高分辨率调频连续波激光绝对测距研究

大空间精密测量在重大装备制造、空间科技、国防工业等方面发挥着重要作用,激光高精度绝对长度测量是大空间精密测量领域的重要研究课题.调频连续波激光测距是近年来激光绝对测距研究的热点,它克服了脉冲法测量分辨率低和相位法激光测距存在2π缠绕模糊度问题的缺点,有着测量精度高、量程大的优点.本文研究了调频连续波激光测距的原理,分析了影响其测距分辨率的主要原因,证明了利用等光频间隔采样来抑制激光调制非线性对测距结果影响的可行性.该方法可以提高测距分辨率,且系统构成简单、实用性强.搭建了光纤调频连续波激光测距系统,并加入了辅助干涉光路对测量信号进行等光频间隔采样.利用该系统进行了测距分辨率实验,实验结果表明,本系统测量分辨率可以达到50μm,测量范围达到了10m.     

A high-power electro-absorption modulated 2d-array laser source for FM/cw scannerless lidar

A scannerless amplitude-modulated (AM) semiconductor lidar (or laser radar) using frequency-modulation continuous wave ranging (FM/cw) techniques is one of the most promising research directions at present. In this lidar system, the high-power AM laser source is a key component of the lidar transmitter. First of all, we present here the architecture of an FM/cw scannerless AM ladar and summarize and analyze various schemes of high-power laser modulation. Then we put forward a new method for obtaining a high-power AM laser output, that is, to compose a two-dimensional (2D) array light source with laser diodes (LDs) and electro-absorption modulation modules (EAMs). Finally, the optical properties of electro-absorption modulated laser arrays are briefly analyzed. The analysis shows that the modulated laser arrays have the features of integration and miniaturization and can realize incoherent light illumination in the far field. This laser array is potentially a more ideal high-power AM laser source applied to an FM/cw scannerless imaging lidar.     

基于重采样技术的调频连续波激光绝对测距高精度及快速测量方法研究

调频连续波激光测距方法可以实现高精度的大尺寸绝对距离测量, 且测量过程无需合作目标, 在大空间坐标精密测量领域有很高的研究价值. 而如何提高测量分辨率和实用化一直是近年来调频连续波激光绝对测距研究的热点. 本文研究了调频连续波激光测距的原理, 基于双光路调频连续波激光测距系统, 提出了通过信号拼接提高测量分辨率的信号处理优化方案, 该方案可以提高测距分辨率, 且可以降低对激光器的性能要求; 提出了可实现高速测量的简易测量方法. 设计加工了双光路光纤调频连续波激光测距系统, 利用该系统进行了测距分辨率及测距误差标定实验, 实验结果表明: 优化方案可以有效地提高测量分辨率和测量效率, 在26 m测量范围内, 测距分辨率达到了50 μm, 测距误差不超过100 μm; 快速测量方案有较高实用价值.     

Simplified small signal gain calculations in free electron lasers

A method proposed recently is used for the evaluation of the small signal gain in various free electron laser configurations. The theory is applied to (a) the wigglerfree free electron laser with a uniform axial guide magnetic field and arbitrary direction of propagation of the amplified radiation and (b) the free electron laser with the axially modulated guide field (lowbitron). It is demonstrated that the new approach simplifies the gain calculation significantly in comparison with the traditional method.     

高重复频率、单模纳秒脉冲全光纤激光器

报道了一种基于主振荡放大技术的全光纤脉冲激光器.种子激光器使用直接调制的单纵模半导体激光器,其输出波长为1 063.8 nm,重复频率100 kHz~10 MHz连续可调谐,光纤放大器采用了多级放大器级联的方法.在重复频率100 kHz、脉冲宽度5 ns时,激光器获得了平均功率为1.2 W,峰值功率为2.4 kW的单横模激光脉冲输出.     

Three-dimensional optical storage in fused silica using modulated femtosecond pulses

Three-dimensional bitwise optical recording with a density of 500 Gb/cm3 in fused silica using a Ti:sapphire femtosecond laser modulated by binary digits is demonstrated. Laser pulses modulation is realized by modulating two circuits of trigger pulses signal which are used to control laser pulses trapping and switching out from cavity, respectively. Bits are optically readout in both a parallel reading (phase-contrast) and a serial reading (confocal-type) methods. The method for modulating laser pulses can also be used in all of pulsed laser systems which operate in cavity-dumping configuration.     

高重复频率、单模纳秒脉冲全光纤激光器

报道了一种基于主振荡放大技术的全光纤脉冲激光器.种子激光器使用直接调制的单纵模半导体激光器,其输出波长为1 063.8 nm,重复频率100 kHz～10 MHz连续可调谐,光纤放大器采用了多级放大器级联的方法.在重复频率100 kHz、脉冲宽度5 ns时,激光器获得了平均功率为1.2 W,峰值功率为2.4 kW的单横模激光脉冲输出.     

Broadband amplitude-modulated laser source with high output power for FM/cw lidar transmitter

We are building a long-range FM/cw nonscanning imaging lidar breadboard. This lidar system achieves ranging based on a frequency modulation/continuous wave (FM/cw) technique, implemented by an amplitude modulated mid-IR diode laser transmitter with a linear frequency modulation (LFM) of the subcarrier. Firstly, various schemes of light beam modulation are analyzed. Secondly, we put forward a laser modulation scheme whose core was formed by a 1.55 μm electro-absorption modulated laser diode (EML) and an erbium-doped optical fiber amplifier (EDFA), then a corresponding experimental system architecture and components for light beam modulation and detection are established. Finally, a corresponding experiment of laser beam modulation is completed for the first time. In our experiment, the EML amplitude is modulated by a 200 MHz to 800 MHz LFM signal, whose amplitude value is 2.05 V. The average output power of the modulated laser obtained in the experiment is 10 W, peak power is 16.4 W, and the average modulation depth is 78%. The results of tests predict that this laser modulation scheme is likely to improve the imaging range of the FM/cw lidar.     

Directly modulated lasers in negative dispersion fiber links

In the paper the impact of frequency chirp of directly modulated lasers is considered in context of high-speed data transmission over negative dispersion fiber. Chirp/dispersion induced signal distortions are described, and detailed investigations of their influence on 10 Gb/s transmission system performance are presented. The desired laser chirp characteristics and optimal driving conditions are determined. It is also demonstrated that directly modulated laser with low chirp offers similar or even better dispersion tolerance than unchirped (externally modulated) source. Experimental verification of described investigations is presented at the end.     

外部光注入的光泵浦自旋垂直腔表面发射激光器中的两个混沌偏振分量对两个复杂形状目标中的多区域精确测距

基于外部光注入的光泵浦自旋垂直腔表面发射激光器(vertical cavity surface-emitting laser,VCSEL)的两个混沌偏振分量,提出了对两个复杂形状目标中的多区域精确测距方案.这里,两个混沌偏振探测波具有飞秒量级快速动态并且被双极性sinc波形调制,使它们具有时空不相关特性.利用这些特性,通过计算多束延时反馈混沌偏振探测波形和与之相对应的参考波形的相关性,实现了对两个复杂形状目标多区域位置矢量精确测量.研究结果表明,对多区域小目标的测距具有非常低的相对误差(低于0.94%).当光电探测器的带宽足够大时,其测距的分辨率达到0.4 mm,并具有很强的抗噪声能力.本文的研究结果在复杂形状目标的精确测距方面具有潜在应用.     

Tailored terahertz pulses from a laser-modulated electron beam

We present a new method to generate steady and tunable, coherent, broadband terahertz radiation from a relativistic electron beam modulated by a femtosecond laser. We have demonstrated this in the electron storage ring at the Advanced Light Source. Interaction of an electron beam with a femtosecond laser pulse copropagating through a wiggler modulates the electron energies within a short slice of the electron bunch with about the same duration of the laser pulse. The bunch develops a longitudinal density perturbation due to the dispersion of electron trajectories, and the resulting hole emits short pulses of temporally and spatially coherent terahertz pulses synchronized to the laser. We present measurements of the intensity and spectra of these pulses. This technique allows tremendous flexibility in shaping the terahertz pulse by appropriate modulation of the laser pulse.     

Experimental investigation of harmonic and subharmonic synchronization of 40 GHz mode-locked quantum-dash laser diodes

Synchronization of a 40 GHz quantum-dash mode-locked (ML) Fabry-Perot laser diode with optically injected pulse streams is experimentally studied. Injected signals consist of nonmodulated and modulated trains of 1.6 ps pulses at various repetition rates, ranging from 10 to 160 GHz and 10 to 160 Gbps, respectively. Subharmonic, fundamental, and harmonic synchronization of the ML laser allows retrieval of stable 40 GHz clock pulses featuring a width of 1.8 ps. Frequency components at 10 and 20 GHz do not create any amplitude modulation on the recovered 40 GHz clock pulses when injecting signals at 10 and 20 GHz/Gbps. In addition, external synchronization of the laser with pulse streams at 80 and 160 GHz/Gbps is sustained despite the absence of significant components at or below 40 GHz.     

强度调制532 nm激光水下测距

激光水下探测在水下目标搜寻、资源勘探等领域具有重要的应用,而散射是激光水下探测面临的主要挑战.载波调制激光雷达具有抗散射、抗干扰的优点,本文利用自行研制的532 nm强度调制激光源,在3 m长的水箱中搭建激光水下探测系统. 532 nm激光源最大输出功率为2.56 W,强度调制范围为10.0 MHz—2.1 GHz,光束发散角约0.5 mrad.通过在水箱中添加氢氧化镁(Mg(OH)_2)粉末,测量了不同浑浊度下水的衰减系数.采用相位测距的方法,目标反射光的调制信号为探测信号,对激光源进行调制的电信号作为参考信号,利用相关运算获得激光的延时时间,进而可以获得水下目标的距离.最大调制频率为500 MHz时,实现了距离为4.3个衰减长度目标的探测,测距误差约12 cm.探测距离越远,测距误差越大,调制频率越高,测距精度越高.     

Nonlinear optical correction of the pulse shape from a directly modulated DFB laser

In this work, the effect of modulation instability (MI) in optical fiber is used to reshape nanosecond pulses form a directly modulated diode laser. Our configuration includes a fiber where MI causes the side lobes in the signal spectrum and a filter at the fiber output rejecting the side lobes. Simulations show abrupt drop of the transmission of the setup if pulse power is above some critical value. We investigated the transmission for fibers with lengths in the range between 62-m and 4.5-km. The critical power was found to be inversely proportional to the fiber length. An average scaled critical power is 2.16 W km. We demonstrated the application of the method for rejection of the transient peak in a directly modulated diode laser.     

Controlled dumping of pulses of laser radiation ranging from MIR to the MM by means of optical semiconductor switching

A simple method to produce sub-ns pulses of laser radiation with wavelength ranging from middle infrared (MIR) to millimeter (MM) is experimentally demonstrated. The system operates with a single OSS (optical semiconductor switching) driven by a ns Nd laser. Pulse control is produced by varying the distance of a mirror from an OSS used as a cavity dumping. The effectiveness of this sub-ns MM pulse generation is larger than the one observed in other schemes. Pulses with 500-ps FWHM and 15-W power were produced at 2.65 mm with a time compression compared to the 2-ns FWHM of the 1.06-μm driving laser. Received: 30 October 2000 / Revised version: 26 January 2001 / Published online: 9 May 2001     

相移法测量寿命的理论分析

本文从理论上分析了利用调制的单模激光场共振激发来测量原子或分子的寿命.结果表明:感生荧光或共振荧光信号与调制信号间的相移主要来自三个过程:(1)由原子的横向弛豫时间及激光线宽决定的激发过程;(2)由激发能级及其他能级上粒子数衰减过程;(3)与系统达到平衡态粒子数分布所需的弛豫时间有关的碰撞激发过程.讨论了在不同的情况下它们的影响,并在分析中考虑了多普勒加宽的影响.     

光学线性调频步进信号合成与高分辨测距演示

利用分立激光器合成大带宽线性调频步进信号(FSCS)激光信号源以实现高分辨测距。依据FSCS信号测距原理,数值计算了激光器各参数对合成FSCS信号的影响及其误差容限。计算表明,在各激光器参数误差中,起始频率和起始时间误差是引起合成FSCS信号测距分辨力恶化的最主要因素。另外,利用一台中心波长1550nm、线宽为150kHz的线性调波长激光器,通过光学斩波方式,模拟产生了合成FSCS信号,进行了高分辨力的测距演示。理论分析与实验结果表明,采用分立激光器合成高分辨力测距FSCS信号是可行的。     

The effect of spatial filtering on non-linear distortions of laser diodes

It is demonstrated for the first time that spatial filtering of partially reflected irradiance of light from a directly modulated semiconductor laser diode produces a weak but appreciable amount of harmonic distortions. The laser diode discussed here is assembled to be misaligned in such a way that there is significant reflection of light from the submount attached to the heat sink. This geometry of laser and submount gives rise to interfered light intensities of simultaneous AM- and FM-modulated outputs in the far-field region. Numerical calculations are presented for a variety of laser structures and FM-modulation parameters. The second-order harmonic distortion is found to disclose multiple peaks with respect to the aperture radius of a receiving optical system.     

Improvement in dispersion tolerance of high-speed fiber optic transmission link by optimization of laser modulation

In this paper, the improvement in dispersion tolerance of high-speed transmission link with directly modulated laser is presented. The method is based on adopting the laser modulation (its amplitude and depth) accordingly to the dispersion of actual link. As it is shown, the tolerance of negative dispersion may be doubled and of positive increased even seven times. The simulations are proved by hardware experiment, in which the extended dispersion tolerance is demonstrated.     

Two Step Deceleration of Cesium Atomic Beam by Frequency Modulated Diode Lasers

Two step white light slowing of a cesium atomic beam was demonstrated by using two kinds of frequency modulated diode lasers. In addition to a frequency modulated free-running diode laser used for the first step of wide range deceleration, a frequency modulated external cavity diode laser (ECDL) with narrower and sharper spectrum was used for the second step deceleration. It was shown that the number of slowed atoms was increased more than twofold by the two step deceleration which kept narrow velocity width of 27 m/s, which is smaller than that by only the single step deceleration.