Fabry-Perot interferometer scanned by geometric phase

A novel method to scan a Fabry-Perot (FP) interferometer is introduced. A geometric phase shifter consisting a rotating half wave plate (HWP) sandwiched between two fixed quarter wave plates is supposed to be confined within a FP cavity. When the HWP is rotated, the axis modes of the FP cavity shift one-way in the spectral space without any mirror scanning. This method will provide a method to measure frequency resolved spectra precisely under the situation that the absorption loss during the round trip changes if a mirror is scanned, such as the absorption by the environmental water vapor can not be ignored.