Nd-doped Yttrium Aluminum Garnet Laser Research Articles

Direct evidence of laser-induced shock wave plasma from organic targets in low pressure He ambient gas, showing the effect of target hardness on its propagation speed and the resulted spectral performance

We report the experimental evidence of shock wave plasma generation by direct observation of the plasma propagation using 68 mJ 1064 nm Nd-doped yttrium aluminum garnet laser irradiation on a relatively soft organic sample in He ambient gas at 2 kPa. The density jump associated with the arrival of the plasma front at a certain position is detected by means of the highly sensitive interferometric technique, in conjunction with the observation of the first appearance of the plasma emission at the same position and time of the plasma front arrival. The result shows that the plasma front moves at the Sedov speed, which is typical of shock wave propagation, yielding an excellent emission spectrum for the harder sample. The shock front speed is further found to decrease in the case of a softer sample, resulting in lesser spectroscopic performance, as demonstrated by the emission spectra measured from mochi samples of different hardnesses. The result of this study is promising for paving the way to extending the laser-induced breakdown spectroscopy application to the much needed spectrochemical analysis of agricultural and food products.

A novel double-pulse laser plasma spectroscopic technique for H analysis in metal samples utilizing transversely excited atmospheric-pressure CO2 laser-induced metastable He atoms

The analysis of hydrogen in a metal sample (zircaloy-4), which is usually difficult to perform using conventional laser-induced breakdown spectroscopy (LIBS) techniques, has been achieved using a double-pulse technique under He gas at atmospheric pressure. In this technique, a transversely excited atmospheric-pressure (TEA) CO2 laser (1.5 J, 200 ns) was focused onto the metal surface to induce a strong He gas plasma whilst simultaneously focussing a Nd-doped yttrium aluminum garnet (Nd:YAG) laser (120 mJ, 8 ns), synchronized with the TEA CO2 laser, onto the metal to ablate atoms into the resulting He gas plasma. The emission spectrum obtained shows a narrow H linewidth with low background intensity and long lifetime emission, thereby indicating that excitation takes place via metastable He atoms. The H emission from H2O can be suppressed by a careful pretreatment involving heating the sample in a vacuum chamber.

Quenching of He-induced intensity enhancement effect in H and D emission produced by Nd-doped yttrium aluminum garnet laser irradiation on solid targets in low pressure helium gas

An experimental study was performed on the N2-induced quenching of He-induced intensity enhancement effect in reduced-pressure plasma emission produced by Nd-YAG irradiation on solid zircaloy and porous fossil samples. The spatial distributions and temporal variations in the emission intensities show pronounced intensity quenching effects on the He I 667.9 nm, H I 656.2 nm, and D I 656.1 nm emission lines in both samples when a tiny amount (5% by volume) of nitrogen was added to helium gas, while leaving the spatial and temporal intensity profiles of the heavier Zr and Ca atoms virtually unaffected. In both cases of different ambient gases, the spatial and temporal variations in the He, H, and D emission intensities exhibit distinct features and changes, which are clearly distinguishable from those observed on the Zr and Ca emission lines, which were mainly produced by the shock-wave induced thermal excitation process. The analysis of these data unambiguously revealed the presence of an additional and distinct “He-assisted” excitation mechanism in the He plasma, which was further suggested to be related to the He metastable excited state. The quenching effect was therefore explained as a consequence of energy depletion of the He metastable excited state triggered by the Penning ionization process induced by the presence of nitrogen. This also explains the relatively insensitive response of the Zr and Ca emission intensity profiles to nitrogen addition despite the increased plasma electron density resulting from the ionization process.

Temperature dependence of magnetoresistivity of cobalt-polytetrafluoroethylene granular composite films

Heterogeneous granular films consisting of ferromagnetic cobalt particles embedded in a polytetrafluoroethylene polymer matrix were fabricated by the pulsed laser deposition technique carried out with a Nd-doped yttrium aluminum garnet laser at 355nm. The samples exhibit a magnetoresistance (MR) change of 4% (at 8kOe) at room temperature. An enhancement of MR to a value of 6% was observed at 20K which is attributed to the increasing order of magnetic moments in the ferromagnetic state at low temperature. Significant loop opening of the MR profile at low temperature and the temperature dependence of MR reveal the interplay of superparamagnetic relaxation behavior and spin-dependent electron tunneling.

Percussion drilling with laser: hole completion criterion

The criterion for ensuring formation of a through hole is an important aspect of laser drilling. Close observation of the process has indicated that drilling of the substrate is achieved in six successive microstages. The stages have been time resolved and the behaviour of laser induced plasma (LIP) during the microstages of drilling has been carefully examined. The LIP behaviour during the percussion laser drilling has been explained with the help of the contemporary understanding of plasma. This paper proposes a drilling completion criterion that is based on the behaviour of LIP during laser percussion drilling of SUPERNI 263A with a Nd-doped yttrium aluminium garnet laser. The completion of formation of a hole is marked by the momentarily increased luminescence of the plasma plume emanating from the laser beam exit side of the workpiece during drilling. The criterion has been experimentally verified. The criterion is versatile and can help in augmenting productivity during manufacturing.

Bandwidth dependence of pulse duration in intra-cavity frequency-doubled lasers: theory and experiment

A model is presented of pulse evolution in broadband intra-cavity frequency-doubled lasers. The model utilizes normalized coupled rate equations for each mode, including terms that represent the loss due to nonlinear mixing between longitudinal modes. The pulse energy, shape, peak power and duration are calculated by numerical solution of these equations. The model shows that the pulse duration depends not only on the initial population inversion, photon lifetime and the effective nonlinear coupling coefficient, as is the case for narrowband lasers, but also on the fundamental bandwidth. A gain-switched Ti:sapphire laser, pumped by a Q-switched Nd-doped yttrium aluminium garnet laser at 532 nm, was frequency doubled using an intra-cavity β-barium borate crystal. The bandwidth was reduced from about 25 nm to about 1.5 nm in two steps using a series of prisms, and the resulting changes in experimental pulse durations and energies agree well with the model.

Influence of probe beam multi-reflection on thermal lens measurements: Application to Nd:YAG rods

In this work time resolved thermal lens measurements were carried out in Nd-doped yttrium aluminum garnet laser crystal. We observed that due to the probe beam multi-reflection the thermal lens signal was influenced by the moving fringes that occurred in front of the detector. By using a heat sink device, this effect was minimized permitting the determination of the temperature coefficient of optical path length, the thermal diffusivity and the thermal conductivity of the sample. As a complementary technique, the thermal relaxation method was used to measure the sample specific heat.

Stabilizing Unstable Steady States Using Multiple Delay Feedback Control

Feedback control with different and independent delay times is introduced and shown to be an efficient method for stabilizing fixed points (equilibria) of dynamical systems. In comparison to other delay based chaos control methods multiple delay feedback control is superior for controlling steady states and works also for relatively large delay times (sometimes unavoidable in experiments due to system dead times). To demonstrate this approach for stabilizing unstable fixed points we present numerical simulations of Chua's circuit and a successful experimental application for stabilizing a chaotic frequency doubled Nd-doped yttrium aluminum garnet laser.

Time-resolved thermal lens determination of the thermo-optical coefficients in Nd-doped yttrium aluminum garnet as a function of temperature

In this work, time-resolved thermal lens measurements are carried out in Nd-doped yttrium aluminum garnet laser crystal as a function of temperature. The temperature coefficient of the optical path length, the thermal diffusivity, and the thermal conductivity were determined in the temperature range between 22°C and 180°C. In addition, the specific heat was measured by the thermal relaxation method in the same temperature range. The results showed that the thermal conductivity and the temperature coefficient of the optical path length are responsible for the strong increase of the thermal lens amplitude when the sample temperature was increased. These results suggest that the time resolved procedure is useful in the study of laser materials and may be an important tool for laser designers.

Intensity coherence of a multimode Nd-doped yttrium aluminum garnet laser

We measure the power spectral densities, at relaxation oscillation frequencies, of the individual longitudinal modes and the total intensity of a Nd-doped yttrium aluminum garnet laser at low pumping rate. We then test relationships between these quantities that are derived from the modal rate equations theory. The theoretical relations for the two mode case are confirmed by the experiment. However, in the three-mode regime, theory and experiments do not agree well.

Laser nitriding of iron with laser pulses from femtosecond to nanosecond pulse duration

Pulsed-laser nitriding is an attractive method to improve metal surface properties, such as hardness, wear, and corrosion resistance, with the advantage of simple experimental setup, rapid treatment, and precise process control. Here, the dependence of the laser nitriding process on the laser pulse duration was investigated over five orders of magnitude in a series of experiments employing pulsed lasers ranging from nanosecond excimer laser pulses (55 ns) and Nd-doped yttrium aluminum garnet (Nd:YAG) laser pulses (8 ns), to ultrashort Ti:sapphire laser pulses (150 fs). The results revealed that for all laser pulse durations and different wavelengths a large nitriding effect was observed. The excimer laser shows the highest nitriding efficiency. The basic processes for the femtosecond pulsed laser are not well understood but seem to result at least partly from processes within the plasma, whereas nanosecond nitriding is based mainly on processes within the liquid/solid surface.

The influence of assist gas on the mechanism of material ejection and removal during laser percussion drilling

This paper elucidates the effects of different gases on the dominant mechanisms of the material ejection and removal and the cooling rate during laser percussion drilling. In particular, work was conducted for the reactive O2 and inert Ar assist gases, in terms of their effects on (a) the behaviour and characteristics of spatter, (b) the recast layer characteristics and (c) the material ejection spark distribution for the drilling of Nimonic 263 alloy sheets using a Nd-doped yttrium aluminium garnet laser. It was shown that using O2 assist gas, the material ejection was characterized with increased liquid particle ejection, coupled with volatile burning, possibly similar to that of nucleation-induced explosions, while the employment of Ar assist gas influenced the material ejection to be dominated by bulk radial liquid ejection.

Measurement of the grain-boundary tensile strength in columnar freshwater ice

A new procedure for measuring the tensile strength of grain boundaries in a columnar ice sample is presented. In this experiment, an ice coating is grown on an Al surface with its columnar grains normal to the interface. The sample is loaded by a compressive stress wave of about 20ns duration, generated by impinging a Nd-doped yttrium aluminium garnet laser pulse on the back side of the Al substrate. The compression stress wave propagates through the ice coating turns tensile after reflecting from its free surface and loads the ice-Al interface normally. Because of the Poisson effect, this interface-normal stress is accompanied by an interface-parallel biaxial stress of a somewhat lower (33% of the axial stress) magnitude. Because the interface strength is much larger than the tensile strength of grain boundaries, this latter stress, even though small, is sufficient to cause the onset of cracking in some of the favourably oriented grain boundaries. The threshold laser energy corresponding to this event is determined by observing the sample under an optical microscope, after each loading. Finally, this threshold laser energy is converted to the local interface stress, causing boundary separation using an elastic wave simulation. Using this procedure, an average across-column grain-boundary tensile strength of 47MPa was determined at-10oC. Since the boundaries are loaded at a strain rate of almost 107 s-1, all inelastic deformations during the boundary decohesion process are essentially suppressed, and thus the measured values are essentially intrinsic depending only on the structure and chemistry of the grain boundary, including the defects, if any.

Energy analysis and quality evaluation of pulsed neodymium-doped yttrium aluminium garnet laser cladding ceramic-metal composite coating on aluminium alloys

This research addressed an ‘energy threshold’ evaluation method for pulsed Nd-doped yttrium aluminium garnet (Nd-YAG) laser cladding. A high-power pulsed Nd-YAG laser was selected as the main equipment to investigate the performance of laser cladding ceramic-metal composite coating on Al alloys. Additionally, the basic characteristics of pulsed Nd-YAG laser for cladding processing were analysed. Three important system parameters of the pulsed Nd-YAG laser were isolated as the key factors that significantly affect cladding quality: Cladding speed, pulse width and laser power (pumping current). The experimental results were carefully analysed and evaluated. Firstly, the evaluation procedures included cladding track observation. The integrity of the tracks was used to judge whether the cladding was successful or inadequate. Secondly, the cross-section of the cladding layer was measured and examined. Three indexes of cladding quality (the thickness, the penetration depth and the shrinkage cavity area in the cladding layer) were analysed. From these evaluations, it was found that the pulse energy is the critical factor for pulsed Nd-YAG laser cladding. The research results also suggest that the minimum pulse energy Ep∗ may be adopted as the ‘energy threshold’ for continuously and successfully cladding the ceramic-metal composite on Al alloys.

High-performance solid-state dye laser based on peryleneorange-doped polycom glass

The new laser dye, perylene orange, doped into polycom glass (sol–gel glass–poly(methyl methacrylate) composite) has been developed into a tunable gain material with high performance. A sample polished to 0.02 μm smoothness and λ/5 flatness, over the central region, was excited in an optical cavity with a second harmonic Q-switched Nd-doped yttrium aluminium garnet laser as a pump source. The solid-state dye laser had a threshold of 53 μJ, a slope efficiency of 72% and a normalized photostability of 50 GJ mol−1. The laser damage thresholds of various host materials were obtained and the damage threshold of polycom glass was found to be 26 J cm−2. The beam profile of a He‒Ne laser transmitted through samples of host material showed that the optical homogeneity of polycom glass is approaching that of pure polymers.

High-performance solid-state dye laser based on perylene-orange-doped polycom glass

The ne laser dye, perylene orange, doped into polycom glass (sol-gel glass-poly(methyl methacrylate) composite) has been developed into a tunable gain material ith high performance.,A sample polished to 0.02μm smoothness and λ/5 flatness, over the central region, as excited in an optical cavity with a second harmonic Q-switched Nd-doped yttrium aluminium garnet laser as a pump source. The solid-state dye laser had a threshold of 53 μJ, a slope efficiency of 72%, and a normalized photostability of 50 GJ mol -1 The laser damage thresholds of various host materials were obtained and the damage threshold of polycom glass was found to be 26 J cm -2 . The beam profile of a He-Ne laser transmitted through samples of host material showed that the optical homogeneity of polycom glass is approaching that of pure polymers.

The processing of SiC/SiC ceramic matrix composites using a pulsed Nd-YAG laser: Part II The effect of process variables

The machining of a composite material consisting of silicon carbide (SiC) fibres in a chemical- vapour-infiltrated SiC matrix has been investigated using a 400 W pulsed Nd-doped yttrium aluminium garnet laser. Part I of this work reported the findings of investigating the influence of the laser pulse parameters, principally with respect to the material removal rate. In this, the analysis concentrates on the quality of the processed surface and considers the effect of processing variables such as the choice and pressure of assist gas and the point of focus of the incident laser beam. Scanning electron microscopy is used to assess the extent of the heat-affected zone and its impact on the structural integrity of the composite. Throughout this study, comparison is made with related ceramic matrix composites (CMCs) that have been investigated. The intention is that, by determining the influential factors in each of the specific cases, observations may be made concerning the laser processing of CMC material systems in general. © 1998 Chapman & Hall

Generation of coherent undulator radiation using a relativistic photoelectron beam with picosecond micropulses.

The coherent effect of undulator radiation is investigated using relativistic photoelectron beam (RPE) produced by the fourth harmonics of a [ital Q]-switched Nd-doped yttrium aluminum garnet (Nd-YAG) laser. The [ital Q]-switched laser pulse contains a burst of 30 ps micropulses superposed with a weak 8 ns pulse. The current and shape of the RPE were controlled by changing the incident laser intensity. The temporal structure of the RPE was measured using the Cherenkov radiation emitted by relativistic electrons impinged on an optical fiber. When the irradiated laser intensity is stronger than 5 MW/cm[sup 2], the wave form of the RPE does not show any microstructure, which is due to the saturation of the RPE current density by space-charge effect. The measured radiation power from the RPE having no microstructure was near the noise level of a microwave diode, even though the current was much larger than that of the RPE having micropulses. With irradiated intensity less than 1 MW/cm[sup 2], the temporal structure of the RPE closely follows that of the laser pulse. The measured power of the undulator radiation generated by micropulses was more than 10[sup 3] times stronger than that of the theoretically predicted incoherent radiation and tendsmore » towards a quadratic dependence on the electron-beam current. The enhancement and tendency is due to coherent radiation emitted by electron-beam micropulses whose typical bunch lengths are comparable to the radiation wavelength (7.4 mm).« less

Coherence and resonance effects in high-order harmonic generation.

We present experimental measurements of harmonic generation in xenon using a 1064-nm 40-psec Nd-doped yttrium aluminum garnet laser. The harmonic yield is studied as a function of the position of the laser focus in the atomic beam. It shows regular oscillations whose period decreases with increasing harmonic order and which can be interpreted as phase-matching effects due to the tight focusing geometry. Moreover, the amplitudes of the oscillations vary rapidly with the laser intensity, a signature of resonance effects in the harmonic generation process.

Test of the special-relativistic Doppler formula at beta =0.84.

We used the relativistic Doppler effect to shift the energy of the fourth-harmonic photons of a Nd-doped yttrium aluminum garnet laser into the range from 1.4 to 15.8 eV in a moving atom's reference frame by varying the angle of intersection theta between the laser beam and an 800-MeV atomic hydrogen (${\mathrm{H}}^{0}$) beam. We measured the intersection angle theta for several well known Lyman transitions (1s to np) in ${\mathrm{H}}^{0}$ and fitted these data by the form E=(${E}_{0}$/${g}_{0}$)\ensuremath{\gamma}(1+\ensuremath{\beta} costheta). The value of ${E}_{0}$/${g}_{0}$ obtained in this manner, compared with the measured value of ${E}_{0}$ for the laser photons, yielded ${g}_{0}$=1.000 04(27) at \ensuremath{\beta}\ensuremath{\simeq}0.84. The prediction of special relativity that ${g}_{0}$=1 is thus verified to an accuracy of 2.7\ifmmode\times\else\texttimes\fi{}${10}^{4}$.