The Preparation And Investigation Of Er³⁺-doped Transparent Glass Ceramics And Glass Ceramics Fibers For 2.7 μm Mid-infrared Emission

Recently,much attention has been paid on the development of mid-infrared（2⁵ μm）fiber lasers owing to their extensive applications in atmospheric communication,medical diagnosis,environmental monitoring,defence security and so forth.Er³⁺ is an ideal emission candidate for 2.7 μm emission due to the transition of 4I11/2 → 4I13/2.In order to obtain efficient mid-infrared emission,it is of vital importance to develop suitable host materials.Nowadays,the host materials for 2.7 μm emission have been primarily focused on glasses and single-crystals.On one hand,although with low phonon energy and excellent luminescence performance,single-crystals are very difficult to prepare fibers.On the other hand,although with excellent fiber-drawing ability,glasses exhibit weak crystal field,which results in weak emission of rare-earth ions.Glass ceramic（GC）,combining the strong crystal field of nanocrystals and excellent fiber-drawing ability of glass matrix,is considered as a promising medium for Er³⁺-doped fiber to produce intense 2.7 μm emission.In present work,a series of compositions and preparation processes were studied to fabricate transparent GCs,and make a study on the 2.7 μm optical properties.In addition,we demonstrated enhanced mid-infrared emission in GC fibers fabricated by novel melt-in-tube technique,and the GC fibers were crystallization controllable.Moreover,the melt-in-tube technique will open a new way for the fabrication of various optical functional GC fibers.Detailed research contents are as follows:（1）Er³⁺-doped transparent GCs were successfully prepared by melt-quenching method.The XRD,Raman spectra and TEM results confirmed that LaOF nanocrystals were precipitated in the glass matrix.Owing to the incorporation of Er³⁺ ions into LaOF nanocrystals with low phonon energy after a proper heat-treatment,enhanced 2.7 μm emission were observed in the GCs compared to that of precursor glass.And the 2.7 μm emission intensity increases significantly with increasing crystallization temperature.Upconversion,near-infrared emissions,and energy transfer mechanism were investigated to understand 2.7 μm emission behaviors.Furthermore,absorption,emission and gain cross sections in GC-640 sample at 2.7 μm were also analyzed,the maximum value of obtained absorption and emission cross sections are 0.69 × 10^-20 cm² and 0.72 ×10^-20 cm²,respectively.（2）Er³⁺-doped transparent tellurite GCs were successfully prepared by melt-quenching method.The XRD results confirmed that PbTe₃O₇ nanocrystals with a diameter of 8 to 30 nm were precipitated in the glass matrix,and a higher Er³⁺-doped concentration as nucleating agent is beneficial to the crystallization.The TEM results revealed the evenly dispersion of PbTe₃O₇ nanocrystals in glass matrix and most of Er³⁺ ions entered into the PbTe₃O₇ nanocrystals after a careful heat treatment.2.7 μm emission were obtained due to the low phonon energy of tellurite glass.Owing to the efficient crystal field,intense 2.7 μm emission were observed in the transparent GCs compared to that of as-prepared glasses and the optimal condition for 2.7 μm emission was 3% Er³⁺-doped tellurite glass heat-treated above 370°C.Furthermore,the prepared tellurite GC possesses high stimulated emission cross section(0.80 × 10^-20 cm²)and a low pumping threshold,which endow the GC with potential applications for mid-infrared laser.（3）Er³⁺-doped transparent germanotellurite glasses and GCs were successfully prepared by melt-quenching method.The DSC results confirmed that the addition of GeO₂ can improve the thermal stability of prepared samples.From FTIR and Raman spectra,it is noted that the addition of GeO₂ increases the content of OH^-and phonon energy of glasses.Moreover,the influence of GeO₂ concentration and heat-treated temperature on the mid-infrared,near-infrared and upconversion emissions were discussed in detail.It is found that the 2.7 μm emission intensity and the fluorescence lifetime decrease with the increase of GeO₂ concentration,and the 2.7 μm emission intensity enhances remarkably with the increase of heat treatment temperature.（4）Transparent GCs containing NaYF₄: Er³⁺ nanocrystals were prepared by melt-quenching method.After a careful heat treatment,most of Er³⁺ preferentially enter into the precipitated NaYF₄ nanocrystals.Morever,enhanced 2.7 μm emission were achieved in GCs under a 980 nm LD excitation.The precursor glass rods were drawn near the soften temperature and they were completely opaque after the drawing process.Therefore,the rod-in-tube method was not suitable for the fabrication of GC fibers.Then GC fibers containing NaYF₄:Er³⁺ nanocrystals were successfully prepared by melt-in-tube method.The obtained precursor fibers,in which the structure can be maintained well,exhibit no obvious element diffusion or crystallization.After a careful heat treatment of precursor glass fiber,NaYF₄ nanocrystals were precipitated evenly in the glass fiber core,and the GC fibers were crystallization controllable.Enhanced 2.7 μm emission and lifetime were observed in the GC fibers under 980 nm LD pumping.The transmission loss of GC fiber at 1310 nm was measured to be 11.81 dB/m.In addition,a theoretical simulation was performed to evaluate the possibility of 2.7 μm laser output.It can be obtained that the optimum length is around 9 m and the output power is 5.46 W for the pump power of 20 W.