Photoelectronic analog-to-digital conversion: sampling and quantizing at 100 Gs/s

In high-speed analog-to-digital conversion of microwave bandwidth signals, jitter on the sampling pulse train limits speed and resolution. The use of a mode-locked laser pulse train to minimize jitter has been previously reported; in such previous study, the laser pulses are converted to electrical pulses through photoconduction. Here, we report initial results on a technique in which the conversion is performed by photoelectron emission. The electron pulse train serves as the sampling comb and is emitted from a photocathode illuminated by 266-nm 1.3-ps laser pulses. Each pulse is deflected by the analog voltage applied at a pair of miniature deflector plates to land on a code plate made up of an array of electron detectors. We report results of sampling and quantizing at 100 Gs/s using a metal photocathode. Our measurements indicate that our analog-to-digital converter achieves a 3-bit resolution. Experiments are now underway that employ 200-fs 730-nm pulses and an AlGaAs/GaAs photocathode and should show at least 4-bit resolution at 100 Gs/s.