Fig. 1. Simulation of a beta barium borate (BBO) based single-stage uOPA. (a) Signal energy of the uOPA as a function of the crystal length and the pump input energy. (b) Comparison of the normalized fluence lineouts after the OPA (solid line) with the input distribution (shaded area) at 1.5 mm. The colors correspond to the legend entries in (a). The input parameters of the signal are t = 66 fs (FWHM, Gaussian), = 1.8 mm (Gaussian) and E = 1 nJ. The input parameters of the pump are t = 1.5 ps (FWHM, Gaussian) and w = 2.14 mm (Gaussian).

Fig. 2. (a) Calculated normalized parametric gain of the uOPA for a pump at 515 nm with an intensity of 80 GW/cm and a 1 mm thick BBO crystal. The pump-to-signal angle defines the phase-matching angle between the signal at 1034 nm and the crystal axis. For this angle, we calculated the normalized SHG-efficiency at every wavelength for the signal (b) and the idler (c). The white dashed and dotted lines mark the spectral range of the signal and idler input wavelengths for a pump-to-signal angle of 2.25° 0.1°. The calculations did not include pump depletion.

Fig. 3. Simulated, normalized fluence lineouts of the signal after the uOPA at an output energy of 1 mJ (solid lines, referred to as ‘OPA’ in the legend) and of the input distribution (red shaded area, referred to as ‘input’ in the legend). Blue lines indicate that 1 mJ of output energy was achievable in this setup. If this was not the case, red lines represent the distribution after the maximum propagation distance of 1.5 mm. The input parameters for this simulation are a pump-to-signal angle of 2.25° and a seed energy of 1 μJ. The pump energy and signal duration have been varied according to the titles on the top and right, respectively. The beam sizes were not changed compared with the simulation in Figure 1.

Fig. 4. Simulated, normalized power, integrated over the spatial profile of the signal after the uOPA at an output energy of 1 mJ (solid lines, referred to as ‘OPA’ in the legend) and of the input distribution (red shaded area, referred to as ‘input’ in the legend). Blue lines indicate that 1 mJ of output energy was achievable in this setup. If this was not the case, red lines represent the distribution after the maximum propagation distance of 1.5 mm. The same simulation parameters as in Figure 3 are used.

Fig. 5. Schematic drawing of the complete uOPA system, including the oscillator (gray box), the pump laser (blue boxes) and the uOPA stages (orange boxes).

Fig. 6. Output energy and signal gain of the uOPA over pump energy in the first stage (a) and the optical-to-optical efficiency of the first stage (b). The dashed curve in (a) represents a fit to the OPA gain in the non-depleting pump regime (see Equation (1)).

Fig. 7. Output energy of the uOPA over pump energy in the second stage (a) and the optical-to-optical efficiency of the second stage (b). The pump energy in the first stage is kept constant at 2.7 mJ corresponding to an input energy of (5.75 1.2) μJ for the second stage.

Fig. 8. Beam diameters after amplification in the second uOPA stage, measured at a threshold of exp(2) (blue data) and via the second moment of the distribution (orange data).

Fig. 9. Exemplary radial lineouts of the beam after the second uOPA stage at different pump energies of 6, 13.5, and 24 mJ: (a) linear scale; (b) log-scale.

Fig. 10. M² measurement of the amplified pulse at an output energy of 1 mJ and pump energy of 24 mJ.

Fig. 11. Full beam profile of the amplified pulse at an output energy of 1 mJ and pump energy of 24 mJ.

Fig. 12. Average shift of the spectral center of gravity by delay (a) and spectra of the uOPA output in dependency of the delay between the pump and seed (b). The blue-shaded region marks the original seed spectrum of the oscillator.

Fig. 13. Stability measurement of the uOPA system over approximately 3 hours in a test laboratory. The colorbar represents the distribution of the energy over a 5 s timespan to visualize the short-term stability. The mean energy of this measurement was 1.3 mJ.

Fig. 14. Contrast measurement of the PHELIX laser before (red area) and after (blue area) implementation of the new uOPA. Several measurements before and after the implementation have been averaged for a clearer picture of the low intensity levels.