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.

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.

A data processing method for correcting the frequency sweeping nonlinearity of tunable laser in an optical frequency domain reflectometry (OFDR) system is presented. The proposed method calibrates the positions of all the data points by the time-scale factor determined by the zero-crossing points of the auxiliary interferometer. Then the signal of the main interferometer is interpolated with the calibrated results. In experiment, a spatial resolution of 0.17 mm over a measurable range of 155m has been obtained even when the optical path difference of the auxiliary interferometer is shorter than that of the main interferometer. This method could resolve the problem that measurable range is limited by the length of delay fiber in the auxiliary interferometer and significantly decrease the requirement of sampling rate.

Distributed optical fiber sensing based on optical frequency domain reflectormeter (OFDR) is a promising technology to achieve a high spatial resolution downwards to several millimeters. To reduce the interaction between the spatial and spectral resolution, a OFDR signal processing method based on Morlet wavelet transform (WT) is proposed to enrich spectrum information especially under a high spatial resolution, and the resolution of spectrum/measured distributed sensing physical quantity can thus be improved. The relationship among distributed sensing parameters and wavelet parameters is demonstrated. And the spectral resolution, spatial interval of the measured gauge and spatial resolution are all manually controllable by adjusting wavelet parameters. To the best knowledge of the authors, it is the first time to report a practical WT method for high performance OFDR. Experimental results indicate that significant improvements, such as strain resolution of 1 le, average repeatability over a full strain range (3000 le) of 4.3 με, and nonlinearity of lower than 0.67 The advantages of this method include high spatial resolution, high spectral resolution, high robustness, and excellent applicability with current OFDR systems.

To enhance the precision manufacture of the microholes of high aspect ratios, a fiber Bragg grating (FBG) probe with a twin fiber structure based on capillary-driven self-assembly fabrication method is developed for a micro-hole-measuring machine to do the high-precision measurement. The sensing principle of the twin FBG probe is investigated through analyzing the strain distribution within the probe under an axial or radial contact displacement. Then, the design and fabrication method of the twin FBG probe is demonstrated. To improve the accuracy of the measurement, both data processing method and measuring method are proposed. A data processing method for acquiring the triggering position with the transformation of the signal domain and multiple fitting is implemented to raise the accuracy of the triggering position. And a method for measuring the diameter of a microhole with perpendicular bisector of the chords and dichotomization scanning is utilized to enhance the determination accuracy of the diameter. Experimental and simulation results indicate that the data processing method and measuring method are robust, and the accuracy of the measurement can be improved.