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.
Pulse pressure measurement, as one of the major home health monitoring technologies, is of great significance for the assessment of blood pressure and recognition of lung cancer. Here we propose and experimentally demonstrate a noninvasive optical fiber pulse sensor, which comprises a simple structure in which a section of thin-core fiber is spliced into another single-mode fiber. Responses of fiber pulse sensor to the simulated pulsations are tested by using a piezoelectric transducer. And a silicone rubber device is designed to ensure that weak pulse signals are detected. The sensors with an inserted thin-core fiber length of 2-3 mm can display remarkable response to human pulse pressure. The signal waveforms detected are almost unaffected by the subject’s movement. 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.
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).
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.
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