A team at King Abdullah University of Science and Technology (KAUST) demonstrated an all-fiber-based approach to generating Bessel beams that could be useful for applications ranging from optical trapping to quantum communications. Bessel beams look quite different from the usual Gaussian light beams found in optics. They possess properties including self-healing and diffraction-free propagation and can carry orbital angular momentum (OAM). This family of beams — also known as 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. The KAUST team experimentally demonstrated that a custom-engineered optical fiber can generate a particular Bessel beam on demand. “Generating Bessel beams using traditional techniques involves space-consuming, expensive optical elements that require precise alignment,” said Innem Reddy, a Ph.D. student in the research group. “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,” Reddy said. The team used two-photon lithography, which enables 3D printing of intricate optical structures to fabricate special beam-shaping elements directly onto the tip of a single-mode optical fiber. The team’s 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. Illustration of the KAUST team's custom-engineered optical fiber, which it fabricated using two-photon lithography. The all-fiber-based approach to generating Bessel beams could be useful for applications ranging from optical trapping to quantum communications. Courtesy of KAUST/Andrea Bertoncini. “Our design allows for the generation of both zeroth- and high-order [Bessel beams] and fully controllable tailoring of the beams’ parameters, such as their diffraction-free propagation distance or the width of their central peak or node,” the researchers said in their paper. “Remarkably, we report for the first time, to the best of our knowledge, the generation of high-order [Bessel beams] from optical fibers.” The team has already used TPL to customize fibers in other ways, including the creation of polarization beamsplitters, 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,” said researcher Carlo Liberale, a research associate in bioscience at KAUST. Although the researchers developed the approach to transforming the standard Gaussian-like beam output from single-mode fibers into more complex optical modes, they said it could also be applied to multimode fibers, to expand their mode conversion possibilities. The researchers anticipate the approach to have a positive impact on applications such as optical and quantum communications, fiber-based sensors, microscopy, spectroscopy, and optical trapping. The research was published in Optica (www.doi.org/10.1364/OPTICA.453839).
