Generating Bessel beams with customised fibre

An all fibre-based approach to generating special optical beans, called Bessel beams, could open up new applications in imaging, optical trapping and communications. Bessel beams look different from the usual Gaussian light beams found in optics — they possess several interesting properties including self-healing, diffraction-free propagation and the ability to 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.

The creation of Bessel beams presents its own challenges, as several bulk optical elements, such as spatial light modulators or cone-shaped axicons, are needed to convert Gaussian beams to Bessel beams. However, Innem Reddy, Andrea Bertoncini and Carlo Liberale from King Abdullah University of Science and Technology (KAUST) have demonstrated that a custom-engineered fibre can do the job and generate a particular Bessel beam on demand.

Reddy, a PhD student in the group, said that generating Bessel beams using traditional techniques involves space-consuming, expensive optical elements that require precise alignment. “By opting for a fibre-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 fibre-based generation of Bessel beams allows innovative applications, such as minimally invasive endoscopic probes, optical coherence tomography, fibre-based optical trapping and manipulation of microscopic particles,” Reddy said.

The researchers used a technique called two-photon lithography (TPL), which enables 3S printing of intricate optical structures to fabricate special beam-shaping elements directly onto the tip of a single-mode optical fibre. 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 work is the latest in a research program that aims to leverage the potential of the TPL technique, where light is used to ‘write’ fine optical structures by solidifying a photoresist. The research findings were published in the journal Optica.

Researchers have already used TPL to customise fibres in other ways, including the creation of polarisation beam splitters, microlens assembles and optical tweezers.

“Fabricating ever more sophisticated optical devices on the end of optical fibres to empower them to deliver complex functionalities is one of the main research directions of our group,” Liberale said.

Image credit: iStock.com/vchal