Watt-Level Visible Continuous-Wave Upconversion Fiber Lasers toward the “Green Gap” Wavelengths of 535–553 nm

The development of visible fiber lasers toward increasing power and variety has facilitated their usage in a rapidly growing number of scientific and industrial applications. Until now, holes in the visible spectral region have remained difficult or expensive to fill. Green laser oscillation in Ho³⁺ active fibers has contributed to bridging the so-called “green gap” between readily accessible spectral ranges, which is difficult to access with conventional semiconductor laser sources. However, green fiber lasers are limited in power because of the lack of high-brightness pump sources. Their wavelength tuning capability is also bottlenecked at short emission wavelengths due to the strong signal reabsorption. In this study, we developed a means of continuously tunable laser operation at the true-green wavelength of 535–553 nm. In a free-running laser operation, an output power up to 0.98 W was achieved at a center wavelength of 543.1 nm, yielding a Gaussian-like beam. This represents the highest power level demonstrated to date from a visible Ho³⁺-doped fiber laser. The results enable a new class of broadband green tunable fiber lasers with a power level on the order of watts and beyond, filling in the “green gap”.