S. Sottini

Geodesic Lenses for Guided Optical Waves

Two-dimensional lenses are essential elements in integrated optics. As an alternative to the techniques already suggested and tested to obtain such optical systems, a different method is reported that consists of extending to thin film optics the principle of configuration lenses already studied for application to microwave antennas. The working principle of such lenses is that the propagation occurs along a curved surface in a two-dimensional Riemann space. The rays are the geodesics of the surface. Two optical geodesic lenses having planar input and output have been constructed and tested. They can be easily inserted in planar film circuits. The advantage of these optical systems is that the substrate can be prepared in advance with the accuracy of glass optics. Then the dielectric film can be deposited with the same method used for planar components.

General solution of the problem of perfect geodesic lenses for integrated optics

The problem of designing waveguide geodesic lenses for integrated optical processing is considered. Geodesic lenses are, at present, the best solution whenever crystals of high refractive index must be used as substrates. A good geodesic lens has to be constituted by a surface of rotation, coupled without discontinuities with the planar optical circuit. The present paper derives, by an analytical procedure, the general expression for the profile of aspherical geodesic lenses able to form perfect geometrical images of the points of two given concentric circles on each other. In particular, the equations that characterize a family of lenses having two conjugate foci external to the lens depression are given. The case of one external and one internal focus is only outlined because it is less important in practice. The advantage of this analytical method is represented by the clear theoretical approach that allows a large flexibility in the lens design without long expensive computations.

Photobleaching of polydiacetylene waveguides: a characterization of the process and patterning of optical elements

The photobleaching process of poly-3butoxyl-carbonyl-methyl-urethane (poly-3BCMU) waveguides by means of an UV lamp and the 488-nm line of an Ar laser is characterized and modeled. The limits of the theory are discussed in light of experimental results, and we stress the role of the oxygen diffusion rate on the process. Finally, we adopt the photobleaching method to pattern a guided-wave micro-optic device and holographic diffraction gratings on spun poly-3BCMU films.

Geodesic optics: new components

In integrated optics, geodesic lenses show some peculiar characteristics that have attracted increasing attention. Such properties also suggested the realization of components other than lenses, that is, of geodesic elements capable of performing operations different from focusing. In this paper we present a new class of geodesic components able to steer, deflect, or spread a guided beam. Such components have been designed following the same analytical procedure. They have no connection apart from the vertex in some cases. A first geodesic corner reflector has been realized in the laboratory. Some preliminary results are shown and discussed.

Reflection Holographic Filters for Compacting Optical Processors

Character recognition experiments, carried out with reflection matched filters, are described. The reflection filters are obtained by means of volume holograms, recorded in photographic emulsion and suitably processed. These filters allow a good discrimination among similar characters and exhibit large positioning tolerances. They turn out to be well comparable with transmission hologram filters. An advantage of reflection filters is that the filtering operation can be performed with one-lens processor, thus reducing by one-half the size and the cost of the processor.

Fabrication tolerances in geodesic lenses: A rule of thumb

A rule of thumb has been derived for geodesic lenses which allows one to evaluate, with a good precision, the change of the focal length as a function of the depth change Delta z_0 due to fabrication errors. This rule of thumb has been tested on a large number of perfect geodesic lenses designed following a relatively simple analytic procedure. Considering the use of geodesic lenses in an integrated data processor with a high resolution detector array, the thumb rule can be used with an accuracy particularly good to evaluate the increase of the beam spot size at a prespecified focal line, due to the error Delta z_0 . It has been confirmed that, in order to build lenses with resolution capability compatible with a 12-{mu} m spacing of the detector array, we have to get a precision control of sim 0.5 {mu} m during the fabrication. Such a tolerance seems to be achievable at least in the near future.

Low noise and good efficiency volume holograms.

Techniques for improving diffraction efficiency by bleaching conventional absorption holograms, recorded as silver images, have been widely described. Unfortunately, the reconstructed image quality tends to be degraded by an increase of noise-light. Moreover experiments have confirmed the tendency of the bleached silver emulsions to darken, and consequently holograms get worse after a long exposure to light. Attempts have been made to minimize these effects by handling photographically recorded holograms with different bleaching baths and different procedures. An alternative solution is represented either by the use of techniques that yield final holograms that do not contain silver salts or by the use of different recording materials, such as dichromated gelatinand photopolymers. In this letter we present a simple bleaching-plus-darkening procedure, applied to volume (Lippmann-Bragg) holograms, recorded on Kodak 649-F photographic plates. Our purpose was to obtain a compromise between high efficiency and low noise together with light insensitivity of the final hologram. The diffraction efficiency was improved by means of a salted bleaching bath, while scattered noise and light sensitivity were minimized by a successive darkening bath. A two-steps process similar to the above mentioned is commonly used in photography to intensify negatives. We used a darkening bath that yields a final hologram with density equal or a little less than the original one. The processing steps for Kodak 649F plates, conventionally exposed are: