Research On Tapered Optical Fiber Probe And Wireless Sensing System

The optical fiber sensoring technology is a new type of sensing technology with the development of the optical fiber communication technology.Compared with the traditional electrical sensors,the optical fiber sensor has the advantages of anti-electromagnetic interference,compact size,high sensitivity,and wide band transmission.Therefore,it has become a focus in the sensor field.In recent years,high sensitivity is a trend of the research and development of the optical fiber device.The research on sensing technology based on the tapered optical fiber has been paid more attention internationanlly,and many researchers have been carring out relevant studies,which have a great application potential in many fields.With the proposal and development of the internet of things,the achievement of intelligent sensing network becomes the pursuit of the scholars.Since the optical fiber sensor network with cable has the disadvantages of poor mobility,low flexibility,poor scalability,flimsy,high cost for installation and maintenance,it is not suitable to monitor space with large scale where the monitoring points are far away from each other.The combination of the wireless sensor network technology and the optical fiber sensing technology absorbing merits of each other can effectively solve above problem and expand their application,which has an important scientific significance and enormous social value.In this paper,the tapered optical fiber probe and the wireless sensing system are studied on the basis of sensing,information processing and wireless communication technique.The key techniques are studied in detail,including the sensor based on the tapered optical fiber coated by low refractive index?RI?material,the sensor based on the tapered optical fiber deposited with high RI nanofilm,wireless sensor nodes based on optic-fiber sensor,the corresponding network topology and protocol,and the network experiments.The main work of this thesis is as follows:1.The adiabatic tapered optical fiber coated by low RI material is proposed.A theoretical model is proposed to study the transmission characteristic and sensing mechanism.The theoretical results show that the local taper angle becomes smaller to satisfy the adiabatic condition with the increase of the ambient RI.With the increase of the coating RI,the ratio of the core power and total power decreases,leading to reduce output power.Additionally,the closer the coating RI is to the cladding RI,the faster the ratio decreases,which means a higher sensitivity.With the increase of coating thickness,the ratio decreases and converges to a constant after the thickness increases to a certain value,where the highest sensitivity emerges.With the decrease of the radius of the taper waist,the sensitivity increases.In the experiment,the tapered fiber temperature sensor is coated by the silicon rubber,and the power demodulation is applied,which has the advantages of simple structure and good stability.The experimental results show that the output power increases with the increase of the temperature,and the sensitivity increases with the decrease of the taper waist radius.The sensitivity of the temperature sensor with 10 ?m taper waist is 14.88 ?W/oC.2.The adiabatic tapered optical fiber deposited by high RI nanofilm is proposed.The theoretical models from two aspects of the ray-optic and mode theory are proposed to study the transmission characteristic and sensing mechanism.The results show that the local taper angle becomes smaller to satisfy the adiabatic condition with the increase of the nanofilm thickness.Based on ray-optic,the optical field confined in the tapered fiber leaks out into the nanofilm and multiple beam interference occurs,which can be regarded as a thin film Fabry-Perot resonator.The transmission presents interference dips at different wavelengths.Based on mode theory,light guided in the fiber is coupled to the lossy mode in the nanofilm,so that lossy mode resonance occurs.The resonance wavelength exhibits red shift with the increase of three key parameters: the nanofilm thickness,the real part of the nanofilm RI and the ambient RI.The resonance depth increases with the the increase of the imaginary part of nanofilm RI or the taper waist length,and the decrease of the taper waist radius.In addition,with the increase of the incident angle,the resonance depth and width decrease,and the resonance wavelength exhibits tiny blue shift.3.Atomic layer deposition?ALD?technology is introduced to deposit high RI nanofilm around the tapered optical fiber,which is simple and effective.The choices of coating materials are various.The special advantages of accurate thickness control,good uniformity and adhesion guarantee the sensing characteristics of the sensor head.The experimental results show that RI sensitivity can be enhanced by depositing high RI nanofilm around the taper waist.In common band,the sensitivity of tapered fiber deposited by Al₂O₃ nanofilm is 6008 nm/RIU for the ambient RI close to 1.33,and 16660 nm/RIU for the ambient RI close to 1.45.The sensitivity of tapered fiber deposited by TiO₂ nanofilm is 7096 nm/RIU for the ambient RI close to 1.33,and 31647 nm/RIU for the ambient RI close to 1.45.The temperature-sensitive silicone gel is coated around the fiber taper with Al₂O₃ nanofilm to fabricate a high sensitivity temperature sensor.The high sensitivity of 2.44 nm/oC is obtained.4.The wireless optic-fiber senor system is realized.The resolution of the temperature sensor node based on the tapered optical fiber coated by low RI material is 0.091 oC,and the resolution of the temperature sensor node based on the tapered optical fiber deposited by Al₂O₃ nanofilm is 0.024 oC.Compared these two kinds of temperature sensors,the former has a wider range of measurement,while the latter has a higher sensitivity and resolution.The electronics components are selected based on their size,power consumption,cost,and reliability.By using module design,the sensor module,the micro processor module,wireless communication module and power supply module of the sensor node are designed and developed.This kind of sensor node can integrate any optical fiber sensor with intensity demodulated to measure a variety of physical quantity.The two-layer network topology is proposed.The bottom-layer is composed of the optic-fiber sensor nodes and the management nodes.The top-layer is composed of the management nodes,the route nodes,and central control node.The hierarchical nested communication protocol with sleep mechanism and low power consumption is designed and realized.The sensing data can be sent to the central control node accurately by the wireless optic-fiber sensor system.Combining the optical fiber sensor with the wireless sensor network,this system has advantages of wireless design and flexible network construction,which are not available in the traditional optical fiber sensing system.The research achievement which has proprietary intellectual property rights can be widely used for distributed sensing and is significative for the engineering application.