Adaptively aligned optical correlator and method

Details Adaptively aligned optical correlator and method Adaptively aligned optical correlator and method

1999-03-16

2001-12-11

Boudreau, Leo (Department: 2721)

Image analysis

Pattern recognition

Template matching

C382S151000, C382S278000, C382S287000, C382S294000, C359S011000, C359S237000, C359S561000

Reexamination Certificate

active

06330361

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to optical image processors or correlator systems which use coherent light, spatial light modulators, and Fourier transform optics to process information or recognize patterns.
DESCRIPTION OF THE RELATED ART
It has long been known that two-dimensional spatial Fourier transform techniques can be used to process images. Pattern recognition, for example, is often facilitated by working with the Fourier transform of a spatially varying image rather than the original image. In particular, optical correlators produce the two dimensional correlation of an input image with a reference image. A high level of correlation at a localized area within the image frame indicates recognition of the reference image in that area. The correlation function is most easily performed by multiplying in frequency domain the Fourier transformation of an image with a “filter,” which is a frequency domain representation of a reference image or images. The resulting (two dimensional) frequency function is then transformed back to the spatial domain. Although such processing can be done digitally, it can be performed much more rapidly by optical Fourier transformation of the image. Such high speed image processing finds numerous applications in diverse fields such as security, aviation, biomedical diagnostics and artificial intelligence.
Optical correlators using two dimensional Fourier transform optics and active, electronic spatial light modulators (SLMs) have been used to perform high speed, two-dimensional pattern recognition. For example, U.S. Pat. No. 5,311,359, to Lucas et al, describes an optical correlator system having a source of coherent light and a plurality of active and passive optical components placed along a folded, zig-zag optical path. The active optical components are an input SLM, a filter SLM, and a pixilated photodetector (usually a CCD device); each of these active elements is typically an array of electronically addressable active pixels, arranged in a two-dimensional matrix. The passive components include reflective focusing mirrors and polarizers. In the patented apparatus, the optical path is a tunnel contained within a block of a transparent ceramic with an extremely low coefficient of temperature expansion (7×10
−8
/° C.).
FIG. 1
shows the prior art optical correlator of the Lucas patent, in which a coherent electromagnetic radiation source (typically a laser)
10
fixed within a solid optical support body
12
produces a collimated and polarized coherent beam of radiation
14
. The beam
14
strikes a pixilated reflective input SLM
16
and is modulated by whatever pattern is electronically imposed on the input SLM
16
. The modulation is spatially distributed. As discussed in the patent, if a magneto-optic SLM is used, the pixels of the SLM are each individually modulated electronically to cause rotation of the polarization of reflected light, as a function of the input signal addressed to that pixel. Such an SLM also requires an input polarizer and an exit polarizer which function as an analyzer (not shown in FIG.
1
). The result is that some pixels of the array effectively absorb more while others reflect more, as a function of the electronic modulating pattern applied. The modulated beam
14
is reflected and propagates to a first focusing mirror
18
. The beam
14
reflects from the first focusing mirror
18
and focuses at a filter SLM
20
.
Provided that the first focusing mirror
18
has the correct focal length to image the input SLM
16
onto the filter SLM
20
, it is a well known consequence of wave optics that the image formed at the plane of the filter SLM
20
will be optically transformed by a spatial Fourier transform into a frequency domain representation F (u,v) of the input image f (x,y) (where x and y are the spatial coordinates of the input SLM
16
and u and v are the spatial coordinates of the filter SLM
20
). It must be understood that the Fourier transform referred to throughout is a Fourier transform of a function (intensity of modulation) varying with position (in this case position on the face of the input SLM
16
). This Fourier transform of a spatially varying function is not to be confused with the Fourier transform of a time varying function, which is more familiar to engineers. The planar surfaces of the SLMs will actually present functions varying in two dimensions; thus the transformed image will be a two-dimensional Fourier transform representing in two dimensions the frequency components of a two-dimensional image.
The Fourier transformed image formed at the filter SLM
20
is further modulated at that focal plane by a two dimensional pattern or “filter.” The filter is typically the previously obtained Fourier transform of some reference, and therefore consists of a pattern of frequency components mapped onto a two dimensional surface. As described in the prior art patent, the correlation of an input image with a reference image can be obtained by taking the product of the Fourier transformed input image and the Fourier transformed reference image. That product F (u,v)*H(u,v) (where H(u,v) is the filter) is then focused by a second focusing mirror
22
onto a photodetector
24
. The resulting image at the photodetector
24
is the Fourier transform of the product image, which yields the correlation of the input image with the reference image (with inversion of the coordinates x and y at the detector). The correlation of the input function and the filter function thus appears at the photodetector
24
as a two-dimensional correlated optical intensity function.
The above described prior correlator must be precisely aligned optically for best operation. The optical components, including the input SLM
16
, the filter SLM
20
, the photodetector
24
, and mirrors
18
and
22
must be adjusted into optical alignment with the beam
14
. The necessary adjustments require great precision and delicacy to bring the assembly into optimal optical alignment. Once aligned, the alignment is often difficult to maintain as it is influenced by thermally driven stresses imposed on the optical assembly and the coherent light source. Mechanical mounting and vibration induced stress may also cause undesirable misalignment effects.
The most critical alignment in the prior correlators, as recognized by Lucas, is the adjustment of the position of the filter SLM
20
relative to the input SLM
16
. This adjustment is critical in the geometry of the Lucas patent. The patented correlator maps a specific spatial frequency component of the input image onto a specific point in the focal plane of the first focusing mirror
18
. For proper operation, each specific input image frequency component must be further modulated (multiplied) by the specific corresponding filter frequency component. This requires that specific frequency components of the input image be mapped onto specific pixels of the filter SLM
20
. For best operation, an even more stringent condition must be met: the specific frequency component of the input image must be mapped onto the center of the corresponding filter pixel. This requires that the position of the filter SLM
20
be adjusted in x, y and z directions relative to the input SLM
16
, with an accuracy typically on the order of only a few microns.
In the prior optical correlators the input and filter SLMs are difficult to precisely position. Once positioned, their relative position is difficult to maintain in the face of inevitable mechanical and thermal stresses and changes in environmental conditions.
SUMMARY OF THE INVENTION
The invention is a method and apparatus for adaptively aligning an optical correlator.
Rather than introducing the coherent beam into an optical assembly at a fixed angle as in prior correlators, the invention uses a beam deflector, which dynamically varies the beam deflection in response to a correction signal. A feedback system detects misalignment and provides a correction signal which causes the beam deflector to bring the optical correlator back into pro

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