228 W CW thulium-doped fiber MOPA laser at 1908 nm with high power stability and near diffraction-limited beam quality

Thulium-doped fiber lasers (TDFLs) at 1908 nm are promising for medical lithotripsy, polymer processing, atmospheric sensing and industrial heating, and act as vital pumps for holmium lasers and ZGP optical parametric oscillators. Yet severe reabsorption below ∼1.94 µm restricts simultaneous high power, stable output and near diffraction-limited beam quality. We present an all-fiber continuous-wave thulium-doped master oscillator power amplifier (MOPA) to overcome these limitations. A linear fiber Bragg grating seed delivers 16.01 W with 50.5% slope efficiency, while the amplifier uses low-absorption gain fiber, an integrated mode field adaptor, and full water cooling to suppress thermal distortion and nonlinear effects. Pumped by 424 W at 793 nm, the system produces 228.3 W maximum output with 53.4% slope efficiency featuring a stable ∼1908 nm wavelength, 0.49–0.57 nm linewidth and fully suppressed amplified spontaneous emission. A 120-minute stability test at average output power of 209.7 W demonstrates a low RMS fluctuation of 0.04%. The measured beam quality factors are M x 2 = 1.08 and M y 2 = 1.06, both below 1.1 and close to the diffraction limit. This laser integrates high power, high stability and near diffraction-limited beam quality at 1908 nm, suitable for medical lithotripsy, polymer welding and mid-infrared conversion.