Spatially resolved spectral interferometry is used to measure the mode content of a Yb-doped photonic-crystal fiber rod amplifier with a 2300 μm2 mode area. The technique, known as S2 imaging, was adapted for the short fiber amplifier at full power and revealed a small amount of a copolarized LP11 mode. Simulations illustrate the potential for weak mode suppression in this fiber and agree qualitatively with the measurements of S2 and M2. Higher-order-mode content depends on the alignment of the input signal at injection and ranged from −18 dB for optimized alignment to −13 dB when the injection alignment was offset along the LP11 axis by 30% of the 55 μm mode-field diameter.
© 2011 Optical Society of AmericaPDF Article