Modeling rare-earth doped microfiber ring lasers

We propose a compact laser configuration based on resonating both the pump and signal light along a microfiber ring doped with active ions. We estimate the minimum Q-factor to obtain lasing and find that values already demonstrated in passive microfiber rings will be sufficient. We model the performance of this device in steady state using rate equations and show that pump resonance can significantly reduce the threshold and increase the quantum efficiency, especially for rings made of materials with weak active ion absorption. Numerical examples for erbium and ytterbium doped devices are presented. Taking into account scattering and coupling losses the optimum pump coupling factor is calculated. The dependences of the quantum efficiency and threshold power on the coupling losses are also investigated. We predict that efficient ytterbium-doped lasers can be obtained with a ring diameter down to a few tens of micrometers.