Stable cavity ring-down spectroscopy enabled by intracavity injection with real-time PID feedback locking for reliable acetylene sensing

Abstract： A high-stability cavity ring-down spectroscopy (CRDS) system for trace acetylene (C₂H₂) detection was demonstrated using intracavity laser injection. The probe laser was directly injected into the optical resonator cavity through a CaF₂ window, completely bypassed the transmission through the high-reflectivity cavity mirror. This laser–cavity coupling method significantly enhances the coupling efficiency of the laser and the stability of its modes. Experimentally, a 3.3-fold enhancement was achieved in the initial ring-down signal amplitude, directly improving the signal-to-noise ratio. A detection limit of 2.9 ppb was achieved by Allan variance analysis, representing a 11.7-fold decrease over that of a conventional cavity mirror-coupled CRDS. On this basis, the laser frequency was actively stabilized using a modulation-free frequency locking method to the center of the C₂H₂ absorption line at 1530.37 nm via a PID feedback loop. Wavelength drift-induced fluctuations in ring-down time were effectively suppressed, and the detection limit was further reduced to 1.4 ppb, yielding a 2.1-fold decrease over the intracavity-injected CRDS without PID control and a 24.2-fold decrease compared to 33.9 ppb of the traditional extracavity-injected CRDS. In summary, this strategy was shown to offer superior stability and sensitivity for high-precision trace gas sensing, and can be directly implemented across a broad class of high-finesse cavity-enhanced techniques, regardless of target molecule or operating wavelength.