碘吸收激光稳频中调制信号的理论计算

激光频率标准是将激光频率锁定于原子或分子的超精细能级间的跃迁频率,从而获得高精密度的激光频率输出。在激光稳频研究工作中,通常将碘分子的吸收谱线采用导数谱、FM光谱和调制转移光谱MTS(ModulationTransferSpectroscopy)进行频率的精密控制。为更好地消除吸收谱线的本底噪声,目前最好的稳频方案是采用调制转移光谱法。它具有高灵敏度、高分辨率和无多普勒背景等特点。从物理学的原理出发,分析了激光对介质的极化机理,导出介质对激光的吸收和色散作用的数学模型,用信号处理的技术从理论上推导了MTS谱线线型,分析了光谱线型的特性,讨论了提高激光频率稳定度的各种情况。     

调制转移光谱光频率标准系统中电光参数的优化研究

根据窄带相位调制光在非线性吸收介质中的近简并四波混频的物理机理,分析了采用电光调制器实现调制转移光谱(MTS)技术的电磁场激励及其动态演化过程,建立了MTS中频率色散谱和吸收谱的理论模型. 研究结果表明:以分子吸收谱线色散谱信号作为频率误差鉴别信号时,系统调制频率取0.72倍谱线线宽附近可以得到最佳的色散信号强度;解调相位的变化对吸收谱的影响很大,但对色散谱而言,解调相位在90°附近的小范围波动对频率误差信号解调效果影响不大;调制度在0.5–1区域可以得到较好的综合稳频效果. 关键词： 光频率标准 调制转移光谱 频率稳定 电光参数优化     

海面微波散射场多普勒谱特性研究

基于粗糙面电磁散射双尺度模型推导给出了海面微波散射场多普勒谱频移和谱宽的理论公式, 在该理论公式的推导过程中同时考虑了大尺度海浪的倾斜调制、遮蔽效应和曲率修正效应等因素的影响. 文中将理论公式计算结果与精确数值结果进行了比较, 并讨论了倾斜调制、遮蔽效应及曲率修正效应等因素对多普勒频移和谱宽的影响, 发现倾斜调制使水平极化散射回波多普勒频移显著增大, 从而导致水平极化回波多普勒频移比垂直极化回波多普勒频移大; 在中等入射角度区域, 遮蔽效应和曲率修正效应对多普勒谱并无显著影响, 而在掠射条件下, 遮蔽效应使得多普勒频移增大、谱宽变窄. 本研究对深入理解动态海面散射场频谱特性具有一定参考意义.     

激光稳频中的高灵敏度光谱技术

高灵敏度谱分析技术是激光稳频系统的关键。通过系统的理论分析,结合国内外研究的稳频系统,详尽介绍了导数谱和ＦＭ 光谱激光稳频技术,并对其进行了分析、比较。两者的根本区别在于ＦＭ 光谱技术具有相对较大的调制频率,这是它们在应用中各具特点的根源：ＦＭ 光谱技术具有更高的灵敏度和噪声抑制能力,可以探测到０－００５ ％ 的微小吸收,而导数谱技术对未知的超精细谱线结构具有较好的兼容性。     

二维粗糙海面散射回波多普勒谱频移及展宽特征

基于二维线性海面模型及粗糙面电磁散射的一阶小斜率近似公式,研究了海面回波各阶多普勒谱的频移特性,得到了多普勒谱频移所对应的理论公式,与经典多普勒频移公式相比考虑到了风速、大尺度波浪轨道运动等因素对多普勒谱频移的影响;同时根据粗糙面的双尺度模型,基于多普勒谱频移的物理机理,给出了求解散射场多普勒谱展宽的理论公式.最后将理论公式计算所得结果与模拟结果及实测数据进行了比较,讨论了风速、入射波频率及入射角等因素对多普勒频移及多普勒谱展宽的影响.通过比较可以发现,给出的理论公式可以对多普勒谱频移和展宽进行一定程度的预测. 关键词： 粗糙海面 电磁散射 多普勒谱     

采用铷原子射频频率调制光谱与调制转移光谱对1560nm激光经波导倍频至780nm进行稳频的比较

采用射频频率调制光谱(RF-FMS)和调制转移光谱(MTS)将1560nm分布反馈激光器经MgO:PPLN波导高效倍频后锁定于87 Rb原子D2线超精细跃迁。比较了两种方法的原理、谱线以及锁频结果。激光器自由运转30min的典型频率起伏为8~10MHz,采用RF-FMS和MTS锁频30min的频率起伏均方根分别为±135kHz和±85kHz,这主要是由于后者的信噪比高,谱线完全无背景。采用MTS方案,搭建了一套结构紧凑、性能稳定的1.5μm光纤通信波段激光稳频系统。     

空气中激光等离子体通道的三次谐波光谱特性研究

对不同条件下强激光在空气中形成等离子体通道的三次谐波光谱特性进行了研究。单脉冲能量12 mJ,脉宽30 fs,重复频率10 Hz,中心波长795 nm的飞秒激光脉冲经0.5 m焦距的凹面镜聚焦,在空气中形成了等离子体通道, 并在前 向观测到谱线半峰全宽（FWHM）为15 nm的三次谐波。随着脉冲啁啾的变化,三次谐波的光谱出现红移或兰移,当激光脉冲附带+1.3×105fs2的二阶色散时,三次谐波谱线红移且谱峰强度增长了两倍。同时 ,通过改变可编程声光色散滤波器（AOPDF）光谱调制的位置（Hole position）,三次谐波的光谱也发生频移。     

注入锁定激光器的边带锁频技术稳频系统优化分析

介绍了边带锁频技术稳频方法,推导出鉴频曲线,对稳频控制过程进行了分析.建立了注入锁定激光器的边带锁频技术稳频系统理论模型,讨论了相关参量对稳频效果的影响并且进行了优化.结果表明,增加入射光强,采用窄线宽的法布里-珀罗以及对法布里-珀罗进行高准确度温控都可以增强稳频效果.提出适用于注入锁定激光器的两种稳频方案并进行比较.     

一种利用布里渊增益谱边带解调提高布里渊光时域反射系统测温精度的方法

测温精度是衡量分布式光纤温度传感系统的一项重要性能指标.本文提出一种通过解调布里渊增益谱边带,以提高布里渊光时域反射仪测温精度的方法.在此基础上,进一步分析并验证了探测光脉冲峰值功率对测温精度的影响.理论分析表明,利用声光调制器的频移特性可产生布里渊增益谱边带,相比于中心峰解调方法,采用布里渊增益谱边带解调法可获得更高的系统信噪比,进而提高系统测温精度.实验结果表明,在相同测量条件下,布里渊增益谱左边带峰值功率较其中心峰峰值功率高3.27 dB,且其-1 dB谱宽比中心峰窄14.5 MHz.对布里渊增益谱左边带进行频率扫描,由于相干探测时参考光的作用以及消除了相干瑞利噪声的影响,系统信噪比提高了4.35 dB,并在10.2 km的传感距离上实现了±0.5?C的测温精度.     

强耦合光机械腔中的简正模式分裂和冷却

研究由辐射压力与驱动Fabry-Perot光学腔相耦合而产生的腔光机械动力学行为. 通过量子朗之万方程具体研究了机械振子的涨落光谱、机械阻尼与共振频移和基态冷却. 随着输入激光功率的增加,振子的涨落光谱呈现简正模式分裂的现象,并且数值模拟结果和实验结果相符合. 同时推导了有效机械阻尼和共振频移. 红移边带导致了机械模的冷却,蓝移边带引起了机械模的放大. 此外,引入一种近似机制来研究振子的基态冷却,并且考虑在解析边带机制下简正模式分裂对机械振子冷却的影响. 最后,数值讨论了初始浴温度、输入激光功率和机械品质因数这三个因素对机械振子冷却的影响. 关键词： 腔光机械 辐射压力 简正模式分裂 冷却     

Spectral line suppression and sideband narrowing via quantum interference

We reveal that for a realistic system, interference effects are obtained such as the suppression of central line and inner sidebands and the narrowing of the outer fluorescence sidebands. For this purpose, we consider a spontaneous decay from an excited state to a metastable state when the excited and metastable states are resonantly coupled to an auxiliary metastable state by a laser field and a microwave field, respectively. The fluorescence spectrum evolves from a five-peaked structure into a doublet of ultrasharp lines as the ratio of the laser field Rabi frequency to the microwave Rabi frequency is decreased. The physical origin is presented in terms of dressed states.     

Time-dependent electron transport in HgTe/CdTe quantum wells

Based on the Floquet theory and Keldysh's nonequilibrium Green's function methods, we study the electron transport through the HgTe/CdTe quantum wells (QWs) irradiated by a monochromatic laser field. We find that when the laser field is applied, the edge states are split into a series of sidebands. When the Fermi level lies among these sidebands, the quantized plateau of the conductance is destroyed. Instead, the conductance versus the radiation frequency exhibits the successive oscillation peaks corresponding to the resonant tunneling through the sidebands of the edge states. The resonant interaction between the quasiparticles and the radiation field opens the gaps in the crossing region of the sidebands, which can be tuned by the radiation strength and frequency. This leads to the shift of the oscillation peaks in the conductance. We also show that the amplitudes of the oscillation peaks in the conductance are governed by the radiation strength and frequency.     

噪声免疫腔增强光外差分子饱和光谱线型的理论分析

噪声免疫腔增强光外差分子光谱(NICE-OHMS)作为世界上最灵敏的光谱技术可以被应用到痕量气体检测、频率标准、原子分子光谱以及超灵敏引力波测量等领域中,高精细度谐振腔吸收池的使用在增长激光与腔内物质相互作用路径的同时,极大的提高了腔内激光功率,这就会饱和低气压下的气态样品吸收线从而获得亚多普勒光谱结构,因此NICE-OHMS技术不仅具有高灵敏、还具有超高分辨的优点。该研究基于光与二能级分子相互作用的密度矩阵理论对NICE-OHMS技术中包含亚多普勒的多普勒展宽光谱线型进行了理论推导,获得了光谱线型的表达式,同时以该表达式对光谱线型进行了数值模拟,其中调制频率、饱和参量、频率调制系数分别设置为384 MHz,10和0.2。由模拟结果可见吸收光谱由两个边带的吸收信号构成,在包络上存在四个亚多普勒饱和结构;色散光谱由载频以及边带的色散三者决定,并在包络上存在五个亚多普勒饱和结构,获得了与已有实验一致的结果。最后重点分析了不同探测相位、不同饱和参量下的NICE-OHMS光谱线型尤其是亚多普勒结构的变化,由于饱和参量按照调制系数分配给载频和边带,因此虽然载频饱和参量很大,但NICE-OHMS吸收光谱幅度变化不大,主要是由于该光谱信号只与边带饱和参量有关,可以看出NICE-OHMS多普勒展宽信号具有饱和效应免疫的特性,与已有实验结果也符合较好。为更进一步的实验研究提供了必要的理论基础。     

A 532 nm molecular iodine optical frequency standard based on modulation transfer spectroscopy

We report construction of an iodine-stabilized laser frequency standard at 532 nm based on modulation transfer spectroscopy(MTS) technology with good reproducibility. A frequency stability of 2.5 × 10~(-14) at 1 s averaging time is achieved,and the frequency reproducibility has a relative uncertainty of 3.5×10~(-13), demonstrating the great stability of our setup.The systematic uncertainty of the iodine-stabilized laser frequency standard is evaluated, especially the contribution of the residual amplitude modulation(RAM). The contribution of the RAM in MTS cannot be evaluated directly. To solve this problem, we theoretically deduce the MTS signal with RAM under large modulation depth, and prove that the nonsymmetric shape of the MTS signal is directly related to the MTS effect. The non-symmetric shape factor r can be calibrated with a frequency comb, and in real experiments, this r value can be obtained by least-squares fitting of the MTS signal,from which we can infer the RAMinduced frequency shift. The full frequency uncertainty is evaluated to be 5.3 k Hz(corresponding to a relative frequency uncertainty of 9.4×10~(-12)). The corrected transition frequency has a difference from the BIPM-recommended value of 2 k Hz, which is within 1σ uncertainty, proving the validity of our evaluation.     

光泵THz激光器输出频率稳定特性研究

在太赫兹(THz)成像、雷达探测、相干通信等许多应用领域中,THz辐射源的频率稳定性是直接影响其应用效果的核心问题之一。基于双光子迁移效应建立了光泵THz激光器输出激光频率漂移的物理模型,推导出THz激光频率漂移的解析计算公式。以光泵甲醇(CH₃OH)为例, 给出了不同压强下的甲醇吸收谱线,定量分析了泵浦光频率漂移和泵浦功率对THz激光频率稳定性的影响,并讨论了THz激光腔内工作气体压强对THz激光频率漂移的影响。研究结果表明：随着泵浦光功率的增加,THz激光频率的漂移量逐渐增加;随着THz腔内工作气体压强升高,THz激光的频率漂移逐渐下降;当泵浦光频率漂移量在一定范围时,将出现THz激光的频率漂移量极值,且泵浦光的频率漂移量等于工作气体吸收谱线宽度的1/4时,THz激光输出的频率漂移达到极值。由此可见,在实际工作中,不仅需要合理选择腔内的工作条件(压强、温度),而且还需要采取措施将泵浦光的频率漂移控制在一定范围以内,以提高THz激光的输出频率稳定性。     

基于相位调制器与Fabry-Perot干涉仪的激光多普勒频移测量方法

提出了一种激光多普勒频移测量方法, 此方法利用正弦相位调制使信号光在原频率成分基础上产生正负一阶边带, 再由Fabry-Perot干涉仪对调制光振幅和相位进行调整, 使其产生固定频率的拍频信号, 利用此拍频信号的振幅随频率变化而变化的性质来进行多普勒频移测量. 通过理论分析证明该方法具有很高的测量精度, 加工装调难度不大, 兼顾了普通相干与非相干探测方法的优势. 另外通过实验证明该方法的正确性与可行性, 并通过与普通非相干方式比较发现该方法在测量精度上可以提高 约1个数量级. 关键词： 多普勒频移 相位调制 Fabry-Perot干涉仪 拍频     

Laser phase and frequency stabilization using an optical resonator

We describe a new and highly effective optical frequency discriminator and laser stabilization system based on signals reflected from a stable Fabry-Perot reference interferometer. High sensitivity for detection of resonance information is achieved by optical heterodyne detection with sidebands produced by rf phase modulation. Physical, optical, and electronic aspects of this discriminator/laser frequency stabilization system are considered in detail. We show that a high-speed domain exists in which the system responds to the phase (rather than frequency) change of the laser; thus with suitable design the servo loop bandwidth is not limited by the cavity response time. We report diagnostic experiments in which a dye laser and gas laser were independently locked to one stable cavity. Because of the precautions employed, the observed sub-100 Hz beat line width shows that the lasers were this stable. Applications of this system of laser stabilization include precision laser spectroscopy and interferometric gravity-wave detectors.     

Spontaneous-emission spectrum of the nonlasing supermodes in semiconductor laser arrays

It is shown that the interference between amplified spontaneous emission into the nonlasing supermode and the laser field of a semiconductor laser array causes spatial hole burning, which couples the dynamics of the amplified spontaneous emission with the laser field. In particular, phase locking between the amplified spontaneous emission and the lasing mode leads to the formation of a spectral triplet composed of in-phase relaxation oscillation sidebands and an out-of-phase line at the lasing frequency.     

Applications of the CSA-amplified PASS experiment

The recently reported CSA-amplified PASS experiment correlates the spinning sidebands at the true spinning frequency omega(r) with the spinning sidebands that would be obtained at the effective spinning frequency omega(r)/N, where N is termed the scaling factor. The experiment is useful for the measurement of small chemical shift anisotropies, for which slow magic-angle spinning frequencies, required to measure several spinning sidebands, can be unstable. We have experimentally evaluated the reliability of this experiment for this application. In particular we have demonstrated that large scaling factors of the order of N=27 may be used, whilst still obtaining accurate chemical shift sideband intensities at the effective spinning frequency from the F(1) projection. Moreover, the sideband intensities are accurately obtained even in the presence of significant pulse imperfections. A second application of the CSA-amplified PASS experiment is the measurement of the chemical shift anisotropy of sites that experience homonuclear dipolar coupling, as may be found in uniformly labelled biological molecules, or for nuclei with a high natural abundance. The effects of homonuclear dipolar coupling on CSA-amplified PASS spectra has been investigated by numerical simulations and are demonstrated using uniformly (13)C enriched l-histidine monohydrochloride monohydrate.     

Multi-sideband Generation in a Femtosecond Cr4+:YAG Laser

We describe the behavior of multi-sidebands in a self-mode-locked femtosecond Cr⁴⁺:YAG laser operating near 1.54 μm. Stokes and anti-Stokes sideband components are extended over 20 THz around the center frequency. An interesting feature here is that when the spectral width of the mode-locked pulse is increased, the specific Stokes sideband near 1.65 μm (a shift of 13.5 THz) is strongly enhanced due to an induced-Raman process in the laser rod. The measured frequency shifts for all sidebands are well explained by four-wave-mixing processes in the laser rod, accompanied by the resonance effect of the soliton and dispersive wave, both of which are affected mainly by cavity dispersions.