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| 1 | 1.3 kW monolithic linearly polarized single-mode master oscillator power amplifier and strategies for mitigating mode instabilities显示文摘We report on the high-power amplification of a 1064 nm linearly polarized laser in an all-fiber polarizationmaintained master oscillator power amplifier,which can operate at an output power level of 1.3 kW.The beam quality(M^2) was measured to be <1.2 at full power operation.The polarization extinction rate of the fiber amplifier was measured to be above 94% before mode instabilities(MIs) set in,which reduced to about 90% after the onset of MI.The power scaling capability of strategies for suppressing MI is analyzed based on a semianalytical model,the theoretical results of which agree with the experimental results.It shows that mitigating MI by coiling the gain fiber is an effective and practical method in standard double-cladding large mode area fiber,and,by tight coiling of the gain fiber to the radius of 5.5 cm,the MI threshold can be increased to three times higher than that without coiling or loose coiling.Experimental studies have been carried out to verify the idea,which has proved that MI was suppressed successfully in the amplifier by proper coiling. | Rumao Tao Pengfei Ma Xiaolin Wang Pu Zhou Zejin Liu | 2015 | Photonics Research2015，3，3： | 15 |
| 2 | High power all-fiberized and narrow-bandwidth MOPA system by tandem pumping strategy for thermally induced mode instability suppression显示文摘An all-fiberized and narrow-bandwidth master oscillator power amplification(MOPA) system with record output power of 4 kW level and slope efficiency of 78% is demonstrated. Tandem pumping strategy is tentatively introduced into the narrow-bandwidth MOPA system for thermally induced mode instability(TMI) suppression. The stimulated Brillouin scattering(SBS) effect is balanced by simply using one-stage phase modulation technique. With different phase modulation signals, SBS limited output powers of 336 W, 1.2 kW and 3.94 kW are respectively achieved with spectral bandwidths accounting for 90% power of ~0.025, 0.17 and ~0.89 nm. Compared with our previous 976 nm pumping system, TMI threshold is overall boosted to be >5 times in which tandem pumping increases the TMI threshold of >3times. The beam quality(M~2 factor) of the output laser is well within 1.5 below the TMI threshold while it is ultimately saturated to be 1.86 with the influence of TMI at maximal output power. Except for SBS and TMI, stimulated Raman scattering(SRS) effect will be another challenge for further power scaling. In such a high power MOPA system, multidetrimental effects(SBS, SRS and TMI) will coexist and may be mutual-coupled, which could provide a well platform for further comprehensively investigating and optimizing the high power, narrow-bandwidth fiber amplifiers. | Pengfei Ma Hu Xiao Daren Meng Wei Liu Rumao Tao Jinyong Leng Yanxing Ma Rongtao Su Pu Zhou Zejin Liu | 2018 | High Power Laser Science and Engineering2018，6，4： | 9 |
| 3 | Experimental study of mode distortion induced by stimulated Raman scattering in high-power fiber amplifiers显示文摘The experimental investigation of mode distortion induced by stimulated Raman scattering(SRS)in a high-power fiber amplifier,which includes the evolutions of optical spectra,spatial beam profiles,and time-frequency characteristics,has been carried out in detail.Temporal-frequency characteristics have been studied for the first time,to the best of our knowledge,by using a low-speed camera and high-speed photodiode traces,which revealed that temporal-frequency characteristics of SRS-induced mode distortion are different from traditional dynamic mode instability(MI).The experimental results show that the output beam profile remains stable before the mode distortion occurs and fluctuates obviously after the onset of SRS-induced MI but on a time scale of seconds,which is much lower than that of Yb-gain-induced MI featuring millisecond-level beam profile fluctuation.It also shows that the mode distortion became measurable in company with the onset of inter-mode four-wave mixing(IM-FWM)when the ratio of Raman light reaches 3%;further,the beam quality factor M2degrades gradually from 1.4 to 2.1 as the ratio of Raman light increases.The mode distortion is accompanied by an obvious temperature increase of the output passive fiber,which further confirms that the mode distortion originates from SRS.The cause of the mode distortion induced by SRS has been explained in the context of core-pumped SRS effect,and the investigation on the accompanying IM-FWM effect indicates that the main content of the SRSinduced high-order mode is the LP21 mode. | QIUHUI CHU QIANG SHU ZENG CHEN FENGYUN LI DONGLIN YAN CHAO GUO HONGHUAN LIN JIANJUN WANG FENG JING CHUANXIANG TANG RUMAO TAO | 2020 | Photonics Research2020，8，4： | 6 |
| 4 | Propagation of high-power fiber laser with high-order-mode content显示文摘Propagation properties of high-power fiber laser with high-order-mode(HOM) content are studied numerically for the first time to the best of our knowledge. The effect of HOM on the propagation property is evaluated by the power in the bucket(PIB) metric. It is shown that PIB is mainly dependent on HOM content rather than the relative phase between the fundamental mode and HOM. The PIB in vacuum is more than 80% when the power fraction of the HOM is controlled to be less than 50% at 5 km. The relative phase has an impact on the peak intensity position and concentration of the far-field intensity distribution. If an adaptive optics system is used to correct the peak intensity deviation, the results indicate that there exists a maximal value of PIB as relative phase increases. Such effect is weakened when propagating in turbulence. Compared to the laser beams without HOM, laser beams with HOM content are less influenced by the turbulence and can reduce average intensity fluctuation. The results may be useful in the design of a high-power fiber laser system. | Rumao Tao Long Huang Pu Zhou Lei Si Zejin Liu | 2015 | Photonics Research2015，3，4： | 5 |
| 5 | Comprehensive investigation on producing high-power orbital angular momentum beams by coherent combining technology显示文摘High-power orbital angular momentum(OAM)beams have distinct advantages in improving capacity and data receiving for free-space optical communication systems at long distances.Utilizing the coherent combination of a beam array technique and helical phase approximation by a piston phase array,we have proposed a generating system for a novel high-power beam carrying OAM,which could overcome the power limitations of a common vortex phase modulator and a single beam.The characteristics of this generating method and the orthogonality of the generated OAM beams with different eigenstates have been theoretically analyzed and verified.Also a high-power OAM beam produced by coherent beam combination(CBC)of a six-element hexagonal fiber amplifier array has been experimentally implemented.Results show that the CBC technique utilized to control the piston phase differences among the array beams has a high efficiency of 96.3%.On the premise of CBC,we have obtained novel vortex beams carrying OAM of±1 by applying an additional piston phase array modulation on the corresponding beam array.The experimental results agree approximately with the theoretical analysis.This work could be beneficial to areas that need high-power OAM beams,such as ultra-long distance free-space optical communications,biomedical treatments,and powerful trapping and manipulation under deep potential wells. | Dong Zhi Tianyue Hou Pengfei Ma Yanxing Ma Pu Zhou Rumao Tao Xiaolin Wang Lei Si | 2019 | High Power Laser Science and Engineering2019，7，2： | 3 |
| 6 | Towards tapered-fiber-based all-fiberized high power narrow linewidth fiber laser显示文摘Yb-doped fiber laser(YDFL) is a promising kind of laser source due to high efficiency, excellent beam quality, robust configuration, diverse applications and so on. The progress of large-mode-area double cladding active fiber and high power laser diodes have been continuously promoting the power scaling of the YDFLs. However, the further power scaling is blocked by some physic issues including nonlinear effect, thermal-induced mode instability(MI), optical damage and so forth. So, newlydesigned fibers with special structures are invented to address or relieve these limitations. The tapered fiber, which has changed mode area along its longitudinal direction, has been demonstrated to benefit suppressing the nonlinear effect, especially the stimulated Brillouin scattering(SBS) effect. In this paper, we will review the previous work done to explore the advantages of the tapered fiber for high power output, and then for the first time to our knowledge, demonstrate an all-fiberized high power narrow linewidth fiber laser based on a homemade tapered Yb-doped fiber(T-YDF). As a result, an output power of 260 W with a narrow linewidth of ~2 GHz is obtained, with SBS effectively suppressed owing to gradually increased mode area of the T-YDF as well as continuously changed Brillouin frequency shift dependent on the core diameter. Further power scaling is limited by MI, and the features of MI are studied in detail. It is pointed out that, the MI seems the primary and durative limitation factor of the tapered fiber amplifier for high power narrow linewidth output due to the gradually increased effective numerical aperture(NA) and the excitation of the high-order modes, even although it has the advantage to reduce other nonlinear effects. At last,worthwhile discussion about the optimization of the T-YDF and the whole fiber laser system for both SBS and MI suppression is conducted based on the previous and our results. It is concluded that the small diameter at the small-diameter end and low optical NA are preferred in the design of a T-YDF to ensure good beam quality and improve the MI threshold. The good splicing quality and proper coiling or bend state are also necessary. | HUANG Long ZHOU ZiChao SHI Chen TAO RuMao MA PengFei WANG XiaoLin ZHOU Pu | 2018 | Science China(Technological Sciences)2018，61，7： | 2 |
| 7 | Monolithic high-average-power linearly polarized nanosecond pulsed fiber laser with near-diffraction-limited beam quality显示文摘An all-fiberized high-average-power narrow linewidth ns pulsed laser with linear polarization is demonstrated. The laser system utilizes a typical master oscillator power amplifier(MOPA) configuration. The stimulated Brillouin scattering(SBS) is effectively suppressed due to the short fiber length and large mode area in the main amplifier, combined with the narrow pulse duration smaller than the phonon lifetime of SBS effect. A maximal output power of 466 W is obtained with a narrow linewidth of ~203.6 MHz, and the corresponding slope efficiency is ~80.3%. The pulse duration is condensed to be ~4 ns after the amplification, corresponding to the peak power of 8.8 kW and the pulse energy of 46.6 μJ. Neardiffraction-limited beam quality with an M2 factor of 1.32 is obtained at the output power of 442 W and the mode instability(MI) is observed at the maximal output power. To the best of our knowledge, this is the highest average output power of the all-fiberized narrow linewidth ns pulsed fiber laser with linear polarization and high beam quality, which is a promising source for the nonlinear frequency conversion, laser lidar, and so on. | Long Huang Pengfei Ma Daren Meng Lei Li Rumao Tao Rongtao Su Yanxing Ma Pu Zhou | 2018 | High Power Laser Science and Engineering2018，6，3： | 2 |
| 8 | Principle and numerical demonstration of high power all-fiber coherent beam combination based on self-imaging effect in a square core fiber显示文摘The self-imaging effect in a square core fiber has been investigated, and an integrated all-fiber combiner has been proposed based on a large mode area double clad fiber, which can be employed to construct high power coherent beam combining sources in the all-fiber format. The influence of various parameters on beam quality(M^(2)) and efficiency of the all-fiber coherent beam combiner has been studied numerically, which reveals that the near diffraction-limited laser beam can be achieved. A principle demonstration of the self-imaging effect has been carried out experimentally in a square core fiber, which proves the feasibility of beam combining with the square fiber, and that it is a promising way to develop high power coherent beam combination sources. | Yuefang Yan Yu Liu Haoyu Zhang Yue Li Yuwei Li Xi Feng Donglin Yan Jianjun Wang Honghuan Lin Feng Jing Wenhui Huang Rumao Tao | 2022 | Photonics Research2022，10，2： | 2 |
| 9 | Coherent beam combination of fiber lasers with a strongly confined tapered self-imaging waveguide:theoretical modeling and simulation显示文摘Coherent beam combination(CBC)of fiber lasers based on self-imaging properties of a strongly confined tapered waveguide(SCTW)is studied systematically.Analytical formulas are derived for the positions,amplitudes,and phases of the N self-images at the output of a SCTW,which are important for quantitative analysis of waveguidebased CBC.The formulas are verified with numerical examples by a finite difference beam propagation method(FDBPM)and the errors of the analytical expressions are studied.This study shows that the analytical formulas agree well with the FDBPM simulation results when the taper angle is less than 1.4°and the phase distortion is less thanλ∕10.The relative errors increase as the taper angle increases.Based on the theoretical model and the FDBPM,we simulated the CBC of fiber laser array and compared the CBC based on the tapered waveguide with that based on the nontapered one.The effects of input beam number,aperture fill factor,and taper angle on the combination performance have been studied.The study reveals that a beam which has near-diffraction limited beam quality(M2≤1.41)and a single beam without side lobe in the far field can be achieved with tapered-waveguide-based CBC.It is shown that beam quality depends on input beam number,aperture fill factor,and taper angle.There exists a best fill factor which will increase as input beam number increases.The tolerance of the system on the fill factor and the taper angle is studied,which is 0.45 | Rumao Tao Xiaolin Wang Hu Xiao Pu Zhou Lei Si | 2013 | Photonics Research2013，1，4： | 2 |
| 10 | Average spreading of finite energy Airy beams in non-Kolmogorov turbulence显示文摘 | Tao Rumao Si Lei Ma Yanxing | 2013 | Opt Laser Eng2013，51，4： | 1 |
| 11 | Toward high-power nonlinear fiber amplifier显示文摘Stimulated Raman scattering(SRS) effect is considered to be one of the main obstacles for power scaling in general-type fiber lasers. Different from previous techniques that aim at suppressing SRS, nonlinear fiber amplifier(NFA), which manipulates and employs the SRS for power scaling in rare-earth-doped fiber, is under intensive research in recent years.In this paper, the authors will present an all-round study on this new kind of high-power fiber amplifier. A theoretical model is proposed based on the rate equation and amplified spontaneous emission(ASE), with random noise taken into account. By numerical solving of the theoretical model, the power scaling potential, heat analysis and advantages in suppressing the undesired backscattering light are quantificationally analyzed for the first time. Then two different types of high-power NFAs are demonstrated individually. Firstly, a laser diode pumped NFA has reached kilowatt output power,and the results agree well with theoretical predictions. Secondly, a tandem-pumped NFA is proposed for the first time and validated experimentally, in which 1.5 kW output power has been achieved. The authors also briefly discuss several new issues relating to the complex nonlinear dynamics that occur in high-power NFAs, which might be interesting topics for future endeavors. | Hanwei Zhang Pu Zhou Hu Xiao Jinyong Leng Rumao Tao Xiaolin Wang Jiangming Xu Xiaojun Xu Zejin Liu | 2018 | High Power Laser Science and Engineering2018，6，3： | 1 |
| 12 | Kilowatt high average power narrow-linewidth nanosecond all-fiber laser显示文摘A high power narrow-linewidth nanosecond all-fiber laser based on the master oscillator power amplifier(MOPA)configuration is demonstrated. A pulsed seed with high repetition rate of 10 MHz was generated by modulating a continuous-wave(CW) single-frequency fiber laser at ~1064 nm by using an electro-optic intensity modulator(EOIM).After multi-stage cascaded power amplification, the average power was boosted to be kilowatt level. The pulses from the main amplifier had a pulse width of ~3 ns and an average/peak power of 913 W/28.6 kW. Further power scaling of the pulses was limited by stimulated Raman scattering(SRS) for the moment, method for SRS suppression and further power scaling was briefly discussed. | Rongtao Su Pu Zhou Xiaolin Wang Rumao Tao Xiaojun Xu | 2014 | High Power Laser Science and Engineering2014，2，1： | 1 |
| 13 | Average spreading of finite energy Airy beams in non-Kolmogorov turbulence 显示文摘 | Rumao Tao Lei Si Yanxing Ma et aZ | 2013 | Optics & Lasers in Engineering2013，51，4： | 1 |
| 14 | Static and dynamic mode evolution in high-power distributed side-coupled cladding-pumped fiber amplifiers显示文摘We present a theoretical study of mode evolution in high-power distributed side-coupled cladding-pumped(DSCCP)fiber amplifiers.A semi-analytical model taking the side-pumping schemes,transverse mode competition,and stimulated thermal Rayleigh scattering into consideration has been built,which can model the static and dynamic mode evolution in high-power DSCCP fiber amplifiers.The mode evolution behavior has been investigated with variation of the fiber amplifier parameters,such as the pump power distribution,the length of the DSCCP fiber,the averaged coupling coefficient,the number of the pump cores and the arrangement of the pump cores.Interestingly,it revealed that static mode evolution induced by transverse mode competition is different from the dynamic evolution induced by stimulated thermal Rayleigh scattering.This shows that the high-order mode experiences a slightly higher gain in DSCCP fiber amplifiers,but the mode instability thresholds for DSCCP fiber amplifiers are higher than those for their end-coupled counterparts.By increasing the pump core number and reducing the averaged coupling coefficient,the mode instability threshold can be increased,which indicates that DSCCP fibers can provide additional mitigation strategies of dynamic mode instability. | Rumao Tao Yu Liu Lianghua Xie Cong Gao Min Li Benjian Shen Shan Huang Honghuan Lin Jianjun Wang Feng Jing | 2021 | High Power Laser Science and Engineering2021，9，4： | 0 |
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