Advances in two-micron lasers for nonlinear conversion into the mid-IR

Efficient high-power 2 mm fiber laser sources play a growing role in direct pumping of nonlinear crystals for frequency conversion into to mid-IR. There is an ongoing progress in fiber development and cavity improvement achieving outstanding laser performance for an efficient optical parametric generation. In this paper, we report on the investigation and characterization of a polarization-maintaining (PM) Thulium-Holmium-codoped triple-clad fiber (THTF). First fundamental studies were performed in a continuous-wave (CW) regime and showed highly promising results as a high power pump source for frequency conversion into the mid-IR. The paper focuses on first fundamental studies and the comparison of laser setups based on a 4.1 m and a 7 m active fiber length. Using the 7 m fiber, the THTF laser delivered an output power of 181 W. The laser had a degree of polarization of 98 %, a slope efficiency of 34.1 %, an optical-to-optical efficiency of 30% and a linewidth of 250 pm centered at 2050 nm. The laser output performance is compared with previous data of a THTF laser with a 4.1 m long fiber.