Marco Hellwing

High-intensity, high-contrast laser pulses generated from the fully diode-pumped Yb:glass laser system POLARIS

We report on the first generation of high-contrast, 164 fs duration pulses from the laser system POLARIS reaching focused peak intensities in excess of 2×10(20) W/cm2. To our knowledge, this is the highest peak intensity reported so far that has been achieved with a diode-pumped, solid-state laser. Several passive contrast enhancement techniques have been specially developed and implemented, achieving a relative prepulse intensity smaller than 10(-8) at t=-30 ps before the main pulse. Furthermore a closed-loop adaptive-optics system has been installed. Together with angular chirp compensation, this method has led to a significant reduction of the focal spot size and an increase of the peak intensity.

16.6 J chirped femtosecond laser pulses from a diode-pumped Yb:CaF2 amplifier.

We report the amplification of laser pulses at a center wavelength of 1034 nm to an energy of 16.6 J from a fully diode-pumped amplifier using Yb:CaF2 as the active medium. Pumped by a total optical power of 300 kW from high-power laser diodes, a gain factor of g=6.1 was achieved in a nine-pass amplifier configuration agreeing with numerical simulations. A measured spectral bandwidth of 10 nm full width at half-maximum promises a bandwidth-limited compression of the pulses down to a duration of 150 fs. These are, to our knowledge, the most energetic laser pulses achieved from a diode-pumped chirped-pulse amplifier so far.

Ultra-high contrast frontend for high peak power fs-lasers at 1030 nm

We present the results from a new frontend within a double-chirped pulse amplification architecture (DCPA) utilizing crossed-polarized wave generation (XPW) for generating ultra-high contrast, 150 μJ-level, femtosecond seed pulses at 1030 nm. These pulses are used in the high energy class diode-pumped laser system Polaris at the Helmholtz Institute in Jena. Within this frontend, laser pulses from a 75 MHz oscillator-pulse train are extracted at a repetition rate of 1 Hz, temporally stretched, amplified and then recompressed reaching a pulse energy of 2 mJ, a bandwidth of 12 nm and 112 fs pulse duration at a center wavelength of 1030 nm. These pulses are temporally filtered via XPW in a holographic-cut BaF₂ crystal, resulting in 150 μJ pulse energy with an efficiency of 13 %. Due to this non-linear filtering, the relative intensity of the amplified spontaneous emission preceding the main pulse is suppressed to 2×10⁻¹³. This is, to the best of our knowledge, the lowest value achieved in a high peak power laser system operating at 1030 nm center wavelength.

High contrast, 86 fs, 35 mJ pulses from a diode-pumped, Yb:glass, double-chirped-pulse amplification laser system.

We demonstrate the generation of 86 fs, 35 mJ, high-contrast laser pulses at 1030 nm with a repetition rate of 1 Hz from a diode-pumped double chirped-pulse amplification setup. The pulses exhibit a spectral bandwidth exceeding 27 nm full width at half-maximum. This could be achieved by using a laser architecture comprising two stages of chirped pulse amplification with a cross-polarized wave generation filter in between, by applying spectral shaping and by increasing the spectral hard-clip of the second stretcher. These are, to the best of our knowledge, the shortest pulses at the mJ level with ultra-high contrast generated with a diode-pumped front end at 1030 nm.

Scaling laser-diode pumped solid-state amplifiers to the petawatt level

Summary form only given. Increased performance of high-power laser-diodes, decline in laser-diode-power cost, development of new lasing materials, progress on key optical components, and novel pulse-compression techniques enable the design of efficient petawatt-class amplifiers based on diode-pumped chirped-pulse amplification technology. New research in high-field plasma physics, laser-pumped X-ray sources, laser fusion, particle accelerators and generators, as well as astro-physics is envisioned. We have developed laser-diode bars for long pulse pumping that deliver more than 200 W peak-power at almost 50% electrical to optical efficiency. Progress in semiconductor technology, heat-sink design, and diode packaging ensures safe pulsed operation for the required lifetime at driving currents of more than 220 A.. The system design of POLARIS (Petawatt Optical Laser Amplifier for Radiation Intensive Experiments) and supporting data is presented, and the prospects and scaleability of diode-pumped high-power laser-technology discussed. The front-end consisting of a Ti:sapphire oscillator, a low-aberration stretcher, and a regenerative amplifier delivers 1 mJ in 2 ns pulses with 14 nm band width centered at 1035 nm.

Tunable filters for precise spectral gain control in ultra-short-pulse laser systems

We present tunable spectral filters (TSFs) as a variable and precisely adjustable method to control the spectral gain of short-pulse laser systems. The TSFs provide a small residual spectral phase and a high damage threshold, and generate no pre- or post-pulses. The method is demonstrated for two different laser materials and can be applied as an intracavity compensation in regenerative amplifiers as well as a method for pre-compensation in high-energy multipass amplifiers. With this method, a full width at half-maximum bandwidth of 23.9 nm could be demonstrated in a diode-pumped, 50 J Yb:CaF2 amplifier.

Multipass amplifiers of POLARIS

Advanced high intensity laser matter interaction experiments always call for optimized laser performance. In order to further enhance the POLARIS laser system, operational at the University of Jena and the Helmholtz-Institute Jena, in particular its energy, bandwidth and focusability, new amplifier technologies have been developed and are reported here. Additionally, existing sections were considerably improved. A new multi-pass amplification stage, which is able to replace two currently used ones, was developed in close collaboration with the MPQ (Garching). The new basic elements of this amplifier are well homogenized pump modules and the application of a successive imaging principle. By operating the amplifier under vacuum conditions a top hat beam profile with an output energy of up to 1.5 J per pulse is foreseen. The already implemented POLARIS amplifier A4 was further improved by adapting an advanced method for the homogenization of the multi-spot composed pump profile. The new method comprises a computer-based evolutionary algorithm which optimizes the position of the different spots regarding its individual size, shape and intensity. The latter allowed a better homogenization of the POLARIS near field profile.

POLARIS - A compact, diode-pumped laser system in the Petawatt regime

POLARIS is a compact diode pumped Yb-phosphate glass laser system. It is designed to reach the petawatt regime with a possible repetition frequency of 0.1 Hz. The first three amplifiers of this system are operating, and already constitute an all diode pumped terawatt system. The POLARIS system is designed to have an output of about 150 J in 150 fs, and should be completed in 2006.

Highest Intensity Diode Pumped Solid State Laser System

We report on the development and the generation of high-contrast, high-intensity laser pulses from the fully diode-pumped laser system POLARIS. The laser is optimized to meet the requirements for experiments on high-intensity laser matter interaction.

Tunable CW and Q-switched operation in Yb:CaF 2 and Yb:SrF 2

Summary form only given. Diode-pumped solid-sate laser systems combined with CPA technique are promising devices for generation of highest peak intensities. With regard to the choice of a suitable gain medium its spectroscopic properties determine the scaling of a laser system towards both highest output pulse energy and shortest pulse duration. In order to saturate the absorption with a minimum number of diode lasers a long fluorescence lifetime of the gain medium is desired. Ytterbium-doped fluorides such as Yb:CaF2 and Yb:SrF2 comprise a broad emission spectrum and a comparable long fluorescence lifetime. At a dopant concentration of 3.3% (atm.) we measured a radiation lifetime of 2.2 ms for Yb:CaF2 and 4.6 ms for Yb:SrF2. It shows absorption and emission cross section whereas the latter is determined by reciprocity method. According to ISO 11254-1 standard the damage threshold of Yb:CaF2 was measured to be 53 J cm-2 at a pulse duration of 10 ns (FWHM) and a center wavelength of 1064 nm. We built a diode-pumped Yb:CaF2 and Yb:SrF2 laser with a stable cavity, a resonator length of 1 m and a glass prism for wavelength tuning. A pulsed driven stack of 25 fast axis collimated diode-laser bars for 100 W (quasi-CW) maximum output power each is applied as pump source. The laser operates in quasi-CW operation with a repetition rate of 1 Hz and a pulse duration of 2 ms. Applying a DKDP Pockels cell a quarter wave plate and a thin film polarizer the laser operates as Q-switched laser and regenerative amplifier respectively. Pulses with a duration of 23 ns and 60 mJ pulse energy were generated in Q-switched operation with Yb:CaF2 as gain medium. As seed-pulse source a commercial Ti:Sa oscillator with 1030 nm center wavelength is used. The hard clip bandwidth after stretching to 2 ns is 32 nm. Regenerative amplification will be demonstrated.