We review techniques to characterize optical data with very high fidelity using the principle of optical sampling based on four-wave mixing in a highly nonlinear fiber. This approach results in sub-ps time resolution without artifacts in the impulse response and also excellent sensitivity allowing statistical analysis. In particular we describe implementations aimed at differentially phase encoded data (such as DQPSK) with all-optical balanced detection, formats requiring the complete signal field recovery (such as QAM) with phase-sensitive sampling, and signals requiring real-time recovery with all-optical real-time sampling. The all-optical approach to characterize optical waveform is shown to be very attractive and practical way to capture the details of various kinds of data and is, in principle, also scalable to extremely high baud rates.
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