tEchnology scan hEads
because we know the position of the galvo, we can automatically turn the laser on and off at the right position.’
In addition, Rekowski says, because the Laser marking, in this case on bottle caps, uses scan heads to position the beam extremely quickly. Image courtesy of Scanlab
is scanning back and forth, the focal point of the beam is making an arc across the scan field. Therefore, with a flat workpiece, you need to be able to flatten that focal field; the beam has to remain focused as it scans across the workpiece.’ In a three-axis system, the f-theta lens is
replaced with a pre-scanning objective system that dynamically focuses the beam along the z-axis. Erwin Wagner, chief technical officer at Raylase, notes that one advantage with incorporating a dynamic lens for the z-axis is that a range of field sizes can be covered. An f-theta objective, on the other hand, provides a fixed focal length, which in turn defines the size of the scanning field. Fabricating f-theta lenses covering larger working areas becomes very expensive, and a dynamic lens for the z-axis increases the field of view. Raylase manufactures scanning systems and components. It has supplied three-axis systems for scanning fields of up to 2 x 2m.
Micromachining
In most laser systems the scan head will be synchronised with a linear positioning stage. ‘To position the beam correctly, the galvo axis and linear axis have to be synchronised,’ explains Daniel Schwab, responsible for project engineering and hardware development at Arges, a laser scan head provider. ‘The encoder signal of the linear stage is transmitted to the scan head controller in order for it to calculate the mirror position and position the beam accurately during the movement of the stage.’ Arges also builds controllers for laser systems to synchronise linear stages and scan heads. The company has recently introduced its Anteater three-axis scanning system, suited to applications like micromachining requiring high-speed scanning. The Anteater scanning system incorporates small mirrors for high-speed processing and can be used with picosecond
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lasers for cold ablation. How well the scan head is synchronised with linear or rotary stages can impact upon the precision of the laser system, which is crucial for micromachining tasks and creating very small features on the part. US company, Aerotech, has released the Nmark CLS scan head controller, aimed at high-end laser micromachining applications. ‘The previous generation of the product, the Nmark SSaM, supports a mature serial communication interface called the XY2100, which is available with almost all commercial scan heads,’ explains Ron Rekowski, advanced automation division manager at Aerotech. ‘The limitation with this serial control protocol is that the exact location of the galvo is unknown. From a programmer’s perspective, the protocol is open loop – the command is sent and you have no idea how long it takes to arrive or what the profile looks like. That’s been a historic problem with galvos.’ To get around this, Rekowski says, galvo suppliers implemented various overheads in their programming such as jump delays, mark delays, and polygon delays. These delays are implemented to ensure the galvo reaches a certain position before switching the laser on or off. There are some inherent shortcomings in
this, according to Rekowski. ‘We come from the high-precision laser processing area, but working with linear positioning stages,’ he says. ‘We always know where the stages are positioned; we have direct access to the feedback of the stages. What we tried to do was develop a product that treated the scanner just like a servo axis. What that required was to ditch the XY2100 serial communication protocol, instead accessing the feedback of the galvo directly. By doing this we know exactly where the galvo is positioned at any given point in time. This means a lot of these programming concepts are redundant. Now,
DECEMBER 2011/JANUARY 2012 l ElEctro optics 19
controller has access to the feedback, the laser can be triggered as a function of position. ‘Historically that’s never been done. Typically, the laser was fired at a constant repetition rate and, as the mirrors accelerated and decelerated, there was a change in overlap of the laser pulses – the pulse rate was fixed, but the material was moving at a variable rate so the spacing of the laser pulses changed.’ The Nmark CLS triggers the laser as a function of position, so when the mirrors accelerate or decelerate the controller will speed up or slow down the trigger rate of the laser to ensure the pulses are positioned precisely.
Ultrafast pulses The emergence of ultrafast laser technology, pulsing on the order of picosecond or femtosecond timescales, is driving the development of scanning systems. Hofner at Scanlab comments that the company is working on scanners for ultra-short pulsed lasers. Ultrafast lasers can be used for cold ablation,
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Visit OPCO Laboratory, Booth 1024, Photonics West 2012, San Francisco, January 24—26
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