基于锥形七芯光纤的紧凑型超声波传感器

Ultrasonic Microfiber Sensor Based on Tapered Seven-core Fiber

提出一种基于锥形七芯光纤的紧凑型光纤超声传感器,并进行了实验验证。该传感器由熔接在两根单模光纤之间的锥形七芯光纤制成,形成单模光纤—锥形七芯光纤—单模光纤的级联结构。由于单模光纤和七芯光纤的纤芯不匹配,容易激发高阶模式,被激发的多种模式的光波继续沿着七芯光纤传播,然后到达锥形区域。由于锥度直径的急剧减小,模式间发生干涉,传感器的灵敏度得到提高。制备了不同直径的锥形七芯光纤,并对其模间干涉和超声测量进行了对比分析。超声波在水中传播时,将周期性地改变周围液体的折射率,基于锥形光纤的倏逝场效应,调制锥形光纤中光波的传输。该超声波传感器具有制作简单、信噪比高、频率响应宽等特点。

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 11μm,19μm and 29μm.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 19μm 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.