A Novel Common Path Interferometric Technique for Vibration Measurement Based on Two Fiber-Optic Couplers

A novel common path demodulation technique for low-coherence fiber-optic sensors for vibration measurement based on two serially connected fused silica fiber-optic directional couplers is presented. A 2
$\,\times\,$
2 coupler is used in the sensor arm and a 3
$\,\times\,$
3 coupler serves as a receiving interferometer, providing at the same time two quadrature signals for the interferometric demodulation. A very stable output signal is obtained after the differentiate-and-cross-multiply procedure with the minimal detectable amplitude of the vibration of about 70 pm/
$\surd$
Hz.