Extraction characteristics of a dual fiber compound cavity

We study experimentally the time dependence, steady state behavior and spectra of a dual fiber-laser compound cavity. Theoretically we confirm the CW and spectral characteristics. This particular cavity is formed with two Er-doped fiber amplifiers, each terminated with a fiber Bragg grating, and coupled through a 50/50 coupler to a common feedback and output coupling element. The experiment and theory show that a low Q, high gain symmetric compound cavity extracts nearly 4 times the power of a component resonator. This extraction is maintained even when there is significant difference in the optical pathlengths of the two component elements. Further, our measurements and theory show that the longitudinal modes of the coupled cavity are distinct from the modes of the component cavities and that the coherence is formed on a mode-by-mode basis using these coupled-cavity modes. The time behavior of the compound cavity shows slow fluctuations, on the order of seconds, consistent with perturbations in the laboratory environment.