The shot rate of Z-Beamlet is primarily limited by thermal lensing in the slab amplifiers. Without active cooling, the recovery time of the cylindrical thermal lens is 3-4 hours (see Fig. 1), depending on the desired focal spot quality. However, the shot rate can be increased by employing active cooling and/or adaptive optics wavefront correction.
Figure 1a: Measured cylindrical thermal lens in the Z-Beamlet amplifiers.
Figure 1b: Decay of thermal lens over time (black curve). Residual thermal lens after mathematical subtraction of cylinder, showing that the laser can safely shoot after about 60 miutes.
We have demonstrated a simple, low cost alternative by deforming a rectangular mirror substrate that is simply supported at two pairs of symmetric line contacts. Figure 2 shows the mechanical implementation of the deformable mirror concept.=
Figure 2: Cross-section view of the single actuator deformable mirror at Z-Beamlet.
The four precision ground dowel pins (J), chosen for their exceptional tolerances, provide the line contacts on the mirror substrate (H) and are located by the housing (E) and piston (F). Force from the primary spring (C), varied by the travel of the linear actuator (A), is transferred precisely to the center of the piston, without moment coupling, by the ball (D), thus balancing the force equally between the dowel pin pairs. This produces a concave cylinder at the reflective substrate face (M) (see Figure 3).
Figure 3: Surface deflection of deformable mirror at a center deflection of 21.2 mm. One can clearly see the cylindrical substrate shape created.
To quantify the performance of the deformable mirror the wavefront taken by a Shack-Hartman sensor was fitted to a 3-dimensional cylindrical shape. This fit function was then subtracted from the measured wavefront data. Figure 4 a) depicts a contour plot of the residual error of the uncorrected shot after mathematical subtraction of the cylindrical fit.
Figure 4a: Contour plot of the residual on shot thermal lens after mathematical subtraction of cylindrical aberration.
Figure 4 b) displays a contour plot of the measured residual wavefront error after compensation with the deformable mirror.
Figure 4b: Contour plot of the measured on shot thermal lens using the deformable mirror corrector.
Missing data points are due to lenslets that were not recognized by the Hartmann sensor software and have therefore been omitted. It can be seen that the measured data agrees very well with the predicted residual beam distortion. This demonstrates that the deformable mirror can in fact fully compensate cylindrical aberration.
More detailed information can be found in:
Jens Schwarz, Marc Ramsey, Ian Smith, Daniel Headley, and John Porter, “Low order adaptive optics on Z-Beamlet using a single actuator deformable mirror” Opt. Comm., 2006, accepted for publication and soon to be published