To meet the demand for ever-increasing data transfer rates in fiber optic communications, there is great interest in space-division multiplexing of light. Mode-division multiplexing, a subset of space-division multiplexing, increases the channel capacity of a system by spatial conversion and coupling of optical signals into the orthogonal modes of a multimode fiber channel. Current mode multiplexers, which rely on directional couplers, photonic lanterns, and free-space optics, can be challenging to fabricate, align, and scale to a large number of modes with low loss and crosstalk. As a result, more resources have to be spent on amplifiers and digital signal processing which increases the cost of the communications system. Metasurfaces offer a solution to this problem and have shown wide success in realizing various optics. Being lithographically patterned with subwavelength-scale dielectric structures, metasurfaces allow full control of the wavefront. We adopt a folded metasurface configuration by coating the dielectric structures with cladding and depositing metal on both sides of a glass substrate. Inside the substrate, light interacts with the metasurface multiple times to yield unprecedented abilities in shaping light. Based on this platform, we demonstrate a metasurface mode multiplexer design with low loss and crosstalk in the telecom. It directly aligns to an array of input fibers and an output fiber and converts each input mode into the orthogonal modes of the latter fiber. Short-distance and long-haul communication systems such as data centers and submarine links, respectively, can benefit from this mode multiplexer.
We study transparent ceramics made of erbium doped yttria, as a candidate material for quantum storage of photons in the telecom wavelength. Samples with different heat processing conditions are compared. Two different species of Er3+ ions with drastically different dephasing properties are identified. In general, samples processed at lower temperature show narrower inhomogeneous and homogeneous linewidths. We also demonstrate both type I and type II waveguide in the material using a femtosecond laser, with propagation loss of 0.22 dB/mm.
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