An all fiber-based approach to generating Bessel beams could open up new applications in imaging, optical trapping and communi­cations. Bessel beams look quite different from the usual Gaussian light beams found in optics. In particular, they possess several interesting properties including self-healing, diffraction-free propa­gation and the ability to carry orbital angular momentum (OAM). This family of vortex beams with a charac­teristic ring-like shape and a dark central region include different orders of beams carrying different values of OAM.

However, the creation of Bessel beams is somewhat inconvenient: several bulk optical elements, such as spatial light modulators or cone-shaped axicons, are needed to convert Gaussian beams to Bessel beams. Now, Innem Reddy, Andrea Berton­cini and Carlo Liberale from King Abdullah University of Science and Technology in Saudi Arabia have experi­mentally demons­trated that a custom-engineered optical fiber can do the job and generate a particular Bessel beam on demand. “Generating Bessel beams using traditional techniques involves space-consuming, expensive optical elements that require precise alignment,” explains Reddy. “By opting for a fiber-based solution, we can obtain a compact Bessel beam generator that is pre-aligned and can deliver these beams even in remote and confined spaces, such as endoscopic appli­cations.”

“In particular, the fiber-based generation of Bessel beams allows innovative applications, such as minimally invasive endoscopic probes, optical coherence tomo­graphy, fiber-based optical trapping and manipulation of micro­scopic particles.” The team’s fiber is a masterpiece of custom engineering. They use two-photon litho­graphy (TPL), which enables 3D printing of intricate optical structures to fabri­cate special beam-shaping elements directly onto the tip of a single-mode optical fiber. Their design has three segments that, collec­tively, efficiently align and transform a conventional Gaussian beam into an annular beam and then, finally, a Bessel beam of the desired order and OAM value.

The team has already used TPL to customize fibers in other ways, including the creation of polari­zation beam splitters, microlens assemblies and optical tweezers. “Fabri­cating ever more sophis­ticated optical devices on the end of optical fibers to empower them to deliver complex functionalities is one of the main research directions of our group,” commented Liberale. (Source: KAUST)

Reference: I. V. A. K. Reddy et al.: 3D-printed fiber-based zeroth- and high-order Bessel beam generator, Optica 9, 645 (2022); DOI: 10.1364/OPTICA.453839

Link: Biological and Environmental Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia