Novel Fiber Fabry-Pérot Interferometric Sensing Structure

Fiber Fabry-Perot interferometric (FFPI) sensors have been widely used due to their ease of fabrication, miniature size, low cost, high sensitivity, and wide range of measurement. However, direct measurement of thin films is limited in the FFPI technique due to the difficulty of immobilization of thin films onto the tip of receptor by traditional Fabry-Perot (F-P) cavities fabricating method. So, a novel method of F-P cavity fabrication is introduced based on the interfere principle for monitoring the thin film immobilization.In this work, light interference principle and the working principle of F-P cavity are introduced in detail, and the classification, feature and application of optical fiber F-P sensors are also mentioned. Parameters of intrinsic Fabry-Perot interferometric structure are analyzed and calculated based on interference principle, and the F-P cavity length is offered.The detailed fabrication and the calculation of cavity length are the important part of this work. The fabrication method adopted is getting internal mirror by using wet etching and fusion splicing on cleaved perfect optical fiber tip. The half-wave loss problem is avoided effectively using this structure. Mixing concentration of etching solution and etching time are selected after repeated experiment, and more ideal internal mirror is obtained by repeated adjustment. Then the sensor tip which forms the other reflecting mirror is cleaved under the microscope device.Compared with intensity demodulation method, phase demodulation method which is more complicated is adopted in the signal processing system. Phase demodulation method is a widely used method currently for that it is with high precision and little influence by light intensity. It can also achieve the demodulation for sensing multiplexing signals. Double wave crest algorithm is used to calculate the F-P cavity length based on F-P interfere principle. And the algorithm is realized by MATLAB program.After the fabrication of the sensing structure and the demodulation of signals, bio-material RNA is immobilized on the sensing tip. The migrated phase is caused by the F-P cavity length which is affected by the thickness of bio-film. Thus, the immobilization of the RNA onto the sensing tip can be obtained by the phase migration.