Study On Crystal Growth Of Erbium-doped Yttrium Gallium Garnet And Its Laser Properties

Laser has the characteristics of high coherence,high directivity,high monochromatism and high brightness.After more than 60 years of research and development,laser has been widely used in national defense,military,optical fiber communication,laser medical treatment and other related fields.Laser can be divided into dye laser,gas laser,semiconductor laser and solid-state laser,among which all-solid-state laser stands out among many kinds of laser due to its good stability of output wavelength,compact structure,small size and high efficiency.As the core material of all solid-state lasers,the performance of laser gain medium plays an important role in the development of lasers.One of the most important methods to improve the performance of solid-state lasers is to develop laser gain media with good optical quality,high damage threshold,high laser conversion efficiency and excellent thermal performance.Garnet structure oxide crystal has a very wide transmittance spectrum,and has excellent mechanical properties,thermal properties and optical properties.As early as in the 1960s,YAG(yttrium aluminum garnet)as the representative of the garnet structure laser crystal has been widely concerned.Up to now,a variety of garnet crystal series have been used as laser gain medium in solid state lasers.The 1.5?1.6 ?m near infrared band laser is in the safety band of human eyes.At the same time,1.6?m laser is in the transparent window of the atmosphere,so in space science research,remote sensing ranging and Doppler wind radar has important application value.The 3?m band mid-infrared laser is located near the strong absorption peak of water and is the main energy region of blackbody radiation at high temperature,so it has great application prospect in laser medical treatment,laser weapon and missile guidance.Erbium(Er3+)ions have abundant energy level structure.The electron transition between 4?11/2?4?132 energy levels can generate the laser in 2.7-3 ?m waveband,and the electron transition between 4?13/2?4?15/2 energy levels can generate the laser in 1.5-1.7?m waveband.At present,erbium-doped ions laser crystals are the main laser gain medium used in 1.6 ?m and 3 ?m solid state lasers.High power and high efficiency laser output can be achieved by using co-band pumping technology in 1.6 ?m lasers.However,high efficiency laser output has only been obtained in the field of fiber lasers.The laser output efficiency of 1.6 ?m solid-state laser needs to be further improved.The main problem facing 3 ?m solid state laser is that the 4?11/2 energy level lifetime of Er3+ion is shorter than that of 4?13/2 energy level,and there is serious self-termination phenomenon in the process of laser oscillation.Meanwhile,When the 976 nm LD is used as the pump source to stimulate 3?m laser,the quantum loss up to 2/3 will be generated,resulting in serious thermal effect.It is difficult to obtain high power and high efficiency laser output.In this paper,based on the theory of energy recovery effect,we explored the matrix materials with better laser performance in the 3 ?m waveband.To solve the above problems,we chosed Er:YGG crystal with lower phonon energy and excellent thermal properties as the laser matrix material.High quality Er:YGG crystals were grown by optical floating region method,and the crystal structure and components were characterized.The thermal and optical properties of the crystal at room temperature were tested.High concentration doped Er:YGG crystal is the laser gain medium to achieve high efficiency 2.82 ?m continuous laser.Low concentration doped Er:YGG crystal is the laser gain medium to achieve high efficiency 1.64 ?m continuous laser.Co:LaMgAl11O19 crystal as a saturable absorber to achieve a 1.64?m passive Q-switched laser.The main work contents are as follows:1.Er:YGG crystals growthIn this paper,the advantages of the growth of Er:YGG crystals by the method of optical floating zone was first analyzed.Large size(Maximum size is ?5×35 mm3)Er:YGG single crystals with different doping concentrations(0.5?5?10?20 ? 30 at.%)and high optical quality were successfully grown.The factors of affecting the crystal quality,including the preparation of raw materials,the preparation of material rods,the rotation and speed of crystal growth,and the problems of dendrite were analyzed.Therefore,a complete set of crystal growth technology was explored.2.Performance characterization of Er:YGG crystals(1)Crystal structure and composition testingIn the aspect of crystal structure test,the XRD pattern of the crystal was measured by X-ray powder diffractometer.The test results show that the XRD diffraction spectrum line is consistent with the standard card diffraction peak of YGG crystal,and it can be known that Er:YGG crystal belongs to Ia3d space group and m3m point group.It is proved that the crystals grown have high quality and single crystal properties.Through powder refinement of Er:YGG crystal,it can be seen that the cell parameters of Er:YGG crystal decrease with the increase of Er3+ doping concentration.The effective separation coefficient of Er and Y elements in the crystal was measured by X-ray fluorescence spectrometry.The test results show that in Er:YGG crystal,the effective separation coefficient of Er3+and Y3+ions is closed to 1,which indicates that each element is evenly distributed in the crystal growth process.The actual doping concentration is consistent with the expected concentration,thus realizing the growth of high-quality crystals with uniform composition.(2)Characterization of thermal properties of crystalsIn terms of thermal properties,the thermal expansion,density,specific heat,thermal diffusion and thermal conductivity of the crystals were measured,and the variation of the above properties with temperature was studied.When the crystal doping concentration is 0.5,5,10,20 and 30 at.%,the corresponding room temperature thermal conductivity is 9.7,7.73,7.52,7.23 and 6.80 W/mK,respectively.It coulde be seen that the thermal conductivity decreases with the increase of doping ion concentration.(3)Characterization of optical properties of crystalsIn terms of absorption spectra,the absorption spectra of crystals at room temperature were measured.The high concentration doped Er:YGG crystals(doped with 5,10,20 and 30 at.%)have a wide absorption range near 970 nm,and the half-peak width is 19 nm.The wider absorption band reduces the need for wavelength stability of the pumped LD.The Er:YGG crystal with doping ion concentration of 0.5 at.%has a strong absorption peak at 1466 nm,the half-peak width is 29 nm,and the absorption cross section is 9.8×10-21cm2.In terms of fluorescence lifetime and room temperature emission spectrum,we found that the energy lifetime of YGG matrix crystal is more sensitive to ion concentration change.Compared with Er:YAG crystal with Er3+ion concentration of 30 at.%,Er:YGG crystal has shorter lower energy lifetime and longer upper energy lifetime,so it has more advantages in achieving 3 ?m laser output.Compared with Er:YAG crystal,Er:YGG crystal with Er3+ion concentration of 0.5 at.%has longer fluorescence lifetime,smaller emission cross section and stronger energy storage capacity.Therefore,Er:YGG crystal is more suitable for passive modulation pulse modulation.3.Laser experiment(1)Er:YGG crystals with doping concentration of 5,10,20 and 30 at.%as the laser gain medium,The laser output with a central wavelength of 2.82 ?m is achieved.The Er:YGG crystal with doping concentration of 10 at.%obtained the maximum laser output power of 1.38 W and the maximum oblique efficiency of 35.4%,which broke through its quantum limit(976/2820=34.6%).(2)Using Er:YGG crystal with doping concentration of 0.5 at.%as the laser gain medium,the dual wavelength continuous laser output at 1641 nm and 1650 nm is obtained.The maximum output power is 3.34 W,and the maximum slant efficiency is 42.1%.According to the absorption and emission cross sections,the effective gain cross sections of Er:YGG crystal with Er3+ion concentration of 0.5 at.%are calculated.It could be seen from the gain spectrum that Er:YGG crystal has a wide gain range.When ?>0.25,the positive gain band between 1640 nm and 1650 nm can be obtained.(3)The Co:LaMgAl11O19 crystal is used as Q switch for the passive Q switching experiment in 1.6 ?m band.In the experiment,three different modulation depth of Co:LaMgAl11O19 crystals were researched:the largest thickness of 1.05 mm Co:LaMgAl11O19 crystal obtained the shortest pulse width of 2.27 ?s,the smallest thickness of 0.35 mm Co:LaMgAl11O19 crystal obtained the maximum pulse energy 15.49 ?J and the highest peak power 4.30 W in the experiment.The central wavelength of the pulsed laser is located at 1642 nm and 1650 nm,and it can be found that the pulsed laser has a narrower emission spectrum than the continuous excitation spectrum.In summary,Er:YGG crystals have excellent thermal and laser properties,which is a kind of laser gain medium with great application prospect.Therefore,Er:YGG crystals have great potential in 1.6?m and 3 ?m solid state lasers.