The demand for distributed acoustic sensors, which are capable of reconstructing the amplitude, frequency, and phase of an acoustic field, is increasing. These sensors are typically realized by phase-sensitive optical time domain reflectometry ( $\Phi$ -OTDR). However, common $\Phi$ -OTDR systems suffer from a fading phenomenon, which causes amplitude fluctuation on Rayleigh backscattering traces, and the scattered light may not have enough intensity in some regions. These areas have low amplitude backscattering that may be even lower than the system noise floor during some periods of time. Therefore, we cannot reconstruct the phase signal properly in such low intensity areas. Systems with multiple frequencies have been proposed to achieve fading suppression. However, online data processing of such schemes remains a challenge, especially for continuous in-field applications. Prediction of fading must be performed in real time. Any delay caused by massive calculations is not acceptable. In this paper, a continuous fading suppression method based on a $\Phi$ -OTDR system with three different probe frequencies is presented as well as a tracking algorithm for selecting the optimum probe signal for any time, which predicts the occurrence of fading before it actually occurs. The performance of the proposed method has been experimentally evaluated and statistically analyzed. The distortion induced by the fading effect could be suppressed to 1.15% under continuous real-time running. The method is highly effective and repeatable, which makes it suitable for practical online purposes.

The performance of distributed optical fiber acoustic sensor as a function of parameters such as the linewidth of the laser, the probe pulse width, and the amplitude and frequency of perturbation is experimentally studied. The aim of this study is to experimentally confirm the outcome of the simulation results obtained previously. It is shown that the experimental and simulation results are in good agreement, and the precision of the sensing system depends on the pulse width and linewidth of the probe pulse, as well as the frequency and amplitude of the perturbation. It is shown that the sensing precision of the system can be enhanced by reducing the width of the probe pulse while increasing the linewidth of the laser.

A sensitivity enhanced temperature sensor with cascaded tapered two-mode fibers (TTMFs) based on the Vernier effect is proposed and experimentally demonstrated. It is confirmed that series connection exhibits higher extinction ratio than parallel one both by theory and experiments, which provides guidance for related experiments. In experiments, two TTMFs have the same single-mode fiber-TTMF-single-mode fiber configuration, while the free spectral ranges (FSRs) are chosen with slightly difference by modifying the parameters in the tapering process. Experimental results show that the proposed temperature sensor possesses sensitivity of −3.348 nm/°C in temperature measurement range from 25 °C to 60°C, 11.3 times sensitivity enhancement in comparison with single TTMF. Benefiting from advantages of high temperature sensitivity, simplicity of manufacture and long distance sensing, this novel sensitivity enhanced temperature sensor can be applied to various particular fields, such as oil wells, coal mines and so on.

Phase-sensitive time-domain reflectometry (Φ-OTDR) can be used for fully distributed long-distance vibration monitoring. There is a fading phenolmenon in the Φ-OTDR, which will cause the signal intensity somewhere to be too low to extract the phase of the signal without distortion. In this paper, the Φ-OTDR based on space-division multiplexing (SDM) is proposed to suppress fading and we used multi-core optical fiber (MCF) to realize SDM. While inheriting the previous optimization strategy, we proposed a strategy based on frequency spectral similarity to process multiple independent signals obtained by SDM. And we compared the two methods. Through the experiments, the distortion rate can be reduced from an average level of 9.34% to less than 2% under continuous running of 270 s, which proves that SDM is a reliable technical route to achieve fading suppression. This method can effectively improve the fading suppression capability of the existed commercial systems.

The senescence of lens epithelial cells (LECs) is a major factor leading to age-related cataract (ARC). ARC results in visual impairment and severe vision loss in elderly patients. However, the specific mechanism of ARC remains unclear, and there are no effective therapeutic agents to halt the formation of ARC. This study aimed to assess the underlying mechanism of the formation of ARC and investigate the potential anti-ageing effect of metformin (MET) on ARC. Male C57BL/6 mice were divided into three groups: the control group having young mice (3 months old, n = 40), the naturally aged group (aged 20 months, n = 60) and the MET group (MET, 20 months, n = 60). Mice in the control and the naturally aged groups were fed a standard purified mouse diet ad libitum and water, whereas those in the MET group were fed chows supplemented with 0.1% MET for 10 months. The transparency of the lens and age-associated proteins p21 and p53 were analysed in the LECs of these three groups. Furthermore, we determined the expressions of the adenosine monophosphate (AMP)-activated protein kinase (AMPK) pathway and the effect of MET on this pathway in LECs during the ageing process of ARC. In addition, the relationship between autophagy and the senescence of LECs and the role of MET in the autophagy of LECs during the ageing process of ARC were examined. Our results indicated that age-related inactivation of the AMPK pathway and impairment of autophagy might contribute to the senescence of LECs and the occurrence of ARC. More importantly, these results demonstrated that MET effectively alleviated the senescence of LECs and the formation of ARC probably via inactivation of the AMPK pathway and augmentation of autophagy. These findings revealed that MET can be exploited as a potentially useful drug for ARC prevention. Our study will help in enlightening the development of innovative strategies for the clinical treatment of ARC.

A distributed optical fiber dynamic strain sensor also known as a distributed acoustic sensor (DAS) based on two-mode fiber is demonstrated. By using φ-OTDR interrogation technique, the backscattered light from higher order modes can be used to fully quantify vibrations along the sensing fiber. In addition, by combining the results obtained from different modes, 2.52dB improvement in noise floor is achieved. These results confirm that few-mode fibers can be used for DAS applications.