Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
2002-02-01
2004-04-27
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C359S321000, C310S311000
Reexamination Certificate
active
06728024
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention is derived from a striking novel discovery, that porous crystalline materials have piezoelectric and piezooptic properties. The present invention, therefore, relates to devices and methods which take advantage of this newly discovered phenomenon. One main use of this discovery is in the field of adaptive optics, e.g., adaptive reflectors such as mirrors, however, other uses, as is further delineated below, are anticipated. Thus, for example, the new technology can find uses in (i) attached fiber optics maneuvering (movement of fiber for focusing, etc.); (ii) attached fiber optic bending for mode control; (iii) attached fiber optics intensity and polarization control; (iv) spatial light modulation (electro optical and opto-optical modulation) by adjustment of specific elements or a whole device; (v) tunneling devices, where the tunneling current is sensitive to distance between elements; (vi) scanning microscopy heads, optical or magnetic disc readers which have to be maneuvered by electric or light signal; and (vii) light or voltage detectors.
Most of the following background discussion, however, focuses on the construction, fabrication, use, advantages and limitations of prior art adaptive mirrors, however, there is no intention to restrict the use of the newly discovered piezoelectric and piezooptic properties of porous crystalline materials to the field of adaptive optics, as many other uses and applications of this core technology are envisaged.
Adaptive optics systems are essentially a servo loop, with a sensitive wave front sensor, a control computer, and a flexible mirror to correct aberrations in a beam of light. Despite large efforts made in the last few decades, progress in deformable mirrors has been slow, and there are only a few kinds available. The high price of these mirrors is an indicator of the problems in their manufacture, such as complex construction, non-repeatability and non-uniformity.
What is required of an adaptive mirror? It has to be agile enough to correct even the strongest and densest wave front fluctuations (usually a few micrometers in stroke), while not contributing errors of its own. The more elements it has the better, ranging between few and thousands of actuators. It has to be quick enough to correct even the fastest variations, while not resonating close to the operational frequency. It has to run on low power to avoid a cumbersome power supply and control system, while not loosing in dynamic range. It has to be small and light enough to mount in a compact space. It has to be fail-safe or at least allow easy correction or replacement of bad elements. It has to have serial command lines to the elements to avoid massively parallel wiring. Mechanically, it needs to be of good optical quality and insensitive to temperature variations, even without active correction. Finally, it ought to have a sound price, which is derived mainly from its construction technology.
There are many other fields where actuation of devices is a part of their operation: communication devices, switching devices, scanning microscopes, printers, and many more depend on mechanical movement of smaller or larger parts as a response to an electronic or optical command. The discussion below will concentrate on adaptive optics (or the slower active optics) as a relevant example: such systems change the path of light beams, their direction or the wave front emanating from them, usually to correct aberrations.
Lets start with the simpler systems, those that can serve as basis for systems that are more complex. Lets define these systems as being able to correct only one mode at a time. The lowest modes would be those which can be defined by one parameter over the correction area. Zernike decomposition, which is common for optical round apertures, has basic modes as follows: (i) piston correction (given value of the wave front), this mode is important only when using a segmented mirror; (ii) tip and tilt (given value of the wave front derivatives in x and y directions; and (iii) defocus (given value of the wave front curvature).
Piston correction is achieved merely by moving the mirror surface up and down, while maintaining its direction. The size of the elements d is small compared to the lateral scale of the aberrations. Mirror movement in parallel to itself is achieved by piston actuators; these actuators are the basis of most deformable mirrors. Mentioned here are some commonly used devices. Comparative designs and analyses have appeared in J. A. Pearson, R. H. Freeman, and Harold C. Reynolds, Jr., ‘Adaptive optical techniques for wave-front correction’, in Applied Optics and Optical Engineering Vol. VII, R. R. Shannon and. C. Wyant, editors, Academic Press, 246-340, 1979; M. A. Ealey, ‘Actuators: design fundamentals, key performance specifications, and parametric trades’, SPIE Vol. 1543, 346-362, 1991; M. A. Ealey and J. A. Wellman, ‘Deformable mirrors: design fundamentals, key performance specifications, and parametric trades’, SPIE 1543, 36-51, 1991; E. N. Ribak, ‘Deformable mirrors’, in Adaptive Optics in Astronomy, NATO ASI Vol. 423, 149-62, 1994; R. K. Tyson, Principles of Adaptive Optics, Academic Press, 1998; R. E. Aldrich, ‘Deformable mirror wavefront correctors’, in Adaptive Optics Engineering Handbook, Marcel Dekker, 2000. The main types of deformable mirrors appear in
FIGS. 1
a-f
, each of which has its limitations.
The most common method of piston correction is by using piezoelectric actuators. These actuators are very convenient since they respond directly and quite linearly to an applied voltage. Their response (for lead zirconium titanate, the most common material) is in the order of 1 micrometer for 1 kV, which is too small to achieve the several microns required to correct for atmospheric turbulence. Only one mirror was used in this configuration: the monolithic piezoelectric mirror[J. S. Feinlieb, S. G. Lipson, and P. E. Cone, ‘Monolithic piezoelectric for wavefront correction’, Appl. Phys. Lett. Vol. 25, 311-315, 1974], where the electrode contacts were drilled through the actuator block almost to the face of the mirror. A number of schemes were devised for better voltage response. In one scheme use is made of the thickness-to-length ratio: instead of applying the voltage along the most responsive direction, it is applied across this direction. By making the piezoelectric material very long and very thin, the transverse response is amplified by this ratio. Since they are constructed from ceramic materials, these actuators can be manufactured in almost any desired shape. For this application they are prepared in a tubular shape, which is convenient for other applications. Another choice is to bond a stack of many thin slabs of the material, and apply low voltage on all of them in cascade. The slabs are combined with their polarizations directions alternating so that application of the voltage is in parallel.
Another piezoelectric material is lead magnesium niobate, with a better (but uni-directional) voltage response and lower hysteresis [G. H. Blackwood, P. A. Davis, and M. A. Ealey, ‘Characterization of MMN:BA electrostrictive plates and SELECT actuators at low temperatures’, SPIE Vol. 1543, 422-429, [Blackwood et al., 1991]. This kind of response is called electrostrictive. However, since it always extends, either under positive or negative voltage, a bias voltage must be applied to it for bidirectional movement.
Another option for actuators is the voice coil, such as used in commercial loud speakers. In this case, a magnetic field drives a coil attached to a piston. The drawback here is the heating created by the flowing current. The magnetostrictive actuator consists of a solenoid within which is the magnetostrictive ferrite whose length changes under magnetic field. Here again the actuator has to be rid of the heat in the solenoid. It is easier to use these actuators in a laser adaptive mirror, since both have to be cooled anyway.
The hydraulic actuator is ab
Choi William
Epps Georgia
G.E. Ehrlich (1995) Ltd.
Technion Research & Development Foundation Ltd.
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