Optical modulator and manufacturing method of optical modulator

Abstract

To provide an optical modulator capable of decreasing intensity of light inputted to each waveguide of a Mach-Zehnder modulation part, and to provide a manufacturing method of the optical modulator.SOLUTION: An optical modulator comprises: a first Mach-Zehnder modulation part having a first waveguide and a second waveguide; a second Mach-Zehnder modulation part having a third waveguide and a fourth waveguide; and a third Mach-Zehnder modulation part which is optically coupled to the first Mach-Zehnder modulation part and the second Mach-Zehnder modulation part. The third Mach-Zehnder modulation part includes: a fifth waveguide having an input end which is optically coupled to an output end of the first waveguide; a sixth waveguide having an input end which is optically coupled to an output end of the second waveguide; a seventh waveguide having an input end which is optically coupled to an output end of the third waveguide; and an eighth waveguide having an input end which is optically coupled to an output end of the fourth waveguide.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to an optical modulator and a method for manufacturing an optical modulator. [Background technology]

[0002] Patent Document 1 discloses an optical modulator. The optical modulator includes a first Mach-Zehnder modulation section and a second Mach-Zehnder modulation section. The first Mach-Zehnder modulation section includes a first waveguide and a second waveguide. The second Mach-Zehnder modulation section includes a third waveguide and a fourth waveguide. Output ends of the first waveguide and the second waveguide are optically coupled to a fifth waveguide via an optical coupler. Output ends of the third waveguide and the fourth waveguide are optically coupled to a sixth waveguide via an optical coupler. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-33042 [Patent Document 2] U.S. Patent No. 9,069,223 Summary of the Invention [Problem to be solved by the invention]

[0004] In the optical modulator of Patent Document 1, light from the input waveguide propagates through the first and second waveguides, and the light propagated through each waveguide is combined by an optical coupler. Similarly, light from the input waveguide propagates through the third and fourth waveguides, and the light propagated through each waveguide is combined by an optical coupler. Then, the light combined by the optical coupler is guided to the fifth and sixth waveguides. In this case, the intensity of the light propagating through the fifth waveguide is the sum of the intensity of the light propagating through the first waveguide and the intensity of the light propagating through the second waveguide. The intensity of the light propagating through the sixth waveguide is the sum of the intensity of the light propagating through the third waveguide and the intensity of the light propagating through the fourth waveguide. Therefore, when a Mach-Zehnder modulation section having the fifth waveguide and the sixth waveguide is used, the intensity of light input to each waveguide of the Mach-Zehnder modulation section becomes higher than the intensity of light propagating through each waveguide from the first waveguide to the fourth waveguide.

[0005] The present disclosure provides an optical modulator capable of reducing the intensity of light input to each waveguide of a Mach-Zehnder modulation section, and a method for manufacturing the optical modulator. [Means for solving the problem]

[0006] An optical modulator according to one aspect of the present disclosure includes a first Mach-Zehnder modulation section having a first waveguide and a second waveguide, a second Mach-Zehnder modulation section having a third waveguide and a fourth waveguide, a third Mach-Zehnder modulation section optically coupled to the first Mach-Zehnder modulation section and the second Mach-Zehnder modulation section, and an optical coupler optically coupled to the third Mach-Zehnder modulation section, wherein the third Mach-Zehnder modulation section includes a fifth waveguide having an input end optically coupled to an output end of the first waveguide, and an output end of the second waveguide. a sixth waveguide having an input end optically coupled to an output end of the third waveguide, a seventh waveguide having an input end optically coupled to an output end of the fourth waveguide, an eighth waveguide having an input end optically coupled to an output end of the fourth waveguide, a first electrode connected to the fifth waveguide and the sixth waveguide, and a second electrode connected to the seventh waveguide and the eighth waveguide, wherein the optical coupler is optically coupled to the output end of the fifth waveguide, the output end of the sixth waveguide, the output end of the seventh waveguide, and the output end of the eighth waveguide. Effect of the Invention

[0007] According to the present disclosure, an optical modulator capable of reducing the intensity of light input to each waveguide of a Mach-Zehnder modulation section and a method for manufacturing the optical modulator are provided. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a plan view illustrating a schematic configuration of an optical modulator according to an embodiment. [Diagram 2] FIG. 2 is a cross-sectional view taken along line II-II in FIG. [Diagram 3] FIG. 3 is a cross-sectional view taken along line III-III in FIG. [Figure 4] FIG. 4 is a diagram showing the relationship between the intensity of light propagating through a waveguide and the voltage that can be applied to an electrode connected to the waveguide. [Diagram 5] FIG. 5 is a flowchart showing a method for manufacturing the optical modulator according to this embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] [Description of the embodiments of the present disclosure] First, the contents of the embodiments of the present disclosure will be listed and described.

[0010] (1) An optical modulator according to one embodiment includes a first Mach-Zehnder modulation unit having a first waveguide and a second waveguide, a second Mach-Zehnder modulation unit having a third waveguide and a fourth waveguide, a third Mach-Zehnder modulation unit optically coupled to the first Mach-Zehnder modulation unit and the second Mach-Zehnder modulation unit, and an optical coupler optically coupled to the third Mach-Zehnder modulation unit, wherein the third Mach-Zehnder modulation unit includes a fifth waveguide having an input end optically coupled to an output end of the first waveguide, and an output end of the second waveguide. a sixth waveguide having an input end optically coupled to an output end of the third waveguide, a seventh waveguide having an input end optically coupled to an output end of the fourth waveguide, an eighth waveguide having an input end optically coupled to an output end of the fourth waveguide, a first electrode connected to the fifth waveguide and the sixth waveguide, and a second electrode connected to the seventh waveguide and the eighth waveguide, wherein the optical coupler is optically coupled to the output end of the fifth waveguide, the output end of the sixth waveguide, the output end of the seventh waveguide, and the output end of the eighth waveguide.

[0011] According to the optical modulator of this embodiment, the intensity of light input to each waveguide (the fifth waveguide to the eighth waveguide) of the third Mach-Zehnder modulation section can be reduced.

[0012] (2) In the above (1), the optical modulator may further include a ninth waveguide connecting the output end of the first waveguide and the input end of the fifth waveguide, and an upper clad of the ninth waveguide may include a semiconductor layer of a first conductivity type, and each of the upper clads of the first waveguide and the fifth waveguide may include a semiconductor layer of a second conductivity type.

[0013] (3) In the above (1) or (2), the optical coupler may include a first optical coupler optically coupled to the output end of the fifth waveguide and the output end of the sixth waveguide, a second optical coupler optically coupled to the output end of the seventh waveguide and the output end of the eighth waveguide, and a third optical coupler optically coupled to the first optical coupler and the second optical coupler.

[0014] (4) A method for manufacturing an optical modulator according to this embodiment includes the steps of preparing an optical modulator according to any one of (1) to (3) above, the optical modulator further including a third electrode connected to the first waveguide, a fourth electrode connected to the second waveguide, a fifth electrode connected to the third waveguide, and a sixth electrode connected to the fourth waveguide; adjusting voltages applied to the third electrode and the fourth electrode; adjusting voltages applied to the fifth electrode and the sixth electrode; adjusting a voltage applied to the first electrode based on a result of adjusting the voltages applied to the third electrode and the fourth electrode; adjusting a voltage applied to the second electrode based on a result of adjusting the voltages applied to the fifth electrode and the sixth electrode; readjusting the voltages applied to the third electrode and the fourth electrode based on a result of adjusting the voltage applied to the first electrode; and readjusting the voltages applied to the fifth electrode and the sixth electrode based on a result of adjusting the voltage applied to the second electrode.

[0015] [Details of the embodiment of the present disclosure] Hereinafter, an embodiment of the present disclosure will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same or equivalent elements are designated by the same reference numerals, and duplicated descriptions are omitted. In the drawings, X-axis, Y-axis, and Z-axis directions that intersect with each other are shown as necessary. For example, the X-axis, Y-axis, and Z-axis directions are perpendicular to each other.

[0016] FIG. 1 is a plan view showing an optical modulator according to the present embodiment. The optical modulator 1 can modulate the intensity or phase of light in optical communication, for example, to generate a modulated signal. The optical modulator 1 may include an input port P1, a substrate 11, an input waveguide W1, waveguides W2, W3, W4, W5, W6, W7, and W8, an optical filter C1, optical couplers C2, C3, C4, C5, C6, C7, C8, and C9, a first optical modulation section M1, a second optical modulation section M2, a first output waveguide W9a, a first output port P2, a second output waveguide W9b, and a second output port P3. The optical modulator 1 may not include the optical couplers C4, C7, and C8, the second optical modulation section M2, the second output waveguide W9b, and the second output port P3.

[0017] The substrate 11 extends along the X-axis direction and the Y-axis direction. When viewed from the Z-axis direction, the substrate 11 may have a substantially rectangular shape. When viewed from the Z-axis direction, an input port P1 is provided at a first end 11a of the substrate 11 in the X-axis direction. The input port P1 is located at the center of the first end 11a of the substrate 11 in the Y-axis direction.

[0018] An input waveguide W1 is provided on the substrate 11. An input end of the input waveguide W1 is connected to an input port P1. For example, the input end of the input waveguide W1 is the input port P1. An output end of the input waveguide W1 is optically coupled to an input end of an optical filter C1. An output end of the optical filter C1 is optically coupled to an input end of a waveguide W2. The optical filter C1 is, for example, a one-input, one-output coupler. An output end of the waveguide W2 is optically coupled to an input end of an optical coupler C2. An output end of the optical coupler C2 is optically coupled to an input end of a waveguide W3 and an input end of a waveguide W4. The optical coupler C2 is, for example, a one-input, two-output MMI (Multi-Mode Interface) coupler.

[0019] The output end of the waveguide W3 is optically coupled to the input end of the optical coupler C3. The two output ends of the optical coupler C3 are optically coupled to the input end of the waveguide W5 and the input end of the waveguide W6, respectively. The output end of the waveguide W4 is optically coupled to the input end of the optical coupler C4. The two output ends of the optical coupler C4 are optically coupled to the input end of the waveguide W7 and the input end of the waveguide W8, respectively. The optical couplers C3 and C4 are, for example, one-input, two-output MMI couplers.

[0020] The output end of waveguide W5 is optically coupled to the input end of optical coupler C5. The output end of waveguide W6 is optically coupled to the input end of optical coupler C6. The output end of waveguide W7 is optically coupled to the input end of optical coupler C7. The output end of waveguide W8 is optically coupled to the input end of optical coupler C8.

[0021] The first optical modulation section M1 and the second optical modulation section M2 are provided on a substrate 11. The first optical modulation section M1 and the second optical modulation section M2 are arranged to sandwich the input waveguide W1 in the Y-axis direction. The first optical modulation section M1 and the second optical modulation section M2 are, for example, IQ optical modulation elements. Each of the first optical modulation section M1 and the second optical modulation section M2 has a first Mach-Zehnder modulation section MZ1, a second Mach-Zehnder modulation section MZ2, and a third Mach-Zehnder modulation section MZ3.

[0022] The first Mach-Zehnder modulation unit MZ1 has a waveguide W10 (first waveguide), a waveguide W11 (second waveguide), an electrode E1a (third electrode), and an electrode E2a (fourth electrode). The waveguide W10 and the waveguide W11 are the first arm waveguide and the second arm waveguide of the first Mach-Zehnder modulation unit MZ1, respectively. The waveguide W10 and the waveguide W11 are provided on a substrate 11 and have a height in the Z-axis direction. An input end of the waveguide W10 and an input end of the waveguide W11 are optically coupled to two output ends of an optical coupler C5, respectively. The optical coupler C5 is, for example, a one-input, two-output MMI coupler.

[0023] The electrode E1a is connected to the waveguide W10. The electrode E1a is provided on the waveguide W10. The electrode E1a applies a voltage to the waveguide W10. The electrode E1a is electrically connected to an electrode pad E1c via a wiring E1b. A voltage is applied to the electrode pad E1c from a power source (not shown).

[0024] The electrode E2a is connected to the waveguide W11. The electrode E2a is provided on the waveguide W11. The electrode E2a applies a voltage to the waveguide W11. The electrode E2a is electrically connected to an electrode pad E2c via a wiring E2b. A voltage is applied to the electrode pad E2c from a power source (not shown).

[0025] The second Mach-Zehnder modulation unit MZ2 has a waveguide W12 (third waveguide), a waveguide W13 (fourth waveguide), an electrode E3a (fifth electrode), and an electrode E4a (sixth electrode). The waveguide W12 and the waveguide W13 are the first arm waveguide and the second arm waveguide of the second Mach-Zehnder modulation unit MZ2, respectively. The waveguide W12 and the waveguide W13 are provided on the substrate 11 and have a height in the Z-axis direction. The input end of the waveguide W12 and the input end of the waveguide W13 are optically coupled to two output ends of an optical coupler C6, respectively. The optical coupler C6 is, for example, a one-input, two-output MMI coupler.

[0026] The electrode E3a is connected to the waveguide W12. The electrode E3a is provided on the waveguide W12. The electrode E3a applies a voltage to the waveguide W12. The electrode E3a is electrically connected to an electrode pad E3c via a wiring E3b. A voltage is applied to the electrode pad E3c from a power source (not shown).

[0027] The electrode E4a is connected to the waveguide W13. The electrode E4a is provided on the waveguide W13. The electrode E4a applies a voltage to the waveguide W13. The electrode E4a is electrically connected to an electrode pad E4c via a wiring E4b. A voltage is applied to the electrode pad E4c from a power source (not shown).

[0028] The third Mach-Zehnder modulation section MZ3 has a waveguide W14 (fifth waveguide), a waveguide W15 (sixth waveguide), a waveguide W16 (seventh waveguide), a waveguide W17 (eighth waveguide), an electrode E5a (first electrode), and an electrode E6a (second electrode). The waveguide W14 and the waveguide W15 extend apart from each other. The waveguide W16 and the waveguide W17 extend apart from each other. The waveguide W14 and the waveguide W15 function as a first arm waveguide of the third Mach-Zehnder modulation section MZ3. The waveguide W16 and the waveguide W17 function as a second arm waveguide of the third Mach-Zehnder modulation section MZ3. The waveguide W14, the waveguide W15, the waveguide W16, and the waveguide W17 are provided on the substrate 11 and have a height in the Z-axis direction.

[0029] The input end of waveguide W14 is optically coupled to the output end of waveguide W10, and the input end of waveguide W15 is optically coupled to the output end of waveguide W11.

[0030] The input end of waveguide W16 is optically coupled to the output end of waveguide W12, and the input end of waveguide W17 is optically coupled to the output end of waveguide W13.

[0031] The electrode E5a is connected to the waveguide W14 and the waveguide W15. The electrode E5a includes a first portion E5a1 provided on the waveguide W14, a second portion E5a2 provided on the waveguide W15, and a third portion E5a3 connecting the first portion and the second portion. The electrode E5a applies a voltage to the waveguide W14 and the waveguide W15. The electrode E5a is electrically connected to the electrode pad E5c via the wiring E5b. A voltage is applied to the electrode pad E5c from a power source (not shown).

[0032] The electrode E6a is connected to the waveguide W16 and the waveguide W17. The electrode E6a includes a first portion E6a1 provided on the waveguide W16, a second portion E6a2 provided on the waveguide W17, and a third portion (not shown) connecting the first portion and the second portion. The electrode E6a applies a voltage to the waveguide W16 and the waveguide W17. The electrode E6a is electrically connected to the electrode pad E6c via the wiring E6b. A voltage is applied to the electrode pad E6c from a power source (not shown).

[0033] The configuration of the waveguides W14, W15, W16, and W17 will be described in more detail with reference to Fig. 2. Fig. 2 is a cross-sectional view taken along line II-II in Fig. 1. Fig. 2 shows cross sections of the waveguides W14, W15, W16, and W17.

[0034] As shown in FIG. 2, each of the waveguides W14, W15, W16, and W17 includes a first semiconductor layer 12 of n-type (first conductivity type) provided on a substrate 11, a core layer 13 provided on the first semiconductor layer 12, a second semiconductor layer 14 of p-type (second conductivity type) provided on the core layer 13, and a third semiconductor layer 15 provided on the second semiconductor layer 14.

[0035] In this embodiment, the fourth semiconductor layers 16 and 17 are provided on the major surface 11c of the substrate 11, spaced apart from each other in the Y-axis direction. The fourth semiconductor layer 16 is provided between the major surface 11c and the first semiconductor layer 12 of the waveguide W14 and the first semiconductor layer 12 of the waveguide W15. The fourth semiconductor layer 17 is provided between the major surface 11c and the first semiconductor layer 12 of the waveguide W16 and the first semiconductor layer 12 of the waveguide W17. The fourth semiconductor layers 16 and 17, the first semiconductor layer 12, the core layer 13, the second semiconductor layer 14, and the third semiconductor layer 15 are provided in this order on the major surface 11c of the substrate 11.

[0036] The first semiconductor layer 12 constitutes the lower cladding of each of the waveguides W14, W15, W16, and W17. The second semiconductor layer 14 constitutes the upper cladding of each of the waveguides W14, W15, W16, and W17. The third semiconductor layer 15 constitutes the upper contact of each of the waveguides W14, W15, W16, and W17. The fourth semiconductor layers 16 and 17 constitute the lower contact of each of the waveguides W14, W15, W16, and W17. The core layer 13 of the waveguide W14, the core layer 13 of the waveguide W15, the core layer 13 of the waveguide W16, and the core layer 13 of the waveguide W17 are arranged to be spaced apart from each other in the Y-axis direction.

[0037] The substrate 11 is, for example, a semi-insulating semiconductor substrate. The substrate 11 includes a III-V compound semiconductor doped with an insulating dopant. The substrate 11 includes, for example, indium phosphide (InP) doped with iron (Fe).

[0038] The first semiconductor layer 12 includes a III-V compound semiconductor doped with an n-type dopant. The first semiconductor layer 12 includes, for example, InP doped with silicon (Si). The second semiconductor layer 14 includes a III-V compound semiconductor doped with a p-type dopant. The second semiconductor layer 14 includes, for example, indium gallium arsenide (InGaAs) or InP doped with zinc (Zn). The third semiconductor layer 15 includes a III-V compound semiconductor doped with a p-type dopant. The third semiconductor layer 15 has a p-type dopant concentration higher than the p-type dopant concentration of the second semiconductor layer 14. The third semiconductor layer 15 may include a semiconductor material different from the semiconductor material of the second semiconductor layer 14. The third semiconductor layer 15 includes, for example, InGaAs or InP doped with silicon (Si). The fourth semiconductor layers 16 and 17 include a III-V compound semiconductor doped with an n-type dopant. The fourth semiconductor layers 16 and 17 have an n-type dopant concentration higher than an n-type dopant concentration of the first semiconductor layer 12. The fourth semiconductor layers 16 and 17 include, for example, InP doped with silicon (Si).

[0039] The core layer 13 is an i-type semiconductor layer, i.e., an undoped semiconductor layer. The core layer 13 may have a multiple quantum well structure. The core layer 13 includes, for example, an aluminum gallium indium arsenide (AlGaInAs)-based III-V group compound semiconductor.

[0040] Resin layers 18, 19 and insulating films 20, 21 are further provided on the main surface 11c of the substrate 11. The resin layer 18 is provided on the main surface 11c. The resin layer 18 is provided so as to bury the waveguides W14, W15, W16, W17 and the fourth semiconductor layers 16, 17. That is, the waveguides W14, W15, W16, W17 and the fourth semiconductor layers 16, 17 are covered with the resin layer 18. The insulating film 20 is provided on the resin layer 18. The resin layer 19 is provided on the insulating film 20. The resin layer 19 is provided so as to bury the electrodes E5a and E6a. That is, the electrodes E5a and E6a are covered with the resin layer 19. The insulating film 21 is provided on the resin layer 19.

[0041] The resin layers 18 and 19 include, for example, benzocyclobutene (BCB). The insulating films 20 and 21 are protective films that include, for example, an inorganic material. The insulating films 20 and 21 are protective films that include, for example, silicon oxide (SiO2). Alternatively, the insulating films 20 and 21 may be protective films that include silicon oxynitride (SiON).

[0042] An electrode E5a is connected to the third semiconductor layer 15 and the second semiconductor layer 14 of the waveguide W14. Similarly, an electrode E5a is connected to the third semiconductor layer 15 and the second semiconductor layer 14 of the waveguide W15. The electrode E5a is in ohmic contact with the corresponding third semiconductor layer 15. The electrode E5a includes, for example, a platinum (Pt) layer, a titanium (Ti) layer, and a gold (Au) layer.

[0043] As described above, the wiring E5b is connected to the electrode E5a. Specifically, as shown in Fig. 2, the wiring E5b penetrates the resin layer 19 and the insulating film 21 and is connected to the electrode E5a. The wiring E5b includes, for example, a titanium tungsten (TiW) layer, a Pt layer, and an Au layer.

[0044] A ground electrode (not shown) is provided on the fourth semiconductor layer 16 at a position separated from the waveguides W14 and W15. The ground electrode includes, for example, an alloy layer of Au, germanium (Ge), and nickel (Ni), a Ti layer, a Pt layer, and an Au layer. The ground electrode is electrically connected to the fourth semiconductor layer 16.

[0045] An electrode E6a is connected to the third semiconductor layer 15 and the second semiconductor layer 14 of the waveguide W16. Similarly, an electrode E6a is connected to the third semiconductor layer 15 and the second semiconductor layer 14 of the waveguide W17. The electrode E6a is in ohmic contact with the corresponding third semiconductor layer 15. The electrode E6a includes, for example, a platinum (Pt) layer, a titanium (Ti) layer, and a gold (Au) layer.

[0046] As described above, the wiring E6b is connected to the electrode E6a. Although not shown, the wiring E6b, like the wiring E5b, penetrates the resin layer 19 and the insulating film 21 and is connected to the electrode E6a. The wiring E6b includes, for example, a titanium tungsten (TiW) layer, a Pt layer, and an Au layer.

[0047] A ground electrode E7 is provided on the fourth semiconductor layer 17 at a position spaced apart from the waveguides W16 and W17. The ground electrode E7 includes, for example, an alloy layer of Au, germanium (Ge), and nickel (Ni), a Ti layer, a Pt layer, and an Au layer. The ground electrode E7 is electrically connected to the fourth semiconductor layer 17.

[0048] In the third Mach-Zehnder modulation section MZ3, voltages are applied between the electrode E5a and the ground electrode and between the electrode E6a and the ground electrode E7. For example, a DC reverse bias voltage and an AC voltage are superimposed and applied between the electrode E5a and the ground electrode and between the electrode E6a and the ground electrode E7. As a result, an electric signal flows between the electrode E5a and the ground electrode and between the electrode E6a and the ground electrode E7. The refractive indexes of the waveguides W14, W15, W16, and W17 are changed by the electric signal. As a result, the phase of the light propagating through the waveguides W14, W15, W16, and W17 is modulated.

[0049] In this embodiment, the waveguide W10 and the waveguide W11 have the same structure as the waveguide W14 and the waveguide W15, respectively. That is, each of the waveguide W10 and the waveguide W11 includes an n-type first semiconductor layer 12 provided on a substrate 11, a core layer 13 provided on the first semiconductor layer 12, a p-type second semiconductor layer 14 provided on the core layer 13, and a third semiconductor layer 15 provided on the second semiconductor layer 14.

[0050] In this embodiment, the waveguide W12 and the waveguide W13 have the same structure as the waveguide W16 and the waveguide W17, respectively. That is, each of the waveguide W12 and the waveguide W13 includes an n-type first semiconductor layer 12 provided on the substrate 11, a core layer 13 provided on the first semiconductor layer 12, a p-type second semiconductor layer 14 provided on the core layer 13, and a third semiconductor layer 15 provided on the second semiconductor layer 14.

[0051] Although not shown in the figure, an electrode E1a is connected to the third semiconductor layer 15 and the second semiconductor layer 14 of the waveguide W10. The electrode E1a is in ohmic contact with the third semiconductor layer 15 of the waveguide W10. Similarly, an electrode E2a is connected to the third semiconductor layer 15 and the second semiconductor layer 14 of the waveguide W11. The electrode E2a is in ohmic contact with the third semiconductor layer 15 of the waveguide W11. The electrodes E1a and E2a include, for example, a platinum (Pt) layer, a titanium (Ti) layer, and a gold (Au) layer.

[0052] An electrode (not shown) is provided on the fourth semiconductor layer 16 at a position spaced apart from each of the waveguides W10 and W11. The electrode includes, for example, an alloy layer of Au, germanium (Ge), and nickel (Ni), a Ti layer, a Pt layer, and an Au layer. The electrode is electrically connected to the fourth semiconductor layer 16.

[0053] In the first Mach-Zehnder modulation section MZ1, a voltage is applied between the electrodes E1a and E2a and an electrode not shown. For example, a DC reverse bias voltage and an AC voltage are superimposed and applied between the electrodes E1a and E2a and an electrode not shown. As a result, an electric signal flows between the electrodes E1a and E2a and an electrode not shown. The refractive index of the waveguides W10 and W11 is changed by the electric signal. As a result, the phase of the light propagating through the waveguides W10 and W11 is modulated.

[0054] Although not shown in the figure, an electrode E3a is connected to the third semiconductor layer 15 and the second semiconductor layer 14 of the waveguide W12. The electrode E3a is in ohmic contact with the third semiconductor layer 15 of the waveguide W12. Similarly, an electrode E4a is connected to the third semiconductor layer 15 and the second semiconductor layer 14 of the waveguide W13. The electrode E4a is in ohmic contact with the third semiconductor layer 15 of the waveguide W13. The electrodes E3a and E4a include, for example, a platinum (Pt) layer, a titanium (Ti) layer, and a gold (Au) layer.

[0055] An electrode (not shown) is provided on the fourth semiconductor layer 17 at a position spaced apart from each of the waveguides W12 and W13. The electrode includes, for example, an alloy layer of Au, germanium (Ge), and nickel (Ni), a Ti layer, a Pt layer, and an Au layer. The electrode is electrically connected to the fourth semiconductor layer 17.

[0056] In the second Mach-Zehnder modulation section MZ2, a voltage is applied between the electrodes E3a and E4a and an electrode not shown. For example, a DC reverse bias voltage and an AC voltage are superimposed and applied between the electrodes E3a and E4a and an electrode not shown. As a result, an electric signal flows between the electrodes E3a and E4a and an electrode not shown. The refractive index of the waveguides W12 and W13 is changed by the electric signal. As a result, the phase of the light propagating through the waveguides W12 and W13 is modulated.

[0057] 1 again. In this embodiment, the optical coupler C9 includes an optical coupler C9a (first optical coupler), an optical coupler C9b (second optical coupler), and an optical coupler C9c (third optical coupler). The optical couplers C9a, C9b, and C9c are disposed closer to the first end 11a of the substrate 11 than the third Mach-Zehnder modulation unit MZ3 in the X-axis direction.

[0058] The optical coupler C9a is optically coupled to the output end of the waveguide W14 and the output end of the waveguide W15. Specifically, the output end of the waveguide W14 and the output end of the waveguide W15 are optically coupled to the two input ends of the optical coupler C9a, respectively.

[0059] The optical coupler C9b is optically coupled to the output end of the waveguide W16 and the output end of the waveguide W17. Specifically, the output end of the waveguide W16 and the output end of the waveguide W17 are optically coupled to the two input ends of the optical coupler C9b, respectively.

[0060] The output end of the optical coupler C9a and the output end of the optical coupler C9b are optically coupled to two input ends of the optical coupler C9c, respectively. That is, the optical coupler C9c is optically coupled to the optical coupler C9a and the optical coupler C9b. The optical coupler C9a and the optical coupler C9b are, for example, two-input, one-output MMI couplers.

[0061] In this embodiment, in the first optical modulation unit M1, two first output waveguides W9a are optically coupled to two output ends of the optical coupler C9c. In the second optical modulation unit M2, two second output waveguides W9b are optically coupled to two output ends of the optical coupler C9c. The optical coupler C9c is, for example, a two-input, two-output MMI coupler. The two first output waveguides W9a are respectively connected to two first output ports P2. For example, each of the output ends of the first output waveguide W9a is the first output port P2. The two second output waveguides W9b are respectively connected to two second output ports P3. For example, each of the output ends of the second output waveguide W9b is the second output port P3.

[0062] The first output port P2 and the second output port P3 are provided at a first end 11a of the substrate 11 in the X-axis direction. The first output port P2 and the second output port P3 are provided so as to sandwich the input port P1 in the Y-axis direction.

[0063] Next, with reference to FIG. 3, the manner in which the waveguides W10, W11, W12, and W13 are connected to the corresponding waveguides W14, W15, W16, and W17 in this embodiment will be described in further detail.

[0064] In this embodiment, the first optical modulation section M1 and the second optical modulation section M2 further include a waveguide W18 (ninth waveguide), W19, W20, and W21 that connect the waveguides W10, W11, W12, and W13 to the corresponding waveguides W14, W15, W16, and W17. FIG. 3 is a cross-sectional view taken along line III-III in FIG. 2. FIG. 3 shows cross sections of the waveguides W10, W14, and W18. In FIG. 3, the configurations (electrodes, etc.) on the waveguides W10, W14, and W18 are omitted. The waveguides W18, W19, W20, and W21 are provided on the substrate 11, and have heights in the Z-axis direction.

[0065] The waveguide W18 connects the output end of the waveguide W10 and the input end of the waveguide W14. Specifically, the output end of the waveguide W10 is connected to the input end of the waveguide W18, and the input end of the waveguide W14 is connected to the output end of the waveguide W18. This optically couples the output end of the waveguide W10 to the input end of the waveguide W14. The output end of the waveguide W10 is optically coupled to the input end of the waveguide W14 via the waveguide W18. That is, the output end of the waveguide W10 and the input end of the waveguide W14 are optically coupled without an optical coupler. The length of the waveguide W18 may be 10 μm or more, or 2000 μm or less. In this embodiment, the length of the waveguide W18 is defined by the length in the X-axis direction.

[0066] 3, the waveguide W18 includes an n-type sixth semiconductor layer 22 provided on the fourth semiconductor layer 16, a core layer 23 provided on the sixth semiconductor layer 22, and an n-type seventh semiconductor layer 24 provided on the core layer 23. No electrodes are connected to the waveguide W18.

[0067] The sixth semiconductor layer 22 constitutes a lower cladding of the waveguide W18. The seventh semiconductor layer 24 constitutes an upper cladding of the waveguide W18. The sixth semiconductor layer 22 and the seventh semiconductor layer 24 include a III-V compound semiconductor doped with an n-type dopant. The sixth semiconductor layer 22 and the seventh semiconductor layer 24 include, for example, InP doped with silicon (Si).

[0068] As described above, the seventh semiconductor layer 24 is an n-type semiconductor layer, and the third semiconductor layer 15 is a p-type semiconductor layer. That is, the upper clad of the waveguide W10, the upper clad of the waveguide W14, and the upper clad of the waveguide W18 are composed of semiconductor layers of mutually different conductivity types.

[0069] The core layer 23 is an i-type semiconductor layer, i.e., an undoped semiconductor layer. The core layer 23 may have a multiple quantum well structure. The core layer 23 includes, for example, an aluminum gallium indium arsenide (AlGaInAs)-based III-V group compound semiconductor.

[0070] The waveguide W19 connects the output end of the waveguide W11 and the input end of the waveguide W15. Specifically, the output end of the waveguide W11 is connected to the input end of the waveguide W19, and the input end of the waveguide W15 is connected to the output end of the waveguide W19. This optically couples the output end of the waveguide W11 to the input end of the waveguide W15. The output end of the waveguide W11 is optically coupled to the input end of the waveguide W15 via the waveguide W19. That is, the output end of the waveguide W11 and the input end of the waveguide W15 are optically coupled without an optical coupler. The length of the waveguide W19 may be the same as the length of the waveguide W18. In this embodiment, the length of the waveguide W19 is defined by the length in the X-axis direction.

[0071] The waveguide W20 connects the output end of the waveguide W12 and the input end of the waveguide W16. Specifically, the output end of the waveguide W12 is connected to the input end of the waveguide W20, and the input end of the waveguide W16 is connected to the output end of the waveguide W20. This optically couples the output end of the waveguide W12 to the input end of the waveguide W16. The output end of the waveguide W12 is optically coupled to the input end of the waveguide W16 via the waveguide W20. That is, the output end of the waveguide W12 and the input end of the waveguide W16 are optically coupled without an optical coupler. The length of the waveguide W20 may be the same as the length of the waveguide W18. In this embodiment, the length of the waveguide W20 is defined by the length in the X-axis direction.

[0072] The waveguide W21 connects the output end of the waveguide W13 and the input end of the waveguide W17. Specifically, the output end of the waveguide W13 is connected to the input end of the waveguide W21, and the input end of the waveguide W17 is connected to the output end of the waveguide W21. This optically couples the output end of the waveguide W13 to the input end of the waveguide W17. The output end of the waveguide W13 is optically coupled to the input end of the waveguide W17 via the waveguide W21. That is, the output end of the waveguide W13 and the input end of the waveguide W17 are optically coupled without an optical coupler. The length of the waveguide W21 may be the same as the length of the waveguide W18. In this embodiment, the length of the waveguide W21 is defined by the length in the X-axis direction.

[0073] In this embodiment, the waveguide W19, the waveguide W20, and the waveguide W21 have the same structure as the waveguide W18. That is, the waveguide W19 includes an n-type sixth semiconductor layer 22 provided on the fourth semiconductor layer 16, a core layer 23 provided on the sixth semiconductor layer 22, and an n-type seventh semiconductor layer 24 provided on the core layer 23. Each of the waveguide W20 and the waveguide W21 includes an n-type sixth semiconductor layer 22 provided on the fourth semiconductor layer 17, a core layer 23 provided on the sixth semiconductor layer 22, and an n-type seventh semiconductor layer 24 provided on the core layer 23.

[0074] Therefore, the upper clad of the waveguide W11, the upper clad of the waveguide W15, and the upper clad of the waveguide W19 are composed of semiconductor layers of different conductivity types. The upper clad of the waveguide W12, the upper clad of the waveguide W16, and the upper clad of the waveguide W20 are composed of semiconductor layers of different conductivity types. The upper clad of the waveguide W13, the upper clad of the waveguide W17, and the upper clad of the waveguide W21 are composed of semiconductor layers of different conductivity types.

[0075] In the optical modulator 1 of this embodiment, the waveguide W14 has an input end optically coupled to the output end of the waveguide W10. The waveguide W15 has an input end optically coupled to the output end of the waveguide W11. The waveguide W16 has an input end optically coupled to the output end of the waveguide W12. The waveguide W17 has an input end optically coupled to the output end of the waveguide W13. That is, the waveguide W10 and the waveguide W14 are optically coupled to each other in a one-to-one relationship. The waveguide W11 and the waveguide W15 are optically coupled to each other in a one-to-one relationship. The waveguide W12 and the waveguide W16 are optically coupled to each other in a one-to-one relationship. The waveguide W13 and the waveguide W17 are optically coupled to each other in a one-to-one relationship. Therefore, the intensity of the light propagating through each of the waveguides W10, W11, W12, and W13 can be made approximately equal to the intensity of the light propagating through each of the waveguides W14, W15, W16, and W17. As a result, the intensity of the light input to each of the waveguides W14, W15, W16, and W17 of the third Mach-Zehnder modulation unit MZ3 can be reduced.

[0076] The magnitude of the voltage that can be applied to the waveguide depends on the intensity of the light propagating through the waveguide. Specifically, as the intensity of the light propagating through the waveguide decreases, the voltage that can be applied to the electrode connected to the waveguide increases. Here, the relationship between the intensity of the light propagating through the waveguide and the voltage that can be applied to the electrode connected to the waveguide will be described in more detail with reference to FIG. 4. FIG. 4 is a diagram showing the relationship between the intensity of the light propagating through the waveguide and the voltage that can be applied to the electrode connected to the waveguide. The vertical axis is the intensity of the light (dBm). The horizontal axis is the voltage (V).

[0077] In FIG. 4, graph G1 shows the relationship between the intensity of light with a wavelength of 1530 nm and the maximum voltage that can be applied to an electrode connected to the waveguide when light at that intensity propagates through the waveguide.

[0078] As shown in graph G1, when the light intensity decreases, the maximum voltage that can be applied to the electrodes increases. For example, when the light intensity decreases by 3 dBm, the maximum voltage that can be applied to the electrodes increases by about 1 V. Therefore, if the intensity of the light propagating through the waveguide can be reduced, the voltage that can be applied to the electrodes connected to the waveguide can be increased, and the modulatable phase range can be expanded.

[0079] As described above, in the optical modulator 1, the intensity of the light input to each of the waveguides W14, W15, W16, and W17 of the third Mach-Zehnder modulation unit MZ3 can be reduced. Therefore, in the optical modulator 1, the modulatable phase can be expanded in the third Mach-Zehnder modulation unit MZ3, compared to an optical modulator in which the third Mach-Zehnder modulation unit has two arm waveguides. Alternatively, in the optical modulator 1, the intensity of the light input to the input waveguide W1 can be increased while maintaining the modulatable phase range equivalent to that of the above optical modulator. As a result, in the optical modulator 1, the intensity of the output light can be increased.

[0080] Next, a method for manufacturing the optical modulator according to this embodiment will be described with reference to Fig. 5. Fig. 5 is a flowchart showing the method for manufacturing the optical modulator according to this embodiment. The optical modulator 1 can be manufactured as follows.

[0081] First, the optical modulator 1 is prepared (step S1). Each waveguide included in the optical modulator 1 can be formed by processing a semiconductor laminate by photolithography and etching. Each electrode included in the optical modulator 1 can be formed by lift-off.

[0082] Next, the voltages applied to the electrodes E1a and E2a are adjusted (step S2). In step S2, the voltages applied to the electrodes E1a and E2a are adjusted so that the intensity of the output light at the first output port P2 and the second output port P3 is maximized.

[0083] In this manufacturing method, the voltages applied to the electrodes E1a and E2a are adjusted, for example, as follows. First, the voltage applied to the electrode E1a is adjusted. At this time, the voltage applied to the electrode E2a is fixed. The voltage applied to the electrode E2a is fixed, for example, at 0V. Next, the intensity of the output light is measured while changing the value of the voltage applied to the electrode E1a within a predetermined range. The range is set, for example, to be greater than or equal to -15V and less than or equal to 0V. Next, the voltage value corresponding to the maximum intensity of the measured output light is recorded.

[0084] Next, the voltage to be applied to the electrode E2a is adjusted in the same manner as the voltage to be applied to the electrode E1a was adjusted. Then, the absolute value of the voltage value recorded at the electrode E1a is compared with the absolute value of the voltage value recorded at the electrode E2a. The voltage value with the smaller absolute value is set as the voltage to be applied to the corresponding electrode, and the voltage to be applied to the other electrode is set to 0V. For example, if the absolute value of the voltage value recorded at the electrode E1a is smaller than the absolute value of the voltage value recorded at the electrode E2a, the voltage to be applied to the electrode E1a is set to the recorded voltage value, and the voltage to be applied to the electrode E2a is set to 0V.

[0085] In step S2, the voltages applied to the electrodes E3a, E4a, and E6a are fixed to an extinction voltage, and the voltage applied to the electrode E5a is fixed. The extinction voltage is set to, for example, −18 V. The voltage applied to the electrode E5a is fixed to, for example, 0 V.

[0086] Next, the voltages applied to the electrodes E3a and E4a are adjusted (step S3). In step S3, the voltages applied to the electrodes E3a and E4a are adjusted so that the intensity of the output light at the first output port P2 and the second output port P3 is maximized. In this manufacturing method, the voltages applied to the electrodes E3a and E4a are adjusted in the same manner as in step S2 in which the voltages applied to the electrodes E1a and E2a are adjusted.

[0087] Next, based on the result of adjusting the voltages applied to the electrodes E1a, E2a, E3a, and E4a (results of steps S2 and S3), the voltages applied to the electrodes E5a and E6a are adjusted (step S4). In step S4, based on the result of adjusting the voltages applied to the electrodes E1a, E2a, E3a, and E4a, the voltages applied to the electrodes E5a and E6a are adjusted so that the intensity of the output light at the first output port P2 and the second output port P3 is maximized.

[0088] In this manufacturing method, the voltages applied to the electrodes E5a and E6a are adjusted, for example, as follows. First, the voltage applied to the electrode E5a is adjusted. At this time, the voltage applied to the electrode E6a is fixed. The voltage applied to the electrode E6a is fixed, for example, to 0V. Next, the intensity of the output light is measured while changing the value of the voltage applied to the electrode E5a within a predetermined range. The range is set, for example, to -15V or more and 0V or less. Next, the voltage value corresponding to the maximum intensity of the measured output light is recorded.

[0089] Next, the voltage to be applied to the electrode E6a is adjusted in the same manner as the voltage to be applied to the electrode E5a is adjusted. Then, the absolute value of the voltage value recorded in the electrode E5a is compared with the absolute value of the voltage value recorded in the electrode E6a. The voltage value with the smaller absolute value is set as the voltage to be applied to the corresponding electrode, and the voltage to be applied to the other electrode is set to 0V. For example, if the absolute value of the voltage value recorded in the electrode E5a is smaller than the absolute value of the voltage value recorded in the electrode E6a, the voltage to be applied to the electrode E5a is set to the recorded voltage value, and the voltage to be applied to the electrode E6a is set to 0V. In addition, in step S4, the voltages to be applied to the electrodes E1a, E2a, E3a, and E4a are set to voltages based on the adjustment results in steps S2 and S3, respectively.

[0090] Next, based on the result of adjusting the voltage applied to the electrodes E5a and E6a, the voltage applied to the electrodes E1a and E2a or the voltage applied to the electrodes E3a and E4a is readjusted (step S5). In step S5, based on the result of adjusting the voltage applied to the electrodes E5a and E6a, the voltage applied to the electrodes E1a and E2a or the voltage applied to the electrodes E3a and E4a is readjusted so that the intensity of the output light at the first output port P2 and the second output port P3 is maximized.

[0091] In this manufacturing method, the voltages applied to the electrodes E1a and E2a or the voltages applied to the electrodes E3a and E4a are readjusted, for example, as follows: When the voltage applied to the electrode E5a is set to the voltage value recorded in step S4 and the voltage applied to the electrode E6a is set to 0 V, the voltages applied to the electrodes E1a and E2a are readjusted.

[0092] Specifically, the electrode E1a and the electrode E2a to which the voltage value recorded in step S2 is set is readjusted. For example, in step S2, when the voltage applied to the electrode E1a is set to the recorded voltage value and the voltage applied to the electrode E2a is set to 0V, the voltage applied to the electrode E1a is readjusted. In this case, first, the intensity of the output light is measured again while changing the voltage value applied to the electrode E1a within a predetermined range. At this time, the voltages applied to the electrodes E2a, E3a, and E4a are set to the voltages adjusted in steps S2 and S3, respectively. The above range is set to, for example, the same range as in step S2. Next, the voltage corresponding to the maximum intensity of the measured output light is set as the voltage applied to the electrode E1a.

[0093] On the other hand, when the voltage applied to electrode E5a is set to 0V and the voltage applied to electrode E6a is set to the voltage value recorded in step S4, the voltages applied to electrodes E3a and E4a are readjusted in the same manner as described above. That is, in step S5, the voltage applied to the electrode corresponding to the electrode to which the voltage value recorded in step S4 is set, out of electrodes E5a and E6a, is readjusted.

[0094] According to this manufacturing method, it is possible to provide the optical modulator 1 capable of reducing the intensity of light input to each of the waveguides W14, W15, W16, and W17 of the third Mach-Zehnder modulation section MZ3.

[0095] In the optical modulator 1, a voltage is applied to the waveguides W14 and W15 by the electrode E5a, and a voltage is applied to the waveguides W16 and W17 by the electrode E6a. In this case, if the optical modulator 1 is manufactured without going through the above-mentioned step S5, even if the same voltage is applied to the waveguides W14 and W15, the amount of modulation of the phase of the light propagating through the waveguides W14 and W15 may vary due to individual differences between the waveguides. The same applies to the waveguides W16 and W17.

[0096] In this manufacturing method, after the voltage applied to the electrodes E5a and E6a is adjusted, the voltage applied to the electrodes E1a and E2a or the voltage applied to the electrodes E3a and E4a is readjusted based on the result of the adjustment. Therefore, by readjusting the voltage applied to the electrodes E1a and E2a, the difference in the amount of modulation of the phase of light due to the individual difference between the waveguides W14 and W15 is eliminated. Similarly, by readjusting the voltage applied to the electrodes E3a and E4a, the difference in the amount of modulation of the phase of light due to the individual difference between the waveguides W16 and W17 is eliminated. As a result, according to this manufacturing method, it is possible to suppress the occurrence of variations in the amount of modulation of the phase of light between the waveguides W14 and W15 and between the waveguides W16 and W17.

[0097] Although the preferred embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to the above embodiments.

[0098] In the above-described embodiment, the third semiconductor layer 15 is a p-type semiconductor layer, and the seventh semiconductor layer 24 is an n-type semiconductor layer. However, the combination of the conductivity type of the third semiconductor layer 15 and the conductivity type of the seventh semiconductor layer 24 is not limited to the above-described combination. That is, the third semiconductor layer 15 may be an n-type semiconductor layer, and the seventh semiconductor layer 24 may be a p-type semiconductor layer.

[0099] In the above-described embodiment, the waveguide W19, the waveguide W20, and the waveguide W21 have the same structure as the waveguide W18. However, the structures of the waveguide W19, the waveguide W20, and the waveguide W21 are not limited to the above-described structures. That is, the waveguide W19, the waveguide W20, and the waveguide W21 may have a structure different from that of the waveguide W18.

[0100] In the above-described embodiment, the output end of waveguide W10 is optically coupled to the input end of waveguide W14 via waveguide W18. The output end of waveguide W11 is optically coupled to the input end of waveguide W15 via waveguide W19. The output end of waveguide W12 is optically coupled to the input end of waveguide W16 via waveguide W20. The output end of waveguide W13 is optically coupled to the input end of waveguide W17 via waveguide W21.

[0101] However, the manner of connection between the waveguides W10, W11, W12, and W13 and the corresponding waveguides W14, W15, W16, and W17 is not limited to the above-mentioned manner. For example, the optical modulator 1 may not include at least one of the waveguides W18, W19, W20, and W21. If the optical modulator 1 does not include the waveguide W18, the output end of the waveguide W10 may be directly connected to the input end of the waveguide W14. Similarly, the output end of the waveguide W11 may be directly connected to the input end of the waveguide W15, the output end of the waveguide W12 may be directly connected to the input end of the waveguide W16, and the output end of the waveguide W13 may be directly connected to the input end of the waveguide W17.

[0102] In the above-described embodiment, the optical coupler C9 includes the optical coupler C9a, the optical coupler C9b, and the optical coupler C9c. However, the optical coupler C9 only needs to be optically coupled to the output end of the waveguide W14, the output end of the waveguide W15, the output end of the waveguide W16, and the output end of the waveguide W17, and may not include the optical coupler C9a, the optical coupler C9b, and the optical coupler C9c. In this case, the input end of the optical coupler C9 may be optically coupled to the output end of the waveguide W14, the output end of the waveguide W15, the output end of the waveguide W16, and the output end of the waveguide W17. That is, the optical coupler C9 may be a 4-input 2-output MMI coupler.

[0103] In the above-described manufacturing method, the voltages applied to the electrodes E1a, E2a, E3a, E4a and the electrodes E5a, E6a are adjusted so that the intensity of the output light at the first output port P2 and the second output port P3 is maximized. However, the voltages applied to the electrodes E1a, E2a, E3a, E4a and the electrodes E5a, E6a do not have to be adjusted so that the intensity of the emitted light is maximized.

[0104] For example, the voltages applied to the electrodes E1a, E2a, E3a, E4a and the electrodes E5a, E6a may be adjusted so that the intensity of the output light at the first output port P2 and the second output port P3 is minimized. Alternatively, the voltages applied to the electrodes E1a, E2a, E3a, E4a and the electrodes E5a, E6a may be adjusted so that the intensity of the output light at the first output port P2 and the second output port P3 is a predetermined value.

[0105] In these cases, in step S2, the voltages applied to the electrodes E1a and E2a may be adjusted so that the intensity of the output light at the first output port P2 and the second output port P3 becomes a minimum or a predetermined value. In step S3, the voltages applied to the electrodes E3a and E4a may be adjusted so that the intensity of the output light at the first output port P2 and the second output port P3 becomes a minimum or a predetermined value. In step S4, based on the result of adjusting the voltages applied to the electrodes E1a, E2a, E3a, and E4a, the voltages applied to the electrodes E5a and E6a may be adjusted so that the intensity of the output light at the first output port P2 and the second output port P3 becomes a minimum or a predetermined value. In step S5, based on the result of adjusting the voltage applied to electrodes E5a and E6a, the voltage applied to electrodes E1a and E2a or the voltage applied to electrodes E3a and E4a may be readjusted so that the intensity of the output light at the first output port P2 and the second output port P3 becomes a minimum or a predetermined value.

[0106] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present invention is defined by the claims, not by the above meaning, and is intended to include all modifications within the scope and meaning equivalent to the claims. [Explanation of symbols]

[0107] 1...Optical modulator 11...Substrate 11a...first end 11c…main surface 12...First semiconductor layer 13…Core layer 14...Second semiconductor layer 15…Third semiconductor layer 16...Fourth semiconductor layer 17…Fourth semiconductor layer 18…Resin layer 19...Resin layer 20...Insulating film 21...Insulating film 22...6th semiconductor layer 23…Core layer 24...7th semiconductor layer C1...Optical filter C2: Optical coupler C3: Optical coupler C4…Optical coupler C5…Optical coupler C6…Optical coupler C7…Optical coupler C8…Optical coupler C9…Optical coupler C9a...Optical coupler (first optical coupler) C9b...Optical coupler (second optical coupler) C9c...Optical coupler (third optical coupler) E1a...electrode (3rd electrode) E1b…Wiring E1c…electrode pad E2a...electrode (4th electrode) E2b…Wiring E2c…electrode pad E3a...electrode (5th electrode) E3b…Wiring E3c…electrode pad E4a...electrode (6th electrode) E4b…Wiring E4c…electrode pad E5a…Electrode (1st electrode) E5b…Wiring E5c…electrode pad E6a...electrode (second electrode) E6b…Wiring E6c…electrode pad E7: Ground electrode G1…Graph M1: First optical modulation section M2: Second optical modulation section MZ1: First Mach-Zehnder Modulation Section MZ2: Second Mach-Zehnder Modulation Section MZ3: Third Mach-Zehnder Modulation Section P1...input port P2: First output port P3: Second output port W1…input waveguide W2…Waveguide W3…Waveguide W4…Waveguide W5…Waveguide W6…Waveguide W7…Waveguide W8…Waveguide W9a…1st output waveguide W9b…2nd output waveguide W10…Waveguide (1st waveguide) W11…Waveguide (2nd waveguide) W12…Waveguide (3rd waveguide) W13…Waveguide (4th waveguide) W14…Waveguide (5th waveguide) W15…Waveguide (6th waveguide) W16…Waveguide (7th waveguide) W17…Waveguide (8th waveguide) W18…Waveguide (9th waveguide) W19…Waveguide W20…Waveguide W21…Waveguide

Claims

1. a first Mach-Zehnder modulation section having a first waveguide and a second waveguide; a second Mach-Zehnder modulation section having a third waveguide and a fourth waveguide; a third Mach-Zehnder modulation section optically coupled to the first Mach-Zehnder modulation section and the second Mach-Zehnder modulation section; an optical coupler optically coupled to the third Mach-Zehnder modulator; Equipped with The third Mach-Zehnder modulation unit a fifth waveguide having an input end optically coupled to the output end of the first waveguide; a sixth waveguide having an input end optically coupled to the output end of the second waveguide; a seventh waveguide having an input end optically coupled to the output end of the third waveguide; an eighth waveguide having an input end optically coupled to the output end of the fourth waveguide; a first electrode connected to the fifth waveguide and the sixth waveguide; a second electrode connected to the seventh waveguide and the eighth waveguide; Equipped with the optical coupler is optically coupled to an output end of the fifth waveguide, an output end of the sixth waveguide, an output end of the seventh waveguide, and an output end of the eighth waveguide.

2. a ninth waveguide connecting the output end of the first waveguide and the input end of the fifth waveguide; an upper cladding of the ninth waveguide includes a semiconductor layer of a first conductivity type; The optical modulator according to claim 1 , wherein an upper cladding of each of the first waveguide and the fifth waveguide includes a semiconductor layer of the second conductivity type.

3. The optical coupler includes: a first optical coupler optically coupled to the output end of the fifth waveguide and the output end of the sixth waveguide; a second optical coupler optically coupled to the output end of the seventh waveguide and the output end of the eighth waveguide; a third optical coupler optically coupled to the first optical coupler and the second optical coupler; The optical modulator according to claim 1 or 2, comprising:

4. A step of preparing the optical modulator according to claim 1 or 2, the optical modulator further comprising: a third electrode connected to the first waveguide, a fourth electrode connected to the second waveguide, a fifth electrode connected to the third waveguide, and a sixth electrode connected to the fourth waveguide; adjusting a voltage applied to the third electrode and the fourth electrode; adjusting a voltage applied to the fifth electrode and the sixth electrode; adjusting the voltages applied to the first electrode and the second electrode based on the results of adjusting the voltages applied to the third electrode, the fourth electrode, the fifth electrode, and the sixth electrode; readjusting the voltages applied to the third electrode and the fourth electrode or the voltages applied to the fifth electrode and the sixth electrode based on a result of adjustment of the voltages applied to the first electrode and the second electrode; A method for manufacturing an optical modulator, comprising:

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