Improving fiber-optic laser beam delivery by incorporating GRADIUM optics

The performance of a fiber-optic laser beam delivery system strongly depends on the fiber and the optics used to image the fiber face on the workpiece. We have compared off-the-shelf homogenous (BK7) and GRADIUM (axial-gradient) singlets to determine what improvement the GRADIUM offers in practice to the typical laser user. The realized benefit for this application, although significant, is much smaller than would be realized by a conventional imaging application. The figure of merit for laser-based materials processing is the 86% energy-enclosure radius, which is not directly supported by commercial ray-tracing software. Therefore empirical rules of thumb are presented to understand when GRADIUM (or any other well-corrected optics) will yield meaningful improvements to the beam delivery system.     

Optical manipulation of microscopic objects by means of vertical-cavity surface-emitting laser array sources

We report on experimental verification of optical trapping using multiple beams generated by a vertical-cavity surface-emitting laser (VCSEL) array. Control of the spatial and temporal emission of a VCSEL array provides flexibility for manipulation of microscopic objects with compact hardware. Simultaneous capture of multiple objects and translation of an object without mechanical movement are demonstrated by an experimental system equipped with 8 x 8 VCSEL array sources. Features and applicability of the method are also discussed.     

铬原子束激光感生荧光稳频技术理论分析

在铬原子光刻中可采用铬原子束激光感生荧光(LIF)来实现激光频率的稳定,以满足原子光刻对激光频率稳定性的要求。激光感生荧光稳频技术除有很高的稳频精度以外,还可以获得相对于原子共振频率的一定失谐量。本文根据已有的实验条件和参数从铬原子束的速率分布、激光频率与荧光斑点中心横向位移的关系、二象限光电探测器测得的误差信号等方面对铬原子束激光感生荧光(LIF)稳频技术作了理论分析。     

Signals of an intermodal fiber interferometer induced by laser frequency modulation

Signals of an intermodal fiber interferometer induced by laser optical frequency modulation are studied. Dependences of signal amplitudes and spectra on the laser frequency deviation are examined theoretically and experimentally for various optical fibers. It is established that the sensitivity of the intermodal fiber interferometer to the laser frequency variation essentially depends on the fiber refractive index profile. The minimal sensitivity corresponds to graded-index (α ≈ 2) multimode optical fibers. Step-index optical fibers (α = ∞) are more sensitive to the laser frequency variation by more than a factor of 100.     

Growth of GaN and AlN thin films by laser induced molecular beam epitaxy

Hexagonal GaN and AlN thin films were grown by laser induced molecular beam epitaxy using Al or Ga metal as target material and N₂as nitrogen source. The films were deposited on sapphire (0001) and SiC (0001) substrates. Epitaxial growth of GaN has been achieved at 730°C and 10⁻³ mbar N₂ pressure. The AlN films were polycrystalline with predominant (0001) orientation.     

Optimization of the arrangement of fiber laser array in beam coherent combining

Based on the expanding of the fundamental mode of a step-index fiber in terms of Laguerre–Gaussian modes, the accurate expression for the beam propagation of a fiber laser in free space is obtained. Thereby, the coherent combining beam of three fiber laser arrays including circular arrangement, square arrangement, and diamond arrangement are numerically analyzed. The study shows that all the beams gradually concentrate centrally on the propagation axis and the highest far-field peak intensity can be obtained by using the circular arrangement. Meanwhile, the far-field intensity of the circular arrangement by using the Laguerre–Gaussian approximation is also compared with that by using the pure Gaussian approximation, which indicates that the pure Gaussian approximation will induce much error in the far-field intensity. Finally, the influence of the radius of a circular fiber laser array on the far-field intensity is studied, of which the result shows that the far-field intensity decreases with increasing radius. Therefore, the fiber laser elements are suggested to be placed as close as possible.     

Fracture toughness improvement of crack tip by irradiation with electron beam

By irradiation on the crack tip of a thin 30 CrMnSi steel plate with electron beam to modify its texture, the fracture toughness can be improved. The effect of such factors as the power density, the diameter of electron beam and the duration of irradiation on the fracture toughness are studied. The fracture toughness was measured with the $\text{R-}\text{curve}$ method.     

光学参数对非相干光纤激光组束效果的影响

提高非相干光纤激光组束功率,需要尽可能大的光栅衍射效率。通过理论分析和数值仿真,结果表明PTR布拉格光栅存在最佳厚度,并且光栅衍射效率随着光栅频率和组束波长带宽的减小而增大。在光纤阵列宽度一定的条件下,存在最佳光栅频率值使得总体衍射效率取极大值。实际应用中应选取光栅频率200～700mm^-1,组束波长带宽0-20nm,在此基础上进一步设计组束光学系统的其他参数。     

Blue-light emission from porous silicon induced by laser irradiation

Porous silicon coated with silica gel emitted light with higher energy after it had been irradiated by a Nd:YAG laser. SEM and XRD measurements demonstrate no crystal-to-amorphous transformation under the irradiation, which is presumably because of good protection of the material by silica coat. FTIR results show that the blue-light emission obtained is due to the formation of Si–C and Si–O–C surface bonds under the laser treatment.     

Enhancement of the thermal lens signal induced by sample matrix absorption of the probe laser beam

The effect of the absorption of the probe laser beam by the sample matrix on the thermal lens signal of a solute was investigated for aqueous solutions of Tb(III), Yb(III), and Nd(III). The measurements were performed with a thermal lens instrument in which the pump and the probe beam were derived from a tunable Ti:sapphire laser. Thermal lens signals were found to be enhanced in the region where the probe beam overlapped with the absorption band of the sample matrix. The observed enhancement was confirmed further with samples of the same solutes (lanthanide ions) but in D20, which does not absorb in the same spectral region as water. The enhancement may be due to the fact that absorption by the sample matrix led to a change in its refractive index and the production of a temperature gradient. In addition to fundamental importance, the observed enhancement can be used to improve the sensitivity of the thermal lens measurements by judiciously selecting a solvent that absorbs the probe laser beam.     

DFB光纤激光器声致弯曲振动研究

对利用DFB（分布反馈式）光纤激光器进行水声探测时的弯曲振动问题进行了分析与实验研究。总结了采用非平衡干涉仪解调系统解调的DFB光纤激光水听器的声压灵敏度计算公式;基于梁的弯曲理论,通过数值方法计算了两端固定的DFB光纤激光器在50Hz～2000Hz频率范围内的声压灵敏度,绘制了该频率范围内的频响曲线：采用振动液枉法对一支DFB光纤激光器在该频率范围内进行了实验研究,实验数据具有良好的可重复性,实验结果与理论分析吻合。表明了细长型结构的DFB光纤激光器在水声场中很容易由于弯曲振动而引入较大的非声压振动的干扰信号,影响其水声探测性能,有必要在DFB光纤激光水听器探头的设计中考虑这一因素。     

Focused ion beam milled pattern structures induced by laser pulse on AgInSbTe phase change films

Uniform and regular pattern structure array were fabricated on carbon (20 nm) /AgInSbTe (200 nm) / glass film by laser direct-writing technology with green and blue laser, respectively. The focused ion beam was used to mill the patterned structures to obtain the morphology inside the structures. The microscopy results indicate that the interior of the structure is hollow and the material is resolidified. Many shrinkage cavities and pores, the main characteristics of rapid solidification, were found inside the structures. The strong convection flow from the center of the molten pool to its periphery was the main cause of microbump formation. Material volume expansion from solid to liquid, and even to gas, also contributed to bump formation.     

利用测头等效法确定激光单点测头的方向

提出了一种利用测头等效法和迭代标定法确定激光单点测头方向的方法。该方法通过求解从激光测头的一维坐标向三维坐标转换的变换矩阵确定激光测头在测量机直角坐标系中的方向。利用测头等效法将激光测头等效为接触式测头,保证它在标定过程中的测量值不变,使其能测量空间的一个固定点,从而确定了求解变换矩阵的“共轭对”;利用迭代标定法保证了“共轭对”的确定满足测头等效条件,使变换矩阵的解(激光束的方向)逐步趋近于理想值。实验表明,当测头的一维测量精度为0.0128mm时,利用变换矩阵转换得到的三维数据的精度为0.0173mm,这证明利用该方法所确定的激光单点测头的方向具有较高的精度。     

Influence of spatiotemporal coupling induced by an ultrashort laser pulse shaper on a focused beam profile

4f pulse shapers have been widely used to temporally manipulate femtosecond optical pulses by spectral filtering. When the temporal waveform is manipulated with a spatial light modulator consisting of segmented pixels, the spatial profile of the output beam also varies because of diffraction at the pixel array, which is known as a spatiotemporal coupling effect. This effect produces a complicated spatio-temporal profile near the focus of the ultrashort pulses, which may affect the interpretation of experimental results obtained with shaped ultrashort pulses. We investigate the spatial intensity distribution at the focus of temporally shaped pulses through ablation experiments. The three-dimensional space-time beam profile is also numerically calculated.     

Growth of doped silicon nanowires by pulsed laser deposition and their analysis by electron beam induced current imaging

Doped silicon nanowires (NWs) were epitaxially grown on silicon substrates by pulsed laser deposition following a vapour-liquid-solid process, in which dopants together with silicon atoms were introduced into the gas phase by laser ablation of lightly and highly doped silicon target material. p-n or p(++)-p junctions located at the NW-silicon substrate interfaces were thus realized. To detect these junctions and visualize them the electron beam induced current technique and two-point probe current-voltage measurements were used, based on nanoprobing individual silicon NWs in a scanning electron microscope. Successful silicon NW doping by pulsed laser deposition of doped target material could experimentally be demonstrated. This doping strategy compared to the commonly used doping from the gas phase during chemical vapour deposition is evaluated essentially with a view to potentially overcoming the limitations of chemical vapour deposition doping, which shows doping inhomogeneities between the top and bottom of the NW as well as between the core and shell of NWs and structural lattice defects, especially when high doping levels are envisaged. The pulsed laser deposition doping technique yields homogeneously doped NWs and the doping level can be controlled by the choice of the target material. As a further benefit, this doping procedure does not require the use of poisonous gases and may be applied to grow not only silicon NWs but also other kinds of doped semiconductor NWs, e.g. group III nitrides or arsenides.     

Review of ultrafine particle generation by laser ablation from solid targets in gas flows

A comparative review is given of laser ablation generation of ultrafine particles (UFPs: size range about 1–100 nm) in gas flows carried out by different research groups. Common features of the laser ablation deposition (LAD) method obtained for different materials (mainly metals or oxides) and lasers are discussed. Special emphasis is given to CO₂-LAD of oxide UFPs (see (1)) for elucidation of basic mechanisms of particle formation and the LAD process.     

Measurement of Y-notch tip plastic zone by the laser speckle technique

This paper presents a laser speckle method for the measurement of plastic zone ahead of a Y-notch tip on metallic specimens. The method employs an unexpanded laser beam to illuminate the test object. Diffracted images are captured on a ground glass and recorded using a CCD camera. The recorded images were digitised and subsequently analysed on a personal computer. The plastic strain at the notch tip was correlated with its corresponding speckle image using a recently proposed parameter known as the integrated peak spectral intensity coefficient. Specimens containing Y-notches of various angles were tested and the resu Its compared well with that predicted by theory.     

Detecting nanometric vibrations of a liquid surface by a metal-coated fiber tip

Optical radiation emitted from a metal-coated fiber tip apex when it emerges from a liquid into the air was measured. A strong enhancement of the output radiation and its strong dependence on the relative position of the tip to a liquid surface were observed. This phenomenon permits in situ recording of nanometric vibrations of the liquid and provides a basis for development of various ultrasensitive vibration detecting sensors.     

Analysis of the effects of aberrations on coherent polarization beam combining of fiber laser beams

In this paper, the influence of aberrations on coherent polarization beam combining (CPBC) of fiber laser beams is analyzed in detail for the first time to our knowledge. The expression of the coherently polarization combined output power is obtained, which contains the geometry and nongeometry factors of the CPBC system. The geometry and nongeometry aberrations are numerically studied to investigate their effects on the combining efficiency. It is found that to ensure the combining efficiency of a CPBC unit (we studied) to be more than 90%, the overlap error (the beams in the polarization beam combiner cannot be superposed entirely) should be controlled to be less than 0.45, the tilt error should be less than 2π/15 rad, and the RMS value of the phase error should be limited to be within λ/10. The generalized methodology that we present offers a good reference for investigating the CPBC system in an extensive way.