Optical plane waveguide

Details Optical plane waveguide Optical plane waveguide

2000-03-03

2001-07-31

Ullah, Akm E. (Department: 2874)

Optical waveguides

Planar optical waveguide

Reexamination Certificate

active

06269211

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical plane waveguide having a core region with a high refractive index provided in a substrate and guiding signal light.
2. Related Background Art
As an optical element for processing, such as branching, an incident light from an input optical fiber and outputting the same to an output optical fiber, an optical plane waveguide is used. The optical plane waveguide comprises a silica substrate provided with a core region having a high refractive index formed therein, and the core region, for example, has a structure for branching the incident light. A light input/output port of the optical plane waveguide is disposed at an end portion of the silica substrate. That is, light incident on the core region of the optical plane waveguide from the input optical fiber propagates through and is branched in the core region, and further the branched light propagates through the core region thereafter so as to be outputted to the output optical fiber.
Recently, the downsizing and integration of the optical plane waveguide are required. For example, a technique disclosed in the Institute of Electronic Information Communication General Assembly C-3-155 in 1998 is for achieving downsizing and further integration for an optical plane waveguide: by increasing the relative refractive index difference of the core region with respect to the substrate to 0.4%, signal light in the core region can be more strongly confined therein, thereby enabling to lessen a radius of curvature of a bent portion of the core region.
SUMMARY OF THE INVENTION
The present inventors have studied the above-mentioned prior art, and consequently found out the following problems. Namely, in the conventional optical plane waveguide, the larger the relative refractive index difference of the core region with respect to the substrate is, the stronger the confinement for signal light in the core region is, and whereby the radius of curvature of the bent portion of the core region can become smaller. However, in order to maintain the stability of the optical characteristics of the optical plane waveguide by restraining generation of a higher order mode light (namely, generation of light other than fundamental mode light), the width and the thickness of the core region should be narrower as the relative refractive index difference of the core region with respect to the substrate is larger. In the case the width and the thickness of the core region are smaller, the mode field diameter of a fundamental mode light propagating through the core region is smaller than that of a fundamental mode light propagating through an optical fiber, and whereby the coupling loss between the optical plane waveguide and the optical fiber increases.
For example, when the cross-section perpendicular to the optical axis of the core region is designed to be a square shape in consideration of the polarization independence, when the following relational expression:
a≦&lgr;c
/(2
n
1
·{square root over (&Dgr;n)}).
is satisfied, wherein, a is the width (or thickness) of the core region, n1 is the refractive index of the core region, &Dgr;n is the relative refractive index difference of the core region with respect to the substrate, and &lgr;c is the cut off wavelength, generation of a higher order mode light can be restrained such that only a fundamental mode light can propagate through the core region. As it can be seen in the formula, in order to restrain the generation of a higher order mode light, as the relative refractive index difference &Dgr;n is larger, the width (or thickness) a of the core region should be smaller.
FIG. 1
is a graph showing the relationship between the relative refractive index difference &Dgr;n of the core region with respect to the substrate and the coupling loss in such a case. The graph shows the coupling losses of the cases with the wavelengths &lgr; 1.3 &mgr;m and 1.55 &mgr;m, and the cutoff wavelengths &lgr;c of 1.3 &mgr;m and 1.55 &mgr;m, respectively. As can be seen from the graph, as the relative refractive index difference &Dgr;n is larger, the width (or thickness) a of the core region should be smaller because the mode field diameter of a fundamental mode light propagating through the core region is smaller than that of a fundamental mode light propagating through an optical fiber and thus the coupling loss is larger. In particular, when the relative refractive index difference &Dgr;n is 0.4% or more, the coupling loss drastically increases.
The present invention has been achieved for solving the above-mentioned problems, and an object thereof is to provide an optical plane waveguide with a small coupling loss with respect to an optical fiber, capable of achieving downsizing and high integration.
An optical plane waveguide according to the present invention comprises a substrate provided with a core region having a higher refractive index than the substrate and one or more side core regions. The core region has a light input/output port at the end face of the substrate. The side core regions are provided along the core region and positioned at a peripheral portion of the substrate where the end face of the substrate is included. Also, the side core regions function for enlarging the mode field diameter of signal light in the direction parallel to the surface of the substrate. In accordance with the optical plane waveguide, since the side core regions are provided at one or both sides of the core region in the vicinity of the end face of the substrate, the mode field diameter of signal light can be enlarged in the direction parallel to the surface of the substrate. Therefore, even in the case the cross-sectional size of the core region is small, the coupling loss between the light input/output port on the end face of the optical plane waveguide and the optical fiber can be reduced because the side core regions are provided for enlarging the mode field diameter of signal light.
Further, in an optical plane waveguide according to the present invention, the widths of the core region and the side core regions are adjusted such that the optical coupling between the core region and the side core regions is improved and the propagation constant of the fundamental mode is changed continuously in the traveling direction, while restraining generation of a higher order mode light. In particular, it is preferable to have a narrow width of the core region and wide widths of the side core regions in a region where the normalized propagation constant of a higher order mode light is 0 or less. In this case, the mode conversion from the fundamental mode to the higher order mode is restrained and accordingly the coupling loss between the light input/output port of the optical plane waveguide and the optical fiber can be reduced.
Moreover, in an optical plane waveguide according to the present invention, each of the side core regions has a width of a predetermined size or more, and the distance between the core region and the side core regions is adjusted such that the propagation constant of the fundamental mode light is changed continuously along its traveling direction. In this case, since each of the side core regions has a width of a predetermined size or more, a sufficient processing accuracy can be ensured so that the effect of providing the side core regions can be obtained stably.
In an optical plane waveguide according to the present invention, the width of the core region is set such that the normalized propagation constant of the higher order mode light can be 0 or less as well as the thickness of the core region is set to be larger than the width of the core region. In this case, since the mode field diameter of the fundamental mode light can be enlarged not only in the lateral direction but also in the vertical direction (thickness direction of the core region) of the substrate, the coupling loss can further be reduced. Furthermore, in the optical plane waveguide, a circuit whose characteristics are not influenced by the higher o

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