【作者】 徐萍；

【导师】 武海斌；

【作者基本信息】 华东师范大学 ， 光学， 2022， 硕士

【摘要】 近年来,超冷原子物理极大推动了量子模拟、量子信息和精密测量等前沿科学研究的进展,逐渐成为物理研究领域的热点。与传统的碱金属超冷原子相比,镧系元素如铒（Er）、铥（Tm）、镝（Dy）等还具有长程的各向异性偶极-偶极相互作用（DDI）,因此镧系超冷原子在研究偶极量子气体方面具有重要的科学意义。铒元素不仅具有丰富的能级结构,还具有不同丰度的同位素,涵盖玻色子和费米子,为研究偶极量子气体的新奇物理现象提供了便利,因此我们选择铒原子为研究对象。在冷原子实验中,激光频率和功率的稳定度是影响实验结果的重要因素,故本文的研究工作围绕超冷铒原子实验激光系统的需求展开。在超冷原子物理实验中,需要将磁光阱内囚禁的原子团装载到偶极阱中进行蒸发冷却获得超冷原子,远失谐偶极阱俘获光的强度起伏会导致阱深的变化,使得阱中的原子被加热而缩短偶极阱的俘获寿命,影响最终的原子数和温度。鉴于我们超冷铒原子系统对低频的强度噪声非常敏感,本文结合声光调制器采用外部伺服反馈射频功率的方法搭建了一套激光功率稳定的实验系统,稳定度达到7.3×10^-5。另外,分别用自制PI电路与商用SIM960反馈锁定光功率后,进行频谱分析对比,发现自制电路在抑制激光低频强度噪声方面有明显优势。在超冷铒原子实验系统中,由于窄线宽的583nm跃迁（自然线宽Γ=2π×190k Hz）具有较低的多普勒冷却温度极限（4.6μK）,所以国际上普遍采用该跃迁作为磁光阱的俘获光。其中,为了维持实验系统长时间稳定运转,窄线宽、高稳定性的583nm光源是激光冷却与囚禁铒原子实验系统中的关键技术之一。基于参考腔的Pound-Drever-Hall（PDH）稳频技术在窄线宽激光稳频中应用非常广泛,但腔内温度起伏、腔体材料内部存在应力等因素的影响,参考腔存在长期漂移。针对以上问题,我们将PDH技术与碘分子调制转移谱结合进行两级锁定,利用PDH稳频技术作为前级反馈将激光器稳定于光学参考腔,抑制短期起伏和压窄激光线宽,以碘分子调制转移谱稳频作为后级反馈,克服光学参考腔不可避免的长期漂移。为了评估两级锁定后激光的长期稳定性,将稳频后的激光与光学频率梳的超稳激光拍频。4小时的拍频监测结果表明,与仅用PDH稳频时的长期漂移205k Hz相比,双系统稳频时最大频率起伏为±12k Hz,满足超冷铒原子实验系统长时间稳定运转的实验需求。本文激光功率、频率的稳定方案均具有普适性,可为其它镧系元素如铥（Tm）、铕（Eu）等超冷偶极量子气体实验系统的搭建提供参考。更多还原

【Abstract】 In recent years,ultracold atomic physics has greatly promoted the progress of quantum simulation,quantum information and precision measurement,so it has gradually become the focus of physics research.In contrast to conventional ultracold alkali atomic gases,lanthanide atoms(e.g,erbium(Er),thulium(Tm),dysprosium(Dy))have exceptionally large magnetic dipole moments due to non-S electronic ground states.Therefore,lanthanides not only have short-range isotropic contact interactions,but also have long-range anisotropic dipole-dipole interactions(DDI).As a result,lanthanides have important scientific significance in the many-body dipolar system.Erbium with rich energy level structure and different abundance of isotopes,including bosons and fermions,offers convenience for studying novel physical phenomena of dipolar quantum gas.Therefore,erbium is an ideal candidate for studying dipolar gases.In an ultracold atomic experiment,it is necessary to load an atom cloud from a magneto-optical trap(MOT)into the far-off resonance optical dipole trap(ODT)for evaporative cooling to obtain a quantum gas.The intensity fluctuation of the dipole far-detuning capture laser will lead to the change of the trap depth,resulting in the heating of atoms and consequently shortening of the capture lifetime,which will affect the final atomic number and temperature.Because of that our ultracold erbium atom system is very sensitive to the intensity noise of low frequency,an experimental system of laser power stability is established in this paper by external servo feedback of radiofrequency power of acousto-optic modulator.In addition,by analyzing and comparing the spectrum of residual power noise of the homemade PI circuit and the commercial SIM960 PID controller,the homemade circuit has obvious advantages in suppressing the low-frequency intensity noise.In the ultracold erbium atom experiment,due to the narrow linewidth of the 583-nm transition(natural linewidth Γ = 2π × 190 k Hz),it has a low Doppler cooling temperature limit(4.6μK),so the 583-nm laser is widely used as the capture light in the magneto-optical trap.In order to maintain the stabilization during running the experiment,the 583-nm laser with narrow linewidth and high stability is one of the key technologies in the laser cooling and trapping erbium atom experimental system.The Pound-Drever-Hall(PDH)technique is one of the most powerful active laser stabilization techniques to narrow the linewidth and suppress the short-term frequency fluctuation,where the laser frequency is locked to the resonance of a reference cavity.However,due to the internal stress of the cavity material and temperature fluctuation,the reference cavity inevitably has long-term drift.In order to solve this problem,we employ a two-stage locking system by combining the PDH technique together with the modulation transfer spectrum(MTS)of iodine molecule.Specifically,the583-nm laser is first pre-stabilized by the reference cavity via the Pound-Drever-Hall(PDH)technique to suppress short-term fluctuation and narrow laser linewidth,while the MTS of iodine molecule serves as the secondary feedback to overcome the inevitable long-term drift of the optical reference cavity.To evaluate the stability of the two-stage locking laser,the stabilized laser beats with an optical frequency comb.During up to 4 hours of monitoring,the long-term drift of laser frequency stabilized only cavity is 205 k Hz.By contrast,the maximum fluctuation is within ±12k Hz in the case of two-stage laser stabilization,which meets the requirements of long-term stable operation of ultracold erbium experiment.The laser power and frequency stabilization schemes in this paper are universally used in the ultracold experiment.Moreover,it may provide a strategy to build up other possible dipolar experimental systems with lanthanides such as europium(Eu)and thulium(Tm).更多还原