Sensing Applications Of High Order Whispering Gallery Modes

Whispering gallery mode(WGM)microcavity has the advantages of high Q factor,high energy density,and easy integration.It has important application value in the field of sensing.At present,WGM microcavities have been widely used in sensing fields,such as temperature sensing,electromagnetic field sensing,gas sensing,and biological sensing.However,in these works,low order WGM with Lorentz lineshape is mostly used for sensing.Compared with traditional low order modes,higher order modes generated by the thin fiber taper coupled microcavity have the advantages of large energy fraction in the liquid core and long penetration distance in the liquid core,so they can greatly enhance the interaction between light and matter.It is expected that the high order modes used for sensing will greatly improve the sensitivity of the sensors and the sensors will be simple and practical.In this paper,we study the sensing application of high order WGM,the details are summarized as follows:Starting from the eigenequation of WGM,the electric field intensity distribution,basic parameters,and sensing parameters of different order modes were analyzed by numerical simulation.Finally,the optimization scheme for the performance of high order mode sensing is analyzed from the perspectives of microcavity size and material.We use thin fiber taper(?1 ?m)coupled WGM microcavity to excite high-order modes for high-sensitivity temperature sensing.In the optofluidic ring resonator,to improve the sensitivity,we use ethanol with higher thermo-optic coefficient as the liquid core,and at the same time,use high-order modes excited by thin fiber taper coupled microcavity to conduct temperature-sensing research.The experimental results agree well with the theoretical results,and the radial order number of the mode used for temperature sensing is estimated to 10.In addition,the temperature sensor can also be used as a temperature tunable optical filter.If it is placed in a ring fiber laser,the temperature-tunable laser can be generated,which can also realize temperature sensing.Compared with the direct transmission spectrum sensing method,the detection range of the laser sensing method is relatively narrow,but because the linewidth of the laser is much smaller than the linewidth of the transmission spectrum,the detection limit is significantly improved.Based on the previous temperature sensing principle,we use thin fiber taper coupled microcavity to excite high-order modes for optical power sensing at 980 nm band.We injected the erbium ion gain medium solution into the microcavity,and coupled the 1550 nm band signal light and the 980 nm band pump light into the microcavity at the same time through a wavelength division multiplexer(WDM).Due to the principle of non-radiation transition,the pump light excites the erbium ions to generate heat and thus the temperature of the liquid core will rise.Then,the two lasers are separated by a WDM.Based on the high-order mode temperature sensing principle,the pump light power is sensed by monitoring the signal light transmission spectrum.Based on the previous temperature sensing principle,we use thin fiber taper coupled microcavity to excite high-order modes for liquid flow rate sensing.We use a temperature control platform to heat the microcavity so that it has a temperature difference from the surrounding environment,and then continuously push the liquid into the microcavity.The flowing liquid changes the temperature of the microcavity wall and the liquid core.Based on the principle of high-order mode temperature sensing,the liquid flow rate is measured by monitoring the transmission spectrum: when the flow rate is low,the mode is blue shift;when the flow rate is high,the mode is red shift.