A photoconductive sampling technique was developed that is well suited to probing cryogenic devices such as mesoscopic devices. Low-temperature-grown GaAs photoconductive switches were embedded in a coplanar waveguide and illuminated with pigtailed optical fibers. The chip was conduction cooled in a liquid helium cryostat. The fiber-coupled switches were designed to minimize external infrared-radiation loading and to obtain efficient optical to electrical conversion. Measurements at 300, 77, and 4.2 K demonstrate very low thermal loading and a temporal resolution of 4.7 ps—arising from dispersion of the femtosecond pulses(&lgr;=810nm)in the optical fiber. ©1997 American Institute of Physics.