Photoconductive sampling is a technique of optical sampling, based on the use of photoconductive switches. On such a switch, an ultrashort laser pulse can close an electrical connection for a very short time. A second photoconductive switch (driven with a delayed laser pulse) may be used for sampling of the resulting signal at a different location: the switch connects the corresponding part of the circuit with some output lead only for a very short time interval, the temporal position of which can be adjusted via an optical delay line.
An alternative sampling technique is electro-optic sampling. Compared with that, photoconductive sampling has the main advantages of allowing the generation of signals with higher frequencies (even without very fast electrical connections) and operating with higher spatial resolution, but it usually requires the fabrication of photoconductive switches at the test points (even though free-moving external probes have been demonstrated) and cannot be used for measuring electric fields outside conductors.
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