JPH11167032A - Bent optical waveguide circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a curved optical waveguide circuit small in size and low in loss. SOLUTION: The light emitted from a light guide part 103 is reflected by a reflecting groove 105 and recoupled with the light guide part 103 by arranging the reflecting groove 105 which has a reflecting function across a gap (g) at the outer peripheral part of the curved light guide part 103 continued through both end parts of an input/output waveguide 102 to/from which an input/output light entering/outgoing.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bent optical waveguide circuit, and more particularly to a bent optical waveguide circuit which guides a light wave propagating through an optical waveguide with low loss. It is something. 2. Description of the Related Art In an optical switch, an optical modulator, or the like using an optical waveguide, a monolithic or hybrid integrated optical device is provided between an optical input / output section and an optical function processing section, or Y.
A bent optical waveguide is often used between an internal optical circuit such as a branch or a directional coupler and an optical function processing unit. The structure of these bent optical waveguides is usually formed in an S-shaped arc or trigonometric shape. FIGS. 7A and 7B show an example of a basic configuration of a bent optical waveguide circuit using a conventional titanium thermal diffusion lithium niobate (: Ti-LiNbO ₃ , hereinafter referred to as “Ti-LN”) optical waveguide. It is shown in b). FIG. 7A shows a curvature radius r.
FIG. 4 is a plan view of a bent optical waveguide in which arcs of FIG. In the figure, reference numeral 701 denotes an LN substrate, which constitutes a clad portion of an optical waveguide. Reference numeral 702 denotes an input / output waveguide, and reference numeral 703 denotes a bent optical waveguide, all of which constitute a core portion (a Ti thermal diffusion portion) of the optical waveguide. Reference numeral 704 denotes input / output light. FIG.
(B) shows the input / output waveguide 702 and the bent optical waveguide 703 slightly shifted in axis. Thus, the field (electric field or magnetic field) distribution of the respective propagating lights is made to substantially match, and the radiation loss of the bent optical waveguide 703 can be reduced. In this case, the normal axis deviation amount Δx ₂ is set to about twice as large as Δx ₁ .
Further, the waveguide width w ₂ is typically in the magnitude almost equal to the output waveguide width w _1. [0004] In the bent optical waveguide circuit according to the prior art as described above, in order to reduce the size, the waveguide length L is reduced while the waveguide distance d is kept constant. Then, there is a disadvantage that the loss in the bent optical waveguide 703 increases. In particular, in the case of the LN optical waveguide, the loss of light tends to increase because the optical confinement intensity of the waveguide (the amount determined by the refractive index difference between the waveguide core and the clad and the core width and thickness) is smaller than that of the semiconductor waveguide or the like. It becomes remarkable. In order to improve this, it is known that, for example, a bent optical waveguide having a trigonometric function may be used instead of the arc shape, but in this case, there is a limit to downsizing. For example, in the case of an ordinary Ti-LN waveguide, when the effective radius of curvature is about 50 mm or less, the loss increases rapidly, and it is difficult to reduce the size and increase the integration. On the other hand, there is a method of arranging a medium having a relatively small refractive index in a cladding region adjacent to the outer peripheral surface of a bent waveguide. However, when this method is applied to a weakly confined waveguide such as a Ti-LN waveguide, the refractive index distribution in the waveguide cross section direction (vertical and horizontal directions in the cross section)
Has an asymmetric configuration, so that the confinement is weaker and the radiation loss is rather large. An object of the present invention is to provide a small-sized and low-loss bent optical waveguide circuit in view of the above prior art. [0007] The structure of the present invention that achieves the above object has the following features. 1) In a bent optical waveguide circuit having a bent optical waveguide in which a part of the optical waveguide is formed in a curved shape or a bent shape, the light is reflected in the vicinity of the outer peripheral surface opposite to the bending direction of the bent optical waveguide region. That the part was arranged. 2) In the bent optical waveguide circuit described in 1) above, the reflecting portion is disposed near the inner peripheral surface on the same side as the bending direction of the bent optical waveguide. 3) In the bent optical waveguide circuit described in 1) or 2) above, the phase of the radiated light reflected by the reflecting portion and recombined with the bent optical waveguide is matched with the phase of the guided light of the bent optical waveguide. , The position of the reflecting portion is set. 4) In the bent optical waveguide circuit described in any one of 1) to 3) above, the width of the bent optical waveguide is made larger than the width of the input / output waveguide, or the bent optical waveguide is connected to the optical axis. The axis is shifted in the direction perpendicular to the axis. Embodiments of the present invention will be described below in detail with reference to the drawings. FIGS. 1A and 1B are diagrams showing a bent optical waveguide circuit according to a first embodiment of the present invention using LiNbO ₃ as a substrate, wherein FIG. 1A is a plan view thereof, and FIG. It is sectional drawing. In both figures, a cladding portion 101 is a cladding portion of an optical waveguide formed of an LN substrate. I / O waveguide 10
Reference numeral 2 denotes a core portion of the optical waveguide formed by diffusing impurities such as titanium (Ti) or proton.
The bent optical waveguide 103 is an S-shaped arc-shaped bent optical waveguide having a radius of curvature r, and is formed by a core portion of the optical waveguide formed by diffusion of an impurity such as titanium (Ti) or proton. The input / output light 104 enters and exits the bent optical waveguide 103 via the input / output waveguide 102. The reflection groove 105 is formed at a position which is bent at the surface of the LN substrate constituting the cladding part 101 and is separated from the optical waveguide circuit 103 by a gap g. Here, a case is shown in which air (functioning as a cladding of a waveguide) is disposed above the LN substrate. The bent optical waveguide 103 is
Basically, it has the same configuration as the prior art shown in FIG. FIG. 2 is an explanatory diagram for explaining the principle of the bent optical waveguide circuit according to the embodiment shown in FIG.
As shown in the figure, the light propagation in the bent optical waveguide 203 is understood as a leakage waveguide phenomenon. That is, a part of the guided light propagating in the optical waveguide is mainly radiated to the outer periphery of the optical waveguide, and the amount of radiation is substantially determined by the confinement effect of the waveguide and the radius of curvature r. As the radius of curvature r decreases, the radiation amount increases and the loss increases. In the present invention, as shown in FIG. 2, the radiated light is reflected by the peripheral surface of the reflection groove 205, and is bent again and returned to the optical waveguide 203. At this time, the wavelength of light, the strength of the confinement effect of the waveguide,
The present invention is characterized in that the gap g is set to an appropriate size so as to match the radius of curvature r, so that the effective optical path lengths of the emitted light and the guided light are matched.
That is, when the radiated light returns to the waveguide again, the phase planes of the radiated light are made substantially coincident with each other so that the light is efficiently coupled and the loss is reduced. In FIG. 2, reference numeral 201 denotes a clad portion. FIG. 3 is a characteristic diagram for explaining the effect of the bent optical waveguide circuit according to the embodiment of the present invention shown in FIG.
9 shows a calculation result using the beam propagation method. Here, in the embodiment shown in FIG. 1, the case where the bent optical waveguide 103 formed by diffusing Ti into the LN substrate by the ordinary thermal diffusion method is analyzed. In order to simplify the calculation, the slab waveguide model analysis by the equivalent refractive index method is performed, assuming that the core layer and the cladding part have a uniform refractive index for the diffusion waveguide. The waveguide width was calculated using w = 6 μm as an example. In FIG. 1, the distance between the input and output waveguides is d = 50 μm, and
5, the radius of curvature r of the S-shaped arc when the length L of the bent optical waveguide 103 is changed using the gap g as a parameter.
4 shows the loss characteristics per unit length of the waveguide with respect to. In FIG. 3, the broken line shows the case where the gap g is sufficiently large, and shows the characteristics of the prior art shown in FIG. As is clear from FIG. 3, in the case of the Ti-LN waveguide, when the gap g is set to about 5 to 10 μm, the curvature radius r can be made smaller than that of the related art while ensuring low loss characteristics. I understand. When the gap g is smaller than 3 μm, the loss increases remarkably. This is because the field distribution of the bent optical waveguide 103 is greatly different from the field distribution of the input / output waveguide 102 due to the influence of the reflection groove 105, and the coupling loss therebetween is increased. This is because, since the refractive index distribution in the direction becomes asymmetric, the confinement becomes weaker and the radiation loss becomes larger as compared with the case where the reflection groove 105 is not provided. On the other hand, when the gap g is about 15 μm or more, the phase conditions of the emitted light and the guided light do not match, and the emitted light does not recombine with the waveguide, and the loss increases. FIG. 4 is a plan view showing a second embodiment of the present invention, in which a gap g ₂ is also provided near the inner peripheral surface on the same side as the bending direction of the S-shaped bent optical waveguide 403. And a reflection groove 406 is arranged. In this case, the light wave radiated to the inner peripheral side of the bent optical waveguide 402 can be re-coupled to the waveguide according to the same principle as in the embodiment shown in FIG. In FIG. 4, reference numeral 401 denotes a clad portion,
Reference numeral 402 denotes an input / output waveguide, and reference numeral 405 denotes a reflection groove, which functionally corresponds to the reflection groove 105 in FIG. Further, g ₁ is functionally corresponding gap to the gap g in FIG. FIG. 5 is a plan view showing a third embodiment of the present invention. The position of the S-shaped bent optical waveguide 503 is shifted by Δx ₁ and Δx _{2 in a} direction perpendicular to the light propagation direction. I have. Also, to further reduce the radiation loss by enhancing the bending beam width w ₂ of the waveguide section 503 slightly larger configuration than the width w ₁ of the input and output waveguides 502, waveguide confinement effect. In FIG. 5, reference numeral 501 denotes a clad portion,
A reflection groove 505 functionally corresponds to the reflection groove 105 in FIG. Further, g is a gap functionally corresponding to the gap g in FIG. FIG. 6A is a plan view showing a fourth embodiment of the present invention. In place of the S-shaped bent optical waveguide portion, a bent optical waveguide 60 formed in a bent linear shape is used.
3 is used. In the present embodiment, as shown in FIG. 6B, the radiated light is totally reflected by the reflection groove 605 and is bent and re-coupled to the optical waveguide 603. Here, the bending angles θ ′ and θ ″ are set according to the confinement strength of the bent optical waveguide 603 and the magnitude of the refractive index of the waveguide material. By appropriately setting the gap g, the phase plane of the radiated light to be recombined can be made to coincide with the phase plane of the guided light, and the same principle as that of the embodiment shown in FIG. In the meantime, the bent optical waveguide according to the conventional technique has a bent portion as shown in Fig. 6 (b), and the guided light at the oblique input / output end faces of the input / output waveguide portion 602 and the bent optical waveguide 603. It is intended to conduct the waveguiding using the refraction effect, but in fact, a part of the wave is radiated mainly toward the outer peripheral portion due to the diffraction phenomenon of the guided light and cannot be recombined.
The loss is large. Incidentally, in FIG.
1 is a cladding part, 602 is an input / output waveguide, and 604 is input / output light. In the above-described embodiments, the case where the operating wavelength is mainly in the 1.5 μm band, an L-shaped bent optical waveguide using LN for the substrate, and a Ti diffused waveguide for the core layer has been described. However, besides this, for example, as a waveguide material,
The present invention can be applied to devices using any optical waveguide material such as a ferroelectric material such as LiTaO ₃ or PLZT, or a semiconductor material, an inorganic material such as glass or quartz, or an organic material such as polyimide according to the wavelength of light. Can be applied. In addition, the same effects as those of the above-described embodiment can be obtained by appropriately setting the size of the gap g up to the reflection groove for the optical waveguide having any curved shape such as a trigonometric function as the curved optical waveguide shape. be able to.
In addition, the same applies when a low-refractive-index medium is arranged instead of the reflecting groove, or the emitted light is reflected by a high-reflective film in which a single layer or a plurality of films having different refractive indices are arranged at an appropriate thickness. In principle, the same effect as in the above-described embodiment can be obtained. As described above, the bent optical waveguide circuit according to the present invention has a reflection groove having a reflection function formed at a position having an appropriate distance at least on the outer peripheral portion of the waveguide. A small and low-loss optical waveguide configuration is made possible.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a first embodiment of the present invention using LiNbO ₃ as a substrate.
FIG. 3 is a diagram showing a bent optical waveguide circuit according to the embodiment of the present invention,
(A) is the top view, (b) is the AA sectional view taken on the line. FIG. 2 is an explanatory diagram for explaining the principle of the bent optical waveguide circuit according to the embodiment shown in FIG. 1; FIG. 3 is a characteristic diagram for explaining the effect of the bent optical waveguide circuit according to the first embodiment of the present invention shown in FIG. 1, and illustrates a beam propagation method for the bent optical waveguide in the 1.55 μm wavelength band. It shows the calculation results used. FIG. 4 is a plan view showing a second embodiment of the present invention. FIG. 5 is a plan view showing a third embodiment of the present invention. FIG. 6 is a diagram showing a fourth embodiment of the present invention, in which (a)
Is a plan view thereof, and (b) is an explanatory view showing a mode of reflection in this. FIG. 7 is a plan view showing a bent optical waveguide circuit according to the related art. [Description of Signs] 101, 201, 401, 501, 601 Clad portions 102, 402, 502, 602 Input / output waveguides 103, 203, 403, 503, 603 Bend optical waveguides 104, 604 Input / output light 105, 205, 405 , 406, 505, 605
Reflection groove

Claims (1)

Claims: 1. In a bent optical waveguide circuit having a bent optical waveguide in which a part of the optical waveguide is formed in a curved shape or a bent shape, the direction is opposite to the direction in which the bent optical waveguide region bends. A bent optical waveguide circuit, wherein a reflecting portion is arranged near an outer peripheral surface. 2. The bent optical waveguide circuit according to claim 1, wherein a reflecting portion is arranged near an inner peripheral surface on the same side as the bending direction of the bent optical waveguide. . 3. The bent optical waveguide circuit according to claim 1 or claim 2, wherein the radiated light reflected by the reflecting portion and recoupled to the bent optical waveguide, and the phase of the guided light of the bent optical waveguide. A bent optical waveguide circuit, wherein the position of the reflection portion is set so that the surfaces are aligned. 4. Any one of claims 1 to 3
Wherein the width of the bent optical waveguide is made larger than the width of the input / output waveguide, or the bent optical waveguide is shifted in the direction perpendicular to the optical axis. Bent optical waveguide circuit.