Fiber-Integrated Phase Change Microsphere System for Character Encoding and Matrix-Vector Multiplication

All-optical modulation and spectral encoding are significant for optical computing and information processing. However, the modulation level and efficiency of the traditional structure are limited by the size and uniformity of the modulation area and the insertion loss caused by on-chip coupling. A fiber optic integrated platform is proposed, which consists of single mode fiber (SMF), graded-index multimode fiber (GRIN-MMF), Ge2Sb2Te5 (GST)-coated microspheres, and a tapered fiber coupler. The interface between the SMF and MMF achieves spatial mode expansion through multimode interference, significantly enlarging the modulation spot and enhancing the light material interaction. The platform exhibits stable and reversible spectral modulation, with transmission fluctuations of 0.29 (amorphous state) and 0.68 dB (crystalline state) under repeated measurements. The GST switching process exhibits a fast response, with crystallization and amorphization times measured at 263 and 164 ns, respectively. By integrating two microspheres, we achieved all-optical encoding of English letters, numbers, and symbols, successfully writing the characters “All-optical”. Furthermore, a dual-microsphere array is configured to realize basic optical logic gates (AND/OR) and 2 × 2 matrix-vector multiplication. This work proposes an architecture for integrating modulation, storage, and computing on a fiber-based photonic platform that paves the way toward scalable neuromorphic and memory photonic systems.