Development of a fiber-optic laser delivery system capable of delivering 213 and 266 nm pulsed Nd:YAG laser radiation for tissue ablation in a fluid environment

Ultraviolet (UV) lasers have the capability to precisely remove tissue via ablation; however, due to strong absorption of the applicable portion the UV spectrum, their surgical use is currently limited to extraocular applications at the air/tissue boundary. Here we report the development and characterization of a fiber-optic laser delivery system capable of outputting high-fluence UV laser pulses to internal tissue surfaces. The system has been developed with a view to intraocular surgical applications and has been demonstrated to ablate ocular tissue at the fluid/tissue boundary. The fifth (213?nm) and fourth(266?nm) harmonics of a Nd:YAG laser were launched into optical fibers using a hollow glass taper to concentrate the beam. Standard and modified silica/silica optical fibers were used, all commercially available. The available energy and fluence as a function of optical fiber length was evaluated and maximized. The maximum fluence available to ablate tissue was affected by the wavelength dependence of the fiber transmission; this maximum fluence was greater for 266?nm pulses (8.4?J/cm2) than for 213?nm pulses (1.4?J/cm2). The type of silica/silica optical fiber used did not affect the transmission efficiency of 266?nm pulses, but transmission of 213?nm pulses was significantly greater through modified silica/silica optical fiber. The optical fiber transmission efficiency of 213?nm pulses decreased as a function of number of pulses transmitted, whereas the transmission efficiency of 266?nm radiation was unchanged. Single pulses have been used to ablate fresh porcine ocular tissue. In summary, we report a method for delivering the fifth (213?nm) and fourth (266?nm) harmonics of a Nd:YAG laser to the surface of immersed tissue, the reliability and stability of the system has been characterized, and proof of concept via tissue ablation of porcine ocular tissue demonstrates the potential for the intraocular surgical application of this technique.     

Etch rate and spectroscopic ablation studies of Er:YAG laser-irradiated dentine

The interaction of a 200-mus Er:YAG laser at 2.94 mum with human molar dentine has been studied by ablation depth rate measurements as well as time-resolved and optical multichannel analyzer emission spectroscopy. Ablation rates indicate a threshold fluence of ~5.2 J cm(-2) for significant material removal with a low-fluence (<20 J cm(-2)) effective optical absorption coefficient of ~700 cm(-1). Deviation from a Beer's law dependence is significant in the range ~20 to ~60 J cm(-2) and indicates a maximum effective plume absorption of ~1200 cm(-1) at ~40 J cm(-2), coinciding with the appearance of strong line and broadband optical emission in the visible region. Time-of-flight emission measurements yield maximum species-resolved ablation velocities of up to ~1.2 x 10(6) cm s(-1), enabling calculation of plasma temperatures. The results suggest that etch-rate characteristics are driven by changes in plume absorption dynamics, which have a strong dependency on incident laser fluence.     

高模量碳纤维增强树脂基复合材料的皮秒激光加工阈值特性

对聚丙烯腈基高模量碳纤维/改性氰酸酯树脂复合材料(M55/BS-4)和一种沥青基高导热碳纤维/树脂基复合材料(K600/5418)的皮秒激光加工阈值和形貌特性进行了研究。通过面积外延法测定并比较了这两种碳纤维复合材料的近红外皮秒激光加工阈值及其阈值孵化效应,并预测了两种复合材料的单脉冲阈值;分析了入射能量通量(0.7~25 J/cm2)及光束扫描速度(0.2~5 m/s)对切口质量的影响规律。结果表明,碳纤维热导率的巨大差异导致不同碳纤维复合材料的加工阈值及形貌存在明显定量差距。使用可获得的最高扫描速度(5 m/s)和3.2倍(~8 J/cm2)单脉冲阈值的加工参数,可使材料的碳纤维和树脂几乎协同去除,加工形貌上表现为切缝入口宽度均匀、切割边缘整齐。使用更高扫描速度并配以合适的加工能量有望进一步提高加工质量。      

Optical evidence for reactive processes when embedding Cu nanoparticles in Al(2)O(3) by pulsed laser deposition

The optical response of nanocomposite thin films formed by Cu nanoparticles (NPs) embedded in amorphous aluminium oxide (Al(2)O(3)) prepared by pulsed laser deposition (PLD) in vacuum is studied in order to investigate the possible existence of reactive processes on the Cu NPs during their covering with Al(2)O(3). The study is performed as a function of the laser fluence on the Al(2)O(3) target (0.6-4.6?J?cm(-2)), while the laser fluence for Cu ablation is kept constant (1.8?J?cm(-2)). The structural analysis of the films shows that they are formed by a high density of NPs with average dimensions in the 4.9-5.9?nm range. The optical response of the films has been followed in situ by real-time reflectivity measurements at 633?nm and after deposition by transmission measurements as a function of wavelength around the surface plasmon resonance (SPR). For low laser fluences on the Al(2)O(3) target, the absorption spectrum is dominated by a well-defined SPR absorption band at 1.9?eV. As the laser fluence is increased, the intensity of the absorption band associated with the SPR decreases and shifts to 2.1?eV. The films deposited at low fluences contain metallic Cu NPs and, as the laser fluence increases sputtering of Cu from the NPs and mixing of the species from the Al(2)O(3) deposition with the Cu from the NPs surface takes place. The latter process leads to the formation of an Al-Cu oxide cover on the Cu NPs. The present results provide evidence for mixing of species from the host and Cu at the surface of the NPs, and it is shown how the degree of mixing depends on the laser fluence used to ablate the Al(2)O(3) host target.     

Spontaneous emission from the C3 radical in carbon plasma

Spontaneous emission measurements are discussed for the Swings transitions of the C(3) radical in laser-generated graphite plasma, and the spectroscopy of the C(3) radical in carbon vapor and plasma is summarized. A review is given of some theoretical calculations and emission spectroscopic investigations are presented. Time-averaged, laser-induced optical breakdown spectra are reported from Nd:YAG laser generated graphite microplasma. In 200-300 Torr of argon and helium, and depending on the specific experimental configuration, a weak emission continuum is observed centered at 400 nm when using a laser fluence of typically 1 J/cm(2). Such continua were not detected in our previous experiments using focused laser radiation. The possibilities for the origin of this continuum are considered.     

Fabrication, characterization and mode locking application of single-walled carbon nanotube/polymer composite saturable absorbers

We present the fabrication of a high optical quality single-walled carbon nanotubes (SWNTs) polyvinyl alcohol (PVA) composite film. The composites demonstrate strong saturable absorption at ～1.5 μm, the spectral range for optical communications. By measuring the nonlinear transmission of a sub-picosecond pump pulse through the film, we were able to deduce a saturation fluence of ～13.9 μJ/cm² and a modulation depth ～16.9% (in absorption) at a high pulse fluence ～200 μJ/cm². Transient saturable absorption is investigated by measuring the transmitted autocorrelation traces at various incident power levels. Observed side-peak suppression indicates a fast recovery time on the scale of ～1 ps for our saturable absorber devices. Furthermore, we use these SWNT-PVA composite saturable absorbers as mode-lockers in an Er³⁺ fiber ring laser and achieve ～560 fs pulse generation with good jitter performance and long term stability. The laser performance is also associated with the parameters of our SWNT based saturable absorber.     

Laser-induced incandescence: excitation intensity

Assumptions of theoretical laser-induced incandescence (LII) models along with possible effects of high-intensity laser light on soot aggregates and the constituent primary particles are discussed in relation to selection of excitation laser fluence. Ex situ visualization of laser-heated soot by use of transmission electron microscopy reveals significant morphological changes (graphitization) induced by pulsed laser heating. Pulsed laser transmission measurements within a premixed laminar sooting flame suggest that soot vaporization occurs for laser fluences greater than 0.5 J/cm(2) at 1064 nm. Radial LII intensity profiles at different axial heights in a laminar ethylene gas jet diffusion flame reveal a wide range of signal levels depending on the laser fluence that is varied over an eight fold range. Results of double-pulse excitation experiments in which a second laser pulse heats in situ the same soot that was heated by a prior laser pulse are detailed. These two-pulse measurements suggest varying degrees of soot structural change for fluences below and above a vaporization threshold of 0.5 J/cm(2) at 1064 nm. Normalization of the radial-resolved LII signals based on integrated intensities, however, yields self-similar profiles. The self-similarity suggests robustness of LII for accurate relative measurement of soot volume fraction despite the morphological changes induced in the soot, variations in soot aggregate and primary particle size, and local gas temperature. Comparison of LII intensity profiles with soot volume fractions (f(v)) derived by light extinction validates LII for quantitative determination of f(v) upon calibration for laser fluences ranging from 0.09 to 0.73 J/cm(2).     

Soot particle disintegration and detection by two-laser excimer laser fragmentation fluorescence spectroscopy

A two-laser technique is used to study laser-particle interactions and the disintegration of soot by high-power UV light. Two separate 20 ns laser pulses irradiate combustion-generated soot nanoparticles with 193 nm photons. The first laser pulse, from 0 to 14.7 J/cm2, photofragments the soot particles and electronically excites the liberated carbon atoms. The second laser pulse, held constant at 13 J/cm2, irradiates the remaining particle fragments and other products of the first laser pulse. The atomic carbon fluorescence at 248 nm produced by the first laser pulse increases linearly with laser fluence from 1 to 6 J/cm2. At higher fluences the signal from atomic carbon saturates. The carbon fluorescence from the second laser pulse decreases as the fluence from the first laser increases, suggesting that the particles fully disintegrate at high laser fluences. We use an energy balance parameter, called the photon/atom ratio, to aid in understanding laser-particle interactions. These results help define the regimes where photofragmentation fluorescence methods quantitatively measure total soot concentrations.     

Gain saturation measurements of ytterbium-doped Sr5(PO4)3 F

We report on the experimental measurement of the saturated gain of Yb(3+):Sr(5)(PO(4))(3)F at the 1047-nm laser line as a function of pump fluence and probe energy. The emission line was accurately modeled as a single homogeneous extraction, yielding values of 6.2 x 10(-20) cm(2) for the emission cross section and 3.3 J/cm(2) for the saturation fluence.     

Application of laser-induced incandescence to the detection of carbon nanotubes and carbon nanofibers

Laser-induced incandescence applied to a heterogeneous, multielement reacting flow is characterized by temporally resolved emission spectra, time-resolved emission at selected detection wavelengths, and fluence dependence. Two-pulse laser measurements are used to further probe the effects of laser-induced changes on the optical signal. Laser fluences above 0.6 J/cm2 at 1064 nm initiate laser-induced vaporization, yielding a lower incandescence intensity, as found through fluence-dependence measurements. Spectrally derived temperatures show that values of excitation laser fluence greater than this value lead to superheated plasmas with temperatures well above the vaporization point of carbon. The temporal evolution of the emission signal at these fluences is consistent with plasma dissipation processes, not incandescence from solidlike structures. Two-pulse laser experiments reveal that other material changes are produced at fluences below the apparent vaporization threshold, leading to nanostructures with different optical and thermal properties.     

Tunable Continuous-Wave Yb:YLF Laser Operation with a Diode-Pumped Chirped-Pulse Amplification System

We have demonstrated a diode-pumped Yb:LiYF(4) (Yb:YLF) laser oscillator for the first time to our knowledge. A wide tuning range of 25 nm and a high slope efficiency of 50% were obtained at a high laser-diode pump intensity of 100 kW/cm(2). Emission and absorption spectra of the Yb:YLF crystal at 8 K represent a wide laser gain width of 38 nm, indicating efficient laser operation similar to that of a four-level laser system with a reduced saturation fluence of 11 J/cm(2).     

Method for determination of the volume of material ejected as molten droplets during visible nanosecond ablation

A novel method is presented for determining the volume of molten material ejected from a substrate as a result of visible pulsed-laser ablation. A 100-microm-wide pulsed-laser light sheet (tau approximately 5 ns, lambda = 532 nm) was used in conjunction with a CCD camera to provide high-speed cross-sectional images of single-pulse ablation of aluminum with a visible nanosecond laser source. Computational analysis of the two-dimensional gray-scale images was used to determine the total volume of material ejected from the substrate in the form of molten droplets. Ablation with dual-wavelength (511- and 578-nm) pulses of 30-ns duration was characterized under various fluence conditions (0-25 J cm(-2)), allowing a quantitative threshold for explosive melt ejection in aluminum to be established at approximately 10 J cm(-2). The temporal evolution of the ejected material showed that, for an incident fluence of approximately 40 J cm(-2), molten-droplet ejection commenced at approximately 400 ns and ceased after approximately 2 micros.     

Numerical and experimental study of the thermal stress of silicon induced by a millisecond laser

A spatial axisymmetric finite element model of single-crystal silicon irradiated by a 1064 nm millisecond laser is used to investigate the thermal stress damage induced by a millisecond laser. The transient temperature field and the thermal stress field for 2 ms laser irradiation with a laser fluence of 254 J/cm(2) are obtained. The numerical simulation results indicate that the hoop stresses along the r axis on the front surface are compressive stress within the laser spot and convert to tensile stress outside the laser spot, while the radial stresses along the r axis on the front surface and on the z axis are compressive stress. The temperature of the irradiated center is the highest temperature obtained, yet the stress is not always highest during laser irradiation. At the end of the laser irradiation, the maximal hoop stress is located at r=0.5 mm and the maximal radial stress is located at r=0.76 mm. The temperature measurement experiments are performed by IR pyrometer. The numerical result of the temperature field is consistent with the experimental result. The damage morphologies of silicon under the action of a 254 J/cm(2) laser are inspected by optical microscope. The cracks are observed initiating at r=0.5 mm and extending along the radial direction.     

Surface damage of extreme-ultraviolet gratings exposed to high-energy 20-fs laser pulses

The damage fluences of gratings for diffraction of ultraviolet radiation, which are used in high-order harmonic generation experiments, have been measured with respect to the fundamental laser beam radiation. We have tested gold and platinum coatings of 40- and 50-nm thickness, respectively, deposited onto fused-silica substrates, after irradiation of high-energy, spatially filtered, 20-fs laser pulses at 780 nm. The damage appears at a fluence of ~0.3 J cm(-2) for gold and at a fluence of ~0.4 J cm(-2) for platinum. Scanning electron microscopy of the irradiated regions revealed different damaging mechanisms for the two coatings.     

Excimer laser-induced microstructural changes of alumina and silicon carbide

Surface treatments with a KrF excimer laser were applied on alumina and silicon carbide ceramic materials. Results on the surface modifications induced by laser were related to the processing parameters: laser fluence (1.8 and 7.5 J/cm²), number of laser pulses (1 to 500), frequency (1 to 120 Hz), pulse duration (25 ns), sample speed under the laser beam and working atmosphere. It was ascertained that alumina can be laser treated under air, while silicon carbide needs an inert atmosphere to avoid surface oxidation. Microstructural analyses of surface and cross section of the laser processed samples evidenced that at low fluence (1.8 J/cm²) the surface of both ceramics is covered by a scale due to melting/resolidification. At high fluence (7.5 J/cm²) there are no continuous scales on the surfaces; material is removed by decomposition/vaporisation and the depth of material removal is linearly dependent on the number of pulses. On alumina surface, a network of microcracks formed, while on silicon carbide different morphologies (flat and rugged areas, deposits of debris and discontinuous thin remelted scales) were detected. The evolution of surface morphology and roughness is discussed with reference to composition, microstructure and physical and optical properties of the two tested ceramics and to laser processing parameters.     

Laser ablative shaping of plastic optical components for phase control

A new scheme for phase control of optical components with laser ablation has been developed. One can ablate the surface shape of optical plastic material coated on a glass plate by using 193-nm laser light to control the transmission wave front. The surface shape is monitored in situ and corrected to attain the desired aberration level. The irradiation fluence is approximately 40 mJ/cm(2), and the ablation depth/pulse is approximately 0.01 mum/pulse for UV-cured resin. A wave-front aberration of 3.0 lambda is reduced to 0.17 lambda for flat surface shaping. For spherical surface generation, an aberration of 2.5 lambda is reduced to 0.2 lambda. The increase in surface roughness is kept within acceptable levels.     

Phototransformation of C60 thin films by UV pulsed laser irradiation: comparative photoacoustic, AFM, and Raman studies

Fullerene C60 films deposited by sublimation were irradiated with Kr-F laser in a wide fluence interval from 15 to 40 mJ/cm2. In situ photoacoustic analysis was applied to study the phase transformation during the irradiation. The results obtained were discussed in conjunction with atomic force microscopy (AFM) and Raman spectroscopy data. It was found that for a irradiation fluence interval from 22 to 30 mJ/cm2, 80% of C60 undergoes photopolymerization (presumably through 2 + 2 cycloaddition). For a laser energy higher than 30 mJ/cm2, a new amorphous carbon phase forms, having a large content of diamond-like, tetra-amorphous carbon (ta-C).     

Measurement of small-signal absorption coefficient and absorption cross section of collagen for 193-nm excimer laser light and the role of collagen in tissue ablation

A 193-nm ArF excimer laser transmission was measured at subablative fluence through varying strength solutions of dissolved collagen, yielding an absorption cross section of 1.14 x 10(-17) cm2 for the peptide bond, which accounts for 96% of the total collagen attenuation that is based on additional transmission measurements through solutions of isolated constituent amino acids. The measured absorption cross sections, in combination with typical corneal tissue composition, yield a predicted corneal tissue absorption coefficient of 16,000 cm(-1). In addition, dry collagen films were prepared and ablation-rate data were recorded as a function of laser fluence. Ablation rates were modeled by use of a Beer-Lambert blow-off model, incorporating a measured ablation threshold and an absorption coefficient that are based on the measured collagen absorption cross section and the film bond density. The measured ablation rates and those predicted by the model were in very good agreement. The experiments suggest that collagen-based absorption coefficients are consistent with predicted corneal tissue ablation rates and previously observed dynamic changes in tissue properties under ablative conditions.     

Optical quality micromachining of glass with focused laser-produced metal plasma etching in the atmosphere

Recent results are reported about the optical quality surface finish obtained on glass substrates with focused laser beam produced metal plasma etching in the atmosphere. The bombardment of high-speed, high-temperature electrons from an underdense plasma on the surface of glass substrates appears to play a dominant role in this process. The effective laser fluence window for this high-quality glass machining on common microscope slides is relatively narrow. With a Corning microslide 2947, and by use of carbon steel as the plasma source, we obtained parameters between 3.5 and 4.5 J/cm2. Above the upper limit, laser-induced optical breakdown occurs in the glass material and leads to the formation of microcracks. Below the lower limit, the process was found to be ineffective. In these experiments highly defined, clean, sharp-edged, 50 x 50 micropit arrays of 15.0-microm diameter with a depth of 3.2 microm and a center-to-center separation of 18.0 microm were fabricated on Corning microslide 2947 glass substrates.     

Desorption/ionization fluence thresholds and improved mass spectral consistency measured using a flattop laser profile in the bioaerosol mass spectrometry of single Bacillus endospores

Bioaerosol mass spectrometry is being developed to analyze and identify biological aerosols in real time. Mass spectra of individual Bacillus endospores were measured with a bipolar aerosol time-of-flight mass spectrometer in which molecular desorption and ionization were produced using a single laser pulse from a Q-switched, frequency-quadrupled Nd:YAG laser that was modified to have an approximately flattop profile. The flattened laser profile allowed the minimum fluence required to desorb and ionize significant numbers of ions from single aerosol particles to be determined. For Bacillus spores, this threshold had a mean value of approximately 1 nJ/microm(2) (0.1 J/cm(2)). Thresholds for individual spores, however, could apparently deviate by 20% or more from the mean. Threshold distributions for clumps of MS2 bacteriophage and bovine serum albumin were subsequently determined. Finally, the flattened profile was observed to increase the reproducibility of single-spore mass spectra. This is consistent with the general conclusions of our earlier paper on the fluence dependence of single-spore mass spectra and is particularly significant because it is expected to enable more robust differentiation and identification of single bioaerosol particles.