Optical devices and optical communication devices

Abstract

To provide an optical device that can ensure a stable branching ratio.SOLUTION: An optical device comprises: a modulator; and a tap coupler. The modulator comprises: an optical waveguide that is formed by a thin film LN (Lithium Niobate) substrate, and in which light passes through; and an electrode that applies voltage to the optical waveguide, and modulates a phase of the light according to an electric field within the optical waveguide depending on the voltage. The tap coupler that has at least a portion of itself formed by the thin film LN substrate branches a portion of the light which passes through the optical waveguide. The tap coupler comprises a delay interferometer that branches a portion of the light passing through with a branching ratio depending on a phase difference of the light which passes through an inside of the tap coupler from the optical waveguide.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an optical device and an optical communication apparatus. [Background technology]

[0002] A conventional optical modulator is composed of, for example, an optical waveguide provided on a substrate and a modulation section provided nearby. The modulation section has a signal electrode and a ground electrode. When a voltage is applied to the signal electrode, an electric field is generated in the optical waveguide. The electric field in the optical waveguide changes the refractive index of the optical waveguide, causing a change in the phase of the light. The optical waveguide forms a Mach-Zehnder interferometer, and the optical output changes depending on the difference in the phase of the light between the optical waveguides.

[0003] The optical modulator integrates, for example, a four-channel Mach-Zehnder modulator. Each Mach-Zehnder interferometer has an RF (Radio Frequency) modulation section and a DC (Direct Current) modulation section. A high-frequency signal with a bandwidth of, for example, several tens of GHz is input to the electrodes of the RF modulation section to perform high-speed modulation. A bias voltage is applied to the electrodes of the DC modulation section, and the bias voltage is adjusted so that the ON / OFF of the electrical signal corresponds to the ON / OFF of the optical signal.

[0004] The optical waveguides of the optical modulator constitute, for example, a Mach-Zehnder interferometer, and output, for example, an XY polarization multiplexed IQ signal due to the optical phase difference between multiple parallel optical waveguides. Then, two channels of the four-channel output are combined to generate two IQ signals, one of which is subjected to polarization rotation and polarization multiplexed by a polarization beam combiner before being output.

[0005] On the other hand, the optical waveguide may be a diffused optical waveguide formed at a position that does not overlap with the signal electrode by diffusing a metal such as titanium from the substrate surface. FIG. 9 is a schematic cross-sectional view showing an example of a conventional DC modulation unit 100. The DC modulation unit 100 shown in FIG. 9 includes an LN substrate 101 made of LN (lithium niobate: LiNbO3) crystal and a diffused optical waveguide 102 formed on the surface of the LN substrate 101. The DC modulation unit 100 further includes a buffer layer 103 that covers the diffused optical waveguide 102 on the LN substrate 101, and an electrode 104 stacked on the buffer layer 103. The electrode 104 includes a signal electrode 104A and a pair of ground electrodes 104B.

[0006] The diffusing optical waveguide 102 is disposed at a position where it does not overlap with the signal electrode 104A and the pair of ground electrodes 104B. The composition and thickness of the buffer layer 103 are determined so as to have a low resistance value in order to suppress DC drift (changes in emitted light over time caused by an applied bias voltage).

[0007] However, this diffused optical waveguide 102 has weak light confinement, resulting in poor electric field application efficiency and a high drive voltage. Therefore, there is a thin-film optical waveguide, in which an optical waveguide using a thin film of LN crystal is formed in a position where it does not overlap with the signal electrode. Thin-film optical waveguides can confine light more strongly than diffused optical waveguides that use diffused metal, improving the electric field application efficiency and reducing the drive voltage.

[0008] Fig. 10 is a schematic cross-sectional view showing an example of a conventional DC modulation section 200. The DC modulation section 200 shown in Fig. 10 has a support substrate 201 made of Si (silicon) or the like, and an intermediate layer 202 laminated on the support substrate 201. Furthermore, the DC modulation section 200 has a thin-film LN substrate 203 laminated on the intermediate layer 202, and a buffer layer 204 made of SiO2 laminated on the thin-film LN substrate 203.

[0009] The thin-film LN substrate 203 is a thin-film optical waveguide 206 having a convex shape that protrudes upward. The thin-film optical waveguide 206 is a rib-type waveguide having a rib 206A and slabs 206B formed on both sides of the rib 206A. The rib 206A and the slab 206B are covered with a buffer layer 204, and a signal electrode 205A (205) and a pair of ground electrodes 205B (205) having a coplanar (CPW) structure are disposed on the surface of the buffer layer 204. That is, the signal electrode 205A and a pair of ground electrodes 205B sandwiching the signal electrode 205A are disposed on the buffer layer 204. The buffer layer 204 can prevent light propagating through the thin-film optical waveguide 206 from being absorbed by the signal electrode 205A and the ground electrodes 205B.

[0010] A convex thin-film optical waveguide 206 is formed on the thin-film LN substrate 203 located between the signal electrode 205A and the ground electrode 205B. Furthermore, a step 204A that covers the entire convex thin-film optical waveguide 206 is also formed on the buffer layer 204 located between the signal electrode 205A and the ground electrode 205B.

[0011] With such a thin film optical waveguide 206, a bias voltage is applied to the signal electrode 205A to generate an electric field, which changes the refractive index of the thin film optical waveguide 206, thereby modulating the light propagating through the thin film optical waveguide 206.

[0012] The DC modulation section 200 includes a feedback circuit that uses a tap coupler to branch off a portion of the output light from the thin-film optical waveguide 206 as monitor light, and adjusts the bias voltage applied to the signal electrode 205A based on the optical intensity of the branched monitor light. The tap coupler used in the feedback circuit is configured as a directional coupler with two waveguides arranged in the thin-film optical waveguide 206. In the feedback circuit, the branching ratio of the tap coupler needs to be stabilized in order to accurately monitor the optical intensity. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Japanese Patent Application Publication No. 61-28925 [Patent Document 2] Japanese Patent Publication No. 2020-134874 Summary of the Invention [Problem to be solved by the invention]

[0014] In the thin-film optical waveguide 206 formed from thin-film LN crystal, light is strongly confined, resulting in light being concentrated in one area. However, in the waveguide of the tap coupler adjacent to the thin-film optical waveguide 206, the directional coupling through which light propagates in the thin-film optical waveguide 206 is weak. Furthermore, since the branching ratio of the tap coupler is highly dependent on the width of the waveguide within the tap coupler, the branching ratio of the tap coupler varies significantly due to the influence of manufacturing errors in the waveguide width. As a result, manufacturing errors in the waveguide width result in large variations in the value of the monitor light branched by the tap coupler.

[0015] The disclosed technology has been made in consideration of the above points, and aims to provide an optical device or the like that can ensure a stable branching ratio. [Means for solving the problem]

[0016] In one aspect, the optical device disclosed herein includes a modulator and a tap coupler. The modulator is formed of a thin-film LN (lithium niobate) substrate and includes an optical waveguide through which light passes and electrodes for applying a voltage to the optical waveguide, modulating the phase of the passing light in response to an electric field in the optical waveguide that corresponds to the voltage. The tap coupler is at least partially formed of the thin-film LN substrate and branches a portion of the light passing through the optical waveguide. The tap coupler includes a delay interferometer that branches a portion of the passing light with a branching ratio that corresponds to the phase difference of the light passing from the optical waveguide through the tap coupler. [Effects of the Invention]

[0017] According to one aspect of the optical device etc. disclosed in the present application, a stable branching ratio can be ensured. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a block diagram showing an example of the configuration of an optical communication device according to this embodiment. [Figure 2] FIG. 2 is a schematic plan view showing an example of the configuration of the optical modulator according to the first embodiment. [Figure 3] FIG. 3 is a schematic plan view showing an example of the configuration of a tap coupler. [Figure 4] FIG. 4 is a schematic cross-sectional view showing an example of the first DC modulation section. [Figure 5] FIG. 5 is a schematic plan view showing an example of the configuration of the tap coupler of the second embodiment. [Figure 6] FIG. 6 is a schematic plan view showing an example of the configuration of the tap coupler of the third embodiment. [Figure 7] FIG. 7 is a schematic plan view showing an example of the configuration of an optical modulator according to a fourth embodiment. [Figure 8] FIG. 8 is a schematic plan view showing an example of the configuration of the tap coupler of the fifth embodiment. [Figure 9] FIG. 9 is a schematic cross-sectional view showing an example of a conventional DC modulation section. [Figure 10] FIG. 10 is a schematic cross-sectional view showing an example of a conventional DC modulation section. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, embodiments of the optical device and the like disclosed in the present application will be described in detail with reference to the drawings. However, the present invention is not limited to these embodiments. [Example]

[0020] FIG. 1 is a block diagram showing an example of the configuration of an optical communication device 1 according to a first embodiment. The optical communication device 1 shown in FIG. 1 is connected to an optical fiber 2A(2) on the output side and an optical fiber 2B(2) on the input side. The optical communication device 1 includes a DSP (Digital Signal Processor) 3, a light source 4, an optical modulator 5, and an optical receiver 6. The DSP 3 is an electrical component that performs digital signal processing. For example, the DSP 3 performs processing such as encoding of transmission data, generates an electrical signal including the transmission data, and outputs the generated electrical signal to the optical modulator 5. The DSP 3 also obtains an electrical signal including reception data from the optical receiver 6 and performs processing such as decoding of the obtained electrical signal to obtain the reception data.

[0021] The light source 4 includes, for example, a laser diode, and generates light of a predetermined wavelength and supplies it to the optical modulator 5 and the optical receiver 6. The optical modulator 5 is an optical device that modulates the light supplied from the light source 4 with an electrical signal output from the DSP 3 and outputs the resulting optical transmission signal to the optical fiber 2A. The optical modulator 5 is, for example, an optical device such as an LN optical modulator that includes an LN (Lithium Niobate: LiNbO3) optical waveguide and a modulation unit. The LN waveguide is formed from an LN crystal substrate. The optical modulator 5 generates an optical transmission signal by modulating the light supplied from the light source 4 with an electrical signal input to the modulation unit as the light propagates through the LN waveguide.

[0022] The optical receiver 6 receives an optical signal from the optical fiber 2B and demodulates the received optical signal using light supplied from the light source 4. The optical receiver 6 then converts the demodulated received optical signal into an electrical signal and outputs the converted electrical signal to the DSP 3.

[0023] Fig. 2 is a schematic plan view showing an example of the configuration of the optical modulator 5 of Example 1. The optical modulator 5 shown in Fig. 2 has an optical fiber 4A from the light source 4 connected to the input side, and an optical fiber 2A for sending a transmission signal connected to the output side. The optical modulator 5 has an input unit 11, a modulating unit 12, an output unit 13, an electrode 14, an RF input unit 15, and an RF termination unit 16.

[0024] The input section 11 has one LN waveguide 11A connected to the optical fiber 4A, and a first branching section 11B. The LN waveguide 11A is a single LN waveguide connected to the optical fiber 4A. The first branching section 11B optically couples the LN waveguide 11A to an LN waveguide 21A in the modulation section 12, and optically branches the light from the LN waveguide 11A at a 1:1 ratio.

[0025] The modulation section 12 has a first forward path section 21, a first return path section 22, a first return path section 23, a second return path section 24, a second forward path section 25, a third return path section 26, and a second return path section 27.

[0026] The first outgoing path section 21 is a section that optically couples the input section 11 and the first folding section 22. The first outgoing path section 21 has two LN waveguides 21A, two second branching sections 21B, four LN waveguides 21C, four third branching sections 21D, eight LN waveguides 21E, and four first DC (Direct Current) modulation sections 21F. The LN waveguide 21A is an LN waveguide that optically couples the first branching section 11B and the second branching section 21B in the input section 11. The second branching section 21B optically couples the one LN waveguide 21A and the two LN waveguides 21C, and optically branches the light from the LN waveguide 21A at a 1:1 ratio. The LN waveguide 21C is an LN waveguide that optically couples the second branching portion 21B and the third branching portion 21D. The third branching portion 21D optically couples the LN waveguide 21C and the two LN waveguides 21E, and optically branches the light from the LN waveguide 21C at a 1:1 ratio.

[0027] The first DC modulation unit 21F is configured with a daughter Mach-Zehnder (MZ) interferometer. The first DC modulation unit 21F has two LN waveguides 21E and an electrode 14. The electrode 14 has a first DC signal electrode 14A1 and a pair of first DC ground electrodes 14B1. The LN waveguide 21E is, for example, a rib-type optical waveguide formed using a thin-film LN substrate 53. The thin-film LN substrate 53 has spontaneous polarization in the Z direction of the crystal axis of the LN crystal, resulting in an internal electric field within the thin-film LN crystal. The light propagation direction of the LN waveguide 21C is the Y-axis direction of the thin-film LN crystal. The first DC modulation unit 21F generates an electric field within each LN waveguide 21E in response to voltage application from the electrode 14 to each LN waveguide 21E, and adjusts the refractive index of each LN waveguide 21E in response to the electric field. Furthermore, the first DC modulation section 21F adjusts the phase of the light passing through each LN waveguide 21E in accordance with the adjustment of the refractive index of each LN waveguide 21E, and modulates the light in accordance with the phase difference of the light.

[0028] The first folded section 22 is a section that optically couples the first outgoing path section 21 and the first returning path section 23. The first folded section 22 has eight LN waveguides 22A. The light propagation direction of the LN waveguides 22A is approximately the Z-axis direction of the thin-film LN crystal.

[0029] The first return path section 23 is a section that optically couples the first return section 22 and the second return section 24. The first return path section 23 has eight LN waveguides 23A and four RF (Radio Frequency) modulation sections 23B.

[0030] The RF modulation unit 23B has two LN waveguides 23A and an electrode 14. The electrode 14 has an RF signal electrode 14A2 and a pair of RF ground electrodes 14B2. The LN waveguide 23A is, for example, a rib-type optical waveguide formed using a thin-film LN substrate 53. The thin-film LN substrate 53 has spontaneous polarization in the Z direction of the crystal axis of the LN crystal, resulting in an internal electric field within the thin-film LN crystal. The light propagates in the LN waveguide 23A along the Y axis of the thin-film LN crystal. The RF modulation unit 23B generates an electric field within each LN waveguide 23A in response to voltage application from the electrode 14 to each LN waveguide 23A, and adjusts the refractive index of each LN waveguide 23A in response to the electric field. Furthermore, the RF modulation unit 23B adjusts the phase of light passing through each LN waveguide 23A in response to the adjustment of the refractive index of each LN waveguide 23A, and modulates the light in response to the phase difference of the light.

[0031] The RF input unit 15 is connected to the RF signal electrode 14A2 in the RF modulation unit 23B and applies a voltage (high frequency signal) to the RF signal electrode 14A2. The RF termination unit 16 is connected to the RF ground electrode 14B2 in the RF modulation unit 23B and is connected to the termination of the RF signal electrode 14A2 to prevent unnecessary reflection of the high frequency signal transmitted by the RF signal electrode 14A2.

[0032] The second folded section 24 is a section that optically couples the first return path section 23 and the second forward path section 25. The second folded section 24 has eight LN waveguides 24A. The light propagation direction of the LN waveguides 24A is approximately the Z-axis direction of the thin-film LN crystal.

[0033] The second outgoing path section 25 is a section that optically couples the second folding back section 24 and the third folding back section 26. The second outgoing path section 25 has four first multiplexing sections 25A, four LN waveguides 25B, two second DC (Direct Current) modulation sections 25C, and two second multiplexing sections 25D. The first multiplexing section 25A optically couples between the two LN waveguides 24A and one LN waveguide 25B, and multiplexes the light from the LN waveguide 24A.

[0034] The second DC modulation unit 25C is configured with a parent-side Mach-Zehnder (MZ) interferometer. The second DC modulation unit 25C has two LN waveguides 25B and an electrode 14. The electrode 14 has a second DC signal electrode 14A3 and a pair of second DC ground electrodes 14B3. The LN waveguide 25B is, for example, a rib-type optical waveguide formed using a thin-film LN substrate 53. The thin-film LN substrate 53 has spontaneous polarization in the Z direction of the crystal axis of the LN crystal, resulting in an internal electric field within the thin-film LN crystal. The light propagates in the LN waveguide 25B in the Y-axis direction of the thin-film LN crystal. The second DC modulation unit 25C generates an electric field in each LN waveguide 25B in response to voltage application from the electrode 14 to each LN waveguide 25B, and adjusts the refractive index of each LN waveguide 25B in response to the electric field. Furthermore, the second DC modulation unit 25C adjusts the phase of the light passing through each LN waveguide 25B in accordance with the adjustment of the refractive index of each LN waveguide 25B, and modulates the light in accordance with the phase difference of the light. The second multiplexing unit 25D optically couples between the two LN waveguides 25B and the one LN waveguide 26A, and multiplexes the light from each LN waveguide 25B.

[0035] The third folding section 26 is a section that optically couples the second outgoing path section 25 and the second returning path section 27. The third folding section 26 has two LN waveguides 26A. The light propagation direction of the LN waveguides 26A is approximately the Z-axis direction of the thin-film LN crystal. The LN waveguides 26A are waveguides that optically couple between the second multiplexing section 25D in the second outgoing path section 25 and the LN waveguide 27A in the second returning path section 27.

[0036] The second return path section 27 is a section that optically couples the third return section 26 and the output section 13. The second return path section 27 has two LN waveguides 27A. The light propagation direction of the LN waveguide 27A is the Y-axis direction of the thin-film LN crystal. The LN waveguide 27A is a waveguide that optically couples the LN waveguide 26A in the third return section 26 and the output section 13.

[0037] The output unit 13 has two LN waveguides 13A, a PR (Polarization Rotator) 13B, and a PBC (Polarization Beam Combiner) 13C. The PR 13B rotates the I signal or Q signal input from one of the second DC modulation units 25C by 90 degrees to obtain a vertically polarized optical signal after the 90-degree rotation. The PR 13B then inputs the vertically polarized optical signal to the PBC 13C. The PBC 13C multiplexes the vertically polarized optical signal from the PR 13B with the horizontally polarized optical signal input from the other second DC modulation unit 25C, and outputs a polarization multiplexed signal.

[0038] A tap coupler 30 is disposed between the LN waveguide 27A in the second return path section 27 and the LN waveguide 13A in the output section 13. The tap coupler 30 is disposed between the LN waveguide 27A and the LN waveguide 13A, and outputs a portion of the light input from the LN waveguide 27A as monitor light, and outputs the remaining light to the LN waveguide 13A in the output section 13. The tap coupler 30 has a delay interferometer 300 that branches a portion of the light passing through the LN waveguide 27A at a branching ratio corresponding to the phase difference of the light passing through the tap coupler 30 from the LN waveguide 27A.

[0039] Fig. 3 is a plan view schematic diagram showing an example of the configuration of the tap coupler 30. The delay interferometer 300 of the tap coupler 30 shown in Fig. 3 includes a first coupler 31, a second coupler 32, a first waveguide 33 and a second waveguide 34 that optically couple the first coupler 31 and the second coupler 32, and a monitor 35.

[0040] The first coupler 31 is an MMI (Multi-Mode Interference) coupler that receives light from the LN waveguide 27A and splits and outputs the light to the first waveguide 33 and the second waveguide 34. There are two types of couplers: directional coupling couplers and MMI couplers. The splitting ratio of a directional coupling coupler is highly dependent on the width of the waveguide, and the splitting ratio changes due to manufacturing errors in the waveguide width. In contrast, the splitting ratio of an MMI coupler is less dependent on the width of the waveguide, and fluctuations in the splitting ratio are minimal even if manufacturing errors occur in the waveguide width. Therefore, an MMI coupler is used for the first coupler 31. The first coupler 31 is an MMI coupler with two inputs and two outputs.

[0041] The first waveguide 33 is formed of a thin-film LN substrate 53 and is a waveguide that inputs one of the beams of light branched from the first coupler 31 to the second coupler 32. The second waveguide 34 is formed of a thin-film LN substrate 53 and is a waveguide that inputs the other beam of light branched from the first coupler 31 to the second coupler 32. The second waveguide 34 has a longer optical length than the first waveguide 33, thereby constituting a delay section. For example, the second waveguide 34 is a curved waveguide that includes a portion whose light propagation direction (approximately the Z-axis direction of the crystal axis) differs from the light propagation direction of the first waveguide 33 (the Y-axis direction of the crystal axis). In other words, the second waveguide 34 is made longer than the waveguide length of the first waveguide 33, for example. As a result, a phase difference occurs between one light passing through the first waveguide 33 and the other light passing through the second waveguide .

[0042] The second coupler 32 multiplexes one light beam input from the first waveguide 33 with the other light beam input from the second waveguide 34, and branches the resulting multiplexed light beam with a phase difference at a branching ratio corresponding to the phase difference. The second coupler 32 outputs a portion of the multiplexed light beam to a monitor 35 as monitor light, and outputs the remaining light beam to the LN waveguide 13A. The second coupler 32 is an MMI coupler. The monitor 35 receives the monitor light branched by the second coupler 32 and outputs the optical intensity of the monitor light beam as a monitor result to the DSP3. The DSP3 adjusts the drive voltages applied to the first DC modulation unit 12F, the RF modulation unit 23B, and the second DC modulation unit 25C based on the monitor result of the monitor 35.

[0043] Next, the configuration of the optical modulator 5 of Example 1 will be specifically described. FIG. 4 is a schematic cross-sectional view showing an example of a first DC modulation unit 21F. For convenience of explanation, the first DC modulation unit 21F shown in FIG. 2 is configured with four MZ interferometers, but the schematic cross-sectional view of FIG. 4 will explain the configuration with one MZ interferometer. The second DC modulation unit 25C is also configured with two MZ interferometers. However, since the configuration per MZ interferometer is identical to that of the first DC modulation unit 21F, identical components are assigned the same reference numerals, and redundant descriptions of the configuration and operation will be omitted. The first DC modulation unit 21F shown in FIG. 4 includes a support substrate 51 and an intermediate layer 52 stacked on the support substrate 51. Furthermore, the first DC modulation unit 21F includes a thin-film LN substrate 53 made of thin-film LN crystal stacked on the intermediate layer 52, a buffer layer 54 stacked on the thin-film LN substrate 53, and electrodes. The electrode 14 includes a first DC signal electrode 14A1 and a pair of first DC ground electrodes 14B1.

[0044] The support substrate 51 is, for example, a substrate made of Si or LN. The intermediate layer 52 is, for example, a layer made of a transparent material such as SiO2 or TiO2 whose refractive index is lower than that of LN. Similarly, the buffer layer 54 is, for example, a layer made of a transparent material such as SiO2 or TiO2 whose refractive index is lower than that of LN.

[0045] The thin-film LN substrate 53 is a thin-film optical waveguide 60 having a protruding stripe that protrudes upward. The thin-film optical waveguide 60 is the LN waveguide 21E of the first DC modulation section 21F. The thin-film optical waveguide 60 is a rib-type optical waveguide having a rib 60A and slabs 60B on both sides of the rib 60A. The rib 60A has an upper surface and sidewall surfaces of the rib 60A. The thin-film optical waveguide 60 is, for example, an LN waveguide 11A, 21A, 21C, 21E, 22A, 23A, 24A, 25B, 26A, 27A, 13A, etc. The thin-film optical waveguide 60 is covered with a buffer layer 54. The buffer layer 54 is provided to prevent light propagating through the thin-film optical waveguide 60 from being absorbed by the electrodes 14.

[0046] The buffer layer 54 covers the upper surfaces of the ribs 60A of the thin film optical waveguide 60 and also covers the slabs 60B of the thin film optical waveguide 60. The first DC signal electrode 14A1 and a pair of first DC ground electrodes 14B1 are disposed on the buffer layer 54.

[0047] The thin film optical waveguide 60 located between the first DC signal electrode 14A1 and the first DC ground electrode 14B1 is a rib 60A within the thin film optical waveguide 60. The thin film optical waveguide 60 in which the first DC signal electrode 14A1 and the first DC ground electrode 14B1 are located is a slab 60B within the thin film optical waveguide 60.

[0048] A thin film optical waveguide 60 made of a thin film LN substrate 53 having a thickness of 0.5 to 3 μm is sandwiched between the intermediate layer 52 and the buffer layer 54. The width of the rib 60A that becomes the thin film optical waveguide 60 is, for example, about 1 to 8 μm.

[0049] The first DC signal electrode 14A1 is made of a metal material such as gold or copper and has a width of 2 to 10 μm and a thickness of 1 to 20 μm. The first DC ground electrode 14B1 is made of a metal material such as gold or copper and has a thickness of 1 μm or more. When a bias voltage corresponding to an electrical signal output from the DSP 3 is applied to the first DC signal electrode 14A1, an electric field is generated in a direction from the first DC signal electrode 14A1 to the first DC ground electrode 14B1, and this electric field is applied to the thin-film optical waveguide 60. As a result, the refractive index of the thin-film optical waveguide 60 changes in response to the application of the electric field to the thin-film optical waveguide 60, making it possible to modulate the light propagating through the thin-film optical waveguide 60.

[0050] The first coupler 31 in the tap coupler 30 branches the light from the thin-film optical waveguide 60 into the first waveguide 33 and the second waveguide 34. The second coupler 32 multiplexes one light from the first waveguide 33 and the other light from the second waveguide 34, branches the multiplexed light at a branching ratio according to the phase difference, and outputs monitor light to the monitor 35, while outputting the remaining light to the thin-film optical waveguide 60 (LN waveguide 13A).

[0051] The monitor 35 receives monitor light from the second coupler 32 according to the branching ratio, and adjusts the drive voltages of the first DC modulation unit 21F, the second DC modulation unit 25C, and the RF modulation unit 23B according to the optical intensity of the monitor light.

[0052] The tap coupler 30 of the first embodiment has a first waveguide 33 and a second waveguide 34 disposed between a first coupler 31 and a second coupler 32. An optical phase difference occurs depending on the difference in optical length between the first waveguide 33 and the second waveguide 34, and the tap coupler 30 sets a branching ratio according to the phase difference. In other words, the branching ratio of the tap coupler 30 is determined by the difference in optical length between the first waveguide 33 and the second waveguide 34, and is therefore less susceptible to manufacturing errors in the width of the waveguides as in the prior art. As a result, even if there is a manufacturing error in the width of the waveguides in the tap coupler 30, which is a thin-film optical waveguide 60 with strong optical confinement, the branching ratio of the tap coupler 30 can be stabilized.

[0053] The tap coupler 30 has a delay interferometer 300 that branches a portion of the light passing through it at a branching ratio that corresponds to the phase difference of the light passing from the thin film optical waveguide 60 to the inside of the tap coupler 30. As a result, even if the thin film optical waveguide 60 has strong light confinement and there is a manufacturing error in the waveguide width inside the tap coupler 30, the branching ratio of the tap coupler 30 can be stabilized.

[0054] The second coupler 32 in the delay interferometer 300 multiplexes one light from the first waveguide 33 with the other light from the second waveguide 34, and branches the combined light with a phase difference at a branching ratio according to the phase difference. As a result, the branching ratio of the tap coupler 30 can be stabilized.

[0055] Furthermore, since the first coupler 31 and the second coupler 32 of the tap coupler 30 are MMI couplers, fluctuations in the branching ratio can be reduced even if manufacturing errors occur in the width of the waveguide.

[0056] In the tap coupler 30 of the first embodiment, the optical lengths of the first waveguide 33 and the second waveguide 34 are changed to adjust the phase difference between the light passing through the first waveguide 33 and the second waveguide 34. As a method for changing the optical lengths of the first waveguide 33 and the second waveguide 34, a method is shown in which the waveguide length of the second waveguide 34 is made longer than the waveguide length of the first waveguide 33 to form a curved shape. However, the shape is not limited to a curved shape, and can be changed as appropriate as long as the waveguide length of the second waveguide 34 is made longer than the first waveguide 33.

[0057] In addition, the tap coupler 30 has been exemplified as setting the branching ratio by differentiating the lengths of the two waveguides 33 and 34, but the branching ratio may also be set by, for example, changing the phase difference by adjusting the drive voltage applied to the waveguides rather than the lengths of the waveguides 33 and 34, and can be changed as appropriate.

[0058] Although the first coupler 31 in the tap coupler 30 in Example 1 is exemplified as an MMI coupler with two inputs and two outputs, it may also be an MMI coupler with one input and two outputs, and such an embodiment will be described below as Example 2. [Example]

[0059] Fig. 5 is a schematic plan view showing an example of the configuration of a tap coupler 30A of Example 2. Note that the same components as those in the optical communication device 1 of Example 1 are denoted by the same reference numerals, and redundant descriptions of the configuration and operation will be omitted. The tap coupler 30A shown in Fig. 5 differs from the tap coupler 30 shown in Fig. 3 in that a first coupler 31A with one input and two inputs is used instead of the first coupler 31 with two inputs and two outputs.

[0060] The waveguide length of the first coupler 31A with one input and two outputs is physically shorter than that of the first coupler 31 with two inputs and two outputs. The input of the first coupler 31 is optically coupled to the thin-film optical waveguide 60, and the light from the thin-film optical waveguide 60 is branched into the first waveguide 33 and the second waveguide 34.

[0061] The tap coupler 30A of the second embodiment uses a first coupler 31A with one input and two outputs instead of the first coupler 31 with two inputs and two outputs, so that the size in the light propagation direction can be made smaller than that of the tap coupler 30 of the first embodiment. As a result, it can contribute to the miniaturization of the optical communication device 1.

[0062] In the tap coupler 30 of Example 1, the second waveguide 34 connecting the first coupler 31 and the second coupler 32 is curved to form a delay section. However, when the shape of the second waveguide 34 is curved, the direction of the electric field in the curved second waveguide 34 deviates from the Z-axis direction of the crystal axis, and therefore the direction of the internal electric field due to spontaneous polarization (Z-axis direction) differs from the direction of the electric field in the second waveguide 34 of the delay interferometer 300. An embodiment of the tap coupler 30 in which the direction of the internal electric field due to spontaneous polarization (Z-axis direction) and the direction of the electric field in the second waveguide 34 of the delay interferometer 300 are made the same will be described below as Example 3. [Example]

[0063] Fig. 6 is a plan view schematic diagram showing an example of the configuration of a tap coupler 30B according to a third embodiment. The tap coupler 30B shown in Fig. 6 differs from the tap coupler 30A shown in Fig. 5 in that the direction of the electric field in the second waveguide 34A is linear, in the Z-axis direction of the crystal axis, the same as the direction of the electric field in the first waveguide 33. The width of the second waveguide 34A is narrower than the width of the first waveguide 33, resulting in an optical length difference.

[0064] The direction of the electric field in the second waveguide 34A is the same as the direction of the electric field in the first waveguide 33 and the direction of the internal electric field due to spontaneous polarization, that is, the Z-axis direction, and therefore the refractive indices of the first waveguide 33 and the second waveguide 34A are stabilized. As a result, even when the electric fields in the first waveguide 33 and the second waveguide 34A are oriented in the same Z-axis direction, the branching ratio of the tap coupler 30 can be stabilized.

[0065] In the tap coupler 30B of the third embodiment, the first waveguide 33 and the second waveguide 34A are linearly oriented so that the electric field directions are aligned with the Z-axis direction of the crystal axis, and the width of the second waveguide 34A is narrower than the width of the first waveguide 33. As a result, even when the electric fields of the first waveguide 33 and the second waveguide 34A are aligned with the same Z-axis direction, the electric field in the waveguide is stabilized, and the branching ratio can be stabilized.

[0066] In the first embodiment, the first waveguide 33 in the tap coupler 30 and the thin-film optical waveguide 60 in the first DC modulation unit 21F have the same light propagation direction, which makes the tap coupler 30 unstable when the temperature changes. The LN crystal in the thin-film LN substrate 53 has anisotropy, and the electro-optic effect in the Z-axis direction is greater than in the X and Y directions. Therefore, in the first DC modulation unit 21F, an electric field is applied in the Z-axis direction to increase modulation efficiency, and the light propagation direction of the thin-film optical waveguide 60 is set to the Y direction.

[0067] On the other hand, when the temperature of the LN crystal changes, the spontaneous polarization in the Z-axis direction changes, which changes the electric field in the first waveguide 33 and the second waveguide 34 of the tap coupler 30, and the refractive index of the first waveguide 33 and the second waveguide 34 changes. As a result, the optical length difference between the first waveguide 33 and the second waveguide 34 constituting the tap coupler 30 changes, and the branching ratio of the tap coupler 30 changes. For example, when the length of the waveguide is L and the refractive index of the waveguide is N, the optical length can be calculated as N × L. Therefore, since the lengths N of the first waveguide 33 and the second waveguide 34 in the tap coupler 30 are different, the optical lengths of the first waveguide 33 and the second waveguide 34 also differ. Therefore, when a temperature change occurs, an optical length difference occurs between the first waveguide 33 and the second waveguide 34, and the branching ratio of the tap coupler 30 also changes.

[0068] Therefore, to deal with such a situation, the tap coupler 30C may be arranged so that the light propagation direction (Y direction) of the thin film optical waveguide 60 of the first DC modulation section 21F is approximately perpendicular to the light propagation direction (Z direction) of the first waveguide 33 and the second waveguide 34 of the tap coupler 30. Such an embodiment will be described below as Example 4. [Example]

[0069] Fig. 7 is a schematic plan view showing an example of the configuration of an optical modulator 5 of Example 4. The optical modulator 5 shown in Fig. 7 differs from the optical modulator 5 shown in Fig. 2 in that the tap coupler 30C is arranged so that the light propagation direction of the tap coupler 30C is approximately perpendicular to the light propagation direction (Y direction) of the first DC modulation section 21F, for example, facing the Z axis direction of the thin-film LN crystal. The tap coupler 30C has a first coupler 31C, a second coupler 32C, a first waveguide 33C, a second waveguide 34C, and a monitor 35C.

[0070] The first coupler 31C is optically coupled to the LN waveguide 27A and is arranged so that the light propagation direction is oriented in the Z-axis direction of the crystal axis. The second coupler 32C is also optically coupled to the LN waveguide 13A and is arranged so that the light propagation direction is oriented in the Z-axis direction of the crystal axis. Furthermore, the first waveguide 33C optically couples the first coupler 31C and the second coupler 32C and is arranged so that the light propagation direction is oriented in the Z-axis direction of the crystal axis. The second waveguide 34C is also optically coupled to the first coupler 31C and the second coupler 32C and is arranged so that the light propagation direction is oriented in the Z-axis direction of the crystal axis.

[0071] When the orientation of the thin-film optical waveguide 60 of the first DC modulation unit 21F, i.e., the light propagation direction, is defined as the Y direction, the electric field direction in the thin-film optical waveguide 60 of the first DC modulation unit 21F is defined as the Z direction. On the other hand, when the orientation of the first waveguide 33C of the tap coupler 30C, i.e., the light propagation direction of the first waveguide 33C, is defined as the Z direction, the electric field direction in the first waveguide 33C of the tap coupler 30C is defined as the Y direction. Furthermore, the direction of the internal electric field due to spontaneous polarization of the thin-film LN crystal is the Z direction, which differs from the electric field direction (Y direction) of the tap coupler 30C. As a result, even when temperature changes occur, the refractive index change of the first waveguide 33C and the second waveguide 34C due to the internal electric field due to spontaneous polarization is small, and therefore the branching ratio of the tap coupler 30C using the delay interferometer 300 can be stabilized.

[0072] The modes of light that propagate through the thin-film optical waveguide 60 include TE mode and TM mode, but the mode of light modulated by the first DC modulator 21F is TE mode. The direction of the electric field of the TE mode propagating in the Y direction is the Z direction, and in the Z direction, the change in refractive index when temperature changes is large. On the other hand, the direction of the electric field of the TE mode propagating in the Z direction is the Y direction, and in the Y direction, the change in refractive index when temperature changes is small. Therefore, when the light propagation direction of the tap coupler 30 is set to the Z direction, the change in refractive index when temperature changes is small, and the branching ratio of the tap coupler 30 can be stabilized.

[0073] The tap coupler 30 of Example 1 is exemplified as being arranged such that the light propagation directions of the first coupler 31, the second coupler 32, the first waveguide 33, and the second waveguide 34 are the same. However, the first waveguide 33 in the tap coupler 30 may be arranged in a direction substantially perpendicular to the light propagation directions of the first coupler 31 and the second coupler 32, and such an embodiment will be described below as Example 5. [Example]

[0074] Fig. 8 is a plan view schematic diagram showing an example of the configuration of a tap coupler 30D according to a fifth embodiment. The tap coupler 30D shown in Fig. 8 is characterized in that a first waveguide 33D and a second waveguide 34D are arranged so that the light propagation directions of the first waveguide 33D and the second waveguide 34D are substantially perpendicular to the light propagation directions of the first coupler 31D and the second coupler 32D. The tap coupler 30D includes a first coupler 31D, a second coupler 32D, a first waveguide 33D, a second waveguide 34D, and a monitor 35D.

[0075] The first coupler 31D is optically coupled to the LN waveguide 27A and is arranged so that the light propagation direction is oriented in the Y-axis direction of the crystal axis. The second coupler 32D is also optically coupled to the LN waveguide 13A and is arranged so that the light propagation direction is oriented in the Y-axis direction of the crystal axis. Furthermore, the first waveguide 33D optically couples the first coupler 31D and the second coupler 32D and is arranged so that the light propagation direction is oriented in the Z-axis direction of the crystal axis. The second waveguide 34D is also optically coupled to the first coupler 31D and the second coupler 32D and is arranged so that the light propagation direction is oriented in the Z-axis direction of the crystal axis.

[0076] In the tap coupler 30D of Example 5, the first coupler 31D, the second coupler 32D, the first waveguide 33D, and the second waveguide 34D are arranged so that the light propagation direction of the first coupler 31D and the second coupler 32D is the Y-axis direction and the light propagation direction of the first waveguide 33D and the second waveguide 34D is the Z-axis direction. As a result, when the light propagation direction of the first coupler 31D and the second coupler 32D is the Y-axis direction, the size of the tap coupler 30D in the Y-axis direction can be reduced.

[0077] In the tap coupler 50D of Example 5, the light propagation direction of the first coupler 31D and the second coupler 32D is the Y-axis direction, and the light propagation direction of the first waveguide 33D and the second waveguide 34D is the Z-axis direction. However, the first coupler 31D, the second coupler 32D, the first waveguide 33D, and the second waveguide 34D may be arranged so that the light propagation direction of the first coupler 31D and the second coupler 32D is the Z-axis direction, and the light propagation direction of the first waveguide 33D and the second waveguide 34D is the Y-axis direction, and this can be modified as appropriate.

[0078] Furthermore, although a rib-type waveguide has been exemplified as the LN waveguide, it is not limited to a rib-type waveguide, and can also be applied to, for example, a channel-type waveguide or a high-mesa waveguide. [Explanation of symbols]

[0079] 1 Optical communication equipment 3 DSP 4 light source 5 Optical Modulator 14 electrodes 30, 30A, 30B, 30C, 30D Tap Coupler 31 First Coupler 32 Second Coupler 33 First waveguide 34 Second Waveguide 35 monitors 60 Thin-film optical waveguide 300 Delay Interferometer

Claims

1. a modulator that includes a first optical waveguide formed of a thin-film LN (Lithium Niobate) substrate and through which light passes, and an electrode that applies a voltage to the first optical waveguide, and that modulates the phase of the light passing through in accordance with an electric field in the first optical waveguide that corresponds to the voltage; a thin-film LN tap coupler that is connected to the first optical waveguide and that branches a part of the light passing through a second optical waveguide formed by the thin-film LN substrate; and The tap coupler includes: a delay interferometer that branches a part of the light passing through the second optical waveguide from the first optical waveguide at a branching ratio according to a phase difference of the light passing through the second optical waveguide within the tap coupler; The delay interferometer comprises: a first coupler that receives the light from the first optical waveguide and branches the received light; a first waveguide in the second optical waveguide that passes one of the lights branched from the first coupler; a second waveguide in the second optical waveguide that passes the other light branched from the first coupler and delays the optical output compared to the first waveguide; a second coupler that multiplexes the one light from the first waveguide and the other light from the second waveguide and branches the multiplexed light having a phase difference at a branching ratio corresponding to the phase difference; and An optical device characterized in that when the propagation direction of the light in the first optical waveguide is the Z-axis direction of the LN crystal of the thin-film LN substrate, the propagation direction of the light in the first waveguide is the Y-axis direction of the LN crystal.

2. The first waveguide comprises:

2. The optical device according to claim 1, wherein the second waveguide has an optical length different from that of the first waveguide.

3. The first waveguide comprises:

3. The optical device according to claim 2, wherein the second waveguide has a different waveguide length from the first waveguide.

4. The second waveguide comprises:

3. The optical device according to claim 1, wherein the optical waveguide is a curved waveguide including a portion whose propagation direction is different from that of the first waveguide.

5. 3. The optical device according to claim 1, wherein the first waveguide and the second waveguide are formed on the thin-film LN substrate, with the second waveguide being oriented in the same direction as the propagation direction of the light in the first waveguide and having a width different from that of the first waveguide.

6. 3. The optical device according to claim 1, wherein the first coupler, the second coupler, the first waveguide, and the second waveguide are arranged so that the propagation direction of the light in the first coupler and the second coupler is perpendicular to the propagation direction of the light in the first waveguide and the second waveguide.

7. The first coupler and the second coupler are 2. The optical device according to claim 1, wherein the optical device is an MMI (Multi Mode Interference) coupler.

8. a processor that performs signal processing on the electrical signal; A light source that generates light; a modulator formed of a thin-film LN (Lithium Niobate) substrate, the modulator having a first optical waveguide through which the light passes and an electrode for applying a voltage to the first optical waveguide, the modulator modulating the phase of the light passing through the first optical waveguide in response to an electric field in the first optical waveguide that corresponds to the voltage; a thin-film LN tap coupler that is connected to the first optical waveguide and that branches a part of the light passing through a second optical waveguide formed by the thin-film LN substrate; and The tap coupler includes: a delay interferometer that branches a part of the light passing through the second optical waveguide from the first optical waveguide at a branching ratio according to a phase difference of the light passing through the second optical waveguide within the tap coupler; The delay interferometer comprises: a first coupler that receives the light from the first optical waveguide and branches the received light; a first waveguide in the second optical waveguide that passes one of the lights branched from the first coupler; a second waveguide in the second optical waveguide that passes the other light branched from the first coupler and delays the optical output compared to the first waveguide; a second coupler that multiplexes the one light from the first waveguide and the other light from the second waveguide and branches the multiplexed light having a phase difference at a branching ratio according to the phase difference, An optical communication device characterized in that when the propagation direction of the light in the first optical waveguide is the Z-axis direction of the LN crystal of the thin-film LN substrate, the propagation direction of the light in the first waveguide is the Y-axis direction of the LN crystal.

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