JLMN-Journal of Laser Micro/Nanoengineering Vol. 19, No. 2, 2024

Planar Light Valve Optical Head for High Throughput 10µm Feature
Size Laser Processing
Gregory Jacob*, Hirofumi Mizuno, Tianbo Liu, Alex Payne, and Lars Eng*

Silicon Light Machines, USA
Corresponding author’s e-mail: gjacob@siliconlight.com

A high-throughput laser micro-machining system has been developed using a programmablemulti-spot modulated line beam capable of >30x throughput enhancement over a single-spot system.
While commercially available lasers have been rapidly growing in output energy and power, singlespot scanning systems cannot take full advantage of these advancements. This system provides highthroughput, high resolution micro-processing on a variety of surfaces (stainless steel, aluminum, andpolymer). This high productivity system is enabled by a high-power MEMS spatial light modulator
called the Planar Light Valve (PLVTM). The PLV is a 1088-pixel device in a linear configuration capable of 10-bit, 100 kHz amplitude modulation. The PLV supports pulse energies of ~1 mJ (1064
nm, 10 ps) and has been used at pulse widths down to 300 femtoseconds. The system can resolvefeatures as small as 10 µm with an edge placement resolution of 2.5 µm. Large area processing isdone by scanning and stitching this line beam across the media using X-Y stages and a unique relay
galvo scanner has been incorporated into the system to allow for faster scanning over a 15 x 15 mm
area.
DOI: 10.2961/jlmn.2024.02.2006
Keywords: PLV, multi-beam, spatial light modulator, high power, scan head, MEMS
1. Introduction
1.1 Laser processing systems
Many manufacturing processes rely on lasers for theirprecision and speed [1-6]. There is a widespread need forhigher throughput systems to meet customer demands andreduce the cost of producing parts. A typical laser processing system consists of a collimated laser beam scannedby galvo mirrors and focused through an f-theta lens. As
laser powers and pulse energies have continued to grow,the single-spot system cannot take full advantage of thelaser output energy, especially when high resolution is required [7]. The highest-performance galvo scanners can
scan at speeds of 30 m/s and polygon scanners can scan as
fast as 90 m/s [8]. But even a 90m/s scan speed can only
utilize a 50W laser with a 10µm spot size. This means thathigher-power lasers cannot be used to their full potential or
are restricted to larger spot sizes which limits their application to lower-resolution applications.
1.2 Multi-beam system architecture
Laser processing systems have begun using multiple
beams to increase the throughput [9-12]. While the beamscanning and delivery system can be replicated to create
multiple beams, this leads to complex software control todivide up the processing work and create seamless interfaces. This approach also has limitations in scalability. In thispaper a more scalable approach is taken of splitting thebeam into many spots that can be modulated at high speed[13,14]. The resulting system requires much slower scan
speeds and can easily take advantage of higher repetition
rates and average powers of lasers. To make user of higher
pulse energy, a straightforward optical magnification

change is required if the output fluence is to be kept constant.
A spatial light modulator (SLM) is used to enable onthe-fly pattern switching of incident laser light [15-18]. To
be performant in high-power applications, the SLM musthave a suitable damage threshold and operate at kHz
switching rates. The Planar Light Valve (PLVTM) was chosen for this system because it can support pulse energies of
up to 920 µJ at 200 fs duration. It can operate in the wavelength range of 355 – 1070 nm and supports switching
speeds of >100 kHz [13,19].
1.3 Planar Light Valve Spatial Light Modulator
The PLV is a MEMS device composed of a tiled arrayof circular pistons enclosed by a static faceplate. In the
short axis, 40 pistons are ganged electrically to make a
1,020 µm tall pixel. The long axis contains 1,088 pixels
which are the width of a single element (25.5 µm) for a
total length of 27.7 mm (Figure 1) [19].
Similar to the Grating Light Valve (GLV), the PLV utilizes a diffraction mechanism to modulate light by directing
it into different diffraction orders which are either passed or
dumped depending on the order (Figure 2) [20,21]. Whenno voltage is placed on the dynamic faceplates, they arelevel with that static portion and create a mirror-like surface. When a voltage is placed on the dynamic portion(shown in red) it is drawn toward the common baseplate
below by a capacitive force, creating a variable depth diffraction grating. The light that reflects off the dynamic andstatic portions will destructively interfere and create diffraction orders. Input light picks up a round-trip phase shiftequal to twice the displacement, thus the maximum interference will occur when at a displacement of λ/4, where λJLMN-Journal of Laser Micro/Nanoengineering Vol. 19, No. 2, 2024

is the wavelength of the light. The optical system is arranged in such a way that light reflected into the 0th order ispassed, while light diffracted into higher orders is blocked
by a spatial filter. In this way, the displacement of a faceplate translates to intensity modulation in the image plane.
The highest intensity in the image plane is seen at 0 displacement and the lowest intensity (dark state) is seen at
λ/4 displacement. The voltage and resulting displacement is
controlled with 10-bit resolution on each channel, whichallows for 1,024 intensity values in the image plane foreach pixel.

Fig. 1 Images of the PLV device showing (a) the entire device
die, (b) a zoomed-in section of the optical area with one device
pixel highlighted (1 element wide by 40 elements tall), and (c) afurther zoomed-in section showing individual elements with the
dynamic part highlighted in red.

2. Methodology
2.1 Optical system
The optical system consists of illumination optics, projection optics, and relay optics (Figure 3). The illumination
optics take a Gaussian beam input from the laser and createa rectangular beam at the PLV with a top-hat profile in bothdimensions. The beam expander is used to resize the beam
to a diameter of 5 mm for the top-hat shaper. The top-hat
shaper and focusing lens create a 4 x 4 mm square top-hat
beam. The cylindrical telescope resizes the square top-hat
beam into a 27 x 1 mm top-hat to match the dimensions of
the PLV. This telescope comprises 3 lenses for the shortaxis and 3 lenses for the long-axis to precisely control thedimensions of the beam on the PLV.
The projection optics create an anamorphic image of
the PLV which is imaged by the galvo relay system to thework surface. The long-axis is imaged with a demagnification of 10.4x, so the pattern on the PLV can be seen at theintermediate image plane as well as at the final output image. The short-axis is focused as tightly as possible to a 10
µm focus with a sinc2 profile. A pickoff mirror sends asmall amount of light to the calibration system, which isused to measure and flatten out any nonuniformities in theillumination. The result is a flat-top line beam that is 2.5
mm long with ~10 μm sinc2 profile in the short axis (Figure4). At any moment, the pattern displayed on the PLV is thepattern that is seen at the work surface, making it intuitive
to place energy exactly where it is needed.

It has a large optical area since the full 40-element
length of each pixel can be used to modulate light. Light isspread out over the entire device to keep the power density
low and condensed into a thin line at the work surface.

Fig. 3 Schematic of the optical system used in the PLV opticalhead.
Fig. 2 Illustration showing (a) device with maximum light goingto 0th order, (b) maximum light going to 1st orders, and (c) an
example optical system with a spatial filter cutting off the 1st orders.

1.4 Optical Head
The optical head was designed as a turn-key solution toquickly start working with a PLV-modulated multi-spot
beam. It provides high-quality beam shaping and imaging
optics in an industrial enclosure, allowing users to focus onlaser processing rather than optical implementation.

Fig. 4 Optical head output beam with example on/off pattern
and all on pattern.

JLMN-Journal of Laser Micro/Nanoengineering Vol. 19, No. 2, 2024

The turn-key version of this system utilizes an integrated 100 W, 460 μJ, 10 ps, 1064 nm laser. It can also be usedwith many other lasers by coupling the light in through an
input window.
2.2 Scanning over media
The PLV provides modulation in the long axis of theline beam. To process a 2D structure the line beam must be
scanned orthogonally to the modulation direction. As thebeam is scanned the pattern is changed to deposit energy
where it is required. The scanning motion can be achievedby moving the beam or by moving the media. Many typesof scanners can be used by themselves or in conjunction
with others. Examples include stages, galvo mirrors, rollto-roll, rotating drum, and spinning disk to name a few.
2.3 Galvo relay optics
The optical head employs a unique integrated galvo
scanning relay system. Whereas a single-spot system needs
only to scan and focus a beam, the galvo relay optics usedhere are designed to image and scan the PLV pattern to thework surface (Figure 5). Two identical f-theta lenses wereused with a separation of 50 mm to create a 1x magnification relay system. The imaging configuration is key to enabling the use of galvos in the system. A focal length of 65
mm was chosen for the f-theta lens to be able to resolve 10
μm features. This results in a scan field of 15 x 15 mm.
Stages with a longer travel can be used for processing
larger areas.

Fig. 6 Example of scan path used when more than one pass isneeded to cover the processing area.

When using XY stages the position encoder signals
from the scanning stage are used to create a trigger signal
to advance the pattern (also called the column) of the PLV(Figure 7). In this way, the PLV pattern can be synchronized with the position of the stages.

Fig. 7 Schematic of signals used for synchronization with stage
scanning.

When using the galvo mirrors a signal is sent from thegalvo controller at the start of the scan (Figure 8). The PLV
controller will then run a collection of patterns (called aframe) at a pre-defined period set on an internal clock.

Fig. 5 Galvo relay system which images and scans the intermediate line beam image.

2.4 Motion synchronization
The scanning motion must be synchronized with the
PLV pattern to deposit energy in the desired locations. If
the pattern being processed is wider than the line beam, thebeam is scanned over the area in multiple passes (Figure 6).
A demo system has been built that can scan the workpieceusing XY stages or scan the beam using the integrated galvos.

Fig. 8 Schematic of signals used for synchronization with galvo
scanning.

2.5 Distortion compensation
One challenge when working with galvos is the distortion that is introduced by the f-theta lens. Ideally, the beamwould be scanned in a straight line when only one axis is inmotion. However, the straight scan paths become curved
due to barrel distortion, and the scans overlap more thefarther away from the center we go (Figure 9).

JLMN-Journal of Laser Micro/Nanoengineering Vol. 19, No. 2, 2024

Fig. 9 Illustration showing ideal scan paths and the scan paths
after being distorted by the f-theta lens.

3. Data and Results
3.1 Resolution
The feature size in the PLV-modulated direction can be
adjusted by setting groups of pixels to the on state (maximum intensity) and the off state (minimum intensity). In
Figure 11 black-marked features are shown for blocks of 12
pixels on, 12 pixels off down to 1 pixel on, 1 pixel off. The
marks created appear well-resolved down to 4 pixels, corresponding to a 9.8 μm beam size. The measured featuresize will depend on the application and laser parameters
being used. When using 3 pixels or less, the spatial frequencies exceed the resolution of the optics and we see
lower contrast marks.

With a single beam, this could be corrected in the galvo
controller. When scanning the multi-spot line beam in thissystem, extra complexity is introduced because the distortion can vary across the line. One approach to compensate
for the distortion is to use distortion data to pre-distort the
input pattern so that a non-distorted pattern results after
processing (Figure 10).

Fig. 11 Black-marked features on SS304 showing well-resolved
spots down to a grouping of 4 PLV pixels.

The horizontal and vertical resolutions were also tested
against scan speed. Figure 12 shows vertical lines madewith blocks of 4 pixels on, and 4 pixels off. Multiple passes
were used at higher scan speeds to darken the image. Thispattern shows well-resolved lines and spaces even at thehighest scan speed of 854 mm/s.
Fig. 10 Illustration showing the concept of pre-distorting theimage to compensate for the inherent distortion in the f-theta lens.
Images from https://learnopencv.com/understanding-lensdistortion/

A matrix of distortion data is required to calculate thepre-distorted image. A denser matrix will lead to more accurate distortion correction, but getting data at each pixel ofthe input image is not possible. So, the data is calculated
for each pixel is calculated using bilinear interpolation.
Each pixel in the input image is mapped to a new pixel inthe distorted image. For example, consider a certain pixel
with measured distortion of -9 µm in X and -10 µm in Y
where each pixel represents 5 µm. This pixel would then be
shifted +2 pixels in X (rounding to the nearest pixel) and+2 pixels in Y when creating the distorted image. After all
pixels are mapped to their new location, some holes are left
in the image. For high-contrast binary images these holes
can simply be filled in by averaging the 4 nearest neighborpixels. For gray-scale images, a different method may berequired.

Fig. 12 Horizontal and vertical line pairs created at different scan
speeds. Well-resolved vertical lines from the PLV pattern are seen
throughout.

Figure 13 shows horizontal lines made by turning theentire PLV array on and off. The modulation frequency is
noted at the left of each image. We see well-resolved lines
up to 117 kHz, but at 150 kHz the lines become significantly lighter indicating that the PLV may not have fully
reached the on state. In this test, the modulation rate is increased by increasing the scan speed. This also leads tolighter features because fewer pulses hit a given area. Thus,
this result should be taken as a rough characterization of
the modulation rate of the PLVJLMN-Journal of Laser Micro/Nanoengineering Vol. 19, No. 2, 2024

3.3 Distortion correction
The distortion was measured by marking 100 μm diameter spots at a 500 μm pitch across the entire scan field. The
actual position was located and plotted against the ideal
position (Figure 16). The difference between these twopoints is the distortion.

Fig. 13 Horizontal lines created at increasing modulation frequencies by setting the PLV to all pixels on state and all pixels off
state.

3.2 Edge placement
The edge placement capabilities work with the resolution to determine how smoothly laser-marked features are
rendered. Though the optics can resolve features as small
as 4 PLV pixels, these features can be shifted to adjacentpixels on the array in steps of 1 pixel. Effectively we havea grid set at 2.5 μm on which we place the spots in our line
beam, allowing us to create very smooth diagonal features(Figure 14).

Fig. 16 Distortion data collected over 16 x 16 mm area.

Fig. 14 Illustration showing how shifting a spot by 1 PLV pixel
as the pattern increments results in smoothly rendered diagonalfeatures.

Using the method of pre-distorting the image discussed
in section 2.5, a square image was marked with and without
distortion correction shown in Figure 17. The corrected
image has noticeably straighter sides than the uncorrected
image. Table 1 shows the measurements of the sides of thesquare images. Before distortion correction, the magnitudeof the average error over the length of the 4 sides was
1.29%. This was reduced to 0.13% after distortion correction.

Different grid sizes were used to mark a sample image
shown in Figure 15. A staircase effect is seen at the coarser
grid sizes, but the finer grid sizes show very smooth features. The processing time is the same regardless of gridsize. The only penalties are that more data and higher triggering frequency are required at finer grid sizes. For black
marking on stainless steel, a typical scan rate of 40 mm/s
with a 2.5 μm grid size requires a trigger frequency of 16
kHz, which is well within the capabilities of the PLV.
Fig. 17 Square image marked without and with distortion
correction.

Fig. 15 Example showing the effect of marking the same image
at different grid sizes.

JLMN-Journal of Laser Micro/Nanoengineering Vol. 19, No. 2, 2024

Table 1

Measurements of square images shown in Figure 17 before and after distortion correction.
Before Correction

After Correction

Dimension

Ideal (µm)

Measured (µm)

Error (%)

Measured (µm)

Error (%)

Top Width

12740

12538

-1.59

12767

0.21

Bottom Width

12740

12562

-1.40

12748

0.06

Left Height

13000

12844

-1.20

13004

0.03

Right Height

13000

12873

-0.98

13029

0.22

The original array of spots was marked with distortion
correction, and the average distortion was reduced by 2.2x,
from 73 μm to 33 μm (Figure 18). Some of the spots were
pushed out too much resulting in a positive displacement
error, so this area of distortion correction has an opportunity for further improvement. In future work, it may be possible to correct large-scale distortion errors in the galvo
controller and make small corrections by pre-distorting theimage, which will help to equalize the overlaps between
adjacent passes of the line beam.

3.4 Process time
A selection of images was marked on 304 stainless steel
to compare against a typical single-spot marking system.
The images shown in Figure 19 range from very dense tovery sparse with a range of feature sizes.

Fig. 19 Selection of images used for throughput comparison.

Processing times for the single-spot system and PLV
multi-spot system are shown in Table 2. The single-spot
markings had an L* of 21.67. The multi-spot system parameters were dialed in to get close to this value with an L*of 22.82. The L* is a measure of the darkness of a sample
with a value closer to 0 indicating a darker sample. A difference of 2.0 is just noticeable to the human eye, so thedarkness of the images generated is very hard to tell apart.
The fine features of images 4 and 5 were noticeably sharperon the multi-spot system. For the multi-spot system, a scan
speed of 45 mm/s was used with a repetition rate of 400
kHz and laser pulse energy of 233 µJ. The resulting peakfluence at the media was 0.6 J/cm2 at the center of theshort-axis profile.

Fig. 18 Location of grid of dots after pre-distorting image.
Table 2

Process times and throughput enhancement for five benchmark images using a single-spot system (50 W, 25 µJ, 3 ps) and the
PLV multi-spot system. On the single-spot system, all processing was done with galvo mirrors. On the multi-spot system,
images smaller than 15 x 15 mm were processed by scanning the galvo mirrors while larger images were processed by only
scanning the stages.
Single-Spot 50W

Multi-spot system 93W#

Image Name

Size (mm)Time (s)Scanner

Time (s)Throughput

1Black Circle

φ 30

264.93

Stage

14.48

18.3x

2

Dot Matrix

9.3 x 9.3

7.11

Galvo

0.72

9.9x

3

QR Code10 x 10

34.04

Galvo

0.97

35.1x

4

SLM Logo

6.6 x 10

14.43

Galvo

0.58

24.9x

5

Small Text

33.4 x 11.4

92.01

Stage

4.84

19.0x

JLMN-Journal of Laser Micro/Nanoengineering Vol. 19, No. 2, 2024

3.5 Engraving rates
The multi-spot system was also used to demonstrateengraving on stainless steel. Rather than just modifying thesurface structure, engraving aims to remove some amountof material. Table 3 below shows the depths ranging from
135 µm to 486 µm and corresponding removal rates.
Table 3Engraving depths and corresponding removal rates.
Laser used was Carbide 1030 nm at 100 kHz, 800 uJ(1.75 J/cm2 peak fluence), and a scan speed of 5mm/s was used

Passes

Depth
(µm)

Time
(sec)

Depth / pass(µm)

Removal Rate
(mm3/min)

33

135

412.5

4.09

1.96

67

259

837.5

3.87

1.86

133

486

1662.5

3.65

1.75

3.6 Sample images
Figures 20-23 show a small sampling of images andmarkings created with the optical head.

Fig. 20 Black-marked image on stainless steel of (a) chess board,(b) and (c) zoomed in showing 10 μm wide unmarked outline
around the knight.

Fig. 21 Golden Palace in Kyoto marked on 68 x 45 stainless steel
plaque.

Fig. 22 Images marked on silicon wafers in collaboration with A.
Brand, Fraunhofer ISE. Laser used was Photonics Industries RX2
at 90 W power, 200 kHz repetition rate and peak fluence of 1.0
J/cm2.

Fig. 23 Images of (a) large black-marked area on stainless steel,(b) visual pyramid effect created with grayscale dithering, and (c)sharp text.

4. Conclusions
An optical head for laser processing has been demonstrated with up to 35x throughput enhancement at >10xresolution (10 µm feature) compared to a 58 µm single spot
system. The throughput can be further scaled with a more
powerful laser. This system is especially well-suited for
applications with densely marked features at high resolution.
Though a large throughput enhancement has alreadybeen demonstrated, new PLV devices are in development
which are estimated to provide >5x throughput enhancement compared to the system discussed here. Other versions of the optical head are also in development to work at
different wavelengths (green and UV), as well as to delivera longer line beam to the media for faster processing.

JLMN-Journal of Laser Micro/Nanoengineering Vol. 19, No. 2, 2024

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