Article Laser Operation of Tm: LuAG Single- Fiber Grown by the Micro-Pulling down Method

Jian Liu 1,2, Jifei Dong 1, Yinyin Wang 1, Hangqi Yuan 1, Qingsong Song 2, Yanyan Xue 2, Jie Xu 1, Peng Liu 1, Dongzhen Li 1 , Kheirreddine Lebbou 3 , Zhanxin Wang 1, Yongguang Zhao 1,* , Xiaodong Xu 1,* and Jun Xu 2,*

1 Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China; [email protected] (J.L.); [email protected] (J.D.); [email protected] (Y.W.); [email protected] (H.Y.); [email protected] (J.X.); [email protected] (P.L.); [email protected] (D.L.); [email protected] (Z.W.) 2 School of Physics Science and Engineering, Institute for Advanced Study, Tongji University, Shanghai 200092, China; [email protected] (Q.S.); [email protected] (Y.X.) 3 Institut Lumière Matière, UMR5306 Université Lyon1-CNRS, Université de Lyon, 69622 Lyon, France; [email protected] * Correspondence: [email protected] (Y.Z.); [email protected] (X.X.); [email protected] (J.X.)

Abstract: Single crystals fiber (SCF) of LuAG doped with 2.0 at.% thulium has been grown by using micro-pulling down (µ-PD) technique. A continuous-wave output power of 2.44 W was achieved with a slope efficiency of 11.7% using a 783-nm diode laser as pump source. The beam quality factors 2  M were 1.14 and 1.67 in the x and y directions, respectively. This is, to the best of our knowledge, the  first report on the Tm:LuAG SCF laser. Using ray tracing analysis, the influence of laser performance

Citation: Liu, J.; Dong, J.; Wang, Y.; by the pump intensity distribution in the SCF was also studied. Yuan, H.; Song, Q.; Xue, Y.; Xu, J.; Liu, P.; Li, D.; Lebbou, K.; et al. Laser Keywords: Tm:LuAG; fiber; mid-infrared laser Operation of Tm: LuAG Single- Crystal Fiber Grown by the Micro-Pulling down Method. Crystals 2021, 11, 898. https://doi.org/ 1. Introduction 10.3390/cryst11080898 Lasers in the 2-µm spectral region attract much attention due to their applications in various fields, such as medical LIDAR system [1] and remote sensing of water vapor Academic Editor: Anna in the atmosphere [2]. To date, rare ions Tm3+ and/or Ho3+ doped materials have been Paola Caricato demonstrated promising candidates for the generation of 2 µm laser. In comparison, Tm3+ doped laser could be directly pumped by commercially available AlGaAs diodes at Received: 10 July 2021 ~790 nm and providing a high quantum yield of almost 200% at a high Tm- level by Accepted: 28 July 2021 Published: 31 July 2021 taking advantages of the two-for-one cross-relaxation excitation process [3]. Lu3Al5O12 (LuAG) similar to YAG crystal, has the same isostructural garnet structure

Publisher’s Note: MDPI stays neutral and had been widely studied for its excellent thermal conductivity and physical prop- µ with regard to jurisdictional claims in erties [4,5]. In 1995, Barnes et al. first reported the Tm:LuAG laser around 2 m with a published maps and institutional affil- relatively low slope efficiency of 7.3% [6]. In 2015, a maximum output power of 4.42 W iations. at 2021.2 nm was obtained, corresponding to a slope efficiency of 49.5% [7]. In 2020, the Tm:LuAG laser produced a 42.5-µJ pulse energy at a 13.3 kHz repetition rate through the self-Q-switching (SQS) technique [8]. For mode-locking operations, pulse durations of 2 ps and 13.6 ps were respectively generated from Tm:LuAG ceramic [9] and crystal [10] lasers. All these results proved that LuAG is a promising host for the 2-µm laser operation when Copyright: © 2021 by the authors. 3+ Licensee MDPI, Basel, Switzerland. doped with Tm ions. µ This article is an open access article The micro-pulling down ( -PD) technique developed from the Czochralski method distributed under the terms and can grow single-crystal fibers with a diameter around 500 µm and length of several dozen conditions of the Creative Commons centimeters. In general, we define crystals elongated along one dimensional body of special Attribution (CC BY) license (https:// (normally cylindrical) geometry and diameter from several microns to approximately 1 mm creativecommons.org/licenses/by/ as single-crystal fiber [11]. However, except for a demonstrated laser operation based on a 4.0/). 2-mm diameter Tm:LuAG crystal rod grown using such µ-PD method [12], there is, to the

Crystals 2021, 11, 898. https://doi.org/10.3390/cryst11080898 https://www.mdpi.com/journal/crystals Crystals 2021, 11, x FOR PEER REVIEW 2 of 7

Crystals 2021, 11, 898 operation based on a 2-mm diameter Tm:LuAG crystal rod grown using such µ2-PD of 7 method [12], there is, to the best of our knowledge, no Tm:LuAG SCF or its based laser operation has been reported so far. Therefore, the laser performance, in particular its bestdependence of our knowledge, on the spatial no Tm:LuAG pumping SCF intensity or its based distribution laser operation in such has typical been SCF, reported is still so far.unknown. Therefore, the laser performance, in particular its dependence on the spatial pumping intensityIn this distribution study, Tm:LuAG in such typicalSCF, with SCF, diameter is still unknown. of ~0.9 mm and length of 190 mm, has beenIn grown this study, by using Tm:LuAG the µ-PD SCF, technique. with diameter Performance of ~0.9 mm of and the length Tm:LuAG of 190 SCF mm, CW has beenlaser grownpumped by by using the the783-nmµ-PD diode technique. laser Performancewas investigated. of the By Tm:LuAG using ray SCF tracing CW laser analysis, pumped the bypump the intensity 783-nm diodedistribution laser was in the investigated. SCF and thus By usingits impact ray tracingon laser analysis, performance the pump were intensityinvestigated. distribution in the SCF and thus its impact on laser performance were investigated.

2. Experiments 2.1. To fabricatefabricate thethe Tm:LuAGTm:LuAG SCF,SCF, thethe initialinitial rawraw materialsmaterials werewere cracklecrackle Tm:LuAGTm:LuAG crystalscrystals growngrown by by the the Czochralski Czochralski method. method. The The Tm Tm concentration concentration was was 2.0 at.%2.0 at.% with with respect re- 3+ to Lu corresponding3+ to the formula (Lu Tm ) Al O . The growth experiments spect to Lu corresponding to the formula0.98 (Lu0.980.02Tm0.023 )3l5All512O12. The growth experiments were conducted in flowing N atmosphere to prevent oxidation of the Ir crucible. The were conducted in flowing N22 atmosphere to prevent oxidation of the Ir crucible. The preparedprepared cracklecrackle waswas meltmelt inin anan iridiumiridium crucible,crucible, andand then,then, thethe meltmelt waswas pulledpulled downdown continuouslycontinuously throughthrough a a capillary capillary channel channel at theat bottomthe bottom of the of crucible. the crucible. An undoped An undoped LuAG crystalLuAG crystal oriented oriented in <111> in direction<111> direction was used was as used the seedas the and seed pulled and downpulled atdown the rate at the of 0.5rate mm/min. of 0.5 mm/min. Through Through a CCD-camera, a CCD-camera, the meniscus the meniscus and the diameterand the diameter of the growing of the crystal were controlled by controlling the radio-frequency heating power. Figure1a shows growing crystal were controlled by controlling the radio-frequency heating power. Fig- the end view of the SCF with a diameter of 0.9 mm. The crystal with a uniform diameter of ure 1a shows the end view of the SCF with a diameter of 0.9 mm. The crystal with a 190 mm in length is shown in Figure1b. uniform diameter of 190 mm in length is shown in Figure 1b. (c)

(b)

Figure 1.1. End facet of Tm:LuAG SCF (a)) as-grownas-grown Tm:LuAGTm:LuAG SCFSCF ((bb)) andand schematicschematic ofof thethe CWCW Tm:LuAGTm:LuAG SCFSCF laserlaser pumped pumped by by a 783-nma 783-nm laser laser diode diode (c), (c IM,), IM, input input mirror; mirror; OC, OC, output output coupler; coupler; DM, dichroicDM, dichroic mirror; mirror; PM,power PM, power meter. meter. 2.2. Experimental Setup 2.2. Experimental Setup The experimental setup of the Tm:LuAG SCF laser is schematically shown in Figure1c. The experimental setup of the Tm:LuAG SCF laser is schematically shown in Figure The pump source was a fiber-coupled (100-µm core with NA = 0.22) laser diode delivering 1c. The pump source was a fiber-coupled (100-μm core with NA = 0.22) laser diode de- a maximum power of 25 W at 783 nm with a measured beam quality, i.e., M2-factor of 34.livering The pumpa maximum beam waspower focused of 25 into W theat 783 crystal nm bywith a couplinga measured lens beam system, quality, where i.e., L1 2 withM -factor a focal of length34. The of pump 25.4 mmbeam was was used focused to collimate into the the crystal pump by light,a coupling and L2 lens with system, focal lengthswhere L1 of with 30, 50, a andfocal 75 length mm wasof 25.4 used mm to was focus used the pumpto collimate beam intothe pump the Tm:LuAG light, and SCF L2 withwith beamfocal spotlengths diameters of 30, 50, of ~116,and 75 196, mm and was 294 usedµm. to The focus input the mirror pump (IM) beam has into a high the transmissionTm:LuAG SCF of aboutwith beam 99% at spot pump diameters wavelength of ~116, and high196, reflectionand 294 μ ofm. more The thaninput 99.9% mirror at the(IM) lasing has a wavelengths. high transmission Several of output about couplers99% at pump (OCs) wavelength with transmission and high of 3%,reflection 5%, 10%, of andmore 14% than were 99.9% used at tothe explore lasing thewavelengths. best laser performance. Several output A 20-mm-longcouplers (OCs) Tm:LuAG with trans- SCF cuttingmission from of 3%, the 5%, fabricated 10%, and entire 14% SCF were was used used to as explore the sample the forbest the laser laser performance. operation. ToA avoid the Fresnel reflection, both its end faces were polished and anti-reflection coated. We have directly measured the single-pass propagation loss using a He–Ne laser and high- − sensitive power meter, giving a loss of 0.048 cm 1. The SCF was mounted in a home-made Crystals 2021, 11, x FOR PEER REVIEW 3 of 7

20-mm-long Tm:LuAG SCF cutting from the fabricated entire SCF was used as the sam- ple for the laser operation. To avoid the Fresnel reflection, both its end faces were pol- Crystals 2021, 11, 898 ished and anti-reflection coated. We have directly measured the single-pass propagation3 of 7 loss using a He–Ne laser and high-sensitive power meter, giving a loss of 0.048 cm−1. The SCF was mounted in a home-made aluminum module and directly water-cooled to 15 °C. A dichroic mirror (DM) was used as a beam splitter for pump and laser beams. aluminum module and directly water-cooled to 15 ◦C. A dichroic mirror (DM) was used as 3.a beamResults splitter and Discussion for pump and laser beams. 3. ResultsAt first, and laser Discussion performances with different pump-guiding conditions were investi- gated. In this case, the focused beam diameters in the SCF were 116 and 196 μm, exhib- At first, laser performances with different pump-guiding conditions were investigated. itingIn this depths case, theof focus focused of beam~0.8 and diameters ~2.4 mm, in therespectively. SCF were 116The and large 196 diµvergencem, exhibiting angle depths (0.15 andof focus 0.05) ofmade ~0.8 it and easy ~2.4 to realize mm, respectively. the pump-guiding. The large Using divergence ray tracing angle analysis (0.15 and[13], 0.05) the stimulatedmade it easy spatial to realize distribution the pump-guiding. of pump light Using (with ray tracingL2, f = analysis30) in the [13 SCF], the is stimulatedshown in Figurespatial 2(a). distribution The pump-guiding of pump light mode (with can L2, bef =clearly 30) in seen. the SCF For isthe shown beam in diameter Figure2 a.ofThe 196 μpump-guidingm, the maximum mode output can be power clearly was seen. 1.24 For W the with beam a diameterslope efficiency of 196 µofm, 6.7% the maximumat an ab- sorbedoutput pump power power was 1.24 of W19 with W, higher a slope than efficiency that of of 0.68 6.7% W at and an absorbed 4.6% obtained pump powerwith the of beam19 W, higherdiameter than of that116 ofμm, 0.68 as W can and be 4.6% seen obtained in Figure with 2(b). the For beam the diameter latter case, of 116theµ outputm. For powerthe latter rose case, slowly the outputwith the power slope rose efficiency slowly withof 1.5% the when slope efficiencythe absorbed of 1.5% pump when power the wasabsorbed beyond pump 15 W. power It seems was beyondthat the 15 intens W. Ite seems pump-guiding that the intense gave pump-guidinga worse laser perfor- gave a manceworse laserin our performance case with the in 20-mm-long our case with Tm:L the 20-mm-longuAG SCF, which Tm:LuAG failed SCF, to meet which our failed expec- to tationsmeet our on expectationspump guiding on as pump that guidingin the traditional as that in glass the traditional fiber lasers. glass fiber lasers.

y [mm] 1 4 2 0 0 z

20 [mm] 1.4 10 (a) L2 = 50 mm 1.2 L2 = 30 mm 1.0

0.8

0.6

0.4 Output power [W] Output 0.2

0.0 0 5 10 15 20 25 (b) Absorbed pump power [W] FigureFigure 2. SimulatedSimulated pump intensity distributiondistribution inin thethe SCFSCF with with L2 L2 ( f(f= = 30 30 mm) mm) by by using using ray ray tracing trac- inganalysis analysis (a), ( anda), and output output power power of Tm:LuAG of Tm:LuAG SCF SCF in different in different pump pump configurations configurations (b). (b).

Thus,Thus, mode-matching maymay stillstill be be a a critical critical factor factor for for power power scaling scaling in such in such relatively rela- tivelyshort SCF,short andSCF, thereafter, and thereafter, pumping pumping configuration configuration with with a spot a diameterspot diameter of 294 of µ294m wasμm wasemployed, employed, which which in principle in principle has has a better a better mode-matching mode-matching with with the the fundamental fundamental laser la- sermode mode (~300 (~300µm μ beamm beam diameter) diameter) along along the the SCF. SCF. In this In this case, case, the focusedthe focused pump pump beam beam had hada depth a depth of focus of focus of ~6 of mm, ~6 mm, and thusand thethus position the position of the of focal the pointfocal point in the in SCF the became SCF be- a camecritical a parameter.critical parameter. Figure3 Figurea shows 3a the shows stimulated the stimulated spatial distribution spatial distribution of pump of light pump in lightthe SCF. in the In SCF. the present In the present work, Lwork, = 1, 4,L 7= mm1, 4, 7 (distance mm (distance between between the input the input end facet end andfac- etfocal and point), focal point), with free with propagation free propagation of 10, 13, of 1610, mm 13, 16 in themm SCF, in the was SCF, performed was performed for the CWfor theregime. CW regime. The laser The performances laser performances for the three for the pumping three pumping configurations configurations were investigated were in- vestigatedwith a physical with cavitya physical length cavity of ~2 length cm and of a 10%~2 cm OC, and as a shown 10% OC, in Figure as shown3b. The in maximumFigure 3b. Theoutput maximum powers foroutput the threepowers cases for were the 2.16,three 2.44, cases and were 1.91 2.16, W, corresponding 2.44, and 1.91 to W, the corre- slope efficiency of 10.2%, 11.7%, and 8.19%. The case of L = 4 mm has the best laser performance and lowest laser threshold, which indicates a trade-off between mode-matching and pump guiding is essential for the present Tm:LuAG SCF laser.

Crystals 2021, 11, x FOR PEER REVIEW 4 of 7

sponding to the slope efficiency of 10.2%, 11.7%, and 8.19%. The case of L = 4 mm has the Crystals 2021, 11, 898 best laser performance and lowest laser threshold, which indicates a trade-off between4 of 7 mode-matching and pump guiding is essential for the present Tm:LuAG SCF laser.

y [mm] 1 4 2 0 0 z 10 20 [mm] (a) 3.0 L=1 mm 2.5 L=4 mm L=7 mm 2.0

1.5

1.0 Output power [W] Output power 0.5

0.0 0 5 10 15 20 25 (b) Absorbed pump power [W]

FigureFigure 3. 3. SimulatedSimulated pump pump intensity intensity distribution distribution in in the the SCF SCF by by using using ray ray tracing tracing analysis analysis (a (a),), and and outputoutput power power of of Tm:LuAG Tm:LuAG SCF for thethe focalfocal pointspoints atat different different positions positions (b (),b), L: L: distance distance between between the theinput input end end facet facet and and focal focal point. point.

TheThe CW CW laser laser operation operation for for different different OC OCss with with a pump a pump beam beam diameter diameter of 294 of μ 294m forµm thefor L the = 4 L mm = 4 mmwas wasthereafter thereafter studied studied in detail in detail.. Figure Figure 4 shows4 shows the theoutput output power power with with re- spectrespect to tothe the absorbed absorbed pump pump power. power. For For va variousrious OCs, OCs, the the output output power power reaches reaches the the highesthighest 2.44 2.44 W W with with the the 10% OC, and decreaseddecreased sharplysharply toto 1.181.18 W W as as the the transmittance transmittance of ofthe the output output coupler coupler increased increased to 14%.to 14%. The The slope slope efficiency efficiency measured measured this timethis time is much is much lower lowerthan inthan a previous in a previous report report about Tm:YAGabout Tm:YAG lasers using lasers a thickusing rod a thick [14] (50%),rod [14] and (50%), also lower and alsothan lower the Tm:YAG than the crystalTm:YAG [12 crystal] (39.1%) [12] grown (39.1%) by grown using theby usingµ-PD techniquethe µ-PD technique in which thein 3+ which790-nm the LD 790-nm was used LD as was pump used power. as pump The relatively power. lowThe concentrationrelatively low of concentration Tm doped may of Tmbe3+ responsible doped may for be the responsible low slope efficiency.for the low We slope know efficiency. that high We Tm know concentration that high doped Tm concentrationwas important doped to realize was two-for-oneimportant to cross-relaxation. realize two-for-one Normally, cross-relaxation. materials with Normally, 4.0 at.% materialsTm doped with were 4.0 used at.% forTm laser doped operation were used [9,10 for], whilelaser operation just 2.0 at.% [9,10], Tm dopedwhile just was 2.0 used at% in Tmour doped experiments. was used The in detailedour experiments. data are alsoThe showndetailed in data Table are1. also shown in Table 1.

Table 1. Maximum output power, slope efficiency, and laser threshold for the Tm:LuAG SCF.

OCs Threshold (W) Maximum Output Power (W) Slope Efficiency (%) 3% 1.13 1.90 9.3 5% 1.15 2.10 10.6 10% 1.23 2.44 11.7 14% 2.20 1.18 6.2

Figure 4. The output power of the Tm:LuAG laser as a function of absorbed pump power with different OCs.

Crystals 2021, 11, 898 5 of 7

Table 1. Maximum output power, slope efficiency, and laser threshold for the Tm:LuAG SCF.

OCs Threshold (W) Maximum Output Power (W) Slope Efficiency (%) 3% 1.13 1.90 9.3 5% 1.15 2.10 10.6 10% 1.23 2.44 11.7 14% 2.20 1.18 6.2

Figure5 shows the laser spectrum of the Tm:LuAG SCF with different OCs. The peak wavelength changed from 2025 nm to 1972 nm. The wavelength red-shift with decrease of the OCs is caused by the enhanced reabsorption effect with lower OC transmission, where a higher population inversion is needed, resulting in a drift of gain spectrum [15]. It is obvious that the multiple wavelengths occurred and the peak wavelength blue-shift rapidly when the OCs increased from 10% to 14%. The change of gain cross sections may be responsible for this phenomenon [16]. Unfortunately, the gain cross section of the Tm:LuAG SCF cannot be depicted for the uneven concentration around the end facet of Tm:LuAG SCF grown by the µ-PD technique [17].

Figure 5. The laser spectrum of Tm:LuAG SCF with different OCs.

The beam profile was also measured at an output power of 2 W by using two plano- convex spherical lens (L3, f = 150 mm, L4, f = 100 mm) and a beam profiler (WinCamD-IR- BB, Dataray Inc). The M2 at two directions were measured to be 1.67 and 1.10 in the x- and y-directions, as shown in Figure6. The relatively larger M2 value along the x-axis is caused by the slight misalignment between the pump and oscillation in this axis, which would introduce more high-order mode. The transverse-mode also can be observed from the 2D beam profile. This phenomenon can be understood since our cavity was plano–plano and no extra transverse-mode-limiting optical elements were applied [16]. Crystals 2021, 11, 898 6 of 7

Figure 6. The beam quality factor measurement of the Tm:LuAG SCF at 2 W, the inset shows the 2D beam profile.

4. Conclusions Tm:LuAG SCF with a diameter of ~0.9 mm and 190 mm in length has been grown by using the µ-PD technique. To the best of our knowledge, the laser performance of Tm:LuAG with a diameter below 1 mm is reported for the first time. The laser operation with different pump-guiding configurations was investigated. The results showed that the pure pump-guiding mode was not a good choice for our Tm:LuAG SCF. Then, the correlation between the free propagation distance and pump-guiding mode was experi- mentally investigated. The highest output power of 2.44 W with slope efficiency of 11.7% was achieved. Additionally, the beam quality factor was calculated to be 1.10 and 1.67 at two directions. The work illustrates that the trade-off between mode-matching and pump guiding is essential for the present Tm:LuAG SCF laser. Further work will focus on the enhancement of the slope efficiency by optimizing the cavity design and the improvement of crystalline quality with higher Tm3+ concentration.

Author Contributions: Methodology, Y.Z.; formal analysis, J.D., Y.W., H.Y. and J.X. (Jun Xu); investi- gation, Q.S., Y.X., J.X. (Jie Xu), P.L. and D.L.; validation, Y.Z.; Data curation, Z.W.; funding acquisition, Y.Z., X.X. and J.X. (Jun Xu); supervision, J.X. (Jun Xu); writing—original draft preparation, J.L.; resources, Y.Z.; writing—review and editing, Y.Z., X.X., J.X. (Jun Xu) and K.L. All authors have read and agreed to the published version of the manuscript. Funding: National Natural Science Foundation of China (62075090, 52032009, 61621001, 61975071) and Natural Science Foundation of Jiangsu Province, China (SBK2019030177). Conflicts of Interest: The authors declare no conflict of interest.

References 1. Scholle, K.; Heumann, E.; Huber, G. Single mode Tm and Tm, Ho: LuAG lasers for LIDAR applications. Laser Phys. Lett. 2004, 1, 285–290. [CrossRef] 2. Taczak, T.M.; Killinger, D.K. Development of a tunable, narrow-linewidth, cw 2066-µm Ho: YLF laser for remote sensing of atmospheric CO2 and H2O. Appl. Opt. 1998, 37, 8460–8476. [CrossRef][PubMed] 3. Honea, E.; Beach, R.; Sutton, S.; Speth, J.; Mitchell, S.; Skidmore, J.; Emanuel, M.; Payne, S. 115-W Tm:YAG diode-pumped solid-state laser. IEEE J. Quantum Electron. 1997, 33, 1592–1600. [CrossRef] 4. Klemens, P.G. Thermal Resistance due to Point Defects at High Temperatures. Phys. Rev. 1960, 119, 507–509. [CrossRef] 5. Ma, C.Y.; Zhu, J.F.; Nan, X.; Hu, Z.Q.Z.; Wen, C.; Long, J.Q.; Yuan, X.Y.; Cao, Y.G. Demonstration and CW laser performances of composite YAG/Nd: LuAG/YAG transparent laser ceramic. J. Alloys Compd. 2017, 727, 912–918. [CrossRef] 6. Barnes, N.P.; Jani, M.G.; Hutcheson, R.L. Diode-pumped, room-temperature Tm: LuAG laser. Appl. Opt. 1995, 34, 4290–4294. [CrossRef][PubMed] Crystals 2021, 11, 898 7 of 7

7. Feng, T.; Yang, K.; Zhao, S. Efficient CW dual-wavelength and passively Q-switched Tm: LuAG lasers. IEEE Photon. Technol. Lett. 2015, 27, 7–10. [CrossRef] 8. Beyatli, E.; Kaya, F.; Bilici, H. Self-Q-switched and multicolor operation of a Tm: LuAG laser. Appl. Opt. 2021, 59, 8247–8252. [CrossRef][PubMed] 9. Wang, Y.C.; Lan, R.J.; Mateos, X.; Li, J.; Li, C.Y.; Suomalainen, S.; Harkonen, A.; Guina, M.; Petrov, V.; Griebner, U. Thulium doped LuAG ceramics for passively mode locked lasers. Opt. Express 2017, 25, 7084–7091. [CrossRef][PubMed] 10. Luan, C.; Yang, K.; Zhao, J.; Zhao, S.; Li, T.; Zhang, H.; He, J.; Song, L.; Dekorsy, T.; Guina, M.; et al. Diode-pumped mode-locked Tm: LuAG laser at 2 µm based on GaSb-SESAM. Opt. Lett. 2017, 42, 839–842. [CrossRef][PubMed] 11. Rudolph, P.; Fukuda, T. Fiber Crystal Growth from the Melt. Cryst. Res. Technol. 1999, 34, 13–40. [CrossRef] 12. Song, Q.S.; Xu, X.D.; Zhou, Z.Y.; Xu, B.; Li, D.Z.; Liu, P.; Xu, J.; Lebbou, K. Laser operation of Tm: LuAG single crystal fiber grown by the micro-pulling down method. IEEE Photon. Technol. Lett. 2018, 30, 1913–1916. [CrossRef] 13. 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][PubMed] 14. Callicoatt, B.; Bennett, G.; Hinckley, M.; Petersen, E.; Schober, A.; Wagner, G. High average power Tm: YAG waveguide laser. In Proceedings of the 2017 Conference on Lasers and Electro-Optics (CLEO), San Jose, CA, USA, 14–19 May 2017. 15. Zhao, Y.; Zhou, W.; Xu, X.; Shen, D. Spectroscopic properties and pulsed laser performance of thulium-doped (Lu,Y)3Al5O12 mixed crystal. Opt. Mater. 2016, 62, 701–705. [CrossRef] 16. Zhao, Y.G.; Wang, L.; Chen, W.D.; Wang, J.L.; Song, Q.S.; Xu, X.D.; Liu, Y.; Shen, D.Y.; Xu, J.; Mateos, X.; et al. 35 W continuous- wave Ho: YAG single-crystal fiber laser. High Power Laser Sci. Eng. 2020, 8, e25. [CrossRef] 17. Xu, X.D.; Lebbou, K.; Moretti, F.; Pauwels, K.; Lecoq, P.; Auffray, E.; Dujardin, C. Ce-doped LuAG single-crystal fibers grown from the melt for high-energy physics. Acta Mater. 2014, 67, 232–238. [CrossRef]