The optical frequency domain reflectometry (OFDR) demodulation technology based on DBR laser and spectral splicing was proposed in our previously studied. Spectral splicing solves the problem of laser mode hopping that is not allowed in the original OFDR demodulation technology. However the detailed analysis of the influence of spectral splicing on the spatial resolution of OFDR didn’t been provided. Based on previous research, the influence of spectral splicing on the spatial resolution of OFDR was analyzed in detail. The effects of single-segment wavelength range, number of spectral splicing segments, and splicing error on spatial resolution of OFDR were simulated. The results indicate that the spatial resolution of OFDR after spectral splicing is determined by the single-segment wavelength range, and it has the same spatial resolution as demodulating only one section of the spectrum. Meanwhile, the spatial resolution is independent of the number of spectral splicing segments and splicing errors (within 10 pm). Therefore, in order to improve the spatial resolution of OFDR, the single-segment wavelength range should be expanded as much as possible. Finally, experimental testing was conducted to verify the effectiveness. 35 spectral segments were achieved using a DBR laser, with each segment having a wavelength range of 1.2nm. By identifying the peak width of the fiber optic connector, it was verified that the spatial resolution of multi-segment spectral splicing and single-segment spectral is the same. However, experiments have also shown that spectral splicing weakens the measurement noise of the system, which is one of its advantages.
KEYWORDS: Optical fibers, Composites, Rayleigh scattering, Signal detection, Reflectometry, Single mode fibers, Lithium, Signal attenuation, Scattering, Detection theory
Optical frequency domain reflectometry technology can achieve high spatial resolution temperature/strain sensing by detecting the back-to-Rayleigh scattered light of the fiber. The fiber is implanted into the composite material to realize the detection of defects, damage, cracks, and debonding of the composite material. However, the implantation of composite materials will cause a large loss of light intensity due to the micro bending of the fiber. Meanwhile, Rayleigh scattering signal of single-mode fiber for communication (the mode field diameter is about 9 μm) is weak, which makes that the signal is submerged in the noise when the fiber is implanted in composite materials. In order to explore influence of fiber mode field diameter on measurement of composite materials, we theoretically analyzed the influence of the mode field diameter on the micro bending loss and scattering intensity. And the Rayleigh scattering intensity of fibers with mode field diameters of 9 μm, 7 μm, and 4.2 μm were tested with optical frequency domain reflectometry system. The results show that the micro bending loss decreases and the Rayleigh scattering intensity increases with reduce of the fiber mode field diameter. Polyimide coated fibers with cladding diameter of 125 μm and mode field diameter of 9 μm and 4.2 μm were implanted in the glass fiber composite materials. The scattering signal of the optical fiber with mode field diameter of 9 μm cannot be recognized when the fiber is implanted in the composite material. However, the fiber with mode field diameter of 4.2 μm achieves the measurement of 31 m.
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