The Pressure Sensing Research On Long-period Fiber Grating

The fiber grating is a kind of diffraction grating which has axial periodic variations of refractive index in fiber core. It possesses many advantages such as small volume, good ability of wavelength selectivity, freedom from nonlinear effects, capability of antielectromagnetic interference, and low loss when being connected to fiber communication systems, etc. And so it attracts great attentions and has been a hot topic in the fields of optical fiber communication and fiber sensing. Many industrial control environments, such as petrochemical pipeline pressure, water level, oil well pressure, gas pipeline and so on, needs long-term and real-time detecting about the pressure. Nowadays,further research on longperiod fiber grating(LPG) as a pressure sensor is more and more popular. Based on mechanically induced long-period grating(MLPG) as pressure sensor, a structure which is combined by LPG and MLPG is proposed. Because of the interference principle, the sensitivity of pressure sensor can be greatly improved. In this thesis, we completed the following research works and also get several innovative achievements.Firstly, the classification and corresponding spectral characteristics of the fiber gratings were introduced, and also the application of fiber grating in communication and sensing were summarized.Secondly, based on Maxwell’s equations, we introduced the mode coupling theory of fiber gratings in details and explained the reason of grating mode coupling. The coupling theory of long-period fiber gratings was derived and its spectrum was also simulated with software.Thirdly,a variety of fabrication methods of long-period fiber gratings were introduced in this chapter, including phase mask writing, amplitude mask writing, point-by-point UV writing, grooving corrosion writing, and mechanical induced writing et al. And the comparative analysis on the advantages and disadvantages of these methods also is done.Besides these, the system of making long-period fiber gratings by using UV laser was investigated, which includes setting up the UV light path and the He-Ne light path used as collimating system. Especially, we developed integrated control software and realized the integrated control of the UV laser and the one-dimensional motorized translation stage when writing a fiber grating. With this software we can easily manufacture different kinds of fiber grating by flexibly setting the relevant parameters, and the production efficiency is greatly improved. In addition, the method of making a Long-period grating mechanically was also investigated, and a new type of mechanical induced template which can flexibly change the grating length and period was proposed.Finally, we investigated on the MLPG pressure sensor,and put forward a new kind of Mach–Zehnder interferometer sensor which is made of MLPG and LPG. The spectral characteristics of the above structure and its sensing characteristic to pressure were analyzed in detail. The experiment results show that such a structure can greatly increase the sensor sensitivity.