JPH0360097B2 - - Google Patents

Description

[Detailed description of the invention] (a) Technical field of the invention The present invention relates to improvements in optical waveguides.

(b) Background of the Technology In optical application fields such as optical communications, miniaturized, high-performance optical waveguides with high light transmission efficiency have been developed with the aim of miniaturizing optical circuit elements and facilitating mass production.

(c) Prior Art and Problems Conventional optical waveguides like this will be explained using FIGS. 1, 2, and 3. 3rd from figure 1
The figures above are plan views showing the structure of a conventional optical waveguide.
The optical waveguide shown in these figures is made by patterning titanium (Ti) and depositing it on an electro-optic crystal substrate 1 made of lithium niobate (LiNbo ₃ ).
By heating at a temperature of about 1000℃ for 5 to 10 hours
Optical waveguides 2, 3, 4 are formed by diffusing Ti into the substrate.
form. The optical waveguide shown in Figure 1 is well known as a directional coupler type switch, and the input line and output line of this type of optical waveguide are single mode waveguides, but the central part has two parallel waveguides. Consisting of waveguides 2A and 2B, this part is designed to excite fundamental mode (symmetrical mode) and primary mode (asymmetrical mode) light, and the light has coupling between the waveguides, causing propagation of the two modes. Due to the speed difference, a phase shift of π allows light to transition from one waveguide, for example 2A, to the other waveguide 2B. The transition of light between the waveguides 2A and 2B is controlled by changing the refractive index outside the waveguides via electrodes (not shown) of various shapes formed on the waveguides. In an optical switch with such an optical waveguide, if the difference in propagation constant between the fundamental mode and the primary mode is increased, the operating voltage will be lowered.
In order to increase the difference in propagation constants, two parallel lines should be brought closer together. In other words, the light seeping out from one line is coupled to the other line, and the optical power is transferred to the other line, but the larger the amount of seepage, the faster the light will transfer to the other line. The closer the waveguides are, the faster the optical power transfers, and in this way, if the refractive index of this part is slightly changed using the electro-optic effect, etc.
That's how quickly you'll move on. However, the conventional example shown in FIG. 1 has the disadvantage that the element length of the element is long. The waveguides shown in FIG. 3 constitute a two-mode switch, and the waveguides 2A and 2B in the center of FIG.
The operating voltage is lower because it is coupled with
In a device having a waveguide with such a structure, the waveguide width is large and it is easy to generate higher-order mode light or radiation mode light. Therefore, even if the fundamental mode light and the first-order mode light are out of phase by π, other higher-order modes The light is in a half-finished state, and the light cannot be emitted to only one waveguide, but the light is also emitted to the other waveguide, resulting in poor crosstalk characteristics.

On the other hand, in order to make it easy to connect optical fibers to the input and output sides of the optical switch, the distance between the input and output ends of the pair of optical waveguides on the input and output sides should be set according to the diameter of the fibers. It is necessary to have.

In the conventional examples shown in Figures 1 and 3, it is possible to design the length and angle of the input and output side optical waveguides and the optical waveguide sandwiched between them separately. As is clear from the figure, the two-mode switch shown in Figure 2 has a structure in which straight lines intersect, so the crossing angle of the intersecting optical waveguides on the input and output sides and the length of the optical waveguide between the crossing positions are dependent. It has become a relationship. This structure is easy to design to excite the fundamental mode and primary mode.

In Fig. 2, the intersection angle of the optical waveguides on the input and output sides is smaller than in Figs. 1 and 3, and the length of the central optical waveguide between the intersection points is the same. If it is made larger, the length of the central optical waveguide between the intersection points will be shorter than in the cases of FIGS. 1 and 3. Since the operating voltage is inversely proportional to the length between the central optical waveguides between the crossing points, in that case the second
In the structure shown in the figure, the operating voltage is higher than in the structures shown in FIGS. 1 and 3.

(d) Object of the Invention It is an object of the present invention to eliminate the above-mentioned drawbacks, to provide an optical waveguide device that operates at low voltage, has a relatively short element length, and has good crosstalk characteristics.

(e) Structure of the Invention According to the present invention, the object is to provide a pair of input-side optical waveguides and a pair of output-side optical waveguides, each of which gradually expands outward and intersects at one end thereof. It consists of a central optical waveguide provided between the intersection points of the waveguides, and the outer linear portion of the input side and output side optical waveguides at each intersection point and the outer linear portion of the central optical waveguide coupled thereto are connected at an obtuse angle. The width of the central part of the central optical waveguide is selected to be smaller than the width of the central optical waveguide at the intersection position, and the outer linear part of the central optical waveguide gradually becomes narrower toward the central part. This is achieved by an optical waveguide characterized by a curved shape.

(f) Embodiment of the Invention An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 4 is a plan view showing one embodiment of the optical waveguide of the present invention, and FIG. 5 is a plan view showing another embodiment of the optical waveguide of the present invention.

As shown in FIG. 4, the optical waveguide of the present invention includes a central optical waveguide 23 sandwiched between a pair of input side optical waveguides 21A, 21B and a pair of output side optical waveguides 22A, 22B. The crossing angle of each optical waveguide on the output side can be made larger than that shown in FIG. The outer linear portions of the output side waveguides 22A and 22B at their respective intersection positions and the outer linear portion of the central optical waveguide are connected at an obtuse angle,
Moreover, the width at this intersection position is larger than the width at the central portion A. Therefore, the length of the central optical waveguide is longer than the conventional optical waveguide between the intersection points when the input side waveguide and the output side waveguide are lined up in a straight line as shown in FIG.

In this way, the central optical waveguide sandwiched between the intersections of the input and output optical waveguides is made longer than the conventional one, so the voltage applied to the electrodes can be low. Narrowing the aperture creates conditions that make it difficult for higher-order modes to be excited, which means that unnecessary higher-order modes are selectively radiated, resulting in less crosstalk than in the past.

In addition, as another embodiment of the present invention, as shown in FIG. b
has a value larger than the width a of each input/output optical waveguide and smaller than the width c of the intersection position of the input/output side optical waveguides. It has been experimentally confirmed that by using such an optical waveguide, there is less crosstalk than in the conventional optical waveguide, and the voltage applied to the electrodes can be lower than that of the conventional optical waveguide.

To form such an optical waveguide, Ti is deposited in a predetermined pattern on a LiNbO ₃ electro-optic crystal substrate using a mask to a thickness of approximately 400 Å and a width of approximately 7 μm, and then diffused at a temperature of 1040°C. to form an optical waveguide with a depth of approximately 5 μm. Then SiO ₂ if necessary
After a film is deposited to form an insulating film, Al metal is deposited in a predetermined pattern to form electrodes and an optical waveguide.

(g) Effects of the Invention As described above, the optical waveguide of the present invention has the advantage of being able to operate at low voltage and provide a high-performance optical waveguide with little crosstalk phenomenon.

[Brief explanation of drawings]

1 to 3 are plan views showing the structure of a conventional optical waveguide, and FIGS. 4 and 5 are plan views showing an embodiment of the optical waveguide of the present invention. In the figure, 1 is a LiNbo ₃ substrate, 2, 2A, 2B,
3, 4, 23, 33, are optical waveguides, especially 21A,
21B, 31A, 31B are input side optical waveguides, 22
A, 22B, 32A, and 32B are the output side optical waveguides A and B are central portions of the central optical waveguide, and a, b, and c are the width dimensions of the optical waveguides.

Claims (1)

[Scope of Claims] 1. A pair of input-side optical waveguides and a pair of output-side optical waveguides, each of which gradually expands outward and intersects at one inner end thereof, and is provided between the intersecting position of both waveguides. The outer linear portion at each intersection position of the input side and output side optical waveguides and the outer linear portion of the central optical waveguide coupled thereto are connected at an obtuse angle, and the central optical waveguide The width of the central part of the waveguide is selected to be smaller than the width of the central optical waveguide at the intersection position, and the outer linear part of the central optical waveguide gradually tapers toward the central part. optical waveguide.