3~5μm稀土离子掺杂中红外光纤激光器的研究进展(特邀)

Research progress in 3-5μm rare earth ion doped mid-infrared fiber lasers(invited)

3~5μm中红外波段是一个极特殊的电磁波谱区间,它不仅覆盖着众多分子与原子的本征吸收峰,同时还是大气透明窗口之一。此波段的激光器在气体探测、生物医疗、国防等众多领域都具有很大的应用前景。文中围绕常用于3~5μm光纤激光产生的三种稀土离子(即Er^(3+)、Ho^(3+)和Dy^(3+)),对基于这些离子掺杂的连续和脉冲中红外光纤激光器的发展现状进行了梳理,最后对3~5μm掺稀土离子光纤激光器的发展进行了展望。

Significance In the mid-infrared band,3-5μm is a very special window,and it covers many intrinsic absorption peaks of molecules and atoms.Moreover,it is one of the transparent windows of the atmosphere.Therefore,lasers working in this band have great application prospects in various fields such as gas detection,material processing,biomedicine,military confrontation and remote sensing.Compared with quantum cascade lasers,solid-state lasers,and optical parametric lasers,fiber lasers have advantages of excellent beam quality,good heat dissipation,easy miniaturization and integration,high conversion efficiency,and good robustness.It stands out in the field of mid-infrared laser and has become a cutting-edge research hotspot in the field of laser.At present,the methods of generating 3-5μm mid-infrared fiber laser can be roughly divided into the following three categories:1)Direct lasing method based on rare earth ion-doped fiber;2)Nonlinear wavelength frequency shift;3)Supercontinuum generation.However,the latter two schemes usually using rare earth doped fiber lasers as the pump sources.In addition,the rare earth ion doped fiber lasers have advantages of high gain,large bandwidth,high nonlinear,easy integration and so on,and have gradually become one of the ideal platforms for mid-infrared.Therefore,the rare earth ion doped fiber lasers are the foundation and core of the development of 3-5μm band laser technology.Progress Three kinds of gain ions,Er^(3+),Ho^(3+)and Dy^(3+),which are commonly used in 3-5μm fiber lasers,are introduced in detail.The current development of continuous and pulsed fluoride fiber lasers based on these ions doping is reviewed,respectively.In recent years,the performance of 3-5μm continuous wave fiber lasers has been greatly improved.The output power of 15 W,10.1 W and 200 mW was achieved in the 3.5μm(Er^(3+):ZBLAN),3.2μm(Dy^(3+):ZBLAN),and 3.9μm(Ho^(3+):InF_(3)),respectively.Moreover,broadly tuning of~700 nm is realized in the spectral range of 2.710-3.415μm for a Dy^(3+):ZBLAN continuous wave laser.On the other hand,with the rapid development of mid-infrared gain-switching,Q-switching technology and related devices,3-5μm shortpulse lasers have made great technical breakthroughs.The largest output power of 1.4 W is achieved in a 3.22μm Er^(3+):ZBLAN fiber.However,there is still a long way to go for further improvement in stability,peak power and pulse energy.Ultra-short pulse fiber lasers with 3-5μm band have achieved breakthroughs,the longest wavelength of 3.61μm is achieved for mode-locked mid-infrared pulses.But the mode-locking pulse performance,including pulse width compression,power/energy improvement,wavelength expansion,noise suppression and stability improvement,still have many problems to be solved and studied.Conclusions and Prospects:In recent years,with the development and maturity of fluoride fiber drawing and doping techniques and the optimization and innovation of pumping mode,significant progress has been made in the field of middle infrared fiber lasers based on rare earth ions doping.The reported rare earth doped midinfrared fiber lasers in the 3-5μm band are reviewed from the perspective of continuous lasers and pulse lasers,respectively.The following trends are summarized:(1)The output power will be further improved.In recent years,continuous fiber lasers over 3μm have achieved output power of 15 W.However,compared with the~2.8μm fiber lasers,the output power of this band still has a large room for improvement.In future studies,the output power can be further increased to tens or even hundreds of watts by further optimizing the pump structure,optimizing the preparation process of fluoride fiber,optimizing the fiber welding technology and the performance of fiber passive devices such as fiber grating and fiber end caps.(2)The wavelength will be further extended to longer wavelengths.For rare-earth ions doped continuous laser,the current recorded wavelength output is 3.92μm,and no further breakthrough has been achieved since 2018,and the field of rare-earth ion doped fiber laser>4μm is still a blank.In future studies,it is worthwhile to further extend the output wavelength by developing new doping ions such as Tb^(3+).(3)Toward all-fiber configuration.There is no doubt that the all-fiber system is in line with the development trend of fiber lasers.In the band of 3-5μm,due to the lack of fiber functional devices and the imperfection of high quality fiber processing technology,the overall all-optical fiber level is low.In the future research,the development of active and passive fiber functional devices for mid-infrared laser generation should be accelerated.It can be predicted that in the near future,high performance,all-fiber 3-5μm rare earth ion-doped midinfrared laser will move from the laboratory to many practical fields,and promote the technical development and progress of industry,medical,national defense and other related fields.