Double-grating frequency shifter for low-coherence heterodyne interferometry

A double-grating frequency shifter for low-coherence heterodyne interferometry is proposed. Compared with conventional frequency shifting methods, the new frequency shifter can generate frequency shift without additional optical path delay between different wavelengths. It also in principle has no scanning range limitation because the scanning mirror moves in a part of an optical system where different wavelength constituents of the light propagate in the same direction. To demonstrate the effectivity of the frequency shifter, a Twyman–Green interferometer was connected with the frequency shifter through an optical fiber. Displacement values measured by the low-coherence heterodyne interferometer have a standard deviation of 0.09µm for a displacement range of 50µm.