EXFO PSO-200 Optical Modulation Analyzer - Manual (Page 250)

Coherent Detection and Sampling Methods
242 PSO-200
PSO-200 Principles of Operation
The sampling is usually conducted by sequential sampling with precise
control of the sampling instant, or by random sampling that builds up the
measured signal from the asynchronous relation between the signal and
sampling frequency. The signal is reconstructed by repositioning the
time-stamped samples on a correct (equivalent-) time position. The
visualization of a time-varying signal in an eye diagram (superimposing the
signal to a limited time frame) provides information on the signal and its
impairments (symbol rate, rise time, timing jitter, intensity noise...).
The PSO-200 is designed to provide the constellation chart, including
transitions, of advanced modulation formats, such as BPSK, QPSK, APSK,
and 16-QAM. In order to achieve distortion-free measurements of the
phase-encoded data, the PSO-200 relies on a pulsed LO with short, clean,
and chirp-free pulses. The sampling frequency is lower than the symbol
rate but is sufficient to track the IF in real time.
PSO-200 Principles of Operation
Coherent detection with receiver phase diversity is required to obtain the
optical field, that is, retrieve the complete optical carrier phase
information. The PSO-200 is based on linear equivalent-time optical
sampling, where the input signal field is mixed with a pulsed LO in a
dual-polarization 90° optical hybrid, as illustrated below.