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.

We present a D-shaped photonic crystal fiber (PCF) plasmon sensor based on a large-hole self-reference channel for applications in biological and chemical sensing. Both the Plasmon’s fundamental linear-polarization LP01 and high-order LP11 modes are excited at the interface between the analyte and the silver film based on results of the numerical calculation method. We obtained the transmitted spectrum of the guided mode LP01 in the D-shaped PCF. We found that the peak spectral wavelength of the sensor increases when the refractive index of the analyte increases. The spectral sensitivity clearly increases with the silver film’s thickness on the sensor’s platform, although it is unaffected by other variations of the PCF’s geometry. We obtained an optimal spectral sensitivity for the sensor of 3191.43nm/RIU for a silver film thickness of 50nm.

We propose a second harmonic phase angle method (WMS- $\theta _{2f}$ ) based on wavelength modulation spectroscopy for trace gas detection. WMS- $\theta _{2f}$ exhibits the advantages of background-free and immunity to light intensity fluctuations. The detection sensitivity is 1-2 orders of magnitude higher than that of the first harmonic phase angle detection method. Moreover, there is room for further improvement in sensitivity by optimizing the linearity of the light intensity modulation of the laser. We verified the feasibility of the WMS- $\theta _{2f}$ method experimentally and theoretically, respectively.