A polarization mode dispersion (PMD) emulator with tuneable first-order PMD and constant second-order PMD

摘要

We design a polarization mode dispersion (PMD) emulator with fixed second-order polarization mode dispersion (SO-PMD) but varying first-order PMD (FO-PMD). The emulator constitutes of an optical delay line (ODL), a polarization controller (PC) and a fixed number of randomly concatenated polarization maintaining fibre (PMF) segments. An understanding of the SO-PMD equation is the first vital step to consider before designing such an emulator. The control of the differential group delay (DGD) statistics with wavelength proves to be the key measure for this design. Results show that the mean DGD (or the mean magnitude of the FO-PMD vector (tau)) of the emulator is biased towards the dominant wavelength-independent tau of the ODL. This is provided the dominant tau is by far greater than FO-PMD contributions from the other cascaded sections. Experimentally it is shown that when the DGD ((DELTA)(tau)) is wavelength-independent due to the absence of mode coupling, or when the wavelength-dependent DGD spectra do not change with time due to fixed mode coupling, there is negligible influence on the SO-PMD. The PC angle is controlled at an angle theta to ensure that the sub-emulator tau is always parallel to the ODL tau. Thus by rotating the mode coupling angle theta, we change the wavelength-dependent DGD spectra thereby ensuring SOPMD variation.