BEAM DELIVERY Gaining focus Mike Delbusso, Laser Mechanisms
Good processing head design is essential to reduce focal shift, a phenomenon caused by the absorption of energy into the focusing lens. As the lens increases in temperature, its index of refraction and radius of curvature changes or distorts and the focus position shifts towards the lens with a different spot size (also described as thermal lensing). All optics absorb some energy from
the laser. The absorption of energy into the lens can increase if the lens material and coating are inadequately rated for the amount of laser power directed onto them, but drastically increases if there are contaminants on the lens. This is a self-perpetuating process whereby debris
on the lens causes a focal shift, creating a bad weld condition that creates even more debris and more focal shift. Most laser processing heads use a
transmissive focus, where the beam passes through a lens to achieve focus. An alternative is to use a mirror, typically a parabola machined from copper, which focuses the beam as it reflects it. Reflective parabolic mirrors have long been used in high-power CO2
applications
because they are more robust, less susceptible to damage and debris, and easier to clean than a transmissive lens, but they also do not exhibit thermal lensing and have virtually no focus shift. Polished, uncoated copper can provide excellent
Scanlab has developed its ExcelliScan
system, which uses digital encoders and innovative servo technology. Te new servo technique more fully exploits scanner dynamics, thus boosting throughput while maintaining maximum linearity and long- term stability – even in constant operation. Another solution approach is to link individual galvanometer axes into more highly integrated systems. ‘And the manufacturing industry is also making contributions – for example, by deploying novel materials,’ commented Hofner.
The power of optics Depending on the process and lasers used, lenses can be a critical portion of the optical train. A flat view lens sitting at the head of a scanning device for a standard CO2
results, but enhanced coatings can give reflectivity of more than 99 per cent for the 1µm wavelength of fibre lasers. Eliminating focal shift through the use
of reflective optics provides stability in laser welding processing heads, but there remains the one issue that plagues all welding applications – the debris that is generated by the process. It has been observed that the 1µm wavelength is far more susceptible to, and far less forgiving of, any debris either on the optical elements or even in the free space air in the beam path. Keeping the optics clean and protected is essential. Visual inspection is not completely adequate as it has been found that contamination can
exist on a sub-micron level, invisible to the naked eye, but still of influence to the laser beam. Frequent inspection by microscope, combined with routine cleaning and replacement of optics, must be a formal procedure. There are a variety of additional
methods for protecting optics – such as cross-flow air knives, purge chambers and reduced apertures – that have been successfully implemented. Airborne debris in the beam path can also cause scatter of the 1µm beam, so the machine design of a laser welding system should incorporate effective fume removal of the welding zone and in some cases, the placement of additional air knives close to the process.
laser, for
example, is relatively simple to make and control. But as Hofner pointed out, in the ultrashort pulse domain where the laser itself is extremely aggressive, every optical element will have some impact on the quality of beam delivery. One company involved in this area is Swiss firm Optotune, which was founded in 2008. Optotune’s initial focus was on machine vision applications such as automated inspection. According to Dr David Stadler, application engineer at Optotune, the company recently intensified its efforts in the direction of laser manipulation, converting its tunable lens for 3D laser processing. ‘For laser processing applications, it was essential to develop the optical feedback that measures the deflection of the lens in-situ and hence allows for precise laser spot control and high long-term stability,’ Stadler
28 LASER SYSTEMS EUROPE ISSUE 29 • WINTER 2015
flexible and natural approach is using optics that resembles the human eye
The more
explained. ‘Certain sectors have to overcome the paradigm of fixed glass optics, which poses many limitations. Te more flexible and natural approach is using optics that resemble the human eye – Optotune’s tunable lenses, for example.’ Te lenses consist of a container
that is filled with an optical fluid and sealed off with an elastic polymer membrane. Te deflection of the lens is proportional to the pressure in the fluid. For laser processing, the company developed its EL-10-42-
OF lens module with a 10mm clear aperture and integrated optical feedback. It is optimised for 532nm and 1,064nm laser wavelengths. Te tunable lenses have an electromagnetic
actuator used to exert pressure on the container. Hence, the focal distance of the lens is controlled by the current flowing through
the coil of the actuator. So far, bulky motorised solutions are used to shiſt the laser spot in z-direction and hence enable 3D laser processing. In a complementary approach without mechanically moving parts, Optotune’s EL-10-42-OF can be integrated directly into an existing system. As Stadler observed, there is a clear trend within the industry towards more flexible, compact and less power-consuming systems. Tunable lenses also have a very broad range
of applications. In general they are beneficial when precise and fast focusing at different working distances is required. Tis is relevant for machine vision applications, microscopy or for laser processing where a laser has to be focused on the workpiece. Like many others, the company is also looking to expand its efforts in the sector of micromachining where the challenge is controlling the laser spot precisely – a challenge that continues to grow as laser power increases.
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www.lasersystemseurope.com
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