The soliton optical phase stability versus amplified spontaneous noise is i
nvestigated in a recirculation loop experiment in the presence of in-line f
ilters, The behavior of phase fluctuations is shown to be in good agreement
with numerical simulations, The phase standard deviation is kept below 0.1
5 x pi for distances up to 8000 km. We, therefore, demonstrate the feasibil
ity of phase-shift keying on solitons for long haul transmission systems.