Bessel beams are one type of non-diffracting laser beam. This simply means that these coherent monochromatic beams do not diverge. In this context, if you consider Gaussian beams which almost all natural laser beams are, these beams also show the non-diffracting property within a specific propagation length known as the Rayleigh range where the value of the Rayleigh range is inversely proportional to the beam’s size. However, the Gaussian beams start to diverge after the Rayleigh range, at a proportional rate to the size of the beam. When it comes to Bessel beams, the Rayleigh range is comparatively larger than that of the Gaussian beams and this is why these beams are considered to be non-divergent and non-diffracting.
Characteristics of the Bessel Beams
Apart from non-divergent and non-diffracting characteristics, there are several other notable characteristics of Bessel beams. For instance, these beams have the ability to self-heal or self-reconstruct after travelling through an obstacle. Whereas, in the case of other beams, including Gaussian beams, an obstacle will cause the beams’ divergence properties to alter. The constitution, mainly the angular spectrum content of a Bessel beam is developed in such a way that the beam gets easily reconstructed after encountering an obstacle, due to an interference effect.
How to Produce a Bessel Beam?
Typically, Bessel beams are not generated naturally in laser cavities. Therefore, we need to produce them in the laboratory by using some methods. There are two main effective methods to get Bessel beams. The first method involves an axicon lens. The axicon lens is basically a cone-shaped prism, the apex of which takes place along the optical axis. The axicon lens Bessel beam was the initial method to be developed. However, this method has various limitations in terms of manufacturing tolerances. The second method involves the use of a DOE or diffractive optical element. This method is comparatively effective and modern. In a diffractive optical element, phase modulation information is stored on small discrete elements called pixels. These pixels hold the encoded phase information. When an input beam comes across the diffractive Bessel beam axicon element, its phase gets changed in a way similar to how it would be if there were an axicon lens present instead. The main benefit of using a diffractive optical element is that it can encode any type of axicon. Therefore, this method has no limitations regarding the angle between the base axicon and the wall. More importantly, the diffractive Bessel beam axicon element always remains flat while an axicon lens has an apex with a badly defined optical function.
Applications
Bessel beams are useful in various applications. Some common applications of Bessel beams are laser beam shaping, such as in 3D printing and lithography, laser material processing, such as glass cutting, drilling, and engraving, and biomedical imaging.
