Transverse mode selection in a Nd-doped fiber amplifier at 910 nm

Selective amplifications of LP₀₁ fundamental mode and higher order modes LP₁₁ and LP₀₁ are demonstrated in a double-pass Nd-doped LMA fiber amplifier operating at 910 nm. A multimode core fiber Bragg grating is employed to select a single guided mode by simply adjusting the wavelength of the seed signal. Although the M² parameter of the output beam from the amplifier was ~2.5 in a single-pass configuration, a double-pass configuration with LP₀₁ mode selection reduces the value of the M² parameter to 1.06 in spite of the multimode nature of the core (V~5). In addition, it is shown that this amplifier configuration permits to lower both the power saturation and the parasitic emission at 1060 nm, which consequently increase the pump-to-signal conversion efficiency at 910 nm.