Optical communication system, optical communication method, optical transmission device, and optical reception device

Abstract

Provided is an optical communication system comprising an optical transmission device and an optical reception device. The optical transmission device includes: a replication unit that replicates an FM signal; a frequency shifter that frequency-shifts a plurality of replicated FM signals so that frequencies of the plurality of FM signals do not overlap; a multiplexer that multiplexes the plurality of frequency-shifted FM signals; and an intensity modulation unit that intensity-modulates the multiplexed signal, converts the modulated signal into an optical signal, and transmits the optical signal. The optical reception device includes: an intensity demodulation unit that performs intensity demodulation on an optical signal received from the optical transmission device and generates an electrical signal; a demultiplexing unit that demultiplexes the electrical signal into a plurality of FM signals; and an FM signal processing unit that restores a signal before FM modulation on the basis of the plurality of demultiplexed FM signals.

Description

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

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

[0002] 2. Description of the Related Art There exists an FM batch transmission system that uses FM batch conversion to convert multi-channel video signals into optical signals, which are transmitted through an optical transmission line, demodulated on the receiving side, and output to a video viewing device such as a TV.

[0003] Toshiaki Shimoba et al., "Study on wideband RF signal transmission system using FM batch conversion method with all-channel phase modulation," IEICE General Conference, 2021. Ryo Miyatake et al., "Optical transmission experiment on FM conversion method with wideband phase modulation," IEICE Communications Express, 2021.

[0004] In an FM bulk transmission system, if there is frequency selectivity on the transmitting or receiving side, even if the transmitting side sends a signal with a constant amplitude, the amplitude of waves of specific frequencies in the signal demodulated on the receiving side will be attenuated. This can cause distortion in the transmitted signal and degrade reception quality.

[0005] The object of the present invention is to improve distortion characteristics.

[0006] One aspect of the present invention is an optical communication system comprising an optical transmitting device and an optical receiving device, wherein the optical transmitting device comprises a duplication unit that duplicates FM signals, a frequency shifter that frequency shifts the multiple FM signals so that the frequencies of the duplicated multiple FM signals do not overlap, a multiplexer that combines the multiple frequency-shifted FM signals, and an intensity modulation unit that intensity-modulates the combined signal, converts it into an optical signal, and transmits the optical signal, and the optical receiving device comprises an intensity demodulation unit that intensity-demodulates the optical signal received from the optical transmitting device and generates an electrical signal, a demultiplexing unit that demultiplexes the electrical signal into multiple FM signals, and an FM signal processing unit that restores the signal before it was FM-modulated by the transmitting side based on the demultiplexed multiple FM signals.

[0007] One aspect of the present invention is an optical communication method having an optical transmitting means that duplicates FM signals, frequency-shifts the duplicated FM signals so that the frequencies of the duplicated FM signals do not overlap, combines the frequency-shifted FM signals, intensity-modulates the combined signal, converts it into an optical signal, and transmits the optical signal; and an optical receiving means that intensity-demodulates the transmitted optical signal, generates an electrical signal, demultiplexes the electrical signal into a plurality of FM signals, and restores the signal before being FM-modulated by the transmitting side based on the demultiplexed FM signals.

[0008] One aspect of the present invention is an optical transmitting device comprising a duplication unit that duplicates an FM signal, a frequency shifter that frequency-shifts the multiple FM signals so that the frequencies of the duplicated multiple FM signals do not overlap, a multiplexer that combines the multiple frequency-shifted FM signals, and an intensity modulation unit that intensity-modulates the combined signal, converts it into an optical signal, and transmits the optical signal.

[0009] One aspect of the present invention is an optical receiving device comprising an intensity demodulation unit that intensity demodulates an optical signal received from the optical transmitting device and generates an electrical signal, a demultiplexing unit that demultiplexes the electrical signal into a plurality of FM signals, and an FM signal processing unit that restores the signal before it was FM modulated by the transmitting side based on the demultiplexed plurality of FM signals.

[0010] The present invention can improve the distortion characteristics.

[0011] FIG. 1 is a diagram illustrating a configuration of an optical communication system according to a first embodiment. FIG. 2 is a diagram illustrating an example of a configuration of an optical transmitting device according to the first embodiment. FIG. 3 is a diagram illustrating an example of a carrier wave and a side wave. FIG. 4 is a diagram illustrating an example of a configuration of an optical receiving device according to the first embodiment. FIG. 5 is a flowchart illustrating an operation of an optical transmitting device. FIG. 6 is a flowchart illustrating an operation of an optical receiving device. FIG. 7 is a diagram illustrating an example of a carrier wave and a side wave selected by an FM signal selecting unit according to a second embodiment. FIG. 8 is a diagram illustrating an example of a configuration of an optical receiving device according to a third embodiment.

[0012] Embodiments of the present invention will be described in detail with reference to the drawings. Fig. 1 is a diagram showing the configuration of an optical communication system 1 according to a first embodiment. The optical communication system 1 is a system for transmitting video by optical signals. The optical communication system 1 includes an optical transmitting device 10, an optical transmission line 20, and an optical receiving device 30.

[0013] The optical transmitter 10 receives a multi-channel video signal from a facility such as a broadcasting station. The optical transmitter 10 frequency-modulates and intensity-modulates the multi-channel video signal, converting it into an optical signal. The optical transmitter 10 transmits the optical signal to the optical receiver 30 via an optical transmission path 20.

[0014] The optical receiving device 30 receives the optical signal from the optical transmitting device 10, and generates a multi-channel video signal by intensity demodulating and FM demodulating the optical signal. The optical receiving device 30 outputs the generated multi-channel video signal to, for example, an external video viewing device 90. The video viewing device is, for example, a television.

[0015] The optical transmitting device 10 will be described in detail below. Fig. 2 is a diagram showing an example of the configuration of the optical transmitting device 10 according to the first embodiment. The optical transmitting device 10 includes an FM modulation unit 11, a replicating unit 12, a frequency shifter 13, a multiplexer 14, and an intensity modulation unit 15.

[0016] The FM modulation unit 11 performs FM modulation on the input multi-channel video signal to generate an FM signal. The multi-channel video signal is a signal obtained by frequency multiplexing video signals having different frequencies for each channel. The video signals for each channel may be, for example, a CATV signal, a BS / CS right-hand circular polarization IF signal, or a BS / CS left-hand circular polarization IF signal.

[0017] FM signals have a carrier wave and sidewaves. The sidewaves are divided into upper sidewaves, which are waves with frequencies higher than the carrier wave frequency, and lower sidewaves, which are waves with frequencies higher than the carrier wave frequency. Furthermore, the upper sidewaves are divided into first upper sidewave, second upper sidewave, etc., in order of frequency proximity to the carrier wave, and the lower sidewaves are divided into first lower sidewave, second lower sidewave, etc., in order of frequency proximity to the carrier wave. The sidewaves of FM signals generally tend to have smaller amplitudes the farther away they are from the carrier wave in frequency. However, even with smaller amplitudes, theoretically, sidewaves can occur infinitely far. In other words, FM signals have a radio bandwidth. However, since signals with infinite bandwidths cannot be handled in practice, a threshold is generally set for the amplitude, and sidewaves with amplitudes smaller than this threshold are ignored as nonexistent. In this specification, sidewaves below a predetermined amplitude threshold are considered nonexistent, and each FM signal is treated as having a finite bandwidth. Furthermore, the waveform of an FM signal varies depending on the modulation index used in FM modulation.

[0018] FIG. 3 shows an example of a carrier wave and sidewaves. The kth upper sidewave is denoted as pk (k is an integer), and the kth lower sidewave is denoted as mk (k is an integer). The "amplitude of an FM signal" refers to the "amplitude of the carrier wave and each sidewave of an FM signal." The instantaneous value of the amplitude of an FM signal may fluctuate when the modulation index is fixed, but the time-averaged value of the amplitude of the FM signal is uniquely determined by the modulation index.

[0019] The replicating unit 12 replicates the FM signal generated by the FM modulating unit 11. The replicating unit 12 is, for example, a coupler, and generates a plurality of identical FM signals.

[0020] The optical transmitting device 10 includes frequency shifters 13 in the same number as the number of FM signals generated by duplication by the duplication unit 12. For example, if N FM signals (N is an integer) are generated by duplication by the duplication unit 12, the optical transmitting device 10 includes N frequency shifters 13-1, 13-2, ... 13-N. The frequency shifters 13 shift the frequencies of the FM signals generated by the duplication unit 12. Here, each frequency shifter 13 shifts the frequency of the FM signal so that the frequency does not overlap with that of FM signals frequency-shifted by other frequency shifters 13.

[0021] For example, the frequency shifter 13-k (k=1, 2, . . . N) shifts the frequency of the FM signal by a frequency shift amount f ｋ The frequency shift amount f ｋ As the value of k increases, f １ <f ２ <...<f Ｎ In this case, the difference between the FM signals to be frequency-shifted is f ２ -f １ , f ３ -f ２ ...f Ｎ -f Ｎ－１ If f is larger than the frequency width of the FM signal, there will be no overlap in the frequencies of the FM signals to be frequency-shifted. The frequency width of the FM signal is the difference in frequency between the upper side wave, which has the largest frequency difference from the carrier wave, and the lower side wave, which has the largest frequency difference from the carrier wave. The frequency shift amount f by each frequency shifter 13 is ｋ may be set in advance based on the frequency width of the FM signal, or may be determined based on the FM signal generated by the FM modulation unit 11.

[0022] The combiner 14 combines the FM signals frequency-shifted by each frequency shifter 13. The combined FM signal is shown in Figure 1. The combined FM signal is a signal in which duplicated FM signals are arranged so that they do not overlap on the frequency axis.

[0023] The intensity modulation unit 15 intensity-modulates the FM signal multiplexed by the multiplexer 14 and converts it into an optical signal. The intensity modulation unit 15 transmits the optical signal to the optical receiving device 30 via the optical transmission line 20.

[0024] Next, a detailed description will be given of the optical receiving device 30. Fig. 4 is a diagram showing an example of the configuration of the optical receiving device 30 according to the first embodiment. The optical receiving device 30 includes an intensity demodulator 31, a branching unit 32, an FM signal processor 33, and a storage unit 35.

[0025] The intensity demodulator 31 demodulates the intensity of the optical signal received from the optical transmitter 10 to generate an electrical signal. The electrical signal generated here is the combined FM signal generated in the optical transmitter 10.

[0026] The demultiplexing unit 32 demultiplexes the electrical signal (combined FM signal) generated by the intensity demodulation unit 31 into a plurality of FM signals of different frequency bands. The demultiplexing unit 32 is, for example, a band-pass filter corresponding to the frequency of each FM signal.

[0027] The FM signal processing unit 33 restores the signal before FM modulation by the optical transmitting device 10 based on the multiple FM signals demultiplexed by the demultiplexing unit 32. The FM signal processing unit 33 includes an FM signal selection unit 331 and an FM demodulation unit 332. The FM signal selection unit 331 selects the FM signal with the highest quality from the multiple FM signals of different frequency bands demultiplexed by the demultiplexing unit 32. The FM signal selection unit 331 compares the ideal value of the FM signal amplitude with the amplitude of each FM signal demultiplexed by the demultiplexing unit 32. The ideal value of the FM signal amplitude is the time-averaged value of the amplitude of the FM signal generated by the FM modulation unit 11 of the optical transmitting device 10. The time-averaged value of the amplitude of the FM signal generated by the FM modulation unit 11 can be calculated based on the modulation index. The time-averaged value of the amplitude of the FM signal is calculated in advance based on, for example, the modulation index and stored in advance in the storage unit 35. Alternatively, the FM signal selection unit 331 may have an algorithm for calculating the time average value of the amplitude of the FM signal based on the modulation index, and may calculate the time average value of the amplitude of the FM signal by inputting the modulation index. Furthermore, in order to identify the frequency band of each FM signal demultiplexed by the demultiplexing unit 32, the amount of frequency shift applied to each FM signal by the frequency shifter 13 is stored in advance in the storage unit 35. The FM signal selection unit 331 compares the amplitude of each FM signal demultiplexed by the demultiplexing unit 32 with the ideal value of the amplitude of the FM signal, using the time average value of the amplitude of the FM signal and the amount of frequency shift applied to each FM signal stored in the storage unit 35.

[0028] The FM signal selection unit 331, for example, calculates the difference between the ideal value of the FM signal amplitude and the amplitude of each FM signal demultiplexed by the demultiplexing unit 32, and selects the FM signal with the smallest difference as the "FM signal of the best quality." Here, the "difference between the ideal value of the FM signal amplitude and the amplitude of the FM signal demultiplexed by the demultiplexing unit 32" refers to the "sum of the differences between the ideal values ​​of the amplitude of each of the carrier and side waves of the FM signal and the amplitude of each of the carrier and side waves of the FM signal demultiplexed by the demultiplexing unit 32." The FM signal selection unit 331, for example, calculates the rate of fluctuation of the amplitude of each FM signal demultiplexed by the demultiplexing unit 32 with respect to the ideal value of the FM signal amplitude, and selects the FM signal with the smallest rate of fluctuation as the "FM signal of the best quality." Here, "the rate of change of the amplitude of the FM signal separated by the separation unit 32 relative to the ideal value of the amplitude of the FM signal" means "the sum of the rates of change of the amplitude of each of the carrier wave and side waves of the FM signal separated by the separation unit 32 relative to the ideal value of the amplitude of each of the carrier wave and side waves of the FM signal."

[0029] In the example shown in Figure 4, the FM signals separated by the branching unit 32 are three signals: a first FM signal, a second FM signal, and a third FM signal. In Figure 4, the ideal amplitude of the FM signals is indicated by a dashed line, and the amplitudes of the first FM signal, the second FM signal, and the third FM signal are indicated by a solid line. In the example shown in Figure 4, distortion and degradation due to frequency selectivity occurs in the frequency band from the third upper sideband (1-p3) of the first FM signal to the first upper sideband (2-p1) of the second FM signal. In the example shown in Figure 4, the FM signal selection unit 331 selects the third FM signal, which is not affected by frequency selectivity and has an amplitude closest to the ideal value, as the FM signal with the best quality among the first FM signal, the second FM signal, and the third FM signal.

[0030] The FM demodulator 332 applies a frequency shift amount in the opposite direction to the frequency shift amount applied by the frequency shifter 13 to the FM signal selected by the FM signal selector 331, thereby restoring the frequency of the selected FM signal to the frequency of the original FM signal. The FM demodulator 332 demodulates the FM signal that has returned to the original frequency. Note that, at the stage when the FM signal selector 331 selects an FM signal, the FM signal selector 331 may apply a frequency shift amount in the opposite direction to the frequency shift amount applied by the frequency shifter 13 to each FM signal demultiplexed by the demultiplexer 32, thereby restoring the frequency of each FM signal to the frequency of the original FM signal.

[0031] The FM demodulation unit 332 demodulates the FM signal to restore the multi-channel video signal, and outputs the restored multi-channel video signal to, for example, an external video playback device.

[0032] 5 is a flowchart showing the operation of the optical transmitter 10. The FM modulator 11 modulates the multi-channel video signal into an FM signal (step S11). The duplicator 12 duplicates the FM signal generated by the FM modulator 11 (step S12). Each frequency shifter 13 frequency-shifts the duplicated FM signal (step S13). The combiner 14 combines the frequency-shifted FM signals (step S14). The intensity modulator 15 intensity-modulates the combined FM signal and converts it into an optical signal (step S15). The optical signal is transmitted to the optical receiver 30 via the optical transmission path 20.

[0033] FIG. 6 is a flowchart showing the operation of the optical receiving device 30. The intensity demodulator 31 intensity-demodulates the received signal (step S21). The electrical signal generated by the intensity demodulation in step S21 is the combined FM signal generated by the optical transmitting device 10. The demultiplexer 32 demultiplexes the electrical signal (combined FM signal) generated by the intensity demodulator 31 into multiple FM signals of different frequency bands (step S22). Multiple FM signals of different frequency bands are generated in step S22. The FM signal selector 331 selects the FM signal with the best quality from the multiple FM signals of different frequency bands demultiplexed by the demultiplexer 32 (step S23). The FM demodulator 332 demodulates the FM signal selected by the FM signal selector 331 (step S24). At this time, the FM demodulator 332 demodulates the selected FM signal by applying a frequency shift in the opposite direction to the frequency shift applied by the transmitting side. As a result, the multi-channel video signals on the transmitting side are restored and output to a video shooting device or the like.

[0034] The FM demodulation unit 332 of the optical receiving device 30 demodulates the FM signal with the best quality and restores the multi-channel video signal, thereby preventing the demodulation of a distorted FM signal, thereby preventing distortion from occurring in the demodulated multi-channel video signal.

[0035] (Second Embodiment) The FM signal selection unit 331 according to the first embodiment selects the FM signal with the best quality from a plurality of FM signals. The FM signal selection unit 331 according to the second embodiment selects the carrier wave and sidewaves with the best quality from a plurality of FM signals and generates an FM signal by combining the selected carrier wave and sidewaves. The FM signal selection unit 331 according to the second embodiment extracts the carrier wave and sidewaves from each FM signal. The FM signal selection unit 331 extracts the carrier wave and sidewaves from each FM signal using, for example, a frequency filter. Note that the demultiplexing unit 32 may extract the carrier wave and sidewaves from each FM signal.

[0036] The FM signal selection unit 331 compares the ideal value of the amplitude of the carrier wave of the FM signal with the amplitude of the carrier wave of the extracted FM signal. The ideal value of the amplitude of the carrier wave of the FM signal is the time-average value of the amplitude of the carrier wave of the FM signal generated by the FM modulation unit 11 of the optical transmission device 10. The time-average value of the amplitude of the carrier wave of the FM signal generated by the FM modulation unit 11 can be calculated using the modulation index. The time-average value of the amplitude of the carrier wave of the FM signal is calculated in advance based on, for example, the modulation index and stored in advance in the storage unit 35. Alternatively, the FM signal selection unit 331 may have an algorithm for calculating the time-average value of the amplitude of the carrier wave of the FM signal based on the modulation index, and may calculate the time-average value of the amplitude of the FM signal by inputting the modulation index.

[0037] Furthermore, in order to identify the carrier frequency of each FM signal, the amount of frequency shift applied to each FM signal by frequency shifter 13 is stored in advance in storage unit 35. FM signal selection unit 331 uses the time average value of the carrier amplitude of the FM signal stored in storage unit 35 and the amount of frequency shift applied to each FM signal to compare the ideal value of the carrier amplitude of the FM signal with the carrier amplitude of each FM signal separated by separation unit 32.

[0038] The FM signal selection unit 331 calculates, for example, the difference between the ideal value of the amplitude of the carrier wave of the FM signal and the amplitude of the carrier wave of each FM signal separated by the separation unit 32, and selects the carrier wave with the smallest difference as the "carrier wave with the best quality." The FM signal selection unit 331 calculates, for example, the fluctuation rate of the amplitude of the carrier wave of each FM signal separated by the separation unit 32 with respect to the ideal value of the amplitude of the carrier wave of the FM signal, and selects the carrier wave with the smallest fluctuation rate as the "carrier wave with the best quality."

[0039] The FM signal selection unit 331 compares the ideal value of the sidewaves of the FM signal with the amplitude of the extracted sidewaves of the FM signal for each sidewave (first upper sidewave, second upper sidewave, ..., first lower sidewave, second lower sidewave, ...). The ideal value of the sidewaves of the FM signal is the time-average value of the amplitude of the sidewaves of the FM signal generated by the FM modulation unit 11 of the optical transmitting device 10. The time-average value of the amplitude of the sidewaves of the FM signal generated by the FM modulation unit 11 can be calculated using the modulation index. The time-average value of the amplitude of the sidewaves of the FM signal may be calculated in advance based on, for example, the modulation index and stored in the storage unit 35 in advance. Alternatively, the FM signal selection unit 331 may have an algorithm for calculating the time-average value of the amplitude of the sidewaves of the FM signal based on the modulation index, and the time-average value of the amplitude of the FM signal may be calculated by inputting the modulation index.

[0040] Furthermore, in order to identify the frequency of the sidebands of each FM signal, the amount of frequency shift applied to each FM signal by frequency shifter 13 is stored in advance in storage unit 35. FM signal selection unit 331 uses the time-averaged value of the amplitude of the sidebands of the FM signals stored in storage unit 35 and the amount of frequency shift applied to each FM signal to compare the ideal value of the amplitude of the sidebands of the FM signal with the amplitude of the sidebands of each FM signal demultiplexed by demultiplexer 32.

[0041] For example, the FM signal selection unit 331 calculates the difference between the ideal value of the amplitude of the sidewaves of the FM signal and the amplitude of the sidewaves of each FM signal separated by the separation unit 32, and selects the sidewave with the smallest difference as the "sidewave with the best quality." For example, the FM signal selection unit 331 calculates the rate of fluctuation of the amplitude of the sidewaves of each FM signal separated by the separation unit 32 with respect to the ideal value of the amplitude of the sidewaves of the FM signal, and selects the sidewave with the smallest rate of fluctuation as the "sidewave with the best quality." In this way, the FM signal selection unit 331 can select the carrier wave and sidewave with the best quality from multiple FM signals.

[0042] The FM signal selection unit 331 applies a frequency shift amount in the opposite direction to the frequency shift amount applied by the frequency shifter 13 to each of the selected carrier wave and sidewave, thereby restoring the frequency of the selected carrier wave or sidewave to the frequency of the carrier wave or sidewave of the original FM signal.The FM signal selection unit 331 then combines the carrier wave and sidewave whose frequencies have been shifted back to generate the "FM signal of the highest quality."A combiner, for example, is used to combine the carrier wave and sidewave.

[0043] FIG. 7 shows an example of carrier waves and sidewaves selected by the FM signal selection unit 331 according to the second embodiment. In the example shown in FIG. 7 , the FM signals separated by the separation unit 32 are three signals: a first FM signal, a second FM signal, and a third FM signal. In FIG. 7 , the ideal amplitude values ​​of the FM signals are indicated by dashed lines, and the amplitudes of the first FM signal, the second FM signal, and the third FM signal are indicated by solid lines. Among the first FM signal, the second FM signal, and the third FM signal, the third upper sidewave and fourth upper sidewave (1-p3, 1-p4) of the first FM signal, the carrier wave and sidewaves of the second FM signal, and the third lower sidewave and fourth lower sidewave (3-m3, 3-m4) of the third FM signal have amplitude changes due to frequency selectivity. At this time, the FM signal selection unit 331 selects the lower sidewaves from the first lower sidewave to the fourth lower sidewave, the carrier wave, the first upper sidewave, and the second upper sidewave from the first FM signal, and selects the third upper sidewave and the fourth upper sidewave from the third FM signal, and generates a new FM signal by combining the selected carrier wave and sidewaves.

[0044] As described above, the FM signal selection unit 331 according to the second embodiment selects the best quality signal not on an FM signal basis but on a carrier wave and sidewave basis, and generates a high-quality FM signal by combining the selected signals. As a result, compared to the optical receiving device 30 according to the first embodiment, the optical receiving device 30 according to the second embodiment can generate a high-quality FM signal even if all of the copied FM signals are affected by frequency selectivity.

[0045] 8 is a diagram showing an example of the configuration of an optical receiving device 30 according to the third embodiment. The FM signal processing unit 33 according to the third embodiment includes N FM demodulation units 332 and a waveform selection unit 333.

[0046] Each FM demodulator 332 according to the third embodiment demodulates the FM signals demultiplexed by the demultiplexer 32 to restore multi-channel video signals. The FM demodulator 332 according to the third embodiment applies a frequency shift amount in the opposite direction to the frequency shift amount applied by the frequency shifter 13 to the demultiplexed FM signals, restores the frequency of the original FM signal, and then demodulates the signal. As a result, the same number of multi-channel video signals as the number of demultiplexed FM signals are restored.

[0047] The waveform selection unit 333 selects the "multi-channel video signal with the best quality" from among the multi-channel video signals restored by each FM demodulation unit 332. The waveform selection unit 333 outputs the selected multi-channel video signal with the best quality to, for example, an external video viewing device.

[0048] The waveform selection unit 333 selects, for example, a multi-channel video signal in which a pre-inserted pilot signal or a known signal can be most accurately restored as the "multi-channel video signal of the best quality." For example, the waveform selection unit 333 selects, for example, a multi-channel video signal with the least block noise as the "multi-channel video signal of the best quality."

[0049] The optical receiving device 30 of the third embodiment demodulates all of the duplicated FM signals and selects the signal with the best quality from the demodulated signals, thereby achieving the same effect as when the optical receiving device 30 of the first embodiment demodulates the FM signal with the best quality.

[0050] Other Embodiments One embodiment of the present invention has been described in detail above with reference to the drawings, but the specific configuration is not limited to that described above, and various design changes and the like can be made within the scope that does not deviate from the gist of the present invention.

[0051] The signal input to the optical transmitter 10 is not limited to a multi-channel video signal, but may be any signal in which signals of different frequencies are frequency-multiplexed.

[0052] A portion of the processing of the optical transmitter 10 and the optical receiver 30 in the above-described embodiments may be implemented by a computer using software. In this case, a program for implementing this function may be recorded on a computer-readable recording medium, and the program may be loaded into a computer system and executed. Note that the term "computer system" as used herein includes hardware such as an OS and peripheral devices. Furthermore, the term "computer-readable recording medium" refers to portable media such as flexible disks, optical magnetic disks, ROMs, and CD-ROMs, as well as storage devices such as hard disks built into a computer system. Furthermore, the term "computer-readable recording medium" may also include media that dynamically store programs for a short period of time, such as communication lines used when transmitting programs via networks such as the Internet or telephone lines, or media that store programs for a fixed period of time, such as volatile memory within the computer system that serves as the server or client. The program may be a program that implements a portion of the above-described functions, or may be a program that can implement the above-described functions in combination with a program already stored in the computer system, or may be implemented using a programmable logic device such as an FPGA (Field Programmable Gate Array).

[0053] 1 Optical communication system, 10 Optical transmitter, 11 FM modulation unit, 12 Replicator, 13 Frequency shifter, 14 Multiplexer, 15 Intensity modulation unit, 20 Optical transmission path, 30 Optical receiver, 31 Intensity demodulator, 32 Branching unit, 33 FM signal processing unit, 331 FM signal selection unit, 332 FM demodulator, 333 Waveform selection unit, 35 Storage unit

Claims

1. An optical communication system comprising an optical transmitting device and an optical receiving device, wherein the optical transmitting device comprises: a duplicating unit that duplicates FM signals; a frequency shifter that frequency shifts the multiple FM signals so that the frequencies of the multiple duplicated FM signals do not overlap; a combiner that combines the multiple frequency-shifted FM signals; and an intensity modulator that intensity-modulates the combined signal, converts it into an optical signal, and transmits the optical signal; and the optical receiving device comprises: an intensity demodulator that intensity-demodulates the optical signal received from the optical transmitting device and generates an electrical signal; a demultiplexer that demultiplexes the electrical signal into multiple FM signals; and an FM signal processor that restores the signal before it was FM-modulated by the transmitting side based on the demultiplexed multiple FM signals.

2. The optical communication system according to claim 1, wherein the FM signal processing unit comprises: an FM signal selection unit that selects the FM signal with the best quality from the plurality of demultiplexed FM signals; and an FM demodulation unit that demodulates the selected FM signal.

3. The optical communication system according to claim 2, wherein the FM signal selection unit selects the best quality carrier wave and each side wave from the plurality of FM signals, and generates an FM signal by combining the selected carrier wave and each side wave.

4. The optical communication system according to claim 1, wherein the FM signal processing unit comprises: an FM demodulation unit that demodulates the plurality of demultiplexed FM signals; and a waveform selection unit that selects the waveform with the best quality from the plurality of demodulated FM signals.

5. The optical communication system according to claim 2, wherein, of the plurality of FM signals, the FM signal having the smallest difference from the ideal value of the FM signal amplitude or the smallest fluctuation rate from the ideal value of the amplitude is selected as the FM signal of the best quality.

6. An optical communication method comprising: an optical transmitting means for duplicating an FM signal, frequency-shifting the multiple FM signals so that the frequencies of the multiple duplicated FM signals do not overlap, combining the multiple frequency-shifted FM signals, intensity-modulating the combined signal, converting it into an optical signal, and transmitting the optical signal; and an optical receiving means for intensity-demodulating the transmitted optical signal to generate an electrical signal, separating the electrical signal into multiple FM signals, and restoring the signal before being FM-modulated by the transmitting side based on the separated multiple FM signals.

7. An optical transmitting device comprising: a duplication unit that duplicates an FM signal; a frequency shifter that frequency-shifts the multiple FM signals so that the frequencies of the duplicated multiple FM signals do not overlap; a multiplexer that combines the multiple frequency-shifted FM signals; and an intensity modulation unit that intensity-modulates the combined signal, converts it into an optical signal, and transmits the optical signal.

8. An optical receiving device comprising: an intensity demodulation unit that intensity-demodulates a received optical signal and generates an electrical signal; a demultiplexing unit that demultiplexes the electrical signal into a plurality of FM signals; and an FM signal processing unit that restores the signal before being FM-modulated by the transmitting side based on the plurality of demultiplexed FM signals.

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