Characterization of an external electro-optic sampling probe:Influence of probe height on distortion of measured voltage pulses

We experimentally and theoretically investigate the distortion that an external electro-optic sampling tip imprints on a measured voltage pulse that propagates along a coplanar stripline. By using the electro-optic effect of the GaAs substrate on which the stripline is fabricated we obtain the undisturbed voltage pulse, which serves as a reference pulse. This allows us to quantitatively compare the voltage pulse measured with the external electro-optic sampling tip to the undisturbed voltage pulse. The sensitivity of the sampling measurement and the distortion of the voltage pulse are determined for different heights of the tip which is lifted off the stripline. The experimental data agree very well with numerical finite difference calculations. We find that small changes of the tip height already influence the measurements strongly. From the experiments we infer an optimum working height of the sampling tip of only 0.5 μm. Our investigations help to improve the accuracy of electro-optic sampling measurements.