Axicon

An axicon is a specialized type of lens that produces a line-shaped focus along its optical axis, instead of a point focus like a conventional lens. This elongated focal region is the defining characteristic of an axicon lens.

Typically, axicons are conical lenses, meaning one surface is flat while the other is conical. The conical surface refracts incoming light rays in such a way that they converge along a line segment on the optical axis. The length and position of this line focus depend on the cone angle of the axicon and the refractive index of the material.

Axicons are used in various applications where an extended focal region is required. Some examples include:

• Laser machining and processing: Creating extended cuts or surface modifications.
• Optical coherence tomography (OCT): Improving depth of field and axial resolution.
• Particle trapping and manipulation: Forming optical tweezers along a line.
• Microscopy: Extending the depth of focus for imaging thick samples.
• Generating Bessel beams: Axicons are commonly used to create approximations of non-diffracting Bessel beams. These beams maintain their shape over a longer distance than a Gaussian beam.
• Alignment: Providing a visual guide for aligning optical components.

The term "axicon" was coined by John McLeod in the 1950s. Variations of the axicon lens exist, including refractive axicons (made of glass or plastic), diffractive axicons (manufactured using diffraction gratings or holograms), and reflective axicons (using mirrors). Each type offers advantages and disadvantages in terms of manufacturing complexity, wavelength range, and efficiency.