Research On Pulsed Mid-infrared Fiber Laser Operating At 2～4 μm

Mid-infrared lasers operating at 2～4μm have attracted widespread attention because of their applications in such as gas monitoring,biomedicine,material processing,infrared countermeasures.Currently,there are many techniques to genenrate laser in this band e.g.,cascade laser,solid-state laser,optical parametric oscillator,optical parametric generation/amplification,fiber laser,etc.Compared to other lasers,fiber laser has the advantages of good beam quality,great heat dissipation,high conversion efficiency,easy integration and miniaturization,etc.The topic of this dissertation is around the recent research hot spot（i.e.,mid-infrared fiber laser）.It has been reported as a focus in 2012 by the top journal Nature Photonics and regarded as one of four potential development directions in the field of mid-infrared photonics.In the recent decade,mid-infrared fiber lasers at 2～4μm have grown fast with the improved technique of infrared fiber drawing and device fabrication.Compared to continuous wave（CW）or quasi CW and ultrashort pulses,short pulses with duration scales ofμsⁿs,as a result of high average power and pulse energy,hold great potential in efficient tissue ablation and cutting,specific material processing,blinding of infrared-guided weapon.However,there are still some issues and gaps that need to be solved and filled.In this dissertation,we will carry out the experimental and theoretical research on short pulse generation at 2～4μm in different rare-earth-ion doped fluoride fiber lasers using Q-switching and gain-swtiching techniques.Firstly,the motivation of developing 2～4μm mid-infrared laser is introduced.The main approaches of generating fiber laser in this band is summarized,in which rare-earth-ion doped fluoride fiber lasers are introduced in detail.The development progress of short pulsed lasers based on these rare-earth-ion doped fluoride fibers is reviewed and the directions in the future are pointed out.Secondly,considering the practical demand of laser surgery for high-performance short pulsed fiber lasers at the single wavelength of³μm and dual wavelengths of³μm and²μm,the experimental research on gain-swtiching is carried out with the help of efficient lasing potential of Ho³⁺-doped ZBLAN fiber at³μm and²μm.Under CW pumping at 1150 nm,dual-wavelengh self-pulsing at³μm and²μm is experimentally observed for the first time.The maximum output powers of 917.5 mW and 414.7 mW are only limited by the pump power.The experimental results and analyses indicate that³μm self-pulsing may be resulted from the saturable absorption of fiber itself owing to the absorption of ⁵I₇ level at the laser wavelength,and the²μm is gain-swithced by the³μm self-pulsing;Under pulse pumping at 1150 nm,four different temporal states of³μm gain-switched pulses are observed for the first time.The influences of pump power（energy）,pump pulse width,repetition rate,and laser wavelength on laser characteristics are studied.The tunable wavelength range of 2.9³μm under gain-switching is achieved;Under simultaneous CW and pulse pumping at 1150 nm,dual-wavelength gain-switched pulses at³μm and²μm are obtained for thre first time.This provides a promising new scheme for designing dual-wavelength medical laser sources in the future.Thirdly,considering the fact that it is difficult for pulsed Ho³⁺-doped ZBLAN fiber lasers to significantly extend the wavelength beyond 3μm,the experimental research on short pulse is carried out based on Dy³⁺-doped ZBLAN fiber with a longer and broader emission band around 3μm.Using PbS nanoparticles as the saturable absorber,the first tunable passively Q-switched Dy³⁺-doped ZBLAN fiber laser pumped at 1090 nm is built up with a tunable wavelength range of 2.71³.08μm.³70 nm tunable range is also the current record of pulsed rare-earth-ion doped fiber lasers at any wavelengths.Moreover,the potential of PbS material in mid-infrared pulse generation is experimentally revealed for the first time;Under pulse pumping at 1090 nm,the first gain-switched Dy³⁺-doped ZBLAN fiber laser is presented with a tunable wavelength range of 2.8³.1μm.Fourthly,for the dual-wavelength cascade pumped³.5μm Er³⁺-doped ZBLAN fiber lasers,the output power of passively Q-switched pulses is low due to the low damage threshold of the used saturable absorbers effective at this wavelength,while further power scaling of gain-switched pulses is also limited due to laser quenching during a single pump pulse when pumping with high energy short pulses.To solve the above issues,the experimental research is carried out.Based on a Fe²⁺:ZnSe crystal with a high damage threshold and a broad operation band around 3.5μm,the first dual-wavelength cascade pumped tunable passively Q-switched Er³⁺-doped ZBLAN fiber laser is demonstrated at the range of 3.4³.7μm.The maximum output power of 583.7 mW,scaled with a factor of almost 5,is only limited by the pump power;Using 1950 nm pulses with aμs scale width and a high repetition rate as the trigger which circumvents the possible laser quenching behavior during a single high energy ns scale pump pulse,a high power dual-wavelength cascade pumped gain-switched Er³⁺-doped ZBLAN fiber laser at³.5μm is presented.The maximum output power of 1.04 W is almost one order of magnitude higher than the previous result.This is also the first watt-level pulsed laser beyond 3μm based on rare-earth-ion doped fibers;The parallel pumping method at 655 nm and 1981 nm is proposed and introduced in an Er³⁺-doped ZBLAN fiber laser to realize³.5μm for the first time.The experimental results indicate the parallel pumping method is more efficient for laser generation than the ordinary cascade pumping method,thus offering a new way for efficient pulsing in the future.Lastly,the current³.9μm Ho³⁺-doped InF₃ fiber laser pumped at 888 nm has a high threshold and a low efficiency,which is not ideal for pulse generation.To solve this issue,the theoretical exploration is performed using the dual-wavlength pumping method at 888nm and 962 nm.Based on rate and power propagation equations,the theoretical model of dual-wavlength pumped Ho³⁺-doped InF₃ fiber laser at³.9μm is built up.By comparing two pump methods numerically,it is found that dual-wavlentth pumping can yield laser more efficiently by the way of efficient population recycling and is an ideal scheme for efficient³.9μm laser geneataion.This will offer a reliable platform for pulse generation at this wavelength in the future.