Numerical design of an efficient Ho³⁺-doped InF₃ fiber laser at ∼3.2 ​μm

In this work, we theoretically unlock the potential of Ho³⁺-doped InF₃ fiber for efficient ∼3.2 ​μm laser generation (from the ⁵F₄,⁵S₂→⁵F₅ transition), by employing a novel dual-wavelength pumping scheme at 1150 ​nm and 980 ​nm, for the first time. Under clad-coupled 1150 ​nm pumping of 5 ​W, ∼3.2 ​μm power of 3.6 ​W has been predicted with the optical-to-optical efficiency of 14.4%. Further efficient power scaling, however, is blocked by the output saturation with 980 ​nm pumping. To alleviate this behavior, the cascaded ⁵I₅→⁵I₆ transition, targeting ∼3.9 ​μm, has been activated simultaneously, therefore accelerating the population circulation between the laser upper level ⁵F₄,⁵S₂ and long-lived ⁵I₆ level under 980 ​nm pumping. As a result, enhanced ∼3.2 ​μm power of 4.68 ​W has been obtained with optical-to-optical efficiency of 15.6%. Meanwhile the ∼3.9 ​μm laser, yielding power of 2.76 ​W with optical-to-optical efficiency of 9.2%, is theoretically achievable as well with a moderate heat load, of which the performance is even better than the prior experimentally and theoretically reported Ho³⁺-doped InF₃ fiber lasers emitting at ∼3.9 ​μm alone. This work demonstrates a versatile platform for laser generation at ∼3.2 ​μm and ∼3.9 ​μm, thus providing the new opportunities for many potential applications, e.g., polymer processing, infrared countermeasures, and free-space communications.