Progress in Research on Visible Rare-Earth-Doped Fiber Lasers:from Continuous Wave to Femtosecond Pulses(Invited)
Significance Visible lasers,with wavelengths ranging from 380 nm to 780 nm,have important applications in the fields of display,biomedicine,precision processing,precision spectroscopy,optical communication,and military defense.Among all the different visible lasers currently available,the rare-earth-doped fiber ones attract considerable attention due to their advantages of high efficiency,excellent performance,compact structure,and maintenance-free nature.In this study,different types of lasers,including visible continuous-wave(CW)fiber lasers,visible Q-switched fiber lasers,and visible mode-locked fiber lasers,are discussed comprehensively,along with their output characteristics.The latest research progress indicates that these lasers can cover the entire visible wavelength range and present different colors,such as blue(～480 nm),cyan(～491 nm),green(～520 nm),yellow(～573 nm),orange(～605 nm),red(～635 nm),and deep-red(～717 nm).The output power approaches 10 W for the all-fiber visible lasers,and the pulse duration of the mode-locked pulse is less than 200 fs.Thus,the all-fiber visible lasers play an increasingly important role in underwater optical communication,material processing,laser welding,and spatiotemporal super-resolution imaging.This study summarizes the progress in the research on visible fiber lasers,which provides a strong basis for any future research and application on visible fiber lasers.Progress With continuous research on fluoride fibers doped with rare-earth metal ions like Pr3+,Ho3+,Er3+,Dy3+,Tm3+,and Nd3+,visible CW fiber lasers,visible Q-switched fiber lasers,and visible mode-locked fiber lasers have been actively developed.After nearly 30 years of development,the outputs of blue,green,yellow,red,and deep-red fiber lasers have been scaled up to Watt-level.Notably,the maximum output powers of red(～635 nm)and green(～521 nm)fiber lasers reach～5 W and～3.6 W,respectively,as shown in Fig.8 and Fig.11.Visible mode-locked fiber lasers have the advantages of higher peak power and shorter response time than visible Q-switched fiber lasers.The development of visible mode-locked fiber lasers has been accelerated by the development of high-performance rare-earth-doped fluoride fibers.In 2020,Zou et al.reported the first all-fiber visible-wavelength(635 nm)passively mode-locked picosecond laser with a pulse duration as short as～96 ps.In the following two years,red-light mode-locked fiber lasers were further developed.As shown in Fig.15,a 635-nm spatiotemporal mode-locking(STML)picosecond fiber laser with the implementation of a Pr3+/Yb3+co-doped few-mode fiber and nonlinear polarization rotation(NPR)technology was reported by Ruan et al.in 2022.By further incorporating a visible ultrafast fiber amplifier,the average power at 635 nm was boosted up to 440 mW,corresponding to a maximum pulse energy and a peak power of 4 nJ and 280 W,respectively,while the pulse duration was shortened to 9 ps.This fills the knowledge gap of STML in the visible fiber lasers.By integrating the NPR scheme into Dy∶ZBLAN and Ho∶ZBLAN fiber lasers,Luo et al.obtained dissipative soliton resonance pulses at～575 nm and～545 nm,respectively.The average output power at 575 nm reached a maximum of～240 mW,which represents an improvement of almost two orders of magnitude compared to those reported for the latest mode-locked visible fiber lasers.The minimal pulse duration at 575 nm is 83 ps as shown in Fig.16.Furthermore,by using a shorter gain fiber(Ho∶ZBLAN),the smallest pulse duration of 19.7 ps is achieved for the ultrafast true-green passively mode-locked fiber laser.The average output power at 545 nm reaches a maximum of～288 mW,thus filling the"green gap"of semiconductor materials.To obtain mode-locked femtosecond pulses in the visible spectrum,a team from the Laval University reported a mode-locked fiber laser with a compressed external cavity that produced ultrafast pulses at 635 nm.The passively mode-locked ring cavity is based on nonlinear polarization evolution in a single-mode Pr3+-doped fluoride fiber and runs in an all-normal dispersion regime.The compressed pulses at 635 nm have a duration of 168 fs,a peak power of 0.73 kW,and a repetition rate of 137 MHz(Fig.17).Furthermore,the pulses directly emitted in a visible fiber oscillator by a phase-biased nonlinear amplifying loop mirror have durations less than 200 fs.Conclusions and Prospects In this study,we review the current progress in research on directly emitting visible fiber lasers prepared from rare-earth-doped fluoride fibers.In summary,among the rare-earth-doped fluoride fiber lasers,the Pr3+-doped one is particularly useful for fabricating visible lasers because it can efficiently produce blue,green,orange,red,and deep-red spectra,pumped by GaN semiconductor laser.With fluoride fibers doped with rare-earth metal ions like Ho3+,Dy3+,Tb3+,Tm3+and Pr3+/Yb3+,the wavelength can cover the entire visible spectrum.Significant progress has been made in the development of CW,Q-switched,and mode-locked fiber lasers.However,there remain some unsolved problems associated with visible fiber lasers,such as high power,large pulse energy,and femtosecond pulse generation.For visible CW fiber lasers,the highest possible output power is～5 W at 635 nm.Further improvement of the output power,beam quality,slope efficiency,and ability to cover more visible wavelengths is the key to promoting the development and application of visible CW fiber lasers.Therefore,the research and numerical simulations of new visible rare-earth fibers with high damage thresholds,high-performance visible fiber devices,visible beam combiners,etc.will be of great significance.For visible pulsed fiber lasers,the highest pulse energy that can be obtained is～3.17 mJ at 543 nm,and the shortest pulse duration is 168 fs at 635 nm.The research on STML,femtosecond pulse generation,all-fiber configuration operating in more visible wavelengths needs to be performed.Improving the pulse energy,average power and stability,and realizing the visible femtosecond all-fiber lasers are key to promoting the development and application of visible pulsed fiber lasers.Therefore,the new visible rare-earth-doped fibers,saturable absorber materials,and mode-locking technologies need to be explored.Through the innovation of breakthrough technologies,we believe that the visible CW/ultrafast fiber lasers will find widespread applications in the fields of biomedicine,optical communication,material processing,optical microscopy,and scientific research in the future owing to their advantages of miniaturization,high performance,maintenance-free nature,and low cost.
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