Femtosecond Laser Machining of Internal Serrated Apertures in Circular Substrates

Introduction

An internal serrated aperture asks a lot of a manufacturing process: every tooth around the bore has to match the next in width, angle, and edge finish, on a substrate that may be too thin to tolerate much handling. That combination is where conventional tooling runs out of room.

This article looks at why that combination is so hard to produce with stamping, micro-milling, or wire EDM, and how femtosecond laser cutting changes the constraints involved, along with the materials and applications where this type of component is used.

What Is an Internal Serrated Aperture?

An internal serrated aperture is a circular part with a toothed, comb-like profile cut into its central opening rather than around its outer edge. Under magnification, the pattern can look strikingly similar to a gear, a ring of evenly spaced points surrounding a central bore, which is why it's sometimes described as a gear-like internal cutout or a toothed internal profile.

It usually isn't a gear. Watch gear teeth mesh with another moving part to transmit motion; the structure discussed here does not. In optical engineering, it is sometimes called a serrated aperture or apodizer, though the manufacturing challenge, cutting a fine, repeatable tooth profile into a thin circular substrate, is the same regardless of which application it serves.

(Femtosecond laser machined internal serrated aperture with precise toothed geometry and clean edges.)

Why Complex Internal Profiles Are Difficult to Manufacture

The difficulty comes from three factors stacking on top of each other: a small feature size, a complex contour, and, frequently, a thin substrate.

Stainless steel apertures for optical applications are often supplied at 0.05mm thickness or less. At that gauge, the material has almost no rigidity of its own. Stamping force or wire EDM tension can distort the part before the cut is even finished, and wire tension becomes harder to manage without distorting the part as the substrate gets thinner.

Traditional Machining vs. Femtosecond Laser for Internal Serrated Aperture Manufacturing

How Femtosecond Laser Enables Precision Serrated Aperture Machining

CAD-Based Complex Geometry Processing

The tooth profile exists first as a CAD file, and the laser follows that path directly. There is no die to fabricate and no cutter to select for the smallest internal radius, so a custom serration pattern can go from drawing to finished part without new tooling.

Minimal Thermal Impact and Clean Edges

A femtosecond pulse interacts with the material for a shorter time than heat needs to diffuse into the surrounding structure. This does not eliminate heat entirely, but it keeps the affected zone narrow enough that edge quality and material properties near the cut stay close to their original state, a meaningful advantage on a 0.05mm substrate where even minor thermal distortion is visible.

Micron-Level Dimensional Control

Positioning accuracy in the ±1μm range makes it possible to hold tight tolerances across dozens of repeated tooth features around a single bore. On a 0.05mm stainless steel aperture, MONO's ML-TRACK femtosecond laser cutting platform has held corner radii as tight as 2μm at the tooth root, a geometry that falls below what conventional micro-milling tools or WEDM wire can practically reproduce. This capability is especially valuable for custom precision components where conventional tooling cannot achieve the required internal geometry.

Femtosecond Laser Machining Process

1. 1CAD Design Review. The drawing, target geometry, and required tolerances are reviewed before any cutting begins.
2. 2Process Parameter Optimization. Laser power, pulse parameters, and scanning strategy are tuned to the specific material and substrate thickness.
3. 3Precision Laser Cutting. The internal profile and tooth-like structure are cut in a single non-contact pass on MONO's femtosecond laser systems.
4. 4Inspection and Verification. Parts are checked under magnification, often at 20x or higher, to confirm dimensional accuracy and edge quality before release.
5. 

Materials and Applications of Internal Serrated Apertures

Femtosecond laser machining processes stainless steel, molybdenum, and tungsten, along with technical ceramics such as silicon carbide and alumina, and glass. Stainless steel is the most common substrate for optical aperture applications, while molybdenum and tungsten see wider use in semiconductor-related components.

• Semiconductor components
• Optical and metrology instruments
• High-power laser beam diagnostics and delivery systems
• Laser beam shaping apertures

Why Choose Femtosecond Laser for Custom Internal Profile Machining?

Non-contact processing removes the tool-wear and tool-access limits that come with stamping or milling. The same flexibility that handles a serrated aperture applies to other complex internal geometries, so a design change means updating the CAD file rather than commissioning a new die. A narrow heat-affected zone rounds out the advantages, which is part of why the process suits both early-stage prototypes and small-batch precision production, including parts thin enough that mechanical processing would risk deforming them before the cut finishes.

FAQ

What is an internal serrated aperture?

An internal serrated aperture is a circular component with a toothed or comb-like profile cut into its central opening. In optical and laser engineering, this structure is also referred to as a serrated aperture.

Is this a microgear, or something else?

Although the internal tooth pattern resembles a gear at first glance, this component does not mesh with or drive another part. It is more accurately described as a serrated aperture, designed to shape a beam's diffraction profile rather than transmit mechanical motion.

Can femtosecond lasers cut complex internal profiles?

Yes. Because the process is beam-based rather than tool-based, femtosecond laser machining can cut internal geometries, including fine serrated or toothed patterns, that would be difficult or impossible to reproduce with a physical die or milling tool. For a closer look at how the two ultrafast processes compare, see how engineers choose between femtosecond and picosecond lasers.

What materials can be processed with femtosecond laser machining?

Femtosecond laser machining is compatible with a wide range of materials, including stainless steel, molybdenum, tungsten, silicon carbide, alumina, and glass.