Limit-Cycle Fiber-Laser Sensing via Optical-Loss-Induced Frequency Conversion

Conventional fiber sensors that rely on intensity modulation often suffer from power fluctuations and reduced signal-to-noise ratio (SNR), which limit their measurement precision. Here, we present a new sensing paradigm based on limit cycle dynamics in a passively Q-switched fiber laser (QSFL), which converts optical loss into measurable frequency shifts through modulation of the oscillation period. The demonstrated QSFL exhibits a stable limit cycle with a 59.43 μs period (16.92 kHz), enabling loss-induced changes to be read out as frequency variations with Hertz-level precision. By incorporating loss sensitive elements, such as a fiber coil and a tapered microfiber, we realize quantitative and high-SNR measurements of rotation angle, curvature, and refractive index. Beat-note linewidth of 5 Hz provides an exceptional figure of merit (FOM) and long-term stability of 16.84 Hz/h. The achieved resolutions are 0.02° over a 20° linear angle range, 4.46×10−4m−1for curvature (0∼0.25 m−1), and the FOM is 2.42×10⁴ for refractive index (1.33∼1.36 RIU). These results establish QSFL limit cycle as a compact and versatile platform that integrates ultra-precise frequency readout, intrinsic noise suppression, and multi-parameter fiber sensing.