The Study Of Seawater Salinity Sensor Based On Microfiber Ring Resonator

One of the development trends of the photonic components is miniaturization, soas to the typical transmission waveguide fiber. One of the differences betweenmicrofiber and fiber is that microfiber has evanescent field. Light transmits in thefiber core by total reflection for the traditional fiber while light transmits around theoutside surface of the fiber for microfiber. The exists of evanescent field make it verysensitive to ambient environment. Besides, the easy fabrication, low cost and higherintegration make it ideal component of sensor. Microfiber ring resonator is one of theimportant components based on the microfiber. We use microfiber ring resonator tomeasure the seawater salinity and combine the micro/nano Optics and ocean detection,which provides a new salinity measurement method. The main work of the artical isas follows:In the first part of this article, we introduce the transmission character ofmicrofiber in the seawater environment and obtain the transmission constant andenergy distribution of field mode. Then the character of microfiber ring resonator isstudied, and because of the absorption of seawater, we studied the dependence of Qon fiber diameter, ring diameter and detection wavelength considering the absorptionloss. The results shows: Q increased with increased fiber diameter and ring diameter,and decreased with the longer wavelength.The seawater salinity sensor with weak temperature dependence is proposed inthe second part of this article. First, we build the embedded sensor model, and thecondition of the weak temperature dependence is studied. The size of microfibercontrols the ratio of thermal effect contributions of different parts, and the thermaleffect of whole system reach zero when the microfiber size is appropriate value. Thesensor becomes insensitive to temperature at this moment. MgF2material is chosen tobe the embedding material considering the thermal effect of seawater. Thesensitivity and detection limit of sensor is studied after the weak temperature dependence is realized. The result shows: Higher sensitivity corresponds to smallercoating thickness, smaller microfiber diameter and longer wavelength. The largerdetection limit corresponds to larger coating thickness. After that we proposed twoother salinity sensors-the bare microfiber ring resonator and the microfiber ringresonator embedded in Teflon-in order to study the effect of embeding matirial onthe ring resonator sensor. We make a comparison among three sensors. We concludethat the sensitivity of bare microfiber ring resonator and the microfiber ring resonatorembedded in MgF2has no much difference and we suggest to choose the later oneconsidering protecting the ring resonator in the practical application. By optimizingparameters of the MgF2-embedded salinity sensor, sensitivity and detection limit canreach to0.02nm/‰and0.18‰when the wavelength is900nm, the coating thicknessis10nm and diameter is0.8μm respectively. At last, in order to study the effects ofdifferent coating style, we analyze the character of the ring resonator with cylindricalcoating and make a comparison with embedded coating style. The results show bothof the sensitivity and detection limit have been improved and can reach0.03nm/‰and0.13‰respectively.At last, we make experiment study on the performance of ring resonator and thesensitivity is obtained with two different ring resonator style: bare ring resonator andring resonator supported by a bar. The results show that it is effective to use themicrofiber ring resonator to measure salinity.