Wavelength selective switch

A wavelength selective switch, comprising:

• an input/output unit including a plurality of input/output portions arranged two-dimensionally for inputting or outputting wavelength-multiplexed signal light;
• a dispersive portion capable of dispersing, along a first plane, the signal light input from the input/output portion in a direction corresponding to the wave length;
• a deflection unit having a plurality of deflection elements for deflecting each of a plurality of signal light beams that are to be dispersed by the dispersive portion into wavelengths, so that the signal light beams are each made incident on different one of the input/output portions depending on the wavelength;
• and
• an ovalization relay optical system disposed between the input/output unit and the dispersive portion, the ovalization relay optical system including at least one first ovalization optical element and at least one second ovalization optical element, the first ovalization optical element having a refractive power in a first direction perpendicular to an optical axis which is larger than a refractive power in a second direction perpendicular to the optical axis and to the first direction, the second ovalization optical element having a refractive power in the second direction which is larger than a refractive power in the first direction, the first ovalization optical element and the second ovalization optical element being arranged so that the first direction becomes perpendicular to the first plane,
• the ovalization relay optical system condensing signal light beams that are to be input from all the input/output portions, onto a first conjugate point, the first conjugate point being closer to the ovalization relay optical system between the deflection unit and the ovalization relay optical system, than any other conjugate points that are conjugate to the deflection unit;
• the ovalization relay optical system making a beam waist forming position along the first direction of signal light beams emitted from the ovalization relay optical system to the dispersive portion side coincide to the first conjugate point;
• the ovalization relay optical system condensing, in the second direction, the signal light beams that are to be input from all the input-output portions onto the first condensing point in the ovalization relay optical system, and making the first condensing point conjugate to the first conjugate point;
• and
• the ovalization relay optical system ovalizing a beam shape of the signal light beams being incident on the deflection element.