JP6795342B2 - Modulation circuit and modulation control method - Google Patents

Description

The present invention relates to a modulation circuit for an optical signal, and more particularly to bias control when modulating an optical signal.

With the increase in communication speed in optical communication networks, it is important to perform highly accurate modulation of optical signals when transmitting optical signals. In an optical modulator such as an LN (Lithium Niobate) modulator used for modulating an optical signal, it is known that the operating point shifts due to a phenomenon such as DC (Direct Current) drift or temperature drift. Therefore, in the LN modulator, in order to optimize the operating point, bias control is performed in which a bias is applied to correct the operating point.

In bias control, the output from the optical modulator is measured, and the application of bias is often controlled based on the measurement result. However, in the pulse position modulation (PPM) method, which is one of the optical signal modulation methods, the pulse is discrete due to the modulation, so that the mark rate of the modulated signal becomes a very small value. Therefore, the measurement result of the signal after modulation tends to vary, and the accuracy of bias control tends to decrease. When the accuracy of bias control is lowered, the optical characteristics such as the power and extinction ratio of the modulated optical signal are deteriorated. Therefore, in order to maintain the characteristics of the optical signal, it is desirable that there is a technique capable of performing bias control with high accuracy even if the mark rate of the optical signal is low, and related techniques are being developed. As a technique for performing bias control with high accuracy without depending on the mark ratio of an optical signal, for example, a technique such as Patent Document 1 is disclosed.

Patent Document 1 relates to a modulation circuit that modulates an optical signal in a state in which CW (Continuous Wave) light is multiplexed on a pulsed optical signal. The modulation circuit of Patent Document 1 modulates a multiplexed signal in which a pulsed optical signal and CW light having a wavelength different from the optical signal are multiplexed by a modulator. The modulated multiplex signal is demultiplexed and the signal derived from the CW light is measured. In the modulation circuit of Patent Document 1, the bias voltage is set based on the measurement result of the signal derived from the CW light, and the bias control of the modulator is performed. Patent Document 1 states that by performing such bias control, it is possible to perform accurate bias control regardless of the mark rate of the waveform input to the modulator.

Japanese Unexamined Patent Publication No. 2016-46567

However, the technique of Patent Document 1 is not sufficient in the following points. The modulation circuit of Patent Document 1 performs bias control based on the measurement result of a signal that is demultiplexed after being modulated in a state where an optical signal used for transmission and CW light having a wavelength different from that of the optical signal are combined. There is. However, the characteristics of optical signals of different wavelengths vary depending on the wavelength when passing through a modulator or other optical element. That is, the amount of change in power at the time of output of two optical signals that have passed through the same modulator or element differs depending on the wavelength. Therefore, there is a possibility that the optimum operating point of the optical signal to be modulated and the operating point estimated from the measurement results of the signals derived from CW light having different wavelengths may deviate from each other. If the optimum operating point of the optical signal to be modulated and the operating point estimated from the measurement result deviate from each other, the accuracy of bias control in the modulator deteriorates. Therefore, the technique of Patent Document 1 is not sufficient as a technique for performing bias control with high accuracy without depending on the mark ratio of an optical signal.

In order to solve the above problems, it is an object of the present invention to obtain a modulation circuit capable of performing bias control with high accuracy without depending on the mark ratio of an optical signal.

In order to solve the above problems, the modulation circuit of the present invention includes a wave combining means, a modulation means, a demultiplexing means, a measuring means, and a bias controlling means. The wave combining means includes a first optical signal having a first wavelength, a first light having a second wavelength different from the first wavelength, and a third light signal having a first wavelength and a third wavelength different from the second wavelength. It combines with the second light of the wavelength and outputs it as a combined wave signal. The modulation means modulates the combined wave signal and outputs it as a modulated signal. The demultiplexing means demultiplexes the modulated signal and outputs it as a first modulated signal of the first wavelength, a second modulated signal of the second wavelength, and a third modulated signal of the third wavelength, respectively. The measuring means measures the amplitude levels of the second modulated signal and the third modulated signal, respectively. The bias control means controls the bias voltage applied when the modulation means modulates the combined wave signal based on the amplitude levels of the second modulation signal and the third modulation signal.

The modulation control method of the present invention is different from the first optical signal of the first wavelength, the first light of the second wavelength different from the first wavelength, and the first wavelength and the second wavelength. It combines with the second light of the third wavelength and outputs it as a combined wave signal. The modulation control method of the present invention modulates a combined wave signal and outputs it as a modulated signal. In the modulation control method of the present invention, the modulation signal is demultiplexed and used as the first modulation signal of the first wavelength, the second modulation signal of the second wavelength, and the third modulation signal of the third wavelength, respectively. Output. The modulation control method of the present invention measures the amplitude levels of the second modulation signal and the third modulation signal, respectively. The modulation control method of the present invention controls the bias voltage applied when the combined wave signal is modulated based on the amplitude levels of the second modulation signal and the third modulation signal.

According to the present invention, bias control can be performed with high accuracy without depending on the mark ratio of an optical signal.

It is a figure which shows the outline of the structure of the 1st Embodiment of this invention. It is a figure which shows the outline of the structure of the 2nd Embodiment of this invention. It is a figure which shows a part of the structure of the circuit of the 2nd Embodiment of this invention. It is a figure which shows the example of the amplitude level of the signal in the 2nd Embodiment of this invention. It is a figure which shows the example of the waveform of the signal in the 2nd Embodiment of this invention.

(First Embodiment)
The first embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 shows an outline of the configuration of the modulation circuit of the present embodiment. The modulation circuit of the present embodiment includes a wave combining means 1, a modulation means 2, a demultiplexing means 3, a measuring means 4, and a bias controlling means 5. The wave combining means 1 includes a first optical signal having a first wavelength, a first light having a second wavelength different from the first wavelength, and a third light having a first wavelength and a second wavelength different from the second wavelength. It is combined with the second light of the wavelength of and output as a combined wave signal. The modulation means 2 modulates the combined wave signal and outputs it as a modulated signal. The demultiplexing means 3 demultiplexes the modulated signal and outputs it as a first modulated signal of the first wavelength, a second modulated signal of the second wavelength, and a third modulated signal of the third wavelength, respectively. .. The measuring means 4 measures the amplitude levels of the second modulated signal and the third modulated signal, respectively. The bias control means 5 controls the bias voltage applied when the modulation means 2 modulates the combined wave signal based on the amplitude levels of the second modulated signal and the third modulated signal.

In the modulation circuit of the present embodiment, the light of the second wavelength and the light of the third wavelength is multiplexed with the optical signal of the first wavelength by the wave combining means 1, and the modulation is performed by the modulation means 2. Further, the signal modulated by the modulation means 2 is demultiplexed by the demultiplexing means 3 and output as a first modulation signal, a second modulation signal and a third modulation signal, respectively. In the modulation circuit of the present embodiment, the measurement means 4 measures the amplitude levels of the first light, the second modulation signal derived from the second light, and the third modulation signal, and the bias control means 5 measures the amplitude levels. , The bias voltage is controlled based on the measurement result. In the modulation circuit of the present embodiment, since the bias voltage is controlled based on the measurement results of the amplitude levels of the signals derived from the first light and the second light, variations in the measurement results are unlikely to occur. Further, in the modulation circuit of the present embodiment, since the measurement result of the amplitude level of the signal derived from two lights having different wavelengths is performed, the influence of the wavelength dependence of the measurement result on the bias control can be suppressed. .. Therefore, in the modulation circuit of the present embodiment, the bias voltage can be accurately controlled. As a result, in the modulation circuit of the present embodiment, the bias control can be performed with high accuracy without depending on the mark rate of the optical signal.

(Second Embodiment)
A second embodiment of the present invention will be described in detail with reference to the drawings. FIG. 2 shows an outline of the configuration of the modulation circuit of the present embodiment. The modulation circuit of the present embodiment is a circuit that outputs an optical signal subjected to pulse position modulation in an LN (Lithium Niobate) modulator. In the LN modulator, a bias voltage is applied to correct the deviation from the optimum operating point that occurs during operation. The modulation circuit of the present embodiment modulates a pulsed optical signal used for signal transmission by combining two continuous wave lights having different wavelengths, and demultiplexes a signal corresponding to the continuous wave wavelength. It is characterized in that the bias voltage is controlled based on the result of measuring the amplitude level.

The configuration of the modulation circuit of this embodiment will be described. The modulation circuit of the present embodiment includes an optical signal output unit 11, a timing signal generation unit 12, a modulation signal generation unit 13, a first optical output unit 14, a second optical output unit 15, and a combined wave. A unit 16, a modulation unit 17, and a separation unit 18 are provided. Further, the modulation circuit of the present embodiment includes a bias control unit 19, a first light receiving unit 20, a second light receiving unit 21, a first signal adding unit 22, a second signal adding unit 23, and the like. A signal inversion unit 24 is further provided.

The optical signal output unit 11 has a function of outputting a pulse signal based on the control signal. The optical signal output unit 11 includes a semiconductor laser and outputs an optical signal having a wavelength of λs as a signal source output signal S14. The wavelength λs is preset. The optical signal output unit 11 controls on / off of the output of the laser beam based on the control signal input as the output timing signal S13 from the first signal addition unit 22, and outputs a pulsed optical signal. The optical signal output from the optical signal output unit 11 as the signal source output signal S14 is sent to the combine unit 16.

The timing signal generation unit 12 outputs a signal having a duty ratio of 0.5 and a predetermined frequency. The timing signal generation unit 12 generates an alternating signal of 0 and 1, that is, a signal in which the High state and the Low state are alternately repeated based on a predetermined frequency, and outputs the signal as the timing signal S12. The timing signal generation unit 12 sends the timing signal S12 to the first signal addition unit 22 and the signal inversion unit 24, respectively.

The modulation signal generation unit 13 has a function of generating a control signal of the modulation unit 17 for modulating an optical signal. The modulation signal generation unit 13 generates a control signal indicating the timing at which the modulation unit 17 modulates the optical signal based on the code data or the like input from the outside. The modulation signal generation unit 13 sends the generated control signal to the first signal addition unit 22 and the second signal addition unit 23 as the modulation timing signal S11.

The first optical output unit 14 has a function of outputting CW (Continuous Wave) light having a wavelength of λ1, that is, continuous wave light which is continuously oscillating light. The first light output unit 14 includes a semiconductor laser and outputs continuous light having a wavelength of λ1. The wavelength λ1 is preset. The first light output unit 14 sends light having a wavelength of λ1 to the combine unit 16 as the first light S15.

The second light output unit 15 has a function of outputting CW light having a wavelength of λ2, that is, continuous wave light which is continuously oscillating light. The second light output unit 15 includes a semiconductor laser and outputs continuous light having a wavelength of λ2. The wavelength λ2 is preset. The second light output unit 15 sends light having a wavelength of λ2 to the combine unit 16 as the second light S16.

In the modulation circuit of this embodiment, the wavelength λs is set to be between the wavelengths λ1 and λ2. Further, in the modulation circuit of the present embodiment, the wavelength λ2 is set to be longer than the wavelength λ1. That is, in the modulation circuit of the present embodiment, the value of each wavelength is set so as to satisfy λ1 <λs <λ2. Further, the output levels of the first light S15 having a wavelength λ1 and the second light S16 having a wavelength λ2 are set to be constant.

The combiner 16 has a function of combining and outputting an input optical signal or the like. The combiner 16 includes an optical signal having a wavelength λs input from the optical signal output unit 11 as a signal source output signal S14 and light having a wavelength λ1 input from the first optical output unit 14 as the first light S15. , Light of wavelength λ2 input from the second light output unit 15 as the second light S16 is combined. The combiner 16 transmits an optical signal obtained by combining the signal source output signal S14, the first light S15, and the second light S16 and multiplexing the wavelengths as the combine signal S17 to the modulation unit 17. For the combiner portion 16, for example, an optical coupler can be used. Further, the merging unit 16 of the present embodiment corresponds to the merging means 1 of the first embodiment.

The modulation unit 17 has a function of modulating the input optical signal. An LN modulator is used for the modulation unit 17 of the present embodiment. The modulation unit 17 of the present embodiment performs pulse position modulation on the optical signal input as the combined wave signal S17 based on the control signal input as the modulation control signal S19, and outputs the modulation signal S21. FIG. 3 shows an outline of the configuration of the modulation unit 17 of the present embodiment. The modulation unit 17 of the present embodiment includes a signal modulation unit 31, a modulation control unit 32, and a bias input unit 33.

The signal modulation unit 31 modulates the signal input from the combine unit 16 as the combine signal S17 based on the control of the modulation control unit 32, and outputs it as the modulation signal S21. The signal modulation unit 31 modulates the optical signal by controlling the voltage applied to the LiNbO ₃ crystal based on the control of the modulation control unit 32. Further, the signal modulation unit 31 corrects the operating point by applying a bias voltage to the LiNbO ₃ crystal based on the control of the bias input unit 33.

The modulation control unit 32 modulates the optical signal by controlling the voltage applied to the LiNbO ₃ crystal in the signal modulation unit 31 based on the control signal input as the modulation control signal S19 from the second signal addition unit 23. .. The modulation control unit 32 stores in advance the relationship between the voltage applied to the LiNbO ₃ crystal in the signal modulation unit 31 and the modulation amount of the optical signal.

The bias input unit 33 applies a bias voltage to the LiNbO ₃ crystal of the signal modulation unit 31 based on the bias control signal S20 sent from the bias control unit 19. The bias input unit 33 controls the bias voltage applied to the LiNbO ₃ crystal based on the value of the bias voltage input from the bias control unit 19 as the bias control signal S20. Further, the modulation unit 17 of the present embodiment corresponds to the modulation means 2 of the first embodiment.

The separation unit 18 has a function of demultiplexing and outputting the input optical signal. The separation unit 18 separates the wavelength-multiplexed optical signal input from the modulation unit 17 as the modulation signal S21 into optical signals having three wavelengths, λs, λ1 and λ2. The modulation unit 17 outputs an optical signal having a wavelength of λs among the separated optical signals as an output signal S22 which is an output signal from the modulation circuit.

The separation unit 18 sends an optical signal having a wavelength of λ1 out of the separated optical signals to the first light receiving unit 20 as the first monitoring signal S23. Further, the separation unit 18 sends an optical signal having a wavelength of λ2 among the separated optical signals to the second light receiving unit 21 as a second monitoring signal S24. For the separation unit 18, for example, a diffraction grating can be used. Further, the separation unit 18 of the present embodiment corresponds to the demultiplexing means 3 of the first embodiment.

The bias control unit 19 has a function of setting a bias voltage when the modulation unit 17 modulates an optical signal. The bias control unit 19 is the amplitude level of the first measurement signal S25 of the wavelength λ1 input from the first light receiving unit 20 and the second measurement signal S26 of the wavelength λ2 input from the second light receiving unit 21. Based on the amplitude level, the amplitude level of the optical signal having the wavelength λs at the time of output of the modulation unit 17 is estimated. The amplitude level refers to the amplitude of the optical signal in the state where the optical signal indicates 1, that is, in the high state. The bias control unit 19 sets the value of the bias voltage based on the estimated amplitude level of the optical signal of the wavelength λs and the set value of the amplitude level of the wavelength λs. The bias control unit 19 stores in advance the relationship between the voltage applied to the LiNbO ₃ crystal in the signal modulation unit 31 and the modulation amount of the optical signal. The bias control unit 19 determines the relationship between the voltage applied to the LiNbO ₃ crystal stored in advance and the modulation amount of the optical signal, and the measurement results input as the first measurement signal S25 and the second measurement signal S26. Based on this, the bias voltage applied to the LiNbO ₃ crystal is controlled. The bias control unit 19 sends the calculated bias voltage value information to the bias input unit 33 of the modulation unit 17 as a bias control signal S20. Further, the bias control unit 19 of the present embodiment corresponds to the bias control means 5 of the first embodiment.

An example of a method of setting a bias voltage value in the modulation circuit of this embodiment will be described with reference to FIG. FIG. 4 is a diagram schematically showing the relationship between the optical signal of each wavelength and the amplitude level as a graph. In the example of FIG. 4, the optical signal having the wavelength λ1 is set so that the amplitude level is n1 (dB) smaller than the optical signal having the wavelength λs. Further, the optical signal having the wavelength λ2 is set so that the amplitude level is n2 (dB) larger than the optical signal having the wavelength λs. The difference between the amplitude levels of the optical signals of wavelengths λ1 and λ2 and the amplitude levels of the optical signals of wavelengths λs is stored in advance by creating a calibration curve or the like based on the actually measured data. In the state as shown in FIG. 4, the bias control unit 19 estimates the amplitude level of the optical signal of the wavelength λs from the amplitude levels of the optical signals of the wavelength λ1 and the wavelength λ2 in advance, so that the bias control unit 19 determines the amplitude level of the optical signal of the wavelength λs. The deviation from the set value of can be calculated. The bias control unit 19 sets the value of the bias voltage based on the deviation from the set value of the amplitude level of the optical signal having the wavelength λs.

The first light receiving unit 20 has a function of measuring the amplitude level of the optical signal having the wavelength λ1. The first light receiving unit 20 converts an optical signal having a wavelength λ1 input as a first monitoring signal S23 from the separating unit 18 into an electric signal. The first light receiving unit 20 sends data indicating the amplitude level of the signal converted into an electric signal to the bias control unit 19 as the first measurement signal S25. The first light receiving unit 20 of the present embodiment can be formed by using a photodiode.

The second light receiving unit 21 has a function of measuring the amplitude level of the optical signal having the wavelength λ2. The second light receiving unit 21 converts an optical signal having a wavelength λ2 input from the separating unit 18 as the second monitoring signal S24 into an electric signal. The second light receiving unit 21 sends data indicating the amplitude level of the signal converted into the electric signal to the bias control unit 19 as the second measurement signal S26. The second light receiving unit 21 of the present embodiment can be formed by using a photodiode. Further, the first light receiving unit 20 and the second light receiving unit 21 of the present embodiment correspond to the measuring means 4 of the first embodiment.

The first signal addition unit 22 adds a signal input as a timing signal S12 from the timing signal generation unit 12 and a signal input as a modulation timing signal S11 from the modulation signal generation unit 13. The first signal addition unit 22 sends the added signal as an output timing signal S13 to the optical signal output unit 11.

The second signal addition unit 23 adds the signal input as the inversion signal S18 from the signal inversion unit 24 and the signal input as the modulation timing signal S11 from the modulation signal generation unit 13. The signal input from the signal inversion unit 24 is a signal obtained by inverting the signal output from the timing signal generation unit 12 in the signal inversion unit 24. The second signal addition unit 23 sends the added signal to the modulation control unit 32 of the modulation unit 17 as the modulation control signal S19.

The signal inversion unit 24 inverts the input signal and outputs it. The signal inversion unit 24 inverts the High state and the Loe state of the signal input as the timing signal S12 from the timing signal generation unit 12. The signal inversion unit 24 sends a signal obtained by inverting the signal input as the timing signal S12 to the second signal addition unit 23 as an inversion signal S18.

The operation of the modulation circuit of this embodiment will be described. When the modulation circuit starts operating, the modulation signal generation unit 13 outputs a signal indicating the modulation timing as the modulation timing signal S11 based on the code data or the like input from the outside. The modulation timing signal S11 output from the modulation signal generation unit 13 is sent to the first signal addition unit 22 and the second signal addition unit 23, respectively.

Further, the timing signal generation unit 12 generates a signal having a duty ratio of 0.5 at a predetermined frequency and outputs it as a timing signal S12. The timing signal S12 output from the timing signal generation unit 12 is sent to the first signal addition unit 22 and the signal inversion unit 24, respectively.

When the modulation timing signal S11 and the timing signal S12 are input, the first signal addition unit 22 adds the two signals and outputs the output timing signal S13. The output timing signal S13 output from the first signal addition unit 22 is sent to the optical signal output unit 11.

When the output timing signal S13 is input, the optical signal output unit 11 outputs an optical signal having a wavelength of λs as a signal source output signal S14 at a transmission timing based on the output timing signal S13. The optical signal output unit 11 outputs light when the output timing signal S13 indicates a high state, and stops the output of light when the output timing signal S13 indicates a low state. The signal source output signal S14 output from the optical signal output unit 11 is sent to the combine unit 16.

Further, the first light output unit 14 generates CW light having a wavelength λ1, that is, continuous wave light, and outputs it as the first light S15. Similarly, the second light output unit 15 generates CW light having a wavelength of λ2, that is, continuous wave light, and outputs it as the second light S16. The first light S15 and the second light S16 output from the first light output unit 14 and the second light output unit 15 are sent to the combine unit 16, respectively.

When the signal source output signal S14, the first light S15 and the second light S16 are input to the combiner 16, the combiner 16 receives the signal source output signal S14, the first light S15 and the second light. The optical signal in which the wavelength of S16 is multiplexed is output as the combined wave signal S17. That is, the combiner unit 16 outputs an optical signal having a wavelength λs, a light having a wavelength λ1 and an optical signal having a wavelength λ2 multiplexed as a combined wave signal S17. The combined wave signal S17 output from the combined wave unit 16 is sent to the modulation unit 17.

FIG. 5 schematically shows a waveform of a signal at each part of the modulation circuit of the present embodiment. In the example of FIG. 5, the modulation unit 17 is shown as "(4) optical signal output unit output", "(5) first optical output unit output", and "(6) second optical output unit output". The signal is wavelength-multiplexed and input.

Further, when the timing signal S12 output from the timing signal generation unit 12 is input to the signal inversion unit 24, the signal inversion unit 24 inverts the signal and outputs it as an inversion signal S18. When the input timing signal S12 is in the Low state, the signal inversion unit 24 outputs a signal in the High state. When the input timing signal S12 is in the High state, the signal inversion unit 24 outputs the signal in the Low state. Further, the inverting signal S18 output from the signal inverting unit 24 is sent to the second signal adding unit 23.

When the modulation timing signal S11 and the inverting signal S18 are input to the second signal addition unit 23, the second signal addition unit 23 adds the modulation timing signal S11 and the inverting signal S18 to form a modulation control signal S19. Output. The modulation control signal S19 output from the second signal addition unit 23 is sent to the modulation unit 17.

When the combined wave signal S17 is input to the modulation unit 17, the modulation unit 17 modulates the combined wave signal S17 based on the modulation control signal S19 and the bias control signal S20, and outputs the modulation signal S21. The modulation signal S21 output from the modulation unit 17 is sent to the separation unit 18.

When the modulated signal S21 is input to the separation unit 18, the separation unit 18 demultiplexes the input optical signal with wavelength multiplexing. Since the modulation signal S21 is a wavelength-multiplexed optical signal having a wavelength λs, a wavelength λ1 and a wavelength λ2, the separation unit 18 demultiplexes the optical signal having the wavelength λs, the wavelength λ1 and the wavelength λ2.

In the example of FIG. 5, in the modulation unit 17, modulation is performed based on the signal shown as “(7) Modulation unit control signal input”, and the signal shown as “(8) Modulation unit output” is used as the modulation signal S21. It is output. In the modulation unit 17, the component of the signal output from the optical signal output unit 11 based on the timing of the signal generated by the timing signal generation unit 12 is canceled by being modulated by the signal inverted by the signal inversion unit 24. Will be done. As a result, the output signal from the modulation unit 17 has a wavelength λ1 modulated by a pulse wave having a wavelength λs based on the timing of the signal generated by the modulation signal generation unit 13 and a signal inverted by the signal inversion unit 24. The first light S15 and the second light S16 having a wavelength of λ2 are multiplexed optical signals.

When the separation unit 18 is demultiplexed into optical signals of each wavelength, the optical signals of each wavelength are shown in FIG. 5 as “(9) Separation unit output (wavelength λs)” and “(10) Separation unit output (wavelength λ1)”. And are output as optical signals shown as "(11) Separation unit output (wavelength λ2)". That is, the light having a wavelength λ1 and a wavelength λ2, which was a continuous wave, is output as an optical signal having a duty ratio of about 0.5 after passing through the modulation unit 17. In the modulation circuit of the present embodiment, the system for monitoring the amplitude level of the optical signal can be improved by using the optical signal having a duty ratio of about 0.5 for measuring the amplitude level. Further, in FIG. 5, the optical signal having the waveform shown as “(9) Separation unit output (wavelength λs)” is output from the modulation circuit as the output signal S22.

When the wavelength-multiplexed modulation signal S21 is demultiplexed, the separation unit 18 outputs an optical signal having a wavelength of λs as an output signal S22. The optical signal of wavelength λs output from the separation unit 18 as the output signal S22 is output to the transmission line or the like as the output signal of the modulation circuit. Further, when the wavelength-multiplexed modulation signal S21 is demultiplexed, the separation unit 18 outputs an optical signal having a wavelength λ1 as a first monitoring signal S23. The first monitoring signal S23 output from the separation unit 18 is sent to the first light receiving unit 20. Further, when the wavelength-multiplexed modulation signal S21 is demultiplexed, the separation unit 18 outputs an optical signal having a wavelength of λ2 as a second monitoring signal S24. The second monitoring signal S24 output from the separation unit 18 is sent to the second light receiving unit 21.

When the first monitoring signal S23 is input to the first light receiving unit 20, the first light receiving unit 20 converts the input optical signal of wavelength λ1 into an electric signal. When the first light receiving unit 20 converts an optical signal having a wavelength of λ1 into an electric signal, the first light receiving unit 20 sends the converted electric signal to the bias control unit 19 as a first measurement signal S25.

When the second monitoring signal S24 is input to the second light receiving unit 21, the second light receiving unit 21 converts the input optical signal having a wavelength λ2 into an electric signal. When the second light receiving unit 21 converts an optical signal having a wavelength of λ2 into an electric signal, the second light receiving unit 21 sends the converted electric signal to the bias control unit 19 as a second measurement signal S26.

When the first measurement signal S25 and the second measurement signal S26 are input to the bias control unit 19, the bias control unit 19 is the modulation unit 17 based on the first measurement signal S25 and the second measurement signal S26. Calculate the value of the applied bias voltage. When calculating the value of the bias voltage, the bias control unit 19 calculates an estimated value of the amplitude level of the optical signal having the wavelength λs based on the amplitude level indicated by the first measurement signal S25 and the second measurement signal S26. To do. The bias control unit 19 biases so as to correct the difference between the estimated value of the amplitude level of the optical signal of the wavelength λs calculated based on the measurement result and the set value of the amplitude level of the optical signal of the wavelength λs stored in advance. Set the voltage value. When the bias control unit 19 calculates the value of the bias voltage, the bias control unit 19 sends information indicating the value of the bias voltage to the modulation unit 17 as the bias control signal S20.

When the bias control signal S20 is input to the modulation unit 17, the modulation unit 17 applies a bias voltage based on the bias control signal S20 when the combined wave signal S17 is modulated. By repeating the above operation, each part of the modulation circuit repeatedly modulates the optical signal and controls the bias. By repeating the above operation, the modulation circuit of the present embodiment can modulate the optical signal at the optimum operating point by bias control.

In the modulation circuit of the present embodiment, the optical signal output from the optical signal output unit 11 is coupled with two continuous lights on the long wavelength side and the short wavelength side of the optical signal and modulated into a wavelength-multiplexed signal. Modulation is applied in unit 17. Further, the wavelength-multiplexed optical signal after the modulation is demultiplexed, and the optical signal having the wavelength corresponding to the two continuous lights is measured. In the modulation circuit of the present embodiment, the bias voltage applied by the modulation unit 17 is determined based on the amplitude level of the optical signal having the wavelength corresponding to the two continuous lights.

In this way, by determining the bias voltage based on the measurement results of continuous light of two wavelengths, modulation can be performed even if there is a wavelength dependence on the change in amplitude level during modulation or when each element passes through. It is possible to accurately estimate the amplitude level of the optical signal used for the transmission to be applied. Further, in the modulation circuit of the present embodiment, since the signal derived from the continuous light whose mark rate does not become a low value even after the modulation is measured, the amplitude level on the output side of the modulation unit 17 is measured with high accuracy. can do. Therefore, in the modulation circuit of the present embodiment, the bias of the modulation unit 17 can be controlled with high accuracy. As a result, the modulation circuit of the present embodiment can perform bias control with high accuracy without depending on the mark ratio of the optical signal.

In the modulation circuit of the second embodiment, the optical signal is modulated by the pulse position modulation method, and the modulation method includes an intensity modulation method, a frequency modulation method, a phase modulation method, or a combination of these modulation methods. Other modulation methods may be used.

In the modulation circuit of the second embodiment, the optical signal is modulated in a state where two lights having different wavelengths are multiplexed as a continuous wave, but the multiplexed light may have three or more wavelengths. Further, in the modulation circuit of the second embodiment, continuous light having two wavelengths including the wavelength λs of the output optical signal is multiplexed, but two or more on the long wavelength side or the short wavelength side of the wavelength λs. The continuous wave light of the wavelength of may be multiplexed. Further, the continuous wave light to be combined with the optical signal having the wavelength λs may be input to the combining portion in a state where the light having a plurality of wavelengths is multiplexed.

1 Combine wave means 2 Modulation means 3 Demultiplexing means 4 Measuring means 5 Bias control means 11 Optical signal output unit 12 Timing signal generation unit 13 Modulation signal generation unit 14 First optical output unit 15 Second optical output unit 16 Wave part 17 Modulation part 18 Separation part 19 Bias control part 20 First light receiving part 21 Second light receiving part 22 First signal addition part 23 Second signal addition part 24 Signal inversion part 31 Signal modulation part 32 Modulation control part 33 Bias input section S11 Modulation timing signal S12 Timing signal S13 Output timing signal S14 Signal source Output signal S15 First light S16 Second light S17 Combined signal S18 Inversion signal S19 Modulation control signal S20 Bias control signal S21 Modulation signal S22 Output Signal S23 1st monitoring signal S24 2nd monitoring signal S25 1st measurement signal S26 2nd measurement signal

Claims (8)

A first optical signal having a first wavelength, a first light having a second wavelength different from the first wavelength, and a third wavelength different from the first wavelength and the second wavelength. Wavelength means that combines the second light and outputs it as a combined wave signal,
A modulation means that modulates the combined wave signal and outputs it as a modulation signal,
The modulation signal is demultiplexed and output as a first modulation signal of the first wavelength, a second modulation signal of the second wavelength, and a third modulation signal of the third wavelength, respectively. Means and
A measuring means for measuring the amplitude levels of the second modulated signal and the third modulated signal, respectively.
A bias control means for controlling a bias voltage applied when the combined wave signal is modulated by the modulation means based on the amplitude levels of the second modulation signal and the third modulation signal.
Equipped with a,
The bias control means adjusts the bias voltage so that the amplitude level of the first modulated signal estimated from the amplitude levels of the second modulated signal and the third modulated signal becomes a preset value. A modulation circuit characterized by being controlled . Light that generates and outputs the first optical signal of the first wavelength based on a signal obtained by adding a modulation timing signal which is a signal indicating a modulation timing and a timing signal having a duty ratio of 0.5. With more signal output means
The modulation circuit according to claim 1 , wherein the modulation means modulates the combined wave signal based on a signal obtained by adding the modulation timing signal and a signal obtained by inverting the timing signal. An optical output means that outputs the first light and the second light so that the amplitude level of the first modulated signal is between the amplitude level of the second modulated signal and the third modulated signal. The modulation circuit according to claim 1 or 2 , further comprising. Said first wavelength, said second wavelength, said third modulation circuit according to any one of claims 1 to 3, characterized in that it is set as the wavelength between the wavelength of. A first optical signal having a first wavelength, a first light having a second wavelength different from the first wavelength, and a third wavelength different from the first wavelength and the second wavelength. Combines with the second light and outputs as a combined wave signal,
The combined wave signal is modulated and output as a modulated signal.
The modulation signal is demultiplexed and output as a first modulation signal of the first wavelength, a second modulation signal of the second wavelength, and a third modulation signal of the third wavelength, respectively.
The amplitude levels of the second modulated signal and the third modulated signal are measured, respectively.
The bias voltage applied when the combined wave signal is modulated is controlled based on the amplitude levels of the second modulated signal and the third modulated signal .
The bias voltage is controlled so that the amplitude level of the first modulated signal estimated from the amplitude levels of the second modulated signal and the third modulated signal becomes a preset value. Modulation control method. The first optical signal having the first wavelength is generated based on a signal obtained by adding a modulation timing signal which is a signal indicating the modulation timing and a timing signal having a duty ratio of 0.5.
The modulation control method according to claim 5 , wherein the modulation control method is characterized in that the combined wave signal is modulated based on a signal obtained by adding the modulation timing signal and the signal obtained by inverting the timing signal. To output the first light and the second light so that the amplitude level of the first modulated signal is between the amplitude level of the second modulated signal and the third modulated signal. The modulation control method according to claim 5 or 6 , characterized in that. Said first wavelength, said second wavelength, said third modulation control method according to any one of claims 5 7, characterized in that it is set as the wavelength between the wavelength of.

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