Liquid lenses to enhance fast, compact 3D focus control

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Optotune has developed a new generation of focus-tunable lenses for 3D laser applications.

With a new generation of focus-tunable lenses for NIR and green wavelengths with integrated optical feedback, Optotune is enabling new applications, mainly in the medical and industrial laser fields.

The demand for controlling the focus of a laser beam in all three axis is ever increasing. Traceability regulations have resulted in a higher need for laser labelling in many industries, from PCB engraving in consumer electronics to the marking of medical implants.

Treatments with laser beams are already well-established solutions in ophthalmology and dermatology, where laser light needs to be carefully (de)focused in three-dimensional space. 

Lasers are also becoming a commodity in additive manufacturing and other emerging markets.

The trend is expected to continue: according to a current report of the Japanese market research specialists Shibuya Data Count, the laser marking market itself is on the course of a steady growth until 2030, with the CAGR of 4-5%.

One common pain point of these applications is fast and precise focus control. The need is to focus either at specific object planes (so called 2.5D focusing) or in arbitrary volume (3D laser applications). Naturally, standard optical configurations came upfront first: galvanometric mirrors were used to provide beam steering in a plane (x-y control), whereas the required z-control was established using translational optics or mechanically moving the object.

These solutions work, but are not ideal in terms of size, speed, limited z-range, cost, or mechanical wear that leads to a reduced lifetime. Consequently, there has been a visible shift towards adaptive optical elements. In these cases, spatial displacement of an optical element is replaced by the change of its shape.

Optical feedback included

Developments at Optotune followed exactly this trend. The Swiss company´s core competences cover the opto-electro-mechanical design of dynamic light controlling components. One of Optotune´s innovative developments is based on employing the well-known voice coil technology, that is for example used in microphones, to control the shape of a polymer lens filled with optical liquid.

The first generation of Optotune´s laser focus-tunable lens EL-10-42-OF has proven to be an excellent solution in many areas of application where the highest standards were needed in terms of laser spot size and scanning speed, especially if volumes are large.

Figure 1: An EL-10-42-OF liquid lens in a configuration for 2.5D laser marking.

To explain the product name: EL designates electrically tunable lenses, 10 the clear aperture in mm, 42 outer diameter of the package in mm, whereas OF stands for Optical Feedback. This focus-tunable lens had been designed for a particular wavelength of 1,064nm that was commonly used in most laser processing applications, and for low- to mid-power (50W) lasers.

At these laser levels it is highly likely to have thermal effects influencing stability and accuracy of the lens´ optical power. For that reason, the developers integrated internal optical feedback in the initial version of the EL-10-42-OF lens that solved this problem. This technology is constantly active during lens operation to stabilize the optical power of the lens and to allow for repeatability as low as 0.02 diopters in the whole focus tuning range between -2.0 and +2.0 diopters. Even if the repeatability is not of importance, the optical feedback remains an important specification in fields such as ophthalmology, where it is mandatory due to regulatory requirements.

Expanding to the NIR and visible range 

With the second generation of the EL-10-42-OF, the Swiss designers have now introduced a new version that covers a much wider wavelength range in the NIR spectrum from 950 to 1,100nm. This is useful because there are laser applications using NIR lasers outside the 1,064nm point. Optotune also introduced a green version of the focus-tunable lens for lasers with a wavelength of 532nm and up to 20W, which particularly attracted customers in ophthalmology.

The expansion of the wavelength range was accompanied by some additional adjustments of the design like optimized optical components such as cover glasses and filters that helped to improve the lens stability and reduced its thermal sensitivity. Optotune has also redesigned the components involved in the optical feedback loop to further improve the system repeatability when switching the laser on or using the lens near the edge of the diopter range. Improvements in the manufacturing processes and additional quality control tests also lead to a better reliability users benefit from. 

Exciting fields of application

Industrial laser processing is a field of application where already the first generation of the EL-10-42-OF focus-tunable lens has proven to be particularly useful for several years now. There are a couple of reasons for that. Firstly, operating at both near-infrared and the visible wavelengths, it could work with a wide range of materials. 

Secondly, 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 compact EL-10-42-OF can easily be added to a system between the laser and the scanning head, being perfectly compatible with pre-installed f-theta lenses that are commonly used for optical field flattening (figure 1). The adapted setup 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 (figure 2).

Figure 2: Marking field and z-ranges represent typical values when a 160mm f-theta lens is used. 

As the speed is not so critical, the EL-10-42-OF for 2.5D applications is typically realized in analog signal configuration, where the analog board controls the lens’ optical power by monitoring its optical feedback and temperature, allowing to jump between large z-axis variations within milliseconds.

Thirdly, true 3D laser processing is a particular challenge in the industry since either speed, z-range or spot uniformity are compromised. Configurations with the EL-10-42-OF, as shown in figure 3, typically supported by a digital controller using the XY2-100 protocol, resolve all these challenges.

Figure 3: 3D laser marking configurations with EL-10-42-OF do not require an f-theta lens anymore. 

They offer simultaneously high speed (up 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 (figure 4). In addition, a configuration like that does not require f-theta lenses as the field flattening is performed by the EL-10-42-OF itself. Only standard off-the-shelf optics are needed to define a starting working distance. 

Figure 4: The laser spot sizes (in µm) remain uniform in the entire marking volume, leading to more accuracy of the markings.

Inline visual inspection with tunable liquid lenses

The laser industry also experiences an increasing demand in laser job inspection. One reason for that trend lies in the necessity for real-time quality monitoring. Another important reason is the growing need for automation. External cameras can partially meet this need, but visual inline solutions with a coaxial setup of inspection path and laser beam path, separated by a beam splitter, are the preferred choice for integration and resolution reasons.

Optotune has approached this issue in two possible ways. In the first configuration (figure 5), a EL-16-40 tunable lens, a sister product to the EL-10-42-OF, is added to a standard camera to perform automatic focusing for inspection. In addition, it can reliably measure the object distance using Optotune´s depth from focus contrast algorithm.

Figure 5: Inline inspection configuration with the EL-16-40 tunable lens

This can be used for repositioning the laser focus to the right plane. Being outside the laser beam path, the EL-16-40 lens can be integrated regardless of the laser power class. The existing setup is minimally affected, as only a simple dichroic mirror needs to be added.

Systems with laser power levels up to 50W can benefit from a configuration where the EL-10-42-OF lens is used for laser processing and the inline inspection at the same time (figure 6). 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.

Figure 6: Inline inspection configuration with the EL-10-42-OF tunable 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 setup using an f-theta lens. On top of that, users benefit from a setup that is far more compact and from reduced overall cost.

Conclusion

Optotune´s new generation of focus-tunable lenses for laser beams in the NIR and visible supplies users with new options to design compact and accurate systems for 3D focus control and laser process monitoring. Those can be installed at less overall cost in emerging 2.5D and 3D laser applications like laser marking and cutting, as well as in ophthalmology, micro-processing, additive manufacturing, and many other fields of use.

Article authored by: Dr Branislav Timotijevic, business development manager at Optotune

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