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“ (UV femtosecond amplifier LLG 50), the available output energy reaches 100 mJ with a subpicosecond pulse duration (UV femtosecond amplifier LLG 100).

Figure 1: Schematic of the interferometric multiplexing