Research On Ho³⁺-doped Solid-state Lasers Diode-pumped At1.1μm

Solid-state lasers operating in2μm eye-safe spectral region are useful for varieties ofapplications such as atmospheric monitoring, optical parametric oscillator pumping andmedical appliance. The emission wavelengths of trivalent holmium ion-based lasers arearound2μm, so many efforts have been taken to these lasers. In recent years, YAG lasertransparent ceramic is getting more and more attention due to its advantages in fabrication,properties and design. High-power Nd:YAG transparent ceramic CW laser has beensuccessfully performed in China. All of these have created excellent conditions for thepreparation, laser output and application of Ho:YAG transparent ceramics. A new fluoridecrystal LuLiF4has also been received more attention in the field of near-infrared lasers.Compared with oxides, fluoride crystals are interesting due to their high transparency in awide range and low phonon-energy which reduces nonradiative relaxation between adjacentenergy levels. Rare earth doped fluoride crystals often possess long lifetimes for certainenergy levels. The LuLiF4crystal has negative thermal dependence of refractive index, whichpartially compensates the positive bulging of the end faces of the laser rod owing to thepositive thermal expansion coefficients, leading to weak overall thermal lensing. What’s more,the crystal growth of LiLuF4should be facilitated because it would not need excess LiF,which could always create small inclusions in the crystal and reduce its optical quality.In this thesis, we investigated the growth, thermal conductivity, spectra and laserperformance of Ho3+doped YAG ceramic and LiLuF4single crystals. The main contents ofthis thesis are as follows:1）The absorption spectrum of1at.%Ho:YAG transparent ceramic sample shows six strong peaks in the range of200~2200nm, whose central wavelengths lie in418nm,453nm,539nm,635nm,1128nm and1906nm respectively. The1128-nm absorption peakmatches the emission wavelength of1.1-μm LD well, which can be utilized for pumpingwith the commercial LDs without requirement of wavelength control technique. Theabsorption coefficient and cross section of primary peak around1128nm is calculated tobe1.0487and2.410-21cm2.2）It shows the fluorescence spectrum from1800to2200nm of Ho:YAG ceramic withdoping concentration of1at.%at room temperature (300K), which was the emissionspectrum of Ho3+:5I7â†'5I8. The FWHM of the emission band of main peak was about80nm with the center wavelength of near2091nm. We present a Ho3+-doped YAG ceramiclaser which is directly diode-pumped at~1.13μm for the first time. Under the absorptionpower of5.8W, the output power was up to1W of continuous wave at the lasingwavelength of2099nm.The laser has a slope efficiency of20.6%with respect toabsorbed power and a beam propagation factor of M2~4. The threshold pump power wasaround1W, indicating pump threshold intensity of350W/cm2for1.13-μm pumping.3）The5I8â†'5I6absorption spectrum of Ho3+:LuLiF(1.0at.%) is spanned by the emissionspectra of1.1-μm laser diode source or of Yb3+-doped lasers specially designed to thelong wavelength range about1147nm. The peak absorption cross sections at1150nm are0.21×10-20cm2and0.29×10-20cm2for σ and Ï€ polarization. The peak emission crosssections at2060nm (Ï€ polarization) and2066nm (σ polarization) are1.3×10-20cm2and0.67×10-20cm2.4）We report a laser operation based on Ho3+-doped LuLiF4single crystal, which representsthe first laser emission of a fluoride crystal slab in the2μm wavelength range directlypumped with1.15-μm laser diode. Theoretical models with considerations of energytransfer processes, including laser re-absorption, cross-relaxation (CR), energy transferupconversion (ETU), and excited state absorption (ESA) are presented. It is found that the cross-relaxation and energy transfer upconversion play an important role for the laserefficiency. Good agreement is achieved between the simulations and experimentalmeasurements. The maximum power of1.4W was generated with the10%outputcoupler, corresponding to a slope efficiency of29%with respect to the absorbed pumppower and the threshold pump power was around0.6W. The output laser beam wasÏ€-polarized with the degree of polarization>25dB and a beam propagation factor of M2~2at the lasing wavelength of2.066μm.5) The spectroscopic properties of Ho3+-doped LiLuF4crystals and Ho3+and Pr3+co-dopedLiLuF4crystal around2.9μm were studied. The effects of Pr3+ions on the fluorescencelifetimes of the5I7and5I6manifolds of Ho3+in LiLuF4crystal were also investigated. Itwas demonstrated that Pr3+depopulates the Ho3+:5I7level and has little influence on theHo3+:5I6level at the same time. The measured lifetime of the5I6manifold in the Ho,Pr:LuLiF4crystal is1.47ms, which is18%shorter when compared with the date for theHo: LuLiF4crystal (1.8ms). On the other hand, the measured lifetime of the5I7manifoldin the Ho, Pr:LuLiF4crystal is1.97ms, which is78%shorter compared to the date for theHo: LuLiF4crystal (16ms). This shortening of the measured lifetime confirms that Pr3+ions are able to depopulate the Ho3+:5I7for2.9μm emission in LLF crystal, which mayinduce the population inversion and facilitate laser operation.