Displacement measurement of sub-nanometer resolution using space-domain active fiber cavity ring-down technology

•A displacement sensor using FSI combined with active FCRD technology is proposed.•A Bi-EDFA is utilized to increase the number of round trips for achieving high resolution.•A sensitivity of 0.05 km−1 ·nm−1 with stability of 1.3 % was achieved.•Using the proposed method, the displacement sensor with resolution of 0.3 nm was achieved. A displacement sensor of sub-nanometer resolution using space-domain active fiber cavity ring-down (FCRD) technology was developed. In our scheme, by combining active FCRD with frequency-shifted interferometry technology, a sensitivity of 0.05 km−1 ·nm−1 was achieved and the stability was improved mostly because this scheme is free from thepowerfluctuationofalight source, benefited to the use of low-noise Bi-EDFA and the differential detection method to reduce the direct current noise. Besides the first sensitivity amplification process is involved in this technique due to the multiple times rounds of interaction between the light pulse and the sensor head in the fiber cavity, the secondary sensitivity amplification process is realized by using a Bi-EDFA to compensate the inherent cavity loss of fiber cavity, which results in a significantly improvement in resolution. The experimental results show that our proposed displacement sensor can achieve a resolution of up to 0.3 nm with stability of 1.3 %.