Resonant electro-optical modulator for high laser-power single-mode fiber relay

We describe the design and performance of a resonant electro-optical modulator, based on stoichiometric lithium tantalate, capable of handling high optical powers and providing a large modulation depth. This phase modulator is part of the single-mode fiber relay used in the ESO VLT Laser Guide Star Facility for adaptive optics to transport 589-nm laser light from the PARSEC dye laser installed in the laser clean room to the laser launch telescope. The purpose of the phase modulator is to broaden the single-frequency PARSEC laser linewidth before the laser light is injected into the single-mode relay fiber. By this the power handling capability of the single-mode fiber is increased by a factor of 3 or 10 W of in-coupled power at 589 nm, while maintaining the excitation efficiency of the mesospheric sodium D2 transition.