Abstract: A first phase modulator was made of a couple of 3 multiplied by 3 multiplied by 22 mm$+3$/ KTP crystals set into an open rectangular coaxial line in such a way that the electric field in the active medium was parallel to the crystallographic Z axis. The line was driven by a single 4 kV, 140 ps FWHM pulse through an inductive connector in the middle, giving rise to damped oscillations at 4 GHz. Phase modulation of laser beams passing through the 44 mm of KTP was due to an efficient Pockels effect owing to similar electric and optical wave velocities in the device. This latter was presented at IAEA Conference, Paris, France November 1994. The main characteristics are reviewed here, however, two noticeable modifications are introduced. The first modification is in the choice of the connector inductance to get the best compromise between voltage transmission and damping rate. The second modification consists in using a second high voltage pulse delayed and synchronized in resonance with the first one to add the respective effects. Nevertheless the combination of both pulses in the same feedthrough involves a reduction of pulse amplitude down to 2.5 kV. Fabry-Perot spectra are shown for 1.5 ns, 1053 nm single frequency laser beams entering the modulator at different working times, less than 20 ns. The measured bandwidths are between 25 and 60 pm but 2 time greater values are expected from a new design driven by two independent pulses of 4 kV. !0
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