摘要： An optical fiber magnetic field sensor based on the birefringence of magnetic fluid is proposed by filling the magnetic fluid into a hollow-core waveguide. The birefringence characteristics of magnetic fluids with different concentrations tuned by magnetic field were investigated under different magnetic fields when the filled hollow-core waveguide is placed in a Sagnac loop. As the magnetic field increases, the internal chain-like structure of the magnetic fluid increases, resulting in changes in birefringence. The experimental results show that the device has a highly linear response to the variation of magnetic field intensity in the range of 0–300 Oe with the maximum sensitivity of 29.2 pm/Oe and resolution of 0.68 Oe. The proposed magnetic field sensor has such advantages as ease of fabrication, simple structure, low cost and it is expected to find potential applications in the magnetic field measurement with high accuracy, optical fiber gyroscopes, magneto-optic modulators and so on. Due to the external field tunability, the device also provides a promising technology for microfluidics research.