Noise-optimized silicon radiometers

This paper describes a new, experimentally verified, noise analysis and the design considerations of the dynamic characteristics of silicon radiometer s. Transimpedance gain, loop gain, and voltage gain were optimized versus f requency for photodiode current meters measuring ac and de optical radiatio n. Silicon radiometers with improved dynamic characteristics were built and tested. The frequency-dependent photocurrent gains were measured. The nois e floor was optimized in an ac measurement mode using photodiodes of differ ent shunt resistance and operational amplifiers with low l/f voltage and cu rrent noise. In the dark (without any signal), the noise floor of the optim ized silicon radiometers was dominated by the Johnson noise of the source r esistance. The Johnson noise was decreased and equalized to the amplified l /f input noise at a 9 Hz chopping frequency and 30 s integration time const ant, resulting in an equivalent root-mean-square (rms) photocurrent noise o f 8 x 10(-17) A. The lowest noise floor of 5 x 10(-17) A, equal to a noise equivalent power (NEP) of 1.4 x 10(-16) W at the 730 nm peak responsivity, was obtained at a 100 s integration time constant. The radiometers, optimiz ed for ac measurements, were tested in a de measurement mode as well. Perfo rmances in ac and de measurement modes were compared. In the ac mode, a ten times shorter (40 s) overall measurement time was needed than in the de mo de (400 s) to obtain the same 10(-16) A noise floor.