Ultrahigh-spectral-purity laser for the VIRGO experiment

We report on the frequency stabilization and on the measurement of the spectral density of frequency fluctuations (range 20 Hz to 20 kHz) and of the Allan variance (0.02 ms to 10(5) s) of a Nd:YAG laser for the VIRGO interferometric detector of gravitational waves. The short-term frequency fluctuations presented here compare favorably with those of other known oscillators. The Allan variance is better than 10(-14) for all measurement times below 0.1 s.