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We present a tunable, hybrid waveguide-fiber optical parametric oscillator (OPO) synchronously pumped by an ultra-fast fiber laser exploiting four-wave mixing (FWM) generated in silicon nitride waveguides. Parametric oscillation results in a 35 dB enhancement of the idler spectral power density in comparison to spontaneous FWM, with the ability of wide wavelength tuning over 86 nm in the O-band. Measurements of the oscillation threshold and the efficiency of the feedback loop reveal how an integration of the OPO on a single silicon nitride chip can be accomplished at standard repetition rates of pump lasers in the order of 100 MHz.

We present tunable waveguide-based optical parametric amplification by four-wave mixing (FWM) in silicon nitride waveguides, with the potential to be set up as an all-integrated device, for narrowband coherent anti-Stokes Raman scattering (CARS) imaging. Signal and idler pulses are generated via FWM with only 3 nJ pump pulse energy and stimulated by using only 4 mW of a continuous-wave seed source, resulting in a 35 dB enhancement of the idler spectral power density in comparison to spontaneous FWM. By using waveguides with different widths and tuning the wavelength of the signal wave seed, idler wavelengths covering the spectral region from 1.1 µm up to 1.6 µm can be generated. The versatility of the chip-based FWM light source is demonstrated by acquiring CARS images.

We demonstrate supercontinuum generation in stoichiometric silicon nitride (Sisub3/subNsub4/subin SiOsub2/sub) integrated optical waveguides, pumped at telecommunication wavelengths. The pump laser is a mode-locked erbium fiber laser at a wavelength of 1.56 µm with a pulse duration of 120 fs. With a waveguide-internal pulse energy of 1.4 nJ and a waveguide with 1.0 µm × 0.9 µm cross section, designed for anomalous dispersion across the 1500 nm telecommunication range, the output spectrum extends from the visible, at around 526 nm, up to the mid-infrared, at least to 2.6 µm, the instrumental limit of our detection. This output spans more than 2.2 octaves (454 THz at the -30 dB level). The measured output spectra agree well with theoretical modeling based on the generalized nonlinear Schrödinger equation. The infrared part of the supercontinuum spectra shifts progressively towards the mid-infrared, well beyond 2.6 µm, by increasing the width of the waveguides.

We report on the first experimental demonstration of the suppression of spontaneous Raman scattering via ground state depletion. The concept of Raman suppression can be used to achieve sub-diffraction-limited resolution in label-free microscopy by exploiting spatially selective signal suppression when imaging a sample with a combination of Gaussian- and donut-shaped beams and reconstructing a resolution-enhanced image from this data. Using a nanosecond pulsed laser source with an emission wavelength of 355 nm, the ground state of tris(bipyridine)ruthenium(II) molecules solved in acetonitrile was depleted and the spontaneous Raman scattering at 355 nm suppressed by nearly 50 %. Based on spectroscopic data retrieved from our experiment, we modeled the Raman image of a scattering center in order to demonstrate the applicability of this effect for superresolution Raman microscopy.

We demonstrate the potential of birefringence-based, all-optical, ultrafast conversion between the transverse modes in integrated optical waveguides by modelling the conversion process by numerically solving the multi-mode coupled nonlinear Schroedinger equations. The observed conversion is induced by a control beam and due to the Kerr effect, resulting in a transient index grating which coherently scatters probe light from one transverse waveguide mode into another. We introduce birefringent phase matching to enable efficient all-optically induced mode conversion at different wavelengths of the control and probe beam. It is shown that tailoring the waveguide geometry can be exploited to explicitly minimize intermodal group delay as well as to maximize the nonlinear coefficient, under the constraint of a phase matching condition. The waveguide geometries investigated here, allow for mode conversion with over two orders of magnitude reduced control pulse energy compared to previous schemes and thereby promise nonlinear mode switching exceeding efficiencies of 90% at switching energies below 1 nJ.

We report ultra-broadband supercontinuum generation in high-confinement Si3N4 integrated optical waveguides. The spectrum extends through the visible (from 470 nm) to the infrared spectral range (2130 nm) comprising a spectral bandwidth wider than 495 THz, which is the widest supercontinuum spectrum generated on a chip.

Simulations on three-dimensional random lasers were performed by finite-difference time-domain integration of Maxwell's equations combined with rate-equations providing gain. We investigated the frequency-dependent emission polarization of random lasers in the far-field of the sample and characterized the influence of anisotropic pumping in orthogonal polarizations. Under weak scattering, the polarization states of random lasing modes were random for isotropic pumping and linear under anisotropic pumping. These findings are in accordance with recent experimental observations. A crossover was observed towards very strong scattering, in which the scattering destroys the pump-induced polarization-anisotropy of the random lasing modes and randomizes (scrambles) the mode-polarization.

We designed an all-fiber mode-locked Erbium laser with optically stabilized repetition rate of 31.4 MHz. The stabilization was achieved by changing the refractive index of an Ytterbium-doped fiber in the resonator via optical pumping at a wavelength of 978 nm; and for long-term stability the local temperature of the fiber was additionally controlled with a thermo-electric element. The repetition rate was stabilized over 12 hours, and an Allan deviation of 2.5 × 10⁻¹² for an averaging time of 1 s could be achieved.

The impact of delayed optical feedback on the supercontinuum noise properties is investigated numerically and experimentally. The supercontinuum is generated by coupling femtosecond laser pulses into a microstructured fiber within a ring resonator, which introduces the optical feedback. The power noise and spectral amplitude noise properties of this feedback system are numerically and experimentally compared with single-pass supercontinuum generation. In a demonstrative experiment via optical feedback the power noise could be reduced by 15 dB and the spectral amplitude noise could be reduced by up to 28 dB.

The delay and phase dependent behavior of a system for supercontinuum generation by using a microstructured fiber within a synchronously pumped ring resonator is presented numerically. The feedback introduced by the resonator led to an interaction of the supercontinuum with the following femtosecond laser pulses and thus to the formation of a nonlinear oscillator. Via the feedback phase different regimes of nonlinear dynamics, such as steady state, period multiplication, limit cycle and chaos can be adjusted systematically. The spectrum within one regime of nonlinear dynamics can additionally be modified independently from the regime of nonlinear dynamics.

We theoretically investigate a scheme to obtain sub-diffraction-limited resolution in coherent anti-Stokes Raman scattering (CARS) microscopy. We find using density matrix calculations that the rise of vibrational (Raman) coherence can be strongly suppressed, and thereby the emission of CARS signals can be significantly reduced, when pre-populating the corresponding vibrational state through an incoherent process. The effectiveness of pre-populating the vibrational state of interest is investigated by considering the excitation of a neighbouring vibrational (control) state through an intense, mid-infrared control laser. We observe that, similar to the processes employed in stimulated emission depletion microscopy, the CARS signal exhibits saturation behaviour if the transition rate between the vibrational and the control state is large. Our approach opens up the possibility of achieving chemically selectivity sub-diffraction-limited spatially resolved imaging.

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.

We report on the generation of wave breaking-free pulses from a passively mode-locked all-fiber Erbium oscillator at 1.55 microm pumped by a Raman fiber assembly operating at 1.48 microm. The extracted pulses show a self-similar behaviour and can be scaled up to an average power of 675 mW only limited by the pump power. The laser operates at the fundamental repetition rate of 108 MHz, therefore the pulse energy is 6.2 nJ. After external compression, the dechirped pulse duration is 64 fs.

A comparison between composite crystalline and ceramic composite Nd:YAG rods for high power diode end-pumping is presented. Laser output power characteristics as well as the thermal lensing properties of the composite laser rods were evaluated. A maximum laser output power of 121 W and an optical-to-optical efficiency of 48 % were achieved by longitudinal pumping with fiber-coupled laser diodes.

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.

We report on the measurement of an Erbium-fiber oscillator's carrier-envelope-offset frequency using an extruded SF6 photonic crystal fiber for the generation of a more than two octave-spanning supercontinuum from 400 nm to beyond 1750 nm. A modified type of f-2f-interferometer was employed, beating the frequency doubled input signal of the fiber oscillator with the supercontinuum to generate the carrier-envelope-offset beat. Controlling the fiber oscillator's pump power with an electronic feedback loop, we phase-locked the carrier-envelope-offset frequency to an external reference source. The resulting residual phase excursions correspond to fractional frequency instabilities of the oscillator's frequency comb of the order of 10(-16) for averaging times longer than 10 s.

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.

We report an all-fiber diode-pumped Erbium-fiber oscillator-amplifier system as a source for supercontinuum generation in a photonic crystal fiber at 1560 nm. The passively mode-locked oscillator-amplifier system provides linearly polarized output pulses of 60 fs and an average output power of 59 mW at a repetition rate of 59.1 MHz. The laser pulses were launched into an extruded SF6-fiber for generation of an ultra-broadband supercontinuum. The evolution of the supercontinuum as a function of launched pulse energy was investigated. With pulse energies of about 200 pJ we observed a more than octave-spanning supercontinuum from 400 nm to beyond 1750 nm.

We present comparative noise figure measurements on neodymium and ytterbium doped double-clad fiber amplifiers. While the neodymium doped amplifier showed the quantum-limited noise figure of 3 dB, the ytterbium doped amplifier exhibited excess noise due to its three-level system. We compared optical and electrical methods with respect to the accuracy and the suitability for high power measurements.

Efficient high-power operation of Cr:LiSAF, Cr:LiSGAF, and Cr:LiCAF lasers pumped by broad-area laser diodes is demonstrated. A maximum slope efficiency of 51 % and output power of 0.55 W was reached at 1.2 W of absorbed pump power, which is the highest output power to date with broad-area laser diode pumping. With the laser design used the onset of thermal quenching in Cr:LiSAF due to high temperatures was pushed to higher pump powers and good mode matching was achieved.

A comparison of composite Nd:YAG laser rod crystals with one, two and three doped segments for high-power diode end-pumping is presented. An approach based on an expansion of the heat generation density and temperature distributions into a Fourier-Bessel basis set for solving the stationary heat conduction equation is used for choosing adequate segment lengths and dopant concentrations. A maximum laser output power of 167.5 W at an optical-to-optical efficiency of 53.6% was achieved by longitudinal pumping a crystal with three doped segments with fibre-coupled laser diodes.

Summary form only given. We report on a widely tunable narrow linewidth ytterbium-doped fiber ring laser device with a linearly polarized output power of 10 W. The Large Mode Area (LMA) core of the 20 m long fiber had a diameter of 10 /spl mu/m and was doped with 1000 ppm ytterbium. The core was transverse single-mode for wavelengths above 1 /spl mu/m. The pump-cladding was D-shaped in order to improve the pump light absorption. As pump source we used a fiber coupled 980 nm laser diode. The small quantum defect between the pump and the laser wavelength enables a very high efficiency. At maximum pump power of nearly 32 W a linearly polarized output power of 10 W with a slope efficiency of 68% with respect to the absorbed pump power at a wavelength of 1080 nm was achieved.

The manipulation of a supercontinuum via delayed optical feedback is investigated experimentally. The supercontinuum is generated in a microstructured fiber and a feedback ring resonator introduces the optical feedback and leads to the formation of different regimes of nonlinear dynamics. Via the feedback phase the optical spectrum and the regimes of nonlinear dynamics can be adjusted systematically. The impact of delay detuning on two different length scales, namely on a sub-wavelength scale and on a larger scale in the order of 10 μm are discussed. Additionally, the adjustment of the optical spectrum without changing the regime of nonlinear dynamics is demonstrated.

A system for supercontinuum generation by using a photonic crystal fiber within a synchronously pumped ring cavity is presented. The feedback led to an interaction of the generated supercontinuum with the following femtosecond laser pulses and thus to the formation of a nonlinear oscillator. The nonlinear dynamical behavior of this system was investigated experimentally and compared with numerical simulations. Steady state, period doubling and higher order multiplication of the repetition rate as well as limit cycle and chaotic behavior were observed in the supercontinuum generating system.

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.

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.

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.

We report on a diode end-pumped composite core-doped ceramic Nd:YAG laser. The ceramic Nd:YAG rod consists of a centrally doped region of 1.5 mm in diameter in a 3 mm rod. An output power of 144 W was achieved with an absorbed pump power of 226 W corresponding to an opt.-opt. efficiency of 64 % by longitudinal pumping with fiber-coupled laser diodes.

We present a high power fundamental mode ring oscillator with four longitudinally diode-pumped laser rods and effective birefringence compensation as a laser source for the next generation of gravitational wave detection. With this setup a linearly polarized output power of 213 W in a nearly diffraction limited beam with an M2 value of 1.15 was demonstrated.

The interferometric gravitational-wave detector LISA requires laser sources with 1W of output power and low frequency and power noise as well as actuators for further power and frequency stabilization. We report on power- and frequency noise measurements of an Yb-doped fiber amplifier seeded by a nonplanar ring oscillator and identify actuators for both power and frequency stabilization of such a system.

The beam quality of large-mode-area fiber amplifiers was investigated at the 10 W power level using a tunable ring cavity, that is also used in the laboratory system of GEO600 as a pre-mode-cleaner for mode filtering. More than 98% of the overall output power were contained within the polarized (200:1) TEM00 mode with an appropriate choice of coiling diameter. With the high sensitivity ring cavity analysis, the beam quality improvement caused by decreasing the coiling diameter was verified, while this could not be seen within conventional M2-measurements. The results are compared with the properties of single-mode fibers.

Q-switching of a wavelength tunable Yb3+-doped double-clad fiber laser by a Tm3+-codoping in the gain fiber is demonstrated. This system showed up to 2.4 W output power, up to 140 kHz repetition rate, a maximum pulse energy of 21.8 microJ and a minimum pulse duration of 1.1 micros. Using a grating pair in Littrow-Littman configuration the emission wavelength was tunable between 1055 nm and 1090 nm. The output radiation showed a maximum spectral linewidth of 4 GHz.

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.

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.