A
wavelength-division multiplexed optical
transmission system to keep the correlation of
data patterns among
wavelength channels to the low level, preventing large XPM and XGM from occurring when the correlation is strong, and assuring a stable transmitting quality. The
wavelength-division multiplexed optical
transmission system for transmitting an optical
signal constructed by a frame has any of (1) a frame phase changing circuit for mutually differing transmitting frame phases between at least two or more
wavelength channels among a wavelength channel group transmitted through the same
optical fiber transmitting line
system, or (2) a scrambling circuit using a data scrambler based on a pseudo
random pattern for mutually differing scrambling patterns between two or more
wavelength channels among a wavelength channel group transmitted through the same
optical fiber transmitting line, or (3) a
dummy data generating circuit for mutually differing
invalid data patterns between at least two or more
wavelength channels in a wavelength channel group transmitted through the same
optical fiber transmitting line when transmitting meaningless data.