Sensitivity analysis of a multimode fiber-optic Sagnac interferometer to high magnetic fields in an MRI scanner environment

This paper investigates the sensitivity of a multimode fiber-optic Sagnac interferometer (MFOSI) to high magnetic fields, using a 1.5 Tesla magnetic resonance imaging (MRI) scanner as the test environment. The MFOSI setup employs sinusoidal phase modulation and lock-in detection to measure the non-reciprocal phase shift between counter-propagating waves. Experimental results revealed an apparent insensitivity of the MFOSI to the applied magnetic field. To account for this behavior, a theoretical model is developed using scattering matrix and generalized coherence matrix formalisms, incorporating mode-coupling effects. The analysis reveals a cross-sensitivity between the Sagnac phase shift and the Faraday rotation angle and highlights a trade-off between sensitivity and bias stability, both strongly influenced by the number of guided modes. It is further shown that under strong mode coupling, the use of unpolarized light improves bias stability but reduces sensitivity. These findings provide valuable insights into the behavior and design of multimode fiber-optic sensors operating in high magnetic field environments, such as MRI systems.