JPH03225301A - Optical branching circuit - Google Patents

Abstract

PURPOSE:To obtain a stable branching ratio even if an optical waveguide of an optical branching circuit for incident light is shortened by making the refractive index of the waveguide for the incident light of the circuit lower than that of optical waveguides of the circuit for exit light. CONSTITUTION:When optical waveguides are formed on a Z-cut LiNbO3 substrate 11, Ti as an impurity for increasing the refractive index of the substrate is diffused by the prescribed widths of the optical waveguides and a prescribed film thickness and then MgO as an impurity for reducing the refractive index is diffused only in a part for a waveguide 12 for incident light before branching. By this double diffusion of Ti and MgO, the waveguide 12 has sufficiently lower intensity of confinement of light than primary mode cutoff. The thickness of the resulting Ti film as waveguides 14, 15 for exit light is set so as to attain intensity of confinement close to primary mode cutoff. Since light is weakly confined in the waveguide 12, the propagation mode distribution of light incident on the waveguide 12 becomes a steady state within the shortest distance and a stable branching ratio is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical branch circuit used in optical integrated circuits and the like, and particularly relates to a waveguide type optical branch circuit.

[Prior art] In optical integrated circuits that realize various functions using optical waveguides formed on a substrate, optical branch circuits are the most basic optical circuit elements, and are used for optical signal distribution, Matsuhatsu Ender type modulators, etc. , branch type optical switches, etc.

A conventional example will be described below by taking a symmetrical two-branch circuit using a Ti-diffused L i N b 03 optical waveguide as an example.

FIG. 3 is a perspective view showing this optical branch circuit.

By diffusing Ti on the L i N b 03 substrate 31, the refractive index is increased and a single mode optical waveguide is formed. The incident light enters the incident optical waveguide 32 from the optical waveguide end face 36 by direct optical fiber coupling or lens coupling. The light propagated through the input optical waveguide 32 is split into two beams by a bifurcation 33, and each beam propagates through output optical waveguides 34 and 35. Since the excess loss of the optical branching circuit increases in proportion to the branching angle θB of the two branches, the angle is made as small as possible.

Furthermore, as a means to reduce excessive loss due to mode conversion at the branch section, the branch section has a tapered structure.
(MasaIIritsu Haruna et.
at; 'Electrooptical Bra
nching Waveguide 5w1tches
and their Application to
LX 40ptical SwitchingNetw
orks': Journal of' Llghtw
ave Technology.

vol, LT-1, NO, l, P223 '83)
, a mode-coupling type branch (Seino et al.; ゛Mode-coupling type Y-branch waveguide.

1986 National Conference of the Institute of Electronics and Communication Engineers, Optical and Radio Division.

There are reports such as lecture number 250). Optical branches with an excess loss of about 0.3 dB have been obtained by these methods.

[Problems to be Solved by the Invention] In conventional optical branch circuits, emphasis is placed on reducing excessive loss in the branch portion. However, in the optical branching circuit, the branching angle θ8 cannot be set large due to the increase in loss, so curved optical waveguides for widening the spacing between the output optical waveguides are practically indispensable. In such a waveguide structure, even if the branch portion has low loss, the radius of curvature of the curved waveguide is increased in order to miniaturize the circuit, which may increase radiation loss. Therefore, the loss of an optical branch circuit should be considered as the sum of the mode conversion loss of the branch and the radiation loss of the curved optical waveguide.

Therefore, when reducing the loss of the optical branch circuit as a whole, reducing the loss at the bent portion is an important factor to consider. It is also known that radiation loss can be reduced by increasing the radius of curvature of the bend in the output optical waveguide, or by increasing the amount of increase in the refractive index of the optical waveguide to strengthen optical confinement within the optical waveguide. It is being

When reducing the radius of curvature of a curved waveguide in order to miniaturize a conventional optical branch circuit, the optical confinement strength of the optical waveguide must be increased to reduce radiation loss at the curved portion. . However, in this case, in the conventional optical branch circuit, the light confinement strength of the incident optical waveguide also becomes large, and the distance required for the light incident from the incident end face to reach a steady state within the optical waveguide becomes long. If the propagation mode branches before reaching a steady state, a stable optical branching circuit cannot be obtained because the branching ratio varies depending on the incident state of the light.Therefore, in order to stabilize the branching ratio, it is necessary to This means that the optical waveguide must be sufficiently long.In other words, in an optical branch circuit with a conventional structure, in order to shorten the length of the bend, the input optical waveguide must be lengthened, which creates a contradiction. Put it away.

[Means for Solving the Problems] Means provided by the present invention to solve the above-mentioned problems is as follows: The incident light is guided by one optical path over a certain length, and the light is guided by the one optical path. It has an optical waveguide that branches light into a plurality of optical paths, and the optical waveguide is a high refractive index optical path that has a higher refractive index than other parts by introducing impurities into a path-like shape on the substrate. In the optical waveguide, at least one of the plurality of optical paths guiding the branched light is an optical branching circuit having a curved part, and
When the part of the optical waveguide that branches the light guided by one optical path into a plurality of optical paths is used as a branching part, the refractive index of the one optical path (incoming optical waveguide) that guides the incident light to the branching part is the branching part. Optical path (output optical waveguide) that guides the light branched at the
It is characterized by a refractive index smaller than that of .

[Function] In the present invention, by increasing the concentration of impurities that increase the refractive index of the output optical waveguide, the guided light can be strongly confined within the optical waveguide and bending radiation loss can be reduced. By introducing another impurity that has the effect of reducing the refractive index into the waveguide, the strength of light confinement is weakened, so that the incident light reaches a steady state over a shorter distance than when the refractive index of the incident optical waveguide is not reduced. do. In other words, according to the present invention, it is possible to independently optimize the optical confinement strength of the input optical waveguide and the output optical waveguide of the optical branch circuit.

Therefore, by applying the present invention, a stable branching ratio can be obtained even if the input optical waveguide is shorter than the conventional one, and the loss can be reduced even if the length of the output optical waveguide is shortened by reducing the radius of curvature of the bend. This makes it possible to create a low waveguide structure, making it possible to realize a small optical branch circuit with low loss.

[Example] Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a perspective view showing a first embodiment of the present invention.

Z-Cut LiNb0. An optical waveguide is formed on the substrate 11 under the following conditions. First, the impurity that increases the refractive index is T1, the optical waveguide width W is W-8 μm, and T before diffusion is
i The film thickness di is set to d and -800A, and T1 is diffused for 8 hours in an air atmosphere at 1050°C. Then, the impurity that reduces the refractive index is MgO, the width W is 8 u rn only in the part of the input waveguide 12 before branching, and the Mg before diffusion is
The O film thickness d2 is set to 450A, and MgO is diffused for 4 hours in an oxygen atmosphere at 950°C.

The input waveguide 12 before branching is set so that the light confinement strength is sufficiently weaker than the first mode cutoff by doubly diffusing Ti, which increases the refractive index, and MgO, which decreases the refractive index. There is.

In addition, the output waveguide 14 after branching in which only Ti is diffused
．． The Ti film thickness d1 of No. 15 is set to have a confinement strength near the first-order mode cutoff. In the optical branch circuit created under the above conditions, the input optical waveguide 12
In this case, light confinement becomes weaker due to the influence of MgO diffusion.

Therefore, the propagation mode distribution of the light that has entered the incident optical waveguide 12 becomes a steady state at the shortest distance, and a stable branching ratio is obtained.

Further, the output waveguides 14 and 15 after branching have a high degree of Tia and the optical confinement of the guided light is stronger than that of the conventional one, so that the radiation loss at the curved portion can be reduced more than that of the conventional one.

In this way, in this embodiment, the optical confinement strength of the input waveguide and the output waveguide to the branch is optimized by selectively doubly diffusing Ti and MgO. Therefore, in the structure of the embodiment shown in FIG. 1, a stable branching ratio can be obtained even if the input optical waveguide is made shorter than the conventional one, and even if the radius of curvature of the output optical waveguide is made smaller than the conventional one, there is no loss. An optical waveguide structure without increase can be achieved. Therefore, if the loss is the same as that of the conventional circuit, this embodiment can be made more compact, or if the size is the same as that of the conventional circuit, an optical branch circuit with lower loss can be realized.

FIG. 2 is a perspective view showing a second embodiment of the invention. In this example, Ti is first diffused with an optical waveguide width W of 8 μm and a Ti film thickness d before diffusion of 80 OA, and then Mg
The width W of the input waveguide 12 before branching is 8 μm, the MgO film thickness d2 before diffusion is 450A, and the optical waveguide branch 23
Diffusion is performed by changing the MgO film thickness from d2 to O in a tapered manner. MgO film thickness taper 100~200ha/mm
If this is done, mode conversion loss due to changes in the refractive index of the optical waveguide will hardly occur, making it possible to realize an optical branching circuit with lower loss.

Although the case of a Ti-diffused L i N b O3 optical waveguide has been explained above as an example, the present invention can also be applied to optical branch circuits formed using impurity-diffused optical waveguides made of other waveguide materials such as silica-based glass and semiconductors. The same effect can be obtained in any case.

The present invention can also be applied when connecting optical branch circuits in multiple stages in a cascade, increasing the impurity concentration that increases the refractive index of the output optical waveguide, and decreasing the refractive index of the input optical waveguide of the next stage optical branch circuit. By diffusing impurities, the optical confinement strength of the input waveguide and output waveguide to the branch can be optimized.

Therefore, even if the input optical waveguide is shortened, a stable branching ratio can be obtained, and even if the radius of curvature of the output optical waveguide is made smaller than that of the conventional optical waveguide, an optical waveguide structure with no increase in loss can be achieved.

[Effects of the Invention] As explained above, according to the present invention, the optical confinement strength is independently controlled by changing the impurity concentration of the input optical waveguide and the output optical waveguide of the optical branching circuit, and the branching ratio of the input optical waveguide is adjusted. It is possible to achieve effects that were not possible with conventional optical branching circuits, such as stabilizing the output waveguide and reducing loss in the output optical waveguide. This makes it possible to realize an optical branching circuit that is much smaller, has lower loss, and provides a more stable optical branching ratio than conventional circuits.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing a first embodiment of the present invention, FIG. 2 is a perspective view showing a second embodiment of the invention, and FIG. 3 is a perspective view showing a conventional optical branch circuit. 11.21.31...LiNbO3 substrate, 12°22
．． 32... Incoming optical waveguide, 13, 23. 33... Optical waveguide branch, 14, 15, 24, 25, 34° 35.
... Output optical waveguide, 36... Optical waveguide end surface.

Claims (1)

[Scope of Claims] An optical waveguide is provided, which guides incident light in one optical path over a certain length and branches the light guided by the one optical path into a plurality of optical paths, and the optical waveguide is formed on a substrate. The optical waveguide is a high refractive index optical confinement path in which impurities are introduced into a path-like shape to have a higher refractive index than other parts, and at least one of the plurality of optical paths guiding the branched light is an optical waveguide. One is an optical branching circuit having a curved bending part, and when the optical waveguide in the part where the light guided by the one optical path is branched into a plurality of optical paths is used as a branching part, the incident light is guided to the branching part. An optical branching circuit characterized in that the refractive index of the one optical path is smaller than the refractive index of the optical path that leads out the light branched at the branching section.