X射线自由电子激光试验装置(以下简称“SXFEL试验装置”)是中国第一台X射线相干光源,其输出波长小于9 nm。这台基于0.84 GeV 直线加速器、以掌握装置相关技术和实验演示种子型自由电子激光(FEL)级联与短波长回声型FEL为主要目标的自由电子激光装置,于2020年11月通过国家验收。本文将介绍SXFEL试验装置的基本情况和主要进展。

A novel optical fiber evanescent-wave (EW) sensing platform combining a taper and a refractive index (RI) gel residue is proposed. The platform includes two identical large core multimode fibers perpendicularly positioned to each other: one for excitation light delivery (i-fiber) and one for EW fluorescent signal collection (r-fiber). One end of the r-fiber is decladded to expose a segment of a cylindrical fiber core terminated with a taper. A small drop of rhodamine 6G (R6G) solution sample is distributed in such a way that it surrounds the side wall of the cylindrical portion of the core and covers the i-fiber end face. The fluorescent signal is recorded under the following conditions: 1) the entire taper is exposed to air; 2) the entire taper is immersed into a large gel block; 3) the taper is covered with a gel residue. A dramatic rise of the fluorescence signal is observed in the third case, which is over 20 times more than the level achieved from the first two cases. We reveal that the intensive mode coupling in the sandwiched air-gel-taper architecture accounts for this phenomenon, which is discussed in detail.

The critical findings associated with end-face total internal reflection (TIR) phenomemon we proved before are reported. In particular, these findings reveal that the end-face-TIR capable rays experience enormous mode mixing when encountering a roughened end face. As a result, 94% of the overall detectable power is contributed by this effect. With a smooth fiber end face, this figure is mere 52%. We interpret the mechanism behind these unusual phenomena and its significance for the performance enhancement of fiber optic evanescent wave sensor.

改变外色散腔半导体激光器的温度、电流和腔长,可实现复盖调谐范围内的连续调谐和准确调谐.本文叙述连续调谐和准确调谐的原理与实验结果.