Miniature piezoelectric translators for optical applications

A piezoelectric translator has a flat ribbon geometry and a large translation length perpendicular to the surface of the bridge. The translator includes a platform supporting the optic or micro-optic element and a slender piezo actuation system for displacing the platform. A position sensing system provides feedback to the actuation system regarding displacement of the platform. The actuation system includes a plurality of bridge actuators wherein each actuator includes a flat piezoelectric ribbon and leaf spring cap bonded to the ribbon, in single- or double-sided geometry. A stacked bridge geometry is also provided and allows increased displacement for a given applied voltage. Two collinearly placed single span bridge actuators or two-span bridge actuators can be used to provide linear translations with micro-rotation control.

TECHNICAL FIELD
 The present invention relates generally to lightweight optics, including
 micro-optics and fiber optics translation. More particularly, the
 invention relates to a piezoelectric translator having a bridge geometry
 and a relatively large translation length.
 BACKGROUND OF THE INVENTION

Background Art
 Modern day aircraft and spacecraft, and particularly modern day military
 platforms, typically make use of a large number of optical components. Due
 to the physical characteristics of light, most electro-optical
 applications require alignment between one or more optical components. For
 example, the most basic function of launching light into an optical fiber
 requires alignment of the fiber with the light source. Typically, the
 light source will be a laser and the application will require high
 accuracy beam pointing. Such a configuration is commonly used in
 applications such as optical scanners, laser designators or projector
 systems. Another type of application involves the positioning of microlens
 arrays, which typically need to be moved for tens to hundreds of microns
 in one or two dimensions. Movement in such small proportions is termed
 "translation" in the optics industry and is the subject of much attention.
 Other applications include laser communications, laser radar and optical
 steering for unmanned airborne vehicles.
 Conventional approaches to translating optical elements have employed piezo
 rod actuators to provide the required displacement. Rod actuators,
 however, are heavy, large, non-planar and bulky. These limitations have
 hindered the development of optical systems for airborne and space
 applications as well as other electro-optical systems requiring
 compactness and a large translation length. Translation length is defined
 by the amount of displacement achievable for a given voltage and rod
 length. Thus, rod actuators require a relatively large rod length for very
 small translation lengths. Typically, translation lengths are on the order
 of one micron per millimeter of rod length for applied fields of about 20
 kV/cm.
 Translation length is also affected by the weight of the optical element
 being displaced. For example, a typical lens array can weigh as little as
 80 grams. This relatively small weight does not require the force
 generated by rod actuators. It is therefore desirable to enable relatively
 large displacements of lightweight optical elements at higher speeds and
 in very compact actuation systems.
 It would further be desirable to provide a method and device for providing
 relatively large displacement of a micro-optic element using a small
 voltage. It will be appreciated that power consumption is often just as
 critical to optical applications as size. Accordingly, a large translation
 length is needed.
 It would also be highly desirable to provide a method and system having
 inherent mechanical amplification. Such a system would allow increased
 compactness and lower voltages. It would also be desirable to provide high
 accuracy feedback to increase the speed of actuation. Conventional rod
 actuators are sluggish due to their size, weight and feedback problems.
 It will be understood that displacement on the order of tens of microns is
 significantly affected by environmental effects. For example, slight
 fluctuations in temperature can result in changes in material properties
 which cause a substantial amount of system noise. It is therefore highly
 desirable to provide a micro-optic translation system with temperature
 compensated designs and/or compact and effective environmental isolation.
 SUMMARY OF THE INVENTION
 The above and other objects are provided by a preferred piezoelectric
 translator and method for translating lightweight optical elements such as
 micro-optics, mini-optics, or fiber optics. The piezoelectric translator
 includes a platform supporting the optical element and a piezo bridge
 actuation system for displacing the platform. A position sensing system
 provides feedback to the actuation system regarding displacement of the
 platform. The actuation system includes a plurality of bridge actuators
 wherein each actuator includes a flat piezoelectric ribbon and a metal
 bridge, similar to a leaf spring, bonded to the ribbon, such that each
 actuator forms a piezoelectric bridge having a slender geometry. Single-,
 and double-span piezoelectric bridges can be constructed for effecting
 pure translation, pure rotation, or mixed translation and rotation.
 Moreover, double-sided piezoelectric bridges can be constructed which can
 substantially increase translation at a given applied voltage. Similarly,
 stacks of single and/or double-sided bridge actuators can be constructed
 to further increase the translation at relatively low voltages.
 The method for translating a lightweight optical element includes the steps
 of supporting the optical element with a platform and displacing the
 platform with a piezo bridge actuation system. Feedback is provided to the
 actuation system based on displacement of the platform. Furthermore, a
 leaf spring cap is bonded to a flat piezoelectric ribbon and the
 combination forms a thin strip, nearly one-dimensional actuator in the
 form of a piezoelectric bridge. The actuator is disposed along a perimeter
 of the platform and actuated to provide positive linear displacement of
 the optical element along a first axis. A second bridge actuator may be
 used to provide push-pull actuation and a greater control of translation
 of the micro-optic element. An alternative method to push-pull actuation
 is to replace one of the bridge actuators with a spring mechanism. Similar
 actuators on adjacent sides will allow full two-dimensional actuation.
 Placement of bridge actuators between the frames that hold micro-optic
 arrays can allow three-dimensional actuation and add optical focus
 control. A specific arrangement of using two independent bridge actuators
 or a double-span bridge actuator on each side of the optical or
 micro-optical element will add the capability to control micro-rotations,
 in addition to pure translation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring to FIG. 1, translation of an optical lens array element 20 is
 shown as the optical element 20 relates to a coupling element 21. While
 the elements are shown as a lens array and an optical fiber, the present
 invention can be applied to LED's, photodetectors, lasers, microlens
 arrays, optical fiber or fiber optic bundles, and other lightweight
 optical components requiring translation. It will be appreciated that as
 light emanates from optical element 20 to coupling element 21, the need to
 translate one of the elements may arise. It will also be appreciated that
 light may similarly emanate from the coupling element 21 to the optical
 element 20. It is important to note that while additional alignment and
 coupling components are typically employed, translation on the order of
 tens to hundreds of microns may still be necessary. Thus, the present
 invention provides actuation of optical element 20 along any desired axis.
 The present invention also provides an actuation device with a nearly
 planar geometry, thereby significantly reducing size and costs associated
 with optical steering and other translation applications.
 Turning now to FIG. 2, a piezoelectric translator 30 for optical element 20
 is shown in accordance with a preferred embodiment of the present
 invention. Generally, piezoelectric translator 30 includes a platform 31
 supporting the optical element 20, a planar piezo actuation system for
 displacing the platform 31, and a position sensing system 40 providing
 feedback to the actuation system regarding displacement of the platform
 31. It will be appreciated that the actuation system includes a plurality
 of bridge actuators 10 and 11 and is capable of translating the optical
 element 20 in one dimension. It will be further appreciated that
 disposition of bridge actuators 10 and 11 on opposing sides of optical
 element 20 allows push-pull actuation. The preferred embodiment therefore
 provides greater control through the use of a plurality of actuators.
 Specifically, for downward movement of optical element 20, the first bridge
 actuator 10 provides positive linear displacement along a first axis 100.
 Similarly, the second bridge actuator 11 provides negative linear
 displacement along the first axis 100. The reverse is true for upward
 movement of optical element 20. Actuation is effected by a change in
 height of each actuator. As will be discussed in greater detail below,
 bridge actuators 10 and 11 are designed to maximize height change with
 respect to applied voltage and thereby achieve a large translation length.
 A plurality of low friction spacers 12 are disposed along the perimeter of
 the platform 31 to serve as a means for aligning displacement of the
 optical element 20 with the first axis 100. Essentially, spacers 12
 prevent twisting of the optical element 20 during translation. A
 temperature controlled housing 14 encloses the optical element 20, the
 platform 31 and the actuation system. The housing 14 has front and back
 apertures for allowing light to interact with the optical element 20. The
 position sensing system 40 preferably has a capacitative sensing mechanism
 for sensing coarse and/or fine displacement of the platform 31.
 Alternatively, interferometric sensing mechanism allows sensing of fine
 displacement. Thus, a nearly planar geometry is provided while at the same
 time allowing lightweight and relatively fast actuation with a large
 translation length.
 The bridge actuators 10 and 11 will now be described in greater detail.
 Turning to FIGS. 4a and 4b, the increased displacement capability and the
 bridge geometry provided by the present invention can be better
 appreciated. A bridge actuator approach is proposed for construction of a
 miniature, nearly one-dimensional translator capable of carrying a
 relatively lightweight optical component on the order of one or a few
 square inches in size. The bridge actuators are able to dither many tens
 to hundreds of microns, each in one dimension, orthogonal to the bridge
 long axis. A multiplicity of such actuators can produce precise one, two
 or three-dimensional translations for the optical elements. Bridge
 actuators are proposed here to provide inherent mechanical amplification.
 A single such actuator 10 can apply forces to micro-optic elements which
 can be spring loaded on the far side of the micro-optic element to provide
 a restoring force. For example, a two-dimensional actuation system using
 spring mechanisms 60 and 61 is shown in FIG. 9. As discussed above, in
 order to provide more control, the preferred embodiment proposes a
 push-pull configuration using two such bridge actuators to apply the
 electro-mechanical forces to the platform.
 Returning now to FIG. 4a, it can be seen that a typical piezo bridge
 actuator 10 is formed from a piezoelectric (PLZT) ribbon 51 bonded to a
 leaf spring 52 made from a metal such as brass. The leaf spring 52 has a
 large elevation at the center and is nearly flat at the ends. This "sag"
 can best be seen in the side view of bridge actuator 10 as height "h". The
 preferred approach would be to pre-form the PLZT ribbon 51 and metal stamp
 the leaf spring 52 to the desired width, with the bonding to be performed
 later. To further increase translation length .DELTA.h, the PLZT ribbon 51
 is electrically polled in the Z-direction and has a very large d.sub.31
 coefficient. A typical calculation for determining the translation length
 is therefore:
 ##EQU1##
 The result of applying an electric field to PLZT ribbon 51 is therefore a
 tangential force that causes the leaf spring 52 to push the platform in
 the desired direction. Each such translator 30 (FIG. 2) of the preferred
 embodiment is therefore very compact, is nearly one-dimensional, and has a
 large translation length due to its inherent mechanical amplification. Up
 to 60.times. has been demonstrated for a single sided bridge and
 120.times. for double-sided bridges. The translator 30 of the present
 invention has numerous applications and can be used in micro-optical
 scanners, optical communications, fiber optic systems or other optical
 systems requiring high accuracy beam pointing via light-weight
 translators. The bridge actuator design determines translation as a
 function of applied voltage. For example, stacked double-sided bridges 55,
 as shown in FIG. 5c, produce substantially higher translation distances at
 relatively lower voltages, when the individual piezoelectric bridges are
 biased in parallel. Similarly, double-sided bridges 54 as shown in FIG. 5b
 increase translation at a given applied voltage as compared with a
 single-sided bridge.
 Returning to FIG. 2, it will be appreciated that additional actuators can
 be disposed adjacent to the first and second actuators 10 and 11 to serve
 the same purpose as spacers 12. Thus, instead of a single actuator
 physically contacting the optical element 20 on each side, two contact
 points will be provided on each side. FIG. 5a provides an example of such
 a dual function integrated into a single double-span bridge actuator 53.
 It will be appreciated that such a configuration would also allow
 heightened control over twist should such movement be desired.
 It will further be appreciated that the preferred translator 30 can be
 readily modified to provide two-dimensional displacement by disposing the
 second bridge actuator along surfaces 32 or 33 of platform 31. Surfaces 32
 and 33 would therefore be perpendicular to a second axis 200. Negative
 linear displacement along the first and second axes 100 and 200 can
 generally be provided by spring mechanisms discussed above.
 Another embodiment of the present invention additionally provides actuation
 in the Z-direction for two micro-optic arrays and is shown in FIGS. 3a-3c
 as third axis 300. Thus, it will be appreciated that the present invention
 can be readily adapted to provide three-dimensional actuation along first
 axis 100', second axis 200' and third axis 300. Platform 31' supports the
 micro-optic element which is stabilized by spring mechanism 34. Actuation
 along third axis 300 is provided by bridge actuator 13, whereas lateral
 actuation is provided by bridge actuators 15, 16, 17 and 18. First and
 second apertures 70 and 80 allow light to interact with the optical
 element 20.
 Turning now to FIGS. 6-8, displacement results will be discussed in greater
 detail. Specifically, FIG. 6 is a plot of actuator displacement versus
 electric field for various actuator heights (h). It will be appreciated
 that smaller actuator heights have resulted,in an increase in
 displacement. FIG. 7 is a plot of various bridge leaf spring thicknesses.
 In accordance with the present invention, lower bridge thicknesses yield
 higher displacements. Finally, FIG. 8 shows the substantial difference in
 performance between a single-sided bridge actuator as shown in FIG. 4a,
 and a double-sided bridge actuator as shown in FIG. 5b, and demonstrates
 displacements as high as 250 microns.
 Those skilled in the art can now appreciate from the foregoing description
 that the broad teachings of the present invention can be implemented in a
 variety of forms. Therefore, while this invention has been described in
 connection with particular examples thereof, the true scope of the
 invention should not be so limited since other modifications will become
 apparent to the skilled practitioner upon a study of the drawings,
 specification and the following claims.