Optical Modulation Circuit

Abstract

To provide a small-sized and low-loss broadband optical modulation circuit in which a high-speed phase modulation part and a low-speed phase modulation part are provided in one modulator structure, and flat frequency modulation characteristics are accomplished from a DC component up to 10 GHz or more.SOLUTION: An optical modulation circuit comprises: an optical modulator constituted of a silicon optical waveguide; and a drive circuit. The optical modulator includes an optical modulator that has in each of two arm waveguides of a Mach-Zehnder interference waveguide: a pn depletion layer-type phase modulation part which bears high-frequency modulation and to which a reverse bias is applied; a pin injection-type phase modulation part which bears low-frequency modulation; and a phase modulation part which bears an arm balance by a heater. The drive circuit includes: a first drive circuit that generates and amplifies a high-frequency signal, applies a bias to the high-frequency signal, and outputs the same to the pn depletion layer-type phase modulation part; a second drive circuit that generates and amplifies a low-frequency signal and outputs the same to the pin injection-type phase modulation part; and a bias adjustment circuit that applies current to the heater.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to optical modulation circuits. [Background technology]

[0002] In a high-output electrical amplifier (generally 50 milliwatts or more into a 50-ohm load), if the cutoff frequency on the low-frequency side of the frequency band is set to 10 kilohertz or less, it is extremely difficult to set the cutoff frequency on the high-frequency side to 10 gigahertz or more. On the other hand, in the case of a silicon modulator, the drive voltage for an optical modulator requires a peak-to-peak value of about 5 volts even in push-pull operation, and an electrical amplifier with an output of 250 milliwatts into a 50-ohm load is required as the driver.

[0003] Furthermore, a reverse bias must be applied for high-speed operation, and a bias T is required to apply a DC bias voltage, but lowering the low-frequency cutoff for high-frequency signals increases the size of the device. As mentioned above, it is difficult to realize a high-output electrical amplifier in the frequency range from 1 kHz to 10 GHz. Therefore, the frequency band of commercially available electrical amplifiers for driving optical modulators has a high cutoff frequency on the low-frequency side, such as a frequency range from 100 kHz to 10 GHz.

[0004] For example, long-distance optical communications utilize modulation and demodulation methods that limit the frequency band of transmission signals to a range from 100 kilohertz to 10 gigahertz. However, optical packet transmission or optical burst transmission methods require optical modulators that can operate at frequencies from low frequencies below 10 kilohertz to high frequencies above 10 gigahertz, since there are no signal periods between packets or burst signals. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 4935093 Summary of the Invention [Problem to be solved by the invention]

[0006] Patent Document 1 discloses a technology for separating a transmission signal into low-speed and high-speed components, amplifying them, and electrically combining them to drive a modulator. Fig. 21 shows an optical modulation circuit 9z as a first comparative example based on the technology disclosed in Patent Document 1. The optical modulation circuit 9z includes a light source 501, an optical modulator 502, a signal source 503, a branching circuit 504, a modulator driver 505, a low-frequency amplifier 506, and an adder circuit 507. The optical modulator 502 is, for example, an optical modulator using lithium niobate. The cutoff frequency on the low-frequency side of the modulator driver 505 is approximately 100 kHz.

[0007] The optical modulation circuit 9z of the first comparative example has a problem with the specific configuration of the adder circuit. For example, addition is possible using a bias T. However, if the high-frequency cutoff frequency from the inductor terminal to the combination terminal of the bias T is fLC and the low-frequency cutoff frequency from the capacitor terminal to the combination terminal is fHC, then fHC≧fLC. Depending on the design of the bias T, it is difficult to set fLC above 10 kHz from the standpoint of noise, and fHC>fLC. In other words, the frequency band between the frequencies fLC and fHC is a frequency band that cannot be combined, which can result in significant waveform distortion depending on the optical signal. Patent Document 1 does not provide detailed descriptions of the adder circuit, and it is unclear how this problem is solved.

[0008] 22 shows an optical modulation circuit 9za of the second comparative example, which is a modification of the first comparative example. The optical modulation circuit 9za includes a second optical modulator 508 in addition to the optical modulation circuit 9z. The optical modulation circuit 9za of the second comparative example generates an optical packet or an optical burst signal by multi-stage modulation of an optical signal using low-speed modulation and high-speed modulation using two optical modulators and separate drive systems for each of the two optical modulators. The optical modulation circuit 9za of the second comparative example has the following problems: the circuit becomes larger due to the multi-stage modulators; the modulator loss increases; and phase synchronization between the modulator driver 505 and the low-frequency amplifier 506 is difficult because separate modulators are driven.

[0009] This disclosure provides a compact, low-loss wideband optical modulation circuit that has flat frequency modulation characteristics from DC components to 10 GHz or more by providing a high-speed phase modulation section and a low-speed phase modulation section in a single modulator structure. [Means for solving the problem]

[0010] In one aspect of the present disclosure, there is provided an optical modulation circuit comprising an optical modulator formed of a silicon optical waveguide and a drive circuit, wherein the optical modulator has a pn depletion layer type phase modulation unit that is responsible for high-frequency modulation and to which a reverse bias is applied, a pin injection type phase modulation unit that is responsible for low-frequency modulation, and a heater-based phase modulation unit that is responsible for arm balance, each of the two arm waveguides of a Mach-Zehnder type interference waveguide, and the drive circuit comprises a first drive circuit that generates and amplifies a high-frequency signal, applies a bias to the high-frequency signal, and outputs it to the pn depletion layer type phase modulation unit, a second drive circuit that generates and amplifies a low-frequency signal, and outputs it to the pin injection type phase modulation unit, and a bias adjustment circuit that applies current to the heater. [Effects of the Invention]

[0011] According to the present disclosure, it is possible to provide a small, low-loss, wide-band optical modulation circuit having flat frequency modulation characteristics over a wide band. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram showing an example of the configuration of an optical modulation circuit according to the first embodiment. [Figure 2] FIG. 2 is a diagram illustrating an optical waveguide of an optical modulator of the optical modulation circuit according to the first embodiment. [Figure 3] FIG. 3 is a diagram illustrating an optical modulator of the optical modulation circuit according to the first embodiment. [Figure 4] FIG. 4 is a cross-sectional view (part 1) of the optical modulator of the optical modulation circuit according to the first embodiment. [Figure 5] FIG. 5 is a cross-sectional view (part 2) of the optical modulator of the optical modulation circuit according to the first embodiment. [Figure 6] FIG. 6 is a cross-sectional view (part 3) of the optical modulator of the optical modulation circuit according to the first embodiment. [Figure 7] FIG. 7 is a cross-sectional view (part 4) of the optical modulator of the optical modulation circuit according to the first embodiment. [Figure 8] FIG. 8 is a cross-sectional view (part 5) of the optical modulator of the optical modulation circuit according to the first embodiment. [Figure 9] FIG. 9 is a cross-sectional view (part 6) of the optical modulator of the optical modulation circuit according to the first embodiment. [Figure 10] FIG. 10 is a cross-sectional view (part 7) of the optical modulator of the optical modulation circuit according to the first embodiment. [Figure 11] FIG. 11 is a diagram (part 1) for explaining the operation of the optical modulation circuit according to the first embodiment. [Figure 12] FIG. 12 is a diagram (part 2) for explaining the operation of the optical modulation circuit according to the first embodiment. [Figure 13] FIG. 13 is a diagram (part 3) for explaining the operation of the optical modulation circuit according to the first embodiment. [Figure 14] FIG. 14 is a diagram (part 4) for explaining the operation of the optical modulation circuit according to the first embodiment. [Figure 15] FIG. 15 is a diagram (part 5) for explaining the operation of the optical modulation circuit according to the first embodiment. [Figure 16] FIG. 16 is a diagram (part 6) for explaining the operation of the optical modulation circuit according to the first embodiment. [Figure 17]FIG. 17 is a diagram (part 7) for explaining the operation of the optical modulation circuit according to the first embodiment. [Figure 18] FIG. 18 is a diagram (part 8) for explaining the operation of the optical modulation circuit according to the first embodiment. [Figure 19] FIG. 19 is a diagram (part 9) for explaining the operation of the optical modulation circuit according to the first embodiment. [Figure 20] FIG. 20 is a diagram illustrating an example of the configuration of an optical modulation circuit according to the second embodiment. [Figure 21] FIG. 21 is a diagram illustrating a configuration example of an optical modulation circuit of a first comparative example. [Figure 22] FIG. 22 is a diagram illustrating a configuration example of an optical modulation circuit of a second comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0013] The optical modulation circuit according to this embodiment will be described in detail below with reference to the drawings.

[0014] <<First Embodiment>> <Optical modulation circuit 1> 1 is a diagram showing an example of the configuration of an optical modulation circuit 1 according to a first embodiment. The optical modulation circuit 1 according to the first embodiment phase-modulates input light La input from an input port Pin, and outputs output light Lb from a Bar port Pbar or output light Lc from a Cross port Pcross. The optical modulation circuit 1 includes an optical modulator 100 and a drive circuit 200.

[0015] [Optical modulator 100] The optical modulator 100 phase-modulates the input light La using a signal from a driving circuit 200. The optical modulator 100 includes a Mach-Zehnder interferometer 101 made of a silicon optical waveguide, a high-speed phase modulation unit 102, a low-speed phase modulation unit 103, and a phase correction unit 104.

[0016] The Mach-Zehnder interferometer 101 has a pair of arm waveguides (see arm A1 and arm A2 in FIG. 2) and converts a phase change between the arms into an amplitude change. That is, the Mach-Zehnder interferometer 101 has a Mach-Zehnder type interference waveguide.

[0017] The high-speed phase modulation unit 102 is responsible for high-frequency modulation and modulates light using an electrical high-frequency signal. The high-speed phase modulation unit 102 has a pn depletion layer type phase modulator. The high-speed phase modulation unit 102 has a modulator 102a that phase-modulates the light propagating through the arm A1 and a modulator 102b that phase-modulates the light propagating through the arm A2.

[0018] The slow phase modulation unit 103 is responsible for low-frequency modulation and modulates light with a low-frequency electrical signal. The slow phase modulation unit 103 has a pin injection type phase modulator. The slow phase modulation unit 103 has a modulator 103a that phase-modulates the light propagating through the arm A1 and a modulator 103b that phase-modulates the light propagating through the arm A2.

[0019] The phase corrector 104 adjusts the balance between the pair of arms (arm balance). Specifically, it adjusts the phase so that output light Lc is not output when output light Lb is output, and so that output light Lb is not output when output light Lc is output. The phase corrector 104 has a corrector 104a that corrects the phase of light propagating through arm A1, and a corrector 104b that corrects the phase of light propagating through arm A2.

[0020] FIG. 2 is a diagram illustrating an optical waveguide of the Mach-Zehnder interferometer 101 in the optical modulator 100 of the optical modulation circuit 1 according to the first embodiment.

[0021] The Mach-Zehnder interferometer 101 includes a waveguide coupler 106 with a branching ratio of 1:1 on each of the input and output sides. The Mach-Zehnder interferometer 101 has a pair of arms, an arm A1 and an arm A2, between the waveguide couplers 106.

[0022] The Mach-Zehnder interferometer 101 has a waveguide 107a at a position corresponding to the high-speed phase modulation unit 102 of the arm A1, and a waveguide 107b at a position corresponding to the high-speed phase modulation unit 102 of the arm A2. Furthermore, the Mach-Zehnder interferometer 101 has a waveguide 108a at a position corresponding to the low-speed phase modulation unit 103 of the arm A1, and a waveguide 108b at a position corresponding to the low-speed phase modulation unit 103 of the arm A2.

[0023] Each of the waveguides 107a, 107b, 108a, and 108b has a rib structure. Each of the waveguides 107a, 107b, 108a, and 108b is connected to the front and rear waveguides 105 via thin-wire-to-rib transition waveguides.

[0024] The following describes the waveguides in each arm of Mach-Zehnder interferometer 101. Mach-Zehnder interferometer 101 has, in arm A1, waveguide 105, waveguide 107a, waveguide 105, waveguide 108a, and waveguide 105, in that order from input-side waveguide coupler 106 to output-side waveguide coupler 106. Mach-Zehnder interferometer 101 also has, in arm A2, waveguide 105, waveguide 107b, waveguide 105, waveguide 108b, and waveguide 105, in that order from input-side waveguide coupler 106 to output-side waveguide coupler 106.

[0025] A description will be given of electrical wiring provided in the Mach-Zehnder interferometer 101. Fig. 3 is a diagram illustrating electrical wiring of the optical modulator 100 of the optical modulation circuit 1 according to the first embodiment.

[0026] The high-speed phase modulation unit 102 includes a signal wiring 112a, a signal wiring 112b, and a ground wiring 109. A bias application circuit 202a is connected to the signal wiring 112a. A bias application circuit 202b is connected to the signal wiring 112b. A high-speed signal is input to each of the signal wiring 112a and the signal wiring 112b. The signal wiring 112a and the ground wiring 109 form a coplanar line. The signal wiring 112b and the ground wiring 109 also form a coplanar line.

[0027] The signal wiring 112a and the ground wiring 109 are connected by a termination resistor 122a. The termination resistor 122a matches impedance with the coplanar line formed by the signal wiring 112a and the ground wiring 109. The signal wiring 112b and the ground wiring 109 are connected by a termination resistor 122b. The termination resistor 122b matches impedance with the coplanar line formed by the signal wiring 112b and the ground wiring 109.

[0028] The slow phase modulation unit 103 includes signal wiring 113a and signal wiring 113b, and ground wiring 133a and ground wiring 133b. The signal wiring 113a and signal wiring 113b are connected to a differential low-frequency signal amplifier circuit 203. A low-speed signal is input to each of the signal wiring 113a and signal wiring 113b.

[0029] The signal wiring 113a and the ground wiring 133a are connected by a matching resistor 123a. The signal wiring 113b and the ground wiring 133b are connected by a matching resistor 123b. Each of the matching resistors 123a and 123b has a resistance of about several tens of ohms.

[0030] The phase corrector 104 includes wiring 114a and wiring 114b, and wiring 134a and wiring 134b. The wiring 114a and wiring 134a are connected to the heater resistor 124a. The wiring 114b and wiring 134b are connected to the heater resistor 124b. When power is supplied to the heater resistor 124a and heater resistor 124b, the waveguide 105 provided near the heater resistor 124a and heater resistor 124b is heated. When the waveguide 105 is heated, the phase of the light propagating through the waveguide 105 changes. By changing the phase of the light propagating through the waveguide 105, the imbalance between the arms is corrected.

[0031] Next, we will explain the cross-sectional shape of the optical modulator 100. In the explanation of this disclosure, since the arms A1 and A2 are symmetrical in shape, we will explain only the arm A1 and omit the explanation of the arm A2.

[0032] 4 to 10 are cross-sectional views of the optical modulator 100 of the optical modulation circuit 1 according to the first embodiment. Specifically, FIG. 4 is a cross-sectional view taken along line AA in FIG. 3. FIG. 5 is a cross-sectional view taken along line BB in FIG. 3. FIG. 6 is a cross-sectional view taken along line CC in FIG. 3. FIG. 7 is a cross-sectional view taken along line DD in FIG. 3. FIG. 8 is a cross-sectional view taken along line EE in FIG. 3. FIG. 9 is a cross-sectional view taken along line FF in FIG. 3. FIG. 10 is a cross-sectional view taken along line GG in FIG. 3.

[0033] 4, which is a cross-sectional view taken along the line AA in FIG. 3, the optical modulator 100 includes a silicon substrate 140. The optical modulator 100 also includes a silicon oxide layer 150, which is a silicon oxide thin film, on the silicon substrate 140.

[0034] As shown in FIG. 4, which is a cross-sectional view taken along the line AA in FIG. 3, a signal wiring 112a and a ground wiring 109 are embedded inside the silicon oxide layer 150. A waveguide 105 is provided below the silicon oxide layer 150 between the signal wiring 112a and the ground wiring 109. The width W1 of the waveguide 105 is, for example, 450 nanometers. The height h1 of the waveguide 105 is, for example, 220 nanometers. The thickness D1 of the silicon oxide layer 150 is, for example, 5000 nanometers.

[0035] Next, FIG. 5, which is a cross-sectional view taken along the line BB in FIG. 3, will be described. The cross-sectional view BB shows part of the high-speed phase modulation unit 102. A rib waveguide 107as is provided below the signal wiring 112a and the ground wiring 109 in the silicon oxide layer 150. The rib waveguide 107as is made of silicon. Propagating light is confined in an area RA of the rib waveguide 107as. A step H2 of the rib structure of the rib waveguide 107as is, for example, 110 nanometers. Furthermore, a width W2 of the rib structure of the rib waveguide 107as is, for example, 600 nanometers.

[0036] Next, FIG. 6, which is a CC cross-sectional view of FIG. 3, will be described. The CC cross-sectional view is a part of the high-speed phase modulation unit 102. An n-type silicon layer 107an made of n-type silicon and a p-type silicon layer 107ap made of p-type silicon are provided below the signal wiring 112a and the ground wiring 109 in the silicon oxide layer 150. In other words, the high-speed phase modulation unit 102 is a pn depletion layer type phase modulation unit. The n-type silicon and p-type silicon are formed by known ion implantation during circuit fabrication. Note that the amount of ion implantation near the light confinement unit in the n-type silicon layer 107an and the p-type silicon layer 107ap may be reduced to reduce light loss.

[0037] The n-type silicon layer 107an is electrically connected to a signal wiring 112a. The p-type silicon layer 107ap is electrically connected to a ground wiring 109. The p-type silicon layer 107ap is grounded via the ground wiring 109. A positive potential is applied to the n-type silicon layer 107an by the signal wiring 112a. Therefore, a reverse bias is applied to the pn junction between the n-type silicon layer 107an and the p-type silicon layer 107ap.

[0038] When a reverse bias is applied to the pn junction between the n-type silicon layer 107an and the p-type silicon layer 107ap, a depletion layer is generated at the pn junction. The generation of the depletion layer causes a change in carrier density at the junction between the n-type silicon layer 107an and the p-type silicon layer 107ap. The refractive index at the junction between the n-type silicon layer 107an and the p-type silicon layer 107ap changes in response to the change in carrier density at the junction between the n-type silicon layer 107an and the p-type silicon layer 107ap. Therefore, the phase of light propagating through the rib waveguide formed by the n-type silicon layer 107an and the p-type silicon layer 107ap is modulated.

[0039] Since the depletion layer responds quickly to the application of voltage to the n-type silicon layer 107an and the p-type silicon layer 107ap, high-speed phase modulation section 102 can perform high-speed modulation of several tens of gigahertz.

[0040] The p-type silicon layer 107ap is an example of a first p-type silicon layer, and the n-type silicon layer 107an is an example of a first n-type silicon layer.

[0041] Next, FIG. 7, which is a DD cross-sectional view of FIG. 3, will be described. The DD cross-sectional view shows a part of the high-speed phase modulation unit 102. A termination resistor 122a is provided below the signal wiring 112a and the ground wiring 109. The termination resistor 122a is made of, for example, titanium nitride. A rib waveguide 107as is provided below the signal wiring 112a and the ground wiring 109 in the silicon oxide layer 150.

[0042] Next, a description will be given of Fig. 8, which is an E-E cross-sectional view of Fig. 3. A waveguide 105 is provided below the silicon oxide layer 150 between the signal wiring 112a and the ground wiring 109. The waveguide 105 is similar to the thin-wire waveguide 105 in Fig. 4.

[0043] Next, FIG. 9, which is an FF cross-sectional view of FIG. 3, will be described. The FF cross-sectional view shows a part of the slow phase modulation unit 103. An n-type silicon layer 108an, a p-type silicon layer 108ap, and an i-type silicon layer 108ai are provided below the signal wiring 112a and the ground wiring 109 in the silicon oxide layer 150. The n-type silicon layer 108an is made of n-type silicon. The p-type silicon layer 108ap is made of p-type silicon. The i-type silicon layer 108ai is made of intrinsic semiconductor silicon. In other words, the slow phase modulation unit 103 is a pin injection type phase modulation unit. The n-type silicon layer 108an, the p-type silicon layer 108ap, and the i-type silicon layer 108ai are connected to the front and rear 105 by thin-wire-to-rib conversion waveguides.

[0044] The n-type silicon layer 108an is electrically connected to a ground wiring 133a. The n-type silicon layer 108an is grounded via the ground wiring 133a. The p-type silicon layer 108ap is electrically connected to a signal wiring 113a. A positive potential is applied to the p-type silicon layer 108ap via the signal wiring 113a. Therefore, a forward current is injected between the n-type silicon layer 108an and the p-type silicon layer 108ap via the i-type silicon layer 108ai. That is, a forward current is injected into the pin structure formed by the p-type silicon layer 108ap, the i-type silicon layer 108ai, and the n-type silicon layer 108an.

[0045] When a current is injected into the pin structure in the forward direction, the refractive index changes due to the carrier plasma effect. Therefore, the phase of the light propagating through the rib waveguide formed by the p-type silicon layer 108ap, i-type silicon layer 108ai, and n-type silicon layer 108an is modulated. The pin-type optical modulation circuit can modulate frequencies from 200 to 300 MHz. Although the maximum modulation speed is lower than that achieved by reverse bias modulation, the optical propagation loss is approximately 1 / 10.

[0046] The p-type silicon layer 108ap is an example of a second p-type silicon layer, and the n-type silicon layer 108an is an example of a second n-type silicon layer.

[0047] Next, a description will be given of Fig. 10, which is a cross-sectional view taken along line GG in Fig. 3. The cross-sectional view GG is a part of the phase correction unit 104. A heater resistor 124a is provided above the waveguide 105. The heater resistor 124a is made of, for example, titanium nitride.

[0048] [Drive circuit 200] The drive circuit 200 generates a drive signal from the signals input from the differential high-frequency signal generation circuit 300 and the differential low-frequency signal generation circuit 400, and outputs the drive signal to the optical modulator 100. The drive circuit 200 also generates a signal to drive a heater that adjusts the imbalance between the arms, and outputs the signal to the optical modulator 100.

[0049] The drive circuit 200 will be described using Figure 1. The drive circuit 200 includes a differential high-frequency signal amplifier circuit 201, bias application circuits 202a and 202b, a differential low-frequency signal amplifier circuit 203, and a heater drive circuit 204. The differential high-frequency signal amplifier circuit 201, bias application circuits 202a and 202b are collectively referred to as a high-frequency drive circuit 205. The high-frequency drive circuit 205 amplifies the high-frequency signal from the differential high-frequency signal generation circuit 300 and applies a bias. The differential high-frequency signal amplifier circuit 201 amplifies the low-frequency signal from the differential low-frequency signal generation circuit 400.

[0050] FIG. 11 shows an example of the output signal V1 from the differential low-frequency signal generation circuit 400. The upper graph in FIG. 11 shows the positive voltage signal V1+ of the output signal V1 from the differential low-frequency signal generation circuit 400, which is a differential signal. The lower graph in FIG. 11 shows the negative voltage signal V1- of the output signal V1 from the differential low-frequency signal generation circuit 400, which is also a differential signal. The vertical axis represents voltage, and the horizontal axis represents time. The positive voltage signal V1+ and the negative voltage signal V1- are in an antiphase relationship. Furthermore, both the positive voltage signal V1+ and the negative voltage signal V1- contain a DC component.

[0051] Fig. 12 shows an example of the output signal V2 from the differential high-frequency signal generation circuit 300. The upper graph in Fig. 12 shows the positive voltage signal V2+ of the output signal V2, which is a differential signal, from the differential high-frequency signal generation circuit 300. The lower graph in Fig. 12 shows the negative voltage signal V2- of the output signal V2, which is also a differential signal, from the differential high-frequency signal generation circuit 300. The vertical axis represents voltage, and the horizontal axis represents time. The positive voltage signal V2+ and the negative voltage signal V2- are in opposite phase to each other.

[0052] Note that the time axis in FIG. 11 and the time axis in FIG. 12 have different scales. Specifically, time T1 in FIG. 11 is 1000 times larger than time T2 in FIG. 12. In other words, the time axis in FIG. 12 is displayed by enlarging the time axis in FIG. 11 by 1000 times. The same applies to the following graphs. To indicate the time axis, time T1 and time T2 are shown on each graph.

[0053] The lower cutoff frequency of differential low-frequency signal amplifier circuit 203 is assumed to be frequency fL1. Also, differential low-frequency signal amplifier circuit 203 has a bandwidth from DC to frequency fH1 (where frequency fH1 is greater than frequency fL1). Amplifiers capable of outputting a large output of 50 milliwatts or more for a load of 50 ohms, from DC to a frequency of 500 MHz, are already commercially available. While differential low-frequency signal amplifier circuit 203 according to the first embodiment is configured as a single circuit, it may also be provided with two circuits that separately amplify each signal of the differential signal.

[0054] Figure 13 shows the output signal of the differential low-frequency signal amplifier circuit 203. The upper graph in Figure 13 shows the positive voltage signal V3+ of the output signal V3 from the differential low-frequency signal amplifier circuit 203. The lower graph in Figure 13 shows the negative voltage signal V3- of the output signal V3 from the differential low-frequency signal amplifier circuit 203. The vertical axis represents voltage, and the horizontal axis represents time.

[0055] The voltage signal V3+ is applied to the signal wiring 113a of the modulator 103a in the slow phase modulation unit 103. The ground wiring 133a of the modulator 103a is grounded. That is, the potential of the ground wiring 133a is 0 volts. The voltage signal V3- is applied to the signal wiring 113b of the modulator 103b in the slow phase modulation unit 103. The ground wiring 133b of the modulator 103b is grounded. That is, the potential of the ground wiring 133b is 0 volts. The peak-to-peak values ​​of the voltage signals V3+ and V3- are each a voltage Vpp1.

[0056] In the modulator 103a, which is a pin injection type phase modulator, the potential of the electrode on the p-type silicon layer 108ap side must be higher than the potential of the electrode on the n-type silicon layer 108an side.

[0057] 14 shows the optical output characteristics of the optical modulator 100. The vertical axis represents the optical intensity of the optical output, and the horizontal axis represents the voltage of the voltage signal V3+. It is assumed that a slight reverse bias voltage of about 0.1 volts is applied to the signal wiring 112a and the signal wiring 112b.

[0058] 15 shows the optical signal waveform from the optical modulator 100 when the voltage signal shown in Fig. 13 is applied. The vertical axis represents the optical intensity of the output light Lb, and the horizontal axis represents time. Line Lb1 represents the waveform of the output light Lb.

[0059] Figure 16 shows an example of the output signal V4+ of the bias application circuit 202a and the output signal V4- of the bias application circuit 202b. The upper graph in Figure 16 shows the output signal V4+ of the bias application circuit 202a. The lower graph in Figure 16 shows the output signal V4- of the bias application circuit 202b. The vertical axis represents voltage, and the horizontal axis represents time.

[0060] Each of the bias application circuits 202a and 202b is, for example, a bias T. Therefore, a bias potential Vs is applied to each of the output signals V4+ and V4-. The peak-to-peak values ​​of each of the output signals V4+ and V4- are voltages Vpp2.

[0061] The output signal V4+ is applied to the n-type silicon layer 107an via the signal wiring 112a. The p-type silicon layer 107ap is grounded via the ground wiring 109. Therefore, the potential of the p-type silicon layer 107ap is 0 volts. The output signal V4- is applied to the n-type silicon layer of the modulator 102b via the signal wiring 112b.

[0062] 17 shows the optical output characteristics of the optical modulator 100. The vertical axis represents the optical intensity of the optical output, and the horizontal axis represents the difference between the output signals V4+ and V4-. At this time, a slight forward bias voltage of about 0.1 volts is applied to the signal wiring 113a and the signal wiring 113b. The optical output characteristics can be adjusted, as indicated by the arrows in the figure, by changing the current injected into the wiring 114a and the wiring 114b, respectively.

[0063] 18 shows the optical signal waveform from the optical modulator 100 when the voltage signal shown in Fig. 16 is applied. The vertical axis represents the optical intensity of the output light Lb, and the horizontal axis represents time. Line Lb2 represents the waveform of the output light Lb.

[0064] The heater drive circuit 204 supplies current to each of the heater resistors 124a and 124b. Each of the heater resistors 124a and 124b generates heat when a current is supplied from the heater drive circuit 204. The amount of heat generated by each of the heater resistors 124a and 124b is appropriately adjusted by the heater drive circuit 204, thereby adjusting the arm balance and adjusting the bias of the output light.

[0065] Next, FIG. 19 shows the optical output when a low-frequency signal is input simultaneously to signal wiring 113a and signal wiring 113b and a high-frequency signal is input simultaneously to signal wiring 112a and signal wiring 112b in phase with each other, and current is appropriately supplied to wiring 114a and wiring 114b to control the operating point. The vertical axis represents the optical intensity of output light Lb, and the horizontal axis represents time. Line Lb3 represents the waveform of output light Lb. As shown in FIG. 19, output light Lb based on the high-frequency signal and the low-frequency signal is output from optical modulator 100.

[0066] The high frequency drive circuit 205 is an example of a first drive circuit, the differential low frequency signal amplifier circuit 203 is an example of a second drive circuit, and the heater drive circuit 204 is an example of a bias adjustment circuit.

[0067] <Actions and Effects> The optical modulation circuit 1 according to the first embodiment can generate ultra-wideband optical packet signals, optical burst signals, etc., containing high-speed frequency components (10 GHz or higher) from DC components. The optical modulation circuit 1 according to the first embodiment uses a single optical modulator, and therefore does not require extra connections compared to wideband optical modulation circuits that use multi-stage optical modulators, making it possible to realize a compact, low-loss wideband optical modulation circuit.

[0068] The optical modulation circuit 1 according to the first embodiment can reduce loss for modulation up to approximately 200 MHz by using a pin injection type phase modulation section. Furthermore, the optical modulation circuit 1 according to the first embodiment can modulate above 200 MHz by using a pn depletion layer type phase modulation section.

[0069] The optical modulation circuit 1 according to the first embodiment can generate a wideband optical signal of 10 GHz or more from a DC component by simultaneously driving a pin injection type phase modulation unit and a pn depletion layer type phase modulation unit. Furthermore, since the optical modulation circuit 1 according to the first embodiment has a pin injection type phase modulation unit and a pn depletion layer type phase modulation unit close to each other, it becomes easy to synchronize the phases of multiple drive signals and can also suppress the occurrence of wander.

[0070] <<Second embodiment>> <Optical modulation circuit 2> The optical modulation circuit 2 according to the second embodiment differs from the optical modulation circuit 1 according to the first embodiment in the input signal and the drive circuit that processes the input signal.

[0071] 20 is a diagram showing an example of the configuration of an optical modulation circuit 2 according to the second embodiment. The optical modulation circuit 2 includes a driving circuit 200A instead of the driving circuit 200 of the optical modulation circuit 1. The driving circuit 200A further includes a frequency separation circuit 209 in addition to the configuration of the driving circuit 200.

[0072] A wideband signal obtained by adding a low-frequency signal and a high-frequency signal is input from wideband signal generator 350 to optical modulation circuit 2. Frequency separation circuit 209 separates the wideband signal input from wideband signal generator 350 into a low-frequency signal and a high-frequency signal. Frequency separation circuit 209 then inputs the separated low-frequency signal and high-frequency signal to differential low-frequency signal amplifier circuit 203 and differential high-frequency signal amplifier circuit 201, respectively.

[0073] The frequency separation circuit 209 can be configured, for example, by connecting a low-pass filter and a high-pass filter in parallel, so that the wide-band cutoff frequency of the low-pass filter and the low-band cutoff frequency of the high-pass filter match.

[0074] The optical modulation circuit 2 of the second embodiment differs from the optical modulation circuit 1 of the first embodiment in the method of generating low-frequency signals and high-frequency signals, but operates in the same manner as the optical modulation circuit 1 of the first embodiment and has the same functions and effects.

[0075] Although the optical modulation circuit has been described above using the embodiments, the present invention is not limited to the above-described embodiments. Various modifications and improvements, such as combinations or substitutions with part or all of other embodiments, are possible within the scope of the present invention. [Explanation of symbols]

[0076] 1, 2 Optical modulation circuit 100 Optical Modulator 101 Mach-Zehnder Interferometer 102 High-speed phase modulation section 102a, 102b Modulators 103 Slow phase modulation section 103a, 103b Modulator 104 Phase correction section 104a, 104b corrector 105 Waveguide 106 Waveguide Coupler 107a, 107b waveguide 107an n-type silicon layer 107ap p-type silicon layer 108a, 108b waveguide 108ai i-type silicon layer 108an n-type silicon layer 108ap p-type silicon layer 109 Ground wiring 112a, 112b signal wiring 113a, 113b signal wiring 114a, 114b wiring 124a, 124b Heater resistors 133a, 133b ground wiring 134a, 134b wiring 200, 200A drive circuit 201 Differential high frequency signal amplifier circuit 202a, 202b bias application circuit 203 Differential low-frequency signal amplifier circuit 204 Heater drive circuit 205 High frequency drive circuit 209 Frequency Separation Circuit 300 Differential high frequency signal generation circuit 350 Wideband Signal Generator 400 Differential low-frequency signal generation circuit A1, A2 arms

Claims

[Claim 1] an optical modulator configured with a silicon optical waveguide; a drive circuit, The optical modulator comprises: In each of the two arm waveguides of the Mach-Zehnder interferometer, a pn depletion layer type phase modulation section that performs high frequency modulation and to which a reverse bias is applied; a pin injection type phase modulation unit that performs low frequency modulation; a phase modulation unit using a heater that is responsible for arm balance, The pn depletion layer type phase modulation portion is a first signal wiring extending in a first direction; a first ground wiring extending along the first direction; a first waveguide, a first rib waveguide, a second rib waveguide, a third rib waveguide, and a second waveguide that are provided in a layer below the first signal wiring and the first ground wiring, that are arranged side by side in the first direction, and that are connected to each other; Equipped with each of the first waveguide and the second waveguide is provided below and between the first signal wiring and the first ground wiring; each of the first rib waveguide and the third rib waveguide is formed of silicon; the second rib waveguide includes a first p-type silicon layer formed of p-type silicon, and a first n-type silicon layer formed of n-type silicon and bonded to the first p-type silicon layer; the first rib waveguide, the second rib waveguide, and the third rib waveguide each have a rib structure including a first rib provided at a position aligned with the first waveguide in the first direction, a second rib provided below the first signal wiring, and a third rib provided below the first ground wiring; the first rib, the second rib, and the third rib in the first rib waveguide are formed from the same silicon; the first p-type silicon layer in the second rib waveguide is connected to the first ground wiring at the third rib in the second rib waveguide; the first n-type silicon layer in the second rib waveguide is connected to the first signal wiring at the second rib in the second rib waveguide; a junction between the first p-type silicon layer and the first n-type silicon layer in the second rib waveguide is provided on the first rib in the second rib waveguide, the first rib, the second rib, and the third rib in the third rib waveguide are formed from the same silicon; The pin injection type phase modulation unit is A second signal wiring; A second ground wiring; a fourth rib waveguide provided in a layer below the second signal wiring and the second ground wiring, and arranged alongside and connected to the second waveguide along the first direction; Equipped with the fourth rib waveguide comprises: a second p-type silicon layer formed of p-type silicon; an i-type silicon layer formed of intrinsic semiconductor silicon and bonded to the second p-type silicon layer; and a second n-type silicon layer formed of n-type silicon and bonded to the i-type silicon layer; the fourth rib waveguide has a rib structure including a fourth rib provided at a position aligned with the second waveguide in the first direction, a fifth rib provided below the second signal wiring, and a sixth rib provided below the second ground wiring; the second p-type silicon layer in the fourth rib waveguide is connected to the second signal wiring at the fifth rib in the fourth rib waveguide; the second n-type silicon layer in the fourth rib waveguide is connected to the second ground wiring at the sixth rib in the fourth rib waveguide; the i-type silicon layer is provided on the fourth rib of the fourth rib waveguide, The phase modulation unit a fourth waveguide arranged alongside and connected to the fourth rib waveguide along the first direction; a heater resistor provided on the fourth waveguide; Equipped with The drive circuit a first drive circuit that amplifies a high frequency signal, applies a bias to the high frequency signal, and outputs the signal to the first signal wiring of each of the two arm waveguides; a second drive circuit that amplifies a low-frequency signal and outputs the amplified low-frequency signal to the second signal wiring of each of the two arm waveguides; a bias adjustment circuit that applies a current to the heater resistor of each of the two arm waveguides; Optical modulation circuit.

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