This PDF file contains the front matter associated with SPIE Proceedings Volume 7843, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

High power and widely tunable operation of Tm-doped silica fiber laser around 2 μm has been demonstrated using volume Bragg grating as the wavelength selection and spectrum narrowing element. The operating wavelength was continuously tunable from 1930 to 1821 nm, with > 52 W output power over a tuning range of 104 nm and a relatively narrow spectra width of < 15 pm. Over 60.8 W of diffraction limited (M² ~ 1.5) CW output power was generated for 137 W of launched pump power, corresponding to a slope efficiency with respect to launched pump power of 46%. Output characteristics with a conventional replica diffraction grating for wavelength selection were also investigated as a comparison with that of using a volume Bragg grating. A maximum output power of 30.6 W at 1963 nm was generated for 79 W of launched pump power and the lasing wavelength could be tuned over 196 nm from 1859 to 2055 nm at output power levels in excess of 20 W. The bandwidth of the laser output was ~ 0.8 nm.

A single-frequency master-oscillator fiber power-amplifier operating at 1991 nm was demonstrated. The seeding laser was a diode-pumped, high efficient, single-longitudinal-mode Tm:YAP laser with a coupled-cavity configuration. 721 mW single-frequency output power was obtained at 1991 nm, the slope efficiency was 46%. The power amplifier was a diode-pumped Tm-doped fiber. 8.6 W single frequency output power was obtained from the amplifier. The maximum output power was limited by the low coupling efficiency of the seeding signal into the fiber core. The strength of ASE in the fiber amplifier with respect to the power of input signal was studied experimentally. High power single-frequency lasers at 2 μm optical region have potential applications on eye-safe coherent lidar and optical remote sensing.

In order to obtain high slope efficiency of 2 μm laser output, it is essential to design operating parameters of fiber laser prior to the experiment. Based on the rate equations, the pump and signal power equations of thulium doped fiber laser have been built. The effects of the cross relaxation coefficient K₃₁₀₁, the pump power filling factor Γ_p, the laser reflectivity of input mirror R₃, the laser reflectivity of output mirror R₄, and the Tm³⁺ concentration N of kilowatt level thulium doped fiber laser on the slope efficiency have been theoretically analyzed by an improved shooting algorithm. The results indicate that the slope efficiency decreases with the increasing laser reflectivity of output mirror, the pump power filling factor. Meanwhile, the slope efficiency increases with the increasing laser reflectivity of input mirror, the doping concentration and the cross relaxation coefficient, but the doping concentration and the cross relaxation coefficient have more obvious influence on the slope efficiency compared with the laser reflectivity of input mirror.

Thulium-doped fiber laser (TFL), which emitted near 2 μm laser beam, has become the latest revolution in highpower fiber laser technology. Further increasing the output power will face great challenges induced by nonlinear effects; coherent beam combining of TFL can increase laser output power while simultaneously maintaining beam quality. In this manuscript, we will present our detailed investigation on coherent beam combining of TFLs. Three different approaches, i.e., interferometric array, mutual injection locking and active phasing based on multi-dithering technique, are employed. In the interferometric array scheme, coherent combining is realized by using an intracavity fiber coupler in an all-fiber laser array configuration. Efficient coherent combining can be achieved by providing sufficient loss discrimination. High combining efficiency of 85% for two fiber laser has been obtained. In mutual injection locking scheme, mutual coherence between the two fiber lasers is established by means of mutual coupling through two 3dB couplers. High combining efficiency of 99% for two fiber laser has been obtained, and the fringe contrast of the intensity pattern at the receiving plane is as high as 93%. In active phasing scheme, when the phase control system is in the closed loop, the fringe contrast of far-field intensity pattern is improved by more than 75 % from 10 % in open loop, and the residual phase error is less than λ/20.

Multiwavelength seed laser can suppress stimulated Brillouin scattering (SBS) and improve the ultimate output power of the fiber laser amplifier. Coherent combining of multiwavelength lasers/amplifiers is a promising way to get much higher total output power than coherent combining of the single frequency lasers/amplifiers. Coherent beam combining of stimulated Brillouin scattering based multiwavelength fiber lasers is proposed and demonstrated. Multiwavelength laser is generated using stimulated Brillouin scattering effect by seeded a 10 kilometer single mode fiber laser with a single frequency laser and phase locking is achieved using stochastic parallel gradient descent (SPGD) algorithm. Experiment results shows that more than 15 wavelengths are generated for the laser. With active phase control, mean power of the main-lobe in close-loop is 1.37 times of that value in open-loop and the visibility of the long exposure interference pattern is 0.37. Scale up this architecture to higher power involves introduction of power amplifiers, increasing channel number of amplifiers and power of each individual amplifier.

Silica-based thulium-doped fiber devices operating around 810 nm would extend the spectral range covered by highpower fiber devices. Using a comprehensive numerical model of the fiber we have shown that efficient lasing at 810 nm can be achieved for specific ranges of the laser cavity parameters in silica-based thulium-doped fibers with enhanced ³H₄lifetime up to 58 μs as measured in our highly alumina-codoped fibers. We present optimization of the thulium-doped fiber and laser cavity parameters and also potential applications of the developed host material in amplifiers and broadband sources.

A high stability high average-power green laser was reported with composite ceramic Nd:YAG as gain material and KTP crystal as frequency doubler. Average output power of 165 W is obtained at a repetition rate of 25kHz with a diodeto- green optical conversion of 14.68% and measured pulse width of 162 ns. For the average output power of about 160 W, the power fluctuation is less than 0.6%. The experimental results show that the green laser system using this novel ceramic Nd:YAG crystal offers better laser performance and output stability

Y₂O₃:5%Er nanocrystalline powder was prepared by low temperature combustion method. The crystal structure and morphology were analyzed by means of XRD and HRTEM. The resultant powders were sintered into translucent ceramics at 1570°C in vacuum for 6 hours. The micrograph of unpolished surface and fracture surfaces showed that the sintered Y₂O₃:Er ceramics with average grain size at about 10μm had homogeneous micro-structure and low pore volume. Under the excitation of 980 nm, 808 nm and 785 nm diode lasers, respectively, very strong green and red upconversion emissions from Er³⁺ ions were observed, the power dependence of upconversion emission intensity was measured to deduce the upconversion mechanism. A trend of upconversion intensity increase first and then decrease with the excitation time was also found for the first time.

Thermal effects have been investigated in the laser diode end-pumped Tm,Ho:YLF solid state laser. Under the condition that the pump light and the output laser are Gaussian distributions, the continuous wave rate equations of the Tm,Ho:YLF laser are given which take into energy transfer upconversion and ground state reabsorption effects. The fractional thermal loading and thermal focal length are obtained for different pump powers by solving the steady state rate equations. The thermal focal length as a function of pump power is measured by the knife edge method experimentally. Furthermore, the experimental results are compared with the theoretical results, and it is found that the theoretical results agree well with the experimental results.

It is always the hot subject to realize the output of high-power laser in the range of 3-5μm wavelength. This rang of wave band has greatly important applications in military because it located in the atmosphere window. Generally there are two ways to obtain this range of laser wavelength. One way is through optical parameter oscillation (OPO) from shorter laser wavelength and the other is through second harmonic generation (SHG) from longer laser wavelength. Firstly, the comparison between tow nonlinear crystals ZnGeP₂ and AgGaSe₂ is conducted for their nonlinear coefficient and damaging threshold in theory. The theoretical results show that the crystal AgGaSe₂ is more suitable for the SHG of pulsed TEA CO₂ laser. When using pulsed TEA CO₂ laser with wavelength of 9.3μm to pumping AgGaSe₂ SHG crystal, the wavelength of 4.65μm is obtained. In the condition of repetition rate 100Hz, the upmost output power of single pulse is up to level of 1W, which corresponding efficiency of SHG is about 6%. The experimental results show that the polarization of laser beam has greatly influence on the SHG output of the crystal. Under the radiation of 3MW/cm² from fundamental wave and the right position for maximal SHG output in the crystal, when polarization of laser beam rotates ±4.5°, the SHG output of energy decrease about 30%. The research of this paper will make a foundation for further development of mid-infrared laser.

The evolution of shock wave generated by discharge in laser chamber is one of the key factors which affect laser beam quality, discharge stability, and repetition rate of TEA gas laser. In this paper, Mach-Zehnder interferometer is applied to observe both the longitudinal and transversal shock waves between electrodes as well as the acoustic waves originated by preionization in the discharge pumping zone of TEA gas laser. By changing the discharge voltage, gas pressure and gas composition concentration, the developing processes in different conditions are compared and analyzed. It is observed that the shock waves originating from cathode is different from the anode's ones even in the symmetric electrode construction. And the carbon dioxide concentration in helium-buffered working gas can affect the speed of the wave obviously. However, the increasing trend of shock wave speed, when increasing discharge voltage or reducing discharge gas pressure, is inconspicuous.

The pump structure greatly influences the characteristics of a diode side-pumped laser. To achieve high absorption efficiency and a homogeneous pump-beam distribution simultaneously, a systemic algorithm has been established to optimize the pump structure, where multiple reflections occur on the internal wall of the reflector inside the pump chamber. A novel design of an efficient, highly reliable, and good beam quality diode side-pumped solid-state laser is presented. Effort has been done to obtain a highly uniform pumping intensity in the active area, which simultaneously reduces the effects of thermal gradient. In this design a novel lens duct configuration is used. By this way a uniform power distribution and a maximum absorption of pump power is resulted. Numerical analysis also indicates the superiority of the design to other methods such as direct and diffusive pumping techniques.

A self-saturated absorption regime is proposed to theoretically investigate the optical bistability of Tm,Ho:YLF laser. Based on this bistability regime, the rate equation model of the optical bistability is established. The optical bistability behaviors of Tm,Ho:YLF laser are obtained by numerical simulating. The relation between laser gain and loss are also analyzed to confirm the rationality of the bistability mechanism. Furthermore, the time characters of the optical bistability are investigated. It is found that the high pulse power and the duration of the pre-pump are two major factors that affect the laser turn-on delay time and their influences on the turn-on delay are analyzed.

Process of laser ultrasonic generation was simulated in detail for confirming the exact time and location ultrasonic produced. The particular generating information in thin-film could help to accurately analyze the ultrasonic characteristics, especially in high pressure physics. Numerical models based on the two-dimensional axis-symmetry was built in cylinder coordinate system and calculated by finite element method (FEM). The duration and penetration-depth of pulse laser were considered instead of point approximation. In addition, parameters of material were set as functions related with temperature. According to the results, strain energy accumulated below the incidence point, so that the maximum amplitude of ultrasonic wave appeared few distances inside of the sample. Non-negligible errors were caused by that distances in velocity calculation. Without amendment, the errors increased with the broadening of pulse width and decreased with the growth of propagation distance. Therefore, the time ultrasonic generated should be fixed a head of the time that laser peak arrived. Furthermore, wave shape were easy to distinguish but inaccurately while the sample were covered with a transparent window.

In all laser pointing systems, boresight and jitter are two fundamental pointing errors arising from vibrations and atmospheric turbulence. The maximum-likelihood estimation¹ advanced by Deva K.borah recently can simultaneously estimate jitter and boresight. Besides some traditional qualities just like high precision and speediness, the maximum-likelihood estimation has a new quality that the performance of this estimator is different when boresight and jitter errors are different. Furthermore, the Monte Carlo simulation results demonstrate that the maximum-likelihood estimation has a higher degree of precision when boresight is bigger than jitter. According to this, a careful analysis is made and some advice is given for the readers who wish to obtain high precise results.

The objective of this research is to develop advanced control methods to improve the bandwidth and tracking precision of the electro-optical fine tracking system using a fast steering mirror (FSM). FSM is the most important part in this control system. The model of FSM is established at the beginning of this paper. Compared with the electro-optical fine tracking system with ground based platform, the electro-optical fine tracking system with movement based platform must be a wide bandwidth and a robustness system. An advanced control method based on internal model control law is developed for electro-optical fine tracking system. The IMC is an advanced algorithm. Theoretically, it can eliminate disturbance completely and make sure output equals to input even there is model error. Moreover, it separates process to the system dynamic characteristic and the object perturbation. Compared with the PID controller, the controller is simpler and the parameter regulation is more convenient and the system is more robust. In addition, we design an improved structure based on classic IMC. The tracking error of the two-port control system is much better than which of the classic IMC. The simulation results indicate that the electro-optical control system based on the internal model control algorithm is very effective. It shows a better performance at the tracing precision and the disturbance suppresses. Thus a new method is provided for the high-performance electro-optical fine tracking system.

In this paper, the spatial and temporal evolutions of the broadband chirped pulse with small-scale self-focusing (SSSF) in nonlinear medium have been investigated by experiments and simulations. Firstly, we studied the spatial modulation growth of broadband pulse by experiment in CS₂ and the laser will produce many small peaks from perturbation modulation. It is found that modulation growth of chirped optical pulses is delayed with the pulse chirp increasing. And then we investigate the temporal evolution of the pulse at the modulation peak and bottom during the SSSF by numerical simulation. The simulation results indicate that pulse width at modulation peak is decreasing with the increasing of modulation peak intensity. But the pulse at modulation peak will broaden when the peak intensity reaches the maximum and then the pulse began to splitting into two parts. However, the pulse width at modulation bottom always broadens and finally reaches to a constant value. The simulation results are in agreement with the experimental results.

In this paper, we firstly preliminary analyzed how to control the collapse position of beam when intensity and beam waist have been varied by the laser self-focusing in nonlinear media and lens-focusing. We obtain the relations of the focusing position with input power and focal length of lens. The length of focusing is inversely proportional to the input power and directly proportional to focal length of lens. Secondly, Based on the nonlinear propagation equation and split-step Fourier method, we investigate how to control the focal distance and beam quality of high-power laser at focusing spot in nonlinear media. We can control the focusing spot at any position by changed power and lens. The numerical simulations is good consistent with theoretical analysis.

In this paper, a system of containing dual deformable mirrors (DMs) is proposed to adaptively conversion of input beam with wave front distortion into near-diffraction-limited flattop beam based on the stochastic parallel gradient descent (SPGD) algorithm. In the analysis, the wave front distortion of the input beam is chosen as the Zernike representation of Kolmogoroff spectrum of turbulence. The whole shaping system is controlled by the SPGD algorithm. One DM adaptively redistributes the intensity of the input beam and the other adaptively compensates the wave front of the output beam. The near-diffraction-limited flattop beams with different parameters are realized by this technique. The near-diffraction-limited square flattop beam retains an flattop intensity distribution without significant diffraction peaks for a working distance of more than 60cm in the near field.

A study of gaseous sulfur dioxide detection in air by Laser Induced Breakdown Spectroscopy(LIBS) is reported. Plasmas were formed in the sulfur dioxide gas, and three lines of sulfur at 560.61nm, 567.77nm and 565.99nm were observed. We found that the most appropriate experimental conditions for LIBS detection on sulfur dioxide gas are: Laser Pulse Energy =100mJ, Gate Time Delay = 2us. A further study was made in detecting sulfur dioxide gas of different concentrations by LIBS. Finally we calculated the detection limit of sulfur dioxide gas is 330ppm.

Laser cladding monitoring is one of the cladding layer quality controlling methods. At present, cladding monitoring usually monitor the temperature or the shape of melting pool. The height deviation between melted and unmelted point is also considered. In this paper, optical signal of plasma which is one of the characteristic signal of plasma has been detected by phototube in cladding. The relationship between blue-violet light intensity and laser power scan velocity has been discussed, and the relevance between quality of layer and intensity has been analysed as well. The results indicate that while laser power growed the intensity increased just at a low rate, and the intensity decreased as scan velocity increased when the power is lower than a definite value. But when the power is greater than this value the intensity will rise as velocity increase. The quality of layer is improved greatly when the intensity value rang from 1.7 μW/cm² to 2.5μW/cm² with a slight fluctuation.

Most of the temperature and stress fields simulations in laser cladding were based on flat surface, while actually cladding may occur on any curved surface. The difference between cladding on flat surface and curved surface is that the latter will result in uneven distribution of laser power. Experiments of laser cladding have been done on different material gears under various technical conditions, and crackles have been observed by SEM. Some factors that affect laser power actually, such as laser shielding, the incident angle of laser and curved surface of gear are all considered. Based the analysis of the shape of layer after cladding and the phase transformaion during cladding, temperature and stress fields of gear surface laser cladding have been simulated by ANSYS in this article. The results indicate that appropriate material matching and base preheating can decrease the likelihood of crackles, even eliminate it.

Shadowgraphs of dynamic processes outside and inside the target during the intense femtosecond laser ablation of silica glass at different energy fluences are recorded. Two material ejections outside the target and two corresponding stress waves inside the target are observed. In particular, a third stress wave can be observed at energy fluence as high as 40 J/cm². The pressure, the temperature, the free electron density, and the ionic components at the laser pulse end are estimated, based on which the mechanical reaction of the laser heated material is investigated. According to our analysis, the first wave is a thermoelastic wave, while the second and the third may be generated subsequently by the mechanical expansions. Besides, the velocities of the stress waves are deduced from the time-resolved shadowgraphs, and it is found that the first stress wave propagates with a velocity greater than the sound velocity, while the second stress wave propagates with a velocity less than the sound velocity. Therefore, the first wave is a supersonic shockwave with a high stress magnitude, while the second may be the plastic stress wave or subsonic shockwave with a lower stress magnitude. Further more, the temporal evolution the second stress wave is investigated, and its velocity is found to increases gradually at large delay times. According to the extrapolation curve, however, it is speculated that the velocity decreases from a high value initially, which could be due to the interaction between the first and second stress waves at small delay times. These results can provide a further support to the theory of highpressure shock phenomena in femtosecond laser ablations.

Fabrication of microstructures embedded in silica glasses using a femtosecond (fs)-laser-assisted chemical etching technique is systematically studied in this work. By scanning the laser pulses inside samples followed by the treatment of 5%-diluted hydrofluoric (HF) acid, groups of straight channels are fabricated and the relationship between the etching rate and processing parameters, including laser power, scanning speed, scanning time and laser polarization, is demonstrated. Based on the optimization of these parameters, complicated microstructures such as channels, cavities and their combinations are manufactured. The work has great potential applications in microelectromechanical systems, biomedical detection and chemical analysis.

The current state and future trend of laser bio-cladding technology are discussed. Laser bio-cladding is used in implants including fabrication of metal scaffolds and bio-coating on the scaffolds. Scaffolds have been fabricated from stainless steel, Co-based alloy or Ti alloy using laser cladding, and new laser-deposited Ti alloys have been developed. Calcium phosphate bioceramic coatings have been deposited on scaffolds with laser to improve the wear resistence and corrosion resistence of implants and to induce bone regeneration. The types of biomaterial devices currently available in the market include replacement heart valve prosthesis, dental implants, hip/knee implants, catheters, pacemakers, oxygenators and vascular grafts. Laser bio-cladding process is attracting more and more attentions of people.

Laser cladding was performed on the ductile cast iron substrate with Ni-base alloy under different process conditions. The cracks were observed. The temperature field and stress field in laser cladding under different process conditions were simulated with ANSYS finite element software. It was found that cracks were influenced by process variables. In certain ranges of laser power and scanning speed, while the other process parameters remain constant, the numbers of cracks increase with laser power increasing. Similarly the number of cracks increases with scanning velocity increasing while the other process parameters remain constant. In comparison with experimental results, the simulation with ANSYS finite element software could help to predict, to some extent, the crack of laser cladded Ni-alloy on ductile cast iron.

Ni60CuMoW alloy power was clad on 45 steel surfaces using a synchronization powder feeding method by 6kW transverse-flow CO₂ laser apparatus. The effect of laser power and heat treatment process on corrosion resistance of the cladding layer was investigated. The microstructure and mechanical property were analyzed by X-ray diffractometer (XRD), scanning electron microscope (SEM), energy depressive X-ray spectroscopy (EDX), microhardness meter and PS-268A electrochemical test equipment. The results show that the cladding layer is mainly composed of γ (Ni, Fe), solid solution (Ni, Cu), compounds Ni₃₁Si₁₂, Cr₅B₃, CrB, Ni₃B, FeNi₃, M₂₃C₆ (Cr₂₃C₆ or (Fe,Ni) ₂₃C₆) phase and a small amount of WC or W₂C. With the increase of laser power, corrosion resistance and microhardness has been greatly improved. Compared with the untreated substrate, the maximum self-corrosion potential of single-pass layer at laser power 3.2 kW in 3.5% NaCl saturated solution increases by 136.2mV, and the lowest corrosion current density decreases by 2 orders of magnitude. The mean microhardness of treated samples raises by 5.17, 4.90 and 4.89 times, respectively. The corrosion potential of multi-pass layer increases by 437.6mV and corrosion current density decreases by one order of magnitude than that of single-pass layer sample. After temper 600°C heat treatment, the primary dendrite and block (or needle) eutectic in cladding coatings become more uniform, the maximum self-corrosion potential increases by 45.5mV and corrosion current density also decreases obviously.

The metal powder flow is sprayed by a coaxial nozzle with carrying gas in laser cladding, and the powder particles are radiated to be melt metal by high power laser beam. The melting metal enters the melting pool on the substrate which is radiated by the same laser beam. The cladding layer forms, when the melting metal cool down. It is crucial to study the influences of different process parameters on the motional and thermal behavior of moving powder particles. In this paper, 2D model about a moving particle's motional and thermal behavior were established. Results show that the increasing mode of the velocities of a powder particle and the carrying gas depends on their initial velocity; the maximum value of the particle velocity appears in the middle of the nozzle exit; and the temperature of the particle rises rapidly and then falls due to the relationship between cold gas, the variation of laser intensity and its velocity and so on. The results are very helpful to design the nozzle and to select process parameters in coaxial laser cladding.

A high-average-power and high-beam-quality diode-side-pumped solid-state laser was designed carefully for laser beam texture. By using of low concentration Nd:YAG crystals with thermally near-unstable resonator design and two-rod birefringence compensation technology, the 1064 nm cw output power of 400 W at pump power of 1170 W was achieved with the beam quality factor M² ~ 15, corresponding to an optical-to-optical conversion efficiency of 34.2%.

Light propagation in a perfect circular double clad fiber (DCF) is numerically investigated by using the coupled mode method with translating the DCF into a single mode fiber (SMF) and an annular core fiber (ACF). The results show that for the coupling coefficients between the LP₀₁ mode of the SMF and the guided modes (LP_0n modes) of the ACF, the high-order mode has a larger coupling coefficient than the low-order mode for the ACF. By using the coupled mode equations the field distribution in the DCF core is calculated, the results show that the power shows a quasi-periodic distribution along the DCF core and its average period increases with increasing wavelength for different DCF parameters. The simulation results agree with the light ray propagation in the DCF.

The dynamic properties of the laser emission are very important in studying the characteristics of the laser and may reveal the underlying operating mechanism. Here we report a more precise measurement of the build-up time of random laser pumped by picosecond pulse laser. The build-up time is defined as the time delay from the peak of the pumping pulse to that of the emission. The random laser is R6G dye solutions with nanometer size TiO ₂ as the scatterer. Various dye concentrations and scatterer density are tried and measured. A specially customized fiber and a streak camera with a spectrometer are employed to make the simultaneous measurement. The fiber has two branches and the lengths of both branches are made equal with a difference of much less than 1 mm. The dispersion of the fiber, which introduces much error in the results, is also measured and later compensated in the following data processing. The streak camera with spectrometer can catch the random laser pulse and the pumping pulse signal in one shot with a resolution of less than 2 picoseconds. The results show that the build-up time changes evidently with the dye concentration, while it changes a little along with the scatterer density. The pulse width almost remains the same in our experiment considering the errors.

A 2.5W superfluorescent fiber source (SFS) and a tunable PCF laser with 74.4 nm tuning range and 5.5 W output power are experimentally demonstrated by utilizing an ytterbium-doped photonic crystal fiber (PCF) as the gain medium. The tunable operation of the laser is realized by rotating a diffraction grating. Effects of the degrees of the cleaved angle at the anti-reflection fiber end on the output characteristics of the laser and the SFS are investigated. A large cleaved angle is beneficial to realize a broad laser tuning range and beneficial to obtain a high power superfluorescent fiber source without parasitic lasing.

In this paper, we report on the near-field distribution of multi-core photonic crystal fiber lasers. The supermodes of photonic crystal fibers with foursquarely and circularly distributed multi-cores are observed. The supermode properties are investigated by using full-vector finite-element method (FEM). The mode operations of our 16-core foursquare-array and 18-core circular-array photonic crystal fiber lasers are simulated by the COMSOL Multiphysics software. The near-field distribution patterns of in-phase supermode are presented.

During the laser cutting of brittle material using controlled fracture technique, thermal stress is used to induce the crack and the material is separated along the moving direction of the laser beam. In order to investigated the process of pulsed Nd:YAG laser thermal stress cutting brittle silicon wafer, a three-dimensional mathematical thermoelastic calculational model which contains a pre-existing crack was established. The temperature field and thermal stress field in the silicon wafer were obtained by using the finite element method. During the pulse duration, the changes of stress intensity factor around crack tip were analyzed. Meanwhile the mechanism of crack propagation was investigated by analyzing the development of the thermal stress field during the cleaving process, and the calculational results in this paper are in agreement with the reported experiment results.

Multicore fiber lasers have larger mode areas, resulting in higher power thresholds for nonlinear processes such as stimulated Raman scattering and stimulated Brillouin scattering. Because of longer distributed distance of the cores, thermal mechanical effects are decreased compared with those of single-core lasers. Therefore, multicore fiber lasers are proposed as a candidate for the power scaling. The progress of multicore fiber lasers is simply introduced. Optical fields propagating in multicore fibers are coupled evanescently, resulting in what are called supermodes. In this article, the coupled-mode theory for analyzing supermode of fiber transmission is introduced. By mean of the theory, assuming under weak-coupling conditions, the supermodes are approximated as linear superposition of modes of individual cores with appropriate coefficients. The near-field mode distributions of some supermodes are numerically calculated, and the corresponding mode distribution patterns are drawn. For making the multicore fiber laser preferentially operate in a particular supermode so that improving beam quality, an in-phase locking method based on self-imaging Talbot external cavity is introduced.

A hybrid laser composed of Ti:chrysoberyl and Ti:sapphire is proposed in this letter. The laser has the similar structure to ordinary Ti:sapphire based lasers except that the host material of the regenerative amplifier is replaced by Ti:chrysoberyl. The Ti:chrysoberyl is employed in this system with the polarization parallel to a axis since then its photoluminescence spectrum has two peaks. A simplified theoretical model for numerical simulation is presented here. The ability of the hybrid amplifier chain to compensate the gain narrowing is obviously demonstrated and the laser is potential to generate ~10fs, >2petawatt pulses according to the numerical simulation.

Laser ultrasonics, a nondestructive test method (NDT), is more and more applied in industrial fields such as crack detection in metal and nonmetallic materials, and size measurement and welded joint examination. According to the thermoelastic theory, a finite element model for laser-generated surface waves on the elastic cylinder material is built and an experimental installation using laser interferometer as the ultrasonic wave receiver is set up to verify the numerical results. By changing the relative distance between the laser source and the surface notch in the computation, the scanning procedure is simulated. The corresponding varieties of amplitudes of the surface waves, which propagate circumferentially on the cylinder material, are presented and the physical mechanisms are analyzed. The influence of the depth of the surface notch is also discussed. The results demonstrate the SLS technique can be applied to detect tiny crack whose depth is smaller than the wavelength of the SAW.

We describe a synchronously-pumped optical parametric oscillator (OPO) based on periodically poled KTiOPO₄(PPKTP). The OPO was pumped by a self-mode-locked Ti:sapphire laser. Its signal wave covers from 1070 nm to 1375 nm, and could be tuned freely by cavity-length tuning, the corresponding idler wave covering from 1.9 μm to 3.2 μm in the mid-infrared region. Red, green and blue visible light were also generated by intra-cavity sum frequency or frequency doubling. The threshold was measured to be about 250 mW at 810 nm pump. A thin glass substrate was inserted to the cavity for output coupling, and the maximal output power was measured to be 26 mW. We adopted ring cavity and linear cavity experimentally. After optimization, the output characteristics of the ring cavity and linear cavity were investigated respectively. The approach to improve the output efficiency was also discussed.

This article gives an overview of airborne LIDAR (laser light detection and ranging) system and its application. By analyzing the transmission and reception process of laser signal, the article constructs a model of echo signal of the LIDAR system, and gives some basic formulas which make up the relationship of signal-to-noise ratio, for example, the received power, the dark noise power and so on. And this article carefully studies and analyzes the impact of some important parameters in the equation on the signal-to-noise ratio, such as the atmospheric transmittance coefficient, the work distance. And the matlab software is used to simulate the detection environment, and obtains a series values of signal-to-noise (SNR) ratio under different circumstances such as sunny day, cloudy day, day, night. And the figures which describe how the SNR of LIDAR system is influenced by the critical factors are shown in the article. Finally according to the series values of signal-to-noise ratio and the figures, the SNR of LIDAR system decreases as the distance increases, and the atmospheric transmittance coefficient caused by bad weather, and also high work temperature drops the SNR. Depending on these conclusions, the LIDAR system will work even better.

The nonlinear imaging effect for small-scale defects with gain defect, which may be caused by the uneven gain in gain media, is investigated through numerically solving the propagation model with the standard split-step fast-Fouriertransform algorithm. Two cases, i.e. defects only causing gain and defects causing both gain and phase modulation, are considered. It is proved that nonlinear images can also be formed. However, when compared to the nonlinear imaging effect for defects causing attenuation, it is found that: firstly, the intensity at the point corresponding to the center of the scatterer experiences an obvious decreasing just before the nonlinear image is formed, contrary to the case the defect causing attenuation; secondly, this decreasing is enhanced as the gain caused by the defect increases; thirdly, there is a distance several centimeters, even more than ten centimeters, between the nonlinear images for gain defects and those for attenuation defects. For defects which cause both gain and phase modulation, above differences are ignorable, but the nonlinear imaging effect has some new properties: firstly, the beam peak intensity on the exit surface of the nonlinear medium is higher than that in the case the defect only causing phase modulation; secondly, the second-order nonlinear image is increased obviously, but the effect of the defect's gain on the image's peak intensity is limited. Besides, the effect of the distance between the defect and the nonlinear medium and that of the power of incident beam are discussed.

We report a pulsed, widely tunable Ti:sapphire laser pumped by an all-solid-state Q-switched intra-cavity frequency-doubled Nd:YAG laser with repetition rate of 7 KHz. Using two dense flint glass prisms as dispersion elements, the output wavelength could be continuously tuned over 675-970 nm, with spectral line-width of 2 nm. Gain-switching characteristics of Ti: sapphire laser shortened the pulse width to 17.6 ns. Well mode matching between pump and laser beam in the sapphire crystal and thermal design of the cavity ensured stable, efficient laser operation. The maximum output power was 6.2 W at 780 nm when the 532 nm pump power was 22 W; corresponding conversion efficiency was 28.2%.

This article demonstrates a diode-pumped pulse laser operation in a flowing fluid host containing Nd-glass particles, transversely pumped by 810nm laser diodes. A series of Laser pulse as the output are observed. The repetition frequency is 1Hz, and the pulse width is about 100μ s , and the maximum pulse energy is up to 2.93mJ, and the average pulse energy is 0.84mJ. Given that fluid circulation offers improved heat management, the realization of laser output of this new fluid state laser verifies a practical way to solve the heat-induced problems in high energy laser systems.

In this paper, a simple and all-fiber frequency comb generator by stimulated Brillouin scattering (SBS) in highly nonlinear fiber (HNLF) is demonstrated. The optical resonator is composed of a segment of highly nonlinear fiber and two optical loop mirrors. Besides these, a segment of 6m-long erbium-doped fiber was placed in the resonator to provided linear gain. A tunable laser and a 980-nm laser diode were used as the Brillouin pump and the EDF pump respectively. When the wavelength and the power of pump lasers are adjusted correctly, dozens of comb lines with comb spacing in 0.075 nm (9.37 GHz) are obtained.

The mode competition mechanism in concentric 4-core and 7-core fiber lasers with large mode area single mode (SM) fiber as in-phase supermode selection component is presented. The coupling coefficient between the fundamental mode in large mode area SM fiber and each supermode in mutlicore fiber is discussed. For individual supermode in multicore fiber, the coupling coefficient is optimized as a function of the core radius of SM fiber as well as the distance between multicore fiber and SM fiber. The optimization results demonstrate that only two supermodes are involved in concentric-type fiber lasing - in-phase and anti-phase supermode, owing to the negligible coupling coefficients of the other supermodes. Furthermore, to achieve the best in-phase supermode selection, the core radius of SM fiber will be optimized for maximum coupling coefficient difference between in-phase supermode and anti-phase supermodes. The numerical results illustrate that in-phase supermode always dominate the output and is the highest when the distance equals zero. Compared to conventional multicore fiber lasers with Talbot cavity, this all-fiber configuration based on large mode area SM fiber has higher-order supermodes more efficiently suppressed and high-brightness output may be achieved.

Experiments of laser transformation hardening were performed with various process parameters on the surface of ductile cast iron block, which is often used for vehicle body. The distribution of microhardness along the case depth direction and the variation of microstructure of hardened track were investigated. Wear test was carried out using a ball-on-flat-type wear-test machine. The results indicate that the hardened layers after the laser surface hardening treatment have excellence wear resistance. It is seen that the surface hardness of ductile cast iron blocks increases greatly at suitable values of process parameters in laser transformation hardening, which satisfies the vehicle dies.

To optimize the performance of Er³⁺/Yb³⁺ co-doped double cladding fiber laser (EY-DCFL), the output characteristics of DBR EY-DCFLs with different parameters are investigated theoretically and experimentally. The output powers as functions of the launched pump power, the gain fiber length, as well as the reflectivity of the output mirror are presented by numerical simulation based on rate equations and power propagation equations. Experimental study on the output power, the spectral properties and the time-domain stability of DBR EY-DCFLs with different reflectivity is carried out. In the optimum condition, up to 2W output power at 1550.8nm is obtained with a slope-efficiency of 53.8% and a 3dB bandwidth of about 0.02nm.

In this paper, the effects of the gas flow and defocusing distance from laser beam focus on powder-feed laser cladding are investigated, and a quantitative expression to describe the free surface of the molten pool is derived. The effect of transporting gas flow on the powder feed rate, and those of the transporting and shielding gas flow on the divergent angle of the gas-powder flow and the characteristics of the formed clad bead are worked out, respectively, while the other process variables being constant. The effect of gas-powder flow on the molten pool is presented as a boundary condition equation for developing the numerical model of the molten pool. The experimental results obtained at variable defocusing amount show that the negative defocusing amount rather than the positive one is suitable for laser cladding and that the defocusing amount influences the molten height below the substrate surface.

It is well known that small-scale self-focusing is one of the major factors to restrict the capability and efficiency of high-power solid-state laser driver. B-integral is used to measure the severity extent of small-scale self-focusing and the typical value of B-integral is less than 1.8 in narrow band high power laser by NIF. We experimentally obtained the relationship of B-integral with spatial contrast for broadband pulse in case of active modulation and random noise modulation, and found that the growth of contrast is still very slow after B-integral up to 1.8 for 12 nm ultrashort pulse laser. So we can consider that the broadband laser is able to unloosen B-Integral criterion and then improve the system loading.

We evaluated characteristics of various shapes of reflector to get uniform and efficient pumping of Nd:YAG rod even with single-side pumping geometry using commercial software ZEMAX and LASCAD. The combined application of the software enabled us to evaluate numerically the laser performance and absorbed pump diode profile. We tried various shape of reflector such as semi-circular, square, elliptical reflectors, and cusp-shape, to get uniform pumping. In the calculation, water flows inside the cooling tube which surrounds the Nd:YAG rod. For further improvement a planarconcave window was placed in front of the diode to enlarge pump beam divergence angle. Through several trial and error, we found optimal pumping chamber configuration for single-side pumping, which gives reasonably uniform and efficient pumping of Nd:YAG rod. The output power and pulsewidth of Q-switched laser output were optimized by varying the laser cavity length and the output coupler reflectivity.

Because of its own structure's limitation, the beam divergence angle of the semiconductor laser is very large. In the fastaxis direction, the beam plays good quality and can be collimated less than 1° by using cylindrical micro-lens. However, in the slow-axis direction, the beam quality is too poor to be collimated to a small pattern. This could limit its application in the fields which need high beam quality. For high-power laser diode array (LDA), external-cavity technique can make all the emitters working in the same wavelength, and can improve their beam quality. In this paper, direct feedback method was employed to achieve cross-injection between 25 emitters of a LD bar by using a stripe mirror. At a certain cavity length, after the reflection of the external-cavity mirror's reflective stripe, the beam with large slow-axis divergence angle will feedback to the spacer region between the emitters and lose its energy, or would exactly feedback to the neighbor emitter. Simultaneously, the beam with small divergence angle would output from the transparent stripe of the external-cavity mirror. The slow-axis divergence of the stacks with two bars was suppressed from 6°to 2°by using this technique.

Among various trials on improving energy conversion efficiency of silicone based photo-voltaic cell, laser doping casts promising future. Several research groups are suggesting their own methods for laser doping. Usually, doping laser is injected inside of narrow phosphoric acid jet. The injected beam propagates through the jet and it grooves surface of silicone cell. The laser energy also heats the silicone surface and phosphorous is penetrated through the surface. In this work, we separate the grooving laser and heating laser. The silicone surface was grooved by a pulsed fiber laser. The spot size of the laser was 50 μm. For surface measurement, grooved with of 200 μm was needed. To have the groove width, we scanned the laser several times. SEM image of the grooved surface showed ripple of the surface. The phosphoric acid was sprayed on the grooved surface. A fiber coupled cw diode laser heated the sprayed Si surface. After heating, the Si was washed thoroughly by deionized water. The depth profile of penetrated phosphorous was examined by using SIMS. The profile indicated that phosphorous was penetrated about 50 nm. Four point measurement of surface resistance also indicated successful laser doping.

The influence of the delivery fibers, drawn by pump combiner or added initiatively, on stimulated Brillouin scattering (SBS) is discussed, based on the rate-equations combining with SBS. The impacts of pump schemes are simulated and the results indicate that the amplifier performance is considerable undermine by the delivery fiber in co- and counter- pumped amplifiers. As a suitable pump scheme for high power all-fiber amplifiers, we focus on the situation in the co-pumped amplifiers, in which the lengths, Brillouin gain coefficient and core diameter of the delivery fiber make great impacts on the performance of amplifiers. The impacts of the temperature gradients along the fiber, which can broaden the SBS gain profile and thereby suppress SBS, on the amplifier performances are discussed. Suitable suppression scheme is proposed to overcome the influences of delivery fiber on the amplifier performance.

A high-temperature oxidation resistant TiN embedded in Ti₃Al intermetallic matrix composite coating was fabricated on titanium alloy Ti6Al4V surface by 6kW transverse-flow CO₂ laser apparatus. The composition, morphology and microstructure of the laser clad TiN/Ti₃Al intermetallic matrix composite coating were characterized by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). In order to evaluate the high-temperature oxidation resistance of the composite coatings and the titanium alloy substrate, isothermal oxidation test was performed in a conventional high-temperature resistance furnace at 600°C and 800°C respectively. The result shows that the laser clad intermetallic composite coating has a rapidly solidified fine microstructure consisting of TiN primary phase (granular-like, flake-like, and dendrites), and uniformly distributed in the Ti₃Al matrix. It indicates that a physical and chemical reaction between the Ti powder and AlN powder occurred completely under the laser irradiation. In addition, the microhardness of the TiN/Ti₃Al intermetallic matrix composite coating is 844HV_0.2, 3.4 times higher than that of the titanium alloy substrate. The high-temperature oxidation resistance test reveals that TiN/Ti₃Al intermetallic matrix composite coating results in the better modification of high-temperature oxidation behavior than the titanium substrate. The excellent high-temperature oxidation resistance of the laser cladding layer is attributed to the formation of the reinforced phase TiN and Al₂O₃, TiO₂ hybrid oxide. Therefore, the laser cladding TiN/Ti₃Al intermetallic matrix composite coating is anticipated to be a promising oxidation resistance surface modification technique for Ti6Al4V alloy.

An approximate calculation method of material's temperature field is presented for pulse laser heat action when the object irradiated by laser beam is regarded as homogeneous medium with constant thermophysical properties. And the feasibility of this method is also proved by experiment. In order to let our investigation results be adapted for laser beam with arbitrary intensity distribution, the approximate expressions of arbitrary distribution of laser beam and the method of determining the thermophysical parameters of the material through the experiments are discussed.

In this paper, the temperature field of nodular cast iron in laser transformation hardening was simulated by using ANSYS software and tested with CCD-based colorimetric temperature measurement method. Firstly, the temperature field was calculated, and the width of the hardened track was estimated according to the calculated result. Then, the temperature field was measured in the presented experiments. After comparison of the calculated results to the experimental ones, it was found that the width of the hardened track can be predicted by the measured temperature field, which would help analyse the effect of process parameters and carry out real-time control in laser transformation hardening process.

As for master-oscillator-power-amplifier (MOPA) laser systems, Aberrations generated by thermal effects, the mirrors surface and optical elements play important roles to deteriorate laser beam quality. To obtain good beam performance, a compact adaptive optics (AO) system for a Nd:YAG zigzag slab amplifier is built. A rectangular tilt mirror (DM) and a 39-element rectangular piezoelectric deformable mirror in combination with a stochastic parallel gradient descent (SPGD) algorithm is introduced for aberrations correction. Experimental results demonstrate that the output beam quality can enhanced greatly at different power level when the AO system is in operation.

We report on the experimental observation of sideband generation in a passively mode-locked erbium-doped fiber laser. The fiber laser has a conventional ring-cavity configuration for passive mode locking based on nonlinear polarization rotation. Self-starting and stable mode-locking operation is easily achieved in the laser. The output soliton pulses have a duration of about 248 fs and a repetition rate of 13.7 MHz at 1565-nm wavelength. Detailed pulse dynamics of the laser is measured under different operation conditions. Dip-type sidebands are observed on the soliton spectra of the laser, which have clearly different characteristics to those of the conventional Kelly sidebands. The soliton operation of the fiber laser is numerically simulated based on the coupled Ginzburg-Landau equations. The simulation results are consistent with the experimental observations, which confirm that dip-type spectral sidebands can appear on the soliton spectra of a uniform soliton-emission fiber laser.

A novel method based on diffraction theory to control the far-field irradiance profile by deformable mirror is presented. Special near-field phase which determines the contour of the focal spot is obtained by a high spatial frequency deformable mirror. Numerical simulations show that, we can control the far-field intensity envelope as CPP by adopting adaptive optics technique when the spatial resolution of deformable mirror is high enough, here 16×16 actuators in 320mm×320mm aperture. The coupling coefficient is an important factor influencing control effect, and its best value range is round 0.6.

A novel method has been proposed to suppress transverse stimulated Raman scattering or transverse stimulated Brillouin scattering by processing the frequency convector edges into arrises. The mode analysis indicates that the residual reflection at the edges decreases rapidly with the decrease of arris angle and the direction of the ray finally reflected back has an angle with the surface of convector. So with this method transverse stimulated Raman scattering or transverse stimulated Brillouin scattering can be suppressed.