Understanding Frequency
If you're coming from CO2 or diode lasers, frequency is the most confusing setting on a fiber. Here's the mental model that makes the rest of the course click.
The definition (and the unit)
Frequency = the number of laser pulses your fiber fires every second. Measured in kilohertz (kHz). Set 50 in Lightburn → 50,000 pulses/second. Set 200 → 200,000 pulses/second.
Every fiber laser is pulse-based. Even though the beam looks continuous, what's actually happening is that the source charges up and releases, over and over, thousands of times per second. Frequency controls how many of those release events happen.
Pulse power: the pizza analogy
Your laser's average power (60W on yours) is divided across all the pulses in a second. More pulses = each one carries less energy.
| Setting | Pulses/sec | Energy per pulse (60W avg) |
|---|---|---|
| 20 kHz | 20,000 | 3.0 mJ |
| 50 kHz | 50,000 | 1.2 mJ |
| 100 kHz | 100,000 | 0.6 mJ |
| 200 kHz | 200,000 | 0.3 mJ |
Same pizza, more guests = smaller slices. Lower frequency = bigger per-pulse punch.
Pulse overlap: the escalator analogy
This is the more important effect, and the one most explanations skip. The galvo is moving across your part as it fires. At low frequency, pulses land far apart (gaps between them). At high frequency, pulses pile up on each other (heavy overlap).
- Low frequency = empty escalator. A few pulses with breathing room between them. The material has time to cool between hits → next pulse hits cool metal → effective ablation, deeper material removal.
- High frequency = Black Friday escalator. Pulses crammed shoulder-to-shoulder, overlapping. Material can't cool between hits → heat accumulates → melting / smoothing / polishing rather than removing.
What each frequency range is good for
| Range | Best uses | Why |
|---|---|---|
| 20-50 kHz | Deep engraving, aggressive removal | Big per-pulse punch + cooling gaps = real cuts |
| 50-100 kHz | General engraving, coin relief | Balance of removal + finish |
| 100-200 kHz | Smooth/polished engraves, anneal | Heat builds, surface smooths |
| 200-1000 kHz | Color annealing, fine plastic marking, photo on metal | Surface heating only, no ablation |
The band map: frequency as sandpaper grit
A complementary way to carry this in your head: treat frequency like sandpaper grit. Lower = coarser, more aggressive removal. Higher = finer, gentler surface work. Long-term production practice on 60W JPT MOPA-class sources settles into four working bands, and as a shop discipline stays inside 40-600 kHz, below 40 you're in derating territory anyway (see the cutoff section below), and above 600 there's nothing left but surface heat.
| Band (kHz) | Job class |
|---|---|
| 40-135 | Metals: fill engraving, depth work, cutting. 55 kHz is a dependable default for all three. |
| 80-160 | Bitmap / 3D Sliced depth engraving (110-135 kHz is the common working zone) |
| 140-280 | Plastics, polymers, pictures, stippling, surface marking |
| 300-600 | Paint / Cerakote removal, annealing, color work |
The bands don't replace the range table above, they overlay it: the ranges explain the physics, the bands tell you where to start dialing for a given job class. Both are starting points for a 60W MOPA-class source; re-grid on your own machine.
Q-switched vs MOPA frequency range
- Q-switched / non-MOPA: narrow band, typically 20-80, 50-100, or 20-100 kHz. Limited by hardware.
- MOPA: 1 kHz to 4000 kHz, the full spectrum. One of the reasons MOPA sources cost more than Q-switched.
Cutoff frequency (read this before going low)
Every MOPA has a manufacturer-specified cutoff frequency below which the controller derates your power for safety. Below cutoff, the pulse cycle becomes unstable and the controller protects the source by quietly scaling power down, even if Lightburn shows 100%.
- For most JPT MOPA sources (60W M7 family, your OMTech): cutoff is around 48 kHz.
- Set freq below 48 kHz and you're getting less than the power Lightburn says.
- The manufacturer publishes a derating chart, find yours and tape it to the wall.
- The takeaway: don't blindly drop to 20 kHz expecting more aggression. 48 kHz at full power often beats 20 kHz at derated power.
The hammer analogy (refined)
- Low frequency = sledgehammer with rest between swings. Big hits, room to breathe, real cratering. Boom. Boom. Boom.
- High frequency = finishing hammer in machine-gun mode. Tiny taps overlapping. Boop-boop-boop-boop-boop. Smooths the surface rather than removing it.
The surprising truth: frequency mostly controls FINISH, not depth
Community testing on identical brass coins (60W MOPA, speed 2000, power 80%, 256 passes) varying ONLY frequency:
| Frequency | Background finish | Depth |
|---|---|---|
| 48 kHz | Very rough, cratered, noisy | Reference |
| 80 kHz | Slightly textured | Similar to 48 |
| 200 kHz | Mirror-smooth, dish-like | Almost the same as 48 |
| 100 kHz | Best balance: smooth + sharp face details | (at 90% power) |
So if you want more depth, change speed (slower) or power (higher) or passes (more). If you want a different finish, change frequency.
The Q-pulse width companion knob
Frequency's close cousin on a MOPA. Pulse width controls how LONG each individual pulse stays on (typically 2-500 ns), independent of how MANY pulses per second.
- Default to 200 ns. For 80% of jobs (engrave, anneal, anodized AL, raw metal), leave it at 200 ns and tune only frequency/power/speed.
- When to drop it (2-30 ns): photo on metal, color annealing on stainless/Ti, plastic marking. Short pulses ablate cleanly without heat-affected damage.
- When to raise it (300-500 ns): dark color marking on stainless/Ti where you need extra heat per pulse to grow a thick oxide layer.
Power scaling rule (30W → 60W → 100W)
If you find a recipe written for a different MOPA wattage, scale it like this:
- 30W → 60W: roughly half the loops, OR roughly double the speed. Same depth, half the time.
- 60W → 30W: double the loops or halve the speed. Same depth, twice the time.
- 60W → 100W: drop loops by ~40% or increase speed ~60%.
- Frequency, pulse width, and line interval don't scale with wattage, they're material-driven, not source-driven.
Sources: "Understanding Fiber Laser Frequency, Explanation + Real Engraving" (YouTube); 30W vs 60W MOPA comparison (YouTube); community testing on JPT MOPA sources.