Lasers for materials processing: specifications and trends

An overview is given of the types of lasers dominating the field of laser materials processing. The most prominent lasers in this field are the CO₂ and the Nd: YAG laser. The domain of CO₂ lasers is applications which demand high laser powers (up to 30 kW are available at present), whereas the domain of Nd:YAG lasers is micro-machining applications. In the kilowatt range of laser output power, the two types of lasers are in competition. New diffusion-cooled CO₂ laser systems are capable of output laser powers of several kilowatts, with good beam qualities, while still being quite compact. The output power and beam quality of Nd:YAG lasers has been improved in recent years, so that Nd:YAG lasers are now an alternative to CO₂ lasers even in the kilowatt range. This is especially true for applications that demand optical fibre transmission of the laser beam, which is possible with Nd:YAG laser light but not with the longerwavelength light emitted by CO₂ lasers. The main problem in solid-state lasers such as Nd:YAG is the thermal lensing effect and damage due to thermal stresses. In order to reduce thermal loading, cooling has to be enhanced. Several alternative geometries have been proposed to reduce thermal loading and, by this, thermal lensing effects. There are now slab and tube geometries which allow much higher output powers than the conventionally used laser rods. A very new scheme proposes a thin slab whose cooled side is also used as one of the laser mirrors, so that thermal gradients occur mainly in the direction of the beam propagation and not perpendicular to it, as is the case in the other geometries. As well as CO₂ and Nd:YAG lasers, semiconductor laser diodes are very promising for direct use of the emitted light or as pump sources for Nd:YAG and other solid-state lasers. When packaging together thousands of single laser diodes, output powers of several kilowatts can be realized. Major problems are collimation of the highly divergent laser beams and cooling of the laser diode bars.     

Critical pump power and cross-section calculation for Yb<Superscript>3+</Superscript>-doped novel inner-clad structures

Rare-earth doped fiber lasers are now considered suitable sources for the 1.0-, 1.5-, and even 2.0-μm regions in a relatively large number of applications. In particular, Yb³⁺-doped fiber lasers have received strong interest due to its high quantum efficiency, broad tunability, and available high output power. In this work, we present results on a newly developed code for the simulation of the critical power that is required when designing double-clad high-power Yb³⁺-doped fiber lasers, which are based on novel inner-clad structures. From our results, we can also estimate σ_ep, σ_el, σ_ap, and σ_al for the aforementioned free-running laser cavities. Finally, we will formulate our predictions for feasible fiber laser cross sections.     

Method of Sellctive Intracavity Absorption Using Dye Lasers

The method of the selective intracavity absorption is based on the high sensitivity of the laser intensity on the absorption of the sample placed into the resonator. The method for Nd-glass laser with inhomogeneous broadening of the gain spectrum was grounded and experimentally checued by the authors of the papers^1,2. Later demands for the spectral region expanding of the method have led to application of dye lnsers for the intracavity measurements. The capability to broadrband emission in the different spectral regions makes of dye lasers the unique instruments for intracavity spectroscopy. A number of the successful experiments on the dye lasers applications to the sensitive detection of Na, da, Ho, Pr, I₂, Br₂ small concentrations was published until now^3–6 At the same time one cannot consider the theoretical analysis of the intracavity absorption characteristics of the dye lasers, i.e, the lasers with homogeneous spectral broadeninti of active media as sufficiently complete and reliable. The theoretical estimation of the sensitivity of the method for CW laser operation was given in papers^7,8. But analytical formulae for sensitivity in reference⁷ are valid only for near-threshold levels of pumping power which does not often correspond to experimental conditions and leaves open the question about potentialities of the method at the higher pumping power (two or more times exceeding the threshold). The authors of the reference⁸ consider the problem at any pumping levels, but their approach is based on the simplified model which seem to be doubtful and requires the special foundation. In particular they suppose the existence in the active media of the number of N_j particles, which interact only with one mode j and do not interact with other laser modes, and also of the number of N_o molecules (so called reservoir) equally interacting with every mode. In the case of the homogeneous spectral broadening such dividing the molecules into types does not seem to be rightful. In addition in the both the gain and loss of all modes (except one) are assumed to be of the same values so the total number of modes is given arbitrary and not connected with resonator, active media and pumping parameters. Because of this reason one cannot calculate correctly the value of the method sensitivity or analyze it's dependence on the parameters.     

Suppression of self-pulsing in Tm:YAlO3 lasers via current feedback

Self-pulsing of continuous-wave Tm:YAlO₃ lasers limits their use for a variety of important applications. We demonstrate for the first time that the pulsing can be suppressed via feedback to the pump diode laser, a technique that is suitable for both external resonator and monolithic lasers. We also show that the optical transfer function of the laser is that of an unstable relaxation oscillator.     

Experimental study of nonlinear mode mixing in dual-wavelength semiconductor lasers

New schemes for the generation of the sum, difference, and double frequencies in dual-wavelength injection lasers, based on an intracavity nonlinear mode mixing owing to a bulk nonlinearity, are experimentally investigated. Namely, the operation of the dual-wavelength laser diodes at the TE₀ and TE₁ modes, butt-joint laser diodes, and interband cascade lasers with tunnel junction are described.     

Grating-Stabilized External Cavity Diode Lasers for Raman Spectroscopy—A Review

Abstract: Conventional Raman techniques require a continuous-wave laser with stabilized wavelength, narrow line width, and sufficient output power. Due to their miniature size and low cost, diode lasers are good choice as light sources for Raman spectroscopy, especially when compact and portable instruments are needed. However, a solitary multimode diode laser has certain drawbacks that limit its use for Raman application. To circumvent these drawbacks, an external cavity can be coupled to the active gain medium of the diode to enhance the laser performance. A grating-based external cavity allows the laser to operate in a single longitudinal mode with greatly reduced line width and stabilized wavelength. This article examines the fundamentals of semiconductor lasers to show the necessity of operating diode lasers in an external cavity for Raman applications. Two feedback grating-based external cavity diode laser (ECDL) designs, viz. Littrow and Littman-Metcalf configurations, are explained. Historic and recent progress in the development of ECDL devices is reported. An updated summary of ECDL-equipped Raman systems applied to fields such as in vivo biomedical studies and in situ process/quality control is provided. Topics on mode-hop-free continuous scanning, wavelength stabilization, and dealing with ambient conditions are discussed.     

舰载人眼安全型激光测距仪

论述人眼安全型激光测距仪的军事价值和实现途径，比较了几种在研的人眼安全激光器，重点阐述首选的舰用喇曼频移激光测距仪的进展、技术特点及装舰情况。     

1.5410.6-μm eyesafe laser rangefinders performance under adverse weather conditions

In this investigation, the performance of a CO₂ laser rangefinder at 10.6 μm and an erbium:glass rangefinder at 1.54 μm are evaluated and compared under various atmospheric conditions. Both systems and the trial site are described. The effect of atmospheric extinction and scattering is discussed. Also, the maximum range capability of the two laser rangefinders under these degraded weather conditions is assessed. The results show that the tested CO₂ laser rangefinder performed better than the Er:glass LRF used for the trials under all weather conditions. However, comparing two state-of-the-art systems does not lead necessarily to the same conclusion.     

Laser applications in thin-film photovoltaics

We review laser applications in thin-film photovoltaics (thin-film Si, CdTe, and Cu(In,Ga)Se₂ solar cells). Lasers are applied in this growing field to manufacture modules, to monitor Si deposition processes, and to characterize opto-electrical properties of thin films. Unlike traditional panels based on crystalline silicon wafers, the individual cells of a thin-film photovoltaic module can be serially interconnected by laser scribing during fabrication. Laser scribing applications are described in detail, while other laser-based fabrication processes, such as laser-induced crystallization and pulsed laser deposition, are briefly reviewed. Lasers are also integrated into various diagnostic tools to analyze the composition of chemical vapors during deposition of Si thin films. Silane (SiH₄), silane radicals (SiH₃, SiH₂, SiH, Si), and Si nanoparticles have all been monitored inside chemical vapor deposition systems. Finally, we review various thin-film characterization methods, in which lasers are implemented.     

Advances in vanadate laser crystals at a lasing wavelength of 1 micrometer

Sapphire, garnet and vanadate crystals are the most prominent optical materials, and vanadates play important roles in optics, especially in lasers and nonlinear optics. Neodymium‐doped yttrium vanadate (Nd:YVO₄) is representative and available commercially. Based on Nd:YVO₄, several vanadate crystals are being developed with the goal of fulfilling the need for differential applications and improvement of certain operational aspects, such as with pulsed lasers or high‐power continuous‐wave lasers. In recent years, some important effects, including energy enhancement, bistability of output performance, self‐Raman frequency shifting, etc., and some novel applications, such as quantum optics, pulsed lasers modulated by the two‐dimensional crystals, etc., have been discovered with vanadates as gain materials. In this paper, the preparation, characterization and laser applications of vanadate laser crystals at the lasing wavelength of 1 micrometer, including YVO₄, GdVO₄, LuVO₄, Gd_xY_1–xVO₄ and Lu_xGd_1–xVO₄ (0 < x < 1) doped with Nd³⁺ and ytterbium (Yb³⁺) are systematically reviewed by highlighting the most recent research progress. Their specific properties are presented, generation mechanisms of novel physical effects are discussed, new applications are given and possible future applications proposed by focusing on some potential strengths.     

高功率皮秒紫外激光器新进展

高功率皮秒紫外激光器在高精密加工、激光医疗、光电对抗和光伏产业等领域有重要应用,近年来成为固体激光新光源研究热点。本文对国内外基于和频技术的高功率皮秒紫外激光器研究新进展进行了归纳和总结。首先,阐述了和频工作原理,介绍了和频产生皮秒紫外激光的非线性晶体;然后,介绍了国内外高功率皮秒紫外激光器的新进展,包括:高功率皮秒紫外激光器、高峰值功率皮秒紫外激光器、高功率和高峰值功率皮秒紫外激光器。最后,展望了高功率皮秒紫外激光器的进一步发展及应用。归纳和总结表明:高功率皮秒紫外激光器在国外较成熟,国内在该领域的研究刚刚起步。光子晶体光纤和碟片激光器输出基频光的皮秒紫外激光器有突出的优势,已成为皮秒紫外激光产业的主力军。     

Comparison of 905 nm and 1550 nm semiconductor laser rangefinders’ performance deterioration due to adverse environmental conditions

Laser rangefinder performance (i.e., maximum range) is strongly affected by environment due to visibility-dependent laser attenuation in the atmosphere and target reflectivity variations induced by surface condition changes (dry vs. wet). Both factors have their unique spectral features which means that rangefinders operating at different wavelengths are affected by specific environmental changes in a different way. Current state of the art TOF (time of flight) semiconductor laser rangefinders are based mainly on two wavelengths: 905 nm and 1550 nm, which results from atmospheric transmission windows and availability of high power pulsed sources. The paper discusses the scope of maximum range degradation of hypothetical 0.9 μm and 1.5 μm rangefinders due to selected water-related environmental effects. Atmospheric extinction spectra were adapted from Standard Atmosphere Model and reflectance fingerprints of various materials have been measured. It is not the aim of the paper to determine in general which wavelength is superior for laser range finding, since a number of criteria could be considered, but to verify their susceptibility to adverse environmental conditions.     

生物医学领域中超连续激光光谱技术的研究进展

超连续谱激光指的是当泵浦激光穿过特殊光波导时,一系列的非线性效应引起入射激光束的光谱展宽,从而输出宽光谱激光束。随着超快激光和光子晶体光纤技术的发展,利用超短脉冲在光子晶体光纤中的传播链产生相干的且亮度高的超连续谱激光成为了一种理想的白光源。自从超连续谱激光源投入应用以来,其应用领域越来越广。尤其在生物医学的细胞、血液等样品分析当中,荧光光谱学、流式细胞仪、共焦显微、光学相干层析等技术都是强有力的分析工具,在采用这些先进技术的科学仪器中,超连续谱激光源成为了一种主要光学部件。首先对超连续谱激光源的国际研究进展作了详细介绍,然后对超连续激光光谱技术在显微成像、流式细胞仪、荧光寿命成像显微、荧光共振能量转移、光学相干层析、共焦显微生物医学分析等生物医学领域中的发展及应用作了综合阐述。对超连续激光光谱技术在非接触式血液制品鉴别的需求、方案及研究进展进行了重点论述,包括覆盖400～2 000 nm光谱范围的光纤化轻型超连续谱激光光源研究;采用超连续谱激光光谱方法探索不同物种血液的种属特征;根据大数据的血液样品光谱特征元数据库分析建立数学模型,利用数学模型实现对血液样品种属光谱学判定;血液鉴别光谱分析便携式整机系统研发等。对超连续激光光谱技术在生物医学领域的应用前景作了展望。     

Diode laser based light sources for biomedical applications

Diode lasers are by far the most efficient lasers currently available. With the ever‐continuing improvement in diode laser technology, this type of laser has become increasingly attractive for a wide range of biomedical applications. Compared to the characteristics of competing laser systems, diode lasers simultaneously offer tunability, high‐power emission and compact size at fairly low cost. Therefore, diode lasers are increasingly preferred in important applications, such as photocoagulation, optical coherence tomography, diffuse optical imaging, fluorescence lifetime imaging, and terahertz imaging. This review provides an overview of the latest development of diode laser technology and systems and their use within selected biomedical applications. 670 nm external cavity diode laser for Raman spectroscopy built on a 13 × 4 mm² microbench (Copyright FBH/Schurian.com ).     

Indigenous development of a 2 kW RF-excited fast axial flow CO<Subscript>2</Subscript> laser

RF-excited fast axial flow CO₂ lasers in kilowatt regime are presently being used for various new scientific applications in addition to laser material processing because of its versatility and superior beam quality. We have indigenously developed a compact 2 kW RF-excited fast axial flow CO₂ laser with moderate beam quality. In this paper the key design features of the laser and the associated high power capacitively coupled RF excitation technique are discussed in detail. Operational characteristics of this system are described along with the experimental findings.     

Watt-level red-emitting diode lasers and modules for display applications

Red-emitting lasers for display applications require high output powers and a high visibility. We demonstrate diode lasers and modules in the red spectral range based on AlGaInP with optical output powers up to 1 W and a nearly diffraction limited beam. These high-luminance light sources based on tapered lasers are well suited for laser TVs and projectors for virtual reality simulators based on the flying spot technology. Additionally, we developed diode lasers with internal distributed Bragg reflector (DBR) surface gratings. These DBR tapered lasers and master-oscillator power-amplifiers based on DBR ridge-waveguide lasers and tapered amplifiers feature high power, single mode emission with coherence lengths up to several meters, which are suitable for the next-generation 3D displays based on holography.     

Standard and phase-matched grazing-incidence and distributed-feedback FIR gas lasers

In this paper we review theoretical and experimental studies on optically pumped 496 m CH₃F DFB lasers of different configurations, including grazing-incidence arrangement and phase matching by a gap in the periodic structure. These configurations combine the simple tuning mechanism of grazing-incidence systems with the high frequency selectivity of DFB. Our theoretical considerations based on coupled-wave theory are concerned with the dispersion relations and resonance conditions of standard and phase-matched DFB and grazing-incidence gas lasers. We have succeeded in calculating the relevant TM coupling coefficients for lasers with rectangular periodic waveguides. For laser cavities with various continuous gratings we have measured the resonant heights and tuning angles of the laser oscillations of first- and second-order DFB. We have found good agreement with theoretical resonance conditions. In order to improve the mode selectivity and to attain single longitudinal mode operation, which is a requirement for semiconductor lasers in many applications, we have introduced variable gaps in the center of the gratings. These provide phase matching and gap modes. We have compared the measured gap modes with our theory and found agreement in specific cases, where the phase-matched cavity implies single-mode laser operation. Our results on standard and phase-matched DFB cavities promise an improvement of the performance of phase-matched semiconductor lasers with respect to small bandwidth and optimized output power.     

The thermal effect in diode-end-pumped continuous-wave 914-nm Nd:YVO4 laser

The thermal effect and the heat generation in diode-end-pumped continuous-wave 914-nm Nd:YVO₄ lasers are investigated in detail. A theoretical model of a diode end-pumped solid-state laser is constructed to analyse the influence of fractional thermal loading on the thermal effect in the Nd:YVO₄ laser based on finite element analysis. The thermal focal lengths and the end-surface deformations of laser rods in Nd:YVO₄ quasi-three-level and four-level lasers are measured and compared with the results obtained by ordinary interferometry for the demonstration of higher thermal loading in 914-nm laser. Finally the fractional thermal loading in the Nd:YVO₄ quasi-three-level laser is calculated by matching the experimental and the simulated end deformations.     

Laser stabilization to an atomic transition using an optically generated dispersive line shape

We report a simple and robust technique to generate a dispersive signal which serves as an error signal to electronically stabilize a monomode continuous-wave laser emitting around an atomic resonance. We explore nonlinear effects in the laser beam propagation through a resonant vapor by way of spatial filtering. The performance of this technique is validated by locking semiconductor lasers to the cesium and rubidium D₂ lines and observing long-term reduction of the emission frequency drifts, making the lasers well adapted for many atomic physics applications.     

四硼酸铝钕晶体及其小型全固态激光器

当前,全固态激光器在向万瓦级大功率方向发展的同时,也在向微型化发展。利用碟片全固态激光器可获得千瓦级以上激光,利用微片全固态激光器则可实现小体积、高密度、中小功率激光输出。获得高功率高密度激光的关键在于激光材料。本文从碟片和微片激光器的发展和对激光基质材料的要求出发,概述自激活激光晶体的研究,特别对四硼酸铝钕[NdAl3(BO3)4,简称NAB]晶体的结构、生长、性质及其作为有应用前景的小型片状激光器的候选材料作了详细的介绍。近期,采用面积为4×4 mm2,厚度为0.39 mm的微片NAB晶体,用885 nm半导体激光器为光源泵浦,获得了4.6 W的1.063 μm激光的有效输出,其斜效率达到64%,充分显示了NAB晶体作为自激活激光晶体在微片激光器中的应用前景。