QUANTUM IMAGES AND CRITICAL FLUCTUATIONS IN THE OPTICAL PARAMETRIC OSCILLATOR BELOW THRESHOLD

In the first part of the paper we define the spatial spectrum of squee zing following an operational procedure, and we show in general how th e spatial structure of squeezed states can be described in terms of co rrelation functions of quadrature components of the electric field. In the second part we formulate an appropriate quantum model for a degen erate optical parametric oscillator (OPO) with plane mirrors, we analy ze it extensively below the threshold for signal generation The correl ation length diverges when the OPO approaches threshold; this picture provides an ideal completion of the analogy with critical phenomena in second-order phase transition at equilibrium. The spatial configurati on of the correlation function exhibits the phenomenon of ''quantum im age:'' the signal held, which is purely generated by quantum noise, sh ows an ordered spatial structure which is observable via the spatial c orrelation function.