A Large-Capacity Optical Correlator by Wavelength-Angle Multiplexing

Abstract

A novel approach of building a large-capacity optical correlator using Vander Lugt's matched spatial filters is introduced. The system capacity is substantially increased by utilizing a wavelength-angle multiplexing scheme. An optical correlator consisting of a tunable dye laser as the light source and an optical dispersive element such as a diffraction grating placed in the input plane is presented. While sequentially tuning the wave-length of the line emitted from the dye laser and rotating the diffraction grating about the optical axis, the input signal spectrum is scanned along a trajectory of concentric rings in the Fourier plane. With this 2-D scanning approach, up to 1000 MSFs can be spatially multiplexed on a single holographic plate due to the more effective use of the system space-bandwidth product. In synthesizing the spatially multiplexed MSFs a monochromatic laser is used instead of a dye laser since the latter generally does not provide adequate coherence length. In order to accommodate wavelength shifting between the filter recordings and signal correlation detections, a compensation technique which includes the scaling of input signal spectrum and reference beam angle according to the wavelength's ratio is utilized during the filter's fabrication. The large capacity of the optical correlator can be used effectively at high speed and accuracy under electronic/computer control for either multiple-signal detections or for the recognition of a single object with scale and orientation variations. Several illustrated experimental demonstrations are also presented.