【作者】 金丽；

【导师】 马龙生； 蒋燕义；

【作者基本信息】 华东师范大学 ， 光学， 2019， 博士

【摘要】 激光频率精密控制是精密光谱和精密测量的基石,激光频率的控制精度在很大程度上决定了精密光谱与测量的分辨率与精度。经过频率精密控制的激光,具有线宽窄、频率稳定度高的特点,它是新一代时间频率标准-原子光钟、高分辨率光谱、引力波探测、低噪声微波源和光学频率合成器等研究的关键部分。这些应用迫切需要频率稳定度更高、线宽更窄的激光。如将窄线宽激光的频率不稳定度从10^-15提高至10^-16,可将光钟的频率稳定度提高一个数量级,从而能使光钟实现10^-18精度的测量时间从10⁶秒（连续11天以上）减小至一天之内。对于精度更高的光钟和用于空间引力波探测的超长（大于10⁸米）激光干涉仪,还需要10^-17甚至10^-18频率不稳定度的窄线宽激光。为了这个研究目标,本论文在实验上实现了频率不稳定度为10^-16的稳频激光,并在理论上探索如何实现10^-18频率不稳定度的窄线宽激光。在实验上,为了突破参考腔的热噪声限制的激光频率不稳定度在10^-15量级,本文搭建了两套热噪声限制的频率不稳定度为1.6 x 10^-16的F-P光学参考腔系统。采用PDH技术将578 nm激光的频率高精度地锁定在参考腔上。为了获得频率不稳定度为10^-16的激光,本论文重点克服了环境振动噪声对激光系统产生的影响。通过数值模拟计算结合反复的实验测量,选择最佳的参考腔结构和支撑方式,最终将参考腔对外界振动的敏感度降低至5 × 10^-10 g^-1以下,此时振动噪声的贡献接近参考腔的热噪声极限。并且,为了提高激光频率的控制精度,设计了低剩余幅度调制的电光调制器,研制了光纤传输位相噪声抑制系统、高稳定度光功率控制系统和高精度激光频率伺服系统,使得这些噪声对激光频率不稳定度的影响均小于10^-16量级。最后,通过两套稳频激光系统之间频率比对测试,证明每台激光的线宽已压缩至0.2 Hz,频率不稳定度达1.8 x 10^-16（1秒积分时间）,已接近光学参考腔热噪声限制的频率不稳定度。由于受参考腔热噪声的限制,目前利用PDH技术将21 cm单晶硅腔冷却至124 K下获得的激光频率不稳定度为4 x 10^-17,为获得10^-18量级频率不稳定度的稳频激光在技术上是一大挑战。于是,本论文提出了基于冷锶原子系统的四波混频过程产生超窄线宽稳频激光的新方案。由于锶原子的钟跃迁能级具有mHz量级的自然线宽,在四波混频过程中在三束入射光相干极化的作用下,可产生线宽窄、频率稳定度高的混频激光。混频光的线宽将受限于钟跃迁能级的自然线宽而不是入射激光的线宽。最后,通过半经典模型的理论分析,当功率不稳定度为10^-4量级的入射光（具有· Hz线宽）与光晶格中密度为1 × 10¹¹ cm^-3的原子相互作用,可产生具有pW量级功率、线宽为4 mHz、频率不稳定度达2 × 10^-18的混频激光。该方案为下一步开展实验研究提供了理论依据。更多还原

【Abstract】 Precision control of laser frequency is the foundation of precision spectroscopy and precision measurement since it determines the resolution and accuracy of precision spectroscopy and precision measurement.Lasers with ultra-narrow linewidth and high frequency stability are the key component for developing optical atomic clocks-the new generation of time/frequency standards,high-resolution spectroscopy,laser interferometers for gravitational wave detection,low-noise microwave sources,and optical frequency synthesizers.These applications urgently require lasers with higher frequency stability and narrower linewidth.Taking optical atomic clocks as an example,when the frequency instability of narrow-linewidth lasers as local oscillators is reduced from 10^-15 to 10^-16,the frequency instability of optical clocks has been improved from 10^-15/√τ to 10 16/√τ（τis the averaging time）,enabling 10^-18 frequency precision in an averaging time of less than one day.In order to develop optical atomic clocks with even better frequency stability and detect gravitational wave in space based on laser interferometer with more than 10⁸-meters-long arms,it is necessary to develop narrow-linewidth lasers with a frequency instability of 10^-17 or even 10^-18.Towards this goal,in this thesis we construct narrow-linewidth lasers with 10^-16 frequency instability experimentally and theorectically investigate the possibilities of 10^-18 laser frequency instability.To obtain a thermal-noise-limited laser frequency instability better than 10^-15,we construct two frequency-stabilized laser systems at 578 nm by stabilizing their frequencies to the resonance of stable optical reference cavities via the Pound-Drever-Hall technique.The reference cavities are designed to have a thermal-noise-limited length instability of 1.6 x 10^-16.Environmental vibration noise has a great effect on the length instability of the reference cavities,and thus the frequency instability of the cavity-stabilized lasers.We obtain the optimal structure and support positions of the reference cavities by combining numerical simulation calculations with experimental measurements.The vibration sensitivities of the optical reference cavities in all directions are reduced to be less than 5 × 10^-10 g^-1.Based on the vibration insensitive supporting,the frequency noise of the cavity-stabilized lasers induced by vibration noise is close to the cavity-thermal-noise-limited frequency noise.In addition,we reduced the laser frequency noise arisen from residual amplitude modulation of EOMs,random phase noise introduced by optical fibers and light power fluctuations below the thermal-noise-limited length instability of the reference cavities.Based on the frequency comparison between two similar laser systems,the most probable linewidth of each laser is measured to be 0.2 Hz,and the fractional frequency instability of each laser system is 1.8 × 10^-16 at 1 s averaging time,approaching to the thermal-noise-induced length instability of the reference cavitiesSo far,the frequency instability of the cavity-stabilized lasers can reach to 4 x 10^-17 level,which are stabilized to independent 21-cm-long silicon Fabry-Perot cavities operated at 124 K.Achievement of the ultrastable lasers with a frequency instability of 10^-18 would be a technical challenge.Thus,we propose a novel scheme for generation of ultra-narrow linewidth laser based on four-wave mixing in cold strontium（Sr）atoms.Benefiting from the natural linewidth of the clock transition of Sr atoms,the mixing laser light can generate with narrow linewidth and high frequency instability.We show that the linewidth of the mixing laser light is ultimately limited by the natural linewidth of the atomic clock transition rather than by the linewidth of the input lasers.Based on the theoretical analysis,it is possible to generate a 4 mHz-linewidth laser light with a frequency instability of 10^-18 and a power of 10-12 W when the input lasers with a relative intensity instability of 10^-4 and a spectral width of 1 Hz interact with Sr atoms with a density of 1 × 10¹¹ cm^-3.The theoretical exploration described here is a basis for the future experimental research.更多还原