Q-Switched Raman Fiber Laser with Molybdenum Disulfide-Based Passive Saturable Absorber

We demonstrate a Q-switched Raman fiber laser using molybdenum disulfide （MoS2） as a saturable absorber （SA）. The SA is assembled by depositing a mechanically exfoliated MoS2 onto a fiber ferrule facet before it is matched with another clean ferrule via a connector. It is inserted in a Raman fiber laser cavity with a total cavity length of about 8kin to generate a Q-switching pulse train operating at 1560.2nm. A 7.7-kin-long dispersion compensating fiber with 584 ps.nm-i km-1 of dispersion is used as a nonlinear gain medium. As the pump power is increased from 395 m W to 422 m W, the repetition rate of the Q-switching pulses can be increased from 132.7 to 137.4 kHz while the pulse width is concurrently decreased from 3.35μs to 3.03μs. The maximum pulse energy of 54.3 nJ is obtained at the maximum pump power of 422 roW. These results show that the mechanically exfoliated MoS2 SA has a great potential to be used for pulse generation in Raman fiber laser systems.

Mid-Infrared Raman Fiber Lasers

As mid-infrared （MIR） lasers show numerous applications in the field of defense, medical, materials processing, and optical communications. Investigation on MIR Raman fiber lasers （RFL） increasingly becomes a hot topic. Compared with traditional silica fiber, fluoride and chalcogenide glass fibers possess higher nonlinear coefficients and excellent MIR transmittances. In this article, the latest development of the MIR RFL using fluoride and chalcogenide glass fibers as gain media are introduced, respectively. This review article mainly focuses on the development of MIR RFLs in aspects of output wavelength, output power and optical efficiency. The prospect of MIR RFLs is also discussed.

High-power linearly-polarized tunable Raman fiber laser

In this study, we demonstrate an all-fiber high-power linearly-polarized tunable Raman fiber laser system. An in- house high-power tunable fiber laser was employed as the pump source. A fiber loop mirror （FLM） serving as a high reflectivity mirror and a flat-cut endface serving as an output coupler were adopted to provide broadband feedback. A piece of 59-m commercial passive fiber was used as the Raman gain medium. The Raman laser had a 27.6 nm tuning range from 1112 nm to 1139.6 nm and a maximum output power of 125.3 W, which corresponds to a conversion efficiency of 79.4%. The polarization extinction ratio （PER） at all operational wavelengths was measured to be over 21 dB. To the best of our knowledge, this is the first report on a hundred-watt level linearly-polarized tunable Raman fiber laser.

High power tunable Raman fiber laser at 1.2μm waveband

Development of a high power fiber laser at special waveband,which is difficult to achieve by conventional rare-earth-doped fibers,is a significant challenge.One of the most common methods for achieving lasing at special wavelength is Raman conversion.Phosphorus-doped fiber(PDF),due to the phosphorus-related large frequency shift Raman peak at 40 THz,is a great choice for large frequency shift Raman conversion.Here,by adopting 150 m large mode area triple-clad PDF as Raman gain medium,and a novel wavelength-selective feedback mechanism to suppress the silica-related Raman emission,we build a high power cladding-pumped Raman fiber laser at 1.2μm waveband.A Raman signal with power up to 735.8 W at 1252.7 nm is obtained.To the best of our knowledge,this is the highest output power ever reported for fiber lasers at 1.2μm waveband.Moreover,by tuning the wavelength of the pump source,a tunable Raman output of more than 450 W over a wavelength range of 1240.6–1252.7 nm is demonstrated.This work proves PDF’s advantage in high power large frequency shift Raman conversion with a cladding pump scheme,thus providing a good solution for a high power laser source at special waveband.

Short-pulsed Raman fiber laser and its dynamics

We provide a perspective review over the recent development of short-pulsed Raman fiber lasers(RFLs),which can provide laser emissions with flexible wavelengths for a variety of applications as well as an excellent platform to investigate various nonlinear pulse dynamics behaviors that cannot be captured in conventional rare-earth(RE)doped counterparts.Various pulse generation techniques have been explored in RFLs.However,the output pulse performance in terms of the pulse energy,duration and stability from short-pulsed RFLs is still inferior to their RE-doped counterparts despite significant advances made over the past few decades.Therefore,more efforts are required to improve these targets.In this review,we present a detailed overview of the short-pulsed RFLs based on different mechanisms from the principle to the experiment,including the Q-switching,gainswitching,mode-locking,synchronous pumping and other innovative techniques.In addition,Raman-induced pulse dynamics in ultrafast RFLs and RE-doped mode-locked fiber lasers(MLFLs)are briefly reviewed.Finally,a perspective outlook for the future ultrafast RFLs is provided based on their potential applications in industrial and scientific research areas.

Cascaded Random Raman Fiber Laser With Low RIN and Wide Wavelength Tunability

Cascaded random Raman fiber lasers(CRRFLs)have been used as a new platform for designing high power and wavelength-agile laser sources.Recently,CRRFL pumped by ytterbium-doped random fiber laser(YRFL)has shown both high power output and low relative intensity noise(RIN).Here,by using a wavelength-and bandwidth-tunable point reflector in YRFL,we experimentally investigate the impacts of YRFL on the spectral and RIN properties of the CRRFL.We verify that the bandwidth of the point reflector in YRFL determines the bandwidth and temporal stability of YRFL.It is found that with an increase in the bandwidth of the point reflector in YRFL from 0.2 nm to 1.4 nm,CRRFL with higher spectral purity and lower RIN can be achieved due to better temporal stability of YRFL pump.By broadening the point reflector’s bandwidth to 1.4 nm,the lasing power,spectral purity,and RIN of the 4th-order random lasing at 1349 nm can reach 3.03 W,96.34%,and–115.19 dB/Hz,respectively.For comparison,the spectral purity and RIN of the 4th-order random lasing with the point reflector’s bandwidth of 0.2 nm are only 91.20%and–107.99 dB/Hz,respectively.Also,we realize a wavelength widely tunable CRRFL pumped by a wavelength-tunable YRFL.This work provides a new platform for the development of ideal distributed Raman amplification pump sources based on CRRFLs with both good temporal stability and wide wavelength tunability,which is of great importance in applications of optical fiber communication and distributed sensing.

Investigation on extreme frequency shift in silica fiber-based high-power Raman fiber laser

In this paper, we experimentally investigated the extreme frequency shift in high-power Raman fiber laser(RFL). The RFL was developed by using a pair of fiber Bragg gratings with fixed and matched central wavelength(1120 nm)combined with a piece of 31-m-long polarization maintaining(PM) passive fiber adopted as Raman gain medium.The pump source was a homemade high-power, linearly polarized(LP) wavelength-tunable master oscillator power amplifier(MOPA) source with ～25 nm tunable working range(1055–1080 nm). High-power and high-efficiency RFL with extreme frequency shift between the pump and Stokes light was explored. It is found that frequency shift located within 10.6 THz and 15.2 THz can ensure efficient Raman lasing, where the conversion efficiency is more than 95% of the maximal value, 71.3%. In addition, a maximum output power of 147.1 W was obtained with an optical efficiency of71.3%, which is the highest power ever reported in LP RFLs to the best of our knowledge.

Hundred-watt level high-order mode all-fiberized random distributed feedback Raman fiber laser with high mode purity

An all-fiberized random distributed feedback Raman fiber laser(RRFL)with LP_(11) mode output at 1134 nm has been demonstrated experimentally,where an intracavity acoustically induced fiber grating is employed for modal switching.The maximum output power of LP_(11) mode is 93.8 W with the modal purity of 82%,calculated by numerical mode decomposition technology based on stochastic parallel-gradient descent algorithm.To our best knowledge,this is the highest output power with high purity of LP_(11) mode generated from the RRFL.This work may pave a path towards advanced fiber lasers with special temporal and spatial characteristics for applications.

Dual-wavelength bidirectional pumped high-power Raman fiber laser

In this paper, we reported both the experimental demonstration and theoretical analysis of a Raman fiber laser based on a master oscillator–power amplifier configuration. The Raman fiber laser adopted the dual-wavelength bidirectional pumping configuration, utilizing 976 nm laser diodes and 1018 nm fiber lasers as the pump sources. A 60-m-long25/400 μm ytterbium-doped fiber was used to convert the power from 1070 to 1124 nm, realizing a maximum power output of 3.7 kW with a 3 dB spectral width of 6.8 nm. Moreover, we developed a multi-frequency model taking into consideration the Raman gain spectrum and amplified spontaneous emission. The calculated spectral broadening of both the forward and backward laser was in good agreement with the experimental results. Finally, a 1.5 kW, 1183 nm second-order Raman fiber laser was further experimentally demonstrated by the addition of a 70-m-long germaniumdoped passive fiber.

STUDY ON THE MICROSTRUCTURE AND PROPERTY OF PET FIBER BY LASER RAMAN MICROSCOPE

In this paper, the microstructure change of one step-draw PET fiber has been studied byvarious methods, such as, Laser Raman Microscope, Wide-angle X-ray, Density-gradient andPolarizing Microscope. The computer has been used to resolve overlapped bands in the Ramanspectra. Then the band changes have been correlated with trans, gauche and stressed trans-conformations indicated by a conformational index. Based on these indices, the relationshipbetween the conformation change of glycol units in the fiber structure and the macromechanicalproperties of fiber is expounded.

Development of Fiber Lasers and Their Applications

Development of fiber lasers, especially the latest progress in high power fiber lasers, and the problems of the commercialization of high power fiber lasers, are discussed. Research on other kinds of fiber lasers, such as microstructure fiber laser, ultrashort pulse fiber lasers, Raman fiber lasers are also involved.

2 kW single-mode fiber laser employing bidirectional-pump scheme

A 2 kW single-mode fiber laser with two cascade home-made cladding light strippers(CLSs) has been demonstrated by employing bidirectional-pump scheme.2.009 k W signal power is obtained when pump power is 2.63 kW and the slope efficiency is 76.6%.Raman Stokes light is less than-47 d B at 2.009 kW even with a 10-m delivery fiber with core/inner cladding diameter of 20 mm/400 mm.The beam quality M^2≤1.2 and the spectral FWHM bandwidth is 4.34 nm.There is no transverse mode instability and the output power stability of ±0.14% is achieved by special thermal management for a more uniform temperature distribution on the Yb-doped gain fiber.

Raman光纤放大器在远距离遥感探测中的应用

首次将Raman光纤放大技术与波长调制技术结合,使用Raman光纤放大器对近红外激光进行了功率放大,并将放大后的激光应用于远距离天然气管道泄漏遥测中.通过测试Raman光纤放大器在1650 nm波段的性能,发现Raman光纤放大器输出功率为P_(out)时,为保证输出信号不发生形变所需的种子光源调制频率f与P_(out)满足特定的线性关系,并以此为基础实现了500 m远距离激光遥感探测,为在机载平台上开展天然气泄漏激光遥测技术研究提奠定了基础.

辅助光对高功率掺镱光纤激光放大器受激拉曼散射效应的抑制作用

提出一种利用辅助光调控高功率掺镱光纤放大器中增益分布,以控制信号光的受激拉曼散射(SRS)增益、提高SRS阈值的方法.基于数值模拟分析了辅助光的波长以及辅助光和信号光的功率比例对放大器SRS阈值的影响规律.计算结果显示,通过引入适当波长和功率的辅助光可以有效提升放大器的SRS阈值.

全光纤结构拉曼振荡器实现近1.8 kW激光输出

拉曼光纤激光器在实现高功率特殊波长激光方面具有独特优势,受到了广泛关注和研究。近年来,随着渐变折射率多模光纤在拉曼激光器中的运用,不仅提高了可注入的泵浦功率,还通过光束净化特性实现了激光亮度增强。

用于可移动量子重力仪的光纤拉曼激光源研究

基于两个窄线宽光纤激光器和自制的锁相控制系统,研制了一个低相位噪声窄线宽拉曼光纤激光源,建立了拉曼光纤激光系统的相位噪声模型,研究了拍频光功率、不同频率参考源对拉曼激光相位噪声的影响。实验结果表明,该拉曼光纤激光源在10 Hz~1 MHz范围内相位噪声功率谱密度可达到^(-11)8 dBc/√Hz,相应的相位噪声为22.7 mrad,将其应用到重力仪中获得的重力测量灵敏度为10.93μGal/√Hz。对拉曼激光源进行连续3 h的相位噪声稳定性测试和连续25 min的频率稳定性测试,得到的相位噪声标准差为0.734 mrad,相应的重力灵敏度标准差仅为0.349μGal/√Hz。当积分时间为1 s时,相位锁定时频率稳定度为2.675×10^(-11),验证了该激光源具有低相位噪声和高稳定性。利用相位噪声模型得到的理论结果与实验结果一致,证实了理论模型的正确性。

大频移范围光纤拉曼增益谱的测量

光纤拉曼增益谱的测量对光纤激光器的设计与优化至关重要。当前光纤拉曼增益谱的测量主要是基于小信号增益法,但该方法测试时间较长,且可测量的频移范围受限于信号源的波长调谐范围,难以获得光纤在大频移范围内的拉曼增益谱。为此,提出了一种利用光纤的自发拉曼散射谱来反演光纤在大频移范围内拉曼增益谱的方法。利用该方法测量了两款光纤——掺磷光纤与掺锗光纤在1~42 THz频移范围内的拉曼增益谱。在1.6~22 THz频移范围内,基于该方法测得的拉曼增益系数与基于小信号增益法测得的结果基本一致。该工作为测量光纤在大频移范围内的拉曼增益谱提供了一种解决方案。

Hundred-watt-level linearly polarized tunable Raman random fiber laser

A high power linearly polarized tunable Raman random fiber laser（RFL） was studied theoretically and experimentally. The parameters required for the system design were obtained through numerical simulation, based on which a hundred-watt-level linearly polarized tunable RFL was successfully demonstrated. The central wavelength can be continuously tuned from 1113.76 to 1137.44 nm, and the output power exceeds 100 W for all of the lasing wavelengths with the polarization extinction ratio（PER） exceeding 20 d B at the maximum output power.Besides, the linewidth, spectral evolution, and temporal dynamics of a specified wavelength（1124.72 nm） were investigated in detail. Moreover, the theoretical results and the experimental results fit well. To the best of our knowledge, this is the first time for a hundred-watt-level linearly polarized tunable RFL ever reported.

基于混沌激光脉冲簇的拉曼分布式光纤传感技术 (特邀)

提出一种基于混沌激光脉冲簇的拉曼分布式光纤传感方案理论模型。该模型利用混沌脉冲簇激光代替传统脉冲激光作为拉曼分布式光纤传感系统的探测信号,采用延时自差分联合时域相关峰判别技术,剥离出传感光纤温度突变区域内混沌拉曼散射信号的光强信息,实现对温度突变区域的精准定位和识别。该方案通过利用混沌脉冲簇探测信号增强混沌参考信号与混沌拉曼散射信号的相关性,可有效抑制系统噪声。此外,该方法也可通过提高耦合光通量来提升系统信噪比,从而解决混沌单脉冲激光拉曼分布式光纤传感系统信噪比较低、传感距离较短的技术瓶颈。利用数值模拟仿真探究了混沌激光脉冲簇中的脉冲个数对系统传感性能(动态范围、信噪比、传感空间分辨率及温度敏感性)的影响,并分析了在混沌激光脉冲簇拉曼分布式光纤传感系统中性能最优值的脉冲个数,可实现15 km传感距离和10 cm空间分辨率的分布式温度传感。

High-brightness all-fiber Raman lasers directly pumped by multimode laser diodes

High-brightness fiber laser sources usually utilize active rare-earth-doped fibers cladding-pumped by multimode laser diodes(LDs), but they operate in limited wavelength ranges. Singlemode-passive-fiber based Raman lasers are able to operate at almost any wavelength being pumped by high-power fiber lasers. One of the interesting possibilities is to directly pump graded-index(GRIN) multimode passive fibers by available high-power multimode LDs at 915–940 nm,thus achieving high-power Raman lasing in the wavelength range of 950–1000 nm, which is problematic for rare-earthdoped fiber lasers. Here we review the latest results on the development of all-fiber high-brightness LD-pumped sources based on GRIN fiber with in-fiber Bragg gratings(FBGs). The mode-selection properties of FBGs inscribed by fs pulses supported by the Raman clean-up effect result in efficient conversion of multimode pump into a high-quality output beam at 9 xx nm. GRIN fibers with core diameters 62.5, 85 and 100 μm are compared. Further scaling capabilities and potential applications of such sources are discussed.