Optical waveguide element, optical modulation device using the same, and optical transmitter

Abstract

To provide an optical waveguide element that can efficiently remove high-order mode light and is easy to manufacture, even when the width and height of an optical waveguide is small.SOLUTION: An optical waveguide element including a substrate 1 on which a rib type optical waveguide 10 is formed includes a slab waveguide SW disposed close to the rib type optical waveguide 10. Roughness of at least a portion (SF1) of an upper surface of the slab waveguide SW is rougher than roughness of a surface (SF0) of a top of the rib type optical waveguide.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an optical waveguide element and an optical modulation device and an optical transmission device using the same, and more particularly to an optical waveguide element including a substrate on which a rib-type optical waveguide is formed. [Background technology]

[0002] In the fields of optical measurement technology and optical communication technology, optical waveguide elements using substrates on which optical waveguides are formed, such as optical modulators, are widely used. Optical waveguides can be formed by thermally diffusing Ti or other materials into substrates with electro-optical effects, such as lithium niobate (LN), but rib-type optical waveguides, in which convex ribs are formed on the substrate surface, have also been proposed. In particular, rib-type optical waveguides can increase the difference in refractive index between the rib and the surrounding space (air), thereby strengthening light confinement.

[0003] The applicant has proposed a method for attenuating higher modes while suppressing loss in the fundamental mode of light waves propagating through a rib-type optical waveguide in Patent Document 1. Specifically, the applicant proposes providing a slab waveguide SW in the vicinity of a rib-type optical waveguide 10 as shown in Fig. 1, and making the height h of the slab waveguide SW lower than the height H of the rib-type optical waveguide 10. The elliptical dotted line OW shows a simplified representation of the mode field diameter (MFD) of the light waves.

[0004] On the other hand, there is a growing need for faster and more compact optical modulators, and driver-integrated optical modulation devices have been proposed that incorporate a driver circuit (amplifier) ​​for amplifying the electrical signal inside the housing that houses the optical waveguide element.

[0005] When incorporating an amplifier into a housing, the amplifier is placed on one end side of the optical waveguide element, so the optical waveguide formed on the substrate needs to have both the input and output parts of the optical waveguide placed on one end face of the substrate (the end face opposite the amplifier). Therefore, the overall shape of the optical waveguide needs to adopt a structure that is bent at least 180 degrees, so it is necessary to strengthen the optical confinement of the optical waveguide.

[0006] As a method for strengthening the optical confinement of the optical waveguide, the width and height of the optical waveguide are set to 1 μm or less. In such a case, it becomes very difficult to manufacture the slab waveguide SW of Patent Document 1, which has a height h smaller than 1 μm, and to precisely position the slab waveguide SW close to the rib-type optical waveguide 10. Moreover, the dimensions disclosed in Patent Document 1 are for a case in which the MFD of the light wave propagating through the optical waveguide is set to about 10 μm. With these dimensions, the optical waveguide itself becomes multi-mode, and there arises a problem that higher-order mode light cannot be removed at all. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 5359750 Summary of the Invention [Problem to be solved by the invention]

[0008] The problem to be solved by the present invention is to provide an optical waveguide element that can solve the above-mentioned problems, can efficiently remove higher-order mode light even when the width and height of the optical waveguide are small, and is easy to manufacture, and also to provide an optical modulation device and an optical transmission device that utilize the optical waveguide element. [Means for solving the problem]

[0009] In order to solve the above problems, the optical waveguide element and the optical modulation device and optical transmission device using the same according to the present invention have the following technical features. (1) An optical waveguide element including a substrate on which a rib-type optical waveguide is formed, the height of the rib-type optical waveguide being set to 1 μm or less and the width to 3 μm or less, and including a slab waveguide disposed adjacent to the rib-type optical waveguide, the slab waveguide being a first slab waveguide that is adjacent to the rib-type optical waveguide and has a height shorter than that of the rib-type optical waveguide. a second slab waveguide disposed adjacent to the first slab waveguide and opposite to the rib-type optical waveguide, the second slab waveguide being higher than the first slab waveguide;The first slab waveguide has an upper surface that is rougher than the top surface of the rib type optical waveguide.

[0011] (2) In the optical waveguide element according to (1) above, , applicable The roughness of the upper surface of the second slab waveguide is set to be greater than the roughness of the top surface of the rib type optical waveguide. (3) The above (1) or In the optical waveguide element described in (2), the distance between the rib type optical waveguide and the second slab waveguide, which sandwich the first slab waveguide, is set to 1 μm or less.

[0012] (4) The above (2) or In the optical waveguide element described in (3), the roughness of the upper surface of the first slab waveguide is set to be the same as the roughness of the upper surface of the second slab waveguide.

[0013] (5) The optical waveguide element according to any one of (1) to (4), Applicable No. 1S The Love waveguide is characterized in that it is disposed on both sides of the rib-type optical waveguide.

[0014] (6) The above (1) In the optical waveguide element according to any one of (1) to (4), the first slab waveguide and the second slab waveguide are disposed on both sides of the rib type optical waveguide.

[0015] (7) In the optical waveguide element according to any one of (1) to (6) above, the slab waveguide is disposed in the vicinity of an input section, a branching section, or a multiplexing section of the rib-type optical waveguide.

[0016] (8) In the optical waveguide element according to any one of (1) to (7), the input and output portions of the rib-type optical waveguide are ApplicableThe slab waveguide is characterized in that it is provided on the same end surface of a substrate, with a light detection means on the substrate for detecting light waves propagating through the rib-type optical waveguide or radiant light emitted from the rib-type optical waveguide, and the slab waveguide is provided between the input portion and the light detection means.

[0017] (9) The optical waveguide element described in any one of (1) to (8) above is an optical modulation device comprising an electrode for modulating an optical wave propagating through the rib-type optical waveguide, the optical waveguide element being housed in a housing, and an optical fiber for inputting or outputting an optical wave to the rib-type optical waveguide.

[0018] (10) The optical modulation device according to (9) above, further comprising an electronic circuit disposed inside the housing for amplifying a modulation signal input to the optical waveguide element.

[0019] (11) An optical transmitter comprising the optical modulation device according to (9) or (10) above, and an electronic circuit for outputting a modulation signal for causing the optical modulation device to perform a modulation operation. Effect of the Invention

[0020] The present invention relates to an optical waveguide element including a substrate on which a rib-type optical waveguide is formed. The height of the rib-type optical waveguide is set to 1 μm or less, and the width is set to 3 μm or less. The optical waveguide element includes a slab waveguide disposed adjacent to the rib-type optical waveguide. The slab waveguide is a first slab waveguide that is adjacent to the rib-type optical waveguide and has a height shorter than that of the rib-type optical waveguide. a second slab waveguide disposed adjacent to the first slab waveguide and opposite to the rib-type optical waveguide, the second slab waveguide being higher than the first slab waveguide; Since the roughness of the upper surface of the first slab waveguide is greater than the roughness of the top surface of the rib type optical waveguide, the higher order modes of the light waves propagating through the rib type optical waveguide are scattered by this rough surface portion, making it possible to efficiently remove them. [Brief description of the drawings]

[0021] [Figure 1] FIG. 1 is a cross-sectional view illustrating a structure of a conventional optical waveguide element. [Diagram 2] FIG. 1 is a cross-sectional view illustrating a first embodiment of an optical waveguide element according to the present invention. [Diagram 3] FIG. 4 is a cross-sectional view illustrating a second embodiment of the optical waveguide element of the present invention. [Figure 4] FIG. 4 is a cross-sectional view illustrating a third embodiment of the optical waveguide element of the present invention. [Diagram 5] FIG. 11 is a cross-sectional view illustrating an optical waveguide element according to a fourth embodiment of the present invention. [Figure 6] FIG. 2 is a plan view for explaining the arrangement position of an unnecessary light removing means in the optical waveguide element of the present invention. [Figure 7] FIG. 2 is a plan view illustrating the positional relationship between an unnecessary light removing means and a light receiving means in the optical waveguide element of the present invention. [Figure 8] 1 is a plan view illustrating an optical modulation device and an optical transmission device according to the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The optical waveguide element of the present invention will be described in detail below using preferred examples. As shown in Figures 2 to 5, the optical waveguide element of the present invention is an optical waveguide element comprising a substrate 1 on which a rib-type optical waveguide 10 is formed, and a slab waveguide SW arranged in close proximity to the rib-type optical waveguide 10, and the roughness of at least a part (SF1, SF2) of the upper surface of the slab waveguide SW is greater than the roughness (SF0) of the top surface of the rib-type optical waveguide.

[0023] The material of the substrate 1 used in the optical waveguide element of the present invention can be a ferroelectric material having an electro-optic effect, specifically, a substrate such as lithium niobate (LN), lithium tantalate (LT), or PLZT (lead lanthanum zirconate titanate), or a vapor-deposited film made of these materials. In addition, various materials such as semiconductor materials and organic materials can also be used as the substrate of the optical waveguide element.

[0024] The thickness of the substrate 1 on which the optical waveguide is formed may be set to 10 μm or less, preferably 5 μm or less, and more preferably 2 μm or less, in order to achieve speed matching between the microwave and light waves of the modulated signal. In such a case, in order to reinforce the mechanical strength of the substrate 1, a holding substrate 11 having a thickness of 0.2 to 1 mm is attached by direct bonding or via an adhesive. In the optical waveguide element of the present invention, the term "substrate on which an optical waveguide is formed" does not simply mean a single substrate, but also includes a concept including an assembly of a thin plate (e.g., having a thickness of 10 μm or less) on which an optical waveguide is formed and a supporting substrate 11 that supports the thin plate. Furthermore, the "substrate on which an optical waveguide is formed" also includes a substrate in which a vapor-deposited film is formed on a supporting substrate and the film (thin film) is processed into the shape of an optical waveguide.

[0025] As a method for forming an optical waveguide in the substrate 1, a rib-type optical waveguide can be formed by etching the portion of the substrate other than the optical waveguide, forming grooves on both sides of the optical waveguide, etc. In addition to the rib-type optical waveguide, it is also possible to use a method of thermally diffusing a high refractive index material such as Ti into the substrate, or a method of forming a high refractive index portion by proton exchange.

[0026] The height of the rib-type optical waveguide is set to 1 μm or less, and the width to 3 um or less, and the input and output parts of the optical waveguide are provided with spot size converters (SSCs) that change the light wave MFD from 1 μm to approximately 3 μm. This makes it possible to suppress optical coupling loss even if the MFD of the optical fiber installed outside the optical waveguide element is 10 μm.

[0027] 2 to 5, the optical waveguide element of the present invention is characterized in that a slab waveguide SW is provided in the vicinity of a rib-type optical waveguide 10, and at least a part of the upper surface (SF1, SF2) of the slab waveguide SW is made rougher than the surface roughness of the top of the rib-type optical waveguide. This configuration makes it possible to form a means (hereinafter referred to as "unnecessary light removing means") for effectively removing unnecessary light such as higher-order mode light while maintaining fundamental mode light for light waves propagating through the rib-type optical waveguide.

[0028] Methods for roughening the top surface of a slab waveguide include locally forming unevenness with an electron beam and roughening the top surface by etching. The etching conditions for the rough surface are set under conditions that generate unevenness rather than the processing conditions for the rib tops. In particular, since the surface can be locally roughened easily by etching, it can be formed much more easily than the unnecessary light removal means of Patent Document 1. The roughness of the rough surface is preferably 10 nm or more in arithmetic mean roughness Ra. The roughness of the top surface of the rib optical waveguide is usually set to 1 nm or less.

[0029] 2, the slab waveguide SW (SW1) disposed adjacent to the rib type optical waveguide 10 has a shape that is lower in height than the rib type optical waveguide. A rough surface is formed on a part (SF1) of the upper surface of the slab waveguide SW, particularly on the part adjacent to the rib type optical waveguide 10. The roughness of this surface (SF1) is set to be rougher than the roughness of the upper surface (SF0) of the top of the rib type optical waveguide 10.

[0030] In FIG. 3, a first slab waveguide (SW1) having a height lower than that of the rib-type optical waveguide is provided close to the rib-type optical waveguide 10. In addition, a second slab waveguide (SW2) is provided close to the first slab waveguide (SW1), disposed on the opposite side of the rib-type optical waveguide 10, and having a height higher than that of the first slab waveguide (SW1). The manufacturing process can be simplified by making the height of the second slab waveguide the same as that of the rib-type optical waveguide. Here, the roughness of the top surface (SF2) of the second slab waveguide (SW2) is set to be rougher than the top surface (SF0) of the rib-type optical waveguide. In FIG. 3, the top surface of the first slab waveguide (SW1) is not roughened.

[0031] The gap G between the rib-type optical waveguide 10 and the second slab waveguide (SW2) is set to 1 μm or less, more preferably 0.5 μm or less. If the gap G is wider, it becomes difficult to efficiently attract and remove higher-order mode light from the light wave propagating through the rib-type optical waveguide 10, so the gap G is set to a predetermined distance or less.

[0032] In Fig. 4, slab waveguides (SW1, SW1') for removing unnecessary light such as high-order mode light are formed on both sides of the rib-type optical waveguide 10. In Fig. 4, the upper surface of the second slab waveguide is not roughened, but as shown in Fig. 5, it is also possible to form rough surfaces on both the first and second slab waveguides. It goes without saying that slab waveguides having rough surfaces as shown in Fig. 2 or 3 can be installed on both sides of the rib-type optical waveguide 10. Furthermore, by setting the roughness of the upper surface (SF1) of the first slab waveguide (SW1) to be the same as that of the upper surface (SF2) of the second slab waveguide (SW2), the manufacturing process can be simplified.

[0033] FIG. 6 is a plan view explaining the location of the unnecessary light removing means (RM1-4). At the input part of the optical waveguide, coupling loss is likely to occur when the input light Lin enters the optical waveguide. For this reason, since a large amount of high-order mode light is generated at the input part of the optical waveguide, it is preferable to efficiently remove it with the unnecessary light removing means RM1. The symbols Lout1 and Lout2 are output lights.

[0034] In addition, in the branching and multiplexing sections of the optical waveguide, in order to properly control the intensity of the branched light and perform the multiplexing smoothly, it is preferable to effectively remove high-order mode light before it enters the branching section or the optical waveguide section. In addition, there are cases where high-order mode light is also present in the branched or multiplexed light, and it is also possible to provide an unnecessary light removing means after the branching or multiplexing section. In Fig. 6, it is possible to form the unnecessary light removing means (RM2-4) before or after the branching or multiplexing section, or from before to after.

[0035] As shown in FIG. 7, the input and output parts of the rib-type optical waveguide 10 are arranged on the same end face of the substrate (the right end face of the substrate 1 in FIG. 7). In addition, a light detection means (light receiving element) PD for detecting the light wave propagating through the rib-type optical waveguide 10 or the radiated light emitted from the rib-type optical waveguide 10 (multiplexing section) is arranged on the substrate 1. In the configuration as shown in FIG. 7, unwanted light that has become a coupling loss at the input part of the optical waveguide easily enters the light receiving element PD and becomes noise. For this reason, it is particularly important to provide unwanted light removing means RM1 at the input part. In FIG. 7, RM1 is arranged from the input light Lin to the end face of the rib-type optical waveguide 10, but it may be arranged between the input light Lin and the PD. Also, it may be provided to block unwanted light emitted from the substrate so that it does not reach the light receiving element PD.

[0036] Furthermore, part of the higher-order mode light removed by the unwanted light removing means is emitted outside the substrate through the rough portions of the substrate. It is also important to configure the removed unwanted light so that it does not enter the light-receiving element PD. FIG. 7 shows unwanted light removing means RM2 to RM4 that are generated at the branching and multiplexing portions of the rib-type optical waveguide 10. In particular, it is preferable that the unwanted light removing means RM4, which is disposed in the upstream of the light-receiving element PD, absorbs unwanted light with electrodes or the like in order to stabilize the operation of the modulator.

[0037] The optical waveguide element of the present invention is provided with a modulation electrode on a substrate 1 for modulating light waves propagating through the optical waveguide, and is housed in a housing CA as shown in Fig. 8. Furthermore, an optical fiber F for inputting and outputting light waves to and from the optical waveguide is provided to configure an optical modulation device MD. The optical fiber can be arranged not only outside the housing CA as shown in Fig. 8, but also introduced and fixed inside the housing via a through-hole penetrating the side wall of the housing.

[0038] An optical transmitter OTA can be configured by connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal S0 that causes the optical modulation device MD to perform a modulation operation to the optical modulation device MD. Since the modulation signal S to be applied to the optical waveguide element needs to be amplified, a driver circuit DRV is used. The driver circuit DRV and digital signal processor DSP can be placed outside the housing CA, but they can also be placed inside the housing CA. In particular, by placing the driver circuit DRV inside the housing, it is possible to further reduce the propagation loss of the modulation signal from the driver circuit. [Industrial Applicability]

[0039] As described above, according to the present invention, it is possible to provide an optical waveguide element that can efficiently remove higher-order mode light even when the width and height of the optical waveguide are small and that is easy to manufacture. It is also possible to provide an optical modulation device and an optical transmission device that use the optical waveguide element. [Explanation of symbols]

[0040] 1 Board 10 Rib-type optical waveguide SW slab waveguide SW1 First slab waveguide SW2 Second slab waveguide Top surface of the SF0 rib-type optical waveguide SF1~2 Top surface of slab waveguide RM1~4 Unnecessary light removal means MD Optical Modulation Device OTA Optical Transmitter

Claims

1. In an optical waveguide element having a substrate on which a rib-type optical waveguide is formed, The height of the rib-type optical waveguide is set to 1 μm or less, and the width is set to 3 μm or less, a slab waveguide disposed adjacent to the rib-type optical waveguide; the slab waveguide comprises a first slab waveguide adjacent to the rib-type optical waveguide and having a height shorter than that of the rib-type optical waveguide, and a second slab waveguide adjacent to the first slab waveguide and disposed on the opposite side to the rib-type optical waveguide and having a height higher than that of the first slab waveguide, wherein the roughness of the upper surface of the first slab waveguide is greater than the roughness of the top surface of the rib-type optical waveguide.

2. 2. The optical waveguide element according to claim 1, wherein the roughness of the upper surface of said second slab waveguide is set to be greater than the roughness of the top surface of said rib type optical waveguide.

3. 3. The optical waveguide element according to claim 1, wherein the distance between the rib type optical waveguide and the second slab waveguide, which sandwich the first slab waveguide, is set to 1 [mu]m or less.

4. 4. The optical waveguide element according to claim 2, wherein the roughness of the upper surface of the first slab waveguide is set to be the same as the roughness of the upper surface of the second slab waveguide.

5. 5. The optical waveguide element according to claim 1, wherein the first slab waveguide is disposed on both sides of the rib type optical waveguide.

6. 5. The optical waveguide element according to claim 1, wherein the first slab waveguide and the second slab waveguide are disposed on both sides of the rib-type optical waveguide.

7. 7. The optical waveguide element according to claim 1, wherein the slab waveguide is disposed in the vicinity of an input portion, a branching portion, or a multiplexing portion of the rib-type optical waveguide.

8. 8. An optical waveguide element according to claim 1, wherein an input portion and an output portion of the rib type optical waveguide are arranged on the same end surface of the substrate, a photodetector means is provided on the substrate for detecting light waves propagating through the rib type optical waveguide or radiated light emitted from the rib type optical waveguide, and the slab waveguide is provided between the input portion and the photodetector means.

9. 9. An optical modulation device comprising an optical waveguide element according to claim 1, comprising an electrode for modulating an optical wave propagating through the rib-type optical waveguide, the optical waveguide element being housed in a housing, and comprising an optical fiber for inputting or outputting an optical wave to the rib-type optical waveguide.

10. 10. The optical modulation device according to claim 9, further comprising an electronic circuit disposed inside said housing for amplifying a modulation signal input to said optical waveguide element.

11. 11. An optical transmitter comprising: an optical modulation device according to claim 9; and an electronic circuit for outputting a modulation signal for causing the optical modulation device to perform a modulation operation.

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