The tasks of this Phase I proposal are designed to establish the feasibility of an optical sensor for real-time, in situ monitoring of the ullage environment of an aircraft fuel tank. This all-optical fuel tank ullage sensor (FTUS) will be unaffected by fuel splashing and sulfur deposits on the sensor. By combining an innovative oxygen and temperature sensor, this project will enable NASA to offer technologies to monitor the performance of OBIGGS (on-board inert gas generating system). In commercial application, it is imperative that an on-board sensor is in place to identify if OBIGGS, which is safety related, is malfunctioning. FTUS represents a platform technology, which can be used across NASA enterprises, where sensitive monitoring of the ambient environment is required. By using single mode optical fiber in the design, ISL will address size, weight, and cost concerns of the aircraft industry. Tasks are proposed to demonstrate the sensor?s ultrahigh sensitivity and immunity to fuel splashes and deposits of sulfur compounds. To assure success, ISL has assembled a project team having a cumulative 60 person-years of experience in the development of opto-chemical sensors.