  Cross-phase modulation instability (XPMI) is experimentally observed in a
fiber ring cavity with net normal dispersion and mode-locked by long fiber
taper. The taper is deposited with reduced graphene oxide, which can decrease
the threshold of XPMI due to the enhanced nonlinearity realized by 8 mm
evanescent field interaction length and strong mode confinement. Experimental
results indicate that the phase matching conditions in two polarization
directions are different, and sidebands with different intensities are
generated. This phase matching condition can be satisfied even the polarization
state of the laser varies greatly under different pump strengths.
