Short pulse laser development
The system architecture for the generation of energetic femtosecond pulses in the UV is displayed below. Pulses out of a commercially available short pulse laser source (e.g. a Ti:sapphire oscillator/amplifier arrangement) are frequency tripled to 248.5 nm and then amplified in a special excimer amplifier module. Depending on custom requirements such a module can be optimized for different performance characteristics.
The generation of high intensities requires high pulse energies, a short pulse duration and excellent beam focusability. In this respect, a fundamental advantage of UV laser sources is their capability to focus the radiation much tighter than IR devices can do (focal area ~ λ2). UV high brightness laser systems apply excimer modules to amplify the frequency converted radiation of conventional short pulse lasers. A specific advantage of excimer amplifiers is offered by their low density amplifying medium, insuring negligible phase front distortions during amplification, thus resulting in excellent output beam quality.
In order to meet the high demands of reaching highest intensities in the UV, a special twin-tube amplifier device was designed with two discharge channels. A spatial filter implemented between the discharge tubes provides very efficient ASE (amplified spontaneous emission) suppression. Furthermore, a grating pulse compressor applied to the pulses in front of the second discharge tube allows control of the temporal pulse profile at relatively modest energy levels. This system architecture results in output pulse energies of up to 50 mJ with a typical pulse duration of 250 fs with excellent beam quality, thus enabling to reach focused intensities of ~ 1019 W/cm2.
High pulse energies
High pulse energies If a specific application requires the highest possible pulse energy out of a UV femtosecond laser, the main emphasis should be placed on the optimization of the energy extraction efficiency in the excimer power amplifier module. This can be accomplished by applying the so called interferometric multiplexing scheme.
The energy storage time of a KrF amplifier is typically 2-3 ns. Consequently, subsequent replenishment of the gain in every 3 ns within the entire gain lifetime (app. 15 ns) of the amplifier is possible. This means, that a train of 2 or 4 femtosecond pulses can be propagated through the amplifier, and all of the pulses will extract the same amount of energy from the amplifier. After passage through the gain module, the pulses are then recombined at the output. This should be done with interferometric precision in order to maintain the femtosecond pulse duration and the diffraction limited beam quality. Standard multiplexing schemes do not offer the required precision; therefore we introduced a new scheme which ensures phase locked superposition of the pulses upon demultiplexing.
A polarizer subdivides the input pulse into orthogonally polarized components, which propagate through the amplifier along the same optical path in reversed directions. Recombination of the pulses occurs at the same polarizer which is used for beam splitting. Applying the multiplexing scheme to an amplifier module of the type „LLG50“, the available output energy reaches 100 mJ with a subpicosecond pulse duration.
S. Szatmári, P. Simon: Interferometric multiplexing scheme for excimer amplifiers, Optics Commun. 98, 181 (1993).
J. Békési, G. Marowsky, S. Szatmári and P. Simon: A 100 mJ table-top short pulse amplifier for 248 nm using interferometric multiplexing, Zeitschrift für Physikalische Chemie, 215, 12, 1543 (2001)
High average power
Many applications (like industrial scale material processing) require a high average power of the applied laser pulses. This can only be realized using high repetition rate systems. To this aim we converted an industrial excimer laser („NovaLine 100“ of Coherent Lambda Physik) into a short pulse amplifier. This model has a sufficiently large discharge volume and delivers an average power of 100 W at a rep rate of 250 Hz in the standard nanosecond mode. With appropriate modifications of the discharge chamber and the high voltage charging circuit, amplification of seed pulses of a frequency converted Ti:sapphire laser beam up to 30 mJ/pulse at repetition rates in excess of 300 Hz has been demonstrated. The resulting average power of ~10 W is the highest value to date achieved by a table-top deep UV femtosecond laser system.
J. Békési, S. Szatmári, P. Simon, G. Marowsky:
Table-Top KrF Amplifier Delivering 270 fs Output Pulses with over 9 W Average Power at 300 Hz, Appl. Phys. B. 75, 521-524 (2002)
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Laser-Laboratorium Göttingen e.V. (LLG)
Head of the Department
Dr. Peter Simon
"Short Pulses / Nanostructures"
Contact person for
Nano Structure Technology:
Dr. Jürgen Ihlemann