Fig. 1. (a) Sectional view of the EPM, depicting the geometry. The boundary of the laser beam path is shown in red. F_in and F_out are the input and output foci, respectively, of the EPM. (b) Image of a machined EPM placed next to a 10 coin of diameter 17.5 mm. (c) Expected fluence in J/cm² on the surface of the EPM when used with the PEARL laser delivering 20 J on target. All dimensions are in millimeters.

Fig. 2. Focal spot images normalized to peak intensity characterizing the performance of the EPMs. (a, b) Focal spot at the input (a) and output (b) of the EPM at the test bench. (c, d) Focal spots at the input (c) and (d) of the EPM at the PEARL laser facility as measured with low-power alignment beam. Field of view in all the images is 20 μm × 20 μm. The diameter of the full width at half maximum (FWHM) of the focal spot (Φ_FWHM) and the fraction of energy enclosed within the FWHM (E_enc) are given in each image.

Fig. 3. (a) Schematic of the setup used on the test bench to characterize the EPMs and benchmark the retro-alignment system. (b) Image of the setup, showing the physical layout of the components.

Fig. 4. Target alignment by monitoring the near field. (a) Schematic of the alignment method. (b) Near-field images during target alignment at the PEARL facility, showing the unobstructed near field, the shadow of the target moving from the right, and the shadow of the near field when the target was at the focus.

Fig. 5. Average brightness of the spot measured on the retro-imaging camera as a function of the target displacement. A displacement of 0 corresponds to the reference location of the target where it was aligned by monitoring the near field.