Quantum noise analysis of frequency-doubling ring lasers: nonlinearity-induced noise enhancement and its dependence on gain profile

Using a traveling-wave model that can treat the spatial evolution of light fields, quantum noise characteristics of frequency-doubling ring lasers are analyzed under single-frequency operation. Systems under consideration are the discrete system, in which a laser crystal and a nonlinear crystal are separately installed, and the self-doubling system, which is equipped with a nonlinear crystal with laser gain. It is predicted that the unsaturated gain profile does not affect noise characteristics of the discrete system: nonlinear loss-induced noise enhancement (NINE), i.e., an increase of the spontaneous emission noise associated with enlarged gain that compensates the nonlinear loss, manifests itself at a specific operating point regardless of the gain profile. In the self-doubling system, on the other hand, the manifestation of NINE will arise for higher harmonic output compared with that in the discrete system. For the self-doubling system, moreover, the profile of the unsaturated gain causes a significant difference in NINE; because NINE is associated with a large field variation, it is effectively suppressed by configuring the gain profile to be flat or increasing along the direction of laser field propagation.