The 20 watts Nd:YAG laser of VIRGO is one of a new generation of ultrastable lasers, and one of the most stable oscillators ever built. Before being injected in the interferometer, the laser beam is modulated, stabilized in frequency and amplitude, and the unwanted higher order resonance modes are removed through a 140 m long resonant “mode cleaner” cavity, leaving only the fundamental TEM00 mode.

The mirrors of VIRGO combine the highest surface quality (better than a hundredth of a micron) with extremely low diffusion and absorption (less than 1 ppm). In collaboration with industry, substrates with exceptional properties of low absorption and high homogeneity have been produced and a unique large size coating facility (2.5x2.5 m) has been built for the realization of ultra low absorption and diffusion coatings. Highly sophisticated metrology has also been developed to control the exceptional performance attained.