Self-injection blue laser interferometric salinometer via configurable beating frequency

Phase-sensitive, ultralow-phase-noise, and frequency-stable optical salinometers have recently been recognized as the ideal solution for overcoming resolution limitations, sensing constraints, and anti-interference. Benefiting from the low transmission loss of 450 nm blue laser in seawater, we propose and demonstrate an interferometric salinity sensing architecture based on an ultra-high quality (Q) factor crystalline whispering gallery mode (WGM) cavity, which enables the construction of a strongly coherent, configurable optical frequency comb (OFC) through a controllable self-injection feedback path, thereby generating a low-phase-noise beat-frequency signal. Free-space laser and tightly confined WGM cavity prism coupling devices are established, enabling precise control of laser nonlinear dynamics within a 6% self-injection feedback power range. Meanwhile, a marine salinity sensing system with a sensitivity of 56.94 mRad/‰@25°C is demonstrated, achieving a minimum Allen deviation salinity sensing limit of 1.85 × 10^-3‰. This work not only provides a technical approach for high-precision salinity detection but also lays the theoretical and experimental foundation for expanding the application of external cavity feedback lasers in extreme environments and high-precision optical sensing.