Optimization of radial angular filter arrays for detecting the angular distribution of light

The Radial Angular Filter Array (RAFA) is a novel silicon micro-machined optical filter for real-time high resolution measurement of the angular distribution of scattered photons. It includes a radially-distributed series of micro-channels facing a focal point that is several millimeters away from the edge of the device. In this work, three RAFA designs were devised to enhance the angular resolution, while lessening the complexity of the output coupling. These new RAFA designs solved issues associated with a previous prototype device, including the signal loss in high angle channels and light leakage beyond the acceptance angle. Typically, channels in the RAFA are 60 μm deep with a minimum length of 10 mm. To characterize the RAFA designs, we used an incoherent broadband source, collimation optics, turbid samples, and a spectrometer. The tests identified which design features resulted in improved performance, including the preferred output coupling structure, the recommended near specular direction blocking range, the choice of constant aspect ratio or solid angle, and other geometrical parameters.