Carsten Fallnich

The GEO 600 gravitational wave detector

The GEO 600 laser interferometer with 600 m armlength is part of a worldwide network of gravitational wave detectors. Due to the use of advanced technologies like multiple pendulum suspensions with a monolithic last stage and signal recycling, the anticipated sensitivity of GEO 600 is close to the initial sensitivity of detectors with several kilometres armlength. This paper describes the subsystems of GEO 600, the status of the detector by September 2001 and the plans towards the first science run.

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.

Femtosecond lasers as novel tool in dental surgery

Abstract There is a proven potential of femtosecond lasers for medical applications like cornea shaping [1] , ear surgery or dental surgery [2] . Minimal invasive treatment of carious tissue has become an increasingly important aspect in modern dentistry. State of the art methods like grinding using turbine-driven drills or ablation by Er:YAG lasers [3] generate mechanical and thermal stress, thus generating micro cracks of several tens of microns in the enamel [4] . These cracks are starting points for new carious attacks and have to be avoided for long term success of the dental treatment. By using femtosecond lasers (1 fs =10 −15 s ) for ablating dental tissue, these drawbacks can be overcome. We have demonstrated that femtosecond laser ablation offers a tool for crack-free generation of cavities in dental tissue. Furthermore, spectral analysis of the laser induced plasma has been used to indicate carious oral tissue. Our latest results on femtosecond laser dentistry will be presented, demonstrating the great potential of this kind of laser technology in medicine.

High power fundamental mode Nd:YAG laser with efficient birefringence compensation.

Experiments on a high-power end-pumped Nd:YAG rod laser with an efficient birefringence compensation will be presented. A linearly polarized output power of 114 W with an M2-value of 1.05 was realized. Furthermore, the from our best knowledge highest injection-locked singlefrequency output power of 87 W in a nearly diffraction-limited beam was demonstrated.

High-peak-power pulses from a cavity-dumped Yb:KY(WO4)2 oscillator

We report generation of 1.35 microJ femtosecond laser pulses with a peak power of 3 MW at 1 MHz repetition rate from a diode-pumped Yb:KY(WO4)2 laser oscillator with cavity dumping. By extracavity compression with a large-mode-area fiber and a prism sequence, we generate ultrashort pulses with a duration of 21 fs and a peak power of 13 MW.

Status of the GEO600 detector

Of all the large interferometric gravitational-wave detectors, the German/British project GEO600 is the only one which uses dual recycling. During the four weeks of the international S4 data-taking run it reached an instrumental duty cycle of 97% with a peak sensitivity of 7 × 10−22 Hz−1/2 at 1 kHz. This paper describes the status during S4 and improvements thereafter.

Chemical and physical side effects at application of ultrashort laser pulses for intrastromal refractive surgery

Focusing femtosecond (fs) laser pulses into transparent media, such as corneal tissue, leads to optical breakdown, generation of a micro-plasma and, thus, a cutting effect inside the tissue - provided the intensity at the focus exceeds a threshold which depends on the pulse duration. For fs pulses, the corresponding threshold pulse energy is reduced by some orders of magnitude compared with ns or ps pulses. At a low pulse energy, thermal and mechanical damage to surrounding tissue is minimized, enabling a highly precise cutting effect inside the tissue while leaving anterior layers unaltered. In the following, investigations concerning aspects of the interaction of ultrashort laser pulses with aqueous media are presented: in particular, detection of pressure transients and bubble formation. For the first time - to our knowledge - the content of the resulting gas bubbles was analysed by gas chromatography, giving evidence of molecular hydrogen. Secondly, the potential of three-dimensional cutting effects within the corneal stroma for refractive surgery applications was evaluated in vitro on freshly enucleated porcine eyeglobes. Laser pulses with a duration of 200-300 fs and energies of 1.5-600 µJ were provided by a mode-locked Ti:sapphire laser with subsequent chirped pulse amplification.

Noncritically phase-matched continuous-wave mode-locked singly resonant optical parametric oscillator synchronously pumped by a Ti:sapphire laser

Efficient operation of a cw mode-locked singly resonant optical parametric oscillator of noncritically phase-matched KTiOPO4 is demonstrated. The optical parametric oscillator is synchronously pumped by an 82-MHz-repetition-rate mode-locked Ti:sapphire laser with pump-pulse lengths of 1.4 ps. We accomplished tuning of the optical parametric oscillator by tuning the wavelength of the pump laser. A variation of the Ti:sapphire laser wavelength in the range 720 nm < λp < 853 nm tunes the signal wave from 1.052 μm < λ < 1.214 μm and the idler wave from 2.286 < λi < 2.871 μm. We achieved stable operation with transform-limited pulses of 1.2-ps length over the entire tuning range. We obtained 700 mW of maximum average output power at the maximum of the tuning curve for the idler and signal waves, corresponding to an efficiency of 42%.

Direct-write subwavelength structuring with femtosecond laser pulses

Direct-write micro- and nanostructuring laser technologies are very important for the fabrication of new materials and multifunctional devices. Using tightly focused femtosecond laser pulses one can produce submicrometer holes and periodic structures in metals, semiconductors, and dielectrics on arbitrarily shaped surfaces. The achievable structure size is not restricted by the diffraction limit. It is determined by material properties and the laser pulse stability. We report investigations of possibilities to use femtosecond laser pulses for nanostructuring of different materials.

Supercontinuum generation with 200 pJ laser pulses in an extruded SF6 fiber at 1560 nm

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.