Optical transmitter control method

Abstract

This optical transmitter has a signal ADC that outputs an electrical signal corresponding to a set value that changes over time, and an optical modulator that generates modulated light using the electrical signal. When the coordinates of modulated light are defined as two-dimensional coordinates on a complex plane including the magnitude of the optical electric field of the modulated light and information on the optical phase of the modulated light, a first lookup table is created in which a correspondence between each set value and coordinates producible by the optical modulator is recorded, and a second lookup table is created in which, for each set of target coordinates of the modulated light during a period of transmission service, the correspondence between the producible coordinates selected from the first lookup table and the set value corresponding to the producible coordinates is recorded. The creation of the first and second tables has been completed before the transmission service is started.

Description

Optical transmitter control method

[0001] The present invention relates to a method for controlling an optical transmitter.

[0002] In high-speed, high-capacity optical transmission systems, optical QAM (Quadrature Amplitude Modulation), which incorporates information into both optical phase and optical intensity, is widely used. To increase transmission capacity, it is crucial to maximize the multi-level nature of the optical QAM signal and to maintain equal distances between each symbol in the constellation.

[0003] IQ optical modulators are widely used to generate optical QAM signals. Typically, IQ optical modulators are configured in a nested structure in which a Mahtzehnder optical modulator is placed on each of the two arms of a Mahtzehnder interferometer.

[0004] Figure 13 shows an example of a conventional configuration of an optical transmitter 7 that generates optical QAM. Here, the IQ optical modulator 70 in the optical transmitter 7 is assumed to be of the lithium niobate type. Continuous wave (CW) light is input to the input terminal of the IQ optical modulator 70. The IQ optical modulator 70 has a first Mahatzehnder type optical modulator 71a and a second Mahatzehnder type optical modulator 71b. Each of these Mahatzehnder type optical modulators has two arms. In other words, the IQ optical modulator 70 has four arms: the first arm 72a-1, the second arm 72a-2, the third arm 72b-1, and the fourth arm 72b-2. The difference in the optical path lengths of these arms, i.e., the optical path difference, needs to be precisely adjusted. When no drive signal is applied, and assuming the wavelength of CW light is λ, the optical path difference between the first arm 72a-1 and the second arm 72a-2 in the first Mahtzehnder optical modulator 71a is set to ±λ / 2 by the first λ / 2 shift unit 73a. Similarly, the optical path difference between the third arm 72b-1 and the fourth arm 72b-2 in the second Mahtzehnder optical modulator 71b is set to ±λ / 2 by the second λ / 2 shift unit 73b. The outputs of the first Mahtzehnder optical modulator 71a and the second Mahtzehnder optical modulator 71b propagate through the fifth arm 74a and the sixth arm 74b, respectively, before being combined, and the optical path differences of these arms are set to ±λ / 4 by the λ / 4 shift unit 75.

[0005] When the above conditions are satisfied, the output light of the IQ optical modulator 70 is extinguished. However, when the above optical path difference is modulated by a drive signal, an optical QAM signal is output from the IQ optical modulator 70. The drive signal needs to be applied to each of the first Mach-Zehnder optical modulator 71a and the second Mach-Zehnder optical modulator 71b. Here, when generating an optical QAM signal with a multilevel number of m ２ , the generation of the drive signal for the first Mach-Zehnder optical modulator 71a will be described based on the conventional configuration.

[0006] The first digital data generation unit 77a generates continuous numerical data having m types of values, and transmits this numerical data to the first signal DAC (digital / analog converter) 78a as a set value. The first signal DAC 78a generates a modulation signal having m types of voltage levels based on the received set value. There is an upper limit to the types of voltages that can be generated by the DAC. For an n-bit DAC, 2 ｎ types of set values can be received, and 2 ｎ types of voltages can be generated. Therefore, when generating an optical QAM signal with a multilevel number of m ２ , the first signal DAC 78a must be an n-bit DAC that satisfies m≤2 ｎ .

[0007] The output of the first signal DAC 78a is input to the first signal differential output amplifier 79a, and modulation signals of the positive phase and the negative phase whose voltage signs are opposite to each other are generated. In FIG. 13, these are indicated by V １ and -V １ . These modulation signals of the positive phase and the negative phase are applied to the first arm drive electrode 76a-1 and the second arm drive electrode 76a-2, respectively, to modulate the optical path lengths of the first arm 72a-1 and the second arm 72a-2. Since V １ is the voltage of the modulation signal, it changes every moment, but the absolute value of its maximum value and the absolute value of its minimum value are set to be equal. Only when V １ is 0 (ground level), the voltages applied to the first arm drive electrode 76a-1 and the second arm drive electrode 76a-2 are the same.

[0008] V １ Increasing the voltage amplitude reduces modulation loss. However, there is an upper limit to this voltage amplitude. When the half-wavelength voltage of the first Mahatzehnder optical modulator 71a and the half-wavelength voltage of the second Mahatzehnder optical modulator 71b are Vπ, the voltage amplitude applied to the first arm drive electrode 76a-1 and the voltage amplitude applied to the second arm drive electrode 76a-2 are set to a maximum of Vπ (2Vπ in differential sum). That is, V １ The absolute value of is set to be less than Vπ / 2. If this voltage amplitude is exceeded, the modulation loss will start to increase due to the sinusoidal characteristics of the Mahatzehnder interferometer. This maximum voltage amplitude (Vπ at each electrode, 2Vπ for the differential sum) is sometimes called full swing.

[0009] Some commercial DACs have either only positive or only negative output voltages. In that case, an offset voltage application circuit must be added to the output of the first signal DAC 78a to obtain a voltage with equal positive and negative values. Conversely, if the IQ optical modulator 70 is a semiconductor type rather than a lithium nyobate type, the first arm drive electrode 76a-1 and the second arm drive electrode 76a-2 must always have a positive voltage (or a negative voltage depending on the semiconductor circuit configuration). Therefore, an offset voltage Vo must be provided, and V １ +V and -V １ An offset voltage application circuit is necessary to ensure that +Vo is always a positive (or negative) voltage. However, to avoid complexity, these offset voltage application circuits are not shown in the drawings of this application.

[0010] The generation of the drive signal for the second Mahatzehnder optical modulator 71b is carried out in the same manner as the generation of the drive signal for the first Mahatzehnder optical modulator 71a, but the numerical sequence generated by the second digital data generation unit 77b must be different from the numerical sequence generated by the first digital data generation unit 77a. That is, the setting value of the second signal DAC 78b is uncorrelated with the setting value of the first signal DAC 78a, and the voltage V generated by the second signal differential output amplifier 79b ２ and -V ２ Voltage V １ and -V １It must be uncorrelated. The voltage V output by the second signal differential output amplifier 79b ２ and -V ２ The modulated signals are applied to the third arm drive electrode 76b-1 and the fourth arm drive electrode 76b-2, respectively, to modulate the optical path lengths of the third arm 72b-1 and the fourth arm 72b-2.

[0011] Optical QAM signal's optical field E ＱＡＭ Expressed as a complex number, it looks like this:

[0012] E １ = {exp(i( π×V １ / Vπ+π / 2))} / 4 (Formula 1) E ２ = {exp(i(-π×V １ / Vπ-π / 2))} / 4 (Formula 2) E ３ = {exp(i( π×V ２ / Vπ+π ))} / 4 (Formula 3) E ４ = {exp(i(-π×V ２ / Vπ ))} / 4 (Formula 4) E ＱＡＭ = E １ +E ２ +E ３ +E ４ (Formula 5)

[0013] Here E １ ~E ４ These are the optical electric fields originating from the first arm 72a-1, the second arm 72a-2, the third arm 72b-1, and the fourth arm 72b-2 in Figure 13, respectively. The 1 / 4 at the end of equations (1) to (4) corresponds to the division of the input light into four arms. Strictly speaking, these equations should include coefficients for the magnitude of the optical electric field of the CW light input to the IQ optical modulator 70, the loss of the optical waveguide, and the insertion loss of the optical multiplexer / demultiplexer, but these are not essential and are omitted in the explanation of this invention. E is expressed as a dimensionless number normalized by the voltage of the drive signal with Vπ. １ ~E ４ It represents.

[0014] The +π / 2 and -π / 2 terms appearing in (Equation 1) and (Equation 2), respectively, correspond to the phase difference generated by the first λ / 2 shift section 73a. However, depending on the sign of the shift amount, the signs of +π / 2 and -π / 2 in (Equation 1) and (Equation 2) may be reversed. Also, the +π term appearing in (Equation 3) corresponds to the phase difference generated by the second λ / 2 shift section 73b and the λ / 4 shift section 75. However, depending on the sign of the shift amount, the +π term may appear in (Equation 4) instead of (Equation 3). If some of these terms are reversed, the resulting constellation will be inverted with respect to the real or imaginary axis or rotated by an integer multiple of 90 degrees on the complex plane. Although these inversions and rotations do not pose a practical problem, in this application, in order to simplify the explanation and maintain consistency between the drawings and equations, E １ ~E ４ The optical field of the photoelectric field is always expressed by equations (1) to (4).

[0015] Figure 14 shows the constellation of the optical 16-QAM signal generated by the conventional optical transmitter 7 shown in Figure 13, plotted on the complex plane. The horizontal axis Re represents the real part, and the vertical axis Im represents the imaginary part. As mentioned above, m ２ When generating an optical QAM signal, m ≤ 2 ｎ An n-bit DAC that satisfies the following conditions is required. Typically, considering the setting of intermediate transition states between symbols, m << 2 ｎ However, to simplify the explanation, m = 2 ｎ Assuming m = 16^0.5 = 4, the first signal DAC 78a and the second signal DAC 78b are 2-bit DACs, and there are four output levels. The DAC settings corresponding to these output levels are represented as 0, 1, 2, and 3. The voltage amplitude of the drive signal received by the first arm drive electrode 76a-1 and the second arm drive electrode 76a-2 is assumed to be in the full swing state described above.

[0016] As shown in Figure 14, the spacing between symbols is not uniform and is concentrated at the four corners. This is due to the sinusoidal characteristics of the Mahatzehnder interferometer. In a typical optical transceiver, a DAC with a much larger number of bits than 2 is used to equalize the spacing between each symbol. １ and V２ By increasing the options and performing the inverse operation of the modulator's sinusoidal characteristics, V can be made to obtain equally spaced symbols on the complex plane. １ and V ２ Choosing this option is also possible.

[0017] The important point in the conventional example shown in Figure 14 is that there is a simple correspondence between the settings of the first signal DAC 78a and the second signal DAC 78b and the coordinates of each symbol in the generated constellation. In the illustrated example, as the setting of the first signal DAC 78a increases, the real part of the symbol increases, and as the setting of the second signal DAC 78b increases, the imaginary part of the symbol increases. Although nonlinearity arising from the sinusoidal characteristics of the modulator must be considered, it is relatively easy to estimate what the setting values ​​of each DAC should be in order to generate the desired symbol.

[0018] In the explanation of the conventional example so far, the first arm 72a-1 and the second arm 72a-2 are given opposing differential signals (±V １ ) is added, and another reciprocal differential signal (±V) is applied to the third arm 72b-1 and the fourth arm 72b-2. ２ The optical QAM signal was generated by adding ). However, our prior patent application describes a configuration in which the optical QAM signal is generated by providing independent drive signals to all of the first to fourth arms of the IQ optical modulator (see, for example, Patent Document 1).

[0019] Figure 15 shows the configuration of the optical transmitter 8 shown in Patent Document 1. The configuration of the IQ optical modulator 70 is the same as in Figure 13. However, it differs in that it generates four mutually independent drive signals for each of the first arm drive electrodes 76a-1, 76a-2, 76b-1, and 76b-2, using the first digital data generation unit 87a-1 and the first signal DAC 88a-1, the second digital data generation unit 87a-2 and the second signal DAC 88a-2, the third digital data generation unit 87b-1 and the third signal DAC 88b-1, the fourth digital data generation unit 87b-2 and the fourth signal DAC 88b-2, respectively. These four drive signals are amplified by the first signal amplifier 89a-1, the second signal amplifier 89a-2, the third signal amplifier 89b-1, and the fourth signal amplifier 89b-2, instead of the first signal differential output amplifier 79a and the second signal differential output amplifier 79b in Figure 13. The voltage of each of these drive signals is V １ ~V ４ This is represented as follows. By adopting this configuration, not only does the number of symbols that can be generated increase dramatically, but compared to the configuration shown in Figure 13, it is also possible to generate symbols at a position further from the origin, thereby reducing modulation loss.

[0020] International Publication No. 2024 / 079894

[0021] However, in the configuration shown in Figure 15, the correspondence between the output level of each DAC and the generated optical field becomes very complex. When using the configuration shown in Figure 15, equations (1) to (5) are rewritten as equations (6) to (10).

[0022] E １ = {exp(i(π×V １ / Vπ+π / 2))} / 4 (Formula 6) E ２ = {exp(i(π×V ２ / Vπ-π / 2))} / 4 (Formula 7) E ３ = {exp(i(π×V ３ / Vπ+π ))} / 4 (Formula 8) E ４ = {exp(i(π×V ４ / Vπ ))} / 4 (Formula 9) E ＱＡＭ = E １ +E ２ +E ３ +E ４ (Formula 10)

[0023] The relationship between the output level of each DAC and the complex components of the generated constellation is explained. For simplicity, as in Figure 14, all signal DACs are 2-bit DACs, and there are four types of output levels. The DAC settings corresponding to these output levels are represented as 0, 1, 2, and 3. １ From V ４ Each of these will have four independent values. Also, similar to Figure 14, V １ From V ４ Let's assume that all of them are driven with a full swing.

[0024] Figure 16 is a table showing the setting values ​​for the first signal DAC 88a-1, the second signal DAC 88a-2, the third signal DAC 88b-1, and the fourth signal DAC 88b-2, along with the real and imaginary parts of the optical QAM signal. In this case, there are 4^4 = 256 possible combinations of the setting values ​​for the first signal DAC 88a-1, the second signal DAC 88a-2, the third signal DAC 88b-1, and the fourth signal DAC 88b-2, but to avoid complexity, only a portion of the table is shown.

[0025] As can be seen from the table in Figure 16, there is overlap in the coordinates of the symbols represented by the real and imaginary parts of the optical field, so the number of symbols that the optical transmitter 8 can generate is 169, which is less than 256. For example, when the setting values ​​for the first signal DAC 88a-1, the second signal DAC 88a-2, the third signal DAC 88b-1, and the fourth signal DAC 88b-2 are 0, 2, 0, 0 and when they are 0, 2, 3, 3, the combination of the real and imaginary parts of the optical field in the generated constellation is the same. Also, as can be seen from the table in Figure 16, the correspondence between the real and imaginary parts of the generated optical field and the setting values ​​of each DAC becomes much more complex compared to Figure 14.

[0026] Figure 17 is a diagram showing the real and imaginary parts of the optical field shown in the table of Figure 16 as a constellation on the complex plane. The numbers in brackets [] in Figure 17 represent the setting values ​​for the first signal DAC88a-1, the second signal DAC88a-2, the third signal DAC88b-1, and the fourth signal DAC88b-2, respectively. In the figure, the symbol indicated by the symbol A is obtained only when the setting values ​​for the first signal DAC88a-1, the second signal DAC88a-2, the third signal DAC88b-1, and the fourth signal DAC88b-2 are 1, 3, 0, and 2, respectively. In the figure, the symbol indicated by the symbol B is obtained only when the setting values ​​for the first signal DAC 88a-1, the second signal DAC 88a-2, the third signal DAC 88b-1, and the fourth signal DAC 88b-2 are 2, 1, 1, and 3, respectively. In contrast, the symbol indicated by the symbol C in the figure is obtained when the setting values ​​for the first signal DAC 88a-1 and the second signal DAC 88a-2 are 0 and 3, respectively, and the setting values ​​for the third signal DAC 88b-1 and the fourth signal DAC 88b-2 are equal. Therefore, there are four possible combinations of DAC setting values ​​that can generate the symbol indicated by the symbol C.

[0027] As these examples show, even if the coordinates of the optical field to be output on the complex plane are determined, the setting values ​​required for each signal DAC are not necessarily uniquely determined, and even if they are uniquely determined, it is not easy to determine the specific values ​​of each setting.

[0028] In view of the above circumstances, the present invention aims to provide an optical transmitter control method that can quickly determine the setting values ​​required by each signal DAC in order to generate an optical field that is as close as possible to the real and imaginary parts of a desired symbol from among the constellation of optical QAM signals to be generated in an IQ optical modulator.

[0029] One aspect of the present invention is a method for controlling an optical transmitter, comprising: a single or more digital-to-analog converter for signals that takes a set value as a numerical sequence that changes over time and outputs an electrical signal corresponding to the set value; and an optical modulator that is driven by the electrical signal output from the digital-to-analog converter for signals and generates modulated light in which either or both of the magnitude and phase of the optical field are modulated, wherein when the coordinates of the modulated light are defined as two-dimensional coordinates on a complex plane including information on the magnitude and phase of the optical field of the modulated light, for each of the multiple types of set values, the method is defined as a method for controlling an optical transmitter, which includes the set value and the coordinates of the modulated light generated by the optical modulator based on the electrical signal output by the single or more digital-to-analog converters according to the set value. The system includes a first table creation step of creating a first lookup table that records the correspondence with the power coordinates, and a second table creation step of selecting the most closely related generateable coordinates from the first lookup table based on predetermined selection criteria for each target coordinate, which is the coordinate of the modulated light to be generated during the transmission service period, and creating a second lookup table that records the correspondence between the selected generateable coordinates and the setting values ​​used when the selected generateable coordinates were generated. The first and second table creation steps are performed prior to the start of the transmission service and are completed before the start of the transmission service.

[0030] The present invention makes it possible to quickly determine the setting values ​​required by each signal DAC in order to generate an optical field that is as close as possible to the real and imaginary parts of the desired symbol from the constellation of optical QAM signals to be generated in the IQ optical modulator.

[0031] This is a flowchart showing the basic processing overview of the embodiments of the present invention. This is a diagram showing the configuration of the control device according to the first embodiment. This is a flowchart showing the Table A creation process of the control device according to the first embodiment. This is a diagram showing an example of Table A according to the first embodiment. This is a flowchart showing the Table B creation process of the control device according to the first embodiment. This is a diagram showing the optical field constellation obtained by the optical transmitter of the first embodiment. This is a diagram showing an example of Table B according to the first embodiment. This is a diagram showing the symbol selection according to the first embodiment. This is a diagram showing an example of Table B according to the first embodiment. This is a flowchart showing the Table B creation process of the control device according to the second embodiment. This is a diagram showing the configuration of the optical transmitter according to the fourth embodiment. This is a diagram showing the optical field constellation generated by the optical transmitter according to the fourth embodiment. This is a diagram showing an example of the configuration of a conventional optical transmitter. This is a diagram showing the optical 16-QAM signal constellation generated by a conventional optical transmitter. This is a diagram showing an example of the configuration of a conventional optical transmitter. This is a diagram showing a table of the setting value of the signal DAC of a conventional optical transmitter and the real and imaginary part values ​​of the generated optical QAM signal. This is a diagram showing the optical field constellation generated by a conventional optical transmitter.

[0032] Embodiments of the present invention will be described below with reference to the drawings. The embodiment of the present invention relates to a method for generating an optical modulation signal using an external modulator. In this embodiment, in an optical transmitter that generates an optical QAM signal by applying non-reciprocal drive signals to a single Mahatzehnder type optical modulator or IQ optical modulator using multiple DACs, it is possible to quickly determine the setting values ​​of each DAC required to generate an optical electric field corresponding to a desired coordinate on the complex plane, or the vicinity of that coordinate. Furthermore, this embodiment is also applicable in a configuration in which a single Mahatzehnder type modulator is used instead of an IQ optical modulator, and non-reciprocal drive signals are applied to the two arms.

[0033] [Basic Mode for Carrying Out the Invention] Figure 1 is a flowchart showing the basic processing overview of an embodiment of the present invention. Before generating an optical QAM signal using an optical transmitter and transmitting the optical QAM signal to the transmission line to start the transmission service, a startup sequence is first performed (step S101). The startup sequence is necessary not only for this embodiment but for optical transmitters in general, and includes processes such as supplying power to the drive amplifier and starting up the CW light source, but the parts that are not directly related to this embodiment will not be explained.

[0034] The control device for the optical transmitter according to this embodiment performs the processes shown in steps S1 to S13 in Figure 3, which will be described later, during the startup sequence period to create Table A (step S102). Table A is a lookup table that lists the coordinates on the complex plane of the optical electric field generated by the optical modulator for all combinations of the setting values ​​of each signal DAC shown in Figure 15. The details of steps S1 to S13 will be described in the first embodiment.

[0035] Next, during the startup sequence period, the control device performs the processes shown in steps S14 to S23 in Figure 5, which will be described later (step S103). In the processes of steps S14 to S23, first, the control device lists the "target coordinates" which include the coordinates on the complex plane of all optical electric fields that are scheduled to be generated during the transmission service period.

[0036] Here, the target coordinates include all the coordinates of the M-level symbols if a multi-level optical QAM signal with M values ​​is being transmitted. Furthermore, if the optical field during transitions between symbols is also set, as when using a Nyquist filter, the coordinates of that optical field must also be included in the target coordinates.

[0037] Next, the control device selects the coordinates closest to each listed target from Table A. Then, the control device reads from Table A the combination of setting values ​​for each signal DAC that can generate the selected coordinates. The control device creates Table B, a lookup table in which the coordinates of each target obtained in this way and the corresponding setting values ​​for each signal DAC are listed on the same row. At this point, the startup sequence of the optical transmitter is terminated and the transmission service is started (step S104). Details of steps S14 to S23 will also be described in the first embodiment.

[0038] The control device determines the coordinates on the complex plane of the optical field to be generated during the transmission service period (step S105). These coordinates are, in the case of an M-value multi-level optical QAM signal, the coordinates of one of the M symbols on the complex plane, and, if there are multiple sampling points in one time slot, the coordinates on the complex plane of the optical field during the transition between the M symbols.

[0039] The control device searches Table B for the coordinates determined in step S105 and reads the setting values ​​of each signal DAC recorded in the same row as those coordinates (step S106). The control device sets the setting values ​​read in step S106 to each signal DAC (step S107).

[0040] The control device loops through the processes from step S105 to step S107 until the end of the transmission service.

[0041] Although the creation of TableB in steps S101 to S103 requires a relatively long processing time, it only needs to be done once during the startup sequence and does not need to be done again during the transmission service period. Alternatively, non-volatile memory may be provided, and TableB may be recorded in the control unit at the time of factory shipment. Furthermore, the control unit may perform any part of the processing shown in Figure 1 based on manual input or other means.

[0042] [First Embodiment] Figure 2 shows the configuration of the control device 1 of this embodiment. The control device 1 includes a storage unit 10, a first table creation unit 11, a second table creation unit 12, and a control unit 13. The storage unit 10 is, for example, a non-volatile memory. The storage unit 10 stores Table A and Table B. Note that either Table A or Table B may be stored in the non-volatile memory. The first table creation unit 11 creates Table A. The second table creation unit 12 creates Table B. The control unit 13 controls the optical transmitter 3 by referring to Table B in the storage unit 10.

[0043] The optical transmitter 3 includes an optical modulator 31 and a drive system 32. The drive system 32 has one or more signal DACs that output electrical signals corresponding to a set value, which is a numerical sequence that changes over time. The optical modulator 31 is driven by the electrical signals output from the signal DACs of the drive system 32 and generates modulated light in which the magnitude of the optical electric field and / or the optical phase are modulated.

[0044] In this embodiment, the optical modulator 31 of the optical transmitter 3 and its drive system 32 are in the configuration shown in Figure 15 and have four independent drive signals. Specifically, the optical modulator 31 is the IQ optical modulator 70 shown in Figure 15, and the drive system 32 consists of a first digital data generation unit 87a-1, a second digital data generation unit 87a-2, a third digital data generation unit 87b-1, a fourth digital data generation unit 87b-2, a first signal DAC 88a-1, a second signal DAC 88a-2, a third signal DAC 88b-1, a fourth signal DAC 88b-2, a first signal amplifier 89a-1, a second signal amplifier 89a-2, a third signal amplifier 89b-1, and a fourth signal amplifier 89b-2. Hereafter, the first signal DAC 88a-1, the second signal DAC 88a-2, the third signal DAC 88b-1, and the fourth signal DAC 88b-2 will be collectively referred to as signal DAC 88, or as the first to fourth signal DAC 88. Furthermore, the first signal amplifier 89a-1, the second signal amplifier 89a-2, the third signal amplifier 89b-1, and the fourth signal amplifier 89b-2 will be collectively referred to as signal amplifier 89, or as the first to fourth signal amplifiers 89.

[0045] In this embodiment, the first to fourth signal DACs 88 are all assumed to be 2-bit DACs, as in Figures 16 and 17. While 2-bit DACs are not practical due to their extremely limited number of levels, they are shown as 2-bit for the sake of simplifying the diagrams. １ ~V ４ The amplitude of each drive signal is also assumed to be at full swing, as in Figure 17.

[0046] Figure 3 is a flowchart illustrating the TableA creation process of the control device 1 in the first embodiment. This flowchart explains the detailed process of step S102 in Figure 1.

[0047] In step S1, the first table creation unit 11 of the control device 1 declares constants and two-dimensional array variables. DAC1_Level, DAC2_Level, DAC3_Level, and DAC4_Level are integer variables. DAC1_Level1 represents the setting value of the output level of the first signal DAC88a-1, DAC1_Level2 represents the setting value of the output level of the second signal DAC88a-2, DAC1_Level3 represents the setting value of the output level of the third signal DAC88b-1, and DAC1_Level4 represents the setting value of the output level of the fourth signal DAC88b-2.

[0048] Typically, DAC settings are integers calculated from 0. Therefore, if the first signal DAC 88a-1 is an n-bit DAC, then DAC1_Level will be assigned an integer from 0 to 2^(n-1).

[0049] DAC1_max, DAC2_max, DAC3_max, and DAC4_max are all integer constants, representing the maximum possible values ​​for DAC1_Level, DAC2_Level, DAC3_Level, and DAC4_Level, respectively. If the first signal DAC88a-1 is an n-bit DAC, then DAC1_max is 2^(n-1). The same applies to the other signal DAC88s.

[0050] TableA is a two-dimensional array variable of real number type. The total number of columns in TableA is the number of signal DACs in the drive system 32 plus 2. In this embodiment, as shown in Figure 15, the drive system 32 has first to fourth signal DACs 88, so the total number of signal DACs is 4, and the number of columns is 6. The total number of rows in TableA is (DAC1_max+1) × (DAC2_max+1) × (DAC3_max+1) × (DAC4_max+1). TableA is stored in the storage unit 10. If all first to fourth signal DACs 88 are n-bit DACs, the required number of rows is (2^n)^4. Other variables and counters will be explained as needed in each step.

[0051] In step S2, the first table creation unit 11 resets the integer counter Ct_A to zero. This counter corresponds to the rows of TableA.

[0052] The processing from step S2 onward forms a quadruple loop. Each of these loops increments DAC1_Level, DAC2_Level, DAC3_Level, and DAC4_Level from 0 in increments of 1. The loop is exited when DAC1_Level reaches DAC1_max, DAC2_Level reaches DAC2_max, DAC3_Level reaches DAC3_max, and DAC4_Level reaches DAC4_max. The loop processing is performed in steps S3 to S6 and steps S10 to S13.

[0053] Specifically, in step S3, the first table creation unit 11 sets DAC4_Level to 0. In step S4, the first table creation unit 11 sets DAC3_Level to 0. In step S5, the first table creation unit 11 sets DAC2_Level to 0. In step S6, the first table creation unit 11 sets DAC1_Level to 0. After performing the processing in steps S7 to S9, the first table creation unit 11 adds 1 to DAC1_Level. If the first table creation unit 11 determines that DAC1_Level is less than or equal to DAC1_max, it repeats the processing from step S7, and if it determines that DAC4_Level exceeds DAC4_max, it performs the processing in step S11 and adds 1 to DAC2_Level. The first table creation unit 11 repeats the process from step S6 if it determines that DAC2_Level is less than or equal to DAC2_max, and if it determines that DAC2_Level exceeds DAC2_max, it performs the process in step S12 and adds 1 to DAC3_Level. The first table creation unit 11 repeats the process from step S5 if it determines that DAC3_Level is less than or equal to DAC3_max, and if it determines that DAC3_Level exceeds DAC3_max, it performs the process in step S13 and adds 1 to DAC4_Level. The first table creation unit 11 repeats the process from step S4 if it determines that DAC4_Level is less than or equal to DAC4_max, and terminates the process if it determines that DAC4_Level exceeds DAC4_max.

[0054] In step S7 within the quadruple loop, the first table creation unit 11 assigns the real and imaginary parts of the optical field generated by the optical modulator 31 to the real-type variables Generated_Re and Generated_Im, respectively, when the setting values ​​of the first signal DAC 88a-1, the second signal DAC 88a-2, the third signal DAC 88b-1, and the fourth signal DAC 88b-2 are DAC1_Level, DAC2_Level, DAC3_Level, and DAC4_Level, respectively. These can be obtained by calculation or by measurement. The measured values ​​may be input, for example, by the input unit of the control device 1 (not shown).

[0055] When Generated_Re and Generated_Im are determined by actual measurement, the power of the CW light input to the optical modulator 31 and the conversion efficiency of the photoelectric conversion element that receives the optical QAM signal are included in the measurement results. However, the dimensions or units of the real and imaginary parts as physical quantities are not important in this embodiment; it is sufficient to know the relative relationship between the setting value of each DAC 88 and the real and imaginary parts of the optical field.

[0056] When Generated_Re and Generated_Im are to be calculated, it is not necessary to actually change the output of each DAC88; they can be calculated using equations (6) to (10). When these equations are used, E ＱＡＭ The real and imaginary parts of are substituted into Generated_Re and Generated_Im, respectively. These values ​​are dimensionless because the voltage is normalized by half-wavelength voltage. In this embodiment, Generated_Re and Generated_Im are calculated using equations (6) to (10). The first table creation unit 11 may calculate Generated_Re and Generated_Im, or the calculation results from another device may be input to the control device 1.

[0057] Equations (6) to (10) are for ideal conditions, but if imperfections such as the nonlinearity of the DAC output or the imbalance between I and Q in the IQ modulator cannot be ignored, correction coefficients may be used to calculate Generated_Re and Generated_Im.

[0058] In step S8 within the quadruple loop, the first table creation unit 11 sets Generated_Re, Generated_Im, and the six values ​​DAC1_Level to DAC4_Level used to determine them into the row indicated by Ct_A in TableA. In step S9, the first table creation unit 11 adds 1 to Ct_A.

[0059] Figure 4 shows an example of TableA obtained after exiting a quadruple loop. In this embodiment, the number of rows is (2^2)^4 = 256, and Ct_A changes from 0 to 255 within the loop, but to avoid complexity, only a portion of it is shown in Figure 4.

[0060] In this embodiment, the configuration of the drive system 32 of the optical modulator 31, the number of bits of the first to fourth signal DAC 88, and V １ ~V ４ The drive amplitudes are all the same as those in Figures 16 and 17. Therefore, the values ​​listed in the table in Figure 4 are the same as those in the table shown in Figure 16, but in Figure 4, the variable names used in the flowchart are added to the table, and the value of Ct_A in the table is shown on the left side of the table.

[0061] In the explanation so far, V １ ~V ４ The case where the drive amplitude is full swing has been explained. However, in general transceivers, it is not always full swing, and in many cases the swing amount is limited to an amplitude smaller than full swing. There are several factors that determine the swing amount. One is the voltage range that the first to fourth signal DACs 88 can output, another is the gain of the first to fourth signal amplifiers 89, and another is the Vπ of the optical modulator 31.

[0062] Of these, the voltage range that the first to fourth signal DACs 88 can output and the Vπ of the optical modulator 31 are fixed values ​​according to the specifications and are not variable. However, it is possible to operate by making the gains of the first to fourth signal amplifiers 89 variable and switching between them as needed to change the swing amount. In such cases, Generated_Re and Generated_Im in Figure 4 will change. For this reason, if a change in the gain of the first to fourth signal amplifiers 89 is anticipated, it is desirable to create multiple Table A accordingly.

[0063] Figure 5 is a flowchart illustrating the TableB creation process of the control device 1 in the first embodiment of the present invention. This flowchart illustrates step S103 in Figure 1.

[0064] In step S14, the second table creation unit 12 of the control device 1 declares constants and two-dimensional array variables. Target_Re_min, Target_Re_max, Target_Im_min, and Target_Im_max are real constants. Target_Re_min, Target_Re_max, Target_Im_min, and Target_Im_max are defined as the minimum real value, maximum real value, minimum imaginary value, and maximum imaginary value of the optical field to be generated during transmission service operation, respectively. These may be defined as physical quantities containing some dimension, or they may be defined as dimensionless numbers normalized by Vπ. However, these must match the definitions used for Generated_Re and Generated_Im in Table A. In this embodiment, as in Figure 4, the above constants are defined by (Equation 6) to (Equation 10), and the voltage is a dimensionless number normalized by half-wavelength voltage.

[0065] Target_Re_min, Target_Re_max, Target_Im_min, and Target_Im_max need to be set to larger values ​​as the swing amount of the drive signal increases, however, due to the sinusoidal characteristics represented by (Equations 6) to (Equations 10), they cannot become unlimitedly large. In this embodiment, assuming a full swing, Target_Re_min = Target_Im_min = -0.75 and Target_Re_max = Target_Im_max = +0.75 are set.

[0066] The integer constants Line_Re and Line_Im represent the total number of real coordinates and the type of imaginary coordinates required during transmission service operation, respectively. In this embodiment, the generation of 16-QAM is assumed. In this case, Line_Re and Line_Im must be 4 or greater. However, when it is necessary to define the optical field for intermediate transitions between symbols, such as when using a Nyquist filter, it is desirable to make Line_Re and Line_Im much larger than 4. In this embodiment, Line_Re = Line_Im = 6.

[0067] Furthermore, when using the Nyquist filter, the optical field may extend outside the constellation during inter-symbol transitions. Therefore, the size of the generated constellation must be smaller than the range defined by Target_Re_min, Target_Re_max, Target_Im_min, and Target_Im_max.

[0068] In step S15, the second table creation unit 12 defines the resolution of the real part Re_step and the resolution of the imaginary part Im_step as follows.

[0069] Re_step←(Target_Re_max-Target_Re_min) / (Line_Re-1)

[0070] Im_step←(Target_Im_max-Target_Im_min) / (Line_Im-1)

[0071] In this embodiment, both Re_step and Im_step are 0.3.

[0072] In step S16, the second table creation unit 12 defines a two-dimensional array TableB having the total number of signal DACs in the drive system 32 plus 2 columns and the number of rows Line_Re × Line_Im. In this embodiment, since the drive system 32 has the first to fourth signal DACs, TableB has 6 columns and 36 rows. TableB is stored in the storage unit 10.

[0073] In step S17, the second table creation unit 12 resets the integer counter Ct_B to zero. This counter corresponds to the rows of TableB.

[0074] The following processes are handled as nested loops. Each of these loops increments Target_Re and Target_Im from Target_Re_min and Target_Im_min, respectively, by increments of Re_step and Im_step, and exits the loop when all reach Target_Re_max and Target_Im_max. The loop processing is performed in steps S18 to S19 and steps S22 to S23.

[0075] Specifically, in step S18, the second table creation unit 12 sets Target_Im to Target_Im_min. In step S19, the second table creation unit 12 sets Target_R to Target_Re_min. After performing the processing in steps S20 to S21, the second table creation unit 12 adds Re_step to Target_R. If the second table creation unit 12 determines that Target_R is less than or equal to Target_R_max, it repeats the processing from step S20, and if it determines that Target_R exceeds Target_R_max, it performs the processing in step S23 and adds Im_step to Target_Im. The second table creation unit 12 repeats the process from step S19 if it determines that Target_Im is less than or equal to Target_Im_max, and terminates the process if it determines that Target_Im exceeds Target_Im_max.

[0076] Figure 6 shows a constellation of the optical electric field obtained by the optical transmitter 3 of this embodiment. In Figure 6, the six types of Target_Re and Target_Im that change within the double loop are shown by vertical and horizontal dashed lines, respectively. Also in Figure 6, the coordinates (Generated_Re, Generated_Im) of all the optical electric fields obtained in this embodiment are shown as a constellation. The arrangement of symbols in the constellation is the same as in Figure 17, but here the symbols are divided into two types, white circles and black circles, according to the criteria described later.

[0077] Returning to Figure 5, we will resume the explanation of the flowchart. In step S20, which is within the nested loop, the second table creation unit 12 searches for the coordinates (Generated_Re, Generated_Im) recorded in each row of TableA that are closest to the target coordinates (Target_Re, Target_Im). The second table creation unit 12 sets the found Generated_Re, Generated_Im, and the six numerical values ​​consisting of DAC1_Level to DAC4_Level recorded in that row of TableA, into the row indicated by Ct_B in TableB. Then, in step S21, the second table creation unit 12 increments Ct_B by 1.

[0078] Figure 7 shows Table B obtained after exiting the double loop in Figure 5. Here, in addition to the two columns Target_Re and Target_Im, and the four columns consisting of DAC1_Level to DAC4_Level, two columns Generated_Re and Generated_Im, which are the actual coordinates of the optical field generated by DAC1_Level to DAC4_Level, are added to the part labeled G for reference. The value of Ct_B, which corresponds to the row number starting from 0, is written to the left of the table. For example, the values ​​of DAC1_Level to DAC4_Level 3, 0, 1, and 0 when Ct_B is 12 are recorded in the row where Ct_A is 19 in Table A.

[0079] In this embodiment, because the number of DAC settings and corresponding output levels is small, the QAM signals that can be generated are limited to simple ones with few multi-level components. In such cases, Re_step and Im_step can be set to be larger, reducing the calculation time for TableA and TableB.

[0080] The black circle symbols in Figure 6 represent 16 selected from the 36 rows of Table B, where the absolute values ​​of Target_Re and Target_Im are 0.45 or less. These selected symbols correspond to the 16 symbols generated by the corresponding combinations of DAC1_Level, DAC2_Level, DAC3_Level, and DAC4_Level. In this embodiment, the number of DAC88 setting values ​​and their corresponding output levels is too small (4), resulting in some distortion, but it can be seen that the grid-like arrangement of symbols necessary for 16-QAM has been selected.

[0081] Now, returning to the flowchart in Figure 1, we will explain how steps S105 to S107 during the transmission service period are performed in this embodiment. Before the startup sequence is completed and the transmission service begins, TableB generated by the processing in steps S14 to S23 is stored in the storage unit 10 of the control device 1.

[0082] In step S105, the control unit 13 of the control device 1 determines the coordinates of the real part X and the imaginary part Y of the optical field to be generated. For example, the control unit 13 may determine these coordinates according to the output from the first digital data generation unit 87a-1, the second digital data generation unit 87a-2, the third digital data generation unit 87b-1, and the fourth digital data generation unit 87b-2. Next, in step S106, the control unit 13 searches Table B stored in the storage unit 10 for (Target_Re, Target_Im) that matches (X, Y). As is clear from Figure 5, Table B is sorted with Target_Re having a higher priority and Target_Im having a lower priority, so this search can be easily performed. Specifically, the control unit 13 performs the following calculation.

[0083] (Y-Target_Im_min) / Im_step×Line_Re+(X-Target_Re_min) / Re_step

[0084] For example, if X = -0.750 and Y = -0.150, the above equation can be calculated as follows.

[0085] (-0.150 + 0.750) / 0.3 × 6 + (-0.750 + 0.750) / 0.3 = 12

[0086] The control unit 13 refers to the row corresponding to Ct_B=12 in Table_B shown in Figure 7, finds (Target_Re, Target_Im) that matches (X, Y), and obtains the values ​​3, 0, 1, and 0, which are DAC1_Level to DAC4_Level, recorded therein. In step S107, the control unit 13 sets each of these values ​​to the first to fourth signal DACs 88. The first to fourth signal DACs 88 generate modulated signals with voltage levels corresponding to the set values.

[0087] As is clear from Figure 6, when generating optical QAM with a higher number of levels, it is necessary to reduce Re_step and Im_step and increase the number of rows in TableB. On the other hand, if the arrangement of the symbols to be generated is limited to the vicinity of the origin, the range of Target_Re_min, Target_Re_max, Target_Im_min, and Target_Im_max can be narrowed to reduce the number of rows in TableB, and some of Generated_Re and Generated_Im may be left unused.

[0088] The control device 1 may create multiple Table A and Table B, or both, depending on the modulation format used for the transmission service or the amplitude of the electrical signal that drives the optical modulator 31.

[0089] [Variations of the First Embodiment] In the explanation so far, prioritizing clarity of the drawings, the first to fourth signal DACs 88 were assumed to be 2-bit DACs. Here, as a more realistic example, consider the case where the first to fourth signal DACs 88 are 4-bit DACs, and DAC1_max = DAC2_max = DAC3_max = DAC4_max = 15, that is, the output level can be set from 0 to 15. V １ ~V ４ As explained previously, the drive amplitude is assumed to be a full swing.

[0090] Figure 8 shows the selection of symbols when the optical modulator 31 is driven by the drive system 32 using the technology described in Patent Document 1. Figure 8(a) shows all the symbols that can be generated. Compared to Figure 6, it can be seen that the individual symbols can be arranged so densely that they are inseparable. In such cases, it is desirable to make Re_step and Im_step smaller than 0.3 used in Figure 6.

[0091] Figure 8(b) shows the case where 256 symbols are selected from the symbols shown in Figure 8(a) and a 256-QAM is generated under the conditions Target_Re_min = Target_Im_min = -0.6, Target_Im_max = Target_Re_max = +0.6, Re_step = Im_step = 0.08, and Line_Re = Line_Im = 16. This figure shows that a constellation at a nearly practical level can be generated.

[0092] Figure 9 shows Table B used for extraction in Figure 8(b). The number of rows is Line_Re × Line_Im = 256 rows. Table A is not shown, but it requires 16^4 = 65536 rows. In Figure 9, the codes G indicate the Generated_Re and Generated_Im found in Table A.

[0093] As is clear from the comparison between FIGS. 6 and 8, it is desirable that the number of bits of the first to fourth signal DACs 88 be as large as possible. However, in that case, the number of rows in Table A increases dramatically, so the time required for the processing in step S20 increases. Although it is a one-time process only during the startup sequence period or at the time of factory shipment and recalculation is not necessary if it is stored in the nonvolatile memory, it is desirable that this calculation time be short if possible. In the second embodiment, this time is shortened.

[0094] [Second Embodiment] In the configuration shown in FIG. 2, when the number of bits of the first to fourth signal DACs 88 is the same and the amplitudes of V １ to V ４ are the same, the distributions of Generated_Re and Generated_Im shown in Table A are symmetric about twofold on the complex plane and overlap with themselves when rotated 180 degrees about the origin of the complex plane. Also, in many cases, the constellation arrangements of 16-QAM and 256-QAM are also symmetric about twofold. This means that when there is a symbol at the coordinates of real part = X and imaginary part = Y, there is also a different symbol at real part = -X and imaginary part = -Y. Therefore, Target_Re and Target_Im listed in Table B should also be selected to be symmetric about twofold on the complex plane, and by using these symmetries, the processing time of step S20 can be shortened.

[0095] In (Equation 6) to (Equation 10), V １ and V ２ are exchanged, and also V ３ and V ４ are exchanged, and the newly obtained optical electric fields are E １ ' to E ４ and E ＱＡＭ ' respectively. ' respectively.

[0096] E １ ' = {exp(i(π × V ２ / Vπ + π / 2))} / 4 (Equation 11) E ２ ' = {exp(i(π × V １ / Vπ - π / 2))} / 4 (Equation 12) E ３ ' = {exp(i(π × V ４ / Vπ + π )))} / 4 (Equation 13) E ４ ’ = {exp(i(π × V ３ / Vπ )))} / 4 (Equation 14) E ＱＡＭ ’ = E １ ’ + E ２ ’ + E ３ ’ + E ４ ’ (Equation 15)

[0097] Comparing (Equations 11) to (Equation 15) with (Equations 6) to (Equation 10), it can be seen that (Equation 11) is (Equation 7) with the optical phase changed by π, (Equation 12) is (Equation 6) with the optical phase changed by π, (Equation 13) is (Equation 9) with the optical phase changed by π, and (Equation 14) is (Equation 8) with the optical phase changed by π. From this, it can be seen that there is a relationship of E １ ’ = -E ２ , E ２ ’ = -E １ , E ３ ’ = -E ４ , E ４ ’ = -E ３ . Thus, it can be seen that there is a relationship of E ＱＡＭ ’ = E ＱＡＭ .

[0098] V １ to V ４ values respectively correspond one-to-one with the set values of the first to fourth signal DACS 88. Therefore, to change the coordinates of the generated optical electric field to coordinates symmetrically located across the origin of the complex plane, the values of DAC1_Level and DAC2_Level may be exchanged, and the values of DAC3_Level and DAC4_Level may be exchanged.

[0099] This is verified using Table B shown in Figure 9. When Ct_B = 1, Target_Re = -0.52 and Target_Im = -0.60. On the other hand, when Ct_B = 254, Target_Re = 0.52 and Target_Im = 0.60. Since both the real and imaginary parts have their signs reversed, these are symmetric coordinates with respect to the origin. When Ct_B = 1, DAC1_Level, DAC2_Level, DAC3_Level, and DAC4_Level are 15, 2, 14, and 0, respectively. On the other hand, when Ct_B = 254, DAC1_Level, DAC2_Level, DAC3_Level, and DAC4_Level are 2, 15, 0, and 14, respectively. From this, we can confirm that when Ct_B = 254, there is no need to check TableA. Instead, the values ​​of DAC1_Level to DAC4_Level recorded in TableB when Ct_B = 1 are copied to Ct_B = 254, and then the values ​​of DAC1_Level and DAC2_Level are swapped, and the values ​​of DAC3_Level and DAC4_Level are swapped.

[0100] Figure 10 is a flowchart illustrating the TableB creation process utilizing this property. The flowchart in Figure 10 shows a means of shortening the time in step S20. By replacing the process in step S20 in Figure 5 with the process shown in the flowchart in Figure 10, it is possible to shorten the time required to create TableB. However, the conditions are that the absolute values ​​of Target_Re_min and Target_Re_max are equal, the absolute values ​​of Target_Im_min and Target_Im_max are equal, and Line_Re and Line_Im are set to be equal.

[0101] In step S20a, the second table creation unit 12 determines whether Target_Im is greater than 0. If the second table creation unit 12 determines that Target_Im is 0 or less, it performs the process in step S20b. This is the same process as step S20 in Figure 5.

[0102] On the other hand, if the second table creation unit 12 determines that Target_Im is greater than 0, it performs the process in step S20c. In step S20c, the second table creation unit 12 first searches TableB for a row that records the coordinates (-Target_Re, -Target_Im). Since Target_Im is increasing sequentially within the loop, a row that records (-Target_Re, -Target_Im) must exist.

[0103] The second table creation unit 12 records the six numerical values ​​consisting of DAC1_Level to DAC4_Level, Target_Re, and Target_Im recorded in the found row in the row indicated by Ct_B in TableB.

[0104] Next, in step S20d, the second table creation unit 12 swaps the values ​​of DAC1_Level and DAC2_Level recorded in the row indicated by Ct_B in TableB, and also swaps the values ​​of DAC3_Level and DAC4_Level recorded in the same row. Since the comparison and referencing with the numerical values ​​recorded in TableA can be omitted in the processing of steps S20c and S20d, the processing time is significantly reduced.

[0105] [Third Embodiment] In the previous embodiments, the drive signal is V １ ~V ４ All of these were independent. However, even when using a conventional drive system as shown in Figure 13, which applies reciprocal drive signals to the first arm 72a-1 and the second arm 72a-2, and also applies reciprocal drive signals to the third arm 72b-1 and the fourth arm 72b-2, the DAC setting method using Table A and Table B described so far may also be applied. That is, the modulator of the optical transmitter 3 shown in Figure 2 is the IQ optical modulator 70 shown in Figure 14, and the drive system of the optical transmitter 3 is the first digital data generation unit 77a, the second digital data generation unit 77b, the first signal DAC 78a, the second signal DAC 78b, the first signal differential output amplifier 79a, and the second signal differential output amplifier 79b shown in Figure 13. In this case, distortion due to the sinusoidal characteristics of the Mahatzehnder interferometer as shown in Figure 14 can be mitigated.

[0106] In this embodiment, the first table creation unit 11 of the control device 1 performs the same processing as the flow of the first embodiment shown in Figure 3, except as follows. That is, in step S1, the first table creation unit 11 declares integer variables DAC1_Level and DAC2_Level. DAC1_Level1 is the setting value for the output level of the first signal DAC 78a, and DAC1_Level2 is the setting value for the output level of the second signal DAC 78b. Furthermore, the first table creation unit 11 declares integer constants DAC1_max and DAC2_max as the maximum values ​​that DAC1_Level can take and the maximum values ​​that DAC2_Level can take, respectively. Furthermore, the first table creation unit 11 creates Table A with 4 columns (2 + 2) and (DAC1_max + 1) × (DAC2_max + 1) rows, corresponding to the number of signal DACs in the drive system 32. The first table creation unit 11 does not perform the two outer loop processing steps S3, S4, S12, and S13. Then, in step S7, the first table creation unit 11 assigns the real and imaginary parts of the optical field generated by the optical modulator 31, i.e., the IQ optical modulator 70, to the real number variables Generated_Re and Generated_Im, respectively, when the setting values ​​of the first signal DAC 78a and the second signal DAC 78b are DAC1_Level and DAC2_Level, respectively. In step S8, the first table creation unit 11 sets the four values, Generated_Re, Generated_Im, and DAC1_Level and DAC2_Level, which were used to determine them, in the row indicated by Ct_A in TableA.

[0107] Furthermore, the second table creation unit 12 of the control device 1 performs the same processing as the flow of the first embodiment shown in Figure 5, except as follows. That is, in step S16, the second table creation unit 12 defines a two-dimensional array TableB having four columns equal to the number of signal DACs in the drive system 32 (2 + 2) and rows equal to Line_Re × Line_Im. Then, in step S20, the second table creation unit 12 searches for the coordinates (Generated_Re, Generated_Im) recorded in each row of TableA that are closest to the target coordinates (Target_Re, Target_Im). The second table creation unit 12 sets the four numerical values, consisting of the found Generated_Re, Generated_Im, and DAC1_Level and DAC2_Level recorded in the corresponding row of TableA, into the row indicated by Ct_B in TableB.

[0108] In step S106, the control unit 13 of the control device 1 sets the first signal DAC 78a to DAC1_Level read from TableB, and sets the second signal DAC 78b to DAC2_Level.

[0109] [Fourth Embodiment] In the embodiments described so far, a configuration in which an IQ optical modulator is used as the optical modulator 31 has been described. However, as also described in Patent Document 1, it is possible to generate an optical QAM signal by applying two independent drive signals to the two arms of a single Mahatzehnder type optical modulator, and this embodiment can also be applied to an optical QAM signal generated in this way.

[0110] Figure 11 shows the configuration of an optical transmitter 9 used as the optical transmitter 3 shown in Figure 2 in the fourth embodiment. The optical transmitter 9 has a single Mahtzehnder type optical modulator 90 and its modulation drive system 95. In the fourth embodiment, the optical transmitter 3 shown in Figure 2 is replaced with the single Mahtzehnder type optical modulator 90 shown in Figure 11, instead of the IQ optical modulator 80 of the modulator drive system shown in Figure 15. Also, the drive system 32 shown in Figure 2 is replaced with the modulation drive system 95 shown in Figure 11. The Mahtzehnder type optical modulator 90 has a first arm 91a, a second arm 91b, a drive electrode 92a for the first arm, a drive electrode 92b for the second arm, and a λ / 2 shift section 93. The modulation drive system 95 includes a first digital data generation unit 97a, a second digital data generation unit 97b, a first signal DAC 98a, a second signal DAC 98b, a first signal amplifier 99a, and a second signal amplifier 99b.

[0111] The CW light input to the Mahatzehnder-type optical modulator 90 is split and output to a first arm 91a and a second arm 91b. The light transmitted through the first arm 91a is applied to the first arm drive electrode 92a. The light transmitted through the second arm 91b is applied to the second arm drive electrode 92b, and the λ / 2 shift unit 93 sets the optical path difference with the first arm 91a to ±λ / 2. The light after propagation through the first arm 91a and the light after propagation through the second arm 91b are combined and output.

[0112] As described above, there are two digital data generation units, two signal DACs, and two signal amplifiers. The first digital data generation unit 97a generates continuous numerical data with m types of values ​​and transmits the generated numerical data to the first signal DAC 98a as a set value. The output of the first signal DAC 98a is input to the first signal amplifier 99a, which drives the voltage V １ The following is generated. Similarly, the second digital data generation unit 97b generates a series of numerical data having m types of values ​​and transmits the generated numerical data to the second signal DAC 98b as a set value. The output of the second signal DAC 98b is input to the second signal amplifier 99b, and a voltage drive signal V ２ This is generated.

[0113] As described above, the independent drive signals V generated by the first signal amplifier 99a and the second signal amplifier 99b １ and drive signal V ２ These are applied to the first arm drive electrode 92a and the second arm drive electrode 92b of a single Mahatzehnder-type optical modulator 90, respectively. As described in Patent Document 1, if the two drive signals are independent of each other and not reciprocal, the generated optical field can have a real part and an imaginary part on the complex plane, and an optical QAM signal can be generated.

[0114] However, in this configuration, as described in Patent Document 1, each drive signal V １ and V ２ The amplitude should preferably be larger than that of a full swing. That is, V １ and V ２ In both cases, it is desirable that the variation range is from +Vπ / 2 to -Vπ / 2, and the amplitude exceeds the range of Vπ. Figure 12 shows the constellation generated by the optical transmitter 9 shown in Figure 11. Here, there are only two DACs, the first signal DAC 98a and the second signal DAC 98b, but these are assumed to be 5-bit DACs. Drive signal V １ and drive signal V ２ Each of these fluctuation ranges is assumed to be from +Vπ to -Vπ, with an amplitude of 2Vπ. This amplitude is referred to as a 200% swing, meaning twice the full swing.

[0115] Similar to the third embodiment, the control device 1 creates four rows of Table A and Table B, the control unit 13 selects M symbols from a large number of symbols that can be generated, and the optical transmitter 3 can generate an M-QAM signal. However, since the optical field generated in this embodiment is concentric, generating a multi-level PSK (Phase Shift Keying) signal is more suitable than generating a rectangular optical QAM signal.

[0116] In previous embodiments, the two columns Generated_Re and Generated_Im recorded in TableA, or the two columns Target_Re and Target_Im recorded in TableB, represent the real and imaginary parts, respectively, and the complex plane was represented in Cartesian coordinates. However, when generating a multi-level PSK signal, it is desirable to use polar coordinates consisting of distance and angle from the origin instead of Cartesian coordinates. In this case, when comparing TableA and TableB, the closest distance and angle from the origin will be sought. However, in the case of a multi-level PSK signal where there is only one type of distance from each symbol to the origin, the tolerance for errors in distance from the origin is high. Therefore, in the comparison process between TableA and TableB, priority can be given to the closest angle, and the priority of distance from the origin can be lowered. Thus, in step S20 of Figure 5, the second table creation unit 12 may select polar coordinates from among the polar coordinates recorded in each row of TableA that are within a predetermined range of distance from the target coordinate, and from the selected candidates, select the polar coordinate that has the angle closest to the angle of the target coordinate.

[0117] [Variations of Embodiments] In the embodiments described so far, the amplitude of each drive signal has been set to full swing (a drive signal with an amplitude of Vπ is applied to each arm electrode) or 200% swing (a drive signal with an amplitude of 2Vπ is applied to each arm electrode). However, it is not necessary to limit the swing amounts to these, and other swing amounts, such as 50% swing, may also be used.

[0118] Furthermore, in the embodiments described so far, the amplitude of each drive signal was assumed to be of the lithium niobate type, but a semiconductor modulator may also be used. However, in that case, as mentioned above, each drive signal V １ ~V ４ An offset voltage Vo must be applied to maintain a positive or negative voltage.

[0119] The control device 1 described above may be implemented using multiple devices. In this case, it is arbitrary which of these multiple devices implements each functional unit of the control device 1. Furthermore, the same functional unit may be implemented using multiple devices. For example, the control device 1 may consist of a table creation device having a storage unit 10, a first table creation unit 11 and a second table creation unit 12, and a transmitter control device having a storage unit 10 and a control unit 13.

[0120] Furthermore, the control device 1 may be configured using a processor such as a CPU (Central Processing Unit) and memory (main memory). The control device 1 functions as a first table creation unit 11, a second table creation unit 12, and a control unit 13 when the processor executes a program. Note that all or part of each function of the control device 1 may be implemented using hardware such as an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array). The above program may be recorded on a computer-readable recording medium. Computer-readable recording media include, for example, portable media such as flexible disks, magneto-optical disks, ROMs, CD-ROMs, semiconductor memory devices (e.g., SSDs), and memory devices such as hard disks and semiconductor memory devices built into a computer system. The above program may be transmitted via a telecommunications line.

[0121] According to the embodiment described above, the control device controls the optical transmitter. The optical transmitter includes one or more digital-to-analog converters for signals that output electrical signals corresponding to a set value, which is a numerical sequence that changes over time, and an optical modulator that is driven by the electrical signals output from the digital-to-analog converters for signals and generates modulated light in which one or both of the magnitude and / or phase of the optical electric field are modulated. The control device includes a first table creation unit and a second table creation unit. The coordinates of the modulated light are defined as two-dimensional coordinates on a complex plane that include information on the magnitude and phase of the optical electric field of the modulated light. For each of the multiple types of set values, the first table creation unit creates a first lookup table that records the correspondence between the set value and the generateable coordinates, which are the coordinates of the modulated light generated by the optical modulator based on the electrical signals output by the one or more digital-to-analog converters for signals according to the set value. The generateable coordinates correspond, for example, to Generated_Re and Generated_Im in the embodiment. The second table creation unit selects the most closely related generateable coordinates from the first lookup table based on predetermined selection criteria for each target coordinate, which is the coordinate of the modulated light to be generated during the transmission service period. It then creates a second lookup table that records the correspondence between the selected generateable coordinates and the setting values ​​used when the selected generateable coordinates were generated. The target coordinates correspond, for example, to Target_Re and Target_Im in the embodiment. The creation of the first and second tables is performed prior to the start of the transmission service and is completed before the start of the transmission service.

[0122] The control device may further include a control unit. The control unit sets a set value read from a second lookup table corresponding to the coordinates of the modulated light generated during the transmission service period into the digital-to-analog converter for the signal.

[0123] Multiple lookup tables may be created for at least one of the first and second lookup tables, depending on the modulation format used for the transmission service or the amplitude of the electrical signal driving the optical modulator, and stored in non-volatile memory.

[0124] If the coordinates of the modulated light are orthogonal coordinates consisting of a real part and an imaginary part, the selection criterion is the selection of the generateable coordinates that are closest to the target coordinates.

[0125] If the coordinates of the modulated light are polar coordinates consisting of distance and angle from the origin, the selection criteria are to select as candidates from the first lookup table any generateable coordinates whose distance from the target coordinate is within a predetermined range, and then select from the selected candidates the generateable coordinate that has the closest angle to the angle of the target coordinate.

[0126] If the distribution of generateable coordinates on the complex plane exhibits symmetry, the control device simplifies part of the table creation process for the first and second lookup tables by reflecting that symmetry in the selection criteria. For example, the second table creation unit selects other generateable coordinates that are symmetric to the generateable coordinates from the second lookup table, and obtains a setting value corresponding to the generateable coordinates based on the setting value recorded in the second lookup table corresponding to the other selected generateable coordinates.

[0127] The optical modulator may include at least one Mahatzehnder interferometer. The optical path lengths of the two arms of the Mahatzehnder interferometer may be modulated uncorrelatedly with respect to each other by different digital-to-analog converters for signals.

[0128] The second lookup table may be sorted according to the magnitude of one or both of the two coordinates that the target coordinates possess.

[0129] While embodiments of this invention have been described in detail above with reference to the drawings, the specific configurations are not limited to these embodiments and include designs and the like that do not depart from the spirit of this invention.

[0130] 1 Control device 3, 7, 8, 9 Optical transmitter 10 Memory unit 11 First table creation unit 12 Second table creation unit 13 Control unit 31 Optical modulator 32 Drive system 70, 80 IQ optical modulator 71a First Mahatzehnder optical modulator 71b Second Mahatzehnder optical modulator 72a-1 First arm 72a-2 Second arm 72b-1 Third arm 72b-2 Fourth arm 73a First λ / 2 shift unit 73b Second λ / 2 shift unit 74a Fifth arm 74b Sixth arm 75 λ / 4 shift unit 76a-1 Drive electrode for first arm 76a-2 Drive electrode for second arm 76b-1 Drive electrode for third arm 76b-2 Drive electrode for fourth arm 77a First digital data generation unit 77b Second digital data generation unit 78a First signal DAC 78b Second signal DAC 79a First differential output amplifier for signals 79b Second differential output amplifier for signals 87a-1 First digital data generation unit 87a-2 Second digital data generation unit 87b-1 Third digital data generation unit 87b-2 Fourth digital data generation unit 88a-1 First signal DAC 88a-2 Second signal DAC 88b-1 Third signal DAC 88b-2 Fourth signal DAC 89a-1 First signal amplifier 89a-2 Second signal amplifier 89b-1 Third signal amplifier 89b-2 Fourth signal amplifier 90 Mahatzehnder type optical modulator 91a First arm 91b Second arm 92a First arm drive electrode 92b Second arm drive electrode 93 λ / 2 shift section 95 Modulation drive system 97a First digital data generation section 97b Second digital data generation section 99a First signal amplifier 99b Second signal amplifier

Claims

1. An optical transmitter control method for controlling an optical transmitter having: a single or more digital-to-analog converters for signals that take a set value as a set value and output an electrical signal corresponding to the set value; and an optical modulator that is driven by the electrical signal output from the digital-to-analog converters for signals and generates modulated light in which either or both of the magnitude and phase of the optical field are modulated, wherein when the coordinates of the modulated light are defined as two-dimensional coordinates on a complex plane including information on the magnitude and phase of the optical field of the modulated light, a first table creation step is created to create a first lookup table that records the correspondence between each of several types of set values ​​and the generateable coordinates which are the coordinates of the modulated light generated by the optical modulator based on the electrical signals output by the single or more digital-to-analog converters for signals according to the set value; A method for controlling an optical transmitter, comprising: a second table creation step, for each target coordinate which is the coordinate of modulated light to be generated during the transmission service period, a second table creation step, which selects the generateable coordinate most closely related to the target coordinate from the first lookup table based on predetermined selection criteria, and creates a second lookup table that records the correspondence between the selected generateable coordinate and the setting value used when the selected generateable coordinate was generated, wherein the first table creation step and the second table creation step are performed prior to the start of the transmission service and are completed before the start of the transmission service.

2. The optical transmitter control method according to claim 1, wherein the coordinates of the modulated light are orthogonal coordinates consisting of a real part and an imaginary part, and the selection criterion is the selection of the generateable coordinate that is closest in distance to the target coordinate.

3. The optical transmitter control method according to claim 1, wherein the coordinates of the modulated light are polar coordinates consisting of distance and angle from the origin, and the selection criterion is to select the generateable coordinates from the first lookup table as candidates whose distance from the target coordinate is within a predetermined range, and from the selected candidates, select the generateable coordinate having the angle closest to the angle of the target coordinate.

4. The optical transmitter control method according to claim 1, wherein the optical modulator includes at least one Mahatzehnder interferometer, and the optical path lengths of the two arms built into the Mahatzehnder interferometer are modulated uncorrelatedly with respect to each other by different digital-to-analog signal converters.

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