Bit-rate variable DPSK demodulation based on cascaded four-wave mixing

We report a demodulator for DPSK signals at variable bit rates based on cascaded four-wave mixing (FWM). The demodulation utilizes two FWM processes in a photonic crystal fiber (PCF) with in-between dispersion in a chirped fiber Bragg grating (CFBG). The first FWM generates a wavelength-tunable idler carrying phase information of the signal. A tunable optical delay between the signal and the idler is then introduced by dispersion. The signal, the idler, and the pump are reflected by the CFBG with a reflectance of 99% back to the PCF to initiate the second FWM process. In the second FWM, the phase relationship between the signal and the one-bit-delayed idler determines an amplification or attenuation of the idler, converting phase modulation to intensity modulation. Error-free demodulations have been successfully demonstrated for both NRZ and RZ-DPSK signals at 5 and 10 Gb/s.