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# An In-Depth Look at Axicons

### What is an Axicon?

An Axicon is a conical prism defined by its alpha (α) and apex angles. Unlike a converging lens (e.g. a plano-convex (PCX), double-convex (DCX), or aspheric lens), which is designed to focus a light source to a single point on the optical axis, an Axicon uses interference to create a focal line along the optical axis (Figure 1). Within the beam overlap region (called the Depth of Focus, DOF), the axicon can replicate the properties of a Bessel beam, a beam comprised of rings equal in power to one another.

### Bessel Beam Features of an Axicon

Unlike a Gaussian beam which deteriorates over distance, a Bessel beam is non-diffracting, maintaining an unchanged transversal distribution as it propagates. Although a true Bessel beam would require an infinite amount of energy to create, an Axicon generates a close approximation with nearly non-diffracting properties within the Axicon's depth of focus (DOF). DOF is a function of the radius of the beam entering the Axicon (R), the Axicon's index of refraction (n), and the alpha angle (α):

(1)$$\text{DOF} = \frac{R}{\left( n - 1\right) \alpha}$$

Beyond the Axicon's depth of focus, a ring of light is formed. The thickness of the ring (t) remains constant and is easily determined; it is equivalent to the radius of the initial beam or one half the initial beam's diameter (db):

(2)$$t = \frac{d_b}{2}$$

The diameter of the ring is proportional to distance; increasing length from lens output to image (L) will increase the diameter of the ring (dr), and decreasing distance will decrease it. As Equation 3 mathematically illustrates, the diameter of the ring is related to twice the length, the tangent of the product of the refractive index (n), and the alpha angle (α).

(3)$$d_r = 2 \cdot L \cdot \tan{\left[ \left(n - 1 \right) \alpha \right]}$$

Figures 2 - 3 are real-world images of the focusing of green laser light by an Axicon; the figures illustrate an Axicon's ability to maintain constant ring thickness and proportional diameter with distance. In Figure 2, the Axicon is positioned at L = 228.6mm and in Figure 3, at L = 355.6mm. The images were created using a laser with a beam diameter of 4mm, a 127mm x 127mm white balance target, and an Axicon with a 20° alpha angle. For both Figure 2 and Figure 3, the thickness of the ring remained 2mm, while the diameter increased from approximately 73.66mm at L = 228.6mm between Axicon output and image to approximately 114.3mm at L = 355.6mm.

### Applications of an Axicon

The unique properties of an Axicon allow for applications in a range of research and medical fields. For example, an Axicon can aid improvements in laser corneal surgery, an outpatient corrective vision surgical procedure in which a laser ablates corneal tissue to correct the eye's refractive state. An Axicon's ability to focus a laser beam into a ring provides increased capability in smoothing and vaporizing the corneal tissue. With the use of a negative and positive Axicon, ring diameter can be adjusted to fit the patient's and surgeon's needs by manipulating the distance between the two axicons.

An Axicon is also beneficial in optical trapping, the use of a laser to create attractive and repulsive forces to manipulate microparticles and cells. The Bessel beam region within the DOF can trap particles on planar surfaces like a microscope slide without focal drift.1 The ring generated just beyond the DOF can also be used to isolate trapped objects.2

### Using Optical Components with an Axicon

In order to replicate a Bessel beam, an Axicon and laser must be aligned so that the laser beam travels along the Axicon's optical axis. A variety of optical components can be used to achieve the alignment and precision needed to do this including lasers, beam expanders, optical lens mounts, and posts and post holders. For example, beam expanders collimate incident laser light and reduce its divergence so that an Axicon can accurately create a ring shaped beam, and optical lens mounts securely hold the Axicon in place and can offer additional degrees of freedom for micron, or sub-micron, alignment.

By focusing a light source in a line along its optical axis, an Axicon creates an approximation of a Bessel beam. Although the ring diameter increases and decreases proportionally to the distance between the Axicon and the image, the ring thickness remains the same. Axicons are ideal for measurement and alignment, research, and medical applications requiring a ring shaped laser output.

### References

1. Axicon Produces Long, Thin Optical Trap.” Laser Focus World, 1 Sept. 2005.
2. Shao, Bing, et al. “Dynamically Adjustable Annular Laser Trapping Based on Axicons.” Applied Optics, vol. 45, no. 25, 2006, pp. 6421–6428.
3. Haw, Weldon W., MD, and Edward E. Manche, MD. "Prospective Study of Photorefractive Keratectomy for Hyperopia Using an Axicon Lens and Erodible Mask." Journal of Refractive Surgery 16, no. 6 (November 1, 2000): 724-30.
4. Mallik, Proteep. "The Axicon." Lecture, December 7, 2005. Accessed November 15, 2010.

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