In this paper, we propose a semiconductor optical amplifier (SOA)-fiber ring laser (FRL) for fiber Bragg grating (FBG) dynamic strain sensing system with an adaptive demodulator based on two-wave mixing (TWM) photorefractive interferometry. Any strain in the FBG is encoded as a wavelength shift of the light reflected by the FBG. The wavelength modulation is perfectly converted to intensity modulation by splitting the light into signal and pump beams and interfering the beams in an photorefractive InP:Fe crystal. The classical beam-combiner was replaced by a dynamic hologram continuously recorded in the InP:Fe crystal. The results demonstrate that TWM interferometer has the characteristics of adaptability and multiplexing. To investigate multiplexability, a three-channel SOA-fiber ring laser sensor system is presented to detect dynamic strain signals from three FBG sensors simultaneously. Experimental results prove that true multiplexing of several FBG dynamic strain sensors with a single adaptive source is feasible. This technique is expected to be suitable for the monitoring of external impact as well as acoustic emission in structures.
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.
Large bridges and industrial equipment may encounter natural disasters (earthquakes, tsunamis, etc.) and man-made effects during their service period. When they are subjected to these external influences, the structure may be deformed or even cracked, and the internal stress of the structure can also cause the occurrence of acoustic emission events. In this work, we report a fiber-optic acoustic emission sensing system using a semiconductor optical amplifier (SOA)-based fiber-ring laser source including a non-tunable fiber Fabry-Perot filter (NTFFPF) to demodulate dynamic signals from fiber Bragg grating (FBG) sensors. The shift in the FBG reflection spectrum caused by external strain is demodulated by the NTFFPF in the ring laser cavity, which ultimately produces an amplified output signal. The proposed system was used to detect the high-frequency acoustic emission signals generated by the piezoelectric buzzer. Experimental results show that this system can demodulate high-frequency acoustic emission signals with a good response and a high signal-to-noise ratio up to 21.6 dB. At the same time, acoustic emission signals generated by an ultrasonic vibrator with a frequency of 40 kHz are detected simultaneously with a FBG sensor and a piezoelectric sensor placed in the middle of a square aluminum plate. The angle-dependent acoustic emission measurement is performed by placing the ultrasonic vibrator at different angles from 0° to 90° in the radial direction of the FBG sensor. The results show that the sensor system can accurately detect the high-frequency acoustic emission signals on the aluminum plate and larger signal amplitude can be obtained when the angle between the ultrasonic vibrator and the FBG sensor axial is in the range of 0-60°. The fiber ring laser sensing system proposed in this paper has application prospects in many aspects, such as acoustic emission source location and ultrasonic detection.
With the rapid development of photonic integrated circuit, waveguide-based electro-optic modulators are widely used in the fields of optical communication, optical signal processing and optical sensors. The Mach-Zehnder modulator is one of the most widely used device structures as a particular kind of optical switching element, which has the advantages of great accuracy and high sensitivity. We investigate two types of Mach-Zehnder modulators (using balanced and unbalanced interferometers) based on lithium niobate (LiNbO3) through theoretical and numerical analysis. The transmission characteristics of the balanced Mach-Zehnder modulator are numerically analyzed while the electric field is applied across the waveguide in one of the arms (or the two arms) of the interferometer, and the transmission characteristics of the unbalanced Mach-Zehnder modulator with different length differences between the two waveguide arms are studied. Numerical calculation results show that the transmission of the waveguide in the Mach-Zehnder structure changes sinusoidally, with alternately switching between port 2 and port 4. The theoretical results in the present work can provide some guidance for developing the practical optical modulator devices.
A non-invasive optical fiber pulse sensor is proposed and experimentally demonstrated. It comprises a simple structure in which a section of thin-core fiber is spliced into another single-mode fiber. And a silicone rubber device is designed to ensure that weak pulse signals are detected. To assess the availability of the optical fiber pulse sensor, a commercial photoplethysmograph is used to measure the pulse of the same subject as a control. The measurement results of the two methods are consistent. The fiber pulse sensor can show a segmented signal in individual pulses, which provides more physiological information. It also possesses the advantages of high sensitivity, simple signal acquisition and processing, easy fabrication, and thus is an ideal candidate for replacing traditional electrical sensor.
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.
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).
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.
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