Foucault knife-edge test for the characterization of mid-spatial frequencies to optimize liquid-cooled Nd:Phosphate split-slab amplifiers

Analysis of mid-spatial frequency wavefront distortions from a liquid-cooled flash-lamp pumped Nd:phosphate laser amplifier

“…As a guideline for large inertial confinement fusion (ICF) lasers, typical values of 2.5 nm root mean square (RMS) in the (1–10 mm) range for the transmitted wavefront are usually considered as an acceptable upper limit for individual amplifier slabs; in addition, a 1D power spectral density (PSD) specification in the form of , where is a constant and the frequency, is also used to avoid frequency peaks that are likely to be amplified by the Kerr effect [ 17 , 18 ] . In the case of a liquid-cooled amplifier slab, these distortions can be reduced below the nanometer RMS level in the (1–10 mm) range by optimizing the coolant distribution without optical pumping thanks to a knife-edge metrology [ 24 ] . Moreover, the same level of wavefront RMS deformation is present for spatial distortions below the millimeter scale, independent of the coolant distribution [ 24 ] .…”

“…In the case of a liquid-cooled amplifier slab, these distortions can be reduced below the nanometer RMS level in the (1–10 mm) range by optimizing the coolant distribution without optical pumping thanks to a knife-edge metrology [ 24 ] . Moreover, the same level of wavefront RMS deformation is present for spatial distortions below the millimeter scale, independent of the coolant distribution [ 24 ] .…”

“…Such distortions are induced by the coolant flow, as reported in Refs. [24, 25]. However, whatever the repetition rate tested, the PSD stays below the acceptable upper limit represented by the broken purple line.

Figure 4 One-dimensional PSD over the (1–10 mm) range calculated from the wavefront measurements.

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“…(ii) A liquid-cooled amplifier test-bed was designed and built to characterize mid- to large-scale spatial frequency distortions in amplified wavefronts and compare these with model predictions [ 23 ] . In addition, knife-edge Foucault measurements were also performed in a single-pass configuration to investigate (0.1–10 mm) small- to mid-scale spatial frequency distortions, in particular those induced by liquid flow [ 24 ] . Here we report on the amplified optical wavefront performances in the mid- to large-spatial-scale range (1–100 mm) of a neodymium phosphate liquid-cooled amplifier cell pumped by flash-lamps built as a test-bed for liquid-cooled amplification.…”