Integrated holographic encoder for wavelength-hopping/time-spreading optical CDMA

Abstract

AWG size and the fact that typically two separate elements provide demultiplexing, color-dependent time delaying, and recombination of delayed wavelength chips even monolithic AWG-based decoders have relatively large footprints [16]. HBR-based coding provides spectral slicing and time delay in one element in a footprint potentially an order of magnitude smaller than the AWG-based approach. Furthermore, precision lithographic fabrication and a novel apodization approach [6] provide unprecedented control over the encoder spectral transfer function through amplitude and phase apodization of individual diffractive elements comprising the HBR. This allows one to spectrally shape wavelength chips to exhibit steep falloff and have flat-top passbands so that they can be placed in immediate spectral adjacency to each other, making possible efficient use of broad-band light sources. The integrated holographic OCDMA encoder is shown in a schematic top view in Fig. 1(a). Two HBRs are heavily interleaved along the input beam direction. HBR 1 (HBR2), consisting of eight interleaved grating segments indicated by dashed (dotted) lines, connects the input port (IN) to output port OUT1 (OUT2) with a port-specific transfer function corresponding to a distinct time-wavelength code. Data encoding occurs by launching a brief pulse whose spectral width equals or exceeds the code spectral width into the input port. The spectral transfer function connecting a given input-output