INTENSITY FEEDBACK EFFECTS ON QUANTUM-LIMITED NOISE

We examine the quantum-limited behavior of an electro-optical intensit y feedback loop and present a simple theory and experimental data show ing excellent agreement. We show that, although the light incident upo n the in-loop detector may be sub-Poissonian, this light has unique pr operties different from those of a freely propagating beam of intensit y squeezed light. We support this by presenting the results of homodyn e measurements of the phase noise of light extracted from the loop. Th e utility of the in-loop field is discussed, and it is shown that in a ll cases in which linear optical components are used, no advantage in signal-to-noise ratio is gained by taking measurements by using this l ight rather than a coherent source. We also discuss effects seen in th e extracted or out-of-loop light. We demonstrate the existence of, and derive an expression for, an optimum gain for suppressing low-level c lassical noise. Conversely, in the high-gain limit, we demonstrate tha t the extra noise seen in the out-of-loop photocurrent that is due to the feedback process is expressible purely in terms of the mean photoc urrents involved. Lastly, we introduce the novel concept of using an i ntensity feedback loop in conjunction with a squeezed source and show that the feedback loop has the capability of electronically transferri ng squeezing from one light beam to another. (C) 1995 Optical Society of America