Study On The Key Technique Of Fiber Laser Spectral Beam Combining

Power scaling ability of a monolithic fiber laser/amplifier is limited by many factors,such as thermal effects,nonlinear effects,mode instability,brightness limitation of pump sources,fiber damage and so on.Beam combining allows for scaling the power of a laser system in a modular approach while preserving the quality of the combined beam.Scaling brightness of the combined beam is the purpose of a laser beam combining system.In order to realize this gain,two issues are strongly required: one is scaling every laser array's output power while maintaining their good-beam-quality;the other one is scaling the array number of beam combined.In this paper,we select one of the typically representative non-coherent beam combining system-spectral beam combining to perform systematically theoretical and experimental study.The thesis mainly comprised of following contents:1.The fiber laser spectral beam combining was analyzed systematically from the perspective of brightness amplification.The coherent beam combining system and spectral beam combining system were unified assessed by the brightness evaluation factor,and the pre-evaluation criteria for the performance of beam combining were given.As important parameters for describing the beam quality,the relationship between brightness,factor and Strehl ratio()was analyzed.The formula depicting the relationship between brightness and Strehl ratio was given,and the brightness amplification formulas of the coherent beam combining system and the spectral beam combining system were derived respectively.The power and brightness scaling capability of the coherent beam combining and spectral beam combining were compared through the brightness amplification formulas.And the results indicated that comparing to coherent beam combining,the SBC system is competent in achieving scaling amplification of power and brightness at the same time.2.From two aspects of beam quality and output power,the influence factors on simultaneous scaling amplification of power and brightness in fiber laser spectral beam combining system based on the dielectric grating were analyzed theoretically.On the one hand,in accordance with the principle of grating spectral combining,the influence factors on beam quality in single fiber laser diffraction in condition of spectral combining modes of single grating and double grating were analyzed and discussed in detail,which laid a theoretical basis for further clear-cut requirement of technical specification for single laser unit in dielectric grating spectral combining system.The numerical calculation results showed that,with appropriate selection of grating parameters and size of the light spot,the fiber laser with sub-nanometer scale of spectral line width can be used for high-power spectral synthesis and maintain good optical quality.Among them,comparing to single-grating spectral combining,the double-grating structure of the spectral combining is more conducive to increase the single laser unit line width and enhance the single laser output power,while increasing the radius of the beam and reducing the thermal load on optical components.On the other hand,starting from discussion of the relationships between the linewidth of single laser and the grating diffraction efficiency,the number of combining units and the resolution of grating,the combining output power and the combining laser beam diameter and the grating thermal load capacity and other related factors,the dielectric grating spectral combining output power and efficiency issues was analyzed,feasibility of using reflective dielectric grating as high power spectral combining device was clarified.3.The experimental research of fiber laser source with high output power and narrow spectral linewidth which is in practical use for spectral combining was conducted,including the fiber laser based on SMS structure,random fiber laser light source based on distributed feedback and single-frequency fiber laser amplifier.The experimental study on the linear cavity structure of the distributed feedback random fiber laser light source based on different feedback mechanism of Rayleigh reflection and ?-phase shift fiber grating,and the ring cavity structure of the laser based on fiber Bragg grating Fabry-Perot cavity were presented respectively.The experimental results illustrated that the fiber laser light source based on SMS structure can realize the kilowatt laser output through single level amplification,with the output bandwidth ~0.3 nm and PER ~15dB,and this single-polarized,high-power fiber laser amplification system with narrow linewidth,can be used as laser light source of dielectric grating spectrum combining.In addition,the DF fibe lase based on ?-shifted fiber gratings and the FBG-FPF-based ring cavity random laser both acquired ~10mW single-frequency laser output.Among them,FBG-FPF-based random laser ring output laser obtained single polarization output,with PER ~15dB.These two kinds of laser structure can be used as seed source for experimental study of further single-frequency fiber laser amplification.By improving the performance of the single unit output in fiber laser combining system,narrowing the spectral width of the unit module,the feasibility of simplifying the system structure is demonstrated.4.Aiming at the dielectric grating fiber laser combining system,a high power spectral combining experiment platform was constructed.The study on output beam quality and combining efficiency of the spectral combining system were conducted,in order to maintain the beam quality and expand number of the combining laser units.Experimental test of beam quality and diffraction efficiency of dielectric gratings based on Watt-level tunable fiber laser was presented.And it is verified that the degradation of the beam quality of the single fiber laser unit occurs in the direction of the grating diffraction,and the degradation of the beam quality is retarded correspondingly with the decrease in the linewidth of the fiber laser unit.At the same time,in order to verify multi-wavelength and wide-spectral combining ability of the dielectric grating used in the combining system,the 45 W class ultra-wideband wavelength interval fiber laser spectral combining experiment was presented.The effects of broad spectral interval on the beam quality and combining efficiency were analyzed,and the experimental results showed the full use of gain bandwidth Yb-doped fiber enables to expand the number of fiber laser combining units effectively to achieve spectral combining with more laser units,besides improving system output power and minimizing the pressure of single unit output.The fiber laser spectral combining experiment of high power and wide wavelength interval was carried out to demonstrate the feasibility of the spectrum combining scheme based on dielectric grating for high power ouput.