Removal of CW pedestal from optical signal using nonlinear element in ring laser

We analyze and simulate numerically a bidirectional intracavity field of a ring laser with a χ(2) nonlinear crystal for sum frequency generation placed within the cavity. Injecting in the nonlinear crystal from one side a beam with a large CW component and a small-amplitude modulation, and from the other side a time-continuous bias beam, we demonstrate numerically that it is possible to reproduce the time-varying portion of the injected light in the circulating modes. This effectively subtracts the CW pedestal from the signal beam. At the same time, the wavelength of the detected light is converted to one that may be more suitable for efficient photo-detection. Our analysis includes rate equations that model two external counter-propagating beams injected in the ring laser cavity and mixed in the nonlinear crystal with the ring laser modes with which they co-propagate.