Optical-Mode Demultiplexing by Optical MIMO Filtering of Spatial Samples

An optical spatial-mode demultiplexer is proposed consisting of an optical sampler taking probes from the aperture of the multimode fiber, which are then launched into an optical multiple-input multiple-output (MIMO) filter. A theoretical analysis of coupler matrices of three sampler options reveals that samplers can be characterized by a mode-dependent sampling loss and sampling crosstalk arising from a lacking orthogonality of sampling vectors of different modes. The theoretical optimum MIMO allpass filter can be realized by a waveguide structure consisting of cascaded and in parallel operating directional couplers. The multiplexer is scalable to higher mode numbers due to the absence of splitting loss.