ANALYSIS: BEAM DELIVERY
“Focus-tunable lenses can supply users with new options to design compact and accurate systems for 3D focus control and laser process monitoring”
Figure 4: Tunable lenses such as Optotune’s EL-10-42-OF enable both inline inspection and laser marking to be performed using one set-up
focus tuning range between -2.0 and +2.0 diopters. Even if repeatability is not of importance, the optical feedback remains an important specification in fields where it is mandatory due to regulatory requirements.
Expanding to the NIR and visible range Demand has since been seen for such focus-tunable lens technology covering a much wider wavelength range in the NIR spectrum, from 950 to 1,100nm, to process a wider range of materials. Hence the development of the second generation of the EL-10-42-OF, which covers laser applications using NIR lasers outside the 1,064nm point. We also saw need for a green version of the focus- tunable lens, for lasers with a wavelength of 532nm and up to 20W of power, which can be used for marking or cutting certain types of organic material. The expansion of the
wavelength range was accompanied by some additional adjustments of the EL-10-42-OF’s design, such as optimised cover glasses and filters that helped to improve the lens stability and reduce its thermal sensitivity. Components involved in the optical feedback loop were also redesigned to further improve the system’s repeatability when switching the laser on or using the lens near the edge of the diopter range.
Control in three dimensions Many marking and engraving laser systems need occasional jumps to a different height to enable 2.5D processes, but users want to avoid bulky and costly translational optics. The compactness of focus-tunable lenses mean they can easily be added to a system between the laser and the scanning head, being compatible with pre- installed f-theta lenses for optical field flattening. This adapted set-up provides
a fast and flexible working distance adjustment over a large z-range, without any need for mechanical stages. As an example, with an f=160mm f-theta lens, a z-range as large as 100mm can be achieved (see figure 1). For 2.5D applications, the lens can be realised in analogue signal configurations, where an analogue board controls its optical power by monitoring optical feedback and temperature, allowing it to jump between large z-axis variations within milliseconds. Thirdly, 3D laser processing is
a particular challenge in industry since either speed, z-range or spot uniformity are typically compromised. As shown in figure 2, configurations using focus-tunable lenses such as the EL-10-42-OF, typically supported by a digital controller using the XY2-100 protocol, resolve all these challenges. They offer simultaneously high speed (up
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to 6m/s), large scan field and z-range (up to 1,000 x 1,000mm), and a constant spot size in the entire volume. In addition, a configuration like that does not require f-theta lenses as the field flattening is performed by the lens itself. Only standard off-the- shelf optics are needed to define a starting working distance.
Inline visual inspection Industry is also showing increasing demand for real-time quality monitoring during laser processing, driven in part by the growing need for automation. External cameras can partially meet this need, but visual inline solutions with a coaxial set-up of the inspection path and laser beam path, separated by a beam splitter, are the preferred choice for integration and resolution reasons. We see two possible
configurations using focus- tunable lenses that can address this challenge. The first (see figure 3), sees an EL-16-40 tunable lens – sister to the EL- 10-42-OF – added to a standard camera to perform automatic focusing for inspection. This can be used for repositioning the laser focus to the right plane. Being outside the laser beam path, the lens can be integrated regardless of the laser power class. The existing set-up is minimally affected, as only a simple dichroic mirror needs to be added.
The second configuration (see
figure 4), which can be used in systems with laser power levels up to 50W, sees the EL-10-42-OF used for both laser processing and the inline inspection at the same time. The inline camera in this case only requires standard camera optics, and the scan head does not have to be equipped with an f-theta lens. This approach leads to laser marking systems with improved accuracy, as it provides constant laser spots and constant image resolution in the entire marking volume. In addition, the marking volume is even increased compared to a set-up using an f-theta lens. On top of that, users benefit from a set-up that is far more compact, and from reduced overall cost.
Marking and more Focus-tunable lenses for laser beams in the infrared, NIR and visible wavelengths can supply users with options to design compact and accurate systems for 3D focus control and laser process monitoring. They can lower overall installation costs in emerging 2.5D and 3D laser applications, such as laser marking and cutting, as well as micro-processing, additive manufacturing and many other fields of use. l
Dr Branislav Timotijevic is business development manager at Optotune
SUMMER 2022 LASER SYSTEMS EUROPE 25
Optotune
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