Single-mode operations in the large flattened mode optical fiber lasers and amplifiers

Transverse mode competitions are compared between the large mode area (LMA) fiber lasers with and without flattened fundamental modes (FMs). Numerical simulations show that the flattened-FM fiber lasers can support single-mode (SM) operation without employing other mode selection methods. This property is useful for designing LMA fiber lasers with SM operation. For the flattened-FM fiber amplifiers, the photonic crystal fiber (PCF) with a flattened FM is proposed to shape the Gaussian beam into a flat-top one, which is coupled into the input end of the active fiber as a seed. The SM operation based on this approach can be obtained.