Open hardware fast high resolution LASER

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This article describes an open-hardware transparent polygon laser diode scanner suited for 3D printing. The Dutch company Hexastorm plans to sell the first open-hardware transparent polygon scanner. The machine is shown in figure 1.

Figure 1

To clarify, the Hexastorm is put in perspective by comparing it with a Fused Filament Fabrication (FFF) printer. The smallest element of an FFF printer is the nozzle which is a circle with a 300 micrometers diameter. In the Hexastorm, this is an elliptical laser spot which is 50 by 60 micrometers. The standard speed of an FFF printer is 50-80 millimeters per second. The Hexastorm is able to reach a spot speed of 67 to 350 meters per second. The maximum scan length is 24 mm. The speed of a whole scan line is 16 to 84 mm/s.

Description

The transparent polygon scanner consists out of a laser diode which is focused directly with an aspherical lens. The bundle refracts through a transparent polygon and is directed to the surface with a 45 degrees first sided mirror. The bundle is deflected by tilting a transparent plate. The position of the laser bundle is monitored by a photo-diode. A Field Programmable Gate Array (FPGA) is used to ensure the correct timing of and stream data to the laser diode. A schematic side view and top view of the system are shown in figure 2 and 3.

Figure 2
Figure 3

A substrate can be solidified by moving the optical head in a snakelike pattern over the substrate as shown in figure 4. A more advanced scanning technology is infinite field of view.

Figure 4

The system has four advantages; it has a high optical quality, it is cost effective, it is scalable for industrial applications and it is open-hardware. The system has a high optical quality as the bundle is in focus over the full scan line and is incident at 90 degrees. The transparent polygon scanner thus gives a flat field projection and the system is telecentric. Details are provided in the physics section. The system is cost effective as it is able to achieve this quality without an expensive f-theta lens. Only two optical elements are needed; an aspherical lens and a transparent polygon. A reflective polygon scanner needs a thick disk as collimated bundles with a larger diameter can be focused too smaller spots. In the transparent polygon scanner shown, the laser is focused before it hits the polygon. As a result, the disk can be made thinner and therewith lighter which reduces the price of the bearing. Thirdly, the system is scalable. The maximum optical power of laser diodes becomes less as their wavelength becomes shorter. As a result, laser diodes need to be combined to give more power. It is, however, not possible to combine more than two lasers into a single bundle without interference. The electromagnetic field only allows for up to two polarizations. Companies like Kleo Halbleitertechnik therefore sell systems which use 288 laser bundles, see video. Companies which use single bundle transparent polygon scanners, like Hexastorm, have a scan angle of 90 degrees. Kleo uses multiple bundles per polygon and therefore has a scan angle of 45 degrees or less. The concept of scan angle is explained in the physics section.

Figure 4
If multiple transparent polygon scanners are used in a combined fashion a substrate can be solidified at once and the snake like pattern is not needed. This might be desirable in industrial production systems.

Finally, the system is open-source and open-hardware. Users cannot only use the technology but also get to own it. This is done to foster user innovation and increase the acceptance of transparent polygon scanners.

Specifications

The Hexastorm, the first single bundle transparent polygon scanner in the world, has the following specifications:

  maximum line width: 24 mm
  typical line width: 8 mm
  wavelength: 405 nm
  rotation frequency: 67-350 Hertz
  spot size: elliptical, 50 (short axis) x 60 (long axis) micrometers**
  cross scanner error: 40 micrometers
  laser driver frequency: 100 kHz
  laser frequency: 50 MHz
  optical power: 300 mW
  facets: 4

The line scan speed of a transparent polygon scanner is not uniform and varies slightly. The scan speed at the center is 80 percent of that at the edges of a scan line for the typical line width. The non-uniform scan speed can be mitigated by the usage of a high speed laser. In the Hexastorm a 405 nm laser diode with a 50 MHz pulse rate is used. At the edge a lower power can be used as it exposes a zone which is often illuminated twice.


Search Report

Earlier scanners can be split into two groups; transparent polygon scanners and reflective polygon scanners. The Netherlands Organization for Applied Scientific Research (TNO) has filed a patent application for a transparent polygon scanner with a plurality of bundles WO 2015/160252 A1. Rik Starmans, the founder of Hexastorm, is listed as a co-inventor in this patent. In the original filing (WO 2015/160252 A1), claim one describes a transparent polygon scanner with one or more radiative sources for providing one or more bundles. On basis of the international search report, claim one was later amended to a plurality of laser diode sources, that is more than one bundle see final US application US2017038690 A1. The transparent polygon scanner proposed in this article is different from TNO in that it uses a single and not multiple optical bundles per transparent polygon. In addition, the rotation axes of the the proposed system are orthogonal and not co-planar with the substrate. The first reflective polygon scanner in additive manufacturing was used by the Institute of Physical and Chemical Research (RIKEN) in 1997. In 2015, Envisiontec got a patent US 9079355 B2 for a reflective polygon scanner in additive manufacturing to protect its Scan, Spin and Selective Photocure (3SP) technology. KLEO Halbleitertechnik sells the Speedlight 2D. The Speedlight 2D is a system which uses 9 reflective polygons and 288 laser diodes to solidify a substrate with a width of 650 mm. The reflective polygon has 32 facets and rotates at a speed of 50.000 rotations per minute, see patent US8314921B2.

Business Case

Competitor Analysis

The market for laser scanning technology is extremely large. Possible applications are; laser direct imaging of printed circuit boards, additive manufacturing, laser cutters, photocopiers and object scanners. Already in the field of 3D printing applications can range from sintering powders to solidifying polymers or egg whites. The analysis was simplified by listing exposure technologies and light sources in the 3D printing and PCB market. This should give the reader a quick overview of what is available.

Alternative Exposure Technologies

The following alternative illumination technologies can be distinguished;

  • Polygon scanner with refractive F-theta lens and one laser bundle
    • Used by: Envisiontec, Orbotech
  • Polygon scanner with reflective F-theta lens and one laser bundle
    • Used by: Next Scan Technology
    • Notes: The f-theta lens is made with reflective instead of refractive optics. This probably makes the lens lighter than a glass alternative. It might also be beneficial to use this at low wavelengths due to absorption.
  • Reflective polygon scanner with multiple laser bundles
    • Used by: Manz
    • Notes: the polygon tilt angle is smaller than 45 degrees, most likely costs 1 million euro's
  • Transparent polygon scanner with multiple laser bundles
    • Used by: LDI Systems
    • Notes: The tool was based at TNO's technology. LDI systems does not currently sell scanners.
  • DMD chip illuminated with LEDs
    • Used by: Ucamco and Prodways Moving
    • Notes: PCBs can be illuminated with multiple wavelengths which can be advantageous for PCB manufacturing. Ucamco uses multiple beamers adjacent to each other. Multiple beamers can be placed adjacent to each other but this expensive, as a result Prodways translates the beamer and illuminates a 45 degrees mirror, see Moving Light technology. If the mirror is illuminated with laser diodes this can result into multiple-slit interference.
  • Galvanometer scanner with Nd:YAG LASER
    • Used by: 3D Systems, Materialise
    • Notes: low power and frequency of Nd:YAG laser, due to inertia galvanometer scanners are slower than polygon scanners
  • Resonant Galvanometer Scanner
    • Notes: Can reach a line speed of 16 kHz, but the line speed is not constant, see 1 and 2.
  • Liquid Crystal Display (MSLA Technology)
    • Used by: Structo
    • Notes: Structo uses an array light source and projects through a digital mask. This technology can be scaled. It is, however, very energy inefficient see technical details. Cooling is a challenge. The technology cannot reach low wavelengths, i.e. below 400 nm. The light engine has to be in close vicinity of the reservoir. This could be an advantage for a Continuous Liquid Interface Processing (CLIP) like technology.

The Grating Light Valve, sold by companies like Silicon Light Machines, can be used for mask-less lithography below 15 micrometers, i.e. 2.5 micrometer features and was omitted. Too few applications of the MEMS scanner developed by Fraunhofer were known to take it into consideration.

Alternative Light Sources

The following light sources have been considered;

  • Light-Emitting Diode (LED)
    • Used by: Ucamco and Prodways
    • Wavelengths: 405, 395, 385, 374, 365
    • Frequency: set by other element in the optical path, e.g. the refresh rate of the DMD chip
    • Power:<4 watt
    • Price: 5 euro's per LED
    • Note: LEDs offer less contrast and depth of field than laser diodes but can be combined as they do not produce coherent light. Texas Instrument seems to have a monopoly on DMD chips. Projection systems are sold by other vendors; for example, the LUXBEAM Lithography System sold by Visitech. Wintechdigital sells the PRO4500 with the following specifications; 5.5 Watts, 405 nm and 58 micrometers for 2500 euro's. DMD chips can handle less optical power at shorter wavelengths. For wavelengths below 405 nm, the power limit is currently 4 W per chip DLP9000UV.
  • Laser Diode (LD)
    • Used by: Manz, Envisiontec
    • Wavelengths: 405, 395, 375 nm
    • Frequency: 50 MHZ
    • Price: 22 euro's at 405 nm, 3870 euro's at 375 nm
    • Power: 0.4 W at 405 nm, 70 mW at 375 nm
    • Cooling: Air is sufficient, SLD3237VF can operate at 80 degrees.
  • Diode-Pumped Solid State Laser (DPSSL)
    • Used by: Orbotech
    • Wavelength: 355 nm
    • Frequency: 80 MHZ
    • Power: 24 W
    • Price: 190k euro's
    • Vendor: Coherent
    • Sizes: LASER 305 x 200 x 1100 mm, power supply 482 x 177 x 505 mm,
  • Nd:YAG LASER
    • Used by: 3D systems and Materialise
    • Wavelength: 355 nm
    • Power: 1 W
    • Frequency: < 1MHZ

The femtosecond laser, which can be used in two-photon polymerization to focus light in space and time and trigger a non-linear reaction, was thought be too expensive for large-area photo-polymerization.

Physics

In the following, an analytical description of the system is given. The section starts with a parameter definition. Hereafter, the following properties are discussed; polygon, spot ,transparent parallel plate and polygon tilt angle. All the equations are also available in a Python script. This script can be used to quickly obtain the properties of the system. The calculations are verified with a numerical simulation.

Parameter Definition

The polygon rotates about its center, i.e. the point inside the polygon that is equidistant from each vertex. The substrate moves under the polygon in a certain direction. The smallest angle between the illumination direction and the substrate movement direction is defined as the polygonal tilt angle.

  • <math>\alpha</math> denotes the static polygonal tilt angle. In the proposed system, this is 90 degrees.
  • <math>\alpha^'</math> denotes the polygonal while scanning (the polygon tilt angle is speed dependent)
  • <math>I</math> is the angle of incidence of the optical beam on the transparent polygon
  • <math>I_{max}</math> is the maximum angle of incidence used during illumination
  • <math>f_{efl}</math> is the effective focal length of the lens used to focus the bundle
  • T defines the thickness of the polygon, T is equal to 2r.
  • r defines the inradius of the polygon
  • a defines the polygon side length
  • R defines the circumradius of the polygon
  • v is the number of vertices of the polygon.
  • n is the refractive index of the polygon, quartz is used with a refractive index of 1.47
  • d is the diameter of the aspherical lens
  • <math>\lambda</math> defines the center wavelength of the laser diode beam

Polygon

Figure 5.

In figure 5, a regular convex polygon is shown with the following parameters; r is the inradius, R is the polygon circumradius and a is the polygon side length. In figure 5, the number of vertices, v, is equal to 8. Earlier, we defined 2r to be equal to T. The number of facets of the polygon has to be even for opposing planes to be parallel. If the number of facets is uneven, there will be an edge crossing during illumination which makes the polygon unsuited for scanning.

  • <math>a=T\cdot tan(\pi/v)</math>
  • <math>R=\dfrac{a}{2 \cdot sin(\pi/v)}</math>
  • The interior angle of a simple polygon with v vertices is <math>180-360/v</math> degrees.

For an octagon, <math>I_{max}=90-(180-\dfrac{360}{v})*0.5</math> which is 22.5 degrees.

Spot

Light emerges from a small optical window from the laser diode and as a result diverges. There are several ways to focus the diverging beam. The choice is a trade-off between spot quality and cost. Two possibilities are discussed; (1) an aspherical lens and (2) with achromatic doublet after collimation with an aspherical lens and circulation with an anamorphic prism pair.

Lens Alignment

The lens alignment is determined by the optical magnification of the whole system. The emission point typically has a size of 0.5 micrometers by 1 micrometers. The size can be measured via the Fraunhofer diffraction pattern. The emission point accuracy is assumed to be +/- 80 micrometers. This was estimated from similar laser diodes. For a 50 micrometers spot, the aspheric lens has to be placed at an accuracy of 3 micrometers. The magnification is 50. The aspheric lens can be purchased mounted with a M9 thread and screwed into position with thumb screw mounted on the lens.

Rayleigh length

The spot is defined to be in focus in twice the Rayleigh length; <math>z_r=\dfrac{2 \pi w_0^2}{M^2 \lambda}</math>. <math>M^2</math> is called the beam quality factor. Most collimated single TE mode laser diode beams have <math>M^2</math> of 1.1 to 1.2, source Sun, Haiyin.

Spot size

The spot size of a collimated and circulated bundle focused by an achromatic doublet is <math>w_0=\dfrac{4\lambda}{\pi}\dfrac{f_{efl}}{D}</math>, where D is the diameter of the collimated bundle. The spot distance is <math>f_{efl}</math> from the achromatic doublet.

The spot distance of a laser diode directly projected by an aspheric lens is given by the thin lens equation; <math>\dfrac{1}{f_{efl}}=\dfrac{1}{s_1}+\dfrac{1}{s_2}</math>. <math>S_1</math> denotes the distance between the laser diode and the aspheric lens. <math>S_2</math> denotes the separation between the aspheric lens and the spot. The magnification, M, is given by <math>M=-\dfrac{s_2}{s_1}=\dfrac{f}{f-s_1}</math>. As the emission point is not square, the spot will be elliptical.

The smallest spot which can be formed is given by the Airy disk. <math>w_0 \approx 1.22\dfrac{s_2 \lambda}{D}</math>. Here D is the width of the collimated beam or divergent beam at the lens.

Transparent Parallel Plate

The properties of a parallel plate are described by Smith and Wyant. The most important properties of a parallel plate are summarized in this section. Typically, a parallel plate is used to transversely shift a collimated bundle. Via Snell's law it can be easily seen that the refracted bundles are parallel with the incident bundles. For a converging beam, a parallel plate also gives a longitudinal focus point displacement away from the source and optical aberrations. The optical aberrations increase if I is increased. Via an analytical calculation it is ensured that the Strehl ratio is above the Rayleigh limit at <math>I_{max}</math>. This effect starts to play a role for spots below 30 micrometers.

Displacement

  • longitudinal displacement <math>=\dfrac{n-1}{n}T</math>
  • transversal displacement <math>=T sin I(1-\sqrt{\dfrac{1-sin^2 I}{n^2-sin^2 I}})</math>
  • The spot speed can be derived by differentiation the transversal displacement with respect to time.

The speed at the center is smaller than the speed at the edges of a projected line. As a result, the amplitude at the center should be smaller and is ideally corrected for by the laser diode driver by varying the pulse frequency or current.

Streh ratio

The optical performance of the system can be evaluated via the Strehl ratio. If the Strehl ratio gets below a limit during illumination, the aberrations will become dominant and the system will not image properly. As a result, it must be ensured via calculation that the Strehl ratio is larger than some acceptable limit, e.g. the Rayleigh limit of 0.71. Literature provides us with the Seidel coefficients of the main aberrations. These are used to determine the Strehl ratio. In the following, a quick overview is given. The f-number or <math>f_\#</math> equals <math>\dfrac{f_{efl}}{D}</math>. A transparent plate does not have a Petzval field curvature aberration. As a result, the projection of the transparent polygon is telecentric. The third order Seidel aberrations are listed below using Wyant. To simplify checking, the equations are listed with the page and equation number in Wyant's book titled Basic Aberrations and Optical testing.

  • spherical wavefront abberation <math>=-\dfrac{T}{f_\#^4}\dfrac{n^2-1}{128n^3}</math>, page 42, equation 72
  • coma <math>=-\dfrac{TU}{f_\#^3}\dfrac{n^2-1}{16n^3}cos\theta</math>, page 44, equation 75
  • astigmatism <math>=-\dfrac{TU}{f_\#^2}\dfrac{n^2-1}{8n^2}cos^2\theta</math>, page 45, equation 77

The coefficients can be used to calculate the optical path difference W, see page 17 and table 2. The optical path difference can be used to calculate the wavefront error <math> \sigma </math>, see equation 62 on page 37. <math>\sigma^2=\dfrac{1}{\pi}\int_{0}^{2\pi}\int_{0}^{1}\{\Delta W(\rho,\theta)-\Delta \overline{W}\}^2 \rho d\rho d\theta</math>

If the wavefront error is converted to lambda wavefront error via <math>\sigma_{\lambda}=\dfrac{\sigma}{\lambda}</math> The Strehl ratio can be calculated with the first three terms of its Taylor series, see Wyant page 39 equation 67. <math>\text{strehl ratio} \approx 1- (2 \pi \sigma)^2 + (2 \pi \sigma)^4/(2!) </math>


Simulation

The result of the equations can be verified with an open optical ray tracing and lens design framework, such as the Python library made by Dr. Jordens named Rayopt. Using this framework, it can be verified that the system is telecentric and the system is in focus over a plane, i.e. it has a flat field projection. In addition it is possible to verify the transversal displacement, longitudinal displacement and the Strehl ratio. This script has also been made available by Hexastorm.

Experiments