Experimental Study of Structured Light Using a Free-electron Laser Oscillator

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Over the past three decades, laser beams with complex amplitude and phase structures, especially orbital angular momentum (OAM) beams, have been extensively investigated. Researchers have found a wide range of applications for OAM beams spanning a vast range of distance scales, from fundamental physics at the atomic level with modified selection rules, to macroscopic use such as optical tweezers, to probing of the universe such as detection of rotating black holes.

While structured light beams in the visible and longer wavelength regimes can be generated using many techniques, at shorter wavelengths, from vacuum ultraviolet to x-rays to gamma rays, it is much more challenging to produce such light beams. In recent years, to generate structured light in the shorter wavelengths, particle accelerator-based light sources, such as magnetic undulators and free-electron lasers (FELs), have been explored as a promising candidate. While the FEL work was mostly limited to single-pass FELs, we recognized that the oscillator FEL is very attractive for producing high-quality OAM beams with high intracavity power. In this work, we report the first experimental generation of a particular kind of structured light, a coherently mixed (CM) OAM beam, using the Duke storage ring FEL. The coherently mixed OAM beams have been generated up to the fourth order. This was made possible by modifying the FEL cavity to obtain cylindrical symmetry, while suppressing the low-order transverse modes. The cavity modification was implemented using a set of specially developed masks, including an annulus mask and a disk mask.

On the other hand, a reliable and rapid assessment of the structured light has a wide range of applications in the laser development, including high-quality OAM beam generation, optical characterization of beam quality and mode contents, and manipulation and correction of distorted OAM beams. While the diagnostic methods for structured light have been widely investigated in long wavelengths during recent years, they are not available for the short-wavelength regimes due to wavelength limitations of optics used. We report here two general diagnostic techniques for structured light: a phase retrieval method for wavefront reconstruction; and a modal analysis method for assessing the mode contents and beam quality of a structured laser beam. These newly developed methods involve very few optics, and in principle, can be used in a wide range of wavelengths, from infrared to visible to UV and x-ray.

The produced coherently mixed OAM FEL beams are found to possess good beam quality, excellent stability and reproducibility, and substantial intracavity power. Using the aforementioned diagnostic techniques, we have analyzed the measured FEL beam images to retrieve the complex wavefront and mode content. These beams have been found to have good mode quality, dominated by two degenerate OAM modes of the same order but opposite helicities. A pulsed mode operation of the OAM FEL beam has also been developed using an external drive, in which the OAM beams exhibit a highly reproducible temporal structure when the pulsing frequency is varied from 1 Hz to 30 Hz.

The development of OAM FEL beams using the storage ring FEL has paved the way for short-wavelength OAM laser beam generation using future FEL oscillators operating in the extreme ultraviolet and x-ray regimes. The operation of the storage ring FEL also paves the way for the generation of OAM gamma-ray beams via Compton scattering.






Liu, Peifan (2021). Experimental Study of Structured Light Using a Free-electron Laser Oscillator. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/23804.


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