From a magneto-optical study on (TMTSF)2ClO4 we determine the SDW gap
as a function of magnetic field and temperature. The transition temper
atures found for the resolved FI-SDW subphases mimic the established p
hase diagram. Furthermore, the data confirm the existence of a collect
ive excitation of the SDW condensate at relatively high frequency, whi
ch implies that this mode is pinned rather strongly to the lattice. In
additon, we observed an interesting relaxation effect near the N=0 ph
ase transition