Optical interleaver

Details Optical interleaver Optical interleaver

2000-01-31

2001-04-03

Palmer, Phan T. H. (Department: 2874)

Optical waveguides

With optical coupler

Plural

C359S199200, C359S199200, C385S039000

Reexamination Certificate

active

06212313

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to optical communications systems. More particularly, it relates to an optical interleaver for multiplexing or de-multiplexing optical signals.
BACKGROUND ART
Optical wavelength division multiplexing (WDM) has gradually become the standard backbone network for fiber optic communication systems. WDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal 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 referred to generically 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 divides an 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, issued to Wu et al. on Dec. 2, 1997, which is incorporated herein by reference for all purposes. A WDM signal
500
containing two different channels
501
,
502
enters interleaver
999
at an input port
11
. 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 channel to produce filtered signals
105
and
106
. For example wavelength filter
61
rotates wavelengths in the first channel
501
by 90° but does not rotate wavelengths in the second channel
502
at all.
The filtered signals
105
and
106
enter a second birefringent element
50
that vertically walks off the first channel into beams
107
,
108
. The second channel forms beams
109
,
110
.
A second wavelength filter
62
then selectively rotates the polarizations of signals
107
,
108
but not signals
109
,
110
thereby producing signals
111
,
112
,
113
,
114
, having polarizations that are parallel each other. A second polarization rotator
41
then rotates the polarizations of signals
111
and
113
, but not
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
41
suitably configured to rotate the polarizations of signals
111
,
113
but not
112
,
114
.
Third birefringent element
70
combines signals
115
and
116
, into the first channel, which is coupled to output port
14
. Birefringent element
70
also combines signals
117
and
118
into the second 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 channels
501
,
502
at ports
13
and
14
respectively, interleaver 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
has certain drawbacks. First, each port requires its own collimator. Three collimators take up space and require a relatively large walk-off distance for the signals. Consequently, birefringent elements
30
,
50
and
70
tend to be both long and wide. Second, the number of components, particularly birefringent elements, tends to make interleaver
999
bulky, expensive and more massive. Generally, the greater the mass of interleaver
999
, the more unstable its operation. Third, the coupling distance, i.e., the distance between port
11
and ports
13
,
14
, tends to be long, which increases insertion losses in interleaver
999
. Furthermore, each of the ports
11
,
13
and
14
requires a separate collimator to couple the signals into and out of optical fibers. This adds the complexity and expense of interleaver
999
.
There is a need, therefore, for an improved optical interleaver that overcomes the above difficulties.
OBJECTS AND ADVANTAGES
Accordingly, it is a primary object of the present invention to provide a compact optical interleaver that uses fewer parts. It is a further object of the invention to provide an interleaver with a single collimator for coupling optical signals to or from two or more optical fibers.
SUMMARY
These objects and advantages are attained by an optical interleaver having a compact design that allows the use of smaller birefringent elements. The interleaver generally comprises a first birefringent element optically coupled to at least two input/output ports. A first polarization rotator is optically coupled to the first birefringent element. A wavelength filter is optically coupled to the first polarization rotator element, and a second birefringent element is optically coupled to the wavelength filter. A second polarization rotator and a reflector are optically coupled to second birefringent element. The reflector reflects an optical signal that travels from the input/output ports through the elements of the interleaver back through these elements in reverse order back to the input/output ports. Usually, the input/output ports are all located on the same side of the first birefringent element. This compact configuration reduces the number of components required for the interleaver and allows the use of shorter birefringent elements. The interleaver may be configured to operate as a multiplexer, a de-multiplexer, or a router.
Embodiments of the interleaver may include a compensation plate that compensates for a phase difference due to different optical path lengths for ordinary and extraordinary rays travelling in the first birefringent element. Alternative embodiments include gaps in the first polarization rotator or wavelength filter to allow beams on either the forward or reverse path to bypass these elements. Such a configuration reduces the number of components, thereby reducing insertion losses and cost.
Further advantages of the various embodiments of the invention are depicted in the drawings and the detailed description that follows.


REFERENCES:
patent: 5606439 (1997-02-01), Wu
patent: 5694233 (1997-12-01), Wu et al.
patent: 5724165 (1998-03-01), Wu
patent: 5796889 (1998-08-01), Xu et al.
patent: 5912748 (1999-06-01), Wu et al.
patent: 5923472 (1999-07-01), Bergmann
patent: 6005697 (1999-12-01), Wu et al.
patent: 6097518 (2000-08-01), Wu et al.

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