An interferometer system including: an optical system arranged to split a radiation beam from a laser source into a first beam along a first optical path and a second beam along a second optical path, and recombine the first beam and the second beam to a recombined beam, a detector to receive the recombined beam and to provide a detector signal based on the received recombined beam, and a processing unit, wherein a first optical path length of the first optical path and a second optical path length of the second optical path have an optical path length difference, and wherein the processing unit is arranged to determine a mode hop of the laser source on the basis of a phase shift in the detector signal.