The theoretically predicted correlation of laser phase fluctuations in $\Lambda$-type interaction schemes is experimentally demonstrated. We show that the mechanism of correlation in a $\Lambda$ scheme is restricted to high-frequency noise components, whereas in a double-$\Lambda$ scheme, due to the laser phase locking in a closed-loop interaction, it extends to all noise frequencies. In this case the correlation is weakly sensitive to coherence losses. Thus the double-$\Lambda$ scheme can be used to correlate electromagnetic fields with carrier frequency differences beyond the GHz regime.

• Received 10 March 2004

DOI:https://doi.org/10.1103/PhysRevLett.93.223601