Interferometric/feedback spatial light modulation system and method

• US 4,854,677 A
• Filed: 12/21/1987
• Issued: 08/08/1989
• Est. Priority Date: 12/21/1987
• Status: Expired due to Term

• Current Assignee: Hughes Electronics Corporation (AT&T, Inc.)
• Original Assignee: Hughes Aircraft Company (Rtx Corporation)
• Inventors: O'Meara, Thomas R.
• Primary Examiner(s): LaRoche, Eugene R.
• Assistant Examiner(s): McCutcheon, Nathan W.
• Application Number:
  US07/135,857
• Time in Patent Office: 596 Days
• Field of Search: 350/354, 350/355, 350/356, 350/357, 350/353
• US Class Current: 359/242
• CPC Class Codes: G02F 1/21 by interference
• CPC Class Codes: G02F 2/00 Demodulating light; Transferring the modulation of modulated light; Frequency-changing of light G02F1/35 takes precedence

Abstract

A spatial light modulator and associated method are disclosed which achieve an intensity-to-intensity spatial transformation between a noncoherent input beam and a coherent output beam. An intermediate intensity-to-phase modulator provides a coherent, spatially phased modulated readout beam that is combined with a coherent reference beam in an interferometer to yield the desired coherent, spatially intensity modulated output beam. The interferometer also produces a feedback beam with a spatial intensity modulation complementary to that of the input and output beams. The feedback beam is combined with the input beam in a feedback loop that significantly improves the system'"'"'s response time, and can also enhance the linearity of its transfer characteristic.

Claims

1. A spatial light modulation system for spatially encoding a coherent readout beam with information from a noncoherent input light beam, comprising:

a light modulator adapted to receive a noncoherent input light beam and a coherent readout light beam, and to spatially encode the input spatial light pattern onto the readout beam,

means responsive to the readout beam for forming a feedback beam having a spatial encoding which varies in a negative fashion with respect to spatial variations in the input beam, and

means for combining the feedback beam with the input beam in a negative feedback loop, thereby enhancing the responsivity of the readout beam to variations in the input beam.

2. The system of claim 1, wherein the light modulator encodes intensity variations in the input beam to phase variations in the readout beam, further comprising means for establishing a reference beam having a predetermined spatial phase relationship to an unencoded readout beam, and means for combining the reference and encoded readout beams to produce an output beam with a spatial intensity pattern corresponding to the input beam intensity pattern, and the feedback beam with a spatial intensity pattern that varies in a manner generally complementary to variations in the output beam pattern.

3. The system of claim 2, wherein the reference beam is established with a generally 90°

spatial phase differential with respect to an unencoded readout beam from the modulator.

4. The system of claim 2, wherein the readout and reference beams are formed from a common beam source.

5. The system of claim 4, wherein the reference and readout beams are established by a laser, a beam splitter and a reflector means, the beam splitter being positioned to split a beam from the laser into readout and reference beams, the modulator being positioned to encode and direct the readout beam back to the beam splitter, the reflector means being positioned to reflect the reference beam back to the beam splitter with said predetermined spatial phase relationship to the readout beam, the beam splitter combining the reflected reference and encoded readout beams into said output and feedback beams.

6. The system of claim 1, wherein the light modulator and the means for forming a feedback beam are selected to produce a feedback beam having spatial intensity variations I_F (x,y) which vary with the input beam spatial intensity variations I_IN (x,y) substantially in accordance with:

space="preserve" listing-type="equation">I.sub.F (x,y)=-I.sub.IN (x,y)G/(1+G),

where G is the loop gain of the feedback system.

7. A spatial light modulation system for obtaining an output coherent spatial light pattern which corresponds to an input noncoherent spatial light intensity pattern, comprising:

a light modulator adapted to receive a noncoherent input optical beam and a coherent readout optical beam, and to spatially encode the intensity pattern of the input beam onto the phase of the readout beam,

means for establishing a coherent reference beam and the coherent readout beam such that the reference beam has as predetermined spatial phase relationship to the unencoded readout beam,

means for combining the reference and readout beams to produce an output beam with a spatial intensity pattern corresponding to the input beam intensity pattern, and

means for providing a coherent feedback beam in combination with the noncoherent input optical beam to the light modulator to provide a combined coherent-noncoherent input to the light modulator.

8. The system of claim 7, wherein the reference beam is established with a generally 90°

spatial phase differential with respect to the unencoded readout beam.

9. The system of claim 7, wherein the readout and reference beams are formed from a common coherent beam source.

10. The system of claim 9, wherein the means for establishing the reference and readout beams, includes the common coherent beam source, a beam splitter and a reflector means, the beam splitter being positioned to split a beam from the common coherent beam source into readout and reference beams, the modulator being positioned to encode and direct the readout beam back to the beam splitter, the reflector means being positioned to reflect the reference beam back to the beam splitter with said predetermined spatial phase relationship to the readout beam, the beam splitter combining the reflected reference and encoded readout beams into said output beam.

11. An intensity-to-intensity spatial light modulation system, comprising:

an amplitude-to-phase spatial light modulator,

means for directing a noncoherent, spatially intensity modulated input beam as an input to the modulator,

means for forming a coherent readout beam,

means for directing the readout beam onto the modulator, the readout beam being reflected from the modulator with a spatial phase modulation corresponding to the input beam'"'"'s spatial intensity modulation,

means for forming a reference beam having a predetermined spatial phase relationship with respect to an unencoded readout beam,

means for combining the reference and encoded readout beams to produce an output beam with a spatial intensity pattern determined by the phase-interference pattern between the reference and readout beams, and a feedback beam with a spatial intensity pattern that varies in a manner generally complementary to variations in the output beam pattern, and

means for combining the feedback beam with the input beam in a negative feedback loop, thereby enhancing the responsivity of the output beam to variations in the input beam.

12. The system of claim 11, wherein the reference and readout beams are established by a laser, a beam splitter and a reflector means, the beam splitter being positioned to split a beam from the laser into readout and reference beams, the modulator being positioned to encode and direct the readout beam back to the beam splitter, the reflector means being positioned to reflect the reference beam back to the beam splitter with said predetermined spatial phase relationship to the readout beam, the beam splitter combining the reflected reference and encoded readout beams into said output and feedback beams.

13. The system of claim 12, wherein the round trip distance for the reference beam between the beam splitter and reflector means differs from the round trip distance for the readout beam between the beam splitter and modulator by approximately 1/4 the laser beam wavelength, thereby establishing a generally 90°

spatial phase differential between the reference beam and an unencoded readout beam the polarity of the phase differential being selected so that the spatial intensity of the output beam varies in positive proportion to that of the input beam.

14. The system of claim 11, wherein the light modulator and the means for forming a feedback beam are selected to produce a feedback beam having spatial intensity variations I_F (x,y) which vary with the input beam spatial intensity variations I_IN (x,y) substantially in accordance with:

space="preserve" listing-type="equation">I.sub.F (x,y)=-I.sub.IN (x,y)G/(1+G),

where G is the loop gain of the feedback system.

15. A method for spatially encoding a coherent readout beam with information from a noncoherent input light beam, comprising:

spatially encoding the input beam spatial light pattern onto a coherent readout beam,

forming a feedback beam having a spatial encoding which varies in a negative fashion with respect to spatial variations in the input beam, and

combining the feedback beam with the input beam in a negative feedback loop to enhance the responsivity of the readout beam to variations in the input beam.

16. The method of claim 15, wherein spatial intensity variations in the input beam are encoded to phase variations in the readout beam, and further comprising the steps of establishing a reference beam having a predetermined spatial phase relationship to an unencoded readout beam, and combining the reference and encoded readout beams to produce an output beam with a spatial intensity pattern corresponding to the input beam intensity pattern and the feedback beam with a spatial intensity pattern that varies in a manner generally complementary to variations in the output beam pattern.

17. The method of claim 16, wherein the reference beam is established with a generally 90°

spatial phase differential with respect to an unencoded readout beam.

18. The method of claim 15, wherein the feedback beam is formed with spatial intensity variations I_F (x,y) which vary with the input beam spatial intensity variations I_IN (x,y) substantially in accordance with:

space="preserve" listing-type="equation">I.sub.F (x,y)=-I.sub.IN (x,y) G/(1+G),

where G is the feedback loop gain.

19. A method for obtaining an output beam with a coherent spatial light pattern that corresponds to the spatial light intensity pattern of a noncherent input beam, comprising:

spatially encoding the intensity pattern of the input beam onto the phase of a coherent readout beam,

establishing a coherent reference beam having a predetermined spatial phase relationship to the readout beam prior to encoding,

combining the reference and readout beams to produce an output beam with a spatial intensity pattern corresponding to the input beam intensity pattern, and

feeding back a feedback beam, with a spatial encoding which varies in a negative fashion with respect to spatial variations in the intensity pattern of the noncoherent input beam, for combination with the input beam to influence said step of spatial encoding.

20. The method of claim 19, wherein the reference beam is established with a generally 90°

spatial phase differential with respect to an unencoded readout beam, and

the output beam is transmitted through a field stop to modify the contrast ratio.