基于频率上转换的509nm真空压缩态光场的产生及优化

The squeezed light has been considered as an important non-classical light because it can break the shot noise limit. Quantum metrology utilizes squeezed lights to improve the precision of measurement devices. The generation of squeezed states at visible wavelengths by using parametric down-conversion exhibits technical difficulties that intense second harmonic fields at ultraviolet wavelengths can lead to high absorption and photorefractive damage. Here strongly squeezed vacuum states at 509nm will be generated by means of frequency up-conversion from the squeezed vacuum states of 1064 nm. In order to improve squeezing degree, firstly, the propagation losses between the optical cavities, non-perfect mode-matching loss and absorption loss of nonlinear crystals will be reduced. Secondly, the optimal schemes of reflectivity of input couplers and cavity length parameters are obtained respectively based on optical resonator theory and Boyd-Kleinman theory; and the frequency conversion efficiency in the four optical cavities is improved based on low pump powers. Finally, the mode-matching degree of squeezed light and local light at 509nm on the balanced beam splitter will be improved and reflection loss of the balanced homodyne detector will be reduced by means of an improved device, the detection efficiency of the photodiodes will thereby be increased. Strongly squeezed vacuum states at 509nm with about 5dB noise suppression will be obtained. The squeezed vacuum states at 509nm can be used either in quantum metrology or in the atomic control, such as the the Rydberg state manipulation of cesium atoms.

压缩态光场是一种能够突破散粒噪声极限的非经典光场，利用压缩光进行光学测量能够提高测量精度。传统的光学参量下转换技术在制备可见光压缩态光场时存在非线性晶体的热吸收及光折变损伤。本项目利用1064nm真空压缩态光场和PPKTP晶体，通过频率上转换技术实现509nm真空压缩态光场的制备。为了提高压缩度，在制备过程中首先减小实验系统中外腔之间光的传输损耗，提高激光到外腔的模式匹配率，减小非线性晶体对光的吸收。其次基于光学谐振腔理论和Boyd-Kleinman理论分别获得入射腔镜反射率和腔长参数的优化方案，提高弱光泵浦下四个光学腔的频率转换效率。最后，实现509nm压缩光与本底光在平衡分束器上的完全模式匹配，并通过改进装置减少平衡零拍探测器的光反射损耗，提高探测效率，最终实现5dB左右的509nm真空压缩光输出。该波段的压缩光可以用于光谱测量及双光子过程中铯原子里德堡态的操纵。

在激光光谱测量领域，可见光波段的真空压缩态光场能够提高测量仪器的非经典灵敏度。本项目利用509nm激光器通过非简并光学参量振荡（NOPO）、倍频（SHG）、阈值以下的简并光学参量振荡（DOPO）及弱光频率上转换（SFG）四个非线性过程制备509nm真空压缩态光场。为了提高压缩度需要提高非线性频率转换效率和减小光损耗。对于NOPO过程，通过晶体和腔长设计、阈值和泵浦倍率优化完成NOPO高效转换的优化方案；实验上实现了三共振和双共振的参量下转换，509nm泵浦光的转换效率分别为28%和37%；由于逆转换严重影响NOPO的稳定运转，我们创新地探索了泵浦光腔增强的双共振NOPO，研究了泵浦光腔增强转换效率随着入射腔镜反射率和输入泵浦倍率的关系，得到泵浦光阻抗匹配是高效率转换的条件的结论；最后我们尝试将NOPO技术与噪声免疫腔增强光外差分子光谱技术结合并应用到激光光谱检测领域。对于SHG过程，通过晶体、腔长设计及基频光到腔的阻抗匹配研究获得SHG高效转换的优化方案；实验上实现了1064nm基频光单共振倍频，倍频转换效率为23.7%。对于阈值以下DOPO过程，通过阈值和归一化泵浦功率对参数增益的影响研究获得了高的参量转换效率；通过分析探测效率、归一化泵浦光功率、腔内损耗、腔的逃逸效率、分析频率等因素对压缩度的影响，形成提高1064nm真空压缩态光场压缩度的方案。对于弱光SFG过程，基于泵浦不消耗近似和外腔共振技术建立了双波长外腔共振弱光和频模型，分析了信号光到腔的阻抗匹配对和频效率的影响，通过优化和频效率和减小腔内损耗完成了高压缩度可见光真空压缩态光场的理论优化方案；实验上实现了636nm双波长外腔共振弱光和频的输出，弱光的最高转换效率可达93%，研究发现通过双共振外腔和频产生的和频光具有强度噪声自抑制的特性，为真空压缩态光场频率上转换的实现奠定了基础。