We have used an ultralow threshold continuous-wave optical parametric oscillator (OPO) to reduce the quantum fluctuations of the reflected pump beam below the shot noise limit. The OPO consisted of a triply resonant cavity containing a periodically poled lithium niobate crystal pumped by a Nd:YAG (yttrium aluminum garnets) laser and giving signal and idler wavelengths close to $2.12\mu \mathrm{m}$ and a threshold as low as $300\mu \mathrm{W}.$ We detected the quantum fluctuations of the pump beam reflected by the OPO using a slightly modified homodyne detection technique. The measured noise reduction was $30\%$ (inferred noise reduction at the output of the OPO $38\%).$

• Received 27 February 2001

DOI:https://doi.org/10.1103/PhysRevA.64.033815