Identifying crossing collagen fibers in human corneal tissues using pSHG images.

Polarization sensitive second harmonic generation (pSHG) microscopy has been used previously to characterize the structure of collagen fibers in corneal samples. Due to the typical organization of the corneal stroma, the information that pSHG provides may be misleading in points where two different collagen fiber bundles orient along different direction crossings. Here, a simulation that illustrates the problem is presented, along with a novel method that is capable of identifying these crossing points. These results can be used to improve the evaluation of corneal collagen structure, and it has been applied to analyze pSHG data acquired from healthy and keratoconic human corneal samples.

Ultrafast photo-excitation dynamics in isolated, neutral water clusters.

Using the efficient nonlinear conversion scheme which was recently developed in our group [M. Beutler, M. Ghotbi, F. Noack, and I. V. Hertel, Opt. Lett. 134, 1491 (2010); M. Ghotbi, M. Beutler, and F. Noack, ibid 35, 3492 (2010)] to provide intense sub-50 fs vacuum ultraviolet laser pulses we have performed the first real time study of ultrafast, photo-induced dynamics in the electronically excited Ã-state of water clusters (H(2)O)(n) and (D(2)O)(n) , n=2-10. Three relevant time scales, 1.8-2.5, 10-30, and 50-150 fs, can be distinguished which-guided by the available theoretical results-are attributed to H (D)-ejection, OH (OD) dissociation, and a nonadiabatic transition through a conical intersection, respectively. While a direct quantitative comparison is only very preliminary, the present results provide a crucial test for future modeling of excited state dynamics in water clusters, and should help to unravel some of the many still unresolved puzzles about water.

Generation of 2.5 µJ vacuum ultraviolet pulses with sub-50 fs duration by noncollinear four-wave mixing in argon.

Generation of sub-50fs vacuum UV pulses with more than 2.5µJ energy at a 1kHz repetition rate is reported. The pulses at 160nm are produced using noncollinear difference-frequency four-wave mixing between the fundamental and third harmonics of an amplified Ti:sapphire laser in argon. While the pulse duration is maintained by increasing the phase-matching pressure, noncollinear interaction improves the conversion efficiency by 1 order of magnitude in comparison with the previous results in collinear geometry.

Broadly tunable, sub-30 fs near-infrared pulses from an optical parametric amplifier based on BiB3O6.

We report the generation of tunable femtosecond pulses in the near-IR using a two-stage, white-light seeded, collinear, femtosecond optical parametric amplifier (OPA). The OPA, based on BiB(3)O(6) crystal in both stages and pumped at 800 nm by a 1 kHz Ti:sapphire laser amplifier, provides sub-30 fs signal pulses across the whole tuning range of 1.15-1.6 microm with minimum duration of 22 fs and maximum signal energy of 80 microJ.

High-energy, sub-30 fs near-IR pulses from a broadband optical parametric amplifier based on collinear interaction in BiB(3)O(6).

We report efficient generation of tunable femtosecond pulses in the near IR using a two stage, white-light seeded, collinear, femtosecond optical parametric amplifier (OPA). The OPA, based on BiB(3)O(6) crystal in both stages and pumped at 807 nm by a 1 kHz Ti:sapphire laser amplifier, provides sub-30 fs signal pulses after compression with energies exceeding 200 microJ, which corresponds to fivefold pulse shortening and approximately 30% internal conversion efficiency in the second stage considering 150 fs pump pulses with 1.5 mJ energy. The corresponding idler pulses with more than 100 microJ have sub-60 fs duration without compression. The first stage alone is capable of producing sub-20 fs pulses near 1400 nm at the microjoule level without using any compression.

Epidemiological study of gastro-oesophageal reflux disease: reflux in spouse as a risk factor.

BACKGROUND: Gastro-oesophageal reflux disease (GERD) is a growing health-care problem with variable distribution. AIM: To assess GERD prevalence and risk factors and their possible correlation with pathophysiology in a population-based study. METHODS: Individuals aged 18-65 years were enrolled through random cluster sampling in Tehran. Previously validated self-administered questionnaires were used. RESULTS: Of the 2500 questionnaires, 2057 were analysed (mean age: 34.8 +/- 13.0 years, 55.1% female). Frequent GERD was seen in 18.2%. Minor symptoms increased prevalence. Female gender (OR: 1.55, 95% CI: 1.01-2.41), BMI >30 kg/m(2) (OR: 1.79, 95% CI: 1.03-3.12), less education (OR: 1.52, 95% CI: 1.02-2.27), smoking (OR: 1.83, 95% CI: 1.12-2.99), NSAID use (OR: 4.23, 95% CI: 1.66-10.74) and GERD in spouse (OR: 1.82, 95% CI: 1.18-2.82) were associated with frequent GERD on multivariable analysis. GERD in first-degree relatives (OR: 1.73, 95% CI: 1.23-2.43) and asthma (OR: 4.09, 95% CI: 1.27-13.15) correlated with infrequent GERD. Minor symptoms correlated with GERD history in first-degree relatives, coffee consumption and NSAID use. Prevalence in the past 3 months was similar to that in the past 12 months (P < 0.05). CONCLUSIONS: Gastro-oesophageal reflux disease is common in Tehran. The association of 'infrequent symptoms' with GERD history in first-degree relatives and 'frequent symptoms' with GERD history in spouse may point to the presence of yet unknown precipitating environmental factors inducing GERD in a genetically susceptible host. Minor GERD symptoms seem to have independent contribution to GERD. Assessing GERD in the past 3 months predicts prevalence in the past year.

Tunable, high-repetition-rate, femtosecond pulse generation in the ultraviolet.

We report efficient generation of tunable femtosecond pulses in the ultraviolet (UV) by intracavity doubling of a visible femtosecond optical parametric oscillator (OPO). The OPO, based on a 400 microm BiB3O6 crystal and pumped at 415 nm in the blue, can provide visible femtosecond signal pulses across 500-710 nm. Using a 500 microm crystal of beta-BaB2O4 internal to the OPO cavity, efficient frequency doubling of the signal pulses into the UV is achieved, providing tunable femtosecond pulses across 250-355 nm with up to 225 mW of average power at 76 MHz. Cross-correlation measurements result in UV pulses with durations down to 132 fs for 180 fs blue pump pulses.

BiB3O6 femtosecond optical parametric oscillator.

We report a femtosecond optical parametric oscillator (OPO) based on the nonlinear material BiB3O6. The OPO is synchronously pumped in the blue by the second harmonic of a Kerr-lens-mode-locked Ti:sapphire laser. It can provide wide and continuous tuning across the entire green-yellow-orange-red spectral range with a single crystal and a single set of mirrors. Using a 500 microm BiB3O6 crystal and collinear type I (e+e->o) phase matching in the optical yz plane, a signal wavelength range of 480-710 nm is demonstrated with angle tuning at room temperature at average output powers of 270 mW. With 220 fs blue pump pulses, near-transform-limited signal pulses of 120 fs duration have been obtained at 76 MHz repetition rate.

990 mW average power, 52% efficient, high-repetition-rate picosecond-pulse generation in the blue with BiB3O6.

Efficient single-pass second-harmonic generation (SHG) of tunable high-repetition-rate picosecond pulses into the blue is reported in the nonlinear crystal BiB3O6. Using 2.4 ps fundamental pulses from a mode-locked Ti:sapphire laser at 76 MHz and a 10 mm crystal cut for type I (e + e --> o) phase matching in the optical yz plane, second-harmonic average powers as high as 990 mW with excellent stability have been generated at 52% conversion efficiency, and a tunable range of 370-450 nm is demonstrated. From measurements of single-pass SHG in the continuous-wave regime an effective nonlinear coefficient of 3.7 pm/V has been verified for BiB3O6, and direct comparison with beta-BaB2O4 confirms a SHG power enhancement of approximately 23% for the same crystal length. Autocorrelation measurements in a 200 microm crystal of beta-BaB2O4 result in durations of 2.8 ps for the second-harmonic blue pulses.

High-average-power femtosecond pulse generation in the blue using BiB3O6.

Efficient generation of tunable femtosecond pulses in the blue is reported in the nonlinear crystal BiB3O6. By use of fundamental pulses from a mode-locked Ti:sapphire laser at 76 MHz, single-pass second-harmonic average powers of as much as 830 mW have been generated at 50% conversion efficiency, and a tunable range of 375-435 nm in the blue is demonstrated. Temporal measurements using cross correlation of the fundamental and second-harmonic pulses in a 100-microm-thick beta-BaB2O4 crystal result in blue pulse durations of 220 fs for 130-fs fundamental pulses. Direct experimental comparison with beta-BaB2O4 confirms the superior performance BiB3O6 for second-harmonic generation of femtosecond pulses.