Stable Top Element | Diffractive Optics

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Diffractive Optical Stable Top Elements

In various industries there is a need to create a spot with uniform intensity and sharp transition regions. Our partner Holo-Or supplies for over a decade custom and stock Top-Hat Elements that accept a well-defined Gaussian beam, and transform this into a uniform intensity spot in a specific work plane.

Over the years customers asked for a design that is less sensitive for the position, orientation and beam quality of the input beam. In order to address this need Holo-Or introduced the Stable-Top design family.

In this application note we emphasize the difference between the traditional Top-Hat design and the newer Stable-Top version.

For general application information including

Operating Principal
Typical Set-Up
Magnification of the Spot
Design Considerations

we wish to refer to our Top-Hat application note: Free Download

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Characteristics

If we compare the Stable Top with the Traditional Top-Hat design we will find the new Stable Top to be:

Less sensitive to misalignment, (X-Y)
Less sensitive for elipticity of the input beam
Less sensitive for the beam quality of the input beam
Less sensitive to the orientation of the input beam
To have a sharper edge

From the negative side one can expect:

Stronger side lobs
Larger ripple in center

Comparison of Simulation (Nominal)

For a specific design we found the transfer region to be 2 times smaller for the new design (defined as the length of the region where the intensity of the spot climbs from 14% till 90% of the maximum). Although this value may differ per design, it is expected to be representative for this design family.

Unlike the traditional Top-Hat where in theory a virtual ripple free intensity profile can be obtained, the Stable-Top will even in theory have a minor ripple in its uniformity as displayed in figure 1.

Simulation intensity
for Stable-Top

When one takes a cross-section of the intensity profile, (displayed in figure 2) one can see that the ripple for this specific design is very small (~2% in our case), and the shape is rather random. This special shape helps to compensate for all kind of mis-alignments.

Simulation intensity
with Traditional Top-Hat Design

Comparing the profile with that of the traditional Top-Hat design (displayed in Fig. 3) on sees the uniformity to be about the same. One can also see that the traditional design has virtually no side lobs, but has a larger transfer region.

Comparison of Misalignment

For a specific design we compared the effects for a few types of misalignments. In figure 4 one can see that the new stable top design keeps a well defined and sharp edge even when the element is not well aligned with the beam.

In figure 5 one can see that the new stable top design keeps close to the desired round shape for an elliptical input beam that is orientated 45 degrees from the axis.