Photon pairs and heralded single photons, obtained from cavity-assisted parametric down-conversion (PDC), play an important role in quantum communications and technology. This motivated a thorough study of the spectral and temporal properties of parametric light, both above the Optical Parametric Oscillator (OPO) threshold, where the semiclassical approach is justified, and deeply below it, where the linear cavity approximation is applicable. The pursuit of a higher two-photon emission rate leads into an interesting intermediate regime where the OPO still operates considerably below the threshold but the nonlinear cavity phenomena cannot be neglected anymore. Here, we investigate this intermediate regime and show that the spectral and temporal properties of the photon pairs, as well as their emission rate, may significantly differ from the widely accepted linear model. The observed phenomena include frequency pulling and broadening in the temporal correlation for the down-converted optical fields. These factors need to be taken into account when devising practical applications of the high-rate cavity-assisted SPDC sources.

中文翻译：

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量子范围内低于阈值的光学参量振荡器的光谱特性的非线性功率依赖性

从腔辅助参数下变频 (PDC) 获得的光子对和前兆单光子在量子通信和技术中发挥着重要作用。这激发了对参量光的光谱和时间特性的彻底研究，无论是在光学参量振荡器 (OPO) 阈值之上（半经典方法是合理的），还是在其深度之下，线性腔近似是适用的。追求更高的双光子发射率导致了一个有趣的中间状态，其中 OPO 仍然在远低于阈值的情况下运行，但不能再忽略非线性腔现象。在这里，我们研究了这个中间状态，并表明光子对的光谱和时间特性，以及它们的发射率，可能与广泛接受的线性模型有很大不同。观察到的现象包括下变频光场的时间相关性中的频率牵引和展宽。在设计高速腔辅助 SPDC 源的实际应用时，需要考虑这些因素。