Optical correlator using ferroelectric liquid crystal spatial light modulators and Fourier transform lenses

• US 5,418,380 A
• Filed: 04/12/1994
• Issued: 05/23/1995
• Est. Priority Date: 04/12/1994
• Status: Expired due to Term

• Current Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Original Assignee: Martin Marietta Corporation
• Inventors: Simon, Darren M., Serati, Steven A.
• Primary Examiner(s): Nelms, David C.
• Assistant Examiner(s): LE, QUE TAN
• Application Number:
  US08/226,461
• Time in Patent Office: 406 Days
• Field of Search: 250/550, 250/561, 250/239, 359/559-561, 382/31, 382/42, 382/32, 382/8, 364/822
• US Class Current: 250/550
• CPC Class Codes: G02B 27/288 Filters employing polarising elements, e.g. Lyot or Solc filters G02B5/3016 takes precedence
• CPC Class Codes: G06E 3/005 using electro-optical or opto-electronic means
• CPC Class Codes: G06V 10/88 Image or video recognition using optical means, e.g. reference filters, holographic masks, frequency domain filters or spatial domain filters

Abstract

An optical correlator uses ferroelectric liquid crystal spatial light modulators (FLC-SLM'"'"'s) in both the reference and filter planes. The SLM'"'"'s include an electrically addressable memory to store images in the form a two-dimensional array of reflective pixels beneath the FLC layer. The SLM'"'"'s selectively rotate the polarization of the light reflected by each pixel in accordance with the stored image. In particular, a laser produces a polarized beam that is directed through a first polarizing beamsplitter and onto the reference SLM. This beamsplitter blocks unmodulated light reflected by the reference SLM and transmits modulated light through a set of Fourier tranform lenses. The resulting beam is directed through a second polarizing beam splitter onto a filter SLM that has been programmed with the complex conjugate of the Fourier transform of a desired target image. Unmodulated light reflected from the filter SLM is blocked by the second polarizing beamsplitter and modulated light is reflected by the second polarizing beamsplitter through a set of inverse Fourier transform lenses. A CCD camera detects any correlation peak produced by the inverse Fourier transform lenses. A computer system downloads images to the SLM'"'"'s and analyzes the correlation peaks detected by the camera. A half-wave plate can be included between both sets of SLM'"'"'s and polarizing beamsplitters to allow manual adjustment of the polarization of the incident beam entering the SLM.

Claims

1. An optical correlator comprising:

a laser for producing a beam of coherent polarized light;

a reference spatial light modulator (SLM) for modulating and reflecting an incident beam of light, said reference SLM having;

(a) means for storing a reference image in the form of a two-dimensional array of pixels; and

(b) means for selectively modulating said incident beam by rotating the polarization of said beam for selected pixels corresponding to said reference image;

a first polarizing beamsplitter for directing said laser beam onto said reference SLM and receiving said beam reflected from said reference SLM, said first beamsplitter being polarized to transmit modulated light reflected from said reference SLM and to block unmodulated light reflected from said reference SLM;

Fourier transform lenses for receiving said modulated light beam transmitted by said first polarizing beamsplitter and producing a Fourier transform of said beam;

a filter SLM for modulating and reflecting an incident beam of light, said filter SLM having;

(a) means for storing a filter image in the form of a two-dimensional array of pixels, said filter image being the complex conjugate of the Fourier transform of a desired image; and

(b) means for selectively modulating said incident beam by rotating the polarization of said beam for selected pixels corresponding to said filter image;

a second polarizing beamsplitter for directing said Fourier transform beam onto said filter SLM and receiving said beam reflected from said filter SLM, said second polarizing beamsplitter being polarized to transmit modulated light reflected from said filter SLM and to block unmodulated light reflected from said filter SLM;

inverse Fourier transform lenses for receiving said modulated light beam transmitted by said second polarizing beamsplitter and producing an inverse Fourier transform of said beam; and

a camera for detecting any correlation peak produced by said inverse Fourier transform lenses.

2. The optical correlator of claim 1, wherein said reference SLM comprises an ferroelectric liquid crystal SLM.

3. The optical correlator of claim 1, wherein said SLM pixels are electrically addressable.

4. The optical correlator of claim 1, further comprising a polarizing filter between said inverse Fourier transform lenses and said camera.

5. The optical correlator of claim 1, further comprising a half-wave plate between said first polarizing beamsplitter and said reference SLM for adjustably aligning the polarization of light entering said reference SLM.

6. The optical correlator of claim 1, further comprising a half-wave plate between said second polarizing beamsplitter and said filter SLM for adjustably aligning the polarization of light entering said filter SLM.

7. The optical correlator of claim 1, further comprising a computer processor connected to said reference SLM and to said image SLM for downloading images to said SLM'"'"'s.

8. The optical correlator of claim 1, wherein said Fourier transform lenses have two focal planes and said reference SLM and said filter SLM are at said focal planes of said Fourier transform lenses, and wherein said inverse Fourier transform lenses have two focal planes and said filter SLM and said camera are at said focal planes of said inverse transform lenses.

9. An optical correlator comprising:

a laser for producing a beam of coherent polarized light;

a reference spatial light modulator (SLM) for modulating and reflecting an incident beam of light, said reference SLM having;

(a) means for storing a reference image in the form of a two-dimensional array of pixels;

(b) a ferroelectric liquid crystal (FLC) layer for selectively modulating said incident beam by rotating the polarization of said beam for selected pixels corresponding to said reference image; and

(c) a reflective backplane beneath said FLC layer for reflecting said incident beam;

a first polarizing beamsplitter for directing said laser beam onto said reference SLM and receiving said beam reflected from said reference SLM, said first beamsplitter being polarized to transmit modulated light reflected from said reference SLM and to block unmodulated light reflected from said reference SLM;

Fourier transform lenses for receiving said modulated light beam transmitted by said first polarizing beamsplitter and producing a Fourier transform of said beam;

a filter SLM for modulating and reflecting an incident beam of light, said filter SLM having;

(a) means for storing a filter image in the form of a two-dimensional array of pixels, said filter image being the complex conjugate of the Fourier transform of a desired image;

(b) a ferroelectric liquid crystal (FLC) layer for selectively modulating said incident beam by rotating the polarization of said beam for selected pixels corresponding to said filter image; and

(c) a reflective backplane beneath said FLC layer for reflecting said incident beam;

a second polarizing beamsplitter for directing said Fourier transform beam onto said filter SLM and receiving said beam reflected from said filter SLM, said second polarizing beamsplitter being polarized to transmit modulated light reflected from said filter SLM and to block unmodulated light reflected from said filter SLM;

inverse Fourier transform lenses for receiving said modulated light beam transmitted by said second polarizing beamsplitter and producing an inverse Fourier transform of said beam;

a CCD camera for detecting any correlation peak produced by said inverse Fourier transform lenses; and

a computer processor connected to said reference SLM and to said filter SLM for downloading images to said SLM'"'"'s, and for analysis of correlation peaks detected by said CCD camera.

10. The optical correlator of claim 9 wherein said SLM pixels are electrically addressable by said computer processor.

11. The optical correlator of claim 9, further comprising a polarizing filter between said inverse Fourier transform lenses and said CCD camera.

12. The optical correlator of claim 9, further comprising a half-wave plate between said first polarized beamsplitter and said reference SLM for adjustably aligning the polarization of light entering said reference SLM.

13. The optical correlator of claim 9, further comprising a half-wave plate between said second polarized beamsplitter and said filter SLM for adjustably aligning the polarization of light entering said filter SLM.

14. The optical correlator of claim 9, wherein said Fourier transform lenses have two focal planes and said reference SLM and said filter SLM are at said focal planes of said Fourier transform lenses, and wherein said inverse Fourier transform lenses have two focal planes and said filter SLM and said camera are at said focal planes of said inverse transform lenses.

15. An optical correlator comprising:

a laser for producing a beam of coherent polarized light;

a reference spatial light modulator (SLM) for modulating and reflecting an incident beam of light, said reference SLM having;

(a) an electrically addressable memory for storing a reference image in an array of pixels forming a reflective backplane; and

(b) a ferroelectric liquid crystal layer (FLC) for selectively modulating said incident beam by rotating the polarization of said beam for selected pixels corresponding to said reference image;

a first polarizing beamsplitter for directing said laser beam onto said reference SLM and receiving said beam reflected from said reference SLM, said first beamsplitter being polarized to transmit modulated light reflected from said reference SLM and to block unmodulated light reflected from said reference SLM;

a first half-wave plate between said first polarized beamsplitter and said reference SLM for adjustably aligning the polarization of said incident beam entering said reference SLM;

Fourier transform lenses for receiving said modulated light beam transmitted by said first polarizing beamsplitter and producing a Fourier transform of said beam;

a filter SLM for modulating and reflecting an incident beam of light, said filter SLM having;

(a) an electrically addressable memory for storing a filter image in a two-dimensional array of pixels forming a reflective backplane, said filter image being the complex conjugate of the Fourier transform of a desired image; and

(b) a ferroelectric liquid crystal (FLC) layer for selectively modulating said incident beam by rotating the polarization of said beam for selected pixels corresponding to said filter image;

a second polarizing beamsplitter for directing said Fourier transform beam onto said filter SLM and receiving said beam reflected from said filter SLM, said second polarizing beamsplitter being polarized to transmit modulated light reflected from said filter SLM and to block unmodulated light reflected from said filter SLM;

a second half-wave plate between said second polarized beamsplitter and said filter SLM for adjustably aligning the polarization of said incident beam entering said filter SLM;

inverse Fourier transform lenses for receiving said modulated light beam transmitted by said second polarizing beamsplitter and producing an inverse Fourier transform of said beam;

a CCD camera for detecting any correlation peak produced by said inverse Fourier transform lenses; and

a computer processor connected to said reference SLM and to said filter SLM for downloading images to said SLM'"'"'s, and for analysis of correlation peaks detected by said CCD camera.

16. The optical correlator of claim 15, further comprising a polarizing filter between said inverse Fourier transform lenses and said CCD camera.

17. The optical correlator of claim 15, wherein said Fourier transform lenses have two focal planes and said reference SLM and said image SLM are at said focal planes of said Fourier transform lenses, and wherein said inverse Fourier transform lenses have two focal planes and said filter SLM and said CCD camera are at said focal planes of said inverse transform lenses.