The active fiber loop ring-down spectroscopy (FLRDS) can achieve a high-finesse fiber cavity by introducing erbium-doped fiber amplifier (EDFA) gain for highly sensitive detection. However, gain saturation behavior of EDFA induces pulse-to-pulse gain fluctuations, which cause the ring-down signal to deviate from the exponential-decay law and greatly destroy the detection accuracy of the active FLRDS. In order to analyze the active FLRDS more accurately, a theoretical model of the active FLRDS considering the gain saturation behavior of EDFA is established. The performance of the active FLRDS system is simulated and discussed by using the model. The dependence of system performance on the initial optical power, the inherent cavity loss, the test-gas concentration and the EDFA location is deeply researched. This model is applicable to the active FLRDS systems in chemical and optical properties as well as thermal and mechanical parameters.

Pr-MOF([Pr(L)(CO₃)_0.5(H₂O)]_n) has been shown to have excellent nonlinear properties and has been successfully applied to an erbium-doped fiber laser.

Calibration-free wavelength modulation spectroscopy (WMS) is an efficient technique for trace gas monitoring. It is widely used due to its resistance to light intensity fluctuations, strong suppression of low-frequency noise, fast response time, and excellent environmental adaptability. The calibration-free WMS often employs the Levenberg–Marquardt algorithm for spectral fitting to retrieve gas characteristics. However, to the best of our knowledge, an analysis of the main factors affecting the operational effectiveness of the Levenberg–Marquardt algorithm in calibration-free WMS has merely been reported. In this paper, we have systematically analyzed the Levenberg–Marquardt algorithm’s operating mechanism in WMS-2f/1f. The results show that the number of parameters and the estimation errors of the initial parameters are the main factors limiting the retrieval accuracy of the algorithm, which provides some important guidelines for the subsequent optimization of the spectral fitting scheme.