How to laser engrave in colour: MOPA on titanium and steel

Colour laser marking is one of those things that looks like magic until you see it happen. There's no ink, no dye, no coating — just a controlled oxide layer grown on the surface of the metal. The colour you see is light interfering with that film, the same physics that makes a soap bubble iridescent.

Why MOPA, specifically

A standard Q-switched fibre laser has a fixed pulse duration (typically ~100 ns). MOPA — Master Oscillator Power Amplifier — lets you tune pulse width from ~2 ns to several hundred. That tuning is what controls how much heat enters the metal, which controls how thick the oxide film grows, which controls what colour your eye sees.

Materials that take colour

• Titanium: the whole palette — blue, purple, gold, rose, green. The cleanest, most reproducible substrate.
• Stainless steel (300 series): blues, browns, golds. Less saturated than titanium but very durable.
• Niobium: saturated colours, popular for jewellery.

Where to start

• Power: 10–25% — far lower than engraving settings.
• Speed: 800–3000 mm/s.
• Frequency: 50–250 kHz — this is the main colour dial.
• Pulse width: 100–250 ns for warm colours, 2–50 ns for blues.
• Defocus: +1 to +3 mm above the surface for softer colour transitions.

Honest expectations

Two limitations to set with clients up front. First, colours shift with viewing angle — that's the physics. Second, two production runs months apart may not match exactly, because the oxide responds to alloy composition. For brand-critical work, mark in batches and lock down the settings sheet.