| Fiber lasers with high conversion efficiency,good beam quality,compact structure,flexible transmission and other advantages,have been widely used in communication,sensing,medical,scientific research,industrial processing,national defense and other fields.High power is an important pursuit of fiber lasers for national defense and industrial applications,but it is severely limited by nonlinear effects mainly of the stimulated Raman scattering(SRS)and transverse mode instability(TMI)effects.Chirped and tilted fiber Bragg grating(CTFBG),as an all-fiber bandstop filter,can be used to suppress SRS.However,there is still a big gap between the bandwidth,power capacity of CTFBG and the requirement of SRS suppression in practical high power fiber laser systems.In view of this,this paper carried out researches on the preparation,testing and application of broadband CTFBG and high power CTFBG.The main contents are as follows:1.Overview of fiber Grating and CTFBG.The characteristics and preparation methods of CTFBG were briefly introduced,and the research progress of CTFBG mitigating SRS was summarized in detail.It is pointed out that the main development directions of CTFBG at present are improving grating spectrum,enhancing power carrying capacity and application research.2.Design,fabrication and application of broadband CTFBG.In view of the current situation that the bandwidth of CTFBG is not enough and limits the SRS suppression effect,a method of cascaded writing is proposed to fabricate the wide-band CTFBG.Theoretical analysis,simulation and design were carried out.UV phase mask exposure method was used to fabricate the wide-band CTFBG on the double-cladding fiber with the core/cladding diameter of 15/130 μm.The prepared cascaded CTFBG was measured with a 3 d B bandwidth of 20.2 nm,a spectral depth of 23 d B at the central wavelength of1132.6 nm,and an insertion loss of about 0.3 d B at 1080 nm.In order to verify the SRS suppression effect of broadband CTFBG,a 5-kW fiber laser test system was built and a comparative experiment was carried out.The results show that increasing the bandwidth of CTFBG can effectively improve the SRS suppression effect.3.Design,fabrication and testing of large-mode-area(LMA)double-cladding(DC)CTFBG.In order to realize the preparation of high power CTFBG,the dynamic scanning method was further proposed,which reduced the thermal load of the grating by increasing the length of the grating area and reduced the heat.At the same time,the grating packag was improved for better heat dissipation efficiency,so that the grating could work stably for a long time with high power.Using this method,CTFBGs were fabricated on LMA DC fibers with core/cladding diameters of 20/400 μm and 25/400 μm,respectively.In the experiments,20/400 CTFBG withstood 2784 W of injected laser,with insertion loss less than 0.3 dB,and no significant temperature rise was observed.The 25/400 CTFBG withstood 4348 W of injected laser,the temperature rise coefficient was only 1.72℃/k W,and the insertion loss was less than 0.3d B.In addition,the 25/400 CTFBG worked stably in a 10-minute burn-in test,and both gratings had the capability of higher power applications.4.Application of LMA DC CTFBG in SRS suppression of high-power narrowspectrum fiber amplifier.The oscillator single-stage amplifier high power fiber lasers were built,and the SRS suppression ratio of 5.8 d B was achieved when 20/400 CTFBG was inserted into a 2-k W narrow-spectrum fiber laser.When 25/400 CTFBG was inserted into a 3-k W narrow-spectrum fiber laser,the SRS suppression ratio of more than 4 d B was obtained,and 136.2 W Stokes light was filtered out.Meanwhile,the beam quality degradation caused by SRS was inhibited.In the next step,the filter bandwidth and depth of CTFBG could be increased and the CTFBG can be integrated on the optical fiber devices such as the output endcap to improve the suppression effect of SRS. |
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