Adaptive optics using the electrooptic effect

An adaptive electroopical lens system for use in optical data storage systems, optical phased arrays, laser or other optical projectors, and raster scanning devices, and the like. The invention provides an electrooptical means for scanning an optical beam or moving an optical storage or retrieval point. Beam movement is achieved electrooptically, by changing the index of refraction of an electrooptical material by controlling electric fields applied thereto. A plurality of electrodes are disposed on one surface of the electrooptic material and a ground electrode is disposed on the other. The electrodes are adapted to apply electric fields derived from a voltage source to the electroopic material that selectively change its index of refraction and provides for a predetermined index of refraction profile along at least one dimension thereof, thus forming a lens. By appropriately forming the electrode pattern and properly controlling the voltages applied thereto, differing lens shapes may be formed. Since the response times of the electrooptic materials employed in the present invention are on the order of nanoseconds (10.sup.-9 sec) or less, the intrinsic response frequency of the lens system is 10.sup.9 Hz or more. The present invention thus increases the data storage and retrieval capacity of optical systems in which it is employed.

BACKGROUND 
The present invention relates generally to adaptive optical systems, and 
more particularly, to an adaptive optical lens system that uses the 
electrooptic effect. 
At present, conventional optical data storage systems use mechanical or 
acoustical means for moving the optical storage or retrieval point. This 
is typically achieved by means of a rotating or oscillating mirror that 
moves a laser beam to read or write data on the storage medium. These 
approaches are limited to less than 10 KHz reading or writing speeds. 
Acoustic means for moving the light beam is limited by the speed of sound 
in solids, typically on the order of 5 km/second. 
Such devices as optical memories, laser projectors and raster scanning 
devices are all implemented using the above-mentioned mechanical or 
acoustical means. Consequently, there are inherent limitations due to the 
mechanical or acoustical devices that limit the speed of the systems in 
which they are used. Simple devices such as galvanometers, and the like, 
are implemented using rotating mirrors, whose response times are limited 
by the response time of the rotating mirror assembly. 
In some applications, such as optical data storage systems and optical 
phased arrays, and the like, it is important to move the focal point very 
rapidly. To increase the reliability of such a system, it is necessary 
that a nonmechanical or nonacoustical approach be provided. 
Accordingly, it would be an improvement in the art to have an 
electronically adjustable, adaptive, optical lens that does not require 
mechanical or acoustical means for controlling its focussing ability. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an adaptive optical lens system 
for use in optical data storage systems, optical phased arrays, laser or 
other optical projectors, and raster scanning devices, and the like, is 
provided that employs electrooptical means for focussing the system. In 
the case of optical storage devices, the present invention provides an 
electrooptical means for moving the optical storage or retrieval point. In 
the case of laser or other optical projectors, and raster scanning 
devices, and the like, the present invention provides an electrooptical 
means for focusing or scanning the image projected thereby. This is 
achieved electrooptically, by changing the index of refraction of a 
material that is achieved by controlling electric fields applied to the 
adaptive optical lens system. 
More particularly, the present invention comprises an adaptive optical lens 
system. The system includes an electrooptic material having first and 
second surfaces and having an index of refraction that is adjustable is 
response to an applied electric field. A first plurality of electrodes is 
disposed on the first surface of the electrooptic material and at least 
one ground electrode is disposed on the second surface of the electrooptic 
material. The electrodes are adapted to apply electric fields to the 
electrooptic material that are adapted to selectively change the index of 
refraction of the electrooptical material across at least one dimension 
thereof. This provides for a predetermined index of refraction profile 
along the at least one dimension and thus forms a lens. The change in 
index of refraction across the one dimension is adapted to change the 
angle at which an optical beam entering the electrooptic lens exits the 
lens. A voltage source is coupled to the first plurality of electrodes and 
the ground electrode for applying predetermined voltages to the plurality 
of electrodes to create the index of refraction profile. 
The present invention may be considered as a continuous optical phased 
array or an electrooptically adaptive optical system, depending upon its 
application. However, unlike a discrete optical phased array, it does not 
require computation speeds of 10.sup.15 Hz. Since the response times of 
the electrooptic materials employed in the present invention are on the 
order of nanoseconds (10.sup.-9 sec) or less, the intrinsic response 
frequency of the lens system is 10.sup.9 Hz or more. The present invention 
thus increases the data storage and retrieval capacity of optical systems 
in which it is employed.

DETAILED DESCRIPTION 
FIG. 1 shows a first embodiment of an electrooptical lens system 10 in 
accordance with the principles of the present invention. The 
electrooptical lens system 10 comprises an electrooptic material 11, such 
as lithium niobate (LiNbO.sub.3) or barium titanate (BaTiO.sub.3), for 
example. A transparent ground electrode 12 is disposed on one surface of 
the electrooptic material 11. The transparent ground electrode 12 may be 
made of indium tin oxide, for example. A plurality of spaced apart, 
parallel, positive electrodes 13 are disposed on a surface of the 
electrooptic material 11 opposite the ground electrode 11. The plurality 
of positive electrodes 13 may also be made of indium tin oxide, for 
example. A voltage source 14 has a negative terminal 15 connected to the 
ground electrode 11 and has a positive terminal 16 connected to each of 
the plurality of positive electrodes 13 by way of a variable resistor 19 
that permits each electrode 13 to have a different potential. 
The first embodiment of the electrooptical lens system 10 operates such 
that the positive electrodes 13 are individually controlled to provide for 
shaping of the index of refraction of the electrooptic material 11 along 
one axis thereof. This provides for the formation of a concave or convex 
cylindrical lens having varying radii of curvature. The various radii of 
curvature is controlled by controlling the various voltages applied to 
individual ones of the plurality of electrodes 13. 
In order to better understand the electrooptical lens system 10 of the 
present invention, the electrooptic effect is defined as the change of the 
index of refraction (.DELTA.n) of certain materials when they are 
subjected to an electric field. For some materials, such as lithium 
niobate (LiNbO.sub.3) or barium titanate (BaTiO.sub.3), the index of 
refraction is quite high. The index of refraction is 30.times.10.sup.-10 
cm/volt and 820.times.10.sup.-10 cm/volt, respectively, for the two cited 
materials. It has been determined experimentally that electric fields of 
&gt;500 kV/centimeter may be used (for LiNbO.sub.3) without dielectric 
breakdown in the material. However, the devices does not require linearity 
to work. Typically the applied voltage is in the range of from 0 
kV/centimeter to 100 kV/centimeter. Therefore index of refraction changes 
of the order of 0.1 are obtainable. 
The focal length change .DELTA.f of a lens made of an electrooptic material 
is given by the equation: .DELTA.f=f(.DELTA.n/n-1), and the deflection 
change .DELTA..theta. of a beam going through a prism, with the proper 
angle .alpha., is given by the equation: 
.DELTA..theta..congruent..DELTA.n.multidot..alpha.. For .DELTA.n=0.1 and 
f=10 cm, n.congruent.2, and .DELTA.f.congruent.1 cm. Consequently, the 
deflection angle change .DELTA..theta. is for angle .alpha.=30 degrees is 
.DELTA..theta.=3 degrees. 
These values are subject to change (increase) depending on the quality of 
materials available. However, the fact that .DELTA..theta. is only 3 
degrees does not imply that the total deflection angle through an optical 
system would be limited to that value. An optical system may be designed 
to magnify the deflection angle significantly. 
FIG. 2 shows a second embodiment of an electrooptical lens system 30 in 
accordance with the principles of the present invention. The 
electrooptical lens system 30 comprises the electrooptic material 11, 
which may comprise lithium niobate (LiNbO.sub.3) or barium titanate 
(BaTiO.sub.3), for example. The transparent ground electrode 12 is 
disposed on one surface of the electrooptic material 11. The plurality of 
spaced apart, parallel, positive electrodes 13 are disposed on a surface 
of the electrooptic material 11 opposite the ground electrode 11. A second 
plurality of spaced apart, parallel, positive electrodes 33 are disposed 
on top of the first plurality of positive electrodes 13 opposite the 
ground electrode 11. The second plurality of positive electrodes 33 are 
oriented orthogonal to the plurality of positive electrodes 13. The 
voltage source 14 has its negative terminal 15 connected to the ground 
electrode 11 and has its positive terminal 16 connected to each of the 
plurality of positive electrodes 13 by way of the variable resistor 19 
that permits each electrode 13 to have a different potential. A second 
voltage source 34 has a negative terminal 35 connected to the ground 
electrode 11 and has a positive terminal 36 connected to each of the 
second plurality of positive electrodes 33 by way of similar variable 
resistors 19. 
The second embodiment of the electrooptical lens system 30 operates in a 
manner similar to the first embodiment, except that the two sets of 
positive electrodes 13, 33 are individually controlled to provide for 
shaping of the index of refraction of the electrooptic material 11 along 
two orthogonal axes thereof. This provides for the formation of a convex 
or concave spherical lens having varying radii of curvature. 
Thus there has been described a new and improved an adaptive optical system 
that is implemented using the electrooptic effect. It is to be understood 
that the above-described embodiment is merely illustrative of some of the 
many specific embodiments which represent applications of the principles 
of the present invention. Clearly, numerous and other arrangements can be 
readily devised by those skilled in the art without departing from the 
scope of the invention.