Optical Fingerprint Sensor Based On Grating Couplers

Portable electronic devices,such as wearable devices,tablets and cellphone which have played increasingly important role in people's life are developing rapidly in recent years.With the increasing demand for information security,fingerprint recognition technology provides beneficial help for financial and privacy security.Their working principle is that,while finger touches,the sensor detect the corresponding physical parameter and converted it to electric quantity,which contains the fingerprint information.There are three types of fingerprint sensor: optical,capacitive and ultrasonic.With the people's pursuit of high efficiency,light weight and low power consumption,the optical fingerprint sensor which is featured with high resolution,low cost and transparent is attracting more and more attentions in academia and industry.Taking full advantages of light coupling and guiding effect of gratings,we proposed a slim and transparent device based on grating couplers.The collimated light is coupled into the waveguide by the in-coupled metallic grating with a propagation angle larger than the total internal reflection angle(TIR)between air and waveguide at first.Then,in the detection region,the light which cannot satisfy TIR between finger and waveguide is absorbed by the ridges of the finger touching the waveguide surface.In this way,the information of the fingerprint is carried by the reflected light where the dark zones corresponds to the ridges.At last,the light is coupled out by the out-coupled metallic grating and is captured by a camera.The main research works are listed below:The first part is the theory of grating-waveguide coupling.We analysis the influence of parameters to the coupling efficiency,including grating pitch,etch depth,duty cycle,height of aluminum film and waveguide.We emphasized on the introduction of wave vector diagram and Rigorous Couple Wave Analysis(RCWA).Furthermore,the process of the light propagating in the waveguide and the way how it scans the finger surface are simulated by the commercial software Light Tools7.2 which traces the light beams based on the Monte Carlo Method,and we get the optimized parameters.The second part is the device fabrication by laser interference lithography and e-beam evaporation using parameters set in the simulations.Then,we measured the coupling spectra which match the simulations relatively well.Furthermore,we captured the fingerprint image by building up corresponding optical path platform.The feasibility of a transparent optical fingerprint sensor is demonstrated.Finally,the fingerprint image was processed with preprocessing algorithms to enhance the quality fingerprint image.Thus we have envisioned that the proposed sensor is applicable for future portable device.