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 — xTool's published reference

xTool publishes reference ranges for the F2 Ultra MOPA source that we use as the starting point for every new colour job. They are intentionally wide — the whole point of the technique is to sweep across them until the colour you want appears, then lock that square in.

Titanium — build a test matrix on the actual stock

Titanium is the most cooperative metal for MOPA colour, but the numbers that produce a given colour are specific to the machine, the lens and the batch of stock. Rather than publish figures that will not transfer, we start from xTool's own F2 Ultra MOPA reference envelope in the software, then sweep frequency and pulse width across a test grid on the same stock as the job and lock in the square that gives the colour we want.

Stainless steel (including 304)

Stainless is a separate family from titanium — the two are not interchangeable, and titanium figures should not be carried over to 304 or any other stainless grade. xTool's recommended workflow for stainless is to build your own test matrix from the official preset rather than rely on a single fixed setting, because power output and heating conditions vary from machine to machine. Sweep frequency first, then pulse width, on a coupon of the same stock.

Brass

• Power: 30–75%.
• Speed: 100–1000 mm/s.
• Lines per cm: 5000.
• Pulse width: 2 ns.
• Frequency: 1950 kHz.

Honest expectations

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