Gaussian Beam Output from a Large-Mode-Area Higher-Order-Mode Fiber

Higher order modes LP_0,m in specially designed fibers have recently shown to offer robust light propagation in record-large effective areas (A_eff) with negligible mode-coupling or bend distortions. Such beams can be generated with greater than 99% coupling-efficiencies using in-fiber gratings. However, this conversion technique may not be suitable at the output of a high-power fiber-laser or amplifier comprising a higher order mode fiber because converting back to the (small-A_eff) fundamental mode of a fiber would lead to high intensities at the fiber output which could destroy the fiber. In this paper, a combination of a phase plate and an optical filtering system was used to convert the output of a HOM fiber into a nearly fundamental Gaussian beam. Simulations show that by only loosing 22%-25% of the light power due to the filtering, i.e. a loss between 0.97 dB and 1.25 dB, the output will be nearly Gaussian.