Folded optical interleaver with optional routing capability

Details Folded optical interleaver with optional routing capability Folded optical interleaver with optional routing capability

2000-10-30

2002-08-27

Spyrou, Cassandra (Department: 2872)

Optical: systems and elements

Polarization without modulation

By relatively adjustable superimposed or in series polarizers

C359S199200, C359S199200

Reexamination Certificate

active

06441961

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to optical communication. More particularly, it relates to optical switches for wavelength division multiplexing.
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 a signal containing two or more different channels according to their wavelength ranges and directs each channel to a different dedicated fiber. An optical interleaver can spatially separate dense WDM (DWDM) or ultra-dense WDM (UWDM) signals into two complementary subsets, each having twice the original channel spacing. 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. A WDM signal
90
containing two different channels
91
,
92
enters interleaver
999
at an input port
11
. A first birefringent element
30
spatially separates WDM signal
90
into horizontal and vertically polarized components
101
and
102
by a horizontal walk-off. Each of component signals
101
and
102
carries the full frequency spectrum of WDM signal
90
.
Components
101
and
102
couple to a polarization rotator
40
, which selectively rotates the polarization state of either signal
101
or
102
by a predefined amount. For example, rotator
40
rotates signal
102
by 90° so that signals
103
,
104
are both horizontally polarized as they exit rotator
40
and 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
91
by 90° but not the second channel
92
. 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
. 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
91
,
92
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, as the mass of interleaver
999
increases, its operation becomes more unstable. 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 to the complexity and expense of interleaver
999
.
Passband wavelength accuracy and channel isolation are important in DWDM and UWDM applications. For accurate passband wavelength, typically 1% of channel spacing, it is necessary to produce wavelength filters with a length that is accurate to within 1% of the wavelength. Furthermore, the angle between the optic axis of the filter and the polarization of the signal must be carefully controlled. These two requirements increase the cost of current interleavers.
Using a folding design can reduce the complexity and expense of an interleaver. A folding design follows the same general principles as interleaver
999
but uses only two birefringent crystals instead of three. A reflector coupled to the second birefringent crystal reflects the light back through the interleaver elements along a reverse path. Thus, the interleaver elements are fewer and can be made smaller, thereby saving space, complexity and cost.
Unfortunately, even folding designs have drawbacks. One of these drawbacks is that in order to assure proper multiplexing, demultiplexing, or routing, beams containing light in the same wavelength range must be on the same “side” of the interleaver to be properly combined. The prior art uses complex polarization rotation schemes and wavelength filters to accomplish this. Unfortunately, these rotation schemes and wavelength filters add to the manufacturing cost of the interleaver and introduce additional insertion losses.
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 folded optical interleaver that uses fewer and/or smaller birefringent elements than in previous designs. It is a further object of the invention to provide a folded interleaver having a routing capability implemented by a single polarization rotation element.
SUMMARY
The above objects and advantages are attained by an inventive optical interleaver apparatus and method. The apparatus comprises a first birefringent element, a first waveplate, a second birefringent element, a polarization rotation device and a retro-reflector. An optic axis of the first birefringent element is oriented such that the first birefringent element separates an input optical signal into first and second beams having complementary polarizations. The first waveplate is optically coupled to the first birefringent element. The first waveplate rotates a polarization of light in a first wavelength range, but not a second wavelength range. The second birefringent element is optically coupled to the first waveplate. The second birefringent element separates the first beam into a third beam containing light in the first wavelength range and a fourth beam containing light in the second wavelength range. The third beam and the fourth beam have complementary polarizations. The second birefringent element also separates the second bea

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