Optical switch and optical waveform monitoring device

The polarization direction of an optical signal is changed by a polarization controller (11) so as to be orthogonal to a main axis of a polarizer. A control pulse generator (12) generates control pulses from control beam with a wavelength which is different from the wavelength of the optical signal. The optical signal and the control pulse are input to a nonlinear optical fiber (14). In the nonlinear optical fiber (14), the optical signal, during a time period in which the optical signal and the control pulse coincide, is amplified with optical parametric amplification around a polarization direction of the control pulse. The optical signal, during the time period in which the optical signal and the control pulse coincide, passes through the polarizer (15).