JPS63147111A - Optical waveguide circuit - Google Patents

Abstract

PURPOSE:To enhance the optical insulating characteristic of an above-mentioned waveguide and to increase the density and integration scale of circuits by determining the thickness direction of a substrate at the X-axis or Y-axis direction of a crystal and decreasing the refractive index in the required parts adjacent to the optical waveguide in the substrate region outside the above-mantioned waveguide by hydrogen atom substitution. CONSTITUTION:The substrate 1 consists of an LiNbO3 crystal and is so formed that the X-axis or Y-axis direction of the crystal is made to coincide with the thickness direction. Ti is diffused from the region of a prescribed circuit pattern to form the optical waveguide 2 having the high refractive index within the substrate. The lower refractive index regions 3A, 3B are provided to a belt shape of a specified width adjacently to the outer peripheral side of both curved paths 2B, 2C over the length from nearly the beginning point of the curve up to the end point. The regions 3A, 3B are formed by substituting Li in the LiNbO3 crystal with H over the regions of the specified depth and width. A transmission loss is thereby greatly decreased and the radius of curvature can be decreased simultaneously in the case of including the curved parts in the waveguide and, therefore, the density ad integration scale of the circuit are increased and the size of the element is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to improvements in optical waveguide circuits using lithium niobate crystals, and particularly to techniques for reducing transmission loss.

[Conventional technology]

Conventionally, titanium (Ti) was added to a substrate made of lithium niobate (LiNbO3) crystal in a specific circuit pattern.
) is known to form an optical waveguide with a high refractive index.

In the above-mentioned optical waveguide circuit, when a branch part or a merging part is provided in the circuit, the waveguide between these parts and the input/output end is made into a curved path so that the optical axis at the input/output end of the waveguide is on the substrate side. A structure is generally adopted in which the direction is changed to be perpendicular to the edge to increase the connection efficiency with the optical fiber.

When a circuit includes a curved path as described above, the smaller the radius of curvature of this curved path, the greater the wave light loss becomes.

On the other hand, miniaturization of equipment in the field of optoelectronics,
With the demand for high-density integration, there is a strong desire for high-density integration of optical waveguide circuits.In order to reduce the circuit length as much as possible, the radius of curvature of the curved path portion of the optical waveguide must be reduced as much as possible. It is necessary to make it smaller.

Furthermore, in order to achieve high-density integration, when multiple optical waveguides are installed in parallel in a circuit, it is desirable to narrow the spacing between these waveguides as much as possible, but if the spacing is made too narrow, it will cause problems between adjacent waveguides. There is a problem in that mutual interference (crosstalk) of propagating light occurs.

As a basic means of suppressing wave light loss at the above locations, there is a method of increasing the difference in refractive index between the waveguide and the region surrounding the waveguide. In an optical waveguide circuit using a LiN1)03 crystal substrate, the following method has been proposed to increase the above-mentioned refractive index difference. In other words, for the curved road portion, the T1 diffusion treatment is performed in two stages, and during the fourth stage treatment, as shown in Fig. 6, the deposition layer 2Q of the diffusion source T1 in the second stage treatment is transferred to the outer circumferential side of the curved road. A method has been proposed in which a waveguide is formed into a sawtooth shape with a continuous base, thereby creating a refraction amount gradient higher on the outer circumferential side than on the inner circumferential side in the waveguide. Also, for example, like the Matsuhatsu Ender type interferometer,
In circuits in which waveguides of the same type are very close to each other, the substrate portion between these waveguides is removed to a certain depth by ion etching, and an air layer is interposed between the sides of the waveguides, thereby increasing the refractive index difference mentioned above. Methods have been proposed to improve the optical insulation of waveguides.

[Problem that the invention seeks to solve]

However, among the conventional methods mentioned above, the former method requires a two-step diffusion treatment at high temperature, and in the first stage diffusion treatment, the deposited layer of the diffusion source T1 is formed in a sawtooth-like and complicated pattern. must be formed.

Furthermore, the above technique is only implemented in a circuit portion of an S-shaped curved waveguide with a small offset fi, and cannot be applied to a circuit requiring stricter conditions such as a right-angled target path.

In addition, the latter method of increasing the optical insulation of the optical waveguide by ion etching requires complicated masking means, and since etching essentially destroys the substrate material, the side surface of the waveguide in contact with the etched area is There is a problem in that microscopic unevenness is generated, which causes scattering loss of propagating light.

Furthermore, in many cases, the etching slot member that can precisely limit etching to a specific, minute area is extremely small, and in such circuits, it is impossible to remove only the substrate portion by etching without adversely affecting the waveguide. It's almost impossible.

The main purpose of the present invention is to improve the optical insulation properties of waveguides in optical waveguide circuits, without increasing optical loss, especially in curved path sections, which are a bottleneck when attempting to achieve high-density integration of circuits. It is an object of the present invention to provide a circuit structure in which the radius of curvature can be made smaller and the spacing can be further narrowed in the vicinity of the waveguide without increasing crosstalk.

[Means for solving problems]

In an optical waveguide circuit in which an optical waveguide is formed on a lithium niobate (LiNbO3) crystal substrate by titanium (Ti) diffusion, the thickness direction of the substrate is the X-axis or Y-axis direction of the crystal, and the substrate area outside the optical waveguide is At least the main parts adjacent to the optical waveguide are replaced with hydrogen atoms to achieve low refraction.

Typical examples of the main parts whose refractive index is lowered by hydrogen atom substitution are, when the waveguide includes a curved path, a band-shaped region of a constant width adjacent to the outer circumference of the curved path, and a band-shaped region with a constant width where multiple waveguides are in close proximity. Examples include an intermediate region between waveguides when the waveguides are arranged.

In addition, the replacement of Ll and H in the LiNbO3 crystal is achieved by masking the substrate surface with an ashes such as an aluminum film with openings in the main parts, and then contacting the substrate surface with a solution of a compound having hydrogen groups such as carboxylic acid. This can be easily done by heating to about 15oO to 300''C.

[For production]

The region in which hydrogen atoms are substituted in the X@ or Y-axis direction of the LiNbO3 substrate crystal has a large refractive index decrease of around o, ott compared to the original crystal, and this results in a hydrogen-substituted low refractive index region that is a light guide. By providing the waveguide on the outer periphery of the curved path or between adjacent waveguides, the optical insulation of the optical waveguide is improved, the radius of curvature of the curved path can be made smaller than before, and the spacing between parallel optical waveguides can be reduced. can be further reduced.

In addition, hydrogen atom substitution in the LiNbO3 crystal proceeds in one direction along the crystal axis and hardly diffuses to the periphery, so the cross section of the substitution region is approximately quadrilateral. It can be provided with a side contour.

The above facilitates high-density integration of optical waveguide circuits.

〔Example〕

The present invention will be described in detail below based on embodiments shown in the drawings.

FIG. 1 is a plan view of essential parts showing an embodiment of the present invention, in which the substrate is made of LiNbO3 crystal and is molded so that the X-axis or Y-axis direction of the crystal coincides with the thickness direction. By diffusing Ti from a predetermined circuit pattern area on the surface of the substrate, an optical waveguide having a higher refractive index than other parts of the substrate is formed in the substrate.

The optical waveguide 2 includes a straight path portion 2A, a first curved path portion 2B, a second curved path portion 2G whose center of curvature is located on the opposite side of 2B, and a second straight path following this. part 2
D is provided, thereby giving a predetermined amount of deviation between the double-sided lines 21.2D.

Adjacent to the outer periphery of both circuit lines 2B and 2G, a low refractive index region 3 is provided in the form of a band with a constant width, extending approximately from the start point to the end point of the curve.

FIG. 2 shows a partial cross section and refractive index distribution taken along line U-U in FIG. 1. As shown in the figure, both the low refractive index regions 3A and 3B are formed to extend below the bottom of the optical waveguide in the depth direction. Low refractive index region 3A.

3B is formed by replacing Ll in the LiNbO3 crystal with H over a region of a certain depth and width, and can be performed, for example, by the following method.

That is, the circuit formation surface of the substrate 1 is coated with a film such as an aluminum film, an opening is formed in a desired region using a well-known photolithography technique, and a compound having a hydrogen group, such as benzoic acid, is brought into contact with the surface through the opening. ,/jO″C
Heat treatment is carried out at a temperature of 1 to 4Z for several minutes to several hours.

With the above hydrogen substitution, the refractive index is around o, oy (AjJn
m wavelength light) decreases.

By the way, the loss at the bend of an optical waveguide is generally A =
It can be represented by the formula: 018Xp (-C2R). Here, R is the radius of curvature of the bend, and C1 and C2 are constants that depend on the lateral confinement effect on the propagating light and are independent of R.

In addition, in the transition section from a straight road to a curved road, A1 =
It is expressed by the formula Q/2R''.

Here, R is the radius of curvature of the curved path, and q is a constant depending on the confinement effect of the waveguide. To explain this based on Figure S, the power distribution 2 of light propagating inside the curved waveguide 2/
In order for 2 to be kept constant from the input end to the output end,
The tangential velocity at each point within the waveguide cross section must be a function of distance from the waveguide center of curvature. However, in reality, the propagation constant is the same from the inner circumference to the outer circumference, so the propagation constant at a certain radius of curvature RC matches that of the free space propagating light, which causes some of the propagating light to exit the waveguide. It is released into the air and becomes a loss.

However, as in the embodiment described above, if regions JA and 3B are provided adjacent to the outer periphery of the curved waveguide and whose refractive index is further lowered than the original refractive index of the substrate by hydrogen substitution, the above-mentioned problem can be achieved. The loss of wave light to the outside of the waveguide is reduced, and the radius of curvature of the curved path can be made smaller accordingly.

For example, in the case of a conventional LiNbO3 substrate T1 diffusion waveguide without the low refractive index region according to the present invention, the allowable limit radius of curvature for a 900-curve path is approximately /cm, whereas according to the present invention, the curvature radius A curved path with a radius of 1 mm is also possible, and even with a radius of curvature of about l1 mm, the transmission loss in a curved path can be significantly reduced compared to the conventional method.

Other embodiments of the present invention are shown in FIGS. 3 and q.

This example prevents crosstalk of propagating light between these waveguides //, /, when the spacing between the parallel optical waveguides //, / is extremely small, such as in the Matsuhatsu Ender interferometer circuit. Therefore, the low refractive index region 13 is provided by replacing the substrate portion between both waveguides with hydrogen to a certain depth as in the previous embodiment.

〔Effect of the invention〕

According to the present invention, when a Ti diffusion waveguide on a LiNbO3 substrate includes a curved path portion, the transmission loss in the portion can be greatly reduced, and at the same time, the radius of curvature of the curved path can be reduced, resulting in high circuit density. Integration and miniaturization of elements can be achieved.

Furthermore, the hydrogen replacement treatment of LiNbO3 crystals does not require expensive special equipment compared to the conventional two-step T1 diffusion treatment or ion etching treatment described above, and can be carried out at low cost and easily, and can be processed rapidly at low temperatures. There is an advantage.

Further, if necessary, the refractive index gradient can be made into a smooth curve distribution by heat treatment after the substitution treatment.

[Brief explanation of the drawing]

Fig. 1 is a plan view showing one embodiment of the present invention;
FIG. 3 is a plan view showing another embodiment of the present invention, FIG. D is a cross-sectional view of the main part of FIG. 3, and FIG. FIG. 6 is a plan view showing a state in which transmission loss occurs in a curved portion of a wave path, and a perspective view showing an example of a conventional method. l...LiNbO3 substrate ko, /ha/ , :=
-Ti diffusion waveguide 2B, jc... Curved path 31.3B, /J... Low refractive index region Patent applicant Nippon Sheet Glass Co., Ltd. Figure 1 Figure 2 Figure 3

Claims (3)

[Claims] (1) In an optical waveguide circuit in which an optical waveguide is formed on a lithium niobate (LiNbO_3) crystal substrate by titanium (Ti) diffusion, the thickness direction of the substrate is the X-axis or Y-axis direction of the crystal, and the substrate is outside the optical waveguide. An optical waveguide circuit characterized in that at least a main part adjacent to an optical waveguide of the region has a low refractive index by hydrogen atom substitution. (2) The optical waveguide circuit according to claim 1, wherein the main portion is a band-shaped region adjacent to the outer circumferential side of a curved path portion provided in the middle of the optical waveguide. (3) The optical waveguide circuit according to claim 1, wherein the main part is an intermediate region between a plurality of optical waveguides arranged closely.