Fabrication Of Tilted Fiber Bragg Grating And Its Application In High-power Fiber Laser

Since the birth of fiber lasers,as their application fields have become more widespread,people have increasingly higher requirements on their output power.At present,the output power of single fiber laser has exceeded 10 kilowatts,but the nonlinear effects such as stimulated Raman scattering(SRS)and stimulated Brillouin scattering(SBS)are still the key factors that limit the further improvement of output power of fiber laser.So far,researchers in the filed of fiber laser have proposed a variety of methods to suppress the effects of SRS and SBS.Although good results have been achieved,they have encountered technical bottlenecks when power is further improved.This dissertation aims to develop a simple and effective suppression method based on tilted fiber grating.The designing,simulation,fabrication and testing of tilted fiber grating,and its experimental research on suppressing SRS and SBS in high-power fiber laser have been carried out in this dissertation.The main contents include:1.Theoretical research and simulation calculations of tilted fiber gratings and chirped tilted fiber gratings were carried out.Based on the coupled mode theory and transfer matrix method,the simulation models of tilted fiber gratings and chirped tilted fiber gratings were established.The output spectral characteristics of tilted fiber gratings and chirped tilted fiber gratings in single mode fiber and large-mode-area double-clad fiber were calculated respectively,and the influences of the tilt angle,refractive index modulation depth and grating length on the transmission spectrum were analyzed in detail.The results show that the tilted fiber gratings and chirped tilted fiber gratings can be used to suppress the SBS and SRS effects respectively,and the design rules of grating parameters in practical applications are clarified,which lays a theoretical foundation for the next grating fabrication.2.The fabrication and testing of tilted fiber gratings and chirped tilted fiber gratings were carried out.A fiber grating fabrication system based on excimer laser and phase mask was built.The tilted fiber gratings and chirped tilted fiber grating were written on the single mode fiber,and chirped tilted fiber grating were firstly written on the large-mode-area double-clad fiber.The spectral characteristics,loss characteristics,temperature drift characteristics,and temperature power characteristics of the fabricated fiber gratings were tested.The results show that the performance parameters of fabricated fiber gratings can meet the basic requirements of SBS and SRS suppression in fiber laser,which provides strong support for subsequent experimental research.3.The theoretical analysis and confirmatory experimental studies of tilted fiber gratings for SRS suppression were carried out.Based on the SRS theory,the feasibility of suppressing SRS by chirped tilted fiber gratings was analyzed in principle.The experimental system was built and a confirmatory experimental study on SRS suppression was carried out in a 1090 nm fiber laser amplifier.The results show that when the chirped fiber grating is placed between the seed source and the amplifier stage,the overall Raman threshold of amplifier system can be improved,and the SRS effect can be effectively suppressed.The spectral suppression ratio can be up to 25 dB.The similiar effect is obtained when the grating inserted between the amplifier stages,and the rejection effect improves as the number of inserted fiber gratings increases.However,the Bragg resonance reflection of chirped tilted fiber gratings may lead to Raman random laser,which is not conducive to further power increasing of system.4.The experiment of SRS suppression in a kilowatt fiber laser amplifier system based on large-mode-area double-clad chirped tilted fiber grating was carried out.The chirped tilted fiber gratings for 1080 nm high-power fiber lasers were designed and fabricated on large-mode-area double-clad fiber,and their effects were verified in bidirectional semiconductor pump fiber system or tandem pumping fiber system respectively.When the chirped tilted fiber grating is inserted between the seed source and the amplifier,the spectral rejection ratio was up to 10 dB at the highest output power of 2.35 kW in the bidirectional semiconductor pump fiber system;in the tandem pumping fiber system,the spectral rejection ratio was up to 15 dB at the output power of 3.3 kW,and the output power with equal intensity of Stokes light increased from less than 3.5 kW to 3.9 kW,and the M~2 factor increased from greater than 2 to about 1.7;when two gratings were inserted,the spectral rejection ratio was up to 15 dB at the output power of 3.9 kW,and the output power with equal intensity of Stokes light further increased to 4.2 kW,the M~2 factor maintained 1.80 and further power enhancing was limited by the pump laser.The experimental results show that in the actual high-power fiber amplifier system,the chirped tilted fiber grating can effectively improve the Raman threshold of whole system,suppress Stokes light generation,and increase the effective output power.Moreover,a more satisfying suppression could be achieved with a serials of two or more chirped tilted fiber gratings applied.For high-power fiber systems limited by SRS,chirped tilted fiber gratings can effectively improve the output power of the system.5.The structure of fiber laser oscillator with SRS suppression based on chirped tilted fiber grating was proposed,and relevant theoretical and experimental research was carried out.The chirped tilted fiber grating for 1080 nm fiber lasers was designed and fabricated on large-mode-area double-clad fiber,and experiments were carried out on fiber oscillators with different pumping scheme.With a CTFBG inserting before the output coupling FBG in the oscillator,obvious decreasing of Stokes power and increasing of Raman threshold was observed,and a maximum SRS suppression ratio more than 10 d B was achieved in spectrum.The experimental results show that the chirped tilted fiber grating can be applied in the fiber oscillator structure for SRS suppression,which can improve the overall Raman threshold of oscillator system and improve the output characteristics.However,its loss characteristics have a great influence on the oscillator,and its redisual Bragg resonance reflection may lead to Raman random laser,which is not conducive to further increase of system power.With the improvement of fiber grating manufacturing technology,a promotion in rejection could be achieved.6.The technical approach to suppress SBS in narrow-linewidth fiber lasers using tilted fiber grating was proposed,and the experimental study was preliminarily carried out.Aiming at the shortcomings of current SBS suppression methods in narrow-linewidth fiber laser,combined with the SBS generation mechanism and the performance characteristics of tilted fiber grating,the feasibility of tilting fiber grating for SBS suppression was analyzed in principle.The specially designed tilted fiber grating can be used as an extremely narrow-band filter,which forms a high loss at the Stokes frequency shift generated by the SBS effect,which is equivalent to the reduction of fiber Brillouin gain peak,thereby improving The SBS threshold serves to suppress the SBS effect in the fiber laser.In order to verify the feasibility of theoretical analysis,a tilted fiber grating for 1.55μm fiber laser was designed and fabricated,and an experimental system for SBS suppression was built.A preliminary experimental study was carried out.The results show that the tilted fiber grating has a good filtering effect on the backward Stokes light generated by the SBS effect.By improving the performance parameters of tilted fiber grating,a better suppression effect can be obtained.In the next step,the tilted fiber grating can be prepared on the large-mode-area double-clad fiber,which can be used for the suppression of SBS in the high-power narrow-linewidth fiber laser,which is of great significance for the further power improvement of the narrow-linewidth fiber laser.