Fluctuations of the laser pointing is a common problem in laser experiments; they cause a temporal variation of the laser energy, as well as reducing the coupling efficiency between the amplifier states. In the current HHG system at the CQOS laboratory, beam pointing jitter can be observed clearly at the output of the second amplifier while the output beam of the first amplifier is quite stable. The drift is likely due to thermal expansion of the optical elements and the vibration of the benches and support structures. In addition, it is likely due to the coupling mirrors between amplifier states which are not steady enough and are easily displaced. By integrating a high frame rate CCD camera with a new, powerful software algorithm, the beam pointing stabilization system for the HHG experiment has been proven to stabilize the laser pointing to less than 0.5 pixels, where the short-term and long-term drifts have been corrected with a sampling rate of 100 Hz.

 

The stabilization system consists of a steering mirror, a piezo motor driver, a camera and a computer that runs the program. The steering mirror (Thorlabs KC1-PZ piezoelectric mirror) is placed at the exit port of the first amplifier, which is controlled by the motor driver (Thorlabs MDT693). The laser beam after the second amplifier is sampled to illuminate on a screen, where the intensity is recorded by the camera (Basler scA640-120gc) and then sent to the computer which runs a LabView based stabilization program.

 

The laser spot on the screen captured by the camera is processed by the National Instruments Vision software package, in which the intensity profiles along the x and y directions are integrated and fitted with a Gaussian function to determine the beam’s centre with a frame rate up to 100 Hz. Then these centroids are compared with the expected values (in the middle of the camera) and the piezo motor’s steering angles are calculated by a PID algorithm. In addition to correcting the beam’s centre error to zero, the PID parameters are carefully optimized to minimize overshoot while maintaining a fast response of the piezoelectric mirror.

 

Without the stabilization, the beam’s centre has drifted about 6 pixels (corresponding to 300 µm or 2.5% of the beam FWHM diameter) in both the x and y directions. This beam continuously shifts throughout the day and without any adjustment, the drift can be as large as 30 pixels. However, the beam centre is locked to zero with an error of less than 0.5 pixels when the auto stabilization is turned on. For a long day operation, the system is able to lock the beam centre as long as the steering mirror is still within the operating limit.