Locking-free, high-sensitivity optical ultrasound sensing based on radio-frequency-domain slope discrimination

High-sensitivity optical ultrasound sensors are traditionally plagued by a fundamental trade-off between detection limit and environmental stability, often necessitating complex active locking to combat thermal drifts. Here, we introduce a self-stabilized sensing architecture based on heterodyne delay-line interferometry. Instead of interrogating a thermally drifting optical resonance, our approach utilizes a heterodyne unbalanced Mach-Zehnder interferometer to down-convert ultrasound-induced optical frequency modulation into the radio-frequency (RF) domain. This "integrate-and-differentiate" scheme-comprising optical phase accumulation followed by electrical slope discrimination-decouples signal demodulation from low-frequency optical drifts, enabling robust operation without feedback control. We demonstrate a noise-equivalent pressure of 1 mPa/Hz^1/2 level. Crucially, this bias-drift-free architecture maintains stable performance against significant thermal fluctuations, offering a practical solution for high-fidelity photoacoustic imaging and long-term monitoring.