Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
1999-02-09
2001-06-05
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S014000, C385S045000
Reexamination Certificate
active
06243516
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is based on, and claims priority to, Japanese application number 10-040113, filed on Feb. 23, 1998, in Japan, and which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical waveguides which merge together and have a branch angle within a specific range.
2. Description of the Related Art
Optical communication systems using fiber optical transmission lines are being used to transmit relatively large amounts of information. However, as users require larger amounts of information to be rapidly transmitted, and as more users are connected to the systems, a further increase in the transmission capacity of optical communication systems is required.
Optical waveguides are being used for this purpose. For example, optical waveguides are being used in optical external modulators to increase modulation rate, and in optical wave filters for wavelength-multiplex communications, to thereby increase transmission capacity of optical communication systems.
Optical waveguides are also used in various types of optical devices for taking measurements.
For such uses of optical waveguides, it is desirable to form optical waveguides in an integrated circuit (typically referred to as a “chip”). Unfortunately, conventional optical waveguides typically have required lengths which are so long that they prevent desired functions from being implemented within a single chip. For example, optical waveguides may have a required length as long as several centimeters. This makes it difficult to implement optical waveguides in a single chip, despite optical waveguide widths as narrow as several micrometers to several tens of micrometers.
In order to circumvent this problem, optical waveguides can be“folded” many times by using waveguide reflectors so as to implement a long length optical waveguide within the confines of a single chip.
For example,
FIG. 1
is a diagram illustrating a conventional optical waveguide having a folded waveguide structure and formed on a single chip as a Mach-Zehnder modulator. (This device can be found, for example, in Institute of Electronics, Information, and Communication Engineers, Electronics Society Conference, C-151, 1995, which is incorporated herein by reference).
Referring now to
FIG. 1
, waveguides
100
make a U turn at one end of the chip via a folded waveguide portion
101
. A reflection-type wave plate
102
is provided where light is reflected. Through a reflection, TE light changes to TM light, and TM light changes to TE light, thereby achieving a modulator which does not discriminate polarization.
In this example, the waveguides are folded in a geometrical manner (folding angle: 9 degrees). Such a configuration has problems in device performance. Namely, when such a simple configuration is employed, a length of a waveguide where light beams meet is rather short. Even when a reflection surface is formed by cutting saw or the like, a displacement as small as 10 &mgr;m may cause a serious deviation from the reflection geometry, thereby creating a large loss. In this example, a loss amounting to 2 dB may be suffered.
When the folding angle is decreased so as to be as small as several degrees, reflected light returns back to a waveguide where the original light came through. This is presents many problems.
In view of the above described problems, a configuration using folded waveguides has never been used in practice.
FIG. 2
is a diagram illustrating a conventional wavelength-filter-insertion type device. (This device can be found in Institute of Electronics, Information, and Communication Engineers, Electronics Society Conference, C-229, 1995, which is incorporated herein by reference.)
Referring now to
FIG. 2
, the device includes waveguides
110
, a 1.55 &mgr;m port
112
, a common port
114
, a dielectric multi-layer filter
116
and a quartz-family optical waveguide
118
formed on a Si substrate
120
. Waveguides
110
are arranged according to reflection geometry, and have a large reflection angle (10° to 40°) to avoid reflected light going back to where it came from. As a result, a position where filter
116
is placed has a tolerance level in the order of micro-meters. Unfortunately, such a small tolerance in device manufacturing precision results in a low yield.
Therefore, waveguide devices having folding configurations are known to exist. The problem is, however, that a process for creating these devices with sufficient precision is not known.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an optical waveguide device which has folded waveguides having a small loss and a high tolerance for precision, thereby enhancing performance that would otherwise be limited by the confines of a chip.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
The foregoing objects of the present invention are achieved by providing an apparatus which includes first and second single mode optical waveguides and a reflector. The first and second optical waveguides merge together into a merging optical waveguide. The reflector is positioned so that light travelling through the first optical waveguide into the merging optical waveguide is reflected by the reflector to travel through the second optical waveguide. A total reflection complementary angle for the light traveling through the first optical waveguide is &thgr;
c
, and a branch angle &thgr;
b
of the first and second optical waveguides is less than or equal to 0.55&thgr;
c
. The first and second optical waveguides are formed on a substrate, and the total reflection complementary angle &thgr;
c
is based on a difference in refractive indexes between the first and second optical waveguides and the substrate.
Objects of the present invention are further achieved by providing an apparatus which includes first, second, third and fourth optical waveguides, and a reflector. The first and second optical waveguides merge together into a merging optical waveguide. The reflector is positioned so that light travelling through the first optical waveguide into the merging optical waveguide is reflected by the reflector to travel through the second optical waveguide. A branch angle &thgr;
b
of the first and second optical waveguides is less than or equal to 0.55&thgr;
c
, where &thgr;
c
is a total reflection complementary angle for the light traveling through the first optical waveguide. The third and fourth optical waveguides are on an opposite side of the reflector as the first and second optical waveguides. The reflector has transmission characteristics and is positioned so that light travelling through the third optical waveguide passes through the reflector and travels to one of the first and second optical waveguides, and so that light travelling through the fourth optical waveguide passes through the reflector and travels to the other of the first and second optical waveguides. The reflector is formed by either an optical waveguide filter, a half-mirror or a polarization mirror.
Objects of the present invention are also achieved by providing first and second optical waveguides which merge together into a merging optical waveguide, where the first and second optical waveguides and the merging optical waveguide are on a semiconductor substrate. A reflector is positioned so that light travels through the first optical waveguide into the merging optical waveguide, then through the merging optical waveguide to the reflector, and is then reflected by the reflector to travel through the second optical waveguide. The reflector is positioned beyond an intersection point of a center line of the first optical waveguide and a center line of the second optical waveguide. In addition, a branch angle &thgr;
b
of the first and second optical waveguides is less than or equal to 0.55&thgr;
c
.
Further, objects of th
Connelly-Cushwa Michelle R.
Fujitsu Limited
Lee John D.
Staas & Halsey , LLP
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