A phase-shifted demodulation technique with a 3 × 3 coupler and ellipse fitting algorithm (EFA) for the interrogation of interferometric sensors is proposed. To reduce the error of the EFA as to measure small phase signals, additional phase modulation is introduced. The additional modulation provides a walk of the operating point along the Lissajous ellipse large enough to permit calculation of the ellipse parameters at every moment. Experimental result shows that this technique demonstrates high accuracy and stability for measuring small phase signals. The setting of this technology expands the application of the EFA in fiber-optic phase demodulation technology.

An ameliorated phase generated carrier demodulation algorithm based on a reference interferometer is proposed. It is theoretically analyzed, numerically simulated, and experimentally verified in this paper. The method of matching the amplitude of the quadrature signal in the reference interferometer algorithm eliminates the harmonic distortion caused by modulation depth fluctuations, suppresses system noise, and improves system stability. In experiments, where the sensed signal amplitude is 0.68 rad, the frequency is 1 kHz, and the modulation depth ranges from 1.5 to 3.5 rad, the ameliorated algorithm has SINAD that reaches 24.5 dB, which is an average improvement of 7.9 dB over the PGC-DCM algorithm based on a reference interferometer and an average improvement of 3 dB over the PGC-Arctan algorithm based on a reference interferometer. The ameliorated algorithm achieves a THD of 0.451%, which consistently maintains the best level of the traditional PGC algorithm based on a reference interferometer. The method provides an effective reference for signal processing in practical applications of fiber-optic sensors.

This study proposes a highly sensitive and stable optical fiber probe based on Vernier effect for high temperature measurement (up to 1000 °C), utilizing photonic crystal fiber (PCF)–based Fabry-Perot interferometers (FPIs). The cascaded FPIs are fabricated by fusion splicing a section of polarization maintaining PCF to a lead-in single-mode fiber, and then a section of temperature-insensitive hollow core PCF is spliced between the PMPCF and a multi-mode fiber. The shift of the spectral envelope is monitored to measure the temperature variation. Experimental results show that the sensitivities of three fabricated probes are as high as 173.43 pm/ °C, 230.53 pm/ °C and 535.16 pm/ °C when operating from room temperature to 1000 °C, which are consistent with theoretical results. The sensitivities are magnified about 13, 19 and 45 times compared with the single FPI. The linearity of the temperature response is as high as 99.73%. The proposed probe has great application prospects due to compactness, high sensitivity and low cost.

A single-wavelength passive phase-shifted demodulation technique for the interrogation of a diaphragm-based fiber sensor is proposed. It is based on the configuration of the external Fabry–Perot interferometer (EFPI) with dual-cavity (DC) and the ellipse-fitting algorithm (EFA). The technique can address two main concerns: the orthogonality of the DC-EFPI must be matched, the direct current values and fringe visibility of the DC–EFPI must be excluded or normalized. Experiment results show that the proposed technique demonstrates high accuracy amplitude measurement of reflective surface vibration with a high dynamic range over 135 dB and good effectiveness with various initial phase differences apart from two areas around 0 and π rad. It is a successful solution for interrogating of diaphragm-based fiber sensors.

Traditional phase generation carrier technology requires to control the modulation depth at an optimal value for accurate demodulation. In this paper, we proposed an algorithm to directly eliminate the real-time phase modulation depth based on multi-harmonics frequency mixing technology for avoiding the influence of phase modulation depth on the demodulation result. Experiments show that when the modulation depth is in the range of 1.5 to 3.5 rad, the SINAD of the improved algorithm can reach 37.1 dB, which is 13.2 dB higher than that of the traditional PGC-Arctan algorithm on average. The THD of the improved algorithm can reach 0.322%, which always maintains the best level of the traditional PGC-Arctan algorithm. The algorithm provides an effective reference for the signal processing of the fiber-optic interferometric sensors in practical applications.