An optical signal processing apparatus has a diffraction grating element, first
condenser lens, Faraday rotation elements, polarization beam splitter, /2
plate, second condenser lens, third condenser lens, and diffraction grating element.
The magnitude of a magnetic field of each of the Faraday rotation elements is controlled
on the basis of an externally input control signal. The rotation angle of the plane
of polarization of signal light is set to 0 or /2 in accordance with the
magnitude of the magnetic field. Each Faraday rotation element receives signal
light having wavelengths 1 to 4, which has
arrived from the first condenser lens, and outputs the signal light as a polarized
light component in the first azimuth (parallel to the z-axis direction) or second
azimuth (parallel to the y-axis direction).
