Temporary Public Description of the PaMIr project

Until the PaMIr project has officially started, which is expected to happen between November 2018 and February 2019, we only publish this introductory description which is written for a non-expert audience.

“Distances are one of the most fundamental properties that humanity uses to describe the world and has learned to measure in an amazing variety of ways using everything from the length of your forearm to the wavelengths of photons or even electrons or atoms.

Fundamental research in particle and accelerator physics has been generating some of the most formidable distance measurement challenges the world has ever seen. The alignment of modern particle physics experiments and particle accellerators demands sub-micron accuracies over meter scales in extremely harsh environments featuring high radiation doses, cryogenic temperatures, high vacuum and extreme demands on miniaturisation.

In modern industrial societies length measurements are deeply embedded into all production processes and the requirements for range, resolution, speed and absolute accuracy are constantly growing. In modern production machines such as CNC mills or lathes these measurements have to be performed not only with micrometer resolution but in these machines there are dozens of distances that have to be measured rapidly and simultaneous while they are also changing at high speeds of meters per second.

In the course of its fundamental research in particle and accelerator physics the department of Physics at the University of Oxford has developed a technology called Frequency Scanning Interferometry (FSI) capable of measuring absolute distances with high accuracy (better than half a micrometer per meter) and high time resolution of 125 million measurements per second. The principles of this technology are described here. This technology is being successfully commercialised with an industrial partner (Etalon AG) and has found many applications in industry and science under the trademark Absolute Multiline™.

The current commercially available state of the Absolute Multiline Technology is however not yet capable of measuring fast moving targets in a continuous form.

Driven by the need for continuous position monitoring in the beam delivery systems of next generation electron positron colliders and a great potential for industrial applications of such a technology alike, the group of Prof Reichold has recently explored a novel method for rapid distance measurements of fast moving targets referred to as Phase Modulation Interferometry (PaMIr).

In the PaMIr technique, the laser light used in the measurement interferometers is phase modulated at high frequencies and the resulting interference signal is demodulated multiple times at different frequencies. From the resultant signals, it is possible to reconstruct the change in length of measurement interferometer. Different modulation and demodulation schemes are possible and more research is needed to find solutions that can optimally measure many interferometers simultaneously.

The PaMIr method is in principle backward compatible with the instrumentation used for FSI measurements and will maintain the unique feature of FSI such as simultaneous measurements of many distances and a low cost per measurement channel. This compatibility allows a measurement to start at an absolute distance measured with FSI, which can then be tracked at high speed with the PaMIr method.

The PaMIr project aims to develop the basic PaMIr principles to a state ready for implementation in a commercial instrument in a collaboration with Etalon AG and VadaTech. To do so we will have to process the rapid flow of data from each PaMIr measurement interferometer with complex algorithms in real-time with a latency below 0.1 milliseconds as demanded by modern production machines.

The PaMIr instrument will have to simultaneously digitise all interferometer signals at a rate 125 million samples per second and feed these data streams into fast parallel processing units based on Field Programmable Gate Arrays (FPGAs). We will program these FPGAs to apply optimised versions of the PaMIr algorithms to all channels in parallel with low latency, which is a formidable task.

To do this efficiently and in a commercially applicable way we need to know and control the hardware of the interferometers, the detection and DAQ systems at all levels which is why Oxford has teamed up with a world leading manufacturer of data acquisition equipment (VadaTech). Together with VadaTech and Etalon we have already developed a first optical readout system for Absolute Multiline Systems. We will build on this development and further improve its performance to meet the PaMIr requirements.

Ultimately we expect to make the technique commercially available for industrial and scientific application alike. The firmware we develop in this projects will also enable the use of our DAQ systems as multi-channel digital lock-in amplifiers or PLL and PID controllers that can find applications in a wide range of research and development environments.