Quantum-mechanical noise characteristics in a doubly resonant optical parametric oscillator

We obtain information about the amplitude fluctuations of both signal and idler beams from a doubly resonant optical parametric oscillator (DRO) without an optical isolator or a frequency stabilizer. To get a total noise power spectrum, we digitized the photocurrents of the cross-polarized twin beams from the DRO and calculated the noise spectra of the two beams from the digitized data; we also calculated the sum and the difference of the two beams. We fabricated a DRO with a KTiOPO4 (KTP) crystal pumped by a single-mode Ar+ laser (514.5 nm). The experimental results of the quantum-mechanical correlations of the twin beams are well fitted by a theoretical prediction [ J. Opt. Soc. Am. B4, 1520 ( 1987)].