Optical: systems and elements – Polarization without modulation – By relatively adjustable superimposed or in series polarizers
Reexamination Certificate
2000-09-01
2002-08-27
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Polarization without modulation
By relatively adjustable superimposed or in series polarizers
C359S199200, C359S490020, C359S490020, C385S036000
Reexamination Certificate
active
06441960
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to optical communications systems. More particularly, it provides a novel class of optical interleavers with substantially minimized dispersion for multiplexing or de-multiplexing of optical signals.
BACKGROUND ART
Wavelength division multiplexing (WDM) has emerged as the standard technique to transmit information in fiber-optic networks. This is because as the bandwidth of fiber data increases, electronic sorting becomes increasingly complex, while wavelength routing becomes ever more practical and elegant.
In a WDM system, each optical fiber simultaneously carries many different communications channels in light of respectively different wavelengths. Each channel is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology.
WDM systems use components generically referred to as optical interleavers to combine, split, or route optical signals of different channels. Interleavers typically fall into one of three categories, multiplexers, de-multiplexers and routers. A multiplexer takes optical signals of different channels from two or more different input ports and combines them so that they may be coupled to an output port for transmission over a single optical fiber. A de-multiplexer performs the opposite process, that is, it decomposes an optical signal containing two or more different channels according to their wavelength ranges and directs each channel to a different dedicated fiber. A router works much the same way as a de-multiplexer; however, a router can selectively direct each channel according to control signals to a desired coupling between an input channel and an output port.
FIG. 1
depicts a typical optical interleaver
999
of the prior art as described in U.S. Pat. No. 5,694,233, which is incorporated herein by reference. A WDM signal
500
containing two different spectral sets
501
,
502
enters interleaver
999
at an input port
11
. AS used herein, the term “spectral set” refers to a particular range of wavelengths or frequencies that defines a unique information signal. A first birefringent element
30
spatially separates WDM signal
500
into horizontal and vertically polarized components
101
and
102
by a horizontal walk-off. Component signals
101
and
102
both carry the full frequency spectrum of the WDM signal
500
.
Components
101
and
102
are coupled to a polarization rotator
40
. The rotator
40
selectively rotates the polarization state of either signal
101
or
102
by a predefined amount. By way of example, in
FIG. 1
signal
102
is rotated by 90° so that signals
103
,
104
exiting rotator
40
are both horizontally polarized when they enter a wavelength filter
61
.
Wavelength filter
61
selectively rotates the polarization of wavelengths in either the first or second spectral set to produce filtered signals
105
and
106
. For example, wavelength filter
61
rotates wavelengths in the first spectral set
501
by 90° but does not rotate wavelengths in the second spectral set
502
at all.
The filtered signals
105
and
106
enter a second birefringent element
50
that vertically walks off the first spectral set into beams
107
,
108
. The second spectral set forms beams
109
,
110
.
A second wavelength filter
62
then selectively rotates the polarizations of signals
107
and
108
, but not signals
109
and
110
, thereby producing signals
111
,
112
,
113
,
114
that have polarizations parallel to each other. A second polarization rotator
41
then rotates the polarizations of signals
111
and
113
, but not the polarizations of signals
112
and
114
. The resulting signals
115
,
116
,
117
, and
118
then enter a third birefringent element
70
. Note that second wavelength filter
62
may alternatively be replaced by a polarization rotator suitably configured to rotate the polarizations of signals
111
and
113
, but not
112
and
114
.
Third birefringent element
70
combines signals
115
and
116
, into the first spectral channel, which is coupled to output port
14
. Birefringent element
70
also combines signals
117
and
118
into the second spectral channel, which is coupled into output port
13
.
As described above, interleaver
999
operates as a de-multiplexer. By operating interleaver
999
in reverse, i.e., starting with spectral sets
501
,
502
at ports
13
and
14
respectively, interleaver
999
operates as a multiplexer. Furthermore, by suitably controlling the polarization rotation induced by rotators
40
and
41
, interleaver
999
may be configured to operate as a router.
Interleaver
999
described above advantageously uses wavelength filters to separate an input WDM optical signal containing two spectral sets by way of different polarization modes and subsequently exploits the birefrigent walk-off effect to spatially separate different polarization modes, thereby de-multiplexing the input WDM optical signal. The use of the wavelength filters and birefrigent materials, however, inadvertently introduces various dispersion effects, which would degrade the performance of fiber-optic networks if uncompensated for. For instance, there is Polarization Mode Dispersion (PMD) known in the art, owing to the fact that different polarization modes traverse different optical path lengths in a birefrigent material. Moreover, since a wavelength filter is typically composed of a stacked plurality of birefrigent waveplates, different wavelengths of light undertake different polarizations in various constituent waveplates of a wavelength filter; and different polarizations subsequently lead to different optical path lengths. Hence, there is also Wavelength-Filter-Induced-Dispersion (WFID) that is both chromatic and polarization-related. Therefore, care must be taken to ensure that various dispersion effects are substantially minimized in an optical interleaver.
As fiber-optic systems rapidly spread as the backbone of modern communications networks, there is a need for optical interleavers in which dispersion effects are properly accounted for. The desired optical interleavers should also have a simple and low-cost assembly.
OBJECTS AND ADVANTAGES
Accordingly it is a principal object of the present invention to provide a line of optical interleavers in which a novel beam-swapping element is utilized. Moreover, efforts are painstakingly made in the optical interleavers of the present invention to minimize various dispersion effects. It is a further object of the present invention to provide methods for constructing these novel optical interleavers.
An advantage of the beam-swapping element of the present invention is that it provides an effective and inexpensive alternative to the second polarization rotator and wavelength filter employed in the prior art optical interleaver as shown in
FIG. 1
, hence rendering a simple and low-cost assembly to an optical interleaver of the present invention. The use of the beam-swapping element further avoids undesirable complications such as dispersion effects. Another significant advantage of the optical interleavers of the present invention is that they present the first kind in the art in which various dispersion effects are substantially minimized. Such characteristics are highly desirable in fiber-optic networks.
These and other objects and advantages will become apparent from the following description and accompanying drawings.
SUMMARY OF THE INVENTION
The present invention provides an optical interleaver comprising a first birefringent element that decomposes and spatially separates an input WDM signal carrying first and second spectral sets into first and second beams with orthogonal polarizations. The first and second spectral sets are substantially complementary. A first wavelength filter, optically coupled to receive the first and second beams, decomposes the first beam into third and fourth beams and the second beam into fifth and sixth beams, by preferentially rotating the polarization
Li Wei-Zhong
Wang Mark S.
Fish and Richardson P.C.
Oplink Communications Inc.
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