Uniformity of concentration factor and back focal length in molded polymer microlens arrays

An array of polymer microlenses, made of 32×32 individual elements spaced by a 50-∞m pitch, is used in connection to an array of 32×32 6-∞m diameter SPADs (Single Photon Avalanche Detector), with the aim of mitigating the loss in sensitivity due to the area fill-factor. The lens array is fabricated by polymer casting in a photoresist replica mold. Results about repeatability of concentration factor and back focal length are reported. At C= 35 and a focal length F=40 ∞m, the spread of concentration is < 6%, and that of BFL is <0.5 ∞m.