This invention relates to an optical transmission system which allows high
quality transmission of signal light where a plurality of signal channels
are multiplexed, and has a configuration that is particularly suitable for
CWDM optical transmission. In the optical transmission system, the
plurality of signal channels propagating through the optical fiber
transmission line are demultiplexed into a signal channel group in the
first wavelength band .LAMBDA..sub.1 and a signal channel group in the
second wavelength band .LAMBDA..sub.2. Then, each signal channel in the
second wavelength band .LAMBDA..sub.2 where the absolute value of
chromatic dispersion is large is dispersion-compensated. When the bit rate
is B (Gb/s) at a specific wavelength in the second wavelength band
.LAMBDA..sub.2 where the total chromatic dispersion in the optical fiber
transmission line and the dispersion compensator is highest, the chromatic
dispersion value at this specific wavelength is set to be grater than 0
(ps/nm) but 7500/B.sup.2 (ps/nm) or less, and is set such that the loss in
the second wavelength band .LAMBDA..sub.2 is lower than the highest loss
in the first wavelength band .LAMBDA..sub.1.
