Measuring the invisible: VSL’s Picodrift interferometer is pushing the limits of precision
Publication
“Precise” is a moving target. For a carpenter, a millimetre might be precise and for a watchmaker it would be a hundredth of a millimetre. But in the high-tech worlds of the semiconductor industry or gravitational wave-hunters, for example, precision means measuring distances smaller than a single atom. This is the domain of nanoscale metrology.
At VSL we have developed a specialised instrument called the Picodrift interferometer. It is designed to measure displacement (movement) with incredible accuracy, resolving shifts as small as 10 picometres. To put that in perspective: if a single human hair were the size of a mountain, 10 picometres would be the size of a pebble!
This level of ultra-precision measurement is now helping to solve critical challenges in industry and science, from stabilizing the massive Einstein Telescope to ensuring the microchips in your phone are manufactured correctly.
The problem with being too precise
Ultra-precise measurements come with their own peculiar challenges. Even the most advanced motion systems with nanometre-level accuracy face a stubborn enemy: drift. Drift is the slow, unwanted movement that happens over time. Glue joints might settle slightly, metal might expand by a fraction due to a tiny temperature change, or a bolted connection might experience “microslip” – shifting by a microscopic amount under load.
To detect and correct these errors, you need a ruler that is vastly more accurate than the thing you are measuring. You need traceable metrology: a measurement chain that links your specific device back to international measurement standards. The Picodrift interferometer provides exactly that, serving as a master ruler for the nanoworld.
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How the Picodrift works, simply put
The Picodrift is an optical instrument that uses laser light to measure distance. It splits a laser beam into two paths. One path bounces off the object being measured; the other acts as a reference. When the beams recombine, they create an interference pattern. If the object moves even a fraction of a wavelength of light, that pattern shifts.
What makes the Picodrift special is the extremely high level of stability. It actually runs two of these measurements simultaneously. One measures the object, while the other measures the air itself to correct for tiny changes in air pressure or humidity, which can bend light and ruin the measurement.
The entire Picodrift setup is housed in a thermally isolated box that keeps temperature changes to within a few thousandths of a degree per hour. This “balanced” design allows VSL to distinguish between actual movement of the sample and false signals caused by the environment, making it one of the most stable interferometric sensors available.
Correcting for “ghost” movements
Even with such a stable design, VSL researchers constantly hunt for intrinsic drift—errors generated by the machine itself. A recent study used complex computer simulations to model every lens, mirror, and mount in the system.
The results were encouraging: they showed that the mechanical stability of the optical components is excellent. The remaining tiny errors are likely caused by the air inside the device or the fibre-optic cables delivering the laser light. By identifying these sources, VSL can refine the instrument further, pushing the boundaries of sub-nanometre accuracy.
Replacing physical objects with “Virtual Standards”
With the Picodrift, VSL is pioneering a different approach called virtual standards calibration. Instead of the traditional method of using a physical (silicon) standard – a block with finely etched grooves – the Picodrift uses a high-tech buzzer: a piezoelectric actuator that moves back and forth. Because this movement is continuously monitored and calibrated by the Picodrift, the moving actuator becomes the standard.
This “virtual standard” is incredibly flexible and very precise below one nanometre. It can mimic a step, a slope, or a rough surface, allowing users to calibrate atomic force microscopes and other optical displacement sensors with much greater versatility than a static physical standard ever could.
One of the most exciting applications for this new technology is the Einstein Telescope. This future underground observatory is designed to detect gravitational waves. For this purpose, the telescope must be completely isolated from the noisy vibration of the Earth. VSL is part of a project called SENVIDET, which aims to develop sensors used to isolate vibrations at extremely low frequencies.
Using the Picodrift as a reference, VSL is helping to validate new sensors that will actively cancel out ground motion for the Einstein Telescope. If the sensors drift or have too large errors, the telescope is blind. The Picodrift ensures that when these sensors detect a movement, it really is moving.
The future of measuring small
From the depths of the Einstein Telescope to the cleanrooms of chip factories, the need for nanoscale metrology is growing. The VSL Picodrift interferometer bridges the gap between scientific theory and industrial reality.
By providing a way to measure nanometre displacements with traceable metrology that is immune to environmental noise, VSL is helping engineers and scientists trust what they see, even when they are looking at things too small to be seen at all.
An article has been published in the magazine Mikroniek about the VSL Picodrift interferometer. You can read it here: Mikroniek number 5 2025.