Optical communication system, optical transmitter, optical receiver, and optical communication method

Abstract

To expand a reception dynamic range of a signal with a simple configuration or process.SOLUTION: An optical communication system includes: a DML (Directly Modulated Laser) that generates an optical signal which is modulated on the basis of an electrical signal and then transmitted via an optical fiber; a wavelength dispersion adding unit that adds a predetermined amount of wavelength dispersion to the optical signal; and an optoelectronic conversion unit that outputs the electrical signal of a level corresponding to intensity of the optical signal to which the wavelength dispersion is added by the wavelength dispersion adding unit.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to an optical communication system, an optical transmitting device, an optical receiving device, and an optical communication method. [Background technology]

[0002] Conventionally, techniques have been developed to improve communication performance in optical communication systems. For example, techniques have been developed to expand the dynamic range of signal reception in analog RoF (Radio over Fiber) systems, which are an example of optical communication systems.

[0003] For example, 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.

[0004] Furthermore, Patent Document 2 (JP 2005-175826 A) 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 converter 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 converter that converts the optical signal transmitted from the transmitting device via the optical fiber into an electrical signal, a control information extractor 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 converter, and a receiving signal controller that controls amplification or attenuation processing to be performed on the electrical signal converted by the optical / electrical converter based on a reception level obtained from the control information extracted by the control information extractor so as to offset the processing performed by the transmitting signal controller. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 10-51391 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-175826 [Non-patent literature]

[0006] [Non-Patent Document 1] Kuo Zhang, and 4 others, “Performance comparison of DML, EML and MZM in dispersion-unmanaged short reach transmissions with digital signal processing”, Research Article, Optics EXPRESS, December 24, 2018, Vol. 26, No. 26, p. 34288-34304 Summary of the Invention [Problem to be solved by the invention]

[0007] The techniques described in Patent Documents 1 and 2 require a complex circuit configuration in order to expand the dynamic range of signal reception.

[0008] The present disclosure has been made to solve the above-mentioned problems, and its purpose is to provide an optical communication system, an optical transmitting device, an optical receiving device, and an optical communication method that are capable of expanding the reception dynamic range of a signal with a simple configuration or processing. [Means for solving the problem]

[0009] The optical communication system of the present disclosure includes a DML (Directly Modulated Laser) that generates an optical signal modulated based on an electrical signal and transmitted through an optical fiber, a chromatic dispersion adding unit that adds a predetermined amount of chromatic dispersion to the optical signal, and an opto-electrical conversion unit that outputs an 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.

[0010] The optical transmitting device of the present disclosure includes a DML that generates an optical signal modulated based on an electrical signal, and a chromatic dispersion adding unit that adds a predetermined amount of chromatic dispersion to the optical signal generated by the DML and outputs the optical signal to an optical fiber.

[0011] The optical receiving device of the present disclosure includes a chromatic dispersion adding unit that adds a predetermined amount of chromatic dispersion to an optical signal generated by DML and received via an optical fiber, and an opto-electrical conversion unit that outputs an 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.

[0012] The optical communication method disclosed herein is an optical communication method in an optical communication system, and includes the steps of generating an optical signal modulated based on an electrical signal using a DML, the optical signal being transmitted via an optical fiber, adding a predetermined amount of chromatic dispersion to the optical signal, and outputting an electrical signal at a level corresponding to the intensity of the optical signal to which the chromatic dispersion has been added.

[0013] One aspect of the present disclosure may be realized not only as an optical transmission device including such a characteristic processing unit, but also as an optical communication method including such characteristic processing steps, or as a semiconductor integrated circuit that realizes part or all of the optical transmission device.

[0014] Furthermore, one aspect of the present disclosure may be realized not only as an optical receiving device including such a characteristic processing unit, but also as an optical communication method including such characteristic processing steps, or as a semiconductor integrated circuit that realizes part or all of the optical receiving device. [Effects of the Invention]

[0015] According to the present disclosure, the dynamic range of received signals can be expanded with a simple configuration and processing. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram illustrating a configuration of an optical communication system according to a first embodiment of the present disclosure. [Figure 2] FIG. 2 is a diagram illustrating a configuration of a signal processing unit in a master station device according to the first embodiment of the present disclosure. [Figure 3]FIG. 3 is a diagram illustrating a configuration of an optical communication system according to a first modification of the first embodiment of the present disclosure. [Figure 4] FIG. 4 is a diagram illustrating a configuration of an optical communication system according to a second modification of the first embodiment of the present disclosure. [Figure 5] FIG. 5 is a diagram showing a procedure for calculating the minimum receiving sensitivity in an optical communication system by simulation. [Figure 6] FIG. 6 is a graph showing the simulation results of the minimum receiving sensitivity in the optical communication system according to the first comparative example. [Figure 7] FIG. 7 is a graph showing a simulation result of the minimum receiving sensitivity in the optical communication system according to the second comparative example. [Figure 8] FIG. 8 is a graph showing a simulation result of the minimum receiving sensitivity in the optical communication system according to the third comparative example. [Figure 9] FIG. 9 is a graph showing a simulation result of the minimum receiving sensitivity in the optical communication system according to the third comparative example. [Figure 10] FIG. 10 is a graph showing a simulation result of the minimum receiving sensitivity in the optical communication system according to the third comparative example. [Figure 11] FIG. 11 is a graph showing a simulation result of the minimum receiving sensitivity in the optical communication system according to the third comparative example. [Figure 12] FIG. 12 is a graph showing a simulation result of the minimum receiving sensitivity in the optical communication system according to the third comparative example. [Figure 13] FIG. 13 is a graph showing a simulation result of the minimum receiving sensitivity in the optical communication system according to the third comparative example. [Figure 14] FIG. 14 is a graph showing a simulation result of the minimum receiving sensitivity in the optical communication system according to the first embodiment of the present disclosure. [Figure 15] FIG. 15 is a graph showing a simulation result of the minimum receiving sensitivity in the optical communication system according to the first embodiment of the present disclosure. [Figure 16]FIG. 16 is a graph showing a simulation result of the minimum receiving sensitivity in the optical communication system according to the first embodiment of the present disclosure. [Figure 17] FIG. 17 is a graph showing a simulation result of the minimum receiving sensitivity in the optical communication system according to the first embodiment of the present disclosure. [Figure 18] FIG. 18 is a graph showing a simulation result of the minimum receiving sensitivity in the optical communication system according to the first embodiment of the present disclosure. [Figure 19] FIG. 19 is a graph showing a simulation result of the minimum receiving sensitivity in the optical communication system according to the first embodiment of the present disclosure. [Figure 20] FIG. 20 is a diagram illustrating an example of a communication sequence in the optical communication system according to the first embodiment of the present disclosure. [Figure 21] FIG. 21 is a diagram illustrating a configuration of an optical communication system according to the second embodiment of the present disclosure. [Figure 22] FIG. 22 is a diagram illustrating an example of a communication sequence in an optical communication system according to the second embodiment of the present disclosure. [Figure 23] FIG. 23 is a diagram illustrating an example of an optical module in an optical communication system according to a modified example of the first embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0017] First, the contents of the embodiments of the present disclosure will be listed and described.

[0018] (1) An optical communication system according to an embodiment of the present disclosure includes a DML that generates an optical signal modulated based on an electrical signal and transmitted through an optical fiber, a chromatic dispersion adding unit that adds a predetermined amount of chromatic dispersion to the optical signal, and an opto-electrical conversion unit that outputs an 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.

[0019] In this way, by adding chromatic dispersion to an optical signal generated using a DML and transmitted through an optical fiber, 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 generate an electrical signal based on an optical signal to which an appropriate amount of chromatic dispersion has been added according to, for example, the length of the optical fiber. Therefore, by utilizing the characteristics of the minimum receiving sensitivity relative to the fiber transmission distance when a DML is used as an optical transmitting device, it is possible to improve the minimum receiving sensitivity of an optical communication system under actual operating conditions. Therefore, it is possible to expand the signal receiving dynamic range with a simple configuration or processing.

[0020] (2) The linewidth enhancement factor of the DML may be 3 or more, and the adiabatic chirp of the optical signal generated by the DML may be 4 GHz or more.

[0021] With this configuration, the dynamic range of the received signal can be further expanded.

[0022] (3) The optical communication system may be used in an analog RoF system.

[0023] Such a configuration makes it possible to realize an analog RoF system with an expanded dynamic range for receiving signals.

[0024] (4) The optical communication system may further include an equalizer that corrects the frequency characteristics of the electrical signal output by the photoelectric conversion unit.

[0025] Such a configuration can further improve the quality of optical communication between devices connected via optical fibers.

[0026] (5) An optical transmitting device according to an embodiment of the present disclosure includes a DML that generates an optical signal modulated based on an electrical signal, and a chromatic dispersion adding unit that adds a predetermined amount of chromatic dispersion to the optical signal generated by the DML and outputs the optical signal to an optical fiber.

[0027] In this way, by adding chromatic dispersion to an optical signal generated using a DML and outputting the signal to an optical fiber, an optical receiving device, for example, can generate an electrical signal based on an optical signal to which an appropriate amount of chromatic dispersion corresponding to the length of the fiber has been added. Therefore, by utilizing the characteristics of the minimum receiving sensitivity relative to the fiber transmission distance when a DML is used as an optical transmitting device, it is possible to improve the minimum receiving sensitivity of an optical communication system under actual operating conditions. Therefore, the signal receiving dynamic range can be expanded with a simple configuration or processing.

[0028] (6) An optical receiving device according to an embodiment of the present disclosure includes a chromatic dispersion adding unit that adds a predetermined amount of chromatic dispersion to an optical signal generated by DML and received via an optical fiber, and an opto-electrical conversion unit that outputs an 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.

[0029] In this way, by adding chromatic dispersion to an optical signal generated using a DML and received via an optical fiber, and outputting an electrical signal with a level corresponding to the intensity of the optical signal to which chromatic dispersion has been added, it is possible to generate an electrical signal based on an optical signal to which an appropriate amount of chromatic dispersion has been added according to, for example, the length of the optical fiber. Therefore, by utilizing the characteristics of the minimum receiving sensitivity relative to the fiber transmission distance when a DML is used as an optical transmitting device, it is possible to improve the minimum receiving sensitivity of an optical communication system under actual operating conditions. Therefore, it is possible to expand the signal receiving dynamic range with a simple configuration or processing.

[0030] (7) An optical communication method according to an embodiment of the present disclosure is an optical communication method in an optical communication system, and includes the steps of generating an optical signal using a DML, the optical signal being modulated based on an electrical signal, the optical signal being transmitted through an optical fiber, adding a predetermined amount of chromatic dispersion to the optical signal, and outputting an electrical signal at a level corresponding to the intensity of the optical signal to which the chromatic dispersion has been added.

[0031] In this way, by adding chromatic dispersion to an optical signal generated using a DML and transmitted through an optical fiber, 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 generate an electrical signal based on an optical signal to which an appropriate amount of chromatic dispersion has been added according to, for example, the length of the optical fiber. Therefore, by utilizing the characteristics of the minimum receiving sensitivity relative to the fiber transmission distance when a DML is used as an optical transmitting device, it is possible to improve the minimum receiving sensitivity of an optical communication system under actual operating conditions. Therefore, it is possible to expand the signal receiving dynamic range with a simple configuration or processing.

[0032] 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.

[0033] First Embodiment [Configuration and basic operation] FIG. 1 is a diagram illustrating a configuration of an optical communication system according to a first embodiment of the present disclosure. Referring to FIG. 1, the optical communication system 301 includes a slave station device 101 and a master station device 201. The slave station device 101 and the master station device 201 are connected to each other via an optical fiber 191. The length of the optical fiber 191 is, for example, 20 km or less. For example, the optical communication system 301 is used in an analog RoF system. The slave station device 101 and the master station device 201 are examples of optical transmitting devices and examples of optical receiving devices.

[0034] For example, the slave station device 101 receives an OFDM (Orthogonal Frequency Division Multiplexing) modulated millimeter wave band radio signal containing communication data from the mobile communication terminal 151 via the antenna 161. The slave station device 101 generates an optical signal based on the received radio signal and outputs the generated optical signal to the optical fiber 191. Note that the slave station device 101 may also be configured to receive a signal containing communication data via a wired connection, generate an optical signal based on the received signal, and output the generated optical signal to the optical fiber 191.

[0035] The master station device 201 receives an optical signal from the slave station device 101 via the optical fiber 191. The master station device 201 generates an electrical signal based on the received optical signal, and obtains communication data from the generated electrical signal.

[0036] (Slave station device) The slave station device 101 includes an amplifier 11, a frequency conversion unit 12, and a DML 13.

[0037] The amplifier 11 amplifies a radio signal received from the mobile communication terminal 151 via the antenna 161 and outputs the amplified signal to the frequency conversion unit 12 .

[0038] The frequency converter 12 converts the radio signal received from the amplifier 11 into an IF (Intermediate Frequency) signal, which is an electrical signal of several gigahertz to tens of gigahertz, and outputs the IF signal to the DML 13 .

[0039] The DML 13 generates an optical signal modulated based on an electrical signal as an optical signal to be transmitted via the optical fiber 191. More specifically, the DML 13 is an optical transmitting device that modulates the optical output by directly modulating the current injected into a semiconductor laser. The DML 13 generates an optical signal by directly modulating the current of the IF signal received from the frequency conversion unit 12. The DML 13 outputs the generated optical signal to the optical fiber 191.

[0040] The optical signal generated by the optical transmitting device includes a frequency chirp Δf(t) expressed by the following equation (1):

number

[0041] where α is the linewidth enhancement factor of the optical transmitting device, k is the adiabatic chirp parameter, P is the optical power of the optical signal generated by the optical transmitting device based on the electrical signal, and Pbias is the optical power of the optical signal generated by the optical transmitting device based on the bias current.

[0042] The optical signal generated by the DML 13 includes a frequency chirp Δf(t) consisting of a transient chirp and an adiabatic chirp fc expressed by the following equation (2):

number

[0043] For example, the linewidth enhancement factor α of the DML 13 is 3 or greater. Also, for example, the adiabatic chirp fc of the optical signal generated by the DML 13 is 4 GHz or greater. The linewidth enhancement factor α of the DML 13 is preferably 6 or greater. The adiabatic chirp fc of the optical signal generated by the DML 13 is preferably 8 GHz or greater, and more preferably 12 GHz or greater.

[0044] (Master station device) The master station 201 includes a chromatic dispersion adder 21, an opto-electric converter 22, an amplifier 23, and a demodulator 24.

[0045] The chromatic dispersion adding unit 21 adds a predetermined amount of chromatic dispersion to the optical signal. That is, the chromatic dispersion adding unit 21 adds a predetermined amount of chromatic dispersion to the optical signal generated by the DML 13 in the slave station device 101 and received from the slave station device 101 via the optical fiber 191. The chromatic dispersion adding unit 21 outputs the optical signal to which chromatic dispersion has been added to the opto-electrical conversion unit 22.

[0046] For example, the chromatic dispersion adding unit 21 adds, to the optical signal received from the slave station device 101 via the optical fiber 191, the amount of chromatic dispersion that the optical signal will experience when transmitted using the optical fiber 191 of X kilometers, i.e., the phase shift φx. In other words, when the length of the optical fiber 191 is L kilometers, the chromatic dispersion adding unit 21 simulates the optical signal received from the slave station device 101 via the optical fiber 191 of (L+X) kilometers, and outputs the simulated signal to the opto-electrical conversion unit 22. X is, for example, 20. X may be set depending on the length of the optical fiber 191 and the frequency F of the electrical signal to be transmitted.

[0047] For example, the chromatic dispersion adding unit 21 is an FBG (Fiber Bragg Grating) including an optical fiber and a diffraction grating formed in the core of the optical fiber. One end of the FBG is connected to the DML 13 in the slave station device 101 via an optical connector (not shown) and an optical fiber 191, and the other end is connected to the photoelectric conversion unit 22 in the master station device 201.

[0048] The photoelectric conversion unit 22 outputs an electrical signal at a level corresponding to the intensity of the optical signal to which chromatic dispersion has been added by the chromatic dispersion adding unit 21. For example, the photoelectric conversion unit 22 receives an optical signal via an FBG, which is the chromatic dispersion adding unit 21, generates an electrical signal by photoelectrically converting the received optical signal, and outputs the generated electrical signal to the amplifier 23.

[0049] The amplifier 23 amplifies the electrical signal received from the photoelectric conversion unit 22 and outputs the amplified signal to the demodulation unit 24 .

[0050] The demodulation unit 24 receives the electrical signal from the amplifier 23 and acquires communication data by demodulating the received electrical signal. For example, the demodulation unit 24 outputs the acquired communication data to a data processing unit (not shown).

[0051] 2 is a diagram illustrating a configuration of a signal processing unit in the master station device according to the first embodiment of the present disclosure. Referring to FIG. 2, the demodulation unit 24 includes an FFT (Fast Fourier Transform) processing unit 25, an equalizer 26, and a signal processing unit 27.

[0052] The FFT processing unit 25 digitally converts the electrical signal received from the amplifier 23, performs FFT processing, and outputs FFT data obtained by the FFT processing to the equalizer 26.

[0053] The equalizer 26 corrects the frequency characteristics of the electrical signal output by the photoelectric conversion unit 22. More specifically, the equalizer 26 receives FFT data from the FFT processing unit 25 and corrects the received FFT data, i.e., performs equalization processing to shape the waveform of each subcarrier indicated by the FFT data. The equalizer 26 outputs the corrected FFT data to the signal processing unit 27.

[0054] The signal processing unit 27 receives the FFT data from the equalizer 26 and performs various signal processing such as demodulation and error correction for each subcarrier on the received FFT data to obtain communication data.

[0055] <Variation 1> 3 is a diagram illustrating a configuration of an optical communication system according to a first modification of the first embodiment of the present disclosure. Referring to FIG. 3, compared to the optical communication system 301, the optical communication system 302 includes multiple slave station devices 101 and includes a master station device 202 instead of the master station device 201. For example, the optical communication system 302 includes slave station devices 101A and 101B, which are slave station devices 101. Note that the optical communication system 302 may also be configured to include three or more slave station devices 101.

[0056] The slave station device 101A and the master station device 202 are connected to each other via an optical fiber 191A, which is also an optical fiber 191. The slave station device 101B and the master station device 202 are connected to each other via an optical fiber 191B, which is also an optical fiber 191.

[0057] The slave station device 101A receives a radio signal from the mobile communication terminal 151A via the antenna 161A, generates an optical signal based on the received radio signal, and outputs it to the optical fiber 191A. Similarly, the slave station device 101B receives a radio signal from the mobile communication terminal 151B via the antenna 161B, generates an optical signal based on the received radio signal, and outputs it to the optical fiber 191B. Hereinafter, the wavelength of the optical signal generated by the slave station device 101A is referred to as λ1, and the wavelength of the optical signal generated by the slave station device 101B is referred to as λ2. λ1 and λ2 may be the same or different.

[0058] The master station device 202 receives an optical signal from the slave station device 101A via the optical fiber 191A, generates an electrical signal based on the received optical signal, and acquires communication data from the generated electrical signal. The master station device 202 also receives an optical signal from the slave station device 101B via the optical fiber 191B, generates an electrical signal based on the received optical signal, and acquires communication data from the generated electrical signal.

[0059] More specifically, the parent station device 202 includes a plurality of chromatic dispersion adding units 21, a plurality of photoelectric conversion units 22, a plurality of amplifiers 23, and a plurality of demodulation units 24, the number of which corresponds to the number of child station devices 101. More specifically, the parent station device 202 includes chromatic dispersion adding units 21A and 21B as the chromatic dispersion adding units 21, photoelectric conversion units 22A and 22B as the photoelectric conversion units 22, amplifiers 23A and 23B as the amplifiers 23, and demodulation units 24A and 24B as the demodulation units 24.

[0060] The chromatic dispersion adding unit 21A adds a predetermined amount of chromatic dispersion to the optical signal generated by the DML 13 in the slave station device 101A. That is, the chromatic dispersion adding unit 21A adds a predetermined amount of chromatic dispersion to the optical signal received from the slave station device 101A via the optical fiber 191A. The chromatic dispersion adding unit 21A outputs the optical signal to which chromatic dispersion has been added to the opto-electrical conversion unit 22A.

[0061] The chromatic dispersion adding unit 21B adds a predetermined amount of chromatic dispersion to the optical signal generated by the DML 13 in the slave station device 101B. That is, the chromatic dispersion adding unit 21B adds a predetermined amount of chromatic dispersion to the optical signal received from the slave station device 101B via the optical fiber 191B. The chromatic dispersion adding unit 21B outputs the optical signal to which chromatic dispersion has been added to the opto-electrical conversion unit 22B.

[0062] For example, the photoelectric conversion unit 22A receives an optical signal via an FBG that is the chromatic dispersion adding unit 21A, and generates an electrical signal by photoelectrically converting the received optical signal, and outputs the electrical signal to the amplifier 23A. Also, for example, the photoelectric conversion unit 22B receives an optical signal via an FBG that is the chromatic dispersion adding unit 21B, and generates an electrical signal by photoelectrically converting the received optical signal, and outputs the electrical signal to the amplifier 23B.

[0063] Amplifier 23A amplifies the electrical signal received from photoelectric conversion unit 22A and outputs the amplified signal to demodulation unit 24A. Amplifier 23B amplifies the electrical signal received from photoelectric conversion unit 22B and outputs the amplified signal to demodulation unit 24B.

[0064] Demodulation unit 24A receives the electrical signal from amplifier 23A and acquires communication data from the received electrical signal. Demodulation unit 24B receives the electrical signal from amplifier 23B and acquires communication data from the received electrical signal.

[0065] <Variation 2> Fig. 4 is a diagram illustrating a configuration of an optical communication system according to Modification 2 of the first embodiment of the present disclosure. With reference to Fig. 4, compared to the optical communication system 302 according to Modification 1, the optical communication system 303 includes a master station device 203 instead of the master station device 202, and further includes an optical coupler 31. The optical communication system 303 performs wavelength division multiplexing (WDM).

[0066] The slave station device 101A and the optical coupler 31 are connected to each other via an optical fiber 191A1, which is also an optical fiber 191. The slave station device 101B and the optical coupler 31 are connected to each other via an optical fiber 191B1, which is also an optical fiber 191. The optical coupler 31 and the master station device 203 are connected to each other via an optical fiber 191C, which is also an optical fiber 191.

[0067] The slave station device 101A receives a radio signal from the mobile communication terminal 151A via the antenna 161A, generates an optical signal based on the received radio signal, and outputs the optical signal to the optical fiber 191A1. The slave station device 101B receives a radio signal from the mobile communication terminal 151B via the antenna 161B, generates an optical signal based on the received radio signal, and outputs the optical signal to the optical fiber 191B1. For example, the wavelength λ1 of the optical signal generated by the slave station device 101A and the wavelength λ2 of the optical signal generated by the slave station device 101B are different from each other.

[0068] The optical coupler 31 multiplexes an optical signal received from the slave station device 101A via the optical fiber 191A1 and an optical signal received from the slave station device 101A via the optical fiber 191B1, and outputs the multiplexed signal to the optical fiber 191C.

[0069] The master station device 203 receives an optical signal from the optical coupler 31 via the optical fiber 191C and demultiplexes the received optical signal into wavelengths. For each demultiplexed optical signal, the master station device 203 generates an electrical signal based on the optical signal and obtains communication data from the generated electrical signal.

[0070] More specifically, the master station device 203 includes a chromatic dispersion adding unit 21 and an optical coupler 41. The master station device 203 also includes a plurality of opto-electrical conversion units 22, a plurality of amplifiers 23, and a plurality of demodulation units 24, the number of which corresponds to the number of slave station devices 101. More specifically, the master station device 203 includes a chromatic dispersion adding unit 21, an optical coupler 41, opto-electrical conversion units 22A and 22B, amplifiers 23A and 23B, and demodulation units 24A and 24B.

[0071] The chromatic dispersion adding unit 21 adds a predetermined amount of chromatic dispersion to the optical signal generated by the DML 13 in the slave station device 101A and the optical signal generated by the DML 13 in the slave station device 101B. That is, the chromatic dispersion adding unit 21 adds a predetermined amount of chromatic dispersion to the optical signal received from the optical coupler 31 via the optical fiber 191C. The chromatic dispersion adding unit 21 outputs the optical signal to which chromatic dispersion has been added to the optical coupler 41.

[0072] The optical coupler 41 demultiplexes and outputs the optical signal received from the chromatic dispersion adding unit 21. More specifically, the optical coupler 41 receives the optical signal via an FBG, which is the chromatic dispersion adding unit 21. Of the received optical signals, the optical coupler 41 outputs the optical signal with wavelength λ1 to the optical fiber 191A2, which is the optical fiber 191, and outputs the optical signal with wavelength λ2 to the optical fiber 191B2, which is the optical fiber 191.

[0073] The photoelectric conversion units 22A and 22B output electrical signals at levels corresponding to the intensity of the optical signal to which chromatic dispersion has been added by the chromatic dispersion adding unit 21. For example, the photoelectric conversion unit 22A receives an optical signal of wavelength λ1 from the optical coupler 41 via the optical fiber 191A2, performs photoelectric conversion on the received optical signal to generate an electrical signal, and outputs the electrical signal to the amplifier 23A. Also, for example, the photoelectric conversion unit 22B receives an optical signal of wavelength λ2 from the optical coupler 41 via the optical fiber 191B2, performs photoelectric conversion on the received optical signal to generate an electrical signal, and outputs the electrical signal to the amplifier 23B.

[0074] Amplifier 23A amplifies the electrical signal received from photoelectric conversion unit 22A and outputs the amplified signal to demodulation unit 24A. Amplifier 23B amplifies the electrical signal received from photoelectric conversion unit 22B and outputs the amplified signal to demodulation unit 24B.

[0075] Demodulation unit 24A receives the electrical signal from amplifier 23A and acquires communication data from the received electrical signal. Demodulation unit 24B receives the electrical signal from amplifier 23B and acquires communication data from the received electrical signal.

[0076] (Receiving sensitivity in optical communication systems) The inventors of the present application calculated the minimum receiving sensitivity in optical communication systems 301, 302, and 303 by simulation. The inventors also calculated the minimum receiving sensitivity in optical communication systems according to comparative examples 1 to 3 by simulation. That is, the optical communication system according to comparative example 1 is an optical communication system that, compared to optical communication system 301, includes an MZM (Mach-Zehnder Modulator) instead of DML 13 as an optical transmitting device and does not include chromatic dispersion adding unit 21. The optical communication system according to comparative example 2 is an optical communication system that, compared to optical communication system 301, includes an EML (Electro-Absorption Modulated Laser) instead of DML 13 as an optical transmitting device and does not include chromatic dispersion adding unit 21. The optical communication system according to comparative example 3 is an optical communication system that, compared to optical communication system 301, does not include chromatic dispersion adding unit 21.

[0077] In the following, minimum receiver sensitivity refers to the power of the electrical signal to be transmitted at the slave station when the EVM (Error Vector Magnitude) of the electrical signal transmitted to the master station is 8 percent. In other words, in an optical communication system, the smaller the minimum receiver sensitivity, the larger the reception dynamic range of the wireless signal.

[0078] FIG. 5 is a diagram showing a procedure for calculating the minimum receiving sensitivity in an optical communication system by simulation.

[0079] 5, in this simulation, first, an OFDM-modulated electrical signal to be transmitted by a slave station device was generated, with a frequency F of 1 GHz, 3 GHz, 5 GHz, 7 GHz, 9 GHz, or 11 GHz, a signal bandwidth of 400 MHz, and a subcarrier spacing of 1920 Hz. Note that frequency F refers to the minimum frequency of the signal bandwidth (step S11).

[0080] Next, in this simulation, an optical signal was generated based on an electrical signal. In the optical communication system, an optical transmitting device generates an optical signal modulated based on an electrical signal. The optical signal generated by the optical transmitting device had an optical transmission power of +5 dBm, an optical wavelength of 1550 nm, and a relative intensity noise of -145 dB / Hz. Furthermore, the optical transmitting device was assumed to perform ideal linear conversion, with an OMI (Optical Modulation Index) of 0.15 when the power of the input electrical signal was 0 dBm, and to have an infinite bandwidth (step S12).

[0081] Next, in this simulation, a phase shift φ1 based on the frequency chirp Δf(t) was added to the generated optical signal, as expressed by the following equation (3): Specifically, the square root of the power of the generated optical signal was calculated, and the calculated value was multiplied by exp(iφ1).

number

[0082] Next, in this simulation, the optical signal to which the phase shift φ1 was added was subjected to FFT processing (step S14).

[0083] Next, in this simulation, a phase shift φ2 expressed by the following equation (4) was added to the optical signal. Specifically, the FFT data obtained by FFT processing was multiplied by exp(iφ2). The phase shift φ2 corresponds to the phase shift that occurs when the optical signal is transmitted from the slave station to the master station via optical fiber.

number

[0084] Here, D is the dispersion parameter. L is the fiber transmission distance [km] of the optical signal, which corresponds to the length of the optical fiber connecting the slave station equipment and the master station equipment. λ is the wavelength [nm] of the optical signal. ω is the angular frequency [rad / sec] of the optical signal. c is the speed of light. In this simulation, the transmission loss in the optical fiber was set to 0.3 dB / km, and the dispersion parameter D in equation (4) was set to 17 picoseconds / (nm km). In this simulation, the fiber transmission distance L in equation (4) was changed from 0 km to 60 km in 5 km intervals (step S15).

[0085] Next, in a simulation of the minimum receiving sensitivity in the optical communication systems 301, 302, and 303, a phase shift φx was added to the optical signal. Specifically, the FFT data multiplied by exp(iφ2) was further multiplied by exp(iφx). The phase shift φx corresponds to the chromatic dispersion added to the optical signal by the chromatic dispersion adding unit 21. Furthermore, in the simulation of the minimum receiving sensitivity in the optical communication systems 301, 302, and 303, the insertion loss of the chromatic dispersion adding unit 21 was set to 2 dB in accordance with the specifications of a general FBG. Note that in the simulation of the minimum receiving sensitivity in the optical communication systems of Comparative Examples 1 to 3 that do not include the chromatic dispersion adding unit 21, no phase shift φx was added to the optical signal (step S16).

[0086] Next, in this simulation, the FFT data was subjected to IFFT (Inverse FFT) processing (step S17), and the absolute value of the optical signal obtained by the IFFT processing was calculated (step S18).

[0087] Next, in this simulation, an electrical signal was generated based on the optical signal, and the generated electrical signal was amplified. In the optical communication system, an electrical signal is generated by the photoelectric conversion unit 22, and the electrical signal is amplified by the amplifier 23. In this simulation, the conversion efficiency of the photoelectric conversion was set to 0.8 A / W, the dark current was set to 1.0 nA, the input-equivalent noise was set to 15 pA / Hz^(1 / 2), ideal linear conversion was performed, and the bandwidth was set to infinite (step S19).

[0088] Next, in this simulation, a predetermined Gaussian noise was added to the amplified electrical signal (step S20).

[0089] Next, in this simulation, the electrical signal to which Gaussian noise was added was subjected to FFT processing, and the FFT data obtained by the FFT processing was subjected to equalization processing. In the optical communication system, the FFT processing is performed by the FFT processing unit 25, and the equalization processing is performed by the equalizer 26 (steps S21 and S22).

[0090] Next, in this simulation, various signal processing operations were performed on the FFT data after the equalization process to obtain communication data, and the EVM was calculated based on the obtained communication data.Then, while adjusting the power of the electrical signal generated in step S11, the power of the electrical signal when the EVM was 8 percent was investigated (step S23).

[0091] Fig. 6 is a graph showing simulation results of the minimum receiving sensitivity in the optical communication system according to Comparative Example 1. In Fig. 6, the horizontal axis represents the fiber transmission distance L [km], and the vertical axis represents the minimum receiving sensitivity [dBm].

[0092] 7 is a graph showing the simulation results of the minimum receiving sensitivity in the optical communication system according to Comparative Example 2. FIG. 7 should be read in the same way as FIG.

[0093] 6 and 7, in the optical communication system according to Comparative Example 1 and the optical communication system according to Comparative Example 2, the minimum receiving sensitivity when the fiber transmission distance L is greater than zero is greater than the minimum receiving sensitivity when the fiber transmission distance L is zero.

[0094] 8 to 13 are graphs showing simulation results of the minimum receiver sensitivity in the optical communication system according to Comparative Example 3. FIG. 8 shows the minimum receiver sensitivity when the linewidth enhancement factor α is set to 3.0 and the adiabatic chirp fc is set to 4 GHz. FIG. 9 shows the minimum receiver sensitivity when the linewidth enhancement factor α is set to 3.0 and the adiabatic chirp fc is set to 8 GHz. FIG. 10 shows the minimum receiver sensitivity when the linewidth enhancement factor α is set to 3.0 and the adiabatic chirp fc is set to 12 GHz. FIG. 11 shows the minimum receiver sensitivity when the linewidth enhancement factor α is set to 6.0 and the adiabatic chirp fc is set to 4 GHz. FIG. 12 shows the minimum receiver sensitivity when the linewidth enhancement factor α is set to 6.0 and the adiabatic chirp fc is set to 8 GHz. FIG. 13 shows the minimum receiver sensitivity when the linewidth enhancement factor α is set to 6.0 and the adiabatic chirp fc is set to 12 GHz. FIGS. 8 to 13 can be interpreted in the same way as FIG. 6.

[0095] 8 to 13, in the optical communication system according to Comparative Example 3, the minimum receiving sensitivity when the fiber transmission distance L is less than 5 km is equal to or greater than the minimum receiving sensitivity of the optical communication systems according to Comparative Examples 1 and 2.

[0096] On the other hand, in the optical communication system according to Comparative Example 3, particularly when the frequency F of the electrical signal to be transmitted is 3 GHz or higher, the minimum receiving sensitivity tends to decrease as the fiber transmission distance L increases in the range of approximately 5 km to 30 km. Depending on the frequency F, the linewidth enhancement factor α, the adiabatic chirp fc, and the fiber transmission distance L, the minimum receiving sensitivity may become smaller than the minimum receiving sensitivity when the fiber transmission distance L is zero.

[0097] However, in optical communication systems that are actually used, the distance between the master station and slave station is often 20 km or less, so it is desirable to improve the minimum receiving sensitivity when the fiber transmission distance L is 20 km in optical communication systems.

[0098] 14 to 19 are graphs showing simulation results of the minimum receiver sensitivity in the optical communication system according to the first embodiment of the present disclosure. FIG. 14 shows the minimum receiver sensitivity when the linewidth enhancement factor α is set to 3.0 and the adiabatic chirp fc is set to 4 GHz. FIG. 15 shows the minimum receiver sensitivity when the linewidth enhancement factor α is set to 3.0 and the adiabatic chirp fc is set to 8 GHz. FIG. 16 shows the minimum receiver sensitivity when the linewidth enhancement factor α is set to 3.0 and the adiabatic chirp fc is set to 12 GHz. FIG. 17 shows the minimum receiver sensitivity when the linewidth enhancement factor α is set to 6.0 and the adiabatic chirp fc is set to 4 GHz. FIG. 18 shows the minimum receiver sensitivity when the linewidth enhancement factor α is set to 6.0 and the adiabatic chirp fc is set to 8 GHz. FIG. 19 shows the minimum receiver sensitivity when the linewidth enhancement factor α is set to 6.0 and the adiabatic chirp fc is set to 12 GHz. 14 to 19 should be viewed in the same way as FIG.

[0099] Referring to FIG. 14, for example, when the linewidth enhancement factor α is “3.0”, the adiabatic chirp fc is 4 GHz, and the frequency F is 11 GHz, the minimum receiving sensitivity in the optical communication systems 301, 302, and 303 is approximately 8.2 dB lower than the minimum receiving sensitivity when the fiber transmission distance L is zero, despite the presence of an insertion loss of 2 dB due to the chromatic dispersion adding unit 21.

[0100] Referring to FIG. 15, for example, when the linewidth enhancement factor α is “3.0”, the adiabatic chirp fc is 8 GHz, and the frequency F is 11 GHz, the minimum receiving sensitivity in the optical communication systems 301, 302, and 303 is approximately 9.6 dB lower than the minimum receiving sensitivity when the fiber transmission distance L is zero, despite the presence of an insertion loss of 2 dB due to the chromatic dispersion adding unit 21.

[0101] 14 to 19, the optical communication systems 301, 302, and 303 have smaller minimum receiving sensitivities when the fiber transmission distance L is 20 km or less, compared to the optical communication system according to Comparative Example 3. More specifically, the minimum receiving sensitivities in the optical communication system according to Comparative Example 3 are smallest when the fiber transmission distance L is 20 km to 35 km, whereas the minimum receiving sensitivities in the optical communication systems 301, 302, and 303 are smallest when the fiber transmission distance L is 5 km to 20 km. That is, the fiber transmission distance L at which the minimum receiving sensitivities in the optical communication systems 301, 302, and 303 are smallest is shifted to a shorter value compared to the fiber transmission distance L at which the minimum receiving sensitivities in the optical communication system according to Comparative Example 3 are smallest.

[0102] Therefore, the minimum receiving sensitivity under actual operating conditions can be improved in the optical communication systems 301, 302, and 303. That is, the receiving dynamic range of radio signals under actual operating conditions can be expanded in the optical communication systems 301, 302, and 303.

[0103] Furthermore, in optical communication systems 301, 302, and 303, the configuration in which optical signals are generated using DML 13 reduces the impact of RF fading, i.e., a decrease in amplitude components, of the electrical signal to be transmitted due to chromatic dispersion in optical fiber 191, compared to a configuration in which an MZM or EML is used instead of DML 13. On the other hand, in a configuration in which optical signals are generated using DML 13, changes in the phase characteristics of the electrical signal to be transmitted due to chromatic dispersion in optical fiber 191 tend to be greater than in a configuration in which an MZM or EML is used instead of DML 13.

[0104] In contrast, the optical communication systems 301, 302, and 303 are configured to use the DML 13 to generate optical signals based on OFDM-modulated radio signals, which allows the master station devices 201, 202, and 203 to correct the phase of the electrical signal to be transmitted with simple processing and configuration, thereby achieving better communication with less influence of RF fading while suppressing the influence of changes in phase characteristics.

[0105] [Operation flow] 20 is a diagram illustrating an example of a communication sequence in the optical communication system 301 according to the first embodiment of the present disclosure.

[0106] 20, first, the slave station device 101 receives a radio signal from the mobile communication terminal 151 via the antenna 161. The slave station device 101 amplifies the received radio signal and converts the amplified radio signal into an IF signal (step S102).

[0107] Next, the DML 13 in the slave station device 101 generates an optical signal by directly modulating the current of the IF signal (step S104).

[0108] Next, the DML 13 in the slave station device 101 outputs the generated optical signal to the optical fiber 191 (step S106).

[0109] Next, the chromatic dispersion adding unit 21 in the parent station device 201 adds, to the optical signal received from the child station device 101 via the optical fiber 191, the amount of chromatic dispersion that the optical signal would experience when transmitted using X kilometers of optical fiber 191 (step S108).

[0110] Next, the photoelectric conversion unit 22 in the master station 201 performs photoelectric conversion on the optical signal to which chromatic dispersion has been added by the chromatic dispersion adding unit 21, thereby generating an electrical signal (step S110).

[0111] Next, the master station 201 amplifies the electrical signal and demodulates the amplified electrical signal to obtain communication data (step S112).

[0112] In the optical communication systems 301, 302, and 303 according to the first embodiment of the present disclosure, the linewidth enhancement factor α of the DML 13 is set to 3 or greater, but this is not limiting. The linewidth enhancement factor α may be less than 3. Even if the linewidth enhancement factor α is less than 3, it may be possible to expand the reception dynamic range of the radio signal depending on the frequency F of the electrical signal to be transmitted and the adiabatic chirp fc of the optical signal generated by the DML 13.

[0113] In the optical communication systems 301, 302, and 303 according to the first embodiment of the present disclosure, the adiabatic chirp fc of the optical signal generated by the DML 13 is set to 4 GHz or higher, but this is not limiting. The adiabatic chirp fc may be set to less than 4 GHz. Even if the adiabatic chirp fc is set to less than 4 GHz, it may be possible to expand the reception dynamic range of the radio signal depending on the frequency F of the electrical signal to be transmitted and the linewidth enhancement factor α of the DML 13.

[0114] Furthermore, although the optical communication systems 301, 302, and 303 according to the first embodiment of the present disclosure are described as being used in an analog RoF system, the present disclosure is not limited to this and may be used in a system other than an analog RoF system.

[0115] Furthermore, in the optical communication systems 301, 302, and 303 according to the first embodiment of the present disclosure, the demodulator 24 in the master station device 201 is configured to include the equalizer 26, but this is not limiting. The demodulator 24 may be configured not to include the equalizer 26. In this case, the signal processor 27 receives unequalized FFT data from the FFT processor 25 and performs various signal processes on the received FFT data to acquire communication data.

[0116] In the optical communication systems 301, 302, and 303 according to the first embodiment of the present disclosure, the DML 13 is an FBG, but this is not limiting. The DML 13 may be a GT (Gires Tournois) etalon or an X-kilometer optical fiber provided as an extra in the master station 201.

[0117] Furthermore, in the optical communication systems 301, 302, and 303 according to the first embodiment of the present disclosure, the slave station device 101 outputs an optical signal to the optical fiber 191, and the master station devices 201, 202, and 203 add chromatic dispersion to the optical signal received from the slave station device 101 via the optical fiber 191 and generate an electrical signal at a level corresponding to the intensity of the optical signal to which chromatic dispersion has been added. However, the present invention is not limited to this. Alternatively, the master station devices 201, 202, and 203 may add chromatic dispersion to the optical signal and output the optical signal to which chromatic dispersion has been added to the optical fiber 191, and the slave station device 101 may generate an electrical signal at a level corresponding to the intensity of the optical signal received from the master station devices 201, 202, and 203 via the optical fiber 191, and transmit a radio signal based on the generated electrical signal via the antenna 161.

[0118] Meanwhile, there is a demand for a technique that can expand the dynamic range of signal reception with a simple configuration or processing.

[0119] The techniques described in Patent Documents 1 and 2 require complex circuit configurations to expand the dynamic range of received signals. Non-Patent Document 1 also presents the results of comparing the characteristics of MZM, DML, and EML. However, Non-Patent Document 1 does not disclose any technology that can expand the dynamic range of received OFDM-modulated signals under actual operating conditions.

[0120] In contrast, in optical communication systems 301, 302, and 303 according to the first embodiment of the present disclosure, the DML 13 generates an optical signal that is modulated based on an electrical signal and transmitted via an optical fiber 191. The chromatic dispersion adding unit 21 adds a predetermined amount of chromatic dispersion to the optical signal generated by the DML 13. The photoelectric conversion unit 22 outputs an electrical signal at a level corresponding to the intensity of the optical signal to which chromatic dispersion has been added by the chromatic dispersion adding unit 21.

[0121] In this way, with a configuration that adds chromatic dispersion to an optical signal generated using the DML 13 and transmitted through the optical fiber 191, and outputs 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 generate an electrical signal based on an optical signal to which an appropriate amount of chromatic dispersion has been added according to, for example, the length of the optical fiber 191. Therefore, by utilizing the characteristics of the minimum receiving sensitivity with respect to the fiber transmission distance when the DML 13 is used as the optical transmitting device, it is possible to improve the minimum receiving sensitivity of the optical communication systems 301, 302, and 303 under actual operating conditions. Therefore, in the optical communication systems 301, 302, and 303 according to the first embodiment of the present disclosure, the receiving dynamic range of signals can be expanded with a simple configuration or processing.

[0122] 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.

[0123] <Second embodiment> This embodiment, as compared with the optical communication systems 301, 302, and 303 according to the first embodiment, relates to an optical communication system 304 that adds chromatic dispersion to an optical signal before it is output to a parent station via an optical fiber 191. Other than the details described below, the optical communication system is the same as the optical communication systems 301, 302, and 303 according to the first embodiment.

[0124] 21 is a diagram illustrating a configuration of an optical communication system according to the second embodiment of the present disclosure. Referring to FIG. 21, compared to the optical communication system 301 according to the first embodiment of the present disclosure, the optical communication system 304 includes a slave station device 102 instead of the slave station device 101 and a master station device 204 instead of the master station device 201.

[0125] (Slave station device) Compared to the slave station device 101, the slave station device 102 further includes a chromatic dispersion adding unit 21.

[0126] The DML 13 generates an optical signal that is modulated based on an electrical signal and is transmitted via the optical fiber 191. More specifically, the DML 13 generates the optical signal by directly modulating the current of the IF signal received from the frequency conversion unit 12. The DML 13 outputs the generated optical signal to the chromatic dispersion adding unit 21.

[0127] The chromatic dispersion adding unit 21 adds a predetermined amount of chromatic dispersion to the optical signal generated by the DML 13 and outputs the signal to the optical fiber 191. For example, the chromatic dispersion adding unit 21 adds, to the optical signal to be transmitted to the master station 204 via the optical fiber 191, the amount of chromatic dispersion that the optical signal will experience when transmitted using the optical fiber 191 that is X kilometers long, i.e., the phase shift φx.

[0128] As described above, for example, the chromatic dispersion adding unit 21 is an FBG including an optical fiber and a diffraction grating formed in the core of the optical fiber. One end of the FBG is connected to the DML 13 in the slave station device 101, and the other end is connected to the opto-electrical converting unit 22 in the master station device 204 via an optical connector (not shown) and an optical fiber 191. The chromatic dispersion adding unit 21 adds chromatic dispersion to the optical signal received from the DML 13 and outputs the signal to the optical fiber 191.

[0129] (Master station device) The master station device 204 does not include the chromatic dispersion adder 21 as compared to the master station device 201 .

[0130] The photoelectric conversion unit 22 outputs an electrical signal at a level corresponding to the intensity of the optical signal to which chromatic dispersion has been added by the chromatic dispersion adding unit 21. For example, the photoelectric conversion unit 22 receives an optical signal from the slave station device 101 via the optical fiber 191, performs photoelectric conversion on the received optical signal to generate an electrical signal, and outputs the generated electrical signal to the amplifier 23.

[0131] The amplifier 23 amplifies the electrical signal received from the photoelectric conversion unit 22 and outputs the amplified signal to the demodulation unit 24. The demodulation unit 24 receives the electrical signal from the amplifier 23 and demodulates the received electrical signal to obtain communication data. For example, the demodulation unit 24 outputs the obtained communication data to a data processing unit (not shown).

[0132] The minimum receiving sensitivity in the optical communication system 304 can be calculated by simulation using the same procedure as for calculating the minimum receiving sensitivity in the optical communication systems 301, 302, and 303, except that the order of adding the phase shift φ2 and the phase shift φx to the optical signal is reversed, and the calculated value is the same as the minimum receiving sensitivity in the optical communication systems 301, 302, and 303. Similarly to the optical communication systems 301, 302, and 303, the optical communication system 304 has a lower minimum receiving sensitivity when the fiber transmission distance L is 20 km or less than that of the optical communication system according to Comparative Example 3. Therefore, the optical communication system 304 can improve the minimum receiving sensitivity under actual operating conditions. That is, the optical communication system 304 can expand the reception dynamic range of wireless signals under actual operating conditions.

[0133] FIG. 22 is a diagram illustrating an example of a communication sequence in an optical communication system according to the second embodiment of the present disclosure.

[0134] 22, first, the slave station device 102 receives a radio signal from the mobile communication terminal 151 via the antenna 161. The slave station device 102 amplifies the received radio signal and converts the amplified radio signal into an IF signal (step S202).

[0135] Next, the DML 13 in the slave station device 102 generates an optical signal by directly modulating the current of the IF signal (step S204).

[0136] Next, the chromatic dispersion adder 21 in the slave station device 102 adds, to the optical signal generated by the DML 13, the amount of chromatic dispersion that the optical signal will experience when transmitted over the X kilometers of optical fiber 191 (step S206).

[0137] Next, the chromatic dispersion adding unit 21 in the slave station device 102 outputs the optical signal to which chromatic dispersion has been added to the optical fiber 191 (step S208).

[0138] Next, the photoelectric conversion unit 22 in the master station device 204 performs photoelectric conversion on the optical signal received from the slave station device 102 via the optical fiber 191 to generate an electric signal (step S210).

[0139] Next, the master station 204 amplifies the electrical signal and demodulates the amplified electrical signal to obtain communication data (step S212).

[0140] In the optical communication system 304 according to the second embodiment of the present disclosure, the slave station device 102 is configured to include a chromatic dispersion adding unit 21, while the master station device 204 is configured not to include a chromatic dispersion adding unit 21. However, the present invention is not limited to this. Both the slave station device 102 and the master station device 204 may be configured to include a chromatic dispersion adding unit 21.

[0141] Furthermore, in the optical communication system 304 according to the second embodiment of the present disclosure, the slave station device 102 adds chromatic dispersion to an optical signal and outputs the optical signal with the chromatic dispersion added to the optical fiber 191, and the master station device 204 generates an electrical signal at a level corresponding to the intensity of the optical signal received from the slave station device 101 via the optical fiber 191. However, the present invention is not limited to this. The master station device 204 may output an optical signal to the optical fiber 191, and the slave station device 102 may add chromatic dispersion to the optical signal received from the master station device 204 via the optical fiber 191, generate an electrical signal at a level corresponding to the intensity of the optical signal with the chromatic dispersion added, and transmit a radio signal based on the generated electrical signal via the antenna 161.

[0142] In the optical communication system 304 according to the second embodiment of the present disclosure, the DML 13 is an FBG, but this is not limiting. The DML 13 may be a GT etalon or an X-kilometer optical fiber provided as an extra in the slave station device 102.

[0143] 23 is a diagram illustrating an example of an optical module in an optical communication system according to a modification of the second embodiment of the present disclosure. Compared to optical communication system 304, optical communication system 305 according to the modification includes optical module 210 instead of DML 13 and chromatic dispersion adding unit 21.

[0144] The optical module 210 includes a main body 211 and an optical connector 212. The main body 211 has a DML 13, a lens 51, and a wavelength dispersion adding section 61. The wavelength dispersion adding section 61 has anti-reflection films 61A and 61B, a GT etalon 61C, and an optical reflection film 61D. The optical module 210 is, for example, a TOSA (Transmitter Optical Sub-Assembly).

[0145] In optical module 210, DML 13 generates an optical signal by directly modulating the current of the IF signal received from frequency conversion unit 12, and outputs the generated optical signal to lens 51. Lens 51 focuses the optical signal output by DML 13 and outputs it to chromatic dispersion adding unit 61.

[0146] The optical signal collected by the lens 51 enters the GT etalon 61C via the anti-reflection coating 61A. The GT etalon 61C adds a predetermined amount of chromatic dispersion to the received optical signal and reflects it. The optical signal is then reflected one or more times between the optical reflection coating 61D and the GT etalon 61C, and is then output to the optical fiber 191 via the anti-reflection coating 61B and the optical connector 212.

[0147] 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.

[0148] The above description includes the following additional features. [Appendix 1] a DML that generates an optical signal modulated based on an electrical signal, the optical signal being transmitted over an optical fiber; a chromatic dispersion adding unit that adds a predetermined amount of chromatic dispersion to the optical signal; an opto-electrical conversion unit that outputs an electrical signal having a level corresponding to the intensity of the optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit; The optical communication system, wherein the wavelength dispersion adding section is an FBG. [Explanation of symbols]

[0149] 11 Amplifier 12 Frequency conversion section 13 DML 21,21A,21B Chromatic dispersion addition section 22, 22A, 22B Photoelectric conversion unit 23, 23A, 23B Amplifier 24, 24A, 24B Demodulation section 25 FFT processing section 26 Equalizer 27 Signal Processing Section 31,41 Optical coupler 51 Lens 61 Chromatic dispersion addition section 61A,61B Anti-reflection coating 61C GT etalon 61D Light-reflecting film 101,101A,101B,102 Slave station device 151, 151A, 151B Mobile communication terminal 161, 161A, 161B Antenna 191, 191A, 191A1, 191A2, 191B, 191B1, 191B2, 191C optical fiber 201,202,203,204 Master station device 210 Optical Module 211 Main body 212 Optical Connector 301, 302, 303, 304 Optical communication systems

Claims

1. a Directly Modulated Laser (DML) that generates an optical signal modulated based on an electrical signal, the optical signal being transmitted via an optical fiber; a chromatic dispersion adding unit that adds a predetermined amount of chromatic dispersion to the optical signal; an opto-electrical conversion unit that outputs an electrical signal having a level corresponding to the intensity of the optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit; The chromatic dispersion adding unit adds chromatic dispersion to the optical signal by an amount set according to the length of the optical fiber and the frequency of the electrical signal used for modulation by the DML.

2. the linewidth enhancement factor of the DML is 3 or more; The optical communication system of claim 1 , wherein the adiabatic chirp of the optical signal generated by the DML is 4 GHz or greater.

3. 3. The optical communication system according to claim 1, which is used in an analog RoF (Radio over Fiber) system.

4. a Directly Modulated Laser (DML) that generates an optical signal modulated based on an electrical signal, the optical signal being transmitted via an optical fiber; a chromatic dispersion adding unit that adds a predetermined amount of chromatic dispersion to the optical signal; an opto-electrical conversion unit that outputs an electrical signal having a level corresponding to the intensity of the optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit; an equalizer that corrects the frequency characteristics of the electrical signal output by the photoelectric conversion unit.

5. a DML that generates a modulated optical signal based on an electrical signal; a chromatic dispersion adding unit that adds a predetermined amount of chromatic dispersion to the optical signal generated by the DML and outputs the optical signal to an optical fiber; The chromatic dispersion adding unit adds chromatic dispersion to the optical signal by an amount set according to the length of the optical fiber and the frequency of the electrical signal.

6. a chromatic dispersion adding unit that adds a predetermined amount of chromatic dispersion to an optical signal generated by a DML and received via an optical fiber; a photoelectric conversion unit that outputs an electrical signal having a level corresponding to the intensity of the optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit, The chromatic dispersion adding unit adds chromatic dispersion to the optical signal by an amount set according to the length of the optical fiber and the frequency of the electrical signal used for modulation by the DML.

7. a chromatic dispersion adding unit that adds a predetermined amount of chromatic dispersion to an optical signal generated by a DML and received via an optical fiber; an opto-electrical conversion unit that outputs an electrical signal having a level corresponding to the intensity of the optical signal to which the chromatic dispersion has been added by the chromatic dispersion adding unit; an equalizer that corrects the frequency characteristics of the electrical signal output by the photoelectric conversion unit.

8. An optical communication method in an optical communication system, comprising: generating an optical signal modulated based on an electrical signal using a DML, the optical signal being transmitted over an optical fiber; adding a predetermined amount of chromatic dispersion to the optical signal; outputting an electrical signal having a level corresponding to the intensity of the optical signal to which the chromatic dispersion has been added; In the step of adding chromatic dispersion, an amount of chromatic dispersion set according to the length of the optical fiber and the frequency of the electrical signal used for modulation by the DML is added to the optical signal.

9. An optical communication method in an optical communication system, comprising: generating an optical signal modulated based on an electrical signal using a DML, the optical signal being transmitted over an optical fiber; adding a predetermined amount of chromatic dispersion to the optical signal; outputting an electrical signal having a level corresponding to the intensity of the optical signal to which the chromatic dispersion has been added; and correcting the frequency characteristics of the output electrical signal using an equalizer.

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