Fabrication And Applications Of Physically-deformed Long-period Fiber Gratings

By periodically changing the physical structure of the fiber,the stress distribution of the fiber will be changed,and the resulting elasto-optic effect can periodically change the refractive index of the fiber,thus forming a long-period fiber grating,which is called physically-deformed long-period fiber gratings?LPFGs?.Compared to laser-exposure-based LPFGs,physically-deformed LPFGs have a lot of advantages and unique features.These include long-term stability,unique sensing performance,stronger polarization correlation,and orbital angular momentum modulation characteristics.Therefore,the study on physically-deformed LPFGs has important theoretical and practical value.In this thesis,two kinds of physically-deformed LPFGs,i.e.,chiral long-period fiber gratings?CLPFGs?and microtapered long-period fiber gratings?MLPFGs?,are studied,which are all based on single-mode fibers?SMFs?.We proposed new fabrication methods and explored the applications of these two kinds of gratings in the field of communication and sensing.The main work contents and achievements of this doctoral thesis are as follows:?1?An axis-offset double-sided rotating method for fabricating CLPFGs is proposed.The key and unique feature of the axis-offset double-sided rotating method is that the axis-offset is intentionally introduced between the two fiber holders in the fabrication device.When the grating is produced by this method,the rotating motors on both sides rotate in the same direction at the same speed.When using the traditional one-sided twisting method to fabricate the CLPFG from an SMF,there is a fatal flaw that cannot control the grating resonance wavelength.Researchers have been searching for a solution,but they have not solved the problem yet that the grating resonance wavelengths are uncontrollable.Our proposed axis-offset double-sided rotating method solves this problem well,which precisely controls the resonance wavelengths of the CLPFG fabricated by SMF,i.e.,this method is capable of obtaining identical resonance wavelengths of the CLPFGs for the same grating period.Moreover,the grating structure is strict,the grating physical strength is high,the fabrication process of grating is simple and the repeatability of grating is good.The axis-offset double-sided rotating method can be applied to different kinds of fibers to fabricate CLPFGs,such as photonic crystal fibers and multi-core fibers.?2?We propose a method for tuning the resonance wavelength of a wavelength-selective coupler?WSC?based on the torsional stress.By applying a torsional stress to a WSC consisting of multiple parallel MLPFGs,the resulting elasto-optic effect changes the resonance wavelength of the gratings,which finally changes the resonance wavelength of the WSC and thus achieves the wavelength tuning.The wavelength tunability is-0.01985 nm/°,and the maximum wavelength tuning range is 6.8 nm,moving from 1569.8 nm to 1563 nm.Moreover,the linear relationship between the resonance wavelength shift and the change in the twist angle is good.In a WSC consisting of two parallel MLPFGs,the maximum peak coupling efficiency detected in the tapping fiber is-9.958 dB.In a WSC consisting of three parallel MLPFGs,the maximum peak coupling efficiencies of the measured spectra in two tapping fibers are-15.437 dB and-14.887 dB,respectively.?3?We investigate the modulation of the optical field orbital angular momentum of a single-helical CLPFG based on SMF.Two interference fields with centrally appearing Y-structures are obtained in the interference experiments between the plane wave and the output optical field of the CLPFG.As a result,the interference field corresponding to the left-handed CLPFG shows a downward Y-structure feature,while the interference field corresponding to the right-handed CLPFG shows an upward Y-structure feature.Two spiral interference fields are obtained in the interference experiments between the spherical wave and the output optical field of the CLPFG.As a result,the helical interference field corresponding to the left-handed CLPFG rotates counterclockwise,while the helical interference field corresponding to the right-handed CLPFG rotates clockwise.The experimental results show that the single-helical CLPFG based on SMF can generate±1-order optical vortex.?4?We develop a new program for fabricating CLPFG based on a fusion splicer platform where a dual-beam CO₂ laser is used as a heating source.The fabricated CLPFGs based on SMFs by the program have advantages of smooth surface and low insertion loss.Torsional stress sensing experiments were performed using fabricated CLPFGs.The torsional sensing sensitivities of all feasible modes of the transmission spectrum have been measured,and the wavelength shift linearity of each mode is good.Compared with the LPFGs torsional sensing experiments which have been reported,the torsional sensing sensitivity of CLPFGs based on SMFs is 6 times higher than that of CLPFGs based on photonic crystal fiber and 5 times higher than that of LPFGs written by UV laser irradiation.?5?We propose a method for fabricating MLPFGs based on the first-softening and second-tapering processes.Differing from the traditional method,in the proposed fabrication process,the fiber is firstly stretched to be subjected the tensile stress,and then the fiber is heated and softened,so that the fiber is pulled in an instant to obtain a reproducible fiber microtaper.In addition,we use two-beam CO₂laser as the heating source to illuminate and soften fiber in two contrary directions.So,the new fabrication method is cleaner,the contamination to the grating is small,the fiber is softened more uniformly,and the grating quality is higher.This method applies the fully automated program that we have developed and stored in the fusion splicer,so it is integrated,stable and easy to operate.