Special Report: Optical Patterns

In the system, the Fourier spectra due to different microlasers can overlap one another if the separations between neighboring microlasers are adjusted to match the period of an SLM in the filter plane. Although a matched filter was synthesized with a single reference pattern, it is duplicated at many periodic locations in the Fourier plane and matches the multiple input spectra. In this way, an efficient space-bandwidth product in the filter plane is increased and sharper correlation peaks are obtained. Such an approach would be extremely useful for increasing the reliability of a conventional optical correlator. The only practical issues here are generation of reference beams that are coherent with object beams to record holograms and the removal of undesired fringes in an autocorrelation peak. The authors suggested a time-division-multiplexing (TDM) scheme as a way to eliminate interference fringes.

12.3.4 Compact and robust incoherent correlator As explained above, the light from a microlaser is highly monochromatic. However, the phases of the light from the microlasers are not locked with each other. These two unique coherence properties (temporally highly coherent and spatially incoherent) make a SELDA an ideal light source for implementing a compact and robust incoherent correlator.

Fig. 12.10. Compact and robust incoherent correlator: (a) system, (b) experimental results.

Fig. 12.10 (a) shows an example of the microlaser-based incoherent correlator [67]. The light from each SEL is collimated by lens L1 and illuminates the hologram to reconstruct holographic images on the output plane which is located at the focal plane of lens L2. The image generated by each SEL is shifted by the amount that corresponds to the position of the SEL. The reconstructed images generated by the light from different SEL's are added up incoherently because each laser operates independently, averaging out the phase-sensitive interference terms. The eventual summation of all the reconstructed images generated by all the SEL's in the input plane gives the correlation between the input and the reference image stored on the hologram. Because the system does not involve any moving parts (e.g., rotating diffuser) or bulky optical components, the whole system can be miniaturized and integrated with semiconductor technologies. Fig. 12.10 (b) shows the correlation output obtained from the SELDA correlator. A holographic filter was fabricated for the input pattern (Bell logo) and was tested for the two input patterns (the Bell logo and a Chinese character meaning light). For the correct input [Bell logo (middle)], a bright autocorrelation peak appears at the center of the correlation output (middle, right). On the other hand, for the incorrect input [Chinese character (bottom)], a cross-correlation is obtained (bottom, right). As shown in the figure, the cross-correlation signal is much weaker than the autocorrelation peak, allowing a satisfactory discrimination between the two input patterns.

12.4 Holographic memory readout with microlasers

12.4.1 Introduction Volume holographic memory has been extensively investigated in the past as a way of massively storing media to allow fast random access page-organized memory [68-72]. This volume holographic memory has reawakened its interest recently because of the immense demand for storage media with fast access and a large storage capacity for applications such as multimedia. However, the holographic system normally requires bulky and complicated beam deflectors to steer the beam direction from one to another, corresponding to the desired page. Moreover, speed and resolution of a beam deflector are quite limited.

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Optical Patterns

 

 

 

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