Wave-front correction of a femtosecond laser using a deformable mirror

Abstract

Typical applications of ultra-high-power femtosecond lasers include precision drilling and surface micro-machining of metals, and micro-structuring of transparent materials. However, high peak-power pulsed lasers are difficult to focus close to the diffraction limit because of aberrations that induce deviations from a perfect spatial wave-front. The sources of these aberrations include thermally induced and nonlinear optical distortions, as well as static distortions such as those introduced by gratings used in chirped-pulse amplification (CPA). A spatially clean beam is desirable to achieve the highest possible intensity on-target, and to minimize the energy deposited outside the central focus. One way to achieve this is to correct the wave-front using an adaptive optical element such as a deformable mirror, a more cost-effective solution than increasing peak intensity by providing further pulse amplification. The wave-front of the femtosecond system is measured using a Hartmann-Shack wave-front sensor, and corrected with a 37-channel deformable membrane mirror used slightly off-axis. The deformable mirror has been tested with a FISBA OPTIK μPhase HR digital interferometer, which is also used to calibrate the performance of the wave-front sensor. The influence of fluctuations of the laser on the measurement is minimised by averaging the centroid positions obtained from several consecutive frames. The distorted wave-front is compared to a reference flat wave-front which is obtained from a collimated laser diode operating at the same wavelength as the femtosecond system. The voltages on the deformable mirror actuators are then set to minimise the difference between the measured and reference wave-fronts using a simple least squares approach. Wave-front sensor and correction software is implemented in Matlab.