Optical waveguides – With optical coupler – Switch
Reexamination Certificate
2002-05-23
2004-07-06
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Switch
C385S018000, C385S024000, C385S033000, C385S037000
Reexamination Certificate
active
06760501
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to optical signal processing (e.g., switching) systems and components therefor, and is particularly directed to an arrangement for correcting for the inherent field curvature of a multi-optical signal focusing element, such as a concave mirror, upon the planar surface of an optical signal control (e.g., switching) device, such as a micro electro-mechanical switch (MEMS) or a liquid crystal array. Correction for this field curvature is accomplished by defining an auxiliary focal plane as a ‘best fit’ approximation of the curved focal plane of the focusing element surface, and positioning the MEMS such that its planar, optical signal-receiving surface intersects the auxiliary focal plane.
In addition, the transmission paths of the optical signals focused by the focusing element are modified, as by means of a transmissive optical wedge, so as to make the auxiliary focal plane effectively coplanar with the optical signal-receiving surface of the MEMS. Such correction is useful in free space devices, such as a configurable optical add/drop multiplexer (COADM), a dynamic gain equalizer (DGE), wavelength blocker or dynamic channel equalizer, a free space switch, or a wavelength switch, where spatially separated beams must be approximately focused on a planar device by an optical element having a curved focal field. It may be noted that two conditions are caused by defocusing at the reflection plane. First, the principal ray is not perpendicular to the processing plane and introduces losses by introducing positional offsets and thus poorly coupling into an output optical fiber. Second, if the beam is defocused at the processing plane, a broader beam (not the focus point) is reflected, so that a broader beam is poorly coupled into the output fiber.
BACKGROUND OF THE INVENTION
As described in the above-referenced '270 application, wavelength division multiplexed (WDM)-based optical communication systems often employ configurable optical add/drop multiplexers (COADMs) to selectively add and/or drop one or more channels (wavelengths) from a composite optical signal stream, as may be transported over a multichannel optical signal waveguide. As a non-limiting example,
FIG. 1
diagrammatically illustrates the general architecture of a COADM of the type disclosed in the '270 application. A COADM is an example of a limited multi-wavelength switch; of course, multiple wavelength switches with larger numbers of channels and greater switching flexibility are also possible.
As shown therein the COADM comprises first and second optical circulators
10
and
20
, respectively coupled to first and second optical waveguides
31
and
32
, and used to separate in/out and add/drop optical signals. In order to interface external optical channels with respect to the COADM, the first circulator
10
has an input or IN port
11
, and an output or OUT EXPRESS port
12
. A third port
13
of the circulator
10
is coupled to the first optical waveguide
31
. Similarly, the second circulator
20
has an input or ADD port
21
, and an output or OUT DROP port
22
. A third port
23
of the circulator
20
is coupled to the second optical waveguide
32
.
A focusing lens
35
is coupled to focus optical signals transported by waveguides
31
and
32
at the focal plane
45
of the lens
35
to be coincident with a point N of the curved focal field LP of the focusing element
40
, such as a reflective surface of revolution, for example, a spherical reflector. Beams
41
and
42
launched from the waveguides
31
and
32
parallel to the axis of the lens
35
(which does not have to be parallel to axis of reflector
40
) emerge from the lens
35
to intersect at a focal point N, where the curved focal field LP of the reflector
40
and the focal plane
45
of lens
35
are substantially coincident. Passing through focal point N, beams
41
and
42
define an opening angle &ggr;.
These two beams are reflected from the spherical reflector
40
upon a dispersive element
50
such as a diffraction grating, which spatially disperses the respective optical frequency components of the two beams along different directions in accordance with their various wavelengths components &lgr;. The diffraction grating may be coplanar with, or it may intersect and be oriented at an angle with respect to the focal plane
45
. The wavelengths dispersed by the diffraction grating are reflected by the spherical reflector
40
, so as to be incident upon the control surface
65
of a beam modifier
60
, such as, but not limited to a digitally controlled micro electro-mechanical switch (MEMS) array, such as one containing a one- or two-dimensional M×N distribution micro-mirrors, located in focal plane
45
, each micro-mirror being selectively positionable in one of two bistable orientations.
For purposes of a reduced complexity illustration, the MEMS array
60
will be assumed to have a 2×2 bypass configuration associated with the four external ports of the two circulators
10
and
20
. Also, in the present example, a respective beam of light includes only first and second wavelengths &lgr;
1
and &lgr;
2
, although more than two wavelengths are typically used in practice. In a first operational state, the MEMS array
60
is operative to cause an ‘express’ signal-associated wavelength, contained in the optical signal that is launched into the IN port
11
of the circulator
10
, to propagate to the EXPRESS port
12
of the same circulator
10
, and a ‘drop’ signal-associated wavelength signal, that is launched into port
11
of the circulator
10
, to propagate to port
22
of circulator
20
in a second operational state. Conversely, a wavelength supplied to port
21
of second circulator
20
propagates to port
22
of the second circulator
20
in the second mode of operation, but is not collected in the first mode of operation.
In operation, an input optical signal beam containing each of the two wavelengths &lgr;
1
and &lgr;
2
is launched into the port
11
of the first optical circulator
10
and is circulated thereby to port
12
and coupled to the optical waveguide
31
. This multi-wavelength beam of light is transmitted through waveguide
31
and is directed therefrom upon the lens
35
in a direction that is substantially parallel to its optical axis. Lens
35
then directs the light beam to the spherical reflector
40
at Point A, from which the beam is reflected, so as to be incident on the diffraction grating
50
at point B, where it is spatially dispersed into two sub-beams of light respectively corresponding to the two wavelengths &lgr;
1
and &lgr;
2
. Each of these sub-beams is incident upon points C
1
, C
2
of the spherical reflector
40
, and is reflected thereby, so as to be incident upon respective ‘micro’ reflectors
61
-D
1
and
62
-D
2
of the MEMS array
60
.
The reflector
61
may be controllably oriented in a first of its two bistable orientations, such that the sub-beam of light corresponding to the first dispersed wavelength &lgr;
1
, is reflected back along the same optical path to the lens
35
, enters waveguide
31
again and propagates therethrough to port
13
of the circulator
10
, where it is circulated to the OUT/EXPRESS port
12
. The reflector
62
, however, is oriented such that the sub-beam of light corresponding to the second dispersed wavelength &lgr;
2
is reflected back along a different optical path. As a consequence, the dropped signal corresponding to the wavelength &lgr;
2
is returned to the lens
35
via point E of the reflector
40
, point F of the diffraction grating
50
and again to point G of the reflector
40
, at an angle opposite to its input angle, so that it enters the waveguide
32
and propagates therethrough to the port
23
of the second circulator
20
, wherein it is circulated to the OUT/DROP port
22
.
Simultaneously with this operation, a second beam of light having wavelength &lgr;
2
may be supplied as an ADDED input beam into the IN/ADD port
21
of the second optical circulator
Bismuth Jacques
Iyer Rajiv
JDS Uniphase Inc.
MacLean Doug
Rojas Omar
Sanghavi Hemang
Teitelbaum Neil
LandOfFree
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