Quantifying direct DQPSK receiver with integrated photodiode array by assessing an adapted common-mode rejection ratio

In this work, a direct DQPSK receiver was fabricated, which comprises a polymer waveguide based delay-line interferometer (DLI); a polymer based optical hybrid, and two monolithic pairs of > 25 GHz bandwidth photodiodes that are vertically coupled to the polymer planar lightwave circuit (PLC) via integrated 45Y mirrors. The common mode rejection ratio (CMRR) is used to characterize the performance of coherent receivers, by indicating the electrical power balance between the balanced detectors. However, the standard CMRR can only be measured when the PDs can be illuminated separately. Also, the standard CMRR does not take into account the errors in the relative phases of the receiver outputs. We introduce an adapted CMRR to characterize the direct receiver, which takes into account the unequal responsivities of the PDs, the uneven split of the input power by the DLI and hybrid, the phase error and the extinction ratio of the DLI and hybrid.