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At least 721 records · Page 40Linked to original sources
PubMed2002

Electron-yield enhancement in a laser-wakefield accelerator driven by asymmetric laser pulses.

The effect of asymmetric laser pulses on electron yield from a laser wakefield accelerator has been experimentally studied using >10(19) cm(-3) plasmas and a 10 TW, >45 fs, Ti:Al2O3 laser. The laser pulse shape was controlled through nonlinear chirp with a grating pair compressor. Pulses (76 fs FWHM) with a steep rise and positive chirp were found to significantly enhance the electron yield compared to pulses with a gentle rise and negative chirp. Theory and simulation show that fast rising pulses can generate larger amplitude wakes that seed the growth of the self-modulation instability, and that frequency chirp is of minimal importance for the experimental parameters.

Journal Article↗
PubMed2006

Verruca vulgaris: pulsed dye laser therapy compared with salicylic acid + pulsed dye laser therapy.

BACKGROUND: In recent years, lasers have been used in verruca therapy and some successful results have been obtained in some types of verruca. In some of these laser studies, peeling processes have been used to increase penetration. OBJECTIVE: This study aimed to compare the effects of pulsed dye laser (PDL) therapy with the effects of PDL therapy following salicylic acid (SA) application. The purpose of the SA application is to increase the laser penetration by decreasing the hyperkeratosis of verruca vulgaris. METHODS: This controlled study included 66 lesions from 19 patients enrolled in the study. PDL was applied to 33 lesions following 30% SA application twice a day for 5 days, and the remaining 33 lesions underwent PDL therapy. PDL was administered in both groups at 4-week intervals varying from one to five sessions. In the comparison of these two groups, the changes in verruca dimensions were recorded in width x length x height (mm3), and the results of the therapy were evaluated statistically. RESULTS: In both PDL and SA + PDL groups, the regressions observed in the size of the lesions were statistically significant at the end of the fifth session compared with those observed at the onset of the therapy (P < 0.05). At the end of the fifth session, the difference between the two groups was not statistically significant (P = 0.451); however, the clearance rate in the SA + PDL group was more statistically significant than that in the PDL group after the second session (P = 0.049). The complete clearance rate in the SA + PDL group was 54.6% after the second session, but 66.4% in the PDL group only after the fifth session. Complete clearance in the SA + PDL group was observed after 2.2 sessions, and in the PDL group after 3.1 sessions (P < 0.05). CONCLUSIONS: Although there was no difference between the clearance rate of the SA + PDL group and that of the PDL group after the fifth session, adding SA to PDL decreased the number of sessions to a large extent.

Adolescent↗
PubMed2003

Development of a pumping laser system for x-ray laser research.

A two-beam chirped-pulse-amplification Nd:glass laser system dedicated to x-ray laser research is described. Each beam provides an output energy of 20 J with a typical pulse duration of 1.3 ps. A prepulse of variable duration is generated by use of a novel, to our knowledge, optical system. A reflection optical system, comprised of an off-axis parabolic mirror and a spherical mirror, produces a line focus with 6-mm length and 165-microm width without chromatic aberration. By use of this pumping laser system, the nickel-like silver x-ray laser at a wavelength of 13.9 nm has been demonstrated.

Journal Article↗
PubMed2003

Imaging of the expansion of femtosecond-laser-produced silicon plasma atoms by off-resonant planar laser-induced fluorescence.

Planar laser-induced fluorescence measurements were used to investigate the expansion dynamics of a femtosecond laser-induced plasma. Temporally and spatially resolved measurements were performed to monitor the atoms that were ablated from a silicon target. A dye laser (lambda = 288.16 nm) was used to excite fluorescence signals. The radiation of an off-resonant transition (Si 390.55 nm) was observed at different distances from the target surface. This allowed easy detection of the ablated Si atoms without problems caused by scattered laser light. Abel inversion was applied to obtain the radial distribution of the Si atoms. The atom distribution in the plasma shows some peculiarities, depending on the crater depth.

Journal Article↗
PubMed2004

Real-time control of ultrafast laser micromachining by laser-induced breakdown spectroscopy.

Ultrafast laser micromachining provides many advantages for precision micromachining. One challenging problem, however, particularly for multilayer and heterogeneous materials, is how to prevent a given material from being ablated, as ultrafast laser micromachining is generally material insensitive. We present a real-time feedback control system for an ultrafast laser micromachining system based on laser-induced breakdown spectroscopy (LIBS). The characteristics of ultrafast LIBS are reviewed and discussed so as to demonstrate the feasibility of the technique. Comparison methods to identify the material emission patterns are developed, and several of the resulting algorithms were implemented into a real-time computer control system. LIBS-controlled micromachining is demonstrated for the fabrication of microheater structures on thermal sprayed materials. Compared with a strictly passive machining process without any such feedback control, the LIBS-based system provides several advantages including less damage to the substrate layer, reduced machining time, and more-uniform machining features.

Journal Article↗
PubMed2004

Spatial chirp and angular dispersion of a laser crystal for a four-mirror cavity Kerr-lens mode-locked laser.

We describe oscillating loops in a laser cavity and optical paths in a laser crystal of different wavelength rays for a four-mirror cavity Kerr-lens mode-locked laser. The relation between different wavelength ray paths and laser resonator parameters is deduced. The analytical expressions of second- and third-order dispersion including angular dispersion of the crystal are presented. The variations of group-delay dispersion (GDD) and third-order dispersion (TOD) with cavity parameters are calculated exactly. The calculation shows that GDD and TOD increase rapidly when the spacing between two folding mirrors approaches the boundary of a cavity stability zone. The rapid dispersion increase influences the mode-locked pulse width and the mode-locked stability.

Journal Article↗
PubMed2003

Actively Q-switched Nd:YVO4 laser using an electro-optic periodically poled lithium niobate crystal as a laser Q-switch.

We demonstrate a low-voltage and fast laser Q-switching by using an electro-optic periodically poled lithium niobate (EO PPLN) crystal. The half-wave voltage measured from the EO PPLN crystal was 0.36 V x d (microm)/L (cm), where d is the electrode separation and L is the electrode length. When a 13-mm-long EO PPLN was used as a laser Q switch at 7-kHz switching rate, we measured an approximately 12-ns pulse width and approximately 0.74-kW laser pulses at 1064-nm wavelength from a diode-pumped Nd:YVO4 laser with continuous 1.2-W pump power at 809-nm wavelength.

Journal Article↗
PubMed2003

Ho:YAG laser intracavity pumped by a diode-pumped Tm:YLF laser.

We report a compact Ho:YAG laser that is intracavity pumped by a diode-pumped Tm:YLF laser. Both lasers exhibit pulse mode behavior. Operating both crystals at room temperature (25 degrees C), we obtained 1.6 W of average output at 2.09 microm from the Ho:YAG laser for 15.4 W of diode power incident upon the Tm:YLF rod and a slope efficiency of 21%.

Journal Article↗
PubMed2003

Spatial laser-wing suppression in saturated laser-induced fluorescence without spatial selection.

Spatial wings of laser beams are of great concern in saturated laser-induced fluorescence. Their contribution to fluorescence is customarily avoided by resolution of laser peaks in the interaction volume. An alternative and versatile approach is formulated, based on the derivative of fluorescence with respect to laser intensity. It turns out that wing-free data are possible, although they are obtained from wing-dependent fluorescence. The advantages of this approach are exact centerline detection and simplicity of the experimental setup and procedure.

Journal Article↗
PubMed2006

Distributed Bragg reflector fiber laser fabricated by femtosecond laser inscription.

A femtosecond laser inscription technique is proposed for the fabrication of a fiber grating laser directly into a nonphotosensitive gain fiber. A distributed Bragg reflector fiber grating laser is realized in a conventional, untreated Er:Yb-codoped fiber as an illustration. Robust, single-mode, single-polarization laser operation at temperatures in excess of 600 degrees C is further achieved without compromising performance. A nonlinear, thermally induced wavelength shift is also observed at elevated temperatures.

Journal Article↗
PubMed2006

Lidar measurement of constituents of microparticles in air by laser-induced breakdown spectroscopy using femtosecond terawatt laser pulses.

We experimentally demonstrated remote sensing of the constituents of microparticles in air by combining laser-induced breakdown spectroscopy (LIBS) and lidar, using femtosecond terawatt laser pulses. Laser pulses of 70 fs duration and 130 mJ energy generated filaments when focused at a focal length of 20 m and the pulses irradiated artificial saltwater aerosols in air at a 10 Hz pulse repetition rate. Na fluorescence was observed remotely at a distance of 16 m using a 318 mm diameter Newtonian telescope, a spectrometer, and an intensified CCD camera. These results show the possibility of remote measurement of the constituents of atmospheric particles, such as aerosols, clouds, and toxic materials, by LIBS-lidar using femtosecond terawatt laser pulses.

Journal Article↗
PubMed1997

Compact tunable Cr:LiSAF laser for infrared matrix-assisted laser desorption/ionization.

A tunable Cr:LiSAF laser-pumped optical parametric oscillator was used for mid-infrared matrix-assisted laser desorption/ionization (MALDI) mass spectrometry experiments. The mass spectra of substance P, bovine insulin and pd(T)10 in the 2.88-2.96 microns range showed excellent single shot signal quality (signal-to-noise-ratio, resolution) but poor shot-to-shot reproducibility. The reproducibility is expected to improve with more stable laser design. No correlation was found between the absorption spectrum of the matrix and the MALDI response.

Animals↗
PubMed2000

Matrix-assisted laser desorption/ionization time-of-flight analysis of Bacillus spores using a 2.94 microm infrared laser.

The performance of infrared (2.94 microm) and ultraviolet (337 nm) lasers were compared for analysis of purified spores of B. subtilis, B. cereus and B. globigii on a four-inch end-cap reflectron time-of-flight instrument. Infrared matrix-assisted laser desorption/ionization (IR-MALDI) mass spectra of these microorganisms displayed a larger number of biomarker peaks above m/z 4000, compared with UV-MALDI. Biomarker peaks were observed at higher m/z values with the IR laser.

Bacillus↗
PubMedDate not supplied

Predictability of the size of laser-induced lesions in T1-Weighted MR images obtained during interstitial laser-induced thermotherapy of benign prostatic hyperplasia.

The purpose of this study was to predict diameters of lesions induced by laser-induced thermotherapy (LITT) of benign prostatic hyperplasia (BPH) from MRI signal/tissue temperature correlations during on-line monitoring with a temperature-sensitive fast low-angle shot (FLASH) sequence. Twenty LITT procedures with Nd:YAG (1,064 nm) and diode (830 nm) lasers were monitored on line with a T1-weighted FLASH sequence at 1.5 Tesla. Interstitial prostate temperature (T) was measured on line in 10 LITT procedures and laser energy deposition in 12. Slopes of linear regression curves for signal intensity (SI) over T were applied to determine SI at 60 degrees C to estimate diameters of intraprostatic LITT lesions. Diameters of unperfused LITT lesion cores in contrast-enhanced T1-weighted images served as gold standards. Linear regression curves with an average slope of -.54% SI/degrees C were obtained in 17 LITT procedures. Correlation coefficients were r = .92-.95 for SI/T and SI/energy deposition. Baseline variation of SI at body temperature was +/-3.9%, corresponding to +/-7 degrees C. Prediction of size (13 lesions) from on-line FLASH imaging was correct in 10 of 13, whereas 3 lesions were overestimated. Prediction of LITT lesion diameters from on-line MRI monitoring is possible with a temperature-sensitive FLASH sequence in the prostate. Accuracy may suffice to assign target regions of interest to tissue locations to be protected from coagulation.

Aged↗
PubMed2001

In vivo laser-induced interstitial thermotherapy of pig liver with a temperature-controlled diode laser and MRI correlation.

BACKGROUND AND OBJECTIVE: To evaluate the efficacy of a new temperature-controlled Diode laser to generate in vivo tissue coagulations with laser-induced interstitial thermotherapy (LITT) in pig liver and MRI-correlation of these necroses. STUDY DESIGN/MATERIALS AND METHODS: Each of four animal subjects received eight treatments at four different constant temperatures (70, 80, 90, and 100 degrees C) and at two out of three different intervals (3, 6, 9 minutes), resulting in a total of 32 lesions. After sacrificing, the liver was examined in an open MRI (0.35T). Tissue necrosis was measured macroscopically and compared with the size measured by MRI. RESULTS: The size of the necrosis was energy dependent with the largest extent at 100 degrees C at 9 minutes (2.1 cm). The correlation between macroscopic lesion size and MRI findings was significant (P > 0.036). Histological workup excluded carbonization in every case. CONCLUSIONS: LITT carried out with the Diode laser provides reproducible necroses of pig liver in vivo. MR-imaging results compared favorably with those obtained by pathologic specimens.

Animals↗
PubMed2005

Peptide patterns of laser dissected human trophoblasts analyzed by matrix-assisted laser desorption/ionisation-time of flight mass spectrometry.

This study describes the first promising steps in the comparison of peptide patterns of laser capture microdissected trophoblast cells obtained from frozen tissue sections in relative low numbers, approximately 125 cells. Trophoblasts were collected by laser capture microdissection from a terme human placenta and dissolved in detergents, sonified, and digested with trypsin. The resulting peptide mixtures were directly analyzed by matrix-assisted laser desorption/ionisation-time of flight mass spectrometry. Using this approach specific peptide patterns that consist on average of approximately 35 peptide peaks for trophoblast cells, and surrounding villous stroma cells could be obtained. From the results it was concluded that trophoblast and surrounding villous stroma cells show exclusive discriminating peptide patterns. In the future this method is potentially suitable for finding specific peptides and identification of proteins that are related to the pathogenesis of trophoblast pregnancy diseases such as preeclampsia.

Detergents↗
PubMed2004

Comparison of laser-induced dissociation and high-energy collision-induced dissociation using matrix-assisted laser desorption/ionization tandem time-of-flight (MALDI-TOF/TOF) for peptide and protein identification.

The fragmentation of peptides under laser-induced dissociation (LID) as well as high-energy collision-induced dissociation (CID) conditions has been investigated. The effect of the different fragmentation mechanisms on the formation of specific fragment ion types and the usability of the resulting spectra, e.g. for high-throughput protein identification, has been evaluated. Also, basic investigations on the influence of the matrix, as well as laser fluence, on the fragment ion formation and the consequences in the spectral appearance are discussed. The preconditions for obtaining 'pure' CID spectra on matrix-assisted laser desorption/ionization tandem time-of-flight (MALDI-TOF/TOF) instruments are evaluated and discussed as well as the differences between LID and CID in the resulting fragment ion types. While containing a wealth of information due to additional fragment ions in comparison with LID, CID spectra are significantly more complex than LID spectra and, due to different fragmentation patterns, the CID spectra are of limited use for protein identification, even under optimized parameter settings, due to significantly lower scores for the individual spectra. Conditions for optimal results regarding protein identification using MALDI-TOF/TOF instruments have been evaluated. For database searches using tandem mass spectrometric data, the use of LID as fragmentation technique in combination with parameter settings supporting the use of internal fragment ions turned out to yield the optimal results.

Amino Acid Sequence↗