SUB-POISSONIAN LIGHT AND PHOTOCURRENT SHOT-NOISE SUPPRESSION IN CLOSED OPTOELECTRONIC LOOP

Photocurrent shot-noise suppression is observed in closed optoelectronic loop below the limit (1 - eta) of standard shot-noise level (SNL) imposed by conventional theory of photodetector with quantum efficiency eta. Experimental results on photocurrent noise are presented for different values of feedback strength and quantum efficiency eta of in-loop photodiode. It is shown that at fixed feedback strength the suppression factor does not depend on eta. Minimum noise spectral density is observed below the SNL by a factor 17.8 (- 12.5 dB). Such large suppression cannot be explained in terms of an ordinary sub-Poissonian light state in the loop. We suggest the concept of an anticorrelation light state, where the light in the loop is described as a non-stationary state with Poissonian quantum fluctuations, and superimposed anticorrelated classical fluctuations. While the degree of noise reduction in the photocurrent can be arbitrary large, the degree of anticorrelation between the classical and quantum fluctuations in the light beam remains limited by the quantum efficiency of the feedback photodiode. Calculations based on this concept are in good agreement with experimental data on super-shot-noise power, obtained for the beam extracted from the loop by a beam splitter.