A spread spectrum approach to diffusive optical spectroscopy and imaging

This work presents the latest simulations and experimental results of an innovative module for functional diffused optical imaging based on double nested encoding of wavelength and position of the emitting light. A spread spectrum approach to NIR (near infrared) diffusive optical imaging was recently presented: a pseudo-random bit sequence (PRBS) is used to modulate the light emitted by a continuous emission (CW) laser diode, before entering a turbid medium; when the coded excitation sequence propagates through the tissue, it is split into a group of components that have different path lengths going into the detector. Correlation of the detected signals with the excitation sequence can pick up each component with a specific delay, with the consequence of obtaining time domain (TD) information using cheaper CW sources. We propose to generalize the approach, adopting a two-level code division multiple access (CDMA) technology in an innovative module which is expected to enhance the performances of diffusive imaging systems. We called this technique WS-CDM (wavelength and space code division multiplexing). We describe the module architecture and show the result of numerical simulations; furthermore, experimental activity aimed at validating the approach with two optical signals and related replicas generated by a fading-affected channel is being carried on and preliminary encouraging results will be shown.