Spatial Mode Demodulation of Multimode Interference Sensors by a “fiber Camera”

Interference due to transverse mode phase differences is the fundamental principle of multimode fiber sensors. Traditional demodulation methods based on spectral domain have limited sensitivity and accuracy due to spectral resolution constraints. To overcome this limitation, we propose a new sensing approach that maps the intermodal interference in spatial domain. We achieve this by sparsely sampling the fiber transverse mode field distribution (MFD) via a multi-core single-mode fiber (MCF), which acts as a cost-effective “fiber camera”. Additionally, spatial mode modulation was introduced to selectively excite different modes in the sensor. By switching the launching cores, different excited modes respond differently for a given environmental perturbation, resulting in a significant improvement in sensing resolution by up to 20 times. Multiplexing of sensor head offers a solution to cross-sensitivity by leveraging different core excitations. The discrimination of temperature and refractive index was demonstrated by use of a deep learning algorithm. Our spatial mode demodulation scheme presents exciting possibilities for low-cost, highly sensitive, and fast MMI sensors.