SENSORS: The Exploration of Novel All-Purpose All-fiber Spectrometer Module for Scalable and Distributed Optical Fiber Sensor System

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Fiber-optic sensors have become one of the most important sensors for structural monitoring. Inparticular, Fiber Bragg Grating (FBG) based sensors used in conjunction with a wavelengthdivision multiplexing (WDM) scheme, where each FBG sensor is dedicated to a sensor channelwhile sharing a common fiber link and interrogation module, have emerged as the most attractiveapproach for implementing large-scale civil engineering monitoring systems. The strength of theFBG sensor lies in its all-fiber nature. It can be easily connected to the interrogation module by afiber link thus bypassing the labor-intensive packaging bottleneck. However, a major obstaclefacing FBG-based sensor technology in field deployment is the lack of a cost-effectiveinterrogation module capable of handling multiple FBG sensors.Here we propose a novel all-fiber spectrometer as the building block for a low-cost interrogationmodule. The proposed all-fiber spectrometer consists of an Acousto-Optic Tunable Filter (AOTF)implemented directly on a 10-30 cm long single-mode fiber and an on-fiber semiconductorphotodetector. The spectrometer is built on our prior research experience in tunable all-fiberdevices for dense WDM fiber-optic communication applications. It combines the merits of an all-fiberdevice and the wide tunability of an AOTF. The all-fiber spectrometer is compact, lightweight, fast, low cost and has a performance comparable to existing grating-based and Fabry-Perotspectrometers. In addition, its non-blocking characteristics enables a distributedinterrogation architecture that offers scalability, interchangeability, and enhanced systemredundancy. Although the primary application of this proposal is on structural sensing, the all-fiberspectrometer can be developed as a hand held portable sensor system for chemical andbiochemical applications.The basic operating principle of our approach has been demonstrated in the laboratory. Using afrequency-shift key (FSK) modulation scheme, we have demonstrated wavelength accuracy of0.02 nm (this corresponds to a strain of 20 ustrain) for a sensor channel spacing of 1.2 nm withroom for additional performance improvement. With further reduction of the filter bandwidth,improvement of the packaging design, particularly thermally stability, we aim at developing aprototype multi-section AOTF spectrometer to be field tested in actual civil engineeringstructures to demonstrate the practicality of this new approach.The project team reflects a multi-disciplinary effort. Dr. H. P. Lee, the PI, is leading a group witha proven research record in tunable all-fiber devices. Dr. M. Feng has a distinguished researchrecord in exploring various sensors for health monitoring of civil engineering structures. Dr. F.G.Shi brings in much needed expertise in packaging design, simulation, and reliability modeling.The research infrastructure is already in place at the PI's home institution, and the CaliforniaDepartment of Transportation will provide in-kind support for field trials on its highways andbridges.The proposed projects are expected to have strong impact on research, education, and potentialtechnology transfer. As reflected from the budget, the project is leaning heavily on supportinggraduate students. Based on our experience at UC Irvine, we also expect active participation ofundergraduate students through the campus-sponsored UROP (undergraduate researchopportunity program) to work with the graduate students and faculty.