Subpicosecond Electro-Optic Sampling Using Coplanar Strip Transmission Lines

Abstract

A sampling technique using the electro-optic effect capable of characterizing subpicosecond electrical signals has recently been demonstrated. In this system 100 fs pulses from a colliding pulse mode-locked laser were used to generate the electrical test signal via a Cr:GaAs photoconductive switch. A second beam of pulses was used to probe the birefringence induced by the electrical pulses as they propagate down a balanced stripline fabricated on LiTa0₃. Signal averaging allowed submillivolt signals to be recovered. The best temporal response of the system was achieved with the velocity matched geometry, which was obtained for a particular angle of incidence of the probe beam on the electro-optic crystal. In this configuration a risetime of 500 fs was obtained. During this work it was observed that strong dispersion effects were taking place when the wavelength of the electromagnetic signal approached the cross-sectional dimensions of the transmission line. Temporal resolution is improved and dispersion effects are reduced as the dimensions of the stripline are reduced. In order to further improve the temporal response of the sampler in this configuration, the thickness would have to be reduced to the order of 10 um, which presents severe fabrication difficulties.