Search

Generation of two types optical supercontinua, structured and structureless ones, has been observed for the first time. The nonlinear processes, responsible for the generation of the optical supercontinua, have been discussed.

Compression of self-trapped femtosecond laser pulses in bulk fused silica at normal dispersion conditions is reported. Improvement of the spatiotemporal pulse shape and formation of soliton-like field distribution is observed.

Based on viewpoint that O2 is the most informative indication of gas degradation in transversely excited atmospheric (TEA) CO2 lasers, we have experimentally investigated the O2 concentration using a 100 Hz, 500 W class TEA‐CO2 laser with a catalytic gas recycler. A simple theoretical model is also proposed to describe the O2 concentration and then compared with experimental data over a wide range of operating conditions. The results show that the model can be used for design of optimized gas recyclers.

Generation and propagation of high-order harmonics in high-pressure (0.5--4 atm) gases by intense, ultrashort laser pulses is numerically investigated. The calculations are performed utilizing a self-consistent solution of the time-dependent Schr\"odinger and one-dimensional wave equations. The results obtained from the second-order and from the reduced first-order wave equations are compared. It is shown that at high pressure and for the high-order harmonics the use of the reduced first-order wave equation is not adequate. At very high pressure (\ensuremath{\sim}4 atm) a new regime of phase matching was observed. The applicability of the reduced first-order wave equation is also discussed.

The interaction of intense ultrashort laser pulses with a gas of argon atoms in numerically investigated. The investigation utilizes a model that solves both the time- dependent Schroedinger equation and the wave equation self- consistently. The time-dependent Schroedinger equation for a single-active electron in the effective static potential and in the potential of electron-laser interaction is solved using alternating direction implicit method. The Maxwell's wave equation is solved using an explicit finite-different technique. We show that, under some conditions, propagation effects significantly clean up the single-atom spectrum providing new spectrum with well-resolved harmonic structure.

Self-compression of high-intensity femtosecond pulses has been observed in a number of atomic and molecular gases and solid bulk material. The evolution of the femtosecond pulse parameters during the self-compression has been studied under a variety of experimental conditions. Generation of spatiotemporal solitons has been achieved by the combined action of self-compression and self-focusing.

To apply annular output beams emitted from an unstable resonator to a multiple-pass cell (MPC) for Raman conversion, we studied the mode-matching condition of non-Gaussian beams to a MPC using beam propagation analysis based on Laguerre-Gaussian functions. During transits of the MPC, the radial profile of an annular beam changes between annular and Airy patterns. Although such behavior indicates that it is impossible to achieve complete mode matching of an annular beam, we found a quasi-mode-matching condition under which the variation of beam size was minimized. The above theoretical analysis was verified experimentally using a CO(2) laser beam prepared for a para-hydrogen Raman laser.

Transverse and longitudinal density disturbances in the discharge volume of a transversely excited atmospheric carbon-dioxide laser are measured using a laser schlieren method, and two-dimensional aerodynamic calculations qualitatively reproduce the temporal histories of the disturbances. Exponential decay of the transverse disturbance between the flattened electrodes lasts for several milliseconds. An efficient damping of the shock wave after discharge is confirmed using an acoustic attenuator based on the Helmholtz resonator design. Accumulated pressure disturbances in the volume due to successive laser shots induce a gradual decrease to the steady-state energy level in a pulse-periodic operation, and it is deduced that the modulational instability in the shot to shot pulse energy results from the longitudinal standing wave.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A variety of infrared rotational Raman lasers developed at RIKEN for the purpose of molecular laserisotope separation are described. One of the major advances is that annular pump beams from anunstable resonator of a TEA C02 power oscillator can be efficiently converted to Stokes beams in a multiple pass cell, which results in an increase in the total efficiency of the para-H2 Raman laser system. The annular pump beams are used to generate second Stokes waves in ortho-D2, which provide anumber of lines different from those of first Stokes waves in para-H2 in the 13-18 jim spectra' range. First Stokes generation using the S(2) transition of ortho-D2 at room temperature is also described.Keywords: stimulated rotational Raman scattering, para-hydrogen, ortho-deuterium, annular beam1. INTRODUCTIONStimulated rotational Raman scattering of a C02 laser in para-H2 is a powerful infrared source in the 13- 18 im spectral range,'3 where there are a number of molecular absorption lines arising from the

Purpose: Malnutrition is common in patients with heart failure. We evaluated the influence of serum albumin concentration on the prognosis of the patients with Acute Decompensated Heart Failure (ADHF). Methods: 728 patients with ADHF were divided into 4 groups by the quartile of serum albumin concentration on admission (Q1: 1.8–3.1g/dl; Q2: 3.2–3.5g/dl; Q3: 3.6–3.8g/dl; Q4: 3.9–4.9g/dl). The short-term and long-term prognoses of 4 groups were investigated retrospectively. Results: In-hospital mortality of Q1, Q2, Q3, and Q4 group were 13.8%, 6.2%, 3.4%, and 3.0%, respectively. During the mean follow-up periods of 569±631 days, 180 patients (24.7%) died for all cause and 105 patients (14.4%) died for cardiac cause. As compared with the highest albumin group (Q4 group), the hazard ratio for all-cause death of the lowest albumin group (Q1 group) was 3.46 (95% CI; 2.24–5.40) in the unadjusted model, and 2.77 (95% CI; 1.79–4.33) in the age- and sex- adjusted model. ![Figure][1] Albumin concentration and prognoses Conclusion: Serum albumin concentration is useful to predict the prognosis of patients with ADHF. [1]: pending:yes

We generated highly phase-matched isolated attosecond pulses for the first time. This result can lead not only to the fundamental pump-and-probe-type experiments but to field-induced recolliding electron spectroscopy based on a single event.

High-pressure CO2 lasers are capable of producing intense beams with continuous frequency tunability in the 9µm and 10µm bands. The linewidth, however, amounts in general to several GHz with multilongitudinal-mode oscillations.1 Although injection locking is widely applied to narrow the linewidth in various lasers, this technique is effective only at the wavelength near the line centers in the high-pressure CO2 lasers because of the lack of a suitable master oscillator for providing tunable injection beams.

We demonstrate high-resolution fluorescence microscopy based on a cyclic sequential multiphoton (CSM) process, which gives rise to fluorescence emission following a sequence of cyclic transitions between the bright and dark states of a fluorophore induced by pump and reverse light. By temporally modulating the reverse intensity, we can extract the fluorescence signal generated through the CSM process. We show that the demodulated fluorescence signal is nonlinearly proportional to the excitation intensities and it gives a higher spatial resolution than that of a confocal microscope.

We demonstrate the measurement of two-photon excitation (TPE) spectra, used not only for fluorescence but also for photoconversion in green-to-red photoconvertible Kaede, using nonlinear Fourier-transform spectroscopy. It was found that in unphotoconverted Kaede, the TPE spectrum for photoconversion is much different to that for green-fluorescence. This is similar to the difference between the one-photon excitation of photoconversion in the neutral form and that of green-fluorescence in the ionized form.

We present measurements of two-photon excitation (TPE) spectra of various fluorescent proteins with nonlinear Fourier-transform spectroscopy. By using an ultrabroadband laser pulse with a spectrum ranging from 700 to 1100 nm, the absolute TPE spectra of six typical fluorescent proteins (SeBFP, Sapphire, eGFP, eCFP, Venus, DsRed) were measured with high spectral resolution.

We demonstrate methods to increase the energy incident on hollow fibers for spectral broadening by self-phase modulation. We used chirped pulses for spectral broadening, lowering the optical intensity to avoid ionization of the gaseous medium. We also used helium as a nonlinear medium and demonstrated the generation of 5.0fs, 5.0mJ pulses at a repetition rate of 1kHz using a pressure gradient hollow-fiber pulse compressor.

We have designed and fabricated a high-transmittance, extremely flat, free-standing extreme ultraviolet (EUV) filter for the wavelength region between 17–50 nm. The Al thin-film filter, which is coated with a SiC film to increase its oxidation tolerance, has a transmittance of 29% at 30 nm. This value is approximately 5 times higher than that of an uncoated filter fabricated using a conventional method.

To minimize urotherial tissue injuries by false laser irradiation during the laser stones fragmentation, a novel fiber-optic analytical system was developed which was able to distinguish urotherial tissues from urinary stones in vivo. This system was composed of the fiber-optic pulsed photo-thermal radiometry (PPTR) system together with a thin-fiber endoscope. The ultraviolet Ar laser was employed as the excitation light source. The catheter of this system was 6F in diameter and consisted of SiO2 glass fiber (400 micrometers (phi) ) as an excitation line, an As-S glass fiber (400 micrometers (phi) ) as a detection line, and a thin-fiber endoscope (610 micrometers (phi) ). A urinary stone was introduced into the canine ureter uneter under general anesthesia. The catheter system was inserted through an opening of the ureter distal to the stone. The e-folding decay time of the PPTR waveform of which optical and/or thermal properties of the tissues and stones were characterized was measured in vivo. The e-folding decay times were significantly different between urinary stones and the canine ureter. These results suggest that the fiber-optic PPTR analysis might be significantly useful procedure to prevent urotherial tissue from false irradiation injuries in clinical laser stone fragmentation.

To overcome the abrupt closure and late restenosis after percutaneous transluminal coronary angioplasty (PTCA), new technology termed 'hot balloon angioplasty' has been considered. A novel laser-heated hot balloon catheter has been developed to address these problems. Its feasibility and acute thermal effects in the porcine coronary arteries in vitro was studied. As the balloon material was teflon (heat-resistant up to 240 degree(s)C). The temperature of the balloon could rise abruptly. So, intima~media were welded while marked thermal injury was not detected in adventitial tissue.

We report on the compression of intense ultrashort laser pulses for the purpose of producing few-cycle optical pulses at the multimillijoule level by using a planar waveguide (PWG). The PWG is composed of two parallel glass slabs separated by a gap of 100 microm and mounted in a gas chamber filled with a noble gas. In comparison with the conventional hollow-fiber-based pulse compression technique, the use of a PWG enables the injection of high-energy ultrashort pulses, because the input laser beam is confined only in the lateral direction perpendicular to the waveguide plane. Using this technique, we demonstrate the generation of 12 fs, 2 mJ laser pulses in an argon-filled PWG.

We have demonstrated the generation of 5 fs, 0.5 TW pulses at 1 kHz repetition rate using a pulse compression technique in a hollow fiber with a pressure gradient. Owing to the excellent beam quality by passing through the hollow fiber, the beam after pulse compression could be focused to a nearly diffraction-limited spot size. We obtained for the first time a peak intensity as high as 5x10(18) W/cm(2) in the 2-cycle regime.

We have investigated the nonlinear susceptibility of atoms induced by high intensity, ultrashort laser pulses using a numerical solution of the time-dependent Schrodinger equation. We found that the instantaneous nonlinear susceptibility becomes saturated at high intensity. We also found that the saturation is closely linked to depletion of the ground state. Based on the numerical results, a simple model that generalizes the nonlinear susceptibility of atoms for high intensity, ultrashort laser pulses is proposed. We also investigated the ionization-induced dipole moment and found that the amplitude of the dipole moment induced by an ionized electron is, in general, smaller than that induced by a free-electron, and is attributable to a residual interaction between the ionized-electron and its parent ion.

We have developed a beam stabilization system for high-power femtosecond lasers operating at a repetition rate of 1kHz. The beam pointing at a hollow fiber input was stabilized to within 1μm rms and the fluctuations of the broadened spectra and power were significantly improved. This technique enables us to increase the input power to a hollow fiber for intense few-cycle pulse generation.

To prevent a false laser irradiation to an urotherial tissue during the laser stone fragmentation, we applied the fiber-optic pulsed photothermal radiometry (PPTR) to discrimination between urinary stones and urotherial tissues. The experiment was made in vitro with the various wet sample of the urinary stones and the canine urotherial tissues. Three different lasers, ultraviolet argon laser(UV Ar laser), visible argon laser(Vis. Ar laser) and carbon monoxide laser(CO laser), were used for excitation light sources of the PF'TR method and compared to obtain clear discrimination between the stones and the tissue. The UV Ar laser gave the best result among them. The wet canine urotherial tissue was clearly differential from the urinary stones by the measurement of the e-folding decay time of the PPTR waveform.

We studied an use of the continuous wave(cw) lasers(infrared and u1tra-violet) to the laser angioplasty by in vitro and an animal experiment in vivo. The use of the cw lasers provides an advantage of the reliable laser energy delivery by a thin flexible fiber. We had already reported the ablation characteristics and the fiber delivery capability of the CO laser. In this paper, we describe an another interesting cw laser for the laser angioplasty. The ablation characteristics of the uv Ar laser of which wavelength was approximately 350nm was studied. To develop an operation procedure of the angioscopic laser angioplasty, the animal experiments in vivo have been done under a visualization by a thin laser angioscope catheter with the CO laser irradiation. The advantage of the visualization during the laser angioplastic procedures are discussed.

CO laser may be efficient for thermal fusion of intima of arterial wall without adventitial tissue damage because of high tissue absorption. To investigate the efficacy of CO laser as a laser bam for laser balloon angioplasty (LBA). CO laser was irradiated to aortic tissue through 3Oim polyethylene membrane and tissue temperature was measured by a thermistor. At 2Owatt/cm2 200joules/cm2 continuous laser exposure (CE), tissue temperature was above 100°C within a depth of 1mm and rapidly decreased to 60 °C or below between 2 and 3mm in depth. Moreover, adventitial temperature could be decreased by changing duty ratio (exposure duration/interval) of intermittent laser exposure (IE) despite of the same laser energy. Light microscopy showed high degree of medial coagulation necrosis in CE, however thermal coagulation was observed only at the surface of intima of aortic tissue in IE at duty ratio 1 / 2. These findings suggested CO laser could coagulate intimal layer with less deep thermal damage compared to Nd- YAG laser and that IE was better for superficial welding than CE at the same energy. We concluded that CO laser might be more efficient as a laser beam for LBA than Nd-YAG laser.

We developed a new percutaneous transluminal coronary angioscopic catheter for visualization of coronary artery.This angioscopic catheter has an inflatable balloon at the distal tip and one - directional angulation mechanism.We performed percutaneous transluminal coronary angioscopy during cardiac catheterization cosecutively in 155 patients. With this angioscope , we could get good'-'fair visualization in 81%(131 of 162 lesions)without major complications.We could investigate the endothelial macropathology of ischemic heart disease such as unstable angina and acute myocardial infarction.

The laser ablation characteristics of the procine menisci by the various laser irradiations were studied. We employed nine laser wavelengthes from ultraviolet(UV) region to infrared(IR) region, pulsed or continuous wave(cw). They were ArF(193nm in wavelength, pulsed), KrF(248nm, pulsed), XeC1(308nm, pulsed), XeF(351nm, pulsed), Ar ion UV(351nm,cw), Ar ion visible(515nm, cw), Nd:YAG(1.06pm, cw), C0(5.3pm, cw) and C02(10.6jim, cw) lasers. All laser irradiation experiments were performed in vitro with a fixed laser beam to the menisci in air circumstance. The ablation rate and damaged layer thickness were measured. In order to compare the ablation characteristics of various laser wavelengths at the same condition, the laser irradiation energy density was kept constant to 60,J/cm2/sec. The ablation rate by the energy density of 1J/cm2 and the damaged layer thickness by the energy density of 1200J/cm2 were measured. The measured menisci ablation rate by ArF excimer laser was 9.5 (ArF 9.5), (KrF 92.1), (XeCl 74.4), (XeF 2.3), (Ar ion UV 64.8), (CO 69.5), (CO2 204.9) )ig/J. The damaged layer thickness of menisci by ArF excimer laser was 119 (ArF 119), (KrF 12), (XeCl 299), (XeF 645), (Ar ion UV 860), (CO 1105), (CO2 1209) pm. This study will investigate if the ablation characteristics depends on the wavelength of lasers and on emission mode (pulsed or cw). In the case of XeC1 excimer laser irradiation experiment, the contact irradiation to menisci in the water circumstance were carried out. The ablation rate using XeC1 excimer laser in water by the contact method was almost equal to that by the non-contact method in air. The damaged layer thickness using XeC1 excimer laser in water was considerably thin compared with that by the non-contact method in air. We expect that this study for menisci laser ablation may be contribute the progress in arthroscopic laser meniscectomy.

Selective fluorescence excitation of specific molecular species is demonstrated by using coherent control of two-photon excitation with supercontinuum pulses generated with a microstructure fiber. Pulse shaping prior to pulse propagation through the fiber is controlled by a self-learning optimization loop so that the highest fluorescence signal contrast between two fluorescent proteins is obtainable. The self-learning optimization loop successfully controls both the optical nonlinarity of the microstructure fiber and the two-photon excitation of the fluorescent proteins.

We investigated the possibility of plasma-induced spectral broadening of high-energy ultrashort laser pulses in a helium-filled multipass cell (MPC) through a series of full numerical simulations of the extended nonlinear Schrodinger equation. It was found that the gas pressure must be set low so that the propagation dynamics can be controlled only by plasma defocusing. Simulations using 100 mJ, 40 fs laser pulses in the MPC, which is 6 m long and has a mirror of 3.1 m radius at each end, showed that if the gas pressure is set within the range of 40-130 Pa, then the relevant spectral broadening can be obtained after five passes, yielding compressed pulses of a 4.7-6.4 fs width. The ratio of the energy of the compressed pulse to the output pulse is found to be within 58-88%.

We demonstrate two complementary types of microscopy using an identical setup for single-pulse coherent anti-Stokes Raman scattering (CARS) imaging, which employs an ultrabroadband laser pulse with a spectral bandwidth of 4800 cm(-1) and enables the suppression of nonresonant CARS signals. One is a novel type of microscopy that uses spectral phase modulation for the selective excitation of a single Raman mode. The selective excitation is achieved by the modulated pulse focusing its difference-frequency spectrum into a narrow spectral region. Another type is Fourier-transform CARS (FT-CARS) microspectroscopy based on the measurement of the CARS spectrum obtained from the Fourier-transform of the interferometric autocorrelation (IAC) signal. Vibrational spectral imaging of chemical and biological samples is demonstrated using the two types of microscopy.

The focusability of multiple high-order harmonics in the extreme-ultraviolet and soft-x-ray regions is described, together with the design and performance of the ellipsoidal mirror used for this purpose. The mirror focuses intense coherent light in the spectral-region from 25 to 40 nm into a 2.4 microm spot size with a focused peak intensity of 6 x 10(13) W/cm2. The focal images indicate that a good beam profile is obtained with a near-Gaussian distribution and a beam quality factor (M2 value) as low as 2.4.

We have proposed and demonstrated a novel approach for generating high-energy extreme-ultraviolet (XUV) continuum radiation. When a two-color laser field consisting of a sub-10-fs fundamental and its parallel-polarized second harmonic was applied to high-order harmonic generation in argon, a continuum spectrum centered at 30 nm was successfully obtained with an energy as high as 10 nJ. This broadband emission indicates the possibility of generating intense single attosecond pulses in the XUV region.

An intense sub-10 fs Ti:sapphire laser system has been developed for investigating ultrafast nonlinear phenomena. Sub-10 fs, multimillijoule pulses were obtained from a laser system consisting of a conventional chirped-pulse amplifier and a subsequent pressure-gradient hollow fiber pulse compressor, without any additional amplification. We describe the design and performance of the laser system together with high-order harmonic generation in the sub-10 fs regime as an example of one of the applications that were made possible by using this laser system.

A proposal for spectral broadening of an intense laser pulse with energy 15 mJ and a pulse duration of 40 fs in an argon-filled hollow fiber, using conjugate pressure-gradient method, is presented. The design, to be referred as conjugate pressure-gradient method, shall consist of a pair of hollow-fiber segments, each with gas pressure that increasingly and decreasingly varies, respectively. It is expected that the occurrence of early self-focusing can be avoided, as well as the elimination of the additional nonlinear processes at hollow fiber exit. Simulation using 15 mJ, 40 fs laser pulses shows that, in the intensity regime where ionization is important, excellent spectral broadening and spectral phase of the pulses can be obtained using the proposed conjugate pressure gradient method. The method therefore shall be useful for the spectral broadening of high-energy, ultrashort laser pulses.

We propose and demonstrate a pulse compression technique using a hollow fiber with a pressure gradient. This technique improves the spatial and spectral qualities of femtosecond laser pulses spectrally broadened by self-phase modulation, and allows an increase of the pulse energy for pulse compression. Using chirped mirrors for dispersion compensation, we have successfully compressed the pulse to less than 10 fs with an energy as high as 5 mJ.

We have developed a single-shot transient-grating autocorrelator that is capable of measuring femtosecond pulses in the ultraviolet region. The single-shot transient-grating method provides a simple and reliable measurement without any of the effects caused by shot-to-shot fluctuations of the pulses. The influence of nonlinear phenomena at high intensities can be observed in pulses amplified by a KrF laser.

We investigate the dynamics of femtosecond laser pulses propagating in a hollow fiber filled with argon, through a full numerical solution of the nonlinear Schrodinger equation. The simulation results show that, if the intensity is low and no ionization takes place, the spatial profile of the beam does not change very much so that its propagation model may be simplified to a one-dimensional model. If the intensity is high and ionization takes place, the spatial dynamics as well as temporal dynamics become very complicated because of self-focusing and defocusing. It is found that, for the same value of the B integral, self-focusing inside a hollow fiber can be substantially suspended by a differential gas pressure technique, where the gas pressure is set to be a minimum at the entrance and then increased with the propagation distance. Numerical simulations show that using such a technique, the energy transmitted during propagation inside hollow fiber is significantly enhanced, and the spatial phase is also improved.

The spatiotemporal dynamics of high-intensity femtosecond laser pulses propagating in argon have been investigated experimentally and the results compared with numerical calculations. After the beam expands and refocuses, the pulse splits in two. While the subcomponent expands immediately, the main component propagates as a filament. The pulse duration of the main component is less than half the initial pulse duration and the spectrum is broadened by a factor of two.

We demonstrate delayed four-wave Raman mixing in hydrogen gas and discuss the polarization of multiple rotational Raman radiation in the sub-100-fs regime. The mechanism of sideband generation through the interaction between a probe pulse and coherence of molecular motions induced by a pump pulse in hydrogen is revealed. One can artificially control the ellipticity of the polarization of the rotational Raman sidebands by changing the pump pulse polarization.

The interaction of intense few-cycle KrF driver pulses with a gas of neutral He atoms is numerically investigated. Calculations are performed by solving the time-dependent Schrödinger equation within a framework of the single-active electron model. It is demonstrated that a single attosecond pulse as well as a train of bursts of subfemtosecond duration can be produced by Fourier synthesis of selected succsesive harmonics. The results are compared to those obtained by the strong field approximation model.

Self-shortening of high-intensity femtosecond pulses in rare gases Ar and Kr has been found at normal dispersion conditions. The self-shortening results in soliton formation due to the medium nonlinearity higher than the third-order, which is the first experimental observation of such type of soliton. A new compression technique, scalable for high-energy femtosecond pulses, has been achieved based on this phenomenon.

High-harmonic generation from He atoms illuminated by high-intensity femtosecond KrF laser pulses is studied. It is shown that single attosecond pulses as well as trains of several attosecond pulses, one per half-a-period, can be generated in the several-cycle regime of the intense driver pulse.