Marking the Subsurface of Glass for Anti-Counterfeiting

For subsurface engraving very short, extremely high energy laser pulses focus below the surface of transparent materials thus locally changing the material structure in the focus of the laser beam. Such subsurface codes can neither be altered nor removed without causing considerable damage.

Product trademark recognition, traceability and anti-counterfeiting solutions are essential for exclusive brands to secure their position in the modern market. Internal laser marking of transparent materials is an attractive solution for a variety of applications. However, the first generation of laser systems, needed a too-long pulse duration of the marking laser, enough to thermally generate micro-cracks in the material.
These micro-cracks or fractures might weaken the glass structure with the risk of glass rapture. As micro-cracks in glass containers are entirely out-of-the-question in the medical and pharmaceutical industry, the newly developed femto-second laser, as a result of the Naginels research project, is of paramount importance for these industries as a means of providing glass containers such as vials and syringes with secure tracking and tracing marks.

The Principle
In a process to mark with laser, a high power laser beam transforms into heat. This energy is absorbed in the material surface to be marked. The absorption results in removal of material, a colour change or foaming in certain plastics.

For example, the anodized metal surface layer is removed to expose the lower layer. In glass, a shallow notch is engraved in the material, creating a frost. On stainless steel, suitable laser parameters result in a localized effect of tempering. In some plastics, the material changes colour. In all cases, the markings are clean, clear and indelible.
Each laser pulse marks a very fine dot, with a diameter of 0.05 mm to 0.2 mm, depending on the lens used. Lines (thickness of 75 to 250μm) are formed by a series of superimposed dots. The laser beam is directed by mirrors mounted on ultra-quick galvanometers to draw lines, shapes, and characters directly on the surface.

The Naginels project
In 2002, six European companies started a research project called Naginels (Non-Aggressive Glass Internal Engraving Laser System), to develop a new laser marking process with a direct application to traceability. One of the partners, Amplitude Systemes, developed the diode pumped Ytterbium doped pulse lasers with typical pulse durations of 100 femto-seconds (1 fs = 10-15 seconds), forming light pulses of only 30 microns long. During this extremely short pulse the energy dissipated cannot be transferred to the surrounding material and this makes the writing process athermal. Therefore, these ultra-fast laser pulses do not induce micro-cracks but only locally change the refractive index of the glass.

Amplitude Systemes' Satsuma laser

These changes in the structure of the glass are invisible to the naked eye, however, their association can be seen with a very high contrast thanks to another optical phenomenon which is the diffraction of light. As this diffraction only occurs in some lighting conditions, Naginels marking is invisible, and can be as small as a piece of hair. Therefore its application can be extended to anti-counterfeiting.

The image resolution is high enough to allow (datamatrix) codes to be written small enough to be invisible. Additional alphanumeric codes can be made available for the consumer to check the authenticity of the product.

According to the company, this technology fully meets the requirements of the industry in terms of traceability, authentication and anti-counterfeiting. Each vial, syringe or container may be marked with an unalterable mark inside the material, guaranteeing the visual authenticity of the product (control by the buyer). Also a coded and hidden element can be added to the marked information to ensure the consolidation of data and of the anti-counterfeiting aspect (control by the authorities). Moreover, for FDA validation, no additive is applied on or inside glass, therefore no new contamination can occur and no additional validation must be performed.

This technology can also be implemented for decoration in perfume bottles as the diffraction effect provide a decomposition of light such as a rainbow does.

To exploit the Naginels technology the six partner companies in the project set up TrackInside S.A., which is responsible for the marketing of the process of marking transparent materials for traceability and anti-counterfeiting.

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