In this paper, a high efficient terahertz source based on n-type gallium phosphide crystal via cryogenic process is investigated through collinear difference frequency generation pumper by 1064 nm Nd:YAG laser and its OPO system. Absorption coefficient of this crystal at THz range shows a dramatic decrease from ~ 50 cm-1 to 0.5 cm--1 as the temperature decreases from 300 k to 80 k. Four times enhancement of the terahertz emission power and much more broad spectra range (~ 0.2- 3.8 THz) has been achieved in this kind of 0.5 mm length gallium phosphide crystal during the whole varied temperature difference frequency generation from 300 k to 80 k. These results indicate that cooling down the crystal temperature is an effective way to improve the terahertz source property, such as terahertz output power and frequency range.
The concentration of iodide (I − ) ions was determined by a fluorescence quenching method; a polymer-tipped optical fiber (PTOF) was used as the sensing head. The polymer tip, made by a free-radical photopolymerization method, combined the advantages of high sensitivity of the tapered fiber with a flexible and simple fabrication process. A sol–gel encapsulated fluorescein, used as the fluorescence indicator, was dip coated on the surface of the PTOF. Experimental results indicate that the backscatter fluorescence intensity collected by PTOF is 9.60 times higher than that of the flat-cut fiber. When the I − ions concentration changed from 6 to 36 mM, the backscatter fluorescence intensity collected by PTOF decreased linearly with a response time of 2 s. We demonstrate a method that can detect remote ion concentration with high repeatability, with stability, and which is simple to carry out. This method can be used in applications such as chemical production and water quality monitoring.
Plastic optical fiber (POF) probe with macro-bending (U-shaped) biconical tapered structure is proposed and optimized for the relative humidity (RH) sensing in this paper. And the probe was coated with a fluorescent moisture-sensitive film containing rhodamine 6G (R6G). Its sensing performance was optimized by changing the probe's curvature radius and biconical tapered transition length. The experimental result shows that its sensitivity and linearity are greatly increased. Also, this probe has good repeatability, photostability, and long-term stability.