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From the coarse movements of a manufacturing plant s conveyer belt to the ultra-fine movements required to image a single cell, the accurate movement of samples, components, and tools is one of the fundamental challenges of industry and science.

What type of movement do you need? The first question to answer is whether it is more important to achieve good repeatability without a sufficiently accurate position feedback, or is it better to know how non-ideal the position is for every single movement with the best possible accuracy to be able to correct the data?

Piezo-based nanopositioners are usually the preferred choice to achieve very precise, repeatable, and reliable motion. The compact nature and the vacuum compatibility mean that these positioners can be utilized in a variety of cramped locations and are especially suited for the usage in vacuum chambers.

What does position accuracy actually mean? Even if the step sizes can be kept at a minimum, how do you know that the motor actually arrived at the desired position? Here is where an encoder comes in. The encoder allows the controller to verify that the stage is at its desired position, and can provide correction if necessary, hence a position feedback. This closed loop is therefore a real- time monitoring of the position, using sensors (encoders) to communicate the position between the piezo drive and its controller.

Can you trust the encoders? There are several types of encoders to consider. The choice usually depends on the environmental requirements, temperature, and light sensitivity, and most importantly, the extent of precision needed. Incorporating the sensor inside the motor itself is a practical solution. However, this creates limitations regarding the achievable repeatability and accuracy. While the encoders usually show reasonable repeatability <50 nm this is not the major restriction of the technology.

The integration inevitably, increases the distance between the measurement location and the Point of Interest (POI) for example the sample on top of the stage. This is especially crucial for multi-axis positioning systems as the individual errors of each encoder lead to amplified positioning uncertainty at the sample. Furthermore, the mounting of the encoders and the assembly

Ultra-precise interferometric closed loop positioning

of the positioner components highly affect the motion accuracy, as the encoders cannot detect or compensate lateral runouts of individual axes in the system. This leads, in combination with high sensitivity to thermal deformations and typical sensor non- linearities of >50 nm, to limited accuracy and usability. Finally, encoders lack reliability in synchrotron typical environments as their lifetime is drastically reduced by radiation. Therefore, the exchange of components often requires unsealing of a chamber, which involves a significant amount of time and leads to high costs.

Advanced closed loop sensing The most accurate closed loop feedback can only be achieved with the most accurate displacement sensing devices, hence an interferometer. The interferometer ideally should have certified accuracy and repeatability, and should be suited for the demanding environments. Furthermore, to reach true nanometre accuracy, the non- linearities associated with all optical sensors, should be actively compensated.

attocube s interferometer IDS3010 fulfills these requirements. In combination with the AMC100 motion controller, attocube offers a user-friendly closed loop positioning solution. The integrated user interface ensures intuitive control of the complete system, while the modular and miniaturized components offer high flexibility. Furthermore, during its initialization, the IDS3010 performs a distance calibration, which provides an absolute distance reference on the micrometre scale. This referencing is strongly dependent on the setup stability and doesn t offer highly accurate absolute position data, however it offers increased usability when initializing the closed loop system.

Highest precision for optical alignment The unique advantages of an interferometer based closed loop motion system are especially relevant in the optics and photonics fields. Optical setups are the key to fabricate and analyse materials on a nanometre level. This however is only possible if the mirrors, lenses and sample are aligned with comparable precision to ensure exact guidance and interaction of the light with the sample. In addition to the higher precision, an external interferometric sensor enables unrivaled long- term stability of the setup by offering drift compensation directly at the POI.

In summary, to achieve true nanometre resolution, today s state-of-the-art optical systems utilize short-wave light, which requires demanding environmental conditions. The guidance of such light requires the ultimate precision in optics alignment. This can only be achieved with closed loop positioning systems utilizing the sensor feedback as the positioning mechanism. Therefore, a sensor like the IDS3010, which is suitable for extreme environments, measures close to the sample, and offers highest resolution is the key to future advancements in sophisticated optics applications.

attocube Tel +49 89 420 797 0 E-mail info@attocube.com www.attocube.com

Left: Interferometric closed-loop control of an xyz-positioner stack using attocube s AMC100 position controller and IDS3010 interferometer. The fibre array and a mirror block are mounted on top of the positioner. The three miniaturized sensor heads measure the xyz-displacement close to the point of interest (= fibre array).