Periodic Pressure Type Fiber Filter Research

The long period fiber grating (LPFG) is one kind of fiber grating with the period ranging from tens microns to hundreds microns. The core mode and special cladding modes of the fiber in the LPFG can be coupled, which induces the characteristics of band-rejection filtering with a bandwidth of several nanometers in the grating. On the other hand, the energy transfer between the core modes in the two LPFGs can be achieved if only the two parallel LPFGs are closely placed together, where the characteristics of band-pass filtering can be obtained. Therefore, two kinds of fiber filters with the characteristic of band-rejection or band-pass can be devised. Recently, LPFG-based filters have become the research hotspot because of their strongpoints of low insertion loss, no reflection and easy connections.The long period fiber grating can be induced by imposing a periodic pressure on a fiber, and the method has the advantages of simple structure low manufacturing cost, adjustable coupling strength and good reconfigurability and so on. In this dissertation the period-pressure induced fiber filters manufactured according the method above-mentioned are researched, the main contents of the dissertation are given as follows:1. The mechanism of period-pressure induced LPFGs is analyzed according to the theory of micro-bending loss and ray propagating theory, and the characteristics of the mode coupling and the transmission spectra are studied based on the coupled mode theory in this dissertation. The result show that the periodic pressure imposed on fiber will change the refractive index of fiber because of the periodic micro-bending effect and the elastic-optic effect, to form a periodic pressure induced LPFG. The refractive index modulation in transverse of the fiber is related to the radial coordinate (r) and angular coordinate(φ). And at the same time, the modulation of the refractive index along axis of the fiber has the same period with the micro-bending owing to the pressure applied on the fiber; There are several loss-vale on the transmission spectra of the LPFG, and the depth of which is decided by coupling coefficient between the core mode and the cladding mode. 2. The design of the filters and experimental analysis are presented. The result of the experiment indicate that the performance of the fiber filters is mainly determined by the magnitude of pressure and the grating structure which includes the grating period, grating length, fiber materials and so on, which manifests in that:1) the operating wavelength is proportional to the grating period, and affected by the fiber material;2) the filtering capabilities of the fiber filters are determined by the pressure, the length of the grating and the materials of the fiber;3) the3dB bandwidth is proportional to the operating wavelength, inverse proportion to the periods of the grating, and has a positive correlation with the coupling coefficient between the core mode and the cladding mode;4) the fiber filters have a certain dgree of the polarization dependence.3. The dispersive characteristics and the automatic pressurizing device are discussed. The results indicate that the fiber filters will induce some dispersion when it is inserted into the optical system, but will not have a serious impact on the system; the automatic pressurizing device make the fiber filter become a kind of application-flexible equipment from a simple filter.