Research On Microfiber-based Refractive Index And Relative Humidity Sensors

That microfiber is invested with the advantages of lightweight,strong evanescent field on its surface and high stability of signal transmission contributes to microfiber becoming one of the most popular waveguides in optical sensing area.In addition,microfiber sensors are with the assets of resistances to high temperature,corrosion and electromagnetic interference,and they could be operated in distance.By far,numerous optical sensors based on microfibers have been studied for the sensing of different physical parameters including refractive index,temperature,humidity,strain,magnetic field and so forth.Among these sensors,the quantified detections of the refractive index(RI)and relative humidity(RH)play an indispensable role both in the human productive activities and in our daily life.Therefore,to develop the highly accurate humidity and refractive humidity sensors is a necessary and significant work.In this paper,accordingly,two kinds of sensors based on microfiber were demonstrated for sensing refractive index and relative humidity respectively.The main works of this paper are list as follow:1.Through tapering a single mode fiber,a microfiber with micro-arched transitions was fabricated by controlling the variances like the tapering speed;tapering time and the relative position of fiber to the heating flame at the same time.The introduction of the micro-arched transition triggers the increase in the proportion of high order modes propagating in the microfiber,which is responsible for the trend that a visible interference pattern would be attained by the microfiber even with a small diameter.Even though the diameter of the microfiber was below 6 ?m,the power extinction in the transmission spectrum was still larger than 10 dB.2.The sensing characteristic of refractive index of the microfiber with micro-arched transition was studied.In our experiment,the relationship between the diameter of the microfiber and sensitivity of it was systematically investigated,and it showed that the decrease in the diameter brought about the increase in its sensitivity.Reducing the diameter of microfiber to 5.6 ?m,we achieved a high RI sensitivity of 2739.8 nm/RIU in the RI range of 1.333–1.3580.Moreover,higher RI sensitivities of 4515.7 nm/RIU and 12,620.0 nm/RIU were attained in the measured ranges of 1.3705–1.3905 and 1.3940–1.3980 respectively.3.The detection of concentrations of protein solutions by a microfiber with micro-ached transitions was demonstrated as one example of its applications.A sample with high RI sensitivity was used to study its response to the concentrations of the bovine serum albumin(BSA)solutions.As the result showed,the sensitivity of the sensor to BSA solutions was 0.3 nm/(mg/mL).4.A high-sensitivity relative humidity(RH)sensor was made by coating the microfiber with a three-dimensional graphene network(3-DGN).The 3-DGN was fabricated by self-assembly of polystyrene(PS)microspheres which were each covered by reduced graphene oxide(rGO)nanosheets.3-DGN serves as the active material.Absorption of water molecules by the 3-DGN layer increases the effective refractive index of the 3-DGN cladding,leading to the decreasing of optical transmission power.Due to the high surface-to-volume ratios and the porous features of of the 3-DGN,which enhance its interaction with the water molecules,this kind of sensor shows great potential in getting high RH sensitivity.5.The response of humidity,the stability and repeatability,and the temperature characteristic,of the microfiber coated with 3-DGN cladding were studied.It turned out that the sensor achieved a high linear humidity sensitivity of-0.224 dB/%RH over the range 50.5–70.6 %RH and-4.118 dB/%RH over the humidity range 79.5–85.0 %RH.Beside,the sensor displayed excellent stability and repeatability.It also insensitive to the change of temperature.6.Two samples(S1)and 2(S2)with different humidity sensitivities were used to investigate response and recovery times of the relative humidity sensor.The humidity sensitivities of S1 and S2 were-2.02 dB/%RH and-0.57 dB/%RH,respectively.The result showed that the response time of S2 was phenomenally shorter than that of S1.The response time of 4.0s of S2 was achieved as the surrounding relative humidity changed from 71.7 %RH to 85.6 %RH.