JP2788762B2 - Optical circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical circuit using an optical waveguide provided on a crystal substrate, and more particularly to an optical circuit formed by crossing optical waveguides.

[Conventional technology]

As optical communication systems become more practical, advanced systems with higher capacity and more functions are required, and new functions such as switching of optical transmission lines, switching, distribution of many optical signals, and merging are added. Is needed. An optical switch is used as a means for obtaining an optical transmission line switching or network switching function. Optical switches currently in practical use are those that mechanically move prisms, mirrors, fibers, etc., and suffer from disadvantages such as low speed, inadequate reliability, and large shapes that are unsuitable for matrix formation. is there. As a means for solving this problem, there is a waveguide type optical switch using an optical waveguide. This has features such as high speed, integration of many elements, and high reliability. Especially lithium diobate (LiNb
A material using a ferroelectric material such as O ₃ ) crystal has features such as low light absorption and low loss, and high efficiency due to a large electro-optic effect. Various types of optical switches such as a sex coupler type optical switch, a total reflection type optical switch or a Mach-Zehnder type optical switch have been reported.

In particular, a large number of integrated matrix optical switches are connected by an optical circuit consisting of an optical waveguide on the substrate between the waveguide-type optical switches, and many studies and reports have been made because they are key devices for the purpose of switching and exchanging optical signals. ing. When applying such a waveguide type matrix optical switch to an actual optical communication system, low loss is important, and for this purpose, it is essential that the optical circuit constituting the matrix optical switch has low loss. It is.

On the other hand, an optical fiber is currently used for distributing and merging optical signals, but is unsuitable for an increase in the number of optical signals to be distributed and merged in the future due to an increase in the number of optical signals. What is being developed as a means for solving this problem is a waveguide type optical circuit also using an optical waveguide. An optical circuit using an optical waveguide formed on a dielectric substrate such as a quartz optical waveguide, a glass plate, and a lithium niobate (LiNbO ₃ ) crystal, which is the same material as the optical fiber, has low light absorption and low loss. Although many reports have been made, it is still practically essential that the optical circuit has low loss when applied to an actual optical communication system.

[Problems to be solved by the invention]

However, conventional optical circuits have not obtained sufficient characteristics with respect to optical circuit loss. FIG. 2 (a), (b)
An example of an optical circuit constructed by intersecting conventional optical waveguides is described in Yutaka Nishimoto's literature, IEICE Technical Report OQE 88
FIG. 4 shows a plan view of the optical circuit of an 8 × 8 matrix optical switch according to −147. In FIG. 2 (a), embedded single-mode optical waveguides 12 and 13 formed by diffusing titanium on a lithium niobate crystal substrate 11 cut out perpendicular to the Z-axis and having a refractive index larger than that of the substrate 11 are formed. Are formed, and the optical waveguides 12 and 13 are several μm apart from each other at the center of the substrate 11.
The directional couplers 14 are formed in close proximity to each other. Second
In FIG. 1A, 64 directional couplers 14 are integrated on the same substrate to form an 8 × 8 matrix optical switch. FIG. 2B is an enlarged plan view of a region where the optical waveguides 12 and 13 connecting the directional couplers 14 intersect.
15 is a control electrode, 17 is incident light, 18 and 19 are outgoing light, and 20 is an intersection. According to the above-mentioned document, the difference in loss due to the connection path of the 8 × 8 matrix optical switch is due to two optical waveguides.
It has been clarified that the loss is caused by the loss at the intersection of 12 and 13 (hereinafter referred to as "crossing loss"), and that it is necessary to reduce the crossing loss in order to achieve low loss. The difference in loss due to the difference between the connection paths is caused by the difference in the number of intersections where the two optical waveguides 12 and 13 intersect in each connection path. That is, according to the above-mentioned document, the crossing loss is about 0.35 dB at one crossing point with respect to the TM polarized light, and at this time, the crossing point of each connection path starts from point 0.
Because it is different from 15 points. Therefore, there is a difference of about 5 dB between the connection paths at the intersections 0 and 15 only by the intersection loss. In order to reduce the device loss, it is necessary to reduce the crossing loss. The guided light propagating in the single mode becomes multimode from the region where the two intersecting waveguides are close to each other to the intersection, and after passing through the intersection, the single mode is again transmitted in the region where the two intersecting waveguides are separated. Becomes As described above, cross loss occurs due to mode conversion and mode coupling at the crossing portion 20, and transfer of guided light to the other waveguide that crosses and is close to the other.

Incidentally, in the directional coupler type optical switch of 64 elements of the 8 × 8 matrix optical switch shown in FIG.
The control electrode 15 is provided on the optical waveguide constituting the directional coupler 14 via a buffer layer for preventing light absorption by the control electrode 15.
Are formed. In FIG. 2B, the optical waveguide 12
As the incident light 17 incident on the optical waveguide 13 propagates through the directional coupler 14, the light energy is gradually transferred to the optical waveguide 13 adjacent thereto, and after passing through the directional coupler 14, almost 10
The energy is shifted by 0% to become the emitted light 18. On the other hand, when a voltage is applied to the control electrode 15, the refractive index of the optical waveguide below the control electrode 15 changes due to the electro-optic effect, and a phase velocity mismatch between the waveguide modes propagating through the optical waveguides 12 and 13 occurs. As a result, the connection between them changes. Using this operation, the propagation path of the guided light is switched.

An object of the present invention is to solve the above-mentioned problems of the conventional optical circuit,
An object of the present invention is to provide a low-loss optical circuit.

[Means for solving the problem]

To achieve the above object, an optical circuit according to the present invention comprises:
A pair of optical waveguides that are formed to intersect the crystal substrate at an acute angle, a concave portion that is formed in the crystal substrate adjacent to an intersection of the optical waveguides, and that is formed on a surface of the crystal substrate. An optical circuit comprising a cladding layer having a refractive index lower than that of the crystal substrate, confining the guided light at the intersection, and reducing an intersection loss occurring at the intersection, wherein the concave portion has two optical waveguides intersecting at the acute angle. Characterized by being formed only in a region sandwiched between the.

[Operation] The optical circuit of the present invention has a concave portion on the crystal substrate excluding the optical waveguide at the intersection where the optical waveguides intersect, and the concave portion is formed in a region where two intersecting optical waveguides are close to each other. Therefore, the cross section of the adjacent optical waveguides has a ridge shape at the periphery of the intersection between the two optical waveguides. According to experiments performed by the present inventor, the crossing loss is reduced by forming the crossing portion in such a structure.

In other words, by making the optical waveguide around the intersection a ridge shape, the cladding layer surrounding the optical waveguide from the transverse direction to the traveling direction of the guided light is smaller than the crystal substrate compared to the intersection in the conventional optical circuit. Further, a substance having a lower refractive index can be obtained. For this reason, compared with the conventional structure, from the region where two intersecting waveguides approach each other to immediately before the intersection, the two adjacent waveguides are blocked by a material having a low refractive index, and the waveguides are guided. Strong confinement of wave light, no multi-mode component is generated, and loss due to mode conversion, loss due to mode coupling, and loss due to transfer of guided light to the other optical waveguide, which are the causes of cross loss described above, are reduced. Therefore, the crossing loss is greatly reduced.

As described above, in the optical circuit of the present invention, the crossing loss can be reduced, and an optical circuit having a crossing portion can be obtained with a lower loss than in the related art.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIGS. 1 (a) and 1 (b) are a plan view and a cross-sectional view of an intersection of an optical waveguide which is an embodiment of an optical circuit according to the present invention. In the figure, a recess 21 is formed in a lithium niobate crystal substrate 11 around an intersection 20 between optical waveguides 12 and 13 and excluding the optical waveguide.
Is formed by dry etching such as an ion beam method, a reactive ion beam method, or a reactive ion method, or wet etching such as a chemical etching method using a solution. .

In the optical circuit of this embodiment, a reactive ion beam etching method was used. That is, after forming an optical waveguide by titanium diffusion in the lithium niobate crystal substrate 11 in the same manner as in the conventional example, the region other than the portion where the concave portion 21 is formed is masked, and the lithium niobate crystal substrate 11 is formed by a reactive ion beam etching method. Was removed by etching.

The depth D of the concave portion 21 in the optical circuit of the present invention is 0.1 μm
With the above, the effect of reducing the crossing loss was recognized. Optical waveguide
The cladding layer 16 that surrounds 12 and 13 from the lateral direction with respect to the traveling direction of the guided light may be made of a material that has a smaller refractive index than the lithium niobate crystal substrate 11 and is transparent. As this material, SiO ₂ , Si ₃ N ₄ , Al ₂ O ₃ or the like was used.

In the conventional optical circuit, the cross loss was about 0.35 dB at one cross section with respect to the TM polarization, whereas in the optical circuit according to the present embodiment, the cross loss was about 0.1 dB, which was significantly smaller than the conventional example. Was. At this time, in FIG.
m and G are 0 μm or more, and the width W of the optical waveguides 12 and 13 is 9 μm.
m, the intersection angle θ of the optical waveguide at the intersection is about 7 degrees (7
Degree), the cladding layer 16 is SiO ₂ and the optical waveguide is thick
It was formed by thermally diffusing 630 Å of titanium at 1050 ° C. for 8 hours.

The optical waveguide forming the optical circuit according to the present invention is not limited to a lithium niobate crystal substrate in which titanium is diffused, but is formed on a glass substrate or a proton exchange optical waveguide using lithium niobate, sapphire, or a Si substrate. It is clear that the present invention can be applied to all optical circuits such as a quartz optical waveguide.

[Effects of the Invention] As described above, the optical circuit according to the present invention can reduce the loss as compared with the conventional optical circuit.

[Brief description of the drawings]

1 (a) is a plan view showing an example of an optical circuit according to the present invention, FIG. 1 (b) is a sectional view taken along line AA 'of FIG. 1 (a), and FIG. 2 (a) shows a conventional optical circuit. FIG. 2B is an enlarged view of a part II in FIG. 2A. 11: Lithium niobate crystal substrate 12, 13: Optical waveguide, 14: Directional coupler 15: Control electrode, 16: Cladding layer 17: Incident light, 18, 19: Outgoing light 20: Intersection, 21: Recess

Claims (1)

(57) [Claims] A pair of optical waveguides formed at an acute angle to intersect with the crystal substrate; a recess formed in the crystal substrate adjacent to an intersection of the optical waveguides; An optical circuit comprising a cladding layer having a lower refractive index than the crystal substrate formed on the surface, confining the guided light at the intersection, and reducing the crossing loss generated at the intersection, wherein the recesses intersect at the acute angle An optical circuit formed only in a region sandwiched between two optical waveguides.