The benefit of focus tunable lenses for laser processing

Date: July 2012
Published by: Industrial Laser Solutions
Link: Online version   

Controlling a beam focus with an electrically tunable lens enables a faster, more compact and reliable laser system

By Dr. Selina Pekarek and Mark Blum

Introduction

In standard technologies of laser processing, axial focusing is achieved with mechanical translation stages, moving either the optics or the target. This approach exhibits limitations concerning speed, reliability and costs. Instead of using complex mechanics, the z-position of a beam focus can be controlled by using a lens with an electrically tunable focus. This allows for fast response times in the range of milliseconds, enabling high-speed processing and fast switching between targets. Furthermore, because of the reduced number of moving parts the overall system can be made more compact and reliable. The tunable-lens technology developed by Optotune is already successfully in use in applications such as laser marking, engraving and surface preparation. 

Working principle of the electrically tunable lenses

The electrically tunable lens from Optotune is a shape-changing lens. The core of the lens consists of a container, which is filled with an optical fluid and sealed off with an elastic polymer membrane. An electromagnetic actuator is integrated in the lens which controls a ring that exerts pressure on the container (see FIGURE 1). The deflection of the lens depends on the pressure in the fluid. Therefore, the focal length of the lens can be controlled by the current flowing through the coil of the actuator. The aperture of Optotune’s EL-10-30 is 10 mm and the focal length can be varied between 20 and 120 mm (see FIGURE 2). The voltages which have to be applied to cover the complete tuning range are small (<5 V). The lens does not exhibit hysteresis and the performance of the lens is not sensitive to different polarizations. Moreover, the electrically tunable lenses enable novel applications due to their short response times in the range of milliseconds (see FIGURE 3). Additionally, the membrane is long-term stable and the lens can be operated over more than 10’000’000 life-cycles. A challenge for the lens by principle is gravity because it can induce a coma when the lens is used in upright position (horizontal optical axis). Therefore, it is recommended to design the optical system with the lens in lying position (vertical optical axis). However, recent optimization of the membrane’s mechanical properties reduced this issue sufficiently, so that the lens is also suitable for high precision applications.

Damage and controlling

Optotune lenses, which are made of low dispersion material, exhibit very low absorption in the wavelength range from 250 nm to 2000 nm. Several tests have been performed with both continuous wave (cw) and pulsed laser sources. The cw laser source tested emitted at the wavelength of 1070 nm with an output power of 200 W and the beam diameter was 3 mm. This yields a damage threshold of at least 2.2 kW/cm2. Using a pulsed laser operating at 1064 nm, with a pulse duration of 20 ns at a repetition rate of 50 kHz, an average power of 10 W was focused onto the tunable lens. With a spot size of 50 m, this yields an energy density of 10J/ cm2. Up to these levels, the lens did not show any sign of damage. The effective damage threshold could not be determined because of the limited power of the available test sources. 

In case of high intensities, the residual absorption can heat up the lens. As a consequence, the fluid expands and the focusing behavior of the lens might change. However, the tunable lens can be easily remote-controlled and therefore, an even higher precision and temperature independent operation can be achieved by using an optical feedback system. 

Applications

Focus tunable lenses are a novel platform technology with many applications. In imaging, the lenses are used for fast focus control and zoom solutions (e.g. in machine vision, microscopy or camera phones). In illumination systems, the lenses provide a compact and efficient way to control divergence (e.g. of an LED spot light). The high damage thresholds of the tunable lenses also open up a wide range of applications in laser processing. While good results have already been obtained in marking, 3D engraving and surface preparation, the same is expected to apply also to micromachining and drilling. In principle, even high power applications such as cutting and welding should be possible. In the following an example of a setup for 3D laser marking is described in more detail.

3D laser processing

FIGURE 4 shows an ideal layout for a 3D laser processing system with an electrically tunable lens, galvo mirrors and an F-Theta lens. The layout is described in more detail in the following. First, the beam size of the laser can be adapted by an optional beam expander if required. Furthermore, the optical system consists of a lens combination of a tunable lens and a fixed concave lens. The electrically tunable lens is a converging lens. However, a negative focal length can be achieved in combination with an offset lens. Therefore, by changing the focal length of the tunable lens, the divergence angle after the two lenses is varied. In combination with the focusing lens at the end of the optical system, this results in an adjustment of the z-position of the focus of e.g. 150 mm. For this adjustment, only the current applied to the tunable lens has to be changed. In contrast, in a traditional setup either a lens or the object plane has to be translated mechanically. A mechanical translation is usually accompanied by less precision, it is more time consuming and less reliable. After the two lenses, the propagation direction of the beam is controlled with galvo mirrors. In combination with an F-Theta lens, the beam can be focused everywhere on the x-y-plane with a good spot quality. With the described setup, the position of the focus can be controlled in all three directions with a minimal amount of mechanics and at high speed. 

Conclusion

In summary, the presented tunable lenses are suitable for high-power applications and can thus boost the avenue for faster and more reliable focusing in laser processing. They are especially suited to replace the traditional mechanical units to adjust the z position of the focus with a high precision and reliability. Furthermore, as a consequence of their fast response times, they allow fast switching in the range of milliseconds.