A Top Hat laser beam, also known as, Flat Top laser beam, is a laser beam whose intensity profile at certain optical plains (typical the far field or the system focus plane) is mostly flat, unlike the typical Gaussian-like intensity profile of most laser beams. The top hat beam has a flat region at the center, and a “transfer region” at the edges of the beam where the energy decays to zero.
The name “top hat” is derived by the energy distribution which is similar to the top hat (or beaver hat) known to be worn by magicians.
Generally, there are several methods of achieving a top hat laser beam profile. One such method of achieving flat top beams is usable with highly coherent, low M2 laser beams. In this method, a top hat beam shaper, also known as a flat top beam shaper, is used to transform a Gaussian beam profile with a known diameter into a focused spot with a well-defined size and shape. The beam shaper is often a Diffractive Optical Element (DOE), especially in high power material processing applications, where precise angles are critical. This DOE is a unique phase free-form that modifies the laser beam phase to achieve a continuous intensity profile as well as a smooth phase profile at the far field, without any speckles.
Another method is the use of highly multimode lasers, and shaping the laser with a flat top homogenizing diffuser to the desired top hat shape. This method works well for less temporally-coherent laser sources with high M2, as the resulting profile can be very smooth, but results in a highly speckled beam for highly coherent laser sources with low M2.
Another method of flat top beam shaping is by using a laser whose output is a flat top. This can be highly effective, but often limits the laser system flexibility as the flat top profile needs to be imaged to the work plane, often requiring complicated optics and large distances.
Top Hat Applications
Top hat laser beam’s unique feature of sharp energy decay is extremely powerful when using laser sources for material processing applications, where accuracy is crucial and a specific energy threshold is required to perform the required process.
Anything above that process threshold is redundant, and anything below it is insufficient and may result in undesired thermal reactions (HAZ- Heat Affected Zone), for example – black scorched edges when cutting wood or cardboard.
Amongst the applications utilizing the benefits of the top hat laser beam are:
- Cutting
- Drilling
- Scribing
- Ablation
- Annealing
and many more.
The top hat laser beam is even more important when considering a system or process where the energy cost is a significant concern, such as when optimizing processing throughput. Thanks to its sharp transfer region and the ability to custom design for a specific process, the top hat beam shaper DOE enables one to optimize the use of their available laser source and utilize the energy in the most effective way to achieve the process, thus enabling higher processing speeds or larger processed areas.
Basic top hat typical setup
The most basic setup of a top hat laser beam will include the laser source, the top hat beam shaper, a focusing optic and the working plane where one positions the workpiece surface to be treated.
The analytical top hat beam shaper component of the system needs to be specifically suited to the system parameters, most importantly:
- Source wavelength
- Input beam diameter
- Desired spot size at the EFL
Holo/Or offers a large variety of standard top hat designs as well as tailored customization capabilities to meet your specific system parameters.
Design Considerations when using top hat in your system
To achieve optimal performance using a top hat beam shaper, one should account for following considerations:
- Input source:
- Single-mode (TEM00) Gaussian beam
- Preferably collimated (if not – collimation is required)
- All optics in the beam path, including mirrors, beam expanders / telescopes, and focusing optics, must be:
- Of high quality with low aberration level to avoid adding wave-front errors
- Larger than the beam diameter (e-2) on the aperture plane by a factor of at least 2 (optimally by a factor of over 2.5)
- Output spot:
- The top hat beam shaper output spot size has to be larger than the diffraction limited spot size by a factor of at least 1.5
- Diffraction limited spot size can be calculated by our calculator using the formula:
Where:
DL is the diffraction limited spot size
M2 is the input beam quality parameter
EFL is the effective focal length
λ is the wavelength
and D is the input beam diameter
TL; DR – Q & A
Q: What is a top hat laser beam?
A: A top hat laser beam is a laser beam whose intensity profile at certain optical plains (typically the far field or the system focus plane) is mostly flat, with a sharp drop to zero beyond certain shape boundaries.
Q: How can I get a Flat Top laser beam?
A: A Flat Top laser beam can be created from a single mode (TEM00) input laser by using a top hat beam shaper optical element and a focusing optic (a lens), or for highly multi-mode input by using a flat-top shaping diffuser.
Q: When is it advised to use a top hat beam shaper?
A: It is advised to use a top hat laser beam in high-power laser systems with a single-mode Gaussian laser source, where a very accurate and well-defined focused spot is required, and energy cost is a material component in the system. Note that are design considerations to be taken when using this component in your system.
Q: What are the top hat applications?
A: Some of the top hat laser most common applications include cutting, drilling, scribing ablation, annealing and more.
Q: How is a top hat beam shaper different than a homogenizer?
A: Both top hat and homogenizer produce a uniform intensity distribution, however their operation principals are quite different: an analytical top hat beam shaper continuously transforms the phase front to create a smooth, speckle-less Flat-Top intensity profile from a single mode input source, while a homogenizer overlays the beam over itself multiple times, creating a mixed beam with a Flat Top envelope suitable for multi-mode lasers where the overlaying will not generate strong speckles.
