(November Series #3) Closed-loop Optimisation of Deformable Mirrors for Laser Beam Aberration Correction

Zoe Wang will discuss our recent pre-print on "Closed-loop Optimisation of Deformable Mirrors for Laser Beam Aberration Correction".

Time: Nov 17th (postponed to Dec 5th), 2023 at 2:00 PM (London Time) [online]

Link: https://us06web.zoom.us/j/81028148713

Abstract: Laser beam optimisation is important for the creation of high intensity laser focal spots used in applications such as laser acceleration of particles from a thin foil target, with potential applications in areas such as hadron therapy. By presenting a closed-loop optimisation of low actuator count deformable mirrors (DM) for correction of spatial phase aberrations in a high power laser beam, we explore optimisation algorithms across systems with 5 and 9 actuators with a 12-bit control system (with $10^{18}$ and 3x$10^{32}$ search spaces) in terms of required lab time and speed and robustness of convergence. Conventional approaches such as genetic algorithms are comparatively brute force and can be inefficient when applied to a large search space. Hence, we also evaluate methods using uncertainty estimates, such as Bayesian Optimisation, that aim to maximise information gain at every experimentation step. We also discuss practical issues, such as methods for efficiently determining a simple single valued metric of laser beam quality with image data from a camera. Averaging noisy 2D visual measurements increases precision, but comes at the cost of slower experimental runs, and so it is important to find a balance between speed, accuracy and robustness against becoming trapped in a local minimum. Overall, the system showcases the challenges of a high throughput closed-loop optimisation system linked to image based measurement, and further work will focus on the integration of physical expert knowledge such as the representation of a laser beam aberration or a correction using a group of actuators described by a Zernike polynomial.