Study On Optofluidic Refractive Index Sensor Based On Large Air Hole Polarization-maintaining Microstructured Optical Fiber Rocking Filter

In recent years,bioscience and biotechnology have developed rapidly.The effective acquisition of information on the composition and content of biochemical substances is a necessary prerequisite for research in the field of bioscience,which relies on efficient and accurate biochemical analysis techniques and tools.The optical biochemical sensor has the advantages of high accuracy and fast response.It can meet the different needs of the biochemical sensing field through the construction of various optical sensor structures,and is favored by researchers.Microfluidic technology is an advanced technology in the field of biochemical analysis.Compared with traditional analytical techniques,it has many outstanding advantages,such as very small sample consumption and fast response.At the beginning of this century,the cross-curricular interests of microfluidic technology and photonics gave birth to the emerging research field of optical microfluidics.The design idea of the optical microfluidic sensor is to consider how to construct the microfluidic channel when designing the optical sensing structure,integrate the optical structure with the microfluidic channel to form a platform for interaction between the optical signal and the microfluids,and realize the measurement of change in biochemical composition of the microfluids.Photonic crystal fiber,also known as microstructured fiber(MOF),is a major innovation in the field of fiber optics in recent years,and it has many excellent properties that are not found in conventional fibers.Especially,the air holes whose cross section are constant in the axial direction of the fiber can directly serve as microfluidic channels,so that the light and the microfluids can interact directly in the optical fiber,which provides a new opportunity and possibility for developing novel optical microfluidic sensors.Compared with traditional optical microfluidic sensors,the microstructured fiber-based microfluidic sensors have advantages of long interaction distance between the light and the microfluids,compact structure,and easy coupling of optical signals.In this work,we propose a novel optofluidic refractive index sensor based on polarization-maintaining microstructured optical fiber Rocking Filter.In the polarization-maintaining microstructure fiber,a periodic back and forth torsion structure is carve into along the axial direction to realize the resonant coupling of the orthogonal polarization modes in the optical fiber.Through polarization detection,we can obtain a transmission spectrum similar to that of a long-period grating,so that obtaining a polarization rocking filter.Based on the coupled mode theory,we simulated the transmission spectrum of the device.A small C-shaped fiber is respectively connected to the connection between the two ends of the device and the single mode fiber,and the tested liquid is flow in and out from the air hole of the microstructure fiber without affecting the optical signal coupling between the single-mode fiber and the microstructure fiber.An all-fiber optical microfluidic refractive index sensor is obtained.We simulated the modal birefringence dispersion curve of the polarization-maintaining microstructure fiber when the refractive index of the microfluids was changed around 1.333 using the finite element analysis method,and then obtained the transmission spectrum of the device for different refractive index values of the microfluids.By tracking the wavelength shift of the spectra,we obtained a very high refractive index sensitivity of 7196.4nm/RIU.We also numerically demonstrated that the optofluidic refractive index sensitivity can be improved to 16754 nm/RIU when the MOF is tapered to half of the initial diameter.