Optical modulator, optical transmitter, and optical transceiver

Abstract

The invention relates to an optical modulator, an optical transmitter and an optical transceiver. The optical modulator includes: a first coupler that splits signal light into two beams of light; the first waveguide is connected to one output end of the first coupler; a second waveguide connected to the other output end of the first coupler; and a second coupler that couples the signal light from the first waveguide and the second waveguide and outputs the coupled light. The first waveguide comprises a first input side arm waveguide, a first output side arm waveguide and a first folded waveguide. The second waveguide comprises a second input side arm waveguide, a second output side arm waveguide and a second folded waveguide. The first input side arm waveguide and the second input side arm waveguide and the first output side arm waveguide and the second output side arm waveguide are waveguides including a material having a high EO characteristic. The first folded waveguide and the second folded waveguide are waveguides comprising a material (SiN) having a low refractive index.

Description

Technical Field

[0001] The implementation methods discussed in this article involve optical modulators, optical transmitters, and optical transceivers. Background Technology

[0002] For example, a thin-film LN modulator is known to use a thin-film LN as an optical modulator, which can perform efficient and high-speed operation. However, in thin-film LN modulators, the modulator length is usually required to be 10 mm or more to achieve high efficiency (low half-wave voltage Vp), and there is a problem of size reduction.

[0003] Therefore, as a means of solving the problems described above, a structure is known in which a thin-film LN modulator made of different materials is integrated on a Si photonic substrate, and a portion of the functionality of an optical modulator, such as a DC phase shifter, is implemented by Si photonics adapted to reduce size (CLEO 2023STh40.5). Furthermore, a structure is known in which a thin-film LN modulator made of different materials is integrated on a Si photonic substrate, and the arm waveguide of the optical modulator is folded to reduce its size in the longitudinal direction (IEEE 17th International Group IV Photonics Conference 2021 10.1109 / IEDM19573.2019.8993510).

[0004] Figure 25 This is a schematic plan view showing an example of a conventional optical modulator 500. Figure 25 The optical modulator 500 shown is a thin-film LN modulator with a folded structure mounted on a Si photonic substrate 501. The optical modulator 500 includes the Si photonic substrate 501, an input multimode interferometer (MMI) 502, a modulator body 503, and a folded portion 504. The input and output 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 optical modulator 500 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 beams, outputs one of the split beams to the first waveguide 520, and outputs the other split beam to the second waveguide 530. Output coupler 540 is a coupler that couples signal light from first waveguide 520 and signal light from second waveguide 530 and outputs the coupled light to output waveguide 502B. Electrode 550 has a GSG structure including a single signal electrode 551 and two ground electrodes 552. Signal electrode 551 includes a straight signal electrode 551A and a folded signal electrode 551B. Each of the ground electrodes 552 includes a straight ground electrode 552A and a folded ground electrode 552B.

[0005] The first waveguide 520 includes a first input waveguide 521, a first input side-arm waveguide 522, a first folded waveguide 523, a first output side-arm waveguide 524, and a first output waveguide 525. The first input waveguide 521 is a waveguide connected between the input coupler 510 and the first input side-arm waveguide 522. The first output waveguide 525 is a waveguide connected between the output coupler 540 and the first output side-arm waveguide 524. The first folded waveguide 523 is a curved waveguide connected between the first input side-arm waveguide 522 and the first output side-arm waveguide 524. The first input side-arm waveguide 522 is a straight-arm waveguide that modulates the signal light by changing the refractive index of the signal light to be guided according to the electrical signal from the signal electrode 551 to the ground electrode 552. The first output side-arm waveguide 524 is a straight-arm waveguide that modulates the signal light by changing the refractive index of the signal light to be guided according to the electrical signal from the signal electrode 551 to the ground electrode 552. Meanwhile, signal electrode 551 is disposed near one side surface of the first input-side waveguide 522 and the first output-side waveguide 524, and ground electrode 552 is disposed near the other side surface of the first input-side waveguide 522 and the first output-side waveguide 524. The first input waveguide 521, the first folded waveguide 523, and the first output waveguide 525 are configured using Si waveguides, and the first input-side waveguide 522 and the first output-side waveguide 524 are constructed of thin-film LN waveguides.

[0006] The second waveguide 530 includes a second input waveguide 531, a second input side-arm waveguide 532, a second folded waveguide 533, a second output side-arm waveguide 534, and a second output waveguide 535. The second input waveguide 531 is a waveguide connected between the input coupler 510 and the second input side-arm waveguide 532. The second output waveguide 535 is a waveguide connected between the output coupler 540 and the second output side-arm waveguide 534. The second folded waveguide 533 is a bent waveguide connected between the second input side-arm waveguide 532 and the second output side-arm waveguide 534. The second input side-arm waveguide 532 is a straight-arm waveguide that modulates the signal light by changing the refractive index of the signal light to be guided according to the electrical signal from the signal electrode 551 to the ground electrode 552. The second output side-arm waveguide 534 is a straight-arm waveguide that modulates the signal light by changing the refractive index of the signal light to be guided according to the electrical signal from the signal electrode 551 to the ground electrode 552. Meanwhile, signal electrode 551 is disposed near one side surface of the second input-side waveguide 532 and the second output-side waveguide 534, and ground electrode 552 is disposed near the other side surface of the second input-side waveguide 532 and the second output-side waveguide 534. The second input waveguide 531, the second folded waveguide 533, and the second output waveguide 535 are configured using Si waveguides, and the second input-side waveguide 532 and the second output-side waveguide 534 are constructed of thin-film LN waveguides.

[0007] Patent Document 1: International Publication No. 2008 / 099950

[0008] Patent Document 2: U.S. Patent Application Publication No. 2022 / 404652

[0009] In the conventional optical modulator 500, the first input-side waveguide 522, the first output-side waveguide 524, the second input-side waveguide 532, and the second output-side waveguide 534 are constructed from thin-film LN waveguides. Furthermore, in the optical modulator 500, the first folded waveguide 523 and the second folded waveguide 533 are configured using Si waveguides. To achieve high-speed operation of the optical modulator 500, the propagation speed of the light passing through the optical modulator 500 needs to be matched with the propagation speed of the electrical signal; that is, speed matching is required, and essentially, it is designed so that the refractive index of the light and the refractive index of the electrical signal are identical.

[0010] Figure 26 This is a diagram illustrating an example of the relationship between the refractive index and waveguide width of a Si waveguide and a signal electrode. In the first folded waveguide 523 and the second folded waveguide 533, a Si waveguide is used, and as shown... Figure 26 As shown, the refractive index of the Si waveguide is approximately 4, and the refractive index of the signal electrode is approximately 1.9. Therefore, a velocity difference exists between the propagation speed of light guided by the Si waveguide and the propagation speed of the electrical signal passing through the signal electrode.

[0011] To ensure speed matching, the optical path length between the light and electrical signals needs to be matched as the product of the waveguide length L and the refractive index n (n×L). Therefore, the electrode length of the signal electrode needs to be increased by 4 / 1.9, which is the ratio of the refractive index, approximately twice the waveguide length. As a result, the size of the folded signal electrode 551B arranged on each side surface of the first folded waveguide 523 and the second folded waveguide 533 increases, thereby increasing the overall size of the optical modulator 500.

[0012] Therefore, one aspect of the embodiments of the present invention is to provide an optical modulator, etc., that can ensure speed matching and reduce the chip size of the optical modulator. Summary of the Invention

[0013] According to one aspect of the embodiments, the optical modulator includes: a substrate including a high-refractive-index waveguide; a first coupler disposed on the substrate and splitting signal light into two beams; a first waveguide disposed on the substrate and connected to one output terminal of the first coupler; a second waveguide disposed on the substrate and connected to the other output terminal of the first coupler; a second coupler disposed on the substrate, coupling signal light from the first waveguide and signal light from the second waveguide, and outputting coupled signal light; and an electrode applying an electrical signal to the first waveguide and the second waveguide. The first waveguide includes: a first input-side arm waveguide connected to the first coupler; a first output-side arm waveguide connected to the second coupler; and a first folded waveguide connected between the first input-side arm waveguide and the first output-side arm waveguide. The second waveguide includes: a second input-side arm waveguide connected to the first coupler; a second output-side arm waveguide connected to the second coupler; and a second folded waveguide connected between the second input-side arm waveguide and the second output-side arm waveguide. Compared to the high-refractive-index waveguide, the first input-side arm waveguide, the second input-side arm waveguide, the first output-side arm waveguide, and the second output-side arm waveguide are waveguides comprising materials with high EO characteristics. Compared to the high-refractive-index waveguide, at least a portion of the first folded waveguide and the second folded waveguide are waveguides comprising materials with low refractive indices. Attached Figure Description

[0014] Figure 1 This is a schematic plan view illustrating an example of an optical modulator according to the first embodiment;

[0015] Figure 2 This is a schematic plan view showing an example of a folded section;

[0016] Figure 3 This is a schematic cross-sectional view showing an example of an optical modulator;

[0017] Figure 4 This is a perspective view showing an example of the first and second folded waveguides at the intersection;

[0018] Figure 5 This is a schematic plan view showing an example of the first input-side conversion unit;

[0019] Figure 6A It shows along Figure 5 A schematic cross-sectional view of an example of a section taken by line AA;

[0020] Figure 6B It shows along Figure 5 A schematic cross-sectional view of an example of a section taken by line BB;

[0021] Figure 6C It shows along Figure 5 A schematic cross-sectional view of an example of a section taken by line CC;

[0022] Figure 6D It shows along Figure 5 A schematic cross-sectional view of an example of a section taken by line DD;

[0023] Figure 7 This is a schematic plan view showing an example of a first-stage conversion unit on the first input side;

[0024] Figure 8A It shows along Figure 7 A schematic cross-sectional view of an example of a section taken by line AA;

[0025] Figure 8B It shows along Figure 7 A schematic cross-sectional view of an example of a section taken by line BB;

[0026] Figure 8C It shows along Figure 7 A schematic cross-sectional view of an example of a section taken by line CC;

[0027] Figure 8D It shows along Figure 7 A schematic cross-sectional view of an example of a section taken by line DD;

[0028] Figure 9 It shows along Figure 1 A schematic cross-sectional view of an example of a section taken by line AA;

[0029] Figure 10 This is a diagram illustrating the relationship between refractive index and waveguide width for Si waveguides, SiN waveguides, and signal electrodes;

[0030] Figure 11 This is a diagram illustrating an example of the comparison results between the optical path length and electrode length of the folded portion in the first embodiment, the first comparative example, and the second comparative example.

[0031] Figure 12 This is a perspective view showing an example of a first folded waveguide and a second folded waveguide at the intersection of the optical modulator of the second embodiment;

[0032] Figure 13 This is a schematic cross-sectional view showing an example of a folded portion of the optical modulator according to the second embodiment;

[0033] Figure 14 This is a schematic plan view showing an example of a first-stage conversion unit and a second-stage conversion unit on the first input side in a first folded waveguide;

[0034] Figure 15A It shows along Figure 14 A schematic cross-sectional view of an example of a section taken by line AA;

[0035] Figure 15B It shows along Figure 14 A schematic cross-sectional view of an example of a section taken by line BB;

[0036] Figure 15C It shows along Figure 14 A schematic cross-sectional view of an example of a section taken by line CC;

[0037] Figure 15D It shows along Figure 14 A schematic cross-sectional view of an example of a section taken by line DD;

[0038] Figure 16 This is a schematic plan view illustrating an example of an optical modulator according to a third embodiment;

[0039] Figure 17 It shows along Figure 16 A schematic cross-sectional view of an example of a section taken by line AA;

[0040] Figure 18 This is a schematic plan view illustrating an example of an optical modulator according to the fourth embodiment;

[0041] Figure 19 This is a diagram illustrating an example of the DP-IQ modulator of the fifth embodiment;

[0042] Figure 20 This is a diagram illustrating an example of an optical transceiver according to the sixth embodiment;

[0043] Figure 21 This is a schematic cross-sectional view showing an example of an optical transceiver;

[0044] Figure 22A This is a schematic cross-sectional view showing an example of a Si photonic substrate after a first forming process;

[0045] Figure 22B This is a schematic cross-sectional view showing an example of a Si photonic substrate after a second forming process;

[0046] Figure 22C This is a schematic cross-sectional view showing an example of a Si photonic substrate after the first removal process;

[0047] Figure 22D This is a schematic cross-sectional view showing an example of a Si photonic substrate after a third forming process;

[0048] Figure 22E This is a schematic cross-sectional view showing an example of a Si photonic substrate after the second removal process;

[0049] Figure 22F This is a schematic cross-sectional view showing an example of a Si photonic substrate after a fourth forming process;

[0050] Figure 22G This is a schematic cross-sectional view showing an example of a Si photonic substrate after the fifth forming process;

[0051] Figure 22H This is a schematic cross-sectional view showing an example of a Si photonic substrate after the sixth forming process;

[0052] Figure 22I This is a schematic cross-sectional view showing an example of a Si photonic substrate processed by the seventh forming process;

[0053] Figure 23 This is a diagram illustrating an example of the optical module of the seventh embodiment;

[0054] Figure 24 This is a diagram illustrating an example of an optical transceiver according to the eighth embodiment;

[0055] Figure 25 This is a schematic plan view showing an example of a conventional optical modulator; and

[0056] Figure 26 This is a diagram illustrating an example of the relationship between the refractive index and waveguide width of a Si waveguide and a signal electrode. Detailed Implementation

[0057] Preferred embodiments of the present invention will be described with reference to the accompanying drawings. However, the present invention is not limited to the embodiments described below. Furthermore, the embodiments described below can be appropriately combined as long as no contradiction arises.

[0058] (a) First embodiment

[0059] Figure 1 This is a schematic plan view showing an example of the optical modulator 1 according to the first embodiment. Figure 1The optical modulator 1 shown is a Mach-Zehnder type thin-film LN optical modulator chip mounted on a Si photonic substrate 2. The optical modulator 1 includes the Si photonic substrate 2, an input / output multimode interferometer (MMI) 3, a modulator body 4, and a folded portion 5. The input / output MMI 3 includes an input waveguide 6 that inputs input light to the modulator body 4, and an output waveguide 7 that outputs signal light from the modulator body 4. The modulator body 4 is a modulation operation unit of the Mach-Zehnder type modulator that performs optical modulation by applying a voltage to the thin-film LN waveguide. The folded portion 5 is the folded portion of the arm waveguide of the optical modulator 1.

[0060] The optical modulator 1 includes 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 splits the signal light from the input waveguide 6, outputting the split light to the first input waveguide 21A and the second input waveguide 31A. The first waveguide 20 is disposed on the Si substrate 71 and connected to one output terminal of the first coupler 10. The second waveguide 30 is disposed on the Si substrate 71 and connected to the other output terminal of the first coupler 10.

[0061] The second coupler 40 is disposed on the Si substrate 71 and couples the signal light from the first output waveguide 25 disposed in the output stage of the first waveguide 20 and the signal light from the second output waveguide 35 disposed in the output stage of the second waveguide 30, and outputs the coupled signal light to the output waveguide 7. The electrode 50 is a GSG electrode that applies electrical signals to the first waveguide 20 and the second waveguide 30.

[0062] The first waveguide 20 includes a first input waveguide 21, a first input side arm waveguide 22, a first folded waveguide 23, a first output side arm waveguide 24, a first output waveguide 25, a first input side modulation unit conversion unit 26, and a first output side modulation unit conversion unit 27. For example, the first input waveguide 21 is a Si waveguide connected between the first coupler 10 and the first input side arm waveguide 22.

[0063] The first input-side waveguide 22 is a straight-arm waveguide made of a high-EO material such as thin-film LN and connected between the first input waveguide 21 and the first folded waveguide 23. The first folded waveguide 23 is a waveguide with a folded structure and connected between the first input-side waveguide 22 and the first output-side waveguide 24. The first output-side waveguide 24 is a straight-arm waveguide made of a high-EO material such as thin-film LN and connected between the first folded waveguide 23 and the first output waveguide 25.

[0064] The first input-side modulation unit conversion unit 26 includes an output terminal of the first input waveguide 21 and an input terminal of the first input-side arm waveguide 22, and allows signal light to be converted between the first input waveguide 21 and the first input-side arm waveguide 22. Furthermore, the first input-side modulation unit conversion unit 26 includes an output terminal of the first input-side arm waveguide 22 and an input terminal of the first folded waveguide 23, and allows signal light to be converted between the first input-side arm waveguide 22 and the first folded waveguide 23.

[0065] The first output-side modulation unit conversion unit 27 includes an output terminal of the first folded waveguide 23 and an input terminal of the first output-side arm waveguide 24, and allows signal light to be converted between the first folded waveguide 23 and the first output-side arm waveguide 24. The first output-side modulation unit conversion unit 27 also includes an output terminal of the first output-side arm waveguide 24 and an input terminal of the first output waveguide 25, and allows signal light to be converted between the first output-side arm waveguide 24 and the first output waveguide 25.

[0066] The second waveguide 30 includes a second input waveguide 31, a second input-side arm waveguide 32, a second folded waveguide 33, a second output-side arm waveguide 34, a second output waveguide 35, a second input-side modulation unit conversion unit 36, and a second output-side modulation unit conversion unit 37. For example, the second input waveguide 31 is a Si waveguide connected between the first coupler 10 and the second input-side arm waveguide 32.

[0067] The second input-side waveguide 32 is a straight-arm waveguide made of a high-EO material such as thin-film LN and connected between the second input waveguide 31 and the second folded waveguide 33. The second folded waveguide 33 is a waveguide with a folded structure and connected between the second input-side waveguide 32 and the second output-side waveguide 34. The second output-side waveguide 34 is a straight-arm waveguide made of a high-EO material such as thin-film LN and connected between the second folded waveguide 33 and the second output waveguide 35.

[0068] The second input-side modulation unit conversion unit 36 ​​includes an output terminal of the second input waveguide 31 and an input terminal of the second input-side arm waveguide 32, and allows signal light to be converted between the second input waveguide 31 and the second input-side arm waveguide 32. The second input-side modulation unit conversion unit 36 ​​also includes an output terminal of the second input-side arm waveguide 32 and an input terminal of the second folded waveguide 33, and allows signal light to be converted between the second input-side arm waveguide 32 and the second folded waveguide 33.

[0069] The second output-side modulation unit conversion unit 37 includes an output terminal of the second folded waveguide 33 and an input terminal of the second output-side arm waveguide 34, and allows signal light to be converted between the second folded waveguide 33 and the second output-side arm waveguide 34. The second output-side modulation unit conversion unit 37 also includes an output terminal of the second output-side arm waveguide 34 and an input terminal of the second output waveguide 35, and allows signal light to be converted between the second output-side arm waveguide 34 and the second output waveguide 35.

[0070] The first input-side waveguide 22, the second input-side waveguide 32, the first output-side waveguide 24, and the second output-side waveguide 34 are waveguides comprising a material (such as LN) having high EO characteristics compared to the Si substrate 71. Furthermore, the first folded waveguide 23 and the second folded waveguide 33 are waveguides comprising a material (such as SiN) having a low refractive index compared to the Si substrate 71.

[0071] The first waveguide 20 includes a first input-side arm waveguide 22 located on the outer periphery of the fold, a first folded waveguide 23 located on the outer periphery of the fold, and a first output-side arm waveguide 24 located on the inner periphery of the fold.

[0072] The second waveguide 30 includes a second input-side arm waveguide 32 located on the inner periphery of the fold, a second folded waveguide 33 located on the inner periphery of the fold, and a second output-side arm waveguide 34 located on the outer periphery of the fold.

[0073] Electrode 50 is an electrode with a GSG structure and includes a signal electrode 51, a first ground electrode 52, and a second ground electrode 53. Signal electrode 51 includes an input signal electrode 51A, an output signal electrode 51B, and a folded signal electrode 51C. Input signal electrode 51A is disposed between the first input-side waveguide 22 and the second input-side waveguide 32, and is electrically connected to the folded signal electrode 51C. Output signal electrode 51B is disposed between the first output-side waveguide 24 and the second output-side waveguide 34, and is electrically connected to the folded signal electrode 51C. Folded signal electrode 51C is disposed between the first folded waveguide 23 and the second folded waveguide 33, and electrically connects the input signal electrode 51A and the output signal electrode 51B.

[0074] The first ground electrode 52 includes a first input-side ground electrode 52A, a first output-side ground electrode 52B, and a first folded ground electrode 52C located on the outer periphery. The first input-side ground electrode 52A is disposed near the side surface of the first input-side arm waveguide 22 on the outer periphery so as to face the input signal electrode 51A, and is electrically connected to the first folded ground electrode 52C. The first output-side ground electrode 52B is disposed near the side surface of the second output-side arm waveguide 34 on the outer periphery so as to face the output signal electrode 51B, and is electrically connected to the first folded ground electrode 52C. The first folded ground electrode 52C is electrically connected between the first input-side ground electrode 52A and the first output-side ground electrode 52B.

[0075] The second ground electrode 53 includes a second input-side ground electrode 53A located on the inner periphery, a second output-side ground electrode 53B located on the inner periphery, and a second folded ground electrode 53C located on the inner periphery. The second input-side ground electrode 53A is arranged near the side surface of the second input-side arm waveguide 32 located on the inner periphery so as to face the input signal electrode 51A, and the second input-side ground electrode 53A is electrically connected to the second folded ground electrode 53C located on the inner periphery. The second output-side ground electrode 53B is arranged near the side surface of the first output-side arm waveguide 24 located on the inner periphery so as to face the output signal electrode 51B, and the second output-side ground electrode 53B is electrically connected to the second folded ground electrode 53C. The second folded ground electrode 53C is electrically connected between the second input-side ground electrode 53A and the second output-side ground electrode 53B.

[0076] Figure 2This is a schematic plan view showing an example of the folded portion 5. The folded portion 5 includes a first folded waveguide 23 and a second folded waveguide 33. The first folded waveguide 23 includes a first input-side high-refractive-index waveguide 61A, a first output-side high-refractive-index waveguide 62A, a first low-refractive-index waveguide 63A, a first input-side first-stage conversion unit 64A1, and a first output-side first-stage conversion unit 64A2. The first input-side high-refractive-index waveguide 61A is formed on a first layer 70A on a Si substrate 71, connected to the first input-side arm waveguide 22, and has a core made of, for example, Si and a small curvature. The first output-side high-refractive-index waveguide 62A is formed on the first layer 70A, connected to the first output-side arm waveguide 24, and has a core made of, for example, Si and a small curvature. The first low-refractive-index waveguide 63A is a straight waveguide formed on a second layer 70B on a Si substrate 71, connecting the first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A, and having a core made of, for example, SiN. The second layer 70B is a high-refractive-index layer. Meanwhile, since the first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A are configured with Si waveguides, they can be folded shortly by bending with a small curvature. The first folded waveguide 23 connects the first input-side arm waveguide 22 and the first output-side arm waveguide 24. The first folded waveguide 23 includes: a first input-side high-refractive-index waveguide 61A, which is connected to the first input-side arm waveguide 22 and serves as a first input-side folded waveguide located on the outer periphery of the fold; and a first output-side high-refractive-index waveguide 62A, which is connected to the first output-side arm waveguide 24 and serves as a first output-side folded waveguide located on the inner periphery of the fold.

[0077] The first input-side first-stage conversion unit 64A1 includes an output terminal of a first input-side high-refractive-index waveguide 61A and an input terminal of a first low-refractive-index waveguide 63A, and allows signal light to be converted between the first input-side high-refractive-index waveguide 61A and the first low-refractive-index waveguide 63A. The first output-side first-stage conversion unit 64A2 includes an output terminal of a first low-refractive-index waveguide 63A and an input terminal of a first output-side high-refractive-index waveguide 62A, and allows signal light to be converted between the first low-refractive-index waveguide 63A and the first output-side high-refractive-index waveguide 62A.

[0078] The second folded waveguide 33 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 conversion unit 64B1, and a second output-side first-stage conversion unit 64B2. The second input-side high-refractive-index waveguide 61B is formed on a first layer 70A on a Si substrate 71, connected to the second input-side arm waveguide 32, and has a core made of, for example, Si and a small curvature. The second output-side high-refractive-index waveguide 62B is formed on the first layer 70A, connected to the second output-side arm waveguide 34, and has a core made of, for example, Si and a small curvature. The second low-refractive-index waveguide 63B is formed on a second layer 70B on a Si substrate 71, connected between the second input-side high-refractive-index waveguide 61B and the second output-side high-refractive-index waveguide 62B, and has a core made of, for example, SiN and is a straight waveguide. Meanwhile, the second input-side high-refractive-index waveguide 61B and the second output-side high-refractive-index waveguide 62B are configured with Si waveguides, thus enabling short-distance folding by bending with a small curvature. A second folded waveguide 33 connects the second input-side arm waveguide 32 and the second output-side arm waveguide 34. The second folded waveguide 33 includes: a second input-side high-refractive-index waveguide 61B, which is connected to the second input-side arm waveguide 32 and serves as a second input-side folded waveguide located on the outer periphery of the fold; and a second output-side high-refractive-index waveguide 62B, which is connected to the second output-side arm waveguide 34 and serves as a second output-side folded waveguide located on the inner periphery of the fold.

[0079] The second input-side first-stage conversion unit 64B1 includes an output terminal of the second input-side high-refractive-index waveguide 61B and an input terminal of the second low-refractive-index waveguide 63B, and allows signal light to be converted between the second input-side high-refractive-index waveguide 61B and the second low-refractive-index waveguide 63B. The second output-side first-stage conversion unit 64B2 includes an output terminal of the second low-refractive-index waveguide 63B and an input terminal of the second output-side high-refractive-index waveguide 62B, and allows signal light to be converted between the second low-refractive-index waveguide 63B and the second output-side high-refractive-index waveguide 62B.

[0080] Figure 3 This is a schematic cross-sectional view showing an example of optical modulator 1. Figure 3The optical modulator 1 shown includes 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 optical modulator 1 includes a first layer 70A, a second layer 70B, and a third layer 70C. The first layer 70A is disposed between the upper cladding layer 73 and the lower cladding layer 72. The second layer 70B is disposed in the lower cladding layer 72 near the upper cladding layer 73. The third layer 70C is disposed in the lower cladding layer 72 near the Si substrate 71. For example, the first layer 70A serves as the core layer of the first output-side waveguide 24. For example, the second layer 70B serves as the 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.

[0081] The second layer 70B is a high-refractive-index layer. For example, the third layer 70C is used as the core layer of the first low-refractive-index waveguide 63A.

[0082] The third layer, 70C, is the first-level low-refractive-index layer.

[0083] Figure 4 This is a perspective view showing an example of the first folded waveguide 23 and the second folded waveguide 33 at the intersection. Figure 4 As shown in the diagram, the first low-refractive-index waveguide 63A, arranged on the third layer 70C and included in the first folded waveguide 23, and the second output-side high-refractive-index waveguide 62B, arranged on the second layer 70B and included in the second folded waveguide 33, intersect each other in a three-dimensional manner in different layers. As a result, the first folded waveguide 23 and the second folded waveguide 33 have low losses and are able to prevent crosstalk between the first folded waveguide 23 and the second folded waveguide 33.

[0084] Figure 5 This is a schematic plan view showing a portion of either the first input-side modulation unit conversion unit 26 or the second input-side modulation unit conversion unit 36. For ease of explanation, either the first input-side modulation unit conversion unit 26 or the second input-side modulation unit conversion unit 36 ​​is shown. However, the first output-side modulation unit conversion unit 27 and the second output-side modulation unit conversion unit 37 have substantially the same structure; therefore, identical components and identical operations are indicated by the same reference numerals, and descriptions of identical components and identical operations will be omitted. Figure 5The first input-side modulation unit conversion unit 26 shown includes an output terminal of a first input waveguide 21 and an input terminal of a first input-side arm waveguide 22. The first input waveguide 21 has a tapered structure, wherein the waveguide width gradually decreases from the input terminal of the first input-side arm waveguide 22 toward the middle portion of the first input-side arm waveguide 22. Furthermore, the second input-side modulation unit conversion unit 36 ​​includes an output terminal of a second input waveguide 31 and an input terminal of a second input-side arm waveguide 32. The second input waveguide 31 has a tapered structure, wherein the waveguide width gradually decreases from the input terminal of the second input-side arm waveguide 32 toward the middle portion of the second input-side arm waveguide 32.

[0085] Figure 6A It shows along Figure 5 The diagram shows a schematic cross-sectional view of an example of a section taken by line AA. Figure 6A A portion of the first input-side modulation unit conversion unit 26 shown includes 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. For example, the second layer 70B in the lower cladding layer 72 is a layer made of 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.

[0086] Figure 6B It shows along Figure 5 The diagram shows a schematic cross-sectional view of an example of a section taken by line BB. Figure 6B A portion of the first input-side modulation unit conversion unit shown includes a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. The second layer 70B in the lower cladding layer 72 is the core layer of the first input waveguide 21, forming the base of a tapered structure with a relatively large width. The first layer 70A in the upper cladding layer 73 serves as the core layer of the first input-side arm waveguide 22. Meanwhile, for example, the first input-side arm waveguide 22 is a thin-film LN waveguide with a ribbed structure.

[0087] Figure 6C It shows along Figure 5 The diagram shows an example of a cross-sectional portion taken by line CC. Figure 6C The input-side modulation unit conversion unit shown includes a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. The second layer 70B in the lower cladding layer 72 is the core layer of the first input waveguide 21, is the end of the tapered structure, and is connected to... Figure 6B The portion shown has a smaller core width compared to the others. The first layer 70A in the upper cladding layer 73 serves as the core layer of the first input-side arm waveguide 22.

[0088] Figure 6D It shows along Figure 5The diagram shows a schematic cross-sectional view of an example of a section taken by line DD. Figure 6D A portion of the first input-side modulation unit conversion unit 26 shown includes a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. In this portion, the second layer 70B is not present in the lower cladding layer 72. The first layer 70A in the upper cladding layer 73 serves as the core layer of the first input-side arm waveguide 22.

[0089] Meanwhile, the first input-side modulation unit conversion unit 26 includes an output terminal of the first input-side arm waveguide 22 and an input terminal of the first input-side high-refractive-index waveguide 61A. The input terminal of the first input-side high-refractive-index waveguide 61A has a tapered structure, wherein the waveguide width gradually decreases from the output terminal of the first input-side arm waveguide 22 toward the middle portion.

[0090] Figure 7 This is a schematic plan view showing an example of the first input-side first-stage conversion unit 64A1. For ease of explanation, the first input-side first-stage conversion unit 64A1 in the first folded waveguide 23 is also shown. However, the second input-side first-stage conversion unit 64B1 in the second folded waveguide 33 has the same structure; therefore, the same components and the same operations are indicated by the same reference numerals, and descriptions of the same components and the same operations will be omitted. Figure 7 The first-input-side first-stage conversion unit 64A1 shown includes an output terminal of a first-input-side high-refractive-index waveguide 61A and an input terminal of a first-input-side low-refractive-index waveguide 63A. The output terminal of the first-input-side high-refractive-index waveguide 61A has a tapered structure, wherein the waveguide width gradually decreases from the input terminal of the first-input-side low-refractive-index waveguide 63A toward the middle portion. The input terminal of the first-input-side low-refractive-index waveguide 63A has a tapered structure, wherein the waveguide width gradually increases from the input terminal 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-input-side low-refractive-index waveguide 63A, and enables optical conversion with low loss.

[0091] Figure 8A It shows along Figure 7 The diagram shows a schematic cross-sectional view of an example of a section taken by line AA. Figure 8A The first folded waveguide 23 shown includes a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. For example, the second layer 70B in the lower cladding layer 72 is a layer made of Si and serves as the core layer of the first input-side high-refractive-index waveguide 61A. In other words, the first input-side high-refractive-index waveguide 61A is a Si waveguide with a channel structure.

[0092] Figure 8B It shows along Figure 7The diagram shows a schematic cross-sectional view of an example of a section taken by line BB. Figure 8B The first input-side first-stage conversion unit 64A1 shown 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 serves as the core layer of the first input-side high-refractive-index waveguide 61A. A third layer 70C in the lower cladding layer 72 serves as the core layer of the first low-refractive-index waveguide 63A. In other words, for example, the first low-refractive-index waveguide 63A is a SiN waveguide with a channel structure. Here, the first input-side high-refractive-index waveguide 61A has a wider width, and the first low-refractive-index waveguide 63A has a narrower width.

[0093] Figure 8C It shows along Figure 7 The diagram shows an example of a cross-sectional portion taken by line CC. Figure 8C The first input-side first-stage conversion unit 64A1 shown includes a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. The second layer 70B in the lower cladding layer 72 serves as the core layer of the first input-side high-refractive-index waveguide 61A. The third layer 70C in the lower cladding layer 72 serves as the core layer of the first low-refractive-index waveguide 63A. Here, the first input-side high-refractive-index waveguide 61A has a narrower width, and the first low-refractive-index waveguide 63A has a wider width.

[0094] Figure 8D It shows along Figure 7 The diagram shows a schematic cross-sectional view of an example of a section taken by line DD. Figure 8D The first folded waveguide 23 shown includes a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. The third layer 70C in the lower cladding layer 72 serves as the core layer of the first low-refractive-index waveguide 63A.

[0095] Meanwhile, the following example is shown, wherein the first input-side first-stage conversion unit 64A1 includes an output terminal of a first input-side high-refractive-index waveguide 61A and an input terminal of a first low-refractive-index waveguide 63A, and the first output-side first-stage conversion unit 64A2 includes an output terminal of a first low-refractive-index waveguide 63A and an input terminal of a first output-side high-refractive-index waveguide 62A. In this case, the input terminal of the first output-side high-refractive-index waveguide 62A has a tapered structure, wherein the waveguide width gradually decreases from the output terminal of the first low-refractive-index waveguide 63A toward the middle portion. The output terminal of the first low-refractive-index waveguide 63A has a tapered structure, wherein the waveguide width gradually increases from the output terminal and remains constant in the middle portion.

[0096] Figure 9 It shows along Figure 1 The diagram shows a schematic cross-sectional view of an example of a section taken by line AA. Figure 9The modulator body 4 shown includes a Si substrate 71, a lower cladding layer 72, an upper cladding layer 73, and electrodes 50. The modulator body 4 includes a first input-side waveguide 22 and a second input-side waveguide 32 in the first layer 70A of the upper cladding layer 73, and a first output-side waveguide 24 and a second output-side waveguide 34 in the first layer 70A. The electrodes 50 arranged beside the upper cladding layer 73 include an input signal electrode 51A, a first input-side ground electrode 52A, a second input-side ground electrode 53A, an output signal electrode 51B, a first output-side ground electrode 52B, and a second output-side ground electrode 53B.

[0097] The input signal electrode 51A is arranged between the first input-side waveguide 22 and the second input-side waveguide 32. The first input-side ground electrode 52A is arranged facing the input signal electrode 51A across the first input-side waveguide 22. The second input-side ground electrode 53A is arranged on the opposite side of the input signal electrode 51A across the second input-side waveguide 32.

[0098] The output signal electrode 51B is arranged between the first output-side waveguide 24 and the second output-side waveguide 34. The first output-side ground electrode 52B is arranged facing the output signal electrode 51B across the first output-side waveguide 24. The second output-side ground electrode 53B is arranged on the opposite side of the output signal electrode 51B across the second output-side waveguide 34.

[0099] The polarization direction X of the modulator body 4 is the same in both the forward and reverse paths of the fold. The first input-side waveguide 22 modulates the signal light from the input signal electrode 51A to the first input-side ground electrode 52A based on the electrical signal in the reverse direction of polarization direction X. The second input-side waveguide 32 modulates the signal light from the input signal electrode 51A to the second input-side ground electrode 53A based on the electrical signal in the forward direction of polarization direction X.

[0100] In contrast, the first output-side waveguide 24 modulates the signal light from the output signal electrode 51B to the first output-side ground electrode 52B based on an electrical signal in the opposite direction of polarization X. The second output-side waveguide 34 modulates the signal light from the output signal electrode 51B to the second output-side ground electrode 53B based on an electrical signal in the positive direction of polarization X.

[0101] In other words, the first input-side waveguide 22 and the first output-side waveguide 24 modulate the signal light based on electrical signals in the same reverse direction. The second input-side waveguide 32 and the second output-side waveguide 34 modulate the signal light based on electrical signals in the same forward direction. Applying electrical signals in the same direction in both the input-side forward path and the output-side reverse path improves modulation efficiency. Furthermore, compared to the first input-side waveguide 22 and the first output-side waveguide 24, the second input-side waveguide 32 and the second output-side waveguide 34 enable push-pull operations with phase changes in opposite directions.

[0102] Figure 10 This diagram illustrates an example of the relationship between refractive index and waveguide width for Si waveguides, SiN waveguides, and signal electrodes. 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. For example, the first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A are Si waveguides. For example, the first low-refractive-index waveguide 63A is a SiN waveguide. Figure 10 As shown, the refractive index of the SiN waveguide is 1.6 to 1.9, which is close to the refractive index of the electrical signal; therefore, a portion of the first folded waveguide 23 is configured using 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 roughly achieved, thereby eliminating or reducing the need for delay electrodes used for velocity matching. While the first folded waveguide 23 has been described, the same effect is achieved with respect to the second folded waveguide 33.

[0103] Figure 11 This diagram illustrates an example of the comparison results between the optical path length and electrode length of the folded portion 5 in the first embodiment, the first comparative example, and the second comparative example. 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 is a Si bent 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 Si bent waveguide including a bent portion R and portions before and after the bent portion R. The first low-refractive-index waveguide 63A is a SiN straight waveguide including a straight portion.

[0104] Furthermore, as a first comparative example, the first folded waveguide includes 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 using a Si waveguide. For ease of explanation, it is assumed that the first folded waveguide of the first comparative example has the same configuration as the first folded waveguide 23 of the first embodiment, but the difference is that the first folded waveguide of the first comparative example is configured using a Si waveguide. The input-side waveguide is a Si curved waveguide including a curved portion and portions before and after the curved portion. The output-side waveguide is a Si curved waveguide including a curved portion and portions before and after the curved portion. The straight waveguide is a Si straight waveguide including a straight portion.

[0105] Furthermore, as a second comparative example, the first folded waveguide includes an input-side waveguide, an output-side waveguide, and a straight waveguide connecting the input-side and output-side waveguides, and is configured using a SiN waveguide. For ease of explanation, it is assumed that the first folded waveguide of the second comparative example has the same configuration as the first folded waveguide 23 of the first embodiment, but the difference is that the first folded waveguide of the second comparative example is configured using a SiN waveguide. The input-side waveguide is a SiN bent waveguide including a bent portion and portions before and after the bent portion. The output-side waveguide is a SiN bent waveguide including a bent portion and portions before and after the bent portion. The straight waveguide is a SiN straight waveguide including a straight portion.

[0106] First, in the first folded waveguide of the first comparative example, when the waveguide length L of the portion before and after the bend in the input-side waveguide is set to 20 micrometers (μm), the optical path length of the portion before and after the bend in the input-side waveguide is 80 μm because the refractive index of the Si waveguide is approximately 4. The optical path length of the portion before and after the bend in the output-side waveguide is also 80 μm. In the case of the Si waveguide, a low-loss bend waveguide can be achieved even with a small radius of curvature; therefore, the radius of curvature of the bend in the input-side waveguide can be reduced to, for example, 10 μm. The refractive index of the Si waveguide is approximately 4, resulting in an optical path length of 63 μm for the bend. The optical path length of the bend in the output-side waveguide is also 63 μm. Furthermore, when the waveguide length of the straight portion is set to 400 μm, the optical path length of the straight portion is 1600 μm because the refractive index of the Si waveguide is approximately 4. As a result, the total optical path length of the first folded waveguide is 1743 μm. To achieve velocity matching between the optical and electrical signals, the product of the refractive index and length (n×L) between the optical waveguide and the electrical wiring needs to be matched. In the configuration of the first comparative example, the refractive index of the electrical signal is approximately 1.9; therefore, setting the electrical wiring length to 917 μm is sufficient to achieve velocity matching. When the first folded waveguide is configured using only Si waveguides, the refractive index of the first folded waveguide increases, and the optical path length increases, so the electrical wiring length needs to be increased in proportion to the refractive index to achieve velocity matching and is set to approximately 0.9 mm.

[0107] In the second comparative example, when the waveguide length L of the portion before and after the bend in the input-side waveguide is set to 20 μm, the optical path length of the portion before and after the bend in the SiN waveguide is 38 μm due to the refractive index of the SiN waveguide being 1.9. The optical path length of the portion before and after the bend in the output-side waveguide is also 38 μm. In the SiN waveguide, the refractive index of the core is relatively low; therefore, in order to bend the waveguide with low loss, the radius of curvature needs to be increased, so the radius of curvature of the bend in the input-side waveguide needs to be set to, for example, 60 μm or greater. The refractive index of the SiN waveguide is 1.9, so the optical path length of the bend is 179 μm. The optical path length of the bend in the output-side waveguide is also 179 μm. Furthermore, when the waveguide length of the straight portion is set to 400 μm, the optical path length of the straight portion is 760 μm due to the refractive index of the SiN waveguide being 1.9. As a result, the optical path length of the first folded waveguide is 977 μm. Similar to the first comparative example, to achieve velocity matching between the optical and electrical signals, the product of the refractive index and length (n×L) between the optical waveguide and the electrical wiring needs to be matched. In the configuration of the second comparative example, the refractive index of the electrical signal is approximately 1.9; therefore, setting the electrical wiring length to 514 μm is sufficient to achieve velocity matching. When the first folded waveguide is configured using only SiN waveguides, the refractive index of the first folded waveguide decreases, but compared to the Si waveguide, the curvature of the bent waveguide needs to be increased from 10 μm to 60 μm. As a result, the actual length of the waveguide increases, thus increasing the electrical wiring length to approximately 515 μm.

[0108] In contrast, in the first folded waveguide 23 of the first embodiment, when the waveguide length L of the portion before and after the curved portion R of the first input-side high-refractive-index waveguide 61A is set to 20 μm, the optical path length of the portion before and after the curved portion R of the first input-side high-refractive-index waveguide 61A is 80 μm due to the refractive index of the Si waveguide being 4. The optical path length of the portion before and after the curved portion R of the first output-side high-refractive-index waveguide 62A is also 80 μm. In the Si waveguide, the radius of curvature of the curved portion R of the first input-side high-refractive-index waveguide 61A can be reduced to 10 μm, and the optical path length of the curved portion R is 63 μm due to the refractive index of the Si waveguide being approximately 4. The optical path length of the curved portion R of the first output-side high-refractive-index waveguide 62A is also 63 μm. In contrast, the waveguide of the straight portion is a SiN waveguide with a low refractive index of 1.9. Therefore, when the waveguide length is set to 400 μm, the optical path length of the straight portion is 760. As a result, the total optical path length of the first folded waveguide 23 is 903. Similar to the first and second comparative examples, to achieve speed matching between the optical and electrical signals, it is necessary to match the product (n×L) of the refractive index and length between the optical waveguide and the electrical wiring. In the configuration of the first embodiment, the refractive index of the electrical signal is approximately 1.9; therefore, setting the electrical wiring length to 475 μm is sufficient to achieve speed matching. In this embodiment, the bent waveguide portions of the first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A are configured with Si waveguides to reduce the actual length, and the straight portion in which a long waveguide is drawn is configured with a SiN waveguide as the first low-refractive-index waveguide 63A. As a result, due to the effect of reducing the optical path length by reducing the actual length of the Si waveguide and the refractive index of the SiN waveguide, the electrode length can be reduced to 475 μm, thereby reducing the electrode length and chip size.

[0109] The folded signal electrode 51C of the first embodiment has an electrode length of 475 μm, the folded signal electrode of the first comparative example has an electrode length of 917 μm, and the folded signal electrode of the second comparative example has an electrode length of 514 μm. Therefore, the electrode length of the folded signal electrode 51C of the first embodiment is reduced, thereby reducing the chip size of the optical modulator 1 compared to the electrode lengths of the first and second comparative examples.

[0110] In the first embodiment, the first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A of the first folded waveguide 23 are configured with Si waveguides that reduce curvature, and the first low-refractive-index waveguide 63A after bending is configured with SiN waveguides that facilitate velocity matching. The length of the Si waveguide portions of the first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A can be reduced, while the length of the SiN waveguide portion of the first low-refractive-index waveguide 63A can be increased, thereby reducing the overall waveguide length and the optical path length. As a result, the electrode length of the folded signal electrode 51C, which needs to be matched with the optical path length of the first folded waveguide 23, can be reduced, thereby reducing the length of the delay waveguide, which leads to a reduction in the size of the folded portion.

[0111] 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 configured with Si waveguides that reduce curvature, and the bent second low-refractive-index waveguide 63B is configured with SiN waveguides that facilitate velocity matching. The lengths of the Si waveguide portions of the second input-side high-refractive-index waveguide 61B and the second output-side high-refractive-index waveguide 62B can be reduced, while the length of the SiN waveguide portion of the second low-refractive-index waveguide 63B can be increased, thus reducing the overall waveguide length and the optical path length. As a result, the electrode length of the folded signal electrode 51C, which needs to be matched with the optical path length of the second folded waveguide 33, can be reduced, thereby reducing the length of the delay waveguide, which leads to a reduction in the size of the folded portion. In other words, velocity matching can be ensured and the chip size of the optical modulator 1 can be reduced.

[0112] Simultaneously, the lengths of the Si waveguide and SiN waveguide are appropriately adjusted by the left and right arms to make the optical path lengths of the first folded waveguide 23 and the second folded waveguide 33 equal. For example, methods to make the lengths of the Si waveguide or SiN waveguide equal are known, but there are no specific restrictions on the method, as long as the optical path lengths are equal.

[0113] In the optical modulator 1 of the first embodiment, the LN waveguide is described as an example of a waveguide made of a high EO material, but the embodiment is not limited to this example, and the same effect can be achieved by waveguides made of high EO materials such as BaTiO3, PLZ or PZT and with a Pockel factor of 10 pm / V or greater.

[0114] Meanwhile, for ease of explanation, Si waveguides are described as examples of high refractive index waveguides and SiN waveguides are described as examples of low refractive index waveguides, but the implementation is not limited to these examples and can be modified appropriately.

[0115] In the first embodiment, the first folded waveguide 23 and the second folded waveguide 33 have been described as having a first output-side high-refractive-index waveguide 62A and a first low-refractive-index waveguide 63A intersecting each other in a three-dimensional manner. However, the embodiment is not limited to this example, and a different embodiment will be described below as a second embodiment. Meanwhile, components identical to those in the optical modulator 1 of the first embodiment are indicated by the same reference numerals, and descriptions of identical components and operations will be omitted.

[0116] (b) Second Embodiment

[0117] Figure 12 This is a perspective view showing an example of a first folded waveguide 23A and a second folded waveguide 33A of an optical modulator 1A according to a second embodiment. The optical modulator 1A of the first embodiment and the optical modulator 1A of the second embodiment differ from each other in that the first folded waveguide 23A and the second folded waveguide 33A are configured to convert the signal light by using three different waveguides, and the distance between the high-refractive-index waveguide and the low-refractive-index waveguide at the intersection of the waveguides is increased.

[0118] The first folded waveguide 23A includes a first input-side high-refractive-index waveguide 61A, a first output-side high-refractive-index waveguide 62A, a first input-side first-stage refractive-index waveguide 63A1, a first output-side first-stage refractive-index waveguide 63A2, and a first second-stage low-refractive-index waveguide 63A3. The first folded waveguide 23A includes a first input-side first-stage conversion unit 64A11, a first input-side second-stage conversion unit 64A31, a first output-side second-stage conversion unit 64A41, and a first output-side first-stage conversion unit 64A21. For example, the first input-side high-refractive-index waveguide 61A is a Si waveguide using the second layer 70B of the lower cladding layer 72 as the core layer and connected to the output end of the first input-side arm waveguide 22. For example, the first output-side high-refractive-index waveguide 62A is a Si waveguide using the second layer 70B as the core layer and connected to the input end of the first output-side arm waveguide 24.

[0119] Figure 13 This is a schematic cross-sectional view showing an example of the folded portion 5A of the optical modulator 1A according to the second embodiment. For example, the first-stage refractive index waveguide 63A1 on the first input side is a SiN waveguide that uses the third layer 70C of the lower cladding layer 72 as the core layer and is indirectly connected to the output end of the high refractive index waveguide 61A on the first input side. For example, the first-stage refractive index waveguide 63A2 on the first output side is a SiN waveguide that uses the third layer 70C as the core layer and is indirectly connected to the input end of the high refractive index waveguide 62A on the first output side.

[0120] For example, the first and second stage low-refractive-index waveguides 63A3 are SiN waveguides that use the fourth layer 70D of the lower cladding layer 72 as the core layer and indirectly connect the first input-side first-stage refractive-index waveguide 63A1 and the first output-side first-stage refractive-index waveguide 63A2. The fourth layer 70D is the second-stage low-refractive-index layer.

[0121] The first input-side first-stage conversion unit 64A11 includes an output terminal of the first input-side high-refractive-index waveguide 61A and an input terminal of the first input-side first-stage refractive-index waveguide 63A1, and allows signal light to be converted between the first input-side high-refractive-index waveguide 61A and the first input-side first-stage refractive-index waveguide 63A1. The first output-side first-stage conversion unit 64A21 includes an output terminal of the first output-side first-stage refractive-index waveguide 63A2 and an input terminal of the first output-side high-refractive-index waveguide 62A, and allows signal light to be converted between the first output-side first-stage refractive-index waveguide 63A2 and the first output-side high-refractive-index waveguide 62A.

[0122] The first input-side second-stage conversion unit 64A31 includes an output terminal of the first input-side first-stage refractive index waveguide 63A1 and an input terminal of the first and second-stage low-refractive index waveguide 63A3, and allows signal light to be converted between the first input-side first-stage refractive index waveguide 63A1 and the first and second-stage low-refractive index waveguide 63A3. The first output-side second-stage conversion unit 64A41 includes an output terminal of the first and second-stage low-refractive index waveguide 63A3 and an input terminal of the first output-side first-stage refractive index waveguide 63A2, and allows signal light to be converted between the first and second-stage low-refractive index waveguide 63A3 and the first output-side first-stage refractive index waveguide 63A2.

[0123] The second folded waveguide 33A includes a second input-side high-refractive-index waveguide 61B, a second output-side high-refractive-index waveguide 62B, a second input-side first-stage low-refractive-index waveguide 63B1, a second output-side first-stage low-refractive-index waveguide 63B2, and a second output-side second-stage low-refractive-index waveguide 63B3. The second folded waveguide 33A also includes a second input-side first-stage conversion unit 64B11, a second input-side second-stage conversion unit 64B31, a second output-side second-stage conversion unit 64B41, and a second output-side first-stage conversion unit 64B21. For example, the second input-side high-refractive-index waveguide 61B is a Si waveguide that uses the second layer 70B of the lower cladding layer 72 as its core layer and is connected to the output end of the second input-side arm waveguide 32. Similarly, the second output-side high-refractive-index waveguide 62B is a Si waveguide that uses the second layer 70B as its core layer and is connected to the input end of the second output-side arm waveguide 34.

[0124] For example, the first-stage low-refractive-index waveguide 63B1 on the second input side is a SiN waveguide that uses the third layer 70C of the lower cladding layer 72 as the core layer and is indirectly connected to the output end of the high-refractive-index waveguide 61B on the second input side. Similarly, the first-stage low-refractive-index waveguide 63B2 on the second output side is a SiN waveguide that uses the third layer 70C as the core layer and is indirectly connected to the input end of the high-refractive-index waveguide 62B on the second output side.

[0125] For example, the second low-refractive-index waveguide 63B3 is a SiN waveguide that uses the fourth layer 70D of the lower cladding layer 72 as the core layer and indirectly connects the first low-refractive-index waveguide 63B1 on the second input side and the first low-refractive-index waveguide 63B2 on the second output side.

[0126] The second input-side first-stage conversion unit 64B11 includes an output terminal of the second input-side high-refractive-index waveguide 61B and an input terminal of the second input-side first-stage low-refractive-index waveguide 63B1, and allows signal light to be converted 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 conversion unit 64B21 includes an output terminal of the second output-side first-stage low-refractive-index waveguide 63B2 and an input terminal of the second output-side high-refractive-index waveguide 62B, and allows signal light to be converted between the second output-side first-stage low-refractive-index waveguide 63B2 and the second output-side high-refractive-index waveguide 62B.

[0127] The second input-side second-stage conversion unit 64B31 includes an input terminal of the second input-side first-stage low-refractive-index waveguide 63B1 and a second-stage low-refractive-index waveguide 63B3, and allows signal light to be converted between the second input-side first-stage low-refractive-index waveguide 63B1 and the second-stage low-refractive-index waveguide 63B3. The second output-side second-stage conversion unit 64B41 includes an output terminal of the second-stage low-refractive-index waveguide 63B3 and an input terminal of the second output-side first-stage low-refractive-index waveguide 63B2, and allows signal light to be converted between the second-stage low-refractive-index waveguide 63B3 and the second output-side first-stage low-refractive-index waveguide 63B2.

[0128] like Figure 13 As shown, the first output-side waveguide 24 is connected to the first output-side high-refractive-index waveguide 62A that spans the second-stage low-refractive-index waveguide 63B3. As a result, compared to the optical modulator 1 of the first embodiment, the distance between the second-stage low-refractive-index waveguide 63B3 and the first output-side high-refractive-index waveguide 62A is increased, thereby preventing losses and crosstalk at the intersection.

[0129] Figure 14This is a schematic plan view showing examples of the first input-side first-stage conversion unit 64A11 and the first input-side second-stage conversion unit 64A31 in the first folded waveguide 23A. For ease of explanation, the first input-side first-stage conversion unit 64A11 and the first input-side second-stage conversion unit 64A31 in the first folded waveguide 23A are shown as examples. However, the second input-side first-stage conversion unit 64B11 and the second input-side second-stage conversion unit 64B31 in the second folded waveguide 33A have the same configuration; therefore, the same components and the same operations are indicated by the same reference numerals, and descriptions of the same components and the same operations will be omitted. Figure 14 The first folded waveguide 23A shown includes a first input-side high-refractive-index waveguide 61A, a first-stage refractive-index waveguide 63A1, and a first- and second-stage low-refractive-index waveguide 63A3. The output end of the first input-side high-refractive-index waveguide 61A has a tapered structure, wherein the waveguide width gradually decreases from the input end of the first-stage refractive-index waveguide 63A1 towards the middle portion. The input end of the first-stage refractive-index waveguide 63A1 has a tapered structure, wherein the waveguide width gradually increases from the input end and remains constant in the middle portion. The output end of the first-stage refractive-index waveguide 63A1 has a tapered structure, wherein the waveguide width gradually increases from the output end and remains constant in the middle portion. The input end of the first- and second-stage low-refractive-index waveguide 63A3 has a tapered structure, wherein the waveguide width gradually increases from the input end and remains constant in the middle portion. As a result, the waveguide width has a tapered structure, thereby enabling optical coupling between the first-stage refractive index waveguide 63A1 and the first and second-stage low-refractive index waveguide 63A3 on the first input side, and achieving optical conversion with low loss.

[0130] Figures 15A to 15D This is a schematic cross-sectional view showing the cross-sectional structure at multiple locations in the second-stage conversion unit 64A31 on the first input side. Figure 15A It shows along Figure 14 The diagram shows a schematic cross-sectional view of an example of a section taken by line AA. Figure 15A A portion of the first folded waveguide 23A shown includes a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. The third layer 70C in the lower cladding layer 72 is, for example, SiN and serves as the core layer of the first-stage refractive index waveguide 63A1 on the first input side. In other words, the first-stage refractive index waveguide 63A1 on the first input side is a SiN waveguide with a channel structure.

[0131] Figure 15B It shows along Figure 14 The diagram shows a schematic cross-sectional view of an example of a section taken by line BB. Figure 15BThe first input-side second-stage conversion unit 64A31 in the first folded waveguide 23A shown 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, for example, SiN and serves as the core layer of the first input-side first-stage refractive index waveguide 63A1. The fourth layer 70D of the lower cladding layer 72 is, for example, SiN and serves as the core layer of the first and second-stage low-refractive-index waveguides 63A3. In other words, the first and second-stage low-refractive-index waveguides 63A3 are SiN waveguides with a channel structure. In this section, the width of the first input-side first-stage refractive-index waveguide 63A1 increases, and the width of the first and second-stage low-refractive-index waveguides 63A3 decreases.

[0132] Figure 15C It shows along Figure 14 The diagram shows an example of a cross-sectional portion taken by line CC. Figure 15C The first input-side second-stage conversion unit 64A31 in the first folded waveguide 23A shown 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, for example, SiN and serves as the core layer of the first input-side first-stage refractive index waveguide 63A1. The fourth layer 70D of the lower cladding layer 72 is, for example, SiN and serves as the core layer of the first and second-stage low-refractive-index waveguides 63A3.

[0133] Figure 15D It shows along Figure 14 The diagram shows a schematic cross-sectional view of an example of a section taken by line DD. Figure 15D A portion of the first folded waveguide 23A shown includes a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. The fourth layer 70D of the lower cladding layer 72 is SiN and serves as the core layer of the first and second stage low refractive index waveguides 63A3.

[0134] The input end of the first output-side high-refractive-index waveguide 62A has a tapered structure, wherein the waveguide width gradually decreases from the output end of the first-stage refractive-index waveguide 63A2 towards the middle portion. The input end of the first-stage refractive-index waveguide 63A2 has a tapered structure, wherein the waveguide width gradually increases from the input end and remains constant in the middle portion. The output end of the first-stage refractive-index waveguide 63A2 has a tapered structure, wherein the waveguide width gradually increases from the output end and remains constant as it approaches the middle portion. The input end of the first and second-stage low-refractive-index waveguide 63A3 has a tapered structure, wherein the waveguide width gradually increases from the input end and remains constant in the middle portion.

[0135] In the second embodiment, 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 are configured using Si waveguides that reduce curvature. Furthermore, the first input-side first-stage refractive-index waveguide 63A1, the first second-stage low-refractive-index waveguide 63A3, and the first output-side first-stage refractive-index waveguide 63A2, after bending, are configured using SiN waveguides that facilitate velocity matching. The length of the Si waveguide portions of the first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A is reduced, while the length of the SiN waveguide portions of the first input-side first-stage refractive-index waveguide 63A1, the first second-stage low-refractive-index waveguide 63A3, and the first output-side first-stage refractive-index waveguide 63A2 is increased. Therefore, the total waveguide length and the optical path length can be reduced. As a result, the electrode length of the folded signal electrode 51C, which needs to be matched with the optical path length of the first folded waveguide 23A, can be reduced, thereby reducing the length of the delay waveguide, which leads to a reduction in the size of the folded portion.

[0136] 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 configured with Si waveguides that reduce curvature. Furthermore, the bent 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 configured with SiN waveguides that facilitate velocity matching. The lengths of 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 reduced, while the lengths of 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 increased. Therefore, the total waveguide length and the optical path length can be reduced. As a result, the electrode length of the folded signal electrode 51C, which needs to be matched with the optical path length of the second folded waveguide 33A, can be reduced, thereby reducing the length of the delay waveguide, which leads to a reduction in the size of the folded portion. In other words, speed matching can be ensured and the chip size of the optical modulator 1A can be reduced.

[0137] Meanwhile, in the optical modulator 1 of the first embodiment, the first input-side waveguide 22 is arranged on the outer periphery, the first output-side waveguide 24 is arranged on the inner periphery, the second input-side waveguide 32 is arranged on the inner periphery, and the second output-side waveguide 34 is arranged on the outer periphery. Therefore, in the optical modulator 1, an example of truncation by the intersection of the first folded waveguide 23 and the second folded waveguide 33 has been described. However, the embodiment is not limited to this example; the first input-side waveguide 22 can be arranged on the outer periphery, the first output-side waveguide 24 can be arranged on the outer periphery, the second input-side waveguide 32 can be arranged on the inner periphery, and the second output-side waveguide 34 can be arranged on the inner periphery. This embodiment will be described below as a third embodiment.

[0138] (c) Third implementation method

[0139] Figure 16 This is a schematic plan view illustrating an example of the optical modulator 1B according to the third embodiment. Meanwhile, components identical to those in the optical modulator 1 of the first embodiment are indicated by the same reference numerals, and descriptions of the same components and operations will be omitted. The optical modulator 1 of the first embodiment and the optical modulator 1B of the third embodiment differ from each other in that there is no intersection between the first folded waveguide 23B and the second folded waveguide 33B, and the polarization direction X is opposite between the first input-side arm waveguide 22 and the second input-side arm waveguide 32 and between the first output-side arm waveguide 24 and the second output-side arm waveguide 34.

[0140] The first waveguide 20 includes a first input-side arm waveguide 22 located on the outer periphery of the fold, a first folded waveguide 23B located on the outer periphery, and a first output-side arm waveguide 24A located on the outer periphery. The second waveguide 30 includes a second input-side arm waveguide 32 located on the inner periphery, a second folded waveguide 33B located on the inner periphery, and a second output-side arm waveguide 34A located on the inner periphery.

[0141] The first folded waveguide 23B includes a first input-side high-refractive-index waveguide 61A, a first output-side high-refractive-index waveguide 62A, a first low-refractive-index waveguide 63A, a first input-side first-stage conversion unit 64A1, and a first output-side first-stage conversion unit 64A2. For example, the first input-side high-refractive-index waveguide 61A is a Si waveguide formed on a first layer 70A on a Si substrate 71 and connected to a first input-side arm waveguide 22 located on the outer periphery. For example, the first output-side high-refractive-index waveguide 62A is a Si waveguide formed on a first layer 70A and connected to a first output-side arm waveguide 24A located on the outer periphery. For example, the first low-refractive-index waveguide 63A is a SiN waveguide formed on a second layer 70B on a Si substrate 71 and connected between the first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A.

[0142] The first input-side first-stage conversion unit 64A1 includes an output terminal of a first input-side high-refractive-index waveguide 61A and an input terminal of a first low-refractive-index waveguide 63A, and allows signal light to be converted between the first input-side high-refractive-index waveguide 61A and the first low-refractive-index waveguide 63A. The first output-side first-stage conversion unit 64A2 includes an output terminal of a first low-refractive-index waveguide 63A and an input terminal of a first output-side high-refractive-index waveguide 62A, and allows signal light to be converted between the first low-refractive-index waveguide 63A and the first output-side high-refractive-index waveguide 62A.

[0143] The second folded waveguide 33B 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 conversion unit 64B1, and a second output-side first-stage conversion unit 64B2. For example, the second input-side high-refractive-index waveguide 61B is a Si waveguide formed on a first layer 70A on a Si substrate 71 and connected to the second input-side arm waveguide 32 located on the inner periphery. For example, the second output-side high-refractive-index waveguide 62B is a Si waveguide formed on the first layer 70A and connected to the second output-side arm waveguide 34A located on the inner periphery. For example, the second low-refractive-index waveguide 63B is a SiN waveguide formed on a second layer 70B on a Si substrate 71 and connected between the second input-side high-refractive-index waveguide 61B and the second output-side high-refractive-index waveguide 62B.

[0144] The second input-side first-stage conversion unit 64B1 includes an output terminal of the second input-side high-refractive-index waveguide 61B and an input terminal of the second low-refractive-index waveguide 63B, and allows signal light to be converted between the second input-side high-refractive-index waveguide 61B and the second low-refractive-index waveguide 63B. The second output-side first-stage conversion unit 64B2 includes an output terminal of the second low-refractive-index waveguide 63B and an input terminal of the second output-side high-refractive-index waveguide 62B, and allows signal light to be converted between the second low-refractive-index waveguide 63B and the second output-side high-refractive-index waveguide 62B.

[0145] Electrode 50 is an electrode with a GSG structure and includes a signal electrode 51, a first ground electrode 52, and a second ground electrode 53. Signal electrode 51 includes an input signal electrode 51A, an output signal electrode 51B, and a folded signal electrode 51C. Input signal electrode 51A is disposed between the first input-side waveguide 22 and the second input-side waveguide 32, and is electrically connected to the folded signal electrode 51C. Output signal electrode 51B is disposed between the first output-side waveguide 24A and the second output-side waveguide 34A, and is electrically connected to the folded signal electrode 51C. Folded signal electrode 51C is electrically connected between the input signal electrode 51A and the output signal electrode 51B.

[0146] The first ground electrode 52 includes a first input-side ground electrode 52A, a first output-side ground electrode 52B, and a first folded ground electrode 52C located on the outer periphery. The first input-side ground electrode 52A is disposed near the side surface of the first input-side arm waveguide 22 on the outer periphery so as to face the input signal electrode 51A, and is electrically connected to the first folded ground electrode 52C. The first output-side ground electrode 52B is disposed near the side surface of the first output-side arm waveguide 24A on the outer periphery so as 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 is electrically connected between the first input-side ground electrode 52A and the first output-side ground electrode 52B.

[0147] The second ground electrode 53 includes a second input-side ground electrode 53A located on the inner periphery, a second output-side ground electrode 53B located on the inner periphery, and a second folded ground electrode 53C located on the inner periphery. The second input-side ground electrode 53A is arranged near the side surface of the second input-side arm waveguide 32 located on the inner periphery so as to face the input signal electrode 51A, and the second input-side ground electrode 53A is electrically connected to the second folded ground electrode 53C. The second output-side ground electrode 53B is arranged near the side surface of the second output-side arm waveguide 34A located on the inner periphery so as to face the output signal electrode 51B, and the second output-side ground electrode 53B is electrically connected to the second folded ground electrode 53C. The second folded ground electrode 53C located on the inner periphery is electrically connected between the second input-side ground electrode 53A and the second output-side ground electrode 53B.

[0148] Figure 17 It shows along Figure 16 The diagram shows a schematic cross-sectional view of an example of a section taken by line AA. Figure 17 The modulator body 4B shown includes a Si substrate 71, a lower cladding layer 72, an upper cladding layer 73, and electrodes 50. The modulator body 4B includes a first input-side waveguide 22 located on the outer periphery and a second input-side waveguide 32 located on the inner periphery, disposed on the first layer 70A of the upper cladding layer 73. The modulator body 4B also includes a first output-side waveguide 24A located on the outer periphery and a second output-side waveguide 34A located on the inner periphery, disposed on the first layer 70A. The electrodes 50 disposed on the upper cladding layer 73 include an input signal electrode 51A, a first input-side ground electrode 52A located on the outer periphery, and a second input-side ground electrode 53A located on the inner periphery. Furthermore, the electrodes 50 include an output signal electrode 51B, a first output-side ground electrode 52B located on the outer periphery, and a second output-side ground electrode 53B located on the inner periphery.

[0149] The input signal electrode 51A is disposed between the first input-side waveguide 22 and the second input-side waveguide 32. The first input-side ground electrode 52A is disposed near the side surface of the first input-side waveguide 22 on the opposite side of the input signal electrode 51A. The second input-side ground electrode 53A is disposed near the side surface of the second input-side waveguide 32 on the opposite side of the input signal electrode 51A.

[0150] The output signal electrode 51B is disposed between the first output side waveguide 24A and the second output side waveguide 34A. The first output side ground electrode 52B is disposed near the side surface of the first output side waveguide 24A on the opposite side of the output signal electrode 51B. The second output side ground electrode 53B is disposed near the side surface of the second output side waveguide 34A on the opposite side of the output signal electrode 51B.

[0151] The polarization direction X of the modulator body 4 is reversed between the forward and reverse paths. The first input-side waveguide 22 modulates the signal light according to the electrical signal in the reverse direction from the input signal electrode 51A to the first input-side ground electrode 52A. The second input-side waveguide 32 modulates the signal light according to the electrical signal in the forward direction from the input signal electrode 51A to the second input-side ground electrode 53A.

[0152] In contrast, the first output-side waveguide 24A modulates the signal light according to an electrical signal in the reverse direction from the output signal electrode 51B to the first output-side ground electrode 52B. The second output-side waveguide 34A modulates the signal light according to an electrical signal in the forward direction from the output signal electrode 51B to the second output-side ground electrode 53B.

[0153] In other words, the first input-side waveguide 22 and the first output-side waveguide 24A modulate the signal light based on electrical signals in the same reverse direction. The second input-side waveguide 32 and the second output-side waveguide 34A modulate the signal light based on electrical signals in the same forward direction. Applying electrical signals in the same direction along both the forward and reverse paths improves modulation efficiency.

[0154] In the optical modulator 1B of the third embodiment, the polarization direction X1 of the thin film LN of the first input-side waveguide 22 and the second input-side waveguide 32 is opposite to the polarization direction X2 of the thin film LN of the first output-side waveguide 24A and the second output-side waveguide 34A. As a result, the modulation electric field and polarization direction are both in opposite directions between the first input-side waveguide 22 and the first output-side waveguide 24A, so that the phase can be modulated in the same direction before and after folding. Similarly, the modulation electric field and polarization direction are both in the same direction between the second input-side waveguide 32 and the second output-side waveguide 34A, so that the phase can be modulated in the same direction before and after folding. In addition, a push-pull operation in which the phase changes in opposite directions along the first input-side waveguide 22 and the first output-side waveguide 24A can be realized.

[0155] Meanwhile, in the optical modulator 1 of the first embodiment, an example of setting a single folded portion has been described; however, multiple folded portions, such as two folded portions, can be set, and this embodiment will be described below as the fourth embodiment.

[0156] (d) Fourth Implementation

[0157] Figure 18 This is a schematic plan view illustrating an example of an optical modulator 1C according to a fourth embodiment. Components identical to those in the optical modulator 1 of the first embodiment are indicated by the same reference numerals, and descriptions of the same components and operations will be omitted. The optical modulator 1C of the fourth embodiment differs from the optical modulator 1 in that it has multiple folded portions (such as two folded portions), signal light is input from one end face D1 of the chip of the optical modulator 1C, and signal light is output from another end face D2 of the optical modulator 1C opposite to one end face D1.

[0158] The optical modulator 1C includes a Si photonic substrate 2, an input MMI 3C, a modulator body 4C, a first folded portion 5C1, a second folded portion 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 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 splits the signal light from the input waveguide 6 into multiple beams, outputting the split beams to the first input waveguide 21A and the second input waveguide 31A.

[0159] The first waveguide 20A is disposed on the Si substrate 71 and connected to one output terminal of the first coupler 10. The second waveguide 30A is disposed on the Si substrate 71 and connected to the other output terminal of the first coupler 10. The second coupler 40 is disposed on the Si substrate 71 and couples 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, and outputs the coupled signal light to the output waveguide 7. The electrode 50 is a GSG electrode that applies electrical signals to the first waveguide 20A and the second waveguide 30A.

[0160] The first waveguide 20A includes a first input waveguide 21A, a first input side arm waveguide 22A, a first input side folded waveguide 81, a first intermediate arm waveguide 82, and a first output side folded waveguide 83. The first waveguide 20A also includes a first output side arm waveguide 24B, a first output waveguide 25B, a first input side modulation unit conversion unit 85, a first intermediate side modulation unit conversion unit 86, and a first output side modulation unit conversion unit 87.

[0161] The first input waveguide 21A is a Si waveguide connected between the first coupler 10 and the first input-side arm waveguide 22A. The first input-side arm waveguide 22A is a straight-arm waveguide made of a thin film LN as a high-EO material and connected between the first input waveguide 21A and the first input-side folded waveguide 81. The first input-side folded waveguide 81 is a waveguide with a folded structure connected between the first input-side arm waveguide 22A and the first intermediate arm waveguide 82. Meanwhile, the first input-side folded waveguide 81 has, for example... Figure 2 The structure is the same as the first folded waveguide 23 shown in the figure.

[0162] The first intermediate arm waveguide 82 is a straight-arm waveguide made of a thin film LN as a high-EO material and connected 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 and connected between the first intermediate arm waveguide 82 and the first output-side arm waveguide 24B. Meanwhile, the first output-side folded waveguide 83 has, for example, a folded structure. Figure 2 The structure is the same as the first folded waveguide 23 shown in the figure.

[0163] The first output-side arm waveguide 24B is a straight-arm waveguide made of thin film LN as a high-EO material and connected between the first output-side folded waveguide 83 and the first output waveguide 25B. The first output waveguide 25B is a Si waveguide connected between the first output-side arm waveguide 24B and the second coupler 40.

[0164] The first input-side modulation unit conversion unit 85 includes an output terminal of the first input waveguide 21A and an input terminal of the first input-side arm waveguide 22A, and allows signal light to be converted between the first input waveguide 21A and the first input-side arm waveguide 22A. Furthermore, the first input-side modulation unit conversion unit 85 includes an output terminal of the first input-side arm waveguide 22A and an input terminal of the first input-side folded waveguide 81, and allows signal light to be converted between the first input-side arm waveguide 22A and the first input-side folded waveguide 81.

[0165] The first intermediate-side modulation unit conversion unit 86 includes an output terminal of the first input-side folded waveguide 81 and an input terminal of the first intermediate-arm waveguide 82, and allows signal light to be converted between the first input-side folded waveguide 81 and the first intermediate-arm waveguide 82. The first intermediate-side modulation unit conversion unit 86 also includes an output terminal of the first intermediate-arm waveguide 82 and an input terminal of the first output-side folded waveguide 83, and allows signal light to be converted between the first intermediate-arm waveguide 82 and the first output-side folded waveguide 83.

[0166] The first output-side modulation unit conversion unit 87 includes an output terminal of the first output-side folded waveguide 83 and an input terminal of the first output-side arm waveguide 24B, and allows signal light to be converted between the first output-side folded waveguide 83 and the first output-side arm waveguide 24B. The first output-side modulation unit conversion unit 87 also includes an output terminal of the first output-side arm waveguide 24B and an input terminal of the first output waveguide 25B, and allows signal light to be converted between the first output-side arm waveguide 24B and the first output waveguide 25B.

[0167] The second waveguide 30A includes a second input waveguide 31A, a second input-side arm waveguide 32A, a second input-side folded waveguide 91, a second intermediate arm waveguide 92, and a second output-side folded waveguide 93. Furthermore, the second waveguide 30A includes a second output-side arm waveguide 34B, a second output waveguide 35B, a second input-side modulation unit conversion unit 94, a second intermediate-side modulation unit conversion unit 95, and a second output-side modulation unit conversion unit 96.

[0168] The second input waveguide 31A is a Si waveguide connected between the first coupler 10 and the second input-side arm waveguide 32A. The second input-side arm waveguide 32A is a straight-arm waveguide made of a thin film LN as a high-EO material and connected between the second input waveguide 31A and the second input-side folded waveguide 91. The second input-side folded waveguide 91 is a waveguide with a folded structure and connected between the second input-side arm waveguide 32A and the second intermediate arm waveguide 92. Meanwhile, the second input-side folded waveguide 91 has, for example, a... Figure 2 The second folded waveguide 33 shown in the figure has the same structure.

[0169] The second intermediate arm waveguide 92 is a straight-arm waveguide made of a thin film LN as a high-EO material and connected between the second input-side folded waveguide 91 and the second output-side folded waveguide 93. The second output-side folded waveguide 93 is a waveguide with a folded structure and connected between the second intermediate arm waveguide 92 and the second output-side arm waveguide 34B. The second output-side arm waveguide 34B is a straight waveguide made of a thin film LN as a high-EO material and connected between the second output-side folded waveguide 93 and the second output-side waveguide 35B.

[0170] The second input-side modulation unit conversion unit 94 includes an output terminal of the second input waveguide 31A and an input terminal of the second input-side arm waveguide 32A, and allows signal light to be converted between the second input waveguide 31A and the second input-side arm waveguide 32A. The second input-side modulation unit conversion unit 94 also includes an output terminal of the second input-side arm waveguide 32A and an input terminal of the second input-side folded waveguide 91, and allows signal light to be converted between the second input-side arm waveguide 32A and the second input-side folded waveguide 91.

[0171] The second intermediate-side modulation unit conversion unit 95 includes an output terminal of the second input-side folded waveguide 91 and an input terminal of the second intermediate-arm waveguide 92, and allows signal light to be converted between the second input-side folded waveguide 91 and the second intermediate-arm waveguide 92. The second intermediate-side modulation unit conversion unit 95 also includes an output terminal of the second intermediate-arm waveguide 92 and an input terminal of the second output-side folded waveguide 93, and allows signal light to be converted between the second intermediate-arm waveguide 92 and the second output-side folded waveguide 93.

[0172] The second output-side modulation unit conversion unit 96 includes an output terminal of the second output-side folded waveguide 93 and an input terminal of the second output-side arm waveguide 34B, and allows signal light to be converted between the second output-side folded waveguide 93 and the second output-side arm waveguide 34B. The second output-side modulation unit conversion unit 96 also includes an output terminal of the second output-side arm waveguide 34B and an input terminal of the second output waveguide 35B, and allows signal light to be converted between the second output-side arm waveguide 34B and the second output waveguide 35B.

[0173] Compared to the high-refractive-index waveguide formed on the Si substrate 71, the first input-side arm waveguide 22A, the second input-side arm waveguide 32A, the first output-side arm waveguide 24B, and the second output-side arm waveguide 34B are waveguides comprising materials with high EO characteristics. Compared to the high-refractive-index waveguide formed on the Si substrate 71, the first intermediate arm waveguide 82 and the second intermediate arm waveguide 92 are waveguides comprising materials with high EO characteristics. Furthermore, compared to the high-refractive-index waveguide formed on the Si substrate 71, the first input-side folded waveguide 81, the second input-side folded waveguide 91, the first output-side folded waveguide 83, and the second output-side folded waveguide 93 are waveguides comprising materials with low refractive index.

[0174] The first waveguide 20A includes a first input-side arm waveguide 22A located on the outer periphery, a first input-side folded waveguide 81 located on the outer periphery, and in... Figure 18 The first intermediate arm waveguide 82 is located on the left side. In addition, the first waveguide 20A includes a first output-side folded waveguide 83 located on the inner periphery and a first output-side arm waveguide 24B located on the inner periphery.

[0175] The second waveguide 30A includes a second input-side arm waveguide 32A located on the inner periphery, a second input-side folded waveguide 91 located on the inner periphery, and... Figure 18 The second intermediate arm waveguide 92 is located on the right side. In addition, the second waveguide 30A includes a second output-side folded waveguide 93 located on the inner periphery and a second output-side arm waveguide 34B located on the outer periphery.

[0176] Signal electrode 51 includes an input signal electrode 51A, an input-side folded signal electrode 51C1, an intermediate-side signal electrode 51D, an output-side folded signal electrode 51C2, and an output signal electrode 51B. Input signal electrode 51A is disposed between the first input-side arm waveguide 22A and the second input-side arm waveguide 32A, and is electrically connected to the input-side folded signal electrode 51C1. Input-side folded signal electrode 51C1 is electrically connected to the intermediate-side signal electrode 51D. Intermediate-side signal electrode 51D is disposed between the first intermediate-side arm waveguide 82 and the second intermediate-side arm waveguide 92, and is electrically connected to the output-side folded signal electrode 51C2. Output-side folded signal electrode 51C2 is electrically connected to the output signal electrode 51B. Output signal electrode 51B is disposed between the first output-side arm waveguide 24B and the second output-side arm waveguide 34B, and is electrically connected to the output-side folded signal electrode 51C2.

[0177] The first ground electrode 52 includes a first input-side ground electrode 52A located on the outer periphery, a first input-side folded ground electrode 52C1 located on the outer periphery, and a first intermediate-side ground electrode 52D. Furthermore, the first ground electrode 52 includes a first output-side folded ground electrode 52C2 located on the inner periphery and a first output-side ground electrode 52B located on the inner periphery. The first input-side ground electrode 52A is arranged near the side surface of the first input-side arm waveguide 22A located on the outer periphery so as to face the input signal electrode 51A, and the first input-side ground electrode 52A is electrically connected to the first input-side folded ground electrode 52C1 located on the outer periphery. The first input-side folded ground electrode 52C1 is electrically connected between the first input-side ground electrode 52A and the first intermediate-side ground electrode 52D. The first intermediate-side ground electrode 52D is arranged on... Figure 18The first intermediate arm waveguide 82, located on the left side, faces the intermediate signal electrode 51D. The first intermediate side ground electrode 52D is electrically connected between the first input-side folded ground electrode 52C1 and the first output-side folded ground electrode 52C2. The first output-side folded ground electrode 52C2 is electrically connected between the first intermediate side ground electrode 52D and the first output-side ground electrode 52B. The first output-side ground electrode 52B is arranged near the side surface of the first output-side arm waveguide 24B on the inner periphery side, facing the output signal electrode 51B, and is electrically connected to the first output-side folded ground electrode 52C2.

[0178] The second ground electrode 53 includes a second input-side ground electrode 53A located on the inner periphery, a second input-side folded ground electrode 53C1 located on the inner periphery, and a second intermediate-side ground electrode 53D located on the inner periphery. Furthermore, the second ground electrode 53 includes a second output-side folded ground electrode 53C2 located on the outer periphery and a second output-side ground electrode 53B located on the outer periphery. The second input-side ground electrode 53A is arranged near the side surface of the second input-side arm waveguide 32A located on the inner periphery so as to face the input signal electrode 51A, and the second input-side ground electrode 53A is electrically connected to the second input-side folded ground electrode 53C1 located on the inner periphery. The second input-side folded ground electrode 53C1 is electrically connected between the second input-side ground electrode 53A and the second intermediate-side ground electrode 53D. The second intermediate-side ground electrode 53D is arranged on... Figure 18 The second intermediate arm waveguide 92, located on the right side, faces the intermediate signal electrode 51D. The second intermediate side ground electrode 53D is electrically connected between the second input-side folded ground electrode 53C1 and the second output-side folded ground electrode 53C2. The second output-side folded ground electrode 53C2 is electrically connected between the second intermediate side ground electrode 53D and the second output-side ground electrode 53B. The second output-side ground electrode 53B is arranged near the side surface of the second output-side arm waveguide 34B on the outer periphery, facing the output signal electrode 51B, and is electrically connected to the second output-side folded ground electrode 53C2.

[0179] The first input-side folded waveguide 81 includes a first input-side high-refractive-index waveguide 61A, a first output-side high-refractive-index waveguide 62A, a first low-refractive-index waveguide 63A, a first input-side first-stage conversion unit 64A1, and a first output-side first-stage conversion unit 64A2. For example, the first input-side high-refractive-index waveguide 61A is a Si waveguide formed on the first layer 70A of the Si substrate 71 and connected to the first input-side arm waveguide 22A. For example, the first output-side high-refractive-index waveguide 62A is a Si waveguide formed on the first layer 70A and connected to the first intermediate arm waveguide 82. For example, the first low-refractive-index waveguide 63A is a SiN waveguide formed on the second layer 70B of the Si substrate 71 and connected between the first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A.

[0180] The first input-side first-stage conversion unit 64A1 includes an output terminal of a first input-side high-refractive-index waveguide 61A and an input terminal of a first low-refractive-index waveguide 63A, and allows signal light to be converted between the first input-side high-refractive-index waveguide 61A and the first low-refractive-index waveguide 63A. The first output-side first-stage conversion unit 64A2 includes an output terminal of a first low-refractive-index waveguide 63A and an input terminal of a first output-side high-refractive-index waveguide 62A, and allows signal light to be converted between the first low-refractive-index waveguide 63A and the first output-side high-refractive-index waveguide 62A.

[0181] 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 conversion unit 64B1, and a second output-side first-stage conversion unit 64B2. For example, the second input-side high-refractive-index waveguide 61B is a Si waveguide formed on a first layer 70A on a Si substrate 71 and connected to the second input-side arm waveguide 32A. For example, the second output-side high-refractive-index waveguide 62B is a Si waveguide formed on the first layer 70A and connected to the second intermediate arm waveguide 92. For example, the second low-refractive-index waveguide 63B is a SiN waveguide formed on a second layer 70B on a Si substrate 71 and connected between the second input-side high-refractive-index waveguide 61B and the second output-side high-refractive-index waveguide 62B.

[0182] The second input-side first-stage conversion unit 64B1 includes an output terminal of the second input-side high-refractive-index waveguide 61B and an input terminal of the second low-refractive-index waveguide 63B, allowing signal light to be converted between the second input-side high-refractive-index waveguide 61B and the second low-refractive-index waveguide 63B. The second output-side first-stage conversion unit 64B2 includes an output terminal of the second low-refractive-index waveguide 63B and an input terminal of the second output-side high-refractive-index waveguide 62B, allowing signal light to be converted 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 spans the second low-refractive-index waveguide 63B and connects between the first low-refractive-index waveguide 63A and the first intermediate arm waveguide 82.

[0183] The first output-side folded waveguide 83 includes a first input-side high-refractive-index waveguide 61A, a first output-side high-refractive-index waveguide 62A, a first low-refractive-index waveguide 63A, a first input-side first-stage conversion unit 64A1, and a first output-side first-stage conversion unit 64A2. For example, the first input-side high-refractive-index waveguide 61A is a Si waveguide formed on a first layer 70A on a Si substrate 71 and connected to a first intermediate arm waveguide 82. For example, the first output-side high-refractive-index waveguide 62A is a Si waveguide formed on a first layer 70A and connected to a first output-side arm waveguide 24B. For example, the first low-refractive-index waveguide 63A is a SiN waveguide formed on a second layer 70B on a Si substrate 71 and connected between the first input-side high-refractive-index waveguide 61A and the first output-side high-refractive-index waveguide 62A.

[0184] The first input-side first-stage conversion unit 64A1 includes an output terminal of a first input-side high-refractive-index waveguide 61A and an input terminal of a first low-refractive-index waveguide 63A, and allows signal light to be converted between the first input-side high-refractive-index waveguide 61A and the first low-refractive-index waveguide 63A. The first output-side first-stage conversion unit 64A2 includes an output terminal of a first low-refractive-index waveguide 63A and an input terminal of a first output-side high-refractive-index waveguide 62A, and allows signal light to be converted between the first low-refractive-index waveguide 63A and the first output-side high-refractive-index waveguide 62A.

[0185] The second output-side folded waveguide 93 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 conversion unit 64B1, and a second output-side first-stage conversion unit 64B2. For example, the second input-side high-refractive-index waveguide 61B is a Si waveguide formed on a first layer 70A on a Si substrate 71 and connected to the second intermediate arm waveguide 92. For example, the second output-side high-refractive-index waveguide 62B is a Si waveguide formed on the first layer 70A and connected to the second output-side arm waveguide 34B. For example, the second low-refractive-index waveguide 63B is a SiN waveguide formed on a second layer 70B on a Si substrate 71 and connected between the second input-side high-refractive-index waveguide 61B and the second output-side high-refractive-index waveguide 62B.

[0186] The second input-side first-stage conversion unit 64B1 includes an output terminal of the second input-side high-refractive-index waveguide 61B and an input terminal of the second low-refractive-index waveguide 63B, allowing signal light to be converted between the second input-side high-refractive-index waveguide 61B and the second low-refractive-index waveguide 63B. The second output-side first-stage conversion unit 64B2 includes an output terminal of the second low-refractive-index waveguide 63B and an input terminal of the second output-side high-refractive-index waveguide 62B, allowing signal light to be converted 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 spans the first low-refractive-index waveguide 63A and connects the second intermediate arm waveguide 92 and the second low-refractive-index waveguide 63B.

[0187] The first input-side waveguide 22A modulates the signal light according to the electrical signal in the reverse direction from the input signal electrode 51A to the first input-side ground electrode 52A. The second input-side waveguide 32A modulates the signal light according to the electrical signal in the forward direction from the input signal electrode 51A to the second input-side ground electrode 53A.

[0188] The first intermediate arm waveguide 82 modulates the signal light according to the electrical signal in the reverse direction 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 according to the electrical signal in the forward direction from the intermediate side signal electrode 51D to the second intermediate side ground electrode 53D.

[0189] The first output-side waveguide 24B modulates the signal light according to the electrical signal in the reverse direction from the output signal electrode 51B to the first output-side ground electrode 52B. The second output-side waveguide 34B modulates the signal light according to the electrical signal in the forward direction from the output signal electrode 51B to the second output-side ground electrode 53B.

[0190] In other words, the first input-side waveguide 22A, the first intermediate-side waveguide 82, and the first output-side waveguide 24B modulate the signal light based on an electrical signal in the same reverse direction. The second input-side waveguide 32A, the second intermediate-side waveguide 92, and the second output-side waveguide 34B modulate the signal light based on an electrical signal in the same forward direction. Applying an electrical signal in the same direction in the forward, intermediate, and reverse paths improves modulation efficiency.

[0191] In the optical modulator 1C of the fourth embodiment, signal light is input from one end face D1 of the optical chip and output from the other end face D2 of the optical chip. As a result, multiple folded optical modulators 1C can be arranged in parallel with a simple layout.

[0192] In the optical modulator 1C, by incorporating two folded sections, the length of the modulator body 4C can be increased without altering the chip length of the optical modulator 1C. As a result, efficient (low Vpi) operation can be achieved.

[0193] Meanwhile, a dual polarization in-phase quadrature (DP-IQ) equipped with four optical modulators 1C of the fourth embodiment can be used, and this embodiment will be described below as the fifth embodiment.

[0194] (e) Fifth Implementation

[0195] Figure 19 This is a diagram illustrating an example of the DP-IQ modulator 1D according to the fifth embodiment. (and...) Figure 18 The same components of the optical modulator 1C shown are indicated by the same reference numerals, and descriptions of the same components and the same operations will be omitted. Figure 19 The DP-IQ modulator 1D shown includes four optical modulators 1C arranged in parallel according to the fourth embodiment. The DP-IQ modulator 1D includes an LD input port 111, a splitting portion 112, an IQ modulator 1D1 for the X-polarization component, an IQ modulator 1D2 for the Y-polarization component, and a polarization rotator (PR) 113. Furthermore, the DP-IQ modulator 1D includes a polarization beam combiner (PBC) 114 and a transmitted light output port 115.

[0196] The splitting section 112 is an XY splitting MMI that optically splits the input light from the input waveguide 6 and outputs the optically split signal light to the IQ modulator 1D1 for the X-polarization component and the IQ modulator 1D2 for the Y-polarization component. The IQ modulator 1D1 for the X-polarization component includes a first splitting section 121, two first DC phase shifters (DCPS) 122, two second splitting sections 123, and four second DCPS 124. The IQ modulator 1D1 for the X-polarization component includes an optical modulator 1C1 for the I-component, an optical modulator 1C2 for the Q-component, and a first multiplexing unit 125.

[0197] The first splitting section 121 in the IQ modulator 1D1 for the X-polarization component is an IQ splitting MMI that optically splits the X-polarization component signal light from the splitting section 112 into I-component signal light and Q-component signal light. The first splitting section 121 outputs the optically split I-component signal light to the first DCPS 122. For example, the first DCPS 122 is a phase shifter that shifts the phase of the I-component signal light, such as a heater for heating a Si waveguide. The first DCPS 122 is arranged directly below the Si waveguide and adjusts the phase of the signal light guided through the Si waveguide by changing the refractive index of the Si waveguide through heating by the heater. The first DCPS 122 outputs the phase-shifted I-component signal light to the second splitting section 123.

[0198] The second splitting section 123 outputs the phase-shifted signal light of the I component from the first DCPS 122 to each of the second DCPS 124. For example, the second DCPS 124 is a phase shifter that shifts the phase of the I component signal light, such as a heater for heating the Si waveguide. The second DCPS 124 is positioned directly below the Si waveguide and adjusts the phase of the signal light guided 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 optical modulator 1C1 of the X-polarized I component. The optical modulator 1C1 modulates the I component signal light of the X-polarized I component and outputs the modulated I component signal light of the X-polarized I component to the first multiplexing unit 125 for the X-polarized component.

[0199] The first splitting section 121 of the IQ modulator 1D1 for the X-polarization component outputs the optically split Q-component signal light to the first DCPS 122. For example, the first DCPS 122 is a phase shifter that shifts the phase of the Q-component signal light, such as a heater for heating a Si waveguide. The first DCPS 122 outputs the phase-shifted Q-component signal light to the second splitting section 123. The second splitting section 123 outputs the phase-shifted Q-component signal light from the first DCPS 122 to each of the second DCPS 124. For example, the second DCPS 124 is a phase shifter that shifts the phase of the Q-component signal light, such as a heater for heating a Si waveguide. The second DCPS 124 outputs the phase-shifted Q-component signal light to the optical modulator 1C2 for the X-polarization component's Q-component. The optical modulator 1C2 modulates the Q-component signal light of the X-polarization component and outputs the modulated Q-component signal light to the first multiplexing unit 125 for the X-polarization component. The first multiplexing unit 125 for the X-polarization component is an IQ coupling MMI that couples the signal light of the I component of the X-polarization component with the signal light of the Q component of the X-polarization component.

[0200] The IQ modulator 1D2 for the Y-polarization component includes a first split section 121, two first DCPS 122, two second split sections 123, and four second DCPS 124. The IQ modulator 1D2 for the Y-polarization component includes an optical modulator 1C3 for the I component, an optical modulator 1C4 for the Q component, and a first multiplexing unit 125.

[0201] The first splitting section 121 in the IQ modulator 1D2 for the Y-polarized component is an IQ splitting MMI that splits the Y-polarized signal light from the splitting section 112 into I-component and Q-component signal light. The first splitting section 121 outputs the optically split I-component signal light to the first DCPS 122. For example, the first DCPS 122 is a phase shifter that shifts the phase of the I-component signal light, such as a heater for heating a Si waveguide. The first DCPS 122 outputs the phase-shifted I-component signal light to the second splitting section 123. The second splitting section 123 outputs the phase-shifted I-component signal light from the first DCPS 122 to each of the second DCPS 124. For example, the second DCPS 124 is a phase shifter that shifts the phase of the I-component signal light, such as a heater for heating a Si waveguide. The second DCPS 124 outputs the phase-shifted I-component signal light to the optical modulator 1C3 for the Y-polarized component. The optical modulator 1C3 for the I component of the Y polarization component modulates the signal light of the I component of the Y polarization component and outputs the modulated signal light of the I component of the Y polarization component to the first multiplexing unit 125 for the Y polarization component.

[0202] The first splitting section 121 of the IQ modulator 1D2 for the Y-polarized component outputs the optically split Q-component signal light to the first DCPS 122. For example, the first DCPS 122 is a phase shifter that shifts the phase of the Q-component signal light, such as a heater for heating a Si waveguide. The first DCPS 122 outputs the phase-shifted Q-component signal light to the second splitting section 123. The second splitting section 123 outputs the phase-shifted Q-component signal light from the first DCPS 122 to each of the second DCPS 124. For example, the second DCPS 124 is a phase shifter that shifts the phase of the Q-component signal light, such as a heater for heating a Si waveguide. The second DCPS 124 outputs the phase-shifted Q-component signal light to the optical modulator 1C4 for the Y-polarized component. The optical modulator 1C4 modulates the Q-component signal light of the Y-polarized component and outputs the modulated Q-component signal light of the Y-polarized component to the first multiplexing unit 125 for the Y-polarized component. The first multiplexing unit 125 for the Y-polarization component is an IQ coupling MMI that couples the signal light of the I component and the signal light of the Q component of the Y-polarization component.

[0203] The first multiplexing unit 125 for the X-polarization component couples the signal light of the I component and the signal light of the Q component of the X-polarization component, and outputs the coupled IQ component signal light of the X-polarization component to PBC 114. The first multiplexing unit 125 for the Y-polarization component couples the signal light of the I component and the signal light of the Q component of the Y-polarization component, and outputs the coupled IQ component signal light of the Y-polarization component to PR 113. PR 113 polarizes and rotates the IQ component signal light of the Y-polarization component, and outputs the polarized and rotated IQ component signal light of the Y-polarization component to PBC 114. PBC 114 couples the IQ component signal light of the X-polarization component with the polarized and rotated IQ component signal light of the Y-polarization component, and outputs the coupled XY polarization component signal light as transmitted light to the transmitted light output port 115.

[0204] The first split portion 121, the second split portion 123, and the first multiplexing unit 125 constituting the optical modulator 1C are configured using silicon photonics with Si waveguides, and the size can be reduced by utilizing the properties of silicon photonics. By forming a portion of the two waveguides included in the optical modulator 1C at the silicon photonic element side, a first DCPS 122 and a second DCPS 124 for properly adjusting the phase of the optical modulator 1C can be implemented by heaters formed on the waveguides. Furthermore, a small-sized and low-power phase shifter can be realized. The first DCPS 122 and the second DCPS 124 using heaters can be used not only to adjust the phase of the optical modulator 1C, but also to adjust the phase between the I and Q channels of the IQ modulator 1D1 (1D2).

[0205] The DP-IQ modulator 1D of the fifth embodiment incorporates an IQ modulator 1D1 for X-polarization and an IQ modulator 1D2 for Y-polarization. As a result, the propagation speed of light in the optical modulator 1C is substantially equal to the propagation speed of the electrical signal, thereby improving the velocity mismatch in the DP-IQ modulator 1D. Therefore, the limitation of the operating frequency band of the DP-IQ modulator 1D can be avoided. Furthermore, the DP-IQ modulator 1D can improve the velocity mismatch while ensuring a longer operating length, thereby reducing the half-wave voltage Vp and improving the modulation efficiency of the DP-IQ modulator 1D.

[0206] The DP-IQ modulator 1D incorporates an IQ modulator 1D1 for X-polarization and an IQ modulator 1D2 for Y-polarization, and each optical modulator 1C in the optical modulator 1C ensures speed matching and reduces the size of the folded portion. As a result, the overall chip size of the DP-IQ modulator 1D can be reduced.

[0207] Meanwhile, in the fifth embodiment, a DP-IQ modulator 1D is shown as an example, but in addition to the DP-IQ modulator 1D, an optical receiver can also be mounted on the Si substrate 71, and this embodiment will be described below as the sixth embodiment.

[0208] (f) Sixth implementation method

[0209] Figure 20 This is a diagram illustrating an example of the optical transceiver 1E according to the sixth embodiment. Meanwhile, with... Figure 19 The same components of the DP-IQ modulator 1D shown are indicated by the same reference numerals, and descriptions of the same components and the same operations will be omitted. Figure 20 The optical transceiver 1E shown is an optical integrated circuit comprising: an optical modulator element 130A, which includes a DP-IQ modulator 1D; and an optical receiver element 130B, which receives DP-QAM signals. In the optical transceiver 1E, the optical modulator element 130A and the optical receiver element 130B are integrated using silicon photonics technology. The optical transceiver 1E includes an LD input port 111, a transmitted light output port 115, and a received light input port 116.

[0210] The receiving optical input port 116 is an optical port arranged on one end face D1 of the optical transceiver 1E and connected between the optical fiber that receives the incoming receiving light (described later) and the optical receiver element 130B. The LD input port 111 is an optical port arranged on one end face D1 of the optical transceiver 1E and connected between the optical modulator element 130A that receives the local oscillator light from a light source (not shown) and the optical receiver element 130B. The transmitted light output port 115 is an optical port arranged on one end face D1 of the optical transceiver 1E and connected between the optical fiber that outputs the transmitted light and the optical modulator element 130A.

[0211] For example, optical modulator element 130A is a DP-IQ modulator 1D. For example, optical receiver element 130B is a coherent receiver. Optical receiver element 130B includes a third split section 131, a fourth split section 132, a polarization beam splitter (PBS) 133, and a polarization rotator (PR) 134. Furthermore, optical receiver element 130B includes a first optical mixing circuit 135A (135) and a second optical mixing circuit 135B (135). Optical receiver element 130B includes four first optical receiving elements 136A (136) and four second optical receiving elements 136B (136).

[0212] The third splitting section 131 is a Tx / Lo splitting MMI that optically splits the light from the light source connected to the LD input port 111. The third splitting section 131 outputs one path of the optically split light to the splitting section 112 in the DP-IQ modulator 1D as the input light source for the optical modulator, and outputs the other path of the optically split light to each of the optical mixing circuits 135 as the local oscillator light for the optical receiver. The fourth splitting section 132 optically splits the local oscillator light from the third splitting section 131 and outputs the split light to each of the optical mixing circuits 135. The PBS 133 splits the light from the receiving light input port 116 into X-polarized receiving light and Y-polarized receiving light, outputting the X-polarized receiving light to the first optical mixing circuit 135A and the Y-polarized receiving light to PR 134. PR 134 polarizes and rotates the Y-polarized receiving light by 90 degrees, and outputs the polarized and rotated Y-polarized receiving light to the second optical mixing circuit 135B.

[0213] The first optical mixing circuit 135A interferes with the local oscillator light using the X-polarized component of the received light to obtain the I and Q components of the optical signal. The first optical mixing circuit 135A outputs the I component optical signal from the X-polarized component to the first optical receiving element 136A, and also outputs the Q component optical signal to the first optical receiving element 136A.

[0214] The second optical mixing circuit 135B interferes with the local oscillator light using the Y-polarized component of the received light to obtain the I and Q components of the optical signal. The second optical mixing circuit 135B outputs the I component optical signal from the Y-polarized component to the second optical receiving element 136B, and also outputs the Q component optical signal to the second optical receiving element 136B.

[0215] For example, the first optical receiving element 136A is a Si photonic photodetector (Ge-PD) that electrically converts the I-component of the X-polarized optical signal from the first optical mixing circuit 135A and outputs the electrical signal of the I-component obtained through electrical conversion. The Ge-PD has a structure in which a Ge layer is arranged in a layer directly below the Si waveguide. Furthermore, the first optical receiving element 136A electrically converts the Q-component of the X-polarized optical signal from the first optical mixing circuit 135A and outputs the electrical signal of the Q-component obtained through electrical conversion.

[0216] For example, the second optical receiving element 136B is a Si photonic Ge-PD, which electrically converts the I component of the Y-polarized optical signal from the second optical mixing circuit 135B and outputs the electrical signal of the I component obtained through electrical conversion. The second optical receiving element 136B also electrically converts the Q component of the Y-polarized optical signal from the second optical mixing circuit 135B and outputs the electrical signal of the Q component obtained through electrical conversion.

[0217] Figure 21 This is a schematic cross-sectional view showing an example of an optical transceiver 1E. Figure 21 The second DCPS 124 in the optical transceiver 1E shown includes 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 includes a Si waveguide 124C disposed on a second layer 70B of the lower cladding layer 72, a heater 124A disposed beneath the Si waveguide 124C in the lower cladding layer 72, and heater terminals 124B connected to both ends of the heater 124A. Simultaneously, the heater terminals 124B are electrically connected to electrode wiring 50A.

[0218] The optical transceiver 1E includes an optical modulator 1C comprising a first input-side modulation unit conversion unit 26, a modulator body 4C, and a first folded portion 5C1. The modulator body 4C comprises 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 on a second layer 70B of the lower cladding layer 72 and a first input-side high-refractive-index waveguide 61A disposed on the second layer 70B. The modulator body 4C also includes a first low-refractive-index waveguide 63A disposed on a third layer 70C of the lower cladding layer 72 and a first input-side arm waveguide 22A disposed on the first layer 70A of the upper cladding layer 73.

[0219] The first optical receiving element 136A in the optical transceiver 1E includes a Si substrate 71, a lower cladding layer 72, and an upper cladding layer 73. Each first optical receiving element 136A includes a Si waveguide 136A3 disposed on a 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 electrode wiring 50A.

[0220] Figure 22A This is a schematic cross-sectional view showing an example of a Si photonic substrate 210A after a first forming process.

[0221] Si photonic substrate 210A includes a Si substrate 211 for Si photonics and a BOX layer laminated on the Si substrate 211.

[0222] 212. A Si waveguide 213 and a SiN waveguide 214 disposed in the BOX layer 212. The Si photonic substrate 210A includes a heater 124A for a second DCPS 124 disposed in the BOX layer 212, and a first light-receiving element 136A disposed in the BOX layer 212. The Si waveguide 213 includes a Si waveguide 124C for the second DCPS 124, a Si waveguide 213 for a high-refractive-index waveguide 62B on the second output side of the optical modulator 1C, and a Si waveguide 136A3 for the first light-receiving element 136A. The Si photonic substrate 210A includes a heater terminal 124B for the heater 124A and a PD terminal 136A2 for the first light-receiving element 136A.

[0223] Figure 22B This is a schematic cross-sectional view showing an example of a Si photonic substrate 210B after a second fabrication process. A Si substrate 221 for supporting the substrate is fabricated. Figure 22A The Si photonic substrate 210A shown is flipped upside down, and the Si substrate 221 used to support the substrate is bonded to the surface of the BOX layer 212, thereby obtaining... Figure 22B The Si photonic substrate 210B, formed using the second process, is shown. An example of a Si substrate 221 as a support substrate is also shown; however, a crystal substrate with a low dielectric constant can be used, and appropriate modifications can be made. When a crystal substrate is used as the support substrate, it is suitable for the high-speed operation of the optical modulator 1C.

[0224] Figure 22C This is a schematic cross-sectional view showing an example of a Si photonic substrate 210C after the first removal process. (The image is obtained from...) Figure 22B The Si photonic substrate 210B shown in the figure has a portion of the Si substrate 211 for Si photonics and a portion of the BOX layer 212 on the Si substrate 211 for Si photonics removed, resulting in the following: Figure 22C The Si photonic substrate 210C shown has undergone the first removal process. In this case, the surface of the BOX layer 212 is removed, so that the Si waveguide is located at a position close to the surface of the BOX layer 212, a few hundred nanometers away.

[0225] Figure 22D This is a schematic cross-sectional view showing an example of a Si photonic substrate 210D formed using a third fabrication process. An LN-side Si substrate 231 with an integrated thin-film LN layer 232 is fabricated. The thin-film LN layer 232 on the LN-side Si substrate 231 is bonded to… Figure 22C On the surface of the BOX layer 212 of the Si photonic substrate 210C shown in the figure, thereby obtaining Figure 22DThe Si photonic substrate 210D shown is formed using a third process. In the previous process, the Si waveguide was located close to the surface of the BOX layer 212, which reduced the difference between the thin-film LN waveguide, which serves as the thin-film LN layer 232, and the Si waveguide, and simplified optical coupling.

[0226] Figure 22E This is a schematic cross-sectional view showing an example of a Si photonic substrate 210E after the second removal process. (The image is obtained from...) Figure 22D The Si photonic substrate 210D shown in the figure has its LN-side Si substrate 231 removed and polished so that the thickness of the thin film LN layer 232 reaches a predetermined thickness (e.g., about 500 nm), resulting in a material with the following properties: Figure 22E The Si photonic substrate 210E shown has undergone the second removal process.

[0227] Figure 22F This is a schematic cross-sectional view showing an example of a Si photonic substrate 210F after a fourth forming process. (The remaining text appears to be incomplete and requires further context.) Figure 22E A portion of the thin-film LN layer 232 on the BOX layer 212 of the Si photonic substrate 210E shown is dry-etched to form a ribbed waveguide 232A, for example, the first input-side arm waveguide 22 of the thin-film LN layer 232. As a result, a waveguide 232A is obtained as shown in the diagram. Figure 22F The Si photonic substrate 210F shown is formed using the fourth process.

[0228] Figure 22G This is a schematic cross-sectional view showing an example of a Si photonic substrate 210G after a fifth forming process. (The image shows the substrate being formed on...) Figure 22F An upper cladding layer 73 is formed on the rib waveguide 232A in the thin film LN layer 232 of the BOX layer 212 of the Si photonic substrate 210F shown in the figure, to obtain the desired effect. Figure 22G The Si photonic substrate 210G shown is processed using the fifth forming process.

[0229] Figure 22H This is a schematic cross-sectional view showing an example of a Si photonic substrate 210H after a sixth forming process. Figure 22G The Si photonic substrate 210G shown includes a via 240 for forming electrode wiring 50A connected to the heater terminal 124B of the heater 124A, a plate for forming a ribbed waveguide 232A of the thin film LN layer 232, and a PD terminal 136A2 for the first light receiving element 136A. As a result, the following is obtained: Figure 22H The Si photonic substrate 210H shown is formed using the sixth process.

[0230] Figure 22I This is a schematic cross-sectional view showing an example of a Si photonic substrate 210I after the seventh forming process. By pointing to... Figure 22HThe Si photonic substrate 210H shown in the figure has electrode material injected into each via 240 to form electrode wiring 50A, thereby obtaining the following: Figure 22I The Si photonic substrate 210I shown is formed using the seventh process.

[0231] In the optical transceiver 1E of the sixth embodiment, the optical modulator element 130A and optical receiver element 130B, including the DP-IQ modulator 1D, are mounted using silicon photonics technology, enabling each optical modulator 1C in the optical modulator 1C to ensure speed matching and reduce the size of the folded portion. As a result, the overall chip size can be reduced in the optical transceiver 1E with the DP-IQ modulator 1D mounted.

[0232] The optical modulator element 130A includes a DP-IQ modulator 1D incorporating multiple optical modulators 1C. Furthermore, the DP-IQ modulator 1D can improve speed mismatch while ensuring a longer operating length, thereby reducing the half-wave voltage Vp and improving the modulation efficiency of the DP-IQ modulator 1D.

[0233] Meanwhile, the following describes an embodiment of the optical module 1F equipped with the optical transceiver 1E of the sixth embodiment as the seventh embodiment.

[0234] (g) Seventh Implementation Method

[0235] Figure 23 This is a diagram illustrating an example of the configuration of the optical module 1F in the seventh embodiment. Meanwhile, components identical to those in the optical transceiver 1E of the sixth embodiment are indicated by the same reference numerals, and descriptions of the same components and operations will be omitted. Figure 23 The optical module 1F shown is a coherent optical subassembly (COSA) that includes an optical transceiver 1E, an optical fiber array 141, a driver (DRV) circuit 142, and a transimpedance amplifier (TIA) circuit 143.

[0236] The fiber array 141 is an array in which fiber F2, which is connected to the transmission light output port 115, fiber F1, which is connected to the LD input port 111, and fiber F3, which is connected to the receiving light input port 116, are connected together.

[0237] DRV circuit 142 is a drive circuit that applies an electrical signal to the signal electrode 51 in each of the optical modulators 1C. TIA circuit 143 is an amplifier that amplifies the electrical signal obtained by the electrical conversion performed by the first optical receiving element 136A and the second optical receiving element 136B, and outputs the amplified electrical signal.

[0238] The optical module 1F of the seventh embodiment includes an optical transceiver 1E in which a plurality of optical modulators 1C are incorporated, such that each of the optical modulators 1C can ensure speed matching and reduce the size of the folded portion. As a result, the overall chip size can be reduced in the optical module 1F in which the optical transceivers 1E are incorporated.

[0239] Meanwhile, the implementation of the optical transceiver 1G equipped with the optical module 1F of the seventh embodiment will be described below as the eighth embodiment.

[0240] (h) Eighth Implementation

[0241] Figure 24 This is a diagram illustrating an example of the optical transceiver 1G of the eighth embodiment. Components identical to those in the optical module 1F of the seventh embodiment are indicated by the same reference numerals, and descriptions of the same components and operations will be omitted. Figure 24 The optical transceiver 1G shown includes a laser diode (LD) 151, an optical module 1F, and a digital signal processor (DSP) 152. The optical transceiver 1G is a compact transceiver conforming to QSFP, OSFP, and other standards. For example, the LD 151 is a light source emitting a laser. 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. For example, the optical modulator element 130A is a DP-IQ modulator 1D, etc. 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 processing of transmitted signals and demodulation processing of received signals).

[0242] DSP 152 performs processing such as encoding on the transmitted data, generates an electrical signal including the transmitted data, and outputs the generated electrical signal to DRV circuit 142. DRV circuit 142 drives optical modulator element 130A according to the electrical signal from DSP 152.

[0243] Optical receiver element 130B performs electrical conversion on the signal light. TIA circuit 143 amplifies the converted electrical signal and outputs the amplified electrical signal to DSP 152. DSP 152 performs processing such as decoding on the electrical signal obtained from TIA circuit 143 and obtains the received data.

[0244] Meanwhile, for ease of explanation, an example has been described in which optical modulator element 130A and optical receiver element 130B are combined in optical transceiver 1G, but optical transmission devices that combine only optical modulator element 130A are also applicable.

[0245] The optical transceiver 1G of the eighth embodiment includes an optical module 1F, which incorporates an optical transceiver 1E with a plurality of optical modulators 1C. Each optical modulator 1C ensures speed matching and reduces the size of the folded portion. As a result, the overall size of the optical transceiver 1G incorporating the optical module 1F can be reduced.

[0246] The components of each unit shown in the accompanying drawings do not always need to be physically configured in the manner illustrated. In other words, the specific distribution and integration of each unit are not limited to those shown in the drawings, and all or some units may be functionally or physically distributed or integrated in any unit, depending on various loads or usage conditions.

[0247] Furthermore, all or any part of the various processing functions implemented by the device can be implemented by a central processing unit (CPU) (or a microcomputer, such as a microprocessor unit (MPU) or a microcontroller unit (MCU)). In addition, all or any part of the various processing functions can be implemented by programs analyzed and executed by the CPU, or by hardware using wired logic.

[0248] One aspect can ensure speed matching and reduce the chip size of the optical modulator.

Claims

1. An optical modulator comprising: a substrate including a high refractive index waveguide; a first coupler arranged on the substrate and splitting signal light into two beams of light; a first waveguide arranged on the substrate and connected to one output end of the first coupler; a second waveguide arranged on the substrate and connected to the other output end of the first coupler; a second coupler arranged on the substrate, coupling signal light from the first waveguide and signal light from the second waveguide, and outputting the coupled signal light; and an electrode applying an electric signal to the first waveguide and the second waveguide, wherein the first waveguide includes: a first input side arm waveguide connected to the first coupler; a first output side arm waveguide connected to the second coupler; and a first folded waveguide connected between the first input side arm waveguide and the first output side arm waveguide, the second waveguide includes: a second input side arm waveguide connected to the first coupler; a second output side arm waveguide connected to the second coupler; and a second folded waveguide connected between the second input side arm waveguide and the second output side arm waveguide, the first input side arm waveguide, the second input side arm waveguide, the first output side arm waveguide, and the second output side arm waveguide are waveguides including a material having high EO characteristics, as compared with the high refractive index waveguide, and at least a part of the first folded waveguide and the second folded waveguide are waveguides including a material having low refractive index, as compared with the high refractive index waveguide.

2. The optical modulator according to claim 1, wherein the first waveguide includes: the first input side arm waveguide located at an outer circumferential side of folding; the first output side arm waveguide located at an inner circumferential side of folding; and the first folded waveguide connected between the first input side arm waveguide and the first output side arm waveguide, the second waveguide includes: the second input side arm waveguide located at an inner circumferential side of folding; the second output side arm waveguide located at an outer circumferential side of folding; and the second folded waveguide connected between the second input side arm waveguide and the second output side arm waveguide.

3. The optical modulator according to claim 2, wherein the first folded waveguide includes: a first input side high refractive index waveguide employing 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 employing the high refractive index layer as a core layer and connected to the first output side arm waveguide; and ​ a first low-index waveguide employing the first-stage low-index layer on the substrate as a core layer and connected between the first input-side high-index waveguide and the first output-side high-index waveguide, the second folded waveguide includes: a second input-side high-index waveguide employing the high-index layer as a core layer and connected to the second input-side arm waveguide; a second output-side high-index waveguide employing the high-index layer as a core layer and connected to the second output-side arm waveguide; and a second low-index waveguide employing the first-stage low-index layer as a core layer and connected between the second input-side high-index waveguide and the second output-side high-index waveguide.

4. The optical modulator of claim 3, wherein the first low-index waveguide allows a transfer of the signal light with respect to the first input-side high-index waveguide and allows a transfer of the signal light with respect to the first output-side high-index waveguide, the second low-index waveguide allows a transfer of the signal light with respect to the second input-side high-index waveguide and allows a transfer of the signal light with respect to the second output-side high-index waveguide, and one of the first input-side high-index waveguide and the second output-side high-index waveguide crosses the low-index waveguide.

5. The optical modulator of claim 2, wherein the first folded waveguide includes: a first input-side high-index waveguide employing a high-index layer on the substrate as a core layer and connected to the first input-side arm waveguide; a first output-side high-index waveguide employing the high-index layer as a core layer and connected to the first output-side arm waveguide; a first input-side first-stage index waveguide employing a first-stage low-index layer on the substrate as a core layer and connected to the first input-side high-index waveguide; a first output-side first-stage index waveguide formed on the first-stage low-index layer and connected to the first output-side high-index waveguide; and a first second-stage low-index waveguide employing a second-stage low-index layer on the substrate as a core layer and connected between the first input-side first-stage index waveguide and the first output-side first-stage index waveguide, the second folded waveguide includes: a second input-side high-index waveguide employing the high-index layer as a core layer and connected to the second input-side arm waveguide; a second output-side high-index waveguide employing the high-index layer as a core layer and connected to the second output-side arm waveguide; and a second low-index waveguide employing the first-stage low-index layer as a core layer and connected between the second input-side high-index waveguide and the second output-side high-index waveguide. a second input-side first-stage low-refractive waveguide that employs the first-stage low-refractive layer as a core layer and is connected to the second input-side high-refractive waveguide; a second output-side first-stage low-refractive waveguide that employs the first-stage low-refractive layer as a core layer and is connected to the second output-side high-refractive waveguide; and a second second-stage low-refractive waveguide that employs the second-stage low-refractive layer as a core layer and is connected between the second input-side first-stage low-refractive waveguide and the second output-side first-stage low-refractive waveguide.

6. The optical modulator according to claim 5, wherein the first second-stage low-refractive waveguide allows the signal light to be transferred with respect to the first input-side first-stage refractive waveguide and allows the signal light to be transferred with respect to the first output-side first-stage refractive waveguide, the second second-stage low-refractive waveguide allows the signal light to be transferred with respect to the second input-side first-stage low-refractive waveguide and allows the signal light to be transferred with respect to the second output-side first-stage low-refractive waveguide, and one of the first input-side high-refractive waveguide and the first output-side high-refractive waveguide crosses one of the first second-stage low-refractive waveguide and the second second-stage low-refractive waveguide.

7. The optical modulator according to claim 1, wherein the first waveguide includes: the first input-side arm waveguide located on the outer peripheral side of the folding; the first folded waveguide located on the outer peripheral side of the folding; and the first output-side arm waveguide located on the outer peripheral side of the folding, and the second waveguide includes: the second input-side arm waveguide located on the inner peripheral side of the folding; the second folded waveguide located on the inner peripheral side of the folding; and the second output-side arm waveguide located on the inner peripheral side of the folding.

8. The optical modulator according to claim 7, wherein the first folded waveguide includes: a first input-side high-refractive waveguide that employs a high-refractive layer on the substrate as a core layer and is connected to the first input-side arm waveguide; a first output-side high-refractive waveguide that is formed on the high-refractive layer and is connected to the first output-side arm waveguide; and a first low-refractive waveguide that employs a low-refractive layer on the substrate as a core layer and is connected between the first input-side high-refractive waveguide and the first output-side high-refractive waveguide, and the second folded waveguide includes: a second input-side high-refractive waveguide that employs the high-refractive layer as a core layer and is connected to the second input-side arm waveguide; a second output-side high-refractive waveguide that employs the high-refractive layer as a core layer and is connected to the second output-side arm waveguide; and a second low refractive index waveguide that employs the low refractive index layer as a core layer and is connected between the second input side high refractive index waveguide and the second output side high refractive index waveguide.

9. The optical modulator according to claim 3, wherein 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 waveguides that employ Si as a core, and the first low refractive index waveguide and the second low refractive index waveguide are waveguides that employ SiN as a core.

10. The optical modulator of claim 1, wherein, the material having high EO properties includes at least one of LiNbO3, BaTiO3, PZT, and PLZT.

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

12. An optical transmitter, the optical transmitter comprising: an optical modulator that modulates light guided in accordance with an electrical signal, wherein the optical modulator comprises: a substrate that includes a high refractive index waveguide; a first coupler that is arranged on the substrate and splits signal light into two beams of light; a first waveguide that is arranged on the substrate and connected to one output of the first coupler; a second waveguide that is arranged on the substrate and connected to the other output of the first coupler; a second coupler that is arranged on the substrate, couples signal light from the first waveguide and signal light from the second waveguide, and outputs the coupled signal light; and an electrode that applies an electrical signal to the first waveguide and the second waveguide, the first waveguide includes: a first input side arm waveguide that is connected to the first coupler; a first output side arm waveguide that is connected to the second coupler; and a first folded waveguide that is connected between the first input side arm waveguide and the first output side arm waveguide, the second waveguide includes: a second input side arm waveguide that is connected to the first coupler; a second output side arm waveguide that is connected to the second coupler; and a second folded waveguide that is connected between the second input side arm waveguide and the second output side arm waveguide, compared to the high refractive index waveguide, the first input side arm waveguide, the second input side arm waveguide, the first output side arm waveguide, and the second output side arm waveguide are waveguides that include a material having high EO properties, and compared to the high refractive index waveguide, at least a portion of the first folded waveguide and the second folded waveguide are waveguides that include a material having low refractive index.

13. An optical transceiver, the optical transceiver comprising: an optical modulator that modulates light guided in accordance with an electrical signal; an optical receiver that converts received signal light into an electrical signal; and an optical transmitter that transmits signal light in accordance with an electrical signal. a signal processor that generates an electric signal to be output to the optical modulator and performs processing on an electric signal obtained from the optical receiver, wherein the optical modulator includes: a substrate including a high refractive index waveguide; a first coupler arranged on the substrate and splitting signal light into two beams of light; a first waveguide arranged on the substrate and connected to one output end of the first coupler; a second waveguide arranged on the substrate and connected to the other output end of the first coupler; a second coupler arranged on the substrate, coupling signal light from the first waveguide and signal light from the second waveguide, and outputting the coupled signal light; and an electrode that applies an electric signal to the first waveguide and the second waveguide, and the first waveguide includes: a first input side arm waveguide connected to the first coupler; a first output side arm waveguide connected to the second coupler; and a first folded waveguide connected between the first input side arm waveguide and the first output side arm waveguide, the second waveguide includes: a second input side arm waveguide connected to the first coupler; a second output side arm waveguide connected to the second coupler; and a second folded waveguide connected between the second input side arm waveguide and the second output side arm waveguide, compared to the high refractive index waveguide, the first input side arm waveguide, the second input side arm waveguide, the first output side arm waveguide, and the second output side arm waveguide are waveguides including a material having high EO characteristics, and compared to the high refractive index waveguide, at least a part of the first folded waveguide and the second folded waveguide are waveguides including a material having low refractive index.

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