Line-of-sight link is widely used in common free-space optical (FSO) laser communications between two fixed locations. While in practical underwater wireless optical communications (UWOC), the environment is relatively complicated. In some scenarios there exist irremovable obstacles, which block the line-of-sight optical link. Fortunately, the air-water interface can function as a natural mirror to enable non-line-of-sight optical link using the total internal reflection. Very recently, twisted light beams carrying orbital angular momentum (OAM) have attracted researchers' great attention to improve the transmission capacity in UWOC. Here, we propose and experimentally demonstrate a non-line-of-sight underwater twisted light transmission link utilizing the total internal reflection at the air-water interface. To overcome the beam fluctuation and drift caused by the change of interface states, we develop a proof-of-concept adaptive feedback system to provide a stable output. Moreover, we study the degrading effects of the slight wind effect, the salinity (turbidity) effect, and the vertical thermal gradient-induced turbulence effect. The results show that the water wave caused by the slight wind causes the most beam drift, the thermal gradient causes the most distortions, and the salinity causes the most power loss.
From metamaterial to metasurface and metadevice, the artificial structure with sub-wavelength scale on diverse platforms offers the ability to shape light in a custom way. The optical fiber is a robust and flexible media that has seen wide applications in optical communications, optical sensing, microscopy, and endoscope imaging. Here, we consider metasurface on a large-core fiber platform for twisting light. Using the designed and fabricated meta-facet fiber, we demonstrate (i) the excitation of both linearly polarized and circularly polarized twisted light (OAM+1, OAM−1) from either meta-facet side or planar-facet side, (ii) phase-front reconstruction of twisted light simply from a tilt interferogram using the Fourier-transform method, and (iii) ultra-broadband response from 1480 to 1640 nm with high phase purity above 93% for twisting light. The demonstrations on meta-facet fiber for twisting ultra-broadband light with high phase purity may open up perspectives to more emerging applications in information, biology, and medical science.
Mid-infrared (2–20 μm) light has been attracting great attention in many areas of science and technology. Beyond the extended wavelength range from visible and near-infrared to mid-infrared, shaping spatial structures may add opportunities to grooming applications of mid-infrared photonics. Here, we design and fabricate a reflection-enhanced plasmonic metasurface and demonstrate efficient generation of structured light with the phase helix and intensity helix at 2 μm. This work includes two distinct aspects. First, structured light (phase helix, intensity helix) generation at 2 μm, which is far beyond the ability of conventional spatial light modulators, is enabled by the metasurface with sub-wavelength engineered structures. Second, the self-referenced intensity helix against environmental noise is generated without using a spatially separated light. The demonstrations may open up advanced perspectives to structured light applications at 2 μm, such as phase helix for communications and non-communications (imaging, sensing) and intensity helix for enhanced microscopy and advanced metrology.
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