Single element vs multi element beam shaper advantages

What is a beam shaper?  Why are beam shapers needed?

Most lasers output a gaussian beam profile, with the fluence high at the spot center and gradually decreasing toward the edges. This is often suboptimal in various laser applications, requiring shaping of the beam. Beam shaping is a process where  the laser beam intensity profile and shape are changed to a distribution that is more suitable to the application requirements. This is often done through the use of a laser beam shaper, a diffractive or refractive component that manipulates the beam, typically placed before the system focus lens. Laser beam shapers belong to two main families: The phase randomizing  beam shaping diffusers, that perform best for multimode shaping,  and the  input dependent  top hat shapers , suitable for single mode input. In this article we focus on input-dependent flat top beam shapers and compare two approaches to achieving flat top: using a single optical element vs. using a multi-element system.

beam shaper quality parameters

A top hat beam shaper can shape the create a flat top profile without scrambling the phase and generating internal speckles. Thus, such shaping can achieve flat top spots or beams with good uniformity and sharp edges but is sensitive to input laser parameters such as beam diameter, laser quality (M2) and the lens system used to focus the beam to the work plane.

Top Hat quality can be characterized by several parameters:

• Efficiency -What part of input laser energy is within the shape, often defined at exp-2 of the flat top level. Most beam shapers have more than 90% efficiency, irrespective of shaping method.
• Transfer region /Edge width- The area were the intensity drops from the flat level to zero is called transfer region, or the edge of the shape.  It is defined in mrad for angular shaping or µm at the focal plane and is a function of both beam shaper design and system parameters.
• Uniformity –While designed to be flat, the shaped beam always has some variation in the flat top energy level, characterized as Peak to valley or peak- to average. The variations are predominantly  a function of system parameters, although beam shaper production tolerances do play a role.

All top hat  quality parameters are derived from a single system parameter, regardless of the specific beam shaping method employed to get a flat top. This parameter is the diffraction limited divergence, called ‘’diffraction limited spot size’’ in focal systems the. A single mode TEM00 gaussian beam with a certain diameter and wavelength has a natural angle of divergence that arises from diffraction even when the beam is perfectly collimated. This divergence and the EFL of the focusing system together determine the spot size when the beam is focused, called the diffraction limited spot size (DL).

Basic Physics thus puts a limit on beam shaping – it is not possible to shape a beam into a shape that is smaller than the DL. This is why DL is the basic scale unit of beam shaping. The ratio of top hat size to DL determines the quality of the shaping. i.e. how “flat top” is the flat top spot. The more DLs there are in the flat top shape, the better the flat top quality. Therefore, any comparisons between different shaping methods must be made for an optical system with the same DL value. i.e., same wavelength, NA and laser quality

Single Element beam shapers

One of the most commonly used types of beam shaper is the single element beam shaper. These are freeform optics that create a pre-designed phase front on the input laser beam, generating a uniform intensity distribution at the far field. Both DOEs and refractive free forms (ROEs, called PT by Holo/Or)  can serve as single element beam shapers, with the main difference being the effect of manufacturing tolerances and sensitivity to wavelength. For most cases , DOEs are optimal when shaping to a small number of diffraction limits (<15DL in x, y), as  the high accuracy of DOE lithographic production methods allows them to achieve good shaping quality . ROEs are better for cases where the shape is larger (>15 DL), or for polychromatic illumination (multiple lasers), as surface error in the free form processing has weaker effects on shaping in such cases. All single element beam shaper types typically operate at a lens system’s waist (focal plane). A typical setup is composed of a laser beam going into a variable beam expander used to adjust diameter, followed by the beam shaper and a focus lens. This sort of simple setup makes single element beam shapers especially suitable for integration into industrial laser systems.

Multi Element beam shapers

Single element beam shapers generate a top hat intensity pattern at focus or far field, but the phase front at this plane is not flat.  A flat phase and flat intensity are important for certain applications where defocus behavior must be symmetrical around the waist. By using more than one element, advanced manipulation of phase and intensity can be performed, including generating a flat top beam with a flat phase profile.  The simplest such configuration is what is known as “collimated top hat” and is a two element system:  The first focal beam shaper element creates a flat top beam at a certain plane, where a second phase  element is placed.  The second element flattens the phase, generating a collimated beam with a flat top intensity pattern. These beams can be useful for applications such as bulk laser sensing in transparent media or other specialized applications, as they maintain their flat top profile over distances that can be up more than X100 times the size of the top hat (i.e., for a 6mm flat top beam, a collimation range of >600mm is reasonable). However, they are often not useful for most industrial beam shaping applications, since they cannot be directly focused to a small flat top spot. To get a small spot, such beams must be imaged at large ratios (often 1:10-1:30), requiring impractically long optical paths that are hard to adjust.   For example, with a typical F-Theta EFL=100mm, such an optical system would have a >3m optical path. More complex multi-surface configurations exist but are generally offer no advantage  for shaping tasks such as shaping a TEM00 gaussian into a flat top beam of any given shape or size. Typically, the more surfaces are used for beam shaping, the higher the cost and the more losses, scattering and aberration are added to the beam.

Summary

Laser beam shaping is a highly useful technique for many laser applications. The basic unit of all beam shapers is the diffraction limited spot size- this determines shaping quality, regardless of what method is used for shaping. Single element beam shapers are highly useful for creating a flat top spot on the worksurface, being easy to integrate and capable of working with the system focusing optics. Multi element beam shapers are rarely needed for most industrial applications, as the flat top beam they generate cannot be focused directly but must be imaged, and multiple elements add cost and reduce performance compared to a single element.