Nonlinear amplification of side-modes in frequency combs.

We investigate how suppressed modes in frequency combs are modified upon frequency doubling and self-phase modulation. We find, both experimentally and by using a simplified model, that these side-modes are amplified relative to the principal comb modes. Whereas frequency doubling increases their relative strength by 6 dB, the growth due to self-phase modulation can be much stronger and generally increases with nonlinear propagation length. Upper limits for this effect are derived in this work. This behavior has implications for high-precision calibration of spectrographs with frequency combs used for example in astronomy. For this application, Fabry-Pérot filter cavities are used to increase the mode spacing to exceed the resolution of the spectrograph. Frequency conversion and/or spectral broadening after non-perfect filtering reamplify the suppressed modes, which can lead to calibration errors.

14 GHz visible supercontinuum generation: calibration sources for astronomical spectrographs.

We report the use of a specially designed tapered photonic crystal fiber to produce a broadband optical spectrum covering the visible spectral range. The pump source is a frequency doubled Yb fiber laser operating at a repetition rate of 14 GHz and emitting sub-5 pJ pulses. We experimentally determine the optimum core diameter and achieve a 235 nm broad spectrum. Numerical simulations are used to identify the underlying mechanisms and explain spectral features. The high repetition rate makes this system a promising candidate for precision calibration of astronomical spectrographs.

Octave-spanning supercontinuum generated in SF6-glass PCF by a 1060 nm mode-locked fibre laser delivering 20 pJ per pulse.

We report the generation of an octave-spanning supercontinuum in SF6-glass photonic crystal fiber using a diode-pumped passively mode-locked fs Yb-fiber laser oscillating at 1060 nm. The pulses (energy up to 500 pJ and duration 60 fs) were launched into a 4 cm length of PCF (core diameter 1.7 microm and zero-dispersion wavelength approximately 1060 nm). Less than 20 pJ of launched pulse energy was sufficient to generate a supercontinuum from 600 nm to 1450 nm, which represents the lowest energy so far reported for generation of an octave-spanning supercontinuum from a 1 microm pump. Since the laser pulse energy scales inversely with the repetition rate, highly compact and efficient sources based on SF6-glass PCF are likely to be especially useful for efficient spectral broadening at high repetition rates (several GHz), such as those needed for the precise calibration of astronomical spectrographs, where a frequency comb spacing >10 GHz is required for the best performance.

[Effect of maternal age on the course of labor--analysis of women over 40 years of age]. / Einfluss des mütterlichen Alters auf den Geburtsverlauf--Analyse bei Frauen über 40 Jahre.

In order to determine the birth risk for pregnant women in the age group > or = 40, the delivery data of 143 pregnant women in this age group were retrospectively evaluated over a 3-year period. Here, 37 of these 143 women (25.9%) were primiparae. The following was examined: The number of prenatal examinations (including ultrasound examinations), the incidence of genetic examinations, the delivery methods with the percentage of surgical deliveries, complications at the time of delivery as well as the fetal outcome with APGAR values, umbilical artery pH, birth weights, neonatal morbidity and mortality. The delivery results were compared with representative populations of women between 20-29 years (n = 2252) and 30-39 years (n = 1980). Pregnancy in older women still ends significantly more often with cesarean section than in younger women. Here, the rate of cesarean sections was 32.7% compared to 21.9% in 30-39-year-olds and 15.8% in 20-29-year-olds. However, parity has an even greater influence on the mode of delivery than age. Only 30.1% of multi-parae over 40 years underwent surgical delivery (vaginal and abdominal) compared to 77.3% of primiparae. It was also found that multiparae more rarely had surgical delivery than younger primiparae (30-39 years 53.3%, 20-29 years 39.3%). Despite the high surgical delivery rate being in the group of primiparae over 40 years, the fetal outcome was comparatively poor, so that the less restrictive indication for surgical delivery seems justified.