Optical modulator, optical transmitter and optical transceiver

Abstract

An optical modulator and the like are provided that can reduce the chip size of the optical modulator while ensuring velocity matching. [Solution] The optical modulator includes a first coupler that branches a signal light into two, a first waveguide connected to one output of the first coupler, a second waveguide connected to the other output of the first coupler, and a second coupler that combines and outputs the signal light from the first and second waveguides. The first waveguide includes a first input arm waveguide, a first output arm waveguide, and a first folded waveguide. The second waveguide includes a second input arm waveguide, a second output arm waveguide, and a second folded waveguide. The first and second input arm waveguides and the first and second output arm waveguides are waveguides containing a material (LN) with high EO characteristics. The first and second folded waveguides are waveguides containing a material (SiN) with a low refractive index.

Description

[Technical Field]

[0001] The present invention relates to an optical modulator, an optical transmitter, and an optical transceiver. [Background technology]

[0002] For example, thin-film LN modulators are known as optical modulators that are highly efficient and capable of high-speed operation. However, thin-film LN modulators generally require a modulator length of 10 mm or more to achieve high efficiency (low half-wave voltage Vπ), and miniaturization remains an issue.

[0003] To solve this problem, a structure has been proposed in which a thin-film LN modulator is integrated on a Si photonics substrate using heterogeneous materials, and some of the functions of an optical modulator, such as a DC phase shifter, are realized using Si photonics, which is suitable for miniaturization (Reference Paper 1). Another structure has been proposed in which a thin-film LN modulator is integrated on a Si photonics substrate using heterogeneous materials, and the arm waveguide of the optical modulator is folded back to reduce the longitudinal size (Reference Paper 2).

[0004] Reference paper 1: CLEO 2023 STh4O.5 Reference paper 2:2021 IEEE 17th International Conference on Group IV Photonics 10.1109 / IEDM19573.2019.8993510

[0005] Fig. 25 is a plan view schematic diagram showing an example of a conventional optical modulator 500. The optical modulator 500 shown in Fig. 25 is a thin-film LN modulator with a folded structure mounted on a Si photonics substrate 501. The optical modulator 500 includes the Si photonics substrate 501, an input MMI (Multi-Mode Interferometer) 502, a modulator body 503, and a folded section 504. The input MMI 502 includes an input waveguide 502A that inputs input light to the modulator body 503, and an output waveguide 502B that outputs signal light from the modulator body 503. The modulator body 503 includes an input coupler 510, a first waveguide 520, a second waveguide 530, an output coupler 540, and an electrode 550. The input coupler 510 splits the input light from the input waveguide 502A into two, outputs one of the split input light to the first waveguide 520, and outputs the other split input light to the second waveguide 530. The output coupler 540 combines the signal light from the first waveguide 520 and the signal light from the second waveguide 530 and outputs the combined signal to the output waveguide 502B. The electrode 550 has a GSG structure consisting of one signal electrode 551 and two ground electrodes 552. The signal electrode 551 has a linear signal electrode 551A and a folded signal electrode 551B. The ground electrode 552 has a linear ground electrode 552A and a folded ground electrode 552B.

[0006] The first waveguide 520 has a first input waveguide 521, a first input arm waveguide 522, a first folded waveguide 523, a first output arm waveguide 524, and a first output waveguide 525. The first input waveguide 521 is a waveguide connecting the input coupler 510 and the first input arm waveguide 522. The first output waveguide 525 is a waveguide connecting the output coupler 540 and the first output arm waveguide 524. The first folded waveguide 523 is a bent waveguide connecting the first input arm waveguide 522 and the first output arm waveguide 524. The first input arm waveguide 522 is a straight arm waveguide that modulates the signal light by changing the refractive index of the signal light guided in response to an electrical signal from the signal electrode 551 to the ground electrode 552. The first output arm waveguide 524 is a linear arm waveguide that modulates the signal light by changing the refractive index of the signal light guided in response to an electrical signal from the signal electrode 551 to the ground electrode 552. The signal electrode 551 is disposed near one side surface of the first input arm waveguide 522 and the first output arm waveguide 524, and the ground electrode 552 is disposed near the other side surface of the first input arm waveguide 522 and the first output arm waveguide 524. The first input waveguide 521, the first folded waveguide 523, and the first output waveguide 525 are formed of Si waveguides, and the first input arm waveguide 522 and the first output arm waveguide 524 are formed of thin-film LN waveguides.

[0007] The second waveguide 530 has a second input waveguide 531, a second input arm waveguide 532, a second folded waveguide 533, a second output arm waveguide 534, and a second output waveguide 535. The second input waveguide 531 is a waveguide connecting the input coupler 510 and the second input arm waveguide 532. The second output waveguide 535 is a waveguide connecting the output coupler 540 and the second output arm waveguide 534. The second folded waveguide 533 is a bent waveguide connecting the second input arm waveguide 532 and the second output arm waveguide 534. The second input arm waveguide 532 is a straight arm waveguide that modulates the signal light by changing the refractive index of the signal light guided in response to an electrical signal from the signal electrode 551 to the ground electrode 552. The second output arm waveguide 534 is a linear arm waveguide that modulates the signal light by changing the refractive index of the signal light guided in response to an electrical signal from the signal electrode 551 to the ground electrode 552. The signal electrode 551 is disposed near one side surface of the second input arm waveguide 532 and the second output arm waveguide 534, and the ground electrode 552 is disposed near the other side surface of the second input arm waveguide 532 and the second output arm waveguide 534. The second input waveguide 531, the second folded waveguide 533, and the second output waveguide 535 are formed of Si waveguides, and the second input arm waveguide 532 and the second output arm waveguide 534 are formed of thin-film LN waveguides. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2008 / 099950 [Patent Document 2] US Patent Application Publication No. 2022 / 404652 Summary of the Invention [Problem to be solved by the invention]

[0009] In the conventional optical modulator 500, the first input arm waveguide 522, the first output arm waveguide 524, the second input arm waveguide 532, and the second output arm waveguide 534 are formed of thin-film LN waveguides. Furthermore, in the optical modulator 500, the first folded waveguide 523 and the second folded waveguide 533 are formed of Si waveguides. In order to achieve high-speed operation of the optical modulator 500, it is necessary to match the propagation speed of light propagating within the optical modulator 500 with the propagation speed of an electrical signal, i.e., to achieve velocity matching, and basically the optical modulator is designed so that the refractive index of light and the refractive index of an electrical signal match.

[0010] 26 is an explanatory diagram showing an example of the relationship between the refractive index and the waveguide width in the Si waveguide and the signal electrode. Since the first folded waveguide 523 and the second folded waveguide 533 use Si waveguides, as shown in FIG. 26, the refractive index of the Si waveguide is about 4, while the refractive index of the signal electrode is about 1.9. Therefore, a speed difference occurs between the propagation speed of light guided through the Si waveguide and the propagation speed of the electrical signal flowing through the signal electrode.

[0011] Therefore, to ensure velocity matching, the optical path length, which is the product (n×L) of the waveguide length L and the refractive index n, must be matched between the light and the electrical signal, so the electrode length of the signal electrode must be made longer than the waveguide length by 4 / 1.9, the ratio of the refractive indexes, that is, approximately twice. As a result, the sizes of the folded signal electrodes 551B arranged on the side surfaces of the first folded waveguide 523 and the second folded waveguide 533, respectively, become larger, and the overall size of the optical modulator 500 also becomes larger.

[0012] In one aspect, an object is to provide an optical modulator or the like that can reduce the chip size of the optical modulator while ensuring velocity matching. [Means for solving the problem]

[0013] An optical modulator according to one embodiment includes a substrate including a high-refractive-index waveguide, a first coupler disposed on the substrate and branching a signal light into two, a first waveguide disposed on the substrate and connected to one output of the first coupler, and a second waveguide disposed on the substrate and connected to the other output of the first coupler. The optical modulator also includes a second coupler disposed on the substrate and multiplexing and outputting the signal light from the first waveguide and the signal light from the second waveguide, and electrodes for applying electrical signals to the first waveguide and the second waveguide. The first waveguide includes a first input arm waveguide connected to the first coupler, a first output arm waveguide connected to the second coupler, and a first folded waveguide connecting the first input arm waveguide and the first output arm waveguide. The second waveguide includes a second input arm waveguide connected to the first coupler, a second output arm waveguide connected to the second coupler, and a second folded waveguide connecting the second input arm waveguide and the second output arm waveguide. The first input arm waveguide, the second input arm waveguide, the first output arm waveguide, and the second output arm waveguide are waveguides containing a material having higher EO characteristics than the high refractive index waveguide. At least a portion of the first folded waveguide and the second folded waveguide is a waveguide containing a material having a lower refractive index than the high refractive index waveguide. [Effects of the Invention]

[0014] According to one aspect, the chip size of the optical modulator can be reduced while ensuring velocity matching. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic plan view showing an example of an optical modulator according to a first embodiment. [Figure 2] FIG. 2 is a schematic plan view showing an example of the folded portion. [Figure 3] FIG. 3 is a cross-sectional view showing an example of an optical modulator. [Figure 4]FIG. 4 is a perspective view showing an example of a first folded waveguide and a second folded waveguide at an intersection. [Figure 5] FIG. 5 is a schematic plan view showing an example of the first input-side transition section. [Figure 6A] FIG. 6A is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line AA shown in FIG. [Figure 6B] FIG. 6B is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line BB shown in FIG. [Figure 6C] FIG. 6C is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line CC shown in FIG. [Figure 6D] FIG. 6D is a cross-sectional view showing an example of the cross section taken along line DD shown in FIG. [Figure 7] FIG. 7 is a schematic plan view showing an example of the first input-side first-stage transition section. [Figure 8A] FIG. 8A is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line AA shown in FIG. [Figure 8B] FIG. 8B is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line BB shown in FIG. [Figure 8C] FIG. 8C is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line CC shown in FIG. [Figure 8D] FIG. 8D is a schematic cross-sectional view showing an example of the cross-sectional portion taken along line DD shown in FIG. [Figure 9] FIG. 9 is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line AA shown in FIG. [Figure 10] FIG. 10 is an explanatory diagram showing an example of the relationship between the refractive index and the waveguide width in a Si waveguide, a SiN waveguide, and a signal electrode. [Figure 11] FIG. 11 is an explanatory diagram showing an example of the comparison results of the optical waveguide length and the electrode length of the folded portion in Example 1 and Comparative Examples 1 and 2. In FIG. [Figure 12] FIG. 12 is a perspective view illustrating an example of a first folded waveguide and a second folded waveguide at an intersection of the optical modulator according to the second embodiment. [Figure 13] FIG. 13 is a cross-sectional view illustrating an example of a folded portion of an optical modulator according to a second embodiment. [Figure 14] FIG. 14 is a schematic plan view showing an example of a first input-side first-stage transition section and a first input-side second-stage transition section in the first folded waveguide. [Figure 15A] FIG. 15A is a cross-sectional view showing an example of the cross section taken along line AA shown in FIG. [Figure 15B] FIG. 15B is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line BB shown in FIG. [Figure 15C] FIG. 15C is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line CC shown in FIG. [Figure 15D] FIG. 15D is a schematic cross-sectional view showing an example of the cross-sectional portion taken along line DD shown in FIG. [Figure 16] FIG. 16 is a schematic plan view illustrating an example of an optical modulator according to a third embodiment. [Figure 17] FIG. 17 is a cross-sectional view showing an example of the cross section taken along line AA shown in FIG. [Figure 18] FIG. 18 is a schematic plan view showing an example of an optical modulator according to a fourth embodiment. [Figure 19] FIG. 19 is an explanatory diagram illustrating an example of a DP-IQ modulator according to a fifth embodiment. [Figure 20] FIG. 20 is an explanatory diagram illustrating an example of an optical transceiver according to a sixth embodiment. [Figure 21] FIG. 21 is a schematic cross-sectional view showing an example of an optical transceiver. [Figure 22A] FIG. 22A is a schematic cross-sectional view showing an example of a Si photo substrate after the first formation step. [Figure 22B] FIG. 22B is a cross-sectional view schematically illustrating an example of a Si photo substrate after the second forming step. [Figure 22C] FIG. 22C is a schematic cross-sectional view showing an example of a Si photo substrate after the first removal step. [Figure 22D] FIG. 22D is a schematic cross-sectional view showing an example of a Si photo substrate after the third forming step. [Figure 22E] FIG. 22E is a schematic cross-sectional view showing an example of a Si photo substrate after the second removal step. [Figure 22F]FIG. 22F is a schematic cross-sectional view showing an example of a Si photo substrate after the fourth forming step. [Figure 22G] FIG. 22G is a schematic cross-sectional view showing an example of a Si photo substrate after the fifth forming step. [Figure 22H] FIG. 22H is a schematic cross-sectional view showing an example of a Si photo substrate after the sixth forming step. [Figure 22I] FIG. 22I is a schematic cross-sectional view showing an example of a Si photo substrate after the seventh forming step. [Figure 23] FIG. 23 is an explanatory diagram illustrating an example of an optical module according to a seventh embodiment. [Figure 24] FIG. 24 is an explanatory diagram illustrating an example of an optical transceiver according to an eighth embodiment. [Figure 25] FIG. 25 is a schematic plan view showing an example of a conventional optical modulator. [Figure 26] FIG. 26 is an explanatory diagram showing an example of the relationship between the refractive index and the waveguide width in a Si waveguide and a signal electrode. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments of the optical modulator and the like disclosed in the present application will be described in detail with reference to the drawings. Note that the present invention is not limited to these embodiments. Furthermore, the examples shown below may be combined as appropriate within the scope of not causing any contradiction. [Example]

[0017] FIG. 1 is a schematic plan view showing an example of an optical modulator 1 according to a first embodiment. The optical modulator 1 shown in FIG. 1 is a modulator chip having a thin-film LN Mach-Zehnder optical modulator mounted on a Si photonics substrate 2. The optical modulator 1 includes the Si photonics substrate 2, an input / output MMI (Multi-Mode Interferometer) 3, a modulator body 4, and a folding unit 5. The input / output MMI 3 includes an input waveguide 6 for inputting input light to the modulator body 4, and an output waveguide 7 for outputting signal light from the modulator body 4. The modulator body 4 is a modulation operation unit of the Mach-Zehnder modulator that performs optical modulation by applying a voltage to the thin-film LN waveguide. The folding unit 5 is a portion where the arm waveguides of the modulator body 4 are folded back.

[0018] The optical modulator 1 has a first coupler 10, a first waveguide 20, a second waveguide 30, a second coupler 40, and an electrode 50. The first coupler 10 is disposed on a Si substrate 71 and is a coupler that branches and outputs signal light from an input waveguide 6 to a first input waveguide 21A and a second input waveguide 31A. The first waveguide 20 is disposed on the Si substrate 71 and is connected to one output of the first coupler 10. The second waveguide 30 is disposed on the Si substrate 71 and is connected to the other output of the first coupler 10.

[0019] The second coupler 40 is disposed on the Si substrate 71, and is a coupler that multiplexes the signal light from the first output waveguide 25 at the output stage of the first waveguide 20 with the signal light from the second output waveguide 35 at the output stage of the second waveguide 30, and outputs the multiplexed signal light to the output waveguide 7. The electrode 50 is a GSG electrode that applies an electric signal to the first waveguide 20 and the second waveguide 30.

[0020] The first waveguide 20 has a first input waveguide 21, a first input arm waveguide 22, a first folded waveguide 23, a first output arm waveguide 24, a first output waveguide 25, a first input modulation section transition section 26, and a first output modulation section transition section 27. The first input waveguide 21 connects between the first coupler 10 and the first input arm waveguide 22, and is, for example, a Si waveguide.

[0021] The first input arm waveguide 22 is a straight arm waveguide made of a high EO material, for example, thin-film LN, that connects the first input waveguide 21 and the first folded waveguide 23. The first folded waveguide 23 is a waveguide with a folded structure that connects the first input arm waveguide 22 and the first output arm waveguide 24. The first output arm waveguide 24 is a straight arm waveguide made of a high EO material, for example, thin-film LN, that connects the first folded waveguide 23 and the first output waveguide 25.

[0022] The first input-side modulation section transition section 26 is composed of the output end of the first input waveguide 21 and the input end of the first input-side arm waveguide 22, and transitions the signal light between the first input waveguide 21 and the first input-side arm waveguide 22. The first input-side modulation section transition section 26 is also composed of the output end of the first input-side arm waveguide 22 and the input end of the first folded waveguide 23, and transitions the signal light between the first input-side arm waveguide 22 and the first folded waveguide 23.

[0023] The first output-side modulation section transition section 27 is composed of the output end of the first folded waveguide 23 and the input end of the first output-side arm waveguide 24, and transitions the signal light between the first folded waveguide 23 and the first output-side arm waveguide 24. The first output-side modulation section transition section 27 is composed of the output end of the first output-side arm waveguide 24 and the input end of the first output waveguide 25, and transitions the signal light between the first output-side arm waveguide 24 and the first output waveguide 25.

[0024] The second waveguide 30 has a second input waveguide 31, a second input arm waveguide 32, a second folded waveguide 33, a second output arm waveguide 34, a second output waveguide 35, a second input modulation section transition section 36, and a second output modulation section transition section 37. The second input waveguide 31 is, for example, a Si waveguide, and connects between the first coupler 10 and the second input arm waveguide 32.

[0025] The second input arm waveguide 32 is a straight arm waveguide made of a high EO material, for example, thin-film LN, that connects the second input waveguide 31 and the second folded waveguide 33. The second folded waveguide 33 is a waveguide with a folded structure that connects the second input arm waveguide 32 and the second output arm waveguide 34. The second output arm waveguide 34 is a straight arm waveguide made of a high EO material, for example, thin-film LN, that connects the second folded waveguide 33 and the second output waveguide 35.

[0026] The second input-side modulation section transition section 36 is composed of the output end of the second input waveguide 31 and the input end of the second input-side arm waveguide 32, and transitions the signal light between the second input waveguide 31 and the second input-side arm waveguide 32. The second input-side modulation section transition section 36 is composed of the output end of the second input-side arm waveguide 32 and the input end of the second folded waveguide 33, and transitions the signal light between the second input-side arm waveguide 32 and the second folded waveguide 33.

[0027] The second output-side modulation section transition section 37 is composed of the output end of the second folded waveguide 33 and the input end of the second output-side arm waveguide 34, and transitions the signal light between the second folded waveguide 33 and the second output-side arm waveguide 34. The second output-side modulation section transition section 37 is composed of the output end of the second output-side arm waveguide 34 and the input end of the second output waveguide 35, and transitions the signal light between the second output-side arm waveguide 34 and the second output waveguide 35.

[0028] The first input arm waveguide 22, the second input arm waveguide 32, the first output arm waveguide 24, and the second output arm waveguide 34 are waveguides containing a material having higher EO characteristics than the Si substrate 71, such as LN. The first folded waveguide 23 and the second folded waveguide 33 are waveguides containing a material having a lower refractive index than the Si substrate 71, such as SiN.

[0029] The first waveguide 20 has a first input arm waveguide 22 on the outer circumferential side of the fold, a first folded waveguide 23 on the outer circumferential side of the fold, and a first output arm waveguide 24 on the inner circumferential side of the fold.

[0030] The second waveguide 30 has a second input arm waveguide 32 on the inner circumferential side of the fold, a second folded waveguide 33 on the inner circumferential side of the fold, and a second output arm waveguide 34 on the outer circumferential side of the fold.

[0031] The electrode 50 has a GSG structure and includes a signal electrode 51, a first ground electrode 52, and a second ground electrode 53. The signal electrode 51 includes an input signal electrode 51A, an output signal electrode 51B, and a folded signal electrode 51C. The input signal electrode 51A is disposed between the first input arm waveguide 22 and the second input arm waveguide 32 and is electrically connected to the folded signal electrode 51C. The output signal electrode 51B is disposed between the first output arm waveguide 24 and the second output arm waveguide 34 and is electrically connected to the folded signal electrode 51C. The folded signal electrode 51C is disposed between the first folded waveguide 23 and the second folded waveguide 33 and is electrically connected between the input signal electrode 51A and the output signal electrode 51B.

[0032] The first ground electrode 52 includes a first input ground electrode 52A on the outer periphery, a first output ground electrode 52B on the outer periphery, and a first folded ground electrode 52C on the outer periphery. The first input ground electrode 52A is disposed near the side surface of the first input arm waveguide 22 on the outer periphery to face the input signal electrode 51A and is electrically connected to the first folded ground electrode 52C on the outer periphery. The first output ground electrode 52B is disposed near the side surface of the second output arm waveguide 34 on the outer periphery to face the output signal electrode 51B and is electrically connected to the first folded ground electrode 52C. The first folded ground electrode 52C electrically connects the first input ground electrode 52A and the first output ground electrode 52B.

[0033] The second ground electrode 53 includes a second input ground electrode 53A located on the inner periphery, a second output ground electrode 53B located on the inner periphery, and a second folded ground electrode 53C located on the inner periphery. The second input ground electrode 53A is disposed near the side surface of the second input arm waveguide 32 located on the inner periphery so as to face the input signal electrode 51A, and is electrically connected to the second folded ground electrode 53C located on the inner periphery. The second output ground electrode 53B is disposed near the side surface of the first output arm waveguide 24 located on the inner periphery so as to face the output signal electrode 51B, and is electrically connected to the second folded ground electrode 53C. The second folded ground electrode 53C electrically connects the second input ground electrode 53A and the second output ground electrode 53B.

[0034] FIG. 2 is a plan view schematic diagram showing an example of the folded section 5. The folded section 5 includes a first folded waveguide 23 and a second folded waveguide 33. The first folded waveguide 23 includes a first input high-refractive-index waveguide 61A, a first output high-refractive-index waveguide 62A, a first low-refractive-index waveguide 63A, a first input first-stage transition section 64A1, and a first output first-stage transition section 64A2. The first input high-refractive-index waveguide 61A is formed on a first layer 70A on a Si substrate 71 and is connected to the first input arm waveguide 22. The first output high-refractive-index waveguide 62A is formed on the first layer 70A and is connected to the first output arm waveguide 24 ... The first low-refractive-index waveguide 63A is formed on the second layer 70B on the Si substrate 71 and connects the first input high-refractive-index waveguide 61A and the first output high-refractive-index waveguide 62A. For example, the first low-refractive-index waveguide 63A is a straight waveguide with a SiN core. The second layer 70B is a high-refractive-index layer. The first input high-refractive-index waveguide 61A and the first output high-refractive-index waveguide 62A are Si waveguides, allowing them to be folded back in a short length with a small curvature. The first folded waveguide 23 connects the first input arm waveguide 22 and the first output arm waveguide 24. The first folded waveguide 23 is connected to the first input arm waveguide 22 and has a first input high-refractive-index waveguide 61A which is a first input folded waveguide on the outer circumferential side of the fold, and a first output high-refractive-index waveguide 62A which is a first output folded waveguide on the inner circumferential side of the fold and is connected to the first output arm waveguide 24.

[0035] First input-side first-stage transition section 64A1 is composed of an output end of first input-side high-refractive-index waveguide 61A and an input end of first low-refractive-index waveguide 63A, and transitions signal light between first input-side high-refractive-index waveguide 61A and first low-refractive-index waveguide 63A. First output-side first-stage transition section 64A2 is composed of an output end of first low-refractive-index waveguide 63A and an input end of first output-side high-refractive-index waveguide 62A, and transitions signal light between first low-refractive-index waveguide 63A and first output-side high-refractive-index waveguide 62A.

[0036] The second folded waveguide 33 includes a second input high-refractive-index waveguide 61B, a second output high-refractive-index waveguide 62B, a second low-refractive-index waveguide 63B, a second input first-stage transition portion 64B1, and a second output first-stage transition portion 64B2. The second input high-refractive-index waveguide 61B is formed on a first layer 70A on a Si substrate 71 and is connected to the second input arm waveguide 32. The second output high-refractive-index waveguide 62B is formed on the first layer 70A and is connected to the second output arm waveguide 34 ... output high-refractive-index waveguide 62B is formed on the first layer 70A and is connected to the second output arm waveguide 34. The second low-refractive-index waveguide 63B is formed on the second layer 70B on the Si substrate 71 and connects the second input high-refractive-index waveguide 61B and the second output high-refractive-index waveguide 62B. For example, the second low-refractive-index waveguide 63B is a straight waveguide with a SiN core. The second input high-refractive-index waveguide 61B and the second output high-refractive-index waveguide 62B are Si waveguides, allowing them to be folded back in a short length with a small curvature. The second folded waveguide 33 connects the second input arm waveguide 32 and the second output arm waveguide 34. The second folded waveguide 33 is connected to the second input arm waveguide 32 and has a second input high-refractive-index waveguide 61B which is a second input folded waveguide on the outer periphery of the fold, and a second output high-refractive-index waveguide 62B which is a second output folded waveguide on the inner periphery of the fold and is connected to the second output arm waveguide 34.

[0037] The second input-side first-stage transition section 64B1 is composed of the output end of the second input-side high-refractive-index waveguide 61B and the input end of the second low-refractive-index waveguide 63B, and transitions the signal light between the second input-side high-refractive-index waveguide 61B and the second low-refractive-index waveguide 63B. The second output-side first-stage transition section 64B2 is composed of the output end of the second low-refractive-index waveguide 63B and the input end of the second output-side high-refractive-index waveguide 62B, and transitions the signal light between the second low-refractive-index waveguide 63B and the second output-side high-refractive-index waveguide 62B. The first output-side arm waveguide 24 and the first low-refractive-index waveguide 63A are connected by the first output-side high-refractive-index waveguide 62A, straddling the second low-refractive-index waveguide 63B.

[0038] FIG. 3 is a cross-sectional schematic diagram showing an example of an optical modulator 1. The optical modulator 1 shown in FIG. 3 has a Si substrate 71, a lower cladding layer 72 stacked on the Si substrate 71, and an upper cladding layer 73 stacked on the lower cladding layer 72. The optical modulator 1 also has a first layer 70A, a second layer 70B, and a third layer 70C. The first layer 70A is a layer disposed between the upper cladding layer 73 and the lower cladding layer 72. The second layer 70B is a layer disposed within the lower cladding layer 72 on a side closer to the upper cladding layer 73. The third layer 70C is a layer disposed within the lower cladding layer 72 on a side closer to the Si substrate 71. The first layer 70A serves as, for example, a core layer of the first output arm waveguide 24. The second layer 70B serves as, for example, a core layer of the first output waveguide 25, the first output-side high-refractive-index waveguide 62A, and the second output-side high-refractive-index waveguide 62B. The second layer 70B is a high-refractive-index layer. The third layer 70C serves as, for example, a core layer of the first low-refractive-index waveguide 63A. The third layer 70C is a first-stage refractive index layer.

[0039] 4 is a perspective view showing an example of the first folded waveguide 23 and the second folded waveguide 33 at the intersection. At the intersection shown in FIG. 4, the first low-refractive-index waveguide 63A in the first folded waveguide 23 arranged on the third layer 70C and the second output-side high-refractive-index waveguide 62B in the second folded waveguide 33 arranged on the second layer 70B intersect three-dimensionally in different layers. As a result, the first folded waveguide 23 and the second folded waveguide 33 have low loss, and crosstalk between the first folded waveguide 23 and the second folded waveguide 33 can be suppressed.

[0040] FIG. 5 is a schematic plan view showing a portion of the first input-side modulation section transition section 26 or the second input-side modulation section transition section 36. For ease of explanation, only the first input-side modulation section transition section 26 or the second input-side modulation section transition section 36 is shown. However, since the first output-side modulation section transition section 27 and the second output-side modulation section transition section 37 have substantially the same structure, the same reference numerals are used to denote the overlapping configurations and operations, and the overlapping configurations and operations will not be described again. The first input-side modulation section transition section 26 shown in FIG. 5 is composed of the output end of the first input waveguide 21 and the input end of the first input arm waveguide 22. The first input waveguide 21 has a tapered structure in which the waveguide width gradually narrows from the input end of the first input arm waveguide 22 toward the middle of the first input arm waveguide 22. The second input-side modulation section transition section 36 is composed of the output end of the second input waveguide 31 and the input end of the second input-side arm waveguide 32. The second input waveguide 31 has a tapered structure in which the waveguide width gradually narrows from the input end of the second input-side arm waveguide 32 toward the middle portion of the second input-side arm waveguide 32.

[0041] Fig. 6A is a cross-sectional schematic diagram showing an example of the cross section taken along line AA in Fig. 5. The portion of the first input-side modulator transition section 26 shown in Fig. 6A has a Si substrate 71, a lower cladding layer 72 laminated on the Si substrate 71, and an upper cladding layer 73 laminated on the lower cladding layer 72. A second layer 70B inside the lower cladding layer 72 is made of, for example, Si, and serves as the core layer of the first input waveguide 21. In other words, the first input waveguide 21 is a Si waveguide with a channel structure.

[0042] 6B is a cross-sectional view showing an example of the cross section taken along line BB in FIG. 5. The portion of the first input-side modulator transition section shown in FIG. 6B includes a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. A second layer 70B in the lower cladding layer 72 is the core layer of the first input waveguide 21, and is wider at the base of the tapered structure. A first layer 70A in the upper cladding layer 73 is the core layer of the first input arm waveguide 22. The first input arm waveguide 22 is, for example, a thin-film LN waveguide with a rib structure.

[0043] 6C is a cross-sectional schematic diagram showing an example of the cross section taken along line CC shown in FIG. 5. The portion of the input-side modulation transition section shown in FIG. 6C has a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. The second layer 70B of the lower cladding layer 72 is the core layer of the first input waveguide 21, and is the tip portion of the tapered structure, with a core width narrower than that of the portion shown in FIG. 6B. The first layer 70A of the upper cladding layer 73 is the core layer of the first input-side arm waveguide 22.

[0044] 6D is a cross-sectional view showing an example of the cross section taken along line DD in FIG. 5. The portion of the first input-side modulator transition section 26 shown in FIG. 6D has a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. In this portion, the second layer 70B does not exist in the lower cladding layer 72. The first layer 70A of the upper cladding layer 73 becomes the core layer of the first input-side arm waveguide 22.

[0045] The first input-side modulation section transition section 26 is composed of the output end of the first input-side arm waveguide 22 and the input end of the first input-side high-refractive-index waveguide 61A. The input end of the first input-side high-refractive-index waveguide 61A has a tapered structure in which the waveguide width gradually narrows from the output end of the first input-side arm waveguide 22 toward the middle.

[0046] FIG. 7 is a schematic plan view showing an example of a first input-side first-stage transition section 64A1. For ease of explanation, the first input-side first-stage transition section 64A1 in the first folded waveguide 23 is shown. However, since the second input-side first-stage transition section 64B1 in the second folded waveguide 33 has a similar structure, the same reference numerals are used to designate the overlapping configurations and operations, and the overlapping configurations and operations will not be described again. The first input-side first-stage transition section 64A1 shown in FIG. 7 is composed of the output end of the first input-side high-refractive-index waveguide 61A and the input end of the first low-refractive-index waveguide 63A. The output end of the first input-side high-refractive-index waveguide 61A has a tapered structure in which the waveguide width gradually narrows from the input end of the first low-refractive-index waveguide 63A toward the middle. The input end of the first low-refractive-index waveguide 63A has a tapered structure in which the waveguide width gradually widens from the input end and remains constant in the middle portion. As a result, the tapered waveguide width facilitates optical coupling between the first input-side high-refractive-index waveguide 61A and the first low-refractive-index waveguide 63A, thereby achieving low-loss optical transition.

[0047] Fig. 8A is a cross-sectional schematic diagram showing an example of the cross section taken along line AA shown in Fig. 7. First folded waveguide 23 shown in Fig. 8A has a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. A second layer 70B inside lower cladding layer 72 is made of, for example, Si, and serves as the core layer of first input-side high-refractive-index waveguide 61A. In other words, first input-side high-refractive-index waveguide 61A is a Si waveguide with a channel structure.

[0048] FIG. 8B is a cross-sectional schematic diagram showing an example of the cross section taken along line BB in FIG. 7. The first input-side first-stage transition section 64A1 shown in FIG. 8B includes a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. A second layer 70B in the lower cladding layer 72 is the core layer of the first input-side high-refractive-index waveguide 61A. A third layer 70C in the lower cladding layer 72 is the core layer of the first low-refractive-index waveguide 63A. That is, the first low-refractive-index waveguide 63A is, for example, a SiN waveguide with a channel structure. Here, the first input-side high-refractive-index waveguide 61A is wide, and the first low-refractive-index waveguide 63A is narrow.

[0049] 8C is a cross-sectional schematic diagram showing an example of the CC cross section shown in FIG. 7. The first input-side first-stage transition portion 64A1 shown in FIG. 8C has a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. A second layer 70B in the lower cladding layer 72 is a core layer of the first input-side high-refractive-index waveguide 61A. A third layer 70C in the lower cladding layer 72 is a core layer of the first low-refractive-index waveguide 63A. Here, the width of the first input-side high-refractive-index waveguide 61A is narrow, and the width of the first low-refractive-index waveguide 63A is wide.

[0050] Fig. 8D is a cross-sectional schematic diagram showing an example of the cross section taken along line DD shown in Fig. 7. The first folded waveguide 23 shown in Fig. 8D has a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. A third layer 70C inside the lower cladding layer 72 is a core layer of the first low-refractive-index waveguide 63A.

[0051] While the first input-side first-stage transition section 64A1 is illustrated as being configured by the output end of the first input-side high-refractive-index waveguide 61A and the input end of the first low-refractive-index waveguide 63A, the first output-side first-stage transition section 64A2 is configured by the output end of the first low-refractive-index waveguide 63A and the input end of the first output-side high-refractive-index waveguide 62A. In this case, the input end of the first output-side high-refractive-index waveguide 62A has a tapered structure in which the waveguide width gradually narrows from the output end of the first low-refractive-index waveguide 63A toward the middle. The output end of the first low-refractive-index waveguide 63A has a tapered structure in which the waveguide width gradually widens from the output end and remains constant in the middle.

[0052] Fig. 9 is a schematic cross-sectional view showing an example of the cross section taken along line AA in Fig. 1. The modulator body 4 shown in Fig. 9 includes a Si substrate 71, a lower cladding layer 72, an upper cladding layer 73, and an electrode 50. The modulator body 4 includes a first input arm waveguide 22 and a second input arm waveguide 32, each having a first layer 70A in the upper cladding layer 73 as its core, and a first output arm waveguide 24 and a second output arm waveguide 34, each having the first layer 70A as its core. The electrode 50 disposed on the upper cladding layer 73 includes an input signal electrode 51A, a first input ground electrode 52A, a second input ground electrode 53A, an output signal electrode 51B, a first output ground electrode 52B, and a second output ground electrode 53B.

[0053] The input signal electrode 51A is disposed between the first input arm waveguide 22 and the second input arm waveguide 32. The first input ground electrode 52A is disposed opposite the input signal electrode 51A across the first input arm waveguide 22. The second input ground electrode 53A is disposed on the opposite side of the second input arm waveguide 32 to the input signal electrode 51A.

[0054] The output signal electrode 51B is disposed between the first output arm waveguide 24 and the second output arm waveguide 34. The first output ground electrode 52B is disposed opposite to the output signal electrode 51B across the first output arm waveguide 24. The second output ground electrode 53B is disposed on the opposite side of the second output arm waveguide 34 to the output signal electrode 51B.

[0055] The polarization direction X of the modulator body 4 is the same on the outward and return paths. The first input arm waveguide 22 modulates the signal light from the input signal electrode 51A to the first input ground electrode 52A in accordance with an electrical signal in the direction opposite to the polarization direction X. The second input arm waveguide 32 modulates the signal light from the input signal electrode 51A to the second input ground electrode 53A in accordance with an electrical signal in the direction forward from the polarization direction X.

[0056] On the other hand, the first output arm waveguide 24 modulates the signal light from the output signal electrode 51B to the first output ground electrode 52B in accordance with an electrical signal in the direction opposite to the polarization direction X. The second output arm waveguide 34 modulates the signal light from the output signal electrode 51B to the second output ground electrode 53B in accordance with an electrical signal in the direction forward of the polarization direction X.

[0057] In other words, the first input arm waveguide 22 and the first output arm waveguide 24 modulate the signal light in response to the same reverse-direction electrical signal. The second input arm waveguide 32 and the second output arm waveguide 34 modulate the signal light in response to the same forward-direction electrical signal. Since electrical signals in the same direction are applied to the input-side forward path and the output-side backward path, modulation efficiency can be improved. Moreover, the second input arm waveguide 32 and the second output arm waveguide 34 can achieve push-pull operation in which the phase changes in the direction opposite to that of the first input arm waveguide 22 and the first output arm waveguide 24.

[0058] FIG. 10 is an explanatory diagram showing an example of the relationship between the refractive index and the waveguide width of a Si waveguide, a SiN waveguide, and a signal electrode. The first folded waveguide 23 includes a first input-side high-refractive-index waveguide 61A, a first output-side high-refractive-index waveguide 62A, and a first low-refractive-index waveguide 63A. The first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A are, for example, Si waveguides. The first low-refractive-index waveguide 63A is, for example, a SiN waveguide. As shown in FIG. 10, the refractive index of a SiN waveguide is 1.6 to 1.9, which is close to the refractive index of an electrical signal, so a portion of the first folded waveguide 23 is made of a SiN waveguide. As a result, even if the electrical wiring length of the folded signal electrode 51C is approximately the same as the waveguide length, velocity matching can be achieved, making it possible to omit or shorten a delay electrode for velocity matching. Although the first folded waveguide 23 has been described, the same effect can be obtained with the second folded waveguide 33 as well.

[0059] 11 is an explanatory diagram showing an example of the comparison results of the optical waveguide length and electrode length of the folded portion 5 in Example 1 and Comparative Examples 1 and 2. The first folded waveguide 23 has a first input-side high-refractive-index waveguide 61A, a first output-side high-refractive-index waveguide 62A, and a first low-refractive-index waveguide 63A. The first input-side high-refractive-index waveguide 61A is a bent Si waveguide including a bent portion R and portions before and after the bent portion R. The first output-side high-refractive-index waveguide 62A is a bent Si waveguide including a bent portion R and portions before and after the bent portion R. The first low-refractive-index waveguide 63A is a straight SiN waveguide including a straight portion.

[0060] Furthermore, in Comparative Example 1, the first folded waveguide has an input-side waveguide, an output-side waveguide, and a straight waveguide connecting the input-side waveguide and the output-side waveguide, and is configured as a Si waveguide. For ease of explanation, the first folded waveguide of Comparative Example 1 has the same configuration as the first folded waveguide 23 of Example 1, but differs in that it is configured as a Si waveguide. The input-side waveguide is a bent Si waveguide including a bent portion and portions before and after the bent portion. The output-side waveguide is a bent Si waveguide including a bent portion and portions before and after the bent portion. The straight waveguide is a straight Si waveguide including a straight portion.

[0061] In addition, in Comparative Example 2, the first folded waveguide has an input-side waveguide, an output-side waveguide, and a straight waveguide connecting the input-side waveguide and the output-side waveguide, and is configured as a SiN waveguide. For ease of explanation, the first folded waveguide of Comparative Example 2 has the same configuration as the first folded waveguide 23 of Example 1, but differs in that it is configured as a SiN waveguide. The input-side waveguide is a bent SiN waveguide including a bent portion and portions before and after the bent portion. The output-side waveguide is a bent SiN waveguide including a bent portion and portions before and after the bent portion. The straight waveguide is a straight SiN waveguide including a straight portion.

[0062] First, in the first folded waveguide of Comparative Example 1, if the waveguide length L of the portion before and after the bent portion of the input-side waveguide is 20 μm, the refractive index of the Si waveguide is approximately 4, so the optical waveguide length of the portion before and after the bent portion of the input-side waveguide is 80 μm. The optical waveguide length of the portion before and after the bent portion of the output-side waveguide is also 80 μm. With a Si waveguide, a bent waveguide can be realized with low loss even with a small radius of curvature. For example, the radius of curvature of the bent portion of the input-side waveguide can be reduced to 10 μm. Since the refractive index of the Si waveguide is approximately 4, the optical waveguide length of the bent portion is 63 μm. The optical waveguide length of the bent portion of the output-side waveguide is also 63 μm. Furthermore, if the waveguide length of the straight portion is 400 μm, the refractive index of the Si waveguide is approximately 4, so the optical waveguide length of the straight portion is 1600 μm. As a result, the total optical waveguide length of the first folded waveguide is 1743 μm. To achieve velocity matching between light and electrical signals, the product of the refractive index and length (n*L) must match between the optical waveguide and the electrical wiring. In the structure of Example 1, the refractive index of the electrical signal is about 1.9, so the electrical wiring length needs to be 917 μm to achieve velocity matching. If the first folded waveguide is only a Si waveguide, the optical path length of the light becomes long due to the high refractive index of the first folded waveguide, and the electrical wiring length needs to be increased by the ratio of the refractive index to achieve velocity matching, which is about 0.9 mm.

[0063] In Comparative Example 2, when the waveguide length L of the input-side waveguide before and after the bent portion is 20 μm, the refractive index of the SiN waveguide is 1.9, so the optical waveguide length of the input-side waveguide before and after the bent portion is 38 μm. The optical waveguide length of the output-side waveguide before and after the bent portion is also 38 μm. Because the refractive index of the core of the SiN waveguide is relatively low, a large radius of curvature is required to bend the waveguide with low loss. For example, the radius of curvature of the bent portion of the input-side waveguide needs to be 60 μm or more. Because the refractive index of the SiN waveguide is 1.9, the optical waveguide length of the bent portion is 179 μm. The optical waveguide length of the bent portion of the output-side waveguide is also 179 μm. Furthermore, when the waveguide length of the straight portion is 400 μm, the refractive index of the SiN waveguide is 1.9, so the optical waveguide length of the straight portion is 760 μm. As a result, the optical waveguide length of the first folded waveguide is 977 μm. As in Comparative Example 1, to achieve velocity matching between light and electrical signals, the product of the refractive index and length (n*L) must be matched between the optical waveguide and the electrical wiring. In the structure of Example 1, the refractive index of the electrical signal is approximately 1.9, so the electrical wiring length needs to be 514 μm to achieve velocity matching. When the first folded waveguide is only a SiN waveguide, the refractive index of the first folded waveguide is lower, but the curvature of the bent waveguide needs to be increased from 10 μm to 60 μm compared to a Si waveguide. As a result, the actual length of the waveguide is longer, and as a result, the electrical wiring length is also longer, reaching approximately 515 μm.

[0064] In contrast, in the first folded waveguide 23 of Example 1, when the waveguide length L of the portions before and after the bent portion R of the first input-side high-refractive-index waveguide 61A is 20 μm, the refractive index of the Si waveguide is 4, so the optical waveguide length of the portions before and after the bent portion R of the first input-side high-refractive-index waveguide 61A is 80 μm. The optical waveguide length of the portions before and after the bent portion R of the first output-side high-refractive-index waveguide 62A is also 80 μm. The radius of curvature of the bent portion R of the first input-side high-refractive-index waveguide 61A can be reduced to 10 μm in a Si waveguide, and the refractive index of the Si waveguide is approximately 4, so the optical waveguide length of the bent portion R is 63 μm. The optical waveguide length of the bent portion R of the first output-side high-refractive-index waveguide 62A is also 63 μm. On the other hand, the waveguide in the straight portion is a SiN waveguide with a low refractive index, and its refractive index is as low as 1.9. Therefore, if the waveguide length is 400 μm, the optical waveguide length of the straight portion is 760 μm. As a result, the total optical waveguide length of the first folded waveguide 23 is 903 μm. As in Comparative Examples 1 and 2, to achieve velocity matching between light and electrical signals, the product of the refractive index and length (n*L) must be matched between the optical waveguide and the electrical wiring. In the structure of Example 1, the refractive index of the electrical signal is approximately 1.9, so the electrical wiring length should be 475 μm to achieve velocity matching. In this example, Si waveguides are used for the curved waveguide portions of the first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A to shorten their actual lengths, and SiN waveguides are used as the first low-refractive-index waveguide 63A for the straight portion where the waveguide is routed long. As a result, the electrode length can be shortened to 475 μm due to the effect of shortening the actual length by the Si waveguide and shortening the optical path length by reducing the refractive index of the SiN waveguide, and by shortening the electrode length, the chip size can be reduced.

[0065] The electrode length of the folded signal electrode 51C in Example 1 is 475 μm, the electrode length of the folded signal electrode in Comparative Example 1 is 917 μm, and the electrode length of the folded signal electrode in Comparative Example 2 is 514 μm. Therefore, the electrode length of the folded signal electrode 51C in Example 1 is shorter, and thus the chip size of the optical modulator 1 can be made smaller than the electrode lengths in Comparative Examples 1 and 2.

[0066] In the first embodiment, the first input high-refractive-index waveguide 61A and the first output high-refractive-index waveguide 62A of the first folded waveguide 23 are Si waveguides that allow for a small curvature, and the first low-refractive-index waveguide 63A after the bend is a SiN waveguide that is advantageous for velocity matching. The Si waveguide portions of the first input high-refractive-index waveguide 61A and the first output high-refractive-index waveguide 62A are short, while the SiN waveguide portion of the first low-refractive-index waveguide 63A is long, thereby shortening the overall length of the waveguide and the optical path length. As a result, the electrode length of the folded signal electrode 51C, which needs to be matched to the optical path length of the first folded waveguide 23, can be shortened, thereby shortening the length of the delay waveguide and, as a result, reducing the size of the folded portion.

[0067] The second input-side high-refractive-index waveguide 61B and the second output-side high-refractive-index waveguide 62B of the second folded waveguide 33 are Si waveguides that allow for small curvatures, while the second low-refractive-index waveguide 63B after the bend is a SiN waveguide that is advantageous for velocity matching. The Si waveguide portions of the second input-side high-refractive-index waveguide 61B and the second output-side high-refractive-index waveguide 62B are short, while the SiN waveguide portion of the second low-refractive-index waveguide 63B is long, thereby shortening the overall length of the waveguide and the optical path length. As a result, the electrode length of the folded signal electrode 51C, which must be matched to the optical path length of the second folded waveguide 33, can be shortened, thereby shortening the length of the delay waveguide and, as a result, reducing the size of the folded portion. In other words, the chip size of the optical modulator 1 can be reduced while maintaining velocity matching.

[0068] The lengths of the Si waveguide and the SiN waveguide are adjusted appropriately in the left and right arms so that the optical path lengths of the first folded waveguide 23 and the second folded waveguide 33 are the same. For example, there is a method of aligning the lengths of the Si waveguides and the SiN waveguides, but there is no limitation to this as long as the optical path lengths are the same.

[0069] In the optical modulator 1 of the first embodiment, an LN waveguide is used as an example of a high EO material waveguide, but the present invention is not limited to this. Similar effects can be obtained with a waveguide made of a high EO material having a Pockels coefficient of 10 pm / V or more, such as BaTiO3, PLZT, or PZT.

[0070] For convenience of explanation, a Si waveguide is used as the high refractive index waveguide and a SiN waveguide is used as the low refractive index waveguide, but the present invention is not limited to these and can be changed as appropriate.

[0071] In the first folded waveguide 23 and the second folded waveguide 33 of Example 1, the first output-side high-refractive-index waveguide 62A and the first low-refractive-index waveguide 63A intersect three-dimensionally. However, the present invention is not limited to this, and an embodiment thereof will be described below as Example 2. Note that the same components as those of the optical modulator 1 of Example 1 are denoted by the same reference numerals, and descriptions of the overlapping components and operations will be omitted. [Example]

[0072] 12 is a perspective view showing an example of the first folded waveguide 23A and the second folded waveguide 33A at the intersection of the optical modulator 1A of the embodiment 2. The optical modulator 1A of the embodiment 2 differs from the optical modulator 1 of the embodiment 1 in that the first folded waveguide 23A and the second folded waveguide 33A are configured using three different layers of waveguides to transition the signal light, and the distance between the high refractive index waveguide and the low refractive index waveguide is increased at the intersection of the waveguides.

[0073] The first folded waveguide 23A includes a first input high-refractive-index waveguide 61A, a first output high-refractive-index waveguide 62A, a first input first-stage low-refractive-index waveguide 63A1, a first output first-stage low-refractive-index waveguide 63A2, and a first second-stage low-refractive-index waveguide 63A3. The first folded waveguide 23A includes a first input first-stage transition portion 64A11, a first input second-stage transition portion 64A31, a first output second-stage transition portion 64A41, and a first output first-stage transition portion 64A21. The first input high-refractive-index waveguide 61A is, for example, a Si waveguide, and is connected to the output end of the first input arm waveguide 22, with the second layer 70B of the lower cladding layer 72 as a core layer. The first output-side high-refractive-index waveguide 62A is, for example, a Si waveguide that has the second layer 70B as a core layer and is connected to the input end of the first output-side arm waveguide 24.

[0074] 13 is a cross-sectional view showing an example of the folded section 5A of the optical modulator 1A according to the second embodiment. The first input-side first-stage low-refractive-index waveguide 63A1 is, for example, a SiN waveguide, which has the third layer 70C of the lower cladding layer 72 as its core layer and is indirectly connected to the output end of the first input-side high-refractive-index waveguide 61A. The first output-side first-stage low-refractive-index waveguide 63A2 is, for example, a SiN waveguide, which has the third layer 70C as its core layer and is indirectly connected to the input end of the first output-side high-refractive-index waveguide 62A.

[0075] The first second-stage low-refractive-index waveguide 63A3 is, for example, a SiN waveguide that indirectly connects the first input-side first-stage low-refractive-index waveguide 63A1 and the first output-side first-stage low-refractive-index waveguide 63A2, and has the fourth layer 70D of the lower cladding layer 72 as its core layer. The fourth layer 70D is a second-stage low-refractive-index layer.

[0076] First input-side first-stage transition section 64A11 is composed of an output end of first input-side high-refractive-index waveguide 61A and an input end of first input-side first-stage low-refractive-index waveguide 63A1, and transitions signal light between first input-side high-refractive-index waveguide 61A and first input-side first-stage low-refractive-index waveguide 63A1. First output-side first-stage transition section 64A21 is composed of an output end of first output-side first-stage low-refractive-index waveguide 63A2 and an input end of first output-side high-refractive-index waveguide 62A, and transitions signal light between first output-side first-stage low-refractive-index waveguide 63A2 and first output-side high-refractive-index waveguide 62A.

[0077] The first input-side second-stage transition section 64A31 is composed of an output end of the first input-side first-stage low-refractive-index waveguide 63A1 and the first second-stage low-refractive-index waveguide 63A3, and transitions the signal light between the first input-side first-stage low-refractive-index waveguide 63A1 and the first second-stage low-refractive-index waveguide 63A3. The first output-side second-stage transition section 64A41 is composed of an output end of the first second-stage low-refractive-index waveguide 63A3 and the input end of the first output-side first-stage low-refractive-index waveguide 63A2, and transitions the signal light between the first second-stage low-refractive-index waveguide 63A3 and the first output-side first-stage low-refractive-index waveguide 63A2.

[0078] The second folded waveguide 33A includes a second input high-refractive-index waveguide 61B, a second output high-refractive-index waveguide 62B, a second input first-stage low-refractive-index waveguide 63B1, a second output first-stage low-refractive-index waveguide 63B2, and a second second-stage low-refractive-index waveguide 63B3. The second folded waveguide 33A includes a second input first-stage transition portion 64B11, a second input second-stage transition portion 64B31, a second output second-stage transition portion 64B41, and a second output first-stage transition portion 64B21. The second input high-refractive-index waveguide 61B is, for example, a Si waveguide, and is connected to the output end of the second input arm waveguide 32 using the second layer 70B of the lower cladding layer 72 as a core layer. The second output-side high-refractive-index waveguide 62B has the second layer 70B as a core layer and is connected to the input end of the second output-side arm waveguide 34, and is, for example, a Si waveguide.

[0079] Second input-side first-stage low-refractive-index waveguide 63B1 is, for example, a SiN waveguide, which has third layer 70C of lower cladding layer 72 as its core layer and is indirectly connected to the output end of second input-side high-refractive-index waveguide 61B. Second output-side first-stage low-refractive-index waveguide 63B2 is, for example, a SiN waveguide, which has third layer 70C as its core layer and is indirectly connected to the input end of second output-side high-refractive-index waveguide 62B.

[0080] The second second-stage low-refractive-index waveguide 63B3 is, for example, a SiN waveguide, which has the fourth layer 70D of the lower cladding layer 72 as a core layer and indirectly connects the second input-side first-stage low-refractive-index waveguide 63B1 and the second output-side first-stage low-refractive-index waveguide 63B2.

[0081] The second input-side first-stage transition section 64B11 is composed of the output end of the second input-side high-refractive-index waveguide 61B and the input end of the second input-side first-stage low-refractive-index waveguide 63B1, and transitions the signal light between the second input-side high-refractive-index waveguide 61B and the second input-side first-stage low-refractive-index waveguide 63B1. The second output-side first-stage transition section 64B21 is composed of the output end of the second output-side first-stage low-refractive-index waveguide 63B2 and the input end of the second output-side high-refractive-index waveguide 62B, and transitions the signal light between the second output-side first-stage low-refractive-index waveguide 63B2 and the second output-side high-refractive-index waveguide 62B.

[0082] The second input-side second-stage transition section 64B31 is composed of an input end of the second input-side first-stage low-refractive-index waveguide 63B1 and a second second-stage low-refractive-index waveguide 63B3, and transitions the signal light between the second input-side first-stage low-refractive-index waveguide 63B1 and the second second-stage low-refractive-index waveguide 63B3. The second output-side second-stage transition section 64B41 is composed of an output end of the second second-stage low-refractive-index waveguide 63B3 and an input end of the second output-side first-stage low-refractive-index waveguide 63B2, and transitions the signal light between the second second-stage low-refractive-index waveguide 63B3 and the second output-side first-stage low-refractive-index waveguide 63B2.

[0083] 13, the first output arm waveguide 24 is connected to the first output high-refractive-index waveguide 62A that straddles the second second-stage low-refractive-index waveguide 63B3. As a result, the distance between the second second-stage low-refractive-index waveguide 63B3 and the first output high-refractive-index waveguide 62A is greater than in the optical modulator 1 of the first embodiment, thereby making it possible to suppress loss and crosstalk at the intersection.

[0084] FIG. 14 is a plan view showing an example of the first input-side first-stage transition section 64A11 and the first input-side second-stage transition section 64A31 in the first folded waveguide 23A. For ease of explanation, the first input-side first-stage transition section 64A11 and the first input-side second-stage transition section 64A31 in the first folded waveguide 23A are shown. However, since the second input-side first-stage transition section 64B11 and the second input-side second-stage transition section 64B31 in the second folded waveguide 33A have similar structures, the same reference numerals are used to designate the same components and operations, and the description of the same components and operations will be omitted. The first folded waveguide 23A shown in FIG. 14 includes a first input-side high-refractive-index waveguide 61A, a first input-side first-stage low-refractive-index waveguide 63A1, and a first second-stage low-refractive-index waveguide 63A3. The output end of the first input-side high-refractive-index waveguide 61A has a tapered structure in which the waveguide width gradually narrows from the input end of the first input-side first-stage low-refractive-index waveguide 63A1 toward the middle. The input end of the first input-side first-stage low-refractive-index waveguide 63A1 has a tapered structure in which the waveguide width gradually widens from the input end and remains constant in the middle. The output end of the first input-side first-stage low-refractive-index waveguide 63A1 has a tapered structure in which the waveguide width gradually widens from the output end and remains constant in the middle. The input end of the first second-stage low-refractive-index waveguide 63A3 has a tapered structure in which the waveguide width gradually widens from the input end and remains constant in the middle. As a result, the tapered waveguide width facilitates optical coupling between the first input-side first-stage low-refractive-index waveguide 63A1 and the first second-stage low-refractive-index waveguide 63A3, thereby achieving low-loss optical transition.

[0085] 15A to 15D are cross-sectional schematic diagrams showing the cross-sectional structure at various locations of the first input-side second-stage transition section 64A31. FIG. 15A is a cross-sectional schematic diagram showing an example of the cross-sectional portion taken along line AA shown in FIG. 14. The portion of the first folded waveguide 23A shown in FIG. 15A has a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. A third layer 70C inside the lower cladding layer 72 is made of, for example, SiN, and serves as the core layer of the first input-side first-stage low-refractive-index waveguide 63A1. That is, the first input-side first-stage low-refractive-index waveguide 63A1 is a SiN waveguide with a channel structure.

[0086] FIG. 15B is a cross-sectional schematic diagram showing an example of the cross section taken along line BB in FIG. 14. The first input-side second-stage transition section 64A31 in the first folded waveguide 23A shown in FIG. 15B includes a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. The third layer 70C of the lower cladding layer 72 is made of, for example, SiN and serves as the core layer of the first input-side first-stage low-refractive-index waveguide 63A1. The fourth layer 70D of the lower cladding layer 72 is made of, for example, SiN and serves as the core layer of the first second-stage low-refractive-index waveguide 63A3. That is, the first second-stage low-refractive-index waveguide 63A3 is a SiN waveguide with a channel structure. In this portion, the first input-side first-stage low-refractive-index waveguide 63A1 is wide, and the first second-stage low-refractive-index waveguide 63A3 is narrow.

[0087] 15C is a schematic cross-sectional view showing an example of the CC cross section shown in FIG. 14. First input-side second-stage transition portion 64A31 in first folded waveguide 23A shown in FIG. 15C includes a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. A third layer 70C of lower cladding layer 72 is made of, for example, SiN and serves as a core layer of first input-side first-stage low-refractive-index waveguide 63A1. A fourth layer 70D of lower cladding layer 72 is made of, for example, SiN and serves as a core layer of first second-stage low-refractive-index waveguide 63A3.

[0088] Fig. 15D is a cross-sectional schematic diagram showing an example of the cross section taken along line DD shown in Fig. 14. The portion of first folded waveguide 23A shown in Fig. 15D has a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. A fourth layer 70D of lower cladding layer 72 is made of SiN and becomes the core layer of first second-stage low refractive index waveguide 63A3.

[0089] The input end of the first output-side high-refractive-index waveguide 62A has a tapered structure in which the waveguide width gradually narrows from the output end of the first output-side first-stage low-refractive-index waveguide 63A2 toward the middle. The input end of the first output-side first-stage low-refractive-index waveguide 63A2 has a tapered structure in which the waveguide width gradually widens from the input end and remains constant in the middle. The output end of the first output-side first-stage low-refractive-index waveguide 63A2 has a tapered structure in which the waveguide width gradually widens from the output end and remains constant as it approaches the middle. The input end of the first second-stage low-refractive-index waveguide 63A3 has a tapered structure in which the waveguide width gradually widens from the input end and remains constant in the middle.

[0090] The first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A of the first folded waveguide 23A of the second embodiment are Si waveguides that allow for a small curvature. Furthermore, the first input-side first-stage low-refractive-index waveguide 63A1, the first second-stage low-refractive-index waveguide 63A3, and the first output-side first-stage low-refractive-index waveguide 63A2 after the bend are SiN waveguides that are advantageous for velocity matching. The Si waveguide portions of the first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A are short, while the SiN waveguide portions of the first input-side first-stage low-refractive-index waveguide 63A1, the first second-stage low-refractive-index waveguide 63A3, and the first output-side first-stage low-refractive-index waveguide 63A2 are long. Therefore, the optical path length can be shortened while the overall length of the waveguide is kept short. As a result, the electrode length of the folded signal electrode 51C, which needs to be matched to the optical path length of the first folded waveguide 23A, can be shortened, thereby shortening the length of the delay waveguide and, as a result, reducing the size of the folded portion.

[0091] The second input-side high-refractive-index waveguide 61B and the second output-side high-refractive-index waveguide 62B of the second folded waveguide 33A are Si waveguides that allow for small curvature. Furthermore, the second input-side first-stage low-refractive-index waveguide 63B1, the second second-stage low-refractive-index waveguide 63B3, and the second output-side first-stage low-refractive-index waveguide 63B2 after the bend are SiN waveguides that are advantageous for velocity matching. The Si waveguide portions of the second input-side high-refractive-index waveguide 61B and the second output-side high-refractive-index waveguide 62B are short, while the SiN waveguide portions of the second input-side first-stage low-refractive-index waveguide 63B1, the second second-stage low-refractive-index waveguide 63B3, and the second output-side first-stage low-refractive-index waveguide 63B2 are long. Therefore, the optical path length can be shortened while the overall length of the waveguide is kept short. As a result, the electrode length of the folded signal electrode 51C, which needs to be matched to the optical path length of the second folded waveguide 33A, can be shortened, thereby shortening the length of the delay waveguide and, as a result, reducing the size of the folded portion. In other words, the chip size of the optical modulator 1A can be reduced while ensuring velocity matching.

[0092] In the optical modulator 1 of Example 1, the first input arm waveguide 22 is arranged on the outer circumferential side, the first output arm waveguide 24 on the inner circumferential side, the second input arm waveguide 32 on the inner circumferential side, and the second output arm waveguide 34 on the outer circumferential side. Therefore, in the optical modulator 1, the first folded waveguide 23 and the second folded waveguide 33 intersect. However, this is not limited to this, and it is also possible to arrange the first input arm waveguide 22 on the outer circumferential side, the first output arm waveguide 24 on the outer circumferential side, the second input arm waveguide 32 on the inner circumferential side, and the second output arm waveguide 34 on the inner circumferential side. Therefore, this embodiment will be described below as Example 3. [Example]

[0093] 16 is a plan view schematic diagram showing an example of an optical modulator 1B according to a third embodiment. The same components as those in the optical modulator 1 of the first embodiment are denoted by the same reference numerals, and explanations of the overlapping components and operations will be omitted. The optical modulator 1 of the first embodiment differs from the optical modulator 1B of the third embodiment in that there is no intersection between the first folded waveguide 23B and the second folded waveguide 33B, and the polarization direction X is reversed between the input arm waveguides 22 and 32 and the output arm waveguides 24 and 34.

[0094] The first waveguide 20 has a first input arm waveguide 22 on the outer circumferential side of the folded portion, a first folded waveguide 23B on the outer circumferential side, and a first output arm waveguide 24A on the outer circumferential side. The second waveguide 30 has a second input arm waveguide 32 on the inner circumferential side, a second folded waveguide 33B on the inner circumferential side, and a second output arm waveguide 34A on the inner circumferential side.

[0095] First folded waveguide 23B includes first input high-refractive-index waveguide 61A, first output high-refractive-index waveguide 62A, first low-refractive-index waveguide 63A, first input first-stage transition section 64A1, and first output first-stage transition section 64A2. First input high-refractive-index waveguide 61A is formed on a first layer 70A on Si substrate 71 and is connected to first input arm waveguide 22 on the outer periphery, e.g., a Si waveguide. First output high-refractive-index waveguide 62A is formed on first layer 70A and is connected to first output arm waveguide 24A on the outer periphery, e.g., a Si waveguide. The first low-refractive-index waveguide 63A is formed on the second layer 70B on the Si substrate 71 and is, for example, a SiN waveguide, connecting between the first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A.

[0096] First input-side first-stage transition section 64A1 is composed of an output end of first input-side high-refractive-index waveguide 61A and an input end of first low-refractive-index waveguide 63A, and transitions signal light between first input-side high-refractive-index waveguide 61A and first low-refractive-index waveguide 63A. First output-side first-stage transition section 64A2 is composed of an output end of first low-refractive-index waveguide 63A and an input end of first output-side high-refractive-index waveguide 62A, and transitions signal light between first low-refractive-index waveguide 63A and first output-side high-refractive-index waveguide 62A.

[0097] The second folded waveguide 33B includes a second input high-refractive-index waveguide 61B, a second output high-refractive-index waveguide 62B, a second low-refractive-index waveguide 63B, a second input first-stage transition portion 64B1, and a second output first-stage transition portion 64B2. The second input high-refractive-index waveguide 61B is formed on a first layer 70A on a Si substrate 71 and is connected to the second input arm waveguide 32 located on the inner periphery. The second output high-refractive-index waveguide 62B is formed on the first layer 70A and is connected to the second output arm waveguide 34A located on the inner periphery. The second low-refractive-index waveguide 63B is formed on the second layer 70B on the Si substrate 71 and connects between the second input-side high-refractive-index waveguide 61B and the second output-side high-refractive-index waveguide 62B, and is, for example, a SiN waveguide.

[0098] The second input-side first-stage transition section 64B1 is composed of the output end of the second input-side high-refractive-index waveguide 61B and the input end of the second low-refractive-index waveguide 63B, and transitions the signal light between the second input-side high-refractive-index waveguide 61B and the second low-refractive-index waveguide 63B. The second output-side first-stage transition section 64B2 is composed of the output end of the second low-refractive-index waveguide 63B and the input end of the second output-side high-refractive-index waveguide 62B, and transitions the signal light between the second low-refractive-index waveguide 63B and the second output-side high-refractive-index waveguide 62B.

[0099] The electrode 50 has a GSG structure and includes a signal electrode 51, a first ground electrode 52, and a second ground electrode 53. The signal electrode 51 includes an input signal electrode 51A, an output signal electrode 51B, and a folded signal electrode 51C. The input signal electrode 51A is disposed between the first input arm waveguide 22 and the second input arm waveguide 32 and is electrically connected to the folded signal electrode 51C. The output signal electrode 51B is disposed between the first output arm waveguide 24A and the second output arm waveguide 34A and is electrically connected to the folded signal electrode 51C. The folded signal electrode 51C electrically connects the input signal electrode 51A and the output signal electrode 51B.

[0100] The first ground electrode 52 includes a first input ground electrode 52A on the outer periphery, a first output ground electrode 52B on the outer periphery, and a first folded ground electrode 52C on the outer periphery. The first input ground electrode 52A is disposed near the side surface of the first input arm waveguide 22 on the outer periphery to face the input signal electrode 51A and is electrically connected to the first folded ground electrode 52C. The first output ground electrode 52B is disposed near the side surface of the first output arm waveguide 24A on the outer periphery to face the output signal electrode 51B and is electrically connected to the first folded ground electrode 52C. The first folded ground electrode 52C on the outer periphery electrically connects the first input ground electrode 52A and the first output ground electrode 52B.

[0101] The second ground electrode 53 includes a second input ground electrode 53A located on the inner periphery, a second output ground electrode 53B located on the inner periphery, and a second folded ground electrode 53C located on the inner periphery. The second input ground electrode 53A is disposed near the side surface of the second input arm waveguide 32 located on the inner periphery so as to face the input signal electrode 51A, and is electrically connected to the second folded ground electrode 53C. The second output ground electrode 53B is disposed near the side surface of the second output arm waveguide 34A located on the inner periphery so as to face the output signal electrode 51B, and is electrically connected to the second folded ground electrode 53C. The second folded ground electrode 53C located on the inner periphery electrically connects the second input ground electrode 53A and the second output ground electrode 53B.

[0102] FIG. 17 is a cross-sectional schematic diagram showing an example of the cross section taken along line AA in FIG. 16. The modulator body 4B shown in FIG. 17 includes a Si substrate 71, a lower cladding layer 72, an upper cladding layer 73, and an electrode 50. The modulator body 4B includes a first input arm waveguide 22 on the outer periphery and a second input arm waveguide 32 on the inner periphery, which are arranged on a first layer 70A of the upper cladding layer 73. The modulator body 4B includes a first output arm waveguide 24A on the outer periphery and a second output arm waveguide 34A on the inner periphery, which are arranged on the first layer 70A. The electrode 50 arranged on the upper cladding layer 73 includes an input signal electrode 51A, a first input ground electrode 52A on the outer periphery, and a second input ground electrode 53A on the inner periphery. Furthermore, the electrode 50 has an output signal electrode 51B, a first output ground electrode 52B on the outer periphery, and a second output ground electrode 53B on the inner periphery.

[0103] The input signal electrode 51A is disposed between the first input arm waveguide 22 and the second input arm waveguide 32. The first input ground electrode 52A is disposed near the side surface of the first input arm waveguide 22 opposite to the input signal electrode 51A. The second input ground electrode 53A is disposed near the side surface of the second input arm waveguide 32 opposite to the input signal electrode 51A.

[0104] The output signal electrode 51B is disposed between the first output arm waveguide 24A and the second output arm waveguide 34A. The first output ground electrode 52B is disposed near the side surface of the first output arm waveguide 24A opposite to the output signal electrode 51B. The second output ground electrode 53B is disposed near the side surface of the second output arm waveguide 34A opposite to the output signal electrode 51B.

[0105] The polarization direction X of the modulator body 4 is opposite between the forward path and the return path. The first input arm waveguide 22 modulates the signal light in response to a backward electrical signal from the input signal electrode 51A to the first input ground electrode 52A. The second input arm waveguide 32 modulates the signal light in response to a forward electrical signal from the input signal electrode 51A to the second input ground electrode 53A.

[0106] On the other hand, the first output arm waveguide 24A modulates the signal light in response to a reverse electrical signal from the output signal electrode 51B to the first output ground electrode 52B, and the second output arm waveguide 34A modulates the signal light in response to a forward electrical signal from the output signal electrode 51B to the second output ground electrode 53B.

[0107] That is, the first input arm waveguide 22 and the first output arm waveguide 24A modulate the signal light in response to the same reverse-direction electrical signal. The second input arm waveguide 32 and the second output arm waveguide 34A modulate the signal light in response to the same forward-direction electrical signal. Since the same direction electrical signal is applied to the outgoing path and the return path, modulation efficiency can be improved.

[0108] The optical modulator 1B of the third embodiment has a structure in which the polarization direction X1 of the thin-film LN of the first input arm waveguide 22 and the second input arm waveguide 32 is opposite to the polarization direction X2 of the thin-film LN of the first output arm waveguide 24A and the second output arm waveguide 34A. As a result, the modulation electric field and polarization direction of the first input arm waveguide 22 and the first output arm waveguide 24A are both opposite, making it possible to modulate the phase in the same direction before and after folding. Similarly, the modulation electric field and polarization direction of the second input arm waveguide 32 and the second output arm waveguide 34A are both forward, making it possible to modulate the phase in the same direction before and after folding. Moreover, it is possible to achieve push-pull operation in which the phase changes in the opposite direction to the first input arm waveguide 22 and the first output arm waveguide 24A.

[0109] Although the optical modulator 1 of Example 1 has been exemplified as having one turn-around point, the number of turn-around points may be an even number, for example, two, and such an embodiment will be described below as Example 4. [Example]

[0110] 18 is a schematic plan view showing an example of an optical modulator 1C of Example 4. Note that the same components as those of the optical modulator 1 of Example 1 are given the same reference numerals, and explanations of the overlapping components and operations will be omitted. The optical modulator 1C of Example 4 differs from the optical modulator 1 in that the number of return points is an even number, for example, two, and the signal light is input from one end face D1 of the chip of the optical modulator 1C and output from the other end face D2 opposite to the one end face D1 of the chip of the optical modulator 1C.

[0111] The optical modulator 1C includes a Si photonics substrate 2, an input MMI 3C, a modulator body 4C, a first folding section 5C1, a second folding section 5C2, and an output MMI 3D. The input MMI 3C includes an input waveguide 6 that inputs light to the optical modulator 1C. The output MMI 3D includes an output waveguide 7 that outputs the signal light from the optical modulator 1C. The optical modulator 1C includes a first coupler 10, a first waveguide 20A, a second waveguide 30A, a second coupler 40, and an electrode 50. The first coupler 10 is disposed on the Si substrate 71 and is a coupler that splits and outputs the signal light from the input waveguide 6 to a first input waveguide 21A and a second input waveguide 31A.

[0112] The first waveguide 20A is disposed on the Si substrate 71 and is connected to one output of the first coupler 10. The second waveguide 30A is disposed on the Si substrate 71 and is connected to the other output of the first coupler 10. The second coupler 40 is disposed on the Si substrate 71 and is a coupler that multiplexes and outputs the signal light from the first output waveguide 25 of the first waveguide 20A and the signal light from the second output waveguide 35 of the second waveguide 30A to the output waveguide 7. The electrode 50 is a GSG electrode that applies an electric signal to the first waveguide 20A and the second waveguide 30A.

[0113] The first waveguide 20A has a first input waveguide 21A, a first input arm waveguide 22A, a first input folded waveguide 81, a first intermediate arm waveguide 82, and a first output folded waveguide 83. The first waveguide 20A has a first output arm waveguide 24B, a first output waveguide 25B, a first input modulation section transition section 85, a first intermediate modulation section transition section 86, and a first output modulation section transition section 87.

[0114] The first input waveguide 21A is a Si waveguide that connects the first coupler 10 and the first input arm waveguide 22A. The first input arm waveguide 22A is a linear arm waveguide made of thin-film LN, a high EO material, that connects the first input waveguide 21A and the first input folded waveguide 81. The first input folded waveguide 81 is a waveguide with a folded structure that connects the first input arm waveguide 22A and the first intermediate arm waveguide 82. The first input folded waveguide 81 has the same structure as the first folded waveguide 23 shown in FIG. 2, for example.

[0115] The first intermediate arm waveguide 82 is a linear arm waveguide made of thin-film LN, which is a high EO material, and connects between the first input-side folded waveguide 81 and the first output-side folded waveguide 83. The first output-side folded waveguide 83 is a waveguide with a folded structure that connects between the first intermediate arm waveguide 82 and the first output-side arm waveguide 24B. The first output-side folded waveguide 83 has, for example, substantially the same structure as the first folded waveguide 23 shown in FIG. 2.

[0116] The first output arm waveguide 24B is a linear arm waveguide made of thin-film LN, which is a high EO material, that connects between the first output folded waveguide 83 and the first output waveguide 25B. The first output waveguide 25B is a Si waveguide that connects between the first output arm waveguide 24B and the second coupler 40.

[0117] The first input-side modulation section transition section 85 is composed of the output end of the first input waveguide 21A and the input end of the first input arm waveguide 22A, and transitions the signal light between the first input waveguide 21A and the first input arm waveguide 22A. The first input-side modulation section transition section 85 is also composed of the output end of the first input arm waveguide 22A and the input end of the first input folded waveguide 81, and transitions the signal light between the first input arm waveguide 22A and the first input folded waveguide 81.

[0118] The first intermediate side modulation section transition section 86 is composed of an output end of the first input side folded waveguide 81 and an input end of the first intermediate arm waveguide 82, and transitions the signal light between the first input side folded waveguide 81 and the first intermediate arm waveguide 82. The first intermediate side modulation section transition section 86 is composed of an output end of the first intermediate arm waveguide 82 and an input end of the first output side folded waveguide 83, and transitions the signal light between the first intermediate arm waveguide 82 and the first output side folded waveguide 83.

[0119] The first output-side modulation section transition section 87 is composed of the output end of the first output-side folded waveguide 83 and the input end of the first output-side arm waveguide 24B, and transitions the signal light between the first output-side folded waveguide 83 and the first output-side arm waveguide 24B. The first output-side modulation section transition section 87 is composed of the output end of the first output-side arm waveguide 24B and the input end of the first output waveguide 25B, and transitions the signal light between the first output-side arm waveguide 24B and the first output waveguide 25B.

[0120] The second waveguide 30A has a second input waveguide 31A, a second input arm waveguide 32A, a second input folded waveguide 91, a second intermediate arm waveguide 92, and a second output folded waveguide 93. The second waveguide 30A further has a second output arm waveguide 34B, a second output waveguide 35B, a second input modulation section transition section 94, a second intermediate modulation section transition section 95, and a second output modulation section transition section 96.

[0121] The second input waveguide 31A is a Si waveguide that connects the first coupler 10 and the second input arm waveguide 32A. The second input arm waveguide 32A is a linear arm waveguide made of thin-film LN, a high EO material, that connects the second input waveguide 31A and the second input folded waveguide 91. The second input folded waveguide 91 is a waveguide with a folded structure that connects the second input arm waveguide 32A and the second intermediate arm waveguide 92. The second input folded waveguide 91 has the same structure as the second folded waveguide 33 shown in FIG. 2, for example.

[0122] The second intermediate arm waveguide 92 is a linear arm waveguide made of thin-film LN, a high EO material, that connects between the second input folded waveguide 91 and the second output folded waveguide 93. The second output folded waveguide 93 is a waveguide with a folded structure that connects between the second intermediate arm waveguide 92 and the second output arm waveguide 34B. The second output arm waveguide 34B is a linear waveguide made of thin-film LN, a high EO material, that connects between the second output folded waveguide 93 and the second output waveguide 35B.

[0123] The second input-side modulation section transition section 94 is composed of the output end of the second input waveguide 31A and the input end of the second input arm waveguide 32A, and transitions the signal light between the second input waveguide 31A and the second input arm waveguide 32A. The second input-side modulation section transition section 94 is composed of the output end of the second input arm waveguide 32A and the input end of the second input-side folded waveguide 91, and transitions the signal light between the second input arm waveguide 32A and the second input-side folded waveguide 91.

[0124] The second intermediate side modulation section transition section 95 is composed of an output end of the second input side folded waveguide 91 and an input end of the second intermediate arm waveguide 92, and transitions the signal light between the second input side folded waveguide 91 and the second intermediate arm waveguide 92. The second intermediate side modulation section transition section 95 is composed of an output end of the second intermediate arm waveguide 92 and an input end of the second output side folded waveguide 93, and transitions the signal light between the second intermediate arm waveguide 92 and the second output side folded waveguide 93.

[0125] The second output-side modulation section transition section 96 is composed of the output end of the second output-side folded waveguide 93 and the input end of the second output-side arm waveguide 34B, and transitions the signal light between the second output-side folded waveguide 93 and the second output-side arm waveguide 34B. The second output-side modulation section transition section 96 is composed of the output end of the second output-side arm waveguide 34B and the input end of the second output waveguide 35B, and transitions the signal light between the second output-side arm waveguide 34B and the second output waveguide 35B.

[0126] The first input arm waveguide 22A, the second input arm waveguide 32A, the first output arm waveguide 24B, and the second output arm waveguide 34B are waveguides containing a material having higher EO characteristics than the high refractive index waveguides formed on the Si substrate 71. The first intermediate arm waveguide 82 and the second intermediate arm waveguide 92 are waveguides containing a material having higher EO characteristics than the high refractive index waveguides formed on the Si substrate 71. Furthermore, the first input folded waveguide 81, the second input folded waveguide 91, the first output folded waveguide 83, and the second output folded waveguide 93 are waveguides containing a material having a lower refractive index than the high refractive index waveguides formed on the Si substrate 71.

[0127] The first waveguide 20A has a first input arm waveguide 22A on the outer circumferential side, a first input folded waveguide 81 on the outer circumferential side, and a first intermediate arm waveguide 82 on the left side in Fig. 18. Furthermore, the first waveguide 20A has a first output folded waveguide 83 on the inner circumferential side, and a first output arm waveguide 24B on the inner circumferential side.

[0128] The second waveguide 30A has a second input arm waveguide 32A on the inner circumferential side, a second input folded waveguide 91 on the inner circumferential side, and a second intermediate arm waveguide 92 on the right side in Fig. 18. Furthermore, the second waveguide 30A has a second output folded waveguide 93 on the inner circumferential side, and a second output arm waveguide 34B on the outer circumferential side.

[0129] The signal electrode 51 includes an input signal electrode 51A, an input folded signal electrode 51C1, an intermediate signal electrode 51D, an output folded signal electrode 51C2, and an output signal electrode 51B. The input signal electrode 51A is disposed between the first input arm waveguide 22A and the second input arm waveguide 32A and is electrically connected to the input folded signal electrode 51C1. The input folded signal electrode 51C1 is electrically connected to the intermediate signal electrode 51D. The intermediate signal electrode 51D is disposed between the first intermediate arm waveguide 82 and the second intermediate arm waveguide 92 and is electrically connected to the output folded signal electrode 51C2. The output folded signal electrode 51C2 is electrically connected to the output signal electrode 51B. The output signal electrode 51B is disposed between the first output arm waveguide 24B and the second output arm waveguide 34B, and is electrically connected to the output folded signal electrode 51C2.

[0130] The first ground electrode 52 includes a first input ground electrode 52A on the outer periphery, a first input folded ground electrode 52C1 on the outer periphery, and a first intermediate ground electrode 52D. The first ground electrode 52 also includes a first output folded ground electrode 52C2 on the inner periphery, and a first output ground electrode 52B on the inner periphery. The first input ground electrode 52A is disposed near the side surface of the first input arm waveguide 22A on the outer periphery, facing the input signal electrode 51A, and is electrically connected to the first input folded ground electrode 52C1 on the outer periphery. The first input folded ground electrode 52C1 electrically connects the first input ground electrode 52A and the first intermediate ground electrode 52D. The first intermediate ground electrode 52D is disposed near the side surface of the first intermediate arm waveguide 82 on the left side in FIG. 18 so as to face the intermediate signal electrode 51D, and electrically connects the first input folded ground electrode 52C1 and the first output folded ground electrode 52C2. The first output folded ground electrode 52C2 electrically connects the first intermediate ground electrode 52D and the first output ground electrode 52B. The first output ground electrode 52B is disposed near the side surface of the first output arm waveguide 24B on the inner periphery so as to face the output signal electrode 51B, and electrically connects to the first output folded ground electrode 52C2.

[0131] The second ground electrode 53 includes a second input ground electrode 53A on the inner periphery, a second input folded ground electrode 53C1 on the inner periphery, and a second intermediate ground electrode 53D on the inner periphery. The second ground electrode 53 further includes a second output folded ground electrode 53C2 on the outer periphery, and a second output ground electrode 53B on the outer periphery. The second input ground electrode 53A is disposed near the side of the second input arm waveguide 32A on the inner periphery, facing the input signal electrode 51A, and is electrically connected to the second input folded ground electrode 53C1 on the inner periphery. The second input folded ground electrode 53C1 electrically connects the second input ground electrode 53A and the second intermediate ground electrode 53D. The second intermediate ground electrode 53D is disposed near the side surface of the second intermediate arm waveguide 92 on the right side in FIG. 18 so as to face the intermediate signal electrode 51D, and electrically connects the second input folded ground electrode 53C1 and the second output folded ground electrode 53C2. The second output folded ground electrode 53C2 electrically connects the second intermediate ground electrode 53D and the second output ground electrode 53B. The second output ground electrode 53B is disposed near the side surface of the second output arm waveguide 34B on the outer periphery so as to face the output signal electrode 51B, and electrically connects to the second output folded ground electrode 53C2.

[0132] The first input folded waveguide 81 includes a first input high-refractive-index waveguide 61A, a first output high-refractive-index waveguide 62A, a first low-refractive-index waveguide 63A, a first input first-stage transition portion 64A1, and a first output first-stage transition portion 64A2. The first input high-refractive-index waveguide 61A is formed on a first layer 70A on a Si substrate 71 and is connected to the first input arm waveguide 22A, and is, for example, a Si waveguide. The first output high-refractive-index waveguide 62A is formed on the first layer 70A and is connected to the first middle arm waveguide 82, and is, for example, a Si waveguide. The first low-refractive-index waveguide 63A is formed on the second layer 70B on the Si substrate 71 and is, for example, a SiN waveguide, connecting between the first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A.

[0133] First input-side first-stage transition section 64A1 is composed of an output end of first input-side high-refractive-index waveguide 61A and an input end of first low-refractive-index waveguide 63A, and transitions signal light between first input-side high-refractive-index waveguide 61A and first low-refractive-index waveguide 63A. First output-side first-stage transition section 64A2 is composed of an output end of first low-refractive-index waveguide 63A and an input end of first output-side high-refractive-index waveguide 62A, and transitions signal light between first low-refractive-index waveguide 63A and first output-side high-refractive-index waveguide 62A.

[0134] The second input-side folded waveguide 91 includes a second input-side high-refractive-index waveguide 61B, a second output-side high-refractive-index waveguide 62B, a second low-refractive-index waveguide 63B, a second input-side first-stage transition portion 64B1, and a second output-side first-stage transition portion 64B2. The second input-side high-refractive-index waveguide 61B is formed on a first layer 70A on the Si substrate 71 and is connected to the second input-side arm waveguide 32A, and is, for example, a Si waveguide. The second output-side high-refractive-index waveguide 62B is formed on the first layer 70A and is connected to the second middle arm waveguide 92, and is, for example, a Si waveguide. The second low-refractive-index waveguide 63B is formed on the second layer 70B on the Si substrate 71 and connects between the second input-side high-refractive-index waveguide 61B and the second output-side high-refractive-index waveguide 62B, and is, for example, a SiN waveguide.

[0135] The second input-side first-stage transition section 64B1 is composed of the output end of the second input-side high-refractive-index waveguide 61B and the input end of the second low-refractive-index waveguide 63B, and transitions the signal light between the second input-side high-refractive-index waveguide 61B and the second low-refractive-index waveguide 63B. The second output-side first-stage transition section 64B2 is composed of the output end of the second low-refractive-index waveguide 63B and the input end of the second output-side high-refractive-index waveguide 62B, and transitions the signal light between the second low-refractive-index waveguide 63B and the second output-side high-refractive-index waveguide 62B. The first output-side high-refractive-index waveguide 62A connects the first low-refractive-index waveguide 63A and the first intermediate arm waveguide 82 across the second low-refractive-index waveguide 63B.

[0136] The first output folded waveguide 83 includes a first input high-refractive-index waveguide 61A, a first output high-refractive-index waveguide 62A, a first low-refractive-index waveguide 63A, a first input first-stage transition portion 64A1, and a first output first-stage transition portion 64A2. The first input high-refractive-index waveguide 61A is formed on a first layer 70A on the Si substrate 71 and is connected to the first intermediate arm waveguide 82, for example, a Si waveguide. The first output high-refractive-index waveguide 62A is formed on the first layer 70A and is connected to the first output arm waveguide 24B, for example, a Si waveguide. The first low-refractive-index waveguide 63A is formed on the second layer 70B on the Si substrate 71 and is, for example, a SiN waveguide, connecting between the first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A.

[0137] First input-side first-stage transition section 64A1 is composed of an output end of first input-side high-refractive-index waveguide 61A and an input end of first low-refractive-index waveguide 63A, and transitions signal light between first input-side high-refractive-index waveguide 61A and first low-refractive-index waveguide 63A. First output-side first-stage transition section 64A2 is composed of an output end of first low-refractive-index waveguide 63A and an input end of first output-side high-refractive-index waveguide 62A, and transitions signal light between first low-refractive-index waveguide 63A and first output-side high-refractive-index waveguide 62A.

[0138] The second output folded waveguide 93 includes a second input high-refractive-index waveguide 61B, a second output high-refractive-index waveguide 62B, a second low-refractive-index waveguide 63B, a second input first-stage transition portion 64B1, and a second output first-stage transition portion 64B2. The second input high-refractive-index waveguide 61B is formed on the first layer 70A on the Si substrate 71 and is connected to the second intermediate arm waveguide 92, for example, a Si waveguide. The second output high-refractive-index waveguide 62B is formed on the first layer 70A and is connected to the second output arm waveguide 34B, for example, a Si waveguide. The second low-refractive-index waveguide 63B is formed on the second layer 70B on the Si substrate 71 and connects between the second input-side high-refractive-index waveguide 61B and the second output-side high-refractive-index waveguide 62B, and is, for example, a SiN waveguide.

[0139] The second input-side first-stage transition section 64B1 is composed of the output end of the second input-side high-refractive-index waveguide 61B and the input end of the second low-refractive-index waveguide 63B, and transitions the signal light between the second input-side high-refractive-index waveguide 61B and the second low-refractive-index waveguide 63B. The second output-side first-stage transition section 64B2 is composed of the output end of the second low-refractive-index waveguide 63B and the input end of the second output-side high-refractive-index waveguide 62B, and transitions the signal light between the second low-refractive-index waveguide 63B and the second output-side high-refractive-index waveguide 62B. The second input-side high-refractive-index waveguide 61B connects the second intermediate arm waveguide 92 and the second low-refractive-index waveguide 63B across the first low-refractive-index waveguide 63A.

[0140] The first input arm waveguide 22A modulates the signal light in response to a reverse electrical signal from the input signal electrode 51A to the first input ground electrode 52A. The second input arm waveguide 32A modulates the signal light in response to a forward electrical signal from the input signal electrode 51A to the second input ground electrode 53A.

[0141] The first intermediate arm waveguide 82 modulates the signal light in response to a reverse electrical signal from the intermediate side signal electrode 51D to the first intermediate side ground electrode 52D. The second intermediate arm waveguide 92 modulates the signal light in response to a forward electrical signal from the intermediate side signal electrode 51D to the second intermediate side ground electrode 53D.

[0142] The first output arm waveguide 24B modulates the signal light in response to a reverse electrical signal from the output signal electrode 51B to the first output ground electrode 52B. The second output arm waveguide 34B modulates the signal light in response to a forward electrical signal from the output signal electrode 51B to the second output ground electrode 53B.

[0143] That is, the first input arm waveguide 22A, the first intermediate arm waveguide 82, and the first output arm waveguide 24B modulate the signal light in response to the same reverse-direction electrical signal. The second input arm waveguide 32A, the second intermediate arm waveguide 92, and the second output arm waveguide 34B modulate the signal light in response to the same forward-direction electrical signal. Since electrical signals in the same direction are applied to the outward, intermediate, and return paths, modulation efficiency can be improved.

[0144] In the optical modulator 1C of the fourth embodiment, the signal light is input from one end face D1 of the optical chip and output from the other end face D2 opposite to the one end face D1 of the optical chip. As a result, it is possible to arrange multiple folded type optical modulators 1C in parallel with a simple layout.

[0145] In the optical modulator 1C, by providing two folding points, the length of the modulator body 4C can be increased without changing the chip length of the optical modulator 1C. As a result, it is possible to achieve highly efficient (low Vpi) operation.

[0146] The optical modulator 1C of the fourth embodiment may be applied to a dual-polarization in-phase quafrature (DP-IQ) modulator equipped with four of them, and this embodiment will be described below as a fifth embodiment. [Example]

[0147] FIG. 19 is an explanatory diagram showing an example of a DP-IQ modulator 1D according to a fifth embodiment. The same components as those of the optical modulator 1C shown in FIG. 18 are denoted by the same reference numerals, and descriptions of the overlapping components and operations will be omitted. The DP-IQ modulator 1D shown in FIG. 19 has four optical modulators 1C according to the fourth embodiment arranged in parallel. The DP-IQ modulator 1D includes an LD input port 111, a branching section 112, an IQ modulator 1D1 for the X polarization component, an IQ modulator 1D2 for the Y polarization component, and a PR (Polarization Rotator) 113. Furthermore, the DP-IQ modulator 1D includes a PBC (Polarization Beam Combiner) 114 and a transmission light output port 115.

[0148] Branching unit 112 is an XY branching MMI that optically branches input light from input waveguide 6 and outputs the branched signal light to IQ modulator 1D1 for the X polarization component and IQ modulator 1D2 for the Y polarization component. IQ modulator 1D1 for the X polarization component has a first branching unit 121, two first DCPSs (Direct Current Phase Shifters) 122, two second branching units 123, and four second DCPSs 124. IQ modulator 1D1 for the X polarization component has optical modulator 1C1 for the I component, optical modulator 1C2 for the Q component, and a first multiplexing unit 125.

[0149] The first branching unit 121 in the X-polarized component IQ modulator 1D1 is an IQ branching MMI that optically branches the X-polarized component signal light from the branching unit 112 into an I-component signal light and a Q-component signal light. The first branching unit 121 outputs the branched I-component signal light to the first DCPS 122. The first DCPS 122 is a phase shifter, such as a heater that heats a Si waveguide, that phase-shifts the I-component signal light. The first DCPS 122 is disposed directly below the Si waveguide and adjusts the phase of the signal light propagating through the Si waveguide by changing the refractive index of the Si waveguide through heater heating. The first DCPS 122 outputs the phase-shifted I-component signal light to the second branching unit 123.

[0150] The second branching unit 123 outputs the phase-shifted I-component signal light from the first DCPS 122 to each second DCPS 124. The second DCPS 124 is a phase shifter, such as a heater that heats a Si waveguide, that phase-shifts the I-component signal light. The second DCPS 124 is disposed directly below the Si waveguide and adjusts the phase of the signal light propagating through the Si waveguide by changing the refractive index of the Si waveguide through heating by the heater. The second DCPS 124 outputs the phase-shifted I-component signal light to the X-polarized I-component optical modulator 1C1. The X-polarized I-component optical modulator 1C1 modulates the X-polarized I-component signal light and outputs the modulated X-polarized I-component signal light to the first X-polarized component multiplexing unit 125.

[0151] The first branching unit 121 in the IQ modulator 1D1 for the X polarization component outputs the branched Q component signal light to the first DCPS 122. The first DCPS 122 is a phase shifter, such as a heater that heats a Si waveguide, that phase-shifts the Q component signal light. The first DCPS 122 outputs the phase-shifted Q component signal light to the second branching unit 123. The second branching unit 123 outputs the phase-shifted Q component signal light from the first DCPS 122 to each second DCPS 124. The second DCPS 124 is a phase shifter, such as a heater that heats a Si waveguide, that phase-shifts the Q component signal light. The second DCPS 124 outputs the phase-shifted Q component signal light to the optical modulator 1C2 for the Q component of the X polarization component. Optical modulator 1C2 for the Q component of the X polarization component modulates the signal light of the Q component of the X polarization component and outputs the modulated signal light of the Q component of the X polarization component to first multiplexing unit 125 for the X polarization component. First multiplexing unit 125 for the X polarization component is an IQ-combined MMI that multiplexes the signal light of the I component of the X polarization component and the signal light of the Q component of the X polarization component.

[0152] The IQ modulator 1D2 for the Y polarization component has a first branching unit 121, two first DCPSs 122, two second branching units 123, and four second DCPSs 124. The IQ modulator 1D2 for the Y polarization component has an optical modulator 1C3 for the I component, an optical modulator 1C4 for the Q component, and a first multiplexing unit 125.

[0153] The first branching unit 121 in the Y polarization component IQ modulator 1D2 is an IQ branching MMI that optically branches the Y polarization component signal light from the branching unit 112 into an I component signal light and a Q component signal light. The first branching unit 121 outputs the branched I component signal light to the first DCPS 122. The first DCPS 122 is a phase shifter, such as a heater that heats a Si waveguide, that phase-shifts the I component signal light. The first DCPS 122 outputs the phase-shifted I component signal light to the second branching unit 123. The second branching unit 123 outputs the phase-shifted I component signal light from the first DCPS 122 to each second DCPS 124. The second DCPS 124 is a phase shifter, such as a heater that heats a Si waveguide, that phase-shifts the I component signal light. The second DCPS 124 outputs the phase-shifted I-component signal light to the Y-polarized I-component optical modulator 1C3. The Y-polarized I-component optical modulator 1C3 modulates the Y-polarized I-component signal light and outputs the modulated Y-polarized I-component signal light to the first Y-polarized component multiplexer 125.

[0154] The first branching unit 121 in the Y-polarized component IQ modulator 1D2 outputs the branched Q-component signal light to the first DCPS 122. The first DCPS 122 is a phase shifter, such as a heater that heats a Si waveguide, that phase-shifts the Q-component signal light. The first DCPS 122 outputs the phase-shifted Q-component signal light to the second branching unit 123. The second branching unit 123 outputs the phase-shifted Q-component signal light from the first DCPS 122 to each second DCPS 124. The second DCPS 124 is a phase shifter, such as a heater that heats a Si waveguide, that phase-shifts the Q-component signal light. The second DCPS 124 outputs the phase-shifted Q-component signal light to the Y-polarized component Q-component optical modulator 1C4. Optical modulator 1C4 for the Q component of the Y polarization component modulates the signal light of the Q component of the Y polarization component and outputs the modulated signal light of the Q component of the Y polarization component to first multiplexing unit 125 for the Y polarization component. First multiplexing unit 125 for the Y polarization component is an IQ-combined MMI that multiplexes the signal light of the I component of the Y polarization component and the signal light of the Q component of the Y polarization component.

[0155] The first multiplexing unit 125 for the X polarization component multiplexes the I-component signal light of the X polarization component and the Q-component signal light of the X polarization component, and outputs the combined IQ-component signal light of the X polarization component to the PBC 114. The first multiplexing unit 125 for the Y polarization component multiplexes the I-component signal light of the Y polarization component and the Q-component signal light of the Y polarization component, and outputs the combined IQ-component signal light of the Y polarization component to the PR 113. The PR 113 rotates the polarization of the IQ-component signal light of the Y polarization component, and outputs the IQ-component signal light of the Y polarization component after the polarization rotation to the PBC 114. The PBC 114 multiplexes the IQ-component signal light of the X polarization component and the IQ-component signal light of the Y polarization component after the polarization rotation, and outputs the combined XY-polarized component signal light to the transmission light output port 115 as transmission light.

[0156] The first branching section 121, the second branching section 123, and the first multiplexing section 125 that make up the optical modulator 1C are constructed using silicon photonics Si waveguides, enabling miniaturization by taking advantage of the characteristics of silicon photonics. By forming portions of the two waveguides that make up the optical modulator 1C on the silicon photonics device side, the first DCPS 122 and the second DCPS 124 that appropriately adjust the phase of the optical modulator 1C can be realized using heaters formed on the waveguides. Furthermore, a compact, low-power phase shifter can be realized. The first DCPS 122 and the second DCPS 124 using heaters can be used not only for phase adjustment of the optical modulator 1C, but also for phase adjustment between the I channel and Q channel of the IQ modulator 1D1 (1D2).

[0157] The DP-IQ modulator 1D of the fifth embodiment incorporates an X-polarized IQ modulator 1D1 and a Y-polarized IQ modulator 1D2. As a result, the propagation speed of the electrical signal and the propagation speed of light in the optical modulator 1C are substantially the same, thereby improving the velocity mismatch in the DP-IQ modulator 1D. This prevents the operating bandwidth of the DP-IQ modulator 1D from being limited. Furthermore, since the DP-IQ modulator 1D improves the velocity mismatch while maintaining a long operating length, it is possible to suppress the half-wave voltage Vπ and improve the modulation efficiency of the DP-IQ modulator 1D.

[0158] The DP-IQ modulator 1D incorporates an X-polarized IQ modulator 1D1 and a Y-polarized IQ modulator 1D2, and each optical modulator 1C can reduce the size of the return part while ensuring velocity matching. As a result, the total chip size of the DP-IQ modulator 1D can also be reduced.

[0159] In addition, in Example 5, for example, a DP-IQ modulator 1D is exemplified, but in addition to the DP-IQ modulator 1D, an optical receiver may also be mounted on the Si substrate 71, and this embodiment will be described below as Example 6. [Example]

[0160] FIG. 20 is an explanatory diagram showing an example of an optical transceiver 1E according to a sixth embodiment. The same components as those of the DP-IQ modulator 1D shown in FIG. 19 are denoted by the same reference numerals, and redundant descriptions of the components and operations will be omitted. The optical transceiver 1E shown in FIG. 20 is an optical integrated circuit having an optical modulator element 130A including the DP-IQ modulator 1D and an optical receiver element 130B that receives a DP-QAM signal. The optical transceiver 1E integrates the optical modulator element 130A and the optical receiver element 130B using silicon photonics technology. The optical transceiver 1E has an LD input port 111, a transmission light output port 115, and a reception light input port 116.

[0161] The received light input port 116 is located at one end face D1 of the optical transceiver 1E and is an optical port that connects an optical fiber (described later) that inputs received light to the optical receiver element 130B. The LD input port 111 is located at one end face D1 of the optical transceiver 1E and is an optical port that connects the optical modulator element 130A and the optical receiver element 130B that input local light from a light source (not shown). The transmitted light output port 115 is located at one end face D1 of the optical transceiver 1E and is an optical port that connects the optical fiber that outputs transmitted light to the optical modulator element 130A.

[0162] The optical modulator element 130A is, for example, a DP-IQ modulator 1D. The optical receiver element 130B is, for example, a coherent receiver. The optical receiver element 130B has a third branching unit 131, a fourth branching unit 132, a PBS (Polarization Beam Splitter) 133, and a PR (Polarization Rotator) 134. Furthermore, the optical receiver element 130B has a first optical hybrid circuit 135A (135) and a second optical hybrid circuit 135B (135). The optical receiver element 130B has four first photodetectors 136A (136) and four second photodetectors 136B (136).

[0163] The third branching unit 131 is a Tx / Lo branching MMI that optically branches light from a light source connected to the LD input port 111. The third branching unit 131 outputs one of the branched light beams to the branching unit 112 in the DP-IQ modulator 1D as an input light source for the optical modulator, and outputs the other to each optical hybrid circuit 135 as local light for the optical receiver. The fourth branching unit 132 branches and outputs the local light from the third branching unit 131 to each optical hybrid circuit 135. The PBS 133 demultiplexes the received light from the received light input port 116 into X-polarized received light and Y-polarized received light, and outputs the X-polarized received light to the first optical hybrid circuit 135A and the Y-polarized received light to the PR 134. The PR 134 rotates the Y-polarized received light by 90 degrees and outputs the Y-polarized received light after the polarization rotation to the second optical hybrid circuit 135B.

[0164] The first optical hybrid circuit 135A causes the X polarization component of the received light to interfere with the local light to obtain I component and Q component optical signals. The first optical hybrid circuit 135A outputs the I component optical signal of the X polarization component to the first photodetector 136A, and also outputs the Q component optical signal to the first photodetector 136A.

[0165] The second optical hybrid circuit 135B causes the Y polarization component of the received light to interfere with the local light to obtain I and Q component optical signals. The second optical hybrid circuit 135B outputs the I component optical signal of the Y polarization component to the second photodetector 136B, and also outputs the Q component optical signal to the second photodetector 136B.

[0166] The first photodetector 136A is, for example, a Si photonics Ge-PD (Photo Detector), which converts the optical signal of the I component of the X polarization component from the first optical hybrid circuit 135A into an electrical signal and outputs the electrical signal of the I component after the electrical conversion. The Ge-PD has a structure in which a Ge layer is disposed directly below a Si waveguide. The first photodetector 136A also converts the optical signal of the Q component of the X polarization component from the first optical hybrid circuit 135A into an electrical signal and outputs the electrical signal of the Q component after the electrical conversion.

[0167] The second photodetector 136B is, for example, a Si photonics Ge-PD, and converts the optical signal of the I component of the Y polarization component from the second optical hybrid circuit 135B into an electrical signal of the I component after the electrical conversion. The second photodetector 136B converts the optical signal of the Q component of the Y polarization component from the second optical hybrid circuit 135B into an electrical signal of the Q component after the electrical conversion.

[0168] Fig. 21 is a cross-sectional view showing an example of an optical transceiver 1E. The second DCPS 124 in the optical transceiver 1E shown in Fig. 21 has a Si substrate 71, a lower cladding layer 72 laminated on the Si substrate 71, and an upper cladding layer 73 laminated on the lower cladding layer 72. The second DCPS 124 has a Si waveguide 124C disposed on a second layer 70B of the lower cladding layer 72, a heater 124A disposed below the Si waveguide 124C in the lower cladding layer 72, and heater terminals 124B connected to both ends of the heater 124A. The heater terminals 124B are electrically connected to the electrode wiring 50A.

[0169] The optical modulator 1C in the optical transceiver 1E includes a first input-side modulation section transition section 26, a modulator body 4C, and a first folded section 5C1. The modulator body 4C includes a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. The modulator body 4C includes a first input waveguide 21A disposed in the second layer 70B of the lower cladding layer 72 and a first input-side high-refractive-index waveguide 61A disposed in the second layer 70B. The modulator body 4C includes a first low-refractive-index waveguide 63A disposed in the third layer 70C of the lower cladding layer 72 and a first input-side arm waveguide 22A disposed in the first layer 70A of the upper cladding layer 73.

[0170] The first photodetector 136A in the optical transceiver 1E has a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. The first photodetector 136A has a Si waveguide 136A3 disposed on the second layer 70B of the lower cladding layer 72, a Ge layer 136A1 disposed below the Si waveguide 136A3, and PD terminals 136A2 connected to both ends of the Si waveguide 136A3. The PD terminals 136A2 are electrically connected to the electrode wiring 50A.

[0171] 22A is a cross-sectional view showing an example of a Si photo substrate 210A after the first formation process. The Si photo substrate 210A includes a Si substrate 211 for a Si photo, a BOX layer 212 stacked on the Si substrate 211, a Si waveguide 213 disposed in the BOX 212 layer, and a SiN waveguide 214 disposed in the BOX layer 212. The Si photo substrate 210A also includes a heater 124A used for a second DCPS 124 disposed in the BOX 212 layer, and a first light-receiving element 136A disposed in the BOX 212 layer. The Si waveguide 213 includes a Si waveguide 124C of the second DCPS 124, a Si waveguide 213 of a second output-side high-refractive-index waveguide 62B of the optical modulator 1C, and a Si waveguide 136A3 of a first light-receiving element 136A. The Si photo substrate 210A has a heater terminal 124B of the heater 124A and a PD terminal 136A2 of the first light receiving element 136A.

[0172] 22B is a cross-sectional view showing an example of a Si photo substrate 210B after the second formation process. A Si substrate 221 for use as a support substrate is prepared. After the Si photo substrate 210A shown in FIG. 22A is turned upside down, the Si substrate 221 for use as a support substrate is bonded to the surface of the BOX layer 212, thereby obtaining the Si photo substrate 210B after the second formation process shown in FIG. 22B. Note that although the Si substrate 221 is used as an example of the support substrate, a quartz substrate with a low dielectric constant may also be used, and this can be changed as appropriate. When a quartz substrate is used as the support substrate, it is suitable for high-speed operation of the optical modulator 1C.

[0173] Fig. 22C is a cross-sectional view showing an example of a Si photo substrate 210C after the first removal step. By removing the Si substrate 211 for the Si photo and a part of the BOX layer 212 on the Si substrate 211 for the Si photo from the Si photo substrate 210B shown in Fig. 22B, the Si photo substrate 210C after the first removal step is obtained as shown in Fig. 22C. At this time, the surface of the BOX layer 212 is removed so that the Si waveguide is located close to a position within several hundred nm from the surface of the BOX layer 212.

[0174] 22D is a cross-sectional view showing an example of a Si photo substrate 210D after the third formation step. An LN-side Si substrate 231 on which a thin-film LN layer 232 is integrated is prepared. The thin-film LN layer 232 on the LN-side Si substrate 231 is bonded to the surface of the BOX layer 212 of the Si photo substrate 210C shown in FIG. 22C, thereby obtaining the Si photo substrate 210D after the third formation step shown in FIG. 22D. In the previous step, the Si waveguide was positioned close to the surface of the BOX layer 212, so the distance between the thin-film LN waveguide (thin-film LN layer 232) and the Si waveguide can be reduced, making it possible to facilitate optical coupling.

[0175] Fig. 22E is a cross-sectional view showing an example of a Si photo-substrate 210E after the second removal step. The LN-side Si substrate 231 is removed from the Si photo-substrate 210D shown in Fig. 22D, and the thin-film LN layer 232 is polished to a predetermined thickness, for example, about 500 nm, to obtain the Si photo-substrate 210E after the second removal step shown in Fig. 22E.

[0176] Fig. 22F is a cross-sectional view showing an example of a Si photo substrate 210F after the fourth formation step. A portion of the thin-film LN layer 232 on the BOX layer 212 of the Si photo substrate 210E shown in Fig. 22E is dry-etched to form a rib-structured waveguide 232A in the thin-film LN layer 232, for example, the first input-side arm waveguide 22. As a result, the Si photo substrate 210F after the fourth formation step shown in Fig. 22D is obtained.

[0177] Fig. 22G is a cross-sectional view showing an example of a Si photo substrate 210G after the fifth formation step. By forming an upper cladding layer 73 on the rib-structured waveguide 232A of the thin-film LN layer 232 formed on the BOX layer 212 of the Si photo substrate 210F shown in Fig. 22F, the Si photo substrate 210G after the fifth formation step shown in Fig. 22G is obtained.

[0178] Fig. 22H is a cross-sectional view showing an example of a Si photo substrate 210H after the sixth formation step. The Si photo substrate 210G shown in Fig. 22G has formed therein heater terminal 124B of heater 124A, a slab of waveguide 232A with a rib structure in thin-film LN layer 232, and vias 240 for forming electrode wiring 50A connected to PD terminal 136A2 of first light receiving element 136A. As a result, the Si photo substrate 210H after the sixth formation step shown in Fig. 22H is obtained.

[0179] Fig. 22I is a cross-sectional view showing an example of a Si photo substrate 210I after the seventh formation step. An electrode material is injected into each via 240 of the Si photo substrate 210I shown in Fig. 22 to form electrode wirings 50A, thereby obtaining the Si photo substrate 210I after the seventh formation step shown in Fig. 22I.

[0180] The optical transceiver 1E of the sixth embodiment is equipped with an optical modulator element 130A including a DP-IQ modulator 1D1 and an optical receiver element 130B using silicon photonics technology, and each optical modulator 1C can reduce the size of the return part while ensuring speed matching. As a result, the optical transceiver 1E equipped with the DP-IQ modulator 1D1 can also reduce the total chip size.

[0181] The optical modulator element 130A has a DP-IQ modulator 1D incorporating multiple optical modulators 1C. Moreover, the DP-IQ modulator 1D improves velocity mismatch while maintaining a long operating length, thereby suppressing the half-wave voltage Vπ and improving the modulation efficiency of the DP-IQ modulator 1D.

[0182] An embodiment of an optical module 1F incorporating the optical transceiver 1E of the sixth embodiment will be described below as a seventh embodiment. [Example]

[0183] 23 is an explanatory diagram showing an example of the configuration of an optical module 1F of Example 7. Note that the same components as those of the optical transceiver 1E of Example 6 are denoted by the same reference numerals, and descriptions of the overlapping configurations and operations will be omitted. The optical module 1F shown in FIG. 23 is a COSA (Coherent Optical Subassembly) having the optical transceiver 1E, a fiber array 141, a DRV (Driver) circuit 142, and a TIA (Transimpedance Amplifier) ​​circuit 143.

[0184] The fiber array 141 is an array that collectively connects an optical fiber F2 connected to the transmission light output port 115, an optical fiber F1 connected to the LD input port 111, and an optical fiber F3 connected to the reception light input port .

[0185] The DRV circuit 142 is a driver circuit that applies an electric signal to the signal electrode 51 in each optical modulator 1C. The TIA circuit 143 is an amplifier that amplifies the electric signals that have been electrically converted by the first light receiving element 136A and the second light receiving element 136B, and outputs the amplified electric signals.

[0186] The optical module 1F of the seventh embodiment includes an optical transceiver 1E incorporating a plurality of optical modulators 1C, and therefore each optical modulator 1C can reduce the size of the return part while ensuring speed matching. As a result, the optical module 1F incorporating the optical transceiver 1E can also reduce the total chip size.

[0187] An embodiment of an optical transceiver 1G equipped with the optical module 1F of the seventh embodiment will be described below as an eighth embodiment. [Example]

[0188] FIG. 24 is an explanatory diagram illustrating an example of an optical transceiver 1G according to an eighth embodiment. The same components as those in the transceiver module of the seventh embodiment are denoted by the same reference numerals, and descriptions of the overlapping components and operations will be omitted. The optical transceiver 1G illustrated in FIG. 24 includes an LD (Laser Diode) 151, an optical module 1F, and a DSP (Digital Signal Processor) 152. The optical transceiver 1G is a compact transceiver conforming to standards such as QSFP and OSFP. The LD 151 is, for example, a light source that emits laser light. The optical module 1F includes an optical transceiver 1E, a DRV circuit 142, and a TIA circuit 143. The optical transceiver 1E includes an optical modulator element 130A and an optical receiver element 130B. The optical modulator element 130A is, for example, a DP-IQ modulator 1D. The DSP 152 controls the entire optical transceiver 1E. The DSP 152 is an electrical component that performs digital signal processing, such as IQ modulation of a transmission signal and demodulation of a reception signal.

[0189] The DSP 152 performs processing such as encoding of the transmission data, generates an electrical signal including the transmission data, and outputs the generated electrical signal to the DRV circuit 142. The DRV circuit 142 drives the optical modulator element 130A in response to the electrical signal from the DSP 152.

[0190] The optical receiver element 130B converts the signal light into an electric signal. The TIA circuit 143 amplifies the converted electric signal and outputs the amplified electric signal to the DSP 152. The DSP 152 performs processing such as decoding of the electric signal obtained from the TIA circuit 143 to obtain received data.

[0191] For convenience of explanation, the optical transceiver 1G has been illustrated as incorporating the optical modulator element 130A and the optical receiver element 130B, but it is also applicable to an optical transmitter incorporating only the optical modulator element 130A.

[0192] The optical transceiver 1G of the seventh embodiment has an optical module 1F including an optical transmitter / receiver 1E with multiple built-in optical modulators 1C. Each optical modulator 1C can reduce the size of the return part while ensuring speed matching. As a result, the total size of the optical transceiver 1G with the built-in optical module 1F can be reduced.

[0193] Furthermore, the components of each unit shown in the figure do not necessarily have to be physically configured as shown in the figure. In other words, the specific form of distribution and integration of each unit is not limited to that shown in the figure, and all or part of them can be functionally or physically distributed and integrated in any unit depending on various loads, usage conditions, etc.

[0194] Furthermore, the various processing functions performed by each device may be executed in whole or in part on a CPU (Central Processing Unit) (or a microcomputer such as an MPU (Micro Processing Unit) or MCU (Micro Controller Unit)). Needless to say, the various processing functions may be executed in whole or in part on a program analyzed and executed by a CPU (or a microcomputer such as an MPU or MCU), or on hardware using wired logic. [Explanation of symbols]

[0195] 1 Optical modulator 10 First Coupler 20 First waveguide 22 first input arm waveguide 23 First folded waveguide 24 first output arm waveguide 30 Second waveguide 32 second input arm waveguide 33 Second folded waveguide 34 second output arm waveguide 40 Second Coupler 50 electrodes 61A First input side high refractive index waveguide 62A First output high refractive index waveguide 63A First low refractive index waveguide 63A1 First input side first stage low refractive index waveguide 63A2 First output side first stage low refractive index waveguide 63A3 First and second stage low refractive index waveguide 61B Second input side high refractive index waveguide 62B Second output high refractive index waveguide 63B Second low refractive index waveguide 63B1 Second input side first stage low refractive index waveguide 63B2 Second output side first stage low refractive index waveguide 63B3 Second stage low refractive index waveguide

Claims

1. an optical modulator comprising: a substrate including a high refractive index waveguide; a first coupler disposed on the substrate and branching a signal light into two; a first waveguide disposed on the substrate and connected to one output of the first coupler; a second waveguide disposed on the substrate and connected to the other output of the first coupler; a second coupler disposed on the substrate and multiplexing and outputting the signal light from the first waveguide and the signal light from the second waveguide; and electrodes for applying electric signals to the first waveguide and the second waveguide, The first waveguide comprises: a first input arm waveguide connected to the first coupler; a first output arm waveguide connected to the second coupler; a first folded waveguide connecting the first input arm waveguide and the first output arm waveguide; The second waveguide comprises: a second input arm waveguide connected to the first coupler; a second output arm waveguide connected to the second coupler; a second folded waveguide connecting the second input arm waveguide and the second output arm waveguide; the first input arm waveguide, the second input arm waveguide, the first output arm waveguide, and the second output arm waveguide are a waveguide including a material having higher EO characteristics than the high refractive index waveguide; At least a portion of the first folded waveguide and the second folded waveguide An optical modulator characterized in that the waveguide contains a material with a lower refractive index than the high refractive index waveguide.

2. The first waveguide comprises: the first input arm waveguide located on the outer circumferential side of the folded portion; the first output arm waveguide located on the inner periphery side of the folded portion; a first folded waveguide connecting the first input arm waveguide and the first output arm waveguide; The second waveguide comprises: the second input arm waveguide located on the inner periphery side of the folded portion; the second output arm waveguide located on the outer circumferential side of the folded portion; a second folded waveguide connecting the second input arm waveguide and the second output arm waveguide; 2. The optical modulator according to claim 1, further comprising:

3. The first folded waveguide comprises: a first input-side high-refractive-index waveguide having a high-refractive-index layer on the substrate as a core layer and connected to the first input-side arm waveguide; a first output-side high-refractive-index waveguide having the high-refractive-index layer as a core layer and connected to the first output-side arm waveguide; a first low refractive index waveguide, the first low refractive index layer on the substrate being a core layer, connecting the first input high refractive index waveguide and the first output high refractive index waveguide; The second folded waveguide comprises: a second input-side high-refractive-index waveguide having the high-refractive-index layer as a core layer and connected to the second input-side arm waveguide; a second output-side high-refractive-index waveguide having the high-refractive-index layer as a core layer and connected to the second output-side arm waveguide; 3. The optical modulator according to claim 2, further comprising: a second low-refractive-index waveguide having the first-stage low-refractive-index layer as a core layer, connecting the second input-side high-refractive-index waveguide and the second output-side high-refractive-index waveguide.

4. The first low refractive index waveguide comprises: The signal light is transferred between the first input-side high refractive index waveguide and the first output-side high refractive index waveguide, and The second low refractive index waveguide comprises: The signal light is transferred between the second input-side high refractive index waveguide and the second output-side high refractive index waveguide, and 4. The optical modulator according to claim 3, wherein either the first input-side high refractive index waveguide or the second output-side high refractive index waveguide straddles a low refractive index waveguide.

5. The first folded waveguide comprises: a first input-side high-refractive-index waveguide having a high-refractive-index layer on the substrate as a core layer and connected to the first input-side arm waveguide; a first output-side high-refractive-index waveguide having the high-refractive-index layer as a core and connected to the first output-side arm waveguide; a first input-side first-stage low-refractive-index waveguide having a first-stage low-refractive-index layer on the substrate as a core layer and connected to the first input-side high-refractive-index waveguide; a first output-side first-stage low refractive index waveguide formed in the first-stage low refractive index layer and connected to the first output-side high refractive index waveguide; a first second-stage low refractive index waveguide, the second-stage low refractive index layer on the substrate being a core layer, connecting the first input-side first-stage low refractive index waveguide and the first output-side first-stage low refractive index waveguide; The second folded waveguide comprises: a second input-side high-refractive-index waveguide having the high-refractive-index layer as a core layer and connected to the second input-side arm waveguide; a second output-side high-refractive-index waveguide having the high-refractive-index layer as a core layer and connected to the second output-side arm waveguide; a second input-side first-stage low-refractive-index waveguide having the first-stage low-refractive-index layer as a core layer and connected to the second input-side high-refractive-index waveguide; a second output-side first-stage low-refractive-index waveguide having the first-stage low-refractive-index layer as a core layer and connected to the second output-side high-refractive-index waveguide; 3. The optical modulator according to claim 2, further comprising: a second second-stage low refractive index waveguide having the second-stage low refractive index layer as a core layer and connecting the second input-side first-stage low refractive index waveguide and the second output-side first-stage low refractive index waveguide.

6. The first second stage low refractive index waveguide comprises: The signal light is transferred between the first input-side first-stage low refractive index waveguide and the first output-side first-stage low refractive index waveguide, and The second second-stage low refractive index waveguide comprises: The signal light is transferred between the second input side first stage low refractive index waveguide and the second output side first stage low refractive index waveguide, and The first input-side high refractive index waveguide or the first output-side high refractive index waveguide is 6. The optical modulator according to claim 5, wherein the first second-stage low refractive index waveguide or the second second-stage low refractive index waveguide is straddled.

7. The first waveguide comprises: the first input arm waveguide located on the outer circumferential side of the folded portion; the first folded waveguide located on the outer periphery of the fold; the first output arm waveguide located on the outer circumferential side of the folded portion, The second waveguide comprises: the second input arm waveguide located on the inner periphery side of the folded portion; the second folded waveguide located on the inner periphery side of the fold; 2. The optical modulator according to claim 1, further comprising: the second output arm waveguide located on the inner periphery side of the folded portion.

8. The first folded waveguide comprises: a first input-side high-refractive-index waveguide having a high-refractive-index layer on the substrate as a core layer and connected to the first input-side arm waveguide; a first output-side high-refractive-index waveguide formed in the high-refractive-index layer and connected to the first output-side arm waveguide; a first low refractive index waveguide, the first low refractive index layer on the substrate being a core layer, connecting the first input high refractive index waveguide and the first output high refractive index waveguide; The second folded waveguide comprises: a second input-side high-refractive-index waveguide having the high-refractive-index layer as a core layer and connected to the second input-side arm waveguide; a second output-side high-refractive-index waveguide having the high-refractive-index layer as a core layer and connected to the second output-side arm waveguide; 8. The optical modulator according to claim 7, further comprising: a second low refractive index waveguide having the low refractive index layer as a core layer and connecting the second input-side high refractive index waveguide and the second output-side high refractive index waveguide.

9. The first input-side high refractive index waveguide, the first output-side high refractive index waveguide, the second input-side high refractive index waveguide, and the second output-side high refractive index waveguide are It is a waveguide with a Si core, The first low refractive index waveguide and the second low refractive index waveguide are 4. The optical modulator according to claim 3, wherein the waveguide has a core made of SiN.

10. The material having high EO properties is LiNbO 3 , BaTiO 3 2. The optical modulator according to claim 1, wherein the optical modulator comprises at least one material selected from the group consisting of PZT and PLZ.

11. 2. The optical modulator according to claim 1, wherein the number of times the first waveguide and the second waveguide are folded is an even number.

12. An optical transmitter including an optical modulator that modulates guided light in response to an electrical signal, The optical modulator comprises: a substrate including a high refractive index waveguide; a first coupler disposed on the substrate and branching a signal light into two; a first waveguide disposed on the substrate and connected to one output of the first coupler; a second waveguide disposed on the substrate and connected to the other output of the first coupler; a second coupler disposed on the substrate and multiplexing and outputting the signal light from the first waveguide and the signal light from the second waveguide; and electrodes for applying electric signals to the first waveguide and the second waveguide; The first waveguide comprises: a first input arm waveguide connected to the first coupler; a first output arm waveguide connected to the second coupler; a first folded waveguide connecting the first input arm waveguide and the first output arm waveguide; The second waveguide comprises: a second input arm waveguide connected to the first coupler; a second output arm waveguide connected to the second coupler; a second folded waveguide connecting the second input arm waveguide and the second output arm waveguide; the first input arm waveguide, the second input arm waveguide, the first output arm waveguide, and the second output arm waveguide are a waveguide including a material having higher EO characteristics than the high refractive index waveguide; At least a portion of the first folded waveguide and the second folded waveguide is An optical transmitter characterized in that the waveguide contains a material with a lower refractive index than the high refractive index waveguide.

13. an optical modulator that modulates guided light in response to an electrical signal; an optical receiver that converts received signal light into an electrical signal; a signal processing unit that generates an electrical signal to be output to the optical modulator and processes the electrical signal obtained from the optical receiver, The optical modulator comprises: a substrate including a high refractive index waveguide; a first coupler disposed on the substrate and branching a signal light into two; a first waveguide disposed on the substrate and connected to one output of the first coupler; a second waveguide disposed on the substrate and connected to the other output of the first coupler; a second coupler disposed on the substrate and multiplexing and outputting the signal light from the first waveguide and the signal light from the second waveguide; and electrodes for applying electric signals to the first waveguide and the second waveguide; The first waveguide comprises: a first input arm waveguide connected to the first coupler; a first output arm waveguide connected to the second coupler; a first folded waveguide connecting the first input arm waveguide and the first output arm waveguide; The second waveguide comprises: a second input arm waveguide connected to the first coupler; a second output arm waveguide connected to the second coupler; a second folded waveguide connecting the second input arm waveguide and the second output arm waveguide; the first input arm waveguide, the second input arm waveguide, the first output arm waveguide, and the second output arm waveguide are a waveguide including a material having higher EO characteristics than the high refractive index waveguide; At least a portion of the first folded waveguide and the second folded waveguide is An optical transceiver, wherein the waveguide includes a material with a lower refractive index than the high refractive index waveguide.

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