Study On Two Novel Long-Period Fiber Gratings

The fabrication method, optical characteristics and corresponding applications based on side-written long period fiber grating (LPFG) and sone gratings with special waveguide structure, such as ultra long period fiber grating (ULPFG), edge-written long period fiber grating (E-LPFG) and so on, induced by high frequency CO2 laser pulses were studied in the past several years, and the research results obtain the initial recognition of domestic and foreign experts. In this thesis, the long period fiber grating with rotary refractive index modulation (R-LPFG) is further researched, and besides, a novel LPFG with special waveguide structure is designed and fabricated. The characteristics and the applications in fiber sensing of the above fiber gratings have been studied by experimental and theory. The main work is given as follows:1. The long period fiber grating with rotary refractive index modulation (R-LPFG) is further studied, and the beat frequency theory is proposed to analyze the transmission characteristics and environmental response characteristics of R-LPFG, for the first time to our knowledge: (1) For R-LPFG with small twist period the resonance peak will be split into two, whose wavelength separation decreases with the twist period; (2) The temperature characteristics, strain characteristics and twist characteristics are studied experimentally, and some particular spectral properties are discovered.2. An ultra-long-period fiber grating with periodic grooves structure (G-ULPFG) is reported for the first time, and the fabrication method, structure feature and environmental response characteristics are studied experimentally: (1) periodic grooves with different depth are carved on the fiber by controlling the exposure location and power value of high frequency CO2 laser pulses; (2) The experimental results of the temperature, strain and environment refractive index characteristics of G-ULPFG show that different resonant peaks of the G-ULPFG have different temperature, strain and refractive index sensitivities, which is similar to the characteristics of ULPFG., but the environmental response sensitivities of G-ULPFG are enhanced due to the edge-groove structure, and in particular, the strain and refractive index sensitivity is much larger than that of conventional ULPFG.3. The applications of the above fiber gratings are primarily studied: (1) Some environmental parameters can be measured simultaneously by using R-LPFG or G-ULPFG; (2) A highly sensitive humidity sensor based on G-ULPFG can be realized due to its high refractive index sensitivity, and the principle is as the following: The G-ULPFG is coated with a new nano-composite hydrogel, which has strong water capacity. This kind of hydrogel can absorb water vapor in the air substantially to make its own refractive index change, and will induce the resonant peak shift of the ULPFG, realizing humidity measurement.