Axicon lenses are kind of non-conventional optical elements that are used for two main purposes, namely:
- To generate ring-shaped beam structures.
- To increase the depth of focus of lasers (in transparent materials).
These two functions highlighted above stem from the fact that axicon lenses generate Bessel-like beams, which are said to be non-diffractive beams, at least to a certain limited extent. When a common laser beam, with a Gaussian profile, for instance, passes through an axicon, the beam radiance pattern is kind of re-ordered into a ring shape. If the axicon is used in conjunction with a lens, there is an elongation effect along the optical axis. This means that the focal spot is maintained for a longer distance compared to the gaussian input beam, which is equivalent to saying that there is an increase in the depth of focus for the system.
The first type of axicon that was invented was the refractive axicon. A refractive axicon looks similar to a plano-convex lens but with a striking difference. The active part of the element can be described as a kind of cone with an apex right at the center. Light is refracted by this structure in a way that a beam with a ring shape appears. Then, with the advent of diffractive optics and its ability to encode any optical function onto the pixels of the diffractive optical element, the diffractive axicon was developed.
The diffractive axicon addresses some unwanted features that are present in the refractive axicon. For instance, in the refractive axicon, due to the presence of the apex and the manufacturing difficulties associated with it, there is a dead zone in which light behaves erratically or is simply blocked. Also, a diffractive axicon does not add optical aberrations to the input beam whilst a refractive axicon will exhibit some degree of it due to the polishing process. With a diffractive axicon, greater control can be exerted over the shape and angles of the axicon function that is being encoded. A diffractive axicon can also be made smaller which opens up the possibility of having arrays of diffractive axicons.
Diffractive axicons are used in many fields. For instance, the ring structure is used in corneal surgery to smooth and process the tissue. Diffractive axicons are also used in laser glass cutting for the generation of Bessel-like beams that create plasma filamentation in the glass over long distances, enabling cutting with a narrow spot over a large glass thickness.