Four-Photon Optical Parametric Noise in Water

Optical parametric noise has been produced in a centrosymmetric medium, water, by a 9-MW ruby laser beam. The wavelengths, in the visible, and the angles of emission agree with the tuning curve calculated for a phase-matched four-photon process. A power of ≈½μW emitted into a 38-Å band is observed in the low-loss wavelength range, in approximate agreement with theory. Very strong idler absorption has little apparent effect on the signal power or bandwidth.