Fast waveform metrology : generation, measurement and application of sub-picosecond electrical pulses
This thesis describes work performed at the National Physical Laboratory to improve the electrical risetime calibration of instruments such as fast sampling oscilloscopes. The majority of the work can be divided into four sections: development of an ultrafast optoelectronic pulse generator; measurement of fast electrical pulses with an electrooptic sampling system; de-embedding of transmission line and transition effects as measured at different calibration reference planes; and calibration of an oscilloscope. The pulse generator is a photoconductive switch based on low-temperature Gallium Arsenide, which has a very fast carrier recombination time. Sub-picosecond electrical pulses are produced by illuminated a planar switch with 200 fs optical pulses from a Ti: sapphire laser system. The pulses are measured using a sampling system with an external electro-optic probe in close proximity to the switch. The electro-optic sampling system, with a temporal resolution better than 500 fs, is used to measure the electrical pulses shape at various positions along the planar transmission line. The results are compared to a pulse propagation model for the line. The effects of different switch geometries are examined. Although the pulse generator produces sub-picosecond pulses near to the point of generation, the pulse is shown to broaden to 7 ps after passing along a length of transmission line and a coplanar-coaxial transition. For a sampling oscilloscope with a coaxial input connector, this effect is significant. Frequency-domain measurements with a network analyser, further electro-optic sampling measurements, and the transmission line model are combined to find the network transfer function of the transition. Using the pulse generator, the electro-optic sampling system and the transition knowledge, a 50 GHz sampling oscilloscope is calibrated. The determination of the instrument step response(nominal risetime 7 ps) is improved from an earlier value of 8.5 -3.5 / +2.9 ps to a new value of 7.4 -2.1 / +1.7 ps with the calibration techniques described.