4.1 W All-Fiber Pr-Doped Deep-Red Laser at 717 nm

As deep-red semiconductor lasers are still in development, high-power deep-red lasers have traditionally relied on excimer or dye lasers, or nonlinear frequency conversion of near-infrared lasers, precluding efficient, compact, and affordable laser systems. Herein, the first compact watt-level all-fiber CW Pr $^{3+}$ -doped laser operating in the deep-red waveband was developed to navigate the challenge. The all-fiber laser consists of a double-clad Pr $^{3+}$ -doped fluoride fiber, two homemade visible fiber dichroic mirrors with a damaged intensity of $>$ 15 MW/cm $^{2}$ , and a 443-nm high-power pump source with a multimode fiber-pigtail output. We exhaustively investigated the influences of the gain fiber length and the reflectivity of output mirrors on the deep-red laser performance by experiments and simulations. A maximum average power of 4.1 W at 717 nm was directly obtained under a pump power of 20.2 W, which is more than five times higher than previously reported. The laser slope efficiency is as high as 22.2 $\%$ , and its power fluctuation is less than 0.93 $\%$ . Such a compact high-power all-fiber deep-red laser holds promise for applications in biophotonics, biomedicine, and UV laser generation.