Optical waveguide elements, optical modulation devices and optical transmission devices

Abstract

An optical waveguide element is provided that suppresses insertion loss related to coupling with optical fibers, etc., while achieving miniaturization of the optical waveguide element. An optical waveguide element, an optical modulation device, and an optical transmission device are provided. The optical waveguide element includes: a ribbed optical waveguide (10) formed of a material (1) having an electro-optic effect; and a reinforcing substrate (2) supporting the optical waveguide. The optical waveguide element is characterized in that one end of the optical waveguide forms a tapered portion (11) whose width tapers toward the end face of the reinforcing substrate, and includes a structure (3) comprising a material having a higher refractive index than the material constituting the reinforcing substrate to cover the tapered portion, and a coating layer (4) comprising a material having a lower refractive index than the material constituting the structure to cover the structure.

Description

Technical Field

[0001] The present invention relates to an optical waveguide element, an optical modulation device using the optical waveguide element, and an optical transmission device, and particularly to an optical waveguide element comprising a ribbed optical waveguide formed of a material having an electro-optic effect, and a reinforcing substrate supporting the optical waveguide. Background Technology

[0002] In recent years, with the increasing amount of information in the information and communication field, there is a growing demand not only for long-distance optical communication but also for high-speed and high-capacity optical communication used between cities or data centers. Moreover, due to space constraints imposed by base stations, the demand for high-speed and miniaturized optical modulators is surging.

[0003] In particular, when miniaturizing optical modulators, the bending radius of the optical waveguide can be reduced by enhancing its optical confinement effect. For example, the directions of the light waves incident on the optical waveguide element and the emitted light waves can be bent by 90 degrees or 180 degrees, thereby enabling the fabrication of optical modulators suitable for miniaturization. To enhance this optical confinement, it is effective to miniaturize the optical waveguide, for example, by setting the mode field diameter (MFD) of the propagated light wave to less than 3 μm.

[0004] LiNbO3 (hereinafter, LN), exhibiting electro-optic effects, demonstrates low distortion and optical loss when converting electrical signals to optical signals, making it suitable for use as a long-distance optical modulator. However, existing optical waveguides have a mean flatness (MFD) of approximately 10 μm and a bending radius of tens of millimeters, hindering miniaturization. Nevertheless, recent advancements in grinding and bonding technologies have made LN thinner sheets possible, and research and development of LN optical waveguide components with an MFD of approximately 1 μm are underway.

[0005] On the other hand, the MFD (Mean Displacement) of optical fibers is around 10 μm. In optical waveguide elements containing micro-waveguides with an MFD of less than 3 μm, when the fiber is directly bonded to the end face of the element when light is incident from the end face, a large insertion loss occurs. To address this issue, research is underway to limit the incident light wave to a spot size of 3 μm or less, and to incorporate a spot size converter (SSC) that is magnified inversely within the chip for the emitted light wave.

[0006] As shown in Patent Documents 1 to 3, a typical SSC uses a tapered shape that expands the width or thickness of the optical waveguide in two or three dimensions toward the end of the waveguide. The advantages of this method include its simple design; however, because expanding the optical waveguide induces multimode, the available designs are limited, making it unsuitable for optical modulators.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: International Publication No. WO2012 / 042708

[0010] Patent Document 2: International Publication No. WO2013 / 146818

[0011] Patent Document 3: Japanese Patent No. 6369036 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] The problem to be solved by the present invention is to address the aforementioned problems and to provide an optical waveguide element that suppresses insertion loss related to coupling with optical fibers, etc., while achieving miniaturization of the optical waveguide element. Furthermore, an optical modulation device and an optical transmission apparatus using the aforementioned optical waveguide element are provided.

[0014] Technical means to solve the problem

[0015] In order to solve the aforementioned problems, the optical waveguide element, the optical modulation device using the optical waveguide element, and the optical transmission device of the present invention have the following technical features.

[0016] (1) An optical waveguide element comprising: a ribbed optical waveguide formed of a material having an electro-optic effect; and a reinforcing substrate supporting the optical waveguide, wherein one end of the optical waveguide forms a tapered portion whose width tapers toward the end face of the reinforcing substrate, and includes a structure comprising a material having a higher refractive index than the material constituting the reinforcing substrate to cover the tapered portion, and a coating layer comprising a material having a lower refractive index than the material constituting the structure to cover the structure.

[0017] (2) The optical waveguide element according to (1) is characterized in that the tapered portion comprises multiple stacked optical waveguides and is configured such that the width of the optical waveguide disposed on the upper side is narrower than the width of the optical waveguide disposed on the lower side.

[0018] (3) The optical waveguide element according to (1) or (2) is characterized in that the coating layer is composed of an adhesive, the coating layer functions as an adhesive layer, and the adhesive layer bonds the upper reinforcing substrate disposed on the upper side of the structure to the reinforcing substrate side on which the optical waveguide and the structure are formed.

[0019] (4) The optical waveguide element according to any one of (1) to (3) is characterized in that the thickness of the reinforcing substrate near the end face of the reinforcing substrate is thinner than the thickness of the reinforcing substrate on the lower side of the tapered portion.

[0020] (5) The optical waveguide element according to any one of (1) to (4) is characterized in that the mode field diameter of the light wave propagating in the optical waveguide is less than 3 μm, and the mode field diameter of the optical fiber connected to the optical waveguide element and relative to the optical wave input or output of the optical wave is 3 μm or more.

[0021] (6) An optical modulation device, characterized in that an optical waveguide element according to any one of (1) to (5) is housed in a housing, and the optical modulation device includes an optical fiber for inputting or outputting optical waves relative to the optical waveguide.

[0022] (7) The optical modulation device according to (6) is characterized in that the optical waveguide element includes a modulation electrode for modulating an optical wave propagating in the optical waveguide, and an electronic circuit for amplifying the modulation signal input to the modulation electrode of the optical waveguide element is provided inside the housing.

[0023] (8) An optical transmitting device, characterized in that it comprises: an optical modulator according to (6) or (7); and an electronic circuit for outputting a modulation signal that causes the optical modulator to perform a modulation operation.

[0024] The effects of the invention

[0025] The present invention provides an optical waveguide element comprising: a ribbed optical waveguide formed of a material having an electro-optic effect; and a reinforcing substrate supporting the optical waveguide. In the optical waveguide element, one end of the optical waveguide forms a tapered portion whose width tapers towards the end face of the reinforcing substrate. A structure comprising a material with a higher refractive index than the material constituting the reinforcing substrate covers the tapered portion. A coating layer comprising a material with a lower refractive index than the material constituting the structure covers the structure. Therefore, the MFD of the optical wave can be reduced or expanded while suppressing the generation of multimode light, thus suppressing insertion loss related to coupling with optical fibers, etc., and is suitable for miniaturization. Attached Figure Description

[0026] Figure 1 This is a perspective view showing an example of the optical waveguide element of the present invention.

[0027] Figure 2 yes Figure 1 A cross-sectional view of the optical waveguide element at point X-X'.

[0028] Figure 3 yes Figure 1A plan view of an optical waveguide element.

[0029] Figure 4 yes Figure 3 Cross-sectional views at each of the dashed lines.

[0030] Figure 5 This diagram illustrates the changes in rib shape caused by the two-stage etching process.

[0031] Figure 6 It was implemented Figure 5 The figure illustrates the shape of the ends of the optical waveguide after two stages of etching.

[0032] Figure 7 This is a perspective view showing another shape of the optical waveguide used in the optical waveguide element of the present invention.

[0033] Figure 8 It means including Figure 7 A three-dimensional view of an optical waveguide element with the shape shown.

[0034] Figure 9 yes Figure 8 A cross-sectional view of the optical waveguide element at point X-X'.

[0035] Figure 10 yes Figure 8 A plan view of an optical waveguide element.

[0036] Figure 11 yes Figure 10 Cross-sectional views at each of the dashed lines.

[0037] Figure 12 This is a perspective view illustrating an embodiment in which the thickness of a portion of the reinforcing substrate of the optical waveguide element of the present invention is reduced.

[0038] Figure 13 This diagram illustrates the portion where the thickness of the reinforcing substrate is reduced.

[0039] Figure 14 yes Figure 13 The cross-sectional view at the dashed line A-A' in (a).

[0040] Figure 15 This is a plan view illustrating the optical modulation device and optical transmission apparatus of the present invention.

[0041] [Explanation of Symbols]

[0042] 1: A layer formed of a material with electro-optic effect / a material with electro-optic effect / a layer constituting an optical waveguide / LiNbO3, a material constituting an optical waveguide

[0043] 2: Reinforced substrate / cladding layer

[0044] 3: Structure / Core

[0045] 4: Coating / cladding layer

[0046] 5: Upper reinforced substrate

[0047] 8: Frame

[0048] 10, 12, 14: Optical waveguide

[0049] 11, 15: Conical part / conical section

[0050] 16: Part

[0051] 20: Reinforced substrate

[0052] A-A', B-B', C-C', X-X': Section lines

[0053] DSP: Digital Signal Processor

[0054] DRV: Driver circuit

[0055] F: Optical fiber

[0056] LN: LiNbO3 exhibiting electro-optic effect

[0057] MD: Optical Modulation Device

[0058] OTA: Optical Transmission Device

[0059] PR1, PR2: Photoresist

[0060] R1, R2, R3: Parts of the thinned reinforcing substrate 20

[0061] RB: Rib

[0062] W1, W2, Y, Z: Width Detailed Implementation

[0063] Hereinafter, the optical waveguide element of the present invention will be described in detail using preferred examples.

[0064] Furthermore, in the following description, the structure of the end of the optical waveguide is described with the exit portion as the center, but the incident portion can also be constructed in the same way.

[0065] like Figures 1 to 4As shown, the optical waveguide element of the present invention includes: a ribbed optical waveguide 10 formed of a material 1 having an electro-optic effect; and a reinforcing substrate 2 supporting the optical waveguide. The optical waveguide element is characterized in that one end of the optical waveguide forms a tapered portion 11 whose width tapers toward the end face of the reinforcing substrate, and includes a structure 3 containing a material having a higher refractive index than the material constituting the reinforcing substrate to cover the tapered portion, and a coating layer 4 containing a material having a lower refractive index than the material constituting the structure to cover the structure.

[0066] As the material used in the optical waveguide element constituting the present invention, ferroelectric materials with electro-optic effects can be utilized. Specifically, substrates such as lithium niobate (LN) or lithium tantalate (LT), lead lanthanum zirconate titanate (PLZT), or epitaxial films formed from these materials can be used. In addition, various materials such as semiconductor materials or organic materials can also be used as substrates for the optical waveguide element.

[0067] The optical waveguide 10 used in this invention has an extremely thin thickness of about a few μm. Methods include mechanically grinding a crystalline substrate such as LN to achieve thinning, or using an epitaxial film such as LN. In the case of an epitaxial film, the epitaxial film is formed, for example, based on the crystal orientation of a single-crystal substrate such as a SiO2 substrate, a sapphire single-crystal substrate, or a silicon single-crystal substrate, using methods such as sputtering, chemical vapor deposition (CVD), or sol-gel methods.

[0068] Because the waveguide layer is thin, a reinforcing substrate 2 is disposed on the back side of the optical waveguide 10 to improve the mechanical strength of the optical waveguide element. The reinforcing substrate 2 can also be made of a material with a lower refractive index than the waveguide layer, such as a SiO2 substrate. Alternatively, the layer 1 constituting the optical waveguide 10 can be directly bonded to the reinforcing substrate 2, or the reinforcing substrate 2 can be used as a base for crystal growth and a layer constituting the epitaxial film of the optical waveguide can be disposed thereon.

[0069] The rib-shaped protrusions constituting the optical waveguide can be formed by dry or wet etching of the layer forming the optical waveguide (e.g., an LN layer). Alternatively, to increase the refractive index of the ribs, a method of thermally diffusing a high-refractive-index material such as Ti at the rib location can also be used.

[0070] The optical waveguide element of the present invention is characterized in that, as Figure 1 or Figure 3As shown, one end (the incident or exit portion of the light wave) of the ribbed optical waveguide 10 adopts a tapered portion 11 whose width tapers towards the end face of the reinforcing substrate 2, a so-called "inverted tapered shape." This is completely different from the tapered shapes of existing Patent Documents 1 to 3, which widen or thicken towards the end. Furthermore, in Figure 1 or Figure 3 The example shown is an inverted conical shape in which the width of the optical waveguide changes, but the present invention is not limited to this shape. Within the range where the difference in refractive index between the core and the cladding can ensure that the optical wave propagates in a single mode, for example, a shape in which the thickness gradually decreases or a shape in which the width decreases while the thickness also decreases can also be used.

[0071] Additionally, a structure 3, comprising a material with a higher refractive index than the material constituting the reinforcing substrate 2, covers the tapered portion with a reduced width. The refractive index of this structure is lower than that of the material constituting the optical waveguide 10. As the material for the structure 3, inorganic materials such as glass or resin-based materials with increased refractive index can be used. Inorganic materials may also be included, taking into account the durability of the SSC.

[0072] If the structure 3 is formed from resin materials such as adhesives or photoresists (permanent photoresists), air bubbles can easily enter the vicinity of the optical waveguide 10 during resin coating. Therefore, it is more preferable to form it by using sputtering or other methods to form a film of inorganic materials.

[0073] Furthermore, to cover structure 3, a coating layer 4 comprising a material with a lower refractive index than the material constituting the structure is provided. The coating layer can be a resin layer such as an adhesive, or it can be an air layer. Additionally, as... Figure 2 As shown, the coating layer 4, composed of an adhesive, can also function as an adhesive layer, bonding the upper reinforcing substrate 5, which is disposed on the upper side of the structure 3, to the reinforcing substrate 2 on which the optical waveguide 10 and the structure 3 are formed. The upper reinforcing substrate 5 improves the mechanical strength of the end face of the optical waveguide element and also serves as a support component when the optical fiber or optical block is directly bonded to the end face. With this support component, the optical axis alignment of the optical fiber and the optical waveguide becomes easier, and connection loss can be reduced even under different MFDs.

[0074] Will Figure 3 The cross-sections from the dashed line A-A' to D-D' are shown in Figure 4 (a) to Figure 4 In (d). Through these structures, from Figure 4 (a) 10-directional ribbed optical waveguide Figure 4 (d) The optical waveguide with structure 3 as the core and the reinforcing substrate 2 and the coating layer 4 as the cladding is gradually switched, that is, from Figure 4(a) towards Figure 4 (d) Gradually switching the optical waveguide also simultaneously achieves the conversion of the beam size of the propagating light wave. For example, for a light wave with an MFD of 1 μm propagating in optical waveguide 10, it is possible to maintain single-mode operation while simultaneously... Figure 4 In the (d) stage, the MFD of the light wave is expanded to about 3μm.

[0075] Here, when fabricating the inverted conical waveguide 11, the etching mask itself needs to be thinned to prevent peeling off in the narrow section. However, if it is thinned, it becomes difficult to fabricate the ribbed waveguide in a single etching process. Therefore, as... Figure 5 As shown, consider dividing into Figure 5 (a) to Figure 5 The etching process of (c) and Figure 5 (d) to Figure 5 The etching process of (f) is carried out in these two stages. However, although the depth (height) of the rib RB can be obtained, the LN is etched at an angle, so the width Y of the optical waveguide changes to the width Z and becomes thinner. Figure 5 The symbols PR1 and PR2 are photoresists. Figure 5 (a) and Figure 5 The dashed line in (d) represents the boundary removed by the resist pattern.

[0076] Figure 6 The following situation is shown: Figure 5 As shown, a resist pattern is disposed on the pattern of the optical waveguide 10 fabricated in one step, and the LN around the optical waveguide 10 is further removed to further deepen the optical waveguide 10. In this case, the portion shown by the diagonal line is further shaved off, and the width of the optical waveguide 12 becomes thinner. The width of the inverted cone shape also becomes drastically thinner, thus increasing the propagation loss.

[0077] In order to maintain the inverted conical shape as designed, in the optical waveguide element of this invention, such as Figure 7 As shown, the tapered portions (11, 15) include multiple stacked optical waveguides (10, 14), and the width W1 of the upper optical waveguide 10 is narrower than the width W2 of the lower optical waveguide 14.

[0078] In order to form such Figure 7 Such an optical waveguide is etched a first time with a resist pattern consistent with the shape of optical waveguide 10, and a second time with a resist pattern consistent with the shape of optical waveguide 14. Furthermore, in Figure 7 (or Figure 1 In the process, etching is performed near the tapered portion until the reinforcing substrate 2 is exposed, but a very thin LN portion may remain.

[0079] Figures 8 to 11 It was adopted Figure 7 The shape of the tapered portion shown is represented by the SSC. (Example: ...) Figure 8 As shown, for LN(1), which is the material constituting the optical waveguide, the portion 16 far from the optical waveguide may remain without being specifically eliminated. Figure 8 The conical part contains two stacked conical parts (11, 15), but is not limited to two parts, and may also have three or more parts.

[0080] Structure 3 is configured to cover the tapered portion of the optical waveguide. Structure 3 is identical to the structure described above. Furthermore, as... Figure 9 As shown, the coating layer 4, the upper reinforcing substrate 5 and Figure 2 The same configuration applies. Of course, it goes without saying that the covering layer 4 can also be replaced with an air layer.

[0081] Will Figure 10 The cross-sectional view at the point from dashed line A-A' to dashed line D-D' is shown in the figure. Figure 11 (a) to Figure 11 In (d). Figure 11 The MFD of the light wave propagating in the optical waveguide 10 of (a) is from Figure 11 (a) towards Figure 11 The (d) gradually changes, and eventually, the MFD of the light wave gradually expands to... Figure 11 An optical waveguide containing a core 3 and cladding portions (2 and 4) as shown in (d).

[0082] Through simulation Figure 1 The shape of SSC and Figure 8 The propagation loss of the SSC was compared. Regarding the refractive indices of each component, it was assumed that the optical waveguide 10 was 2.138, the reinforcing substrate 2 was 1.494, the structure 3 was 1.575, and the coating layer 4 was 1.542. (Construction) Figure 1 The thickness of layer 1 of the optical waveguide material is set to 0.2 μm, the thickness of the optical waveguide 10 protruding from the surface of layer 1 is set to 0.6 μm, and the width of the optical waveguide 10 is set to 1.3 μm. Assuming the structure... Figure 8 The width of the optical waveguide in the tapered portion 11 is 1.3 μm, and the width of the optical waveguide constituting the tapered portion 15 is 3.0 μm.

[0083] The simulation results confirm that, for example Figure 8 Compared to the case with only one tapered section, the propagation loss is improved from 0.41 to 0.05 (Loss / dB) in the case with two tapered sections. In addition, if the coupling loss of the optical fiber is also considered, the transmission loss of the light wave propagating from the optical waveguide 10 through the SSC section into the optical fiber is improved from 0.73 to 0.32 (Loss / dB).

[0084] In addition, such as Figure 12 and Figure 13 As shown, it is also possible to strengthen the area near the end face of the substrate ( Figure 12 The thickness of the reinforcing substrate 20 (on the lower right side) is configured to be equal to or thinner than the thickness of the reinforcing substrate 2 on the lower side of the tapered portion 15. Figure 13 The thinned portion of the reinforcing substrate 20 (the area where the reinforcing substrate 2 is cut deeper) is represented by the shaded areas R1 to R3. This is not limited to only... Figure 13 In case (a) the strengthening substrate 2 is thinned near the end face of the optical waveguide element, such as Figure 13 As shown in (c), the reinforcing substrate 2 can also be made thin according to the shape of the tapered portion 15.

[0085] Figure 14 yes Figure 13 The cross-sectional view at the dashed line A-A' in (a). Figure 14 The hollow cross shown in the figure is a symbol representing the overlapping position of the front end of the tapered portion 11 of the optical waveguide. By setting it in this way, the reinforcing substrate 2 can be made thinner, and the position of the tapered portion 11, which is the front end of the optical waveguide 10, can be adjusted to the center position of the optical waveguide with the structure 3 as the core, thereby reducing propagation loss.

[0086] In the optical waveguide element of the present invention, the mode field diameter (MFD) of the light wave propagating in the optical waveguide within the element is less than 3 μm (e.g., about 1 μm), and the mode field diameter of the optical fiber connected to the optical waveguide element relative to the input or output light wave within the element is 3 μm or more (e.g., 10 μm). Therefore, the radius of curvature of the optical waveguide within the optical waveguide element can be reduced while using conventional optical fibers, thereby contributing to the miniaturization of the optical waveguide element.

[0087] The optical waveguide element of the present invention is provided with a modulation electrode for modulating the light wave propagating in the optical waveguide 10, and as follows: Figure 15 It is thus housed within the housing 8. Furthermore, by providing an optical fiber F that supplies input and output light waves relative to the optical waveguide, an optical modulation device MD can be constructed. Figure 15 In this design, the optical fiber is introduced into the frame through a through-hole in the side wall and directly connected to the optical waveguide element. The optical waveguide element and the optical fiber can also be optically connected via a space optical system.

[0088] An optical transmitting device (OTA) can be constructed by connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal to the optical modulator MD. Since the modulation signal applied to the optical waveguide element needs to be amplified, a driver circuit (DRV) is used. The driver circuit (DRV) and the digital signal processor (DSP) can be configured either outside or inside the housing 8. In particular, configuring the driver circuit (DRV) inside the housing further reduces the propagation loss of the modulation signal from the driver circuit.

[0089] Industrial availability

[0090] As explained above, the present invention provides an optical waveguide element that can suppress insertion loss related to coupling with optical fibers, etc., while achieving miniaturization of the optical waveguide element. Furthermore, it also provides an optical modulation device and an optical transmission apparatus using the aforementioned optical waveguide element.

Claims

1. An optical waveguide element, comprising: Rib-shaped optical waveguides are formed from materials with electro-optic effects; and a reinforcing substrate to support the optical waveguide, wherein the optical waveguide element is characterized in that, One end of the optical waveguide forms a tapered portion whose width tapers toward the end face of the reinforcing substrate. A structure comprising a material having a higher refractive index than the material constituting the reinforcing substrate is used to cover the tapered portion. A coating layer comprising a material having a lower refractive index than the material constituting the structure is configured to cover the structure, wherein the end face of the reinforcing substrate has a region where the tapered portion is not configured, the region having a first portion aligned with the tapered portion and a second portion located on both sides of the first portion, wherein the thickness of the reinforcing substrate in the first portion is thinner than the thickness of the reinforcing substrate on the lower side of the tapered portion, and the thickness of the reinforcing substrate in the second portion is equal to the thickness of the reinforcing substrate on the lower side of the tapered portion.

2. The optical waveguide element according to claim 1, characterized in that, The tapered portion comprises multiple stacked optical waveguides, and the width of the optical waveguide disposed on the upper side is narrower than the width of the optical waveguide disposed on the lower side.

3. The optical waveguide element according to claim 1 or 2, characterized in that, The coating layer is composed of an adhesive and functions as an adhesive layer, which bonds the upper reinforcing substrate disposed on the upper side of the structure to the reinforcing substrate side on which the optical waveguide and the structure are formed.

4. The optical waveguide element according to claim 1 or 2, characterized in that, The mode field diameter of the light wave propagating in the optical waveguide is less than 3 μm, and the mode field diameter of the optical fiber connected to the optical waveguide element and relative to the input or output light wave of the optical waveguide is more than 3 μm.

5. An optical modulation device, characterized in that, The optical waveguide element as described in any one of claims 1 to 4 is housed within a housing, and the optical modulation device includes an optical fiber that inputs or outputs optical waves relative to the optical waveguide.

6. The optical modulation device according to claim 5, characterized in that, The optical waveguide element includes a modulation electrode for modulating light waves propagating in the optical waveguide, and an electronic circuit inside the housing amplifies the modulation signal input to the modulation electrode of the optical waveguide element.

7. An optical transmitting device, characterized in that... It comprises: an optical modulation device as described in claim 5 or 6; and electronic circuitry that outputs a modulation signal that causes the optical modulation device to perform a modulation operation.

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