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.     

Laser-diode pumped self-Q-switched microchip lasers

Optical properties of Cr,Yb:YAG, Cr,Nd:YAG crystals, and composite Yb:YAG/Cr:YAG ceramics self-Q-switched solid-state laser materials are presented. The merits of these self-Q-switched laser materials are given and the potentials of such lasers can be chosen by the applications. Cr,Yb:YAG and composite Yb:YAG/Cr:YAG ceramics self-Q-switched laser are conducted. Although several tens of kW peak power can be obtained with a monolithic microchip Cr,Yb:YAG laser, the experimental results show that the performance of this laser is limited by the absorption of Cr⁴⁺ ions at a pump wavelength of 940 nm and strong fluorescence quenching at high Cr concentration. Composite Yb:YAG/Cr:YAG ceramics are more suitable to realize high pulse energy and peak power (up to MW level) with optimized lasing and Q-switching parts. In addition, the instabilities induced by the multi-longitudinal mode competition in Cr,Nd:YAG and Cr,Yb:YAG microchip lasers are addressed. The different gain bandwidths of Yb:YAG and Nd:YAG play an important role in the instability of the output laser pulse trains. Stable laser pulses from the Cr,Yb:YAG microchip laser were obtained due to the antiphase dynamics. For the Cr,Nd:YAG microchip laser, the instability caused by the multi-longitudinal mode competition is an intrinsic property. Different transverse patterns were observed in Cr,Nd:YAG microchip lasers when a pump beam with larger diameter was used. Saturated inversion population distribution inside the gain medium plays an important role in the transverse pattern formation. Different transverse patterns were reconstructed by combining different sets of the Hermite-Gaussian modes.     

LD端面泵浦Cr4:YAG被动调Q激光器输出特性研究

对激光二极管端面泵浦Cr⁴⁺:YAG被动调QNd:YAG激光器输出特性进行了实验研究.实验研究发现,激光器输出功率及脉冲重复频率随谐振腔长度增大而增大.为解释这一实验现象,测量了泵浦光斑在激光晶体内尺寸,同时计算了激光晶体及Cr⁴⁺:YAG晶体内的基模激光光斑半径随谐振腔长度变化.分析结果表明:激光晶体内泵浦光斑尺寸远小于激光晶体内基模光斑半径,腔模间交叠效率较低;当腔长增加时,激光晶体内的基模激光光斑减小,腔模间交叠效率增加,从而导致输出功率及脉冲重复频率随腔长增加而增加;另外,Cr⁴⁺:YAG晶体内光斑半径也随谐振腔长度减小,引起Cr⁴⁺:YAG晶体漂白时间缩短,导致脉冲重复频率随腔长增加而增加.     

Radially polarized laser beam from a Nd:YAG laser cavity with a c-cut YVO4 crystal

We demonstrate the generation of a radially polarized beam by simply inserting an undoped c-cut YVO₄ crystal into a Nd:YAG laser cavity. In a hemispherical cavity, the cylindrically symmetric, positive birefringence of the YVO₄ crystal extends the stability limit of the cavity length for an extraordinary ray (radial polarization) compared to an ordinary one (azimuthal polarization). By adjusting the cavity length, a radially polarized beam with an output power up to 1 W was selectively obtained. In addition, a higher-order transverse mode was also generated by arranging the cavity design. The method demonstrated in this paper can be readily applied to laser systems with an isotropic laser medium. PACS 42.60.Da; 42.25.Lc; 42.25.Ja     

A novel prism beam-shaping laser diode bar end-pumped TEM<Subscript>00</Subscript> mode Nd:YVO<Subscript>4</Subscript> laser

A beam-shaping diode end-pumped TEM₀₀ mode CW Nd:YVO₄ laser is presented. A special beam-shaping element made up of isosceles right-angled prism pieces is adopted to realize beam symmetric. Two cylinder lenses are used to couple the shaped beam into a 3 × 3 × 9 mm Nd:YVO₄ crystal with 0.3 at % neodymium doping. By using this laser system, we have achieved 6.1 W CW laser operated in single transverse mode at 1064 nm with 95.2% reshaping efficiency and 25.6% optical-optical conversion efficiency.     

Single higher-order transverse mode operation of a radially polarized Nd:YAG laser using an annularly reflectivity-modulated photonic crystal coupler

We demonstrate single higher-order transverse mode operation of a radially polarized Nd:YAG laser by using a polarization-selective and reflectivity-modulated output coupler. A narrow annular region with low reflectivity fabricated in a photonic crystal mirror behaves so as to select a higher-order transverse mode of a cylindrically symmetric laser beam. A double-ring-shaped radially polarized TM(02) mode beam is stably generated.     

Emission characteristics of debris from CO<Subscript>2</Subscript> and Nd:YAG laser-produced tin plasmas for extreme ultraviolet lithography light source

We describe a comparative study of the emission characteristics of debris from CO₂ and Nd:YAG laser-produced tin plasmas for developing an extreme-ultraviolet (EUV) lithography light source. Tin (Sn) ions and droplets emitted from a Sn plasma produced by a CO₂ laser or an Nd:YAG laser were detected using Faraday cups and quartz crystal microbalance (QCM) detectors, respectively. The droplets were also monitored by using silicon substrates as witness plates. The results showed higher ion kinetic energy and lower particle emission for the CO₂ laser than the Nd:YAG laser for the same laser energy (50 mJ). The average ion energy was 2.2 keV for the CO₂ laser-produced plasma (LPP), and 0.6 keV for the Nd:YAG LPP. The debris accumulation of the CO₂ LPP detected by the QCM detectors, however, was less than one fourth of that of the Nd:YAG LPP for the same laser energy. Using ion energy data, the mirror lifetime is estimated for the CO₂ and Nd:YAG lasers. In both cases, the upper limit of the number of shots was of the order of 10⁶. PACS  52.38.DX; 52.38.Ph; 52.38.Mf     

Conversion of a high-order mode beam into a nearly Gaussian beam by use of a single interferometric element

We present a new, compact, and practical optical mode converter that efficiently transforms a high-order Hermite-Gaussian (HG) laser beam into a nearly Gaussian beam. The mode converter is based on coherently adding different transverse parts of the high-order mode beam by use of a single planar interferometric element. The method, configuration, and experimental results obtained with a pulsed Nd:YAG HG TEM10 laser beam are presented. The results reveal that the efficiency of conversion of a HG beam to a nearly Gaussian beam can be as high as 90%.     

LD端面泵浦Cr~4∶YAG被动调Q激光器输出特性研究

对激光二极管端面泵浦Cr4+∶YAG被动调Q Nd∶YAG激光器输出特性进行了实验研究.实验研究发现,激光器输出功率及脉冲重复频率随谐振腔长度增大而增大.为解释这一实验现象,测量了泵浦光斑在激光晶体内尺寸,同时计算了激光晶体及Cr4+∶YAG晶体内的基模激光光斑半径随谐振腔长度变化.分析结果表明:激光晶体内泵浦光斑尺寸远小于激光晶体内基模光斑半径,腔模间交叠效率较低;当腔长增加时,激光晶体内的基模激光光斑减小,腔模间交叠效率增加,从而导致输出功率及脉冲重复频率随腔长增加而增加;另外,Cr4+∶YAG晶体内光斑半径也随谐振腔长度减小,引起Cr4+∶YAG晶体漂白时间缩短,导致脉冲重复频率随腔长增加而增加.     

Comparison of mechanisms and effects of Nd:YAG and CO2 laser cleaning of titanium alloys

The paper compares laser cleaning trials performed using a Q-switched Nd:YAG laser, λ = 1.064 μm and a continuous wave (CW) CO₂ laser, λ = 10.6 μm, applied to aerospace-grade, contaminated titanium alloys. The mechanisms for cleaning using each laser system are modelled to determine the mode and extent of contaminant removal. The model results are then compared with the surface chemistry and micro-structural results from the cleaning trials performed. The results show the dominant cleaning process for Nd:YAG cleaning to be by evaporation of the contaminant via conduction through surface heating, while for CO₂ laser cleaning the small fraction of the beam coupling directly with the contaminant is sufficient for direct heating and selective evaporation. The results for experimental cleaning, electron beam (EB) welding and diffusion bonding align well with the model, particularly when secondary reactions are taken into account.     

Yellow light generation by frequency doubling of a diode-pumped Nd:YAG laser

We demonstrate the generation of TEM₀₀ mode yellow light in critically type II phase-matched KTiOPO₄ (KTP) with intracavity frequency doubling of a diode-pumped Nd:YAG laser at room temperature. After a 150 μm thick etalon have been inserted into the cavity, the stability and beam quality of the second harmonic generation (SHG) is enhanced. A continuous wave (CW) TEM₀₀ mode output power of 1.67 W at 556 nm is obtained at a pump level of 16 W. The total optical to optical conversion efficiency is about 10.44%. To the best of our knowledge, this is the first Watt-level yellow light generation by frequency doubling of Nd:YAG laser.     

Characterization of laser processing of single‐crystal natural diamonds using micro‐Raman spectroscopic investigations

The application of lasers for processing diamond has revolutionized the diamond industry and its applications in microelectronics, microelectromechanical system (MEMS) and microoptoelectromechanical system (MOEMS) technologies. The process quality can be evaluated using spectroscopic techniques. In the present investigation, four different types of Q‐switched solid‐state lasers (with different beam parameters), namely, a lamp‐pumped Nd:YAG laser operating at 1064 nm, a lamp‐pumped Nd:YAG laser operating at second harmonically generated 532 nm, a diode‐pumped Nd:YVO₄ laser operating at 1064 nm and a diode‐pumped Nd:YAG laser operating at 1064 nm, have been employed for the processing of a single‐crystal, gem‐quality, natural diamond. The main objective behind the selection of these lasers with different beam parameters was to study the effect of wavelength, pulse width, pulse energy, peak power and beam quality factor (M² factor) on various aspects of processing (such as microcracking, material loss and cut surface quality) and their relative merits and demerits. The overall weight loss of the diamond and formation of microcracks during processing have been studied for the above four cases. The characteristics of the graphite formed during processing, elemental analysis, surface morphology of the cut surface and process dynamics have been studied using micro‐Raman spectroscopy and scanning electron microscopy (SEM). We observed that laser cutting of single‐crystal diamonds used for industrial applications can be accomplished without microcracking or surface distortion using Q‐switched Nd:YAG lasers. This allows direct processing without extensive postgrinding and polishing stages. Very efficient diamond processing is possible using diode‐pumped lasers, which results in the lowest possible breakage rate, good accuracy, good surface finish and low weight loss. From the micro‐Raman and SEM studies, it is concluded that the surface quality obtained is superior when diode‐pumped Nd:YVO₄ laser is used, owing to its extremely high peak power. The maximum graphite content is observed while processing with lamp‐pumped Nd:YAG laser at 532 nm. An overall comparison of all the laser sources leads to the conclusion that diode‐pumped Nd:YAG laser is a superior option for the efficient processing of natural diamond crystals. Copyright © 2006 John Wiley & Sons, Ltd.     

High peak power Nd:YAG laser pumped by 600-W diode laser stack

The Q-switched laser with triangle slab made of Nd:YAG crystal side pumped by 600-W quasi-cw diode laser stack has been designed. The multimode (M²≈2.6) output energy of about 42 mJ was demonstrated in free running mode for110-mJ pump energy. In Q-switch experiments, the KDDP Pockels cell was placed between the slab and rear mirror in plane-plane cavity with output coupler of 84% transmission. The energy of 8 mJ in 2.1-ns pulse duration was obtained for near TEM₀₀ output beam. For passive Q-switching by means of Cr:YAG crystal of 12.6% unsaturated transmission, the energy of 5.1 mJ in 2.5-ns pulse duration was obtained for output beam close to TEM₀₀ mode.     

500.5-nm blue-green laser based on sum-frequency mixing of diode pumped neodymium lasers

We report for the first time a continuous-wave (CW) coherent radiation at 500.5 nm by intracavity sum-frequency generation of 1063 nm Nd:GdVO₄ laser and 946 nm Nd:YAG laser. Blue-green laser is obtained by using a doubly cavity, type-II critical phase matching KTiOPO₄ (KTP) crystal sum-frequency mixing. With total pump power of 27.8 W, TEM₀₀ mode blue-green laser at 500.5 nm of 421 mW is obtained. At the output power level of 421 mW, the blue-green power stability is better than 2.8% and laser beam quality M ² factor is 1.37.     

High efficiency TEM00 mode, diode-pumped, conduction-cooled, Nd:YAG zig-zag slab laser

We report a compact, conduction-cooled, highly efficient, continuous wave (CW) Nd:YAG slab laser in diode-side-pumped geometry. To achieve high efficiency, a novel laser head for Nd:YAG slab has been developed. For an absorbed pump power of 27.6 W, maximum output power of 10.4 W in multimode and 8.2 W in near-diffraction-limited beam quality has been obtained. Slope and optical-to-optical conversion efficiencies are 45.3% and 37.7% in multimode with beam quality factors (M²) in x and y directions equal to 32 and 8, respectively. TEM₀₀ mode operation was achieved in a hybrid resonator with slope and optical-to-optical conversion efficiencies of 43.2% and 29.7%, respectively. Beam quality factors in x and y directions are ?1.5 and ?1.6 for the whole output power range. The laser radiation was linearly polarized and polarization contrast ratios are >1200:1 in the multimode and 1800:1 in the TEM₀₀ mode operation. In passive Q-switching with Cr⁴⁺:YAG crystal of 68% initial transmission, 18 ns pulsewidth has been achieved with an average power of 2 W at a repetition rate of 16 kHz.     

Propagation parameters of the beam extracted from a diode pumped Nd:YAG ceramic slab laser with a hybrid stable–unstable resonator

We analyze the propagation properties of the beam extracted from a diode pumped ceramic Nd:YAG slab laser adopting a hybrid stable–unstable resonator. Such a resonator produces a beam characterized by an Hermite–Gauss mixture-like distribution in one transverse direction and a hard-edge diffracted distribution on the other transverse direction. The beam propagation parameters M_x² and M_y² are measured for different values of the diodes driving current. We obtain a beam parameter product smaller than 3 mm mrad in both transverse directions and in the whole range of powers, up to an extraction of 220 W in a QCW regime.     

Pulsed infrared radiation transmission through chalcogenide glass fibers

Two different kinds of chalcogenide glass IR fibers were evaluated relative to transmission of pulsed IR radiation produced by several laser sources in the wavelength range from 1 to 10 μm. Fibers composed either from As-Se-Te or from As₂S₃ glass, of 250, 500, 750 and 1000 μm and 250, 750 and 1000 μm core diameters were studied, respectively. Attenuation measurements were obtained as a function of the laser energy input and as a function of curvature, wherever this was possible. The output beam quality was also studied using a beam profiler. The lasers used were a Q-switched Nd:YAG laser, emitting at 1.06 μm, a free-running or Q-switched Er:YAG laser emitting at 2.94 μm and a tunable pulsed CO₂ laser emitting in the range of 9.3-10.6 μm. The fibers exhibited better behavior when tested with the Er:YAG laser and they were found fragile in pulsed radiation from the Nd:YAG and the CO₂ laser. The output beam profiles generally showed a central multi-spiking energy distribution.     

All-solid-state doubly resonant sum-frequency mixing laser at 555 nm

We report for the first time a coherent radiation at 555 nm by intracavity sum-frequency generation of 946 nm Nd:YAG laser and 1343 nm Nd:LuVO₄ laser. Yellow-green laser is obtained by using a doubly folded cavity, type-II critical phase matching KTP crystal sum-frequency mixing. With total pump power of 31.9 W (13.7 W pump power for 1343 nm Nd:LuVO₄ laser and 18.2 W pump power for 946 nm Nd:YAG laser), TEM₀₀ mode yellow-green laser at 555 nm of 2.35 W is obtained.     

A comparison between characteristics of various laser-based denim fading processes

Our experiments show that the laser fading process has similar, even better results, in some cases, than the conventional technologies of indigo-dyed denim. Using the beams from Nd:YAG laser (1064 nm and its second harmonic 532 nm), CTH:YAG laser (2.09 μm), CO₂ laser (10.6 μm) and various laser beam parameters we found the optimum fluence and power density in order to obtain a similar fading appearance like the one obtained by conventional processes. The purpose of this work was to determine the effectiveness of laser-based technologies and to measure the wear characteristics of the processed textile like colorfastness, strength resistance, tearing strength, and dimensional changes.     

Optimal ablation of fluorine-doped tin oxide (FTO) thin film layers adopting a simple pulsed Nd:YAG laser with TEM00 mode

In material processing, a laser system with optimal laser parameters has been considered to be significant. Especially, the laser ablation technology is thought to be very important for fabricating a dye-sensitized solar cell (DSSC) module with good quality. Moreover, the TEM₀₀ mode laser beam is the most dominant factor to decide the incident photon to current conversion efficiency (IPCE) characteristics. In order to get the TEM₀₀ mode, a pin-hole is inserted within a simple pulsed Nd:YAG laser resonator. And the spatial field distribution is measured by using three pin-hole diameters of 1.6, 2.0 and 4.0 mm, respectively. At that moment, each case has the same laser beam energy by adjusting the discharge voltage and pulse per second (pps). From those results, it is known that the pin-hole size of 1.6 mm has the perfect TEM₀₀ mode. In addition, at the charging voltage of 1000 V, 10 pps, the feeding speed of 6.08 mm/s and the overlapping rate (OL) of 62%, the scanning electron microscope (SEM) photograph of fluorine-doped tin oxide (FTO) thin film layers shows the best ablation trace.