tEchnology DiFFrActiVE optics
light in order Keeping
A multi-line pattern generated by a DoE projected onto a computer mouse. the bright spot in the centre is the zero order, which can provide a reference point for 3D measurements Image courtesy of Holoeye Photonics
Manipulating light by diffraction can be highly desirable in applications ranging from industrial laser processing to interferometry, as greg Blackman discovers
T
he properties of diffraction, which describe how waves propagate after encountering an obstacle, are applied to great effect in many optical systems.
Diffractive optical elements (DOEs) can maximise the potential of coherent laser light, with one optical component often replacing what would otherwise be a complicated optical setup of refractive and reflective lenses to achieve the same effect. A top-hat beam shaper, for example, which operates by diffraction, can turn a standard Gaussian beam into a top-hat profile, straightening the sides and flattening the top to give a sharp, uniform beam. Laser marking systems make use of these beam shapers to generate sharp, clear print without blurry edges. A laser beam can also be split into patterns and
arrays by passing it through a single diffractive element, something that would be difficult to achieve with other optics. Homogenisers, also operating by diffraction, reduce laser beam hotspots and provide a uniform output. Diffraction is a phenomenon resulting from waves (in the context of optical systems, light
20 ElEctro optics l MARCH 2011
waves) interacting with an object, either bending round a corner or spreading out through an opening. Incident light waves impacting on a surface, such as diffraction grating, will propagate out and interfere with one another, cancelling out or adding together as they become in and out of phase with one another. In this way, specific patterns of light can be created depending on the optical design. ‘Using DOEs, a transformation function can be
defined to create almost any pattern of light from a single laser beam,’ explains Israel Grossinger, president of Israeli company Holo-Or, which designs and manufactures DOEs. ‘A Gaussian beam can be transformed into a top-hat, for instance, or the beam can be split into anything from 2 to 1,000 separate beams either in 1D or 2D. All these things are very difficult to do with conventional optics. Because we can define the transformation function with diffractive optics, we can have an output that is very different from the input.’
One of the original products Holo-Or
developed 20 years ago was a DOE used to correct chromatic aberrations between a CO2
laser and its
red HeNe aiming beam as used in surgical laser systems. The DOE lines up the CO2
laser, which
is invisible to the human eye, in exactly the same focal plane as its aiming beam, a necessity if the laser system is to be used for surgery. ‘This was one of the first products Holo-Or designed, and now many companies manufacturing medical laser systems use diffractive optics,’ comments Grossinger. The DOEs are used not only to correct chromatic aberrations, but also to create homogenised spots and create patterned images on the tissue. ‘Diffractive optics can be designed for
particular focal lengths according to the customer’s specifications, rather than having to use various optics to change focal lengths and recollimate the light,’ explains Bernhard Russell, technical sales engineer at UK company, Laser Components, which supplies diffractive optics, including those from Holo-Or. ‘Also, because there is light lost at each surface of an optic, replacing many optics with one DOE increases the efficiency of the optical system.’ DOEs can change the shape of the beam from
a single spot to a more complex shape, which can provide benefits for producing a high-quality weld, as Daniel Lloyd, development engineer at UK DOE manufacturer Laser Optical Engineering, notes: ‘With this [complex] beam shape, material properties can be controlled within a weld a lot better, allowing materials like plastic powders to be welded rather than simply fused, as is the case with many rapid manufactured parts. Fully
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