U.S. patent application number 09/928237 was filed with the patent office on 2002-05-23 for multi-wavelength cross-connect optical switch.
Invention is credited to Bhattarai, Amal R., Heritage, Jonathan P., Solgaard, Olav.
Application Number | 20020061160 09/928237 |
Document ID | / |
Family ID | 21810393 |
Filed Date | 2002-05-23 |
United States Patent
Application |
20020061160 |
Kind Code |
A1 |
Solgaard, Olav ; et
al. |
May 23, 2002 |
Multi-wavelength cross-connect optical switch
Abstract
A cross-connect switch for fiber-optic communication networks
employing a wavelength dispersive element, such as a grating, and a
stack of regular (non-wavelength selective) cross bar switches
using two-dimensional arrays of micromachined, electrically
actuated, individually-tiltable, controlled deflection
micro-mirrors for providing multiport switching capability for a
plurality of wavelengths. Using a one-dimensional micromirror
array, a fiber-optic based MEMS switched spectrometer that does not
require mechanical motion of bulk components or large diode arrays
can be constructed with readout capability for WDM network
diagnosis or for general purpose spectroscopic applications.
Inventors: |
Solgaard, Olav; (Davis,
CA) ; Heritage, Jonathan P.; (Davis, CA) ;
Bhattarai, Amal R.; (Davis, CA) |
Correspondence
Address: |
John P. O'Banion
O'BANION & RITCHEY LLP
Suite 1550
400 Capitol Mall
Sacramento
CA
95814
US
|
Family ID: |
21810393 |
Appl. No.: |
09/928237 |
Filed: |
August 10, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09928237 |
Aug 10, 2001 |
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09618320 |
Jul 18, 2000 |
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09618320 |
Jul 18, 2000 |
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09022591 |
Feb 12, 1998 |
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60038172 |
Feb 13, 1997 |
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Current U.S.
Class: |
385/18 ; 356/326;
385/17 |
Current CPC
Class: |
G02B 6/35 20130101; G02B
6/3516 20130101; G02B 6/3556 20130101; G02B 6/357 20130101; H04Q
11/0005 20130101; G02B 6/3512 20130101; G02B 6/3584 20130101; G02B
6/3582 20130101; G02B 6/3534 20130101; G02B 6/356 20130101; H04Q
2011/0024 20130101; H04Q 2011/0083 20130101; G02B 6/3546 20130101;
G02B 6/3592 20130101; G02B 6/3518 20130101; G02B 6/3566 20130101;
G02B 6/3542 20130101; G02B 6/32 20130101; G01J 3/28 20130101; H04Q
2011/0026 20130101 |
Class at
Publication: |
385/18 ; 385/17;
356/326 |
International
Class: |
G02B 006/35; G01J
003/28 |
Claims
What is claimed is:
1. A fiber optic switch, comprising an array of actuated mirrors
for switching optic signals from a plurality of input optic fibers
onto a plurality of output optic fibers.
2. A fiber optic switch as recited in claim 1, wherein separate
wavelength channels on each input optic fiber are switched
independently by said array of mirrors.
3. A fiber optic switch as recited in claim 1, further comprising a
plurality of optical elements for positioning optical beams from
said input optic fibers onto said array of mirrors.
4. A fiber optic switch as recited in claim 1, further comprising a
plurality of optical elements for positioning optical beams
reflected from said array of mirrors onto said output fibers.
5. A fiber optic switch as recited in claim 1, further comprising:
(a) a wavelength dispersive element; and (b) a plurality of lenses
associated with said first wavelength dispersive element; (c)
wherein said wavelength dispersive element and said plurality of
lenses position optical beams from said input optical fibers onto
said array of mirrors.
6. A fiber optic switch as recited in claim 5, wherein said
wavelength dispersive element comprises a diffraction grating.
7. A fiber optic switch as recited in claim 5, wherein said
wavelength dispersive element comprises a prism.
8. A fiber optic switch as recited in claim 5, further comprising:
(a) a second wavelength dispersive element; and (b) a second
plurality of lenses associated with said second wavelength
dispersive element; (c) wherein said second wavelength dispersive
element and said second plurality of lenses position optical beams
reflected by said array of mirrors onto said output optic
fibers.
9. A fiber optic switch as recited in claim 8, wherein at least one
of said wavelength dispersive elements comprises a diffraction
grating.
10. A fiber optic switch as recited in claim 8, wherein at least
one of said wavelength dispersive elements comprises a prism.
11. A fiber optic switch, comprising: (a) an array of actuated
mirrors for switching optic signals from a plurality of input optic
fibers onto a plurality of output optic fibers; (b) a wavelength
dispersive element; and (c) a plurality of lenses associated with
said wavelength dispersive element; (d) wherein said wavelength
dispersive element and said plurality of lenses position optical
beams from said input optical fibers onto said array of
mirrors.
12. A fiber optic switch as recited in claim 12, wherein separate
wavelength channels on each input optic fiber are switched
independently by said array of mirrors.
13. A fiber optic switch as recited in claim 1 1, wherein said
wavelength dispersive element comprises a diffraction grating.
14. A fiber optic switch as recited in claim 11, wherein said
wavelength dispersive element comprises a prism.
15. A fiber optic switch as recited in claim 11, further
comprising: (a) a second wavelength dispersive element; and (b) a
second plurality of lenses associated with said second wavelength
dispersive element; (c) wherein said second wavelength dispersive
element and said second plurality of lenses position optical beams
reflected by said array of mirrors onto said output optic
fibers.
16. A fiber optic switch as recited in claim 15, wherein at least
one of said wavelength dispersive elements comprises a diffraction
grating.
17. A fiber optic switch as recited in claim 15, wherein at least
one of said wavelength dispersive elements comprises a prism.
18. A fiber optic switch, comprising: (a) an array of actuated
mirrors for switching optic signals from a plurality of input optic
fibers onto a plurality of output optic fibers; (b) a first
wavelength dispersive element; (c) a first plurality of lenses
associated with said first wavelength dispersive element; (d) a
second wavelength dispersive element; and (e) a second plurality of
lenses associated with said second wavelength dispersive element;
(f) wherein said first wavelength dispersive element and said first
plurality of lenses position optical beams from said input optical
fibers onto said array of mirrors and wherein said second
wavelength dispersive element and said second plurality of lenses
position optical beams reflected by said array of mirrors onto said
output optic fibers.
19. A fiber optic switch as recited in claim 18, wherein separate
wavelength channels on each input optic fiber are switched
independently by said array of mirrors.
20. A fiber optic switch as recited in claim 18, wherein at least
one of said wavelength dispersive elements comprises a diffraction
grating.
21. A fiber optic switch as recited in claim 18, wherein at least
one of said wavelength dispersive elements comprises a prism.
22. A fiber optic switch, comprising: (a) a plurality of input
optic fibers; (b) a plurality of output optic fibers: (c) an array
of actuated mirrors for switching optic signals from said input
optic fibers onto said output optic fibers; (d) a first wavelength
dispersive element; (e) a first plurality of lenses associated with
said first wavelength dispersive element; (f) a second wavelength
dispersive element; and (g) a second plurality of lenses associated
with said second wavelength dispersive element; (h) wherein said
first wavelength dispersive element and said first plurality of
lenses position optical beams from said input optical fibers onto
said array of mirrors and wherein said second wavelength dispersive
element and said second plurality of lenses position optical beams
reflected by said array of mirrors onto said output optic
fibers.
23. A fiber optic switch as recited in claim 22, wherein separate
wavelength channels on each input optic fiber are switched
independently by said array of mirrors.
24. A fiber optic switch as recited in claim 22, wherein at least
one of said wave length dispersive elements comprises a diffraction
grating.
25. A fiber optic switch as recited in claim 22, wherein at least
one of said wavelength dispersive elements comprises a prism.
26. A fiber optic spectrometer, comprising: (a) a fiber optic input
path; (b) a fiber optic output path; (c) a detector; and (d) an
array of actuated mirrors for switching optic signals from said
fiber optic input path to said fiber optic output path or said
detector.
27. A fiber optic spectrometer as recited in claim 26, further
comprising: (a) a wavelength dispersive element; and (b) a lens
associated with said wavelength dispersive element; (c) wherein
said wavelength dispersive element and said lens positions optical
beams from said fiber optic input path onto said array of
mirrors.
28. A fiber optic spectrometer as recited in claim 27, wherein said
fiber optic input path and said fiber optic output path share a
single optic fiber.
29. A fiber optic spectrometer as recited in claim 27, wherein said
fiber optic input path and said fiber optic output path are carried
by separate optic fibers.
30. A fiber optic spectrometer, comprising: (a) an input optic
fiber; (b) an output optic fiber; (c) a detector; (d) an array of
actuated mirrors for switching optic signals from said input optic
fibers onto said output optic fiber or said detector; (e) a
wavelength dispersive element; and (f) a lens associated with said
wavelength dispersive element; (g) wherein said wavelength
dispersive element and said lens positions optical beams from said
input optical fiber onto said array of mirrors.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. provisional
application serial No. 60/038,172 filed on Feb. 13, 1997.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable
REFERENCE TO A MICROFICHE APPENDIX
[0003] Not Applicable
BACKGROUND OF THE INVENTION
[0004] 1 . Field of the Invention
[0005] This invention relates to a cross-connect switch for
fiber-optic communication networks including wavelength division
multiplexed (WDM) networks, and more particularly to such an
optical switch using a matrix of individually tiltable
micro-mirrors.
[0006] 2. Description of the Background Art
[0007] Multi-port, multi-wavelength cross-connect optical switches
with characteristics of large cross-talk rejection and flat
passband response have been desired for use in wavelength-division
multiplexed (WDM) networks. Four-port multi-wavelength cross-bar
switches based on the acousto-optic tunable filter have been
described ("Integrated Acoustically-tuned Optical Filters for
Filtering and Switching Applications," D. A. Smith, et al., IEEE
Ultrasonics Symposium Proceedings, IEEE, New York, 1991, pp.
547-558), but they presently suffer from certain fundamental
limitations including poor cross-talk rejection and an inability to
be easily scaled to a larger number of ports. Attempts are being
made to address to this problem by dilating the switch fabric, both
in wavelength and in switch number, to provide improved cross-talk
rejection and to expand the number of switched ports so as to
provide an add-drop capability to the 2.times.2 switch fabric. This
strategy, however, adds to switch complexity and cost. Recently,
Patel and Silberberg disclosed a device employing compactly
packaged, free-space optical paths that admit multiple WDM channels
spatially differentiated by gratings and lenses ("Liquid Crystal
and Grating-Based Multiple-Wavelength Cross-Connect Switch," IEEE
Photonics Technology Letters, Vol. 7, pp. 514-516, 1995). This
device, however, is limited to four (2.times.2) ports since it
relies on the two-state nature of polarized light.
BRIEF SUMMARY OF THE INVENTION
[0008] It is therefore an object of this invention to provide an
improved multi-wavelength cross-connect optical switch which is
scalable in port number beyond 2.times.2.
[0009] Another object of the invention to provide such an optical
switch which can be produced by known technology.
[0010] Another object of this invention to provide such an optical
switch with high performance characteristics such as basic low
loss, high cross-talk rejection and flat passband
characteristics.
[0011] Another object of the invention is to provide a fiber-optic
switch using two arrays of actuated mirrors to switch or rearrange
signals from N input fibers onto N output fibers, where the number
of fibers, N, can be two, or substantially larger than 2.
[0012] Another object of the invention is to provide a fiber-optic
switch using 1-D arrays of actuated mirrors.
[0013] Another object of the invention is to provide a fiber-optic
switch using 2-D arrays of actuated mirrors.
[0014] Another object of the invention is to provide a fiber-optic
switch using mirror arrays (1-D or 2-D) fabricated using
micromachining technology.
[0015] Another object of the invention is to provide a fiber-optic
switch using mirror arrays (1-D or 2-D) fabricated using
polysilicon surface micromachining technology.
[0016] Another object of the invention is to provide a fiber-optic
switch using arrays (1-D or 2-D) of micromirrors suspended by
torsion bars and fabricated using polysilicon surface
micromachining technology.
[0017] Another object of the invention is to provide a fiber-optic
switch with no lens or other beam forming or imaging optical device
or system between the mirror arrays.
[0018] Another object of the invention is to provide a fiber-optic
switch using macroscopic optical elements to image or position the
optical beams from the input fibers onto the mirror arrays, and
likewise using macroscopic optical elements to image or position
the optical beams from the mirror arrays onto the output
fibers.
[0019] Another object of the invention is to provide a fiber-optic
switch using microoptics to image or position the optical beams
from the input fibers onto the mirror arrays, and likewise using
microoptics to image or position the optical beams from the mirror
arrays onto the output fibers.
[0020] Another object of the invention is to provide a fiber-optic
switch using a combination of macrooptics and microoptics to image
or position the optical beams from the input fibers onto the mirror
arrays, and likewise using combination of macrooptics and
microoptics to image or position the optical beams from the mirror
arrays onto the output fibers.
[0021] Another object of invention is to provide a fiber-optic
switch in which the components (fibers, gratings, lenses and mirror
arrays) are combined or integrated to a working switch using
Silicon-Optical-Bench technology.
[0022] Another object of the invention is to provide a fiber-optic
switch using 2-D arrays of actuated mirrors and dispersive elements
to switch or rearrange signals from N input fibers onto N output
fibers in such a fashion that the separate wavelength channels on
each input fiber are switched independently.
[0023] Another object of the invention is to provide a fiber-optic
switch as described above, using diffraction gratings as wavelength
dispersive elements.
[0024] Another object of the invention is to provide a fiber-optic
switch as described above, using micromachined diffraction gratings
as wavelength dispersive elements.
[0025] Another object of the invention is to provide a fiber-optic
switch using fiber Bragg gratings as wavelength dispersive
elements.
[0026] Another object of the invention is to provide a fiber-optic
switch using prisms as wavelength dispersive elements.
[0027] Another object of the invention is to provide a fiber-optic
based MEMS switched spectrometer that does not require mechanical
motion of bulk components nor large diode arrays, with readout
capability for WDM network diagnosis.
[0028] Another object of the invention is to provide a fiber-optic
based MEMS switched spectrometer that does not require mechanical
motion of bulk components nor large diode arrays, with readout
capability for general purpose spectroscopic applications.
[0029] Further objects and advantages of the invention will be
brought out in the following portions of the specification, wherein
the detailed description is for the purpose of fully disclosing
preferred embodiments of the invention without placing limitations
thereon.
[0030] An optical switch embodying this invention, with which the
above and other objects can be accomplished, may be characterized
as comprising a wavelength dispersive element, such as a grating,
and a stack of regular (non-wavelength selective) cross bar
switches using a pair of two-dimensional arrays of micromachined,
electrically actuated, controlled deflection micro-mirrors for
providing multiport switching capability for a plurality of
wavelengths.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The invention will be more fully understood by reference to
the following drawings which are for illustrative purposes
only:
[0032] FIG. 1 is a schematic diagram of an optical switch in
accordance with the present invention.
[0033] FIG. 2 is a schematic plan view of a single layer of the
switching matrix portion of the optical switch shown in FIG. 1.
[0034] FIG. 3 is a diagrammatic plan view of a row of individually
tiltable micro-mirrors employed in the switch array portion of the
optical switch shown in FIG. 1.
[0035] FIG. 4 is a schematic diagram showing switching matrix
geometry in accordance with the present invention.
[0036] FIG. 5 is a schematic sectional view of a grating made in
silicon by anisotropic etching.
[0037] FIG. 6 is a schematic diagram of an alternative embodiment
of the optical switch shown in FIG. 1 employing a mirror in the
symmetry plane.
[0038] FIG. 7 is a schematic plan view of a single layer of the
switching matrix portion of the optical switch shown in FIG. 6.
[0039] FIG. 8 is a schematic diagram of a WDM spectrometer
employing an optical switch in accordance with the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0040] Referring more specifically to the drawings, for
illustrative purposes the present invention is embodied in the
apparatus generally shown in FIG. 1 through FIG. 8, where like
reference numerals denote like parts. It will be appreciated that
the apparatus may vary as to configuration and as to details of the
parts without departing from the basic concepts as disclosed
herein.
[0041] Referring first to FIG. 1, a multi-port (N.times.N ports),
multi-wavelength (M wavelength) WDM cross-connect switch 10
embodying this invention is schematically shown where, in the
example shown, N=3. In this switch 10, the wavelength channels 12a,
12b, 12c of three input fibers 14a, 14b, 14c are collimated and
spatially dispersed by a first (or input) diffraction grating-lens
system 16. The grating-lens system 16 separates the wavelength
channels in a direction perpendicular to the plane of the paper,
and the dispersed wavelength channels are then focused onto a
corresponding layer 18a, 18b, 18c of a spatial micromechanical
switching matrix 20. The spatially reorganized wavelength channels
are finally collimated and recombined by a second (or output)
diffraction grating-lens system 22 onto three output fibers 24a,
24b, 24c. The input and output lens systems are each composed of a
lenslet array 26 (32) and a pair of bulk lenses 28, 30 (34, 36 )
such that the spot size and the spot separation on the switching
matrix 20 can be individually controlled. Two quarter-wave plates
38, 40 are inserted symmetrically around the micromechanical
switching matrix 20 to compensate for the polarization sensitivity
of the gratings 42, 44, respectively.
[0042] Referring now to FIG. 2, a schematic plan view of a single
layer 18 a of the switching matrix 20 of FIG. 1 is shown. As can be
seen in FIG. 2, six micromirrors 46a through 46f are arranged in
two arrays 48a, 48b that can be individually controlled so as to
optically "couple" any of the three input fibers 14a, 14b, 14c to
any of the three output fibers 24a, 24b, 24c. Referring also to
FIG. 3, an example of the structural configuration of a row of
individually tiltable micromirrors on the switch array can be seen.
As shown in FIG. 3, each mirror 46a 46b, 46c is suspended by a pair
of torsion bars or flexing beams 50a, 50b attached to posts 52a,
52b, respectively. Note that each mirror also includes a landing
electrode 54 on which the mirrors land when they are deflected all
the way down to the substrate.
[0043] The advantages of this switching matrix arrangement include
low cross talk because cross coupling between channels must go
through two mirrors, both of which are set up to reject cross talk,
low polarization sensitivity, and scalability to larger numbers of
input fibers than two (which is the limit for polarization-based
switches). The preferred fabrication technology for the micromirror
arrays is surface micromachining as disclosed, for example, in
Journal of Vacuum Science and Technology B, Vol. 6, pp.
1809-1813,1988 because they can be made by this method as small as
the optical design will allow, they can be batch-fabricated
inexpensively, they can be integrated with on-chip micromachinery
from materials such as polysilicon and they can be miniaturized so
as to reduce the cost of packaging.
[0044] Referring now to FIG. 4, there are several factors to
consider for designing the switching matrix 20. FIG. 4 shows an
example of two mirror arrays 56a, 56b where each array comprises a
plurality of mirrors, Na through Nn. The basic parameters are the
Gaussian beam radium (.omega..sub.0) at the center of the switch,
the mirror size in the horizontal direction (s), the distance
between the mirror arrays (p), the incident angle on the mirror
arrays (.beta.), and the maximum deflection of the micromirrors
(t). The maximum angular deflection (.alpha.) of the optical beams
is dependent on the maximum deflection and the mirror size (in the
horizontal direction), or tan .alpha.=4 t/s (the optical deflection
being twice that of the mechanical deflection and the mirrors being
tethered at their center points). Conditions that must be satisfied
include: (i) the optical beams must be sufficiently separated on
the mirrors to keep cross-talk at reasonable levels; (ii) the sizes
of the arrays must be small enough such that no shadowing occurs;
and (iii) the path length differences through the switch must not
introduce significant variations in insertion loss.
[0045] If a Gaussian-beam formalism is used, the cross coupling of
two parallel beams of radius .omega..sub.0 offset by d is given as
exp{-(d/.omega..sub.0)2}. If this should be less than 40 dB, for
example, the ratio C=d/.omega..sub.0 must be larger than 3. This
minimum ratio applies to both fiber-channel (horizontal) separation
and wavelength channel (vertical) separation. Since the beam radii
are larger at the mirrors due to diffraction than at the focus, the
above conclusion does not strictly apply, but it may be required
that the spacing of the beams must be at least three times larger
than the optical beam radius also at the mirrors. This requirement
also reduces the losses due to aperture effects to insignificant
levels. The minimum mirror spacing (which is larger or equal to the
mirror size) is then expressed as:
s=C.omega./cos .beta.=C(.omega..sub.0/cos
.beta.){1+(.lambda.p/2.pi..omega- ..sub.0
.sup.2).sup.2}.sup.1/2
[0046] where C is a factor greater than 3, .omega. is the beam size
at the mirrors, and .lambda. is the wavelength of the light. This
requirement, together with the restrictions on angular deflection,
puts an upper limit on the number of fiber channels (=N) in the
switch. If the maximum deflection angle is assumed small, the
maximum number of channels is:
N=1+{4.pi.cos .beta./C.sup.2.lambda.}t
[0047] The corresponding array separation and mirror spacing
are:
p=2.pi..omega..sub.0.sup.2/.lambda.
[0048] and
s=2.sup.1/2C.omega..sub.0/cos .beta.
[0049] If C=3, .beta.=0.3 radian and .lambda.=1.55 micron, N is
given by N=1 +0.86 t where t is in micron. The conclusion is that
the simple geometry of FIG. 4 can be used to design relatively
large switches. Using known surface micromachining techniques,
switches with three fiber channels can be fabricated with a total
sacrificial layer thickness of 2.75 micron. Larger switching
matrices will require thicker sacrificial layers, such as 8.1
micron for N=8 and 17.4 micron for N=16. These thicknesses can be
obtained by simply using thicker sacrificial layers or by using
out-of-plane structures, or through a combination of these.
[0050] The minimum beam radium .omega. is determined from the
requirement that the first mirror array should not obstruct the
beams after they are reflected from the second mirror, and that the
second mirror array should not obstruct the beams before they reach
the second mirror. If N is maximized, C=3, .sym.=0.3 radian and
.lambda.=1.55 micron, the beam radium in micron must be larger than
the number of fiber channels, or .omega..sub.0>N micron. The
requirement of maximum path length difference is less restrictive
(.omega..sub.0>0.8(N-1) micron). In general, the mirrors should
be made as small as possible because miniaturization of the mirrors
leads to increased resonance frequencies, lower voltage
requirements and better stability. For N=3, .omega..sub.0 should
preferably be on the order of 3 micron, leading to a mirror spacing
of s=13.3 micron. Mirrors of this size, as well as mirrors suitable
for larger matrices are easily fabricated by a standard surface
micromachining process. This implies that micromachined switching
matrices can be scaled to large numbers of fiber channels (e.g.,
N=16).
[0051] The minimum mirror separation in the wavelength-channel
dimension may be smaller than the mirror separation s given above
by the factor of cos .beta. because the mirrors are not tilted in
that direction. The spacing, however, may be larger because there
is no switching in the wavelength direction and there is no maximum
angle requirement. It may be preferred to add 20 to 30 micron of
space between the mirrors for mirror posts (see FIG. 3) and
addressing lines. For a switch with N=3 and s=110 micron, for
example, a preferred separation in the wavelength dimension may be
on the order of 130 micron. If the Littrow configuration is used,
the required size of the Gaussian-beam radium on the grating
is:
.omega..sub.g=C.sub.w.lambda..sup.2/2.pi..DELTA..lambda.tan
.theta..sub.c
[0052] where C.omega..sub.w is the ratio of beam separation to beam
radium in the wavelength dimension, .DELTA..lambda. is the
wavelength separation between channels and .theta..sub.c is the
incident angle (which equals the diffraction angle in the Littrow
configuration). With C.sub.w=5.2 (25 micron beam radium and 130
micron wavelength-channel separation), k=1.55 micron,
.DELTA..lambda.=1.6 nm ("the MONET standard") and .theta..sub.c=45
degrees, .omega..sub.g is 1 .2 mm (the order of diffraction m being
1). This means that the long side of the grating perpendicular to
the grooves must be on the order of 5.3 mm and the focal length
f.sub.2 of the second lens (the one between the grating and the
microswitches) should be about 60.8 mm.
[0053] The rotation of the grating about an axis perpendicular to
the optical axis and the grating grooves require a corresponding
reduction of the grating period by .LAMBDA.=m.lambda. cos .phi.(2
sin .theta..sub.0) where .phi. is the rotation angle of the
grating. If m=1, .lambda.=1.55 micron, .theta..sub.0=45 degrees and
.phi.=30 degrees, the grating period is 0.95 micron.
[0054] In a simple system without the microlens arrays shown in
FIG. 1, the magnification of the input plane onto the switching
matrix is given by the ratio of the length f.sub.2 of the second
lens to the focal length f.sub.1 of the first lens (e.g., lens 28
between the lenslets 26 and grating 42). The role of the lenslet is
to allow a different magnification for the mode size and for the
mode spacing. In the optimized geometry described above, the ratio
of the mode separation to mode radius is 2.sup.{fraction (1/2
)}C=4.24. If the fibers are as close together as practically
possible (e.g., 125 micron equal to the fiber diameter), the ratio
on the input is 125/5=25. The mode radius therefore must be
magnified 5.9 times more than the mode spacing. This can be
accomplished in several ways.
[0055] According to one method, lenslets are used to magnify the
fiber modes without changing the mode separation. The lenslets are
placed less than one focal length in front of the fibers to form an
imaginary magnified image of the fiber mode. Ideally, the lenslet
diameters should be comparable to or smaller than the fiber
diameter, allowing the minimum fiber separation and a short focal
length of the first lens to be maintained. According to another
method, the fiber mode is expanded adiabatically over the last part
of its length by a heat treatment so as to out-diffuse the core.
Mode size increase on the order of 4 times can be accomplished.
[0056] According to still another method, the fibers are thinned
down or microprisms are used to bring the modes closer together
without changing the mode size.
[0057] The first two methods require the same magnification or
reduction of the input field. The third method has the advantage
that less reduction is needed, leading to smaller systems. If N=3,
.omega..sub.0=25 micron, s=110 micron and f.sub.2=60.8 mm, the
required magnification is 0.84. If lenslets of 125 micron diameter
are used, the required focal length f.sub.1, of the first lens is
72.4 mm. If lenslets of 200 micron diameter are used, the required
magnification is 0.525 and the required focal length f.sub.1, of
the first lens is 115.8 mm.
[0058] Parameters of a switch according to one embodiment of this
invention with three input channels and three output channels are
summarized in Table 1.
[0059] Referring again to FIG. 3, the switching matrix design shown
is compatible with the MUMP (the Multiuser Micro Electro-Mechanical
System Process at the Microelectronics Center of North Carolina)
and its design rules. The full switching matrix includes two arrays
each with eight rows of the mirrors shown. The mirrors are actuated
by an electrostatic field applied between the mirror and an
electrode underneath (not shown). Each of the mirrors in the
switching matrix has three states, but the mirrors in the three
rows do not operate identically. The central mirror may send the
beam to either side, while the outer mirrors only deflect to one
side. According to one design, the two on the sides are mirror
images of each other, the center mirror being either in the flat
state (no voltage applied) or brought down to the point where it
touches the substrate on either side. The electrode under the
central mirror is split in two to allow it to tilt either way. The
side mirrors also have a state that is half way between the flat
state and the fully pulled-down state. This may be achieved by
having continuous control over the mirror angles. Although this is
complicated by the electromechanical instability of parallel plate
capacitors because, as the voltage on the plates is increased, the
capacitance goes up and this leads to a spontaneous pulling down of
the mirror when the voltage is increased past a certain value, this
effect can be avoided either by controlling the charge rather than
the voltage on the capacitors, or by using an electrode geometry
that pushes the instability point past the angle to be accessed.
Charge control utilizes the full range of motion of the mirrors but
complicates the driver circuitry for the switch. It may be
preferable to use electrode geometry to achieve the required number
of states.
[0060] When the MUMP process is used, the mirror size has a lower
limit imposed by the minimum cross section that can be defined in
this process. To achieve large tilt angles without too much
deflection of the rotation axis of the mirror, the flexing beams
must be kept short. The shorter the flexing beam, the larger the
mirrors must be for the electrostatic force to be able to pull the
mirrors down. Calculations show that the two side mirrors have the
required angle when pulled down if use is made of beams that are 15
to 20 micron long, depending on the exact value of the material
constants. For the voltage requirement to be acceptable, the mirror
size must be on the order of 100.times.100 micron. The beams of the
central mirror may be slightly longer, the maximum angle for the
central mirror being half that of the side mirrors. The geometry as
shown in FIG. 3 ensures that the side mirrors can be tilted to half
their maximum angles before reaching the electrostatic instability.
The corresponding resonance frequencies are on the order of 20 to
50 Khz. FIG. 3 shows one of 8 layers of micromirrors in the
switching matrix described above; that is, the mirrors are
separated by 110 micron in the horizontal (fiber-channel) direction
and by 130 micron in the vertical (wavelength-channel) direction.
Two such arrays make up a switching matrix as shown in FIG. 2. The
mirrors are shown on landing electrodes, that are shorted to the
mirrors, when deflected all the way down to the substrate.
Addressing lines and shorts are not shown in FIG. 3.
[0061] The whole switch may be fabricated in so-called
silicon-optical bench technology. The lenses, the switching arrays
and the in/out modules may be integrated, but commercially
available dielectric gratings are too bulky for this technology.
Microgratings may be developed, based on anisotropic etching of
silicon. Etching of the <100> surface of silicon through
rectangular etch masks that are aligned with the [111] directions
of the substrate, creates V-shaped grooves defined by the
<111> crystallographic planes of silicon. An array of such
V-grooves 56 constitutes a grating that can be used in the Littrow
configuration as shown in FIG. 5. The spacing between grooves can
be made arbitrarily small (e.g., 1 micron) by under-etching the
mask and subsequently removing this layer. The Littrow angle, which
is determined by the crystalline planes of silicon, is equal to
54.7 degrees. By taking advantage of the well-defined shape of a
unit cell of the grating, it is possible to obtain high diffraction
efficiency in higher order diffraction modes, the V-grooves
constituting a blazed grating operated on higher orders.
Higher-order operation means that the wavelength dependence of the
diffraction efficiency increases. It can be shown that with 8
wavelengths separated by 1.6 nm centered at 1.55 micron and the
grating geometry of FIG. 5, the diffraction order can be increased
to m=15 with less than 1 % variation in the diffraction efficiency
between wavelength channels. With m=15, .lambda.=1.55 micron,
.theta..sub.0=54.7 degrees and .phi.=30 degrees, the grating period
is calculated to be .LAMBDA.=12.3 micron. V-grooves with this
spacing can be easily patterned and etched by using standard
micromachining technology. V-groove gratings fabricated in this way
can be metallized and used as gratings directly, or be used as a
mold for polysilicon microgratings that can be rotated out of the
plane by using micro-hinges. A potentially very important
additional advantage of higher-order gratings, as described here,
is the reduced polarization sensitivity as compared to first-order
gratings. Since the periodicity is much larger than the wavelength,
the diffraction will be close to being independent of polarization.
Still, two wave plates may be used to compensate for residual
polarization sensitivity due to oblique angle reflections.
[0062] Referring now to FIG. 6 and FIG. 7, the fiber-optic switch,
being symmetric about it's center, can be implemented with a
symmetry mirror 58 in the symmetry plane 60. This essentially cuts
the component count in half. The output channels may either be on
the input fibers and separable by optical rotators (not shown) or
on a separate output fiber array (not shown) that is placed above
the input array. In the latter case, the micromirror array 62 and
the symmetry mirror 58 are slightly tilted about an axis, such that
the light is directed to the output fiber array.
[0063] It will be appreciated that the fiber-optic switch of the
present invention can serve network functions other than a
traditional N.times.N.times.M (where M is the number of wavelengths
in a WDM system).
[0064] It will further be appreciated that the fiber-optic switch
of the present invention can be used in connection with diagnostic
tools for WDM networks. WDM network management systems and software
require knowledge of the state of the entire network. Knowledge of
the state of the many distributed optical channels is especially
important. The manager requires confirmation of whether or not a
channel is active, it power and how much a channel optical
frequency has drifted as well as the noise power level.
Furthermore, this knowledge permits management software to
identify, isolate, and potentially rectify or route around physical
layer faults.
[0065] Clearly network management requires numerous channel
spectrometers that may be deployed in large number at affordable
prices. Traditionally, such spectrometers are fabricated from
gratings and lenses while an electronically scanned diode array or
a mechanically scanned grating provide spectral information.
Unfortunately, diode array technology at 1.55 microns, and 1.3
microns, the preferred communications wavelengths, is immature.
Such arrays are much more costly and unreliable than those
available for the visible spectral region. Moving gratings are
bulky and commercial system based on this approach are too costly
for wide spread use in production networks.
[0066] Accordingly, a variation of the network switch described
above can be employed to provide the desired functionality.
Referring to FIG. 8, an in-line WDM spectrometer 64 in accordance
with the present invention is shown. In the embodiment shown in
FIG. 8, WDM optical signals 66 emanating from an input fiber 68
would be collimated and diffracted from the grating 70 forming a
high resolution spatially dispersed spectrum at the lens 72 focal
plane. A single MEMS switch array 74 would be placed at the lens
focal plane, thus permitting the deflection of individual optical
channels or portions of a channel by one or more mirrors in the
array. A quarter-wave plate 76 is inserted symmetrically around the
switching array 74 to compensate for the polarization sensitivity
of the gratings 70. A single infrared diode detector 78 and
focusing lens (not shown) would be placed in the return path of the
reflected, and suitably displaced, return beam after a second
grating diffraction from grating 70. A full spectrum can then
obtained by scanning a deflection across the mirror array. Hence, a
synchronized readout of the single photo diode yields the full
spectrum to the management software.
[0067] The input fiber 68 and output fiber 80 can be arranged in a
variety of ways. For example, they can be arranged side by side in
the plane as shown in FIG. 8. An alternative would be to place the
output fiber over or under the input fiber. A further alternative
would be to use the same fiber for the input and output paths, and
separate the signals using an optical circulator or the like.
[0068] The micromirror array 74 is preferably a one-dimensional
array with one mirror per wavelength. Each mirror would thus
operate in one of two states; the mirror either sends its
corresponding wavelength to the output fiber 78 or is tilted
(actuated) so that its corresponding wavelength is sent to the
detector 76. In normal operation, only one of the mirrors is set up
to deflect its wavelength to the detector.
[0069] Note also that, if the output fiber if removed and replaced
by a beam sink (not shown), the spectrometer will still function
although such an embodiment could not be used in an in-line
application.
[0070] The invention has been described above by way of only a
limited number of examples, but the invention is not intended to be
limited by these examples. Many modifications and variations, as
well as design changes to significantly reduce the overall size,
are possible within the scope of this invention. For example, the
beam radius in the switching matrix may be reduced. The
no-obstruction criterion allows a change to .omega..sub.0=5 micron
for a 4-fiber switch, and this allows the focal length of both
lenses to be reduced by a factor of 5. As another example, the
micromachining technology may be improved to place the mirrors
closer. The posts (as shown in FIG. 3) and addressing lines may be
moved under the mirrors to reduce the beam radium on the grating by
a factor of 1.2. Together with the increased diffraction angle of a
micromachine grating (say, to 54.7 degrees from 45 degrees), the
focal lengths of the lenses can be reduced by a factor of 1.7. As a
third example, the fiber modes may be brought closer together on
the in/out modules. If the mode spacing is reduced from 200 micron
to 22.2 micron, the first and second lenses may have the same focal
length (such that magnification =1). In addition, advanced designs
may reduce the number of required components. The switch may be
designed so as to be foldable about its symmetry point such that
the same lenses and the same grating will be used both on the input
and output sides. In summary, it is to be understood that all such
modifications and variations that may be apparent to an ordinary
person skilled in the art are intended to be within the scope of
this invention.
[0071] Although the description above contains many specificities,
these should not be construed as limiting the scope of the
invention but as merely providing illustrations of some of the
presently preferred embodiments of this invention. Thus the scope
of this invention should be determined by the appended claims and
their legal equivalents.
1 TABLE 1 Components Parameter Values Input/output fibers Fiber
channels: 3 Standard single mode fiber Input-output MONET standard:
wavelengths Center wavelength: 1.55 micron Wavelength channels: 8
Wavelength separation: 1.6 nm Lenslets Ball lens,
200-micron-diameter n < 1.6 First lens Bulk lens f.sub.1 = 125.5
mm Grating Period: 0.95 micron Diffraction angle: 45 degrees Size:
>6 mm Second Lens Bulk lens f.sub.2 = 78.5 mm Switching matrix N
= 3 Mirror spacing: Fiber dimension: 110 micron Wavelength
dimension: 130 micron Array spacing: 2.5 mm Thickness of
sacrificial layer: 2.75 micron
* * * * *