A short pulse, free running, Nd : YAG laser for the cleaning of stone cultural heritage

This paper presents a Nd : YAG laser operating in free running (FR) regime, with a pulse duration (20 μs) shorter than conventional systems (>200 μs), mainly developed for applications in laser cleaning of stones, especially for the restoration of cultural heritage. The system was also optimized to achieve high energy and low divergence, for easy coupling with optical fibers. The unusual pulse temporal regime induces a spiky behavior of the laser output which could also help in the application. Details on the technologies for the flashlamps power supplies, including the discharge circuits needed to achieve the short pulses, are given. Application trials on artworks and artificial samples are also discussed. Results show that the intermediate pulse duration avoids the mechanical damage induced by the photomechanical effect of Q-switch lasers and the thermal damage, as superficial melting, usually induced by long pulse FR lasers.     

Quasi-CW diode pumped Nd:KGW laser at low temperature

We have studied the laser performance of Nd: KGW and Nd: YAG rods by monitoring the 1.06 μm laser emission at 78.5 K and 300 K. Our observations indicate that the laser efficiency from Nd: KGW is much higher than from Nd: YAG at these two temperatures. The laser threshold in Nd: KGW is lower at 78.5 K than at 300 K but it is always lower than that in Nd:YAG.     

Laser cleaning of gilded wood: A comparative study of colour variations induced by irradiation at different wavelengths

There is a growing interest by art conservators for laser cleaning of wood artworks, since traditional cleaning with chemical solvents can be a source of decay, due to the prolonged action of chemicals after the restoration. In this experiment we used excimer and Nd:YAG lasers, emitting radiation in the ultraviolet (248 nm), visible (532 nm) and near infrared (1064 nm), to investigate the effect of laser interaction on gilded wood samples at different wavelengths. Increasing fluence levels were tested to assess threshold values both for surface damage and colour changes. Detailed colorimetric analyses of the irradiated samples show that cleaning effectiveness is related to the emission wavelength, the fluence and the number of pulses.     

Experimental study of high-power pulse side-pumped Nd:YAG laser

The paper reports on the characterization of a compact and simple side-pumped 0.538 J×100 Hz pulse Nd:YAG laser. A side-pumping configuration with 100 laser diode bars is used in the laser head. We also experimentally studied the laser performance of the diode-pumped Nd:YAG laser head in the free running and Q-switched operation under different repetition rates.     

Comparative evaluation of UV-vis-IR Nd:YAG laser cleaning of beeswax layers on granite substrates

The beeswax treatment applied in the sixties to prevent rain water from penetrating the outer stone surface of valuable granitic Galician monuments is contributing to the acceleration of the superficial degradation process of these monuments. At present, the northern sector of the renaissance frieze in the Cloister of the Cathedral of Santiago de Compostela is one of the most representative examples.Conventional wax removal methods (water, chemical and mechanical cleaning) can possibly destruct important details of the relief. Therefore laser removal is considered as a good alternative.In this work, we report systematic investigations of the effect of laser cleaning at different Nd:YAG laser wavelengths (266, 355, 532 and 1064 nm) on representative samples of the real historical surfaces.Laser removal of beeswax on granite at neither of the four wavelengths of the Nd:YAG laser is not a layer by layer removal process. For each irradiance and wavelength there is a maximum thickness that can be completely removed by a single pulse. Above this thickness the waxy material is not removed, although it undergoes thermal modifications; since the fraction of radiation that reaches the granite substrate is not enough to trigger the ejection of material.Our results show that the wax-granite interface plays a fundamental role in granite cleaning, and when the wax is weakened by absorption of radiation at 266 nm, the removal process becomes more efficient.     

Investigation into absorption of the incident laser beam during Nd:YAG laser processing of metals

Laser materials processing is highly affected by the existence of surface plasma. The absorption of surface plasma during drilling alters the power intensity distribution of the incident laser beam across the irradiated spot. The present study is carried out to measure the electron number density and temperature using a Langmuir probe while a mathematical formulation is conducted for the absorption coefficients due to electron-ion, electron-neutral atom collisions, inverse Bremsstrahlung, and photoionization processes. Consequently, a computer program is developed to compute the relevant absorption coefficients as well as the overall absorption coefficient. The laser power intensity distribution before and after the plasma absorption is computed at a plane 2.6 mm above the workpiece surface. It is found that 13% of the reduction occurs in the incident laser output power intensity at this plane in the plasma.     

Cutting by a high power laser at a long distance without an assist gas for dismantling

As the applications of laser processing progress, new fields of use are being investigated, including dismantling with power lasers. To fulfil our dismantling requirements we propose a new laser method that we have called the laser dismantling (LD) process. This cutting method uses a high-power laser at a long distance, without an assist gas, and with a focal length of the system of 1 m to 10 m. Precision and accuracy in the process are not the same as for laser cutting for production and assembly. The first application of the laser dismantling process, on which we demonstrate our method, is the dismantling of obsolete nuclear plants with remote controlled, or automatic, robots in irradiated environments. For our demonstrator, the beam from a Nd:YAG laser was focused by a multimode optical fibre. The objectives of this paper are: to discuss the criteria for determining the theoretical feasibility of LD; to discuss issues related to future industrial implementation by introducing the process's basic principles; and to compare LD with classical laser processing, which differs not only in the consideration of cutting quality and speed, but also in the cutting irregularities that could be accepted.     

Mathematical modelling of an Nd: YAG laser: peak power and pulse width

In a previous paper, the author presented an empirically derived mathematical model of a pulse pumped laser which allowed calculation of the output energy from the laser—from the mirrors-only resonator to the fully assembled Q-switched state. In this paper the model is developed further by deriving an expression for the peak output power. New coupling and loss time constant are stated. The validity of the model is demonstrated by the ability of calculate the laser parameters from a measured pulse shape, knowing the rod dimensions and the pump energy at which the pulse was measured.     

High power Nd:YAG slab laser sidepumped by diode laser array

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Transmission properties of all-silica fibres for high-power Nd:YAG lasers

The transmission properties of two different types of all-silica fibres, step index and graded-index fibres, are investigated using a multimode high-power cw Nd:YAG rod laser. The reflections from the fibre surface back into the laser cavity affect the laser power and the beam parameters, waist diameter and divergence. A set-up for simultaneously measuring these values in front of and behind a fibre is presented. The maximum laser power transmitted by fibres is dependent upon the beam parameters, the fibre design and the surface conditions. An upper limit for the maximum divergence which can be coupled into a fibre with a certain numerical aperture without loss is given. The beam parameters behind the fibre, related to the input parameters, are presented.     

Reduction of beam divergence of a thermal lensing insensitive Nd:YAG laser

In a previous paper we proposed an active resonator designed especially for Nd:YAG lasers. The beam generated by this resonator had a flat-top beam profile and a divergence that was insensitive to the thermal lensing effect. The beam generated by this resonator was, however, too divergent to be focused into a fibre. With the help of numerical simulations, we have found a way to reduce this divergence, and to render the far-field of the laser beam fine enough to be coupled into a fibre with 600 μm core and NA = 0.2.     

Analysis and modelling of high power laser beam position sensing

A new theory using a rotating convex mirror to reflect part of a laser beam into a pyro-electric sensor in order to detect the high power laser beam position is addressed in this research. The experiments were performed on the Nd: YAG laser system. The sensing performance was tested with the average power up to 50 W, duration time 1 ms and duration frequency 360 Hz. The results of static experiments show that the sensing system designed and developed in this research may correctly detect the Nd: YAG laser beam with the system parameters varied. The results of dynamic experiments show that the average sensing accuracy of beam position is up to 90% with a wide variation of laser system parameters.     

An automatically controlled Nd:YAG laser with variable pulse width

The development of a high-energy, pulsed Nd:YAG laser system for materials-processing and medical applications is reported here. A variable pulse width in the range of 0.3–10 ms and a variable pulse repetition rate up to 50 pps are provided. An automatic operation system using a microprocessor-based driver/ controller enables safe operation of the laser system and automatic material processing when integrated in a laser/robot system.     

Nd:YAG laser delivery system utilizing GRIN rod lenses as focusing-collimating optics

A sequential electromechanical system capable of delivering the radiation coming from a medium power Nd:YAG laser to an array of multimode optical fibres is presented. The system utilizes GRIN-rod microlenses as focusing-collimating optics. A focusing unit positioned at the fibre distal-end, which can work in two different configurations, has also been designed and constructed.     

Relative performance of a 1.06 μm laser with various Nd doped crystals

Laser emission at 1.06 μm has been observed from Nd: doped YAG, KGW and SGGM crystals pumped by a linear Xe-gas filled flashlamp in the input energy range of 1–25 J. The intrinsic slope efficiency has been determined to be 8.84%, 2.25% and 1.25% from Nd: KGW (Nd: 3% atomic), Nd: YAG (Nd: 1.0% atomic) and Nd: SGGM (Nd: 4% atomic), respectively.     

A Nd: YAG laser with a flat-top beam profile and constant divergence

An active resonator designed especially for Nd: YAG lasers is proposed. The expected performance of this resonator is the generation of a laser beam with a flat-topped distribution and a divergence invariant under the thermal lensing effect, such that it could be applied to surface treatment without special optical or mechanical elements. The beam quality and efficiency of such a resonator are presented and discussed in comparison with a parallel-plane resonator as adopted by most industrial Nd: YAG lasers.     

A kilowatt level diode-side-pumped QCW Nd:YAG ceramic laser

We demonstrate a kilowatt level Quasi-continuous-wave (QCW) diode-side-pumped Nd:YAG ceramic laser at 1064 nm. The laser system adopts a master oscillator power amplifier scheme (MOPA). The master oscillator contains two diode-pumped laser modules. Under the pump power of 2000 W, an output power of 686 W was obtained. After amplified by an identical ceramic laser module, a maximum output power of 1020 W was obtained under a total incident pump power of 3433 W, corresponding to an optical-optical conversion efficiency of 29.7%. At the maximal output power, the repetition frequency was measured to be 1 kHz and the pulse width was 114 μs. To the best of our knowledge, this is the first time to realize QCW side-pumped Nd:YAG ceramic laser system with output power above 1 kW.     

透明Nd:YAG激光陶瓷的研究与应用进展

综述了近年来,特别是近两年来国内外透明Nd:YAG激光陶瓷的研究与应用情况,重点对我国的透明Nd:YAG激光陶瓷研究近况进行了分析,指出了透明Nd:YAG激光陶瓷是目前陶瓷研究的热点。提出了透明Nd:YAG激光陶瓷研究中尚存在的问题和我国在此领域中与国外研究的巨大差距。分析指出:我国研究透明Nd:YAG激光陶瓷的各个院所之间,应该强强联合,优势互补,对该领域的理论和制备工艺进行更为深入的研究。     

Limit analysis for laser removal of micron contaminant colloidal silicon dioxide particles from the super-smooth optical glass substrate by pulse Nd:YAG laser

Multimode Nd:YAG pulse laser was applied to remove micron and submicron particles by vaporizing a thin paint film pre-coated on super-smooth optical substrate surface. By analyzing the poor absorption of the optical glass substrate to the irradiative Nd:YAG pulse laser, the removal mechanism of contaminated colloidal particles from the super-smooth surface through vaporization of a volatile solid film is described. A limit analysis was proposed to determine the lower and the upper threshold of laser fluence for cleaning the SiO₂ contaminants from super-smooth K8 optical substrate. Relevant experiments on laser cleaning of micron-polishing particles from super-smooth K8 optical substrate confirmed the usefulness of this method in assisting the selection of effective cleaning fluence for accomplishing high cleanliness, which was in a range of 80–90% of the predicted upper threshold.     

Diffraction losses of Nd:YAG and Yb:YAG laser crystals

Based on space-dependent rate equations, the lowest threshold input power for a diode end-pumped solid-state laser is obtained for the pump spot size w_p→0. However, as the pump beam waist is decreased, the thermally induced effects in the laser rod would be very high. Diffraction losses caused by radial and tangential variations of refractive index have been analyzed and compared for the Nd : YAG and the Yb : YAG at room temperature (300 K) and liquid-nitrogen temperature (77 K).