| Fiber lasers have been widely used in defense military,industrial processing and biomedical fields due to its compact structure,high conversion efficiency,good beam quality and convenient thermal management.Different applications have their appointed requirements on the output power,wavelength and linewidth of the laser.Thereinto,highpower,narrow-linewidth and long-wavelength fiber lasers with wavelengths ranging from 1100 nm to 1200 nm have many applications.For example,high-power,narrowlinewidth fiber lasers emitting at 1178 nm have an important application in sodium laser guided stars.Besides,the yellow-orange light obtained by frequency doubling of the narrow-linewidth 1120 nm fiber laser plays an important role in many applications such as flow cytometry,laser photocoagulation,microscopic imaging,and optical trap cooling.There are three main ways to realize the long-wavelength fiber laser ranging from 1100 nm to 1200 nm according to the classification of gain sources: firstly,the Yb-doped fiber laser based on the real-level transition,whose power scaling would be limited by amplified spontaneous emission and self-oscillation because of the gain competition resulted from the level structure of Yb-ion;secondly,the Raman fiber laser based on nonlinear effect,whose power scaling would be limited by stimulated Brillouin scattering and high-order Raman effect due to the long fiber length(usually more than 100 meters)used in the laser;thirdly,the hybrid-gain fiber laser based on both Yb-ion gain and Raman gain.The power boosting potential of this technology solution has been verified in widelinewidth lasers.However,its development towards high-power,narrow-linewidth lasers still faces serious technical challenges,which are embodied in the fact that hybrid-gain fiber amplifiers need to be implemented by forward-pumping.However,the research results have shown that the linewidth of the output would be broadened when the Raman gain is involved in the amplification process,which leads to the difficulty in realizing high-power,narrow-linewidth output lasing in long-wavelength.By summarizing the prior technical proposal,we select high-power narrowlinewidth hybrid-gain fiber amplifiers as the research object.The influences of pumping methods and seed characteristics on the time-frequency characteristics of the output are considered in the hybrid-gain fiber amplifiers.After that,systematical study are carried on to analyze the physical mechanism of spectral broadening in this technical solution.Afterwards,the feasibility and basic conditions of high-power,narrow-linewidth and long-wavelength fiber laser are demonstrated.We propose an effective technical scheme to realize high-power,narrow-linewidth and long-wavelength fiber laser through analysis of the physical mechanism.At last,the high-power,narrow-linewidth hybrid-gain fiber amplifier is realized based on a reasonable experimental structure.The specific content is as follows:1?The theoretical and experimental studies on the output characteristics and linewidth evolution mechanism of single-frequency Raman fiber amplifier are carried out.The influence of pump methods and pump time-domain on the output power,linewidth and time-domain characteristics are demonstrated,which indicates the feasibility and necessary conditions for linewidth maintaining and power amplifying in single-frequency Raman fiber amplifiers by forward pumping.Firstly,a 1120 nm single-frequency Raman fiber amplifier platform is built up to study the effects of pump methods and pump timedomain on the output characteristics of the laser.The experimental results show that: by using pump sources with unstable time-domain,the linewidth would be maintained in backward pumping,and the pump laser time-domain fluctuations have no effect on the output power and efficiency;but in forward pumping,there is obvious spectral broadening in the output,and the output power and efficiency would be improved with the increase of the time-domain fluctuations.In addition,the spectral broadening of the output laser is positively correlated with the fluctuations of the pump laser time-domain.In other words,the spectral broadening of the output laser would be weakened with the decrease of the time-domain fluctuations.Therefore,in forward pumping,the pump light must be a time-frequency stabilized fiber laser in order to maintain the linewidth.Based on the above idea,we take the single-frequency Raman fiber amplifier emitting at 1120 nm as a carrier to realize the single-frequency Raman fiber amplifier in forward pumping for the first time,by using a narrow-linewidth pump source with a stable time-domain.We demonstrate that by using stable time-domain pumping technology in the Raman fiber amplifier,the linewidth could be maintained effectively in forward pumping.2?The theoretical study on the gain dynamics and output characteristics of the highpower narrow-linewidth hybrid-gain fiber amplifier is carried out,and the feasibility of power scaling and linewidth maintaining by using both the Yb-ion gain and the Raman gain is demonstrated.The specific results are as follows: firstly,the noise transfer characteristics of the Raman pump light in the hybrid-gain fiber amplifier are analyzed in detail,and the influence of the time-domain characteristics of the Raman pump light on the linewidth evolution of the amplifier is explained.The results show that in the hybridgain fiber amplifier,the noise transmission of the Raman pump light is similar to the allpass filter,and the strong fluctuations would be completely transmitted to the output signal light,which causes the linewidth broadening.Therefore,the Raman pumping light with stabilized time-domain is one of the necessary conditions for the high-power,narrow-linewidth hybrid-gain fiber amplifier.Secondly,the 976 nm laser and the 1018 nm laser are used as different initial pump lights,respectively,and the noise transfer characteristics of the initial pump light in the hybrid-gain fiber amplifier are analyzed in detail,and the influence of the time-domain characteristics of the initial pump light on the linewidth evolution of the amplifier is explained.The results show that for the conventional 976 nm pumping,the output laser linewidth is insensitive to the noise of the initial pump light;however,for the 1018 nm pumping,the initial pump light noise is transmitted to the output signal light,and in turn,the output laser spectrum is significantly broadened.Thirdly,the SBS threshold characteristics and second-order Stokes optical threshold characteristics of the high-power narrow-linewidth hybrid-gain fiber amplifiers and the backward-pumping narrow-linewidth Raman fiber amplifiers are contrastively analyzed.The potential of power scaling in the high-power narrow-linewidth hybrid-gain fiber amplifiers is illuminated.3?Experimental researches on high-power narrow-linewidth hybrid-gain fiber amplifier are carried out,and the narrow-linewidth long-wavelength fiber laser above kilowatt level is demonstrated for the first time.It is confirmed the feasibility that highpower,narrow-linewidth long-wavelength fiber laser is realized by hybrid-gain fiber amplifier.Firstly,a hundred-watt narrow-linewidth hybrid-gain fiber amplifier was built.The effects of seed power ratio,total seed power and gain fiber length on the output characteristics of the laser were studied experimentally,and the output power of hundredwatt level with no linewidth broadening is realized.Subsequently,a kilowatt-scale narrow-linewidth hybrid-gain fiber amplifier is built seeded by the kilowatt-level narrowlinewidth hybrid-gain fiber amplifier.The influence of the seed power ratio,the total seed power,and the amplified spontaneous emission in the seed laser on the output characteristics are studied experimentally.Finally,we achieve the kilowatt-level narrowlinewidth long-wavelength output with near-diffraction limit beam quality. |
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