1.  Field of the Invention
The present invention relates generally to optical beam steering and particularly to an optical system utilizing the electro-optical properties of liquid crystals to steer a light beam relative to a light-receiving end of an optical waveguide such as an optical fiber. 
2.  Description of the Related Art
Nxc3x97N optical cross connect switches used, for example, in telecommunications systems, optically couple any one of N input optical fibers to any one of N output optical fibers.  Such switches comprise examples of systems that make use of various techniques for deflecting or steering a light beam emitted from a light source, in this case the light-emitting end of a selected one of the N input fibers, to the light-receiving end of an optical waveguide, here in the form of a selected one (and in some cases, more than one) of the N output fibers. 
One conventional optical beam steering technique applies diffractive beam steering that exploits the electro-optical properties of liquid crystals.  This approach is analogous to the use of phased-array antennas for directing microwave radiation in radar systems and is the subject of a number of patents, for example, U.S. Pat.  No.  5,093,747 to Dorschner and U.S. Pat.  No.  5,963,682 to Dorschner, et al.  These patents disclose a liquid crystal beam steering device comprising an optical beam phase shifting liquid crystal cell having a pair of spaced apart, parallel, superposed windows optically transparent at the wavelengths of interest.  The pair of windows have inner, confronting surfaces.  An electrically conductive, optically transparent (or reflective) common or ground plane electrode is affixed to the inner surface of one of the windows.  A plurality of parallel, electrically conductive, optically transparent stripe electrodes are affixed to the internal surface of the other window.  The space between the confronting electrode-bearing surfaces of the windows is filled with a layer of liquid crystal molecules, typically of the long, thin, rod-like organic type of the nematic phase.  A periodic staircase waveform comprising voltage steps applied to the stripe electrodes creates corresponding local variations in the refractive index of the liquid crystal layer in such a manner as to form a diffraction grating of adjustable period.  Thus, the wavefront of a light beam that emerges from the liquid crystal cell is tilted with respect to that of the incident wavefront.  In this fashion, the optical beam phase shifting liquid crystal cell provides controllable beam steering as a function of the electrical potentials applied to the stripe electrodes.  This conventional phased-array beam steering technique is digital in nature, with discrete voltages applied to the stripe electrodes (each representing a pixel) producing a stepped or staircase variation in the refractive index of the liquid crystal layer and a corresponding stepped or staircase variation in the optical phase delay during the transit of a light beam through the liquid crystal layer. 
A drawback of striped electrode liquid crystal cell beam steering devices is that they tend to introduce crosstalk due to diffraction.  Another disadvantage of striped electrode beam steerers systems is that they limit the available beam steering angles to discrete angular increments.  This limitation results from the digital nature of these devices as well as from the striped electrode interconnection and drive schemes.  In a conventional striped electrode liquid crystal cell, not all of the electrode elements are electrically independent; rather, every nth electrode is typically connected together to form periodically repeating electrode series or subarrays.  The addressable beamsteering angles are restricted to those that correspond to integer multiples of 360 degree (2xcfx80) phase ramps across each electrode subarray.  Although large subarrays can accommodate many possible integer factors (and thus many steerable angles), the steerable angle is still limited to discrete increments; it is not continuously variable. 
U.S. Pat.  No.  4,852,962 discloses an optical fiber switch for switching light from one input fiber to any one of several output fibers.  The switch includes a light deflection or steering cell consisting of two glass plates, nematic liquid crystal material between the glass plates, and spacers and electrodes formed on both sides of elongated electrode holders disposed along opposite edges of the glass plates, that is, along opposite sides of the light beam whose direction is to be controlled.  Each electrode pair is controlled by an independent voltage source.  The ""962 patent states that when the intensity of the electric fields produced on the two sides of the beams differ, different orientations of the nematic crystal material occur across the beams, and depending on the difference in the field strength between the two electrode pairs the light beams are deflected through a larger or smaller angle.  The deflections of the light is said to determine which one of the multiplicity of output fibers receives the light emitted form the single input fiber.  A xe2x80x9ccontinuous gradientxe2x80x9d of refractive index is said to be formed across the light beams, but the ""962 patent does not make clear how this is achieved, nor does the patent describe the distribution of the xe2x80x9cgradientxe2x80x9d, for example, whether it is linear. 
Liquid crystal beam steering devices of the prior art deflect a light beam to couple a selected input optical fiber with a selected one (or more) output fiber(s).  None of those devices deal with the problem of aligning the light beam relative to the light-receiving end(s) of the selected output fiber(s) to correct, for example, for the angle of the incident or emitted beam or for lens imperfections.  In an Nxc3x97N optical switch, the focal points of the light beams passing through the coupling lenses must precisely align with the receiving end surfaces of the output optical fibers to minimize coupling losses.  A misalignment of even a few microns substantially reduces the optical coupling efficiency and, as N increases even modestly, the number of different combinations of connections (N!)  becomes very large.  Thus, to mechanically fix the relative positions of the fiber ends, microlenses and other optical elements so that each input fiber is properly aligned optically with all of the output fibers would be extremely difficult and costly. 
FIG.  1 is a graph showing the rapid decrease in coupling efficiency as a function of fiber position error for a commonly used communication optical fiber, namely, a single mode fiber having a core diameter of 8 microns and a numerical aperture of 0.092.  For example, it will be seen that for a fiber misalignment of 4.5 microns, the coupling efficiency is halved.  Spatial registration precision of 1-2 microns and 1/10xc2x0 of angular precision are required for adequate coupling efficiency. 
It is therefore an overall object of the present invention to provide an improved optical beam steering device. 
It is another object of the invention to provide an optical beam steering device that improves the optical coupling efficiency between a source of the beam and a receiving optical waveguide. 
It is a further object of the present invention to provide an optical beam steering device that improves the optical coupling efficiency between an input optical waveguide and an output optical waveguide. 
It is a further object of the present invention to provide an optical beam steering device that improves the optical coupling efficiency between an input optical waveguide and an output optical waveguide by correcting for imperfections in one or more lenses disposed in the optical path and/or for misalignments of the input and/or output optical waveguides. 
It is a further object of the present invention to provide an optical beam steering device that improves the optical coupling efficiency between an input optical waveguide and an output optical waveguide while allowing relaxation of the mechanical tolerances of the various optical components disposed along the optical path. 
It is yet another, and more specific, object of the present invention to provide a liquid crystal beam steering device that improves the optical coupling efficiency between the input and output optical fibers of an optical switch by controlling the position of the focal point of a light beam emitted from an input fiber relative to the light-receiving end of an output fiber. 
It is still a further object of the present invention to provide a liquid crystal beam steering device that improves the optical coupling efficiency between the input and output optical fibers of an optical switch by precisely aligning the focal point of a light beam emitted from an input fiber with the light-receiving end of an output fiber thereby permitting relaxation of the mechanical tolerances of the switch structure. 
In accordance with the broader aspects of the present invention, there is provided an apparatus for optically coupling a light source with a light-receiving end face of an output optical waveguide along an optical path, in which the apparatus comprises a lens positioned in the optical path for focusing a light beam emitted from the light source at a focal point on the light-receiving end face of the output optical waveguide; and an adaptive coupler positioned in the optical path, the adaptive coupler being responsive to a beam steering control signal for steering the focal point relative to the light-receiving end face of the output optical waveguide to align the focal point with said light-receiving end face.  In one form of the invention, the light source comprises a light-emitting end face of an input optical waveguide and the input and output optical waveguides comprise optical fibers. 
In accordance with a more specific aspect of the present invention, there is provided an adaptive coupler for steering an optical beam, the adaptive coupler comprising a pair of transparent substrates having confronting, parallel inner faces, the inner face of one of the pair of substrates carrying a beam intercepting, optically transparent, constant potential (typically ground) electrode and the inner face of the other of the pair of substrates carrying an electrically resistive, beam intercepting, optically transparent film; a pair of spaced apart electrodes in electrical contact with the film for applying a potential gradient along the film; and an electro-optical phase shifting medium disposed between the confronting inner faces of the pair of substrates.  The electro-optical phase shifting medium comprises a liquid crystal medium, and preferably a dual frequency liquid crystal medium enabling high speed redirection of the optical beam.  The optically transparent film may comprise a material having an electrical resistivity, preferably zinc oxide doped with alumina. 
As is known, the application of an electric field to a layer of liquid crystal molecules affects the retardance and hence the speed of light therethrough.  Thus, the application of a linear voltage gradient along the aforementioned film on the inner face of the one substrate results in a linearly varying electric field between that film and the constant potential electrode creating a corresponding linear variation in the refractive index of the liquid crystal medium and a linear wavefront tilt in the intercepting optical beam. 
In accordance with yet another aspect of the present invention, 2-dimensional beam steering may be provided by stacking two adaptive couplers with their field gradients crossed.  Thus, the adaptive coupler described above may further include a second pair of transparent substrates having confronting, parallel inner faces, the second pair of substrates being disposed parallel with the first-mentioned pair of substrates, the inner face of one of the second pair of substrates carrying a beam intercepting, optically transparent electrode and the inner face of the other of the second pair of substrates carrying an electrically resistive, beam intercepting, optically transparent film, preferably of a material such as doped zinc oxide; a pair of spaced apart electrodes in contact with the film for applying a potential gradient across the film in a direction perpendicular to the direction of the first-mentioned potential gradient; and an electro-optical phase shifting medium, preferably a dual frequency liquid crystal, disposed between the confronting faces of the second pair of substrates. 
In accordance with one specific, exemplary application of the present invention, there is provided an optical switch for optically coupling the light-emitting end of a selected one of a plurality of input optical waveguides, such as optical fibers, with the light-receiving end of a selected one of a plurality of output optical waveguides, such as optical fibers.  The optical switch comprises a switch array for selectively deflecting a light beam from the light-emitting end of the selected one of the plurality of input optical waveguides to the light-receiving end of the selected one of the plurality of output optical waveguides.  The switch further comprises a lens for focusing the light at a focal point on the light-receiving end of the selected one of the plurality of output optical waveguides.  Last, the switch includes an adaptive coupler, as already described, for steering the focal point relative to the light-receiving end of the selected one of the plurality of output optical waveguides to align the focal point with the light-receiving end.  Other forms of optical waveguide switches and couplers are of course possible; a selected one of multiple input waveguides may be coupled to a single output waveguide, and vice versa.  In all instances, the adaptive coupler of the present invention corrects for system inaccuracies such as beam misalignments and lens aberrations to improve optical coupling efficiency.