Research On Fabrication And Characteristics Of Long Period Fiber Grating Based On Twin-core Structured Fiber

Through a long process of evolution, the development of human society has now entered a highly information-oriented era. The information technology has become the major trend of contemporary development, and the degree of information has also become an important characteristic to evaluate the level of the country's modernization. With more and more high-technology products going into people's lives, there is more demand for information on high-speed and large-capacity information for transmission. These have been the targets for many researchers. As an important part of the optical communication field, optical fiber and various types optical device play a main role in the rapid development of information technology. Long period fiber grating has been widely used in the field of optical communication and sensing applications due to its low insertion loss and easy manufacture technics.The objective of this work is to study the fabrication method of long period fiber gratings using improved CO2 laser exposure technology for traditional single core single mode fiber and special twin-core fiber. The transmission spectrum and sensing characteristics of these gratings have also been investigated and demonstrated. The main works include:1. We have designed a set of three-dimensional fiber-optic rotary coaxial control system to meet different needs of long period fiber grating fabrication for single side exposure, multi-side exposure and rotating exposure using CO2 laser pulse, and achieve the testing immediately for strain and bending after the fabrication of LPFG.2. The symmetric exposure method for long period fiber grating fabrication is proposed and demonstrated, it put CO2 laser focused on one side of the fiber cross section and then the other side with the same energy and exposure parameters. This make the fiber refractive index change periodicity twice in order. Through the observation of transmission spectra, we find that the spectra will be weaken at the second side with the exposure time increasing, and the resonant wavelength will shift to long-wave direction.3. The long period fiber grating fabricated by asymmetric thermal pulses stress method and it transmission spectra characteristics and sensing characteristics are investigated and demonstrated. According to the theoretical analysis for the action of asymmetric thermal pulse stress, we obtained that the resonant peak of LPFG will shift to short or long-wave direction with different angle of two asymmetric exposure directions. The experimental results were agreed with the theoretical analysis, and the critical angle difference for the resonant peak change trend is also obtained. These characteristics can be used to regulate and control the wavelength of LPFG in the writing process.4. Based on the couple-mode theory, a theoretical mold for discripting the long period fiber grating in an offset core fiber is given. We have simulated the cladding refractive index changes with different angles. Based on this result, the twin-core fiber long period fiber grating has been proposed and fabricated, this device includes two parallel long period fiber grating integrated in one optical fiber. The couple-mode theory model of the twin-core long period fiber grating is established and the spectra characteristics are also investigated by simulation and experiments.5. The twin-core long period fiber grating fabricated using CO2 laser are carried out by single side exposure and prestressed torsion exposure method, respectively. We obtain that the transmission spectrum of the twin-LPFG fabricated by sigle side exposure is strongly depends on the exposure direction around the twin-core fiber and the prestressed torsion exposure method can significantly enhance the grating writing efficiency.