The Study Of Mechanically Induced Long Period Fiber Grating In Polarization-maintaining Fiber

Long-period fiber grating (LPFG) is a kind of transmission grating. Unlike fiber Bragg grating (FBG), LPFG can couple fundamental guided mode to discrete co-propagating cladding modes. LPFG have many advantages such as easy fabrication, low insertion loss, low back-reflection, polarization independence and integrated volume etc. They can act as fiber mode converters, fiber band-rejection filters, EDFA gain flattening filters and fiber sensors. All these characters present a broad application field for LPFG in the field of optical fiber communications and fiber sensors.The optical properties of the optical fiber core would be changed eternally when it is exposed to UV light. Using this photosensitive property, most of the LPFG are made to have a periodical index change along the fiber core. This traditional writing method of the LPFG is popular, lots of the gratings can be achieved easily. Once the gratings have been fabricated, their spectra can hardly be tuned. These reasons lead to the grating is poor flexibility. In this paper, via the photo-elasticity effect, we demonstrate that a flexible technique that periodically wrapped brass wires can be used to produce a periodical stain, mechanically inducing a LPFG in a fiber. This method is simple, tunable and reconfigurable.In this thesis, mechanically-induced LPFG in PMF is investigated. Its fabrication method, general characteristics and potential applications are discussed through the theoretical and experiment. The main contents of the thesis are as follows: Theory:1. Introductions are given to the recent developments of the fiber gratings and the applications of the different kinds of the fiber gratings. Especially the fabrication methods, fabrication principles and their applications.2. Based on the Coupling Mode Theory, theoretical analysis of the principles of operation for LPFG and LPFG in the PMF were made. The main process of simulating transmission spectral characteristics of above them, discussions are given on the influence of the period of LPFG, the length of LPFG, and so on, thus establishing the basis for writing LPFG.Experimentation:1. A novel method to mechanically induce LPFG with brass wires is proposed. Because the index modulation period in a LPFG can be as large as hundreds of micrometers, it is possible to induce it mechanically. By pressing the fiber between a periodically brass wires grooved plate and a flat plate. Via the photo-elastic effect, the pressure points induce a periodic index modulation.2. Discussions are given on the influence of the applied pressures and the period of LPFG to the transmission spectra. Higher pressures result in stronger mode coupling and thus generally in deeper notches. When the pressure was increased, the attenuation of the notch decreased. Comparatively, when the pressure was decreased, the attenuation of the notch increased. The position of the peak loss wavelength is tunable by adjusting the angle of the fiber on the brass wires, the number and the diameter of the brass wires. Compared with the traditionally methods, this method is erasable, reconfigurable and inexpensive. Its spectrum can be tuned easily by tuning the applied pressures and by changing the different brass wires. This new type of grating would have potential applications in gain flattening as band-rejection filters and in interrogation of FBG lying on the slopes of the resonant band.3. Based on above experiment, we changed the type of fiber, using the polarization-maintaining fiber (PMF) and made the novel LPFG. Due to the intrinsic stress-induced birefringence in PMF, the x- and y- polarization modes are split and each one has a different effective index. Different effective refractive indexes could lead to different central wavelength of gratings. In other words, the spectral response of the LPFG written in PMF consists of two combs corresponding to orthogonal linear polarizations. The spectral interval between the combs could be changed by varying the fiber birefringence. Because of the particularity of the LPFG in the PMF, we proposed some special applications in fiber sensors.