A compact sensor utilizing off-axis integrated cavity output spectroscopy at a wavelength of 1.389 μm was developed to measure the δ¹⁸O and δ²H isotopic ratios in liquid water samples within the natural concentration range. Allan variance analysis indicated an optimal averaging time of approximately 21 s. The stability and long-term reproducibility of the laser system were assessed using 1-second averaging over 75 min. The minimum detectable absorption (MDA) was determined to be approximately 8.4 × 10⁻⁴ Hz⁻¹/² with a 17 s averaging time. To assess accuracy and reproducibility, the setup was tested within the natural concentration range (δ²H: -400 to 0‰ and δ¹⁸O: -54 to 0‰) using five standard samples, achieving an average accuracy and reproducibility of better than 2.5‰ for δ²H and 0.5‰ for δ¹⁸O.
