High-performance SAW dispersive delay line using reflective thin metal dot arrays

The first U-path surface-acoustic-wave (SAW) reflective array compressor (RAC) employing reflective arrays of thin metal dots on lithium niobate is reported. The thin rectangular metal dots are made together with the transducers in a single photolithographic step. Performance comparable with that of large time-bandwidth, conventional, grooved RACs is demonstrated, in good agreement with the design.     

Opto-electronic high order feedback neural network

A novel neural network design, in which nonlinearities are created by feedback, is described. It is called the HOFNET. The design is suitable for optical implementation because it is tolerant of the limited dynamic ranges present in optical systems. An optical system with electronic feedback was constructed and its operation is described.<>     

Variable numerical-aperture temporal-coherence measurement of resonant-cavity LEDs

The first interferometric measurements of temporal-coherence length variation with numerical aperture (NA) are described for 650 nm, resonant-cavity light-emitting diodes (LEDs) agreeing with spectrally derived results. The interferometrically measured coherence length (22 /spl mu/m to 32 /spl mu/m) reduced by 37% for a 0.42 increase in NA. For a larger range of NA (0-1), this would give coherence lengths (10 /spl mu/m-40 /spl mu/m) lying in the gap between that of conventional LEDs (/spl sim/5 /spl mu/m) and superluminescent diodes (/spl sim/60 /spl mu/m).     

High-sensitivity detection of narrowband light in a more intense broadband background using coherence interferogram phase

This paper describes an optical interferometric detection technique, known as the interferogram phase step shift, which detects narrowband, coherent, and partially coherent light in more intense broadband incoherent background light using changes in the phase gradient with the optical path difference of the coherence interferogram to distinguish the bandwidth or coherence of the signal from that of the background. The detection sensitivity is assessed experimentally by measuring the smallest signal-to-background ratio or signal-to-clutter ratio (SCR), which causes a detectable change in the self-coherence interferogram phase. This minimum detectable SCR (MDSCR) is measured for the multimode He-Ne laser, resonant-cavity light-emitting diode (LED), narrowband-filtered white light, and LED signal sources in a more intense tungsten-halogen-lamp white-light background. The highest MDSCRs to date, to the authors' knowledge, are -46.42 dB for coherent light and -31.96 dB for partially coherent light, which exceed those of existing automatic single-domain techniques by 18.97 and 4.51 dB with system input dynamic ranges of 19.24 and 11.39 dB, respectively. The sensitivity dependence on the signal-to-system bandwidth ratio and on the relative offset of their central wavelengths is also assessed, and optimum values are identified.     

Spatial uniformity measurement of SAW convolvers

For correlation of coded waveforms, the nonlinear interaction process in a SAW convolver should ideally give an amplitude and phase independent of position. A new experimental method of measuring this spatial uniformity used CW test waveforms and gave a resolution of 25 ns?a considerably higher resolution than previously attained.     

Spatioangular-multiplexing scheme for dense holographic storage

A novel multiplexing scheme for dense holographic storage in photorefractive crystals, spatioangular multiplexing, is described in detail. Compared with spatial multiplexing, spatioangular multiplexing increases the storage capacity by fully utilizing the volume of the storage medium. On the other hand, spatioangular multiplexing reduces the number of holograms overlapping any one hologram in a given volume and so increases the diffraction efficiency achievable as compared with angular multiplexing. Using this scheme, we succeeded in storing 756 high-resolution patterns in an Fe:LiNbO(3) crystal of volume 1 cm(3), with an average diffraction efficiency of 0.5%. This large database is designed for practical use in a novel associative-memory system, called a high-order feedback neural network.     

Narrow-aperture chirp transducers for SAW convolvers

Narrow-aperture SAW beams can be generated efficiently using chirp transducers, thus avoiding the need for beam compression in convolvers. The transducers can be designed such as to compensate for phase errors due to dispersive SAW propagation. Experimental confirmation was obtained using convolvers with 120 MHz bandwidth and 16 ?s interaction length, and a bilinearity factor of ?65 dBm was achieved.     

Insertion Loss and Misalignment Tolerance in Multimode Tapered Waveguide Bends

Experimental measurements of laterally tapered multimode waveguide bends fabricated photolithographically on FR4 printed circuit board establish that the product of the mean insertion loss (in linear units) and the mean source misalignment tolerance is a constant which depends only on the taper ratio TR (input width/output width) and not on the radius of curvature R. The minimum loss of 0.78 dB occurs in the special case of TR = 0.8, R = 14 mm. Together, these form waveguide layout design rules for board-to-board and chip-to-chip optical interconnects.