JPH03119304A - Optical circuit - Google Patents

Abstract

PURPOSE:To easily form an optical circuit with low loss by providing a dielectric film which has a less refractive index than a crystal substrate at the intersection part of optical waveguides. CONSTITUTION:The dielectric film 16 which has the less refractive index than the crystal substrate 11 is formed at the intersection part 20 where the optical waveguides 12 and 13 crystallized on the crystal substrate 11 cross each other. In this case, the equivalent refractive index of the intersection part where the dielectric film 16 is loaded is larger than that of the optical waveguides which are obtained by diffusing titanium. Therefore, the confinment of waveguide light of the optical waveguides is increased at the intersection part and loss due to mode conversion, loss due to mode coupling, and loss due to the movement of waveguide light to the other adjacent optical waveguides which are factors of intersection loss are reduced respectively. Consequently, the insertion loss is reduced greatly and the dielectric film can easily be formed by utilizing a buffer layer such as an SiO2 film and an AI2O3 film formed so as to prevent light from being absorbed by a control electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical circuit using optical waveguides provided on a crystal substrate, and more particularly to an optical circuit constructed by intersecting optical waveguides.

[Conventional technology]

As the practical use of optical communication systems progresses, advanced systems with larger capacity and multiple functions are required, and new functions such as switching and exchanging optical transmission lines and distributing and merging numerous optical signals are being added. is needed. Optical switches are used as means for switching optical transmission lines and providing network switching functions. Optical switches currently in use mechanically move prisms, mirrors, fibers, etc., and are slow and have insufficient reliability.
The disadvantage is that the shape is large, making it unsuitable for matrix formation. A waveguide type optical switch using an optical waveguide is being developed as a means to solve this problem, and has features such as high speed, integration of multiple elements, and high reliability. In particular, those using ferroelectric materials such as lithium niobate (LiNb0s) crystals have features such as low light absorption and low loss, and high efficiency because they have a large electro-optic effect. Various types of optical switches have been reported in the past, such as a directional coupler type optical switch, a total internal reflection type optical switch, and a Matsuhatsu Enda type optical switch.

In particular, many studies and reports have been made on matrix optical switches, which connect waveguide optical switches with optical circuits made of optical waveguides on a substrate, and are a key device for the purpose of switching and exchanging optical signals. ing. When applying such a waveguide matrix optical switch to an actual optical communication system, it is important to have low loss, and for this purpose, it is essential that the optical circuit that makes up the matrix optical switch has low loss. It is.

On the other hand, optical fibers are currently used to distribute and combine optical signals, but as the number of optical signals to be distributed and combined increases in the future, optical switches will become larger. It's inappropriate. A waveguide type optical circuit using an optical waveguide is currently being developed as a means to solve this problem. A silica-based optical waveguide, a glass plate, and lithium niobate (LiNb0), which are the same materials as optical fibers, are used.
．． ) Many reports have been made of optical circuits using optical waveguides formed on dielectric substrates such as crystals, as they have low optical absorption and low loss, but it is still difficult to apply them to actual optical communication systems. It is practically essential that optical circuits have low loss.

[Problems to be Solved by the Invention] However, conventional optical circuits do not have sufficient characteristics regarding optical circuit loss. Figure 2(a).

(b) is an example of an optical circuit constructed by crossing conventional optical waveguides, as shown in the literature by Yutaka Hyakumoto et al., Technical Report of the Institute of Electronics, Communication and Information Engineers, ○QE, 88-147, an optical circuit of an 8×8 matrix optical switch. In FIG. 2(a), which shows a plan view, a titanium film is formed on a lithium niobate crystal substrate 11 cut perpendicularly to the Z axis, and then thermally diffused to make the refractive index larger than that of the substrate. Embedded single mode optical waveguides 12 and 13 are formed. The optical waveguides 12 and 13 are close to each other within several μm at the center of the substrate, forming a directional coupler 14. Figure 2(a)
In this case, the directional coupler 14 has 64 elements integrated on the same substrate to form an 8.times.8 matrix optical switch.

FIG. 2(b) is an enlarged plan view of the area where the optical waveguides 12 and 13 that connect the directional couplers 14 intersect.

According to the above-mentioned literature, the difference in loss due to the connection bus of an 8×8 matrix optical switch is due to the two optical waveguides 12 and 1.
It has been clarified that this is caused by loss at the intersection of No. 3 (hereinafter referred to as "crossing loss"), and that it is necessary to reduce the crossing loss in order to reduce the loss. The difference in loss due to the difference in the connection buses described above is due to the difference in the number of intersections where the two optical waveguides 12 and 13 intersect in each connection bus. That is, according to the above-mentioned literature, the crossing loss is about 0.35 dB at one crossing point for TM polarized light, and this is because each connection path has different crossing points from 0 to 15 points. Therefore, the intersections it has are 0 and 1.
A problem arises in that a difference in loss of approximately 5 dB occurs between the five connection buses due to cross loss alone. In order to reduce the difference in loss due to the difference in connection buses and obtain a practical optical switch, it is necessary to reduce the crossing loss.

The guided light that has propagated in single mode becomes multimode from the area where the two intersecting waveguides are close to the intersection, and after passing through the intersection, it becomes single mode again in the area where the two intersecting waveguides are separated. becomes. Therefore, cross loss occurs due to mode conversion at the cross section 20, mode coupling, and transfer of the guided light to the other adjacent waveguide that intersects.

By the way, if the refractive index of the diamond-shaped crossing part 20 shown by diagonal lines in FIG. It has already been introduced in many documents (for example, the document APPLI by T. Kurokawa et al.
ED 0PTICS, Vol. 16. Na4゜Apri
l 1977, p. 1033). On the other hand, in order to partially increase the refractive index of an optical waveguide formed by thermal diffusion of a titanium film, multiple titanium film formation steps and lithography are required, as described in Japanese Patent Publication No. 61-25642. There is a method of partially increasing the film thickness of titanium protein.

Using this technology, as shown in FIG. 3, by forming two layers of titanium film only on the diamond-shaped intersection 20,
It is possible to form an optical circuit in which the refractive index of the waveguides is higher than that of the waveguides 12 and 13. Here, FIG. 3(a) is a plan view of a pattern in which two layers of titanium film are formed only at the diamond-shaped intersections 20, and FIG. 3(b) is a cross-sectional view taken along line AA' in FIG. 3(a). be. However, the above-mentioned technique has a problem in that it requires multiple titanium film formation steps that are expensive and take a long time to form before forming an optical circuit having intersections.

In addition, in each directional coupler type optical switch of 64 elements of the 8×8 matrix optical switch shown in FIG. 2(a),
A control electrode 1 is provided on the optical waveguide constituting the directional coupler 14.
A control electrode 15 is formed with a buffer layer interposed therebetween to prevent light absorption by the rays 5. In FIG. 2(b), as the incident light 17 that has entered the optical waveguide 12 propagates through the directional coupler 14, the optical energy gradually transfers to the adjacent optical waveguide 13 and passes through the directional coupler 14. After that, almost 100% of the energy is transferred to the optical waveguide I ¥813 and becomes 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 under the control electrode 15 changes due to the electro-optic effect, causing phase velocity mismatch between the waveguide modes propagating in the optical waveguides 12 and 13. occurs, and the bonding state between the two changes. This operation is used to switch the propagation path of the guided light.

The purpose 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 an optical circuit that has low loss and can be easily formed.

[Means to solve the problem]

In the optical circuit of the present invention, in an optical circuit constituted by a plurality of optical waveguides formed on a crystal substrate, which intersect with each other, the intersection portions where the optical waveguides intersect with each other have a refractive index lower than that of the crystal substrate. It is formed with a dielectric film.

[Effect]

Since the optical circuit of the present invention has a dielectric film having a lower refractive index than the crystal substrate at the intersection where the optical waveguides intersect, the isolateral refractive index of the intersection loaded with the dielectric film is such that titanium is diffused. It is higher than the optical waveguide obtained by Therefore, compared to the conventional structure, the optical waveguide has stronger confinement of guided light at the intersection, and multimode components are not generated at the intersection.
Or, even if it does occur, its proportion is small, and the loss due to mode conversion, loss due to mode coupling, and loss due to transition of guided light to the other adjacent optical waveguide, which are the causes of crossover loss mentioned above, are reduced. Losses are significantly reduced.

Here, the titanium film forming process for forming the optical circuit of the present invention is performed only once, and the dielectric film uses a buffer layer such as a 5i02 film or an Al2O3 film formed to prevent light absorption by the control electrode. It can be easily formed by

As described above, in the optical circuit of the present invention, crossing loss can be reduced, and an optical circuit having crossing parts can be easily obtained with lower loss than conventional optical circuits.

〔Example〕

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

FIGS. 1(a) and 1(b) show a plan view and a sectional view of an intersection of optical waveguides, which is an embodiment of the optical circuit according to the present invention.

The dielectric film 16 in this embodiment is the optical waveguide 12.13.
Regarding the 5i02 sputtered film with a thickness of approximately 2800 angstroms formed on the entire surface of the lithium niobate crystal substrate 11 on which
It was formed by etching away the other parts. That is, after forming the SiO□ sputtered film, the control electrode 1
5 (see FIG. 2) and the diamond-shaped intersection 20 were masked, and the remaining portions were removed by wet etching using a mixed solution of ammonium fluoride and hydrofluoric acid.

In the conventional optical circuit, the crossing loss was about 0j5 dB at one crossing point for 7M polarized light, whereas in the optical circuit according to this embodiment, the crossing loss was about 0.1 dB, which is significantly smaller than the conventional example. Ta. Further, the optical circuit of this example could be easily formed by simply adding an etching removal process of the 5i02 sputtered film by lithography. At this time, the width W of the optical waveguides 12 and 13 is 9 μm, the thickness H of the dielectric film 16 is 2800 angstroms, and the crossing angle θ of the optical waveguides is 9 μm.
is about 7 degrees (but not less than 7 degrees), and the optical waveguide was formed by thermally diffusing titanium with a thickness of 630 angstroms on the lithium niobate crystal substrate 11 at 1050° C. for 8 hours.

Note that the optical waveguide forming the optical circuit according to the present invention is not limited to one in which titanium is diffused on a lithium niobate crystal substrate, but can also be formed on a glass substrate, a proton exchange optical waveguide using lithium niobate, a sapphire, or an St substrate. It is obvious that the present invention can be applied to all optical circuits including quartz-based optical waveguides.

〔Effect of the invention〕

As described above, in the optical circuit according to the present invention, an optical circuit having a crossing portion can be easily obtained with low loss compared to a conventional optical circuit.

[Brief explanation of drawings]

FIGS. 1(a) and 1(b) are a plan view and a sectional view showing an example of an optical circuit according to the present invention, and FIG. 2(a) is a sectional view of the optical circuit according to the present invention. (b) is a plan view showing an example of a conventional optical circuit;
Figures (a) and (b) are a plan view and a cross-sectional view of a pattern in which two layers of titanium films are formed at intersections. 11... Lithium niobate crystal substrate, 12.13...
- Optical waveguide, 14... Directional coupler, 15... Control electrode, 16... Dielectric film, 17... Incident light, 18.
... Outgoing light, 20... Intersection.

Claims (1)

[Claims] An optical circuit configured by a plurality of optical waveguides formed on a crystal substrate intersecting each other, characterized in that a dielectric film having a lower refractive index than the crystal substrate is provided at the intersection where the optical waveguides intersect with each other. optical circuit.