JP5757557B2 - Optical modulator having multiple Mach-Zehnder structures capable of bias point adjustment - Google Patents

Description

  The present invention relates to an optical modulator having a plurality of Mach-Zehnder structures capable of adjusting a bias point.

  Mach-Zehnder (MZ) optical modulators are widely used in the field of optical communications. For example, Japanese Patent No. 3867148 discloses an optical modulator having an MZ waveguide.

Japanese Patent No. 3867148

  In recent years, it has become possible to generate a complex optical signal by integrating a plurality of MZ structures. On the other hand, a method for setting each bias point of the integrated MZ structure to a desired state has not been established.

  Therefore, the present invention provides a method for setting each bias point of an MZ structure in a desired state even for an optical modulator having a plurality of MZ structures. Another object of the present invention is to provide an optical modulator that can stably output a modulation signal such as a QPSK signal, an FSK signal, a QAM signal, or an SSB signal.

  In the first aspect of the present invention, a phase modulator is installed after a multiplexing unit of a certain sub-MZ, and a dithering signal is input to the phase modulator, and a bias voltage applied to the sub-MZ is adjusted. Therefore, it is based on the knowledge that the optimal bias situation can be grasped.

  An optical modulation system according to a first aspect includes a first branching unit 1, a first waveguide 2 branched from the first branching unit 1, and a Mach-Zehnder interferometer provided in the first waveguide 2. And a first waveguide 3 that is branched from the first branching unit 1 and is different from the first waveguide 2, and is joined to the first waveguide 2 at the multiplexing point 4. This is an optical modulation system having two waveguides 5 and a second modulator 6 having a Mach-Zehnder interferometer provided in the second waveguide 5.

  The first modulator 3 includes a first main Mach-Zehnder waveguide 7a, a first sub-Mach-Zehnder waveguide 8a provided on one arm of the first main Mach-Zehnder waveguide 7a, For phase-modulating the light that has passed through the combined portion of the second sub Mach-Zehnder waveguide 9a provided in the remaining arm of one main Mach-Zehnder waveguide 7a and the first sub-Mach-Zehnder waveguide 8a. A first phase modulator 10a and a second phase modulator 11a for performing phase modulation on the light that has passed through the multiplexing portion of the second sub Mach-Zehnder waveguide 9a are provided.

  On the other hand, the second modulator 6 includes a second main Mach-Zehnder waveguide 7b, a third sub-Mach-Zehnder waveguide 8b provided on one arm of the second main Mach-Zehnder waveguide 7b, For phase-modulating the light that has passed through the combined portion of the fourth sub Mach-Zehnder waveguide 9b and the third sub-Mach-Zehnder waveguide 8b provided in the remaining arm of the second main Mach-Zehnder waveguide 7b. A third phase modulator 10b and a fourth phase modulator 11b for performing phase modulation on the light that has passed through the multiplexing portion of the fourth sub Mach-Zehnder waveguide 9b are provided.

  The optical modulation system includes a first sub-Mach-Zehnder waveguide 8a, a second sub-Mach-Zehnder waveguide 9a, a third sub-Mach-Zehnder waveguide 8b, a fourth sub-Mach-Zehnder waveguide 9b, and a first phase modulation. A power supply system for supplying a voltage to be applied to the power supply system 10a, the second phase modulator 11a, the third phase modulator 10b, and the fourth phase modulator 11b, and a control signal to the power supply system. Includes a control unit having a computer for changing the voltage output from the signal and a detection unit for detecting an output from the multiplexing point 4.

  The control unit applies a dithering signal to at least one of the first phase modulator 10a, the second phase modulator 11a, the third phase modulator 10b, and the fourth phase modulator 11b. Instructs the power supply system to sweep the bias voltage applied to the sub-Mach-Zehnder waveguide existing before the phase modulator to which the dithering signal is applied and to the phase modulator to which the dithering signal is applied. The components derived from the dithering signal are extracted from the detection information detected by the bias sweep command means and the detection unit. The components derived from the dithering signal extracted by the dithering signal extraction unit and the dithering signal extraction unit. And an optimum bias voltage value obtaining means for obtaining a bias voltage value in which the fluctuation of the value falls within a predetermined range.

  Thereby, the optical modulation system according to the first aspect of the present invention includes the first sub-Mach-Zehnder waveguide 8a, the second sub-Mach-Zehnder waveguide 9a, the third sub-Mach-Zehnder waveguide 8b, and the fourth sub-Mach-Zehnder waveguide 8b. The optimum value of the bias voltage applied to any one of the sub Mach-Zehnder waveguides 9b can be obtained.

  In a preferred embodiment of the first aspect, the first modulator 3 is a first QPSK modulator that outputs a four-phase phase shift keying QPSK signal, and the second modulator 6 is a first QPSK modulator. This is a second QPSK modulator 6 that outputs a QPSK signal having an amplitude smaller than that of the output signal from.

  In a preferred embodiment of the first aspect, the light modulation system is a plurality of waveguides branched from the first branching section 1, and is other than the first waveguide 2 and the second waveguide 5. Provided on one arm of the main Mach-Zehnder waveguide, the main Mach-Zehnder waveguide provided on the plurality of waveguides, the waveguide that is combined with the first waveguide 2 and the second waveguide 5 at the multiplexing point 4 The sub-Mach-Zehnder waveguide, the sub-Mach-Zehnder waveguide provided in the remaining arm of the main Mach-Zehnder waveguide, and the light passing through the combined portion of one of the sub-Mach-Zehnder waveguides. A phase modulator and a phase modulator for performing phase modulation on the light that has passed through the multiplexing portion of the remaining sub-Mach-Zehnder waveguide. The power supply system of the optical modulation system of this aspect supplies a voltage to be applied to all of the sub Mach-Zehnder waveguides and supplies a voltage to all of the phase modulators. The dithering signal instructing means of the control unit is a means for issuing a command to the power supply system so as to apply the dithering signal to at least one of the phase modulators.

  A preferred embodiment of the first aspect relates to a bias adjustment method using the above light modulation system. In this method, a dithering signal is applied to at least one of the first phase modulator 10a, the second phase modulator 11a, the third phase modulator 10b, and the fourth phase modulator 11b.

  Thereafter, the detection unit detects the output from the multiplexing unit 4 while sweeping the bias voltage applied to the sub Mach-Zehnder waveguide that outputs the optical signal to the phase modulator to which the dithering signal is applied. Then, the detection information detected by the detection unit is input to the control unit.

  Then, the dithering signal extraction unit of the control unit extracts a component derived from the dithering signal based on the input detection information. Then, the optimum bias voltage value acquisition means of the control unit obtains a bias voltage value at which the fluctuation of the component derived from the dithering signal falls within a predetermined range. In this way, this bias adjustment method is the first sub-Mach-Zehnder waveguide 8a, the second sub-Mach-Zehnder waveguide 9a, the third sub-Mach-Zehnder waveguide 8b, and the fourth sub-Mach-Zehnder waveguide. The optimum value of the bias voltage applied to any of 9b can be obtained.

  The second aspect of the present invention is based on the knowledge that an optimum bias situation can be grasped by applying a dithering signal to a certain sub-MZ waveguide and performing lock-in detection on a frequency component twice the dithering signal. . Unlike the first aspect, the optical modulation system according to the second aspect does not require a phase modulator after each sub-MZ waveguide.

  In the third aspect of the present invention, a sine wave electric signal is input to all MZ interferometers except the MZ interferometer in a state where a dithering signal is input to a certain MZ interferometer. This sine wave electric signal is different in frequency from the dithering signal. This is because the component derived from the dithering signal is extracted from the output light. The light modulation system according to the third aspect of the present invention is obtained by appropriately adjusting the light modulation system according to the first aspect of the present invention and the second aspect of the present invention. Can be operated.

  All the above aspects can be applied to an optical modulation system having M × N sub Mach-Zehnder waveguides. That is, in this case, one arm of the first main Mach-Zehnder waveguide 7a further has M−1 sub-Mach-Zehnder waveguides. The remaining arms of the first main Mach-Zehnder waveguide 7a further have M-1 sub Mach-Zehnder waveguides. One arm of the second main Mach-Zehnder waveguide 7b further has M−1 sub-Mach-Zehnder waveguides. The second main Mach-Zehnder waveguide 7b further has M−1 sub Mach-Zehnder waveguides. The optical modulation system has (N−4) waveguides each having M sub Mach-Zehnder waveguides connected to the first branching unit 1 and the multiplexing point 4. A specific example of this is the light modulation system shown in FIG. 5 or FIG. In the system shown in FIG. 6, the number N of columns is 2N. In the example shown in FIG. 6, there are N rows of waveguides having M sub Mach-Zehnder waveguides. In the example shown in FIG. 5 and FIG. 6, the bias voltage applied to all the sub Mach-Zehnder waveguides is adjusted by replacing one of the columns as one arm of the first main Mach-Zehnder waveguide 7a. can do. M is an integer of 1 or more, and N is an integer of 4 or more.

  According to the present invention, it is possible to provide a method for setting each bias point of an MZ structure in a desired state even for an optical modulator having a plurality of MZ structures.

FIG. 1 is a diagram illustrating a configuration example of an optical modulation system according to a first aspect. FIG. 2 is a schematic diagram of an optical modulation system having a plurality of MZ structures. FIG. 3 is a diagram showing a change in optical output when a dithering signal is applied to the phase modulator. FIG. 4 is a diagram for explaining a voltage applied to the Mach-Zehnder waveguide. FIG. 5 is a diagram illustrating a configuration example of an optical modulation system. FIG. 6 is a diagram illustrating a configuration example of an optical modulation system. FIG. 7 is a conceptual diagram showing an example of a system in which a phase modulator is provided in the previous stage of the Mach-Zehnder waveguide. FIG. 8 is a conceptual diagram showing an example of a system in which electrodes are provided on both arms of a sub Mach-Zehnder waveguide. FIG. 9 is a conceptual diagram showing an example of a system in which a phase modulator is provided in the subsequent stage of the sub Mach-Zehnder waveguide. FIG. 10 is a diagram for explaining the optical output when a sinusoidal voltage is input to the two-electrode Mach-Zehnder modulator. FIG. 11 is a diagram for explaining temporal changes in outputs of two adjustment target sub Mach-Zehnder modulators in a system according to the present invention.

Description of Principle of Bias Adjustment Method The first aspect of the present invention is that a phase modulator is installed after the multiplexing section of a sub-MZ (the latter stage), and a dithering signal is input to the phase modulator. This is based on the knowledge that the optimum bias situation can be grasped by adjusting the bias voltage applied to the MZ.

In this specification, an example of a “dithering signal” is a sine wave signal whose frequency is sufficiently lower than the bit rate. Examples of dithering signal amplitude is 1/50 or 1/3 of the half-wave voltage V _[pi modulator, or V _[pi 1/20 or 1/5 and the like.

  FIG. 1 is a diagram illustrating a configuration example of an optical modulation system according to a first aspect. As shown in FIG. 1, the light modulation system according to the first aspect of the present invention includes a first branch portion 1, a first waveguide 2 branched from the first branch portion 1, A first modulator 3 having a Mach-Zehnder interferometer provided in the waveguide 2 and a waveguide different from the first waveguide 2 branched from the first branching unit 1, A second waveguide 5 that is joined to the waveguide 2 at the multiplexing point 4 and a second modulator 6 that has a Mach-Zehnder interferometer provided in the second waveguide 5 are included.

  In the example shown in FIG. 1, a system having two main Mach-Zehnder interferometers is drawn. However, the light modulation system of the present invention may further include a plurality of main Mach-Zehnder interferometers. The manufacturing method and functions of the Mach-Zehnder interferometer are already known. For this reason, in the present invention, known configurations and manufacturing methods can be employed as appropriate.

  The first modulator 3 forms a so-called nested Mach-Zehnder interferometer. An optical modulator having such an interferometer is already known in Patent Document 1 and the like. That is, the first modulator 3 includes a first main Mach-Zehnder waveguide 7a, a first sub-Mach-Zehnder waveguide 8a provided on one arm of the first main Mach-Zehnder waveguide 7a, And a second sub Mach-Zehnder waveguide 9a provided on the remaining arm of one main Mach-Zehnder waveguide 7a.

  On the other hand, the optical modulator according to this aspect includes a first phase modulator 10a for performing phase modulation on the light that has passed through the multiplexing portion of the first sub Mach-Zehnder waveguide 8a, and a second sub-Mach-Zehnder waveguide. And a second phase modulator 11a for performing phase modulation on the light that has passed through the multiplexing portion of the waveguide 9a. These phase modulators are provided between the multiplexing unit of each sub Mach-Zehnder waveguide and the multiplexing unit 12a of the first main Mach-Zehnder waveguide (after the sub-Mach-Zehnder waveguide).

  On the other hand, the second modulator 6 includes a second main Mach-Zehnder waveguide 7b, a third sub-Mach-Zehnder waveguide 8b provided on one arm of the second main Mach-Zehnder waveguide 7b, And a fourth sub Mach-Zehnder waveguide 9b provided on the remaining arm of the second main Mach-Zehnder waveguide 7b. The second modulator 6 includes a third phase modulator 10b for performing phase modulation on the light that has passed through the multiplexing portion of the third sub Mach-Zehnder waveguide 8b, and a fourth sub-Mach-Zehnder waveguide. And a fourth phase modulator 11b for performing phase modulation on the light that has passed through the multiplexing portion 9b.

  This optical modulation system includes a first sub-Mach-Zehnder waveguide 8a, a second sub-Mach-Zehnder waveguide 9a, a third sub-Mach-Zehnder waveguide 8b, a fourth sub-Mach-Zehnder waveguide 9b, and a first phase. A power supply system that supplies voltages to be applied to the modulator 10a, the second phase modulator 11a, the third phase modulator 10b, and the fourth phase modulator 11b, and a power supply system by supplying a control signal to the power supply system. It has a control part which has a computer which changes the voltage which a system outputs, and a detection part which detects the output from multiplexing point 4.

  The control unit includes a dithering signal instruction unit, a dithering signal extraction unit, and an optimum bias voltage value acquisition unit. The control unit includes a computer. The computer has an input / output unit, a control device, an arithmetic device, and a storage device. These elements are connected so that information can be exchanged by a bus or the like. The storage device may store various types of information and a control program. For example, predetermined information (for example, operation information from a pointing device) is input from the input / output unit. Then, the control device reads a control program stored in the storage device. Then, according to a control command from the control program, information stored in the storage device is read as appropriate, and a predetermined calculation is performed by the calculation device. In this way, predetermined processing can be achieved.

  The dithering signal indicating means applies a dithering signal to at least one of the first phase modulator 10a, the second phase modulator 11a, the third phase modulator 10b, and the fourth phase modulator 11b. This is a device for issuing commands to the power supply system. For example, predetermined information is input to the control unit from the input / output unit of the control unit. Then, a control part reads the arithmetic program memorize | stored in the memory | storage device. Then, an appropriate calculation process is performed to output an instruction to generate a predetermined voltage in the power supply system.

  The bias sweep command means issues a command to the power supply system so as to sweep the bias voltage applied to the sub Mach-Zehnder waveguide existing in the preceding stage of the phase modulator. An example of the sub Mach-Zehnder waveguide existing in the preceding stage of the phase modulator is the first sub-Mach-Zehnder waveguide 8a for the first phase modulator 10a. The control unit stores information related to the phase modulator to which the dithering signal is applied. Then, the control unit reads information related to the phase modulator to which the stored dithering signal is applied, and issues a command to the power supply system to sweep the bias voltage applied to the corresponding sub Mach-Zehnder waveguide.

  On the other hand, the detector detects the output light from the light modulation system. Information detected by the detection unit is applied to the control unit. And a dithering signal extraction part extracts the component originating in a dithering signal from the detection information which the detection part detected.

  The component derived from the dithering signal has a variation derived from the frequency of the dithering signal. For this reason, the component derived from the dithering signal can be extracted by extracting the variation derived from the frequency of the dithering signal from the detection signal.

  The optimum bias voltage value acquisition means is a device for obtaining a bias voltage value extracted by the dithering signal extraction unit so that the fluctuation of the component derived from the dithering signal falls within a predetermined range. The storage unit stores a predetermined threshold value. On the other hand, the control unit uses the information regarding the component derived from the extracted dithering signal, and the calculation unit calculates the fluctuation. And a control part reads a predetermined | prescribed threshold value from a memory | storage part, and compares the fluctuation | variation of the component originating in the dithering signal calculated | required in the salt remainder part, and a threshold value. Thus, the optimum bias voltage value can be obtained by using the light modulation system according to the first aspect of the present invention. The specific numerical value for “within a predetermined range” varies depending on the system. Therefore, for example, an actual measurement may be performed once to determine an approximate value in the range of the value, and the value may be stored in the storage unit. This process is the same below.

  Note that the optimum bias voltage value may be obtained by obtaining a bias voltage value that minimizes a change in the component derived from the extracted dithering signal.

  Thereby, the optical modulation system according to the first aspect of the present invention includes the first sub-Mach-Zehnder waveguide 8a, the second sub-Mach-Zehnder waveguide 9a, the third sub-Mach-Zehnder waveguide 8b, and the fourth sub-Mach-Zehnder waveguide 8b. The optimum value of the bias voltage applied to any one of the sub Mach-Zehnder waveguides 9b can be obtained.

  In a preferred embodiment of the first aspect, the first modulator 3 is a first QPSK modulator that outputs a four-phase phase shift keying QPSK signal, and the second modulator 6 is a first QPSK modulator. This is a second QPSK modulator 6 that outputs a QPSK signal having an amplitude smaller than that of the output signal from. Since such a configuration is employed, a quadrature amplitude modulation signal can be generated as disclosed in International Publication WO2009-37794.

  In a preferred embodiment of the first aspect, the light modulation system is a plurality of waveguides branched from the first branching section 1, and is other than the first waveguide 2 and the second waveguide 5. Provided on one arm of the main Mach-Zehnder waveguide, the main Mach-Zehnder waveguide provided on the plurality of waveguides, the waveguide that is combined with the first waveguide 2 and the second waveguide 5 at the multiplexing point 4 The sub-Mach-Zehnder waveguide, the sub-Mach-Zehnder waveguide provided in the remaining arm of the main Mach-Zehnder waveguide, and the light passing through the combined portion of one of the sub-Mach-Zehnder waveguides. A phase modulator and a phase modulator for performing phase modulation on the light that has passed through the multiplexing portion of the remaining sub-Mach-Zehnder waveguide. The power supply system of the optical modulation system of this aspect supplies a voltage to be applied to all of the sub Mach-Zehnder waveguides and supplies a voltage to all of the phase modulators. The dithering signal instructing means of the control unit is a means for issuing a command to the power supply system so as to apply the dithering signal to at least one of the phase modulators.

  A preferred embodiment of the first aspect relates to a bias adjustment method using the above light modulation system. In this method, a dithering signal is applied to at least one of the first phase modulator 10a, the second phase modulator 11a, the third phase modulator 10b, and the fourth phase modulator 11b.

  Thereafter, the detection unit detects the output from the multiplexing unit 4 while sweeping the bias voltage applied to the sub Mach-Zehnder waveguide that outputs the optical signal to the phase modulator to which the dithering signal is applied. Then, the detection information detected by the detection unit is input to the control unit.

  Then, the dithering signal extraction unit of the control unit extracts a component derived from the dithering signal based on the input detection information. Then, the optimum bias voltage value acquisition means of the control unit obtains a bias voltage value at which the fluctuation of the component derived from the dithering signal falls within a predetermined range. In this way, this bias adjustment method is the first sub-Mach-Zehnder waveguide 8a, the second sub-Mach-Zehnder waveguide 9a, the third sub-Mach-Zehnder waveguide 8b, and the fourth sub-Mach-Zehnder waveguide. The optimum value of the bias voltage applied to any of 9b can be obtained.

  FIG. 2 is a schematic diagram of an optical modulation system having a plurality of MZ structures. In the light modulation system shown in FIG. 2, N nested Mach-Zehnder waveguides are arranged in parallel. Each nested Mach-Zehnder waveguide has two sub Mach-Zehnder waveguides. And it has a phase modulator in the latter part of each sub MZ waveguide. Then, the bias voltage applied to the sub-MZ is adjusted with the dithering signal input to the phase modulator.

FIG. 3 is a diagram showing a change in optical output when a dithering signal is applied to the phase modulator. As shown in FIG. 3, when the bias state of the first sub-MZ waveguide is other than the bottom (stable state), the output from the first sub-MZ waveguide becomes strong. Of the fluctuations included in the output signal of the optical modulator, the fluctuation due to the dithering signal becomes large. On the other hand, when the bias state of the first sub-MZ waveguide is the bottom (stable state), the output from the first sub-MZ waveguide is small. Of the fluctuations included in the output signal of the optical modulator, the fluctuation due to the dithering signal is reduced. Therefore, a stable bias state can be obtained by controlling the fluctuation of the component derived from the dithering signal to be within a predetermined range.
In the example shown in FIG. 3, the bias point is set at the bottom. However, a stable state can be obtained even when the bias point is set to other than the bottom. The bias point can be set to the top by observing the fluctuation component of the optical output due to the dithering signal and adjusting the bias voltage so that the fluctuation component becomes large. Therefore, the bias point can be set to the quadrature by adjusting the bias voltage so that it is in between.

  According to the second aspect of the present invention, a dithering signal is applied to a certain sub-MZ waveguide, and a frequency component (for example, a doubled frequency component) n times (n is an integer of 2 or more) of the dithering signal. Based on the knowledge that the optimal bias situation can be grasped by lock-in detection. Unlike the first aspect, the optical modulation system according to the second aspect does not require a phase modulator after each sub-MZ waveguide.

FIG. 4 is a diagram for explaining a voltage applied to the Mach-Zehnder waveguide. Let θ be the phase change due to the application of voltage. The amplitude of the input light is E ₀ and the amplitude of the output light is E. _Let ω _{d be} the angular frequency of the dithering signal. The amplitude of the AC voltage applied to the MZ interferometer and Vm, the voltage of the DC voltage V _0. Let _Vπ be the half-wave voltage of the modulator. Then, the phase change θ due to the output light E and voltage application can be expressed as follows.

Therefore, E / E ₀ can be expressed as follows.

The amplitude E of the output light can be observed by direct detection. From the above equation, the offset component, ω _d component, and 2ω _d component can be expressed as follows.

From the above equation, it can be seen that when Vm / Vπ is substantially less than 1, the output signal E is dominated by the carrier signal, the primary component, and the −1st order component. Moreover, to mainly contribute to 2 [omega _d component of the signal obtained by direct detection is found to be beat signal between 1 and -1 order. That, 2 [omega _d component of the output signal by adjusting the theta _{DC (direct} current voltage V ₀₎ to be within a predetermined range, it is possible to obtain a stable bias condition.

  The second aspect of the present invention includes a first branching unit 1, a first waveguide 2 branched from the first branching unit 1, and a Mach-Zehnder interferometer provided in the first waveguide 2. The first modulator 3 is a waveguide different from the first waveguide 2 branched from the first branching unit 1, and is a second waveguide that is combined with the first waveguide 2 at the multiplexing point 4. The present invention relates to an optical modulation system having a waveguide 5 and a second modulator 6 having a Mach-Zehnder interferometer provided in the second waveguide 5.

  The first modulator 3 includes a first main Mach-Zehnder waveguide 7a, a first sub-Mach-Zehnder waveguide 8a provided on one arm of the first main Mach-Zehnder waveguide 7a, And a second sub Mach-Zehnder waveguide 9a provided on the remaining arm of one main Mach-Zehnder waveguide 7a.

  The second modulator 6 includes a second main Mach-Zehnder waveguide 7b, a third sub-Mach-Zehnder waveguide 8b provided on one arm of the second main Mach-Zehnder waveguide 7b, A fourth sub Mach-Zehnder waveguide 9b provided on the remaining arm of the main Mach-Zehnder waveguide 7b.

  The light modulation system according to the second aspect includes a first sub Mach-Zehnder waveguide 8a, a second sub-Mach-Zehnder waveguide 9a, a third sub-Mach-Zehnder waveguide 8b, and a fourth sub-Mach-Zehnder waveguide. A power supply system for supplying a voltage to be applied to 9b, a control unit having a computer for changing a voltage output from the power supply system by supplying a control signal to the power supply system, and a detection for detecting an output from the multiplexing point 4 And a part.

  As described above, the basic configuration of the optical modulation system related to the second aspect is the same as the basic configuration of the optical modulation system related to the first aspect, except that there is no phase modulator after the sub Mach-Zehnder waveguide. is there.

  The control unit includes a dithering signal instruction unit, a bias sweep command unit, a dithering related signal extraction unit, and an optimum bias voltage value acquisition unit.

  In the light modulation system according to the first aspect, the dithering signal is applied to the phase modulator. On the other hand, the light modulation system according to the second aspect applies the dithering signal to the sub Mach-Zehnder waveguide. More specifically, a dithering signal is applied to the bias electrode of the sub Mach-Zehnder waveguide.

  That is, the dithering signal indicating means includes the first sub Mach-Zehnder waveguide 8a, the second sub-Mach-Zehnder waveguide 9a, the third sub-Mach-Zehnder waveguide 8b, and the fourth sub-Mach-Zehnder waveguide 9b. Commands the power supply system to apply a dithering signal to at least one.

  The bias sweep command means instructs the power supply system to sweep the bias voltage applied to the sub Mach-Zehnder waveguide to which the dithering signal is applied. The control unit stores information related to the sub Mach-Zehnder waveguide to which the dithering signal is applied or the sub-Mach-Zehnder waveguide to which the dithering signal is applied in the storage unit. Then, information relating to the sub Mach-Zehnder waveguide is read from the storage unit, and a command is issued to the power supply system to sweep the bias voltage applied to the information relating to the sub-Mach-Zehnder waveguide.

  The dithering-related signal extraction unit extracts a frequency component twice the dithering signal from the detection information detected by the detection unit. That is, the control unit is connected to the detection unit. The detection unit can detect the output of the light modulation system. For example, the control unit is set so that lock-in detection can be performed so as to extract a frequency component twice the dithering signal in advance by operating the detection unit. For this reason, the control unit can extract a frequency component twice the dithering signal.

  The optimum bias voltage value acquisition means obtains a bias voltage value extracted by the dithering related signal extraction unit so that a frequency component twice the dithering signal is within a predetermined range. That is, the storage device of the control unit stores threshold values (upper limit and lower limit). And a control part reads the threshold value memorize | stored in the memory | storage device. Then, the control unit causes the calculation unit to compare the threshold value with a frequency component twice the measured dithering signal. In this way, the control unit determines the magnitude relationship between the frequency component twice the dithering signal and the threshold value.

  Accordingly, the light modulation system according to the second aspect includes the first sub Mach-Zehnder waveguide 8a, the second sub-Mach-Zehnder waveguide 9a, the third sub-Mach-Zehnder waveguide 8b, and the fourth sub-Mach-Zehnder. The optimum value of the bias voltage applied to any one of the waveguides 9b can be obtained.

  The operation for optimizing the bias voltage may be continuously performed during the operation of the optical modulation system, and the optimization of the bias voltage may be automatically performed.

  In a preferred embodiment of the optical modulation system according to the second aspect, the first modulator 3 is a first QPSK modulator that outputs a four-phase phase shift keying QPSK signal, and the second modulator 6 is a first modulator This is a second QPSK modulator 6 that outputs a QPSK signal having a smaller amplitude than the output signal from the QPSK modulator.

  A preferred embodiment of the light modulation system relating to the second aspect is a plurality of waveguides branched from the first branching section 1, other than the first waveguide 2 and the second waveguide 5, 1 waveguide 2 and the second waveguide 5 are combined at the multiplexing point 4, a main Mach-Zehnder waveguide provided in a plurality of waveguides, and provided on one arm of the main Mach-Zehnder waveguide. And a sub Mach-Zehnder waveguide provided on the remaining arm of the main Mach-Zehnder waveguide. The power supply system of this optical modulation system supplies a voltage to be applied to all of the sub Mach-Zehnder waveguides. The dithering signal instructing means of the control unit is a means for issuing a command to the power supply system so as to apply the dithering signal to at least one of the sub Mach-Zehnder waveguides.

  In the third aspect of the present invention, a sine wave electric signal is input to all MZ interferometers except the MZ interferometer in a state where a dithering signal is input to a certain MZ interferometer. For example, the amplitude condition is 2.405 ((πVm) / (2Vπ) = 2.405). This value is the zero of the first order Bessel function of order 0. This sine wave electric signal is different in frequency from the dithering signal. This is because the component derived from the dithering signal is extracted from the output light. The light modulation system according to the third aspect of the present invention is obtained by appropriately adjusting the light modulation system according to the first aspect of the present invention and the second aspect of the present invention. Can be operated.

  Under this condition, the square value of the carrier component is much larger than the square value of the primary component or the square value of the −1st component. Then, it can be said that the offset component is a component in which the first term (n = 0) is dominant.

Therefore, a stable bias state can be obtained by adjusting the bias voltage and adjusting θ _DC (DC voltage V ₀ ) so that the offset component is within a predetermined range.

  The third aspect of the present invention includes a first branching unit 1, a first waveguide 2 branched from the first branching unit 1, and a Mach-Zehnder interferometer provided in the first waveguide 2. The first modulator 3 is a waveguide different from the first waveguide 2 branched from the first branching unit 1, and is a second waveguide that is combined with the first waveguide 2 at the multiplexing point 4. The present invention relates to an optical modulation system having a waveguide 5 and a second modulator 6 having a Mach-Zehnder interferometer provided in the second waveguide 5.

  The first modulator 3 includes a first main Mach-Zehnder waveguide 7a, a first sub-Mach-Zehnder waveguide 8a provided on one arm of the first main Mach-Zehnder waveguide 7a, And a second sub Mach-Zehnder waveguide 9a provided on the remaining arm of the main Mach-Zehnder waveguide 7a.

  The second modulator 6 includes a second main Mach-Zehnder waveguide 7b, a third sub-Mach-Zehnder waveguide 8b provided on one arm of the second main Mach-Zehnder waveguide 7b, A fourth sub Mach-Zehnder waveguide 9b provided on the remaining arm of the main Mach-Zehnder waveguide 7b.

  This optical modulation system includes voltages applied to the first sub Mach-Zehnder waveguide 8a, the second sub-Mach-Zehnder waveguide 9a, the third sub-Mach-Zehnder waveguide 8b, and the fourth sub-Mach-Zehnder waveguide 9b. A power supply system that supplies power, a control unit having a computer that changes a voltage output from the power supply system by giving a control signal to the power supply system, and a detection unit that detects an output from the multiplexing point 4 Have.

  The control unit includes a dithering signal instruction unit, a sine wave electric signal instruction unit, a bias sweep command unit, an offset component extraction unit, and an optimum bias voltage value acquisition unit.

  The dithering signal indicating means includes at least one of the first sub Mach-Zehnder waveguide 8a, the second sub-Mach-Zehnder waveguide 9a, the third sub-Mach-Zehnder waveguide 8b, and the fourth sub-Mach-Zehnder waveguide 9b. Instruct the power supply system to apply the dithering signal.

  The sine wave electric signal indicating means includes dithering signal indicating means and sine wave electric signals to all sub Mach-Zehnder waveguides other than the sub Mach-Zehnder waveguide that the dithering signal indicating means instructs to apply the dithering signal. A command is issued to the power supply system to input.

  At this time, adjustment is performed so that the carrier component (J0 component) of the optical output detected by the detector is only derived from the sub Mach-Zehnder waveguide to which the dithering signal is applied. This adjustment can be achieved by controlling the sinusoidal electric signal applied to each sub Mach-Zehnder waveguide by the control unit that receives the measurement result from the detector.

  The bias sweep command means instructs the power supply system to sweep the bias voltage applied to the sub Mach-Zehnder waveguide to which the dithering signal is applied.

  The offset component extraction unit extracts the offset component from the detection information detected by the detection unit. Under this condition, the offset component is a component whose intensity does not change even if the intensity of the dithering signal changes. The intensity of the component derived from the dithering signal changes with a certain period. Therefore, an offset component can be extracted by extracting a component whose intensity changes at a frequency different from the intensity change of the dithering signal or a component whose intensity does not change.

  The optimum bias voltage value acquisition means obtains a bias voltage value at which the offset component falls within a predetermined range. The control unit has a storage device, and threshold values (upper and lower limit values) are stored in the storage device. The control unit reads the threshold value and causes the calculation unit to compare with the offset component.

  Thereby, this optical modulation system includes the first sub Mach-Zehnder waveguide 8a, the second sub-Mach-Zehnder waveguide 9a, the third sub-Mach-Zehnder waveguide 8b, and the fourth sub-Mach-Zehnder waveguide 9b. The optimum value of the bias voltage applied to either can be obtained.

  In a preferred embodiment of the optical modulation system according to the third aspect, the first modulator 3 is a first QPSK modulator that outputs a four-phase phase shift keying QPSK signal, and the second modulator 6 is a first modulator. This is a second QPSK modulator 6 that outputs a QPSK signal having a smaller amplitude than the output signal from the QPSK modulator.

  A preferred embodiment of the light modulation system relating to the third aspect is a plurality of waveguides branched from the first branching section 1, other than the first waveguide 2 and the second waveguide 5, 1 waveguide 2 and the second waveguide 5 are combined at the multiplexing point 4, a main Mach-Zehnder waveguide provided in a plurality of waveguides, and provided on one arm of the main Mach-Zehnder waveguide. Phase modulation for phase modulation of the light that has passed through the sub-Mach-Zehnder waveguide, the sub-Mach-Zehnder waveguide provided on the remaining arm of the main Mach-Zehnder waveguide, and the combined portion of one of the sub-Mach-Zehnder waveguides A modulator and a phase modulator for performing phase modulation on the light that has passed through the multiplexing portion of the remaining sub Mach-Zehnder waveguide.

  The power supply system supplies the voltage to be applied to all of the sub Mach-Zehnder waveguides and supplies all the voltages to be applied to the phase modulator.

  The dithering signal indicating means of the control unit is a means for issuing a command to the power supply system so as to apply the dithering signal to at least one of the phase modulators.

Application Examples of the Invention So far, an optical modulation system has been described in which one sub Mach-Zehnder waveguide exists in two arms of the main Mach-Zehnder waveguide. The present invention can also be used for an optical modulation system in which the main Mach-Zehnder waveguide has a plurality of arms and an optical modulation system in which a plurality of sub-Mach-Zehnder waveguides are provided in each arm.

  FIG. 5 shows an example in which the first branch portion 1 branches into a plurality of (N) arms, and a plurality of (M) sub Mach-Zehnder waveguides are provided in each arm. In this example, there are M × N sub Mach-Zehnder waveguides.

  In FIG. 6, the first branch portion 1 branches to a plurality of (N) arms. Each branched arm is further connected to N main Mach-Zehnder waveguides. Each main Mach-Zehnder waveguide has two arms. Each arm of the main Mach-Zehnder waveguide is provided with M sub-Mach-Zehnder waveguides.

  The optical modulation system according to the first aspect is provided with a phase modulator in each of the N arms in FIG. In FIG. 6, a phase modulator is provided in each of the 2N arms.

A bias adjustment method based on the first aspect will be described by extracting one row (MZ _{1,1 to} MZ _{1 , M} ) of the parallel modulator structure. Furthermore, in the following description, the case where the bias voltage of the i-th (i is 1 to M) Mach-Zehnder waveguide is adjusted will be described as an example.

In this case, applying a dithering signal to the phase modulator PM ₁ present in the first column. Then, the fluctuation component of the optical output due to the dithering signal is observed. The bias voltages of all MZ _{1, i} (i: _{1 to} M) are adjusted so that the fluctuation of the component derived from the dithering signal of the optical output is maximized. This is because all the bias points of the 1st to nth Mach-Zehnder waveguides excluding the i-th MZ waveguide are located other than the null point. On the other hand, when any of the i-th MZ waveguide 1 to k (k ≦ M) -th MZM is at the null point, the fluctuation of the optical output becomes small regardless of the bias point of MZ1 _{, i} .

Then, the fluctuation component of the optical output due to the dithering signal is observed, and the bias voltage of MZ _{1, i} is adjusted so that the fluctuation component becomes maximum, minimum, or an intermediate value thereof. Then, the bias points of MZ _{1 and i} can be set to top, bottom, and quadrature, respectively.

  In the example shown in FIGS. 5 and 6, the phase modulator is installed in the subsequent stage of the Mach-Zehnder waveguide (the last MZM in the column) from the multiplexing unit 4. In this case, the bias voltages of all MZMs included in this column can be adjusted. On the other hand, the phase modulator that applies the dithering signal may exist at an intermediate position of the column (for example, between the kth and k + 1th MZM).

  Next, an example in which the system shown in FIG. 5 or 6 is used for the second aspect will be described. As described above, in the second aspect of the present invention, the phase modulator depicted in FIGS. 5 and 6 is unnecessary.

The bias voltages of all MZ _{1, i} (i: _{1 to} M) are adjusted so that the fluctuation of the component derived from the dithering signal of the optical output is maximized. This is because all the bias points of the 1st to nth Mach-Zehnder waveguides excluding the i-th MZ waveguide are located other than the null point.

Thereafter, a dithering signal is applied to MZM (MZ _{1, i} ) whose bias voltage is to be adjusted. Then, a frequency component twice the dithering signal is detected. Thereby, the bias voltage of the MZM for which the bias voltage is desired to be adjusted can be adjusted.

  Next, an example in which the system shown in FIG. 5 or 6 is used for the third aspect will be described. As described above, in the third aspect of the present invention, the phase modulator depicted in FIGS. 5 and 6 is unnecessary.

The bias voltages of all MZ _{1, i} (i: _{1 to} M) are adjusted so that the fluctuation of the component derived from the dithering signal of the optical output is maximized. This is because all the bias points of the 1st to nth Mach-Zehnder waveguides excluding the i-th MZ waveguide are located other than the null point.

Then, a dithering signal is applied to the MZM whose bias voltage is to be adjusted. On the other hand, for each MZM included in the same column to which no dithering signal is applied, one MZM is selected from each dependent connection group and driven by a large amplitude electric signal. For example, MZ _2n , MZ _3n , MZ _4n,. . . Are sequentially driven by a large amplitude electric signal. Then, the bias voltage is adjusted while monitoring a light output component (offset) that is constant regardless of the dithering signal. In this way, the bias voltage of the target MZM can be adjusted.

(Slides 4 and 5)
Next, an example of a system according to the fourth aspect of the present invention will be described. In this example, a phase modulator is provided before the MZM. FIG. 7 is a conceptual diagram showing an example of a system in which a phase modulator is provided in the previous stage of the Mach-Zehnder waveguide. Since the basic configuration of this system is the same as the system described above, the description is omitted and repeated.

  In this system, a phase modulator is provided in front of each sub Mach-Zehnder waveguide. An electrode for applying a bias voltage to each sub-Mach-Zehnder waveguide is provided so that the power supply system can apply a bias voltage only to an arbitrary sub-Mach-Zehnder waveguide.

  In this system, for example, when adjusting the bias voltage of the first sub-Mach-Zehnder waveguide, the dithering signal is applied to the first phase modulator located in the preceding stage of the first sub-Mach-Zehnder waveguide. The bias voltage of the first sub Mach-Zehnder waveguide can be adjusted by sweeping the bias voltage of the first sub-Mach-Zehnder waveguide.

  For example, in this system, it is preferable to first adjust any sub-Mach-Zehnder waveguide other than the target whose bias voltage is to be adjusted to be in a state other than the bottom. Normally, the bias voltage of at least one of the sub Mach-Zehnder waveguides is other than the bottom, so that the above state can be obtained without requiring any special operation.

  In this state, when the bias voltage of the first sub Mach-Zehnder waveguide is applied while applying the dithering signal to the first phase modulator located in the preceding stage of the first Sub-Mach-Zehnder waveguide, FIG. The output as shown is obtained.

  That is, the component derived from the dithering signal is extracted from the output signal. The component derived from the dithering signal is accompanied by a variation caused by the dithering signal. For this reason, by separating and extracting components with such fluctuations from the output signal, components derived from the dithering signal can be automatically extracted on the computer.

  While sweeping the bias voltage of one sub Mach-Zehnder waveguide, the amplitude intensity of the component derived from the dithering signal in the output is stored in the storage unit together with the value of the bias voltage. After the sweep operation is completed, the amplitude intensity stored in the storage unit is read, and the value of the bias voltage when the amplitude intensity is the lowest is read. In this way, an optimum value of the bias voltage in a certain state can be obtained. That is, the bias voltage of the first sub Mach-Zehnder waveguide can be set to the bottom state by setting the bias voltage so that the fluctuation of the component derived from the dithering signal in the output is minimized.

  This concept can be applied to a system having N parallel main Mach-Zehnder waveguides, each main Mach-Zehnder waveguide having M sub-Mach-Zehnder waveguides. That is, the present invention can preferably perform bias adjustment in a system having at least three or more sub Mach-Zehnder waveguides.

(Slide 6 pages)
Next, an example of a system according to the fifth aspect of the present invention will be described. In this example, electrodes are provided on both arms of the sub Mach-Zehnder waveguide. A modulation signal can be applied to these electrodes. FIG. 8 is a conceptual diagram showing an example of a system in which electrodes are provided on both arms of a sub Mach-Zehnder waveguide. Since the basic configuration of this system is the same as the system described above, the description is omitted and repeated.

  In this system, for example, when the bias voltage of the first sub-Mach-Zehnder waveguide is adjusted, the first electrode and the second electrode located on both arms of the first sub-Mach-Zehnder waveguide are dithered in phase. Apply ring signal. This system can bring the bias voltage of the first sub Mach-Zehnder waveguide to the bottom state by using the same adjustment method as described above.

(Slide 7 pages)
Next, an example of a system according to the sixth aspect of the present invention will be described. In this example, electrodes for applying a modulation signal to both arms of the sub Mach-Zehnder waveguide are provided. In this example, it is preferable that a phase modulator is provided after the sub Mach-Zehnder waveguide. In this system, even if the phase modulator is provided in front of each sub Mach-Zehnder waveguide, the same processing can be performed. FIG. 9 is a conceptual diagram showing an example of a system in which a phase modulator is provided in the subsequent stage of the sub Mach-Zehnder waveguide. Since the basic configuration of this system is the same as the system described above, the description is omitted and repeated.

In this system, an electrode for applying a modulation signal is provided to both arms of the sub Mach-Zehnder waveguide, and a voltage can be applied to an arbitrary electrode. This system is adapted to apply a voltage to an arbitrary phase modulator. As shown in FIG. 10, in this example, it is possible to apply a signal in which the DC voltage (V ₀ ) and the modulation voltage (Vm) are combined to the electrodes provided on both arms of each sub Mach-Zehnder waveguide.

  In this system, it is preferable to obtain the value of Vπ in each Mach-Zehnder waveguide prior to bias adjustment. A method for obtaining the value of Vπ in the Mach-Zehnder waveguide is already known. For this reason, according to a known method, the value of Vπ in the Mach-Zehnder waveguide is obtained and stored in the storage unit, and the value stored in the storage unit is read out appropriately and used for the arithmetic processing.

  The principle of adjusting the bias voltage of this system is as shown in FIG. In this system, for example, when adjusting the bias voltage of the first sub Mach-Zehnder waveguide, a dithering signal is applied to the electrodes of both arms of the first sub-Mach-Zehnder waveguide, respectively. At this time, the dithering signal applied to both arms is adjusted to be 1/20 to 1/5 (for example, 1/10) of Vπ. The phase difference between the dithering signals applied to both arms is set to 180 °. Direct detection or square detection of output. At this time, the offset component, the component that depends on the angular frequency of the dithering signal, and the component that depends on the angular frequency twice that of the dithering signal are as shown in FIG.

  The output signal is dominated by a carrier wave (carrier signal) and ± 1st order components. On the other hand, a component that depends on twice the angular frequency of the dithering signal obtained by direct detection is mainly a beat between the primary signal and the −1st order signal. That is, the DC voltage applied to the electrodes may be controlled so that the component depending on the angular frequency twice as large as the dithering signal becomes the largest.

(Slide page 10)
Next, an example of a system according to the seventh aspect of the present invention will be described. In this example, electrodes for applying a modulation signal to both arms of the sub Mach-Zehnder waveguide are provided. In this example, it is preferable that a phase modulator is provided after the sub Mach-Zehnder waveguide. In this system, even if the phase modulator is provided in front of each sub Mach-Zehnder waveguide, the same processing can be performed. FIG. 9 is a conceptual diagram showing an example of a system in which a phase modulator is provided in the subsequent stage of the sub Mach-Zehnder waveguide. Since the basic configuration of this system is the same as the system described above, the description is omitted and repeated.

  In this system, an electrode for applying a modulation signal is provided to both arms of the sub Mach-Zehnder waveguide, and a voltage can be applied to an arbitrary electrode. This system is adapted to apply a voltage to an arbitrary phase modulator. The phase modulator provided in the subsequent stage of the multiplexing portion of each sub Mach-Zehnder waveguide may be driven independently or may be push-pull operated.

  In this system, it is preferable to obtain the value of Vπ in each Mach-Zehnder waveguide prior to bias adjustment. A method for obtaining the value of Vπ in the Mach-Zehnder waveguide is already known. For this reason, according to a known method, the value of Vπ in the Mach-Zehnder waveguide is obtained and stored in the storage unit, and the value stored in the storage unit is read out appropriately and used for the arithmetic processing.

In this system, for example, when the bias voltage of the first sub-Mach-Zehnder waveguide is adjusted, the first electrode and the second electrode located on both arms of the first sub-Mach-Zehnder waveguide are dithered in phase. Apply ring signal. At this time, the dithering signal applied to both arms is adjusted to be 1/20 to 1/5 (for example, 1/10) of Vπ. A sine wave electric signal is input to all the Mach-Zehnder waveguides other than the first sub-Mach-Zehnder waveguide. The frequency of the sine wave electrical signal shall be different from the frequency of the dithering signal. In this case, it is preferable that the frequency of the sine wave electric signal does not become an integer multiple or a fraction of the frequency of the dithering signal. Further, it is preferable that the frequency of the sine wave electric signal and the frequency of the dithering signal are not slightly different. Then, the amplitude of the sine wave electric signal is adjusted to be about 2.405 (= πV _H / 2Vπ). Then, the carrier component of the output from the entire system (J ₀ component) is only the those from the first sub Mach-Zehnder waveguide.

  For this reason, the optical output is directly detected, and an invariant component (offset component) with respect to the dithering signal is evaluated. The DC voltage value (V0) is adjusted so that the offset component is minimized. Thereby, the bias voltage can be adjusted.

In order to directly detect the optical output, for example, a photodetector (PD) that does not include the frequency of the large amplitude sine wave as a band is used, or a filter is passed through the output of the photodetector to detect the large amplitude sine wave component. Just do it.
This system can bring the bias voltage of the first sub Mach-Zehnder waveguide to the bottom state by using the same adjustment method as described above.

(Slide 12 pages)
Next, an example of a system according to the eighth aspect of the present invention will be described. In this example, electrodes for applying a modulation signal to both arms of the sub Mach-Zehnder waveguide are provided. In this example, it is preferable that a phase modulator is provided after the sub Mach-Zehnder waveguide. In this system, even if the phase modulator is provided in front of each sub Mach-Zehnder waveguide, the same processing can be performed. FIG. 9 is a conceptual diagram showing an example of a system in which a phase modulator is provided in the subsequent stage of the sub Mach-Zehnder waveguide. Since the basic configuration of this system is the same as the system described above, the description is omitted and repeated.

This system adjusts the phase difference of light through PM _n−1 and PM _n (the bias voltage of these phase modulators). This adjustment method is preferably performed after the system has set the bias of each sub Mach-Zehnder waveguide to null, for example, according to the method described above. Note that PM _n−1 and PM _n are preferably phase modulators included in one main Mach-Zehnder waveguide.

  In this system, an electrode for applying a modulation signal is provided to both arms of the sub Mach-Zehnder waveguide, and a voltage can be applied to an arbitrary electrode. This system is adapted to apply a voltage to an arbitrary phase modulator. The phase modulator provided in the subsequent stage of the multiplexing portion of each sub Mach-Zehnder waveguide may be driven independently or may be push-pull operated.

In this system, for example, the bias of each sub Mach-Zehnder waveguide is set to null. Thereafter, the phase modulators to be adjusted are PM _n−1 and PM _n, and the sub Mach-Zehnder waveguides existing in the previous stage of the respective phase modulators are MZ _n−1 and MZ _n . In this case, by applying a dithering signal having the same frequency and MZ _n-1 and MZ _n, it modulates the light intensity. At this time, the electric amplitude of the applied voltage is set to be equal to or less than Vπ of each MZ. When MZ _n-1 and MZ _n are two-electrode Mach-Zehnder waveguides having electrodes on both arms, two dithering signals applied to the two electrodes of each Mach-Zehnder waveguide are reversed. Let it be a phase. Then, the voltage applied to PM _n−1 and PM _n is changed (or swept), and the change in the light output is observed.

The peak-to-peak value of the combined output from MZ _n-1 and MZ _n depends on the phase difference of light passing through PM _n-1 and PM _n .

  This system can also be applied to the system shown in FIG. Furthermore, even if the number of MZs included in each row is not the same number (M), it can be used. In either case, the bias voltage of the column other than the column subject to bias adjustment is set to the bottom, and the bias voltage of the column subject to bias adjustment is set to other than the bottom. In addition, by performing the adjustment method described above, for example, the bias between the columns to be adjusted can be adjusted to the bottom.

  This system can be similarly used in the case of the system shown in FIG. 6 or a multistage nested Mach-Zehnder waveguide.

(Slide 17 pages)
Next, an example of a system according to the ninth aspect of the present invention will be described. In this example, electrodes for applying a modulation signal to both arms of the sub Mach-Zehnder waveguide are provided. In this example, it is preferable that a phase modulator is provided after the sub Mach-Zehnder waveguide. In this system, even if the phase modulator is provided in front of each sub Mach-Zehnder waveguide, the same processing can be performed. FIG. 9 is a conceptual diagram showing an example of a system in which a phase modulator is provided in the subsequent stage of the sub Mach-Zehnder waveguide. Since the basic configuration of this system is the same as the system described above, the description is omitted and repeated.

This system adjusts the phase difference of light through PM _n−1 and PM _n (the bias voltage of these phase modulators). The phase modulators to be adjusted are PM _n−1 and PM _n, and the sub Mach-Zehnder waveguides existing in the previous stage of the respective phase modulators are MZ _n−1 and MZ _n . PM _n−1 and PM _n are preferably phase modulators included in one main Mach-Zehnder waveguide.

  In this system, an electrode for applying a modulation signal is provided to both arms of the sub Mach-Zehnder waveguide, and a voltage can be applied to an arbitrary electrode. This system is adapted to apply a voltage to an arbitrary phase modulator. The phase modulator provided in the subsequent stage of the multiplexing portion of each sub Mach-Zehnder waveguide may be driven independently or may be push-pull operated.

  In this system, it is preferable to obtain the value of Vπ in each Mach-Zehnder waveguide prior to bias adjustment. A method for obtaining the value of Vπ in the Mach-Zehnder waveguide is already known. For this reason, according to a known method, the value of Vπ in the Mach-Zehnder waveguide is obtained and stored in the storage unit, and the value stored in the storage unit is read out appropriately and used for the arithmetic processing.

In this system, adjustment is performed so that the biases of MZ _n−1 and MZ _n are not the null point (bottom). The bias state of the sub Mach-Zehnder waveguide other than _{MZ n-1} and MZ _n is arbitrary.

In this system, it applies a sine-wave electric signal to the sub Mach-Zehnder waveguide other than MZ _n-1 and MZ _n. This sine wave electric signal is preferably a large amplitude sine wave vibration. That is, the amplitude of the sine wave electric signal is adjusted to be about 2.405 (= πV _H / 2Vπ). The bias voltage of PM _n−1 and PM _n is swept with a sine wave, a triangular wave, a sawtooth wave, or a dithering signal, or the applied voltage is changed using a DC voltage. Measure the output from the system, and extract only the components derived from the bias voltage. Since the component derived from the bias voltage is a component that changes in conjunction with the frequency of the applied signal, it can be extracted by frequency analysis of the output signal.

The relationship between the phasor display and the light intensity of this system is as shown in FIG. A _n is the absolute value of the amplitude of the light output of MZ _n. The point where the component derived from the bias voltage has the maximum value is the in-phase bias voltage, and the point where the component has the minimum value is the reverse phase.

  The present invention can be used in the field of optical information communication.

DESCRIPTION OF SYMBOLS 1 1st branch part 2 1st waveguide 3 1st modulator 4 Combined point 5 2nd waveguide 6 2nd modulator 7a 1st main Mach-Zehnder waveguide 7b 2nd main Mach-Zehnder Waveguide 8a First sub-Mach-Zehnder waveguide 8b Third sub-Mach-Zehnder waveguide 9a Second sub-Mach-Zehnder waveguide 9b Fourth sub-Mach-Zehnder waveguide 10a First phase modulator 10b Third phase Modulator 11a Second phase modulator 11b Fourth phase modulator

Claims (7)

A first branch (1);
A first waveguide (2) branched from the first branch (1);
A first modulator (3) having a Mach-Zehnder interferometer provided in the first waveguide (2);
The waveguide is branched from the first branch (1) and is different from the first waveguide (2), and is combined with the first waveguide (2) at the multiplexing point (4). A second waveguide (5);
A light modulation system comprising a second modulator (6) having a Mach-Zehnder interferometer provided in the second waveguide (5),

The first modulator (3) is:
A first main Mach-Zehnder waveguide (7a);
A first sub Mach-Zehnder waveguide (8a) provided on one arm of the first main Mach-Zehnder waveguide (7a);
A second sub Mach-Zehnder waveguide (9a) provided on the remaining arm of the first main Mach-Zehnder waveguide (7a);
A first phase modulator (10a) for performing phase modulation on the light that has passed through the multiplexing portion of the first sub Mach-Zehnder waveguide (8a);
A second phase modulator (11a) for performing phase modulation on the light that has passed through the multiplexing portion of the second sub Mach-Zehnder waveguide (9a);
Comprising

The second modulator (6)
A second main Mach-Zehnder waveguide (7b);
A third sub Mach-Zehnder waveguide (8b) provided on one arm of the second main Mach-Zehnder waveguide (7b);
A fourth sub Mach-Zehnder waveguide (9b) provided on the remaining arm of the second main Mach-Zehnder waveguide (7b);
A third phase modulator (10b) for performing phase modulation on the light that has passed through the multiplexing portion of the third sub Mach-Zehnder waveguide (8b);
A fourth phase modulator (11b) for performing phase modulation on the light that has passed through the multiplexing portion of the fourth sub Mach-Zehnder waveguide (9b);
Comprising

The light modulation system comprises:
The first sub-Mach-Zehnder waveguide (8a), the second sub-Mach-Zehnder waveguide (9a), the third sub-Mach-Zehnder waveguide (8b), the fourth sub-Mach-Zehnder waveguide (9b) ), Voltage applied to the first phase modulator (10a), the second phase modulator (11a), the third phase modulator (10b), and the fourth phase modulator (11b). A power supply system for supplying
A control unit having a computer for changing a voltage output from the power supply system by giving a control signal to the power supply system;
A detector for detecting an output from the multiplexing point (4);
Have

The controller is
Dithering to at least one of the first phase modulator (10a), the second phase modulator (11a), the third phase modulator (10b), and the fourth phase modulator (11b) A dithering signal indicating means for instructing the power supply system to apply a signal;
A bias sweep command means for commanding a power supply system to sweep a bias voltage applied to a sub-Mach-Zehnder waveguide existing in a stage preceding the phase modulator to which the dithering signal is applied;
A dithering signal extraction unit for extracting a component derived from the dithering signal from detection information detected by the detection unit;
An optimum bias voltage value obtaining means for obtaining a bias voltage value extracted by the dithering signal extraction unit and in which a variation in a component derived from the dithering signal falls within a predetermined range;
Have
While storing the bias voltage of the sub-Mach-Zehnder waveguide existing in the preceding stage of the phase modulator to which the dithering signal is applied, the amplitude intensity of the component derived from the dithering signal is stored in the storage unit together with the value of the bias voltage Then, after the sweep is completed, the amplitude intensity stored in the storage unit is read, and the value of the bias voltage when the amplitude intensity is the lowest is read,

Thus, the first sub Mach-Zehnder waveguide (8a), the second sub-Mach-Zehnder waveguide (9a), the third sub-Mach-Zehnder waveguide (8b), and the fourth sub-Mach-Zehnder waveguide An optimum value of the bias voltage applied to any one of the waveguides (9b) can be obtained.

Light modulation system.
The first modulator (3) is a first QPSK modulator that outputs a four-phase phase shift keying (QPSK) signal;
The second modulator (6) is a second QPSK modulator (6) that outputs a QPSK signal having a smaller amplitude than an output signal from the first QPSK modulator.
The light modulation system according to claim 1.
The light modulation system comprises:
A plurality of waveguides branched from the first branching section (1) other than the first waveguide (2) and the second waveguide (5); A waveguide that joins the waveguide (2) and the second waveguide (5) and the multiplexing point (4);

A main Mach-Zehnder waveguide provided in the plurality of waveguides; a sub-Mach-Zehnder waveguide provided in one arm of the main Mach-Zehnder waveguide; and a remaining arm of the main Mach-Zehnder waveguide. A sub-Mach-Zehnder waveguide, a fifth phase modulator for performing phase modulation on the light that has passed through the combining portion of the one sub-Mach-Zehnder waveguide, and a combining portion of the remaining sub-Mach-Zehnder waveguide A sixth phase modulator for performing phase modulation on the light having passed through
Have
The power supply system supplies a voltage to be applied to all of the sub Mach-Zehnder waveguides, and supplies a voltage to all of the phase modulators.

The dithering signal instruction means of the control unit is:
The light modulation system according to claim 1, wherein the light modulation system is a means for instructing a power supply system to apply a dithering signal to at least one of the fifth phase modulator and the sixth phase modulator.
One arm of the first main Mach-Zehnder waveguide (7a) further has (M-1) sub-Mach-Zehnder waveguides,
The remaining arms of the first main Mach-Zehnder waveguide (7a) further have (M-1) sub-Mach-Zehnder waveguides,
One arm of the second main Mach-Zehnder waveguide (7b) further has (M−1) sub-Mach-Zehnder waveguides,
The second main Mach-Zehnder waveguide (7b) further has (M−1) sub-Mach-Zehnder waveguides,
The light modulation system has N waveguides each having M sub Mach-Zehnder waveguides connected to the first branch (1) and the multiplexing point (4).
The light modulation system according to claim 1.
One arm of the first main Mach-Zehnder waveguide (7a) further has (M-1) sub-Mach-Zehnder waveguides,
The remaining arms of the first main Mach-Zehnder waveguide (7a) further have (M-1) sub-Mach-Zehnder waveguides,
One arm of the second main Mach-Zehnder waveguide (7b) further has (M−1) sub-Mach-Zehnder waveguides,
The second main Mach-Zehnder waveguide (7b) further has (M−1) sub-Mach-Zehnder waveguides,
The light modulation system includes N waveguides each having M sub Mach-Zehnder waveguides connected to the first branch (1) and the multiplexing point (4),

The dithering signal instruction means includes:
Instructing the power supply system to apply a dithering signal to at least one of the phase modulators included in the light modulation system;
When applying a dithering signal to two or more phase modulators, the power supply system is instructed to apply a dithering signal having a different frequency.
The light modulation system according to claim 1.
A first branch (1);
A first waveguide (2) branched from the first branch (1);
A first modulator (3) having a Mach-Zehnder interferometer provided in the first waveguide (2);
The waveguide is branched from the first branch (1) and is different from the first waveguide (2), and is combined with the first waveguide (2) at the multiplexing point (4). A second waveguide (5);
A method for adjusting a bias of an optical modulation system, comprising: a second modulator (6) having a Mach-Zehnder interferometer provided in the second waveguide (5),

The first modulator (3) is:
A first main Mach-Zehnder waveguide (7a);
A first sub Mach-Zehnder waveguide (8a) provided on one arm of the first main Mach-Zehnder waveguide (7a);
A second sub Mach-Zehnder waveguide (9a) provided on the remaining arm of the first main Mach-Zehnder waveguide (7a);
A first phase modulator (10a) for performing phase modulation on the light that has passed through the multiplexing portion of the first sub Mach-Zehnder waveguide (8a);
A second phase modulator (11a) for performing phase modulation on the light that has passed through the multiplexing portion of the second sub Mach-Zehnder waveguide (9a);
Comprising

The second modulator (6)
A second main Mach-Zehnder waveguide (7b);
A third sub Mach-Zehnder waveguide (8b) provided on one arm of the second main Mach-Zehnder waveguide (7b);
A fourth sub Mach-Zehnder waveguide (9b) provided on the remaining arm of the second main Mach-Zehnder waveguide (7b);
A third phase modulator (10b) for performing phase modulation on the light that has passed through the multiplexing portion of the third sub Mach-Zehnder waveguide (8b);
A fourth phase modulator (11b) for performing phase modulation on the light that has passed through the multiplexing portion of the fourth sub Mach-Zehnder waveguide (9b);
Comprising

The light modulation system comprises:
The first sub-Mach-Zehnder waveguide (8a), the second sub-Mach-Zehnder waveguide (9a), the third sub-Mach-Zehnder waveguide (8b), the fourth sub-Mach-Zehnder waveguide (9b) ), Voltage applied to the first phase modulator (10a), the second phase modulator (11a), the third phase modulator (10b), and the fourth phase modulator (11b). A power supply system for supplying
A control unit having a computer for changing a voltage output from the power supply system by giving a control signal to the power supply system;
A detector for detecting an output from the multiplexing point (4);
And

The controller is
Dithering to at least one of the first phase modulator (10a), the second phase modulator (11a), the third phase modulator (10b), and the fourth phase modulator (11b) A dithering signal indicating means for instructing the power supply system to apply a signal;
A bias sweep command means for commanding a power supply system to sweep a bias voltage applied to a sub Mach-Zehnder waveguide that outputs an optical signal to a phase modulator to which the dithering signal is applied;
A dithering signal extraction unit for extracting a component derived from the dithering signal from detection information detected by the detection unit;
An optimum bias voltage value obtaining means for obtaining a bias voltage value extracted by the dithering signal extraction unit and in which a variation in a component derived from the dithering signal falls within a predetermined range;
Have

Dithering to at least one of the first phase modulator (10a), the second phase modulator (11a), the third phase modulator (10b), and the fourth phase modulator (11b) Applying a signal;

The detection unit detects the output from the multiplexing unit (4) while sweeping the bias voltage applied to the sub Mach-Zehnder waveguide that outputs the optical signal to the phase modulator to which the dithering signal is applied. When,

Detection information detected by the detection unit is input to the control unit;

A step in which a dithering signal extraction unit of the control unit extracts a component derived from the dithering signal based on the input detection information and stores the amplitude intensity in a storage unit together with a value of a bias voltage;

An optimum bias voltage value acquisition means of the control unit reads out the amplitude intensity stored in the storage unit after the sweep is completed, and reads out the value of the bias voltage when the amplitude intensity is the lowest;

Including

Thus, the first sub Mach-Zehnder waveguide (8a), the second sub-Mach-Zehnder waveguide (9a), the third sub-Mach-Zehnder waveguide (8b), and the fourth sub-Mach-Zehnder waveguide An optimum value of the bias voltage applied to any one of the waveguides (9b) can be obtained.

Bias adjustment method for light modulation system.
A first branch (1);
A first waveguide (2) branched from the first branch (1);
A first modulator (3) having a Mach-Zehnder interferometer provided in the first waveguide (2);
The waveguide is branched from the first branch (1) and is different from the first waveguide (2), and is combined with the first waveguide (2) at the multiplexing point (4). A second waveguide (5);
A light modulation system comprising a second modulator (6) having a Mach-Zehnder interferometer provided in the second waveguide (5),

The first modulator (3) is:
A first main Mach-Zehnder waveguide (7a);
A first sub Mach-Zehnder waveguide (8a) provided on one arm of the first main Mach-Zehnder waveguide (7a);
A second sub Mach-Zehnder waveguide (9a) provided on the remaining arm of the first main Mach-Zehnder waveguide (7a);
A seventh phase modulator for performing phase modulation on the light input to the first sub Mach-Zehnder waveguide (8a);
An eighth phase modulator for performing phase modulation on the light input to the second sub Mach-Zehnder waveguide (9a);
Comprising

The second modulator (6)
A second main Mach-Zehnder waveguide (7b);
A third sub Mach-Zehnder waveguide (8b) provided on one arm of the second main Mach-Zehnder waveguide (7b);
A fourth sub Mach-Zehnder waveguide (9b) provided on the remaining arm of the second main Mach-Zehnder waveguide (7b);
A ninth phase modulator for performing phase modulation on the light input to the third sub Mach-Zehnder waveguide (8b);
A tenth phase modulator for performing phase modulation on the light input to the fourth sub Mach-Zehnder waveguide (9b);
Comprising

The light modulation system comprises:
The first sub-Mach-Zehnder waveguide (8a), the second sub-Mach-Zehnder waveguide (9a), the third sub-Mach-Zehnder waveguide (8b), the fourth sub-Mach-Zehnder waveguide (9b) ), A power supply system for supplying a voltage to be applied to the seventh phase modulator, the eighth phase modulator, the ninth phase modulator, and the tenth phase modulator;
A control unit having a computer for changing a voltage output from the power supply system by giving a control signal to the power supply system;
A detector for detecting an output from the multiplexing point (4);
Have

The controller is
Command the power supply system to apply a dithering signal to at least one of the seventh phase modulator, the eighth phase modulator, the ninth phase modulator, and the tenth phase modulator. A dithering signal indicating means for outputting;
A bias sweep command means for commanding a power supply system to sweep a bias voltage applied to a sub-Mach-Zehnder waveguide existing in a stage preceding the phase modulator to which the dithering signal is applied;
A dithering signal extraction unit for extracting a component derived from the dithering signal from detection information detected by the detection unit;
An optimum bias voltage value obtaining means for obtaining a bias voltage value extracted by the dithering signal extraction unit and in which a variation in a component derived from the dithering signal falls within a predetermined range;
Have
While storing the bias voltage of the sub-Mach-Zehnder waveguide existing in the preceding stage of the phase modulator to which the dithering signal is applied, the amplitude intensity of the component derived from the dithering signal is stored in the storage unit together with the value of the bias voltage Then, after the sweep is completed, the amplitude intensity stored in the storage unit is read, and the value of the bias voltage when the amplitude intensity is the lowest is read,

Thus, the first sub Mach-Zehnder waveguide (8a), the second sub-Mach-Zehnder waveguide (9a), the third sub-Mach-Zehnder waveguide (8b), and the fourth sub-Mach-Zehnder waveguide An optimum value of the bias voltage applied to any one of the waveguides (9b) can be obtained.

Light modulation system.

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