Semiconductor laser optical amplifiers for multichannel coherentoptical transmission

The application of semiconductor laser optical amplifiers in multichannel coherent optical transmission systems is investigated. The amplifiers considered (λ=1.3 μm) exhibit a gain of 24 dB at a grain ripple <2 dB and a 3-dB bandwidth of about 4000 GHz. The characteristics of these amplifiers and transmission experiments with these amplifiers are described. The investigations concern noise accumulation in an amplifier chain, generation of echoes due to backward gain in cascaded amplifiers, crosstalk in multichannel transmission, and the effect of gain saturation due to spontaneous emission. A good fit is shown between the advantages of multichannel coherent optical transmission systems and the properties of semiconductor laser optical amplifiers, which are very promising for future long-haul optical transmission systems     

Signal linewidth broadening due to nonlinear Kerr effect inlong-haul coherent systems using cascaded optical amplifiers

Signal spectral linewidth broadening due to the interaction between nonlinear Kerr effect and spontaneous emission from optical amplifiers in long-haul coherent optical fiber communication systems using optical amplifiers is examined theoretically and experimentally. By theoretical consideration and recirculating-loop experiments, it is shown that this effect becomes an essential limitation on maximum transmission length in transoceanic coherent optical fiber submarine cable systems     

基于光学技术的太赫兹相干辐射源研究

由于太赫兹辐射的独特性质和潜在的应用价值, 国内外关于太赫兹波的产生和探测的研究正呈现日益繁荣的景象, 目前太赫兹相干辐射源的研究已成为太赫兹技术领域最重要的前沿课题之一。介绍了产生太赫兹相干辐射的三种主要途径：一是光学技术, 它从高频向低频发展, 其代表为太赫兹激光器, 如气体激光器、半导体激光器和量子级联激光器等; 二是电子学技术, 它由低频向高频发展, 如微波管、固体微波源等; 三是光电子技术, 其频率由1 THz向两侧展宽, 采用超快激光脉冲触发产生太赫兹脉冲。设计了基于光学技术的太赫兹相干辐射系统, 该装置根据气体振转能级跃迁原理, 采用高压直流激励方式产生受激辐射, 波导管谐振腔体, 工作气体为N2, CD4和D2, 经过优化设计, 预计可以产生1.54 THz和1.58 THz的波连续输出。     

Optically pumped CW dimer lasers

Bound-bound electronic transitions in simple molecules are generally suited to realize efficient multiline laser oscillation in the visible and ultraviolet spectral region. By means of optical excitation with argon and krypton lasers, CW laser oscillation could be obtained for various homonuclear diatomic (dimer) molecules such as Li2, Na2, K2, Bi2, S2, Te2, and I2, with emission of several hundred laser lines in the spectral range of about 400-1350 nm. The principles of these lasers and the general dependence of threshold and output power on temperature, pressure, length of vapor zone, and some other parameters is discussed. To achieve satisfactory CW operation, low quenching losses for the upper laser level population and a sufficiently fast relaxation of the population of the lower laser level are necessary. Under optimum operation conditions, efficiencies up to 15 percent, multiline output powers up to 400 mW, and single line-single frequency output powers up to 200 mW were achieved. These dimer lasers are three-level laser systems. In case of coherent optical excitation, two-photon or Raman-type processes contribute to the amplification process. Due to these mechanisms the forward direction is strongly favored and in a ring laser system spontaneous unidirectional oscillation is obtained. By means of a suitable three-level model, analytical and numerical calculations of gain profiles are performed and compared with experiments. These optically pumped molecular lasers are suited for various spectroscopic and kinetic investigations, for frequency standards or as simple and efficient systems to convert pump laser radiation into other spectral regions.     

新型光纤激光雷达的结构理论分析与作用距离计算

为了改善当前激光雷达存在的对振动敏感,体积庞大,对人眼不安全等问题,综合国内外激光雷达研究现状的基础上.给出了一种新型光纤激光雷达的结构.该雷达利用光纤激光器作为光源,光路中大量采用了光纤器件,使得系统体积大大减小.采用相干探测的原理,根据激光雷达作用距离方程,给出了信噪比与光纤激光雷达作用距离的关系.根据所选的器件参数,通过理论计算,在探测概率为0.9,虚警概率小于10-4的情况下,得到了2 km的最大作用距离.所设计的光纤激光雷达体积小,结构紧凑、新颖,具有良好的探测能力.     

Interactions between closely coupled GaAs injection lasers

Interaction effects between injection lasers have been experimentally studied at 2°K and 77°K under pulse conditions in a single block configuration which permits close optical coupling between the lasers. This close coupling made previously reported effects an order of magnitude more prominent and revealed a new effect. The reduction of the threshold current of one laser by the influence of the coherent beam of a second laser by as much as a factor five has been observed. With respect to the quenching of one laser by a second laser, a quenching ratio (defined as the ratio of the quenched power to the quenching power) of 0.7 was obtained. A new effect observed was the reduction of the threshold current of one laser by the influence of the spontaneous emission from a second laser. A reduction of 10 per cent is typical for this effect. A qualitative discussion treating the two lasers as one optical cavity is given to incorporate these effects into one coherent picture. Logical device applications are also discussed.     

Nondegenerate Phase-Locking of a Semiconductor Laser by Sideband Injection

A semiconductor laser rate equation theory is presented that describes sideband injection locking under both weak optical injection and current modulation. By simultaneous optical injection and current modulation, control of both the phase and the frequency of a semiconductor laser is demonstrated. The phase-locked semiconductor laser operates at a different frequency to the optical injection source, with a frequency-difference given by the current modulation frequency. This method can be used to produce broadband sources, such as those producing ultrashort pulses and those required for coherent control, or to create high-frequency electronic oscillator sources with phase control by interference beating     

An Analysis of Nonlinear Harmonic Generation in High Gain Free-Electron Laser

The odd harmonic evolution in the free-electron laser (FEL) employing planar undulator is studied analytically. The evolution equation of the harmonic is deduced, the dependence relation of the harmonics interactions is revealed, and both the linear and the nonlinear harmonic evolutions can be described. The analytical formula of the nonlinear harmonic power in self-amplified spontaneous emission FELs is given. For the saturation power of the harmonic, an estimation formula is given. The harmonic evolution equation for a high-gain harmonic generation (HGHG) FEL is also deduced, and in the fundamental case, it gives the optical field equation of HGHG, which describes the optical field evolution from the coherent enhancement to the exponential gain until the saturation occurrence.     

Lasers and Their Application to Television Systems

This paper will first discuss characteristics of the different laser sources with particular emphasis on the display output power needs. There are a number of nonlinear optical effects that appear useful to display systems; of particular interest for displays are the effects related to the controllable change of the wavelength of coherent radiation incident on the nonlinear media. The nonlinear techniques to be discussed are those related to harmonic generation, Raman laser action, and optical parametric amplification and oscillation. Calculations on the parametric oscillation effect show that a one watt blue or UV laser can effectively provide the optical pumping which would result in a very appreciable tunable optical output over the visible spectrum.     

卫星相干光通信跟瞄误差对链路性能的影响

以星间相干激光通信链路的接收机相干效率为研究对象,分析了动态跟瞄误差对相干效率的影响,将相干效率引入到星间相干激光通信链路方程,推导了跟瞄误差波前失配条件下的相干光通信链路的误码率概率表达式,对卫星激光通信链路中跟瞄对准误差导致的相干效率下降、链路误码性能劣化情况进行了仿真计算。结果表明,由于跟瞄对准误差波前失配的影响,相干激光通信接收终端具有一个优化的接收孔径,在本仿真条件下,LEO-GEO星间链路动态跟瞄误差为0.5μrad时,优化接收孔径为0.5 m,链路可获得最佳误码性能。     

1341.4nmNd:YAP激光在KTP晶体中三倍频得到447.1nm蓝色相干辐射

本文报道用LiIO_3晶体将1341.4nmNd:YAP脉冲激光倍频产生的670.7nm辐射和1341.4nm激光在KTP晶体中和频得到447.1nm蓝色相干辐射。计算了上述非线性过程的相位匹配条件和有效非线性系数,计算与实验结果相当吻合。     

The Birth of Lightwave Technology and Its Implications to Microwaves

Ever since the invention of lasers in the late 1950's, the use of coherent optical radiation for communication and signal processing with multigigahertz bandwidth has been a major research goal in electron devices, microwaves, quantum electronics, and optics. In order to realize that goal, the key issues that need to be addressed include a) how to transmit optical radiation with low propagation loss and signal distortion, b) how to effectively interface optical devices with electronic devices, c) how to modulate, multiplex, switch, and detect optical radiation at such high data rates, and d) how to solve the materials and fabrication technology problems. In the early 1960's, research on optical communications was concerned primarily with transmission of laser radiation through the atmosphere and the pipes.     

8μm～14μm长波红外相干辐射技术

8μm~14μm长波红外相干辐射在大气激光遥感与通信、高分辨分子指纹光谱及国家安全等诸多领域具有重要应用价值。概述了几种可获得8μm~14μm相干辐射的光源, 主要有CO₂气体激光器、半导体量子级联激光器、红外自由电子激光器、红外超连续谱光源和非线性变频中红外固体激光器等。分别介绍了不同技术的基本原理及国内外进展, 并分析比较了各自的优缺点、发展方向及应用适用范围。其中, 非线性变频中红外固体激光器具有8μm~14μm连续调谐, 可飞秒、皮秒、纳秒及连续波运转, 重频1Hz~GHz可调的突出优势, 且全固化结构、可靠性高、光束质量好, 将成为发展实用化与精密化长波红外相干辐射光源的最佳选择。     

Fast all-optical flip-flop memory exploiting the electric field nonlinearity of coherent laser amplifiers

The coherent nonlinear feedback in an integrated optical flip-flop is studied in order to demonstrate device applications of laser amplifiers operated above the laser threshold. The nonlinear feedback is provided by the stabilizing fields of the laser amplifiers, interfering with coherent optical reference fields. In both stable states of the flip-flop, the nonlinear feedback allows the laser amplifiers to maintain an ever present photon population in the laser modes and to have an approximately constant carrier population. This enables faster response (switching times below 25 ps are reached in this work)than in conventional active optical flip-flop structures. The operation of the flip-flop is studied using a rate equation model accounting for the carrier densities and the complex electric fields in the different cavity modes of the laser amplifiers.     

Trapped radiation in injection lasers

The effect of trapped radiation on the stimulated emission threshold and power in injection lasers is investigated theoretically and experimentally. The density of trapped radiation is determined by the relation between the gain and the loss coefficients for this radiation in the active region. The last value depends upon treatment of the cavity sidewalls and laser diode geometry. The absorption of spontaneous emission in the active diode region provides for an additional optical excitation of the crystal and can result in a decrease of threshold current. The GaAs laser diodes with Fabry-Perot cavities and four-ended resonators were studied. The lasing threshold for the axial modes is increased with the diode width and that for the nonaxial radiating modes is decreased with it. The luminescence spectra from the cavity ends and sidewalls are investigated as a function of diode length and current density. It is shown that the gained luminescence can be an essential source of energy losses in injection lasers and leads to limitations of dimensions of laser diodes with planar p-n-junctions. Various methods for suppression of harmful radiation and for increase of the stimulated emission power in injection lasers are discussed.     

Spectra of gain, absolute refractive index, and wave-guideconfinement factor of strained single quantum-well GaInP laser fromspontaneous emission

Both unamplified and amplified spontaneous emission was measured from a compressively-strained single quantum-well GaInP laser. By using a novel and self-consistent technique briefly described in the text, the data were used to systematically determine the gain-peak energy, quasi-Fermi level separation, intrinsic optical loss, absolute refractive index, and optical waveguide confinement factor and gain, including the dispersion of refractive index for a given current. During this process, there is no need for a knowledge of parameters that are difficult to experimentally check with enough accuracy. Furthermore, the effect of dispersion of the refractive index on the optical waveguide confinement factor is discussed     

Thermodynamics of laser systems--Part I

The laser is treated as a thermodynamic machine where production of radiation is the result of entropy creating spontaneous flows of energy and polarization through the system. One of the laws of thermodynamics governing flows of extensive properties is the symmetry principle of Curie and Prigogine. It determines how much flows are coupled. Going back to the first principles of statistical mechanics and using aforementioned symmetry requirements, a set of alternative Bloch equations is derived. The impact of the new formulation on laser physics is developed. It leads to a set of new conclusions for spontaneous pulsating regimes, for the spectral line shape of coherent radiation, for saturation effects, etc.     

546 km, 140 Mbit/s FSK coherent transmission experiment through 10 cascaded semiconductor laser amplifiers

The effect of the spontaneous emission noise on coherent systems in a multistage semiconductor laser amplifier repeater system is experimentally examined. By appropriate adjustment of the input power level to each amplifier, 546 km FSK transmission of 140 Mbit/s using 10 cascaded amplifiers is demonstrated without power penalty.<>     

光学介质对CO2激光相干雷达光束偏振态的影响

讨论了红外相干激光雷达中光学介质对偏振光束的影响程度,包括光束分束片和反射镜片的影响,给出了定量的结果。表明在相干激光雷达系统中,光学镜片的选取与放置将影响激光束的偏振性质,在系统应用中应仔细考虑。     

Two-mode optical pumping of a laser

The coherent optical pumping of a laser is investigated by means of semiclassical laser theory with the assumption that either the pump source or the laser output is multimode. All radiation modes are allowed to reach saturating levels. A multitude of interesting effects result from induced population pulsations. As a particular application we study numerically the optical pumping of a single-mode homogeneously broadened FIR laser with a two-mode pump. We find that at least for symmetric pumping higher FIR intensities can be obtained with two, instead of one, pump modes.