Axicon

An axicon is a specialized type of lens which has a conical surface. An axicon transforms a laser beam into a ring shaped distribution. They can be convex or concave and be made of any optical material. The combination with other axicons or lenses allows a wide variety of beam patterns to be generated. It can be used to turn a Gaussian beam into non-diffractive Bessel-like beams. Axicons were first proposed in 1954 by John McLeod.

Axicons are used in atomic traps and for generating plasma in linear accelerators. They are used in eye surgery in cases where a ring-shaped spot is useful.

The Axicon is usually characterized by the ratio of the diameter of the ring to the distance from the lens tip to image plane d/l.

Single axicons are usually used to generate an annular light distribution which is laterally constant along the optical axis over a certain range. This special feature results from the generation of (non-diffracting) Bessel-like beams with properties mainly determined by the Axicon angle α.

There are two areas of interest for a variety of applications: a long range with an almost constant intensity distribution (a) and a ring-shaped distant field intensity distribution (b). The distance (a) depends on the angle α of the Axicon and the diameter (ØEP) of the incident beam. The diameter of the annular distant field intensity distribution (b) is proportional to the length l. The width of the ring is about half the diameter of the incident beam.

One application of axicons is in telescopes, where the usual spherical objective is replaced by an axicon. Such a telescope can be simultaneously in focus for targets at distances from less than a meter to infinity, without making any adjustments. It can be used to simultaneously view two or more small sources placed along the line of sight.

Axicons can be used in laser eye surgery. Their ability to focus a laser beam into a ring is useful in surgery for smoothing and ablating corneal tissue. Using a combination of positive and negative axicons, the diameter of the ring of light can be adjusted to obtain the best performance.

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