Spectrum registration and spatial calibration are presented and demonstrated in optical frequency domain reflectometry (OFDR) based sensing system for achieving a wide strain measurement range. In both dynamic and static measurement case the feasibility of the proposed method is evaluated. First, spectrum registration is demonstrated to demodulate strain value, which increases the data acquisition rate dozens of times and also keep the high measurement range. A measurement rate up to 800 Hz is achieved. Second, the general difficulty in large strain measurement is analyzed. The combination of spatial calibration and spectrum registration is therefore proposed to reduce the mismatch of the spatial segments in the case of large strain variation. The local measurement spectrum segment extracted by a narrow box window for spectrum registration is proposed to maintain a high spatial resolution. Strain exceeding 7000 $\mu\epsilon$ is demodulated with a gauge length of 5 mm accurately. Last, GPU technology is used to accelerate the data processing rate in the demodulation process, which makes it more hopeful for the practical application.

The decrease of overlap between the reference spectrum and measurement spectrum (ReS and MeS) is found to limit the strain measurable range of the optical frequency domain reflectometer (OFDR) and signal-to-noise ratio (SNR) of cross-correlation results. In this Letter, the local similar characteristics of Rayleigh scattering (RS) fingerprint spectrum are discovered and, thus, an improved OFDR based on local similarity of RS spectrum is proposed to ensure the feasibility of the OFDR during measuring a large distribution strain with a high spatial resolution. The normalized length and location of the local spectrum are optimized to obtain a high spectral similarity between the ReS and MeS and achieve a robust suppression on cross-correlation fake peaks and multi-peaks. By this method, four times enhancement of similarity is experimentally achieved, compared to conventional OFDR methods when the measured distribution strain varies from 0 to 3000 μϵ with a 3 mm long fiber gauge. The experimental results verify that it effectively avoids influences of fake peaks and multi-peaks. The advantages of this method are a large strain measurable range, robust performance, a high SNR, and applicability with current OFDR systems.

We use a spectrum interpolation technique to improve the distributed strain measurement accuracy in a Rayleigh-scatter-based optical frequency domain reflectometry sensing system. We demonstrate that strain accuracy is not limited by the “uncertainty principle” that exists in the time–frequency analysis. Different interpolation methods are investigated and used to improve the accuracy of peak position of the cross-correlation and, therefore, improve the accuracy of the strain. Interpolation implemented by padding zeros on one side of the windowed data in the spatial domain, before the inverse fast Fourier transform, is found to have the best accuracy. Using this method, the strain accuracy and resolution are both improved without decreasing the spatial resolution. The strain of 3 μϵ within the spatial resolution of 1 cm at the position of 21.4 m is distinguished, and the measurement uncertainty is 3.3 μϵ.

Fiber deformation measurement is crucial in the structure health monitor, which reflects the stress, strain or deformation of the structure where the fiber is laid. The principle of the fiber deformation measurement by the optical phase tracking in OFDR scheme is described and the data processing algorithm is demonstrated in details to receive accurate results. The differential phase is acquired by making a difference between the phase spectrum of the reference and measurement states. A robust phase unwrapping method based on the density distribution is proposed to get the whole unwrapped low-noise phase of the fiber. Additionally, the phase decoherence caused by the scatter unit misalignment induced by the fiber deformation is analyzed and the phase correction method is proposed to improve the phase coherence. The fiber deformation can be tracked along the fiber with a resolution of tens of microns, depending on the set parameters of the OFDR system. Moreover, the strain distribution is also derived from the differential phase. In the experiments, the measurements of the standard single-mode fiber with uniform and non-uniform deformation have been accomplished in a spatial resolution of 40 μm. The noise is about 0.1 μm in a stretching range of about 40 μm. Simultaneously, hundreds of microstrain are obtained and show a high accuracy with a resolution of the level of millimeter.

In this paper, a high resolution and response speed interrogation method based on a reflective-matched Fiber Bragg Grating (FBG) scheme is investigated in detail. The nonlinear problem of the reflective-matched FBG sensing interrogation scheme is solved by establishing and optimizing the mathematical model. A mechanical adjustment to optimize the interrogation method by tuning the central wavelength of the reference FBG to improve the stability and anti-temperature perturbation performance is investigated. To satisfy the measurement requirements of optical and electric signal processing, a well- designed acquisition circuit board is prepared, and experiments on the performance of the interrogation method are carried out. The experimental results indicate that the optical power resolution of the acquisition circuit border is better than 8 pW, and the stability of the interrogation method with the mechanical adjustment can reach 0.06%. Moreover, the nonlinearity of the interrogation method is 3.3% in the measurable range of 60 pm; the influence of temperature is significantly reduced to 9.5%; the wavelength resolution and response speed can achieve values of 0.3 pm and 500 kHz, respectively.

Tuning nonlinearity of the laser is the main source of deterioration of the spatial resolution in optical frequency-domain reflectometry (OFDR) system. In this paper, we develop methods for tuning nonlinearity correction in an OFDR system from the aspect of data acquisition and post-processing. An external clock based on a zero-crossing detection is researched and implemented using a customized circuit. Equal-spacing frequency sampling is, therefore, achieved in real-time. The zero-crossing detection for the beating frequency of 20 MHz is achieved. The maximum sensing distance can reach the same length of the auxiliary interferometer. Moreover, a nonlinearity correction method based on the self-reference method is proposed. The auxiliary interferometer is no longer necessary in this scheme. The tuning information of the laser is extracted by a strong reflectivity point at the end of the measured fiber. The tuning information is then used to resample the raw signal, and the nonlinearity correction can be achieved. The spatial resolution test and the distributed strain measurement test were both performed based on this nonlinearity correction method. The results validated the feasibility of the proposed method. This method reduces the hardware and data burden for the system and has potential value for system integration and miniaturization.