Spatially resolved spectral interferometry is used to measure the mode content of a Yb-doped photonic-crystal fiber rod amplifier with a mode area. The technique, known as imaging, was adapted for the short fiber amplifier at full power and revealed a small amount of a copolarized mode. Simulations illustrate the potential for weak mode suppression in this fiber and agree qualitatively with the measurements of and . Higher-order-mode content depends on the alignment of the input signal at injection and ranged from for optimized alignment to when the injection alignment was offset along the axis by 30% of the mode-field diameter.
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