Fiber Bragg grating (FBG) sensors are popular sensing elements and have a wide application of strain monitoring in the area of structural health monitoring, medical and aerospace due to the features of electromagnetic interference resistance, high sensitivity and simplicity. Here, a simple intensity demodulation configuration based on a semiconductor ring laser is proposed for FBG dynamic strain sensing system. Due to the characteristics of semiconductor optical amplifier, it can act as the gain medium as well as light source. An arrayed waveguide grating module is adapted to be the wavelength demodulator. It is feasible for this configuration to respond when FBG is subjected to dynamic strains at a high frequency. Additionally, a simultaneous dual-channel interrogation system is in detail discussed. This interrogation scheme can be widely utilized in structure health monitoring because of its low insertion loss, high stability and low cost.
An intensity-modulated optical fiber sensor is presented for static strain and vibration monitoring, which is fabricated by splicing a small section thin-core fiber between two standard single-mode fibers. Static strain measurement is performed using a simple cantilever system and a referenced fiber Bragg grating for sensing strain. The results show that optical loss increases with the rising strain for TCF sensor and the maximum optical loss is 0.133 dB. The dynamic response measurement of the cantilever vibration is demonstrated. The experimentally measured vibration frequency range is from 1 Hz to 200 Hz. The developed thin-core fiber sensor has the advantage of no complex demodulation, cost efficient and simple in structure, which is a potential monitoring method for large-scale construction, mechanical equipment, aerospace, and even earth activities.
Arrayed waveguide grating (AWG) has been widely used as a multiplexer in FBG demodulation system because of its high stability, low loss and fast read-write ability. They substitute expensive and vibration fragile spectrometers. In this paper, we compare two kinds of AWG demodulation systems experimentally. One is a multi-channel ultrasonic sensor system using fiber ring laser based on erbium-doped fiber amplifier (EDFA) and arrayed waveguide grating (AWG) as the intensity demodulator. And another is a one-way amplified system based on EDFA. When the external dynamic strains are applied on the FBG sensor, the central wavelength of the FBG will move between two adjacent channels of the AWG. Therefore, the modulation of the central wavelength of the FBG is converted to the amplitude modulation of the output of the two adjacent channels. Experimental results show that the multi-channel ultrasonic sensor system of one-way amplified configuration based on EDFA is more stable and can test high-frequency dynamic strain stably. The ultrasonic signal in water is successfully detected through one-way amplifier configuration.
We propose and experimentally demonstrate a multiplexing methodology for ultrasonic sensors based on fiber Bragg gratings (FBGs) that are included in the laser cavity of a semiconductor optical amplifier (SOA)-based fiber-ring laser system coupled with a fiber Fabry-Pérot (FFP) filter. The fiber ring laser (FRL) consists of an SOA as a gain medium and of FBGs as wavelength selection elements. We experimentally fabricate a dual-wavelength fiber ring laser and confirm stable oscillation outputs of the laser source. And ultrasonic signals generated from the piezoelectric transducers (PZTs) source are successfully detected. Such a multiplexed fiber-optic ultrasonic sensor system may be used for acoustic emission (AE) detection for structural health monitoring (SHM).
We present a comparative assessment of several refractometric optical fiber platforms based on reflective long-period fiber gratings (RLPGs), which was fabricated by combining the long-period gratings with the different fiber optic reflectors such as fiber optic retroreflector, Faraday rotator mirror, and Sagnac fiber loop mirror. In the experiment the refractive index (RI) of liquid was measured with RLPGs. It was found that the reflection spectrum remained the resonant dip without interference in fringe and the resonant wavelength appeared obvious blue shift with the increase of external RI. The spectral depth was reduced about 20 dB after a fiber optic reflector was configured. The simulation result of the resonance wavelength change with the refractive index of the liquid was also given. Meanwhile, the sensitivity of surrounding temperature has been considered. During the temperature measurement process, the intensity hardly changed with temperature. The absolute error of LPG was 1.15dB, and the absolute error of RLPG was about 0.2. The refractometric optical fiber platforms with the configuration of RLPGs had several advantages such as longer sensing distance, RLPG operation mode and non-interference fringe. Combined with the advantages, the sensor structure can not only be applied to measure the RI of glycerol/water solutions, but also be widely used to the measurement of toxic chemical liquid based on the fiber characteristic of resistance to hostile environments, especially far away from the toxic source.