Optical devices

Abstract

The present invention provides an optical device that can suppress the propagation loss of light even when the distance of the optical waveguide is increased or the radius of curvature of the curved section in the path is reduced. [Solution] The optical device comprises a substrate, a waveguide layer having a slab portion provided in contact with the substrate and a ridge portion protruding from the slab portion, a buffer layer provided so as to cover the waveguide layer and containing SiO2 as its main component, adjusted to a refractive index of 1.2 or more and less than 1.4, and a protective layer provided so as to cover the buffer layer and suppressing changes in the refractive index of the buffer layer.

Description

Technical Field

[0001] The present invention relates to an optical device.

Background Art

[0002] Optical devices in which an optical waveguide is provided on a substrate include an optical modulation device that converts an electrical signal into an optical signal, a coupler device that mixes and emits incident RGB laser light, and the like. The optical waveguide provided in such an optical device often has a partially curved path in many cases because of the requirement to accommodate a long path in a small chip (for example, see Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] When a curved portion is provided as an optical waveguide and a path that turns back is adopted, the space efficiency increases if the radius of curvature of the curved portion is reduced. However, for example, in the case of a general optical device in which a waveguide layer is formed of lithium niobate (LN) with a refractive index of 2.2 and is covered with a protective layer of silicon dioxide (SiO2) with a refractive index of 1.5, when the light traveling through the optical waveguide is incident on the interface at an incident angle less than about 42°, it leaks to the protective layer side. That is, the longer the distance of the optical waveguide or the smaller the radius of curvature of the curved portion, the greater the propagation loss of light.

[0005] The present invention has been made to solve such problems, and provides an optical device capable of suppressing the propagation loss of light traveling even when the distance of the optical waveguide is increased or the radius of curvature of the curved portion in the path is reduced.

Means for Solving the Problems

[0006] An optical device according to one aspect of the present invention comprises a substrate, a waveguide layer having a slab portion provided in contact with the substrate and a ridge portion protruding from the slab portion, a buffer layer provided so as to cover the waveguide layer and containing SiO2 as its main component, adjusted to a refractive index of 1.2 or more and less than 1.4, and a protective layer provided so as to cover the buffer layer and suppressing changes in the refractive index of the buffer layer.

[0007] In the above optical device, the protective layer may be made of a light-transmitting material having a refractive index greater than that of the buffer layer. Furthermore, the protective layer preferably has a thickness of 500 nm or more. In that case, the buffer layer preferably has a thickness of 10 nm or more and less than 300 nm.

[0008] Furthermore, the waveguide layer of the above optical device may contain lithium niobate as its main component. In addition, in the above optical device, the angle formed by the sidewall of the ridge portion with respect to the reference plane of the substrate may be 70° or more and 90° or less. The slab portion may also be formed such that its thickness gradually decreases as it moves away from the position where it intersects with the sidewall of the ridge portion. The SiO2 of the buffer layer may also contain Si-H groups. The protective layer may contain one of the following materials: M-Si-O system (where M is at least one of Al, Zr, Hf, La, Ba, Bi, Ti, Ca, Mo, or In), SiN, SiON, or SiO2 with a refractive index of 1.5 or more. In addition, the buffer layer and protective layer in the above optical device may be provided in a region corresponding to the curved path formed by the ridge portion. [Effects of the Invention]

[0009] The present invention provides an optical device that can suppress the propagation loss of light even when the distance of the optical waveguide is increased or the radius of curvature of the curved portion in the path is reduced. [Brief explanation of the drawing]

[0010] [Figure 1]This figure shows the overall configuration of an optical modulation device, which is one type of optical device. [Figure 2] This diagram shows the configuration of an optical modulation element used in an optical modulation device. [Figure 3] This is a cross-sectional view of the laminated board at line III-III. [Figure 4] This diagram schematically shows the configuration of a plasma CVD apparatus for forming a buffer layer. [Figure 5] This is a cross-sectional view of a laminate having a waveguide layer according to a modified example. [Modes for carrying out the invention]

[0011] Embodiments of the present invention will be described with reference to the attached drawings. In each drawing, components with the same reference numerals have the same or similar configuration. Furthermore, in each drawing, if there are multiple structures with the same or similar configuration, reference numerals may be assigned to some of them, while the same reference numerals may be omitted to avoid complexity. Note that the invention claimed is not limited to the following embodiments. Also, not all of the configurations described in the embodiments are necessarily essential as means to solve the problem.

[0012] Figure 1 shows the overall configuration of an optical modulation device 100, which is one embodiment of an optical device. The optical modulation device 100 mainly consists of a package 101 in which an optical modulation element 102 and a relay substrate 103 are hermetically sealed. The optical modulation element 102 is, for example, a DP-QPSK modulator. The optical modulation device 100 also has a plurality of signal pins 105 for inputting high-frequency electrical signals used to modulate the optical modulation element 102, and a feedthrough section 104 for introducing these signal pins 105 into the package 101. Furthermore, the optical modulation device 100 has an input optical fiber 111 for inputting light into the package 101, and an output optical fiber 118 for guiding the light modulated by the optical modulation element 102 to the outside of the package 101, on the same side of the package 101.

[0013] The input optical fiber 111 and the output optical fiber 118 are fixed to the package 101, respectively, via support members 112 and 116. The light input from the input optical fiber 111 is collimated by a lens 113 located within the support 112, and then input to the optical modulation element 102 via lens 114. Alternatively, the end face of the input optical fiber 111 may be directly connected to the input section of the optical waveguide described later, without using a lens. The optical modulation device 100 also has an optical unit 115 that polarization-combines the two modulated lights output from the optical modulation element 102. The polarization-combined light output from the optical unit 115 is focused by a lens 117 located within a support 116 and led to an output optical fiber 118. The relay board 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 to one end of the signal electrode of the optical modulation element 102, for example, by wire bonding. The optical modulation device 100 also includes a plurality of terminators 109 having a predetermined impedance within the package 101.

[0014] Figure 2 shows the configuration 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 the laminate 200 and performs DP-QPSK modulation at, for example, 200 gigabits per second (200G). The laminated structure of the laminate 200 will be described in detail later, but the optical waveguide 210 is mainly a ridge portion that protrudes from the waveguide layer contained in the laminate 200 and is formed as a path for guiding light on the plane of the laminate 200.

[0015] In this embodiment, the laminated board 200 is formed in a rectangular shape. As shown in the figure, the direction of the long side is defined as the X-axis direction, the direction of the short side as the Y-axis direction, and the direction of the thickness as the Z-axis direction. In subsequent drawings, the orientation of the laminated board 200 is indicated by the same coordinate axes.

[0016] The optical waveguide 210 has an input section 210a that receives input light from the input optical fiber 111 on one short side of the laminated board 200. Then, it extends in the X-axis direction and reaches the branching section 210c through a semicircular curved section 210b that converts the traveling direction by 180°. The branching section 210c is provided in three stages, and the single optical waveguide 210 branches into eight.

[0017] The eight branched optical waveguides 210 form Mach-Zehnder type optical waveguides 242 (the portions surrounded by the dashed lines) with two adjacent parallel waveguides as a set. Further, a nested Mach-Zehnder type optical waveguide 241 (the portion surrounded by the two-dot chain line) is formed with two adjacent Mach-Zehnder type optical waveguides 242 as a set. The nested Mach-Zehnder type optical waveguide 241 QPSK-modulates each of the branched input lights and then combines the modulated lights at the combining section 210d. The lights output from the two nested Mach-Zehnder type optical waveguides 241 are respectively output from the two output sections 210e provided on the same short side of the laminated board 200 as the input section 210a. The lights respectively output from the two output sections 210e are polarization-combined by the optical unit 115 and combined into one optical beam.

[0018] On the laminated board 200, signal electrodes 220 for performing modulation operations are provided for each of the four Mach-Zehnder type optical waveguides 242 that constitute the nested Mach-Zehnder type optical waveguide 241. One end of each signal electrode 220 is connected to the respective output terminals of the relay substrate 103. Also, the other end of each signal electrode 220 is connected to the terminator 109. Thereby, the high-frequency electrical signal input from the relay substrate 103 propagates through the signal electrode 220 as a traveling wave and modulates the lights propagating through the Mach-Zehnder type optical waveguide 242. Also, the optical modulator 102 is provided with a plurality of bias electrodes 230 for compensating for fluctuations in the bias point due to DC drift. Each bias electrode 230 is applied with a DC or low-frequency electrical signal to compensate for fluctuations in the bias point of the Mach-Zehnder type optical waveguide 242.

[0019] FIG. 3 is a cross-sectional view taken along the line III-III of the laminated plate 200 shown in FIG. 2. The laminated plate 200 mainly includes 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. Although not shown in the cross-section along the line III-III, there are also locations where the signal electrode 220 and the bias electrode 230 are laminated as described above.

[0020] As the substrate 201, for example, a Si substrate or a sapphire substrate is used. The waveguide layer 202 has a slab portion 211 provided in contact with the substrate 201 and a ridge portion that protrudes convexly from the slab portion 211 and mainly functions as an optical waveguide 210. The waveguide layer 202 is formed of a lithium niobate (LN: refractive index 2.2) film mainly composed of lithium niobate having a higher refractive index than other materials. A part of it is partially removed by an etching process or the like, and the region protected by the mask remains as the ridge portion.

[0021] In the ridge portion formed in this way, the angle θ formed by the side wall 210f with respect 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 formed angle θ is within such a range, it is easy to laminate the buffer layer 203 on the waveguide layer 202.

[0022] The buffer layer 203 is a thin film containing SiO2 whose refractive index is adjusted to 1.2 or more and less than 1.4 as the main component, although the specific film formation method will be described later, and is formed so as to cover the upper surface of the waveguide layer 202. The thickness d of the buffer layer 203 bThe wavelength is adjusted to be between 10 nm and less than 300 nm. By superimposing such a buffer layer 203 on the waveguide layer 202, light traveling through the optical waveguide 210 undergoes total internal reflection as long as the angle with respect to the interface between the waveguide layer 202 and the buffer layer 203 is less than approximately 36° (when the refractive index of the buffer layer 203 is 1.3), thus suppressing propagation loss more effectively than with a conventional buffer layer 203. Therefore, a longer optical waveguide 210 can be designed than before. Furthermore, if the allowable amount of propagation loss is set within a certain range, the radius of curvature of the curved portion 210b can be reduced compared to conventional designs, improving the design flexibility of the laminate 200.

[0023] On the other hand, SiO2 adjusted to a refractive index of 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 a typical refractive index of around 1.5. Therefore, in this embodiment, a protective layer 204 is further laminated to cover the upper surface of the buffer layer 203. The protective layer 204 is made of a material that suppresses changes in the refractive index of the buffer layer 203, and in this embodiment, one of the following materials is used: an M-Si-O system material (where M is at least one of Al, Zr, Hf, La, Ba, Bi, Ti, Ca, Mo, or In), SiN, SiON, or SiO2 with a refractive index of 1.5 or higher. With this configuration, a buffer layer with a lower refractive index compared to conventional materials can be realized.

[0024] Furthermore, the material of the protective layer 204 may be a light-absorbing material. If a light-absorbing material is used for the protective layer 204, it can absorb light that leaks from the optical waveguide 210 through the buffer layer 203 and further into the protective layer 204, thus contributing to the prevention of stray light. Alternatively, the material of the protective layer 204 may be a light-transmitting material having a refractive index greater than that of the buffer layer 203. If a light-transmitting material having a refractive index greater than that of the buffer layer 203 is used for the protective layer 204, it is possible to 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 and further into the protective layer 204 being reflected at the interface of the protective layer 204 and returning to the buffer layer 203 and optical waveguide 210. For example, SiN, LaSiO2, and SiO2 with a refractive index of 1.5 or higher are light-transmitting materials having a refractive index greater than that of the buffer layer 203.

[0025] Also, the thickness d of the protective layer 204 p From the viewpoint of protecting the buffer layer 203, the thickness d of the buffer layer 203 is considered. b It is preferable that the thickness be greater than or equal to 500 nm. The signal electrode 220 and bias electrode 230 mentioned above are provided on top of the upper surface of the protective layer 204.

[0026] Figure 4 is a schematic diagram showing the configuration of the plasma CVD apparatus 500 for forming the buffer layer 203. The plasma CVD apparatus 500 mainly comprises 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 system 570.

[0027] The laminate 200, before the formation of the buffer layer 203, is placed on the lower electrode 552 and heated by a heater 553 energized by a DC power supply 562. The chamber 510 is evacuated using an exhaust device 570 through an exhaust pipe 540. Ar gas is then 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 are energized by a high-frequency power supply 561. A reaction gas containing SiH4 is then supplied into the chamber 510 from the first supply pipe 520, and a reaction gas containing NH3 is supplied from the second supply pipe 530, forming a buffer layer 203 on the waveguide layer 202 of the laminate 200, mainly composed of an SiO2 layer containing Si-H groups and having a refractive index of 1.2 or more and less than 1.4. After that, annealing is performed to remove residual stress.

[0028] In this embodiment, an example of forming the buffer layer 203 using a plasma CVD apparatus 500 has been described, 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.

[0029] Although the laminate 200 according to this embodiment has been described above, the configuration of the laminate 200 can be changed in various ways. For example, in the above example, the upper surface of the slab 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 is a cross-sectional view of a laminate 200' having a waveguide layer 202' according to a modified example, corresponding to line III-III in Figure 2.

[0030] As shown in the figure, in the waveguide layer 202', the cross-sectional shape of the optical waveguide 210 that forms the ridge is the same as that of the waveguide layer 202 described above, but the slab portion 211' is configured such that its thickness gradually decreases as it moves away from the point where it intersects with the side wall of the ridge. That is, a raised portion 211a is formed around the ridge. With such a configuration, propagation loss can be suppressed even for light that crosses from the optical waveguide 210 to the slab portion 211' in the optical beam cross-section.

[0031] Furthermore, in the laminate 200 described above, the buffer layer 203 covers the entire waveguide layer 202, and the protective layer 204 covers the entire buffer layer 203. However, the buffer layer and protective layer may be configured to cover only the region corresponding to the curved path, such as the semicircular curved portion 210b of the optical waveguide 210. Even with such a configuration, propagation loss can be suppressed, especially in curved portions where such loss tends to be large.

[0032] Although the optical modulation device 100 according to this embodiment has been described above, optical devices comprising a substrate, waveguide layer, buffer layer, and protective layer as described above are not limited to optical modulation devices and can be applied to a variety of applications. For example, they can be applied to optical mixing devices for projectors, which receive separately adjusted RGB laser beams, mix them in an optical waveguide, and output light of an arbitrary color. [Explanation of symbols]

[0033] 100…Optical modulation device, 101…Package, 102…Optical modulation element, 103…Intermediate board, 104…Feedthrough section, 105…Input pin, 109…Terminator, 111…Input optical fiber, 112, 116…Support, 113, 114, 117…Lens, 115…Optical unit, 118…Output optical fiber, 200, 200'…Laminate, 201…Substrate, 201a…Reference plane, 202, 202'…Waveguide layer, 203, 203'…Buffer layer, 204, 204'…Protective layer, 210…Optical waveguide, 210a…Input section, 210b…Bay Curved section, 210c…Branching section, 210d…Confluence section, 210e…Output section, 210f…Side wall, 211, 211'…Slab section, 211a…Raised section, 220…Signal electrode, 230…Bias electrode, 241…Nested Mach-Zehnder optical waveguide, 242…Mach-Zehnder optical waveguide, 500…Plasma CVD apparatus, 501…Plasma, 510…Chamber, 520…First supply pipe, 530…Second supply pipe, 540…Exhaust pipe, 551…Upper electrode, 552…Lower electrode, 553…Heater, 561…High-frequency power supply, 562…DC power supply, 570…Exhaust system

Claims

1. circuit board and A waveguide layer having a slab portion provided in contact with the substrate and a ridge portion provided protruding from the slab portion, A SiO2 molecule is provided to cover the waveguide layer, with a refractive index adjusted to be between 1.2 and less than 1.4 as its main component. 2 A buffer layer including, A protective layer is provided so as to cover the buffer layer and to suppress changes in the refractive index of the buffer layer. An optical device equipped with the following features.

2. The optical device according to claim 1, wherein the protective layer is made of a light-transmitting material having 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 500 nm or more.

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

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

6. The optical device according to claim 1, wherein the angle formed by the side wall of the ridge portion with respect to 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 slab portion gradually decreases as it moves away from the position where it intersects with the side wall of the ridge portion.

8. The SiO of the buffer layer 2 The optical device according to claim 1, comprising a Si-H group.

9. The protective layer is made of an M-Si-O system (where M is at least one of Al, Zr, Hf, La, Ba, Bi, Ti, Ca, Mo, or In), SiN, SiON, or SiO with a refractive index of 1.5 or higher. 2 The optical device according to claim 1, comprising any of the following materials.

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

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