Enhancement of visible emission in an Ho³⁺-doped fluorozirconate fiber using dual-wavelength pumping at 44X nm

Optical absorption and fluorescence spectra in the visible spectral range were investigated for an Ho3+-doped fluorozirconate fiber. From the absorption spectrum, the Judd-Ofelt (J-O) intensity parameters, Omega(2), Omega(4), and Omega(6), were calculated along with the spectroscopic parameters, including radiative decay rates (A(JJ)'), radiative branching ratios (beta(JJ)'), and radiative lifetimes (tau(rad)) for the main emission levels, including I-5(7), I-5(6), F-5(5), F-5(4)+S-5(2), F-5(3), and K-3(7)+(5)G(4) of Ho3+ ions. The excited-state absorption (ESA) mechanism of blue diode-pumped visible emission for an Ho3+-doped fluorozirconate fiber was investigated for the first time under blue pumping at 44X nm. Emission bands originating from K-3(7)+(5)G(4) levels at peak wavelengths of 579 and 680 nm show a two-step excitation mechanism. These two emissions are enabled by ESA for both I-5(6) and I-5(7) levels, and the process to the K-3(7)+(5)G(4) levels after nonradiative decay from the (5)G(2) level is ESA from the I-5(6) level, whereas the process to the K-3(7)+(5)G(4) levels after nonradiative decay from the H-3(6) level is ESA from the I-5(7) level. It has been revealed that enhancement of visible emission was enabled by introducing the ESA at 440 nm in an Ho3+-doped fluorozirconate fiber under dual-wavelength pumping at 449 + 440 nm.