Infrastructure Technology Institute
Many modern and retrofitted bridges incorporate a structural health monitoring (SHM) system within their design. An integral part of this system is the sensors that send data to the DAQ. Northwestern University’s Center for Quality Engineering & Failure Prevention and ITI jointly are working of developing a pair of new fiber optic based sensors to monitor both internal moisture in a structure and vibration of a structure. These sensors utilize photonic crystal fibers (PCFs) that are based on long-period gratings (LPGs).
The monitoring of internal moisture or humidity is possible using the behaviors of modal transition in the PCF-LPG where the air channels in PCF cladding are azimuthally coated with two types of nanostructure polymers as primary and secondary coatings by electrostatic self-assembly deposition process. The primary coating does not affect the PCF-LPG parameters such as grating resonance wavelengths and their intensities that can be used for sensing, but it does increase the sensitivity to refractive index of chemical analytes in the air channels. The secondary coating is for selective absorption of analyte molecules of interest. These two coatings significantly modify the cladding mode distribution of PCF-LPG and enhance the evanescent wave interaction with the external environment, resulting in a highly sensitive and selective chemical sensor. These sensors can be tailored to detect moisture or sodium chloride that can ultimately accelerate the corrosion process of a bridge structure.
The monitoring of vibration with fiber-optics has been accomplished using a PCF Mach-Zehnder interferometer (MZI). The MZI consists of two LPG’s with identical transmission spectra are written to the PCF. This process excites the core mode to couple out the cladding modes. The first LPG can couple a part of core mode to selected cladding modes. After the two light beams travel at different speeds over a certain length of the core and cladding, the cladding modes will be recoupled back to the core when they meet the second LPG, thus making interference between the core mode and cladding modes. The dynamic strain is interrogated by the PCF-MZI that is attached on a spring-mass system as shown in figure 1. The combination of the PCF-MZI and spring-mass system operates as an accelerometer. The shift of interference fringe can be measured by a photo detector and the transformed analog voltage signal is proportional to the acceleration of the sensor head.
Fiber-optic accelerometers have many advantages over their electrical counterparts. Fiber based accelerometers have the capability of multiplexing to reduce or possibly eliminate cabling and they are able to transmit optical signals over kilometer range distances making them ideal for large bridge structures. They are also immune to electromagnetic interference and insensitive to temperature.