Study Of Cladding Mode Resonant Wavelength Tuning Sensitivity Of Fiber Gratings

With the growth of the optical networks, the design of the switch node is more and more important. The being design has limitation in many aspects. Since fiber grating can connect with the optical network perfectly and has a small cost. Therefore, it is highly meaningful to design the switching node based on fiber gratings.Fiber gratings can be divided into fiber Bragg grating and long period fiber grating. Both can have resonant peak in its transmission spectrum. This dissertation mainly studies wavelength tuning of fiber gratings based on cladding mode coupling. The object of the study is to reach a large tuning sensitivity and a small bandwidth. Meanwhile, it also mentions some ways to improve the spectrum.Firstly, this thesis analyzes the refractive index modulation model of asymmetric LPFG in detail. The erosion experiment shows that through pulse broadening, the laser will induce an average index change in the fiber cladding and a periodical index change in the fiber core. This explanation also accords with the mode splitting phenomenon which is caused by high order cladding modes coupling. Moreover, the experiment also shows that by erosion part of the fiber cladding, this LPFG will have a larger resonant dip and a smaller 3dB bandwidth.Then, this thesis theoretically analyzes the resonant wavelength tuning sensitivity of LPFG cladding modes coupling. The study shows that for a certain wavelength range, the LPFG should have a small cladding radius and meanwhile a large cladding mode order which is controlled by the grating pitch. Besides, the resonant dip will have a large range shift towards the short wavelength when the environment refractive index changes around the modes transition point. The result shows that by selecting a proper environment refractive index point, the LPFG can work as a high sensitive wavelength tuning device.Finally, in order to reach an even smaller bandwidth, the FBG based cladding mode resonant wavelength tuning is studied as the extension of the LPFG wavelength tuning. The simulation result shows that after reducing the cladding radius and inducing with the HRI layer and mode transition point, FBG can be used to sensing a large scope of environment refractive index.