锶原子三重态谱线的观测及在光钟中的应用

实验中通过互组跃迁689 nm激光抽运形成三重态最低能态原子布居,引入688 nm激光改变三重态最低能态间的原子布居,利用抽运光与探测光空间分离的方法观测碱土金属锶原子的三重态能态间跃迁(5s6s)³S₁ → (5s5p)³P_j(j=0,1,2)的吸收谱线,对应三条跃迁线的激光波段为679 nm, 688 nm和707 nm.探测三重态原子跃迁谱线可以用于锶原子冷却中再抽运光707 nm和679 nm激光频率的直接锁定,相比于通常利用的腔传递技术,可以把再抽运光频率锁定在原子跃迁谱线上,有利于提高锶原子冷却中俘获原子数目的长期稳定性.     

不同波长的激光器通过法布里-珀罗腔相对于铯原子谱线的锁定

采用共焦法布里珀罗腔(CFP)作为桥梁,可以实现不在原子、分子跃迁线附近的单频激光器相对于原子、分子跃迁线的锁定,从而可以有效地抑制激光频率的漂移。在实验中通过射频频率调制光谱技术结合饱和吸收光谱(SAS)将自制852nm光栅外腔反馈半导体激光器锁定到铯6S1/2Fg=4-6P3/2Fe=4、5交叉线上,通过Pound-Drever-Hall(PDH)射频边带技术将作为桥梁的共焦法布里珀罗腔锁定在852nm激光频率上。再通过PDH方法将830nm和908nm两台远离铯原子D2线的外腔半导体激光器同时锁定在作为桥梁的共焦法布里珀罗腔上,实现了830nm和908nm两台激光器相对于铯原子跃迁线的锁定。由锁定后的误差信号估算,20s内852nm激光器相对于铯原子Fg=4-Fe=4、5交叉线的频率起伏小于±540kHz,830nm、908nm激光器相对于共焦法布里珀罗腔的频率起伏分别小于±340kHz和±60kHz,共焦法布里珀罗腔相对于852nm激光的频率起伏小于±550kHz。     

采用偏振光谱对外腔半导体激光器稳频时反馈环路带宽的影响

在采用磁光阱实现单个铯原子俘获的实验中, 运用无调制偏振光谱方法将光栅外腔半导体激光器(提供冷却/俘获光)的频率锁定在铯原子6S1/2 F=4→6P3/2 F′=5的超精细跃迁线上.采用偏振光谱技术得到的类色散曲线作为鉴频信号, 并同时对光栅外腔半导体激光器的电流调制端口和光栅外腔的压电陶瓷电压调制端口进行反馈, 以拓展反馈环路的带宽, 实现激光器的频率锁定.与通常的饱和吸收光谱稳频技术相比, 激光频率锁定之后的频率稳定度得到了明显改善.在取样时间τ=300 s时, 阿仑方差σy(τ)=4.6×10-12.     

铷原子双共振激发态光抽运光谱及其在1.5μm半导体激光器稳频中的应用

本文介绍分别采用双共振光抽运(DROP)和光学双共振(OODR)光谱技术获得铷原子激发态5P_3/2-4D_3/2 (4D_5/2)之间的超精细跃迁光谱.与传统的OODR光谱相比,DROP光谱在信噪比、线宽等方面具有明显的优势.当1 529 nm光栅外腔半导体激光器的频率采用DROP光谱锁定于～87Rb原子的5P_3/2(F'=3) -4D_3/2(F"=3)超精细跃迁线时,300 s内典型的残余频率起伏为～0.65 MHz;明显优于采用OODR光谱锁频的结果(300 s内典型的残余频率起伏为～1.8 MHz).     

锶原子二级Doppler冷却及温度的测量

为了减小锶原子跃迁谱线的多普勒增宽及频移,需要对锶原子进行激光冷却以降低它的速度,而一级冷却只能将原子温度降低至mK量级,这样的原子其速度过大而无法有效地装载至光晶格中,因此必须进行二级冷却.锶原子存在单重态与三重态(5s2)1S0-(5s5p)3P1间互组跃迁,利用与其跃迁波长在689 nm的窄线宽激光对锶原子进一步冷却,可将锶原子团温度降低至μK量级.利用时序有效、准确地控制磁场和光场与原子相互作用时间,通过飞行时间法对锶冷原子温度进行了测算.实验中应用计算机精确控制磁光阱区域中冷原子团下落时间,EMCCD记录冷原子团初始时刻和下落20 ms后的状态.经过分析计算二级冷却温度为4.39 μK,不确定度仅为0.19 μK,二级冷原子团数目约为1.2×10 7.低温二级冷却锶原子温度及原子数目的获得为锶光钟跃迁信号的信噪比估计提供实验参考,也是实现高精度时间频率标准的前提.     

声光偏频亚多普勒光谱无调制激光频率锁定

将激光器锁定到合适的参考频率标准上 ,可以有效地改善激光器的频率稳定性。采用两个声光调制器(AOM) ,使铯原子D2 线饱和吸收光谱分别发生Ω±Δ绝对频移 ;通过改变射频压控振荡器 (RFVCO)的Vf 端口直流电压调节相对频移间隔Δ ,当相对频率间隔选择合适时两信号相减得到了类色散型鉴频曲线。实验中实现了85 2nm光栅外腔半导体激光器相对于铯原子D2 线6S1/2 F =4 6P3 /2 F′ =5超精细跃迁线 (中心频率ν0 )的无调制偏频锁定 (锁定后中心频率ν0 +Ω ,偏频量为Ω)。由闭环锁定后的误差信号估计 ,5 0s内典型的频率起伏小于± 2 70kHz ,较相同时间段内激光器自由运转时的频率起伏 14MHz有显著的改善。该方法可避免对激光器直接进行频率调制的常规饱和吸收锁频方案所引入的额外频率噪声和强度噪声。     

锶原子互组跃迁荧光探测系统的研制

锶原子单态和三重态间的互组跃迁(5s2)1S0-(5s5p)3P1辐射率远小于一般的电偶极跃迁,共振跃迁荧光信号微弱。介绍了一种应用于探测该互组跃迁荧光谱的直流偏置探测器。该探测器选用极低输入偏置电流运算放大器作为前置放大,具有信噪比高、增益高、偏置可调等优点。此探测器探测增益为106 V/W量级,-3dB带宽为1MHz。实验中利用该探测器对锶原子互组跃迁(5s2)1S0-(5s5p)3P1微弱共振荧光进行探测,获得信噪比很好的共振荧光谱,且无直流偏置,并由此获得高信噪比的鉴频曲线。应用该探测器观测到了饱和荧光谱线以及对应的鉴频曲线,可用于689nm激光锁频,应用于锶光钟系统。     

基于电磁诱导透明效应的无调制里德堡跃迁激光锁频

我们实验上展示了利用室温Rb原子蒸汽池中的电磁诱导透明光谱实现里德堡跃迁激光无调制频率锁定方法。在Rb原子5S_(1/2),5P_(3/2),10D_(3/2)组成的级联三能级系统中,耦合激光工作于原子从中间态5P_(3/2)和里德堡态10D_(3/2)跃迁,探测激光工作于耦合基态5S_(1/2)到中间态5P_(3/2)的跃迁。通过扫描耦合光的频率并测量探测光的光强变化实现EIT信号的监测。我们对探测光进行频率调制,通过解调EIT光谱获得误差信号,并将误差信号经由PID控制电路反馈至激光器进而实现耦合激光频率的锁定。利用这种方法实现了耦合激光的无调制锁频,激光锁定后的残余频率起伏约为1 MHz。这种锁定方法对实现里德堡态原子的高效制备具有重要意义。     

光梳主动滤波放大实现锶原子光钟二级冷却光源

提出一种结合注入锁定技术的主动滤波放大方法,将光梳直接注入锁定至光栅外腔半导体激光器,产生窄线宽激光光源,该光源可以用于锶原子光钟二级冷却.实验中,将中心波长为689 nm,带宽为10 nm的光梳种子光源注入689 nm光栅式外腔半导体激光器,通过半导体增益光谱与半导体光栅外腔,从飞秒光梳的多个纵模梳齿中挑选出一个纵模模式来进行增益放大,再通过模式竞争,实现单纵模连续光输出;同时,光梳的重复频率锁定在线宽为赫兹量级的698 nm超稳激光光源上,因此,注入锁定后输出的窄线宽激光也继承了超稳激光光源的光谱特性.利用得到的输出功率为12 mW的689 nm窄线宽激光光源实现了88Sr原子光钟的二级冷却过程,最终获得温度为3μK,原子数约为5×10~6的冷原子团.该方法可拓展至原子光钟其他光源的获得,从而实现原子光钟的集成化和小型化.     

锶玻色子的“魔术”波长光晶格装载实验研究

光晶格中性原子光钟的不确定度已达到10^-18量级. 本文介绍了碱土金属锶原子玻色子⁸⁸Sr在“魔术”波长处的一维光晶格装载, 实现冷锶原子的囚禁并使锶原子的钟跃迁能级(5s²) ¹S₀-(5s5p) ³P₀在此波长处的交流斯塔克光频移一致. 实验中半导体激光器产生“魔术”光波长(813 nm), 通过实验搭建光学驻波场并获得晶格激光聚焦光束, 束腰半径为38 μm. 经过一级冷却和二级冷却后温度约为2 μK的冷锶原子被此“魔术”波长光晶格囚禁. 通过实验测量得到锶原子玻色子⁸⁸Sr光晶格寿命为270 ms, 数目约为1.2×10⁵, 温度在3.5 μK左右, 此外研究了晶格光功率对晶格囚禁原子数目及温度的影响作用. 原子的光晶格装载为后续的钟跃迁提供了长的探测时间, 为进一步的光钟闭环提供了实验基础.     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

Measurement of nonlinear coefficient of optical fiber based on small chirped soliton transmission

We measure the waveform and phase curves of short optical pulses before and after transmission over different lengths of fibers by use of the pulse analyzer with the frequency-resolved optical gating (FROG),and numerically simulate pulse evolution under the experimental conditions.The nonlinear coefficient of the fiber is given by comparing the experimental results with the numerical ones.Difference between the experiment and numerical simulation is analyzed.     

Resonance scattering of canonical elastic shells in absorbing fluid medium

Resonance scattering of elastic spherical shell and cylindrical shell while the surrounding fluid medium has absorption is studied. The normal mode solution derived using exact elastic theory and the separation of variables is still applicable. However, the scattering form function has to be modified for the absorbing medium, otherwise the unreasonable result would be obtained. The backscattering form function in the absorbing medium is redefined, and the form function of elastic spherical and cylindrical shell with vacuum or solid matter filled is calculated in various absorption conditions. The results show that the absorption of surrounding fluid leads to notable attenuation of the coincidence resonances in the mid-frequency, but it has a little influence on the low-frequency resonance scattering induced by the filler inside the shell.     

Single-beam self-referenced phase-sensitive surface plasmon resonance sensor with high detection resolution

A versatile and low-cost single-beam self-referenced phase-sensitive surface plasmon resonance(SPR)sensing system with ultra-high resolution performance is presented.The system exhibits a root-mean-square phase fluctuation of ±0.0028.over a period of 45 min.i.e.a resolution of±5.2×10-9 refractive index units.The enhanced performance has been achieved through the incorporation of three design elements:a true single-beam configuration enabling complete self-referencing so that only the phase change associated with SPR gets detected,a differential measurement scheme to eliminate spurious signals not related to the sensor response,and the elimination of retardation drifts by incorporating temperature stabilization in the liquid crystal phase modulato .Our design should bring the detection sensitivity of non-labeling SPR biosensing closer to that achievable by conventional fluorescence-based techniques.     

Influences of SiO2 protective layers and annealing on the laser-induced damage threshold of Ta2O5 films

Ta2O5 films are prepared on BK7 substrates with conventional electron beam evaporation deposition.The effects of SiO2 protective layers and annealing on the laser-induced damage threshold (LIDT) of the films are investigated.The results show that SiO2 protective layers exert little influence on the electric field intensity(EFI)distribution,microstructure and microdefect density but increase the absorption slightly.Annealing iS effective on decreasing the microdefect density and the absorption of the films.Both SiO2 protective layers and annealing are beneficial to the damage resistance of the films and the latter is more effective to improve the LIDT.Moreover,the maximal LIDT of Ta2O5 films is achieved by the combination of SiO2 protective layers and annealing.     

A stereo matching algorithm using multi-peak candidate matches and geometric constraints

Gray cross correlation matching technique is adopted to extract candidate matches with gray cross correla- tion coefficients less than some certain range of maximal correlation coefficient called multi-peak candidate matches. Multi-peak candidates are extracted corresponding to three closest feature points at first. The corresponding multi-peak candidate matches are used to construct the model polygon. Correspondence is determined based on the local geometric relations between the three feature points and the multi-peak candidates. The disparity test and the global consistency checkout are applied to eliminate the remaining ambiguous matches that are not removed by the local geometric relational test. Experimental results show that the proposed algorithm is feasible and accurate.     

Fiber Modal Index Measurements Based on Fiber Gratings

A novel method based on fiber gratings for measuring the effective indices of fiber modes is proposed. The effective index difference between the core mode and a cladding mode was obtained by analyzing the interference fringe of a pair of cascaded long-period fiber gratings. In order to extract the core mode index from the measured index difference, an index matching oil immersion method is proposed. By analyzing the interaction between the cladding mode and the oil applied on the cladding surface, the mode order and the effective index of the involved cladding mode might be calculated. Experimental results about the interference fringe shifts induced by the oil index and the oil-applied length are also presented.     

Optical Pulse Compression Schemes That Use Nonlinear Bragg Gratings

Nonlinear optical pulse compression of picosecond pulses typically requires long lengths of optical fiber and multiple components. Periodic structures, such as fiber Bragg gratings, are highly dispersive at wavelengths outside of the photonic bandgap. This implies that such gratings can be used as very short all-fiber compressors. In this paper a number of such compression schemes are reviewed involving uniform and nonuniform fiber Bragg gratings, relying on both soliton and nonsoliton compression principles. Experimental results supporting the corresponding underlying theory are also presented. Finally, an extension of one of the compression schemes is shown, which allows the generation of adjustable high-repetition rate soliton trains.