The shaping is effective for pre-compensation of gain profile non-uniformities to achieve a flat output profile. To properly shape the beam profile for high-energy amplification, we use the apodization method, which involves a serrated aperture and a vacuum spatial filter. As Figure 3 shows, a serrated aperture is mounted on an x, y stage to position in the beam centroid. A 4-f relay system images the beam from the serrated aperture plane (position a) to the plane of the 25 mm amplifier (position c). A mode camera is set up to image the amplifier plane to study the beam shape with respect to the change of aperture or pinhole size.
Figure 4 shows the serrated aperture that we currently choose to put in the beamline. It is a steel plate with 90 teeth of ID 8.57 mm and OD 10.57 mm. The modulated near-field image (Fig. 4-a lower) shows the teeth biting into the edge of the beam. Figure 3 (b) shows the unshaped beam profiles (without serrated aperture and pinhole) imaged at the plane of the 25 mm amplifier. The beam width increases from 18 mm to 19.8 mm when the 25 mm rod is pumped. It is clear that The beam has some vertical stripes in the beam caused by a crystal defect from the YFE front end. After shaping, the beam profile is much smoother and smaller (figure 4-c). The beam size increases from 15 mm to 16 mm with pumping.
The current serrated aperture of teeth ID 8.57 mm is close to the YFE beam size 9.5 mm. The current pinhole size is 0.75mm in diameter. For further shaping, we have other serrated apertures and pinholes of smaller IDs as backups, which will further reduce the beam size and output energy.
Figure 3. The apodization method involves a serrated aperture (red circle a) and a vacuum spatial filter to remove modulated high-frequency mode through a pinhole of ID 0.75 mm (red circle b). The 4-f relay system (L1 and L2) images the plane a to plane c. A camera is set up to image the beam at plane c. This image is used to study the beam shape with respect to various serrated apertures.
Figure 4. The comparison of beam profiles with and without spatial shaping: (a) the serrated aperture drawing (upper) that is currently used in the beamline and the modulated beam profile (lower). (b) the original, unshaped beam profile (upper) and the unshaped beam profile being pumped by the 25 mm amplifier (lower). (c) the shaped beam profile (upper) and the shaped beam profile being pumped by the 25 mm amplifier (lower).