We studied a method of measuring upper critical field (H c2 ) of a superconductor based on the width of ∆H = ∆B region, which appears in the superconductor that volume defects are many and dominant. Here we present the basic concept and details of the method. Although H c2 of a superconductor is fixed according to kind of the superconductor, it is difficult to measure H c2 experimentally, and the results are different depending on the experimental conditions. H c2 was calculated from the theory that pinned fluxes at volume defects are picked out and move into an inside of the superconductor when their arrangement is the same as that of H c2 state of the superconductor. H c2 of MgB 2 obtained by the method was 65.4 Tesla at 0 K. The reason that H c2 obtained by the method is closer to ultimate H c2 is based on that ∆F pinning /∆F pickout is more than 4 when pinned fluxes at volume defects of 163 nm radius are picked out. The method will help to find the ultimate H c2 of volume defect-dominating superconductors.
We report on efficient collinear optical parametric generation (OPG) with gain band ranging from 1400 to 2600 nm in a 2 cm-long periodically poled lithium niobate (PPLN) crystal. Such an ultra-broad gain band was obtained by choosing the pump wavelength at 933 nm, at which the group-velocities of the signal and the idler match near the degeneracy point. High OPG efficiency was obtained by quasi-phase matching (QPM). The ultra-broadband OPG led to efficient collinear RGB generation from a single PPLN crystal at a fixed pump wavelength. The green and red beams were found to be originating from high-order QPM sum-frequency generation between the pump and selected frequencies in the OPG band, while the blue beam was high-order QPM second-harmonic generation of the pump.
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