Contact author:SHAO Zhihua (1987-), male, associate professor, Ph.D. degree, mainly focuses on optical fiber sensing technology. Email: zhshao@nwu.edu.cn
Acta Photonica Sinica, Volume. 51, Issue 3, 0306006(2022)
Ultrasonic Microfiber Sensor Based on Tapered Seven-core Fiber
Traditional detection approaches usually employ Piezoelectric Transducers (PZTs) as the ultrasonic source and receiver. However, these current-driven transducers have some inherent drawbacks (susceptibility to electromagnetic interference, narrowband frequency response, and not resistant to high temperature and corrosion). Fiber-optic sensors have attracted significant attention in ultrasonic detecting owing to their outstanding advantages, such as: small size, easy reuse, wideband frequency response, and immunity to electromagnetic interference. The majority of fiber-optic ultrasonic sensors has based on fiber Bragg gratings and Fabry-perot interferometers. However, frequency response range of ultrasonic sensors based on FBGs is relatively narrow. The Fabry-perot interferometers ultrasonic sensors generally consist of a diaphragm and a fiber-optic end-face as two reflectors. Nevertheless, the complex preparation of diaphragm materials, poor chemical stability, and heat resistance, limit the application of sensor. In this study, a compact fiber-optic ultrasonic sensor based on a Tapered Seven-core Fiber (TSCF) is proposed and experimentally demonstrated. This proposed sensor has the advantages of easy fabrication, compact structure, and high sensitivity. The sensor comprises a TSCF sandwiched between two Single-mode Fibers (SMFs), forming a cascade structure of SMF-TSCF-SMF. The SCF (YOFC, MC1010-A, China) is used to make ultrasonic sensors. A commercial fiber fusion splicer (Fujikura, FSM-80C) is used to fabricate the SMF-TSCF-SMF structure. Thereafter, the optical fiber fused biconical taper system (FBTZolix) is used to taper the SCF into the TSCF with diameters of 11 μm, 19 μm and 29 μm. A certain prestress is applied to keep the SCF tight and straight during the fused tapering process. High order modes are easily excited owing to the core mismatch of SMF and Seven-core Fiber (SCF). The excited multiple modes continue to propagate along the SCF and then arrive at the tapered region. These transmission spectra exhibited multiple interference peaks. This is because complex optical modes are excited and are involved in mode interference. Therefore, these transmission spectra are not in a standard sinusoidal pattern, but become more irregular. Due to the sharply reduced taper diameter (as thin as several micrometers), the core distances are largely decreased and the evanescent fields are extended simultaneously. Thus, it is sufficient to induce diverse inter-modal coupling at the abrupt taper, including the mode coupling among cores, and coupling and recoupling of the cladding-to-core modes. Highly sensitive mode interferences are obtained. For the TSCF, the ultrasonic wavelength is much longer than the taper diameter and shorter than the fiber length. The fiber taper is axially constrained, that is, the axial elongation of the fiber taper can be neglected. The core and cladding diameters in the tapered region become thinner, and the TSCF has an obvious effect on evanescent waves. When the sensor is immersed in water, the Ultrasonic Wave (UW) signal periodically changes the refractive index of the surrounding liquid and modulates the transmission spectrum according to the evanescent-field interaction between the liquid and the transmitting light. Meanwhile, due to the effect of evanescent field, the light energy transmitted in the fiber can penetrate into the surrounding medium, resulting in energy reduction. Thus, the TSCF sensor with a diameter taper of 19 μm is used as the receiving source of ultrasonic signals. Driven by a function generator, the PZT (SIUI, 1Z20SJ50DJ) separately emits a 1 MHz continuous wave with a voltage amplitude of 10 V as the ultrasonic source. The edge filtering method is used to demodulate the ultrasonic signal received by the TSCF sensor. A tunable laser (Santec-710) with a 100 kHz linewidth and 0.1 pm tunable resolution was used as the light source. The output power of the tunable laser was 20 mW. The photodetector (New Focus, Model 2117) with a bandwidth of 10 MHz converts the optical signal into a voltage signal, which is finally monitored by an oscilloscope (RIGOL, DS2302A). The bandpass filter built into the photodetector has a frequency range of 500 kHz to 3 MHz, which is used to shield the surrounding noise. UW detection is processed in water at room temperature, which provides an almost constant temperature environment around the sensor. The sensor directly faces the emitting end of PZT with a separation of 2.5 cm. The continuous signals exhibit good uniformity and stability in the time domain. The peak-to-peak voltage of TSCF is about 0.4 V.
Contact author:SHAO Zhihua (1987-), male, associate professor, Ph.D. degree, mainly focuses on optical fiber sensing technology. Email: zhshao@nwu.edu.cn
0 Introduction
Traditional detection approaches usually employ the Piezoelectric Transducers (PZTs)as the signal source and receiver [
The majority of fiber-optic ultrasonic sensors are based on Fiber Bragg Gratings (FBGs)[
In this paper,a compact fiber-optic ultrasonic sensor based on TSCFs is proposed and experimentally demonstrated. The tapered region of TSCFs has a strong evanescent field effect,which largely overlaps with the ultrasonic coupling agent (water). The applied ultrasonic field changes the water density and then shifts the interferometric fringe of the transmission spectrum. Herein TSCFs in different diameters are fabricated,and their mode interferences and ultrasonic measurements are comparatively analyzed.
1 Sensor fabrication and principle
The sensor comprises a TSCF sandwiched between two Single-Mode Fibers (SMFs),forming a cascade structure of SMF-TSCF-SMF,as shown in
Figure 1.The proposed TSCF sensor
A commercial fiber fusion machine (Fujikura,FSM-80C)is used to fabricate the SMF-TSCF-SMF structure with a 9-mm-long seven-core fiber. Then a fiber flame taper (FBT-Zolix)is used to taper the SCF into TSCF in different diameters. A certain prestress is applied to keep the SCF tight and straight during the fused tapering process. To obtain a uniform and symmetrical TSCF,the tapering step is divided into two steps:setting the velocity and displacement of sliders to 2.6 mm/s and 0.5 mm,respectively,and then increasing the two drawing parameters to 3.6 mm/s and 2.5 mm separately. The tapering parameters can be adjusted as needed to fabricate TSCFs in different diameters. In this paper,SCF (YOFC,MC1010-A,China)is used to make ultrasonic sensors. The six cores of the seven-core fiber are located on the six corners of the regular hexagon,and the center of the hexagon is the middle core of the seven-core fiber. The diameter of the SCF is 150 μm,and the distance between adjacent cores is 42 μm. Taper fibers are fabricated with diameters of 11 μm,19 μm and 29 μm,and the micrographs of the tapered areas are shown in
Figure 2.Transmission spectra of the SMF-TSCF-SMF structure with 29 μm diameter taper,19 μm diameter taper and 11 μm diameter taper
The transmission spectra of TSCF with diameters of 11 μm and 19 μm in water and air are shown in
Figure 3.Transmission spectra of TSCF with different diameters before and after entering water
where
On the other hand,the core and cladding diameters in the tapering region become thinner due to the tapering. The light in core can be transmitted along with the surface of the tapered SCF due to the evanescent wave [
where
where
2 Experimental results and discussions
The fiber-optic ultrasonic detection system is shown in
Figure 4.Schematic diagram of the ultrasonic detection system
As expected,the continuous UW signal presents good uniform and stability in time domain,as shown in
Figure 5.1 MHz sensor ultrasonic response
To demonstrate the stability of the sensor,the repetition frequency of the signal generator is fixed at 100 Hz and the driving amplitude is 4 V to drive PZT to emit ultrasonic with a frequency of 1MHz. The TSCF sensor response sequence is collected at room temperature,as shown in
Figure 6.Sensor stability test
The response to UW of sensor is performed at the different distances between PZT and sensor. In the experiment,two devices are hold in the same axis,and the sensor is moved along the axis by moving stage. UWs are recorded in the range of 1 cm to 5 cm with a separation of 0.5 cm. As shown in
Figure 7.Experimental measurement
The sensor also measures UWs at several frequencies of 250 kHz and 5 MHz,and the results are shown in
Figure 8.Ultrasonic response of sensor
Figure 9.Spectrum response characteristics of sensor to pulsed UWs with frequency of 500 kHz and 5 MHz
The change of sensor time domain signal with PZT driving voltage is shown in the
Figure 10.Variation of sensor response amplitude with ultrasonic intensity
3 Conclusion
In conclusion,a compact fiber-optic taper sensor based on Tapered Seven-Core Fiber (TSCF)is designed for high frequency Ultrasonic Wave (UW)detection. Due to the tapered region of TSCFs has a strong evanescent field effect,when placing the sensor in water,the UW signal periodically changes the refractive index of the surrounding liquid and modulates the transmission spectrum according to the evanescent-field interaction between the liquid and the transmitting light. The TSCF with different diameter is analyzed theoretically,and the TSCF with region diameter of 19 μm is selected for experimental test. By testing the response of the TSCF to ultrasonic signals with different frequencies and intensities,it is proved that the sensor has good ultrasonic response characteristics.
Get Citation
Copy Citation Text
Xi YANG, Huanhuan YIN, Zhihua SHAO, Xueguang QIAO. Ultrasonic Microfiber Sensor Based on Tapered Seven-core Fiber[J]. Acta Photonica Sinica, 2022, 51(3): 0306006
Category: Fiber Optics and Optical Communications
Received: Oct. 13, 2021
Accepted: Jan. 7, 2022
Published Online: Apr. 8, 2022
The Author Email: QIAO Xueguang (xgqiao@nwu.edu.cn)