Optical techniques for broadband microwave signal generation and detection have been developed to characterize monolithic microwave integrated circuits (MMICs). Emphasis is on the enhancement of measurement accuracy and the identification of limitations. De-embedded complex S-parameters are derived from time-domain data obtained from both a GaAs photoconductive switch and electro-optic sampling of a Ka-band MMIC power amplifier. These parameters are directly compared with those measured from a network analyzer of the same circuit.