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Narrow linewidth external cavity semiconductor lasers (NLECSLs) have many important applications, such as spectroscopy, metrology, biomedicine, holography, space laser communication, laser lidar and coherent detection, etc. Due to their high coherence, low phase-frequency noise, high monochromaticity and wide wavelength tuning potential, NLECSLs have attracted much attention for their merits. In this paper, three main device structures for achieving NLECSLs are reviewed and compared in detail, such as free space bulk diffraction grating external cavity structure, waveguide external cavity structure and confocal Fabry–Perot cavity structure of NLECSLs. The Littrow structure and Littman structure of NLECSLs are introduced from the free space bulk diffraction grating external cavity structure of NLECSLs. The fiber Bragg grating external cavity structure and silicon based waveguide external cavity structure of NLECSLs are introduced from the waveguide external cavity structure of NLECSLs. The results show that the confocal Fabry–Perot cavity structure of NLECSLs is a potential way to realize a lower than tens Hz narrow linewidth laser output.

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Laser sensing and vision sensing smart canes can improve the convenience of travel for the visually impaired, but for the present, most of the system functions of laser sensing and vision sensing smart canes are still defective. Guide equipment and smart blind canes are introduced and classified first, and the smart blind canes based on vision sensing, laser sensing and laser vision sensing are investigated, respectively, and the research status of laser vision sensing smart blind canes is sorted out. The advantages and disadvantages of various laser vision sensing smart blind canes are summarized, especially the research development of laser vision fusion as the core of new smart canes. The future development prospects of laser vision sensing smart blind cane are overviewed, to boost the development of laser vision sensing smart blind cane, to provide safe and efficient travel guarantee for the visually impaired.

Introduction: Previous studies have shown that applying external vibrational load of a certain frequency and intensity can accelerate the tooth movement, thus shorten orthodontic treatment time. It is believed that the optimal frequency is correlative to the modal mode of the target toothperiodontal ligament (PDL)-bone complex (TPBC). This study aims to investigate modal mode, natural frequencies included, of the TPBC using finite element (FE) method. Methods: A three dimensional TPBC model of a maxillary central incisor were reconstructed from cone beam CT images. A FE model was then created in Ansys 14.5. The tooth, PDL, and bone were meshed with 10-node hexahedral elements (SOLID 187) with the numbers of the elements equaled to 17,803, 4,137, and 32,577, respectively. All parts were considered homogeneous and isotropic. The material properties (Young's Module (Pa), Poisson's ratio and Density (kg/m 3 ) as: 2e10, 0.31 and 1.36e3 for tooth, 4.7e5, 0.4 and 1.1e3 for PDL, and 3.5e9, 0.33 and 1.4e3 for bone, respectively. Fixed constraint was set for the alveolar bone. PCG Lanczos method was used in the modal analysis. Six order modes were included in the computation. Result: The first six order modal mode of the TPBC were obtained. The corresponding natural frequencies were 702, 1049, 2713, 2805, 3026, and 3530 Hz, respectively. Different tooth movement patterns, including translation, rotation, extrusion/intrusion, were observed in these modal modes. Conclusion: The proposed method is able to obtain the natural frequencies and modal modes of a TPBC. The results can be used as reference for developing vibrational device to accelerate orthodontic tooth movement.