Extrinsic Fabry-Perot Interferometer (EFPI)

The construction of the Extrinsic Fabry-Perot Interferometer (EFPI) is first described by Murphy et al. The Fabry-Perot (FP) cavity is formed by a single mode fiber and a multimode fiber sealed inside a hollow core tube.

The single mode fiber acts as the inputoutput fiber whereas the multimode fiber is used solely as a reflector. The hollow core tube acts as a guide tube in addition to protecting the cavity from external elements. Light enters the single mode fiber and it is partially reflected from the first glass-air interface as R1. The transmitted light travels through the cavity, is reflected from the second air-glass interface and enters the single mode fiber again as R2. These reflections then interfere in the single mode iber. The output depends on the difference in the optical path lengths traveled by the two interfering waves. The effect of subsequent reflections inside the cavity are negligible.