Patent ID: 7525665

Claim:
An interferometer comprising: a light source; a measurement arm; a reference arm; a polarising beamsplitter arranged to receive light from the light source and split the light into a measurement beam and a reference beam having different polarizations, the measurement beam and the reference beam passing around the measurement arm and reference arm, respectively, and returning to the beamsplitter; and a detector that receives the measurement beam and the reference beam from the beamsplitter and that reacts to interference of light from the reference beam and the measurement beam; and a device that at least one of removes and separates out an error beam, said error beam caused by leakage of light from one of said measurement and reference beams into the other of said measurement and reference beams, or by leakage of light back towards the light source; wherein the device that at least one of removes and separates said error beam is provided in the measurement arm or reference arm.