Abstract: Method and system for converting a first light beam having a first frequency into a second light beam having a second frequency. A pump photon having (angular) frequency 3&ohgr; (or 4&ohgr;) is provided within an optical cavity, defined by two mirrors that are fully reflecting at a frequency 2&ohgr;. In a first approach, the pump photon is received and down-converted (3&ohgr;→2&ohgr;+&ohgr;) by a first nonlinear crystal, and the 2&ohgr; photon is further down-converted (2&ohgr;→&ohgr;+&ohgr;) by a second nonlinear crystal), to produce three photons (&ohgr;+&ohgr;+&ohgr;) that exit from the cavity. In a second approach, the pump photon is received and down-converted (4&ohgr;→2&ohgr;+2&ohgr;) by a first nonlinear crystal, and one or both of the 2&ohgr; photons is further down-converted (2&ohgr;→&ohgr;+&ohgr;) by a second nonlinear crystal).