Fig. 1. Schematic diagram of the laser system amplifier and free space propagation. Subjects of Sections 2.1–2.3 of this paper are identified from bottom to top. The 61 amplifying fibers are represented in red coming out from a set of nine cooled plates between which they are distributed before converging to form a bundle within the laser head (Section 2.1). While a small fraction (in green) of the light is diverted toward a measurement path dedicated at the servo loop control for CBC, the main beam (in red) undergoes a spatial selection (Section 2.2) followed by a temporal selection within the pulse train (Section 2.3).

Fig. 2. Three-dimensional printed water-cooled laser head hosting sleeves (white zirconium cylinder) in a V-shaped holder. Ferule-equipped fiber ends are inserted into the sleeves for accurate alignment. Downstream of the laser head, the water-cooled copper microlens array mount can be seen, while upstream the laser head, the nine water-cooled fiber amplifier supporting plates can be observed. (a) Back side and (b) front side views; (c) front side with Yb-doped fiber fluorescence.

Fig. 3. CAD view of the 61-hole plate collecting the unabsorbed 976 nm CW pump light refracted through the fiber end-cap (a). The cooling channel is split into two subchannels to maximize surface exchange and optimize heat removal. Thermal modeling shows a 14°C thermal gradient for single-channel cooling ducts (b), while it reveals a 10°C thermal gradient for split channel cooling ducts (c). A heat source of approximately 1 W per hole was considered in the model.

Fig. 4. Temperature evolution over a 2-hour period recorded through PT100 thermal probes fixed at an amplifier plate (red) and the laser head (blue). Far fields recorded in the pinhole plane are displayed a t = 10, 80 and 130 minutes. The pump was set at 5 A at t = 8 minutes. The repetition rate is 55 MHz and a 50% stable efficiency is recorded, leading to approximately 220 W average power beam after the pinhole.

Fig. 5. Far fields recorded in the pinhole plane in kW average power regime (bottom left values). The top three images are obtained at a 55 MHz repetition rate over a 50-minute period. The bottom three images are obtained at 55 MHz, 1 MHz and 429 kHz repetition rates, respectively. Energy in the main lobe is given as well as peak power after compression at 350 fs.

Fig. 6. (a) Stainless-steel pinhole with the onset of melting at 440 W average power operation. (b) Impact of surrounding lobes on the tungsten pinhole mount. (c) ZrO₂ coated pinhole. (d) CAD view illustrating surrounding beam deviation toward a beam dump.

Fig. 7. AR/AR coated 9° angular segment in an AR/HR 120 mm diameter disk.

Fig. 8. (a) Pulse train within the burst at 333 kHz. (b) Motor and mirror assembly (front right) and Leysop 1000:1 extinction ratio Pockels cell housing (back left). (c) 31 Hz burst train. (d) Single burst.

Fig. 9. Pockels cell output pulse train recorded with a Thorlabs DET08CFC 5 GHz photodiode when applying a 111 μs, 7 kV voltage square signal onto the KD*P electrodes. A series of 37 pulses can be observed. The contrast exceeds 1:100, evaluation being limited by the residual noise level on the photodiode. A few erratic parasitic signals appear on the trailing edge.

Fig. 10. Single pulse extraction from the 31 Hz burst.