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.
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.
High-speed wavelength interrogation technology for fiber Bragg grating (FBG) has attracted increasing attention in recent decades. Dynamic population gratings, formed in the rare-earth doped fibers, can be worked as adaptive beamsplitters in the adaptive interferometric detection configurations based on two-wave mixing (TWM). In combination the advantages of a semiconductor optical amplifier-based fiber ring laser (SOAFRL) sensors and fast response of dynamic population gratings demodulation in TWM system, in this work, we propose a demodulator for fiber Bragg grating (FBG) sensor using an interferometer based on transient two-wave mixing via dynamic population gratings in saturable Er-doped fiber (EDF). The proposed simple and robust configuration has an all-fiber design based on commercially available elements which makes it promising for applications in optical fiber ultrasonic sensors. Experimental results show that the SOAFRL is stable and can stably respond to dynamic signals with high frequencies.
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.
In this paper, we theoretically and experimentally demonstrate a dynamic strain sensor system utilizing a semiconductor optical amplifier (SOA)-based fiber-ring laser(FRL).The outside of the laser cavity consisting of a fiber FabryPerot(FFP) filter as an intensity demodulator. The SOA-based FRL is incorporating the fiber Bragg gratings(FBGs) as wavelength selective components for fiber lasers.The optical signals reflected from the FBGs are detected by photodetectors (PDs) after filtering by the FFP filter and band-pass filter (BPF). The change of the external dynamic strain will cause the spectral-shift of the reflected light of the FBG which can be dynamically monitored by the change of the output light intensity from the filter.The experimental results show that the sensing system we proposed here has a good response to the dynamic strain signal.In the meantime,we also simulated the spectra of the FFP filter and the FBG,and then we obtained the optimal response range near the peak of the spectrum of the FFP filter.The system demonstrated here has a simple structure and low cost,which make it attractive for dynamic strain detection in structure health monitoring.
Dynamic population gratings recorded in rare-earth-doped optical fibers, which are promising substitutes of photorefractive crystals for adaptive interferometric detections of mechanical vibrations and laser-induced ultrasound especially in industrial conditions, is formed by two counter-propagating mutually coherent laser recording waves via local saturation of the fiber optical absorption or gain (in optically pumped fibers). The dynamic population gratings are very attractive for different applications such as single-frequency cw fiber lasers, tunable narrow-band fiber optical filters, fiber optical sensors, adaptive interferometers, etc. The detection configuration will have an all-fiber design and will be based only on commercially available elements. In this work, we report an optical fiber adaptive vibrometer based on dynamic population grating in Er-doped optical fiber. A linear interferometer utilized for adaptive detection of mechanical vibrations. The two-wave mixing signal appears here as a result of nonlinear interaction between the direct wave <i>R </i>and the back propagating phase modulated wave<i> S</i>, which is reflected from the vibrating surface of a piezoelectric vibrating mirror. This all-fiber detection system has a fast response, is easy to prepare, which can be a potential method for detection of mechanical vibrations.