Research On Mid-infrared Laser Based On The Rare-earth Ions Doped Calcium Fluoride And Strontium Fluoride Crystals

Mid-infrared laser in the 2-3?m region,located in the“atmospheric window”and near the strong absorption peak of water,has important application prospect in the environmental gas monitoring,medicine and space communication.In recent years,mid-infrared solid-state laser technologies based on rare-earth doped ions crystals have been developing rapidly,and mid-infrared laser based on rare-earth doped ions crystals directly pumped by laser diode?LD?,eliminating the intermediate frequency conversion,with the advantages of simple structure,good beam quality,and high conversion efficiency,is an important technical way to achieve efficient and high-power 2-3?m laser output.The research of mid-infrared laser technology and devices have become an important direction and hotspot in the laser field.Innovative mid-infrared laser technology is expected to expand the application of mid-infrared laser,resulting in unpredictable scientific discoveries.Thulium(Tm³⁺)and Erbium(Er³⁺)ions doped crystals are commonly used as laser gain medium to produce mid-infrared 2-3?m laser,and its pump sources are located in the mature band of commercial LD.However,due to the limitations of laser materials and technology,the development of mid-infrared 2-3?m laser directly pumped by LD is relatively backward,and relevant theories and experiments attracted a significant attention by researchers.“Every novel materials brings its own applications for devices”,with the development of solid-state laser materials,Calcium fluoride?CaF₂?and Strontium fluoride crystals?SrF₂?with low phonon energy and fluoride structure have shown unique advantages in the mid-infrared band,and can achieve efficient laser output with a very low doping concentration of rare earth ions,which provides the conditions for the development of compact mid-infrared ultrafast laser.This paper focuses on the inadequate development but promising application of pulsed laser generation technology in 2-3?m band,which combined with crystal physics and materials science,and analyzes the spectral characteristics of rare-earth ions doped CaF₂/SrF₂ fluoride crystals,cluster effect and energy transfer process in theoretically and experimentally,and the laser output characteristics was characterizatied,which lays a foundation for the development of compact and efficient mid-infrared all-solid-state laser,meanwhile providing a certain reference for the optimization of the crystal growth.The main research work of this paper are as follows:1.The characteristics,application and research status of mid-infrared pulsed laser in 2-3?m band were analyzed.The mode-locking technique for ultra-fast laser and Q-switching technique for short pulse laser were described.The design method of all-solid-state mode-locked laser resonator was analyzed.The research progress of Tm³⁺/Er³⁺doped wide-spectrum laser crystals and the new saturable absorber materials applied in the mid-infrared band in recent years were investigated.2.Research on the characteristics of short pulsed laser and ultra-short pulsed laser based on Tm³⁺:CaF₂ crystal:using LD with the central wavelength of 792 nm as a pump source,the continuous wave?CW?laser with nearly twice the quantum efficiency was realized in a 3at.% Tm³⁺:CaF₂ crystal;a birefringent filter was used as the tuning element to obtain a wide continuously tuning laser output;by using the new type of silver nanoparticles as a saturable absorber,a 2?m passively Q-switched laser of Tm³⁺:CaF₂ crystal was realized.The dynamic mechanism of 2?m CW mode-locked laser in Tm³⁺:CaF₂ crystal was further studied theoretically and experimentally;with a semiconductor saturable absorption mirror,the stable operation of 2?m ultra-fast pulsed laser in Tm³⁺:CaF₂ crystal was realized for the first time.3.The CW laser characteristics of Er:CaF₂/SrF₂ crystals under different excitation pumping wavelengths were studied.Based on the traditional pump mechanism?the active particles arepumped to the upper level or above?,the CW laser output characteristics of Er³⁺ Er:CaF₂/SrF₂ crystals were studied with a 980 nm LD as pump source,obtaining the maximum output power of 1.41 W and a slope efficiency of 40%.Based on the upconversion pumping scheme?based on the upconversion of radiation absorbed into an energy level lying below the lasing state?,a remarkable CW laser output was obtained with a 1532 nm LD as pump source,obtaining a maximum output power of about 205 mW and the slope efficiency of 9.5%.Further exploration through the cooperative pumping schemes,it is expected to achieve efficient CW mid-infrared laser.4.Mid-infrared self-Q-switched Er,Pr:CaF₂ diode-pumped laser performance was studied.Using a 3at.%Er,0.03at.%Pr:CaF₂ crystal as a gain medium,the CW laser performance was characterized comprehensively,and a diode-end-pumped self-Q-switched pulsed laser was achieved for the first time.The laser is free from any active or passive modulators in cavity,depending on the energy level properties,potentially promising for a micro and integrated diode-pumped mid-infrared laser system.5.Mid-infrared dual-wavelength Er:SrF₂ dual-wavelength pulsed lasers based on black phosphorus and bismuthene absorbers were studied.Black phosphorus?BP?and Bismuth nanomaterial?Bi-NSs?saturable absorbers were successfully prepared and used as modulators in mid-infrared Er:SrF₂ laser respectively,obtaining stable dual-wavelength pulsed laser output.Combining with the theoretical analysis,the modulation mechanism of the new two-dimensional material in the mid-infrared region was studied,which provided a basis for the research and development of commercial mid-infrared modulator and an effective technical approach for the development of low-cost 2.8?m miniaturized pulsed laser.6.Mid-infrared pulsed laser based on MXene saturable absorber was studied.A novel MXene saturable absorber was successfully prepared,and its morphology and nonlinear optical parameters were characterized.By using it as a modulator in 2.8?m Er:CaF₂-SrF₂ laser,stable Q-switched and Q-switched mode-locked lasers were obtained.MXene saturable absorber was used as a mode-locked modulator in mid-infrared laser devices,which is expected to realize stable CW mode-locked laser.7.2.8?m ultra-fast pulsed lasers based on a Er:CaF₂-SrF₂ mixed crystal was investigated.Using a SESAM,a stable Q-switched mode-locked pulsed laser operation was realized for the first time in Er:CaF₂-SrF₂ mixed crystal,with a maximum average output power of 125 mW,and the corresponding pulse repetition rate and pulse width of 136.3 MHz and 1.78 ns respectively.Considering that SESAM in the mid-infrared region is not mature and there are few parameters to choose from for commercial products,a mid-infrared self-mode-locked laser resonator was designed based on Kerr-lens mode-locked technology.Under the condition that no modulator was added into the cavity and no aperture was inserted into the cavity,a LD-pumped Kerr-lens mode-locked Er:CaF₂-SrF₂ laser was realized.8.The laser output characteristics of Er:CaF₂ single crystal fiber in 2.8?m were studied and analyzed.The single crystal material with fiber shape is a new medium between the traditional crystal material and the quartz fiber,and the diameter is usually less than 1mm.The Er:CaF₂ single crystal fiber rod was successfully grown,and the doping concentration of rare-earth ions was optimized.Combined with good thermal management technology,an efficient continuous laser output of the single crystal fiber was realized in the mid-infrared region for the first time with a very low doping concentration of Er³⁺?⁰.5at.%?.