Free-space self-interference microresonator with tunable coupling regimes

The device of free-space self-interference microresonator coupled by fiber tapers is proposed. Different from sensors with similar structures on-chip and benefit to the combination between microcavity and optical fiber sensing, the coupling regime is adjustable and a separated sensing area from coupling regions is available. This method makes it feasible to optimize coupling efficiency in detection and broaden the scope of application in dissipative sensing. The transmission spectrum exhibits a distinct phenomenon under a long optical path of sensing arm, including quasi-sinusoidal modulation and profile split. Based on that, transmission characteristics are analyzed by theoretical stimulations and the subsequent experiments are in good agreement with the theory. In addition, we estimate the sensing performance of this device and that the sensitivity can reach −4.76 dB/(10−7 RIU) with a 1 m sensing arm theoretically when applied in monitoring the refractive index change, which is almost an order of magnitude larger than the previously reported data. Besides being adjustable, compact, and efficient, this device shows great potential in the precision measurement and expands the applicable measurement field of similar structures, such as pull pressure that cannot be detected using bus straight waveguides.