PaMIr: Phase Modulation Interferometry (resubmission)

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.

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.

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 2.7 million measurements per second. This technology is being successfully commercialised with an industrial partner (Etalon AG) and has found many applications in industry and science. The current commercially available state of FSI is however not yet capable of measuring fast moving targets in a continuous form.

The group of Prof Reichold at the University of Oxford has recently developed 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.

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 and VadaTech. To do so we will have to analyse the rapid flow of data from each PaMIr measurement interferometer with complex algorithms in an on-line way with a latency below 0.1 milliseconds as needed for applications in 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 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 DAQ system 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 FSI. We will build on this development and further improve its performance to meet the PaMIr requirements.

Ultimately we expect to license the PaMIr technology to Etalon AG to make the technique commercially available for industrial and scientific application alike and to license the firmware we develop to VadaTech to allow the use of their DAQ systems as multi-channel digital lock-in amplifiers or PLL and PID controllers.