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When using single-photon detector array lidar to detect ships in a foggy environment, the multiple scattering between laser and sea fog, sea surface and ships leads to a large number of fake images in the reconstructed depth images, which seriously limits the ability to detect and recognize ships. Here, we propose a three-dimensional imaging algorithm suitable for ships in fog with a single-photon detector array lidar. This algorithm can eliminate fake images caused by multiple reflections of the sea surface in a foggy environment. Firstly, the algorithm uses a dual-Gamma estimation algorithm to eliminate the influence of non-signal photons, and uses a pulse-like selection algorithm to accurately extract the target echo waveform. Secondly, based on the characteristics of the target echo, the principal component analysis (PCA) and K-means algorithm are used to cluster the features of the separated echo signals. Finally, the result of feature classification is used to achieve efficient fake images removal. We show that the proposed algorithm can more accurately reconstruct the depth image of a ship with a distance of 1.72 km in fog environment. Compared with the traditional algorithms, the proposed algorithm can suppress the sea surface reflection while retaining the most target contour. It has been successfully demonstrated for small ships at different distances. This research is expected to improve the detection and recognition ability of single-photon detector array lidar for small targets in complex scenes. [ABSTRACT FROM AUTHOR]

A fiber laser curvature sensor based on dual-wavelength gain competitive intensity difference demodulation is proposed and experimentally demonstrated. Here a linear cavity dual-wavelength erbium-doped fiber laser (DWEDFL) is build with two filter components i.e. fiber Bragg gratings (FBG) with very close center wavelengths. Since the dual wavelengths are sharing the same gain medium erbium-doped fiber (EDF), gain competition mechanism is formed with sensitive feature to external measurand curvature. Besides, the curvature information is demodulated by subtracting the power of the dual wavelengths. Experimental results show a total sensitivity as high as -57.22 dB/m−1. Our scheme not only improves the measuring sensitivity, but also eliminates the effects of source power jitter and external disturbances for example temperature shift since they do almost the same impact on the dual wavelengths. [ABSTRACT FROM AUTHOR]

A relative humidity (RH) sensor based on femtosecond laser induced spiral micro-structured single mode-multimode-single mode (FISM-SMS) fiber structure was experimentally demonstrated. The spiral micro-groove ablated multimode fiber of the FISM-SMS fiber structure was coated with RH sensitive polyvinyl alcohol (PVA) whose refractive index varies as a function of ambient humidity level. Due to the FISM-SMS fiber structure highly sensitive to ambient refractive index, transmission spectrum of the RH sensor changed greatly at different ambient relative humidity. RH response of the sensor was tested by putting it into a homemade climatic chamber whose humidity level can be adjusted by a series of saturated salt solutions. Experimental results show that a RH sensitivity of ∼650 pm/%RH with linearity of ∼0.99 when humidity level varies from 35 % RH to 70 % RH. Besides, reversibility and stability make the sensor a good candidate for RH sensing. [ABSTRACT FROM AUTHOR]