WO2017126039A1 - Émetteur optique, système de communication optique, et procédé de communication optique - Google Patents

Émetteur optique, système de communication optique, et procédé de communication optique Download PDF

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Publication number
WO2017126039A1
WO2017126039A1 PCT/JP2016/051489 JP2016051489W WO2017126039A1 WO 2017126039 A1 WO2017126039 A1 WO 2017126039A1 JP 2016051489 W JP2016051489 W JP 2016051489W WO 2017126039 A1 WO2017126039 A1 WO 2017126039A1
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Prior art keywords
optical
signal
phase
modulation unit
unit
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PCT/JP2016/051489
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English (en)
Japanese (ja)
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光子 中村
健太郎 榎
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三菱電機株式会社
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Priority to PCT/JP2016/051489 priority Critical patent/WO2017126039A1/fr
Priority to JP2017562202A priority patent/JP6563040B2/ja
Publication of WO2017126039A1 publication Critical patent/WO2017126039A1/fr

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/516—Details of coding or modulation

Definitions

  • the present invention relates to an optical transmitter, an optical communication system, and an optical communication method.
  • a polarization multiplexing phase modulation method using digital coherent technology is adopted as an optical transmission method.
  • Crosstalk becomes a limiting factor of transmission distance.
  • a Mach-Zehnder type lithium niobate (LiNbO 3 ) optical modulator (hereinafter referred to as “Mach-Zehnder modulator”) can be used.
  • Patent Document 2 discloses a first drive signal and a second drive signal that respectively drive two arms (optical waveguides) of a Mach-Zehnder modulator in an optical transmitter including an optical phase modulation unit incorporating a Mach-Zehnder modulator. Discloses a technique for optimizing the optical wavelength chirp applied to the transmitted optical signal by adjusting the amplitude and phase of the signal.
  • Patent Document 3 discloses an optical transmitter including an optical phase modulation unit including a Mach-Zehnder modulator and an optical filter, in which the optical phase modulation unit differentially pre-processes a single longitudinal mode optical signal generated from a light source.
  • a technique is disclosed in which optical phase modulation is performed using a code NRZ (non-return to zero) signal, and an optical filter converts the optical phase-modulated signal into an RZ optical intensity modulation signal.
  • Patent Document 4 includes a quadrature phase shift modulator (QPSK modulator) incorporating a Mach-Zehnder modulator, and in an optical transmitter that transmits a modulated optical signal, the modulated optical signal is received from the opposite optical receiver. Is disclosed, and a transmission parameter is controlled so as to reduce the error rate.
  • QPSK modulator quadrature phase shift modulator
  • a device for converting the data transmission system to RZ can be realized by serially connecting a Mach-Zehnder modulator and a digital modulator such as an optical phase modulator such as a dual polarization QPSK modulator. That is, by using a Mach-Zehnder modulator as an external modulator of the digital modulator, it becomes possible to transmit an RZ-modulated optical signal.
  • the Mach-Zehnder modulator bifurcates the input optical signal and gives a phase difference between the optical signals propagating through the two arms, thereby modulating the intensity of the optical signal after branching, and converting the optical RZ signal to Generate.
  • the optical RZ signal generated in this way is modulated by a digital modulator such as a dual polarization quadrature phase shift keying modulator (DP-QPSK modulator), thereby transmitting an RZ-modulated optical signal.
  • DP-QPSK modulator dual polarization quadrature phase shift keying modulator
  • the Mach-Zehnder modulator has a phase shift and an amplitude shift different from the desired optical signal between the two arms due to variations in accuracy at the time of manufacture, and the rise time of the generated optical RZ signal (hereinafter referred to as “rise pulse”).
  • the time Tr "and the fall time (hereinafter referred to as" fall time Tf ") are asymmetric.
  • the rise time Tr and fall time Tf of the optical RZ signal are asymmetric
  • Conduction is performed in a state where the phase between the Mach-Zehnder modulator and the subsequent digital modulator is shifted.
  • the extraction performance of the clock signal is reduced in the digital signal processing unit such as the digital demodulator on the optical receiver side, and the number of errors is increased by about 10 2 to 10 5 compared with the normal time.
  • bit error rate bit error rate: BER
  • Patent Documents 2-4 are not related to an optical transmitter having a configuration in which a digital modulator is connected to a subsequent stage of a Mach-Zehnder modulator as an RZ modulator, and cannot solve such problems. Absent.
  • the technique described in Patent Document 2 is a technique for optimizing the optical wavelength chirp applied to the transmission optical signal, and cannot solve the above-described problems.
  • the technique described in Patent Document 3 is a technique for reducing the chromatic dispersion of an optical transmission medium and transmission quality degradation caused by the interaction between the chromatic dispersion and the nonlinear optical effect, and can solve the above-described problems. is not.
  • the technique described in Patent Document 4 is a technique for controlling transmission parameters used in the QPSK modulator to reduce the error rate of the modulated optical signal transmitted from the QPSK modulator. This is not a technique for improving the code error rate at the time of reception on the optical receiver side in an optical transmitter having a configuration in which a modulator is installed.
  • An object of the present invention is to provide an optical communication system and an optical communication method in which the optical transmitter transmits a modulated optical signal to the optical receiver.
  • An optical transmitter modulates a Mach-Zehnder type RZ modulation unit that generates an optical RZ signal, which is a return-to-zero optical signal, and modulates the optical RZ signal based on transmission target data.
  • a digital modulation unit that generates an optical signal and outputs the modulated optical signal, a monitoring unit that monitors a state of the modulated optical signal output from the digital modulation unit, and the RZ based on the monitoring result
  • a control unit that controls at least one of the phase and the amplitude of the optical RZ signal generated by the modulation unit.
  • An optical communication system includes an optical transmitter and an optical receiver that receives the modulated optical signal output from the optical transmitter.
  • An optical communication method provides a modulated optical signal generated by an optical transmitter having a Mach-Zehnder type RZ modulation unit that generates an optical RZ signal that is a return-to-zero optical signal.
  • an undesired phase or amplitude shift between optical signals propagating through two arms in the Mach-Zehnder type RZ modulation unit is possible to reduce the code error rate after reception and maintain high optical transmission performance.
  • FIG. 3 is a diagram illustrating an example of a relationship between a clock signal and an inverted clock signal used in the RZ modulation unit and an output signal (optical RZ signal) from the RZ modulation unit in the optical communication system of FIG. 2.
  • FIG. 4 is a diagram illustrating an example of an optical RZ signal when the phase of the inverted clock signal is shifted in the relationship of FIG. 3.
  • FIG. 4 is a diagram illustrating an example of an optical RZ signal when the phase and amplitude of an inverted clock signal change in the relationship of FIG. 3.
  • FIG. 3 is a flowchart illustrating an example of processing in the optical communication system in FIG. 2. It is a flowchart which shows the process following the flowchart of FIG. It is a block diagram which shows the example of 1 structure of the optical communication system which concerns on Embodiment 2 of this invention. It is a flowchart which shows an example of the process in the optical communication system of FIG.
  • FIG. 9 is a conceptual diagram illustrating a signal waveform output from a digital modulation unit in the optical communication system of FIG. 8.
  • FIG. 9 is a conceptual diagram for explaining a difference in code error rate depending on presence / absence of an undesired deviation in amplitude and phase between optical signals propagating through two arms of an RZ modulation unit in the optical communication system of FIG. 8.
  • FIG. 9 is a conceptual diagram for explaining a difference in code error rate depending on presence / absence of an undesired deviation in amplitude and phase between optical signals propagating through two arms of an RZ modulation unit in the optical communication system of FIG. 8.
  • FIG. 9 is a conceptual diagram for explaining a difference in optical output power of an optical transmitter depending on presence / absence of an undesired deviation in amplitude and phase between optical signals propagating through two arms of an RZ modulation unit in the optical communication system of FIG. 8. .
  • FIG. 9 is a conceptual diagram for explaining a difference in signal intensity between optical signals propagating through two arms of the RZ modulator in the optical communication system of FIG.
  • It is a block diagram which shows the example of 1 structure of the optical communication system which concerns on Embodiment 3 of this invention.
  • optical transmitter according to an embodiment of the present invention, an optical communication system (optical transmission system) including the optical transmitter and an optical receiver, and optical communication in which the optical transmitter transmits a signal to the optical receiver
  • the method will be described with reference to the drawings.
  • the optical communication system according to the present invention is particularly applicable to a metro network that connects cities (between base stations installed in each city) or a core network that connects continents (between land-based terminals installed on each continent). Yes, but not limited to this.
  • FIG. 1 is a diagram illustrating a configuration of an optical communication system according to the first comparative example.
  • the optical transmitter 100 and the optical receiver 200 are connected via an optical transmission medium 300 so as to communicate with each other.
  • the optical transmitter 100 includes a light source 110, an RZ modulation unit 120, and a digital modulation unit 130, and transmits a modulated optical signal to the optical receiver 200 via the optical transmission medium 300.
  • the light source 110 includes a laser diode (LD) that oscillates a continuous wave and outputs continuous light.
  • the output continuous light is unmodulated.
  • the RZ modulator 120 includes a two-electrode Mach-Zehnder modulator 121, an attenuator (ATT) 122a for a clock signal, an attenuator 122b for an inverted clock signal, and a phase shifter (phase shifter: PS) for a clock signal.
  • ATT attenuator
  • PS phase shifter
  • the clock generation unit 124 includes a clock oscillator that generates a clock signal (CLK) 124a that is an electrical signal indicating a clock and an inverted clock signal (inverted CLK) 124b that indicates an inverted clock thereof.
  • CLK clock signal
  • inverted CLK inverted clock signal
  • the phase shifter 123a shifts the phase of the clock signal 124a and outputs it to the attenuator 122a.
  • the attenuator 122a attenuates the amplitude of the input electric signal and outputs it to the arm of the Mach-Zehnder modulator 121 (hereinafter also referred to as “first arm”).
  • the phase shifter 123b shifts the phase of the inverted clock signal 124b and outputs it to the attenuator 122b.
  • the attenuator 122b attenuates the amplitude of the input electric signal and outputs it to the arm of the Mach-Zehnder modulator 121 (hereinafter also referred to as “second arm”).
  • the Mach-Zehnder modulator 121 branches the continuous light output from the light source 110 to pass through the first arm and the second arm, and is input as a control signal to the first arm and the second arm as described above.
  • the phases of the lights propagating through the first arm and the second arm are made different from each other, and then both lights are multiplexed.
  • the Mach-Zehnder modulator 121 generates the optical RZ signal in this way and outputs it to the digital modulation unit 130.
  • the digital modulation unit 130 is connected to the subsequent stage of the RZ modulation unit 120, and includes a dual polarization quadrature phase shift (DP-QPSK) modulator 131, a phase shift unit 132, and a data output unit 133.
  • the phase shifter 132 outputs a control signal that maintains the phase of the optical RZ signal as it is (in other words, shifts by 0 degrees) to the DP-QPSK modulator 131, and sets the phase of the optical RZ signal to 90.
  • a second phase shifter 132b that outputs a control signal to the DP-QPSK modulator 131, and a third phase shifter 132c that outputs a control signal to shift the phase of the optical RZ signal by 180 degrees to the DP-QPSK modulator 131.
  • a fourth phase shifter 132 d that outputs a control signal for shifting the phase of the optical RZ signal to 270 degrees to the DP-QPSK modulator 131.
  • the first to fourth phase shifters 132a to 132d are also simply referred to as “phase shifters 132a to 132d”.
  • the data output unit 133 distributes the values of transmission target data for each component of XI (X polarization in-phase), XQ (X polarization orthogonal), YI (Y polarization in-phase), and YQ (Y polarization orthogonal) Output to the phase shifters 132a to 132d.
  • the phase shifters 132a to 132d compensate for data skew (phase shift) between XI, XQ, YI, and YQ.
  • the DP-QPSK modulator 131 is connected to the subsequent stage of the RZ modulator 120, and the optical RZ signal output from the Mach-Zehnder modulator 121 of the RZ modulator 120 is based on the control signals output from the phase shifters 132a to 132d. Then, a modulated optical signal is generated by performing sequential modulation.
  • the DP-QPSK modulator 131 may incorporate a Mach-Zehnder modulator for modulation.
  • the control signals output from the phase shifters 132a to 132d are electric signals for changing the phase of light in the two arms of the built-in Mach-Zehnder modulator.
  • the digital modulator 130 outputs the modulated optical signal modulated by the DP-QPSK modulator 131 to the optical receiver 200 via the optical transmission medium 300.
  • An optical transmission unit (optical transmission / reception unit) (not shown) is provided at the subsequent stage of the digital modulation unit 130, and the optical transmission unit outputs the modulated optical signal to the optical receiver 200.
  • the optical transmitter 100 includes the RZ modulator 120 and the DP-QPSK modulator 131 to perform RZ-DP-QPSK modulation using the polarization multiplexing phase modulation method.
  • the optical receiver 200 receives the modulated optical signal transmitted from the optical transmitter 100 and performs demodulation processing.
  • the output light waveform of the two-electrode type Mach-Zehnder modulator 121 is distorted, and this distortion may cause the reception characteristics of the optical receiver 200 to deteriorate.
  • the amplitude and phase of the optical signal propagating through the two arms of the Mach-Zehnder modulator 121 may be different from those desired due to manufacturing variations of the Mach-Zehnder modulator 121 or environmental changes such as ambient temperature changes.
  • the rise time Tr and the fall time Tf of the modulated optical RZ signal are asymmetric.
  • the optical transmitter in the optical communication system according to the first embodiment compensates for an undesired phase shift or amplitude shift between the optical signals propagating through the two arms of the two-electrode Mach-Zehnder modulator with high accuracy. , To prevent reception characteristics degradation.
  • An optical communication system according to Embodiment 1 an optical transmitter in the optical communication system, and a control procedure (algorithm) suitable for the optical transmitter will be described below.
  • FIG. 2 is a block diagram illustrating a configuration example of the optical communication system according to the first embodiment.
  • the optical communication system according to Embodiment 1 includes an optical transmitter 1 and an optical receiver 2 connected to the optical transmitter 1 via an optical transmission medium 3.
  • the optical transmitter 1 includes a light source 10, an RZ modulation unit 20, and a digital modulation unit 30, and transmits a modulated optical signal to the optical receiver 2 via an optical transmission medium 3 such as an optical cable.
  • the light source 10 outputs continuous light similarly to the light source 110 shown in FIG.
  • the RZ modulation unit 20 is a Mach-Zehnder type RZ modulation unit that generates an optical RZ signal that is a return-to-zero optical signal from the light output from the light source 10.
  • the RZ modulation unit 20 can include a two-electrode Mach-Zehnder modulator 21, a signal adjustment unit 22, and a clock generation unit 24.
  • the optical RZ signal generated by the RZ modulation unit 20 is not limited to a single-stream optical RZ signal (a signal having an RZ format), but may be an optical RZ signal having another format such as a CS-RZ format.
  • the optical RZ signal generated by the RZ modulation unit 20 may be a signal synchronized with the modulation unit of the digital modulation unit 30. For example, when data is modulated in symbol units as in the DP-QPSK modulator 31. Can be a signal synchronized with the symbol.
  • the signal adjustment unit 22 includes an attenuator 22a and a phase shifter 22b, and adjusts the generated optical RZ signal.
  • the clock generator 24 generates a clock signal (CLK) 24a and an inverted clock signal (inverted CLK) 24b, similarly to the clock generator 124 shown in FIG.
  • CLK clock signal
  • inverted CLK inverted clock signal
  • the clock signal 24 a and the inverted clock signal 24 b are examples of two electric signals (also referred to as “first electric signal” and “second electric signal”) input to the bipolar Mach-Zehnder modulator 21.
  • the clock generator 24 outputs the clock signal 24a to the first arm of the Mach-Zehnder modulator 21, and outputs the inverted clock signal 24b to the phase shifter 22b.
  • the phase shifter 22b shifts the phase of the inverted clock signal 24b and outputs it to the attenuator 22a.
  • the attenuator 22 a attenuates the amplitude of the electrical signal output from the phase shifter 22 b and outputs the attenuated signal to the second arm of the Mach-Zehnder modulator 21.
  • the connection order of the attenuator 22a and the phase shifter 22b may be the reverse of the case of FIG.
  • the amount to be shifted in the phase shifter 22b and the amount to be attenuated in the attenuator 22a can be controlled by the control unit 50.
  • the attenuator 22a controls the amplitude of the electric signal input to one arm (second arm or first arm) of the Mach-Zehnder modulator 21.
  • the phase shifter 22b controls the phase of the electric signal input to the one arm in order to control the relative phase between the two arms of the Mach-Zehnder modulator 21.
  • FIG. 2 an example in which the inverted clock signal 24b is adopted as an electrical signal to be controlled in amplitude and phase is described.
  • an attenuator and a phase shifter may be arranged so that the electrical signal becomes the clock signal 24a. .
  • the effects of the first embodiment and other embodiments described later can be similarly obtained as long as the operation ranges of the attenuator and the phase shifter can be secured. .
  • Comparing the first configuration and the second configuration when the first configuration is adopted, it is necessary to consider the insertion loss and phase shift of the attenuator and the phase shifter. There is an effect that the number of parts can be halved compared to the case where the second configuration is adopted.
  • the control parameter in the control unit 50 described later can be half (two) as compared with the case where the first configuration is adopted.
  • the Mach-Zehnder modulator 21 propagates (passes) the branched light to the two arms by branching the continuous light output from the light source 10, and the electric signal input to the two arms as a control signal as described above. Thus, the phases of the light propagating through the two arms are made different from each other, and then both the lights are multiplexed.
  • the Mach-Zehnder modulator 21 generates the optical RZ signal in this way and outputs it to the digital modulator 30.
  • the optical RZ signal generated here is adjusted by the attenuator 22a and the phase shifter 22b as described above.
  • the RZ modulation unit 20 will be described on the assumption that it has a two-electrode type Mach-Zehnder modulator 21, and other embodiments will be described based on this assumption as well.
  • the RZ modulation unit 20 is not limited to such a configuration, and has a plurality of optical waveguides (arms), and a Mach-Zehnder type that generates an optical RZ signal by causing a phase difference of light between the two arms.
  • Any RZ modulator may be used.
  • the description will be made on the assumption that the Mach-Zehnder modulator 21 is a two-electrode type, but the number of electrodes may be three or more.
  • an LN-type Mach-Zehnder modulator using lithium niobate (LiNbO 3 ) can be applied. It may be.
  • the digital modulation unit 30 is connected to the subsequent stage of the RZ modulation unit 20, generates a modulated optical signal by modulating the optical RZ signal output from the RZ modulation unit 20 based on transmission target data, and generates the modulated optical signal. Output.
  • the digital modulation unit 30 may include a DP-QPSK modulator 31, a phase shift unit 32, and a data output unit 33.
  • the digital modulation unit 30 outputs data to be transmitted using at least one of the clock signal 24a and the inverted clock signal 24b generated by the clock generation unit 24 as a clock signal, and performs digital modulation. In this way, the RZ modulation unit 20 and the digital modulation unit 30 can be synchronized.
  • the clock generation unit 24 may be disposed outside the RZ modulation unit 20.
  • the phase shifter 32 is a first phase shifter having the same functions as the first phase shifter 132a, the second phase shifter 132b, the third phase shifter 132c, and the fourth phase shifter 132d shown in FIG. 32a, a second phase shifter 32b, a third phase shifter 32c, and a fourth phase shifter 32d.
  • the first to fourth phase shifters 32a to 32d are also simply referred to as “phase shifters 32a to 32d”.
  • the data output unit 33 distributes the value of the transmission target data for each of XI, XQ, YI, and YQ.
  • the phase shifters 32a to 32d compensate for data skew (phase shift) between XI, XQ, YI, and YQ, and perform DP-QPSK modulation on the control signal.
  • the phase shifters 32a to 32d compensate for data skew (phase shift) between XI, XQ, YI, and YQ, and perform DP-Q
  • the DP-QPSK modulator 31 is connected to the subsequent stage of the RZ modulation unit 20, and based on the control signals output from the phase shifters 32a to 32d, the optical RZ signal output from the Mach-Zehnder modulator 21 of the RZ modulation unit 20 is used. Then, a modulated optical signal is generated by performing sequential modulation.
  • the optical transmitter 1 transmits a modulated optical signal modulated and output by the DP-QPSK modulator 31 to the optical receiver 2 via the optical transmission medium 3 after the digital modulator 30 (not shown). Prepared). However, this transmission unit may be a part of the digital modulation unit 30.
  • the digital modulation unit 30 is not limited to the one having the DP-QPSK modulator 31 as illustrated, that is, the one that modulates in the DP-QPSK system.
  • the digital modulation unit 30 is different in the number of modulation levels or the type of modulation itself such as 8PSK system, 16PSK system, DQPSK system, D8PSK system, D16PSK system, quadrature amplitude modulation (QAM) system that modulates the optical RZ signal. You may have a modulator corresponding to another kind of digital modulation system.
  • the optical transmission medium 3 uses a fiber having no polarization maintaining function.
  • the optical receiver 2 receives the modulated optical signal transmitted from the optical transmitter 1 via the optical transmission medium 3, and performs demodulation processing. Therefore, the optical receiver 2 includes a receiving unit and a digital demodulating unit (not shown). Moreover, you may comprise the optical receiver 2 so that it may have the main control part (not shown) which controls the whole.
  • the optical transmitter 1 further includes a monitoring unit 40 and a control unit 50.
  • the control unit 50 may be a main control unit that controls the entire optical transmitter 1.
  • the monitoring unit 40 monitors the state of the modulated optical signal output from the digital modulation unit 30.
  • the control unit 50 controls at least one of the phase and the amplitude of the optical RZ signal generated by the RZ modulation unit 20 based on the result of monitoring by the monitoring unit 40.
  • Such control is realized by the control unit 50 controlling at least one of the first electric signal exemplified by the clock signal and the second electric signal exemplified by the inverted clock signal based on the monitoring result.
  • the control unit 50 controls the first electric signal based on the monitoring result, such as controlling the amplitude of the first electric signal and controlling the phase of the second electric signal based on the monitoring result.
  • the amplitude of the first electric signal, the phase of the first electric signal, the amplitude of the second electric signal, and the phase of the second electric signal may be controlled.
  • control unit 50 is a phase and amplitude control unit that controls both the phase and the amplitude of the optical RZ signal generated by the RZ modulation unit 20 will be described.
  • the monitoring unit 40 includes a BER (code error rate) monitor 41.
  • the BER monitor 41 is an example of a receiving unit (BER receiving unit) that receives information indicating the BER when the modulated optical signal is received by the optical receiver 2 from the optical receiver 2.
  • This receiving unit can be integrated with the transmitting unit provided in the subsequent stage of the digital modulation unit 30 and provided in the optical transmitter 1 as an optical transmission unit (optical transmission / reception unit).
  • This transmission unit is preferably received via the optical transmission medium 3 that transmits the modulated optical signal, and particularly preferably receives information indicating the BER on the same channel as the channel that transmits the modulated optical signal.
  • the optical receiver 2 needs to transmit information indicating the BER to the optical transmitter 1 and includes a transmission unit for that purpose.
  • the transmitter on the optical receiver 2 side can be integrated with the receiver on the optical receiver 2 side and provided in the optical receiver 2 as an optical transmission unit.
  • a plurality of components in the optical transmitter 1 can be configured as one device.
  • the digital modulation unit 30, the monitoring unit 40, the transmission control unit in the subsequent transmission unit of the digital modulation unit 30, and the clock generation unit 24 include one digital signal processing circuit (for example, an IC for digital signal processing). Can be mounted on the optical transmitter 1.
  • the monitoring unit 40 monitors the BER with the BER monitor 41 in order to monitor the state of the modulated optical signal output from the digital modulation unit 30. That is, the monitoring unit 40 acquires the BER, and the control unit 50 controls the phase and amplitude of the optical RZ signal generated by the RZ modulation unit 20 based on the BER. As described above, the monitoring unit 40 and the control unit 50 perform control (feedback control) of the optical RZ signal based on the BER.
  • the control unit 50 controls the phase and amplitude of the optical RZ signal generated by the RZ modulation unit 20 so that the BER that is the result of monitoring by the monitoring unit 40 becomes small.
  • the control unit 50 shifts the amplitude amount (suppression amount) attenuated by the attenuator 22a in the RZ modulation unit 20 and the phase by the phase shifter 22b so that the BER becomes small. Control the shift amount.
  • an example is given in which both the phase and amplitude of the optical RZ signal are controlled, but it is only necessary to control so that the BER, which is the result of monitoring, is small, and the control target may be at least one of phase and amplitude. .
  • FIG. 3 is a diagram illustrating an example of the relationship between the clock signal 24 a and the inverted clock signal 24 b used in the RZ modulation unit 20 and the output signal (optical RZ signal) from the RZ modulation unit 20.
  • 4 is a diagram illustrating an example of an optical RZ signal when the phase of the inverted clock signal 24b is shifted (shifted) in the relationship of FIG. 3
  • FIG. 5 is a phase of the inverted clock signal 24b in the relationship of FIG. It is a figure which shows an example of the optical RZ signal when an amplitude changes.
  • the two-electrode Mach-Zehnder modulator 21 modulates unmodulated light output from the light source 10 based on the clock signal 24a output from the clock generator 24 and the inverted clock signal output from the attenuator 22a (RZ Pulsed modulation) and output to the digital modulator 30.
  • the inverted clock signal input to the Mach-Zehnder modulator 21 is a signal obtained by shifting the phase of the inverted clock signal 24b generated by the clock generator 24 by the phase shifter 22b and then changing the amplitude by the attenuator 22a. is there.
  • the clock generator 24 generates a clock signal 24a illustrated as CLK and an inverted clock signal 24b illustrated by a symbol in which CLK is overlined.
  • the RZ modulation unit 20 outputs an optical RZ signal as shown in FIG.
  • the phase shift is performed by the phase shifter 22b without changing the amplitude of the inverted clock signal 24b in a state where such an optical RZ signal is output, for example, the rise time Tr and the rise time Tr illustrated in FIG.
  • An optical RZ signal with asymmetric downstream time Tf is output.
  • the amplitude and phase of the inverted clock signal 24b are changed by the attenuator 22a and the phase shifter 22b, for example, the rise time Tr and the fall time Tf as illustrated in FIG.
  • An RZ signal is output.
  • the RZ modulation unit 20 is changed to that shown in FIG. 3 by shifting the inverted clock signal 24b in the reverse direction by the phase shifter 22b.
  • An optical RZ signal having no asymmetry as shown can be output to the digital modulator 30.
  • the inverted clock signal 24b is shifted in the reverse direction by the phase shifter 22b and the amplitude is changed in the reverse direction by the attenuator 22a.
  • the RZ modulator 20 can output an optical RZ signal having no asymmetry as shown in FIG. 3 to the digital modulator 30.
  • the DP-QPSK modulator 31 receives an optical RZ signal (an RZ pulsed optical signal), and performs polarization and phase modulation of the optical RZ signal.
  • the DP-QPSK modulator 31, for example, separates the received optical RZ signal into X polarization and Y polarization, and controls the X polarization input from, for example, the first phase shifter 32a and the third phase shifter 32c. Sequential modulation is performed based on the signal, and sequential modulation is performed based on the control signal input from the remaining second phase shifter 32b and the fourth phase shifter 32d for the Y polarization.
  • the DP-QPSK modulator 31 combines the modulated X-polarized wave and Y-polarized wave, and outputs the combined signal to the subsequent transmission unit (optical transmission unit).
  • the modulated optical signal output in this way is a signal obtained by phase-modulating the optical RZ signal output from the RZ modulation unit 20 based on transmission target data.
  • the method of polarization and phase modulation in the DP-QPSK modulator 31 is not limited to this.
  • the optical transmission unit following the DP-QPSK modulator 31 transmits the modulated optical signal input from the DP-QPSK modulator 31 via the optical transmission medium 3.
  • the waveform of the modulated optical signal that is actually transmitted is not particularly illustrated, for example, since the optical RZ signal illustrated in FIGS. 3 to 5 is phase-modulated based on polarization and transmission target data, the RZ modulation unit The waveform of the optical RZ signal output from 20 is reflected, and basically the phase of the optical RZ signal is shifted according to the transmission target data. Therefore, when the waveform of the optical RZ signal is the waveform illustrated in FIG. 4 or 5, the waveform of the modulated optical signal transmitted from the optical transmitter 1 is also equal, so that the rise time Tr The downstream time Tf becomes asymmetric.
  • the optical receiver 2 obtains transmission target data by coherently detecting the received modulated optical signal and demodulating the signal light.
  • the optical receiver 2 calculates the BER based on the demodulated error count, and transmits the calculation result (information indicating BER) to the optical transmitter 1 via the optical transmission medium 3.
  • This information may be automatically transmitted by the optical transmitter 1 as confirmation of reception of the modulated optical signal.
  • the BER monitor 41 requests information indicating the BER and responds to the request. This information may be returned as The BER monitor 41 receives the information indicating the BER and outputs the received information indicating the BER to the control unit 50.
  • the optical receiver 2 may transmit information indicating an error count after demodulation to the optical transmitter 1 as information indicating the BER.
  • the BER monitor 41 calculates the BER based on the received information indicating the error count, and outputs the calculation result (information indicating the BER) to the control unit 50.
  • control unit 50 controls the attenuator 22a and the phase shifter 22b to adjust the phase and amplitude of the inverted clock signal 24b input to one arm of the Mach-Zehnder modulator 21 so that the BER becomes small.
  • the control unit 50 increases or decreases (changes) the phase and amplitude of the inverted clock signal 24b input to the Mach-Zehnder modulator 21 so that the BER becomes small.
  • the phase and amplitude are controlled according to the calculated amount of change in the attenuator 22a and the phase shifter 22b in the signal adjustment unit 22.
  • the amount of phase change refers to the amount of phase shift in the phase shifter 22b
  • the amount of amplitude change refers to the amount of amplitude suppression in the attenuator 22a.
  • control unit 50 sequentially changes the change amount in the attenuator 22a and the change amount in the phase shifter 22b in accordance with the BER, and adjusts the amplitude and phase of the inverted clock signal 24b to be input to the Mach-Zehnder modulator 21.
  • the optical waveform output from the Mach-Zehnder modulator 21 changes as described with reference to FIGS. 3 to 5, and the waveform of the modulated optical signal output from the digital modulation unit 30 in accordance with the change.
  • the waveform of the optical signal transmitted to the optical receiver 2 changes, and as a result, the BER after reception by the optical receiver 2 is improved or deteriorated.
  • the control unit 50 changes the phase and amplitude in the same direction as the previous change (increase direction if the previous is an increase direction, decrease if the previous is a decrease direction).
  • the direction is determined to be opposite to the previous change, and the attenuator 22a and the phase shifter 22b are controlled based on the determination.
  • the control unit 50 can control the phase and amplitude of the inverted clock signal 24b input to the Mach-Zehnder modulator 21 so that the BER monitored by the BER monitor 41 decreases.
  • a change value (control value) that minimizes the BER can be found.
  • control unit 50 adjusts the change value of the inverted clock signal so as to minimize the BER (controls the attenuator 22a and the phase shifter 22b).
  • BER controls the attenuator 22a and the phase shifter 22b.
  • the control unit 50 sets the change amount (suppression amount) of the attenuator 22 a and the change amount (shift amount) of the phase shifter 22 b of the signal adjustment unit 22 to arbitrary reference values. (Step S1). Next, the control unit 50 executes a rough adjustment process (the processes in steps S2 to S5) as a pre-process.
  • the control unit 50 simultaneously changes the shift amounts in the phase shifters 32a to 32d of the DP-QPSK modulator 31 from the original set values (for example, reference values exemplified by 0 degrees, 90 degrees, 180 degrees, and 270 degrees) by + 1 / Increase by m [deg] (step S2).
  • m is an arbitrary integer.
  • the control unit 50 acquires the power (optical output power) of the entire wavelength band (or a part of the wavelength band) of the modulated optical signal output from the DP-QPSK modulator 31 (step S3).
  • the control unit 50 stores information indicating the acquired optical output power in an internal memory in association with information indicating how many times the optical output power has been increased in step S2.
  • This information is information indicating the number of times of increase, and here is k which is an integer of m or less.
  • the optical output power can be obtained by providing the optical transmitter 1 with a detector that performs the detection.
  • a detector for example, a photodetector (PD) 43, which will be described later in Embodiment 3, can be cited.
  • control unit 50 determines whether or not m / m [deg] processing has been completed (step S4). If NO, the control unit 50 returns to step S2 and repeats steps S2 to S4. .
  • the control unit 50 extracts k corresponding to the maximum value of the optical output power from the memory when YES is obtained in Step S4, and a shift amount (+ k / m [deg] that maximizes the optical output power). ) And shifting the original set values of the phase shifters 32a to 32d (for example, 0 degrees for the first phase shifter 32a, 90 degrees for the second phase shifter 32b, etc.) by the shift amount, A shift amount is set (step S5).
  • the optical transmitter 1 starts transmission of the modulated optical signal to the opposite optical receiver 2, and accordingly, reception by the optical receiver 2 is started (step S6).
  • the optical receiver 2 calculates a BER at the time of reception, and feeds back information indicating the BER to the optical transmitter 1.
  • the BER monitor 41 receives the information and passes it to the control unit 50, and the control unit 50 receives the information indicating the BER (step S7).
  • the feedback of BER in step S7 is performed sequentially.
  • control unit 50 increases the suppression amount in the attenuator 22a of the signal adjustment unit 22 by A [dB] from the reference value determined in Step S1 (Step S8).
  • the value A is a positive value determined in advance.
  • the attenuator 22a increases the suppression amount (decreases the amplitude).
  • the control unit 50 determines whether or not the value indicating the BER received through the BER monitor 41 as a result of this control has improved (that is, has become smaller) (step S9). Returning to the process of S8, the same process is repeated. On the other hand, in the case of NO in step S9, the control unit 50 performs the process of the reverse tendency, that is, the process of reducing the suppression amount in the attenuator 22a by A [dB] from the current value (step S10). Thereafter, the control unit 50 obtains information indicating the BER again and performs the same determination as in step S9 (step S11). If YES, the control unit 50 returns to the process of step S10 and repeats the same process.
  • step S11 that is, when the BER does not improve
  • the control unit 50 sets the suppression amount in the attenuator 22a to the current value (that is, the BER does not improve) as in step S8.
  • a [dB] is increased from the value at the time (step S12).
  • the control unit 50 determines the suppression amount in the attenuator 22a so as to minimize the BER, and is output from the phase shifter 22b with the determined suppression value.
  • the attenuator 22a is controlled so that the amplitude of the signal is suppressed. Thereby, the adjustment of the suppression amount in the attenuator 22a is completed.
  • the control unit 50 increases the shift amount in the phase shifter 22b of the signal adjustment unit 22 by B [deg] from the reference value determined in step S1 (step S13).
  • the value B is a positive value determined in advance.
  • the direction in which the phase of the inverted clock signal 24b that has passed through the phase shifter 22b is advanced in terms of time will be described as a positive direction.
  • the phase shifter 22b uses a value obtained by adding B [deg] to the reference value as the shift amount.
  • step S9 the control unit 50 determines whether or not the value indicating the BER received via the BER monitor 41 has improved as a result of this control (step S14). Return to the process and repeat the same process.
  • step S14 the control unit 50 performs the process of the reverse tendency, that is, the process of reducing the shift amount in the phase shifter 22b by B [deg] from the current value (step S15). Thereafter, the control unit 50 obtains information indicating the BER again and makes the same determination as in step S9 (step S16). If YES, the control unit 50 returns to the process of step S15 and repeats the same process.
  • step S16 that is, when the BER is not improved
  • the control unit 50 sets the shift amount in the phase shifter 22b to the current value (that is, the BER is improved) as in step S13.
  • the value is incremented by B [deg] from the value at the time of disappearance (step S17), and the process is terminated.
  • the control unit 50 determines the shift amount in the phase shifter 22b that minimizes the BER, and the phase of the inverted clock signal 24b is determined by the determined shift amount.
  • the phase shifter 22b is controlled so as to be shifted. Thereby, the adjustment of the shift amount in the phase shifter 22b is completed.
  • steps S2 to S5 can be omitted.
  • steps S8 to S12 and the processes in steps S13 to S17 may be reversed.
  • the suppression amount of the attenuator 22a in the signal adjustment unit 22 in step S8 and the shift amount of the phase shifter 22b in the signal adjustment unit 22 in step S13 were changed in the positive direction at the initial stage. At least one of them can be negative in the first place.
  • the value A is a predetermined value.
  • steps S9 and S11 the control unit 50 determines the degree of improvement or deterioration of the BER, and adaptively sets the value A according to the determination result. You may make it change to.
  • step S14 and S16 the control unit 50 may determine the degree of improvement or deterioration of the BER, and adaptively change the value B according to the determination result. .
  • the optical RZ signal generated based on the BER after reception.
  • An undesired phase or amplitude between optical signals propagating through the two arms in the RZ modulator 20 that may be caused by manufacturing variations in the RZ modulator 20 and changes in the installation environment by controlling at least one of the amplitude and phase of the RZ Can be compensated with high accuracy.
  • the degradation of the BER after reception caused by the manufacturing variation of the RZ modulation unit 20 and the change of the installation environment is reduced (the BER after reception is improved), and the optical transmission performance is maintained high. can do.
  • another optical device such as an optical amplifier is connected between the optical transmitter 1 and the optical receiver 2
  • the deterioration of the BER is reduced based on the BER at the time of reception (that is, the BER is improved). )be able to.
  • the first embodiment for example, between the first electric signal exemplified by the clock signal and the second electric signal exemplified by the inverted clock signal, as in the waveforms shown in FIGS.
  • adjustment is possible when there is a large deviation that can be confirmed by looking at the waveform.
  • the waveform shift is as small as about 2 ps (picoseconds) (about 0.3 UI [Unit Interval]). Therefore, in the first embodiment, even when the high-precision RZ modulation unit 20 is provided so that the deviation between the optical signals in the two arms is small and the asymmetry between the rise time Tr and the fall time Tf is small, the asymmetry is also provided.
  • control unit 50 can control according to the performance of the RZ modulation unit 20 by setting the value A and the value B to be smaller as the undesired deviation between the optical signals in the two arms of the RZ modulation unit 20 is smaller. become.
  • the control unit 50 controls both the phase and the amplitude of at least one of the first electric signal and the second electric signal input to the Mach-Zehnder modulator 21.
  • the asymmetry between the rise time Tr and the fall time Tf of the optical RZ signal is mainly caused by the phase shift, when the phase is shifted, the degree of this asymmetry changes depending on the amplitude shift. It is. Then, the clock extraction performance in the optical receiver 2 is deteriorated due to the asymmetry of the optical RZ signal.
  • the first electric signal and the second electric signal according to the BER at the time of reception.
  • control and necessary monitoring are not limited to the connection test stage, but are adapted to environmental changes such as disconnection and deterioration of the optical transmission medium 3 such as an optical cable, and setting change of the opposing optical receiver 2. For this reason, it is desirable to carry out this operation at any time (for example, periodically) during operation of this optical communication system. It should be noted that such control and monitoring necessary for it are not necessarily performed frequently (that is, at a high cycle) at short intervals during the operation of the optical communication system.
  • the monitoring unit 40 selects the BER.
  • the symbol error rate may be monitored, or the BER and the symbol error rate may be monitored. Even when the symbol error rate is monitored, at least one of the phase and the amplitude of the optical RZ signal generated by the RZ modulation unit 20 is controlled so that the symbol error rate is reduced as in the case of monitoring the BER. It is preferable.
  • the optical communication system according to Embodiment 1 includes an optical transmitter 1 and an optical receiver 2, and the optical transmitter 1 modulates to the optical receiver 2 via the optical transmission medium 3.
  • the optical communication method according to the first embodiment is a method for transmitting a modulated optical signal from the optical transmitter 1 to the optical receiver 2 via the optical transmission medium 3, and the optical transmitter 1 includes a digital modulator.
  • the optical transmitter 1 monitors at least the phase and amplitude of the optical RZ signal generated by the RZ modulation unit 20 based on the monitoring result of the monitoring unit 40. Control one.
  • the optical transmitter 1 modulates the optical RZ signal based on the transmission target data, and the Mach-Zehnder type RZ modulation unit 20 that generates the optical RZ signal from the light output from the light source 10. And a digital modulator 30 that generates a modulated optical signal and outputs the modulated optical signal.
  • the monitoring target in the monitoring unit 40 is BER as described above.
  • Such an optical communication system can transmit not only information indicating BER but also data indicating general information (transmission target data) from the optical receiver 2 side to the optical transmitter 1 side. That is, it is preferable to configure so that bidirectional communication is possible.
  • the optical communication system according to the first embodiment preferably includes a plurality of optical communication devices, and each optical communication device has both the function of the optical transmitter 1 and the function of the optical receiver 2.
  • one optical communication device will be referred to as an optical communication device as it is, and an optical communication device on the opposite side (for example, the opposite station side) will be referred to as an opposite device.
  • This optical communication device is a device that performs optical communication with the opposite device via the optical transmission medium 3.
  • the optical communication system according to Embodiment 1 can be applied to a core network and a metro network that require high optical transmission performance.
  • the optical communication device and the opposite device can be installed apart from each other by, for example, about 500 km to 5000 km.
  • another optical device such as an optical amplifier may be interposed between the optical communication device and the opposite device.
  • This optical communication device includes an RZ modulation unit 20, a digital modulation unit 30 that outputs a modulated optical signal (referred to as a first modulated optical signal), and a transmission unit that transmits the first modulated optical signal to the opposite device.
  • a receiving unit that receives a second modulated optical signal that is a modulated optical signal transmitted from the opposite device via the optical transmission medium 3, and a monitoring unit 40 that monitors the state of the first modulated optical signal;
  • a control unit 50 that controls at least one of the phase and the amplitude of the optical RZ signal generated by the RZ modulation unit 20 based on the result of monitoring by the monitoring unit 40.
  • the transmission unit and the reception unit in this optical communication apparatus correspond to those described above as the optical transmission unit, but the reception unit can receive information other than information indicating the BER as the second modulated optical signal.
  • the monitoring unit 40 receives information indicating the first BER, which is the BER when the first modulated optical signal is received by the opposing device, from the opposing device by the receiving unit, and As the state monitoring, the first BER is monitored.
  • the optical communication device has a function of transmitting information indicating the BER (information indicating the second BER) when the second modulated optical signal is received from the opposite device to the opposite device.
  • the optical communication apparatus further includes a calculation unit that calculates information indicating a second BER that is a BER of the second modulated optical signal received by the reception unit, and the transmission unit is the calculation unit. Information indicating the calculated second BER is transmitted to the opposite device.
  • the opposite device also has the same configuration as the optical communication device described above.
  • FIG. 8 is a block diagram illustrating a configuration example of the optical communication system according to the second embodiment. 8, parts having the same or corresponding functions as those in FIG. 2 are denoted by the same reference numerals as those used in FIG.
  • the difference between the second embodiment and the first embodiment will be described.
  • various examples described in the first embodiment can be applied to the second embodiment.
  • the optical transmitter 1a according to the second embodiment is obtained by replacing the control unit 50 with the control unit 50a in the optical transmitter 1 shown in FIG.
  • the control unit 50 a has the function of the control unit 50 and also performs control of the phase shift unit 32 based on information indicating the BER received from the BER monitor 41. That is, in the first embodiment, at least one of the phase and the amplitude of the electric signal input to the Mach-Zehnder modulator 21 is controlled after receiving the BER, but in the second embodiment, the BER is input to the DP-QPSK modulator 31. Similarly, the phase of the optical RZ signal to be changed according to the transmission target data is controlled based on the BER.
  • the digital modulation unit 30 changes the modulated optical signal by changing at least the phase of the optical RZ signal generated by the RZ modulation unit 20 according to the transmission target data.
  • the control part 50a changes the change amount (phase shift amount) when the digital modulation part 30 changes the phase of the optical RZ signal produced
  • the control unit 50a preferably controls the amount of change so that the BER becomes small.
  • the above change amount indicates an amount by which the phase shifters 32a to 32d in the phase shift unit 32 shift the optical RZ signal by the control signal to the DP-QPSK modulator 31. Therefore, the amount of change is the shift amount determined as the reference value in the phase shifters 32a to 32d (in the case where the processing of FIG. 6 is adopted, the result of the further rough adjustment processing in steps S2 to S5 being performed). This corresponds to a value obtained by further changing (shift amount). That is, the control unit 50a controls such a shift amount to change according to the BER.
  • FIG. 9 is a flowchart showing an example of processing in the optical communication system of FIG. 8, and FIG. 10 is a conceptual diagram showing signal waveforms output from the digital modulation unit 30 in the optical communication system of FIG.
  • Step S30 includes rough adjustment processing for the digital modulation unit 30 as described with reference to steps S2 to S5 shown in FIG.
  • the attenuator 22a is set to an attenuation amount (suppression amount) that improves the BER
  • the phase shifter 22b is set to a shift amount that improves the BER.
  • the control unit 50 a adjusts the phase shift unit 32 of the digital modulation unit 30.
  • the adjustment of the change amount in the phase shift unit 32 is performed in this order for the second phase shifter 32b, the third phase shifter 32c, and the fourth phase shifter 32d with the first phase shifter 32a as a reference.
  • the control unit 50a receives the BER feedback from the optical receiver 2 in step S7 of FIG.
  • the control unit 50a sets the first phase shifter 32a to a reference (reference device) that does not change the phase shift amount from the current set value (step S31), and sets the second phase shifter 32b as an adjustment target.
  • Select step S32.
  • the control unit 50a changes the change amount (phase shift amount) in the second phase shifter 32b by C [deg] in the plus direction (direction in which the phase is advanced) (step S33), obtains feedback thereof, and obtains the BER. It is determined whether or not has been improved (becomes smaller) (step S34).
  • the value C is a positive value determined in advance.
  • step S34 the control unit 50a returns to the process in step S33 and repeats the same process. If NO in step S34, the control unit 50a changes the phase shift amount in the second phase shifter 32b to be adjusted by C [deg] in the minus direction (direction in which the phase is delayed) (step S35). Next, the control unit 50a obtains the feedback and determines whether or not the BER has been improved (step S36). If YES in step S36, the process returns to step S35 and the same process is repeated. On the other hand, if NO in step S36 (if BER does not improve), the phase shift amount in the second phase shifter 32b is increased by C [deg] (step S37). In step S37, the control unit 50a increases the shift amount of the phase to be adjusted by C [deg] when the BER does not improve. This completes the adjustment of the second phase shifter 32b.
  • step S38 determines whether or not adjustments for all adjustment targets have been completed. If YES, the process ends. If NO in step S38, the next adjustment target (the third phase shifter 32c next to the second phase shifter 32b) is selected (step S39), the process returns to step S33, and the processes of steps S33 to S38 are performed. repeat.
  • the control unit 50a determines NO in Step S38, selects the fourth phase shifter 32d that is the next adjustment target in Step S38, and performs the process of Step S33. Returning to step S33, steps S33 to S38 are repeated.
  • the control unit 50a determines NO in Step S38 and ends the process. Thus, the adjustment of the phase shift unit 32 is completed.
  • the overlapping degree of the waveform of the modulated optical signal output from the digital modulation unit 30 becomes large (the waveform is thin), and the optical receiver 2 side It becomes easy to detect.
  • the reference value of the phase shift amount of the phase shifters 32a to 32d for example, 0 degrees for the first phase shifter 32a, 90 degrees for the second phase shifter 32b, and 180 degrees for the third phase shifter 32c.
  • the difference in deviation from the second phase shifter 32d is 270 degrees
  • the degree of overlap becomes smaller and the waveform becomes thicker as shown in the upper waveform of FIG.
  • the first phase shifter 32a, the second phase shifter 32b, the third phase shifter 32c, and the fourth phase shifter 32d receive the optical RZ signal when the signal is input from the data output unit 33.
  • the description is based on the assumption that a control signal that shifts the phase is output to the DP-QPSK modulator 31.
  • the phase shifters 32a to 32d provide, as this control signal, a signal obtained by actually shifting a prescribed pulse signal, which is a prescribed electrical signal, according to the phase shift amount, for example, to the DP-QPSK modulator 31. (This example can also be applied to the first embodiment).
  • the control of the change amount according to the BER as described above is performed by the control unit 50a with respect to the phase shifters 32a to 32d. This can be realized by performing control that changes the phase of the signal in accordance with the BER.
  • FIG. 11 is a conceptual diagram for explaining a difference in BER depending on the presence or absence of an undesired deviation in amplitude and phase between optical signals propagating through the two arms of the RZ modulation unit 20 in the optical communication system of FIG. 12 is a conceptual diagram for explaining the difference in the optical output power of the optical transmitter 1 depending on the presence or absence of the deviation
  • FIG. 13 is a conceptual diagram for explaining the difference in signal intensity (spectrum) due to the presence or absence of the deviation. is there.
  • the minimum counter BER (minimum value of the counter BER of the graph b2) is about 10 2 to 10 5 larger than the minimum counter BER (minimum value of the graph b1) when the above-described deviation does not occur. Deteriorates.
  • the optical output power of the optical transmitter 1a is modulated by a modulator regardless of the presence or absence of the deviation, as shown by the graph p1 (graph when there is no deviation) and the graph p2 (graph when there is a deviation).
  • the graph p1 and the graph p2 coincide with each other only depending on the phase difference between them. Therefore, the modulation loss becomes large at the point where the BER is minimized (the point where the phase difference between the modulators is deviated and the graph b2 takes the minimum value) when the above-described deviation occurs.
  • the optical output power of the optical transmitter 1a is reduced.
  • the optical transmitter 1a controls the attenuator 22a and the phase shifter 22b based on the counter BER, so that the first input to the two arms of the Mach-Zehnder modulator 21 is performed. Eliminating amplitude and phase shifts between one electrical signal and a second electrical signal, thereby eliminating unwanted amplitude and phase shifts that can occur between optical signals propagating through the two arms.
  • the phase difference between the Mach-Zehnder modulator 21 and the digital modulator 30 is adjusted based on the counter BER. Therefore, according to the second embodiment, in addition to the effect of improving the opposing BER according to the first embodiment, as shown in the graph b1, the graph p1, and the graph si1 in FIGS. 11 to 13, the optical transmitter 1a The optical output power of the modulated optical signal during reception at the optical receiver 2 side is less likely to appear.
  • the symbol error rate can be monitored instead of BER or in combination with BER. Even in this case, it is preferable to control the change amount according to the symbol error rate, and it is particularly preferable to control the change amount so that the symbol error rate becomes small, as in the case of BER monitoring.
  • the optical communication device, the optical communication system, and the optical communication method corresponding to the second embodiment can be realized in the same manner as in the first embodiment.
  • a plurality of optical communication devices each having the functions of the optical transmitter 1a and the optical receiver 2 are provided, and optical communication can be performed between the plurality of optical communication devices.
  • FIG. 14 is a block diagram showing a configuration example of an optical communication system according to Embodiment 3 of the present invention. 14, parts having the same or corresponding functions as those in FIG. 2 are denoted by the same reference numerals as those used in FIG.
  • the difference between the third embodiment and the first embodiment will be described. However, various examples described in the first and second embodiments can be applied to the second embodiment.
  • the optical transmitter 1b includes a transmission unit that transmits the modulated optical signal output from the digital modulation unit 30 to the optical receiver 2 via the optical transmission medium 3.
  • the monitoring unit 40b has a detection unit that detects the signal intensity (power) of a part of the wavelength band (that is, part of the frequency band) of the modulated optical signal transmitted to the optical receiver 2. This detector detects the power (energy per unit time) in a part of the wavelength band of the spectrum of the modulated optical signal (modulation spectrum).
  • the detection unit includes a bandpass filter (BPF) 42 that passes a part of the wavelength band of the modulated optical signal, and the signal intensity (modulated light) of the modulated optical signal after passing through the BPF 42.
  • a photodetector (photodetector: PD) 43 that detects a signal intensity of a wavelength extracted from the signal by the BPF 42.
  • the monitoring unit 40b may include a power meter that is a thermal effect type PD as the PD 43.
  • the portion cut out by the BPF 42 (the above-mentioned part of the wavelength band) has an amplitude shift or a phase shift between the first electric signal and the second electric signal input to the two arms of the Mach-Zehnder modulator 21. (Ie, when there is a phase or amplitude shift different from what is desired between the optical signals propagating through the two arms), the suppressed part of the spectral sidelobes, This is the valley that occurs from the center to the left and right.
  • the wavelength section (wavelength range) of this portion varies depending on the transmission speed and modulation degree of optical communication.
  • the BPF 42 may pass all of this portion that is predetermined according to the transmission speed and the modulation degree. Further, in the BPF 42, using the graph si2 in FIG. 13, for example, only the wavelength sections ⁇ 1 and ⁇ 2 may be passed, or all side lobes other than the main lobe may be passed.
  • the optical transmitter 1b installs a branching coupler 3a in the optical transmission medium 3 in order to input the modulated optical signal output from the digital modulation unit 30 to the BPF 42, and the optical transmission medium 3 is connected by the coupler 3a. It branches into an optical transmission medium 3c connected to the optical receiver 2 side and an optical transmission medium 3b connected to the BPF 42 side.
  • the optical transmitter 1b may employ a configuration in which leaked light from the optical transmission medium 3 is input to the BPF 42 instead of adopting a configuration in which the output is branched by the coupler 3a.
  • the monitoring unit 40b monitors the signal intensity of the modulated optical signal by the detection unit exemplified by the BPF 42 and the PD 43 as a monitoring of the state of the modulated optical signal output from the digital modulation unit 30, and controls the monitoring result.
  • the control unit 50b receives information indicating the signal intensity as a result of monitoring from the monitoring unit 40b, and controls the signal adjusting unit 22 based on the information.
  • the control method of the signal adjustment unit 22 is the same as that in the first embodiment except that information indicating the signal strength is used instead of the information indicating the BER in the first embodiment.
  • control unit 50b causes the phase and amplitude of the optical RZ signal generated by the RZ modulation unit 20 so that the signal intensity as a result of monitoring by the monitoring unit 40b increases (more preferably, maximizes). It is preferable to control at least one of the above. Further, the control unit 50b preferably controls the change amount when the digital modulation unit 30 changes the phase of the optical RZ signal generated by the RZ modulation unit 20 according to the transmission target data, according to the signal intensity. . In this case, the digital modulation unit 30 generates a modulated optical signal by changing at least the phase of the optical RZ signal generated by the RZ modulation unit 20 according to transmission target data. Moreover, it is preferable that the control part 50b controls the said change amount so that signal strength may become large (more preferably it will become the maximum).
  • FIG. 15 is a flowchart showing an example of processing in the optical communication system of FIG.
  • the optical transmitter 1b executes steps S1 to S5 of FIG. 6 (step S50).
  • the coarse adjustment processing for the digital modulation unit 30 is completed, and a shift amount (+ k / m [deg]) that maximizes the optical output power is determined, and the phase shifters 32a to 32d correspond to the shift amount.
  • the shift amount is set by shifting the original set value (reference value in the phase shifters 32a to 32d).
  • the PD 43 outputs (feeds back) information indicating the power (signal intensity) of a part of the wavelength band of the modulated optical signal input via the BPF 42 to the control unit 50b (step S51). Also here, it is assumed that the power feedback in step S51 is performed sequentially.
  • the processing procedure after Step S51 is the same as Steps S8 to S17 in FIGS. Note that steps S8 to S17 in the processes shown in FIGS. 6 and 7 correspond to steps S52 to S61 in the process shown in FIG. However, in the process shown in FIG. 15, in the process shown in FIG. 6 and FIG.
  • the control unit 50b determines the suppression amount in the attenuator 22a so as to maximize the above power, and outputs from the phase shifter 22b with the determined suppression value.
  • the attenuator 22a is controlled so that the amplitude of the received signal is suppressed. Thereby, the adjustment of the suppression amount in the attenuator 22a is completed.
  • the control unit 50b determines the shift amount in the phase shifter 22b so as to maximize the power, and the inverted clock signal is determined with the determined shift amount.
  • the phase shifter 22b is controlled so that the phase of 24b is shifted. Thereby, the adjustment of the shift amount in the phase shifter 22b is completed.
  • the optical transmitter 1b according to Embodiment 3 feeds back the output light from the digital modulation unit 30 and inputs between the first electric signal and the second electric signal input to the two arms.
  • the optical transmitter 1b By eliminating the difference in amplitude and phase, an undesired difference in amplitude and phase generated between the optical signals propagating through the two arms can be eliminated. Therefore, according to the third embodiment, in addition to the effect of the first embodiment, it is possible to improve the counter BER only by the control in the optical transmitter 1b, and the BER described in the first embodiment is shown. It is not necessary for the optical receiver 2 to transmit information, and its function can be omitted from the optical receiver 2.
  • the optical communication system according to the third embodiment can employ a configuration for controlling the phase shift unit 32 as in the optical communication system according to the second embodiment. That is, the control unit 50b can be configured to receive the monitoring result of the monitoring unit 40b and control not only the signal adjustment unit 22 but also the phase shift unit 32 based on the monitoring result.
  • the method of controlling the signal adjustment unit 22 is as described in the processing example shown in FIG.
  • information indicating the opposite BER as a result of monitoring is replaced with information indicating the signal strength output from the PD 43. Is.
  • the optical communication system according to the first or second embodiment can be applied to the optical communication system according to the third embodiment as it is.
  • the signal adjustment unit 22 is controlled based on both the information indicating the corresponding BER obtained from the BER monitor 41 and the information indicating the signal strength obtained from the PD 43 (for example, the inverted clock signal 24b).
  • the signal adjustment unit 22 and the phase shift unit 32 are controlled (for example, the value of the suppression amount and the shift amount of the inverted clock signal 24b).
  • the value of the change amount in the phase shift unit 32 may be determined and controlled based on the determination).
  • FIG. 16 is a hardware configuration diagram illustrating a configuration of a modification of the optical transmitters 1, 1 a, 1 b or the optical communication device according to the first to third embodiments.
  • Each of the optical transmitters 1, 1 a, 1 b and any one of the optical transmitters 1, 1 a, 1 b shown in FIGS. 2, 8, and 14 includes a memory 61 as a storage device that stores a program as software, and a memory 61. It can implement
  • the part excluding the Mach-Zehnder modulator 21 in the RZ modulation unit 20, the digital modulation unit 30, the monitoring unit 40, and the control units 50, 50a, 50b execute programs. It can be realized by the processor 62. Such a program can be distributed by storing it in a non-temporary recording medium (for example, an optical disk, a semiconductor memory, a magnetic disk, etc.) and distributing it, or it can be stored in a server device and distributed via the Internet. Can be made.
  • a non-temporary recording medium for example, an optical disk, a semiconductor memory, a magnetic disk, etc.
  • Light source 20 RZ modulator, 21 Mach-Zehnder modulator, 22 Signal adjuster, 22a Attenuator, 22b Phase shifter, 24 clock generator, 24a clock signal, 24b inverted clock signal, 30 digital modulator, 31 DP-QPSK modulator, 32 phase shifter, 32a, 32b, 32c

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
  • Optical Communication System (AREA)

Abstract

L'invention concerne un émetteur optique (1) qui comporte : une unité de modulation RZ du type Mach-Zehnder (20) qui génère un signal RZ optique ; une unité de modulation numérique (30), qui génère un signal optique modulé par modulation du signal RZ optique sur la base de données à transmettre, et qui délivre le signal optique modulé ; une unité de surveillance (40) qui surveille l'état du signal optique modulé délivré à partir de l'unité de modulation numérique (30) ; et une unité de commande (50) qui commande, sur la base des résultats de la surveillance, la phase et/ou l'amplitude du signal RZ optique devant être généré par l'unité de modulation RZ (20). Par conséquent, un taux d'erreurs de bit après réception est amélioré par l'émetteur optique (1).
PCT/JP2016/051489 2016-01-20 2016-01-20 Émetteur optique, système de communication optique, et procédé de communication optique WO2017126039A1 (fr)

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PCT/JP2016/051489 WO2017126039A1 (fr) 2016-01-20 2016-01-20 Émetteur optique, système de communication optique, et procédé de communication optique
JP2017562202A JP6563040B2 (ja) 2016-01-20 2016-01-20 光送信器、光通信システム、及び光通信方法

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2003279912A (ja) * 2002-03-26 2003-10-02 Fujitsu Ltd 光変調器の制御装置
JP2004294883A (ja) * 2003-03-27 2004-10-21 Fujitsu Ltd 光変調器の制御装置
WO2011114753A1 (fr) * 2010-03-19 2011-09-22 日本電信電話株式会社 Modulateur optique
JP2012222733A (ja) * 2011-04-13 2012-11-12 Fujitsu Ltd スキュー低減方法および光伝送システム

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP4838775B2 (ja) * 2007-07-20 2011-12-14 Ｎｔｔエレクトロニクス株式会社 光送信回路

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2003279912A (ja) * 2002-03-26 2003-10-02 Fujitsu Ltd 光変調器の制御装置
JP2004294883A (ja) * 2003-03-27 2004-10-21 Fujitsu Ltd 光変調器の制御装置
WO2011114753A1 (fr) * 2010-03-19 2011-09-22 日本電信電話株式会社 Modulateur optique
JP2012222733A (ja) * 2011-04-13 2012-11-12 Fujitsu Ltd スキュー低減方法および光伝送システム

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