AYASE Top-Hat Beam Shaper TECHNOLOGY

Similar to the conventional Diffractive Optical Element (DOE) technology, it uses light diffraction to convert a Gaussian beam into a top-hat beam (uniform intensity distribution), but has the following unique features:

An optical element with these features together has never existed before. The Top-Hat Beam Shaper can be used to convert into a light beam of

• line
• square-shaped

and also can be customized to:

• Resizing the incident Gaussian beam and top hat image
• Beam conversion to circular or rectangular
• Intensity distribution other than top hat

Comparison with Conventional DOE

Multi-wavelength Support

Conventional DOE diffracts light through a staircase-shaped structure. The diffraction efficiency is maximized when height of each staircase step is just an integer fraction of the wavelength, but it is not possible to achieve the desired diffraction at indecisive wavelengths. On the other hand, the CSDOE is not limited to any particular wavelength because it consists of a smooth aspheric surface. The specific calculation of the difference is shown in Figure-1.

The right side of the figure shows a conventional DOE (staircase type 16 steps, using IFTA method) designed for a wavelength of 532nm, and the left side shows the top-hat converted intensity distribution of the Top-Hat Beam Shaper.
At the design wavelength of 532nm (green line), the intensity distribution is top-hat, but at 405nm (shorter) and 702nm (longer), the intensity distribution profile is completely different in the conventional DOE, while the Top-Hat Beam Shaper maintains almost the same top-hat intensity distribution.

Here is a movie comparing how the intensity distribution profiles change by continuously changing the wavelength.

Fake Spots

The conventional DOE is achieved by processing the surface of the element into a staircase shape of approximately the same size as the wavelength. However, it is difficult to achieve a fine staircase shape, which inevitably results in rounded corners and uneven heights. As a result, unwanted diffraction, which does not appear at the design phase, occurs in the actual device. On the other hand, the Top-Hat Beam Shaper is made of a smooth aspheric surface, as mentioned above. The aspheric shapes are produced by a precision cutting or grinding machines, leaving no room for error such as staircases. This means that the Top-Hat Beam Shaper is completely free of fake spots.

Efficiency

Due to the absence of fake spots, all the energy of the incident Gaussian beam converges into the top-hat profile. Therefore, in principle, the diffraction efficiency is 100%. Even if the geometry is ideal, the efficiency of a conventional DOE varies depending on the number of stairs. Also, as the number of steps increases, the machining process becomes more complex and there is more room for error.

Comparison with Lineman

Ayase’s Lineman module is another product that makes light lines with top-hat intensity distribution; it spreads the light from a very small module to generate long, uniform lines of light with a uniform intensity distribution.

Differences in Operating Principles

The result of ray tracing calculation of the 60 °Lineman lens is shown in Figure - 3.

The density of the rays is controlled so that the intensity is uniform at the image plane as shown in the figure 3. This density change is not sensitive to wavelength, allowing Lineman to operate over a wide range of wavelength.

On the other hand, the Top-Hat Beam Shaper is designed to generate a top-hat intensity distribution at the focal plane that is converged by the lens. Figure-4 shows the result of ray tracing in the same way. In the figure, a converging lens with a focal length of 100mm is also drawn, but unlike the Lineman lens, the rays seem to travel almost straight ahead, and the Top-Hat Beam Shaper seems to have almost no effect.

Figure-5 shows a magnified view of the rays around the focus of the converging lens. All rays should be focused into small region, but the rays seem to have broken up.

Ray tracing calculation does not reflect the nature of light as a wave, and thus looks like a meaningless ray of light. However, if the calculation with diffraction is taken into account (Fresnel diffraction-based calculation), the top-hat intensity profile appears at the focal point (d = 0mm) as shown in Figure 6.

Thus, the Top-Hat Beam Shaper has a different operating principle from that of Lineman, and is designed to take advantage of the properties of light as a wave, just like a conventional DOE. On the other hand, unlike conventional DOEs, the Top-Hat Beam Shaper is not sensitive to wavelength, as is the case with Lineman.

Fabrication Technology

As optical elements, Lineman and Top-Hat Beam Shaper are manufactured in exactly the same way.

In other words, the mold is processed into an aspheric shape and the shape is transferred to the surface of the glass or plastic by thermal deformation process. This technology is called glass pressing or plastic molding.

As a result, the Top-Hat Beam Shaper has a characteristic of highly stable top-hat transformation with exactly the same accuracy and repeatability as Lineman.