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Raman spectroscopy has been widely used in various fields due to its unique and superior properties.

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Metasurfaces provide a remarkable platform to manipulate over phase, amplitude, and polarization flexibly and precisely. Bifocal metalens draws great research interest due to its ability of converging wavefronts to different focal positions horizontally and longitudinally. However, as wavelength of incident light changes, chromatic aberration will cause the focal lengths reliance on the incident wavelength, which will affect the performance of metasurface, especially for longitudinal bifocal metalens. In this work, a broadband achromatic longitudinal bifocal metalens (BALBM) based on single nanofin unit cell is demonstrated. Pancharatnam-Berry (PB) phase is used to converge the incident light. Cross commixed sequence distribution (CCSD) is introduced to control the positions of focal points FLand FRwhen left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) incident. Propagation phase is used to compensate the phase difference caused by chromatic aberration. Simulation results show that in the continuous wavelength range from 500 nm to 700 nm, the focal point shifts caused by chromatic dispersion are reduced 81% for FL and 83% for FR, respectively. The focal length variations are stabilized to 6.21% for FLand 4.8% for FRcomparing with the focal lengths at the initial wavelength 500 nm. The proposed BALBM brings advances to bifocal metasurfaces in versatile application areas including machine vision, optical computed tomography and microimaging.

An optical fiber inclinometer applying modal interference based on an abrupt tapering cascading a peanutshape configuration is proposed. The abrupt taper splits part of the core mode into cladding modes and the interference occurs when the peanut-shape recombines the cladding modes and core mode together. Four inclinometers with different lengths have been fabricated and characterized, and the high sensitivities of −1.76 and −4.49 nm/°are obtained in the angle range of 0°-3°and 3°-6.66°, respectively.Index Terms-Optical fiber inclinometer, fiber tapering, peanut-shape, modal interferometer.