Questions about laser sources often go beyond laser power. Users may want to know the pulse duration, maximum pulse energy, numerical aperture of the optical system, or even the diameter of the beam where it reaches the workpiece.

These specifications are closely related to the way a laser deposits energy into a material. They also have a bearing on engraving depth, edge definition, and the ability to reproduce small details.

For fiber laser marking, four parameters are particularly useful to understand: pulse duration, pulse energy, numerical aperture and spot diameter. Each describes a different part of the laser system, and none of them should be considered in isolation.

Pulse Duration

Pulse duration is simply the time for which one laser pulse lasts. Depending on the laser source, it may be expressed in nanoseconds (ns), picoseconds (ps), or other units of time.

em-smart fiber laser marking machines use Q-switched fiber laser sources. Published specifications for some models give a repetition frequency of around 30–60 kHz.

This number should not be confused with pulse duration. A repetition frequency of 30 kHz means that the laser can produce up to 30,000 pulses per second. It does not tell us how long each pulse lasts. The pulse width is determined by the design and operating characteristics of the laser source itself, so the source datasheet is the appropriate reference when an exact value is required.

Q-switched lasers are widely used where short, high-peak-power pulses are needed. Their pulse duration can vary considerably between different laser architectures. General technical information on this principle is available from RP Photonics.

Why does pulse duration matter during marking? If a similar amount of energy is delivered over a shorter period, the instantaneous or peak power becomes higher. Depending on the material and the rest of the process parameters, this can help limit unnecessary heat spreading and preserve small features.

Take a stainless steel nameplate with small lettering or a QR code. When the laser parameters are properly matched to the material, the edges of the characters can remain clean, and the narrow parts of the pattern are less likely to merge. This is one of the situations where pulse characteristics become visible in the finished mark rather than remaining just a specification on a datasheet.

Maximum Pulse Energy

Pulse energy describes how much energy is contained in an individual laser pulse. The value is normally given in millijoules (mJ) or microjoules (μJ).

For a simple estimate, pulse energy can be calculated from average power and repetition frequency:

Pulse Energy ≈ Average Power ÷ Repetition Frequency

• 20 W ÷ 30 kHz ≈ 0.67 mJ
• 20 W ÷ 60 kHz ≈ 0.33 mJ
• 30 W ÷ 30 kHz ≈ 1.00 mJ
• 30 W ÷ 60 kHz ≈ 0.50 mJ

These figures are calculated average energy per pulse. They are useful for understanding the relationship between power and frequency, but they should not be presented as the laser source's specified maximum pulse energy. Actual pulse characteristics depend on the source design and its operating conditions.

At the same average power, a lower repetition frequency means more energy is available in each pulse. This is one reason frequency is often adjusted when the goal shifts from surface marking toward stronger material removal.

Consider deep engraving on brass or stainless steel. With suitable power, scanning speed, and hatch settings, reducing the repetition frequency can increase the energy delivered by each pulse. Over multiple passes, this can contribute to more effective material removal and greater engraving depth.

The result is not controlled by pulse energy alone. Material composition, focus, scanning speed, hatch spacing, and the number of passes all play a role.

Numerical Aperture

Numerical aperture, commonly written as NA, is an optical parameter that describes the angular range over which an optical system can accept or focus light.

It is expressed as:

NA = n × sin α

Here, n is the refractive index of the medium and α is the half-angle of the light cone.

In a laser marking machine, the focusing optics determine how the incoming beam is concentrated onto the workpiece. This affects the size of the focused spot as well as the depth over which the beam remains reasonably well focused.

F-Theta lenses are commonly used in galvo-based laser marking systems. Their optical design allows the focused beam to be scanned across a defined working field while maintaining controlled focusing characteristics.

The actual NA is determined by the optical configuration and lens design. A marking field such as 110 × 110 mm is not enough to establish the numerical aperture on its own. If a precise NA value is needed, the specification of the particular lens should be checked.

A small engraving area provides a useful real-world example. When adding a tiny logo or lettering to a ring, the available surface is limited, and the engraved features may only occupy a few millimeters. Good focusing performance helps keep those small lines and characters distinguishable rather than producing a blurred or oversized mark.

For a more detailed explanation of numerical aperture and its role in optical systems, see Edmund Optics' technical reference.

Spot Diameter

Spot diameter is the approximate size of the focused laser beam where it meets the workpiece.

A simplified optical relationship is:

Spot Diameter ≈ (4 × M² × λ × f) ÷ (π × D)

The variables in this relationship are:

• M² — beam quality factor
• λ — laser wavelength
• f — focal length
• D — incident beam diameter

Fiber laser marking systems commonly operate at a wavelength of approximately 1064 nm.

In practice, the final spot is affected by more than the theoretical equation. Beam quality, diffraction, focal length, incident beam diameter, and optical aberrations can all change the result. This is why a calculated spot size should be treated as an approximation rather than as a direct measurement of the machine's marking capability.

With a 110 × 110 mm marking configuration, the focused spot may fall within the tens-of-micrometers range, depending on the source and optical setup.

Spot diameter should also be kept separate from specifications such as minimum line width or minimum character height. A machine may, for instance, specify a minimum line width of 0.02 mm, a minimum character height of 0.15 mm, and repeatability of ±0.002 mm. These figures describe marking performance under specified conditions; they are not simply different ways of stating the laser spot diameter.

A metal card with a small QR code is a straightforward example. The code consists of many small square modules packed into a relatively limited area. A well-focused beam makes it easier to reproduce these modules and the fine lines between them without excessive spreading.

How to Check the Parameters

Not every laser parameter can be calculated from the machine's headline specifications. The source model, optical components, and actual machine configuration all need to be considered.

When checking a machine, start with the manufacturer specifications and the information on the laser source. The lens model can also be useful when investigating the optical side of the system.

Laser marking software can provide another part of the picture. In LightBurn, users can work with marking parameters such as frequency and other process settings. These settings are useful for controlling the marking process, but they do not replace the laser source datasheet when the question concerns pulse duration, maximum pulse energy, or numerical aperture.

If a particular source or lens specification is not publicly available, the most reliable option is to ask the manufacturer or technical support team. Providing the exact machine model and laser source information will make it easier to identify the relevant specifications.

For em-smart equipment, technical questions can be sent to service@em-smart.com.

Parameter Overview

Parameter	What it describes	Main effect
Pulse Duration	Length of an individual pulse	Peak power and thermal behavior
Pulse Energy	Energy contained in one pulse	Material removal and engraving depth
Numerical Aperture	Focusing characteristics of the optical system	Spot size and focal depth
Spot Diameter	Approximate focused beam size on the workpiece	Fine detail and line definition

These parameters are connected, but one does not automatically determine the final marking result.

For instance, a smaller spot can help with fine details, but it does not by itself guarantee deeper engraving. Likewise, higher pulse energy may increase material removal without being the best choice for a very fine pattern. The actual result comes from the interaction between the laser source, optics, process parameters, and material.

Practical Marking Considerations

For fine engraving, the focus position and optical setup should be matched to the size of the details being marked. Scanning speed, frequency, and other process settings then need to be adjusted according to the material.

When deeper engraving is required, a lower repetition frequency can provide more energy per pulse at the same average power. Multiple passes and appropriate scanning parameters are normally used together to control material removal.

For smaller marking areas where finer detail is more important than field size, a shorter focal-length F-Theta lens, such as a 70 mm configuration where supported, can be considered. The choice involves a trade-off between marking field and focusing characteristics, so it should be based on the application rather than treated as a universal setting.

In practice, test marking remains the most reliable way to determine the right combination of parameters. Different metals can respond quite differently even when the nominal laser settings are identical.

Conclusion

Pulse duration, pulse energy, numerical aperture, and spot diameter describe different parts of the laser marking process.

Pulse duration concerns how long each pulse lasts. Pulse energy tells us how much energy is carried by each pulse. Numerical aperture describes an important characteristic of the focusing optics, while spot diameter is related to the size of the focused beam on the workpiece.

Understanding these terms makes it easier to interpret laser specifications and, more importantly, to connect them with actual marking results. For applications involving fine text, QR codes, or deep engraving, the useful question is rarely which single parameter is “best”. The better approach is to look at how the laser source, optical system, marking settings, and material work together.