Optical Interferometers and Their Application in Scientific Research: The research group for application in molecular, biomedical and material science at ELI Beamlines needs to control the delay of synchronized laser beams down to femtosecond precision. The aim of this student project was to apply interferometry in femtosecond delay line stage tests for atomic, molecular, and optical sciences and for coherent diffractive imaging. For this purpose, a Michelson Interferometer was built using ELI in-house resources. Using a bam splitter, a laser beam was split into two arms and then recollected to create an interference pattern. This pattern was detected by a fast 2GHz Thorlabs photodiode, DET025A/M, with à 400-1000 nm sensitivity range and rise/fall times of 150 ps, in order to calibrate a femtosecond delay line stage. The Aerotech PRO190SL/ SLE was used as the translation linear stage. This stage served as the main component of the delay line tests. Further, Attocube sensors based on the Fabry-Perot Interferometer technique were added to the set-up to improve the precision and reliability of the calibration method. The advantages, disadvantages and limits of this method as well as lessons learned will be discussed from both an educational and scientific point of view.