Steven K. Case

Optical elements with ultrahigh spatial-frequency surface corrugations

In this paper we discuss the properties of optical elements fabricated by holographically recording an ultrahigh spatial-frequency pattern in a photoresist mask followed by reactive ion etching to transfer this pattern into the surface of a quartz substrate. Such optical elements are environmentally durable, potentially easy to replicate, and exhibit diffraction efficiencies in excess of 85%. In addition, two other properties are reported for the first time. Such elements at normal (0°) incidence are antireflective, with broadband reflection coefficients as low as 0.035%. Also, the elements exhibit artificially produced birefringence making them useful as wave plates. These results may be particularly significant in the UV and IR, where damage-resistant antireflection coatings and transparent birefringent materials may not exist.

Three Dimensional Inspection Using Multistripe Structured Light

Structured light vision systems offer a practical solution to many 3-D inspection problems. This paper examines the trade-offs involved in the design of 3-D inspection systems that use triangulation. General equations describing the behavior of such systems are derived. The implementation of a multistripe system is discussed. Problems common among structured light systems are explored, and alternative solutions are suggested. Experimental results from a prototype system are presented.

Multifacet holographic optical elements for wave front transformations

A new type of holographic optical element combines some of the flexibility of computer-generated holograms with the high light efficiency of volume phase holograms to produce optical elements capable of arbitrary illumination transformations with nearly 100% light efficiency. The optical element is recorded by subdividing a volume hologram film surface into numerous small areas (facets), each of which is individually exposed. A final optical system consisting of two dichromated gelatin holograms in series is demonstrated. The first hologram spatially redistributes the incident light, and the second hologram produces a desired phase front on the redistributed light.

High-efficiency hybrid computer-generated holograms.

A hybrid hologram is constructed which combines the flexibility of a computer-generated hologram (CGH) with the high-diffraction efficiency of volume phase holograms. The new hologram is constructed by using a spatially filtered wave front from a conventional 246- x 256-cell CGH as the object wave for the recording of a volume hologram. Theoretical and experimental results show that simultaneous high-diffraction efficiency and reconstruction fidelity are possible.

Coordinate Transformations Via Multifacet Holographic Optical Elements

Arbitrarily prescribed space variant operations can be carried out with high efficiency by multifacet holographic optical elements recorded in dichromated gelatin. In this paper, such elements are used to perform coordinate transformations from Cartesian to polar coordinates. Theoretical calculations of resolution limits and experimental results are given.

Fourier processing in the object plane

Optical Fourier processing is carried out by placing volume-hologram filters in the object plane of an optical system instead of in the conventional Fourier plane. This new type of Fourier processing is analyzed, and experimental results of optical differentiation by a volume-hologram filter are shown.

Image formation by multifacet holograms

A method is described in which arbitrary images are formed via Fresnel diffraction. In this method, a multifacet hologram is used to deflect small square areas of light from a given spatial location in the input plane to an arbitrarily prescribed location in the output plane, producing an image composed of small square patches of light. Five different variations of multifaceted holography are presented. Volume phase holograms are used in the deflection process resulting in nearly 100% efficiency.

3-D Vision System Analysis and Design

Structured light vision systems offer a practical solution to many 3-D inspection problems. This paper examines the tradeoffs involved in the design of 3-D inspection systems that use triangulation. General equations describing the behavior of such systems are derived. Problems common among structured light systems are explored and alternative solutions presented. The implementation of a multi-stripe system is discussed.

Partitioned Holographic Optical Elements

A new type of holographic optical element is demonstrated which combines some of the flexibility of computer generated holograms with the large space-bandwidth product and high diffraction efficiency of interferometrically recorded volume phase holograms. The optical element is recorded by subdividing a volume hologram film surface into numerous small areas (facets), each of which is individually exposed under computer control. Each facet is used to produce a portion of the desired final wavefront. This new method allows the rapid production of high efficiency, customized, space-variant optical elements. A holographic cylindrical lens which has greater than 90% light efficiency and forms a line focus on a curved surface is demonstrated.

Holographic Beamsplitter For Multiple Plane Wave Formation With Matched Phasefronts

The design and fabrication of a holographic beamsplitter that will produce multiple phase and amplitude matched output beams from a collimated laser beam is discussed. Multiple exposures are recorded in a dichromated gelatin (DCG) plate to make the output beams seem to emanate from a single point with high diffraction efficiency and low scattering. During fabrication, a technique was developed to verify that the relative phasefronts of all the output beams are matched. The holograting can be designed such that any relative phase, amplitude and angle between the output beams as well as different recording/readout wavelengths can be specified for a variety of applications. A deterministic method of calculating the optimum Bragg angles for the recording beams has been developed to compensate for the wavelength shift between the recording laser and the readout laser. A special emphasis is given to the application of using a laser diode for readout. Specifically, a double grating method is utilized for making the element less sensitive to wavelength drift common to laser diodes. A single small component has been fabricated with output beams in-line with respect to the input beam axis such that it can be configured into compact and rugged optical systems.