JP2014010188A - Optical intensity modulation apparatus using mach-zehnder optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical intensity modulation apparatus that prepares no table of operating conditions in advance, does not know the input wavelength during operation, and yet can adapt to operating conditions even when operation conditions vary over time.SOLUTION: An optical intensity modulation apparatus that uses a Mach-Zehnder modulator in which two optical waveguides have a characteristic that the phase difference between the two optical waveguides varies non-linearly relative to applied voltage comprises: the Mach-Zehnder modulator with one or both of the two optical waveguides including modulation electrodes to which a data signal over which a modulation electrode bias voltage is superposed is applied; and an automatic half-wavelength voltage control circuit consisting of a minute modulation signal superposing circuit that superposes a minute modulation signal over either one of the electrodes, a light branching circuit that branches part of output light of the modulator over which the minute modulation signal is superposed, a minute modulation signal detecting circuit that detects a minute modulation signal component from the branched output light, and a modulation electrode bias voltage control circuit that so controls the modulation electrode bias voltage as to minimize the detected minute modulation signal.

Description

  The present invention relates to a light intensity modulation device using a Mach-Zehnder modulator used in optical fiber communication.

  In optical fiber communication, a Mach-Zehnder (MZ) type optical modulator is used as an external modulator for code-modulating light. Conventionally, a Mach-Zehnder modulator using lithium niobate for an optical waveguide has been used, but recently, the use of a Mach-Zehnder modulator using a semiconductor material for the optical waveguide has been studied. The semiconductor Mach-Zehnder modulator has a feature that it is smaller than a modulator using lithium niobate, and is a promising technology for realizing miniaturization of an optical transmission device. This is realized by the configuration of Non-Patent Document 1 and Non-Patent Document 2.

  A conventional light intensity modulation device using a semiconductor Mach-Zehnder modulator will be described with reference to Patent Document 1.

  FIG. 1 is a schematic diagram of a conventional light intensity modulation device using a semiconductor Mach-Zehnder modulator, and is a diagram in which FIG. 3 of Patent Document 1 is rewritten. First, the configuration of the semiconductor Mach-Zehnder modulator 10 will be described. Of the two optical waveguides constituting the interferometer of the Mach-Zehnder optical modulator, the modulation electrode 11 to which the high-speed data signal voltage biased with the modulation electrode bias voltage is applied is applied to the optical waveguide constituting the first arm 1. In order to adjust the phase difference of the interferometer, the optical waveguide constituting the second arm 2 is provided with a phase difference adjustment electrode 12 to which a phase difference adjustment voltage is applied.

  The semiconductor Mach-Zehnder modulator has a characteristic that the phase difference of the light of the interferometer of the Mach-Zehnder type optical modulator increases nonlinearly with respect to the voltage applied to the electrode. Therefore, the characteristic peculiar to the semiconductor Mach-Zehnder modulator due to this characteristic will be described.

  2 shows a voltage of an electrode on an arm of a semiconductor Mach-Zehnder modulator (in the semiconductor Mach-Zehnder modulator, a reverse bias voltage is applied to a semiconductor PN junction in order to apply a voltage. Although a negative voltage may be applied depending on the direction, it causes confusion with respect to the magnitude of the voltage. Therefore, in the explanation of the present invention, the absolute value of the voltage is used to explain between the arms). It is a characteristic view of the phase difference (phase difference between arms) of light. Here, the horizontal axis represents the voltage of the modulation electrode of the first arm 1 or the voltage of the phase difference adjusting electrode of the second arm 2, and the vertical axis represents the light propagating through the first arm 1 and the second arm 2. This is the phase difference from the propagating light.

  When the voltage of the two electrodes of the semiconductor Mach-Zehnder modulator in FIG. 1 is 0, if the first arm 1 and the second arm 2 are configured to be equal, the light propagating through the first arm 1 is The light having the same phase as the light propagating through the second arm 2 has a phase difference of zero. Here, when the voltage of the first arm 1 is increased from 0, the refractive index of light propagating through the first arm 1 changes. As a result, the light propagating through the first arm 1 has a phase difference with respect to the light propagating through the second arm 2. The phase difference increases as the voltage of the first arm 1 increases. FIG. 2 shows this state.

  The change in output light intensity at this time is shown in FIG. It is the characteristic view which showed the relationship between the voltage of the electrode on the arm of a semiconductor Mach-Zehnder modulator, and optical output. Called extinction characteristics. The horizontal axis represents the voltage of the modulation electrode of the first arm 1 or the voltage of the phase difference adjustment electrode of the second arm 2, and the vertical axis represents the output light intensity of the optical modulator constituting the Mach-Zehnder optical interferometer. When the voltage of the two electrodes of the semiconductor Mach-Zehnder modulator is 0, the phase difference is 0. Therefore, when both are combined at the output end of the Mach-Zehnder interferometer, modulated light having the same intensity as the input light is generated. The output light intensity is maximized, and the on-level of the output light when the light modulation operation is performed is output.

  When the voltage of the first arm 1 is increased from 0, the output light intensity decreases, and the phase difference between the light propagating through the first arm 1 and the light propagating through the second arm 2 at a certain input voltage. Becomes exactly π, and when both are combined at the output end of the Mach-Zehnder interferometer, they cancel each other and are extinguished. In this way, the off level of the output light is output. Here, the difference between the voltage at which the output light is turned on and the voltage at which the output light is turned off is referred to as a half-wave voltage Vπ.

  When the voltage is further increased, the phase difference increases from π to increase the output light intensity, and when the phase difference is exactly 2π, the output light intensity becomes the maximum again and the on level is output. FIG. 3 also shows this state. Since the output light intensity changes in accordance with the value of the input voltage in this way, the Mach-Zehnder modulator can change (modulate) the output light intensity by changing the input voltage (input electric signal). it can.

  Similarly, the output light intensity can be changed by changing the voltage for the second arm 2 as well. It is also possible to apply a voltage to the first arm 1 and the second arm 2 simultaneously. Note that when the voltage is applied to the first arm 1 and there is a phase difference, if the voltage of the second arm 2 is increased from 0 to approach the voltage of the first arm 1, the phase difference is reduced. There is a need.

  A Mach-Zehnder modulator with an optical waveguide made of lithium niobate uses the optical effect of the Pockels effect, so the phase difference changes linearly with respect to the input voltage, and the relationship between the voltage and the phase difference is represented by a straight line. The However, in the semiconductor Mach-Zehnder modulator, as shown in FIG. 2, the phase difference of light propagating between the arms increases non-linearly with respect to the input voltage, and the gradient increases as the input voltage increases. Become. That is, it is expressed by a curve that is convex downward and monotonously increases.

  This non-linear characteristic is, for example, an electro-optic effect due to a Franz-Keldish effect that occurs in an optical waveguide in which the core layer of the optical waveguide is made of a semiconductor bulk material and the bandcap wavelength is set slightly shorter than the signal wavelength, or This appears when the electro-optic effect due to the quantum confined Stark effect generated in the optical waveguide in which the core layer of the optical waveguide is constituted by a multiple quantum well layer (MQW). Since the semiconductor Mach-Zehnder modulator uses the Franz Keldisch effect or the quantum confined Stark effect in addition to the Pockels effect as a cause of the electro-optic effect that causes a change in refractive index when a voltage is applied, nonlinear characteristics occur. Due to the non-linear characteristic with respect to the input voltage, the extinction curve in FIG. 3 shows that the intervals between the peaks and valleys are gradually getting closer. Further, due to this nonlinear phase difference characteristic, the half-wave voltage Vπ decreases as the input voltage increases.

  Further, as can be seen from the characteristics when the wavelength of the input light is changed from 1530 nm to 1560 nm in FIG. 2, there is an input light wavelength dependency, and the slope becomes gentle as the wavelength of the input light is changed from a short wave to a long wave. It has become. Further, as can be seen from the characteristics when the wavelength of the input light is changed from 1530 nm to 1560 nm in FIG. 3, the interval between the peaks and the valleys becomes wider as the wavelength of the input light is changed from a short wave to a long wave. That is, the half-wave voltage Vπ increases as the wavelength of the input light changes from a short wave to a long wave.

  On the other hand, in the Mach-Zehnder modulator using lithium niobate, since the phase difference changes linearly with respect to the input voltage, the half-wave voltage Vπ does not depend on the voltage. Further, there is no wavelength dependency of the half-wave voltage Vπ.

  As described above, the semiconductor Mach-Zehnder modulator has voltage dependency and input light wavelength dependency which are not found in the Mach-Zehnder modulator using lithium niobate.

  The principle of performing ON / OFF modulation of light with a high-speed data signal using a semiconductor Mach-Zehnder modulator will be described with reference to FIG. The extinction characteristic is shown in the lower left of the figure. In order to avoid applying a forward bias, a bias voltage Vb larger than half of the amplitude of the high-speed data signal is applied to the modulation electrode, and a high-speed data signal having the amplitude equal to the half-wave voltage Vπ is superimposed. The voltage Va of the phase difference adjusting electrode is set so that the level is turned off and the high level is turned on. That is, the modulation electrode bias voltage is set to Vb, and the phase difference is adjusted so that the output light intensity is in the center between the voltage at which the output light intensity is turned on and the voltage at which it is turned off in the extinction characteristic in the state where the high-speed data signal is not superimposed (during non-modulation). The voltage Va of the electrode is adjusted. When the phase difference adjusting electrode pressure Va is controlled in this way, the high-speed data signal voltage changes centering on the bias voltage of the modulation electrode. The output turns ON / OFF. Since the light intensity changes around the unmodulated position, this point is called an operating point. In particular, an operating point when ON / OFF modulation is performed is called a QUAD point.

  In this way, ON / OFF modulation of light by a high-speed data signal is realized. It is also called NRZ (Non Return To Zero) modulation.

  In general, an optical modulator is required to output an optical signal having a predetermined amplitude. However, when the half-wave voltage Vπ becomes larger or smaller than the high-speed data signal voltage, there is a problem that the amplitude of the optical signal that is modulated and output deviates from a predetermined value according to the principle shown in FIG. Arise.

  As described above, in order to perform ON / OFF modulation, it is necessary to make the amplitude of the high-speed data signal equal to the half-wave voltage Vπ.

  However, as described above, in the semiconductor modulator, when the input wavelength changes, the half-wave voltage Vπ changes. In addition, the bias voltage Vb needs to be set to the QUAD point of the extinction characteristic, but when the input wavelength changes, the phase difference changes and the voltage at the QUAD point also changes. Therefore, when the input wavelength changes, the high-level and low-level voltages of the high-speed data signal are shifted from the positions of the on-voltage and off-voltage, respectively. For the above reasons, when the input wavelength changes, the output optical signal waveform changes under the same driving conditions (the same bias voltage and the same modulation amplitude).

  With respect to this problem, Patent Document 1 solves the problem by changing and setting the modulation electrode bias voltage Vb and the phase difference adjustment voltage Va according to the wavelength of the input light, and with the same modulation amplitude regardless of the wavelength. The same output signal waveform is obtained.

  Specifically, in Patent Document 1, the light intensity modulation device controls a modulation electrode bias voltage Vb supplied from a DC power supply according to the wavelength of input light input to the optical modulator, as shown in FIG. A bias voltage control circuit (first arm control circuit) 20 and a phase difference adjustment voltage control circuit (second arm control circuit) 30 for controlling the phase difference adjustment voltage Va supplied from the DC power supply according to the wavelength of the input light are provided. Configured.

  In the bias voltage control circuit (first arm control circuit) 20, the bias voltage control unit 23 refers to the reference table including the correspondence table 23 a between the wavelength of the input light and the bias voltage Vb, and corresponds to the wavelength of the input light. The optimum bias voltage Vb determined in advance is read out, and the DC power supply 22 is controlled so as to obtain the read out bias voltage. The phase difference adjustment voltage control unit 32 in the phase difference adjustment voltage control circuit (second arm control circuit) 30 refers to a reference table including a correspondence table 32a between the wavelength of the input light and the phase difference adjustment voltage Va. Then, the optimum phase difference adjustment voltage Va determined in advance is read, and the DC power supply 31 is controlled so as to become the read phase difference adjustment voltage.

  However, this method has a problem that a table needs to be created in advance for all wavelengths used as input light.

  In Patent Document 1, an electronic circuit that performs a calculation of a predetermined rule is provided, and the above-described value may be calculated based on the wavelength of input light by the electronic circuit. In this case, it is not necessary to create a table for all wavelengths, but there is another problem that it is necessary to measure parameters that are the basis of the calculation rule in advance. In order to obtain this data, there is no change in the need for measurement in advance.

  Furthermore, in the method of Patent Document 1, it is necessary to know the wavelength of the input light by some means during operation. If you don't know the wavelength, you don't know what voltage to set.

  Furthermore, since the phase difference and half-wave voltage Vπ of the semiconductor Mach-Zehnder modulator change not only with the input wavelength but also with the input optical power and element temperature, it is necessary to create a table under the same conditions as the operating state. If the operating state such as optical power or element temperature is changed, the table must be taken again.

  In addition, long-term operation for several years may cause changes over time such as an increase in series resistance and an increase in leakage current in semiconductor devices and drive circuits. When the half-wave voltage Vπ changes due to changes over time, or when the drive amplitude of the drive circuit changes due to changes over time, the drive amplitude becomes excessive or insufficient. Therefore, the modulation characteristic is deteriorated.

  In general, since the high-speed data signal voltage has a larger loss than the DC voltage, if a high-speed data signal applied to the modulator is generated with an amplitude equal to the voltage obtained from the DC characteristics, the connection cable, connection RF waveguide, connection Due to the high frequency loss of the wire or the like, the voltage actually applied to the element becomes insufficient from the generated voltage. Therefore, the loss must be measured or estimated, and the amplitude of the high-speed data signal must be larger than the half-wave voltage Vπ obtained from the DC characteristics. However, since the loss cannot always be compensated accurately, It is not known whether the amplitude of the high-speed data signal is applied at a desired voltage in the actual device. That is, it is difficult to set the amplitude of the high-speed data signal from the DC extinction characteristic measurement value.

  In Non-Patent Document 3, an approximate expression is obtained by fitting the extinction characteristic on the assumption that the phase difference characteristic is expressed by a quadratic expression of voltage. In addition, the bias voltage at the time of modulation is calculated by estimating that the high-frequency loss is 1/3. This realizes a modulation operation that does not require manual adjustment over the entire wavelength region. In this method, the calculation method is specifically shown, and the number of parameters to be accumulated is small, but the same problem that the extinction characteristic needs to be measured in advance under use conditions in order to obtain the calculation formula. have.

  Patent Document 2 discloses an apparatus and method for automatically controlling an operating point to a QUAD point, assuming a Mach-Zehnder modulator using lithium niobate. If this method is used, fluctuations in the operating point due to not only the wavelength but also other influences can be corrected and controlled. However, since the change in the half-wave voltage Vπ due to the wavelength cannot be corrected, it is necessary to set the modulation electrode bias voltage using the table and the relational expression as described above. Further, it cannot adapt to changes in the half-wave voltage Vπ that are not represented by a table or a relational expression such as a temperature change or deterioration with time, and the modulation characteristics deteriorate.

  Patent Document 3 discloses an apparatus that controls the amplitude of a high-speed data signal so as to be equal to the amplitude of the high-speed data signal when the half-wave voltage Vπ changes. However, according to this method, it is necessary to use a drive device that can change the drive amplitude of the high-speed data signal by the control signal, and manufacturing such a drive device is costly and complicated. Will be invited.

Japanese Patent No. 4272585 Japanese Patent No. 2642499 JP 7-46190 A

I. Betty, MG Boudreau, R. Longone, RA Griffin, L. Laugley, A. Maestri, A. Pujol, B. Pugh, `` Zero Chirp 10 Gb / s MQW InP Mach-Zehnder Transmitter with Full-Band Tunability '', OFC2007, OWH6 (2007). Eiichi Yamada, Akira Ohki, Nobuhiro Kikuchi, Yasuo Shibata, Takako Yasui, Kei Watanabe, Hiroyuki Ishii, Ryuzo Iga, and Hiromi Oohashi, `` Full C-band 40-Gbit / s DPSK Tunable Transmitter Module Developed by Hybrid Integration of Tunable Laser and InP npin Mach-Zehnder Modulator '' OFC2010, OFU4 (2010). Mitsuteru Ishikawa, Ken Tsuzuki, Nobuhiro Kikuchi, Kazuo Kasaya, Yasuo Shibata, Hiroyuki Ishii, Hiromi Oohashi, and Hiroshi Yasaka, "10-Gb / s Full C-band Operation of InP Mach-Zehnder Modulator Co-packaged with Tunable Laser Array under Constant Modulation Voltage ", OECC2008 WeR-3 (2008).

  The optical modulator control devices of Patent Document 1 and Non-Patent Document 1 have the problem that it is necessary to create a table or an arithmetic expression in advance under operating conditions, and it is necessary to know the wavelength of input light during operation. There is a problem that the modulation characteristic changes when the operating condition changes due to a change with time.

  Combining Patent Document 2 and Patent Document 3 can compensate for a change in operating point and a change in half-wave voltage Vπ, but there is a problem that it is necessary to change the driving amplitude of a high-speed data signal.

  The present invention has been made in view of such problems, and the object of the present invention is to know the input wavelength during operation without creating an operation condition table in advance while keeping the drive amplitude of the high-speed data signal constant. There is also a need to provide a light intensity modulation device that adapts to the operating state even when the operating conditions change due to changes over time and does not change the modulation characteristics.

  In order to achieve such an object, the present invention provides a Mach-Zehnder having a characteristic that a phase difference between two optical waveguides constituting a Mach-Zehnder interferometer changes nonlinearly with respect to an applied voltage. A light intensity modulation device for turning on / off an optical signal using a modulator, wherein one or both of the two optical waveguides include a modulation electrode to which a data signal biased with a modulation electrode bias voltage is applied. The Mach-Zehnder modulator, a minute modulation signal superimposing circuit that superimposes a minute modulation signal on any one of the electrodes of the two optical waveguides, and output light of the modulator on which the minute modulation signal is superimposed. An optical branch circuit that branches a part, a minute modulation signal detection circuit that detects a minute modulation signal component from the branched output light, and a detected minute modulation signal to be minimized Serial characterized by comprising an automatic half-wave voltage control circuit composed of a modulation electrode bias voltage control circuit for controlling the modulating electrode bias voltage.

  The invention according to claim 2 is an optical intensity modulation device using the Mach-Zehnder optical modulator according to claim 1, wherein the Mach-Zehnder modulator modulates one of the two optical waveguides. A modulation electrode to which a data signal voltage biased with an electrode bias voltage is applied, and a phase difference adjustment electrode controlled by an automatic operating point control circuit in the other of the two optical waveguides. Features.

  A third aspect of the present invention is a light intensity modulation device using the Mach-Zehnder optical modulator according to the first aspect, wherein the Mach-Zehnder modulator includes a first optical waveguide out of the two optical waveguides. A first modulation electrode to which a first data signal voltage biased with a first modulation electrode bias voltage is applied, the second optical waveguide being biased with a second modulation electrode bias voltage, A second modulation electrode to which a data signal voltage having a phase opposite to that of the first data signal voltage is applied, and at least one of the two optical waveguides has a phase difference of an interferometer separately from the modulation electrode. It is a differential drive type Mach-Zehnder modulator including a phase difference adjusting electrode to be adjusted.

  According to a fourth aspect of the present invention, an optical signal is turned on / off using a Mach-Zehnder modulator having a characteristic in which a phase difference between two optical waveguides constituting a Mach-Zehnder interferometer changes nonlinearly with respect to an applied voltage. An intensity modulation device, wherein the modulation electrode is applied to one optical waveguide of the two optical waveguides, and the ground electrode is applied to the other optical waveguide so as to apply a data signal between the modulation electrode and the ground electrode. A phase difference controlled by an operating point control circuit in one or both of the two optical waveguides, including a modulation electrode bias voltage application electrode connected to a common bottom surface of the two optical waveguides A single drive push-pull configuration type Mach-Zehnder modulator including an adjustment electrode and a micro modulation signal for superimposing a minute modulation signal on any one of the electrodes of the two optical waveguides. A modulation signal superimposing circuit, an optical branching circuit for branching a part of the output light of the modulator on which the micromodulation signal is superimposed, a micromodulation signal detecting circuit for detecting a micromodulation signal component from the branched output light, And an automatic half-wave voltage control circuit including a modulation electrode bias voltage control circuit that controls the modulation electrode bias voltage so that the detected minute modulation signal is minimized.

  According to a fifth aspect of the present invention, there is provided a light intensity modulation device using the Mach-Zehnder optical modulator according to any one of the first to fourth aspects, wherein the light further includes an automatic operating point control circuit. In the intensity modulation device, the automatic operating point control circuit performs half-wave voltage control and operating point control simultaneously using different modulation frequencies, or alternately using the same modulation frequency. To do.

  As described above, according to the present invention, since the half-wave voltage Vπ of the semiconductor Mach-Zehnder modulator is controlled according to the operating state, it is not necessary to prepare an operating condition table in advance and to know the input wavelength. In addition, even if there is a change over time, it has the effect that it operates adaptively and the modulation characteristics do not change, and it is not necessary to change the drive amplitude of the high-speed data signal at a constant level, so it is low cost with a simple configuration A high-frequency drive circuit can be used.

It is a schematic diagram of a conventional light intensity modulation device using a semiconductor Mach-Zehnder modulator. It is a figure which shows the relationship of the phase difference of the light between arms with respect to an electrode voltage. It is a figure which shows the relationship of the output light intensity with respect to an electrode voltage. It is a figure explaining ON / OFF modulation of light by a high-speed data signal. 1 is a block diagram of a light intensity modulation device according to a first embodiment of the present invention. (A) is a figure explaining the principle of automatic half wavelength voltage control in case half wavelength voltage V (pi) is equal to a data signal amplitude, (b) is when half wavelength voltage V (pi) is larger than data signal amplitude. It is a figure explaining the principle of automatic half wavelength voltage control, (c) is a figure explaining the principle of automatic half wavelength voltage control in case half wavelength voltage V (pi) is smaller than a data signal amplitude. It is a figure which shows the bias voltage dependence of half wavelength voltage V (pi). It is a block diagram of the light intensity modulation apparatus which is the 2nd Embodiment of this invention. It is a block diagram of the light intensity modulation apparatus which is the 3rd Embodiment of this invention. It is a block diagram of the light intensity modulation apparatus which is the 4th Embodiment of this invention. It is a block diagram of the light intensity modulation apparatus which is the 5th Embodiment of this invention. It is a block diagram of the light intensity modulation apparatus which is the 6th Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 5 is a block diagram of the light intensity modulation apparatus according to the first embodiment of the present invention.

  Light that performs ON / OFF modulation using a Mach-Zehnder modulator 10 having a characteristic that the phase difference between the two optical waveguides of the optical waveguide 3 constituting the Mach-Zehnder optical interferometer changes nonlinearly with respect to the voltage applied to the optical waveguide. This is a configuration for explaining the operating principle of the light intensity modulation device that is an intensity modulation device and includes an automatic half wavelength voltage control circuit 50 that automatically controls the half wavelength voltage Vπ.

  First, a configuration for performing ON / OFF modulation using the Mach-Zehnder modulator 10 will be described.

  In the Mach-Zehnder modulator 10, an optical waveguide 3 constituting a Mach-Zehnder type optical interferometer is formed on a substrate by the first arm 1 and the second arm 2, and the Mach-Zehnder modulator 10 is configured. The input light input to the input end of the Mach-Zehnder modulator 10 branches into two in the optical waveguide 3 constituting the Mach-Zehnder type optical interferometer, and the optical waveguide and the second arm constituting the first arm 1. 2 is propagated through the optical waveguide, and these branched lights are combined and output from the output terminal of the Mach-Zehnder modulator 10 as output light. In the optical waveguide constituting the first arm 1 and the optical waveguide constituting the second arm 2, a plurality of electrodes for applying a voltage to the upper surface of the optical waveguide are formed. The bottom surface or substrate of the optical waveguide is connected to the ground electrode, and a voltage is applied between the top electrode and the substrate. As a result, an electric field is applied to the optical waveguide sandwiched between the upper electrode and the substrate, and a phase difference is generated due to the electro-optic effect. The light intensity output from the Mach-Zehnder modulator 10 changes according to the generated phase difference, and the light modulation operation is realized. It is assumed that the optical waveguide structures of the first arm 1 and the second arm 2 are equal, and the Mach-Zehnder modulator has a characteristic that the generated phase difference changes nonlinearly with respect to the applied voltage.

  In order to perform ON / OFF modulation with a high-speed data signal, the high-speed data signal generated from the high-speed data signal source 41 and amplified to a constant amplitude by the wide-band amplifier 42 is input to the RF terminal of the bias T circuit 43, and the modulation electrode The bias voltage is biased, output from the output terminal, and applied to the modulation electrode 11 formed on the first arm 1. In order to perform optimal modulation, the amplitude of the high-speed data signal needs to be equal to the half-wave voltage Vπ. Although not shown here, it is desirable that the signal applied to the modulation electrode 11 is terminated inside the modulator or outside via an output terminal.

  The first phase difference adjusting electrode 12-1 and the second phase difference adjusting electrode 12-2 formed on either or both of the first arm 1 and the second arm 2 have an operating point as a QUAD point. In order to adjust the phase difference of the interferometer for control, the operating point control circuit 60 adjusts the phase difference adjusting electrode so that the operating point is adjusted to be the center of the optical amplitude at the center of the driving amplitude of the high-speed data signal. A voltage is applied.

  When each of the two optical waveguides is provided with a phase difference adjusting electrode, either or both of them are controlled. In other words, one phase difference adjustment electrode may be grounded and only one phase difference adjustment electrode may be controlled. If both are controlled, the maximum adjustment range of each phase difference adjustment electrode may be halved. FIG. 5 shows a configuration in which both of the two optical waveguides are provided with phase difference adjusting electrodes and both of the phase difference adjusting electrodes are controlled.

  By configuring as described above, ON / OFF modulation using a high-speed data signal is realized.

  Next, the automatic half-wave voltage control circuit 50 that is a feature of the present invention will be described. The automatic half-wave voltage control circuit 50 controls the half-wave voltage Vπ so that the amplitude of the high-speed data signal is equal to the half-wave voltage Vπ. The automatic half-wave voltage control circuit 50 includes a dither signal (oscillation signal, minute modulation signal) detection circuit 51, a modulation electrode bias voltage control circuit 52, a dither signal superposition circuit 53, and an addition circuit 54.

  The dither signal generated from the dither signal superimposing circuit 53 is superimposed on the modulation electrode bias voltage generated from the modulation electrode bias voltage control circuit 52 by the adding circuit 54, input to the DC terminal of the bias T circuit 43, and output from the output terminal. And applied to the modulation electrode 11. Here, the dither signal is superimposed on the modulation electrode 11, but any one of the two arms of the optical waveguide 3 constituting the Mach-Zehnder type optical interferometer may be superimposed. The phase difference adjusting electrode 12-1, the second phase difference adjusting electrode 12-2, and the like may be superimposed on each other.

  The optical output signal on which the dither signal is superimposed is partly branched by the optical branch circuit 14 optically coupled to the output terminal of the Mach-Zehnder modulator 10 and input to the dither signal detection circuit 51. The dither signal detection circuit 51 includes a photodetector 51-1 (which includes a photodiode and an amplifier) and a phase comparison circuit 51-2 (also referred to as a synchronous detection circuit) (which includes a multiplication circuit and a low-pass filter). Are). The optical signal is detected by a photodiode built in the photodetector 51-1 of the dither signal detection circuit 51, and the dither signal is amplified by a built-in amplifier. The amplified dither signal is multiplied by the dither signal generated from the dither signal superimposing circuit 53 by the multiplication circuit built in the phase comparison circuit 51-2, and the high-frequency component by the low-pass filter built in the phase comparison circuit 51-2. Is removed, converted into a voltage corresponding to the phase difference between the two dither signals, and output, thereby realizing phase comparison. The modulation electrode bias voltage control circuit 52 controls the bias voltage of the modulation electrode 11 so that the output signal of the phase comparison circuit 51-2 based on the phase comparison result is minimized. In this way, a feedback circuit is constituted by the dither signal superimposing circuit 53, the optical modulator 10, the optical branching circuit 14, the dither signal detecting circuit 51, and the modulation electrode bias voltage control circuit 52, thereby realizing half-wave voltage control.

  In FIG. 5, the same voltage as the modulation electrode bias voltage is applied to the electrode 13 provided in the second arm 2 so that the operating point does not shift due to the increase or decrease of the modulation electrode bias voltage to the modulation electrode 11. doing.

  As explained in the conventional example, the extinction ratio (on / off ratio) is insufficient if the drive amplitude of the high-speed data signal is less than the half-wave voltage Vπ during modulation, and the signal waveform is reversed when the drive amplitude exceeds the half-wave voltage Vπ. The extinction ratio deteriorates. Therefore, it is necessary to control the drive amplitude to be equal to the half-wave voltage Vπ by adjusting the drive amplitude or changing the half-wave voltage Vπ, but the present invention always makes the drive amplitude constant, It is characterized by automatically controlling the half-wave voltage Vπ.

  FIG. 6 is a diagram for explaining the principle of automatic half-wave voltage control. 6A shows a case where the half-wave voltage Vπ is equal to the data signal amplitude, FIG. 6B shows a case where the half-wave voltage Vπ is larger than the data signal amplitude, and FIG. The smaller case is shown. Separately, it is assumed that the operating point is always controlled to the QUAD point so that the center of the driving amplitude of the high-speed data signal becomes the center of the optical amplitude by the method described above. The operating point is indicated by ●.

A high-speed data signal is applied to the Mach-Zehnder modulator, and modulated signal light is generated at the optical output. Further, the extinction characteristic of the Mach-Zehnder modulator is changed by the dither signal applied to one of the electrodes (modulation electrode 11 in FIG. 5) from the dither signal (micromodulation signal) superimposing circuit 53. In FIG. 6, the change of the extinction characteristic due to the dither signal is shown as three extinction characteristics as the relationship between the voltage and the output light intensity, and the dither signal has the maximum amplitude to negative, zero, and the maximum positive amplitude, respectively. This is shown schematically. As a result, the dither signal is superimposed on the modulation signal at the optical output of the Mach-Zehnder modulator. Therefore, a part of the optical output is branched and converted into an electrical signal using a photodiode or the like, and a dither signal is detected. Here are placed the modulation frequency of the dither signal and f _0.

When the driving amplitude of the high-speed data signal is optimal and equal to the half-wave voltage Vπ, no f ₀ component is generated in the optical output as shown in FIG. When the drive amplitude is too small, an f ₀ component having a phase opposite to that of the dither signal is generated in the optical output as shown in FIG. Conversely, when the drive amplitude is too large, an f ₀ component in phase with the dither signal is generated in the optical signal as shown in FIG. Therefore, by comparing the phase of the dither signal of frequency f ₀ from the dither signal superimposing circuit and the optical output dither signal (synchronous detection), a voltage corresponding to the magnitude of the drive amplitude is generated. In the electrode bias control circuit, when the synchronous detection output voltage is negative, the half-wave voltage Vπ is controlled to be small, and when it is positive, the half-wave voltage Vπ is controlled to be large. When Vπ is optimal, the synchronous detection output is zero, and the drive amplitude of the high-speed data signal is equal to the half-wave voltage Vπ. More specifically, using the relationship between the modulation electrode bias voltage and half-wave voltage Vπ, which will be described later, when the synchronous detection output voltage is negative, the modulation electrode bias voltage increases so that the half-wave voltage Vπ decreases. If positive, the modulation electrode bias voltage is controlled to be small so that the half-wave voltage Vπ is large.

  Here, the transmission characteristic in which the transmitted light increases when the reverse bias voltage increases in FIG. 6 (upward-right characteristic in the figure) is described. However, the transmission characteristic that the transmitted light decreases when the reverse bias voltage increases (downward-right characteristic). ), The direction (positive / negative) of the detected dither signal is reversed. In this case, the direction of control is reversed, but the synchronous detection output component is still controlled to be zero.

  In this description, the dither signal is superimposed on the same arm as the modulation signal. However, the dither signal may be superimposed on the opposite arm. In this case, the detected dither signal direction (positive / negative) is reversed, but synchronous detection output is performed. There is no change in controlling the component to be zero.

  The principle of changing the half-wave voltage Vπ will be described. For example, in an InP semiconductor Mach-Zehnder modulator, the optical waveguide is composed of an InP-based semiconductor optical waveguide, and the core layer of the optical waveguide uses a semiconductor whose band gap wavelength is shorter than the wavelength of the input light. Phase modulation based on the refractive index change caused by the effect, or phase modulation based on the refractive index change caused by the quantum confined Stark effect using multiple quantum wells in the core layer of the optical waveguide is used. Therefore, the phase difference of light increases non-linearly with respect to the applied voltage, and approximately the sum of a term linear to the applied voltage and a term proportional to the square. Thus, the half-wave voltage Vπ of the semiconductor Mach-Zehnder modulator depends on the bias voltage, and the half-wave voltage Vπ can be changed by controlling the bias voltage of the modulation electrode.

  FIG. 7 schematically shows the bias dependence of the half-wave voltage Vπ. The figure is drawn with the signal wavelength as a parameter. It is the figure represented based on the characteristic view of the phase difference of the light between the arms shown in FIG. As can be seen from the figure, the half-wave voltage Vπ decreases monotonously with respect to the voltage. Therefore, in order to reduce the half-wave voltage Vπ, the bias voltage of the modulation electrode 11 is increased and the half-wave voltage Vπ is increased. Is sufficient to reduce the bias voltage of the modulation electrode 11. In this way, by controlling the bias voltage in accordance with the synchronous detection output, the half-wave voltage Vπ can be changed to make the high-speed data signal amplitude equal to the half-wave voltage Vπ.

  The essence of the present invention is that the half-wave voltage Vπ can be changed by changing the modulation electrode bias voltage as a result of the characteristic that the phase difference changes nonlinearly with respect to the applied voltage. FIG. 7 shows a characteristic diagram using the wavelength as a parameter. However, the relationship between the voltage and the phase difference does not need to change depending on the wavelength, and the present invention can be implemented even if the half-wave voltage Vπ does not change depending on the wavelength.

  If the optical waveguide is made of a material containing a component whose phase change due to voltage application changes nonlinearly, it is necessary to control the half-wave voltage Vπ, and conversely, the applied voltage is controlled. Therefore, it is necessary to pay attention to the fact that the half-wave voltage Vπ can be made an optimum value. Since the phase change is proportional to the refractive index change, a material including a component whose phase change varies nonlinearly includes, for example, a material including a component whose refractive index change is proportional to the square of the applied electric field, or a phase change caused by saturation. For example, a material containing a component proportional to the square root of the applied voltage. In this way, in a material in which the phase change due to voltage application changes nonlinearly, the half-wave voltage Vπ changes depending on the voltage, so by controlling the bias voltage, the high-speed data signal amplitude and the half-wave voltage Vπ are equal. Can be controlled.

  Based on the above operation principle, ON / OFF modulation is performed using the Mach-Zehnder modulator 10 having the characteristic that the phase difference between the two optical waveguides of the optical waveguide 3 constituting the Mach-Zehnder type optical interferometer changes nonlinearly with respect to the applied voltage. A light intensity modulation device that performs optical modulation including a modulation electrode 11 to which a high-speed data signal biased with a modulation electrode bias voltage is applied to one or both of the two optical waveguides of the Mach-Zehnder modulator. It can be seen that the high-speed data signal amplitude and the half-wave voltage Vπ can be controlled to be equal by controlling the modulation electrode bias voltage using the automatic half-wave voltage control circuit 50 which is a feature of the present invention.

  It supplements about an operating point control circuit. In FIG. 5, the same voltage is applied to the electrode 13 provided in the second arm 2 for correcting the phase difference so that the operating point does not shift due to increase / decrease in the modulation electrode bias voltage of the modulation electrode 11. Yes.

  When this configuration is not used, the operating point shifts when the modulation electrode voltage is changed. Therefore, the operating point is always controlled using an automatic operating point control circuit such as that disclosed in Patent Document 2. By performing half-wave voltage control and operating point control simultaneously using different dither modulation frequencies, half-wave voltage control and operating point control can be realized.

  Alternatively, half-wave voltage control and operating point control can be realized by alternately performing half-wave voltage control and operating point control. Furthermore, if half-wave voltage control and operating point control are alternately performed using the same dither frequency, the dither frequency generator and the dither signal detection circuit can be shared, and the number of parts and the cost can be reduced.

  It supplements about a phase difference adjustment electrode. Here, a method of adjusting the phase difference of the interferometer by biasing the phase difference adjusting electrode to a reverse bias and controlling the voltage is shown. However, as in Non-Patent Document 2, the phase difference adjusting electrode is used for current injection. When the phase difference is adjusted by the above, the configuration and operation can be realized in the same manner by simply changing the voltage application to current injection.

  The phase difference adjustment electrode is biased forward, and the current is controlled by injecting a current into the phase difference adjustment electrode. The refractive index change due to the plasma effect due to the current injection is controlled, and the phase difference of the Mach-Zehnder optical interferometer is controlled. Adjust. When the bias is forward biased and the current is injected into the phase difference adjusting electrode, the modulation efficiency is high, so that there is an advantage that the electrode length of the phase difference adjusting electrode can be shortened. Therefore, controlling the phase difference adjusting electrode includes both cases where the phase difference adjusting electrode is reverse-biased to control the voltage and forward bias is used to control the injection current.

  It supplements about the case where a phase difference adjustment electrode is provided in both arms. In the case where both optical waveguides are provided with phase difference adjusting electrodes and both are controlled, the phase difference adjusting electrode is biased to a reverse bias and the phase difference of the interferometer is adjusted by controlling the voltage. This is equivalent to controlling the direction in which the absolute value of the voltage of the first phase difference adjusting electrode is decreased and controlling in the direction to increase the absolute value of the voltage of the second phase difference adjusting electrode. Therefore, for example, when the absolute value of the voltage of the first phase difference adjusting electrode is controlled to be reduced, the voltage of the second phase difference adjusting electrode must be controlled in a direction exceeding 0V. Control may be performed in the direction of increasing the absolute value of. Similarly, in the case where both of the two optical waveguides are provided with a phase difference adjusting electrode and both are controlled, the phase difference adjusting electrode is biased to a forward bias, and a current is injected into the phase difference adjusting electrode. When controlling the current, controlling the refractive index change due to the plasma effect caused by the current injection, and adjusting the phase difference of the Mach-Zehnder interferometer, control the current in the first phase difference adjusting electrode to be reduced. Is equivalent to controlling the current of the second phase difference adjusting electrode in a direction to increase the current. Therefore, for example, when the current of the first phase difference adjustment electrode is controlled to be reduced, the current of the second phase difference adjustment electrode is increased if the current must be controlled in a direction exceeding 0 mA. What is necessary is just to control to the direction to do.

  It supplements about a photodiode. Although the photodiode is illustrated as being configured separately from the Mach-Zehnder modulator, it may be integrated on a semiconductor substrate. When monolithically integrated on a semiconductor substrate, the output waveguide itself can absorb part of the output light and operate as a photodiode.

  In the above description, the output-side multiplexer constituting the Mach-Zehnder optical interferometer has been described as being configured with a 2 × 1 multiplexer, but may be configured with a 2 × 2 multiplexer. In that case, a part of the output side port may be branched, or all or a part of the non-output side port may be branched and used for dither signal detection. The ratio of the signal detected at the output port and the signal detected at the non-output port may be used for dither signal detection.

  As described above, control can be performed so that the amplitude of the high-speed data signal and the half-wave voltage Vπ coincide.

  FIG. 8 is a block diagram of an optical modulator control device according to the second embodiment of the present invention.

  A high-speed data signal is applied to the modulation electrode 11 of one arm of the Mach-Zehnder modulator to perform ON / OFF modulation, and the operating point is controlled using the electrode of the other arm as the phase difference adjustment electrode 12. The phase difference between the two optical waveguides of the optical waveguide 3 constituting the Mach-Zehnder optical interferometer has a characteristic that changes nonlinearly with respect to the applied voltage to the optical waveguide. The wavelength voltage Vπ is automatically controlled.

  First, a configuration for performing ON / OFF modulation using the Mach-Zehnder modulator 10-2 will be described.

  In the Mach-Zehnder modulator 10-2, an optical waveguide 3 constituting a Mach-Zehnder type optical interferometer is formed on the substrate by the first arm 1 and the second arm 2, and the Mach-Zehnder modulator 10-2 is formed. . The input light input to the input end of the Mach-Zehnder modulator 10-2 is branched into two in the optical waveguide, and the optical waveguide constituting the first arm 1 and the optical waveguide constituting the second arm 2 are provided. Propagating, and these branched lights are combined and output as output light from the output terminal of the Mach-Zehnder modulator 10-2. A modulation electrode 11 to which a high-speed data signal is applied is formed on the upper surface of the optical waveguide constituting the first arm 1, and a phase difference adjusting electrode 12 is formed on the upper surface of the optical waveguide constituting the second arm 2. ing. The bottom surface or substrate of the optical waveguide is connected to the ground electrode, and a voltage is applied between the top electrode and the substrate. As a result, an electric field is applied to the optical waveguide sandwiched between the upper electrode and the substrate, and a phase difference is generated due to the electro-optic effect. The light intensity output from the Mach-Zehnder modulator 10-2 changes according to the generated phase difference, and the light modulation operation is realized. Here, it is assumed that the present Mach-Zehnder modulator has a characteristic that the generated phase difference changes nonlinearly with respect to the applied voltage.

  In order to perform ON / OFF modulation with a high-speed data signal, the high-speed data signal generated from the high-speed data signal source 41 and amplified to a constant amplitude by the wide-band amplifier 42 is input to the RF terminal of the bias T circuit 43, The bias voltage is biased, output from the output terminal of the bias T circuit 43, and applied to the modulation electrode 11. In order to perform optimal modulation, the amplitude of the high-speed data signal needs to be equal to the half-wave voltage Vπ. Although not shown here, it is desirable that the high-speed data signal is terminated inside the modulator or outside via an output terminal.

  Further, the second arm 2 is formed with a phase difference adjusting electrode 12 for adjusting the phase difference of the interferometer in order to control the operating point to the QUAD point, and at the center of the driving amplitude of the high-speed data signal, A phase difference adjusting electrode voltage is applied by the automatic operating point control circuit 61 so as to be the center of the amplitude. For this, for example, an automatic operating point control circuit as in Patent Document 2 is used. In order to perform automatic operation point control using the method of Patent Document 2, it is necessary to minutely modulate the amplitude of the high-speed data signal by the automatic operation point control circuit 61. Further, since it is necessary to detect the dither signal from the optical output, in FIG. 8, the dither signal is extracted from the photodetector 51-1 of the automatic half-wave voltage control circuit 50 and input to the automatic operating point control circuit 61. Yes. Further, without using the automatic operating point control circuit 61, using the operating point control circuit 60 for controlling the operating point so as to be the center of the optical amplitude at the center of the driving amplitude of the high-speed data signal manually or using a table. May be.

  By configuring as described above, ON / OFF modulation using a high-speed data signal is realized.

  Next, the automatic half-wave voltage control circuit 50, which is a feature of the present invention, realizes half-wave voltage control with the same configuration as that of the first embodiment. Therefore, the description of the configuration and operation is omitted. In FIG. 8, the dither signal is applied to the modulation electrode 11 using the dither signal superimposing circuit 53. Here, although the dither signal is superimposed on the modulation electrode 11, any one of the two arms of the optical waveguide 3 constituting the Mach-Zehnder type optical interferometer may be superimposed on either, and the phase difference It may be superimposed on the adjustment electrode 12.

  As described above, in the Mach-Zehnder modulator 10-2 having this configuration, the amplitude of the high-speed data signal and the half-wave voltage Vπ can be controlled to coincide with each other.

  FIG. 9 shows a block diagram of a light intensity modulation apparatus according to the third embodiment of the present invention.

  A structure in which modulation electrodes are provided in both arms of the Mach-Zehnder modulator 10-3, and each of the differential high-speed data signals is applied to them to modulate light ON / OFF (differential drive push-pull configuration or dual Called a drive push-pull configuration). Further, in addition to the modulation electrode, a phase difference adjusting electrode for controlling the operating point is separately provided. The phase difference between the two optical waveguides of the optical waveguide 3 constituting the Mach-Zehnder type optical interferometer has a characteristic that changes nonlinearly with respect to the voltage applied to the optical waveguide. In this configuration, the voltage Vπ is automatically controlled.

  First, a configuration for performing ON / OFF modulation using the Mach-Zehnder modulator 10-3 will be described. In the Mach-Zehnder modulator 10-3, an optical waveguide 3 constituting a Mach-Zehnder type optical interferometer is formed on the substrate by the first arm 1 and the second arm 2, and the Mach-Zehnder modulator 10-3 is configured. . The input light input to the input end of the Mach-Zehnder modulator 10-3 is branched into two in the optical waveguide, and the optical waveguide constituting the first arm 1 and the optical waveguide constituting the second arm 2 are provided. Propagating, and the branched lights are combined and output as output light from the output terminal of the Mach-Zehnder modulator 10-3. The optical waveguide constituting the first arm 1 includes the first modulation electrode 11-1, the first phase difference adjusting electrode 12-1, and the second arm 2 for applying a voltage to the optical waveguide. In the optical waveguide, a second modulation electrode 11-2 and a second phase difference adjusting electrode 12-2 for applying a voltage to the optical waveguide are formed on the upper surface. The bottom surface or substrate of the optical waveguide is connected to the ground electrode, and a voltage is applied between the top electrode and the substrate. As a result, an electric field is applied to the optical waveguide sandwiched between the upper electrode and the substrate, and a phase difference is generated due to the electro-optic effect. The light intensity output from the Mach-Zehnder modulator 10-3 changes according to the generated phase difference, and the light modulation operation is realized. The optical waveguide structures of the first arm 1 and the second arm 2 are equal, and the lengths of the first modulation electrode 11-1 and the second modulation electrode 11-2 are configured to be equal. The Mach-Zehnder modulator has a characteristic that the generated phase difference changes nonlinearly with respect to the applied voltage.

  In order to perform ON / OFF modulation with a high-speed data signal, the high-speed data signal generated from the high-speed data signal source 41 is amplified by the differential output broadband amplifier 42-2, and the first output of the differential output broadband amplifier 42-2 is amplified. The high-speed data output signal is input to the RF terminal of the first bias T circuit 43-1, is biased with the modulation electrode bias voltage, is output from the output terminal of the first bias T circuit 43-1, and is subjected to Mach-Zehnder modulation. The second high-speed data output signal, which is applied to the first modulation electrode 11-1 of the detector and is the opposite-phase output of the differential output broadband amplifier 42-2, is the RF of the second bias T circuit 43-2. Is input to the terminal, biased with the modulation electrode bias voltage, output from the output terminal of the second bias T circuit 43-2, and applied to the second modulation electrode 11-2 of the Mach-Zehnder modulator. It is.

  It should be noted that the first modulation electrode bias voltage and the second modulation electrode bias voltage are equal. As a result, the half-wave voltage Vπ of the first modulation electrode becomes equal to the half-wave voltage Vπ of the second modulation electrode. The amplitude of the first high-speed data signal and the amplitude of the second high-speed data signal are set equal. Thus, since the first and second data signals have opposite phases and the same amplitude, and the half-wave voltages Vπ of the first and second modulation electrodes are set to be equal, the semiconductor Mach-Zehnder modulator is set. The chirp of the signal waveform output from is canceled and becomes zero. In order to perform optimum modulation, the sum of the amplitudes of the high-speed data signals needs to be equal to the half-wave voltage Vπ. Therefore, in this setting, the amplitudes of the first and second high-speed data signals are equal to the half-wave voltage Vπ. Equal to half.

  Further, the first phase difference adjusting electrode 12-1 or the second phase difference adjusting electrode 12-for adjusting the phase difference of the interferometer is provided on the upper surface of either the first arm 1 or the second arm 2 or both. 2 is formed. The operating point of the first phase difference adjusting electrode 12-1 and the second phase difference adjusting electrode 12-2 is adjusted so as to be the center of the optical amplitude at the center of the driving amplitude of the high-speed data signal. The phase difference adjusting electrode voltage is applied by the operating point control circuit 60. A configuration in which the operating point is always controlled using the automatic operating point control circuit 61 as in Patent Document 2 may be employed.

  By configuring as described above, ON / OFF modulation using a high-speed data signal is realized.

  Next, the automatic half-wave voltage control circuit 50, which is a feature of the present invention, has the same configuration as that of the first embodiment and realizes half-wave voltage control. Therefore, the description of the configuration and operation is omitted. It is a feature of this embodiment that the first modulation electrode bias voltage and the second modulation electrode bias voltage are controlled equally. A dither signal is applied to the first modulation electrode 11-1 using the dither signal superimposing circuit 53. Here, the dither signal is superimposed on the first modulation electrode 11-1, but if it is one of the two arms of the optical waveguide 3 constituting the Mach-Zehnder interferometer, it is superimposed on either. Alternatively, it may be superimposed on the phase difference adjusting electrode.

  As described above, in the Mach-Zehnder modulator 10-3 having this configuration, the amplitude of the high-speed data signal and the half-wave voltage Vπ can be controlled to coincide with each other.

  FIG. 10 is a block diagram of a light intensity modulation device according to the fourth embodiment of the present invention.

  A structure in which the modulation electrode 11 is provided on one arm of the two arms of the Mach-Zehnder modulator and the ground electrode 15 is provided on the other arm, and a high-speed data signal is applied between them to modulate the light ON / OFF (single drive). Push-pull configuration). In order to apply a bias to the modulation electrode 11 by applying a voltage to the substrate (more specifically, the bottom surface common to the optical waveguide), the modulation electrode bias voltage application electrode 16 connected to the bottom surface common to the optical waveguide or the substrate is provided. I have. Further, in addition to the modulation electrode 11, a first phase difference adjustment electrode 12-1 and a second phase difference adjustment electrode 12-2 for separately controlling the operating point are provided. The phase difference between the two optical waveguides of the optical waveguide 3 constituting the Mach-Zehnder optical interferometer has a characteristic that changes nonlinearly with respect to the applied voltage to the optical waveguide. In this configuration, the wavelength voltage Vπ is automatically controlled.

  First, a configuration for performing ON / OFF modulation using the Mach-Zehnder modulator 10-4 will be described.

  In the Mach-Zehnder modulator 10-4, an optical waveguide 3 constituting a Mach-Zehnder type optical interferometer is formed on the substrate by the first arm 1 and the second arm 2, and the Mach-Zehnder modulator 10-4 is configured. . The input light input to the input end of the Mach-Zehnder modulator 10-4 is branched into two in the optical waveguide, and the optical waveguide constituting the first arm 1 and the optical waveguide constituting the second arm 2 are Propagated, and these branched lights are combined and output as output light from the output terminal of the Mach-Zehnder modulator 10-4. The modulation electrode 11 and the first phase difference adjusting electrode 12-1 are provided on the upper surface of the optical waveguide constituting the first arm 1, and the ground electrode 15 and the second electrode are provided on the upper surface of the optical waveguide constituting the second arm 2. The phase difference adjusting electrode 12-2 is formed. The substrate is not grounded, and a high-speed data signal applies a voltage between the modulation electrode 11 and the ground electrode 15. The bottom surface (substrate) of the optical waveguide is connected to a modulation electrode bias voltage application electrode 16 for applying a bias to the modulation electrode 11. The output of the automatic operating point control circuit 50 is applied to the modulation electrode bias voltage application electrode 16 via the inductor 45. An electric field is applied to the optical waveguide sandwiched between the upper electrode and the substrate, and a phase difference is generated due to the electro-optic effect. When a high-speed data signal is applied, a single drive push-pull configuration is adopted in which an electric field having the same magnitude is applied in the opposite direction between the modulation electrode 11 and the substrate and the ground electrode 15 and the substrate. The light intensity output from the Mach-Zehnder modulator 10-4 changes according to the generated phase difference, and the light modulation operation is realized. The optical waveguide structures of the first arm 1 and the second arm 2 are the same, and the lengths of the modulation electrode 11 and the ground electrode 15 are configured to be equal. The Mach-Zehnder modulator has a characteristic that the generated phase difference changes nonlinearly with respect to the applied voltage.

  In order to perform ON / OFF modulation with a high-speed data signal, a high-speed data signal generated from the high-speed data signal source 41 and amplified to a constant amplitude by the broadband amplifier 42 is applied to the modulation electrode 11. In order to perform optimal modulation, the amplitude of the high-speed data signal needs to be equal to the half-wave voltage Vπ. Although not shown here, it is desirable that the high-speed data signal is terminated between the modulation electrode 11 and the ground, inside the modulator, or externally via an output terminal.

  Further, either or both of the first arm 1 and the second arm 2 are provided with a first phase difference adjusting electrode 12-1 for adjusting the phase difference of the interferometer in order to control the operating point to the QUAD point, or A second phase difference adjusting electrode 12-2 is formed. Here, the phase difference is adjusted by the automatic operating point control circuit 61 to the first phase difference adjusting electrode 12-1 so that the operating point is controlled so that the center of the optical amplitude is at the center of the driving amplitude of the high-speed data signal. An electrode voltage is applied. For this, for example, an automatic operating point control circuit as in Patent Document 2 is used. In order to realize the automatic operation point control of Patent Document 2, it is necessary to minutely modulate the amplitude of the high-speed data signal by the automatic operation point control circuit 61. Further, since it is necessary to detect the dither signal from the optical output, the dither signal is extracted from the photodetector 51-1 and input to the automatic operating point control circuit 61 in FIG. Further, without using the automatic operating point control circuit 61, using the operating point control circuit 60 for controlling the operating point so as to be the center of the optical amplitude at the center of the driving amplitude of the high-speed data signal manually or using a table. May be.

  By configuring as described above, ON / OFF modulation using a high-speed data signal is realized.

  The automatic half-wave voltage control circuit 50, which is a feature of the present invention, realizes half-wave voltage control in the same manner as in the second embodiment. A dither signal is applied to the second phase difference adjusting electrode 12-2 using the dither signal superimposing circuit 53. Here, the dither signal is superimposed on the second phase difference adjusting electrode 12-2, but if it is one of the two arms of the optical waveguide 3 constituting the Mach-Zehnder interferometer, the ground electrode 15 is used. It may be superimposed on any other than, and may be superimposed on the modulation electrode.

  As described above, the Mach-Zehnder modulator 10-4 having this configuration can be controlled so that the amplitude of the high-speed data signal matches the half-wave voltage Vπ.

  FIG. 11 is a block diagram of an optical modulator control apparatus according to the fifth embodiment of the present invention. This is a light intensity modulation device provided with an automatic half-wave voltage control circuit, and further includes a light intensity modulation device provided with an automatic operating point control circuit 70, which is one of its embodiments.

  Since the Mach-Zehnder modulator 10-3, the circuit for performing ON / OFF modulation with a high-speed data signal, and the automatic half-wave voltage control circuit 50 have the same configuration and operation as those of the third embodiment, the description of the configuration and operation is omitted. . Thus, automatic half-wave voltage control is realized.

  The configuration and operation of the automatic operating point control circuit 70 will be described.

  The automatic operating point control circuit 70 automatically controls the operating point so as to be the center of the optical amplitude at the center of the driving amplitude of the high-speed data signal. The automatic operating point control circuit 70 includes a second phase comparison circuit 71, a phase difference adjustment electrode voltage control circuit 72, a second dither signal superimposing circuit 73, and a second addition circuit 74.

  The dither signal generated by the second dither signal superimposing circuit 73 is superimposed on the modulation electrode bias voltage generated by the modulation electrode bias voltage control circuit 52 by the second addition circuit 74 and further generated by the dither signal superimposing circuit 53. The dither signal is superimposed by the adder circuit 54, input to the DC terminal of the first bias T circuit 43-1, output from the output terminal of the first bias T circuit 43-1, and the first modulation electrode 11-. 1 is applied. The output of the second adder circuit 74 is input to the DC terminal of the second bias T circuit 43-2, output from the output terminal of the second bias T circuit 43-2, and the second modulation electrode 11-. 2 is applied. The second dither signal superimposed on the first modulation electrode 11-1 and the second modulation electrode 11-2 so that the operating point is not shifted by the dither signal generated by the second dither signal superimposing circuit 73. The dither signals generated by the superimposing circuit 73 have the same amplitude. Thereby, the dither signal of the automatic operating point control circuit is superimposed on the optical output signal.

  The optical output signal on which the dither signal of the automatic operating point control circuit is superimposed is partly branched by the optical branch circuit 14 optically coupled to the output terminal of the Mach-Zehnder modulator 10-3, and the automatic half-wave voltage control circuit Detection is performed by a photodiode built in the photodetector 51-1 of the 50 dither signal detection circuit 51, and the dither signal is amplified by a built-in amplifier. In this embodiment, the photodetector 51-1 is used both for an automatic half-wave voltage control circuit and an automatic operating point control circuit. The amplified dither signal is multiplied by the dither signal generated from the second dither signal superimposing circuit 73 by the multiplication circuit built in the second phase comparison circuit 71 and built in the second phase comparison circuit 71. A high-frequency component is removed by the low-pass filter, and the voltage is converted into a voltage corresponding to the phase difference between the two dither signals and output to realize phase comparison. The applied voltage of the first phase difference adjustment electrode 12-1 is controlled by the phase difference adjustment electrode voltage control circuit 72 so that the output signal of the second phase comparison circuit 71 based on the phase comparison result is minimized. Instead, the voltage applied to the second phase difference adjusting electrode 12-2 or both may be controlled. Thus, a feedback circuit including the second dither signal superimposing circuit 73, the optical modulator 10, the optical branching circuit 14, the photodetector 51-1, the second phase comparison circuit 71, and the phase difference adjustment electrode voltage control circuit 72 is configured. Thus, automatic operation point control is realized.

  By performing automatic half-wave voltage control and automatic operating point control simultaneously using different dither modulation frequencies, automatic half-wave voltage control and automatic operating point control can be realized simultaneously.

  Further, even if the dither modulation frequency of the automatic half-wave voltage control and the automatic operating point control is the same, the half-wave voltage control and the operating point control can be realized by alternately performing the half-wave voltage control and the operating point control.

  FIG. 12 is a block diagram of an optical modulator control device according to the sixth embodiment of the present invention.

  If the automatic half-wave voltage control circuit and the automatic operating point control circuit share the dither signal detection circuit and the dither signal superposition circuit, and automatic half-wave voltage control and automatic operating point control are performed alternately, the number of parts and the cost Can be reduced. This embodiment shares part of the automatic half-wave voltage control circuit and the automatic operating point control circuit, and includes an automatic half-wave voltage / operating point control circuit 80 that can realize automatic half-wave voltage control and automatic operating point control. It is an intensity modulation device.

  The Mach-Zehnder modulator 10-3 and the circuit for performing ON / OFF modulation with a high-speed data signal have the same configuration and operation as those of the fifth embodiment, and thus the description of the configuration and operation is omitted.

  The automatic half-wave voltage / operating point control circuit 80 includes a dither signal detection circuit 81, a modulation electrode bias voltage control circuit 82-1, a phase difference adjustment electrode voltage control circuit 82-2, a dither signal superposition circuit 83, an addition circuit 84, and an output. A switching circuit 85 and an input switching circuit 86 are included. The dither signal detection circuit 81 includes a photodetector 81-1 (which includes a photodiode and an amplifier) and a phase comparison circuit 81-2 (which includes a multiplication circuit and a low-pass filter). It is configured.

  First, the configuration and operation when the automatic half-wave voltage / operating point control circuit 80 functions as automatic half-wave voltage control will be described. In this case, the output of the output switching circuit 85 is input to the modulation electrode bias voltage control circuit 82-1, and the input of the input switching circuit 86 is the output of the modulation electrode bias voltage control circuit 82-1. This is the same configuration and operation as the automatic half-wave voltage control circuit 50 in the fifth embodiment, and controls the half-wave voltage Vπ so that the amplitude of the high-speed data signal and the half-wave voltage Vπ are equal.

  The dither signal generated by the dither signal superimposing circuit 83 is superimposed on the modulation electrode bias voltage generated by the modulation electrode bias voltage control circuit 82-1 by the adding circuit 84, and applied to the DC terminal of the first bias T circuit 43-1. It is inputted, outputted from the output terminal of the first bias T circuit 43-1, and applied to the first modulation electrode 11-1. The modulation electrode bias voltage generated by the modulation electrode bias voltage control circuit 82-1 is input to the DC terminal of the second bias T circuit 43-2 and output from the output terminal of the second bias T circuit 43-2. And applied to the second modulation electrode 11-2. Therefore, the same modulation electrode bias voltage is applied to the first modulation electrode 11-1 and the second modulation electrode 11-2, and the dither signal is superimposed only on the first modulation electrode 11-1. As a result, a dither signal for automatic half-wave voltage control is superimposed on the optical output signal.

  The optical output signal on which the dither signal is superimposed is partly branched by the optical branch circuit 14 optically coupled to the output terminal of the Mach-Zehnder modulator 10-3, and the photodetector 81-1 of the dither signal detection circuit 81 is split. The dither signal is amplified by a built-in amplifier. The amplified dither signal is multiplied by the dither signal generated by the dither signal superimposing circuit 83 by the multiplication circuit built in the phase comparison circuit 81-2, and the high-frequency component by the low-pass filter built in the phase comparison circuit 81-2. Is removed, converted into a voltage corresponding to the phase difference between the two dither signals, and output to the input of the output switching circuit 85 to realize phase comparison.

  The output signal of the phase comparison circuit 81-2 based on the phase comparison result is input to the modulation electrode bias voltage control circuit 82-1 by the output switching circuit 85, and the modulation electrode bias voltage control circuit so that this signal is minimized. 82-1 controls the equal modulation electrode bias voltage of the first modulation electrode 11-1 and the second modulation electrode 11-2. In this way, a feedback circuit is constituted by the dither signal superimposing circuit 83, the Mach-Zehnder optical modulator 10-3, the optical branching circuit 14, the dither signal detecting circuit 81, and the modulation electrode bias voltage control circuit 82-1. Is realized.

  Next, the configuration and operation when the automatic half-wave voltage / operating point control circuit 80 functions as automatic operating point control will be described. This is a case where the output of the output switching circuit 85 is input to the phase difference adjusting electrode voltage control circuit 82-2 and the input of the input switching circuit 86 is the output of the adding circuit 84. The configuration and operation are the same as those of the automatic operating point control circuit 70 in the fifth embodiment, and the operating point is controlled so as to be the center of the optical amplitude at the center of the driving amplitude of the high-speed data signal.

  The dither signal generated by the dither signal superimposing circuit 83 is superimposed on the modulation electrode bias voltage generated by the modulation electrode bias voltage control circuit 82-1 by the adding circuit 84, and applied to the DC terminal of the first bias T circuit 43-1. It is inputted, outputted from the output terminal of the first bias T circuit 43-1, and applied to the first modulation electrode 11-1. Since the input of the input switching circuit 86 is switched to the terminal connected to the output of the adding circuit 84, the dither signal generated by the dither signal superimposing circuit 83 is converted by the adding circuit 84 into the modulation electrode bias voltage control circuit 82-1. Is superimposed on the modulation electrode bias voltage generated by the second bias T circuit 43-2, input to the DC terminal of the second bias T circuit 43-2, output from the output terminal of the second bias T circuit 43-2, and output to the second modulation electrode 11. -2. Therefore, the same modulation electrode bias voltage on which the same dither signal is superimposed is applied to the first modulation electrode 11-1 and the second modulation electrode 11-2. Thereby, the dither signal for automatic operation point control is superimposed on the optical output signal.

  Since the configuration and operation until the phase comparison is realized by the optical output signal on which the dither signal is superimposed are the same as in the case of working as automatic half-wave voltage control, the description thereof is omitted. The output signal of the phase comparison circuit 81-2 based on the phase comparison result is input to the phase difference adjustment electrode voltage control circuit 82-2 by the output switching circuit 85, and the phase difference adjustment electrode voltage is set so that this signal is minimized. The control circuit 82-2 controls the voltage applied to the first phase difference adjusting electrode 12-1. Instead, the voltage applied to the second phase difference adjusting electrode 12-2 or both may be controlled. Thus, a feedback circuit is constituted by the dither signal superimposing circuit 83, the Mach-Zehnder optical modulator 10-3, the optical branching circuit 14, the dither signal detecting circuit 81, and the phase difference adjusting electrode voltage control circuit 82-2, and automatic operation point control is thereby performed. Is realized.

  Therefore, by adopting such a configuration, the output switching circuit 85 and the input switching circuit 86 are switched in conjunction, and automatic half-wave voltage control and automatic operating point control are alternately performed, so that automatic half-wave voltage control and automatic operating point are alternately performed. Even if the dither modulation frequency for control is the same, automatic half-wave voltage control and automatic operating point control can be realized.

  “Embodiment 5” and “Embodiment 6” are implemented for the semiconductor Mach-Zehnder modulator 10-3, but the same applies to other types of semiconductor Mach-Zehnder modulators such as the semiconductor Mach-Zehnder modulator 10-4 of FIG. Can be implemented.

DESCRIPTION OF SYMBOLS 1 1st arm 2 2nd arm 3 Optical waveguide which comprises Mach-Zehnder type | mold optical interferometer 10, 10-2, 10-3, 10-4 Mach-Zehnder modulator 11 Modulation electrode 11-1 1st modulation electrode 11- 2 Second modulation electrode 12 Phase difference adjustment electrode 12-1 First phase difference adjustment electrode 12-2 Second phase difference adjustment electrode 13 Electrode 14 Optical branch circuit 15 Ground electrode 16 Modulation electrode bias voltage application electrode 20 Bias voltage Control circuit 22, 31 DC power supply 23 Bias voltage control unit 23a, 32a Corresponding table 30 Phase difference adjustment voltage control circuit 32 Phase difference adjustment voltage control unit 41 High-speed data signal source 42 Broadband amplifier 42-2 Differential output wideband amplifier 43 Bias T Circuit 43-1 First bias T circuit 43-2 Second bias T circuit 44 Capacitor 45 Inductor 0 automatic half-wave voltage control circuit 51 dither signal detection circuit 51-1 photodetector 51-2 phase comparison circuit 52 modulation electrode bias voltage control circuit 53 dither signal superposition circuit 54 addition circuit 60 operating point control circuit 61 automatic operating point control circuit 70 Automatic Half Wavelength Voltage Control Circuit 71 Second Phase Comparison Circuit 72 Modulation Electrode Bias Voltage Control Circuit 73 Second Dither Signal Superposition Circuit 74 Second Adder Circuit 80 Automatic Half Wavelength Voltage / Operating Point Control Circuit 81 Dither Signal Detection Circuit 81-1 Photo detector 81-2 Phase comparison circuit 82-1 Modulation electrode bias voltage control circuit 82-2 Phase difference adjustment electrode voltage control circuit 83 Dither signal superimposition circuit 84 Adder circuit 85 Output switching circuit 86 Input switching circuit

Claims (5)

A light intensity modulation device that performs ON / OFF of an optical signal using a Mach-Zehnder modulator having a characteristic that a phase difference between two optical waveguides constituting a Mach-Zehnder interferometer changes nonlinearly with respect to an applied voltage,
The Mach-Zehnder modulator including a modulation electrode to which a data signal biased with a modulation electrode bias voltage is applied to one or both of the two optical waveguides;
A minute modulation signal superimposing circuit that superimposes a minute modulation signal on any one of the electrodes of the two optical waveguides, and an optical branch that branches a part of the output light of the modulator on which the minute modulation signal is superimposed A circuit, a minute modulation signal detection circuit for detecting a minute modulation signal component from the branched output light, and a modulation electrode bias voltage control circuit for controlling the modulation electrode bias voltage so that the detected minute modulation signal is minimized. An optical intensity modulation device using a Mach-Zehnder optical modulator, comprising: an automatic half-wave voltage control circuit comprising: The Mach-Zehnder modulator is
A modulation electrode to which a data signal voltage biased with a modulation electrode bias voltage is applied to one of the two optical waveguides;
2. The light intensity modulation using a Mach-Zehnder optical modulator according to claim 1, wherein a phase difference adjusting electrode controlled by an automatic operating point control circuit is included in the other of the two optical waveguides. apparatus. The Mach-Zehnder modulator is
The first optical waveguide of the two optical waveguides includes a first modulation electrode to which a first data signal voltage biased with a first modulation electrode bias voltage is applied,
The second optical waveguide includes a second modulation electrode biased with a second modulation electrode bias voltage and applied with a data signal voltage having a phase opposite to the first data signal voltage,
Of the two optical waveguides, at least one optical waveguide is a differential drive type Mach-Zehnder modulator including a phase difference adjusting electrode for adjusting a phase difference of an interferometer separately from the modulation electrode. A light intensity modulation device using the Mach-Zehnder light modulator according to claim 1. A light intensity modulation device that performs ON / OFF of an optical signal using a Mach-Zehnder modulator having a characteristic that a phase difference between two optical waveguides constituting a Mach-Zehnder interferometer changes nonlinearly with respect to an applied voltage,
Including the modulation electrode in one optical waveguide of the two optical waveguides and the ground electrode in the other optical waveguide so as to apply a data signal between the modulation electrode and the ground electrode;
A modulation electrode bias voltage application electrode connected to a common bottom surface of the two optical waveguides;
A single-drive push-pull configuration type Mach-Zehnder modulator including a phase difference adjusting electrode controlled by an operating point control circuit in one or both of the two optical waveguides;
A minute modulation signal superimposing circuit that superimposes a minute modulation signal on any one of the electrodes of the two optical waveguides, and an optical branch that branches a part of the output light of the modulator on which the minute modulation signal is superimposed A circuit, a minute modulation signal detection circuit for detecting a minute modulation signal component from the branched output light, and a modulation electrode bias voltage control circuit for controlling the modulation electrode bias voltage so that the detected minute modulation signal is minimized. An optical intensity modulation device using a Mach-Zehnder optical modulator, comprising: an automatic half-wave voltage control circuit comprising: The light intensity modulation device further comprising an automatic operating point control circuit,
The automatic operating point control circuit is:
The half-wave voltage control and the operating point control are simultaneously performed using different modulation frequencies, or alternately performed using the same modulation frequency. Light intensity modulation device using the Mach-Zehnder light modulator.

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