Sagnac fiber interferometer with the dynamic population grating formed in the rare-earth doped fiber is proposed for homodyne adaptive detection of optical phase modulation. The configuration is shown to be a simple all-optical fiber sensor suitable for linear high sensitivity detection of mechanical vibrations, acoustic signals, thermo-optic effect etc. Theoretical consideration shows that the quadratic response of this interferometric configuration associated with the amplitude dynamic grating is observed in the reflected wave mainly, while the recorded phase grating results in the linear energy exchange between the transmitted and reflected from the Sagnac loop light waves. Experiments with the erbium- and ytterbium-doped fiber based adaptive Sagnac configurations (with the operation wavelengths 1485 and 1,064 nm respectively) of the fiber accelerometers confirmed these general conclusions and demonstrated sensitivity of the fiber based interferometric configurations (~3 10⁻⁵ rad/Hz¹/²) governed basically by the noise of the utilized lasers.

In this Letter, we report a novel, to the best of our knowledge, and simple approach for passive quadrature-phase demodulation of relatively long multiplexed interferometers based on two-channel coherence correlation reflectometry. Two-wavelength channels are generated using a single unmodulated CW-DFB diode laser and an acousto-optic frequency shifter. The introduced frequency shift determines the optical lengths of the interferometers. In our experiments, all interferometers have the same optical length of 32 cm corresponding to the π/2 phase difference between channel signals. An additional fiber delay line was introduced between channels to destroy coherence between initial and frequency-shifted channels. Demultiplexing of channels and sensors was performed using correlation-based signal processing. Amplitudes of cross correlation peaks obtained for both channels were used to extract the interferometric phase for each interferometer. Phase demodulation of relatively long multiplexed interferometers is experimentally demonstrated. Experimental results prove that the proposed technique is suitable for interrogating a serial array of relatively long interferometers dynamically modulated with phase excursions exceeding 2π. Simultaneous interrogation and phase demodulation were experimentally demonstrated using an in-line array of low-finesse Fabry–Perot interferometric sensors