Aiming at the problems of Doppler-induced error and slow measurement rate in frequency-swept interferometry, a novel dynamic clearance measuring method is proposed. By combining frequency-swept and frequency-fixed interferometry, and using fusion processing of interferometric signals, this method not only can eliminate the Doppler-induced error, but also can give the dynamic clearance value at each sampling point. To verify this method, an all fiber-optic measurement system was constructed, and experiments under 500 Hz triangular and 4 kHz sinusoidal vibrations were carried out. Experimental results show that the dynamic clearance can be reconstructed accurately.

To realize accurate dynamic axial clearance measurement in high-speed rotating machinery using basic Frequency-swept interferometry (FSI), a novel algorithm of Doppler error suppression is proposed. In this algorithm, we first extract the incremental phase of the FSI signal by Hilbert transform and use it to construct a virtual clearance variation. Then, using the periodic characteristics of rotor rotation, optimized initial clearance can be obtained. Eventually, the dynamic axial clearance at each sampling point can be recovered with the optimized initial clearance and the Doppler error is greatly suppressed. The feasibility of this method is tested and verified by both simulation and experiment. For a 4 kHz rotation simulating target located at 5 mm away, the experimental results show that the dynamic clearance measurement error is less than 2.057 µm, which meets the measurement accuracy required for most high-speed rotating machinery.

If the measurand changes during the spectrum acquisition process, it easily leads to the failure of the classic demodulation algorithms of low-finesse optical fiber Fabry-Perot (FP) sensors. To address this problem, a novel demodulation model is proposed based on the definition of the instantaneous frequency. The proposed model establishes the relationship between the optical path length (OPL) of the FP cavity and the instantaneous frequency distribution of the FP interference spectrum. The link between the classic FFT algorithm and this model is discussed, and it is found that this model can be viewed as a generalized form of the FFT algorithm. Based on this model, the Doppler-induced demodulation error is analyzed. The analysis uncovers that the average frequency of the FP interference spectrum should be used for the evaluation of the error, and the error is proportional to the variation of OPL during the spectrum acquisition period. Further, numerical simulation and an experiment were carried out to verify the proposed model, and results show that the proposed model is effective for the dynamic low-finesse FP cavity. It is the first time that the idea of instantaneous frequency is introduced for the FP demodulation, and this model provides us a new way to cope with the FP sensing signal.

Spectral distortions are complex and unpredictable in Fiber Bragg Grating (FBG) sensing applications. An inappropriate choice of FBG demodulation algorithm will lead to unacceptable results in practice. Although many demodulation algorithms have been developed for distorted spectra, they may be limited for some types of spectral distortion. Until now, no model of FBG spectral distortion has been proposed to test different demodulation algorithms. Here, we fulfill this aim by introducing a comprehensive model with asymmetrical distortion, multi-peaks distortion, interferometric noise, background noise and random noise. Using numerical simulations, we evaluate the performance of eight most typical demodulation algorithms with this model. We show that different algorithms should be used within specific ranges of spectral distortions. Altogether, this study provides a guide map as useful reference for selecting an appropriate FBG demodulation algorithm for practical applications. Furthermore, our proposed model paves the way for developing more reliable FBG demodulation algorithms for different spectral distortions.

The optical path length (OPL) of the Fabry-Perot cavity may change during the spectrum acquisition; it introduces demodulationerror or even causes demodulation failure based on traditional algorithms. To address this problem, the relationship between the dynamic OPL and instantaneous frequency is investigated. Demodulation of the dynamic OPL based on instantaneous frequency analysis is proposed. By combining spectrogram (SP) and Wigner-Ville Distribution (WVD), an effective time-frequency (TF) analysis method is investigated to calculate the dynamic OPL at each sampling point in a period of a sweeping laser. Experimental results show that the demodulation speed is 4.8 MHz at a 2 KHz sweeping rate, the max deviation of a sinusoidal variation of OPL is around 0.14 μm, and the max deviation of a triangular variation of OPL is around 0.13 μm.

The Doppler effect of frequency-swept interferometry (FSI) is often seen as an obstacle to the dynamic ranging accuracy. However, the potential of this obstacle is rarely noticed and used. In this paper, by combining the periodical characteristics of the rotational Doppler effect, an FSI-based multi-parameter measurement method for the rotor is proposed. Through the establishment of the rotational Doppler formula of FSI, it is found that the frequency, direct component, and amplitude of the dynamic distance given by FSI can be utilized to estimate the angular velocity, axial clearance, and tilt angle of the rotor, respectively. A rotor platform and a fiber-optic FSI system were constructed, and a series of experiments were carried out to verify the proposed method. The experimental results showed that the relative errors of the measured axial clearance, angular velocity, and tilt angle were less than 3.5%. This work provides a new perspective on the multi-parameter measurement of the rotor and makes it possible to directly perform multi-parameter measurement inside the space-confined rotating machinery as only a single small-size fiber-optic probe is needed.