Generating highly squeezed hybrid Laguerre-Gauss modes in large-Fresnel-number degenerate optical parametric oscillators

We theoretically describe the quantum properties of a large-Fresnel-number degenerate optical parametric oscillator (DOPO) with spherical mirrors that is pumped by a Gaussian beam and is tuned at the subharmonic frequency to a given transverse-mode family. We first analyze the classical problem and find that only the Laguerre-Gauss modes with lowest orbital angular momentum (OAM) are amplified above threshold. The transverse symmetry of the classically emitted signal field depends on the family index f. If f is even the lowest available OAM is zero and emission occurs in a radially symmetric mode; on the contrary, if f is odd the lowest available OAM is 1 and the emitted signal field has the shape of a hybrid Laguerre-Gauss mode (a linear combination of the two Laguerre-Gauss modes with OAM equal to +/- 1) that breaks the rotational invariance of the system. Next we focus on the squeezing properties of this DOPO model. As for the modes with lowest OAM we demonstrate that their quantum properties (in the linear approximation) are equal to the standard single-mode DOPO (for even f) or to the recently analyzed DOPO tuned to its first transverse-mode family [C. Navarrete-Benlloch, E. Roldan, and G. J. de Valcarcel, Phys. Rev. Lett. 100, 203601 (2008)]. Concerning the rest of (classically empty) modes (having larger OAM) we find that combinations of Laguerre-Gauss modes with opposite OAM (hybrid Laguerre-Gauss modes) exhibit quadrature squeezing. This property is independent of the even or odd character of the family index and hence has nothing to do with the symmetry of the classically emitted signal field. Noticeably the amount of squeezing does not depend on the pump level (it is thus noncritical squeezing) and can be arbitrarily large for the lower OAM nonamplified modes.