JP2006345156A - Optical communication system and optical communication method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical communication system and an optical communication method wherein a receiver side improves deterioration in accuracy of a received optical signal. <P>SOLUTION: A transmitter 2 inserts a reference signal to data signals for every prescribed period (or every prescribed data amount), the reference signal including three pulses with different amplitudes including at least the maximum value and the minimum value of the data signals, and a receiver 3 corrects the received data signals on the basis of the three received pulses to compensate transmission characteristics (nonlinear characteristics) of an optical fiber line 90. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical communication system and an optical communication method for performing communication based on a multi-value optical signal corresponding to a light amount.

In recent years, with the widespread use of digital broadcasting and streaming video distribution, high-speed communication technology is required even in the home, and therefore, technical development related to optical communication is widely performed.
For example, in Patent Document 1 below, data transmission of an optical signal that enables high-density transmission by amplitude-modulating the data signal on the reception side based on the amplitude of the reference signal added to the data signal on the transmission side. The technology is disclosed.

JP 2000-244586 A

  By the way, when optical communication is performed, the accuracy of received data on the receiving side is likely to deteriorate due to the following factors associated with the transmission system. However, Patent Document 1 does not disclose a technique for improving this accuracy deterioration.

(1) Signal loss in an optical fiber Especially in the case of long-distance communication, the loss of a transmission signal by an optical fiber is large. Particularly in the case of a burst signal, it is difficult to control the level of the received signal to a constant value.
(2) Physical change of optical fiber, etc. The optical signal reception level changes due to relatively gradual environmental changes such as temperature fluctuations, and also due to changes in physical characteristics such as bending and pulling of optical fibers. Vary between.

  The present invention has been made in view of the above-described viewpoints, and an object thereof is to provide an optical communication system and an optical communication method capable of improving accuracy deterioration of an optical signal received on a receiving side.

  In order to overcome the above problems, a first aspect of the present invention is an optical communication system that performs communication based on a multilevel optical signal corresponding to a light amount between a transmission device and a reception device, wherein the transmission device Transmits a reference signal having predetermined three values including at least a maximum value and a minimum value among the multiple values in addition to the data signal for each predetermined period, and the receiving device receives the reference signal based on the reference signal The optical communication system is configured to synchronize for sampling the data signal and correct the received data signal according to a received optical signal level corresponding to the predetermined three values.

  In order to overcome the above-described problems, a second aspect of the present invention is an optical communication method for performing communication between a transmission device and a reception device based on a multilevel optical signal corresponding to a light amount, wherein the transmission device In addition to the data signal, a first step of transmitting a reference signal having a predetermined three values including at least a maximum value and a minimum value among the multiple values for each predetermined period; and the receiving device based on the reference signal And a second step of synchronizing the received data signal for sampling, and a third step of correcting the received data signal by the receiving device based on a received optical signal level corresponding to the predetermined three values. And an optical communication method.

  In order to overcome the above-described problems, a third aspect of the present invention is an optical communication system that performs communication based on a multilevel optical signal corresponding to a light amount between a transmission device and a reception device, wherein the transmission device Generates and transmits a data signal based on the first reference clock, and has a data rate higher than that of the first reference clock and the same sample value by the first reference clock. The reference signal including a plurality of pulses is transmitted every first period, and the receiving device is configured to generate a reference clock whose sample value of the plurality of pulses becomes the first sample value based on at least one of the plurality of pulses. A second reference clock having the same frequency as that of the first reference clock, and a reference signal for sampling the data signal that has received the second reference clock; That is an optical communication system.

  In order to overcome the above-described problem, a fourth aspect of the present invention is an optical communication method for performing communication between a transmission device and a reception device based on a multilevel optical signal corresponding to a light amount, and the transmission device A first step of generating and transmitting a data signal based on a first reference clock, and the transmission device has a data rate higher than that of the first reference clock and a sample value based on the first reference clock. A second step of transmitting a reference signal including a plurality of pulses having the same first sample value for each predetermined period, and the receiver is configured to sample the plurality of pulses based on at least one pulse of the plurality of pulses. A third step of generating a second reference clock having the same frequency as the first reference clock, the reference clock having a value equal to the first sample value; and the receiving device receives the data signal A fourth step, the receiving apparatus, a fifth step of sampling the said data signal received by said second reference clock, is optical communication method having.

  According to the present invention, it is possible to improve accuracy degradation of an optical signal received on the receiving side.

Hereinafter, an embodiment of an optical communication system according to the present invention will be described with reference to the accompanying drawings.
<First Embodiment>
FIG. 1 is a diagram illustrating a configuration of an optical communication system 1 according to the first embodiment.

The optical communication system 1 according to the embodiment includes a data processing device 4 that generates data to be transmitted, a transmission device 2 that transmits data, and a reception device 3 that receives data.
The data processing device 4 is a device that generates desired digital data, such as a computer device or an image generation device. In FIG. 1, the data processing device 4 generates serial data DATA as such digital data and transmits it to the transmission device 2. In the optical communication system 1 of the present embodiment, it is assumed that serial data DATA is transmitted to the transmission device 2 in a MAC (Media Access Control) frame format, for example.
In addition, the data processing device 4 transmits a clock signal CLK serving as an operation reference to the transmission device 2.

In the transmission device 2, the data of the MAC frame transmitted from the data processing device 4 is processed in units of 8 bits in order from the LSB. The 8-bit data is photoelectrically converted and an optical signal is sent to the optical fiber line 90. This optical signal is a multi-value signal corresponding to the amount of light (light intensity).
In addition, the transmission device 2 generates its own operation clock signal CLK1 from the clock signal CLK acquired from the data processing device 4, and processes the MAC frame and generates various signals to be transmitted based on the clock signal CLK1. Do.
In the following description, data obtained from the data processing device 4 and converted from data to be transmitted into an optical signal is referred to as a data signal. In addition to this data signal, the transmitter 2 generates a reference signal as an optical signal for its own LD (laser diode) power control and clock recovery at the receiving side, as will be described later, and this reference signal is used as data. The signal is added to the signal and transmitted to the receiver 3.

  The interval of the reference signal can be set as appropriate, but it depends on the allowable level of clock deviation between the transmitter 2 and the receiver 3, the rate of fluctuation of loss due to the optical fiber line 90, and the rate of fluctuation of LD power accompanying temperature change. Thus, the upper limit required for the system is determined.

The receiving device 3 includes a photodiode that receives the optical signal transmitted from the transmitting device 2.
In the reception device 3, the clock signal CLK2 is reproduced based on the pulse included in the reference signal received from the transmission device 2, and the received data signal is sampled based on the reproduced clock signal CLK2.

Next, a specific configuration of the transmission device 2 will be described.
FIG. 2 is a diagram illustrating an example of a circuit configuration of the transmission device 2.

  As shown in FIG. 2, the transmission device 2 includes a serial / parallel converter 21, an LD power controller 22, a current controller 23, a resolution setting unit 24, a digital / analog converter 25, a laser diode L1 as a light emitting element, A photodiode P2 is provided as a light receiving element for monitoring.

In order to process the MAC frame received from the data processing device 4 in units of 8 bits, the serial / parallel converter 21 converts the serial data DATA into 8-bit (data D0 to D7) parallel signals in synchronization with the clock signal CLK1. Convert. Further, a signal corresponding to the level of the 8-bit data D0 to D7 is given to the current control unit 23.
In FIG. 2, for example, when the data D7 is “1”, the signal S211 becomes “L level (low level)”, and the signal S212 becomes “H level”. For example, when the data D7 is “0”, the signal S211 becomes “H level” and the signal S212 becomes “L level”. The same applies to the other data D0 to D6.

The current control unit 23 controls the bias current of the laser diode L1 and the control circuits 230 to 237 that control the current corresponding to the 8-bit output (data D0 to D7) of the serial / parallel conversion unit 21 to flow to the laser diode L1. And a current control circuit 238 to be generated.
As shown in FIG. 2, since the current control circuits 230 to 237 corresponding to the data D0 to D7 have the same configuration, only the current control circuit 237 will be described below.

As shown in FIG. 2, the current control circuit 237 includes a pull-up resistor R1, emitter-common transistors Q1 and Q2, a transistor Q3 as a constant current source, an operational amplifier OP1, and a resistor R2.
When the data D7 is “1”, the signal S211 is at L level and the signal S212 is at H level, so that the transistor Q1 is turned off and the transistor Q2 is turned on. Therefore, a current substantially equal to the emitter current of the transistor Q3, which is a constant current source, is pulled from the laser diode L1 side via the transistor Q2.
When the data D7 is “0”, the signal S211 is at the H level and the signal S212 is at the L level, so that the transistor Q1 is turned on and the transistor Q2 is turned off. Therefore, no current is pulled from the laser diode L1 side.

  When the data D7 is “1”, the current flowing through the emitter of the transistor Q3, which is a constant current source, is set by a signal S7 set to one of the input terminals of the operational amplifier OP1. That is, since the output terminal of the operational amplifier OP1 is fed back to the input side via the base of the transistor Q3, in the balanced state, the other voltage level of the input terminal of the operational amplifier OP1 is equivalent to the signal S7, and the emitter current I7. Is (voltage level of S7) / R2.

The resolution setting unit 24 includes a digital / analog converter 241 and a plurality of operational amplifiers 242, converts the adjustment signal S _mod generated by the LD power control unit 22 into an analog signal S7, and also converts the voltage of the analog signal S7. The level is halved in order. Thereby, signals S6, S5,..., S0 are generated in order.
That is, S6 = (1/2) * S7, S5 = (1/2) * S6, S4 = (1/2) * S5, S3 = (1/2) * S4, S2 = (1/2) * S3, S1 = (1/2) × S2, and S0 = (1/2) × S1.

As described above, the current control circuits 230 to 237 of the current control unit 23 generate the constant currents I0 to I7 corresponding to the voltage levels of the signals S0 to S7 when the corresponding data D0 to D7 is “1”. Is done. Then, by setting the signals S7, S6,..., S0 described above, the current values of the constant currents I7, I6,.
That is, I6 = (1/2) × I7, I5 = (1/2) × I6, I4 = (1/2) × I5, I3 = (1/2) × I4, I2 = (1/2) × I3, I1 = (1/2) × I2, and I0 = (1/2) × I1.

The digital / analog converter 25 converts the constant bias signal _Sbias generated by the LD power control unit 22 into an analog signal S8.
In response to the analog signal S8, the current control circuit 238 of the current control unit 23 generates a constant current I8. The constant current I8 is a current for securing an optical signal having a constant light amount without depending on the serial data DATA input to the serial / parallel converter 21.

A current obtained by integrating constant currents flowing through each current control circuit of the current control unit 23 flows through the laser diode L1. That is, the current _{I L1} flowing to the laser diode L1 is I0 + I1 + I2 + I3 + I4 + I5 + I6 + I7 + I8. As a result, a multi-value current corresponding to the 8-bit parallel data D0 to D7 flows through the laser diode L1 as a light emitting element, and a multi-value optical signal is transmitted.
As described above, in the transmission device 2, each current control circuit assigns a bit weight to each bit of the parallel data D <b> 0 to D <b> 7, and an amplitude-multiplexed multilevel optical signal is transmitted to the optical fiber line 90. .

The photodiode P2 and the LD power control unit 22 constitute a feedback loop for LD power control. That is, the optical signal generated by the laser diode L1 is received by the photodiode P2, photoelectrically converted and fed back to the LD power control unit 22, and the adjustment signal S _mod is changed according to the value.
The LD power control is a calibration process of the transmission apparatus 2 that is periodically performed to compensate for a change in output characteristics of the laser diode caused by an environmental change such as a temperature change. Usually, LD power control is performed at long intervals of 1 second to 10 seconds.
After the LD power control is completed, data to be transmitted (MAC frame data in units of 8 bits) is given to the serial / parallel converter 21. At this time, the signal of the adjustment signal S _mod of the LD power controller 22 The level is fixed and the feedback loop does not function.

Next, a reference signal transmitted by the transmission device 2 will be described.
The reference signal is inserted into a data signal of a predetermined data unit such as an 8-bit unit of the MAC frame, and the purpose thereof is LD power control and clock signal reproduction on the receiving side.
As described above, the LD power control may be performed every long period of 1 second to 10 seconds in order to cope with environmental changes. However, the clock signal regeneration on the receiving side is performed by bending or pulling the optical fiber line 90. For example, for a 1 gigabit / second optical signal corresponding to a standard such as Gigabit Ethernet (registered trademark), it is once every 100 samples (once every 100 nanoseconds). It must be done at very short intervals.

FIG. 3 is an optical signal waveform diagram showing an example of a reference signal, where (a) shows a case where LD power control is performed, and (b) shows a case where LD power control is not performed.
As shown in FIG. 3, when LD power control is performed (FIG. 3 (a)), it takes time until the output of the laser diode L1 is stabilized particularly when the power is turned on or reset. In order to perform control, a long period is required as a reference signal. On the other hand, when the LD power control is not performed (FIG. 3B), the reference signal is a pulse for reproducing the clock signal on the receiving device 3 side (there is at least one for clock reproduction). Is sufficient, and it does not require a long period of time as a reference signal.
Thus, the reference signal is a signal having a different signal period depending on the transmission timing.

  In the optical communication system 1 according to the present embodiment, a further purpose of the reference signal is to transmit a base signal for calibration of the optical signal on the receiving side. Even if the amount of light of the transmitted optical signal is the same due to a change in transmission characteristics (non-linear characteristics) affected by physical factors (bending and pulling) of the optical fiber line 90, the optical signal is light on the receiving side. The signal reception level may change greatly. In order to compensate for this change, the process of adjusting the reception level on the reception side is calibration.

In the optical communication system 1 according to the present embodiment, the reference signal has a ternary pulse including the maximum value and the minimum value of the optical signal as the data signal in order to correct the nonlinear characteristic of the transmission characteristic. Selection of the digital value corresponding to these three values needs to be performed in advance between the transmission device 2 and the reception device 3 in terms of the communication protocol.
FIG. 4 is a diagram illustrating a waveform example of a reference signal including three pulses corresponding to predetermined three digital values. As shown in FIG. 4, a reference signal including three pulses of amplitude levels a, b, and c corresponding to three digital values is transmitted to the receiving device 3 and used for correction (calibration) of the data signal. Is done.

Next, a specific configuration of the receiving device 3 will be described.
FIG. 5 is a diagram illustrating an example of a circuit configuration of the receiving device 3.

  As illustrated in FIG. 5, the reception device 3 includes a photodiode P1 as a light receiving element, a resistor R3, a buffer amplifier 31, a clock reproduction unit 32, an analog / digital converter (ADC) 33, and a data adjustment unit 34.

The buffer amplifier 31 takes in the signal photoelectrically converted by the photodiode P1 and the resistor R3, and generates an analog signal S31.
Based on the reference signal received from the transmission device 2, the clock recovery unit 32 generates a clock signal CLK 2 that is the same as the operation clock signal CLK 1 of the transmission device 2. The clock regeneration unit 32 includes a clock generator that generates a clock signal having the same frequency as that of the clock signal CLK1, and is based on a rising edge or a falling edge of at least one pulse among a plurality of pulses included in the reference signal. The clock signal CLK2 that is the same as the clock signal CLK1 is generated by adjusting the phase of the clock signal generated by the clock generator.

  The analog / digital converter 33 is supplied with the clock signal CLK2 from the clock reproduction unit 32, and converts the output analog signal S31 of the buffer amplifier 31 into a digital signal S33 based on the clock signal CLK2.

The data adjustment unit 34 decodes the digital signal S33 corresponding to the optical signal (data signal) amplitude-multiplexed in units of 8 bits, converts the digital signal S33 into the original 8-bit data D0 to D7, and converts the converted digital data into the converted digital data. Calibrate against it.
In the calibration, the reception level (output of the analog / digital converter 33) of the base signal for calibration (for example, three pulses of amplitude levels a, b, and c as shown in FIG. 4) included in the reference signal. A conversion formula for correcting the non-linear transmission characteristic is determined based on the digital value) and the ternary digital value determined in advance by the communication protocol. Then, the data adjustment unit 34 corrects the digital value of the data signal received thereafter by this conversion formula.
Various methods such as an interpolation method using a polynomial (for example, a spline interpolation method) can be applied to the conversion formula determination method based on the three points.

Next, the operation of the optical communication system 1 based on the reference signal will be described with reference to FIG.
6A and 6B are timing charts for explaining the operation of the optical communication system 1, where FIG. 6A is a transmitted / received optical signal waveform, FIG. 6B is an operation phase of the optical communication system 1, and FIG. 2, (d) shows the operation phase of the receiving device 3. In FIG. 6A, it is assumed that there is no signal change due to a communication delay or the like between the transmitted optical signal and the received optical signal.

First, immediately after the system is started at time t0, as shown in FIG. 6A, the output characteristics of the laser diode L1 of the transmission device 2 are unstable, and the reception device 3 operates based on the transmission signal during that period. The waiting time is set according to the protocol so as not to perform the calibration (FIG. 6D).
Simultaneously with the start-up, the transmitter 2 starts LD power control. As a result, the value of the current flowing through the laser diode L1 is adjusted so that an optical signal having a constant light amount is transmitted with respect to the digital signal to be transmitted.
The reference signal is transmitted at times t1 to t3, but the receiving device 3 can perform synchronous reproduction of the clock signal CLK2 even while the transmitting device 2 performs LD power control. That is, the receiving device 3 performs clock recovery when it recognizes the rising edge or falling edge of the pulse included in the reference signal.

  Then, as shown in FIG. 6D, when the waiting time elapses, the receiving apparatus 3 performs calibration. Although not shown in FIG. 6 (a), the reference signal has three pulses having different amplitude levels corresponding to digital values predetermined in the protocol as shown in FIG. In the apparatus 3, a conversion formula for correcting the data signal is determined based on these three pulses. And it correct | amends with the conversion type | formula with respect to the data signal transmitted after the time t3. This compensates for nonlinear transmission characteristics in the optical fiber line 90.

  The reference signal shown in FIG. 6 is inserted at a very short interval such as once every 100 samples (once every 100 nanoseconds) in the case of an optical signal of 1 gigabit / second corresponding to a standard such as Gigabit Ethernet. In the receiving device 3, the data signal is corrected each time.

As described above, in the optical communication system 1 according to the present embodiment, in the transmission apparatus 2, a reference signal having three pulses having different amplitudes including at least the maximum value and the minimum value of the data signal is predetermined as the data signal. Insertion is performed every period (or a predetermined amount of data), and the reception device 3 corrects the reception data signal based on the received three pulses, thereby compensating the transmission characteristic (nonlinear characteristic) of the optical fiber line 90. .
Accordingly, it is possible to correct the data signal by following the physical change of the optical fiber line 90 that fluctuates in a short period, such as bending or pulling, in a very short period, and as a result, the accuracy of the data signal is maintained high. be able to.

<Second Embodiment>
Next, a second embodiment of the present invention will be described.
The optical communication system according to the present embodiment is characterized by a reference signal and a receiving device that receives the reference signal, as compared with the optical communication system 1 according to the first embodiment. In addition, the structure of the optical communication system in this embodiment can apply FIG.1,2,5 demonstrated in 1st Embodiment as it is.

FIG. 7 is a diagram illustrating a waveform of a reference signal according to the present embodiment. The arrows in the figure indicate the sampling timing on the receiving device 3 side. 7A and 7B show the same reference signal waveform, but the sampling timing is different.
As shown in FIG. 7A, the reference signal in the present embodiment is a pulse with a higher data rate than the data signal. For example, in the example shown in FIG. 7A, the data rate of the reference signal is twice that of the data signal. Further, as shown in FIG. 7A, a plurality of pulses are set such that the sampling results on the receiving device 3 side with respect to this reference signal all have a constant value (first sample value of the present invention). In FIG. 7A, the constant value is the maximum value of the data signal, but is not limited to this, and any value may be used as long as it is constant.

The receiving device 3 receives the reference signal, generates a clock signal CLK2 having a period indicated by an arrow in the figure, and performs sampling for analog / digital conversion. That is, the clock signal CLK2 becomes a pulse that rises (or falls) in the middle of the rising edge and falling edge of the received pulse (reference signal).
In the present embodiment, the clock recovery unit 32 (see FIG. 5) of the reception device 3 captures the timing of the rising edge and the falling edge of the received pulse, and rises (or falls) between these timings. The clock signal CLK2 for sampling the data signal is regenerated by adjusting the phase of the clock signal generated by the internal clock generator.
When the clock signal CLK2 can be completely reproduced on the receiving device 3 side in this way, the clock signal CLK2 (second reference clock of the present invention) rises (or falls) at the timing indicated by the arrow in FIG. In addition, the clock signal has the same cycle as that of the clock signal CLK1 on the transmission side (first reference clock of the present invention).

  FIG. 7B shows a state in which the sampling timing on the receiving device 3 side is shifted due to a change in characteristics of the clock transmitter on the receiving device 3 side, a change with time, or a change in characteristics of the transmission system. In a state where the sampling is shifted, the sampling result changes from the maximum value to the minimum value, unlike the case of FIG. Thereby, the receiving device 3 recognizes that the sampling is shifted. Note that the sampling result after the shift is not limited to the minimum value of the data signal as shown in FIG. 7B, but may be a value different from the sampling result before the shift.

  The reference signal in this embodiment is a pulse with a higher data rate compared to the data signal. Even if the sampling is shifted, the sampling shift is recognized earlier than the erroneous sampling occurs for the data signal. It is to do. Therefore, as long as the sampling theorem is followed, the higher the data rate of the reference signal, the smaller the sampling deviation can be recognized. In this case, it goes without saying that the sampling frequency on the receiving device 3 side must be increased as necessary.

When the sampling deviation is recognized, the receiving device 3 requests to retransmit the reference signal or sets the reference signal insertion interval (transmission interval) to be shortened. As a result, it is possible to minimize sampling deviation on the system.
Further, in the case where a sampling deviation is not recognized a predetermined number of times with respect to the reference signal, the reference signal transmission interval may be set longer in the system. This saves the trouble of processing unnecessary reference signals on the receiving device 3 side.

  As described above, according to the optical communication system according to the present embodiment, the receiving device 3 can recognize the deviation of its own sampling before erroneous sampling of the data signal occurs due to the sampling of the reference signal. , Can take the necessary action. Therefore, high accuracy of the data signal can be maintained regardless of sampling deviation (clock deviation) caused by the transmission system or the like.

The embodiments of the present invention are not limited to the above-described embodiments, and those skilled in the art can make various modifications without departing from the scope of the present invention. For example, the optical communication method according to the embodiment can be applied to optical space transmission.
Further, the reference signal and its processing described in the first and second embodiments can be realized in combination. For example, the reference signal having a high data rate in the second embodiment can include three pulses with different levels for calibration.

It is a figure which shows the structure of the optical communication system which concerns on embodiment. It is a figure which shows an example of the circuit structure of the transmitter in embodiment. It is an optical signal waveform diagram which shows an example of a reference signal. It is a figure which shows the example of a waveform of the reference signal containing the pulse corresponding to predetermined three digital values. It is a figure which shows an example of the circuit structure of the receiver in embodiment. It is a timing chart for demonstrating operation | movement of the optical communication system which concerns on embodiment. It is a figure which shows the waveform of the reference signal which concerns on other embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Optical communication system, 2 ... Transmission apparatus, 21 ... Serial / parallel conversion part, 22 ... LD power control part, 23 ... Current control part, 24 ... Resolution setting part, 25 ... Digital-analog converter (DAC), 3 DESCRIPTION OF SYMBOLS Receiving device 31 ... Buffer amplifier 32 ... Clock regeneration part 33 ... Analog-digital converter (ADC) 34 ... Data adjustment part L1 ... Laser diode, P1, P2 ... Photodiode, 4 ... Data processing device, 90: Optical fiber line.

Claims (7)

An optical communication system that performs communication between a transmission device and a reception device based on a multi-value optical signal according to the amount of light,
The transmitter transmits a reference signal having a predetermined three values including at least a maximum value and a minimum value among the multi-values in addition to the data signal every predetermined period,
The receiving device synchronizes for sampling the received data signal based on the reference signal, and corrects the received data signal according to a received optical signal level corresponding to the predetermined three values. Communications system. An optical communication method for performing communication between a transmission device and a reception device based on a multi-value optical signal corresponding to a light amount,
A first step in which the transmitting device transmits a reference signal having a predetermined three values including at least a maximum value and a minimum value among the multi-values in addition to the data signal, for each predetermined period;
A second step in which the receiving device synchronizes for sampling the received data signal based on the reference signal;
A third step in which the receiving device corrects the received data signal according to a received optical signal level corresponding to the predetermined three values;
An optical communication method comprising: An optical communication system that performs communication between a transmission device and a reception device based on a multi-value optical signal according to the amount of light,
The transmitter is
A plurality of data signals are generated and transmitted based on the first reference clock, the data rate is higher than that of the first reference clock, and the sample values based on the first reference clock are the same first sample value. Transmitting a reference signal including a pulse every first period;
The receiving device is:
Based on at least one of the plurality of pulses, a reference clock in which a sample value of the plurality of pulses becomes the first sample value, and a second reference clock having the same frequency as the first reference clock is generated, An optical communication system that uses the second reference clock as a reference signal for sampling the received data signal. When the receiving device receives a new reference signal after generating the second reference clock, the receiving device samples a plurality of pulses included in the reference signal based on the second reference clock, and does not all become the first sample value. The optical communication system according to claim 3, wherein the transmission interval of the reference signal is shorter than the first period on the condition. When the receiving device receives a new reference signal after generating the second reference clock, the receiving device samples a plurality of pulses included in the reference signal based on the second reference clock, and does not all become the first sample value. The optical communication system according to claim 3, wherein re-transmission of the reference signal is requested on the condition. When the receiving device receives a new reference signal after generating the second reference clock, the receiving device samples a plurality of pulses included in the reference signal based on the second reference clock, and all the values become the first sample value. The optical communication system according to claim 3, wherein a transmission interval of the reference signal is set longer than the first period on condition that the reference signal is continued a predetermined number of times. An optical communication method for performing communication between a transmission device and a reception device based on a multi-value optical signal corresponding to a light amount,
A first step in which the transmitting device generates and transmits a data signal based on a first reference clock;
The transmitter transmits a reference signal including a plurality of pulses having a data rate higher than that of the first reference clock and having the same sample value based on the first reference clock as the first sample value every predetermined period. A second step;
The receiving device is a reference clock whose sample value of the plurality of pulses becomes the first sample value based on at least one pulse of the plurality of pulses, and a second reference having the same frequency as the first reference clock. A third step of generating a clock;
A fourth step in which the receiving device receives the data signal;
A fifth step in which the receiving device samples the data signal received by the second reference clock;
An optical communication method comprising:

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