Application Of Long Period Fiber Gratings In Raman Suppression Of High Power Fiber Lasers

Fiber lasers are widely used in the fields of industrial manufacture,medical,optical communication and national defense because of their advantages,such as high conversion efficiency,excellent beam quality,compact configuration and convenient heat management.Although the output power of a single fiber laser has exceeded 20 k W,the nonlinear effects such as stimulated Raman scattering（SRS）and transverse mode instability（TMI）are still the major factors that prevent further improvement of output performance.This dissertation describes a simple,effective,and low-cost method for SRS suppression in high power fiber lasers based on long period fiber gratings（LPFGs）.The design,simulation,fabrication and testing of LPFGs were investigated in this paper,and the fabricated LPFGs were used in high-power fiber lasers for SRS suppression.1.Theoretical analysis and simulation calculations of LPFG were carried out.The simulation models of LPFGs were established based on the coupled mode theory.The formulation of coupling constant in three-layer fiber has been demonstrated by computational procedure.The transmission spectra of LPFG in single mode fiber were calculated,and the influences of the period,refractive index modulation and grating length on the transmission spectra were analyzed in detail,which provide a theoretical basis for the fabrication.2.The fabrication and testing of LPFG were carried out.A new CO₂laser scanning system based on a two-axis galvanometer has been built up for the fabrication of LPFGs.The LPFGs were written on the large-mode-area（LMA）double-clad fiber（DCF）with core/cladding diameter of 10/130μm.The characteristics of transmission spectra,insertion losses,temperature sensitivity,strain sensitivity and thermal effect of the fabricated LPFGs were investigated.Experimental results show that all the characteristics make these LPFGs ideal candidates for SRS suppression in high power fiber lasers,which provides strong support for subsequent experimental research.3.The theoretical analysis,simulation calculations and experimental research of LPFG for SRS suppression were carried out.The feasibility of SRS suppression through LPFGs was analyzed theoretically and we constructed a high power fiber amplifier and oscillator to experimentally validate the effects of LPFGs.For the fiber amplifier,when LPFG was inserted between the seed and amplification stage of the fiber amplifier,the SRS threshold was improved significantly and the suppression ratio of SRS was about 13d B at 5 k W output power,which greatly improved the purity of the output laser.For the fiber oscillator,we constructed a 1080 nm fiber oscillator used as a verification system and relevant SRS inhibition experiments were carried out.The fabricated LPFG was used for filtering and suppressing Raman signals in fiber oscillators and evident inhibition effects were achieved.Results showed that the filtering ratio of Stokes light beyond 16d B and the purity of signal light was improved significantly with the insertion of LPFG.Then the LPFG was used for SRS suppression within the cavity of the oscillator for the first time.Experimental results show that a maximum SRS suppression ratio of 18 d B was achieved and the extractable Raman-free output power was increased by about 13%with the insertion of LPFG.4.In order to broaden the bandwidth of SRS suppression,wide bandwidth cascaded LPFGs were proposed,designed and fabricated.Based on the improved fabrication system,the cascaded LPFGs with wider and deeper resonant peak were experimentally validated.Through analyzing the variation of the spectra of cascaded LPFGs,the spectrum of cascaded LPFGs can be regarded as a superposition of spectra of all the sub-LPFGs.The insertion losses and thermal coefficients of cascaded LPFGs were investigated.Results showed that these wide bandwidth fiber filters were capable of being used in high power fiber lasers.5.The performance of SRS suppression in a high power fiber laser system based on cascaded LPFGs was carried out.A 5 k W level oscillator one-stage power fiber laser amplifier was established and several comparative experiments demonstrated the superiority of cascaded LPFGs.Experimental results showed that cascaded LPFGs can suppress SRS more effectively than single LPFG in high power fiber amplifiers.The SRS suppression in high power fiber laser oscillators through different types of LPFGs were carried out.Experimental results showed that the SRS threshold and the purity of signal light can be improved with the insertion of LPFGs.Compared with single LPFG,cascaded LPFGs can suppress the SRS more effectively.Additionally,the frequency shift of Raman scattering was analyzed theoretically based on Raman gain spectrum and the transmission spectra of cascaded LPFGs.This novel phenomenon demonstrated the validity of cascaded LPFGs on SRS suppression.A cascaded LPFGs including three sub-LPFGs were proposed and fabricated to suppress the first and the second order of SRS.Experimental results showed that the cascaded LPFGs with 3-d B bandwidth of 115 nm can inhibit the first two orders of SRS simultaneously,and improve the purity of signal power.Cascaded LPFGs broke through the limitation of bandwidth of single LPFG,which also broadened the application potential of LPFGs in the SRS suppression.