Research On High Power Mid-infrared Erbium-doped Fluoride Fiber Laser

Mid-infrared（MIR）lasers have important applications in many fields such as national defense,laser surgery,material processing,and spectroscopy.Large amounts of matters,such as biological tissues,organic compounds,and polymers,have strong absorption in MIR range.Because of the existence of atmosphere transparent window,lasers of 3-5μm particularly have their special advantages over communication,photoelectric countermeasure,and remote sensing applications.In recent years,MIR fluoride fiber laser is developed very fast.It has attracted grate attention for its high beam quality,compact volume,high energy transfer efficient,and good heat dissipation effect.Fluoride fiber laser becomes a compatible MIR laser source.In this paper,the erbium doped fluoride(Er³⁺:ZBLAN)fiber laser is taken as the research object,and the related research on power upscaling is carried out.First,the application scenarios of MIR laser and the requirements of these applications for laser source are introduced.The methods of generation MIR laser are summarized,and the advantages of fluoride fiber laser are stated.The research progress of fluoride fiber laser is introduced,and the develop history and state-of-the-art progresses of 2.8μm and 3.5μm Er³⁺:ZBLAN fiber laser,which is the research object of this paper,is reviewed in detail.Then,the numerical simulation of 2.8μm Er³⁺:ZBLAN fiber laser is carried out based on the rate equations.The traditional model of four energy levels is expanded to eight,and the solving efficiency of the model is improved.By numerical simulation,parameters are optimized,such as the length of the fiber,the core diameter of the fiber,the reflectivity of the laser cavity,and the dopant concentration of the fiber.The results play a guiding role in realizing high power fluoride fiber laser.A numerical simulation of 3.5μm Er³⁺:ZBLAN fiber laser is also carried out.Focusing on the problem that traditional dual-wavelength pump（DWP）scheme has difficult to achieve core pumping setup,dual-wavelength-clad pump（DWCP）scheme is put forward.The possibility ofDWCP is computed by the numerical simulation,and optimization of parameters are performed.Based on the results of rate equations,the solution of heat conduction equations of Er³⁺:ZBLAN fiber laser is improved,and a heat simulation model is built up,which lays a theoretical foundation for the heat dissipation design of fiber laser.Afterwards,the power upscaling of continuous wave（CW）fluoride fiber laser is experimentally researched.By the mechanical design of cooling and drying structure,a passive cooling compact CW 2.8μm Er³⁺:ZBLAN fiber laser is built.It has a length by width by height of 300×290×120 mm,and an output power of 4.63 W.The optical-optical efficiency is 25.2%.By bidirectional pumping,a 3.5μm Er³⁺:ZBLAN fiber laser is built,with an output power of 2.32 W and an optical-optical efficiency of 10.33%.Moreover,spectra beam combining（SBC）is used to combine the output from several fiber lasers for further power upscaling.According to the geometric optics and wave optics theory,the coupling efficiency of external cavity and the output laser beam quality of SBC are driven.The equations revealed the difficulties of applying SBC to MIR wavelengths.With the theoretical analyzes,a proof-of-principle experiment is conducted to achieve the SBC of three 2.8μm Er³⁺:ZBLAN fiber lasers.The output power is 2.14 W,and the combination efficiency is 68%,with a spectrum width of 6.4nm.The beam quality M² in x and y directions are 2.3 and 2.0 respectively.A telescope system is used to compress the spectrum and improve the beam quality.Improving methods to realize SBC of fluoride fiber lasers with high power,high efficiency and high expansibility are given based on theoretical and experimental results.Finally,the peak power of the Er³⁺:ZBLAN fiber lasers are boosted by pulsed output.From the prospective of material heat damage,the light path design and the substrate of the saturable absorber（SA）is improved.A 2.8μm passively Q-switched Er³⁺:ZBLAN fiber laser is made with a 2-D Mo S₂ SA.The average output power is over 1 W,and the peak power is 5.5 W.Compared to the former reported Mo S₂ based2.8μm Q-switched laser,the average power is improved over 6 times and the peak power is raised by 122%.From the prospective of material improving,a Mn doped nanoparticle Fe₃O₄ SA is fabricated by vapor deposition.Based on this SA,a 2.8μm passively Q-switched Er³⁺:ZBLAN fiber laser is achieved with an average power of774 m W and a peak power of 5.1 W.Compared to the results of undoped SA,the output power is improved 6 times and the damage threshold is more than doubled.Form the application requirements of tunable spectrum,a wavelength tunable 2.8μm passively Q-switched Er³⁺:ZBLAN fiber laser is performed with the same Fe₃O₄ SA.The wavelength tunable range is 2710-2810 nm.The average output power at the center wavelength is 188 m W,with a peak power of 1.27 W.Compared with the former reported wavelength tunable Q-switched lasers at 3μm wavelength range,the average power and peak power are improved by 69%and 76%respectively.The mode-locking regime of the laser is also achievable at the wavelength of 2760 nm,with an average power of 295 mW.