ABSTRACT
This paper explores the potential of optical-based systems, specifically pseudo-non-diffractive beams, as an alternative for alignment. The study focuses on structured laser beams and hollow structured laser beams, which exhibit lower divergence and enhanced detection capabilities. The research objective is to analyze and compare centroiding algorithms in terms of accuracy and robustness to noise. The study compares the gamma-corrected and threshold-corrected center of gravity and correlation template matching. It also introduces a polarization-based algorithm.
ABSTRACT
The Structured Laser Beam (SLB) is a pseudo-non-diffracting laser beam that shares many characteristics with a Bessel beam. However, it can theoretically propagate over an unlimited distance while maintaining an extremely low inner core divergence of only 0.01 mrad. This makes it a promising candidate for precise long-distance alignment applications such as the alignment of particle accelerator components at CERN. In this work, a novel method to assess the symmetrical wavefront aberrations induced by an SLB generator is presented. Our approach is based on the analysis of a single-intensity distribution of an SLB. The coefficients of the Zernike polynomials are estimated using artificial intelligence before least-squares fitting is used to refine the result. This approach ensures that the fitting avoids local minima. This method provides a novel way to analyze the optical aberrations induced by the SLB generator.
ABSTRACT
The alignment of particle accelerators demands a dedicated measurement system based on a straight-line reference. This straight line can be provided by a laser beam. The alignment then involves accurately measuring the offset of accelerator components with respect to this light path. In order to be efficient, the laser beam needs to serve as a stable and straight reference for distances of several hundreds of meters. The attainable accuracy depends, among other parameters, on the laser spot size, which should ideally change very little over the distances at which the alignment system needs to operate. Due to the significant divergence of Gaussian laser beams, we propose using a structured laser beam (SLB) for alignment. Its transversal intensity profile is similar to a Bessel beam and consists of an intense inner core (IC) and concentric rings. The divergence of the IC, i.e., the growth of its size with distance, can be limited to 10µrad using a favorable generator configuration. Thus an SLB may be suitable as a straight-line reference for long-distance alignment applications. However, the SLB is distorted if obstructions cover parts of the outermost ring (OR) of the beam within, which should therefore also be small. In this paper, we investigate the relationship between the size of the IC and OR depending on the design parameters of the SLB generator. We use numerical simulations and experiments with different generators over distances up to 50â m to analyze the transversal intensity profile and wavefronts of different SLBs. The results indicate the general suitability of an SLB as a reference line for long-distance alignment but also expose tradeoffs between small IC and small OR. The findings outlined in the paper help to describe the optimal SLB parameters for given conditions.