JP2014146915A - Digital optical transmitter, optical communication system, and digital optical transmission method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a digital optical transmitter, an optical communication system, and a digital optical transmission method, capable of easily compensating an I-Q imbalance caused by changes in transmission conditions such as wavelength switching and temperature fluctuation.SOLUTION: A digital optical transmitter 10 comprises: an I-Q balance control unit 20 which generates a control signal corresponding to transmission conditions; a transmission data sequence arithmetic processing unit 30 which generates first data and second data on the basis of the control signal; an optical branching unit 40 which branches an optical signal into two optical signals; a first optical modulator 51 which optically modulates one of the branched optical signals on the basis of the first data and outputs the first optical signal; a second optical modulator 52 which optically modulates the other optical signal on the basis of the second data and outputs the second optical signal; and a coupling unit 60 which couples the first optical signal and the second optical signal at a prescribed phase difference φ. The control signal cancels a deviation amount from the phase difference φ.

Description

  The present invention relates to a digital optical transmitter, an optical communication system, and a digital optical transmission method, and more particularly to a digital optical transmitter, an optical communication system, and a digital optical transmission method that perform digital coherent optical communication.

  With the explosive demand for broadband multimedia communication services such as the Internet and video distribution, the introduction of long-distance, large-capacity and highly reliable optical fiber communication systems is progressing. In an optical fiber communication system, it is important to reduce the installation cost of an optical fiber, which is an optical transmission line, and to increase the transmission band utilization efficiency per optical fiber. For this reason, the importance of digital coherent optical communication technology using a digital optical transceiver is increasing.

  Digital coherent optical communication improves reception sensitivity by 3 to 6 dB compared to analog optical transceivers using modulation methods such as OOK (on-off keying), which are widely applied in large-capacity optical communication systems. At the same time, waveform distortion compensation, such as chromatic dispersion compensation and polarization dispersion compensation, can be compensated by digital signal processing (DSP) on the transmission side or reception side. Therefore, digital coherent optical communication can realize improvement in performance and cost reduction of the communication device. In such a digital optical transmitter, the output of the D / A converter is used in order to perform multilevel modulation using QPSK (quadure phase shift keying) modulation, QAM (quadraure amplitude modulation) modulation, or pre-equalization. The used optical modulator capable of arbitrary waveform modulation is used.

  As an optical modulator used in a digital optical transmitter, for example, an MZ optical modulator in which an optical waveguide type optical phase modulator is incorporated in an optical waveguide type Mach-Zehnder (hereinafter referred to as MZ) interferometer is used. It is done. In the MZ type optical modulator, various optical variable signals such as intensity modulation and phase modulation can be generated by adjusting the applied voltage and the configuration of the interferometer.

  In Patent Documents 1 and 2, MZ in which an optical waveguide is formed on the surface of a substrate made of an electro-optic crystal whose refractive index changes in proportion to an applied electric field strength such as lithium niobate (LiNbO 3, hereinafter referred to as LN). A type optical modulator is disclosed. However, optical modulation using an LN modulator requires automatic bias control (ABC) to compensate for DC drift that occurs in the optical waveguide.

  Therefore, in Patent Document 1, in ABC when using DQPSK (differential QPSK) modulation, a phase shift is detected by superimposing a low-frequency signal on an electrode to which a DC bias is applied, and ABC is performed by feedback control. A technique is disclosed. Patent Document 2 discloses a method of measuring changes in a phase shift voltage and an optical spectrum ratio using an optical spectrum analyzer in an optical transmitter using an LN optical modulator, and adjusting a phase shift based on the measurement result. Is disclosed.

  On the other hand, Patent Document 3 discloses an optical waveguide type MZ optical modulator using a semiconductor such as gallium arsenide (GaAs) or indium phosphide (InP) as an optical element that can be driven even at a low voltage. In an optical waveguide using a semiconductor, ABC like an LN modulator is not required, but deviation from the optimum setting value occurs due to wavelength switching or temperature fluctuation. Further, in Patent Document 3, as a method for compensating for the deviation from the optimum setting value, the low-frequency signal is superimposed, the spectrum of the output signal is monitored, and feedback control is performed to adjust the voltage applied to the divided electrodes. The technique to do is disclosed.

JP 2007-82094 A JP 2012-42796 A WO2011 / 043079 Publication

  In digital optical transmitters, DC imbalances such as DC drift that occurs when using an LN optical modulator, wavelength switching that occurs when a semiconductor optical modulator is used, and phase shifts due to temperature fluctuations, etc. It is necessary to compensate by feedback control or shifting various parameters. When a multi-level modulation signal such as QAM or a pre-equalization signal using a complicated transmission waveform is used, the transmission waveform is distorted without sufficient control accuracy, resulting in a deterioration in reception performance.

  The present invention has been made in view of the above problems, and can easily compensate for IQ imbalance caused by changes in transmission conditions such as wavelength switching and temperature fluctuation, and can maintain transmission signal quality suitably. An optical transmitter, an optical communication system, and a digital optical transmission method are provided.

  In order to achieve the above object, a digital optical transmitter according to the present invention includes an IQ balance control unit that generates a control signal according to a transmission condition, and first and second data based on the control signal. A transmission data sequence calculation processing unit for generating two series of data, an optical branching unit for branching and outputting the input optical signal, and optically modulating one of the branched optical signals based on the first data; A first optical modulator that outputs as a first optical signal, a second optical modulator that optically modulates the other branched optical signal based on the second data, and outputs the second optical signal as a second optical signal; And a coupling unit that couples the first optical signal and the second optical signal with a predetermined phase difference φ, and the control signal is a signal for performing processing for canceling the amount of deviation from the phase difference φ. It is characterized by that.

  To achieve the above object, an optical communication system according to the present invention is wavelength-multiplexed with the above-described digital optical transmitter that transmits a transmission signal of wavelength λ1, a digital optical receiver that receives a reception signal of wavelength λ2, and And a ROADM device that multiplexes the transmission signal of wavelength λ1 transmitted to the optical signal and separates and outputs the reception signal of wavelength λ2 from the wavelength-multiplexed optical signal, and the digital optical transmitter has the wavelength λ1 A pre-equalization process corresponding to the above is performed.

  In order to achieve the above object, the digital optical transmission method according to the present invention generates a control signal according to transmission conditions, and performs pre-equalization processing for canceling the deviation amount from the phase difference φ based on the control signal. The first data and the second data are generated, the input optical signal is branched into two and output, and one of the branched optical signals is optically modulated based on the first data to generate the first light The optical signal is output as a signal and the other optical signal is optically modulated based on the second data and output as a second optical signal, and the first optical signal and the second optical signal are combined with a predetermined phase difference φ. To do.

  The digital optical transmitter, the optical communication system, and the digital optical transmission method according to the present invention easily compensate for IQ imbalance caused by changes in transmission conditions such as wavelength switching and temperature fluctuations, and keep transmission signal quality favorable. be able to.

1 is a configuration diagram of a digital optical transmitter 10 according to a first embodiment of the present invention. It is a block diagram of the digital optical transmitter 100 which concerns on the 2nd Embodiment of this invention. It is a block diagram of the digital optical transmitter 300 which concerns on the modification of the 2nd Embodiment of this invention. It is a block diagram of another MZ type IQ optical modulator 301B which concerns on the modification of the 2nd Embodiment of this invention. It is a block diagram of the digital optical transmitter 500 which concerns on the 3rd Embodiment of this invention. It is a system configuration | structure figure of the optical communication system 600 which concerns on the 4th Embodiment of this invention.

(First embodiment)
A first embodiment according to the present invention will be described. A block diagram of a digital optical transmitter according to this embodiment is shown in FIG. In FIG. 1, a digital optical transmitter 10 according to the present embodiment includes an IQ balance control unit 20, a transmission data sequence calculation processing unit 30, an optical branching unit 40, a first optical modulator 51, and a second optical modulation. And a coupling portion 60.

  The IQ balance control unit 20 generates a control signal corresponding to the transmission condition and outputs the control signal to the transmission data sequence calculation processing unit 30. The IQ balance control unit 20 according to the present embodiment calculates a deviation amount Δφ from the phase difference φ based on the characteristics of the first optical modulator 51 and the second optical modulator 52, and calculates the calculated deviation amount. A control signal for adding a compensation amount −Δφ for canceling Δφ to two series of data is generated.

  Specifically, the IQ balance control unit 20 calculates the deviation Δφ from the phase difference φ based on the wavelength λ of the optical signal handled by the optical modulators 51 and 52 and the temperature T of the optical modulators 51 and 52. Calculate. Note that a lookup table in which a plurality of deviation amounts Δφ calculated in advance for each wavelength λ of the optical signal and the temperature T of the optical modulators 51 and 52 is registered is stored, and the IQ balance control unit 20 The shift amount Δφ corresponding to the wavelength λ and the measured temperature T of the optical modulators 51 and 52 can be extracted from the lookup table, and a control signal can be generated based on the extracted shift amount Δφ.

  The transmission data series calculation processing unit 30 generates two series of data of the first data and the second data based on the control signal input from the IQ balance control unit 20, and each of the first optical modulators 51. And output to the second optical modulator 52. By using the control signal, the first data and the second data are given a phase difference of a compensation amount −Δφ for canceling the shift amount Δφ from the phase difference φ generated in the coupling unit 60. Hereinafter, giving the compensation amount −Δφ to the first data and the second data is referred to as pre-equalization processing.

  The optical branching unit 40 divides an optical signal output from a light source (not shown) into two, and outputs one to the first optical modulator 51 and the other to the second optical modulator 52.

  The first optical modulator 51 optically modulates one optical signal input from the optical branching unit 40 based on the first data input from the transmission data sequence calculation processing unit 30, and outputs the first optical signal as a first optical signal. . The second optical modulator 52 optically modulates the other optical signal input from the optical branching unit 40 based on the second data input from the transmission data sequence calculation processing unit 30, and outputs the second optical signal as a second optical signal. . Here, the first optical signal is an I-ch optical signal, and the second optical signal is a Q-ch optical signal.

  The combining unit 60 combines the first optical signal output from the first optical modulator 51 and the second optical signal output from the second optical modulator 52 with a predetermined phase difference φ. And output as a transmission signal. Here, since the optical modulators 51 and 52 have wavelength dependency and temperature dependency, a phase difference (deviation amount Δφ) corresponding to the wavelength and temperature is generated between the first optical signal and the second optical signal. To do. However, in the present embodiment, a compensation amount −Δφ for canceling the shift amount Δφ is given in advance to the first data and the second data input to the optical modulator 51 and the optical modulator 52. Accordingly, the first optical signal and the second optical modulator 52 are combined with a desired phase difference φ in the combining unit 60, and a transmission signal having a favorable quality is output.

  As described above, in the digital optical transmitter 10 according to the present embodiment, the IQ balance control unit 20 generates the control signal based on the characteristics of the optical modulators 51 and 52, and the transmission data sequence calculation processing unit 30 Two series of data are generated based on the control signal. Based on the control signal, the two series of data input to the optical modulators 51 and 52 are given a compensation amount −Δφ for canceling the shift amount Δφ.

  Therefore, due to the characteristics of the optical modulators 51 and 52, even when a phase difference is given in the coupling unit 60, even if there is a deviation of Δφ from the predetermined phase difference φ, the deviation amount Δφ is compensated by the compensation amount −Δφ given in advance. A transmission signal having a desired phase difference φ can be obtained. That is, it is possible to provide the digital optical transmitter 10 that can easily compensate for IQ imbalance caused by changes in transmission conditions such as wavelength switching and temperature fluctuations and can maintain transmission signal quality suitably.

(Second Embodiment)
A second embodiment will be described. FIG. 2 shows a block diagram of the digital optical transmitter according to this embodiment. In FIG. 2, a digital optical transmitter 100 according to the present embodiment includes a transmission data sequence calculation processing unit 101, an encoder 102, an IQ imbalance compensation circuit 103, an IQ balance control unit 104, a digital-analog conversion. DAC (digital to analog converter) 105, driver amplifier 106, MZ type I-Q optical modulator 107, I-ch optical modulator 108, Q-ch optical modulator 109, phase shift unit phase shifter 110, A light source 111 and a control unit 112 are provided.

  Here, in FIG. 2, when there are a plurality of units having the same function, the reference numerals are assigned with -1, -2, etc., for distinction. In the description, for example, “D / A converter 105-1”, “D / A converter 105-2”, and the like are described as “D / A converter 105” unless there is a particular need for distinction.

  Transmission data DATA1 and DATA2 (data string) are input to the transmission data sequence calculation processing unit 101. The data sequence input to the transmission data sequence calculation processing unit 101 is encoded in the encoder 102 in accordance with the modulation scheme of the transmission signal, and then the IQ balance control circuit 103 performs IQ balance control. IQ imbalance compensation is performed according to the control signal from the unit 104. Then, it is separated into an I-ch (in-phase channel) signal and a Q-ch (quadture-channel) signal and output from the IQ imbalance compensation circuit 103. The IQ imbalance compensation circuit 103 and the IQ balance control unit 104 will be described later.

  The I-ch signal and Q-ch signal output from the I-Q imbalance compensation circuit 103 are converted from digital electric signals to analog electric signals in the DACs 105-1 and 105-2, respectively, and then driver amplifier 106 -1 and 106-2, and is output to the I-ch optical modulator 108 and the Q-ch optical modulator 109.

  On the other hand, the optical signal output from the light source 111 is input to the MZ type IQ optical modulator 107. The MZ type IQ optical modulator 107 includes an I-ch optical modulator 108, a Q-ch optical modulator 109, and a phase shift unit phase shifter 110.

  The optical signal input to the MZ type I-Q optical modulator 107 is branched into two optical signals passing through the I-ch optical waveguide and the Q-ch optical waveguide along the optical waveguide, one of which is for I-ch. The light passes through the optical modulator 108, and the other passes through the Q-ch optical modulator 109 and the phase shifter phase shifter 110. At this time, the optical signals passing through the I-ch optical modulator 108 and the Q-ch optical modulator 109 are subjected to optical modulation according to the electrical signals output from the driver amplifiers 106-1 and 106-2. Is done. The phase of the optical signal that has passed through the Q-ch optical modulator 109 changes by π / 2 in the phase shifter phase shifter 110. The light that has passed through the I-ch optical modulator 108 and the light that has passed through the Q-ch optical modulator 109 and the phase shifter phase shifter 110 are combined at the output stage, and MZ-type IQ optical modulation is performed. Is output from the transmitter 107 as a transmission signal.

  As the I-ch optical modulator 108 and the Q-ch optical modulator 109, an LN optical modulator or a split electrode type MZ optical modulator as described in Patent Document 1 and Patent Document 2 is used. Can do. Therefore, the MZ type I-Q optical modulator 107 can use a configuration in which LN optical modulators and divided electrode type MZ type optical modulators are arranged in a nest type. Here, the entire MZ type I-Q optical modulator 107 is the parent MZ type optical modulator, the I-ch optical modulator 108 and the Q-ch optical modulator 109 are the child MZ type modulator. And so on.

  In FIG. 2, the phase shifter phase shifter 110 is disposed on the same optical waveguide as the Q-ch optical modulator 109, but it is not necessarily required to be disposed on the same waveguide as the Q-ch optical modulator 109. Alternatively, it may be arranged in the same waveguide as the I-ch optical modulator 108. Further, a phase shifter is arranged in both the I-ch and Q-ch waveguides so that the phase difference between the light passing through the I-ch optical waveguide and the Q-ch optical waveguide is relatively π / 2. May be. In this case, bias control of the IQ balance control unit 104 described later is performed so that the phase difference between the light passing through the I-ch optical waveguide and the Q-ch optical waveguide is relatively π / 2.

  The control unit 112 switches the wavelength of the light source 111. In the present embodiment, when the wavelength is switched, the control unit 112 further notifies the later-described IQ balance control unit 104 of the wavelength switching.

Next, the detailed operation of the IQ balance control unit 104 will be described. The I-Q balance control unit 104 applies a DC bias voltage _{Vπ / 2} to the phase shift unit phase shifter 110 so that the phase difference of light passing through the I-ch optical waveguide and the Q-ch optical waveguide becomes _{π / 2.} Is applied (bias control).

In general, the optimum value of V _{π / 2} applied to the phase shifter phase shifter 110 varies depending on the temperature T of the MZ type IQ optical modulator 107 or the light source wavelength λ. For this reason, when the temperature T changes due to secular change or when the light source wavelength λ is switched in a ROADM (reconfigurable optical add / drop multiplexer) system or the like, the optimum V _{π / 2} fluctuates, and the I-ch optical waveguide is changed. The phase difference of the Q-ch optical waveguide is π / 2 + Δφ (T, λ).

When V _{π / 2} is not adjusted, the output signal from the MZ type I-Q optical modulator 107 is a distorted signal in which the I-ch component and the Q-ch component are not orthogonally combined and maintained in an orthogonal state. As sent.

Here, since Δφ (T, λ) is a function of the temperature T and the light source wavelength λ, Δφ can be uniquely determined if T and λ are designated. Therefore, the IQ balance control unit 104 further determines the compensation amount −Δφ according to the designated T and λ, and generates a control signal generated based on the determined compensation amount −Δφ as an IQ imbalance compensation circuit. The IQ imbalance compensation circuit 103 that outputs the signal 103 calculates the signal waveforms of the I-ch data and the Q-ch data based on the control signal from the IQ balance control unit 104. That is, in the I-Q imbalance compensation circuit 103, pre-equalization processing for generating a phase difference of -Δφ between the drive waveform of the I-ch optical modulator 108 and the drive waveform of the Q-ch optical modulator 109 (I-Q imbalance compensation) is performed.

  By applying a phase difference of −Δφ to the I-ch signal and the Q-ch signal in advance by the pre-equalization process, the MZ type I-Q optical modulator 107 correctly corrects the I-ch component and the Q-ch component. A suitable output signal combined with the orthogonal state is output.

  Furthermore, when the wavelength of the light source 111 is switched by the control unit 112, the IQ balance control unit 104 of the present embodiment switches the compensation amount −Δφ according to the light source wavelength λ. The IQ balance control unit 104 holds in advance a suitable compensation amount −Δφ corresponding to the values of T and λ, and is preset from the values of T and λ when the digital optical transmitter 100 is started up. A suitable compensation amount −Δφ is selected, and a control signal is generated based on the selected compensation amount −Δφ. For this preset setting, a lookup table prepared in advance can be used.

  Note that since the variations in T and λ are slow variations (switching) over time, it is not always necessary to use preset setting values. The IQ balance control unit 104 performs a suitable compensation amount −Δφ by training or exploratory control with a relatively slow response speed using signal quality information such as an eye opening of a transmission signal, an error rate of a reception signal, and the like. Can also be determined.

  Here, the IQ balance control unit 104 and the control unit 112 may be mounted together with the transmission data sequence calculation processing unit 101 on a chip such as an LSI (Large Scale Integration). In addition, instead of generating the control signal in the IQ balance control unit 104 or the control unit 112, an external signal from an FPGA (Field Programmable Gate Array), a microcomputer, or a PC can be used.

  Next, detailed operation of the IQ imbalance compensation circuit 103 will be described. Consider the output signal of the MZ type IQ optical modulator 107 when the phase shift amount of the phase shifter phase shifter 110 is π / 2 + Δφ.

The output signal E ″ _I + jE ″ _Q of the MZ type I-Q optical modulator 107 is obtained by using the electric field component E ′ _{I of the I} -ch optical waveguide and the electric field component E ′ _Q of the Q-ch optical waveguide by using E ′ _I + Exp {j (π / 2 + Δφ)} E ′ _Q = (E ′ _I −sin ΔφE ′ _Q ) + j cos ΔφE ′ _Q (j is an imaginary unit).

従って、Ｉ−Ｑインバランス補償回路１０３では（１）式の逆演算として（２）式の演算を行えば良い。

すなわち、符号化器１０２からの出力信号のＩ−ｃｈ成分がＥ_Ｉ、Ｑ−ｃｈ成分がＥ_Ｑであるとき、Ｉ−Ｑインバランス補償回路１０３は、Ｉ−ｃｈ成分としてＥ’_Ｉ＝Ｅ_Ｉ+（ｓｉｎΔφ／ｃｏｓΔφ）Ｅ_Ｑ、Ｑ−ｃｈ成分としてＥ’_Ｑ＝Ｅ_Ｑ／ｃｏｓΔφを出力すれば良い。 Accordingly, when the output signal of the MZ type IQ optical modulator 107 is expressed in vector, it can be expressed as in equation (1).

Therefore, the IQ imbalance compensation circuit 103 may perform the calculation of equation (2) as the inverse calculation of equation (1).

That is, when the I-ch component of the output signal from the encoder 102 is E _I and the Q-ch component is E _Q , the IQ imbalance compensation circuit 103 uses E ′ _I = E as the I-ch component. _I + (sin Δφ / cos Δφ) E _Q , E ′ _Q = E _Q / cos Δφ may be output as the Q-ch component.

  The IQ imbalance compensation according to the equations (1) and (2) is an example of an ideal case. Additional waveform distortion occurs when it is not ideal due to variation in characteristics of analog devices such as the DAC 105, the driver amplifier 106, and the MZ type I-Q optical modulator 107 and band limitation. For this reason, it is not always necessary to strictly perform the compensation by the equations (1) and (2). By performing compensation substantially in accordance with the equations (1) and (2), adjustment is performed so that the signal quality information such as the eye opening of the transmission signal and the error rate of the received signal are suitable.

  As described above, in the digital optical transmitter 100 of the present embodiment, the IQ imbalance compensation circuit 103 is a control generated from the compensation amount −Δφ determined by the IQ balance control unit 104 according to T and λ. Based on the signal, pre-equalization processing (I-Q imbalance compensation) is performed. In this case, even when the IQ imbalance occurs in the MZ type IQ optical modulator 107 due to temperature fluctuation or wavelength switching, the IQ imbalance compensation circuit 103 performs the IQ imbalance compensation in advance. A suitable transmission signal can be transmitted.

  In FIG. 2, only the encoder 102 and the IQ imbalance compensation circuit 103 are representatively shown as internal functions of the transmission data sequence calculation processing unit 101. However, in an actual digital transmitter, a spectrum shaping circuit is described. Various transmission digital signal processing functions such as a frequency characteristic adjustment circuit and a pre-equalization circuit are implemented.

  If a semiconductor optical modulator is used as the MZ type I-Q optical modulator 107, the DC drift in the optical waveguide generated by the LN modulator can be ignored in principle. Therefore, it is not necessary to perform automatic control such as ABC on the phase shifter phase shifter 110, and IQ imbalance compensation can be performed more stably.

  Further, in the above description, the case where the phase difference given by the phase shift unit 110 is π / 2 (orthogonal) is described, but a value other than π / 2 (for example, φ) can be set according to the modulation method used. . In that case, the IQ imbalance compensation circuit 103 performs arithmetic processing so as to compensate for the phase shift amount Δφ from the phase difference φ of light passing through the predetermined I-ch optical waveguide and Q-optical waveguide. .

(Modification of the second embodiment)
FIG. 3 shows the configuration of a digital optical transmitter according to a modification of the second embodiment. In the digital optical transmitter 300 of this embodiment, an MZ type IQ optical modulator 301 is used instead of the MZ type IQ optical modulator 107 of the digital optical transmitter of FIG. Here, the same components as those of the digital optical transmitter 100 of the second embodiment shown in FIG. 2 are denoted by the same reference numerals, and the description thereof is omitted.

  The MZ type IQ optical modulator 301 includes an optical branching MMI (Multi-Mode Interference) coupler 302, an I-ch optical modulator 303, a Q-ch optical modulator 304, and an optical multiplexing MMI coupler 305.

  After the optical signal from the light source 111 is input to the MZ type IQ optical modulator 301, the optical branching MMI coupler 302 causes a relative phase difference φin between the output of the I-ch optical waveguide and the output of the Q-ch optical waveguide. Granted and branched.

  The branched I-ch signal and Q-ch signal are subjected to desired optical modulation by the I-ch optical modulator 303 and the Q-ch optical modulator 304, respectively, and then to the optical multiplexing MMI coupler 305, respectively. Entered. The optical multiplexing MMI coupler 305 optically combines the optical signal that has passed through the I-ch optical waveguide and the optical signal that has passed through the Q-ch optical waveguide with a relative phase difference φout. At this time, an orthogonal state between the I-ch signal and the Q-ch signal can be realized by designing the optical branching MMI coupler 302 and the optical multiplexing MMI coupler 305 so that φin + φout = π / 2.

  However, in φin and φout, variation Δφ from π / 2, which is an ideal value, occurs due to variation in temperature T and switching of light source wavelength λ. Therefore, the IQ balance control unit 104 determines Δφ using T and λ, generates a control signal for canceling the determined Δφ, and outputs the control signal to the IQ imbalance compensation circuit 103. Since the operation of the IQ imbalance compensation circuit 103 is the same as that of the digital optical transmitter 100 of the second embodiment, the description thereof is omitted.

Note that, similarly to the digital optical transmitter 100 according to the second embodiment, a Y-branch optical waveguide can be used instead of the optical branch MMI coupler 302. A configuration diagram of the MZ type IQ optical modulator 301B in this case is shown in FIG. In the Y-branch optical waveguide, the output of the I-ch optical waveguide and the output of the Q-ch optical waveguide are branched in the same phase, so that a phase difference π / 2 is given by the optical multiplexing MMI coupler 305B. Also _in the configuration of FIG. 4, φ _in = 0 and φ _out = π / 2. Therefore, as in the digital transmitter 300 of FIG. 3, the IQ imbalance compensation circuit performs pre-equalization processing (IQ imbalance). Compensation) makes it possible to adjust the transmission signal stably and with high quality even when transmission conditions such as wavelength switching and temperature fluctuation occur.

(Third embodiment)
A third embodiment will be described. FIG. 5 shows the configuration of the digital optical transmitter according to this embodiment. In FIG. 5, a digital optical transmitter 500 includes an encoder 501, m I-ch driver amplifiers 502, n Q-ch driver amplifiers 503, an MZ type I-Q optical modulator 504, and I-chs. Divided electrode type optical modulator 505, Q-ch divided electrode type optical modulator 506, m I-ch divided electrodes 507, n Q-ch divided electrodes 508, I-ch phase shifter 509, Q-ch A phase shifter 510, a light source 511, an IQ balance control unit 512, and a control unit 513 are provided.

  Here, in FIG. 5, when there are a plurality of units having the same function, the reference numerals are assigned with -1, -2, etc., for distinction. In the description, “encoder 501-1”, “encoder 501-2”, and the like are described as “encoder 501”, for example, unless it is necessary to distinguish between them.

  In FIG. 5, an encoder 501-1 converts an I-ch transmission digital signal into a binary or multi-value analog signal according to the modulation method of the transmission optical signal. The encoder 501-2 converts the Q-ch transmission digital signal into a binary or multi-value analog signal according to the modulation method of the transmission optical signal. The analog electric signals converted by the encoder 501 are amplified by the driver amplifier 502 and the driver amplifier 503 and input to the I-ch split-electrode optical modulator 505 and the Q-ch split-electrode optical modulator 506, respectively. Is done.

  The MZ type IQ optical modulator 504 includes an I-ch split electrode type optical modulator 505, a Q-ch split electrode type optical modulator 506, an I-ch phase shifter 509, and a Q-ch phase shifter 510. Composed. The optical signal output from the light source 511 is input to the MZ type I-Q optical modulator 504 and then branched into two optical signals passing through the I-ch optical waveguide and the Q-ch optical waveguide along the optical waveguide. One passes through the I-ch split-electrode type optical modulator 505 and the I-ch phase shifter 509, and the other passes through the Q-ch split-electrode type optical modulator 506 and the Q-ch phase shifter 510.

At this time, the optical signals passing through the I-ch split-electrode type optical modulator 505 and the Q-ch split-electrode type optical modulator 506 are I-ch driver amplifiers 502-1 to 502-m and Q-, respectively. Optical modulation is performed in accordance with the electrical signals output from the ch driver amplifiers 503-1 to 503-n. The I-ch phase shifter 509 and the Q-ch phase shifter 510 are fixed so that the phase difference between the optical signal passing through the I-ch optical waveguide and the optical signal passing through the Q-ch optical waveguide becomes π / 2. Voltages V _Ifix and V _Qfix are applied, respectively.

  The lights that have passed through the I-ch optical waveguide and the Q-ch optical waveguide are combined at the output stage, and output from the MZ type IQ optical modulator 504 as a transmission signal. The detailed configurations of the I-ch split-electrode type optical modulator 505 and the Q-ch split-electrode type optical modulator 506 are disclosed in Japanese Patent Application Laid-Open No. 2002-260, and will not be described.

Next, detailed operation of the IQ balance control unit 512 will be described. From the I-Q balance control unit 512, the fixed voltages V _Ifix and V _Qfix are _set to the I-ch phase so that the phase difference of light passing through the I-ch optical waveguide and the Q-ch optical waveguide becomes π / 2. Applied to the counter 509 and the Q-ch phase shifter 510, respectively. However, the optimum value of _{V iFIX} and _{V Qfix} general, because it changes depending on the temperature T or signal wavelength λ of the MZ-type I-Q optical modulator 504, and when the temperature T changes due to aging, When the light source wavelength λ is switched in a ROADM (reconfigurable optical add / drop multiplexer) system or the like, the optimum V _Ifix and V _Qfix change, and the phase difference between the I-ch optical waveguide and the Q-ch optical waveguide is π / 2 + Δφ (T, λ). Therefore, when V _Ifix and V _Qfix are not adjusted, the output signal of the MZ type I-Q optical modulator 504 is distorted without being combined with the I-ch component and the Q-ch component maintaining the correct orthogonal state. It is transmitted as a signal.

Here, since Δφ (T, λ) is generally a function of T and λ, if T and λ are specified, Δφ can be uniquely determined. Therefore, according to the specified T and λ, Δφ is compensated, and V _Ifix and V _Qfix are set so that the phase difference of light passing through the I-ch optical waveguide and the Q-ch optical waveguide becomes π / 2. can do. At this time, for the setting values of V _Ifix and V _Qfix, a setting value suitable for compensating Δφ is measured in advance, and preset values are obtained from the values of T and λ when the digital transmitter 500 is started up. Therefore, a suitable setting value can be selected.

Further, it is desirable that the preset setting is simply set using a lookup table prepared in advance. In addition, since variations in T and λ are slow variations (switching) over time, it is not always necessary to use preset setting values. Signal quality information such as eye opening of a transmission signal, error rate of a reception signal, etc. Suitable V _Ifix and V _Qfix can also be determined by training or exploratory control with relatively slow response speed using. Further, when the wavelength switching control is performed from the control unit 513, the installation value of the IQ balance control unit 512 can be switched simultaneously with the wavelength switching of the light source 511.

(Fourth embodiment)
A fourth embodiment will be described. The configuration of the optical communication system according to this embodiment is shown in FIG. In FIG. 6, the optical communication system 600 includes a ROADM (reconfigurable optical add / drop multiplexer) device 601, a digital optical transmitter 100, a control unit 112, an optical modulation unit 602, and a digital optical receiver 603.

  In FIG. 6, when there are a plurality of units having the same function, the reference numerals are assigned with −1, −2 etc. to distinguish them. In the description, for example, “ROADM device 601-1”, “ROADM device 601-2” and the like are described as “ROADM device 601” unless otherwise distinguished. Also, the same components as those of the digital optical transmitter 100 of FIG.

  The optical communication system 600 shows a configuration example when the digital optical transmitter 100 according to the second embodiment is applied to a ROADM system. In the optical communication system 600 configured in a ring shape, when a wavelength-multiplexed optical signal passes through the ROADM device 601-1, a transmission signal having a wavelength λ1 specified by control from the system is transmitted from the digital optical transmitter 100. Transmitted, and multiplexed (Add / MUX) and transmitted in ROADM 601-1. On the other hand, the ROADM device 601-1 separates (Drop / DEMUX) the received signal of the wavelength λ2 specified by the control from the system, and then receives it by the digital optical receiver 603.

  Next, the operation of the digital optical transmitter 100 will be described in detail. The digital transmitter 100 is configured in the same manner as the digital optical transmitter 100 according to the second embodiment illustrated in FIG. 2, and includes a control unit 112 and an optical modulation unit 602. That is, the optical modulation unit 602 includes a transmission data sequence calculation processing unit 101, an IQ balance control unit 104, a DAC 105, a driver amplifier 106, an MZ type IQ optical modulator 107, and a light source 111.

  The control unit 112 switches the suitable compensation amount −Δφ of the IQ balance control unit 104 of the MZ type IQ optical modulator 107 based on the wavelength λ1 and the temperature T specified by the control from the system. By this switching, the digital optical transmitter 100 can transmit the transmission optical signal having the wavelength λ1 at high speed and stably without performing complicated feedback control or the like.

  Therefore, in the optical communication system 600, a suitable optical communication system can be provided even when the transmission wavelength λ1 of the digital optical transmitter 100 is switched by system control accompanying switching of the transmission path. Although the ring-shaped ROADM system has been described with reference to FIG. 6, the same effect can be obtained when the network topology is a mesh network or a linear network.

  As described above, the digital optical transmitter of the present invention suitably adjusts the IQ imbalance of the MZ type IQ optical modulator even when the transmission wavelength of the digital optical transmitter is changed or switched. Therefore, it is possible to transmit a suitable transmission signal at high speed and stably without performing complicated feedback control or the like. In addition, an optical communication system using the digital optical transmitter of the present invention has high-speed and stable switching performance and system performance even when wavelength switching by system control is performed by path switching or the like.

  The present invention is not limited to the above-described embodiment, and design changes and the like within a range not departing from the gist of the present invention are included in the present invention.

DESCRIPTION OF SYMBOLS 10 Digital optical transmitter 20 IQ balance control part 30 Transmission data series calculation process part 40 Optical branch part 51 1st optical modulator 52 2nd optical modulator 60 Combining part 100, 300, 500 Digital optical transmitter 600 Optical communication system 101 Transmission data sequence calculation processing unit 102 Encoder 103 IQ imbalance compensation circuit 104, 512 IQ balance control unit 105 Digital-analog converter (DAC)
106 Driver amplifier 107, 301, 504 MZ type I-Q optical modulator 108, 303 I-ch optical modulator 109, 304 Q-ch optical modulator 110 Phase shift unit phase shifter 111, 511 Light source 112, 513 Control Section 302 Optical branching MMI coupler 305 Optical multiplexing MMI coupler 501 Encoder 502 I-ch driver amplifier 503 Q-ch driver amplifier 505 I-ch split-electrode optical modulator 506 Q-ch split-electrode optical modulator 507 I-ch divided electrode 508 Q-ch divided electrode 509 I-ch phase shifter 510 Q-ch phase shifter 601 ROADM device 602 Optical modulation unit 603 Digital optical receiver

Claims (10)

An IQ balance control unit for generating a control signal according to transmission conditions;
Based on the control signal, a transmission data sequence calculation processing unit that generates two series of data of the first data and the second data;
An optical branching unit for branching and outputting the input optical signal;
A first optical modulator that optically modulates one of the branched optical signals based on the first data and outputs the first optical signal as a first optical signal;
A second optical modulator that optically modulates the other branched optical signal based on the second data and outputs the second optical signal as a second optical signal;
A coupling unit that couples the first optical signal and the second optical signal with a predetermined phase difference φ;
With
The digital optical transmitter according to claim 1, wherein the control signal is a signal for performing processing for canceling an amount of deviation from the phase difference φ. The digital optical transmitter according to claim 1, wherein the IQ balance control unit generates the control signal according to characteristics of the first and second optical modulators. The digital optical transmitter according to claim 2, wherein the characteristics include wavelength characteristics or temperature characteristics of the first and second optical modulators. The IQ balance control unit holds a lookup table in which a plurality of compensation amounts are registered according to the wavelength of the optical signal or the temperature of the IQ balance control unit, and the compensation amount extracted from the lookup table The digital optical transmitter according to claim 3, wherein the control signal is generated based on. The transmission data series calculation processing unit
An encoder that encodes data according to a modulation scheme;
An IQ imbalance compensation circuit that performs the pre-equalization process on the encoded data;
The digital optical transmitter according to any one of claims 1 to 4, further comprising: The optical branching unit is an MMI coupler that splits the input optical signal into two by giving a phase difference φin.
The coupling unit is an MMI coupler that couples the first optical signal and the second optical signal with a phase difference φout = φ−φin instead of a phase difference φ.
The digital optical transmitter according to claim 1. A light source that generates an optical signal and outputs the optical signal to the optical branching unit;
A control unit that obtains the wavelength of the optical signal generated by the light source and the temperatures of the first optical modulator and the second optical modulator and outputs them to the IQ balance control unit as the transmission condition;
The digital optical transmitter according to any one of claims 1 to 6, further comprising: The digital optical transmitter according to any one of claims 1 to 7, wherein the first and second optical modulators are semiconductor optical modulators or split electrode optical modulators. The digital optical transmitter according to any one of claims 1 to 8, which transmits a transmission signal having a wavelength λ1.
A digital optical receiver for receiving a reception signal of wavelength λ2,
A ROADM device that multiplexes the transmitted signal of wavelength λ1 to the wavelength-multiplexed optical signal and separates and outputs the received signal of wavelength λ2 from the wavelength-multiplexed optical signal;
With
An optical communication system, wherein the digital optical transmitter performs pre-equalization processing according to a wavelength λ1. Generate a control signal according to the transmission conditions,
Generating the first data and the second data subjected to the pre-equalization processing for canceling the shift amount from the phase difference φ based on the control signal;
The input optical signal is split into two and output.
One of the branched optical signals is optically modulated based on the first data and output as a first optical signal, and the other optical signal is optically modulated based on the second data and second Output as an optical signal,
Combining the first optical signal and the second optical signal with a predetermined phase difference φ;
Digital optical transmission method.

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