Free-running Er Research Articles

High average power (105 W) Fe:ZnSe laser pumped by radiation of laser diode side-pumped Er:YAG lasers.

A high average power re-frequency operation Fe:ZnSe laser using laser diode side-pumped free-running Er:YAG lasers as activating sources is presented. Two pieces of subsurface layer doped Fe:ZnSe polycrystal are adoptive in a reflective resonator configuration and face-cooled by liquid nitrogen. A maximal Fe:ZnSe laser power of 105 W at a wavelength of 4.1 μm is achieved upon pumping by ten home-made Er:YAG lasers with fiber coupled output working at a frequency of 250 Hz and a pulse duration of ∼420 μs. Corresponding to the maximum Fe:ZnSe laser power, the optical-optical efficiency and slope efficiency with respect to the absorbed pump power are 43% and 44% respectively. The beam quality factor M2 is measured to be 3.4. To the best of our knowledge, it is the highest output average power of an Fe:ZnSe laser reported.

Quantitative spectroscopic characterization of laser-induced effects on oil paint films using 213, 266, or 2940 nm

Removal of aged varnish layers from easel paintings is a challenging task requiring methodologies capable of preventing any alteration of the underlying painted surface. Laser-ablation approaches using the fourth and fifth harmonics of a Q-Switched Nd:YAG (266 and 213 nm, 15 ns) and Free-Running Er:YAG laser (2940 nm, 160 µs pulses) can allow for very thin removal rates but their discrimination of the underlying paint film and self-termination potential are very limited. Thus, there is a need to develop suitable analytical methods for promptly identifying the possible occurrence of any undesired effect on the paint layer beneath the varnish, which is intentionally or unintentionally irradiated during the laser thinning treatment. Such side effects depend on the irradiation parameters and the specific material features of the pictorial film. Linseed oil paint models were prepared using lead white, vermilion, chrome yellow, ultramarine, Prussian blue and zinc white. Two distinct pigment volume concentrations (PVC) were used to investigate the dependence of the laser-induced alterations on oil binder volume ratio: low PVC (gloss paint) and high PVC (matte paint). After determining threshold exposure limits for each laser wavelength, laser-induced effects slightly above the threshold fluence were qualitatively and quantitatively assessed using FTIR and µ-Raman spectroscopies. Results showed that IR-reflectance bands of glossy and matte films exhibited appreciable shifts and distortion upon laser irradiation while reductions of the signal-to-background ratio were observed in µRaman spectra. The developed method, based on the processing of a number of spectral acquisitions, enabled for the first time the quantitative assessment and comparison of the corresponding undesired laser-induced effects in the outermost painted layer.

Er:YAG Laser Cleaning of Painted Surfaces: Functional Considerations to Improve Efficacy and Reduce Side Effects

The restoration of paintings always involves the removal of darkened superficial layers, which are mainly due to dust deposition and aged varnishes. As cleaning is an irreversible and invasive treatment, physical methods (i.e., laser cleaning) instead of chemical ones are frequently suggested to reduce side effects on pictorial layers. Among the most employed laser systems, the free-running Er:YAG laser is considered very suitable for fine arts cleaning. This laser works at 2.94 μm, at which only –OH and –NH bonds in molecules are excited. This character can become a disadvantage when pigments with these functional groups are present. To understand the potential of the Er:YAG laser in such situations or in the presence of degradable pigments, the effectiveness of varnish removal from paintings prepared with egg yolk as the binder and cinnabar and lead white as the pigments were systematically investigated. Different cleaning conditions were used, and a hyperspectral sensor was innovatively used as a rapid, in situ and non-destructive technique to assess the effects of laser ablation, besides microscopic analysis. Though results obtained show all these pigments are sensitive to this laser radiation, satisfactory cleaning can be achieved without damaging the pictorial layer. The best cleaning conditions were 0.5 W of power (50 mJ and 10 Hz for energy and frequency), with 2-propanol as the wetting agent.

In vitro study on nanosecond-pulsed Q-switched Er:YAG laser-induced selective removal for caries dentin

Since 2000, Federation Dentaire Internationale has advocated for “minimal intervention” dental treatments. The dental field has demanded less-invasive caries treatment methods. However, recent caries treatments using a microsecond-pulsed free-running Er:YAG laser with a pulse duration of 200–300 μs have issues with non-selectivity for caries due to the thermal effect caused by the laser. To improve the selectivity by reducing the thermal effect, the ablation characteristics of a nanosecond-pulsed Er:YAG laser are investigated. A Q-switched Er:YAG laser with a rotating mirror Q-switching system attached to a flash lamp-pumped Er:YAG laser and a free-running Er:YAG laser were used. The pulse duration of the Q-switched Er:YAG laser was 80–130 ns, while that of the free-running Er:YAG laser was around 200 μs. The pulse repetition rate of the free-running Er:YAG laser was constant at 10 Hz, but that of the Q-switched Er:YAG laser fluctuated between 10 and 23 Hz. The fluence was 6, 10, or 14 J/cm2. The samples were bovine sound dentin slices and caries models. Caries models were prepared by immersing sound dentin slice samples into a 0.1-M lactic acid solution for 24 h at 37 °C. The samples were irradiated with the two lasers horizontally without water spray for 1, 2, or 4 s. Laser irradiation experiments indicate that the selectivity towards caries occurs at the fluence of 6 J/cm2 without water spray. The ablation depth of a demineralized dentin slice by the Q-switched Er:YAG laser is about three times that by the free-running Er:YAG laser at the fluence of 10 J/cm2. In addition, the Q-switched Er:YAG laser suppresses the thermal effect reducing the range of the thermal melting area. Caries dentin may be selectively removed without water spray using the Q-switched Er:YAG laser with suppressing dental pulp necrosis because the temperature increase is below 5 °C. The ablation depths by the Q-switched Er:YAG are about three times larger than those using the free-running Er:YAG laser.

Nd:YAG and Er:YAG laser cleaning to remove Circinaria hoffmanniana (Lichenes, Ascomycota) from schist located in the Côa Valley Archaeological Park

Lichens are part of the dominant colonizer crusts of rock-art on schist in the Côa Valley Archaeological Park (NE Portugal). Among the several species observed at the site, Circinaria hoffmanniana is one of the most extensive on the rock surfaces. Given the historic value of the park's open-air prehistoric rock-art, which has been designated UNESCO World Heritage, an effective, precise, selective and controllable cleaning method for the engravings is required, in order to extract the lichen while avoiding any grain extraction and/or physical changes in the surface. Based on these requirements, 2 different lasers commonly used in the conservation and restoration of cultural heritage were applied in one scan, with the aim of obtaining a satisfactory level of cleaning as quickly as possible. The 2 lasers were: a Q-switched Nd:YAG (pulse duration of 6 ns) and a free-running Er:YAG (pulse duration of 250 μs), working at IR radiation of 1064 nm and 2940 nm respectively. The cleaning was evaluated by assessing the extraction rate and the damage induced on the schist using a multianalytical methodology based on stereomicroscopy, colour measurements in CIELAB and CIELCH colour spaces, Fourier transform infrared spectroscopy, Raman spectroscopy and scanning electron microscopy. Further studies should be performed to evaluate the tertiary bioreceptivity of the treated surface.

Study of a room temperature, monocrystalline Fe:ZnSe laser, pumped by a high-energy, free-running Er:YAG laser

The energy and spectral characteristics of an Fe:ZnSe laser, pumped by 300 µs pulses of a 2.94 µm flash-lamp pumped free-running Er:YAG laser, are studied in the temperature range of 5 °C–18 °C. At 5 °C, the output energy reaches 1.6 J with a slope efficiency of 25% with respect to the absorbed pump energy. In a nonselective cavity, the emission spectrum demonstrates a shift to the longwave region with a decrease in the transmittance of the output coupler.

Erbium:Ytterbium co-doped large-core fiber laser with continuous wave output power over 20 W at 1609.02 nm

We report a high-power cladding-pumped large-core Er,Yb co-doped fiber laser operating at 1609.02 nm. A maximum output power of 20.52 W was obtained for the launched pump power of 99.5 W, corresponding to a slope efficiency of −23.5% with respect to the launched pump power. This is, to the best of our knowledge, the highest output power reported from free-running Er,Yb co-doped laser oscillator architecture at 1609.02 nm long wavelength.

High-energy diode side-pumped Er:LiYF4 laser.

This work presents a novel diode side-pumping scheme for Er:YLF with a characteristic laser emission at 2.81μm. Average output powers greater than 10W and corresponding pulse energies exceeding 100mJ with a high slope efficiency of 18.7% are achieved. The Er:YLF laser efficiently operates at room-temperature cooling water and maintains a good beam quality of M2<12 in the major axis and M2<6 in the minor axis of the beam profile. The laser cavity is studied by means of Findlay-Clay analysis, and the operation at characteristic pump parameters is investigated. In addition, a dynamic laser rate equation simulation is introduced for quantitative and qualitative comparison. This compact and scalable free-running Er:YLF laser provides an effective building block for a subsequent Q-switching concept.

Mid-infrared Er:ZBLAN fiber laser reaching 3.68 μm wavelength

We report a continuous-wave Er:ZBLAN fiber laser with the operation wavelength reaching 3.68 μm. The mid-infrared Er:ZBLAN fiber laser is pumped with the dual-wavelength sources consisting of a commercial laser diode at 970 nm and a homemade Tm-doped fiber laser at 1973 nm. By increasing the launched pump power at 1973 nm, the laser wavelength can be switched from 3.52 to 3.68 μm. The maximum output power of 0.85 W is obtained with a slope efficiency of 25.14% with respect to the 1973 nm pump power. In the experiment, the laser emission at 3.68 μm is obtained with a significant power of 0.62 W, which is the longest emission wavelength in free-running Er:ZBLAN fiber lasers.

Hard tooth tissue removal by short and long Er:YAG or Er,Cr:YSGG mid-infrared laser radiation

Hard dental tissue removal by laser radiation is an alternative treatment to conventional dental-drilling procedures. The advantages of this therapy are fast and localized treatment of hard dental tissue and painlessness. The most effective systems for those purposes are Er-lasers generating radiation at wavelengths of around 3 μm. The aim of this study was qualitative and quantitative examination of human dentin and ivory tissue removal by pulsed free-running (FR) and Q-switched (QSW) Er:YAG and Er,Cr:YSGG laser radiations. From the obtained results it follows that generally Er:YAG laser has lower threshold for the tissue removal in both FR and QSW regimes. Furthermore, the FR Er:YAG and Er,Cr:YSGG radiation can be effective for both dentin and ivory ablation and can prepare smooth cavities without side effects. The QSW regime is useful preferably for precise ablation of a starting tooth defect and for the part of the tooth very close to the gum. This regime is excellent for micro-preparation or for tooth treatment of children.

Investigation of Fe : ZnSe laser in pulsed and repetitively pulsed regimes

The characteristics of a Fe : ZnSe laser pumped by a single-pulse free-running Er : YAG laser and a repetitively pulsed HF laser are presented. An output energy of is achieved in the case of liquid-nitrogen cooling of the : ZnSe active laser element longitudinally pumped by an Er:YAG laser with a pulse duration of and an energy up to . The laser efficiency with respect to the absorbed energy is 47%. The output pulse energy at room temperature is . The decrease in the output energy is explained by a strong temperature dependence of the upper laser level lifetime and by pulsed heating of the active element. The temperature dependence of the upper laser level lifetime is used to determine the pump parameters needed to achieve high pulse energies at room temperature. Stable repetitively-pulsed operation of the : ZnSe laser at room temperature with an average power of and a maximum pulse energy of is achieved upon pumping by a train of HF laser pulses with a repetition rate of .

Resonant Infrared and Ultraviolet Matrix-Assisted Pulsed Laser Evaporation of Titanium Oxide/Graphene Oxide Composites: A Comparative Study

Pure titanium dioxide (TiO2) and graphene oxide (GO) as well as TiO2/GO composite structures were grown by matrix-assisted pulsed laser evaporation (MAPLE) in a controlled oxygen atmosphere. The MAPLE target dispersions were prepared using distilled water as solvent matrix, with TiO2 nanoparticles (NPs) and GO platelets serving as host materials. Two laser sources, a free-running IR Er:YAG (λ = 2940 nm, τfwhm ≅ 350 μs, ν = 10 Hz) and a UV KrF* excimer (λ = 248 nm, τfwhm ≅ 25 ns, ν = 10 Hz) laser, were used for the transfer and immobilization experiments by infrared (IR)- and ultraviolet (UV)-MAPLE, respectively. The potential physical mechanisms implied in both the IR- and UV-MAPLE processes are discussed, based on numerical simulations of temperature evolution of the distilled water matrix, TiO2 NPs, and GO platelets. Our results demonstrate the effectiveness of IR- and UV-MAPLE processes for the immobilization of nanoentities onto solid substrates. During IR-MAPLE, the laser radiation is primarily absor...

High peak power Q-switched Er:YAG laser with two polarizers and its ablation performance for hard dental tissues.

An electro-optically Q-switched high-energy Er:YAG laser with two polarizers is proposed. By using two Al(2)O(3) polarizing plates and a LiNbO(3) crystal with Brewster angle, the polarization efficiency is significantly improved. As a result, 226 mJ pulse energy with 62 ns pulse width is achieved at the repetition rate of 3 Hz, the corresponding peak power is 3.6 MW. To our knowledge, such a high peak power has not been reported in literature. With our designed laser, in-vitro teeth were irradiated under Q-switched and free-running modes. Results of a laser ablation experiment on hard dental tissue with the high-peak-power laser demonstrates that the Q-switched Er:YAG laser has higher ablation precision and less thermal damage than the free-running Er:YAG laser.

Absolute dual-comb spectroscopy at 1.55 μm by free-running Er:fiber lasers

We report on a compact scheme for absolute referencing and coherent averaging for dual-comb based spectrometers, exploiting a single continuous-wave (CW) laser in a transfer oscillator configuration. The same CW laser is used for both absolute calibration of the optical frequency axis and the generation of a correction signal which is used for a real-time jitter compensation in a fully electrical feed-forward scheme. The technique is applied to a near-infrared spectrometer based on a pair of free-running mode-locked Er:fiber lasers, allowing to perform real-time absolute-frequency measurements over an optical bandwidth of more than 25 nm, with coherent interferogram averaging over 1-s acquisition time, leading to a signal-to-noise ratio improvement of 29 dB over the 50 μs single shot acquisition. Using 10-cm single pass cell, a value of 1.9 × 10−4 cm−1 Hz−0.5 noise-equivalent-absorption over 1 s integration time is obtained, which can be further scaled down with a multi-pass or resonant cavity. The adoption of a single CW laser, together with the absence of optical locks, and the full-fiber design makes this spectrometer a robust and compact system to be employed in gas-sensing applications.

Study of a 2-J pulsed Fe:ZnSe 4-μm laser

We report the results of a study of a pulsed Fe:ZnSe laser for temperatures ranging from 85 to 295 K. With a free-running Er:YAG laser, operating at 2.94 μm, as a pump source, a maximum output energy of 2.1 J at 85 K was produced at a wavelength of 4.1 μm. The optical-to-optical efficiency of the laser was 35% at maximum energy while the maximum absorbed energy slope efficiency was as high as 51%. By operating at 245 K (a temperature which can be easily reached using thermoelectric coolers) we achieved 1.3 J of output energy from an Fe:ZnSe laser, with a slope efficiency of 29% (23% optical-to-optical efficiency). The maximum output energy reached 42 mJ at room temperature (295 K). The temperature dependence of the lifetime of the 5T2 energy level of Fe2+ in ZnSe was studied in the 275–365 K temperature range. The activation energy for nonradiative relaxation was found to be 2235 cm−1.

Comparison between laser-induced photoemissions and phototransmission of hard tissues using fibre-coupled Nd:YAG and Er3+-doped fibre lasers

During pulsed laser irradiation of dental enamel, laser-induced photoemissions result from the laser-tissue interaction through mechanisms including fluorescence and plasma formation. Fluorescence induced by non-ablative laser light interaction has been used in tissue diagnosis, but the photoemission signal accompanying higher power ablative processes may also be used to provide real-time monitoring of the laser-tissue interaction. The spectral characteristics of the photoemission signals from normal and carious tooth enamel induced by two different pulsed lasers were examined. The radiation sources compared were a high-power extra-long Q-switched Nd:YAG laser operating at a wavelength of 1,066 nm giving pulses (with pulse durations in the range 200-250 μs) in the near infrared and a free-running Er(3+)-doped ZBLAN fibre laser operating at a wavelength near 3 μm with similar pulse durations in the mid-infrared region. The photoemission spectra produced during pulsed laser irradiation of enamel samples were recorded using a high-resolution spectrometer with a CCD array detector that enabled an optical resolution as high as 0.02 nm (FWHM). The spectral and time-dependence of the laser-induced photoemission due to thermal emission and plasma formation were detected during pulsed laser irradiation of hard tissues and were used to distinguish between normal and carious teeth. The use of these effects to distinguish between hard and soft biological tissues during photothermal ablation with a pulsed Nd:YAG laser or an Er fibre laser appears feasible. The real-time spectrally resolved phototransmission spectrum produced during pulsed Nd:YAG laser irradiation of human tooth enamel samples was recorded, with a (normalized) relative transmission coefficient of 1 (100%) for normal teeth and 0.6 (60%) for the carious teeth. The photoemission signal accompanying ablative events may also be used to provide real-time monitoring of the laser-tissue interaction.

Quantitative morphological evaluation of laser ablation on calculus using full-field optical coherence microscopy

The quantitative morphological evaluation at high resolution is of significance for the study of laser-tissue interaction. In this paper, a full-field optical coherence microscopy (OCM) system with high resolution of ∼2 μm was developed to investigate the ablation on urinary calculus by a free-running Er:YAG laser. We studied the morphological variation quantitatively corresponding to change of energy setting of the Er:YAG laser. The experimental results show that the full-field OCM enables quantitative evaluation of the morphological shape of craters and material removal, and particularly the fine structure. We also built a heat conduction model to simulate the process of laser-calculus interaction by using finite element method. Through the simulation, the removal region of the calculus was calculated according to the temperature distribution. As a result, the depth, width, volume, and the cross-sectional profile of the crater in calculus measured by full-field OCM matched well with the theoretical results based on the heat conduction model. Both experimental and theoretical results confirm that the thermal interaction is the dominant effect in the ablation of calculus by Er:YAG laser, demonstrating the effectiveness of full-field OCM in studying laser-tissue interactions.

Mid-infrared radiation transmission through fluoride glass multimode optical fibers

Measurement of the transmission laser radiation from Q-switched and free-running Er:YAG lasers operating at 2.94 μm through fluoride glass multimode optical fibers of 448, 465 and 620 μm core diameter were performed. Attenuation measurements were obtained as a function of the laser energy input, of the bending angle and of the radius of curvature. The fibers exhibited low attenuation under straight and bent conditions. The output beam quality was also studied using the appropriate beam profiler. The output beam profiles showed a central multi-spiked energy distribution.

Chemical, morphological and chromatic behavior of mural paintings under Er:YAG laser irradiation

Several pigments (malachite CuCO3⋅Cu(OH)2, azurite 2CuCO3⋅Cu(OH)2, yellow ochre (goethite α-FeOOH, gypsum CaSO4⋅2H2O), St. John’s white CaCO3 formed from slaked lime) and respective mural paintings specimens were subjected to the free-running Er:YAG laser radiation in order to study their damage thresholds, in a broad range of laser fluences, both in dry and wet conditions. The specimens’ damage thresholds were evaluated by spectroscopic methods, colorimetric measurements and microscopic observation. The pigments containing –OH groups were found to be more sensitive than St. John’s white; hence the most sensitive paint layers in dry conditions are those containing malachite, azurite (both 1.3 J/cm2) and yellow ochre (2.5 J/cm2) as compared to the ones containing St. John’s white (15.2 J/cm2). The presence of wetting agents (w.a.) attenuated the pigments chemical alteration. The damage thresholds of all the paint layers, in presence of w.a., were found to be around 2.5 J/cm2. The alteration was caused by thermo-mechanical damage and by binding medium ablation of a fresco and a secco prepared specimens, respectively.

Investigation of the Er:YAG Laser at 2.94 μm to Remove Lichens Growing on Stone

The analysis of the surface ablation of the removal of lichens from stone by use of a free-running pulsed Er:YAG laser (erbium-doped yttrium, aluminium, garnet crystal) at a wavelength of 2.94 μm demonstrated the complete destruction of the lichen cell wall. In this paper these results are confirmed, and experiments are described which seek to determine the physical/chemical mechanism of the ablation process using pyrolysis gas chromatography–mass spectrometry (pyrolysis GC-MS), highperformance liquid chromatography (HPLC), Fourier transform infrared spectroscopy (FTIR) and fluorescence microscopy.