<title>From computer-generated holograms toward partially coherent optical signal processors</title>

Abstract

Optical signal processing has its roots in the experiments of Lord Rayleigh, Abbe and Porter that were the first that dealt with the spectrum of an image. This historical path of revolution has been followed years later by the extraordinary work of A. W. Lohmann in the optical data processing during the last 40 years. The new innovations and the future possibilities that are to be opened up in the new millennium show his dominant signature. Some recent projects that enlighten the future ofthe optical signal processing field are described in this presentation. The invention of the computer generated holograms (CGH) was a giant leap for the optical signal processing field. Filters and holograms that previously were generated by direct holographic recording means, have been all of a sudden replaced by synthetic functions designed and realized by digital computers. It was the first interface between digital computers and optical systems. Such an approach led to the design of opto-electronic systems that operate in perfect synergy where each element is utilized for what it does best. Along the years, the field of computer-generated holography has been expanded to what is known as: diffractive optical elements. Techniques like kinoforms, binary optics, on-axis CGH, etc. have been developed for addressing the growing application list ofsuch elements. CGHs highly affected the optical signal-processing field. For example, various new processing techniques were created and applied for invariant pattern recognition (Circular Harmonics (CH), Synthetic Discriminate Filters (SDF) etc). The next innovation wave reached the shores of the optical data processing community in the 80s when A. W. Lohmann presented the optical interconnections as the next challenge of optical data processing. Many configurations were discussed, investigated and applied for optical processing (perfect shuffle, omega net, cross over etc.). In the 90s A. W. Lohmann was a key player in a new revolution in optical processing where optics was used as a transformation tool. New transformations were invented and realized by optical means as for instance Fractional Fourier Transform, Wigner distribution, Fractional Hubert and Hartley Transforms etc. Those were applied for various signal-processing applications and used also in digital processing. In the new millenium optics adapts itself to the binary mode of operation that is common in computer systems. This trend becomes feasible also due to the impressive progress in the opto-electrical interface devices such as the spatial light modulators, light sources such as VCSELs and detectors such as photo-diodes. These new achievements permit also the operation of opto-electronic systems at extremely high rates. It is evident that in the next years of the millenium optical data processing field will continue to grow, develop and replace additional processing modules in the digital computation world. Without much doubt those will be accompanied and innovated by the scientific assistance foundation and guidance of A. W. Lohmann. In this paper we will focus on optical processing of partially coherent light. This field is mostly interesting and relevant since it includes both the aspects of data processing and the optical design skills that insure its promising industrial future.