An exact analytical solution for the evolution of stimulated Raman cro
sstalk in a singlemode WDM fiber system is presented utilizing the tri
angular approximation to the Raman gain spectrum. This approach provid
es a rigorous expression for crosstalk impairment. In the limit of sma
ll interchannel power exchange, our result reduces to Chraplyvy's prev
ious estimate. Our formulation can be employed to predict the Raman pe
nalty in future multiwavelength fiber communication systems.