At first glance, the basic challenge is the disk thick- ness needed for efficient heat dissipation, because the conventional scheme involves low absorbance of the active disk material. However, the pump scheme allows one to find an elegant solution to this problem. The pro- cess of excitation in the active material of the disk can be represented in the form of multi-channel optical confi- guration [4,5] consisting of a parabolic mirror and a pump deflection system. The parabolic mirror focuses a colli- mated beam of the laser cavity containing an active ele- ment. After partial absorption of radiation passing through the disk body, some energy of the beam is reflected from a highly reflective mirror located on the backside of the disk. As a result of multiple reflections through prisms and the parabolic mirror in general we deal with 20 itera- tions of radiation passes through the body of the disk, which ensures high efficiency of absorption of light en- ergy of the pump beam. The optical scheme of such re- sonators tolerates limitations on the brightness of the pump source. Generally, pumping of a high average po- wer disklaser does not require a beam of high optical quality. That is why both laser diodes and rather complex laser diode structures with a uniform intensity distribu- tion can be used in this case. In terms of the practical use laser diodes with a uniform intensity distribution are more preferable due to much lower energy consumption per one watt of the pump power.
In order to obtain more detailed dynamic information about metallic plume, we used the high-speed photography to record the color images of metallic plume in a highpowerdisklaser welding process. High-speed photography is an effective method and is widely used in welding measurement, it can accurately capture and monitor the metallic plume information. The important characteristic features of plumes could be extracted from these color images [10,11]. Usually, the more energy a weldment absorbs, the bigger the metallic plume is. Here, the area of metallic plume was used as the characteristic parameter and the short-time Fourier transform was applied to obtain the time-frequency characteristics of plume. Also, the hierarchical clustering was used to analyze the plume characteristics and finally a clustering curve was plotted. Welding experimental results showed that in a definite parameter combination, the 6th fitting curve of the metallic plume frequency characteristic clustering could effectively reflect the fluctuation trend of the weld bead width.
The principle of the disklaser operation is based on the use of an active element in the form of a disk with a cooled surface. High cooling efficiency of the laser medium is provided by a large area of the disk surface, which is im- portant from the point of view of the heat transfer process. Therefore, the average power in the beam can reach quite high values. It is important to note that due to effective heat transfer within the disk area there is no thermal lens effect, characteristic of ‘rod’ and ‘slab’ (optical range) geometries of the active element. In this case, cooling is performed through a side surface of the active element; a two-dimensional heat flow, forming a parabolic profile of thermal distortion, propagates through this side surface. The temperature dependence of the refractive index in this case leads to the emergence of a strong thermal lens effect. Such a lens deteriorates the directivity of the output and signifi- cantly limits the effectiveness of its action. The thin disk, in contrast, is cooled through a thin contact region on one side and generates a one-dimensional axial heat flow on the other side. As a result, the temperature gradient is distributed parallel to the laser beam, which does not result in the thermal lens effect. In practice, a thin laser element in high-powerdisk lasers is either connected to a porous heat sink or is cooled by forced convection [2,3].
With increasing power levels produced by fibre sources in CW and pulsed operation, photodarkening has appeared as another limiting factor for further power scaling. Photodarkening can be described by a temporal decay of average output power. The decay rate depends on various parameters. This decay is due to increased absorption of the signal in the Yb-doped doped core and results in a reduction of the device lifetime. The underlying physical process for doped fibre degradation could be attributed to the formation of colour centres or other photoinduced structural transformations in the silica glass host  associated to defect sites in the glass. The induced absorption is more pronounced for visible wavelengths and tails out to 1.1 µm. It was observed that the photodarkening rate has a 7 th order dependence on the populaton inversion . For this reason, high-energy pulsed system with an average population inversion of 40-50% can degrade 10 4 -10 7 faster than standard CW fibre laser operating with 5-10% population inversion. Photodarkening effects can be reduced by employing a counter propagative pumping configuration since it provides a more uniform population inversion than co-propagative pumping. With increasing interest for photodarkening, fibre manufacturers have also started to work on solutions to overcome this issue. This can be done by improving the fabrication process to avoid formation of defects in the glass matrix. Liekki has demonstrated improved performance by fabricating YDF with so- called Direct Nanoparticle Deposition (DND) . Another technique consists in minimising Yb clustering by increasing the aluminium content in the fibre composition [32, 33].
1.1. Laser-Induced Breakdown Spectroscopy. LIBS is a form of optical emission spectroscopy utilizing a high-powered, pulsed laser. The short pulse is focused onto a sample surface where it creates a high-power density in a localized area. A plasma is created from the ablated material . Plasma formation is a complex process, which is still under investiga- tion. The current models for ablation describing the change in thermal properties of the sample material are adequate for this study . The models for nanosecond laser ablation state that when a laser hits a target, with enough high energy, the surface heats up, melts, vaporizes, and ionizes creating a plasma. If there is additional energy in the laser pulse, the plasma absorbs it, increasing the plasma temperature. Fur- ther heating of the surface of the target can occur if the plasma is hot enough [6, 7]. For a more detailed discussion on plasma ablation, refer to  and the references therein.
Rare-earth doped silica fibers have been used for many years to create continuous-wave lasers, and Er-doped fiber amplifiers are now widely used in telecommunications. In addition, cladding pumped fiber allows the efficient conversion of multimode radiation from highpower, low cost, broad-stripe semiconductor laser diodes into the single-mode emission of fiber lasers. With its broad gain bandwidth and high optical conversion efficiency, Yb-doped silica fiber represents an attractive medium for the generation and amplification of high energy ultrashort optical pulses. However, these potential advantages of Yb-doped silica fiber as a gain and nonlinear medium for mode-locked lasers and ultrashort pulse amplifiers have been less well studied, and it was not until 1999 that significant research interest first appeared in Yb-fiber chirp pulse amplifier (CPA) systems. This thesis describes the development of the first practical and stable, femtosecond, Yb-fiber oscillator, and of an Yb-fiber amplifier based CPA system (pulses ~10 µJ, <500 fs). Novel aspects of the system include the use of a high extinction ratio Electro-Optic modulator for pulse selection, and the development of a compact chirped-fiber- Bragg-grating (CFBG) pulse stretcher that provides both 2 nd and 3 rd order chirp compensation. Recently published theoretical results have demonstrated that the asymptotic solution for ultrashort pulses in a high gain fiber amplifier is a linearly chirped pulse, which can therefore be recompressed with a standard grating compressor. This thesis reports the first experimental comparison of nonlinear pulse evolution towards the asymptotic form using a cascaded amplifier system. The ‘direct amplification’ system was constructed by removing the CPA stretcher grating, which also enabled the use of a less dispersive and more compact compressor. Further system development should lead to the generation of ultrashort pulses at high average power levels and >100 kHz repetition rates.
Laser delivery System: Transmission of laser cavity to the tissue is provided by one of three deviced articulated arms optical fibers or micromanipulators (16) Articulated arms direct laser energy from the laser cavity to the desired location through a series of hollow rigid tubes with reflective of the laser wavelength being transmitted so that coherence and power are maintained allowing the fine focusing of the exiting beam a hand piece at the end of the arm contains a lens that focuses the beam. (16) Several limitation remain in articulated arm systems despite recent advances to improve their mobility articulated arms are somewhat cumbersome to use in a clinical setting. the mirrors are easily misaligned when either the laser device or the articulated arm is disturbed. despite the aforementioned limitation carbon dioxide or Er. YAG lasers almost exclusively utitize articulated arms because their infrared wavelength are not transmissible along currently available optical fibers (16) Both articulated arms and optical fibers can be coupled through a microscope with a micromanipulator which provides a controlled means of moving the laser beam across the surface of human skin. the micromanipulator can also be coupled with a computer for completely preprogrammed and precise skin irradiation. (18)
In Figure 7, spectrum and device vertical far − field characteristics test result in 3 A working current were giv- en in (a) and (b). Peak wavelength of the device is 1059.4 nm, the spectral half width (FWHM) is 3 nm. As a result of the large optical cavity structure, in vertical direction 95% of peak vertical divergence angle is 49.8 ˚, 50% peak divergence angle is only 24.6 ˚, this is about to 30˚ for conventional semiconductor laser.
Mathematical modeling and simulation have been de- veloped and used to understand and gain insight into the various mechanisms which take place during heating of a rotating workpiece using a moving laser beam. Transient three-dimensional model of a rotating workpiece under- going laser heating and material removal was developed and used for different materials including mainly ceram- ics as well as titanium alloys [19-23]. However, research work for a moving Gaussian laser heat source to predict depth of heat-affected zone and temperature distribution in a Cartesian coordinates of hard steels is far from com- plete and more research work is required.
technologies in laser pulse generation and intensities rising to a critical level 1 , ions in the MeV/nucleon range became accessible starting in the early 2000s [91, 33, 108]. Since then, the maximum laser intensity achieved in the lab- oratories worldwide has almost stagnated, and so did the maximum kinetic energies. Many efforts have aimed to optimize the laser-target interaction with the goal to optimize the particle beam for potential applications. Moti- vations range from cheap and compact proton sources for radiographic can- cer therapy [4, 98], to injection sources for conventional accelerators  and many more. Laser ion acceleration is still a young and explorative field, however it finds itself at the transition from proof of concept experiments to first applications using the generated ions rather than just exploring how to produce them. The Center for Advanced Light and Application (CALA) facility, which is in the final construction phase during the time of this thesis (2018) is an example of a facility specifically aimed at the advancement of the application of laser-driven particle beams. While there are still obstacles to overcome in source generation, the laser ion acceleration has come to a point, where technological developments play a key role and can decide on successful operation.
The fact that light is reflected from a thin plasma layer formed on the substrate sur- face means that the surface can be curved to induce focusing (just as in conventional solid state optics). By appropriate choice of the surface curvature, an incident focusing laser beam can be made to focus with an even smaller F/#. A focusing plasma mirror (FPM) of ellipsoidal geometry with two foci, such that demagnification of a focal spot occurs from one focal position to the other, satisfies the need to have off-axis focusing to ensure that the target is not blocking the incoming laser beam. Such an optic is attractive not only because of the increase in peak laser intensity achievable, but also because it improves pulse intensity contrast in the same way as a PPM. The use of such an optic was first demonstrated in a proof-of-principle experiment on a terawatt (TW) level laser system reported by Kon et al  and Nakatsutsumi et al , whereby a F/0.4 FPM was devel- oped and achieved a five-fold reduction in focal spot size compared to the spot formed by a conventional F/2.7 off-axis parabolic (OAP) mirror. The intensity enhancement was in- directly diagnosed by measurement of the maximum energy of protons accelerated from a thin target foil positioned at the FPM beam focus.
Aluminium and its alloys are high potential non-ferrous materials used universally. These alloys are of great interest to a wide range of industries because they exhibit quite an attractive variety of favorable mechanical and chemical properties. Aluminium and its alloys exhibit excellent mechanical strength, low specific weight, good formability and have relatively low cost and thus are widely used for industrial applications. The automotive industry is the dominant market for aluminium products throughout the world. Cylinder heads, engine blocks, closure panels, such as boots, trunk lids and door panels. Structural components made from aluminium and its alloys are vital to the aerospace industry. Inspire of all the attractive properties exhibited by these alloys, industrial applications are still limited. The surface properties of aluminium alloys are insufficient for many engineering requirements. Aluminium alloys possess poor tribological properties and load bearing capacity due to their low melting points, low hardness and weak interatomic bond.
The optical spectrum at the output of the MOPA is shown in Fig. 4(a). At 60 W average output power the signal peak is 25 dB above the peak of the background ASE. There was no spectral broadening observed confirming that the nonlinear effects are minimized in the large core amplifier fiber of the MOPA. The pulse quality after amplification was verified using autocorrelation measurements shown in fig. 4(b). The input pulse duration of 4.5 ps, determined from the input autocorrelation trace, is maintained after amplification to 60 W. The low level pedestal observed on the output autocorrelation trace is from a small coupling of the first-order mode into higher- order modes within the amplifier fiber. We have also used the modulator to reduce the repetition rate of the pulses from 10 GHz down to 10 MHz to investigate the peak power scaling effects in this amplifier system. Peak powers of 1.3 MW have been obtained, however further work is required to optimize the preamplifiers to improve the OSNR for lower repetition rates.
Kim et al (2001) have proposed a new power supply for pulsed Nd:YAG laser adopting zero crossing control (ZCC) method which is simple and compact in design. In this power supply, SCR was turned on at zero point of input AC voltage by the method of zero crossing control (ZCC). In 2002, a new real time multi-discharge method (RTMD) was reported. This method uses real-time one-chip microcomputer that can turn on the flashlamp with a precision of up to 1µs and thus can create diverse pulse shapes and strength, in addition to longer pulse (Hong et al, 2002).
essed into 4 lm-wide mesa stripe devices. The 1.5–2:5 mm long lasers with high- and antireflection coatings on the rear and front facets lase either at the GS (around 1265 nm, 50 mA threshold at the pulsed pump) or simultaneously at the GS and ES (around 1190 nm, 1:5 A threshold at the pulsed pump) in the whole range of pumping. Short-pulsed electrical pumping with a total pulse duration of 30 ns was used to achieve high output power operation and avoid the effect of overheating on the output pulse shape. Pulses of 5 ns rise-time were obtained from a highpower digital pulse source (up to 2A current), and the laser output was detected using a high-speed pin detector with a cut-off fre- quency of 30 GHz and a 50 GHz digital oscilloscope.
Entire process of (the single atom response of) HHG (for a single hydrogen atom) can be described by 3-dimensional Schr¨ odinger equation (Eqn. (2.3)). Several numerical methods have been invented to eﬃciently and precisely solve the equation [73–76]. Remember that the typical eﬃciency of HHG around the cut-oﬀ region would be far below 10 − 6 . Therefore Eqn. (2.3) has to be evaluated within an accuracy better than 10 − 6 for a detailed discussion of the cut-oﬀ region. We developed a code to fully numerically solve the HHG process based on the Time-Dependent Generalized Pseudo-Spectral (TDGPS) method invented by Chu et al. . Here we brieﬂy outline the method. The hydrogen potential is modiﬁed to be conﬁned in a ﬁnite-size box with an inﬁnitely high potential barrier in order to obtain discretized energy levels for ionized states. The electron wave function is expanded with energy eigenfunctions of the potential that consist of bound and ionized states. The time- evolution of the wavefunction is calculated with the split-operator method where each time- step contains the time-evolution due to an interaction free Hamiltonian and the interaction Hamiltonian. The dipole moment is calculated from so obtained wavefunctions for each time step. The high harmonic spectrum is obtained by taking the Fourier transform of the dipole moment. Fig. 2.5 shows the obtained high harmonic spectrum. For this example the following parameters are used in the simulation:
Remote laser welding integrated with a scanner offers a number of advantages over conventional laser welding, for example, high flexibility, reduced cycle time, high speed production of sound welds with consistent quality, a high degree of automation, good accessibility to the weld joints due to the long focal length, cost savings through the use of less materials and reduced mainte- nance demands; low distortion from a smaller heat input, and small floor space requirements [25,26].
repetition rate of 1 Hz. In our experiment the energy was attenuated to about 12 J on target with enhanced temporal contrast using a re-collimating single plasma mir- ror . Using plastic foil targets with a thickness of about 200 nm, the laser drives a TNSA proton source with cut-off energies in the range of 30 MeV. The pulsed, tunable solenoid magnet [18, 68] was positioned 80 mm behind the target and was therefore able to collect the high energetic part of the beam without particle loss. It acted as chromatic lens and was used to generate a focus of a desired central energy at the irradiation site in air about two meters downstream of the target. The energy bandwidth of the transported proton bunch amounted to about 20% (FWHM) at the focus position. For the presented experiment, a central proton energy of 15.4 MeV was focused into the I-BEAT detector, corresponding to a solenoid current of 12 kA leading to a magnetic field of about 10 T, accordingly. This proton energy has been chosen to optimize the signal-to-noise ratio recorded by the I-BEAT detector. The increased ion energy herein enabled a comparison with an RCF stack (EBT3 Gafchromic film, calibrated with an X-ray tube). Fig. 5.14 shows a shot for a design energy of about 16 MeV and its comparison to an RCF stack in two consecutive shots (small shot-to-shot fluctuations are ignored). This energy setting has been chosen to optimize the signal-to-noise ratio recorded by the I-BEAT detector. Fig. 5.14b shows the measured signal and the simulated signal using the reconstructed spectrum of Fig. 5.14c as an input. Fig. 5.14d validates that I-BEAT can reconstruct the depth dose distribution quantitatively. The depth resolution (horizontal spacing between data points) of I-BEAT is due to the sampling rate and transfer function and the error bar due to the limited band width of the transducer (10 MHz). The dose- error of I-BEAT results mainly from the calibration (e.g. fluctuations in the particle number) at the Tandem accelerator and electronic noise in the measured signal. For the RCF stack (EBT3-films) an overall accuracy in dose determination better than 5% can be obtained. However, due to higher Linear Energy Transfer (LET) in the Bragg peak region (with respect to the plateau region) of a mono-energetic proton beam, quenching effects might occur, which can lead to higher dose uncertainties. The gray bar in Fig. 5.14d t RCF illustrates the water equivalent thickness and thus
However, the packaging of a high-powerlaser diode is time and effort consuming. The consideration places not only on the thermal challenges, the mechanical integrity, the electrical coupling, the excessively-induced bonding stress, but also on the optical stringent alignment in order to achieve high optical coupling between the very small facet areas (few microns square) with the pigtail fibre.
Much of the progress achieved in the development of these powerful ultrafast laser systems has been driven by the growing industrial market of ultrafast lasers for high-precision material processing. Cold ablation of a large variety of materials can be driven at higher speeds with high repetition rate, high energy ultrafast lasers. In fact, many of the above-mentioned state-of-the-art laser systems have been demonstrated in an industrial R&D environment. In this field, the advantages of kW ultra- fast lasers are starting to be demonstrated: for example, efficient processing of carbon fiber reinforced polymer (CFRP) with negligible thermal damage was demon- strated, which was enabled by the application of a thin- disk multipass amplifiers system that generates ps pulses at an average output power exceeding 1 kW . More recently, very high productivity surface structuring of steel for antibacterial applications has been demon- strated using the same amplifier system . Addition- ally, new findings appear to show that glass cutting can still be scaled significantly further .