Optical circuit element

Abstract

Provided is an optical circuit element (10) capable of preventing stray light propagating in a portion of the optical circuit element (10) including a substrate (11) from being emitted to the outside. The optical circuit element (10) has a substrate (11), an optical waveguide layer (12) formed on one face of the substrate (11), and a protective layer (13) formed so as to overlap the optical waveguide layer (12), characterized in that the optical waveguide layer (12) has an optical waveguide (14) that propagates light, a groove portion (15) reaching a position deeper than the one face is formed from a surface of the protective layer (13) toward the substrate (11), and an optical absorption layer (16) covering at least a bottom face and a side face of the groove portion (15) is provided.

Description

Technical Field

[0001] This disclosure relates to optical circuit elements having optical waveguides.

[0002] This application claims priority based on Japanese Patent Application No. 2020-167655, filed on October 2, 2020, the contents of which are incorporated herein by reference. Background Technology

[0003] With the widespread adoption of the Internet, communication volume has increased dramatically, making fiber optic communication extremely important. Fiber optic communication converts electrical signals into optical signals and transmits them through optical fibers, featuring wide bandwidth, low loss, and strong noise immunity.

[0004] As methods for converting electrical signals into optical signals, direct modulation using semiconductor lasers and external modulation using optical modulators are known. Direct modulation is low-cost as it does not require an optical modulator, but it has limitations in high-speed modulation. External optical modulation is typically used in high-speed and long-distance applications.

[0005] As an optical modulator, an optical modulator with an optical waveguide formed by diffusion of Ti (titanium) near the surface of a lithium niobate single-crystal substrate has been put into practical use. High-speed optical modulators with speeds of 40 Gb / s and above have been commercialized, but they have a significant drawback, such as a relatively long total length of about 10 cm.

[0006] In this regard, Patent Document 1 discloses an optical waveguide element having a cross-section composed of a ridge with a ridge shape, the ridge being composed of a combination of a first part and a second part, having a two-level ridge structure with the first part formed on the upper surface of the second part, and the width of the second ridge being more than 5 times the width of the first ridge.

[0007] Such optical waveguide elements can be miniaturized. However, due to this miniaturization, during the alignment process—aligning the optical axis between the light emitter and the end of the waveguide—uncoupled light components are easily generated. These light components propagate in the portion of the waveguide outside the waveguide itself. After multiple reflections at the end face, a portion is input to the photodetector, leading to stray light. Stray light propagating within the waveguide element can hinder photodetector alignment, causing increased connection loss or poor connection. This is especially problematic when using visible light as the light source, as the waveguide becomes smaller, thus amplifying the impact of stray light.

[0008] To prevent stray light from reaching the photodetector within such an optical waveguide element, for example, Patent Document 2 discloses an optical integrated circuit element in which a groove penetrating the optical waveguide layer is provided at a location other than the waveguide portion that guides light, and the side of the groove is inclined relative to a surface perpendicular to the element surface. The optical integrated circuit element disclosed in Patent Document 2 also discloses forming a material that absorbs light propagating in the optical waveguide layer in a manner that at least covers the side of the groove.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: Japanese Patent No. 6369147.

[0012] Patent Document 2: Japanese Patent Application Publication No. 11-52154. Summary of the Invention

[0013] The problem that the invention aims to solve

[0014] However, in the technology disclosed in Patent Document 2, the groove for blocking stray light is formed above the surface of the substrate. Therefore, although it is effective in blocking stray light propagating in the cladding, it has the problem that it cannot block stray light propagating in the substrate.

[0015] The technology disclosed herein was developed with this condition in mind, and its purpose is to provide an optical circuit element capable of preventing stray light propagating in the substrate portion of the optical circuit element from escaping to the outside.

[0016] Technical means for solving problems

[0017] In one aspect of this disclosure, an optical circuit element is provided, having a substrate, an optical waveguide layer formed on one side of the substrate, and a protective layer overlapping the optical waveguide layer. The optical waveguide layer has an optical waveguide for propagating light, and a groove is formed from the surface of the protective layer toward the substrate, reaching a depth deeper than the one side. The protective layer has a light-absorbing layer that at least covers the bottom and side surfaces of the groove.

[0018] Invention Effects

[0019] According to the technology disclosed herein, it is possible to provide an optical circuit element that prevents stray light propagating in the substrate portion of the optical circuit element from escaping to the outside. Attached Figure Description

[0020] Figure 1 This is a top view of the optical circuit element of the first embodiment of the present invention as viewed from above.

[0021] Figure 2 It is along Figure 1A sectional view cut along line A-A'.

[0022] Figure 3 It is along Figure 1 A sectional view cut along line B-B'.

[0023] Figure 4 This is a cross-sectional view showing another example of the shape of the groove.

[0024] Figure 5 This is a cross-sectional view showing another example of the formation of a light-absorbing layer.

[0025] Figure 6 This is a top view of the optical circuit element of the second embodiment of the present invention viewed from above.

[0026] Figure 7 This is a top view of the optical circuit element of the third embodiment of the present invention viewed from above.

[0027] Figure 8 This is a top view of the optical circuit element of the fourth embodiment of the present invention viewed from above.

[0028] Figure 9 It is along Figure 8 A sectional view cut along the C-C' line.

[0029] Figure 10 This is a top view of the optical circuit element of the fifth embodiment of the present invention viewed from above.

[0030] Figure 11 This is a top view showing the Mach-Zehnder type optical modulator (optical circuit element) according to the sixth embodiment of the present invention. Detailed Implementation

[0031] Hereinafter, an optical circuit element applying an embodiment of the present disclosure will be described with reference to the accompanying drawings. Furthermore, the embodiments shown below are described in detail to better understand the spirit of the invention, and unless otherwise specified, they are not intended to limit the scope of the present disclosure. Additionally, the accompanying drawings used in the following description are sometimes enlarged representations of key parts for ease of understanding of the features of the present disclosure, and the dimensions and ratios of the constituent elements may not be identical to the actual dimensions.

[0032] (First Implementation)

[0033] Figure 1 This is a top view of the optical circuit element of the first embodiment of the present invention as viewed from above. Figure 2 It is along Figure 1 A sectional view cut along line A-A'. Figure 3 It is along Figure 1 A sectional view cut along line B-B'.

[0034] The optical circuit element 10 of the first embodiment has an optical waveguide 14. For example, light incident from the input end IN of the optical waveguide 14 (incident light) propagates in the optical waveguide 14 and is emitted from the output end OUT (emitted light). The light source (light emitter) S can be arranged close to the input end IN of the optical waveguide 14, for example.

[0035] The optical circuit element 10 has a substrate 11, a waveguide layer 12 overlappingly formed on one surface 11a of the substrate 11, and a protective layer 13 overlappingly formed on the waveguide layer 12. An optical waveguide 14 is formed on the waveguide layer 12, which has a ridge portion 17 with a ridge shape (convex shape) in cross section.

[0036] The ridge 17 protrudes from the surface 12a of the waveguide layer 12 and extends along the optical waveguide 14.

[0037] Such a ridge 17 can be integrally formed using the same material as the waveguide layer 12, such as lithium niobate (LiNbO3).

[0038] The cross-sectional shape of the ridge 17 is not limited as long as it is capable of guiding light waves; for example, it can be dome-shaped, triangular, or rectangular. Furthermore, in this embodiment, an optical waveguide 14 is formed in the waveguide layer 12 via the ridge 17, but it is also possible to form the optical waveguide 14 in the waveguide layer 12 without forming a ridge 17 that protrudes from the surface 12a of the waveguide layer 12. For example, by doping the waveguide layer 12 made of lithium niobate with titanium (Ti), an optical waveguide 14 without ridges can also be formed.

[0039] The waveguide layer 12 is preferably composed of a lithium niobate (LiNbO3) film, for example. Lithium niobate has a large electro-optic constant, making it suitable as a constituent material for optical devices such as optical modulators. Hereinafter, the structure of the present invention when the waveguide layer 12 is a lithium niobate film will be described in detail.

[0040] As for substrate 11, there are no particular limitations as long as its refractive index is lower than that of the lithium niobate film, but a substrate that allows the lithium niobate film to be formed as an epitaxial film is preferred. As an example of substrate 11, a sapphire single crystal substrate or a silicon single crystal substrate is preferred. The crystal orientation of the single crystal substrate is not particularly limited.

[0041] The lithium niobate film constituting the waveguide layer 12 has the property of being easily formed as a c-axis oriented epitaxial film relative to single-crystal substrates of various crystal orientations. The c-axis oriented lithium niobate film has a three-dimensional symmetry, so it is desirable that the single-crystal substrate of the substrate also has the same symmetry. In the case of a sapphire single-crystal substrate, a substrate with the c-plane is preferred, and in the case of a silicon single-crystal substrate, a substrate with the (111) plane is preferred.

[0042] Here, an epitaxial film is a film whose crystal orientation is aligned with that of a substrate or base film relative to the substrate. When the in-plane of the film is defined as the X-Y plane and the film thickness direction is defined as the Z-axis, the crystals are aligned in the X, Y, and Z axis directions. For example, it can be identified as an epitaxial film by first confirming the peak intensity at the orientation position based on 2θ-θ X-ray diffraction and then confirming the poles.

[0043] Specifically, when performing measurements based on 2θ-θ X-ray diffraction, the intensity of all peaks other than the target plane should be 10% or less, preferably 5% or less, of the maximum peak intensity of the target plane. For example, in a c-axis oriented epitaxial film of lithium niobate, the intensity of peaks other than the (00L) plane should be 10% or less, preferably 5% or less, of the maximum peak intensity of the (00L) plane. (00L) is a general term for the equivalent planes such as (001) and (002).

[0044] Next, in the pole determination, it is necessary to observe the poles. In the confirmation condition of the peak intensity at the first orientation position mentioned above, it only indicates the orientation in one direction. Even if the first condition is obtained, if the crystal orientation is not aligned in the plane, the intensity of X-rays is not high at a specific angular position, and the pole cannot be observed. Lithium niobate has a trigonal crystal structure, so there are 3 poles in LiNbO3(014) in a single crystal. It is known that in the case of lithium niobate film, the crystal is epitaxially grown in a so-called bicrystalline state with symmetrical bonding when the crystal rotates 180° around the c-axis. In this case, the three poles are symmetrically bonded to two, so there are 6 poles. In addition, when a lithium niobate film is formed on a silicon single crystal substrate with (100) plane, the substrate is 4 times symmetrical, so 4×3=12 poles can be observed. Furthermore, in this embodiment, the lithium niobate film epitaxially grown in a bicrystalline state is also included in the epitaxial film.

[0045] The lithium niobate film constituting waveguide layer 12 has the composition LixNbAyOz. A represents an element other than Li, Nb, and O. x is 0.5–1.2, preferably 0.9–1.05. y is 0–0.5. z is 1.5–4, preferably 2.5–3.5. 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, Ce, etc., or combinations of two or more.

[0046] It is desirable that the thickness of the lithium niobate film constituting waveguide layer 12 is less than 2 μm.

[0047] When the film thickness exceeds this level, there are concerns about the difficulty in forming a high-quality film. If the lithium niobate film is too thin, the light containment within the film weakens, raising concerns about increased stray light leakage from the substrate or buffer layer. Even when an electric field is applied to the lithium niobate film, the effective refractive index change of the optical waveguides (1a, 1b) may be small. Therefore, it is desirable for the lithium niobate film to have a thickness of at least 1 / 10 of the wavelength of the light being used.

[0048] As a method for forming the lithium niobate film constituting the waveguide layer 12, it is desirable to utilize film formation methods such as sputtering, CVD, and sol-gel methods. The c-axis is oriented perpendicularly to the main surface of the single-crystal substrate constituting the substrate 11, and by applying an electric field parallel to the c-axis, the optical refractive index changes proportionally to the electric field. When sapphire is used as the single-crystal substrate constituting the substrate 11, the lithium niobate film can be epitaxially grown directly on the sapphire single-crystal substrate.

[0049] When silicon is used as the single-crystal substrate constituting substrate 11, a lithium niobate film is formed by epitaxial growth via a cladding layer (not shown). As the cladding layer (not shown), a material with a lower refractive index than the lithium niobate film and suitable for epitaxial growth is used. For example, when Y₂O₃ is used as the cladding layer (not shown), a high-quality lithium niobate film can be formed.

[0050] Furthermore, as a method for forming the lithium niobate film constituting the waveguide layer 12, a method of grinding a thin lithium niobate single-crystal substrate is also known. Although this method has advantages such as obtaining the same properties as a single crystal, it is difficult to process thin films with a thickness of less than 2 μm. As described above, in the present invention, the lithium niobate film is formed by film deposition, thus enabling mass production and facilitating large-diameter applications.

[0051] The protective layer 13 covers the upper surface of the waveguide layer 12, which includes the ridge 17. The material of the protective layer 13 is not particularly limited, but a material with a refractive index lower than that of the waveguide layer 12 can be used, such as silicon oxide (SiO2) and aluminum oxide (Al2O3).

[0052] In addition, as a protective layer 13, materials such as magnesium fluoride (MgF2), lanthanum oxide (La2O3), zinc oxide (ZnO), hafnium oxide (HfO2), magnesium oxide (MgO), yttrium oxide (Y2O3), calcium fluoride (CaF2), indium oxide (In2O3), or mixtures thereof may also be used.

[0053] The thickness of the protective layer 13 can be as small as 0.2 to 1 μm. In addition, in this embodiment, the protective layer 13 covers the entire upper surface of the waveguide layer 12, but it can also be configured to selectively cover only the area near the upper surface of the optical waveguide 14 formed on the waveguide layer 12.

[0054] A groove 15 is formed in the optical circuit element 10. In this embodiment, the groove 15 is formed on a portion of both sides of the optical waveguide 14 formed in the waveguide layer 12. The groove 15 in this embodiment is rectangular, for example, a rectangle when viewed from one side of the substrate. In addition, the groove 15 is formed as an inverted trapezoid in cross-sectional shape in the thickness direction (stack direction) t of the optical circuit element 10, and the side surface 15a of the groove 15 is formed as an inclined surface that is inclined relative to the thickness direction t.

[0055] The groove 15 is formed from the surface 13a of the protective layer 13 toward the substrate 11 to a depth greater than one surface 11a of the substrate 11. That is, the bottom surface 15b of the groove 15 is formed at a position where it enters the interior of the substrate from one surface 11a of the substrate 11, and the substrate 11 is recessed in the thickness direction t at the part where the groove 15 is formed.

[0056] Furthermore, in this embodiment, the side surface 15a of the groove 15 is an inclined surface that is inclined at a predetermined angle θ relative to the thickness direction t, but for example, Figure 4 As shown, the groove 15 can also be formed in a rectangular manner as the cross-sectional shape of the optical circuit element 10 in the thickness direction t, and the side surface 15a of the groove 15 is formed in a manner that is a vertical surface along the thickness direction t.

[0057] The depth of the portion of substrate 11 into which the groove 15 is cut along the thickness direction t from one surface 11a of substrate 11, i.e., the gap d between one surface 11a of substrate 11 and the bottom surface of the groove 15, can be set according to the wavelength of light propagating in the optical waveguide 14. That is, the gap d only needs to be set to more than half the wavelength of light propagating in the optical waveguide 14. For example, if the wavelength of light propagating in the optical waveguide 14 is 520 nm, the groove 15 can be formed with a gap d of 260 nm or more.

[0058] Between the two grooves 15, starting from the bottom surface 15b of the groove 15, the substrate 11, the waveguide layer 12 on the ridge 17 where the optical waveguide 14 is formed, and the protective layer 13 are formed to extend in a dam-like shape with a narrow width.

[0059] A light-absorbing layer 16 is formed in the groove 15, covering the bottom surface 15b and the side surface 15a of the groove 15. In this embodiment, the light-absorbing layer 16 is formed to cover not only the bottom surface 15b and the side surface 15a of the groove 15, but also the surface 13a of the protective layer 13. Alternatively, the light-absorbing layer 16 may also have a structure that does not cover the surface 13a of the protective layer 13.

[0060] The light-absorbing layer 16 is made of a material that absorbs light propagating in the optical waveguide 14. The material constituting the light-absorbing layer 16 can be selected according to the wavelength of the light propagating in the optical waveguide 14. For example, if the light propagating in the optical waveguide 14 is visible light, a material capable of absorbing and blocking light in the visible light wavelength range can be used, such as a resin material containing visible light-absorbing pigments including C, Si, Ge, anthocyanin compounds, azo compounds, diphenylmethane compounds, triphenylmethane compounds, etc.; semiconductors such as In and Ga; oxides or nitrides composed of Ti, Ni, Cr, Fe, Nb, Ta, Zn, W, Mo, or alloys thereof. Furthermore, if the light propagating in the optical waveguide 14 is infrared light, a material capable of absorbing and blocking light in the infrared wavelength range can be used, such as a resin material containing infrared-absorbing pigments including anthocyanin compounds, diammonium compounds, squaric acid cyanide compounds, etc.

[0061] The light absorption layer 16 can be formed in such a way that it is thick enough to absorb, for example, more than 50% of the stray light P incident on the light absorption layer 16. Thus, the stray light P can be absorbed during the passage of the light absorption layer 16 formed on one side 15a of the groove 15 and the light absorption layer 16 formed on the other side 15a.

[0062] In addition to being formed on the bottom surface 15b and side surface 15a of the groove portion 15 with a predetermined thickness as in this embodiment, the light absorption layer 16 can also be formed, for example, as in this embodiment. Figure 5 As shown in the diagram. In Figure 5 In this structure, a light-absorbing layer 16 is formed by completely filling the groove 15, which includes the bottom surface 15b and the side surface 15a. By configuring it in this way, stray light P can be absorbed more reliably.

[0063] According to the first embodiment of the optical circuit element 10 with the various structures described above, for example, during the alignment process of aligning the optical axis between the light source (light emitter) S that introduces light into the optical waveguide 14 and the input end IN of the optical waveguide 14, light components that are not coupled to the optical waveguide 14 may sometimes be generated. Such light components that are not coupled to the optical waveguide 14 become the portion outside the optical waveguide 14 within the optical circuit element 10, for example, becoming stray light P propagating near one surface 11a of the substrate 11 and in the protective layer 13. In the optical circuit element 10 of this embodiment, when such stray light P reaches the formation position of the groove portion 15, the stray light P is absorbed by the light absorption layer 16.

[0064] In particular, the groove 15 is formed along the thickness direction t to a position deeper than one surface 11a of the substrate 11, so that stray light P propagating near one surface 11a of the substrate 11 is reliably absorbed by the light absorption layer 16 formed in the groove 15.

[0065] By using the groove 15 and the light-absorbing layer 16 formed thereon, stray light P is absorbed and blocked, thereby preventing stray light P from entering the photodetector (not shown) located at the output end OUT of the optical waveguide 14. This prevents increased connection loss or poor connection from interfering with the alignment of the photodetector during the alignment process.

[0066] Furthermore, stray light P can also be blocked by appropriately setting the tilt angle θ of the side surface 15a of the slot 15. For example, when stray light P is incident from the protective layer 13 into the space (air layer) of the slot 15, if the refractive index of air is set to 1 and the refractive index of the protective layer 13 is set to approximately 3.5, total internal reflection occurs at the interface when the incident angle of stray light P at the interface between the protective layer 13 and the air is 15° or more due to this refractive index difference. When the incident angle of stray light P onto the side surface 15a of the slot 15 is 15° or more, and the tilt angle θ of the side surface 15a is ±15° or more, the reflectivity of stray light P becomes 100%, and stray light P is completely reflected to the upper or lower side of the optical circuit element 10 and removed.

[0067] (Second Implementation)

[0068] Next, the optical circuit element of the second embodiment of the present invention will be described. Furthermore, in the following embodiments, structures identical to those in the first embodiment described above will be labeled with the same numbers, and repeated descriptions will be omitted.

[0069] Figure 6 This is a top view of the optical circuit element of the second embodiment of the present invention viewed from above.

[0070] In this embodiment, the optical circuit element 20 has multiple slots (five in this embodiment) 25A, 25B, 25C, 25D, and 25E formed at intervals on both sides of the optical waveguide 14 along its extension direction. On one surface 11a of the substrate 11 (see reference 11a) viewed from above... Figure 2 When ), each groove 25A to 25E is formed into a rectangle (rectangle) with the same shape as each other.

[0071] In this embodiment, the grooves 25A to 25E are formed in such a way that the intervals G1 between grooves 25A and 25B, G2 between grooves 25B and 25C, G3 between grooves 25C and 25D, and G4 between grooves 25D and 25E are all different.

[0072] Furthermore, the sum of the intervals between any two slots 25A to 25E is different from the sum of the intervals between any two slots 25A to 25E. For example, the sum of interval G1 + interval G3 is different from the sum of interval G2 + interval G4. Also, for example, the sum of interval G2 + interval G3 + interval G4 is different from the sum of interval G1 + interval G3 + interval G4.

[0073] When multiple slots 25A to 25E are regularly arranged at equal intervals, there is a concern that stray light reflected regularly may be amplified. However, by making the intervals between adjacent slots 25A to 25E different as in this embodiment, it is possible to prevent stray light from being regularly reflected and amplified by each other. With the multiple slots 25A to 25E and the light absorption layer 16 covering these slots, stray light P can be reliably absorbed and blocked.

[0074] (Third Implementation)

[0075] Next, the optical circuit element of the third embodiment of the present invention will be described. Furthermore, in the following embodiments, structures identical to those in the first embodiment described above will be labeled with the same numbers, and repeated descriptions will be omitted.

[0076] Figure 7 This is a top view of the optical circuit element of the third embodiment of the present invention viewed from above.

[0077] In this embodiment, the optical circuit element 30 has multiple slots (five in this embodiment) 35A, 35B, 35C, 35D, and 35E formed on both sides of the optical waveguide 14 at equal intervals. On one surface 11a of the substrate 11 (see reference 11a) viewed from above... Figure 2 When ), each groove 25A to 25E is formed into a rectangle.

[0078] In this embodiment, the grooves 35A to 35E are formed in a manner in which the widths W1 to W5 of each groove 35A to groove 35E along the extension direction of the optical waveguide 14 are completely different.

[0079] Furthermore, the sum of the widths W1 to W5 of any of the slots 35A to 35E is different from the sum of the widths W1 to W5 of any of the other slots 35A to 35E. For example, the sum of width W1 + width W3 is different from the sum of width W2 + width W4. Also, for example, the sum of width W1 + width W3 + width W5 is different from the sum of width W1 + width W2 + width W4.

[0080] When the widths of the multiple slots 35A to 35E are equal, there is a concern that stray light will be amplified by regular reflection. However, by making the widths of adjacent slots 35A to 35E different as in this embodiment, it is possible to prevent stray light from being amplified by regular reflection. With the multiple slots 35A to 35E and the light absorption layer 16 covering these slots, stray light P can be reliably absorbed and blocked.

[0081] (Fourth Implementation)

[0082] Next, the optical circuit element of the fourth embodiment of the present invention will be described. Furthermore, in the following embodiments, structures identical to those in the first embodiment described above will be labeled with the same numbers, and repeated descriptions will be omitted.

[0083] Figure 8 This is a top view of the optical circuit element of the fourth embodiment of the present invention viewed from above. Figure 9 It is along Figure 8 A sectional view cut along the C-C' line.

[0084] In the optical circuit element 40 of this embodiment, the optical waveguide 14 is composed of a straight portion 14L extending in a straight line and a bent portion 14R bending from the straight portion 14L.

[0085] Furthermore, at the two straight sections 14L, multiple grooves 45, 45... are formed on both sides of the straight section 14L, and the inner surface (side surface, bottom surface) of the groove 45 is covered by the light absorption layer 16.

[0086] In addition, on the imaginary extension line Q1 of the straight portion L1 extending in a direction separate from the bending direction of the bending portion 14R at the connection between the straight portion L1 and the bent portion 14R of the optical waveguide 14, a plurality of grooves 45, 45... are also formed, and the inner surface (side surface, bottom surface) of the groove 45 is covered by the light absorption layer 16.

[0087] According to the optical circuit element 40 with this structure, when light propagating in the optical waveguide 14 enters the bent portion 14R from the straight portion L1, it bends along the bent portion 14R. Conversely, stray light P propagating in the substrate 11 and protective layer 13 does not bend at the formation position of the bent portion 14R and travels directly in a straight line. Then, the straight-traveling stray light P is absorbed by a plurality of slots 45, 45... formed on the imaginary extension line Q1 of the straight portion L1 and the light-absorbing layer 16 covering these slots. Therefore, according to the optical circuit element 40 of this embodiment, the stray light P traveling in a straight line at the formation position of the bent portion 14R of the optical waveguide 14 will not be emitted to the outside of the optical circuit element 40, which can prevent the increase of connection loss or the occurrence of poor connection due to interference with the alignment of the photodetector during, for example, the alignment process.

[0088] (Fifth Implementation)

[0089] Next, the optical circuit element of the fifth embodiment of the present invention will be described. Furthermore, in the following embodiments, structures identical to those in the fourth embodiment described above will be labeled with the same numbers, and repeated descriptions will be omitted.

[0090] Figure 10 This is a top view of the optical circuit element of the fifth embodiment of the present invention viewed from above.

[0091] In the optical circuit element 50 of this embodiment, the optical waveguide 14 is composed of a straight portion 14L extending in a straight line and a bent portion 14R bending from the straight portion 14L.

[0092] Furthermore, at the two straight sections 14L, multiple grooves 45, 45... are formed on both sides of the straight section 14L, and the inner surface (side surface, bottom surface) of the groove 45 is covered by the light absorption layer 16.

[0093] In addition, along the curved outer periphery of the curved portion 14R of the optical waveguide 14, a plurality of curved grooves 55, 55... are formed, and the inner surface (side surface, bottom surface) of the groove 55 is covered by the light absorption layer 16.

[0094] According to the optical circuit element 50 with this structure, when light propagating in the optical waveguide 14 enters the curved portion 14R from the straight portion L1, it bends along the curved portion 14R. In contrast, stray light P propagating in the substrate 11 and the protective layer 13 does not bend at the formation position of the curved portion 14R and travels directly in a straight line. Then, the straight-traveling stray light P is absorbed by a plurality of curved grooves 55, 55... formed along the curved outer periphery of the curved portion 14R and the light-absorbing layer 16 covering these grooves.

[0095] As an example, according to the structure of this embodiment, for instance, when the wavelength of the light incident on the optical waveguide 14 is set to 520 nm, and the light absorption layer 16 is formed from a Si film, the light absorption coefficient of Si is 1.35 × 10⁻⁶. 5 cm －1 Therefore, even if the thickness of the light absorption layer 16 is 100 nm, by arranging five slots 55, stray light can be attenuated to about 26% of its intensity before incidence during the entire period of stray light transmission through the light absorption layer 16 formed in the five slots 55.

[0096] Therefore, according to the optical circuit element 50 of this embodiment, stray light P that travels in a straight line at the formation position of the bend 14R of the optical waveguide 14 will not be emitted to the outside of the optical circuit element 40, which can prevent the connection loss from increasing or the connection from being poor due to interference with the alignment of the photodetector during, for example, the alignment process.

[0097] (Sixth Implementation Method)

[0098] Next, the optical circuit element of the sixth embodiment of the present invention will be described. Furthermore, in the following embodiments, structures identical to those in the first embodiment described above will be labeled with the same numbers, and repeated descriptions will be omitted. In this embodiment, a Mach-Zehnder type optical modulator will be described as an example of an optical circuit element.

[0099] Figure 11 This is a top view showing the Mach-Zehnder type optical modulator (optical circuit element) according to the sixth embodiment of the present invention.

[0100] The optical modulator (optical circuit element) 60 is a device that modulates the light propagating within the optical waveguide 14 by applying a voltage to the Mach-Zehnder interferometer formed by the optical waveguide 14. The optical waveguide 14 is branched into two optical waveguides 14S1 and 14S2 at the branch points E1 and E2, forming a dual-electrode structure with one, i.e., two, first electrodes 67a and 67b respectively on the optical waveguides 14S1 and 14S2.

[0101] This optical modulator 60 uses terminating resistors 69 to connect two first electrodes 67a and 67b and second electrodes 68a, 68b, and 68c, which function as traveling wave electrodes. The second electrodes 68a, 68b, and 68c are set as ground electrodes. Complementary signals with the same absolute value but different phases relative to the two first electrodes 67a and 67b are input from the input sides 64a and 64b of the first electrodes 67a and 67b of the optical modulator 60. The lithium niobate film constituting the substrate exhibits an electro-optic effect; therefore, by applying an electric field to the optical waveguides 14S1 and 14S2, the refractive indices of the optical waveguides (14S1 and 14S2) change as +Δn and -Δn, respectively, resulting in a change in the phase difference between the optical waveguides (14S1 and 14S2). Through this change in phase difference, intensity-modulated signal light is output from the optical waveguide 14S4 on the output side D2 of the optical modulator 60.

[0102] In this optical modulator 60, grooves 65, 65... are formed on both sides of the optical waveguide 14S3 on the input side D1, on both sides of the optical waveguide 14S4 on the output side D2, on the inner side of branch E1, on the outer side of branch E2, on the outer side of the bends E3 and E4 of the optical waveguide 14S1, and on the outer side of the bends E5 and E6 of the optical waveguide 14S2. Furthermore, the inner surfaces (side surfaces and bottom surfaces) of these grooves 65, 65... are covered by a light-absorbing layer (not shown).

[0103] With this structure, stray light generated in the optical waveguides 14S1 to 14S4 of the optical modulator 60 is absorbed and blocked by the slots 65, 65... and the light absorption layer. Therefore, stray light will not be emitted to the outside of the optical modulator 60, and malfunctions caused by light other than signal light (stray light) can be prevented.

[0104] (Structure Example)

[0105] As a structural example, an optical circuit element has a substrate, an optical waveguide layer formed on one side of the substrate, and a protective layer formed overlappingly on the optical waveguide layer. The optical waveguide layer has an optical waveguide for propagating light, and a groove is formed from the surface of the protective layer toward the substrate, reaching a depth deeper than the one side. The protective layer has a light-absorbing layer that at least covers the bottom and side surfaces of the groove.

[0106] As a structural example, the light-absorbing layer is formed by filling the entire groove.

[0107] As a structural example, multiple grooves are formed at intervals along one side of the substrate.

[0108] As a structural example, the grooves are formed in three or more ways, extending approximately parallel to each other, and the grooves are formed in such a way that the spacing between any two adjacent grooves is different from the spacing between any two adjacent grooves.

[0109] As a structural example, the grooves are formed in four or more ways, extending approximately parallel to each other, and the grooves are formed in a way that the sum of the intervals between any two grooves is different from the sum of the intervals between any two grooves.

[0110] As a structural example, the groove is formed in such a way that the width of the groove is different from the width of the adjacent groove.

[0111] As a structural example, the grooves are formed in three or more ways, extending approximately parallel to each other, and the sum of the widths of any of the grooves is different from the sum of the widths of any other groove.

[0112] As a structural example, the optical waveguide has a curved portion that bends from the straight portion, and the groove portion is arranged to intersect with the imaginary extension of the straight portion.

[0113] As a structural example, the optical waveguide has a curved portion that bends from a straight portion, and the groove is formed to extend in a curved manner along the curved portion.

[0114] As a structural example, the optical waveguide layer contains lithium niobate.

[0115] As a structural example, the optical waveguide is ridge-shaped.

[0116] As a structural example, the light-absorbing layer is made of Si.

[0117] The foregoing has described one embodiment of the technology disclosed herein, but this embodiment is provided by way of example and is not intended to limit the scope of the invention. This embodiment can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. When this embodiment and its modifications are included within the scope or spirit of the invention, they are also included within the scope of the invention as described in the claims and their equivalents.

[0118] Explanation of reference numerals in the attached figures

[0119] 10… Optical circuit components

[0120] 11…Substrate

[0121] 12…waveguide layer

[0122] 13…protective layer

[0123] 14… Optical waveguide

[0124] 15…groove section

[0125] 15a…side view

[0126] 15b…bottom

[0127] 16…light absorption layer

[0128] 17…Spine

Claims

1. An optical circuit element comprising a substrate, an optical waveguide layer formed on one side of the substrate, and a protective layer overlapping the optical waveguide layer, characterized in that, The optical waveguide layer has an optical waveguide that enables light to propagate. A groove is formed from the surface of the protective layer toward the substrate, extending to a depth greater than the surface of the protective layer. It has a light-absorbing layer that covers at least the bottom and sides of the groove. The thickness of the optical waveguide layer is greater than 1 / 10 of the wavelength of the light and less than 2 μm. The thickness of the protective layer is 0.2~1μm. The gap between one surface of the substrate and the bottom surface of the groove is set to be more than half the wavelength of the light.

2. The optical circuit element according to claim 1, characterized in that, The light-absorbing layer is formed by completely burying the groove.

3. The optical circuit element according to claim 1 or 2, characterized in that, The grooves are formed in multiple spaces spaced apart from each other along one side of the substrate.

4. The optical circuit element according to claim 3, characterized in that, The grooves are formed in three or more portions that extend in a generally parallel manner. The grooves are formed in a manner in which the spacing between any two adjacent grooves is different from the spacing between any two adjacent grooves.

5. The optical circuit element according to claim 3, characterized in that, The grooves are formed in four or more ways, extending in a manner that is generally parallel to each other. The grooves are formed in a manner that differs from the sum of the intervals between any of the grooves.

6. The optical circuit element according to claim 3, characterized in that, The groove is formed in such a way that the width of the groove is different from the width of the adjacent groove.

7. The optical circuit element according to claim 6, characterized in that, The grooves are formed in three or more portions that extend in a generally parallel manner. The groove is formed in a manner that differs from the sum of the widths of any other groove.

8. The optical circuit element according to any one of claims 1 to 7, characterized in that, The optical waveguide has a curved portion that bends from the straight portion, and the groove portion is configured to intersect with the imaginary extension of the straight portion.

9. The optical circuit element according to any one of claims 1 to 8, characterized in that, The optical waveguide has a curved portion that bends from a straight portion, and the groove is formed to extend in a curved manner along the curved portion.

10. The optical circuit element according to any one of claims 1 to 9, characterized in that, The optical waveguide layer contains lithium niobate.

11. The optical circuit element according to any one of claims 1 to 10, characterized in that, The optical waveguide is ridge-shaped.

12. The optical circuit element according to any one of claims 1 to 11, characterized in that, The light-absorbing layer is made of Si.

Images