JP5195998B2 - Optical device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce loss of microwave. <P>SOLUTION: A first ridge is defined by a first groove and a second groove, and an inner waveguide exists in the ridge. A second ridge is defined by the second groove and a third groove, and an outer waveguide exists in the ridge. A third ridge is defined by the third groove and a fourth groove. There are joints where the first waveguide and the second waveguide join. Ridge width Wa is defined as the width of the first ridge, second ridge and third ridge existing in a light input side waveguide section which is a parallel waveguide section extending to a point where light reaches one of the joints and a light output side waveguide section which is a parallel waveguide section where light is transmitted from the other joint. Ridge width Wb is defined as the width of the first ridge, second ridge and third ridge existing in the vicinity of the joints. Ridge width Wc is defined as the width of the first ridge, the second ridge and the third ridge existing in a bent waveguide section which is a waveguide section where the second waveguide is located. The ridge widths are formed to satisfy Wa=Wc&gt;Wb. <P>COPYRIGHT: (C)2012,JPO&amp;INPIT

Description

  The present invention relates to an optical device, and more particularly to an optical device in which an optical waveguide is formed on a dielectric substrate to perform optical communication control.

  There is an optical modulator as an optical device widely used in optical communication. An optical modulator is a device that converts an electrical signal into an optical signal by forming an optical waveguide on a substrate and performing external modulation that changes the amount of light absorbed on the optical waveguide by the voltage applied to the optical waveguide. is there.

FIG. 27 is a diagram illustrating an optical modulator, and FIG. 28 is a cross-sectional view of the optical modulator. In the optical modulator 100, a metal film such as Ti (titanium) is formed on a part of the crystal substrate 101 using LiNbO ₃ (or LiTaO ₂ ) and thermally diffused, or proton exchange is performed in benzoic acid after patterning. Thus, the Mach-Zehnder interferometer type optical waveguide 110 is formed.

  The optical waveguide 110 includes an incident waveguide 111, parallel waveguides 112a and 112b, and an output waveguide 113. The signal electrode 102 is provided on the parallel waveguide 112a, and the ground electrode 103 is provided on the parallel waveguide 112b. To form a coplanar electrode.

  When using a Z cut substrate, patterning is performed by placing an electrode directly above the waveguide, as shown in FIG. 28, in order to use a change in refractive index due to an electric field in the Z direction. In addition, a buffer layer 104 is provided between the crystal substrate 101, the signal electrode 102, and the ground electrode 103 in order to prevent light propagating through the parallel waveguides 112 a and 112 b from being absorbed by the signal electrode 102 and the ground electrode 103. .

  When the optical modulator 100 is driven at high speed, the terminal of the signal electrode 102 and the ground electrode 103 is connected by a resistor R0 to form a traveling wave electrode, and a microwave signal is applied from the input side. At this time, the refractive indexes of the parallel waveguides 112a and 112b change as + Δna and −Δnb, respectively, due to the electric field, and the phase difference between the parallel waveguides 112a and 112b changes, so that the Mach-Zehnder interference causes the output from the output waveguide 113. The intensity-modulated signal light is output.

  Further, by changing the cross-sectional shape of the electrode, the effective refractive index of the microwave is controlled, and the speed of the light and the microwave is matched to obtain a high-speed optical response characteristic. Note that the optical modulator 100 used for commercial use is generally driven at a bit rate of 10 Gb / s or 40 Gb / s per channel.

  On the other hand, as a device to which the optical modulator 100 is applied, there is an RZ modulator that generates an optical signal of RZ (Return to Zero: an encoding format that returns to 0 level even if the data value is 0 or 1). is there.

FIG. 29 is a diagram showing an RZ modulator. The RZ modulator 50 has a configuration in which two Mach-Zehnder interferometer type optical waveguides are connected by a folded waveguide 53.
The optical waveguide includes an incident waveguide 51, parallel waveguides 52a-1, 52b-1, a folded waveguide 53, parallel waveguides 52a-2, 52b-2, and an output waveguide 54. The two Mach-Zehnder modulators 5a, 5 b is connected by a folded waveguide 53.

  A signal electrode 55-1 is provided on the parallel waveguide 52a-1, and a ground electrode 56 is provided on the parallel waveguide 52b-1. A signal electrode 55-2 is provided on the parallel waveguide 52a-2, and a ground electrode 56 is provided on the parallel waveguide 52b-2.

  FIG. 30 is a diagram showing the folded waveguide 53. The folded waveguide 53 includes a straight waveguide 53a in a straight portion and an arc-shaped bent waveguide 53b having a small radius of curvature, and the bent waveguide 53b is required to have a small optical loss due to radiation. .

  Here, when continuous light is incident on the incident waveguide 51 of the Mach-Zehnder modulator 5a, the light is branched into Y and the light flows on the parallel waveguides 52a-1 and 52b-1. At this time, an NRZ (Non Return to Zero) electric signal is input to the signal electrode 55-1, and the Mach-Zehnder modulator 5a is driven, so that an NRZ optical signal is generated at the multiplexing unit of the Mach-Zehnder modulator 5a.

  Further, the NRZ optical signal enters the Mach-Zehnder modulator 5b via the folded waveguide 53 and is branched into Y, and then flows through the parallel waveguides 52a-2 and 52b-2. At this time, a clock electric signal is input to the signal electrode 55-2 and the Mach-Zehnder modulator 5b is driven to generate an RZ-modulated optical signal at the multiplexing portion of the Mach-Zehnder modulator 5b. RZ optical signal can be obtained from

  In this way, by connecting the two Mach-Zehnder modulators 5a and 5b via the folded waveguide 53, the elongated Mach-Zehnder modulators 5a and 5b can be arranged in parallel, thereby realizing a compact device structure. It becomes possible.

  As a conventional optical modulator having a folded waveguide, a technique has been proposed in which a groove is provided on the outer peripheral side of the folded waveguide to suppress radiation loss generated in the folded waveguide (see Patent Document 1).

JP 2004-287093 A (paragraph numbers [0012], [0013], FIG. 1)

  For example, when Ti is diffused at a high temperature of 1000 ° C. or higher on the surface of the substrate, the optical waveguide allows the light to be confined and propagated because the refractive index of the metal portion becomes higher than the surroundings. Is.

  Ti diffusion or proton exchange that can reduce propagation loss is generally used as an optical waveguide manufacturing method, but even optical waveguides manufactured by these methods cannot be said to have sufficient light confinement. There is a drawback that radiation loss occurs at the bent portion of the optical waveguide.

  For example, in the above-described RZ modulator 50, when light propagates through the folded waveguide 53, in particular, the curved waveguide 53b having a large curvature (having a small radius of curvature), light emission appears remarkably outside the curved waveguide 53b. End up.

  For this reason, in the above-described prior art (Japanese Patent Application Laid-Open No. 2004-287093), a groove is dug in the substrate on the outer periphery side of the folded waveguide 53 to suppress radiation loss. FIG. 31 is a diagram showing a configuration in which a groove is provided on the outer periphery of the folded waveguide 53, and FIG. 32 is a cross-sectional view of the folded waveguide 53 and the groove.

  The substrate on the outer peripheral side of the folded waveguide 53 is dug down by etching or the like to generate the groove 57. In addition, a buffer layer 58 is provided on the side surface of the groove 57 in order to prevent scattering loss due to the rough surface of the groove. With such a structure, light confinement is strengthened to prevent radiation loss.

  However, in the prior art as described above, when the device is manufactured, if the positional relationship between the pattern of the folded waveguide 53 and the pattern of the groove 57 deviates from the design value due to a manufacturing error, the radiation loss increases, which is desired. There was a problem that the quality of can not be maintained.

  FIG. 33 is a diagram showing an increase in radiation loss caused by a positional shift between the folded waveguide 53 and the groove 57. In FIG. The vertical axis represents the loss dB, and the horizontal axis represents the deviation μm of the groove 57. This shows an increasing tendency of radiation loss when the groove 57 is displaced in the vertical direction with respect to the folded waveguide 53.

  When the shift of the groove 57 is 0 μm, the loss is 1.4 dB, which is a desired design value. Further, the groove 57 is in the plus direction from 0 μm (the direction in which the waveguide on the light output side of the folded waveguide 53 and the groove 57 approach each other, and the waveguide on the light input side of the folded waveguide 53 and the groove 57 are separated), or minus When the direction shifts (the direction in which the waveguide on the light input side of the folded waveguide 53 approaches the groove 57 and the waveguide on the light output side of the folded waveguide 53 separates from the groove 57), the radiation loss becomes a parabolic shape. It can be seen that increases.

  FIG. 34 is a diagram showing the cause of radiation loss when the positional relationship between the folded waveguide 53 and the groove 57 is shifted. If the relative position between the folded waveguide 53 and the groove 57 deviates from the design value, the light mode is not matched at the coupling portion between the straight waveguide 53a and the bent waveguide 53b (for example, when propagating through the straight waveguide 53a). A mismatch between the optical axis of the light beam and the optical axis of the light beam when propagating through the curved waveguide 53b) causes scattering and increases radiation loss.

  As described above, the configuration in which the groove 57 is provided on the outer periphery of the folded waveguide 53 shown in the prior art can ideally suppress the radiation loss, but the design value is not necessarily used when manufacturing the device. Therefore, if it deviates from the design value due to a manufacturing error, there is a problem that the radiation loss increases and the quality deteriorates. Therefore, there is a need for an improvement measure that effectively increases the tolerance for manufacturing errors and effectively suppresses radiation loss even when there are manufacturing errors.

  The present invention has been made in view of the above points, and even when the relative position between the folded waveguide and the groove provided on the outer periphery of the folded waveguide has an error, it is manufactured even when it is deviated from the design value. An object of the present invention is to provide an optical device that suppresses an increase in radiation loss.

  In order to solve the above problems, an optical device is provided. The optical device includes an incident waveguide that divides incident light into two, a first waveguide that is a linear shape or a waveguide having a curved shape with a curvature radius of a certain value or more, and a radius of curvature that is greater than that of the first waveguide. Is composed of a second waveguide that is a waveguide having a small curved shape, and is composed of an inner waveguide that is a bifurcated inner waveguide, the first waveguide, and the second waveguide An outer waveguide that is an outer waveguide that is branched into two, and an output waveguide that combines and emits light propagating through the inner waveguide and light propagating through the outer waveguide, and A Mach-Zehnder interferometer-type folded waveguide that propagates signal light, a signal electrode formed on the outer waveguide, a ground electrode formed on both sides of the signal electrode, and the inner waveguide. A first groove which is a groove formed in the substrate along the inner side A second groove which is a groove formed in the substrate along the outer side of the inner waveguide and the inner side of the outer waveguide, and a third groove which is formed in the substrate along the outer side of the outer waveguide. And a fourth groove which is a groove formed in the substrate along the outside of the third groove, and is formed by the first groove and the second groove, and the inner waveguide is formed. The first ridge is formed by the second groove and the third groove, and the second ridge is formed by the second groove and the third groove, and the third groove and the fourth groove are formed by the outer waveguide. The third ridge is defined as a ridge formed by the above-described structure, and is a parallel waveguide area until light reaches the coupling portion with respect to the coupling portion between the first waveguide and the second waveguide. The optical input side waveguide area and the optical output side waveguide area, which is a parallel waveguide area in which light is propagated from the coupling portion, exist. The width of each of the first ridge, the second ridge, and the third ridge is Wa, and the first ridge, the second ridge, and the second ridge existing in the vicinity of the coupling portion The ridge width of each of the three ridges is Wb, and the first ridge, the second ridge, and the third ridge are present in the bent waveguide area, which is the waveguide area where the second waveguide is located. When the respective ridge widths are Wc, the ridge widths are formed so that Wa = Wc> Wb.

  Suppresses the increase in optical radiation loss even when the curved waveguide with a small radius of curvature and the groove provided on the outer circumference of the waveguide have an error that causes an error in the relative position. It becomes possible to do. In addition, since the characteristic impedance of the electrode can be maintained at a constant value, it is possible to reduce the loss of the microwave.

It is a figure which shows the structure of an optical device. It is a figure which shows the coupling | bond part periphery of the optical input side of a linear waveguide and a bending waveguide. It is a figure which shows the cross section of the coupling | bond part by the side of light input. It is a figure which shows the coupling | bond part periphery of the optical output side of a linear waveguide and a bending waveguide. It is a figure which shows the cross section of the coupling | bond part on the optical output side. It is a figure which shows the state which does not have a shift | offset | difference with a return | turnback waveguide and an outer groove | channel without an inner groove. It is a figure which shows the state which has an inner groove and there is no shift | offset | difference with a return | turnback waveguide and an outer groove. It is a figure which shows the state which does not have an inner groove and has shifted | deviated to the return | turnback waveguide and an outer groove. It is a figure which shows the state which has an inner groove | channel and the deviation has arisen in the return | turnback waveguide and the outer groove | channel. It is a figure which shows the state which does not have an inner groove and has shifted | deviated to the return | turnback waveguide and an outer groove. It is a figure which shows the state which has an inner groove | channel and the deviation has arisen in the return | turnback waveguide and the outer groove | channel. It is a figure which shows the increase in the radiation loss which arises with respect to the shift | offset | difference of the positional relationship of a return | turnback waveguide and an outer groove | channel. It is a figure which shows the state which enlarged the thickness of the buffer layer of a ridge side surface. It is a figure which shows the relationship between a curvature radius and radiation loss. It is a figure which shows a mode field diameter. It is a figure which shows a ridge width and a ridge depth. It is a figure which shows the coupling | bond part cross section which made the whole ridge upper part the optical waveguide. It is a figure which shows an extending | stretching inner groove. It is a figure which shows the influence which the ratio of the outer periphery of an extending | stretching inner groove and the inner periphery of a bending waveguide exerts on a loss. It is a figure which shows the structure which made the shape of the linear waveguide taper shape in the coupling part vicinity. It is a figure which shows an optical modulator. It is a figure which shows an optical modulator. It is a figure which shows the coupling | bond part vicinity of a straight waveguide and a bending waveguide. It is a figure which shows the cross section of the coupling | bond part vicinity. It is a figure which shows the cross section of the coupling | bond part vicinity. It is a figure which shows the cross section of the coupling | bond part vicinity. It is a figure which shows an optical modulator. It is sectional drawing of an optical modulator. It is a figure which shows a RZ modulator. It is a figure which shows a return | turnback waveguide. It is a figure which shows the structure by which the groove | channel was provided in the outer periphery of the return | turnback waveguide. It is sectional drawing of a return | turnback waveguide and a groove | channel. It is a figure which shows the increase in the radiation loss which arises with respect to the shift | offset | difference of the positional relationship of a return | turnback waveguide and a groove | channel. It is a figure which shows the cause which a radiation loss arises when the positional relationship of a return | turnback waveguide and a groove | channel shifts | deviates.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a configuration of an optical device. The optical device 1 is a device that performs optical communication control such as light modulation by providing an optical waveguide on a dielectric substrate 300 such as an LN (LiNbO ₃ ) crystal substrate.

  In the drawing, only the folded waveguide 10 is shown as the optical waveguide, and other waveguide portions such as the incident / outgoing waveguide are not shown (the optical waveguide of the optical device 1 is taken as an example of the optical modulator). The whole structure will be described later with reference to FIG.

  The folded waveguide 10 includes a first waveguide 11 and a second waveguide 12. The first waveguide 11 is a waveguide having a linear shape or a curved shape having a curvature radius of a certain value or more (for example, 4 mm or more). Hereinafter, the first waveguide 11 is referred to as a straight waveguide 11.

  The second waveguide 12 is a waveguide having an arcuate curved shape having a smaller radius of curvature than the straight waveguide 11, that is, a large curvature. Hereinafter, the second waveguide 12 is referred to as a curved waveguide 12.

  The outer groove 21 is a groove formed by digging down the substrate 300 along the outer periphery of the folded waveguide 10. The light input side inner groove 22 a is a groove provided inside the straight waveguide 11 and formed by digging down the substrate 300 in the vicinity of the light input side coupling portion between the straight waveguide 11 and the bent waveguide 12. The light output side inner groove 22 b is a groove provided inside the straight waveguide 11 and formed by digging down the substrate 300 in the vicinity of the light output side coupling portion between the straight waveguide 11 and the bent waveguide 12.

  FIG. 2 is a diagram showing the periphery of the coupling portion on the light input side between the straight waveguide 11 and the curved waveguide 12, and FIG. 3 is a diagram showing a cross section of the coupling portion on the light input side. An outer groove 21 is formed on the outer periphery of the folded waveguide 10, and an optical input side inner groove 22 a is formed in the vicinity of the coupling portion on the light input side between the linear waveguide 11 and the bent waveguide 12, whereby the linear waveguide 11. The vicinity of the coupling portion on the light input side with the bent waveguide 12 has a ridge structure as shown in FIG. That is, a groove (outer groove 21 and light input side inner groove 22a) is provided on both sides of the folded waveguide 10 at the coupling portion A1-A2 between the straight waveguide 11 and the bent waveguide 12, thereby forming a ridge.

  Further, in the vicinity of the coupling portion on the light input side, a distance B1 between the outer periphery of the straight waveguide 11 and the inner periphery of the outer groove 21, and a distance B2 between the inner periphery of the straight waveguide 11 and the outer periphery of the light input side inner groove 22a. Means that the outer groove 21 and the light input side inner groove 22a are formed in a tapered shape so as to be continuously narrowed in the light traveling direction.

In addition, when the fine roughness exists in the side surface of the outer groove | channel 21 and the light input side inner groove | channel 22a, the scattering loss of light generate | occur | produces by the roughness. In order to reduce this scattering loss, a buffer layer 40 is provided on the side surfaces of the outer groove 21 and the light input side inner groove 22a. The buffer layer 40 is made of a material having a refractive index smaller than that of the substrate 300 and transparent to light propagating in the waveguide, for example, SiO ₂ .

  FIG. 4 is a view showing the vicinity of the coupling portion on the light output side between the straight waveguide 11 and the curved waveguide 12, and FIG. 5 is a diagram showing a cross section of the coupling portion on the light output side. An outer groove 21 is formed on the outer periphery of the folded waveguide 10, and a light output side inner groove 22 b is formed in the vicinity of the light output side coupling portion between the straight waveguide 11 and the bent waveguide 12. The vicinity of the coupling portion on the light output side with the bent waveguide 12 has a ridge structure as shown in FIG. That is, grooves (outer grooves 21 and light output side inner grooves 22b) are provided on both sides of the folded waveguide 10 at the coupling portion A1-A2 between the straight waveguide 11 and the bent waveguide 12, thereby forming a ridge.

  Further, in the vicinity of the coupling portion on the light output side, a distance B3 between the outer periphery of the straight waveguide 11 and the inner periphery of the outer groove 21, and a distance B4 between the inner periphery of the straight waveguide 11 and the outer periphery of the light output side inner groove 22b. Means that the outer groove 21 and the light output side inner groove 22b are formed in a tapered shape so as to be continuously widened in the light traveling direction. As in FIG. 3, the buffer layer 40 is also provided on the side surface of the light output side inner groove 22b.

  Next, a difference between a configuration in which the inner groove is not provided (for example, a configuration as in the related art (Japanese Patent Laid-Open No. 2004-287093)) and a configuration in which the inner groove is provided as in the optical device 1 will be described in detail. . Since the coupling part on the light input side and the coupling part on the light output side have the same configuration, only the coupling part on the light input side is shown and described in the following description. Further, when the light input side inner groove 22a and the light output side inner groove 22b are described without distinction, they are simply referred to as the inner groove 22.

(1) A state in which there is no deviation between the folded waveguide 10 and the outer groove 21 (when there is no manufacturing error)
FIG. 6 is a diagram showing a state in which there is no inner groove 22 and there is no displacement between the folded waveguide 10 and the outer groove 21. The mode of light propagating through the straight waveguide 11 and the mode of light propagating through the bent waveguide 12 are designed so as to substantially match at the coupling portion, and the positional relationship between the folded waveguide 10 and the outer groove 21 is designed. If manufactured according to the value, the radiation loss can be reduced (note that the mode of light is for the 0th order light mode).

  FIG. 7 is a diagram showing a state in which there is an inner groove 22 and there is no displacement between the folded waveguide 10 and the outer groove 21. If a state in which the folded waveguide 10 and the outer groove 21 are not displaced can be manufactured, the radiation loss is reduced.

(2) The folded waveguide 10 is separated from the outer groove 21 (when there is a manufacturing error)
FIG. 8 is a diagram showing a state in which there is no inner groove 22 and the folded waveguide 10 and the outer groove 21 are displaced. The state where the space | interval of the return | turnback waveguide 10 and the outer groove | channel 21 has left | separated from the design value is shown.

  In the bent waveguide 12, the light confinement in the outer groove 21 has a stronger influence than the light confinement in the bent waveguide itself, so that the light mode is distributed along the side surface of the outer groove 21. The image when the light is bent and flows through the waveguide 12 is an image that propagates in the form of the left side of the light traveling direction so that the end portion of the light mode is in contact with the side surface of the outer groove 21.

  Therefore, when designing the outer groove 21, the light propagating through the straight waveguide 11 and the bent waveguide 12 when the mode end of the light propagating through the bent waveguide 12 contacts the side surface of the outer groove 21. Is formed by obtaining the position of the outer groove 21 so that the optical axis does not mismatch at the coupling portion (for example, in FIG. 6 described above, the outer groove 21 is positioned as designed. Is the case).

  On the other hand, as shown in FIG. 8, when the distance between the folded waveguide 10 and the outer groove 21 is larger than the design value, there is no change in light propagating through the straight waveguide 11 (in the straight waveguide 11, a straight line The center of the waveguide 11 becomes the center of light, and there is no change in this state). Since the mode end of the light in the bent waveguide 12 propagates in contact with the side surface of the outer groove 21, the bent waveguide 12 and the outer groove are transmitted. When the distance from 21 becomes larger than a predetermined value, the light flowing through the bent waveguide 12 propagates closer to the left side in the light traveling direction than in the case of FIG. For this reason, mismatch occurs between the optical axis of the light when propagating through the straight waveguide 11 and the optical axis of the light when propagating through the bent waveguide 12, and the radiation loss increases.

  FIG. 9 is a diagram showing a state in which there is an inner groove 22 and a deviation occurs between the folded waveguide 10 and the outer groove 21. The state where the space | interval of the return | turnback waveguide 10 and the outer groove | channel 21 has left | separated from the design value is shown.

  The light traveling on the straight waveguide 11 toward the coupling portion is guided by the inner groove 22 and distributed along the side surface of the outer groove 21. As an image, the light traveling toward the coupling portion on the straight waveguide 11 is pushed up to the left of the light traveling direction (upward in FIG. 9) due to the presence of the inner groove 22.

  The light propagating through the curved waveguide 12 propagates along the outer groove 21 side. However, by providing the inner groove 22 at the position shown in FIG. 9, the light propagating through the straight waveguide 11 also moves toward the outer groove 21 side. It will be sent. For this reason, it is possible to correct the axial misalignment of the light at the coupling portion, and to reduce the radiation loss.

(3) A state in which the outer groove 21 approaches the folded waveguide 10 (when there is a manufacturing error)
FIG. 10 is a diagram showing a state in which there is no inner groove 22 and the folded waveguide 10 and the outer groove 21 are displaced. The state where the space | interval of the return | turnback waveguide 10 and the outer groove | channel 21 has approached from the design value is shown.

  As shown in FIG. 10, if a part of the folded waveguide 10 is scraped by the outer groove 21, the width of the waveguide is reduced. Then, light confinement is weakened in the narrowed portion. This is particularly noticeable in the straight waveguide, and the mode field diameter of the light in the straight waveguide 11 is greatly expanded. On the other hand, in the bent waveguide 12, since the influence by the confinement of the outer groove 21 is large, the change in the mode field diameter of light is small.

  For this reason, the mode field diameter of the light when propagating through the straight waveguide 11 is different from the mode field diameter of the light when propagating through the bent waveguide 12, and the radiation loss increases due to the mismatch of the optical axes. (The mode field diameter will be described later with reference to FIG. 15).

  FIG. 11 is a diagram showing a state in which there is an inner groove 22 and a deviation occurs between the folded waveguide 10 and the outer groove 21. The state where the space | interval of the return | turnback waveguide 10 and the outer groove | channel 21 has approached from the design value is shown.

  When the inner groove 22 is present at the position shown in FIG. 11, the spread of the mode field diameter of the light flowing through the straight waveguide 11 can be suppressed. For this reason, the optical axis of the light when propagating through the straight waveguide 11 and the optical axis of the light when propagating through the bent waveguide 12 coincide with each other, and radiation loss can be reduced.

  FIG. 12 is a diagram illustrating an increase in radiation loss caused by a positional shift between the folded waveguide 10 and the outer groove 21. The vertical axis represents the loss dB, and the horizontal axis represents the deviation μm of the outer groove 21. It can be seen that when the inner groove 22 is provided, the radiation loss is reduced even if the folded waveguide 10 and the outer groove 21 are displaced, and the increase in the radiation loss is suppressed.

  As described above, according to the optical device 1, the outer groove 21 is provided on the outer periphery of the folded waveguide 10, and the inner peripheral side of the folded waveguide 10 in the vicinity of the coupling portion between the straight waveguide 11 and the bent waveguide 12. Is provided with an inner groove 22 to form a ridge structure on the light input side and the light output side of the folded waveguide 10.

  With such a configuration, even when the relative positional relationship between the outer groove 21 and the folded waveguide 10 is shifted, it is possible to prevent an increase in radiation loss, increase tolerance against process errors, and achieve high-quality optical communication. Can be realized.

Next, other features (manufacturing features or structural features) of the optical device 1 will be described in the following (a1) to (g1).
(A1) When forming the outer grooves 21 and the inner grooves 22 on the substrate 300 (Z-cut LN substrate), after forming an optical waveguide (including the folded waveguide 10) on the surface of the substrate 300, dry etching or the like Thus, the outer groove 21 and the inner groove 22 are formed in the substrate 300, and the buffer layer 40 is further provided.

Further, in order to reduce the radiation loss, a manufacturing method in which the propagation loss of the optical waveguide is small and the confinement is strong is desired. Therefore, an optical waveguide is formed by Ti diffusion or proton exchange.
(B1) In order to strengthen the light confinement in the horizontal direction by the ridge, the refractive indexes on both sides of the ridge must be small. Further, in order to reduce the mode field diameter, it is necessary to reduce the ridge width to some extent. However, if the ridge width is too small, there is a possibility that the ridge side surface is affected by stress.

Therefore, in the vicinity of the coupling portion between the straight waveguide 11 and the bent waveguide 12, both sides of the ridge are made to be gas. As the gas, air or N ₂ gas is used. Specifically, when an optical module incorporating the optical device 1 is manufactured, it may be manufactured by filling the module with air or N ₂ gas.

Since gases such as air and N ₂ gas have a low refractive index, they can contribute to the enhancement of light confinement in the horizontal direction by the ridge, and even if the ridge width is narrow, the influence of stress is affected by the gas filling pressure. It becomes possible to reduce.

  (C1) When the buffer layer 40 is thicker or has a refractive index closer to LN, the scattering loss can be suppressed, but at the same time, the light confinement becomes weak, so that the radiation loss in the bent waveguide 12 increases. In addition, since the scattering loss increases at a portion where the bent waveguide 12 and the outer groove 21 approach each other, the scattering loss increases in the vicinity of the coupling portion between the straight waveguide 11 and the bent waveguide 12.

  Therefore, the buffer layer 40 on the side surface of the ridge is thickened only in the vicinity of the coupling portion between the straight waveguide 11 and the bent waveguide 12. Further, the refractive index of the buffer layer 40 on the side surface of the ridge is increased only in the vicinity of the coupling portion between the straight waveguide 11 and the bent waveguide 12. By doing so, the scattering loss in the vicinity of the coupling portion is suppressed. FIG. 13 shows a state where the thickness of the buffer layer 40 on the side surface of the ridge is increased.

  (D1) FIG. 14 is a diagram showing the relationship between the radius of curvature and the radiation loss. The vertical axis represents the radiation loss dB, and the horizontal axis represents the radius of curvature mm. The radiation loss when the radius of curvature of the curved waveguide 12 without the outer groove 21 changes and the radiation loss when the radius of curvature of the curved waveguide 12 with the outer groove 21 changes are shown.

  From FIG. 14, when the radius of curvature is 4 mm or less, the radiation loss when the outer groove 21 is present is smaller than the radiation loss when the outer groove 21 is absent, and the effect of reducing the radiation loss by the outer groove 21 is the curvature. It can be seen that the radius appears below 4 mm. When the curvature radius is 4 mm or more, scattering loss may occur due to roughness of the side surface of the groove, and the loss may increase.

  That is, assuming that the radius of curvature of the bent waveguide 12 is R, the outer groove 21 is required when R ≦ 4 mm, and the outer groove 21 is not required when R> 4 mm. Therefore, the folded waveguide 10 of the optical device 1 is manufactured so that the radius of curvature of the bent waveguide 12 is 4 mm or less and the radius of curvature of the straight waveguide 11 exceeds 4 mm.

  (E1) At the coupling portion between the straight waveguide 11 and the bent waveguide 12, the mode field diameter (MFD) of light is about 10 μm or less. FIG. 15 shows the mode field diameter. The vertical axis represents the light intensity, and the horizontal axis represents the diameter of the waveguide (waveguide diameter).

  The signal light propagates as a spread radiation light. MFD is an index representing the extent of light distribution with respect to the waveguide diameter. The light intensity distribution is a curve having a Gaussian distribution shape, the light intensity is highest at the center of the waveguide, and the light intensity decreases toward the outside of the waveguide.

For such a curve, when the maximum value of the light intensity is 1, 1 / e ² of the maximum value 1 at the center of the waveguide (e is the base of the natural logarithm (= 2.718...)) (About 13.5%) is generally defined as the mode field diameter.

  On the other hand, the peak position of the optical power in the vertical direction (substrate depth direction) of the coupling portion between the straight waveguide 11 and the curved waveguide 12 is about 3 μm lower than the substrate surface (ridge upper surface). Therefore, at the coupling portion between the straight waveguide 11 and the bent waveguide 12, the ridge width is set to 10 μm or less, and the ridge depth is set to 3 μm or less. FIG. 16 shows the ridge width and ridge depth.

  (F1) Since the curved waveguide 12 has a small radius of curvature, light confinement is weak. At this time, since the mode field diameter of light is widened, a mode mismatch with the straight waveguide 11 occurs. In order to prevent this, the bent waveguide 12 is formed so that the width of the bent waveguide 12 is larger than the width of the straight waveguide 11 in the vicinity of the coupling portion between the straight waveguide 11 and the bent waveguide 12.

  (G1) In the vicinity of the coupling portion between the straight waveguide 11 and the bent waveguide 12, since both sides are grooves, light confinement becomes strong, so that a multimode waveguide is formed. When the 0th-order mode light is coupled to the higher-order mode, a loss occurs when the light is re-coupled to the single-mode waveguide. Therefore, the distance from the straight waveguide 11 to the outer groove 21 is set to be equal to the distance from the straight waveguide 11 to the inner groove 22 (the outer groove 21 and the inner groove 22 with the straight waveguide 11 in between. To be symmetrical).

  That is, as described above with reference to FIG. 2, in the vicinity of the coupling portion on the light input side, the distance B1 between the straight waveguide 11 and the outer groove 21 and the distance B2 between the straight waveguide 11 and the light input side inner groove 22a are The outer groove 21 and the light input side inner groove 22a are formed in a tapered shape so that they continuously narrow in the light traveling direction. At this time, the intervals B1 and B2 are symmetrically equal. To form.

  Similarly, as described above with reference to FIG. 4, in the vicinity of the coupling portion on the light output side, the distance B3 between the linear waveguide 11 and the outer groove 21 and the distance B4 between the straight waveguide 11 and the light output side inner groove 22b The outer groove 21 and the light output side inner groove 22b are formed in a tapered shape so that they continuously widen in the light traveling direction. At this time, the intervals B3 and B4 are symmetrically equal. To form. With such a configuration, the symmetry of light propagating through the straight waveguide 11 becomes good, and the generation of higher-order mode excitation can be prevented.

Next, as modifications of the optical device 1, modifications (a2) to (d2) will be described below.
(A2) FIG. 17 is a view showing a cross section of the coupling portion in which the entire ridge upper part is an optical waveguide. In the cross section near the coupling portion between the straight waveguide 11 and the bent waveguide 12, the entire upper portion of the ridge is an optical waveguide (folded waveguide 10).

  By adopting such a shape, it is possible to further increase the tolerance against the positional deviation between the outer groove 21 and the folded waveguide 10. If the width of the optical waveguide is made larger than the width of the ridge at the coupling portion between the straight waveguide 11 and the bent waveguide 12, the entire inside of the ridge becomes a waveguide region even if the outer groove 21 is slightly shifted. This is because the refractive index distribution in the inside does not change.

(B2) The bent waveguide 12 is formed in contact with the outer groove 21. Thereby, tolerance can be made higher with respect to the deviation between the outer groove 21 and the bent waveguide 12.
(C2) FIG. 18 is a view showing an extending inner groove. In the inner groove 22 described above, a scattering loss may occur depending on the angle of the inner groove 22. In order to prevent this, the inner groove 22 extends toward the inner peripheral side of the bent waveguide 12 to form an extended inner groove 22-1 which is an extended portion, and the inner groove 22- extends from the inner periphery of the bent waveguide 12. It forms so that the distance (width | variety) to the outer periphery of 1 may change continuously.

  That is, if it is on the light input side, it will be bent on the light output side so that the width from the inner periphery of the bent waveguide 12 to the outer periphery of the extending inner groove is continuously increased with respect to the light traveling direction. The width from the inner periphery of the waveguide 12 to the outer periphery of the extending inner groove is formed so as to be continuously narrow with respect to the light traveling direction.

  Here, if the outer periphery of the extending inner groove 22-1 is too small compared to the inner periphery of the waveguide 12, the effect of the extending inner groove 22-1 cannot be obtained. Scattering loss due to side roughness increases radiation loss.

  FIG. 19 is a diagram showing the influence of the ratio of the outer periphery of the extending inner groove 22-1 and the inner periphery of the bent waveguide 12 on the loss. Loss of the bent waveguide 12 when the relative position of the outer groove 21 with respect to the folded waveguide 10 is as designed (positional deviation 0) and when it is deviated 1.5 μm from the design value (positional deviation 1.5 μm). Is shown. The vertical axis represents the loss dB of the bent waveguide 12, and the horizontal axis represents the ratio of the outer periphery of the extended inner groove 22-1 to the inner periphery of the bent waveguide 12 (the outer periphery of the extended inner groove 22-1 / the inner periphery of the bent waveguide 12). ).

  When the positional deviation is 0, the loss increases when the ratio (the outer circumference of the extending inner groove 22-1 / the inner circumference of the bent waveguide 12) is smaller than 0.3, and 0.6 when the positional deviation is 1.5 μm. This shows that the loss increases. From this relationship, it is desirable that the radius of the outer periphery of the extended inner groove 22-1 extending toward the inner periphery of the bent waveguide 12 is designed to be 30 to 60% of the radius of the inner periphery of the bent waveguide 12. .

  (D2) FIG. 20 is a diagram showing a configuration in which the shape of the linear waveguide 11 is tapered in the vicinity of the coupling portion. In the vicinity of the coupling portion between the straight waveguide 11 and the bent waveguide 12, the light mode is designed to be matched. Therefore, it may be necessary to change the width of the linear waveguide 11 near the coupling portion and away from the coupling portion.

  In such a case, as shown in FIG. 20, the linear waveguide 11a changes the width of the waveguide in a taper shape toward the vicinity of the coupling portion so that the waveguide width gradually increases as it approaches the coupling portion. Shape. With such a configuration, the waveguide width can be changed without increasing the loss.

  Next, an optical modulator to which the optical device 1 is applied will be described. FIG. 21 is a diagram showing an optical modulator. The configuration of the optical device 1 is applied to the RZ modulator described above with reference to FIG. The optical modulator 3 has a configuration in which two Mach-Zehnder interferometer type optical waveguides are connected on a substrate 30 by a folded waveguide 10.

  The optical waveguide includes an optical input side Mach-Zehnder interferometer type waveguide 3a, a folded waveguide 10, and an optical output side Mach-Zehnder interferometer type waveguide 3b. The optical input side Mach-Zehnder interferometer-type waveguide 3 a is composed of an incident waveguide 31 and optical input side parallel waveguides 32 a-1 and 32 b-1, and the folded waveguide 10 is composed of a straight waveguide 11 and a curved waveguide 12. The optical output side Mach-Zehnder interferometer-type waveguide 3b includes optical output side parallel waveguides 32a-2 and 32b-2 and an output waveguide.

  A signal electrode 35-1 is provided on the light input side parallel waveguide 32a-1, and a ground electrode 36 is provided on the light input side parallel waveguide 32b-1. A signal electrode 35-2 is provided on the light output side parallel waveguide 32a-2, and a ground electrode 36 is provided on the light output side parallel waveguide 32b-2.

  The outer groove 21 is formed by digging down the substrate 30 along the outer periphery of the folded waveguide 10. The light input side inner groove 22 a is provided inside the straight waveguide 11 and is formed by digging down the substrate 30 in the vicinity of the light input side coupling portion between the straight waveguide 11 and the curved waveguide 12. The light output side inner groove 22 b is provided inside the straight waveguide 11 and is formed by digging down the substrate 30 in the vicinity of the light output side coupling portion between the straight waveguide 11 and the bent waveguide 12.

  As described above, in the optical modulator 3, the outer waveguide 21, the light input side inner groove 22 a, and the light output side inner groove 22 b are provided to the folded waveguide 10, so that the bent waveguide 12 and the outer groove 21 are provided. Even when an error occurs in the relative position and manufacturing is performed out of a desired design value, an increase in radiation loss can be suppressed and high-quality light modulation can be performed.

  FIG. 22 shows an optical modulator. The optical modulator 4 is a device having a configuration in which an optical waveguide of one Mach-Zehnder modulator is folded. In the optical modulator 4, a Mach-Zehnder interferometer-type folded waveguide 4-1, a signal electrode 45, a ground electrode 46, and grooves m1 to m4 (first to fourth grooves) are formed on a substrate 4a.

  The Mach-Zehnder interferometer-type optical waveguide 4-1 includes an incident waveguide 41, an inner waveguide 42, an outer waveguide 43, and an output waveguide 44. The incident waveguide 41 is a waveguide that divides incident light into two.

  The inner waveguide 42 includes a straight waveguide 42a that is a waveguide having a linear shape or a curved shape with a curvature radius equal to or greater than a certain value, and a bent guide that is a waveguide having a curved shape having a smaller curvature radius than the straight waveguide 42a. This is a waveguide that is configured from the waveguide 42b and is located inside one of the two branches.

  The outer waveguide 43 is composed of a straight waveguide 43a and a bent waveguide 43b, and is a waveguide located outside the other branched into two. The exit waveguide 44 is a waveguide that combines and emits light propagating through the inner waveguide 42 and light propagating through the outer waveguide 43.

  The signal electrode 45 is provided on the outer waveguide 43, and the ground electrode 46 is provided on both sides of the signal electrode 45. The groove m1 is a groove formed by digging down the substrate 4a along the inner periphery of the inner waveguide 42. The groove m2 is a groove formed by digging down the substrate 4a along the outer periphery of the inner waveguide 42 and the inner periphery of the outer waveguide 43. The groove m3 is a groove formed by digging down the substrate 4a along the outer periphery of the outer waveguide 43. The groove m4 is a groove formed by digging down the substrate 4a along the outer periphery of the groove m3.

  FIG. 23 is a view showing the vicinity of a coupling portion between a straight waveguide and a bent waveguide. The vicinity of the coupling portion between the straight waveguide 42a and the curved waveguide 42b of the inner waveguide 42 and the vicinity of the coupling portion between the linear waveguide 43a and the curved waveguide 43b of the outer waveguide 43 are shown.

  24-26 is a figure which shows the cross section of a coupling | bond part vicinity. 24 shows a cross section taken along A1-A2 in FIG. 23, FIG. 25 shows a cross section taken along B1-B2 in FIG. 23, and FIG. 26 shows a cross section taken along C1-C2 in FIG.

  The optical modulator 4 forms a ridge structure by providing grooves on both sides of the straight waveguides 42a and 43a and on the entire sides of the curved waveguides 42b and 43b. A buffer layer 47 is provided on the surface of the substrate 4a.

  When forming the grooves, as shown in the sectional view, two grooves are provided on both sides of the signal electrode 45, that is, the grooves m1 and m2 are provided inside the signal electrode 45, and the grooves m3 and m4 are provided outside. The ridge shape is provided so as to be symmetric about the signal electrode 45.

  On the other hand, a ridge formed by the groove m1 and the groove m2 and having the inner waveguide 42 is ridge R1, and a ridge formed by the groove m2 and the groove m3 and having the outer waveguide 43 is ridge R2, and the grooves m3 and m4. The ridge R3 is defined as the ridge R3, and the parallel waveguide area until the light reaches the coupling portion with respect to the coupling portion between the straight waveguides 42a and 43a and the curved waveguides 42b and 43b is the optical input side waveguide. A parallel waveguide area where light is propagated from the coupling portion is a light output side waveguide area.

  At this time, the ridge widths of the ridge R1, ridge R2 and ridge R3 existing in the optical input side waveguide section and the optical output side waveguide section are set to Wa, and the ridge R1, ridge R2 and ridge existing in the vicinity of the coupling portion. When each ridge width of R3 is Wb and each ridge width of ridge R1, ridge R2 and ridge R3 existing in the bent waveguide area where the bent waveguides 42b and 43b are located is Wc, Wa = Wc> Wb A ridge width is formed in each waveguide section so as to be (FIG. 23).

  With this configuration, the symmetry of the electrodes (signal electrode 45 and ground electrode 46) is improved, and the characteristic impedance of the electrodes can be maintained at a constant value, so that the loss of microwaves can be reduced. become.

  On the other hand, in the case of forming an optical waveguide, the straight waveguides 42a and 43a are formed so that the waveguide width is smaller than the ridge width (FIG. 24), and the straight waveguides 42a and 43a are coupled to the bent waveguides 42b and 43b. In the vicinity of the portion, the waveguide width is formed larger than the ridge width (FIG. 25), and the curved waveguides 42b and 43b are formed so as to be offset toward the outside of the substrate 4a on the top surface of the ridge. By forming in this way, radiation loss is further reduced.

  Further, in the vicinity of B1-B2 of the coupling portion, the ridge width becomes narrow and the signal electrode 45 on the ridge may be easily detached from the ridge due to a process error or the like. To prevent this, the linear waveguides 42a and 43a A structure in which the width of the signal electrode 45 may be narrowed only in the vicinity of the coupling portion (B1-B2) with the bent waveguides 42b and 43b.

  Furthermore, the characteristic impedance of the electrode depends on the width and height of the ridge as well as the width of the signal electrode 45, the thickness of the electrode, the distance between the signal electrode 45 and the ground electrode 46, and the like. In a portion where the width of the ridge is narrow or the width of the signal electrode 45 is narrow, the impedance is higher than in other portions. For this reason, the microwave is reflected by the discontinuity of impedance, and the modulation characteristics may be deteriorated.

  Therefore, in order to prevent this, the electrode is thickened only in the vicinity of the coupling portion between the straight waveguides 42a and 43a and the curved waveguides 42b and 43b, and the gap between the signal electrode 45 and the ground electrode 46 is narrowed. It is effective to take at least one countermeasure such as forming ~ m4 shallow, or thinning the buffer layer 47 between the substrate 4a and the electrode.

(Appendix 1) In an optical device having an optical waveguide,
A first waveguide which is a waveguide having a linear shape or a curved shape having a curvature radius equal to or greater than a certain value, and a second waveguide which is a waveguide having a curved shape having a smaller radius of curvature than the first waveguide. And a folded waveguide formed on the substrate,
An outer groove that is a groove formed in the substrate along the outside of the folded waveguide;
A light input side inner groove which is a groove formed in the substrate in the vicinity of a light input side coupling portion between the first waveguide and the second waveguide, which is provided inside the first waveguide;
A light output side inner groove which is a groove formed in the substrate in the vicinity of a light output side coupling portion between the first waveguide and the second waveguide, which is provided inside the first waveguide;
With
By forming the outer groove and the light input side inner groove, the vicinity of the light input side coupling portion between the first waveguide and the second waveguide has a ridge structure,
By forming the outer groove and the light output side inner groove, the vicinity of the light output side coupling portion between the first waveguide and the second waveguide has a ridge structure.
An optical device characterized by that.

(Appendix 2) In the vicinity of the coupling portion on the light input side, the distance between the outer periphery of the first waveguide and the inner periphery of the outer groove, the inner periphery of the first waveguide, and the inner groove of the light input side The distance from the outer periphery of the outer groove and the light input side inner groove are formed so as to continuously narrow toward the light traveling direction,
In the vicinity of the coupling portion on the light output side, the distance between the outer periphery of the first waveguide and the inner periphery of the outer groove, and the distance between the inner periphery of the first waveguide and the outer periphery of the light output side inner groove. And forming the outer groove and the light output side inner groove so as to be continuously widened in the light traveling direction,
The optical device according to appendix 1, wherein:

(Supplementary Note 3) In the vicinity of the coupling portion on the light input side, the distance between the outer periphery of the first waveguide and the inner periphery of the outer groove, the inner periphery of the first waveguide, and the inner groove of the light input side The distance from the outer circumference of the
In the vicinity of the coupling portion on the light output side, the distance between the outer periphery of the first waveguide and the inner periphery of the outer groove, and the distance between the inner periphery of the first waveguide and the outer periphery of the light output side inner groove. Are formed to be symmetrically equal,
The optical device according to Supplementary Note 2, wherein

  (Appendix 4) When forming a buffer layer on the surface of the substrate, only the ridge in the vicinity of the coupling portion on the light input side and the coupling portion on the light output side of the first waveguide and the second waveguide, The optical device according to appendix 1, wherein the buffer layer on the side surface of the ridge is thickened and the refractive index is increased.

  (Supplementary Note 5) The supplementary note 1 is characterized in that both sides of the ridge near the coupling portion on the light input side and the coupling portion on the light output side of the first waveguide and the second waveguide are filled with gas. The optical device described.

(Supplementary note 6) The optical device according to supplementary note 1, wherein the curvature radius of the second waveguide is set to 4 mm or less, and the curvature radius of the first waveguide exceeds 4 mm.
(Supplementary Note 7) The width of the ridge in the vicinity of the coupling portion on the light input side and the coupling portion on the light output side of the first waveguide and the second waveguide is 10 μm or less, and the depth of the ridge is 3 μm or less. The optical device according to appendix 1, wherein the optical device is formed to be

  (Supplementary Note 8) In the vicinity of the coupling portion on the optical input side and the coupling portion on the optical output side of the first waveguide and the second waveguide, the width of the second waveguide is the first waveguide. The optical device according to appendix 1, wherein the optical device is formed so as to be larger than a width of the waveguide.

(Additional remark 9) The optical device of Additional remark 1 characterized by making the whole ridge upper part of the coupling | bond part vicinity of a said 1st waveguide and a said 2nd waveguide into a waveguide.
(Supplementary note 10) The optical device according to supplementary note 1, wherein the outer groove is in contact with the second waveguide.

(Additional remark 11) The said optical input side inner groove | channel is extended to the inner peripheral side of the said 2nd waveguide,
For the light input side extended inner groove which is the extended inner groove portion, the width from the inner periphery of the second waveguide to the outer periphery of the light input side extended inner groove is continuous with respect to the light traveling direction. To be wide,
Extending the optical output side inner groove to the inner peripheral side of the second waveguide;
For the light output side extended inner groove which is the extended inner groove portion, the width from the inner periphery of the second waveguide to the outer periphery of the light output side extended inner groove is continuous with respect to the light traveling direction. Forming to be narrow,
The optical device according to appendix 1, wherein:

  (Additional remark 12) The curvature radius of the outer periphery of the said optical input side extending | stretching inner groove and the said optical output side extending | stretching inner groove is 30%-60% of the curvature radius of the inner periphery of a said 2nd waveguide. The optical device according to appendix 11.

(Supplementary note 13) The first waveguide on the light input side is continuously guided toward the light input side coupling portion between the first waveguide and the second waveguide with respect to the light traveling direction. Changing the width of the first waveguide into a tapered shape so that the waveguide width is widened;
The first waveguide on the light output side has a narrow waveguide width continuously with increasing distance from the coupling portion on the light output side between the first waveguide and the second waveguide with respect to the light traveling direction. The width of the first waveguide is changed in a tapered shape so that
The optical device according to appendix 1, wherein:

(Supplementary Note 14) In an optical device that performs optical modulation,
A Mach-Zehnder interferometer-type optical waveguide, which is located on the light input side, and a Mach-Zehnder interferometer-type optical waveguide;
A Mach-Zehnder interferometer-type optical waveguide, which is located on the light output side, and a Mach-Zehnder interferometer-type optical waveguide;
A first waveguide which is a waveguide having a linear shape or a curved shape having a curvature radius equal to or greater than a certain value, and a second waveguide which is a waveguide having a curved shape having a smaller radius of curvature than the first waveguide. And a folded waveguide that folds back the light propagating through the optical input side Mach-Zehnder interferometer type optical waveguide to the optical output side Mach-Zehnder interferometer type optical waveguide, and
An outer groove that is a groove formed in the substrate along the outside of the folded waveguide;
A light input side inner groove which is a groove formed in the substrate in the vicinity of a light input side coupling portion between the first waveguide and the second waveguide, which is provided inside the first waveguide;
A light output side inner groove which is a groove formed in the substrate in the vicinity of a light output side coupling portion between the first waveguide and the second waveguide, which is provided inside the first waveguide;
An optical device comprising:

(Supplementary Note 15) In an optical device that performs optical modulation,
An incident waveguide that divides incident light into two, a first waveguide that is a linear shape or a curved shape with a curvature radius equal to or greater than a certain value, and a curved shape that has a smaller curvature radius than the first waveguide. A second waveguide which is a waveguide having an inner waveguide which is an inner waveguide which is branched in two, and a second branch which is composed of the first waveguide and the second waveguide. An outer waveguide, which is an outer waveguide, and an output waveguide that combines and emits the light propagating through the inner waveguide and the light propagating through the outer waveguide, and is formed on a substrate to generate a signal A Mach-Zehnder interferometer-type folded waveguide that propagates light;
A signal electrode formed on the outer waveguide;
A ground electrode formed on both sides of the signal electrode;
A first groove which is a groove formed in the substrate along the inner side of the inner waveguide;
A second groove which is a groove formed in the substrate along the outer side of the inner waveguide and the inner side of the outer waveguide;
A third groove which is a groove formed in the substrate along the outside of the outer waveguide;
A fourth groove which is a groove formed in the substrate along the outside of the third groove;
An optical device comprising:

(Supplementary Note 16) A ridge formed by the first groove and the second groove and having the inner waveguide is formed by the first ridge, the second groove and the third groove, and the outer side. The ridge in which the waveguide exists is the second ridge, the ridge formed by the third groove and the fourth groove is the third ridge,
A light input side waveguide section that is a parallel waveguide section until light reaches the coupling section with respect to the coupling section between the first waveguide and the second waveguide, and light from the coupling section Wa is the ridge width of each of the first ridge, the second ridge, and the third ridge existing in the optical output-side waveguide section, which is a parallel waveguide section through which light is propagated,
Wb is the ridge width of each of the first ridge, the second ridge, and the third ridge present in the vicinity of the coupling portion;
When the ridge width of each of the first ridge, the second ridge, and the third ridge existing in the bent waveguide area, which is the waveguide area where the second waveguide is located, is Wc,
16. The optical device according to appendix 15, wherein the ridge width is formed such that Wa = Wc> Wb.

(Supplementary note 17) The optical device according to claim 15, wherein the width of the signal electrode is narrowed only in the vicinity of the coupling portion between the first waveguide and the second waveguide.
(Supplementary Note 18) The first waveguide formed in parallel on the substrate is formed so that a waveguide width of the first waveguide is smaller than a ridge width.
The waveguide width of the first waveguide and the waveguide width of the second waveguide located near the coupling portion between the first waveguide and the second waveguide are formed larger than the ridge width. And
The second waveguide of the waveguide portion having a small curvature radius is formed by being offset toward the outer side of the substrate on the upper surface of the ridge.
The optical device according to supplementary note 15, wherein

(Supplementary note 19) The optical device according to supplementary note 15, wherein the height of the signal electrode and the ground electrode is increased only in the vicinity of the coupling portion between the first waveguide and the second waveguide.
(Supplementary note 20) The optical device according to supplementary note 15, wherein an interval between the signal electrode and the ground electrode is narrowed only in the vicinity of a coupling portion between the first waveguide and the second waveguide.

  (Supplementary note 21) The first groove, the second groove, the third groove, and the fourth groove are formed shallowly only in the vicinity of the coupling portion between the first waveguide and the second waveguide. The optical device according to supplementary note 15, wherein

  (Supplementary note 22) When forming a buffer layer on the surface of the substrate, the buffer layer between the substrate and the signal electrode is formed thin, and the buffer layer between the substrate and the ground electrode is thinned. The optical device according to appendix 15, wherein the optical device is formed.

DESCRIPTION OF SYMBOLS 1 Optical device 10 Folding waveguide 11 1st waveguide 12 2nd waveguide 21 Outer groove 22a Light input side inner groove 22b Light output side inner groove 30 Substrate

Claims (5)

In an optical device that performs light modulation,
An incident waveguide that divides incident light into two, a first waveguide that is a linear shape or a curved shape with a curvature radius equal to or greater than a certain value, and a curved shape that has a smaller curvature radius than the first waveguide. A second waveguide which is a waveguide having an inner waveguide which is an inner waveguide which is branched in two, and a second branch which is composed of the first waveguide and the second waveguide. An outer waveguide, which is an outer waveguide, and an output waveguide that combines and emits the light propagating through the inner waveguide and the light propagating through the outer waveguide, and is formed on a substrate to generate a signal A Mach-Zehnder interferometer-type folded waveguide that propagates light;
A signal electrode formed on the outer waveguide;
A ground electrode formed on both sides of the signal electrode;
A first groove which is a groove formed in the substrate along the inner side of the inner waveguide;
A second groove which is a groove formed in the substrate along the outer side of the inner waveguide and the inner side of the outer waveguide;
A third groove which is a groove formed in the substrate along the outside of the outer waveguide;
A fourth groove which is a groove formed in the substrate along the outside of the third groove;
With
A ridge formed by the first groove and the second groove and having the inner waveguide is formed by the first ridge, and the second groove and the third groove are formed by the outer waveguide. A ridge to be formed is a second ridge, a ridge formed by the third groove and the fourth groove is a third ridge,
A light input side waveguide section that is a parallel waveguide section until light reaches the coupling section with respect to the coupling section between the first waveguide and the second waveguide, and light from the coupling section Wa is the ridge width of each of the first ridge, the second ridge, and the third ridge existing in the optical output-side waveguide section, which is a parallel waveguide section through which light is propagated,
Wb is the ridge width of each of the first ridge, the second ridge, and the third ridge present in the vicinity of the coupling portion;
When the ridge width of each of the first ridge, the second ridge, and the third ridge existing in the bent waveguide area, which is the waveguide area where the second waveguide is located, is Wc,
An optical device characterized in that a ridge width is formed such that Wa = Wc> Wb.   2. The optical device according to claim 1, wherein the width of the signal electrode is narrowed only in the vicinity of the coupling portion between the first waveguide and the second waveguide. The first waveguide formed in parallel on the substrate is formed so that a waveguide width of the first waveguide is smaller than a ridge width;
The waveguide width of the first waveguide and the waveguide width of the second waveguide located near the coupling portion between the first waveguide and the second waveguide are formed larger than the ridge width. And
The second waveguide of the waveguide portion having a small radius of curvature is formed by being offset toward the outer side of the substrate on the upper surface of the ridge.
The optical device according to claim 1.   2. The optical device according to claim 1, wherein the height of the signal electrode and the ground electrode is increased only in the vicinity of the coupling portion between the first waveguide and the second waveguide.   2. The optical device according to claim 1, wherein an interval between the signal electrode and the ground electrode is narrowed only in the vicinity of a coupling portion between the first waveguide and the second waveguide.

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