Optical device

Abstract

Provided is an optical device capable of suppressing propagation loss of traveling light even if the distance of an optical waveguide is increased or the radius of curvature of a curved portion in a path is reduced. The optical device includes: a substrate; a waveguide layer having a plate portion provided in contact with the substrate and a ridge portion provided so as to protrude from the plate portion; a buffer layer that is provided so as to cover the waveguide layer and contains, as a main component, SiO2 having a refractive index adjusted to 1.2 or more but less than 1.4; and a protective layer that is provided so as to cover the buffer layer, and that suppresses a change in the refractive index of the buffer layer.

Description

Technical Field

[0001] This invention relates to an optical device. Background Technology

[0002] Optical devices with optical waveguides disposed on a substrate include optical modulation devices that convert electrical signals into optical signals and coupling devices that mix and emit incident RGB laser light. Since the optical waveguides disposed in such optical devices aim to accommodate a long path within a small chip, they often have locally curved paths (for example, see Patent Document 1).

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Publication No. 2023-522151 Summary of the Invention

[0006] The technical problem that the invention aims to solve

[0007] In optical waveguides employing a path with bends and folds, reducing the radius of curvature of the bend improves spatial efficiency. However, in a typical optical device, for example, where the waveguide layer is formed of lithium niobate (LN) with a refractive index of 2.2 and covered by a protective layer of silicon dioxide (SiO2) with a refractive index of 1.5, light traveling in the waveguide will leak towards the protective layer if it is incident on the boundary surface at an angle of less than approximately 42°. In other words, the longer the waveguide or the smaller the radius of curvature of the bend, the greater the light propagation loss.

[0008] This invention was made to solve such a problem by providing an optical device that can suppress the propagation loss of traveling light even when the distance of the optical waveguide is extended or the radius of curvature of the bend in the path is reduced.

[0009] Technical means for solving technical problems

[0010] An optical device according to one aspect of the present invention comprises: a substrate; a waveguide layer having a plate portion disposed in contact with the substrate and a ridge portion protruding from the plate portion; a buffer layer disposed to cover the waveguide layer and comprising SiO2 as the main component having a refractive index adjusted to be 1.2 or higher and less than 1.4; and a protective layer disposed to cover the buffer layer and suppressing changes in the refractive index of the buffer layer.

[0011] In the aforementioned optical devices, the protective layer can also be constructed from a light-transmitting material having a refractive index higher than that of the buffer layer. Furthermore, the protective layer preferably has a thickness of 500 nm or more. In this case, the buffer layer preferably has a thickness of 10 nm or more and less than 300 nm.

[0012] Furthermore, the waveguide layer of the aforementioned optical device may contain lithium niobate as the main component. Additionally, in the aforementioned optical device, the angle between the sidewall of the ridge and the reference plane of the substrate may be 70° or more and 90° or less. Furthermore, the plate portion may be formed such that its thickness gradually decreases with increasing distance from the position where it intersects with the sidewall of the ridge. Furthermore, the SiO2 in the buffer layer may contain Si-H based materials. Furthermore, the protective layer may contain any one of the following raw materials: M-Si-O system (M being at least one of Al, Zr, Hf, La, Ba, Bi, Ti, Ca, Mo, In), SiN, SiON, or SiO2 with a refractive index of 1.5 or higher. Furthermore, the buffer layer and protective layer in the aforementioned optical device may also be disposed in regions corresponding to the curved path formed by the ridge.

[0013] The effects of the invention

[0014] According to the present invention, an optical device is provided that can suppress the propagation loss of traveling light even if the distance of the optical waveguide is extended or the radius of curvature of the bend in the path is reduced. Attached Figure Description

[0015] Figure 1 This is a diagram showing the overall structure of an optical modulation device as one type of optical device.

[0016] Figure 2 This is a diagram showing the structure of the optical modulation element used in an optical modulation device.

[0017] Figure 3 This is a cross-sectional view of the laminated plate (section III-III).

[0018] Figure 4 This is a diagram that roughly illustrates the structure of a plasma CVD device that forms a buffer layer.

[0019] Figure 5 This is a cross-sectional view of a waveguide layer stack with a modified version.

[0020] Explanation of symbols:

[0021] 100…Optical modulation device, 101…Package, 102…Optical modulation element, 103…Relay substrate, 104…Feedthrough section, 105…Input pin, 109…Terminator, 111…Input fiber, 112, 116…Support member, 113, 114, 117…Lens, 115…Optical unit, 118…Output fiber, 200, 200'…Laminated board, 201…Substrate, 201a…Reference plane, 202, 202'…Waveguide layer, 203, 203'…Buffer layer, 204, 204'…Protective layer, 210…Optical waveguide, 210a…Input section, 210b…Bend section, 210… c…branch section, 210d…merging section, 210e…output section, 210f…sidewall, 211, 211'…plate section, 211a…protrusion section, 220…signal electrode, 230…bias electrode, 241…nested Mach-Zehnder type optical waveguide, 242…Mach-Zehnder type optical waveguide, 500…plasma CVD device, 501…plasma, 510…chamber, 520…first supply tube, 530…second supply tube, 540…exhaust tube, 551…upper electrode, 552…lower electrode, 553…heater, 561…high frequency power supply, 562…DC power supply, 570…exhaust device. Detailed Implementation

[0022] The embodiments of the present invention will be described with reference to the accompanying drawings. In the drawings, components labeled with the same symbols have the same or identical structure. Furthermore, in the drawings, when multiple structures with the same or identical structure exist, to avoid complexity, sometimes only some are labeled with symbols, and the other identical symbols are omitted. Additionally, the invention described in the claims is not limited to the following embodiments. Furthermore, all structures described in the embodiments are not necessarily essential technical means for solving the technical problem.

[0023] Figure 1 This diagram illustrates the overall structure of an optical modulator 100, which is one type of optical device. The optical modulator 100 is mainly composed of a package 101 that hermetically seals an optical modulator 102 and a relay substrate 103. The optical modulator 102 is, for example, a DP-QPSK modulator.

[0024] The optical modulator 100 also has a plurality of signal pins 105 for inputting high-frequency electrical signals used for modulation of the optical modulator 102 and a feedthrough 104 for guiding these signal pins 105 into the package 101. Additionally, the optical modulator 100 has an input optical fiber 111 for inputting light into the package 101 and an output optical fiber 118 on the same side of the package 101 for guiding light modulated by the optical modulator 102 to the outside of the package 101.

[0025] The input fiber 111 and the output fiber 118 are respectively fixed to the package 101 via support members 112 and 116, which serve as fixing components. The light input from the input fiber 111 is collimated by the lens 113 disposed within the support member 112, and then input to the optical modulation element 102 via the lens 114. Alternatively, the end face of the input fiber 111 can be directly connected to the input section of the optical waveguide, which will be described later, without using a lens.

[0026] The optical modulator 100 also has an optical unit 115 for polarization combining two modulated lights output from the optical modulator 102. The polarization-combined light output from the optical unit 115 is focused by a lens 117 disposed within a support 116 and led out through an output optical fiber 118.

[0027] The relay substrate 103 relays the high-frequency electrical signal input from the signal pin 105 to the optical modulation element 102 via a conductor pattern (not shown). This conductor pattern is connected, for example, to one end of the signal electrode of the optical modulation element 102 via wire bonding. Additionally, the optical modulation device 100 includes a plurality of terminators 109 with defined impedances within the package 101.

[0028] Figure 2 This diagram illustrates the structure of the optical modulation element 102 used in the optical modulation device 100. The optical modulation element 102 has an optical waveguide 210 formed as part of a laminate 200, for example, performing DP-QPSK modulation at 200 gigabits per second (200G). The laminate structure of the laminate 200 will be described in detail later. The optical waveguide 210 is a ridge mainly projecting toward the waveguide layer contained in the laminate 200, forming a path for guiding light on the plane of the laminate 200.

[0029] In this embodiment, the laminate 200 is formed as a rectangle. As shown in the figure, the direction of the long side is defined as the X-axis, the direction of the short side is defined as the Y-axis, and the thickness direction is defined as the Z-axis. Furthermore, the same coordinate axes will also be shown in the following figures to indicate the orientation of the laminate 200.

[0030] The optical waveguide 210 has an input section 210a on one short side of the laminate 200 for receiving input light from the input optical fiber 111. Furthermore, the optical waveguide 210 reaches a branch section 210c via a semi-circular bend 210b extending along the X-axis and changing its direction of travel by 180°. The branch section 210c is configured in three stages, with eight branches per optical waveguide 210.

[0031] Eight-branched optical waveguides 210 form a Mach-Zehnder type optical waveguide 242 (the portion surrounded by the dotted line) that groups two adjacent parallel waveguides together. Furthermore, nested Mach-Zehnder type optical waveguides 241 (the portion surrounded by the double-dotted line) are formed, also grouping two adjacent Mach-Zehnder type optical waveguides 242 together. After QPSK modulation of the input light after branching, the nested Mach-Zehnder type optical waveguides 241 cause the modulated light to merge at the confluence section 210d. The light output from the two nested Mach-Zehnder type optical waveguides 241, like the input section 210a, is output from two output sections 210e located on the same short side of the laminate 200. The light output from the two output sections 210e is polarized and combined into a single beam by optical unit 115.

[0032] Signal electrodes 220 are provided on the laminate 200 to modulate the four Mach-Zehnder waveguides 242 constituting the nested Mach-Zehnder waveguide 241. One end of each signal electrode 220 is connected to a respective output terminal of the relay substrate 103. The other end of each signal electrode 220 is connected to the terminator 109. Thus, the high-frequency electrical signal input from the relay substrate 103 becomes a traveling wave and propagates in the signal electrodes 220, modulating the light propagating in each Mach-Zehnder waveguide 242.

[0033] In addition, the optical modulation element 102 is provided with a plurality of bias electrodes 230 for compensating for the bias point variation caused by DC drift. Each bias electrode 230 is applied a DC or low-frequency electrical signal to compensate for the bias point variation of the Mach-Zehnder type optical waveguide 242.

[0034] Figure 3 yes Figure 2 The diagram shows a cross-sectional view (III-III) of the laminate 200. The laminate 200 mainly consists of a substrate 201, a waveguide layer 202 laminated on the substrate 201, a buffer layer 203 laminated on the waveguide layer 202, and a protective layer 204 laminated on the buffer layer 203. Additionally, although not shown in cross-section III-III, as described above, there are portions where signal electrodes 220 and bias electrodes 230 are laminated.

[0035] The substrate 201 may be a Si substrate or a sapphire substrate. The waveguide layer 202 has a plate portion 211 that is in contact with the substrate 201 and a ridge portion that protrudes from the plate portion 211 and mainly functions as an optical waveguide 210. The waveguide layer 202 is formed of a lithium niobate film whose main component is lithium niobate (LN: refractive index 2.2), which has a higher refractive index than other materials. A portion of it is partially removed by etching or the like, and the area protected by a mask remains as the ridge portion.

[0036] In the ridge thus formed, the angle θ formed by its sidewall 210f relative to the reference plane 201a (XY plane) which is the surface of the substrate 201 is preferably 70° or more and 90° or less. If the angle θ is in such a range, it is easy to stack the buffer layer 203 on the waveguide layer 202.

[0037] Regarding the buffer layer 203, the specific film formation method will be described in detail later. It is a thin film containing SiO2 as the main component, with the refractive index adjusted to be greater than 1.2 and less than 1.4, and is formed to cover the upper surface of the waveguide layer 202. The thickness d of the buffer layer 203 is... b The wavelength is adjusted to be greater than 10 nm and less than 300 nm. If such a buffer layer 203 is overlapped with the waveguide layer 202, light traveling in the optical waveguide 210 will undergo total internal reflection as long as the angle with respect to the boundary between the waveguide layer 202 and the buffer layer 203 is less than approximately 36° (in the case of a buffer layer 203 with a refractive index of 1.3). Therefore, propagation loss can be suppressed compared to conventional buffer layers 203. Thus, a longer optical waveguide 210 can be designed than before. Furthermore, if the allowable propagation loss is set within a certain range, the radius of curvature of the bend 210b can be reduced compared to the past, increasing the design freedom of the laminate 200.

[0038] On the other hand, SiO2 with a refractive index adjusted to 1.2 or higher and less than 1.4 is prone to deterioration over time when exposed to the atmosphere, and its refractive index rises to around 1.5, which is the general refractive index. Therefore, in this embodiment, a protective layer 204 is further stacked to cover the upper surface of the buffer layer 203. The protective layer 204 is formed from a raw material that suppresses the refractive index change of the buffer layer 203. In this embodiment, any one of the following raw materials is used: M-Si-O system (M is at least one of Al, Zr, Hf, La, Ba, Bi, Ti, Ca, Mo, In), SiN, SiON, or SiO2 with a refractive index of 1.5 or higher. With this structure, a buffer layer with a lower refractive index than conventional raw materials can be achieved.

[0039] Alternatively, the material of the protective layer 204 can also be a light-absorbing material. If the protective layer 204 is made of a light-absorbing material, it can absorb light leaking from the optical waveguide 210 through the buffer layer 203 into the protective layer 204, thus helping to prevent stray light. Alternatively, the material of the protective layer 204 can also be a light-transmitting material with a refractive index greater than that of the buffer layer 203. If the protective layer 204 is made of a light-transmitting material with a refractive index greater than that of the buffer layer 203, it can significantly suppress the instability of light passing through the optical waveguide 210 caused by light leaking from the optical waveguide 210 through the buffer layer 203 into the protective layer 204 being reflected at the boundary surface of the protective layer 204 and returning to the buffer layer 203 and the optical waveguide 210. For example, SiN, LaSiO2, and SiO2 with a refractive index of 1.5 or higher are equivalent to light-transmitting materials with a refractive index greater than that of the buffer layer 203.

[0040] Furthermore, from the viewpoint of protecting the buffer layer 203, the thickness d of the protective layer 204 p The preferred thickness d of the buffer layer 203 b The thickness is preferably 500 nm or more. Furthermore, the signal electrode 220, bias electrode 230, and the upper surface of the protective layer 204 are disposed overlapping each other.

[0041] Figure 4 This is a schematic diagram showing the structure of a plasma CVD apparatus 500 that forms a buffer layer 203. The plasma CVD apparatus 500 mainly includes a chamber 510, an upper electrode 551, a lower electrode 552, a heater 553, a high-frequency power supply 561, a DC power supply 562, and an exhaust device 570.

[0042] Before the formation of the buffer layer 203, the laminate 200 is positioned on the lower electrode 552 and heated by a heater 553 powered by a DC power supply 562. Vacuum exhaust is performed inside the chamber 510 via an exhaust pipe 540 using an exhaust device 570. Then, Ar gas is supplied into the chamber 510 from the first supply pipe 520, generating plasma 501 between the upper electrode 551 and the lower electrode 552, which is applied by a high-frequency power supply 561. Next, a reaction gas containing SiH4 is supplied into the chamber 510 from the first supply pipe 520, and a reaction gas containing NH3 is supplied into the chamber 510 from the second supply pipe 530. A buffer layer 203 containing Si-H based components, with a refractive index of 1.2 or higher and less than 1.4, is formed on the waveguide layer 202 of the laminate 200. The buffer layer 203 is primarily composed of a SiO2 layer with a refractive index of 1.2 or higher and less than 1.4. Afterward, an annealing process is performed to remove residual stress.

[0043] Furthermore, this embodiment describes an example of forming a buffer layer 203 using a plasma CVD apparatus 500, but the method for forming the buffer layer 203 is not limited to this. The buffer layer 203 can also be formed using methods such as atmospheric pressure CVD or thermal CVD.

[0044] The laminate 200 of this embodiment has been described above, but the structure of the laminate 200 can be modified in various ways. For example, in the example above, the upper surface of the plate portion 211 constituting the waveguide layer 202 is formed to be parallel to the reference plane 201a (XY plane) of the substrate 201, but it can also be tilted. Figure 5 It is a laminate of waveguide layer 202' with modified form and Figure 2 The cross-sectional view corresponding to III-III.

[0045] As shown in the figure, in waveguide layer 202', the cross-sectional shape of the optical waveguide 210, which serves as the ridge, is the same as that of waveguide layer 202 described above, but the plate portion 211' is configured such that its thickness gradually decreases as it moves away from the position where it intersects with the sidewall of the ridge. That is, a ridge portion 211a is formed around the ridge. With this structure, even light that crosses from the optical waveguide 210 to the plate portion 211' in the beam cross section can suppress propagation loss.

[0046] Furthermore, in the above-described laminate 200, the buffer layer 203 covers the entire waveguide layer 202, and the protective layer 204 covers the entire buffer layer 203. However, it can also be configured, like the semi-circular bend 210b in the optical waveguide 210, where the buffer layer and protective layer only cover the area corresponding to the bend path. Even with such a structure, propagation loss can be suppressed, especially in the bend where it is prone to increase.

[0047] The above describes the optical modulation device 100 of this embodiment. However, optical devices having the substrate, waveguide layer, buffer layer, and protective layer described above are not limited to optical modulation devices and can be used in various applications. For example, they can also be applied to optical mixing devices for projectors, where RGB lasers are incident and adjusted separately and mixed in an optical waveguide to output light of any color.

Claims

1. An optical device, wherein, have: substrate; A waveguide layer having a plate portion disposed in contact with the substrate and a ridge portion protruding from the plate portion; A buffer layer, disposed to cover the waveguide layer, and comprising SiO2 as the main component with a refractive index adjusted to be greater than 1.2 and less than 1.4; and A protective layer is provided to cover the buffer layer and to suppress changes in the refractive index of the buffer layer.

2. The optical device according to claim 1, wherein, The protective layer is made of a light-transmitting raw material with a refractive index greater than that of the buffer layer.

3. The optical device according to claim 1, wherein, The protective layer has a thickness of more than 500 nm.

4. The optical device according to claim 3, wherein, The buffer layer has a thickness of more than 10 nm and less than 300 nm.

5. The optical device according to claim 1, wherein, The waveguide layer contains lithium niobate as the main component.

6. The optical device according to claim 1, wherein, The angle between the sidewall of the ridge and the reference plane of the substrate is 70° or more and 90° or less.

7. The optical device according to claim 1, wherein, The thickness of the plate gradually decreases as it moves away from the position where it intersects with the sidewall of the ridge.

8. The optical device according to claim 1, wherein, The SiO2 in the buffer layer contains Si-H groups.

9. The optical device according to claim 1, wherein, The protective layer comprises any one of the following raw materials: M-Si-O system, SiN, SiON, and SiO2 with a refractive index of 1.5 or higher, wherein M is at least one of Al, Zr, Hf, La, Ba, Bi, Ti, Ca, Mo, and In.

10. The optical device according to claim 1, wherein, The buffer layer and the protective layer are disposed in the region corresponding to the curved path formed by the ridge.

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