optical modulator

Abstract

[Technical Problem] Improve the deterioration of the extinction ratio caused by the asymmetry of a pair of optical waveguides. [Solution] An optical modulator (1) includes a Mach-Zehnder optical waveguide (2) having first and second waveguides (5A, 5B) arranged in parallel with each other, and a signal electrode for controlling the phase of light propagating in the Mach-Zehnder optical waveguide (2). The first and second waveguides (5A, 5B) have a first section (Z) in which the line width of the second waveguide (5B) is narrower than the line width of the first waveguide (5A). A1 ), and a second interval (Z ) in which the line width of the first waveguide (5A) is narrower than the line width of the second waveguide (5B) A2 ). The first interval (Z A1 ) and the second interval (Z A2 ) switches at the bending portion (5C1, 5C2).

Description

Technical Field

[0001] The present invention relates to an optical modulator, and in particular to a waveguide structure of a Mach-Zehnder optical modulator. Background Art

[0002] With the rapid growth of the internet and the rapid increase in communication volume, fiber-optic communications have become increasingly important. Fiber-optic communications convert electrical signals into optical signals and transmit them through optical fibers. They offer high bandwidth, low loss, and strong noise immunity.

[0003] Known methods for converting electrical signals into optical signals include direct modulation using semiconductor lasers and external modulation using optical modulators. Direct modulation requires no optical modulator and is cost-effective, but has limitations in high-speed modulation. Therefore, external modulation is used for high-speed, long-distance applications.

[0004] As an optical modulator, a Mach-Zehnder optical modulator, in which an optical waveguide is formed by diffusing Ti (titanium) near the surface of a lithium niobate single crystal substrate, has been put into practical use (see, for example, Patent Document 1). A Mach-Zehnder optical modulator utilizes an optical waveguide (Mach-Zehnder optical waveguide) with a Mach-Zehnder interferometer structure—a technique in which light emitted from a single light source is split into two beams that pass through different paths and then reunite to produce interference. High-speed optical modulators exceeding 40 Gb / s are already commercially available, but they suffer from a significant drawback of being approximately 10 cm in length.

[0005] In contrast, Patent Document 2 discloses a Mach-Zehnder optical modulator using a c-axis-oriented lithium niobate film. Compared to optical modulators using a lithium niobate single crystal substrate, optical modulators using a lithium niobate film can achieve significant miniaturization and lower driving voltage.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent No. 5488226

[0009] Patent Document 2: Japanese Patent No. 6456662 Summary of the Invention

[0010] Technical problem to be solved by the invention

[0011] The linewidths of the two parallel waveguides that comprise a Mach-Zehnder waveguide can sometimes be asymmetric due to factors such as the manufacturing process. In this case, the effective refractive index differs between the two waveguides, causing wavelength-dependence in the light propagation characteristics. When this wavelength-dependence is present, even if the operating wavelength of the input and output light is extinct, background light mixed with the input and output light at wavelengths other than the operating wavelength will still be guided. Consequently, the light intensity in the OFF state increases, deteriorating the extinction ratio.

[0012] Therefore, an object of the present invention is to provide an optical modulator capable of improving the deterioration of the extinction ratio caused by the asymmetry of a pair of optical waveguides.

[0013] Technical means to solve the problem

[0014] The inventors of the present application have conducted in-depth research on methods for improving the deterioration of the extinction ratio caused by the difference in effective refractive index between a pair of optical waveguides. As a result, they found that by making the length of the wide linewidth interval of one optical waveguide and the length of the wide linewidth interval of the other optical waveguide substantially equal, the effective refractive indexes between the pair of optical waveguides can be made equal, thereby improving the extinction ratio.

[0015] The present invention is made based on this technical discovery. The optical modulator of the present invention is characterized in that it includes: a Mach-Zehnder optical waveguide, which includes: an input waveguide, a beam splitting portion for splitting the light propagating in the input waveguide, a first and a second waveguide extending from the beam splitting portion and arranged parallel to each other, a beam combining portion for combining the light propagating in the first and second waveguides, and an output waveguide for propagating the light output from the beam combining portion; and a signal electrode, which controls the phase of the light propagating in the Mach-Zehnder optical waveguide, wherein the first and second waveguides have: a first interval in which the line width of the second waveguide is narrower than the line width of the first waveguide; and a second interval in which the line width of the first waveguide is narrower than the line width of the second waveguide, and the first interval and the second interval are switched at a curved portion.

[0016] According to the present invention, it is possible to improve the deterioration of the extinction ratio caused by the asymmetry of the first and second waveguides constituting a pair of optical waveguides.

[0017] In the present invention, the first and second waveguides preferably have a folded structure comprising alternating straight sections and curved sections, with the first section and the second section switching at the curved section. By forming the section where the line widths of the first and second waveguides constituting the Mach-Zehnder optical waveguide interchange in a folded shape, the line width can be continuously varied, thereby preventing optical loss and achieving miniaturization of the optical modulator.

[0018] In the present invention, it is preferable that the first and second waveguides have an even number of the bent portions, thereby making it possible to make the lengths of the bent portions of the first and second waveguides constituting the Mach-Zehnder waveguide equal to each other.

[0019] In the present invention, the first and second waveguides preferably include: first to third linear portions arranged parallel to each other; a first curved portion connecting the first linear portion and the second linear portion; and a second curved portion connecting the second linear portion and the third linear portion. The first section is provided between the first and third linear portions, and the second section is provided within the second linear portion. The first and second sections switch between the first and second curved portions. This allows for an optical modulator with improved symmetry between the first and second waveguides.

[0020] In the present invention, the waveguide is preferably a ridge waveguide formed by forming a lithium niobate film formed on a substrate into a ridge shape. By using a ridge waveguide made of a lithium niobate film, a curved portion can be formed with a large curvature, which can reduce the size of the optical modulator.

[0021] Preferably, the optical modulator of the present invention includes a buffer layer covering at least the upper surfaces of the first and second waveguides, and the signal electrode faces the upper surface of the first waveguide via the buffer layer. This enables realization of an optical modulator with low drive voltage and excellent electro-optical characteristics.

[0022] Effects of the Invention

[0023] According to the present invention, it is possible to provide an optical modulator capable of improving the deterioration of the extinction ratio caused by the asymmetry of a pair of optical waveguides. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a schematic plan view showing the structure of the optical modulator according to the first embodiment of the present invention, and only the optical waveguide is shown.

[0025] Figure 2 This is a schematic plan view showing the structure of the optical modulator according to the first embodiment of the present invention, and illustrates the entire optical modulator including the traveling-wave electrodes.

[0026] Figure 3 (a) and (b) are schematic diagrams for explaining the improvement of the asymmetry of a pair of optical waveguides.

[0027] Figure 4 (a) to (c) are Figure 1 and Figure 2 The schematic cross-sectional view of the light modulator shown in FIG. Figure 1 and Figure 2 The cross-sectional view along the Xa-Xa line, (b) is the cross-sectional view along the Xa-Xa line, Figure 1and Figure 2 The cross-sectional view along the Xb-Xb line, (c) is the cross-sectional view along the Xb-Xb line, Figure 1 and Figure 2 Cross-sectional view of the Xc-Xc line.

[0028] Figure 5 (a) and (b) are schematic plan views showing the structure of an optical modulator according to a second embodiment of the present invention, wherein (a) shows only the optical waveguide, and (b) shows the entire optical modulator including the traveling-wave electrode.

[0029] Figure 6 (a) and (b) are Figure 5 The schematic cross-sectional views of the optical modulator shown in (a) and (b) are as follows: Figure 5 The cross-sectional view of the Xa-Xa line of (a) and (b), (b) is along the Figure 5 Cross-sectional views along the Xb-Xb line in (a) and (b).

[0030] Figure 7 is the second constant line width interval Z A2 Length L A2 Relative to the first line width constant interval Z A1 Length L A1 The relationship between the ratio of the optical modulator and the extinction ratio is shown in the coordinate diagram. The horizontal axis represents L A2 / L A1 , the vertical axis represents the extinction ratio at the working wavelength of 1550nm. DETAILED DESCRIPTION

[0031] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0032] Figure 1 and Figure 2 is a schematic plan view showing the structure of an optical modulator according to a first embodiment of the present invention. Figure 1 Only the optical waveguide is shown. Figure 2 The entire optical modulator including the traveling wave electrodes is shown.

[0033] like Figure 1 and Figure 2 As shown, the optical modulator 1 of this embodiment includes: a Mach-Zehnder optical waveguide 2, which is formed on a substrate 10 and includes first and second waveguides 5A and 5B arranged parallel to each other; a signal electrode 21, which is arranged so as to overlap with the first waveguide 5A when viewed from above; a first ground electrode 22, which is arranged so as to overlap with the second waveguide 5B when viewed from above; and a second ground electrode 23, which is arranged on the side opposite to the first ground electrode 22 when viewed from the signal electrode 21.

[0034] The Mach-Zehnder optical waveguide 2 is an optical waveguide having a Mach-Zehnder interferometer structure, and has, from the optical input port 1i to the optical output port 1o, an input waveguide 3 consisting of an optical waveguide, a beam splitting section 4 for splitting the light propagating in the input waveguide 3, a pair of parallel waveguides 5 consisting of a first and a second waveguide 5A, 5B extending from the beam splitting section 4, a beam combining section 6 for combining the light propagating in the first and the second waveguides 5A, 5B, and an output waveguide 7 consisting of an optical waveguide for propagating the light output from the beam combining section 6.

[0035] The input waveguide 3 extending from the optical input port 1i is connected to one end of the first and second waveguides 5A and 5B via the beam splitter 4, and the other ends of the first and second waveguides 5A and 5B are connected to the output waveguide 7 via the beam combiner 6. in The light is input to the beam splitter 4 through the input waveguide 3, split by the beam splitter 4, and combined by the beam combiner 6 after traveling through the first and second waveguides 5A and 5B. The light is then output from the optical output port 1o as modulated light S through the output waveguide 7. out is output.

[0036] In this embodiment, the Mach-Zehnder waveguide 2 is composed of a combination of straight sections and curved sections. Specifically, the first and second waveguides 5A and 5B of the Mach-Zehnder waveguide 2 have first to third straight sections 5S1 to 5S3 arranged parallel to each other, a first curved section 5C1 connecting the first straight section 5S1 and the second straight section 5S2, and a second curved section 5C2 connecting the second straight section 5S2 and the third straight section 5S3. The first and second curved sections 5C1 and 5C2 have a folded shape that changes the direction of travel of the optical waveguide 180 degrees, forming concentric semicircular shapes. Thus, the first and second waveguides 5A and 5B have a folded structure composed of an alternating combination of straight sections and curved sections.

[0037] The first to third straight sections 5S1 to 5S3 of the first and second waveguides 5A and 5B of the Mach-Zehnder optical waveguide 2 extend along the longitudinal direction (Y direction) of the substrate 10. The optical input port 1i is provided at one end of the substrate 10 in this longitudinal direction, and the optical output port 1o is provided at the other end of the substrate 10 in this longitudinal direction. In this embodiment, the first and second waveguides 5A and 5B preferably have an even number of curved sections 5C. That is, the first and second waveguides 5A and 5B preferably have 2n+1 (where n is a positive integer) straight sections 5S and 2n curved sections 5C. This allows the total length of the first waveguide 5A and the total length of the second waveguide 5B within the multiple curved sections 5C to be equal, thereby improving the symmetry between the first waveguide 5A and the second waveguide 5B.

[0038] In the above structure, the input light S input to the optical input port 1i inIt is input to one end of the first straight section 5S1, travels from one end to the other end of the first straight section 5S1, turns back at the first curved section 5C1, and travels from one end to the other end of the second straight section 5S2 in the opposite direction of the first straight section 5S1. Then, it turns back at the second curved section 5C2 and travels from one end to the other end of the third straight section 5S3 in the same direction as the first straight section 5S1. out The light is output from the optical output port 1o.

[0039] The signal electrode 21 (first control electrode) is provided along a portion of the first waveguide 5A and is positioned between the first ground electrode 22 and the second ground electrode 23 in a plan view. The first ground electrode 22 (second control electrode) is provided along a portion of the second waveguide 5B. One end of the signal electrode 21 serves as an RF signal input port, and the other end is connected to the first and second ground electrodes 22 and 23, respectively, via a terminal resistor 24. Thus, the signal electrode 21 and the first and second ground electrodes 22 and 23 function as coplanar traveling-wave electrodes.

[0040] An electrical signal (modulation signal) is input to the RF signal input port of the signal electrode 21. The first and second waveguides 5A and 5B are made of an electro-optical material, typically lithium niobate. Therefore, applying an electric field to the first and second waveguides 5A and 5B causes the refractive indexes of the first and second waveguides 5A and 5B to change by +Δn and -Δn, respectively, thereby changing the phase difference between the pair of optical waveguides. Signal light modulated by this phase difference change is output from the optical output port 1o. If the phase difference between the two beams propagating through the first and second waveguides 5A and 5B is large, the two beams cancel each other out, resulting in an OFF state. If the phase difference is small, the intensities of the two beams increase, resulting in an ON state (connected state). The extinction ratio is the ratio of the light intensity when the optical modulator 1 is in the ON state to the light intensity when it is in the OFF state. If the ON state is bright and the OFF state is dim, the extinction ratio is high; if the ON state is dim and the OFF state is bright, the extinction ratio is low.

[0041] To apply a DC bias, a pair of bias electrodes (not shown) may be provided at a position overlapping the first and second waveguides 5A and 5B when viewed from above. One end of the pair of bias electrodes serves as an input port for the DC bias. The region where the pair of bias electrodes are formed can be located closer to the optical input port 1i of the Mach-Zehnder optical waveguide 2 than the region where the signal electrode 21 is formed, or closer to the optical output port 1o. Alternatively, the bias electrodes can be omitted, and a modulated signal pre-superimposed with a DC bias can be input to the RF signal input port.

[0042] The Mach-Zehnder waveguide 2 of the present embodiment is characterized in that the line widths of first and second waveguides 5A and 5B branched from the input waveguide 3 and provided parallel to each other differ depending on their positions in the longitudinal direction.

[0043] In the parallel waveguide 5 composed of the combination of the first and second waveguides 5A and 5B, the first line width constant intervals Z are sequentially provided from the optical input port 1i side to the optical output port 1o side. A1 , the first line width transition interval Z B1 , the second line width constant interval Z A2 , the second line width transition interval Z B2 and the first line width constant interval Z A1 In particular, the first straight line portion 5S1 and the third straight line portion 5S3 constitute the first constant line width interval Z A1 The second straight line portion 5S2 constitutes the second constant line width interval Z A2 In addition, the first curved portion 5C1 constitutes the first line width transition zone Z B1 The second curved portion 5C2 constitutes the second line width transition zone Z B2 .

[0044] Here, the first line width constant interval Z A1 (First section) is a section in which the line widths of the first and second waveguides 5A and 5B are constant and the line width of the second waveguide 5B is narrower than the line width of the first waveguide 5A. A2 (Second interval) is an interval in which the line widths of the first and second waveguides 5A and 5B are constant and the line width of the first waveguide 5A is narrower than the line width of the second waveguide 5B. B1 The line width transition interval Z is the interval where the line widths of the first waveguide 5A and the second waveguide 5B change. B2 This is the interval in which the line widths of the first waveguide 5A and the second waveguide 5B change. In addition, the constant line width interval does not necessarily require a completely constant line width, and includes a case where the line width change is relatively small compared to the line width transition interval.

[0045] For the first constant line width interval Z A1 The line width W of the first waveguide 5A in a1 、W a3 For example, as long as the line width is relatively wide in relation to the second waveguide 5B (W a1 <W b1 、W a3 <W b3 ), there is no need to be in the second line width constant interval Z A2 The line width W of the first waveguide 5A in a2 That is, the first line width constant interval Z of the first waveguide 5A is relatively wide. A1 Line width W in a1 and the second line width constant interval Z A2 Line width W in a2 Even if there is Wa1 <W a2 This relationship also applies to the second waveguide 5B.

[0046] The first line width constant interval Z A1 The total length (L1+L5) is preferably equal to the second line width constant interval Z A2 That is, preferably, the length (L1+L5) of the portion with a relatively wider line width in the first waveguide 5A compared to the second waveguide 5B and the length (L3) of the portion with a relatively narrow line width are equal, and the length (L3) of the portion with a relatively wider line width in the second waveguide 5B compared to the first waveguide 5A and the length (L1+L5) of the portion with a relatively narrow line width are equal. More preferably, the first and second line width transition intervals Z B1 , Z B2 The total length of the first waveguide 5A and the total length of the second waveguide 5B are also equal. A1 and the second line width constant interval Z A2 The lengths do not need to be exactly the same and can vary slightly.

[0047] Figure 3 (a) and (b) are schematic diagrams for explaining the improvement of the asymmetry of a pair of optical waveguides.

[0048] When the waveguide layer 11 on the substrate 10 is processed into a ridge shape to form a plurality of linear waveguides extending in the Y direction and parallel to each other, the line width of the linear waveguides may vary due to the manufacturing process. Figure 3 As shown in (a), among the six linear waveguides SL1 to SL6 extending in the Y direction, the line width W of the linear waveguide SL1 closest to one end side (origin P0) in the X direction may be SL1 The line width W of the six linear waveguides SL1 to SL6 is the widest and the farther away from one end in the X direction, the narrower the line width. SL1 ~W SL6 Satisfy W SL1 >W SL2 >W SL3 >W SL4 >W SL5 >W SL6 In this case, since the line widths of two linear waveguides adjacent to each other in the X direction selected from these linear waveguides are always different, the effective refractive indexes of the two linear waveguides are also different, thereby deteriorating the extinction ratio of the optical modulation element.

[0049] However, if Figure 3 As shown in (b), one end of the linear waveguide SL1 is e 11 One end of the straight waveguide SL4 14Connect one end of the straight waveguide SL2 through the curved waveguide CL1. 12 One end of the linear waveguide SL3 13 Connect the other end of the straight waveguide SL3 through the curved waveguide CL2. 23 The other end of the linear waveguide SL6 26 Connect the other end of the straight waveguide SL4 through the curved waveguide CL3. 24 The other end of the linear waveguide SL5 25 By connecting by bending waveguide CL4, in this case, a relatively wide line width section and a relatively narrow line width section can be mixed on one optical waveguide. Figure 3 As shown, by making the total length of the section with a relatively wider linewidth and the total length of the section with a relatively narrower linewidth in each of the first and second waveguides 5A and 5B substantially equal, the asymmetry of the first and second waveguides 5A and 5B can be offset. Consequently, the effective refractive indexes of the pair of optical waveguides can be made uniform, thereby improving the extinction ratio of the optical modulator 1.

[0050] Figure 4 (a) to (c) are Figure 1 and Figure 2 The schematic cross-sectional view of the light modulator 1 is shown in FIG. 1 , wherein (a) is a cross-sectional view of the light modulator 1 along Figure 1 and Figure 2 The cross-sectional view along the Xa-Xa line, (b) is the cross-sectional view along the Xa-Xa line, Figure 1 and Figure 2 The cross-sectional view along the Xb-Xb line, (c) is the cross-sectional view along the Xb-Xb line, Figure 1 and Figure 2 Cross-sectional view of the Xc-Xc line.

[0051] like Figure 4 As shown in (a) to (c) in FIG. 1 , the optical modulator 1 of this embodiment has a multilayer structure in which a substrate 10 , a waveguide layer 11 , a protective layer 12 , a buffer layer 13 , an insulating layer 14 , and an electrode layer 15 are stacked in this order.

[0052] The substrate 10 is, for example, a sapphire substrate, and a waveguide layer 11 composed of a lithium niobate film is formed on the surface of the substrate 10. The waveguide layer 11 includes first and second waveguides 5A and 5B (ridge waveguides) formed by a ridge 11r. The width of the first and second waveguides 5A and 5B can be, for example, approximately 1 μm. The waveguide layer 11 can be made of any electro-optical material, but is preferably made of lithium niobate (LiNbO3). Lithium niobate has a large electro-optical constant and is therefore a preferred material for optical devices such as optical modulators. If the width of the ridge 11r is non-uniform in the Z-axis direction, the average value of the width is used as the linewidth.

[0053] The protective layer 12 is formed in an area that does not overlap with the first and second waveguides 5A and 5B when viewed from above. The protective layer 12 covers the entire area of the upper surface of the waveguide layer 11 where the ridge 11r is not formed. The side surfaces of the ridge 11r are also covered by the protective layer 12, thereby preventing scattering loss caused by the roughness of the side surfaces of the ridge 11r. The thickness of the protective layer 12 is approximately the same as the height of the ridge 11r of the waveguide layer 11. The material of the protective layer 12 is not particularly limited; for example, silicon oxide (SiO2) can be used.

[0054] To prevent light propagating through the first and second waveguides 5A and 5B from being absorbed by the signal electrode 21 or the first ground electrode 22, a buffer layer 13 is formed to cover the upper surface of the ridge 11r that constitutes the first and second waveguides 5A and 5B. The buffer layer 13 is preferably made of a material with a lower refractive index than the waveguide layer 11 and high transparency. The thickness of the buffer layer 13 directly above the ridge 11r is preferably between 0.3 μm and 3 μm. To reduce light absorption by the electrodes, the buffer layer 13 should be as thick as possible. To apply a high electric field to the first and second waveguides 5A and 5B, the buffer layer 13 should be as thin as possible. Because there is a trade-off between light absorption by the electrodes and the applied voltage, it is important to select a material with a high dielectric constant and a low refractive index to strike a balance between the two.

[0055] In this embodiment, the buffer layer 13 covers not only the upper surfaces of the first and second waveguides 5A and 5B but also the entire base surface, including the upper surface of the protective layer 12. However, the buffer layer 13 may be patterned so as to selectively cover only the vicinity of the upper surfaces of the first and second waveguides 5A and 5B. Alternatively, the protective layer 12 may be omitted, and the buffer layer 13 may be formed directly on the entire upper surface of the waveguide layer 11.

[0056] The insulating layer 14 is provided to form a step on the lower surface of the traveling-wave electrode. An opening (slit) is formed in the region of the insulating layer 14 that overlaps the first and second waveguides 5A and 5B, exposing the upper surface of the buffer layer 13. By embedding a portion of the electrode layer 15 within this opening, a step is formed on the lower surface of the signal electrode 21 and the first ground electrode 22. The thickness of the insulating layer 14 is preferably 1 μm or greater. A thickness of 1 μm or greater achieves the effect of providing a step on the lower surface of the signal electrode 21 and the first ground electrode 22.

[0057] The electrode layer 15 includes a signal electrode 21, a first ground electrode 22, and a second ground electrode 23. The signal electrode 21 (first control electrode) is provided to overlap the ridge 11r corresponding to the first waveguide 5A in order to modulate light propagating through the first waveguide 5A. It faces the first waveguide 5A across the buffer layer 13. The first ground electrode 22 (second control electrode) is provided to overlap the ridge 11r corresponding to the second waveguide 5B in order to modulate light propagating through the second waveguide 5B. It faces the second waveguide 5B across the buffer layer 13. The second ground electrode 23 is provided on the opposite side of the signal electrode 21 from the first ground electrode 22.

[0058] The signal electrode 21 has a two-layer structure, comprising an upper portion 21a formed in the electrode layer 15 and a lower portion 21b embedded in an opening extending through the insulating layer 14. The lower portion 21b of the signal electrode 21 is located at the end of the upper portion 21a of the signal electrode 21 that is closest to the first ground electrode 22. Therefore, the lower surface of the lower portion 21b of the signal electrode 21 is located closer to the first ground electrode 22 than the lower surface of the upper portion 21a. With this structure, the lower surface of the lower portion 21b of the signal electrode 21 contacts the upper surface of the buffer layer 13 above the first waveguide 5A, covering the first waveguide 5A via the buffer layer 13. The lower surface of the upper portion 21a of the signal electrode 21 is located above the lower surface of the lower portion 21b and does not contact the buffer layer 13.

[0059] The first ground electrode 22 also has a two-layer structure, comprising an upper portion 22a formed in the electrode layer 15 and a lower portion 22b embedded in an opening extending through the insulating layer 14. The lower portion 22b of the first ground electrode 22 is located at the end of the upper portion 22a of the first ground electrode 22 that is closer to the signal electrode 21. Therefore, the lower surface of the upper portion 22a of the first ground electrode 22 is located closer to the signal electrode 21 than the lower surface of the lower portion 22b. With this structure, the lower surface of the lower portion 22b of the first ground electrode 22 contacts the upper surface of the buffer layer 13 above the second waveguide 5B, covering the second waveguide 5B via the buffer layer 13. The lower surface of the upper portion 22a of the first ground electrode 22 is located above the lower surface of the lower portion 22b and does not contact the buffer layer 13.

[0060] The second ground electrode 23 is provided on the opposite side of the first ground electrode 22 across the signal electrode 21. The second ground electrode 23 has a single-layer structure consisting only of a conductor provided in the electrode layer 15, but may also have a double-layer structure like the signal electrode 21 or the first ground electrode 22.

[0061] As mentioned above, the line width of a pair of optical waveguides may be asymmetric due to the manufacturing process, which may lead to a deterioration of the extinction ratio. A1 like Figure 4 (a) and Figure 4 As shown in (c), the line width W of the first waveguide 5A is a1 、W a3 Than the line width W of the second waveguide 5B b1 、W b3 width, and conversely in the second line width constant interval Z A2 like Figure 4 As shown in (b), the line width W of the second waveguide 5B is b2 Than the line width W of the first waveguide 5A a2 width, and make the first line width constant in interval Z A1 The length of the second line width constant interval Z A2 The lengths of the two waveguides are substantially the same, so the symmetry of the line widths of the pair of waveguides can be improved to improve the extinction ratio.

[0062] Next, the structure of the optical modulator 1 in the case where the waveguide layer 11 is formed of a lithium niobate film will be described in detail.

[0063] The substrate 10 is not particularly limited as long as its refractive index is lower than that of the lithium niobate film. It is preferably a substrate capable of forming a lithium niobate film as an epitaxial film, and is particularly preferably a sapphire single crystal substrate or a silicon single crystal substrate. The crystal orientation of the single crystal substrate is not particularly limited. The lithium niobate film has the property of being easily formed as a c-axis oriented epitaxial film for single crystal substrates of various crystal orientations. The c-axis oriented lithium niobate film has a three-dimensional symmetry, so it is ideal that the underlying single crystal substrate also has the same symmetry. In the case of a sapphire single crystal substrate, a c-plane substrate is preferred, and in the case of a silicon single crystal substrate, a (111) plane substrate is preferred.

[0064] Here, an epitaxial film refers to a film whose crystal orientation is uniformly aligned with the underlying substrate or base film. When the film's in-plane is defined as the XY plane and the film thickness is defined as the Z axis, the crystals are uniformly aligned along the X, Y, and Z axes. For example, confirmation of an epitaxial film can be achieved by first confirming the peak intensity at the orientation position using θ-2θ X-ray diffraction and then confirming the poles.

[0065] Specifically, in the first step of θ-2θ X-ray diffraction measurement, the peak intensities of all planes other than the target plane (the target plane) must be less than 10%, and preferably less than 5%, of the maximum peak intensity of the target plane. For example, in a c-axis-oriented epitaxial film of lithium niobate, the peak intensities of all planes other than the (00L) plane must be less than 10%, and preferably less than 5%, of the maximum peak intensity of the (00L) plane. (00L) is a general term for planes equivalent to (001) or (002).

[0066] In the second-step pole measurement, it is necessary to be able to observe the poles. Under the conditions of the peak intensity at the orientation confirmation position in the first step, the orientation in only one direction can be indicated. Even if the conditions of the first step are met, if the crystal orientation is inconsistent within the plane, the X-ray intensity will not increase at a specific angle position, and the poles cannot be observed. LiNbO3 has a trigonal crystal structure, so there are three poles in the LiNbO3 (014) single crystal. It is known that in the case of lithium niobate films, epitaxial growth is carried out in a so-called twin state—a state obtained by symmetrical bonding of crystals rotated 180 degrees around the c-axis. In this case, since the two are bonded in a state of three poles symmetrically, there are six poles. In addition, when the lithium niobate film is formed on a silicon single crystal substrate with a (100) plane, since the substrate is four-fold symmetrical, 4×3=12 poles can be observed. In addition, in the present invention, lithium niobate films epitaxially grown in a twin state are also included in the epitaxial film.

[0067] The composition of the lithium niobate film is LixNbAyOz. A represents an element other than Li, Nb, and O. x is 0.5 to 1.2, preferably 0.9 to 1.05. y is 0 to 0.5. z is 1.5 to 4, preferably 2.5 to 3.5. Elements representing A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, and Ce, and combinations of two or more are also acceptable.

[0068] Ideally, the lithium niobate film has a thickness of 2 μm or less. This is because if the film thickness exceeds 2 μm, it becomes difficult to form a high-quality film. On the other hand, if the lithium niobate film is too thin, the film's ability to confine light becomes weak, allowing light to leak into substrate 10 or buffer layer 13. Furthermore, the change in the effective refractive index of the optical waveguide when an electric field is applied may be reduced. Therefore, the lithium niobate film's thickness is ideally at least approximately 1 / 10 the wavelength of the light being used.

[0069] The thickness of the lithium niobate film in the electric field application region is preferably 1 μm or more, particularly preferably 1.4 μm or more. This is because, when the wavelength λ of light is 1550 nm, which is used in optical communication systems, if the thickness of the lithium niobate film is less than 1 μm, the half-wave voltage Vπ increases sharply, making it difficult to reduce the half-wave voltage Vπ to a practical voltage value of 3 V or less. This is because if the film thickness is thin, the confinement of light in the lithium niobate film becomes weak, and the electro-optical effect becomes substantially smaller. On the other hand, if the film thickness is 1.5 μm or more, the confinement of light becomes sufficiently strong, so even if the film thickness is further increased, Vπ remains almost unchanged. As described above, when the thickness of the lithium niobate film is 1 μm or more, the driving voltage or propagation loss can be reduced.

[0070] As a method for forming a lithium niobate film, it is ideal to utilize a film forming method such as a sputtering method, a CVD method, or a sol-gel method. The c-axis of lithium niobate is oriented perpendicular to the main surface of the substrate 10, and by applying an electric field parallel to the c-axis, the optical refractive index changes in proportion to the electric field. When sapphire is used as a single crystal substrate, the lithium niobate film can be epitaxially grown directly on the sapphire single crystal substrate. When silicon is used as a single crystal substrate, the lithium niobate film is formed by epitaxial growth via a coating layer (not shown). As a coating layer, a material having a lower refractive index than that of the lithium niobate film and suitable for epitaxial growth is used. For example, when Y2O3 is used as a coating layer (not shown), a high-quality lithium niobate film can be formed.

[0071] Also known methods for forming lithium niobate films include thinning and polishing a lithium niobate single crystal substrate or slicing a lithium niobate single crystal substrate. These methods have the advantage of achieving properties similar to those of a single crystal and can be applied to the present invention.

[0072] As described above, the optical modulator 1 of this embodiment includes: a Mach-Zehnder optical waveguide 2 including first and second waveguides 5A and 5B branching from a single input waveguide 3 and arranged parallel to each other; and a traveling wave electrode for applying an electric field to the first and second waveguides 5A and 5B, wherein the wiring section of the first and second waveguides 5A and 5B includes a first constant linewidth section Z in which the linewidth of the second waveguide 5B is wider than the linewidth of the first waveguide 5A. A1 , a second constant line width interval Z in which the line width of the first waveguide 5A is wider than the line width of the second waveguide 5B A2 , and set in the first constant line width interval Z A1 and the second line width constant interval Z A2 The line width transition zone Z where the line width of the waveguide changes B1 , Z B2 , the first line width constant interval Z A1 The length of the second line width constant interval Z A2 The lengths of the two optical waveguides are equal, thus canceling out any asymmetry in the pair of optical waveguides caused by the manufacturing process. This improves the deterioration of the extinction ratio caused by background light outside the operating wavelength being in a guided state when the operating wavelength of the input and output light is in an extinction state.

[0073] Figure 5 (a) and (b) are schematic plan views showing the structure of an optical modulator according to a second embodiment of the present invention, wherein (a) shows only the optical waveguide, and (b) shows the entire optical modulator including the traveling wave electrode. Figure 6 (a) and (b) are Figure 5 (a) and (b) are schematic cross-sectional views of the optical modulator 1, wherein (a) is a cross-sectional view of the optical modulator 1 along the optical path. Figure 5The cross-sectional views of (a) and (b) along the Xa-Xa line, (b) is a cross-sectional view along the Xa-Xa line. Figure 5 Cross-sectional views along the Xb-Xb line in (a) and (b).

[0074] like Figure 5 As shown in (a) and (b), the optical modulator 1 of this embodiment is characterized in that the first and second waveguides 5A and 5B of the Mach-Zehnder waveguide 2 are composed only of straight line portions. Figure 6 As shown in (a) and (b), the optical modulator 1 has a multilayer structure in which a substrate 10, a waveguide layer 11, a protective layer 12, a buffer layer 13, an insulating layer 14, and an electrode layer 15 are stacked in this order. This structure is the same as that of the first embodiment.

[0075] In this embodiment, the first constant line width interval Z A1 The line widths of the first and second waveguides 5A and 5B are constant, and the line width of the second waveguide 5B is narrower than the line width of the first waveguide 5A. A2 The line width of the first and second waveguides 5A and 5B is constant and the line width of the first waveguide 5A is narrower than the line width of the second waveguide 5B. B1 The line width transition interval Z is the interval where the line widths of the first waveguide 5A and the second waveguide 5B change. B2 This is a section where the line widths of the first waveguide 5A and the second waveguide 5B change.

[0076] As in the first embodiment, it is preferable that the first constant line width interval Z A1 The total length (L1+L5) and the second line width constant interval Z A2 That is, preferably, the length (L1+L5) of the portion with a relatively wider line width in the first waveguide 5A compared to the second waveguide 5B and the length (L3) of the portion with a relatively narrow line width are equal, and the length (L3) of the portion with a relatively wider line width in the second waveguide 5B compared to the first waveguide 5A and the length (L1+L5) of the portion with a relatively narrow line width are equal. More preferably, the first and second line width transition intervals Z B1 , Z B2 The total length of the first waveguide 5A and the total length of the second waveguide 5B are also equal. A1 and the second line width constant interval Z A2 The lengths of the first waveguide 5A and the second waveguide 5B do not need to be completely the same and may be slightly different. According to this embodiment, the symmetry of the first waveguide 5A and the second waveguide 5B can be improved to improve the extinction ratio.

[0077] In conventional optical modulators, the line width of one of the first and second waveguides 5A and 5B constituting the Mach-Zehnder waveguide 2 may be slightly wider than the line width of the other throughout its entire length, resulting in a deteriorated extinction ratio due to the uneven distribution (asymmetry) of the line width in the longitudinal direction of the optical waveguide. However, in this embodiment, the length of the portion with a relatively wider line width and the length of the portion with a relatively narrower line width in each of the first and second waveguides 5A and 5B are equalized. This improves the symmetry between the first and second waveguides 5A and 5B, thereby improving the extinction ratio.

[0078] Preferred embodiments of the present invention have been described above, but the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention, and such modifications are naturally included in the scope of the present invention.

[0079] For example, while the above embodiment illustrates a single-drive optical modulator, a so-called dual-drive optical modulator may also be employed, and the electrode structure is not particularly limited. Furthermore, while the above embodiment employs a single-channel structure in which a single Mach-Zehnder optical modulator is disposed on substrate 10, a multi-channel array structure comprising multiple Mach-Zehnder optical modulators may also be employed.

[0080] Example

[0081] The first constant line width interval Z in the first and second waveguides 5A and 5B constituting the Mach-Zehnder optical waveguide 2 is obtained. A1 and the second line width constant interval Z A2 The effect of different lengths on the extinction ratio.

[0082] When the first line width constant interval Z A1 The length is L A1 The optical path difference Δnd1 is 5000 nm, and the second line width constant interval Z A2 The length is L A2 The optical path difference Δnd2 = 5000nm × L A2 / L A1 When the optical path difference of the Mach-Zehnder waveguide is Δnd 12 =Δnd1-Δnd2 and the extinction ratio (Ext ratio) at the working wavelength of 1550nm is related to L A2 / L A1 The relationship is shown in Figure 7 in the coordinate diagram.

[0083] according to Figure 7 It can be seen that when the extinction ratio specification of the optical modulator is 28dB or above, L A2 / L A1>0.5, the specification is met. That is, when the optical path difference Δnd in the constant line width range is 5000nm, as long as L A2 / L A1 >0.5, the desired extinction ratio can be obtained, and L A1 =L A2 .

[0084] Description of Reference Numerals

[0085] 1. Optical modulator

[0086] 1i Optical input port

[0087] 1o optical output port

[0088] 2 Mach-Zehnder waveguide

[0089] 3 Input waveguide

[0090] 4 beam splitter

[0091] 5 Parallel waveguides

[0092] 5A First Waveguide

[0093] 5B Second waveguide

[0094] 5C1 First bend

[0095] 5C2 Second bend

[0096] 5S1 First straight line

[0097] 5S2 Second straight line

[0098] 5S3 Third straight line

[0099] 6. Beam-binding section

[0100] 7 Output waveguide

[0101] 10 substrate

[0102] 11 Waveguide layer

[0103] 11r ridge

[0104] 12 protective layer

[0105] 13 Buffer layer

[0106] 14 Insulation layer

[0107] 15 Electrode layer

[0108] 21 Signal electrode (first control electrode)

[0109] 21a Upper portion of the signal electrode

[0110] 21b Lower layer of signal electrode

[0111] 22 First ground electrode (second control electrode)

[0112] 22a Upper portion of the first ground electrode

[0113] 22b Lower layer portion of the first ground electrode

[0114] 23 Second ground electrode

[0115] 24 terminal resistors

[0116] CL1~CL4 curved waveguide

[0117] SL1~SL6 linear waveguide

[0118] S in Input light

[0119] S out Modulated light (output light)

[0120] Z A1 First line width constant interval (first interval)

[0121] Z A2 Second line width constant interval (second interval)

[0122] Z B1 First line width transition interval

[0123] Z B2 Second line width transition interval

Claims

1. An optical modulator, comprising: A Mach-Zehnder optical waveguide comprising: an input waveguide, a beam splitting portion for splitting light propagating in the input waveguide, first and second waveguides extending from the beam splitting portion and arranged parallel to each other, a beam combining portion for combining light propagating in the first and second waveguides, and an output waveguide for propagating light output from the beam combining portion; a signal electrode that controls the phase of light propagating in the Mach-Zehnder optical waveguide, wherein the first and second waveguides have: A first interval in which the line width of the second waveguide is narrower than the line width of the first waveguide; and a second interval in which the line width of the first waveguide is narrower than the line width of the second waveguide, The first section and the second section are connected by a line width transition section as a curved portion. said first and second waveguides having an even number of said bends, The line width is continuously changed from the line width of the first section to the line width of the second section over the entire length of the curved portion, or the line width is continuously changed from the line width of the second section to the line width of the first section. The entire length of the first section is equal to the entire length of the second section.

2. The optical modulator according to claim 1, wherein The first and second waveguides have: First to third straight line portions arranged parallel to each other; a first curved portion connecting the first straight portion and the second straight portion; and a second curved portion connecting the second straight portion and the third straight portion, The first section and the second section are switched at the first curved portion and the second curved portion.

3. The optical modulator according to claim 1 or 2, wherein: The waveguide is a ridge waveguide obtained by forming a lithium niobate film formed on a substrate into a ridge shape.

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