360-Degree Star Coupler for Detecting Sub-Wavelength Features

We propose a novel 360-degree star coupler. We show theoretically how this coupler can detect perfectly conducting cylinders with radii significantly smaller than a wavelength, in the far field. We experimentally demonstrate detection of a metal probe using the 360-degree star coupler in a silica planar lightwave circuit. With improvements, this coupler may be able to perform sub-wavelength 2-dimensional (2-D) imaging.