The Threshold of Phase Locking in the System of Two Multi-Junction Superconducting Loops

Coherent behavior (phase locking) in the system of two superconducting loops containing arrays of Josephson junctions and influencing each other by means of mutual inductance was numerically investigated. It is shown that under certain conditions the mutual interaction between loops can result in total phase-locking of the system. The threshold of the locking state depends on the strength of the interaction (mutual inductance). It is found that the basic behavior of the system in the phase-locked state can be explained by means of simple model of single multi-junction loop with an effective inductance which depends on mutual inductance. It is shown that synchronization in this system is possible and the mutual interaction increases the stability of phase locking in arrays.     

A system of phase synchronization of a spin-transfer nano-oscillator

We have studied the dynamics of a filterless phase-synchronization system (PSS) of a spin-transfer nano-oscillator (STNO) generating microwave oscillations in a broad range of frequencies under the effect of direct current and external magnetic field. Bifurcations in the system caused by a change in the frequency detuning of synchronized oscillations are considered. Bands of phase locking and synchronism are determined. The existence of a phase-locking band in the filterless PSS of STNOs basically distinguishes these oscillators from other types of microwave generators.     

Distributed Brillouin sensor system based on offset locking of two distributed feedback lasers

An offset locking technique, which uses an external optical delay line to tune the distributed feedback (DFB) laser frequency and a proportional-integral-derivative (PID) controller to lock the tuned frequency, is proposed for the first time, to the best of our knowledge, in the distributed Brillouin sensor system. This method provides large tuning range (greater than 1 GHz), high tuning speed (less than 100 mus per frequency step), and frequency tuning is independent of the laser frequency and power. The two DFB lasers are phase locked at the Brillouin frequency using a hardware PID controller. Using this offset locking with optical delay line, we demonstrated a high signal-to-noise ratio of 32 dB, which allows 1 m spatial resolution and better than 0.6 MHz frequency measurement accuracy (equivalent to 0.5 degrees C temperature resolution or 8 microepsilon strain resolution) over kilometers sensing length. The bias of the electro-optic modulator is controlled by a lock-in amplifier to provide high temperature or strain measurement accuracy.     

Compact laser transmitter delivering a long-range infrared beam aligned with a monitoring visible beam

A compact laser transmitter, which takes advantage of an optical subassembly module, was proposed and demonstrated, providing precisely aligned collinear IR and visible beams. The collimated IR beam acts as a long-range projectile for simulated combat, carrying an optical pulsed signal, whereas the visible beam plays the role of tracking the IR beam. The proposed laser transmitter utilizes IR (λ(1)=905 nm) and visible (λ(2)=660 nm) light sources, a fiber-optic collimator, and a beam combiner, which includes a wavelength division multiplexing (WDM) filter in conjunction with optical fiber. The device was built via the laser welding technique and then evaluated by investigating the characteristics of the generated light beams. The IR collimated beam produced had a Gaussian profile and a divergence angle of ~1.3 mrad, and the visible monitoring beam was appropriately collimated to be readily discernible in the vicinity of the transmitter. The two beams were highly aligned within an angle of 0.004 deg as anticipated. Finally, we performed a practical outdoor field test to assess the IR beam with the help of a receiver. An effective trajectory was observed ranging up to 660 m with an overall detectable beam width of ~60 cm.     

Three-phase phase-lock loop for distorted utilities

A novel three-phase phase-locked loop (PLL) structure suitable for phase and angular frequency derivation from distorted ac utility voltages is presented. The proposed PLL has a simple structure; a conventional three-phase PLL followed by a proportional-integral (Pl)-controlled moving average filter together with a phase-locking algorithm. The objective of the proposed technique is to capture the fundamental phase angle and angular frequency of three-phase clean, distorted, balanced or unbalanced ac utilities. The PLL gives fast, accurate angular frequency and phase locking and is robust to utility distortion such as line notching, random noise, voltage imbalance, phase loss, phase imbalance, harmonics, dc offsets and frequency variation. The analysis presented substantiates the immunity of the proposed PLL to unbalanced and distorted utility conditions. The PLL technique is simulated and digital signal processor (DSP)-implemented for a three-phase system to verify the analytical results. The simulated and experimental results, for numerous utility conditions, demonstrate its phase-tracking ability, whereas the conventional technique fails to lock accurately in highly distorted, three-phase grid-connected operation.     

Theory of Injection Locking for Large Magnetization Motion in Spin-Transfer Nano-Oscillators

We study magnetization dynamics in spin-transfer devices subject to DC and microwave injected currents. When the frequency of the injected current is sufficiently close to the self-oscillation frequency of the device, phase-locking occurs. This phenomenon is theoretically studied by using Landau-Lifshitz equation with Slonczewski spin-torque term. By exploiting separation of time scales and using averaging technique, we derive equations which are applicable to the study of phase-locking for arbitrary large magnetization motion. The stability diagram in the (detuning, ac current)-plane is determined and it is shown that phase locking is hysteretic at sufficiently large ac currents.     

Optical retrodirective tracking system approach using an array of phase conjugators for communication and power transmission

A new concept for a retrodirective tracking system applicable for communication and power transmission is proposed. In the proposed concept, the power transmitter utilizes a receiver's pilot signal to obtain information about its direction by conjugating the signal's phase inside a nonlinear medium. Power is therefore transmitted back to the receiver by the phase-conjugated signal beam. The power can be concentrated by an array of phase conjugators, which provides a large aperture so that the intensity can be increased on the receiver's photovoltaic panels compared to a single element. Controlling the phase and the direction of the readout beams in the four-wave-mixing process provides control over the interference pattern, its position, and its size. A numerical analysis is given for the phase and spot size control, and measurements with two Co-doped Sr(x)Ba(1-x)Nb(2)O(6) (Co:SBN) crystals confirm the occurrence of interference that is achieved for the case of two beams.     

Experiments with active phase matching of parallel-amplified Multiline HF laser beams by a phase-locked Mach-Zehnder interferometer

Active phase matching of multiline HF laser beams by means of a phase-locked Mach-Zehnder interferometer was demonstrated by locking the interferometer to the central interference fringe at zero optical path length difference. The central fringe could be found by varying the spectral content of the input beam. Laser amplification in one leg of the interferometer decreased fringe visibility without adversely affecting locking. Single-line fringe patterns produced by an array spectrometer (while the interferometer was operated in its scanning mode) were analyzed to show that no significant dispersion occurred in the amplifier. The techniques developed have potential for measuring dispersion mismatch between larger parallel amplifiers. These experiments demonstrated in principle that a number of multiline HF amplified beams can be recombined and phase-matched to produce a high beam quality output beam.     

集成锁相环在激光脉冲调制系统中的应用

锁相环由鉴相器、环路滤波器及压控震荡晶体组成,是一个能跟踪输入信号频率和相位的闭环自动控制系统。研制的激光脉冲调制系统采用锁相技术,以单片、集成锁相环代替分立元件,实现了片内鉴频和鉴相的功能。研制的腔倒空驱动器能够输出 4MHz, 800kHz, 400kHz, 80kHz,40kHz, 8kHz, 4kHz, 800Hz, 400Hz 等不同重复频率的脉冲信号,输出功率达到瓦级,满足了声光布拉格池的要求。该激光脉冲调制系统已经应用在皮秒时间相关单光子计数光谱仪系统中,取得了比较理想的效果。     

Phase Locking of CO(2) Lasers

A method of phase locking two CO(2) lasers by radiation exchange is presented. This phase-locking was achieved by use of a copper prism as a beam folding device in the resonators and extraction of the output radiation by a common output coupler. Energy exchange led to a phase-locked state if several locking conditions were fulfilled. The amount of radiation injected from one resonator to the second cavity could be adjusted by movement of the prism. The influence of the strength of coupling on the locking range was studied. The beat signal between the two unlocked lasers could be measured, whereas in the case of phase-locked operation twice the intensity was detected. Despite the inclusion of several assumptions, a simplified mathematical model delivered good agreement between calculated and experimental results.     

Arbitrary phasing technique for two-dimensional coherent laser array based on an active segmented mirror

We introduce a novel (to the best of our knowledge) phasing technique for a coherent laser array. We have accomplished arbitrary phasing in the interval 0-π. A seven-channel laser array experiment is built for verification. A custom-made beam arraying structure is designed to arrange beamlets into a two-dimensional hexagonal array. In the phase-locking loop, the wavefront sensing is performed interferometrically. An active segmented mirror is used for phasing, and the control signals are generated by the proportional control algorithm. In experiment, all the beamlets have been properly phased, and the experiment of inertia-free beam steering has been accomplished.     

基于光学锁相环的高稳定度激光稳频方法研究

报道了一种基于光学锁相环的高稳定度激光稳频方法,用于提高可调谐外腔半导体激光器(TECDL)的频率稳定度和准确度。自行研制的光学锁相环电路采用数字鉴相与差分运算相结合的方式获得高灵敏度的鉴频鉴相误差信号,并通过高速模拟PID实现整个系统的闭环锁定。利用该光学锁相环系统进行了TECDL偏频锁定至光学频率梳(OFC)的实验,实验结果表明环路锁定后拍频频率波动在±0.3Hz范围内,偏置频率为50MHz时,光学锁相环系统在1s和1000s积分时间的相对阿伦方差分别为1.5×10^-9和8.5×10^-13。系统锁定后,拍频线宽由500kHz压缩至2kHz。该研究表明采用基于光学锁相环的激光稳频方法可以实现亚Hz级的激光频差控制,通过将TECDL偏频锁定至高稳定度的参考激光源可显著提高其频率稳定度,使其能够满足超精密测量、冷原子/离子干涉测量等领域对激光频率稳定度和准确度的要求。     

Infrared Mueller matrix acquisition and preprocessing system

An analog Mueller matrix acquisition and preprocessing system (AMMS) was developed for a photopolarimetric-based sensor with 9.1-12.0 microm optical bandwidth, which is the middle infrared wavelength-tunable region of sensor transmitter and "fingerprint" spectral band for chemical-biological (analyte) standoff detection. AMMS facilitates delivery of two alternate polarization-modulated CO(2) laser beams onto subject analyte that excite/relax molecular vibrational resonance in its analytic mass, primes the photoelastic-modulation engine of the sensor, establishes optimum throughput radiance per backscattering cross section, acquires Mueller elements modulo two laser beams in hexadecimal format, preprocesses (normalize, subtract, filter) these data, and formats the results into digitized identification metrics. Feed forwarding of formatted Mueller matrix metrics through an optimally trained and validated neural network provides pattern recognition and type classification of interrogated analyte.     

Simultaneous injection locking of couples of high-power broad-area lasers driven by a common current source

We experimentally demonstrate the simultaneous injection locking of pairs of high-power broad-area laser diodes in a 19-laser array driven by a common current source. Each pair is injection locked by use of a single-mode low-power semiconductor laser. The frequency and phase locking are verified bythe optical spectrum and the interference pattern between the injection-locked lasers. The influence of frequency detuning on the (simultaneous) injection behavior has been experimentally clarified. We validate a necessary condition for the injection locking of a broad-area laser array.     

Mode-locking external-cavity laser-diode sensor for displacement measurements of technical surfaces

A novel laser sensor for position measurements of technical solid-state surfaces is proposed. An external Fabry-Perot laser cavity is assembled by use of an antireflection-coated laser diode together with the technical surface. Mode locking results from pumping the laser diode synchronously to the mode spacing of the cavity. The laser cavity length, i.e., the distance to the measurement object, is determined by evaluation of the modulation transfer function of the cavity by means of a phase-locked loop. The mode-locking external-cavity laser sensor incorporates a resonance effect that results in highly resolving position and displacement measurements. More than a factor-of-10 higher resolution than with conventional nonresonant sensing principles is achieved. Results of the displacement measurements of various technical surfaces are reported. Experimental and theoretical investigations are in good agreement.     

Phase locking of CO(2) lasers by the use of diffraction effects

A technique to phase lock CO(2) lasers with spatially separated active media was investigated. Only reflective optics (except for the output coupler) were used, in view of applications of this method in the field of high-power lasers. Phase locking was established when the beams of two resonator branches were made to propagate very close to each other along the so-called coupling path. As a result of diffraction effects both resonators were exchanging energy, establishing a phase-locked operation mode when several locking conditions were fulfilled. A maximum coupling coefficient (the ratio between the diffracted intensity in the second cavity and the intercavity intensity in the first resonator) of 2.6% could be achieved. Because phase locking was highly dependent on the difference between the two resonator lengths, a length control that uses a piezoelectric translator connected to one of the resonator mirrors was used. To detect phase locking, the intensity maximum of the interference pattern of the two laser beams was monitored with a fast detector. By application of a ramp signal to the piezoelectric translator and detection of the peak intensity, the locking range could be measured. Up to a mismatch of the resonator lengths of λ/130, locking could be maintained. The measurements were compared with results of a computer simulation with Huygens-Fresnel integrals to describe diffraction and the one-dimensional Maxwellian equations to calculate supermodes and to analyze their stability. The numerical results showed an excellent agreement with the measured values.     

单腔双光梳激光器及其应用研究

双光梳技术凭借其超高的频率分辨力与检测精度成为当今科研领域探索的重点。本文分别介绍了利用电光调制、微谐振腔、相位锁定两台独立的锁模激光器、单腔双光梳激光器产生双光梳的方法,重点阐述了基于单腔双波长激光器的双光梳产生方法集成度高、结构较简单、成本低的优势;之后详细介绍了目前单腔双光梳激光器中,双波长腔内运转的脉冲波形复用法、空间复用法、方向复用法、波长复用法和偏振复用法的工作原理及最前沿的研究进展;最后,具体分析了单腔双光梳在光谱学、测距、光纤传感等领域的应用情况,并展望未来单腔双光梳的研究与发展方向,为推动单腔双光梳技术性能的进一步提升和广泛应用提供借鉴。     

Laser Interferometer for Space Gravitational Waves Detection and Earth Gravity Mapping

The idea of using space laser interferometer to measure the relative displacement change between two satellites has been considered for space gravitational waves detection and Earth gravity filed mapping in recent years. Some investigations on the key issues of laser interferometer in our working team have been presented in this paper. An on-ground laser interferometer prototype used for the demonstration of satellite-to-satellite ranging has been constructed, which is equipped with phasemeter, laser pointing modulation and laser phase-locking control. The experimental results show that path-length measurement sensitivity of the laser interferometer reaches 200 pm/√ Hz, and phase measurement precision achieves 2π × 10^??5 rad/√ Hz, and laser pointing modulation precision is better than 80 nrad/√ Hz, and laser phase-locking control precision attains 2π × 10^??4 rad/√ Hz within the frequency regime of 1 mHz–1 Hz. All of these demonstrate that the proposed laser interferometer has very promising feasibility to meet the requirement of the Taiji, TianQin and Space Advanced Gravity Measurement (SAGM) missions which are put forward by Chinese scientists.     

Lidar measurements taken with a large-aperture liquid mirror. 1. Rayleigh-scatter system

A lidar system has been built to measure atmospheric-density fluctuations and the temperature in the upper stratosphere, the mesosphere, and the lower thermosphere, measurements that are important for an understanding of climate and weather phenomena. This lidar system, the Purple Crow Lidar, uses two transmitter beams to obtain atmospheric returns resulting from Rayleigh scattering and sodium-resonance fluorescence. The Rayleigh-scatter transmitter is a Nd:YAG laser that generates 600 mJ/pulse at the second-harmonic frequency, with a 20-Hz pulse-repetition rate. The sodium-resonance-fluorescence transmitter is a Nd:YAG-pumped ring dye laser with a sufficiently narrow bandwidth to measure the line shape of the sodium D(2) line. The receiver is a 2.65-m-diameter liquid-mercury mirror. A container holding the mercury is spun at 10 rpm to produce a parabolic surface of high quality and reflectivity. Test results are presented which demonstrate that the mirror behaves like a conventional glass mirror of the same size. With this mirror, the lidar system's performance is within 10% of theoretical expectations. Furthermore, the liquid mirror has proved itself reliable over a wide range of environmental conditions. The use of such a large mirror presented several engineering challenges involving the passage of light through the system and detector linearity, both of which are critical for accurate retrieval of atmospheric temperatures. These issues and their associated uncertainties are documented in detail. It is shown that the Rayleigh-scatter lidar system can reliably and routinely measure atmospheric-density fluctuations and temperatures at high temporal and spatial resolutions.     

A Semi-analytic Approach for Coherent Laser Radar System Efficiency

Abstract

A nearest Gaussian approximation (NGA) is proposed to approximate any shape for a single mode laser beam by a Gaussian shape. The application considered is a determination of the system efficiency in heterodyne coherent laser radar (HCLR). For an actual beam its NGA is defined by three parameters: the waist spot size and location, and an amplitude coefficient. These parameters are computed by a maximization of the norm of the scalar product written for the actual and Gaussian beams. In the case of the truncated Gaussian beam, particularly relevant to HCLR, the waist location can be analytically calculated, and only two parameters remain unknown: the waist spot size and amplitude coefficient. Using numerical applications, it is shown the NGA is in good agreement with Fresnel integral solution. The NGA combines a good accuracy and capability of analytical solutions. It can treat a variation in system efficiency owing to a misalignment angle between the transmitter and local oscillator.