Operation Principle of Diffractive Optical Elements
The different types of Diffractive Optics (beam-splitters, pattern generators, kinoforms, beam shapers and gratings) utilize a microstructure surface relief profile for their optical function. Light transmitted by a DOE can be reshaped to almost any desired distribution, just by diffraction and the subsequent propagation. The DOE only encodes the shape of the desired intensity pattern, but maintains other parameters of the incident light source (e.g. beam size, divergence, polarization).
Due to their design flexibility, DOEs can have optical functions that can otherwise not be achieved at all, or only with complicated optical systems. Moreover, compared to refractive optical elements, DOEs are typically much thinner and lighter, making them an attractive replacement in a number of applications.
In most scenarios the incident laser beam is collimated, or focused to a fixed distance behind the DOE. HOLOEYE can offer solutions also for non-collimated or partially collimated light sources. To do so, the DOE microstructure combine pattern generation with focal power. In addition, the focusing properties of HOLOEYE’s DOEs are not limited to fixed working distances. Instead, our pattern generators can provide required focal power to achieve focusing of the laser to strongly inclined target planes. In fact, the best focus location per diffraction direction can be matched to almost arbitrary surfaces.
Diffractive Beam Splitter
A Diffractive Beam Splitter splits the incident laser beam into a 1-dimensional or 2-dimensional array of beams. Typically diffractive beam splitters are used in combination with a focusing lens. If so, the output beam array becomes an array of focused spots at a certain distance behind the lens.
The arrangement of the spots is not limited to arrays in perpendicular x-y lattices. Also hexagonal or irregular lattices are possible. For more complex arrangement of spots, like for structured light pseudo-random spot patterns, the diffractive beam-splitters can also be referred to as Diffractive Pattern generators.
Diffractive Pattern Generators
With Diffractive Optics complex patterns with a very high depth of field can be created.
The pattern comprises of many spots, which may overlap so that the element could be referred to as a Diffractive Diffusor, or still be visibly as individual spots, so that the element could be referred to as a Diffractive Beam-splitter.
Due to the high accuracy of the microstructures, the diffraction angles can be extremely precise, in particular when using a frequency stabilized laser source.
With Diffractive Diffusers flexible shaping of the emitted angular power distribution of various light sources can be achieved. Diffractive Diffusers can be best used with VCSEL arrays, because they consist of many individual incoherent laser emitters.
As a result, the angular far field diffracted light distribution is much less affected from interference-caused intensity modulations, and more uniform light distributions are obtained.
With tailored diffractive diffusers, HOLOEYE is able to create various light distributions for the application wavelength. By suppressing the zero order diffraction to well below 1% compared to the incident light even for large diffraction angles, the desired profiles can be obtained in very good approximation.
Diffractive Beam Shapers
An incident laser beam of ideally Gaussian intensity profile is transformed into a desired intensity profile at the target plane or workpiece. In most cases, the target is a uniform (‘flat-top’) circular or rectangular beam profile. Other shapes and non-uniform profiles can be obtained as well.
For a custom development, precise information about the input beam intensity and phase profile is required. For beams with high beam quality of M²<1.3, the phase profile is sufficiently described by the radius of curvature of its wavefront.