Optical waveguides – Planar optical waveguide – Thin film optical waveguide
Reexamination Certificate
2001-12-27
2004-12-28
Healy, Brian (Department: 2874)
Optical waveguides
Planar optical waveguide
Thin film optical waveguide
C385S122000, C385S124000, C385S129000, C385S131000, C385S132000, C385S141000, C385S142000, C065S385000, C438S031000
Reexamination Certificate
active
06836608
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to planar optical waveguides used for optical communication, optical information processing or the like, and to methods for manufacturing the same. In particular, the present invention relates to planar optical waveguides made of polymer material and to methods for manufacturing the same.
As the technology of optical information processing advances and optical communication systems are put into practice, there is a need for the development of a variety of components for optical communications, such as optical transmission lines, semiconductor lasers and photodetectors. Of these, optical transmission lines transmitting optical signals are especially important, and necessary requirements are that the optical losses are small, and manufacturing is easy.
As optical transmission lines, there are quartz-based optical transmission lines, which are based on quartz, and organic polymer optical transmission lines, which are based on an organic polymer material. Among these, even though in optical transmission lines having a planar optical waveguide that is organic polymer based (polymer waveguide), the transparency, heat resistance and performance are poorer than that of quartz-based transmission lines, their flexibility is excellent and they can be easily formed into transparent films, and they are promising with regard to their low number of manufacturing steps, their low cost, etc.
As the material for polymer optical waveguides, many polymer materials, from fluorinating polyimides to polymethylmethacrylate, silicone resins and epoxy resins, have been disclosed, for example in Japanese Laid-Open Publication No. 09-251113, or by Shin Hikida, Saburo Imamura in “DENSHI ZAIRYO”, page 32, Feb. 1996 issue, and by Tohru Maruno in “OYO BUTURI”, vol. 68, 1
st
issue (1999) among others.
Referring to
FIGS. 8A and 8B
, the following is an explanation of a conventional polymer-based planar optical waveguide.
FIGS. 8A and 8B
schematically show the cross-sectional structure of the planar optical waveguide.
In the conventional planar optical waveguide shown in
FIG. 8A
, a lower cladding layer
200
having a groove with quadrilateral cross section is formed on a substrate
100
, and a core layer
300
made of an organic polymer material is filled into this groove. Furthermore, an upper cladding layer
400
is formed such that it completely covers the core layer
300
.
In an alternative conventional planar optical waveguide as shown in
FIG. 8B
, a lower cladding layer
201
is formed on a substrate
101
, a core layer
301
made of an organic material with a quadrilateral cross section is formed on the lower cladding layer
201
, and an upper cladding layer
401
is formed such that it completely buries the core layer
301
.
If the cross section of the core layers
300
and
301
is quadrilateral as in these conventional optical waveguides, there is the possibility that the optical path length of the light guided by reflection along the optical waveguide becomes longer than necessary. Furthermore, optical transmission losses and distortions occur at the boundaries between the different sides of the quadrilateral. Therefore, it is desirable that the cross-sectional shape of the core layer is circular.
However, in conventional planar optical waveguides, the cross-sectional shape of the core layer is quadrilateral due to manufacturing considerations. In the approach shown in
FIG. 8A
, the groove of the lower cladding layer
200
is formed by etching, so that the cross-sectional shape of the core layer
300
becomes quadrilateral. Similarly, also in the approach shown in
FIG. 8B
, the core layer
301
itself is formed by etching, so that the cross-sectional shape of the core layer
301
becomes quadrilateral.
The applicant of the present application has investigated several approaches for making the cross section of the optical waveguide circular, and disclosed them for example in Japanese Patent Application No. 2000-180648. However, in these approaches, a process that is completely different from existing processes is used, which creates the new problem of high costs.
On the other hand, to realize an optical transmission line with low optical losses, a uniform transparency without optical dispersion and high-quality film forming properties are desired for the polymer material of the optical waveguide that serves as the core layer. To realize a core layer with high transparency, bulky molecular components are introduced into the polymer in conventional polymer optical waveguides, in order to prevent inter-molecular stacking and crystallization and to achieve amorphousness. However, this approach causes the problem that the polymer matrix tends to become brittle.
In view of these problems, it is a first object of the present invention to provide a planar optical waveguide having a substantially circular cross section with an easy process and a method for manufacturing the same. It is a second object of the present invention to provide a polymer optical waveguide with more uniform transparency and with excellent durability, adhesiveness, etc.
SUMMARY OF THE INVENTION
A first planar optical waveguide in accordance with the present invention comprises a layered film formed on a substrate, and an optical waveguide core formed in the layered film, wherein a cross section of the optical waveguide core is substantially quadrilateral, wherein a dopant layer including refractive index-lowering molecules is provided around the optical waveguide core having a substantially quadrilateral cross section, and wherein the refractive index-lowering molecules included in the dopant layer are unevenly distributed in the optical waveguide core with a concentration that is higher toward outer sides and corners of the optical waveguide core, whereby a graded-index optical waveguide is constituted. It should be noted that in the present specification, “around” does not necessarily mean “encircling” but is mainly used in the sense of “disposed to the side of” or “disposed in the vicinity of”.
In a preferred embodiment, the dopant layer is formed on the substrate, and the optical waveguide core is formed on the dopant layer.
In a preferred embodiment, the dopant layer is formed on an upper side of the optical waveguide core.
It is preferable that the optical waveguide core includes a polymer material, the refractive index-lowering molecules include fluorinated compatible molecules whose fluorine concentration is higher than that of the polymer material, and the fluorinated compatible molecules are reacted with reactive groups included in the polymer material to immobilize the fluorinated compatible molecules by chemical bonding.
It is preferable that the polymer material is at least one fluorinated polymer material selected from the group consisting of fluorinated polyimide, fluorinated polysiloxane and fluorinated polymethacrylate resins, and the refractive index-lowering molecules include fluorinated compatible molecules whose fluorine concentration is higher than that of the fluorinated polymer material.
A first method for manufacturing a planar optical waveguide in accordance with the present invention includes (a) a step of forming a first dopant film including refractive index-lowering molecules on a substrate, (b) a step of forming a thin film to serve as optical waveguide core on the substrate, and subsequently forming an optical waveguide core with substantially quadrilateral cross section by etching the thin film, (c) a step of forming a second dopant layer including refractive index-lowering molecules on an upper side of the optical waveguide core with substantially quadrilateral cross section, and (d) a step of doping the refractive index-lowering molecules from the first and second dopant layers into the optical waveguide core with substantially quadrilateral cross section, whereby the refractive index-lowering molecules is distributed unevenly with a concentration that is higher toward outer sides and corners of the optical waveguide core.
In a prefera
Kishimoto Yoshio
Mitsuda Masahiro
Healy Brian
Matsushita Electric - Industrial Co., Ltd.
McDermott Will & Emery LLP
Petkovsek Daniel
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