Spectral periodicity in soliton explosions on a broadband mode-locked Yb fiber laser using time-stretch spectroscopy.

Experimentally, we demonstrate, to the best of our knowledge, the first observation of periodic spectrum changing via soliton explosion in a passively mode-locked fiber laser by a nonlinear polarization evolution. Using time stretch to capture 7220 consecutive single-shot spectra over a 100 µs time window in real time, the soliton explosions appear in a transition between two different mode-locking states. Simultaneous measurements of spectrum and pulse energy at three different output points in the laser cavity show that the soliton explosion's dynamics are related to residual dispersion. This study improves the understanding of pulse formation and evolution in the unstable mode-locking regime of lasers.

Glaucomatous changes in lamina pores shape within the lamina cribrosa using wide bandwidth, femtosecond mode-locked laser OCT.

PURPOSE: The lamina cribrosa (LC) is known to play a critical role in the pathogenesis of glaucoma. Although it has been reported that striae-shaped or slit-shaped lamina pores are more frequent in eyes with primary open angle glaucoma (POAG), this observation is based only on fundus photography. The primary object of this study is to perform layer-by-layer comparisons of the shape of lamina pores within the LC in vivo. DESIGN: Cross-sectional study. METHODS: Optic nerve head B-scans were obtained using custom-made broad-wavelength optical coherence tomography with a mode-locked laser. A total of 300 single B-scans per eye were obtained and three-dimensional images were rendered from these image sequences to obtain 2-µm thin-slice en face images of the LC. Elongation indices (EIs) of the lamina pores were measured from the anterior surface (AS) of the LC to the deeper layers in 40-µm increments. RESULTS: Thirteen eyes from 10 primary open angle glaucoma (POAG) patients of mean deviation -15.2 (-16.5, -12.9) (median [25,75 percentile]) dB and 10 eyes from 7 normal controls were studied. Although the EI value was not significantly different between the superior, temporal and inferior regions of the LC at any depth level in either group, it was greater at the AS than at the 40 µm and 80 µm depth levels (P < .001) in both groups, and was greater in the POAG group only at the AS and 40 µm depth level (P ≤ .05). After adjustment for age and refraction, the effects of depth and presence of POAG on the EI value remained significant. Also, the severity of glaucoma and depth were significant factors associated with EI in multivariate analysis. CONCLUSIONS: Elongation of lamina pores was significantly more evident at the anterior surface and the 40-µm depth level of the LC in POAG eyes than in normal eyes, suggesting that nerve fiber bundles passing through the LC were under greater stress in the anterior layers of the LC.

Vertical asymmetry of lamina cribrosa tilt angles using wide bandwidth, femtosecond mode-locked laser OCT; effect of myopia and glaucoma.

PURPOSE: Morphological features of the lamina cribrosa (LC) and optic disc may be important in the pathogenesis of glaucoma and myopic neuropathy. We therefore performed a cross-sectional study of patients with glaucoma and myopic neuropathy to evaluate vertical asymmetry of LC tilt angles (LCTAs) from Bruch's membrane opening (BMO). MATERIAL AND METHODS: Forty-six control eyes and 35 primary open-angle glaucoma (POAG) eyes were included. A raster scanning protocol with 300 single B-scans (without averaging) were obtained using optical coherence tomography with a wide-bandwidth, femtosecond mode-locked (ML) laser. Superior temporal to inferior nasal (ST) direction and inferior temporal to superior nasal (IT) direction (±45° rotation with a horizontal line) lines were drawn, and the angle between the inner edge of the BMO plane and the best fitting line for the anterior LC plane was measured as the LCTA. The generalized estimating equation was used to analyze the eye-derived data. RESULTS: Although no significant differences in either ST-LTCAs or IT-LTCAs were observed between the glaucoma group and non-glaucoma group, the IT-LCTAs were found to be significantly greater than the ST-LCTA in both the glaucoma and non-glaucoma groups (P < 0.001). After adjustment for other potential confounding factors by multivariate analysis, greater refractive errors were significantly correlated with IT-LCTAs. CONCLUSIONS: Vertical asymmetry of the LC tilting from the BMO plane exists in both normal and POAG eyes, and correlates with the degree of myopia.

Two-color high-harmonic generation in plasmas: efficiency dependence on the generating particle properties.

The high-order harmonic generation (HHG) in silver, gold, and zinc plasma plumes irradiated by orthogonally polarized two-color field is studied theoretically and experimentally. We find an increase of the HHG efficiency in comparison with the single-color case, which essentially depends on the plasma species and harmonic order. An increase of more than an order of magnitude is observed for silver plasma, whereas for gold and zinc it is lower; these results are reproduced in our calculations that include both propagation and microscopic response studies. We show that the widely used theoretical approach assuming the 1s ground state of the generating particle fails to reproduce the experimental results; the agreement is achieved in our theory using the actual quantum numbers of the outer electron of the generating particles. Moreover, our theoretical studies highlight the redistribution of the electronic density in the continuum wave packet as an important aspect of the HHG enhancement in the two-color orthogonally polarized fields with comparable intensities: in the single-color field the electronic trajectories with almost zero return energy are the most populated ones; in the two-color case the total field maximum can be shifted in time so that the trajectories with high return energies (in particular, the cut-off trajectory) become the most populated ones.

Application of mid-infrared pulses for quasi-phase-matching of high-order harmonics in silver plasma.

We demonstrate the quasi-phase-matching of a group of harmonics generated in Ag multi-jet plasma using tunable pulses in the region of 1160 - 1540 nm and their second harmonic emission. The numerical treatment of this effect includes microscopic description of the harmonic generation, propagation of the pump pulse, and the propagation of the generated harmonics. We obtained more than 30-fold growth of harmonics at the conditions of quasi-phase-matching in the region of 35 nm using eight-jet plasma compared with the case of imperforated plasma.

Generation of broadband noise-like pulse from Yb-doped fiber laser ring cavity.

We have demonstrated a generation of the noise-like pulse (NLP) with broadband spectrum in a nonlinear polarized evolution-based passive mode-locked Yb-doped fiber (YDF) ring laser. At the cavity dispersion of near zero, the NLP with spectrum bandwidth up to 131 nm (FWHM) was obtained at a central wavelength of 1070 nm with output power of 136 mW and 80 MHz repetition rate. To our best knowledge, this spectrum bandwidth of NLP is the broadest among the reported YDF lasers. The autocorrelation function of pulse contained the short (30 fs) and long (4.6 ps) components. This short coherence light source is well suited for the optical coherent tomography used for ophthalmology at a wavelength of ∼1000 nm.

Three-dimensional optic nerve head images using optical coherence tomography with a broad bandwidth, femtosecond, and mode-locked laser.

PURPOSE: The aim of this study was to demonstrate the fine laminar structure of the optic nerve head (ONH), in vivo, using a broad wavelength, ultra-high resolution, and optically coherent tomography (OCT) system. METHODS: This high-resolution OCT system, based on a 200 nm bandwidth spectrometer and an 8 femtosecond ultra-short, mode-locked, coherent laser light source, enabled in vivo cross-sectional ONH imaging with 2.0 µm axial resolution. A total of 300 optic disc B-scans, which consisted of 300 × 2048 pixels, were obtained in 10 µm steps. Three-dimensional images were rendered from these images to obtain n face images of the optic disc. Fundus photography, scanning laser ophthalmoscopy (SLO), and standard OCT were also performed for all subjects. RESULTS: Thirty-six eyes of normal subjects and ten eyes of glaucoma patients with mean age of 40.0 ± 10.0 years were enrolled in this study. Sequential en face images, from the ONH surface to deeper layers, were reconstructed in 2.0 µm steps. Observation of the images indicated variations in the shape and arrangement of the lamina pores at different depths. Clear lamina pores were identified by this technique in 44 eyes, compared with the fundus camera (identified in six eyes), SLO (identified in 14 eyes), and standard OCT (identified in 24 eyes) (all comparisons, p < 0.001). CONCLUSIONS: The fine structure of the ONH could be resolved in vivo using our OCT, providing improved imaging that can be used in research and clinical applications for a better characterization of the anatomical and pathological features associated with glaucoma.

Correlation between lamina cribrosa tilt angles, myopia and glaucoma using OCT with a wide bandwidth femtosecond mode-locked laser.

PURPOSE: To measure horizontal and vertical lamina cribrosa (LC) tilt angles and investigate associated factors using prototype optical coherence tomography (OCT) with a broad wavelength laser light source. DESIGN: Cross sectional study. METHODS: Twenty-eight no glaucoma eyes (from 15 subjects) and 25 glaucoma eyes (from 14 patients) were enrolled. A total of 300 optic nerve head B-scans were obtained in 10 µm steps and the inner edge of Bruch's membrane opening (BMO) was identified as the reference plane. The vertical and horizontal angles between BMO line and approximate the best-fitting line for the surface of the LC were measured and potential associated factors were estimated with univariate and multivariate logistic regression analyses. RESULTS: The median (interquartile range) horizontal and vertical tilt angles were 7.10 (2.43-11.45) degrees and 4.15 (2.60-6.85) degrees in eyes without glaucoma and 8.50 (4.40-14.10) degrees and 9.30 (6.90-14.15) degrees in glaucoma eyes, respectively. The refractive errors had a statistically significant association with horizontal LC tilt angles (coefficients, -1.53 per diopter) and glaucoma had a significant correlation with vertical tilt angles (coefficients, 6.56) using multiple logistic regression analysis (p<0.001). CONCLUSIONS: OCT allowed evaluation of the internal tilting of the LC compared with the BMO. The horizontal internal LC tilt angle was correlated with refractive errors, corresponding to myopic physiological changes, and vertical internal LC tilt was correlated with glaucoma, corresponding to glaucomatous pathological changes. These parameters have important implications for investigation of the correlation between myopia, glaucoma and LC morphological features.

Fabrication of two-dimensional periodic nanostructures by two-beam interference of femtosecond pulses.

Two-dimensional periodic nanostructures on ZnO crystal surface were fabricated by two-beam interference of 790 nm femtosecond laser. The long period is, as usually reported, determined by the interference pattern of two laser beams. Surprisingly, there is another short periodic nanostructures with periods of 220-270 nm embedding in the long periodic structures. We studied the periods, orientation, and the evolution of the short periodic nanostructures, and found them analogous to the self-organized nanostructures induced by single fs laser beam.

Development of a compact efficient 10 Hz 20 TW Ti:sapphire laser system with a 1 kHz regenerative amplifier.

We have constructed a compact efficient Ti:sapphire laser system that generates 30 fs, 630 mJ pulses at a repetition rate of 10 Hz. A new geometry for a single-stage multipass power amplifier is proposed that greatly weakens and even makes use of thermal lensing. Such geometry can realize high output in a single-stage power amplifier; otherwise at least two-stage power amplifiers are required. The new configuration simplifies the laser system and reduces the cost. The key point in this design is that the beam spot size evolution is considered in combination with the pulse amplification.

Intense exact resonance enhancement of single-high-harmonic from an antimony ion by using Ti:Sapphire laser at 37 nm.

We demonstrated the intensity enhancement of a single high-order harmonic at a wavelength of 37.67 nm using the lowly ionized antimony laser-ablation plume. The conversion efficiency of this harmonic was 2.5x10(-5) and the output energy was 0.3 muJ. Such an enhancement of single-harmonic was caused by the multiphoton resonance with the strong radiative transition of the Sb II ions. The cutoff energy of the harmonics generated in Sb plasma was 86 eV (55(th) harmonic).

Seventy-first harmonic generation from doubly charged ions in preformed laser-ablation vanadium plume at 110 eV.

We have demonstrated a generation of 71(st) harmonic at a photon energy of 110 eV (wavelength 11.2 nm) using low ionized vanadium ions in a laser-ablation plume. The conversion efficiency of this harmonic was estimated to be 1.6x10(-7). Such high harmonic generation occurred from the interaction of a femtosecond laser pulse with a doubly charged ion, which has high third ionization potential. This work shows the extension of HHG cutoff energy can be achieved by using the doubly charged ion.

Anomalous enhancement of a single high-order harmonic by using a laser-ablation tin plume at 47 nm.

We have successfully demonstrated intensity enhancement of a single high-order harmonic at a wavelength of 46.76 nm by using low-ionized tin ions in a laser-ablation plume as the nonlinear medium. The ablation plume was produced by irradiating a solid tin target with a 10 mJ energy picosecond laser pulse. Strong 17th-harmonic generation at a wavelength of 46.76 nm was observed with a conversion efficiency of about 10(-4). We attribute the strong enhancement of the single high-order harmonic to multiphoton resonance with a strong radiative transition of the Sn II ion.

Strong resonance enhancement of a single harmonic generated in the extreme ultraviolet range.

We report the demonstration of strong resonance enhancement of a single high-order harmonic in the extreme ultraviolet (XUV) region generated from the interaction of a femtosecond pulse with low-ionized In ablation. A strong 13th harmonic (61.2 nm) of Ti:sapphire laser radiation with output intensity almost two orders of magnitude higher than neighboring harmonics was observed in these studies. The high conversion efficiency of the 13th harmonic (8 x 10(-5)) is attributed to multiple collisions of electron trajectories with the origin due to multiphoton resonance with the In ion.

Third-harmonic generation in air by use of femtosecond radiation in tight-focusing conditions.

Third-harmonic generation (THG) in air in tight-focusing conditions is presented. Variation of the pulse duration supplied optimal conditions for THG by femtosecond pulses that varied in the range 110-1300 fs. We show that this third-harmonic generation was caused by Kerr-induced phase variations of fundamental and harmonic beams. Various characteristics (laser intensity, focusing conditions, pulse duration, air pressure, etc.) of THG in air were analyzed to optimize this process. The THG conversion efficiency of 795 nm, 300 fs radiation was 1 x 10(-3). The harmonic radiation did not show considerable disruption of its spectral and spatial distribution in tight focusing conditions for intensities as high as 5 x 10(14) W cm(-2).

High-order harmonic generation from boron plasma in the extreme-ultraviolet range.

We demonstrate the generation of up to the 63rd harmonic (lambda = 12.6 nm) of a Ti:sapphire laser pulse (150 fs, 10 mJ), using a prepulse- (210-ps,24-mJ) produced boron plasma as the nonlinear medium. The influence of various parameters on the harmonic conversion efficiency was analyzed. The steep decrease in intensity for low-order harmonics (up to 19th order) was followed by a plateau. Typical conversion efficiencies were evaluated to be 10(-4) (for a 3rd harmonic) to 10(-7) (within the plateau region). Harmonic generation appeared to be efficient for the plasma that comprised neutral atoms and singly ionized boron.

Development and applications of a compact hybrid tabletop terawatt chirped-pulse amplification Ti:sapphire-Nd:glass laser for x-ray lasing and harmonic generation.

A hybrid tabletop (2 m x 3 m) terawatt chirped-pulse amplification Ti:sapphire-Nd:glass laser (1054 nm, 475 fs, 500 mJ, 9 x 10(17) W cm(-2)) has been developed for laser-matter experiments. An overall gain factor of 10(10) was achieved for the laser. The results of laser applications in the studies of soft-x-ray Ni-like Mo 18.9-nm lasing and of harmonic generation from solid surfaces are presented.

Effects of pump propagation and absorption on the gain distribution of longitudinally pumped Ni-like molybdenum x-ray lasers.

We reveal from simulations that in longitudinally pumped Ni-like Mo x-ray lasers, deformation of the temporal gain profile can occur, causing the x-ray laser pulse to have a steep rising edge. This is shown to be due to the rapid change in the inverse bremsstrahlung absorption when the main picosecond pulse pumps the cold preplasma.