JP2014123044A - Optical waveguide circuit and fabrication method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve such problems that when an optical directional coupler is constituted by using a high-mesa waveguide, confinement of light is so intense that the light is hardly coupled between waveguides, which increases a coupling length, and that even when a pseudo-ridge type configuration is employed to form a ridge structure in only a part between waveguides and to form a high-mesa structure outside both of the waveguides, a gap between the waveguides is required to be fabricated with accuracy of normally about sub-micrometers, and therefore, fluctuations in a circuit dimension due to a fabrication error greatly influence characteristics of the directional coupler.SOLUTION: An optical waveguide circuit is provided, which has a structure for compensating drawbacks of a high-mesa structure by leaving a thin clad (thin film clad layer) above a core of a directional coupler having a pseudo-ridge structure of conventional technology, wherein the thin film clad layer is made of the same material as a clad of the directional coupler. By forming the thin film clad above the core, the coupling length of the directional coupler is decreased, which improves tolerance to a fabrication error (an allowable error) of the circuit. A fabrication method of the optical waveguide circuit is also provided.

Description

  The present invention relates to an optical waveguide circuit, and more particularly to an optical directional coupler capable of changing a light branching ratio.

  With the widespread use of new Internet communication terminals such as smartphones, the use of optical communication that supports mobile phones and the Internet has also exploded. There is a demand for higher speed and larger capacity for optical communication, and there are various demands for optical devices used for optical communication. In particular, splitters for branching optical signals, MMI (Multi Mode Interference) circuits, directional couplers, and optical waveguide circuits for demultiplexing each wavelength such as arrayed waveguide diffraction grating (AWG) are important. It has become.

  The optical directional coupler is a circuit that branches an optical signal by bringing two optical waveguides close to each other. The branching ratio can be changed by changing the length in which the two optical waveguides are close to each other. The optical directional coupler is used not only as a single unit but also as a basic component of various optical circuits such as a Mach-Zehnder interferometer.

  Against the background of the above-mentioned demand for optical communication, there is an increasing need to realize a monolithic integrated circuit in which various optical circuits are integrated on a single substrate for the purpose of downsizing optical devices and increasing the capacity of a light source chip for optical communication. ing. Realizing various functions and increasing communication capacity by integrating optical active devices such as semiconductor lasers and optical modulators and optical passive devices such as optical waveguide circuits on a single substrate be able to. These devices are manufactured using a compound semiconductor material such as InP or GaAs, which is a direct transition semiconductor that can easily realize a semiconductor laser.

  When an optical passive device is formed on a compound semiconductor substrate such as InP or GaAs, optical waveguides for guiding light are roughly classified into three types: embedded waveguides, ridge waveguides, and high mesa waveguides. Types are used. The embedded waveguide has a structure in which a semiconductor core having a large refractive index is surrounded by a semiconductor material having a lower refractive index, and is employed in many semiconductor lasers. The ridge-type waveguide has a structure in which the low refractive index cladding is left only on the light guiding portion, and there is an advantage that the substrate and the waveguide core are not required to be processed. The high mesa waveguide is a waveguide that penetrates to the upper cladding, the core, and the lower cladding (substrate) and performs etching to form a high mesa structure.

  Each of these three types of waveguides has advantages and disadvantages, but when considering optical signal routing on a compound semiconductor substrate, a high-mesa waveguide with low bending loss due to strong confinement is used. It is preferable to use it. By using a high mesa waveguide, the area of the optical passive device can be made smaller, and the integrated device can be miniaturized.

T. Fujisawa, S. Kanazawa, H. Ishii, N. Nunoya, Y. Kawaguchi, A. Ohki, N. Fujiwara, K. Takahata, R. Iga, F. Kano, and H. Oohashi, “1.3μm, 4 × 25-Gbit / s, monolithically integrated light source for metro area 100-Gbit / s Ethernet, ”IEEE Photonics Technology Letters, vol. 23, no. 6, pp.356-358, Mar. 2011.

  However, when an optical directional coupler is configured using a high mesa waveguide, the strong light confinement becomes a problem. Since light confinement is strong, light is hardly coupled between the waveguides, and the coupling length is increased, and the advantage of downsizing using the high mesa waveguide is reduced. One solution is a ridge structure in which only the coupling part of the directional coupler is formed, or a pseudo ridge in which only the waveguide is formed with a ridge structure and the outside of both waveguides is formed with a high mesa structure. A configuration of the mold can be considered. Even if such a structure is adopted, the gap between the waveguides usually has to be produced with an accuracy of about submicron, and a slight fluctuation in the circuit dimensions due to the production error causes the characteristics of the directional coupler (for example, It appears as a large difference in bond length.

  The present invention has been made in view of such problems, and in manufacturing a quasi-ridge type directional coupler, the coupling length is made shorter than that of the prior art, and a directional coupler against manufacturing errors is provided. An object of the present invention is to provide an optical waveguide circuit having improved tolerance of characteristics and a manufacturing method thereof.

  In order to achieve such an object, the present invention provides an optical waveguide circuit having a laminated structure in which a core layer and a cladding layer are sequentially laminated on a substrate. A first optical waveguide formed on a ridge portion composed of a part and having a waveguide core composed of the core layer and an upper clad composed of the clad layer; and A first optical waveguide disposed substantially parallel to the first optical waveguide, the second optical waveguide having a waveguide core composed of the core layer and an upper clad composed of the cladding layer, and Each waveguide core of the second optical waveguide is continuous, and a thin film clad made of the same material as the cladding layer is provided at least on a part of the pre-continuous waveguide core. The first optical waveguide and the second optical waveguide are etched in such a manner that each side surface on the opposite side forms a surface substantially perpendicular to the laminated surface through the cladding layer and the core layer, respectively. An optical waveguide circuit characterized by having a structure.

  Preferably, the substrate is manufactured using a compound semiconductor material such as InP or GaAs. The optical waveguide circuit of the present invention can also be included in a monolithic integrated circuit in which various optical circuits are integrated on one substrate.

The invention according to claim 2 is the optical waveguide circuit according to claim 1, wherein the clad layer of the first optical waveguide and the second optical waveguide is provided between the thin film clad and the clad layer. It further comprises an etching stop layer used for etching the substrate.
The invention according to claim 3 is the optical waveguide circuit according to claim 2, wherein the thickness of the etching stop layer is in the range of 5 to 50 nm.

  A fourth aspect of the present invention is the optical waveguide circuit according to the second or third aspect, wherein the etching stop layer is formed by being laminated between the core layer and the clad layer. .

  A fifth aspect of the present invention is the optical waveguide circuit according to any one of the first to fourth aspects, wherein the thin film clad is laminated between the core layer and the clad layer separately from the clad layer. It is characterized by being formed.

  A sixth aspect of the present invention is the optical waveguide circuit according to any one of the first to fifth aspects, wherein the first optical waveguide and the second optical waveguide constitute a pseudo-ridge type optical directional coupler. It is characterized by that.

  According to a seventh aspect of the present invention, in a method of manufacturing an optical waveguide circuit including a pseudo-ridge type optical directional coupler, a core layer, a thin film cladding layer, an etching stop layer, and a cladding layer are sequentially stacked on a substrate. A first step of fabricating a laminated substrate; a second step of etching the cladding layer to define a coupling portion of an optical directional coupler to the etching stop layer; and a second step of removing the etching stop layer. And a fourth step of forming a mask for patterning the high-mesa waveguide so as to cover the region including the coupling portion, and etching through the mask. A first step of forming a first optical waveguide and a second optical waveguide, wherein the first optical waveguide and the second optical waveguide are opposed to each other. Each of the non-side surfaces has a mesa structure etched to form a plane generally perpendicular to the stacking plane through the cladding layer and the core layer, respectively. Each of the waveguide cores of the first optical waveguide and the second optical waveguide is continuous, and the thin film cladding made of the same material as the cladding layer is formed on at least a part of the preceding continuous waveguide core. A method for producing an optical waveguide circuit comprising:

  The invention according to claim 8 is the method of manufacturing the optical waveguide circuit according to claim 7, wherein the thickness of the etching stop layer is in a range of 5 to 50 nm, and the third step is performed by wet etching. It is characterized by that. The first optical waveguide and the second optical waveguide constitute a pseudo-ridge type optical directional coupler.

  According to the present invention, it is possible to construct a small-sized optical waveguide circuit that is easier to manufacture. It is possible to realize further increase in speed and capacity of optical communication.

FIG. 1 is a view showing a laminated structure of semiconductors used in the optical directional coupler of the present invention. FIG. 2 is a diagram showing the structure of the optical directional coupler of the present invention. FIG. 3 is a diagram for explaining a method of manufacturing an optical directional coupler according to the present invention. FIG. 4 is a diagram showing a cross-sectional structure of an optical directional coupler having a pseudo-ridge structure according to the present invention and the prior art. FIG. 5 is a diagram showing the relationship between the thin film cladding thickness and the coupling length in the optical directional coupler of the present invention. FIG. 6 is a diagram showing a cross-sectional structure of an (MMI waveguide) when the thin-film clad layer is extremely thick in the present invention. FIG. 7 is a diagram for explaining the etching of the interval portion in the optical directional coupler having a pseudo-ridge structure. FIG. 8 is a diagram showing the gap length dependence of the coupling length in the conventional structure and the optical directional coupler of the present invention.

  The optical waveguide circuit of the present invention has a structure that compensates for the shortcomings of the high mesa structure by leaving a thin clad (thin clad layer) of the same material as the clad above the core of the coupling portion of the directional coupler of the prior art pseudo-ridge structure. It is characterized by having By providing the thin film cladding on the core, the coupling length of the directional coupler is further shortened, the circuit size is reduced, and the tolerance (allowable error) of the optical directional coupler characteristic to the circuit fabrication error is improved.

  The optical waveguide circuit of the present invention is formed on a ridge portion formed from a part of the substrate in an optical waveguide circuit configured in a laminated structure in which a core layer and a cladding layer are sequentially laminated on a substrate, A first optical waveguide having a waveguide core made of the core layer and an upper clad made of the clad layer; and disposed substantially parallel to the first optical waveguide on the ridge portion; And a second optical waveguide having a waveguide core composed of layers and an upper clad composed of the cladding layer. Each of the waveguide cores of the first optical waveguide and the second optical waveguide is continuous, and a thin film cladding made of the same material as the cladding layer is formed on at least a part of the previous continuous waveguide core. And the first optical waveguide and the second optical waveguide are configured such that the side surfaces that are not opposed to each other form surfaces that are substantially perpendicular to the stacked surface through the cladding layer and the core layer, respectively. It has an etched mesa structure.

  The thin film clad may be formed by being laminated between the core layer and the clad layer separately from the clad layer. Moreover, you may form directly from a clad layer.

  Hereinafter, the configuration and manufacturing method of an optical waveguide circuit including the optical directional coupler of the present invention will be described in detail. In the following description, the invention will be described as an invention of an optical directional coupler for the sake of simplicity. However, the characteristics of the present invention can be applied to an optical waveguide circuit including an optical directional coupler, and are passive circuits having other functions. The present invention can also be applied to an optical integrated circuit combined with an active circuit.

  The configuration of the optical directional coupler of the present invention is shown in FIG. 4A, in contrast to the configuration of the prior art optical directional coupler of FIG. 4B described later. The optical directional coupler 400 of the present invention has a thin film clad layer 405 on the core layer 402. Further, an etching stop layer 406 for forming the thin film cladding layer 405 is also provided. Before describing the configuration of the optical directional coupler of the present invention, first, a basic semiconductor multilayer structure will be described.

FIG. 1 is a view showing a laminated structure of semiconductors used in the optical directional coupler of the present invention. FIG. 1 is a cross-sectional view of a semiconductor laminated structure before producing an optical directional coupler, as viewed from the side of the laminated surface. On an n-type InP substrate 101, an InGaAsP bulk core 102 having a thickness of 300 nm and a band gap wavelength of 1.15 μm, lattice-matched to the substrate 101, an InP thin film cladding 103 having a thickness Δ ₁ , and an InGaAsP having a thickness Δ ₂ The etching stop layer 104 and an InP upper cladding layer 105 having a thickness of 2 μm including a contact layer (not shown). The composition of InGaAsP in the etching stop layer 104 is the same as that of the bulk core 102.

  FIG. 2 shows a structural diagram of the optical directional coupler of the present invention. 2A is a top view viewed from a direction perpendicular to the substrate surface (xy plane), and FIG. 2B is a cross-sectional view of the light traveling direction (x axis) of the optical waveguide. . As shown in FIG. 2A, the optical directional coupler includes an input waveguide section 201, a directional coupler section 202, and an output waveguide section 203. In FIG. 2A, both sides of the two high mesa waveguides 204 and 205 that penetrate from the input waveguide section 201 to the output waveguide section 203 appear to be a space, but the substrate surface 101 exists. Please note that. 2B shows a cross-sectional view (yz plane) of the aa ′ portion in the input waveguide portion 201 and a cross-sectional view of the bb ′ portion in the directional coupler portion 202 (y−). z planes) are shown respectively. As can be seen from the cross-sectional view of the bb ′ section, in the directional coupler section 202, the two waveguides have a pseudo-ridge structure. Further, in the directional coupler unit 202, the core layer 207 is continuous over the two waveguides at the coupling portion between the two ridge waveguides. Furthermore, on the core layer, a thin film clad layer 206 unique to the optical directional coupler of the present invention is provided between two waveguides.

  FIG. 3 is a diagram for explaining a method of manufacturing an optical directional coupler according to the present invention. Steps (steps) proceed in the order of (a) to (e) in FIG. 3. In each step, the right side is a top view (xy plane) and the left side is a light traveling direction (x-axis direction) from the input waveguide portion side. The sectional view (yz plane) containing the aa 'line which looked at is shown. The optical directional coupler of the present invention is manufactured by the following procedure based on the semiconductor laminated structure shown in FIG. The semiconductor stacked structure in FIG. 3 is obtained by epitaxially growing an n-type InP substrate 301 on a n-type InP 301 substrate, and a core layer 302, a thin-film cladding layer 303, an etching stop layer 304, and an upper cladding layer 305 in this order. It is formed by stacking.

  As shown in FIG. 3A, first, a mask 306 is formed on the upper cladding layer 305 in order to pattern the directional coupler portion of the pseudo-ridge structure by photolithography. Next, the upper cladding layer 305 is etched halfway by dry etching, and then wet etching using a hydrochloric acid-based solution (etching up to the etching stop layer 304 to obtain the state shown in FIG. 3B). It reaches.

  Further, as shown in FIG. 3C, the etching stop layer 303 at the joint between the two waveguides 305a and 305b in the process of production is removed using a sulfuric acid-based solution. Next, as shown in FIG. 3D, a mask 307 for patterning the high mesa waveguide is formed so as to cover the coupling portion. Thereafter, as shown in FIG. 3E, a directional coupler portion having a pseudo-ridge structure, its input high mesa waveguide, and its output high mesa waveguide are formed by dry etching.

  Therefore, the present invention also has a side as a method for manufacturing an optical waveguide circuit. That is, as a different aspect of the present invention, a method for producing an optical waveguide circuit including a pseudo-ridge type optical directional coupler is provided. In this method, a core layer, a thin film clad layer, an etching stop layer, and a cladding layer are sequentially laminated on a substrate to produce a laminated substrate, and the cladding layer is etched to form the etching stop layer. A second step of defining a coupling portion of the optical directional coupler, a third step of removing the etching stop layer, and patterning the high mesa waveguide so as to cover the region including the coupling portion A fourth step of forming the mask.

  Furthermore, the present invention is a fifth step of forming a first optical waveguide and a second optical waveguide arranged substantially in parallel by etching through the mask, wherein the first optical waveguide is formed. The waveguide and the second optical waveguide have a mesa structure that is etched so that each side surface on the opposite side forms a surface substantially perpendicular to the laminated surface through the cladding layer and the core layer, respectively. A fifth step. Each of the waveguide cores of the first optical waveguide and the second optical waveguide is continuous, and the thin film is made of the same material as the cladding layer on at least a part of the preceding continuous waveguide core It has a cladding.

  Here, the thickness of the etching stop layer 304 has a minimum value that can be controlled in the epitaxial growth process. The minimum thickness that can be formed with good reproducibility by epitaxial growth is about 5 nm. However, if the controllable thickness can be further reduced, the thickness may be further reduced. On the other hand, if the etching stop layer 304 is too thick, light that should be concentrated on the core layer 302 leaks to the etching stop layer 304, thereby increasing the coupling length of the optical directional coupler. Therefore, it is desirable that the thickness of the etching stop layer 304 be a level that does not affect the mode field of light, as will be specifically described below.

If the thickness of the etching stop layer 304 is negligibly small compared to the thickness of the core layer 302, light leakage to the etching stop layer 304 is small and the mode field is not affected. If the ratio of the confinement ratio in the etching stop layer 304 is 10% or less with respect to the confinement ratio of the total optical power in the core layer 302, the influence of the etching stop layer 304 on the mode field can be ignored. In this embodiment, when the thickness of the etching stop layer 304 is 50 nm, the ratio of light confinement to the core layer 302 is 50%, and the ratio of light confinement to the etching stop layer 304 is 5%. At this time, the ratio of light confinement to the core layer and light confinement to the etching stop layer is 10%. Therefore, the upper limit of the thickness of the etching stop layer is 50 nm. After all, the thickness of the etching stop layer is specifically in the range of about 5 nm to 50 nm. In the examples of the present specification, the thickness Δ ₂ of the etching stop layer is 5 nm. The definition of the coupling length of the directional coupler will be described later.

  FIG. 4 is a diagram showing a cross-sectional structure of the optical directional coupler having a pseudo-ridge structure according to the present invention and the prior art. (A) shows the optical directional coupler of this invention, (b) shows the optical directional coupler of a prior art. In the configuration of the prior art in FIG. 4B, the ridge portion is provided on the substrate 401, and the cores of the two waveguides are continuous. A thin film clad 403 made of the same material as the clad layer is further provided on the continuous core. The width of each waveguide mesa is W, and the gap between the two mesas is gap. The prior art structure of FIG. 4B does not have the thin film cladding 403 and the etching stop layer 404 that are characteristic in the structure of the directional coupler of the present invention.

FIG. 5 is a diagram showing the relationship between the thickness of the thin-film cladding layer and the coupling length in the optical directional coupler of the present invention. The horizontal axis represents the thickness Δ ₁ of the thin film cladding layer 403 in FIG. 4A, and the vertical axis represents the coupling length. Here, the mesa width W of each waveguide is 1.5 μm, the gap is 0.5 μm, and the wavelength used is 1.3 μm. Coupling length is defined as the length of a directional coupler such that light input from one port of the input waveguide of the directional coupler is completely output to the opposite port of the output waveguide ( (Also called complete bond length), which can be obtained by the following equation (1).

Here, Δn _eff represents a difference between the effective refractive index of the fundamental mode and the effective refractive index of the first higher-order mode. As can be seen from FIG. 5, the coupling length is shortened by increasing the thickness Δ ₁ of the thin film cladding layer 403. Therefore, it can be seen that the size of the directional coupler can be reduced by increasing the thickness of the thin-film cladding layer 403. However, when the thickness of the thin film clad layer 403 is too large, the configuration of the two high mesa waveguides to be coupled approaches a multimode interference type (MMI) waveguide as shown in FIG.

  In the MMI waveguide, it is difficult to adjust the branching ratio to the two output waveguides. Therefore, when an optical directional coupler is to be configured, the thickness of the thin-film cladding layer 403 is desirably 0.1 μm or less in the configuration of the optical directional coupler of this embodiment. When the thickness of the thin clad layer is 0.1 μm or less, the coupling length can be reduced to about half that of the conventional optical directional coupler, and the optical directional coupling to the monolithic integrated circuit is possible. Including a device has a great effect on circuit miniaturization.

  In the above description, the effect that the coupling length can be shortened by setting the thickness of the thin film cladding layer 403 to a range of 0.1 μm or less has been described. However, the length of the coupling portion of the two waveguides in the directional coupler actually used is often used in a state shorter than the state of the definition of coupling length (complete coupling length). Thus, even when the length of the portion where the two waveguides are coupled in the optical directional coupler is shorter than the coupling length, the configuration including the thin-film cladding layer of the present invention similarly applies. An optical integrated circuit or the like can be miniaturized.

  As described above, the effect of the present invention resides in that a thin film cladding layer is provided on a continuous core layer in a coupling portion between two high mesa waveguides. 2, 3, and 4 have all been described as including an etch stop layer, but according to other fabrication methods that can form a thin film cladding layer on a continuous core layer, the etch stop layer is not necessarily Note that this is not necessary. For example, in the dry etching stage of FIG. 3B, the thickness corresponding to the thin-film cladding layer 303 is left and the cladding layer 305 is removed to a necessary depth, so that etching is performed when wet etching is not performed. The stop layer 304 is not necessary. Further, after the clad layer 305 is dry-etched, the step can be measured and wet etching can be performed until a desired thickness corresponding to the thin-film clad layer remains.

  In any of the above cases, it is not necessary to previously form the thin film clad 304 layer in the laminated structure, and the portion corresponding to the thin film clad layer may be formed directly from the clad layer 305. However, it is more preferable to provide the etching stop layer 304 in terms of controllability of the thickness of the thin film cladding layer.

  Further, in the top view (a) of FIG. 2, it has been described that the thin-film cladding layer 206 is present in all regions between the two waveguides 204 and 205 of the directional coupler unit 202. However, even when the thin clad layer 206 is present in at least a part of the region between the two high mesa waveguides 204 and 205, the coupling length can be shortened as compared with the optical directional coupler having the conventional structure. . When a monolithic integrated circuit includes an optical directional coupler, there is a certain effect on circuit miniaturization.

  In the directional coupler, in order to control the branching ratio at the output port, it is extremely important to control the distance (hereinafter, gap length) between the two high mesa waveguides in the coupling portion.

  FIG. 7 is a diagram for explaining the etching of the interval portion in the optical directional coupler having a pseudo-ridge structure. In the directional coupler of the present invention, the gap is as small as about 0.5 μm, and wet etching is used to produce the end face at the joint as described with reference to FIG. For this reason, as shown in FIG. 7, the etching proceeds in the lateral direction of the mesa on the opposite side surfaces of the two mesa structures 702 and 704, and the actual gap width of the coupling portion becomes wide.

  FIG. 8 is a diagram showing the gap length dependence of the coupling length in the prior art and the optical directional coupler of the present invention. The curve labeled (a) is for the prior art optical directional coupler, and the curve labeled (b) is for the optical directional coupler of the present invention, with the gap length coupled on the horizontal axis and the vertical axis on the vertical axis, respectively. Taking a long. As for the conventional optical directional coupler (a), it can be seen that as the gap length increases, the coupling length increases in a quadratic function.

  On the other hand, in the optical directional coupler of the present invention of (b), when the thickness of the thin film cladding is 0.1 μm, the gap length dependence of the coupling length is smaller than that of the prior art of (a). As the gap length increases, the bond length increases approximately linearly. The increase in the coupling length when the gap length value is changed from 0.5 μm to 1 μm was 2.8 times in the conventional optical directional coupler, whereas in the optical directional coupler of the present invention, The rate of change in bond length is small, 1.94 times. Therefore, even when a gap length error occurs in the etching process during the production of the mesa structure, according to the optical directional coupler of the present invention, the coupling length variation sensitivity can be suppressed as compared with the configuration according to the prior art. . That is, it is possible to suppress the influence of fluctuations in the dimensions of the optical circuit due to the manufacturing error of the gap between the waveguides on the characteristics (for example, the coupling length) of the optical directional coupler. An optical waveguide circuit with improved tolerance of optical circuit characteristics against fabrication errors can be provided.

  As described above in detail, by using the optical directional coupler of the present invention in an optical integrated circuit, it is smaller and easier to manufacture than the pseudo-ridge type directional coupler having the structure of the prior art. It is possible to provide a directional coupler and improve the tolerance against the characteristic variation of the optical directional coupler due to manufacturing errors.

  The present invention is generally applicable to an optical communication system. In particular, it can be used for an optical waveguide circuit.

101, 301, 401, 601, 701 Substrate 102, 207, 302, 402 Core layer 103, 206, 303, 403 Thin film cladding layer 104, 304, 404 Etching stop layer 105, 305, 405 Upper cladding layer 201 Input waveguide section 202 Directional coupler 203 Output waveguide 204, 205, 305a, 305b High mesa waveguide 306, 307 Mask 400 Optical directional coupler

Claims (8)

In an optical waveguide circuit configured in a laminated structure in which a core layer and a cladding layer are sequentially laminated on a substrate,
A first optical waveguide formed on a ridge portion constituted by a part of the substrate, and having a waveguide core constituted by the core layer and an upper clad constituted by the clad layer;
A second optical waveguide disposed on the ridge portion substantially in parallel with the first optical waveguide and having a waveguide core composed of the core layer and an upper clad composed of the cladding layer;
Each of the waveguide cores of the first optical waveguide and the second optical waveguide is continuous, and a thin film cladding made of the same material as the cladding layer is formed on at least a part of the previous continuous waveguide core. With
The first optical waveguide and the second optical waveguide are etched so that the side surfaces that are not opposed to each other form surfaces that are substantially perpendicular to the stacked surface through the cladding layer and the core layer, respectively. An optical waveguide circuit characterized by having a mesa structure.   An etching stop layer used for etching the cladding layer of the first optical waveguide and the second optical waveguide is further provided between the thin film cladding and the cladding layer. Item 4. The optical waveguide circuit according to Item 1.   The optical waveguide circuit according to claim 2, wherein the thickness of the etching stop layer is in the range of 5 to 50 nm.   The optical waveguide circuit according to claim 2, wherein the etching stop layer is formed by being laminated between the core layer and the clad layer.   5. The optical waveguide circuit according to claim 1, wherein the thin film cladding is formed by being laminated between the core layer and the cladding layer separately from the cladding layer. 6.   6. The optical waveguide circuit according to claim 1, wherein the first optical waveguide and the second optical waveguide constitute a pseudo-ridge type optical directional coupler. In a method of manufacturing an optical waveguide circuit including a pseudo-ridge type optical directional coupler,
A first step in which a core layer, a thin film clad layer, an etching stop layer, and a clad layer are sequentially laminated on the substrate to produce a laminated substrate;
Etching the cladding layer to define a coupling portion of an optical directional coupler to the etch stop layer;
A third step of removing the etch stop layer;
A fourth step of forming a mask for patterning the high mesa waveguide so as to cover the region including the coupling portion;
Etching through the mask is a fifth step of forming the first optical waveguide and the second optical waveguide arranged substantially in parallel, wherein the first optical waveguide and the second optical waveguide are formed. The waveguide has a mesa structure in which each side surface on the non-opposite side is etched so as to form a surface substantially perpendicular to the laminated surface through the cladding layer and the core layer, respectively, With
Each of the waveguide cores of the first optical waveguide and the second optical waveguide is continuous, and the thin film is made of the same material as the cladding layer on at least a part of the preceding continuous waveguide core A method for producing an optical waveguide circuit comprising a clad.   The method according to claim 7, wherein the etching stop layer has a thickness in a range of 5 to 50 nm, and the third step is performed by wet etching.

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