Optical tunnel-trap detector for radiometric measurements

We have designed and evaluated an optical trap detector that is simple to fabricate and convenient to use. The trap geometry is a triangular tube similar to a kaleidoscope, with two photodiodes in place of a mirror on each side. One version incorporates six 10 mm × 10 mm silicon photodiodes in a detector having a 7 mm diameter aperture. Spectral responsivity measurement results indicate that the quantum efficiency, evaluated at 10 nm increments from 450 nm to 950 nm, is greater than 99 %. Another version incorporates six 18 mm × 18 mm silicon photodiodes in a detector having a 12 mm diameter aperture. Spectral responsivity measurement results from this instrument are similar to those from the other one, but the quantum efficiency is lower. Spatial uniformity measurement results for the larger detector indicate that, over the 12 mm diameter area, response variations are less than 0.05 %. We have evaluated several fields-of-view and confirmed that, in a special case, the detector geometry will accommodate an input beam having a divergence as large as 14°. Design considerations are briefly described and may be used to create similar, scaled versions with other photodiode types and sizes.