Optical tunnel-trap detector for radiometric measurements

We have designed and evaluated an optical trap detector that is simple to f abricate and convenient to use. The trap geometry is a triangular tube simi lar to a kaleidoscope, with two photodiodes in place of a mirror on each si de. One version incorporates six 10 mm x 10 mm silicon photodiodes in a det ector having a 7 mm diameter. aperture. Spectral responsivity measurement r esults indicate that the quantum efficiency, evaluated at 10 nm increments from 450 nm to 950 nm, is greater than 99 %. Another version incorporates s ix 18 mm x 18 mm silicon photodiodes in a detector having a 12 mm diameter aperture. Spectral responsivity measurement results from this instrument ar e similar to those from the other one, but the quantum efficiency is lower. Spatial uniformity measurement results for the larger detector indicate th at, 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 diverge nce as large as 14 degrees. Design considerations are briefly described and may be used to create similar, scaled versions with other photodiode types and sizes.