Interferometer module

Method of determining a displacement of a measurement mirror relative to a reference mirror using an interferometer module adapted for emitting a measurement beam and an associated reference beam towards a respective measurement mirror and reference mirror, said interferometer module comprising:

• a beam combiner for combining a reflection of said measurement beam and said reference beam into a combined beam,
• a non-polarizing beam splitter for splitting said combined beam into a first split beam and a second split beam, each split beam comprising a component of said reference beam and said measurement beam,
• a first polarizing beam splitter, for splitting said first split beam into a first polarized beam having a first polarization and a second polarized beam having a second polarization,
• a second polarizing beam splitter, for splitting said second split beam into a third polarized beam having a third polarization and a fourth polarized beam having a fourth polarization,
• a first, second, third and fourth detector for detecting a beam energy of said first, second, third and fourth polarized beams respectively,
• wherein said polarizations of said first, second, third and fourth polarized beams are different polarizations, said method comprising the steps of:
• measuring the energies of said first, second, third and fourth polarized beams at said first, second, third and fourth detector respectively,
• providing a first displacement signal as a difference between said first and second measured energy, and providing a second displacement signal as a difference between said third and fourth measured energy.