Squeezed state of light at doubled frequency in an external ring cavity

A physical interpretation of the formation of squeezed light at the doubled frequency is proposed. A quantum theory of second-harmonic generation in a ring cavity, with a resonance at the second harmonic, is developed. The efficiency of squeezing quantum fluctuations of the quadrature components is considered as a function of the mirror reflection coefficients and of the efficiency of conversion to the second harmonic in one cavity trip.