Wavelength selective switch

A wavelength selective switch, comprising:

• a spectroscopic element to separate input light and providing angular dispersion depending on wavelengths;
• a collective lens to gather light output from the spectroscopic element;
• a fixture member to fix one end of the collective lens;
• and
• a movable reflection block which comprises a plurality of mirrors arranged in a direction of the angular dispersion made by the spectroscopic element, changes the angles of the mirrors in a direction differing from the direction of angular dispersion, and reflects light output from the collective lens;
• wherein the collective lens is fixed at one end by the fixture member with respect to the direction of angular dispersion of the light output from the spectroscopic element,
• wherein the collective lens is fixed at an end in a direction in which the angle of the light output from the spectroscopic element changes in accordance with the spectroscopic element's temperature characteristics when the temperature rises, and
• wherein the collective lens expands with heat in a direction in which the collective lens is not fixed when the temperature changes, and the angle of the light output from the collective lens is changed in a direction opposite to the direction in which the angle of the light output from the spectroscopic element changes so as to correct a deviation in light gathering positions on the mirrors, of the light output from the spectroscopic element, depending on the temperature characteristics.