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We report a high-data-rate two-dimensional photodetector array (2D-PDA) operated over 10 GHz for compact receiver systems in multicore fiber (MCF), few-mode fiber (FMF), and FMF-MCF, which can be directly coupled to multiparallel beams from the MCF and FMF. The 2D-PDA (pixel size of 20-30 µm, pixel number of 32) were designed and characterized for the frequency response and crosstalk. In the on-wafer probing test, the averaged 3-dB bandwidth of 11.2 GHz and 19.8 GHz were measured for 30-µm and 20-µm pixels, respectively. In the MCF direct-coupling demonstration with the 2D-PDA, the selected 16 pixels could work well at 25 Gb/s (nonreturn-to-zero: NRZ). By using triangularshape-aligned seven-core fiber and the square-shape-aligned seven cores in the 2D-PDA, the coupling efficiency and alignment tolerance were investigated for the universal MCF photoreceiver. In the demonstration of the mode-division multiplexing signal detection with 2D-PDA, the multiplexed three-mode signal with 20-Gbps quadrature phase-shift keying was directly coupled to the 2D-PDA. Through the electrical phase matching system with the multipleinput multiple-output function, the three-mode signal (20 Gbps in each mode) could be successfully detected and separated error-free.

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The formation of polygonal satellite vortices due to the rotation of the fluid by a bottom disk is studied. The results obtained for a normal fluid (water) are compared with that observed for pure electron plasma in Malmberg-Penning traps. The comparison shows similar patterns of vortices produced in two completely different systems, providing the experimental evidence of the direct analogy between the polygonal satellite vortices observed in a normal fluid and the vortices in pure electron plasma.

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We have studied Josephson coupling in a-axis YBa2Cu3O7−δ/Pr1−xYxBa2Cu3O7−δ /YBa2Cu3O7−δ sandwich-type superconductor/normal/superconductor (SNS) junctions. Growth parameters for the films were optimized for smoothness of the barrier interfaces and the films were well characterized by scanning electron microscopy, atomic force microscopy, and cross-sectional transmission electron microscopy. Transport of the barrier material was highly dependent upon the doping: for x=0, the resistivity followed a T−1/4 behavior and was ∼105 Ω cm at 4.2 K; for x=0.45, the resistivity was nearly metallic and was 10−1 Ω cm at 4.2 K. In the small junction regime W<λj, the critical current scaled with junction area, and was modulated by a magnetic field. We have compared our results to other SNS work and found that the coupling strength of the barrier increases with the conductivity of the barrier material. Possible sources of the coupling are discussed.

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