Demonstration of an All-Fiber Ultra-Low Numerical Aperture Ytterbium-Doped Large Mode Area Fiber in a Master Oscillator Power Amplifier Configuration Above 1 kW Power Level

We demonstrate an all-fiber ultra-low numerical aperture high power fiber laser system operating in the continuous-wave regime at a central wavelength of 1080 nm. A special ytterbium-doped fiber preform has been designed and fabricated by using modified chemical vapor deposition technique with the deposition percentages of 0.03mol% of $\text{Yb}_{2} \text{O}_{3}$ , 1.8mol% of $\text{Al}_{2}\text{O}_{3}$ , and 2.1mol% of $\text{P}_{2}\text{O}_{5}$ . Then it has been drawn to obtain an active fiber having core/cladding diameters of 26 μm/410 μm respectively. The numerical aperture of the fiber has been first predicted as 0.034 from refractive index profile of the preform. Afterward, this number has been verified with a simple test setup by altering the bending diameter of the fiber and the excitation conditions of it. To test the high power performance, a laser system has been constructed in a master oscillator power amplifier configuration, and ∼ 80 W seed signal power is amplified to 1.05 kW. The quality of the laser output beam has been measured in terms of $\text{M}^2$ value along both x and y coordinates as 1.11 and 1.16 respectively.