In the WDM optical transmission, optical signals propagated through an optical
transmission line are supplied to a circulator of a wavelength dispersion compensator.
The optical signals supplied to an input terminal of the circulator are transmitted
to an input and output terminal of the circulator, and inputted to fiber gratings
of the reflection type. The optical signals having a specified wavelength is reflected
by one of the fiber gratings, again inputted to the input and output terminal of
the circulator, and transmitted to the output terminal of the same. Since the incident
optical signals are reflected by the fiber gratings situated at different positions,
there arises the difference in the time spent in traveling to and from the fiber
grating between the optical signals. Accordingly, the wavelength dispersions of
the optical signals can be compensated by suitably selecting the positions of the
fiber gratings.