.

[0012] Figure 2 A table showing the various parameters used in simulating the CNR of the RF signal in the optical communication system according to the first embodiment of the present invention.

[0013] Figure 3 This is a graph illustrating an example of the simulation results of the CNR of an RF signal in an optical communication system according to the first embodiment of the present invention.

[0014] Figure 4 This is a graph illustrating an example of the simulation results of the CNR of an RF signal in an optical communication system according to the first embodiment of the present invention.

[0015] Figure 5 This diagram illustrates an example of a communication sequence in an optical communication system according to the first embodiment of the present invention.

[0016] Figure 6 This is a flowchart illustrating the construction method of the optical communication system according to the first embodiment of the present invention.

[0017] Figure 7A table showing the various parameters used in simulating the CNR of an RF signal in an optical communication system according to a variation of the first embodiment of the present invention.

[0018] Figure 8 A graph illustrating an example of the simulation results of the CNR of an RF signal in an optical communication system according to a variation of the first embodiment of the present invention.

[0019] Figure 9 A graph illustrating an example of the simulation results of the CNR of an RF signal in an optical communication system according to a variation of the first embodiment of the present invention.

[0020] Figure 10 This diagram illustrates the structure of an optical communication system according to a second embodiment of the present invention.

[0021] Figure 11 This diagram illustrates the structure of an optical communication system according to a modified example of the second embodiment of the present invention.

[0022] Figure 12 This diagram illustrates the structure of an optical communication system according to a modified example of the second embodiment of the present invention.

[0023] Figure 13 This diagram illustrates the structure of an optical communication system according to the third embodiment of the present invention. Detailed Implementation

[0024] Previously, technologies were developed to improve the communication quality of optical communication systems.

[0025] [The problem the invention aims to solve]

[0026] In the technologies described in Patent Documents 1 and 2, complex circuit structures are required to improve communication quality.

[0027] This invention was made to solve the above-mentioned problems, and its purpose is to provide an optical communication system, optical transmission method, and construction method that can improve the communication quality of an optical communication system with a simple structure.

[0028] [Effects of the Invention]

[0029] According to the present invention, the communication quality of an optical communication system can be improved with a simple structure.

[0030] [Description of Embodiments of the Invention]

[0031] First, examples will be given to illustrate the embodiments of the present invention.

[0032] (1) The optical communication system according to the embodiments of the present invention includes: a laser source; an optical modulator that generates an optical signal by modulating light from the laser source based on a first electrical signal; a dispersion addition unit that adds a predetermined amount of dispersion to the optical signal generated by the optical modulator; and a photoelectric conversion unit that generates a second electrical signal with a level corresponding to the intensity of the optical signal to which the dispersion has been added by the dispersion addition unit.

[0033] By adding dispersion to an externally modulated optical signal and outputting an electrical signal with a level corresponding to the intensity of the dispersed optical signal, noise components, such as those caused by the relative intensity noise of a laser source, can be attenuated. Therefore, compared to structures that increase the modulation depth or use a laser source with low relative intensity noise, this structure is unaffected by the nonlinearity of the photoelectric conversion unit and does not increase the power consumption of the optical modulator, allowing for improved signal reception sensitivity in optical communication systems with a simple structure. Thus, the communication quality of optical communication systems can be improved with a simple structure.

[0034] (2) In (1) above, the dispersion addition unit may also add a amount of dispersion to the optical signal such that the CNR (Carrier to Noise Ratio) of the second electrical signal is above a predetermined value.

[0035] With this structure, noise components can be reduced to less than a specified value by adding dispersion to the optical signal, thereby improving the communication quality of the optical communication system.

[0036] (3) In (2) above, the specified value may also be a value set based on the CNR of the third electrical signal when the photoelectric conversion unit generates the third electrical signal, which is an electrical signal with a level corresponding to the intensity of the light signal generated by the light modulator without added dispersion.

[0037] Such a structure, for example, compared to a structure where the transmission distance of an optical signal is zero, can improve the CNR of an electrical signal and enhance the communication quality of an optical communication system.

[0038] (4) In any of (1) to (3) above, the dispersion addition part may also be an optical transmission medium disposed between the optical modulator and the photoelectric conversion part.

[0039] With this structure, dispersion can be added to the optical signal in a simple way within the optical signal transmission line.

[0040] (5) In any of (1) to (4) above, the optical communication system may include: a first laser source as the laser source; a second laser source as the laser source; a first optical modulator as the optical modulator; a second optical modulator as the optical modulator; a first photoelectric conversion unit as the photoelectric conversion unit; and a second photoelectric conversion unit as the photoelectric conversion unit, wherein the first optical modulator can generate a downlink optical signal as the optical signal by modulating light from the first laser source, and the second optical modulator can generate an uplink optical signal as the optical signal by modulating light from the second laser source, wherein the dispersion The dispersion-adding unit can add the dispersion to the downlink optical signal generated by the first optical modulator and the uplink optical signal generated by the second optical modulator. The first photoelectric conversion unit can generate a second electrical signal with a level corresponding to the intensity of the downlink optical signal to which the dispersion has been added by the dispersion-adding unit. The second photoelectric conversion unit can generate a second electrical signal with a level corresponding to the intensity of the uplink optical signal to which the dispersion has been added by the dispersion-adding unit. The wavelengths of the downlink optical signal and the uplink optical signal can be 1530 nm or more and 1565 nm or less, and the difference between the wavelengths of the downlink optical signal and the uplink optical signal can be 20 nm or less.

[0041] With this structure, noise components in optical signals can be further reduced in optical communication systems using the C-band (Conventional-band) for bidirectional communication.

[0042] (6) In any of (1) to (4) above, the optical communication system may include: a first laser source as the laser source; a second laser source as the laser source; a first optical modulator as the optical modulator; a second optical modulator as the optical modulator; a first photoelectric conversion unit as the photoelectric conversion unit; and a second photoelectric conversion unit as the photoelectric conversion unit. The first optical modulator can generate a downlink optical signal as the optical signal by modulating the light from the first laser source, and the second optical modulator can generate an uplink optical signal as the optical signal by modulating the light from the second laser source. The dispersion addition unit can also add light generated by the first optical modulator. The dispersion is added to the downlink optical signal and the uplink optical signal generated by the second optical modulator. The first photoelectric conversion unit can generate a second electrical signal with a level corresponding to the intensity of the downlink optical signal with the dispersion added by the dispersion addition unit. The second photoelectric conversion unit can generate a second electrical signal with a level corresponding to the intensity of the uplink optical signal with the dispersion added by the dispersion addition unit. The wavelengths of the downlink optical signal and the uplink optical signal can be above 1260nm and below 1360nm. The difference between the wavelengths of the downlink optical signal and the uplink optical signal can also be below 5nm. The length of the optical fiber used to transmit the downlink optical signal and the uplink optical signal can be below 5km.

[0043] This structure further reduces noise components in the optical signal in optical communication systems using the O-band (Original-band) for bidirectional communication. Furthermore, it suppresses the increase in optical signal attenuation and ensures a CNR above a specified value.

[0044] (7) In any of (1) to (6) above, the optical communication system may have: a splitting unit that splits the optical signal to which the dispersion has been added by the dispersion adding unit, and the optical communication system may have a plurality of photoelectric conversion units, each of which can generate a second electrical signal with a level corresponding to the intensity of the optical signal after being split by the splitting unit.

[0045] This structure enables improved communication quality in optical communication systems that perform one-to-many communication.

[0046] (8) The optical communication method involved in the embodiments of the present invention is an optical communication method with a laser light source optical communication system. The optical communication method includes: a step of generating an optical signal by modulating light from the laser light source based on a first electrical signal; a step of adding a predetermined amount of dispersion to the generated optical signal; and a step of generating a second electrical signal with a level corresponding to the intensity of the optical signal with added dispersion.

[0047] By adding dispersion to an externally modulated optical signal and outputting an electrical signal with a level corresponding to the intensity of the dispersed optical signal, noise components, such as those caused by the relative intensity noise of a laser source, can be attenuated. Therefore, compared to structures that increase the modulation depth or use a laser source with low relative intensity noise, this method is unaffected by the nonlinearity of the photoelectric conversion unit and does not increase the power consumption of the optical modulator, allowing for improved signal reception sensitivity in optical communication systems with a simpler structure. Thus, the communication quality of optical communication systems can be improved with a simpler structure.

[0048] (9) The construction method involved in the embodiments of the present invention is a construction method for an optical communication system, the optical communication system having: a laser source; an optical modulator that modulates light from the laser source based on a first electrical signal to generate an optical signal; and a photoelectric conversion unit that generates a second electrical signal with a level corresponding to the intensity of the optical signal, the construction method comprising: a step of obtaining the effective length of the optical fiber between the optical modulator and the photoelectric conversion unit; a step of determining, based on the obtained effective length, a dispersion amount to be added to the optical signal to make the CNR of the second electrical signal above a predetermined value; and a step of connecting a dispersion addition unit that adds the determined dispersion amount to the optical fiber.

[0049] By determining the dispersion amount based on the effective length of the optical fiber and connecting a dispersion-adding unit to the optical fiber, the noise component, such as that caused by the relative intensity noise of a laser source, can be attenuated in an optical communication system with the added dispersion-adding unit. Therefore, compared to structures that increase the modulation index or use a laser source with low relative intensity noise, this method is unaffected by the nonlinearity of the photoelectric conversion unit and does not increase the power consumption of the optical modulator, allowing for improved reception sensitivity of electrical signals in an optical communication system with a simple structure. Thus, the communication quality of an optical communication system can be improved with a simple structure.

[0050] Hereinafter, embodiments of the present invention will be described using the accompanying drawings. Furthermore, the same or equivalent parts in the drawings will be labeled with the same reference numerals, and their descriptions will be omitted. In addition, at least some of the embodiments described below can be combined in any way.

[0051] <First Implementation>

[0052] [Structure and Basic Movements]

[0053] Figure 1 This diagram illustrates the structure of an optical communication system according to a first embodiment of the present invention. (Refer to...) Figure 1 The optical communication system 301 includes a master station device 101 and a slave station device 201. The master station device 101 and the slave station device 201 are connected to each other via an optical fiber 191 with a length of La kilometers. For example, the optical fiber 191 is a standard single-mode optical fiber. The length La is, for example, less than 1 kilometer. In this specification, "standard single-mode optical fiber" refers to single-mode optical fiber conforming to G.652.B or G.652.D as defined by the ITU-T (International Telecommunication Union Telecommunication Standardization Sector). The zero-dispersion wavelength of standard single-mode optical fiber is around 1300 nm.

[0054] The master station device 101 includes: an RF signal generation unit 10; an LD11A as an LD (Laser Diode) 11; an optical modulator 12A as an optical modulator 12; an optical coupler 13A as an optical coupler 13; a PD14A as a PD (PhotoDiode) 14; an amplifier 15A as an amplifier 15; an optical fiber 16; and an adapter 17. The optical fiber 16 is an example of a dispersion-adding unit.

[0055] The slave device 201 includes: LD11B as LD11; optical modulator 12B as optical modulator 12; optical coupler 13B as optical coupler 13; PD14B as PD14; and amplifier 15B as amplifier 15.

[0056] For example, fiber 16 is an optical transmission medium provided in the transmission line between optical modulator 12A in master station device 101 and PD14B in slave station device 201, and also in the transmission line between optical modulator 12B in master station device 101 and PD14A in slave station device 201. More specifically, fiber 16 is a standard single-mode optical fiber with a length of Lb kilometers additionally provided in master station device 101. Adapter 17 is a connector that connects fiber 16 to fiber 191.

[0057] LD11 is an example of a laser source. LD11A is an example of a first laser source. LD11B is an example of a second laser 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 a photoelectric conversion unit. PD14A is an example of a first photoelectric conversion unit. PD14B is an example of a second photoelectric conversion unit.

[0058] The master station device 101 and the slave station device 201 transmit and receive optical signals containing analog signals through optical fiber 191.

[0059] (Downlink optical signal)

[0060] The RF signal generation unit 10 in the master station device 101 receives a digital signal containing communication data from a base station device not shown. The master station device 101 generates an RF signal, for example, in the millimeter-wave band, by performing analog-to-analog conversion on the received digital signal. The RF signal generated by the RF signal generation unit 10 is an example of a first electrical signal. The RF signal generation unit 10 outputs the generated RF signal to the optical modulator 12A.

[0061] LD11A outputs light with wavelength λ1 to optical modulator 12A. For example, LD11A outputs C-band light to optical modulator 12A. That is, wavelength λ1 is above 1530nm and below 1565nm. As an example, wavelength λ1 is 1565nm.

[0062] Optical modulator 12A generates a downlink optical signal OPd by modulating light from LD11A based on an RF signal. More specifically, optical modulator 12A is an external modulator following MZM (Mach-Zehnder Modulator) or EML (Electro-Absorption Modulated Laser). Optical modulator 12A generates a downlink optical signal OPd with wavelength λ1 by modulating light received from LD11A based on an RF signal received from RF signal generation unit 10. Optical modulator 12A transmits the generated downlink optical signal OPd to slave device 201 via optical coupler 13A and optical fibers 16 and 191.

[0063] Optical fiber 16 adds a predetermined amount of dispersion to the downlink optical signal Opd generated by optical modulator 12A. More specifically, the downlink optical signal OPd generated by optical modulator 12A is transmitted through optical fiber 16, thereby adding a amount of dispersion corresponding to the length Lb of optical fiber 16.

[0064] In addition, the downlink optical signal OPd is transmitted from the master station device 101 to the slave station device 201 through the optical fiber 191, thereby adding a amount of dispersion corresponding to the length La of the optical fiber 191.

[0065] The PD14B in slave device 201 generates an RF signal with a level corresponding to the intensity of the downlink optical signal OPd, which has been dispersed by optical fibers 16 and 191. More specifically, PD14B receives the downlink optical signal OPd from master device 101 via optical fiber 191 and optical coupler 13B, and performs photoelectric conversion on the received downlink optical signal OPd to generate the RF signal. The RF signal generated by PD14B is an example of a second electrical signal. PD14B outputs the generated RF signal to amplifier 15B.

[0066] Amplifier 15B amplifies the RF signal received from PD14B. The amplified RF signal is then transmitted via antenna to a mobile communication terminal (not shown).

[0067] (Uplink optical signal)

[0068] The LD11B in the slave device 201 outputs light with wavelength λ2 to the optical modulator 12B. For example, the LD11B outputs C-band light to the optical modulator 12B. That is, the wavelength λ2 is 1530 nm or more and 1565 nm or less. For example, when the fiber length Sum, which is the sum of the length Lb of fiber 16 and the length La of fiber 191, is a value within the range Rg1 described later, the difference between wavelength λ1 and wavelength λ2 is 20 nm or less. As an example, wavelength λ2 is 1545 nm.

[0069] Optical modulator 12B generates an uplink optical signal OPU by modulating light from LD11B based on an RF signal. More specifically, optical modulator 12B is an external modulator conforming to MZM or EML. Optical modulator 12B receives a millimeter-wave band RF signal containing communication data via an antenna from a mobile communication terminal (not shown). Optical modulator 12B generates an uplink optical signal OPU with wavelength λ2 by modulating light received from LD11B based on the RF signal received from the mobile communication terminal. Optical modulator 12B transmits the generated uplink optical signal OPU to master station device 101 via optical coupler 13B and optical fiber 191.

[0070] The uplink optical signal OPU is transmitted from the slave device 201 to the master device 101 through the optical fiber 191, thereby adding a amount of dispersion corresponding to the length La of the optical fiber 191.

[0071] In the master station device 101, the optical fiber 16 adds a predetermined amount of dispersion to the uplink optical signal OPU generated by the optical modulator 12B. More specifically, the uplink optical signal OPU generated by the optical modulator 12B is transmitted through the optical fiber 16, thereby having a dispersion amount corresponding to the length Lb of the optical fiber 16 added to it.

[0072] PD14A generates an RF signal with a level corresponding to the intensity of the uplink optical signal OPU, which has been dispersed by fiber 16. More specifically, PD14A receives the uplink optical signal OPU from slave device 201 via fibers 191 and 16 and optical coupler 13A, and performs photoelectric conversion on the received uplink optical signal OPU to generate the RF signal. The RF signal generated by PD14A is an example of a second electrical signal. PD14A outputs the generated RF signal to amplifier 15A.

[0073] Amplifier 15A amplifies the RF signal received from PD14A. Amplifier 15A then transmits the amplified RF signal to a base station device (not shown).

[0074] (CNR of RF signal)

[0075] The CNR of the RF signal amplified by amplifier 15 is represented by the following mathematical formula (1).

[0076] [Mathematical Expression 1]

[0077]

[0078] Where BN is the signal bandwidth of the RF signal. RIN is the relative intensity noise of LD11. m is the optical modulation degree. R is the photoelectric conversion efficiency of PD14. Pr is the optical received power of PD14. e is the elementary charge. Id is the dark current contained in the RF signal output by PD14. Ie is the input equivalent noise of amplifier 15. H1 is the transfer function representing the input-output characteristics of LD11. H2 is the transfer function representing the input-output characteristics of optical modulator 12.

[0079] The transfer functions H1 and H2 are represented by the following mathematical expressions (2) and (3), respectively.

[0080] [Mathematical Expression 2]

[0081]

[0082] [Mathematical Expression 3]

[0083]

[0084] Where αLD is the transient chirp of LD11. αEX is the transient chirp of optical modulator 12. ω is the center frequency of the RF signal received by optical modulator 12. For example, ω is the average of the center frequency of the bandwidth of the RF signal received by optical modulator 12A and the center frequency of the bandwidth of the RF signal received by optical modulator 12B. ωc is the adiabatic chirp of LD11. j is the imaginary unit. The phase θ in transfer functions H1 and H2 is represented by the following mathematical formula (4).

[0085] [Mathematical Expression 4]

[0086]

[0087] Where L is the transmission distance of the optical signal in a standard single-mode fiber. D is the dispersion parameter. λ is the wavelength of the optical signal transmitted through a standard single-mode fiber. c is the speed of light in a vacuum.

[0088] Figure 2 A table showing the various parameters used in simulating the CNR of the RF signal in the optical communication system according to the first embodiment of the present invention.

[0089] Figure 3 and Figure 4 This is a graph illustrating an example of simulation results for the CNR of an RF signal in an optical communication system according to the first embodiment of the present invention. Figure 3 and Figure 4 In the diagram, the horizontal axis represents the transmission distance L [km], and the vertical axis represents the CNR [dB]. Figure 3 To indicate that it is in use Figure 2 Simulation results show the relationship between the transmission distance L of the optical signal transmitted through a standard single-mode fiber and the CNR of the RF signal amplified by amplifier 15 under various parameters. Figure 4 To indicate in relation to Figure 3 The simulation results shown are compared with the simulation results of the relationship between the transmission distance L of the optical signal transmitted through a standard single-mode fiber and the CNR of the RF signal amplified by amplifier 15 when RIN is set to -160dB / Hz.

[0090] Reference Figure 3 and Figure 4The CNR of the RF signal amplified by amplifier 15 reaches its maximum at transmission distances L1, L3, and L5, and its minimum at transmission distances L2, L4, and L6. This is because, at transmission distances L1, L3, and L5, the amplitude noise component contained in the optical signal due to the relative intensity noise RIN of LD11 is converted into a phase component and attenuated by the dispersion of fibers 16 and 191. L1 is the minimum value among the transmission distances L where the CNR reaches its maximum. L2 is the second smallest value among the transmission distances L where the CNR reaches its maximum. L3 is the third smallest value among the transmission distances L where the CNR reaches its maximum.

[0091] For example, fiber 16 adds a amount of dispersion to the optical signal such that the CNR of the RF signal generated by PD14 is above a specified threshold Th1.

[0092] The threshold Th1 is set based on a reference value Vs, which is the CNR of the RF signal when PD14 generates an RF signal with an intensity level corresponding to the undispersed optical signal generated by optical modulator 12. The reference value Vs is the CNR of the RF signal amplified by amplifier 15 when the lengths Lb and La of fiber 16 and fiber 191 are zero and directly connected to optical couplers 13A and 13B. For example, the threshold Th1 is a value 6 dB smaller than the reference value Vs. Alternatively, the threshold Th1 can be a value 3 dB smaller than the reference value Vs, or it can be the same as the reference value Vs.

[0093] For example, the total length of fiber 16 (Lb) and fiber 191 (La), i.e., the fiber length (Sum), is set to a CNR value range Rg1, Rg2, and Rg3 that is above the threshold Th1. The value ranges Rg1, Rg2, and Rg3 respectively encompass L1, L2, and L3. Hereinafter, each value range Rg1, Rg2, and Rg3 will also be referred to as the value range Rg.

[0094] More specifically, when the length Lb of fiber 16 is not within the numerical range Rg of the length La of fiber 191, the length Sum of fiber 16 is set to be within the smallest numerical range Rg that is greater than the length La.

[0095] For example, if the length Lb of fiber 16 is less than L1 km in the case that the length La of fiber 191 is less than L1 km, the length Sum of fiber 16 is set in a way that is closer to L1 km.

[0096] In addition, the length La of optical fiber 16 is set such that, for example, the length La of optical fiber 16 is 5 kilometers, the length Sum of optical fiber is set to be closer to the value of L2 kilometers.

[0097] [Work Process]

[0098] Figure 5 This diagram illustrates an example of a communication sequence in an optical communication system according to the first embodiment of the present invention.

[0099] Reference Figure 5 First, the master station device 101 generates a millimeter-wave band RF signal by performing analog conversion on the digital signal received from the base station device (step S11).

[0100] Next, the master station device 101 modulates the light from LD11A based on the RF signal to generate a downlink optical signal OPd (step S12).

[0101] Next, the master station device 101 sends the generated downlink optical signal OPd to the slave station device 201 via optical fibers 16 and 191. The downlink optical signal OPd is transmitted through optical fibers 16 and 191, thereby adding a amount of dispersion corresponding to the length Lb of optical fiber 16 and the length La of optical fiber 191 (step S13).

[0102] Next, the slave device 201 generates an RF signal by photoelectric conversion of the downlink optical signal OPd received from the master device 101 (step S14).

[0103] Next, the slave device 201 amplifies the generated RF signal and sends the amplified RF signal to a mobile communication terminal (not shown) via an antenna (step S15).

[0104] Next, the slave device 201 receives a millimeter-wave RF signal containing communication data via an antenna from a mobile communication terminal (not shown) (step S16).

[0105] Next, the slave device 201 modulates the light from the LD11B based on the RF signal to generate an uplink optical signal OPU (step S17).

[0106] Next, the slave device 201 sends the generated uplink optical signal OPU to the master device 101 via optical fiber 191. The uplink optical signal OPU is transmitted through optical fibers 191 and 16, thereby adding a amount of dispersion corresponding to the length La of optical fiber 191 and the length Lb of optical fiber 16 (step S18).

[0107] Next, the master station device 101 performs photoelectric conversion on the uplink optical signal OPU received from the slave station device 201 to generate an RF signal (step S19).

[0108] Next, the master station device 101 amplifies the generated RF signal and sends the amplified RF signal to the base station device (not shown) (step S20).

[0109] Figure 6 This is a flowchart illustrating the construction method of the optical communication system according to the first embodiment of the present invention.

[0110] Reference Figure 6 First, the installer of the optical communication system 301 obtains the effective length of the laid optical fiber 191 in the optical communication system 301 without optical fiber 16. More specifically, the installer, for example, transmits a measurement optical signal from the master station device 101 to the slave station device 201 via optical fiber 191, and calculates the length La of optical fiber 191 based on the measurement result of the CNR of the RF signal generated in the slave station device 201. Alternatively, the installer can also actually measure the length La of optical fiber 191 (step S31).

[0111] Next, based on the simulation results of the effective length of fiber 191 and CNR, the installers determine the amount of dispersion to be added to the optical signal in order to make the CNR above the threshold Th1. More specifically, the installers determine the length Lb of fiber 16 when the fiber length Sum is a value within the minimum numerical range Rg greater than the length La (step S32).

[0112] Next, the installer will connect the fiber 16 with the determined dispersion to the fiber 191. More specifically, the installer will connect the fiber 16 of the determined length Lb between the fiber 191 and the optical coupler 13A via the adapter 17 (step S33).

[0113] Alternatively, in step S31, before laying the optical fiber 191, the operator can obtain the effective length of the optical fiber 191 when it is laid between the master station device 101 and the slave station device 201 based on the distance between the setting position of the master station device 101 and the setting position of the slave station device 201, instead of obtaining the effective length of the already laid optical fiber 191.

[0114] Furthermore, the installer can determine the length Lb of the fiber 16 in step S32 without using the simulation results of CNR. More specifically, CNR becomes a maximum when the transfer function H1 is zero, provided that the transient chirp αEX of the optical modulator 12 is sufficiently small compared to the transient chirp αLD of the LD11. Furthermore, the transfer function H1 can be approximated by the following mathematical formula (5) when the adiabatic chirp ωc of the LD11 is sufficiently small compared to the center frequency ω of the RF signal.

[0115] [Mathematical Expression 5]

[0116]

[0117] Moreover, the fiber length Sum when the transfer function H1 is zero is represented by the following mathematical formula (6).

[0118] [Mathematical Expression 6]

[0119]

[0120] Where n is a natural number. For example, the construction worker determines the fiber length Sum and the fiber 16 length Lb based on mathematical formula (6).

[0121] Furthermore, although the master station device 101 in the optical communication system 301 according to the embodiment of the present invention is configured to have an optical fiber 16 and an adapter 17, it is not limited thereto. Instead of the master station device 101, the slave station device 201 may also be configured to have an optical fiber 16 and an adapter 17.

[0122] Furthermore, although the optical modulator 12 in the optical communication system 301 according to the embodiments of the present invention is configured to generate an optical signal by modulating light from the LD 11 based on an RF signal, it is not limited thereto. The optical modulator 12 may also be configured to generate an optical signal by modulating light from the LD 11 based on an electrical signal containing an RF signal and, for example, a digital control signal of 1 Gbps or less. In this case, in the optical communication system 301, since the frequency of the RF signal is sufficiently large compared to the frequency of the control signal, the effect of the dispersion added to the optical signal through 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.

[0123] Furthermore, although in the optical communication system 301 according to the embodiments of the present invention, the optical fiber 16 is configured to add a amount of dispersion to the optical signal that makes the CNR of the RF signal above the threshold Th1, it is not limited thereto. The optical fiber 16 may also be configured to add a amount of dispersion to the optical signal that can improve the CNR compared to the case where the optical fiber 16 is not provided.

[0124] Furthermore, although the optical communication system 301 according to the embodiment of the present invention employs a configuration in which the threshold Th1 is set based on a reference value Vs, it is not limited thereto. The threshold Th1 may also be set based on the average value of the CNR of the RF signal in the simulation results, rather than on the reference value Vs.

[0125] Furthermore, although the optical communication system 301 according to the embodiments of the present invention is configured to have an optical fiber 16, it is not limited thereto. The optical communication system 301 may also be configured to have an FBG (Fiber Bragg Grating) instead of an 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 an optical coupler 13, and the other end is connected to an optical fiber 191 via an adapter 17.

[0126] Furthermore, although the optical communication system 301 according to the embodiments of the present invention employs the following configuration: simulation is performed using the wavelength λ1 of the downlink optical signal as the wavelength λ of the optical signal transmitted through a standard single-mode fiber, and the fiber length Sum is set based on the simulation results, it is not limited to this. The optical communication system 301 may also employ the following configuration: simulation is performed using the wavelength λ2 of the uplink optical signal instead of the wavelength λ1 of the downlink optical signal as the wavelength λ of the optical signal transmitted through a standard single-mode fiber, and the fiber length Sum is set based on the simulation results. Furthermore, the optical communication system 301 may also employ the following configuration: simulation is performed using the average value λave of wavelengths λ1 and λ2 as the wavelength λ of the optical signal transmitted through a standard single-mode fiber, and the fiber length Sum is set based on the simulation results. Furthermore, in the optical communication system 301, where at least one of wavelengths λ1 and λ2 can be changed, the average of the maximum and minimum values ​​of three or more wavelengths that can be used as wavelengths λ1 and λ2 may also be used.

[0127] Furthermore, although LD11A and 11B are configured to output C-band light in the optical communication system 301 according to the embodiments of the present invention, they are not limited thereto. LD11A and 11B can also be configured to output O-band light. That is, wavelengths λ1 and λ2 can also 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 wavelength λ1 and wavelength λ2 is 5 nm or less. Furthermore, in this case, considering the attenuation of the optical signal in optical fibers 16 and 191, the length La of optical fiber 191 is preferably less than the minimum value of the transmission distance L when the CNR is at its minimum. As an example, the length La of optical fiber 191 is, for example, 5 kilometers or less.

[0128] Figure 7 A table showing the various parameters used to simulate the CNR of the RF signal in the optical communication system according to a variation of the first embodiment of the present invention.

[0129] Figure 8 and Figure 9This is an example illustrating the simulation results of the CNR of an RF signal in an optical communication system according to a variation of the first embodiment of the present invention. Figure 8 and Figure 9 In the diagram, the horizontal axis represents the transmission distance L [km], and the vertical axis represents the CNR [dB]. Figure 8 To indicate the use Figure 7 Simulation results showing the relationship between the transmission distance L of the optical signal in optical fibers 16 and 191 and the CNR of the RF signal amplified by amplifier 15 under various parameters shown. Figure 9 To indicate in relation to Figure 8 The simulation results shown are compared with the simulation results of the relationship between the transmission distance L of the optical signal in optical fibers 16 and 191 and the CNR of the RF signal amplified by amplifier 15 when RIN is set to -160dB / Hz.

[0130] Reference Figure 8 and Figure 9 The CNR amplified by amplifier 15 reaches a minimum at a transmission distance L of L7. For example, fiber optic 16 adds a amount of dispersion to the optical signal such that the CNR of the RF signal generated by PD14 is above a specified threshold Th2. Figure 8 and Figure 9 In the simulation results shown, with Figure 3 and Figure 4 Compared to the simulation results shown, the transmission distance L is longer when the CNR is at its maximum. Therefore, in the optical communication system 301 using the O-band, for example, when the length La of the optical fiber 191 is L7 or more, the length Lb of the optical fiber 16 used to ensure a CNR above the specified value becomes longer compared to the case using the C-band. Therefore, the length La of the optical fiber 191 is preferably less than L7, and more preferably less than 5 kilometers.

[0131] Next, other embodiments of the present invention will be described using the accompanying drawings. Furthermore, the same or equivalent parts in the drawings will be labeled with the same reference numerals, and their descriptions will be omitted.

[0132] <Second Implementation>

[0133] Compared to the optical communication system 301 of the first embodiment, this embodiment relates to an optical communication system 302 having a plurality of slave devices 201. Except as described below, it is the same as the optical communication system 301 of the first embodiment.

[0134] Figure 10 This diagram illustrates the structure of an optical communication system according to a second embodiment of the present invention. (Refer to...) Figure 10Compared with optical communication system 301, optical communication system 302 has a master station device 102 to replace master station device 101, and also has a slave station device 202 and a splitter 24.

[0135] Compared to the master station device 101, the master station device 102 also has an LD11C as LD11, an optical modulator 12C as optical modulator 12, a PD14C as PD14, and an amplifier 15C as amplifier 15, and has an optical coupler 23A instead of an optical coupler 13A.

[0136] Compared with slave device 201, slave device 202 has LD11D as LD11 instead of LD11B, optical modulator 12D as optical modulator 12 instead of optical modulator 12B, optical coupler 13D as optical coupler 13 instead of optical coupler 13B, PB14D as PB14 instead of PD14B, and amplifier 15D as amplifier 15 instead of amplifier 15B.

[0137] The RF signal generation unit 20 in the master station device 102 receives a digital signal containing communication data from a base station device not shown. The master station device 101 generates an RF signal, for example, in the millimeter-wave band, by performing analog-to-analog conversion on the received digital signal. The RF signal generated by the RF signal generation unit 10 is an example of a first electrical signal. The RF signal generation unit 10 outputs the generated RF signal to the optical modulator 12C.

[0138] LD11C outputs light at wavelength λ3 to optical modulator 12C. For example, LD11C outputs C-band light to optical modulator 12C. That is, wavelength λ1 is above 1530nm and below 1565nm.

[0139] Optical modulator 12C modulates light from LD 11C based on an RF signal to generate a downlink optical signal OPd. Optical modulator 12C transmits the generated downlink optical signal OPd to splitter 24 via optical coupler 23A and optical fibers 16 and 191. Splitter 24 is an example of a splitting section.

[0140] In addition, the optical modulator 12A transmits the downlink optical signal OPd to the splitter 24 via the optical coupler 23A and optical fibers 16 and 191.

[0141] Splitter 24 splits the downlink optical signal Opd, which has been amplified by optical fibers 16 and 191. More specifically, splitter 24 outputs the downlink optical signal OPd sent by optical modulator 12 in master station device 102 to slave station devices 201 and 202.

[0142] The PD14B in the slave device 201 generates an RF signal with a level corresponding to the intensity of the downlink optical signal OPd after being split by the splitter 24. More specifically, the PD14B receives the downlink optical signal OPd with wavelength λ1 after being split by the splitter 24 via the optocoupler 13B, and generates the RF signal by photoelectric conversion of the received downlink optical signal OPd. The PD14B outputs the generated RF signal to the amplifier 15B.

[0143] Amplifier 15B amplifies the RF signal received from PD14B. The amplified RF signal is then transmitted via an antenna to a mobile communication terminal (not shown).

[0144] The PD14D in the slave device 202 generates an RF signal with a level corresponding to the intensity of the downlink optical signal OPd after being split by the splitter 24. More specifically, the PD14D receives the downlink optical signal OPd with wavelength λ3 after being split by the splitter 24 via the optocoupler 13D, and generates an RF signal by photoelectric conversion of the received downlink optical signal OPd. The PD14D outputs the generated RF signal to the amplifier 15D.

[0145] Amplifier 15D amplifies the RF signal received from PD14D. The amplified RF signal is then transmitted via an antenna to a mobile communication terminal (not shown).

[0146] The LD11D in the slave device 202 outputs light with wavelength λ4 to the optical modulator 12D. For example, the LD11D outputs C-band light to the optical modulator 12D. That is, the wavelength λ4 is above 1530nm and below 1565nm.

[0147] Optical modulator 12D modulates the light from LD11D based on the RF signal to generate an uplink optical signal OPU with wavelength λ4. Optical modulator 12D transmits the generated uplink optical signal OPU to master station device 102 via optical coupler 13D, splitter 24 and optical fiber 191.

[0148] Optical modulator 12B modulates the light from LD11B based on the RF signal to generate an uplink optical signal OPU with wavelength λ2, and sends the generated uplink optical signal OPU to the master station device 102 via optical coupler 13B, splitter 24 and optical fiber 191.

[0149] In the master station device 102, PD14A receives an uplink optical signal OPU with wavelength λ2 from the slave station device 201 via splitter 24, optical fibers 191 and 16, and optical coupler 23A, and generates an RF signal with a level corresponding to the intensity of the received uplink optical signal OPU. PD14A outputs the generated RF signal to amplifier 15A. Amplifier 15A amplifies the RF signal received from PD14A and transmits the amplified RF signal to a base station device (not shown).

[0150] PD14C receives the uplink optical signal OPU with wavelength λ4 from slave device 201 via splitter 24, optical fibers 191 and 16, and optical coupler 23A, and generates an RF signal with a level corresponding to the intensity of the received uplink optical signal OPU. PD14C outputs the generated RF signal to amplifier 15C. Amplifier 15C amplifies the RF signal received from PD14C and transmits the amplified RF signal to a base station device (not shown).

[0151] For example, the difference between the transmission distance L between the master station device 102 and the slave station device 201 and the transmission distance L between the master station device 102 and the slave station device 202 is less than 300 meters. Furthermore, in the case where the LD11 outputs O-band light, the difference between the transmission distance L between the master station device 102 and the slave station device 201 and the transmission distance L between the master station device 102 and the slave station device 202 is less than 1 kilometer.

[0152] The installer of the optical communication system 302 determines the length Lb of the optical fiber 16 for slave devices 201 and 202 when the optical fiber length Sum is within the numerical range Rg, and calculates the average value Lbave of the determined length Lb. Then, the installer connects the optical fiber 16 of the determined average value Lbave length between the optical fiber 191 and the optical coupler 23A via adapter 17.

[0153] Figure 11 This diagram illustrates the structure of an optical communication system according to a modified example of the second embodiment of the present invention. (Refer to...) Figure 11 Compared with optical communication system 302, optical communication system 303 has slave devices 203 and 204 to replace slave devices 201 and 202, and has an optical coupler 25 to replace splitter 24.

[0154] Compared to slave device 201, slave device 203 does not have optical coupler 13B. Compared to slave device 202, slave device 204 does not have optical coupler 13D.

[0155] The slave device 203's PD14B receives the downlink optical signal OPd with wavelength λ1 from the master device 102 via optical fiber 191 and optical coupler 25, and performs photoelectric conversion on the received downlink optical signal OPd to generate an RF signal.

[0156] The PD14D in the slave device 204 receives the downlink optical signal OPd with wavelength λ3 from the master device 102 via optical fiber 191 and optical coupler 25, and performs photoelectric conversion on the received downlink optical signal OPd to generate an RF signal.

[0157] The optical modulator 12B in the slave device 203 modulates the light from the LD11D based on the RF signal to generate an uplink optical signal OPU with a wavelength of λ2, and sends the generated uplink optical signal OPU to the master device 102 via the optical coupler 25 and the optical fiber 191.

[0158] The optical modulator 12D in the slave device 204 modulates the light from the LD11D based on the RF signal to generate an uplink optical signal OPU with a wavelength of λ4. The generated uplink optical signal OPU is then transmitted to the master device 102 via the optical coupler 25 and the optical fiber 191.

[0159] Figure 12 This diagram illustrates the structure of an optical communication system according to a modified example of the second embodiment of the present invention. (Refer to...) Figure 12 Compared with optical communication system 302, optical communication system 304 has a master station device 103 to replace master station device 102.

[0160] Compared to the master station device 101, the master station device 103 has an optical coupler 33A instead of an optical coupler 23A.

[0161] The optical modulator 12A in the master station device 103 sends the downlink optical signal OPd to the splitter 24 via the optical coupler 33A and optical fibers 16 and 191.

[0162] In slave device 201, PD14B receives downlink optical signal OPd with wavelength λ1 after being split by splitter 24 via optical coupler 13B, and performs photoelectric conversion on the received downlink optical signal OPd to generate RF signal.

[0163] The PD14D in the slave device 202 receives the downlink optical signal OPd with wavelength λ1 after being split by the splitter 24 via the optical coupler 13D, and performs photoelectric conversion on the received downlink optical signal OPd to generate an RF signal.

[0164] In the main station device 103, PD14A receives uplink optical signals OPU with wavelengths λ2 and λ4 via splitter 24, optical fibers 191 and 16 and optical coupler 33A. The received uplink optical signals OPU are photoelectrically converted to generate RF signals.

[0165] Next, other embodiments of the present invention will be described using the accompanying drawings. Furthermore, the same or equivalent parts in the drawings will be labeled with the same reference numerals, and their descriptions will be omitted.

[0166] <Third Implementation Method>

[0167] Compared to the optical communication system 301 of the first embodiment, this embodiment relates to an optical communication system 305 that performs FSO (Free Space Optics) communication. Except as described below, it is the same as the optical communication system 301 of the first embodiment.

[0168] Figure 13 This diagram illustrates the structure of an optical communication system according to a third embodiment of the present invention. (Refer to...) Figure 13 Compared with optical communication system 301, optical communication system 305 has a master station device 105 to replace master station device 101 and a slave station device 205 to replace slave station device 201.

[0169] Compared to the master station device 101, the master station device 105 does not have the adapter 17 and also has the lens 51A. Compared to the slave station device 201, the slave station device 205 also has the lens 51B.

[0170] The optical modulator 12A in the master station device 105 modulates the light from the LD11A to generate a downlink optical signal OPd with wavelength λ1. The optical modulator 12A transmits the generated downlink optical signal OPd to the slave station device 205 via the optical coupler 13A, the optical fiber 16 and the lens 51A.

[0171] In slave device 205, PD14B receives downlink optical signal OPd from master device 105 via lens 51B and optical coupler 13B, performs photoelectric conversion on the received downlink optical signal OPd, and generates RF signal.

[0172] The optical modulator 12B in the slave device 205 modulates the light from the LD 11B to generate a downlink optical signal OPd with a wavelength of λ2. The optical modulator 12B transmits the generated downlink optical signal OPd to the master device 105 via the optical coupler 13B and the lens 51B.

[0173] The PD14A in the master station device 105 receives the downlink optical signal OPd from the slave station device 205 via the lens 51A, the optical fiber 16 and the optical coupler 13A, and performs photoelectric conversion on the received downlink optical signal OPd to generate an RF signal.

[0174] In optical communication system 305, compared with optical communication system 301, it is not necessary to consider the length La of optical fiber 191, so the length Lb of optical fiber 16 can be determined simply and more accurately.

[0175] It should be considered that the above embodiments are illustrative in all respects and not restrictive. The scope of the invention is set forth not in the foregoing description but in the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0176] Each process (function) in the above-described embodiments can be implemented by a processing circuit including one or more processors. Besides the one or more processors, the processing circuit can also be composed of integrated circuits combining one or more memories, various analog circuits, and various digital circuits. The one or more memories store programs (commands) that cause the one or more processors to execute the above processes. The one or more processors can execute the above processes according to the programs read from the one or more memories, or according to logic circuits pre-designed to execute the above processes. The processors can be various processors suitable for controlling computers, such as CPUs (Central Processing Units), GPUs (Graphics Processing Units), DSPs (Digital Signal Processors), FPGAs (Field Programmable Gate Arrays), and ASICs (Application Specific Integrated Circuits). Alternatively, multiple physically separate processors can cooperate to execute the above processes. For example, the aforementioned processors, which are mounted on multiple physically separate computers, can cooperate with each other via networks such as LAN (Local Area Network), WAN (Wide Area Network), and the Internet to execute the aforementioned processes. The aforementioned programs can be installed into the aforementioned memory from external server devices via the aforementioned networks, or they can be circulated in the form of storage media such as CD-ROM (Compact Disc Read Only Memory), DVD-ROM (Digital Versatile Disk Read Only Memory), and semiconductor memory, and installed into the aforementioned memory from the aforementioned storage media.

[0177] The above description includes the features of the following notes.

[0178] [Note 1]

[0179] An optical communication system having:

[0180] Laser source;

[0181] An optical modulator that generates an optical signal by modulating light from the laser source based on a first electrical signal;

[0182] A dispersion-adding unit that adds a predetermined amount of dispersion to the optical signal generated by the optical modulator; and

[0183] The photoelectric conversion unit generates a second electrical signal with a level corresponding to the intensity of the light signal to which the dispersion has been added by the dispersion addition unit.

[0184] The dispersion-adding section is an optical fiber that adds a certain amount of dispersion to the optical signal, causing the CNR of the second electrical signal to be above a predetermined value.

[0185] Explanation of reference numerals in the attached figures

[0186] 10, 20: RF signal generation unit;

[0187] 11, 11A, 11B, 11C, 11D: LD;

[0188] 12, 12A, 12B, 12C, 12D: Optical modulators;

[0189] 13, 13A, 13B, 13C, 13D, 23A, 25, 33A: Optical couplers;

[0190] 14, 14A, 14B, 14C, 14D: PD;

[0191] 15, 15A, 15B, 15C, 15D: Amplifiers;

[0192] 16: Optical fiber;

[0193] 17: Adapter;

[0194] 24: Switch;

[0195] 51A, 51B: Lenses;

[0196] 101, 102, 103, 105: Main station equipment;

[0197] 191: Optical fiber;

[0198] 201, 202, 203, 204, 205: Slave station devices;

[0199] 301, 302, 303, 304, 305: Optical communication systems;

[0200] Vs: Baseline value;

[0201] Th1: Threshold;

[0202] Rg, Rg1, Rg2, Rg3: Numerical range.

Claims

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

2. The optical communication system according to claim 1, wherein, The dispersion addition unit adds an amount of dispersion to the optical signal such that the CNR (Carrier to Noise Ratio) of the second electrical signal becomes a predetermined value or higher.

3. The optical communication system according to claim 2, wherein, The specified value is a value set based on the CNR of the third electrical signal when the photoelectric conversion unit generates the third electrical signal, which is an electrical signal with a level corresponding to the intensity of the light signal without added dispersion generated by the light modulator.

4. The optical communication system according to any one of claims 1 to 3, wherein, The dispersion-adding part is an optical transmission medium disposed between the optical modulator and the photoelectric conversion part.

5. The optical communication system according to any one of claims 1 to 4, wherein, The optical communication system includes: a first laser source as the laser source; a second laser source as the laser source; a first optical modulator as the optical modulator; and a second optical modulator as the optical modulator. The first photoelectric conversion unit is the photoelectric conversion unit described above; And a second photoelectric conversion unit, which is the photoelectric conversion unit. The first optical modulator generates a downlink optical signal as the optical signal by modulating the light from the first laser source. The second optical modulator generates an uplink optical signal as the optical signal by modulating the light from the second laser source. The dispersion-adding unit adds the dispersion to the downlink optical signal generated by the first optical modulator and the uplink optical signal generated by the second optical modulator. The first photoelectric conversion unit generates a second electrical signal with a level corresponding to the intensity of the downlink optical signal to which the dispersion has been added by the dispersion addition unit. The second photoelectric conversion unit generates a second electrical signal with a level corresponding to the intensity of the uplink optical signal to which the dispersion has been added by the dispersion addition unit. The wavelengths of the downlink optical signal and the uplink optical signal are both above 1530nm and below 1565nm. The wavelength difference between the downlink optical signal and the uplink optical signal is less than 20 nm.

6. The optical communication system according to any one of claims 1 to 4, wherein, The optical communication system includes: a first laser source as the laser source; a second laser source as the laser source; a first optical modulator as the optical modulator; and a second optical modulator as the optical modulator. The first photoelectric conversion unit is the photoelectric conversion unit described above; And a second photoelectric conversion unit, which is the photoelectric conversion unit. The first optical modulator generates a downlink optical signal as the optical signal by modulating the light from the first laser source. The second optical modulator generates an uplink optical signal as the optical signal by modulating the light from the second laser source. The dispersion-adding unit adds the dispersion to the downlink optical signal generated by the first optical modulator and the uplink optical signal generated by the second optical modulator. The first photoelectric conversion unit generates a second electrical signal with a level corresponding to the intensity of the downlink optical signal to which the dispersion has been added by the dispersion addition unit. The second photoelectric conversion unit generates a second electrical signal with a level corresponding to the intensity of the uplink optical signal to which the dispersion has been added by the dispersion addition unit. The wavelengths of the downlink optical signal and the uplink optical signal are both above 1260nm and below 1360nm. The difference between the wavelength of the downlink optical signal and the wavelength of the uplink optical signal is less than 5 nm. The length of the optical fiber used to transmit the downlink optical signal and the uplink optical signal is less than 5 km.

7. The optical communication system according to any one of claims 1 to 6, wherein, The optical communication system further includes a splitting unit, which splits the optical signal to which the dispersion has been added by the dispersion-adding unit. The optical communication system has multiple photoelectric conversion units. Each photoelectric conversion unit generates a second electrical signal with a level corresponding to the intensity of the optical signal after it has been split by the splitter.

8. An optical communication method, which is an optical communication method for an optical communication system with a laser light source, the optical communication method comprising: The step of generating an optical signal by modulating light from the laser source based on a first electrical signal; The step of adding a predetermined amount of dispersion to the generated optical signal; as well as The step of generating a second electrical signal with a level corresponding to the intensity of the light signal with added dispersion.

9. A method for constructing an optical communication system, the optical communication system comprising: a laser source; and an optical modulator that generates an optical signal by modulating light from the laser source based on a first electrical signal. And a photoelectric conversion unit that generates a second electrical signal with a level corresponding to the intensity of the optical signal. The construction method includes: The step of obtaining the effective length of the optical fiber between the optical modulator and the photoelectric conversion unit; Based on the obtained effective length, the step of determining the amount of dispersion added to the optical signal to make the CNR of the second electrical signal above a predetermined value; and The step of connecting the dispersion-adding part, which adds the determined dispersion amount, to the optical fiber.

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