Patent ID: 7801451

Claim:
An intensity modulation/differential phase modulation conversion circuit comprising: an optical phase modulation element; a one-bit delay element; optical directional couplers; and an optical phase modulation circuit comprising: an optical phase modulation element; an optical band pass filter; and an optical directional coupler; wherein control light with a wavelength of λ C is coupled with probe light with a wavelength of λ P using the optical directional coupler and input to the optical phase modulation element, the probe light that has been phase modulated by the optical phase modulation element is extracted through the optical band pass filter, and the phase of the probe light is modulated in response to the intensity of the control light; and wherein the control light is given as the control light delayed by a first optical directional coupler and the one-bit delay element; the branching light from the first optical directional coupler is coupled with probe light using a second optical directional coupler; the probe light given as the coupled light output from the second optical directional coupler is phase modulated by control signals via the first optical phase modulation element to give modulated signals; the delayed control light is given as the control light of the optical phase modulation circuit; the phase modulated signals are given as the probe light of the optical phase modulation circuit; and intensity on/off keying modulation of the control light is converted to differential phase shift keying modulation of the probe light.