Selective injection locking of a multi-mode semiconductor laser to a multi-frequency reference beam

Injection locking is a well known and commonly used method for coherent light amplification. Usually injection locking is obtained on a single-mode laser injected by a single-frequency seeding beam. In this work we show that selective injection locking of a single-frequency may also be achieved on a multi-mode semiconductor laser injected by a multi-frequency seeding beam, if the slave laser provides sufficient frequency filtering. This selective injection locking condition depends critically on the frequency detuning between the free-running slave emission frequency and each injected frequency component. Stable selective injection locking to a set of three seeding components separated by 1.2 GHz is obtained. This system provides an amplification up to 37 dB of each component. This result suggests that, using distinct slave lasers for each frequency line, a set of mutually coherent high-power radiation modes can be tuned in the GHz frequency domain.     

光纤飞秒光学频率梳的研制及绝对光学频率测量

实验利用商品光纤飞秒激光器,自行构建了一套完整的光学频率梳系统,并获得了约30 dB信噪比的系统频移(f_ceo)信号.实现了光频梳重复频率(f_rep)信号及系统频移(f_ceo)信号的高稳定度锁定,并通过实验验证了光频梳锁定的跟踪精度.基于此稳定光频梳完成了对1064 nm碘稳频Nd:YAG固体激光器的绝对频率测量.实验结果表明,f_rep的跟踪精度在100 s取样时间时优于3.7×10^-14,测量得到的1064 nm激光器绝对频率为:281630111757362 Hz.这一测量结果与国际计量委员会(CIPM)给出的国际推荐值符合到不确定度之内. 关键词： 光纤光频梳 稳频 锁相技术 光学频率计量     

High sensitivity HCN laser interferometer for plasma electron density measurements

This paper describes a high sensitivity 337 μm HCN laser interferometer for plasma electron density measurements. The plasma phase shift is transferred to a low frequency signal obtained by slightly shifting the frequency of the 337 μm radiation in the reference beam of the interferometer. The frequency shift is produced by diffracting the radiation of a cylindrical rotating blazed grating. A phase shift is deduced from the time variation of the zero crossings of the low frequency signal, giving a result which is independent of amplitude fluctuations. Using pyroelectric detectors, the interferometer has a time resolution of 100 μsec and a sensitivity of 10^-2 fringe.     

非消声水池中低频换能器测量的空间域处理方法

提出了一种在非消声水池中低频换能器测量的空间域处理方法。该方法是在发射和接收换能器保持相对位置不变的情况下,通过多次无规则变换其在有限水域中的位置,让同频反射声的干扰贡献随机和无规则化,最终将上述所有位置接收的信号同步叠加而将同频反射声干扰消除,从而提取出了所需的直达波波形信号,克服了传统的脉冲测量技术受同频反射声干扰无法提取直达信号的问题。在5 m×7 m×6 m非消声水池,测量换能器样品,测量频率范围是400~2500 Hz,测量得到的结果与湖上测试结果相比,两者基本一致。该方法的实现,大大的提高了非消声水池的低频换能器测试能力,有效测量频率下限比一般脉冲测试方法降低一个数量级。      

High temperature Raman scattering and phase transition in EuAlO3

Polarized Raman spectra of EuAlO₃ have been obtained at various temperatures from 77–1500 K. Three phonon modes are observed to soften in frequency with increasing temperature. Discontinuity in the frequency temperature dependence appears at 1420 K. This result is consistent with a first order phase transition. Analysis of the Raman data shows evidence of a coupling between the soft mode of Bg(XY) symmetry and one mode of same symmetry.     

激光光源线宽对外差探测性能的影响

本文根据统计理论分析了激光线宽对外差探测结果的影响, 并讨论了激光线宽对杨氏干涉条纹对比度的影响. 本文基于解析结果做了数值仿真, 所得结果表明激光线宽增加时, 外差探测方式仍可以检测到中频信号, 但在线宽较大时, 受噪声影响无法准确地提取到中频频率. 为验证理论分析结果, 使用线宽为1 MHz的激光光源进行了8.1 km外场实验, 实验结果与数值分析结论一致, 即不会因为线宽增加而无法检测到中频信号. 文中所得结论对于外差探测光源的选择有重要指导意义, 因此根据测量目标的特性和测量要求, 可按照文中结论评估光源的线宽指标.     

T=0 Green's function for the exciton

The excitons as electronic excitations of an insulator are obtained from the full manybody Hamiltonian by means of T=0Green's function techniques. Exciton amplitudes and energies are obtained via the vertex-part equation from an eigenvalue equation which in the Wannier limit is the effective-mass equation. A formula is obtained for the frequency dependent effective interaction whose static limit (ω=0) goes over into the Haken-Schottky dielectric-constant interaction for excitons of large electron-hole separations. A simple criterion will result to answer the question on when one is allowed to replace the frequency dependent interaction by its ω=0 limit.     

86-km optical link with a resolution of 2 × 10<Superscript>-18</Superscript> for RF frequency transfer

RF frequency transfer over an urban 86 km fibre has been demonstrated with a resolution of 2×10^-18 at one day measuring time using an optical compensator. This result is obtained with a reference carrier frequency of 1 GHz, and a rapid scrambling of the polarisation state of the input light in order to reduce the sensitivity to the polarisation mode dispersion in the fibre. The limitation due to the fibre chromatic dispersion associated with the laser frequency fluctuations is highlighted and analyzed. A preliminary test of an extended compensated link over 186 km using optical amplifiers gives a resolution below 10^-17 at 1 day.     

飞秒钛宝石光学频率梳的精密锁定

经相位锁定后的飞秒钛宝石光学频率梳已经广泛用于绝对光频的测量，这是光频标领域一个革命性的突破.在自建的90MHz飞秒钛宝石激光器的基础上首先采用光子晶体光纤将其光谱展宽到一个光倍频程，接着利用锁相环技术分别将重复频率和载波包络频移同时高精度地锁定到一台稳定度为6×10^－14的Cs钟上，进而得到了稳定度相同的飞秒光学频率梳.     

共线型声光可调谐滤波器非互易效应研究

通过创建超声频率与衍射效率的动态方程,分析了声光器件在不同射频驱动频率下衍射效率的分布情况,并给出相应表达式.提出了在100 MHz射频处也存在与高于1 GHz处类似非互易现象.进一步通过比例系数δF/ΔF的引入,在此基础上分别利用波长532 nm和632.8 nm激光器,在二氧化碲晶体中进行非互易效应实验.实验得到传输带宽改变达到总衍射带宽的50%～60%,与理论计算值基本吻合.研究结果表明,实际中可将非互易效应大规模应用于包括定向耦合器（光路由器）在内的光电器件的开发.     

Calculation of the optimal imaging parameters for frequency modulation atomic force microscopy

True atomic resolution of conductors and insulators is now routinely obtained in vacuum by frequency modulation atomic force microscopy. So far, the imaging parameters (i.e., eigenfrequency, stiffness and oscillation amplitude of the cantilever, frequency shift) which result in optimal spatial resolution for a given cantilever and sample have been found empirically. Here, we calculate the optimal set of parameters from first principles as a function of the tip–sample system. The result shows that the either the acquisition rate or the signal-to-noise ratio could be increased by up to two orders of magnitude by using stiffer cantilevers and smaller amplitudes than are in use today.     

Time-dependent directionality of cooperative emission after short pulse excitation

Cooperative emission from a sphere of N two-level atoms excited by a short pulse is shown to have a time-dependent angular distribution, with the possibility of reversal in the dominant direction of emission. This is a result of the different values of the frequency shifts and decay rates associated with the various collective eigenmodes of the atomic system obtained in a scalar photon model.     

Influence of strong laser fields on the inverse bremsstrahlung collision frequency

A new model is described for the absorption of laser light by a plasma. Two issues important for high laser intensity are considered: changes in the electron velocity distribution and the absorption non-linearity. The effects of anisotropy and non-Maxwellian distribution function are analysed with a hard sphere model. It is shown that anisotropy alters the electron-ion collision frequency by an amount which depends on the ratio of the temperatures in direction of and perpendicular to the laser field.The effect of high laser intensity is considered assuming Coulomb collisions, and a new expression obtained for the collision frequency as a function of quiver velocity. It will be shown that the result is in agreement with the Spitzer result in the case of small fields and with the result of Catto and Speziale in the case of very strong fields, but shows an unexpected maximum of the collision frequency in an intermediate regime.     

Vibration of a membrane having a circular outer boundary and an eccentric circular inner boundary

In this paper, a method of solving vibration problems for a membrane having a circular outer boundary and an eccentric circular inner boundary is presented. The frequency equation for the membrane is given, and the dependence of the natural frequency on the eccentricity is obtained. It is shown that the result obtained from this analysis in the limit of the eccentricity being zero coincides with that of a membrane with a concentric circular inner boundary.     

基于快速傅里叶变换的条纹图像处理研究

为提高船体扭转角变形测量精度,对双光栅干涉测量法产生的莫尔条纹采用快速傅里叶变换频域处理。采集不同宽度的莫尔条纹图像,分别对图像进行快速傅里叶变换,对比分析图像频谱,得到实际莫尔条纹图像的有效频率信息。分析滤波器的选择原则,选用陷波低通滤波器,去除频域图像中高频噪声,获得莫尔条纹有效信息。实验结果表明,莫尔条纹的频域处理精度远高于空域处理结果,使扭转角测量精度提高到1.2″。     

In VivoLongitudinally Detected ESR Measurements at Microwave Regions of 300, 700, and 900 MHz in Rats Treated with a Nitroxide Radical

A signal detector of longitudinally detected ESR (LODESR) is independent of the resonant frequency. We developed anin vivoLODESR spectrometer operating in the regions of 300, 700, and 900 MHz. Using this apparatus, we estimated signal intensities at different operating frequencies obtained from non- or high-dielectric loss phantoms that contained nitroxide radical solutions and from live rats that had received a nitroxide radical. Our result, higher signal intensities in the high-dielectric loss samples (such as physiological saline solution and animals) at a lower frequency, shows that the influence of a decrease in dielectric loss dominates over the signal reduction caused by smaller Zeeman splitting. We believe that this finding strongly supports anin vivoESR resonant frequency that tends to be low.     

The temperature dependence of intensity and the function of the transition dipole moment for the induced optical absorption bands of oxygen in the region of the Herzberg photodissociation continuum (200–230 nm)

Application of coherent interaction of laser light with a focused ultrasonic wave to the technique of acoustooptic visualization in multiple-scattering media is discussed. By analyzing spatial distribution of the optical radiation modulated by ultrasound (the photocurrent at ultrasonic frequency), images of large-sized inhomogeneities embedded into the scattering medium have been obtained. A light-absorbing half-plane and a square with sides of 5 mm were used as the inhomogeneities. The visualization was performed under optimal conditions for measuring the alternating photocurrent calculated for the proposed model of coherent interaction between the laser and acoustic beams (the Raman-Nath diffraction). The alternating current at the ultrasonic frequency was obtained as a result of mixing the waves of the diffraction fields on the detector’s photocathode. All experimental values were obtained from a single measurement without averaging the alternating photocurrent at the ultrasonic frequency of 3 MHz, with the scattering parameter varying up to μL≤37.5, where μ is the extinction coefficient and L is the thickness of the scattering medium along the laser beam axis. The measured quantities varied in the course of the measurements by more than 10 orders of magnitude.     

实心砖墙对电磁波的衰减特性

应用模式传输线理论分析了多层周期介质实心砖墙对电磁波的衰减特性。计算得到了电磁波穿透实心砖墙的透射反射系数与入射角和频率的关系曲线,数值计算的结果与软件仿真的结果符合较好。利用电磁波测量系统测量得到了实心砖墙对电磁波衰减随频率（2~8 GHz）的变化关系。综合考虑数值计算和实验测量的结果,得到频率为2~8 GHz时,实心砖墙对电磁波的衰减为15~25 dB。     

W-band gunn second subharmonically locked oscillator

In this paper, the averaging method is used to analyse the performance of second subharmonically injection locked Gunn oscillator. Some useful expressions such as the locking range, output response, output impedance of nonlinear device in fundamental and subharmonic frequency are obtained. a W — band subharmonically locked Gunn oscillator is developed and experimental result demonstrates the validity of this analysis.     

A STUDY ON REAL-TIME LOW-FREQUENCY SPATIAL FILTERING PROCESS OF AN OPTICAL IMAGE USING LiNbO3:Fe CRYSTAL

The optimum image processing result of low-frequency filtering was obtained when LiNbO₃:Fe crystal as a spatial filter was placed at a certain position behind the spatial-frequency spectral plane of an optical Fourier transform system, which corresponds to the minimum transmittance ratio in Z-scan curve. The experimental results show that self-defocusing due to photorefractive negative-lens effect is the main factor responsible for the low- frequency filtering.