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.
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