Low-noise photodiode detector for optical fluctuation diagnostics

The beam emission spectroscopy optical fluctuation diagnostic requires the highest possible quantum efficiency detector at 656 nm to minimize the photon statistical baseline limit to the detectable fluctuation level. A photoconductive photodiode detector with an extremely low-noise preamplifier and a reactive feedback circuit provides quantum efficiencies up to 70%--80% for a useful frequency range of at least 0--150 kHz with incident powers of ∼10 nW. The diodes are chosen for negligible leakage current and hence do not require active cooling. These detectors have provided increase in the sensitivity to plasma fluctuation amplitude by a factor of ∼14 over photomultipliers and a factor of 4 over large area avalanche photodiodes