Abstract
We propose and demonstrate a new method for phaselocking the signal fields emitted above threshold by a nondegenerate, type-II optical parametric oscillator (OPO). This method is based on the observation that amplitude modulation of the pump beam produces a related modulation of the frequency difference of the OPO signals via the temperature-tuning of the index of refraction in the nonlinear crystal. We successfully use pump modulation as a correction for phase-difference locking of the OPO signals and observe a 1 kHz beat note stable over more than 10 s, both figures solely limited by the measurement time. This method eliminates the need for applying electronic phase-correction signals directly to the nonlinear crystal which caused crystal damage in a previous phaselocking technique.
Highlights
Ultrastable quantum optics is a tool of choice for precision measurements at the ultimate quantum limit
The orthogonally polarized twin beams emitted above threshold by a type-II optical parametric oscillator (OPO) were electronically phaselocked to each other using the tuning of the OPO via the electro-optic (EO) effect of its Na:KTiOPO4 (Na:KTP) nonlinear crystal
Note that this classical noise reduction on the phase difference occurred in the DC-10 kHz servo bandwidth, while the quantum noise reduction on the amplitude difference took place within the OPO cavity linewidth which extended to several MHz
Summary
Ultrastable quantum optics is a tool of choice for precision measurements at the ultimate quantum limit. The necessity for such a high voltage arose from our observation of an unexpected decrease of the EO effect at low frequencies (below 1 kHz) of the phaselock error signal. This phaselock method is based on the observation that a variation of the intensity or polarization of the OPO pump beam yields a change in the frequency difference of the signal beams via the photothermal effect [15]. Residual absorption of this light leads to a thermal change in the OPO’s nonlinear crystal, which varies the crystal’s refractive indices and leads to a change in the frequency difference between the twin beams output by the OPO
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