Objective Polarization-maintaining ytterbium-doped fiber(PMYDF)lasers with smaller core diameters can suppress transverse mode instability(TMI)more effectively.However,reducing the core size also decreases the stimulated Brillouin scattering(SBS)threshold,which is a major challenge for narrow linewidth(＜10 GHz)linear-polarized PMYDF lasers.To suppress SBS while still suppressing TMI,larger core fibers are a better choice.In this study,SBS is effectively suppressed in a 1.6-kW 8-GHz spectral width linear-polarized fiber laser,adopting gain fiber with 25-μm core diameter and a white-noise-broadened phase-modulated single-frequency laser(PMSFL).In addition,TMI is effectively suppressed by careful application of the fiber-coiling method.The results indicate that 25-μm core diameter PMYDF has great application potential for high-power ultra-narrow linewidth(＜10 GHz)linear-polarized fiber lasers.Methods The seed source used in this study is a white-noise-broadened PMSFL.The seed power is initially amplified from～20 mW to～20 W by a two-stage pre-amplifier,and then injected into a primary amplifier.The primary amplifier uses a 25-μm core diameter PMYDF with a gain-fiber length of～5.5 m,a mode field area of～340 μm2,and 976-nm pump light absorption of～2 dB/m.The design of the primary amplifier is based on a backward-pumping scheme,and six non-wavelength-stabilized 976-nm fiber-coupled semiconductor lasers(LDs)with a maximum output power of＞400 W serve as the pumping sources.After amplification,the output laser beam is stripped by a cladding power stripper(CPS)to remove the cladding light,and then output through a fiber endcap.The total length of the output fiber is～1.5 m.Between the pre-and primary amplifiers,a high power fiber isolator(ISO)with a backward power monitoring port(TAP)is used to isolate and leak the backward light.A mode-field adapter(MFA)is inserted between the pre-and primary amplifiers to realize mode-field matching between different fibers.The laser system has an all-fiber structure.The laser power is directly measured using a power meter.A prism is used to attenuate the output laser beam,and a laser spectrum analyzer is used to measure the spectrum of the attenuated laser beam.After the output laser beam is collimated,the P and S waves are split using a polarization beam splitter(PBS)and their respective power levels are measured using power meters and used to calculate the polarization extinction ratio(PER).The output laser beam is collimated and attenuated,and its beam quality is measured using a beam-quality analyzer.Results and Discussions Initially,the primary amplifier gain fiber is coiled to a diameter of 12 cm.When the output power of the primary amplifier reaches 1 kW,the slope efficiency is significantly reduced,indicating that the TMI threshold has been reached.To suppress TMI,the gain fiber coiling diameter is decreased to 9 cm,and the laser power increases to 1.2 kW.However,there is still a significant reduction in slope efficiency.When the coiling diameter of the gain fiber is decreased further to 8 cm,the laser output power reaches 1.6 kW without obvious slope-efficiency reduction.The total injected pump power of the primary amplifier is 2400 W,and the corresponding optical conversion efficiency is～66.5％[Fig.2(a)].As the fiber coiling diameter decreases from 12 cm to 8 cm,the TMI is effectively suppressed[Fig.2(b)].At full output power,the backward power leaked from the ISO TAP is 0.59 W,and the proportion of the output laser power is～0.037％.This indicates that the output power of the system has considerable potential to increase before reaching the SBS threshold[Fig.2(a)].Based on the theoretical model established to calculate the stimulated thermal Rayleigh scattering effect(STRS)in the PMYDF,the energy-coupling process between the fundamental mode(FM)and high-order mode(HOM)for different gain fiber coiling diameters is simulated according to the fiber and pump parameters adopted in this experiment.The results show that as the fiber coiling diameter decreases from 12 cm to 8 cm,the nonlinear energy coupling process from the FM to the HOM is significantly suppressed(Fig.3).The measured laser wavelength at full output power is～1053.15 nm and the 3 dB linewidth is～0.0294 nm(Fig.4).The transverse beam quality factor(Mx2),longitudinal beam quality factor(My2),and PER measured at the full laser power are 1.090,1.166,and 96％(～14 dB),respectively(Fig.6).Conclusions A 1053-nm narrow spectral width linear-polarized fiber laser system is constructed based on the master oscillator power amplifier structure using a white-noise-modulated single-frequency laser as the seed source.In the primary amplifier,selection of a suitable coiling diameter for the gain fiber coiling in combination with the use of a 976-nm backward pumping scheme enables the successful suppression of the TMI effect.Finally,a 1.6-kW,8-GHz spectral width linear-polarized fiber laser emitting at 1053 nm is achieved,with Mx2,My2,and polarization extinction ratio of 1.090,1.166,and～14 dB,respectively.No obvious SBS is observed during the experiment.
fiber laserslinear polarizationnarrow linewidthhigh power
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