A new scheme for coherently connecting optical frequencies in a 3:1 ratio has been demonstrated. To phase lock a Nd:YAG laser at 1064 nm with a CO overtone laser at 3192 nm, we generated their difference frequency in RbTiOAsO4 (RTA) and beat it against the second harmonic of 3192 nm that was generated in AgGaSe2.
Olivier Pfister, Manfred Mürtz, Joseph S. Wells, Leo Hollberg, and James T. Murray, "Division by 3 of optical frequencies by use of difference-frequency generation in noncritically phase-matched RbTiOAsO4," Opt. Lett. 21, 1387-1389 (1996)