Mode control in a 7.2 kW near-single-mode monolithic fiber amplifier employing the gain-tailoring effect

In this paper, we investigate the performance of a high-power confined doped fiber amplifier to achieve high power and high beam quality. The impact of various factors, including core diameter, doping ratio, and numerical aperture, on the gain-tailoring effect is explored to establish fiber parameters for attaining near single mode beam quality at high power, taking into account variations in coiling diameters. Based on simulation results, a confined doped fiber with a low numerical aperture, featuring a core/cladding diameter of 30/250 μm and a relative doping ratio of 0.8, was fabricated in-house and utilized in a tandem-pumped fiber amplifier. The experimental results coincide well with the simulation results except for discrepancies arising from the imperfect refractive index profile of the fiber. An output power of 7.2 kW with the M² factor of 1.55 was obtained. This work can provide guidelines for designing 10 kW level single-mode fiber lasers employing confined doped fiber.