Numerical demonstration of 3.5 μm laser generation in Tm/Er distributed-doped fiber pumped by LDs

We demonstrate theoretically the 3.5 μm laser generation based on Tm-assisted self-generated 1980 nm laser side-pumping. The gain medium is a double-clad fluoride fiber with a Tm-doped inner cladding and an Er-doped core. A 793 nm laser diode (LD) pumps Tm ions in the inner cladding to generate a 1980 nm laser, replacing the dedicated external 1980 nm fiber pump source. Combined with a 980 nm LD, Er ions in the core are enabled to generate a 3.5 μm laser. When pumped with sufficient power at 980 nm and a fixed 20 W power at 793 nm, and given a core absorption of 1 dB/m at 1980 nm, the output power of the 3.5 μm laser remains ∼4.9 W in different Er-doped concentrations. Compared with the traditional method, this approach achieves more uniform population inversion along the fiber and a slightly higher 1980 nm to 3.5 μm laser conversion efficiency. This research represents a novel method for simplifying 3.5 μm laser systems and could theoretically improve power scaling via cladding pumping.