  Modulation instability with subsideband generation induced by periodic power
variation in soliton fiber ring lasers is reported. We found that different
wavelength shifts of subsideband generation are related to different periodic
power variation. The period of power variation and wavelength shifts of
subsideband can be changed by altering the linear cavity phase delay. It is
also found that the periodic power variation is caused by the interaction
between the nonuniform polarization state of the circulating light and the
polarizer in the laser cavity.
