Bessel beams are different from other types of beams due to their non-diffracting and other special properties. The unique properties of Bessel beams make them useful for plenty of laser applications. A coherent monochromatic beam with the non-diffracting property doesn’t spread out much over a distance that is much larger than what regular beams typically do. To understand this, we need to consider what happen to a normal laser beam focused by a lens:
For Gaussian beams focused by a lens, around the waist of the beam there is a range of defocus where the beam width stays stable , called the Rayleigh range or focal depth, and this range is directly related to the focused spot size. For Bessel-like beams, on the other hand, the waist size remains almost the same over a distance of many Rayleigh ranges, thus there is no dependance of the depth of focus on spot size with such beams.
Characteristics of Bessel Beams
The term “Bessel beam” has come from the Bessel functions of order n that describe the intensity pattern of these beams. The role of the Bessel-like functions is to produce concentric rings around a central lobe. The beam’s propagation distance increases when the number of rings increases, as each ring is focussed in turn to become the central lobe at a different position along the axis . However, due to this expansion, the energy in the central lobe decreases.
Whereas Gaussian beams with small initial sizes diverge quickly, the intensity of the Bessel beams remains the same over longer distances due to their non-divergent and non-diffracting properties.
Bessel Beam Axicon: How to Create Bessel Beams?
We must understand that Bessel beams are not naturally occurring laser beams. These beams are the result of artificial creation in the laboratory. We can use two different methods to produce Bessel beams. The first method that the scientists developed for the generation of these beams involves a cone-shaped prism called an axicon lens. The apex of this lens is placed along the optical axis. However, this method has several drawbacks, including issues in manufacturing tolerances and undefined areas in the apex.
Another method involves the use of diffractive optical elements. This is the most precise Bessel beam generation method. These optical elements, called diffractive axicons. involve an assortment of discrete rings, or “teeth” that create a phase modulation. The interesting thing is that the phase modification of the beams after passing through the diffractive Bessel beam axicon elements is identical to what would happen after passing through a traditional axicon lens. Thus, the versatility of the diffractive method removes constraints related to the angle on the base axicon when it comes to interaction with the wall. Another advantage of using a diffractive axicon element is that, unlike a refractive axicon, it has a non-defined non-apex area which is suitable for manipulating small input beams.