Low-Cost Real-Time Fiber-Optic Distance Sensing for OPM-based Biomagnetic Measurements

Background: Accurate tracking of the distance between magnetic sensors and biological tissue is particularly relevant to biomagnetic measurements in which stable wearable sensor configurations are not feasible, such as optically pumped magnetometer-based magnetomyography (OPM-MMG). In these measurements, movement-induced changes in source-to-sensor geometry can affect the recorded signal nonlinearly and complicate motion artifact control. Because components used near optically pumped magnetometers must be nonmagnetic to minimize magnetic interference, a compact, nonmagnetic method is needed for real-time monitoring of sensor-to-surface distance. Methods: Low-cost, nonmagnetic polymer fiber sensors (Keyence FS-N11MN with FU-6F fibers) were evaluated for real-time distance measurement using a fully automated linear stage over a range of 0-30 mm and at controlled incidence angles from 50 to 90 degrees. The sensors generated analog output voltages from 1 to 5 V, sampled at 2048 Hz, with a response time of 16 ms. Custom scripts were used for signal averaging, noise estimation, uncertainty weighting, calibration, and distance reconstruction. Results: Voltage decreased nonlinearly with increasing distance. For the pooled data obtained at 80 and 90 degrees, weighted Hill1 fits over 0-10 mm and 0-30 mm yielded adjusted R-squared values greater than 0.9990. Across the five sensors, the mean absolute reconstruction error was 0.0282 mm when both calibration and evaluation were restricted to 0-10 mm, and 0.1310 mm when calibration over 0-30 mm was evaluated over 10-30 mm. Periodic modulation was observed beyond 10 mm, remained present when Sensor 1 was operated independently, and was consistent with the discrete internal signal levels of the sensor. Conclusion: Low-cost, nonmagnetic polymer fiber sensing may provide a compact, real-time method for monitoring sensor-to-surface distance over 0-10 mm in optically pumped magnetometer-based biomagnetic measurements. Extension across the full 0-30 mm range will require range-specific calibration and further validation using skin-facing surfaces.