What is an Axicon and what does it do?
An axicon is an optical element with a conical surface that transforms a collimated laser beam into a focal line and a non-diffracting ring-shaped beam.
Creates a Focal Line: Unlike a normal lens that focuses light to a single point, an axicon refracts light to create a focal line or a series of points along the optical axis.This is achieved through the interference of light waves refracted by the conical surface.
Converts a Collimated Beam into a Ring: When the light beam is projected onto a surface in the "far field" (beyond the depth of focus), the axicon produces a ring-shaped beam. The diameter of this ring increases with the distance from the lens, while the thickness of the ring remains constant
Specifications
Incoming Light: A collimated beam of light, with a diameter of 2R, enters the flat side of the axicon.
Refraction: The conical surface of the axicon refracts the light rays inwards at a constant angle. Instead of converging to a single point, all the refracted rays are directed towards a segment of the central axis.
Line Focus (DOF): The region where these refracted rays overlap is where the line focus is formed. This entire region of overlap is defined as the Depth of Focus (DOF).
Output Profile: Beyond the DOF, the refracted rays no longer overlap on the central axis and begin to diverge. This divergence causes the beam to lose its Bessel profile and become a ring-shaped beam with an outer diameter of dr and a ring thickness of t at the observation plane.
2R: The input beam diameter
α: The physical angle of Axicon
DOF: Depth of focus, where Bessel beam is formed
L: The distance from Axicon to image plane
t: The output ring thickness (line width)
dr: The output ring diameter
Physical Angle(α)
The physical angle of an axicon is the base angle of its conical surface, which determines the angle at which light rays are deflected and the resulting properties of the beam it generates.
Physical Angle (α) : 0.5° ~ 40° (Typical：0.5°, 1°, 2°, 5°, 10°, 20°, 40°)
Ring Diameter
You can calculate the ring diameter of an axicon using the following formula:
Dr=2L x tan[(n−1)α]
Key Variables
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Dr is the diameter of the ring.
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L is the distance from the axicon to the plane where you are measuring the ring.
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n is the refractive index of the axicon lens material.
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α is the physical angle (base angle) of the axicon's conical surface.
The larger the physical angle (α), the larger the diameter of the concentric rings formed after the same propagation distance (L).
DOF (Depth of the Focus)
The Depth of Focus (DOF) for an axicon is the specific distance along the optical axis where the lens generates a non-diffracting, "focal line" with a nearly constant spot size and high intensity.
How to Calculate the Axicon DOF
The simplified formula for calculating the depth of focus for a plano-conical axicon lens with a small refraction angle is:
DOF ≈ R/(n−1)α
Where:
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R is the radius of the input laser beam.
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n is the refractive index of the axicon lens material.
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α (alpha) is the base angle of the axicon's conical surface.
The larger the physical angle (α), the shorter the depth of focus (DOF) of the Bessel beam; the smaller the physical angle (α), the longer the depth of focus.
(assuming axicon refractive index,n=1.51; input beam radius,r0 = 1mm)
Material of Axicon
The refractive index of an axicon lens is determined by its material. The larger the refractive index of the axicon, the shorter the depth of focus (DOF) of the Bessel beam.
(assuming axicon physical angle,α=2°; input beam radius,r0 = 1mm)
Mounting
Self Centering Mount
A self-centering mount uses spring-loaded jaws to automatically grip the axicon by its edge, ensuring its optical axis is precisely aligned with the mount's mechanical center.
Common Setups
Beam Expander + Axicon
This is a very common and effective setup. The primary purpose of placing a beam expander before an axicon is to increase the diameter of the input beam. This has two major effects on the output from the axicon:
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Increased Depth of Focus (DOF): The DOF of an axicon is directly proportional to the radius of the input beam. By using a beam expander to magnify the input beam, you can significantly increase the length of the focal line created by the axicon. This is crucial for applications that require a long, non-diffracting beam, such as laser alignment over long distances or deep-cutting applications.
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Increased Ring Thickness: For the ring-shaped beam formed in the far-field, its thickness is proportional to the input beam's diameter. Therefore, a larger input beam from the expander will produce a thicker, more powerful ring, which is beneficial for tasks like laser drilling and machining. *The ring thickness of a beam generated by a simple refractive axicon is approximately equal to the radius of the input laser beam
Plano Convex Lense + Axicon
When a collimated laser beam passes through this setup, the axicon first transforms the beam into a conical shape. The plano-convex lens, which is a standard focusing lens, then takes this conical light and converges it to a precise annular focus (a ring) at a set distance. This allows you to create a high-intensity, ring-shaped spot on a workpiece or target, which is invaluable for certain applications.
Axicon + Axicon (Same Specifications)
The most critical and useful feature of this setup is the ability to tune the diameter of the output ring beam by adjusting the distance between the two axicons.
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As you move the axicons closer together: The conical beam from the first axicon hits the second axicon at a smaller diameter. This results in a smaller diameter for the output collimated ring beam.
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As you move the axicons farther apart: The conical beam expands more before reaching the second axicon. This results in a larger diameter for the output collimated ring beam.
Advantages of Using Axicon
Why Bessel Beams Excel in 3D Microscopy
Using a Bessel beam (from an axicon) in 3D microscopy offers several key advantages over a Gaussian beam, optimized for imaging speed and sample health.
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Extended Depth of Focus: A single Bessel beam scan can image a large depth of a sample at once, drastically increasing 3D scanning speed compared to the multiple z-scans required by a Gaussian beam.
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"Self-Healing" Property: The Bessel beam can pass through scattering objects like biological tissue without its central core being destroyed, allowing for clearer imaging deeper into the sample.
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Reduced Phototoxicity: The Bessel beam's lower peak intensity (as energy is distributed across a core and rings) minimizes damage to living cells and reduces photobleaching of fluorescent dyes.
Using Bessel Beams for Laser Cutting
Compared to a Gaussian beam, the main benefit of using a Bessel beam for cutting is its ability to cut deeper and with higher quality, especially in transparent materials.
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Extended Depth of Focus: A Bessel beam maintains its focus and intensity over a much longer distance than a Gaussian beam. This allows for single-pass cutting of thick materials, which would require multiple passes with a Gaussian beam. This property is particularly valuable for applications like "stealth dicing" where the laser modifies the material internally.
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"Self-Healing" Property: The Bessel beam can pass through small defects or debris in the material without its central core being destroyed. This helps maintain a consistent cut quality, particularly when cutting deep channels or grooves.
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