Optical Injection Phase-lock Loop for Sub-100 fϵ/√Hz Resolution Fiber Laser Strain Sensor

Distributed-feedback fiber lasers (DFB-FLs) based sensors have achieved great performance in strain sensing for realizing the resolution to pico-level, while higher strain resolution is still very challenging. In this paper, we propose an optical injection phase-lock loop (OIPLL) for ultrahigh resolution DFB-FL-based strain sensor. A narrow linewidth laser tracks the gained Pound-Drever-Hall (PDH) error signal enabling self-compensation for quasi-static strain sensing, which is generated from DFB-FL by combing the optical injection-locking (OIL) and PDH technique. Meanwhile, with an acousto-optic modulator's (AOM's) frequency shifter from the narrow linewidth laser, a sideband laser is beating with the DFB-FL to frequency-offset lock to a helical bandpass filter (HBPF), as an absolute frequency reference, through an optical phase-locked loop (OPLL) based on amplitude detection (AmD). In strain experiments, a high quasi-strain spectral resolution of 7.89 pϵ/√Hz at 10 Hz with OIL path is obtained, and an ultrahigh dynamic strain spectral resolution of 124.88 fϵ/√Hz at 100 Hz and better than 100 fϵ/√Hz over 1 kHz with OPLL path is achieved at the same time. Under the quasi-static strain operating, the temporal dynamic strain resolution of 99.7 pϵ in 60 s is realized. Due to the high strain resolution, the proposed strain sensor appears to be extremely auspicious for applications in geophysical research.