Source: official account: comprehensive research on optics and semiconductors
Diffractive Optical Elements (DOE), also known as binary optical devices, are mainly used for laser beam shaping, such as homogenization, collimation, focusing, and forming specific patterns. The use of diffractive optical elements for beam shaping is a very convenient, flexible, and powerful beam shaping method developed in recent years. DOE can be applied to various types of input lasers (such as single-mode Gaussian lasers, multi-mode lasers, etc.), forming specified spot shapes and intensity distributions on the laser focal plane, and can also achieve specific intensity distributions in the direction of laser propagation. Typical functions include: generating flat top distributed circular or rectangular light spots; Generate a linear distribution of light spots; Homogenize non-uniform multimode laser; Generate circular and multi ring light spot distributions; Generate one-dimensional and two-dimensional multi beam laser distributions; Form multi focus and long focal depth distribution in the transmission direction. With the urgent demand for laser beam transformation in domestic and international markets, various solutions based on diffractive optics are increasingly valued by the market, especially in the fields of flat top shaping and multi focus cutting, which have core independent intellectual property rights. Not only that, components and modules such as beam splitting, uniform light, long focal depth, vortex beam, dot matrix, and characteristic patterns also have important applications in various fields.
Diffraction is the phenomenon of light propagating around obstacles, also known as diffraction. Geometric optics believes that light propagates in a uniform medium along a straight line, so diffraction ontradicts the prediction of optical paths by geometric optics. But the principle of diffraction is actually easy to understand. Since light is a wave, we can use sound waves as an analogy. Any point on the wavefront can be a source, generating new waveforms that continue to propagate outward. Therefore, light that "bypasses" obstacles is equivalent to a point on the wavefront "derived" from it. This is why this phenomenon is called diffraction. For the principle of diffraction, please refer to the Huygens Fresnel diffraction theory mentioned in the previous article "Laser Special Topic (8): Theoretical Knowledge of Optical Resonant Resonators". The optical shaping principle of DOE is based on the diffraction phenomenon of light waves. Traditional optical components, such as lenses and mirrors, mainly manipulate light waves through refraction and reflection, while DOE achieves precise control over the direction, phase, and amplitude of light wave propagation by diffracting light waves through its special structure. Specifically, DOE modulates the wavefront of incident light through subtle changes in its surface structure. These structures can be regular or designed according to specific algorithms to produce the desired diffraction patterns. DOE manufacturing typically employs semiconductor processing techniques such as photolithography and etching to achieve nanometer or submicron level structural accuracy. When light passes through DOE, according to diffractive optics
Classification of DOE applications
DOE can be mainly divided into: shaping, beam splitting, multi focus, structured light, and other special beam generation based on their applications.
DOE for beam shaping can achieve specified spot shapes (square, polygon, strip, ring, circular, etc.) and energy distributions (such as flat top, Gaussian, ring, m-beam generator, diffractive cone lens, etc.) on the working surface. The ring generator is used to generate a circular intensity distribution spot. Commonly used ring generators include vortex phase plates, diffraction cone lenses, and multi ring generators. Cone lenses are widely used in laser processing to generate Bessel beams for achieving larger depth of focus. By applying diffractive optical technology on a conical lens, collimated light can be transformed into a cone surface for transmission. Through lens imaging, a circular light spot can be achieved. If used for point light sources, it can form focal lines distributed along the axis. Adjusting the beam diameter and the position of the diffraction cone lens can achieve rings of different diameters and thicknesses. A diffraction beam splitter divides a collimated beam into multiple beams arranged in one or two dimensions, with each beam retaining its original characteristics (structured light, on the other hand, is not responsible for generating structured light and does not guarantee the preservation of the original beam characteristics), and exiting at different angles. A diffraction beam splitter is essentially a grating structure, and its exit angle satisfies the grating equation. By carefully designing binary or multivariate diffraction unit structures, energy distribution between various outputs can be achieved (while ordinary gratings cannot achieve arbitrary distribution). Complex diffraction beam splitters can generate wide field illumination at large angles and specific patterns of light spot distribution. For example, a Daman grating is a binary phase grating with a special aperture function, which produces a Fraunhofer diffraction pattern of a certain number of equal intensity light spots on the incident light wave, completely avoiding the uneven distribution of spectral point intensity caused by the envelope of function intensity in general amplitude gratings. Compared with other diffraction structures of optical beam splitters, Dammann grating belongs to the Fourier transform type of beam splitter, which has the advantages of uniform light intensity of the spot array, not affected by the distribution of incident light waves, and can generate arbitrarily arranged lattice. Structured light refers to point, line, and surface structured light. A structured light generator can generate various customized light intensity distributions, such as shape, pattern, period, etc. By transmitting structured light onto uneven surfaces and measuring the deformation of their light intensity distribution, the depth and motion of different positions of the target can be calculated. Structured light generators have broad application prospects in 3D imaging (such as facial recognition), 3D sensing (such as autonomous driving LiDAR), machine vision, and computational vision.
Specific application examples:
Flat top beam shaper, Gaussian light leveling top light DOE
A flat top beam shaper, also known as a focusing type flat top beam shaper or a laser flat top spot lens, is used to obtain a flat top spot with uniform energy distribution, steep boundaries, and a specific shape. The size of the light spot is generally in the range of tens of micrometers to hundreds of micrometers. The shape of the light spot can be circular, rectangular, square, straight, elliptical, or customized, with excellent energy uniformity, which can achieve various high-precision laser processing applications.
2. M-wave diffraction optical element
The function of M-wave diffraction optical elements is to shape the incident laser into an M-shape, which refers to the uniform sliding of the laser spot in a certain direction, and the energy distribution after integration is uniform, forming a narrow straight line with uniform energy distribution. The characteristic of M-Shape laser spot makes it particularly suitable for laser material processing.
3. C-shaped beam shaping element
C-shaped beam shaper components are mainly used in the field of laser welding processing technology. The C-shaped strength distribution has advantages in width/depth ratio, reducing oxidation and eliminating hot cracks by giving bubbles a place to escape from the weld seam.
4. Multi focal transmission DOE, long focal depth DOE
As shown on the left in the figure below, multi focal DOE uses incident laser beams that are focused on different areas of the lens to simultaneously obtain multiple focal points in the axial propagation direction. The energy and spacing of each focal point are almost equal, making it particularly suitable for deep processing of transparent materials, such as laser deep cutting, invisible cutting, and cutting of 0.1-1mm glass or sapphire. As shown on the right in the figure below, a long focal depth DOE can generate a focal point near the focal length of the incident laser with nearly uniform energy distribution and a focal depth of several tens of micrometers to several millimeters. The laser focus can be stretched several tens of times longer than before, while the width remains basically unchanged. A long focal depth spot with uniform energy is particularly suitable for deep cutting of materials. Long focal depth DOE and multifocal DOE complement each other and are particularly suitable for the field of laser material processing. For example, laser cutting of transparent materials (glass, sapphire, etc.), laser metrology, laser process monitoring/monitoring, microscopes, etc.
5. Beam homogenizer (homogenizer mirror/homogenizer plate)
A beam homogenizer, also known as a homogenizer mirror or homogenizer plate, is used to evenly disperse the laser beam at a larger angle, thereby evenly distributing the laser energy over a larger range. Laser homogenizer and flat top beam shaper both form a laser spot with uniform energy distribution, and the difference between the two lies in the different angles of the outgoing beam, resulting in different spot sizes. The spot size obtained by flat top beam shaper is generally in the micrometer range, while the laser homogenizer outputs a uniform spot in the millimeter range.
6. Diffraction beam splitter
A diffraction beam splitter (lattice beam splitter) is one of the most basic diffractive optical elements, which functions to split a single incident light into several or more beams, and each beam has the characteristics of the original beam (except for its power and propagation angle changes, without changing the diameter, divergence angle, and wavefront distribution of the initial beam). The output of the beam splitter can be arranged in one dimension, two dimensions, or a line spot array, and the arrangement can be completely customized by the user.
7. Spiral phase plate, laser vortex lens
Spiral phase plate (Vortex Lens) is a commonly used DOE component for obtaining continuous phase changes. Its function is to generate a small diameter circular light spot (Vortex), which looks like a vortex like circle, also known as vortex light. Spiral phase plate is the most direct and simple method to obtain vortex light, which can conveniently design the diameter and topological charge of vortex light to meet the actual needs of users.
8. Diffraction axis cone mirror, circular ring laser generator, multi ring laser generator, concentric ring laser lens
As shown on the left in the figure below, a diffractive axicon, also known as a ring axicon laser generator, is used to transform the laser beam into a ring after passing through it. The laser energy is distributed in a ring/circle manner, where the width of the ring is generally very narrow, unlike the laser vortex lens, which produces light spots with a certain width. As shown on the right in the figure below, the multi ring laser generator can convert the incident beam into multiple concentric rings, whose radii are controllable and their widths are narrow, similar to a ring laser generator. The diameter, quantity, separation angle between rings, and energy distribution of each ring can be customized to obtain various concentric ring laser patterns.
How to choose DOE components
Generally speaking, the following principles should be noted before choosing to use DOE components:
1. The light beam generated by diffractive optical elements cannot violate the propagation law of light; The specific light intensity distribution constructed can only exist within a certain depth of field range. Therefore, when using it, the required spot shape, size, working distance, depth of field, etc. cannot be achieved simultaneously, and a balance needs to be made.
2. Diffraction optical elements are usually designed based on the wavelength, beam aperture, beam mode (M2), and near-field intensity distribution of the laser, so these parameters should be accurately measured before selection. Using parameters that do not match the design parameters will result in poor performance or even inability to use.
3. Diffraction optical elements are sensitive to the angle of incident light and require good optical path adjustment accuracy and stability.
4. Most diffractive optical elements precisely control the wavefront phase of the incident laser, so other components in the optical path such as reflective/transmissive lenses and lenses need to use high-precision, low aberration devices, otherwise it will affect the final effect.
5. Like conventional transmissive optical elements, diffractive optical elements can be made of infrared materials such as quartz, glass, gemstones, plastics and resins, ZnSe, etc., or coated with anti reflective films according to different wavelength and laser intensity requirements.
6. The dispersion of diffractive optical devices is not related to glass materials, but only to wavelength, that is, regardless of the refractive index of the materials that make up the diffractive device, as long as they are in the same wavelength band, they have the same dispersion ability. Furthermore, the Abbe number of diffractive optical devices is opposite in sign to that of traditional glass, and its absolute value is smaller than that of traditional glass materials, indicating that diffractive optical devices have strong dispersion. Further analysis shows that the partial dispersion caused by diffraction of diffractive optical elements is quite different from that caused by refraction of optical materials. Traditional optical materials show greater dispersion in the short wave band and smaller dispersion in the long wave band, while the changes of diffractive optical elements are just the opposite, which is conducive to the correction of the secondary spectrum.
7. The thermal expansion coefficient of refractive elements is determined by the linear expansion coefficient and refractive index temperature coefficient of the optical material, while the thermal expansion coefficient of diffractive optical elements is only related to the linear expansion coefficient of the substrate material, and is not related to the refractive index characteristics of the material.
Disclaimer: This article is reproduced or adapted online, and the copyright belongs to the original author. The content of the article is the author's personal opinion. Reproduction is only intended to convey a different viewpoint and does not represent the company's endorsement or support of that viewpoint. If you have any objections, please feel free to contact us.