Optical: systems and elements – Polarization without modulation – Polarization using a time invariant electric – magnetic – or...
Reexamination Certificate
2000-05-18
2003-03-25
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Polarization without modulation
Polarization using a time invariant electric, magnetic, or...
C359S494010, C359S490020, C359S490020, C359S506000, C359S199200, C385S011000, C385S024000
Reexamination Certificate
active
06538815
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical circulators utilized in lightwave communication systems and more particularly to bi-directional optical circulators within which a first subset of two mutually exclusive subsets of signals propagates in a logical clockwise direction and the second subset of the two mutually exclusive subsets of signals propagates in a logical counterclockwise direction opposite to the clockwise direction. The present invention further relates to wavelength division multiplexers and de-multiplexers in lightwave communications systems and, more particularly, to multiplexers and de-multiplexers utilizing bi-directional optical circulators.
2. Description of the Related Art
The optical circulator is a non-reciprocal multi-port device that has some similarities to an optical isolator but is more generally applicable.
FIG. 1
illustrates the operation of a generalized four-port optical circulator
100
of the prior art. Light that enters the circulator at port A
102
exits the optical circulator
100
at port B
104
. However, light that enters the optical circulator at port B
104
does not travel to port A
102
but instead exits at port C
106
. Similarly, light entering the port C
106
exits only at port D
108
, and light entering port D
108
exits only at the port A
102
. In general, given a set of n equivalent optical input/output ports comprising a certain logical sequence within an optical circulator, light inputted to any port is outputted from the logical next port in the sequence and is prevented from being output from any other port. Since a light signal will only travel only one way through any two consecutive ports of an optical circulator, such ports, in effect, comprise an optical isolator. By installing a reflector at one port of a generalized n-port optical circulator (where n≧4) such that light outputted from the port is reflected back into the same port, the circulator may then be utilized as an (n−1)-port circulator. Furthermore, by blocking or failing to utilize one port of a generalized n-port optical circulator (where n≧4), the device may be used as an (n−1)-port quasi-circulator.
The main application of optical circulators is in bi-directional optical fiber communications in which two signals at the same wavelength may simultaneously propagate in opposite directions through a single fiber. In this way, optical circulators permit a doubling of the bit carrying capacity of an existing unidirectional fiber optic communication link since optical circulators can permit full duplex communication on a single fiber optic link.
FIG. 2
illustrates a common method by which a pair of conventional optical circulators can be used to provide simultaneous, bi-directional communication on a single fiber optic link. Two three-port optical circulators,
110
and
112
, are installed at opposite ends of a fiber optic link
114
. Each circulator comprises three ports, with ports A
1
116
; B
1
118
and C
1
120
located on circulator
110
and ports A
2
122
, B
2
124
and C
2
126
located on circulator
112
. For optical circulators
111
and
112
, communication transmitters
128
and
130
are optically coupled to port A
1
116
and A
2
122
, respectively, the common fiber link
114
is optically coupled to port B
1
118
and B
2
124
, respectively, and communication receivers
132
and
134
are optically coupled to port C
1
120
and port C
2
126
, respectively. Because of the signal light re-direction properties of the optical circulators discussed above, light emitted from each transmitter
128
and
130
is launched into the fiber link
114
from opposite ports B
1
118
and B
2
124
in opposite directions. At the end of each respective path, the two optical circulators
110
,
112
separate incoming signals from outgoing signals, so that the transmitters
128
,
130
and receivers
132
,
134
do not interfere with each other.
A more complex bi-directional optical communications system using a star architecture and optical circulators located, together with other components, on customers' premises, is disclosed in U.S. Pat. No. 5,572,612, which is incorporated herein by reference.
Optical circulators have many other applications in fiber optic communications systems.
FIG. 3
illustrates an apparatus disclosed in U.S. Pat. No. 5,822,095 in which an optical add/drop multiplexer is constructed using two optical circulators and an intervening optical filter. U.S. Pat. No. 5,822,095 is incorporated herein by reference.
In
FIG. 3
herein, which corresponds to
FIG. 1
of U.S. Pat. No. 5,822,095, the wavelength components &lgr;
1
to &lgr;
n
, of an input n-wave signal are received by an input optical fiber
136
and transmitted through an optical circulator
138
and an optical fiber
140
to an optical bandpass filter
144
. The bandpass filter
144
allows a specific wavelength &lgr;
1
to pass but rejects the other wavelengths &lgr;
2
to &lgr;
n
. The rejected wavelengths are reflected by the bandpass filter
144
back to circulator
138
which re-directs them to output fiber
146
. Meanwhile, the signal at wavelength &lgr;
1
passes through filter
144
to another optical fiber
148
and thenceforth into another optical circulator
150
which then drops it to an output optical fiber
152
. Furthermore, another signal component with the same wavelength &lgr;
1
is simultaneously added and is introduced from another input optical fiber
154
interfaced to the second optical circulator
150
. The second optical circulator
150
directs the added second signal to the bandpass filter
144
in the reverse direction from that of the dropped signal. After the second signal with wavelength &lgr;
1
passes through the bandpass filter
144
, it is mixed with the rejected wavelengths &lgr;
2
to &lgr;
n
from the first input fiber at the first optical circulator
138
and is outputted, along with these other rejected wavelengths, via output fiber
146
.
In U.S. Pat. Nos. 5,383,686 and 5,825,520, which are both incorporated herein by reference, optical wavelength multiplexers and de-multiplexers are disclosed in which one or more Bragg grating reflectors are used in conjunction with one or more optical circulators. An example of one such de-multiplexer
155
and multiplexer
157
, as disclosed in U.S. Pat. No. 5,825,520, is illustrated herein in
FIGS. 4
a
and
4
b
, respectively.
FIGS. 4
a
and
4
b
herein correspond to the
FIGS. 3
a
and
2
, respectively, of U.S. Pat. No. 5,825,520, and are explained briefly herein.
In the de-multiplexer
155
of
FIG. 4
a
, an input signal
159
is de-multiplexed into two output signals,
164
and
166
. In the multiplexer
157
of
FIG. 4
b
, input signals
167
and
169
are multiplexed into output signals
170
.
The grating reflector, such as grating reflector
156
of
FIG. 4
a
or grating reflector
158
of
FIG. 4
b
, typically comprises an in-fiber Bragg grating reflector and specifically reflects only one signal at a specific target wavelength. When coupled to the intermediate port
160
of a three-port optical circulator, such as circulator
162
of
FIG. 4
a
, then only the target wavelength will be returned back to the circulator
162
and thereby directed to the output fiber
164
via port
165
of circulator
162
. All other signals at different wavelengths will pass through the grating reflector
156
and then be output via the intermediate port
160
through fiber
166
. In this fashion, the target signal is separated from all other signals. In similar fashion, as shown in
FIG. 4
b
, if port
168
is used as input for the signal at wavelength &lgr;
j
, port
160
is used as the input for the signal at &lgr;
i
, and a grating reflector
158
specific to wavelength &lgr;
j
is coupled to port
160
, then the two signals are multiplexed and outputted through port
165
to output fiber
170
.
As disclosed in U.S. Pat. No. 5,748,349, which is herein incorporated by reference, an optical add/drop multiplexer may be constructed by
Avanex Corporation
Jr. John Juba
Spyrou Cassandra
Staas & Halsey , LLP
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