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.

We implement a BOTDR sensor that combines the complementary coding with the fast Fourier transform (FFT) technique for high-performance distributed sensing. The employment of the complementary coding provides an enhanced signal-to-noise ratio of the sensing system, which leads to high accuracy measurement. Meanwhile, FFT technique in BOTDR is combined to reduce the measurement time sharply compared to the classical frequency sweeping technique. In addition, a pre-depletion two-wavelength probe pulse is proposed to suppress the distortion of the coding probe pulse induced by EDFA. Experiments are carried out beyond 10 km single-mode fiber, and the results show the capabilities of the proposed scheme to achieve 2 m spatial resolution with 0.37 MHz frequency uncertainty which corresponds to ∼0.37 °C temperature resolution or ∼7.4 με strain resolution. The measurement time can be more than tens of times faster than traditional frequency sweeping method in theory.

Triangulation laser range scanning, which has been wildly used in various applications, can reconstruct the 3D geometric of the object with high precision by processing the image of laser stripe. The unbiased line extractor proposed by Steger is one of the most commonly used algorithms in laser stripe center extraction for its precision and robustness. Therefore, it is of great significance to assess the statistical performance of the Steger method when it is applied on laser stripe with Gaussian intensity profile. In this paper, a statistical behavior analysis for the laser stripe center extractor based on Steger method has been carried out. Relationships between center extraction precision, image quality and stripe characteristics have been examined analytically. Optimal scale of Gaussian smoothing kernel can be determined for each laser stripe image to achieve the highest precision according to the derived formula. Flexible three-step noise estimation procedure has been proposed to evaluate the center extraction precision of a typical triangulation laser scanning system by simply referring to the acquired images. The validity of our analysis has been verified by experiments on both artificial and natural images.

To verify the feasibility of Brillouin optical time domain reflector (BOTDR) applied in space station, three kinds of commercial fibers, YOFC fiber (about 1km), Corning fiber (about 1km) and Fujikura fibers (about 1.5km), have been radiated by Co60 in radiation experiment. Based on the Brillouin Gain Spectrum (BGS) along sensing fiber, we analyze power curve of Brillouin backscattered light, Brillouin Frequency Shift (BFS) along sensing fiber and spatial resolution carefully. The results indicate that radiation has little effect on BFS when signal-to-noise ratio (SNR) is large enough. However, since radiation will lead to radiation-induced attenuation, the amplitude of BGS will decrease accordingly. And if the SNR becomes small enough, the accurate BFS cannot be obtained. For the stressed fiber section of BOTDR, the result shows that radiation has little effect on the strain coefficient at the total radiation dose of 200Gy, but radiation will deteriorate the spatial resolution of BOTDR with the increase of radiation dose. These results are good references for the introduction of BOTDR to space station.

We propose a plasmonic nano-optical conveyor belt for peristaltic transport of nano-particles. Instead of illumination from the top, waveguide-coupled excitation is used for trapping particles with a higher degree of precision and flexibility. Graded nano-rods with individual dimensions coded to have resonance at specific wavelengths are incorporated along the waveguide in order to produce spatially addressable hot spots. Consequently, by switching the excitation wavelength sequentially, particles can be transported to adjacent optical traps along the waveguide. The feasibility of this design is analyzed using three-dimensional finite-difference time-domain and Maxwell stress tensor methods. Simulation results show that this system is capable of exciting addressable traps and moving particles in a peristaltic fashion with tens of nanometers resolution. It is the first, to the best of our knowledge, report about a nano-optical conveyor belt with waveguide-coupled excitation, which is very important for scalability and on-chip integration. The proposed approach offers a new design direction for integrated waveguide-based optical manipulation devices and its application in large scale lab-on-a-chip integration.

We demonstrate an enhanced distributed acoustic sensing (DAS) system based on ultra-weak fiber Bragg grating (UWFBG) array and self-heterodyne detection. By using the interaction process between nearby UWFBGs, the distribution of acoustic field along the sensing fiber could be fully captured. Self-heterodyne detection was applied, leading to much lower noise density level for the proposed sensing system. The experimental results have shown that the acoustic field could be reconstructed with high fidelity even at the rear end of a 42 km long sensing fiber with the spatial resolution of 11.7 m. The proposed sensing system has a linear intensity response of R2=0.9988, while the peak-to-peak value (VPP) of dynamic strain was tuned from 9.28 n $\epislon$ to 176.96 n $\epsilon$ . The root-mean-square (RMS) measurement error of our sensing system was less than 1.32 n $\epsilon$ . The frequency response range of the proposed system was from 8 Hz to 1 kHz. The proposed sensing system offered a promising solution for the performance enhancement of DAS system that could achieve high spatial resolution, high sensing sensitivity, long sensing distance and dead zone free measurement at the same time.