If you turn on your pocket torch for just a second and point it towards the sky, your beam reaches all the way to the moon. How fast would you need to switch the torch on and off in order for the beam to be shorter than the thickness of a human hair? It's not something you could accomplish with your thumb, that much is certain. Ultra-short beams, or pulses, of this order of magnitude are emitted by so-called femtosecond lasers, which split the laser light into compressed, high-energy wave packets. They can be used to work on any material – from the cornea of the human eye to super-hard ceramics – with micron accuracy. The French manufacturer of precision devices, ISP System, produces the actuators with which the prisms, mirrors and fi lters in such high-performance lasers are precisely aligned so that the light pulses reach the right point with the right power. Reliable drive is ensured by the stepper motors from FAULHABER.
How a laser functions is explained, in principle, by the word itself, as it is an acronym for Light Amplification by Stimulated Emission of Radiation. Explained in simplified terms, the light is amplified and bundled in the laser through the input of energy and optical tricks. The result is the thin but powerful beam which can read CDs and barcodes, treat cancerous tumours or weld metal. Needed for the optical tricks are, among other things, prisms, filters and mirrors. These split the beam, concentrate certain wavelengths and – for amplification – reflect the beam back and forth many times between two mirrors. While the constant beam of, e.g., a laser pointer is relatively weak, the energy in pulsed lasers is highly compressed into small packets by manipulating the frequencies and delaying the emission of light.
It doesn't get much shorter
Lasers with pulses in the attosecond range (0.000 000 000 000 000 001 seconds = 10-18 sec) have already been used in research. With their ultra-short pulses, even extremely fast dynamic processes in the atomic nucleus can be made visible. For industrial or medical uses, the pulses can be a bit longer, lasting a fraction of the time it takes to blink. A femtosecond (10-15 sec) is one quadrillionth of a second. In this fraction of a blink of the eye, the light travels a path of just 0.3 microns – a human hair is about two hundred times thicker. Femtosecond lasers are today state-ofthe-art and are used in many areas. These include multiphoton microscopy, microsurgery, the processing of the finest structures, e.g., in medical technology, chemical analysis or in forgery-proof micro-marking.
The femtosecond laser can generate up to a hundred million laser pulses per second. The material struck by these pulses has no time to melt – it is transformed directly to the gaseous state and can simply be extracted. Layers that are just a few millionths of a millimetre (nanometre) can thereby be removed with high accuracy without producing melting residues or heating neighbouring material. Material properties such as homogeneity, absorption capacity, vaporisation temperature or hardness play practically no role – the laser can be fired at nearly anything.
This "cold processing" leaves behind no residues and does not affect the quality of the work piece. It is only a matter of selecting the correct pulse duration, pulse energy, pulse frequency and correct focussing in order to achieve the desired results. And this is where the actuators from ISP System come into play. They move the prisms, filters and mirrors inside of the laser and give the light pulse its highly defined quality. Three different types of drive come into question here: electromagnetic, piezoelectric and mechanical. "In some application areas, such as research, the first two drive types can draw on their particular strengths. In day-to-day industrial use, however, mechanically driven actuators are superior in many respects", says Sébastien Theis from ISP System, who qualifies this with: "provided the devices are of suitable quality and first-class motors are used. They must function very precisely, but their drive electronics cannot be too complex."
It is here, in particular, that piezoelectric and electromagnetic drives have a decisive disadvantage: they can only achieve their high precision in a closed control loop. This means that a measurement unit (sensor) is needed that measures the movement and passes the values on to the drive electronics, which then adjust the movement accordingly. The closed control loop means a relatively high level of complexity and requires additional electronics. This makes the solution not only more expensive, but also considerably more complex and bulkier.
Stepper motors, on the other hand, can also be operated in an open control loop, do not need a sensor and are, for these reasons alone, more compact. "The motor itself counts the steps. Using this value, it is possible to derive a very exact position. Thanks to the quality of the FAULHABER motors, we can be certain that no step is lost, provided there are no obstacles in the way. Even then, it is sufficient to move the drive back to the zero position to obtain a reliable calibration", explains Sébastien Theis.
Undeviating, even without power
Another advantage of the stepper motor is that it reliably holds its position even without power. During operation in the enclosed laser box, this is important because each laser pulse is accompanied by an electromagnetic discharge. A closed control loop, consisting of actuator, sensor and drive electronics, can only function if power is supplied. In such a unit, the electrical discharge can cause interference. This, in turn, can cause, e.g., an adjustable mirror to move out of its specified position. This would be fatal for the precision of the laser if used, for example, to make a correction to the cornea of a short-sighted eye. With the stepper motor, which remains fixed in place when there is no power, such interference is, fortunately, not possible.
In addition to these design-related characteris tics and the high quality, there were other reasons why ISP System selected motors from FAULHABER: "We found no other manufacturer who could as easily and as quickly accommodate our requirements. We can select from an incomparably wide range of motors and gearheads. Moreover, FAULHABER is able to specifically develop motors according to our requirements. Thanks to short delivery times, we can respond very quickly to the requests of our customers. And we always have highly competent contacts who know our technology and who contribute to its further development", says Sébastien Theis.