WO2018116344A1 - Optical transmitter and waveform distortion correction method - Google Patents

Abstract

An optical transmitter comprising: a digital signal processing unit (110) for generating transmission data signals including training signals that are clock signals configured by use of alternating patterns using binary bit sequences; DACs (120A-120D) for converting the transmission data signals to analog signals; drivers (121A-121D) for amplifying the analog signals to generate driving signals; a light source (122) for emitting a carrier light; I/Q modulators (123X, 123Y) for modulating the carrier light on the basis of the driving signals, thereby generating optical modulated signals; PDs (126X, 126Y) for generating intensity signals indicating the intensities of the optical modulated signals; and ADCs (127X, 127Y) for converting the intensity signals to digital signals. The digital signal processing unit (110) compares the training signals with the digital signals, thereby detecting the waveform distortions of the optical modulated signals and then correcting the waveform distortions.

Description

Optical transmitter and waveform distortion correction method

The present invention relates to an optical transmitter and a waveform distortion correction method, and more particularly to an optical transmitter and a waveform distortion correction method for correcting waveform distortion of an optical modulation signal.

With the recent increase in demand for data communication, it is required to realize high-speed and large-capacity optical transmission. Therefore, a technique for transmitting a plurality of bits in one symbol time has been proposed, and in particular, in multilevel modulation, a plurality of bits can be transmitted in one symbol. For example, optical transmission systems using QPSK (Quadrature Phase Shift Keying) and m-QAM (Quadrature Amplitude Modulation) have been put into practical use. m in m-QAM indicates the number of signal points, and is 16 or 64 or the like. In polarization multiplexing, it is possible to transmit signals using two polarizations orthogonal to each other.

Multi-level modulation is realized in recent years by digital signal processing. For example, the optical transmitter includes a digital signal processing unit and an I / Q (In-phase / Quadrature) modulator. The digital signal processing unit generates a drive signal from the transmission data, and the I / Q modulator generates an optical modulation signal from the carrier light using the drive signal generated in the digital signal processing unit. This optical modulation signal is transmitted through an optical fiber transmission line, demodulated in an optical receiver, and transmission data is reproduced.

In multi-level modulation with a large number of bits per symbol, the inter-symbol distance becomes shorter as the multi-level number increases, and thus the demand for waveform distortion of the optical modulation signal becomes severe. For example, the causes of waveform distortion include insufficient analog bandwidth of the driver and I / Q modulator included in the optical transmitter, harmonic distortion in the driver, and the like. When discriminating symbols in an optical transmission system using QPSK, waveform distortion due to intensity change does not affect the deterioration of reception characteristics, but in the case of an optical transmission system using m-QAM, it depends on the intensity of the optical modulation signal. Since the symbol is also discriminated, a symbol error occurs due to an intensity change due to waveform distortion, and the reception characteristics in the optical receiver deteriorate.

Therefore, in order to improve the signal quality of the optical modulation signal, it is necessary to detect the waveform distortion of the optical modulation signal generated by the optical transmitter and correct the distortion. In recent years, techniques related to detection and correction of waveform distortion have been actively proposed. For example, Patent Document 1 discloses a technique using a training signal (hereinafter, referred to as a TN signal) composed of a specific bit string having a constant intensity. Is described.

Specifically, in the technique described in Patent Document 1 (hereinafter, conventional technique), the optical transmitter has a PD (Photo Detector) as a light receiver and an ADC (Analog to Digital Converter), and is a digital signal. The processing unit generates a TN signal composed of a specific bit string having a constant strength and inserts it into the transmission data. The generated TN signal is received by the PD, and an intensity signal is generated. The intensity signal is converted into a digital signal by the ADC and input to the digital signal processing unit. The intensity signal input to the digital signal processing unit has waveform distortion caused by, for example, the analog band shortage of the driver and the I / Q modulator mentioned above and the harmonic distortion of the driver. Therefore, the distortion amount is detected by comparing the TN signal inserted into the transmission signal with the intensity signal, and the waveform distortion is corrected by digital signal processing so as to minimize the distortion amount.

JP 2016-72942 A

The waveform distortion of the optical modulation signal generated in the optical transmitter can be detected using, for example, a digital coherent receiver. However, when a new dedicated digital coherent receiver is provided to detect waveform distortion, the cost of the entire optical transmission system increases due to the increase in the cost of the optical transmitter.

Therefore, by using a digital coherent receiver provided in the opposite apparatus in the optical transmission system, it is possible to detect waveform distortion of the optical modulation signal without increasing the cost. However, in this configuration, the detected waveform distortion includes not only the waveform distortion generated in the optical transmitter but also the waveform distortion generated in the transmission path. Therefore, as a method for detecting and correcting the waveform distortion generated in the optical transmitter, signal processing inside the optical transmitter can be considered as described in Patent Document 1.

However, in the technique described in Patent Document 1, the bit string constituting the TN signal and the bit string constituting the intensity signal are compared bit by bit in the detection of waveform distortion. In this case, if the PD band and the ADC sampling rate are equal to or less than the baud rate of the transmission signal, the intensity signal generated by the PD cannot be correctly generated bit by bit, and it is difficult to detect waveform distortion. There are challenges.

Therefore, an object of the present invention is to make it possible to detect waveform distortion even when the band of the optical receiver and the sampling rate of the ADC are equal to or less than the baud rate of the transmission signal.

An optical transmitter according to a first aspect of the present invention includes a digital signal processing unit that generates a transmission data signal including a training signal, which is a clock signal configured by an alternating pattern of binary bit strings, and the transmission data signal. A first controller that converts the signal into an analog signal; a driver that amplifies the analog signal to generate a drive signal; a light source that emits carrier light; and the carrier light that is modulated based on the drive signal, thereby modulating light. A modulator that generates a signal, a light receiver that generates an intensity signal indicating the intensity of the light modulation signal, and a second controller that converts the intensity signal into a digital signal, and the digital signal processing unit includes: Comparing the training signal and the digital signal to detect waveform distortion of the optical modulation signal and correcting the waveform distortion; And butterflies.

An optical transmitter according to a second aspect of the present invention includes a digital signal processing unit that generates a transmission data signal including a clock signal composed of an alternating pattern of binary bit strings, and converts the transmission data signal into an analog signal A first controller that amplifies the analog signal and adjusts a waveform of the analog signal to generate a drive signal, a light source that emits carrier light, and the carrier light based on the drive signal From the modulator that generates the optical modulation signal, the light receiver that generates the intensity signal indicating the intensity of the optical modulation signal, and the intensity signal that indicates the intensity of the portion corresponding to the clock signal. A distortion identifying unit that identifies a higher harmonic component than a predetermined frequency; and the analog signal wave so as to suppress the higher harmonic component to the driver. It is to adjust, characterized in that and a distortion correcting unit for correcting the waveform distortion of the optical modulation signal.

The waveform distortion correction method according to the first aspect of the present invention generates a transmission data signal including a training signal that is a clock signal composed of an alternating pattern of binary bit strings, and converts the transmission data signal into an analog signal. Amplifying the analog signal to generate a drive signal, modulating carrier light based on the drive signal, generating an optical modulation signal, generating an intensity signal indicating the intensity of the optical modulation signal, The intensity signal is converted into a digital signal, and the waveform distortion of the optical modulation signal is detected by comparing the training signal and the digital signal, and the waveform distortion is corrected.

The waveform distortion correction method according to the second aspect of the present invention generates a transmission data signal including a clock signal composed of an alternating pattern of binary bit strings, converts the transmission data signal into an analog signal, A signal is amplified to generate a drive signal, a carrier light is modulated based on the drive signal, an optical modulation signal is generated, an intensity signal indicating the intensity of the optical modulation signal is generated, and the clock From the intensity signal indicating the intensity of the portion corresponding to the signal, specify a harmonic component higher than a predetermined frequency, and by adjusting the waveform of the analog signal to suppress the harmonic component, It is characterized by correcting waveform distortion of the optical modulation signal.

According to one aspect of the present invention, a bit sequence constituting a TN signal to be inserted into transmission data is set as an alternating pattern of a specific period, and a clock signal having a specific frequency is generated, whereby the optical receiver band and the ADC sampling rate are transmitted. Even when the baud rate is lower than the signal, waveform distortion can be detected.

2 is a block diagram schematically showing a configuration of an optical transmitter according to Embodiments 1 and 2. FIG. (A) And (b) is the schematic for demonstrating the frequency characteristic of the gain in Embodiment 1, and its correction | amendment. 3 is a schematic diagram showing a frame configuration of transmission data in Embodiment 1. FIG. (A)-(f) is the schematic which shows the time waveform of the distortion detection signal in Embodiment 1, and the time waveform of the optical output of an I / Q modulator. In Embodiment 1, it is the schematic which shows the time waveform of the optical output of an I / Q modulator when the zone | band of PD is smaller than the baud rate of a transmission data signal. 6 is a schematic diagram illustrating a configuration of a transmission data frame in Embodiment 2. FIG. (A)-(c) is the schematic which shows the time waveform of the TN signal in Embodiment 2, and the time waveform of the optical output of an I / Q modulator. 6 is a block diagram schematically showing a configuration of an optical transmitter according to a third embodiment. FIG. 10 is a schematic diagram illustrating a configuration of a transmission data frame in Embodiment 3. FIG. (A)-(c) is the schematic which shows the time waveform of the clock signal in Embodiment 3. FIG. (A) And (b) is the schematic which shows the hardware structural example in Embodiment 1-3.

Embodiment 1 FIG.

FIG. 1 is a block diagram schematically showing a configuration of an optical transmitter 100 according to the first embodiment.
In the first embodiment, it is assumed that the gain varies with frequency in the frequency characteristics of the gain due to a shortage of analog bands of the driver and the I / Q modulator. Further, it is assumed that the optical transmitter 100 is in operation in the optical transmission system.

The optical transmitter 100 includes a digital signal processing unit 110, a DAC (Digital to Analog Converter) 120A to 120D as a first controller, drivers 121A to 121D, a light source 122, and an I / Q modulator as a modulator. 123X, 123Y, PBC (Polarization Beam Combiner: polarization combiner) 124, couplers 125X, 125Y as optical branching units, PDs 126X, 126Y as light receivers, and ADCs 127X, 127Y as second controllers Prepare.

The digital signal processing unit 110 generates a transmission data signal from the data signal DS by digital signal processing. In the first embodiment, the optical transmitter 100 transmits data by a polarization multiplexing method. Therefore, the digital signal processing unit 110 transmits the transmission data signal EX (XI1, XQ1) corresponding to the data transmitted using the X polarization and the transmission data signal corresponding to the data transmitted using the Y polarization. EY (YI1, YQ1) is generated. In the following description, the I phase and Q phase of the I / Q modulator 123X are referred to as channel XI and channel XQ, respectively, the I phase and Q phase of the I / Q modulator 123Y are referred to as channel YI and channel YQ, respectively.

The DACs 120A to 120D convert the transmission data signals XI1, XQ1, YI1, and YQ1 generated by the digital signal processing unit 110 into analog signals.
The drivers 121A to 121D amplify the transmission data signals XI1, XQ1, YI1, and YQ1 converted into analog signals by the DACs 120A to 120D, and generate drive signals XI2, XQ2, YI2, and YQ2.
The light source 122 generates continuous light (carrier light) having a predetermined frequency.

The I / Q modulator 123X modulates the continuous light generated by the light source 122 with the drive signals XI2 and XQ2, and generates an optical modulation signal X1.
The I / Q modulator 123Y modulates the continuous light generated by the light source 122 with the drive signals YI2 and YQ2, and generates an optical modulation signal Y1.
The PBC 124 combines the optical modulation signal X1 generated by the I / Q modulator 123X and the optical modulation signal Y1 generated by the I / Q modulator 123Y to generate a polarization multiplexed optical signal.

The coupler 125X branches the optical modulation signal X1 and inputs it to the PD 126X.
The coupler 125Y branches the optical modulation signal Y1 and inputs it to the PD 126Y.
The PD 126X generates an intensity signal X2 representing the intensity of the light modulation signal X1 by direct detection.
The PD 126Y generates an intensity signal Y2 representing the intensity of the light modulation signal Y1 by direct detection.
Note that the PDs 126X and 126Y both include a photodiode that converts an optical signal into an electrical signal.
The ADC 127X converts the intensity signal X2 generated by the PD 126X into a digital signal.
The ADC 127Y converts the intensity signal Y2 generated by the PD 126Y into a digital signal.

Then, the digital signal processing unit 110 detects the waveform distortion of the optical modulation signal by comparing the training signal and the digital signal, and corrects the waveform distortion.
The digital signal processing unit 110 includes a mapper 111, a TN signal insertion unit 112, a TN signal generation unit 113, a distortion identification unit 114, and a frequency domain correction unit 115.

The mapper 111 generates a processed data signal from the data signal DS input to the optical transmitter 100. Specifically, the mapper 111 performs processing data signals OX (OXI, OXQ) corresponding to data transmitted using X polarization and processing data signals corresponding to data transmitted using Y polarization. OY (OYI, OYQ) is generated. The processed data signals OX and OY can be expressed by the following equations (1) and (2).
OX = OXI + jOXQ (1)
OY = OYI + jOYQ (2)

The TN signal generation unit 113 generates a TN signal used to detect waveform distortion of the optical modulation signals X1 and Y1. The TN signal is a clock signal composed of an alternating pattern of binary bit strings.
The TN signal insertion unit 112 inserts the TN signal generated by the TN signal generation unit 113 into the processing data signals OXI, OXQ, OYI, and OYQ generated by the mapper 111, thereby inserting the insertion data signals IXI, IXQ, and IYI. , IYQ is generated.
Although details of the TN signal generated by the TN signal generation unit 113 will be described later, the TN signal generated by the TN signal generation unit 113 is processed by the TN signal insertion unit 112 as processed data signals OXI, OXQ, OYI, and OYQ. It is stored in a plurality of transmission data frames constituting the same.

Based on the transmission data signal into which the TN signal is inserted, the optical modulation signals X1 and Y1 generated by the I / Q modulators 123X and 123Y include components corresponding to the TN signal. Therefore, the intensity signals X2 and Y2 generated by the PDs 126X and 126Y also include a component corresponding to the TN signal. The intensity signals X2 and Y2 including components corresponding to the TN signal are converted into digital signals by the ADCs 127X and 127Y and input to the digital signal processing unit 110.

The distortion specifying unit 114 compares the intensity signals X2 and Y2 converted into digital signals by the ADCs 127X and 127Y and the TN signal generated by the TN signal generation unit 113 for each frequency, thereby calculating the intensity signal X2 for the frequency. , Y2 gain is detected. The distortion specifying unit 114 specifies a gain frequency characteristic as shown in FIG. 2A, for example, by obtaining a gain with respect to a frequency from a plurality of transmission data frames.

The frequency domain correction unit 115 is a distortion correction unit that corrects the gain for each frequency in the insertion data signals IXI, IXQ, IYI, and IYQ based on the frequency characteristics of the gain specified by the distortion specification unit 114. For example, the frequency domain correction unit 115 corrects the frequency characteristic of the gain shown in FIG. 2A according to the frequency characteristic of the gain specified by the distortion specifying unit 114, that is, as shown in FIG. Set the appropriate characteristics. Thereby, the fluctuation | variation by the frequency characteristic of a gain is correct | amended.

As a method of correcting the frequency characteristics of the gain in the frequency domain correction unit 115, for example, an FIR (Finite Impulse Response) filter can be considered. When the FIR filter is used, the distortion specifying unit 114 sets the multiplier of the FIR filter so that the frequency characteristic of the gain specified by the distortion specifying unit 114 has a constant gain regardless of the frequency. In this example, the use of the FIR filter in the frequency domain correction unit 115 will be described as an example. However, any method may be used as long as the frequency characteristic of the gain can be corrected.

FIG. 3 is a diagram showing a frame structure of transmission data in the first embodiment.
The TN signals a to d are used to detect waveform distortion generated in the channels XI, XQ, YI, and YQ, respectively.

TN signal a is used to detect waveform distortion occurring in channel XI. Therefore, the TN signal a includes a distortion detection signal TXI used to detect waveform distortion generated in the channel XI, and is inserted into the processing data signal OXI of the channel XI. Details of the bit string constituting the distortion detection signal TXI will be described later. Then, “0 (zero)” is stored in the other channels XQ, YI, and YQ. Here, “0” indicates that the drive signals XQ2, YI2, and YQ2 input to the I / Q modulators 123X and 123Y are “0”, and the I / Q modulators 123X and 123Y are driven. This indicates that optical signals corresponding to the signals XQ2, YI2, and YQ2 are not output.

Similarly, the TN signal b is used to correct waveform distortion generated in the channel XQ, the distortion detection signal TXQ is inserted into the processed data signal OXQ of the channel XQ, and “0” is stored in the other channels. The The same applies to the TN signal c and the TN signal d, which are used to correct waveform distortion generated in the channel YI and the channel YQ, respectively, and distortion detection is performed on the processed data signal OYI and the processed data signal OYQ of the channel YI. The signal TYI and the distortion detection signal TYQ are inserted, and “0” is stored in the other channels.

In FIG. 3, in a plurality of transmission data frames constituting the processed data signals OXI, OXQ, OYI, OYQ, TN signals a to d are inserted at regular intervals between areas for storing the data signals a to f. .
In FIG. 3, as an example, the TN signals a to d are inserted into the processing data signals OXI, OXQ, OYI, and OYQ at regular intervals. It may be inserted at a position. For example, all of the TN signals a to d may be inserted immediately after the synchronization signal, or all of the TN signals a to d may be inserted continuously at the end of the transmission data frame.

In addition, the TN signal insertion unit 112 can insert a TN signal having a different frequency each time a synchronization signal is detected.
In the first embodiment, since TN signals are inserted at regular intervals, for example, the digital signal processing unit 110 notifies the PD 126X, 126Y of the timing at which the synchronization signal is inserted, thereby causing the PD 126X, 126Y. Can generate intensity signals X2 and Y2 indicating the intensity of only the light modulation signal of the portion corresponding to the training signal.

Next, the bit string of the distortion detection signal included in the TN signal will be described. Here, as an example, the distortion detection signal TXI included in the TN signal a will be described.

4A to 4F are schematic diagrams showing the time waveform of the distortion detection signal TXI and the time waveform of the optical output of the I / Q modulator 123X.
Note that “drive voltage = 0” illustrated in FIGS. 4A, 4C, and 4E indicates that the I / Q modulator 123X operates at the extinction point. Further, the distortion detection signal TXI is composed of a positive value “1” and a negative value “−1”, and each of them is an alternating pattern in which several bits are alternately repeated to generate clock signals having different frequencies. In other words, the TN signal generation unit 113 can change the frequency of the clock signal by changing the number of consecutive bits having the same value in the alternating pattern.

FIG. 4A is a diagram illustrating a time waveform of the distortion detection signal TXI included in the TN signal a generated by the digital signal processing unit 110 and converted into an analog signal by the DAC 120A. In FIG. 4A, the vertical axis represents drive voltage and the horizontal axis represents time.
Symbols a1 to a8 in FIG. 4A indicate a bit string of the distortion detection signal TXI, which is generated by the TN signal generation unit 113, and inserted into the processed data signal OXI by the TN signal insertion unit 112. The bit string of the distortion detection signal TXI in FIG. 4A is an alternating pattern clock signal that alternately repeats a positive value “1” and a negative value “−1” bit by bit.

FIG. 4B is a diagram illustrating a time waveform of the optical output of the I / Q modulator 123X when the distortion detection signal TXI is input as the drive signal XI2. In FIG. 4B, the vertical axis indicates the light intensity in the X polarization, and the horizontal axis indicates the time.
As shown in FIG. 4B, the light intensity of the optical modulation signal X1 is maximized at the timing at which the bits a1 to a8 shown in FIG. 4A, that is, the bits constituting the distortion detection signal TXI are inserted. At the timing when the value Pa is taken and the drive voltage becomes 0, the light intensity of the light modulation signal X1 becomes the minimum value Pq. The minimum value Pq indicates the operation at the extinction point of the optical modulator.

FIG. 4C shows a time waveform of the distortion detection signal TXI included in the TN signal e. In FIG.4 (c), the vertical axis | shaft has shown the drive voltage and the horizontal axis has shown time. In addition, the bit string of the distortion detection signal in FIG. 4C is an alternating pattern clock signal in which the positive value “1” and the negative value “−1” are alternately repeated by 2 bits.

FIG. 4D is a diagram showing a time waveform of the optical output of the I / Q modulator 123X when the distortion detection signal TXI is input as the drive signal XI2, similarly to FIG. 4B. In FIG. 4D, the vertical axis represents the light intensity in the X polarization, and the horizontal axis represents time. The light intensity of the light modulation signal X1 takes the maximum value Pb, and the light intensity of the light modulation signal X1 becomes the minimum value Pq at the timing when the drive voltage becomes zero.

FIG. 4E is a diagram showing a time waveform of the distortion detection signal TXI included in the TN signal f. In FIG. 4 (e), the vertical axis represents drive voltage and the horizontal axis represents time. In addition, the bit string of the distortion detection signal in FIG. 4E is an alternating pattern clock signal in which a positive value “1” and a negative value “−1” are alternately repeated every 3 bits.

FIG. 4F shows a time waveform of the optical output of the I / Q modulator 123X when the distortion detection signal TXI is input as the drive signal XI2, similarly to FIGS. 4B and 4D. It is. In FIG. 4F, the vertical axis indicates the light intensity in the X polarization, and the horizontal axis indicates the time. The light intensity of the light modulation signal X1 takes the maximum value Pc, and the light intensity of the light modulation signal X1 becomes the minimum value Pq at the timing when the drive voltage becomes zero.

Here, in the bit string shown in FIG. 4A, the frequency interval between the code a1 and the code a2 is equal to the baud rate of the transmission data signal. For example, when the baud rate of the transmission data signal is 32 Gbaud, the frequency interval between the code a1 and the code a2 is 32 GHz. Therefore, the distortion detection signal TXI shown in FIG. 4A is a 16 GHz clock signal. Similarly, the distortion detection signal TXI shown in FIG. 4C is an 8 GHz clock signal, and FIG. The distortion detection signal TXI shown in FIG. 5 becomes a 5.3 GHz clock signal.

Here, consider the light intensity of the clock signal. Assume that the frequency characteristic of the gain when the distortion detection signal TXI inserted into the processed data signal OXI passes through the driver 121A and the I / Q modulator 123X is as shown in FIG. In this case, since the gain is low when the frequency of the clock signal is high, the light intensity of the optical modulation signal X1 is small, and when the frequency of the clock signal is low, the gain is high, and thus the optical modulation signal X1 The light intensity increases.

The distortion detection signal TXI is received by the PD 126X as described above, and the intensity signal X2 is generated. As shown in the prior art, if the band of the PD 126X is as large as the baud rate of the transmission data signal, it is possible to generate the intensity signal X2 that changes for each bit.

However, when the band of the PD 126X is smaller than the baud rate of the transmission data signal, the intensity signal X2 for each bit cannot be generated. Therefore, the intensity signal X2 having a constant value as the time average of the light intensity of the light modulation signal X1 is Generated by PD126X. In such a case, for example, the time waveform of the optical output of the I / Q modulator 123X when the distortion detection signal TXI shown in FIG. 4A is input as the drive signals XI2 and XQ2 is shown in FIG. Be like that.

Therefore, in the ADC 127X, the intensity signal X2 of the distortion detection signal TXI is obtained by performing the process of averaging the intensity level of the intensity signal X2 to a constant value regardless of the band of the PD 126X and the frequency of the clock signal. It takes a constant value with respect to the frequency of the signal TXI. This value is converted into a digital signal and input to the distortion specifying unit 114.

The distortion identification unit 114 receives the frequency and intensity of the distortion detection signal TXI from the TN signal generation unit 113. Then, the distortion specifying unit 114 calculates the gain by taking the difference between the intensity of the distortion detection signal TXI and the intensity of the intensity signal X2. The distortion specifying unit 114 can obtain the relationship between the frequency and the gain from the calculated gain and the frequency of the distortion detection signal TXI, and calculates the frequency characteristic of the gain by performing this operation for each transmission data frame. be able to.

The frequency domain correction unit 115 performs correction in the frequency domain so that the gain of the intensity signal X2 is constant regardless of the frequency, based on the obtained frequency characteristics of the gain. For example, when the gain characteristic of the frequency shown in FIG. 2A is obtained, correction for increasing the gain in the high frequency band as shown in FIG. 2B is performed so as to correct the gain characteristic. Thereby, a constant gain can be obtained regardless of the frequency.

In the first embodiment, the time for transmitting the distortion detection signal TXI is ensured to be a time that can be detected even in the case of the PD 126X and 126Y having a small band. For example, the PD 126X having a band of 1 GHz, If it is 126Y, the distortion detection signal TXI may be transmitted using a time of 1 ns. In addition, the sampling rates of the ADCs 127X and 127Y may be equivalent to the bands of the PDs 126X and 126Y.

In the first embodiment, since the frequency of the clock signal constituting the distortion detection signal has a maximum frequency that is half the baud rate of the transmission data signal, the gain of the frequency of the baud rate of the transmission data signal is specified. I can't. However, the gain of the frequency of the original baud rate can be specified by increasing the baud rate of the transmission data signal. That is, when specifying the gain of the frequency of 32 GHz, the baud rate of the transmission data signal may be set to 64 Gbaud or more, which is twice or more.

Consider the distortion detection signal TXQ included in the TN signal b, the distortion detection signal TYI included in the TN signal c, and the distortion detection signal TYQ included in the TN signal d in the same way as the distortion detection signal TXI included in the TN signal a. Can do.

In the first embodiment, the PDs 126X and 126Y are provided in the subsequent stage of the I / Q modulators 123X and 123Y, but the first embodiment is not limited to such an example. For example, a PD may be provided in the subsequent stage of the PBC 124 as in the prior art. Even in this case, even if the PD band and the ADC sampling rate are smaller than the baud rate of the transmission data signal, the waveform distortion can be detected and corrected with the same accuracy as in the first embodiment.

In Embodiment 1, the distortion detection signal included in the TN signal is inserted into only one channel. However, since the intensity signal can be generated for each of the X polarization and the Y polarization with the configuration shown in FIG. 1, two distortion detection signals having different polarizations are included for one TN signal. It may be. For example, in the transmission data frame shown in FIG. 2, the TN signal a may include the distortion detection signals TXI and TYI. In this case, the PD 126X and 126Y generate the intensity signals X2 and Y2, respectively, and the ADCs 127X and 127Y convert the signals into digital signals and input them to the distortion specifying unit 114. As a result, the time for inserting the TN signal into the processed data signal can be halved.

In the first embodiment, the distortion detection signal has an alternating pattern in which a positive value “1” and a negative value “−1” are alternately repeated every several bits. Thereby, even when the bands of the PDs 126X and 126Y and the sampling rates of the ADCs 127X and 127Y are small, the gain can be made constant regardless of the frequency level.

Embodiment 2. FIG.
As shown in FIG. 1, the optical transmitter 200 according to the second embodiment includes a digital signal processing unit 210, DACs 120A to 120D, drivers 121A to 121D, a light source 122, an I / Q modulator 123X. 123Y, PBC 124, couplers 125X and 125Y, PDs 126X and 126Y, and ADCs 127X and 127Y.
The optical transmitter 200 according to the second embodiment is configured in the same manner as the optical transmitter 100 according to the first embodiment except for the digital signal processing unit 210.

In the first embodiment, during the operation of the optical transmitter 100, the bit sequence of the TN signal is a clock signal having an alternating pattern in which a positive value “1” and a negative value “−1” are alternately repeated every several bits. Thus, it is possible to calculate the frequency characteristics of the gain using a plurality of TN signals, and detect and correct distortion by using the PDs 126X and 126Y having a band smaller than the baud rate of the transmission data signal or the ADCs 127X and 127Y having a low sampling rate. did.
In the second embodiment, as a technique for calculating and correcting the frequency characteristic of the gain in more detail, a technique for detecting and correcting waveform distortion when assuming a startup time before operating the optical transmitter 200 will be described. To do.

The digital signal processing unit 210 includes a mapper 211, a TN signal insertion unit 212, a TN signal generation unit 113, a distortion identification unit 114, and a frequency domain correction unit 115. The digital signal processing unit 210 in the second embodiment is configured in the same manner as the digital signal processing unit 110 in the first embodiment except for the mapper 211 and the TN signal insertion unit 212.

The mapper 211 generates a processed data signal OXI, OXQ, OYI, OYQ by processing a predetermined data signal using polarization. Before the operation of the optical transmitter 200 is started, no data signal is input to the optical transmitter 200, so the mapper 211 uses a predetermined data signal. The predetermined data signal may be stored in a memory (not shown) in the digital signal processing unit 210, or may be generated by the mapper 211, for example.

The TN signal insertion unit 212 inserts the TN signal generated by the TN signal generation unit 113 into the processing data signals OXI, OXQ, OYI, and OYQ generated by the mapper 211, thereby inserting the insertion data signals IXI, IXQ, and IYI. , IYQ is generated. In the second embodiment, the configuration of the transmission data frame is different from that of the first embodiment.

FIG. 6 is a schematic diagram showing a configuration of a transmission data frame in the second embodiment.
As shown in FIG. 6, the transmission data frame includes a synchronization signal and a TN signal. Further, the TN signal a to the TN signal d have n (n is an integer of 1 or more) distortion detection signals having different bit string configurations. The integer n may be determined in advance according to the frequency for correcting the waveform distortion.

In the second embodiment, since the start-up time before operating the optical transmitter is assumed, the area for storing the data signal in the transmission data frame in the first embodiment shown in FIG. It can be used as a storage area.

The TN signal a is used to detect waveform distortion generated in the channel XI, and includes n distortion detection signals TXI. The same applies to the TN signal b, the TN signal c, and the TN signal d, which are used to detect waveform distortion generated in the channel XQ, the channel YI, and the channel YQ, respectively, and the distortion detection signal TXQ, the distortion detection signal TYI, and N distortion detection signals TYQ are included.

As shown in FIG. 6, in the second embodiment, in a plurality of transmission data frames constituting the processed data signals OXI, OXQ, OYI, and OYQ, TN signals a to d are stored in areas for storing data signals. Has been inserted.

Next, a bit string of the distortion detection signal TXI included in the TN signal a will be described.
7A to 7C are schematic diagrams showing the time waveform of the TN signal a and the time waveform of the optical output of the I / Q modulator 123X.

FIG. 7A shows a time waveform of the TN signal a. FIG. 7A shows the drive voltage on the vertical axis and time on the horizontal axis.
The distortion detection signal TXI1 is an alternating pattern in which a positive value “1” and a negative value “−1” are alternately repeated bit by bit, and the distortion detection signal TXI2 is a positive value “1” and a negative value “−1”. And the distortion detection signal TXIn is a bit string composed of an alternating pattern in which a positive value “1” and a negative value “−1” are alternately repeated n bits. By configuring the bit string as described above, the distortion detection signals TXI1 to TXIn can be clock signals having different frequencies, and these are inserted into the transmission data frame.

FIG. 7B is a diagram illustrating a time waveform of the TN signal a included in the optical modulation signal X1 by the I / Q modulator 123X. In FIG. 7B, the vertical axis indicates the light intensity in the X polarization, and the horizontal axis indicates the time.
In the case of the frequency gain characteristic as shown in FIG. 2A, the gain in the high frequency band is lower than the gain in the low frequency band. Therefore, as shown in FIG. 7B, when the number of consecutive bits with the same sign is small, that is, when the frequency of the clock signal is high, the gain decreases, so as shown in FIG. Strength decreases. Conversely, when the number of consecutive bits with the same sign is large, that is, when the frequency of the clock signal is low, the light intensity increases because the decrease in gain is small.

The TN signal a generated as described above is included in the intensity signal X2 generated by the PD 126X. Here, as in the first embodiment, when the frequency of the distortion detection signal is larger than the band of the PD 126X, the intensity signal X2 that is constant as the time average of the light intensity of the light modulation signal X1 is generated. When the frequency of the distortion detection signal is smaller than the band of the PD 126X, the intensity signal X2 that is correctly generated for each bit is generated.

Therefore, as in the case of the first embodiment, the ADC 127X performs the process of averaging the intensity level of the intensity signal X2 to obtain a constant value, so that the intensity signal X2 of the distortion detection signal regardless of the band of the PD 126X. Is a constant value with respect to frequency. The ADC 127X converts the value obtained thereby into a digital signal and inputs the digital signal to the digital signal processing unit 210. In the digital signal processing unit 210, the distortion is specified by the distortion specifying unit 114.

The distortion specifying unit 114 obtains the intensity and frequency information of the n distortion detection signals TXI1 to TXIn included in the TN signal a from the TN signal generation unit 113, and compares the information with the intensity signal X2 input from the ADC 127X. The frequency characteristic of gain is calculated. Then, based on the calculated frequency characteristic of the gain, the frequency domain correction unit 115 performs a setting for correcting the gain characteristic so that the gain is constant regardless of the frequency. As a result of performing such distortion correction, the time waveform of the light of the TN signal a after distortion correction is as shown in FIG.

This time, the TN signal a has been described as an example, but the frequency characteristics of the gain can be calculated and corrected by performing the same operation for the TN signal b to the TN signal d.

This time, as in the first embodiment, the PDs 126X and 126Y are provided in the subsequent stage of the I / Q modulators 123X and 123Y of the respective polarization components. However, as in the first embodiment, the PBC 124 is the same as in the prior art. A PD may be provided in the subsequent stage. Even in this case, it is possible to calculate and correct the gain frequency characteristic with the same accuracy.

In the second embodiment, by performing distortion correction when the optical transmitter 200 is started up, the area of the transmission data signal in the transmission data frame can also be used as the area of the TN signal. As a result, the frequency characteristics of the gains of the optical modulation signals X1 and Y1 can be calculated with higher accuracy, and the gain can be made constant regardless of the frequency.

Embodiment 3 FIG.
FIG. 8 is a block diagram schematically showing the configuration of the optical transmitter 300 according to the third embodiment.
The optical transmitter 300 includes a digital signal processor 310, DACs 120A to 120D, drivers 321A to 321D, a light source 122, I / Q modulators 123X and 123Y, a PBC 124, couplers 125X and 125Y, and PDs 126X and 126Y. And a distortion specifying unit 314 and a distortion correcting unit 328.
The optical transmitter 300 according to the third embodiment has the same configuration as the optical transmitter 100 according to the first embodiment, except for the digital signal processing unit 310, the drivers 321A to 321D, the distortion specifying unit 314, and the distortion correcting unit 328. Has been.

In the first and second embodiments, clock signal intensity signals X2 and Y2 included in the optical modulation signals X1 and Y1 generated by the I / Q modulators 123X and 123Y using a clock signal having a plurality of frequencies are used. The frequency characteristic of the gain of the optical modulation signal was calculated and corrected so that the gain was constant regardless of the frequency using an FIR filter or the like.
In the third embodiment, the configuration and operation of the optical transmitter 300 for the purpose of correcting the harmonic distortion generated by the drivers 321A to 321D included in the optical transmitter 300 will be described. In the third embodiment, the case where the optical transmitter 300 is started up will be described as in the second embodiment.

The digital signal processing unit 310 generates transmission data signals XI1 #, XQ1 #, YI1 #, and YQ1 # from the data signal DS by digital signal processing.
The digital signal processing unit 310 includes a mapper 311, a TN signal insertion unit 112, and a TN signal generation unit 313. The digital signal processing unit 310 according to the third embodiment does not include the distortion specifying unit 114 and the frequency domain correction unit 115 included in the digital signal processing unit 110 according to the first embodiment. Further, the processing of the mapper 311 and the TN signal generation unit 313 is different from the processing of the TN signal generation unit 113 in the first embodiment.

The mapper 311 generates a processed data signal OXI, OXQ, OYI, OYQ by processing a predetermined data signal using polarization. Since the data signal is not input to the optical transmitter 300 before the operation of the optical transmitter 300 is started, the mapper 311 uses a predetermined data signal as in the second embodiment.

As will be described later, the TN signal generation unit 313 generates a clock signal having a specific frequency by configuring a bit string having a specific pattern.
The TN signal insertion unit 112 inserts the clock signal generated by the TN signal generation unit 113 into the processed data signals OXI, OXQ, OYI, and OYQ, thereby transmitting the transmission data signals XI1 #, XQ1 #, YI1 #, and YQ1 #. Generate.

The transmission data signals XI1 #, XQ1 #, YI1 #, and YQ1 # into which the clock signal is inserted by the TN signal insertion unit 112 are converted into analog signals by the DACs 120A to 120D. Note that the analog signals output from the DACs 120A to 120D are sine waves.
Transmission data signals XI1 #, XQ1 #, YI1 #, YQ1 # converted into analog signals by DACs 120A to 120D are input to drivers 321A to 321D. The drivers 321A to 321D amplify the transmission data signals XI1 #, XQ1 #, YI1 #, and YQ1 #, adjust the waveform by changing the operating point of the FET inside the driver, etc., and drive signals XI2, XQ2, YI2 and YQ2 are generated. As a result, the drivers 321A to 321D can reduce waveform distortion when converted into an optical signal.

The I / Q modulators 123X and 123Y receive the carrier light generated by the light source 122 and generate optical modulation signals X1 and Y1 based on the drive signals XI2, XQ2, YI2, and YQ2.
The PBC 124 combines the polarizations of the optical modulation signals X1 and Y1 and sends them to the transmission line.

Here, the optical modulation signals X1 and Y1 generated by the I / Q modulators 123X and 123Y are branched by the couplers 125X and 125Y arranged at the subsequent stage of the I / Q modulators 123X and 123Y, respectively, and PDs 126X and 126Y are obtained. Is input.
The PDs 126X and 126Y generate intensity signals X2 and Y2 based on the input light modulation signals X1 and Y1.

The distortion specifying unit 314 specifies the harmonic component included in the intensity signals X2 and Y2 by referring to the clock signal generated by the TN signal generating unit 113.
The distortion correction unit 328 adjusts the parameters of the drivers 321A to 321D so that the harmonic component specified by the distortion specifying unit 314 is minimized, thereby driving signals XI2, XQ2, Harmonic components included in YI2 and YQ2 are suppressed. The parameters of the drivers 321A to 321D here include, for example, a bias voltage applied to a terminal for adjusting a gain of an output amplitude with respect to an input amplitude, a cross point of an output waveform, and the like. The details of the operations of the distortion specifying unit 314 and the distortion correcting unit 328 will be described later.

FIG. 9 is a schematic diagram showing a configuration of a transmission data frame in the third embodiment.
As described above, in the third embodiment, similarly to the second embodiment, assuming that the optical transmitter 300 is started up, the TN signal generation unit 113 also generates an area where the transmission data signal is originally stored. The clock signals TXICLK, TXQCLK, TYICLK, and TYQCLK having a specific frequency are stored. The clock signals TXICLK, TXQCLK, TYICLK, and TYQCLK are used to specify the harmonic distortion of the drivers 121A, 121B, 121C, and 121D, and are inserted into the processed data signals OXI, OXQ, OYI, and OYQ. For example, when the clock signal TXICLK is inserted into the processing data signal OXI, “0 (zero)” is stored in the other channels.

As shown in FIG. 9, in the third embodiment, in a plurality of transmission data frames constituting the processed data signals OXI, OXQ, OYI, OYQ, clock signals TXICLK, TXQCLK, TYICLK and TYQCLK are inserted.

Next, the configuration of the bit string of the clock signal generated by the TN signal generation unit 313 will be described. Here, as an example, the clock signal TXICLK inserted into the processing data signal OXI will be described.

The clock signal TXICLK is composed of a positive value “1” and a negative value “−1”, and each is alternately repeated several bits. For example, when the baud rate of the transmission data signal is 32 Gbaud, a 3.2 GHz clock signal can be generated by alternately repeating a positive value “1” by 5 bits and a negative value “−1” by 5 bits. it can. However, the frequency of the generated clock signal needs to be smaller than the band of the PD 126X provided at the subsequent stage of the I / Q modulator 123X.

Although the clock signal TXICLK has been described here, the clock signals TXQCLK, TYICLK, and TYQCLK are also clock signals having the same bit string configuration as the clock signal TXICLK. The clock signals TXICLK, TXQCLK, TYICLK, and TYQCLK may be clock signals having different frequencies or may be clock signals having the same frequency as long as the frequency is lower than the band of the light receiver. .

Next, details of operations of the distortion specifying unit 314 and the distortion correcting unit 328 will be described.
Here, as an example, a comparison between the clock signal TXICLK included in the intensity signal X2 generated by the PD 126X and the clock signal TXICLK generated by the TN signal generation unit 113 will be described.

The distortion specifying unit 314 specifies a harmonic component included in the intensity signal X2 generated by the PD 126X based on the clock signal TXICLK generated by the TN signal generating unit 313. As a specifying method, for example, synchronous detection can be cited. In general, it is known that the frequency of the harmonic component is an odd multiple of the frequency of the original signal. Therefore, when the harmonic component included in the intensity signal X2 is identified by synchronous detection using the clock signal TXICLK generated by the TN signal generation unit 313 as a reference signal, the distortion identification unit 314 first determines the frequency of the clock signal TXICLK. Is converted to an odd multiple of the desired frequency, ie, the frequency of the clock signal, and then synchronous detection is performed. The distortion specifying unit 314 can specify the harmonic component by operating in this way. Since the harmonic component has a plurality of frequency components, the distortion specifying unit 314 can specify the components of the respective frequencies by performing processing for converting the frequency of the clock signal with respect to the respective frequencies.

Based on the distortion amount specified by the distortion specifying unit 314, the distortion correction unit 328 changes the setting of the parameter (adjustment value) of the driver 321A so that the distortion amount is minimized. Examples of the parameters here include the bias voltage applied to the adjustment terminal that determines the performance such as the gain of the output amplitude with respect to the input amplitude of the driver 321A and the cross point of the output waveform, as described above.

FIGS. 10A to 10C are schematic diagrams showing time waveforms of clock signals in the third embodiment.
FIG. 10A is a schematic diagram showing a time waveform of the clock signal TXICLK input to the driver 321A. FIG. 10A shows the driving voltage on the vertical axis and time on the horizontal axis.
FIG. 10B is a schematic diagram illustrating a time waveform of the clock signal TXICLK output from the driver 321A when distortion correction is not performed. FIG. 10B shows the driving voltage on the vertical axis and time on the horizontal axis. Since the harmonic distortion generated in the driver 321A is not corrected, the driver 321A cannot linearly amplify the clock signal shown in FIG. 10A as the input signal, and the output waveform is the maximum value of the output of the driver 321A. Becomes a waveform saturated at a constant value.
FIG. 10C is a schematic diagram illustrating a time waveform of the clock signal TXICLK output from the driver 321A when distortion correction is performed by the distortion correction unit 328 based on the distortion amount specified by the distortion specifying unit 314. It is. FIG. 10C shows the driving voltage on the vertical axis and time on the horizontal axis. Since the waveform shown in FIG. 10C is corrected so as to reduce the harmonic distortion generated by the driver 321A, it is amplified linearly when the time waveform shown in FIG. 10A is input. Output time waveform.

In the above, the method of specifying the harmonic component using the synchronous detection in the distortion specifying unit 314 has been described, but any method may be used as long as the harmonic component can be specified. .
As another method for identifying the harmonic component, for example, there is a method of comparing the intensity signal X2 and the clock signal TXICLK in the frequency domain. In this method, the harmonic component contained in the intensity signal X2 can be specified by performing Fourier transform on the intensity signal X2 and the clock signal TXICLK by FFT and performing a subtraction process.
As another method, there is a method using a high-pass filter that passes only a signal having a frequency larger than the frequency of the original clock signal TXICLK with respect to the intensity signal X2. When the high-pass filter is used, only the harmonic component of the clock signal TXICLK included in the intensity signal X can be specified. Therefore, if only the frequency of the clock signal TXICLK generated by the TN signal generation unit 313 is transmitted to the distortion specifying unit 314, it is not necessary to input the clock signal TXICLK to the distortion specifying unit 314.

Here, the case where the clock signal TXICLK is used as the operation example of the distortion specifying unit 314 and the distortion correction unit 328 has been described. However, the clock signals TXQCLK, TYICLK, and TYQCLK are the same as the clock signal TXICLK. That is, the distortion identifying unit 314 identifies the harmonic component by comparing the intensity signals X2 and Y2 generated by the PDs 126X and 126Y with the clock signal generated by the TN signal generating unit 113, and the identification result Based on this, the harmonic distortion generated by the drivers 321b, 321C, and 321D can be corrected.

In the third embodiment, when the optical transmitter 300 is started up, a clock signal with a specific frequency is used, and the intensity signals X2 and Y2 generated by the PDs 126X and 126Y are compared with the original clock signal to thereby generate harmonic components. It was set as the structure which detects. Based on this detection result, the harmonic distortion can be corrected by adjusting the bias voltage value of the adjustment terminal of the driver so as to reduce the harmonic component.

FIGS. 11A and 11B are schematic diagrams illustrating hardware configuration examples.
A part or all of the digital signal processing units 110 and 210, the distortion specifying unit 314, and the distortion correcting unit 328 described above may be a single circuit or a composite circuit as shown in FIG. , A programmed processor, a parallel programmed processor, an ASIC (Application Specific Integrated Circuits) or an FPGA (Field Programmable Gate Array).

In addition, a part or all of the digital signal processing units 110 and 210, the distortion specifying unit 314, and the distortion correcting unit 328 are stored in the memory 11 and the memory 11, for example, as illustrated in FIG. It can also be configured with a processor 12 such as a CPU (Central Processing Unit) that executes the program being executed. Such a program may be provided through a network, or may be provided by being recorded on a recording medium.

100, 200, 300 optical transmitter, 110, 210, 310 digital signal processing unit, 111, 211, 311 mapper, 112, 212 TN signal insertion unit, 113, 313 TN signal generation unit, 114, 314 distortion identification unit, 115 Frequency domain correction unit, 120A, 120B, 120C, 120D DAC, 121A, 121B, 121C, 121D, 321A, 321B, 321C, 321D driver, 122 light source, 123X, 123Y I / Q modulator, 124 PBC, 125X, 125Y coupler 126X, 126Y PD, 127X, 127Y ADC, 328 distortion correction unit.

Claims (18)

1. A digital signal processing unit that generates a transmission data signal including a training signal that is a clock signal configured by an alternating pattern of binary bit strings;
   A first controller for converting the transmission data signal into an analog signal;
   A driver that amplifies the analog signal to generate a drive signal;
   A light source that emits carrier light;
   A modulator that generates an optical modulation signal by modulating the carrier light based on the drive signal;
   A light receiver for generating an intensity signal indicating the intensity of the light modulation signal;
   A second controller for converting the intensity signal into a digital signal;
   The optical signal processing unit detects the waveform distortion of the optical modulation signal by comparing the training signal and the digital signal, and corrects the waveform distortion.
2. The optical transmitter according to claim 1, wherein the second controller generates the digital signal so as to indicate a value obtained by averaging the intensity indicated by the intensity signal.
3. The digital signal processing unit detects the waveform distortion for each frequency by changing the frequency of the clock signal by changing the number of consecutive bits having the same value in the alternating pattern. Item 3. The optical transmitter according to Item 1 or 2.
4. The transmission data signal includes a plurality of synchronization signals,
   The optical transmitter according to claim 3, wherein the digital signal processing unit changes the frequency and includes the training signal in the transmission data signal each time the plurality of synchronization signals are detected.
5. The optical transmitter according to claim 4, wherein the optical receiver generates the intensity signal so as to indicate an intensity of only a portion corresponding to the training signal in the optical modulation signal.
6. The digital signal processor is
   A mapper that generates a processed data signal by processing the data signal input to the optical transmitter using polarization; and
   A training signal generator for generating the training signal;
   A training signal insertion unit for generating an insertion data signal by inserting the training signal into the processed data signal;
   By comparing the clock signal and the digital signal for each frequency, a distortion specifying unit that specifies a frequency characteristic of gain,
   The distortion correction part which produces | generates the said transmission data signal by correct | amending a gain for every frequency in the said insertion data signal based on the frequency characteristic of the said gain is provided. The optical transmitter according to any one of the above.
7. The optical transmission according to claim 6, wherein the training signal insertion unit inserts the training signal between areas in which the data signal is stored in a plurality of transmission data frames constituting the processed data signal. vessel.
8. The digital signal processor is
   A mapper that generates a processed data signal by processing a predetermined data signal using polarization; and
   A training signal generator for generating the training signal;
   A training signal insertion unit for generating an insertion data signal by inserting the training signal into the processed data signal;
   By comparing the clock signal and the digital signal for each frequency, a distortion specifying unit that specifies a frequency characteristic of gain,
   The distortion correction part which produces | generates the said transmission data signal by correct | amending a gain for every frequency in the said insertion data signal based on the frequency characteristic of the said gain is provided. The optical transmitter according to any one of the above.
9. The optical transmitter according to claim 8, wherein the training signal insertion unit inserts the training signal into an area in which the data signal is stored in a plurality of transmission data frames constituting the processed data signal.
10. The distortion correction unit is an FIR (Finite Impulse Response) filter that sets a multiplier parameter so as to correct the gain for each frequency based on the frequency characteristic of the gain specified by the distortion specifying unit. An optical transmitter according to any one of claims 6 to 9.
11. A digital signal processing unit for generating a transmission data signal including a clock signal composed of an alternating pattern of binary bit strings;
    A first controller for converting the transmission data signal into an analog signal;
    A driver that generates a drive signal by amplifying the analog signal and adjusting the waveform of the analog signal;
    A light source that emits carrier light;
    A modulator that generates an optical modulation signal by modulating the carrier light based on the drive signal;
    A light receiver for generating an intensity signal indicating the intensity of the light modulation signal;
    From the intensity signal indicating the intensity of the portion corresponding to the clock signal, a distortion specifying unit that specifies a higher harmonic component than a predetermined frequency,
    An optical transmitter comprising: a distortion correction unit that corrects a waveform distortion of the optical modulation signal by causing the driver to adjust a waveform of the analog signal so as to suppress the harmonic component.
12. The optical transmitter according to claim 11, wherein the distortion specifying unit specifies the harmonic component by comparing the clock signal and the intensity signal.
13. The optical transmitter according to claim 12, wherein the distortion specifying unit specifies the harmonic component by synchronously detecting the clock signal and the intensity signal.
14. The distortion specifying unit specifies the harmonic component by converting the clock signal and the intensity signal into a frequency domain by FFT (Fast Fourier Transform) and performing a subtraction process. 13. The optical transmitter according to 12.
15. The optical transmission according to claim 12, wherein the distortion specifying unit specifies the harmonic component from the intensity signal by using a high-pass filter that passes only a band higher than the frequency of the clock signal. vessel.
16. The optical transmitter according to any one of claims 11 to 15, wherein the distortion correction unit controls the driver so that the harmonic component is minimized.
17. Generating a transmission data signal including a training signal, which is a clock signal composed of an alternating pattern of binary bit strings;
    Converting the transmission data signal into an analog signal;
    Amplifying the analog signal to generate a drive signal;
    By modulating the carrier light based on the drive signal, an optical modulation signal is generated,
    Generating an intensity signal indicating the intensity of the light modulation signal;
    Converting the intensity signal into a digital signal;
    A waveform distortion correction method comprising: comparing the training signal and the digital signal to detect waveform distortion of the optical modulation signal and correcting the waveform distortion.
18. Generating a transmission data signal including a clock signal composed of an alternating pattern of binary bit strings;
    Converting the transmission data signal into an analog signal;
    Amplifying the analog signal to generate a drive signal,
    By modulating the carrier light based on the drive signal, an optical modulation signal is generated,
    Generating an intensity signal indicating the intensity of the light modulation signal;
    From the intensity signal indicating the intensity of the portion corresponding to the clock signal, identify harmonic components higher than a predetermined frequency,
    A waveform distortion correction method, wherein the waveform distortion of the optical modulation signal is corrected by adjusting the waveform of the analog signal so as to suppress the harmonic component.

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