JP-7856744-B2 - Optical modulation element

JP7856744B2JP 7856744 B2JP7856744 B2JP 7856744B2JP-7856744-B2

Inventors

原裕貴
菊川隆

Assignees

ＴＤＫ株式会社

Dates

Publication Date: 20260511
Application Date: 20220323

Claims (6)

circuit board and It consists of a lithium niobate film having a ridge portion disposed on the substrate, and first and second waveguide portions arranged parallel to each other, A buffer layer arranged on the first and second waveguide sections, A first dielectric layer provided on the buffer layer, The system comprises first and second signal electrodes provided along the first and second waveguide sections, Each of the first and second signal electrodes is, In a plan view, the solid line portion is provided on the outside of the first and second waveguide sections and is continuously provided in the direction of propagation, A dashed line portion is provided inside the pair of solid line portions, in a position that overlaps with the first and second waveguide portions in a plan view, and is provided intermittently in the direction of propagation. It has a connecting portion that connects the solid line portion and the dashed line portion, The dashed portion has an upper portion provided on the first dielectric layer and a lower portion embedded in a trench pattern provided on the first dielectric layer. The width of the lower layer is wider than the width of the ridge and narrower than the width of the upper layer. The thickness of the upper layer of the solid line portion, the dashed line portion, and the connecting portion is 6 μm. The length of the aforementioned connection portion is 2 μm or more and 8 μm or less. The maximum distance between the two ends of the connecting portion in the direction of travel is more than half the length of the line segment pattern that constitutes the dashed portion. The first and second waveguide sections each have a straight section and a curved section. The optical modulation element is characterized in that the dashed portions of the first and second signal electrodes overlap with the first and second waveguide portions in a plan view in both the straight portion and the curved portion.
The optical modulation element according to claim 1, wherein the width of the solid line portion is wider than the width of the dashed line portion.
The optical modulation element according to claim 1 or 2, further comprising a second dielectric layer covering the exposed surfaces of the first and second signal electrodes.
The optical modulation element according to claim 3, wherein the solid line portion, the dashed line portion, and the connecting portion are arranged so that their respective upper surfaces are on the same plane.
The optical modulation element according to claim 3 or 4, wherein the second dielectric layer is a resin or SiN.
The optical modulation element according to any one of claims 1 to 5, wherein the spacing of the dashed lines is 1 μm or more and 50 μm or less.

Description

This disclosure relates to an optical modulation element. With the spread of the internet, the volume of data traffic has increased dramatically, making fiber optic communication extremely important. Fiber optic communication converts electrical signals into optical signals and transmits these optical signals through optical fibers, offering features such as wide bandwidth, low loss, and resistance to noise. Two methods for converting electrical signals into optical signals are known: direct modulation using semiconductor lasers and external modulation using optical modulators. Direct modulation does not require an optical modulator and is low-cost, but it has limitations in high-speed modulation, so external modulation is used for high-speed, long-distance applications. As for optical modulators, those that form optical waveguides near the surface of a lithium niobate single crystal substrate by Ti (titanium) diffusion have been put into practical use. High-speed optical modulators exceeding 40 Gb/s have been commercialized, but a major drawback is their long length of around 10 cm. In contrast, Patent Document 1 discloses a Mach-Zehnder type optical modulator in which a c-axis oriented lithium niobate film is formed on a sapphire single crystal substrate by epitaxial growth, and this lithium niobate film is used as an optical waveguide. Optical modulators using lithium niobate films can be significantly miniaturized and driven at lower voltages compared to optical modulators using lithium niobate single crystal substrates. However, achieving both a reduction in high-frequency loss and a lower driving voltage remains a challenge for further broadbanding. Furthermore, velocity matching between light and microwaves is necessary for broadbanding. Regarding the speed matching of light and microwaves, for example, Patent Document 2 discloses a low-speed wave electrode structure configured by adding thin fins to a pair of substantially parallel conductor strips so that the pair of conductor strips are capacitively coupled. This electrode structure substantially increases the capacitance per unit length between the strips, thereby slowing down the phase velocity of the microwave signal. Thus, speed matching of light and microwaves becomes possible. Furthermore, Patent Document 3 describes an electro-optical device including a waveguide for transmitting optical signals and electrodes for transmitting microwaves. The waveguide includes at least one optical material having an electro-optical effect, and the electrodes include a channel region and a plurality of extensions protruding from the channel region. Since the electrode extensions are closer to a part of the waveguide than the channel region, it is possible to reduce the velocity mismatch between the optical signal and the microwave signal. Patent Document 3 also discloses a so-called GSSG electrode structure in which a pair of ground electrodes (G) are arranged outside a pair of signal electrodes (S). Patent No. 6456662Special Publication No. 6-510378U.S. Patent Application Publication No. 2021/0157177 Figure 1 is a schematic plan view showing the configuration of an optical modulation element according to a first embodiment of the present disclosure, where (a) shows an optical waveguide pattern and (b) shows an electrode pattern superimposed on the optical waveguide pattern.Figures 2(a) and 2(b) are schematic cross-sectional views of the optical modulation element shown in Figure 1, where Figure 2(a) is a cross-sectional view along the line X1 - X1 in Figure 1(b), and Figure 2(b) is a cross-sectional view along the line X2 - X2 in Figure 1(b).Figure 3 is a schematic plan view showing the configuration of the first and second signal electrodes in detail.Figure 4 is a substantially cross-sectional view showing the structure of an optical modulation element according to a second embodiment of the present disclosure.Figure 5 is a schematic plan view showing the structure of an optical modulation element according to a third embodiment of the present disclosure.Figure 6 is a substantially cross-sectional view showing the structure of an optical modulation element according to a fourth embodiment of the present disclosure.Figure 7 is a schematic plan view showing the configuration of an optical modulation element according to a fifth embodiment of the present disclosure.Figure 8 is a graph showing the relationship between the connection region length Ld and the effective refractive index Nm of microwaves.Figure 9 is a graph showing the relationship between the width SH of the connection and the effective refractive index Nm of microwaves.Figure 10 is a graph showing the relationship between the electrode layer thickness T and the effective refractive index Nm of microwaves. Preferred embodiments of this disclosure will be described in detail below with reference to the attached drawings. Figures 1(a) and 1(b) are schematic plan views showing the configuration of