Influence of fiber mode field diameter on measurement of composite material using optical frequency domain reflectometry

Abstract

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.