Bessel beams are a distinct type of beams with some important properties that make them very appealing for a variety of laser applications. Unlike the more common Gaussian beams, Bessel beams do not diverge. This means that the beam maintains the same size at different distances. This non-divergence property is also construed as a non-diffracting phenomena in light beams. Gaussian beams can exhibit a certain degree of non-divergence as specified by their Rayleigh range. But this Rayleigh range is proportional to the square of the beam waist. Thus, for a small beam waist, the Rayleigh range is also very small. This explains why in a high NA microscope lens the depth of focus is very limited. In contrast, a Bessel beam has a much larger depth of focus than that obtained with a Gaussian beam. Likewise, a Bessel beam in free space propagation can remain collimated for much larger distances.
The name Bessel beam is derived from the fact that the irradiance profile of these beams is characterised by a Bessel function of order n. These functions exhibit a central lobe and concentric rings. The irradiance is distributed evenly across the rings and the central lobe. With a higher number of rings, the propagation distance increases at the expense of a lower energy in the central lobe.
Another very interesting property of Bessel beams is their capacity to self-reconstruct when an obstacle is placed along their elongated focus. As expected, the beam casts a shadow immediately after the obstacle but after some defined distance the beam reconstructs itself. This curious property can be explained by the fact that a Bessel beam is composed by a set of waves propagating on a cone.
Bessel beams are an alternative solution to the Helmholtz wave equation, just like the more common Gaussian beams. Bessel beams, however, must be generated somehow artificially. One simple method for generating a Bessel beam is to use an annular aperture and a lens in the path of a collimated beam. In this case the throughput is compromised as much of the light of the original beam is blocked by the aperture. A more efficient way is to use an axicon lens which is a conical type of lens with an apex at the front. This will help to generate Bessel beam axicon. A third method is to use a diffractive axicon element in which the phase imparted by an equivalent axicon is encoded into a discrete phase structure. This method provides a much more accurate phase profile, with no tip undefined zone and no variability in the axicon angles, making it especially suitable for precise laser applications.
Among the applications in which Bessel beams have proven effective are light sheet fluorescence microscopy, optical tweezers and laser glass cutting, to name a few. In optical tweezers a Bessel beam can trap long thin objects, like E. Coli. Furthermore, the inherent ring structure of the Bessel beam allows trapping both high and low refractive index particles at the same time. In light sheet fluorescence microscopy, in which a sample is illuminated with a light sheet, a Bessel beam can improve the penetration of this light sheet as the effect of scattering particles in the sample is diminished due to the aforementioned self-reconstructing property.
