Why Use A Flat-top Laser Beam?

Why use a flat-top laser beam?

Most laser beams are Gaussian, although in some cases it is beneficial to have a non-Gaussian irradiance profile. The symmetrical irradiance profile of a Gaussian beam decreases as the distance from the center of the laser beam cross section increases. A top-hat beam has a constant irradiance profile through the cross section of the laser beam (Figure 1). Some applications benefit from constant intensity over a given area, including processing of semiconductor wafers, nonlinear frequency conversion at high power levels, and materials processing. Top-hat beams generally produce more accurate and predictable results, such as cleaner cuts and sharper edges, than Gaussian beams, but they come with additional system complexity and cost.

Figure 1: Gaussian beams waste energy through excess energy above the threshold required for the application and energy below the threshold outside the Gaussian beam. Flat-top beams are more efficient because they exceed the threshold while minimizing wasted energy.

Gaussian beam

High-quality single-mode lasers produce a low-order Gaussian irradiance distribution, the TEM00 mode. A Gaussian laser beam with the same average optical power as a flat-top laser beam has twice the peak flux (Figure 2). Gaussian beams remain invariant under transformations; therefore, as the beam propagates through the system, the beam profile remains Gaussian even if the beam size changes. This is because the Fourier transform of a Gaussian function is another Gaussian function. Light undergoes a Fourier transform by propagating to infinity or by focusing through a perfect lens.

Figure 2: Gaussian beam and flat-top beam at the same optical power, showing that the peak intensity of the Gaussian beam is twice that of the flat-top beam

Gaussian lasers are more common and cost-effective than other lasers, but they have several disadvantages, such as their “wings,” or areas of low intensity extending from the usable center portion of the beam. If the intensity of the wings of a Gaussian beam is below the threshold required for the application, it often results in wasted energy (Figure 1). They can also cause damage to surrounding areas and extend the heat-affected zone, which is detrimental in applications such as laser surgery and precision material processing. Because of the extended heat-affected zone of a Gaussian beam, the accuracy of cutting or shaping fine features using a Gaussian beam will be lower than with a flat-top beam, making the flat-top beam a better choice for such applications.

Flat-top beam

One way to assess how close an actual beam is to an ideal flat-top beam is through the flatness factor (Fη). This is determined by dividing the average irradiance value by the maximum irradiance value stated in ISO 13694.

The absence of flanks and steeper edge transitions in a flat-top beam delivers more efficient energy delivery and creates a smaller heat-affected zone. This is beneficial in a wide range of applications where high accuracy and minimizing damage to surrounding areas are a priority. In metrology applications, such as laser-induced damage threshold (LIDT) testing, the uniform and well-defined profile of a flat-top beam reduces measurement uncertainty and statistical variance. The uniform illumination provided by a flat-top beam also benefits a wide range of applications, such as fluorescence microscopy, holography, and interferometry.

Flat-top beams are not as cost-effective as Gaussian beams because additional beam shaping components are required to convert the output of the laser into a flat-top beam (Figure 3). This beam shaping component can be built into the laser source or placed external to the laser. These additional beam shaping components are sensitive to x-y alignment and depend on the input beam diameter. Flat-top laser beams also do not remain constant under transformations; therefore, the beam profile of the incident flat-top beam is not naturally preserved as the beam propagates. The Fourier transform of the flat-top function is an Airy disk function, which means that a flat-top beam will eventually evolve into an Airy disk.

Figure 3: Animation of the beam profile transitioning between Gaussian and flat-top distributions

How to achieve a flat-top beam

In some low-cost and low-performance systems, Gaussian beams are physically truncated using an aperture to create a pseudo flat top. This wastes energy from the Gaussian flanks, but can be effective when cost is a driving factor. For applications that require higher performance and efficient use of expensive laser energy, beam shaping optics are used to transform the Gaussian beam profile into a flat top beam profile. Refractive beam shapers allow for uniformity of intensity distribution and a flat phase front. Refractive flat top beam shapers produce collimated flat top beams with nearly 100% efficiency and no internal focusing, allowing for high power input beams (Figures 4 and 5). Their optical design makes holography, microscopy, and system integration suitable, especially at long distances.

Figure 4: Using a flat-top beam shaper to refractionally shape an incident Gaussian beam into a flat-top profile

Figure 5: Experimental intensity profile of the input Gaussian beam (left) and the output flat-top beam leaving the flat-top beam shaper (right)

Another type of beam shaper converts an input Gaussian beam into a collimated Airy disk profile. After focusing through a diffraction-limited lens, this forms a flat-top focal spot (Figure 6). The compact design and threading of these components make them easy to integrate into a variety of systems. They also have an efficiency of nearly 100%, making them suitable for applications that require a flat-top profile at the focal point, such as lithography, micromachining, and microwelding.

Figure 6: A flat-top beam shaper converts the input Gaussian beam profile into an Airy disk profile, resulting in a flat-top beam profile after passing through the focusing optics.

In addition to refractive beam shapers, other types include reflective, holographic, and diffractive designs.