Pulse contrast

The pulse contrast of high-power lasers is the ratio between the maximum intensity of the laser pulse and the intensity at a fixed point in time:

${\displaystyle K={\frac {I_{\mathrm {max} }}{I(t)}}}$.

On a time scale of picoseconds that is interesting for high-power lasers, the pulse contrast can no longer be measured by photodiodes and fast evaluation electronics. Instead, the intensity profile can be determined by autocorrelation , i.e. H. by scanning the laser pulse with itself.

Schematic representation of the pulse characteristics of high-power lasers

There are several causes for intensities before and after the main pulse. Post-pulses are mostly caused by internal reflections on glass surfaces, the mirroring of which is not ideal and thus causes different delay paths. Pre-pulses in the time domain of a few picoseconds are caused by reflections of the main pulse on surfaces with different delay paths in regenerative amplifiers . Pre-pulses in the nanosecond range are generated by the transmission of a so-called pulse picker, which uses Pockels cells and polarizers to reduce the repetition rate of the laser after the oscillator. Also on a time scale of nanoseconds before the main pulse, the contrast is negatively influenced by the amplified spontaneous emission (ASE for short). The ASE is caused by the fluorescence of the active medium and is difficult to avoid. If the picosecond contrast drops to the intensity level of the ASE, this is an indication of incomplete compensation of the spectral phase.

In the nanosecond range, the pulse contrast can be adjusted using fast optical switches, e.g. B. Pockels cells, can be improved directly by about 3 orders of magnitude. On shorter time scales, an improvement in contrast can be achieved through non-linear processes. This includes, for example, the generation of the second harmonic, the process of cross polarized wave generation (XPW) and the use of a saturable absorber . With very high pulse energies, the use of a plasma mirror is most efficient.

literature

• Christian Rödel: Plasma mirror to improve the contrast of terawatt laser pulses for the generation of high harmonics on surfaces. Diploma thesis, University of Jena, 2009 ( PDF ).