Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2000-01-21
2002-05-28
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S028000, C385S050000
Reexamination Certificate
active
06396984
ABSTRACT:
CLAIM OF PRIORITY
This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C §119 from an application; entitled Mode Shape Converter, Method For Fabricating the Mode Shape Converter And Integrated Optical Device Using The Mode Shape Converter earlier filed in the Korean Industrial Property Office on Jan. 21 1999, and there duly assigned Serial No. 99-1764 by that Office.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a mode shape converter, a method for fabricating the mode shape converter, and an integrated optical device using the mode shape converter, and more particularly to a mode shape converter arranged at an input or output terminal of an optical device and adapted to couple lights inputted into or outputted from the optical device via optical fibers. The present invention also relates to a method for fabricating such a mode shape converter, and an integrated optical device using the mode shape converter.
2. Description of the Prior Art
An integrated optical technique is a technique for integrating a variety of optical devices using waveguides on one substrate. Using such an integrated optical technique, it is possible to easily integrate a multi-functional optical device having a complicated structure on a limited small area because the alignment of unit optical devices can be easily made.
Examples, incorporated by reference herein, of waveguide structure implementing an integrated optical device are disclosed in U.S. Pat. No. 5142,596 to Kiminori Mizuuchi et al. entitled Tapered Light Wave guide And Wavelength Converting Element Using Tile Same; U.S. Pat. No. 5,391,869 to Robert W. Ade et al. entitled Single-Side Growth Reflection-Based Waveguide-Integrated Photodetector; U.S. Pat. No. 5,910,012 to Takeshi Takeuchi entitled Waveguide Type Semiconductor Photodetecting Device Method For Fabricating; and U.S. Pat. No. 5,078,516 to Elyahou Kapon et al. entitled Tapered Rib Waveguides.
A rib waveguide is a channel waveguide fabricated by partially etching a planar waveguide. Such rib waveguides have various advantages as follows. First, it is possible to select respective refractive indices of a core and a clad within a wide range. Second, it is possible to fabricate a single-mode waveguide having a large cross-sectional area irrespective of a refractive index difference between the core and clad. Third, it is possible to easily adjust optical characteristics such as a mode distribution and a propagation constants under the condition in which an etched depth is used as a major process parameter. Fourth, a precise pattern can be obtained, as compared to rectangular waveguides. This is because the etched depth in the rib waveguide is less than those of the rectangular waveguides. Fifth, it is possible to reduce damages occurring during an etching process for a core layer, for example, errors in pattern size caused by an anisotropic etching, a cracking phenomenon occurring during the etching process for a layer having stresses, and damages caused by a re-accumulation of by-products formed during the etching process.
In spite of such advantages, the above mentioned rib waveguide has a disadvantage in that a very large coupling loss is generated when an optical fiber is coupled to the waveguide of the optical device. Single-mode optical fibers have a circular mode distribution having an aspect ratio of 1:1 while having a relatively large size, for example, about 10 &mgr;m. On the other hand, rib waveguides have an oval mode distribution in which its horizontal width is larger than its vertical width. In many cases, the mode distribution size of such a rib waveguide is also larger than those of the single mode optical fibers. For this reason, there is a misalignment in mode shape at the connection between a rib waveguide and an optical fiber. Due to such a mode shape misalignment, an optical wave encounters with a discontinuity while passing through the connection, so that it involves a coupling loss while being reflected or scattered. In order to solve this problem, a mode shape converter is arranged at the input or output terminal of the integrated optical device to which an optical fiber is coupled. The mode shape converter serves to conduct the function for slowly converting the mode of the optical fiber into a mode shape suitable for execution of the functions of the optical device, thereby achieving a reduction in coupling loss.
FIG. 1
is a perspective view illustrating the structure of a conventional mode shape converter disclosed in U.S. Pat. No. 5,078,516. She mode shape converter shown in
FIG. 1
includes a first waveguide
100
, a second waveguide
102
, and a substrate
104
. In
FIG. 1
, the reference numeral
106
denotes an input terminal whereas the reference numeral
108
denotes an output terminal. The reference numeral
110
represents respective refractive indices of the first waveguide
100
, second waveguide
102
, and substrate
104
. The first waveguide
100
is designed to have a small mode size suitable for execution of the functions of an optical device to which the mode shape converter is coupled. The second waveguide
102
is designed to have a refractive index less than that of the first waveguide
100
while having a large mode size to obtain an advantageous input/output coupling with an optical fiber. The input terminal
106
has a waveguide constituted only by the second waveguide
102
. This second waveguide
102
uses air as its upper clad while using the substrate
104
as its lower clad in order to confine optical waves in a depth direction. In order to confine optical waves in a longitudinal direction, the second waveguide
102
, which serves as a core, is partially etched to have a rib waveguide structure.
The output terminal
108
has a waveguide constituted only by the first waveguide
100
. The first waveguide
100
of the output terminal
108
has a strip loaded waveguide structure different from the rib waveguide structure of the input terminal
106
. The first waveguide
100
uses air as its upper clad while using the second waveguide
102
as its lower clad.
A mode conversion region is defined between the input and output terminals
106
and
108
in order to convert a mode coupled after being inputted from the optical fiber to the optical device into a mode shape suitable for execution of the functions of the optical device without any loss of the coupled mode. The rib waveguide having a large mode size is converted into the strip loaded waveguide having a small mode size by the mode conversion region. A light guided through the mode shape converter is slowly shifted toward the first waveguide
100
because the first waveguide
100
has a refractive index higher than that of the second waveguide
102
even though the widths of both the first and second waveguides
100
and
102
increase. When the guided light reaches the output terminal
108
, the power thereof is mainly concentrated toward the first waveguide
100
.
FIG. 2
a
is a diagram illustrating a mode profile of the input terminal
106
in the above mentioned mode shape converter whereas
FIG. 2
b
is a diagram illustrating a mode profile of the output terminal
108
in the mode shape converter.
However, the integrated optical device provided with the above mentioned mode shape converter has problems as follows. First, the fabrication is troublesome because it is necessary to use two cores made of different materials, and the first waveguide should be precisely formed on the second waveguide. Second, there is a limitation in minimizing the coupling loss of the optical device to an optical fiber having a circular mode because the input terminal
106
has a rib waveguide structure having an oval waveguide mode even though it has a large mode size. Third, since the mode shape converter uses a down-tapering structure in order to increase the mode size of the input-end waveguide, its waveguide taper increases in length. An increase in transmission loss occurs during the mode conve
Cho Jung-Hwan
Kim Duk-Bong
Rhee Tae-Hyung
Yi Sang-Yun
Bushnell , Esq. Robert E.
Lee John D.
Sam-Sung Electronics Co., Ltd.
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