Electro-optic sampling of high-speed devices and integrated circuits

The operating speeds of the fastest electronic devices and integrated circuits (ICs) have surpassed the capabilities of conventional electronic measurement instrumentation. Electro-optic sampling is an optical probing technique which has ultrashort temporal resolution and is capable of noninvasively probing ICs at internal nodes. This technique is voltage-sensitive because it relies upon the electric field produced by the signal voltage on the device under test (DUT). The electric field (and hence the voltage) can be sampled because it produces birefringence in an electro-optic crystal which changes the state of polarization of an ultrashort-duration optical probe pulse that propagates through the electro-optic crystal. The electro-optic crystal is the substrate of the DUT for direct probing, is a crystal on a separate test structure for hybrid probing, and is a separate crystal placed above the DUT for external probing. Temporal resolution below 1 ps and a sensitivity below 0.1 mV/√Hz have been demonstrated (though not in the same experiment). The principles of electro-optic sampling are reviewed in this paper. Selected applications for measurement of high-speed waveforms in discrete devices and in ICs are presented.