Optical waveguides – Polarization without modulation
Reexamination Certificate
2000-01-27
2002-03-05
Ngo, Hung N. (Department: 2874)
Optical waveguides
Polarization without modulation
C385S011000, C385S034000, C359S490020
Reexamination Certificate
active
06353691
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical technology, and more particularly to a method and system for splitting or combining an optical signal.
BACKGROUND OF THE INVENTION
Conventional optical signals are often desired to be split or combined based on the polarization of the signal. In order to perform either of these functions, a conventional splitter/combiner is often used.
FIG. 1
depicts a conventional splitter/combiner
10
. The conventional splitter/combiner
10
includes fiber
12
at one end and fibers
36
and
46
at the opposite end. The fibers
36
and
46
are polarization-maintaining fibers. Consequently, an optical signal travelling over one of the fibers
36
and
46
will maintain its polarization. The conventional splitter/combiner
10
includes collimators
13
,
30
and
40
for collimating the optical signals carried on fibers
12
,
36
and
46
, respectively. Each conventional collimator
13
,
30
and
40
typically includes a capillary
14
,
34
and
44
, respectively. The capillaries
14
,
34
and
44
hold each fiber
12
,
36
and
46
, respectively. The collimators
13
,
30
and
40
also include lenses
16
,
32
and
42
, respectively. The lenses
16
,
32
and
42
are used in conjunction with the capillaries
14
,
16
and
44
, respectively, for collimating the optical signals carried on fibers
12
,
36
and
46
, respectively.
The conventional splitter/combiner
10
also includes a large birefringent crystal
20
. A birefringent crystal has the property that light polarized in one direction (“ordinary signal”) is deflected based on a different index of refraction than light polarized in a second, perpendicular direction (“extraordinary signal”). Any unpolarized optical signal can represented by an optical signal polarized in two perpendicular directions. Here, the optical signal can be decomposed into two portions polarized in the plane of the page and out of the plane of the page. The birefringent crystal
20
is oriented so that one portion of the optical signal polarized in out of the plane of the page and perpendicular to the optic axis
21
(“conventional ordinary signal”) is transmitted undeflected. The birefringent crystal
20
also deflects a remaining portion of the optical signal polarized in another direction (“conventional extraordinary signal”).
FIG. 1
depicts the conventional ordinary signal
24
and the conventional extraordinary signal
22
traveling through the birefringent crystal
20
. As depicted in
FIG. 1
, the ordinary signal is polarized in a direction perpendicular to the optic axis
21
of the birefringent crystal
20
. Furthermore, as depicted in
FIG. 1
, the optical signal enters the birefringent crystal
20
in a direction perpendicular to the face of the birefringent crystal
20
. Thus, the angle of incidence of the optical signal, measured from a direction perpendicular to the face of the birefringent crystal
20
, is zero degrees.
The conventional splitter/combiner
10
splits the optical signal input at the fiber
12
into two signals, based on the polarization of the signals. The signal from the input fiber
12
is collimated by the collimator
13
and transmitted to the birefringent crystal
20
. The birefringent crystal
20
splits the input signal to the conventional ordinary signal
24
and the conventional extraordinary signal
22
. Because of the orientation of the birefringent crystal
20
with respect to the collimator
13
and the direction of the optic axis
21
, the conventional extraordinary signal
22
is transmitted at an angle, while the conventional ordinary signal
24
is transmitted undeflected. Thus, at the end of the birefringent crystal
20
closer to the fibers
36
and
46
, the conventional ordinary signal
24
and the conventional extraordinary signal
22
are separated by a distance. The conventional ordinary signal
24
and the conventional extraordinary signal
22
are output from the birefringent crystal
20
in parallel. The conventional extraordinary signal
22
is provided to collimator
30
and output over polarization-maintaining fiber
36
. Similarly, the conventional ordinary signal
24
is transmitted to the collimator
40
and output over the polarization-maintaining fiber
46
.
When the conventional splitter/combiner
10
is to combine signals, the signals are input at fibers
36
and
46
. The polarizations of the input signals provided over the fibers
36
and
46
are preferably that of the conventional extraordinary signal
22
and conventional ordinary signal
24
, respectively. Because of the orientation of the birefringent crystal
20
, the conventional extraordinary signal
22
and conventional ordinary signal
24
are combined. The combined signal is then transmitted to the collimator
13
and output via the fiber
12
. Thus, the conventional splitter/combiner
10
can thus split or combine optical signals.
Although the conventional splitter/combiner
10
functions, one of ordinary skill in the art will readily recognize that the conventional splitter/combiner
10
is large and expensive. The birefringent crystal
10
is made large to develop a sufficient space between the conventional ordinary signal
24
and the conventional extraordinary signal
22
. Furthermore, multiple collimators
13
,
30
and
40
for the fibers
12
,
36
and
46
consume space. The birefringent crystal is also costly due to its size. Furthermore, the number of components, such as the use of three collimators
13
,
30
and
40
also add to the cost of the conventional splitter/combiner
10
. The conventional splitter/combiner
10
is, therefore, large and expensive.
Accordingly, what is needed is a system and method for providing a splitter/combiner that is smaller and less expensive. The present invention addresses such a need.
SUMMARY OF THE INVENTION
The present invention provides a method and system for providing a splitter/combiner. The splitter/combiner comprises a first birefringent wedge having a first optic axis and a first wedge angle and a second birefringent wedge having a second optic axis and a second wedge angle. The second birefringent wedge is optically coupled with the first birefringent wedge. The second wedge angle is complementary to the first wedge angle. The second optic axis is perpendicular to the first optic axis such that an extraordinary signal for the first birefringent wedge is an ordinary signal for the second birefringent wedge. The first and second birefringent wedges establish a first path for a first portion of a first optical signal and establish a second path for a second portion of the first optical signal. The first path and the second path are separated by an angle. The first optical signal travels from the first birefringent wedge to the second birefringent wedge. The first birefringent wedge and the second birefringent wedge also combine a second optical signal travelling along the first path with a third optical signal travelling along the second path. The second optical signal and the third optical signal travel from the second birefringent wedge to the first birefringent wedge.
According to the system and method disclosed herein, the present invention provides a splitter/combiner which can be made smaller and more cheaply than conventional splitter/combiners.
REFERENCES:
patent: 4650289 (1987-03-01), Kuwahara
patent: 5291571 (1994-03-01), Kunikane et al.
patent: 5909310 (1999-06-01), Li et al.
patent: 6205270 (2001-03-01), Cao
Li Wei Zhong
Yang Yanfeng
Ngo Hung N.
Oplink Communications Inc.
Sawyer Law Group LLP
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