Research On Short Pulse And Ultrashort Pulse Lasers Based On Tm³⁺-doped Fluoride Crystals

In recent years,all-solid-state lasers based on artificial crystals materials have developed rapidly to practical applications.Among them,the development of infrared laser crystals suitable for laser diode（LD）pumped for human eye safety,medical treatment,remote sensing,optical communication and other applications has become one of the mainstream directions of laser materials.The research on laser technology and devices based on crystals of this band is one of the hot topics in the current laser field.The 2 μm "eye-safe" laser has transmission rate in the atmosphere,which provides great market potential for remote sensing detection,space optical communication,LIDAR,and orther applications.At the same time,2 μm laser is located near the strong absorption peak of water molecules,and its penetration ability in human tissue is weak.It can cut soft tissue efficiently and accurately with small damage range and fast coagulation,thus becoming an ideal light source for medical surgery.In addition,2 μm laser is also an ideal pump source for abtaining long-wave mid-infrared laser.LD pumped Tm³⁺-doped fluoride gain medium is an important technical means to realize 2 μm laser.The study of miniature and efficient short and ultrashort pulse laser technology and devices at 2 μm is one of the significant development directions of solid-state laser domain.The emergence of novel materials at 2 μm band has brought new opportunities to the research of lasers.In this paper,based on the comprehensive characterization of physical and spectral properties of Tm³⁺-doped crystals and Tm³⁺-doped single crystal fibers,the output characteristics of short pulse and ultrashort pulse lasers of LD pumped Tm³⁺-doped fluoride laser materials are studied theoretically and experimentally for the first time.Starting from the study of the dynamic mechanism of continuous-wave mode-locked lasers,the feasibility of using novel saturable absorber as a modulator to realize stable broadband ultrafast laser operation in 2 μm is explored.The prototypes of LD pumped Q-switched and mode-locked solid-state laser experimental system near 2 μm were built,and the stable operations of efficient short pulse and ultrashort pulse laser at 2 μm were realized.It provides definite reference for the development of 2 μm ultrafast laser devices with independent intellectual property rights,high efficiency,miniaturization and compact structure.The main research contents of this paper are as follows:1.The applications of 2 μm laser in medical treatment,LIDAR,remote sensing and industrial processing were described,and the technical means for generating 2 μm laser were analyzed.Starting from the energy level of Tm³⁺-ion,the characteristics and advantages of Tm laser were analyzed,and the physical and optical properties of Tm³⁺-doped laser materials used in this paper were introduced.The research status of 2 μm ultrafast lasers was investigated,and the technical scheme for the research on a new generation of LD pumped Tm³⁺-doped ultrafast lasers at 2 μm was proposed.2.LD pumped Tm:YLF short pulse lasers based on novel saturable absorbers.The emergence of two-dimensional materials has brought opportunities for the research of 2μm pulse lasers.In this paser,two novel carbon-based saturable absorbers,graphene oxide and graphdiyne,were successfully prepared,and the nonlinear optical characterization was carried out at 2 μm.As optical modulators,the LD pumped Tm:YLF passively Q-switched pulse laser output were successfully realized.In the graphene oxide modulation experiment,the high repetition rate laser operation with central wavelength of 1928.23 nm was realized,and the average output power was 379 mW.In the graphdiyne modulation laser,the efficient operation of pulse laser was achieved,with an average output power of 1.29 W,a corresponding repetition rate of 91.58 kHz and a central wavelength of 1908.41 nm.The experimental results demonstrate that this kind of saturable absorption modulator devices has potential to achieve high power and high repetition rate pulse lasers at 2 μm.3.LD pumped Q-switched laser performance based on wide spectrum disordered Tm,La:CaF₂ crystal.Acousto-optic Q-switched technology is an effective technological means to obtain high energy pulse laser output.In this study,using acousto-optic active Q-switched technology,the stable operation of high energy short pulse dual-wavelength laser of Tm,La:CaF₂ crystal was achieved.The dual-wavelength were centered at 1881.7 nm and 1888.5 nm,respectively.The maximum output power was 944 mW,and single pulse energy was up to 1.89 mJ.Additionally,passively Q-switched laser operation of Tm,La:CaF₂ crystal was realized.Based on single walled carbon nanotube as modulator,the compact LD pumped Tm,La:CaF₂ pulse laser output was obtained.The central wavelength of passively Q-switched laser was 1904.6 nm.4.All-solid-state LD pumped Tm,La:CaF₂ ultrafast laser operation at 1886.11 nm was realized.Based on the laser dynamics theory,the mechanism of semiconductor saturable absorption mirror（SESAM）continuous-wave mode-locking at 2 μm was investigated,and the stability criteria for realizing stable continuous-wave mode-locking and suppressing Q-switched mode-locking was also analyzed.Mathcad software was used to design and simulate the laser resonator.The high stability operation of 1886.11 nm SESAM mode-locked laser was realized for the first time by using 792 nm LD pumped wide spectrum Tm,La:CaF₂ crystal.The signal-to-noise ratio was up to 76.6 dBm,and the repetition rate was 71.35 MHz.5.The characteristics of 2 μm ultrafast laser based on graphdiyne saturable absorber were studied,realizing the Q-switched mode-locked laser operation for the first time.Under the Q-switched envelope,the repetition rate of mode-locked pulse was 71.6 MHz,the central wavelength of the Q-switched mode-locked laser was at 1887.9 nm,and the full width at half maximum of the spectrum was up to 8.2 nm.6.LD pumped Tm:CaF₂ single-crystal fiber self-Q-switched laser.Single-crystal fiber takes into account the advantages of bulk single crystal and glass fiber,and has good thermal management performance and large specific surface area.So it has become a promising laser gain medium for the next generation of high power and miniaturized solid-state lasers.On the basis of the comprehensive characterization of continuous-wave laser in Tm:CaF₂ single-crystal fiber,a compact and efficient LD pumped self-Q-switched pulse laser output was realized in a 12 mm linear plane-concave laser resonator.At this time,no additional modulation elements were introduced in the cavity.The maximum average output power was 1.17 W,and the slope efficiency was as high as 56.01%.The successful realization of efficient watt-level Tm:CaF₂ single-crystal fiber spulse laser provides a new research scheme for compact Tm pulse laser at 2 μm,which has important practical value.Meanwhile,the instantaneous output spectrum of the self-Q-switched laser varies randomly with time in the range of 1960-1990 nm,which was speculated to be caused by the reabsorption effect of Tm³⁺-ion.7.LD pumped ultrashort pulse laser performance based on Tm:CaF₂ single-crystal fiber.By using birefringent plate as the tuning element,the output characteristics of Tm:CaF₂ single-crystal fiber tuning laser were studied.The wide tuning range from 1859 nm to 2040 nm about 181 nm was realized with smooth tuning curve,which proves that Tm:CaF₂ single-crystal fiber has the potential to generate ultrashort pulse lasers.Based on SESAM as passicely mode-locked element,and applied to LD pumped Tm:CaF₂ single-crystal fiber laser,the operation of 1893.9 nm continuous-wave mode-locked laser in Tm:CaF₂ single-crystal fiber was achieved.This is the first report on the wavelength tuning and ultrashort pulse laser performance of single-crystal fiber,which further confirms that Tm:CaF₂ single-crystal fiber is an ideal laser material for ultrashort pulse laser at 2 μm.