Study On Femtosecond Laser Fabrication And Characteristics Of Few-mode Fiber Bragg Grating

In recent years,space division multiplexing?SDM?optical transmission communication systems based on few-mode fiber?FMF?and multi-core fibers?MCF?have made tremendous research progress due to the limitation of bandwidth of single-mode optical fibers.Fiber Bragg grating?FBG?is one of the most widely used devices in optical fiber communication.The fabrication of FBG in FMF has great research value for the development of SDM communication system.On the other hand,FBG has many excellent characteristics,such as good wavelength selectivity,small volume,small insertion loss,anti-electromagnetic interference,corrosion resistance,electrical insulation,and quasi-distributed sensing.Based on the few-mode transmission characteristic of the few-mode fiber,FBG fabricated on few-mode fibers has great application value in the fields of fiber sensing and fiber laser.In this paper,based on the femtosecond laser phase mask method and the direct writing method of FBG fabrication,FBG was fabricated on the twin-core few-mode fiber and few-mode fiber.The sensing characteristics of twin-core few-mode fiber grating were studied,and the grating was applied to the bending sensing.Based on the few-mode transmission characteristics of the few-mode fibers,the generation and regulation characteristics of the orbital angular momentum modes of the line-by-line few-mode FBG?LBL-FMFBG?and chiral few-mode FBG?CFMFBG?fabricated on the few-mode fibers by femtosecond laser direct writing were studied.The polarization characteristics of two kinds of few-mode FBGs were also studied.The experiment shows that the CFMFBG have circular birefringence characteristics.The main contents of this paper are as follows:1.The femtosecond laser phase mask method and direct writing fabrication method of FBG were studied.Femtosecond laser phase mask and direct writing grating fabrication systems were built respectively.The spectral characteristics and polarization-dependent loss of single-mode fiber grating fabricated by two methods were studied.Based on femtosecond laser direct writing method,a variety of single-mode fiber gratings were fabricated flexibly,including uniform FBG,chirped FBG,phase-shifted FBG and a series of superstructure FBG.The fabrication techniques of various gratings have been explored.Finally,the superstructure fiber gratings fabricated by direct writing method was proposed for channel coding.The location of each grating segment of superstructure FBG was reasonably distributed,and superstructure FBG was coded by using the location information of each grating segment combined with the time domain reflectance spectrum of superstructure FBG.Furthermore,the encoding of superstructure FBG can be encrypted by using the frequency domain information of each grating segment of superstructure FBG.2.,The FBG was selectively fabricated in one core of a twin-core few-mode fiber?TC-FMF?using 800 nm femtosecond laser phase mask method,and its mode coupling characteristics were studied.The temperature and bending response characteristics of twin-core few-mode fiber Bragg gratings?TC-FM FBG?were studied.The temperature sensitivity of the three resonant peaks of the grating is 9.86,10.31 and 9.55 pm/°C,respectively,in the temperature range of 20¹00°C.In terms of bending response,the sensitivity of the grating to bending is directional.When the plane of the two core axes coincides with the bending plane,the bending sensitivity of the grating reaches the maximum of-37.41 pm/m^-1,and can distinguish between positive bending and negative bending.Therefore,the twin-core-few-mode fiber grating can be used for bending sensing in one-dimensional direction in a constant temperature environment.3.Line-by-line FBG_A and FBG_B with different periods were written on core_A and core_B of TC-FMF at the same fiber-axial position by femtosecond laser direct-writing grating fabrication system.And then,The two ends of the two-core low-mode fiber are respectively spliced with two single-mode fibers on the same side of the 3 dB coupler,and that the coreA and coreB are respectively aligned with the cores of the two single-mode fibers,to achieve the simultaneous detection of spectrum of FBG_A and FBG_B.The temperature and bending response characteristics of the grating were studied.By using difference method of resonant wavelengths of FBG_A and FBG_B,it was found that the bending sensitivity of line-by-line twin-core few-mode FBG is doubled compared with the bending sensitivity of the FBG fabricated on a single core of TC-FMF by phase mask method,and the influence of ambient temperature on bending sensing is eliminated.4.Femtosecond laser direct writing grating fabrication system was used to fabricate LBL-FMFBG and CFMFBG.The spectral characteristics of LBL-FMFBG and CFMFBG were studied.The polarization characteristics of LBL-FMFBG and CFMFBG were tested by polarization analysis system.The circular birefringence characteristics of the CFMFBG have been found in the experience,which leads to the spectral separation of right-handed and left-handed circularly polarized light.Furthermore,the generation and control characteristics of orbital angular momentum?OA?of LBL-FMFBG and CFMFBG were studied based on the interference between reflected light of grating and reference light?spherical wave?by using a field measurement system.It was found that under the condition of specific polarized light input,when input light were coupled to reverse transmission LP₁₁ mode by the LBL-FMFBG and the CFMFBG,the corresponding reflected light is OAM beam,and the topological charge is not zero.By adjusting the polarization state of input light or the polarizer angle of reflected light beam,the?1 OAM regulation can be achieved.