Precision spectroscopy and frequency stabilization using a compact dual-mode operation cavity-enhanced absorption spectrometer at 1550 nm

The development of precision frequency references at 1550 nm is limited by scarce molecular transition options for fiber-optic communications and coherent LiDAR applications. We identify the 3₂₁ (101) ← 3₁₂ (000) ro-vibrational transition of HD¹⁶O at 1549.8639 nm as a metrologically viable candidate, offering complementary capabilities to existing BIPM-recommended standards. A compact dual-mode operation cavity-enhanced absorption spectrometer (30 cm × 30 cm footprint) employing spectral characterization and active locking enables precision spectroscopy and laser stabilization via differential piezoelectric transducer (PZT) feedback for cavity length control. Using wavelength-modulated cavity-enhanced saturated absorption spectroscopy with optical frequency comb calibration, we determine the absolute transition frequency to be 193,431,476,145.8(12) kHz. The system achieves 2 × 10⁻¹² frequency stability at 512 s integration time and sustains <20 kHz frequency deviation over 10-hour continuous operation with a cavity leaking rate of 0.18 Pa/hour performance rivaling conventional acetylene-based references in this spectral band. This work establishes HD¹⁶O transitions as practical frequency references for field-deployable wavelength stabilization in next-generation photonic systems operating at telecom wavelengths.