High-accuracy calibration technique for passive pre-alignment of a laser autofocus system (English)

The development of laser driven plasma accelerators with high average power in solids requires simple and robust autofocus systems to determine, with accuracy, the focal position with respect to a rapidly refreshing target. Trial-and-error processes used in single shot experiment are not useful in this scenario. In this work we present a passive pre-alignment procedure to calibrate an autofocus setup, for fast refreshing solid target system, based on an astigmatic optical system. The calibration technique uses a hard aperture and a photodetector, located at the target position, to find the focal point with an experimental error, standard deviation, of less than 5 μm, which represents our translation stage resolution, achieved for a Rayleigh length of 35 μm. A geometrical analysis and numerical simulation have been done to demonstrate the effectiveness of the calibration technique and the accuracy of the astigmatic autofocus system. System accuracy can be improved with a better resolution of the translation stage.