crystals Article Laser Operation of Tm: LuAG Single-Crystal 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 (m-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; single crystal 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-doping 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 m-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-nmm-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. Crystal Growth 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.