Optical correlator

Optical absorption-based correlation spectroscopy is a means of selectively detecting, or "fingerprinting", gases, in this work, of industrial importance. Optical correlation spectroscopy methods of gas detection are... 

Fig. 26. Photograph of a compact version of the optical correlator for security verification that uses photorefractive polymers...

In the second part of this paper we describe the photoconductive properties of a polysilane and the realization and performances of an organic spatial light modulator for optical correlation using this polymer. 

That SPAs can have both electrical I/O as well as optical I/O renders them potentially useful to any application that interfaces between optical and electronic realms. Such applications are more numerous than already discussed in this section. The emerging SPA technology will likely result in new applications emerging for years to come. Some additional applications not previously mentioned are the parallel-processing planes of pipeline architectures (useful for pixel-level image processing) the various levels of optical neural networks the input, filter, and/or output planes of optical correlators and optical phased-array beam steering. 

Nagayama N, Yokoyama M. Optical correlation system by use of photorefractive polymer. Mol Cryst Liq Cryst Sci Tech Sect A 1999 327 19-22. 

Fig. 6.65 Optical correlator with translation-retroreflecting prism and second-harmonic generation...

All these devices, which are called optical correlators, measure the correlation between the field amplitude Eit) or the intensity lit) at the time t and its values Eit + r) or lit + x) at a later time. These correlations are mathematically expressed by normalized correlation functions of order k. The normalized first order correlation function... 

During recent years the development of fast photodetectors has made impressive progress. For example, PIN photodiodes (Sect. 4.5) are available with a rise time of 20 ps [11.100]. However, until now the only detector that reaches a time resolution slightly below Ips is the streak camera [11.101]. Femtosecond pulses can be measured with optical correlation techniques, even if the detector itself is much slower. Since such correlation methods represent the standard technique for measuring of ultrashort pulses, we will discuss them in more detail. 

Liquid crystal modulators, placed on the plane, where the Fourier image of the object is formed (Fig. 8.10), are widely used as one of the important parts of optical correlators [60] used, in particular, in real-time object tracking systems [61]. Liquid crystal modulators could be made as a-Si H-FLC light valves [62] or TV modules [61, 63], working in a special video feedback regime. 

PhotMefractive (PR) polymers are multifunctional field-responsive materials that combine photoconducting and electro-optic propoties. hi this charter, we will review the basics of photorefiactivity in polymers and liquid crystals and describe recent advances that have led to highly efficient materials. These advances in material development enable a variety of photonic aiqilications including optical correlators for security verification. 

This work was supported by the US Office of Naval Research (ONR) through the MURl Center CAMP, by the US National Science Foundation (NSF), by an international CNRS/NI travel grant, by the USAF Office of Scientific Research (AFOSR), and by aNATO travel grant. E. H. is a postdoctoral fellow of the Fund for Scierrtific Research-Flanders (Belgium). We acknowledge coUabrnation with D. D. Stede, J. L. Maldonado, and B. L. Volodin from the University of Arizona, and with Prof. B. Javidi from the Uruvoaity of Connecticut for the work on tte optical correlator. 

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