A humidity fiber optic sensor based on phase-shifted (PS) fiber Bragg Gratings (FBG) is demonstrated in this paper. The sensor (PS-FBG) is coated with a moisture sensitive polyimide. When this thin coating is exposed to moisture its swells, hence inducing tensile stress on the PS-FBG and affecting its Bragg wavelength. Due to its intrinsic properties, the PS-FBG sensor presents the same trend of wavelength variation as standard fiber Bragg Grating sensor but with higher measurement resolution, and reliable measurements can be obtained in different humidity and temperature environments. This paper assesses the suitability, including sensitivity and response time, of the phaseshifted FBG sensor approach for humidity sensing. By monitoring this change, it is demonstrated that the humidity level of the environment can be accurately monitored.
Load monitoring and damage detection are the two critical aspects of aircraft structural health monitoring (SHM). The
load monitoring is achieved by detecting the local strain changes while the damage detection is generally accomplished
by the monitoring of acoustic signal changes. It would be ideal that a single sensor can perform both tasks
simultaneously. This paper reports the demonstration of using fiber Bragg gratings to monitor load and acoustic signal
applied on a simulated aircraft structure. The results clearly show that a single fiber Bragg grating sensor with short
grating length has the potential to perform both tasks simultaneously.
Structural Health Monitoring (SHM) has been identified as an area of significant potential for advanced aircraft
maintenance programs that ensure continued airworthiness, enhanced operational safety and reduced life cycle cost.
Several sensors and sensory systems have been developed for the implementation of such health monitoring capability.
Among a wide range of developed technologies, fiber optic sensor technology, in particular fiber Bragg grating based
emerged as one of the most promising for aircraft structural applications. This paper is set to explore the suitability of
using a new Fiber Bragg Grating sensor (FBG) system developed for operation in two modes, low and high speed
sensing modes, respectively. The suitability of the system for potential use in aircraft load monitoring and damage
detection applications has been demonstrated. Results from FBG sensor system were in good agreement with results
from conventional resistive strain gauges, validating this capability for load monitoring. For damage detection, the FBG
sensor system was able to detect acoustic waves generated 52 inches (1.32 m) away. The initial results, obtained in a full
stale experimentation, demonstrate the potential of using FBG sensors for both load monitoring and damage detection in
aircraft environment.
We report a miniaturized wavelength interrogator for the static and dynamic strain fiber Bragg grating sensors. The
developed interrogator is based on a monolithically integrated echelle diffractive grating and works in two independent
modes, one for the static strain measurement and the other for dynamic strain measurement. The proposed interrogator is
evaluated by measureing a static strain of 400 με and a dynamic strain of 200 Hz with a peak-to-peak amplitude of 200
με. Initial results are excellent and show that these two interrogation modes can be potentially performed
simultaneously.
A wavelength interrogation system based on an arrayed-waveguide-grating (AWG) is evaluated. The transmission
wavelengths are capable of shifting by thermally scanning the AWG. By employing this AWG wavelength tunability
scheme, the center wavelength of a long-period-grating (LPG) sensor is precisely interrogated. The theoretical principle
and the experimental result are presented in this paper. An interrogated wavelength resolution of 0.5 pm is achieved by
introducing this technique. Furthermore, a palm-size LPG sensor interrogator based on a chip integrated with an AWG
demultiplexer, a photodiode, a signal processing module and an electronic controlling module is proposed.
An intensity interrogation system for a long-period-grating (LPG) -based fiber sensor using a linear combination of two Gaussian functions as the curve fitting function is proposed and demonstrated. The selected resonant dip of the LPG transmission spectrum can be reconstructed with the curve fitting function based on the measured intensities at different wavelengths. Thus, the center wavelength and the minimum transmission value of the resonant dip can be interrogated simultaneously. The center wavelength is obtained by calculating the first-order derivative of the fitting function, which is 1562.74 nm compared to 1562.3 nm directly measured using an optical spectrum analyzer (OSA). The minimum transmission value is obtained directly from the fitting curve, which is -35.6 dBm is compared to -34.2 dBm directly measured from an OSA. An arrayed waveguide grating (AWG) is supposed to be adopted for the intensity measurement. However, the experiment is carried out using a tunable optical filter to approve the concept due to the unavailability of a suitable AWG at the time of experiment.
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