On the Gaussian approximation and margin measurements for optical DPSK systems with balanced detection

Based on the computed exact probability density functions (PDFs), we analyze the conventional Gaussian approximation (GA) method and the thresholding-based bit error rate (BER) measurement method in back-to-back optical differential phase shift keying (DPSK) systems with balanced detection for the purpose of margin measurement in noise-loading experiments. We show that the GA method fails when used with DPSK systems in contrast to its success in the case of on-off keyed (OOK) systems, and as a result, the thresholding method is not as robust for DPSK systems as it is for OOK systems. Still, our results show that the thresholding method, when using reasonably small threshold bias, can measure large performance margins for DPSK systems with a wide range of E_b/N₀ (the energy bit to noise power spectral density ratio) and optical bandpass filter (BPF) bandwidth, and receiver imperfections such as the PIN responsivity imbalance and frequency detuning. We further discuss briefly on BER estimation for nonlinear fiber transmission.