JP5163234B2 - Optical waveguide - Google Patents

Description

  The present invention relates to a photoelectric conversion element, an optical waveguide used in an optical integrated circuit in which they are integrated, and a manufacturing method thereof.

  In recent years, in the fields of optical communication represented by regional networks, intercity networks, etc., and optical interconnections applied to servers, routers, etc., multiple optical signal demultiplexing / multiplexing, transmission / reception operations An optical integrated circuit that performs optical signal processing such as is used. As the range of use expands, it is expected to provide a compact, low power, and low cost optical integrated circuit.

  In an optical integrated circuit, a planar optical waveguide is normally used as an optical signal transmission path. In a planar optical waveguide, a core / cladding waveguide generally has a substantially rectangular cross-sectional shape, and a structure in which the top / bottom surface and both side surfaces are covered with a cladding is adopted. Yes. When an inorganic material having optical transparency for signal light is used as the core material, in the conventional planar optical waveguide, the core layer is flatly formed on the lower clad layer and etched to form a substantially rectangular cross-sectional shape. Forming a core having In this core formation step, for example, an etching mask corresponding to the planar pattern of the core layer is produced by using exposure and development with a photosensitive material. Next, the core layer is etched by etching such as RIE (Reactive Ion Etching) using the produced etching mask, so that the cross-sectional shape of the core is rectangular. On the other hand, when an organic polymer material having optical transparency to signal light is used as a core material, for example, when the organic polymer material itself is a photosensitive material, exposure and development are used to obtain a desired plane. A core layer having a pattern is formed. Thereafter, an upper clad layer is formed so as to cover both side surfaces and the upper surface of the formed core. Therefore, in the core / cladding structure manufactured by the above-described method, both side surfaces of the core are in contact with the same cladding material, and the horizontal direction is symmetrical.

  In addition to the structure in which the core formed by the etching process is covered with a clad layer, the following is reported as a conventional optical waveguide structure. As a planar optical waveguide structure, a structure in which a core layer is formed by epitaxial growth so that a groove is formed in a substrate or a lower clad portion and the groove is embedded, and a manufacturing method thereof are disclosed (for example, Patent Documents) 1 (FIGS. 1-6)). Further, as shown in FIG. 4 of Patent Document 2, after the core is formed by etching, an intermediate layer having a refractive index different from that of the upper cladding layer is covered with the lower cladding layer and the upper surface and both side surfaces of the core. In addition, a structure in which a uniform and thin film thickness is formed and then covered with an upper clad layer is also reported.

In the case of a planar optical waveguide having a structure in which a rectangular core is surrounded by a clad, since it includes a straight waveguide and a curved waveguide region, loss due to the following factors occurs. First, in a curved waveguide having a configuration in which both side surfaces of the core are covered with a core material having the same refractive index, a difference occurs in the optical path length between the inner peripheral side and the outer peripheral side in the propagation path that draws an arc of curvature _R0. As a result, the effective refractive index distribution is deformed (see Non-Patent Document 1 (FIG. 8.17)). Accordingly, when the propagation mode of the propagating light is a single mode, the center (maximum) of the light intensity distribution in the waveguide mode of the light propagating through the curved waveguide region is shifted to the outer peripheral side, resulting in the curvature. Radiation loss (uniform bending loss) of the curved waveguide itself having a constant R ₀ occurs.

In classical optics, when light is incident from a high refractive index medium to a low refractive index medium at an interface having a refractive index difference, if the incident angle θ _i is greater than or equal to the critical angle θ _C , the low refractive index medium side is reached. The transmitted light disappears and is totally reflected. In the straight waveguide region, light incident on the core / cladding interface is mainly propagated at a critical angle θ _C or more determined by the refractive index n _{core of the core} layer and the refractive index n _{clad of the} cladding layer. On the other hand, in a curved waveguide having a constant curvature R ₀ , for example, light incident on the outer peripheral side surface from the core side under the condition of incident angle θ _i = θ _C propagates in the tangential direction at the incident point. Will do. Therefore, the light incident under the condition of the incident angle θ _i = θ _C is in a state of oozing out from the outer peripheral side surface describing the circular arc to the cladding layer. Thus, in a curved waveguide having a constant curvature R ₀ , the light component that oozes out from the outer peripheral side surface that draws an arc into the cladding layer becomes a loss. On the other hand, also on the inner peripheral side surface, the light incident under the condition of the incident angle θ _i = θ _C propagates in the tangential direction at the incident point. Accordingly, the light incident under the condition of the incident angle θ _i = θ _C travels from the inner peripheral side surface that draws an arc toward the center of the core.

In a curved waveguide in which the width of the core layer is W and the center of the curved waveguide describes an arc having a curvature R ₀ , the optical path length δL ₀ at the center is δL ₀ = R ₀ · δΘ when bending a minute angle δΘ. is there. On the other hand, the optical path length is (R ₀ −δr) · δΘ at a position (R ₀ −δr) separated from the center by δr on the inner circumference side, and at a position (R ₀ + δr) separated by δr on the outer circumference side. The optical path length is (R ₀ + δr) · δΘ. Therefore, the phase change of the light having the wavelength λ _{0 in} the vacuum that occurs when turning the minute angle δΘ is 2π · (R ₀ · δΘ) / (λ ₀ / n) at the center (R ₀ ) of the curved waveguide. _core ), position (R ₀ −δr), 2π · {(R ₀ −δr) · δΘ} / (λ ₀ / n _core ), position (R ₀ + δr), 2π · {(R ₀ + δr) · It is approximately shown as δΘ} / (λ ₀ / n _core ). In other words, the effective refractive index, the center of the curved waveguides (R _0), n _core, in the position _{(R 0 -δr), n core} · (R 0 -δr) / R 0, the position (R ₀ + Δr) is approximately shown as n _core · (R ₀ + δr) / R ₀ . Similarly, the effective refractive index of the cladding layer is n _clad · (R ₀ −Δr) / R _{0 at a} position (R ₀ −Δr) separated by Δr on the inner peripheral side of the center (R ₀ ) of the core layer. At a position (R ₀ + Δr) separated by Δr on the outer peripheral side, it is approximately indicated as n _clad · (R ₀ + Δr) / R ₀ . When the waveguide mode of propagating light is a single mode, the electromagnetic field distribution of light propagating in the above-described curved waveguide showing the effective refractive index distribution is the core layer (maximum position) of the distribution. Is shifted from the center (R ₀ ) to the outer peripheral side (see Non-Patent Document 1 (FIG. 8.18)). Accordingly, when the waveguide mode of the propagating light is a single mode, the light intensity distribution in the waveguide mode of the light propagating through the curved waveguide having a constant curvature R ₀ is the center of the waveguide width (R ₀ ). The distribution center (maximum) is shown on the outer peripheral side.

  In addition, when the propagation mode of the propagating light is a single mode, the center (maximum) of the light intensity distribution in the waveguide mode of the light propagating through the curved waveguide region is shifted to the outer peripheral side. Radiation loss (mode conversion loss) also occurs at the coupling portion between the straight waveguide and the curved waveguide, or at the coupling portion between the curved waveguides having different curvatures.

In classical optics, light incident on the condition where the incident angle θ _i = θ _{C at} the position following the outer peripheral side surface of the curved waveguide at the coupling portion between the linear waveguide and the curved waveguide is as follows. Proceed along the side of the core. For this reason, when reaching the curved waveguide side, a state of oozing out from the outer peripheral side surface describing the arc into the cladding layer is obtained. When the curved waveguide region following the coupling portion is reached, the light component that oozes out from the outer peripheral side surface that draws the arc to the cladding layer becomes a loss. Similarly, at the joint between curved waveguides with different curvatures, when a curved waveguide with a small curvature is connected after a curved waveguide with a large curvature, or the bending direction of the front and rear curved waveguides (the center of curvature) When the S-shaped curved waveguide is configured, the light component that oozes out from the outer peripheral side surface that draws the arc to the cladding layer when reaching the subsequent curved waveguide region is: Loss.

Further, when the waveguide mode of propagating light is a single mode, the center (maximum) of the electromagnetic field distribution is derived from the effective refractive index distribution in the curved waveguide described above, and the center of the core layer ( R ₀ ) is shifted to the outer peripheral side. At that time, in the curved waveguide, the electromagnetic field oozing out into the cladding layer on the outer peripheral side of the core layer is relatively increased (see Non-Patent Document 1 (FIG. 8.18)). At that time, at the coupling portion between the straight waveguide and the curved waveguide, the center (maximum) of the electromagnetic field distribution does not match, so that a loss similar to the off-axis loss occurs. Similarly, at the joint between curved waveguides with different curvatures, when a curved waveguide with a small curvature is connected after a curved waveguide with a large curvature, or the bending direction of the front and rear curved waveguides (the center of curvature) When the S-shaped curved waveguide is configured differently, the center of the electromagnetic field distribution (maximum) does not match at the coupling portion, so that the loss is similar to the off-axis loss. Occurs.

In a conventional planar optical waveguide having a structure in which both side surfaces of the core are covered with a core material having the same refractive index, the refractive index n _{core of the} core layer is suppressed in order to suppress uniform bending loss in a curved waveguide having a constant curvature R _0. And a method of increasing the difference between the refractive indexes n _clad of the clad layer is used. For example, the refractive index n _clad of the cladding layer is made the same as the conventional one, and the refractive index n _core of the core layer is increased. At this time, the difference between the optical path length on the inner circumference side of the curved waveguide and the optical path length on the outer circumference side becomes relatively small, so that the deformation of the effective refractive index distribution is relatively suppressed. Therefore, the maximum of the light intensity distribution in the waveguide mode of light propagating in the curved waveguide region tends to shift to the outer peripheral side, but the shift amount is reduced, and at the same time, the cladding layer extends from the outer peripheral side surface of the core. The part that oozes out to the side is also reduced. As a result, in the process of propagating through the curved waveguide region, loss due to light leaching from the outer peripheral side surface of the core to the cladding layer side is suppressed.

In addition, as a means of reducing radiation loss (mode conversion loss) at the coupling portion between the straight waveguide and the curved waveguide, an offset is added to the waveguide so as to follow an effective waveguide mode distribution (light intensity distribution). Means are disclosed (for example, see Non-Patent Document 1 (FIG. 8.18)). Specifically, at the coupling portion, the inner peripheral side surface of the core of the curved waveguide region is moved to the center of curvature rather than the side surface of the straight waveguide region, and the outer peripheral side surface of the core of the curved waveguide region is moved. In this state, the linear waveguide is moved to the center side of the core rather than the side surface. Therefore, the center of the core of the curved waveguide region is shifted to the center of curvature from the center of the core of the straight waveguide. When the waveguide mode of propagating light is a single mode, the waveguide mode distribution (light intensity distribution) shows a single peak, and the maximum position of this peak is the waveguide mode distribution (light intensity distribution). It can be regarded as the center. At this time, when no “offset” is provided, the center of the effective waveguide mode distribution (light intensity distribution) in the curved waveguide region is shifted from the center of the core to the outer peripheral side. On the other hand, the center of the core itself is moved to the inner circumference side (curvature center side) by the “offset”, so that it matches the center of the effective waveguide mode distribution (light intensity distribution) in the straight waveguide And can. As a result, the loss of light intensity propagated from the straight waveguide side to the curved waveguide region is relatively reduced.
JP-A-2005-300678 JP 2004-264689 A Yasuo Kokubun, “Lightwave Engineering”, Kyoritsu Shuppan, June 10, 1999, pages 250-253.

  In the core formation process in a conventional optical waveguide that employs a channel-type waveguide structure, the core layer is flatly formed on the lower clad layer, and the planar pattern of the core layer is formed using exposure development using a photosensitive material. An etching mask corresponding to is prepared. Next, the core layer is etched by etching such as RIE using the produced etching mask, so that the cross-sectional shape of the core is rectangular. The upper cladding layer is formed so as to cover both side surfaces and the upper surface of the core having a rectangular cross-sectional shape to be manufactured. Therefore, in the core / cladding structure, the refractive index profile in the horizontal direction is symmetric with respect to the center of the core. When a curved waveguide is formed by a core / clad structure having this refractive index distribution, the effective refractive index distribution is deformed, and when light propagates in the curved waveguide, the waveguide mode changes from the center of the core to the outer periphery. Shift to the side. Accordingly, when the waveguide mode of light is a single mode, the light intensity distribution on the inner peripheral side and the outer peripheral side is asymmetric with respect to the center of the waveguide mode distribution (light intensity distribution). In particular, the skirt of the distribution on the outer peripheral side has a shape that exudes considerably from the outer peripheral side wall of the core to the outside. This asymmetry of the waveguide mode and the shape that oozes out to the outer periphery are the radiation loss of the curved waveguide itself (uniform bending loss), and the radiation loss at the joint with a straight waveguide or a waveguide with a different curvature. (Mode conversion loss) occurs.

The uniform bending loss in the curved waveguide is mainly caused by the diffusion of light from the outer peripheral side wall of the core to the outside. For example, the ratio of the refractive index n _{core of the core} to the refractive index n _clad of the _clad on the side wall surface on the outer peripheral side of the core: reducing the critical angle θ _C determined by n _clad / n _core reduces the uniform bending loss. Is made. Therefore, in the conventional optical waveguide, when the refractive index n _clad of the cladding layer is the same, the relative refractive index difference between the refractive index n _{core of the core} and the refractive index n _clad of the cladding: Δ = (n _core ² −n _clad ² ) / By increasing ( _{2.n core} ² ), that is, by reducing the ratio: n _clad / n _core , the critical angle θ _C is reduced and the uniform bending loss is reduced. In this method, it is necessary to use a material having a higher refractive index as a light-transmitting material used for forming the core. At this time, the thickness D and the width W of the rectangular cross-sectional core that satisfies the single mode condition are selected based on the wavelength of light propagating in the core: (λ ₀ / n _core ). When the refractive index n _core is higher, the core thickness D and width W are smaller. For example, by increasing the refractive index n _core of the core, the ratio of the refractive index n _clad of a refractive index n _core and cladding of the core: How to reduce the n _clad / n _core, albeit an essential means, use Possible high refractive index materials are limited, and it is necessary to reduce the size of the core, which presents a problem in terms of processing accuracy.

For example, as a technique for reducing the mode conversion loss at the connection between a straight waveguide and a curved waveguide, conventionally, when the waveguide mode is a single mode, the waveguide mode distribution when light propagates through the straight waveguide In order to make the center of the waveguide and the center of the waveguide mode distribution when light propagates through the curved waveguide coincide with each other, means for providing an “offset” at the connection portion is used. The radius of curvature of the core center of the curved waveguide: R ₀ , the width of the core: W, and the “offset” amount: δR ₀ is, for example, in the order of {1/8 · (W ² / R ₀ )}. Therefore, the problem is inherent in the processing accuracy. Furthermore, an appropriate “offset” amount: δR ₀ is a radius of curvature of the core center of the curved waveguide: R ₀ , a core width: W, a core refractive index n _core , and a clad refractive index n _clad . It is necessary to design based on the parameters, and a considerable amount of numerical simulation calculation is required.

  The present invention solves the above problems, and the present invention provides a planar optical waveguide that meets the following objectives.

  The first object of the present invention is to facilitate various characteristics including a horizontal refractive index distribution and an effective horizontal refractive index distribution of a waveguide composed of a core / clad in a planar optical waveguide. It is an object of the present invention to provide an optical waveguide having a structure that can be adjusted.

In addition, a further object of the present invention is to use, for example, a means for increasing the ratio of the refractive index n _{core of the core} and the refractive index n _clad of the cladding: n _clad / n _core when applied to a curved waveguide. To adjust the refractive index profile in the horizontal direction of the waveguide composed of the core / cladding instead of reducing the uniform bending loss, reducing the diffusion of light from the sidewall surface on the outer peripheral side of the core to the outside Therefore, an object of the present invention is to provide an optical waveguide that achieves uniform bending loss reduction.

  A further object of the present invention is to maintain the waveguide mode in a single mode, and to connect each of the optical waveguides at the connection portion between the straight waveguide and the curved waveguide or between the curved waveguides having different curvatures. In order to reduce the mode conversion loss by matching the centers in the effective waveguide mode distribution when light propagates through the core, it is configured with a core / cladding rather than providing an “offset” at the connection By adjusting the refractive index distribution in the horizontal direction of the waveguide, the centers in the effective waveguide mode distribution when light propagates through each optical waveguide are made to coincide with each other, reducing the mode conversion loss. It is to provide an optical waveguide.

  In order to solve the above-mentioned problems, the refractive index distribution in the horizontal direction of the waveguide constituted by the core / cladding, in particular, the effective refractive index of the core region in the vicinity of the horizontal interface with the cladding is adjusted. Found that it was effective. Specifically, when forming a planar optical waveguide composed of a substrate, a lower clad portion, a core portion, and an upper clad portion formed on the substrate, the cross section that forms a light propagation path and has a rectangular shape It is effective to provide an adjustment unit having a function of adjusting the effective refractive index of the core region in the vicinity of the interface between the core region and the clad portion in contact with both side surfaces thereof. In particular, in a curved waveguide, the effective refractive index distribution in the core region having a rectangular cross section is a continuous effective refractive index change. For the purpose of compensating and canceling this continuous refractive index change, The inventors have found that it is desirable that the effective refractive index change using the adjusting unit is also continuous.

  The present invention has been completed based on these findings.

That is, the optical waveguide according to the first aspect of the present invention is
And the substrate, are formed on the substrate, a lower cladding portion of the refractive index n _clad1, the core part of the refractive index n _core, an optical waveguide made comprises an upper cladding portion of the refractive index n _clad2,
The optical waveguide is a planar optical waveguide comprising a rectangular groove region having a width W and a depth D, constituting a light propagation path,
The lower surface, left side surface, and right side surface of the groove region having a rectangular cross section constitute an interface in contact with the lower cladding part,
With respect to the wavelength lambda ₀ of the light in vacuum propagating in the optical waveguide, the refractive index n _clad1 the lower cladding portion, the refractive index n _core, the refractive index n _clad2 the upper cladding portion of the core portion, n _core> n _clad1 and n _core > n _clad2 are selected to satisfy the relationship,
The cross-sectional shape of the core part is
The upper surface of the core part is in contact with the upper cladding part to form a flat interface,
The lower surface of the core portion is in contact with the lower cladding portion at least in the groove region, and forms a flat interface,
One of the left side and the right side of the groove region is
It is composed of an interface where the lower cladding part and the first adjusting part having a refractive index n ₁ are in contact with each other.
In the lower surface of the groove region, the first adjustment portion is in contact with the lower cladding portion and forms a flat interface having a width W ₁ .
In the groove region, the first adjustment portion forms an interface in contact with the core portion,
The interface between the first adjustment portion and the core portion intersects the lower surface of the groove region at an inclination angle θ ₁ , and the inclination angle θ ₁ is selected as θ ₁ <90 °. ,
The cross-sectional shape of the first adjustment portion includes an interface where the lower cladding portion and the first adjustment portion are in contact with each other on one side surface of the groove region, an interface where the first adjustment portion and the core portion are in contact, and the groove region. Is formed by an interface in contact with the first adjustment portion and the lower clad portion,
The left side of the groove region, the remaining one of the right side,
It is composed of an interface where the lower clad part and the second adjusting part having a refractive index n ₂ are in contact with each other.
In the lower surface of the groove region, the second adjustment portion is in contact with the lower cladding portion and forms a flat interface having a width W ₂ .
In the groove region, the second adjustment part forms an interface in contact with the core part,
The interface between the second adjusting portion and the core portion intersects the lower surface of the groove region at an inclination angle θ ₂ , and the inclination angle θ ₂ is selected as θ ₂ <90 °. ,
The cross-sectional shape of the second adjusting portion is such that, on the other side surface of the groove region, either the lower cladding portion or the upper cladding portion is in contact with the second adjusting portion, and the second adjusting portion and the core portion are in contact with each other. The optical waveguide is characterized by being formed by the interface and the interface in contact with the second adjustment part and the lower cladding part in the lower surface of the groove region.

that time,
The refractive index n ₁ of the first adjustment unit is
At least satisfy the relationship of n ₁ > n _clad1 with respect to the refractive index n _clad1 of the lower cladding part,
For the refractive index n _core of the core part, the relationship of n ₁ ≠ n _core is satisfied,
The refractive index n ₂ of the second adjustment unit is
At least satisfy the relationship of n ₂ > n _clad1 with respect to the refractive index n _clad1 of the lower cladding part,
A configuration satisfying the relationship of n ₂ ≠ n _core with respect to the refractive index n _core of the core portion can be selected.

Moreover,
The refractive index n ₁ of the first adjustment unit is
At least satisfy the relationship of n ₁ > n _clad1 with respect to the refractive index n _clad1 of the lower cladding part,
For the refractive index n _core of the core part, the relationship of n ₁ ≠ n _core is satisfied,
The refractive index n ₂ of the second adjustment unit is
At least satisfy the relationship of n ₂ > n _clad1 with respect to the refractive index n _clad1 of the lower cladding part,
A configuration satisfying the relationship of n ₂ = n _core with respect to the refractive index n _core of the core portion;
Or
The refractive index n ₁ of the first adjustment unit is
At least satisfy the relationship of n ₁ > n _clad1 with respect to the refractive index n _clad1 of the lower cladding part,
For the refractive index n _core of the core part, the relationship of n ₁ = n _core is satisfied,
The refractive index n ₂ of the second adjustment unit is
At least satisfy the relationship of n ₂ > n _clad1 with respect to the refractive index n _clad1 of the lower cladding part,
A configuration satisfying the relationship of n ₂ ≠ n _core with respect to the refractive index n _core of the core portion can also be selected.

For example, when it is necessary to form different states on the left and right of the light propagation path, such as a curved waveguide,
The refractive index n ₁ of the first adjustment unit and the refractive index n ₂ of the second adjustment unit are
A configuration that satisfies at least the relationship of n ₁ ≠ n ₂ is selected.

In the curved waveguide according to the first aspect of the present invention, for example,
The refractive index n ₁ of the first adjustment unit is
At least satisfy the relationship of n ₁ > n _clad1 with respect to the refractive index n _clad1 of the lower cladding part,
For the refractive index n _core of the core part, the relationship of n ₁ ≠ n _core is satisfied,
The refractive index n ₂ of the second adjustment unit is
At least satisfy the relationship of n ₂ > n _clad1 with respect to the refractive index n _clad1 of the lower cladding part,
Relative refractive index n _core of the core part, on the selected configuration meets a relation of n ₂ ≠ n _core,
The planar optical waveguide is a curved waveguide;
In the curved waveguide,
A first adjustment portion having a refractive index n ₁ is provided on the outer peripheral side surface of the groove region.
A second adjusting portion having a refractive index n ₂ is disposed on the inner peripheral side surface of the groove region;
The refractive index n ₁ of the first adjustment unit and the refractive index n ₂ of the second adjustment unit are
It is preferable to adopt a configuration selected so as to satisfy the relationship of n ₂ > n _core > n ₁ with respect to the refractive index n _core of the core portion.

In addition, the method for manufacturing the optical waveguide according to the first aspect of the present invention described above is as follows.
As its manufacturing process,
_Forming a lower clad portion of refractive index n _clad1 on a substrate;
A step of forming a rectangular groove region having a cross section of width W and depth D on the upper surface of the lower clad portion;
A first adjustment member is selectively formed on one side of the groove region so as to cover the side surface and the lower surface of the groove region with a width W ₁ from the side surface on one side of the first adjustment unit, Forming a first adjusting portion having a refractive index n ₁ ;
A second adjustment material is selectively formed on the side surface of the groove region so that the left side surface and the right side surface of the groove region are covered with the side surface and the lower surface of the groove region with a width W ₂ from the side surface. Forming a film and forming a second adjusting portion having a refractive index n ₂ ;
In the groove region, a core portion having a refractive index n _core is formed so as to cover the film formation surface of the first adjustment portion, the film formation surface of the second adjustment portion, and the lower surface of the groove region, and to fill the groove region. Forming a film and forming an interface where the core portion and the first adjustment portion are in contact, and an interface where the core portion and the second adjustment portion are in contact;
A step of forming an upper clad portion of refractive index n _clad2 so as to cover the upper surface of the core portion, the upper surface of the first adjustment portion, the upper surface of the second adjustment portion, and the upper surface of the lower clad portion; An optical waveguide manufacturing method characterized by the following.

The optical waveguide according to the second aspect of the present invention is
And the substrate, are formed on the substrate, a lower cladding portion of the refractive index n _clad1, the core part of the refractive index n _core, an optical waveguide made comprises an upper cladding portion of the refractive index n _clad2,
The optical waveguide is a planar optical waveguide comprising a rectangular groove region having a width W and a depth D, constituting a light propagation path,
The lower surface, left side surface, and right side surface of the groove region having a rectangular cross section constitute an interface in contact with the lower cladding part,
With respect to the wavelength lambda ₀ of the light in vacuum propagating in the optical waveguide, the refractive index n _clad1 the lower cladding portion, the refractive index n _core, the refractive index n _clad2 the upper cladding portion of the core portion, n _core> n _clad1 and n _core > n _clad2 are selected to satisfy the relationship,
The cross-sectional shape of the core part is
The upper surface of the core part is in contact with the upper cladding part to form a flat interface,
The lower surface of the core part is in contact with the lower cladding part in at least the region where the cross section is rectangular, and forms a flat interface,
One of the left side and the right side of the groove region is
It is composed of an interface where the lower cladding part and the first adjusting part having a refractive index n ₁ are in contact with each other.
In the lower surface of the groove region, the first adjustment portion is in contact with the lower cladding portion and forms a flat interface having a width W ₁ .
In the groove region, the first adjustment portion forms an interface in contact with the core portion,
The interface between the first adjustment portion and the core portion intersects the lower surface of the groove region at an inclination angle θ ₁ , and the inclination angle θ ₁ is selected as θ ₁ <90 °. ,
The cross-sectional shape of the first adjustment part is such that, on one side surface of the groove region, an interface between the lower cladding part and the first adjustment part, an interface between the first adjustment part and the core part, and the groove In the lower surface of the region, formed by an interface in contact with the first adjustment portion and the lower cladding portion,
The left side of the groove region, the remaining one of the right side,
An optical waveguide comprising an interface where a lower cladding part and a core part are in contact with each other.

that time,
The refractive index n ₁ of the first adjustment unit is
At least satisfy the relationship of n ₁ > n _clad1 with respect to the refractive index n _clad1 of the lower cladding part,
It is desirable to have a configuration satisfying the relationship of n ₁ ≠ n _core with respect to the refractive index n _core of the core portion.

In the curved waveguide according to the second aspect of the present invention, for example,
The refractive index n ₁ of the first adjustment unit is
At least satisfy the relationship of n ₁ > n _clad1 with respect to the refractive index n _clad1 of the lower cladding part,
Relative refractive index n _core of the core part, on the selected configuration meets a relation of n ₁ ≠ n _core,
The planar optical waveguide is a curved waveguide;
In the curved waveguide,
A first adjustment portion having a refractive index n ₁ is disposed on the outer peripheral side surface of the groove region,
The refractive index n ₁ of the first adjustment unit is
A configuration selected so as to satisfy the relationship of n _core > n ₁ with respect to the refractive index n _core of the core portion;
Or
The planar optical waveguide is a curved waveguide;
In the curved waveguide,
A first adjusting portion having a refractive index n ₁ is disposed on the inner peripheral side surface of the groove region;
The refractive index n ₁ of the first adjustment unit is
It is preferable to adopt a configuration selected so as to satisfy the relationship of n _core <n ₁ with respect to the refractive index n _core of the core portion.

In addition, the method for manufacturing the optical waveguide according to the second aspect of the present invention described above is as follows.
As its manufacturing process,
_Forming a lower clad portion of refractive index n _clad1 on a substrate;
A step of forming a rectangular groove region having a cross section of width W and depth D on the upper surface of the lower clad portion;
A first adjustment member is selectively formed on one side of the groove region so as to cover the side surface and the lower surface of the groove region with a width W ₁ from the side surface on one side of the first adjustment unit, Forming a first adjusting portion having a refractive index n ₁ ;
In the groove area, the left side surface of the groove area, while remaining on the right side, the deposition surface of the first adjusting portion, and covers the lower surface of the groove area to fill the trench region, the refractive index n _core Forming a core part and forming an interface between the core part and the first adjustment part;
An optical waveguide manufacturing method comprising a step of forming a film of an upper cladding portion of a refractive index n _clad2 so as to cover an upper surface of a core portion and an upper surface of a lower cladding portion.

  Furthermore, the optical waveguide according to the present invention is particularly effective when applied to a curved waveguide. For example, in the optical waveguide having the form described below, it is preferable to use it for the curved waveguide portion.

For example,
An optical waveguide in which a straight waveguide portion and a curved waveguide portion are connected,
The straight waveguide portion is
A substrate, a lower clad portion having a refractive index n _clad1 formed on the substrate, a core portion having a refractive index n _core , and an upper clad portion having a refractive index n _clad2 ;
A planar optical waveguide comprising a rectangular groove region having a width W and a depth D, constituting a light propagation path,
The lower surface, left side surface, and right side surface of the groove region having a rectangular cross section constitute an interface in contact with the lower cladding part,
With respect to the wavelength lambda ₀ of the light in vacuum propagating in the optical waveguide, the refractive index n _clad1 the lower cladding portion, the refractive index n _core, the refractive index n _clad2 the upper cladding portion of the core portion, n _core> n _clad1 and n _core > n _clad2 are selected to satisfy the relationship,
The cross-sectional shape of the core part is
The upper surface of the core part is in contact with the upper cladding part to form a flat interface,
The lower surface of the core portion is in contact with the lower cladding portion at least in the groove region, and forms a flat interface,
Both the left side surface and the right side surface of the groove region,
It consists of the interface where the lower cladding and the core are in contact;
The curved waveguide portion is the above-described curved waveguide according to the first aspect of the present invention or the curved waveguide according to the second aspect of the present invention;
The connection between the straight waveguide portion and the curved waveguide portion is as follows:
In the straight waveguide portion, a rectangular groove region having a cross section having a width W and a depth D, which forms a light propagation path, and in the curved waveguide portion, a cross section having a width W, which forms a light propagation path. In addition, a rectangular groove region having a depth D can be connected to each other in a form in which the lower surface, the left side surface, and the right side surface thereof coincide with each other.

For example,
An optical waveguide formed by connecting two curved waveguide portions having different curvatures,
The two curved waveguide portions having different curvatures are respectively the curved waveguide according to the first aspect of the present invention or the curved waveguide according to the second aspect of the present invention;
The connection between the two curved waveguide sections is
In one curved waveguide portion, a light propagation path is formed. A rectangular groove region having a cross section of width W and depth D, and in the other curved waveguide portion, a light propagation path is formed. A rectangular groove region having a width W and a depth D is connected to each other in a form in which the lower surface, the left side surface, and the right side surface of the two are coincident with each other.

For example,
An optical waveguide formed by connecting two curved waveguide portions having different bending directions,
The two curved waveguide portions having different bending directions are respectively the curved waveguide according to the first aspect of the present invention or the curved waveguide according to the second aspect of the present invention;
The connection between the two curved waveguide sections is
In one curved waveguide portion, a light propagation path is formed. A rectangular groove region having a cross section of width W and depth D, and in the other curved waveguide portion, a light propagation path is formed. A rectangular groove region having a width W and a depth D is connected to each other in a form in which the lower surface, the left side surface, and the right side surface of the two are coincident with each other.

  In the optical waveguide according to the present invention, in the core / clad structure constituting the planar optical waveguide, the optically and electromagnetically characteristic of the light transmissive material forming the core portion at the horizontal boundary between the core region and the clad. By arranging different light transmissive materials, for example, the refractive index distribution in the horizontal direction of the waveguide, particularly the effective refractive index of the core region near the horizontal interface with the cladding can be easily adjusted. . In particular, in the effective refractive index distribution in the horizontal direction of the waveguide, a distribution in which the effective refractive index continuously changes from the horizontal boundary with the cladding toward the center of the core region is easily formed. can do.

For example, when the waveguide mode is a single mode, in the conventional curved waveguide, the effective refractive index decreases on the inner peripheral side and the effective refractive index on the outer peripheral side as compared with the central portion of the core region. Due to the increase, when light propagates in the waveguide, the center of the mode distribution shifts from the center of the core region to the outer peripheral side. On the other hand, according to the present invention, an adjustment portion having a refractive index intermediate between the refractive index n _{core of the core} and the refractive index n _clad of the cladding is provided at the boundary on the outer peripheral side of the core region. The effective refractive index can be made higher than the effective refractive index on the outer peripheral side. In addition, for example, according to the present invention, an effective refraction at the inner peripheral side can be achieved relatively by providing an adjustment portion having a refractive index higher than the refractive index n _{core of the} core at the boundary on the inner peripheral side of the core region. The refractive index can be higher than the effective refractive index on the outer peripheral side. By achieving the refractive index distribution as described above, when the light propagates in the waveguide, the center of the mode distribution is prevented from shifting from the center of the core region to the outer peripheral side, and the center of the mode distribution is reduced. It is possible to match the center of the core region. At that time, the effective refractive index distribution of the core region is relatively in a state where the effective refractive index on the inner peripheral side is higher than the effective refractive index on the outer peripheral side, and the center of the mode distribution is at the core region. As a result, the bleeding of light from the outer peripheral side of the core region to the cladding side is suppressed.

First, when the waveguide mode is a single mode, the optical waveguide according to the present invention is applied to a curved waveguide, and when light propagates in the waveguide, the center of the mode distribution is the outer periphery side from the center of the core region. By suppressing the phenomenon of shifting to a straight waveguide, the center of the mode distribution in the straight waveguide is matched with the center of the mode distribution in the curved waveguide without performing "offset" correction when connecting the straight waveguide and the curved waveguide. It becomes possible. As a result, it is possible to suppress mode conversion loss that occurs at the connecting portion between the straight waveguide and the curved waveguide. It is also possible to relatively suppress uniform bending loss that occurs when light propagates through the curved waveguide. The present invention makes it possible to suppress uniform bending loss in a curved waveguide without using a means for reducing the ratio of the refractive index n _{core of the core} and the refractive index n _clad of the _clad : n _clad / n _core. Therefore, by applying the optical waveguide according to the present invention, it is possible to suppress an increase in uniform bending loss and to produce a curved waveguide having a small curvature. That is, by applying the optical waveguide according to the present invention, an increase in mode conversion loss and uniform bending loss can be suppressed, and the planar optical waveguide including the straight waveguide region and the curved waveguide region can be miniaturized.

  Next, the optical waveguide of the present invention will be described in detail with reference to the drawings, taking a typical embodiment as an example. Each embodiment illustrated below is an example of the best mode of the present invention, but the present invention is not limited to these illustrated modes.

(First embodiment)
A planar optical waveguide according to a first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a cross-sectional view schematically showing the structure of an optical waveguide structure 1, which is an example of the structure of a planar optical waveguide according to the first embodiment. As illustrated in FIG. 1, the optical waveguide structure 1 used in the first embodiment includes a lower clad portion 31 formed on a substrate 2, a groove formed on the upper surface of the lower clad portion 31, and one of the groove portions. Of the first adjustment portion 41 formed on the side wall portion of the groove, the second adjustment portion 42 formed on the other side wall portion of the groove portion, the core portion 32 formed at the center portion of the groove, the lower cladding portion 31 and the core portion 32 The upper clad portion 33 is formed so as to cover the upper surface.

The cross-sectional shape of the groove part itself formed on the upper surface of the lower cladding part 31 is a rectangular shape having a depth: D and a width: W. The first adjustment portion 41 is in contact with one side wall surface of the groove portion and a part of the bottom surface of the groove portion. The interface where the first adjusting portion 41 and the core portion 32 are in contact with each other is inclined with respect to the bottom surface of the groove portion, and the inclination angle θ ₁ is set to a constant angle. The second adjustment portion 42 is in contact with the other side wall surface of the groove portion and a part of the bottom surface of the groove portion. The interface where the first adjusting portion 42 and the core portion 32 are in contact with each other is inclined with respect to the bottom surface of the groove portion, and the inclination angle θ ₂ is set to a constant angle.

In the optical waveguide structure 1 illustrated in FIG. 1, the first adjustment portion 41, the core portion 32, and the second adjustment portion 42 occupying the inside of the groove portion, and the lower cladding portion 31 and the upper cladding portion 33 that surround the periphery thereof. A refractive index waveguide structure is configured by utilizing a relative refractive index difference provided therebetween. The lower clad part 31 is manufactured with a thickness: T _clad1 , but since a groove part with a depth: D is formed from the upper surface, the thickness left under the core part 32 is (T _clad1 -D). Become.

For example, when a semiconductor substrate is used as the substrate 2 on which the lower cladding portion 31 is formed, the refractive index: n _sub of the substrate 2 is generally more than the refractive index: n _core of the light transmissive material forming the core portion 32. Get higher. At that time, when the remaining thickness (T _clad1 -D) is thin, the diffusion of light to the substrate 2 shows a significant influence. In order to eliminate the influence caused by the diffusion of light to the substrate 2, the lower cladding portion 31 according to the depth: D of the groove portion so that the remaining thickness: (T _clad1 -D) is sufficiently thick. Thickness: Select T _clad1 .

The material used for forming the core part 32, the lower clad part 31, and the upper clad part 33 is a material exhibiting light transmittance with respect to the wavelength λ ₀ of the guided light in vacuum. In addition, the material used for forming the first adjustment unit 41 and the second adjustment unit 42 is also a material that transmits light with respect to the wavelength λ ₀ of the guided light in vacuum. At that time, since the first adjusting portion 41 and the second adjusting portion 42 are formed so as to cover the side wall surface of the groove portion, a film forming method capable of selectively covering and depositing on the vertical side surface is adopted. It is desirable to select a light transmissive material that can be deposited. For example, ion beam sputtering, aerosol deposition, or the like can be used as a film forming method capable of selectively covering and depositing on the vertical side surface. It is desirable to select a material used for forming the second adjustment unit 42.

In particular, the core 32, the first adjustment unit 41, and the second adjustment unit 42 are usually caused by absorption (or inelastic light scattering / elastic light scattering) when light having a wavelength of λ _{0 in} the vacuum propagates. A light-transmitting material that does not substantially exhibit transmitted light intensity attenuation is utilized. Further, when light having a wavelength of λ _{0 in} a vacuum propagates to the lower clad part 31 and the upper clad part 33 as well, attenuation of transmitted light intensity due to absorption (or inelastic light scattering / elastic light scattering). A light-transmitting material that does not substantially exhibit the above is utilized.

Thereafter, the core part 32, the first adjustment part 41, the second adjustment part 42, the lower cladding part 31 and the upper cladding part 33 all absorb the wavelength λ ₀ of the guided light in vacuum. An explanation will be given by taking as an example an embodiment using a light-transmitting material that does not show attenuation of transmitted light intensity due to (or inelastic light scattering / elastic light scattering). The refractive index in vacuum: ratio n _0: is defined as n / n _0, the value of the specific refractive index n _r, hereinafter referred to as the refractive index of each material.

Wavelength in vacuum of the guided light being: at lambda _0, the refractive index of the core portion 32: n _core, the refractive index n _clad2 the upper cladding portion 33, the refractive index n _clad1 the lower cladding 31, n _core> n A light transmissive material used for forming the core portion 32, the lower clad portion 31, and the upper clad portion 33 is selected so as to satisfy the relationship of _clad1 , n _core > n _clad2 .

In the vertical direction, light is confined by the relative refractive index difference between the lower clad part 31 constituting the bottom surface of the groove part, the upper clad part 33 covering the upper surface, and the core part 32. Usually, the refractive index n _clad1 refractive index n _clad2 and the lower cladding portion 31 of the upper cladding portion 33 is to be equal, for selecting light transmissive material utilized in the formation of the lower cladding portion 31 and the upper cladding portion 33 . At that time, the interface of the lower cladding portion 31 and the core portion 32, the critical angle at the interface of the upper cladding portion 33 and the core portion 32: theta _Cdown, theta _{Cup respectively, sinθ Cdown = (n clad1 /} n core), sinθ Cup = (N _clad2 / n _core ) is satisfied. Usually, the condition of n _clad2 = n _clad1 is selected, so that θ _Cdown = θ _Cup .

In the conventional planar optical waveguide in which the core portion 32 occupies the entire groove portion, the interface between the lower cladding portion 31 and the core portion 32 is formed on both side wall surfaces of the groove portion, and the critical angle at the interface: θ _Cside is sin θ _Cside = (n _clad1 / n _core ) is satisfied. That is, θ _Cside = θ _Cdown . In classical optics, the incident angle: θ _i <θ _Cside with respect to the interface between the lower cladding part 31 and the core part 32, and the light incident from the core part 32 side is refracted at this interface, and the lower cladding part 31 Go to the side. Incident angle: θ _i ≧ θ _Cside , and light incident from the core portion 32 side cannot pass through this interface and travel to the lower cladding portion 31 side. Exceptionally, when the incident angle is θ _i = θ _Cside , light incident from the core portion 32 side oozes out to the lower cladding portion 31 side having a low refractive index and effectively travels in the tangential direction of the interface at the incident point. An optical component (evanescent wave) is generated.

When propagating light having a wavelength of λ _{0 in} a vacuum through a planar optical waveguide manufactured using this groove, the light intensity distribution of the propagating light satisfies the single mode condition. In particular, it is preferable to select the cross-sectional size; depth: D, and width: W of the rectangular groove so that the vertical light intensity distribution and the horizontal light intensity distribution are in a single mode.

  When achieving the single mode condition, the cross-sectional size of the rectangular groove; depth: D, width: W may be W> D or W ≦ D. In the first to fifth embodiments, the configuration selected in the range of W> D is taken as an example, and the operation principle and effects of the present invention are described.

In an example of the planar optical waveguide according to the first embodiment, at least the refractive index of the light-transmitting material used for forming the first adjustment unit 41: n ₁ and the light transmission used for forming the second adjustment unit 42. The refractive index of the material: n ₂ is selected to be higher than the refractive index n _clad1 of the lower cladding part 31. That is, the selection is made so as to satisfy the relationship of n ₁ > n _clad1 and n ₂ > n _clad1 . Further, at least one of the refractive index of the light transmissive material used for forming the first adjusting portion 41: n ₁ and the refractive index of the light transmissive material used for forming the second adjusting portion 42: n ₂ is the core portion. Refractive index of 32: A value different from n _core is selected. Usually, selection is made so as to satisfy the relationship of n ₁ ≠ n _core and n ₂ ≠ n _core . Therefore, in general, the light-transmitting material used for forming the first adjustment portion 41 and the light-transmitting material used for forming the second adjustment portion 42 include the light-transmitting material used for forming the core portion 32. Use light transmissive materials with different electrical and optical properties.

Next, an aspect in which the planar optical waveguide according to the first embodiment is used as a curved waveguide will be described with reference to FIG. 2 regarding the refractive index structure of the optical waveguide structure 1 and its operating principle. The refractive index structure of the optical waveguide structure 1 illustrated in FIG. 2 is an aspect in which the center of curvature of the curved waveguide is located on the side wall side where the first adjustment unit 41 is provided. At this time, when the curved waveguide is formed, the locus drawn by the center of the groove is an arc shape having a radius of curvature R ₀ . Accordingly, the side wall surface on the inner peripheral side of the groove has an arc shape with a radius of curvature: R ₀ − (1/2 · W), and the side wall surface on the outer peripheral side has a radius of curvature: R ₀ + (1/2 · W). ) Arc shape.

FIG. 2A shows a refractive index structure in the cross section AA ′ in the optical waveguide structure 1 schematically shown in FIG. That is, the refractive index: n ₁ of the first adjusting portion 41 formed on the inner peripheral side wall portion of the groove portion is higher than the refractive index: n _core of the core portion 32 and is formed on the outer peripheral side wall portion of the groove portion. that the refractive index of the second adjusting portion 42: n ₂ is the refractive index of the core portion 32: is selected lower than the n _core. Therefore, the relationship is n ₁ > n _core > n ₂ > n _clad1 . Since the groove portion is formed on the upper surface of the lower clad portion 31, the side wall surface on the inner peripheral side of the groove portion is the interface between the light transmissive material forming the lower clad portion 31 and the first adjustment portion 41, the outer peripheral side. The wall surface is an interface between the light transmissive material forming the lower cladding part 31 and the first adjustment part 41. Therefore, the critical angle: θ _Cin on the inner peripheral side wall surface of the groove satisfies sin θ _Cin = (n _clad1 / n ₁ ). The critical angle θ _Cout on the inner peripheral side wall surface of the groove satisfies sin θ _Cout = (n _clad1 / n ₂ ). Therefore, θ _Cin > θ _Cside > θ _Cout .

Similarly, when an arc-shaped curved waveguide having a radius of curvature R ₀ is formed by a conventional planar optical waveguide in which the core portion 32 occupies the entire groove portion, when light propagates in the curved waveguide, the horizontal direction The light intensity distribution becomes a distribution showing a maximum on the outer peripheral side from the center of the groove. As long as the groove width: W is sufficiently small with respect to the radius of curvature: R ₀ and W / R ₀ << 1 is satisfied, the effective refractive index distribution of a minute region including this groove is not patented. As described in Document 1, the shape is inclined in proportion to the distance r from the center of curvature. Specifically, when light travels along a circular arc having a small angle δθ along the curved waveguide, on the inner peripheral side, if the distance from the center of curvature is r = R ₀ −δr, the optical path length is δθ · ( R ₀ −δr), and on the outer peripheral side, if the distance from the center of curvature is r = R ₀ + δr, the optical path length is δθ · (R ₀ + δr). At that time, the phase change Δψ shown when the light having a wavelength of λ ₀ in the vacuum advances in the optical path length in the medium having the refractive index n is:
On the inner circumference side, Δψ _in = 2π · δθ · (R ₀ −δr) / (λ ₀ / n),
At the center of the groove, Δψ _center = 2π · δθ · R ₀ / (λ ₀ / n),
On the outer peripheral side, Δψ _out = 2π · δθ · (R ₀ + δr) / (λ ₀ / n).

Therefore, the effective refractive index n _eff is
On the inner circumference side, Δψ _in = 2π · δθ · R ₀ / (λ ₀ / n _in ) = 2π · δθ · (R ₀ −δr) / (λ ₀ / n),
At the center of the groove, Δψ _center = 2π · δθ · R ₀ / (λ ₀ / n _center ) = 2π · δθ · R ₀ / (λ ₀ / n),
On the outer peripheral side, Δψ _out = 2π · δθ · R ₀ / (λ ₀ / n _out ) = 2π · δθ · (R ₀ + δr) / (λ ₀ / n)
It is calculated approximately so as to satisfy. That is, the effective refractive index n _eff is
On the inner circumference side, n _in = n · (R ₀ −δr) / R ₀ ,
At the center of the groove, n _center = n,
On the outer peripheral side, n _out = n · (R ₀ + δr) / R ₀
It is expressed approximately.

  In this effective refractive index distribution, the effective refractive index on the outer peripheral side is higher than the inner peripheral side in the clad part and groove, and the electromagnetic field intensity distribution (light intensity distribution) of the propagating light. The maximum is a shape shifted from the center of the groove portion to the outer peripheral side.

In the optical waveguide structure 1 schematically shown in FIG. 1, the interface between the first adjustment portion 41 formed on the inner peripheral side wall portion and the core portion 32 is inclined with respect to the bottom surface of the groove portion. Angle: θ ₁ is set to a constant angle. Assuming that the width of the first adjusting portion 41 that contacts the bottom surface of the groove portion is W ₁ , the inclination angle θ ₁ satisfies tan θ ₁ = D / W ₁ . The interface between the second adjusting portion 42 formed on the outer peripheral side wall portion and the core portion 32 is inclined with respect to the bottom surface of the groove portion, and the inclination angle θ ₂ is set to a constant angle. . Assuming that the width of the second adjusting portion 42 that contacts the bottom surface of the groove portion is W ₂ , the inclination angle θ ₂ satisfies tan θ ₂ = D / W ₂ . Of course, W ₁ + W ₂ ≦ W, and normally, W ₁ ≦ (1/2) · W and W ₂ ≦ (1/2) · W.

In the optical waveguide structure 1, the effective refractive index distribution is as follows.
On the inner peripheral side wall surface: r = R ₀ − (1/2 · W)
In the lower cladding part 31, n _clad1-eff = n _clad1 · {R ₀ − (1/2 · W)} / R ₀ ;
In the first adjustment unit 41, n _1−eff = n ₁ · {R ₀ − (1/2 · W)} / R ₀ ;
At the position of W ₁ from the side wall surface on the inner peripheral side: r = R ₀ − (1/2 · W) + W ₁ ,
In the core unit 32, n _core-eff = n _core · {R ₀ − (1/2 · W) + W ₁ } / R ₀ ;
At the position of W ₂ from the outer peripheral side wall surface: r = R ₀ + (1/2 · W) −W ₂
In the core section 32, n _core-eff = n _core · {R ₀ + (1/2 · W) −W ₂ } / R ₀ ;
On the outer peripheral side wall surface: r = R ₀ + (1/2 · W)
In the lower cladding part 31, n _clad1-eff = n _clad1 · {R ₀ + (1/2 · W)} / R ₀ ;
In the second adjustment unit 42, n _2−eff = n ₂ · {R ₀ + (1/2 · W)} / R ₀
In the groove portion, the refractive index distribution exists in the depth direction, and the inner peripheral region: R ₀ − (1/2 · W) <r <R ₀ − (1/2 · W) + W ₁ At each position r, the effective refractive index n _eff (r) is n ₁ · {R ₀ − (1/2 · W)} / R ₀ > n _eff (r)> n _core · {R ₀ − ( It shows a continuous change of 1/2 · W) + W ₁ } / R ₀ . On the other hand, in the region on the outer peripheral side: R ₀ + (1/2 · W) −W ₂ <r <R ₀ + (1/2 · W), the effective refractive index n _eff (r) at each position r. N _core · {R ₀ + (1/2 · W) −W ₂ } / R ₀ > n _eff (r)> n ₂ · {R ₀ + (1/2 · W)} / R ₀ Change.

  Therefore, in the optical waveguide structure 1, when the above effective refractive index distribution is employed, the effective refractive index on the outer peripheral side is higher than that on the inner peripheral side in the lower cladding part 31, but in the inside of the groove part, A situation has been achieved in which the effective refractive index on the inner peripheral side is relatively higher than that on the outer peripheral side. Therefore, when the waveguide mode is a single mode, the center (maximum) of the electromagnetic field intensity distribution (light intensity distribution) of the propagating light is not in a shape that shifts from the center of the groove to the outer peripheral side, but is positioned at the center of the groove. It can be in a state to do.

FIG. 2B shows a light intensity distribution in a curved waveguide having a conventional structure; a conventional mode distribution 34; a light intensity distribution in a curved waveguide using the optical waveguide structure 1; and a mode distribution 35 in the present invention. Are shown schematically in comparison. In the conventional mode distribution 34, the center (maximum) of the electromagnetic field intensity distribution (light intensity distribution) of the propagating light is shifted to the outer peripheral side from the center of the groove part. The oozing from the side wall to the cladding on the outer peripheral side shows a considerable ratio. On the other hand, in the mode distribution 35 according to the present invention, the center (maximum) of the electromagnetic field intensity distribution (light intensity distribution) of the propagating light is positioned at the center (R ₀ ) of the groove part, so that the outer circumference of the groove part is increased. The seepage from the side wall to the outer cladding is relatively suppressed.

2A, the width W ₁ in which the first adjustment portion 41 formed on the inner peripheral side wall portion is formed, and the second adjustment portion 42 formed on the outer peripheral side wall portion are formed. An example in which the widths W ₂ are equal is illustrated. In other words, when the condition of W ₁ = W ₂ is satisfied, the maximum of the electromagnetic field intensity distribution (light intensity distribution) of the light propagating in the curved waveguide is positioned at the center of the groove. , the refractive index of the core portion 32: for n _core, the refractive index of the first adjusting portion 41: n ₁ and the refractive index of the second adjusting portion 42: the n ₂ illustrates a properly selected one embodiment.

Of course, even when W ₁ ≠ W ₂ , for example, when W ₁ > W ₂ , the maximum of the electromagnetic field intensity distribution (light intensity distribution) of light propagating in the curved waveguide is at the center of the groove. as a state positioned, the refractive index of the core portion 32: for n _core, the refractive index of the first adjusting portion 41: n ₁ and the refractive index of the second adjusting portion 42: n ₂ properly selecting the Is possible. Specifically, regarding the second adjustment portion 42 formed on the outer peripheral side wall portion, when (n ₂ , W ₂ ) is selected in the same manner as in FIG. 2A, the width formed of the first adjustment portion 41 When W ₁ is widened to (W ₁ + ΔW ₁ ), the refractive index n ₁ of the first adjustment unit 41 is reduced to (n ₁ −Δn ₁ ) to propagate in the curved waveguide. The state where the maximum of the electromagnetic field intensity distribution (light intensity distribution) of light is located at the center of the groove can be achieved. At that time, (n ₁ −n _core ) · W ₁ and (n ₁ −Δn ₁ −n _core ) · (W ₁ + ΔW ₁ ) are substantially equal.

In addition, the first adjustment portion 41 formed on the inner peripheral side wall portion compensates for a decrease in the effective refractive index of the core portion that occurs on the inner peripheral side from the center of the groove portion, while it is formed on the outer peripheral side wall portion. The second adjusting portion 42 that functions to cancel the increase in effective refractive index of the core portion that occurs on the outer peripheral side from the center of the groove portion. Therefore, (n ₁ −n _core ) · W ₁ and (n _core −n ₂ ) · W ₂ are approximately the same as {n · (W / 2) / R ₀ } · (W / 2).

As described above, when an arc-shaped curved waveguide having a radius of curvature of R ₀ is configured using the planar optical waveguide of the optical waveguide structure 1, the first adjusting unit 41 and the center of curvature are Among the second adjusting portions 42, those provided on the inner peripheral side of the groove portion are higher than the refractive index of the core portion 32: n _core, and those provided on the outer peripheral side of the groove portion are based on the refractive index of the core portion 32: n _core . To be low. Therefore, in the example shown in FIG. 2, the case where the center of curvature is on the first adjustment unit 41 side has been described. However, when the center of curvature is on the second adjustment unit 42 side, it can be reversed. Not too long.

  Next, a method for manufacturing a planar optical waveguide having the optical waveguide structure 1 will be described with reference to FIG. FIG. 3 is a cross-sectional view illustrating a process of manufacturing the optical waveguide structure 1 shown in FIG.

  First, as shown in FIG. 3A, a lower clad portion 31 is formed on a substrate 2 made of Si or the like. After forming the light transmissive material constituting the lower cladding portion 31, a groove portion having a rectangular cross section having a width W and a depth D is formed by etching such as RIE.

  Subsequently, as illustrated in FIG. 3B, the first adjustment portion 41 is formed on one side wall portion of the groove portion. For film formation of the first adjustment unit 41, it is optimal to use a method capable of selectively covering and forming a film on the vertical side surface of the groove, such as an aerosol position method or ion beam sputtering.

  Subsequently, as illustrated in FIG. 3C, the second adjustment portion 42 is formed on the side wall portion of the groove portion on the opposite side of the first adjustment portion 41. The film formation method used for film formation of the second adjustment unit 42 is the same as the method of film formation of the first adjustment unit 41.

  Subsequently, as shown in FIG. 3D, the core portion 32 is formed so as to fill the groove portion. For the formation of the core portion 32, a film formation method similar to the film formation method used for film formation of the first adjustment portion 41 and the second adjustment portion 42 can be used. Further, a lift-off method in which only the core portion 32 is selectively embedded, or a film is formed so as to cover the entire upper surface of the lower cladding portion 31 and then polished, so that the upper surface of the core portion 32 is the lower cladding portion 31. A step of adjusting the flatness of the upper surface of the core portion 32 to the flatness of the upper surface of the lower cladding portion 31 may be added as appropriate, such as flattening so as to be the same level as the upper surface of the lower clad.

  And as shown in FIG.3 (e), the upper clad part 33 is formed so that the upper surface of the core part 32 and the lower clad part 31 may be coat | covered.

  As described above, in the planar optical waveguide according to the first embodiment of the present invention, when the core portion 32 is formed in the groove portion, as illustrated in the optical waveguide structure 1 shown in FIG. By providing the first adjusting portion 41 and the second adjusting portion 42 in the same, materials having different characteristics can be disposed on the horizontal boundary of the cladding portion. Therefore, it is possible to realize an optical waveguide capable of easily adjusting characteristics such as the refractive index on the groove portion side at the horizontal boundary of the cladding portion.

  For example, when the waveguide mode is a single mode, when producing a curved waveguide, the refractive index has an intermediate refractive index between the core part and the cladding part near the horizontal boundary between the core part and the cladding part on the outer peripheral side. By providing the adjustment unit, the center (maximum position) of the waveguide mode of light propagating through the curved waveguide can be shifted in the direction of the center of curvature. By utilizing this function (feature), it is possible to align the center of the groove part where the core part is formed and the center (maximum position) of the waveguide mode in the curved waveguide.

  The planar optical waveguide according to the first embodiment shows a channel-type waveguide structure in which the upper and lower sides and the left and right sides of the core portion formed in the groove portion are surrounded by a clad. Instead of the channel-type waveguide structure, the present invention may be applied to a planar optical waveguide having an inverted ridge structure in which the core portion 32 is formed not only on the groove portion but also on the entire lower surface of the upper cladding portion 33. That is, even in the reverse ridge structure, by providing the first adjustment part 41 and the second adjustment part 42 on the side wall part of the groove part, the core part and the clad part form a horizontal boundary on the side wall surface of the groove part. In other words, the adjustment unit for adjusting the effective refractive index is present.

  In the above example, the aspect in which the first embodiment of the present invention is applied to the adjustment of the waveguide mode distribution of light in the curved waveguide has been described. On the other hand, when the characteristics of the first adjustment unit 41 and the second adjustment unit 42 formed on the side wall portion of the groove, for example, the optical and electromagnetic characteristics are the same, adjustment of mode distribution such as spot size conversion is attempted. It can be applied to the construction of functional optical waveguides.

(Second Embodiment)
A planar optical waveguide according to a second embodiment of the present invention will be described with reference to FIG. FIG. 4 is a cross-sectional view schematically showing the structure of the optical waveguide structure 1A, which is an example of the structure of the planar optical waveguide according to the second embodiment. As illustrated in FIG. 4, the optical waveguide structure 1 </ b> A used in the second embodiment includes a lower clad part 31 formed on the substrate 2, a groove formed on the upper surface of the lower clad part 31, and one side of the groove part. The first adjustment portion 41 is formed on the core portion 32, the core portion 32 is formed at the center of the groove, the lower cladding portion 31, and the upper cladding portion 33 formed so as to cover the upper surface of the core portion 32.

The cross-sectional shape of the groove part itself is rectangular, and the first adjustment part 41 is in contact with the side wall surface on one side of the groove part and a part of the bottom surface of the groove part. The interface where the first adjusting portion 41 and the core portion 32 are in contact with each other is inclined with respect to the bottom surface of the groove portion, and the inclination angle θ ₁ is set to a constant angle. Assuming that the width of the first adjusting portion 41 that contacts the bottom surface of the groove portion is W ₁ , the inclination angle θ ₁ satisfies tan θ ₁ = D / W ₁ .

When this optical waveguide structure 1A is employed in the curved waveguide region, the groove is formed without providing an “offset” at the junction with the straight waveguide region. Therefore, also in the curved waveguide region, the groove depth D formed on the upper surface of the lower cladding portion 31 and the groove width W are the same as those in the straight waveguide region. Further, the side wall on the inner peripheral side of the groove part and the side wall on the outer peripheral side of the groove part in the curved waveguide region are connected to the two side walls of the groove part in the linear waveguide region without providing a step. At that time, the lower clad part 31, the upper clad part 33, and the core part 32 are integrally formed using the same material in the linear waveguide region and the curved waveguide region, respectively. That is, the curved waveguide region, the refractive index of the core portion 32: n _core, the refractive index n _clad2 the upper cladding portion 33, the refractive index n _clad1 the lower cladding 31, core portion constituting the straight waveguide, upper cladding This is the same value as the refractive index of the lower and lower cladding portions. Usually, the refractive index n _clad1 refractive index n _clad2 and the lower cladding portion 31 of the upper cladding portion 33 is selected to be equal.

The first adjustment unit 41 is formed of a material having characteristics different from those of the core unit 32. For example, the refractive index: n ₁ of the first adjustment unit 41 is different from the refractive index: n _core of the core unit 32. For example, in the curved waveguide region, when the first adjustment unit 41 is formed on the curvature center side (inner peripheral side) of the groove, the refractive index of the first adjustment unit 41 is lower than the refractive index of the core unit 32: n _core. Ratio: Select a material so that n ₁ is high. Therefore, the refractive index n _clad1 of the lower cladding part 31, the refractive index of the core part 32: n _core , and the refractive index of the first adjusting part 41: n ₁ satisfy the relationship of n ₁ > n _core > n _clad1 .

On the contrary, in the curved waveguide region, when the first adjustment unit 41 is formed on the side (outer peripheral side) different from the center of curvature of the groove portion, the first adjustment unit 41 is smaller than the refractive index of the core unit 32: n _core. The material is selected such that the refractive index of n ₁ is low. At that time, the material is selected so that the refractive index n ₁ of the first adjustment unit 41 is higher than the refractive index n _clad1 of the lower cladding part 31. Therefore, the refractive index n _clad1 of the lower cladding part 31, the refractive index of the core part 32: n _core , and the refractive index: n ₁ of the first adjustment part 41 satisfy the relationship of n _core > n ₁ > n _clad1 .

Next, an aspect in which the planar optical waveguide according to the second embodiment is used as a curved waveguide will be described with reference to FIG. 4 regarding the refractive index structure of the optical waveguide structure 1A and its operating principle. The refractive index structure of the optical waveguide structure 1A illustrated in FIG. 4 is an aspect in which the center of curvature of the curved waveguide is located on the side wall side where the first adjustment unit 41 is provided and satisfies the relationship of n ₁ > n _core > n _{clad 1} It is. At this time, when the curved waveguide is formed, the locus drawn by the center of the groove is an arc shape having a radius of curvature R ₀ . Accordingly, the side wall surface on the inner peripheral side of the groove has an arc shape with a radius of curvature: R ₀ − (1/2 · W), and the side wall surface on the outer peripheral side has a radius of curvature: R ₀ + (1/2 · W). ) Arc shape.

In a range where the groove width: W is sufficiently small with respect to the radius of curvature: R ₀ and W / R ₀ << 1 is satisfied, the effective refractive index distribution of a minute region including the groove is as follows: As an approximate notation.

For example, in an aspect in which the first adjustment portion 41 is formed on the curvature center side (inner peripheral side) of the groove portion,
On the inner peripheral side wall surface: r = R ₀ − (1/2 · W)
In the lower cladding part 31, n _clad1-eff = n _clad1 · {R ₀ − (1/2 · W)} / R ₀ ;
In the first adjustment unit 41, n _1−eff = n ₁ · {R ₀ − (1/2 · W)} / R ₀ ;
At the position of W ₁ from the side wall surface on the inner peripheral side: r = R ₀ − (1/2 · W) + W ₁ ,
In the core unit 32, n _core-eff = n _core · {R ₀ − (1/2 · W) + W ₁ } / R ₀ ;
On the outer peripheral side wall surface: r = R ₀ + (1/2 · W)
In the lower cladding part 31, n _clad1-eff = n _clad1 · {R ₀ + (1/2 · W)} / R ₀ ;
In the core unit 32, n _core-eff = n _core · {R ₀ + (1/2 · W)} / R ₀
In the groove portion, the refractive index distribution exists in the depth direction, and the inner peripheral region: R ₀ − (1/2 · W) <r <R ₀ − (1/2 · W) + W ₁ At each position r, the effective refractive index n _eff (r) is n _core · {R ₀ − (1/2 · W) + W ₁ } / R ₀ <n _eff (r) <n ₁ · {R _It shows a continuous change of ₀₋ (1/2 · W)} / R ₀ .

Therefore, in the optical waveguide structure 1A, when the above effective refractive index distribution is adopted, the effective refractive index on the outer peripheral side is higher than that on the inner peripheral side in the lower cladding part 31, but in the inside of the groove part, It is possible to achieve a situation in which the effective refractive index on the inner peripheral side is relatively higher than that on the outer peripheral side. In this case, when the waveguide mode is a single mode, the center (maximum) of the electromagnetic field intensity distribution (light intensity distribution) of the propagating light is not a shape that shifts from the center of the groove part to the outer peripheral side. It can be in a state located at the center (R ₀ ).

In addition, the first adjustment portion 41 formed on the inner peripheral side wall portion reduces the effective refractive index of the core portion generated on the inner peripheral side from the center of the groove portion and the core portion generated on the outer peripheral side from the center of the groove portion. Due to the increase in the effective refractive index, it has a function to compensate for a state in which the effective refractive index is relatively lower on the inner peripheral side than the groove center compared to the outer peripheral side from the groove center. . Therefore, (n ₁ −n _core ) · W ₁ is approximately equal to 2 · {n · (W / 2) / R ₀ } · (W / 2).

  Further, the production of the optical waveguide structure 1A includes the step of forming the second adjustment portion 42 after forming only the first adjustment portion 41 on one side wall of the groove portion in the manufacturing process of producing the optical waveguide structure 1. This can be achieved by omitting and changing the step of forming the core portion 32 so as to fill the groove portion. Alternatively, in the manufacturing process for manufacturing the optical waveguide structure 1, the material for forming the second adjustment part 42 may be the same material as the material for forming the core part 32 in the step of forming the second adjustment part 42. The production of the optical waveguide structure 1A can be achieved.

  In addition, when the film is formed on the normal upper surface, the materials of the first and second adjustment portions and the core portion are made the same structure in materials having different film formation rates on the groove side surfaces, and the optical waveguide structure 1 is used in the manufacturing method. Or it is good also as a preparation method of 1A.

(Third embodiment)
A planar optical waveguide according to a third embodiment of the present invention will be described with reference to FIG. FIG. 5 is a cross-sectional view schematically showing the structure of the optical waveguide structure 1B, which is an example of the structure of the planar optical waveguide according to the third embodiment.

  As illustrated in the cross-sectional view of FIG. 5, the optical waveguide structure 1B used in the third embodiment is a planar optical waveguide having a ring shape. The optical waveguide structure 1B has a cross-sectional structure illustrated in FIG. That is, the lower clad part 31 formed on the substrate 2, the groove formed on the upper surface of the lower clad part 31, the first adjustment part 41 formed on one side of the groove part, and the core part 32 formed in the center part of the groove. The upper cladding portion 33 is formed so as to cover the upper surfaces of the lower cladding portion 31 and the core portion 32. In that case, the 1st adjustment part 41 formed in the one side of a groove part is formed in the outer peripheral side of the ring-shaped groove part.

The refractive index n _core of the core portion 32 is selected to be higher than the refractive index n _clad2 of the upper cladding portion 33 and the refractive index n _{clad1 of} the lower cladding portion 31. The first adjustment unit 41 is formed of a material having characteristics different from those of the core unit 32. The material is selected so that the refractive index: n ₁ of the first adjusting portion 41 formed on the outer peripheral side of the groove portion is lower than the refractive index: n _core of the core portion 32. At that time, the material is selected so that the refractive index n ₁ of the first adjustment unit 41 is higher than the refractive index n _clad1 of the lower cladding part 31. Therefore, the refractive index n _clad1 of the lower cladding part 31, the refractive index of the core part 32: n _core , and the refractive index: n ₁ of the first adjustment part 41 satisfy the relationship of n _core > n ₁ > n _clad1 .

In this ring-shaped planar optical waveguide, the trajectory passing through the center of the ring-shaped groove is a perfect circle having a curvature (radius): R ₀ . At that time, the cross section of the groove formed on the upper surface of the lower cladding part 31 is a rectangle having a groove depth: D and a groove width: W. The 1st adjustment part 41 formed in the outer peripheral side is in contact with the side wall surface of the outer peripheral side of a groove part, and a part of bottom face of a groove part. The interface where the first adjusting portion 41 and the core portion 32 are in contact with each other is inclined with respect to the bottom surface of the groove portion, and the inclination angle θ ₁ is set to a constant angle. Assuming that the width of the first adjusting portion 41 that contacts the bottom surface of the groove portion is W ₁ , the inclination angle θ ₁ satisfies tan θ ₁ = D / W ₁ .

The inner peripheral sidewall surface of the groove, the refractive index: the n _core of the core portion 32 is in contact the lower cladding portion 31 of the refractive index n _clad1, critical angle theta _Cin in the side wall surface of the inner peripheral side, sin [theta _Cin = (N _clad1 / n _core ) is satisfied. On the outer peripheral side wall surface of the groove, the first adjusting portion 41 having a refractive index n 1 is in _{contact with} the lower cladding portion 31 of the refractive index n clad ₁ , and the critical angle θ _Cout on the outer peripheral side wall surface is sin θ _Cout. = satisfy (n _clad1 / n _1). The critical angle θ _{C1 at} the interface between the core portion 32 having a refractive index: n _core and the first adjusting portion 41 having a refractive index: n ₁ satisfies sin θ _C1 = (n ₁ / n _core ). Therefore, θ _Cout > θ _C1 > θ _Cin .

Radiation loss (uniform bending loss) due to oozing from the outer peripheral side wall surface to the lower cladding portion 31 on the outer peripheral side in the ring-shaped planar optical waveguide not provided with the first adjustment portion 41 is the outer peripheral side. This corresponds to the radiation loss of the light component incident from the core portion 31 side at an incident angle θ _i ≦ θ _Cin . On the other hand, in a ring-shaped planar optical waveguide provided with the first adjusting portion 41 having a refractive index of n ₁ , radiation loss (one) caused by oozing from the outer peripheral side wall surface to the lower cladding portion 31 on the outer peripheral side. The bending loss) is an incident angle θ _i ≦ θ _Cout from the first adjustment unit 41 side with respect to the side wall surface on the outer peripheral side of the light component oozing from the core unit 31 side to the first adjustment unit 41 side. This corresponds to the radiation loss of the incident light component.

Next, an aspect of using the planar optical waveguide according to the third embodiment as a ring-shaped waveguide will be described with reference to FIG. 5 regarding the refractive index structure of the optical waveguide structure 1B and its operating principle. The refractive index structure of the optical waveguide structure 1B illustrated in FIG. 5 is such that the center of curvature of the curved waveguide is located on the side opposite to the side wall on which the first adjustment unit 41 is provided, and the relationship of n _core > n ₁ > n _{clad 1} It is a mode to satisfy. At this time, when the curved waveguide is formed, the locus drawn by the center of the groove is an arc shape having a radius of curvature R ₀ . Accordingly, the side wall surface on the inner peripheral side of the groove has an arc shape with a radius of curvature: R ₀ − (1/2 · W), and the side wall surface on the outer peripheral side has a radius of curvature: R ₀ + (1/2 · W). ) Arc shape.

In a range where the groove width: W is sufficiently small with respect to the radius of curvature: R ₀ and W / R ₀ << 1 is satisfied, the effective refractive index distribution of a minute region including the groove is as follows: Are expressed approximately.
On the inner peripheral side wall surface: r = R ₀ − (1/2 · W)
In the lower cladding part 31, n _clad1-eff = n _clad1 · {R ₀ − (1/2 · W)} / R ₀ ;
In the core unit 32, n _core-eff = n _core · {R ₀ − (1/2 · W)} / R ₀ ;
At the position of W ₁ from the side wall surface on the outer peripheral side: r = R ₀ + (1/2 · W) −W ₁
In the core unit 32, n _core-eff = n _core · {R ₀ + (1/2 · W) −W ₁ } / R ₀ ;
On the outer peripheral side wall surface: r = R ₀ + (1/2 · W)
In the lower cladding part 31, n _clad1-eff = n _clad1 · {R ₀ + (1/2 · W)} / R ₀ ;
In the first adjustment unit 41, n _1−eff = n ₁ · {R ₀ + (1/2 · W)} / R ₀
In the groove portion, a refractive index profile exists in the depth direction, and the outer peripheral region: R ₀ + (1/2 · W) −W ₁ <r <R ₀ + (1/2 · W) At each position r, the effective refractive index n _eff (r) is n _core · {R ₀ + (1/2 · W) −W ₁ } / R ₀ > n _eff (r)> n ₁ · {R _It shows a continuous change of 0+ (1/2 · W)} / R ₀ .

Therefore, when the above effective refractive index distribution is employed in the optical waveguide structure 1B, the effective refractive index on the outer peripheral side is higher than that on the inner peripheral side in the lower cladding part 31, but in the inside of the groove part, It is possible to achieve a situation in which the effective refractive index on the inner peripheral side is relatively higher than that on the outer peripheral side. In this case, when the waveguide mode is a single mode, the center (maximum) of the electromagnetic field intensity distribution (light intensity distribution) of the propagating light is not a shape that shifts from the center of the groove part to the outer peripheral side. It can be in a state located at the center (R ₀ ).

Temporarily, when light having a symmetric light intensity distribution showing the maximum light intensity is incident on the center of the groove portion of the ring-shaped planar optical waveguide, the light intensity of the light propagating through the ring-shaped planar optical waveguide Consider the distribution. The light intensity distribution of the light propagating through the ring-shaped planar optical waveguide not provided with the first adjustment unit 41 corresponds to the conventional mode distribution 34 shown in FIG. The light intensity distribution has a maximum on the outer peripheral side from the center. On the other hand, in the ring-shaped planar optical waveguide in which the first adjustment portion 41 having the refractive index: n ₁ is provided on the outer peripheral side with the inclination angle: θ ₁ , the light oozes from the core portion 31 side to the first adjustment portion 41 side. Ingredients are suppressed. Therefore, the maximum position of the light intensity distribution of the light propagating through the ring-shaped planar optical waveguide is shifted relatively inward as compared with the conventional mode distribution 34 shown in FIG. It will be a thing. As a result, in the light intensity distribution of the light propagating through the ring-shaped planar optical waveguide provided with the first adjusting portion 41 having the refractive index: n ₁ on the outer peripheral side, it is distributed outside from the side wall surface on the outer peripheral side of the groove portion. In the conventional mode distribution 34, the sum of the light intensities to be reduced is lower than the sum of the light intensities distributed outside the outer peripheral side wall surface of the groove. In other words, by providing the first adjusting portion 41 having a refractive index n ₁ on the outer peripheral side with an inclination angle θ ₁ , the lower cladding portion 31 on the outer peripheral side from the side wall surface on the outer peripheral side in the ring-shaped planar optical waveguide. It is possible to suppress the radiation loss due to the oozing out into the surface.

In addition, the first adjustment portion 41 formed on the outer peripheral side wall portion reduces the effective refractive index of the core portion generated on the inner peripheral side from the center of the groove portion and the effective portion of the core portion generated on the outer peripheral side from the center of the groove portion. In comparison with the inner peripheral side from the center of the groove portion, the function of canceling out the state where the effective refractive index is higher at the outer peripheral side than the center of the groove portion is relatively achieved due to the increase in the refractive index. Therefore, (n _core −n ₁ ) · W ₁ is approximately equal to 2 · {n · (W / 2) / R ₀ } · (W / 2).

In FIG. 5, as an example of the light intensity distribution of light propagating in a ring-shaped planar optical waveguide in which the first adjusting portion 41 having a refractive index: n ₁ is provided on the outer peripheral side at an inclination angle: θ ₁ When the mode is a single mode, the mode in which the center (maximum position) of the light intensity distribution (mode distribution) in the horizontal direction coincides with the center position of the groove is illustrated.

Instead of the above configuration, a ring-shaped planar optical waveguide in which the second adjustment portion 42 having a refractive index: n ₁ higher than the refractive index: n _core of the core portion 32 is provided at an inclination angle: θ ₂ on the inner peripheral side. In this embodiment, the center (maximum position) of the light intensity distribution (mode distribution) in the horizontal direction can be made to coincide with the center position of the groove. Furthermore, the refractive index of the core portion 32: the refractive index higher than n _core : the second adjusting portion 42 with n ₁ is provided on the inner peripheral side, and the refractive index of the core portion 32 lower than refractive index: n _core on the outer peripheral side: It is also possible to adopt a form of a ring-shaped planar optical waveguide provided with the n ₁ first adjusting portion 41.

As described above, when the waveguide mode is the single mode, in the optical waveguide structure 1B, when propagating through the ring-shaped waveguide, the center (maximum position) of the waveguide mode of light is changed to the conventional ring-shaped mode. The center of curvature can be shifted from the center (maximum position) of the waveguide mode in the waveguide. Utilizing this advantage, when propagating through a ring-shaped waveguide, the ratio of the portion of the light guide mode that oozes out from the outer peripheral side wall of the groove portion to the outer peripheral portion is relatively suppressed. Is possible. In this case, the uniform bending loss can be reduced without increasing the relative refractive index difference between the core portion 32 and the lower cladding portion 31 as in the conventional ring-shaped waveguide. Further, when the relative refractive index difference between the core portion 32 and the lower clad portion 31 is the same (n _core ² −n _{clad 1} ² ) / ( _{2.n core} ² ), it is as uniform as a conventional ring-shaped waveguide. In the range of bending loss, the radius of curvature (R ₀ ) of the ring shape can be made smaller.

  The ring-shaped planar optical waveguide shown in FIG. 5 constitutes a closed light propagation path. Since the inside of this closed light propagation path is not provided with a mechanism for generating light, it is necessary to supply light to be propagated from the outside. For example, a directional coupler or a multimode interferometer coupler can be used as means for injecting light to be propagated from the outside into the closed light propagation path. Conversely, a directional coupler or a multimode interferometer coupler can also be used as means for extracting the light propagating through the closed light propagation path to the outside.

(Fourth embodiment)
A planar optical waveguide according to a fourth embodiment of the present invention will be described with reference to FIG. FIG. 6 is a cross-sectional view schematically showing the structure of the optical waveguide structure 1C, which is an example of the structure of the planar optical waveguide according to the fourth embodiment. As illustrated in the cross-sectional view of FIG. 6, the optical waveguide structure 1 </ b> C used in the fourth embodiment has a waveguide structure in which a straight waveguide 302 and a curved waveguide 301 are connected.

The straight waveguide 302 includes a lower clad part 31 formed on the substrate 2, a groove formed on the upper surface of the lower clad part 31, a core part 32 formed in the groove part, and an upper surface of the lower clad part 31 and the core part 32. The upper clad part 33 is formed so as to cover it. The cross section of the groove formed on the upper surface of the lower cladding part 31 is a rectangle having a groove depth: D and a groove width: W. At that time, as shown in FIG. 1, the internal structure of the groove portion is embedded in the first adjustment portion 41 and the second adjustment portion 42 and the central portion of the groove, which are formed so as to be in contact with the side walls of the groove portion, respectively. is composed of a core portion 32 formed to the refractive index of the first adjusting portion 41: n _1S, refractive index of the second adjusting portion 42: n _2S and the refractive index of the core portion 32: n _core is equal (N _1S = n _core = n _2S ). Of course, in the straight waveguide 302, the refractive index n _core of the core portion 32 is selected to be higher than the refractive index n _clad2 of the upper cladding portion 33 and the refractive index n _{clad1 of} the lower cladding portion 31.

  On the other hand, the curved waveguide 301 has a cross-sectional structure illustrated in FIG. That is, the lower clad part 31 formed on the substrate 2, the groove formed on the upper surface of the lower clad part 31, the first adjustment part 41 formed on one side of the groove part, and the core part 32 formed in the center part of the groove. The upper cladding portion 33 is formed so as to cover the upper surfaces of the lower cladding portion 31 and the core portion 32. In that case, the 1st adjustment part 41 formed in the one side of a groove part is formed in the outer peripheral side of the groove part utilized for the curved waveguide 301. FIG.

The lower clad part 31 and the lower clad part 31 of the curved waveguide 301 are integrally formed with the lower clad part 31 and the lower clad part 31 of the linear waveguide 302, respectively. Further, the core portion 32 of the curved waveguide 301 is formed integrally with the core portion 32 of the straight waveguide 302. Therefore, also in the curved waveguide 301, the refractive index: n _core of the core portion 32 is selected to be higher than the refractive index n _clad2 of the upper cladding portion 33 and the refractive index n _{clad1 of} the lower cladding portion 31.

In the curved waveguide 301, the first adjustment portion 41 formed on the outer peripheral side of the groove portion is formed of a material having characteristics different from those of the core portion 32. Specifically, the material is selected so that the refractive index: n ₁ of the first adjusting portion 41 formed on the outer peripheral side of the groove portion is lower than the refractive index: n _core of the core portion 32. At that time, the material is selected so that the refractive index n ₁ of the first adjustment unit 41 is higher than the refractive index n _clad1 of the lower cladding part 31. Therefore, the refractive index n _clad1 of the lower cladding part 31, the refractive index of the core part 32: n _core , and the refractive index: n ₁ of the first adjustment part 41 satisfy the relationship of n _core > n ₁ > n _clad1 .

In the curved waveguide 301, the trajectory through which the center of the groove passes has an arc shape with a curvature (radius): _R0 . At this time, the cross section of the arc-shaped groove is a rectangle having a groove depth: D and a groove width: W. In the connection portion between the curved waveguide 301 and the straight waveguide 302 that are connected without providing an “offset”, the side wall surface of the arc-shaped groove portion and the side wall surface of the linear groove portion are connected without a step.

The 1st adjustment part 41 formed in the outer peripheral side is in contact with the side wall surface of the outer peripheral side of a groove part, and a part of bottom face of a groove part. The interface where the first adjusting portion 41 and the core portion 32 are in contact with each other is inclined with respect to the bottom surface of the groove portion, and the inclination angle θ ₁ is set to a constant angle. Assuming that the width of the first adjusting portion 41 that contacts the bottom surface of the groove portion is W ₁ , the inclination angle θ ₁ satisfies tan θ ₁ = D / W ₁ .

In the region of the straight waveguide 302, the inside of the rectangular groove portion has the refractive index: n _core of the core portion 32, and has a structure in contact with the lower cladding portion 31 of the refractive index n _clad1 on both side walls. Yes. Therefore, in the region of the straight waveguide 302, the horizontal refractive index distribution is symmetrical with respect to the center of the rectangular groove. Therefore, when the waveguide mode is a single mode, the horizontal light intensity distribution (mode distribution) is rectangular in the process of light propagating through the straight waveguide 302 as illustrated in FIG. A symmetrical distribution is shown in which the light intensity is maximized at the center of the groove.

  On the other hand, light of a symmetric light intensity distribution showing the maximum of light intensity is incident on the center of the groove portion from the straight waveguide 302 to the curved waveguide 301 region composed of an arc-shaped planar optical waveguide. Then, the light intensity distribution of the light propagating through the arc-shaped planar optical waveguide will be considered.

On the inner peripheral side wall surface of the arc-shaped groove, the core portion 32 of refractive index: n _core and the lower cladding portion 31 of the refractive index n _clad1 are in contact, and the critical angle θ _Cin on the inner peripheral side wall surface is , _Sin θ _Cin = (n _clad1 / n _core ). On the outer peripheral side wall surface of the groove, the first adjusting portion 41 having a refractive index n 1 is in _{contact with} the lower cladding portion 31 of the refractive index n clad ₁ , and the critical angle θ _Cout on the outer peripheral side wall surface is sin θ _Cout. = satisfy (n _clad1 / n _1). The critical angle θ _{C1 at} the interface between the core portion 32 having a refractive index: n _core and the first adjusting portion 41 having a refractive index: n ₁ satisfies sin θ _C1 = (n ₁ / n _core ). Therefore, θ _Cout > θ _C1 > θ _Cin .

The component that oozes out from the outer peripheral side wall surface into the lower cladding portion 31 on the outer peripheral side in the arc-shaped planar optical waveguide not provided with the first adjusting portion 41 is incident angle θ on the outer peripheral side wall surface. It corresponds to the light component incident from the core portion 31 side with _i ≦ θ _Cin . On the other hand, the component that exudes from the outer peripheral side wall surface to the lower cladding portion 31 on the outer peripheral side in the arc-shaped planar optical waveguide provided with the first adjusting portion 41 of refractive index: n ₁ is the core portion 31 side. Among the light components that ooze out from the first adjustment unit 41 to the first adjustment unit 41 side, the light component is incident on the outer peripheral side wall surface from the first adjustment unit 41 side at an incident angle θ _i ≦ θ _Cout . The light component that oozes from the core part 31 side to the first adjustment part 41 side is incident on the interface between the core part 31 and the first adjustment part 41 at an incident angle θ _i ≦ θ _C1 from the core part 31 side. Corresponds to the ingredients.

Next, an aspect of using the planar optical waveguide according to the fourth embodiment as a curved waveguide will be described with reference to FIG. 6 regarding the refractive index structure of the optical waveguide structure 1C and its operating principle. In the refractive index structure of the optical waveguide structure 1B illustrated in FIG. 6, the center of curvature of the curved waveguide is located on the side opposite to the side wall on which the first adjustment unit 41 is provided, and the relationship of n _core > n ₁ > n _{clad 1 is satisfied} . It is a mode to satisfy. At this time, when the curved waveguide is formed, the locus drawn by the center of the groove is an arc shape having a radius of curvature R ₀ . Accordingly, the side wall surface on the inner peripheral side of the groove has an arc shape with a radius of curvature: R ₀ − (1/2 · W), and the side wall surface on the outer peripheral side has a radius of curvature: R ₀ + (1/2 · W). ) Arc shape.

In a range where the groove width: W is sufficiently small with respect to the radius of curvature: R ₀ and W / R ₀ << 1 is satisfied, the effective refractive index distribution of a minute region including the groove is as follows: Are expressed approximately.
On the inner peripheral side wall surface: r = R ₀ − (1/2 · W)
In the lower cladding part 31, n _clad1-eff = n _clad1 · {R ₀ − (1/2 · W)} / R ₀ ;
In the core unit 32, n _core-eff = n _core · {R ₀ − (1/2 · W)} / R ₀ ;
At the position of W ₁ from the side wall surface on the outer peripheral side: r = R ₀ + (1/2 · W) −W ₁
In the core unit 32, n _core-eff = n _core · {R ₀ + (1/2 · W) −W ₁ } / R ₀ ;
On the outer peripheral side wall surface: r = R ₀ + (1/2 · W)
In the lower cladding part 31, n _clad1-eff = n _clad1 · {R ₀ + (1/2 · W)} / R ₀ ;
In the first adjustment unit 41, n _1−eff = n ₁ · {R ₀ + (1/2 · W)} / R ₀
In the groove portion, a refractive index profile exists in the depth direction, and the outer peripheral region: R ₀ + (1/2 · W) −W ₁ <r <R ₀ + (1/2 · W) At each position r, the effective refractive index n _eff (r) is n _core · {R ₀ + (1/2 · W) −W ₁ } / R ₀ > n _eff (r)> n ₁ · {R _It shows a continuous change of 0+ (1/2 · W)} / R ₀ .

Therefore, when the above effective refractive index distribution is adopted in the optical waveguide structure 1C, the effective refractive index on the outer peripheral side is higher in the lower cladding portion 31 than in the inner peripheral side in the curved waveguide 301. It is possible to achieve a situation where the effective refractive index on the inner peripheral side is relatively higher than that on the outer peripheral side inside the groove. In this case, when the waveguide mode is a single mode, the center (maximum) of the electromagnetic field intensity distribution (light intensity distribution) of the propagating light is not a shape that shifts from the center of the groove part to the outer peripheral side. It can be in a state located at the center (R ₀ ).

If a planar optical waveguide structure in which the first adjustment unit 41 is not provided is adopted, when the waveguide mode is a single mode, the light intensity distribution of light propagating in the circular arc-shaped planar optical waveguide is as shown in FIG. The center (maximum) of the light intensity distribution is provided on the outer peripheral side from the center of the core portion 32 so as to correspond to the conventional mode distribution 34 shown in FIG. On the other hand, in the ring-shaped planar optical waveguide in which the first adjustment portion 41 having the refractive index: n ₁ is provided on the outer peripheral side with the inclination angle: θ ₁ , the light oozes from the core portion 31 side to the first adjustment portion 41 side. Ingredients are suppressed. Therefore, when the waveguide mode is a single mode, the position of the center (maximum) of the light intensity distribution of the light propagating through the arc-shaped planar optical waveguide is the conventional mode distribution 34 shown in FIG. As compared with the above, it is relatively shifted to the inner peripheral side.

In addition, the first adjustment portion 41 formed on the outer peripheral side wall portion reduces the effective refractive index of the core portion generated on the inner peripheral side from the center of the groove portion and the effective portion of the core portion generated on the outer peripheral side from the center of the groove portion. In comparison with the inner peripheral side from the center of the groove portion, the function of canceling out the state where the effective refractive index is higher at the outer peripheral side than the center of the groove portion is relatively achieved due to the increase in the refractive index. Therefore, (n _core −n ₁ ) · W ₁ is approximately equal to 2 · {n · (W / 2) / R ₀ } · (W / 2).

In FIG. 6, when the waveguide mode is a single mode, the light propagates through a curved waveguide 301 formed by an arc-shaped planar optical waveguide provided with a first adjusting portion 41 having a refractive index of n ₁ on the outer peripheral side. As an example of the light intensity distribution of the light to be transmitted, a mode in which the center (maximum position) of the light intensity distribution (mode distribution) in the horizontal direction coincides with the center position of the groove portion is illustrated. In this aspect, the center (maximum position) of the horizontal light intensity distribution of light propagating through the straight waveguide 302 and the horizontal light intensity distribution of light propagating through the curved waveguide 301 are both It coincides with the center position of the groove. In other words, an “offset” is provided at the coupling portion between the straight waveguide and the curved waveguide, and the center (maximum position) of the light intensity distribution in the horizontal direction of the light propagating through the straight waveguide is propagated through the curved waveguide. Similar to the case where the centers (maximum positions) of the light intensity distributions in the horizontal direction of light are matched, in the embodiment shown in FIG. 6, the radiation loss (mode conversion loss) at the coupling portion between the straight waveguide and the curved waveguide is also reduced. Can be reduced.

In the aspect illustrated in FIG. 6, the cross-sectional structure of the curved waveguide 301 uses a configuration in which the first adjustment portion 41 having a refractive index n ₁ lower than the refractive index n _core of the core portion 32 is provided on the outer peripheral side of the groove portion. doing. Instead, as the cross-sectional structure of the curved waveguide 301, the first adjusting portion 41 having a refractive index n ₁ lower than the refractive index n _core of the core portion 32 is provided on the outer peripheral side of the groove portion on the inner peripheral side of the groove portion. It is also possible to employ a configuration in which the second adjustment section 42 having a refractive index n ₂ higher than the refractive index n _core of the core section 32 is provided. Alternatively, as the cross-sectional structure of the curved waveguide 301, a configuration in which the first adjusting portion 41 having a refractive index n ₁ higher than the refractive index n _core of the core portion 32 can be adopted on the inner peripheral side of the groove portion.

  As described above, in the optical waveguide structure 1C, the central axes of the mode distributions of the curved waveguide and the straight waveguide can be matched, so that the straight waveguide and the curved waveguide are connected without performing “offset” correction. In the form, mode conversion loss can be suppressed.

(Fifth embodiment)
A planar optical waveguide according to the fifth embodiment of the present invention will be described with reference to FIG. FIG. 7 is a cross-sectional view schematically showing the structure of an optical waveguide structure 1D, which is an example of the structure of a planar optical waveguide according to the fifth embodiment. As shown in the cross-sectional view of FIG. 7, the optical waveguide structure 1D used in the fifth embodiment has a waveguide structure in which a straight waveguide 302, a straight waveguide 303, and a straight waveguide 304 are connected. doing.

The straight waveguide 302 includes a lower clad part 31 formed on the substrate 2, a groove formed on the upper surface of the lower clad part 31, a core part 32 formed in the groove part, and an upper surface of the lower clad part 31 and the core part 32. The upper clad part 33 is formed so as to cover it. The cross section of the groove formed on the upper surface of the lower cladding part 31 is a rectangle having a groove depth: D and a groove width: W. The rectangular groove portion is occupied by the core portion 32. At this time, in the straight waveguide 302, the refractive index: n _core of the core portion 32 is selected to be higher than the refractive index n _clad2 of the upper cladding portion 33 and the refractive index n _{clad1 of} the lower cladding portion 31.

  Therefore, in the region of the straight waveguide 302, the horizontal refractive index distribution is symmetrical with respect to the center of the rectangular groove. Therefore, as illustrated in FIG. 7, in the process of light propagating through the straight waveguide 302, the horizontal light intensity distribution (mode distribution) shows the maximum light intensity at the center of the rectangular groove. Shows a symmetrical distribution.

The linear waveguide 302 area, the sidewall surface of the groove, the refractive index: the lower cladding portion 31 of the core portion 32 and the refractive index n _clad1 of n _core is in contact, the critical angle theta _CS1 in the side wall surface, sin [theta _CS1 = (N _clad1 / n _core ) is satisfied. Accordingly, light incident at an incident angle θ _i <θ _CS1 from the core portion 32 side with respect to the side wall surface passes through the side wall surface of the groove portion and is dissipated to the lower cladding portion 31. As a result, in the process in which light propagates through the straight waveguide 302, the light component incident at the incident angle θ _i ≧ θ _CS1 from the core portion 32 side with respect to the side wall surface is confined in the core portion 32. .

  In the next straight waveguide 303 as well, the cross section of the groove formed on the upper surface of the lower cladding part 31 is a rectangle having a groove depth: D and a groove width: W. As shown in FIG. 1, the internal structure of the groove is formed so as to embed the first adjustment part 41 and the second adjustment part 42 and the central part of the groove, which are formed so as to be in contact with the side walls of the groove, respectively. The core part 32 is configured.

In the straight waveguide 303, both the first adjustment part 41 and the second adjustment part 42 formed on the side wall side of the groove part are formed of a material having different characteristics from the core part 32. Specifically, the refractive index: n _1A of the first adjustment part 41 formed on one side of the groove part than the refractive index: n _core of the core part 32 and the refractive index: n _2A of the second adjustment part 42 are: Select materials so that both are low. At that time, materials are selected so that the refractive index: n _1A of the first adjustment unit 41 and the refractive index: n _2A of the second adjustment unit 42 are both higher than the refractive index n _clad1 of the lower cladding part 31. Therefore, the refractive index n _clad1 of the lower cladding part 31, the refractive index of the core part 32: n _core , the refractive index of the first adjusting part 41: n _1A and the refractive index of the second adjusting part 42: n _2A are n The relationship of _core > n _1A > n _clad1 and n _core > n _2A > n _clad1 is satisfied.

The 1st adjustment part 41 formed in the one side of the groove part is in contact with the side wall surface of a groove part, and a part of bottom face of a groove part. The interface where the first adjusting portion 41 and the core portion 32 are in contact with each other is inclined with respect to the bottom surface of the groove portion, and the inclination angle θ _1A is set to a constant angle. Assuming that the width of the first adjusting portion 41 contacting the bottom surface of the groove portion is W _1A , the inclination angle θ _1A satisfies tan θ _1A = D / W _1A . Moreover, the 2nd adjustment part 42 formed in the other one side of a groove part is in contact with the side wall surface of a groove part, and a part of bottom face of a groove part. The interface where the second adjustment portion 42 and the core portion 32 are in contact with each other is inclined with respect to the bottom surface of the groove portion, and the inclination angle θ _2A is set to a constant angle. Assuming that the width of the second adjusting portion 42 contacting the bottom surface of the groove portion is W _2A , the inclination angle θ _2A satisfies tan θ _2A = D / W _2A .

Also in the region of the straight waveguide 303, the horizontal refractive index distribution is symmetrical with respect to the center of the rectangular groove. Specifically, the refractive index: n _1A of the first adjustment unit 41 and the refractive index: n _2A of the second adjustment unit 42 are made equal, and the inclination angles: θ _1A and θ _2A are made equal. Therefore, when the waveguide mode is a single mode, the light intensity distribution (mode distribution) in the horizontal direction is rectangular in the process of light propagating through the linear waveguide 303 as illustrated in FIG. A symmetrical distribution is shown in which the light intensity is maximized at the center of the groove.

In the straight waveguide 303 region, the first adjusting portion 41 having a refractive index of n _1A or the second adjusting portion having a refractive index of n _2A is in _{contact with} the lower cladding portion 31 of the refractive index n _{clad1 on} the side wall surface of the groove. and which, critical angle at the side wall surface: theta _CSA1, theta _{CSA 2 is, sinθ CSA1 = (n clad1 /} n 1A), sinθ CSA2 = satisfy (n _clad1 / n _2A). since it is n _1A = n _2A, the critical angle at the side wall surface has a θ _CSA1 = θ _CSA2. Thus, for the side wall surface, light incident at the first adjusting portion 41 (or the second adjusting portion 42) incident angle theta _i from <theta _CSA1 (or θ _i <θ _CSA2) passes through the side wall surfaces of the groove Then, it dissipates to the lower cladding part 31.

  Further, in the straight waveguide 304 as well, the cross section of the groove formed on the upper surface of the lower cladding part 31 is a rectangle having a groove depth: D and a groove width: W. As shown in FIG. 1, the internal structure of the groove is formed so as to embed the first adjustment part 41 and the second adjustment part 42 and the central part of the groove, which are formed so as to be in contact with the side walls of the groove, respectively. The core part 32 is configured.

In the straight waveguide 304 as well, the first adjustment part 41 and the second adjustment part 42 formed on the side wall side of the groove part are both made of a material having different characteristics from the core part 32. Specifically, the refractive index: n _1BA of the first adjustment part 41 formed on one side of the groove part than the refractive index: n _core of the core part 32, and the refractive index: n _2B of the second adjustment part 42 are: Select materials so that both are low. At that time, materials are selected so that the refractive index: n _1B of the first adjustment unit 41 and the refractive index: n _2B of the second adjustment unit 42 are both higher than the refractive index n _clad1 of the lower cladding part 31. Therefore, the refractive index n _clad1 of the lower cladding part 31, the refractive index of the core part 32: n _core , the refractive index of the first adjustment part 41: n _1B, and the refractive index of the second adjustment part 42: n _2B are n The relationship of _core > n _1B > n _clad1 and n _core > n _2B > n _clad1 is satisfied.

The 1st adjustment part 41 formed in the one side of the groove part is in contact with the side wall surface of a groove part, and a part of bottom face of a groove part. The interface between the first adjusting portion 41 and the core portion 32 is inclined with respect to the bottom surface of the groove portion, and the inclination angle θ _1B is set to a constant angle. Assuming that the width of the first adjusting portion 41 that contacts the bottom surface of the groove portion is W _1B , the inclination angle θ _1B satisfies tan θ _1B = D / W _1B . Moreover, the 2nd adjustment part 42 formed in the other one side of a groove part is in contact with the side wall surface of a groove part, and a part of bottom face of a groove part. The interface where the second adjusting portion 42 and the core portion 32 are in contact with each other is inclined with respect to the bottom surface of the groove portion, and the inclination angle θ _2B is set to a constant angle. Assuming that the width of the second adjusting portion 42 that contacts the bottom surface of the groove portion is W _2B , the inclination angle θ _2B satisfies tan θ _2B = D / W _2B .

Also in the region of the straight waveguide 304, the horizontal refractive index distribution is symmetrical with respect to the center of the rectangular groove. Specifically, the refractive index: n _1B of the first adjustment unit 41 and the refractive index: n _2B of the second adjustment unit 42 are made equal, and the inclination angles: θ _1B and θ _2B are made equal. Therefore, when the waveguide mode is a single mode, as illustrated in FIG. 7, the light intensity distribution (mode distribution) in the horizontal direction in the process of light propagating through the straight waveguide 304 is rectangular. A symmetrical distribution is shown in which the light intensity is maximized at the center of the groove.

At that time, the refractive index: n _1B of the first adjustment unit 41 and the refractive index: n _2B of the second adjustment unit 42 in the region of the straight waveguide 304 are the same as those of the first adjustment unit 41 in the region of the straight waveguide 303. The refractive index: n _1A is equal to the refractive index of the second adjusting section 42: n _2A . On the other hand, the inclination angles: θ _1B and θ _2B in the region of the straight waveguide 304 are smaller than the inclination angles: θ _1A and θ _2A in the region of the straight waveguide 303.

In the region of the straight waveguide 304, on the side wall surface of the groove portion, the first adjustment portion 41 having a refractive index: n _1B or the second adjustment portion 42 having a refractive index: n _2B and the lower cladding portion 31 of the refractive index n _clad1 are formed. in contact, the critical angle at the side wall surface: theta _CSB1, theta _{CSB2 is, sinθ CSB1 = (n clad1 /} n 1B), sinθ CSB2 = satisfy (n _clad1 / n _2B). since it is _{n 1B = n 2B = n 1A} = n 2A, the critical angle at the side wall surface has a _{θ CSB1 = θ CSB2 = θ CSA1} = θ CSA2. Thus, for the side wall surface, light incident at the first adjusting portion 41 (or the second adjusting portion 42) incident angle theta _i from <theta _CSB1 (or θ _i <θ _CSB2) passes through the side wall surfaces of the groove Then, it dissipates to the lower cladding part 31.

  When the waveguide mode is a single mode, as illustrated in FIG. 7, in the process in which light propagates through the straight waveguide 302, the horizontal light intensity distribution (mode distribution) of the rectangular groove is The peak of the light intensity is shown at the center, and a relatively sharp peak shape with few light components existing outside the side wall surfaces on both sides is shown. Even in the process of propagating through the next-stage straight waveguide 303, the horizontal light intensity distribution (mode distribution) shows a symmetrical distribution, but the ratio of the light components existing outside the side wall surfaces on both sides increases. , Relatively broadening of the peak shape. Further, even in the process of propagating through the straight waveguide 304, the light intensity distribution (mode distribution) in the horizontal direction is a symmetrical distribution, but the ratio of the light components existing outside the side wall surfaces on both sides further increases. Further, the decrease in the maximum value of the light intensity also proceeds remarkably. As a result, the relative peak shape is greatly expanded. When the width (full width at half maximum) that is ½ of the maximum value of the light intensity is used as an index of the spot size, light propagates sequentially through the straight waveguide 302, the straight waveguide 303, and the straight waveguide 304. In the process of doing so, the spot size gradually increases.

In the aspect illustrated in FIG. 7, the effective refractive index distribution in the groove portion in the linear waveguide 302, the linear waveguide 303, and the linear waveguide 304 is as follows. First, in the region of the straight waveguide 302, the refractive index in the groove is constant at the refractive index of the core 32: n _core between both side walls of the groove.

In the region of the straight waveguide 303, the width of W _1A from the side wall surface on which the first adjustment portion 41 is formed is linear from the refractive index n _1A of the first adjustment portion 41 on the side wall surface to the width of W _1A. Further, the effective refractive index increases up to the refractive index of the core portion 32: n _core . Similarly, in the width of W _2A from the side wall surface on which the second adjustment portion 42 is formed, the refractive index of the second adjustment portion 42 on the side wall surface: n _2A , linearly with the width of W _2A of the core portion 32. Refractive index: The effective refractive index increases up to n _core . The central portion of the groove, the width portion of (W−W _1A −W _2A ) is the refractive index of the core portion 32: n _core .

In the region of the straight waveguide 304, the width of W _1B from the side wall surface on which the first adjustment portion 41 is formed is linear from the refractive index n _1B of the first adjustment portion 41 on the side wall surface to the width of W _1B. Further, the effective refractive index increases up to the refractive index of the core portion 32: n _core . Similarly, in the width of W _2B from the side wall surface on which the second adjustment portion 42 is formed, the refractive index of the second adjustment portion 42 on the side wall surface: n _2B , linearly with the width of W _2B of the core portion 32. Refractive index: The effective refractive index increases up to n _core . The center portion of the groove, the width portion of (W−W _1B −W _2B ) is the refractive index of the core portion 32: n _core .

Instead of the above-described embodiment, the width W _1B of the first adjustment unit 41 and the width W _2B of the second adjustment unit 42 in the straight waveguide 304 are set as the width W _1B of the first adjustment unit 41 in the straight waveguide 303. _1A is equal to the width W _{2A of} the second adjustment unit 42, and the refractive indexes n _1B and n _2B of the first adjustment unit 41 and the second adjustment unit 42 in the linear waveguide 304 are set as the first adjustment in the linear waveguide 303. The refractive index of the part 41 and the second adjustment part 42 may be lower than n _1A and n _2A . Also in this aspect, when the width (full width at half maximum) that is half the light intensity maximum value is used as an index of the spot size, the light is linear waveguide 302, linear waveguide 303, linear waveguide. In the process of sequentially propagating with 304, the spot size gradually increases.

  Furthermore, the width of the first adjustment unit 41 and the width of the second adjustment unit 42 and the refractive index of the first adjustment unit 41 and the second adjustment unit 42 may be changed simultaneously along the pointer. In the process in which light sequentially propagates through the straight waveguide 302, the straight waveguide 303, and the straight waveguide 304, the spot size gradually increases.

As described above, the linear waveguide provided with the same width of the first and second adjustment portions having the same refractive index on both side walls of the groove portion without changing the width W of the groove portion itself; W ₁ = W ₂ Can be connected to form a spot size converter. At this time, the spot size to be converted can be adjusted by adjusting the widths W ₁ and W ₂ of the _first and second adjusting portions provided on both side walls of the groove.

  The optical waveguide according to the present invention can be used, for example, when an optical integrated circuit used for optical interconnection represented by an intra-area network, an inter-city network, etc., and an optical interconnection applied to a server / router, etc. It can be used as a planar optical waveguide including a curved waveguide region that functions as an optical propagation path in the process of optical signal processing such as optical signal demultiplexing / multiplexing and transmission / reception operations.

It is sectional drawing which shows typically an example of the structure of the optical waveguide concerning the 1st Embodiment of this invention. It is a figure which shows typically the refractive index structure of the optical waveguide concerning the 1st Embodiment of this invention, and the light intensity distribution in a curve waveguide. It is sectional drawing which shows typically an example of the manufacturing process of the optical waveguide concerning the 1st Embodiment of this invention. It is sectional drawing which shows typically an example of the structure of the optical waveguide concerning the 2nd Embodiment of this invention. It is a figure which shows typically an example of the structure of the optical waveguide concerning the 3rd Embodiment of this invention. It is a figure which shows typically an example of the structure of the optical waveguide concerning the 4th Embodiment of this invention. It is a figure which shows typically an example of the structure of the optical waveguide by the 5th Embodiment of this invention.

Explanation of symbols

1, 1A, 1B, 1C, 1D Optical waveguide structure 2 Substrate 3 Optical waveguide 301 Curved waveguides 302, 303, 304 Linear waveguide 31, 51 Lower cladding part 32, 52 Core part 33, 53 Upper cladding part 34 Conventional structure Mode distribution 35 of a curved waveguide in an optical waveguide Mode distribution 41 of a curved waveguide in an optical waveguide according to the present invention 41 First adjustment unit 42 Second adjustment unit

Claims (9)

And the substrate, are formed on the substrate, a lower cladding portion of the refractive index n _clad1, the core part of the refractive index n _core, an optical waveguide made comprises an upper cladding portion of the refractive index n _clad2,
The optical waveguide is a planar optical waveguide comprising a rectangular groove region having a width W and a depth D, constituting a light propagation path,
The lower surface, left side surface, and right side surface of the groove region having a rectangular cross section constitute an interface in contact with the lower cladding part,
With respect to the wavelength lambda ₀ of the light in vacuum propagating in the optical waveguide, the refractive index n _clad1 the lower cladding portion, the refractive index n _core, the refractive index n _clad2 the upper cladding portion of the core portion, n _core> n _clad1 and n _core > n _clad2 are selected to satisfy the relationship,
The cross-sectional shape of the core part is
The upper surface of the core part is in contact with the upper cladding part to form a flat interface,
The lower surface of the core portion is in contact with the lower cladding portion at least in the groove region, and forms a flat interface,
One of the left side and the right side of the groove region is
It is composed of an interface where the lower cladding part and the first adjusting part having a refractive index n ₁ are in contact
In the lower surface of the groove region, the first adjustment portion is in contact with the lower cladding portion and forms a flat interface having a width W ₁ .
In the groove region, the first adjustment portion forms an interface in contact with the core portion,
The interface between the first adjustment portion and the core portion intersects the lower surface of the groove region at an inclination angle θ ₁ , and the inclination angle θ ₁ is selected as θ ₁ <90 °. ,
The cross-sectional shape of the first adjustment portion includes an interface where the lower cladding portion and the first adjustment portion are in contact with each other on one side surface of the groove region, an interface where the first adjustment portion and the core portion are in contact, and the groove region. Is formed by an interface in contact with the first adjustment portion and the lower clad portion,
The left side of the groove region, the remaining one of the right side,
It is composed of an interface where the lower clad part and the second adjustment part having a refractive index n ₂ are in contact
In the lower surface of the groove region, the second adjustment portion is in contact with the lower cladding portion and forms a flat interface having a width W ₂ .
In the groove region, the second adjustment part forms an interface in contact with the core part,
The interface between the second adjusting portion and the core portion intersects the lower surface of the groove region at an inclination angle θ ₂ , and the inclination angle θ ₂ is selected as θ ₂ <90 °. ,
The cross-sectional shape of the second adjusting portion is such that, on the other side surface of the groove region, either the lower cladding portion or the upper cladding portion is in contact with the second adjusting portion, and the second adjusting portion and the core portion are in contact with each other. An interface, and an interface in contact with the second adjustment portion and the lower cladding portion in the lower surface of the groove region;
The refractive index n ₁ of the first adjustment unit is
At least satisfy the relationship of n ₁ > n _clad1 with respect to the refractive index n _clad1 of the lower cladding part ,
For the refractive index n _core of the core part, the relationship of n ₁ ≠ n _core is satisfied,
The refractive index n ₂ of the second adjustment unit is
At least satisfy the relationship of n ₂ > n _clad1 with respect to the refractive index n _clad1 of the lower cladding part ,
_Satisfies the relationship of n ₂ ≠ n _core with respect to the refractive index n _core of the core part ;
The refractive index n ₁ of the first adjustment unit and the refractive index n ₂ of the second adjustment unit are
Satisfy at least the relationship of n ₁ ≠ n ₂ ,
The planar optical waveguide is a curved waveguide;
In the curved waveguide,
A first adjustment portion having a refractive index n ₁ is provided on the outer peripheral side surface of the groove region.
A second adjusting portion having a refractive index n ₂ is disposed on the inner peripheral side surface of the groove region;
The refractive index n ₁ of the first adjustment unit and the refractive index n ₂ of the second adjustment unit are
An optical waveguide characterized by being selected so as to satisfy a relationship of n ₂ > n _core > n ₁ with respect to the refractive index n _core of the core portion. And the substrate, are formed on the substrate, a lower cladding portion of the refractive index n _clad1, the core part of the refractive index n _core, an optical waveguide made comprises an upper cladding portion of the refractive index n _clad2,
The optical waveguide is a planar optical waveguide comprising a rectangular groove region having a width W and a depth D, constituting a light propagation path,
The lower surface, left side surface, and right side surface of the groove region having a rectangular cross section constitute an interface in contact with the lower cladding part,
With respect to the wavelength lambda ₀ of the light in vacuum propagating in the optical waveguide, the refractive index n _clad1 the lower cladding portion, the refractive index n _core, the refractive index n _clad2 the upper cladding portion of the core portion, n _core> n _clad1 and n _core > n _clad2 are selected to satisfy the relationship,
The cross-sectional shape of the core part is
The upper surface of the core part is in contact with the upper cladding part to form a flat interface,
The lower surface of the core part is in contact with the lower cladding part in at least the region where the cross section is rectangular, and forms a flat interface,
One of the left side and the right side of the groove region is
It is composed of an interface where the lower cladding part and the first adjusting part having a refractive index n ₁ are in contact
In the lower surface of the groove region, the first adjustment portion is in contact with the lower cladding portion and forms a flat interface having a width W ₁ .
In the groove region, the first adjustment portion forms an interface in contact with the core portion,
The interface between the first adjustment portion and the core portion intersects the lower surface of the groove region at an inclination angle θ ₁ , and the inclination angle θ ₁ is selected as θ ₁ <90 °. ,
The cross-sectional shape of the first adjustment portion is such that, on one side surface of the groove region, an interface between the lower cladding portion and the first adjustment portion, an interface between the first adjustment portion and the core portion, and the groove In the lower surface of the region, formed by an interface in contact with the first adjustment portion and the lower cladding portion,
The left side of the groove region, the remaining one of the right side,
It consists of the interface where the lower cladding and the core are in contact;
The refractive index n ₁ of the first adjustment unit is
At least satisfy the relationship of n ₁ > n _clad1 with respect to the refractive index n _clad1 of the lower cladding part ,
_Satisfies the relationship of n ₁ ≠ n _core with respect to the refractive index n _core of the core part ;
The planar optical waveguide is a curved waveguide;
In the curved waveguide,
A first adjustment portion having a refractive index n ₁ is disposed on the outer peripheral side surface of the groove region,
The refractive index n ₁ of the first adjustment unit is
An optical waveguide characterized by being selected so as to satisfy a relationship of n _core > n ₁ with respect to the refractive index n _core of the core portion. And the substrate, are formed on the substrate, a lower cladding portion of the refractive index n _clad1, the core part of the refractive index n _core, an optical waveguide made comprises an upper cladding portion of the refractive index n _clad2,
The optical waveguide is a planar optical waveguide comprising a rectangular groove region having a width W and a depth D, constituting a light propagation path,
The lower surface, left side surface, and right side surface of the groove region having a rectangular cross section constitute an interface in contact with the lower cladding part,
With respect to the wavelength lambda ₀ of the light in vacuum propagating in the optical waveguide, the refractive index n _clad1 the lower cladding portion, the refractive index n _core, the refractive index n _clad2 the upper cladding portion of the core portion, n _core> n _clad1 and n _core > n _clad2 are selected to satisfy the relationship,
The cross-sectional shape of the core part is
The upper surface of the core part is in contact with the upper cladding part to form a flat interface,
The lower surface of the core part is in contact with the lower cladding part in at least the region where the cross section is rectangular, and forms a flat interface,
One of the left side and the right side of the groove region is
It is composed of an interface where the lower cladding part and the first adjusting part having a refractive index n ₁ are in contact
In the lower surface of the groove region, the first adjustment portion is in contact with the lower cladding portion and forms a flat interface having a width W ₁ .
In the groove region, the first adjustment portion forms an interface in contact with the core portion,
The interface between the first adjustment portion and the core portion intersects the lower surface of the groove region at an inclination angle θ ₁ , and the inclination angle θ ₁ is selected as θ ₁ <90 °. ,
The cross-sectional shape of the first adjustment portion is such that, on one side surface of the groove region, an interface between the lower cladding portion and the first adjustment portion, an interface between the first adjustment portion and the core portion, and the groove In the lower surface of the region, formed by an interface in contact with the first adjustment portion and the lower cladding portion,
The left side of the groove region, the remaining one of the right side,
It consists of the interface where the lower cladding and the core are in contact;
The refractive index n ₁ of the first adjustment unit is
At least satisfy the relationship of n ₁ > n _clad1 with respect to the refractive index n _clad1 of the lower cladding part ,
_Satisfies the relationship of n ₁ ≠ n _core with respect to the refractive index n _core of the core part ;
The planar optical waveguide is a curved waveguide;
In the curved waveguide,
A first adjusting portion having a refractive index n ₁ is disposed on the inner peripheral side surface of the groove region;
The refractive index n ₁ of the first adjustment unit is
An optical waveguide characterized by being selected so as to satisfy the relationship of n _core <n ₁ with respect to the refractive index n _core of the core portion. An optical waveguide in which a straight waveguide portion and a curved waveguide portion are connected,
The straight waveguide portion is
A substrate, a lower clad portion having a refractive index n _clad1 formed on the substrate, a core portion having a refractive index n _core , and an upper clad portion having a refractive index n _clad2 ;
A planar optical waveguide comprising a rectangular groove region having a width W and a depth D, constituting a light propagation path,
The lower surface, left side surface, and right side surface of the groove region having a rectangular cross section constitute an interface in contact with the lower cladding part,
With respect to the wavelength lambda ₀ of the light in vacuum propagating in the optical waveguide, the refractive index n _clad1 the lower cladding portion, the refractive index n _core, the refractive index n _clad2 the upper cladding portion of the core portion, n _core> n _clad1 and n _core > n _clad2 are selected to satisfy the relationship,
The cross-sectional shape of the core part is
The upper surface of the core part is in contact with the upper cladding part to form a flat interface,
The lower surface of the core portion is in contact with the lower cladding portion at least in the groove region, and forms a flat interface,
Both the left side surface and the right side surface of the groove region,
It consists of the interface where the lower cladding and the core are in contact;
The curved waveguide portion is the optical waveguide according to any one of claims 1, 2 , and 3;
The connection between the straight waveguide portion and the curved waveguide portion is as follows:
In the straight waveguide portion, a cross-section having a width W and a depth D forming a light propagation path and a cross-section having a width W forming a light propagation path in the curved waveguide portion. An optical waveguide characterized in that a rectangular groove region having a depth D is connected in such a manner that the lower surface, the left side surface, and the right side surface of both are matched. An optical waveguide formed by connecting two curved waveguide portions having different curvatures,
The two curved waveguide portions having different curvatures are optical waveguides according to any one of claims 1, 2 , and 3;
The connection between the two curved waveguide sections is
In one curved waveguide portion, a light propagation path is formed. A rectangular groove region having a cross section of width W and depth D, and in the other curved waveguide portion, a light propagation path is formed. An optical waveguide characterized in that a rectangular groove region having a width W and a depth D is connected in such a manner that the lower surface, the left side surface, and the right side surface of the two regions coincide with each other. An optical waveguide formed by connecting two curved waveguide portions having different bending directions,
The two curved waveguide portions having different bending directions are optical waveguides according to any one of claims 1, 2 , and 3;
The connection between the two curved waveguide sections is
In one curved waveguide portion, a light propagation path is formed. A rectangular groove region having a cross section of width W and depth D, and in the other curved waveguide portion, a light propagation path is formed. An optical waveguide characterized in that a rectangular groove region having a width W and a depth D is connected in such a manner that the lower surface, the left side surface, and the right side surface of the two regions coincide with each other. In the rectangular groove region having a cross section of width W and depth D,
The optical waveguide according to any one of claims 1 to 6, wherein the width W and the depth D are selected in a range of W> D. A method for producing an optical waveguide according to claim 1, comprising:
_Forming a lower clad portion of refractive index n _clad1 on a substrate;
A step of forming a rectangular groove region having a cross section of width W and depth D on the upper surface of the lower clad portion;
A first adjustment member is selectively formed on one side of the groove region so as to cover the side surface and the lower surface of the groove region with a width W ₁ from the side surface on one side of the first adjustment unit, Forming a first adjusting portion having a refractive index n ₁ ;
A second adjustment material is selectively formed on the side surface of the groove region so that the left side surface and the right side surface of the groove region are covered with the side surface and the lower surface of the groove region with a width W ₂ from the side surface. And a step of forming a second adjusting portion having a refractive index n ₂ ;
In the groove region, a core portion having a refractive index n _core is formed so as to cover the film formation surface of the first adjustment portion, the film formation surface of the second adjustment portion, and the lower surface of the groove region, and to fill the groove region. Forming a film and forming an interface where the core portion and the first adjustment portion are in contact, and an interface where the core portion and the second adjustment portion are in contact;
An optical waveguide manufacturing method comprising a step of forming a film of an upper clad portion of a refractive index n _clad2 so as to cover an upper surface of a core portion and an upper surface of a lower clad portion. A method of manufacturing an optical waveguide according to claim 2 or claim 3 ,
_Forming a lower clad portion of refractive index n _clad1 on a substrate;
A step of forming a rectangular groove region having a cross section of width W and depth D on the upper surface of the lower clad portion;
A first adjustment member is selectively formed on one side of the groove region so as to cover the side surface and the lower surface of the groove region with a width W ₁ from the side surface on one side of the first adjustment unit, Forming a first adjusting portion having a refractive index n ₁ ;
In the groove area, the left side surface of the groove area, while remaining on the right side, the deposition surface of the first adjusting portion, and covers the lower surface of the groove area to fill the trench region, the refractive index n _core Forming a core part and forming an interface between the core part and the first adjustment part;
An optical waveguide manufacturing method comprising a step of forming a film of an upper clad portion of a refractive index n _clad2 so as to cover an upper surface of a core portion and an upper surface of a lower clad portion.

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