Torsional vibration provides critical information for assessing the health of rotating shafts, whereas contact sensors can be difficult to deploy and may suffer from signal contamination in industrial environments. This study proposes a non-contact torsional vibration monitoring framework targeting High-Voltage Direct Current (HVDC) converter-station auxiliary drives, using a dual-beam differential laser Doppler vibrometer (LDV) and entropy-based lightweight learning. Two spatially separated poi