Dual Wavelength Pumping Scheme for Directly Diode-Pumped 75X-nm Fiber Lasers

For decades, researchers have been interested in developing efficient rare-earth-doped laser sources using dual-wavelength pumping. However, dual-wavelength pumping of rare-earth-doped lasers and amplifiers are currently restricted to pump wavelengths longer than the visible. Extending the use of dual-wavelength pumping for directly diode-pumped rare-earth-doped lasers is of great importance for both academic and practical purposes. We have developed the first blue dual-wavelength pumping of Holmium-doped ZrF4-BaF2-LaF3-AlF3-NaF (Ho:ZBLAN) fiber lasers operating at 75X nm. A dual-wavelength pumping method was proposed by pumping two separate wavelengths of 449 and 442 nm into a Ho:ZBLAN fiber to populate the upper laser state F-5(4) + S-5(2) and depopulate the lower laser state I-5(7), respectively. The influence of the 449 and/or 442 nm pumping schemes on the efficiency and threshold of the 75X nm laser was studied by varying the active fiber length. For dual-wavelength pumping with blue laser diodes, a shorter fiber and a lower 449 nm pump power are favoured. Our approach expands the applicability of dual-wavelength pumping to visible laser systems with energy-level structures consisting of long-lived lower-level bottlenecks.