Short-Pulse-Width Repetitively Q-Switched ~2.7-Μm Er:Y2O3 Ceramic Laser

applied sciences

Article Short-Pulse-Width Repetitively Q-Switched ~2.7-µm Er:Y2O3 Ceramic Laser

Xiaojing Ren 1, Yong Wang 2, Jian Zhang 2, Dingyuan Tang 2 and Deyuan Shen 1,* 1 Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China; [email protected] 2 Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China; [email protected] (Y.W.); [email protected] (J.Z.); [email protected] (D.T.) * Correspondence: [email protected]; Tel.: +86-21-6564-2159

Received: 16 October 2017; Accepted: 17 November 2017; Published: 22 November 2017

Abstract: A short-pulse-width repetitively Q-switched 2.7-µm Er:Y2O3 ceramic laser is demonstrated using a specially designed mechanical switch, a metal plate carved with slits of both slit-width and duty-cycle optimized. With a 20% transmission output coupler, stable pulse trains with durations (full-width at half-maximum, FWHM) of 27–38 ns were generated with a repetition rate within the range of 0.26–4 kHz. The peak power at a 0.26 kHz repetition rate was ~3 kW.

Keywords: laser materials; mid-infrared lasers; rare-earth solid-state lasers

1. Introduction Laser radiation at ~2.7-µm is important for practical applications and scientific research. Being regions of water absorption and molecular fingerprints, laser sources at ~2.7-µm are useful for biomedical therapy [1,2] and atmospheric sensing [3]. In addition, these lasers are utilized for generating 3–5-µm laser emission [4,5], which corresponds to an atmosphere transparent window. Lasers at ~2.7-µm also facilitate studies of laser materials that is suitable to generate deep-infrared lasing [6,7]. Both the applications and research require versatile ~2.7-µm laser sources with short pulse duration and high repetition rate. A simple approach to generate ~2.7-µm pulses is Q-switching ion-based (as Er3+, Ho3+, Dy3+ and 2+ 4 4 Cr ) lasers. Er-based lasers operating on the transition between I11/2 and I13/2 energy levels are most often utilized because of the mature pump sources of flashlamps and ~976-nm laser diodes (LDs). With 50 atom % Er:YAG (Y3Al5O12), 30 atom % Er:YSGG (Y3Sc2Ga3O12), and 15 atom % Er:YLF (LiYF4) laser materials, ~2.7-µm pulses at durations of tens of nanoseconds have been obtained. However, the repetition rates of these lasers are generally limited to several Hertz due to the severely thermal effects generated during laser operations [8–11]. Laser pulses at ~2.7-µm with repetition rates at the kilohertz scale have been generated from Er:ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) fiber lasers, which are more thermally advanced [12,13]. However, the pulse durations are hundreds of nanoseconds because fibers have limited energy storage capability. So far, short pulses at ~2.7-µm with high repetition rates are rare. The use of Er-based sesquioxides is promising in terms of obtaining ~2.7-µm laser pulses with short pulse durations and high repetition rates. As ~2.7-µm laser oscillation can be realized from Er-based sesquioxides with low doping concentrations (lower than ~7 atom %) [14,15], the thermal effects generated during laser operation are greatly alleviated. In addition, sesquioxides have high thermal conductivity, which decreases slightly with increasing doping concentration [16]. Furthermore, Er-based sesquioxides have long ~2.7-µm fluorescence lifetimes (e.g., an order of magnitude longer than that of Er:YAG), making them beneficial for energy storage [17]. Unfortunately, sesquioxides have extremely high melting points (>2400 ◦C), imposing serious challenges for traditional single-crystal-growth

Appl. Sci. 2017, 7, 1201; doi:10.3390/app7111201 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 1201 2 of 7

Appl. Sci. 2017, 7, 1201 2 of 7 approach. In this aspect, polycrystalline ceramics are superior to single crystals since they can be sintered at muchUnfortunately, lower temperatures. sesquioxides Moreover, have extremely transparent high ceramics melting havepoints advantages (>2400 °C), over imposing crystals inserious terms of theirchallenges rapid fabrication for traditional in large single-crystal-growth scale and composite approach. structures, In this flexible aspect, doping polycrystalline concentrations, ceramics and are good thermo-mechanicalsuperior to single properties crystals [si18nce]. Recently, they can sesquioxide be sintered ceramics at much have lower been temperatures. successfully fabricatedMoreover, and mainlytransparent explored ceramics to realize have continuous-wave advantages over (CW) crystals laser in operationterms of their at ~2.7- rapidµm fabrication. A passively in large Q-switched scale and composite structures, flexible doping concentrations, and good thermo-mechanical properties ~2.7-µm Er:Y2O3 ceramic laser with a pulse duration of 4.47 µs and a pulse repetition rate of 12.6 kHz [18]. Recently, sesquioxide ceramics have been successfully fabricated and mainly explored to realize was realized, aiming to demonstrate the broadband availability of black-phosphorus [19]. For actively continuous-wave (CW) laser operation at ~2.7-μm. A passively Q-switched ~2.7-μm Er:Y2O3 ceramic Q-switched operation, an acousto-optically Q-switched Er:Y O ceramic laser was demonstrated, laser with a pulse duration of 4.47 μs and a pulse repetition rate2 of3 12.6 kHz was realized, aiming to generating pulses with durations of 41–190 ns in the range of 0.3–10 kHz [20]. Due to the scarcity of demonstrate the broadband availability of black-phosphorus [19]. For actively Q-switched operation, ~2.7-µm acousto-optic and electric-optic Q-switches, mechanical Q-switching is commonly used in a an acousto-optically Q-switched Er:Y2O3 ceramic laser was demonstrated, generating pulses with µ wavelengthdurations rangeof 41–190 of ~2.7-ns in them [range21,22], of which 0.3–10 does kHz not[20]. require Due to highthe scarcity voltages of ~2.7- or driveμm acousto-optic power and has avoidableand electric-optic insert losses. Q-switches, mechanical Q-switching is commonly used in a wavelength range of ~2.7-Hereμm we[21,22], report which a short-pulse-width does not require high repetitively voltages or Q-switched drive power Er:Y and2 Ohas3 ceramicavoidable laser insert at losses. ~2.7-µ m using aHere specially we report designed a short-pulse-width mechanical switch, repetitively a metal Q-switched plate carved Er:Y2O with3 ceramic slits laser of both at ~2.7- slit-widthμm andusing duty-cycle a specially optimized. designed The mechanical laser performances switch, a metal with plate output carved couplers with slits (OCs) of both of 5%,slit-width 8% and and 20% transmissionsduty-cycle optimized. are compared The in laser both performances CW and Q-switched with output operation couplers modes. (OCs) In theof CW5%, operation8% and 20% mode, thetransmissions 8% transmission are compared OC yields in an both output CW powerand Q-switched of over 1.8 operation W. In the modes. Q-switched In the CW operation operation mode, themode, 20% transmissionthe 8% transmission OC yields OC yields pulse an trains output with po durationswer of over (FWHM) 1.8 W. In ofthe 27–38 Q-switched ns and operation energies of 80.8–27.5mode, theµJ with20% transmission repetition rates OC yields in the pulse range trains of 0.26–4 with durations kHz. The (FWHM) corresponding of 27–38 peakns and power energies at the 0.26of kHz80.8–27.5 repetition μJ with rate repetition is ~3 kW. rates To in thethe bestrange of of our 0.26–4 knowledge, kHz. The corresponding the laser offers peak the power shortest at the pulse durations0.26 kHz among repetition ~2.7- rateµm Q-switchedis ~3 kW. To lasers the best with of pulse our knowledge, repetition frequency the laser offers (PRF) the above shortest several pulse Hertz. durations among ~2.7-μm Q-switched lasers with pulse repetition frequency (PRF) above several Hertz. 2. Experimental Details 2. Experimental Details Figure1 shows the schematic layout of the pulsed Er:Y 2O3 ceramic laser. A pig-tailed ~976-nm Figure 1 shows the schematic layout of the pulsed Er:Y2O3 ceramic laser. A pig-tailed ~976-nm LD with a fiber core diameter of 105-µm and a numerical aperture (NA) of less than 0.15 was used LD with a fiber core diameter of 105-μm and a numerical aperture (NA) of less than 0.15 was used to to pump the Er:Y2O3 ceramic. The pump light was focused into the Er:Y2O3 sample with a ~420-µm pump the Er:Y2O3 ceramic. The pump light was focused into the Er:Y2O3 sample with a ~420-μm spot spot diameter through a 25-mm-focal-length lens F1 and a 100-mm-focal-length lens F2. The confocal diameter through a 25-mm-focal-length lens F1 and a 100-mm-focal-length lens F2. The confocal parameterparameter was was estimated estimated to to be be ~26 ~26 mm.mm. The physical length length of of the the plane–plane plane–plane cavity cavity was was 28 mm. 28 mm. TheThe input input coupler coupler (IC) (IC) was was high-transmission high-transmission coated coated at ~976at ~976 nm nm (T > (T 98%) > 98%) and and high-reflectivity high-reflectivity coated at ~2.7-coatedµm at (R~2.7- >μ 99.8%).m (R > 99.8%). Three flatThree mirrors flat mirrors with transmissionswith transmissions of 5%, of 5%, 8%, 8%, and and 20% 20% at at ~2.7- ~2.7-µmμm and highand transmissions high transmissions at ~976 at ~976 nm werenm were employed employed as as OCs. OCs. The The Er:Y Er:Y22O3 ceramicceramic (developed (developed at Jiangsu at Jiangsu NormalNormal university) university) was was synthesized synthesized by theby solid-statethe solid-state reaction reaction method method and vacuum and vacuum sintering sintering followed byfollowed hot isostatic by hot pressing isostatic [14 pressing]. The ceramic[14]. The sample ceramic had sample a dimension had a dimension of 2 × 3of× 2 12× 3 mm× 12 mm and and an Er-ion an concentrationEr-ion concentration of 7 atom of %7 atom and % was and uncoated. was uncoated. It was It was mounted mounted in ain copper a copper block block that that was was cooledcooled by waterby atwater ~13 at◦C ~13 to allow°C to forallow effective for effective heat removal. heat removal. A dichroic A dichroic mirror (DM)mirror coated (DM) withcoated high with reflectivity high at thereflectivity laser wavelength at the laser and wavelength high transmission and high attr theansmission pump wavelengthat the pump was wavelength placed between was placed the OC andbetween the detector the OC to and filter the out detector the unabsorbed to filter out pump the unabsorbed power. pump power.

FigureFigure 1. Schematic1. Schematic setup setup of of the the Q-switched Q-switched Er:YEr:Y2OO33 ceramicceramic laser. laser. LD: LD: laser laser diode; diode; IC: IC: input input coupler; coupler; OC:OC: output output coupler; coupler; DM: DM: dichroic dichroic mirror. mirror. Appl. Sci. 2017, 7, 1201 3 of 7 Appl. Sci. 2017, 7, 1201 3 of 7

TheThe mechanical mechanical Q-switch Q-switch was was a a1-mm-thick 1-mm-thick rotating rotating metal plate carvedcarved with with sagittal sagittal rectangular rectangular slitsslits (see (see the the inset inset of Figure of Figure 1),1 of), ofwhich which the the rotating rotating speed speed and and slit slit number number were were variable variable from from 1 1to to 134 rounds134 rounds per second per second and 1 and to 130, to 30,respectively. respectively. It was It was placed placed close close to to the ceramicceramic sample,sample, where where the the diameterdiameter of ofthe the laser laser mode mode was was calculated calculated to tobe be ~470- ~470-μm.µm. The The modulation modulation frequency frequency was was adjusted adjusted by changingby changing the slit the number slit number or (and) or (and) the rotating the rotating speed. speed. The Theslit width slit width was was optimized optimized to be to 0.6 be 0.6mm. mm. The dutyThe ratio duty of ratio the Q-switch of the Q-switch was continually was continually tunable tunable by moving by moving the plate the platealong along its radial its radial direction. direction. Using suchUsing a mechanical such a mechanical Q-switch considerably Q-switch considerably increased th increasede time for thethe timeinversion for the population inversion to population accumulate. to accumulate. 3. Results 3. Results A comparison of CW laser performances with 5%, 8% and 20% transmissions OCs of the Er:Y2O3 mediumA was comparison first made. of CW Figure laser performances 2 shows dependences with 5%, 8% of and output 20% transmissions powers on the OCs absorbed of the Er:Y pump2O3 medium was first made. Figure2 shows dependences of output powers on the absorbed pump power. power. The absorbed pump power was calculated by multiplying the incident pump power by the The absorbed pump power was calculated by multiplying the incident pump power by the absorption absorption efficiency, which was measured under non-lasing condition (without existence of the laser efficiency, which was measured under non-lasing condition (without existence of the laser resonator) to resonator) to be ~90%. The threshold pump powers for the 5%, 8% and 20% transmission OCs were be ~90%. The threshold pump powers for the 5%, 8% and 20% transmission OCs were ~1.1 W, ~1.4 W, ~1.1 W, ~1.4 W, and ~3 W, respectively. At pump powers of less than 14 W, the output powers and ~3 W, respectively. At pump powers of less than 14 W, the output powers increased linearly with increasedthe pump linearly power with for all the three pump OCs, power and the for 8% all OCthree yielded OCs, theand highest the 8% slope OC yielded efficiency the of highest 10.2% with slope efficiencyrespect toof absorbed10.2% with pump respect power. to absorbed When the pu pumpmp power. power wasWhen above the 14pump W, the power laser becamewas above slightly 14 W, theless laser efficient became due slightly to the increasedless efficient thermally due to inducedthe increased losses. thermally Nevertheless, induced an output losses. power Nevertheless, of over an 1.8output W was power obtained of over at 191.8 W W of was pump obtained power at with 19 W the of 8% pump transmission power with OC. the According 8% transmission to the laser OC. Accordingproperties to with the different laser properties output couplers, with thedifferent distributed output cavity couplers, losses (excluding the distributed the output cavity coupling losses (excludingloss) were the calculated output tocoupling be ~1.67% loss) cm were−1 using calculated the well-known to be ~1.67% Findlay–Clay cm−1 using method, the comprisingwell-known Findlay–Claythe thermally method, induced comprising losses and the the thermally losses of the induced cavity mirrorslosses and and the ceramic losses sample of the (~1.2%cavity mirrors cm−1 −1 andestimated ceramic thoughsample the (~1.2% in-line cm transmissionestimated spectrum though ofthe a 1.3-mm-longin-line transmission Er:Y2O3 sample).spectrum The of attainablea 1.3-mm- longoutput Er:Y2 powerO3 sample). should The be readilyattainable increased output by power improving should the be cooling readily system increased and reducingby improving passive the coolinglosses system of the ceramicand reducing sample. passive losses of the ceramic sample.

FigureFigure 2. Continuous-wave 2. Continuous-wave (CW) (CW) output output po powerwer vs. vs. absorbed absorbed pump pump power withwith different different output output couplers couplers..

TheThe Q-switched Q-switched operation performancesperformances were were studied studied with thewith same the output same couplers.output couplers. An HgCdTe An HgCdTedetector detector (PVM-10.6, (PVM-10.6, VigoSystem VigoSystem S. A) with a riseS. A) time with of 1.5 a nsrise and time an oscilloscope of 1.5 ns (DSO104A,and an oscilloscope Keysight) (DSO104A,with a bandwidth Keysight) of 1with GHz a werebandwidth utilized of to 1 measure GHz were thepulse utilized characters. to measure The thresholdthe pulse for characters. Q-switched The thresholdoperation for was Q-switched the same operation as that for was the CWthe same operation. as that Not for too the far CW above operation. the threshold, Not too stablefar above pulse the threshold,trains were stable observed pulse fortrains all three were OCs. observed The pulse for charactersall three OCs. were thenThe optimizedpulse characters by changing were thethen optimizeddistance by between changing the the center distance of the between mechanical the cent switcher of and the themechanical optical axes switch of the and cavity, the optical and the axes of theoptimal cavity, distance and the was optimal found distance to be ~50 was mm. found The to pulse be ~50 duration mm. The and pulse pulse duration energy and as functions pulse energy of as thefunctions pulse repetitionof the pulse frequency repetition for allfrequency three OCs for are all shownthree OCs in Figures are shown3 and 4in. WithFigures the 5%3 and and 4. 8% With thetransmission 5% and 8% OCs,transmission single pulses OCs, with single 40 ns pulses duration with and 40 30.8 ns µdurationJ energy, andand 3730.8 ns durationμJ energy, and and 46.1 37µ Jns durationenergy and were 46.1 obtained μJ energy at a 0.26were kHz obtained pulse repetition at a 0.26 kHz frequency pulse (correspondingrepetition frequency to the rotating(corresponding speed of to the rotating speed of 130 rounds per seconds and a slit number of 2) under a pump power of 3.2 W. With the increase in repetition frequency, the pulse duration increased while the pulse energy decreased. Nevertheless, pulse durations of less than 50 ns were obtained in the range of 0.26–4 kHz with both OCs. When further increasing the pump power, multi-pulse operation occurred. With the Appl. Sci. 2017, 7, 1201 4 of 7

130 rounds per seconds and a slit number of 2) under a pump power of 3.2 W. With the increase in Appl.repetition Sci. 2017, frequency,7, 1201 the pulse duration increased while the pulse energy decreased. Nevertheless,4 of 7 Appl.pulse Sci. durations 2017, 7, 1201 of less than 50 ns were obtained in the range of 0.26–4 kHz with both OCs. When4 of 7 20%further transmission increasing OC, the shorter pump power,pulse duration multi-pulse and operation larger pulse occurred. energy With were the achieved. 20% transmission This is because OC, 20% transmission OC, shorter pulse duration and larger pulse energy were achieved. This is because a shorterhigher pulseoutput duration coupling and can larger delay pulse the energy occurrence were achieved. of multi-pulse This is because operation a higher in mechanically output coupling Q- a higher output coupling can delay the occurrence of multi-pulse operation in mechanically Q- switchedcan delay lasers. the occurrenceIn this case, of multi-pulse multi-pulse operat operationion occurred in mechanically when the Q-switched pump power lasers. was Inabove this 10 case, W. switched lasers. In this case, multi-pulse operation occurred when the pump power was above 10 W. Undermulti-pulse 10 W of operation pump power, occurred the whenpulse theduration pump incr powereased was from above 27 10to W.38 ns, Under whereas 10 W the of pump pulse power,energy Under 10 W of pump power, the pulse duration increased from 27 to 38 ns, whereas the pulse energy decreasedthe pulse durationfrom 80.8 increased to 27.5 from μJ with 27 to 38increasing ns, whereas the the repetition pulse energy frequency decreased from from 0.26 80.8 to to 27.54 kHz,µJ decreased from 80.8 to 27.5 μJ with increasing the repetition frequency from 0.26 to 4 kHz, correspondingwith increasing to the~3 kW repetition peak power frequency at 0.26 from kHz 0.26 repetition to 4 kHz, frequency. corresponding The optical–optical to ~3 kW peak conversion power at corresponding to ~3 kW peak power at 0.26 kHz repetition frequency. The optical–optical conversion efficiencies0.26 kHz repetition in the Q-switched frequency. mode The optical–optical could be effe conversionctively improved efﬁciencies using in the a larger Q-switched plate modecarved could with efficiencies in the Q-switched mode could be effectively improved using a larger plate carved with morebe effectively slits but the improved same turn-off using a larger area and/or plate carved a moto withr with more higher slits but rotating the same speed turn-off to make area and/or the laser a more slits but the same turn-off area and/or a motor with higher rotating speed to make the laser operatemotor withat a higher rotatingrepetition speed rate. to makeImprovement the laser operatein the Q-switched at a higher repetition efficiencies rate. is Improvement also achievable in operate at a higher repetition rate. Improvement in the Q-switched efficiencies is also achievable throughthe Q-switched optimizing efﬁciencies the pump is also wavelength achievable to through avoid the optimizing excited state the pump absorption wavelength of the to upper avoid laser the throughexcited state optimizing absorption the ofpump the upper wavelength laser level to duringavoid the the excited pumping state process. absorption The short of the pulse upper durations laser levellevel during during the the pumpingpumping process.process. TheThe shortshort pulse durations achievedachieved inin ourour experimentexperiment are are a a achieved in our experiment are a consequence of the good energy storage of Er:Y2O3, in which losses consequence of the good energy storage of Er:Y2O3, in which losses of excited ions on the upper laser consequenceof excited ions of onthe the good upper energy laser storage level caused of Er:Y by2O3 multiphonon, in which losses relaxation of excited and ions spontaneous on the upper emission laser level caused by multiphonon relaxation and spontaneous emission are small. levelare small. caused by multiphonon relaxation and spontaneous emission are small.

FigureFigureFigure 3. 3.3. Pulse PulsePulse duration duration duration vs.vs. vs. pulsepulse pulse repetitionrepetition repetition fr frequencyequency withwith different different output outputoutput couplers. couplers.couplers.

Figure 4. Pulse energy vs. pulse repetition frequency with different output couplers. FigureFigure 4.4. PulsePulse energy energy vs. vs. pulse pulse repetition repetition freq frequencyuency with different outputoutput couplers.couplers. Figures 5 and 6 show the typical single pulse profile and pulse train recorded at 0.26 kHz Figures 5 and 6 show the typical single pulse profile and pulse train recorded at 0.26 kHz repetitionFigures rate5 and with6 show the 20% the transmission typical single OC pulse unde profiler 10 andW of pulse pump train power. recorded The atpulse 0.26 profile kHz repetition shows a repetition rate with the 20% transmission OC under 10 W of pump power. The pulse profile shows a fairlyrate with symmetric the 20% shape transmission and the time OC underspacing 10 betwee W of pumpn two adjacent power. Thepulses pulse remains profile nearly shows fixed a fairly for fairlythesymmetric given symmetric repetition shape shape and rate the and with time the no spacing time noticeable spacing between timing betwee two jitter. adjacentn two The adjacent pulses amplitude remains pulses fluc nearlytuationsremains fixed werenearly for estimated the fixed given for thetorepetition givenbe less repetition than rate 10%. with rate no noticeablewith no noticeable timing jitter. timing The jitter. amplitude The amplitude fluctuations fluc weretuations estimated were toestimated be less tothan be less 10%. than 10%. Appl. Sci. 2017, 7, 1201 5 of 7 Appl. Sci. 2017, 7, 1201 5 of 7 Appl.Appl. Sci. Sci. 2017 2017, 7,, 7 1201, 1201 5 5of of 7 7

Figure 5. Typical pulse profile recorded at 0.26 kHz pulse repetition frequency (PRF) under 10 W of Figure 5. Typical pulse profile recorded at 0.26 kHz pulse repetition frequency (PRF) under 10 W of absorbedFigureFigure 5. 5. TypicalpumpTypical power. pulse pulse profile proﬁle recorded recorded at at 0.26 0.26 kHz pu pulselse repetition frequencyfrequency (PRF)(PRF) underunder 1010 W W of of absorbed pump power. absorbedabsorbed pump pump power. power.

FigureFigureFigure 6. 6. 6.Pulse PulsePulse train train train recorded recorded recorded at at 0.26 0.26 kHzkHz PRPR PRFF under under 10 W of absorbed absorbed pumppump power. power. Figure 6. Pulse train recorded at 0.26 kHz PRF under 10 W of absorbed pump power. TheThe optical optical spectrum spectrum of of the the Q-switched Q-switched Er:Y Er:Y22OO33 ceramicceramicceramic laser laser withwith the the 20% 20% transmission transmissiontransmission OC OC was was foundfoundThe to to opticalvary vary slightly slightlyspectrum with with of the the Q-switchedrepetition repetition rate rate Er:Y or or2O pump pump3 ceramic level.level. laser Figure Figure with 7 77the shows shsh 20%owsows the transmissionthethe typical typicaltypical emissionemission emission OC was wavelengthfoundwavelength to vary measured measured measuredslightly with by byby a a Fouriera the FourierFourier repetition transform transformtransform rate spectrum or spectrumspectrum pump analyzer level. analyzer withFigure a withwith resolution 7 sh aowsa resolutionresolution ofthe 7.5 typical GHz ofof (OSA205,7.5emission7.5 GHzGHz (OSA205,wavelength(OSA205,Thorlabs, Thorlabs, Newton,Thorlabs, measured Newton, NJ, Newton, by USA). a NJ, FourierNJ, It wasUSA). USA). centered transform It It waswas at centeredcentered 2716.5spectrum nm at 2716.5 with analyzer a nmnm linewidth withwithwith aa oflinewidthalinewidth resolution 0.12 nm. of of 0.12 0.12of 7.5nm. nm. GHz (OSA205, Thorlabs, Newton, NJ, USA). It was centered at 2716.5 nm with a linewidth of 0.12 nm.

Figure 7. Spectrum of the Q-switched Er:Y2O3 laser. FigureFigure 7. 7. SpectrumSpectrum of of the the Q-switched Er:Y22O3 laser. Figure 7. Spectrum of the Q-switched Er:Y2O3 laser. 4. Conclusions 4. 4.Conclusions Conclusions 4. ConclusionsIn summary, we demonstrated a short-pulse-width repetitively Q-switched Er:Y2O3 ceramic In summary, we demonstrated a short-pulse-width repetitively Q-switched Er:Y2O3 ceramic laser Inat summary,~2.7-μm using we demonstrated a specially adesigned short-pulse-width mechanical repetitively switch. Stable Q-switched pulse Er:Ytrains2O with3 ceramic 27–38 laser ns laser Inat summary,~2.7-µ μm using we demonstrated a specially designed a short-pu mechanicallse-width switch. repetitively Stable Q-switched pulse trains Er:Y with2O 327–38 ceramic ns durationsat ~2.7- m and using energies a specially of 80.8–27.5 designed μJ were mechanical obtained switch. with a Stable pulse pulse repetition trains frequency with 27–38 within ns durations a range durationslaser at ~2.7- andμ energiesm using ofa 80.8–27.5speciallyµ μdesignedJ were obtained mechanical with aswitch. pulse repetitionStable pulse frequency trains withinwith 27–38 a range ns ofand 0.26 energies to 4 kHz. of 80.8–27.5A peak powerJ were of ~3 obtained kW was with realized a pulse at 0.26 repetition kHz. The frequency results presented within a range here reveal of 0.26 the to durations and energies of 80.8–27.5 μJ were obtained with a pulse repetition frequency within a range ofpotential4 0.26 kHz. to A4 peakkHz.of Er-doped powerA peak of sesquioxidepower ~3 kW of was ~3 ceramikW realized wascs atrealizedin 0.26 generating kHz. at 0.26 The short kHz. results pulses The presented results at ~2.7- presented hereμm. reveal Improvements here the potentialreveal inthe of 0.26 to 4 kHz. A peak power of ~3 kW was realized at 0.26 kHz. The resultsµ presented here reveal the potentialtermsof Er-doped of of pulse Er-doped sesquioxide energy sesquioxideshould ceramics be possible cerami in generating bycs furtherin generating short optimizing pulses short atthe pulses ~2.7- transmissionm. at Improvements~2.7- μofm. the Improvements output in termscoupler. of in termspotentialpulse of energy pulse of Er-doped energy should should besesquioxide possible be possible by cerami further bycs further optimizingin generating optimizing the short transmission the pulses transmission at of ~2.7- the outputμ ofm. the Improvements coupler.output coupler. in terms of pulse energy should be possible by further optimizing the transmission of the output coupler. Appl. Sci. 2017, 7, 1201 6 of 7

Acknowledgments: This work was supported by the National Science Foundation of China (NSFC) (61177045, 11274144), NSAF (U1430111), and a project funded by the Priority Academic Development of Jiangsu Higher Education Institutions (PAPD). Author Contributions: Jian Zhang, Dingyuan Tang and Deyuan Shen conceived and designed the experiments; Xiaojing Ren and Yong Wang performed the experiments, analyzed the data, and wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Guidotti, R.; Merigo, E.; Fornaini, C.; Rocca, J.P.; Medioni, E.; Vescovi, P. Er:YAG 2,940-nm laser fiber in endodontic treatment: A help in removing smear layer. Lasers Med. Sci. 2014, 29, 69–75. [CrossRef][PubMed] 2. Zhao, Y.; Yin, Y.; Tao, L.; Nie, P.; Tang, Y.; Zhu, M. Er:YAG laser versus scaling and root planing as alternative or adjuvant for chronic periodontitis treatment: A systematic review. J. Clin. Periodontol. 2014, 41, 1069–1079. [CrossRef][PubMed] 3. Ayerden, N.P.; Mandon, J.; Ghaderi, M.; Harren, F.J.; Wolffenbuttel, R.F. A photonic microsystem for hydrocarbon gas analysis by mid-infrared absorption spectroscopy. In Proceedings of the 2017 IEEE 30th International Conference on Micro Electro Mechanical Systems (MEMS), Las Vegas, NV, USA, 22–26 January 2017; pp. 1052–1055. 4. Gauthier, J.C.; Fortin, V.; Carree, J.Y.; Poulain, S.; Poulain, M.; Vallee, R.; Bernier, M. Mid-IR supercontinuum from 2.4 to 5.4 µm in a low-loss fluoroindate fiber. Opt. Lett. 2016, 41, 1756–1759. [CrossRef][PubMed] 5. Robichaud, L.R.; Fortin, V.; Gauthier, J.C.; Chatigny, S.; Couillard, J.F.; Delarosbil, J.L.; Vallee, R.; Bernier, M.

Compact 3–8 µm supercontinuum generation in a low-loss As2Se3 step-index fiber. Opt. Lett. 2016, 41, 4605–4608. [CrossRef][PubMed] 6. Vodopyanov, K.L.; Makasyuk, I.; Schunemann, P.G. Grating tunable 4–14 µm GaAs optical parametric oscillator pumped at 3 µm. Opt. Express 2014, 22, 4131–4136. [CrossRef][PubMed] 7. Frolov, M.P.; Korostelin, Y.V.; Kozlovsky, V.I.; Podmar’kov, Y.P.; Savinova, S.A.; Skasyrsky, Y.K. 3 J pulsed Fe:ZnS laser tunable from 3.44 to 4.19 µm. Laser Phys. Lett. 2015, 12, 055001. [CrossRef] 8. Zajac, A.; Skorczakowski, M.; Swiderski, J.; Nyga, P. Electrooptically Q-switched mid-infrared Er:YAG laser for medical applications. Opt. Express 2004, 12, 5125–5130. [CrossRef][PubMed] 9. Högele, A.; Hörbe, G.; Lubatschowski, H.; Welling, H.; Ertmer, W. 2.70 µm CrEr:YSGG laser with high output energy and FTIR-Q-switch. Opt. Commun. 1996, 125, 90–94. [CrossRef] 10. Maak, P.; Jakab, L.; Richter, P.; Eichler, H.J.; Liu, B. Efficient acousto-optic Q switching of Er:YSGG lasers at 2.79-µm wavelength. Appl. Opt. 2000, 39, 3053–3059. [CrossRef][PubMed] 3+ 11. Wyss, C.H.R.; Luthy, W.; Weber, H.P. Modulation and single-spike switching of a diode-pumped Er :LiYF4 laser at 2.8 µm. IEEE J. Quantum Electron. 1998, 34, 1041–1045. [CrossRef] 12. Qin, Z.; Xie, G.; Zhang, H.; Zhao, C.; Yuan, P.; Wen, S.; Qian, L. Black phosphorus as saturable absorber for the Q-switched Er:ZBLAN fiber laser at 2.8 µm. Opt. Express 2015, 23, 24714–24718. [CrossRef][PubMed] 13. Shen, Y.; Wang, Y.; Luan, K.; Huang, K.; Tao, M.; Chen, H.; Yi, A.; Feng, G.; Si, J. Watt-level passively Q-switched heavily Er3+-doped ZBLAN fiber laser with a semiconductor saturable absorber mirror. Sci. Rep. 2016, 6, 26659. [CrossRef][PubMed] 14. Qiao, X.B.; Huang, H.T.; Yang, H.; Zhang, L.; Wang, L.; Shen, D.Y.; Zhang, J.; Tang, D.Y. Fabrication, optical 3+ properties and LD-pumped 2.7 µm laser performance of low Er concentration doped Lu2O3 transparent ceramics. J. Alloys Compd. 2015, 640, 51–55. [CrossRef]

15. Sanamyan, T. Diode pumped cascade Er:Y2O3 laser. Laser Phys. Lett. 2015, 12, 125804. [CrossRef] 16. Krankel, C. Rare-Earth-Doped Sesquioxides for Diode-Pumped High-Power Lasers in the 1-, 2-, and 3-µm Spectral Range. IEEE J. Sel. Top. Quantum Electron. 2015, 21, 1602013. [CrossRef] 3+ 17. Sanamyan, T.; Simmons, J.; Dubinskii, M. Er -doped Y2O3 ceramic laser at ~2.7 µm with direct diode pumping of the upper laser level. Laser Phys. Lett. 2010, 7, 206–209. [CrossRef] 18. Ikesue, A.; Aung, Y.L. Ceramic laser materials. Nat. Photonics 2008, 2, 721–727. [CrossRef] 19. Kong, L.C.; Qin, Z.P.; Xie, G.Q.; Guo, Z.N.; Zhang, H.; Yuan, P.; Qian, L.J. Black phosphorus as broadband saturable absorber for pulsed lasers from 1 µm to 2.7 µm wavelength. Laser Phys. Lett. 2016, 13, 045801. [CrossRef] Appl. Sci. 2017, 7, 1201 7 of 7

20. Ren, X.J.; Wang, Y.; Fan, X.L.; Zhang, J.; Tang, D.Y.; Shen, D.Y. High-peak-power acousto-optically Q-switched

Er:Y2O3 ceramic laser at ~2.7 µm. IEEE Photonics J. 2017, 9, 1–6. [CrossRef] 21. Skórczakowski, M.; Pichola, W.; Šwiderski, J.; Nyga, P.; Galecki, L.; Maciejewska, M.; Kasprzak, J. 30 mJ,

TEM00, high repetition rate, mechanically Q-switched Er:YAG laser operating at 2940 nm. Opto-Electron. Rev. 2011, 19, 206–210. [CrossRef] 22. Murphy, F.J.; Arbabzadah, E.A.; Bak, A.O.; Amrania, H.; Damzen, M.J.; Phillips, C.C. Optical chopper Q-switching for ﬂashlamp-pumped Er,Cr:YSGG lasers. Laser Phys. Lett. 2015, 12, 045802. [CrossRef]

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