A distributed optical fiber sensing system merged Mach-Zehnder interferometer and phase sensitive optical time domain reflectometer (φ-OTDR) system for vibration measurement with high-frequency response and high spatial resolution is demonstrated, where modulated pulses are proposed to be used as sensing source. Frequency response and location information are obtained by Mach-Zehnder interferometer and φ-OTDR technology, respectively. In order to simulate high-frequency vibration of crack of cable and civil structure, experiments on detection of piezoelectric transducer and pencil-break are carried out. Spatial resolution of 5 m and the maximum frequency response of ~3 MHz are achieved in 1064 m fiber link when the narrow pulse width is 50 ns.

We report a highly sensitive refractive index (RI) sensor based on three cascaded single-mode fiber tapers, in which a weak taper is sandwiched between the two tapers to improve the sensitivity of the sensor. Experimental results show that the sensitivity of the device is $0.286\mathrm{nm}$ for a 0.01 RI change, which is about four times higher than that of the normal two-cascaded-taper-based Mach–Zehnder interferometer. In addition, the sensitivity of the device could be enhanced by tapering a longer and thinner middle weak taper. Such kinds of low-cost and highly sensitive fiber-optic RI sensors would find applications in chemical or biochemical sensing fields.

In-line fiber optic interferometers have attracted intensive attention for their potential sensing applications in refractive index, temperature, pressure and strain measurement, etc. Typical in-line fiber-optic interferometers are of two types: Fabry-Perot interferometers and core-cladding-mode interferometers. It’s known that the in-line fiber optic interferometers based on single-mode fibers can exhibit compact structures, easy fabrication and low cost. In this paper, we review two kinds of typical in-line fiber optic interferometers formed in single-mode fibers fabricated with different post-processing techniques. Also, some recently reported specific technologies for fabricating such fiber optic interferometers are presented.

Pollutions caused by heavy metals has become a wordwide concerned issue. Herein, CsPbBr ₃ perovskite quantum dots are used as fluorescent probe for selective detection of copper ions in organic phase with a limit of detection (LOD) down to 0.1 nM.

Laser linewidth narrowing down to kHz or even Hz is an important topic in areas like clock synchronization technology, laser radars, quantum optics, and high-precision detection. Conventional decoherence measurement methods like delayed self-heterodyne/homodyne interferometry cannot measure such narrow linewidths accurately. This is because a broadening of the Gaussian spectrum, which hides the laser’s intrinsic Lorentzian linewidth, cannot be avoided. Here, we introduce a new method using the strong coherent envelope to characterize the laser’s intrinsic linewidth through self-coherent detection. This method can eliminate the effect of the broadened Gaussian spectrum induced by the 1/f frequency noise. We analyze, in detail, the relationship between intrinsic laser linewidth, contrast difference with the second peak and the second trough (CDSPST) of the strong coherent envelope, and the length of the delaying fiber. The correct length for the delaying fiber can be chosen by combining the estimated laser linewidth (Δf_est) with a specific CDSPST (ΔS) to obtain the accurate laser linewidth (Δf). Our results indicate that this method can be used as an accurate detection tool for measurements of narrow or super-narrow linewidths.

Facing the gap in the unsupervised construction of health indicator (HI) with a uniform failure threshold, a new unsupervised HI construction approach is developed. First, the distribution of the raw vibration signal is estimated by the Gaussian mixture model, then a distribution contact ratio metric (DCRM) is designed to compute the distance between two arbitrary distributions. With DCRM, a distribution contact ratio metric health indicator (DCRHI) is innovatively constructed for well representing the degradation process and obtaining a uniform failure threshold. Next, aiming at the challenge of prediction under limited samples, a novel consolidated memory gated recurrent unit (CMGRU) is proposed by making full use of the historical state information, and it can effectively slow down the forgetting speed of important trend information. Combing the proposed DCRHI and CMGRU, a novel remaining useful life (RUL) prediction methodology is put forward for enhancing the predictive performance. Via two public bearing datasets, several contrast experiments are implemented, and the comparative results show that DCRHI can better describe the degradation process of bearing than other typical unsupervised HIs, and CMGRU has a stronger prediction ability than other classical time series processing networks. Thus, the proposed methodology has great application value in the RUL prediction.