30-watt level, 1.7 µm first-order Raman fiber laser with more than 99% spectral purity

The 1.7 μm laser serves as a key pump source for efficient 3 μm mid-infrared laser output. In this work, we present a high-power, high-spectral-purity first-order Stokes Raman fiber laser operating at 1680 nm, pumped by a homemade 1565 nm master oscillator power amplifier (MOPA). The key to our design is the use of a broadband chirped fiber Bragg grating (CFBG, which is of a 25.5 nm bandwidth) as the Raman cavity mirror, which effectively mitigates the spectral mismatch induced by nonlinear broadening at high power levels. As a result, we achieved an output power of 30.2 W at 1680 nm with a spectral purity of 99.2% and a slope efficiency of 72.9% relative to the launched pump power. The laser also exhibited excellent power stability, with fluctuations below 1% over one hour of continuous operation. To the best of our knowledge, this represents the highest power reported for a first-order Stokes Raman fiber laser at 1.7 μm with a purity exceeding 99%. This work provides a highly stable and power-scalable solution for the generation of high-power 1.7 μm lasers. The high spectral purity makes this pump source promising for the all-fiber integration of mid-infrared fiber lasers.