I use a Cloudray MOPA fiber laser to make diffraction gratings, multi-color images from bitmaps, and almost-holograms on stainless steel sheets. I inspect the gratings with an electron microscope, and also create images made from diffraction grating "pixels" where each pixel has controllable pitch and angle.

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Summary: Making Diffractive/Structural-Color Patterns on Stainless Steel with a MOPA Fiber Laser (Applied Science)

What’s demonstrated

• A method to produce vivid, angle-dependent colors and diffractive effects on stainless steel using a MOPA fiber laser engraver.
• Two related effects:
  1. Structural color via thin-film oxide thickness on stainless steel (destructive/constructive interference filters parts of the spectrum).
  2. Diffraction grating patterns by engraving closely spaced scan lines with controlled pitch.

Key ideas & mechanisms

• MOPA architecture = Master Oscillator Power Amplifier: precise control of pulse rate and per-pulse energy, enabling repeatable surface modification.
• Color formation (stainless steel):
  • Laser heating grows a controlled oxide layer of specific thickness.
  • The oxide acts as a thin film: wavelength-dependent interference removes portions of the spectrum; the remaining light produces a perceived color.
• Diffraction grating:
  • Parallel engraved lines with sub-wavelength to few-micron pitch diffract light; the grating period sets the observed color at a given view angle.
  • Varying the pitch across the artwork steers colors to the viewer.

Practical setup & workflow

• Uses a galvo-style fiber laser marker with motorized Z/auto-focus (depth gauge measures work distance; Z-axis motor sets focus).
• LightBurn is used to assign settings per color/layer and to import SVG artwork.
• Typical approach:
  1. Run a material test (color chart) holding everything constant except pulse frequency, mapping frequency to resulting colors on the same stainless stock.
  2. In LightBurn, assign layers in the SVG to the tested frequency “swatches” to place specific colors.
  3. For diffractive images, generate toolpaths that vary line spacing (pitch) as a function of position; import the resulting vectors to engrave.

Parameters (as shown/mentioned)

• Pulse frequency strongly affects outcome; examples include sweeps around ~100 kHz, ~200 kHz, ~300 kHz, up to ~800 kHz.
• Power kept modest for interference-color passes (example given: ~20% power for color work), higher power reserved for cutting.
• Speed/spacing:
  • For oxide color: speed mainly balances heat input with frequency and power.
  • For grating effects: scan speed plus pulse repetition determine effective line spacing; fine control of the pitch is crucial.

Observations & results

• The same stainless coupon shows distinct, reproducible colors at different frequencies when other settings are held constant.
• Angular dependence is strong for both oxide-color and diffractive pieces; colors shift with viewing and lighting angles.
• Complex images can be produced by pitch-modulated gratings; despite simple input art, interference of multiple passes/regions can make patterns look intricate.
• Photographing the pieces is tricky because color is view-angle sensitive; small angle changes can alter perceived hue/intensity.

Tips, constraints, cautions

• Start with low power and increase gradually to avoid excessive heat-affected zones or surface damage.
• Maintain consistent focus (motorized Z/depth gauge helps); small defocus changes color and grating efficiency.
• Keep parameters consistent across tests (same alloy, finish, focus, environment) so color charts map reliably to final art.
• For grating work, ensure stable galvo timing and predictable step spacing; small timing errors accumulate into visible artifacts.

Software/data handling

• Large vector files (tens of MB SVGs) were handled; LightBurn managed big inputs reliably.
• Simple scripts were used to compute position-dependent grating pitch so the “right” color is diffracted toward the viewer across the image.

Limitations noted

• View-angle sensitivity: results are spectacular in person but hard to capture consistently in photos.
• Tuning required for photo-realistic halftones; more experimentation needed to balance oxide color vs. grating effects without unintended interactions.

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