Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
1999-02-26
2001-05-22
Phan, James (Department: 2872)
Optical waveguides
With optical coupler
Particular coupling structure
C385S039000
Reexamination Certificate
active
06236784
ABSTRACT:
FIELD OF THE INVENTION
The present invention of the present application relates to an optical waveguide and a lightwave circuit.
BACKGROUND OF THE INVENTION
A study of a lightwave circuit in which optical waveguides, Y branch optical waveguides, and directional couplers each composed of silica (glass) or polymer or the like are brought into integration on a substrate, has recently been brought into activation with a view toward aiming at reductions in the sizes and costs of optical parts. The Y branch optical waveguide is important as an element which constitutes the lightwave circuit. In order to reduce its branch excess loss, Y branch optical waveguides having various structures have been discussed.
While a symmetric optical waveguide whose branching ratio is 1:1, has been usually discussed as a Y branch optical waveguide, an asymmetric Y branch optical waveguide having a different branching ratio is required according to uses. An asymmetric Y branch using a silica (glass) waveguide has been described in, for example, “NTT R&D”, Vol. 46, No.5, 1997, pp.473-485 (Article 1) or “Proceedings of the 1995 Electronics Society Conference of the Institute of Electronics, Information and Communication Engineers”, SC-1-15, pp.337-338 (Article 2). An optical transmission and reception module excellent in received sensitivity and a splitter having an arbitrary number of branches have been implemented by using such an asymmetric Y branch optical waveguide.
Described specifically, the former (Article 1) example has an input waveguide, a tapered waveguide for extending or spreading incident light, and two output waveguides. Further, the branching ratio is rendered asymmetric by shifting the input waveguide and the tapered waveguide from a central axis by a predetermined value. In the latter (Article 2) example on the other hand, the width of a core at each input terminal or end of a branch optical waveguide is set as W
1
and the widths of cores at two output terminals or ends are respectively set as W
0
. Further, a tapered waveguide is provided in which each core width gradually spreads from W
1
to W
0
along a light traveling direction L. The branching ratio is made asymmetric by setting an increase ratio dW/dL of each core width to different values with two waveguides which are caused to branch off.
SUMMARY OF THE INVENTION
Problems on the hitherto-used prior art will be made clear prior to the description of the invention of the present application. Namely, when the asymmetric Y branch optical waveguide described as the conventional example is actually fabricated, the following problems arise.
In the former (Article 1), a tip of a clearance or interval defined between the output waveguides is brought into rounded form (whose width is about 3 &mgr;m) because of the resolution of photolithography and side etching or the like which occurs upon core patterning. Light incident to each output waveguide has a large light intensity at a central portion. Thus, since the light is scattered at the round portion, a radiation loss occurs in a Y branch. The radiation loss is relatively small and negligible in an optical waveguide in which a refractive index difference between each core and a clad is relatively small (e.g., when the refractive index difference is less than or equal to 0.3%). However, when a waveguide having the large refractive index difference is used (e.g., when the refractive index difference is 0.45% or above), the radiation loss becomes extremely great, thus leading to practical trouble.
On the other hand, in the latter (Article 2), an asymmetric Y branch is designed so as not to produce a radiation loss excessively by taking a wedge-shaped structure even if the width between waveguides is finite. However, since a portion strong in light or light intensity is incident to and scattered at a slit portion, a large radiation loss eventually occurs in a waveguide in which a refractive index difference between each core and a clad is large. Further, the width of each input end of the branch waveguide become narrowed to a half extent as compared with other portions. Thus, when the waveguide is formed by a weak material such as polymer or the like, a region for this is apt to have a possibility of the waveguide being cracked by stress or the like. Further, when a Y branch is actually fabricated, defects such as voids (bubbles) might occur in a narrow portion between the branch waveguides upon forming an upper clad layer and embedding the Y branch therein. Therefore, since a portion strongest in light falls on or hits the defects even in the case of any conventional asymmetric Y branches, a large radiation loss occurs.
An object of the present invention is to provide an asymmetric Y branch optical waveguide which provides less radiation losses and has a stable branching ratio, and a lightwave circuit using the asymmetric Y branch optical waveguide. The present asymmetric Y branch optical waveguide and the lightwave circuit using it are useful for use in an optical transmission system or an optical network or the like.
The present inventors have reconsidered various configurations of the Y branch optical waveguide over its entirety with a view toward solving the problems of the aforementioned conventional example. With respect to the invention of the present application, the present inventors have paid attention to a multi-mode interference (multi-mode interference: hereinafter abbreviated as “MMI”) type Y branch optical waveguide as an optical waveguide, and considered the fabrication of an asymmetric Y branch by using it. With respect to the MMI type Y branch, a symmetric (1:1) Y branch is known as will be described later. However, an asymmetric Y branch is not known. The present inventors have carried out an extensive investigation about the MMI type Y branch and have found out that an asymmetric Y branch optical waveguide having less radiation losses and a stable branching ratio could be obtained by contriving its structure.
Namely, an asymmetric Y branch optical waveguide according to the invention of the present application comprises an input waveguide for entering light therein, two output waveguides for outputting the light therefrom, and a multi-mode waveguide which is disposed between the input waveguide and the two output waveguides and generates a plurality of mode lights therefrom, and wherein the multi-mode waveguide is made asymmetric with respect to a center line extending in the direction of an optical axis.
Here, the configuration of asymmetry can be obtained by the following methods, for example. The first is a method of setting the width of one of entrances or entrance portions of a multi-mode waveguide, which are divided by a center line so as to be smaller than that of its corresponding exit portion of the multi-mode waveguide. The second is a method of setting the width of one of intermediate portions of the multi-mode waveguide, which are divided by the center line so as to be smaller than that of its corresponding exit portion of the multi-mode waveguide. On the other hand, even in the case of either of the first and second, the exit portions of the multi-mode waveguide are set symmetrically with respect to the center line extending in the direction of the optical axis.
Further, an embodiment of another asymmetric Y branch optical waveguide according to the invention of the present application comprises an input waveguide for entering light therein, two output waveguides for outputting the light therefrom, and a multi-mode waveguide which is disposed between the input waveguide and the two output waveguides and generates a plurality of mode lights therefrom, and wherein the distances between sides of core portions of the multi-mode waveguide and a center line differ from each other at least at a portion with respect to the direction of traveling of the light.
When it is desired to allow the distances between the sides of the core portions and the center line to differ from each other, the distance is changed into form curved with respect to the traveling directi
Hitachi , Ltd.
Kenyon & Kenyon
Phan James
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