Influences of CO2 laser annealing on mechanical and laser-induced damage properties of ZrO2 thin films

Zirconium dioxide (ZrO₂) thin films were deposited on BK7 glass substrates by the electron beam evaporation method. A continuous wave CO₂ laser was used to anneal the ZrO₂ thin films to investigate whether beneficial changes could be produced. After annealing at different laser scanning speeds by CO₂ laser, weak absorption of the coatings was measured by the surface thermal lensing (STL) technique, and then laser-induced damage threshold (LIDT) was also determined. It was found that the weak absorption decreased first, while the laser scanning speed is below some value, then increased. The LIDT of the ZrO₂ coatings decreased greatly when the laser scanning speeds were below some value. A Nomarski microscope was employed to map the damage morphology, and it was found that the damage behavior was defect-initiated both for annealed and as-deposited samples. The influences of post-deposition CO₂ laser annealing on the structural and mechanical properties of the films have also been investigated by X-ray diffraction and ZYGO interferometer. It was found that the microstructure of the ZrO₂ films did not change. The residual stress in ZrO₂ films showed a tendency from tensile to compressive after CO₂ laser annealing, and the variation quantity of the residual stress increased with decreasing laser scanning speed. The residual stress may be mitigated to some extent at proper treatment parameters.     

飞秒激光制作铌酸锂光波导的研究

针对Z切割铌酸锂晶体在中心波长为800 nm、重复频率为76 MHz的飞秒脉冲激光下,利用放大倍数为40、数值孔径为0.65的会聚透镜聚焦,研究了扫描速度和扫描次数的变化对刻写光波导结构的影响,分析了波导结构变化的原因,并测试了1×2光波导功分器的通光性能.实验结果表明:在采用横向扫描、聚焦深度为350~400 μm、扫描速度为100 μm/s和激光功率为250 mW时,获得较理想的铌酸锂光波导,1×2光波导功分器的输出功率分布基本达到均分.     

Experimental research on axisymmetric folded-combined cavity CO2 laser

Rationality and feasibility of axisymmetric folded-combined cavity CO₂ laser which is reported in this paper have been proved by principle experiments. The laser lays a foundation to manufacture higher power CO₂ lasers whose output power can easily reach 1 kW. Faculas at different positions in free space and facula of nine beams which pass through the beam focusing optics have been obtained. Analyses of the experimental results are made. These analyses lay a foundation for beam transformation, transmission and shaping.     

Surface evolution and laser damage resistance of CO2 laser irradiated area of fused silica

A 10.6 μm CO₂ laser has been reported to effectively mitigate the laser damage growth of fused silica. Two zones of the laser irradiated area are defined in this work: the distorted zone and the laser affected zone. The parameters of the two zones are studied at different CO₂ laser beam sizes, irradiation times, and powers by microscopy, profilometry, and photoelastic method. The results show that the diameter of laser affected zone is almost completely determined by the laser beam size and the distorted zone is associated with the mitigation range of CO₂ laser beam. The diameter and depth of the distorted zone increase as the laser power and irradiation time increase. The depth grows exponentially depending on the irradiation time. The maximum residual stress discrepancy is located near the boundary of the laser affected zone. The laser damage resistance test results show that the distorted zone and the laser affected zone have a better damage resistance than the original substrate.     

CO2 laser heating of surfaces: Melt pool formation at surface

The melt pool formation during the heating of titanium and steel surfaces by a moving CO₂ laser beam is examined. The repetitive pulses are introduced in the simulations and the Marangoni effect in the melt pool is incorporated in the model study. The influence of laser scanning speed and the laser intensity parameter on the melt pool size is also considered. The enthalpy–porosity method is adopted to account for the phase change in the irradiated spot. It is found that the influence of laser scanning speed on the melt pool size is considerable, which is more pronounced for laser beam parameter β=1. The melt pool size is smaller for stainless steel as compared to that corresponding to titanium.     

Surface treatment with linearly polarized laser beam at oblique incidence

An effective method for surface heat treatment with 10.6 μm linear polarized laser beam at oblique incidence is reported. A circular focused laser spot on the workpiece surface, simultaneously with 2.2–4 times increasing of the absorption are obtained in the 70–80° range of the incidence angle. The main element of the experimental setup is the astigmatic focusing head which focalize the laser beam into an elliptical spot of ellipticity >3 at normal incidence. At a proper incidence angle (obtained by the focusing head tilting) the focused laser spot on the work piece surface gets a circular form and p-state of polarization is achieved.We performed laser heat treatment (transformation hardening, surface remelting) of the uncoated surface, as well as the alloying and cladding processes by powder injection. An enhancement of the processing efficiency was obtained; in this way the investment and operation costs for surface treatment with CO₂ laser can be significantly reduced. Several technical advantages concerning the pollution of the focusing optical components, powder jet flowing and reflected radiation by the work piece surface are obtained.     

CO2 laser beam modulating for surface texturing machining

Laser texturing is a novel technique that may be used to texture a cold roller in the process of manufacturing high quality steel sheets. With the aim of improving the quality of the textured roller by using a CO₂ laser, a new laser beam modulating device is proposed. An optical beam expander with a fast rotating chopper system is designed. The laser pulse is split into two parts by the chopper blades; one is the preheating pulse that is reflected onto optical loop mirrors; the other is the directly transmitted pulse that creates the craters at the preheated spots. The preheating beam focus spot size and position can be adjusted. The focusing characteristics and optical parameter compensation for the flying optics are investigated. The heat transfer and melt process of laser texturing are numerically simulated. The effects of the double pulses on the texturing are analyzed. The effect of preheating the sample ahead of the laser texturing pulse is examined. The surface profile and bump hardness show improvements by using this approach.     

Design considerations and scaling laws for high power convective cooled CW CO2 lasers

Various criteria for designing high power convective cooled CO₂ lasers have been discussed. Considering the saturation intensity, optical damage threshold of the optical resonator components and the small-signal gain, the scaling laws for designing high power CW CO₂ lasers have been established. In transverse flow CO₂ lasers having discharge of square cross-section, the discharge lengthL and its widthW for a specific laser powerP (Watt) and gas flow velocityV (cm/s) can be given byL = 1.4 x 10⁴ p ^1/2 V ^-1 cms andW = 0.04P ^1/2 cms. The optimum transmitivity of the output coupler is found to be almost constant (about 60%), independent of the small signal gain and laser power. In fast axial flow CO₂ lasers the gas flow should be divided into several discharge tubes to maintain the flow velocity within sonic limit. The discharge length in this type of laser does not depend explicitly on the laser power, instead it depends on the input power density in the discharge and the gas flow velocity. Various considerations for ensuring better laser beam quality are also discussed.     

Measurement and direct regulation of the velocity of a fast beam using doppler-tuned laser photon-atom interactions

An optical method is described that uses the Doppler shift to measure the velocity of a fast particle beam merged with a photon beam from a laser. The Doppler-tuned resonance signal is used in a feedback loop to regulate directly the velocity of the beam. The method is demonstrated using a beam of ⁴He atoms pumped by CO₂ laser photons into high Rydberg pure quantum states. Applications to other areas of fast beam physical research are discussed.     

Novel colliding plasma pulse clipper and optical isolator for high power infra-red lasers

The properties of a high density plasma focus produced inside a ring target with an Nd laser have been investigated. Studies of the axial propagation of a CO₂ laser beam through the plasma focus have shown that the device can be used either as a fast pulse clipper or as an optical isolator for high power infra-red lasers. Self-focussing, strong refraction and defocussing and non-classical absorption of the CO₂ laser beam have all been observed.     

CO2/CO2 isotopic ratio measurements with a continuous-wave quantum cascade laser in exhaled breath

A laser spectrometer based on a continuous-wave thermoelectrically-cooled distributed feedback quantum cascade laser at ∼2308 cm⁻¹ has been evaluated for measurement of ¹³CO₂/¹²CO₂ isotopic ratio (δ¹³C) changes in exhaled breath samples and in CO₂ gas flows in the concentration range 1-5%. Mid-infrared CO₂ absorption spectra were measured in a 54.2-cm long optical cell using balanced detection whereby the beam passing through the cell was ratioed against a reference beam split-off from the main beam before the cell. Signal-to-noise ratios (SNR) were estimated for CO₂ concentration measurements determined from either absorption peak amplitude or absorption peak area. The highest SNR were achieved in the measurements based upon a fitted absorption peak area. Typical short-term δ¹³C precisions of 1.1⁰/₀₀ (1-s integration time) and 0.5⁰/₀₀ (8-12-s integration time) were estimated from the two-sample (Allan) variance plots of data recorded in the optical cell at a pressure of 20 Torr and with no active temperature stabilization of the cell and gas flow. The best precision of 0.12⁰/₀₀ was achieved for averaging 80 successive 1-s integration time measurements.     

Evaluation of fracture toughness of ZrO2 and Si3N4 engineering ceramics following CO2 and fibre laser surface treatment

The fracture toughness property (K_1C) of Si₃N₄ and ZrO₂ engineering ceramics was investigated by means of CO₂ and a fibre laser surface treatment. Near surface modifications in the hardness were investigated by employing the Vickers indentation method. Crack lengths and the crack geometry were then measured by using the optical microscopy. A co-ordinate measuring machine was used to investigate the diamond indentations and to measure the lengths of the cracks. Thereafter, computational and analytical methods were employed to determine the K_1C. An increase in the K_1C of both ceramics was found by the CO₂ and the fibre laser surface treatment in comparison to the as-received surfaces. The K_1C of the CO₂ laser radiated surface of the Si₃N₄ was over 3% higher in comparison to that of the fibre laser treated surface. This was by softening of the near surface layer of the Si₃N₄ which comprised of lowering of hardness, which in turn increased the crack resistance. The effects were not similar in ZrO₂ ceramic to that of the Si₃N₄ as the fibre laser radiation in this case had produced an increase of 34% compared to that of the CO₂ laser radiation. This occurred due to propagation of lower crack resulting from the Vickers indentation test during the fibre laser surface treatment which inherently affected the end K_1C through an induced compressive stress layer. The K_1C modification of the two ceramics treated by the CO₂ and the fibre laser was also believed to be influenced by the different laser wavelength and its absorption co-efficient, the beam delivery system as well as the differences in the brightness of the two lasers used.     

Novel considerations of functions of helium and nitrogen in a fast flow CO2 laser

In a research of fast axial flow CO₂ laser sustained by 150 kHz silent discharge, we found the optimized gas mixing ratio was CO₂:N₂:He=1:22:5 or the content of helium was only about 18%. This result upset the situation of common CO₂ lasers in which the most important laser gas is helium. An explanation of our particular results and supporting experimental evidence are given.     

Effect of focusing geometry on the continuous optical discharge properties

We studied the effect of the laser beam focusing geometry on the continuous optical discharge (COD) properties. We used a full two-dimensional radiative gas-dynamic model for the COD, maintained by a vertical CO₂ laser beam in free air atmosphere, in the Earth's gravitational field. The model takes into account all of the factors that are of importance in laser-sustained plasma processes, and uses realistic quasi optics to describe the laser radiation propagation. Results are presented for the optical discharge parameters as functions of applied laser power and degree (f-number) to which the laser beam is focused.     

Temperature prediction for CO2 laser heating of moving glass rods

This paper presents a heat transfer model to calculate the temperature field in moving glass rods heated by a CO₂ laser. Conduction and radiation heat transfer in radial and axial directions are taken into account in the current model. The Rosseland diffusion approximation is incorporated to analyze the radiation heat transfer in the glass rod. A two-band model is used to simulate the spectral property of the glass. Results of the simulation show that glass rods of sufficiently large optical thickness should be treated as a semitransparent medium for radiative transfer, and it is reasonably accurate to assume it to be opaque to CO₂ laser irradiation. It has been shown that the resulting temperature profile is strongly dependent on the laser parameters, i.e., the size of laser beam and the power of the laser. The diameter and speed of the moving glass rod are also important in determining the temperature field although the convective heat transfer coefficient between the glass rod and the environment has little effect.     

Divergence of far-infrared laser beam and collimation for Galilean and Keplerian system designs

Local confinement of the electromagnetic fields in a waveguide is a boundary condition problem which is often discussed as an example of wave propagation. A wave can also propagate in the air in a region without boundaries. In this case, the beam of electromagnetic waves is merely diverging as no condition of total internal reflection exists in a uniform medium such as air. We develop equations predicting the proper separation distance between a pair of lenses to collimate a far-infrared laser beam for both a Galilean and Keplerian telescopic systems. For a typical CO₂ laser Gaussian beam at λ=10.6 μm, it is found that the separation distance is much more than the simple sum of the focal lengths for both lenses used in a given telescopic system. This substantial increase in the separation distance between the pair of lenses is mostly due to the divergence of the emitting radiation of the CO₂ laser in the air, which is large compared to that of other commercially available laser sources. Some experimental results are given for both the Galilean and Keplerian telescopes and the discrepancies are compared from the calculations. A simple methodology is also described to test the collimation of the invisible CO₂ laser beam after it is transmitted through the pair of collimated ZnSe lenses. Good estimations for the separation distance between a pair of collimation lenses and beam magnifications are very useful when focusing a beam of CO₂ laser to a very small spot in various designs, where high-power density is required. Computations for the separation distance and magnification are found to be very close to the experimental values reported with a discrepancy of less than 4%.     

Data preparation and fabrication of DOE using electron-beam lithography

The application of electron-beam lithography to the generation of diffractive optical elements topology is examined. The formula for the estimation of exposure data volume for variable-shaped electron-beam lithography is presented as a function of diffractive optical element parameters and approximation accuracy. A special software dedicated to preparing exposure data for fabrication of diffractive optical elements is developed. Diffractive optical elements with an artificial refractive index are manufactured with a feature size much less than the wavelength. Design and experimental results on photomasks fabrication for an optical element focusing radiation into a ring with pregiven parameters are presented. The photomask set is manufactured for a reflecting optical element focusing the high power CO₂ laser beam into two points with required parameters for laser welding.     

Case hardening of steel by a CO2 laser beam

Case hardening of a carbon steel, Armco Iron, an alloyed steel and a high-speed steel was carried out with a CO₂ laser beam. The indispensibility of an absorptive coating in the laser heat treatment was established. Four coatings capable of absorbing CO₂ laser radiation of a 10.6 μm wave length were examined. The influence of spot size, power and scanning speed on the depth of hardening were studied. Processing at optimal conditions yielded a hardening depth of up to 0.7 mm and a surface hardness of up to 220 H_v.     

会聚激光扫描铌酸锂晶体写入光波导时的最佳曝光间距

对利用激光精细加工技术在铌酸锂(LiNbO₃)晶体中写入光波导时的最佳曝光间距进行了详细地理论分析和实验研究.通过数值求解简化后的光折变动力学方程组，对会聚激光束沿不同方向扫描晶体时的最佳曝光间距进行了详细的数值模拟.结果表明：最佳曝光间距的选取与写入光束的扫描方向无关；当聚焦激光束平行于光轴扫描晶体时，得不到波导结构，但可以采用“三明治”辐照方式得到对称的折射率分布.采用会聚的绿激光束扫描掺铁铌酸锂(LiNbO₃：Fe)晶体进行了实验研究，晶体中的光致折射率变 关键词： 激光精细加工 最佳曝光间距 光波导 3晶体')" href="#">LiNbO₃晶体     

Lifetime measurement in138Ba+ using superimposed ion and laser beams

As a feasibility test for nanosecond time resolution in the superimposed beam geometry, we remeasured the lifetime of the 6p ²P_3/2 level of¹³⁸Ba⁺ by a new technique. Merging an accelerated ion beam with an argon laser beam, optical resonance was achieved over a very short path length by Doppler tuning the ion velocity within a short post acceleration zone. The final results is τ=(6.31±0.05) ns which is in good agreement with a previous high precision measurement by Andrä. Implications for the possible observation of coherent transient effects in fast ion beams are briefly discussed.