Flexible And Transparent Fingerprint Sensing System Based On Grating Waveguide Coupler

Biometric technologies are becoming more and more popular in mobile terminals due to its high security and convenience.In a variety of biometric technology,fingerprint recognition is the most mature and widely used biometric technology,because of slim dimension and low cost.Nowadays,there are capacitive,optical and ultrasonic three typical types of fingerprint sensing technology.Recently,with the pursuit of high-screen-ratio and "fullscreen" display terminals,the opaque capacitive fingerprint sensor module has been abandoned by manufacturers due to it cannot be integrate in display.Subsequently,optical and ultrasonic under-display fingerprint technology were rapidly developed.However,the optical fingerprint recognition can only be used in OLED screen with a small fixed area;the ultrasonic fingerprint recognition has not been widely implemented due to high cost and timedelayed.Meanwhile,the emergence of flexible displays and wearable concept mobile devices indicate that "flexibility" is an indispensable feature of the newgeneration displays.Correspondingly,the fingerprint identification module,as a crucial bio-identification component,also need to be flexible and wearable.Aiming at the development of the bezel-less and flexible terminal devices,we proposed a grating-waveguide-coupled fingerprint sensing system based on polydimethylsiloxane(PDMS),which can capture the fingerprint on the whole touch screen while keeping the transparency of display.Meanwhile,it can maintain the fingerprint sensing function under a certain bending condition.This work provides a useful reference for the fingerprint sensing of flexible and high-screen-ratio display terminals.The main research works are listed as below:A flexible and transparent fingerprint sensing system based on grating waveguide coupler was designed.The characteristics and feasibility of fingerprint sensing on flexible waveguides under bending conditions were first explored and verified by the ray tracing simulation software(Light Tools).The influence of parameters to the coupling efficiency were analyzed by using grating coupling equation and finite-difference time-domain method(FDTD).Based on theories and simulation results,a PDMS waveguide with grating structure coated with high refractive index materials was fabricated and characterized.The measured coupling efficiencies of the gratings match with the simulations well.Experiments have verified the feasibility of large-area fingerprint sensing on transparent waveguides,compared the effects of different bending directions on fingerprint images,and obtained that when the bending axis is perpendicular to the direction of the grating lines,the effect on fingerprint sensing is minimal.Finally,we use image sensor to capture fingerprint images and preprocess them in MATLAB to obtain fingerprint minutiae.Compared with the traditional prism-based technology,the fingerprint minutiae coincidence rate of our designed system is 61%.