Passively Q-Switched KTA Cascaded Raman Laser with 234 and 671 Cm−1 Shifts

applied sciences

Article Passively Q-Switched KTA Cascaded Raman Laser with 234 and 671 cm−1 Shifts

Zhi Xie 1,2, Senhao Lou 2, Yanmin Duan 2, Zhihong Li 2, Limin Chen 1, Hongyan Wang 3, Yaoju Zhang 2 and Haiyong Zhu 2,*

1 College of Mechanical and Electronic Engineering, Fujian Agriculture and Forestry University, Fuzhou 350002, China; [email protected] (Z.X.); [email protected] (L.C.) 2 Wenzhou Key Laboratory of Micro-Nano Optoelectronic Devices, Wenzhou University, Wenzhou 325035, China; [email protected] (S.L.); [email protected] (Y.D.); [email protected] (Z.L.); [email protected] (Y.Z.) 3 Crystech Inc., Qingdao 266100, China; [email protected] * Correspondence: [email protected]

Abstract: A compact KTA cascaded Raman system driven by a passively Q-switched Nd:YAG/Cr4+:YAG laser at 1064 nm was demonstrated for the first time. The output spectra with different cavity lengths were measured. Two strong lines with similar intensity were achieved with a 9 cm length cavity. One is the first-Stokes at 1146.8 nm with a Raman shift of 671 cm−1, and the other is the Stokes at 1178.2 nm with mixed Raman shifts of 234 cm−1 and 671 cm−1. At the shorter cavity length of 5 cm, the output Stokes lines with high intensity were still at 1146.8 nm and 1178.2 nm, but the intensity of 1178.2 nm was higher than that of 1146.8 nm. The maximum average output power of 540 mW was obtained at the incident pump power of 10.5 W with the pulse repetition frequency of 14.5 kHz and the pulse width around 1.1 ns. This compact passively Q-switched KTA cascaded Raman laser can yield multi-Stokes waves, which enrich laser output spectra and hold potential applications for Citation: Xie, Z.; Lou, S.; Duan, Y.; Li, Z.; Chen, L.; Wang, H.; Zhang, Y.; remote sensing and terahertz generation. Zhu, H. Passively Q-Switched KTA Cascaded Raman Laser with 234 and Keywords: raman laser; KTA crystal; passively Q-switch 671 cm−1 Shifts. Appl. Sci. 2021, 11, 6895. https://doi.org/10.3390/ app11156895 1. Introduction Academic Editor: Yung-Fu Chen Owing to its high optical damage threshold, broad transparency range, and high nonlinear coefficient together with the low ionic conductivity, potassium titanyle arsenate Received: 4 July 2021 (KTA) is considered to be a promising crystal for nonlinear optics applications. For the Accepted: 20 July 2021 past few decades, KTA has been extensively used to yield efficient eye-safe laser and mid- Published: 27 July 2021 infrared laser via optical parametric oscillation [1–4]. Moreover, KTA has also attracted increasing attention due to its potential application for efficient stimulated Raman laser Publisher’s Note: MDPI stays neutral operation [5–7]. As an effective way to generate coherent light with new wavelengths, with regard to jurisdictional claims in stimulated Raman scattering (SRS) has merits for preparing simple, compact, and robust published maps and institutional affil- laser sources [8–10]. Diverse crystals have been adopted for efficient Raman lasers, such as iations. vanadates [11–13], tungstates [14–17], and molybdates [18]. Different from these commonly used Raman crystals, KTA and its isomorphs possess large Raman gain coefficient and small Raman shifts [19–22], such that it can generate a variety of high order Stokes lasers by cascade SRS. Copyright: © 2021 by the authors. In 1991, Watson first investigated the KTA Raman spectrum and identified a strong Licensee MDPI, Basel, Switzerland. Raman shift of 234 cm−1 [5]. Based on the intracavity pumped KTA using a Nd:YAG/KTA This article is an open access article Raman configuration, Liu et al. studied the first and second-Stokes lasers at 1091 nm and distributed under the terms and 1120 nm, respectively [6,21]. Lan et al. also surveyed the Raman laser of 1091 nm using conditions of the Creative Commons a passively Q-switched Nd:YAG/KTA configuration, and a 250 mW power was realized Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ with the conversion efficiency of 3.3% [22]. Based on the Nd:YAP/KTA configuration, 4.0/). Huang et al. reported a simultaneous dual-wavelength eye-safe Raman laser [23]. Liu et al.

Appl. Sci. 2021, 11, 6895. https://doi.org/10.3390/app11156895 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 9

Appl. Sci. 2021, 11, 6895 2 of 8 Huang et al. reported a simultaneous dual-wavelength eye-safe Raman laser [23]. Liu et al. reported the second harmonic generation (SHG) laser converted from the first-Stokes in KTA Raman with the shift of 671 cm−1 [24]. Our group also studied the multi-Stokes wavesreported from the cascade second Raman harmonic conversion generation based (SHG) on lasera KTA converted crystal [25]. from From the first-Stokesthe main Ra- in −1 manKTA shifts Raman of with 671 thecm− shift1 and of 234 671 cm cm−1 in[24 the]. OurKTA group crystal, also the studied gains the of multi-Stokeshigh order Stokes waves wavesfrom cascade can enrich Raman laser conversion spectral basedlines which on a KTA cannot crystal be [25directly]. From produced the main Ramanfrom the shifts laser of −1 −1 crystal671 cm dopedand with 234 cm rare earthin the ions KTA [26]. crystal, In addition, the gains via of highthe processes order Stokes of the waves SHG can and enrich sum frequencylaser spectral generation lines which (SFG), cannot the Stokes be directly waves produced can produce from thevisible laser light crystal which doped has withim- portantrare earth applications ions [26]. In in addition, remote sensing, via the processes visual displays, of the SHG and and medical sum frequency treatments generation [27,28]. The(SFG), simultaneous the Stokes wavesmulti-order can produce Stokes waves visible ou lighttput which has also has been important reported applications to be a pro- in spectiveremote sensing,pumping visual source displays, for the andterahertz medical (THz) treatments wave generation [27,28]. The employing simultaneous the nonlin- multi- earorder optics Stokes method waves [29]. output has also been reported to be a prospective pumping source for the terahertz (THz) wave generation employing the nonlinear optics method [29]. A solid-state laser based on Cr4+:YAG passively Q-switching has the advantages of A solid-state laser based on Cr4+:YAG passively Q-switching has the advantages of compactness and low cost for high pulse repetition frequency pulse laser operation [30– compactness and low cost for high pulse repetition frequency pulse laser operation [30–32]. 32]. Therefore, a passively Q-switched cascaded Raman laser could be a compact and ef- Therefore, a passively Q-switched cascaded Raman laser could be a compact and efficient ficient laser system for Stokes laser output. In this work, for the first time, multi-Stokes laser system for Stokes laser output. In this work, for the first time, multi-Stokes waves from waves from passively Q-switched KTA cascaded Raman lasers were studied. A passively Q-switched KTA cascaded Raman lasers were studied. A Nd:YAG/Cr4+:YAG Nd:YAG/Cr4+:YAG composite crystal was used as the saturable absorber. Under a pump- composite crystal was used as the saturable absorber. Under a pumping power of 10.5 W ing power of 10.5 W and pulse repetition frequency of 14.5 kHz (for the setup with a 5 cm and pulse repetition frequency of 14.5 kHz (for the setup with a 5 cm length cavity) the length cavity) the largest average output power of 540 mW was realized with a pulse largest average output power of 540 mW was realized with a pulse width of about 1.1 ns. width of about 1.1 ns. The measured spectra show various Stokes lines in the range of The measured spectra show various Stokes lines in the range of 1050~1200 nm, which 1050~1200 nm, which were converted via the Raman shifts of 234 cm−1 and 671 cm−1, and were converted via the Raman shifts of 234 cm−1 and 671 cm−1, and two strong lines were two strong lines were located at 1146.8 nm and 1178.2 nm. located at 1146.8 nm and 1178.2 nm.

2.2. Experimental Experimental Setup Setup KTAKTA is an anisotropicanisotropic opticaloptical crystal crystal of of the th orthotropice orthotropic crystal crystal system. system. According According to theto thepolarization-dependent polarization-dependent spontaneous spontaneous Raman Raman scattering scattering spectra spectra of x-axisof x-axis cut cut KTA KTA crystals, crystals,strong strong Raman Raman scattering scattering with with two two main main Raman Raman shifts shifts at 234at 234 cm cm−1−and1 and 671 671 cm cm−−11 can be be realizedrealized from from the A1 (ZZ) geometry.geometry. Therefore,Therefore, an an x-axis x-axis cut cut KTA KTA crystal crystal of of4 ×4 ×4 4× × 20 mm 3 dimensiondimension was employed asas thethe Raman Raman medium medium to to satisfy satisfy the the X(ZZ)X X(ZZ)X Raman Raman conﬁguration, configuration,according according to Porto to Porto notations. notations. The The KTA KTA crystal crystal was was placed placed in a in compact a compact plane-concave plane-con- cavecavity cavity and drivenand driven by the by passively the passively Q-switched Q-switched Nd:YAG Nd:YAG laser at 1064laser nm. at 1064 Figure nm.1 shows Figure the 1 showsdiagrammatic the diagrammatic sketch of the sketch passively of the Q-switched passively Q-switched KTA cascaded KTA Raman cascaded laser experimentalRaman laser experimentalsetup. The Nd:YAG setup. crystalThe Nd:YAG composited crystal with composited Cr4+:YAG crystalwith Cr in4+:YAG order tocrystal make in the order system to makemore the compact system and more easy compact to dissipate and heat, easy andto dissipate these two heat, crystals and werethese used two tocrystals serve aswere the usedlaser to gain serve medium as the andlaser the gain saturable medium absorber and the forsaturable passively absorber Q-switching, for passively respectively. Q-switch- The ing,Nd:YAG respectively. crystal with The theNd:YAG size of crystal 3 × 3 × with7 mm the3 sizeis 1.0 of at.% 3 × 3 doped, × 7 mm and3 is the1.0 Crat.%4+:YAG doped, crystal and thewith Cr size4+:YAG of 3 crystal× 3 × 3with mm 3sizehas of an 3 initial× 3 × 3 transmission mm3 has an ofinitial about transmission 85%. While of the about optics 85%. was Whileclosed the set optics with each was other,closed the set totalwith cavity each othe lengthr, the was total about cavity 50 mm.length was about 50 mm.

FigureFigure 1. 1. ExperimentalExperimental setup setup of of the the passively passively Q-switched Q-switched KTA KTA cascaded cascaded Raman Raman laser. laser.

The pumping source is a fiber-coupled laser diode array operating at 808 nm, which has a fiber core diameter of 200 µm and a numerical aperture of 0.22. The pump beam was re-imaged by two 808 nm anti-reflection (AR) coated achromatic lenses with focal lengths Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 9

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The pumping source is a fiber-coupled laser diode array operating at 808 nm, which has a fiber core diameter of 200 μm and a numerical aperture of 0.22. The pump beam was re-imagedof 50 and by 80two mm, 808 nm obtaining anti-reflection a beam (AR) waist coated of about achromatic 320 µm lenses in the with Nd:YAG focal lengths crystal. Both the of 50Nd:YAG/Cr and 80 mm,4+ obtaining:YAG composite a beam waist crystal of about and the320 KTAμm in crystal the Nd:YAG were crystal. wrapped Both with the indium foil Nd:YAG/Crand mounted4+:YAG in composite a thermoelectric-cooled crystal and the KTA copper crystal block were withwrapped the temperaturewith indium foil kept at about and20 mounted◦C throughout in a thermoelectric-cooled the experiments. copper Both block end-faces with the of thetemperature composite kept crystal at about and the KTA 20 crystal°C throughout were AR the coated experime fornts. the Both fundamental end-faces andof the Stokes composite wavelengths crystal and from the 1000KTA to 1200 nm, crystal were AR coated for the fundamental and Stokes wavelengths from 1000 to 1200 and the pump light incident face of the Nd:YAG crystal was AR coated at 808 nm. nm, and the pump light incident face of the Nd:YAG crystal was AR coated at 808 nm. 3. Results and Discussion 3. Results and Discussion TableTable 1 lists1 lists Stokes Stokes wavelengths wavelengths with contri withbuted contributed Raman shifts. Raman It shifts.can be seen It can that be seen that therethere are aremany many Stokes Stokes wavelengths wavelengths ranging ranging from 1050 from to 1050 1200 to nm, 1200 which nm, were which yielded were yielded via −1 −1 viatwo two RamanRaman shifts of of 234 234 cm cm−1 andand 671671 cm− cm1. The .fundamental The fundamental and Raman and Ramanlaser oscilla- laser oscillation tionshared shared the the identicalidentical plane-concave plane-concave linear linear cavity cavity constituted constituted by the by input the inputmirror mirror (IM) (IM) and andthe the output output coupler (OC) (OC) mirror. mirror. The The IM IMwas was high-transmission high-transmission (HT, T (HT,> 98%) T coated > 98%) coated at at 808808 nm nm and and high-reflection high-reﬂection (HR, (HR, R > 99.9%) R > 99.9%) coated coatedfrom 1000 from to 1200 1000 nm. to 1200The OC nm. mirror The OC mirror withwith a radius a radius curvature curvature of 100 of mm 100 was mm HR was coated HR coatedfor the range for the of range1050~1130 of 1050~1130 nm. The nm. The transmittancetransmittance curve curve of the of output the output coupler coupler is displayed is displayed in Figure in 2. FigureWith the2. wavelength With the wavelength largerlarger than than 1130 1130 nm, nm,the transmittance the transmittance increased, increased, reaching reaching 1.5% at 1147 1.5% nm at and 1147 67% nm atand 67% at 11821182 nm. nm. Table 1. Stokes wavelengths with contributed Raman shifts. Table 1. Stokes wavelengths with contributed Raman shifts. Wavelength Contributed Raman Shifts Wavelength Contributed Raman Shifts (nm) ωR1 = 234 cm−1, ωR2 = 671 cm−1 −1 −1 (nm) ωR1 = 234 cm , ωR2 = 671 cm 1092.0 ωR1 1120.7 1092.0 2ωR1 ωR1 ω 1146.8 1120.7ωR2 2 R1 1146.8 ω 1150.4 3ωR1 R2 1150.4 3ωR1 1178.2 ωR2 + ωR1 1178.2 ωR2 + ωR1 1182.4 4ωR1 1182.4 4ωR1

FigureFigure 2. Transmittance 2. Transmittance curve curveof the ofOC the mirror OC for mirror different fordifferent wavelengths. wavelengths.

TheThe KTA KTA Raman Raman laser laseroutput output characteristics characteristics were optimized were optimized by adjusting by the adjusting pump the pump powerpower and and cavity cavity length. length. The maximum The maximum power of power the output of the was output achieved was under achieved a pump under a pump powerpower of 10.5 of 10.5W. The W. Thedata dataof the of output the output power powerwere detected were detectedusing the usingthermal the sensor thermal sensor power meter (PM310D, Thorlabs INC). Figure3 exhibits the average output power for two cavity lengths of 5 cm and 9 cm, with respect to the incident pump power. It is shown that the Raman threshold for a 5 cm length cavity was around 3.4 W, and for a 9 cm length cavity it was around 4.9 W. For the two cavity length setups, the average output powers both raised with the increase of incident pump power, provided with the slope efﬁciency of 7.6% for the 5 cm length cavity and 8.0% for the 9 cm length cavity. A lower Raman threshold for 5 cm length cavity beneﬁted from its low cavity loss with the short and compact laser Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 9

power meter (PM310D, Thorlabs INC). Figure 3 exhibits the average output power for two cavity lengths of 5 cm and 9 cm, with respect to the incident pump power. It is shown that the Raman threshold for a 5 cm length cavity was around 3.4 W, and for a 9 cm length Appl. Sci. 2021, 11, 6895 cavity it was around 4.9 W. For the two cavity length setups, the average output powers 4 of 8 both raised with the increase of incident pump power, provided with the slope efficiency of 7.6% for the 5 cm length cavity and 8.0 % for the 9 cm length cavity. A lower Raman threshold for 5 cm length cavity benefited from its low cavity loss with the short and com- pactcavity, laser andcavity, the and 5 cm the length5 cm length cavity cavity is more is more suitable suitable for for high high output output powerpower than than the 9 cm thelength 9 cm length cavity cavity with anwith increase an increase of pump of pump power. power. Under Under an an incident incident pump pump powerpower of 10.5 W, ofthe 10.5 obtained W, the obtained maximum maximum output output powers powers were were 530 mW 530 formW the for setupthe setup with with 5 cm 5 cm length cavity, lengthand 320cavity, mW and for 320 the mW setup for the with setup 9 cm with length 9 cm cavity. length cavity. The power The power instability instability at the at maximum theoutput maximum for these output two for lengththese two cavities length were cavities better were than better 5%. than 5%.

Figure 3. Average output power versus incident pump power of the Raman lasers with the cavity Figure 3. Average output power versus incident pump power of the Raman lasers with the cavity lengthslengths of 5 of cm 5 cmand and 9 cm. 9 cm.

FigureFigure 4 displays4 displays the laser the laser spectra spectra with the with 5 cm the and 5 cm 9 cm and cavity 9 cm lengths, cavity respectively. lengths, respectively. TheThe data data were were measured measured at the at pump the pump power power of 10.5 ofW 10.5using W a usinggrating a monochromator grating monochromator (Omni-(Omni-λ500λ500 model, model, ZOLIX, ZOLIX, Beijing, Beijing, China). China). According According to the wavelengths to the wavelengths and Raman and Raman shiftsshifts shown shown in Table in Table 1, the1, thespectra spectra lines linescan be can identified. be identified. For the Forsetup the with setup the with9 cm the 9 cm lengthlength cavity, cavity, its spectrum its spectrum contains contains two strong two strong lines with lines similar with intensity, similar intensity, which are whichthe are the first-Stokesfirst-Stokes at 1146.8 at 1146.8 nm nmwith with the Raman the Raman shift of shift 671 ofcm 671−1, and cm the−1, Stokes and the at Stokes1178.2 nm at 1178.2 nm withwith mixed mixed Raman Raman shifts shifts of 234 of cm 234−1 and cm −6711 and cm−1 671. Four cm low−1 .intensity Four low lines intensity were also lines de- were also tected.detected. They Theyare the are fundamental the fundamental wave at wave1064 nm, at 1064the first, nm, second the first, and second third-Stokes and third-Stokesat −1 1092.0,at 1092.0, 1120.7, 1120.7, and 1150.4 and nm, 1150.4 respectively, nm, respectively, with a Raman with shift a Raman of 234 cm shift. For of the 234 setup cm− 1. For the with 5 cm length cavity, due to the lower loss caused by the shorter cavity length, the low setup with 5 cm length cavity, due to the lower loss caused by the shorter cavity length, order Stokes were converted to higher order Stokes. The first-Stokes at 1092.0 nm and the second-Stokesthe low order at Stokes1120.7 nm were became converted negligible to higher, while orderthe intensity Stokes. of The 1178.2 first-Stokes nm was sig- at 1092.0 nm nificantlyand the enhanced, second-Stokes and more at 1120.7obvious nm third-Stokes became (1150.4 negligible, nm) and while fourth-Stokes the intensity (1182.4 of 1178.2 nm nm)was waves significantly with Raman enhanced, shift of and234 cm more−1 were obvious also observed. third-Stokes However, (1150.4 the nm) output and was fourth-Stokes (1182.4 nm) waves with Raman shift of 234 cm−1 were also observed. However, the output was still dominated by the Stokes waves at 1178.2 nm and 1146.8 nm. Therefore, different cavity lengths also resulted in the difference of output spectra, which might be mainly attributed to the intensity distribution changing in the cavity. The pulse width and pulse repetition frequency with different incident pump powers were obtained using the free-space InGaAs photodetector and exhibited on the 500 MHz oscilloscope (DPO3052B model). In view of the fact that the measured multi-Stokes wavelengths were very close to each other, and they cannot be segregated in our investigation, only the whole temporal pulse profiles of the Stokes waves were provided. As shown in Figure5, with the incident pump power changed from 3.9 W to 10 W, the pulse repetition frequency went up from 2.9 kHz to 14.5 kHz, and the pulse energy rose from 21 µJ to 36 µJ except for a small decline when the incident pump changed from 4.5 W to 5.2 W. The pulse Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 9

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still dominated by the Stokes waves at 1178.2 nm and 1146.8 nm. Therefore, different cavity lengths also resulted in the difference of output spectra, which might be mainly at- tributedrepetition to the frequency intensity wasdistribution not as stable changing when in the the cavity. instability reached about 25%, which might be caused by the competition of multi-Stokes generation.

Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 9 FigureFigure 4. 4.RamanRaman laser laser spectra spectra measured measured under underthe incide thent incident pump power pump of power10.5 W ofwith 10.5 the W cavity with the cavity lengthslengths of of5 cm 5 cm and and 9 cm. 9 cm.

The pulse width and pulse repetition frequency with different incident pump powers were obtained using the free-space InGaAs photodetector and exhibited on the 500 MHz oscilloscope (DPO3052B model). In view of the fact that the measured multi-Stokes wavelengths were very close to each other, and they cannot be segregated in our investigation, only the whole temporal pulse profiles of the Stokes waves were provided. As shown in Figure 5, with the incident pump power changed from 3.9 W to 10 W, the pulse repetition frequency went up from 2.9 kHz to 14.5 kHz, and the pulse energy rose from 21 μJ to 36 μJ except for a small decline when the incident pump changed from 4.5 W to 5.2 W. The pulse repetition frequency was not as stable when the instability reached about 25%, which might be caused by the competition of multi-Stokes generation.

FigureFigure 5. 5. PulsePulse repetition repetition frequency frequency (PRF) (PRF) and puls ande pulseenergy energy versus versusincident incident pump power. pump power.

TheThe measured measured pulse pulse trains trains and andtemporal temporal pulse pulseprofiles proﬁles at the incident at the incident pump power pump power ofof 10.5 10.5 W W are are displayed displayed in Figure in Figure 6. The6. Thepulse pulse width width was around was around 1.1 ns, 1.1and ns,the andpulse the pulse repetition frequency was 14.5 kHz. A smaller value of pulse width than 1.1 ns might be repetition frequency was 14.5 kHz. A smaller value of pulse width than 1.1 ns might be more accurate, since the critical time resolution of the used 500 MHz oscilloscope was more accurate, since the critical time resolution of the used 500 MHz oscilloscope was limited. We also measured the pulse width of weak leaked fundamental laser at 1064 nm, andlimited. the obtained We also value measured was around the pulse 11 ns. width Therefore, of weak significant leaked pulse fundamental shortening laserwas de- at 1064 nm, tectedand the for obtainedthis KTA Raman value waslaser aroundwhich is 11similar ns. to Therefore, the fact that signiﬁcant achieved in pulse SrMoO shortening4 Ra- was man laser [33].

Figure 6. Temporal pulse profiles and pulse trains under the incident pump power of 10.5 W. Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 9

Figure 5. Pulse repetition frequency (PRF) and pulse energy versus incident pump power.

The measured pulse trains and temporal pulse profiles at the incident pump power of 10.5 W are displayed in Figure 6. The pulse width was around 1.1 ns, and the pulse Appl. Sci. 2021, 11, 6895 repetition frequency was 14.5 kHz. A smaller value of pulse width than 1.1 ns might be 6 of 8 more accurate, since the critical time resolution of the used 500 MHz oscilloscope was limited. We also measured the pulse width of weak leaked fundamental laser at 1064 nm, and the obtained value was around 11 ns. Therefore, significant pulse shortening was de- tecteddetected for this for KTA this KTARaman Raman laser which laser is which similar is to similar the fact to that the achieved fact that in achievedSrMoO4 Ra- in SrMoO4 manRaman laser laser [33]. [33].

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FigureFigure 6. 6.TemporalTemporal pulse pulse profiles profiles and andpulse pulse trains trains under underthe incident the incident pump power pump of power 10.5 W. of 10.5 W. The spectra of the weak visible light accompanying with the Stokes wave output have beenThe measured spectra of usingthe weak a fibervisible optic light accomp spectrometeranying with (AvaSpec-3648 the Stokes wave model). output The have measured beenspectra measured are shown using ina fiber Figure optic7. Itspectrom can beeter seen (AvaSpec-3648 that the main model). lines are The located measured at 573.4 nm spectraand 574.3 are shown nm, and in Figure the former 7. It can can be be seen assigned that the to main the SHG lines ofare 1146.8 located nm, at 573.4 while nm the latter is and 574.3 nm, and the former can be assigned to the SHG of 1146.8 nm, while the latter is from the SFG of 1146.8 nm and 1150.4 nm with the phase matching angle θ of 82~83◦ in the from the SFG of 1146.8 nm and 1150.4 nm with the phase matching angle θ of 82~83° in thex-z x-z plane plane of of KTA KTA crystal.crystal. These These weak weak visi visibleble lights lights possessed possessed an output an output power powerof about of about a adozen dozen milliwatts, which which should should be ascribed be ascribed to the to fact the that fact the that phase the matching phase matching directions directions ofof SHG SHG and and SFG SFG for forthe theStokes Stokes waves waves in the in cavi thety cavitywere close were to closethe x-axis. to the To x-axis. our surprise, To our surprise, thethe SFG SFG light light (567 (567 nm) nm) of 1120.7 of 1120.7 nm and nm 1146.8 and 1146.8 nm was nm not was observed. not observed. As known As from known the from the calculation,calculation, the the SFG SFG matching matching angle angle between between these two these wavelengths two wavelengths is closer isto closerthe x-axis to the x-axis ofof the the KTA KTA crystal crystal compared compared with with those those for 573.4 for 573.4nm and nm 574.3 and nm 574.3 light nm generation. light generation. The The absenceabsence of of567 567 nm nm SFG SFG light light could could be attributed be attributed to the light to the conversion light conversion from 1120.7 from nm 1120.7 to nm to 1150.41150.4 nm nm due due toto high high reflection reflection of the of the reso resonantnant cavity cavity for 1120.7 for 1120.7 nm. Therefore, nm. Therefore, the 567 the 567 nm nmSFG SFG light light was was negligible negligible inin contrast to to the the lines lines of of573.4 573.4 and and 574.3 574.3 nm. nm.

FigureFigure 7. 7.SpectraSpectra of weak of weak visible visible light lightirradiated irradiated from the from KTA the crystal. KTA crystal.

4. Conclusions In summary, a compact passively Q-switched KTA cascaded Raman laser was demonstrated for the first time. A composite crystal of Nd:YAG/Cr4+:YAG was used for passively Q-switched fundamental laser generation at 1064 nm. The obtained multi- Stokes laser wavelengths range from 1090 to 1200 nm. The output spectra with two different cavity lengths were measured. Two strong lines with similar intensity were achieved with a 9 cm length cavity. One is the first-Stokes at 1146.8 nm with Raman shift of 671 cm−1, and the other is the Stokes at 1178.2 nm with mixed Raman shifts of 234 cm−1 and 671 cm−1. For the setup with a shorter length cavity of 5 cm, some conversions from low order Stokes to higher order Stokes occurred due to the lower loss caused by the shorter cavity length, and the output Stokes waves with high intensity were still at 1146.8 nm and 1178.2 nm. However, the intensity of 1178.2 nm was obviously higher than that of 1146.8 nm. Under an incident pump power of 10.5 W, the measured maximum average output power was 540 mW. The measured pulse width was around 1.1 ns, and the pulse repetition frequency was 14.5 kHz. This compact passively Q-switched KTA cascaded Raman laser can yield multi-Stokes waves, which enrich laser output spectra lines and hold potential applications for remote sensing and terahertz generation. Appl. Sci. 2021, 11, 6895 7 of 8

4. Conclusions In summary, a compact passively Q-switched KTA cascaded Raman laser was demonstrated for the ﬁrst time. A composite crystal of Nd:YAG/Cr4+:YAG was used for passively Q-switched fundamental laser generation at 1064 nm. The obtained multi-Stokes laser wavelengths range from 1090 to 1200 nm. The output spectra with two different cavity lengths were measured. Two strong lines with similar intensity were achieved with a 9 cm length cavity. One is the ﬁrst-Stokes at 1146.8 nm with Raman shift of 671 cm−1, and the other is the Stokes at 1178.2 nm with mixed Raman shifts of 234 cm−1 and 671 cm−1. For the setup with a shorter length cavity of 5 cm, some conversions from low order Stokes to higher order Stokes occurred due to the lower loss caused by the shorter cavity length, and the output Stokes waves with high intensity were still at 1146.8 nm and 1178.2 nm. However, the intensity of 1178.2 nm was obviously higher than that of 1146.8 nm. Under an incident pump power of 10.5 W, the measured maximum average output power was 540 mW. The measured pulse width was around 1.1 ns, and the pulse repetition frequency was 14.5 kHz. This compact passively Q-switched KTA cascaded Raman laser can yield multi-Stokes waves, which enrich laser output spectra lines and hold potential applications for remote sensing and terahertz generation.

Author Contributions: Conceptualization, Z.X., Y.D., and H.Z.; validation, Z.X. and S.L.; formal analysis, Z.X., S.L., Z.L., Y.Z., and H.Z.; resources, H.W.; writing—original draft preparation, Z.X. and S.L.; writing—review and editing, Y.D., Z.L., L.C., Y.Z. and H.Z.; supervision, Y.D. and H.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Zhejiang Provincial Natural Science Foundation of China under (Grant No. LY19F050012), the National Natural Science Foundation of China under (Grant No. 62075167 and No. 61905180), the public welfare projects of Wenzhou city under (Grant No. G2020013 and No. S20180015), and the Natural Science Foundation of Fujian Province (Grant No. 2019J01403). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: All of the data reported in the paper are presented in the main text. Any other data will be provided on request. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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