EXFO PSO-200 Optical Modulation Analyzer - Manual (Page 253)

Coherent Detection and Sampling Methods
Optical Modulation Analyzer 245
Signal Processing Algorithms
Polarization Demultiplexing
The polarization-diversity scheme employed in the PSO-200 makes it
polarization independent for single-polarization transmission (even if the
SOP of the signal varies in time), but it may also be applied to
polarization-multiplexed (also referred to as “dual polarization” - DP)
transmission, where the software can separate the two orthogonal SOPs (X
and Y) and display the corresponding two constellation charts.
The figure below shows typical results from a 28 GBd DP-QPSK signal.
Time-Base Reconstruction
The PSO-200 is not a real-time sampling scope, hence the time base is
unknown, especially since the sampling frequency and symbol rate are
independent and essentially considered as free-running.
X constellation X I-eye diagram
Y constellation X phase pattern (512 symbols) X magnitude eye diagram