Research On The Fabrication And Characteristics Of The Mode Coupling Fiber Devices Based On The Laser-induced Gratings

Fiber gratings act as optical passive device to realize mode coupling in optical fiber,and have potential applications in the field of optical fiber communications and fiber sensing.With the increasing flexibility of optical fiber preparation technics,optical coupling devices based on specialty fiber play important roles in the next generation of optical fiber sensing and optical fiber communication system.This thesis focuses on the theoretical analysis and experimental research of mode coupling devices based on the gratings,including fiber core and cladding mode coupling.Long-period fiber gratings(LPFGs)are fabricated in the thinned cladding fiber(TCF)and few-mode fiber(FMF)using CO2-laser,respectively,and tilted fiber Bragg grating(TFBG)is fabricated in the FMF using ultraviolet laser.Subsequently,the spectrum,sensing and mode-coupling characteristics of the fabricated gratings were studied,and their application in optical fiber communication and optical fiber sensing were explored.The main research contents are listed as follows:1.We propose a cladding mode fiber sensor based on LPFG written in the TCF.The sensing characteristics of conventional and over-coupled LPFG are investigated experimentally.The theoretical analysis and experiments show that cladding modes are highly sensitive to the surrounding refractive index(SRI)changes.LP06 mode resonance achieve a high index sensitivity of 7366.6 nm/RIU,which is much higher than that of the same order cladding mode of the conventional LPFGs.The temperature and bending sensitivities of this promising sensor are 70.85 pm/? and 2.32 dB/m-1,respectively.Tilted LPFG(TLPFG)with different periods and different tilt angles were fabricated in the TCF,and the SRI sensing characteristics of TLPFG are comparatively studied.The bending properties of TLPFG are experimentally investigated.The wavelength interval between adjacent resonance dips became smaller as the curvature increases,and the curvature sensitivity of the wavelength interval is-3.66 nm/m-1,which has potential applications in practical measurement.2.All-fiber mode converters based on LPFG in the two-mode fiber are demonstrated.Both non-tilted LPFGs and TLPFGs were fabricated to provide the light coupling between LP01 mode and LP11 core mode with a coupling efficiency of more than 99%.The writing efficiency and the bandwidth of the LPFG mode converter can be adjusted by changing the tilt angle of the TLPFGs.The torsion sensitivity of non-tilted LPFGs and TLPFG mode converters are 0.37 nm/(rad/m)and 0.50 nm/(rad/m),respectively.Two orthogonal vector modes(the HE21even and HE21odd modes)and corresponding orbital angular momentum(OAM)modes with topological charge of± 1 are successfully obtained at the resonance wavelength.The proposed LPFG mode converter could be used as not only a high efficiency wavelength tunable mode converter in the mode division multiplexing system but also a high sensitive torsion sensor in the field of optical sensing.3.We propose all-fiber mode converters based on LPFG written in the four-mode fiber.Mode conversion between the fundamental core mode and different higher-order core modes(LP11,LP21,and LP02 modes)can be realized via single LPFG with an efficiency of 99%at the resonance wavelength.Moreover,optimized mode conversion between the LP01 and LP21 modes can be realized by cascading two LPFGs with different grating pitches.The maximum conversion efficiency is-99.5%at 1553 nm.The OAM modes with different topological charges(± 1,±2)are demonstrated experimentally.5.A method for generating optical vortex beams(OVB)from an all-fiber laser is reported.FMF-LPFG is used to act as an intracavity mode converter enable the OVB operation.Selection of the wavefront handedness can be achieved simply by adjusting the intracavity polarization controller.The laser achieved?50 mW of output power for both first-order and second-order vortex modes generation with the corresponding slope efficiencies of 8.12%and 7.91%,respectively.The linearly polarized orbital angular momentum beams with l= ±1 were characterized experimentally by adjusting a polarizer at the laser output.6.We demonstrate broad-band and low-loss 3-mode and 6-mode mode scramblers employing FMF-LPFG for space-division multiplexing.Step-index(SI)FMF are used to avoid mode coupling to the cladding modes.We characterize the mode scramblers using a swept-wavelength interferometer.Mode-dependent loss(MDL)and modal transfer matrices over C+L band are present.The demonstrated mode scramblers are with negligible mode-dependent loss(MDL)and insertion loss(IL).7.A directional curvature sensor based on TFBG in the FMF is demonstrated.The eigenmodes of LPoi and LP11 mode groups are simulated along with the fiber bending.The directional curvature sensor is based on the LP11 mode resonance in the TFBG.For curvature from 4.883 to 7.625 m-1,the curvature sensitivities at direction of 0° and 90°are measured to be-2.67 and 0.128dB/m-1,respectively.The temperature variation barely affects the resonance depth of LP11 mode.The proposed curvature sensor clearly demonstrates the potential to simultaneous directional curvature and temperature measurement with the resolutions of 9.15x 104 m-1 and 0.952 ?,respectively.8.A mode rotator based on the reflective asymmetric core modes of TFBG in the FMF is proposed.The asymmetric modes rotates with the fiber twisting.The rotation sensitivity is related to the position of TFBG,and the mode rotation sensitivity in our experiment is obtained to be 5.07 °/(rad·m-1).The proposed mode rotator can be used in the MDM system and applied as an optical specklegram sensor.9.A reflective all-fiber mode converter based on FMF-TFBG is demonstrated.By adjusting the reflective LP11,LP21 and LP31 modes,OAM modes with topological charges of ±1,±2,±3 can be generated at different resonant wavelengths.The LP mode conversion efficiencies provided by TFBG are larger than 90%.The TFBG in FMF is promising for all-fiber mode selection and mode filter in the OVB or OAM applications.