On-chip phase-shift induced control of supercontinuum generation in a dual-core Si3N4 waveguide.

We investigate on-chip spectral control of supercontinuum generation, taking advantage of the additional spatial degree of freedom in strongly-coupled dual-core waveguides. Using numerical integration of the multi-mode generalized nonlinear Schrödinger equation, we show that, with proper waveguide cross-section design, selective excitation of supermodes can vary the dispersion to its extremes, i.e., all-normal or anomalous dispersion can be selected via phase shifting in a Mach-Zehnder input circuit. The resulting control allows to provide vastly different supercontinuum spectra with the same waveguide circuit.

Hybrid-integrated diode laser in the visible spectral range.

Generating visible light with wide tunability and high coherence based on photonic integrated circuits is of high interest for applications in biophotonics, precision metrology, and quantum technology. Here we present, to our knowledge, the first demonstration of a hybrid-integrated diode laser in the visible spectral range. Using an AlGaInP optical amplifier coupled to a low-loss Si3N4 feedback circuit based on microring resonators, we obtain a spectral coverage of 10.8 nm around 684.4 nm wavelength with up to 4.8 mW output power. The measured intrinsic linewidth is 2.3±0.2kHz.

Saturated Raman scattering for sub-diffraction-limited imaging.

We present a scheme for a sub-diffraction-limited Raman microscope. The scheme combines the concept from stimulated depletion microscopy with femtosecond stimulated Raman scattering. The suppression of the Raman signal in a three-beam setup with only two involved wavelength-components was accomplished by the saturation of the Raman scattering. A reduction of the Raman signal of up to 79% could be measured with only a single Raman resonance involved. Based on this signal suppression, a resolution enhancement by a factor of 2 could be verified in a first proof-of-concept measurement, opening up a pathway toward label-free sub-diffraction-limited imaging.

In situ visualization of damaged DNA in human sperm by Raman microspectroscopy.

BACKGROUND: Beyond determining the percentage of damaged sperm, current methods of DNA assessment are of limited clinical utility as they render the sample unusable. We evaluated Raman microspectroscopy, a laser-based non-invasive technique that provides detailed chemical 'fingerprints' of cells and which potentially could be used for nuclear DNA-based sperm selection. METHODS: Eight healthy donors provided ejaculates. After system optimization, a minimum of 200 air-dried sperm/sample/donor, prior to/and after UVB irradiation, were assessed by two observers. Spectra were analysed by Principal Component, Spectral Angle and Wavelet Analyses. RESULTS: Spectra provided a chemical map delineating each sperm head region. Principal Component Analysis showed clear separation between spectra from UV-irradiated and untreated samples whilst averaged data identified two regions of interest (1040 and 1400 cm(-1)). Local spectral analysis around the DNA PO(4) backbone peak (1042 cm(-1)), showed that changes in this region were indicative of DNA damage. Wavelet decomposition confirmed both the 1042 cm(-1) shift and a second UVB susceptible region (1400-1600 cm(-1)) corresponding to protein-DNA interactions. No difference was found between observer measurements. CONCLUSIONS: Raman microspectroscopy can provide accurate and reproducible assessment of sperm DNA structure and the sites and location of damage.

Beam position stabilization for a confocal multiphoton microscope.

The influence of beam-pointing on scanning confocal microscopy is investigated. The beam displacement is measured using a quadrant photodiode, and the apparent movement of a sub-micron-sized particle observed by second-harmonic microscopy is linked to the beam displacement. A simple beam-pointing stabilization is implemented, and improvement of beam stability by three orders of magnitude on long time scales is achieved.

Optically induced long-period fiber gratings for guided mode conversion in few-mode fibers.

We propose and demonstrate guided mode conversion by an optically induced long-period fiber grating (OLPG). The beating of two guided modes excited by a nanosecond pulsed laser beam is used to induce a refractive index grating via the Kerr effect. This grating converts a counter-propagating continuous-wave beam from the LP(01) to the LP(11) mode. Numerical simulations using a beam propagation method show that a full conversion is possible. Experimentally, a mode conversion with an efficiency of about 50% is firstly demonstrated.

Modeling of transverse mode interaction in large-mode-area fiber amplifiers.

We model the transverse mode interaction in a large-mode-area fiber amplifier by solving the Fresnel wave equation including local gain saturation. In order to calculate the electric field distribution we apply a finite difference beam propagation method, which is followed by the derivation of the modal powers and modal polarization states. A polarization dependent mode amplification is found that is in good agreement with recent experimental results.

Interaction of transverse modes in a single-frequency few-mode fiber amplifier caused by local gain saturation.

We report on the behavior of modal polarization states in a single-frequency, ytterbium-doped, few-mode fiber amplifier. Experimental data show that the polarization of the individual transverse modes depends on the pump power and that the modes tend towards orthogonally polarized states with increasing gain. The observations can be explained by local gain saturation that favors the amplification of differently polarized modes.

Resonant nonlinearity in high-energy Er3+-fiber chirped-pulse-amplifiers.

We present experimental results, which show that the up-chirp of dispersively stretched femtosecond pulses decreases linearly with increasing pulse energy after amplification in Erbium-doped fibers. For 6 microJ output pulses a nonlinear dispersion of -410(6) fs(2) was measured. This nonlinear dispersive effect is attributable to the resonant dispersion of the Erbium-ions and the decrease of the inversion during pulse amplification and was about one order of magnitude larger than predicted by the literature. Most likely this deviation is attributable to the complex population dynamics of the Er3+-ions during pulse amplification, since in the literature the relation between the refractive index and Er3+-inversion was described for a quasi-static population distribution. Due to the high resonant dispersion the required compressor dispersion for minimum output pulse duration depends strongly on the output pulse energy in Erbium-doped fiber-based chirped-pulse-amplifier set-ups.

High-power ultra-broadband mode-locked Yb3+-fiber laser with 118 nm bandwidth.

A diode-pumped mode-locked double clad Yb3+-fiber ring laser with up to 118 nm bandwidth, a repetition rate of 7.6 MHz and a maximum output power of 2.8 W is presented. The broad bandwidth is achieved by suppressing the long wavelength components in the dispersive grating line of the resonator.

Modulation-free sub-Doppler laser frequency stabilization to molecular iodine with a common-path, two-color interferometer.

Recently, a simple common-path, two-color interferometer has been used for Doppler-free saturated dispersion spectroscopy of iodine. We have used such a set-up to stabilize a Nd:YAG laser for the first time, to our knowledge. This method requires only a small number of low-cost optical components compared to frequency modulation techniques.We have measured a root Allan variance of 5 . 10(-12) for 0.2 s, and below 5 . 10 (-11) for integration times up to 300 s.

Novel suppression scheme for Brillouin scattering.

We demonstrate a new scheme for the efficient suppression of Brillouin scattering of a single-frequency laser source in a 72 m-long Neodymium-doped fiber amplifier by simultaneous amplification of two seed lasers separated in wavelength by two times the Brillouin-shift. This scheme can be independently employed in addition to conventional methods of suppressing stimulated Brillouin scattering enabling further power scaling of existing systems.

Sub-diffraction limited structuring of solid targets with femtosecond laser pulses.

Possibilities to produce sub-diffraction limited structures in thin metal films and bulk dielectric materials using femtosecond laser pulses are investigated. The physics of ultrashort pulse laser ablation of solids is outlined. Results on the fabrication of sub-micrometer structures in 100-200 nm chrome-coated surfaces by direct ablative writing are reported. Polarization maintaining optical waveguides produced by femtosecond laser pulses inside crystalline quartz are demonstrated.

Microsecond pulsed optical parametric oscillator pumped by a Q-switched fiber laser.

We report on what is to our knowledge the first optical parametric oscillator (OPO) pumped by microsecond pulses from a wavelength-tunable solid-state laser. The singly resonant OPO (SRO) is based on a periodically poled LiNbO3 crystal and pumped with 2.1-micros-long pulses from an actively Q-switched Yb fiber laser. At an average fiber laser power of 3.6 W, the SRO generates 1.9-micros-long pulses with a repetition rate of 25 kHz and an average power of 560 mW at 3360 nm. The SRO was tuned from 1518 to 1634 nm (signal) and from 3145 to 3689 nm (idler) via the crystal temperature and poling period. By all-electronic tuning of the fiber laser wavelength over 19 nm, tuning of the mid-infrared idler wavelength over 195 nm was achieved.

Rapidly tunable continuous-wave optical parametric oscillator pumped by a fiber laser.

We report on rapid, all-electronically controlled wavelength tuning of a continuous-wave (cw) optical parametric oscillator (OPO) pumped by an ytterbium fiber laser. The OPO is singly resonant for the signal wave and consists of a 40-mm-long periodically poled lithium niobate crystal in a four-mirror ring cavity. By tuning of the fiber-laser wavelength over 33 nm through an intracavity acousto-optic tunable filter, the OPO idler wavelength is tuned from 3160 to 3500 nm in 330 micros, corresponding to an idler frequency-tuning speed of 28 THz/ms. At a fiber-laser power of 6.6 W at 1074 nm, the singly resonant OPO generates 1.13-W cw idler radiation at 3200 nm.

Electrode geometries for periodic poling of ferroelectric materials.

We have realized novel electrode configurations for efficient periodic poling of ferroelectric crystals by applying a simple patterning method based on laser-induced material ablation. The new geometries were theoretically investigated and compared with conventional configurations. A systematic optimization of the patterning according to the ablated profile led to high-quality quasi-phase-matched structures in lithium niobate with a grating period of 20 mum . The periodically poled samples were characterized by use of third-order second-harmonic generation of a Nd:YAG laser.

All-solid-state neodymium-based single-frequency master-oscillator fiber power-amplifier system emitting 5.5 W of radiation at 1064 nm.

We demonstrate a master-oscillator fiber power-amplifier system consisting of a diode-pumped monolithic nonplanar ring oscillator as the master oscillator and a Nd:glass double-clad fiber as the power amplifier. The system emits up to 5.5 W of single-frequency radiation at a wavelength of 1064 nm with an M(2) value of ~1.1 . The optical emission spectrum is investigated with respect to the background of residual amplified spontaneous emission. Spectrally resolved amplitude-noise behavior is examined. Further power-scaling possibilities are discussed.

Fiber-laser-pumped continuous-wave singly resonant optical parametric oscillator.

We report on what is to our knowledge the first continuous-wave (cw) optical parametric oscillator (OPO) that is pumped by a tunable fiber laser. The OPO is singly resonant for the signal wave and consists of a 40-mm-long periodically poled LiNbO(3) crystal in a four-mirror ring cavity. At a pump power of 8.3 W provided by the wavelength-tunable Yb-doped fiber laser, the singly resonant OPO generates 1.9 W of 3200-nm cw idler radiation. The singly resonant OPO was tuned from 1515 to 1633 nm (signal) and from 3057 to 3574 nm (idler) by means of the crystal temperature and poling period. We obtained a wide idler tuning range, from 2980 to 3700 mn, by tuning the wavelength of the fiber laser from 1032 to 1095 nm.