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

University Of California-Irvine, Irvine CA

Investigators

Abstract

Fiber-optic sensors have become one of the most important sensors for structural monitoring. In particular, Fiber Bragg Grating (FBG) based sensors used in conjunction with a wavelength division multiplexing (WDM) scheme, where each FBG sensor is dedicated to a sensor channel while sharing a common fiber link and interrogation module, have emerged as the most attractive approach for implementing large-scale civil engineering monitoring systems. The strength of the FBG sensor lies in its all-fiber nature. It can be easily connected to the interrogation module by a fiber link thus bypassing the labor-intensive packaging bottleneck. However, a major obstacle facing FBG-based sensor technology in field deployment is the lack of a cost-effective interrogation module capable of handling multiple FBG sensors. Here we propose a novel all-fiber spectrometer as the building block for a low-cost interrogation module. 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 semiconductor photodetector. The spectrometer is built on our prior research experience in tunable all-fiber devices for dense WDM fiber-optic communication applications. It combines the merits of an all-fiber device and the wide tunability of an AOTF. The all-fiber spectrometer is compact, light weight, fast, low cost and has a performance comparable to existing grating-based and Fabry-Perot spectrometers. In addition, its non-blocking characteristics enables a distributed interrogation architecture that offers scalability, interchangeability, and enhanced system redundancy. Although the primary application of this proposal is on structural sensing, the all-fiber spectrometer can be developed as a hand held portable sensor system for chemical and biochemical applications. The basic operating principle of our approach has been demonstrated in the laboratory. Using a frequency-shift key (FSK) modulation scheme, we have demonstrated wavelength accuracy of 0.02 nm (this corresponds to a strain of 20 ustrain) for a sensor channel spacing of 1.2 nm with room for additional performance improvement. With further reduction of the filter bandwidth, improvement of the packaging design, particularly thermally stability, we aim at developing a prototype multi-section AOTF spectrometer to be field tested in actual civil engineering structures 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 with a proven research record in tunable all-fiber devices. Dr. M. Feng has a distinguished research record 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 California Department of Transportation will provide in-kind support for field trials on its highways and bridges. The proposed projects are expected to have strong impact on research, education, and potential technology transfer. As reflected from the budget, the project is leaning heavily on supporting graduate students. Based on our experience at UC Irvine, we also expect active participation of undergraduate students through the campus-sponsored UROP (undergraduate research opportunity program) to work with the graduate students and faculty.