Improvement of Transverse Offset-Based Single-Mode-Multimode-Single-Mode-Fiber Structure for DPSK Signal Demodulation

In this work, we develop a novel all-fiber Mach-Zehnder interferometer (MZI), which is based on a commercial graded-index multimode fiber mismatch spliced between two single-mode fibers. Compared to the common single-mode-multimode-single-mode-fiber (SMS) structure with all fibers aligned perfectly, theoretical analysis shows that more power is coupled into the LP11 mode while less power is coupled into the LP01 mode, resulting in power balance between these two modes and high interference extinction ratio (ER). Experiments were also carried out to evaluate the performance of the proposed MZI, and results of transmission spectra showed that interference ER as high as 18 dB can be obtained. Finally, we demonstrate, through simulation, that our proposed MZI can be used for DPSK signal demodulation. Receiver performances are evaluated through back-to-back sensitivity, narrowband optical filtering and differential group delay, with results showing that our proposed DPSK receiver can provide comparable performance as the conventional receiver.