A MOPA (master oscillator power amplifier) fiber laser looks like any other fiber marker, but it adds a control the others lack: pulse width. Learning what that one setting does is the difference between copying recipes and being able to fix them.
What MOPA changes
On a standard Q-switched fiber source the pulse width is fixed, so the shape of each pulse is tied to the frequency you choose. A MOPA separates them: you set the pulse width independently, from a few nanoseconds to several hundred on many sources, and can run the high frequencies that colour marking needs.
The four settings that matter
Speed, power, frequency and pulse width. An EZCAD pen holds all four, which is why the free EZCAD test card puts every cell on its own pen and writes a CSV of each pen's values. Line spacing, the number of passes and defocus come next, and they matter almost as much on colour work.
How pulse width and frequency behave
At the same average power, a higher frequency means less energy in each pulse, and a longer pulse spreads that energy over more time, putting more heat into the material around each spot. More heat grows thicker oxide on stainless and titanium, which is where colour comes from, and darker, deeper marks on many metals. Shorter pulses keep the heat tighter, which gives finer edges and is gentler on plastics and thin coatings.
What a MOPA is good at
Colour on stainless steel and titanium is the headline: the MOPA colour settings guide lists starting recipes from straw gold to green. On polymer, such as a magazine body, a fiber laser bleaches or foams the plastic instead, so every mark is lighter than the part. PMAG TONES in the Fabricator keeps a design to the tones a laser can make: bleach white, light grey and tan on fiber, plus bone and smoke grey on MOPA.
Test like you mean it
Change one variable at a time and keep the rest fixed. The MOPA colour predictor suggests a starting recipe with its line spacing and defocus; bracket it with the EZCAD test card, or with the native .xcs sample card for an xTool F2 Ultra, which sweeps power against frequency at 200 ns. Clean the metal with isopropyl alcohol first and label the plate before marking it.
Keep a notebook
Write down the source, lens, field size, focus and the four settings next to a photo of every accepted result. The next job on that material then starts from your own evidence instead of a forum post.
Frequently asked questions: Fiber / MOPA
Is a MOPA better than a standard Q-switched fiber?
It is more flexible, because pulse width becomes a setting. That is what makes colour marking on stainless steel possible. For simple marks on metal, both kinds of source can do the job; test on your own parts before deciding a MOPA is needed.
What pulse width should I start with?
Start from your source maker's guidance or a recorded baseline for your material, then sweep it. For reference, the colour recipes in the MOPA colour guide run from about 42 ns for green to about 180 ns for straw gold on stainless 304, and the xTool F2 Ultra sample card sweeps power against frequency at 200 ns.
Why do my results differ from published settings?
Sources, lenses and field sizes differ, and so do metal batches and surface finishes. A setting is only portable as a starting point for a grid. Change one variable at a time and record the lens, focus and field position with the result.