Optical: systems and elements – Polarization without modulation – Polarization using a time invariant electric – magnetic – or...
Reexamination Certificate
2001-09-21
2003-02-18
Juba, Jr., John (Department: 2872)
Optical: systems and elements
Polarization without modulation
Polarization using a time invariant electric, magnetic, or...
C359S494010, C359S490020, C359S506000, C359S578000, C359S579000, C359S900000, C349S018000, C349S198000
Reexamination Certificate
active
06522467
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical systems, and more particularly to a method and system for providing a liquid crystal filter that is insensitive to the polarization of the light.
BACKGROUND OF THE INVENTION
Polarization insensitive liquid crystal tunable filters are used to filter particular wavelengths of light.
FIG. 1
depicts one such conventional polarization insensitive liquid crystal tunable filter system
10
. This conventional system
10
includes an input signal
12
provided on a fiber, a single fiber collimator
14
, a first polarization beam splitter
16
, a first right angle prism
22
, a first half wave plate
24
, a conventional liquid crystal filter unit
26
, a second half wave plate
28
, a second right angle prism
30
, a second polarization beam splitter
32
and a second single fiber collimator
34
that provides an output signal
36
. The first polarization beam splitter
16
splits the input optical signal
12
into two polarization components
18
and
20
with parallel and perpendicular polarization direction relative to the plane of the page, respectively. The first component
18
is transmitted through the first half wave plate
24
with its polarization rotated 90 degree, filtered by the conventional liquid crystal tunable filter unit
26
at region
25
, then reflected by the second right angle prism
30
to the second polarization beam splitter
32
and then reflected to the output. Similarly, the second component
20
is reflected by the first right angle prism
22
, filtered by the conventional liquid crystal tunable filter unit
26
at region
27
and then transmitted through the second half wave plate
28
with its polarization rotated 90 degree. Then the second component
20
is provided to and transmitted by the second polarization beam splitter
32
. Thus, the first component
18
and the second component
20
are recombined to a single optical signal and output as conventional filtered optical signal
36
through single fiber collimator
34
.
FIGS. 2A and 2B
depict front and side views, respectively, of the conventional liquid crystal tunable filter unit
26
, which includes two glass slides
48
A and
48
B two transparent electrodes
44
A and
44
B, high reflective coatings
40
A and
40
B, liquid crystal
46
and spacers
42
A and
42
B. The two spacers
42
A and
42
B set the distance between the high reflective coatings
40
A and
40
B. The elliptical shaped liquid crystal molecules
47
are aligned in the vertical direction and can be oriented clockwise or counter clockwise with the voltage applied on the electrode
44
A and
44
B. Since the polarization direction of first component
18
and second component
20
are aligned with that of liquid crystal at regions
25
and
27
, both components
18
and
20
experience a refractive index or optical phase delay change with the changes of voltage applied on the electrode
44
A and
44
B. The optical phase delay change for both components
18
and
20
provides center wavelength tuning on the combined optical signal
36
.
Although the conventional liquid crystal tunable filter system
10
functions, one of ordinary skill in the art will readily recognize that there are drawbacks. In particular, the conventional liquid crystal tunable filter system
10
has a polarization separation in its transmitted spectrum. Because of the glass slides
48
A and
48
B are not perfectly parallel, the thickness of the liquid crystal
46
varies across the length of the conventional liquid crystal tunable filter unit
26
. In addition, the density of the liquid crystal
46
may vary-across the length of the liquid crystal tunable filter. Furthermore, as depicted in
FIG. 1
, the first component
18
and the second component
20
pass through the conventional liquid crystal tunable filter unit
26
at different positions
25
and
27
. As a result, the first component
18
and the second component
20
can experience different thicknesses and densities of liquid crystal that introduce a difference on the optical phase delay of the components
18
and
20
.
FIG. 3
is a graph
50
depicting the spectra
52
,
54
and
56
, respectively, for the components
18
and
20
and the output signal
34
. The difference in their optical phase delay makes the optical spectra
52
and
54
of the transmitted first component
18
and the second component
20
, respectively, separated and the optical spectrum
56
of the output signal
36
broadened with a higher insertion loss. This optical spectrum separation between first and second component
18
and
20
is called “polarization spectrum separation”, which makes the conventional liquid crystal tunable filter system
10
have the following the drawbacks: a relatively high insertion loss, an increased polarization dependent loss and a broadened width for the pass band.
Accordingly, what is needed is a system and method for providing a liquid crystal filter system that is insensitive to the polarization of light. The present invention addresses such a need.
SUMMARY OF THE INVENTION
The present invention provides a method and system for filtering an optical signal. The method and system comprise providing a polarization beam splitter, first and second rotators, a liquid crystal tunable filter unit, and first and second prisms. The polarization beam splitter splits the optical signal into a first component having a first polarization and a second component having a second polarization. The first rotator rotates a polarization 45 degrees in a first direction, receives the first component from the beam splitter and is located between the liquid crystal tunable filter unit and the polarization beam splitter. The liquid crystal molecules of the liquid crystal tunable filter unit are aligned 45 degrees relative to a vertical direction. The liquid crystal tunable filter unit receives the first component from the first rotator, provides the second component to the first rotator and is between the first rotator and the second rotator. The second rotator rotates the polarization 45 degrees opposite to the first direction, receives the first component from the first rotator and provides the second component to the liquid crystal tunable filter unit. The first prism is located in proximity to the second rotator, receives the first component from the second rotator and provides the second component to the second rotator. The second prism is in proximity to the beam splitter, receives the first component from the first prism and provides the second component from the first beam splitter to the first prism.
According to the system and method disclosed herein, the present invention provides a liquid crystal tunable filter system that is insensitive to the polarization of the input signal.
REFERENCES:
patent: 5111321 (1992-05-01), Patel
patent: 5321539 (1994-06-01), Hirabayashi et al.
patent: 5710655 (1998-01-01), Rumbaugh et al.
patent: 6075647 (2000-06-01), Braun et al.
patent: 6081367 (2000-06-01), Yokoyama et al.
Li Yiqiang
Wang Yongjian
AC Photonics, Inc.
Jr. John Juba
Sawyer Law Group LLP
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