Bi-Res-Net-Enabled High-Accuracy Fiber-Optic Speckle Sensor for Simultaneous Seawater Temperature and Salinity Monitoring

We present a low-cost speckle sensing system based on cascaded eccentric single-mode–multimode fiber structure for simultaneous temperature-salinity measurement in deep-sea cold seeps. The system integrates a CMOS camera with a dual-branch bifurcated residual neural network (Bi-Res-Net), achieving 0.140‰ salinity RMSE (44% accuracy improvement) and 0.179°C temperature RMSE, while preventing error propagation from sequential compensation. With a 20-fold cost reduction ($1,660) compared to conventional CTD systems, this spectrometer-free solution enables traceable thermohaline monitoring, even in uncalibrated conditions (0.447‰ RMSE). Our hardware-algorithm co-design advances extreme-environment sensing for marine applications.