The SFDM/WDM Method Of Fiber-Optic Fabry-Perot Sensors

As an important member of optical fiber sensors family, fiber Fabry-Perot interferometric sensors are being intensively studied as they have many advantages over conventional electrical sensors, such as immunity to electromagnetic interference, capability of responding to a wide variety of measurands, high resolution, high accuracy, small size, etc. Fabry-Perot sensors have been commercialized and widely used in health monitoring for composite materials, large civil engineering infrastructures (e.g. bridges and dams), space aircrafts, airplanes, oil fields, engines, etc., which would lead to the realization of so-called smart materials and structures. However, Fabry-Perot sensors have one intrinsic disadvantage i.e. they are difficult to multiplex. As the result, the cost of the sensor system with a large number of Fabry-Perot sensors is normally high. Hence, we would find some ways to overcome the problem.First,a method based on the coarse wavelength-division-multiplexing technology (CWDM) is proposed to overcome the intrinsic disadvantage of Fabry-Perot sensors. Each Fabry-Perot sensor will occupy a wavelength window predetermined by a CWDM device. This system improves the multiplexing capability of Fabry-Perot sensors by a factor of four. A strain measurement accuracy of better than±2.5μεhas been achieved, which can meet the practical requirement well.A novel method based on the combination of coarse wavelength division multiplexing (CWDM) and spatial frequency division multiplexing (SFDM) is proposed to overcome the disadvantage of Fabry-Perot interferometric sensors. In this work, more than 10 Fizeau strain sensors can be multiplexed using SFDM in each wavelength channel predetermined by CWDM. This method can multiplex a large number of sensors of up to 100 in principle and hence can reduce the cost of the whole sensor system greatly. Also, this hybrid method can take the advantages of both SFDM and CWDM when applied to the multiplexing of a large number of sensors, i.e. excellent multiplexing capability when using SFDM, and good efficiency in using every part of the optical power of the broadband source with CWDM.In order to enhance the number of multiplexed F-P sensors to realize strain monitoring of large engineering structures, intrinsic fiber Fabry-Perot interferometric sensors (IFPIs) are taken into account in this work. The cavity length of an IFPI can be as long as tens of mm hence the sensor number that can be multiplexed is much more...