Partial DPSK (PDPSK): Influence of MZI delay mismatch in high bitrate dispersive channels

Tool Used: OptSim

The objectives of this application note are to: 1. study a partial DPSK system and 2. study influence of MZI delay mismatch in high bitrate dispersive channels.

The conventional DPSK receiver has delay between the two arms in the asymmetric Mach-Zehnder (AMZ) interferometer filter set to the bit period. Any mismatch in the delay between the two arms due to receiver imperfection gives corresponding change in the free spectral range of the AMZ filter transfer function. When this relative delay is less than the bit period, it results in what's called "partial" DPSK. 

The figure below depicts the OptSim layout for RZ-DPSK:

Layout for RZ-DPSK | Synopsys

A 40 Gbps DPSK signal passes through the RZ pulse carver at the transmitter. The transmitted RZ DPSK spectrum is shown below. 

RZ DPSK spectrum | Synopsys

In presence of non-zero dispersion, the delay mismatch of less than one bit gives better performance than the delay mismatch of more than one bit as evident from the curve below. It is interesting to observe that the optimum performance in presence of dispersion does not correspond to zero mismatch in AMZ delay.

AMZ delay | Synopsys

As shown in the OSNR penalty plot below, in presence of increasing dispersion (chromatic and/or PMD), partial DPSK (with delay less than one bit period) gives better performance than classical DPSK (where bit delay mismatch is zero).

Classical DPSK | Synopsys