Do You Know The Classification Of DOE Lasers? (Part 1)

Diffractive Optical Element (DOE) Lasers is a new optical element that is developing vigorously in recent years. DOE usually adopts a micro-nano etching process to form two-dimensional distribution of diffraction elements, each diffraction element can have a specific morphology, refractive index, etc., to fine-regulate the phase distribution of laser wavefront. The laser diffracted after passing through each diffraction unit and interfered at a certain distance (usually infinity or the focal plane of the lens), forming a specific distribution of light intensity.

Figure 1: A) schematic usage of diffractive optical elements; B) Outline; C) Surface microstructure schematic

After the advent of diffraction optical elements in high-power laser, laser processing, laser medical treatment, microscopic imaging, LiDAR, structured light lighting, laser display, and other fields to show a huge application potential, its advantages are mainly:

1) High efficiency. The accurately designed diffraction unit structure can ensure that nearly 100% of the laser energy is projected on the required pattern, and the efficiency is much higher than that of masks and other means.

2) Easy to use. Diffractive optical elements have a very small size and weight and can be used when inserted into the optical path. In most cases, it can be used with standard lenses, field mirrors, microscopic objectives, etc.

3) Flexibility. Thanks to the rapid development of micro and nano processing technology, DOE can be customized for different lasers or different target light intensity/phase distributions. At the same time, the light path structure of the DOE application is very simple, and different lenses can be used to achieve light spots of different geometric sizes.

As a new type of optical device, it is necessary to understand its characteristics when selecting/using diffractive optical elements.

2. the basic principles of diffractive optical element selection

According to different uses, DOE can usually be divided into beam shaping, beam splitting, structured light, multi-focus, other special beam generation, and so on. Each category has different principles, design, and application characteristics. In general, the following principles need to be noted before choosing to use DOE components:

1) The beam generated by the diffractive optical element can not violate the propagation law of light; The specific light intensity distribution constructed by it can only exist within a certain depth of field. Therefore, when in use, the required spot topography, size, working distance, depth of field, etc., sometimes cannot be both, and trade-offs need to be made;

2) Diffractive optical elements are usually designed according to the laser wavelength, beam aperture, beam mode (M2), and near-field intensity distribution, so these parameters should be measured more accurately before selection. The mismatch between the use parameters and the design parameters will lead to poor use effect or even unused;

3) Diffractive optical elements are sensitive to the Angle of incident light and require better optical path adjustment accuracy and stability;

4) Most diffractive optical elements precisely regulate the wavefront phase of the incident laser, so other components in the optical path such as the inverse/transmission lens, lens, etc., should use high-precision and low-wave difference devices, otherwise, it will affect the final effect;

5) As with conventional transmission optical elements, according to the requirements of different wavelengths and laser intensity, diffraction optical elements can be made of quartz, glass, gems, plastics and resins, ZnSe, and other infrared materials, and can also be plated with anti-reflection film.

3. Beam-shaping element

Beam shaping with DOE can achieve the specified spot shape (square, polygon, strip, ring, and circle, etc.) and energy distribution (such as flat top, Gaussian, ring, M-type, etc.) on the working surface.

1) Top hat generator

Flat-top distribution is used in various scenes such as laser medical beauty, laser processing, surface treatment, etc. Flat-top beam generators can transform single-transverse mode lasers (Gaussian distribution, M2< 1.3) Transform into a circular, square, strip, and other uniform light intensity and clear edge distribution.

① Flat-top beam generator features:

· Suitable for single transverse mode Gaussian beam, M2 < 1.3;

· The flat-top generator has the best effect when placed on the waist of the Gaussian beam;

· The flat-top generator cannot produce spots with a scale smaller than the diffraction limit, usually 1.5 to 5 times the diffraction limit;

· When the flat-top generator is in use, the optical element requires low wave difference, and the effective aperture should be more than twice the waist diameter of the incident beam, preferably 2.5 times;

· The target beam shape and intensity distribution can only be maintained within a certain distance range, usually half of the spot size;

· Sensitive to incident light diameter, incident light center position, incident Angle, etc.

② Main applications of flat-top beam generator:

· Laser processing and treatment: microhole, drilling, welding, cutting, marking, corrosion

· Medicine and Beauty

· Laser display

· Marking and printing。

2) Optical Diffuser/ Homogenizer

The beam homogenizer can also produce a variety of shapes and uniform distribution of energy (or specific distribution) of light spots. Unlike the flat-top beam generator, which turns the Gaussian beam into a flat-top distribution, the beam homogenizer homogenizes the non-uniform and irregularly distributed spots. Flat top beam generator for single mode (M2< 1.3) Laser use, beam homogenizer for multi-mode laser homogenization effect is better.

Beam homogenizers usually use the "diffusion angle" to characterize the divergence ability of the collimated beam after passing through the device. Lenses of different focal lengths can be selected to achieve different projection areas.

①Use characteristics of beam homogenizer:

· Insensitive to the vertical placement and lateral deviation;

· The deviation of incidence Angle will lead to a slight increase in zero order;

· Insensitive to incident light size and polarization; No special requirements for the quality of optical components;

· The homogenization effect of a single-mode laser with a small M2 is not good, and there are interference fringes, but the pattern edge is clear; The homogenization effect of a multimode laser with a large M2 is very good, but the edge is slightly blurred.

FIG.  Homogenizing effect of beam homogenizer on single-mode (left) and multi-mode (right) laser

For single-mode laser homogenization requirements, it is generally recommended to use a flat-top generator, in the case that the flat-top generator cannot be used (such as the spot M2 is small, but the intensity distribution is irregular).

②Main applications of beam homogenizer:

· Laser light intensity homogenization and shaping

· Processing and treatment: drilling, melting, marking, marking, welding

· Medical beauty

· Beam shaping of excimer laser

· Heat spot suppression

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