铷87原子双光子跃迁光谱稳频特性研究
Rubidium (Rb) atomic two-photon spectra have attracted great attention in connection with small atomic frequency standards due to their narrow linewidth, absence of Doppler background, and broadening characteristics. In the past few decades, extensive research has been conducted on Rb atomic two-photon spectroscopy. As early as the 1990s, F. Nez et al. measured the absolute frequency of the two-photon transition with an uncertainty of 1.3×10-11. In 1994, Y. Millerioux et al. locked two lasers to the relevant hyperfine levels using Rb atomic two-photon transitions and achieved an instability of 3×10-13 in the 2000 s. In 2000, J.E. Bernard et al. used a frequency-doubled 1556 nm laser to precisely measure the two-photon transition frequency with a stability of 4×10-13 in 200 s. In 2020, Vincent Maurice et al. demonstrated a two-photon transition frequency standard on a micro-optical substrate using a miniature gas cell, achieving an instability of 2.9×10-12 at 450 mW power for 1 s. In 2021, Zachary L. Newman et al. reported a two-photon frequency standard at NIST with an instability of 1.8×10-13 in 100 s averaging time. The Rb two-photon optical frequency standard has the advantages of compactness and high precision, and with the support of micro-comb technology, it is expected to be adaptable to a wider range of application scenarios to become the next-generation high-performance atomic clock. Therefore, it is necessary to investigate this two-photon optical reference with compact volume and high performance.
We conducted a two-photon fluorescence spectroscopy experiment using a high-purity 87Rb vapor cell and a 778.1 nm laser. The laser was generated by an external cavity diode laser (ECDL) and stabilized by direct current modulation. The laser was split into two beams by a polarization beam splitter (PBS) and coupled into single-mode polarization-maintaining fibers. One beam was used to excite the atoms in the vapor cell, which was heated to 110 ℃, and the other beam was used as a reference for the beat frequency measurement with an optical frequency comb. The fluorescence signal was detected by a photomultiplier tube (PMT) and amplified by a trans-impedance amplifier (TIA) and lock-in amplifier. The laser frequency was locked to the zero-crossing point of the error signal using a laser servo device. The experimental setup was fixed on an optical bench with no adjustable components so as to reduce the influence of optical alignment. We used a Glan-Taylor prism to maintain polarization, two focusing lenses to enhance the fluorescence signal, a high-reflectivity mirror, collecting lenses, a high-precision heating system, and an interference filter to optimize the fluorescence signal with a high signal-to-noise ratio and a magnetic shield to minimize the Zeeman effect.
We obtained the fluorescence spectra and error signals of the two-photon transition 5S1/2-5D5/2 at 420 nm in 87Rb atoms using an external cavity diode laser (Fig.3). The laser frequency was scanned near the resonance and modulated by a sinusoidal current. We measured the dependence of the fluorescence on the laser power from 10 mW to 28.89 mW (Fig.4) and on the temperature of the Rb cell from 100 ℃ to 120 ℃. We determined the frequency shift coefficient of -7.11 kHz/mW (Fig.6), which shows a linear relationship between the optical power and the optical frequency shift over a range of optical power. We recorded the frequency distribution in two different situations (Fig.7) which shows that the beat frequency after locking is more stable than that before locking. Figure 8 illustrates the schematic diagram of the beam-focusing system. The alignment is shown near the focal point with the reflected light undeflected (left) and deflected by 0.005° (right). We tested the relation between the modulation width and the frequency shift (Fig.9). The Allan deviation of the beat frequency reached 1.50×10-12 at an averaging time of 1 s and 2.88×10-13 at 500 s (Fig.10).
A high-stability optical frequency reference based on the two-photon transition in 87Rb is developed and characterized. The system parameters such as laser power, temperature of the 87Rb cell, and modulation width are optimized for the locking performance. The key factors that limit the stability of the two-photon optical frequency reference are identified, including the signal-to-noise ratio of the spectrum, the internal modulation noise, the optical alignment of the counter-propagating beams, and the environmental sensitivity of the system structure. The two-photon optical frequency reference achieves a stability improvement of 1‒2 orders of magnitude over the conventional saturated absorption optical frequency reference and also reaches a high level among similar experimental schemes. To further reduce the frequency drift caused by environmental disturbances, future work can use low thermal expansion coefficient glass for the base and bracket of the optical components. Smaller Rb cell and optical elements are good ways to compress the optical path size. Ensuring a vacuum on the physical platform is another efficient way to decrease the influence of the environment. External modulation methods can also help to improve the system's performance.
1 引言
铷原子双光子光谱以谱线线宽窄、无多普勒本底和展宽等特点成为小型原子频标的选择之一。在过去的几十年中,铷原子双光子光频标得到了广泛研究。早在20世纪90年代,Nez等[1]就已经对铷原子双光子光谱进行了研究,测量了双光子跃迁的绝对频率,测量的不确定度为1.3×10-11。1994年,Millerioux等[2]基于铷原子双光子跃迁对相关超精细能级进行了锁定,并使用两台锁定在双光子跃迁能级上的激光器拍频,2000 s稳定度达到了3×10-13。2000年,Bernard等[3]使用1556 nm激光器倍频实现了双光子跃迁频率的精密测量,200 s稳定度为4×10-13。2020年,Maurice等[4]使用微型气室,在微型光学基板上构建了双光子跃迁频率标准,其在功率为450 mW时的1 s稳定度达到了2.9×10-12。2021年,美国国家标准与技术研究院(NIST)的Newman等[5]搭建的双光子光频标在100 s平均时间内实现了1.8×10-13的稳定度。铷的双光子光学频率标准利用其紧凑和高精度的优势,在微型光梳技术[6]的配合下,将会适应更多的应用模式与场景,有望成为新一代小型化高性能原子钟。
笔者利用778 nm外腔半导体激光器(ECDL)激发87Rb原子使其发生双光子跃迁,进而产生420 nm荧光信号,通过荧光信号将激光器频率锁定在双光子跃迁谱线上;构建了基于87Rb原子
2 基本原理与实验设置
2.1 基于双光子跃迁的420 nm荧光光谱理论
对于双光子跃迁,设两束对射光的频率分别为
由原子运动带来的多普勒频移为
对于直接双光子跃迁方式,
对于短期频率稳定度[11],
式中:
将外腔半导体激光器的波长通过调节光栅调整到778.1 nm,并将87Rb的
在没有外场、基态没有极化,而且不考虑超精细结构的情况下,态
式中:
由
87Rb原子
是与渡越时间相关的频率,其中
定义
因此,在设计光学系统时,既要增加光功率密度,提高谱线的信噪比,同时也要考虑小光斑由于增加光功率密度[15]而产生的谱线增宽。温度升高可使气室内87Rb原子的数密度增加,提高信噪比,但同时也会导致跃迁时间频率增加,从而使得谱线展宽。因此,选择合适的加热温度十分重要。
增加光功率也会导致谱线增宽,当
2.2 双光子光学频率参考实验设置
实验装置包括光路和后续信号处理两部分,如
图 2. 87Rb原子双光子光学频率参考示意图。(a)光路与激光稳频系统;(b)光学平台结构图;(c)物理系统结构图
Fig. 2. 87Rb two-photon optical frequency reference schematic. (a) Optical path and laser locking system; (b) structure of the optical bench; (c) structure of the physical system
3 稳频结果与分析
首先使用
图 3. 87Rb 5S1/2( )-5D5/2( ),420 nm双光子荧光谱线和误差信号
Fig. 3. 87Rb 5S1/2( )-5D5/2( ),420 nm two-photon fluorescence spectrum and related error signal
由
调整物理系统输入的778.1 nm激光功率,获得了10~28.89 mW光功率范围内双光子荧光谱线5S1/2(F=2)-5D5/2(F′=4,3,2,1)峰的极值与光功率的关系,如
图 4. 双光子荧光谱线5S1/2( )-5D5/2 ( )峰的极值与光功率的关系
Fig. 4. Relation of the 5S1/2( )-5D5/2 ( ) peak of two-photon fluorescence spectrum and optical power
接着测试不同温度下的荧光强度。将激光功率设为27 mW,并调节87Rb气室的温度,待气室温度稳定后对谱线信号进行采集。
由
接着进行光频移测试实验。使用
图 6. 光功率导致的频率偏移(散点为拍频数据,实线为拍频数据的拟合曲线)
Fig. 6. Frequency shift due to optical power (The dots and solid line indicate the beat frequency data and fitted shift, respectively)
从
在频率稳定度测试实验中使用锁定在87Rb双光子谱线上的ECDL和锁定在GPS驯服超稳晶振上的飞秒光学频率梳拍频。考虑到信噪比和线宽的影响,实验最终采用20 mW的光功率、110 ℃的温度和20 kHz的调制频率。比较了含有和不含有可调节光学元件的两种物理系统中拍频频率分布随时间的变化情况。
图 7. 拍频实验得到的频率分布(使用可调节光学元件的物理系统(A)和使用无可调节部件的物理系统(B)进行拍频的结果)
Fig. 7. Frequency distribution obtained by the beat frequency experiment ( the physical system with (A) or without (B) adjustable components for the beat frequency, respectively)
由于本实验装置不具备精密调节元件角度的条件,为了分析系统元件因不同位置处温度分布不均匀而导致的微小形变,对本实验的物理系统的对准情况进行了模拟,如
图 8. 物理系统光束聚焦示意图(下图展示了反射光未偏转(左)和偏转0.005°(右)时,焦点附近的对准情况)
Fig. 8. Schematic diagram of the physical system beam focus (The lower figures show the alignment near the focal point with the reflected light undeflected (left) and deflected by 0.005° (right))
此外,还测试了调制宽度对拍频频率的影响。将调制电压峰峰值对应的激光频率调制范围
使用具有螺栓锁紧结构的物理系统进行了2.5 h的拍频实验,拍频结果如
图 10. 2.5 h拍频实验中系统的Allan方差
Fig. 10. Allan variance of our system in the 2.5 h beat frequency experiment
4 结论
介绍了一种基于87Rb的双光子跃迁的高稳定性光学频率参考,并对系统参数如光功率、87Rb气室温度、调制宽度进行了优化。这些参数对于系统的锁定效果十分重要。此外,一些关键因素,如谱线的信噪比、内调制噪声、物理系统中对射光线聚焦光斑的光学对准、系统结构的环境敏感性等制约着双光子光学频率参考的稳定度。在双光子跃迁能级窄线宽的基础上,利用高信噪比荧光信号和螺栓固定结构,使双光子光学频率参考的稳定度比目前主流的基于饱和吸收的光学频率参考高1~2个数量级,在同类型实验方案中处于较高水平。为了进一步减小由环境影响造成的频率漂移,可以采用热膨胀系数更低的微晶玻璃制成的底座和支架来固定光学元件,利用尺寸更小的铷泡和光学元器件压缩光路尺寸,减小由环境扰动造成的光学对准劣化,进一步提高系统的稳定性。同时,对物理平台抽真空、使用外调制等方法也可在一定程度上提高系统的性能。
[1] Nez F, Biraben F, Felder R, et al. Optical frequency determination of the hyperfine components of the 5S1/2-5D3/2 two-photon transitions in rubidium[J]. Optics Communications, 1993, 102(5/6): 432-438.
[2] Millerioux Y, Touahri D, Hilico L, et al. Towards an accurate frequency standard at λ778 nm using a laser diode stabilized on a hyperfine component of the Doppler-free two-photon transitions in rubidium[J]. Optics Communications, 1994, 108(1/2/3): 91-96.
[3] Bernard J E, Madej A A, Siemsen K J, et al. Absolute frequency measurement of a laser at 1556 nm locked to the 5S1/2-5D5/2 two-photon transition in 87Rb[J]. Optics Communications, 2000, 173(1/2/3/4/5/6): 357-364.
[4] Maurice V, Newman Z L, Dickerson S, et al. Miniaturized optical frequency reference for next-generation portable optical clocks[J]. Optics Express, 2020, 28(17): 24708-24720.
[5] Newman Z L, Maurice V, Fredrick C, et al. High-performance, compact optical standard[J]. Optics Letters, 2021, 46(18): 4702-4705.
[6] Newman Z L, Maurice V, Drake T, et al. Architecture for the photonic integration of an optical atomic clock[J]. Optica, 2019, 6(5): 680-685.
[7] 洪毅, 侯霞, 陈迪俊, 等. 基于Rb87调制转移光谱稳频技术研究[J]. 中国激光, 2021, 48(21): 2101003.
[8] 亓航航, 杨博文, 赵浩杰, 等. 应用于积分球冷原子钟的窄线宽激光稳频系统[J]. 激光与光电子学进展, 2023, 60(15): 1514008.
[9] 项静峰, 王利国, 任伟, 等. 采用射频调制实现对单频激光器频率噪声的抑制[J]. 中国激光, 2017, 44(5): 0501009.
[10] 范鹏瑞. 基于铷原子双光子跃迁高分辨光谱研究[D]. 太原: 山西大学, 2017.
FanP R. Investigation on high resolution two-photon transition spectroscopy of rubidium atom[D]. Taiyuan: Shanxi University, 2017.
[11] 涂建辉, 梁耀廷, 陆昉, 等. 提高铷原子频标短期稳定度的研究[J]. 宇航计测技术, 2011, 31(4): 56-58.
Tu J H, Liang Y T, Lu F, et al. Research on improving the short-term stability of rubidium frequency standard[J]. Journal of Astronautic Metrology and Measurement, 2011, 31(4): 56-58.
[12] 戴作耀, 雷体仁. 无多普勒双光子跃迁几率的计算[J]. 原子与分子物理学报, 1990, 7(S1): 165-167.
Dai Z Y, Lei T R. Calculation of two-photon transition probability without Doppler[J]. Chinese Journal of Atomic and Molecular Physics, 1990, 7(S1): 165-167.
[13] Grynberg G, Cagnac B. Doppler-free multiphotonic spectroscopy[J]. Reports on Progress in Physics, 1977, 40(7): 791-841.
[14] Sheng D, Pérez Galván A, Orozco L A. Lifetime measurements of the 5D states of rubidium[J]. Physical Review A, 2008, 78(6): 062506.
[15] Thomas J E, Kelly M J, Monchalin J P, et al. Transit-time effects in power-broadened Doppler-free saturation resonances[J]. Physical Review A, 1977, 15(6): 2356-2365.
[16] Terra O, Hussein H. An ultra-stable optical frequency standard for telecommunication purposes based upon the 5S1/2→5D5/2 two-photon transition in rubidium[J]. Applied Physics B, 2016, 122(2): 27.
[17] Zhang S Y, Wu J T, Zhang Y L, et al. Direct frequency comb optical frequency standard based on two-photon transitions of thermal atoms[J]. Scientific Reports, 2015, 5: 15114.
孟一鸣, 项静峰, 徐斌, 李彪, 万金银, 任伟, 邓思敏达, 张迪, 吕德胜. 铷87原子双光子跃迁光谱稳频特性研究[J]. 中国激光, 2023, 50(23): 2301013. Yiming Meng, Jingfeng Xiang, Bin Xu, Biao Li, Jinyin Wan, Wei Ren, Siminda Deng, Di Zhang, Lü Desheng. Frequency Stabilization Characteristics of 87Rb Two‑Photon Transition Spectrum[J]. Chinese Journal of Lasers, 2023, 50(23): 2301013.