Research On Optical Fiber Fabry-Perot Sensor System Based Microwave Demodulation

As a typical optical fiber single-sensor with excellent performance,the fiber-optic Fabry-Perot(F-P)sensor can accurately measure strain,pressure,refractive-index and so many other parameters.Due to the wide range of reflected spectrum of the fiber-optic F-P sensor,it is difficult to rely on wavelength division multiplexing to realize large-capacity multiplexing.Therefore,the large-capacity multiplexing technology for the fiber-optic F-P sensor has become a problem to be solved.Focusing on the above problems,the thesis is based on the sensing concept of optical carrier based microwave interferometry and propose a set of microwave photonics sensing system,that successfully demodulated the fiber-optic F-P sensor and realized large-capacity multiplexed fiber-optic F-P sensors.The outline of main work as follows.(1)The sensitivity of the conventional fiber-optic F-P sensor depends on the length of F-P cavity and is the same with the sensitivity of the in-line fiber etalon,so that the sensitivity of F-P sensor is basically a constant value.In order to overcome the sensitivity limitation of conventional fiber-optic F-P sensors,a long active length,arbitrary length of F-P cavity,and low intrinsic temperature sensitivity fiber-optic F-P sensor is implemented in this research,which has more advantages than traditional F-P sensor,such as faster and easier fabrication,higher reproducibility,capacity of mass production and excellent performance in sensing.(2)According to the fundamental theory of microwave and light about transmission and interference and combining their advantages,it established that the sensing concept of optical carrier based microwave interferometry.Through the principle of demodulation and simulation fully illustrate the feasibility of the scheme.On the basis of it,we demonstrated a microwave photonics sensing system with a signal-to-noise ratio of 120 dB and a spatial resolution in centimeters and introduced the key components in the system.It provides some outstanding features,including remarkable signal quality,insensitivity to the types of optical waveguides,low polarization dependence and relieved fabrication requirements.(3)Taking advantages of a microwave photonics sensing system,two typical fiber-optic F-P sensors are used to demonstrate the demodulation and multiplexed performance of the system.As for optical fiber extrinsic Fabry-Perot sensors,have high sensitivity and easy fabrication,and the parallel multiplexed demodulation method can be used to complete multi-parameter sensing measurement at the same time.For optical fiber intrinsic Fabry-Perot sensors,which cannot be demodulated by the traditional methods.The microwave photonics sensing system can not only demodulate it well,but also realize the large-capacity cascaded sensing.We experimentally validated the scheme using 13 cascaded weak-reflection optical fiber intrinsic F-P sensors.The microwave signals are used to quickly,synchronously and accurately resolve the position and reflectivity of each sensor along the optical fiber,and also demodulate required information from a large number of sensors in time.The reported method has the advantages of multiplexing much more fiber-optic F-P sensors because it is not limited by the bandwidth of the optical source,and it is expected that the proposed multiplexing technique may find many applications for quasi-distributed and distributed measurement of various physical,chemical and biological parameters.