The proposal describes the fundamental development and response quantification of sensitive, lightweight optical sensors for ultrasonic monitoring applications. The principal research components centre on providing a comprehensive theoretical and experimental understanding of the basic interactions between ultrasonic strain fields and optical fibre bragg grating (FBG) sensors. Two potential exploitation examples of the technology provide the background and application context for this research: ultrasonic beamforming in sonar arrays, and acoustic emission detection in structural health monitoring. These areas were carefully selected as they encompass the typical amplitude range of ultrasonic signals commonly encountered in engineering applications (in transmit sonar arrays the displacement fields are of high amplitude, often many 10's of nanometres, whereas in acoustic emission applications, the displacement field amplitudes may be lower than 100 picometres). Letters of support from THALES Underwater Systems (Sonar systems) and AIRBUS UK (Structural Health Monitoring) are included to help demonstrate the value of this work. Of course the opportunities for ultrasonic array monitoring are not confined to sonar systems. The increasing use of complex coded sequence actuation for ultrasonic arrays demonstrates a growing demand for improved ranging accuracy and resolution in sonar, non destructive testing and medical ultrasound fields. The potential for a lightweight, non-intrusive ultrasound field monitoring capability in such arrays provides a unique capability to provide absolute (calibrated against optical wavelength) measurement of the amplitude and phase characteristics at the output of these arrays. Such measurements facilitate calibration, optimisation of beamforming algorithms, and the capability to continuously monitor real-time changes under operational conditions. If successful the research will enable a step change for both areas of application in addition to related fields.