Single-Longitudinal-Mode Laser at 1123 Nm Based on a Twisted-Mode Cavity

Article Single-Longitudinal-Mode Laser at 1123 nm Based on a Twisted-Mode Cavity

Yang Liu, Sasa Zhang, Zhenhua Cong, Shaojie Men , Chen Guan, Yongyao Xie and Zhaojun Liu *

School of Information Science & Engineering and Shandong Provincial Key Laboratory of Laser Technology and Application, Shandong University, Qingdao 266237, China; [email protected] (Y.L.); [email protected] (S.Z.); [email protected] (Z.C.); [email protected] (S.M.); [email protected] (C.G.); [email protected] (Y.X.) * Correspondence: [email protected]

Abstract: A single-longitudinal-mode (SLM) Neodymiun-doped Yttrium Aluminium Garnet (Nd:YAG) laser at 1123 nm was first demonstrated with a twisted-mode (TM) cavity. By eliminating the spatial hole burning phenomenon, a stable SLM 1123 nm laser output was obtained. An efficient TM cavity was designed based on the Brewster plate with high reflection under s-polarization radiation. At an incident pump power of 7.65 W, the maximum output power of 689 mW was obtained. The corresponding optical conversion efficiency was about 9%. The center wavelength was 1122.58 nm and the line-width was <140 MHz.

Keywords: 1123 nm radiation; single longitudinal mode; twisted mode cavity; high efﬁciency

1. Introduction Owing to the single-longitudinal-mode (SLM) characteristic, single-frequency lasers have many distinctive applications in detection, medicine and cold atom physics [1–4]. In addition, single-frequency lasers also play an indispensable role in many emerging scientific research Citation: Liu, Y.; Zhang, S.; Cong, Z.; works [5–7]. Therefore, single-frequency lasers have attracted intense attention and have Men, S.; Guan, C.; Xie, Y.; Liu, Z. been widely researched [8–16]. Recently, the SLM 1123 nm laser attracted the attention of our Single-Longitudinal-Mode Laser at research group because it can motivate thulium up-conversion fiber lasers to produce blue 1123 nm Based on a Twisted-Mode laser emission [17]. Furthermore, a single-frequency yellow–green laser with a wavelength Cavity. Crystals 2021, 11, 58. https:// of 561 nm (second-harmonic radiation of a 1123 nm laser) has been considered the ideal doi.org/10.3390/cryst11010058 source for confocal microscopy flow cytometry and other bioimaging applications [18]. It is

Received: 18 December 2020 also optimal for the treatment of complex ophthalmic diseases and has wide applications Accepted: 12 January 2021 in biomedical applications [19]. To date, Neodymiun-doped Yttrium Aluminium Garnet 3/2 11/2 Published: 13 January 2021 (Nd:YAG) lasers operating in the Stark component of the 4F -4I transition have been proven to be a practical approach to generate 1123 nm lasers [20–25]. However, since the Publisher’s Note: MDPI stays neu- stimulated emission cross-section for the 1123 nm transition is approximately 15 times smaller tral with regard to jurisdictional clai- than that for the 1064 nm line [26], research into SLM 1123 nm lasers has been scarce and is ms in published maps and institutio- almost based on ring cavity technology. In 1999, the first SLM 1123 nm laser was reported nal afﬁliations. by Moore et al. with a ring cavity [20]. An output power of 180 mW was obtained at an incident pump power of 5.6 W. In 2007, with a monolithic nonplanar ring cavity, Zang et al. enhanced the output power of the SLM 1123 nm laser to 1.25 W [21]. However, the typical ring cavity always requires expensive elements such as an isolator, and in the monolithic Copyright: © 2021 by the authors. Li- nonplanar ring cavity, the special design and machining of the gain medium is inevitable. censee MDPI, Basel, Switzerland. Compared with ring cavity technology, twisted-mode (TM) cavity technology is simpler and This article is an open access article distributed under the terms and con- has a lower cost. In this technology, a pair of quarter-wave plates (QWPs) on both sides of the ditions of the Creative Commons At- gain medium and a polarization beamsplitter plate (PBP) in front of the output mirror are tribution (CC BY) license (https:// used to control the polarization state of the intra-cavity beam. The function of the QWPs is to creativecommons.org/licenses/by/ keep the forward-propagation beam and backward-propagation beam circularly polarized 4.0/). and orthogonally oriented in the gain medium. According to [8], the total energy density is

Crystals 2021, 11, 58. https://doi.org/10.3390/cryst11010058 https://www.mdpi.com/journal/crystals Crystals 2021, 11, x FOR PEER REVIEW 2 of 7

polarization state of the intra-cavity beam. The function of the QWPs is to keep the for- Crystals 2021 11 , , 58 ward-propagation beam and backward-propagation beam circularly polarized and2 or- of 7 thogonally oriented in the gain medium. According to [8], the total energy density is spa- tially uniform along the gain medium in this case; thus, there is no spatial hole burning alongspatially the uniformgain medium along theand gain the laser medium can inbe thisoperated case; thus, in single-longitudinal there is no spatial mode. hole burning How- ever,along to the our gain best medium knowledge, and the no laser research can be on operated an SLM in 1123 single-longitudinal nm laser based mode. on TM However, cavity technologyto our best knowledge,has been reported. no research on an SLM 1123 nm laser based on TM cavity technology has beenThe typical reported. TM cavity has a linear structure, and in 2011, Gao et al. designed an L- type cavityThe typical and increased TM cavity the has output a linear power structure, of a and2 μm in SLM 2011, laser Gao etfrom al. designed514 mW to an 1.46 L-type W [27].cavity In andthis increasedwork, a typical the output linear power TM cavity of a 2andµm an SLM L-type laser TM from cavity 514 mWwere to both 1.46 used W [27 to]. generateIn this work, an SLM a typical 1123 linearnm output. TM cavity A 1 at.% and anNd L-type-doped TM Nd:YAG cavity werecrystal both was used selected to generate as the gainan SLM medium. 1123 nmThe output.results indicated A 1 at.% that Nd-doped the L-type Nd:YAG TM cavity crystal showed was selectedbetter performance. as the gain Atmedium. an incident The results pump indicated power of that 7.65 the W, L-type the maximum TM cavity output showed power better of performance. 689 mW was At ob- an tainedincident with pump a corresponding power of 7.65 optical W, the conversion maximum efficiency output power of 9%. of The 689 center mW was wavelength obtained andwith line-width a corresponding were 1122.58 optical nm conversion and <140 efficiencyMHz, respectively. of 9%. The The center beam wavelength quality factors and 2 (line-widthM2) in horizontal were 1122.58 and vertical nm and directions <140 MHz, were respectively. 1.58 and 1.41, The respectively. beam quality The factors polarization (M ) in ratiohorizontal was measured and vertical to be directions >650:1. To were the best 1.58 of and our 1.41, knowledge, respectively. this Thewas polarizationthe first time ratiothat thewas SLM measured laser at to 1123 be >650:1. nm was To reported the best with of our a TM knowledge, cavity. this was the first time that the SLM laser at 1123 nm was reported with a TM cavity. 2. Experimental Setup 2. Experimental Setup The experimental setup of the SLM 1123 nm lasers is shown in Figure 1. A homemade The experimental setup of the SLM 1123 nm lasers is shown in Figure1. A homemade continuous-wave (CW) fiber-coupled laser diode (LD) with a fiber core diameter of 400 continuous-wave (CW) fiber-coupled laser diode (LD) with a fiber core diameter of 400 µm μm and a numerical aperture of 0.22 was used as the pump source. The output beam of and a numerical aperture of 0.22 was used as the pump source. The output beam of the LD the LD was re-imaged by the coupling lens (CL) into the laser crystal. The re-imaging ratio was re-imaged by the coupling lens (CL) into the laser crystal. The re-imaging ratio was was 1:1.5. The TM cavity consisted of two cavity mirrors, two QWPs and a PBP (PBP 1150- 1:1.5. The TM cavity consisted of two cavity mirrors, two QWPs and a PBP (PBP 1150-780, 780, Union Optics, China). The rear mirror M1 was a plane mirror. The entrance surface Union Optics, China). The rear mirror M1 was a plane mirror. The entrance surface was was coated for anti-reflection (AR) at 808 nm (R < 0.2%). The other surface was coated for coated for anti-reflection (AR) at 808 nm (R < 0.2%). The other surface was coated for high highreflection reflection (HR) (HR) at 1123 at 1123 nm ( Rnm> 99.8%)(R > 99.8%) and ARand at AR 808 at nm 808 (R nm< 0.2%).(R < 0.2%). A pair A ofpair zero-order of zero- orderQWPs QWPs (QWP1 (QWP1 and QWP2) and QWP2) at 1123 at nm 1123 were nm placedwere placed on each on side each of side the Nd:YAGof the Nd:YAG crystal. crystal.The principal The principal axes of theaxes QWPs of the wereQWPs oriented were oriented with their with fast their axes fast perpendicular axes perpendicular to each toother each and other at 45and◦ to at the 45° oscillating to the oscillating polarization polarization direction. direction. An Nd:YAG An Nd:YAG crystal crystal with 1 with at.% 1Nd-doped at.% Nd-doped and dimensions and dimensions of Φ5 mm of Φ×5 8mm mm × served 8 mm asserved the gain as the medium. gain medium. The Nd:YAG The Nd:YAGcrystal was crystal water-cooled was water-cooled with the temperature with the temperature maintained maintained at 18 ◦C. One at 18 PBP °C. was One placed PBP wasat Brewster’s placed at angleBrewster’s behind angle QWP2. behind The QWP2 output. couplerThe output (OC) coupler M2 was (OC) also M2 a plane was mirroralso a planewith amirror transmission with a transmission of 2% at 1123 of nm, 2% of at 5% 1123 at nm, 1112 of nm 5% and at 1112 of 4% nm at 1116and of nm 4% and at 1116 high nmtransmission and high t (HT)ransmission at 1064 nm(HT) and at 1064 1319 nm nm, and as shown 1319 nm, in the as shown inset of in Figure the inset 3. Two of Figure cavity 3.configurations Two cavity configurations were used in the were experiments. used in the The experiments. linear TM cavity, The aslinear shown TM in cavity, Figure 1asa, shownwas based in Figure on the 1a, PBP was with based HT on under the PBPp-polarization with HT under radiation. p-polarization The L-type radiation. TM cavity, The L- as typeshown TM in cavity, Figure as1b, shown was based in Figure on the 1b PBP, was with based HR on under the PBPs-polarization with HR under radiation. s-polariza- Both tioncavity radiation. lengths Both of the cavity two configurations lengths of the weretwo configurations about 70 mm. were about 70 mm.

808 nm LD 808 nm LD

1123 nm (a) (b) M1 M2 M1 CL CL M2 Nd:YAG PBP 1123 nm Nd:YAG PBP

QWP1 QWP2 QWP1 QWP2 FigureFigure 1. 1. TheThe experimental experimental setup setup of of the the single-lon single-longitudinal-modegitudinal-mode (SLM) (SLM) 1123 1123 nm nm lasers, lasers, (a ()a linear) linear twisted-modetwisted-mode (TM) (TM) cavity, cavity, ( (bb)) L-type L-type TM TM cavity. cavity. LD, LD, laser laser diode; diode; CL, CL, coupling coupling lens; lens; QWP, QWP, quarter quarter wavewave plate; PBP, polarization beamsplitter plate.

3.3. Experimental Experimental Results Results and and Discussions Discussions To begin with, we compared the output power of the two TM cavities versus the incident pump power, and the results are shown in Figure2. The output power was measured by a power meter (PM10, Molectron, America) connected to a Molectron device (EPM2000, Molectron, America). For the linear TM cavity, the threshold pump power was 4.57 W. The output power increased linearly with the incident pump power, and the slope Crystals 2021, 11, x FOR PEER REVIEW 3 of 7

To begin with, we compared the output power of the two TM cavities versus the incident pump power, and the results are shown in Figure 2. The output power was meas- Crystals 2021, 11, 58 ured by a power meter (PM10, Molectron, America) connected to a Molectron device 3 of 7 (EPM2000, Molectron, America). For the linear TM cavity, the threshold pump power was 4.57 W. The output power increased linearly with the incident pump power, and the slope efficiency was about 11%. At an incident pump power of 8.9 W, the maximum output efficiency was about 11%. At an incident pump power of 8.9 W, the maximum output power power of 497 mW was obtained. With an L-type TM cavity, the threshold was 3.4 W lower of 497 mW was obtained. With an L-type TM cavity, the threshold was 3.4 W lower than than that in aa linearlinear TMTM cavity,cavity, andand aa higherhigher outputoutput powerpower andand slopeslope efficiency efficiency were were obtained. obtained.At At an an incident incident pump pump power power of of 7.65 7.65 W, W, the the output output power power was was measuredmeasured toto bebe 689689 mW. mW. TheThe slopeslope efficiencyefficiency waswas 16%, which was 5% higher than that in in the the linear linear TM TM cavity. cavity. The The betterbetter performance performance of of the the L-type L-type TM ca cavityvity was mainly due to thethe commercialcommercial PBPPBP with with a transmission of of >95% >95% under under p-polarizationp-polarization radiation radiation and and a reflection a reflection of >99% of >99% under under s-polarizations-polarization radiation. radiation. With the With laser the with laser an with L-type an cavity L-type (s cavity-polarization (s-polarization radiation radiation oscillation),oscillation), the loss theof the loss cavity of the was cavity reduced. was reduced. Thus, the Thus, threshold the threshold value was value decreased was decreased and the opticaland the conversion optical conversion efficiency efficiency was improved. was improved. For the Forsame the reason, same reason, in order in orderto ensure to ensure the outputthe output laser laserwas always was always in single-longitudinal in single-longitudinal mode, mode, the maximum the maximum pump pump power power in in the L-typethe L-type cavity cavity was lower was lower than thanthat in that linear in linear cavity. cavity. In addition, In addition, the transmission the transmission of the of the OC wasOC not was optimal. not optimal. In future In future work, work, if the if transmission the transmission of the of OC the can OC be can optimized, be optimized, we we believebelieve that the that output the output power power and the and slop thee slope efficiency efficiency could could be further be further improved. improved.

FigureFigure 2. The 2. averageThe average output output power power of the of1123 the nm 1123 laser nm versus laser versus the incident the incident pump pump power. power.

In theIn following the following experiments, experiments, all the all results the results were were measured measured at an at anincident incident pump pump power powerof of 7.65 7.65 W W with with an an L-type L-type TM TM cavity. cavity. TheThe laserlaser emissionemission spectrumspectrum was observed with a with wide-rangea wide-range optical optical spectrum spectrum analyzer analyzer (AQ6315A, (AQ6315A, Yokogawa, Yokogawa, Japan). Japan). The The wavelength wave- span lengthand span scanning and scanning resolution resolution were set were to 1000–1500 set to 1000–1500 nm and nm 0.5 and nm, 0.5 respectively. nm, respectively. As shown in As shownFigure in3, Figure only one 3, only laser one line laser located line at lo 1123cated nm at was1123 observed. nm was observed. The HT coatings The HT at coat- 1064 nm ings atand 1064 1319 nm nm and effectively 1319 nm effectively suppressed su theppressed oscillations the oscillations of these two of wavelengthsthese two wave- with high lengthsgain. with In high addition, gain.although In addition, the althou emissiongh the cross emission sections cross at 1112 sections nm, 1116 at 1112 nm nm, and 1123 nm 1116 werenm and similar 1123 innm the were Nd:YAG similar crystal in the [ 28Nd],:YAG the transmissions crystal [28], the at 1112 transmissions nm and 1116 at 1112 nm of the nm andOC 1116 were nm higher of the than OC at were 1123 higher nm. Therefore, than at 1123 the lasernm. Therefore, lines at 1112 the nm laser and lines 1116 at nm did not 1112 oscillate.nm and 1116 The nm transmission did not oscillate. curve asThe a functiontransmission of wavelength curve as a function for the OC of wave- is given in the lengthinset for ofthe Figure OC is3 given. in the inset of Figure 3. The longitudinal mode characteristic and line width were measured with a wavelength meter (WS-7, Highﬁnesse, Germany). The resolution of the wavelength meter was 10 MHz. The results are shown in Figure4. The center wavelength was 1122.58 nm in a vacuum and the line width was less than 0.6 pm (<140 MHz). Considering the refractive index of the crystal and PBP, the effective cavity length was about 80 mm. As a result, the longitudinal mode interval could be calculated to be ~1.87 GHz [29]. The line width was much lower than the cavity’s longitudinal mode interval, and there was only one set of interference patterns, as shown in Figure4. All the data illustrate that the TM laser was operating in single-longitudinal mode.

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100

60 T=5%@1112 nm T% 40 T=4%@1114 nm T=2%@1123 nm

CrystalsCrystals 20212021, 11,,11 x ,FOR 58 PEER REVIEW 20 4 of4 7 of 7

0 800 900 1000 1100 1200 1300 1400 λ(nm)

100

60 T=5%@1112 nm T% 40 T=4%@1114 nm Figure 3. The output spectrum of the L-typeT=2%@1123 TM nm cavity laser. The inset shows the transmission

curve as a function of waveleng20 th for the output coupler (OC).

0 The longitudinal800 mode 900 characteristic 1000 1100 1200 and 1300 140line0 width were measured with a wavelength meter (WS-7, Highfinesse, Germany).λ(nm) The resolution of the wavelength meter was 10 MHz. The results are shown in Figure 4. The center wavelength was 1122.58 nm in a vacuum and the line width was less than 0.6 pm (<140 MHz). Considering the refractive index of the crystal and PBP, the effective cavity length was about 80 mm. As a result, the longitudinal mode interval could be calculated to be ~1.87 GHz [29]. The line width was

much lower than the cavity’s longitudinal mode interval, and there was only one set of FigureFigureinterference 3. The 3. The output output patterns, spectrum spectrum as ofshown ofthe the L-type in L-type Figure TM TM cavi 4. cavity tyAll laser. laser.the Thedata The inset insetillustrate shows shows thethat the transmission transmissionthe TM laser curve was curve as a function of wavelength for the output coupler (OC). asoperating a function in of single-longitudinal wavelength for the output mode. coupler (OC). The longitudinal mode characteristic and line width were measured with a wavelength meter (WS-7, Highfinesse, Germany). The resolution of the wavelength meter was 10 MHz. The results are shown in Figure 4. The center wavelength was 1122.58 nm in a vacuum and the line width was less than 0.6 pm (<140 MHz). Considering the refractive index of the crystal and PBP, the effective cavity length was about 80 mm. As a result, the longitudinal mode interval could be calculated to be ~1.87 GHz [29]. The line width was much lower than the cavity’s longitudinal mode interval, and there was only one set of interference patterns, as shown in Figure 4. All the data illustrate that the TM laser was operating in single-longitudinal mode.

FigureFigure 4. 4.The The longitudinal longitudinal mode mode characteristic characteristic of of the the 1123 1123 nm nm laser laser at at 7.65 7.65 W W incident incident pump pump power. power. With an incident pump power of 7.65 W, we also measured the polarization charac- teristicWith and anM 2incidentof the SLM pump 1123 power nm laser.of 7.65 By W, using we also a half-wave measured plate the polarization (HWP) and charac- a PBP, weteristic measured and M the2 of polarization the SLM 1123 characteristics. nm laser. By Figureusing a5 showshalf-wave the powerplate (HWP) behind and the a PBP PBP, aswe a measured function of the the polarization HWP’s rotation characteristics. angle. The Figure maximum 5 shows power the andpower minimum behind the power PBP wereas a 650function mW andof the <1 mW,HWP’s respectively. rotation an Thegle. total The power maximum before power the PBP and was minimum about 680 power mW. Thewere measurement 650 mW and results <1 mW, indicated respectively. that the The transmissions total power before of PBP the at pPBP-polarization was about and 680 s-polarizationmW. The measurement were >95% results and <0.14%, indicated respectively. that the transmissions Thus, the actual of extinctionPBP at p-polarization ratio could

beand calculated s-polarization to be >678:1 were >95% and the and polarization <0.14%, respecti ratio ofvely. the Thus, SLM 1123the actual nm laser extinction was >650:1. ratio FigureThecould 4.M The 2bemeasurement longitudinalcalculated to mode was be >678:1 performedcharacteristic and the by of polarization focusingthe 1123 nm the laser ratio beam at of 7.65 of the theW SLM incident 1123 1123 nm pump nm laser laser onto was a power. charge>650:1. coupled The M2 device measurement (CCD) camera was performed by using a by biconvex focusing lens the (f beam= 100 mm).of the By1123 measuring nm laser theonto laser a charge spot radius coupled and device ﬁtting (CCD) the experimental camera by results, using a we biconvex could calculate lens (f = the 100M mm).2 of the By With an incident pump power of 7.65 W, we also measured the polarization charac- 1123measuring nm laser. the As laser shown spot in radius Figure and6, the fitting values th ofe experimentalM2 in horizontal results, and we vertical could directions calculate teristic and M2 of the SLM 1123 nm laser. By using a half-wave plate (HWP) and a PBP, were 1.58 and 1.41, respectively. we measured the polarization characteristics. Figure 5 shows the power behind the PBP as a function of the HWP’s rotation angle. The maximum power and minimum power were 650 mW and <1 mW, respectively. The total power before the PBP was about 680 mW. The measurement results indicated that the transmissions of PBP at p-polarization and s-polarization were >95% and <0.14%, respectively. Thus, the actual extinction ratio could be calculated to be >678:1 and the polarization ratio of the SLM 1123 nm laser was >650:1. The M2 measurement was performed by focusing the beam of the 1123 nm laser onto a charge coupled device (CCD) camera by using a biconvex lens (f = 100 mm). By measuring the laser spot radius and fitting the experimental results, we could calculate

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Crystals 2021, 11, 58 5 of 7 thethe M M2 2of of the the 1123 1123 nm nm laser. laser. As As shown shown in in Figure Figure 6, 6, the the values values of of M M2 2in in horizontal horizontal and and verticalvertical directions directions were were 1.58 1.58 and and 1.41, 1.41, respectively. respectively.

9090 700 700 120120 6060 600600 500500 400400 150150 3030 300300 200200 100100 00180180 00 100100 200200

Output power, mW power, Output 300 Output power, mW power, Output 300 400400 210210 330330 500500 600600 240 300 700700 240 300 270 270 FigureFigureFigure 5. 5.5. The TheThe polarization polarizationpolarization characteristic characteristiccharacteristic of ofof the thethe SLM SLMSLM 1123 11231123 nm nmnm laser laserlaser at atat 7.65 7.657.65 W WW incident incidentincident pump pumppump power. power.power.

0.180.18

M2x=1.582 0.160.16 M x=1.58 2 M My=1.412y=1.41 0.140.14

0.120.12

0.100.10 Spot size, mm Spot size, mm 0.08 size, Spot 0.08

0.060.06

0.040.04

-5-5 0 0 5 5 10 10 15 15 20 20 25 25 30 30 35 35 Distance, mm Distance, mm .

FigureFigureFigure 6. 6.6. The The beam beam quality quality of of of the the the SLM SLM SLM 1123 1123 1123 nm nm nm laser laser laser at at at7.65 7.65 7.65 W W Wincident incident incident pump pump pump power. power. power. The The The insets insets insets showshowshow the thethe laser laserlaser spot spotspot of ofof the thethe 1123 11231123 nm nmnm laser laserlaser at atat different differentdifferent positions. positions.positions. 4. Conclusions 4.4. Conclusions Conclusions In summary, SLM 1123 nm laser emission has been realized by eliminating the spatial InIn summary, summary, SLM SLM 1123 1123 nm nm laser laser emission emission has has been been realized realized by by eliminating eliminating the the spatial spatial hole burning phenomenon. An efficient TM cavity was designed based on the Brewster holeplatehole burning burning with high phenomenon. phenomenon. reflection underAn An efficient efficients-polarization TM TM ca cavityvity radiation. was was designed designed The centerbased based wavelengthon on the the Brewster Brewster was plate1122.58plate withwith nm, highhigh and reflection thereflection line width underunder was s-polarizations-polarization <140 MHz. The radiation.radiation. maximum TheThe output centercenter power wavelengthwavelength of 689 was mWwas 1122.58was1122.58 obtained nm, nm, and and under the the line line an incidentwidth width was was pump <140 <140 powerMHz. MHz. The ofThe 7.65 maximum maximum W. The output slopeoutput efficiency power power of of was689 689 mW 16%.mW wasInwas addition, obtained obtained the under under polarization an an incident incident ratio pump pump of the power power SLM 1123nmof of 7.65 7.65 W. W. laser The The was slope slope >650:1 efficiency efficiency and the was wasM 216%. 16%.values In In 2 2 addition,inaddition, horizontal the the polarization polarization and vertical ratio ratio directions of of the the SLM wereSLM 1123nm 1.581123nm and laser laser 1.41, was was respectively. >650:1 >650:1 and and To the the the M M best values values of our in in horizontalknowledge,horizontal andand this verticalvertical is the first directionsdirections time that werewere the 1.58 SLM1.58 andand laser 1.41,1.41, at 1123 respectively.respectively. nm was reported ToTo thethe bestbest with ofof a our TMour knowledge,cavity.knowledge, In the this this future, is is the the wefirst first will time time undertake that that the the SL moreSLMM laser laser research at at 1123 1123 on nm thenm TMwas was cavityreported reported SLM with with 1123 a a TM nmTM cavity.lasercavity. to In In improve the the future, future, the outputwe we will will performance, undertake undertake mo more suchre research asresearch optimizing on on the the the TM TM transmissions cavity cavity SLM SLM of1123 1123 the nm OC,nm laseroptimizinglaser to to improve improve the selection the the output output of performance, the performance, gain medium such such (Nd-doped as as optimizing optimizing concentration, the the transmissions transmissions length, of category),of the the OC, OC, optimizingoptimizingoptimizing the thethe selection pumpselection source of of the the (spectral gain gain medium medium width, (Nd-doped spot(Nd-doped size on concentration, concentration, the gain medium) length, length, and category), category), inserting optimizingetalonsoptimizing or gratingsthe the pump pump in source thesource cavity (spectral (spectral to further width, width, compress spot spot size size the on on line the the width.gain gain medium) medium) and and inserting inserting etalonsetalons or or gratings gratings in in the the cavity cavity to to further further compress compress the the line line width. width. Author Contributions: Conceptualization, Y.L.; methodology, Y.L.; validation, Y.L. and C.G.; formal analysis,Author Contributions: S.Z.; investigation, Conceptualization, Y.L.; resources, Z.L.; Y.L.; data methodology, curation, Y.X.; Y.L.; writing—original validation, Y.L. draft and prepara- C.G.; tion,formal Y.L.; analysis, writing—review S.Z.; investigation, and editing, Y.L.; Z.L. resources, and S.M.; Z.L.; funding data cura acquisition,tion, Y.X.; Z.L. writing—original and Z.C. All authors draft have read and agreed to the published version of the manuscript.

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Funding: This research was funded by the Natural Science Foundation of China, grant number 62075117 and 62075116, Joint Foundation of the Ministry of Education, grant number 6141A02022430, Natural Science Foundation of Shandong Province, grant number ZR2019MF039. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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