Research On The Broadband Few-mode Fiber Long-period Grating And Its Application In Mode-locked Fiber Lasers

Structured light is a kind of beam who has a special distribution in intensity,phase and polarization in spacial dimension,which has a lot of applications in particle manipulation,high-resolution imaging,material processing,near-field optics and high capacity optical communications.These structured light are usually obtained by mode conversion.As a light propagation medium,fiber has a lot of advantages such as simple structure,low transmission loss,resistant to bending,corrosion and electromagnetic.Further,the cylindrical symmetric makes the fiber an excellent candidate for the generation and propagation of structured light.Generating structured light with different orders and types using all-fiber systems has attracted researchers’interests in the past decades.In order to achieve the generation,propagation and application of these higher-order modes in fiber,the mode converter is necessary.Long-period fiber grating（LPFG）is a good choice for its convenient fabrication,high conversion efficiency and low insertion loss.With the development of mode-division multiplex system and the increasing requirements on the higher-order pulsed lasers,there is a huge demand on the LPFG mode converter to generate different higher-order modes as well as a broader working bandwidth.However,the LPFG mode converters reported at present are mainly focusing on the first-order mode generation.It is still limited to generate second-or higher-order with high efficiency,low insertion loss and high robustness.Further,the working bandwidth is mainly measure by the 10 d B bandwidth（the corresponding conversion efficiency is 90%）,which means that the LPFG mode converter who has a higher conversion efficiency and broader working bandwidth still need to be further developed.In this paper,we are focusing on to generate different higher-order modes with high conversion efficiency,low insertion loss under broadband using LPFG.In the researching process,we are aiming to revealing the refractive index（RI）modulation,optimizing the fabrication process and exploring the application in mode-locked fiber laser of LPFG.These works are supported by the national natural science foundation of China and National Key R&D Program.The main research content and results of this dissertation are listed as follows:1.We made a deep research on the mode coupling between fundamental mode and second-order mode both in theoretical analyzation and experiment in order to enhance the coupling efficiency and lower the insertion loss.By analyzing the grating mechanism,RI modulation area and spectral characteristics of LPFG fabricated by CO₂laser,we figured out the difficulty in the effective generation of second-order beam and concluded that an angular RI modulation area change could enhance the coupling efficiency between the two modes.In experiment,we proposed to fabricate the LPFG with a preset twist in order to achieve an angular RI modulation area change.Finally,we achieved an effective generation of second-order beam by fabricating the LPFG with a preset twist.The dip in spectrum is 22.83 d B（corresponding conversion efficiency is 99.47%）and the insertion is only 0.24 d B.Besides,the CO₂ laser exposure did a neglectable damage on the fiber which means that the LPFG mode converter is of high robustness.This fabrication method significantly reduces the difficulty of the fabrication of LPFGs second-order mode converter.2.We proposed three kinds of cascading LPFG in order to achieve a broadband mode conversion.The three kinds of cascading LPFGs are chirped cascading LPFG,dispersion compensated cascading LPFG and angular RI modulated cascading LPFG.All these cascading LPFGs could achieve a broadband mode conversion as well as a low insertion loss.In experiment,the chirped cascading LPFG has a 10 d B bandwidth of 184.21 nm and a 13 d B（corresponding conversion efficiency 95%）bandwidth of145.10 nm,which greatly improved the conversion of chirped LPFG.The dispersion compensated cascading LPFG has a 15 d B bandwidth of 147.96 nm,which is the best result in 15 d B bandwidth,to our best knowledge.The angular RI modulated cascading LPFG is the first time,to our best knowledge,to achieve a broadband mode conversion for second-order OAM mode with a 15 d B bandwidth of 103.24 nm.3.Using the broadband LPFGs mode converter,we built all-fiber mode-locked fiber lasers with higher-order mode output.We achieved first-order cylindrical vector（CV）traditional soliton in 2μm waveband and 1550 nm waveband.Besides,we achieved the first-and second-order CV dissipative soliton at L-band,the 3 d B bandwidth of the output spectra are 47.69 nm and 33.18 nm,respectively.Furthermore,it is the first time,to our best knowledge,to detect the time-domin characteristics using time-stretched dispersion Fourier transform technology.The experimental results show that the broadband LPFG mode converter can maintain the same time-domin characteristics and only change the transverse mode field distribution.