Hyperbolic phase function used in a spatial light modulator for flat top focus generation.

We report on a novel transversal beam shaping approach to convert a Gaussian beam to a flat top focus profile by employing a hyperbolic phase function in a liquid crystal on silicon (LCOS) spatial light modulator (SLM). The well-known Powell lens approach is compared to ours. The analytically obtained phase function for the flat top focus conversion problem is compared to our approach theoretically and experimentally. A strategy for the generation of squared and circular top hat foci is derived.

Efficient burst mode amplifier for ultra-short pulses based on cryogenically cooled Yb³âº:CaF2.

We present a novel approach for the amplification of high peak power femtosecond laser pulses at a high repetition rate. This approach is based on an all-diode pumped burst mode laser scheme. In this scheme, pulse bursts with a total duration between 1 and 2 ms are be generated and amplified. They contain 50 to 2000 individual pulses equally spaced in time. The individual pulses have an initial duration of 350 fs and are stretched to 50 ps prior to amplification. The amplifier stage is based on Yb3+:CaF2 cooled to 100 K. In this amplifier, a total output energy in excess of 600 mJ per burst at a repetition rate of 10 Hz is demonstrated. For lower repetition rates the total output energy per burst can be scaled up to 915 mJ using a longer pump duration. This corresponds to an efficiency as high as 25% of extracted energy from absorbed pump energy. This is the highest efficiency, which has so far been demonstrated for a pulsed Yb3+:CaF2 amplifier.

Heating of non-ferromagnetic total hip endoprostheses during magnetic resonance imaging in an animal model.

To determine the heating effects of magnetic resonance imaging (MRI) on non-ferromagnetic total hip endoprostheses, hip endoprostheses were tested in vitro and in an ex vivo animal model. The MRI protocol on a 1.5 T scanner consisted of five sequences with a total duration of 23 min. The maximum heating of prosthetic material was 0.17 K in vitro and 0.90 K ex vivo; the maximum heating of bone surrounding tissue was 0.73 K ex vivo. Slight heating of the prostheses and the surrounding tissue was noted, which should not have any side effects in patients with titanium total hip endoprostheses.