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SHARP Report Summary

Project ID: HPRI-CT-2001-50037
Country: Germany

OPCPA contrast level

In CPA lasers, amplified spontaneous emission (ASE) forms a pedestal underneath the main pulse. The ASE originates in the front-end amplifiers. By using appropriately gated Pockels cell (PC) shutters, its duration can be restricted to less than 10ns. This is still long even versus the duration of the stretched main pulse that is typically a few hundred ps long. Due to its incoherence, the ASE cannot be compressed and hence leaves the compressor unchanged.

In multi-TW titanium:sapphire lasers, the ratio of the peak intensity of the compressed main pulse and the ASE intensity is of the 1019W/cm², the ASE intensityorder of 107.

When the main pulse is focused to reaches 1013W/cm². For solid targets, this value is sufficiently large to create a preplasma with which the main pulse interacts. An important parameter in this context is the scale length of the preplasma. In order to investigate its effect, the ASE preceding the main pulse has to be controlled.

Following the philosophy of minimal invasion into the laser facility, this can be achieved in a relatively simple manner by incorporating an ultrafast PC shutter into the beamline. Since the rise time of a PC scales with the diameter of the KDP crystal, the diameter of the main pulse should be small. To meet this requirement, the shutter is best placed after the amplifier in which the main contribution to the ASE originates.

In our ATLAS (Advanced Titanium: Sapphire Laser) facility, this is the regenerative amplifier. After this amplifier, the beam diameter is expanded from 2mm to 6mm to avoid damage in the Glan prisms as well as in the slow and fast PCs. The first slow shutter consists of the first two small Glan prisms with the PC3 placed in-between and has a 12-ns long transmission window.

Hence the ASE can be maximally 12 ns long. However, actually it is shorter because the main pulse is positioned a few ns behind the leading edge of the window.

Reported by

Max Planck Institute
10 hans Kopferman sreasse 1
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