Study of fundamental mode output of a coiled multimode Yb ³⁺-doped double-cladding fiber amplifier

The composition of modes in double-clad multimode active fiber is studied, The bending influence on numerical apertures of fibers is analyzed by means of geometrical optics, When the optical fiber is bent, different modes have different bending losses, A numerical simulation was used to calculate the bending losses of LP₀₁ and LP₁₁ in different bending radii, Optimum bending radius was chosen to inhibit the high-order modes so that the fundamental mode signal amplification was assured, A 10 meters long Yb ^3--doped double-cladding fiber with core diameter of 30μm and core numerical aperture of 0.06 was used to verify our theoretical calculation. The fiber amplifier was pumped by a diode laser array emitting at 976nm. A non-planar ring oscillator with cw single frequency output at 1064 nm was used as signal source. By coiling the double-cladding fiber, the amplified signal of fundamental mode was obtained. When the pump power was 49.6 W and the signal power was 200 mW, 16.1W of single frequency output power was obtained, the amplification factor was 80.5, The spectra of output signals before and after amplification were measured with Fabry-Perot interferometer, Fundamental mode output is verified.