H. M. Escamilla

Design and fabrication of random phase diffusers for extending the depth of focus

We present a method for designing non-absorbing optical diffusers that, when illuminated by a converging beam, produce a specified intensity distribution along the optical axis. To evaluate the performance of the diffusers in imaging systems we calculate the three-dimensional distribution of the mean intensity in the neighborhood of focus. We find that the diffusers can be used as depth-of-focus extenders. We also propose and implement a method of fabricating the designed diffusers on photoresist-coated plates and present some experimental results obtained with the fabricated diffusers.

Photofabrication of random achromatic optical diffusers for uniform illumination

We propose a method of designing two-dimensional random surfaces that scatter light uniformly within a specified range of angles and produce no scattering outside that range. The method is first tested by means of computer simulations. Then a procedure for fabricating such structures on photoresist is described, and light-scattering measurements with the fabricated samples are presented. The results validate the design procedure and show that the fabrication method is feasible.

Annular phase-only mask for high focal depth

We present an annularly distributed phase-only mask that generates an axial irradiance distribution identical to the point spread function of the Dowski-Cathey lens [Appl. Opt. 34, 1859 (1995)]. This apodizer has an ambiguity function with a bow-tie effect, and consequently its axial modulation transfer function varies slowly with spherical aberration.

Electrically controlled Fresnel zone plates made from ring-shaped 180° domains

We present high efficiency, low noise electrically-controlled Fresnel phase zone plates that were made by creating ring-shaped 180 degrees ferroelectric domains in a lithium niobate wafer. The primary focal lengths of these lenses ranged from 5 to 43 cm, and the light-gathering efficiency was over 37%, very close to the maximum theoretical value of 40.5%.

Fabrication of probe tips for reflection scanning near-field optical microscopes: Chemical etching and heating-pulling methods

Work is described on the fabrication of uncoated probe tips for reflection scanning near-field optical microscopes. A chemical etching process with a rotator, and a simple device to make the tips by a heating-pulling method without a heating laser are introduced. Both techniques allow some manipulation of the tips. Merits and demerits of these two methods are discussed. Finally, it is suggested that observation of the far field scattering pattern of the tips can be used to check the usefulness of the tips.

Interaction of two optical beams at a symmetric random surface

Recently it was shown that the incoherent part of light scattered from random surfaces with even profiles displays a well-defined peak in the specular direction. This effect has been termed specular enhancement.Here we present an experimental and theoretical study of the scattering of two coherent optical beams from a symmetric random surface. It is found that, in addition to the two sharp peaks corresponding to the enhancements in the specular directions, there is a third peak that is due to the interaction of the two optical beams at the surface.

Electro-optic vortex-producing lenses using spiral-shaped ferroelectric domains.

We present electrically controlled wavefront modulators that simultaneously focus and introduce vorticity to an incident beam. These modulators are made out of spiral-shaped 180 degrees ferroelectric domains in lithium niobate; they have a virtually instantaneous response time, withstand high power and can be used throughout the transparency region of the material (0.4 - 5 microm).

Observation of satellite peaks and dips in the scattering of light in a double-pass geometry.

We report the experimental observation of enhanced backscattering and satellite peaks and dips in light scattered by a system that involves double passage of waves through a random-phase screen and a birefringent crystal.

Nonreciprocal effects in the double passage of light through a random diffuser and a birefringent crystal

We present a theoretical and experimental study of the scattering of light by double passage through a system that consists of a strong diffuser, a piece of birefringent crystal, and a plane mirror. We show that this arrangement can produce not only enhanced backscattering and satellite peaks but also satellite dips in the angular distribution of the mean intensity. The experiments are in agreement with theoretical results based on scalar diffraction theory in the paraxial approximation.

IMAGE FORMATION IN THE DOUBLE-PASSAGE SCATTERING CONFIGURATION

Image formation due to the enhanced backscattering of light in a double-passage scattering configuration is considered. It is shown that the system mean intensity response to a coherently illuminated object consists of a coherent image of the object superposed on a uniform intensity background. The point-spread function of the system is formally identical to the point-spread function of an aberration-free conventional optical system with the same numerical aperture. We show, however, that the relative strength of the background increases with the number of independent object regions, reducing rapidly the contrast of the image. Experimental support for our conclusions is provided.