IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 15, AUGUST 1, 2015
1649
Phase-Shifted Terahertz Fiber Bragg Grating for Strain Sensing With Large Dynamic Range Gerald Hefferman, Zhen Chen, Lei Yuan, and Tao Wei Abstract— Recent advances in optical fiber sensing techniques have demonstrated the utility of terahertz (THz) gratings as a modality for strain and temperature sensing. However, these techniques remain reliant on the use of higher order resonant peaks, enhancing their sensitivity at the cost of limited dynamic range. The use of a lower order resonant peak for sensing can lead to a larger dynamic range at the cost of accuracy. This letter reports a π-phase-shifted THz fiber Bragg grating, fabricated using a femtosecond laser, capable of detecting changes in strain over a substantially larger dynamic range than previously reported methods with improved accuracy. A second THz grating without a π-phase-shifted structure, but otherwise identically constructed, was interrogated in series on the same optical fiber. The two devices were simultaneously experimentally investigated using a strain test (∼1 mε in total), and the results presented in this letter. In addition, the theoretical models of the devices were created, which closely matched experimentally observed device physics. Index Terms— Terahertz Bragg grating, interferometry, optical fiber sensor.
I. I NTRODUCTION
O
PTICAL fiber sensors have a unique set of characteristics that make them particularly useful as strain and temperature sensors [1]–[8]. Their chemical stability and immunity to electromagnetic interference make them ideally suited to sensing in harsh environments, while the ease with which multiple sensors can be fabricated in series and simultaneously interrogated allows for multiplexed sensing across considerable distances (∼km), making optical fiber sensors a viable solution in structural health monitoring, energy, and aerospace applications [9]–[12]. Fiber Bragg gratings in particular have demonstrated their utility in these areas, and are a fundamental element of many optical fiber sensing techniques. Recently, fiber Bragg gratings fabricated with grating structures corresponding to the terahertz range (THz FBGs) have demonstrated additional Manuscript received April 20, 2015; revised May 12, 2015; accepted May 13, 2015. Date of publication June 1, 2015; date of current version July 7, 2015. This work was supported by the Division of Computing and Communication Foundations through the National Science Foundation under Grant CCF-1439011. G. Hefferman is with the Warren Alpert Medical School of Brown University, Providence, RI 02903 USA, and also with the Department of Electrical, Computer and Biomedical Engineering, University of Rhode Island, Kingston, RI 02881 USA (e-mail:
[email protected]). Z. Chen and T. Wei are with the Department of Electrical, Computer and Biomedical Engineering, University of Rhode Island, Kingston, RI 02881 USA (e-mail:
[email protected];
[email protected]). L. Yuan is with the Department of Electrical and Computer Engineering, Clemson University, Clemson, SC 29634 USA (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2015.2433682
characteristics that make them particularly well-suited to applications requiring large-scale, multiplexed sensing techniques [13], [14]. Chief among these advantages are their narrow interrogation bandwidth (hundreds of gigahertz) and their high spatial resolution (
