All‐Fiber Microsensor of Polarization at Single‐Photon Level Aided by Deep Learning

Abstract Polarization of light carries vital information in numerous scientific disciplines, including biomedical imaging, optical diagnostics, and environmental sensing. However, accurate polarization measurements in constrained spaces, under low‐light conditions, and at high speeds remain a severe challenge. A compact, all‐fiber polarization sensor capable of single‐shot, real‐time operation with single‐photon sensitivity and long‐term stability is presented. The sensor leverages intermodal interference within a short segment of a few‐mode fiber, coupled to an array of single‐photon detectors. The recorded detections are processed by a neural network, enabling precise reconstruction of complete polarization information for fully and partially polarized states. This robust architecture allows for thousands of polarization state measurements per second while achieving exceptional accuracy with Stokes error below 0.01, and even lower at higher photon fluxes. This technology is demonstrated through diverse experimental scenarios, such as resolving structural details in biological tissues with a spatial resolution of 6 µm, characterizing rapid polarization transitions, and monitoring micro‐scale birefringence in living or moving specimens.