Study On Supercontinuum Generation In Step-index Fluoride Fibers

Supercontinuum(SC)laser sources operating in the mid-infrared(MIR)region have the advantages of broadband spectrum,and high spatial coherence compared with the traditional light sources,thus could be applied in fields of spectroscopy,biomedicine,environmental sensing,homeland security,and so on.High-power MIR-SC lasers based on fluoride fibers have been heavily researched in the past decade,yet most research was focused on passive ZBLAN(ZrF₄-BaF₂-LaF₃-AlF₃-NaF)-fiber based ones and those SC lasers need further power-scaling.Researches on SC lasers based on active ZBLAN fibers or passive InF₃ fibers are still rare and the output power based on these two types of fibers is rather limited.In fact,there is still some room for improvement in output power even for the heavily-researched passive-ZBLAN-fiber-based SC lasers.In this thesis,detailed study was carried out on high-power MIR-SC lasers based on step-index fluoride fibers,including theoretical and experimental research on high-power MIR-SC lasers based on Ho³⁺:ZBLAN fiber,Er³⁺:ZBLAN fiber,passive ZBLAN fiber,and passive InF₃ fiber.The main contents are as follows.Theoretical studies on SC generation in fluoride fibers were carried out and some conclusions were drawn.The strong absorption peak around 3.8?m is the main reason which restrict further spectral extension in Ho³⁺:ZBLAN fiber;the transmission window of the ZBLAN fiber sets the ultimate limitation to the spectral extension of those ZBLAN-fiber-based SC lasers while passive InF₃ fiber is a good candidate for obtaining MIR-SC covering the 3-5?m waveband;using pump laser with longer wavelength would benefit SC generation both by lowering the demand of pump pulse peak power and improving the power conversion efficiency.The conclusion mentioned above guided the experiments in the following.A technical scheme on in-amplifier spectrally-flat MIR-SC generation-“the cascaded amplification and frequency shift”was put forward,and spectrally-flat MIR-SC generation in Ho³⁺:ZBLAN fiber amplifier was experimentally realized based on such scheme.A broadband seed laser based on germania fiber with spectral coverage of 2-3?m and a holmium-doped ZBLAN fiber amplifier were built,and the spectral and power characteristics with respect to different signal pulse repetition rate were carefully investigated.A MIR-SC with spectral coverage of 2.8-3.9?m and maximal output power of 411 m W was obtained,with a 3-dB spectral coverage of 2.93-3.70?m.Such result is the first report on MIR-SC generation based on a holmium-doped ZBLAN fiber amplifier.It was noted that the spectral limit of such Ho³⁺:ZBLAN fiber based MIR-SC laser was due to the excited state absorption(⁵I₆?⁵I₅).In-amplifier MIR-SC generation in large-mode-area Er³⁺:ZBLAN fiber amplifier was investigated.Spectral and power performance of the generated SC with respect to the variations of signal pulse repetition rate and active fiber length were researched under germania-fiber-based and passive-ZBLAN-fiber-based broadband seed lasers.An MIR-SC with spectral coverage of 2.70-4.25?m and output power of 1.75 W was obtained in Er³⁺:ZBLAN fiber amplifier seeded by a germania-fiber-based SC laser,with a optical-to-optical efficiency of 20.5%.When the seed laser was changed to a passive-ZBLAN-fiber-based SC laser,the signal coupling efficiency and the effective signal power was obviously improved,and the optical-to-optical conversion efficiency of the Er³⁺:ZBLAN fiber amplifier was enhanced to a maximal value of 33.0%.The obtained MIR-SC showed a maximal average power of 2.08 W and a best spectral coverage of 2.70-4.25?m.Such output power and optical-to-optical efficiency represent the record level ever reported in ZBLAN fiber amplifier.High-power 2-4?m MIR-SC laser sources based on passive ZBLAN fibers were investigated.Technically,the mode-field-adaptor between the pigtail of the pump laser and the ZBLAN fiber was no longer necessary by using self-designed ZBLAN fiber,thus the coupling loss from the pump laser to the ZBLAN fiber was greatly reduced.Fiber endcap made of multimode AlF₃ fiber was fabricated for end-facet protection.30-W SC lasers with spectral coverage of 1.9-3.4?m were achieved in a piece of 2-m-long ZBLAN fiber with core diameter of 10?m,pumped by a thulium-doped fiber amplifier seeded by a picosecond mode-locked fiber laser or a 2-2.5?m thulium-doped fiber based SC laser.Such result was the first report on 30-W-level SC generation in fluoride fibers.ZBLAN fiber with core diameter of 13.5?m was designed and a piece of 19-m-long fiber was used in experiment.An MIR-SC with average power of 20.6 W and a spectrum of 1.92-4.29?m was demonstrated.The SC has a 10-dB spectral coverage of 2.01-4.08?m.Such result is the first report on 20-W-level ZBLAN-fiber based MIR-SC lasers with 10-dB long wavelength edge exceeding 4?m.Experimental researches on high-power MIR-SC generation based on passive InF₃fibers were carried out.Based on the technique of low-loss fusion splicing between silica and InF₃ fibers,and fluoride fiber end-facet protection technique,high-power SC lasers were achieved,and spectral as well as power performance of the generated SC were investigated.A 1.35 W and all-fiber-integrated spectrally-flat MIR-SC with spectral coverage of 1.5-5.2?m was obtained using a broadband thulium-doped fiber based SC laser covering the 2-2.5?m waveband.The 3-dB spectral coverage was 2.14-4.8?m.This is the first report on silica-InF₃ fiber fusion splicing and endcap of InF₃ fiber,and is also the first one on watt-level SC lasers based on InF₃ fiber.When the pump laser was replaced by a thulium-doped fiber amplifier seeded by a picosecond mode-locked fiber laser a 11.3 W and all-fiberized SC with ultra-broadband spectral coverage of 0.8-4.7?m was obtained,which is the first demonstration on 10-watt level MIR-SC generation in InF₃ fibers.Furthermore,a spectrally-flat 11.8 W SC with spectrum of 1.9-4.9?m was achieved by pumping a piece of InF₃ fiber with a high-power 2-2.5?m thulium-doped fiber based SC laser.The 10-dB spectral coverage of the obtained SC was 2.05-4.60?m,and the obtained average power beyond 3.8?m was 2.18 W.The obtained SC power beyond 3.8?m is also the record SC power ever reported.