Low Loss Tapered Fiber Waveguide Modulator for Crew Cognitive State Monitoring (CSM)

Many crew-related errors in aviation and astronautics are caused by hazardous cognitive states including overstress, disengagement, high fatigue and ineffective crew coordination. Safety can be improved by monitoring and predicting these cognitive states in a non-intrusive manner and designing mitigation strategies. Measuring hemoglobin concentration changes in the brain with functional Near Infrared Spectroscopy (fNIRS) is a promising technique for monitoring cognitive state and optimizing human performance during both space and aviation operations. A compact, wearable fNIRS system would provide an innovative early warning system during long duration missions to detect and prevent vigilance decrements in pilots and astronauts. During FY17 a fNIRS device was designed and built at GRC for human flight simulator testing by LaRC beginning Nov 2018. This device uses a bulk modulator because of its higher efficiency and optical output. As a parallel effort, a waveguide modulator was also built which implemented the fNIRS modulation techniques in polarization-maintaining (PM) fiber-pigtailed waveguide form, which is necessary for the device to be miniaturized into a robust system for clinical and field use. But the 90% optical loss of commercial waveguide modulators reduces the optical output of the system below detection limits. The goal of this effort is to decrease the loss of the waveguide modulators from 90% to <10% by tapering PM optical fibers to better couple with the waveguide internal to the modulator, overcoming the intrinsic limiting factor on power throughput to obtain virtually lossless waveguide modulation. The compelling and motivating vision for modifying a Mach-Zehnder interferometric waveguide modulator in this manner is that it enables field-configurable fNIRS instrumentation without the need for the tedious and time-consuming optical alignment required by a bulk modulator. Investing in this effort will enable a working system prototype suitable for field use. This work will leverage prior Center Innovation Fund (CIF) fNIRS waveguide modulator research as well as the Vytran Fiber Processing Workstation obtained from the 2017 Laboratory Investment Fund (LIF). Final testing will be performed using a novel active phantom developed during our previous CIF award which simulates the optical properties of brain tissue combined with a circulating blood simulant. The goal is to enable a miniaturized, robust and easily configurable fNIRS unit required for field use by reducing waveguide modulator optical loss from current state of the art (SOA) 90% to less than 10%, reducing laser power and PMT requirements.