The Raman time constant (T-R) used in the generalized nonlinear Schrodinger
equation is determined experimentally at 1550 mm based on the Raman self-f
requency shift in a standard single-mode fiber. The effective value of the
T-R as measured is found to be 3.0 fs. Detailed error analysis shows that t
he uncertainty in the measurement is less than +/-1.0 fs, The measured valu
e of the T-R is approximately half of what is commonly used at present in t
heoretical simulations, Determined experimentally in this work the accurate
value of T-R = 3.0 fs is essential in modeling pulse propagation in optica
l fibers.