Wed. Jul 24th, 2024

Bessel beams are beams of light that do not diffract. In other words, a Bessel beam can remain with the same diameter extent for a long distance. To put this into context, a single mode Gaussian beam, which is the most common type of beam in laser systems, will remain collimated for a very small distance and then will start increasing its size linearly with distance. The region where the Gaussian beam is said to be constant in size is referred to as the Rayleigh range. Thus, in a Bessel beam this Rayleigh range is considerably longer than the Gaussian beam and hence a Bessel is said to be non-diffracting. Another remarkable property of Bessel beams is their ability to self-heal when they encounter an obstacle along their path.

Bessel beams do not occur naturally, but they can be obtained by a proper transformation from a Gaussian beam. Such transformation can be carried out by using a Bessel beam axicon. A Bessel beam axicon can either be an axicon lens, in which a piece of glass is cut in such way that resembles a cone with its apex, or it can also be a diffractive Bessel beam axicon, in which the function of the axicon is encoded into a discrete array of modulating elements or pixels. From these two options, the most versatile alternative is to use a diffractive Bessel beam axicon as it is more resilient to manufacturing errors and it is also flat and lightweight. It also avoids the discontinuity of the apex as well as the thermal dependence on performance.

A diffractive axicon can be then used in many applications that could benefit from a Bessel beam. One important consequence of being non-diffracting is that when the beam is focused by a lens, the focal spot is elongated along the optical axis which increases substantially the depth of field of the optical system. This can be beneficial in microscopy, for example. In fact, there is a technique called Light Sheet Fluorescence microscopy that harnesses both the increased depth of field and the self-healing properties from Bessel beams.

Another application for a diffractive Bessel beam axicon is in glass cutting. In this application a laser beam is meant to deliver the same irradiance along the thickness of a glass substrate. The process could consist in simply moving the piece axially, but it will be more practical to first transform the Gaussian beam into a Bessel beam and then there will be no need to move anything.

By admin