George Michael Morris

Experimental demonstration of resonant anomalies in diffraction from two-dimensional gratings.

Resonant effects in diffraction from two-dimensional dielectric gratings is demonstrated experimentally. A two-layer structure, which consists of a uniform guiding layer and a grating layer, yielded symmetric, low-sideband resonance that is suitable for narrow-band filter applications. Excellent agreement between measured and calculated spectral and angular dependence is obtained.

Fundamental limits of optical superresolution

Increasing the resolving power of optical systems beyond the limits imposed by diffraction, or superresolution, has considerable theoretical as well as practical interest. Several schemes have been proposed to achieve superresolution with reasonable success, but there are no criteria that enable one to determine what improvement can ultimately be attained for a certain level of resolution. We have determined fundamental limits imposed on the performance of any superresolution strategy. A brief analysis indicates that current optical-superresolution techniques can still have their performance considerably augmented.

Spectral shifts produced by source correlations.

Statistical correlations of the light emitted by a partially coherent source can produce frequency shifts in the spectrum observed in the far field if the correlation function of the emitted radiation does not satisfy a certain scaling law. A Fourier achromat is used to generate a secondary source in which the degree of spectral coherence is independent of wavelength; i.e., it violates the scaling law. The spectrum detected in the far zone of the secondary source is, in general, found to be displaced in frequency and distorted relative to the spectrum measured at the secondary source. The displacement can be toward the higher frequencies or the lower frequencies depending on the direction of observation.

Frequency-plane filtering with an achromatic optical transform.

A holographic matched filter has been recorded in the frequency plane of an achromatic optical transform configuration. Sandwich-type zone plates and an achromatic doublet are used to form the transform system, which is in cascade with the hologram filter and an achromatic-fringe interferometer. Correlation experiments are reported in which a large space-bandwidth and broad spectral range are obtained.

Matched filtering using band-limited illumination

A holographic matched filter and an achromatic-fringe interferometer are combined to form an optical correlator that works with nonlaser sources. Pattern-recognition capabilities using band-limited illumination are illustrated in a dollar-bill-recognition experiment.

AN INTERFEROMETRIC EFFECT WITH SEMICONDUCTORS IN THE MILLIMETER WAVE REGION.

The reflectance of millimeter waves has been studied for a planar arrangement of a semiconductor panel, an air space, and a metal reflector. For a given wavelength, by adjusting mechanical dimensions and material properties, we observed complete absorption. In these types of systems, changes of conductivity of the semiconductor panel resulted in variations of reflectance. New devices using this physical effect are suggested, such as a device for amplitude modulation of millimeter and submillimeter waves, a simplified interferometer, and an arrangement for conversion of millimeter- and submillimeter-wave images into visual displays.

Diffraction by serrated apertures

Diffraction of monochromatic light by rough apertures is analyzed. Correlation functions are derived for the electric field and for the intensity in the optical transform plane. The expectations calculated are over an ensemble of edges; their roughness is described in terms of a second-order density and associated characteristic function. It is shown that in-plane roughness causes a speckle pattern. The analytical details are markedly different from the more usual case in which speckle is caused by longitudinal phase delay across an extended aperture. Detailed solutions are presented for serrated gaps and edges. Both space and wavelength dependences are included and solutions for cross correlations of electric field and intensity are obtained. These are valid for arbitrary roughness and correlation coefficient. Experiments are described contrasting the optical transforms of serrated and sharp edges. Good qualitative agreement is obtained with the theory. The serration causes a damping of the major spike in the edge transform and it leads to considerable scattering of the radiation.

Computer-generated holography in photorefractive materials

We describe a method to create and reconstruct computer-generated Fourier-transform holograms in photorefractive materials. Holographic reconstructions are obtained from computer-generated data by imaging computer-generated holograms into a BaTiO3 crystal, using either spatially coherent or incoherent light. The intensity variations in the images produce a phase hologram in the crystal by the photorefractive effect. The diffraction efficiency of the holographic reconstructions depends on imaging system f number, crystal orientation, and read-beam polarization. Because these holograms are recorded by using (incoherent or coherent) images rather than coherent interference patterns, our system provides a new technique for the holographic display of computer-generated information.

Image correlation at low light levels

Low-light-level input images are cross correlated with a classical intensity reference image that is stored in computer memory. Experimental measurements of the correlation signal, obtained using a two-dimensional, photon-counting detector and position-computing electronics, are reported. The experimental results are in good agreement with theoretical predictions.

Speckle in achromatic-Fourier-transform systems

Polychromatic speckle in the output plane of an achromatic-Fourier-transform system is analyzed. The degree of speckle correlation is calculated and is found to factorize into spectral and spatial parts. Under conditions for sufficiently fine and rough phase modulation of the complex field amplitude in the object plane, the polychromatic intensity in a paraxial region of the transform plane is shown to be spatially wide-sense stationary and approximately ergodic. For normally distributed height statistics, the intensity is approximately spectrally stationary as well. Our calculations indicate that achromatic-transform-plane speckle should be useful for measuring surface roughness and that an achromatic-transform system may be useful in extending the usable stellar bandwidth in stellar speckle interferometry.