Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2002-02-14
2004-04-27
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S052000, C385S090000
Reexamination Certificate
active
06728449
ABSTRACT:
BACKGROUND
Planar lightwave circuits such as waveguide gratings and optical switches control the routing of optical signals. In order to accomplish this control or routing, input and output optical fibers are connected to the planar lightwave circuit (PLC). A convenient method to align and manage more than one fiber is to use fiber assemblies.
FIG. 1A
shows the example of an optical switch
100
with four fiber assemblies
110
A,
110
B,
110
C, and
110
D for four sets of optical fibers
112
. The four fiber assemblies
110
A,
110
B,
110
C, and
110
D are connected to an optical plate
120
that forms a PLC. In this example, fiber assemblies
110
A and
110
D include optical fibers
112
that input optical signals to optical plate
120
and fiber assemblies
110
B and
110
C include optical fibers
112
that receive optical signals output from optical plate
120
.
In general, optical plate
120
can be made of any material in which optical waveguides can be created. These materials generally have low optical loss for the target wavelengths and a refractive index profile can be created perpendicular to the propagation direction so as to guide the light. In the example of
FIG. 1A
, optical plate
120
is made of an optical material such as fused silica that is selectively doped with impurities to form optical waveguides, but waveguides can be formed in other structures such as in semiconductor lasers and Lithium Niobate (LiNbO
3
) modulators.
Optical plate
120
includes two example sets of optical waveguides
122
and
124
. Optical waveguides
122
are aligned with optical fibers
112
in fiber assemblies
110
A and
10
C, and optical waveguides
124
are aligned with fibers
112
in fiber assemblies
110
B and
110
C. Switching sites
126
that select the paths of the optical signals are at the intersections of optical waveguides
122
and
124
.
In operation, switching sites
126
can be individually turned on or off so that an optical signal input to an optical waveguide
122
or
124
either reflects at one of the switching sites
126
along the waveguide
122
or
124
into another optical waveguide
124
or
122
or passes through every switching site
126
along the optical waveguide
122
or
124
. In one specific implementation, each switching site
126
includes a trench in optical plate
120
that is either filled with a liquid to make the switching site
126
transparent or filled with a gas bubble to make the switching site
126
reflective. An integrated circuit (not shown) underlying optical plate
120
can selectively heat the liquid in a particular switching site
126
to create the gas bubble that turns on that switching site
126
and makes that switching site
126
reflective.
Optical switch
100
can route an optical signal from an optical fiber
112
in fiber assembly
110
A, for example, into any of the optical fibers
112
in fiber assembly
1110
B by making the appropriate switching sites
126
reflective. Alternatively, if none of the switching sites
126
along the optical waveguide
122
are reflective, the optical signal from the optical fiber
112
in fiber assembly
110
A passes through optical plate
120
to an optical fiber
112
in the opposite fiber assembly
110
C.
Proper operation of optical switch
100
requires that the spacing of optical fibers
112
on each fiber assembly
110
A,
110
B,
110
C, or
110
D match the spacing of input/output areas for the corresponding optical waveguides
122
or
124
. Additionally, the optical fibers
112
must be precisely aligned with optical waveguides
122
or
124
and with optical fibers
112
in other fiber assemblies to achieve maximum performance. Fabricating and aligning fiber assemblies with the required precision can present difficulties because waveguides
122
and
124
have typical dimensions of about 10 &mgr;m or less and a standard optical fiber
112
has a diameter of 125 &mgr;m and a core 10 &mgr;m in diameter. The cores of the optical fibers
112
carry the optical signals and must be aligned for transfer of optical signals to or from the corresponding waveguide. Accordingly, for maximum performance the spacing and alignment of the optical fibers
112
typically must be accurate to within a few tenths of a micron.
FIG. 1B
shows a cross-sectional view of a fiber assembly
110
. Fiber assembly
110
includes a substrate
115
having v-grooves
116
in which optical fibers
112
reside. Substrate
115
is typically made of the same material as the optical plate (e.g., fused silica) to provide a matching coefficient of thermal expansion (CTE), but other materials such as silicon can also be used.
Precision machining of substrate
115
can produce v-grooves
116
with consistent shape and spacing. Such machining can use, for example, step and repeat techniques that grind a v-groove
116
in substrate
115
then move substrate
115
the required distance for grinding the next v-groove
116
in substrate
115
. Equipment including a precision stage that positions substrate
115
for grinding can achieve the required precision for the spacing of v-grooves
116
. However, separate mechanical operations such as cutting an edge
118
of substrate
115
generally require remounting substrate
115
on different equipment, which introduces variations greater than the required alignment precision. Accordingly, the position of edge
118
of substrate
115
relative to v-grooves
116
may vary by ±25 &mgr;m.
An exemplary process for aligning fiber assemblies
110
A,
110
B,
110
C, and
110
D with optical plate
120
as in
FIG. 1A
includes a coarse alignment process and a fine alignment process. The coarse alignment process aligns fiber assemblies
110
A,
110
B,
110
C, and
110
D and optical plate
120
with sufficient precision to provide some light flow through the required paths. A fine alignment process measures the intensity of output optical signals and adjusts the positions and orientations of assemblies
120
to maximize optical power flow through switch
100
. Fine alignment can be computer controlled using known “hill climbing” algorithms that find the optimal position and orientation for the fiber assemblies
110
A,
110
B,
110
C, and
110
D.
Coarse alignment of an assembly
110
and an optical plate
120
aligns the cores
114
of optical fibers
112
with respective optical waveguides
122
or
124
in optical plate
120
so that optical signals flow through optical switch
100
. Coarse alignment initially relies on identifying and matching physical features of fiber assembly
110
and optical plate
120
. However, cores
114
, which are to be aligned, are indistinguishable from other portions of optical fibers
112
, and the optical fibers
112
, which have their protective sheathes removed for accurate assembly, are transparent and therefore difficult to identify using machine or human vision. Features such as v-grooves
116
or their edges are similarly difficult to identify, particularly when substrate
115
is transparent. Separate mechanically made features such as edges
118
of substrate
115
, which may be easier to identify, are subject to variations much greater than those required in the coarse alignment.
The difficulties in identifying reliable reference features for coarse alignment typically means that the coarse alignment is conducted manually. Additionally, an alignment based solely on the apparent location of the features often fails to provide adequate optical power transmission for the fine alignment process. Accordingly, the coarse alignment must further include a search process that systematically shifts or reorients the fiber assemblies until achieving a configuration with sufficient optical power transmission for the fine alignment process. Such coarse alignment procedures can take an hour or more, while computer-controlled fine alignment can typically be completed in two to ten minutes. Accordingly, structures and techniques are sought that can reduce the time required for aligning fiber assemblies in optical switches or other PLCs.
SUMMARY
In a
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