Efficient UV-visible emission enabled by 532 nm CW excitation in an Ho 3+ -doped ZBLAN fiber

Abstract

Rare-earth-doped ZBLAN (ZrF 4 -BaF 2 -LaF 3 -AlF 3 -NaF) fibers have evolved to become promising candidates for efficient UV-visible emission because of their low phonon energy and low optical losses, as well as their well-defined absorption bands. We investigate the efficient emission of UV-visible light in a low-concentration (0.1 mol%) Ho 3+ -doped ZBLAN fiber excited by a 532 nm CW laser. In addition to the direct populating of the thermalized 5 F 4 + 5 S 2 levels by ground-state absorption, the upconversion processes responsible for UV-visible emission from the higher emitting levels, 3 P 1 + 3 D 3 , 3 K 7 + 5 G 4 , 5 G 5 , and 5 F 3 , of the Ho 3+ ions are examined using excited-state absorption. The dependence of UV-visible fluorescence intensity on launched green pump power is experimentally determined, confirming the one-photon and two-photon characters of the observed processes. We theoretically investigate the excitation power dependence of the population density for nine Ho 3+ levels based on a rate equation model. This qualitative model has shown a good agreement with the measured power dependence of UV-visible emission. Moreover, the emission cross-sections for blue, green, red, and deep-red light in the visible region are measured using the Füchtbauer–Ladenburg method and corroborated by McCumber theory, and the corresponding gain coefficients are derived. We propose an alternative approach to achieve efficient UV-visible emission in an Ho 3+ -doped ZBLAN fiber using a cost-effective, high-brightness 532 nm laser.