Research On Devices Based On Twin-core Long Period Fiber Grating

Multicore fiber(MCF)can use space division multiplexing to increase the transmission capacity of light.The sensors based on multi core fibers have many excellent characteristics,such as small size,compact structure,high integration,multi-parameter measurements.Twin core fiber(TCF),as a kind of multi-core fiber,has become a hot spot in the research fields of fiber sensing and fiber devices due to its relatively simple structure,fabrication process and easy device integration.Fiber grating has many characteristics including high sensitivity,corrosion resistance and electromagnetic radiation resistance.Compared with fiber Bragg grating(FBG),long period fiber grating(LPFG)can more easily detect the change of external physical quantities by coupling the core mode to the forward propagating cladding mode.As a result,LPFG has been regarded as one of the most important optical fiber passive components at present.The research of new optical devices based on special fiber gratings is of great significance in the fields of optical communication and sensing.In this thesis,optical fiber devices have been fabricated based on symmetric twin-core fiber,in which LPFG was written into the twin-core fiber to make the sensor.The sensitivities of the sensor to temperature,stress and bending parameters have been studied.The performance of the sensor has been optimized by improving the write parameters and changing fiber orientation.Then the twin-core fiber was treated with hydrogen to enhance the coupling efficiency of the twin-core fiber for realizing the coupler.Based on the theoretical and experimental studies of the dual-core fiber,the understanding of the fiber devices has been further improved and the work offers a new scheme for the realization of the multi-core devices.The main research work of this thesis and the corresponding research results are as follows:(1)By using the high frequency CO2 laser system,we have successfully written LPFG into the twin-core fiber.A sensor based on the twin-core long-period fiber grating has been fabricated The sensing sensitivity of the two cores to temperature is 70.8 pm/? and 72.9 pm/?,respectively.The sensing sensitivity of the axial stress is-0.2 pm/?? and-0.3 pm/??,respectively.The maximum sensing sensitivity of the bending in all directions is only 0.55 nm/m-1,to some extent with bending resistance.(2)Based on the coupling mode theory and the coupling mode equation of the twin-core fiber,the modes of twin-core fiber have been theoretically studied by use of finite element method,and the corresponding mode field distributions have been given.It is noted that the resonance peak of twin-core LPFG is generated by the coupling of fundamental mode(LP01)and cladding mode(LP11).The hydrogen-loaded the optical fiber has been investigated in detail.The optical fiber photosensitivity has been enhanced and thus more cladding modes have been excited when the LPFG is written.Two identical LPFG have been independently written into the two cores of the hydrogen-loading twin-core fiber.In addition,a grating auxiliary coupler based on the twin-core fiber has been fabricated.The coupling efficiency is up to 48%,and its feasibility has been analyzed according to theoretical simulations and experiment results.The work provides a promising idea to write LPFG independently in multicore fiber and promotes the further development of fiber optic devices.