Investigation Of Vector Mode Conversion Based On Novel Special Few Mode Fiber Gratings

In the current rapid development modern communication era,the pressure of demand outstripping of communication capacity is greatly alleviated due to the appearance of mode division multiplexing(MDM)communication system.In the MDM communication system,the mode in optical fiber could be employed as the individual transmission channel,and it can multiply the transmission capacity of the fiber communication.The MDM system makes it possible to enter a new superspeed and superlarge capacity informationalized communication time for us.The mode excitation and mode conversion technologies are the key technologies in MDM system,and they have been the hot topics of research in recent years.As the key components in MDM,the mode converters(MCs)have been reported in a wide variety in recent years.Among them,the MC based on fiber grating has been paid much attention for researchers since it has many advantages in high efficiency,low loss,flexible operation etc.Currently,most of the reports on the fiber grating MCs focus on the mode conversion of linear polarization(LP)modes.The vector mode conversions(VMC)based on fiber gratings have been reported rarely.Nonetheless,the vector modes in optical fiber have the widespread application in the fields of the novel fiber lasers,the generation of OAM modes and OAM multiplexing transmission in optical fibers and the vector modes division multiplexing and the others.The broad application prospects of vector modes make it more and more urgent to the demand of the technology of achieving the specific vector mode in optical fibers.Under this research background,a series of vector mode conversion approaches based on novel special few mode fiber gratings are proposed in this thesis.The grating devices based on the special fibers which have the peculiarity of the vector modes separation can be used as the efficient vector mode converters.It has the potential application values and the promising applications in MDM communication system and the related fields of the vector modes in optical fibers.The major innovative results and research results in this thesis are summarized as follows:1.A vector mode conversion approach by utilizing tilted fiber Bragg grating(TFBG)written in the air-core fiber is proposed.The transmission properties of vector modes in the air-core fiber are analyzed.The results show this fiber can support the transmission of the first four vector modes,and it has the property of the degeneracy separation of vector modes.The vector mode conversion characteristic based on the tilted air-core fiber Bragg grating is investigated.The research suggests that the highest conversion efficiencies can be achieved when ?= 2°.2.The contrastive studies on the vector mode coupling properties based on asymmetric step-index fiber Bragg grating(FBG)and asymmetric vortex FBG are carried out.The phase matching condition and the effects of attenuation coefficient and grating modulation depth on the reflection properties are analyzed.The results suggest that the vector modes in asymmetric step-index FBG and asymmetric vortex FBG can achieve the highest conversion efficiencies when the attenuation coefficient ? =0.5?m^-1,0.3?m^-1,respectively.The mode conversion properties of the homemade asymmetric vortex FBG are researched.The consistency of the theoretical and experimental results is verified.3.A vector mode conversion approach by utilizing tilted asymmetric fiber Bragg grating(TAFBG)written in ring-core fiber is proposed.The evolution law of reflection properties with the simultaneous effects of tilt angle and asymmetric attenuation coefficient is analyzed emphatically.The research results suggest that when ? = 0?m^-1,? = 2°,corresponding to the case of TFBG,the high order vector modes have the maximum reflection peak values.4.The vector mode coupling properties of TFBG in ring-core fiber with different excited vector modes are investigated.The phase matching condition under different excited vector modes is researched emphatically.The research results indicate the reflectivities and resonance wavelengths between the excited and output vector modes both present the reversibility relationship.In addition,the single polarization excited vector modes can only convert to the single polarization same type vector modes.Moreover,the vector modes with high conversion efficiencies or high mode purities can be achieved at several specific tilt angles.5.Two vector mode converters based on tilted long period grating(LPG)in the vortex fiber and mechanically induced microbend LPG writing in the inverse-parabolic graded index fiber are proposed,respectively.The selection method of grating period,the phase matching property,the effects of tilt angle,grating modulation depth and microbend amplitude on the transmission spectrum and coupling coefficients are analyzed.The research results suggest that when ? = 83.5°,84°,88°,the transmission peak values of TM₀₁,HE₂₁ and TE₀₁ modes in tilted LPG reach the maximum values,respectively.When microbend amplitude ?=83.5°,84°,88°,the high order vector modes in mechanically induced microbend LPG have the maximum conversion efficiencies.