Real-valued intensity transmission matrix based multimode fiber beamshaping for biophotonics applications.

We present a real-valued intensity transmission matrix (RVITM) framework that enables phase-free structured illumination control through multimode fibers, overcoming long-standing challenges of modal scrambling from dispersion and bending. Unlike previous TM-based approaches that require complex-field measurements or interferometric detection, our method relies solely on 16 × 16 Hadamard intensity inputs, enabling deterministic synthesis of circularly symmetric illumination patterns that remain stable under fiber bending. To the best of our knowledge, this is the first demonstration of RVITM-based ring illumination through a multimode fiber for Raman spectroscopy and the first proof-of-concept application to inverse spatially offset Raman spectroscopy (iSORS). Using a 3 mm-radius ring pattern for iSORS of a two-layer phantom, the subsurface-to-surface Raman peak ratio was enhanced by 1.5 compared with point illumination, confirming improved depth sensitivity without added system complexity. RVITM offers a calibration-friendly and mechanically robust route to fiber-delivered structured illumination, advancing minimally invasive spectroscopy and imaging applications.