Theoretical investigation on semiconductor lasers with passivewaveguides

Semiconductor lasers with vertically integrated passive waveguides are theoretically studied using the coupled mode theory and exact calculation. Formulas for the threshold current density and the far-field patterns are derived. The physical concepts of the modulation of the beam divergence by passive waveguides are given. The exact calculated results show that the beam divergence can be greatly improved by paying a price of only a slight increase of the threshold current density. The operation mode selection is discussed. Attention is also paid to the appearance of side lobes for very narrow far-field patterns. Discussions are given for device design     

Single-Mode Emission From Vertical-Cavity Surface-Emitting Lasers With Low-Index Defects

Mode confinement to low-index "defect" regions of a two-dimensional square photonic lattice has been studied as a means to realize single-spatial-mode vertical-cavity surface-emitting lasers (VCSELs). A selective etching process with two-step metal-organic chemical vapor deposition is used to fabricate the low-index defect VCSELs. A device with a 9-mum-wide aperture demonstrates 2.9-mW continuous-wave (CW) output power. The near diffraction-limited single-lobe far-field and high-resolution spectral analyses confirm single fundamental defect mode emission. Comparison studies between pulsed and CW operation indicate that thermal effects have a significant impact on the modal performance of the devices. Single-mode emission with more than 8 mW of peak pulsed power has been observed with short pulsewidth operation (0.1 mus, 2%). Improvements in thermal management are required to achieve higher CW single-mode emission in these devices     

新型板条激光器的离轴混合腔模场计算

报道了一种新型双板条离轴混合腔激光器。这种激光器结构通过改变传统的冷却方式和采用特殊的谐振腔设计，将使从第一块介质板条高温一侧出射的激光对称地进入另一块板条的低温一侧，从而可对由于温度分布不均匀造成的波面畸变进行一定程度的自校正，减少热效应的影响，可望提高激光器的输出功率和光束质量。利用快速傅里叶变换（FFT）对这种激光器的近场、远场以及相位等模场特性进行了数值计算。分析了波面畸变对输出光束质量的影响，并与常规双板条激光器进行了比较，结果表明这种新型双板条离轴混合腔激光器可以实现一定程度的波面畸变自补偿，从而获得更好的输出光束质量。     

Analysis and performance of channeled-substrate-planar double-heterojunction lasers with geometrical asymmetries

The effects of geometrical asymmetries on the optoelectronic properties of CSP-DH lasers using self-consistent calculations of the optical field and the electron-hole distribution in the active layer are analyzed and compared with device measurements. Laser properties modeled include gain profile, threshold, slope efficiency, near field, and far field. This analysis shows that small geometrical asymmetries due to device fabrication can produce significant changes in the optical and electrical properties of CSP-DH lasers, especially at high-output power levels. For example, a 0.5 μm misalignment of the Zn diffusion with respect to the substrate channel can produce lateral near-field and far-field shifts of 0.6 μm and 2.5°, respectively, and limit single spatial mode operation to about 30 mW.     

Feedback effects in tapered broad-area semiconductor lasers andamplifiers

A detailed experimental investigation of the effect of optical feedback upon the operation of high-power broad area InGaAs strained quantum well semiconductor lasers is presented. In particular, we examine the effect optical feedback has upon the beam quality of high-brightness lasers employing a tapered electrical contact. The far-field of such lasers is shown to degrade rapidly with power feedback ratios exceeding -30 dB, leading to a significantly reduced power in the central diffraction-limited far-field lobe. Far-field degradation is accompanied by dynamic instability and spectral broadening. Good beam quality and low intensity noise can be recovered, however, by rotating the feedback polarization     

Coupled mode analysis of wavefront properties of phased array diode lasers

A coupled mode analysis of phased array diode lasers is used to calculate the far-field wavefront properties of the array modes. It has been found that phase variation across a far-field lobe of the array is less than that of the single emitter.     

Characterizing the beam properties of terahertz quantum-cascade lasers

Terahertz quantum-cascade lasers (QCLs) are very promising radiation sources for many scientific and commercial applications. Shaping and characterizing the beam profile of a QCL is crucial for any of these applications. Usually the beam profile should be as close as possible to a fundamental Gaussian TEM₀₀ mode. In order to completely characterize the laser beam the power and the wavefront have to be measured. We describe methods for characterizing the beam properties of QCLs. Several QCLs with single-plasmon waveguide and emission frequencies between 2 and 5 THz are investigated. The beam profiles of these lasers are shaped into almost fundamental Gaussian modes using dedicated lenses. The beam propagation factor M² is as low as 1.2. The wavefront is measured along the axis of propagation with a THz Hartmann sensor. Its curvature behaves as expected for a Gaussian beam. The applied methods can be transferred to any other THz beam.     

波导结构对905 nm隧道结级联半导体激光器的光束质量和功率的影响

近年来,激光雷达应用对探测距离和灵敏度提出了更高的要求。905 nm半导体激光器作为其理想光源也亟待提升峰值功率与光束质量。在这个背景下,基于非对称大光腔结构研究了不同增益区类型和波导结构对905 nm隧道结脉冲半导体激光器的光束质量和功率效率的影响。通过优化增益区类型和波导结构降低了体电阻和内损耗;增强了限制载流子泄露的能力,提高了器件在高电流下工作的峰值功率和电光效率;通过提高高阶模对基模的阈值增益比值,抑制高阶模式激射,降低了远场发散角。在此基础上,研制的800 μm腔长、200 μm条宽的四有源区半导体激光器在100 ns脉冲宽度、1 kHz重复频率的脉冲功率测试中,41.6 A的脉冲电流强度下实现了峰值功率输出177 W;垂直于PN结方向单模激射,远场发散角半高全宽为24.3°。     

High brightness single-mode ridge laser utilizing buried heterostructure defined by quantum-well intermixing

The authors demonstrate a novel high brightness single-lateral mode ridge laser using quantum well intermixing to form a buried heterostructure. Increased discrimination between the fundamental and higher order modes can be achieved using the buried heterostructure to reduce the width of the gain section, enhancing fundamental mode operation. This allows the ridge width to be increased while maintaining fundamental mode operation, hence reducing the optical intensity at the facet and increasing the optical power before mirror degradation. Standard and novel buried heterostructure ridge lasers of 5-/spl mu/m width are compared; far-field beam profiles clearly show improved modal stability for the novel structure.     

激光非均匀性和内光路热效应对远场特性的影响

高功率高能激光在光束控制系统(或称为内光路)中的传输距离比长程大气传输要短得多，但高功率密度的激光束在通过内光路时的热效应对远场光束质量会有显著影响。采用自编的四维仿真程序，详细计算了沿x方向呈线性变化的环状高功率激光束通过内光路的传输。用像散参数、桶中功率(PIB)和峰值光强位置描述了远场光束质量。研究表明，内光路的热效应(热晕)使得光束的峰值光强和可聚焦能力下降，降低了远场的光束质量，初始光束的非均匀性会影响光束的可聚焦能力并引起像散。值得指出的是，热晕是一种非线性效应，会影响光强分布和引起远场峰值光强位置的移动，对此给出了物理诠释，并用数值计算证实了物理分析。     

Single high-order transverse mode surface-emitting laser with controlled far-field pattern

Single transverse mode vertical-cavity surface-emitting lasers (VCSELs) with high output power are important for high-speed data links, optical recording, laser processing, and so on. We demonstrate a single high-order transverse mode VCSEL with narrow trenches formed on its top surface by using a focused ion beam. The formation of four straight cross trenches resulted in the selection of a stable LP/sub 41/ mode oscillation. A large far-field angle of the LP/sub 41/ mode, which causes a low coupling efficiency even with a multimode fiber, was drastically reduced by loading a phase shift layer. A single-lobe far-field pattern with low scattering loss was realized by loading a spatial phase shift of SiO/sub 2/. We evaluated a coupling efficiency with a single-mode fiber including its alignment tolerance. The controlled far-field pattern enables lens-free direct coupling with a single-mode fiber even for large active-area VCSELs.     

高能激光及其采样后大气传输中湍流及热晕效应的比较

通过数值模拟，比较了高能激光及其采样后在湍流大气中传输时远场中央长曝光斑的环同能量半径与扩展等情况。结果表明，在仅有湍流时两者保持不变，而热晕会导致光斑的进一步扩展；采样器的透光比越低，热晕对光斑的扩展影响越小；未采样高能激光的远场中央光斑相对于高能激光经采样后的远场中央光斑的扩展，可归因于热晕效应。因此，利用光束波前采样器对高能激光及其采样后在湍流大气中传输时的远场光斑特性进行比较，就可以诊断高能激光大气传输热晕效应。     

液晶闪耀光栅波前相位测量方法

电控液晶光栅在卫星激光通信、激光雷达的光束指向应用中具有体积小、光束灵活可控的优势。而获取电控液晶光栅对光波前的相位调制分布则是评价液晶光栅性能的关键。提出液晶光栅调制波前相位的共轭移相干涉测量技术,通过实验对比验证了实测远场分布与近场相位计算远场分布的一致性,由此可实现由近场相位测量的光束指向控制新方法。     

Continuous wave thin plate Nd:glass laser

CW (continuous-wave) operation of end-pumped four-level glass lasers is modeled to obtain expressions for threshold, slope efficiency, and figures of merit. The thermal effects ultimately limiting performance in these lasers is also discussed. Basic design consideration for optimum performance of an end-pumped CW Nd:glass laser intended for mode locking are presented. On the basis of these investigations, a CW Nd:glass laser containing a heavily doped thin phosphate glass plate as the gain material has been built. The plate was inserted into a folded cavity and pumped longitudinally by a Kr laser at 0.8 μm. With 2.5% output coupling the pump threshold and the slope efficiency well above threshold were measured to be 10.5 mW and 36%, respectively. These values agree reasonably with theoretical predictions. Experimental confirmation of the predicted thermal limiting effects are also obtained     

线阵光纤激光相干合成角度扫描控制方法研究

基于光的干涉理论,对多芯线阵光纤激光通过相干合成进行角度扫描的原理进行了研究。利用相干合成模型,用MATLAB分别对信号无噪声与含有随机相位噪声时的合成效果进行了数值仿真计算,分析了通过主动相位控制对合成光进行角度扫描的可行性;基于MOPA结构光纤激光线列搭建了相干合成角度扫描控制系统。通过实验验证了在多路光纤激光完成锁相后,若依次等比改变各芯激光的相位可以实现主极大条纹连续的角度扫描,扫描范围与模型中计算所得结果基本一致,实验结果与理论计算相符。该方法的探索研究对线列光纤激光的相干合成应用拓展有一定的指导意义。     

无波前传感自适应光学神经网络控制方法

针对无波前探测自适应光学系统,研究了基于神经网络的波前控制方法。建立了无波前探测自适应光学仿真模型,分别采用卷积神经网络(Convolution Neural Network,CNN)和普通神经网络(General Neural Network,GNN)作为控制算法,远场光斑图像为神经网络的输入信号,一定阶数的泽尼克模式系数为神经网络的输出,分析了波前校正效果。仿真结果表明,经过充分训练后的神经网络可以快速、准确地从远场光斑图像中复原出入射波前的低阶泽尼克模式系数,CNN的效果优于GNN,二者的损失函数值分别为0.015 8和0.037 6。相比于传统的迭代式寻优控制方法,神经网络控制方法能够基于远场光斑图像快速得到控制信号,在实时性方面有明显优势。     

Comprehensive and fully self-consistent modeling of modern semiconductor lasers

The fully self-consistent model of modern semiconductor lasers used to design their advanced structures and to understand more deeply their properties is given in the present paper. Operation of semiconductor lasers depends not only on many optical, electrical, thermal, recombination, and sometimes mechanical phenomena taking place within their volumes but also on numerous mutual interactions between these phenomena. Their experimental investigation is quite complex, mostly because of miniature device sizes. Therefore, the most convenient and exact method to analyze expected laser operation and to determine laser optimal structures for various applications is to examine the details of their performance with the aid of a simulation of laser operation in various considered conditions. Such a simulation of an operation of semiconductor lasers is presented in this paper in a full complexity of all mutual interactions between the above individual physical processes. In particular, the hole-burning effect has been discussed. The impacts on laser performance introduced by oxide apertures (their sizes and localization) have been analyzed in detail. Also, some important details concerning the operation of various types of semiconductor lasers are discussed. The results of some applications of semiconductor lasers are shown for successive laser structures.     

Spatial coherence and thermal lensing in broad-area semiconductor lasers

Injection profiled broad-area edge-emitting semiconductor lasers demonstrate single transverse mode operation and near-diffraction-limited beam output when driven by pulsed pump current. Thermal effects arising from CW operation induce filamentary dynamics, thus degrading the beam. Transition from the stable nonthermal to the unstable CW regime is analyzed experimentally and numerically, and techniques to improve beam quality in the thermal regime, based on feedback or thermal profiling, are proposed.     

MBE growth and characteristics of periodic index separate confinement heterostructure InGaAs quantum-well lasers

We have used solid-source molecular beam epitaxy (MBE) to grow InGaAs quantum-well lasers emitting at 980nm in a novel configuration of periodic index separate confinement heterostructure (PINSCH). Periodic multilayers (GaAs/AlGaAs) are utilized as optical confinement layers to reduce the transverse beam divergence as well as to increase the maximum output power. The multilayers are grown by temperature modulation MBE without any shutter operation. The heterointerfaces in the multilayers are linearly graded such that the energy barrier heights are greatly decreased. This has led to a drastic reduction in the series resistance which is essential in the performance of high output power. The 5μm × 750μm device has far-field angles of 10° by 20°, a threshold current of 45 mA, an external differential quantum efficiency of 1.15 mW/mA (90%), and an output power of 620 mW, all measured at room temperature under CW operation. A record high fiber coupling efficiency of 51% has been achieved and more than 130 mW of power is coupled into a 5μm-core single mode fiber.     

Experimental study of thermal lensing of Nd：YAG laser

Beamfocusing and higher-order wavefront distortiondue to heat dissipationinalaser gain mediumarei mpor-tant considerationsinthe design of solid-statelasers be-cause wavefront distortion can profoundly influence theperformance of a givenlaser design.Althou…