Abstract
At the present state as described in the last chapter, the PFS is able to deliver 4.9 TW, few-cycle pulses which are already used for applications. To boost the peak power further towards the envisaged petawatt level enabling experiments with unprecedented pulse parameters, an upgrade of the PFS system is currently under construction. The core element of this upgrade is a third OPA stage whose operation requires major modifications to the current system. In the following, the most important considerations and experimental tests that were carried out to prepare for this step will be described.
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Notes
- 1.
The higher nonlinearity of LBO compared to DKDP enables at an identical gain the use of thinner crystals and therefore yields a lower B-integral and better phase-matching conditions which result in a broader amplified bandwidth.
- 2.
To achieve the same focal spot size for both wavelengths, the diameter of the iris was adjusted.
- 3.
As a test, we temporally stretched our pump pulses by about a factor of 4 resulting in an increase of the damage threshold fluence by a factor of \(\sim \)2. Interestingly this result is in accordance with the \(\sqrt{\tau }\) scaling which is supposed to apply only for pulses longer then few tens of picoseconds [7].
- 4.
Note that the self-focusing-induced damage threshold ratio of 2–3 between front and back surface which we observed in our measurements represents an upper limit: as we spatially filter the pump beams in the real system, hot-spots smaller than \(\sim \)1 mm are removed and self-focusing is less dramatic compared to the 180 \(\upmu \)m beam size used for damage threshold measurements.
- 5.
By “weighted mean fluence” we refer to the average fluence value of a beam, weighted with the generated second-harmonic energy: For any non-flat-top beam shape, there is a range of fluences F within the clear aperture of the crystal which can be described by a fluence distribution \(\mathcal {D}(F)\), normalized to \(\int _0^\infty \mathcal {D}(F) \, \mathrm {d}F = 1\). Assuming a constant SHG efficiency, it is \(F_\mathrm {SHG} \propto F\) and one obtains for the energy distribution (sloppily: the “energy fraction transported by the respective fluence”): \(E_\mathrm {SHG}(F) \propto \mathcal {D}(F)\,F\). Hence, the weighted mean fluence is \(\bar{F} = \frac{\int _0^\infty E_\mathrm {SHG}(F)\, F \, \mathrm {d}F}{\int _0^\infty E_\mathrm {SHG}(F) \, \mathrm {d}F} = \frac{\int _0^\infty \mathcal {D}(F)\, F^2 \, \mathrm {d}F}{\int _0^\infty \mathcal {D}(F)\, F \, \mathrm {d}F} \). For a 6th-order Gaussian beam and a crystal aperture of 1.15 \(\times \) FWHM this yields \(\bar{F} = {0.82}{}\,F_\mathrm {max}\) (\({0.88}{}\,F_\mathrm {max}\) for the non-saturated case where the SHG efficiency increases linearly with F).
- 6.
In principle one could also tilt the signal pulse front but since this beam is to be used for further experiments and since a PFT implies angular dispersion and distorts the beam focus, this is not an option for the PFS system.
- 7.
This approach slightly overestimates the effective mismatch as most pump energy is confined to the central part of the beam where bending angles are smaller than in the outer parts.
- 8.
It should be noted here that the common test setup with a double pass through the grating at Littrow angle does not provide meaningful information regarding surface flatness as in this configuration any small deformation is intrinsically compensated and does not appear in the interferogram (for details about his effect see [10]).
- 9.
One might notice that within the tolerance, \(\varepsilon _2\) is equal to \(\varepsilon _3\). This is due to the fact that the summed up telescope magnifications planned for the 1 J and 9 J beams from the compressor to the OPA stages are quasi-identical. As the setup, however, might change in the future, an independent optimization is nevertheless essential.
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Kessel, A. (2018). Preparations for a Third OPA Stage. In: Generation and Parametric Amplification of Few‐Cycle Light Pulses at Relativistic Intensities. Springer Theses. Springer, Cham. https://doi.org/10.1007/978-3-319-92843-2_6
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