Diffractive OpticsDOE Used for laser material processing and medical aesthetics

Diffractive Optics

Diffractive optical element（DOE）Using microstructure design to alter the phase of the propagating light. Reasonable design of the microstructure on the surface of optical diffraction elements can enable the output of any light intensity distribution that conforms to the design when specific light is input.DOETechnology has achieved many functions and optical operations that are not feasible with traditional optical systems. In many applications, these technologies greatly improve system performance. Diffraction optical schemes have many advantages, such as high efficiency, high precision, small size, low weight, and most importantly, they are flexible in meeting various application requirements.

DOEProduct: Beam splitter and beam shaper.

Beam splitterDOEUsed to split a single laser beam into several beams, each beam having the same characteristics as the incident beam (except for power and propagation angle). According to the diffraction pattern of the beam splitter,A beam splitter can generate1Dimensional beam array（1×N）Or2Dimensional beam matrix（M×N）. splitterDOEThe incident light beam can also be divided into different spot distributions, such as circular, random patterns, hexagonal arrays, etc. The beam splitter needs to be used with monochromatic light (such as laser beams), and different beam splitters have specific wavelengths and separation angles between specific output beams.

Beam shaping can convert near Gaussian beams into uniform circular, rectangular, square, and linear beams on the working surface, with edge contours (light intensity distribution). Very clear, and the beam shaper can achieve uniform output intensity distribution, allowing for even surface treatment during laser processing and preventing overexposure or underexposure in specific areas. In addition, the light spot has a steep transition area, thus forming a clear boundary between the processed and unprocessed areas. The beam shaper series includes homogenizers,top-hatVortex lens (spiral phase plate) and diffractive axicon.

DOETypical Applications

With the continuous increase of laser power, many user optical components of integrated systems may not be able to withstand high-power lasers. Therefore, the laser-induced damage threshold（LIDTperhapsLDT）The parameters become an important factor when selecting optical components. The high damage threshold of diffractive optical elements makes them an ideal choice for high-power industrial systems and applications. Both laser material processing applications and laser based medical aesthetics require high-power lasers.

picture1Different beam splitter spot distributions, from left to right:5×5Array, random, hexagonal array, circular

picture2The results of different beam shapes, from left to right: homogenizer, flat top light, vortex lens, and diffraction prism

Application of Diffraction Optical Elements in Laser Material Processing

Recently, the development of new laser systems for industrial demand has increased. Many new processes have been developed, and many traditional processing techniques have been replaced by laser processing techniques. Laser material processing accounts for a significant portion of the entire laser market,DOEPlays an important role in providing laser beam forming that adapts to the process. Laser beam forming and homogenization technology are essential steps for optimizing many laser material processing applications.DOEUsually used in laser ablation and laser processing systems, laser drilling, laser cutting, and other processing to form small feature structures on the surface.

DOELaser based cosmetic treatment

With the use of laser technology becoming an indispensable tool in the field of medical aesthetics, the ability to control laser output is becoming increasingly important.DOEProvides a unique solution that allows the beam to operate in multiple ways while maintaining the lightweight of the components. Beauty treatments typically use high-power lasers. Require uniform and precise laser exposure with sharp edges and high efficiency. This is the ideal solution for beam shaping using diffractive optical devices.DOECommonly used for laser hair removal, laser tattoo removal, skin repair, skin regeneration, and more.

Diffraction optical element - beam splitter

The working principle of a beam splitter is very simple. According to the customer's system requirements, the collimated input beam and output beam are separated at different angles from the beam splitterDOECome out, the separation angle isDOEThe separation angle was determined during the design phase and is very accurate (error)<0.03mRad）. The separation of light beams is designed for the far-field. Therefore, as the light beamDOEAfterwards, they continued to spread and became more clear.

picture3splitterDOEBasic settings,EFL =Effective focal length,m =The order of multiple points (points), θs=The separation angle between two focal points,d = 2The distance (spacing) between focal points, θf= Full angle,D =The length of the light spot array

picture4 1×6Multiple points propagate in dispersive media

The generated multi spot with "zero order" does not diffract and the beam follows the laws of reflection and refraction. For a standard beam splitter with an odd number of beams, the separation angle is of order+1And order0The angle between them (order)0It is the expected beam of light. For a standard beam splitter with an even number of beams, the separation angle is+1Step and-1The angle between orders (zero order is not the desired beam).

Diffraction Optical Elements - UseDOEBeam shaping

A diffraction beam shaper is a phase element that converts a Gaussian input beam into a uniform spot with sharp edges at a specific working distance. Each beam shaper can only be used under specific optical conditions, which is a unique set of optical system parameters: wavelength, input beam size, working distance, and output spot size.

The most basic settings in beam shaper applications include lasers, diffractive beam shaper components, and surfaces to be processed.

Flat top beam shaper

The top hat beam shaper is used to convert a nearly Gaussian incident laser beam into a uniform intensity spot of circular, rectangular, square, linear, or other shapes, with high-quality sharp edges in a specific working plane. To achieve high-quality beam shaper performance, the laser output should be single-mode（TEM00），M2price<1.3.

By using a beam shaper, uniform light spots can be left on the surface of the object to be processed, and specific areas can be prevented from overexposure or underexposure on the surface. In addition, the characteristic of this spot is the sharp transition area, which forms a clear boundary between the processed area and the untreated area. The top hat beam shaper has high efficiency (usually> 95%Excellent uniformity (usually ±)5%Steep transition zone and high laser damage threshold. In addition, the top hat beam shaper is sensitive to input beam size, working distance, and component displacement. flattopDOEUsually used for laser material processing applications (laser ablation, laser cutting, laser drilling), aesthetic treatments (tattoos and hair removal), scientific applications (flow cytometry), and so on.

Homogenizer - Beam shaper

Optical homogenizerDOEConvert a single-mode or multi-mode input beam into a well-defined output beam characterized by a desired shape and uniform flat top intensity. The most common shapes obtained by diffusers are circular, square, rectangular, elliptical, and hexagonal. Meanwhile, images of almost any shape can be designed. The edge of a diffuse beam is usually steep and determinable. The ratio between the input divergence angle and the diffusion angle of the homogenizer determines the ratio between the transition region and the homogenization region of the output beam. In order to achieve the ideal intensity distribution of the beam in the far field or focal plane,DOEThe homogenizer splits the incident light into a semi random direction and a half random direction. This method can design components that can produce arbitrary shapes and have precise output angles and sizes under uniform light intensity conditions. The performance of the diffuser largely depends on the parameters of the incident beam, and in addition, by using highM2The input beam can achieve higher uniformity (Figure)7）. Homogenizer beam shaper is insensitive to beam size, displacement, and component tilt. It provides a high laser damage threshold, while uniformity and efficiency vary with the design. homogenizerDOEUsually used for laser material processing applications (laser welding, laser brazing), aesthetic treatments (tattoos)/Hair removal, body contouring, etc.

picture5Beam shaperDOEBasic settings,d =Forming spot size,D =Beam diameter,EFL =Effective focal length.

picture6Top cap strength distribution, left: square, right: circular

picture7Homogenizer performance based onM2Change, left:M2 = 1On the right:M2 = 10 picture8Vortex lensDOEStaircase phase

Diffraction optical element - spiral phase plate

Vortex lensDOEConvert Gaussian input distribution into a circular energy ring. Spiral phase plate is a unique optical element whose structure is entirely composed of spiral or helical phase, and its purpose is to control the phase of the transmitted light beam. The total etching depth from the top to the bottom of the "staircase" is a function of the design wavelength and the optical index of the substrate. Under normal conditions, the depth is of the same order of magnitude as the design wavelength. Therefore, each vortex phase plate is wavelength specific. Optical vortices require input of collimated single-mode（TEM00）Gaussian input beam, and it converts it intoTEM01Axisymmetric mode.

There are two significant advantages to using a larger input beam diameter. Firstly, the larger beam slightly reduces the outputDOESensitivity of alignment tolerance. Secondly, a larger input beam diameter will be able to generate smaller vortex points, which is often the expected result in many applications. Vortex lenses have high efficiency (usually> 90%And a lower damage threshold. It has sensitivity to component displacement and rotation. Vortex lensDOEUsually used for material processing applications (welding), optical communication (optical mode conversion and generation), scientific applications（STEDMicroscopes, optical tweezers, etc.

Summary:

In recent years, diffractive optical elements have become a mature and widely used technology.DOEThe technology is mainly applied in beam shaping and beam splitting. It is mainly applied in fields such as laser material processing, medical aesthetics, and scientific applications, and has a large market, accounting for a large part of the entire laser application market. Due to the continuous increase in laser power and strict requirements for accuracy,DOEThe high laser damage threshold and high precision make it an effective solution to solve laser application problems.