Optimization Design Of Narrow Linewidth Fiber Grating And Spectrum Control Method Of Fiber Laser

Narrow Linewidth Fiber Bragg Grating(NLFBG)is a new type of optical passive device which makes periodic refractive index modulation in the core of optical fiber by such techniques as UV photolithography.It mainly modulates the longitudinal mode of the laser,i.e.,controlling the linewidth of the laser,and acts as a resonator mirror.The advantages of narrowband reflection and full compatibility with optical fiber make it especially suitable for narrowlinewidth fiber lasers.However,fiber lasers based on narrow-linewidth fiber gratings are prone to produce stimulated Raman scattering(SRS)which becomes the main factor to restrain the performance of narrow-linewidth fiber lasers from greatly improving with its increasing demand for power.Therefore,how to suppress the SRS of high power narrow-linewidth fiber lasers to improve the linewidth of its spectra greatly has become a major problem crying for solution in the field of high-power lasers.To solve this problem,in this thesis,the design and production of NLFBG is optimized and designed to suppress SRS effect in fiber laser to improve laser linewidth according to the theory of fiber laser and fiber grating and the mechanism of SRS.At the same time,in order to improve the power bearing characteristics of fiber grating,an annealing method combining high and low temperature is proposed.The contents of this thesis are as follow.In view of the problem of SRS effect caused by over high power of fiber laser,this paper proposes a method of using a new type of NLFBG to suppress SRS effect in fiber lasers,which puts Chirped Tilted Fiber Bragg Grating(CTFBG)and traditional NLFBG in series according to the theoretical knowledge of fiber lasers and the mechanism of SRS effect,and it also makes studies on the design idea and mathematical model of the new type of NLFBG.According to the basic theory and design ideas,the improved NLFBG is simulated,and a new NLFBG with SRS suppression effect is designed by combining the theory and simulation.In order to write this improved NLFBG accurately,a closed-loop high-precision fiber grating writing system with on-line real-time monitoring system is built in this paper.Meanwhile,in order to improve the power bearing characteristics of the improved NLFBG,we studied the annealing process and testing method of the fiber grating.Finally,in order to verify the improvement effect of the improved NLFBG on the output spectral linewidth of the fiber laser system,the fiber laser of single-cavity oscillation and MOPA type fiber laser are experimentally studied.The experimental results show that the effect of the improved NLFBG on suppressing SRS in the output spectrum of the fiber laser of single-cavity oscillation is more than 20 dB(99%).For MOPA system,adding improved NLFBG directly to the amplifier of MOPA system has better suppression effect on SRS than adding to the seed.According to statistics,the effect of SRS suppression by adding the improved NLFBG to the amplifier of MOPA system is close to 20 dB(99%).