DPSK signals demodulation based on a graded-index multimode fiber mismatch spliced between two single-mode fibers

Differential phase shift keying (DPSK) signal has been shown as a robust solution for next-generation optical transmission systems. One key device enabling such systems is the delay interferometer, converting a phase signal into an intensity modulated signal to be detected by a photodiode. Usually, a Mach-Zehnder interferometer (MZI) is used for DPSK signals demodulation. In this work, we develop an alternative all-fiber MZI, which is based on a graded-index multimode fiber (MMF) mismatch spliced between two single-mode fibers. Interferometer performance is analyzed through both theory and experiment. Experimental results of transmission spectrums show that interference extinction ratio as high as 18 dB is obtained. Finally, we demonstrate, through simulation, that our proposed all-fiber MZI can be used for DPSK signals demodulation. Receiver sensitivity of -22.5 dBm at a bit error rate of 10-15 is obtained in the simulation for detecting a 40 Gbps DPSK signal, which is 1.3±0.2 dB penalty compared to the conventional receiver.