Abstract
Higher order modes LP0,m in specially designed fibers have recently shown to offer robust light propagation in record- large effective areas (Aeff) with negligible mode-coupling or bend distortions [1]. Such beams can be generated with greater than 99% coupling-efficiencies using in-fiber gratings [1]. 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 (smalk-Aeff) fundamental mode of a fiber would lead to high intensities at the fiber output which could destroy the fiber. Since for many applications a Gaussian output is desired, other techniques have to be used to convert the higher order mode with a quite high M2 beam quality value into a nearly Gaussian output with M2≈l. One possibility is to use a binary phase plate which corrects the phase jumps of n which occur at the zeros of the intensity of the higher order mode. In [2] it has been shown that binary phase plates alone cannot improve the M2 value. But in this paper, we use a combination of a phase plate and an optical filtering system to convert the output of a HOM fiber into a nearly fundamental Gaussian beam. Of course, this means that some of the light power is lost in the filtering system. But, 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. The figure shows as example the simulation of the conversion of a LPog mode (M2=13.99) into a nearly Gaussian beam (M2=1.04). In this example, 75% of the incident light power are conserved (by neglecting absorption in the lenses of the filtering system).
© 2007 IEEE
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