FSI Vacuum Heads

When components are put under vacuum and/or cryogenic temperatures, the properties and dimensions of the materials change. This can result in unforeseen consequences such as displacement etc., which in turn can cause reduced performance. Models can be applied to simulate these effects, but for complex devices and systems, it is necessary to actually measure the effects, both to correct these effects and to improve the models. For this reason, the CERN metrology group has developed the Frequency Scanning Interferometry (FSI) Vacuum Heads, in particular for the HL-LHC project to monitor changes of position of the crab cavities as they are put under vacuum and cryogenic conditions.

The device is essentially an FSI measurement collimating optical system built into an interface between atmospheric conditions and a vacuum and/or cryogenic environment, enabling FSI measurement without impact of refraction index of laser transmission mediums. Conventionally, such measurements would be done through a strong glass window.

By using a device such as the FSI head we increase  accuracy because refraction issues (from light passing through glass windows at vacuum interface) are removed or reduced. The laser beam for FSI is provided by fibre into the device. Moreover, thanks to fiducialisation techniques – the in-head optics optical path datum can be known out of the FSI head.

Projects using this technology today:

Non contact cold mass measurements are planned for HL-LHC inner triplets and crab cavities.

Advantages & Applications


  • FSI measurement with high absolute precision within closed vacuum vessels/cryostats.
  • Refraction issues caused by light passing though glass windows are removed.

Possible Application Areas

  • Contactless measurement of objects with different temperatures, allowing for heat conduction effects minimizing.
  • Very high precision alignment for aerospace applications.



  • Compatible with absolute  and relative interferometric systems.
  • Uncertainty: typical 10-15µm (depending of fiducialisation method of the head).
  • Measurement distance in vacuum: 0.2m – several meters  (internal construction might be easily adopted to user need).
  • Fibre length – up to several hundred meters (depends on interferometric system limitation).
Mateusz Sosin