Spatially resolved spectral interferometry is used to measure the mode content of a Yb-doped photonic-crystal fiber rod amplifier with a $2300{\mathrm{\mu m}}^2$ mode area. The technique, known as $S^2$ imaging, was adapted for the short fiber amplifier at full power and revealed a small amount of a copolarized ${\mathrm{LP}}_{11}$ mode. Simulations illustrate the potential for weak mode suppression in this fiber and agree qualitatively with the measurements of $S^2$ and $M^2$ . Higher-order-mode content depends on the alignment of the input signal at injection and ranged from $-18\mathrm{dB}$ for optimized alignment to $-13\mathrm{dB}$ when the injection alignment was offset along the ${\mathrm{LP}}_{11}$ axis by 30% of the $55\mathrm{\mu m}$ mode-field diameter.