JP7768551B2 - Optical modulator and optical modulation method - Google Patents

Description

The present invention relates to an optical modulator and an optical modulation method that enable wideband modulation.

In recent years, the amount of data traffic in communication networks has been increasing. As a result, optical communication systems that transmit and receive large amounts of data require wideband optical modulators that can transmit and receive data at rates exceeding 100 Gbps. The bandwidth characteristics of an optical modulator are determined based on its driving method. To date, no driving method has been realized that can drive an optical modulator in a band above 100 GHz. For conventional optical modulators, two driving methods have been proposed:

The first driving method uses traveling wave electrodes to perform optical modulation through the interaction between co-propagating electrical and optical signals (see, for example, Non-Patent Document 1 and Non-Patent Document 2). The second driving method segments the modulator, configuring each segment as a lumped constant circuit, and driving each individually to perform optical modulation (see Non-Patent Document 3, Patent Document 1, and Patent Document 2). For example, Patent Document 1 describes a Mach-Zehnder optical modulator. The optical modulation device described in Patent Document 1 has a Mach-Zehnder optical waveguide and a driving circuit that applies an electric field to the optical waveguide to change the phase of the guided light, and the driving circuit is divided into multiple segments to divide and reduce the electrical capacitance.

U.S. Pat. No. 1,078,2543 International Publication No. 2016/029836

J. Lin et al., “Single-carrier 72 GBaud 32QAM and 84 GBaud 16QAM transmission using a SiP IQ modulator with joint digital-optical pre-compensation,” Opt. Express, vol. 27, no. 4, pp. 5610-5619, Feb. 2019. Mingbo He, Mengyue Xu, Yuxuan Ren, et al., “High-performance hybrid silicon and lithium niobate Mach-Zehnder modulators for 100 Gbit/s and beyond” Asia Communications and Photonics Conference (ACPC) 2019 OSA Technical Digest (Optica Publishing Group, 2019), paper T4H.3 Benjamin G. Lee, Nicolas Dupuis, Renato Rimolo-Donadio, et al., “Driver-integrated 56-Gb/s segmented electrode silicon Mach Zehnder modulator using optical-domain equalization” Optical Fiber Communication Conference Jan. 2017

In order to further increase the speed and capacity of data transmission and reception, it is desirable, for example, to drive an optical modulator at a wideband modulation frequency as well as a high-band modulation frequency. Both of the two conventional driving methods make it difficult to design for a wider bandwidth. With traveling wave driving, there are issues with wideband impedance matching and optical-RF speed matching, making it difficult to improve the bandwidth. For example, when a silicon modulator is driven by a traveling wave, the modulation characteristics are limited to a maximum bandwidth of approximately 40 GHz (see non-patent document 1). While LN modulators have been able to achieve frequencies up to approximately 70 GHz, achieving a bandwidth greater than this is extremely difficult.

The following issues exist with the segmented drive method for the drive circuit that performs optical modulation on the optical waveguide. (1) When the modulator phase shifter is segmented, the capacitance of each segment decreases in proportion to the number of segments, but the resistance increases in inverse proportion, resulting in the RC bandwidth of each segment remaining unchanged and making it impossible to support a wide bandwidth. Furthermore, even if all segments are set to the most ideal conditions, the modulator bandwidth depends on the RC bandwidth of each segment, making it impossible to expand beyond the RC bandwidth of each segment. For example, even when using the segmented drive method described in Non-Patent Document 3 for a silicon modulator, modulation operation is limited to approximately 56 Gbps.

(2) A driving method that segments and divides the driving circuit that performs optical modulation on the optical waveguide requires multiple drivers to input drive signals according to the number of segments, which makes it impossible to use a general-purpose single RF input driver and complicates the driver design. Furthermore, as the number of segments increases, the number of drivers required for the driving circuit also increases, which complicates modulator control and increases costs.

The two conventional drive methods do not include a method for widening the bandwidth, and no method for driving a modulator with a high bandwidth that exceeds the RC bandwidth has been disclosed. For example, the optical modulator described in Patent Document 1 has not yet proposed modulation in a high bandwidth. Furthermore, as with the optical modulator described in Patent Document 1, the drive frequency of a drive circuit divided into multiple segments depends on the RC bandwidth (cutoff frequency) of the circuit configured in each segment, which poses the issue of being unable to perform modulation beyond the designed RC bandwidth.

The present invention aims to provide an optical modulator and optical modulation method that can output modulated light over a wide bandwidth and can be driven to modulate over a wide bandwidth.

One aspect of the present invention is an optical modulator comprising: an optical waveguide that guides input light; an optical modulation unit provided in the optical waveguide that modulates the guided light; an optical output waveguide that outputs output light modulated by the optical modulation unit; and a driver that applies an electric field to the optical waveguide in the optical modulation unit based on an input drive signal to modulate the input light in accordance with the frequency band of the input drive signal. The optical modulation unit comprises a plurality of optical waveguide elements that divide the optical waveguide into a plurality of regions. The driver unit comprises a plurality of driver circuits connected in series corresponding to each region containing the optical waveguide elements and configured to individually drive each of the optical waveguide elements for different predetermined frequency bands. Based on the input of the input drive signal having a desired frequency band, each driver circuit individually applies a non-uniform electric field to each of the optical waveguide elements in accordance with their arrangement position and their contribution to the modulation amount for the desired frequency band, thereby providing non-uniform modulation amounts to each of the optical waveguide elements, and outputting the modulated output light from the optical output waveguide.

The present invention makes it possible to output modulated light over a wide bandwidth.

1 is a diagram illustrating a configuration of an optical modulator according to an embodiment of the present invention. FIG. 2 is a diagram showing the configuration of an electrical passive circuit provided in a drive circuit. FIG. 10 is a diagram showing the contribution to modulation of multiple drive circuits connected in series. FIG. 1 illustrates an example configuration of a drive circuit having four segments driven by a voltage-based input drive signal. FIG. 10 is a diagram showing the calculation results of the bandwidth of an optical modulator having a driving circuit. FIG. 1 illustrates an example configuration of a drive circuit having four segments driven by a voltage-based input drive signal. FIG. 10 is a diagram showing the calculation results of the bandwidth of an optical modulator having a driving circuit. FIG. 1 illustrates an example configuration of a drive circuit having eight segments driven by a voltage-based input drive signal. FIG. 10 is a diagram showing the calculation results of the bandwidth of an optical modulator having a driving circuit. FIG. 1 is a diagram showing the configuration of a drive circuit driven by a current-based input drive signal. FIG. 10 is a diagram showing the calculation results of the bandwidth of an optical modulator having a driving circuit. FIG. 1 illustrates an example configuration of a drive circuit having eight segments driven by a current-based input drive signal. FIG. 10 is a diagram showing the calculation results of the bandwidth of an optical modulator having a driving circuit. FIG. 10 is a diagram showing the calculation results of the bandwidth of an optical modulator having a driving circuit. FIG. 1 illustrates a configuration of a drive circuit provided with a delay line and driven by a voltage-based input drive signal. FIG. 10 is a diagram illustrating the frequency characteristics of a delay line. FIG. 10 is a diagram showing the calculation results of the bandwidth of an optical modulator having a driving circuit. FIG. 1 is a diagram showing the configuration of a drive circuit provided with a delay line and driven by a current-based input drive signal. FIG. 10 is a diagram illustrating the frequency characteristics of a delay line. FIG. 10 is a diagram showing the calculation results of the bandwidth of an optical modulator having a driving circuit. 10 is a flowchart showing the flow of processing of an optical modulation method executed in the optical modulator.

Embodiments of an optical modulator and an optical modulation method according to the present invention will be described below with reference to the drawings.

As shown in FIG. 1, the optical modulator 1 is a Mach-Zehnder type optical modulator. The optical modulator 1 includes an optical waveguide that guides light and an optical modulation unit that applies an electric field to the optical waveguide to modulate the guided light. The optical waveguide includes, for example, an optical input waveguide 2 that guides input light, an optical branching unit 3 provided downstream of the optical input waveguide 2, a first optical waveguide 4 and a second optical waveguide 5 that branch off downstream of the optical branching unit 3, an optical coupler 6 provided downstream of the first optical waveguide 4 and the second optical waveguide 5, and an optical output waveguide 7 provided downstream of the optical coupler 6. The optical modulation unit includes, for example, a first optical modulation unit provided on the first optical waveguide 4 side and a second optical modulation unit provided on the second optical waveguide 5 side.

The first optical modulation unit is, for example, composed of a portion of the first optical waveguide 4 that is modulated by a first drive circuit 8 that applies an electric field to the first optical waveguide 4. The first optical modulation unit includes, for example, a plurality of optical waveguide elements 4-n (n is an integer greater than or equal to 0) that divide the first optical waveguide 4 into multiple regions. The second optical modulation unit is, for example, composed of a portion of the second optical waveguide 5 that is modulated by a second drive circuit 9 that applies an electric field to the second optical waveguide 5. The second optical modulation unit includes, for example, a plurality of optical waveguide elements 5-n that divide the second optical waveguide 5 into multiple regions. The second drive circuit 9 has the same configuration as the first drive circuit 8 and is arranged in an axisymmetric relationship with the first drive circuit 8. The second drive circuit 9 is arranged to apply an electric field in the opposite direction to that of the first drive circuit 8.

Continuous wave (CW) light is input to the optical input waveguide 2, for example, from a light source (not shown). The light source outputs, for example, laser light. The light guided through the optical input waveguide 2 is branched into first and second branched lights by the optical brancher 3. The first branched light is input to the first optical waveguide 4, and the second branched light is input to the second optical waveguide 5. The first branched light is output downstream without undergoing phase modulation when a first electric field based on a first voltage V0 is applied to the first optical waveguide 4 by the first driving circuit 8. The second branched light is output downstream without undergoing phase modulation when a first electric field based on a first voltage V0 is applied to the second optical waveguide 5 by the second driving circuit 9. The first and second branched lights are input to the optical coupler 6 and optically coupled. At this time, there is no phase difference between the first and second branched lights, and therefore they are output without interference. The output light is input to the optical output waveguide 7, guided there, and output downstream as output light.

The first branched light undergoes phase modulation and is output downstream when a second electric field based on a second voltage V1 is applied to the first optical waveguide 4 by the first driving circuit 8. The second branched light undergoes phase modulation and is output downstream when a second electric field based on a second voltage V1 is applied to the second optical waveguide 5 by the second driving circuit 9. The first branched light and the second branched light are input to the optical coupler 6 and optically coupled. At this time, the first branched light and the second branched light are each phase modulated to create a phase difference of 180°, and they interfere with each other, preventing light from being output downstream from the optical output waveguide 7.

When output light is output from the optical output waveguide 7, the signal is set to 1, and when output light is not output from the optical output waveguide 7, the signal is set to 0. An optical pulse signal is generated corresponding to the respective time lengths of 1 and 0. The first drive circuit 8 and second drive circuit 9 switch between the first voltage V0 and the second voltage V1 in a predetermined frequency band based on an input drive signal (RF (Radio Frequency) signal). The input drive signal is an electrical signal that switches the input power based on this frequency band. The light of a predetermined frequency that is modulated based on the input drive signal and output from the optical output waveguide 7 is called modulated light.

The configuration of the drive unit will be described below. In the following explanation, the configuration of the first drive circuit 8 will be described as a representative of the first drive circuit 8 and second drive circuit 9 that make up the drive unit. The first drive circuit 8 applies an electric field to the optical waveguide in the first optical modulation unit based on the input drive signal input from the signal input unit 10, modulating the input light according to the frequency band of the input drive signal. The first drive circuit 8 is driven based on the input drive signal whose voltage amplitude has been adjusted. The first drive circuit 8 includes multiple drive circuits 8-n connected in series to correspond to each region (also called a segment) containing each optical waveguide element 4-n. Each drive circuit 8-n is configured to be driven based on an input drive signal of a different predetermined frequency band. The drive circuits 8-n are configured to be driven individually in accordance with the optical waveguide element 4-n.

The driving circuit 8-n includes, for example, a phase shifter Dn that applies an electric field to the first optical waveguide 4. In the drawings, the phase shifter Dn is shown as a PN diode. By driving the phase shifter Dn with a reverse bias, the diode can be approximated as an RC series circuit. The phase shifter Dn may also be replaced with an RC series circuit. The phase shifter Dn is provided with a parallel resistor Tn that terminates each segment. Between adjacent driving circuits 8-n, an electrical passive circuit Pn (n is an integer greater than or equal to 1) is provided, which is driven based on an input driving signal in a predetermined frequency band. The driving circuit 8-n is driven in conjunction with the operation of the electrical passive circuit Pn.

The electrical passive circuit Pn is formed by an RC circuit having passive elements such as resistors and capacitors (see Figure 2(A)). In the electrical passive circuit Pn, the inductance of the electrical wiring is not zero, so it may be configured to include inductance. Therefore, the electrical passive circuit Pn may be formed by an RCL circuit in which an inductor is provided in an RC circuit (see Figure 2(B)). The electrical passive circuit Pn is a filter circuit in which the capacitance of the capacitor C, the resistance of the resistor R, and the inductance of the inductor L are set so that it operates in response to an input drive signal in a specified frequency band.

As shown in Figure 3, each electrical passive circuit Pn is configured to individually drive each drive circuit 8-n in accordance with the arrangement position and the contribution of a specific frequency band to the desired frequency band, based on the input drive signal having the desired frequency band input from the signal input unit 10.

Each driver circuit 8-n individually applies a non-uniform electric field according to the drive amount based on the contribution of the electrical passive circuit Pn set according to the arrangement position to each optical waveguide element 4-n, thereby providing non-uniform modulation amounts to each optical waveguide element 4-n. The light modulated by each driver circuit 8-n is output from the optical output waveguide as output light modulated to the desired frequency band.

The multiple drive circuits 8-n and corresponding electrical passive circuits Pn are arranged, for example, in order of the magnitude of their contribution to the specified frequency band in which they are driven, and are connected in series. The drive circuits 8-n and corresponding electrical passive circuits Pn can also operate in a random arrangement that is not connected in series in order of the magnitude of their contribution to the specified frequency band in which they are driven.

The first driver circuit 8 is configured as a band self-adaptive driver circuit with the above-described structure. Band self-adaptation means that the contribution to modulation is made uneven depending on the contribution of the specific frequency band that the circuit itself has to the frequency band of the input drive signal, depending on the position of the segment corresponding to the modulator's phase shifter Dn.

The first drive circuit 8 is configured so that the contribution of the upstream segment in the light propagation direction to the optical modulation of the low-frequency band input drive signal is greater. In this case, the first drive circuit 8 is configured so that the contribution of each segment to the optical modulation of the low-frequency band input drive signal decreases as you move downstream along the light propagation direction.

In contrast, the first drive circuit 8 is configured so that the contribution of the upstream segment in the light propagation direction to the optical modulation of the high-frequency input drive signal is smaller. In this case, the first drive circuit 8 is configured so that the contribution of each segment to the optical modulation of the high-frequency input drive signal increases toward the downstream side along the light propagation direction. With this configuration, the first drive circuit 8 automatically balances the drive voltages operating in each segment of the modulator according to frequency. With this configuration, the first drive circuit 8 exceeds the inherent RC characteristics of the modulator, enabling the realization of an optical modulator with an ultra-high bandwidth, for example, of 100 GHz or more. Furthermore, with this configuration, the input drive signal (RF signal) input to the signal input unit 10 can be generated by a general-purpose single driver, simplifying the driver design.

Figure 4 shows a specific circuit configuration of the first drive circuit 8. In each drive circuit 8-n (n = 0 to 3) in each segment (seg1 to 4), the phase shifter Dn is configured as a series RC circuit. The electrical passive circuit Pn corresponding to the drive circuit 8-n is also configured as a series RC circuit. The electro-optical (EO) conversion band of the modulator is the band of the voltage amplitude Vc of the input drive signal divided by the capacitor C, and is set to directly correspond to the EO conversion band.

Figure 5 shows the calculated bandwidth of optical modulator 1. Conventional silicon optical modulators are driven in a bandwidth of approximately 40-50 GHz, for example. This is roughly equivalent to the RC bandwidth of seg1, with a resistance value of R = 12 and a capacitance of C = 0.3 pF ((= 1/2πRC = 44.2 GHz (approximately 45 GHz)). With the driving method for conventional optical modulators, they are driven in the bandwidth of seg1's voltage amplitude Vc, and even if the driving circuit is divided, each stage is driven in the same bandwidth, so the overall bandwidth of the optical modulator does not exceed 45 GHz.

In contrast, when the optical modulator 1 according to the present invention is configured with the drive circuit 8-n shown in Figure 3, an overall bandwidth expanded to 100 GHz, approximately double that of conventional methods, is obtained. With the optical modulator 1, the contribution of each segment (seg2, seg3, seg4) located downstream of the first segment (seg1) in the first drive circuit 8 increases toward the downstream side as the frequency of the input drive signal increases, and the voltage amplitude Vc increases accordingly, resulting in a bandwidth expansion effect.

Figure 6 shows another specific circuit configuration of the first drive circuit 8. In each drive circuit 8-n (n = 0 to 3) in each segment (seg1 to 4), the phase shifter Dn is configured based on a series RC circuit. Furthermore, the electrical passive circuit Pn (n = 1 to 3) corresponding to the drive circuit 8-n is configured as a series RCL circuit with an inductor.

Figure 7 shows the calculation results for the bandwidth of optical modulator 1. Conventional silicon optical modulators are driven within the bandwidth of the voltage amplitude Vc of segment 1, and even when the drive circuit is divided, each stage is driven within the same bandwidth, so the overall bandwidth of the optical modulator does not exceed 45 GHz. In contrast, when optical modulator 1 according to the present invention is configured with drive circuit 8-n shown in Figure 6, the maximum voltage amplitude Vc in the last segment seg4 is 100 GHz, expanding the overall bandwidth to approximately 120 GHz, approximately 2.7 times that of the conventional system. With optical modulator 1, the electrical passive circuit Pn corresponding to drive circuit 8-n is configured as a series RCL circuit with an inductor, thereby achieving a bandwidth expansion effect.

Figure 8 shows another specific circuit configuration of the first drive circuit 8. The first drive circuit 8 has eight segments (seg1 to seg8). In each drive circuit 8-n (n = 0 to 7), the phase shifter Dn is configured based on a series RC circuit. The electrical passive circuit Pn (n = 1 to 7) corresponding to the drive circuit 8-n is configured as a series RCL circuit.

Figure 9 shows the calculation results for the bandwidth of optical modulator 1. With conventional silicon optical modulators, the overall bandwidth of the optical modulator does not exceed 45 GHz. In contrast, when optical modulator 1 according to the present invention is configured with drive circuit 8-n as shown in Figure 8, the maximum voltage amplitude Vc in the last segment seg4 is 120 GHz, expanding the overall bandwidth to approximately 166 GHz, approximately 3.7 times that of the conventional configuration. With optical modulator 1, the bandwidth can be expanded by configuring eight stages of pairs in drive circuit 8-n.

As shown in Figure 10, the first drive circuit 8 may be driven based on an input drive signal with an adjusted current amplitude. Figure 11 shows the calculation results for the bandwidth of the optical modulator 1. When the optical modulator 1 according to the present invention is configured using the drive circuit 8-n shown in Figure 10, the overall bandwidth can be expanded to approximately 142 GHz.

As shown in Figure 12, the first drive circuit 8, which is driven based on the input drive signal, may be configured with even more stages to adjust the current amplitude. Figures 13 and 14 show the calculation results for the bandwidth of the optical modulator 1. When the optical modulator 1 according to the present invention is configured using the drive circuit 8-n shown in Figure 10, the overall bandwidth can be expanded to approximately 142.4 GHz. When driving the first drive circuit 8 based on an input drive signal with adjusted current amplitude, it is advantageous to increase the number of segments.

As shown in Figure 15, the electrical passive circuit Pn may be provided with a delay line Yn that delays the propagation of an input drive signal with an adjusted voltage amplitude. By providing the delay line Yn, the phase in each drive circuit 8-n can be aligned. Figure 16 shows the frequency characteristics of the delay line Yn (set to approximately 50 GHz when voltage driven). Figure 17 shows the calculation results for the bandwidth of the optical modulator 1. When the optical modulator 1 according to the present invention is configured with the drive circuit 8-n shown in Figure 15, the overall bandwidth can be expanded to approximately 119 GHz.

As shown in FIG. 18, the electrical passive circuit Pn may be provided with a delay line Yn that delays the propagation of the input drive signal whose current amplitude has been adjusted. By providing the delay line Yn, the phases in each drive circuit 8-n can be aligned.

Figure 19 shows the frequency characteristics of delay line Yn (set to approximately 10 GHz when driven by current). Figure 20 shows the calculation results for the bandwidth of optical modulator 1. When optical modulator 1 according to the present invention is configured with drive circuit 8-n shown in Figure 18, the overall bandwidth can be expanded to approximately 110 GHz.

Figure 21 shows the processing flow of the optical modulation method executed in the optical modulator 1. Below, we will explain the processing of the optical modulation method in the first optical waveguide 4, representing the first optical waveguide 4 out of the first and second optical waveguides 4 and 5. Input light is guided into the first optical waveguide 4 (step S100). An input drive signal having a desired frequency band is input to the first driver 8 (step S102). Based on the input of the input drive signal, multiple drive circuits 8-n connected in series corresponding to each region in the first optical waveguide 4, which is divided into multiple regions including multiple optical waveguide elements 4-n, are individually driven (step S104).

At this time, based on the input of the input drive signal, each drive circuit 8-n is individually driven based on the arrangement position and the drive amount corresponding to the contribution of the modulation amount to the desired frequency band, and an uneven electric field is individually applied to each optical waveguide element 4-n according to the drive amount based on the contribution, thereby unevenly modulating each optical waveguide element (step S106). Modulated output light is output from the optical output waveguide 7 (step S108).

As described above, the optical modulator 1 allows multiple optical waveguide elements 4-n to be individually driven by multiple driver circuits 8-n, individually applying a non-uniform electric field to each optical waveguide element 4-n, and providing non-uniform modulation amounts to each optical waveguide element. The optical modulator 1 automatically balances the drive voltages applied to each segment by multiple driver circuits 8-n according to frequency, exceeding the inherent RC characteristics of the modulator and achieving broadband and high-bandwidth modulation. The optical modulator 1 allows multiple driver circuits to be driven using a single driver that inputs a single input drive signal (RF signal), simplifying the device configuration.

The above describes one embodiment of the present invention, but the present invention is not limited to the above embodiment and can be modified as appropriate without departing from the spirit of the invention.

1 Optical modulator 4-n Optical waveguide element 5-n Optical waveguide element 7 Optical output waveguide 8-n Driving circuit C Capacitor L Inductor Pn Electrical passive circuit R Resistor Yn Delay line

Claims (7)

an optical waveguide for guiding input light;
an optical modulation unit provided in the optical waveguide and modulating the guided light;
an optical output waveguide that outputs the output light modulated by the optical modulation unit;
a driver that applies an electric field to the optical waveguide in the optical modulation unit based on an input drive signal to modulate the input light in accordance with a frequency band of the input drive signal,
the optical modulation section includes a plurality of optical waveguide elements that divide the optical waveguide into a plurality of regions,
The drive unit is
a plurality of drive circuits arranged in a line corresponding to each region including the optical waveguide elements , and provided to individually drive each of the optical waveguide elements for different predetermined frequency bands;
a plurality of electrical passive circuits formed by RC circuits having resistors and capacitors and electrically connected in series;
Equipped with
based on the input of the input drive signal having a desired frequency band, a non-uniform electric field is individually applied to each of the optical waveguide elements in accordance with the arrangement position of each of the drive circuits and the degree of contribution of the modulation amount to the desired frequency band, thereby non-uniformly applying modulation amounts to each of the optical waveguide elements;
outputting the modulated output light from the optical output waveguide;
the input drive signal is input to the plurality of electrical passive circuits electrically connected in series;
The optical modulator has the driving circuits electrically connected between adjacent electrically passive circuits electrically connected in series . the electrical passive circuit is formed by an RCL circuit in which an inductor is provided in the RC circuit;
2. The optical modulator according to claim 1 . 3. The optical modulator according to claim 1 , wherein the electrical passive circuit is provided with a delay line in the RC circuit. the plurality of electrical passive circuits are electrically connected in series in order of the magnitude of the predetermined frequency band;
4. The optical modulator according to claim 1. The optical modulator according to claim 1 , wherein the driving section is driven based on the input driving signal whose voltage amplitude has been adjusted. The optical modulator according to claim 1 , wherein the driving section is driven based on the input driving signal whose current amplitude has been adjusted. guiding input light into an optical waveguide that guides light;
an input drive signal having a desired frequency band is input to a drive unit that modulates the input light by applying an electric field to the optical waveguide in accordance with the frequency band of the input drive signal;
a plurality of drive circuits arranged in a line corresponding to each region including a plurality of optical waveguide elements obtained by dividing the optical waveguide into a plurality of regions, and each drive circuit being provided corresponding to each of the optical waveguide elements for each of different predetermined frequency bands;
based on the input of the input drive signal, a non-uniform electric field is individually applied to each of the optical waveguide elements in accordance with the arrangement position of each of the drive circuits and the degree of contribution of the modulation amount to the desired frequency band, thereby non-uniformly applying modulation amounts to each of the optical waveguide elements;
outputting the modulated output light from an optical output waveguide that outputs the output light ;
the input drive signal is input to a plurality of electrical passive circuits formed by RC circuits having resistors and capacitors and electrically connected in series in the drive unit;
The optical modulation method , wherein the driving circuits are electrically connected between adjacent electrically series-connected electrical passive circuits .