Curved ridge optical waveguide, optical device, and method of manufacturing optical waveguide

0001This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No.<patcit id="pcit0001" dnum="JP2003079116A"><text>2003-079116, filed on March 20, 2003</text></patcit>, the entire contents of which are incorporated herein by reference.

0002The present invention relates to an optical waveguide that is used for optical communication. Particularly, the present invention relates to an optical waveguide of a diffusion type waveguide capable of reducing a curvature of a curved portion and capable of making an optical device small.

0003An optical device formed with an optical waveguide is provided using an electrooptic crystal such as an LN substrate (LiNbO3: lithium niobate), forming a metal film such as titanium (Ti) on a part of the crystal substrate, and thermally diffusing the metal film. To connect a plurality of optical devices each formed with the optical waveguide, a curved optical waveguide is formed on a part of the optical waveguide. Based on the provision of the curved optical waveguide, the direction of a straight optical waveguide can be turned. Therefore, it becomes possible to make the optical device small, and, particularly, make a size in its length direction small. An apparatus on which the optical waveguide is mounted can also be provided in compact.

0004When a curved waveguide is formed on a part of the optical waveguide, an optical loss (i.e., a radiation loss) that occurs at the curved waveguide portion becomes a problem. In order to suppress the loss at the curved waveguide portion, a technique is so far proposed to combine light that is radiated at the curved optical waveguide portion again on the optical waveguide, by providing a reflector on the external periphery of the curved optical waveguide (see Japanese Patent Application Laid-open No. <patcit id="pcit0002" dnum="JPH11167032B"><text>H11-167032</text></patcit>, for example).

0005One example of the optical device that is configured using the optical waveguide will be explained. <figref idref="f0010">Fig. 16</figref> is a top plan view of an optical modulator that illustrates an example of the conventional optical device. This optical modulator 20 is a Mach-Zehnder interruption type optical modulator, and has a straight optical waveguide. A data line 22 that is supplied with an electronic signal such as DC is formed to have a predetermined length L0 in its longitudinal direction on a substrate 21. Both ends of the data line 22 are led out to one side of the substrate 21 respectively, and signal electrodes 22a and 22b are formed. A ground electrode 23 is formed on other partition of the substrate 21 excluding the data line 22. An optical waveguide 24 is disposed linearly along the data line 22, and two optical waveguides 24 are disposed in parallel between branch portions 24a and 24b.

0006When the electronic signal is supplied to the data line 22 and also when the optical signal is supplied to the two optical waveguides 24, a phase difference due to mutual interference can be generated between the two optical waveguides 24 in a state that a speed of a progressive wave of the electronic signal (microwave) that is supplied to the data line 22 and a speed of an optical wave that flows through the two optical waveguides 24 are matched. In order to generate the phase difference, a predetermined work length (corresponding to the length L0) is necessary as a length of the parallel two optical waveguides 24. A few centimeters are necessary for the length of L0.

0007However, even when the conventional technique is used, the curvature of the curved optical waveguide cannot be set small. For example, when the curvature of the curved optical waveguide is set small to a few millimeters, the recombining of the light becomes insufficient, and the optical loss increases remarkably. When the curvature of the curved optical waveguide cannot be set small, the total optical waveguide cannot be made small, which makes it impossible to make small the optical device that is formed with the optical waveguide or to integrate the optical device. For example, even when a curved optical waveguide is formed on a part of the optical waveguide of the optical device as shown in <figref idref="f0010">Fig. 16</figref>, the optical device cannot be made compact.

0008The present invention has been achieved in order to solve the above problems, and has an object of providing a compact optical waveguide.

0009United States patent application <patcit id="pcit0003" dnum="US20020146190A1"><text>US 2002/0146190 A1</text></patcit> provides an optical device which can lower the drive voltage which is used as one of the parameters for performance evaluation of the optical device. The optical device includes a substrate having first and second ridges, first and second grooves, a third groove, and first and second banks formed thereon, a Mach-Zehnder optical waveguide formed on the substrate, and an electrode set including first and second signal electrodes and a grounding electrode for controlling light which propagates in the optical waveguide.

0010United States patent No. <patcit id="pcit0004" dnum="US6304685B1"><text>US 6,304,685 B1</text></patcit> relates to a device that is a broadband, electro-optic modulator comprising a coplanar waveguide electrode structure on an exemplary Mach-Zehnder interferometer. The modulator is formed on a lithium niobate substrate that is etched so as to form ridges upon which a gold centre electrode and two gold grounded waveguides are deposited upon a buffer layer of silicon dioxide (S_iO₂) to form a coplanar waveguide electrode structure having a waveguide mode for receiving an electrical signal propagating therethrough in a first direction with a second phase velocity to phase modulate an optical light in the optical waveguide.

0011European patent application <patcit id="pcit0005" dnum="EP1154293A"><text>EP 1154293</text></patcit> describes an optical waveguide type directional coupler and a small optical waveguide circuit using the same. An optical coupling part is provided in which a first optical waveguide and a second optical waveguide are arranged side by side to come close each other at the middle part in the longitudinal direction of the optical waveguides. At least one of the incident side of the first optical waveguide and the incident side of the second optical waveguide is adapted to be a light input part to the optical coupling part. At least one of the outgoing side of the first optical waveguide and the outgoing side of the second optical waveguide is adapted to be a light output part. The first and second optical waveguides in the optical coupling part are formed to be curved waveguide parts that project to the other optical waveguide side.

0012US patent application <patcit id="pcit0006" dnum="US20020031304A"><text>US 2002/0031304</text></patcit> describes that a silicon rib waveguide device includes a mode filter section serially connected to a curved rib section. The curved section has a width large enough to support multimode transmission while the filter section has a straight waveguide of smaller rib width supporting only single mode transmission. A tapered section connects the curved section to the straight section and an outwardly tapered waveguide is connected to the opposite end of the straight section.

0013US patent application <patcit id="pcit0007" dnum="US5524165A"><text>US 5524165</text></patcit> describes a method for optimizing a path length of an optical guide and optical guide obtained by this method. This method for optimizing an optical guide with a nominal width Po having at any point a variable and continuous curve C and a widening delta p with respect to this width Po is <b>characterized in that</b> it comprises a definition stage for widening delta p the guide as a variable function f of the curve C, this function delta p=f(C) being a continuous function.

0014European Patent Application <patcit id="pcit0008" dnum="EP0905836A2"><text>EP 0 905 836 A2</text></patcit> relates to deep native oxide confined ridge waveguide semiconductor lasers fabricated by etching and wet oxidation. The upper cladding layer is partially etched forming a ridge and a native oxide layer is wet oxidized from the remaining upper cladding layer and the active region outside the ridge. The deep native oxide layer provides strong optical confinement to the ridge waveguide. Alternately, the active region can be narrower than the ridge waveguide in the laser structure. The ridge waveguide semiconductor laser structures with native oxide layers can also be curved geometry lasers such as ring lasers.

0015International Patent Application <patcit id="pcit0009" dnum="WO0235267A1"><text>WO 02/35267 A1</text></patcit> relates to a hybrid waveguide structure comprising a combination of buried waveguide sections and ridge waveguide sections on the same substrate, sharing a common core layer. The buried waveguide sections provide the low index contrast desirable for couplers and other device components, whereas the ridge waveguide sections provide the high index contrast needed for efficient low-loss tightly curved waveguides. The devices are fabricate starting from a low index contrast buried waveguide. Cladding material is then selectively removed by etching down from an upper surface either side of the waveguide core to a lower surface. This forms an enhanced index contrast ridge section of the waveguide.

0016It is an object of the present invention to at least solve the problems in the conventional technology.

0017According to one aspect of the present invention there is provided an optical waveguide according to appended independent claim 1.According to a further aspect of the present invention there is provided a method of forming an optical waveguide according to appended independent claim 16.

0018Preferred embodiments are defined in the appended dependent claims.

0019All the accompanying drawings relate to certain aspects of the claimed invention but do not represent embodiments according to the invention. <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> is a top plan view of an optical waveguide according to a first arrangement;</li><li><figref idref="f0001">Fig. 2</figref> is a cross-sectional view of the optical waveguide shown in <figref idref="f0001">Fig. 1</figref>;</li><li><figref idref="f0002">Fig. 3</figref> is a graph indicating the optical loss of the optical waveguide;</li><li><figref idref="f0003">Figs. 4A to 4D</figref> are illustrates for process of manufacturing the optical waveguide;</li><li><figref idref="f0004">Fig. 5</figref> is a cross-sectional view of a modification of the optical waveguide;</li><li><figref idref="f0004">Fig. 6</figref> is a top plan view of another modification of the optical waveguide;</li><li><figref idref="f0005">Fig. 7</figref> is a partial top plan view of a coupler of the optical waveguide;</li><li><figref idref="f0005">Fig. 8</figref> is a top plan view of still another modification of the optical waveguide;</li><li><figref idref="f0006">Fig. 9</figref> is a top plan view of still another modification of the optical waveguide;</li><li><figref idref="f0006">Fig. 10</figref> is a top plan view of still another modification of the optical waveguide;</li><li><figref idref="f0007">Fig. 11</figref> is a top plan view of still another modification of the optical waveguide;</li><li><figref idref="f0007">Fig. 12</figref> is a top plan view of still another modification of the optical waveguide;</li><li><figref idref="f0008">Fig. 13</figref> is a top plan view of an optical modulator according to an optical device;</li><li><figref idref="f0009">Figs. 14A and 14B</figref> are cross-sectional views of the optical device shown in <figref idref="f0008">Fig. 13</figref>;</li><li><figref idref="f0010">Fig. 15</figref> is a top plan view of another example of the optical modulator; and</li><li><figref idref="f0010">Fig. 16</figref> is a top plan view of a conventional optical modulator.</li></ul>

0020An optical device for performing phase modulation according to an arrangement includes an optical waveguide and a signal electrode. The optical waveguide then includes a curved ridge structure that is formed on a substrate so as to have a curvature in a longitudinal direction of the curved ridge structure; a curved optical path that is formed in and along the curved ridge structure; and a buffer layer that covers a side of the ridge structure and that has a lower refractive index than a refractive index of the substrate. The signal electrode is disposed on and along the optical waveguide.

0021A method of manufacturing an optical waveguide according to an arrangement includes forming a pattern of titanium on a substrate, the pattern including a curved pattern for forming a curved optical path; thermally diffusing the pattern at a high temperature; forming a ridge structure by etching the substrate positioned at a side of the curved pattern, along a shape of the curved pattern; and forming a buffer layer on a side of the ridge structure, the buffer layer being made of a material that has a lower refractive index than a refractive index of the substrate.

0022A method of manufacturing an optical waveguide according to an arrangement includes forming a proton exchange pattern on a substrate by proton exchange, the pattern including a curved pattern for forming a curved optical path; forming a ridge structure by etching the substrate positioned at a side of the curved pattern, along a shape of the curved pattern; and forming a buffer layer on a side of the ridge structure, the buffer layer being made of a material that has a lower refractive index than a refractive index of the substrate.

0023An optical waveguide according to a first arrangement will be explained. <figref idref="f0001">Fig. 1</figref> is a top plan view of the optical waveguide according to the first arrangement. <figref idref="f0001">Fig. 2</figref> is a cross-sectional view of the optical waveguide shown in <figref idref="f0001">Fig. 1. Fig. 1</figref> mainly illustrates a curved portion as a key portion of the optical waveguide. An optical waveguide 1 is a titanium diffusion lithium niobate (Ti-LiNbO3) diffusion waveguide. A curved portion 1 a shown in the drawing has an arc shape with a central angle of 180 degrees. A pattern width L of the optical waveguide 1 is set to 5 to 9 micrometers to have a small mode field of light. An optical wave that propagates through the optical waveguide 1 is generally a single mode (hereinafter, "SM") light, but the optical propagation mode of the optical waveguide 1 according to this invention is not limited to the SM.

0024As shown in <figref idref="f0001">Fig. 2</figref>, a substrate 2 is dug out to form a ridge structure 3 at both sides of at least the curved portion 1 a of the optical waveguide 1. The substrate 2 is etched to form the ridge structure 3. The power of the light that propagates through the optical waveguide 1 becomes a maximum at a position of about 3 micrometers from the surface. Therefore, a depth d of the ridge structure 3 that is formed by etching is set to equal to or larger than 3 micrometers. In the ridge structure 3, air (refractive index 1) corresponds to a cladding. A difference between a refractive index of the substrate 2 (refractive index 2.2) in a lateral direction with respect to a light proceeding direction and the refractive index of air becomes large, and a curvature of the curved portion 1a becomes small. Even when a curvature of the curved portion 1a becomes small, a radiation loss can be suppressed.

0025However, the ridge structure 3 that is formed by digging down the substrate 2 has a rough surface and not smooth on the side surface 3a. When the side surface 3a facing the curved portion 1a of the optical waveguide 1 is rough, an optical loss occurs at the curved portion 1a. In other words, when the side surface 3a is rough, a scattering loss becomes large among the optical loss. In order to prevent the scattering loss, a film-shaped buffer layer 4 is formed on the surface of the side surface 3a of the ridge structure 3. For the buffer layer 4, a material having a smaller refractive index than that of the substrate 2 can be used such as silicon oxide (SiO2) and titanium oxide (TiO2).

0026In order to confine the optical field with the side surface 3a of the ridge structure 3, the width W of the ridge structure 3 is preferably equal to or smaller than the width L of the optical waveguide 1 plus 6 micrometers (i.e., both sides are equal to or less than 3 micrometers from the center of the optical waveguide 1 respectively).

0027<figref idref="f0002">Fig. 3</figref> is a graph indicating the optical loss of the optical waveguide. In the drawing, the abscissa represents a curvature (a radius) R of the curved portion 1a of the optical waveguide 1, and the ordinate represents the optical loss (excessive loss). As shown in the drawing, in the configuration having the ridge structure 3 and the buffer layer 4 provided like in the optical waveguide 1 according to the present invention (the circle symbol in the drawing), the optical loss can be suppressed even when the curvature R is set small to 4 millimeters or below. On the other hand, in the configuration having the ridge structure 3 and the buffer layer 4 not formed like in the conventional optical waveguide (the square symbol in the drawing), the optical loss is large in the total range of the curvature R equal to or smaller than 4 millimeters. In the configuration having only the ridge structure 3 provided without providing the buffer layer 4 (the triangle symbol in the drawing), the optical loss can be suppressed as compared with the conventional method.

0028<figref idref="f0003">Figs. 4A to 4D</figref> are illustrates for process of manufacturing the optical waveguide according to claim 16. In the process of manufacturing the optical waveguide 1, a Ti pattern is first formed as shown in <figref idref="f0003">Fig. 4A</figref>. After Ti is deposited on the whole surface of the LN substrate 2, a resist pattern is formed along the layout of the optical waveguide 1 including the ridge structure 1a. Thereafter, the optical waveguide 1 is formed by etching.

0029A thermal diffusion is then carried out as shown in <figref idref="f0003">Fig. 4B</figref>. By thermally diffusing the optical waveguide 1 at a high temperature, the optical waveguide 1 of Ti is thermally diffused to have a cross section in approximately a semicircle shape inside the substrate 2. In this state, the optical waveguide 1 has a highest refractive index at approximately the center of the circle, and can guide the wave in confinement. In this process, the optical waveguide 1 can be formed by proton exchange in benzoic acid, in place of carrying out the thermal diffusion.

0030Thereafter, the etching is carried out as shown in <figref idref="f0003">Fig. 4C</figref>. As described above, the ridge structure 3 is formed by digging out the substrate 2 positioned at least at the curved portion 1 a of the optical waveguide 1 by etching. This etching process may be a reactive ion etching (RIE) or the like. In this etching, the width W and the height d required for the ridge structure 3 are satisfied based on the pattern width L (refer to <figref idref="f0001">Fig. 2</figref>) of the optical waveguide 1.

0031Thereafter, a buffer layer is formed as shown in <figref idref="f0003">Fig. 4D</figref>. The buffer layer 4 is formed to cover the side surface 3a of the ridge structure 3 by sputtering or the like. In the above steps, the optical waveguide 1 can be formed in the state that the ridge structure 3 and the buffer layer 4 are formed on the substrate 2.

0032In the above explanation, while the ridge structure 3 and the buffer layer 4 are provided on only the ridge structure 1 a of the optical waveguide 1, the ridge structure 3 and the buffer layer 4 may also be provided on a straight line portion that continues from the curved portion 1a. Further, while the optical waveguide 1 shown in <figref idref="f0001">Fig. 1</figref> has the ridge structure 1a configured in the arc shape having a central angle of 180 degrees, the curve angle of the curved portion 1 a is not limited to this configuration. For example, an output of the optical wave can be directed to a different direction from the input direction at an angle of 90 degrees or above.

0033Modifications of the optical waveguide 1 will be explained below. <figref idref="f0004">Fig. 5</figref> is a cross-sectional view of a modification of the optical waveguide. In the example shown in <figref idref="f0004">Fig. 5</figref>, the width L of the optical waveguide 1 coincides with the width of the ridge structure 3. This optical waveguide 1 is formed by expanding the width of the resist pattern to be larger than the width W of the ridge structure 3 at the time of forming the Ti pattern of the optical waveguide 1, and then executing a diffusion at the diffusion time so that the diffusion depth is approximately constant in a normal direction of the substrate. The optical waveguide 1 formed in this way has a flat surface as compared with the cross-sectional shape shown in <figref idref="f0001">Fig. 2</figref>.

0034<figref idref="f0004">Fig. 6</figref> is a top plan view of another modification of the optical waveguide. At the time of configuring the optical waveguide 1 by connecting the curved portion 1 a with the straight line portion 1 b, a pattern width L1 of the straight line portion 1 b and a pattern width L2 of the ridge structure 1 a may be differentiated in order to minimize the optical loss at respective positions. In this case, a coupler 5 is provided at a coupling portion between the straight line portion 1b and the curved portion 1 a. The coupler 5 shown in <figref idref="f0004">Fig. 6</figref> is formed to have a stage. <figref idref="f0005">Fig. 7</figref> is a partial top plan view of another example of a configuration of the transformation section. As shown in <figref idref="f0005">Fig. 7</figref>, the coupler 5 may be formed in tapered shape. When the coupler 5 is an optical path whose width gradually changes from a width of the straight line portion 1 b to a width of the curved portion 1a, the coupling loss at the coupler 5 can be reduced.

0035<figref idref="f0005">Fig. 8</figref> is a top plan view of still another modification of the optical waveguide. In the configuration shown in <figref idref="f0005">Fig. 8</figref>, the position of the curved portion 1a of the optical waveguide 1 is shifted to the inside relative to the ridge structure 3. The position of the curved portion 1a is not limited to pass through the center of the ridge structure 3 as shown in <figref idref="f0001">Fig. 1</figref>. In the example shown in <figref idref="f0005">Fig. 8</figref>, the curved portion 1a having the pattern width L is shifted toward the inside from the center of the ridge structure 3 having the width W. In this state, the internal periphery of the curved portion 1a is close to or is connected to the side surface 3a of the inside ridge structure 3. On the other hand, the external periphery of the curved portion 1a is separated from the side surface 3a of the outside ridge structure 3 by a predetermined distance. As explained above, the curved portion 1a may be shifted to the outside, in addition to a configuration of shifting the curved portion 1a to the inside of the ridge structure 3.

0036In this case, the shift is carried out by a predetermined amount to a direction of reducing the radiation loss. Particularly, as shown in <figref idref="f0005">Fig. 8</figref>, the radiation loss can be suppressed by setting a distance between the curved portion 1a and the side surface 3a of the ridge structure 3 that is disposed at the external peripheral side of the curved portion 1a. In general, optical losses that are important in the optical waveguide are the radiation loss and the scattering loss. In the above configuration, when the width W of the ridge structure 3 is made smaller, the influence of the radiation loss can be decreased, but the influence of the scattering loss increases. On the other hand, when the width W of the ridge structure 3 is made larger, the influence of the scattering loss can be decreased, but the influence of the radiation loss increases. The scattering loss occurs because of the roughness of the side surface 3a of the ridge structure 3 as described above. Based on this condition, the shift amount capable of suppressing the radiation loss without having the influence of the scattering loss can be set.

0037<figref idref="f0006">Fig. 9</figref> is a top plan view of still another modification of the optical waveguide. In the configuration shown in <figref idref="f0006">Fig. 9</figref>, the center position of the curved portion 1a and the center position of the straight line portion 1b are shifted at the portion of the coupler 5. In general, at the coupling portion between the straight line portion and the curved portion of the optical waveguide and at the coupling portion between the curved portions, the coupling loss (offset) occurs due to the difference between the optical propagation state (field) of the straight line portion and the optical field of the curved portion. In order to suppress the coupling loss, a configuration of shifting the mutual coupling by a predetermined amount is employed (for example, see Japanese Patent Application Laid-open No. <patcit id="pcit0010" dnum="JPH11167032B"><text>H11-167032</text></patcit>). Based on the provision of the shift of center, the coupling loss can be suppressed by making the distributions of the field (or magnetic field) of the respective propagation light approximately equal. When the center position of the curved portion 1a and the center position of the straight line portion 1b are shifted at the position of the coupler 5 like in the configuration as shown in <figref idref="f0006">Fig. 9</figref>, the coupling loss and the radiation loss of the curved portion 1 a can be suppressed. <figref idref="f0006">Fig. 9</figref> also illustrates the configuration of shifting the center position of the curved portion 1a relative to the center position of the ridge structure 3 (refer to <figref idref="f0005">Fig. 8</figref>). In this way, the scattering loss and the radiation loss can be suppressed more.

0038<figref idref="f0006">Fig. 10</figref> is a top plan view of still another modification of the optical waveguide. The configuration shown in <figref idref="f0006">Fig. 10</figref> is a modification of the ridge structure 3. In the configuration shown in <figref idref="f0006">Fig. 10</figref>, a stage having a predetermined shift amount is formed in the ridge structure 3 at the coupling portion between the curved portion 1a and the straight line portion 1b of the optical waveguide 1. As shown in <figref idref="f0006">Fig. 10</figref>, the width of the curved portion 1a and that of the straight line portion 1b that constitute the optical waveguide 1 are constant. <figref idref="f0006">Fig. 10</figref> also illustrates the configuration of shifting the center position of the curved portion 1a relative to the center position of the ridge structure 3, in a similar manner to that shown in <figref idref="f0006">Fig. 9</figref>. Further, the center position of the straight line portion 1b of the optical waveguide 1 coincides with the center position of the ridge structures 3c and 3d respectively that are provided at both sides of the straight line portion 1b. With this arrangement, a stage having a predetermined shift amount is formed at the coupling portion between the ridge structure 3 of the curved portion 1a and the ridge structures 3c and 3d of the straight line portion 1b of the optical waveguide 1. As explained above, the ridge structure 3 provided at the side of the optical waveguide 1 can be formed individually by matching the optimum layout state at the curved portion 1a with the optimum layout state at the straight line portion 1b.

0039<figref idref="f0007">Fig. 11</figref> is a top plan view of still another modification of the optical waveguide. The configuration shown in <figref idref="f0007">Fig. 11</figref> is a modification of the ridge structure 3. When the ridge structure 3 is formed at only the curved portion 1a of the optical waveguide 1 as shown in <figref idref="f0001">Fig. 1</figref>, a coupling loss may occur at a starting point of the ridge, that is, at the coupling portion between the straight line portion 1b and the curved portion 1a of the optical waveguide 1 (i.e., the position of the coupler 5). In this case, as shown in the drawing, the ridge structure 3 is not provided at only the curved portion 1a but is also stretched as extension portions 3c and 3d to a direction of the straight line portion 1b by a predetermined amount. The extension portions 3c and 3d are formed to gradually leave the straight line portion 1b as they leave the position of the coupling portion. In the example shown in <figref idref="f0007">Fig. 11</figref>, the extension portion 3c positioned at the inside of the optical waveguide 1 is formed toward the inside at same curvature as that of the ridge structure 3. The extension portion 3d positioned at the outside of the optical waveguide 1 has the same curvature as that of the extension 3c, and is formed toward the outside. As explained above, when the extension portions 3c and 3d are formed, the occurrence of the coupling loss at the coupling portion (i.e., the coupling portion between the straight line portion 1b and the curved portion 1a) can be suppressed, even when the ridge structure 3 is partly provided on the curved portion 1a.

0040<figref idref="f0007">Fig. 12</figref> is a top plan view of still another modification of the optical waveguide. In the configuration shown in <figref idref="f0007">Fig. 12</figref>, out of the extension portions explained with reference to <figref idref="f0007">Fig. 11</figref>, only the external extension portion 3d is provided. As shown in <figref idref="f0007">Fig. 12</figref>, when the extension portion 3d is provided on the ridge structure 3 that is positioned at the outside of the optical waveguide 1, the coupling loss can be minimized.

0041An optical device that uses the optical waveguide 1 explained in the first arrangement will be explained. The optical device that uses the optical waveguide 1 includes an optical switch and an optical modulator.

0042<figref idref="f0008">Fig. 13</figref> is a top plan view of an optical modulator as an optical device. This optical modulator has a curved portion at a part of the optical waveguide that is provided in the conventional Mach-Zehnder interference type optical modulator shown in <figref idref="f0010">Fig. 16</figref>.

0043On a substrate 11 of an optical modulator 10 shown in <figref idref="f0008">Fig. 13</figref>, a data line 12, signal electrodes 12a and 12b, and a ground electrode 13 are provided. The data line 12 has a length that satisfies the length of the operation length L0, and has a curved portion that is curved in approximately a U-shape. The optical waveguide 1 is disposed along the data line 12, and branch portions 1c and 1d are formed on a portion that is superimposed with the data line 12. Two optical waveguides are disposed in parallel between these branch portions 1c and 1d. The optical waveguide 1 has the curved portion 1a and the straight line portion 1b in a similar manner to that of the first arrangement. These branch portions 1a and 1b become an interaction section between light and microwave.

0044<figref idref="f0009">Figs. 14A and 14B</figref> are cross-sectional views of the configuration shown in <figref idref="f0008">Fig. 13</figref>. As shown in <figref idref="f0009">Figs. 14A and 14B</figref>, the ridge structure 3 and the buffer layer 4 are provided over the whole length of the optical waveguide 1. The width W of the ridge structure 3 is constant at the curved portion 1a, thereby to make it possible to align the speed of the microwave with the speed of light over the whole region of the interaction section. As shown in <figref idref="f0009">Fig. 14A</figref>, the center position of the ridge structure 3 coincides with the center position of the optical waveguide 1, at the portion of the straight line portion 1b of the optical waveguide 1 (cross-section of A-A). Further, as shown in <figref idref="f0009">Fig. 14B</figref>, the center position of the optical waveguide 1 is disposed with a predetermined shift amount from the center position of the ridge structure 3 to a direction of the external ridge structure 3, at the portion of the curved portion 1a of the optical waveguide 1 (cross-section of B-B). With this arrangement, the radiation loss at the portion of the curved portion 1a can be suppressed. The curved portion 1a of the optical waveguide 1 can also be shifted by a predetermined amount to the internal periphery side of the ridge structure 3 when the radiation loss can be reduced in this direction, in a similar manner to that explained above with reference to <figref idref="f0005">Fig. 8</figref>.

0045According to the optical modulator 10 explained above, when the optical waveguide 1 is provided on the curved portion 1 a that is turned by 180 degrees, the length in the lateral direction can be made shorter than the conventional length shown in <figref idref="f0010">Fig. 16</figref>, and the device can be made compact.

0046<figref idref="f0010">Fig. 15</figref> is a top plan view of another configuration example of the optical modulator as an optical device. The configuration is different from that shown in <figref idref="f0008">Fig. 13</figref> in that the curvature of the curved portion 1a of the optical waveguide 1 is set large. Further, the total width (in the vertical direction in the drawing) is set small. In order to take a large marginal length P to connect a connector at the portions of the signal electrodes 12a and 12b, the straight line portions 1b of the optical waveguide 1 for input and output are set closer to each other. Correspondingly, approximately a half of a part of the straight line portion 1b facing the curved portion 1a side is not linear, and forms an arc portion 1e that connects between the curved portion 1 a and the straight line portion 1b.

0047As explained above, the optical waveguide 1 can be formed freely within a range of the size of the optical device (substrate 2) to match the size (vertical and lateral sizes) of the optical device and conditions of connector connection and the like. In the configuration shown in <figref idref="f0010">Fig. 15</figref>, the ridge structure 3 and the buffer layer 4 are also formed along the optical waveguide 1 as shown in <figref idref="f0009">Fig. 14</figref>. With this arrangement, the optical loss (radiation loss and scattering loss) can be suppressed without depending on the shape of the optical waveguide 1.

0048The optical waveguide 1 is explained taking the example of the configuration of an approximately the U-shape so that the input direction of an optical signal is turned to the output direction of the optical signal by 180 degrees at one position. When one turning is provided at one position of the optical waveguide 1 as shown in <figref idref="f0008">Fig. 13</figref> and <figref idref="f0010">Fig. 15</figref>, the light input section and the output section of the optical waveguide 1 are disposed on the same one side surface of the optical device. The optical waveguide 1 can also have a configuration of approximately an S-shape having a turning at two positions or a turning at three or more positions. When the number of turning of the optical waveguide 1 increases, the work length L0 can be set long, and a variable region of a phase difference can be increased. When the turning is provided by an even number of times, the light input section and the output section of the optical waveguide 1 are disposed on different side surfaces of the optical device.

0049While a Mach-Zehnder type optical modulator is explained above, the optical waveguide 1 can also be applied to a phase modulator. The configuration of the phase modulator is different from that of the optical modulator only in that one optical waveguide is provided without using branch portions. This phase modulator also has electrodes disposed on the optical waveguide 1. When the curved portion 1a, the ridge structure 3, and the buffer layer 4 described above are provided on the optical waveguide 1 that is provided on the phase modulator, the optical loss can be suppressed and the device can be made compact.

0050The optical waveguide 1 explained above can also be applied to other electronic device such as an optical switch in addition to the optical modulator such as the phase modulator. The optical loss in the electronic device can also be suppressed, and the device can be made compact.

0051According to the present invention, a curved portion is formed on an optical waveguide. Even when a curvature of the curved portion is small, there is an effect that optical loss can be suppressed, and that the optical waveguide and an optical device using the optical waveguide can be made compact.

0052Although aspects of the invention has been described with respect to specific arrangements for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the scope of protection.

0053According to a further arrangement an optical waveguide is formed on a substrate, comprising a curved ridge structure that is formed on the substrate so as to have a curvature in a longitudinal direction of the curved ridge structure, a curved optical path that is formed along the curved ridge structure, and a buffer layer that covers a side of the curved ridge structure and that has a lower refractive index than a refractive index of the substrate.

0054According to a further arrangement the curved optical path is formed on a top side of the curved ridge structure.

0055According to a further arrangement the curved ridge structure is formed outside the curved ridge structure.

0056According to a further arrangement the buffer layer is silicon oxide.

0057According to a further arrangement the substrate is made from lithium niobate.

0058According to a further arrangement the curved optical path has a curvature of not more than 4 millimeters.

0059According to a further arrangement the curved optical path has a width in a range from 5 micrometers to 9 micrometers.

0060According to a further arrangement a width of the curved optical path is equal to a width of the curved ridge structure.

0061According to a further arrangement the curved optical path has a turning angle in a range from 90 degrees to 180 degrees.

0062According to a further arrangement a center line of the curved optical path is shifted from a center line of the curved ridge structure.

0063According to a further arrangement the curved ridge structure has a width of at most 6 micrometers plus a width of the curved portion.

0064According to a further arrangement the curved ridge structure has a height of at least 3 micrometers.

0065According to a further arrangement the optical waveguide further comprises a straight optical path that is coupled with the curved optical path, a second ridge structure that is formed on the substrate along the straight optical path, and a second buffer layer that covers a side of the second ridge structure and that has a lower refractive index than a refractive index of the substrate, wherein the straight optical path is formed in and along the second ridge structure, and the second ridge structure has a width increased along the straight optical path.

0066According to a further arrangement the optical waveguide further comprises an optical path that is coupled with the curved optical path and that has a width different from a width of the curved optical path, and a coupler that optically couples the curved optical path with the optical path.

0067According to a further arrangement the coupler is an optical path whose width gradually changes from a width of the curved optical path to a width of the optical path. According to a further advantageous embodiment the optical waveguide further comprises an optical path that is coupled with the curved optical path and that has a width different from a width of the curved optical path, wherein the optical path is coupled with the curved optical path so that an axis of the optical path is shifted from an axis of the curved optical path.

0068According to a further arrangement the optical waveguide farther comprises a straight optical path that is coupled with the curved optical path, a second ridge structure that is formed on the substrate along the straight optical path, and second buffer layer that covers a side of the second ridge structure and that has a lower refractive index than a refractive index of the substrate, wherein the straight optical path is formed in and along the second ridge structure, and the second ridge structure is connected to the curved ridge structure so that an axis of the second ridge structure is shifted from an axis of the curved ridge structure.

0069According to a further arrangement an optical device for performing phase modulation comprises an optical waveguide that includes a curved ridge structure that is formed on a substrate so as to have a curvature in a longitudinal direction of the curved ridge structure, a curved optical path that is formed in and along the curved ridge structure, and a buffer layer that covers a side of the ridge structure and that has a lower refractive index than a refractive index of the substrate, and a signal electrode that is disposed on and along the optical waveguide.

0070According to a further arrangement the optical waveguid includes two optical paths that are parallel to each other, and the signal electrode is disposed on and along each of the two optical paths.

0071According t further arrangement the optical waveguide further includes a second ridge structure that is formed on the substrate straight in a longitudinal direction of the second ridge structure, a straight optical path that is coupled with the curved optical path and that is formed in and along the second ridge structure, and a second buffer layer that covers a side of the second ridge structure and that has a lower refractive index than a refractive index of the substrate, optical signals transmitted through the curved optical path and the straight optical path interact with electric signals transmitted through the signal electrode, and the curved ridge structure and the second ridge structure have a constant width.

0072According to a further arrangement the optical waveguide further includes a first straight optical path that has an input end and a first connecting end that is coupled with one end of the curved optical path, and a second straight optical path that has an output end and a second connecting end is coupled with another end of the curved optical path, and that is parallel to the first straight optical path, and the input end and the output end are disposed on a side of the substrate.

0073According to a further arrangement the curved optical path has a first curved portion and a second curved portion, the first curved portion has a constant curvature, and the second curved portion has a curvature gradually changing to be connected with one of the first connecting end and the second connecting end.

0074According to a further arrangement a method of manufacturing an optical waveguide comprises forming a pattern of titanium on a substrate, the pattern including a curved pattern for forming a curved optical path, thermally diffusing the pattern at a high temperature, forming a ridge structure by etching the substrate positioned at a side of the curved pattern, along a shape of the curved pattern, and forming a buffer layer on a side of the ridge structure, the buffer layer being made of a material that has a lower refractive index than a refractive index of the substrate.

0075According to a further arrangement a method of manufacturing an optical waveguide comprises forming a proton exchange pattern on a substrate by proton exchange, the pattern including a curved pattern for forming a curved optical path, forming a ridge structure by etching the substrate positioned at a side of the curved pattern, along a shape of the curved pattern, and forming a buffer layer on a side of the ridge structure, the buffer layer being made of a material that has a lower refractive index than a refractive index of the substrate.