In a particle accelerator, two particle beams are accelerated and collided at one or several physics
detectors. The machine detector interface (MDI) is the place where the accelerator ring is linked
to the physics detector. It is the area where the alignment requirements are the most stringent, and
the MDI of the lepton version of the Future Circular Collider (FCC-ee) is no exception. It will
implement a complex design and alignment requirement for typical components in the MDI are
of the order of 30 m. Until now, no working solutions have been proposed for the alignment of
such MDI. In this contribution, two different systems are studied: the Surveillance d’Ouvrages par
Fibre Optique (Structural Monitoring by Optical Fibers, SOFO) and the in-lined multiplexed and
distributed frequency scanning interferometry (FSI). Both systems allow individual and simultaneous
distance measurements of multiple portions along a fiber, and a new system using the FSI technology
is proposed for the alignment monitoring. Using a network of helical shaped fiber allows the
measurement of deformations if a sufficient amount of fibers is placed on the cylinder. With a sensor
measurement accuracy of 1 μm, 3D coordinates of points at the end of a 4 m long and 0.25 m radius
cylinder (FCC-ee MDI dimensions) can be determined at an accuracy under 5 μm. This system will
provide a link between the alignment of the final focusing quadrupoles and the rest of the machine.

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