What is a Flat Top Laser Beam Shaper?
Most Laser beams have a Gaussian profile, i.e. they have higher energy in the center, with a slow decay with increasing radius. This behavior is often sub-optimal, for example if the laser beam is used for a process with a certain energy threshold; a lot of energy is wasted by being either unnecessarily above or below this value. This issue can be solved by using flat top beam shapers, also called top hat beam shapers. Flat top beam shapers are optical elements that convert an incoming laser beam with a given intensity profile into a flat top beam profile, i.e. a profile where the energy is spread equally over the beam profile.
How Does a Flat Top beam shaper work?
There are in general two types of Flat Top shapers, and the choice between them be made based on the temporal coherence level of the laser. Analytical Top Hat beam shapers are suitable for coherent, single mode input, and are usually designed for a TEM00 Gaussian input beam with M2<1.3 . They work by a continuous transformation of the phase front, without mixing or overlaying any section of the beam, thus they create a smooth, speckle-less Flat-Top intensity profile, with very high conversion efficiency. To achieve optimal shaping, these Top Hat shapers require a well-defined input beam diameter, good centering accuracy and laser-grade, aberration free focusing and relay optics with a clear aperture of at least X2 input beam diameter.
The second type of flat top beam shapers is flat top diffusers. These elements work by overlaying the beam over itself multiple times, creating a mixed beam with a Flat Top envelope. They are more suitable for multimode, less coherent lasers, where this overlaying does not generate strong speckling. Their sharpness tends to be lower than Analytical shapers, but they are significantly less sensitive to system tolerances and not sensitive to input beam parameters. In this article we focus on the Analytical Top Hat shapers, as the Diffusers are extensively discussed in a separate article.
Types of Analytic Top Hat beam shapers
Top Hat shapers are either refractive or diffractive optics. Refractive beam shapers are based on high accuracy freeform surfaces that refract the incoming laser beam to create the desired flat top intensity profile. Diffractive beam shapers are windows patterned by a diffractive pattern that, for the design wavelength, creates the requires free-form phase to shape the laser into a Top Hat at the far-field (or lens focal plane) .
The main characteristics of Diffractive Top Hat beam shapers compared to refractive ones are:
- Diffractive Top Hat shapers have absolute angular accuracy, enabling small shaped flat tops of down to 1.5 diffraction limits, while refractive Top Hat shapers typically have some tolerance on shaping angle due to production processes.
- Diffractive Stable Top Hat elements have a significantly sharper transfer region compared to refractive Top Hats.
- Diffractive Top Hat shapers can be thin windows, while refractive ones as a rule must have some thickness (typically >3mm) for the accurate processing to the required freeform.
- Diffractive Top Hat shapers are monochromatic, i.e designed for a single wavelength, while refractive ones are polychromatic, i.e work for a range of wavelengths.
How does one work with a diffractive Gaussian to Top Hat beam shaper?
Holo/Or has an extensive installation guide and application notes, aimed at facilitating diffractive Top Hat use. All analytical Top Hat types are sensitive to tolerances, and require careful work and pre-alignment. The main requirements to use Top Hat shapers are:
- Single wavelength, tolerance ±2%
- Single mode TEM00 laser with M2<1.3
- Beam diameter as designed, tolerance ±10%
- Accurate focus – defocus tolerance in Z is approximately the Top Hat size in the focal plane.
- Clear aperture at least X2 input beam diameter, for all optics from the DOE top hat shaper to the focal plane.
Applications of laser Beam Shapers
Top Hat beam shapers are extensively used in the laser industry, mostly in material processing applications.
The applications include:
- Laser Micro machining
- Laser Drilling
- Laser scribing/ ablation
- Lithography
- Laser surface treatments(peening, hardening, annealing)
- Wafer inspection, wafer laser dicing
TL;DR - Q&A summary
Q: What is a laser Flat Top Beam Shaper?
A: A beam shaper in an optical component that turns an input laser beam into a flat angular distribution with a pre-defined shape. i.e a beam where the intensity is the same everywhere inside the shape.
Q: What shapes can a Flat Top shaper make?
A: A flat top beam shaper can create round, square, rectangle and line shapes. Holo/Or’s diffractive shapers are not limited by these shapes, and can create any arbitrary distribution, such as several rectangles or even letters.
Q: What are the types of beam shapers?
A: For Single mode, coherent laser shaping, analytical beam shapers are optimal- either diffractive or refractive. For less coherent lasers, flat top diffusers are optimal.
Q: What are the advantages of Diffractive flat top beam shapers?
A: Diffractive Top Hat shapers offer absolute angular accuracy, enabling shaping to dimensions of a few times the diffraction limited spot size. Such small shapes are highly useful in micro- machining and other precise laser material processing
Q: Where are flat top laser beam shapers used for?
A: flat top laser beam shapers are used in a variety of laser applications, mostly in laser material processing. These include micromachining, drilling, cutting, scribing, surface treatments and others.
