Fiber Laser Frequency Explained: What kHz Actually Does

What is frequency on a fiber laser?

Frequency is the number of laser pulses being requested every second. In LightBurn, frequency is typically entered in kHz, or kilohertz.

That means:

A simple way to translate the number in LightBurn is to add three zeros. If you enter 50 kHz, think 50,000 pulses per second.

Think of a fiber laser as a pulsed system

A fiber laser may look like it is producing a continuous beam while it is engraving, but it is operating through a rapid sequence of individual pulses. The laser stores energy, releases it in a pulse, then repeats that process extremely quickly.

Frequency determines how many of those pulses are packed into the same one-second window.

Low frequency vs. high frequency

The two characteristics I find most useful when thinking about frequency are pulse energy and pulse overlap.

Lower frequency: fewer, harder hits

At a lower frequency, fewer pulses are being delivered during that one-second interval. In practical terms, each individual pulse can carry more energy, and there is more space between pulses.

That combination tends to favor more aggressive material removal. Each pulse hits hard, the material has more time to cool before the next hit, and the engraving can take on a rougher or more cratered texture.

Higher frequency: more, gentler, overlapping hits

At a higher frequency, many more pulses are being delivered during the same amount of time. Individual pulses tend to be less energetic, but they arrive in much quicker succession and overlap more heavily.

That increased overlap keeps more heat concentrated near the surface. Depending on the rest of the settings and the material, that can favor smoother engraving, polishing, fine marking, annealing, and color-marking effects.

Why pulse overlap matters so much

Imagine putting people onto the same escalator during two different situations. On an empty weekday, there may be several steps between people. On Black Friday, people pack together because many more have to occupy the same escalator at the same time.

Frequency works similarly. If the laser needs to fit 20,000 pulses into one second, there is more separation between pulses than if it needs to fit 200,000 into that same second.

With less overlap, the material gets more time to cool between impacts. With more overlap, heat accumulates more easily on the surface.

Why higher frequency usually means less energy per pulse

One simplified way to understand pulse energy is to think about the available average laser power being divided across the requested number of pulses.

If a 50W laser is operating at full average power and you ask it for 50,000 pulses per second, that power is divided across fewer pulses than if you ask it for 100,000 pulses per second.

The pizza analogy from the video works well: the pizza does not get bigger when more people show up. If you divide the same pizza among more people, each person gets a smaller slice.

MOPA vs. non-MOPA frequency range

Your available frequency range depends heavily on the laser source.

A conventional Q-switched or non-MOPA fiber laser usually gives you a relatively narrow frequency range. Depending on the specific source, wattage, and manufacturer, you might see something like 20–80 kHz, 20–100 kHz, or 50–100 kHz.

A MOPA source can provide a much wider range. On my 60W JPT MOPA, for example, the selectable range extends far beyond what I normally need for engraving.

That wider range is one reason MOPA sources offer more flexibility for processes such as fine surface work, annealing, polishing, and some color-marking applications.

Do not ignore cutoff frequency

Just because LightBurn lets you type a low frequency value does not necessarily mean your laser source can deliver full requested power at that value.

My laser has a manufacturer-defined cutoff frequency. Below that point, the controller may reduce available power because the source cannot maintain the requested pulse behavior safely and stably.

For my setup, I used 48 kHz as the lowest test value because that is where I could still get the full requested engraving power. Your source may be different, so refer to the specifications or frequency/power chart for your own laser source rather than copying someone else's minimum value.

The LightBurn coin test

To make the effect easier to see, I engraved the same AI-generated depth-map design on three brass coins and changed only the main engraving frequency.

The test was run on my 60W JPT MOPA using LightBurn's 3D Sliced image mode. The main settings shown in the video were:

1. Speed: 2,000 mm/s
2. Power: 80%
3. Q-pulse width: 200 ns
4. Image resolution: 900 DPI
5. Passes: 256
6. Main frequencies tested: 48 kHz, 80 kHz, and 200 kHz

A cleanup pass was also used periodically, so this should be treated as a controlled comparison from one specific machine and file—not as a universal recipe.

48 kHz result: aggressive and textured

The 48 kHz coin had the roughest background of the three. The surface looked heavily textured, almost as though that texture had been added intentionally.

That result matched the low-frequency behavior discussed earlier: stronger individual pulses, more spacing between pulses, and more aggressive cratering of the brass.

80 kHz result: a middle ground

At 80 kHz, the background was noticeably smoother than the 48 kHz result, although some texture was still visible.

This was close to the range I had normally used for coin engraving because it offered a balance between strong material removal and a cleaner surface.

200 kHz result: surprisingly smooth

The 200 kHz test produced the biggest visual surprise. The background became dramatically smoother—almost dish-like—because of the much heavier pulse overlap and greater heat accumulation at the surface.

I expected the much higher frequency to sacrifice a lot more engraving depth than it actually did. In this test, the difference in depth was smaller than expected, while the surface finish changed substantially.

The follow-up: 100 kHz at 90% power

After seeing how smooth the 200 kHz result looked, I ran one more coin. I increased power from 80% to 90% and set the main frequency to 100 kHz.

That final attempt gave me the combination I liked best: a smoother background while still preserving good detail and depth. After applying brass black, it became the frequency range I planned to use as a new starting point for that particular coin workflow.

So what frequency should you use?

There is no single correct frequency for every fiber laser job. A useful starting framework is:

1. Lower frequency: consider it when you want aggressive ablation and material removal.
2. Middle frequencies: useful when you want a balance between removal and surface quality.
3. Higher frequency: useful when you want more overlap, smoother surface behavior, fine marking, annealing, polishing, or other heat-driven effects.

Then test inside the safe operating range of your own laser source. The exact number that works on one machine may not translate directly to another source, wattage, lens, material, or pulse-width setting.

Using frequency in LightBurn

In LightBurn, frequency is one of the core parameters you will adjust in your layer settings for a galvo fiber laser. On a MOPA system, you may also have a separate Q-pulse width control, giving you another variable that interacts with frequency and power.

If you are still learning how power, speed, frequency, pulse width, line interval, and the other basic controls fit together, Fiber Laser Ignite is my free LightBurn-focused fiber laser course. The course walks through the core settings before moving into real engraving projects.

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