Regulation And Application Of Fiber Based F-P Cavity With An Asymmetric Spectrum

Devices based on Fabry-Perot(F-P)interferometer have important applications in sensing fields,and the performance of a fiber F-P sensor largely depends on the spectral characteristics.The interference spectra of the common F-P cavities conform to the standard Lorentzian profile,and the linear spectral region is only about π/2 phase range at the orthogonal operating point.The linear spectral region adopted for intensity demodulation of the sensor is limited.Nevertheless,there is little research on the spectral characteristic,such as the asymmetry shape which are actually often observed in practical experiments.In this thesis,the theoretical and experimental studies of a optical fiber F-P interferometer are presented.The innovations of this thesis are as follows.A theoretical model affecting the spectral shape variation is established,and an analytical formula governing the shape of reflection spectrum is given.By introducing a composite film on the end of the fiber,the asymmetric spectrum of the fiber based F-P cavity can be regulated through adjusting the structure parameters of the composite film and the coupling efficiency related to cavity length.A miniature of fiber F-P cavity with sawtooth spectrum is proposed for static pressure and dynamic pressure sensing based on a dual-wavelength intensity demodulation scheme,which have been demonstrated to break through the contradiction between sensitivity and response range of traditional sensors.The main research work of this thesis are listed as follows:1.Theoretical model and analytical formula governing the spectral variation of a fiber-based F-P cavity are established.The propagation and interference of light beam in a standard F-P cavity are analyzed theoretically,and the theoretical model affecting the spectral shape is provided.The influence of fiber tip as a reflection surface of the F-P cavity on the interference spectrum is analyzed,and the analytical formula predicting the reflection spectrum of F-P cavity is given.The law of spectral shape change is summarized.By adjusting the materials and structural parameters of the composite film,the analytical parameters in the analytical formula can be adjusted,which further affect the shape of the reflection spectrum of the fiber F-P interferometer as demonstrated by the simulation results.2.Experimental preparation of a fiber-based F-P cavity with asymmetric spectral and its sensing head are introduced.Two kinds of composite film of Pt/Ge and Au/Cr were prepared on the end surface of fiber respectively,together with which the fiber FP cavity is formed with a gold film mirror.The thickness influence of the two kinds of composite films on the spectral shape of the F-P cavity is studied.The assembly of fiber and circular sensing film with different diameter is proposed for preparing the miniature of fiber optics sensor based on F-P cavity exhibiting saw-tooth spectrum.3.The sensing performance of the miniature of fiber optics sensor with saw-tooth spectrum is carried out.The static and dynamic pressure testing system based on a dualwavelength intensity demodulation scheme are built to characterize the sensing performance of fiber F-P interferometer with saw-tooth spectrum.The experimental results show that,compared with the traditional F-P cavity,the proposed fiber F-P cavity has two different spectral regions,which are steep region and smooth region but having large linear spectral range.The simultaneous demodulation of these two spectral regions can help to achieve both high sensitivity and large response range,which can break through the restriction between sensitivity and response range of traditional sensors to a certain extent.