Study On The Characteristics Of Mode Splitting Induced By The Coupling Of Whispering Gallery Mode Microspheres

Whispering Gallery Mode(WGM)microcavities have attracted a lot of researchers in the field of sensor design due to its advantages of high Q value,small modal volume and geometric configuration.At present,WGM microcavities have been widely used in temperature sensing and solution concentration sensing,but the detection process may still be plagued by low reliability,noise crosstalk and other problems.Dual resonant microcavities coupling provides a new way to solve these problems.In particular,the mode splitting induced by the coupling of two microcavities provides a new platform for optical sensing.In this paper,the characteristics of mode splitting induced by the coupling of WGM microspheres have been analyzed,the main work is as follows:Firstly,the laser threshold characteristics of WGM microspheres were characterized and analyzed.On the premise of only TM modes,two microspheres with the same resonant peaks were bonded on the substrate by UV curing adhesive with low refractive index to fabricate a coupling bi-spheres system.Secondly,the bi-spheres system was placed under the microscope objective and excited by 532 nm pump pulse light source.The stable mode splitting spectrum could be obtained,and the coherent evanescent field distribution could be obtained by simulation.Then,by heating the bi-spheres system uniformly,it could be observed that the two branches of the mode splitting spectrum show good sensing characteristics,and the sensitivity of multi-wavelength sensing is about 61 pm/°C.It should be noted that the WGM splitting size remains unchanged when the temperature changes uniformly.Later,when we studied the nonuniform temperature tuning of the bi-spheres system,the obvious anti-crossing phenomenon appeared in the mode splitting spectrum.On the one hand,it verified the strong coupling characteristics,on the other hand,it provided a new idea for the thermal uniformity detection of substrate materials.In addition,due to the slit effect,the solution can be retained between the micro-gaps of the dual spheres for a long time.Using the sensitive characteristics of the coupling point of the bi-spheres,different concentrations of ethanol droplets were added to realize the purpose of solution concentration sensing.After adding ethanol drops between the bi-spheres,the "lower branch" of the splitting mode has no obvious wavelength shift,while the "upper branch" has a larger wavelength shift.The simulation results were in good agreement with the experimental results,that is,with the increase of ethanol concentration,the WGM splitting size showed a sensitivity of ?70 pm/M.Compared with the sensing effect of single microsphere,the sensitivity of the bi-spheres system was about 1.4 times than that of single microsphere,and the noise crosstalk caused by temperature fluctuation was eliminated.