Used as a dopant in laser crystals and optical fibers to enable visible light emission, particularly in solid-state lasers operating in the red and green spectral ranges. Employed in fiber amplifiers for telecommunications and signal processing due to its unique luminescent properties. Applied in phosphors for display technologies and lighting systems, especially in developing color-converting materials for LEDs. Utilized in research for quantum memory and photonic devices owing to its long-lived excited states. Also found in sensors and bioimaging probes where near-infrared or upconversion emission is required.