Article Optimized Growth and Application of Yb:LuAG Single- Fibers by Micro-Pulling-Down Technique

Anye Wang, Jian Zhang * , Shuai Ye, Xiaofei Ma, Baiyi Wu, Siyuan Wang, Feifei Wang, Tao Wang, Baitao Zhang and Zhitai Jia

State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China; [email protected] (A.W.); [email protected] (S.Y.); [email protected] (X.M.); [email protected] (B.W.); [email protected] (S.W.); [email protected] (F.W.); [email protected] (T.W.); [email protected] (B.Z.); [email protected] (Z.J.) * Correspondence: [email protected]

Abstract: Single-crystal fibers (SCFs) have a great application potential in high-power due to 3+ their excellent performance. In this work, high-quality and crack-free Yb :Lu3Al5O12 (Yb:LuAG) SCFs were successfully fabricated by the micro-pulling-down (µ-PD) technology. Based on the laser micrometer and the X-ray Laue diffraction results, these Yb:LuAG SCFs have a less than 5% diameter fluctuation and good along the axial direction. More importantly, the distribution of Yb ions is proved to be uniform by electron probe microanalysis (EPMA) and the scanning electron microscope (SEM). In the laser experiment, the continuous-wave (CW) output power using a 1 mm diameter Yb:LuAG single-crystal fiber is determined to be 1.96 W, at the central wavelength of 1047 nm, corresponding to a slope efficiency of 13.55%. Meanwhile, by applying a 3 mm diameter Yb:LuAG SCF, we obtain a 4.7 W CW laser output at 1049 nm with the slope efficiency of 22.17%. 2  The beam quality factor M is less than 1.1 in both conditions, indicating a good optical quality of the  grown fiber. Our results show that the Yb:LuAG SCF is a potential -state laser gain medium for Citation: Wang, A.; Zhang, J.; Ye, S.; 1 µm high-power lasers. Ma, X.; Wu, B.; Wang, S.; Wang, F.; Wang, T.; Zhang, B.; Jia, Z. Optimized Keywords: single-crystal fiber; micro-pulling-down; Yb:LuAG; CW laser Growth and Laser Application of Yb:LuAG Single-Crystal Fibers by Micro-Pulling-Down Technique. Crystals 2021, 11, 78. https:// 1. Introduction doi.org/10.3390/cryst11020078 In recent years, as the most widely investigated laser system, solid-state lasers have become a research hotspot due to their high laser gain coefficient, high photoelectric Received: 5 January 2021 conversion efficiency, stable working medium and compact laser system [1–4]. The laser Accepted: 18 January 2021 Published: 20 January 2021 gain medium has a pronounced influence on the performance of the lasers. Subject to the nature and structure of the traditional solid-state gain medium, mainly including bulk

Publisher’s Note: MDPI stays neutral crystals, glass and ceramics, a further increase in the laser output power is limited. In spite with regard to jurisdictional claims in of that, the invention and manufacture of a glass fiber laser, which is considered as the published maps and institutional affil- “third-generation laser”, have brought new development ideas for solid-state lasers. Due iations. to the ideal beam quality, high conversion efficiency, being maintenance-free, high stability and small size, a single glass fiber laser can achieve a 10 kW CW laser output. However, due to the low thermal conductivity and lower mechanical strength, the output power of glass fiber lasers is also limited. Therefore, it is necessary to find a new type of laser gain medium [5,6]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Single-crystal fibers (SCFs) are a kind of one-dimensional laser gain medium between This article is an open access article traditional bulk crystals and glass fibers. They combine the advantages of bulk crystals distributed under the terms and and glass fibers, such as good physical properties and excellent thermal management conditions of the Creative Commons performances. Therefore, they are expected to solve the current problems encountered Attribution (CC BY) license (https:// by glass fiber lasers. Compared with silica fibers, SCFs have higher thermal conductivity, creativecommons.org/licenses/by/ a higher rare-earth ion concentration, a lower nonlinear effect and a higher laser 4.0/). damage threshold. Their theoretical laser output power is more than 50 times that of

Crystals 2021, 11, 78. https://doi.org/10.3390/cryst11020078 https://www.mdpi.com/journal/crystals Crystals 2021, 11, 78 2 of 10

traditional silica glass fibers [7]. In 2012, Xavier Delen et al. obtained a 251 W CW laser 3+ output using a Yb :Y3Al5O12 (Yb:YAG) SCF grown by µ-PD technology [8]. As for the laser gain medium, rare-earth garnet crystals are good candidates because they have excellent physical and mechanical properties. They belong to the cubic crystal system, Ia3d space group, and representative crystals include Y3Al5O12 (YAG), Lu3Al5O12 (LuAG) and Y3Sc2Ga3O12 (YSGG). LuAG with a melting point of 2333 K and a density of 6.67 g/cm3 is a kind of garnet structure crystal with excellent thermal and optical properties, a high laser damage threshold and high thermal conductivity. Compared with YAG, the thermal conductivity of LuAG crystals is less affected by the increasing concentration of doping ions, which is conducive to achieve the miniaturization and integration of lasers [9–15]. In addition, Yb:LuAG has a larger effective emission cross-sectional area than Yb:YAG, which is more suitable for high-power solid-state lasers [16]. Yb:LuAG crystals are considered to be high-quality laser gain mediums for 1 µm band lasers. The Yb3+ absorption band is located at 900~1000 nm, which can be effectively coupled with an InGaAs laser diode without strictly controlling the temperature; compared with Nd3+, Yb3+ has a weaker concentration quenching effect, which is also beneficial to the miniaturization of lasers. Furthermore, its absorption wavelength (900–980 µm) and emission wavelength (980–1100 µm) reduce the thermal load effect, which leads to high quantum efficiency almost up to 90%. Meanwhile, considering that Yb3+ laser crystals have 2 2 only the ground state F7/2 and the excited state F5/2, and there are no other laser state 2 energy levels above F5/2, parasitic effects such as up-conversion, excited-state absorption and relaxation oscillations can be avoided to reduce laser energy loss [17–19]. In recent years, some progress has been achieved in the research of 1 micron band lasers for Yb:LuAG SCFs. In 2009, the University of Lyon achieved a continuous laser output of 3.3 W with 100 W pump power by using a 1 mm diameter Yb:LuAG fiber. In 2012, the University of Pisa reached a high efficiency of 32% with a 3 mm diameter Yb:LuAG SCF, but the output power was only 23 mW [20,21]. Therefore, further research on laser application of Yb:LuAG SCFs is of great significance. At present, the main growth methods of SCFs include the laser-heated pedestal growth (LHPG) method and µ-PD technology. The LHPG method has the advantages of high growth speed without crucibles. However, due to the large temperature gradient at the growth interface, the as-grown crystal fibers have large thermal stress, resulting in the decrease in optical uniformity. Compared with LHPG, our self-designed µ-PD device improves the homogeneity of melt components and makes the shape of the fiber controllable. In addition, the temperature gradient during SCF growth can be reduced by using the after-heater and temperature field, so the crystals have a higher laser damage threshold [22–24]. In this work, we successfully grow high-quality SCFs using µ-PD technology. The systematical characterizations on the obtained Yb:LuAG SCF prove its low diameter fluctuation, good crystallinity and uniformity of doped ions. The continuous- wave laser performance of the Yb:LuAG SCFs is also investigated.

2. Materials and Methods 2.1. Growth of Single-Crystal Fibers 3+ The raw materials of Yb :LuAG were synthesized by a solid-state reaction. Yb2O3, Lu2O3 and Al2O3 oxide powders of 4N purity were weighed according to a stoichiometric ratio of YbxLu(3−x)Al5O12 (x = 0.21, 0.3). The powders were ground in an agate mortar for 20~30 min to ensure uniform mixing. Then, the powder materials were pressed into wafers under the pressure of 50 Mpa and sintering was carried out in a muffle furnace at 1873 K for 24 h. High-quality Yb:LuAG single-crystal fibers were grown by the self-developed µ-PD growth equipment. The schematic diagram is shown in Figure1. About 3~5 g sintered polycrystalline materials was put into an iridium crucible and melted by radio frequency (RF) induction heating (power: ~3850 W). The melt remained stable in the capillary at the bottom of the crucible under the force of gravity and surface tension. A high-quality LuAG Crystals 2021, 11, x 3 of 10 Crystals 2021, 11, x 3 of 10

(RF) induction heating (power: ~3850 W). The melt remained stable in the capillary at the bottom(RF) induction of the crucible heating under (power: the force ~3850 of W).gravity The andmelt surface remained tension. stable A inhigh-quality the capillary LuAG at the crystalbottom (1 of× 1the × 30crucible mm3) underin the the<111> force direction of gravity was and used surface as seed. tension. The Aseed high-quality crystal rose LuAG to thecrystal bottom (1 of× 1 the × 30 crucible mm3) into contactthe <111> the direction melt. After was the used melt as fully seed. contacted The seed the crystal seed rosecrys- to talthe and bottom the melting of the cruciblezone was to stable,contact the the seed melt crystal. After thestarted melt to fully be pulled contacted downward the seed and crys- thetal single-crystal and the melting fiber zone started was to stable, grow. theIn order seed crystalto avoid started oxidation to be of pulled the iridium downward crucible, and wethe vacuumed single-crystal the furnacefiber started chamber to grow. to below In order 30 toPa avoid and then oxidation filled ofit thewith iridium dry argon crucible, to atmosphericwe vacuumed pressure the furnace for crystal chamber growth. to below The 30different Pa and stages then filled of the it withYb:LuAG dry argon SCFs’ to growthatmospheric process pressure could be forobserved crystal in growth. real time Th usinge different a CCD stagescamera of through the Yb:LuAG the observa- SCFs’ tiongrowth window process (Figure could 2). be observed in real time using a CCD camera through the observa- tionThe window iridium (Figure after-heater 2). with a small size (Φ2 and Φ4 mm) of the observation win- dow andThe two iridium layers after-heater of ceramic withthermal a small insula sizetion (Φ were2 and used Φ4 mm) to reduce of the the observation temperature win- gradientdow and during two layerscrystal of growth. ceramic Crucibles thermal withinsula 1tion and were3 mm used nozzles to reduce were used the temperaturefor crystal Crystals 2021, 11, 78 3 of 10 growth.gradient A slowduring crystal growth. rate Crucibles is conducive with to 1 improving and 3 mm thenozzles optical were quality used and for crystalcrys- tallinity.growth. In A this slow way, crystal the speedgrowth used rate in is thisconducive work was to improving 1~3 mm/h. the When optical the 3quality mm diameter and crys- fibertallinity. is grown, In this the way, growth the speedrate of used >3 mm/h in this will work reduce was 1~3the mm/h.crystallinity. When Accordingthe 3 mm diameter to our experience,fiber is grown, thecrystal growth the growth (1 rate× 1 ×ofrate 301 mm/hof mm >33 mm/h) is in enou the will <111>gh reduce to obtain direction the high-quality crystallinity. was used crystals. asAccording seed. TheDuring to seed our crystal rose theexperience, growth process, theto thegrowth it bottom is necessary rate of of the 1 mm/hto crucible increa is se toenou the contactgh powers to theobtain according melt. high-quality After to the the melt crystalcrystals. fully growth During contacted the seed speedthe growth and fiber process,crystal diameter, andit is theasnecessary to melting balance to zone increaheat was loss.se the stable, powers the seedaccording crystal to started the crystal to be growth pulled downward speedAn andexcellent fiberand diameter,Yb:LuAG the single-crystal asSCF to wasbalance fibergrown heat started successfully loss. to grow. by In the order μ-PD to technique. avoid oxidation As it is of the iridium exhibitedAn inexcellent Figurecrucible, Yb:LuAG3, the we length vacuumed SCF of was the the grownobtained furnace successfully crystal chamber fiber by to exceeds the below μ-PD 30100 technique. Pa mm and with then As a filleddi- it is it with dry ameterexhibited of about in argonFigure 1 mm. to 3, atmosphericThethe lengthcrystal offiber pressure the presen obtained forts crystalhigh crystal transparency growth. fiber exceeds The and different 100 there mm are stages with no ob-a of di- the Yb:LuAG viousameter inclusions of aboutSCFs’ and 1 mm. growthbubbles The process inside.crystal couldfiber presen be observedts high intransparency real time using and there a CCD are camera no ob- through the vious inclusionsobservation and bubbles window inside. (Figure 2).

Figure 1. Schematic diagram (a) and equipment photo (b) of the micro-pulling-down (μ-PD) tech- Figure 1. Schematic diagram (a) and equipment photo (b) of the micro-pulling-down (µ-PD) technique. nique.Figure 1. Schematic diagram (a) and equipment photo (b) of the micro-pulling-down (μ-PD) tech- nique.

FigureFigure 2. 2. Real-timeReal-time images images of of seeding seeding (a (,ab,b) and) and stable stable crystal crystal growth growth (c (,cd,d) )by by the the CCD CCD camera. camera. Figure 2. Real-time images of seeding (a,b) and stable crystal growth (c,d) by the CCD camera. The iridium after-heater with a small size (Φ2 and Φ4 mm) of the observation window and two layers of ceramic thermal insulation were used to reduce the temperature gradient during crystal growth. Crucibles with 1 and 3 mm nozzles were used for crystal growth. A slow crystal growth rate is conducive to improving the optical quality and crystallinity. In this way, the speed used in this work was 1~3 mm/h. When the 3 mm diameter fiber is grown, the growth rate of >3 mm/h will reduce the crystallinity. According to our experience, the growth rate of 1 mm/h is enough to obtain high-quality crystals. During the growth process, it is necessary to increase the powers according to the crystal growth

speed and fiber diameter, as to balance heat loss. An excellent Yb:LuAG SCF was grown successfully by the µ-PD technique. As it is exhibited in Figure3, the length of the obtained crystal fiber exceeds 100 mm with a diameter of about 1 mm. The crystal fiber presents high transparency and there are no obvious inclusions and bubbles inside. Crystals 2021, 11, 78 4 of 10 Crystals 2021, 11, x 4 of 10

Figure 3. As-grown Yb:LuAG single-crystal fiber. Figure 3. As-grown Yb:LuAG single-crystal fiber.

2.2. Diameter Fluctuation 2.2. Diameter Fluctuation The diameter fluctuation is one of the vital factors for evaluating the quality of SCFs, The diameter fluctuation is one of the vital factors for evaluating the quality of SCFs, indicating the stability of the control system and the growth process. In order to obtain indicating the stability of the control system and the growth process. In order to obtain the diameter fluctuation of the Yb:LuAG SCF, one 1 mm diameter SCF was measured by the diameter fluctuation of the Yb:LuAG SCF, one 1 mm diameter SCF was measured by a a laser micrometer at every 1 mm along the fiber and, in total, 100 point data were col- laser micrometer at every 1 mm along the fiber and, in total, 100 point data were collected. lected. 2.3. Laue Back-Reflection Measurements 2.3. Laue Back-Reflection Measurements The growth direction and crystallinity of fibers were measured by a real-time Laue The growthback diffractometer direction and withcrystallinity a real-time of fibers back-reflection were measured Laue camera by a real-time system (Multiwire Laue MWL back diffractometer120 with with Northstar a real-time software) back-reflection along the axial Laue and camera radial system directions (Multiwire of the MWL Yb:LuAG single- 120 with Northstarcrystal fibers. software) along the axial and radial directions of the Yb:LuAG single- crystal fibers. 2.4. The Concentration and Distribution of Yb3+ 3+ 2.4. The ConcentrationThe doping and Distribution ion concentration of Yb of crystals was measured by an X-ray fluorescence The dopingspectrometer ion concentration (Rigaku, zsx-primus of crystals II) was (Rigaku, measured Matsubara-cho by an X-ray Akishima-shi, fluorescence Tokyo, Japan). spectrometerThe (Rigaku, test was zsx-primus carried out II) by (Rig pressingaku, Matsubara-cho a crystal sample Akishima-shi, with a thickness Tokyo, of 2 mmJa- into boric pan). The testacid. was In carried addition, out by the pressing ion distribution a crystal ofsample the end with face a thickness and longitudinal of 2 mm into direction was boric acid. Inmeasured addition, by the a scanningion distribution electron of microscope the end face (G300 and longitudinal FE-SEM System) direction and was electron probe measured bymicroanalysis a scanning electron (EPMA-1720H, microscope Shimadzu, (G300 FE-SEM Kyoto, Japan), System) respectively. and electron The probe sample length microanalysiswas (EPMA-1720H, about 1 mm, and Shim theadzu, end face Kyoto, was Japan), polished respectively. for the mapping The sample test. We length selected an area was about 1mm,of 150 and× 150 theµ mend2 on face the was end polished face randomly for the to mapping perform test. the YbWe3+ selectedSEM mapping an area test and the of 150 × 150EPMA μm2 on line the scan end testface of randomly Yb3+ was to carried perform out the along Yb3+ the SEM fiber mapping axial direction. test and the EPMA line scan test of Yb3+ was carried out along the fiber axial direction. 2.5. Spectroscopy 2.5. Spectroscopy The absorption spectrum was measured using a crystal wafer with two end faces pol- The absorptionished. The spectrum spectrum was data measured were recorded using by a an crystal Agilent wafer Cary with 7000 UMStwo end at room faces temperature polished. Thein thespectrum range of data 880–1050 were recorded nm. by an Agilent Cary 7000 UMS at room tem- perature in the range of 880–1050 nm. 2.6. Laser Experiments 2.6. Laser ExperimentsIn order to study the laser performance of the prepared Yb:LuAG SCFs, a continuous- wave laser experiment was designed. The laser experimental setup is shown in Figure4 . In order to study the laser performance of the prepared Yb:LuAG SCFs, a continuous- The pump source was a 940 nm semiconductor laser with a core diameter of 200 µm and wave laser experiment was designed. The laser experimental setup is shown in Figure 4. a numerical aperture (NA) of 0.22. The magnification ratio of the focusing system was The pump source was a 940 nm semiconductor laser with a core diameter of 200 μm and 1:1 and the resonant cavity length was 14 cm. The plane input mirror (IM) was coated a numericalwith aperture high transmittance(NA) of 0.22. The at the magnification pump wavelength ratio of (940–980 the focusing nm) system and high was reflectivity 1:1 from and the resonant1000 to cavity 1100 nm.length The was output 14 cm. mirror The plane (OM) input was partiallymirror (IM) reflective-coated was coated with from 1000 to high transmittance1100 nm at with the thepump transmittance wavelength of (940–980 5%, 10% nm) and and 30%. high The reflectivity crystal was from wrapped 1000 in indium to 1100 nm.foil The and output placed mirror in a (OM) was radiator, partially and reflective-coated the water-cooling from temperature 1000 to 1100 was nm set to 17.5 ◦C with the transmittanceto reduce the of thermal5%, 10% effect and 30%. generated The crystal during was the wrapped experiment. in indium Yb:LuAG foil and SCFs with the placed in a coppersizes of radiator,Φ1 mm ×and8 mmthe water-coolin and Φ3 mmg× temperature8 mm were usedwas set in to the 17.5 experiment. °C to reduce the thermal effect generated during the experiment. Yb:LuAG SCFs with the sizes of Ф1 mm × 8 mm and Ф3 mm × 8 mm were used in the experiment.

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Crystals 2021, 11, x 5 of 10 Crystals 2021, 11, 78 5 of 10

Figure 4. A schematic diagram of the Yb:LuAG single-crystal fiber (SCF) laser setup. Figure 4. A schematic diagram of the Yb:LuAG single-crystal fiber (SCF) laser setup. Figure 4. A schematic diagram of the Yb:LuAG single-crystal fiber (SCF) laser setup. 3. Results and Discussion 3. Results and Discussion 3.3.1.3.1. Results Diameter Diameter and Fluctuation FluctuationDiscussion 3.1. DiameterAsAs mentioned mentioned Fluctuation above, above, the the uniformity uniformity of of the the fiber fiber diameter diameter greatly greatly influences influences its its ap- ap- plication.plication.As mentioned The The uniform uniform above, diameter diameter the uniformity and and the the regula of regular ther shapefiber shape diameter are conducive are conducive greatly to achievinginfluences to achieving the its totalap- the plication.reflectiontotal reflection The of the uniform of pump the pumpdiameter light and light and reducing and the reducing regula the lightr theshape transmission light are transmission conducive loss. to Compared loss. achieving Compared with the total glass with reflectionfibers,glass fibers, the of greater the the pump greater viscosity light viscosity and of the reducingof SCF the makes SCF the makeslight it more transmission it moredifficult difficult to loss. control toCompared control the diameter. the with diameter. glass Im- fibers,properImproper the parameters greater parameters viscosity in the in thegrowth of growththe SCF process processmakes such it such more as the as difficult the growth growth to power control power and the and the diameter. the vibration vibration Im- of propertheof the mechanical parameters mechanical device in device the will growth will cause cause process instability instability such in as in the the melting meltinggrowth zone.power zone. ThroughThrough and the thevibration the damping of thetreatmenttreatment mechanical of of the the device mechanical mechanical will cause devices devices instability and and the the in optimization optimization the melting of ofzone. the the growthThrough growth parameters, parameters, the damping Yb: Yb: treatmentLuAGLuAG single-crystal of the mechanical fibersfibers withwith devices aa relativelyrelatively and theuniform uniformoptimization diameter diameter of the were were growth obtained. obtained. parameters, The The diameter diame- Yb: LuAGterfluctuation fluctuation single-crystal of the of LuAGthe fibers LuAG single-crystal with single-crystal a relatively fiber isuniformfiber shown is shown indiameter Figure in5 wereFigure. The obtained. average 5. The fiberaverage The diameter diame- fiber terdiameteris 988.3fluctuationµ m,is 988.3 and of thethe μm, measuredLuAG and thesingle-crystal maximummeasured and maximumfiber minimum is shown and diameters minimumin Figure are 5.diameters 1015.6The average and are 978.6 1015.6fiberµm, diameterandrespectively. 978.6 is μ 988.3m, The respectively. μ diameterm, and the fluctuation The measured diameter of maximum the fluctuat Yb:LuAGion an ofd SCF theminimum isYb:LuAG 3.7%, diameters which SCF isis beneficial3.7%, are 1015.6 which for andisconducting beneficial 978.6 μm, fiberfor respectively. conducting laser experiments. The fiber diameter laser experiments. fluctuation of the Yb:LuAG SCF is 3.7%, which is beneficial for conducting fiber laser experiments.

FigureFigure 5. 5. TheThe diameter diameter fluctuations fluctuations of of the the Yb-doped Yb-doped LuAG LuAG SCF. SCF. Figure3.2. Laue 5. The Back-Reflection diameter fluctuations Measurements of the Yb-doped LuAG SCF. 3.2. Laue Back-Reflection Measurements Laue back-reflection measurement is usually used for crystal orientation and crys- 3.2. Laue Back-Reflection Measurements tallinityLaue tests. back-reflection The X-ray measurement beam is 0.5 is mm usually in diameter, used for andcrystal the orientation distance between and crystal- the linitycrystalLaue tests. and back-reflection The detector X-ray is beam 125 measurement mm. is 0.5Figure mm isin6 a,busually diameter,show used the and for test the crystal principles. distance orientation between It is demonstrated and the crystal- crystal and detector is 125 mm. Figure 6a,b show the test principles. It is demonstrated in Figure linityin Figure tests.6 aThe1,a 2X-raythat thebeam diffraction is 0.5 mm spots in di ofameter, the fiber and end the facedistance are clearbetween and the the crystal growth and6adirection1 ,adetector2 that ofthe is the diffraction125 crystal mm. Figure is spots determined 6a,b of the show fiber as <111>.the end test face Theprinciples. are direction clear It and is has demonstrated the a better growth optical direction in Figure quality of 6athethan1,a crystal2 that the otherthe is determineddiffraction directions spots as in <111>. cubic of the crystalThe fiber directio systems. end nface has When are a better clear the opticaland fiber the moves quality growth along than direction the the axis other of to thethe crystal test different is determined areas, theas <111>. diffraction The directio spots aren has clear a better and consistent,optical quality as can than be the observed other

Crystals 2021, 11, x 6 of 10

Crystals 2021, 11, 78 6 of 10 directions in cubic crystal systems. When the fiber moves along the axis to the test differ- ent areas, the diffraction spots are clear and consistent, as can be observed from Figure

6b1,b2. These resultsfrom illustrate Figure6b that1,b2 .the These LuAG results crystal illustrate fiber has that good the LuAG crystallinity crystal fiberand a has con- good crystallinity sistent orientation.and a consistent orientation.

Figure 6. (aFigure,b) Schematic 6. (a,b) Schematic diagrams ofdiagrams the Laue of back-reflection the Laue back-reflection measurements. measurements. (a1,a2) Characteristic (a1,a2) Characteristic Laue back-reflection pattern andLaue crystal back-reflection orientation ofpattern fiber endand face.crystal (b 1orientation,b2) Characteristic of fiber end Laue face. back-reflection (b1,b2) Characteristic patterns atLaue different positions with the X-rayback-reflection beam hitting patterns the SCF. at different positions with the X-ray beam hitting the SCF.

3.3. The Concentration3.3. The and Concentration Distribution andof Yb Distribution3+ of Yb3+ At the low dopingAt concentration, the low doping the concentration, fluorescence the lifetime fluorescence of Yb:LuAG lifetime increases of Yb:LuAG with increases with the rise in the dopingthe rise concentration, in the doping while concentration, the radiation while capture the radiation and the capture fluorescence and the fluorescence quenching effectsquenching occur at effectsa high occurdoping at level a high [25]. doping The concentration level [25]. The of concentration doping ions ofin doping ions in this experimentthis was experiment characterized was by characterized X-ray fluorescence by X-ray (XRF), fluorescence as shown (XRF), in Table as shown1. The in Table1. The concentration ofconcentration Yb3+ in 1 and of3 mm Yb3+ diameterin 1 and as-grown 3 mm diameter LuAG as-grown fiber samples LuAG is fiber7.26% samples and is 7.26% and 10.71%, respectively.10.71%, The respectively. calculated segregation The calculated coefficient segregation is close coefficient to 1. Compared is close to with 1. Compared with other crystal growthother methods, crystal growth the segregation methods, thecoefficient segregation of μ-PD coefficient technology of µ-PD is higher. technology is higher. In addition, the distribution of doped ions in the crystal fibers plays an important role in their laserTable performance. 1. The Yb3+ Thedopant aggregation concentration of doped of the Yb:LuAG ions will SCF. bring in the non-uni- form heat which is generated during the laser experiment process, consequently reducing Samples Al (Mass%) Lu (Mass%) Yb (Mass%) Yb/Lu (at%) the laser damage threshold of the crystal. The raw material melts evenly in the crucible when SCFs are grownΦ 1by mm the Yb:LuAG μ-PD method, SCF which can 10.6 further improve 83.4 the uniformity 5.99 of 7.26 the composition. TheΦ Yb3 mm3+ ion Yb:LuAG distribution SCF of the LuAG 16.6 SCF was measured 75.4 by EPMA 7.99 and 10.71 SEM, as shown in Figure 7. The results reflect that the Yb3+ is relatively evenly distributed in the LuAG SCF. In addition, the distribution of doped ions in the crystal fibers plays an important role in their laser performance. The aggregation of doped ions will bring in the non-uniform Table 1. The Yb3+heat dopant which concentration is generated of the during Yb:LuAG the SCF. laser experiment process, consequently reducing the laser damage threshold of the crystal. The raw material melts evenly in the crucible when Samples Al (Mass%) Lu (Mass%) Yb (Mass%) Yb/Lu (at%) SCFs are grown by the µ-PD method, which can further improve the uniformity of the Ф1 mm Yb:LuAG SCF 10.6 83.4 5.99 7.26 composition. The Yb3+ ion distribution of the LuAG SCF was measured by EPMA and Ф3 mm Yb:LuAGSEM, SCF as shown in16.6 Figure 7. The75.4 results reflect that7.99 the Yb 3+ is relatively10.71 evenly distributed in the LuAG SCF.

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CrystalsCrystals 20212021, ,1111, ,x 78 77 of of 10 10

Figure 7. The axial and radial (inset) distributions of Yb3+ in the LuAG fiber. FigureFigure 7. 7. TheThe axial axial and and radial radial (inset) (inset) distributions distributions of of Yb Yb3+3+ inin the the LuAG LuAG fiber. fiber. 3.4. Spectral Property 3.4.3.4. Spectral Spectral Property Property The absorption spectrum of the Yb:LuAG crystal between 880 and 1050 nm is shown The absorption spectrum of the Yb:LuAG crystal between 880 and 1050 nm is shown in FigureThe absorption8. The main spectrum absorption of thepeaks Yb:LuAG are located crystal at 938 between and 969 880 nm, and respectively, 1050 nm is shown corre- in Figure8. The2 main2 absorption peaks are located3+ at 938 and 969 nm, respectively, spondingin Figure 8.to The the mainF7/2→ absorption2 F5/2 energy2 peaks level aretransition located of at Yb 938 .and The 969 absorption3+ nm, respectively, intensity corre-at the corresponding to2 the 2F7/2→ F5/2 energy level transition3+ of Yb . The absorption intensity wavelengthsponding to of the 938 F 7/2nm→ isF 5/2higher energy than level that transition at 969 nm of with Yb the. The absorption absorption coefficient intensity of at 9.87 the at the wavelength of 938 nm is higher than that at 969 nm with the absorption coefficient of cmwavelength−1, which providesof 938 nm a isbasis higher for thethan selection that at 969 of the nm pump with thesource. absorption Further, coefficient a wider absorp- of 9.87 9.87 cm−1, which provides a basis for the selection of the pump source. Further, a wider tioncm−1 ,peak which is providesbeneficial a basisto improving for the selection the pu ofmping the pump efficiency, source. and Further, the excitation a wider absorp- of the absorption peak is beneficial to improving the pumping efficiency, and the excitation of the tion peak is beneficial to improving the pumping efficiency, and the excitation of the pumppump wavelength wavelength of of the the matched matched laser laser diode diode (LD)pump (LD)pump source source can can be be obtained obtained without without pump wavelength of the matched laser diode (LD)pump source can be obtained without strictstrict temperature temperature control. control. strict temperature control.

Figure 8. The absorption spectrum of the Yb:LuAG crystal in the wavelength range of 880–1050 nm. Figure 8. The absorption spectrum of the Yb:LuAG crystal in the wavelength range of 880–1050 nm.Figure3.5. Laser 8. The Experiments absorption spectrum of the Yb:LuAG crystal in the wavelength range of 880–1050 nm. Preliminary laser output verification of the as-grown Yb:LuAG SCFs was performed. 3.5. Laser Experiments 3.5.With Laser the Experiments output couplers of 5% and 30%, the maximum continuous-wave (CW) output laser powerPreliminary obtained bylaser using output the 1mmverification diameter of the Yb:LuAG as-grown fiber Yb:LuAG is 1.53 and SCFs 1.96 was W, performed. respectively, WithwithPreliminary the the centraloutput wavelengthscouplerslaser output of 5% ofverification 1032and and30%, 1047of the the nmmaximum as-grown (Figure9 Yb:LuAGcontinuous-wave). Figure 10 SCFsa shows was (CW) theperformed. function output laserWithof output powerthe output laser obtained power couplers by and using of absorption 5% the and 1mm 30%, pump diamet the power. maximumer Yb:LuAG The slopecontinuous-wave fiber efficiencies is 1.53 and are (CW) 1.96 10.83% outputW, andre- spectively,laser13.55%, power respectively, with obtained the central correspondingby using wavelengths the 1mm to an of diamet optical 1032 ander conversion Yb:LuAG 1047 nm efficiency (Figurefiber is 9).1.53 of Figure 6.99% and 1.96 and10a shows 10.09%.W, re- spectively,The laser beam with the quality central factor wavelengths M2 at thehighest of 1032 outputand 1047 power nm (Figure is 1.08 with9). Figure a good 10a Gaussian shows

Crystals 2021, 11, x 8 of 10 Crystals 2021, 11, x 8 of 10

Crystals 2021, 11, x 8 of 10

thethe functionfunction ofof outputoutput laserlaser powerpower andand absorpabsorptiontion pumppump power.power. TheThe slopeslope efficienciesefficiencies areare 10.83%10.83% andand 13.55%,13.55%, respectively,respectively, correspondincorrespondingg toto anan opticaloptical conversionconversion efficiencyefficiency ofof Crystals 2021, 11,the 78 function of output laser power and absorption pump2 power. The slope efficiencies are 8 of 10 6.99%6.99% andand 10.09%.10.09%. TheThe laserlaser beambeam qualityquality factorfactor MM2 atat thethe highesthighest outputoutput powerpower isis 1.081.08 10.83%with a goodand 13.55%,Gaussian respectively, distribution, correspondin indicating theg excellentto an optical optical conversion quality of theefficiency Yb:LuAG of with a good Gaussian distribution, indicating the excellent2 optical quality of the Yb:LuAG 6.99%SCF (Figure and 10.09%. 10b). A The maximum laser beam output quality power factor of 4.7 M W at isthe achieved highest withoutput a 3 powermm diameter is 1.08 withSCF a(Figure good Gaussian 10b). A maximum distribution, output indicating power the of excellent4.7 W is achieved optical quality with a of 3 themm Yb:LuAG diameter Yb:LuAGYb:LuAG SCFSCF atatdistribution, anan absorbedabsorbed indicating pumppump powerpower the excellent ofof 26.2826.28 optical WW (Figure(Figure quality 11).11). of It theIt isis Yb:LuAG promisingpromising SCF toto (Figure 10b). A SCF (Figure 10b). A maximum output power of 4.7 W is achieved with a 3 mm diameter achieve a highermaximum laser output output power power through of 4.7 a Whigher is achieved pump power. with a 3In mm addition, diameter further Yb:LuAG SCF at an Yb:LuAGachieve a SCFhigher at laseran absorbed output powerpump throughpower of a higher26.28 W pump (Figure power. 11). InIt addition,is promising further to optimizationoptimization ofof absorbedthethe laserlaser system pumpsystem power andand coatingcoating of 26.28 ofof W crystalscrystals (Figure alsoalso 11 ). playplay It is anan promising importantimportant to rolerole achieve inin a higher laser achieve a higher laser output power through a higher pump power. In addition, further achieving high-poweroutput and power high-efficiency through a higher lasers. pump power. In addition, further optimization of the laser optimizationachieving high-power of the laser and system high-efficiency and coating lasers. of crystals also play an important role in system and coating of crystals also play an important role in achieving high-power and achieving high-power and high-efficiency lasers. high-efficiency lasers.

Figure 9. The laser spectra of the Yb:LuAG SCF with the central wavelengths of 1032 (a) and 1047 Figure 9. The laser spectra of the Yb:LuAG SCF with the central wavelengths of 1032 (a) and 1047 nmnm ((bb).). FigureFigure 9. 9.The The laser laser spectra spectra of of the the Yb:LuAG Yb:LuAG SCF SCF with with the the central central wavelengths wavelengths of of 1032 1032 ( a()a) and and 1047 1047 nm (b). nm (b).

Figure 10.FigureThe continuous-wave 10. The continuous-wave (CW) laser (CW) performance laser performance (a) and the (a quality) and the of quality the laser of beamthe laser (b) atbeam the maximum(b) power Figure 10. The continuous-wave (CW) laser performance (a) and the quality of the laser beam (b) output ofat theat thethe 1 mm maximummaximum diameter powerpower Yb:LuAG outputoutput SCF. ofof thethe 11 mmmm diameterdiameter Yb:LuAGYb:LuAG SCF.SCF. Figure 10. The continuous-wave (CW) laser performance (a) and the quality of the laser beam (b) at the maximum power output of the 1 mm diameter Yb:LuAG SCF.

Figure 11. The CW laser performance (a) and the quality of the laser beam (b) at the maximum power output of the 3 mm Figure 11. The CW laser performance (a) and the quality of the laser beam (b) at the maximum diameter Yb:LuAGFigure 11. SCF. The CW laser performance (a) and the quality of the laser beam (b) at the maximum powerpower outputoutput ofof thethe 33 mmmm diameterdiameter Yb:LuAGYb:LuAG SCF.SCF. Figure 11. The CW laser performance (a) and the quality of the laser beam (b) at the maximum power output of the4. 3 Conclusions mm diameter Yb:LuAG SCF. In this paper, transparent and non-cracked Yb:LuAG single-crystal fibers were suc- cessfully grown through the µ-PD technique. The diameter fluctuation of the crystal fiber is less than 5%, suggesting that the growth conditions are well optimized and the mechanical Crystals 2021, 11, 78 9 of 10

control system is relatively stable. Hence, this µ-PD technique shows a great prospect to be widely applied for Yb:LuAG SCF growth. The Laue back-scattering technology confirms that the fibers have good crystallinity. EPMA measurements prove that the Yb3+ ions are uniformly distributed in the LuAG fiber, which is crucial to the laser experiment. As a result, the maximum CW output power of the 1 mm Yb:LuAG SCF at 1032 and 1047 nm reaches 1.53 and 1.96 W, respectively, with the beam factor M2 of 1.08. In addition, a 4.7 W CW laser at 1049 nm is obtained by using a 3 mm diameter Yb:LuAG fiber with the slope efficiency of 22.17%. All these results indicate that Yb:LuAG SCFs are a promising solid-state laser gain material for high-power laser applications.

Author Contributions: Conceptualization, A.W. and J.Z.; methodology, A.W. and B.W.; validation, A.W.; formal analysis, A.W. and S.W.; investigation, A.W., S.Y., X.M. and F.W.; data curation, A.W.; writing—original draft preparation, A.W.; writing—review and editing, A.W., T.W. and J.Z.; project administration, J.Z. and B.Z.; funding acquisition, J.Z. and Z.J. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Key Research and Development Program of China (2016YFB1102201), the Key Research and Development Program of Shandong Province (2018JMRH0207, 2018CXGC0410), the National Natural Science Foundation of China (51932004, 61975098, 61975095), the Natural Science Foundation of Shandong Province (ZR2018PEM007), the 111 Project 2.0 (Grant No: BP2018013) and the Open Funds of the State Key Laboratory of Rare Earth Resource Utilization (RERU2019010). Conflicts of Interest: The authors declare no conflict of interest.

References 1. Extance, A. Military technology: Laser Weapons Get Real. Nat. News 2015, 521, 408–410. [CrossRef][PubMed] 2. Brenier, A.; Wu, Y.; Fu, P.; Guo, R.; Jing, F. Evidence of self-frequency doubling from two inequivalent Nd3+ centers in the 3+ La2CaB10O19:Nd bifunctional crystal. J. Appl. Phys. 2005, 98, 123528. [CrossRef] 3. Yu, H.H.; Zhang, H.; Wang, Z.; Wang, J.Y.; Yu, Y.G.; Jiang, M.H. Continuous-wave laser performance of Nd:LuxGd1-xVO4 operating at 1.34 µm. Laser Phys. Lett. 2008, 5, 181–184. [CrossRef] 4. Zuo, C.H.; Zhang, B.T.; He, J.L.; Dong, X.L.; Yang, K.J.; Huang, H.T.; Xu, J.L.; Zhao, S.; Dong, C.M.; Tao, X.T. The acousto-optical Q-switched Nd: GGG laser. Laser Phys. Lett. 2008, 5, 719–721. [CrossRef] 5. Nilsson, J.; Payne, D.N. High-Power Fiber Lasers. Science 2011, 332, 921–922. [CrossRef][PubMed] 6. Kratky, A.; Schuöcker, D.; Liedl, G. Processing with kW fibre lasers: Advantages and limits. In Proceedings of the XVII International Symposium on Gas Flow and Chemical Lasers and High Power Lasers, Lisbon, Portugal, 15–19 September 2008; SPIE: Bellingham, WA, USA, 2008; Volume 7131, p. 71311X. 7. Parthasarathy, T.A.; Hay, R.S.; Fair, G.; Hopkins, F.K. Predicted performance limits of yttrium aluminum garnet fiber lasers. Opt. Eng. 2010, 49, 094302. [CrossRef] 8. Délen, X.; Piehler, S.; Didierjean, J.; Aubry, N.; Voss, A.; Ahmed, M.A.; Graf, T.; Balembois, F.; Georges, P. 250 W single-crystal fiber Yb:YAG laser. Opt. Lett. 2012, 37, 2898–2900. [CrossRef] 9. Kalaycioglu, H.; Sennaroglu, A.; Kurt, A.; Özen, G. Spectroscopic analysis of Tm3+:LuAG. J. Phys. Condens. Matter 2007, 19, 036208. [CrossRef] 10. Petrosyan, A.G.; Shirinyan, G.O. Peculiarities of of rare-earth garnets from non-stoichiometric melts. Inorg. Mater. 1993, 29, 258–261. 11. Beil, K.; Fredrich-Thornton, S.T.; Tellkamp, F.; Peters, R.; Huber, G. Thermal and laser properties of Yb:LuAG for kW thin disk lasers. Opt. Express 2010, 18, 20712–20722. [CrossRef] 12. Veselský, K.; Šulc, J.; Jelínková, H.; Nejezchleb, K.; Škoda, V. Yb Doping Concentration and Temperature Influence on Yb:LuAG Thermal Lensing. In Proceedings of the SPIE LASE: Solid State Lasers XXV: Technology and Devices, San Francisco, CA, USA, 15–18 February 2016; SPIE: Bellingham, WA, USA, 2016. 13. Brenier, A.; Guyot, Y.; Cañibano, H.; Boulon, G.; Ródenas, A.; Jaque, D.; Eganyan, A.; Petrosyan, A.G. Growth, spectroscopic, and 3+ laser properties of Yb -doped Lu3Al5O12 garnet crystal. J. Opt. Soc. Am. B 2006, 23, 676–683. [CrossRef] 14. Dong, J.; Ueda, K.I.; Kaminskii, A.A. Efficient passively Q-switched Yb: LuAG microchip laser. Opt. Lett. 2007, 32, 3266–3268. [CrossRef][PubMed] 15. Euler, F.; Bruce, J.A. Oxygen coordinates of compounds with garnet structure. Acta Crystallogr. 1965, 19, 971–978. [CrossRef] 16. Nakao, H.; Shirakawa, A.; Ueda, K.; Yagi, H.; Yanagitani, T.; Weichelt, B.; Wentsch, K.; Ahmed, M.A.; Graf, T. Yb3+ -Doped Lu3Al5O12 Ceramic Thin-Disk Laser. In Proceedings of the Conference on Lasers and Electro-Optics Pacific Rim, Kyoto, Japan, 30 June–4 July 2013. 17. Klemens, P.G. Thermal Resistance due to Point Defects at High Temperatures. Phys. Rev. 1960, 119, 507–509. [CrossRef] Crystals 2021, 11, 78 10 of 10

18. Mu, X.; Meissner, S.K.; E Meissner, H. Laser diode pumped high efficiency Yb:YAG crystalline fiber waveguide lasers. In Proceedings of the SPIE LASE: Solid State Lasers XXIV: Technology and Devices, San Francisco, CA, USA, 8–10 February 2015; SPIE: Bellingham, WA, USA, 2015; Volume 9342, p. 934209. 19. Johnson, L.F.; Geusic, J.E.; Van Uiter, L.G. Coherent oscillations from Tm3+, Ho3+, Yb3+ and Er3+ ions in yttrium aluminum garnet. Appl. Phys. Lett. 1965, 7, 127–129. [CrossRef] 20. Sangla, D.; Aubry, N.; Nehari, A.; Brenier, A.; Tillement, O.; Lebbou, K.; Balembois, F.; Georges, P.; Perrodin, D.; Didierjean, J. Yb-doped Lu3Al5O12 bers single crystals grown under stationary stable state for laser application. J. Cryst. Growth 2009, 312, 125–130. [CrossRef] 21. Veronesi, S.; Zhang, Y.; Tonelli, M.; Agnesi, A.; Greborio, A.; Pirzio, F.; Reali, G. Spectroscopy and efficient laser emission of Yb3+: LuAG single crystal grown by µ-PD. Opt. Commun. 2012, 285, 315–321. [CrossRef] 22. Wang, T.; Zhang, J.; Zhang, N.; Wang, S.; Wu, B.; Jia, Z.; Tao, X. The characteristics of high-quality Yb:YAG single crystal fibers grown by a LHPG method and the effects of their discoloration. RSC Adv. 2019, 9, 22567–22575. [CrossRef] 23. Che, Z.; Zhang, J.; Wu, B.; Hu, Q.; Jia, Z. Investigation of Y2.1Er0.9(ScxGa1−x)5O12 Matrix Components on the Spectral Prop-erties around 3.0 µm by Micro-Pulling-Down Method. Crystals 2019, 9, 138. [CrossRef] 24. Wu, B.; Nie, H.; Wang, A.; Zhang, J.; Jia, Z.; Zhang, B.; Fu, X.; Hu, Q.; He, J.; Tao, X. Factors influencing optical uniformity of YAG single-crystal fiber grown by micro-pulling-down technology. Cryst. Eng. Comm. 2019, 21, 6929–6934. [CrossRef] 3+ 25. Toncelli, A.; Alshourbagy, M.; Tonelli, M. Optical properties of Yb doped Lu3Al5O12 crystal fibers grown by µ-pulling down technique. J. App. Phys. 2008, 104, 350–676. [CrossRef]