Research On Refractive Index Sensing Based On Subwavelength Grating Microring Resonator

With the development of science and technology,the development of silicon-based integrated photonics is in full steam,because silicon-based photonic devices are compatible with microelectronics processing technology,have small size and are easy to integrate.Especially in the field of optical sensing,silicon-based integrated devices meet the development needs of the next generation of on-chip sensing systems and show great development potential.As one of the basic components of silicon-based device library,silicon-based microring resonator is favored by scientists in the field of sensing because of its compact structure and excellent resonance performance.However,in traditional strip waveguide microring resonators,most of the light is confined inside the waveguide and the external evanescent field is weak,so the interaction between the light and the substance to be measured is weak,resulting in low sensitivity of the device.By modifying the waveguide structure of the microring resonator,the interaction between the light and the substance to be measured can be effectively enhanced,such as changing the strip waveguide to slot waveguide,photonic crystal waveguide,and etc..The sensitivity can be effectively improved in this way,but the detection range is limited by the free spectral range of the microring resonator.In order to broaden the detection range,vernier method and dispersion method are proposed to form the envelope spectrum for sensing.However,the present sensitivity is still low.Therefore,it is urgent to study the refractive index sensor with both high sensitivity and wide detection range.A microring resonator based on subwavelength grating structure is proposed for sensing in this paper.On the one hand,the subwavelength grating waveguide can concentrate the light in the substance to be measured and thus enhance the interaction between the light and the substance,so as to improve the sensitivity.On the other hand,the loss and dispersion characteristics of the subwavelength grating waveguide can be flexibly adjust and used to achieve the envelope spectrum.The sensing function can be realized by tracking the drift of the envelope spectrum with the change of the refractive index.This scheme can effectively expand the detection range and remove the limitation of the free spectral range of the microring resonator.To verify the above scheme,this paper investigates the mode field,loss,and dispersion properties of the subwavelength grating waveguide by using the plane wave expansion method and the finite difference time domain method.Then we study the loss characteristics and coupling characteristics of the subwavelength grating microring resonator,namely the round-trip amplitude transfer factor and the amplitude self-coupling coefficient.The transfer matrix method is used to obtain the envelope spectrum of the subwavelength grating microring resonator.Finally,the refractive index sensing is realized by tracking the change of the envelope spectrum with the refractive index of the substance to be measured.In this paper,the envelope spectrum of subwavelength grating microring resonator is used to achieve a sensitivity of 613 nm/RIU and a wide detection range of 0.5RIU.In order to analyze the sensing performance of the subwavelength grating microring resonator,we firstly analyze the influence factors of the round-trip amplitude transfer factor and the amplitude self-coupling coefficient by using finite difference time domain method.Then,we analyze the influence of the round-trip amplitude transfer factor and the amplitude self-coupling coefficient on the sensing performance.Finally,we give the sensitivity formula of the subwavelength grating microring resonator.After analysis,we can conclude that the sensitivity is not only related to the dispersion characteristics of the round-trip amplitude transfer factor and the amplitude self-coupling coefficient,but also related to their changes with the refractive index of the substabce to be measured.This formula provides a solid theoretical basis for improving the sensing performance of the subwavelength grating microring resonator.The proposed subwavelength grating microring resonator can be used to realize the refractive index sensor with both high sensitivity and wide detection range,which can provide a theoretical basis for the development of integrated refractive index sensor,and a strong candidate for the key component of next generation on-chip sensing system.