The microfiber loop resonator: theory, experiment, and application

This paper describes the theory of a microfiber loop resonator (MLR) and experimentally demonstrates a high quality factor MLR in free space. The MLR is fabricated from the ∼1-μm diameter waist of a biconical fiber taper using the CO₂ laser indirect heating technique. The high coupling efficiency of an MLR is achieved through an adiabatically slow variation of the microfiber diameter in the coupling region. An MLR-loaded Q-factor of 120 000 and an intrinsic Q-factor of 630 000 were demonstrated. As an application, the performance of an MLR as an ultrafast direct contact temperature sensor is also demonstrated. The MLR heating/cooling relaxation time was measured to be ∼3 μs, in good agreement with the developed theory.