Characteristics of Bessel beams
An Axicon, also known as an Axicon lens to the optics community, refers to a family of optics often used to transform laser beams into Bessel-like beams, a family of nondiffracting beams with interesting and useful characteristics. Bessel beams, when focused by an external lens, create a region of elongated focus that has the same diffraction limited spot size as the lens, but can be many multiples of the Rayleigh Range. This allows one to bypass the built-in limit on the depth of focus of lenses, where the tighter one focuses the shorter the Depth of Focus (DOF) becomes.
Applications of Bessel beams
Axicon Bessel beams are used in a variety of laser applications, where the focus media is transparent at the laser wavelength. One such field is laser glass cutting, often done with single mode ultrashort pulse lasers in the IR and green wavelengths. Glasses that must be cut are often 0.3-1.5mm thick, and process energy density often requires the laser to be tightly focused to spots of <10μm over the entire glass depth. This can only be achieved by two methods – mechanically moving the write head, thus reducing cut speed, or increasing the DOF using Bessel Beams. The Bessel Beam method is preferable both in throughput and reliability aspects, as it has no moving parts in Z direction.
Another application for Bessel beams is in Light sheet fluorescent microscopy, where a large DOF Gaussian Bessel beam is scanned over the sample, creating a light sheet that can detect fluorescent-bound particles or cells that are flowing in a microfluidics channel. This method enables high resolution and flow rates, while maintaining good accuracy.
Types of Axicon Bessel Beam generators
There are generally two types of Bessel Beams generator: refractive Axicons and Diffractive Axicons. Refractive axicons are Glass cones that are ground at a certain angle. While simple and affordable, the grinding production process imposes severe limits to their usefulness as Bessel beam generators:
- Base angles cannot be too small (0.5 degrees is the typical limit).
- Angles have tolerances, resulting in unstable performance.
- The cone tip is rounded and badly defined, increasing losses and zero order, especially for smaller beams.
- Polishing must be high-precision to avoid adding phase noise and ruining the Bessel Beam phase.
The other type of axicon lens Bessel Beam generators are Diffractive Axicons. Diffractive Axicons are flat windows with a micron-deep etched pattern on their surface that generate the same function as a refractive axicon for the design wavelength. They have several advantages compared to the refractive option:
- Absolute angular accuracy, no tolerances on Axicon angles.
- No dead zone in the axicon tip – all the area has the same optical performance.
- Phase is accurate and without noise, Bessel generation is perfect.
- Angles can be as small as desired, enabling direct DOF region control without any need for magnification.
- No thermal sensitivity – performance is constant under all conditions.
- Diffractive Bessel beams Axicons are flat, thin and lightweight optical elements posing a more compact and elegant solution, easier to integrate compared to refractive Axicons.
Holo/Or's Solutions for DOF increase
Holo/Or has many years of experience with Bessel Beam generation – we have a wide selection of single element Diffractive Axicon lenses, suitable for accurate, aberration free Bessel beam formation. We have also developed a more optimized focal shaping solution based on special Bessel-like beams, our DeepCleave glass cutting Module. This Module combines the performance of a High NA laser objective and a special Bessel beam DOF element, to achieve a Flat-Top focal region with very little energy outside the focal depth range. This makes it a more efficient solution compared to standard, single element Diffractive Axicons that generate a standard Bessel beam.
