Measurement of polarisation-dependent phase shift in DPSK demodulator

The recent availability of DPSK demodulators has spurred interest in phase-modulated analogue RF photonic links, but phase shifts due to birefringence can cause transmission impairments. Presented are measurements of the polarisation-dependent phase shift in a DPSK demodulator created by the relative angle (in Stokes space) between the launch SOP and the demodulator´s eigenstate. This phase shift is connected to earlier theoretical predictions, and results are compared to a vendor´s specifications for three models. Since imperfect phase modulators may cause modulation of the polarisation at the signal frequency, these impairments may limit RF photonic link performance.