Waveform metrology provides the key to understand and analyze the waveform behavior of different systems and environments. To ensure reliable measurements with high precision, this is particularly crucial for calibrating and characterizing electrical systems, like high-speed sampling oscilloscopes, spectrum analyzers, waveform generators, etc. Additionally, full waveform metrology techniques make it possible to derive parameters from devices operating in the time domain like oscilloscopes, as well as devices operating in the frequency domain such as vector network analyzers. This ensures a comprehensive system analysis with a complete transfer function with the full measurement uncertainty represented by a covariance matrix. For high-bandwidth signals, especially for Terahertz (THz) or sub-THz communications, photonics-assisted signal processing is advantageous as it offers a much wider bandwidth compared to its electrical counterpart with no electromagnetic interference. As such, it is important to determine the transfer function (TF) of a complete optical sampling system adapting techniques from waveform metrology. Therefore, in this chapter, we perform the metrological evaluation of the optical system in terms of non-idealities like amplitude ripple, sideband suppression ratio (SSR), and spectrum roll-off factor \(\beta \) and study parameters like the root mean square error (RMSE), signal-to-noise and distortion ratio (SINAD), and the effective number of bits (ENOB).