Fabrication and characterization of chalcogenide glass microsphere lasers operating at 2 μm
Authors: Hu Bo , Wu Yue-Hao , Zheng Yu-Lu , Dai Shi-Xun ,   Advanced Technology Research Institute, Laboratory of Infrared Materials and Devices, Ningbo University, Ningbo 315211, China,   Key Laboratory of Photoelectric Materials and Devices of Zhejiang Province, Ningbo 315211, China
Published: December 2020

Microsphere lasers operating at the <inline-formula><tex-math id="M6">\begin{document}$2\;{\text{μ}}{\rm{m}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181817_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181817_M6.png"/></alternatives></inline-formula> band have important applications in the fields of bio-medical sensing, laser radars, narrow linewidth optical filtering, and air-pollution monitoring. In this work, we utilize a novel type of chalcogenide glass, whose composition is Ge-Ga-Sb-S or 2S2G, to fabricate microsphere lasers. Compared with chalcogenide glasses used in previous microsphere lasers, this 2S2G glass is environmentally friendly. It also has a lower melting temperature and a higher characterization temperature, implying that 2S2G microspheres can be fabricated at lower temperatures and the crystallization problem happening in the sphere-forming process can be mitigated. A <inline-formula><tex-math id="Z-20190304120007-6">\begin{document}$\text{Tm}^{3+}\text{-}\text{Ho}^{3+} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181817_Z-20190304120007-6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181817_Z-20190304120007-6.png"/></alternatives></inline-formula> co-doping scheme is applied to the 2S2G glass, so that fluorescence light at ～<inline-formula><tex-math id="M7">\begin{document}$2\;{\text{μ}}{\rm{m}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181817_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181817_M7.png"/></alternatives></inline-formula> can be obtained from the bulk glass. Owing to the superior properties of the 2S2G glass, we can utilize a droplet method to mass-produce hundreds of high-quality 2S2S microspheres in one experimental run. The diameters of microspheres fabricated in this work fall in a range of 50−<inline-formula><tex-math id="M8">\begin{document}$250\;{\text{μ}}{\rm{m}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181817_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181817_M8.png"/></alternatives></inline-formula> and typical quality factors (<i>Q</i> factor) of microspheres are higher than 10<sup>5</sup>. As a representative example, we characterize the optical properties of a <inline-formula><tex-math id="M9">\begin{document}$205.82\;{\text{μ}}{\rm{m}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181817_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181817_M9.png"/></alternatives></inline-formula> diameter 2S2G microsphere. This microsphere is placed in contact with a silica fiber taper, so that the pump light can be evanescently introduced into the microsphere and the fluorescence light can be evanescently collected from the microsphere. A commercial laser diode (808 nm) is used as a pump source and an optical spectral analyzer is used to measure the transmission spectra of the microsphere/fiber taper coupling system. Apparent whispering gallery mode patterns in the ～<inline-formula><tex-math id="M10">\begin{document}$2\;{\text{μ}}{\rm{m}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181817_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181817_M10.png"/></alternatives></inline-formula> band can be noted in the transmission spectra of the coupling system. When the pump power increases beyond a threshold of 0.848 mW, a lasing peak at 2080.54 nm can be obtained from the coupling system. Experimental results presented in this work show that this 2S2G chalcogenide glass is a promising base material for fabricating various active optical/photonic devices in the middle-wavelength and long-wavelength infrared spectra.