  We present measurements of optical interband absorption in the fractional
quantum Hall regime in a GaAs quantum well in the range 0 < nu < 1. We
investigate the mechanism of singlet trion absorption, and show that its
circular dichroism can be used as a probe of the spin polarization of the
ground state of the two-dimensional electron system (2DES). We find that at nu
= 1/3 the 2DES is fully spin-polarized. Increasing the filling factor results
in a gradual depolarization, with a sharp minimum in the dichroism near nu =
2/3. We find that in the range 0.5 < nu < 0.85 the 2DES remains partially
polarized for the broad range of magnetic fields from 2.75 to 11 Tesla. This is
consistent with the presence of a mixture of polarized and depolarized regions.
