  We show theoretically and demonstrate experimentally that collapsing
elliptically-polarized laser beams experience a nonlinear ellipse rotation that
is highly sensitive to small fluctuations in the input power. For arbitrarily
small fluctuations in the input power and after a sufficiently large
propagation distance, the polarization angle becomes uniformly distributed in
[0, 2$\pi$] from shot-to-shot. We term this novel phenomenon 'loss of
polarization'. We perform experiments in fused-silica glass, nitrogen gas and
water, and observe a significant increase in the fluctuations of the output
polarization angle for elliptically-polarized femtosecond pulses as the power
is increased beyond the critical power for self-focusing. We also show
numerically and confirm experimentally that this effect is more prominent in
the anomalous group-velocity dispersion (GVD) regime compared to the normal-GVD
regime due to the extended lengths of the filaments for the former. Such
effects could play an important role in intense-field light-matter interactions
in which elliptically-polarized pulses are utilized.
