R. Bödefeld

High-efficiency dielectric reflection gratings: design, fabrication, and analysis

We report on reflection gratings produced entirely of dielectric materials. This gives the opportunity to enhance the laser damage threshold over that occurring in conventional metal gratings used for chirped-pulse-amplification, high-power lasers. The design of the system combines a dielectric mirror and a well-defined corrugated top layer to obtain optimum results. The rules that have to be considered for the design optimization are described. We optimized the parameters of a dielectric grating with a binary structure and theoretically obtained 100% reflectivity for the -1 order in the Littrow mounting for a 45 degrees angle of incidence. Subsequently we fabricated gratings by structuring a low-refractive-index top layer of a multilayer stack with electron-beam lithography. The multilayer system was fabricated by conventional sputtering techniques onto a flat fused-silica substrate. The parameters of the device were measured and controlled by light scatterometer equipment. We measured 97% diffraction efficiency in the -1 order and damage thresholds of 4.4 and 0.18 J/cm(2) with 5-ns and 1-ps laser pulses, respectively, at a wavelength of 532 nm in working conditions.

Alignment of a tiled-grating compressor in a high-power chirped-pulse amplification laser system

We present a novel technique to align a tiled grating in all five relevant degrees of freedom utilized in the compressor of the high-power chirped-pulse amplification laser system POLARIS at the Institute for Optics and Quantum Electronics, Jena, Germany. With this technique, alignment errors of the two gratings with respect to each other can be detected with an accuracy of 1 microrad for the rotational and 40 nm for the translational degrees of freedom. This is well sufficient to recompress 1030 nm pulses, which were stretched to 2.2 ns before amplification, to their bandwith limit of 150 fs.

All-reflective, highly accurate polarization rotator for high-power short-pulse laser systems

We present the setup of a polarization rotating device and its adaption for high-power short-pulse laser systems. Compared to conventional halfwave plates, the all-reflective principle using three zero-phase shift mirrors provides a higher accuracy and a higher damage threshold. Since plan-parallel plates, e.g. these halfwave plates, generate postpulses, which could lead to the generation of prepulses during the subsequent laser chain, the presented device avoids parasitic pulses and is therefore the preferable alternative for high-contrast applications. Moreover the device is easily scalable for large beam diameters and its spectral reflectivity can be adjusted by an appropriate mirror coating to be well suited for ultra-short laser pulses.

Full characterization of the amplified spontaneous emission from a diode-pumped high-power laser system

We present the first complete temporal and spatial characterization of the amplified spontaneous emission (ASE) of laser radiation generated by a diode-pumped high-power laser system. The ASE of the different amplifiers was measured independently from the main pulse and was characterized within a time window of -10ms ≤ t ≤ 10ms and an accuracy of up to 15fs around the main pulse. Furthermore, the focusability and the energy of the ASE from each amplifier was measured after recompression. Using our analysis method, the laser components, which need to be optimized for a further improvement of the laser contrast, can be identified. This will be essential for laser-matter interaction experiments requiring a minimized ASE intensity or fluence.

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.

Contrast improvement by prepulse suppression in cascaded amplifier cavities

We introduce a method to suppress prepulses of pulse picking systems due to the limited extinction ratio of polarization gating systems. By matching the round trip times of the oscillator and the subsequent regenerative amplifiers, leaking pulses are hidden below the temporal intensity pedestal of the main pulse. With this method, prepulses at the temporal position equal to the time difference of the round trip times of the cavities could be suppressed completely.

Comparative damage study on ytterbium-doped materials for diode-pumped high energy lasers

We report on a comparative study of the damage threshold of ytterbium-doped laser materials which are important for diode-pumped, high-energy class short pulse lasers. Both surface and bulk damage thresholds at the lasing wavelength of 1030 nm were investigated. A pulse duration of 6.4 ns was chosen which allows a scaling of the damage threshold for gain media in q-switched lasers as well as chirped-pulse amplifiers. In order to achieve comparability and repeatability of the damage measurements the surface preparation of the used samples was kept constant. Furthermore, the correlation of the bulk damage threshold and the UV absorption spectra was analyzed.

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.

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.

POLARIS 200-TW phased-array grating compressor

Here, the authors present new results obtained by installing this optical arrangement in a vacuum chamber to be able to handle Joule-level pulses of the diode-pumped POLARIS laser system.