Ultra-low threshold operation and pump squeezing with a triply-resonant CW OPO using periodically poled lithium niobate

Quasi-phase matched crystals are very promising materials for nonlinear optics because of their high nonlinear coefficient. We have built a CW triply resonant OPO using a periodically poled lithium niobate crystal. It is pumped at 1.06 /spl mu/m and produces signal and idler beams around degeneracy in the range 2-2.3 /spl mu/m. Tunability is achieved by mode hopping when the temperature is varied. This device has an ultra-low threshold of 300 /spl mu/W. It can be operated in a stable way over long period of time. Due to the so-called cascaded nonlinearities, OPO can efficiently reduce the fluctuations of the pump beam reflected by the cavity without reducing too much its intensity. We have been able to measure on the reflected pump beam a noise reduction of 30% (not corrected for detection efficiency), corresponding to 38% reduction of the quantum fluctuations below shot noise at 4 times above threshold. Our set-up constitutes an efficient and simple "quantum noise eater" for laser beam around 1 /spl mu/m in the mW range.