Frequency modulated liquid crystal beamsteering device

• US 6,456,419 B1
• Filed: 09/29/2000
• Issued: 09/24/2002
• Est. Priority Date: 09/29/2000
• Status: Active Grant

• Current Assignee: Teledyne Scientific & Imaging, LLC (Teledyne Technologies Incorporated)
• Original Assignee: Innovative Technology Licensing LLC
• Inventors: Zhuang, Zhiming, Winker, Bruce K.
• Primary Examiner(s): Ben, Loha
• Application Number:
  US09/675,300
• Time in Patent Office: 725 Days
• Field of Search: 359/279, 359/315, 359/316, 359/318, 359/319, 359/209, 359/245, 359/252, 359/246, 359/254, 359/256, 359/573, 349/140, 349/193, 349/202, 349/33, 349/56, 349/141, 250/227.17, 385/14, 385/130
• US Class Current: 359/279
• CPC Class Codes: G02B 27/0087 Phased arrays
• CPC Class Codes: G02F 1/1392 using a field-induced sign-reversal of the dielectric anisotropy
• CPC Class Codes: G02F 1/292 by controlled diffraction or phased-array beam steering controlled diffraction for optical switching G02F1/31

Abstract

An electrical exciting circuit produces a plurality of oscillating electrical excitations, each at an independently controllable frequency. A set of drive electrodes are distributed in an array, and connected so that each receives a respective one of the excitations. A dual frequency liquid crystal (DFLC) material is arranged in the path of a coherent light beam and is disposed in proximity to the set of drive electrodes so as to receive electrical excitations. The DFLC has a dielectric coefficient which varies locally in relation to the frequency of the local electrical excitation received. The voltages and at least two frequencies of the excitations are controlled so as to produce a desired profile of the dielectric coefficient (for at least one polarization) and a corresponding optical phase delay profile for the coherent beam. Preferably, a novel reflective groundplane is included to improve optical reflective efficiency.

Claims

1. An electro-optical spatial light modulator, for imposing a spatial phase-modulation on a coherent light beam, comprising:

a set of drive electrodes, distributed in an array, with each electrode arranged to receive a respective oscillating electrical excitation;

a liquid crystal material, arranged to receive the coherent light beam and disposed in proximity to said set of drive electrodes so as to receive electrical excitations in local regions from said drive electrodes, said liquid crystal material having a dielectric coefficient which varies in said local regions in relation to the frequency of the local electrical excitation received by said regions; and

a transparent top electrode, disposed on a first surface of said liquid crystal material and arranged to receive and transmit the coherent beam;

wherein said drive electrodes comprise flat, reflective electrodes distributed in a pattern across a second surface, separated from said first surface by said liquid crystal material so that said first and second surfaces define a layer of liquid crystal material;

and wherein said excitations are oscillating electrical potentials applied between said transparent top electrode and said drive electrodes, thereby establishing oscillating electrical fields in said liquid crystal layer.

2. The spatial light modulator of claim 1, further comprising:

an optically reflective, electrically conductive groundplane, said groundplane displaced behind said electrodes at a distance substantially equal to an integer multiple of one half wavelength for the coherent beam.

3. The spatial light modulator of claim 2, wherein said groundplane is divided into multiple optically reflective conductors separated from one another by non-conductive areas, said reflective conductors staggered with respect to said drive electrodes to receive and reflect radiation from the coherent beam which passes between said drive electrodes.

4. An electro-optical spatial light modulator, for imposing a spatial phase-modulation on a coherent light beam, comprising:

a set of drive electrodes, distributed in an array, with each electrode arranged to receive a respective oscillating electrical excitation; and

a liquid crystal material, arranged to receive the coherent light beam and disposed in proximity to said set of drive electrodes so as to receive electrical excitations in local regions from said drive electrodes, said liquid crystal material having a dielectric coefficient which varies in said local regions in relation to the frequency of the local electrical excitation received by said regions;

wherein said liquid crystal material comprises a nematic liquid crystal with a crossover point in its relation between dielectric anisotropy and driving frequency;

said crossover point being the frequency at which said dielectric anisotropy changes sign; and

dielectric anisotropy defined as the difference between the dielectric coefficients for electric fields which are (a) parallel to, and (b) perpendicular to the nematic director of the liquid crystal.

5. A method for electro-optically steering a coherent light beam by imposing an optical phase delay gradient across the beam, comprising:

providing a thin layer of frequency responsive liquid crystal material, arranged in the path of the beam;

said liquid crystal material having a dielectric coefficient which varies locally in response to the frequency of a locally applied, oscillating electromagnetic field; and

driving said liquid crystal material with a plurality of independent electrical excitations dispersed spatially across said layer, thereby producing a spatial variation of the dielectric index across said layer in at least one direction;

wherein said plurality of electrical excitations are dispersed across said liquid crystal layer in a two-dimensional matrix, thereby allowing creation of a gradient along a vector in two dimensions, for steering a beam by two independent, non-coplanar angles; and

wherein said excitations applied to said liquid crystal are pre-defined to produce a phase shift of approximately (x times b) modulo 2π

, where x is a position index assigning position to an individual electrode, and b is a phase gradient in the x direction, thereby imposing on the beam a phase ramp which varies between 0 and 2π

in a modulo 2π

pattern.

6. A method for electro-optically steering a coherent light beam by imposing an optical phase delay gradient across the beam, comprising:

providing a thin layer of frequency responsive liquid crystal material, arranged in the path of the beam;

said liquid crystal material having a dielectric coefficient which varies locally in response to the frequency of a locally applied, oscillating electromagnetic field; and

driving said liquid crystal material with a plurality of independent electrical excitations dispersed spatially across said layer, thereby producing a spatial variation of the dielectric index across said layer in at least one direction;

wherein said plurality of electrical excitations includes at least two excitations with two respective frequencies;

thereby producing a gradient of optical phase delay for the coherent beam which traverses said liquid crystal layer, to steer the beam;

wherein said plurality of independent electrical excitations drive said liquid crystal material via respective reflective electrodes which are disposed on a surface of said liquid crystal material layer and patterned in a two-dimensional array, and wherein the excitations applied to said liquid crystal are defined to produce a further phase shift gradient in a second direction of approximately (y times c) modulo 2π

, where y is a second, independent position index assigning a second position component to an individual electrode, and c is a phase gradient in the y direction, thereby imposing on the beam a phase ramp which varies between 0 and 2n in a modulo 2π

pattern.

7. A device for electro-optically steering a coherent light beam by imposing an optical phase delay gradient across the beam, comprising:

a transparent top electrode, disposed to receive and transmit the beam;

a set of reflective drive electrodes, arranged to receive and reflect the beam;

a liquid crystal layer, located in the path of the beam and between said top electrode and said set of reflective drive electrodes;

said liquid crystal layer having a dielectric coefficient which varies in relation to the frequency of an oscillating electromagnetic field applied across said layer between said top and said reflective drive electrodes;

an electrical exciting circuit with a plurality of outputs connected to respective ones of said drive electrodes, with at least two of said outputs independently variable in frequency to induce a gradient of optical phase delay across said liquid crystal layer.

8. The device of claim 7, further comprising a reflective groundplane disposed beneath said drive electrodes.

9. The device of claim 8, wherein said reflective groundplane comprises:

a transparent material, immediately underlying said drive electrodes and having a thickness substantially equal to an integer multiple of wavelengths for the coherent beam; and

at least one reflective groundplane electrode underlying said transparent material, for modifying the fringe field of said drive electrodes and for reflecting the beam.

10. The device of claim 9, wherein said groundplane electrode and said top transparent electrode are at a common potential with respect to said drive electrodes, and said drive electrodes independently oscillate in potential with respect to said common potential.

11. The device of claim 7, wherein said liquid crystal layer comprises a nematic liquid crystal with a crossover point in its relation between dielectric anisotropy and driving frequency;

said crossover point being the frequency at which said dielectric anisotropy changes sign; and

dielectric anisotropy defined as the difference between the dielectric coefficients for electric fields which are (a) parallel to, and (b) perpendicular to the nematic director of the liquid crystal.

12. The device of claim 7, wherein said liquid crystal layer comprises a nematic liquid crystal having a dielectric coefficient parallel to the nematic director.