Microdynamical fiber-optic switch and method of switching using same

A microdynamical optical switch includes a piezoelectric actuator disposed on a substrate, a mirror securely mechanically coupled to the actuator, an input connection port, and a plurality of output connection ports. The actuator displaces the mirror along a mirror displacement path such that the mirror, which is oriented at a 45.degree. angle to the path of the incident light, deflects light passing from the input connection port into an output connection port. The actuator includes a plurality of piezoelectric bars mechanically coupled together in series in a meander line geometry such that the cumulative deflection of the piezoelectric bars is used to displace the mirror. The amount of displacement of the actuator is governed by a controllable voltage source, which applies a voltage across each of the piezoelectric bars. In one embodiment the microdynamical switch has a 1.times.n arrangement, in which light entering through one input connection port is selectively directed to one of n output connection ports. In an alternative embodiment, the switch has a 2.times.2 arrangement in which light beams entering through two ports are individually selectively directed to one of two respective outlet ports.

Field of the Invention 
This invention relates generally to optical processing systems and more 
particularly to optical switches employed in fiber optic networks. 
BACKGROUND OF THE INVENTION 
Optical switches are devices used to direct (or steer) optical beams in a 
desired direction. Optical switches typically have input ports to receive 
optical signals from one or more channels and two or more output ports 
into which the optical signal can be directed. Such switches are commonly 
referred to be the number of input ports and the number of output ports; 
for example a switch having one input port and two output ports is 
referred to as a 1.times.2 switch; a switch with "n" output ports would be 
designated as a 1.times.n switch. Similarly, if the switch has 2 input 
ports and 2 output ports, it is referred to as a 2.times.2 switch. Optical 
switching occurs when an input beam is directed from its input port to a 
selected output port. 
Optical signals are commonly carried in optical fibers, which provide a 
compact and efficient light channel through which the optical signals can 
pass. Efficient switching of optical signals between respective fibers is 
necessary in most optical processing systems or networks to achieve the 
desired routing of the signals. Desirable performance characteristics for 
fiber optic switches (i.e., optical switches directing signals between 
respective optical fibers) include having low loss (e.g., &lt;1 db), low 
cross-talk (i.e., high channel isolation, e.g. &gt;50 db), relatively fast 
switching speeds (e.g., tens of microseconds for a 1.times.2 or 2.times.2 
switch), polarization-independent operation, and rugged construction, so 
that the switch can be operated in high temperature and relatively dirty 
environments. Such performance parameters would enable fiber optic 
switches to be used in a number of applications, such as phased array 
radars, communication systems, and engine sensor networks (e.g., in 
aircraft engines). 
Conventional fiber optic switches do not provide these desired operating 
characteristics. Most conventional fiber optic switches are either lithium 
niobate based switches or liquid crystal switches. The operation of 
lithium niobate based switches is polarization sensitive, and thus these 
switches necessarily require the use of polarization-preserving optical 
fibers, and also require careful input/output waveguide mode matching in 
the optical system or network. Lithium niobate based switches are 
relatively lossy (about 5 db optical insertion loss per switch) and 
provide only a moderate degree of channel isolation (e.g., 20 db). These 
characteristics, coupled with the fact that such switches require 
intricate fabricating processes to form the optical waveguides in the 
switch and couple those waveguides to optical fibers, make the use of such 
switches in large optical signal processing systems, such as phased array 
antennas that require thousands of switches arranged in cascade 
configurations, impractical. Liquid crystal switches offer relatively high 
on/off ratios (e.g., 36 db) and relatively low (e.g., less than 1 db) 
optical insertion losses, but typically require processing systems using 
polarized light. Additionally, liquid crystal switches cannot be operated 
in high temperature environments (e.g., &gt;450.degree. C.) or dirty 
environments, such as encountered in turbine engines. 
It is accordingly an object of this invention to provide a rugged optical 
switch capable of efficient operation at high temperatures. 
It is a further object of this invention to provide an optical switch 
having a polarization independent and wavelength independent switching 
mechanism. 
Another object of this invention is to provide an optical switch having 
high channel isolation, fast switching speeds, and low optical insertion 
loss. 
It is yet a further object of this invention to provide a readily 
fabricated switch that is compact and robust. 
SUMMARY OF THE INVENTION 
In accordance with this invention, a microdynamical optical switch includes 
a silicon substrate on which the switch architecture is disposed. A 
piezoelectric actuator is disposed on the substrate and is mechanically 
coupled to a mirror such that the mirror is displaced along a mirror 
displacement path in correspondence to deflection of the piezoelectric 
actuator. The switch has at least one optical input connection port and a 
plurality of optical output connection ports, the input connection port 
and the output connection ports being disposed in a spaced relationship so 
that light passing from the input connection port is directed to a 
selected output connection port in dependence on the position of the 
mirror along the mirror displacement path. 
The piezoelectric actuator is advantageously a meander line microactuator 
including a plurality of piezoelectric bars mechanically coupled together 
in series and electrically coupled together in parallel. A controllable 
voltage source is electrically coupled to the microactuator such that the 
deflection of the microactuator corresponds to the amplitude of the 
voltage applied. The mirror is securely coupled to the microactuator by an 
actuating arm such that the deflection of the microactuator causes the 
mirror to be displaced along the mirror displacement path. The mirror has 
one or more optically reflective surfaces, each of which is oriented at 
substantially a 45.degree. angle to the path of the light passing from the 
optical input port such that light passing from the optical input port 
strikes the mirror and is reflected to pass along a path that is 
substantially at right angles to the path of the light incident on the 
mirror. 
In an optical switch in which there is only one optical input port, the 
path along which the mirror is displaced is substantially aligned with the 
path of the incident light emanating from the optical input port. The 
optical output ports are disposed at 90.degree. angles to the mirror 
displacement path such that the mirror can deflect light emanating from 
the input port by 90.degree. into an output port. The particular output 
port that receives the light is determined by the location of the mirror 
along its displacement path. In a switch having two input ports and two 
output ports, the ports are arranged in a cross-pattern such that when the 
mirror is displaced outside the path of the light, the light passes from 
either input port directly into a respective output port aligned on the 
same axis; when the mirror is shifted along its displacement path so that 
the light emanating from the input ports strikes the mirror, the light 
from each respective input port is deflected by 90.degree. and enters the 
other output port, i.e., a different output port from the one into which 
the light entered when the mirror was displaced outside the path of light 
emanating from the input port.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a 1.times.n microdynamical fiber-optic switch 100 
disposed on a substrate 102. In accordance with this invention, switch 100 
comprises an input optical connection port 105 and a plurality of output 
optical connection ports 110, a piezoelectric actuator 120, and a mirror 
assembly 150 that is securely mechanically coupled to actuator 120. 
Substrate 102 is typically a silicon wafer or similar substrate material on 
which the various materials necessary to form the piezoelectric actuator 
can be readily deposited and patterned. Such a silicon wafer is also 
readily micromachined to produce grooves into which the various optical 
fibers carrying signals to and from the switch can be securely fastened. 
Optical input connection port 105 advantageously comprises a single mode 
optical fiber which is fastened to substrate 102 and switch 100 so that 
all light emanating from the connection port propagates along 
substantially the same axis. Each optical output port 110.sub.1, 
110.sub.2, . . . 110.sub.n, similarly also comprises a single mode optical 
fiber fastened to substrate 102 and switch 100. The optical fibers are 
advantageously positioned in silicon V-grooves etched into substrate 102. 
Each optical output port is aligned along an axis that is oriented at 
substantially right angles with respect to the axis of input port 
connection 105. 
Piezoelectric actuator 120 is coupled to a voltage source 130 that can be 
controlled to vary the amplitude of the voltage applied to the 
piezoelectric actuator. Voltage source 130 typically supplies direct 
current (dc); alternatively a time-varying or alternating voltage source 
can be used. Actuator 120 is preferably a meander line microactuator 
comprising at least one, and typically a plurality of, piezoelectric bars 
(also known as unimorphs) 124 mechanically coupled together in series in a 
meander line geometry (or configuration). Both ends of the meander line 
geometry of piezoelectric bars are securely attached to anchors 125a, 125b 
respectively, which anchors provide a foundation that remains stationary 
with respect to the substrate when the piezoelectric bars expand or 
contract in response to applied voltages. The piezoelectric bars comprise 
a thin film of piezoelectric material such as lead zirconate titanate 
(Pb(Zr.sub.1 Ti)O.sub.3, aluminum nitride (AIN), zinc oxide (ZnO), lead 
titanate (PbTiO.sub.3), or the like, and are disposed on a polycrystalline 
silicon mechanical support structure 126, which also serves as a ground 
electrode. The maximum temperature at which the piezoelectric actuator can 
operate is a function of the material which comprises the unimorphs; for 
example aluminum nitride can operate in temperatures up to about 
700.degree. C., zinc oxide up to about 500.degree. C., lead zirconate 
titanate up to about 380.degree.-400.degree. C., and lead titanate up to 
about 340.degree. C. The piezoelectric bars are micromachined to produce 
the desired dimensions and linkages between bars. A typical piezoelectric 
bar has a thickness of about 2 .mu.m, a width of about 40 .mu.m, and a 
length of about 500 .mu.m. Electrodes 127, 128 are disposed along the 
opposing lengthwise faces of each piezoelectric bar, and are connected to 
voltage source 130 so as to be oppositely polarized. Electrodes 127, 128 
comprise a conductive material such as aluminum. 
Application of voltage across piezoelectric material results in stresses or 
strains in the piezoelectric material that cause the bar to expand or 
contract. The piezoelectric polarity between bars is alternated to cause 
linear expansion in one bar and linear contraction in an adjacent bar. 
When the geometry of piezoelectric bars 124 is mechanically attached to 
substrate 102 by anchors 125 at selected points, these stresses or strains 
can be used to cause individual piezoelectric bars to be deflected in 
predetermined directions. The geometry of the plurality of the bars and 
the mechanical connections between them is used to generate an actuator 
displacement that is the sum of the individual displacement of each bar. 
For example, piezoelectric bars can be connected in geometrics which 
result in either a horizontal linear displacement with respect to the 
substrate (a folded path geometry) or a vertical linear displacement with 
respect to the substrate (a ladder-like geometry). In either case, the 
piezoelectric bars at each end of the meander line geometry are typically 
anchored so that they do not move with respect to the substrate, and the 
intermediate bars are mechanically connected in series in a fashion to 
generate the desired collective displacement in response to applied 
voltages. Piezoelectric thin film actuators of the type advantageously 
used in this invention are discussed in the article entitled 
"High-Displacement Piezoelectric Actuation Utilizing a Meander Line 
Geometry-Part I-Experimental Characteristics" by W.P. Robbins, D.L. Polla, 
et al., IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency 
Control, Vol. 38, September 1991, which is incorporated herein by 
reference. 
Mirror assembly 150 comprises an actuator arm 152 which is affixed to 
piezoelectric actuator 120 and to a mirror 154. Mirror 154 is rigidly 
attached to actuator arm 152 such that the orientation of the mirror with 
respect to the actuator arm is fixed, and actuator arm 152 is attached to 
actuator 120 such that substantially the only motion imparted to mirror 
154 when actuator 120 is operated is linear displacement along a mirror 
displacement path 156. Mirror 154 has an optically reflective surface, 
comprising, for example, gold electroplated to an underlying foundation 
having a smooth surface. The reflective surface of the mirror is oriented 
at substantially a 45.degree. angle to the path of the light passing from 
input connection port 105 such that light incident on the mirror is 
reflected onto a path substantially 90.degree. from the path of the light 
emanating from the input connection port. The path of the reflected light 
causes the light to enter one of the output connection ports 110. Actuator 
arm 152 is typically linked to the piezoelectric bar substantially in the 
center of the meander line geometry in order to provide the greatest range 
of displacement of mirror assembly 150. 
In operation, the application of a voltage to piezoelectric actuator 120 
results in deflection of piezoelectric bars 124, in turn causing actuator 
arm 152 is translated in a substantially linear motion so that mirror 154 
to be displaced along displacement path 156. Actuator 120 is disposed so 
that displacement path 156 is substantially aligned with the axis of the 
input connection port, i.e., the mirror displacement path is substantially 
colinear and coincident with the center of the beam of light emerging from 
input connection port 105. Thus, as mirror 154 is displaced along mirror 
displacement path 156 by actuator 120 by a distance corresponding to the 
voltage applied to the actuator, light passing from input connection port 
105 will be deflected (i.e., reflected) into a selected one of the output 
connection ports 110 dependent upon where along the mirror displacement 
path the mirror is positioned. Thus, as illustrated in FIG. 1, when the 
piezoelectric actuator is not deflected, mirror 154 is disposed closest to 
actuator 120 and light entering through input connection port 105 is 
deflected into output connection port 110.sub.1. When actuator 120 is 
fully deflected, the mirror is disposed farthest away from actuator 120 
(in the position shown in phantom as 154' in FIG. 1), and the light 
entering through input connection port 105 is deflected into output 
connection port 110.sub.n. Similarly, by controlling the voltage applied 
to piezoelectric actuator 120 to achieve the desired deflection, mirror 
154 can be selectively positioned along mirror displacement path 156 at 
locations to reflect light from input port 105 into a selected one of the 
output connection ports 110. 
In a typical 1.times.2 microdynamical optical switch in accordance with 
this invention, the optical input and output connection port comprise 
single mode optical fibers. Such fibers commonly have relatively small 
core sizes (about 3 .mu.m) and the mirror need be moved only a small 
predetermined distance, e.g., 10 .mu.m, in order to accomplish the 
switching operation. Such a 10 .mu.m displacement is readily obtained by 
coupling actuator arm 152 directly to piezoelectric actuator 120. This 
coupling arrangement results in relatively fast switching times, allowing 
the switch to change positions within several microseconds. 
As long as the distance that the light must traverse between input 
connection port 105 and one of the output connection ports 110 is 
relatively short, e.g., less than about 50 .mu.m, there is little optical 
coupling loss due to free-space propagation beam spreading. If the 
arrangement of the switch requires longer distances between input ports 
and output ports, optical collimating devices (not shown) such as GRIN 
(graded index) lenses or SELFOC (self focussing) lenses are coupled to the 
input and output fibers to minimize free-space propagation beam spreading. 
GRIN lenses, for example, typically have a diameter in the range of about 
1 mm to 2 mm, which necessitates that the input and output connection 
ports be placed at least that distance apart. Such a spacing between 
fibers further necessitates larger displacements of the mirror along the 
mirror displacement path (e.g., several millimeters) to operate the 
switch, which in turn requires the use of levers or the like (not shown) 
to allow the microactuator to produce sufficient displacement of the 
mirror. 
FIG. 2 illustrates a 2.times.2 optical switch 200 which is readily 
fabricated using two 1.times.2 microdynamical switches. Two by two optical 
switch 200 comprises first and second 1.times.2 microdynamical fiber optic 
switches 210, 220 and first and second passive fiber-optic couplers 230, 
240. A first switch 210 comprises a first optical input connection port 
204, and a second switch 220 comprises a second optical input connection 
port 206. First and second 1.times.2 optical switches preferably comprise 
microdynamical optical switches having piezoelectric actuators as 
described above with respect to FIG. 1. The two output ports of first 
optical switch 210 are respectively optically coupled to a first channel 
optical fiber 212 and a second channel optical fiber 214. Similarly, the 
two output ports of second optical switch 220 are respectively optically 
coupled to a first channel optical fiber 222 and a second channel optical 
fiber 224. 
First passive fiber-optic coupler 230 is coupler to first switch first 
channel optical fiber 212 and to second switch first channel optical fiber 
222. Second passive fiber-optic coupler 240 is optically coupled to first 
switch second channel optical fiber 214 and to second switch second 
channel optical fiber 224. First passive fiber-optic coupler 230 is 
arranged such that optical signals received from optical fibers 212, 222 
are combined to form one signal. A 2.times.2 switch first channel output 
connection 250 is optically coupled to passive coupler 230 to receive the 
combined optical signal therefrom. Similarly, second passive fiber-optic 
coupler 240 is arranged such that optical signals received from optical 
fibers 214, 224 are combined to form a single signal. A 2.times.2 switch 
second channel output connection 260 is optically coupled to passive 
coupler 240 to receive the combined optical signal therefrom. 
A 2.times.2 microdynamical fiber optic switch 300 representing a further 
embodiment of the present invention is illustrated in FIG. 3. In 
accordance with this invention, switch 300 disposed on a substrate 302 
comprises a piezoelectric actuator 320 and a voltage source 330 that can 
be controlled to vary the amplitude of voltage applied to the 
piezoelectric actuator. Voltage source 330 typically is a direct current 
voltage source; alternatively, a time-varying or alternating current 
voltage source can be used. An actuator arm 352 is securely mechanically 
coupled to piezoelectric actuator 320 and to a double sided mirror 354. 
The operation of the actuator is identical in all material respects to the 
operation of the actuator described above with respect to the device 
illustrated in FIG. 1. Application of control voltages to actuator 320 
causes linear displacement of actuator arm 352 and attached mirror 354 
along a mirror displacement path 356. 
Switch 300 further comprises a first input connection port 305, a second 
input connection port 307, a first output connection port 310, and a 
second output connection port 312. These input and output connection ports 
are arranged in a crossed geometry such that the axis of first input port 
connection 305 is substantially aligned with the axis of first output 
connection port 310, and further is substantially orthogonal to the axis 
of second output connection port 312. Similarly, the axis of second input 
connection port 307 is substantially aligned with the axis of second 
output connection port 312. The light paths between the respective input 
and output connection ports cross at a crossover point 318, which point is 
located along mirror displacement path 356. 
In operation, the mirror has two positions which correspond to the 
deflection of the actuator 320. In one position, mirror 354 is not 
interposed at the crossover point in the light paths, thus allowing input 
light signals to pass between the respective first and second input and 
output connection ports uninterrupted. When actuator 320 is deflected, 
mirror 354 is interposed in the light paths between the input and output 
connection ports. The plane of each reflective side of double sided mirror 
354 is positioned to be at substantially a 45.degree. angle to the path of 
the light beams emerging from each of the input connection ports. The 
light beams incident on each side of the mirror are deflected to a path 
substantially 90.degree. from the original path that the respective light 
beams were travelling such that the light beam is deflected into the 
output port connection other than the one into which it was directed when 
the mirror was not interposed in the light path. 
The distance the light beam travels between an input connection port and an 
output connection port (regardless of whether the beam is reflected by the 
mirror) is advantageously about 50 .mu.m or less. When the light beams 
travel a distance of 50 .mu.m or less between input and output ports, 
free-space propagation beam spreading is relatively small and allows 
efficient input/output coupling between respective input and output 
optical fibers, thus providing relatively high channel isolation with low 
insertion loss. For example, these desirable characteristics are obtained 
with an optical switch 300 having input and output connection ports 305, 
307, 310, and 312 comprising single mode optical fibers having a core 
diameter of about 3.2 .mu.m at about 514 nm, a numerical aperture of about 
0.11, and a mode field width of about 3.7. In such an arrangement, mirror 
354 advantageously has a diameter of about 12.5 .mu.m and is translated by 
actuator 320 along a mirror displacement path about 10 .mu.m in length. A 
mirror displacement path of about 10 .mu.m also provides relatively fast 
switching times (i.e., tens of microseconds) between the two positions of 
the switch. 
Alternatively, if the distance a light beam travels between an input and an 
output connection port is greater than about 50 .mu.m (such as may be 
necessary if the size or number of input connection ports in increased), 
collimating devices, such as GRIN or SELFOC lenses (not shown), are 
advantageously coupled to the input and output connection ports so that 
the light beam passing through free space is collimated and thus 
experiences less inter-fiber free space propagation beam spreading. As 
noted above, use of such collimating devices may necessitate larger 
displacements of the mirror along a the mirror displacement path (e.g., 
several millimeters) to operate the switch, which in turn requires the use 
of levers or the like (not shown) to allow the microactuator to produce 
sufficient displacement of the mirror. 
Actuator arm 352 and attached mirror 354 are illustrated in FIG. 3 in the 
position in which mirror 354 is disposed in the light path between the 
respective input and output connection ports. When the switch is operated 
to change from this switching position, actuator arm 352 is translated 
along the mirror displacement path substantially parallel to the plane 
formed by the input and output connection ports until mirror 354 is no 
longer disposed in the light path between the respective input and output 
connection ports. 
In all of the embodiments of the present invention discussed above, the 
direction of light propagating through the switch is reversible. In other 
words, light can enter the switch through input port connections and be 
directed to output connection ports, but there is no structural impediment 
in the invention that would prevent light signals passing from the 
designed output port connections to an input port connection, if such 
reversible operation was so desired. Thus the use of the words "input" and 
"output" in the description of the invention is for ease of discussion and 
does not limit the manner in which the invention is employed. 
While only certain features of the invention have been illustrated and 
described herein, many modifications and changes will occur to those 
skilled in the art. It is, therefore, to be understood that the appended 
claims are intended to cover all such modifications and changes as fall 
within the true spirit of the invention.